LIBRARY Connecticut Agricultural College
|
Vol 2.4:5 3^ |
|
|
Class No. ff y 0 |
//9y7 |
|
Cost |
|
|
Date Ijal . 1 'L |
19 1 |
..■fF
BOOK 570.H917 c. 1
HUNTER # NEW CIVIC BIOLOGY
3 T1S3 001373bM fl
r< /
This Book may be kept out
TWO WEEKS
only and is subject to a fine of TWO GENTS a day thereafter. It will be due on the day indicated below.
JAN 3 191^
Digitized by the Internet Archive in 2013
http://archive.org/details/newcivicbiologypOOhunt
Farmhouse and favorable environment in the coiintrv,
Favorable city environment : two-family houses in a residential section of
large city.
IV
NEW CIVIC BIOLOGY
Presented in Problems
BY
GEORGE WILLIAM HUNTER, Ph.D.
ADJUNCT PROFESSOR OF BIOLOGICAL SCIENCE
POMONA COLLEGE, CLARBMONT, CALIFORNIA
FORMERLY UEAD OP THE DEPARTMENT OF BIOLOGY DEWITT CLINTON
HIGH SCHOOL, NEW YORK; LATER PROFESSOR OF BIOLOGY
KNOX COLLEGE, GALESBURG, ILLINOIS
AMERICAN BOOK COMPANY
NEW YORK CINCINNATI CHICAGO
BOSTON ATLANTA
Copyright, 1914, by GEORGE WILLIAM HUNTER.
Copyright, 1926, by AMERICAN BOOK COMPANY.
HUNTER, NEW CIVIC BIOLOGY.
X(,X3S-
MADE IN U.S.A.
FOREWORD
DuKiNG the past few years, the views of educators on the place of biological science in the secondary school have become more definite. The report of the Committee of the National Education Association on the ^'Reorganization of Science in Secondary Schools " is beginning to make its impress on the minds of thinking teachers of science. The junior high school movement, with the accompanying improvement in the teaching of elementary science, is giving a background of science phenomena that the children of a decade ago did not have. Health teaching and environmental science teaching have produced certain fundamental science con- cepts on which a course in biological science may be built.
While the place of biological science is not fixed in all parts of this country, the tendency is well marked to place it in the tenth year of the school curriculum.^ Recognizing this, New Civic Biology has attempted to build on the unorganized science facts that the high school pupil has already assimilated. The trend toward better health and citizenship building has been recognized, and it is hoped that this book will work toward the ideal develop- ment of efficient, thinking citizens.
A course in biology in the secondary school must be determined by other factors than the mere training of the teacher. It must reach the capabilities of the student, it must appeal to his interest ; it must interpret his environment. And most important of all, it must, by means of the vehicle of the problem and the project, train him in the technique of thinking. Ideas, not types, should be the ultimate development of the laboratory work. As Dr. Walter so well points out, the environmental conditions are often more important than the type. Most important, too, are the
^ See " The Place of Science in the Secondary School," G. W. Hunter, School Review, May-June, 1925.
vii
viii FOREWORD
applications of modern biology in the daily life of the student, for it is this sort of work that has the greatest appeal.
The chief difficulty is not so much in knowing what to teach as in knowing what not to teach. The topics included in this book are those considered most vital in a well-rounded course in ele- mentary biology directed toward civic betterment. The physio- logical functions of plants and animals, the hygiene of the indi' vidual within the community, conservation and the betterment of plant and animal products, the big underlying biological concepts on which societ}^ is built, have all been used to the end that the pupil rudij become a better, stronger, and more unselfish citizen.
At the beginning of each of the following chapters (except the first and the last) are a series of suggested problems. These should serve as a review of the chapter for both teacher and stu- dent. They constitute, to a degree, the ke}^ which opens the way to the understanding of the chapter. Following the problems are laboratory suggestions, many of which are worked out fully in Hunter's Laboratory Problems in Civic Biology.
At the end of each chapter is a list of books which have proved their usefulness either as problem and project references for stu- dents or as aids to the teacher. Most of the books mentioned are within the means of the small school.
For a general introduction to physiological biology, Sedgwick and Wilson, General Biology, Henr}^ Holt and Company ; Needliam, General Biology, Comstock Publishing Company ; and Shull, Prin- ciples of Animal Biology, The IMcGraw Hill Company, are most useful and inspiring books.
One book stands out from the pedagogical standpoint as most helpful. It is Twiss, Principles of Science Teaching, The Mac- millan Company. Other books of value from the teacher's stand- point are Curtis, Investigations in the Teaching of Science, P. Blakiston's Son and Compam^ ; Frank, " How to Teach General Science, P. Blakiston's Son and Company ; Hodge, Nature Study and Life, Ginn and Company ; Downing, Teaching Science in the Schools, University of Chicago Press ; Brownell and Wade, The Teaching of Science, The Century Company ; Ganong, The Teach- ing Botanist, The Macmillan Company; and Eikenberry, The Teaching of General Science, University of Chicago Press.
FOREWORD ix
Every biology teacher should also have access to the Science News Letter, published by Science Service, School Science and Mathematics, Turton and Warner, Chicago, and the General Science Quarterly, W. G. Whitman, Salem, Mass., as all these pub- lications contain much of value to the secondary school teacher of science.
The sincere thanks of the author are extended to all who by advice and suggestions have helped make this book possible. The following have read the manuscript in its entirety : Annah P. Hazen, Head of the Department of Biology in the Eastern Dis- trict High School, City of New York ; Leshe L. Hunt, Department of Biology, Galesburg High School, Galesburg, 111. ; George W. Hunter, III, Assistant in Biology, University of Illinois, Urbana, 111. ; Rosemary F. Mullen, Head of the Department of Biology, Washington Irving High School, City of New York; Mary E. Robb, Department of Biology, Hyde Park High School, Chicago, 111. ; Jesse M. Shaver, Department of Biology, Peabody College, Nashville, Tenn. ; Professor A. C. Walton, Department of Biology, Knox College; and Dr. Frank M. Wheat, Head of the Depart- ment of Biology, George Washington High School, City of New York. Dr. Henry B. Ward, Head of the Department of Zoology of the University of Illinois, and Mr. Robert Sterling Yard, Executive Secretary of the National Parks Association, have also offered valuable suggestions on certain parts of the book.
Thanks are due, also, to Professor E. B. Wilson, Mr. William C. Barbour, Dr. John A. Sampson, W. C. Stevens, and Wilham Beebe; to the United States Department of Agriculture; the New York Aquarium; the Charity Organization Society; and especially to the American Museum of Natural History, for per- mission to copy and use certain photographs and cuts which have been found useful in teaching. Dr. Frank M. Wheat made most of the line drawings prepared for this book and has given many valua,ble suggestions.
CONTENTS
CHAPTER PAGE
I. The General Problem — Some Reasons for the Study
OF Biology ......... 1
PART I. LIVING THINGS IN RELATION TO THEIR ENVIRONMENT AND TO EACH OTHER
11. The Environment of Plants and Animals ... 7 HI. Living Things and the Environment .... 13 IV. The Interrelations of Plants and Animals . . 24
PART II. LIFE PROCESSES IN LIVING THINGS. GREEN PLANTS AS LIVING ORGANISMS
V. The Building Material of Living Things ... 43
VI. Plant Growth and Nutrition. Causes of Growth . 51 VII. Organs of Nutrition. Roots in Relation to the
Soil 63
VIII. How Green Plants Make Food 75
IX. The Circulation and Final Uses of Food by Plants 87
PART HI. GENERAL RELATIONS BETWEEN PLANTS AND
ANIMALS
X. The Simplest Organisms 95
XI. The Relations of Plants to Animals .... 102
PART IV. ANIMALS AS LIVING ORGANISMS
XII. Animal Organisms. The Human Machine . . . 107
XIII. A Study of Foods and Dietaries 118
XIV. Dangers from Food Adulteration, Alcohol, and
Drugs 138
XV. How Food is Prepared for Body Uses . . . 146
XVI. The Blood and its Circulation 162
XVII. Respiration and Excretion 177
xii CONTENTS
PART V. RESPONSES IN PLANTS AND ANIMALS
CHAPTER PAGE
XVIII. How Body Control is Brought About. . . . 192 XIX. How Habits are Formed 208
PART VI. REPRODUCTION AND CLASSIFICATION
XX. Reproduction in Plants and Animals . . . .219 XXL Classification of Plants and Animals .... 234
PART VII. MAN'S CONTROL OF HIS ENVIRONMENT IN RELATION TO HEALTH
XXII. Bacteria and Disease 253
XXIII. How We Fight Bacterial Diseases .... 268
XXIV. The Relations of Animals to Disease .... 278 XXV. Man's Improvement of his Environment . . . 292
PART VIII. MAN'S CONTROL OF HIS ENVIRONMENT IN RELATION TO WEALTH
XXVI. Our Forests, their Uses and Need of Protection . 313 XXVII. The Value of Green Plants to Man . . . .323 XXVIII. Plants without Chlorophyll in their Relation to
Man 334
XXIX. The Economic Importance of Animals .... 347 XXX. Conservation and its Lessons 368
PART IX. MAN'S CONTROL OF HIS ENVIRONMENT IN RE- LATION TO EVIPROVEMENT OF PLANTS AND ANIMALS
XXXI. Plant and Animal Breeding 380
XXXII. The Improvement of the Human Race . . . 394 XXXIII. Some Great Names in Biology 405
Glossary . . . • 415
Weights, Measures, Laboratory Equipment . . 429 Index 431
A NEW CIVIC BIOLOGY
CHAPTER I
THE GENERAL PROBLEM — SOME REASONS FOR THE STUDY OF BIOLOGY
The Study of Biology. — The word hioVogy comes from two Greek words, hios (Hfe) and logos (word or study). Biology, then, is the study of things that are aHve, both plants and animals. And since man is the highest and most important of all living creatures, it is only fitting that emphasis should be placed chiefly on the science underlying man's health and well-being.
Biology is a modern science ; it has found its way into most high schools, and an. increasingly large number of girls and boys are engaged in its study every year. These questions might well be asked by any of the students : Why do I take up the study of biology? Of what practical value is it to me? Besides the dis- cipline it gives me, is there anything that I can take away which will help me in my future life ?
Knowing about Nature is Worth While. — Most of us know something about biology. We are constantly meeting or playing with or collecting living things. We visit the '^ zoo," we have pets, or gardens, we read the papers and magazines ; thus we have some knowledge. But this knowledge is often not very accurate.^ It is worth while from the standpoint of pleasure in one's life to^ know a little about the varied forms of life that one may meet on a walk in the fields or a stroll along the ocean beach. Even for the pleasure it gives us, we should study biology.
Physiology and Hygiene. — If the study of biology will give us a better understanding of our own bodies and their care, then it cer- tainly is of use to us. That phase of biology known as physioVogy deals with the uses of the parts of a plant or an animal ; human physiology and hygiene (hi'ji-en) deal with the uses and care of the parts of the human body. The prevention of sickness is due in a large part to the study of hygiene. It is estimated that over
1
REASONS FOR THE STUDY OF BIOLOGY
twenty-five per cent of the deaths that occur yearly in this country could be averted if all people lived in a hygienic manner. In its application to the life of each of us, as a member of a family,
as a student in school, and as a future citizen, a knowl- edge of hygiene is of the greatest importance.
The Uses of Plants to Man. — But there are other reasons why an educated person should know some- thing about biology. We do not always realize that if it were not for the green plants, there would be no animals on the earth. There is a wonderful bal- ance of life on the earth, maintained by the energy of the sun. We shall see later that green plants, like factories, turn raw mate- rials into food products and thus furnish food to ani- mals. Even the meat-eat- ing animals feed upon those that feed upon plants. Plants furnish man with the greater part of his food in the form of grains and cereals, fruits and nuts, edible roots and leaves; they provide his domesti- cated animals with food ; they give him timber for his houses and wood and coal for his fires ; they pro\dde him with pulp wood, from which he makes his paper, and oak galls, from which he can make ink. Some of man's clothing and the thread with which it is sewed together come from
Blossoms and bolls of the cotton plant. More cloth is made of cotton than of any other fiber.
IMPORTANCE OF PLANTS AND ANIMALS 3
fiber-producing plants. Most medicines, beverages, flavoring ex- tracts, and spices are plant products, while plants are used in hun- dreds of ways in the arts and trades, yielding varnishes, dyestuffs, rubber, and other products.
Bacteria in their Relation to Man. — In still another way, cer- tain plants vitally affect mankind. Tiny plants, called bacteria, so small that millions can exist in a single drop of fluid, exist almost everywhere about us, — in water, soil, food, and air. They play a tremendous part in shaping the destiny of man on the earth. They help him in that they act as scavengers, causing things to decay ; thus they remove the dead bodies of plants and animals from the surface of the earth, and turn tliis material back to the ground ; they assist the tanner ; they help make cheese and butter ; they improve the soil for crop growing, so the farmer can- not do without them. But likewise they sometimes spoil our meat and fish, our vegetables and fruits ; they also sour our milk, and may make our canned goods spoil. Worst of all, they cause many diseases, such as typhoid, tuberculosis, pneumonia, and colds. It is estimated that half the deaths that occur each year are caused by these plants. So important are the bacteria that a subdivision of biology, called hacterioV ogy , has been named for them, and hundreds of scientists are devoting their lives to the study of bac- teria and their control. The greatest of all bacteriologists, Louis Pasteur (pas-ttir')/ once said, ''It is within the power of man to cause all parasitic diseases [most of which are caused by bacteria] to disappear from the world." His prophecy is gradually being fulfilled. Each year sees some disease such as diphtheria, typhoid, or scarlet fever conquered or brought under better control through scientific knowledge, and it may be the lot of some boys or girls who read this book to do their share in such work.
The Harm done by Some Animals. — Animals also play an im- portant part in the world in causing and in carrjdng diseases. Ani- mals that cause disease are usually tiny and live in other animals as parasites; that is, they get their living from the hosts on which they feed and in so doing may cause disease or the death of the hosts. Among the diseases caused by parasitic animals are ma- laria, yellow fever, sleeping sickness, and the hookworm disease.
1 The diacritic marks are those used in the Webster school dictionaries.
4 REASONS FOR THE STUDY OF BIOLOGY
Animals also carry disease germs ; flies, mosquitoes, rats, and cats are well known as spreaders of diseases.
From a monetary standpoint, insects do much harm. It is esti- mated that in this country alone in 1924 they were responsible for considerably over $2,000,000,000 worth of damage by eating crops, stored food, and other things.
The Uses of Animals to Man. — We all know the uses man has made of the domesticated animals for food and as beasts of
Cows in a model stable, with milking machines in operation. There are many state laws and city ordinances for the regulation of dairies and the sale and distribution of milk.
burden. But many other uses are found for animal products, and materials made from animals. Wool, furs, leather, hides, feathers, and silk are examples. The arts make use of ivory, tor- toise shell, corals, and mother-of-pearl ; from animals come per- fumes and oils, glue, and various other commodities.
Relations of Plants and Animals. — Most plants and animals stand in a relation of mutual helpfulness, plants providing food and shelter for animals, and animals giving off waste materials useful to plants in the making of food. We also learn that plants and animals need the same conditions in their surroundings in order to live : water, air, food, a favorable temperature, and usually
KELATIONS OF PLANTS AND ANIMALS 5
light. The Hfe processes of both plants and animals are essentially the same, and the living matter in a tree is as much alive as is the living matter in a fish, a dog, or a man.
Plants and animals are living things, taking what they can from their surroundings ; they enter into competition with one another, and those which are best fitted for life outstrip the others. Each kind of animal and plant tends to vary from its nearest relatives in all details of structure. The strong may hand down to their offspring characteristics which make them winners. Health and strength of body and mind are factors which tell in winning.
Man has made use of this message of nature, and has dei^eloped improved breeds of horses, cattle, and other domestic animals. Plant breeders, likewise, have selected plants or seeds that have shown a tendency to improve, and thus have raised liardier and more fruitful domesticated plants. Man's dominion over the liv- ing things of the earth is tremendous. This is due to his under- standing of the principles which underlie the science of biology.
Photo from Neiv York State Conservation Commission
How does the forest help the stream ?
The Importance of Forests. — Still another reason why we should study biolog;)^ is that we may work intelligently for the conserva- tion of our natural resources, especially of our forests. The living forest is valued for its beauty and its health-giving properties,
6 REASONS FOR THE STUDY OF BIOLOGY
and because it holds water in the earth. It keeps the water from drying out of the soil quickly on hot days and from running off quickly on rainy days. Thus a more even supply of water is given to our rivers, and freshets are prevented. Regions that have been deforested, such as parts of China, Italy, and France, are novv subject to floods and are barren in many places. On the forests depend our supply of timber, much of our water power, and the commercial importance of navigable rivers.
Biology in Relation to Society. — The study of biology also ought to make us better men and women by teaching us that unselfish- ness exists in the natural world as well as among the highest members of society. Animals, lowly and complex, sacrifice their comfort and their very lives for their young. In insect colonies the welfare of the individual is given up for the best interests of the community. The law of mutual give and take, of sacrifice for the common good, is seen everywhere. This should teach us, as we come to take our places in society, to be willing to give up our individual pleasure or selfish gain for the good of the com- munity in which we live. Thus the application of biological prin- ciples will benefit society.
Biology in Relation to Citizenship. — Finally, proper qualifica- tions for citizenship are the biggest return we uisij expect from our high school career. How can a man or woman become an intelligent voter without at least some understanding of biology? Think of the number of times the average voter is asked to ex- ercise judgment on matters concerning the health and wealth of the community. How can he do this without some knowledge of the facts ? And how can he exercise judgment if he has no previous training? Science, especially through experiments, helps us to make intelligent judgments. Through the projects and experi- ments you carry out, your ability to think straight as a citizen will be aided. And when we realize that in the last half century the application of biology to health has lengthened the average span of life ten years ; when we see that applications of biology in industry have made the life of the average workman far safer than before, we understand the need of real knowledge of applied biol- ogy. It is the purpose of this book to help the boys and girls of to-day to become healthy, intelligent, thinking voters of to-morrow
PART I. LIVING THINGS IN RELATION TO THEIR ENVIRONMENT AND TO EACH OTHER
CHAPTER II
THE ENVIRONMENT OF PLANTS AND ANIMALS
Problems : ^ To discover some of the factors of the environment of plants and animals.
To discover the chemical nature of the environment, and of plants and animals.
Laboratory Suggestions ^
Demonstration. The composition of the air. Demonstration. The separation of water into its elements. Laboratory experiment. To determine the solubility of different sub- stances.
Demonstration. Oxidation of carbon and the test for carbon dioxide.
Environment. — Each one of us, no matter where he lives, comes in contact with certain surroundings. Air is everywhere around us. Light and heat are necessary to us, so much so that we use artificial hght at night and artificial heat in winter. Out of doors we walk on the soil of a farm or a village, or on a city street, with its dirty and hard paving stones. Water and food are a necessary part of our surroundings. All these factors — air, light, heat, soil, water, food, and other things — together make up our environment.
But we must not think of the environment as simply the room in which we sit, or the town in which we live. Sunlight, on which our very existence depends, comes from the center of our solar system, 93,000,000 miles away. The water which we use in our kitchens may have been piped scores of miles to us : witness the water supplies of Los Angeles and of New York. The food we use
^ The Problems stated at the beginning of each chapter constitute, in a way, a key to the text of the chapter. The Laboratory Suggestions are to be used at the dis- cretion of the teacher.
7
ENVIRONMENT OF PLANTS AND ANIMALS
may have come from the deeps of the sea or the far-away tropics. Our environment, in a certain sense, includes anything that may affect us in that place where we happen to be; this, of course, includes all other living things, plant or animal, that may come in contact with us during our lives in a given locality.
All animals, and all plants as well, are surrounded by and use the factors of their environment. In order to live, the potted plant in the window, the goldfish in the aquarium, your pet dog at home, all need air, water, light, a certain amount of heat, and food. The Physicist's and Chemist's View of the Environment. — Most of us have had some introduction to science and know that
water, air, soil and rocks, the bodies of living things, in short, anything that occupies space, is called by the physicist matter. The chemist in his turn resolves all matter into ninety-odd sim- ple substances called chemical elements. We know that air surrounds us, that it has a pres- sure of 15 pounds to the square inch at sea level, and that, as we go up from the earth's sur- face, there is less and less air, until it ultimately disappears at a height of about 200 miles from the earth's surface. We know also that the air is composed chiefly of two elements, oxygen (ok'si-jen) and nitrogen (ni'tr6-jen), there being about one part of oxygen to four of nitrogen. Air is a mixture of these gases, together with water vapor, carbon dioxide (a chemical compound), and other gases in very small proportions.
