SCIENCE OF
PLANT LIFE

Tra n s e a u

BIOLOGY

LIBRARY

6

SCIENCE OF PLANT LIFE

A HIGH SCHOOL BOTANY

TREATING OF THE PLANT AND ITS
RELATION TO THE ENVIRONMENT

NEW-WORLD SCIENCE SERIES

Edited by John W. Ritchie

SCIENCE FOR BEGINNERS

By Delos Fall
TREES, STARS, AND BIRDS

By Edwin Lincoln Moseley
COMMON SCIENCE

By Carleton W. Washbume
HUMAN PHYSIOLOGY

By John W. Ritchie
SANITATION AND PHYSIOLOGY

By John W. Ritchie

LABORATORY MANUAL FOR USE WITH
"HUMAN PHYSIOLOGY"

By Carl Hartman

EXERCISE AND REVIEW BOOK IN BIOLOGY

By/. G. Blaisdell
PERSONAL HYGIENE AND HOME NURSING

By Louisa C. Lippitt
SCIENCE OF PLANT LIFE

By Edgar Nelson Transeau

ZOOLOGY

By T. D. A. Cockerell
EXPERIMENTAL ORGANIC CHEMISTRY

By Augustus P. West

NEW-WORLD SCIENCE SERIES

Edited by John W. Ritchie

SCIENCE
OF PLANT LIFE

A High School Botany

Treating of the Plant and Its Relation

to the Environment

by
Rdgar Nelson Trans eau, Ph. £);

Professor of Botany, The Ohio State University

ILLUSTRATED

with engravings, diagrams, and maps
and with 120 original drawings by

Robert J. Sim

Yonkers-on-Hudson, New York

WORLD BOOK COMPANY

1921

WORLD BOOK COMPANY

THE HOUSE OF APPLIED KNOWLEDGE BIOLOGY

'

Established, 1905, by Caspar W. Hodgson
YONKERS-ON-HUDSON, NEW YORK
2126 PRAIRIE AVENUE, CHICAGO

World Book Company offers Science of Plant
Life for high school use. The book em-
bodies a response to the attractiveness of
plant life, a steadfast scientific spirit that
yields neither to sentiment nor to utility,
and an appreciation of the fact that the fun-
damental reason for giving botany a place in
our general scheme of education is that it is
the natural scientific background for the
great plant-producing arts. It is a text that
accords well with our motto, " Books that
apply the world's knowledge to the world's
needs "

MAIN LIBRARY — •'.-. TUtfEDEPT.

NWSS:TSPL-4

Copyright, 1919, by World Book Company

Copyright in Great Britain

All rights reserved

K •.

PREFACE

THE most important question that confronts the teacher of
elementary botany is the selection of the subject matter for
the course. Shall the work consist chiefly of the naming of
plants and of learning the meanings of terms descriptive of
plants and of plant organs, as was common 30 years ago?
Does a course emphasizing a study of the anatomy of plant
organs provide the best introduction to the subject of botany
for pupils of secondary school grade? Have those courses
been satisfactory in which the work centered about the evolu-
tionary development of the plant kingdom, with studies of
the reproductive organs and life histories of types representa-
tive of the great plant groups, or shall we teach physiology
and ecology? Shall we select as material for study wild
plants, often obscure and unfamiliar to the pupil, because
they show certain structures significant in determining the
relationships of plants, or shall we use the familiar plants
of the farm and garden, on which man depends in large part
for his livelihood, to exemplify botanical principles? Is it
best to try to give a " practical" turn to the course by insert-
ing chapters from other subjects like agriculture, forestry, and
plant breeding ; or shall the course be kept within the strict
confines of botany and the relation of botanical facts and
principles to plant production be shown by appropriate refer-
ences and illustrations ? Upon the answers given to these
questions the content of the course to be offered will largely
depend.

The author is one of those who think that our work in
botany should serve as a basis for agriculture, horticulture,
and forestry, just as physics and chemistry form the natural
background of our manufacturing and industrial life. His teach-
ing experience has led him to believe, moreover, that a very

465784

vi Preface

general course in botany that presents and organizes the more
important facts of plant physiology, morphology, ecology, and
economics is interesting to pupils of high school age and is
valuable to any one who later engages in the growing of
plants. The fundamental aim of ihvs text, therefore, is to give
the pupil an understanding of how a plant lives and is affected
by its environment. The nutrition of the plant is the central
theme. Sufficient anatomy and morphology have been intro-
duced to make possible a discussion of the important plant
processes, including reproduction. The environment of
plants and their adjustments to various environmental fac-
tors have been discussed because a knowledge of these subjects
is essential to an understanding of many agricultural practices.
Attention is called to the uses that are made of plants and
plant materials and to the applications of botanical prin-
ciples in plant production in order that the economic im-
portance of plants and of a knowledge of plant life may be
evident. In the final chapter an attempt is made to give
the pupil an understanding of what is meant by the term
" evolution" — to present a definition rather than a discussion.
The most valuable part of any course in botany is the study
of plants in the field and laboratory. It is through the work
with the plants themselves that the teacher has the best
opportunity to give pupils the insight into plant life that will
justify the expenditure of their time and energy on the course.
The laboratory work has been fairly well standardized in
secondary schools, and the methods of handling classes are
well known. This is not true of field work, although it is
no more difficult than laboratory work and is no less im-
portant. Two methods of conducting field observation have
been eminently successful. One is to take the class into the

Preface vii

field to study a particular topic, — not to make random ob-
servations on plants. A second method is to give each pupil
a carefully prepared outline containing questions and direc-
tions by which he can make a trip and discover for himself
the answers to the questions. Both these methods deserve to
be used far more than they are.

This book has been written to supplement laboratory and
field work with plants, not to take the place of such work.
Suggestions for laboratory and field work will be found pre-
ceding each chapter, but these suggestions include more work
than can be accomplished in most high school courses. The
teacher should, therefore, select those exercises that are best
suited to the needs of a particular class, and which are best
adapted to the laboratory equipment and the plant material
available.

ACKNOWLEDGMENTS

THE author wishes to thank many friends and students who
have made suggestions regarding the book. Especially is he
under obligation to the following, who have critically read the
manuscript or the proofs : Dr. Harris M. Benedict, University of
Cincinnati; Dr. J. M. Lewis, University of Texas; Dr. Burton
E. Livingston, Johns Hopkins University ; Dr. S. O. Mast, Johns
Hopkins University; Dr. Charles A. Shull, University of Ken-
tucky; Dr. A. B. Stout, New York Botanical Garden; Dr. G.
H. Transeau, Columbus, Ohio; to his colleagues at Ohio State
University, especially Dr. Jay B. Park, Dr. H. C. Sampson,
Dr. A. E. Waller, Professor W. G. Stover, and Mr. Paul Sears ;
to Mr. G. W. Salisbury, Principal of Atchison County High
School, Emngham, Kansas; Mr. J. R. Locke and Miss Edith
E. Pettee, Highland Park High School, Detroit, Michigan ; Miss
Ella M. Bennett, High School, Ann Arbor, Michigan; Miss Lucy
M. Phelon, High School of Commerce, Springfield, Massachu-
setts ; Miss Ruth Jackson, High School, Wichita, Kansas ; and
Miss Maude Flynn, High School, Columbus, Ohio.

viii

CONTENTS

CHAPTER

PAGE

1. PLANT LIFE IN GENERAL ... i

2. LEAVES AND THEIR STRUCTURES . .13

3. THE MANUFACTURE OF FOOD . . . . -25

4. LEAVES IN RELATION TO LIGHT . 37

5. THE WATER RELATIONS OF LEAVES . . 48

6. LEAF COLORATION AND THE FALL OF LEAVES . . 63

7. DIGESTION, TRANSFER, AND ACCUMULATION OF FOODS . 74

8. THE UTILIZATION OF FOODS .... .82

9. HERBS, SHRUBS, AND TREES . . 94

10. STEMS AND THEIR EXTERNAL FEATURES . . 102

11. THE STRUCTURES AND PROCESSES OF STEMS . . .118

12. THE ENVIRONMENT OF PLANTS ... . 139

13. ECOLOGICAL GROUPS OF STEMS . . . . . . 154

14. THE STRUCTURES AND PROCESSES OF ROOTS . . .166

15. ROOTS AND THEIR ENVIRONMENT 182

1 6. REPRODUCTION IN FLOWERING PLANTS: FLOWERS, FRUITS,

AND SEEDS 197

17. REPRODUCTION IN RELATION TO AGRICULTURE . . .217

18. THE ALG^E 234

19. BACTERIA AND FUNGI 249

20. LIVERWORTS AND MOSSES . , 272

21. THE FERNS AND THEIR ALLIES 283

22. SEED PLANTS: GYMNOSPERMS . . ' . . . . 294

23. SEED PLANTS: ANGIOSPERMS 303

24. THE EVOLUTION OF PLANTS 318

INDEX . 331

Flower in the crannied wall,

I pluck you out of the crannies,

I hold you here, root and all, in my hand,

Little flower — but if I could understand

What you are, root and all, and all in all,

I should know what God and man is.

TENNYSON

SCIENCE OF PLANT LIFE

CHAPTER ONE

PLANT LIFE IN GENERAL
PLANTS FROM OUR STANDPOINT

PROBABLY all of us make our most frequent contact with
nature through plants, and no part of our environment ap-
peals to us more. The city dweller responds to the attrac-
tion of plants by growing a few flowers in a window box.
The suburban resident finds great pleasure in his lawn and
trees. The farmer enjoys the annual miracle of transforming
his bare fields into acres of productive wheat and corn ; and
the forester delights in his work with the largest and most
imposing of plants. This universal challenge of plant life
is reflected in our literature; the stories of our childhood
days and the novels and verse of our maturer years are
made vivid by their backgrounds of garden and meadow, or
of forest and desert.

The importance of plants. There are many reasons for
studying and understanding plants besides the fact that
they afford us pleasure.

(1) Plants furnish all the food there is in the world. Of all
living beings, green plants alone are able to organize the
simple materials found in the air, water, and soil into the
complex substances which all plants and animals must have
for food. The part of our own food which is not derived
from plants comes from animals that directly or indirectly
feed upon plants.

(2) By far the greater part of all the fabrics we use in the
making of clothing is woven out of cotton, linen, and other

2 Science of Plant Life

plant fibers ; and wool and silk come from animals that feed
on plants.

(3) The trees supply the lumber that is used for the con-
struction of most houses, and even when houses are built
of stone or brick, wood is employed in finishing the interiors
and in making the furniture. Wood is used also in the
manufacture of paper, and in almost countless other ways.

(4) Most houses are heated in winter by the burning of
wood, coal, or gas. The energy used in the driving of nearly
all machinery is derived from wood, coal, petroleum, and
natural gas. When we burn wood, we release, in the form
of heat, the great store of energy which the tree obtained
from the sunlight during its lifetime. When we burn coal,
petroleum, or natural gas, we release energy which plants
accumulated from the sunlight of millions of years ago.

(5) Certain small plants have other and quite different re-
lations to human beings, and their activities are of the greatest
consequence to man. These particular plants, the bacteria,
are so minute that they can be seen only by the use of the
microscope. Some of them take nitrogen from the air and
build it into compounds that enrich the soil. Others render
a useful service by breaking down the dead bodies and waste
materials of plants and animals and converting them into
substances that can be used by green plants in the making
of foods. Still other kinds of bacteria cause many of the
diseases to which plants and animals are subject, and it is
necessary for us to learn about them in order that we may
avoid or destroy them.

We see, therefore, that plants contribute to the pleasure
of life ; that, directly or indirectly, they furnish us with food
and provide most of our clothing and shelter ; that they sup-

Plant Life in General

trees and shrubs.

ply the greater part of the energy used in heating and lighting
and in running machinery ; and that as friends or foes, cer-
tain microscopic plants affect the health and well-being of
animals and men. Since plant life is essential to our very
existence, we should have a truly vital interest in the science
of botany, which has grown out of the accumulated mass of
information about plants.

Botany as a science. A science is a body of classified and
systematized facts. The lore of the gardener, the forester,
and the casual student of plants may have value, but it be-
comes a science only when it has been organized into a system
whose principles have been tested and verified by experience,
observation, or experiment. Botany is the science that
treats of the structures, life histories, physiological processes,
distribution, and classification of plants. It is one of the two
divisions of biology; zoology, the science of animal life, is

4 Science of Plant Life

the other. Because so many of the fundamental processes
and structures in plants and animals are similar, it is pos-
sible to include all living beings in the single science of biology.
Yet because plants and animals differ in many important
ways, botany and zoology may well be considered as distinct
sciences.

How a knowledge of botany helps us. A knowledge of
botany contributes directly to our enjoyment of life, because
the more we know about plants, the more interest and mean-
ing we find in every bit of vegetation. Botany helps us to
understand the animal world, also, for plants and animals
are so nearly related that much of what we learn in botany
applies, with slight modifications, to animals as well as to
plants.

Botany has great practical value also ; it furnishes the
scientific basis for many of the most important of human oc-
cupations. For example, agriculture and horticulture are
arts dealing with the methods of field and garden crop pro-
duction; they tell how and when a crop shall be planted,
cared for, and harvested ; but it is botany that furnishes the
scientific knowledge on which these arts are based and explains
the principles underlying the practices of the gardener and the
farmer. Botany does not tell us of the methods to be used
by the forester in developing timber; but no forester can
practice intelligently or invent new and better methods with-
out understanding the principles of botany. How to make
a city or farm dwelling sanitary is an engineering problem ;
but the engineer must be thoroughly familiar with at least
the part of botany which deals with the bacteria, if he is to
be intelligent in his work. Botany, therefore, is of great
practical importance in our daily life ; without a knowledge

Plant Life in General

FIG. 2. A perennially beautiful garden.

of it many of our most important arts can be practiced only
in a cumbersome way.

Nothing has as yet been said about our natural curiosity
as an incentive to the study of plants. Yet the desire to
know and understand is so strongly developed in all human
beings that it has probably had more to do with the growth
of the sciences than all other influences combined. We
naturally want to understand why things are as we find them ;
we are ever seeking explanations of peculiar objects and un-
usual happenings that come to our attention. The study of
botany will help to satisfy our wholesome curiosity about
plants, and will direct our inquiries into profitable channels.

Newspapers and magazines often publish accounts of
strange plants or of unusual plant habits. We read that wheat
found in the tombs of ancient Egypt, where it had been buried
for many centuries, was still alive. It is reported that a tree

Science of Plant Life

FIG. 3. The vegetation more than the house gives character to this
suburban home.

that captures men and large animals and feeds on them has
been found in central Africa. We are informed that a spine-
less cactus has been produced from a spiny one " by treating
it kindly." Such published accounts and the descriptions and
explanations that accompany them are purposely wrapped
in mystery, and tend to give wrong impressions of plant
life. Even a little knowledge of botany will keep one from
being misled by such statements. In common with all the
other sciences, botany enables us to recognize the truth, and so
helps us to avoid errors due to mistaken observation, im-
perfect knowledge, or willful attempts to deceive.

PLANTS FROM THEIR OWN STANDPOINT

Thus far plants have been discussed in relation to man ;
they have been considered as objects of interest, and as a

Plant Life in General 7

part of man's environment that may promote or interfere
with his welfare. Of course plants do not grow, or flower,
or fruit for the sake of the animals and man. The processes
of plants are carried on and their structures developed solely
to meet their own needs. The goal of plant life is the de-
velopment of the individual and the production of young
for the continuance of the species. A plant is successful in
nature, therefore, (i) when it secures nourishment for its
complete development, and (2) when it produces offspring
and thus insures the reproduction of its kind.

Plants as living things. It is important for the beginner in
the study of botany to realize that plants are living things.
Because animals walk, or fly, or swim about, we are accus-
tomed to think of movement as the necessary evidence of
life. To one who has given no thought to the subject, a tree

FIG. 4. Rope-making scene in a Philippine village. Out of plant materials the Fili-
pinos make houses, mats, cloth, boats, and a great variety of household utensils.

8 Science of Plant Life

may seem more akin to the stones among which it is rooted
than to the animals that live about it. But when we study
living beings, we find that there are activities other than
movement, such as respiration and growth, that are regularly
associated with life. As we shall see later, these processes
take place in plants the same as in animals and are evidence
that plants are as truly alive as are animals.

A plant definitely related to its environment. The roots
of the ordinary green plant penetrate the soil in all directions
from the base of the plant, and enable it to take up water
and mineral substances. The stem commonly grows upward
and supports the leaves. Thus the leaves are displayed to
sunlight and are in contact with the oxygen needed for res-
piration and the carbon dioxid required for the making of
food. Each part of the plant is related to its environment in
such a way that its natural processes and the life of the plant
as a whole may be carried on.

Mutual dependence of the parts of a plant. The roots,
stems, and leaves, together, make up the plant's machinery
of nutrition, and upon the efficiency with which the work of
each part is done depends the successful nourishment of the
plant. The chemical nature of the soil, the amount of water
it contains, and its other characteristics, may facilitate or
hinder the work of the roots. This may in turn aid or inter-
fere with the work of the leaves, and as a result the whole
plant may flourish or be dwarfed. Likewise, the leaves may
be exposed to favorable or unfavorable conditions of light
and moisture, and their work may be accelerated or 'retarded.
This in turn affects the stem and the roots, and the whole
plant shows its abundant nutrition or its lack of food.

Under favorable conditions of light and moisture corn may

Plant Life in General

FIG. 5. Plants furnish the primary food supply of the world. The photograph
shows a rice-harvesting scene in the Philippine Islands.

be grown in sand ; but because sand does not provide the
plant with the necessary mineral substances, the corn will
not be of normal growth and will fail to produce good seed.
Under the same conditions of light and moisture, a rich gar-
den soil would produce a normal plant. Or, if during the
winter months corn is grown in the richest of soil in a green-
house, it attains only a small size and may fail to produce good
seeds, because in this case the amount of light is not sufficient.
Hence, when we discuss the relations of any particular part
of a plant to the energy-supplying and nutritive processes,
we must ever keep in mind the interrelation and mutual de-
pendence of all parts of the plant. As no part of the
human body lives an independent life but is dependent for

io Science of Plant Life

its welfare on the activities of the other parts, so the life
of each part of a plant is bound up with the life of the plant
as a whole.

Reproduction an essential process in plant life. Plants,
to be successful, not only must maintain themselves but they
must reproduce themselves. Some of them do this by fur-
ther development of a part of the parent body, as the tuber
of a potato or the runner of a strawberry plant. In many
plants, however, reproduction takes place only through the
production of flowers, fruits, and seeds. These structures in
one way or another are concerned with the production of a
small, undeveloped plant within the seed, and it is upon the
further growth of this young plant that the production of
another generation of that particular kind of plant depends.
A sunflower may develop a tall stem and a large leaf area,
but unless it flowers and produces good seed no young plants
can be grown from it. If it were the only sunflower in ex-
istence, there could be no more sunflowers after its death.
The process of reproduction must, therefore, be considered
as an essential one in plants; without it plant life would
soon disappear from the earth.

Suggestions for Laboratory and Field Work to Precede
Chapter Two

1. Review: physical and chemical changes; molecules and
atoms ; elements and compounds ; solids, liquids, and gases. If
the pupils have had no introductory science, a few simple demon-
strations should be given to make these topics clear.

2. Study a complete plant, noting the roots, stems, leaves,
flowers, fruits, and seeds. Many of the common weeds show all
these parts clearly. Examine the plant, not primarily to learn the
names of the parts, but in order to get a conception of it as a living
thing that is very different from the air and the soil in which it
grows. Find out what the pupils already know concerning the
plant's requirements, its manner of growth, and the functions of
its several parts.

3. Study a number of leaves from different plants, and make
drawings to illustrate a variety of leaf forms, simple and compound
leaves, and the parallel and net arrangements of the veins. Note
the thickness, texture, color, and surface in each of the different
specimens. Note also that the veins reach every part of the
leaf and connect through the petiole with the interior of the plant
stem.

4. Study a skeletonized leaf to make clear the arrangement of
veins and their intimate relation to the mesophyll cells. A nastur-
tium leaf that has been decolorized in alcohol may be studied under
the microscope to show the small veins and the fine ramifications
of the vessels.

5. Dissect a leaf from an Easter lily, live-for-ever, or Wandering
Jew. Study especially the relative positions of the epidermis,
mesophyll, and veins.

6. Study with a microscope cells from the epidermis, dis-
tinguishing the cell wall, protoplasm, nucleus, cytoplasm, and
vacuole. The epidermis from an onion scale is good material
for the study of the cell parts.

7. Draw some cells from a leaf of moss, such as Mnium, after
it has been in good light for several hours. Note cell walls and
chloroplasts. Staining with a weak iodin solution will show the
starch grains within the chloroplasts.

12 Science of Plant Life

8. By experiment determine that light is necessary for the
development and maintenance of the green color in leaves.

9. Study a freshly cut cross-section of a leaf, mounted in water,
to show the detailed structure and the intimate connection between
the epidermis, mesophyll, and veins. Prepared sections will show
details of the stomata and intercellular spaces.

10. Take a field trip to study the variety of leaf forms.

CHAPTER TWO

LEAVES AND THEIR STRUCTURES

THE leaves of plants are their most conspicuous part. The
summer landscape owes its color to them ; and even when we
look at a near-by plant, the
leaves attract most of our
attention and the plant stem,
like the staff of a flag, is likely
to be overlooked. The promi-
nence of leaves is not the result
of chance, for leaves manufac-
ture food and sunlight is neces-
sary for this process. In this
chapter we shall study the
structure of a leaf, and in sub-
sequent chapters we shall dis-
cuss the work of the leaves and
the processes that take place
within these important organs
of the plant.

The parts of a leaf. If we
examine a leaf closely, we see
that it consists of a broad, thin blade, marked into small
divisions by veins. The vein near the middle of the blade is
commonly larger than the others and is called the midrib.
In some forms of leaves there are several prominent veins,
which we may call the principal veins. In general, the small-
est veins form a network uniting with the larger ones, and
these in turn connect with the midrib or with the principal
veins. These large veins are smallest at the apex or outer
end of the leaf, and gradually become larger toward the base

13

Stipul

FIG. 6. Leaf of tulip tree, showing
parts of a complete leaf.

Science of Plant Life

Margin

of the blade. They continue down through the petiole or
leafstalk into the interior of the stem. At the base of the
Apex petiole there is in many leaves
a pair of small appendages, the
stipules. These are usually un-
important structures, but oc-
casionally they are large and
bladelike, and supplement the
blade, or even take its place, in
food manufacture. The primary
divisions of the leaf are the blade,
the petiole, and the stipules.

The leaf made up of tissues.
The soft green tissue essential
to food production is found
chiefly in the blade of the leaf.
This may be shown by dissect-
ing a fleshy leaf like that of the common houseleek or the
live-for-ever. Cutting across the blade of such a leaf, we

Lateral vein

FIG. 7. Leaf of vinca, showing the
parts of the blade.

FIG. 8. Leaves showing variety of form and venation: A, orange; B, peach;
C, bamboo ; D, nasturtium ; E, poplar ; F, lantana ; G, begonia.

Leaves and Their Structures 15

find that there is a skin covering it above and below. The
skin is readily stripped off, leaving the interior of the leaf

FIG. 9. Divided and compound leaves: A, buckeye; B, oxalis; C, avens;
D, celandine ; E, cliff fern ; F, dandelion.

as a green, granular mass of cells with veins running through
it in all directions. The skin is called the epidermis, or
epidermal tissue (Greek: epi, upon, and derma, skin). The
green part is the mesophyll tissue (Greek : meso, middle, and
phyll, leaf). The veins consist of three tissues, the water-
conducting, food-conducting, and mechanical tissues. The
blade therefore commonly contains five tissues : the epidermis
and mesophyll, and the three tissues of the veins..

Cells. When any one of the tissues of a leaf or other living
part of a plant is magnified under a microscope, it is seen to
be composed of small parts built together in much the same
way as the little chambers in a honeycomb. These small
parts are the plant cells (Fig. 1 1) . Each cell consists of a small
mass of jellylike living matter, the protoplasm, which is inclosed
by a firm, transparent wall. The protoplasm is divided into
a denser round or oval body, the nucleus, and a more liquid
portion, the cytoplasm. The nucleus is of great importance ;

i6

Science of Plant Life

the cell dies when it is removed, and it is thought to control
many of the activities that go on within the cell. The cells

FIG. 10. Leaves with prominent stipules : pea, black willow, red clover,
Japanese quince, rose.

are the structural units of plants. Figure 1 2 shows how a leaf
is built of cells of different sizes and shapes.

The cytoplasm makes up the bulk of the living matter of
a cell, but in mature plant cells most of the space inclosed by
the cell wall is occupied by one or more vacuoles or cavities
containing the cell sap. This is water with sugars, mineral
salts, acids, and other substances dissolved in it. Lying
within the cytoplasm are structures called plastids, small
bodies that contain food substances and coloring matters.

The cell wall. The wall which surrounds the cell is com-
posed of a transparent material called cellulose. Its impor-
tance lies in the fact that it gives firmness to the cell. It

Leaves and Their Structures

supports the soft cytoplasm as the wax of the honeycomb sup-
ports the honey within, and it helps to give stiffness to all
parts of the plant. You have
seen pure cellulose in the form
of cotton. Filter paper and
most book papers are made of
cellulose fibers derived from
wood. Water passes freely
through the cellulose walls of
plant cells, as do most substances
that are dissolved in the water.
Animals, as well as plants, are
composed of cells ; but the
animal cell, instead of^ having a
stiff cellulose wall like a plant

cell, has a soft wall, or, as in the FlG' '.'• ^ f' A

moss leaf; B is from a squash- vine
Case Of nerve Cells and white hair; C is a starch-filled cell from a

blood corpuscles, it may lack a P°tato tuber' and D is a cel1 from

the palisade layer of a leaf. E shows

wall entirely. Consequently, the a ceu in cross section,
tissues of animals (except the

skeletal tissues) are usually softer and more pliable than
plant tissues. This makes it easy for an animal to bend and
to move about. The difference in cell walls and in the
pliability of tissues is so general throughout the plant and
animal kingdoms, that it is one of the important distinc-
tions between plants and animals.

The epidermis and the stomata. The cells of the epidermis
are flat, irregularly shaped, closely united, and, for the most
part, colorless. The cell walls on the side of the epidermis
which is exposed to the air become thickened with a waxlike
material called cutin, which forms a layer over the surface of

i8

Science of Plant Life

the leaf. This layer is called the cuticle. It is useful to the
plant because water does not pass through it readily, and it

epide

4J

M

b

Vfeter-coiF
ducting tissu<

Food- con-
ducting tissue

Bundle sheatW Guard cell

;

Lower
epidermis

Stoma^7*><fF -f
FIG. 12. Model of a small piece of vinca leaf, showing cells and tissues.

protects the plant from water loss. It may be compared to
the enamel covering of oilcloth and acts in much the same
way. The cuticle is useful to the plant also because it serves
as a first line of defense against disease germs. The impor-
tance of the epidermis as a protective covering for the delicate
inner tissues of the plant may be judged from the drying and
decay that follow the breaking of the thin epidermal coat of
an apple or a pear.

Scattered among the colorless cells of the epidermis are
pairs of small, crescent-shaped green cells, the guard cells.
Each pair of these surrounds a small opening or pore, the

Leaves and Their Structures

stoma (Greek: stoma, mouth; plural, stomata),1 which is
opened or closed by the expansion or contraction of the guard

FIGS. 13 and 14. Upper epidermis of "Wandering Jew" (Zebrina) leaf, on the left, and
lower epidermis, on the right. St is a stoma, G a guard cell, and Sc a subsidiary cell.
The stomata are found only on the lower surface of the leaf.

cells. The stomata are very important, for they connect the
air spaces among the cells inside the leaf with the external
atmosphere. When open, they allow the exchange of water
vapor and other gases through the epidermis; and when
closed, they complete the barrier to gas movements in either
direction. In many plants stomata occur only on the lower
surfaces of the leaves ; but in some plants they are found on
both the upper and lower leaf surfaces.

The mesophyll. The mesophyll tissue is composed of the
soft, thin- walled cells that lie among the veins in the in-
terior of the leaf. In most leaves there is beneath the upper
epidermis one or more palisade layers, which are composed of

1 Stomata are so small that 2500 of them have an area about equivalent to
that of an ordinary pin hole. They are so numerous, however, that they
occupy about ^ of the area of the average leaf. On a square centimeter
of the lower surface of a sunflower leaf there are about 150,000 of them.

