A Civic Biology, Presented in Problems
by George W. Hunter · English · Gutenberg #39969 · 803,751 chars · public domain (US)
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Title: A Civic Biology, Presented in Problems
Author: George W. Hunter
Release date: June 11, 2012 [eBook #39969]
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK A CIVIC BIOLOGY, PRESENTED IN PROBLEMS ***
Produced by Mark C. Orton, Carol Ann Brown, and the Online
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[Illustration: Compare the unfavorable artificial environment of
a crowded city with the more favorable environment of the
country.]
A CIVIC BIOLOGY
Presented in Problems
BY
GEORGE WILLIAM HUNTER, A.M.
HEAD OF THE DEPARTMENT OF BIOLOGY, DE WITT CLINTON
HIGH SCHOOL, CITY OF NEW YORK.
AUTHOR OF "ELEMENTS OF BIOLOGY," "ESSENTIALS OF
BIOLOGY," ETC.
[Illustration: Printer's Logo]
AMERICAN BOOK COMPANY
NEW YORK CINCINNATI CHICAGO
COPYRIGHT, 1914, BY
GEORGE WILLIAM HUNTER.
COPYRIGHT, 1914, IN GREAT BRITAIN.
* * * * *
HUNTER, CIVIC BIOLOGY.
W. P. 3
Dedicated
TO MY
FELLOW TEACHERS
OF THE DEPARTMENT OF BIOLOGY
IN THE DE WITT CLINTON HIGH SCHOOL
WHOSE CAPABLE, EARNEST, UNSELFISH
AND INSPIRING AID HAS MADE
THIS BOOK POSSIBLE
FOREWORD TO TEACHERS
A course in biology given to beginners in the secondary school should have
certain aims. These aims must be determined to a degree, first, by the
capabilities of the pupils, second, by their native interests, and, third,
by the environment of the pupils.
The boy or girl of average ability upon admission to the secondary school
is not a thinking individual. The training given up to this time, with but
rare exceptions, has been in the forming of simple concepts. These concepts
have been reached didactically and empirically. Drill and memory work have
been the pedagogic vehicles. Even the elementary science work given has
resulted at the best in an interpretation of some of the common factors in
the pupil's environment, and a widening of the meaning of some of his
concepts. Therefore, the first science of the secondary school, elementary
biology, should be primarily the vehicle by which the child is taught to
solve problems and to think straight in so doing. No other subject is more
capable of logical development. No subject is more vital because of its
relation to the vital things in the life of the child. A series of
experiments and demonstrations, discussed and applied as definite concrete
problems which have arisen within the child's horizon, will develop power
in thinking more surely than any other subject in the first year of the
secondary school.
But in our eagerness to develop the power of logical thinking we must not
lose sight of the previous training of our pupil. Up to this time the
method of induction, that handmaiden of logical thought, has been almost
unknown. Concepts have been formed deductively by a series of comparisons.
All concepts have been handed down by the authority of the teacher or the
text; the inductive search for the unknown is as yet a closed book. It is
unwise, then, to directly introduce the pupil to the method of induction
with a series of printed directions which, though definite in the mind of
the teacher because of his wider horizon, mean little or nothing as a
definite problem to the pupil. The child must be brought to the
appreciation of the problem through the deductive method, by a comparison
of the future problem with some definite concrete experience within his own
field of vision. Then by the inductive experiment, still led by a series of
oral questions, he comes to the real end of the experiment, the conclusion,
with the true spirit of the investigator. The result is tested in the light
of past experiment and a generalization is formed which means something to
the pupil.
For the above reason the laboratory problems, which naturally precede the
textbook work, should be separated from the subject matter of the text. A
textbook in biology should serve to verify the student's observations made
in the laboratory, it should round out his concept or generalization by
adding such material as he cannot readily observe and it should give the
student directly such information as he cannot be expected to gain directly
or indirectly through his laboratory experience. For these reasons the
laboratory manual has been separated from the text.
"The laboratory method was such an emancipation from the
old-time bookish slavery of pre-laboratory days that we may
have been inclined to overdo it and to subject ourselves to
a new slavery. It should never be forgotten that the
laboratory is simply a means to the end; that the dominant
thing should be a consistent chain of ideas which the
laboratory may serve to elucidate. When, however, the
laboratory assumes the first place and other phases of the
course are made explanatory to it, we have taken, in my
mind, an attitude fundamentally wrong. The question is, not
what _types_ may be taken up in the laboratory to be fitted
into the general scheme afterwards, but what _ideas_ are
most worth while to be worked out and developed in the
laboratory, if that happens to be the best way of doing it,
or if not, some other way to be adopted with perfect
freedom. Too often our course of study of an animal or plant
takes the easiest rather than the most illuminating path.
What is easier, for instance, particularly with large
classes of restless pupils who apparently need to be kept in
a condition of uniform occupation, than to kill a supply of
animals, preferably as near alike as possible, and set the
pupils to work drawing the dead remains? This method is
usually supplemented by a series of questions concerning the
remains which are sure to keep the pupils busy a while
longer, perhaps until the bell strikes, and which usually
are so planned as to anticipate any ideas that might
naturally crop up in the pupil's mind during the drawing
exercise.
"Such an abuse of the laboratory idea is all wrong and
should be avoided. The ideal laboratory ought to be a
retreat for rainy days; a substitute for out of doors; a
clearing house of ideas brought in from the outside. Any
course in biology which can be confined within four walls,
even if these walls be of a modern, well-equipped
laboratory, is in some measure a failure. Living things, to
be appreciated and correctly interpreted, must be seen and
studied in the open where they will be encountered
throughout life. _The place where an animal or plant is
found is just as important a characteristic as its shape or
function._ Impossible field excursions with large classes
within school hours, which only bring confusion to
_inflexible_ school programs, are not necessary to
accomplish this result. Properly administered, it is without
doubt one of our most efficient devices for developing
biological ideas, but the laboratory should be kept in its
proper relation to the other means at our disposal and never
be allowed to degenerate either into a place for vacuous
drawing exercises or a biological morgue where dead remains
are viewed."--_Dr. H. E. Walter._
For the sake of the pupil the number of technical and scientific terms has
been reduced to a minimum. The language has been made as simple as possible
and the problems made to hinge upon material already known, by hearsay at
least, to the pupil. So far as consistent with a well-rounded course in the
essentials of biological science, the interests of the children have been
kept in the foreground. In a recent questionnaire sent out by the author
and answered by over three thousand children studying biology in the
secondary schools of Connecticut, Massachusetts, New Jersey, and New York
by far the greatest number gave as the most interesting topics those
relating to the care and functions of the human body and the control and
betterment of the environment. As would be expected, boys have different
biological interests from girls, and children in rural schools wish to
study different topics from those in congested districts in large
communities. The time has come when we must frankly recognize these
interests and adapt the content of our courses in biology to interpret the
_immediate_ world of the pupil.
With this end in view the following pages have been written. This book
shows boys and girls living in an urban community how they may best live
within their own environment and how they may cooperate with the civic
authorities for the betterment of their environment. A logical course is
built up around the topics which appeal to the average normal boy or girl,
topics given in a logical sequence so as to work out the solution of
problems bearing on the ultimate problem of the entire course, that of
preparation for citizenship in the largest sense.
Seasonal use of materials has been kept in mind in outlining this course.
Field trips, when properly organized and later used as a basis for
discussion in the classroom, make a firm foundation on which to build the
superstructure of a course in biology. The normal environment, its relation
to the artificial environment of the city, the relations of mutual give and
take existing between plants and animals, are better shown by means of
field trips than in any other way. Field and museum trips are enjoyed by
the pupils as well. These result in interest and in better work. The course
is worked up around certain great biological principles; hence insects may
be studied when abundant in the fall in connection with their relations to
green plants and especially in their relation to flowers. In the winter
months material available for the laboratory is used. Saprophytic and
parasitic organisms, wild plants in the household, are studied in their
relations to mankind, both as destroyers of food, property and life and as
man's invaluable friends. The economic phase of biology may well be taken
up during the winter months, thus gaining variety in subject matter and in
method of treatment. The apparent emphasis placed upon economic material in
the following pages is not real. It has been found that material so given
makes for variety, as it may be assigned as a topical reading lesson or
simply used as reference when needed. Cyclic work in the study of life
phenomena and of the needs of organisms for oxygen, food, and reproduction
culminates, as it rightly should, in the study of life-processes of man and
man's relation to his environment.
In a course in biology the difficulty comes not so much in knowing what to
teach as in knowing what _not_ to teach. The author believes that he has
made a selection of the topics most vital in a well-rounded course in
elementary biology directed toward civic betterment. The physiological
functions of plants and animals, the hygiene of the individual within the
community, conservation and the betterment of existing plant and animal
products, the big underlying biological concepts on which society is built,
have all been used to the end that the pupil will become a better, stronger
and more unselfish citizen. The "spiral" or cyclic method of treatment has
been used throughout, the purpose being to ultimately build up a number of
well-rounded concepts by constant repetition but with constantly varied
viewpoint.
The sincere thanks of the author is extended to all who have helped make
this book possible, and especially to the members of the Department of
Biology in the De Witt Clinton High School. Most of the men there have
directly or indirectly contributed their time and ideas to help make this
book worth more to teachers and pupils. The following have read the
manuscript in its entirety and have offered much valuable constructive
criticism: Dr. Herbert E. Walter, Professor of Zoology in Brown University;
Miss Elsie Kupfer, Head of the Department of Biology in Wadleigh High
School; George C. Wood, of the Department of Biology in the Boys' High
School, Brooklyn; Edgar A. Bedford, Head of Department of Biology in the
Stuyvesant High School; George E. Hewitt, George T. Hastings, John D.
McCarthy, and Frank M. Wheat, all of the Department of Biology in the De
Witt Clinton High School.
Thanks are due, also, to Professor E. B. Wilson, Professor G. N. Calkins,
Mr. William C. Barbour, Dr. John A. Sampson, W. C. Stevens, and C. W.
Beebe, Dr. Alvin Davison, and Dr. Frank Overton; to the United States
Department of Agriculture; the New York Aquarium; the Charity Organization
Society; and the American Museum of Natural History, for permission to copy
and use certain photographs and cuts which have been found useful in
teaching. Dr. Charles H. Morse and Dr. Lucius J. Mason, of the De Witt
Clinton High School, prepared the hygiene outline in the appendix. Frank M.
Wheat and my former pupil, John W. Teitz, now a teacher in the school, made
many of the line drawings and took several of the photographs of
experiments prepared for this book. To them especially I wish to express my
thanks.
At the end of each of the following chapters is a list of books which have
proved their use either as reference reading for students or as aids to the
teacher. Most of the books mentioned are within the means of the small
school. Two sets are expensive: one, _The Natural History of Plants_, by
Kerner, translated by Oliver, published by Henry Holt and Company, in two
volumes, at $11; the other, _Plant Geography upon a Physiological Basis_,
by Schimper, published by the Clarendon Press, $12; but both works are
invaluable for reference.
For a general introduction to physiological biology, Parker, _Elementary
Biology_, The Macmillan Company; Sedgwick and Wilson, _General Biology_,
Henry Holt and Company; Verworn, _General Physiology_, The Macmillan
Company; and Needham, _General Biology_, Comstock Publishing Company, are
most useful and inspiring books.
Two books stand out from the pedagogical standpoint as by far the most
helpful of their kind on the market. No teacher of botany or zoology can
afford to be without them. They are: Lloyd and Bigelow, _The Teaching of
Biology_, Longmans, Green, and Company, and C. F. Hodge, _Nature Study and
Life_, Ginn and Company. Other books of value from the teacher's standpoint
are: Ganong, _The Teaching Botanist_, The Macmillan Company; L. H. Bailey,
_The Nature Study Idea_, Doubleday, Page, and Company; and McMurry's _How
to Study_, Houghton Mifflin Company.
CONTENTS
CHAPTER PAGE
FOREWORD TO TEACHERS 7
I. SOME REASONS FOR THE STUDY OF BIOLOGY 15
II. THE ENVIRONMENT OF PLANTS AND ANIMALS 19
III. THE INTERRELATIONS OF PLANTS AND ANIMALS 28
IV. THE FUNCTIONS AND COMPOSITION OF LIVING THINGS 47
V. PLANT GROWTH AND NUTRITION--THE CAUSES OF GROWTH 58
VI. THE ORGANS OF NUTRITION IN PLANTS--THE SOIL AND
ITS RELATION TO ROOTS 71
VII. PLANT GROWTH AND NUTRITION--PLANTS MAKE FOOD 84
VIII. PLANT GROWTH AND NUTRITION--THE CIRCULATION AND
FINAL USES OF FOOD BY PLANTS 97
IX. OUR FORESTS, THEIR USES AND THE NECESSITY OF THEIR
PROTECTION 105
X. THE ECONOMIC RELATION OF GREEN PLANTS TO MAN 117
XI. PLANTS WITHOUT CHLOROPHYLL IN THEIR RELATION TO
MAN 130
XII. THE RELATIONS OF PLANTS TO ANIMALS 159
XIII. SINGLE-CELLED ANIMALS CONSIDERED AS ORGANISMS 166
XIV. DIVISION OF LABOR, THE VARIOUS FORMS OF PLANTS AND
ANIMALS 173
XV. THE ECONOMIC IMPORTANCE OF ANIMALS 197
XVI. AN INTRODUCTORY STUDY OF VERTEBRATES 232
XVII. HEREDITY, VARIATION, PLANT AND ANIMAL BREEDING 249
XVIII. THE HUMAN MACHINE AND ITS NEEDS 266
XIX. FOODS AND DIETARIES 272
XX. DIGESTION AND ABSORPTION 296
XXI. THE BLOOD AND ITS CIRCULATION 313
XXII. RESPIRATION AND EXCRETION 329
XXIII. BODY CONTROL AND HABIT FORMATION 348
XXIV. MAN'S IMPROVEMENT OF HIS ENVIRONMENT 373
XXV. SOME GREAT NAMES IN BIOLOGY 398
APPENDIX 407
SUGGESTED COURSE WITH TIME ALLOTMENT AND SEQUENCE
OF TOPICS FOR COURSE BEGINNING IN FALL 407
SUGGESTED SYLLABUS FOR COURSE IN BIOLOGY BEGINNING
IN FEBRUARY AND ENDING THE NEXT JANUARY 411
HYGIENE OUTLINE 415
WEIGHTS, MEASURES, AND TEMPERATURES 417
SUGGESTIONS FOR LABORATORY EQUIPMENT 418
INDEX 419
A CIVIC BIOLOGY
I. THE GENERAL PROBLEM--SOME REASONS FOR THE STUDY OF BIOLOGY
What is Biology?--_Biology is the study of living beings, both plant and
animal._ Inasmuch as man is an animal, the study of biology includes the
study of man in his relations to the plants and the animals which surround
him. Most important of all is that branch of biology which treats of the
mechanism we call the human body,--of its parts and their uses, and its
repair. This subject we call _human physiology_.
Why study Biology?--Although biology is a very modern science, it has found
its way into most high schools; and an increasingly large number of girls
and boys are yearly engaged in its study. 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 discipline it gives me, is
there anything that I can take away which will help me in my future life?
Human Physiology.--The answer to this question is plain. If the study of
biology will give us a better understanding of our own bodies and their
care, then it certainly is of use to us. That phase of biology known as
_physiology_ deals with the uses of the parts of a plant or animal; human
physiology and hygiene deal with the uses and care of the parts of the
human animal. The prevention of sickness is due in a large part to the
study of hygiene. It is estimated that over 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 lives of each of us,
as a member of our family, as a member of the school we attend, and as a
future citizen, a knowledge of hygiene is of the greatest importance.
Relations of Plants to Animals.--But there are other reasons why an
educated person should know something about biology. We do not always
realize that if it were not for the green plants, there would be no animals
on the earth. Green plants furnish food to animals. Even the meat-eating
animals feed upon those that feed upon plants. How the plants manufacture
this food and the relation they bear to animals will be discussed in later
chapters. 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 domesticated animals with food; they give him timber for his
houses and wood and coal for his fires; they provide him with pulp wood,
from which he makes his paper, and oak galls, from which he may make ink.
Much of man's clothing and the thread with which it is sewed together come
from fiber-producing plants. Most medicines, beverages, flavoring extracts,
and spices are plant products, while plants are made use of in hundreds of
ways in the useful arts and trades, producing varnishes, dyestuffs, rubber,
and other products.
Bacteria in their Relation to Man.--In still another way, certain 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 the 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 this 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 cannot do
without them. But they likewise sometimes spoil our meat and fish, and our
vegetables and fruits; they sour our milk, and may make our canned goods
spoil. Worst of all, they cause diseases, among others tuberculosis, a
disease so harmful as to be called the "white plague." Fully one half of
all yearly deaths are caused by these plants. So important are the bacteria
that a sub-division of biology, called _bacteriology_, has been named
after them, and hundreds of scientists are devoting their lives to the
study of bacteria and their control. The greatest of all bacteriologists,
Louis Pasteur, once said, "It is within the power of man to cause all
parasitic diseases (diseases mostly caused by bacteria) to disappear from
the world." His prophecy is gradually being fulfilled, and it may be the
lot of some boys or girls who read this book to do their share in helping
to bring this condition of affairs about.