Again, by means of the apparatus shown on page 9, the chem- ist and physicist, working together, have proved that water is a combination of the chemical elements oxygen and hydrogen (hi'dr6- jen). In this case, however, the elements are bound together so closely that they form a substance called water. This substance, which is always composed of a definite proportion of two parts of
Experiment to show the amount of oxy- gen in the air. A before, and B after the phosphorus p is lighted. The white fumes formed by the combination of oxygen with the burning phosphorus settle and are dissolved in the water. How high does the water rise in the jar? Why?
PHYSICIST'S AND CHEMIST'S VIEW
9
hydrogen to one part of oxygen, is expressed by the formula H2O by the chemist and is called a chemical compound.
Both oxygen and hydrogen are colorless, tasteless, and odorless, but hydrogen differs from oxygen by igniting with a slight explosion when it is mixed with a little air and a burning match or splinter is introduced in it. As it burns, drops of water are formed, show- ing that the hydrogen is uniting again with oxygen to form water. Hydrogen has a great chemical affinity for other elements ; hence it is usually found in nature combined with other ele- ments, as, for example, with oxygen in water.
The Place of Water in the Environment. — Water, in the form of rain, snow, or ice, or in river, lake, or sea, forms a very important part of our environment. It car- ries soil or mineral sub- stances, sometimes as sedi- ment, sometimes in solution. The water of the ocean holds salts in solution. If sea water is boiled until it is all evaporated, the salts will remain. If water is poured over them, they will dissolve or become solutes (s6-lutsO . In other words, the salts become divided into very minute particles which distribute them- selves through the liquid. But there are great differences in the solubility of substances. Some, like common salt or sugar, are very soluble ; others, such as lime and iron, very insoluble. Pure water containing no solutes is rarely found in nature.
Oxidation. — Oxygen has the very important property of unit- ing with many other substances. The chemical union of oxygen with another substance is called oxida'tion. Rapid oxidation pro- duces a flame or light. Oxidation, either rapid or slow, may take place wherever there is uncombined oxygen. This fact has great significance in the understanding of important problems of biol- ogy. An example of slow oxidation is seen in the rusting of an iron nail. If the rust and nail are weighed, the total weight
Apparatus for separating water into hydro- gen H and oxygen O: c, copper wire, p, plati- num wire soldered to the copper, with insula- tion so that no copper is exposed in this tube. A few drops of sulphuric acid should be added to the water, to facilitate the action of the electric current.
10 ENVIRONMENT OF PLANTS AND ANIMALS
will be more than that of the original nail. Do you see why? Rust is iron oxide and is formed by the union of iron and oxygen. The slow oxidation of many chemical compounds is constantly taking place in nature and is a part of the process of decay and of brealdng down of complex materials into simpler forms.
The Composition of the. Soil. — The covering of the earth is composed of a mantle of soil and rock. The rock, by the work of wind, frost, heat, water, plants, and animals, gradually breaks down into small bits to form the soil mantle. This is inorganic soil, which is formed usually of several of the elements found in rocks, such as cal'cium, so'dium, magne'sium, siVicon, yotas'sium, and iron, all combined with oxygen.
A visit to the woods or to a well-kept garden shows us that there is another kind of soil than the inorganic soil just mentioned. This
is the rich, dark soil containing hu'mus. Humus is made up largely of dead organic matter, the decayed remains of plants and animals. If we could test the chemical elements to be found in humus, we should find nitrogen, hydrogen, oxygen, and also car- bon, an important element found in all organic matter.
Carbon. — Carbon is found in many conditions in nature. It is found in the bodies of plants and animals, and in coal (fossil plants), and it exists in a nearly pure state in the diamond. The presence of carbon can usually be detected by partly burning or charring a substance ; if carbon is present, some of it remains as a black substance without taste or odor. Carbon may be col- lected by allowing a candle flame to burn in contact with the under side of a sheet of glass. The black deposit is almost pure carbon. The Result of the Oxidation of Carbon. — We have seen that a candle contains carbon. We can also easily show that part of this carbon unites with the oxygen of the air when a candle burns. If a candle is burned in a closed jar it soon goes out. If we now
Percentages of chemical elements in the human body.
COMPOSITION OF PLANTS AND ANIMALS 11
|
In |
In |
|
Sea |
Blood |
|
Wafer |
Serum |
test the gas in the closed jar by shaking it with Hme water, a color- less liquid/ we find that a milky white precipitate is formed. This indicates the presence of a gas called carbon dioxide (CO2), which is formed by the union of one part of carbon with two of oxygen of the air. If lime- water is shaken in a jar of fresh air little or no precipitate will be formed. Most of the car- bon dioxide, therefore, must have come from the burning candle.
The Composition of Plants ^
and Animals. — The soil and /yf /?»
Calcium^i
Sodium
Potass'm
Chlorine
other things in nature are largely composed of chemical compounds, a few like water, iron rust, and table salt being simple, but the greater number being very complex. Rocks, humus, organic food sub- stances, and the bodies of plants and animals are all composed of chemical compounds. Pro- fessor H. F. Osborne of Colum- bia University has pointed out that the chemical substances found in sea water agree very closely with those in the human ^J;^^ blood. There are other facts Rfomjnfi. also which prove that the same
5O4
3059
3.79
5527
Uh
39.00
45.0
12.0
Sodium
-Ma^nesiuw
Kdlcium
^Potdssium
Chlorine
CO3
P.Os
0^
Percentages of chemical substances in sea chemical elements found in the water and in blood serum. (After Osborn.)
environment somehow or other
become organized into the tremendously complex stuff of which
we find living plants and animals composed. These elements
^Limewater can be made by shaking a piece of quicklime the size of your fist in about two quarts of water. Filter the limewater and keep it in bottles well corked.
12 ENVIRONMENT OF PLANTS AND ANIMALS
are principally carbon, hydrogen, oxygen, and nitrogen with ten or more others in very minute proportions. It is logical to believe that living things use the chemical elements in their surroundings and in some wonderful manner build up their own bodies from the materials found in their environment. How this is done we shall learn in later chapters.
Summary. — This chapter has been in the nature of a review of your elementary science ; but it should give you a slightly differ- ent view of the environment because it is considered from the standpoint of living things. We have seen that the chemist's elements and compounds, which give us the factors of the environ- ment, air, water, soil, and food, become a part of living things, which, when they die, are decomposed to form a part of the soil. How this is and why, future chapters will explain.
Problem Questions
1 . What are the factors of thb environment ? Why are they so called ?
2. Are there any factors in the environment which are unnecessary to ani- mals ? To plants ?
3. How are compounds formed? How broken up?
4. Compare rapid and slow oxidation in all respects.
5. Name some compounds found in soil. In water.
6. What is meant by solution ?
7. What proof have we that living things use the factors of their environ- ment?
Problem and Project References
Hunter, Laboratory Problems in Civic Biology. American Book Company. Hunter and Whitman, Civic Science in Home and Community. American
Book Company. Broadhurst, Home and Community Hygiene, Chap. XVI. J. B. Lippincott
Company. Burkett, Stevens, and Hill, Agriculture for Beginners. Ginn and Company. Brigham and McFarlane, Essentials of Geography. • American Book Company. Weed, Chemistry in the Home American Book Company.
CHAPTER III LIVING THINGS AND THE ENVIRONMENT
Problems : What is " being alive "f
What are tropismsf
Of what value are tropisms to a living thing f
What are adaptations f
In what respects has man modified his environment?
Laboratory Suggestions
Home work. The study of a living plant and a living animal. List functions, likenesses, differences.
Demonstration of some tropisms, plant and animal.
Field trip. Visit to a museum or botanical garden or zoological park for the study of habitat groups.
A study of some simple adaptations.
A survey to discover how man has modified his environment.
A Living Plant and a Living Animal compared. — A walk in the fields or a vacant lot on a day in early autumn will give us first- hand acquaintance with many common plants which, because of their ability to grow under somewhat unfavorable conditions, are called weeds. Such plants as the dandelion, butter and eggs, and shepherd's purse, are particularly well fitted by nature to produce many of their kind, and also are able to thrive under conditions which would not easily support life in other less hardy plants. Feeding on these and other plants, are several kinds of animals, most of them insects.
If we attempt to compare, for example, a grasshopper with the plant on which it feeds, we at once see several points of likeness and of difference. Both plant and insect are made up of parts, each of which, as the stem of the plant, or the leg of the insect, appears to be distinct, but is a part of the whole living plant or animal. These parts, such as the leaves of the plant, or the legs of the insect, are used by the plant or the animal for definite purposes,
X3
14 LIVING THINGS AND THE ENVIRONMENT
For example, as we shall see later, the leaf, because of its structure and position on the plant, is fitted to receive and use the sun's light, while the legs of the insect will be found to be jointed, often provided with claws or hooks for holding to a support, and to be capable of movement. Such parts of a living plant or animal, each having a separate work to do, are called organs. Thus plants and animals are spoken of as or'ganisms.
What is being Alive ? — We are all aware, through a study of elementary sci- ence, that matter may assume several forms. Water, for example, when cooled sufficiently, becomes ice, or, if heated to the boiling point, becomes vapor. In order to be changed it has to be acted upon by outside forces. But when a plant or an animal grows, or moves, or in some other way manifests energy, that manifestation comes from within the organism.
We think of things as alive when they do something. Yet water may turn a wheel and generate electricity, which has a force that is capable of ''doing something. " Such a force may set off a blast of dynamite. Another example is a flash of lightning, which may destroy a tree. It is not easy to distinguish exactly what it is to be alive, any more than it is easy to tell what electricity is or what radioac- tivity is. Electricity is a servant of man, but the greatest expert cannot tell what the force actually is. Life is a manifestation of forces, like a flame or electricity. Every living thing, as we shall see later, is like a steam engine or any other machine, in that it is a medium used for the transformation of energy. So we had best start by trying to see how living things act in their normal environment when outside forces influence them.
Response to Stimuli an Indication of Life. — One of the world's greatest biologists, Jacques Loeb (zhak lob), some years ago at- tempted to prove that all living things are more or less auto-
A weed. Notice different parts
the
INDICATIONS OF LIFE 15
matically controlled by the factors of their environment. He assumed that all living matter is sensitive and that it responds or reacts to the forces of its environment, in very definite ways. These forces we call stim'uli (sing, stimulus) ; the response to such a stimulus we call a tro'pism. Loeb and his followers have shown quite conclusively that living matter responds very definitely to temperature, touch, chemical substances, electricity, and various other factors of the environment. Response is e\ddently one indi- cation of being alive. It is a means by which plants and animals adjust themselves to the favorable or unfavorable factors of their environment.
Living Things show Activity. — Response to stimuli means activity or movement. But movement in living things is brought about by changes within the organism, while the movement of an engine is brought about by the force of burning coal or ex- ploding gasoline. Evidently there is a difference here, although it is not easy to explain, in our present state of knowledge.
Method of Growth in Living Things. — The most outstanding difference between the living organism and the non-hving engine lies in their methods of growth. An expert mechanic may build an engine, but no one has ever made a living thing. Growth takes place in a crystal or a limestone stalagmite in a caA^e or in the familiar example of the icicle, but it is simply accretion or gradual addition of non-living material on the outside. But in a plant or animal a mysterious growth takes place through taking into the body various substances quite unlike the body material. The making over of materials in a living body to form the living stuff is called assimilation.
Reproduction in Living Things. — Another striking attribute of living matter is its ability to reproduce its kind. Living plants, by seeds, sprouts, buds, or cuttings, form new li\dng plants, and we all know that some animals hatch from eggs and others are born alive. This act of reproduction is another activity by means of which we can tell that an organism is alive. But we can no more say what assimilation or growth or reproduction is than we can tell what electricity is. They are activities of living things.
How Plants and Animals react to the Primary Factors of their Environment. — Water. It is a weU-known fact that most living
iO LIVING THINGS AND THE ENVIRONMENT
things need water, in order to sustain life. The roots of green plants grow toward a source of water. Some animals appear to be stimulated to move toward water, whereas others move away from moisture. In the words of science, they show hydrot'ropism, and are positively or negatively hydrotrop'ic. Water is of so much importance to man that from the time of the Caesars until now" he has spent enormous sums of money to bring pure water to his cities. The United States government has spent millions of dollars on irrigation to bring the water needed to support life in the western desert lands.
Light. Light is another important factor of the environment. A study of the leaves on any green plant growing near a window
will convince one that the stems of such plants grow toward the light, and that the leaves are held in such positions that they get a maximum amount of sun- light. All green plants are thus influenced by the sun. Other plants which are not green seem either indiffer- ent or negatively influ- enced by the stimulus of light. The direction, as well as the intensity of light, is an important factor. Animals may or may not be attracted by light. A moth, for example, will fly toward a flame ; an earthworm will move away from light. Movements toward or away from light are known as positive or negative heliot'ropism (Gr. helios, sun) or photofropism. Some animals prefer a moderate or weak intensity of light and live in shady forests or jungles, prowling about at night. ■ Others seem to need strong light. Man himself is most comfortable and works most efficiently in a moderate intensity of light.
Gravity. Another factor influencing both plants and animals is gravity. Roots of plants, for example, grow downward and are thus said to be positively geotrop^ic. The stem, on the other hand, grows upward; it is negatively geotropic. Many animals show this response to gravity — geot'ropism — in very definite ways.
The effect of light upon a growing plant.
HOW PLANTS AND ANIMALS REACT 17
Food or Chemical Substances. We shall see later that plants are greatly influenced by the presence or absence of chemical sub- stances in the soil. Since such substances are absorbed by the plant and later built into the organism, we can easily see that responses of this sort are of the utmost importance. As we well know, animals, including man, are much influenced by some kinds of chemical substances that we call foods, and may be seriously affected by other combinations of chemicals called poisons. Re- sponse to chemical substances is called chemot'ropism.
Temperature. Living things are affected by heat or the absence of it. Animals and small plants that are able to move in the water frequently go from a cooler to a warmer part of the fluid, or away from a temperature that becomes unfavorable to their ex- istence. They are thus ssiid to show thermotropism. In cold weather green plants either die or temporarily suspend their life activities, becoming dormant. Likewise, small animals, such as insects, may be killed by cold or may hibernate under stones or boards. Their life activities are slowed down until the coming of warm weather. Bears and some other large animals go to sleep during the winter and awake, thin and hungry, on the approach of warm weather. Ani- mals and plants used to certain temperatures are killed if removed from them. Even man, one of the most adaptable of all animals, cannot stand great changes without discomfort and sometimes death. He heats his houses in winter and sometimes cools them in summer so as to have the amount of heat most acceptable to him, i.e., about 70° Fahrenheit.
The Value of Tropisms. — A study of hundreds of experiments with plants and animals shows us that tropisms are of the greatest use to them. Response to a favorable stimulus results in placing the living plant or animal where it can better succeed in the world. And in general, tropisms bring the organism into adjustment with its environment.
The Environment determines the Kinds of Animals and Plants within it. — In our study of geography we learned that certain luxuriant growths of trees and climbing plants are characteristic of the tropics, with their moist, warm climate. The tropical jungle is often a tangle of long climbing plants, the leaves festooned over the trunks of tall trees, while the jungle floor is covered with a
18 LIVING THINGS AND THE ENVIRONMENT
lower growth of shade-loving plants. Animal life abounds, al- though insects and birds predominate in these tropical regions.
Photo Galloway
Dense tropical jungle in the Amazon Valley.
Most of US live in conditions of temperate climate, with its almost complete absence of plant growth in winter but v^ith luxuriance of life in summer. No boy or girl can fail to notice that temperature must play a very important part in determin- ing not only what will grow in a given locality, but also how it will grow.
ADAPTATIONS IN PLANTS AND ANIMALS 19
As we go northward it is still more evident that temperature plays an important part in determining the kind and amount of plant growth. A glance at the picture will show this. The factors of the environment evidently determine the kind of life to be found in a given locality. If, for example, temperate forms of life were intro- duced by man into the tropics, they would either die or gradually change so as to become fitted to live in their new environment. English sheep with long wool soon died when removed to Cuba,
|
1 |
|||||
|
1 |
ii |
"i |
|||
|
WBMjM^ |
'i |
||||
|
1 |
1 |
/ |
|||
|
H |
1 |
, . ^ ^ X T'. t>y,. |
, //x/^^^<!^ |
x^^ /^'^^^ |
'"'A |
Photo Galloway
Vegetation in northern Alaska, where no trees grow. The reindeer feed on grasses and lichens.
where the climate is very warm. They were not fitted or adapted to live in their changed environment.
Adaptations. — Not only are plants and animals fitted to live under certain conditions, but each part of the body may be fitted to do certain work. I notice that as I write these words the fingers of my right hand grasp the pen firmly and the hand and arm exe- cute some very complicated movements. This they are able to do because of the free movement given through the arrangement of the delicate bones of the wrist and fingers, their attachment to the bones of the arm, a wonderful complex of muscles which move the bones, and a directing nervous system which plans the work. Because of the peculiar fitnesses in the structure of the
20 LIVING THINGS AND THE ENVIRONMENT
hand for this work we say it is adapted to its function of grasping objects. Each part of a plant or animal is usually suited for some particular work. The root of a green plant, for example, is fitted to take in water by having tiny absorbing structures growing from it. The stems have pipes or tubes to convey liquids up and down and are strong enough to support the leafy part of the plant. The thin, flat leaves are arranged to receive a very large amount of
sunlight and to act as solar engines. Each part of a plant does work, and is fitted, by means of cer- tain structures, to do that work. The lungs of a land animal are adapted for taking oxygen from the air, while the gills of a fish can take their sup- ply only from water; that is, only from the air that is dissolved in water. It is because of such adap- tations that living things are able to do their work within their particular environment.
Plants and Animals and their Natural En- vironment.— Those of us who have tried to keep potted plants in the schoolroom know how difficult it is to keep them healthy. Dust, foreign gases in the air, lack of moisture, and other causes make the artificial environment in which they are placed unsuitable for them. A goldfish placed in a small glass jar with no food and no green water plants soon dies. The artificial environment lacks something that the fish needs. Each plant and animal is limited to a certain en- vironment because of certain individual needs which can be pro- vided for only by that particular environment.
A natural barrier across a stream. No trout would be found above this fall. Why not?
CHANGES IN ENVIRONMENT
21
Changes in Environment. — Most plants and animals do not change their environment. Trees, green plants of all kinds, and some animals remain fixed in one spot practically all their hves. Certain tiny plants and most animals move from place to place, either in air, in water, on the earth or in the earth, but they main- tain relatively the same conditions in environment. A high moun- tain chain with intense cold at the upper altitudes would be a barrier over which, for example, a soft-bodied animal like a worm or a snail could not travel. Certain species of trout are found on the western side of the continental divide and other species on the eastern side. Fish will migrate up a stream until they come to a fall too high for them to jump. There they must stop be- cause their environment limits them.
Man in his Environ- ment. — Man, while he is like other animals in requiring heat, light, water, and food, differs from them in that he has come to live in a more or less artificial environ- ment. Men who lived on the earth thousands of years ago did not wear clothes or have elaborate homes of wood, brick, or stone. They did not use fire, nor did they eat cooked foods. In short, by slow degrees, civilized man has come to live in an en- vironment changed from that of other animals. He has learned to build houses and to use fire. The living together of men in communities has caused certain needs to develop. Many things can be supplied in common, as water, milk, and fuel. Wastes
H. NSW CIV. BIOL. — 3
An unfavorable city environment. Compare with the favorable city environment shown in the frontispiece.
22 LIVING THINGS AND THE ENVIRONMENT
of all kinds in a town or city have to be disposed of. Houses have come to be placed close together, or piled one on top of another, as in modern apartment buildings. Fields and trees, in fact most aspects of country outdoor life have virtually disappeared in a large city. City-dwelling man has come to live in an artificial environment.
Care and Improvement of One's Environment. — Man can modify or change his surroundings by making this artificial envi- ronment favorable to live in. He can heat his dwellings in winter and cool them in summer so as to maintain a moderate and nearly constant temperature. He can see that his dwellings have win- dows to let light and air pass in and out. He can have light at night and shade from intense light by day. He can have a sys- tem of pure water supply and drains or sewers to carry awaj^ his wastes. He can plan parks and playgrounds so that the city folk may have breathing spaces, as do their more fortunate neigh- bors in the smaller towns. He can see to it that people ill with *' catching " or communicable diseases are isolated or quarantined from others. Best of all, he is slowly learning to control the harm done by the tiny parasites, plant and animal, that cause and spread diseases. This care of the artificial environment is known as sani- tation, while the care of the individual for himself within the envi- ronment is known as hygiene. It will be the chief aim of this book to show girls and boys how they may become good citizens through the proper control of personal hygiene and sanitation.