20 Science of Plant Life

elongated cells standing close together, as is shown in Figure
12. The remainder of the mesophyll tissue is made up of
ovoid or irregularly shaped cells joined quite loosely, so that
air spaces are left between them. In fact, a much larger
part of the surfaces of these cells is in contact with air spaces
than with other cells. The air spaces within the leaf are con-
tinuous, and through them the oxygen and carbon dioxid of
the atmosphere can reach every cell in the leaf. We shall
see later that the differences in the epidermal and mesophyll
cells, and in the way they are arranged, are definitely related
to the different processes carried on by each of them.

The veins. The veins in a leaf branch again and again,
forming a fine meshwork through all its parts. Each vein is
composed of a bundle of water-conducting and food-con-
ducting tissues surrounded by a bundle sheath. The water-
conducting tissues are located in the upper side of the vein.
These tissues are made up of long, cylindrical cells placed
end to end. Usually the inner walls of these cells have
spiral thickenings, and sometimes the end walls of the cells
are absorbed, leaving continuous tubes or vessels several
cells in length. After the growth of the cells is completed, the
living protoplasm within them dies, and the dead cases of the
cells, with their porous walls, lie within the leaf like bundles of
very fine pipes. Through these vessels, the water and mineral
salts that are absorbed by the roots pass into the leaf to
supply its living cells. The supplies of water and mineral
salts pass out through the walls of the water-conducting ves-
sels into the cells that adjoin them, and then from these they
pass to the other cells of the leaf.

The food-conducting tissues, or vessels, lie below the water-
conducting vessels within the leaf veins. They provide an

Leaves and Their Structures

21

elaborate system of channels by which the surplus foods
manufactured in the leaf are distributed throughout the

FIGS. 15 and 16. Upper and lower epidermis of vinca leaf.

plant. The foods pass from the mesophyll cells into this
food-conducting tissue, and then down through the petiole
of the leaf to the living cells of the stem and roots.

In the smaller veins the bundle sheath is a layer of meso-
phyll cells. In the larger veins it contains one or more layers
of thick-walled cells, which act as a mechanical or supporting
tissue. The mechanical tissue is rigid and gives stiffness to
the leaf.

Cells, tissues, and organs. We see, then, that the actual
work of the plant is done in its cells, of which there are many
millions, and it is the sum of the life and work of all these
cells that makes up the life and work of the plant as a whole.
All cells carry on certain fundamental life processes like
respiration and the assimilation of food, but most cells are
especially adapted to some particular work that is carried on
for the benefit of the plant as a whole. Cells that have the
same special function are similar in structure and are generally

22 Science of Plant Life

grouped together. Such groups of cells with like functions
are called tissues. The epidermis of a leaf, for example, is a
tissue covering the mesophyll and veins.

In order to do its work a tissue needs a source of supplies
and a means of disposing of its products. Hence the group-
ing of the tissues may be mutually advantageous. When
several kinds of tissues are arranged together so that by their
cooperation they can carry on some general function of the
plant, they form an organ. The leaf, for example, is an organ
especially concerned with the manufacture of food. It is
made up, as we have seen, of five different tissues, each com-
posed of thousands of cells.

The chloroplasts. Of the several structures found within
the mesophyll cells, the most important in the primary process
of food manufacture are the chloroplasts. These are round
or lens-shaped bodies which contain a green coloring matter
called chlorophyll. They are composed of living material
and belong to a group of structures called plastids, that are
found in the cytoplasm of all plant cells. Cells may contain
many or only a few chloroplasts, and these may be located
deep within the leaf or near its surface (Fig. 17). Since
the chloroplasts are the special apparatus for the manu-
facture of food, the amount of food produced by a plant
under any given conditions is roughly proportional to their
number.

The chlorophyll. Chlorophyll is held in the chloroplasts
in much the same way that water is held in a sponge. It
stains the chloroplasts green, and it may be removed from
them by putting the leaf in alcohol, in which the chlorophyll
is soluble. After the chlorophyll is dissolved, the chloroplasts
remain in the cell, but they are colorless and the leaf is white

Leaves and Their Structures

or yellowish instead of green.
Light is usually necessary to
the development of chlorophyll.
The white sprouts on potatoes
in a dark cellar, the blanching
of celery when the lower part of
the leaves is covered, and the
whitening of grass under a board,
are familiar evidences of this
fact. In the inner tissues of
plants and in the underground
parts, the plastids are usually

Colorless ; but in many plants FIG. 17. Part of a moss leaf that is

these parts become green if they comP°sed of a sing*e layer of cells-
are exposed to the light. This is why potatoes that grow at
the surface of the soil are likely to be green.

Suggestions for Laboratory Work to Precede Chapter Three

Answer the following questions by means of experiments :

1. Do leaves grow toward the light ?

2. How is the blade moved into its position with reference to
light?

3. Is light necessary for starch manufacture?

4. Is chlorophyll necessary for starch manufacture?

5. Does the manufactured starch remain in the leaves ?

24

CHAPTER THREE

THE MANUFACTURE OF FOOD

You will probably remember from your study of physiology
that all the foods used by animals belong to three classes
of chemical substances : carbohydrates, fats, and proteins.
These same classes of substances constitute the food of plants.
A grain of corn contains a supply of Starch, oil, and pro-
tein for the young plant, and these same foods that are used
by animals are accumulated in many plants. The difference
in the nutrition of plants and animals lies, then, not in any
differences in the foods used, but in the way their foods are
secured. In this chapter the manner in which plants obtain
their foods will be discussed.

Plants the source of all food. Mineral soils and the air
do not contain any of the substances that we class as foods.
Yet green plants may grow luxuriantly on mineral soils. It
follows, therefore, that green plants are able to manufacture
their own foods. They can synthesize, or build together,
simple substances that they obtain from the soil and air
into the complex foods that they require. Animals lack
this power. They must have foods that have already been
built up, rather than the simple materials of which foods are
made. These foods they secure either directly or indirectly
from plants. The ability of plants to manufacture com-
plex foods from simple substances brings up several ques-
tions :

What is the method by which, plants produce food ? Just
what parts of the plants do the work ? What constitutes the
machinery ? Out of what materials is the food manufactured ?
How is the energy supplied? And what are the conditions
under which the process goes on ?

25

26 Science of Plant Life

Photosynthesis. The primary step in the making of food
is the building of carbohydrates through the process called
photosynthesis (Greek : photos, light, and synthesis, putting
together). In this process carbon dioxid from the air and
water from the soil are brought together in the chloroplasts
and united to form carbohydrates. Sugar is the first abun-
dant product, but being soluble in the water of the cell, it is
quite invisible. In most plants a large part of the sugar is
rapidly changed to starch, and as the starch is insoluble in
water, it accumulates temporarily in the chloroplasts in the
form of little grains which may readily be seen with a mi-
croscope. There is a very simple test for the presence of
starch. A solution of iodin stains most substances yellower
brown, but it stains starch blue or purple. So any object
that contains starch — a cell, a leaf, or a piece of cloth — will
be colored purple if iodin is applied to it. *

Light and photosynthesis. If we take a leaf from a plant
that has been in the dark for two days, place the leaf in warm
alcohol to remove the chlorophyll, and then put it in a solution
of iodin, it is stained yellow. This proves the absence of
starch. If the plant is then put in the light for a few hours,
a leaf tested in the same way will be colored purple, showing
that starch is present. Evidently light is necessary for photo-
synthesis. It is not surprising to find that light is so effective
in building up compounds in the green parts of plants, for it
is a powerful agent in causing chemical changes. You may
be familiar with its use in photography. The film and the
printing paper have on them a layer of gelatin containing
certain chemicals. Exposure to the light for even a fraction
of a second effects in these chemically treated surfaces changes
which may be seen when the film or paper is developed.

The Manufacture of Food 27

Many chemical substances kept in drug stores must be pro-
tected from the light; otherwise they soon change their
composition and become different substances.

Chlorophyll necessary for photosynthesis. By using a
plant with variegated leaves, the iodin test will show that
the white parts form no starch. Since starch is formed only
in the green part of the blade, it is evident that chlorophyll
is necessary for photosynthesis. Any green part of a plant can
carry on photosynthesis, but the principal food factories are
the leaves.

Effects of temperature on photosynthesis. The effects of
temperature on photosynthesis may be demonstrated by tak-
ing plants that have been in the dark long enough for the
starch to be removed from the leaves and placing them in
the light, under different temperature conditions. Such tests
will show that the ordinary summer temperatures are most
favorable for photosynthesis, and that when the temperature
falls nearly to the freezing point photosynthesis decreases
rapidly or ceases entirely.

Materials and products. Experiments have shown that
the materials used in photosynthesis are carbon dioxid and
waten Carbon dioxid is a gas that makes up about three out
of every 10,000 parts of the air. Its molecule contains one
atom of carbon and two atoms of oxygen (CC^) . Water, which
the plant gets from the soil, has two atoms of hydrogen and
one atom of oxygen in every molecule (H2O). The carbo-
hydrates made in photosynthesis from the carbon dioxid and
water contain these same elements.1 The simple sugars,

1 Carbohydrates include many substances commonly classified as sugars,
starches, and celluloses. The simple sugars, glucose and fructose, have a
formula CeH^Oe. The double sugars like sucrose (cane and beet sugar) and

28 Science of Plant Life

like glucose, which are the first abundant products of photo-
synthesis, contain six atoms of carbon, twelve atoms of hydro-
gen, and six atoms of oxygen in each molecule. For every
molecule of glucose manufactured, therefore, it would require
six molecules of carbon dioxid to furnish the carbon and six
molecules of water to provide the hydrogen. These amounts
of water and carbon dioxid, however, contain eighteen atoms of
oxygen, twelve more than are needed for the making of glucose,

6CO2+6H2O = C6Hi2O6+i2O.

We should therefore expect oxygen to be given off from
leaves during photosynthesis. That this actually happens
may easily be shown by inverting under water a bundle of
the branches of some water plant, like Elodea, with the cut
ends placed under the mouth of a test tube that is filled with
water (Fig. 18). When exposed to the light for a day, the
tube will be partly filled with gas. By testing with a glowing
match or splinter, the gas may be shown to be mostly oxygen.1

maltose (malt sugar) may be built up by combining two simple sugars,

glucose fructose cane sugar water

one molecule of water being lost in the process ; and they may be split up into
two of the simple sugars by the addition of a molecule of water. The starches
and celluloses are formed by combining many molecules of sugar and removing
as many molecules of water as there are molecules of sugar used. Conse-
quently their formulas are (C6HioO5)n, in which n represents a rather large
number. The starches and celluloses may also be split up into simple sugars
by adding the required number of molecules of water. This last process is
the one by which corn sirup (glucose) is made from corn starch. The process
may be represented by the equation,

(C6Hi005)n + »(H20) ->»(C6Hi206).

starch water glucose

1 Water containing a considerable amount of dissolved carbon dioxid should
be used in this experiment, so that photosynthesis may go on rapidly. For
this reason pond water is better than tap water.

The Manufacture of Food

29

How the supplies are obtained. Every industrial workshop
must constantly be provided with the raw materials needed
in the manufacture of its prod-
uct. Likewise the leaf must be
supplied with the substances
that it uses in the making of
food. These necessary supplies
come to the leaf through the
veins and the stomata. The
water passes into the leaf
through the water-conducting
tissue of the veins. The supply
of carbon dioxid reaches the
cells of the mesophyll through
the stomata and the intercel-
lular spaces. When the stomata
are closed, very little carbon
dioxid can enter, and at such
times the process of photosyn-
thesis is of necessity greatly
retarded.

How the products and wastes
are removed. The manufac-
ture of carbohydrates in the FlG- l8- Experiment to show the giv-

. ing off of oxygen from a water plant

leaf goes on only during the (Eiodea) during photosynthesis.
hours of sunlight ; the removal

of food goes on at all times. The food-conducting tissue of
the veins furnishes the outlet for the product, which is trans-
ferred in the form of sugar. During the day the rate of
manufacture is so much greater than the rate of removal
of food, that starch and sugar accumulate. During the

Science of Plant Life

night the movement of food into the stem nearly empties
the leaf, and by early morning the cells are again in good

condition for food manu-
facture. The waste prod-
uct, oxygen, passes from
the cells to the intercellu-
lar spaces and out through
the stomata to the atmos-
phere.

A leaf, then, is carrying
on photosynthesis at its

A maple leaf and the sugar and maple fuil Capacity Only when
sirup equivalent to the amount it could manu- there is Sunlight, a f avor-
facture in a season. All drawn to the same able temperature, and an

abundant water supply,

and when the stomata are open. Even under these condi-
tions the work may be interfered with if more than a certain
amount of the products accumulate in the cells.

The amount of the product. The amount of carbohydrates
produced in photosynthesis varies so greatly in different plants
and under dissimilar conditions that it is very difficult to
make a general estimate of it. The result of many experi-
ments shows that under favorable conditions a square meter
of leaf surface makes on an average about i gram of car-
bohydrate per hour. At this rate a square meter of leaf sur-
face in midsummer would require 2 months to produce food
equivalent to that consumed by the average man in a day.
This average rate of carbohydrate manufacture may also be
expressed by saying that the leaf makes enough sugar
in a summer to cover it with a layer i millimeter thick
(Fig. 19).

The Manufacture of Food

Summary of photosynthesis. We may summarize the facts
we have learned regarding photosynthesis by likening it to
a manufacturing process of human invention :

The factory

The workrooms
The machinery

The energy

The raw materials

The supply department

The products

The forwarding department
The waste material

The working hours

is the green tissue, . especially
that of the leaves,
are the cells. .

is the chloroplasts and the
chlorophyll,
is the sunlight.

are the carbon dioxid and water
(C02andH20).

is the stomata and intercellular
spaces, and the water-conduct-
ing tissue.

are carbohydrates : sugars
(C6Hi2O6) and starches
(C6H1005)n.

is the food-conducting tissue,
and it works both day and night,
is oxygen, which escapes through
the intercellular spaces and the
stomata.
are all the hours of sunlight.

The production of fats. In addition to carbohydrates,
plants make and use two other classes of foods : fats and
proteins. The fats are quite similar to the carbohydrates in
composition. They contain the same chemical elements :
carbon^ hydrogen^ and oxygea. The proportion of the
oxygen to carbon, however, is smaller. At ordinary temper-
atures fats occur in plants both as solids and liquids. The

32 Science of Plant Life

liquid fats are commonly called oils. They are probably
made directly from the carbohydrates. As there appears
to be present in the cell no special fat-producing apparatus
for bringing about this chemical change, it is probably effected
by the protoplasm and fat can therefore be formed in any
living part of the plant. Although fats are widely distributed
in the plant body, they are especially abundant in seeds and
fruits. Some of the commonest fats and oils of commerce
derived from plants are corn oil, coconut oil, cottonseed oil,
linseed oil, castor oil, olive oil, peanut oil, and cocoa butter.

The making and use of proteins. The proteins are the third
class of foods. They too are constructed in large part from
the carbohydrates ; but their molecules are vastly more com-
plex than are the molecules of carbohydrates and fats, and,
in addition to carbon, hydrogen, and oxygen, they contain
the elements nitrogen and sulfur, and occasionally phos-
phorus. In protein synthesis the amount of sulfur and
phosphorus consumed is small, but a very large amount of
nitrogen is required. Furthermore, nitrogen in the gaseous
condition in which it occurs in the air does not readily unite
with other substances ; so, although it makes up four fifths
of the atmosphere, green plants cannot take it directly from
the air. For the nitrogen needed for protein-making they
must depend, therefore, on the supply which comes from the
soil in the form of nitrates. This is carried to the cells
with the water that is absorbed by the roots. Protein syn-
thesis, like the synthesis of fats, is probably effected by the
protoplasm. It may occur in nearly all parts of a plant, but
it takes place for the most part in the leaves. Proteins, be-
cause of their complex composition, are especially used in
building up and repairing the protoplasm. They are trans-

The Manufacture of Food 33

ported from the leaves by the food-conducting tissue of the
bundles.

Soy beans

Wheatflour

Corn meal .,

ice

175 127 97 T 6.8

FIG. 20. Percentage of protein in various foods.

The most expensive portion of the diet of human beings is
the proteins. Figure 20 shows that in soy beans we possess
the richest source of protein. It also shows why the soy
bean is one of the most important of foods in the Asiatic
nations, where animal foods are very limited. One dollar
will buy several times as much protein in soy beans as it
will in any other plant or animal food. However, recent ex-
periments in animal feeding have shown that for mainte-
nance and growth some proteins are more valuable pound
for pound than others.

Amount of food produced per acre. Since the food supply
of all living beings depends primarily upon these synthetic
processes that are carried on in plants, it is of interest to
inquire how much food may be derived from an average acre
of land when planted to different crops. It must be remem-

34

Science of Plant Life

bered that the plants that produce this food take a consider-
able part for their own maintenance, and that the part which
the farmer harvests is the plants' surplus. The table given
below shows the average yield per acre; its food value cal-
culated in Calories ; l and the number of men that i acre
planted to different crops might feed for i day, assuming that
each man requires 3000 Calories per day.

YIELD PER

ACRE

MILLIONS OF

NUMBER OF MEN

Bushels

Pounds

EQUIVALENT

FED FOR i DA*

Corn . .
Sweet potatoes . . . .
Irish potatoes . . . .
Wheat

35
no

IOO
20

1960
5940
6000

I 2OO

3-1

2.8

•9
8

IOOO

900
600
600

Rice
Soy beans .... . -
Beans

40

16
14

H54
960
840

• 7
•5
.1

56o
500
375

If the plant products of an average acre are fed to cattle,
the dressed beef produced amounts to only 125 pounds,
yielding an energy equivalent to the food of 43 men for i day.
If transformed into pork, the yield is 273 pounds, or sufficient
food for 220 men for i day.2 This shows the great loss of
energy that results when plant foods are converted into meat
before they reach the human consumer. It is evident that as
the human family becomes larger and food becomes scarcer,
we shall have to take more and more of our foods directly
from plants.

1 A Calorie is the amount of heat necessary to raise the temperature of i kilo
of water i degree Centigrade.

* United States Department of Agriculture, Farmers' Bulletin No. 877.

The Manufacture of Food 35

PROBLEMS

1. How do the white parts of a variegated leaf get food?

2. Occasionally in a field of young corn a stalk that lacks chlorophyll will be
found. This white stalk dies as soon as the food supply in the grain from

which it came is exhausted. Why?

3. Geraniums with variegated leaves occasionally produce branches that are
entirely white. A noted horticultural firm offered $1000 to any one of its
gardeners who would root one of these branches and thus produce a white-
leafed geranium. What was the chance for success ? Why ?

Suggestions for Laboratory and Field Work to Precede
Chapter Four

1. Examine one or more plants from each of the following groups,
noting the arrangement of leaves on vertical and inclined stems :

Corn, elm, hackberry, wheat, rye, canna, bamboo.
Lilac, viburnum, catalpa, ash, maple.
Willow, poplar, pine, spruce, peach, apple, sunflower.
Cat-tail, iris, onion, hyacinth, yucca, century plant.

2. In the field, study some of the following plants, noting the
positions their leaves take with reference to light :

Sugar maple, elm, beech, oak, catalpa, Virginia creeper,

Boston ivy.

Tulip tree, cot tonwood, birch, willow, spirea, barberry.
Which cast the more complete shade, trees whose leaves are
sensitive or insensitive to light?

3. On several of the following plants study the leaf positions, to
determine how the blades reach their positions when mature :

Sunflower, mallow, Virginia creeper, Boston ivy, English
ivy, wild prickly lettuce, nasturtium, ragweed, compass
plant, water lily. Other plants of the locality will furnish
additional material for study.

4. Examine submerged plants like pondweeds, Elodea, Cera-
tophyllum, and water buttercups, and compare their leaves with
those of land plants.

5. Sections of leaves of different kinds may be used to show vari-
ations in internal structures.

6. Experiments :

Grow geraniums and nasturtiums with one-sided illumination.
Mark the original position of the leaves with stakes and note
changes in position.

Try the effects of light and darkness on the positions of leaves of
sensitive plants, beans, or sweet clover. Black paper covers may
be used to exclude the light.

CHAPTER FOUR

Note the alternate

arrangement of the leaves.

LEAVES IN RELATION TO LIGHT

THE leaf, as we have seen, must receive light in order to

produce food. Leaves are variously arranged on stems, and

stems have all sorts of positions.

Many of these leaf arrangements and

stem positions are not advantageous

for the display of leaves to the light.

The leaf, however, and especially the

petiole, is so influenced by light dur-
ing its development, that the leaf

when mature has the best possible

position with respect to light. The

raised leaves of the pumpkin, the FIG. 21. Vertical branch of

mosaics of leaves formed on the sides

of buildings by the* Boston ivy, and

the successive tiers of leaves on a beech tree illustrate

different arrangements by which large numbers of leaves

are efficiently displayed to the light.
The arrangement of leaves on stems. Leaves develop

from somewhat thickened places on the stems, called the

nodes. Each node
may bear one, two,
or several leaves.
According to the
number of leaves
that the node
bears, the leaf ar-
rangement is desig-

FIG. 22. Horizontal branch of magnolia. Compare leaf

positions with those of Figure 21. Opposite,

37

Science of Plant Life

In the alternate arrangement each node bears one leaf.
This is also spoken of as the spiral arrangement, because a
line drawn through successive leaf bases
forms a spiral about the stem. Some-
times, as in the corn plant, the spiral
passes half around the stem in going from
one node to the next (page 50). In other
plants, like the sedges, the spiral passes
but a third around the stem between
nodes. In several of our common fruit
trees, as the apple and the peach, the
spiral between nodes passes two fifths
around the stem. These variations of
the spiral arrangement are called the two-
ranked (Fig. 28), three-ranked (Fig. 23),
and five-ranked
arrangements.

F,G.23. Asedge(D»«- I* the Opposite

chiwn), showing three- arrangement two

ranked arrangement of leayes QCCUr at
the leaves.

each node. The

leaves at successive nodes, how-
ever, are at right angles to each
other (Fig. 25), giving four ranks
of leaves. The maple, ash, dog-
wood, and lilac furnish examples
of the opposite arrangement. In
the whorled arrangement the
leaves are in a circle about the

. T .. , FIG. 24. Indian cucumber root,

node. The Indian cucumber root showin4g the whorled arrangement
(Medeola) and the wood lily of the leaves.

Leaves in Relation to Light

39

(page 310) furnish excellent examples of the whorled arrange-
ment.

However, it is only on upright
stems which receive the light equally
on all sides, that the blades take
their normal positions directly out
from the nodes. If an erect shoot
be placed in an inclined position, it
is easy to see that the leaves are no
longer well displayed to the light.
As may be readily seen by examin-
ing the branches of trees and the
stems of trailing plants, horizontal
or inclined stems become twisted
during development because of un-
equal illumination (Fig. 26). The
twisting of the stems brings the
leaves into better positions to re- FlG- 25- Vertical branch of dog-

. . wood, showing the opposite ar-

CClVe hght, but it often obscures the rangement of the leaves.

normal arrangement of the leaves.

The positions of leaves with reference to light. If leaves
are moderately sensitive to light, they assume a position

FIG. 26. Horizontal branch of dogwood. Compare with Figure 25.

Science of Plant Life

approximately at right angles to the line along which the
greatest amount of light reaches them. Consequently the
leaves on most of our common trees, shrubs, and herbs tend to
take an approximately horizontal position. The sugar maple
and the horse-chestnut are examples of trees whose leaves
are displayed in this manner. In the cottonwood and tulip
tree the leaves are only slightly sensitive to light, and the
result is that their leaves assume a great variety of positions.

If leaves are extremely sensi-
tive to light, the blades may
turn toward the sun in the
early morning and follow the
sun throughout the day, always
keeping the broad face of the
leaf to the light. The leaves
of the common mallow move
in this way.

Compass plants. There is
another class of plants which
are sensitive to light, but
which respond to it in a very
different manner. These are
the so-called compass plants,
of which the wild prickly let-
tuce is a widely distributed
example. In sunny situations
the leaves of these plants tend
to take positions edgewise to
FIG. 27. Prickly lettuce plant: 4, viewed the direction of the most in-

from west; B, viewed from south. Drawn ^^Q j- ht As ^ sumight
from a specimen grown under exposure . .

to bright sunlight. is most intense at noon, it is

Leaves in Relation to Light

only in the morning and late afternoon that the flat sides of
the leaves are perpendicular to the sun's rays. This response
to the light also places most of the leaves in a vertical north-
and-south plane and suggests the name " compass plant."
When grown in partial shade, the leaves of these same plants
are horizontal. Hence it is clear that the position of their
leaves in sunny situations is the result of light conditions.

How the blade attains its position with reference to the light.
The attainment of its position by the leaf blade is partly ac-
complished, as has been noted, by the bending and twisting
of the plant stem during its development. To a much greater
extent the blade owes its position to the bending, twisting, and
elongating of the petiole. In-
deed, this is the particular ad-
vantage of the petiole. Its
length and direction of growth
are for the most part deter-
mined by the way in which the
light falls on the blade during
the period of development. An
examination of a branch of a
maple will disclose how the
lengthening and bending of the
petioles help to fit each leaf
into a position where it will
receive the light.

Vertical leaves. In a number
of common plants, including
the iris, cat- tail, calamus, and
many grasses, the leaves are ,

/ ° FIG. 28. Ins, showing leaves held in

Vertical because they are held vertical position by the sheathing bases.

Science of Plant Life

FIG. 29. Guinea grass, a plant grown in the tropics for fodder. Note the vertical
leaves and the large amount of leaf surface exposed by the plant to the light.

in this position by their sheathing bases rather than because
of a response to light. These plants usually occur in dense
growths, and the vertical position of the leaves permits the
light to penetrate to their bases. This has the advantage of
allowing photosynthesis to go on throughout the entire
length of the leaves.

Differences in vertical and horizontal leaves. Vertical
leaves differ from horizontal leaves in several particulars :

In vertical leaves the mesophyll may be composed of
spongy tissue, or it may be composed entirely of palisade
cells. More rarely there are palisade layers on both sides,
with a spongy layer between. In contrast, a horizontal leaf
usually has a palisade layer beneath the upper epidermis, and
the lower portion of the mesophyll is composed of loosely
arranged cells. In vertical leaves stomata usually occur on

Leaves in Relation to Light 43

both surfaces, while in most horizontal leaves the stomata
are confined to the lower surface (page 18). Vertical leaves

FiG. 30. Various positions taken by leaflets of lima bean : A , position in intense
light ; B, position in diffuse light ; C, position in darkness.

are likely to be of the same color on both surfaces, while
horizontal leaves are generally of a darker green on the upper
surface.

The difference in the color of the two sides of a horizontal
leaf is due to the presence of a larger amount of chlorophyll
in the compact palisade layers of the mesophyll than in the
loose spongy layers beneath. In vertical leaves, the similarity
of structure in the mesophyll on each side, and the fact that
both surfaces of the leaf are equally illuminated, account for
the sameness of color of the two surfaces.

Motile leaves. The leaves of which we have been speaking
have their positions rather definitely fixed when they reach
maturity. There is another class of leaves, however, in which
the positions of the blades are not fixed but are changed ac-
cording to the intensity and direction of light. A familiar
example is the roadside sweet clover. At night the three

44

Science of Plant Life

leaflets of the compound leaf droop downward from the peti-
ole ; in the medium light of a cloudy day they are held per-
pendicular to the light; in
the most intense sunlight
the blades are raised above
the petiole until they are
edgewise and point toward
the light. Some observa-
tion of bean seedlings, which
may readily be grown in
the laboratory, will be in-
structive in this connection.
Other examples of motile
leaves may be seen in the
honey locust, the leaflets of
which fold upward at night,
and in white clover, oxalis,
and the red-bud tree. The
leaflets of the sensitive
plant vary their positions
according to light intensity,
and also when touched or

FIG. 31. Sensitive plant. The leaves on the
left side are in normal positions ; those on the
right side have been touched and the leaflets
have folded together wholly or in part and
the petioles have folded toward the stem.
P is the pulvinus.

injured in any way (Fig.

31).