The Relation of Animals to Man.--Animals also play an important part in the
world in causing and carrying disease. Animals that cause disease are
usually tiny, and live in other animals as _parasites_; that is, they get
their living from their hosts on which they feed. Among the diseases caused
by parasitic animals are malaria, yellow fever, the sleeping sickness, and
the hookworm disease. Animals also _carry_ disease, especially the flies
and mosquitoes; rats and other animals are also well known as spreaders of
disease.
From a money standpoint, animals called insects do much harm. It is
estimated that in this country alone they are annually responsible for
$800,000,000 worth of damage by eating crops, forest trees, stored food,
and other material wealth.
The Uses of Animals to Man.--We all know the uses man has made of the
domesticated animals for food and as beasts of 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, tortoise shell, corals, and mother-of-pearl; from animals come
perfumes and oils, glue, lard, and butter; animals produce honey, wax,
milk, eggs, and various other commodities.
The Conservation of our Natural Resources.--Still another reason why we
should study biology is that we may work understandingly for the
conservation of our natural resources, especially of our forests. The
forest, aside from its beauty and its health-giving properties, holds
water in the earth. It keeps the water from drying out of the earth on hot
days and from running off on rainy days. Thus a more even supply of water
is given to our rivers, and thus freshets are prevented. Countries that
have been deforested, such as China, Italy, and parts of France, are now
subject to floods, and are in many places barren. On the forests depend our
supply of timber, our future water power, and the future commercial
importance of cities which, like New York, are located at the mouths of our
navigable rivers.
Plants and Animals mutually Helpful.--Most plants and animals stand in an
attitude of mutual helpfulness to one another, plants providing food and
shelter for animals; 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 light. The life processes of both plants
and animals are essentially the same, and the living matter of a tree is as
much alive as is the living matter in a fish, a dog, or a man.
Biology in its Relation to Society.--Again, the study of biology should be
part of the education of every boy and girl, because society itself is
founded upon the principles which biology teaches. Plants and animals are
living things, taking what they can from their surroundings; they enter
into competition with one another, and those which are the best fitted for
life outstrip the others. Animals and plants tend to vary each from its
nearest relative in all details of structure. The strong may thus hand down
to their offspring the characteristics which make them the 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 developed improved
breeds of horses, cattle, and other domestic animals. Plant breeders have
likewise selected the plants or seeds that have varied toward better
plants, and thus have stocked the earth with hardier and more fruitful
domesticated plants. Man's dominion over the living things of the earth is
tremendous. This is due to his understanding the principles which underlie
the science of biology.
Finally the study of biology ought to make us better men and women by
teaching us that unselfishness 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 the insect communities 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 community in which we live. Thus the
application of biological principles will benefit society.
II. THE ENVIRONMENT OF PLANTS AND ANIMALS
_Problem.--To discover some of the factors of the environment of plants and
animals._
_(a) Environment of a plant._
_(b) Environment of an animal._
_(c) Home environment of a girl or boy._
LABORATORY SUGGESTIONS
_Laboratory demonstrations._--Factors of the environment of
a living plant or animal in the vivarium.
_Home exercise._--The study of the factors making up my own
environment and how I can aid in their control.
Environment.--Each one of us, no matter where he lives, comes in contact
with certain surroundings. Air is everywhere around us; light is necessary
to us, so much so that we use artificial light at night. The city street,
with its dirty and hard paving stones, has come to take the place of the
soil of the village or farm. Water and food are a necessary part of our
surroundings. Our clothing, useful to maintain a certain temperature, must
also be included. All these things--air, light, heat, water, food--together
make up our _environment_.
[Illustration: An unfavorable city environment.]
All other animals, and all plants as well, are surrounded by and use
practically the same things from their environment as we do. The potted
plant in the window, the goldfish in the aquarium, your pet dog at home,
all use, as we will later prove, the factors of their environment in the
same manner. Air, water, light, a certain amount of heat, soil to live in
or on, and food form parts of the surroundings of _every_ living thing.
[Illustration: An experiment that shows the air contains about four fifths
nitrogen.]
[Illustration: Apparatus for separating water by means of an electric
current into the two elements, hydrogen and oxygen.]
The Same Elements found in Plants and Animals as in their Environment.--It
has been found by chemists that the plants and animals as well as their
environment may be reduced to about eighty very simple substances known as
_chemical elements_. For example, the air is made up largely of two
elements, _oxygen_ and _nitrogen_. Water, by means of an electric current,
may be broken up into two elements, _oxygen_ and _hydrogen_. The elements
in water are combined to make a _chemical compound_. The oxygen and
nitrogen of the air are not so united, but exist as separate gases. If we
were to study the chemistry of the bodies of plants and animals and of
their foods, we would find them to be made up of certain chemical elements
combined in various complex compounds. These elements are principally
_carbon_, _hydrogen_, _oxygen_, _nitrogen_, and perhaps a dozen others in
very minute proportions. But the same elements present in the living things
might also be found in the environment, for example, water, food, the air,
and the soil. 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 will learn in later chapters.
[Illustration: Chart to show the percentage of chemical elements in the
human body.]
What Plants and Animals take from their Environment. Air.--It is a
self-evident fact that animals need air. Even those living in the water use
the air dissolved in the water. A fish placed in an air-tight jar will soon
die. It will be proven later that plants also need air in order to live.
[Illustration: The effect of water upon the growth of trees. These trees
were all planted at the same time in soil that is sandy and uniform. They
are watered by a small stream which runs from left to right in the picture.
Most of the water soaks into the ground before reaching the last trees.]
Water.--We all know that water must form part of the environment of plants
and animals. It is a matter of common knowledge that pets need water to
drink; so do other animals. Every one knows we must water a potted plant if
we expect it to grow. 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 is spending millions
of dollars at the present time to bring by irrigation the water needed to
support life in the western desert lands.
[Illustration: The effect of light upon a growing plant.]
Light as Condition of the Environment.--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 such plants grow toward the light. All green
plants are thus influenced by the sun. Other plants which are not green
seem either indifferent or are negatively influenced (move away from) the
source of light. 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.
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 much and
strong light. And man himself enjoys only moderate intensity of light and
heat. Look at the shady side of a city street on any hot day to prove this
statement.
Heat.--Animals and plants are both affected by heat or the absence of it.
In cold weather green plants either die or their life activities are
temporarily suspended,--the plant becomes _dormant_. Likewise small
animals, such as insects, may be killed by cold or they may _hibernate_
under stones or boards. Their life activities are stilled until the coming
of warm weather. Bears and other large animals go to sleep during the
winter and awake thin and active at the approach of warm weather. Animals
or plants used to certain temperatures are killed if removed from those
temperatures. Even man, the most adaptable of all animals, cannot stand
great changes without discomfort and sometimes death. He heats his houses
in winter and cools them in summer so as to have the amount of heat most
acceptable to him, _i.e._ about 70 deg. Fahrenheit.
[Illustration: Vegetation in Northern Russia. The trees in this picture are
nearly one hundred years old. They live under conditions of extreme cold
most of the year.]
The Environment determines the Kind of Animals and Plants within It.--In
our study of geography we learned that certain luxuriant growths of trees
and climbing plants were characteristic of the tropics with its moist, warm
climate. No one would expect to find living there the hardy stunted plants
of the arctic region. Nor would we expect to find the same kinds of animal
life in warm regions as in cold. The surroundings determine the kind of
living things there. Plants or animals _fitted to live_ in a given locality
will probably be found there if they have had an opportunity to reach that
locality. If, for example, temperate forms of life were introduced by man
into the tropics, they would either die or they would gradually change so
as to become fitted to live in their new environment. Sheep with long wool
fitted to live in England, when removed to Cuba, where conditions of
greater heat exist, soon died because they were not fitted or _adapted_ to
live in their changed environment.
[Illustration: Plant life in a moist tropical forest. Notice the air plants
to the left and the resurrection ferns on the tree trunk.]
Adaptations.--Plants and animals are not only 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 execute 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 fitness in the structure of the hand for this work
we say it is adapted to its function of grasping objects. Each part of a
plant or animal is usually fitted for some particular work. The root of a
green plant, for example, is fitted to take in water by having tiny
absorbing organs 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. Each part of a plant does work, and is fitted, by means of certain
structures, to do that work. It is because of these adaptations that living
things are able to do their work within their particular environment.
Plants and Animals and their Natural Environment.--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 or no green water
plants soon seeks the surface of the water, and if the water is not changed
frequently so as to supply air the fish will die. Again the artificial
environment lacks something that the fish needs. Each plant and animal is
limited to a certain environment because of certain individual needs which
make the surroundings fit for it to live in.
[Illustration: A natural barrier on a stream. No trout would be found above
this fall. Why not?]
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 lives. Certain tiny plants and most
animals move from place to place, either in air, water, on the earth or in
the earth, but they maintain relatively the same conditions in environment.
Birds are perhaps the most striking exception, for some may fly thousands
of miles from their summer homes to winter in the south. Other animals,
too, migrate from place to place, but not usually where there are great
changes in the surroundings. A high mountain chain with intense cold at the
upper altitudes would be a barrier over which, for example, a bear, a deer,
or a snail could not travel. Fish like trout will migrate up a stream until
they come to a fall too high for them to jump. There they must stop because
their environment limits them.
[Illustration: A new apartment house, with out-of-door sleeping porch.]
Man in his Environment.--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 environment. Men who lived on the earth
thousands of year ago did not wear clothes or have elaborate homes of wood
or 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 a changed
environment from that of other animals. The living together of men in
communities has caused certain needs to develop. Many things can be
supplied in common, as water, milk, foods. Wastes of all kinds have to be
disposed of in a town or city. Houses have come to be placed close
together, or piled on top of each other, as in the modern apartment. Fields
and trees, all outdoor life, has practically disappeared. 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 environment favorable to live in. He
may heat his dwellings in winter and cool them in summer so as to maintain
a moderate and nearly constant temperature. He may see that his dwellings
have windows so as to let light and air pass in and out. He may have light
at night and shade by day from intense light. He may have a system of pure
water supply and may see that drains or sewers carry away his wastes. He
may see to it that people ill with "catching" or _infectious_ diseases are
isolated or _quarantined_ from others. This care of the artificial
environment is known as _sanitation_, while the care of the _individual_
for himself within the environment is known as _hygiene_. It will be the
chief end of this book to show girls and boys how they may become good
citizens through the proper control of personal hygiene and sanitation.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American
Book Company.
Hough and Sedgwick, _Elements of Hygiene and Sanitation_.
Ginn and Company.
Jordan and Kellogg, _Animal Life_. Appleton.
Sharpe, _A Laboratory Manual for the Solution of Problems in
Biology_, _p. 95_. American Book Company.
Tolman, _Hygiene for the Worker_. American Book Company.
ADVANCED
Allen, _Civics and Health_. Ginn and Company.
III. THE INTERRELATIONS OF PLANTS AND ANIMALS
_Problem.--To discover the general interrelations of green plants and
animals._
_(a) Plants as homes for insects._
_(b) Plants as food for insects._
_(c) Insects as pollinating agents._
LABORATORY SUGGESTIONS
_A field trip_:--Object: to collect common insects and study
their general characteristics; to study the food and shelter
relation of plant and insects. 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 simple insect,
identification of parts--drawing. Examination and
identification of some orders of insects.
_Laboratory demonstration._--Life history of monarch and
some other butterflies or moths.
_Laboratory exercise._--Study of simple flower--emphasis on
work of essential organs, drawing.
_Laboratory exercise._--Study of mutual adaptations in a
given insect and a given flower, _e.g._ butter and eggs and
bumble bee.
_Demonstration of examples of insect pollination._
The Object of a Field Trip.--Many of us live in the city, where the crowded
streets, the closely packed apartments, and the city playgrounds form our
environment. It is very artificial at best. To understand better the
_normal environment_ of plants or animals we should go into the country.
Failing in this, an overgrown city lot or a park will give us much more
closely the environment as it touches some animals lower than man. We must
then remember that in learning something of the natural environment of
other living creatures we may better understand our own environment and our
relation to it.
On any bright warm day in the fall we will find insects swarming everywhere
in any vacant lot or the less cultivated parts of a city park.
Grasshoppers, butterflies alighting now and then on the flowers, brightly
marked hornets, bees busily working over the purple asters or golden rod,
and many other forms hidden away on the leaves or stems of plants may be
seen. If we were to select for observation some partially decayed tree, we
would find it also inhabited. Beetles would be found boring through its
bark and wood, while caterpillars (the young stages of butterflies or
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 would go to work to identify
some of the common forms we would be likely to find on plants. Then a
little later we will find out what they are doing on these plants.
[Illustration: An insect viewed from the side. Notice the head, thorax, and
abdomen. What other characters do you find?]
How to tell an Insect.--A bee is a good example 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 middle part called the _thorax_,
and a hind portion, jointed and hairy, the _abdomen_. We cannot escape
noting the fact that this insect has wings with which it flies and that it
also has legs. The three pairs of legs, which are jointed and provided with
tiny hooks at the end, are attached to the thorax. Two pairs of delicate
wings are attached to the upper or _dorsal_ side of the thorax. The thorax
and indeed the entire body, is covered with a hard shell of material
similar to a cow's horn, there being no skeleton inside for the attachment
of muscles. If we carefully watch the abdomen of a living bee, we notice it
move up and down quite regularly. The animal is breathing through tiny
breathing holes called _spiracles_, placed along the side of the thorax and
abdomen. Bees also have compound eyes. Wings are not found on all insects,
but all the other characters just given are marks of the great group of
animals we call _insects_.
[Illustration: Part of the compound eye of an insect (highly magnified).]
Forms to be looked for on a Field Trip.--Inasmuch as there are over 360,000
different species or kinds of insects, it is evident that it would be a
hopeless task for us even to think of recognizing all of them. But we can
learn to recognize 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 count
on finding members from six groups or _orders_ of insects. These may be
known by the following characters.
The order _Hymenoptera_ (membrane wing) to which the bees, wasps, and ants
belong is the only insect group the members of which are provided with true
stings. This sting is placed in a sheath at the extreme hind end of the
abdomen. Other characteristics, which show them to be insects, have been
given above.
Butterflies or moths will be found hovering over flowers. They belong to
the order _Lepidoptera_ (scale wings). This name is given to them because
their wings are covered with tiny scales, which fit into little sockets on
the wing 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 small, and
one pair is often inconspicuous. These insects may be seen to take liquid
food through a long tubelike organ, called the _proboscis_, which they keep
rolled up under the head when not in use. The young of the butterfly or
moth are known as _caterpillars_ 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 _Orthoptera_
(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.
[Illustration: Forms of life to be met on a field trip. _A_, The red-legged
locust, one of the _Orthoptera_; _o_, the egg-layer, about natural size.
_B_, the honey bee, one of the _Hymenoptera_, about natural size. _C_, a
bug, one of the _Hemiptera_, about natural size. _D_, a butterfly, an
example of the _Lepidoptera_, slightly reduced. _E_, a house fly, an
example of the _Diptera_, about twice natural size. _F_, an orb-weaving
spider, about half natural size. (This is not an insect, note the number of
legs.) _G_, a beetle, slightly reduced, one of the _Coleoptera_.]
Another group of insects sometimes found on flowers in the fall are flies.
They belong to the order _Diptera_ (two wings). These insects 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 _Hemiptera_ (half wings), have a jointed
proboscis which points backward between the front legs. They are usually
small and may or may not have wings.
The beetles or _Coleoptera_ (sheath wings), often mistaken for bugs by the
uneducated, have the first pair of hardened wings meeting in a straight
line in the middle of the back, the second pair of wings being covered by
them. Beetles are frequently found on goldenrod blossoms in the fall.
Other forms of life, especially _spiders_, which have four pairs of walking
legs, _centipedes_ and _millepedes_, both of which are wormlike and have
many pairs of legs, may be found.
Try to discover members of the six different orders named above. Collect
specimens and bring them to the laboratory for identification.
Why do Insects live on Plants?--We 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 among and upon flowering green plants.
The Life History of the Milkweed Butterfly.--If it is possible to find on
our trip some growing milkweed, we are quite likely to find hovering near,
a golden brown and black butterfly, the monarch or milkweed butterfly
(_Anosia plexippus_). Its body, as in all insects, is composed of three
regions. The monarch frequents the milkweed in order to lay eggs there.
This she may be found doing 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 underside of milkweed leaves. Some
wonderful instinct leads the animal to deposit the eggs on the milkweed,
for the young feed upon no other plant. The eggs hatch out in four or five
days into rapid-growing wormlike caterpillars, each of which will shed its
skin several times before it becomes full size. These caterpillars possess,
in addition to the three pairs of true legs, additional pairs of _prolegs_
or caterpillar legs. The animal at this stage is known as a _larva_.