Summary. — We have found the life activities to be reactions to stimuli. As a result, plants and animals show definite responses that are called tropisms, and it appears that by means of these tropisms living things are better able to succeed in the world. They are, in a sense, creatures of the environment, for the sur- roundings determine the types which can live in any particular place. Plants and animals are also fitted or adapted to live in certain conditions by having their parts modified or changed so as to fit better the conditions in which they live. Man, how- ever, is able to change the conditions of his environment, or else to move to a more favorable one. In this sense, he is the most adapt- able of all living creatures, and as such, controls his living con- ditions.
QUESTIONS AND REFERENCES 23
Problem Questions
1. How does a living plant or animal differ from a stone?
2. How are tropisms of value to plants ? To animals ? Give specific ex- amples.
3. How might animals do without light? Could they do without other factors of the environment ?
4. How might movements help in the life of a plant or animal ?
5. Give some examples of fitness or adaptation in a bean plant. In a tree. In a dog or cat.
6. What do we mean by zonal distribution of plants and animals? Look this up in one of the books of reference.
7. Under what unusual conditions do the Eskimos of the far North live ? How do they adapt themselves to their environment?
8. Give instances of man's modification of his environment in your town. In your home.
Problem and Project References
Hunter, Laboratory Problems in Civic Biology. American Book Company. Allen, Civics and Health. Ginn and Company.
Hough and Sedgwick, Hygiene and Sanitation. Ginn and Company. Huntington and Gushing, Principles of Human Geography. John Wiley and
Sons. Jordan and Kellogg, Animal Life. D. Appleton and Company. Loeb, Forced Movements, Tropisms and Animal Conduct. J. B. Lippincott
Company. Transeau, General Botany. World Book Company.
CHAPTER IV THE INTERRELATIONS OF PLANTS AND ANIMALS
Problems: To study insects — their structure, development, food, and homes.
To study flowers — their structure and method of reproduction.
To find out how insects and plants are adapted to aid each other; how flowers are pollinated.
To learn how seeds are dispersed, and the importance of seed dis- persal.
Laboratory Suggestions
A field trip. Object : to collect common insects and study their gen- eral characteristics ; to study the food and shelter relations of plants and in- sects. The pollination of flowers should also be carefully studied so as to give the pupil a general viewpoint as an introduction to the study of biology.
Laboratory exercise. Examination of a simple insect, identification of parts, drawing. Examination and identification of some orders of insects.
Laboratory demonstration. Life history of monarch butterfly and some other butterflies or moths.
Laboratory exercise. Study of a simple flower : emphasis on the work of the essential organs ; drawing.
Laboratory exercise. Study of the mutual adaptations in a given insect and a given flower, e.g. bumblebee and butter and eggs.
Demonstration. Examples of insect pollination.
Laboratory or field exercise. Study of seed dispersal.
The Object of a Field Trip. — Many of us live in a city, where the crowded streets, the closely packed apartments, and the city playgrounds form our immediate environnient. Others of us live in a small town or in the real country. To understand the nor- mal environment of plants and animals we should go into the country. Failing in this, an overgrown city lot or a park will give us the environment as it touches some animals lower than man. We must remember that in learning something of the natural en-^dronment of other living creatures we may better understand our own environment and our relation to it.
24
HOW TO TELL AN INSECT
25
Antenna
On any bright warm day in the fall we shall find insects swarming in a vacant lot or in a city park. Grasshoppers, butterflies alight- ing now and then on the flowers, brightly marked hornets, bees busily working over purple asters or goldenrod, and many other forms more or less hidden away on the leaves or stems of plants, may be seen. If we select for observation some partly decayed tree, we find it also inhabited. Beetles will be found boring through its bark and wood, while caterpillars (the young stages of butterflies and moths) are feeding on its leaves or building homes in its branches. Everywhere above, on, and under ground may be noticed small forms of life, many of them insects. Let us first see how we should go to work to identify some of the common forms we are likely to find on plants. Then a little later we shall find out what they are doing on these plants.^
How to tell an Insect. — A bee is a good ex- ample of the group of animals we call insects. If we examine its body carefully, we notice that it has three regions : a front part or head, a mid- dle part called the tho'rax, which is divided into three parts or seg- ments, and a hind portion, segmented and hairy, the dbdo'men. The three pairs of legs, which are jointed and provided with tiny hooks at the end, are attached to the thorax. How many joints can you find in each leg? Two pairs of delicate wings are attached to the upper or dorsal side of the thorax. The entire body has a tough covering or exoskeVeton composed of chitin (ki'tin), a substance chemically much like a cow's horn. This exoskeleton in the bee is partly covered with tiny hairs which form a vesture over the body. The muscles, which provide for movement, are fastened to the exo- skeleton, for there is no internal skeleton. If we watch the abdomen
^ If the teacher desires, Chapter XXI may be used at this point.
A bee viewed from the side. Notice the head, thorax, and abdomen. What other parts do you find? What structures, if any, are joined to each of these three body di\'isions?
20 INTERRELATIONS OF PLANTS AND ANIMALS
Head of a bee (side view) , showing com- pound eye.
of a living bee, we notice it moves up and down quite regularly. The animal is breathing through tiny holes called spir'acles, placed along the sides of the thorax and abdomen. Bees have compound eyes composed of numerous units, each of which acts as a tiny camera. Bees are provided also with a pair of j ointed feelers called anten'nce. Wings are not found on all insects, nor is a vesture ; but the other structures just given are marks of the great group of animals we call insects.
Forms to be looked for on a Field Trip. — Inasmuch as there are more than 450,000 different species or kinds of insects, it is evi- dent that it would be a hopeless task for us even to attempt to recognize all of them. But we can learn to distinguish a few examples of the common forms that might be met on a field trip. In the fields, on grass, or on flowering plants we may find members from at least six of the twenty orders of insects. These may be known by the following characters :
The order Hymeno'p'tera (membrane wings), to which the bees, wasps, and ants belong, is the only insect order of which some of the members are pro- vided with true stings. This sting is placed in a sheath at the extreme hind end of the abdo- men. Other structures, which show them to be insects, have been given above.
Butterflies and moths will be found hovering
over flowers. They belong to the order Lepidop'tera (scale wings). This name is given to them because their wings are covered with tiny scales, which fit into little sockets much as shingles are placed on a roof. The dust which comes off on the fingers when one catches a butterfly is composed of these scales. The wings are always large and usually brightly colored ; the legs are small, and one pair of
Worker
(jueen
Hymenoptera — bee.
Drone
FORMS OF INSECTS
27
Youngkrya Qcoon
Lepidoptera — stages in the development of the silk moth.
them is often inconspicuous. These insects may be seen to take Hquid food through a long tubehke organ, called the prohos'cis, which they keep rolled up under the head when not in use. During de- velopment the young pass through a stage in which they are known as cater- pillars or larvce and feed on plants by means of a pair of hard jaws.
Grasshoppers, found almost everywhere, and crickets, black grasshopper-like insects often found under stones, belong to the order Orthop'tera (straight wings). Members of this group may-
usually be distinguished by their strong, jumping hind legs, by their chewing or biting mouth parts, and by the fact that the hind wings are folded up under the somewhat stiffer front wings.
Another group of insects some- times found on flowers in the fall are flies. They belong to the order Dij^'tera (two wings). These in- sects are usually rather small and have a single pair of gauzy wings. Flies are of much importance to man because certain of their number are disease carriers.
Bugs, members of the order Hemip'tera (half wings), have a jointed proboscis which is used for piercing and sucking. They are usually small and may or may not have wings.
The beetles or Coleop'tera (sheath wings), often jj^^. mistaken for bugs by the uninformed, have two bedbug.
Orthoptera — 1, cricket; 2, cock- roach ; 3, grasshopper.
28 INTERRELATIONS OF PLANTS AND ANIMALS
pairs of wings, but the first pair form hard covers meeting in a straight Hne in the middle of the back, the second pair of wings, when at rest, being covered by them. Beetles are frequently
found on goldenrod blossoms. Try to discover members of the six different orders named above. Collect specimens and bring them to the labo- ratory for identification.
Other animals which may be
found are spiders, with four
of walking legs, and
Weevil. Ladybird. Calasoma beetle. Coleoptera.
pairs
centipedes and millepedes, both of which
are wormlike and have many pairs of legs.
Why do Insects live on Plants? — We
Centipede.
Spider.
have found insect life abundant on living green plants, some visiting flowers, others hidden away on the stalks or leaves of the plants. Let us next try to find out why insects live upon flowering green plants.
The Life History of the Monarch Butterfly. — If it is possible to find some milkweed on our trip, we are quite likely to find hover- ing near, a golden brown and black butterfly, the monarch or milk- weed butterfly (Anosia plexippus). Its body, as in all insects, is composed of three regions. The female monarch frequents the milkweed in order to lay eggs ; she may be found doing this at almost any time from June until September.
Egg and Larva. The eggs, tiny hat-shaped dots a twentieth of an inch in length, are fastened singly to the under side of milkweed leaves. Sorae wonderful instinct leads this butterfly to deposit ber
LIFE HISTORY OF A BUTTERFLY
29
eggs on the milkweed, for the young feed upon no other plant. The eggs hatch out in four or five days into rapidly growing wormlike caterpillars, each of which will shed its skin several times be- fore it is full grown. These caterpillars pos- sess, in addition to the three pairs of true legs, additional pairs of prolegs or cater- pillar legs. The ani- mal at this stage is known as a larva.
Formation of Pupa. After a life of a few weeks at most, the caterpillar stops eat- ing and begins to spin a tiny mat of silk upon a leaf or stem. It at- taches itself to this web by the last pair of prolegs, sheds its skin again, and hangs there in the dormant stage known as the chrys^alis or pupa. This is a resting stage during which the body changes from a
caterpillar to a butterfly.
The Adult. After a week or more of inactivity in the pupa state, the outer skin is split along the back, and the adult butterfly emerges. At first the wings are soft and much smaller than in the adult. Within fifteen minutes to half an hour after the butterfly emerges, however, the wings are full-sized, having been pumped full of blood and air, and the insect, after her marriage flight, is ready to follow her instinct to deposit, her eggs on a milkweed plant.
Monarch butterfly : adults, larvae, and pupa on their food plant, the milkweed. (From a photograph loaned by the American Museum of Natural History.)
Spiracle
Head
True fegs Caterpillar of
Prolep
moth, the squash borer.
30 INTERRELATIONS OF PLANTS AND ANIMALS
This life history gives us an example of what is known as a complete metamor' yJiosis or change of form.
Plants furnish Insects with Food. — The insects which we have seen on our field trip feed on the green plants among which they live. Each insect has its own favorite food plant or plants, and in many cases the eggs are laid on the plant so that the young may have food close at hand. Some insects prefer the rotted wood of trees. An American zoologist, Packard, has listed 462 species of insects that live upon oak trees alone. Everyivhere animals are
engaged in taking their nourishment from plants, and millions of dollars of damage is done every year to gardens, fruits, and cereal crops by in- sects.
All Animals depend on Green Plants. — ^ Insects in their turn are the food of birds ; cats and dogs may kill birds ; lions and tigers live on large de- fenseless animals such as deer or cattle. And finally, man eats the bodies of both plants and animals. But if we reduce this search for food to its final limit, we see that green plants provide all the food for animals. For the lion or tiger eats the deer which feeds upon grass or green shoots of young trees, and the cat eats the bird that lives on weed seeds or on insects that eat plants. Green plants supply the food of the world.
Homes and Shelter. — On a field trip no one can fail to observe that plants often give animals a home. The grass shelters grass- hoppers and smaller insects which can be obtained by sweeping through the grass with an insect net. Some insects, such as the tent caterpillar, build their homes in the trees or bushes on which
Trees with leaves destroyed by insects, in the Yosemite Valley, California.
USE AND STRUCTURE OF A FLOWER 31
they feed, while others tunnel through the wood, making homes there. Spiders build webs on plants, often using the leaves for shelter. Birds nest in trees, and many other wild animals use the forest as their home. Man has learned to use many kinds of plant products to aid him in making his home, wood and various fibers being the most important of these products.
What Animals do for Plants. — So far it has seemed as if green plants benefited animals and received nothing in return. We shall see later that plants and animals together form a balance of life on the earth and that each is necessary for the other. Certain sub- stances found in the body wastes from animals are necessary to the life of a green plant.
Insects and Flowers. — Certain other problems can be worked out in the fall of the year. One of these is the biological inter- relation between insects and flowers. It is easy on a field trip to find insects lighting upon flowers. They evidently have a reason for doing this.
The Use and Structure of a Flower. — It is a matter of common knowledge that flowers form fruits and that fruits contain seeds. Flowers, then, are not merely things of beauty, but are very impor- tant parts of plants. On our field trip we saw many flowers and noticed that they are of various shapes, colors, and sizes. It will now be our problem first to learn to know the parts of a flower, and then to find out how they are fitted to attract and receive insect visitors.
The Floral Envelope. — The expanded portion of the flower stalk, which holds the parts of the flower, is called the receptacle. The green leaflike parts covering the unopened flower, when taken together, are called the ca'lyx. Each of these parts is a se'pal. The more brightly colored structures are the pet'als. Together they form the coroVla. The calyx and corolla together are called the floral envelope.
The corolla is of importance in making the flower conspicuous. Frequently the petals or corolla have bright marks or dots which lead down to the base of the cup of the flower, where a sweet fluid called nectar is secreted by nectar glands. It is principally this food substance, later made into honey by bees, that makes flowers attractive to insects.
32 INTERRELATIONS OF PLANTS AND ANIMALS
The Essential Organs. — A flower, however, could have no sepals or petals and still do the work for which it exists. The essential organs of the flower are within the floral envelope. They consist of the sta'mens and pis' til (or pistils), the latter being in the
center of the flower. The structures with the knobbed ends are called stamens. In a single stamen the boxlike part at the end is the anther; the stalk which holds the anther is called the fiVament. The anther is in reality a hollow box which produces a large number of little grains called pollen. Each pistil is composed of a rather stout base called the o'vary and a more or less lengthened por- tion rising from the ovary called the style. The upper end of the style, which in most cases is broadened, is called the stigma. The free end of the stigma usually secretes a sweet fluid in which grains of pollen from flowers of the same kind can grow. Insects as Pollinating Agents. — Insects often visit flowers to obtain pollen as well as nectar. In so doing they may transfer some of the poflen from one flower to another of the same kind. This transfer of pollen, called cross-pollination, is of the greatest use to the plant, as we shall see later. Sir John
Ovule
Cross sec f ion of ovary
A simple flower, seen from above {A) and in sec- tion {B) ; and its separate parts.
Sepdl Petal Stdmsn Mil
STRUCTURE OF A BEE
33
Lub'bock tested bees and wasps to see how many trips they made daily from their homes to the flowers, and found that a wasp went out on 1 16 visits during a work- femur
Coxa Coxd^
Trocbsnfer
Trochankr Coxa I ^fmur
tfibia Trochankr
Tarsus, 5 parh Left froni le§ Middle le^ hind le^
Legs of a bee.
Outer pigment I cell-
ing day of 16 hours, while a bee
made almost as many visits
and worked almost as long as
the wasp. It is evident that
in the course of so many trips
to the fields a bee must light
on hundreds of flowers.
Adaptations in a Bee. — When
a plant or animal structure is
fitted to do a certain kind of
work, we say it is adapted to do that work. If we look closely
at a bee, we find the body and legs more or less covered with tiny
hairs, many of them branched. The joints in the legs of the bee
adapt it for complicated movements ; the arrangement of stiff hairs along the edge of a concavity in one of the joints of the hindmost pair forms a structure called the pollen basket, adapted to hold pollen. Bees collect pollen and force it into this concavity by means of a pollen press (us- ually called the wax shears) located be- tween the tibia and metatar'sus of the hind pair of legs. (See figure above.) Pol- len obtained by the bee in this way is taken to the hive to be used as food. But while the insect is gathering pollen for itself, some is caught on the hairs and other projections on the body or legs and is carried from flower to flower. The value of this to a flower we shall see later. The Sight of the Bumblebee. — The large eyes located on the sides of the head are made up of a large number of little
units, called ommatid'ia (sing, ommatidium) , each one of which is
considered to be a very simple eye. The large eyes are therefore
-Crystalline lens -Crystalline cone
Xiorneal pigment cell
■Retinal cell Nucleus
"-Nerve
The compound eye is made of many units, each called an ommatidium.
34 INTERRELATIONS OF PLANTS AND ANIMALS
called the compound eyes. Some insects have only compound eyes, some only simple e^^s, but most insects have both. The simple ej^'es of the bee may be found by a careful observer be- tween and above the compound eyes.
Insects can distinguish certain differences in color ; they can see moving objects, but the}^ do not seem to be able to make out form well. On the other hand, the}^ appear to have an extremely well- developed sense of smell. Insects can perceive at a great distance odors which to the human nose are not recognizable. Night-flying insects, especiall}^, find flowers by odor rather than by color.
Mouth Parts of the Bee. — The mouth of the bee is adapted to take in pollen and nectar, and is used for some of the purposes for which man would use the hands and j
fingers. The honeybee laps or sucks ^^ C^~^ nectar from flowers, it chews the pol- ^^Ejrf ibr md]
^^mple eye
Simpk eye
The head of a bee, front and side views.
Mouth parts of a grasshopper,
for comparison with those of a bee. Ihr, labrum, or upper lip ; md, mandibles, the biting jaws; hyp, hypopharnyx, or tongue ; 7713;, maxillae or under jaws; m.p. maxillary palps, sensory organs ; lab, labium or under lip — with l.p. labial palps.
len, and it uses part of the mouth as a trow^el in making the honey- comb. The mouth parts may be seen in action by watching a bee on a well-opened flower.
Butter and Eggs {Linaria vulgaris). — From July to October the very abundant weed called butter and eggs may be found especially along roadsides and in sunny fields. It bears a tall and conspicuous cluster of yellow and orange flowers.
The corolla projects into a spur on the lower side; an upper two-parted lip shuts down upon a lower three-parted lip. The four stamens are in pairs, two long and two short.
POLLINATION OF FLOWERS
35
Certain parts of the corolla are more brightly colored than the rest of the flower. Butter and eggs is visited by bumblebees, which are guided by the orange lip to alight just where they can push their way into the flower. The bee, seeking the nectar se- creted in the spur, brushes its head and thorax against the stamens. It may then, as it pushes down after nectar, leave some pollen upon the pistil, thus ef- fecting self-pollination, which is the transfer of pollen from the anthers to the stigma of the same flower. Later, in visiting another flower of the same kind, the bee may leave some more of the pollen on the pistil of that second flower. Cross-pollination is the transfer of pollen from the anthers of one flower to the stigma of another flower of the same kind, — some say only if the two flowers are on dif- ferent plants.
History of the Discov- eries regarding Pollination of Flowers. — Although the ancient Greek and Roman naturalists had some vague ideas on the subject of pol- lination, it was not until the first part of the nine- teenth century that a book appeared in which a German scientist, Conrad Spreng'el, worked out the fact that the structure of cer- tain flowers seems to be adapted to the visits of insects in that it offers easy foothold, sweet odor, and desirable food in the shape of pollen and nectar. Sprengel further discovered the fact that pollen can be and is carried by the insect visitors from the
Diagram to show how a bee polHnates but- ter and eggs. A bumblebee, upon entering the flower, rubs its head against the long pair of anthers (A), then continuing to press into the flower so as to reach the nectar at N, it brushes against the stigma >S, thus pollinating the flower.