The change of position in
motile leaves is brought
about by changes in the water content of the cells on oppo-
site sides of a special organ called the pulvinus (Fig. 32),
which is located at the base of the leaflets. This device may
readily be studied in the leaf of the bean.
The leaves of shade plants. As may be observed by a trip

Leaves in Relation to Light

45

to the woods, the leaves of plants growing in the shade are
usually darker and more bluish-green than the leaves of
plants growing in full sunlight.
This difference in color is ac-
counted for in part by the
amount of chlorophyll near the
surface and in part by a slight
difference in the color of the
chlorophyll itself (page 63).
In a few shade plants the depth
of the green color is increased
by the presence of chloroplasts
in the epidermal 'cells. Shade
plants are not subjected to
drying, as are plants growing
in exposed situations, and gen- FlG 32 Pulvinus and section of pul.

erally Speaking their leaves are vinus from leaf of sensitive plant, both

broad and thin. The leaves
of these plants differ further
from the ordinary leaf in that causins the cells Partially to coIlaPse-

The pressure of the cells on the side B
the CUtlde IS leSS developed, then forces the leaf downward.

the mesophyll is composed al-
most entirely of spongy tissue, and usually stomata are
present on both surfaces of the leaf.

Submerged leaves. Every one who has gone fishing or
rowing knows that a great deal of sunlight is reflected from the
surface of water. This means that the amount of light that
penetrates the surface is reduced by just the amount that is
reflected. The penetration of the water by the sun's rays is
further interfered with by the fine sediment that clouds our
ponds and lakes. Every one who has dived and opened his

enlarged. When the leaf is touched, the
water in the cells on the side A passes
outward into the intercellular spaces,

46 Science of Plant Life

eyes under water knows that it is dark at a comparatively
slight depth. Hence submerged plants always grow in light
of reduced intensity. They receive an amount of light com-
parable to that received by the shade plants found in forest
ravines. Submerged leaves, too, are of very soft texture,
and are quite without mechanical tissue in the veins, so that
they are unable to support themselves when lifted from the
water. They are kept upright in the water by their buoyancy,
which is due to the greatly enlarged air spaces among the
mesophyll cells.

Summary. Light has a marked effect upon the positions,
the color, and the structures of leaves. Leaves tend to be
placed directly outward from the nodes to which they are
attached, but light affects them during their development,
and most leaves come to occupy positions that have more
relation to the light than to the stem which bears them. Some
leaves vary their positions constantly as the light, changes.

Leaves of plants grown in weak light, including those of
submerged plants, usually have the mesophyll made up wholly
or largely of spongy cells. Leaves of plants grown in intense
light usually have the mesophyll made up wholly or largely
of palisade cells.

PROBLEMS

1. Why do house plants flourish best at south windows in the winter time ?

2. What part of full sunlight is received by a plant that stands near a window?

3. Why do gardeners shade lettuce plants in midsummer?

4. What other condition, besides light intensity, is affected by shading ?

Suggestions for Laboratory Work to Precede Chapter Five

1. Cut a leaf from some plant, preferably under water. Then
immerse the cut end in red ink or eosin and let it stand for some
time, watching the movement of the red color in the veins. Thin
leaves, like those of balsam and nasturtium, are especially suited
to this experiment. If thick leaves are used, they may be split
with a knife in order to show the veins.

2. Study the surface coverings — cuticle, wax, and hairs — on
a series of leaves like the mullein, century plant, bur oak, tomato,
and geranium.

3. Compare leaves of water plants and moist soil plants with
those of desert forms like the yucca, agave, and cactus.

4. Examine free-hand sections of leaves or prepared microscope
slides that will show variations in stomata, compactness of tissue,
development of air chambers, and water storage.

5. Determine the relative time required for the wilting and
complete drying out of several kinds of leaves, using leaves of
different texture, thickness, and surface covering.

6. Take three similar leaves from a single plant. Cover with a
thin layer of vaselin the upper surface of one, the lower surface of
another, and both surfaces of the third. Determine the relative
time required for the leaves to wilt and also to dry out completely.

7. Experiments :

Show that water is given off by plants, by covering a plant with
a bell jar.

Ascertain the amount of water transpired by a potted plant, by
covering the pot and soil with melted paraffin and weighing the
plant at intervals.

47

CHAPTER FIVE

THE WATER RELATIONS OF LEAVES

DURING a prolonged drought in Illinois, in 1914, oats in some
places failed to attain a height of more than 4 inches and
produced practically no grain, and corn which should have
averaged 10 feet in height reached only 5 feet in many fields,
and yielded only half the normal amount of grain. In the
four great corn-growing states there must be 3 inches of
rainfall in July for the best yield of corn ; and if the rainfall
during July is 2-J inches instead of 3, it is estimated that at
normal prices there is an average loss of $5 an acre, or a total
loss of $150,000,000. Those who cultivate plants know from
experience the importance of a sufficient water supply in the
production of crops, and the reason why the water supply is
important will be apparent when we understand the uses
made of water by the plant.

Why water is necessary to a plant. The active protoplasm
of all plant cells is in a semiliquid condition. More than
90 per cent of its weight is made up of water, and in consist-
ency it closely resembles white of egg. The several parts of
the protoplasm — the cytoplasm, the nucleus, and the plas-
tids — differ somewhat in their water content, but all of
them must be nearly saturated with water to carry on the
life processes. When the amount of water in the cell
falls much below this point, the protoplasm becomes rigid
and its processes are retarded. In many plants the pro-
toplasm may even die if the water content is greatly
reduced. Water is necessary for the life of the protoplasm of
plant cells.

Water is one of the materials used in the production- of
carbohydrate. Without it the process of photosynthesis,

48

The Water Relations of Leaves 49

upon which the world depends for its supply of food, cannot
be carried on. Water is necessary for photosynthesis.

Substances can enter plants only when they are in solution.
Both the gases and the mineral compounds 'that are used
by the plant in its various processes must be in solution in
water before they can be absorbed or pass from one cell to
another within the plant. Indirectly as well as directly water
is necessary to photosynthesis ; for water keeps the mesophyll
cells wet and thus makes it possible for the carbon dioxid to
enter the cells. Water is necessary for the absorption' of min-
erals and gases and for the transfer of materials within the plant.

Vacuoles the reservoirs of the cells. The vacuoles inside
the cytoplasm are minute reservoirs within the cells. They
contain the cell sap, which consists of water holding in solu-
tion sugars, mineral salts, and acids. The relation of the
vacuoles to the protoplasm is most important, for the pro-
toplasm can secrete excess substances into the vacuole or
remove substances from it as they are needed. In many
industrial establishments individual machines are provided
with small boxes or trays, some to hold raw materials and
others to receive the manufactured product. The vacuoles
have the same function as these storage boxes : they hold a
supply of the raw material for the use of the protoplasm, and
they receive some of the products that result from the ac-
tivities within the cell.

Transpiration. If we expose a wet cloth to the air, the
water evaporates ; that is, it changes from a liquid to a
vapor and passes off into the atmosphere. The same thing
happens when a plant is exposed to the air. The mesophyll
cells of the leaf are continually giving up water vapor to the
intercellular spaces, from which, if the stomata are open,

Science of Plant Life

this vapor passes out into the atmosphere. The epidermis of
the leaf also allows some water to pass through it, but in land
plants this is a relatively small amount, because the cuticle
hinders the process (page 54). The giving off of water vapor

from plants is called transpiration.

The loss of water in the form of
vapor is a process that takes place in
animals as well as in plants. If you
hold your hand near a windowpane
on a cool day, a halo of minute water
drops condenses on the glass. These
water particles come from the moist
cells of your skin. If you blow on
a glass, water collects even more
abundantly. The vapor in the
breath is water that has evaporated
from the moist cells of the lungs.

The amount of water transpired
by plants. The amount of water
given off in transpiration is surpris-
ingly large. During its lifetime, a
well- watered corn plant may give off
4 or 5 gallons of water. A sunflower
plant may transpire more than 18
gallons. The water given off by a
FIG. 33. Com plant, and bottle field of wheat during its entire

of water equivalent to that tran- period of development WOUld COVCr
spired by the plant during its life- ,, ~ , , , , ,. r .,

time. (All drawn to the same the field to a depth of 4 Or 5 inches.

scale.) In eastern Colorado the For the best growth of plants, there-

amount.

soil enough water to permit them to

The Water Relations of Leaves 51

take what they need for transpiration. When we consider
that the quantity of water transpired by wheat in cultiva-
tion is one fifth to one eighth of the rainfall of the central
United States, we begin to realize how large a fraction of
all the water that falls on the soil is actually used by the
plants. In all rainfall, some water runs off the soil without
penetrating the surface, some evaporates from the soil surface
itself, and some sinks below the level of the plant roots. Con-
sequently, it is only when there are abundant rains, dis-
tributed throughout the growing season, that the amount
of water needed by the plants for their best development is
available in the upper layers of the soil. It has been shown
by experiment that for the production of every pound of
solid matter in the above-ground parts of crop plants, from
300 to 500 pounds of water are required in the central United
States, and that from 400 to 1000 pounds are needed on the
plains of Colorado. The amount of water used in transpi-
ration is, therefore, many times the amount used in the manu-
facture of food.

Water supply and crop yields. Knowing these water re-
quirements, it is easy to understand why droughts are so dis-
astrous to crops. When the rainfall is slight, not only is the
amount of water that can be secured by the plant from the
soil reduced, but the sunshine is brighter and the air is
usually drier, so that transpiration from the plant is in-
creased. It is in part because of the water requirement of
crop plants that bottom lands — lands along streams in
the bottoms of valleys — are more valuable for growing
crops than are uplands. There the underground water is
nearer the surface, and keeps the supply for plants more
nearly constant.

52 Science of Plant Life

The balance between transpiration and absorption. The

amount of water in the cells of the plant as a whole is de-
termined largely by two processes : (i) the rate of absorption
- the taking of water from the soil ; and (2) the rate of
transpiration. The relation between these two rates de-
termines the water balance inside the plant. If the tran-
spiration is rapid and absorption is slow, internal drought
results and the plant may wilt. If the transpiration is slow
and the water intake is rapid, the cells will be filled to their
utmost capacity.

Importance of the water balance. Of all the factors that
influence the growth of plants and modify the form, size, and
structure of leaves, the water content of the cells is the most
important. Abundant water permits a plant to grow to its
greatest height, and permits the leaves to attain their largest
size and number. Long-continued internal drought may
cause the plant to be dwarfed and the leaves to be small and
few in number. In the river bottom the bur oak may develop
into a magnificent tree 100 feet in height, while on the dry
uplands it may attain only a stunted growth of less than 15
feet. An average leaf on a large tree will have twice the area
of a leaf on a stunted one, and the number of leaves on the
larger tree will be many times the number on the smaller.

In the summer, when the soil is dry and the air is hot,
transpiration may cause the leaves to lose water so rapidly
that they droop, and we say that the plant is wilted. Water
has passed out of the cells of the leaf faster than the water-
conducting tissue has brought in water to replace it, and the
cells are no longer distended and firm. They are like a foot-
ball that is only partly inflated. After a heavy shower the
plants quickly recover, because the water available in the

The Water Relations of Leaves

53

soil has been increased and more water is taken into the plant.
The shower has also covered the leaves with a film of water
and made the air moist around them, and this reduces the
water loss. Under these conditions, the cells of the plant
quickly become turgid, — that is, become fully distended
with water, — and the leaves recover their firmness. The
leaves of many plants like lettuce, pumpkin, and ragweed
depend for their firmness almost entirely upon the turgidity
of the leaf cells.

The balance between the rate of water supply
and the rate of water loss is the most important
water relation of the plant.

The water balance illustrated. The in-
ternal water balance of the plant may be
crudely illustrated by a glass tube with an
inverted porous porcelain cup sealed to one
end and with a stopcock attached near the
other end. If the cup and tube are filled
with water and the open lower end of the
tube is placed in a dish of mercury, the
mercury will rise as the water evaporates
through the porous surface of the cup. If we
nearly close the bottom of the tube by means
of the stopcock, the rate at which the mer-
cury rises is diminished. This is because the
evaporation is decreased as the amount of
water supplied to the cup is lessened. If we
open the stopcock, but cover the outside of
the cup with a thin layer of some substance ing up of water in
like wax, which does not allow water to pass a ^ee through tran-

\ t spiration from the

through it freely, the rate of evaporation will leaves.

54

Science of Plant Life

again be checked. This time the water can pass through the
tube freely, but it cannot evaporate through the cup so
rapidly because of the wax covering.

How plants are adjusted to maintain the water balance.
Plants become modified in many ways in response to the con-
ditions of water supply and water loss under which they grow.
Among the adjustments that help plants to maintain an
advantageous water balance under dry conditions are :

(i) Thickened cuticle and "bloom." The cuticle of a leaf
checks transpiration as does the wax film in the experiment,
and in plants of dry climates the cuticle may be so thick as
to reduce transpiration through the
epidermis to almost nothing. There
are many plants which secrete, in
addition to the cuticle, particles of
wax on their leaves or other parts.
This is the so-called " bloom " which
may be seen on the leaves of the
houseleek and cabbage and on the
fruits of the grape, plum, and blue-
berry. The bloom consists of a layer
of wax particles scattered thickly over
the surface of a leaf or fruit. It forms
a layer that is nearly impervious to
water and helps to reduce water loss
E.s.ciements through the epidermis.
FIG. 35. Vertical sections of (2) Compact leaves. A plant may
leaves of Mertensia showing b adjusted to an inadequate

differences in structure when

growing in moist, shaded water supply by the development of

situation (above), and when leayes ^.^ compact tlSSUCS. In SUCh
growing in dry, intensely

lighted situation (below). leaves the intercellular spaces are much

The Water Relations of Leaves 55

reduced, so that evaporation from the mesophyll cells is
greatly lessened. In extreme cases the mesophyll cells are
all of the compact palisade type, which leaves the minimum of
air space within the leaf.

(3) Small leaf area. A third way in which plants become
adjusted to dry conditions is by a decrease in the total leaf
area. When a plant is brought into the house in autumn,
it drops a number of leaves. The air inside most houses
being much drier than the air outside, transpiration is greatly
increased. As the water supply remains about the same,
the dropping of a few leaves restores the internal water bal-
ance of the plant. Some trees, like the cottonwood, shed
part of their leaves during a summer drought. If a wet
period follows, more leaves may be added, and in this way a
nearly uniform water balance is maintained.

That plants growing under moist conditions have larger
leaves and more leaves than the same kinds of plants growing
under dry conditions has been noted by every one. The
contrast may be observed by comparing weeds that grow along
the base of a railroad embankment or the high bank of a stream
with those that grow near the top.

Transplanting and the water balance. When the skillful
gardener transplants a tree, he cuts off a number of branches
to reduce the number of leaves, in order that the plant may
not dry out before new water-absorbing roots are developed.
Before lettuce, tomato, and cabbages are lifted for trans-
planting, the plants should be watered and allowed to become
turgid ; water should be poured into the holes in which they
are placed, before the soil is closed in around the plants. It
is customary also to cover the plants with boards or paper
covers so as to reduce the transpiration. Maintaining the

56 Science of Plant Life

water balance in transplanted plants may prevent the loss of
many of them and may save weeks of delay in the maturing
of the crop.

The water balance and plant habitats. The place where
a plant grows naturally is called its habitat. The willow grows
beside a stream and the cactus grows in the desert, each in its
natural habitat. If we put the willow in the desert and the
cactus on a wet stream bank, both die. This means that
the conditions that make up each habitat are favorable to
one kind of plant and not to another. The conditions in-
clude not only the kind of soil and the amount of soil water,
but also the evaporative power of the air. In selecting
plants that may live in a particular habitat, the great im-
portance of the dryness or the moistness of the air is to be kept
in mind. Plants whose leaves are soft and transpire water
rapidly can succeed only in moist air, while those that have a
low transpiration rate can maintain a suitable water balance
only in a dry atmosphere. This is one of the reasons why on
a southern slope we find a set of plants that are different from
those on the northern slope.

Recent studies have shown that the
leaves of plants growing near the bottom
of a ravine transpire water 10 to 20
times as fast as do those of plants
growing higher up on an adjoining
southern slope. Doubtless, each year
E.s.ciements seeds of plants that grow in the low
of KTfSta §«! germinate on the upper part of
plant. The upper figure the slope ; but each year the plants

shows an aerial leaf, the ^^ spring from those seec}s are elimi-
lower figure a submerged

leaf. nated through their inability to get the

The Water Relations of Leaves

57

water needed for their
high rate of transpira-
tion. There are plants
like the dandelion that
can adjust themselves
to both these condi-
tions. Most plants,
however, cannot do this,
and those with a high
transpiration rate die
off on a dry hillside,
while those with a low
transpiration rate sur-
vive. This indicates
only one of the factors
which must be taken
into account in the selec-
tion Of plants for par- FlG" 37' Submerged plants. From left to right:
. eelgrass (Vallisneria), naiad (Najas), water weed

ticular habitats ; Other (Elodea), and pond weed (Potamogeton).

factors will be con-
sidered in connection with the study of stems and roots.

Submerged and floating leaves. An examination of a
submerged leaf of any pondweed shows that it has no stomatal
openings. The floating leaves of water lilies and other pond
plants have stomata only on the upper surfaces. Evidently
submerged plants have no transpiration. It is also certain
that they get their carbon dioxid directly from the water
through the epidermis, for carbon dioxid is found dissolved in
pond waters, often in larger proportion than in the air.

In water-lily leaves the upper surface is covered by a
cuticle that is not readily made wet, and it has stomata that

Science of Plant Life

do not open until the
leaf is above water. If
the leaves are raised
entirely above the sur-
face of the water, as
sometimes happens
when the plants are
crowded, both surfaces
develop stomata.

Desert plants and
water storage. In the
desert, where the air is
very dry and the scanty
rainfall is confined to
one or two periods in
the year, plants have
very great difficulty in
securing water. The
perennial plants have
various ways of con-
serving water from one
rainy period to the
next. The barrel cac-
tus has no leaves at
all, and the stem is a
thick cylinder composed largely of water-storage tissue; it
may live without water for 2 years or longer. Some of the
desert shrubs have leaves during the rainy periods only, and
these are shed as soon as the drought comes. Still others,
like the agaves, have very thick, leathery leaves with much
internal water-storage tissue and a very low transpiration rate.

FIG. 38. Desert plants, including forms of Cereus,
prickly pear (Opuntia), Yucca, Euphorbia, and cen-
tury plant (Agave).

The Water Relations of Leaves

59

Adjustment to desert conditions by ability to withstand
drying. Another group of plants is adjusted to desert condi-
tions by being able to with-
stand complete drying. The
resurrection plant (Fig. 39)
of Texas is an example of
this group. During the
rainy season it is green and
has its many scale-leafed
branches spread out for food
manufacture and growth.
When drought comes, the
plant dries out completely
and its branches curl upward
until it is in the form of a
ball. In this condition it
may be blown about by the FIG
wind and remain dormant
for weeks and months, all
of its physiological processes
having ceased. When the plant again becomes wet it unfolds,
and its processes begin anew. In the eastern United States we
find plants of this same type in the lichens, mosses, and small
ferns that grow on the bark of trees and on bare, dry rocks.

Plants classified according to their water relations. In the
preceding paragraphs the importance of the water require-
ments of plants has been made clear. We have seen that the
internal water balance of the plant is of great importance in
modifying its physiological processes and the size and struc-
ture of its organs. Three great classes of plants are dis-
tinguished on the basis of their water relations :

Resurrection plant (Selaginella),
in growing condition (above), and the same
plant in a dry and dormant condition
(below). '

6o

Science of Plant Life

(1) The plants that natu-
rally live where the evapora-
tive power of the air is intense
and the available water is
limited are called xerophytes
(Greek : xeros, dry, and phy-
ton, plant). These are the
plants that are adjusted to a
nearly continuous dearth of
water; the cacti, agaves,
yuccas, and sagebrush of our
Western plains and deserts
are striking representatives.
In the eastern United States
there are less marked ex-
amples of xerophytes in the
plants that live on dry cliffs

FIG. 40. Evergreen trees growing under and sand beaches, and in the

extreme xerophytic conditions in Oregon. mosses and HchenS that gTOW
These Pillars of Hercules" are by the

side of the Lincoln Highway. on trees and rocks.

(2) The plants that live

partly or wholly submerged in the water are known as
hydrophytes (Greek : hudor, water, and phyton, plant) . These
plants have an excessive water supply, and transpiration is
reduced or entirely wanting. In this class are included the
water lilies, pondweeds, cat-tails, bulrushes, and many sedges.
They are the common plants of fresh-water ponds, swamps,
and marshes throughout the world.

(3) Between these extremes are the mesophytes (Greek:
meso, middle, and phyton, plant), by far the largest class of
plants. They have a medium rate of transpiration and grow

The Water Relations of Leaves 61

best with a moderate water supply. In this group are in-
cluded the plants that yield most of our garden, field, and
meadow crops ; also most of the forms that are found in the
maple, beech, and elm forests of the Eastern states, and in
the fir and spruce forests of the canons and bottom lands of
the Western states.

Xerophytes, hydrophytes, and mesophytes are readily dis-
tinguished as groups because of their great differences of
habitat and appearance. But it is not always easy to de-
cide whether a particular plant is a xerophyte, hydrophyte,
or mesophyte, because we find all gradations of form among
plants of the three classes. Nevertheless, these terms are
useful in describing the water relations of many plants.

PROBLEMS

1. How do plants that are wilted in the late afternoon of a hot summer day
recover their firmness during the night, even though there is no rain ?

2. Where, near your home, do mesophytes, xerophytes, and hydrophytes occur?

3. In what regions of the United States are mesophytes most common?

4. In what parts of the United States are xerophytes abundant?

5. In what parts of the United States are hydrophytes common ? What states
have very few hydrophytes?

6. What xerophytes furnish useful products to man and animals?

Suggestions for Laboratory Work to Precede Chapter Six

1. Collect from various plants leaves showing the autumn
colors. Make a record of the colors found in different kinds of
trees.

2. Look for different colors assumed by leaves of the same kind
of tree when growing in shade and in bright sunlight.

3. Look for the red color in leaves that have been partly shaded.
What is the effect of the shade?

4. Cut longitudinal sections through the stem and base of a leaf
that is about ready to fall, and note the abscission layer. .

5. Examine some evergreens and determine the number of
years they hold their leaves. Differences in color of bark will help
to distinguish the growths of different years.

6. Make a chlorophyll extract by placing leaves in warm alcohol.
Dilute the alcohol with a little water, and add enough benzine to
double the volume of the solution. Then shake thoroughly, and
allow to stand until the benzine has separated. The benzine will
contain the yellow pigments, while the chlorophyll will remain in
the alcohol.

62

CHAPTER SIX

LEAF COLORATION AND THE FALL OF LEAVES

IN spring and summer the most prominent feature of the
landscape is the green color of the vegetation. The most
striking feature in autumn is the varied colors of the foliage
on the trees and shrubs. In the northern provinces of Canada
most of the trees are evergreen, and the most abun-
dant deciduous trees, like the aspen, birch, and tamarack,
merely turn yellow. But in our Northern states the vivid
greens of the sugar maple, white oak, gum, and sumac dis-
appear in a blaze of red that contrasts strongly with the
yellows of the hickory, linden, and poplar and with the dark
greens of the hemlock, spruce, and pine. Every one who has
seen the colors of autumn woods and the annual falling of the
leaves must have wondered what processes go on within the
leaves to bring about these changes.

The pigments in green leaves. We can best approach the
matter of autumn colors by inquiring into the composition of
the pigments that give the color to the leaves of deciduous
trees in summer and to the leaves of evergreen trees through-
out the year. The most abundant of these pigments is chlo-
rophyll (Greek : Mows, green, and phyll, leaf), which is bright
green in color. In addition to chlorophyll, two other pigments,
one yellow and one orange, are found in a green leaf. These
three pigments may exist quite independently of one another.1

1 The coloring matter in a green leaf is composed of about 66 per cent green
pigment (chlorophyll) ; 23 per cent yellow pigment (xanthophyll) ; and 10 per
cent orange pigment (carotin, so named because of its abundance in the carrot).
The green pigment is not a simple substance, however, but a mixture of two
kinds of chlorophyll, one of which is blue-green and the other yellow-green.
The depth of the green color in a leaf depends in part on the proportions in
which these various pigments are combined.

63

64 Science of Plant Life

In the chloroplasts all three are present at the same time,
so that we cannot distinguish them under the microscope.
As the three are soluble in alcohol, the presence of the
yellow and orange pigment does not become apparent when
the coloring matter is extracted from leaves by means of
alcohol. The chlorophyll within a leaf is constantly breaking
down, and new chlorophyll is formed in the chloroplasts to
take its place.

Conditions affecting the development of the pigments.
Chlorophyll is produced only in the presence of light, but the
yellow and orange pigments develop in the dark as well as in
the light. When we lay a board on grass or shut out the
light to blanch the leaves of celery, the green color gives way
to yellow or orange. Likewise seedlings grown in the dark
and the inner leaves of head lettuce show a yellow but not a
green hue ; and when the light is cut off from a green leaf,
the green pigment disappears, leaving the yellow pigments
visible.1 These facts make it clear that the yellow pigments
do not require light to develop, while the green pigment does.

There are a number of conditions besides absence of light
that result in the partial or complete disappearance of the
green pigment, but these affect various plants quite differ-
ently. Low temperature, drought, and injuries and diseases
of various kinds may interfere with the nutrition of the leaf ;
even a slight decrease in light may do so. All these factors
tend to affect the green pigment more than the yellow and
orange. Although these same influences — low temperature,
drought, reduced light, injuries, and diseases — may be ef-
fective at other seasons, they become generally operative in
iate summer and autumn. Hence it is at this time of the year

1 A number of evergreens are exceptions to this rule.

Leaf Coloration and the Fall of Leaves 65

that the green pigment disappears from the leaves of most
deciduous plants and unmasks the yellow pigments in the
chloroplasts. There is every gradation in the readiness with
which the green pigment disappears from the leaves of differ-
ent species of deciduous trees, from the cottonwood, in which
the leaves become yellow during a midsummer drought, to
the peach, in which they may still be vivid green when shed.
In evergreens the chlorophyll is less sensitive and external
conditions are not so effective in causing changes in the color
of the leaves.

The red pigment. The red colors of autumn leaves are not
due to changes in the content of the chloroplasts, but to the
formation in the cell sap of a red pigment called anthocyan.
This same pigment is present in the cells of many young leaves
in early spring. It occurs also in beets, in red cabbage, in the
petioles and veins of many different kinds of leaves, in the
Coleus and other foliage plants, and in many flowers. The
presence of anthocyan in the cell sap makes the whole cell
red, and any or all of the cells may develop the pigment.

The development of the most brilliant red coloring of au-
tumn is commonly ascribed to the action of frost. This ex-
planation is probably incorrect, for careful observation indi-
cates that the color is most intense when a moderately low
temperature is accompanied by bright sunshine. In warm,
cloudy autumns the colors are more likely to be dull, with the
yellows predominant. That sunlight is important in the
development of the red pigment may be shown also by an
examination of a leaf that has been closely shaded by another.
The pigment stops so abruptly where the shade begins that
a perfect print of the uppermost leaf results.

The red colors of the fruits of peaches, apples, and pears

66 Science of Plant Life

likewise are due to anthocyan. Here again we may see the
effects of sunlight on the intensity of color by comparing fruits
from the brightly illuminated top of the tree with others from
the shaded under parts. The apples grown in the Northwest-
ern states are more brilliant in color than the same varieties
grown in the Eastern states, and this higher coloration is
probably due to exposure to more intense light.

Among different plants there is much variation in the
amount of light that is required for the development of an-
thocyan colors. This accounts for the great variation in the
brilliancy of autumn coloration in different years. One
autumn affords light conditions which promote the forma-
tion of anthocyan in only a few trees and shrubs ; another
autumn furnishes conditions so favorable that many plants
become brilliant.

The brown colors. In some trees the leaves turn brown
immediately after the loss of their chlorophyll. In other
trees the leaves may first turn yellow or red, and then grad-
ually assume the shades of brown. These brown colors re-
sult from chemical and physical changes in the substances
within the leaf. Just what the processes are is not fully
understood, but it is reasonably certain that tannins and
tannic acid are connected with the making of the brown pig-
ments. The dead bark of trees also turns brown, probably
because of chemical processes within it similar to those which
take place in the leaves.

White leaves. One occasionally finds on plants leaves
that are wholly or partly white. This is simply the natural
color of living plant tissues that lack chlorophyll or other
pigments. The protoplasm, cell sap, and cell walls are
transparent and colorless. The presence of air spaces among

Leaf Coloration and the Fall of Leaves 67

the cells makes these tissues appear white. The ice crystals

of which snow is composed are transparent, but the numerous

air spaces between the crystals reflect

the light and cause it to appear white.

Ice likewise becomes white when it is

filled with minute air bubbles. The

white colors of leaves and flowers merely

show the natural appearance of the

parts in the absence of chlorophyll and

other pigments.