Formation of Pupa.--After a life of a few weeks at most, the caterpillar
stops eating and begins to spin a tiny mat of silk upon a leaf or stem. It
attaches itself to this web by the last pair of prolegs, and there hangs in
the dormant stage known as the _chrysalis_ or _pupa_. This is a resting
stage during which the body changes from a caterpillar to a butterfly.
[Illustration: Monarch butterfly: adults, larvae, and pupa on their food
plant, the milkweed. (From a photograph loaned by the American Museum of
Natural History.)]
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 little insect
is ready after her wedding flight to follow her instinct to deposit her
eggs on a milkweed plant.
Plants furnish Insects with Food.--Food is the most important factor of any
animal's environment. The insects which we have seen on our field trip feed
on the green plants among which they live. Each insect has its own
particular favorite food plant or plants, and in many cases the eggs of the
insect are laid on the food plant so that the young may have food close at
hand. Some insects prefer the rotted wood of trees. An American zoologist,
Packard, has estimated that over 450 kinds of insects live upon oak trees
alone. Everywhere 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 insects.
[Illustration: Damage done by insects. These trees have been killed by
boring insects.]
All Animals depend on Green Plants.--But insects in their turn are the food
of birds; cats and dogs may kill birds; lions or tigers live on still
larger defenseless animals as deer or cattle. And finally comes man, who
eats the bodies of both plants and animals. But if we reduce this search
after 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, or the cat eats the bird that lives
on weed seeds. Green plants supply the food of the world. Later by
experiment we will prove this.
Homes and Shelter.--After a field trip no one can escape the knowledge that
plants often give animals a home. The grass shelters millions of
grasshoppers and countless hordes of other small insects which can be
obtained by sweeping through the grass with an insect net. Some insects
build their homes in the trees or bushes on which 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 come to use all kinds of
plant products to aid him in making his home, wood and various fibers being
the most important of these.
What do Animals do for Plants?--So far it has seemed that green plants
benefit animals and receive nothing in return. We will later see that
plants and animals _together_ form a balance of life on the earth and that
one is necessary for the other. Certain substances 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 interrelations 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. To find out why they
go there and what they do when there, it will be first necessary for us to
study flowers with the idea of finding out what the insects get from them,
and what the flowers get from the insects.
[Illustration: A section of a flower, cut lengthwise. In the center find
the pistil with the ovary containing a number of ovules. Around this organ
notice a circle of stalked structures, the stamens; the knobs at the end
contain pollen. The outer circles of parts are called the petals and
sepals, as we go from the inside outward.]
The Use and Structure of a Flower.--It is a matter of common knowledge that
flowers form fruits and that fruits contain seeds. They are, then, very
important parts of certain plants. Our field trip shows us that flowers 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 find out how they are fitted
to attract and receive insect visitors.
The Floral Envelope.--In a flower 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 are called the sepals._
Together they form the _calyx_.
_The more brightly colored structures are the petals._ Together they form
the _corolla_. The corolla is of importance, as we shall see later, 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 made and secreted. It is principally this
food substance, later made into honey by bees, that makes flowers
attractive to insects.
The Essential Organs.--A flower, however, could live without sepals or
petals and still do the work for which it exists. Certain _essential
organs_ of the flower are within the so-called floral envelope. They
consist of the _stamens_ and _pistil_, 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 _filament_. 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 _ovary_, and a
more or less lengthened portion rising from the ovary called the _style_.
The upper end of the style, which in some cases is somewhat 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 pollen
from one flower to another of the same kind. This transfer of pollen,
called _pollination_, is of the greatest use to the plant, as we will later
prove. No one who sees a hive of bees with their wonderful communal life
can fail to see that these insects play a great part in the life of the
flowers near the hive. A famous observer named Sir John Lubbock tested bees
and wasps to see how many trips they made daily from their homes to the
flowers, and found that the wasp went out on 116 visits during a working
day of 16 hours, while the bee made but a few less visits, and worked only
a little less time than the wasp worked. It is evident that in the course
of so many trips to the fields a bee must light on hundreds of flowers.
[Illustration: Bumblebees. _a_, queen; _b_, worker; _c_, drone.]
Adaptations in a Bee.--If we look closely at the bee, we find the body and
legs more or less covered with tiny hairs; especially are these hairs found
on the legs. _When a plant or animal structure is fitted to do a certain
kind of work, we say it is adapted to do that work._ The joints in the leg
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 leg forms a
structure well adapted to hold pollen. In this way pollen is collected by
the bee and taken to the hive to be used as food. But while gathering
pollen for itself, the dust is caught on the hairs and other projections on
the body or legs and is thus carried from flower to flower. The value of
this to a flower we will see later.
Field Work.--Is Color or Odor in a Flower an Attraction to an Insect?--Sir
John Lubbock tried an experiment which it would pay a number of careful
pupils to repeat. He placed a few drops of honey on glass slips and placed
them over papers of various colors. In this way he found that the honeybee,
for example, could evidently distinguish different colors. Bees seemed to
prefer blue to any other color. Flowers of a yellow or flesh color were
preferred by flies. It would be of considerable interest for some student
to work out this problem with our native bees and with other insects by
using paper flowers and honey or sirup. Test the keenness of sight in
insects by placing a white object (a white golf ball will do) in the grass
and see how many insects will alight on it. Try to work out some method by
which you can decide whether a given insect is attracted to a flower by
odor alone.
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, each of which is
considered to be a very simple eye. The large eyes are therefore called the
_compound eyes_. All insects are provided with compound eyes, with simple
eyes, or in most cases with both. The simple eyes of the bee may be found
by a careful observer between and above the compound eyes.
Insects can, as we have already learned, distinguish differences in color
at some distance; they can see _moving_ objects, but they do not seem to be
able to make out form well. To make up for this, they appear to have an
extremely well-developed sense of smell. Insects can distinguish at a great
distance odors which to the human nose are indistinguishable. Night-flying
insects, especially, find the flowers by the odor rather than by color.
[Illustration: The head of a bee. _A_, antennae or "feelers"; _E_, compound
eye; _S_, simple eye; _M_, mouth parts; _T_, tongue.]
Mouth Parts of the Bee.--The mouth of the bee is adapted to take in the
foods we have mentioned, and is used for the purposes for which man would
use the hands and fingers. The honeybee laps or sucks nectar from flowers,
it chews the pollen, and it uses part of the mouth as a trowel in making
the honeycomb. The uses of the mouth parts may be made out by watching a
bee on a well-opened flower.
Suggestions for Field Work.--In any locality where flowers are abundant,
try to answer the following questions: How many bees visit the
locality in ten minutes? How many other insects alight on the flowers?
Do bees visit flowers of the same kinds in succession, or fly from one
flower on a given plant to another on a plant of a different kind? If the
bee lights on a flower cluster, does it visit more than one flower in the
same cluster? How does a bee alight? Exactly what does the bee do
when it alights?
[Illustration: Flower cluster of "butter and eggs."]
Butter and Eggs (_Linaria vulgaris_).--From July to October this very
abundant weed may be found especially along roadsides and in sunny fields.
The flower cluster forms a tall and conspicuous cluster of orange and
yellow 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.
[Illustration: Diagram to show how the bee pollinates "butter and eggs."
The 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. Inasmuch as bees visit other flowers in the same
cluster, cross-pollination would also be likely. Why?]
Certain parts of the corolla are more brightly colored than the rest of the
flower. This color is a guide to insects. Butter and eggs is visited most
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 secreted in
the spur, brushes his head and shoulders against the stamens. He may then,
as he pushes down after nectar, leave some pollen upon the pistil, thus
assisting in _self-pollination_. Visiting another flower of the cluster, it
would be an easy matter accidentally to transfer this pollen to the stigma
of another flower. In this way pollen is carried by the insect to another
flower of the same kind. This is known as _cross-pollination_. _By
pollination we mean the transfer of pollen from an anther to the stigma of
a flower. Self-pollination is the transfer of pollen from the anther to the
stigma of the same flower; cross-pollination is the transfer of pollen from
the anthers of one flower to the stigma of another flower on the same or
another plant of the same kind._
[Illustration: A wild orchid, a flower of the type from which Charles
Darwin worked out his theory of cross-pollination by insects.]
History of the Discoveries regarding Pollination of Flowers.--Although the
ancient Greek and Roman naturalists had some vague ideas on the subject of
pollination, it was not until the first part of the nineteenth century that
a book appeared in which a German named Conrad Sprengel worked out the
facts that the structure of certain flowers seemed to be adapted to the
visits of insects. Certain facilities were offered to an insect in the way
of easy foothold, sweet odor, and especially food in the shape of pollen
and nectar, the latter a sweet-tasting substance manufactured by certain
parts of the flower known as the nectar glands. Sprengel further discovered
the fact that pollen could be and was carried by the insect visitors from
the anthers of the 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 upon cross-pollination. The growth of the pollen on the
stigma of the flower results eventually in the production of seeds, and
thus new plants. Many species of flowers are self-pollinated and do not do
so well in seed production if cross-pollinated, but Charles Darwin found
that some flowers which were self-pollinated did not produce so many seeds,
and that the plants which grew from their seeds were 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 tended to
_vary_ more than those grown from self-pollinated seed. This has an
important bearing, as we shall see later, in the production of new
varieties of plants. Microscopic examination of the stigma at the time of
pollination also shows that the pollen from another flower usually
germinates before the pollen which has fallen from the anthers of the same
flower. This latter fact alone in most cases renders it unlikely 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 were
adapted or fitted to be cross-pollinated by insects. He also found that, in
the case of flowers that were inconspicuous in appearance, often a
compensation appeared in the odor which rendered them attractive to certain
insects. The so-called carrion flowers, pollinated by flies, are examples,
the odor in this case being like decayed flesh. Other flowers open at
night, are white, and provided with a powerful scent. Thus they attract
night-flying moths and other insects.
Other Examples of Mutual Aid between Flowers and Insects.--Many other
examples of adaptations to secure cross-pollination by means of the visits
of insects might be given. The mountain laurel, which makes our hillsides
so beautiful in late spring, 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.
[Illustration: The condition of stamens and pistils on the spiked
loosestrife (_Lythrum salicaria_).]
In some flowers, as shown by the primroses or primula of our hothouses, the
stamens and pistils are of different lengths in different flowers. Short
styles and long or high-placed filaments are found in one flower, and long
styles with short or low-placed filaments in the other. Pollination will be
effected only when some of the pollen from a low-placed anther reaches the
stigma of a short-styled flower, or when the pollen from a high anther is
placed upon a long-styled pistil. There are, as in the case of the
loosestrife, flowers having pistils and stamens of three lengths. Pollen
only grows on pistils of the same length as the stamens from which it came.
The milkweed or butterfly weed already mentioned is another example of a
flower adapted to insect pollination.[1]
Footnote 1: For an excellent account of cross-pollination of this
flower, the reader is referred to W. C. Stevens, _Introduction to
Botany_. Orchids are well known to botanists as showing some very
wonderful adaptations. A classic easily read is Darwin, _On the
Fertilization of Orchids_.
[Illustration: The pronuba moth within the yucca flower.]
A very remarkable instance of insect help is found in the pollination of
the yucca, a semitropical lily which lives in deserts (to be seen in most
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
_pronuba_, after gathering pollen from an anther, deposits an egg in the
ovary of the pistil, and then rubs its load of pollen over the stigma of
the flower. The young hatch out and feed on the young seeds which have
grown because of the pollen placed on the stigma by the mother. The baby
caterpillars eat some of the developing seeds and later bore out of the
seed pod and escape to the ground, leaving the plant to develop the
remaining seeds without further molestation.
[Illustration: The pronuba pollinating the pistil of the yucca.]
The fig insect (_Blastophaga grossorum_) is another member of the insect
tribe that is of considerable economic importance. It is only in recent
years that the fruit growers of California have discovered that the
fertilization of the female flowers is brought about by a gallfly which
bores into the young fruit. By importing the gallflies it has been possible
to grow figs where for many years it was believed that the climate
prevented figs from ripening.
[Illustration: Pod of yucca showing where the young pronubas escaped.]
Other Flower Visitors.--Other insects besides those already mentioned are
pollen carriers for flowers. Among the most useful are moths and
butterflies. Projecting from each side of the head of a butterfly is a
fluffy structure, the palp. 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 also carry pollen.
[Illustration: A humming bird about to cross-pollinate a lily.]
Flies and some other insects are agents in cross-pollination. Humming birds
are also active agents in some flowers. Snails are said in rare instances
to carry pollen. Man and the domesticated animals undoubtedly frequently
pollinate flowers by brushing past them through the fields.
[Illustration: A cornfield showing staminate and pistillate flowers, the
latter having become grains of corn. An ear of corn is a bunch of ripened
fruits.]
Pollination by the Wind.--Not all flowers are dependent upon insects or
other animals for cross-pollination. Many of the earliest of spring flowers
appear almost before the insects do. Such flowers 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 almost entirely by the wind.
Flowers pollinated by the wind are generally inconspicuous and often lack a
corolla. The anthers are exposed to the wind and provided with much pollen,
while the surface of the stigma may be long and feathery. Such flowers may
also lack odor, nectar, and bright color. Can you tell why?
Imperfect Flowers.--Some flowers, the wind-pollinated ones in particular,
are imperfect; that is, they lack either stamens or pistils. Again, in some
cases, imperfect flowers having stamens only are alone 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.
[Illustration: The flower of "Lady Washington" geranium, in which stamens
and pistil ripen at different times, thus insuring cross-pollination. _A_,
flower with ripe stamens; _B_, flower with stamens withered and ripe
pistil.]
Other Cases.--The stamens and pistil ripen at different times in some
flowers. The "Lady Washington" geranium, a common house plant, shows this
condition. Here also cross-pollination must take place if seeds are to be
formed.
Summary.--If we now collect our observations upon flowers with a view to
making a summary of the different devices flowers have assumed to prevent
self-pollination and to secure cross-pollination, we find that they are as
follows:--
_(1) The stamens and pistils may be found in separate flowers, either on
the same or on different plants._
_(2) The stamens may produce pollen before the pistil is ready to receive
it, or vice versa._
_(3) The stamens and pistils may be so placed with reference to each other
that pollination can be brought about only by outside assistance._
Artificial Cross-pollination and its Practical Benefits to Man.--Artificial
cross-pollination is practiced by plant breeders and can easily be tried in
the laboratory or at home. First the anthers must be carefully removed from
the bud of the flower so as to eliminate all possibility of
self-pollination. The flower must then be covered so as to prevent access
of pollen from without; when the ovary is sufficiently developed, pollen
from another flower, having the characters desired, is placed on the stigma
and the flower again covered to prevent any other pollen reaching the
flower. The seeds from this flower when planted _may_ give rise to plants
with the best characters of each of the plants which contributed to the
making of the seeds.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American
Book Company.
Andrews, _A Practical Course in Botany_, pages 214-249.
American Book Company.
Atkinson, _First Studies of Plant Life_, Chaps. XXV-XXVI.
Ginn and Company.
Coulter, _Plant Life and Plant Uses_, pages 301-322.
American Book Company.
Dana, _Plants and their Children_, pages 187-255. American
Book Company.
Lubbock, _Flowers, Fruits, and Leaves_, Part I. The
Macmillan Company.
Needham, _General Biology_, pages 1-50. The Comstalk
Publishing Company.
Newell, _A Reader in Botany_, Part II, pages 1-96. Ginn and
Company.
Sharpe, _A Laboratory Manual in Biology_, pages 43-48.
American Book Company.
ADVANCED
Bailey, _Plant Breeding_. The Macmillan Company.
Campbell, _Lectures on the Evolution of Plants_. The
Macmillan Company.
Coulter, Barnes, and Cowles, _A Textbook of Botany_, Part
II. American Book Company.
Darwin, _Different Forms of Flowers on Plants of the Same
Species_, D. Appleton 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.
Lubbock, _British Wild Flowers_. The Macmillan Company.
Mueller, _The Fertilization of Flowers_. The Macmillan
Company.
IV. THE FUNCTIONS AND COMPOSITION OF LIVING THINGS
_Problems.--To discover the functions of living matter._
_(a) In a living plant._
_(b) In a living animal._
LABORATORY SUGGESTIONS
_Laboratory study of a living plant._--Any whole plant may
be used; a weed is preferable.
_Laboratory demonstration or home study._--The functions of
a living animal.
_Demonstration._--The growth of pollen tubes.
_Laboratory exercise._--The growth of the mature ovary into
the fruit, _e.g._ bean or pea pod.
A Living Plant and a Living Animal Compared.--A walk into the fields or any
vacant lot on a day in the early fall 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--the
dandelion, butter and eggs, the shepherd's purse--are particularly well
fitted by nature to produce many of their kind, and by this means drive out
other plants which cannot do this so well. On these or other plants we find
feeding several kinds of animals, usually insects.
If we attempt to compare, for example, a grasshopper with the plant on
which it feeds, we see several points of likeness and difference at once.