36 INTERRELATIONS OF PLANTS AND ANIMALS
anthers of a flower to its stigma. It was not until the middle of the nineteenth century, however, that an Englishman, Charles Darwin, applied Sprengel's discoveries on the relation of insects to flowers by his investigations concerning cross-pollination. The growth of the pollen on the stigma of the flower is a necessary step in the production of seeds, and thus of new plants. Many kinds of flowers are self -pollinated and do not do so well in seed production if cross-pollinated, but Darwin found that most flowers which are self-pollinated do not produce so many seeds, and that the plants which grow from their seeds are smaller and weaker than
plants from seeds produced by cross-pollinated flowers of the same kind. He also found that plants grown from cross-pollinated seeds tend to vary more than those grown from self-pol- linated seed. This has an important bearing, as we shall see later, in the pro- duction of new varieties of plants. Microscopic ex- amination of the stigma at the time of pollination also shows that the pollen from another flower usu- ally germinates more rap- idly than the pollen which falls from the anthers of the same flower. This latter fact alone in most cases renders it un- likeh^ for a flower to produce seeds by its own pollen. Darwin worked for years on the pollination of many insect-visited flowers, and discovered in almost every case that showy, sweet-scented, or otherwise attractive flowers are adapted to be cross-pollinated by insects. He also found that, for flowers that are inconspicuous in appearance, often a compensation appears in an odor which renders them attractive to certain insects. The so-called carrion flowers, pollinated by flies, are examples, the odor in this case
Two different -wild orchids. Flowers of this t>T3e were used hx Charles Darwin to work out iis theory of cross-pollination by insects.
DEVICES TO SECURE CROSS-POLLINATION
being like that of decayed flesh. Other flowers open at night and are white, besides having a powerful scent. Thus they attract night-flying moths and certain other insects.
Other Devices to secure Cross-Pollination. — There are many other examples of adaptations to secure cross-pollination by means of the visits of insects. The mountain laurel shows a remarkable adaptation in having the anthers of the stamens caught in little pockets of the corolla. The weight of the visiting insect on the corolla releases the anther from the pocket in which it rests so that it springs up, dusting the body of the visitor with pollen.
In some plants, self-pollination is prevented by certain devices, as in the primroses, in which the stamens and pistils are of different lengths in different flowers. Short styles and long filaments with high- placed anthers are found in some flowers, and long styles and short filaments with low-placed anthers in others. Pollination is most likely to be effected by some of the pollen from a low-placed anther reaching the stigma of a short-styled flower, or by the pollen from a high anther being placed upon a long-styled pistil. There are, as in the case of the spiked loosestrife, flowers hav- ing pistils and stamens of three lengths. Pollen grows best on pistils of the same length as the stamens from which it came. The stamens and pistil ripen at different times in some flowers. The " Lady Washington " gera- nium, a common house plant, shows this condition. Here cross- pollination must take place if seeds are to be formed.^
Special Adaptations between Flowers and Insects. — A very remarkable instance of insect help is found in the pollination of the yuc'ca, a semitropical lily which lives in deserts (to be seen in most
The condition of stamens and pistils on the spiked loosestrife.
1 For an excellent account of cross-pollination of milkweed, the reader is re- ferred to W. C. Stevens, Introduction to Botany. Orchids are well known to botan- ists as showing some very wonderful adaptations. A classic easily read is Darwin, On the Fertilization of Orchids.
H. NEW CIV. BIOL. — 4
38 INTERRELATIONS OF PLANTS AND ANIMALS
The pronuba moth within the yucca flower.
botanic gardens). In this flower the stigmatic surface is above the anther, and the pollen is sticky and cannot be transferred except by insect aid. This is accomplished in a remarkable manner. A little moth, called the pro'nuha, after gathering pollen from an anther, flies away with this load to another flower, there de- posits an egg in the ovary of the pistil, and then rubs its load of pollen over the stigma of the flower. When the egg hatches, the caterpillar feeds on some of the young seeds which have , ^ grown because of the pollen placed on
/V^ y the stigma by the mother. Later it
bores out of the seed pod and escapes to the ground, leaving the plant to de- velop the remaining seeds without further molestation.
The fig insect {Blastophaga gros- sorum) is another member of the insect tribe that is of considerable economic importance. The fertili- zation of the flowers of the fig tree is brought about by a wasp which bores into the young fruit. By importing the wasps to California it was made possible to grow figs where for many years it was believed that the climate prevented them from ripening.
Other Visitors to Flowers. — Among other useful pollen carriers for flowers are butterflies. Projecting from each side of the head of a butterfly or a moth is a fluffy structure, called thepaZp. This collects and carries a large amount of pollen, which is deposited upon the stigmas of other flowers when the butterfly pushes its head down into the flower tube after nectar. The scales and hairs on the wings, legs, and body of a butterfly also carry pollen.
Humming birds, like bees, cross-pollinate some flowers while seeking nectar and insects.
POLLINATION BY THE WIND
39
Flies, too, are agents in cross-pollination. Humming birds also are active agents in some flowers. Snails are said in rare in- stances to carry pollen. Undoubtedly, man and the domesticated animals frequently pollinate flowers by brushing past them while walking over the fields.
Pollination by the Wind. — Not all flowers are dependent upon insects or other animals for cross-pollination. Many of the earliest spring flowers appear almost before the insects do, and are dependent upon the wind for carrying pollen from the stamens of one flower to the pistil of another. Most of our common trees, oak, poplar, maple, and others, are cross-pollinated by the wind.
Flowers pollinated by the wind are generally inconspicuous and often lack a corolla. Their anthers are exposed to the wind and provided with much pollen, while the surface of their stigmas may be long and feathery. Such flowers may also lack odor, nectar, and bright color. Can you tell why?
Imperfect Flowers. — Some flowers, especially those depending upon the wind for pollination, are imperfect ; that is, they lack either stamens or pis- tils. The corn plant is an example. Again, in some cases, imperfect flowers having stamens only are found on one plant, while those flowers having pistils only are found on another plant of the same kind. In such flowers, cross-pollination must of necessity follow. Many of our common trees are examples.
The Necessity of Fruit and Seed Dispersal to a Plant. — We have seen that the chief reason for flowers, from the plant's standpoint, is to produce fruits Avhich contain seeds. The scatter- ing of fruits and seeds is absolutely necessary in order that colonies of plants may reach new localities. It is evident that plants
The corn plant has staminate flowers at the top of the stalk and pistillate flowers at the side. (White circles were painted on the photograph, to show the location of the pistil- late flowers.)
40 INTERRELATIONS OF PLANTS AND ANIMALS
best fitted to scatter their seeds, or to place their fruits contain- ing seeds some Httle distance from the parent plants, are the ones which will spread most rapidly. A plant, in order to advance into new territory, must first get its seeds there. Plants which are best fitted to do this are the most widely distributed on the earth.
Seeds and fruits transported by the wind : 1, milkweed ; 2, ash ; 3, maple ; 4, dan- delion ; 5, clematis ; 6, elm ; 7, basswood ; 8, thistle.
How Seeds and Fruits are scattered. — Seed dispersal is accom- plished in many different ways. Some plants produce enormous numbers of seeds which may or may not have special devices to aid in their scattering. Most weeds are thus started in '^ pastures new." Some prolific plants, like the milkweed, have seeds with a little tuft of hairlike down which allows them to be carried by the wind. Others, as the omnipresent dandelion, have their fruits
Fruits transported by animals: 1, beggar-tick; 2, tick trefoil; 3, Spanish needle i 4, cocklebm- ; 5, sand bur ; 6, small flowered agrimony.
provided with a similar structure, the pappus. Some plants, as the burdock and cocklebur, have fruits provided with tiny hooks which stick to the hair of animals, thus securing transportation. Most fleshy fruits contain indigestible seeds, so that when the fruits are eaten by animals the seeds are passed off from the body unharmed and may, if favorably placed, grow. Nuts of various kinds are often carried off by squirrels, buried, and for-
HOW SEEDS AND FRUITS ARE SCATTERED 41
gotten, to grow later. Such are a few of the ways in which seeds are scattered. All other things being equal, the plants best equipped to scatter seeds or fruits will drive out other plants in a given locality. Because of their adaptations these plants are likely to be very numerous, and for that reason some of them are likely to survive when unfavorable conditions come. Such plants are well exemplified in the weeds of the grass plots and gardens.
The development of an apple. Notice that in this fruit additional parts besides the ovary (o) become part of the fruit. Certain outer parts of the flower, the sepals (s) and receptacle, become the fleshy part of the fruit, while the ovary becomes the core. Stages numbered 2 to 6 are in the order of development.
Summary. — This chapter has brought out several important facts. First, plant life and animal life are closely interrelated, in that insects feed upon plants, make their homes in them, lay their eggs on their leaves or in their bodies, and in other ways depend on them. But the plant often gets something in return from this close association ; many flowers can form seeds only after pollina- tion by an insect visitor. Finally, although the plant may scatter its seeds without outside aid, we find many cases where animals are of assistance in this dispersal, again showing an interrelationship without which certain plants might be doomed to extinction.
Problem Questions *
1. What is a normal environment ?
2. How would you distinguish an insect from other animals?
3. What are the distinguishing characteristics of several orders of insects?
42 INTERRELATIONS OF PLANTS AND ANIMALS
4. \\Tiat is meant by a life historj^ ?
5. How do plants benefit insects ?
6. How do insects benefit plants ?
7. "What uses have the parts of a flower?
8. What kinds of pollination do we find and how is each brought about ?
9. Name a flower and an insect and work out the interrelations they show.
10. What agent other than insects carries pollen from one flower to another ? What adaptations do we find in wind-pollinated flowers ?
11. Can j^ou give any instances of interrelationships between plants and animals which result in the scattering of seeds ?
12. How has man made use of these interrelations? (Be specific.) What might man do to make more use of such interrelations ? Can you give any examples of "give and take" in human interrelationships?
Problem and Project References
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Part II. American Book Company.
Dana, Plants and Their Children, pages 187, 255. American Book Companj^
Darwin, Different Forms of Flowers on Plants of the Same Species. D. Apple- ton and Company.
Darwin, Fertilization in the Vegetable Kingdom, Chaps. I and II. D. Appleton and Company.
Darwin, Orchids Fertilized by Insects. D. Appleton and Company.
Densmore, General Botany. Ginn and Compan3\
Dickerson, Moths and Butterflies. Ginn and Company.
Lubbock, Flowers, Fruits, and Leaves, Part I. The Macmillan Company.
Lutz, Field Book of Insects. G. P. Putnam's Sons.
Needham, General Biology, pages 1-50. Comstock Publishing Company.
Stevens, Introduction to Botany. D. C. Heath and Company.
Transeau, General Botany. World Book Company.
PART n. LIFE PROCESSES IN LIVING THINGS. GREEN PLANTS AS LIVING ORGANISMS
CHAPTER V
THE BUILDING MATERIAL OF LIVING THINGS
Problems : To study cells in both plants and animals and find, the ways in which they are alike and different. To study the parts of a typical cell. To learn how cells produce other cells.
Laboeatory Suggestions.
Examination of cells from inside of cheek. Examination of cells from onion epidermis. Demonstration of elodea and protoplasmic movement. Demonstration of stained sea urchin or starfish egg to show chromosomes. Demonstration of dividing sea urchin or other eggs to show phases of cell division.
Many Forms of Plants and Animals. — It is common knowledge that many kinds of green plants have roots, a stem, and leaves and bear at some time flowers, which give rise to fruits containing seeds. If a seed is planted, we know it will grow into a young plant of the same kind as produced it. But there are plants which do not form seeds, such as the mosses and ferns, and there are still others that are not green in color and may be too small to see with the unaided eye.
Animals, too, as we know, are very diverse in size and appear- ance. A visit to a '' zoo " or a museum gives us some idea of the multitude of forms and their varied places of habitat. Insects, of which we know a little, are very numerous and very different from one another. And in animal life, as well as among the plants, we find forms so minute that we have to call the com- pound microscope to our aid in order to see them.
43
44 BUILDING MATERIAL OF LIVING THINGS
How Cells were first found. — We have already found that we can distinguish life through its activities. A living organism is sensitive to stimuli, it moves, it grows by assimilating or changing over substances into the same chemical composition as itself, and it reproduces its kind. Is there any other way in which we
can distinguish living from Ufe- less matter?
A little over two hundred years ago, a Dutchman, Anton van Leeuwenhoek (la Ven-hook), made a collection of crude magnifying instruments that were the beginnings of our modern microscope. With these instruments he was able to see tiny organisms swimming in drops of pond water, and it is even thought that he first saw living bacteria. From this be- ginning a very complete knowl- edge has been gained concerning the building material of living A compound microscope is used to Organisms. An English doctor,
magiiify small objects from about 40_to -^^^^^^ Hooke, examined COrk, 500 tunes. The upright tube contams ' '
lenses. The object is placed on the stage which is the bark of an Oak
thr/the"oS. too'ldnf ?own "Sto* tree, and found it was made up the lenses, one should see the object of tiny compartments, like little
magnified in a clear light. ^^^^^^ ^j^-^j^ ^ie called Cells, a
term which is now universally used for the unit of structure in living things.
Protoplasm. — This name cell is not quite descriptive. Hooke saw the dead walls around the spaces that during the life of the plant contained living matter. But it was not until more recent times that biologists found that the content of the cell is the important living substance. This living material has been named pro'toplasm (Gr. protos, first; plasma, formative material). While we rarely see it or feel it, nevertheless observation has shown it to be always present where there is life. It is a sticky, semi-fluid
STRUCTURE OF A CELL
45
Cell from mouth,with stained nu- cleus.
substance, somewhat like white of egg in consistency. Under the microscope it seems to be either granular or made of tiny bubbles floating in a more fluid medium, or it sometimes appears to be made up of delicate fibrils or threads, forming a network of infinite complexity. But it is always found making up the structure of living things, just as bricks make up the structure of a wall or a house.
Structure of a Cell. — One of the easiest ways to obtain cells from your own body is to wash your hands carefully and then scrape the inside of your cheek with your clean finger nail. Place a tiny bit of the scrapings on a glass slide, add a drop of dilute blue fountain pen ink, place a cover glass ovsr it, and examine with a microscope. You will find a number of cells, more or less rounded in appearance, and more or less stained by the blue dye. A care- ful examination will show three distinct parts : an outer covering, which is the cell membrane, the cell body filled with protoplasm, and a more deeply stained portion of the protoplasm called the nu^cleus.
Plant cells are equally easy to see. If we peel the skin from one of the fleshy leaves forming an onion bulb, mount it in water to which is added a drop of dilute tincture of iodine, and examine it under a microscope, we find that this skin or epider'mis is also made of cells. Plant cells, however, differ from animal cells in that they have a deli- cate wood wall outside the membrane.
If we examine the leaves of a green plant we find other structures within the cells. Examination of the delicate leaves of the elo'dea, a water plant used in aquariums, shows a more or less regular arrangement of the cells. But each cell shows many large spaces or vac'uoles, which are filled with a non-living fliiid instead of protoplasm. Forming a part of the protoplasm are many small ovoid bodies, most of which are green in color. These are the chloroplasts (klo'r6-plasts) or chlorophyll (klo'r6-fil) bodies (Gr. chloros, green; phyllon, leaf). We shall
Profophsm
Cells of the epidermis of an onion.
46 BUILDING MATERIAL OF LIVING THINGS
see later that they are of the utmost importance to each one of us, as it is by means of the action of the sun upon them that food is manufactured in the green parts of plants.
In the elodea cells, an interesting phenomenon may be observed. The protoplasm in the cell body is seen to be constantly in motion, flowing slowly in the direction of the arrows shown in the diagram. This streaming of protoplasm is one of the manifestations of life within the cell. In many cells this movement may be observed
|
/ f«^;*-.,3^^V--^ |
|
|
#:-r.^i |
|
|
.j?|: ■■■;■,/.--• :^i- |
— |
|
/ ^''-^^■■--1^ |
|
|
'/, ;^'. :-/^x.:.''* |
|
|
? ^f^W |
::::. |
|
' iv :v^:a |
|
|
'■-,\:~- '.., V-. "9©- |
|
|
>A----.--vV^; |
|
|
, W» :l.--^:«Hv |
|
|
;-^^-: Vy: .:^o-? |
|
|
•.«.,-■ ■■' •.■■:-o\---,- |
|
|
-»'• ^-^^%,.^ |
|
|
V »* : 11^^ ^'i |
-_. |
|
1 « « -.■ ■ -'v^i;/*:- |
|
|
' v«t v :^-«- |
|
|
; :-^":^-:ii |
|
Cell wall (cellulose)
Cell membrane
...Nucleus Nucleolus
.Chloroplasi"
IX
A cell of elodea, a plant. The arrows show the "streaming" of the proto- plasm. White spots show vacuoles.
.Cell membrane
.Cytoplasm
•■Cenfrosomes Nucleolus
JM-I^ucleus ^,M..PIasfld
"Vacuole
Diagram of a typical animal cell. In the nucleus, the dark thread-like struc- tures are stained chromosomes.
and we have reason to believe that the protoplasm in most living cells is in motion, thus affording a circulation of the cell contents. If we now examine a specially prepared and stained cell, for ex- ample, the egg cell of a worm or a frog, we shall find that the nu- cleus, when stained with certain dyes, shows numerous small deeply stained bodies within it. These structures are called chro- mosomes (kro'm6-somz ; Gr. chroma, color; soma, body), or color- bearing bodies. The number of these chromosomes in each body cell of a given kind of plant or animal is always the same. The chromosomes are supposed to be the bearers of the qualities which can be handed down from parent to offspring; in other words,
TISSUES AND ORGANS
47
the inheritable quahties or characters which make the offspring
liice its parents.
Tissues and Organs. — The cells which form certain parts of
the veins, the flat blade, or other portions of a leaf, are found in
groups or aggregations, and are more or less alike in size and
shape. Such a collection of cells is called a tissue. Examples of
tissues in animals are the cells covering the outside of the body,
forming the skin or epidermal tissue ; muscle tissue, which
produces movement ; bony tissue, which forms the framework
to which the muscles are ^ ■ ,
^Epidermis
Falisade kyer A vein Spongy tissue Air spBce
attached ; and there are many others.
Collections of tissues which act together in the performance of work form organs. Such an organ is a leaf, made of supporting cells, green cells, spongy cells, etc., or the human arm, with its bony sup- porting tissue, its nerves and muscles, its blood ves- sels and connective tissue.
How Cells form Others. — Cells grow to a certain size and then split into two new cells. In this process, which is of very great importance in the growth of both plants and animals, the nucleus divides first ; the halves separate and go to opposite ends of the cell. The chromosomes divide at the same time, each splitting lengthwise and the parts go in equal numbers to each of the two new nuclei formed from the old nucleus. In this way the matter in the chromosomes is divided equally between the two new nuclei. Then the rest of the protoplasm separates, and two new cells are formed. This process is known as cell division- The usual method of cell division is very complicated
A vein with woody bundles
Diagram of a small part of a leaf, partly in section, greatly magnified to show cells in this organ.
48
BUILDING MATERIAL OF LIVING THINGS
This type of cell division is called mitosis, and may be divided into four stages : the prophase, shown by 1, 2, 3, 4, 5 ; the metaphase (6) ; the anaphase (7, 8) ; and the telephase (9) . A material in the nucleus (n) called chromatin (ch) generally separates out into a thread (2), a band (3), and then breaks into parts, called chromosomes (4, cs). These arrange themselves at the center of the cell (5). Then the chromo- somes split lengthwise (6) and pass one half to each end of the cell as shown in 7 and 8. A new nuclear membrane (n m) , and a new nucleolus {ns) — are formed in 9. Compare 9, the telephase, with 1, the prophase. Two little dots (1, c) in the cytoplasm are called centrosomes. These separate from each other but are connected by tiny threads, known as spindle fibers (3. st). Look for the centro- somes and spindle fibers in all the diagrams.
CELLS OF VARIOUS SIZES AND SHAPES 49
in both plant and animal cells. You will note from the diagram that the division results in the placing of an equal number of chromosomes in each of the two new cells formed.
Cells of Various Sizes and Shapes. — Plant cells and animal cells are of very diverse shapes and sizes. There are cells so large that they can easily be seen with the unaided eye ; for example, the root hairs of plants and eggs of some animals. On the other hand, certain cells, like the bacteria, are so minute that several million might be present in a few drops of milk. The forms of cells are extremely varied in different tissues ; they may have the shape of cubes, columns, spheres, or flat plates, or may be extremely irregular in outline. One kind of tissue cell, found in man, has a body so small as to be quite invisible to the naked eye, although it has a prolongation several feet in length. Such are some of the cells of the nervous system of man and large animals such as the ox, elephant, and whale.
Varying Sizes of Living Things. — Plant cells and animal cells may live alone, or they may form collections of cells. Some plants are so simple in structure as to be formed of only one kind of cells. Usually living organisms are composed of many groups of different kinds of cells. Such collections of cells may form organisms so tiny as to be barely visible to the eye ; as, for instance, some of the small flowerless plants or many of the tiny animals living in fresh water or salt water. On the other hand, among animals, the bulk of the elephant and whale, and among plants, the big trees of Cali- fornia, stand out as notable examples. The large plants and ani- mals are made up of more, not necessarily larger, cells.