The causes of leaf fall. There are
two distinct stages in the process by
which plants drop their leaves : (i) the FIG. 41. Longitudinal sec-
formation at the base of the petiole of *?n .of ^ase .of ,petiole'

r showing abscission layer at

two or more plates of thin-walled cells, beginning of leaf fall,
known as the abscission layer; this takes
place during the development of the leaves and may require
weeks or months for completion ; and (2) the actual separa-
tion of the cells of the abscission layer, which is brought
about by the softening or dissolving of the middle layer of
the walls of the abscission cells. This stage of the process
may take place within a few hours, or at most within a few
days.

The plant is protected from disease and water loss at the
scars left by the falling leaves, through the addition of woody
and corky materials to the cell walls beneath the abscission
layer. This protective layer is formed in some kinds of
plants before the leaf drops, in other plants after the leaf has
fallen.

Conditions promoting leaf fall. After an abscission layer
has developed, there are many climatic and soil conditions

68

Science of Plant Life

that may accelerate the

FIG. 42. Shagbark hickory twig.
A is the bud scales of the terminal
bud of the previous year, B sev-
eral petioles remaining attached
after leaf fall, and C the terminal
bud that will develop the follow-
ing spring. Drawn from a speci-
men collected in December.

falling of the leaves. Among these
are low temperature, reduced light
intensity, and any disturbance of the
water relations of the plant which
results in internal drought. Disease
and insect injuries to the blade fre-
quently bring about abscission.

Leaves contain food materials
when they fall. The materials used
in building the cell walls in a leaf
are lost to the tree when the leaf
falls, and the fallen leaves still re-
tain considerable amounts of starch,
sugar, and protein. In the autumn,
however, photosynthesis declines,
and the amount of food lost by a
deciduous tree through leaf fall is
small in comparison with the quan-
tity that has accumulated in other
parts of the plant.

Abscission in compound leaves.
In many compound leaves, like the
horse-chestnut, ash, and hickory,
abscission first takes place at the
base of each leaflet. Later the
petiole is cut off from the stem in
the same way. Consequently the
leaflets fall first and the petioles
later. In the king-nut hickory the
petiole remains attached to the tree
through the following year (Fig. 42).

Leaf Coloration and the Fall of Leaves

Self-pruning. A large number of our common trees, like
the cottonwood, maple, and elm, develop abscission layers
which cut off twigs and some-
times branches an inch in thick-
ness. In these trees we have twig
fall as well as leaf fall. The
falling of flowers, and of fruits
like apples and nuts, is due to
the abscission layers formed in
the stems.

Evergreen and deciduous trees.
In the Northern states many
persons have come to think that
the evergreen habit is associated
only with needle leaves, because
in the North the evergreens are

FIG. 43. Abscission of branches

mostly of the needle-leafed type. of cottonwood. Twigs and small

But in the Southern States there branches as well as leaves and fruits
, , , - , , , ., are cut off by the formation of abscis-

are many broad-leafed trees, like sion layers
the magnolia, rhododendron, and

holly, that are also evergreen. Moreover, the tamaracks of
the North and the bald cypress of the South furnish examples
of needle-leafed trees that are deciduous. If we include the
shrubs, there are many broad-leafed plants, both in the North
and in the South, that have the evergreen habit. In the tropics
most of the trees are evergreen, and almost all have broad leaves.
It must be noted that even in the case of evergreens individual
leaves remain on the trees for only a limited number of years.
The leaves of the evergreens are quite different structurally
from the leaves of deciduous trees. The evergreens must be
able to withstand freezing and thawing, and also the dry

7o

Science of Plant Life

Lee Moorhouse

FIG. 44. Deciduous trees in winter, on
Umatilla Indian Reservation, Oregon.

Caspar W . Hodgson

FIG. 45. Evergreen trees in winter, in
Yosemite Valley.

Leaf Coloration and the Fall of Leaves 71

winds of winter, which cause transpiration even when the
ground is frozen. This suggests that their usual transpiration
rate must be very low in comparison with that of the decidu-
ous trees.

Evergreen versus deciduous habit. In temperate regions,
where there are great changes in temperature and moisture,
the deciduous and the evergreen habit each has certain ad-
vantages. The advantages of the evergreen habit are: (i)
that the leaves can manufacture food even when the tem-
perature is low ; (2) that with their low water requirement,
evergreens can withstand drier conditions throughout the
year ; (3) that the tree does not waste so much material each
year in the construction of a complete set of new leaves.
The disadvantages of the evergreen habit are : (i) that the
heavy cuticle and compact tissues which aid in conserving
water interfere with rapid photosynthesis ; (2) that the lower
rate of food manufacture prevents rapid growth ; (3) that the
leaves lose in efficiency by their longer service on the trees.

The advantages of the deciduous habit are : (i) that the
leaves, being renewed each year, are more efficient organs of
food manufacture ; (2) that the leaves, with less cuticle and
with tissues less compact, are better fitted for rapid food
manufacture ; (3) that the total leaf area may be much
larger than in the case of the evergreens ; (4) that the trees
are better fitted to withstand the winter drought, because
at that season the entire tree is covered with cork. The dis-
advantages of the deciduous habit are: (i) that the food-
manufacturing season is only from 5 to 8 months, as com-
pared with from 8 to 10 months in the evergreens ; (2) that
each year a large amount of food material is needed to make
an entirely new set of leaves.

72 Science of Plant Life

Finally, we must observe that there are in trees all gradations
between the deciduous and evergreen habits. In the rainy
tropics there are many delicate-leafed evergreens. In the dry
tropics the evergreens have thick, fleshy leaves, or they may
be quite leafless. ' Some plants, like the holly and the Virginia
creeper, may be of the deciduous habit in the North and of the
evergreen habit in the South. Some deciduous trees, like the
cherry, when planted within the tropics become evergreen;
while the magnolia, which is evergreen in the Southern
states, becomes deciduous when grown in a colder climate.
Evidently leaf habits are adjustments to climatic conditions,
especially to conditions of temperature and moisture.

PROBLEMS

1. What are the commonest evergreen trees and shrubs of your locality?

2. What trees of your vicinity develop leaves earliest in the spring? What
trees put out their leaves last ?

3. What trees develop flowers before putting out their leaves?

4. What trees drop their leaves first in autumn ? What trees drop their leaves
last in autumn?

5. What trees shed part of their leaves during the summer?

6. During how many months does each of these trees carry on food manu-
facture?

7. During how many months do the evergreen trees of your locality manufac-
ture food?

8. For how many months do the three leading crop plants of your locality carry
on photosynthesis?

Suggestions for Laboratory Work to Precede Chapter Seven

1. Make a thin paste by boiling a little starch in water. Test
a small portion of the paste with iodin. Test for sugar with Bene-
dict's solution l ; then add diastase solution (malt extract pur-
chased at the drug store may be used) to the starch paste ; after
it has stood in a warm place for some hours, test it again for starch
and .for sugar. What is meant by digestion of starch? What
enzyme brings this about?

2. Test for starch a leaf from a green plant that has been in
sunlight by placing it (i) in boiling water for one minute, (2) in
hot alcohol until bleached, and (3) in an iodin solution. If starch
is present, put the plant in the dark, after cutting off one of its
leaves. Place the severed leaf in a covered dish containing moist
filter paper, and set the dish in the dark. On the following day
test the severed leaf for starch ; also test for starch a leaf freshly
cut from the plant that was placed in the dark. What do the
tests show regarding the movement of carbohydrate out of leaves ?

3. Test seeds of corn, bean, and almond for starch, by placing a
drop of iodin solution on a freshly cut section of each seed.

4. Test the same kinds of seeds for oil by scraping the seeds
and crushing the scrapings on a sheet of white paper. Oil makes
the paper translucent. Which contains the most oil ?

5. Test the same kinds of seeds for protein by placing a drop of
nitric acid on a section of each seed. Nitric acid stains protein
yellow.

6. If time permits, other organs of the plant may be tested
for food in the same way, and starch grains may be studied under
the microscope. Scrapings from potato, the root of the canna,
and various grains will furnish material for the study.

1 Benedict's solution: Dissolve 173 gm. copper acetate and 100 gm. anhy-
drous sodium carbonate in 600 cc. of distilled water. Filter, and add water
until the total solution is 850 cc. Dissolve 17.3 gm. of copper sulfate in 100 cc.
of water and dilute to 150 cc. Then mix the two solutions, which together will
make a liter of Benedict's solution. It is ready for use immediately and keeps
indefinitely. To make a test for sugar, add 3 cc. of Benedict's solution to i cc.
of the solution to be tested, and boil. If sugar is present, a red or yellow pre-
cipitate will appear.

73

CHAPTER SEVEN

DIGESTION, TRANSFER, AND ACCUMULATION OF FOODS

WE have seen that starch is formed in the leaves of a plant
when it is exposed to light. We have also learned by ex-
periment that starch disappears from leaves at night, but
that if a leaf is removed from a plant it will still contain starch
the next day. Furthermore, in many plants like the potato,
turnip, or corn, we find starch in parts of the plant far re-
moved from the leaves. These facts indicate that the starch
is transferred from the leaves and is accumulated in the
stems, roots, or seeds. In the present chapter we shall learn
how this is done.

Digestion of starch. Starch is insoluble in water. It does
not dissolve in the cell sap, and the starch within the cells is
not divided into particles small enough to pass through the
cell walls, before it can be moved from one part of the plant
to another, or even from one cell to another, it must be changed
into some substance that is soluble. The process of changing
starch into a soluble substance has been carefully studied ;
and we know that starch is first converted into maltose and
that the maltose is further split into glucose (page 28). Glu-
cose is readily soluble in water and consequently can be passed
from cell to cell and so transferred to any part of the plant.
The changing of insoluble substances like starch into simpler
soluble substances like glucose is called digestion. Unlike
animals, plants have no special organs of digestion. All
their living cells are capable of digesting the insoluble sub-
stances that are required for their nutrition.

Digestion brought about by enzymes. Digestion is brought
about by substances called enzymes. These are produced by
the living protoplasm of the cells. A large number of differ-

74

Digestion, Transfer, and Accumulation of Foods 75

ent kinds of enzymes have been recognized in plants ; each
enzyme digests only one particular kind of food, and there
must be a different enzyme to digest each kind of food within
the cell. The enzyme which digests starch is called diastase.
The enzyme that digests fats is called lipase. There are other
enzymes which act upon the insoluble forms of protein and
render them soluble. It seems probable that enzymes are
concerned in the principal activities of all living cells. With-
out them there could be none of the rapid changes in foods
that are necessary for the transfer of foods within the plant
and for carrying on the other processes described in this and
the next chapter.

It is interesting to know that if an enzyme is put in a test
tube, with the appropriate food substance, it will bring about
digestion the same as if it were in the living cell. This proves
that digestion is not directly carried on by the living protoplasm,
and that to be digested, foods do not need to be in contact
with living matter. It requires but a very minute quantity
of enzyme to digest a large amount of the particular food upon
which it acts ; for example, a preparation of an enzyme ex-
tracted from the pancreas of an animal was found to digest
2,000,000 times its weight of starch. The amount of diastase,
therefore, that is needed in a mesophyll cell in order to trans-
form to sugar the starch in that particular cell, is so small that
it cannot be measured.

Accumulation of food. A healthy plant usually manu-
factures more food than it uses immediately. In the potato,
surplus food is carried to underground stems, the tubers, and
is there stored. Turnips and beets are examples of plants
that accumulate excess food in their roots. In the maple, it
accumulates in the branches, trunk, and roots. In the cab-

Science of Plant Life

U. S. Dept. of Agriculture

FIG. 46. An orchard in California. The excess food of the tr.
accumulated in the fruits.

bage, food is stored in the cluster of leaves at the top of the
stem. In corn and cereals, the excess food finally accumu-
lates in the grain. In the century plant, a considerable part
of the excess food is stored in the thick, fleshy leaves; the
process of accumulation may go on from 20 to 30 years, and
the total quantity of food stored may amount to many pounds.
In nature such accumulated foods are used for another season's
growth of the plant or in starting the growth of its offspring.
Before insoluble foods are transferred in the plant, they are
digested or made soluble by enzymes. When these soluble
substances accumulate in the cells of storage organs, they fre-
quently are transformed again into an insoluble form. For
example, starch formed in potato leaves is transferred through
the plant to the underground tubers in the form of glucose,

Digestion, Transfer, and Accumulation of Foods 77

U. S. Dept. of Agriculture (E. L. Adams)

FIG. 47. A field of shocked rice in California. The surplus food of the rice plant
accumulates in the seeds. These are more used than any other one article of diet
by man.

and there it accumulates in the cells in the form of starch. It
is believed that the same enzymes which change the starch to
glucose, under suitable conditions change the glucose back
again to starch, and that, in general, the enzymes that digest
foods are the agents that build them up again into the more
complex insoluble forms.

Kinds of food accumulated. In any given plant in which
food is accumulated, protein, carbohydrate, and fat are all
present. Depending on the plant, however, the amount of
any one of these may be very great or it may be so small as
to be practically negligible. In the sugar cane and sugar beet
the excess food occurs mainly in the form of cane sugar (su-
crose). In the potato it is almost wholly starch. The grains
of wheat, oats, and rice contain mostly starch, but also some
protein. In sweet corn there are both sugar and starch; in
field corn there is mostly starch. In both kinds of corn there
are measurable quantities of protein and oil. In the soy bean

Science of Plant Life

Bruce Fink

FIG. 48. A sugar-cane field in Porto Rico. The excess food is accumulated in
the stems in the form of sugar.

and peanut, there are large quantities of both protein and oil.
In the seeds of the coconut, flax, and cotton there is a large
proportion of oil.

Food-accumulating plants and agriculture. Throughout
the long history of agriculture the principal purposes of
agriculturists have been : (i) the discovery of plants that
accumulate large amounts of food; (2) the improvement
of these plants by selection of seed from the most productive
or otherwise most desirable individuals ; (3) improvement of
the methods of cultivation to provide conditions of growth
that will cause the largest possible amounts of foods to be
accumulated in the plants. Sweet corn was developed through
the selection of seeds with a view to securing plants having a

Digestion, Transfer, and Accumulation of Foods 79

large sugar content ; field corn was developed through the
selection of seeds for starch content. The original sugar

P2.2

78.3

W 84.6

Ash, 1.4

BANANA

FIG. 49. Diagrams showing percentages of water, protein, fat, carbohydrate, and ash
in various plant organs. Note the low percentage of water in seeds as well as the
similarity of content of the potato and the banana.

beets contained less thaji 5 per cent of sugar ; because of
careful seed selection and intelligent fitting of the environ-
ment to the plants, the best quality of beets grown today con-
tains as high as 22 per cent. The original potato tubers
weighed about an ounce; potatoes weighing several pounds
are now grown, and a plant may produce ten times as many
tubers as are produced by the original plant.

The testing of wild species of plants for possible cultiva-
tion and use as sources of food, and the introduction of species
cultivated in other countries, is being carried on by govern-
ments in all parts of the world. Through its agricultural ex-
plorers the United States has probably accomplished more

8o Science of Plant Life

in this particular field than have all other countries com-
bined. Examples of plants that have been successfully in-
troduced into this country through the efforts of government
experts are the drought-resistant wheat, dasheens, figs, dates,
guavas, kafir corn, milo, and new varieties of oranges.

PROBLEMS

1. In the recent Yearbooks of the Department of Agriculture, look up the
accounts of new plant introductions.

2. Why is it important that the United States should grow within its own
boundaries as great a variety as possible of cereals, fruits, and other food-
producing plants?

Suggestions for Laboratory Work to Precede Chapter Eight

1. Generate some carbon dioxid by pouring hydrochloric acid
on marble in a flask with a long delivery tube. Pass some of the
gas through clear limewater in a small beaker. What is the
effect ?

2. Blow your breath through a tube into another small beaker
containing limewater. Is the effect the same?

3. Arrange each of two small flasks with a two-hole stopper
and two tubes as shown in Figure 50. In the first flask put
flowers; in the second put clear limewater. After they have
stood for a few minutes, draw air from the first flask through the
limewater in the second. Note the effect. Does the respiration
of plants produce the same gas as the respiration of animals ?

4. Mark young leaves of fern, corn seedlings, and nasturtium
as suggested in the text, and study the growth of the three kinds
of leaves.

5. Show by experiment that oxygen is necessary for the growth
of seeds.

3i

CHAPTER EIGHT

THE UTILIZATION OF FOODS

IN the preceding chapters we have seen how food is manu-
factured by the plant, how it is made soluble and is trans-
ferred, and how the excess food is accumulated in various
organs of the plant. Food is finally utilized by the cells in
respiration, assimilation, and growth. In this chapter we shall
learn the meanings of these terms and study the changes that
the foods undergo in connection with the production of
energy, the making of protoplasm, and the growth of the
cells.

Energy necessary to plant cells. In order to do work,
each machine in a manufacturing establishment must be
supplied with energy, and every living cell in a plant requires
energy for carrying on its work of repair, growth, and move-
ment. In manufacturing establishments the energy is usually
generated at one place and is then transmitted by means of
shafts and belts or wires and motors to all parts of the fac-
tory. The plant cannot transmit energy from one part
to another, but it can and does send food to all its living cells,
and from this food each cell generates within itself the energy
that it needs.

Respiration. A steam engine is supplied with energy by
the oxidation of fuel beneath the boiler that is connected
with it. A cell is supplied with energy by the oxidation of
food within it. The process by which the cells obtain energy
through the oxidation of foods is called respiration. In the
process oxygen is absorbed and carbon dioxid is given off.
Respiration takes place in all living cells, and to carry on this
necessary process all parts of the plant must be supplied with
oxygen. The leaves and stems of land plants obtain their

82

The Utilization of Foods

oxygen from the atmosphere, and the roots from the air that
is in the soil. Wet soils are unsuited to the growth of many

plants, not because of the w — ^

water present, but because of
the lack of a sufficient oxygen
supply for the roots. Drain-
age is a valuable agricultural
practice not only because it
removes excess water, but
also because it draws air
(oxygen) into the soil. When
the farmer breaks the crust

on the surface, he is making it possible for more oxygen to
reach the roots of his crop.

The plant and the process of respiration may be compared to
a manufacturing establishment and the work that goes on in it.

The power stations are every living cell of root, stem, and

i__r

FIG. 50. Carbon dioxid is given off from
the respiration of the flowers.

The machinery
The fuel
The process
The product
The waste

leaf.

is the protoplasm,
is foods, especially carbohydrates,
is the combining of food and oxygen,
is energy,
is carbon dioxid and water.

The working hours are twenty-four a day.

Respiration and photosynthesis contrasted. In photo-
synthesis, carbon dioxid and water are combined to form the
complex molecules of carbohydrates, and a large number of
atoms of oxygen are set free in the process. When, in res-
piration, the complex carbohydrate molecules are again
combined with oxygen, simple molecules of carbon dioxid

84 Science of Plant Life

and water are formed. In photosynthesis, the energy of the
sunlight is used in building up the carbohydrates. The
energy is stored in the carbohydrates, and it may be released
by changing them back to the simple substances out of which
they were made. When we wind up a clock spring, we put
energy into the tightened coil. When the spring is allowed
to uncoil, this energy is released and turns the wheels of the
clock. So in photosynthesis the energy is stored in the car-
bohydrates, and this energy is released in the process of res-
piration and used in the life processes of the cell.

In photosynthesis In respiration

Oxygen is released. Oxygen is consumed.

Energy is accumulated. Energy is released.

Simple molecules are built up Complex molecules are broken

into complex ones. down into simple ones.

Plants accumulate food and Plants consume food and de-
increase in weight. crease in weight.

Comparative rates of respiration. The rate of respiration
is greatest where there is rapid growth, as in germinating
seeds, opening flowers, and ripening fruits. In some of these
it is much more rapid, bulk for bulk, than in animals. The
lowest rates of respiration occur in seeds and other dormant
structures; and there is comparatively little respiration in
woody stems and other hard parts in which there are only a
few living cells.

Respiration and the shipping of fruits and vegetables. How
important the recognition of the respiratory requirement of
living cells is, may be illustrated by the difficulties that have
been met with in shipping fruits and bulbs. Peaches, during
shipment, sometimes develop brownish spots where they

The Utilization of Foods 85

touch each other. These spots were formerly thought to be
due to jarring in transportation, but they are now known to
be caused by packing the peaches so closely that the air does
not have full access to all the fruit. The respiration of the
cells at the points of contact is in consequence interfered
with, and these cells are suffocated and gradually die. Ships
with specially ventilated holds are used in importing bulbs
from Holland and fruits from the tropics. The building of
ventilated holds came as a result of the death of several men
who attempted to unload a cargo of bulbs from an unventilated
bottom.

Assimilation. Another part of the food constructed by the
plant is used for repair and for building additional protoplasm.
A machine, if allowed to stand idle and uncared for, gradually
goes to pieces. The cell is a very delicate mechanism, built
of highly complex substances, and the protoplasm of the ac-
tive cell requires constant repair. The foods that most nearly
approach protoplasm in chemical composition are the pro-
teins. Naturally these are the foods most readily changed
into protoplasm and are the ones mainly used in the process
of assimilation. Assimilation may be defined as the process
through which the living protoplasm is repaired or new proto-
plasm built up by the use of foods. Assimilation takes place
in all living cells.

Growth. The enlargement of plants or the development of
new structures is called growth. The fact about plant life that
is most familiar to all is that when a live seed is planted in
the soil it germinates, and that from it there develops a seed-
ling which continues to enlarge for a longer or shorter time,
depending on the plant and the conditions of growth. The
period of growth may be a month, as in the radish in mid-

86 Science of Plant Life

summer, or it may be hundreds of years, as in some trees. In
the process of growth vast quantities of food are consumed.
During the early stages of a plant's development most of the
food it manufactures is used in this way. In order to grow,
a plant must make new protoplasm, develop new cell walls,
and thicken and strengthen old cell walls. Growth requires
not only food, but energy as well. Indeed, a considerable
part of the energy derived from respiration is used in growth.
We might expect assimilation and food consumption to be
most active in young growing parts, and that this is the case
has many times been verified by experiment. Growth takes
place through the enlargement of cells already present in the
plant, through cell division, and through modification of cells
without enlargement.

The making of the cell walls. The wall that surrounds each
cell of the plant is composed largely of a substance called
cellulose, which is secreted by the living protoplasm. When
the cell is growing, its wall is exceedingly thin and it stretches
as the cell enlarges. When a cell divides, a new wall is formed
between the two parts. As the cell grows older, new layers
of cellulose and allied substances are added. In some tis-
sues, as in the shells of nuts, the walls become so thick as to
occupy most of the volume of the cell. In other tissues, like
the mesophyll of leaves, the cell walls always remain thin.

Chemically, cellulose is a carbohydrate, closely related to
sugar and starch. Sugar and starch are the plant foods that
are mainly used in its manufacture, just as the proteins are
mainly used in the building of new protoplasm. Cotton
fiber is pure cellulose, and exemplifies the strength, lack
of color, and insolubility in water that are characteristic of
cellulose.

The Utilization of Foods 87

Conditions for growth. The conditions most favorable for
growth are abundant water supply and warm temperatures,

FIG. 51. Results of an experiment to show effects of light and moisture on the growth
of potato shoots : A , light but no water ; B, light and water ; C, water but no light.

such as normally occur in summer. For the growth of the
plant as a whole, strong light is favorable because it increases
the supply of food. For the growth of leaves in particular,
medium light is generally most favorable. In darkness the
blades of many plants do not expand, and in very intense
light they do not expand fully because of excessive water
loss.

The growing regions of leaves. By watching the develop-
ment of leaves on any common herb, or on the trees in spring,
we can see that growth takes place rapidly ; also, that growth
ceases when the leaves have developed to a certain rather
definite size. After the leaf is mature, further enlargement
will not take place, no matter how favorable to growth the
external conditions may be. The question arises, do all parts
of the leaf enlarge equally, or do some parts grow more than
others ? There is one characteristic of growing tissue that will

88

Science of Plant Life

A

help us in answering this inquiry : young tissue is very tender,
and easily broken, while old tissue is stronger and firmer.

Fern leaves grow at the
apex. The fern leaf is one
that may be studied in this
connection, for the growing
portion is not only tender
but coiled up, and its unfold-
ing may be noted from day
to day by marking with India
ink the successive positions
of the coil. In the Boston
fern, which is so commonly cul-
tivated as a window plant, the
leaf may continue to unfold
for weeks, if the water supply
is adequate and other condi-
tions are favorable . Evidently
FIG. 52. Growing regions (shaded por- m the ferns the growing region

tions) of leaves : A . leaf of fern ; B. grass . •, 1,111

leaf; and C, leaf of sunflower. 1S at the aPCX and the °ldest

part of the leaf is the base.

Growth in the leaves of seed plants. The flowering plants
have either parallel- veined leaves or net-veined leaves, and
the place of growth in these two types of leaves is different.
In parallel- veined leaves, like those of the members of the
grass family, the growth is at the base. What boy or girl,
walking through a field of timothy or wheat, has not pulled
the leaves from the stems? The leaves always break near
the base. If you have tasted the broken end, you will recall
that it was sweet and tender. The breaking near the base,
and the sweetness there, indicate that the growing region of

The Utilization of Foods

U. S. Dept. of Agriculture
FIG. 53. A tobacco field in Connecticut. The plants are grown for their leaves.

the grass leaf is at the base. A more exact determination of
the growing region may be made by marking a young grass
leaf into equal spaces with India ink. This will show that as
the leaf develops, it is continually pushed upward and outward
from the node where it is attached. This mode of growth is
characteristic not only of grasses but of many other plants
having parallel- veined leaves.

Net-veined leaves. The net- veined leaves develop differ-
ently from the leaves of either ferns or grasses. An exami-
nation of a growing net- veined leaf will show that all parts are
equally firm. The best method of study is to mark the young
leaf into equal squares by means of two series of parallel lines
at right angles to each other. A geranium or nasturtium leaf
serves well for this purpose. After several days it will be
seen that the only change has been an increase in size of the

9o

Science of Plant Life

V. S. Dept. of Agriculture

FIG. 54. Cutting leaves from the sisal (a form of century plant) for making the
fiber used in the manufacture of binder twine in Yucatan.

FIG. 55. Drying sisal fiber in Yucatan.

U. S. Dept. of Agriculture

The Utilization of Foods

squares. The lines in each
direction are still parallel.
This indicates that all
parts of the blade are
growing equally.

These facts regarding
the growth of leaves may
be summarized in a some-
what different way. In the
ferns the last part of the
leaf to mature is the apex.
In parallel-veined leaves a
region near the base is still
in a growing condition
after the other parts are
mature. In net- veined
leaves all parts of the blade
mature at the same time.

Leaves as sources of commercial products. Many plants
are grown or collected for the sake of their leaves. The
most nourishing part of forage crops like hay and alfalfa is
the leaves. Lettuce, celery, and Swiss chard are important
garden leaf crops. The leaves of the tobacco plant furnish
the basis of a world- wide industry. The conductive bundles
from .the leaves of certain Mexican agaves furnish the sisal
fiber used in the manufacture of binder twine. Manila fiber
is made from the bundles of the leaves of a Philippine banana.
The leaves of the tropical palm, raffia, are used by gardeners
for tying up plants, and by others for the making of orna-
mental baskets. Other tropical palms furnish the fibers
from which Panama hats are woven. The eelgrass, which

Bruce Fink

FIG. 56. Hat palm, Porto Rico. The leaves
are split into strips and are used in weaving
hats.

92 Science of Plant Life

grows in shallow water along our coasts, has been found to
be one of the best materials for packing the sound-proof,
fire-proof, and heat-proof walls of apartment houses, factories,
and cold-storage warehouses. Cocain, caff em, digitalis, the
oils of mint and wintergreen, and other substances used in
medicine are derived from the leaves of plants.

Suggestions for Laboratory and Field Work to Precede
Chapter Nine

1. Examine the stem systems of an herb, a shrub, and a tree
and make sketches of them, preferably in the field.

2. Field trip to study herbs, shrubs, and trees, noting the great
variety of forms in each group. A visit to a greenhouse will permit
the study of a variety of tropical species and of methods of prop-
agating plants.

3. Field trip for the study of annuals, biennials, and perennials.
Old fields, gardens, roadsides, and woods will show a variety of
forms. In the case of perennials the underground parts are as
interesting and important as the above-ground shoots. How does
each of these types of plants pass the winter?