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 which is a
part of the whole living plant or animal. Each part of the living plant or
animal which has a separate work to do is called an _organ_. Thus plants
and animals are spoken of as living _organisms_.
[Illustration: A weed--notice the unfavorable environment.]
Functions of the Parts of a Plant.--We are all familiar with the parts of a
plant,--the root, stem, leaves, flowers, and fruit. But we may not know so
much about their uses to the plant. Each of these structures differs from
every other part, and each has a separate work or function to perform for
the plant. _The root holds the plant firmly in the ground and takes in
water and mineral matter from the soil; the stem holds the leaves up to the
light and acts as a pathway for fluids between the root and leaves; the
leaves, under certain conditions, manufacture food for the plant and
breathe; the flowers form the fruits; the fruits hold the seeds, which in
turn hold young plants which are capable of reproducing adult plants of the
same kind._
The Functions of an Animal.--As we have already seen, the grasshopper has a
head, a jointed body composed of a middle and a hind part, three pairs of
jointed legs, and two pairs of wings. Obviously, the wings and legs are
used for movement; a careful watching of the hind part of the animal shows
us that breathing movements are taking place; a bit of grass placed before
it may be eaten, the tiny black jaws biting little pieces out of the grass.
If disturbed, the insect hops away, and if we try to get it, it jumps or
flies away, evidently seeing us before we can grasp it. Hundreds of little
grasshoppers on the grass indicate that the grasshopper can reproduce its
own kind, but in other respects the animal seems quite unlike the plant.
The animal moves, breathes, feeds, and has sensation, while _apparently_
the plant does none of these. It will be the purpose of later chapters to
prove that the functions of plants and animals are in many respects similar
and that _both plants_ and _animals breathe_, _feed_, and _reproduce_.
[Illustration: Section through the blade of a leaf. _e_, cells of the upper
surface; _d_, cells of the lower surface; _i_, air spaces in the leaf; _v_,
vein in cross sections; _p_, green cells.]
Organs.--If we look carefully at the organ of a plant called a leaf, we
find that the materials of which it is composed do not appear to be
everywhere the same. The leaf is much thinner and more delicate in some
parts than in others. Holding the flat, expanded blade away from the branch
is a little stalk, which extends into the blade of the leaf. Here it splits
up into a network of tiny "veins" which evidently form a framework for the
flat blade somewhat as the sticks of a kite hold the paper in place. If we
examine under the compound microscope a thin section cut across the leaf,
we shall find that the veins as well as the other parts are made up of many
tiny boxlike units of various sizes and shapes. These smallest units of
building material of the plant or animal disclosed by the compound
microscope are called _cells_. The organs of a plant or animal are built of
these tiny structures.
[Illustration: Several cells of _Elodea_, a water plant. _chl._,
chlorophyll bodies; _c.s._, cell sap; _c.w._, cell wall; _n._, nucleus;
_p._, protoplasm. The arrows show the direction of the protoplasmic
movement.]
Tissues.[2]--The cells which form certain parts of the veins, the flat
blade, or other portions of the plant, are often found in groups or
collections, the cells of which are more or less alike in size and shape.
Such a collection of cells is called a _tissue_. Examples of tissues are
the cells covering the outside of the human body, the muscle cells, which
collectively allow of movement, bony tissues which form the framework to
which the muscles are attached, and many others.
Footnote 2: _To the Teacher._--Any simple plant or animal
tissue can be used to demonstrate the cell. Epidermal cells
may be stripped from the body of the frog or obtained by
scraping the inside of one's mouth. The thin skin from an
onion stained with tincture of iodine shows well, as do thin
sections of a young stem, as the bean or pea. One of the
best places to study a tissue and the cells of which it is
composed is in the leaf of a green water plant, _Elodea_. In
this plant the cells are large, and not only their outline,
but the movement of the living matter within the cells, may
easily be seen, and the parts described in the next
paragraph can be demonstrated.
[Illustration: A cell. _ch._, chromosomes; _c.w._, cell wall; _n._,
nucleus; _p._, protoplasm.]
Cells.--_A cell may be defined as a tiny mass of living matter containing a
nucleus, either living alone or forming a unit of the building material of
a living thing._ The living matter of which all cells are formed is known
as _protoplasm_ (formed from two Greek words meaning _first form_). If we
examine under a compound microscope a small bit of the water plant
_Elodea_, we see a number of structures resembling bricks in a wall. Each
"brick," however, is really a plant cell bounded by a thin wall. If we look
carefully, we can see that the material inside of this wall is slowly
moving and is carrying around in its substance a number of little green
bodies. This moving substance is living matter, the protoplasm of the cell.
The green bodies (the _chlorophyll_ bodies) we shall learn more about
later; they are found only in plant cells. All plant and animal cells
appear to be alike in the fact that every living cell possesses a structure
known as the _nucleus_ (pl. _nuclei_), which is found within the body of
the cell. This nucleus is not easy to find in the cells of _Elodea_. Within
the nucleus of all cells are found certain bodies called _chromosomes_.
These chromosomes in a given plant or animal are always constant in number.
These chromosomes are supposed to be the bearers of the qualities which we
believe can be handed down from plant to plant and from animal to animal,
in other words, the inheritable qualities which make the offspring like its
parents.
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
chromosomes also divide, each splitting lengthwise and the parts going in
equal numbers to each of the two cells formed from the old cell. In this
way the matter in the chromosomes is divided equally between the two new
cells. Then the rest of the protoplasm separates, and two new cells are
formed. This process is known as _fission_. It is the usual method of
growth found in the tissues of plants and animals.
[Illustration: Stages in the division of one cell to form two. Which part
of the cell divides first? What seems to become of the chromosomes?]
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, cells may be so minute, as in
the case of the plant cells named bacteria, that several million might be
present in a few drops of milk. The forms of cells may be extremely varied
in different tissues; they may assume the form of cubes, columns, spheres,
flat plates, or may be extremely irregular in shape. 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 other large animals, 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 several groups of different kinds of cells. It is
only necessary to call attention to the fact that 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
California, stand out as notable examples. The large plants and animals are
made up of _more_, not necessarily larger, cells.
What Protoplasm can Do.--It responds to influences or stimulation from
without its own substance. Both plants and animals are sensitive to touch
or stimulation by light, heat or cold, certain chemical substances,
gravity, and electricity. Green plants turn toward the source of light.
Some animals are attracted to light and others repelled by it; the
earthworm is an example of the latter. _Protoplasm is thus said to be
irritable._
_Protoplasm has the power to contract and to move._ Muscular movement is a
familiar instance of this power. Movement may also take place in plants.
Some plants fold up their leaves at night; others, like the sensitive
plant, fold their leaflets when touched.
_Protoplasm can form new living matter out of food._ To do this, food
materials must be absorbed into the cells of the living organism. To make
protoplasm, it is evident that the same chemical elements must enter into
the composition of the food substances as are found in living matter. The
simplest plants and animals have this wonderful power as certainly
developed as the most complex forms of life.
_Protoplasm, be it in plant or animal, breathes and throws off waste
materials._ When a living thing does work oxygen unites with food in the
body; the food is burned or _oxidized_ and work is done by means of the
energy released from the food. The waste materials are _excreted_ or passed
out. Plants and animals alike pass off the carbon dioxide which results
from the oxidation of food and of parts of their own bodies. Animals
eliminate wastes containing nitrogen through the skin and the kidneys.
_Protoplasm can reproduce, that is, form other matter like itself._ New
plants are constantly appearing to take the places of those that die. The
supply of living things upon the earth is not decreasing; reproduction is
constantly taking place. In a general way it is possible to say that plants
and animals reproduce in a very similar manner.
The Importance of Reproduction.--Reproduction is the final process that
plants and animals are called upon to perform. Without the formation of
_new_ living things no progress would be possible on the earth. We have
found that insects help flowering plants in this process. Let us now see
exactly what happens when pollen is placed by the bee on the stigma of
another flower of the same kind. To understand this process of reproduction
in flowers, we must first study carefully pollen grains from the anther of
some growing flower.
[Illustration: Pollen grains of different shapes and sizes.]
Pollen.--Pollen grains of various flowers, when seen under the microscope,
differ greatly in form and appearance. Some are relatively large, some
small, some rough, others smooth, some spherical, and others angular. They
all agree, however, in having a thick wall, with a thin membrane under it,
the whole inclosing a mass of protoplasm. At an early stage the pollen
grain contains but a single cell. A little later, however, two nuclei may
be found in the protoplasm. Hence we know that at least two cells exist
there, one of which is called the sperm cell; its nucleus is the sperm
nucleus.
[Illustration: A pollen grain greatly magnified. Two nuclei are found (_n_,
_n'_) at this stage of its growth.]
[Illustration: Three stages in the germination of the pollen grain. The
nuclei in the tube in (3) are the sperm nuclei. Drawn under the compound
microscope.]
Growth of Pollen Grains.--Under certain conditions a pollen grain will grow
or germinate. This growth can be artificially produced in the laboratory by
sprinkling pollen from well-opened flowers of sweet pea or nasturtium on a
solution of 15 parts of sugar to 100 of water. Left for a few hours in a
warm and moist place and then examined under the microscope, the grains of
pollen will be found to have germinated, a long, threadlike mass of
protoplasm growing from it into the sugar solution. The presence of this
sugar solution was sufficient to induce growth. When the pollen grain
germinates, the nuclei enter the threadlike growth (this growth is called
the pollen tube; see Figure). One of the nuclei which grows into the pollen
tube is known as the _sperm nucleus_.
[Illustration: Fertilization of the ovule. A flower cut down lengthwise
(only one side shown). The pollen tube is seen entering the ovule. _a_,
anther; _f_, filament; _pg_, pollen grain; _s_, stigmatic surface; _pt_,
pollen tube; _st_, style; _o_, ovary; _m_, micropyle; _sp_, space within
ovary; _e_, egg cell; _P_, petal; _S_, sepal.]
Fertilization of the Flower.--If we cut the pistil of a large flower (as a
lily) lengthwise, we notice that the style appears to be composed of rather
spongy material in the interior; the ovary is hollow and is seen to contain
a number of rounded structures which appear to grow out from the wall of
the ovary. These are the _ovules_. The ovules, under certain conditions,
will become _seeds_. An explanation of these conditions may be had if we
examine, under the microscope, a very thin section of a pistil, on which
pollen has begun to germinate. The central part of the style is found to be
either hollow or composed of a soft tissue through which the pollen tube
can easily grow. Upon germination, the pollen tube grows downward through
the spongy center of the style, follows the path of least resistance to the
space within the ovary, and there enters the ovule. It is believed that
some chemical influence thus attracts the pollen tube. When it reaches the
ovary, the sperm cell penetrates an ovule by making its way through a
little hole called the _micropyle_. It then grows toward a clear bit of
protoplasm known as the _embryo sac_. The embryo sac is an ovoid space,
microscopic in size, filled with semifluid protoplasm containing several
nuclei. (See Figure.) _One of the nuclei, with the protoplasm immediately
surrounding it, is called the egg cell._ It is this cell that the sperm
nucleus of the pollen tube grows toward; ultimately the sperm nucleus
reaches the egg nucleus and unites with it. _The two nuclei, after coming
together, unite to form a single cell. This process is known as
fertilization._ This single cell formed by the union of the pollen tube
cell or sperm and the egg cell is now called a _fertilized egg_.
Development of Ovule into Seed.--_The primary reason for the existence of a
flower is that it may produce seeds from which future plants will grow.
After fertilization the ovule grows into a seed._ The first beginning of
the growth of the seed takes place at the moment of fertilization. From
that time on there is a growth of the fertilized egg within the ovule which
makes a baby plant called the _embryo_. _The embryo will give rise to the
adult plant._
[Illustration: The fruit of the locust, a bean-like fruit. _p_, the
attachment to the placenta; _s_, the stigma.]
A Typical Fruit,--the Pea or Bean Pod.--If a withered flower of any one of
the pea or bean family is examined carefully, it will be found that the
pistil of the flower continues to grow after the rest of the flower
withers. If we remove the pistil from such a flower and examine it
carefully, we find that it is the ovary that has enlarged. The space within
the ovary has become nearly filled with a number of nearly ovoid bodies,
attached along one edge of the inner wall. These we recognize as the young
seeds.
The pod of a bean, pea, or locust illustrates well the growth from the
flower. The pod, which is in reality a ripened ovary with other parts of
the pistil attached to it, is considered as a _fruit_. By definition, _a
fruit is a ripened ovary and its contents together with any parts of the
flower that may be attached to it_. The chief use of the fruit to the
flower is to hold and to protect the seeds; it may ultimately distribute
them where they can reproduce young plants.
[Illustration: 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
1 to 7 are in the order of development.]
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 which contain seeds. Reproduction and the ultimate scattering of
fruits and seeds are absolutely necessary in order that colonies of plants
may reach new localities. It is evident that plants best fitted to scatter
their seeds, or place fruits containing the seeds some little distance from
the parent plants, are the ones which will spread most rapidly. A plant, if
it is to advance into new territory, must get its seeds there first. Plants
which are best fitted to do this are the most widely distributed on the
earth.
How Seeds and Fruits are Scattered.--Seed dispersal is accomplished 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_ provided with a similar structure, the pappus. Some plants, as the
burdock and clotbur, have fruits provided with tiny hooks which stick to
the hair of animals, thus proving a means of 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
animals, buried, and forgotten, 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 are those which will drive out other
plants in a given locality. Because of their adaptations they are likely to
be very numerous, and when unfavorable conditions come, for that reason, if
for no other, are likely to survive. Such plants are best exemplified in
the weeds of the grassplots and gardens.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American
Book Company.
Andrews, _A Practical Course in Botany_, pages 250-270.
American Book Company.
Atkinson, _First Studies of Plant Life_, Chaps. XXV-XXVI.
Ginn and Company.
Bailey, _Lessons with Plants_, Part III, pages 131-250. The
Macmillan Company.
Coulter, _Plant Life and Plant Uses_. American Book Company.
Dana, _Plants and their Children_, pages 187-255. American
Book Company.
Lubbock, _Flowers, Fruit, and Leaves_, Part I. The Macmillan
Company.
Newell, _A Reader in Botany_, Part II, pages 1-96. Ginn and
Company.
ADVANCED
Bailey, _Plant Breeding_. The Macmillan Company.
Campbell, _Lectures on the Evolution of Plants_. The
Macmillan Company.
Coulter, Barnes, and Cowles, _A Textbook of Botany_, Part
II. American Book Company.
Darwin, _Different Forms of Flowers on Plants of the Same
Species_. Appleton.
Darwin, _Fertilization in the Vegetable Kingdom_, Chaps. I
and II. Appleton.
Darwin, _Orchids Fertilized by Insects_. D. Appleton and
Company.
Mueller, _The Fertilization of Flowers_. The Macmillan
Company.
V. PLANT GROWTH AND NUTRITION. CAUSES OF GROWTH
_Problem.--What causes a young plant to grow?_
_(a) The relation of the young plant to its food supply._
_(b) The outside conditions necessary for germination._
_(c) What the young plant does with its food supply._
_(d) How a plant or animal is able to use its food supply._
_(e) How a plant or animal prepares food to use in various parts
of the body._
LABORATORY SUGGESTIONS
_Laboratory exercise._--Examination of bean in pod.
Examination and identification of parts of bean seed.
_Laboratory demonstration._--Tests for the nutrients:
starch, fats or oils, protein.
_Laboratory demonstration._--Proof that such foods exist in
bean.
_Home work._--Test of various common foods for nutrients.
Tabulate results.
_Extra home work by selected pupils._--Factors necessary for
germination of bean. Demonstration of experiments to class.
_Demonstration._--Oxidation of candle in closed jar. Test
with lime water for products of oxidation.
_Demonstration._--Proof that materials are oxidized within
the human body.
_Demonstration._--Oxidation takes place in growing seeds.
Test for oxidation products. Oxygen necessary for
germination.
_Laboratory exercise._--Examination of corn on cob, the corn
grain, longitudinal sections of corn grain stained with
iodine to show that embryo is distinct from 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.
What makes a Seed Grow.--The general problem of the pages that follow will
be to explain how the baby plant, or _embryo_, formed in the seed as the
result of the fertilization of the egg cell, is able to grow into an adult
plant. Two sets of factors 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 water and heat.
[Illustration: Three views of a kidney bean, the lower one having one
cotyledon removed to show the hypocotyl and plumule.]
If we open a bean pod, we find the seeds lying along one edge of the pod,
each attached by a little stalk to the inner wall of the ovary. If we pull
a single bean from its attachment, we find that the stalk leaves a scar on
the coat of the bean; this scar is called the _hilum_. The tiny hole near
the hilum is called the _micropyle_. Turn back to the figure (page 54)
showing the ovule in the ovary. Find there the little hole through which
the pollen tube reached the embryo sac. This hole is identical with the
micropyle in the seed. The thick outer coat (the _testa_) is easily removed
from a soaked bean, the delicate coat under it easily escaping notice. The
seed separates into two parts; these are called the _cotyledons_. If you
pull apart the cotyledons very carefully, you find certain other structures
between them. The rodlike part is called the _hypocotyl_ (meaning _under
the cotyledons_). This will later form the root (and part of the stem) of
the young bean plant. The first true leaves, very tiny structures, are
folded together between the cotyledons. That part of the plant above the
cotyledons is known as the _plumule_ or _epicotyl_ (meaning _above the
cotyledons_). 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.