Summary. — This chapter has shown us that the units of build- ing material in living things are called cells. These structures vary greatly in size, shape, and number ; but the size of an individual has little or no bearing on the size of the cells of which it is made.
Animal cells are simply tiny bits of protoplasm, each containing a nucleus and surrounded by a delicate living covering called a membrane. Plant cells as a rule have a cellulose (woody) wall out- side the membrane. This is not alive, but is made by the activity of the protoplasm of the cell. Plant cells also contain large vacu- oles and, if green, chloroplasts (chlorophyll bodies).
The nucleus is evidently the center of activity in the cell. When,
50 BUILDING MATERIAL OF LIVING THINGS
through the assimilation of food materials, a cell has grown to a certain bulk, fixed in each form of animals and plants, the nucleus divides. The chromosomes divide, an equal number going to each new nucleus. Then the cell body divides, and two smaller cells result. This is cell division.
Problem Questions
1. Where and how does growth take place in the body of a plant or animal?
2. In what respects are plant and animal cells alike? How are they dif- ferent ?
3. Why do cells divide, instead of growing larger and larger in size?
4. What is protoplasm ? Whj^ can we not see it in our bodies ?
5. What changes take place in the cell when it divides ? Study the dia- grams carefully.
6. What kinds of tissues can j^ou find in your own body without the use of the compound microscope ? Can you find any plant tissues ?
Problem and Project References
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Densmore, General Botany. Ginn and Company.
Locy, Biology and Its Makers. Henry Holt and Company.
Newman, Outlines of General Zoology. The Macmillan Company.
Wilson, The Cell in Development and Heredity. The Macmillan Company.
CHAPTER VI
PLANT GROWTH AND NUTRITION. CAUSES OF
GROWTH
Problems : What causes a young plant to grow f What is the relation of the young plant to its food supply ? What are the outside conditions necessary for germination f What does the young plant do with its food supply f How is a plant or animal able to use its food supply f How does a plant or animal prepare food to use in various parts of the body?
Laboratory Suggestions
Laboratory/ exercise. Examination of a bean in the pod. Examination and identification of parts of a bean seed.
Laboratory demonstration. Tests for the nutrients : starch, fats or oils, protein. Proof that such nutrients exist in beans.
Home work. Test of various common foods for nutrients. Tabulate re- sults.
Home work. Char (partly burn) various kinds of common foods to find if carbon is present.
Extra home work hy selected pupils. Factors necessary for germination of beans. Demonstration of experiments to class.
Demonstration. Proof that materials are oxidized within the human body.
Demonstration. Oxidation takes place in growing seeds. Test for oxida- tion products. Oxygen necessary for germination.
Laboratory exercise. Examination of corn on the cob, the corn grain, lon- gitudinal sections of a corn grain stained with iodine to show that the embryo is distinct from the food supply.
Demonstration. Test for grape sugar.
Demonstration. Grape sugar present in growing corn grain.
Demonstration. The action of diastase on starch. Conditions necessary for action of diastase.
How Seeds are formed. — We have seen that the pollination of flowers results in the growth of the fruit containing the seeds of a plant. A bean pod is an example of one kind of fruit technically known as a leg'ume. Each seed in the pod contains a young plant or em^bryo.
51
52
PLANT GROWTH AND NUTRITION
'Remains of sf-i^a and sf/le
Funiculus
Microp/le Hilum
What makes a Seed grow. — The general purpose of the pages that follow will be to explain how the baby plant, or embryo,
is able to grow into an adult plant. Two sets of factoi's are necessary for its growth : first, the presence of food to give the young plant a start ; second, certain stimulating factors outside the young plant, such as air, moisture, and warmth.
If we open a bean pod, we find the seeds lying along one edge of the pod, each one attached to the inner wall by a little stalk. If we pull a single bean seed from its attach- ment, we see that the stalk leaves a scar on the coat of the bean: this scar is called the hi'lum. The thick outer coat {testa) is readily removed from a soaked bean, the delicate coat under it easily escaping notice. The seed separates into two parts ; these are called the cotyle'dons. If jqm pull apart the cotyledons very care- fully, you find certain other structures between them. The rod- like part is called the hypocofyl (meaning under the cotyledons). This will later form the root (and part of the stem) <:c^///^c/o/7^...=«..===— =...,,^re./a
of the young bean plant. The first true leaves, very ^'"^'■/^^ tiny structures, are folded together between the coty- ledons, and are known as the plu'mule or epicot'yl (meaning above the cotyle- dons) . All the parts of the seed within the seed coats together form the embryo or young plant. A bean seed contains, then, a tiny plant protected by a tough coat.
-Calyx 'Receptacle
Bean pod and enlarged bean. The pistil of the flower of the bean plant becomes the fruit. The ovules develop into the seeds, and the egg cell becomes the embryo.
Hilum
A section through the bean shows that the embryo is made up of undeveloped leaves, the plumule ; an undeveloped shoot, the hypocotyl ; and nourishing structures, the cotyledons.
FOODS 53
Food in the Cotyledons. — The problem now before us is to find out how the embryo of the bean is adapted to grow into an adult plant. Up to this stage of its existence it has had the advantage of food and protection from the parent plant. Now it must begin the battle of life alone. We shall find in all our work with plants and animals that the problem of food supply is one of the most important problems to be solved by the growing organism. Let us see if the embryo is able to get a start in life (which many animals get in the egg) from food provided for it within its own body.
What are Foods? — We have some knowledge of foods in our daily life. We eat meat, vegetables, fruits, and cereals ; we know that they have come from the bodies of plants or animals. That such foods are organic (of living origin) there can be no doubt. But we could not live without water, which is inorganic (of non- living origin) ; and experiments have proved that both plants and animals need certain compounds of iron, potassium, sodium, and other mineral salts, in order to live. It is evident, then, that foods may be organic or inorganic.
Organic Nutrients. — Organic foods are made up of two kinds of substances, the nu'trients and the wastes or refuse. The organic nutrients are classed in four groups :
Carhohy'drates are the simplest of the very complex chemical compounds called organic nutrients. They are composed of car- bon, hydrogen, and oxygen, the two latter elements in a propor- tion to form water. Starch (CeHioOs) and grape sugar (C6H12O6) are common examples of carbohydrates.
Fats and Oils are, like carbohydrates, composed of carbon, hydrogen, and oxygen, but in some proportion which enables them to unite readily with oxygen.
Proteins (pro'te-inz) are the most complex of all nutrients in their composition, and have, besides carbon, oxygen, and hydro- gen, the element nitrogen and minute quantities of other elements.
Vitamins (vi'td-mmz), a very important group which will be discussed in a later chapter.
Test for Starch. — If we boil water with some laundry starch m a test tube, then cool it and add to the mixture two or three
B. NEW CIV. BIOL. — 5
54
PLANT GROWTH AND NUTRITION
drops of iodine solution/ we find that the mixture in the test tube turns purple or deep blue. It has been learned after many experi- ments that starch, but no other known substance, is turned purple
Colors seen in test for starch,
A, before; B, after, a grain of corn has been tested with iodine.
or dark blue by iodine. Therefore, iodine solution is used as a test for the presence of starch.
Starch in the Bean. — If we mash up a little piece of a bean coty- ledon which has been previously soaked in water, and test with iodine solution, the characteristic blue-black color appears, show- ing the presence of starch. If a little of the stained material is mounted in water on a glass slide under the compound microscope, we shall find that the starch is in the form of little ovoid bodies called starch grains (figure, page 60). The starch grains and other food products are made use of by the embryo.
Test for Oils. — If a substance is rubbed on brown paper or is placed on paper and then warmed in an oven, the presence of oil will be shown by a translucent spot on the paper .^
Protein in the Bean. — Another nutrient present in the bean cotyledon is protein. Several tests are used to detect the presence of this nutrient. The following is one of the best known :
Place in a test tube the substance to be tested ; for example, a
1 Iodine solution is made by simply adding a few crystals of iodine to 95 per cent alcohol ; or, better, take by weight 1 gram of iodine crystals, i gram of iodide of potassium, and dilute to a dark brown coior in weak alcohol (35 per cent) or distilled water.
2 The proportion of oil in beans is small, ft is better to try this test on a walnut.
TESTS FOR NUTRIENTS
55
bit of hard-boiled white of egg. Pour over it a little strong (80 per cent) nitric acid and heat gently. Note the color that appears — a lemon yellow. If a little ammonium hydrate is
n
-h Nitric acid
+ Hesi
Ammonium liydrate
\^
Colors seen in test for protein. There are two distinct steps in this test.
A, corn grain before the test; B, when treated with nitric acid ; C, at completion of test, after treatment with ammonium hydrate.
added (preferably after washing the egg in water), the color changes to a deep orange. This change shows that a protein is present.
If the protein is in a liquid state, its presence may be proved by heating, for when it coagulates or thickens, as does the white of an egg when boiled, protein in the form of an alhu'min is present.
Another characteristic protein test easily made at home is burning the substance. If it gives off the odor of burning feath- ers or leather, then protein forms part of its composition.
A test of the cotyledon of a bean with nitric acid and ammonium hydrate shows us the presence of protein. Beans are found by many tests to contain about 23 per cent of protein, 59 per cent of carbohydrates, and 2 per cent of oils. The young plant within a bean is thus shown to be well supplied with nourishment until it is able to take care of itself. In this respect it is somewhat like a young animal within the egg, — a bird or fish, for example.
Germination of the Bean. — If dry seeds are planted in dry saw- dust or dry earth, they will not grow. A moderate supply of water must be given to them. If seeds are kept in a freezing tempera- ture or at a very high temperature, no growth will take place. A moderate temperature and a moderate water supply are most favorable for their development.
56
PLANT GROWTH AND NUTRITION
Developed fiypocof /I-
stages in the germination of the kidney bean.
If some beans are planted so that we can make a record of their
growth, we shall find the first signs of germination to be the break- ing of the testa and the pushing outward of the hypocotyl to form the first root, which grows downward. A later stage shows the hypoco- tyl forming an arch and dragging the bulky cotyledons after it. The stem, as soon as it is released from the ground, straightens up. The cotyledons open, and between them the budlike plumule or epi-
cotyl grows upward, forming the first true leaves and all of the
stem above the cotyledons.
As growth continues, we
notice that the cotyledons
become smaller and smaller,
until their food contents are
completely absorbed into
the young plant. The young
plant now has roots and
leaves and is able to care
for itself and may be said
to have passed through the
stages of germination. What makes an Engine
go. — If we examine the
sawdust or soil in which the
seeds are growing, we find
it forced up by the growing
seeds. Evidently work was
done ; in other words, energy was released by the /Seeds. A
familiar example of release of energy is seen in an engme. Coal
Experiment to show the function of the cotyledons of the pea: a, plant with both cotyledons, b, with one removed, c, with both removed. A, at end of one week; B, at end of three weeks.
OXIDATIOIN OF FOOD
57
is placed in the firebox and lighted, and the lower door of the furnace is opened so as to make a draft of air which will reach the coal. You know the result. The coal burns, heat is re- leased, causing the water in the boiler to make steam, the engine wheels to turn, and work to be done. Let us see what happens from the chemical standpoint.
Coal, Organic Matter. — Coal is formed largely from dead plants, long ago pressed into its present hard form. It contains a large amount of the chemical element carbon. We have al- ready observed (page 11) one of the effects of the oxidation of carbon as proved by the limewater test. Let us now apply this test to the oxidation of food substances in our own bodies.
Oxidation in our Bodies. — If we expel the air from our lungs through a tube into a bottle of limewater, we notice that the lime- water becomes milky. Evidently carbon dioxide is formed in our own bodies. In fact, the heat of the body (98.6° Fahrenheit) is due to oxidation within the body. Food is also oxidized within the human body to release energy for our daily work. In fact, all living things, both plant and animal, release energy as the result of oxidation of food within their cells. Let us prove this by an experiment with some peas.
Food oxidized in Ger- minating Seeds. — If we take equal numbers of soaked peas, placed in two bottles, one tightly stoppered, the other hav- ing no stopper, both bottles being exposed to identical conditions of light, temperature, and moisture, we find that the seeds in both bottles start to germinate, but that those in the closed bottle soon stop, while those in the open jar continue to grow.
Experiment that shows the necessity for air in germination.
58 PLANT GROWTH AND NUTRITION
Why did not the seeds in the covered jar germinate? To answer this question, let us carefully remove the stopper from the closed jar and insert a lighted candle. The candle goes out at once. The surer test of limewater shows the presence of carbon dioxide in the jar. The carbon of the foodstuffs of the pea united with the oxygen of the air, forming carbon dioxide. Growth stopped as soon as the oxygen was exhausted. The presence of carbon dioxide in the jar is an indication that a very important process which we associate with animals rather than with plants, that of respiration, is taking place. The seed, in order to release the energy locked up in its food supply, must have oxygen, so that the oxidation of the food may take place. Hence a constant supply of fresh air is an important factor in germination. It is important that air should penetrate between the grains of soil around a seed. Frequent stirring of the soil makes it easier for air to reach the seed.
Structure of a Grain of Corn. — Examination of a well-soaked grain of corn discloses a difference in the two flat sides of the grain. A light-colored area found on one surface marks the po- sition of the embryo ; the rest of the grain contains the food sup- ply. The interesting thing to remember here is that the food supply is outside of the embryo.
^ A grain cut lengthwise perpendicular to
^'^ the flat side and then dipped in weak iodine Cotyledon shows two distinct parts, an area containing Plumule considerable starch, the en'dosperm, and the embryo or young plant. Careful inspection shows the hypocotyl and plumule (the latter pointing toward the free end of the grain) Hypocotyl and a part surrounding them, the single coty-
ledon (see figure). Here again we have an Section of corn grain, ^^ample of a fitting for future needs, for in this fruit the one seed has at hand all the food material necessary for rapid growth, although the food is here outside the embryo.
Endosperm the Food Supply of Corn. — We find that the one cotyledon of the corn grain does not serve the same purpose to the young plant as do the two cotyledons of the bean. Although we find a little starch in the corn cotyledon, still it is evident from
TEST FOR GRAPE SUGAR
59
our tests that the endosperm is the chief source of food supply. The study of a thin section of the corn grain under the compound microscope shows us that the starch grains in the endosperm are large and regular in size. When the embryo has grown a little, an examination shows that the starch grains near the edge of the coty- ledon are much smaller and quite irregular, having large holes in them. We know that the germinating grain has a much sweeter taste than that which is not growing. This is noticed in sprouting barley or malt. We shall find later that, in order to make use of starchy food, a plant or an animal must in some manner change it to sugar. This change is necessary, because starch will not dis- solve in water, though sugar will ; and in fluid form substances can pass from cell to cell in the plant and thus go where they are needed. A Test for Grape Sugar. — Place in a test tube the substance to be tested and heat it in a little water so as to dissolve the sugar.
liliii
+
V
^^
HEAT
Colors seen in test for grape sugar.
A, a dry corn grain; B, a germinated corn grain, tested for grape sugar.
Add to the fluid twice its bulk of Fehling's solution. ^ Heat the mixture, which should now have a blue color, in the test tube. If grape sugar ^ is present in considerable quantity, the contents of the tube will turn first a greenish, then a yellow, and finally a brick- red color. Smaller amounts will show kss decided red. No other
^ Directions for making Fehling's solution, and Benedict's solution (page 60), will be found in Hunter's Laboratory Problems iii Civic Biology.
2 Grape sugar, or glucose, is a simple kind of sugar found in many plants.
60 PLANT GROWTH AND NUTRITION
food substance than grape sugar and certain other sugars will give this reaction.^ If Benedict's test is used, a colored precipitate will appear in the test tube after boiling, if grape sugar is present. Starch changed to Grape Sugar in the Corn. — That starch is changed to grape sugar in the germinating corn grain can easily
be shown. First, cut length- wise through the embryos of half a dozen grains of corn and test with Fehling's solution to show that no grape sugar is present. Then test in the same way some lAfheBf Oaf Besn Corn Pes ^^^:^^^ ^j^^^ have just begun to
Starch grains, magnified. • , i ^u •^^ '
germmate, and they will give a reaction showing the presence of sugar along the edge of the cotyledon and between it and the endosperm.
Digestion. — This change of starch to grape sugar in the corn is due to a process called digestion. If you chew for a little time a bit of unsweetened cracker — which we know contains starch — it will begin to taste sweet, and if the chewed cracker is tested with Fehling's solution, some of the starch will be found to have changed to grape sugar. Here, again, the process of digestion has taken place. Both in the corn and in the mouth, this change is brought about by the action of chemical substances known as digestive ferments, or enzymes (en'zimz). Such substances have the power under certain conditions to change insoluble foods — solids — into soluble substances. The result is that foods which before diges- tion would not dissolve in water will dissolve after being digested.
The Action of Diastase on Starch. — The enzyme found in the cotyledon of the corn, which changes starch to grape sugar, is called diastase (dfd-stas) . It may be separated from the cotyledon and is prepared by chemists for use in the form of a powder.
To a little starch in half a cup of water add a very little diastase (1 gram) and put the vessel containing the mixture in a warm place, where the temperature will remain nearly constant at about 98° Fahrenheit. Testing part of the contents at the end of half an hour, for starch and for grape sugar, we find both of them present. If the rest of the mixture is tested the next morning, it will be
1 Ordinary cane sugar or beet sugar will not give this reaction.
DIGESTION IN PLANTS AND ANIMALS
61
found that the starch has been completely changed to grape sugar. Starch and warm water alone under similar conditions will not react to the test for grape sugar.
Digestion has the Same Purpose in Plants and in Animals. — In our own bodies we know that solid foods taken into the mouth are broken up by the teeth and moistened by saliva. If we could follow that food, we should find that eventually it became part of the blood. It was made soluble by digestion, and in a liquid form was absorbed into the blood. Once a part of the body, the food is used either to release energy or to build up the body.
A Summary of Nutrients and Their Tests
|
Name |
Chemical Composition |
Test |
|
Starch |
Contains Carbon (C) Hydrogen (H) Oxygen (0) |
Solution of iodine turns it dark blue. |
|
Grape sugar |
Contains Carbon (C) Hydrogen (H) Oxygen (0) |
Forms brick-red precipitate when heated to boiling with Fehling's solution. Forms greenish, yellow, or red precipitate when boiled with Benedict's solution. |
|
Fats and oils |
Contain Carbon (C) Hydrogen (H) Oxygen (0) |
Leave a grease spot on paper after heating. May be extracted by mashing up substance with ether. |
|
Proteins |
Contain Carbon (C) Hydrogen (H) Oxygen (0) Nitrogen (N) and usually Sulphur (S) and other elements |
Turn yellow when heated with strong nitric acid, and then turn orange after addition of ammonium hydrate. |
|
Burning test (odor) Coagulation test (white of egg) |
||
|
Mineral matter |
Many elements, espe- cially Sodium (Na), Calcium (Ca), Iron (Fe) |
Remains as grayish ash after burning food in hot flame for long period. |
|
Water |
Hydrogen (H) Oxygen (0) |
Passes off from food when heated, as water vapor, and can be collected on cold metal or glass, as drops of water. |
62 PLANT GROWTH AND NUTRITION
Summary. — We have learned :
1. That seeds, in order to grow, must possess a food supply either in or around their embryos.
2. That this food supply contains starch, fats, and proteins.
3. That this food supply must be oxidized before energy is released.
4. That in cases where the food is not stored at the point where it is to be used, the food must be digested, so that it may be transported from one part of the plant to another.
The life processes of plants and animals, so far, may be considered as alike ; they feed, take in oxygen, release energy, and grow.
Problem Questions
1. What conditions outside a seed are necessary to make it grow? What conditions inside the seed?
2. What are organic foods? Inorganic foods? How do plants differ from animals in the use of food ?
3. Of what use might food tests be to a boy or girl ?
4. Compare an engine with a plant or an animal. In what ways are they alike?
5. How do corn grains and bean seeds differ? In what respects are they aUke?
6. What is digestion? How is it brought about? Of what use is it to a plant ? To an animal ?
Problem and Project References
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Atkinson, First Studies of Plant Life. Ginn and Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Part I. American Book
Company. Dana, Plants and Their Children. American Book Company. Densmore, General Botany. Ginn and Company. Duggar, Plant Physiology. The Macmillan Company. Hodge, Nature Study and Life, Chap. XXX. Ginn and Company. Lubbock, Flowers, Fruits, and Leaves. The Macmillan Company. Moore and Halligan, Plant Production. American Book Company. Transeau, General Botany. World Book Company. United States Department of Agriculture Year Books will give project refer-
CHAPTER VII
ORGANS OF NUTRITION. ROOTS IN RELATION TO THE SOIL
Problems : What is soil and what does a plant get from it? What determines the direction of growth of roots f How is a root fitted for the work it has to do f How do roots absorb water and soil salts? What is diffusion? What is osmosis?