93

CHAPTER NINE

HERBS, SHRUBS, AND TREES

EVERY one who has occasion to grow plants needs to know
something about the length of life of the plants he is concerned
with, and he must know also whether they have herbaceous
or woody stems. For example, suppose a farmer wishes to
determine whether it will be more profitable to grow sweet
clover or alfalfa in a certain field. Before planting either of
these crops, he should know that one of them is a biennial and
the other a perennial, because all his plans for handling the
crop will depend on this information. Or suppose that an-
other man wishes to have a permanent border of flowering
plants about his lawn to obstruct the view of some unattrac-
tive fields or buildings. He can choose wisely from among
the hundreds of plants listed in nursery catalogs only when he
has definite information about the longevity of the plants
and as to whether they are herbs, shrubs, or trees. A clear
understanding of the classification of plants on the basis of
their length of life, their woodiness, and their tendency to
form single large trunks or a number of stems is helpful also
in any study of the structure and processes of stems.

Longevity of plants. Plants differ greatly in their length
of life. To indicate the length of the natural life periods, the
terms annual, biennial, and perennial are commonly applied
to plants. It is important that these terms be clearly under-
stood before the subject of the tissues and their arrangements
in stems is taken up, because in each of these length-of-life
classes certain characteristics are associated with the form
and development of the stem.

Annuals. Most of our common garden vegetables and
field crops are started from seeds in early spring. The seeds

94

Herbs, Shrubs, and Trees

95

germinate ; roots and shoots develop ; and by midsummer or
autumn, flowers and fruits are produced and new seeds, which
contain the beginning of another generation
of plants, are formed. Then the plants die.
The period from seed germination to seed pro- ^

duction is called the life period. If it is com- Jj|^
pleted within a single growing season, the plant
is called an annual (Latin : annus, year) . Corn,
lettuce, radishes, beans, pumpkins, morning-
glories, and ragweeds are familiar annual plants.
Biennials. During the first season some
plants develop only leaves and roots and a very
short stem. The root is usually large and accu-
mulates a considerable amount of food. In the
second season growth is re- .

newed, and there is developed
an upright stem with leaves,
flowers, fruits, and seeds.
These plants which pass a
winter season during their
vegetative development, and
whose life period includes two
different growing seasons, are
called biennials (Latin : bien-
nium, space of two years). The seeds of some common
weeds, like the shepherd's purse, evening primrose, and wild
lettuce, germinate in August or September, and a little rosette
of leaves is formed close to the ground. Food accumulates in
the root until winter comes. The following spring the plants
make rapid growth, and by midsummer they have blossomed,
produced seed, and died. In spite of the fact that their whole

FIG. 57. Moth mullein, a biennial: first
season rosettes (in foreground) and
mature plant.

96 Science of Plant Life

life is passed within a twelvemonth period, these plants are
called biennials, because their life period covers parts of two
vegetative seasons.

The term annual or biennial as applied to plants, therefore,
does not imply any definite length of life in months. Wheat
may be grown either as an annual or as a biennial, depending
upon whether it is planted in the spring or in the fall.1 Shep-
herd's purse and wild lettuce not infrequently live as annuals
in nature. The commonest biennials of the garden are beets,
carrots, parsnips, turnips, and cabbage. In the first four,
large amounts of food are accumulated in the roots; in the
cabbage the food is stored in the enormous terminal bud, the
" head." These stores of food are used in the production of
seeds the following year.

Usually biennials and annuals are herbs. Biennials, like
annuals, are comparatively small in size, and die after flowers
and seeds have been produced.

Perennials. Perennials (Latin : perennis, lasting through
the year) are plants that live for a number of years. Some of
them, as for example certain grasses, produce seed during the
first and succeeding years. Other perennials, like alfalfa, form
seed at the end of the second and succeeding seasons. Trees
and shrubs usually require several seasons' growth before
seeds are produced. The century plant of our southwestern
deserts develops vegetatively for 25 or 30 years before it pro-
duces a flowering stem and seeds. Then it behaves like an
annual or a biennial, for as soon as the seeds are mature the
whole plant dies. This calls our attention to the interesting

1 When wheat is planted in the spring and grown as an annual, it is called
" spring wheat." When it is planted in the fall and grown as a biennial, it is
called " winter wheat." The same variety may be grown either as spring 01
winter wheat, but in practice different varieties are grown.

Herbs, Shrubs, and Trees

97

fact that in annuals, biennials, and a few perennials there is
no well-marked period of senility or old age. They die sud-
denly at maturity, immediately after
their period of greatest vigor. Trees
and shrubs, on the contrary, have a
distinct period of old age in which
the physiological processes are slowed
down gradually until the plants suc-
cumb to diseases and unfavorable
conditions which they could have
withstood in youth.

Perennials classified according to
the persistent parts. All perennials
add new leaves, new stems, and new
roots each year; but they may be
classified roughly according to the
parts that persist from one season to
the next.

Evergreen trees and shrubs are
perennial in all parts of the plant FIG. 58. Century plant (Agave),

-, T-^ . , JIT showing rosette of fleshy leaves

body. Deciduous tees and shrubs and flowering stem It is a
are perennial in their stems and roots, perennial, but, like an annual
Many herbaceous perennials, like the °r a bienni^' .ll dies w en ll

•* flowers and fruits.

cat- tails, swamp mallows, peonies,

trilliums, and bananas, have annual above-ground stems but
perennial underground stems and roots. Dahlias and sweet
potatoes have perennial roots. Potatoes and the Jerusalem
artichoke (a kind of sunflower) have perennial thickened
underground stems (tubers). Tulips and hyacinths have
perennial underground stems (bulbs). These examples show
that perennial plants have many different ways of bridging

98

Science of Plant Life

FIG. 59. Group of California Big Trees (Se-
quoia gigantea). An idea of their size and
height can be gained by comparing them
with the man standing at the left of the picture.

over unfavorable seasons
like periods of cold or
drought.

There seems to be no
limit to the length of life
of some perennial herbs,
like ferns, the May apple,
Solomon's seal, and cer-
tain grasses and mints.
The older parts die each
year, and new parts form
at the other ends of the
underground stems. The
plants change their loca-
tions slightly each year,
one end of the stem grow-
ing forward and the other
end dying away. There
is no apparent reason why
such plants should not
live indefinitely, perhaps
longer than the oldest
trees; but no one part of
the plant lives for a long
time.

Herbs, shrubs, and trees. Shrubs and trees have woody
stems. The stems of herbs lack woody tissues. Our garden
and field crops are all herbaceous plants. Their stems con-
tain no woody tissue and, for this reason, in temperate climates
the above-ground parts live but a single growing season.

The principal difference between shrubs and trees lies in

Herbs, Shrubs, and Trees

99

the fact that shrubs develop numerous slender above-ground
stems from a single base, while trees develop a single stem or
trunk. This distinction may be
expressed in another way by
saying that shrubs branch un-
derground, while trees branch
only above ground. Most
shrubs are less than 10 feet in
height, but some, like the stag-
horn sumac, may reach a height
of 20 feet. Most trees are be-
tween 25 and 200 feet in height,
but the eucalyptus tree of Aus-
tralia and the giant sequoias of
California are over 300 feet in
height, and the massive trunks
of the latter may be more than FlG. 6o. Japanese dwarf pine. Some

30 feet in diameter. However, of these sma11 Potted trees are a cen-

the distinction between herbs,

shrubs, and trees is not one of size. Herbaceous plants, like
the corn and sunflower, may reach a height of over 15 feet
and the banana a height of 30 feet, while some shrubs are only
a few inches in height and some of the dwarf trees of Japan
that are a century old are less than 5 feet in height (Fig. 60).
Plant characteristics and the plant-producing arts. The
differences in the habits of growth, longevity, and materials
stored by plants has led to specialization among those who
grow plants. For many evident reasons the most important
art of growing plants is agriculture. The farmer deals entirely
with herbs and largely with annuals, though biennials and
perennials may be grown for forage crops. He is for the

ioo Science of Plant Life

most part concerned with plants that accumulate foods in a
highly concentrated form.

The growing of trees to create forests for the production of
timber, fuel, and pulp wood is the field of silviculture. The
silviculturist specializes on those trees that accumulate cel-
lulose in the most usable form.

Horticulture embraces a wider range of plants, but in actual
practice a horticulturist usually specializes on plants having
somewhat similar habits. The growing of food-producing
shrubs and trees represents one division of horticulture.
The object sought is the production of fruits containing
pleasantly flavored substances stored in cells with the thin-
nest possible cell walls. The vegetable grower specializes
on annuals and biennial herbs that accumulate both food
and flavors, and to a less extent on perennials, like asparagus
and rhubarb. Floriculture deals with all classes of plants
and has for its object the production of attractive flowers
and foliage. It reaches its highest development in landscape
architecture, in which masses of vegetation are arranged to
beautify a landscape.

Suggestions for Laboratory and Field Wo*-k -to.
Chapter Ten

1. Make accurate drawings of small portions of herbaceous and
woody stems, noting the various surface features. Goldenrod,
sunflower, aster, gourd, buckeye, box elder, raspberry, elm, ailan-
thus, and hawthorn are particularly suitable. Label the terminal
buds, lateral buds, leaf scars, bundle scars, lenticels, and bud scars.

2. Study branches of spruce and apple, to show growth of dif-
ferent years.

3. Place some twigs of soft maple, ailanthus, willow, and cotton-
wood in water. Make a series of drawings to show stages in the
opening of the buds.

4. Make sketches of several trees to show the deliquescent and
excurrent types of branching.

101

. CHAPTER TEN

STEMS AND THEIR EXTERNAL FEATURES

THE stem usually forms the axis of the plant and bears the
leaves, flowers, and fruits. Plants showing all degrees of
stem branching are found, from the unbranched palm and
corn to the finely divided asparagus and elm. In most
plants the stems are upright, aerial structures ; but in
some plants they lie on the surface of the soil, in other plants
the main stem is underground and only the branches rise
above the surface, and still other plants have the entire stem
underground. The upright stem is the common type and has
many advantages over a horizontal stem. At its lower end
it bears one or more roots that connect the plant with the soil.

Advantages of upright stems.
The photographer uses light to
effect chemical changes in pho-
tographic papers and plates,
and the plant uses light to bring
about chemical changes in its
green tissues. The photog-
rapher who uses sunlight for
his work usually locates his
studio at the top of a tall build-
ing, because there he avoids
the shadows of near-by build-
ings and secures a more con-
stant exposure to light. The
same advantages come to the
plant that has its leaves raised
well above surrounding plants :
the leaves are in less danger of

FIG. 61. Sunflower and burdock, show-
ing advantage of upright stem in leaf
display.

102

Stems and Their External Features

103

being shaded, and each day
they are exposed to the sun-
shine during a longer period.
The tall plant has an
additional advantage in
being able to expose to the
light a greater leaf area
over a given space of
ground, because it can dis-
play several or many layers
of leaves one above the
other. The rosette of
leaves formed by the bur-
dock illustrates the possi-
bilities of leaf display near
the soil. A large sunflower
plant covers no greater soil
area than a burdock, but
it is able to expose to the
sunlight several times as
great a leaf area, because

the sunflower leaves are placed at several different levels (Fig.
6 1 ) . Trees have the greatest stem development and the greatest
leaf display. Rosette plants, like the dandelions and plantains,
represent the opposite extreme of slight stem development,
small leaf area, and a poor leaf display. One advantage in a
tall, upright stem is that it holds the leaves up to the light and
thereby makes possible a greater leaf display.

Plants with upright stems have advantages also in connec-
tion with reproduction and seed dispersal. Wind-pollinated
flowers, like those of corn or the pine, are better exposed to

Edwin Hale Lincoln

FIG. 62. An American elm in the Berkshire
Hills, Massachusetts. The tall stem of such
a plant makes possible the display of a large
number of leaves to the light and facilitates
the production and dispersal of seed.

104 Science of Plant Life

air currents when they are borne on an upright stem, and the
display of insect-pollinated flowers high up on a stem is
advantageous because the flowers are more readily found
by insects. Moreover, seeds are likely to be distributed
more widely when they fall from tall plants. For example,
a maple seed dropped from a height of 50 feet is exposed
to air currents during its fall and is almost sure to reach
the ground at some distance from the point directly below
the starting place. A second advantage in an upright stem
is that it facilitates the production and dispersal of seed.

The advantages of the upright stem are all dependent on its
capacity to support other organs. The stem must be strong
enough to support leaves, flowers, and fruits. The city sky-
scraper needs first of all a strong framework about which the
building is constructed. The stems of tall, erect plants must
be correspondingly strengthened by a mechanical structure.
The base of a tree is much smaller in proportion to its height
than that of the tallest and narrowest building, and it is pos-
sible for trees to reach great heights only because their stems
are composed in large part of supporting tissues that have
great strength. Mechanical or supporting tissues are neces-
sary in upright stems.

Advantages of horizontal stems. Horizontal stems have
little or no mechanical tissue, and they display leaves to the
light advantageously only when they grow in the open. There
are advantages in stems of this type, however, because by
growing horizontally on the soil or beneath the surface of the
soil they spread the plant ; because they are in contact with
the soil and may take root at frequent intervals ; and because
they are better protected than upright stems during the win-
ter and other unfavorable seasons.

Stems and Their External Features 105

Conductive tissues necessary in stems. The simplest land
plants are very small and grow flat on the soil in wet places.
They are constantly in contact with the moist soil, and their
cells can be supplied almost directly with water and mineral
salts. In such plants a conductive system is not necessary ;
but if the leaves of a plant are to be raised into the air, water
for transpiration must not only be supplied to them con-
tinuously, but at times it must be supplied in great quantity.
Because of this fact, a plant that raises its leaves even a few
inches above the soil must possess conductive tissues, and
when large numbers of leaves are raised 200 or 300 feet into
the air, a very extensive water-conducting system is neces-
sary. The water-conducting tissues are of vital importance in
the stems of complex plants.

The roots and stems require a continuous supply of food for
repairing old cells and for building new ones. Since the foods
are manufactured primarily in the leaves, there must be food-
conducting tissues that are adequate to carry them to all
parts of the stem and roots. The food-conducting tissues
also transfer food from the leaves to the seeds and growing
parts, and when food has accumulated in the stem or roots
it may pass up through the conductive tissues of the stem to
other parts of the plant. Food-conducting tissues are neces-
sary in stems to transfer food within the plants.

The stem as a place of food accumulation. Because of the
volume of the stem it is natural that excess food should ac-
cumulate in it. Under favorable conditions, foods for which
the plant has no immediate need are continuously passing
from the leaves into the stem. Consequently, in our larger
plants the stem is the place of temporary storage and the center
from which foods are distributed as needed. In some plants

106 Science of Plant Life

the process of accumulation becomes one of the most im-
portant functions of the stem, and it may include the ac-
cumulation of water as well as of food (page 75).

Stems as photosynthetic organs. In plants with little or
no leaf display, the stems may do all or most of the photo-
synthetic work. The night-blooming cereus and other cac-
tuses, and asparagus and equisetum, have no leaves. The
green stems of most herbaceous plants contribute at least a
part of the carbohydrates, and the young twigs of many
woody plants are green and carry on photosynthetic work.

Physiologically, then, the stem is an organ that supports
and displays leaves to the light ; aids reproduction by ele-
vating the flowers and seeds ; conducts water to the leaves for
transpiration and photosynthesis ; carries food to its own
living cells and to those of the roots ; is a place for the tem-
porary accumulation of food materials ; and may carry on
photosynthesis.

External features of woody stems. On a woody stem nodes,
leaf scars, buds, and lenticels may be seen. The nodes are the
places where the leaves arise, and they are the most prominent
external feature of stems. The arrangement of leaves at the
nodes has already been discussed (page 37). In addition to
the leaf, the node gives rise to one or more buds, just above
the place of leaf attachment, in the so-called axil (Latin :
axilla, armpit) of the leaf. The leaf scars are markings on the
stem where leaves have fallen. The part of a stern between
two nodes is called an internode. The lenticels are small,
dotlike elevations scattered over the surfaces of the internodes.

Buds. Stems and branches produce leaves only once. We
are accustomed to speak of deciduous trees clothing themselves
with a new set of leaves each spring, as though the branches

Stems and Their External Features

107

of the previous year put forth a new set of leaves to replace

those lost the preceding autumn. As a matter of fact, when

we look at a deciduous tree

in winter, we see branches

and twigs, all of which have

borne leaves and none of

which will ever bear leaves

again. The possibility of

producing new foliage lies

in the development of new

branches and twigs. This

is the function of the buds ;

from them the new growth

of each year takes its rise.

The buds of many tropi-
cal plants are like those we
see at the tops of the stems
of garden vegetables. Such
a bud consists of the stem's
growing point and the un-
developed leaves, with no
special coverings of any
kind. These naked buds
occur also on the under-
ground stems of some of
our Northern plants. A
simple sort of bud covering,
which is common in the FlG 63 Twigs of smilax M)> buckeye (B}>

tropics, is made by the fold- and tree-of -heaven (Ailanthus) (C). The

ing together of the Stipules. ^ SCalCS are Designated by «; b and h are

°4 * ^ buds; c is a leaf scar, d a bundle scar, e a

This type Of bud Covering lenticel, / a terminal bud scar, and g a tendril

io8 Science of Plant Life

may be seen in the tulip tree and the magnolias of temperate
climates. The buds of most temperate perennials are covered
with scales. Not infrequently the scales are further covered
with matted hairs and secretions of wax and resin. These all
tend to make the bud coverings impervious to water. In this
way the tender growing parts are protected from excessive loss
of water during the winter and during the still more critical
stage in early spring when the buds are opening.

We are likely to think of buds as being formed at about the
time when the leaves fall from the trees. A good observer,
however, will have noted that the buds begin to develop when
the leaves unfold in spring, and that they grow all summer
long. Because of the prominence of the leaves, the buds are
obscured somewhat during the summer months, and become
conspicuous only after the leaves are gone,

The opening of buds. When the warm weather of spring-
time comes, the innermost bud scales begin to grow and
expand. Sometimes the outer scales are pushed off ; some-
times they elongate and grow like the inner ones. But the
scales quickly reach their full growth, and soon they are cut
off by the formation of an abscission layer at the base of each.
In the buds of a few plants all the scales are dead and are
pushed off by the growth of the stem and leaves inside. The
expansion of bud scales and leaves takes place almost wholly
through the enlargement of cells 'already formed. Within
the bud the minute leaf cells absorb water and develop large
vacuoles. The expansion of these cells results in the en-
largement and spreading of the leaves. Material for the study
of the different habits of bud expansion may be secured in
winter by bringing branches of different kinds of trees into a
warm room and placing them in water until the leaves expand.

Stems and Their External Features 109

Kinds of buds. Every bud contains the growing point of a
stem. In addition, most buds contain the beginnings of
foliage leaves ; that is, the leaves have already begun to de-
velop on the sides of the young stem within the bud. Some
buds, as for example many of those on the maples and elms,
contain the beginnings of flowers. Other buds, like some of
those of the catalpa and the horse-chestnut, contain both
leaves and a flower cluster. Bulbs are really a special under-
ground form of bud, and they are similar in structure to other
buds. We shall consider bulbs when we come to the study
of underground stems.

Bud development and plant form. Buds which occur at
the ends of stems are called terminal buds ; those which occur
at the nodes are called lateral buds. This classification is
useful because only a part of the buds on a stem ever de-
velop and because the form of a plant depends on which set
of buds develops more freely and grows more rapidly. In
most plants, the terminal bud simply extends a stem or
branch ; the lateral buds produce new branches. Plants
with very strong terminal buds tend to become columnar in
form, like the large, unbranched sunflowers of the garden or
like the spruce and palm among trees. Plants with strong
lateral buds tend to branch continually and to become bushy
in form, like the lilac and hydrangea. There are all grada-
tions between these extremes in the development of the
terminal and lateral buds, and in the resulting plant forms.

In many roses the shoots from the base of the stem develop
only through their terminal buds the first year. The shoot is
thus extended to great length by the first season's growth.
The following year the lateral buds develop, and the long
shoot becomes highly branched. As these lateral branches

no

Science of Plant Life

FIG. 64. Date palms in fruit, on an oasis in an Algerian desert. The terminal
bud continues its development year after year and builds up the long, un-
branched stem of the tree. (From photo U. S. Dept. of Agriculture)

Stems and Their External Features m

FIG. 65. Pine trees on Wood River in Oregon. The strong terminal bud
continues its development, and the excurrent stem is the result.

bear the flowers and produce them abundantly only once, we
can promote flowering in these roses by trimming away each
year all but the long, unbranched shoots. In many other
shrubs, as spiraea, barberry, and privet, a few strong lateral
buds at the surface of the soil develop each year. This ac-
counts for the basal branching of these plants.

Excurrent and deliquescent stems. When trees have
strong terminal buds, the main stem extends to the top and
is called excurrent (Latin: excurrens, running out). The
spruce has a strong terminal bud, and just beneath it several
smaller lateral buds. The terminal bud grows upward, and
the lateral buds grow outward, forming a whorl of branches
at the base of the season's growth. This is repeated each
year, the terminal shoot lengthening the stem, and the lateral
buds adding a new whorl of branches. Consequently each

112

Science of Plant Life

,v

FIG. 66. A hackberry in Illinois. The de-
velopment of the lateral buds results in the
formation of a deliquescent stem.

year's growth is marked
by a whorl of branches,
and the age of a tree may
readily be estimated by
counting the number of
whorls on the stem. Since
the oldest branches are
nearest the ground they
are the longest, and the
tree becomes cone-shaped
as it grows.

The terminal buds of the
elm tree seldom survive the
winter. The lateral buds
develop, and the main stem
divides and subdivides un-
til it is lost in the crown

of the trees. This gradual dissolving of the trunk into a
spray of terminal branchlets suggested the name deliquescent
(Latin : deliquescens, dissolving) for this type of stem. We
see, therefore, that the excurrent type of stem depends on
the continual development of terminal buds, while the deli-
quescent type depends on the growth of lateral buds. Con-
sequently we may modify the forms of plants in cultivation
by trimming them and so forcing the growth of certain buds.
Lawn trees and shrubs are grown either for shade or for
ornamental effects. We secure shade by trimming off the
terminal buds and so causing many of the lateral buds to
develop into branches and thus form a denser crown. Orna-
mental effects are secured by trimming plants so that they
will be in artistic harmony with their surroundings.

Stems and Their External Features 113

U. S. Dept. of Agriculture (J. Craig)

FIG. 67. Baldwin apple orchard, showing trees with the branches thinned to
increase the production of fruit.

Fruit trees and grapes have been found to produce more
fruit, and fruit of a better quality, when the number of branches
is small. A smaller number of branches on a tree secures an
open crown and permits the sunlight to penetrate to every
leaf, and the removal of some of the branches forces the de-
velopment of flower buds which might remain dormant if
the terminal and branch buds were allowed to grow uninter-
ruptedly. In grape culture, only four or five branches are
allowed to remain on a vine each year, and these branches are
shortened. This insures full development for a few of the
flowering branches and the production of the best quality of
fruit.

Leaf scars and bud scars. The leaf scars on some plants
are round ; on others they are narrow lines ; on most plants
they are crescent-shaped. Usually they are smooth, except

114

Science of Plant Life

for small, dotlike markings. These markings are bundle
scars; they show where the bundles of conductive and me-
chanical tissue extended outward from the stem into the
petiole. The shape of the leaf scar and the arrangement of
the bundle scars are so characteristic for many kinds of trees
that they may serve to identify the tree in winter.

The bud scales also leave scars when they drop. These
scars are usually numerous and so closely crowded that they
form a roughened ring about the stem. The terminal-bud
scars occur at intervals, surrounding the stem or branch. The
lateral-bud scars occur only at the bases of the branches and
twigs.

Determining annual growth of shoots from terminal-bud
scars. Since the terminal bud marks the end of each year's

growth, the terminal-bud scars
mark off a perennial stem into
segments, each of which repre-
sents the growth of a single
year. Often an interesting
life history is suggested by the
varying length of the intervals
between the bud scars on a
particular stem. By a study
of these intervals we can de-
termine the seasons that were
favorable and those that were
unfavorable on account of
drought, excessive rain, attacks
of insects, or some other cause.
In the pines and spruces the

FIG. 68. Norway spruce, showing whorls .

of branches at end of each year's growth, years growth IS marked Ott

Stems and Their External Features 115

not only by the bud scars, but also by whorls of branches.
Differences in the color of the bark and in its texture will also
help to distinguish successive annual stem segments in most
trees.

Lenticels. All living cells require energy. This is largely
obtained from respiration. Therefore, in addition to a con-
stant food supply, the cells of the stem must have access to
oxygen. As in the leaves the oxygen is supplied through the
intercellular spaces, so in stems there must be sufficient in-
tercellular spaces to permit oxygen to diffuse inward and
carbon dioxid to diffuse outward. There must also be open-
ings through the epidermis or bark to connect these inter-
cellular spaces with the outside atmosphere.

The young green stems of all plants have stomata. Peren-
nial stems, however, soon develop a corky layer beneath the
epidermis, which cuts the cells in the interior of the stem off
from the stomata. While this layer is developing, masses of
round, loose cells form beneath some of the stomata, pushing
out and tearing the epidermis above them. These open places
are the lenticels. They permit gas exchanges, and in older
stems take the place of the stomata. The lenticels of most
trees and shrubs are closed in the late autumn by the growth
of a thin layer of cork beneath them. The following spring
loose cells are again formed at the same point, the cork is
burst open, and the lenticels again permit gas exchanges.

In the cherry and birch the lenticels persist for many years
and become elongated transversely, forming granular rings
part way around the' stem. In the trunks of thick-barked
trees, the lenticels occur in the furrows of the bark.

n6 Science of Plant Life

PROBLEMS

1. What advantage in resisting wind have tall, columnar tree trunks over
equally tall smokestacks or monuments? What disadvantage?

2. What are the perennial food-producing plants of your locality?

3. Find out how your local gardeners trim their grapevines, berry bushes, and
fruit trees. Secure definite information for five of these plants, and deter-
mine the reasons underlying the practices.

4. What are the best trees for street planting in your locality? What trees
now planted there are objectionable? Why?

5. Compare a tree growing in an open field with one of the same species grow-
ing in the woods. Account for the differences in arrangement of branches
and leaves.

6. Which will furnish the better lumber, a tree grown in the open, or one grown
in the forest? Why?

Suggestions for Laboratory Work to Precede Chapter Eleven

1. Sketch cross sections of some monocot stems like corn,
bamboo, asparagus, lily, or wheat. Note particularly the arrange-
ments of the bundles. Cut a longitudinal section through a node,
and note how the bundles unite in the node.

2. Sketch a cross section of a peach branch that is an inch or
two in diameter. Note pith, wood, bark, cortex, cortical paren-
chyma, cork, cambium, annual rings, and pith rays.

3. Sketch a cross section of a pine stem and note the parts, as
in the peach.

4. With a microscope, examine thin sections of monocot, dicot,
and conifer stems, and note differences in the structures of the
bundles.

5. Examine sections of tree trunks and determine the number of
annual rings in the sap wood and in the heart wood. What fraction
of a large tree trunk is alive ?

6. Bring a branch of a tree to the laboratory and try to set up
cleft and whip grafts. Also try to set a bud. Repeat the opera-
tion on trees out of doors and see whether or not the cions will live.

7. Show the region of growth by marking the upper end of a
stem with transverse lines at equal intervals.

8. Show the effect of gravity on the direction of growth by
changing the position of a growing stem with reference to gravity.

9. Show the effect of light on the direction of stem growth by
placing a plant where light can reach it from only one direction.

117

CHAPTER ELEVEN

THE STRUCTURES AND PROCESSES OF STEMS

IF we study the development of a stem from a bud, we find
that the growing point is made up of very minute cells, all of
which are practically alike. These cells divide to make other
cells like themselves, and the lower ones begin to enlarge. In
this way the growing point is pushed forward and the di-
ameter of the stem increased. Upon these two processes, cell
division and cell enlargement, the growth of the stem depends.
Then certain groups of cells begin to take on special forms.
The cells that are to form the bundles elongate ; some of them
develop woody walls. Others elongate but remain thin- walled,
and these form the food-conducting tissue. The other tis-
sues of the stem are composed of cells which have enlarged
and have become rounded or variously angled, and which
have their walls more or less thickened. These cells form the
pith or soft inner part of the stem, and the cortex or outer
portion. In this way the various tissues of stems arise from
the small uniform cells of the growing point.