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
always the most important problem 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.
Organic Nutrients.--Organic foods (those which come from living sources)
are made up of two kinds of substances, the _nutrients_ or food substances
and _wastes_ or _refuse_. An egg, for example, contains the white and the
yolk, composed of nutrients, and the shell, which is waste. The organic
nutrients are classed in three groups.
_Carbohydrates_, foods which contain carbon, hydrogen, and oxygen in a
certain fixed proportion (C{6}H{10}O{5} is an example). They are the
simplest of these very complex chemical compounds we call organic
nutrients. Starch and sugar are common examples of carbohydrates.
_Fats and Oils._--These foods are also composed of carbon, hydrogen, and
oxygen in a proportion which enables them to unite readily with oxygen.
_Proteins._--A third group of organic foods, proteins, are the most complex
of all in their composition, and have, besides carbon, oxygen, and
hydrogen, the element nitrogen and minute quantities of other elements.
[Illustration: Starch grains in the cells of a potato tuber.]
Test for Starch.--If we boil water with a piece of laundry starch in a test
tube, then cool it and add to the mixture two or three drops of iodine
solution,[3] we find that the mixture in the test tube turns purple or deep
blue. It has been discovered by experiment that starch, and no _other known
substance_, will be turned purple or dark blue by iodine. Therefore, iodine
solution has come to be used as a test for the presence of starch.
Footnote 3: Iodine solution is made by simply adding a few
crystals of the element iodine to 95 per cent alcohol; or,
better, take by weight 1 gram of iodine crystals, 2/3 gram
of iodide of potassium, and dilute to a dark brown color in
weak alcohol (35 per cent) or distilled water.
[Illustration: Test for Starch.]
Starch in the Bean.--If we mash up a little piece of a bean cotyledon which
has been previously soaked in water, and test for starch with iodine
solution, the characteristic blue-black color appears, showing the presence
of the starch. If a little of the stained material is mounted in water on a
glass slide under the compound microscope, you will find that the starch is
in the form of little ovoid bodies called _starch grains_. The starch
grains and other food products are made use of by the growing plant.
Test for Oils.--If the substance believed to contain oil is rubbed on brown
paper or is placed on paper and then heated in an oven, the presence of oil
will be known by a translucent spot on the paper.
[Illustration: Test for protein.]
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 bit of
hard-boiled egg. Pour over it a little strong (60 per cent) nitric acid and
heat gently. Note the color that appears--a lemon yellow. If the egg is
washed in water and a little ammonium hydrate added, the color changes to a
deep orange, showing 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 _albumin_ is present.
Another characteristic protein test easily made at home is burning the
substance. If it burns with the odor of burning feathers or leather, then
protein forms part of its composition.[4]
Footnote 4: Other tests somewhat more reliable, but much
more delicate, are the biuret test and test with Millon's
reagent.
A test of the cotyledon of a bean for protein food with nitric acid and
ammonium hydrate shows us the presence of this food. Beans are found by
actual test to contain about 23 per cent of protein, 59 per cent of
carbohydrates, and about 2 per cent oils. The young plant within a pea or
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.
Beans and Peas as Food for Man.--So much food is stored in legumes (as
beans and peas) that man has come to consider them a very valuable and
cheap source of food. Study carefully the following table:--
NUTRIENTS FURNISHED FOR TEN CENTS IN BEANS AND PEAS AT
CERTAIN PRICES PER POUND
=========================================================================
| | TEN CENTS WILL PAY FOR
| PRICES |------------------------------------------
FOOD MATERIALS | PER | TOTAL | | |
AS PURCHASED | POUND | FOOD | PROTEIN | FAT |CARBOHYDRATES
| |MATERIAL | | |
---------------------|--------|---------|---------|--------|-------------
| _Cents_| _Pounds_| _Pounds_|_Pounds_| _Pounds_
Kidney beans, dried | 5 | 2.00 | 0.45 | 0.04 | 1.19
Lima beans, fresh, | | | | |
shelled | 8 | 1.25 | .04 | -- | .12
Lima beans, dried | 6 | 1.67 | .30 | .03 | 1.10
String beans, fresh, | | | | |
30 cents per peck | 3 | 3.33 | .07 | .01 | .23
Beans, baked, canned | 5 | 2.00 | .14 | .05 | .39
Lentils, dried | 10 | 1.00 | .26 | .01 | .59
Peas, green, in pod, | | | | |
30 cents per peck | 3 | 3.33 | .12 | .01 | .33
Peas, dried | 4 | 2.50 | .62 | .03 | 1.55
=========================================================================
[Illustration: A series of early stages in the germination of the kidney
bean.]
Germination of the Bean.--If dry seeds are planted in sawdust or earth,
they will not grow. A moderate supply of water must be given to them. If
seeds were to be kept in a freezing temperature or at a very high
temperature, no growth would take place. A moderate temperature and a
moderate water supply are most favorable for their development.
[Illustration: Bean seedlings. The older seedlings at the left have used up
all of the food supply in the cotyledons.]
If some beans were planted so that we might make a record of their growth,
we would find the first signs of germination to be the breaking of the
testa and the pushing outward of the hypocotyl to form the first root. A
little later the hypocotyl begins to curve downward. A later stage shows
the hypocotyl lifting the cotyledon upward. In consequence the hypocotyl
forms an arch, dragging after it the bulky cotyledons. The stem, as soon as
it is released from the ground, straightens out. From between the
cotyledons the budlike plumule or epicotyl 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 is
now 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 seed. Evidently work
was done; in other words, _energy_ was released by the seeds. A familiar
example of release of energy is seen in an engine. Coal is placed in the
firebox and lighted, the lower door of the furnace is then opened so as to
make a draft of air which will reach the coal. You know the result. The
coal burns, heat is given off, 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 made largely from dead plants, long since
pressed into its present hard form. It contains a large amount of a
chemical element called carbon, the presence of which is characteristic of
all organic material.
[Illustration: The limewater test. The tube at the right shows the effect
of the carbon dioxide.]
Oxidation, its Results.--When things containing carbon are lighted, they
burn. If we place a lighted candle which contains carbon in a closed glass
jar, the candle soon goes out. If we then carefully test the air in the jar
with a substance known as _limewater_,[5] the latter, when shaken up with
the air in the jar, turns milky. This test proves the presence in the jar
of a gas, known as _carbon dioxide_. This gas is formed by the carbon of
the candle uniting with the oxygen in the air. When the oxygen of the air
in the jar was used up, the flame went out, showing that oxygen is
necessary to make a thing burn. This uniting of oxygen with some other
substance is called _oxidation_.
Footnote 5: Limewater can be made by shaking up a piece of
quicklime the size of your fist in about two quarts of
water. Filter or strain the limewater into bottles and it is
ready for use.
[Illustration: Diagram to show that when a piece of wood is burned it forms
water and carbon dioxide.]
Oxidation possible without a Flame.--But a flame is not necessary for
oxidation. Iron, if left in a damp place, becomes rusty. A union between
the oxygen in the water or air and the iron makes what is known as iron
oxide or rust. This is an example of _slow oxidation_.
Oxidation in our Bodies.--If we expel the air from our lungs through a tube
into a bottle of limewater, we notice the limewater becomes milky.
Evidently carbon dioxide is formed in our own bodies and oxidation takes
place there. Is it fair to believe that the heat of our body (for example,
98.6 deg. Fahrenheit under the tongue) is due to oxidation within the body, and
that the work we do results from this chemical process. If so, what is
oxidized?
Energy comes from Foods.--From the foregoing experiment it is evident that
food is oxidized within the human body to release energy for our daily
work. Is it not logical to suppose that all living things, both plant and
animal, release energy as the result of oxidation of foods within their
cells? Let us see if this is true in the case of the pea.
Food oxidized in Germinating Seeds.--If we take equal numbers of soaked
peas, placed in two bottles, one tightly stoppered, the other having 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 almost as well as similar seeds placed in an
open dish would.
[Illustration: Experiment that shows the necessity for air in germination.]
Why did not the seeds in the covered jar germinate? To answer this
question, let us carefully remove the stopper from the stoppered 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 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. The frequent stirring of the soil enables the air to
reach the seed. Air also acts upon some materials in the soil and puts them
in a form that the germinating seed can use. This necessity for oxygen
shows us at least one reason why the farmer plows and harrows a field and
one important use of the earthworm. Explain.
[Illustration: A grain of corn cut lengthwise. _C_, cotyledon; _E_,
endosperm; _H_, hypocotyl; _P_, plumule.]
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 position of the embryo; the rest of the
grain contains the food supply. 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 shows two distinct parts, an area containing considerable
starch, the _endosperm_, and the embryo or young plant. Careful inspection
shows the hypocotyl and plumule (the latter pointing toward the free end of
the grain) and a part surrounding them, the _single_ cotyledon (see
Figure). Here again we have an example 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.
[Illustration: Longitudinal section of young ear of corn. _O_, the fruits;
_S_, the stigmas; _SH_, the sheath-like leaves; _ST_, the flower stalk.
(After Sargent.)]
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 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 grain has begun to grow,
examination shows that the starch grains near the edge of the cotyledon 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 later find
that, in order to make use of starchy food, a plant or animal must in some
manner change it over to sugar. This change is necessary, because starch
will not dissolve in water, while sugar will; in this form substances can
pass from cell to cell in the plant and thus distribute the food where it
is needed.
[Illustration: Test for grape sugar.]
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. Add to the fluid
twice its bulk of Fehling's solution,[6] which has been previously
prepared. 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 yellow, and finally a brick-red
color. Smaller amounts will show less decided red. No other substance than
sugar will give this reaction. If Benedict's test[7] is used, a colored
precipitate will appear in the test tube after boiling.
Footnotes 6 and 7: Directions for making these solutions
will be found in Hunter's _Laboratory Problems in Civic
Biology_.
Starch changed to Grape Sugar in the Corn.--That starch is being changed to
grape sugar in the germinating corn grain can easily be shown if we cut
lengthwise through the embryos of half a dozen grains of corn that have
just begun to germinate, place them in a test tube with some Fehling's
solution, and heat almost to the boiling point. They will be found to 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 a process
of _digestion_. If you chew a bit of unsweetened cracker in the mouth for a
little time, it will begin to taste sweet, and if the chewed cracker, which
we know contains starch, is tested with Fehling's solution, some of the
starch will be found to have changed to grape sugar. Here, again, a process
of digestion has taken place. In both the corn and in the mouth, the change
is brought about by the action of peculiar substances known as digestive
ferments, or _enzymes_. Such substances have the power under certain
conditions to change insoluble foods--solids--into soluble
substances--liquids. The result is that substances which before digestion
would not dissolve in water now will dissolve.
[Illustration: A germinating corn grain. _C_, cotyledon; _H_, growing root
(_hypocotyl_); _P_, growing stem (_plumule_); _S_, endosperm; _d.s._,
digested starch; _p.r._, primary root; _s.r._, secondary root; _r.h._, root
hairs.]
The Action of Diastase on Starch.--The enzyme found in the cotyledon of the
corn, which changes starch to grape sugar, is called _diastase_. It may be
separated from the cotyledon and used in the form of a powder.
To a little starch in half a cup of water we add a very little (1 gram) of
diastase and put the vessel containing the mixture in a warm place, where
the temperature will remain nearly constant at about 98 deg. Fahrenheit. On
testing part of the contents at the end of half an hour, and the remainder
the next morning, for starch and for grape sugar, we find from the morning
test that the starch has been almost 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 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 would find that
eventually it became part of the blood. It was made soluble by digestion,
and in a liquid form was able to reach the blood. Once a part of the body,
the food is used either to release energy or to build up the body.
Summary.--We have seen:
1. That seeds, in order to grow, must possess a food supply either in or
around their bodies.
2. That this food supply must be oxidized before energy is released.
3. That in cases where the food is not stored at the point where it is to
be oxidized the food must be digested so that it may be transported from
one part to another in the same plant.
The life processes of plants and animals, so far, may be considered as
alike; they both feed, breathe (oxidize their food), do work, and grow.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American
Book Company.
Andrews, _A Practical Course in Botany_, pages 1-21.
American Book Company.
Atkinson, _First Studies of Plant Life_, Chap. XXX. Ginn and
Company.
Bailey, _Botany_, Chaps. XX, XXX. The Macmillan Company.
Beal, _Seed Dispersal_. Ginn and Company.
Bergen and Davis, _Principles of Botany_, Chaps. XX, XXX.
Ginn and Company.
Coulter, _Plant Life and Plant Uses_. American Book Company.
Dana, _Plants and their Children_. American Book Company.
Mayne and Hatch, _High School Agriculture_. American Book
Company.
Lubbock, _Flowers, Fruits, and Leaves_. The Macmillan
Company.
Newell, _Reader in Botany_, pages 24-49. Ginn and Company.
Sharpe, _A Laboratory Manual in Biology_, pages 55-65.
American Book Company.
ADVANCED
Bailey, _The Evolution of our Native Fruits_. The Macmillan
Company.
Bailey, _Plant Breeding_. The Macmillan Company.
Coulter, Barnes, and Cowles, _A Textbook of Botany_, Vol. I.
American Book Company.
De Candolle, _Origin of Cultivated Plants_. D. Appleton and
Company.
Duggar, _Plant Physiology_. The Macmillan Company.
Farmers' Bulletins, Nos. 78, 86, 225, 344. U. S. Department
of Agriculture.
Hodge, _Nature Study and Life_, Chaps. X, XX. Ginn and Company.
Kerner (translated by Oliver), _Natural History of Plants_.
Henry Holt and Company, 4 vols. Vol. II, Part 2.
Sargent, _Corn Plants_. Houghton, Mifflin, and Company.
VI. THE ORGANS OF NUTRITION IN PLANTS--THE SOIL AND ITS RELATION TO THE
ROOTS
_Problem.--What a plant takes from the soil and how it gets it._
_(a) What determines the direction of growth of roots?_
_(b) How is the root built?_
_(d) What is in the soil that a root might take out?_
_(e) Why is nitrogen necessary, and how is it obtained?_
LABORATORY SUGGESTIONS
_Demonstration_.--Roots of bean or pea.
_Demonstration or home experiment_.--Response of root to
gravity and to water. What part of root is most responsive?
_Laboratory work_.--Root hairs, radish or corn, position on
root, gross structure only. Drawing.
_Demonstration._--Root hair under compound microscope.
_Demonstration._--Apparatus illustrating osmosis.
_Demonstration or a home experiment._--Organic matter
present in soil.
_Demonstration._--Root tubercles of legume.
_Demonstration._--Nutrients present in some roots.
Uses of the Root.--If one of the seedlings of the bean spoken of in the
last chapter 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.
The development of a bean seedling has shown us that the root grows first.
_One of the most important functions of the root to a young seed plant is
that of a holdfast, an anchor to fasten it in the place where it is to
develop._ It has many other uses, as the taking in of water with the
mineral and organic matter dissolved therein, the storage of food,
climbing, etc. All functions other than the first one stated arise after
the young plant has begun to develop.
[Illustration: A root system, showing primary and secondary roots.]
Root System.--If you dig up a young bean seedling and carefully 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 primary root are called
_secondary_, or _tertiary_, depending on their relation to the first root
developed.
Downward Growth of Root. Influence 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
by a simple experiment.
[Illustration: Revolve this figure in the direction of the arrows to see if
the roots of the radish respond to gravity.]
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 roots now will be found to have
turned in response to the change in position, that part of the root near
the growing point being the most sensitive to the change. This experiment
seems to indicate that the roots are influenced to grow downward by the
force of gravity.
Experiments to determine the Influence of Moisture on a Growing Root.--The
objection might well be interposed that possibly the roots in the pocket
garden[8] grew downward after water. That moisture has an influence on the
growing root is easily proved.
Footnote 8: _The Pocket Garden._--A very convenient form of
pocket germinator may be made as follows. Obtain two cleaned
four by five negatives (window glass will do); place one
flat on the table and place on this half a dozen pieces of
colored blotting paper cut to a size a little less than the
glass. Now cut four thin strips of wood to fit on the glass
just outside of the paper. Next moisten the blotter, place
on it some well-soaked radish, mustard seeds or barley
grains, and cover with the other glass. The whole box thus
made should be bound together with bicycle tape. Seeds will
germinate in this box and with care may live for two weeks
or more.
Plant bird seed, mustard or radish seed in the underside of a sponge, which
should be kept wet, and may be suspended by a string under a bell jar in
the schoolroom window. Note whether the roots leave the sponge to grow
downward, or if the moisture in the sponge is sufficient to counterbalance
the force of gravity.
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 by roots._
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, a
cloverlike plant used for hay in the Western states, often penetrate the
soil after water for a distance of ten to twenty feet below the surface of
the ground.