Laboratory Suggestions
Laboratory or Home Experiment. How to determine the presence of organic matter in soil.
Laboratory demonstration. To test the capacity of soils for holding moisture.
Home experiment. To show the effect of gravity on a growing root.
Laboratory demonstration. Structure of a root in cross section.
Laboratory exercise. Roots and root hairs.
Laboratory demonstration. To show that roots give off acid.
Laboratory demonstration. To show diffusion and osmosis.
Use of the Root. — If one of the seedlings of the bean is allowed to grow in sawdust and is given light, air, and water, sooner or later it will die. Soil is part of its natural environment, and the roots which come in contact with the soil are very important. It is the purpose of this chapter to find out just how the young plant is fitted to get what it needs from this part of its environment; namely, the soil.
Composition of Soil. — As any one knows, the soil is composed of different substances in different localities. Contrast the black soil of Minnesota or Illinois with the sandy soil of Maine or California, or the red clay of Virginia. If we examine a small mass of garden soil carefully, we find that it is composed of numer- ous particles of varying size and weight. Between these particles, if the soil is not caked and hard packed, we can find tiny spaces, which are formed and enlarged when the soil is tilled. They allow the penetration of air and water. If we examine soil under the
63
64
ROOTS
microscope, we find considerable water clinging to the soil particles and forming a delicate film around each particle.
Under the microscope, also, most soils are seen to contain par- ticles of different kinds. Some are tiny pieces of rock, like those still being formed where solid rock is exposed to the weather. Rain, cold, and ice, working alternately with heat, chip off pieces of rock. These pieces in time may be worn smaller b}' the action of ^4nds, running water, and in some places by glaciers. These processes of soil making are aided by oxidation. A glance at crum- bling stones will give you an example of this, in the yellow oxide of iron (rust) disclosed. So by slow degrees the earth became covered
Inorganic soil is being formed b\ weatliering.
Forests help to cover inorganic soil -^dth an organic coating.
with a coating of what we call inorganic soil. Later, generation after generation of tiny plants and animals which lived in the soil died, and their remains formed the first organic materials of the soil.
You are all familiar with the difference between so-called rich soil and poor soil. The dark soil contains more dead plant and animal matter, which forms the portion called humus.
Humus contains Organic Matter. — It is eas}^ to prove that black soil contains organic matter, for if equal weights of care- full}^ dried humus and of soil from a sandy road are heated red- hot for some time and then reweighed, the humus will be found to have lost considerably in weight, and the sandy soil to have lost very little. The material left after heating is inorganic materiaL the organic matter having been burned out.
MATERIALS IN THE SOIL 65
Capacity of Soil for holding Water. — Soil containing organic materials holds water much more readily than inorganic soil, as a simple experiment shows. If we fill vessels of equal size (such as the one shown in the figure) with gravel, sand, barren soil, rich loam, leaf mold, and pulverized leaves — all dry — then pour equal amounts of water on them and measure all that runs through, the water that has been retained will represent the water supply that plants could draw on from such soil.
Soil Water a Solution of Mineral Salts. — Water, as it passes through the soil, gradually dissolves very minute portions of the chemical compounds of which the soil is composed, so that soil water is really a dilute solution of mineral salts.
Capillarity. — Water moves against the force of gravity by means of a physical phenomenon, that of capillar' ity. We know that water will rise against Apparatus for the force of gravity between two closely placed glass +^^?^? ^°^l ^^^^ plates or in a tube of very small bore. This is due to the fact that the particles or molecules of water are attracted to or adhere to the glass. Soil water adheres in the same way to the soil particles, and thus rises in the ground.
Nitrogen in a Usable Form Necessary for Growth of Plants. — A chemical element needed by the plant to make protoplasm is nitrogen, but this element cannot be taken in an uncombined state from either soil water or air. It is usually obtained from the organic matter in the soil, where it exists with other substances in the form of ni'trates. Ammonia and other organic compounds which con- tain nitrogen are then changed by microscopic plants called bacteria, first into nitrites and then into nitrates,
A Plant needs Mineral Matter to make Living Matter. — Liv- ing matter (protoplasm) , besides containing the chemical elements carbon, hydrogen, oxygen, and nitrogen, contains very minute proportions of other elements which make up the basis of certain minerals. These are calcium, sulphur, iron, potassium, magne- sium, phosphorus, sodium, and chlorine.
That plants will not grow well without certain of these mineral substances^ can be proved by the growth of seedlings in a so-called
1 See Hunter's Laboratory Problems in Civic Biology for list of ingredients.
66
ROOTS
nutrient solution. If certain ingredients are left out of this solution, the plants placed in it will not live.
/ "3 T 2 3
Effect of the lack of certain chemical elements on the growth of plants. The three bottles contain (l) distilled water, (2) all the nutrients, (3) all the nu- trients except potassium nitrate. The second half, B, shows the same seedlings one week later than A. Add other bottles omitting in turn calcium sulphate, calcium phosphate, magnesium sulphate, sodium chloride, iron. Why the difference in amount of growth ?
Root System. — If you dig up a young bean seedling and care- fully wash the dirt from the roots, you will see that a long root is developed as a continuation of the hypocotyl. This root is called the primary root. Other smaller roots which grow from the pri- mary root are called secondary, and the roots growing from the latter are called tertiary roots. Evidently the root acts as an anchor for the plant, but does it have other uses?
Downward Growth of Root. In« fluence of Gravity. — Most of the roots examined take a more or less downward direction. We are all familiar with the fact that the force we call gravity influences life upon this earth to a great degree. Does gravity act on the growing root? This question may be answered
A root system, showmg primary, / , '^
secondary, and tertiary roots. by a simple experiment.
GROWTH OF ROOTS
67
Plant mustard or radish seeds in a pocket garden, place it on one edge, and allow the seeds to germinate until the root has grown to a length of about half an inch. Then turn it at right angles to the first position and allow it to remain for one day undisturbed. The root tips will be found to have turned in response to the change in position, and to point down again. That part of the root near the growing point is the one most sensitive to the change. Thi? experiment indicates that the roots are influenced to grow downward by the force of gravity and that the growing point is most responsive to this stimulus. Roots are positively geotropic.
Root of a radish in a pocket garden that is turned in different positions at intervals of a day or more.
Water a Factor which determines the Course taken by Roots. — Water, as well as the force of gravity, has much to do with the direction taken hy roots. If radish seeds germinate on the under side of a moist sponge suspended in the air, their roots will turn against gravity and cling to the wet surface of the sponge. Water is always found below the surface of the ground, but sometimes at a great depth. Most trees and all grasses have a greater area of surface exposed by the roots than by the branches. The roots of alfalfa and sugar beets, in our Western States, often penetrate the soil for a dis- tance of ten to twenty feet below the surface, until they reach that part of the soil which is always moist with underground water.
The Fine Structure of a Root.^ — Let us now examine a root with the aid of the compound microscope, in order to see how it is fitted for the work of taking in and circulating soil water. We find the root to be made up of cells, the walls of which are rather thin. Over the lower end of the root is found a collection of cells, most of which are dead, loosely arranged so as to form a cap over
^ Sections of tradescantia roots are excellent for demonstration of these structures.
68
ROOTS
the growing tip. This is evidently an adaptation which protects the young and actively growing cells just under the root cap. In the body of the root a central cylinder of wood can easily be distin- guished from the surrounding cortex. In a longitudinal section a series of tubelike structures may be found within the central cylinder. These structures are made up of cells which have grown together end to end, the long axis of the cells running the length of
Central cylinder Food passes down here
Walerpdsses up here
■Epidermis Corf-ex
Root cap
Diagram of section of a root tip, showing structure.
^ood/ bundle
Diagram of a root tip, showing root hairs, greatly magnified.
the main root. In their development these cells have grown to- gether in such a manner as to lose their small connecting ends, and now form continuous hollow tubes with rather strong walls. Other cells have developed greatly thickened walls, which give mechani- cal support to the tubelike cells. Collections of such tubes and supporting woody cells together make up what are known as fihrovas' cular bundles in the wood.
Root hairs. — Careful examination of the root of one of the seedlings of mustard, radish, or barley grown in a pocket germi- nator shows a covering of tiny fuzzy structures, at most 3 to 4
ROOT HAIRS
69
Plumuh
Roof- hairs
Root hairs on a corn
seedling-
millimeters in length, called root hairs. They vary in length according to their position on the root, the most and the longest root hairs being found some distance back from the tip. They are outgrowths of the outer layer of the root, the epidermis, and are of very great im- portance to ths living plant.
Structure of a Root Hair. — A single root hair examined under a compound microscope will be found to be a long, threadlike struc- ture, almost colorless in appearance. The cell wall, which is very flexible and thin, is made up of cellulose, through which fluids may easily pass. Clinging close to the cell wall is the protoplasm of the cell, the outer border forming a very delicate membrane. The interior of the root hair contains many vacuoles, or spaces, filled with a fluid called cell sap. Forming a part of the living proto- plasm of the root hair, sometimes in the hairlike prolongation and sometimes in that part of the cell which forms the epidermis, is found a nucleus. The nucleus, the membrane, and the rest of the protoplasm are alive ; the cell wall, formed by the living matter
in the cell, is dead. The root hair is a living plant cell with a membrane and wall so delicate that water and dissolved mineral sub- stances from the soil can pass through them into the interior of the root.
The Root Hairs take More than Water out of the Soil. — If a root con- taining a fringe of root hairs is washed carefully, it will be found to have little particles of soil still clinging to it. Examined under the microscope, these particles of soil seem to be cemented to the sticky surface of the root hair. The soil contains, besides a number of chemical compounds of
merfilm •Soil particle
Diagram of a root hair, with adjacent root cells and particles of soil.
70
ROOTS
various mineral substances, — lime, potash, iron, silica, and many others, — a considerable amount of organic material. Acids of various kinds are present in the soil. These acids so act upon certain of the mineral substances that they become dissolved in the water which is absorbed by the root hairs. Root hairs also give off small amounts of acid, which assist in dissolving minerals. An interesting experiment may be shown to prove this. A solution of
phenol phthaVein loses its color when an acid is added to it. If the roots of a growing pea are placed in a tube con- taining some of this solution, very sUghtly alkaline, the latter will soon change from a dehcate pink to a color- less solution.
It is eas3^ to ssly that the delicate root hairs absorb water, but it is much more difficult to understand the process, be- cause it involves the understanding of certain physical phenomena. But since absorption is a process common to both plants and animal cells and is of vitsl importance, let us study it carefully.
Diffusion. — We all know that certain substances, such as the odor of tobacco smoke or the perfumes of flowers, pass rapidly from the point where they are given off and tend to spread in all direc- tions through the air. The odor of the orange blossoms in California is a memor}' to those who have driven near the orange groves. Substances which will dissolve in liquids will also diffuse through the liquids. A httle powdered e'osin placed in a glass of water will soon make a glass of red ink, so completely does the eosin become dissolved and diffused through the liquid. In the diffusion of both gases and liquids particles of the substance pass from the place where they are most concentrated to where they are less concentrated, or lacking, the rate of travel being much slower in Hquids than in gases.
Imbibition. — The passage of water from point to point by
Effect of root hairs on phe- nolphthalein solution. The change of color indicates the presence of acid.
OSMOSIS
71
capillarity does not account for soil water getting inside the cell. It has to go through the cellulose wall and the delicate membrane within. The walls of cells, like wood, absorb soil water readily by a process known as imbibition. This brings the soil water in con- tact with the cell membrane. Inside the cell membrane is a liquid which would diffuse freely with the soil water if the membrane were removed. But a membrane acts peculiarly toward diffusing substances. An experiment will help us to understand this.
Osmosis. — If we carefully break away part of the shell of an egg so as to expose the delicate skin or membrane underneath, we have a picture of the relation of the cell membrane (like the egg skin) to the cell wall (like the egg shell) . If this egg is placed in a glass of cold water, within a short time the membrane will bulge out, showing that water has passed into the egg through the membrane. If, however, we test the water in the glass for protein, the organic sub- stance of which white of egg is composed, we shall find none. Evi- dently the egg membrane will per- mit the passage of water but not of protein. Such a membrane is said to be semi-yer'meable. It is this kind of membrane that surrounds plant and animal cells. It will permit certain substances such as water to pass through it readily in either direction, and it will per- mit certain substances in solution to pass less readily, while still other substances will not be permitted to pass through at all.
Another experiment will help us. If we take a thistle tube, fill the lower end with a solution of grape sugar and water, then tie tightly over it an animal membrane (such as pig's bladder), and place the apparatus in water, as shown in the figure on the following page, we notice that after a very little time the fluid in the thistle tube begins to rise. Evidently water passes into the tube more rapidly than the substance inside can pass out. If we could see the separate particles, or molecules, of the water and of the solution of water and sugar, they would be found to arrange them- selves on each side of the membrane so as to cover it completely. But since the water molecules diffuse easily through the membrane
Osmosis through the membrane of an egg.
72
ROOTS
n
|
! |
|
|
- |
1 |
|
"~ |
— ~_^— i| |
From the text on. pages 71, 72 explain why the water rises in one tube.
and the sugar molecules do not diffuse easily, it will be seen that the inner side of the membrane does not present so much space
for the diffusion of water par- ticles as does the other side. Hence the flow of water into the tube is more rapid than the flow out of the tube, and the water gradually rises in the thistle tube. This diffusion of water through a semipermeable mem- brane is known as osmo'sis. It will be seen that the greater flow of water particles or mole- cules is from the point of greater concentration of water to the point of lesser concentration of water; hence it is a true diffusion. And since the solution Tvithin the thistle tube is inclosed, the process causes a pressure by the solution within these closed walls. This is knowTi as osmot'ic pres- sure. This pressure, if continued, would burst the egg membrane in the experiment first noted and is a very important force in cir- culating the water in the root hair.
Why the Root Hair absorbs Water and Soil Salts. — The wall of the root hair readily takes in water and dissolved soil salts by im- bibition. The outer edge of the protoplasm forms a semi-per- meable membrane, which, while allowing water and mineral salts in solution to diffuse toward the inside, will not allow the diffusion outward of the sugar and other soluble materials mthin the cell. Hence an inward flow of soil water is started. As soon as the outer cells have increased their holdings of soil water, an osmosis inward is started because the water tends to flow from the place of its greater concentration to the place of lesser concentration. Mineral salts in solution are carried along with the water so that the needed soil substances are carried along from cell to cell, until they reach the small tubes of the central cjdinder. Here other factors help the water up in the root ; of these, capillarity and the pull exerted by evaporation from the upper parts of the plant are believed to be the most important. We shall learn more about this later.
IMPORTANCE OF DIFFUSION AND OSMOSIS 73^
Physiological Importance of Diffusion and Osmosis. — The
processes of diffusion and osmosis are of great importance not only to a plant, but also to an animal. Foods are digested in the food tube of an animal ; that is, they are changed into a soluble form so that they may pass through the walls of the food tube and become part of the blood. The inner lining of part of the food tube is thrown into millions of little fingerlike projections called villi, which look somewhat, in size at least, like root hairs. These fingerlike processes are (unlike a root hair) made up of many cells, but they serve the same purpose as the root hairs, for they absorb liquid food into the blood. This process of absorption is not entirely understood, but is largely by diffusion and osmosis. With- out these processes we should be unable to use most of the food we eat.
Summary. — This chapter has first shown us that rocks, the original earth material, have been broken down into the fragments we call inorganic soil. To this soil have been added, through the process of decay, the bodies of plants and animals which once covered the earth. Plants take out of the soil, water and soluble salts which are used by the plant in making food and eventually living matter.
The structures by means of which the soil water is absorbed are called root hairs. These are elongated projections from cells of the outer covering of the root.
The methods by which the fluids are taken into the root hairs and circulated through the cells of the root are known as diffusion and osmosis. Since the membrane of the root hair and other cells is semi-permeable, allowing the passage of some substances but not of others, a flow of soil water is established toward the in- side of the cell, because the membrane prevents the substances in the cell sap from flowing out. Thus osmotic pressure is established and roots are able to take in large amounts of water and soil salts.
Problem Questions
1. What are the chief differences between "poor" and "rich" soil?
2. How is soil able to hold water?
3. How are roots adapted to do their work?
4. What part of the root is most sensitive to gravity? Prove your answer by experiment.
74 ROOTS
5. Where do root hairs grow most abundantly? Where are they largest?
6. How could you prove that root hairs give off acid?
7. Distinguish clearly between diffusion and osmosis.
8. What is meant by osmotic pressure and how might it be brought about ?
Problem and Project References
Hunter, Laboratory Problems in Civic Biology. American Book Company. Coulter, Barnes, and Cowles, A Textbook of Botany, Part II. American Book
Company. Duggar, Plant Physiology. The MacmiUan Company. Goodale, Physiological Botany. American Book Company. Transeau, General Botany. World Book Company.
CHAPTER VIII HOW GREEN PLANTS MAKE FOOD
Problems: To study the structure of a leaf in order to find out how moisture is given off.
To find the reaction of leaves to light.
To study photosynthesis : the conditions and materials necessary, and the hy -product.
To learn what other functions are performed hy leaves.
Laboratory Suggestions
Demonstration. The passage of fluids up the stem.
Demonstration. Water vapor given off by a plant in sunHght. Loss of weight due to transpiration measured. Laboratory exercise.
(a) Gross structure of a leaf.
(6) Study of stomata and lower epidermis under microscope.
(c) Study of cross section to show cells and air spaces. Demonstration. Reaction of leaves to light. Demonstration. Light necessary in starch making. Demonstration. Chlorophyll necessary in starch making. Demonstration. Air necessary in starch making. Demonstration. Oxygen a by-product of starch making.
What becomes of the Water taken in by the Roots ? — We have seen that more than pure water is absorbed through the root hairs into the roots. What becomes of this water and the other sub- stances that have been absorbed? This question may be partly answered by the following experiments.
Passage of Fluids up the Stem. — If young growing shoots from bean or pea seedlings are placed in red ink (eosin) and left in the sun for a few hours, some of the red ink will be found to have passed up the stem.
Water given off by Evaporation from Leaves. — Take some well- watered potted green plant, as a geranium or hydrangea, cover the pot with sheet rubber, fastening the rubber close to the stem of the plant. Next weigh the plant with the pot. Then cover it with a tall bell jar and place the apparatus in the sun. Id a short
75
76
HOW GREEN PLANTS MAKE FOOD
time drops of moisture are seen to gather on the inside of the jar. If after a few hours we weigh the potted plant again, we find it weighs less than be- fore. Obviously the loss comes from the water vapor which has escaped from stem, or leaves, or both.
The Structure of a Leaf. — In the ex- periment with the red ink and young shoots we shall find that the fluid has gone out into the skeleton or framework of the leaf. Let us now examine a leaf more carefully. It shows usually (1) a flat, broad Made, which may take almost any conceivable shape ; (2) a stalk, o:- pefiole, which spreads out into veins in the blade ; (3) stip'ules, a pair of out- growths from the petiole at its base. In many leaves the stipules fall off early. Some leaves are compound, that is, each of the little leaflike parts is in reality a section of the leaf blade which is so
deeply indented that it is cut away to the midrib or central vein,
as in the rose leaf shown in the figure below.
The Cell Structure of a Leaf. — The outer covering of a leaf, on
both the upper and the lower surfaces, is called the epidermis, and
is composed of large (in
dicotyledons, irregular) cells.
The under surface of most
leaves, as seen through a micro- scope, shows many tiny oval
openings, called sto'mata (sing.
sto'ma). Two guard cells,
usually kidney-shaped, are
found, one on each side of a
stoma. By a change in the
shape of these cells the stoma
is made larger or smaller.
Experiment to prove that water vapor is given off from a green plant.
Compound leaf of rose, showing stipules si.