Stem structures and plant groups. There are three groups
of seed plants that we wish to distinguish at this time because
the stems of the plants that belong to these groups differ
fundamentally. These groups are : (i) the conifers, or cone-
bearing trees, like pines, spruces, firs, and cedars, that have
scale or needle leaves and are for the most part evergreen;
(2) the monocotyledonous plants (monocots), or plants with
parallel- veined leaves, like the grasses, lilies, cannas, orchids,
and palms; and (3) dicotyledonous plants (dicots), or plants
with net- veined leaves, like oaks, maples, sunflowers, asters,
and clovers.

The stems of the plants belonging to these three groups

118

The Structures and Processes of Stems 119

Epidermis and
cuticle

FIG. 69. Stem of moonseed vine, showing tissues and their arrangement.

120 Science of Plant Life

differ in (i) the kinds of tissue making up the bundles, and
(2) the arrangement of the bundles in the stem. We shall
first study the bundles and the arrangement in a dicot stem,
and then we shall learn how the stems of monocots and
conifers differ from those of dicots.

The structure of a dicot stem. When a dicot stem is cut
across, the bundles are seen to be arranged in a ring. The
cylinder of tissue lying inside the bundle cylinder is the pith ;
outside the bundle cylinder is the cortex ; and covering the
cortex is an epidermis very similar to that of leaves (Fig. 69).
In older and harder stems the outer cortical cells have thick
walls and form a corky or hard outer covering that replaces
the epidermis. The pith and the inner part of the cortex
are made up of rounded, thin- walled cells called parenchyma.
In annuals and young perennials the cortical parenchyma con-
tains chlorophyll and resembles the mesophyll of the leaf in
appearance and function. It is this tissue that forms the
inner " green bark " of twigs and gives the green color to the
stems and branches of herbaceous plants.

There are, then, four distinct layers in dicot stems : (i) on
the outside is the epidermis; (2) from the epidermis to the
bundles is the cortex; (3) inside the cortex is the hollow
bundle-cylinder ; (4) the pith forms the axis of the stem, fill-
ing the space inside the cylinder of bundles (Fig. 69) .

Between the bundles of the dicot stem there are strands of
parenchyma cells that connect the pith parenchyma with the
cortical parenchyma. These are the pith rays. They con-
vey food across the stem, and with the other parenchyma
cells form a complex tissue system in which excess foods
accumulate and from which they later move to other parts of
the plant.

The Structures and Processes of Stems 121

General structure of the dicot bundle. The bundles in a
plant stem terminate above in the veins of the leaves, and
below they connect with the bundles of the roots. . In the
dicot stem these bundles contain four tissues : (i) the water-
conducting tissue, (2) the food-conducting tissue, (3) the
cambium, and (4) the mechanical tissue. The cambium is a
layer of thin-walled cells that lies lengthwise in the bundle and
separates the inner water-conducting tissue from the outer
food-conducting tissue.

Tissues of the dicot bundle. The water-conducting tissue
contains long, tubelike vessels made up of cylindrical cells
joined end to end, often for considerable distances without
end walls between them. These tubes (tracheae) usually
have heavy walls marked by spiral and lattice-form thicken-
ings. When mature they are empty of protoplasm. In
other words, they are the coverings of dead cells joined to-
gether to form tubes usually several inches, more rarely several
feet, in length. Mixed with them are smaller and shorter
tubes, and cylindrical cells that retain the cell contents.
All together these tissues form the passageway for the move-
ment of water and mineral salts, and sometimes sugar, to all
parts of the plant. The general direction of the water move-
ment in this tissue is upward, because the lifting of the water
is brought about principally by transpiration from the leaves
(page 134).

The food-conducting tissue differs from the water-conducting
tissue in being composed of smaller, thin-walled cells, all of
which retain their living protoplasm. The largest of these
cells are set end to end, and the end walls have holes in them
like the top of a salt shaker. These rows of cells, therefore,
form tubes with sievelike cross walls in them, and on this

122

Science of Plant Life

FIG. 70. Cross section of a portion of
rootstock of calamus, photographed
through a microscope. The circular
areas are the cells which are filled with
starch.

account they are called sieve
tubes. Through the openings
in the sieve plate the proto-
plasm is continuous from cell to
cell, and through these tubes
the foods pass from one part
of the plant to another. Be-
cause the cells of the stem and
root are supplied with food
manufactured in the leaves, it
is often said that the move-
ment of foods is downward in
a plant. In reality, the direc-
tion of the food current is not
so fixed as is that of the water

current. Food moves toward any part of the plant where it
is being used or is being accumulated. For example, in mid-
summer when a tree is in full leaf and the season's growth
has practically been completed, food moves out of the smaller
branches into the larger branches and the trunk. In the
spring, when leaves and blossoms are developing, food is
being used in the twigs, and the direction of the movement
of food materials is reversed.

The mechanical tissue is made up of cylindrical or spindle-
shaped cells with very heavy walls. Indeed, the walls at
maturity may be so thick as to render the cells almost
solid. Ordinary cellulose is not very hard, but the walls
of the mechanical tissue are hardened and thickened
by a deposit of a substance called lignin. The differ-
ence between hard and soft woods is for the most part
due to the thickening of the walls of the mechanical cells ;

The Structures and Processes of Stems 123

secondarily it is due to changes in the walls themselves
(lignification) .

Annual rings

Tracheae

Pith

Pith

ravs

FIG. 71. Block of oak wood magnified to show the arrangement of the various
tissues which produce the patterns on polished wood surfaces. (Diagrammatic.)

Mechanical tissue is found on both the water-conducting
and food-conducting sides of the bundle. On the food-con-
ducting side it lies outside the food-conducting tissue, and is
made up of long, exceedingly slender, nearly solid, spindle-
shaped cells. These cells are called bast fibers, and the tissue

124

Science of Plant Life

bamboo stem photographed through a
microscope. The large openings in each
bundle are the water-conducting tubes.

that is made up of them is
called the bast. Bast may be
seen in the stringy fibers on a
grapevine or in the bark of
trees. It is the bast fibers
from flax, hemp, jute, and
other dicotyledonous plants
that are used in the manufac-
ture of thread and cordage.

The cells of the mechanical

FIG. 72. Cross section of a portion of tissue on the water-conducting

side of the bundle are some-
what shorter and thicker than
the bast fibers. They are

known as wood fibers and make up what is properly called
the wood, although in most dicots the wood fibers are mixed
with the water-conducting vessels, and the whole inner part
of the bundle is known as wood. In woody plants this me-
chanical tissue is present in abundance and forms the bulk
of the stem. The lumber that is obtained from dicotyledon-
ous trees is derived from the inner parts of the bundles and
is made up of wood fibers and water-conducting tissues.

The cambium is a layer of soft tissue between the two sides
of the bundle. It is the principal growing tissue of the dicot
stem. Growth takes place in it by the longitudinal division
of the cells. On its inner face the cells of the cambium layer
change into water-conducting cells or wood fibers ; on its
outer face they change into food-conducting cells or bast
fibers. In this way the bundles of perennial dicots enlarge
from year to year, and this causes the stem to increase in
thickness. In a tree the cambium cells form a continuous

The Structures and Processes of Stems

FIG. 73. Cross section of portion of a
rattan stem photographed through a
microscope. Note the ring of bundles
near the outside, with the heavy-walled
mechanical tissue and the scattered
bundles within.

layer between the wood and
the bark, and the diameter is
increased by the addition of
successive layers of tissues
built by these cells.

Every one who has made
willow or hickory whistles has
become acquainted with the
cambium. In early spring the
cambium cells are dividing
actively, and the cambium
layer can be broken by tap-
ping on the bark. The whole
bark can then be readily
stripped from the wood.

The monocot stem. The monocot stem, like dicot and
conifer stems, is bounded externally by an epidermis which
closely resembles that of the leaf. The groundwork of the
stem is made up of parenchyma, which is commonly called
the pith. The parenchyma is usually composed of thin-
walled cells, and is the principal tissue for the temporary ac-
cumulation of foods ; from it the sugar solution is obtained
when the stems of sorghum and sugar cane (Fig. 48) are crushed.
In a monocot stem the bundles are scattered, instead of being
arranged in a cylinder as they are in a dicot stem. In the
hollow stems of grasses they are scattered through the cyl-
inder of parenchyma tissue ; in a cornstalk, a shoot of as-
paragus, or the trunk of a palm they are distributed through
the whole stem. As in the dicot and conifer bundles, the
water-conducting tissue is on the side next the center of the
stem, and the food-conducting tissue is on the side toward

126

Science of Plant Life

the epidermis. The scattered arrangement of the bundles
in the pith may easily be seen in a stalk of corn.

FIG. 74. Plantation of abaca, a form of banana, from the petioles
of which Manila fiber is obtained. Abaca flourishes only in the
Philippines. The fiber is used chiefly in the manufacture of ropes.

The monocot bundle. The monocot bundle differs from the
dicot bundle in that it lacks a cambium layer. It is frequently
called a closed bundle, because in the absence of cambium

The Structures and Processes of Stems 127

Bureau of Agriculture, P. I.

FIG. 75. Stripping abaca for fiber. The long petioles are pulled under toothed
knives which scrape the soft tissues from the bundles. Abaca is a monocot, and
the fiber is composed of an entire bundle.

tissue the bundle cannot increase in size, and there can be no
growth of the monocot stem through the multiplication of
cambium cells. The dicot bundle, on the other hand, is
spoken of as open, because there is a cambium layer between
its water-conducting and food-conducting tissues and the
bundle can increase in size. The monocot bundle differs
further from the dicot bundle, in that its mechanical tissues
form a complete sheath about the food and water conducting
parts. It is as though the bast of the outer part of the dicot
bundle and the wood of the inner part were joined at the sides
of the bundle to make a sheath about the conducting tissues.
The fibers like sisal and Manila hemp, that are derived
from the monocots, are usually coarser than the fibers derived
from dicots, because the monocot fibers are entire bundles,

128 Science of Plant Life

while the dicot fibers are made up of only the strand of bast
cells from the food-conducting side. The bundle sheaths
are usually thicker in the bundles near the outer part of the
monocot stem. In fact, in some monocots like rattan and
bamboo, the sheaths of adjacent outer bundles may join each
other and thus form a hard layer beneath the epidermis

(Fig. 73)-

The structures of conifer stems. The conifers, like the di-
cots, have their bundles arranged in a hollow cylinder. In
structure these bundles are somewhat similar to those of
dicots, except that the wood and water-conducting cells are
not distinct. The wood cells form the water-conducting tis-
sue as well as the mechanical tissue. In keeping with their
double function, the cells (tracheids) are thick-walled and
spindle-shaped, with numerous thin places, or pits, in two of
the walls. Because of this structure, the stem retains its
rigidity and still permits the ready passage of water and min-
eral salts.

Growth of stems. The limit of growth of a stem is not so
definite as that of leaves. The length and the diameter of
a stem depend largely upon the conditions under which the
plant lives, the available water supply, amount of light,
temperature, and quality of the soil. Along a dry roadside
a ragweed may complete its development with a stem less
than 6 inches long, while in a rich bottom-land field the same
plant might have reached a height of 15 feet.

The growth in length takes place at the apex of a stem, the
growing point being located in the terminal bud. The grow-
ing region extends back from the tip, sometimes for only a
fraction of an inch, more rarely, as in rapidly growing vines,
a foot or two. If we mark the upper portion of a growing

The Structures and Processes of Stems 129

stem into equal spaces, we may observe on the following day
that the uppermost space has elongated the most. The ad-
joining spaces below are less and less elongated. This indi-
cates that the greater part of cell division and enlargement
takes place very near the tip (the growing point), but that
some growth takes place in the cell layers for a certain dis-
tance below the end of the stem (the growing region).

Annual stems increase in thickness until the plant matures.
This increase in size is brought about by the enlargement of
cells and by the formation of additional cells. Shrubs and
trees increase in thickness each growing season. This is often
called secondary growth ; as we have seen, it is brought about
by the continued growth of the cambium. This layer of cells
produces new water-conducting tissue and wood fibers on its
inner side, and it produces food-conducting tissue and bast
fibers on its outer side. As growth proceeds from year to
year, annual rings mark the successive additions to the wood.
The bark also develops annual layers, but in most woody
plants these are much thinner and less conspicuous than the
annual layers of the wood. Further, since growth takes place
inside the cortex, the cortex is continually being split and
broken. The outer layers may die and after a few years will
be gradually weathered off. The ridges and grooves of the
bark show how much too small the outer bark is to cover the
more recently formed wood. Smooth, thin-barked trees lose
their bark very rapidly. Trees with bark that is thick and
has large ridges are the ones that hold their bark more tena-
ciously. But in all large trees the bark contains only a part
of the cortical layers that have actually been formed ; much
material has scaled off and fallen away. It should be noted
that as a tree increases in diameter, the annual rings of wood

130 Science of Plant Life

in the stem are each year farther removed from the corre-
sponding annual layers of the bark.

Annual monocots increase in thickness through the enlarge-
ment of the bundles and by the multiplication and enlarge-
ment of the pith cells. Perennial monocots, like the bamboo
and asparagus, have underground stems to which new and
thicker stem segments are added each year. The aerial, erect
branches never increase in size after they are once mature ;
but the erect branches from old underground stems are from
the beginning much thicker than those from young plants.
Consequently, no little bamboo rod could ever grow into a
bamboo beam. No large bamboo beam was ever a slender
rod. These aerial branches come out of the ground nearly
as thick as they will be when mature. Asparagus plants are
several years old before the underground stems send up thick,
upright branches suitable for marketing.

Heartwood and sapwood. As the trunks of trees increase
in thickness, all the living cells toward the center of the stem
gradually die. The wood usually changes in color after the
death of these cells. In a peach tree only the outer three or
four annual rings may be alive. In a walnut trunk 2 feet in
diameter, all but the outer 2 inches may be dead. The dead
wood still helps to support the enormous weight of the tree
top, but it has nothing to do with the conduction of water and
substances in solution. This inner dead wood is called the
heartwood; the outer living wood is called the sapwood. The
heartwood in many species of trees is much more valuable
than the sapwood for lumber, because of its color and greater
durability.

Stems in relation to gravity. The direction of growth of
stems is for the most part determined by gravity. The erect

The Structures and Processes of Stems 131

type of stem grows upright even in darkness. If these stems
are laid horizontally, the younger parts will grow faster on
the lower side and the stem will again become erect. This
response of a plant organ to gravity is called geotropism.1
If the response is in the direction of the pull — that is, toward
the earth — as in the case of a primary root, the organ is
said to be positively geotropic. If the response is in the op-
posite direction, as in most stems, the organ is said to be
negatively geotropic. If the response is sidewise, as in many
branches, the organ is said to be transversely geotropic.

Stems in relation to light. Light affects also the direction
of growth of stems, as the plant grown at a window will show.
Most stems grow toward the strongest light. Response to
light is called phototropism, and most stems are positively
photo tropic. The stems of some prostrate plants are held
close to the ground by their response to light, as is proved by
the fact that if a shade is placed over them, the stems become
erect. Some of the common doorweeds of paths and waste
places are examples. There are some prostrate stems, of
course, that lie flat on the ground because of a lack of me-
chanical tissue to hold them upright.

The direction of growth in branches is a compromise be-
tween the response to light and the response to gravity. In
some trees, like the spruce, the direction of growth in the
branches is in some way controlled by the main stem. If the
top of the main stem is cut off, one or two of the lateral branches
become negatively geotropic instead of transversely geotropic.

1 The responses of organs to external influences like gravity and light are
called tropisms (Greek : trope, turning). "Geotropism" means literally a
turning toward or away from the earth; "phototropism," a turning toward
or away from light; "hydrotropism," a turning toward or away from water.

132 Science of Plant Life

This means that they will grow erect instead of horizontally
and will take the place of the main stem.

Grafting and budding. In the propagation of many
varieties of fruit trees it has been found that seeds are not
satisfactory. Most of our cultivated fruit trees are so highly
variable that their seedlings are not like the parent plants
in quality of fruit. Horticulturists long ago learned to over-
come this difficulty by grafting a twig from the desired variety
of tree on a seedling of a similar tree. The graf ted-in branch
then becomes the top of the tree, and the fruit it bears is like
that of the tree from which it came.

In grafting, the plant that furnishes the root is called the
stock. The twig that is attached to it is called the cion (Fig.
76). In cleft grafting, the top of the stock is cut off. The
stock is then split and two cions with chisel-shaped ends are
placed in the cleft, one on either side, so that the cambium of the
cion is in close contact with the cambium of the stock. The wound
is covered with wax to prevent the drying out of the tissues.
If the cambium tissues are in perfect contact, they will soon
unite. New tissue will grow under the wax and finally cover
the wound. If both cions grow, the weaker one is removed.

Whip grafting is the common method of uniting cions to
small seedlings. Usually this is done at or below the surface
of the soil. Both cion and stock are cut obliquely, and each
is split. The upper half of the oblique end of the cion is
pushed into the cleft of the stock and is bound firmly in pi ace
with raffia or twine (Fig. 76). Again, the success of the graft
depends upon the contact between the cambium of the cion and
the cambium of the stock.

In budding, a T-shaped cut is made on the side of the stock,
through the cortex, down to the cambium. A bud from a

The Structures and Processes of Stems 133

tree of the desired variety, with a small oval piece of wood
and bark attached, is slipped down inside the cortex of the

FIG. 76. Methods of grafting and budding. At left, whip grafting; in middle, cleft
grafting; at right, budding. A is the cion, and B the stock. C shows the cion and
stock joined.

stock and tied firmly in place (Fig. 76). This places the two
cambium layers in contact ; the two pieces unite, and the bud
develops into a branch. The stock is then trimmed, so that
only the branch from the cion bud remains.

Grafting is commonly done in the spring ; budding, in the
early fall. The fruit produced on grafted or budded trees is
usually like that of the cion, regardless of the variety of stock.
However, there are cases in which the cion is modified by the
stock. Discussions of these cases may be found in books on
horticulture. Grafting is usually possible only between
closely related species of plants. Sometimes, however, plants
that are more remotely related may be grafted on each other,
as for example tomato, tobacco, potato, and nightshade, or
the pear, apple, and quince.

134

Science of Plant Life

The lifting of water in

physiology of plants has

FIG. 77. Experiment to show the
lifting power of transpiration and
evaporation. Both tubes were filled
with boiled water and placed in a dish
of mercury. In -C the mercury has
been drawn up by transpiration from
a branch of arbor- vitae (A ) ; in D, by
evaporation from a porous cup (B).

is pulled upward into the
Transpiration is greatest

stems. Nothing concerning the
interested more people than the
transport of water from the soil
to the topmost leaves of trees.
Yet in spite of much observation
and experiment, the process is
still only partially explained.

There can be no doubt that
one of the principal factors in
the rise of sap is the evapora-
tion of water from the leaves.
As the water evaporates from
the cells of the mesophyll in
transpiration, water is drawn
from the adjoining water-con-
ducting tissue of the veins into
these cells to take its place.
Water inclosed in tubes has a
high cohesive power ; that is, it
holds together like a solid. If a
pull is exerted on the upper end
of a column of water in the
vessels of a tree, the column
holds together like a cord or
wire, and the whole column is
pulled upward. As the water
at the upper end of the water-
conducting tissue moves into the
mesophyll cells, additional water
blades, petioles, and stems,
and the largest amounts of water

The Structures and Processes of Stems 135

are being lifted in trees during the summer. If at this season
a hole is bored into the trunk of a tree and an air-tight con-
nection made between this hole and a tube that has its lower
end in a vessel of water, the water is drawn into the stem, not
forced out. This indicates that there is more pull on the water
from above than there is pressure from below. It is known
also that there may be currents moving downward in one
layer of the wood and upward in another, although the gen-
eral direction of water transport is upward to the leaves. It
is certain that the roots do not force the water up into the
tops of trees.

The primary factor, then, in the rise of sap is transpiration ;
the second factor is the drawing of water from the water-
conducting tissue by the mesophyll cells to replace that lost
through transpiration ; the third factor is the cohesion of
water columns in the long strands of water-conducting tissue,
which makes it transmit the pull from the mesophyll cells
all the way down to the roots. In the chapter on roots we
shall learn how the water passes from the soil into the roots,
and to what extent the roots aid in the lifting of water.

The pulling up of water by transpiration is exemplified
when cut flowers are placed in a vase containing water. That
water is drawn up into the flowers may be shown by placing
the stems of white flowers in water colored with red ink.

The flow of maple sap. The water in stems always con-
tains a certain amount of sugar in addition to mineral salts.
In the maple, in the early spring when the days are warm but
freezing still occurs at night, great quantities of sugar pass
into the water-conducting tissue. This sugar comes from the
pith rays and other tissues where it accumulated in the form
of starch during the preceding growing season. With the

136 Science of Plant Life

coming of warmer weather the starch is digested, and the sugar
formed from it diffuses into the water-conducting vessels.

The earlier sap is the richest and apparently comes largely
from the upper parts of the tree. The last sap is more dilute
and probably comes from the roots. The positive pressure
that produces the flow occurs only during the day ; at night
it becomes negative and the sap flow ceases. The causes of the
pressure are only partly known. A portion of it is due to
the expansion of gas bubbles within the tree, but this gas
expansion accounts for only a small part of the pressure.

Whether the flow shall continue for weeks or stop after a
few days is determined by weather conditions ; but just how
the ' several weather factors like changes in temperature or
rainfall bring about the increase and decrease of pressure, is
unknown. Even under the most favorable conditions it is
not possible to draw out of a tree more than 5 per cent of the
food that it contains.

A flow of sap somewhat similar to that in the maples occurs
in many species of trees, as in the birch, butternut, and
hornbeam. Sugar and sirup are made from the hard maples.

PROBLEMS

1. A tree increases in height at the rate of 2 feet a year. When the tree is
5 years old, a nail is driven into the trunk 2 feet from the ground. How far
from the ground will the nail be when the tree is 10 years old?

2. Why is the timber from monocot stems less useful for building purposes
than that from dicot stems?

3. What are the woods principally used in your locality for interior finishing
and for making shingles, posts, and flooring? What properties do these
woods have that fit them for these special uses?

4. What kinds of wood are used in the making of furniture ? Why?

5. How is a log quarter-sawed? Why is quarter-sawed lumber more valuable
than that cut otherwise?

The Structures and Processes of Stems 137

6. Why is lumber seasoned by piling it in lumber yards for months or years
before it is used ?

7. What is charcoal? How is it manufactured? What are its uses?

8. Why is charcoal made from peach and apricot pits and the shells of nuts pre-
ferred as an absorbent in gas masks?

9. What commercial products are made by the distillation of wood?

10. What substances are used to preserve railroad ties and posts from decay ?
From what are these substances derived?

Suggestions for Field Work to Precede Chapter Twelve

1. A field trip to a greenhouse will help to make clear the im-
portant factors of the environment. Observe how the water,
light, temperature, soil, animal, and plant factors are taken care
of. (Animal factors: scale, aphids, mealy bugs, and white flies.
Plant factors: overcrowding, bacterial and fungous diseases.)

2. In the field, compare the environmental factors in a lake or
swamp with those on a hilltop or slope. What are the notable
effects of these factors on leaves, stems, and roots? Write the
results of your observations in the form of a table.

3. Examine trees growing in the open to see how the wind
affects the shape of the crown. Is the trunk in the center of the
crown? What is the direction of the prevailing wind in your
locality? In what direction are the branches of trees generally
longest in your locality ? Shortest ? What is the relation of this
branch development to the direction of the prevailing wind ?

4. Field trip to a dense wood. Study (i) the environmental
conditions of the large trees; (2) of the young trees; (3) of the
climbing plants ; (4) of the low herbs ; (5) of the mosses, lichens,
or ferns on the tree trunks ; and (6) of the mosses, lichens, or ferns
growing on the soil. Write your notes in the form of a table show-
ing relative amounts of light, water, and mineral salts available
to plants belonging to each of the above groups, and relative dry-
ness of air to which they are exposed.

138

CHAPTER TWELVE

THE ENVIRONMENT OF PLANTS

EVEN the most casual observation suggests that plants are
greatly affected by the conditions under which they grow.
The grasslands, the forests, and the deserts of North America
are occupied by different plants because of the difference of
conditions in these areas. The white oak growing in the open
has broad, spreading branches and a large crown, while in the
forest it is more columnar and has a crown at the upper level
of the forest. The sugar maple becomes a great tree or re-
mains a mere shrub according to whether it grows in rich soil
or in a crevice of a rock. The internal structure of a plant
is also modified by the external conditions under which it
grows ; the leaves of many plants which are thin and tender
when there is an abundant water supply, become thick and
leathery when grown under conditions of drought. Since
conditions of growth show so great an effect upon the form,
size, structure, abundance, and distribution of plants, we should
know the principal factors that make up a plant's environ-
ment before going further in the study of the plant itself.
It is especially important to understand these factors, because
the purpose of a great part of agricultural practice is to
modify the environment of the plants that are being grown.

Definition of environment. By the environment of a plant
is meant the complex of all those influences outside the plant
which directly or indirectly affect its physiological processes,
its structures, and its development and propagation. These
influences are numerous and are usually spoken of as factors.
The natural habitat of a plant is a combination of environ-
mental factors favorable to the complete development of the
plant. The factors include the chemical and physical prop-

139

140

Science of Plant Life

erties of the soil and the air surrounding the plant; also
light, gravity, and the influences of animals and other plants
(Fig. 78).

FIG. 78. Diagram illustrating the environment of land plants.

The soil factors. If a plant is to grow in a soil, the soil
must furnish it with sufficient water for transpiration and the
manufacture of food. At the same time, the soil must not be

The Environment of Plants

141

Irrigation Division, U. S. Dept. oj Agriculture

FIG. 79. Irrigated and unirrigated sugar cane, showing the value of sufficient
water in the growing of this crop.

so filled with water as to exclude air from the roots. In order
that the roots may penetrate the soil readily, it should not be
too resistant ; but it should be compact enough to afford the
plant a firm anchorage.

The soil must supply also certain indispensable chemical
elements used by plants in the manufacture of food.
These elements are potassium, calcium, magnesium, phos-
phorus, nitrogen, iron, and sulfur. In soils that contain
insufficient amounts of any of these substances the growth of
plants is hindered, and certain plants are excluded from such
a soil. One of the purposes in using chemical fertilizers is
to add to soils or to liberate in them sufficient quantities of
all the elements essential to the vigorous growth of crop
plants. It should be remembered that in addition to the

142

Science of Plant Life

seven elements mentioned above, plants use, for the manu-
facture of food, carbon, hydrogen, and oxygen, which they
derive from water and carbon dioxid.

Most plants grow best when the soil is neutral or slightly
alkaline. Red clover, alfalfa, and blue grass, for example,
cannot withstand an acid soil. By the addition of lime, acid
soils may be neutralized or made alkaline. This explains
the common practice of putting lime or wood ashes on lawns
where a growth of blue grass is desired. Some plants are
favored by an acid soil. Cranberries, blueberries, and redtop
grass are examples of such plants. In arid regions, the
evaporation of water may cause salts to accumulate in, the
surface layers of the soil to such an extent that most or all
plants are excluded.

Another soil factor of great importance is humus. This

U. S. Dept. of Agriculture

FIG. 80. A Kansas cornfield. The soil is rich in humus, and the plants attain a
height of 12 to 14 feet.

The Environment of Plants

143

FIG. 81. The results of deep tillage and shallow tillage.

material, which gives the brown and black colors to rich
agricultural land, is composed of the partially decayed re-
mains of plants. Leaves and other plant organs that fall
to the ground are slowly changed and broken up by bacteria
and other agencies until only the brown, powdery humus
remains.

Humus favors plant growth by increasing the water-hold-
ing capacity of the soil and so rendering the water supply
more uniform throughout the growing season. It improves
the physical properties of the soil by making it mellow. Hu-
mus also makes it possible for bacteria and other organisms
that increase fertility to live within the soil.

In agricultural literature the importance of the soil factors
is emphasized. Plowing, harrowing, disking, and cultivating
are methods of keeping the soil in good physical condition.
Adding manures to increase the humus, and adding nitrates,
phosphates, and other salts as fertilizers, are methods of im-

144 Science of Plant Life

proving the chemical composition of the soil and to a less
extent its physical condition. Irrigating the land where the
water supply is ina4equate, and under draining it where the
soil moisture is excessive, are further means of improving soil
conditions for the growth of crops. The soil requires these
special attentions for the growth of domesticated plants be-
cause they must not only live, as do wild plants, but they
must, in one form or another, yield a profitable return.

Atmospheric water. The water in the air affects plants
directly in several ways. The moistness or dry ness of the
air determines whether less water or more is required for
transpiration, and the amount of water precipitated from the
air in the form of rain determines to a large extent the amount
of water available in the soil. Atmospheric water conderised
in the form of fog and cloud reduces transpiration and also
lessens the amount of light that reaches the plant.