Fine Structure of a Root.[9]--When we examine a delicate root in thin
longitudinal section under the compound microscope, we find the entire root
to be made up of cells, the walls of which are uniformly 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 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 can
easily be distinguished from the surrounding cells. In a longitudinal
section a series of tubelike structures may be found within the central
cylinder. These structures are cells which have grown together at the small
end, the long axis of the cells running the length of the main root. In
their development the cells mentioned have grown together in such a manner
as to lose their small ends, and now form continuous hollow tubes with
rather strong walls. Other cells have come to develop greatly thickened
walls; these cells give mechanical support to the tubelike cells.
Collections of such tubes and supporting woody cells together make up what
are known as _fibrovascular bundles_.
Footnote 9: Sections of tradescantia roots are excellent for
demonstration of these structures.
[Illustration: Cross section of a young taproot; _a_, _a_, root hairs; _b_,
outer layer of bark; _c_, inner layer of bark; _d_, wood or central
cylinder.]
Root Hairs.--Careful examination of the root of one of the seedlings of
mustard, radish, or barley grown in the pocket germinator shows a covering
of tiny fuzzy structures. These structures are very minute, at most 3 to 4
millimeters in length. They vary in length according to their position on
the root, the most and the longest root hairs being found near the point
marked _R. H._ in the figure. These structures are outgrowths of the outer
layer of the root (the _epidermis_), and are of very great importance to
the living plant.
[Illustration: Young embryo of corn, showing root hairs (_R. H._) and
growing stem (_P._).]
Structure of a Root Hair.--A single root hair examined under a compound
microscope will be found to be a long, round structure, almost colorless in
appearance. The wall, which is very flexible and thin, is made up of
cellulose, a substance somewhat like wood in chemical composition, through
which fluids may easily pass. Clinging close to the cell wall is the
protoplasm of the cell. The interior of the root hair is more or less
filled with a fluid called _cell sap_. Forming a part of the living
protoplasm 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 protoplasm and nucleus 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 wall so delicate that water and mineral substances from the
soil can pass through it into the interior of the root.
[Illustration: Diagram of a root hair; _CS_, cell sap; _CW_, cell wall;
_P_, protoplasm; _N_, nucleus; _S_, particles of soil.]
How the Root absorbs Water.--The process by which the root hair takes up
soil water can better be understood if we make an artificial root hair
large enough to be easily seen. An egg with part of the outer shell removed
so as to expose the soft skinlike membrane underneath is an example.
Better, an artificial root hair may be _made_ in the following way. Pour
some soft celloidin into a test tube; carefully revolve the test tube so
that an even film of celloidin dries on the inside. This membrane is
removed, filled with white of egg, and tied over the end of a rubber cork
in which a glass tube has previously been inserted. When placed in water,
it gives a very accurate picture of the root hair at work. After a short
time water begins to rise in the tube, having passed through the film of
celloidin. If grape sugar, salt, or some other substance which will
dissolve in water were placed in the water outside the artificial root
hair, it could soon be proved by test to pass through the wall and into the
liquid inside.
Osmosis.--To explain this process we must remember that gases and liquids
of different densities, when separated by a membrane, tend to flow toward
each other and mingle, the greater flow always being in the direction of
the denser medium. _The process by which two gases or fluids, separated by
a membrane, tend to pass through the membrane and mingle with each other,
is called osmosis._ The method by which the root hairs take up soil water
is exactly the same process. It is by osmosis. The white of the egg is the
best possible substitute for living matter; the celloidin membrane
separating the egg from the water is much like the delicate membrane-like
wall which separates the protoplasm of the root hair from the water in the
soil surrounding it. The fluid in the root hair is denser than the soil
water; hence the greater flow is toward the interior of the root hair.[10]
Footnote 10: For an excellent elementary discussion of
osmosis see Moore, _Physiology of Man and Other Animals_.
Henry Holt and Company.
[Illustration: The soil particles are each surrounded with a delicate film
of water. How might the root hairs take up this water?]
Passage of Soil Water within the Root.--We have already seen that in an
exchange of fluids by osmosis the greater flow is always toward the denser
fluid. Thus it is that the root hairs take in more fluid than they give up.
The cell sap, which partly fills the interior of the root hair, is a fluid
of greater density than the water outside in the soil. When the root hairs
become filled with water, the density of the cell sap is lessened, and the
cells of the epidermis are thus in a position to pass along their supply of
water to the cells next to them and nearer to the center of the root. These
cells, in turn, become less dense than their inside neighbors, and so the
transfer of water goes on until the water at last reaches the central
cylinder. Here it is passed over to the tubes of the woody bundles and
started up the stem. The pressure created by this process of osmosis is
sufficient to send water up the stem to a distance, in some plants, of 25
to 30 feet. Cases are on record of water having been raised in the birch a
distance of 85 feet.
Physiological Importance of Osmosis.--It is not an exaggeration to say that
osmosis is a process not only of great importance to a plant, but to an
animal as well. 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 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 largely by osmosis. Without
the process of osmosis we should be unable to use much of the food we eat.
Composition of Soil.--If we examine a mass of ordinary loam carefully, we
find that it is composed of numerous particles of varying size and weight.
Between these particles, if the soil is not caked and hard packed, we can
find tiny spaces. In well-tilled soil these spaces are constantly being
formed and enlarged. They allow air and water to penetrate the soil. If we
examine soil under the microscope, we find considerable water clinging to
the soil particles and forming a delicate film around each particle. In
this manner most of the water is held in the soil.
[Illustration: Inorganic soil is being formed by weathering.]
How Water is held in Soil.--To understand what comes in with the soil
water, it will be necessary to find out a little more about soil.
Scientists who have made the subject of the composition of the earth a
study, tell us that once upon a time at least a part of the earth was
molten. Later, it cooled into solid rock. Soil making began when the ice
and frost, working alternately with the heat, chipped off pieces of rock.
These pieces in time became ground into fragments by action of ice,
glaciers, running water, or the atmosphere. This process is called
weathering. Weathering is aided by oxidation. A glance at almost any
crumbling stones will convince you of this, because of the yellow oxide of
iron (rust) disclosed. So by slow degrees this earth became covered 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.
[Illustration: This picture shows how the forests help to cover the
inorganic soil with an organic coating. Explain how.]
You are all familiar with the difference between the so-called rich soil
and poor soil. The dark soil contains more dead plant and animal matter,
which forms the portion called _humus_.
[Illustration: Apparatus for testing the capacity of soils to take in and
retain moisture.]
Humus contains Organic Matter.--It is an easy matter to prove that black
soil contains organic matter, for if an equal weight of carefully dried
humus and soil from a sandy road is 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.
Soil containing organic materials holds water much more readily than
inorganic soil, as a glance at the accompanying figure shows. If we fill
each of the vessels with a given weight (say 100 grams each) of gravel,
sand, barren soil, rich loam, leaf mold, and 25 grams of dry, pulverized
leaves, then pour equal amounts of water (100 c.c.) on each 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.
[Illustration: Soil particles cling to root hairs. Why?]
The Root Hairs take more than Water out of the Soil.--If a root containing
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
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. An interesting
experiment may be shown (see Figure on page 80) to prove this. A solution
of _phenolphthalein_ loses its color when an acid is added to it. If a
growing pea be placed in a tube containing some of this solution the latter
will quickly change from a rose pink to a colorless solution.
A Plant needs Mineral Matter to Make Living Matter.--Living matter
(protoplasm), besides containing the chemical elements carbon, hydrogen,
oxygen, and nitrogen, contains a very minute proportion of various elements
which make up the basis of certain minerals. These are calcium (lime),
sulphur, iron, potassium, magnesium, 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 nutrient
solution.[11] Such a solution contains all the mineral matter that a plant
uses for food. If certain ingredients are left out of this solution, the
plants placed in it will not live.
Footnote 11: See Hunter's _Laboratory Problems in Civic
Biology_ for list of ingredients.
[Illustration: Effect of root hairs on phenolphthalein solution. The change
of color indicates the presence of acid.]
Nitrogen in a Usable Form necessary for Growth of Plants.--A chemical
element needed by the plant to make protoplasm is nitrogen. The air can be
proven by experiment to be made up of about four fifths nitrogen, but this
element cannot be taken from either soil, water, or air in a pure state,
but is usually obtained from the organic matter in the soil, where it
exists with other substances in the form of _nitrates_. Ammonia and other
organic compounds which contain nitrogen are changed by two groups of
little plants called _bacteria_, first into nitrites and then nitrates.[12]
Footnote 12: It has recently been discovered that under some
conditions these bacteria are preyed upon by tiny one-celled
animals (_protozoa_) living in the soil and are so reduced
in numbers that they cannot do their work effectively. If,
then, the soil is heated artificially or treated with
antiseptics so as to kill the protozoa, the bacteria which
escape multiply so rapidly as to make the land much richer
than before.
[Illustration: Diagram to show how the nitrogen-fixing bacteria prepare
nitrogen for use by plants; _t_, tubercles.]
Relation of Bacteria to Free Nitrogen.--It has been known since the time of
the Romans that the growth of clover, peas, beans, and other legumes in
soil causes it to become more favorable for growth of other plants. The
reason for this has been discovered in late years. On the roots of the
plants mentioned are found little swellings or nodules; in the nodules
exist millions of bacteria, which take nitrogen from the atmosphere and fix
it so that it can be used by the plant; that is, they assist in forming
nitrates for the plants to use. Only these bacteria, of all the living
plants, have the power to take the free nitrogen from the air and make it
over into a form that can be used by the roots. As all the compounds of
nitrogen are used over and over again, first by plants, then as food for
animals, eventually returning to the soil again, or in part being turned
into free nitrogen, it is evident that any _new_ supply of usable nitrogen
must come by means of these nitrogen-fixing bacteria.
Rotation of Crops.--The facts mentioned above are made use of by careful
farmers who wish to make as much as possible from a given area of ground in
a given time. Such plants as are hosts for the nitrogen-fixing bacteria are
planted early in the season. Later these plants are plowed in and a second
crop is planted. The latter grows quickly and luxuriantly because of the
nitrates left in the soil by the bacteria which lived with the first crop.
For this reason, clover is often grown on land in which it is proposed to
plant corn, the nitrogen left in the soil thus giving nourishment to the
young corn plants. In scientifically managed farms, different crops are
planted in a given field on different years so that one crop may replace
some of the elements taken from the soil by the previous crop. This is
known as rotation of crops.[13] The annual yield of the average farm may
thus be greatly increased.
Footnote 13: That crop rotation is not primarily a process
to conserve the fertility of the soil, but is a sanitary
measure to prevent infection of the soil, is the latest
belief of the scientist.
[Illustration: Nitrogen in the soil is necessary for plants. Explain from
this diagram how nitrogen is put into the soil by some plants and taken out
by others.]
Five of the elements necessary to the life of the plant which may be taken
out of the soil by constant use are calcium, nitrogen, phosphorus,
potassium, and sulphur. Several methods are used by the farmer to prevent
the exhaustion of these and other raw food materials from the soil. One
method known as _fallowing_ is to allow the soil to remain idle until
bacteria and oxidation have renewed the chemical materials used by the
plants. This is an expensive method, if land is dear. The most common
method of enriching soil is by means of fertilizing material rich in plant
food. Manure is most frequently used, but many artificial fertilizers, most
of which contain nitrogen in the form of some nitrate, are used, because
they can be more easily transported and sold. Such are ground bone, guano
(bird manure), nitrate of soda, and many others. These also contain other
important raw food materials for plants, especially potash and phosphoric
acid. Both of these substances are made soluble so as to be taken into the
roots by the action of the carbon dioxide in the soil.
The Indirect Relation of this to the City Dweller.--All of us living in the
city are aware of the importance of fresh vegetables, brought in from the
neighboring market gardens. But we sometimes forget that our great staple
crops, wheat and other cereals, potatoes, fruits of all kinds, our cotton
crop, and all plants we make use of grow directly in proportion to the
amount of raw food materials they take in through the roots. When we also
remember that many industries within the cities, as mills, bakeries, and
the like, as well as the earnings of our railways and steamship lines, are
largely dependent on the abundance of the crops, we may recognize the
importance of what we have read in this chapter.
Food Storage in Roots of Commercial Importance.--Some plants, as the
parsnip, carrot, and radish, produce no seed until the second year, storing
food in the roots the first year and using it to get an early start the
following spring, so as to be better able to produce seeds when the time
comes. This food storage in roots is of much practical value to mankind.
Many of our commonest garden vegetables, as those mentioned above, and the
beet, turnip, oyster plant, sweet potato and many others, are of value
because of the food stored. The sugar beet has, in Europe especially,
become the basis of a great industry.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American
Book Company.
Bigelow, _Applied Biology_. The Macmillan Company.
Coulter, _Plant Life and Plant Uses_, Chaps. III, IV.
American Book Company.
Mayne and Hatch, _High School Agriculture_. American Book
Company.
Moore, _The Physiology of Man and Other Animals_. Henry Holt
and Company.
Sharpe, _Laboratory Manual in Biology_, pp. 73-87. American
Book Company.
ADVANCED
Coulter, Barnes, and Cowles, _A Textbook of Botany_, Part
II. Amer. Book Co.
Duggar, _Plant Physiology_. The Macmillan Company.
Goodale, _Physiological Botany_. American Book Company.
Green, _Vegetable Physiology_, Chaps. V, VI. J. and A.
Churchill.
Kerner-Oliver, _Natural History of Plants_. Henry Holt and
Company.
MacDougal, _Plant Physiology_. Longmans, Green, and Company.
VII. PLANT GROWTH AND NUTRITION--PLANTS MAKE FOOD
_Problem.--Where, when, and how do green plants make food?_
_(a) How and why is moisture given off from leaves?_
_(b) What is the reaction of leaves to light?_
_(c) What is made in green leaves in the sunlight?_
_(d) What by-products are given off in the above process?_
_(e) Other functions of leaves._
LABORATORY SUGGESTIONS
_Demonstration._--Water given off by plant in sunlight. Loss
of weight due to transpiration measured.
_Laboratory exercise._--
(_a_) Gross structure of a leaf.
(_b_) Study of stoma 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 to starch making.
_Demonstration._--Air necessary to 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 has been absorbed through the root hairs into the roots.
What becomes of this water and the other substances that have been
absorbed? This question may be partly answered by the following
experiments.
[Illustration: Apple twigs split to show the course of colored water up the
stem.]
Passage of Fluids up the Stem.--If any young growing shoots (young
seedlings of corn or pea, or the older stems of garden balsam,
touch-me-not, or sunflower) are placed in red ink (eosin), and left in the
sun for a few hours, the red ink will be found to have passed up the stem.
If such stems were examined carefully, it would be seen that the colored
fluid is confined to collections of woody tubes immediately under the inner
bark. Water evidently rises in that part of the stem we call the wood.
[Illustration: Experiment to prove that water is given off through the
leaves of a green plant.]
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. In a few minutes drops of moisture are seen to gather on the
_inside_ of the jar. If we now weigh the potted plant, we find it weighs
less than before. Obviously the loss comes from the water lost, and
evidently this water escapes as vapor from either the stem or leaves.
[Illustration: The skeleton of a leaf. _M.R._, the midrib; _P._, the
leafstalk; _V._, the veins.]
The Structure of a Leaf.--In the experiment with the red ink mentioned
above we will 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 _blade_, which may take almost any conceivable
shape; (2) a _stem_ which spreads out in the blade (3) in a number of
_veins_.
[Illustration: Section through the blade of a leaf as seen under the
compound microscope. _S_, air spaces, which communicate with the outside
air; _V_, vein in cross section; _S.T._, breathing hole (stoma); _E_, outer
layer of cells; _P_, green cells.]
The Cell Structure of a Leaf.--The under surface of a leaf seen under the
microscope usually shows numbers of tiny oval openings. These are called
_stomata_ (singular _stoma_). Two cells, usually kidney-shaped, are found,
one on each side of the opening. These are the _guard cells_. By change in
shape of these cells the opening of the stoma is made larger or smaller.
Larger irregular cells form the _epidermis_, or outer covering of the leaf.
Study of the leaf in cross section shows that these stomata open directly
into air chambers which penetrate between and around the loosely arranged
cells composing the underpart of the leaf. The upper surface of leaves
sometimes contains stomata, but more often they are lacking. The under
surface of an oak leaf of ordinary size contains about 2,000,000 stomata.
Under the upper epidermis is a layer of green cells closely packed together
(called collectively the _palisade layer_). These cells are more or less
columnar in shape. Under these are several rows of rather loosely placed
cells just mentioned. These are 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 tubes of the stem out into the
blade of the leaf.
Evaporation of Water.--During the day an enormous amount of water is taken
up by the roots and passed out through the leaves. So great is this excess
at times that a small grass plant on a summer's day evaporates more than
its own weight in water. This would make nearly half a ton of water
delivered to the air during twenty-four hours by a grass plot twenty-five
by one hundred feet, the size of the average city lot. According to Ward,
an oak tree may pass off two hundred and twenty-six times its own weight in
water during the season from June to October.