STRUCTURE OF A LEAF
77
Study of the leaf in cross section shows that the stomata open directly into air chambers which penetrate between and around the loosely arranged cells of spongy tissue composing the under part of the leaf. The upper surface of leaves sometimes contains stomata, but more often it does not. The under surface of an oak leaf of or- dinary size contains about 2,000,000 stomata. Under the upper epidermis is a layer of green cells closely packed to- gether (called collectively the palisade layer) . These cells are more or less columnar in shape. Under them are several rows of the loosely placed cells called collectively the spongy tissue. If we happen to have a section cut through a vein, we find this composed of a number of tubes made up of, and strengthened by, thick- walled cells. The veins are evidently a continuation of the fibrovascular bundles of the
stem out into the blade of
'4r
Opening
Guard cell
[p'idermalcdl
Stoma opening into yr
air spdce in leaf / C
Stomata open
Stomata and guard cells, greatly magnified.
Eplderms Palisdde layer
Vein Spongy tissue
Lower epidermis
Stoma
the leaf (figure, page 47).
Evaporation of Water. — ■ During the day an enor- mous amount of water is taken up by the roots and ' passed out through the leaves in the form of vapor. So rapid is this evaporation, or transpiration, in a small grass plant, that the water evaporated in a day may weigh more than the plant. It is estimated that nearly half a ton of water may be delivered to the air during twenty-four hours by a grass plot 25 by 100 feet, the size of the average city lot. It is estimated that a corn plant in the Central West passes out from its body more than forty gallons of water during its lifetime. Fields of wheat transpire nearly 20 per cent of the total rainfall on
Diagram of section through the blade of a leaf, seen under a compound microscope.
78 HOW GREEN PLANTS MAKE FOOD
their area. The amount of water lost by plants through evapo- ration is many times more than the amount that goes into making food and living matter.
Experiment to show through which surface of a leaf water vapor passes off: Remove two leaves of the same size from some large-leaved plant, as a mullein or a rubber plant. Cover the upper surface of one leaf and the lower surface of the other with vaseline. The leaf stalk of each should be covered with wax or vase- line, and the two leaves exactly balanced on the pans of a balance placed in a warm and sunny window. Within an hour the leaf having its upper surface covered with vaseline will show a loss of weight.
Factors in Transpiration. — The amount of water lost from a plant varies greatly under different conditions. The humidity of the air, its temperature, and the temperature of the plant all affect the rate of transpiration. The stomata also tend to close under some conditions, thus helping to prevent evaporation. Recent experiments indicate that the plant probably has some con- trol over the stomata. The stomata are usually closed at night but remain open from shortly after sunrise until late in the afternoon. They begin to close in the middle of the afternoon, and thus decrease the amount of water lost in the latter part of the day. Plants droop or wilt on hot dry days because they cannot obtain water rapidly enough from the soil to make up for the loss through the leaves. Hairs on the leaf surface, waterproofing of outer cells, a decrease in leaf area, close grouping of leaves to
EFFECT OF LIGHT ON PLANTS
79
prevent evaporation, the absence of leaves, as in the cactus, and the turning of leaves edgewise to light are all modifications which help to hold water in the body of the plant.
Green Plants Food Makers. — We have already seen that green plants are the great food makers for themselves and for animals. We are now ready to learn how green plants make food.
The Sun a Source of Energy. — We know the sun is the source of most of the energy that is received on this earth in the form of heat and light. Every one knows the power of a ^^ burning glass." Solar engines have not come into any great use as yet, because fuel is cheaper, but some day we undoubtedly shall harness the energy of the sun in everyday work. Experiments have shown that as much as 80 per cent of the radiant energy falling on certain green leaves is absorbed. Part of this energy is used by the leaf ; but part is changed to heat, raises the temperature of the leaf, and is lost to the air if the air is cooler than the leaf. Regulation of this temperature is obtained in much the same way as in our own bodies, by evaporation of water. We sweat; the leaf passes off water vapor, largely through the stomata.
Effect of Light on Plants. — In young plants which have been grown in total dark- ness, no green color is found in either stems or leaves, the latter often being reduced to mere scales. The stems are long and more or less reclining.
Two stages in an experiment to show that green plants grow toward the light.
We can explain the changed condition of the seedling grown in the dark only by assuming that lack of light has some effect on the protoplasm of the seedling and induces the growth of the stem. If seedlings have been grow- ing on a window sill, or where the light comes in from one side, you have doubtless noticed that the stem grows toward the source of light and the leaves tend to arrange themselves so as to receive as much light as possible on their upper surfaces. The experiment pictured shows the effect of light very plainly. A hole was cut in
80
HOW GREEN PLANTS MAKE FOOD
one end of a cigar box and barriers were erected in the interior of the box so that the seeds planted in the sawdust received their hght by an indirect course. The young seedUng in this case responded to the influence of the stimuhis of hght so that it grew out finall}^ through the hole in the box into the open air. This growth of the stem to the light is of very great importance to a growing plant, because food making depends largely on the amount of sunlight the leaves receive.
Effect of Light on Leaf Arrangement. — It is a matter of common knowledge that green leaves turn toward the light. Place growing pea seedlings, oxalis, or any other plants of rapid growth near a window which receives full sunlight. Within a short time the leaves will be found in positions to receive the most sunlight possible. Careful observation of any plants growing outdoors shows us that in almost every case the leaves are so arranged as to get much sunlight. The ivy climbing up a wall, the morning- glory, the dandelion, and the bur-
*}^
I
dock, all show different arrange- ments of leaves, each presenting a large surface to the light. Leaves are often definitely ar- ranged, each fitting in between ■^^ 'I others so as to present their
H^^ J upper surface to the sun. Such
an arrangement is known as a leaf mosaic. In the case of the dandelion, a rosette or whorled cluster of leaves is found. In the horse-chestnut, where the leaves come out opposite each other, the older leaves have longer petioles than the younger ones. In the mullein the entire plant forms a cone. The old leaves near the bottom have long stalks, and the little ones near the apex come out close to the main stalk. In every case each leaf receives a large amount of light. Other modifications of these forms may easily be found on a field trip or in a study of house plants.
A dandelion, showing a whorled arrar.ge ment of long irregular leaves.
STARCH MADE BY A GREEN LEAF 81
Starch made by a Green Leaf. — Remember that the upper surface of the leaf is placed toward the sun and that the leaf must be thought of as a solar engine.
If we examine the palisade layer of the leaf, we find cells which are almost cylindrical in form. In the protoplasm of these cells are found a number of tiny green bodies, the chloroplasts or chlo- ^^^^-^ X^
rophyll bodies . If the leaf is placed in wood alcohol, we find that the bodies still remain, but that the color is extracted, going into the , alcohol and giving to it a beautiful green color. The chloroplasts are, indeed, simply part of the proto- plasm of the cell colored green.
rrn IT r xi X J. Variegated leaves of tradescantia.
Ihese bodies are oi the greatest
importance directly to plants and indirectly to animals. The chloroplasts, by means of the energy received from the sun, manu- facture sugars and starches out of certain raw materials obtained from the soil and the air. These raw materials are soil water, which is passed up from the roots through the bundles of tubes into the veins of the leaf, and carbon dioxide, which is taken in through the stomata or pores. A plant with variegated leaves, as the tradescantia or " wandering Jew," makes starch only in the green part of the leaf, though these raw materials reach all parts of the leaf.
The change of color of leaves in autumn seems to be due to loss of chlorophyll, plus the formation of a red pigment in the cells. It is probably not frost that causes leaves to turn but rather a combination of lower temperature with other factors.
Light and Air Necessary for Starch Making. — If we pin strips of black cloth, such as alpaca, over portions of several leaves of a gi-owing hydrangea which has previously been placed in a dark room for a few hours, and then put the plant in direct sunlight for an hour or two, we are ready to test the leaves for starch. We remove the partly covered leaves, boil them to stop further changes, and extract the chlorophyll with wood alcohol (because the green color of the chlorophyll interferes with the blue color of
82
HOW GREEN PLANTS MAKE FOOD
the starch test). A test then shows that starch is present only in the portions of the leaves exposed to sunlight. From this experi- ment we infer that the sun has something to do with starch making
Diagram of experiment to show that sunlight is necessary for starch making. Read the text carefully and then explain this diagram.
in a leaf. The necessity of air for starch making may also easily be proved, for parts of leaves in a plant treal^d as in the previous experiment, if covered with vaseline, will be found to contain no starch, while the parts of the leaf without vaseline, but exposed to the sun and air, do contain starch. The part of the air used in starch making is carbon dioxide, which is always present in the atmosphere in very small amounts, less than 4 parts in 10,000 in fresh air.
Air is necessary for the process of starch making in a leaf, not only because carbon dioxide gas is absorbed but also because the leaf is alive and must have oxygen in order to do its work. This oxygen it takes from the air.
Comparison of Starch Making and Milling. — The manufacture of starch by the green leaf is not easily understood. The process has been compared to the work of a mill. In this case the mill is the chlorophyll of the leaf. The sun furnishes the motive power, the chloroplasts constitute the machinery, and soil water and carbon dioxide are the raw products taken into the mill. The manufactured product is starch,^ and a certain by-product (corresponding to the waste in a mill) is also given out. This by-product is oxygen. To understand the process better, we must refer to the diagram of a small portion of the leaf (page 77) . Here we find that the cells of the green layer of the leaf, under the upper epidermis, perform most of the work. The carbon dioxide is
1 A simple sugar is manufactured and then transformed into starch.
COMPARISON OF STARCH MAKING AND MILLING 83
taken in through the stomata and reaches the green cells by way of the intercellular spaces and by diffusion from cell to cell. Water reaches the green cells through the veins. It then passes into the cells and there becomes part of the cell sap. The light of the sun easily penetrates the cells of the palisade layer, giving the energy needed to make the starch. This whole process is a very delicate one, and will take place only when external conditions are favor able «
Air
Food
Compare the work of this part of a leaf (in starch making) with that of a mill.
For example, too much heat or too little heat stops starch making in the leaf. This building up of starch, with the release of oxy- gen by the chloroplasts in the presence of sunlight, is called yho- tosyn'thesis}
Manufacture of Fats. — Inasmuch as tiny droplets of oil (or fat) are found inside the chlorophyll bodies in the leaf, we believe that fats, too, are made there, probably by a transformation of the starch already manufactured.
Protein Making and its Relation to the Making of Living Mat- ter. — Protein is a part of the food which is necessary to form
^ The process of photosynthesis is very complicated. It probably consists of three steps : first, the taking in of the raw materials, COo and H2O ; second, the rear- rangement of the elements within the green cells ; and third, the formation of very simple sugars, then of more complex sugars, and finally of starch. The process appears to be under the control of enzymes of different kinds, which cause these progressive changes to take place. Enzymes also change the sugars, which are carried to other parts of the plant for storage, into insoluble starch.
84
HOW GREEN PLANTS MAKE FOOD
protoplasm. It is present in the leaf and is found also in the stem and root. Proteins can be manufactm^ed in any of the cells of green plants where starches or sugars and certain salts are found, the presence of light not seeming to be a necessary factor. How they are manufactured is a matter of conjecture. The minerals brought up in the soil water form part of their composition,
and starch or sugar gives three elements (C, H, and 0). The element nitrogen is taken up by the roots as a nitrate (nitrogen in combination with oxygen and some other element, usually with lime or potash) . Proteins are probably not made directly into proto- plasm in the leaf, but are stored by the cells of the plant and used when needed, either to form new cells in growth or to repair waste. ^\Tiile plants and animals obtain their food in different ways, they prob- ably make it into living sub- stance {assimilate it) in the same manner. Foods serve exactly the same purposes in they either are used to build living mattei
A rock split by a growing tiee
Functions of Food.
plants and in animals or they are burned (oxidized) to furnish energy (power to do work) . If you doubt that a plant exerts energy, note how the roots of a tree bore their way through the hardest soil, and how^ stems or roots of trees often split the hardest rocks, as illustrated in the figure. Starch Making and its Relation to Human Welfare. — Leaves which have been in darkness show starch to be present soon after exposure to light. A corn plant may send almost half an ounce of reserve food into the ears in a single day. The formation of fruit and the growth of grain, potatoes, and other food crops, show the economic importance of the work of green leaves. Not only do plants make their own food and store it away, but they m^ke food
GREEN PLANTS GIVE OFF OXYGEN
85
for animals as well ; and the food is stored in such a stable form that it can be kept and sent to all parts of the world. Animals, herbivorous and flesh-eating, man himself, all are dependent upon the starch-making processes of the green plant for the ultimate source of their food. When we consider that in 1924 in the United States the total value of all farm crops was more than $12,000,000,000, and when we realize that these products came from the air and soil through the energy of the sun, we may realize why the study of plant biology is of great importance.
Green Plants give off Oxygen in Sun- light. — In still another way green plants are of direct use to animal life. During the process of starch making, oxygen is given off as a by-product. This may easily be proved by the following experiment. Place any green water plant in a battery jar partly filled with water,i cover the plants with a glass funnel, and mount a test tube full of water over the mouth of the funnel. Then place the apparatus in a warm sunny window. Bubbles of gas are seen to rise from the plant. After two or three hours of hot sun, enough of the gas may be obtained by displacement of the water to prove, by the rapid oxidation of a glowing splinter of wood in the gas, that oxygen is present.
That oxygen is given off as a by-product by green plants is a fact of far-reaching importance. The green covering of the earth is giving to animals an element that they must have, while the animals in their turn are supplying to the plants carbon dioxide, a compound used in food making. Thus a widespread relation of mutual helpfulness exists between plants and animals.
Experiment to show that oxygen is given off by green plants in sunlight.
^ Water contains air in solution, including some carbon dioxide, but the amount may be too small. Immediate success with this experiment will be obtained if the water has been previously charged with carbon dioxide.
H. NEW CIV. BIOL. — 7
86 HOW GREEN PLANTS MAKE FOOD
Respiration by Leaves. — All living things require oxygen. It is by means of the oxidation of food materials within the plant's body that the energy used in growth and movement is released. A plant takes in air with its oxygen largely through the stomata of the leaves, to a less extent through the lenticels in the stem, and through the roots. Thus rapidly growing tissues receive the oxygen necessary for them to perform their work. One of the prod- ucts of oxidation in the form of carbon dioxide is also passed off through these same organs. It can be shown by experiment that a plant uses up oxygen in the darkness ; in the light the amount of oxygen given off as a by-product in the process of starch making is much greater than the amount used by the plant.
Summary. — From the above paragraphs it is seen that a leaf performs the following functions : (1) respiration, or the taking in of oxygen and passing ofT of carbon dioxide ; (2) photosynthesis, or starch making, with the incidental passing out of oxygen ; (3) for- mation of proteins, with their digestion and assimilation to form new tissues ; and (4) the transpiration of water.
Problem Questions
1. Why is it necessary for fluids to pass up a stem into the leaves?
2. Of what use to man is the evaporation of water from leaves?
3. Why does the amount of transpiration vary?
4. Explain the process of photosynthesis.
5. In what respects is this process of value to man?
6. Should green plants be kept in a sick room at night? In the daytime? Explain.
7. Do plants breathe ? How?
Problem and Project References
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Calkins, Biology. Henry Holt and Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Part II and Vol. II.
American Book Company. Densmore, General Botany. Ginn and Company. Duggar, Plant Physiology. The Macmillan Company. Gager, Fundamentals of Botany. P. Blakiston's Son and Company. Goodale, Physiological Botany, pages 337-353 and 409-424. American Book
Company. Transeau, General Botany. World Book Company.
CHAPTER IX
THE CIRCULATION AND FINAL USES OF FOOD BY PLANTS
Problems : To find out where plants store food and what use is made of it.
To study the structure of stems and the passage of liquids up and down them.
To find out how plants digest and assimilate food.
Laboratory Suggestions
Laboratory exercise. The structure (cross section) of a woody stem. Demonstration. To show that food passes downward in the bark. Demonstration. To show the condition of food passing through the stem. Demonstration. Plants with special digestive organs.
The Circulation and Final Uses of Food in Green Plants. — We
have seen that cells of green plants make food — especially the cells that are in the leaves. But all parts of the bodies of plants grow. Roots, stems, leaves, flowers, and fruits grow. Seeds are store- houses of food. We must now examine the stem of some plant in order to see how food is distributed, stored, and finally used in the various parts of the plant.
The Structure of a Dicotyledonous or Woody Stem. — If we cut a cross section through a young willow or apple stem, we find it shows three distinct regions. The center is occupied by the spongy, soft pith ; surrounding this is found the rather tough wood, while the outermost area is hark. More careful study of the bark reveals the presence of three layers — an outer layer, a middle green layer, and an inner fibrous layer. The inner layer is made up largely of tough fiberlike cells known as hast fibers. The most important parts of this inner bark, so far as the plant is concerned, are many tubelike structures known as sieve tuhes. These are long rows of living cells, having perforated sievelike ends.
87
88 CIRCULATION AND USES OF PLANT FOOD
Through these cells food materials pass downward from the upper part of the plant, where they are manufactured.
In the wood will be noticed (see figure) a number of lines called med'ullary rays, or pith rays, radiating outward from the pith
toward the bark. These are Cambium layer yy Annudl rings thin plates of pith which sepa- ^hrdys rate the wood into a number of wedge-shaped masses . The masses of wood contain many elongated cells, which, placed end to end, form thousands of little tubes connecting the leaves with the roots. In addition to these are many thick- walled cells, which give strength to the mass of wood. The bundles of tubes with their surrounding hard-walled cells are the continuation of the bundles of tubes which are found in the root. In sec- tions of wood which have taken several years to grow, we find so-called annual rings. The distance between one ring and the next (see figure) usually represents the amount of growth in one year. Growth takes place from a layer of actively dividing cells, known as the cam'hium layer. This layer forms wood cells from its inner surface and bark from its outer surface. Thus new wood is formed as a distinct ring around the old wood.
Use of the Outer Bark. — The outer bark of a tree is protective. The cells are dead, but the heavy woody skeletons prevent the evap- oration of fluids from within. The bark also protects the tree from attack of plants or animals which might harm it. Most trees are provided with a layer of corky cells. This layer in the cork oak is thick enough to be of commercial importance.
There are small breathing holes known as len'ticels scattered through the surface of the bark. These can be seen easily in a young stem of apple, beech, or horse-chestnut.
Diagram of a twig of box elder three years old, showing three annual growth rings. The radiating lines which cross the wood represent the pith rays or medullary rays.
PROOF THAT FOOD PASSES DOWN THE STEM 89
L
Proof that Food passes down the Stem. — If a freshly cut wil- low twig is placed in water, roots soon begin to develop from that
part of the stem which is under
water. If now the stem is girdled
by removing the bark in a ring just
above where the roots are growing,
the latter will eventually die, and
new roots will appear above the
girdled area. The passage of food
materials takes place in a downward
direction just outside the wood in
the layer of bark which contains the
bast fibers and sieve tubes. This
experiment with the twig explains
why trees die when girdled so as to
cut the sieve tubes of the inner bark.
Many of the canoe birches of our
forests are thus killed, girdled by
thoughtless visitors. In the same
manner mice and other gnawing
animals kill fruit trees. To a much
smaller extent food substances are
conducted also in the wood itself, and food passes from the inner
bark to the center of the tree by way of the pith rays. If the pith rays are tested for foods, it is found that much starch is stored in this part of the tree trunk.
Structure of a Monocotyledonous Stem. — A piece of cornstalk examined carefully in cross section and longitudi- nal section shows us that the main bulk of the stalk is made up of pith, through which are scattered numerous stringy, tough structures called fihrovascular bundles. The latter are the woody bundles of tubes which in this stem pass through the pith and run into the leaves,.
Experiment to show that food material passes down in the inner bark.
A broken cornstalk, with cross section (at left) : A^, node ; R, r, rind ; P, p, pith ; FV, fv, fihrovascular bundle.
90 CIRCULATION AND USES OF PLANT FOOD
where (in young specimens) they may be followed as veins. The outside of the corn stem is formed of large numbers of fibrovascular bundles, which, closely packed together, form a hard, tough outer rind. Thus the woody material on the outside gives mechanical support to an otherwise spongy stem.
Comparison in the Growth of a Dicotyledonous and a Monocoty- ledonous Stem. — In a young dicotyledonous stem, cut in cross
Phloem Cambium layer Xylem
Food made in leaves passes down fhrough — the inner bark
Materials taken in by roof pass up the stem in this region
Diagram to show the areas in a plant through which the raw food materials pass up the stem and food materials pass down.
section, the woody bundles are arranged as a ring near the outer edge of the stem (see figure). These bundles grow both toward the outside and toward the center of the stem from an actively dividing layer of cells. This layer in older stems becomes a com- plete ring under the bark and is called the cambium layer. On the outside of the cambium layer is found the phlo'em, or portion con- taining the sieve tubes which bear elaborated food toward the
DIGESTION IN PLANTS 91
roots. On the inside is found the xylem (zi'lem), or woody tubes that carry water upward.