The distribution of rainfall through the year is of the great-
est importance to vegetation. When the period of heaviest
rainfall coincides with the hottest part of the year, the con-
ditions are best for the rapid growth of plants. If the rain-

FIG. 82. Cross sections of kernels of hard or macaroni wheat. This wheat is grown in
dry regions and is valued because of its large content of protein. In the figures the
flinty or high protein parts are shaded and the soft or starchy parts are white. When
the wheat is grown under the conditions of dry farming, the protein content is highest
04); when regularly irrigated, the same wheat produces soft, starchy grains (C). An
intermediate condition is shown by B. This exemplifies the effect of the water balance
on the composition of a grain.

The Environment of Plants

145

146 Science of Plant Life

fall is scanty during the time of highest temperatures, plants
are hindered in their growth, and only xerophytes may be
able to withstand the conditions.

The temperature factor. As one goes north or south from
the equator, the temperatures of the soil and air decrease.
Increasing altitude in mountains brings about the same effects.
Temperature directly influences the rate of all plant pro-
cesses, and most plants grow best under certain rather fixed
temperature conditions. For tropical plants, air temperatures
above 90 degrees F. are most favorable. Temperate plants de-
velop best at between 60 and 90 degrees F. Arctic and alpine
plants grow at temperatures but little above the freezing point.

Air temperatures are greatly influenced by air drainage.
Cold air is heavier than warm air ; consequently it accumulates
in low grounds and reduces the temperature there. Frost
occurs later in the spring and earlier in the autumn in low
places than on hills. Crop plants like beans, that are easily
injured by frost, can be planted earlier and grown later on
uplands. Peach orchards are more profitable on uplands
than in valley bottoms, because on the uplands they are more
likely to escape late spring frosts.

The time during which the temperature remains above the
freezing point is the growing season. In the tropics this ex-
tends throughout the year. In arctic and alpine regions it
may be reduced to 2 or 3 months. The temperature of the
air and the length of the growing season determine the amount
of food a plant may manufacture, and consequently the
amount of growth.

Soil temperatures also are important. Dark-colored soils
are warmer than light-colored soils, because they absorb the
sun's rays more readily. Well-drained soils are wanner than

The Environment of Plants

DATE WHEN HARVEST OF SPRING OATS BEGI]
A0e.ii

MAY

FiG. 84. Map showing date of oat harvest in different parts of the United States ;
an example of the effect of temperature on the maturing of plants.

wet soils, (i) because less heat is required to raise their tem-
peratures, and (2) because the temperature of a wet soil is
lowered by the constant evaporation of water. The most
valuable farm lands are those with dark-colored, well-drained
soils. On north slopes, soils do not warm up so rapidly in the
spring, and plants growing there start their growth. later than
do those on the south slopes of the same hills. Peach growers
prefer not only uplands but north slopes. Why?

148

Science of Plant Life

FIG. 85. Effects of wind on trees along the Bay of Fundy, Nova Scotia.

The light. The amount of light available to a plant de-
pends primarily upon the intensity of the sunshine. This is
greatest in the tropics and least at the poles. The total
amount of light is influenced also by the length of the day.
At the equator the daylight lasts 12 hours; at the pole,
the light continues all summer. So tropical plants have in-
tense light during half of each day, while arctic plants have
weak light continuously through the growing season.

Locally the light is modified by clouds and fogs. These are
much more prevalent along the seacoast than inland, and are
particularly common along the Pacific coast from Alaska to
California. The slope of the land, especially in mountain
regions, may increase or decrease the intensity and the length
of daylight. Finally, plants may have their light reduced or
cut off by trees or other objects.

The Environment of Plants

149

Gravity. Gravity is an im-
portant environmental factor,
largely because of its influence
on the direction of growth in
stems, roots, and other organs
(page 131). Light influences
also the growth of the various
parts of the plant. Conse-
quently the position of the
aerial organs of plants is to a
large extent determined by the
combined influences of light
and gravity.

Wind. Winds and air cur-
rents are of importance, as
they affect the rate of tran-
spiration or modify the tem-
perature. It may take 10
minutes for your wet hands to
dry in still air, but if you hold

them before an electric fan they will be dry in 2 or 3 minutes.
The passing of an increased volume of air over a wet surface
increases evaporation, and wind affects the transpiration of
plants in the same way. In drying your hands before a fan,
notice also the cooling effect of the breeze. Leaves are cooled
by transpiration in the same way. Winds also may cause
important modifications in the forms of plants, and occasion-
ally violent winds may destroy large areas of timber and
crops.

Animals. Leaf-eating insects, such as the potato beetle,
injure the plant by destroying the food-making apparatus.

W. S. Cooper

FIG. 86. Western white pine on Long's
Peak, Colorado, showing the effects of
violent winds and of wind-driven snow.

150 Science of Plant Life

It is estimated that grasshoppers and leaf-hoppers often eat
as much of the grass in a pasture as do the farm animals.
Plant lice and scale insects remove the sap from the cells of
the tender growing parts and may kill the entire plant. Other
animals, like the earthworm, favor the growth of plants by
loosening the soil and promoting the change of fallen leaves
to humus. Herbivorous (Latin : herba, herb, and vorare, to
eat) wild animals, like the rabbits, squirrels, and deer, markedly
affect natural vegetation, while the domesticated cattle, sheep,
and hogs to a large extent determine what plants can survive
in pastures and grazing lands. Man, more than all other
animals put together, has modified the natural vegetation of
the earth. In some cases he has destroyed it ; in other cases
he has encouraged and protected it. Most of all, he has
selected certain plants and made of them the food supply of
the world.

Other plants as an environmental factor. Other plants,
such as weeds growing among cultivated crops, may modify
the environment of plants by shading them, by removing water
from the soil, and possibly by producing poisonous substances
in the soil. Or a plant may directly affect another plant
by growing on it and taking its nourishment from it. For
example, the mistletoe grows on trees and injures them. Corn
smut and wheat rust live on corn and wheat and decrease
or prevent the production of grain.

The complexity of the environment. The environment of
plants is made up of many factors, and the factors them-
selves are more or less dependent upon each other. Conse-
quently it is often difficult to determine definitely the cause
of a particular effect that is undoubtedly produced by some-
thing in the environment. But as our knowledge of botany

The Environment of Plants 151

advances, we are able to relate more and more of the effects
that we observe in plants to definite factors in their environ-
ment. The farmer, gardener, or forester is often able to use
this knowledge in making the environment more favorable for
the plants which he grows.

Subdivisions of botany. Three of the great subdivisions of
botany are morphology, physiology, and ecology. The study
of the structures of plants is plant morphology (Greek morphos,
form, and logos, study of). The study of the processes of
plants such as transpiration and photosynthesis is plant
physiology (Greek: physis, nature, and logos, study). The
study of plant structures and processes in relation to the en-
vironment, or as they are modified by the factors that make up
the environment, is called ecology (Greek : oikos, home, and
logos], or ecological botany. Since the environment deter-
mines what plants can live in a particular place, ecology in-
cludes also the study of the distribution of plants on the
earth's surface and attempts to account for that distribution.
For this reason the environmental factors that affect plants
are also spoken of as ecological factors ; and the changes in
structure that adjust plants to particular habitats may be
called ecological modifications.

PROBLEMS

1. What is the principal factor that limits the growing of oranges in this coun-
try to Florida and southern California?

2. Why are beans planted a month later than peas?

3. What environmental factor determines that melons and squashes should
not be planted until late spring?

4. What factors in the environment make it necessary to spray potatoes ?

5. What is the principal factor limiting the production of crops on the Western
plains?

6. Why are plants absent from the larger sand dunes?

152 Science of Plant Life

7. Why are plants absent from sea beaches and from the beaches of large
lakes?

8. Why do trees along streams have their longest branches out over the water ?

9. Why do trees along streams frequently lean outward over the water?

10. Why do trees on steep hillsides have their shortest branches toward the
hill?

11. What are the most favorable environmental conditions for the growing of
rice, celery, onions, dates, bananas, sugar cane, guayule, sisal, tulips, and
pecan trees?

12. How do the plants and fruits of blackberries and dewberries grown in
bottom lands differ from those grown on dry hillsides? Why?

Suggestions for Laboratory Work to Precede Chapter Thirteen

1. Sketch a few twining stems. Note the twist of the stem as
shown by surface markings.

2. Sketch examples of tendrils. The Boston ivy, gourd, pea,
sweet pea, and grape are readily obtainable. Is the spiral con-1
tinuously coiled in one direction ? Why ?

3. Dissect a cactus stem. Note the nodes, internal water-
storage and food-storage tissue, and green tissue.

4. Make drawings of a rootstock, corm, bulb, and tuber. Divide
each of them into halves by cutting through the longitudinal axis.
Compare their advantages as organs of propagation and storage.

5. Examine stems of water hyacinth, bulrush, and pondweed,
noting air chambers.

6. Examine a cross section of a grapevine or a rattan stem.
Note the large size of the water tubes.

CHAPTER THIRTEEN

ECOLOGICAL GROUPS OF STEMS

IN the tenth chapter we discussed the normal structures of
the upright stems of land plants. It was pointed out that
each of the three great groups of seed plants has a charac-
teristic arrangement of its tissue systems. The essential
tissues, like the water-conducting, food-conducting, storage,
and mechanical tissues, are present in all.

Stem structures and habitats. Plants growing in different
habitats, as ponds, swamps, and deserts, have very different
stem structures. The stem of a leafless desert plant must of
necessity be different from that of a leafy submerged plant.
The tropical climber has a stem quite unlike that of a plant
whose main stem is underground. These differences con-
sist not so much in the arrangement of the several tissues as
in modifications in their amounts and proportions.

Stems of mesophytes. The native plants of the eastern
United States grow under medium conditions of moisture,
light, and temperature ; and they are characterized by large
leaf area, leaves of soft texture, and much-branched stems.
The vegetation culminates in the forests of the rich, well-
watered soils of the river valleys. Here may be found oaks,
walnuts, elms, and sycamores from 100 to 150 feet in height
and with trunks from 4 to 14 feet in diameter.

In the moist canons of the Sierras of California, the giant
sequoia reaches heights of from 250 to 320 feet above the
ground, with extreme trunk diameters of 35 feet. This tree
is the largest and is perhaps the oldest of all living things.
The redwood, its near relative, grows in the fog-abounding
ravines of the Coast Ranges. Its trunk does not attain a
diameter of more than 28 feet, but it surpasses the giant

154

Ecological Groups of Stems

I5S

sequoia in height. Really to appreciate the size of these trees
you should pace off a distance equal to the diameter of the
trunk and calculate how many times the height of your school
building a giant sequoia is. Then try to imagine how one
of the Big Trees would look if it stood in your school yard.

Stems of climbing plants.

Among mesophytes are
many vines with exceed-
ingly long, slender stems.
The Virginia creeper, wild
cucumber, and grape have
stems from 50 to 300 feet
in length. These long stems
enable them to spread their
leaves over the tops of large
trees . Climbers may attach
themselves to their support
FIG. 87. Tendrils of wild cucumber. Note by twining about it by ten-

the coiling of the tendril by which the plant is drils Or bv SUDDOrtive TOOtS.

drawn nearer the support, and the reversal of ^ , ., . , . ,

the spiral in different parts of the tendril. Tendrils are Specialized <M>

Is there always a reversal in coiled tendrils? gans developed in place of

156

Science of Plant Life

I branches or leaves. They re-
spond to contact with a sup-
port by coiling tightly about it.
After attaching themselves,
they develop mechanical tissue
which gives the plant a firmer
support. In some vines, like
the Boston ivy, the tendrils
have at their tips sensitive disks
which become cemented to the
support. This type of tendril is
especially effective in taking
hold of the bark of trees, rock
cliffs, and walls (Fig. 87).

In the tropics, climbing stems
may attain a length of more
than looofeet. Thus the water
transpired by the terminal
leaves has to be carried for
about a fifth of a mile within
the plant. This suggests the

need of an efficient conductive system in a climbing plant, and
explains why the bulk of its slender stem is made up of con-
ductive tissue.

Stems of hydrophytes. We have seen that submerged
leaves have distinctive forms and characteristic internal
structures. Under- water stems also differ from those of
land plants. In floating plants like duckweed, water hya-
cinth, and Salvinia, the stems are short. Their conductive
systems are poorly developed, and they are practically without
mechanical tissue.

FIG. 88. Climbing stems on a tree
trunk in a tropical forest.

Ecological Groups of Stems 157

Other hydrophytes, like the pond weeds and water lilies,
are rooted in the soil, and their stems bear submerged or
floating leaves. The stems have little or no mechanical tis-
sue. As compared with land plants, the conductive system
is much reduced. Many of these hydrophytes develop
underground rootstocks and tubers. For this reason the
plants commonly grow in masses.

A third group of hydrophytes are those like the cat-tails,
rushes, bulrushes, and sedges, whose roots and stem bases
may be under water, while the upper parts are exposed to the
air. These plants have both the conductive and mechanical
tissues well developed. This is in keeping with the fact that
such plants are exposed to the action of wind and wave and
to the conditions that bring about normal transpiration.

The most distinctive feature of submerged stems is the
presence of large air chambers extending throughout their
length. When the stems are broken open, the tissues are
seen to occupy much less space than the air cavities. We may
properly speak of " intercellular spaces " inmesophytic stems ;
in describing hydrophytes, the term " air cavities " is more
appropriate. They buoy up the plant and provide an internal
atmosphere for gas exchanges between the leaves and roots.

Stems of xerophytes. The xerophytes are the character-
istic plants of deserts and dry plains. They occupy sand
dunes and sand plains along the Atlantic coast and on the
shores of the Great Lakes. They may be found locally on
rock cliffs and on dry, exposed hilltops, situations in which a
reduced water supply in the soil is accompanied by atmos-
pheric conditions that promote rapid transpiration and in
which the plants are periodically or continuously subjected
to drought. Plants that thrive in these habitats show a re-

Science of Plant Life

FIG. 89. A desert scene in Arizona.

Caspar W. Hodgson

duced leaf area or a complete absence of leaves. The stems
also may be reduced in size and amount of branching.

The cactuses represent extreme examples of this type of
plant. Leaves are wanting ; and the stems are columnar,
often ridged and fluted, and always thick and fleshy. The
photosynthetic work in cactuses is done by the cortical tis-
sue. As the green surface is small compared with the green
surface in mesophytic plants, food manufacture is slower
and growth is correspondingly less. Some of the cactuses of
Mexico attain heights of 60 feet. The cactus form points
clearly to one of the most characteristic features of desert
plants ; namely, water storage. A single plant may contain
from 15 to 20 gallons of water. As the plant loses moisture
so slowly, it may continue to live for several years without
an additional supply of water. Other desert plants that
accumulate water in the leaves were mentioned on page 58.

Ecological Groups of Stems 159

Underground stems. Many plants belonging to each of the
three ecological groups, xerophytes, mesophytes, and hydro-
phytes, possess underground stems. Underground stems are
particularly useful as storage places for accumulated food and
water, and as organs for propagating the plant (page 217).

The commonest type of underground stem is the rootstock.
Rootstocks are horizontally growing stems, from which the
aerial stems arise. They may be slender, or thick and fleshy.
Usually they have small scale leaves and buds at the nodes,
and roots that arise from the nodes or from the entire under
surface. The presence of nodes is the external feature of
underground stems that distinguishes them from roots.

In many of the grasses and grasslike plants rootstocks de-
velop rapidly in all directions, sending up erect branches at
short intervals. The rootstocks and their accompanying
roots soon become mixed with those of adjoining plants,
finally forming a closely interwoven mat which is the " turf "
of lawns and meadows. Turf -forming grasses are often of
great value for holding in place the soil of embankments,
dikes, and levees. In these plants the rootstocks are mainly
useful in spreading or extending the plant. Bermuda grass
and Johnson grass are sometimes troublesome weeds because
of their extensive rootstock system. The sand-reed grass
has been planted extensively in Europe and in America to
hold drifting sand in place, and to prevent the sand from in-
vading towns and cultivated fields.

In plants like the May apple, Solomon's seal, and yellow
water lily, the rootstock not only causes the plant to spread,
but it also accumulates a part of the food manufactured each
season and thus serves as a storage organ. It is this store of
food and the readiness with which the rootstock sends up

i6o

Science of Plant Life

shoots, that make the bindweed so difficult to eradicate
from cultivated fields, gardens, and hedgerows.

A short, upright, fleshy
rootstock, like that of the
jack-in-the-pulpit, Cala-
dium (elephant's ear), or
gladiolus, is called a corm.
Corms contain large
amounts of food, and by
the development of their
lateral buds may serve
to reproduce the plant as
well as to carry it over
the winter. The dasheen,
a tropical plant which
resembles the Caladium,
and which has recently
been introduced into the

FIG. 90. Dasheen and edible conns produced United States, has an

by it. The dasheen is related to the common ^^ QQTm ^ ^ ^

elephant s ear or Calad^um, and is extensively

grown in the tropics for food. In the states important SOUrce of food.

along the Gulf Coast it is being introduced as a A bulb ig ft flesny Un-
food plant.

derground bud, made up

of a short stem covered with several layers of thick scales in
which food is stored. Tulips, hyacinths, and onions are
commonly propagated by means of bulbs.

By planting bulbs of the tulip in autumn, we can have
flowers early in the following spring, whereas if we planted the
seeds, we should have to wait several years for flowers. Fur-
thermore, tulips do not grow well except in a very moist
climate, and the development of large, vigorous bulbs is im-

Ecological Groups of Stems

161

FIG. 91. Amaryllis bulb.

possible in most parts of the United States. For this reason
nearly all our tulip bulbs are brought from Holland. The
importation of bulbs from countries
where they grow particularly well is
an important industry and enables
us to have many flowers which can-
not be so successfully propagated in
our climate.

Tubers are the enormously thick-
ened ends of short underground
stems. The potato, the Jerusalem
artichoke, the dahlia, and the com-
mon white water lily develop tubers.
The scale leaves of the ordinary
rootstock are in tubers reduced to ridges, and, the buds them-
selves to mere points. The scales and buds together form
the eyes of tubers. Tubers serve the same purposes for the
plant as do other fleshy underground stems : surplus food
accumulates in them, and by them the plant is multiplied.
The potato tuber has become one of the most important
sources of food for man.

Commercial products derived from stems. We are all
familiar with the important products derived from the trunks
of trees. Lumbering is one of the most important industries
of the United States. Closely associated with it are the
furniture industry, which uses the hardwoods, — walnut,
oak, maple, sycamore, and birch, — and the wood-pulp in-
dustry, which utilizes soft woods — such as spruce and
poplar — in the making of paper. The Southern pines fur-
nish rosin and turpentine ; the bark of oaks and hemlocks sup-
plies tannic acid for the manufacture of leather ; and Span-

162

Science of Plant Life

U. S. Forest Service

FIG. 02. Southern long-leaf pines tapped for resin.

Ecological Groups of Stems

ish oak bark provides the
cork of commerce.

The stem of the rattan
palm, which is one of the
very longest of tropical
climbers, furnishes the cane
for chairs and materials for
basketry and wicker furni-
ture. In this country
baskets and furniture are
made from shoots of the
osier willows.

Stem vegetables of im-
portance as food for human
beings include the potato,
Jerusalem artichoke, as-
paragus, dasheen, and
kohl-rabi.

Sorghum and sugar cane
furnish a considerable part
of the sugar and sirup of

Bureau of Agriculture, P. 7.
FIG. 93. Tapping para rubber trees, in the
Malay States, to obtain the milky juice from
which crude rubber is made.

The maple is the

commerce,
source of a delightfully flavored sugar.

The crude rubber used in the manufacture of shoes, gar-
ments, and tires is furnished by the stem of the desert plant,
guayule, and the sap of various tropical rubber trees. Many
substances used in medicine are derived from stems directly
or by distillation. Wintergreen oil may be obtained from the
twigs of the sweet birch, and camphor from the stem and
branches of the camphor tree.

The bast fibers of flax, jute, and hemp furnish material for
the manufacture of linen, cordage, coarse fabrics, and rugs.

164

Science of Plant Life

Bureau of A gricullure, P. I.
FIG. 94. Marketing bamboo, Philippine Islands.

Cornstalks not only furnish food (ensilage) for cattle, but from
them is made the packing used inside the armored walls of
war vessels.

PROBLEMS

1. What tree is most valuable to the natives of the tropics? Why?

2. Where are the principal forest reserves of the United States located? May
trees be lumbered on forest reserves?

3. What kinds of work are carried on by the United States Forest Service ?

4. Will twining stems twine about a horizontal wire ?

5. For your vicinity, make a list of the plants that climb (i) by twining ; (2) by
means of tendrils ; (3) by means of roots.

Suggestions for Laboratory and Field Work to Precede
Chapter Fourteen

1. Demonstration of imbibition with gelatin, wood, and a dry
leaf.

2. Demonstration of osmosis with thistle tube and animal mem-
brane ; or with diffusion shell or porous cup ; or with carrot root
and glass tube.

3. Study the root system of a seedling or 'weed. Note primary,
secondary, and adventitious roots. Make a drawing of the root
system.

4. In the field, study the roots of some plants growing in dry
sand ; in clay ; and in swampy low ground.

5. Study distribution and development of root hairs in seedlings.
Wheat and oat seedlings are very satisfactory for this purpose.

6. With a microscope examine a young root. Try to make out
the root cap ; the epidermis ; the cortex ; the vascular axis.

7. Longitudinal sections of a root tip will show stages in cell
division and cell enlargement. Note changes in nucleus, cytoplasm,
and vacuole.

165

CHAPTER FOURTEEN

THE STRUCTURES AND PROCESSES OF ROOTS

IN preceding chapters we have learned that leaves manu-
facture food in the presence of light ; that their exposure to

air facilitates the entrance and
exit of carbon dioxid and oxy-

Sen; and that by beinS raised
and displayed on erect stems
their efficiency is increased.

The support of leaves by
stems makes necessary the de-
velopment of mechanical tissue
in the stems. The display of
leaves high above the water
supply of the soil requires a
conductive system capable of
FIG. 95. Dandelion plant, showing the raising water and mineral salts

primary tap root and its branches, the from tne roots ancj of carrying
secondary roots.

food away from the leaves.

Consequently plants that expose great numbers of leaves to
the light must develop large and strong stems. The stems,
in turn, must be firmly anchored in the soil, and they must
also be supplied with the water and mineral substances that
press up through them to the leaves. Anchorage and ab-
sorption are the particular functions of roots, though they
carry on other processes also, such as conduction of water,
transfer of food materials, accumulation of food, respiration,
and growth.

Classification of roots. The root of a well-developed bean
seedling will show the essential features of roots. There is
the primary root, extending downwards from the base of the

1 06

The Structures and Processes of Roots

stem. On its sides are numerous secondary roots which
extend at right angles, or grow obliquely downwards. Un-
like stems, roots possess no
definite nodes from which
branches arise. A second-
ary root may originate at
any point on the primary
root.

In many seedlings there
are also roots that develop
from the first node of the
stem. All roots arising
from stems and leaves are
called adventitious roots.
The "prop roots" that
develop from the lower
nodes of corn stems and
the roots that grow from
" cuttings " are familiar
examples. Adventitious
roots develop also from the
stems of many plants like
the poison ivy and trumpet

Creeper and act as holdfasts pIG. 96. Stages in the development of a corn
in Supporting these Climbers seedling. P is the primary root, S a secondary

root, and A an adventitious root from the first

on trees and walls. Adven- node of the stem,
titious roots may arise also

at any point on a primary or secondary root, following in-
juries. For example, when we plant pieces of horseradish
or dandelion roots, adventitious roots develop. During the
dry season in deserts the younger parts of the root systems

i68

Science of Plant Life

of some plants like the cactuses are killed, and when the next
wet season comes, many adventitious roots develop from the
parts of the root system still alive.
In desert plants these new adven-
titious roots do most of the absorb-
ing work during the moist season.

Root hairs. The young roots of
land plants generally bear root hairs.
These are delicate elongations of the
epidermal cells of the root. They
are especially concerned
with the absorption of Iff
water and mineral salts,
and their presence in-
creases the absorbing
surface of the root from
two to fifty times. Since T

* FIG. 97. Stages m the growth of an onion seed-

the rate Of absorption ling, showing the lifting and shedding of the seed

depends in part Upon coats and the development of the primary and

. secondary roots.

the surface area in con-
tact with the soil water, the advantage in root hairs is evident.
Root hairs are usually short-lived structures, their duration
being best measured in days. They begin to develop at a
short distance from the tip of the root. Farther back they
have attained full length, and beyond this they are in a dy-
ing or dead condition. Thus from day to day the zone of
root hairs moves forward with the growth in length of the
root. This brings the root hairs continually in contact with
new supplies of water and food materials in the soil. As a
plant enlarges, its root system becomes more complex through
repeated branching and the elongation of the branches. Most

The Structures and Processes of Roots 169

of the absorption occurs in the root-hair zone, and this is
continually moved farther and farther from the base of the
stem. In large trees this zone may
be many feet from the base of the
trunk.

Advantages in spreading roots.
The spreading of the root branches
in all directions not only enables
the roots to come in contact with
more water and mineral salts, but
as the woody tissue develops, it
helps to anchor the plant more
firmly. It is an interesting fact

that when stems are subjected to

bending by winds or other agencies,

the mechanical tissue of the root

develops to a greater extent than

ordinarily.

The principal tissues of roots.

In the root, the water-conducting

tissue and the wood form the

central axis. Surrounding this is

a layer of food-conducting tissue.

In perennial roots there is a cam-
bium layer between the central FIG. 98. Enlarged view of the end

axis and the food-conducting tissue. of a root' showin& root caP> growing

^ , . j , , f , , , . region, and root hairs.

Outside the food-conducting tissue

is the cortex, extending to the epidermis. Perennial roots
like those of trees soon lose their epidermis ; later the cortex
also disappears. The continued thickening of the wood and
of the water-conducting and food-conducting tissues results

Growing
point

.Region, of
elongation

Root cap

170

Science of Plant Life

in the death of the outermost layers of the root and the
formation of a bark very similar to that of tree trunks.

Absorption. The
passage of water and
other substances into a
body is called absorp-
tion. Of all the pro-
cesses that take place
in plants, absorption is
the one most commonly
associated with roots.
In addition to water,
roots absorb the min-
eral substances found
tissue''

" ; W-tft/T 'Epidermis
Cortex.

Rod-con-
ducting

Water-con-
ducting
vessel

Growing
point

in plants, and they
absorb a part, or all,
of the oxygen needed
for respiration by their
own cells. Water and
other substances not
only enter the external
cells of a plant by ab-
sorption, but they pass
from cell to cell within
the plant by the same
process. A dead root
in the soil may take
up water and become
saturated; but only a
living root can absorb water rapidly enough to furnish an
adequate supply to the living parts above the soil. The

Root cap

FIG. 99. Diagram of a root tip, showing the
tissues and their arrangement.

The Structures and Processes of Roots 171

water supply of plants, therefore, depends upon the presence
of living cells in the roots.

Three physical processes are involved in absorption in
plants. These processes are diffusion, imbibition, and osmosis.
Their action may be demonstrated to a large extent by the
use of physical apparatus.

Diffusion. If a small dish of ether is exposed in a room, in
a few minutes the odor of the ether may be noticed in all
parts of the room. Even if there were no air currents, the
ether would evaporate ; that is, particles of ether would rise
from the surface of the liquid, pass out of the dish, and move
through the room in every direction. This is an example of
the diffusion of a vapor. The vapor is concentrated in the
dish and the particles move outward into the room where
there is none ; that is, the particles move from the place
where the concentration is greatest to where it is less. After
the ether has evaporated, the vapor tends to become evenly
distributed throughout the room.

Similarly, if a few crystals of copper sulfate are placed in
the bottom of a vessel of water, particles of the copper sulfate
diffuse through the water. The crystals are blue in color,
and as diffusion proceeds, the water in the vessel gradually
becomes blue. The direction of the movement is again from
the place where the diffusing substance is most concentrated
to where it is less concentrated. The particles pass from the
place where they are most abundant to where there are fewer
of them, and this process is continued until they are evenly
distributed throughout the water.

Diffusion of a gas or vapor is very rapid. Diffusion of a
dissolved substance is slow, but the distances that substances
must travel in plant cells are very small. Oxygen and carbon

172 Science of Plant Life

dioxid, when once dissolved in the water of cells, move
about partly by diffusion.