From which Surface of the Leaf is Water Lost?--In order to find out whether
water is passed out from any particular part of the leaf, we may remove two
leaves of the same size and weight from some large-leaved plant[14]--a
mullein was used for the illustrations given below--and cover the upper
surface of one leaf and the lower surface of the other with vaseline. The
leaf stalks of each should be covered with wax or vaseline, and the two
leaves exactly balanced on the pans of a balance which has previously been
placed in a warm and sunny place. Within an hour the leaf which has the
upper surface covered with vaseline will show a loss of weight. Examination
of the surface of a mullein leaf shows us that the _lower surface of the
leaf is provided with stomata_. It is through these organs, then, that
water is passed out from the tissues of the leaf.
Footnote 14: The "rubber plant" leaf is an easily obtainable
and excellent demonstration.
[Illustration: Experiment to show through which surface of a leaf water
passes off.]
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. But there seems to be no certain regulation
of this water loss. Consequently 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.
[Illustration: Diagrams of a stoma. _a_, surface view of a closed stoma;
_b_, the same stoma opened. (After Hanson.) _c_, diagrams of a transverse
section through a stoma, dotted lines indicate the closed position of the
guard cells, the heavy lines the open condition. (After Schwendener.)]
Green Plants Food Makers.--We have previously stated that green plants are
the great food makers for themselves and for animals. We are now ready to
attack the problem of how green plants _make_ food.
The Sun a Source of Energy.--We all know the sun is a source of most of the
energy that is released on this earth in the form of heat or light. Every
boy 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
will directly harness the energy of the sun in everyday work. Actual
experiments have shown that vast amounts of energy are given to the earth.
When the sun is highest in the sky, energy equivalent to one hundred horse
power is received by a plot of land twenty-five by one hundred feet, the
size of a city lot. Plants receive and use much of this energy by means of
their leaves.
Effect of Light on Plants.--In young plants which have been grown in total
darkness, 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. We can explain the changed condition of the seedling grown in
the dark only by assuming that light has some effect on the protoplasm of
the seedling and induces the growth of the green part of the plant. If
seedlings have been growing on a window sill, or where the light comes in
from one side, you have doubtless noticed that the stem and leaves of the
seedlings incline in the direction from which the light comes. The
experiment pictured shows this effect of light very plainly. A hole was cut
in 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 light by an
indirect course. The young seedling in this case responded to the influence
of the stimulus of light so as to grow out finally 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, as we shall see later, food
making depends largely on the amount of sunlight the leaves receive.
[Illustration: Two stages in an experiment to show that green plants grow
toward the light.]
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 are found to be in positions
to receive the most sunlight possible. Careful observation of any plant
growing outdoors shows us that in almost every case the leaves are so
disposed as to get much sunlight. The ivy climbing up the wall, the
morning-glory, the dandelion, and the burdock all show different
arrangements of leaves, each presenting a large surface to the light.
Leaves are often definitely arranged, fitting in between one another so as
to present their 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
young 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
any field trip.
[Illustration: A lily, showing long narrow leaves.]
[Illustration: The dandelion, showing a whorled arrangement of long
irregular leaves.]
Starch made by a Green Leaf.--If we examine the palisade layer of the leaf,
we find cells which are almost cylindrical in form. In the protoplasm of
such cells are found a number of little green-colored bodies, which are
known as _chloroplasts_ or _chlorophyll bodies_. If we place the leaf 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 protoplasm of the cell
colored green. These bodies are of the greatest importance directly to
plants and indirectly to animals. _The chloroplasts, by means of the energy
received from the sun, manufacture starch out of certain raw materials._
These raw materials are soil water, which is passed up through the bundles
of tubes into the veins of the leaf from the roots, and carbon dioxide,
which is taken in through the stomata or pores, which dot the under surface
of the leaf. A plant with variegated leaves, as the coleus, makes starch
only in the green part of the leaf, even though these raw materials reach
all parts of the leaf.
[Illustration: An experiment to show the effect of excluding light (but not
air) from the leaves of a green plant. The result of this experiment is
seen in the next picture. (Experiment performed by C. Dobbins and A.
Schwartz.)]
[Illustration: Starchless area in a leaf caused by excluding sunlight by
means of a strip of black cloth.]
Light and Air necessary for Starch Making.--If we pin strips of black
cloth, such as alpaca, over some of the leaves of a growing 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 for
starch. We then remove some of the covered leaves and extract the
chlorophyll with wood alcohol (because the green color of the chlorophyll
interferes with the blue color of the starch test). A test then shows that
starch is present only in the portions of the leaves exposed to sunlight.
From this experiment we infer that the sun has something to do with starch
making in a leaf. The necessity of a part of the air (carbon dioxide) for
starch making may also easily be proved, for the parts of leaves covered
with vaseline will be found to contain no starch, while parts of the leaf
without vaseline, but exposed to the sun and air, do contain starch.
[Illustration: Diagram to show starch making. Read the text carefully and
then explain this diagram.]
Air is necessary for the process of starch making in a leaf, not only
because carbon dioxide gas is absorbed (there are from three to four parts
in ten thousand present in the atmosphere), but also because the leaf is
alive and must have oxygen in order to do work. This oxygen it takes from
the air around it.
[Illustration: Diagram to illustrate the formation of starch in a leaf.]
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
milling of grain. In this case the mill is the green part 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 fully, we must refer to a small portion of the leaf
shown below. 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
taken in through the stomata and reaches the green cells by way of the
intercellular spaces and by osmosis from cell to cell. Water reaches the
green cells through the veins. It then passes into the cells by osmosis,
and there becomes part of the cell sap. The light of the sun easily
penetrates to 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 favorable. For example, too much
heat or too little heat stops starch making in the leaf. This building up
of food and the release of oxygen by the plant in the presence of sunlight
is called _photosynthesis_.
[Illustration: Diagram (after Stevens) to illustrate the processes of
breathing and food making in the cells of a green leaf in the sunlight.]
Manufacture of Fats.--Inasmuch as tiny droplets of oil 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 Matter.--Protein
material is a food which is necessary to form protoplasm. Protein food is
present in the leaf, and is found in the stem or root as well. Proteins can
apparently be manufactured in any of the cells of green plants, the
presence of light not seeming to be a necessary factor. How it is
manufactured is a matter of conjecture. The minerals brought up in the soil
water form part of its composition, and starch or grape sugar give three
elements (C, H, and O). The element nitrogen is taken up by the roots as a
nitrate (nitrogen in combination with lime or potash). Proteins are
probably not made directly into protoplasm 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. While plants and animals obtain their food in
different ways, they probably make it into living substance (_assimilate_
it) in exactly the same manner.
[Illustration: An example of how a tree may exert energy. This rock has
been split by the growing tree.]
Foods serve exactly the same purposes in plants and in animals; they either
build living matter 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 open the hardest rocks, as illustrated in the figure
above.
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 sends 10 to 15 grams of reserve material into the ears in a single
day. The formation of fruit, and especially the growth of the grain fields,
show the economic importance of this fact. Not only do plants make their
own food and store it away, but they make food for animals as well. And the
food is stored in such a stable form that it may be sent to all parts of
the world in the form of grain or other fruits. 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
remember that in 1913 in the United States the total value of all farm
crops was over $6,000,000,000, and when we realize that these products came
from the air and soil through the energy of the sun, we may begin to
realize why as city boys and girls the study of plant biology is of
importance to us.
[Illustration: Experiment to show that oxygen is given off by green plants
in the sunlight.]
Green Plants give off Oxygen in Sunlight.--In still another way green
plants are of direct use to us in the city. During this process of
starch-making oxygen is given off as a by-product. This may easily be
proven by the following experiment.[15] Place any green water plant in a
battery jar partly filled with water, 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 can
be obtained by displacement of the water to make the oxygen test.
Footnote 15: Immediate success with this experiment will be
obtained if the water has been previously charged with
carbon dioxide.
That oxygen is given off as a by-product by green plants is a fact of
far-reaching importance. City parks are true "breathing spaces." 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.
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 oxygen largely
through the stomata of the leaves, to a less extent through the _lenticels_
or breathing holes in the stem, and through the roots. Thus rapidly growing
tissues receive the oxygen necessary for them to perform their work. The
products of oxidation in the form of carbon dioxide are 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, of course, 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) breathing, or the taking in of oxygen and passing
off of carbon dioxide; (2) starch-making, with the incidental passing out
of oxygen; (3) formation of proteins, with their digestion and assimilation
to form new tissues; and (4) the transpiration of water.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American
Book Company.
Andrews, _A Practical Course in Botany_, pages 160-177.
American Book Company.
Coulter, _A Textbook of Botany_, pages 5-40. D. Appleton and
Company.
Coulter, _Plant Life and Plant Uses_. American Book Company.
Dana, _Plants and their Children_, pages 135-185. American
Book Company.
Sharpe, _A Laboratory Manual in Biology_, pages 90-102.
American Book Company.
Stevens, _Introduction to Botany_, pages 81-99. D. C. Heath
and Company.
ADVANCED
Clement, _Plant Physiology and Ecology_. Henry Holt and
Company.
Coulter, Barnes, and Cowles, _A Textbook of Botany_, Part
II, and Vol. II. American Book Company.
Darwin, _Insectivorous Plants_. D. Appleton and Company.
Duggar, _Plant Physiology_. The Macmillan Company.
Goodale, _Physiological Botany_, pages 337-353 and 409-424.
American Book Company.
Green, _Vegetable Physiology_. J. and A. Churchill.
Lubbock, _Flowers, Fruits, and Leaves_, last part. The
Macmillan Company.
MacDougal, _Practical Textbook of Plant Physiology_.
Longmans, Green, and Company.
Report of the Division of Forestry, U.S. Department of
Agriculture, 1899.
Ward, _The Oak_. D. Appleton and Company.
VIII. PLANT GROWTH AND NUTRITION--THE CIRCULATION AND FINAL USES OF FOOD BY
PLANTS
_Problem.--How green plants store and use the food they make._
_(a) What are the organs of circulation?_
_(b) How and where does food circulate?_
_(c) How does the plant assimilate its 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 Foods in Green Plants.--We have seen that
cells of green plants make food and that such cells are mostly in the
leaves. But _all_ parts of the bodies of plants grow. Roots, stems, leaves,
flowers, and fruits grow. Seeds are storehouses 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 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 _bark_. 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 latter usually brown in color.
This layer is made up largely of tough fiberlike cells known as _bast_
fibers. The most important parts of this inner bark, so far as the plant is
concerned, are many tubelike structures known as _sieve tubes_. These are
long rows of living cells, having perforated sievelike ends. Through these
cells food materials pass downward from the upper part of the plant, where
they are manufactured.
[Illustration: Section of a twig of box elder three years old, showing
three annual growth rings. The radiating lines (_m_) which cross the wood
(_w_) represent the pith rays, the principal ones extending from the pith
in the center to the cortex or bark. (From Coulter's _Plant Relations_.)]
In the wood will be noticed (see Figure) a number of lines radiating
outward from the pith toward the bark. These are thin plates of pith which
separate the wood into a number of wedge-shaped masses. These masses of
wood are composed of 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
sections 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 an actively dividing layer of cells, known as the _cambium 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, the heavy woody skeletons serving to keep out cold and dryness,
as well as prevent the evaporation of fluids from within. The bark also
protects the tree from attack of other 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. The function of
the corky layer in preventing evaporation is well seen in the case of the
potato, which is a true stem, though found underground. If two potatoes of
equal weight are balanced on the scales, the skin having been peeled from
one, the peeled potato will be found to lose weight rapidly. This is due to
loss of water, which is held in by the skin of the unpeeled potato (see
right hand figure below).
There are also small breathing holes known as _lenticels_ scattered through
the surface of the bark. These can easily be seen in a young woody stem of
apple, beech, or horse-chestnut.
[Illustration: Experiment to show that the skin of the potato (a stem)
retards evaporation.]
Proof that Food passes down the Stem.--If freshly cut willow twigs are
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 food material
necessary for the outgrowth of roots evidently comes from above, and 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 willow explains why it is that trees die
when girdled so as to cut the sieve tubes of the inner bark. The food
supply is cut off from the protoplasm of the cells in the part of the tree
below the cut area. Many of the canoe birches of our Adirondack forest are
thus killed, girdled by thoughtless visitors. In the same manner mice and
other gnawing animals kill fruit trees. Food substances are also conducted
to a much less extent in the wood itself, and food passes from the inner
bark to the center of the tree by way of the pith plates. This can be
proved by testing for starch in the pith plates of young stems. It is found
that much starch is stored in this part of the tree trunk.
[Illustration: Experiment to show that food material passes down in the
inner bark.]
In what Form does Food pass through the Stem?--We have already seen that
materials in solution (those substances which will dissolve in the water)
will pass from cell to cell by the process of osmosis. This is shown in the
experiment illustrated in the figure. Two thistle tubes are partly filled,
one with starch and water, the other with sugar and water, and a piece of
parchment paper is tied over the end of each. The lower ends of both tubes
are placed in a glass dish under water. After twenty-four hours, the water
in the dish is tested for starch, and then for sugar. We find that only the
sugar, which has been dissolved by the water, can pass through the
membrane.
[Illustration: Experiment to show osmosis of sugar (right hand tube) and
non-osmosis of starch (left hand tube).]
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. It must be changed to a soluble form, for otherwise it could not
pass through the delicate cell membranes. This is accomplished by the
process of _digestion_. We have already seen that starch is changed to
grape sugar in the corn by the action of a substance (an enzyme) called
_diastase_. 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 in a soluble form until it comes to a place
where food storage is to take place, then it can be transformed to 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.
In a similar manner, protein seems to be changed and transferred to various
parts of the plant. Some forms of protein substance are _soluble_ and
others _insoluble_ in water. 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. In a plant, 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 fruit or seeds of a
plant, and be stored there.
[Illustration: Diagram to show the areas in a plant through which the raw
food materials pass up the stem and food materials pass down.]
What forces Water up the Stem.--We have seen that the process of osmosis is
responsible for taking in soil water, and that the enormous absorbing
surface exposed by the root hairs makes possible the absorption of a large
amount of water. Frequently this is more than the weight of the plant in
every twenty-four hours.
Experiments have been made which show that at certain times in the year
this water is in some way forced up the tiny tubes of the stem. During the
spring season, in young and rapidly growing trees, water has been proved to
rise to a height of nearly ninety feet. The force that causes this rise of
water in stems is known as _root pressure_.
The greatest factor, however, is transpiration of water from leaves. This
evaporation of water in the form of vapor seems to result in a kind of
suction on the column of water in the stem. In the fall, after the leaves
have gone, much less water is taken in by roots, showing that an intimate
relation exists between the leaves and the root.
Summary of the Functions of Green Plants.--The processes which we have just
described (with the exception of food making) are those which occur in the
lives of any plant or animal. All 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. 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.
[Illustration: Leaf of sundew closing over a captured insect.]
[Illustration: The Venus fly trap, showing open and closed leaves.]
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 insects by means of
enzymes poured out by the same hairs. Another plant, the Venus fly trap,
catches insects in a sensitive leaf which folds up and holds the insect
fast until enzymes 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. In this respect plants are like
those animals which have certain organs in the body set apart for the
digestion of food.
Assimilation.--The assimilation of foods, or making of foods into living
matter, is a process we know very little about. 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 we have not yet solved.
Excretion.--The waste and repair of living matter seems to take place 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.
[Illustration: The embryos of (_a_) the morning glory, (_b_) the barberry,
(_c_) the potato, (_d_) the four o'clock, showing the position of their
food supply. (After Gray.)]
Reproduction.--Finally, both plants and animals have organs of
reproduction. We have seen that the flower gives rise, after pollination,
to a fruit which holds the seeds. These seeds hold the _embryo_. Thus the
young plant is doubly protected for a time and is finally thrown off in the
seed with enough food to give it a start in life. In much the same way we
will find that animals reproduce, either by laying eggs which contain an
_embryo_ and food to start it in life or, as in the higher animals, by
holding 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.
The Life Cycle.--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 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 the _life cycle_.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American
Book Company.
Andrews, _A Practical Course in Botany_, pages 112-127.
American Book Company.
Atkinson, _First Studies of Plant Life_, Chaps. IV, V, VI,
VIII, XXI. Ginn.
Coulter, _Plant Life and Plant Uses_, Chap. V. American Book
Company.
Dana, _Plants and their Children_, pages 99-129. American
Book Company.
Mayne and Hatch, _High School Agriculture_. American Book
Company.
Hodge, _Nature Study and Life_, Chaps. IX, X, XI. Ginn and
Company.
MacDougal, _The Nature and Work of Plants_. The Macmillan
Company.
ADVANCED
Apgar, _Trees of the United States_, Chaps. II, V, VI.
American Book Company.
Coulter, Barnes, and Cowles, _A Textbook of Botany_, Vol. I.
American Book Company.
Duggar, _Plant Physiology_. The Macmillan Company.