In the monocotyledonous stem the bundles are scattered, lack the cambium, and increase in number as the stem grows older. They contain sieve tubes on the inside and water-bearing tubes in their outer part.
What causes Water to rise in a Stem. — • We have already seen that osmosis is responsible for getting water inside the root, and that the pressure exerted by this water {root pressure) is frequently capable of forcing fluids a considerable distance up a living stem. But during most of the year root pressure plays a very unimportant part in this phenomenon. It has been found that in small tubes, such as we find in wood, the cohesive force of molecules of water is very great. Also a very large amount of water is evaporated every day through the stomata. This, according to Ganong, averages about 50 grams per square meter of leaf surface in daylight and about 10 grams in darkness, almost half a ton of water being evap- orated from a large tree on a warm summer's day. This evapora- tion causes a pull on the volume of water in the fibrovascular bundles and is an important factor in the rise of fluids in stems.
Digestion. — Much of the food made in the leaves is stored in the form of starch. But starch, being insoluble, cannot be passed from cell to cell in a plant. In our study of the root hair we found that substances in solution {solutes) will pass from cell to cell by diffusion. In our study of a growing seedling we found that a solid food substance, starch, was digested in the corn grain by an enzyme, thus becoming a diffusible substance which could pass from cell to cell. This process of digestion seemingly may take place in all living parts of the plant, although most of it is done in the leaves. In the bodies of all animals, including man, starchy foods are changed in a similar manner, but by other enzymes, into soluble grape sugar.
The food material may be passed along in a soluble form until it comes to a place where food storage is to take place, and then it can be transformed again by the action of a reversible enzyme into an insoluble form (starch, for example) ; later, when needed by the plant in growth, it may again be transformed and sent in a soluble form through the stem to the place where it will be used.
92 CIRCULATION AND USES OF PLANT FOOD
In a similar manner, protein seems to be changed and trans- ferred to various parts of the plant. Some forms of protein are soluble and others insoluble in Avater. White of egg, for example, is slightly soluble, but can be rendered insoluble by heating it so that it coagulates. Insoluble proteins are digested within the plant; how and where is but slightly understood. Soluble proteins pass down the sieve tubes in the bast and then may be stored in the bast or medullary rays of the wood in an insoluble form, or they may pass into the root, fruit, or seeds of a plant, and be stored there. This stored food becomes of immense value to mankind, for it forms not only our cereal, potato, and other crops, but also our fruits of all kinds.
Plants with Special Digestive Organs. — Some plants have special organs of digestion. One of these, the sundew, has leaves
which are covered on one side with tiny glandular hairs. These attract insects and later serve to catch and digest the nitrogenous matter of these in- sects by means of enzymes /^/ ^'# ' '.>. I poured out by the same hairs.
§ i Another plant, the Venus's fly-
Three modified leaves, which get nitro- trap, catches insects in a seusi-
gen from captured insects: 1 pitcher ^^^^ j^^f ^^j^-^j^ £^1^3 ^^^
pJant; 2, sundew; 3, Venus s flytrap. i i i i • o '^
holds the msect fast until en- Z3anes poured out by the leaf slowly digest it. Still others, called pitcher plants, use as food the decayed bodies of insects which fall into their cuplike leaves and die there. These few plants are somewhat like those animals which have certain organs in the body set apart for the digestion of food.
Summary. — The raw^ materials which have been absorbed by the roots pass through conducting tubes up the stem to the leaves, where they are manufactured into food. This food passes down the stem, as a liquid, in the sieve tubes, until it reaches a place where it is used to build tissue or is changed to a solid form and stored.
Summary of the Functions of Plants and Animals. — The processes which have just been described; with the exception of food making,
FUNCTIONS OF PLANTS AND ANIMALS
93
are those which occur in the life of any plant or animal. Both plants and animals breathe; they oxidize their foods to release energy, carbon dioxide being given off as the result of the union of the carbon in the foods with the oxygen of the air (or of the air dis- solved in water) . Both plants and animals digest their food ; plants may do this in the cells of the root, stem, and leaf. Digestion must always occur so that food can be moved in a soluble condition from cell to cell in the plant's body, and it must take place in an animal for precisely the same reason.
Assimilation. — The assimilation of foods, or making of foods into living matter, is a process about which very little is known. We know it takes place in the living cells of plants and animals. But how foods are changed into living matter is one of the mysteries of life which have not yet been solved.
Excretion. — With the building and repair processes there is always waste, in both plants and animals. When living plants breathe, they give off carbon dioxide. In the process of starch making, oxygen might be considered the waste product. Water is evaporated from leaves and stems. The leaves fall and carry away waste mineral substances which they contain.
Reproduction. — Finally, both plants and animals have organs of reproduction. We have seen that the flower gives rise, after
m»
Poppy Pine Street pohfo
Section through seeds, showing embryos.
Lorn'
pollination, to a fruit which holds the seeds. Each seed holds an embryo. Thus the young plant is doubly protected for a time and is finally thrown off with enough food to give it a start in life. In much the same way we shall find that animals re- produce, either by laying eggs each of which contains an embryo and food to start it in life or, as in the higher animals, by holding
94 CIRCULATION AND USES OF PLANT FOOD
and protecting the embryo within the body of the mother until it is born, a helpless little creature, to be tenderly nourished by the mother until able to care for itself.
Life History. — Ultimately both plants and animals grow old and die. Some plants, for example the pea or bean, live but a season ; others, such as the big trees of California, live for hundreds of years. Some animals, certain insects, exist as adults but a day, while the elephant is said to live almost two hundred years. The span of life from the time the plant or animal begins to grow until it dies is known as its life history (or sometimes its life cycle) .
Pkoblem Questions
1. What proof have we of the passage of water and food substances up and down the stem ?
2. How do "monocot" and "dicot" stems differ?
3. How does a stem get air?
4. What causes water to rise against gravity in a stem?
5. How and where are foods digested in plants ?
6. Where are foods stored, and why ?
7. What are the life processes of a green plant?
8. Of what value are green plants to man ?
Problem and Project References
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Densmore, General Botany. Ginn and Company.
Dana, Plants and Their Children, pages 99-129. American Book Company.
Duggar, Plant Physiology. The Macmillan Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Vol. I. American Book
Company. Gager, Fundamentals of Botany. P. Blakiston's Son and Company. Ganong, The Teaching Botanist. The Macmillan Company. Mayne and Hatch, High School Agriculture. American Book Company. Hodge, Nature Study and Life, Chaps. IX, X, XI. Ginn and Company. Transeau, General Botany. World Book Company. United States Department of Agriculture, Yearbooks, for project work.
PART m. GENERAL RELATIONS BETWEEN PLANTS
AND ANIMALS
CHAPTER X
THE SIMPLEST ORGANISMS
Problems: To study the simplest plants and animals and find in what ways they are alike and different.
To find out where they live and how each performs some of the functions necessary to life.
To understand what is meant hy the " cell as a unit.^'
Laboratory Suggestions
Laboratory study. Study of pleurococcus, or any unicellular green plant. Laboratory study. Study of amoeba or Paramecium. Laboratory study. To make a hay infusion.
The Simplest Organisms. — The preceding division of this book has shown us that green plants are food-building organisms ; that, as such, they are of the greatest value to mankind. They are also living organisms, for they breathe, take in food, digest it, pass it through the body so that all parts may have nourishment and throw off waste materials. The fact that they manufacture the organic food substances for themselves makes them different from animals.
Some plants, however, are not green and so do not make their own food. Such are the many examples of fungi (fun-ji) which live in forests or fields, or which, in the form of common molds, are household pests. Our previous study of science has given us some knowledge of ^' germs " or bacteria, the lowest form of colorless plant.
As a matter of fact, it is extremely difficult for biologists to make any hard and fast distinction between the simplest plants and the simplest animals. There are many single-celled plants and many single-celled animals. If the cell is green, it is not always safe to
95
96 THE SIMPLEST ORGANISMS
call it a plant, because some animal cells appear to have small green plastids within them ; and the microscope reveals the pres- ence ot ehloroplasts within these animal cells. Also we might suppose that movement is an indication of an animal form. Here again we should find ourselves at fault, for there are many kinds of motile plant cells, while some animal cells are immotile.
The best indication of plant or animal cell structure seems to be in the external layer of the cell. Plant cells usually have a cel- lulose wall in addition to a cell membrane, while animal cells usu- ally are surrounded by a membrane only. The internal structure of the two kinds of cells also shows some differences.
Bacteria as Simple Plants. — We have seen that perhaps the simplest plants are the bacteria. They are so tiny that, whether » o in the form of a ball (coccus) , a rod (haciVlus) , or
'0 ^^c<^' a spiral (spiriVlum) the details of cell structure
are not well understood. They may have a ^^' ' wall of nitrogenous material, and sometimes
1 -y 9 • ■//• ^^ addition there is a sheath of gelatinous mat-
^^ ter. A nucleus probably is present, although
"iliahd bacilli ^^® nuclear matter may be scattered through the cell. Some bacteria have motile organs, in the form of threads of protoplasm called ''^"//A c r ' c^7'^a, or longer ones called flageVla, with l\^ bacilli °''"^ which they move in fluids. They multiply rapidly by simply dividing in the middle to
Forms of bacteria. /? x n t r t_i j-x'
lorm two new cells. In unfavorable conditions they may form spores, which are resting cells with a heavy wall secreted about them. Such cells may resist dryness or heat, — even boiling, — for a considerable time. As we shall prove in later studies, bacteria, like animals, need organic food and favor- able conditions of the environment in order to grow.
Pleurococcus. — A typical one-celled plant, however, would contain green coloring matter or chlorophyll, and would have the power to manufacture its own food under conditions giving it a moderate temperature, a supply of water, oxygen, carbon dioxide, and sunlight. Such a simple plant is the pleurococ'cus, seen on the shady side of trees, stones, and city houses. This plant would meet one definition of a cell, as it is a minute mass of protoplasm
A HAY INFUSION
97
containing a nucleus. It is surrounded by a wall of a woody material formed by the activity of the living matter within the cell. It also contains a lobed mass of protoplasm which is colored green, the chloroplast. Such is a simple plant cell.
A Hay Infusion. — An ex- ample of the close relation between plants and animals may be seen in the study of a hay infusion. If we place a wisp of hay or straw in a small glass jar nearly full of water, and leave it for a few days in a warm room, certain changes are seen to take place in the contents of the jar : The water gets cloudy and darker in color, and a scum appears on the surface. If some of this scum is examined under the compound microscope, it will be found to consist almost entirely of bacteria. These bacteria aid in the decay which, as the unpleasant odor from the jar testifies, is be-
A spherkdl digs
Pleurococcus. This one-celled green plant may live singly or in groups.
Bacfer/a
A psramecJum
Life in a late stage of a hay infusion.
ginning to take place. Bacteria flourish wherever the food supply is abundant. The water within the jar has come to contain much of the food material which was once within the leaf of grass, — organic nutrients, starch, sugar, and proteins, formed in the leaf by the action of the sun on the chlorophyll of the leaf, and now released into the water by the breaking down of the walls of the
J:
98 THE SIMPLEST ORGANISMS
cells in the leaf. The bacteria themselves release this food from the hay by causing it to decay.
Where to find Paramecia. — If we examine the surface of a hsij infusion, we find a scum formed of bacteria, and a mass of whitish tiny dots collected along the edge of the jar close to the sur- face of the water. More attentive observation shows us that these objects move, and that they are never found far from the surface. They are one-celled animals of several species, but among them we are almost sure to find a slipper-shaped cell, the parame'cium. The Structure of a Paramecium. — The cell body is almost transparent and consists of semifluid protoplasm which has a
granular, grayish appearance under the
^j^Peiiide microscope. This protoplasm appears to
^Contractile vacuole ^^ bounded by a very delicate peVlicle or
covering through which project numer-
Micronuchus ous delicate threads of protoplasm called
Mouth cilia. The locomotion of the Paramecium
\ ^^ - f^ood vacuole is caused by the movement of these cilia,
. . , which lash the water like a multitude of
. "- ,'"' ContracHle vacuole ^^^J ^^^^- The cilia also send particles of
^ food along a groove into a funnel-like
%/,i'^' opening, the gullet, on one side of the cell.
A Paramecium. Once inside the cell body, the particles of
food materials are gathered into little balls within the almost
transparent protoplasm. These masses of food are inclosed in a
little bubble-like area called a vacuole, containing fluid. Two larger
vacuoles may be found; these are the contractile vacuoles; their
purpose seems to be to pass off Hquid waste material from the cell
body. This is done by pulsation of the vacuole, which ultimately
bursts, passing fluid waste to the outside. Solid wastes are passed
out of the cell through an a^nal opening, in somewhat the same
manner. No breathing organs are seen, because diffusion of
oxygen and carbon dioxide may take place anywhere through the
cell membrane. The nucleus of the cell is not easily visible in
living specimens. In a cell that has been stained it has been found
to be a double structure, consisting of a large and a small portion,
called, respectively, the macronucleus and the micronucleus}
^ Some species of paramecia have two micronuclei.
PARAMECIUM — AMOEBA
99
Mouth
Micronudeus
Macronucleus
Reproduction of Paramecium. — Sometimes a Paramecium may
be found in the act of dividing into two by the process known
as fission. Each of the new cells contains half
of the original cell. The original cell may thus
form in succession many hundreds of cells in every
respect like the original parent cell. A process
which appears to stimulate reproduction is called
conjugation. Of this more will be told later. Amoeba.^ — In order to understand more fully
the life of a simple bit of protoplasm, let us take
up the study of the amoe'ha, a type of the simplest
form of animal life. Unlike the plant and animal
cells we have examined, the amoeba has no fixed Paramecium dividing
form. Viewed under the compound microscope, ^
it has the appearance of an irregular mass of granular protoplasm.
Its form is constantly changing as it moves about. This is due to
the pushing out of tiny pro- ^^ '^^-^ '^ ""^"^ ^x-^^""^^ ^i jections of the protoplasm
of the cell, called pseudopo- dia (su-d6-po'di-d ; false
How an amcBba moves (side view).
Contractile vacuole...!
Nucleus.. Food
feet). The locomotion is accomplished by a streaming or flowing of the semifluid protoplasm. The pseudopodia push forward in the direction in which the animal is to go, and the rest of the body fol- lows. In the central part of the cell is the nucleus, a spherical structure, seen more easily when the animals are killed and stained.
Although but a single cell, still the amoeba responds to the stimulus of food when it is near at hand. Food may be taken into the body at any point, the semifluid protoplasm sim- Pseudopodium.
Living amoeba seen through a 1 Amoebae may be obtained from the hay in- microscope,
fusion, from the dead leaves in the bottom of
small pools, from the same source in fresh-water aquariums, from the roots of duck- weed or other small water plants, or from green algae growing in quiet localities. No sure method of obtaining them can be given, except to procure them from some good supply house when needed.
Ectoplasm
fndoplasm
100
THE SIMPLEST ORGANISMS
ply rolling over and engulfing the food material. Within the body, as in the Paramecium, the food becomes inclosed within a fluid space or vacuole. The protoplasm has the power to take out such material as it can use to form new protoplasm or to give energy. Circulation of food material is accomplished by the constant streaming of the protoplasm within the cell.
The cell absorbs oxygen from the water by diffusion through its delicate membrane, giving up carbon dioxide in return. Thus the cell '^ breathes " through any part of its body covering.
Waste nitrogenous products formed within the cell when work is done are passed out by means of a contractile vacuole.
The amoeba, like other one-celled organisms, reproduces by the process of fission. A single cell divides by splitting into two others,
Pseudopodium
'^-'Profoplssm
AmcBba, showing the changes which take place during division of a cell.
each of which resembles the parent cell, except that they are smaller. When these become the size of the parent amoeba, they in turn divide. This is an example of asexual reproduction.
When conditions unfavorable for life come, the amoeba, like some one-celled plants, encysts itself within a membranous wall. In this condition it may become dried and be blown through the air. Upon return to a favorable environment, it begins life again, as before. In this respect it resembles the spore of a plant.
The Cell as a Unit. — In the daily life of a one-celled animal we find the single cell performing all the vital activities which we shall later find the many-celled animal is able to perform. In the amoeba no definite parts of the cell appear to be set off to perform certain functions ; but any part of the cell can take in food, absorb oxygen, change the food into protoplasm, and excrete the waste material. The single cell is, in fact, an organism able to carry on
THE CELL AS A UNIT 101
the business of living almost as effectually as a very complex animal.
Summary. — This chapter has shown us that the simplest plants and animals are composed of a single cell, but that, nevertheless, they are organisms. Plant cells differ from animal cells in struc- ture and function. But the bacteria and other colorless plants are more like animals than plants, in that they do not make food, but destroy it. Some one-celled organisms, such as Paramecium, are complex in structure, while others, such as bacteria, are very simple. It is probable that bacteria are the lowest forms of life known.
Problem Questions
1. Why is a single cell considered an organism?
2. How do single cells absorb food? Digest food? Are these processes different in plant and animal cells? (Look this up in some good reference books.)
3. List the characters of a number of plant and animal cells. How are they ahke and how different ?
Problem and Project References
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Calkins, Protozoa. Lemcke and Buechner.
Hegner, Introduction to Zoology. The Macmillan Company.
Needham, General Biology. Comstock Publishing Company.
Needham and Lloyd, Life of Inland Waters. Comstock Publishing Company.
Sedgwick and Wilson, General Biology. Henry Holt and Company.
Ward and Whipple, Manual of Freshwater Biology. John Wiley and Sons.
H. new civ. BIOL. — 8
CHAPTER XI
THE RELATIONS OF PLANTS TO ANIMALS
Problems: To determine the general biological relations existing between plants and animals, as shown in a balanced aquarium. To understand the meaning of symbiosis.
Suggestions for Laboratory Work
Demonstration of life in a "balanced" and in an "unbalanced" aquarium. Determination of factors causing balance.
Demonstration of some examples of symbiosis.
Study of a Balanced Aquarium. — Perhaps the best way for us to understand the interrelation between plants and animals is to
study an aquarium in which plants and animals live and in which a balance has been es- tablished between the plant life on one side and animal life on the other. Aquariums containing green pond weeds, either floating or rooted, a few snails, some tiny animals known as water fleas, and a fish or two, if kept near a light win- dow, will show this relation.
We have seen that green plants, in favorable conditions of sunlight, heat, moisture, and with a supply of raw food materials, give off oxygen as a by-product whfle manufacturing food in their green cells. We know the necessary raw materials for starch manufacture are carbon dioxide and water, while nitrogenous material is necessary
102
A balanced aquarium. Explain the term "balanced."
STUDY OF A BALANCED AQUARIUM
103
for the making of proteins within the plant. In previous experi- ments we have proved that carbon dioxide is given off by hving things when oxidation occurs in the body. The crawHng snails and the swimming fish give off carbon dioxide, which is dissolved in the water ; the plants themselves, at all times, oxidize food within their bodies, and so must pass off some carbon dioxide. The green plants in the daytime use up the carbon dioxide obtained from the various sources and, with the water which they take in, manu- facture starch. While this process is going on, oxygen is given off to the water of the aquarium, and this free oxygen is used by the animals there.
The plants are continually growing; but the snails and fish eat parts of the plants. Thus the plant life gives food to the animals within the aquarium. The animals give off certain nitrogenous wastes. These ma- terials, with other nitrogenous matter from dead animals and parts of the plants, form part of the raw material used for protein manufacture in the plant. This nitrogenous matter is prepared for use by several different kinds of bacteria which first break the dead bodies down and then give the material to the plants in the form of soluble nitrates. The green plants manufacture food, the animals eat the plants and give off carbon dioxide and nitro- genous waste, from which the plants in turn make food and living matter. The plants give oxygen to the animals, and the animals give carbon dioxide to the plants. Thus a balance exists between the plants and animals in the aquarium. Make a table to show this balance.
Relations between Green Plants and Animals. — What goes on in the aquarium is an example of the relation existing between green plants and animals. Everywhere in the world green plants are making food which becomes, sooner or later, the food of animals.
The carbon and oxygen cycle in the balanced aquarium. Trace by means of the arrows the carbon from the time plants take it in as CO2 until animals give it off. Show what happens to the oxygen.
104
RELATIONS OF PLANTS TO ANIMALS
Energy
(Sun)
This diagram shows that plants and animals on the earth hold the same re- lation to each other as plants and ani-