Ganong, _The Teaching Botanist_. The Macmillan Company.
Goebel, _Organography of Plants_, Part V. Clarendon Press.
Goodale, _Physiological Botany_. American Book Company.
Gray, _Structural Botany_, Chap. V. American Book Company.
Kerner-Oliver, _Natural History of Plants_. Henry Holt and
Company.
Strasburger, Noll, Schenck, and Karston, _A Textbook of
Botany_. The Macmillan Company.
Ward, _The Oak_. D. Appleton and Company.
Yearbook, U. S. Department of Agriculture, 1894, 1895,
1898-1910.
IX. OUR FORESTS, THEIR USES AND THE NECESSITY FOR THEIR PROTECTION
_Problem.--Man's relations to forests._
_(a) What is the value of forests to man?_
_(b) What can man do to prevent forest destruction?_
LABORATORY SUGGESTIONS
Demonstration of some uses of wood. Optional exercise on
structure of wood. Method of cutting determined by
examination. Home work on study of furniture trim, etc.
Visit to Museum to study some economic uses of wood.
Visit to Museum or field trip to learn some common trees.
[Illustration: A forest in North Carolina. (U. S. G. S.)]
The Economic Value of Trees. Protection and Regulation of Water
Supply.--Trees form a protective covering for parts of the earth's surface.
They prevent soil from being washed away, and they hold moisture in the
ground. The devastation of immense areas in China and considerable damage
by floods in parts of Switzerland, France, and in Pennsylvania has resulted
where the forest covering has been removed. No one who has tramped through
our Adirondack forest can escape noticing the differences in the condition
of streams surrounded by forest and those which flow through areas from
which trees have been cut. The latter streams often dry up entirely in hot
weather, while the forest-shaded stream has a never failing supply of
crystal water.
[Illustration: Working to prevent erosion after the removal of the forest
in the French Alps.]
[Illustration: Erosion at Sayre, Pennsylvania, by the Chemung River.
(Photograph by W. C. Barbour.)]
The city of New York owes much of its importance to its position at the
mouth of a great river with a harbor large enough to float the navies of
the world. This river is supplied with water largely from the Adirondack
and Catskill forests. Should these forests be destroyed, it is not
impossible that the frequent freshets which would follow would so fill the
Hudson River with silt and debris that the ship channels in the bay,
already costing the government hundreds of thousands of dollars a year to
keep dredged, would become too shallow for ships. If this _should_ occur,
the greatest city in this country would soon lose its place and become of
second-rate importance.
The story of how this very thing happened to the old Greek
city of Poseidonia is graphically told in the following lines:--
"It was such a strange, tremendous story, that of the Greek
Poseidonia, later the Roman Paestum. Long ago those
adventuring mariners from Greece had seized the fertile
plain, which at that time was covered with forests of great
oak and watered by two clear and shining rivers. They drove
the Italian natives back into the distant hills, for the
white man's burden even then included the taking of all the
desirable things that were being wasted by incompetent
natives, and they brought over colonists--whom the
philosophers and moralists at home maligned, no doubt, in
the same pleasant fashion of our own day. And the colonists
cut down the oaks, and plowed the land, and built cities,
and made harbors, and finally dusted their busy hands and
busy souls of the grime of labor and wrought splendid
temples in honor of the benign gods who had given them the
possessions of the Italians and filled them with power and
fatness.
"Every once in so often the natives looked lustfully down
from the hills upon this fatness, made an armed snatch at
it, were driven back with bloody contumely, and the heaping
of riches upon riches went on. And more and more the oaks
were cut down--mark that! for the stories of nations are so
inextricably bound up with the stories of trees--until all
the plain was cleared and tilled; and then the foothills
were denuded, and the wave of destruction crept up the
mountain sides, and they, too, were left naked to the sun
and the rains.
"At first these rains, sweeping down torrentially,
unhindered by the lost forests, only enriched the plain with
the long-hoarded sweetness of the trees; but by and by the
living rivers grew heavy and thick, vomiting mud into the
ever shallowing harbors, and the land soured with the
undrained stagnant water. Commerce turned more and more to
deeper ports, and mosquitoes began to breed in the brackish
soil that was making fast between the city and the sea.
"Who of all those powerful landowners and rich merchants
could ever have dreamed that little buzzing insects could
sting a great city to death? But they did. Fevers grew more
and more prevalent. The malaria haunted population went more
and more languidly about their business. The natives, hardy
and vigorous in the hills, were but feebly repulsed.
Carthage demanded tribute, and Rome took it, and changed the
city's name from Poseidonia to Paestum. After Rome grew weak,
Saracen corsairs came in by sea and grasped the slackly
defended riches, and the little winged poisoners of the
night struck again and again, until grass grew in the
streets, and the wharves crumbled where they stood. Finally,
the wretched remnant of a great people wandered away into
the more wholesome hills, the marshes rotted in the heat and
grew up in coarse reeds where corn and vine had flourished,
and the city melted back into the wasted earth."[16]
Footnote 16: Elizabeth Bisland and Anne Hoyt, _Seekers in
Sicily_. John Lane Company.
[Illustration: Result of deforestation in China. This land has been ruined
by erosion. (Carnegie Institution Research in China.)]
Prevention of Erosion by Covering of Organic Soil.--We have shown how
ungoverned streams might dig out soil and carry it far from its original
source. Examples of what streams have done may be seen in the deltas formed
at the mouths of great rivers. The forest prevents this by holding the
water supply and letting it out gradually. This it does by covering the
inorganic soil with humus or decayed organic material. In this way the
forest floor becomes like a sponge, holding water through long periods of
drought. The roots of the trees, too, help hold the soil in place. The
gradual evaporation of water through the stomata of the leaves cools the
atmosphere, and this tends to precipitate the moisture in the air.
Eventually the dead bodies of the trees themselves are added to the organic
covering, and new trees take their place.
[Illustration: The forest regions of the United States.]
Other Uses of the Forest.--In some localities forests are used as
windbreaks and to protect mountain towns against avalanches. In winter they
moderate the cold, and in summer reduce the heat and lessen the danger from
storms. Birds nesting in the woods protect many valuable plants which
otherwise might be destroyed by insects.
Forests have great commercial importance. Pyrogallic and other acids are
obtained from trees, as are tar, creosote, resin, turpentine, and many
useful oils. The making of maple sirup and sugar forms a profitable
industry in several states.
The Forest Regions of the United States.--The combined area of all the
forests in the United States, exclusive of Alaska, is about 500,000,000
acres. This seemingly immense area is rapidly decreasing in acreage and in
quality, thanks to the demands of an increasing population, a woeful
ignorance on the part of the owners of the land, and wastefulness on the
part of cutters and users alike.
A glance at the map on page 109 shows the distribution of our principal
forests. Washington ranks first in the production of lumber. Here the great
Douglas fir, one of the "evergreens," forms the chief source of supply. In
the Southern states, especially Louisiana and Mississippi, yellow pine and
cypress are the trees most lumbered.
Which states produce the most hardwoods? From which states do we get most
of our yellow pine, spruce, red fir, redwood? Where are the heaviest
forests of the United States?
[Illustration: Transportation of lumber in the West. A logging train.]
[Illustration: Transportation of lumber in the East. Logs are mostly
floated down rivers to the mills.]
Uses of Wood.--Even in this day of coal, wood is still by far the most used
fuel. It is useful in building. It outlasts iron under water, in addition
to being durable and light. It is cheap and, with care of the forests,
inexhaustible, while our mineral wealth may some day be used up. Distilled
wood gives wood alcohol. Partially burned wood is charcoal. In our forests
much of the soft wood (the cone-bearing trees, spruce, balsam, hemlock, and
pine), and poplars, aspens, basswood, with some other species, make paper
pulp. The daily newspaper and cheap books are responsible for inroads on
our forests which cannot well be repaired. It is not necessary to take the
largest trees to make pulp wood. Hence many young trees of not more than
six inches in diameter are sacrificed. Of the hundreds of species of trees
in our forests, the conifers are probably most sought after for lumber.
Pine, especially, is probably used more extensively than any other wood. It
is used in all heavy construction work, frames of houses, bridges, masts,
spars and timber of ships, floors, railway ties, and many other purposes.
Cedar is used for shingles, cabinetwork, lead pencils, etc.; hemlock and
spruce for heavy timbers and, as we have seen, for paper pulp. Another use
for our lumber, especially odds and ends of all kinds, is in the
packing-box industry. It is estimated that nearly 50 per cent of all lumber
cut ultimately finds its way into the construction of boxes. Hemlock bark
is used for tanning.
The hard woods--ash, basswood, beech, birch, cherry, chestnut, elm, maple,
oak, and walnut--are used largely for the "trim" of our houses, for
manufacture of furniture, wagon or car work, and endless other purposes.
[Illustration: Diagrams of sections of timber. _a_, cross section; _b_,
radial; _c_, tangential. (From Pinchot, U. S. Dept. of Agriculture.)]
Methods of cutting Timber.--A glance at the diagram of the sections of
timber shows us that a tree may be cut radially through the middle of the
trunk or tangentially to the middle portion. Most lumber is cut
tangentially. In wood cut in this manner the yearly rings take a more or
less irregular course. The grain in wood is caused by the fibers not taking
straight lines in their course in the tree trunk. In many cases the fibers
of the wood take a spiral course up the trunk, or they may wave outward to
form little projections. Boards cut out of such a piece of wood will show
the effect seen in many of the school desks, where the annual rings appear
to form elliptical markings. Quite a difference in color and structure is
often seen between the heartwood, composed of the dead walls of cells
occupying the central part of the tree trunk, and the sapwood, the living
part of the stem.
[Illustration: Section of a tree trunk showing knot.]
Knots.--Knots, as can be seen from the diagram, are branches which at one
time started in their outward growth and were for some reason killed.
Later, the tree, continuing in its outward growth, surrounded them and
covered them up. A dead limb should be pruned before such growth occurs.
The markings in bird's-eye maple are caused by buds which have not
developed, and have been overgrown with the wood of the tree.
Destruction of the Forest.--_By Waste in Cutting._--Man is responsible for
the destruction of one of this nation's most valuable assets. This is
primarily due to wrong and wasteful lumbering. Hundreds of thousands of
dollars' worth of lumber is left to rot annually because the lumbermen do
not cut the trees close enough to the ground, or because through careless
felling of trees many other smaller trees are injured. There is great waste
in the mills. In fact, man wastes in every step from the forest to the
finished product.
[Illustration: A forest in the far west totally destroyed by fire and
wasteful lumbering.]
_By Fire._--Indirectly, man is responsible for fire, one of the greatest
enemies of the forest. Most of the great forest fires of recent years, the
losses from which total in the hundreds of millions, have been due either
to railroads or to carelessness in making fires in the woods. It is
estimated that in forest lands traversed by railroads from 25 per cent to
90 per cent of the fires are caused by coal-burning locomotives. For this
reason laws have been made in New York State requiring locomotives passing
through the Adirondack forest preserve to burn oil instead of coal. This
has resulted in a considerable reduction in the number of fires. In
addition to the loss in timber, the fires often burn out the organic matter
in the soil (the "duff") forming the forest floor, thus preventing the
growth of forest there for many years to come. In New York and other states
fires are fought by an organized corps of fire wardens, whose duty it is to
watch the forest and to fight forest fires.
Other Enemies.--Other enemies of the forest are numerous fungus plants,
insect parasites which bore into the wood or destroy the leaves, and
grazing animals, particularly sheep. Wind and snow also annually kill many
trees.
Forestry.--In some parts of central Europe, the value of the forests was
seen as early as the year 1300 A.D., and many towns consequently bought up
the surrounding forests. The city of Zurich has owned forests in its
vicinity for at least 600 years and has found them a profitable investment.
In this country only recently has the importance of preserving and caring
for our forests been noted by our government. Now, however, we have a
Forest Survey of the Department of Agriculture and numerous state and
university schools of forestry which are rapidly teaching the people of
this country the best methods for the preservation of our forests. The
Federal government has set aside a number of tracts of mountain forest in
some of the Western states, making a total area of over 167,000,000 acres.
New York has established for the same purpose the Adirondack Park, with
nearly 1,500,000 acres of timberland. Pennsylvania has one of 700,000
acres, and many other states have followed their example.
[Illustration: The forest primeval. Trees are killing each other in the
struggle for light and air.]
[Illustration: A German beech forest. The trees are kept thinned out so as
to allow the young trees to get a start. Contrast this with the picture
above.]
Methods for Keeping and Protecting the Forests.--Forests should be kept
thinned. Too many trees are as bad as too few. They struggle with one
another for foothold and light, which only a few can enjoy. In cutting the
forest, it should be considered as a harvest. The oldest trees are the
"ripe grain," the younger trees being left to grow to maturity. Several
methods of renewing the forest are in use in this country. (1) Trees may be
cut down and young ones allowed to sprout from cut stumps. This is called
coppice growth. This growth is well seen in parts of New Jersey. (2) Areas
or strips may be cut out so that seeds from neighboring trees are carried
there to start new growth. (3) Forests may be artificially planted. Two
seedlings planted for every tree cut is a rule followed in Europe. (4) The
most economical method is that shown in the lower picture on page 114,
where the largest trees are thinned out over a large area so as to make
room for the younger ones to grow up. The greatest dangers to the forests
are from fire and from careless cutting, and these dangers may be kept in
check by the efficient work of our national and state foresters.
[Illustration: We must protect our city trees. This tree was badly wounded
by being gnawed by a horse.]
A City's Need for Trees.--The city of Paris, well known as one of the most
beautiful of European capitals, spends over $100,000 annually in caring for
and replacing some of the 90,000 trees owned by the city. All over the
United States the city governments are beginning to realize what European
cities have long known, that trees are of great value to a city. They are
now following the example of European cities by planting trees and by
protecting the trees after they are planted. Thousands of city trees are
annually killed by horses which gnaw the bark. This may be prevented by
proper protection of the trunk by means of screens or wire guards. Chicago
has appointed a city forester, who has given the following excellent
reasons why trees should be planted in the city:--
(1) Trees are beautiful in form and color, inspiring a constant
appreciation of nature.
(2) Trees enhance the beauty of architecture.
(3) Trees create sentiment, love of country, state, city, and home.
(4) Trees have an educational influence upon citizens of all ages,
especially children.
(5) Trees encourage outdoor life.
(6) Trees purify the air.
(7) Trees cool the air in summer and radiate warmth in winter.
(8) Trees improve climate and conserve soil and moisture.
(9) Trees furnish resting places and shelter for birds.
(10) Trees increase the value of real estate.
(11) Trees protect the pavement from the heat of the sun.
(12) Trees counteract adverse conditions of city life.
Let us all try to make Arbor Day what it should be, a day for
caring for and planting trees, for thus we may preserve this most
important heritage of our nation.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American
Book Company.
Mayne and Hatch, _High School Agriculture_. American Book
Company.
Murrill, _Shade Trees_, Bul. 205, Cornell University
Agricultural Experiment Station.
Pinchot, _A Primer of Forestry_, Division of Forestry, U. S.
Department of Agriculture.
ADVANCED
Apgar, _Trees of the United States_, Chaps. II, V, VI.
American Book Company.
Coulter, Barnes, and Cowles, _A Textbook of Botany_, Part I
and Vol. II. American Book Company.
Goebel, _Organography of Plants_, Part V. Clarendon Press.
Strasburger, Noll, Schenck, and Karston, _A Textbook of
Botany_. The Macmillan Company.
Ward, _Timber and Some of its Diseases_. The Macmillan
Company.
Yearbook, U. S. Department of Agriculture, Division of
Forestry, Buls. 7, 10, 13, 16, 17, 18, 20, 26, 27.
X. THE ECONOMIC RELATION OF GREEN PLANTS TO MAN
_Problems.--How green plants are useful to man._
_(a) As food._
_(b) For clothing._
_(c) Other uses._
_How green plants are harmful to man._
SUGGESTED LABORATORY WORK
If a commercial museum is available, a trip should be planned to work
over the topics in this chapter. The school collection may well
include most of the examples mentioned, both of useful and harmful
plants.
A study of weeds and poisonous plants should be taken up in actual
laboratory work, either by collection and identification or by
demonstration.
Green Plants have a "Dollar and Cents" Value.--To the girl or boy living in
the city green plants seem to have little direct value. Although we see
vegetables for sale in stores and we know that fruits have a money value,
we are apt to forget that the wealth of our nation depends more upon its
crops than it does on its manufactories and business houses. The economic
or "dollars and cents" value of plants is enormous and far too great for us
to comprehend in terms of figures.
We have already seen some of the uses to mankind of the products of the
forest; let us now consider some other plant products.
[Illustration:
Cabbage Onions Lettuce
Leaves used as food.]
Leaves as Food.--Grazing animals feed almost entirely on tender shoots or
leaves, blades of grass, and other herbage. Certain leaves and buds are
used by man as food. Lettuce, beet tops, kale, spinach, broccoli, are
examples. A cabbage head is nothing but a big bud wh
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