Imbibition. The process of imbibition may be illustrated by
placing a sheet of gelatin in water. Dry gelatin is a hard,
brittle, partly transparent solid. After it has been in water
for a few minutes, it will be found to have increased in weight
and in length, breadth, and thickness. The gelatin, instead
of being brittle, is now soft and pliable ; it is also more trans-
parent than it was.

The increase in size and weight is explained by the fact that
particles of water have forced their way between the particles
of the gelatin, spreading them apart. Since the gelatin par-
ticles have been forced farther apart, the gelatin is more pli-
able and the particles cling to one another less firmly. Hence
when a piece of dry wood is put into water, it imbibes water
and swells. The cell walls of a root, like wood, are largely
composed of cellulose and they take up water in the same way.
When dry seeds are placed in water, they imbibe water and
increase in size. Indeed, most organic substances have
the property of imbibing water and swelling. Imbibition is
a form of diffusion that results in swelling. Compare the
size of a sponge when dry with its size after it has been soaked
in water and squeezed as dry as possible.

When a piece of wood becomes saturated, it stops taking
up water. If, however, the water were being removed from
the inside, more would continue to pass into the wood. This
is exactly what happens in the root of a living plant. The
external cells of the root are in contact with the water of the
soil. Inside the root the water is being used and removed by
being drawn up through the stem to the leaves. More water
then passes into the cell walls and protoplasm to take the place

The Structures and Processes of Roots 173

of that which is drawn away, and this keeps the amount of
water in the cells of the root nearly constant.

Osmosis. The third physical process that aids in the
absorption of water is osmosis. If an animal membrane, as
a piece of bladder, is tied over the broad end of a thistle tube
and the bulb of the tube is immersed in water, the water will
gradually pass through the membrane. The membrane is
permeable to water ; that is, it allows water to pass through
its minute pores. The water continues to move through until
its level is the same inside and outside (Fig. 100).

When the water level is the same inside and outside the
tube, one might think that the water particles were at rest.
This is not the case. Water particles are still passing both
into the thistle tube and out -of it through the membrane.
The rate is the same in both directions, however, and so the
water level within the tube remains unchanged.

If we put a little sugar into the thistle tube, something dif-
ferent happens, as is shown by the fact that the liquid in the
tube begins to rise. Evidently, more water is passing through
the membrane into the tube than is passing out, and this
change has been brought about by the presence of the sugar.
Perhaps we can get a mental picture of what causes this dif-
ference from the diagram in Figure 101. The membrane (C)
allows water molecules to pass through it freely, but it per-
mits scarcely any of the sugar molecules to pass. The outer
side of the membrane is completely covered with water mole-
cules (B), tending to diffuse through the membrane. The
inner side (A) is only partly covered with water molecules,
since part of the area is occupied by sugar molecules. Con-
sequently there are fewer water particles on the side A tend-
ing to diffuse outward than there are on the side B tending to

174

Science of Plant Life

diffuse inward. We may say that the water is more con-
centrated outside the tube than inside, so water passes from

~ - ~WATER~ -

~ -=1 WATER **•

- WATER+SUGARJ —

/ • li/ A T-rr r* D

FIG. 100. Diagrams to illustrate
the passage of water through a
membrane : A, molecule of inside
water; B, molecule of outside
water; C, membrane.

FIG. 101. Diagram to illustrate
osmosis: A, sugar molecule;
B, water molecule; C, selectively
permeable membrane.

the place of greater concentration to the place of less con-
centration. Moreover, sugar is a highly soluble substance ;
that is, it has a great affinity for water, and the sugar particles
tend to hold the water particles in contact with them inside
the thistle tube. The sugar, like the water, tends to pass from
the place of greatest concentration but is restrained by the
membrane from moving outward.

If we close the upper end of the thistle tube, the water will
continue to rise and compress the inclosed air. The pres-
sure developed under these conditions is called osmotic pres-
sure. If a large amount of sugar is put inside the tube, the
water will rise rapidly and exert great pressure. If only a

The Structures and Processes of Roots 175

small amount of sugar is present inside the tube, the water
will rise slowly and exert but little pressure.

When a membrane permits water or other substances to
pass through, it is said to be permeable to that substance.
For example, animal membranes are permeable to water and
to various dyes. A membrane that allows one substance to
pass through it, but retards the passage of another substance,
is said to be selectively permeable. The membrane on the
thistle tube is selectively permeable, because it allows the
passage of water but restrains the sugar that is dissolved in
the water. The membranes in root cells are permeable to
water, but they do not allow sugar and many other sub-
stances found inside the cells to pass out.

The conditions for osmosis as it occurs in plants, then, in-
clude a selectively permeable membrane between two bodies of
water, one of which contains a dissolved substance that does not
pass through the membrane readily.

Osmosis in roots. The cellulose walls of plant cells are
permeable to water and to most of the substances that dis-
solve in water ; but the layer of cytoplasm inside the cell wall
forms a selectively permeable membrane about the cell con-
tents. The cell sap in the vacuole may contain sugar and other
dissolved substances, just as the water in the thistle tube con-
tained sugar. The outer cells of the root are in contact with
the soil water. Hence the water passes into these cells in
the same way as into the thistle tube. In a similar manner
water may move from one cell to another and replace the
water that is being carried to the stem and leaves through the
conductive tissue. The path of the water from the epidermal
cells is through the cells of the cortex to the water-conducting
vessels in the interior of the root.

176

Science of Plant Life

Absorption and rise of sap in plants. Attention was called
to the fact that transpiration exerts a pull on the water in

the conducting tissue of the
leaves (page 134). This pull is
transmitted to the water-con-
ducting tissues of the stem and
root. So a fourth factor enters
into the absorption of water by
the roots : the pull on the water
in the cells of the root is in-
directly due to transpiration
from the leaves. Large trees
have been kept alive for days
by placing the cut-off trunks in
water. This shows that suffi-
cient water to maintain the
water balance of the plant for
at least several days may be
lifted in a plant by the pull of
transpiration without the aid of
roots. It is of practical interest

FIG. 102. Expenment to illustrate the

water balance in a plant. The entire to knOW that CUt flowers Will last

apparatus is filled with water, and A
and C are immersed in water. The

jf

ends Qf

.

water is absorbed by osmosis into the stems are bent over into a vessel

porous cup A, and evaporated from the an(J cut un(ier the Water. If CUt
cup B. The rate of evaporation is

faster than the rate of absorption, as is in the air, air bubbles get into
shown by the fall of the mercury in the ^he Water-Conducting tubes and
outer end of the tube C.

prevent the subsequent move-

ment of water into them. Air bubbles already in stems that
have been cut in the air may sometimes be removed by cutting
off an inch or two of the lower ends of the stems under water,

The Structures and Processes of Roots

177

Root pressure. If a number of well- watered plants are cut
off just above the soil, some of them will exude water for a
day or two. Experiments have
shown that the sap may in some
cases be forced out with pres-
sure sufficient to raise water 30
or 40 feet. This pressure is
called root pressure. When such
pressures exist in plants, they
probably aid in the lifting of
water in stems. Under these cir-
cumstances transpiration pulls
on the columns of water in the
water-conducting vessels from
the top, and root pressure pushes
on them from below. Extensive
experiments have shown, how-
ever, that root pressure is inter- -J>------

mittent. It may exist at one FlG" I0f A plant with its stem cut in

two and connected again with a tube
time and not at another, and similar to that shown in Figure 102.

when transpiration is most active In this case the roots are absorbins

water more rapidly than the leaves are

and the largest Volumes Of Water transpiring it, since the mercury at D is
are being raised in a plant, root Pushed away from the plant. By set-
ting the plant in bright sunshine, the

pressure is wanting entirely. transpiration may be increased. The

Because of all these facts, it is mercury is then almost immediately

generally believed that root d™- toward the plant.

pressure is not a necessary factor in the raising of water in

stems.

Imbibition and osmosis lead to the development of root
pressure, and they are partly responsible for the flow of
maple sap (page 135). Grapevines pruned in the spring exude

178 Science of Plant Life

water for days afterward as a result of root pressure. On a
small scale the same thing may some times be seen when well-
watered begonias and fuchsias are cut off near the soil.

Food conduction. The transfer of food takes place in the
food-conducting tissue of roots in the same way as in stems
and leaves. Substances that are to be transferred must be
in a soluble form, and they are usually in a comparatively
simple form. Starch, for example, is transferred as glucose,
and protein and fats are broken down into simpler compounds
before they are moved from one part of the plant to another.
Diffusion is the principal process that brings about food
conduction.

The movement of a substance into or out of a cell depends
upon the permeability of the cell protoplasm to that par-
ticular substance ; if the cytoplasm will not permit the sub-
stance to pass through, it cannot enter or leave a cell. The
direction of the movement of foods may change from time to
time, as is shown by the fact that sugar and soluble proteins
may move down into the root during one season and up out
of the root at another season. For example, in the turnip or beet
the excess food made by the leaves during the first summer
passes downward into the roots ; the next year, food passes
upward from the roots to the developing stems and leaves.
This may be due to changes in the permeability of the cells or
to changes in the foods stored in the cells.

These changes in the behavior of organs, tissues, and cells
are clear evidences of life. In physical apparatus the behavior
is fixed and a process soon comes to a standstill. In living
things changes are continually taking place in the living mat-
ter itself, and these bring about continual changes in the
processes that are going on.

The Structures and Processes of Roots 179

Accumulation of food in roots. Food accumulates in the
roots of many plants, notably in those of biennials like the
beet, carrot, turnip, and salsify. The sweet potato and the
dahlia are examples of perennials with large storage roots.
The most common forms in which carbohydrates accumulate
in roots are starch and sugar. Starch as a storage material
has the advantages of being insoluble and more concentrated
than sugar. When growth begins anew, starch is readily
converted (digested) into sugar (page 74).

Respiration in roots. Respiration must go on in the living
cells of the roots just as in the other living parts of the plant.
This process requires a constant supply of oxygen. In ob-
taining oxygen as well as in obtaining water, the division of
the roots into numerous fine branches is an advantage, be-
cause it exposes a large surface to the soil air and the soil
water. Some plants are easily injured by the lack of oxygen
in the soil ; if water stands on the soil and excludes the air,
the roots gradually suffocate. Suffocation of a part of the
roots interferes with other root processes besides respiration,
and the whole plant suffers. For example, you may have
seen yellow, sickly corn in low fields where water has stood
for some time. Such plants may recover if the soil is drained.
Water plants and swamp plants can grow in poorly aerated
soils because the roots are able to secure oxygen through the
internal air spaces of the plants.

Under normal conditions the energy liberated by respira-
tion in roots is largely used in growth and in overcoming the
resistance of the soil. During the life of a plant the roots,
like the stem, continue to develop new branches.

The growth of roots. In growing through the soil, the tip
of a root is continually pushed against and between sharp-

180 Science of Plant Life

angled soil particles. If a root tip or a longitudinal section
of one is examined under a microscope, it may readily be seen
that the growing point is not at the very end, as in stems,
but is covered by a root cap. The growing region of the root
extends a few centimeters back from the growing point (Fig. 98) .
Therefore, as the root elongates, the root cap is pushed for-
ward and is abraded by the soil particles. This injures the
outer cells of the cap, but as they are being renewed from
within, the cap is always present as a protection for the grow-
ing point.

At the growing point the cells are small, similar in form, and
filled with protoplasm. They are constantly dividing and
forming new cells. As the cells grow older and enlarge, they
assume the mature cell form of the particular tissue to which
they belong. The protoplasm of mature cells is merely a
lining inside the cell wall. Most of the cell space is occupied
by a water solution of sugar, soluble proteins, salts, and acids.
Sometimes starch grains are present (Fig. 99, page 170).

The three stages of growth are characterized by cell division,
cell enlargement, and the fixation or thickening of the cell
walls. They are very similar in all plant organs. But no-
where can they be seen so readily as in the longitudinal section
of a young root.

Suggestions for Laboratory Work to Precede Chapter Fifteen

1. Grow seedlings on blotting paper or cotton, between two
glass plates. Keep the plates in one position until the roots are
2 or 3 inches long. Then set the plates at right angles to the
previous position. Note the change in the direction of growth of
primary and of secondary roots.

2. Replace' the bottom of a cigar box with mosquito netting,
and place on the netting mustard or radish seeds that have pre-
viously been soaked in water. Then fill the box with wet moss or
cotton. Suspend the box so that the bottom rests at an angle of
45 degrees. Note the direction of growth of the roots as they
develop. Do they respond more to gravity or to moisture ?

3. Study roots of floating or submerged plants. Note air
chambers.

4. Study aerial roots of climbers like Boston ivy and trumpet
creeper. Study the prop roots of mature corn.

5. Sketch and study roots of carrot, parsnip, or radish. Test
for starch.

6. Sketch and study root nodules and roots of either clover or
beans.

7. Study the arrangement of roots on a bulb or corm. Onion,
hyacinth, gladiolus, or crocus may be used.

181

CHAPTER FIFTEEN

ROOTS AND THEIR ENVIRONMENT

ROOTS commonly develop in soil. Floating plants, however,
develop roots in water ; and many plants in the moist tropics

develop roots in the air. Some
of our own climbing plants have
aerial roots. Corresponding
with the differences in their en-
vironments, there are marked
differences in the structures
and processes of roots.

The response of roots to
gravity. The downward
growth of a primary root is
a response to gravity. The
primary root is positively geo-
tropic; it turns toward the
earth. We must clearly un-
derstand, however, that gravity
does not pull the root into the
soil. The soil particles are
heavier than the roots, and
this means that gravity pulls
P. more on them than it does on

FIG. 104. Young and old stems of Bos-
ton ivy. The young stems are held by the TOOtS. A TOOt will even
means of tendrils, the older stems by downward into mercury,

means of adventitious aerial roots. .

which is 15 times as heavy as

the root. Gravity merely determines the direction of
growth. The penetration of the soil is due to growth pres-
sure ; the cells of the root multiply and enlarge, forcing the
tip downward. Secondary roots also respond to the pull of

182

Roots and Their Environment 183

gravity, but they tend to grow at right angles to its direc-
tion rather than directly toward it, as do primary roots.
Secondary roots are transversely geo tropic.

The response of roots to light. Roots tend to grow away
from strong light. They are negatively photo tropic. This
tendency may be observed in the aerial roots of ivies and
other climbers. No matter from what surface on the stem
they arise, they curve around away from the light. This is
advantageous to a climbing plant, for it brings the root to
the supporting tree or wall.

The response of roots to water. Particles of water, in dif-
fusing from wet places in the soil to places that are drier, also
affect the direction of root growth. A root in drier soil will
turn toward the direction from which water particles strike
it. Continued growth will then bring the root into the
moister soil. This response of a root is of great advantage
to the plant. It is sometimes stated that roots " seek " the
water. In reality, roots grow toward moist places only when
water diffusing from the moist places reaches them and so
controls the direction of growth. They do not seek the water,
but they are turned to the moist soil by the water itself.

The distribution of roots in the soil. Another factor that
determines the distribution of roots in soil is the oxygen sup-
ply. Various plants have different requirements, but all roots
doubtless require oxygen for growth. Those who have seen
stumps pulled from the land know that the roots go deep in
upland sandy soils ; that they do not go so deep in heavy clay
soils ; and that they are just beneath the surface in swamp
and bog land. The principal reason why one finds the roots
near the surface in swamps is that these roots were the only
ones that continued to live and grow. The roots that in times

184 Science of Plant Life

of drought penetrated to greater depths were killed off —
suffocated — when the water stood at higher levels. The
distribution of roots in the soil, therefore, is determined prin-
cipally by the combined influences of gravity, water, and
oxygen. Water and gravity control the direction of growth,
and the oxygen supply determines whether or not growth can
take place or the roots survive.

In the plains of eastern Kansas, the roots of plants may
penetrate certain soils to depths of from 15 to 20 feet. The
absorbing parts of these roots actually reach the water table ;
that is, they reach the level at which the soil is saturated,
or the level to which water would rise in a well.

Two or more species of plants are sometimes found associ-
ated in dry regions, and locally in dry habitats, because their
roots get their water at different levels and hence do not
compete with each other. For example, in our Southern
deserts the giant cactus commonly grows with the creosote
bush. The former plant obtains its water from the super-
ficial layers of the soil, while the latter obtains its water at
deeper levels. The roots of lawn grass are very superficial,
and lawn grass suffers from drought much sooner than do
the deeper-rooted dandelion and English plantain that occur
with it as weeds.

In dry regions where plants compete with one another, suc-
cess comes mostly to those that secure a sufficient water
supply. In moist regions success in competition between
plants depends chiefly on ability to reach the light or with-
stand shade.

The pressure of growth. The pressure exerted by roots in
penetrating the soil may be very great, amounting to hun-
dreds of pounds to the square inch. This is readily appreci-

Roots and Their Environment 185

ated when one sees cement sidewalks broken and large rocks
moved by the growth of roots under them. Growth pres-

FIG. 105. Fern leaves pushing upward through a cement sidewalk.
(After G. E. Stone.}

sure is just as powerful in stems and other growing parts.
Fleshy roots like those of the radish and turnip sometimes
force themselves partly out of the ground by the thickening
of the upper portion.

Root contraction. As roots mature, they may contract
in length and so draw the base of the stem a slight distance
into the soil. In this way crevice plants on cliffs are con-
tinually held firmly in place, in spite of the wearing away of
the cliff face by .erosion. In the same way the crowns of
clover and plantain roots that have been lifted up by frosts
may be drawn into the soil, and small bulbs and tubers, many
of which are formed at higher levels than the parent bulbs,
may be pulled deeper into the soil by root contraction.

Root duration. The roots of various plants are annual,
biennial, or perennial. Perennial plants may have either
annual or perennial roots, just as they may have either an-
nual or perennial aerial stems. Plants with bulbs, tubers, or
corms grow a new set of roots each year. Plants with root-
stocks, like the May apple and Solomon's seal, generally
have roots that last for several years. Shrubs and trees also
have perennial roots. We must be sure to understand, how-

i86

Science of Plant Life

U. S. Forest Service

FIG. 106. Typical section of a mountain slope in western North Carolina, after
removal of forest. The binding effects of the roots have been removed, and the erosion
of the soil is so rapid that it is difficult for seedlings to take hold. When the forest was
cut, enough young trees should have been left to hold the soil and start a new lumber crop.

ever, that even in perennial roots the work of absorption is
for the most part done by the new roots which are added each
year. Most biennials, like the common evening primrose
and wild carrot (page 315), have fleshy roots in which food
accumulates during the first year. This food is used in the
rapid development of the plant during the second season.

Ecological types of roots. Most of the root characteristics
thus far described are those of the roots of mesophytes. In
hydrophytes, or water plants, the roots are notably smaller
and less branched than in mesophytes. They absorb water
and mineral substances from the soil even when the plants

Roots and Their Environment

are totally submerged. The roots of hydrophytes, like the
leaves and stems, are remarkable for the presence of internal
air cavities (page 156).

When the roots of
land plants (mesophytes)
extend into well-aerated
water, they develop in-
numerable branches, dif- >
fering in this respect - •
very markedly from the
roots of hydrophytes.
On account of this fact,
roots of trees, especially
those of willow and cot-
ton wood, that enter V
drain pipes and tiles
often develop masses of i
fine branches that ob- — '
struct the flow of the ••-
water even when the

entering root is not FlG I07 Holdfast roots of trumpet creeper, de-
thicker than the lead in veloped from the nodes. These roots are perennial

a pencil. The banks of and may lengthen and branch for several years*
streams are often protected from erosion by the mat of roots
developed along the water's edge. This is why willows are
planted on levees.

In moderately dry regions the roots of xerophytes may
penetrate to considerable depths, but in deserts many of the
largest plants have only small root systems, spread in the
upper layers of the soil (page 184).

Climbing plants, like the Virginia creeper, poison ivy,

i88

Science of Plant Life

FIG. i 08.

Bureau of Science, P. I.
Epiphytes on the branches of trees in the rainy tropics.

Roots and Their Environment

189

Boston ivy, and trumpet creeper, develop holdfast roots which
help to support the vines on trees, walls, and rocks. By
forcing their way into minute pores and crevices, they hold
the plant firmly in place. Usually the roots die at the end of
the first season, but in the trumpet creeper they are perennial.
In the tropics some of the large climbing plants have holdfast
roots by which they attach themselves, and long, cordlike
roots that extend downward through the air until they strike
the soil and become absorbent roots.

Epiphytes. A plant that lives perched on another plant is
an epiphyte (Greek: epi, upon, and phyton, plant). Mosses
and lichens are the most common epiphytes in temperate
regions, but in the rainy
tropics and along our own
Southern coast many flower-
ing plants live attached to the
branches of trees. They usu-
ally have leathery leaves and
a low transpiration rate.
Many have water-storage
tissue in fleshy stems or in
thickened leaves. Others are
called tank epiphytes, because
they catch water in the axils of
the leaves or in pitcher-like
leaves. Epiphytes cling to
the supporting tree by means
of roots that act both as hold-
fasts and water-absorbing

Organs. They do not take ffIGh J09' Florida epiphyte

It belongs to the Uromelia or pineapple

their nourishment from the family.

190

Science of Plant Life

plants on which they grow
(page 249), but depend for
their water upon the evenly
distributed rainfall and for
their mineral substances upon
dust and the decay of the
bark on which they live.

Epiphytes are pronounced
xerophytes, for there is prob-
ably no habitat in which it
is more difficult to maintain
a water balance than the one
in which they live. It is not
surprising, therefore, to find
that among the epiphytic
plants of the West Indies
there are several species of
cactus. Among epiphytes

there are many species of ferns, and many species belong
to two families of flowering plants, the Bromelias and
orchids. The Bromelias are related to the pineapple and
have leaves of the same type. The orchids have flowers
remarkable for their shapes and colors, and have the dis-
tinction of being the highest priced of all flowering plants.
The long moss of Florida, a flowering plant, is perhaps the
best known of American epiphytes. It is an extreme form
and is devoid of roots. The roots of many epiphytes contain
chlorophyll and assist in the manufacture of food.

Roots and transplanting. Only a few years ago it was
thought impossible to transplant large trees or even medium-
sized conifers. Today trees of large size are dug up, trans-

FIG. no. An epiphytic orchid.

Roots and Their Environment

191

U. S. Forest Service

FlG. in. Live-oak tree draped with Spanish moss. This epiphyte, which is
common along the Gulf Coast, has no roots. It is a flowering plant belonging to
the Bromelia family.

ported many miles, and replanted successfully. Even whole
hedgerows several feet in height are transplanted without
injury. This advance in the art of tree moving is a fine ex-
ample of the application of a knowledge of root physiology
to practical problems.

We have learned that the absorbing part of the roots is
mostly in the root-hair zone near the root tips. Formerly
when a tree was dug up for transplanting, all the roots were
cut off 3 or 4 feet from the base of the stem. This operation
destroyed practically all the absorbing organs, and the tree
could not absorb water from the soil until a new set of roots
had developed. Meanwhile it suffered from extreme drought
and not infrequently died.

192

Science of Plant Life

FIG. 112. By using great care to pre-
serve the root system a tree may be
transplanted even when in leaf. Some
hours before the tree is lifted it is thor-
oughly watered in order that the leaves
and other parts may have in them as
large a supply of water as possible.

FIG. 113. The long roots are carefully
laid bare, leaving a ball of earth close to
the trunk of the tree . B ef ore this is done
the hole where the tree is to be planted
should be made ready. It should be so
dug that the tree will not be set deeper
than it was in its original location.

FIG. 114. The roots are wrapped to
keep them from drying, and tied to the
trunk of the tree. The natural environ-
ment of the roots is the moist soil, and it is
very important that they be kept moist ;
even a few minutes' drying may be fatal
to the delicate rootlets.

FIG. 115. The tree is placed on a sled
and securely fastened. It is then hauled
to the new location, where the roots are
carefully spread out and covered with
soil. Fertile topsoil and not the raw
subsoil should be used for covering the
roots.

Roots and Their Environment

193

FIG. 1 1 6. The newly planted tree is
supported by ropes or wires until the
roots can take' hold and anchor it
securely. The soil should be packed
firmly about the roots to facilitate the
absorption of water and allow the quick
development of rootlets and root hairs.

FIG. 117. The tree must be kept well
watered (but not flooded) for a time, for
many of the absorbing rootlets have been
lost and there is danger that the leaves
will not receive an adequate water sup-
ply. By this method large trees are suc-
cessfully transplanted when in full leaf.

Success in transplanting is attained by gradually trimming
the roots months before the tree is moved, and by loosening
the soil near the tree so as to develop a mass of absorbing
roots near the base of the stem. When the tree is lifted, the
roots are not cut off, but as many as possible of them are
carefully removed from the soil. The small roots of trees are
killed by drying, and for this reason they are protected from
wilting by being bound up in wet moss. Sometimes the trees
are loosened somewhat in the autumn and moved during the
winter, together with much of the frozen soil surrounding
the roots. Successful transplanting depends upon reducing
temporarily the loss of water by trimming the top, preserving
the absorbing roots, and exercising care in handling both
roots and stems so that they may not be injured. (See also
Page 55.)

194

Science of Plant Life

Roots in relation to bacteria and fungi. The roots of many
plants have bacteria or fungi growing about them or inside

them. The best-known crop
plants belonging to this group
are the clover, cowpea, and
alfalfa; their roots develop
small nodules in which cer-
tain kinds of bacteria change
nitrogen of the air into nitro-
gen compounds which may
be used by the plants. More
information about these bac-
teria will be found in a later
chapter (page 258).

Many of our trees and
shrubs have fungi surround-
ing their roots. The beech

FIG. 118. Roots of soy bean, showing nodules tree for example, flourishes
containing nitrogen-fixing bacteria.

only when it grows under

such conditions. The difficulty in transplanting azaleas,
laurels, and rhododendrons from the woods to our lawns lies
largely in supplying conditions favorable to the fungi that
invest the roots. It is easy to supply the proper shade and
water conditions for the shrubs, but it is difficult to furnish
soil conditions favorable to the life of the fungi. The trans-
planting of these shrubs is therefore most frequently success-
ful when they are planted in large bodies of soil brought with
them from their natural habitat. Just how the fungi aid the
plant is not understood ; that they are essential is very clear.
Commercial uses of roots. The fleshy roots of the sweet
potato, yam, turnip, carrot, beet, celeriac, and salsify are

Roots and Their Environment 195

important sources of food. The sugar beet is the most im-
portant domestic source of sugar in the United States. The
roots of sassafras, rhubarb, ginseng, aconite, and ipecac are
used in medicine. In the past many other roots were collected
for the same purpose. The roots of plants are used by man
much less than are stems and leaves. Perhaps roots are in
general less useful ; but the failure to utilize them more gen-
erally is in part accounted for by the difficulty of harvesting
them and by our lack of knowledge concerning uses that
might be made of them.

PROBLEMS

1. What states produce sugar from the sugar beet?

2. What states produce sweet potatoes?

3. How are roots made use of in the jetties at the mouth of the Mississippi
River?

4. Why do trees along city streets frequently die when the street is paved or
cement walks are laid inside the curb ?

5. Why does the filling in of wooded land with a 3 or 4 foot layer of soil kill
trees?

6. What flood-plain trees will continue to live in an area that is flooded with
water for a year or more, as by the building of a dam? Why? Why do
other trees die?

Suggestions for Laboratory and Field Work to Precede
Chapter Sixteen

1. Examine and make diagrams of several types of flower
clusters, such as spike, catkin, umbel, raceme, and panicle.

2. Draw several flowers of different types. Label pistil,
stamen, corolla, calyx, petal, sepal, receptacle, peduncle, anther,
filament, ovulary, style, and stigma. Note differences between
monocot and dicot flowers.

3. Examine pollen under a microscope, and germinate grains
in sugar solutions. Try various strengths from 5 to 10 per cent.
Place pollen in a drop of the solution on a micro slide, cover with
cover glass, and keep in a moist chamber. The pollen tubes develop
by the following day.

4. Examine the stigmas of flowers for pollen that has ger-
minated.

5. Soak different kinds of seeds in water and study them,
noting embryo, endosperm, seed coats, cotyledons, hypocotyl, and
plumule.

6. Study mature pine cones with seeds in place, noting the
number of the seeds and their relation to the scales. Dissect a
seed and note the seed coats, endosperm, and embryo.

7. Germinate some seeds and follow the stages in germination.

8. Collect as many kinds of fruits as possible. Study them
from the standpoint of their contents, their origin from the flower,
and their methods of dispersal. Draw as many as convenient.

9. Field trip to study the variety of flowers and flower clusters.

10. Methods of pollination may best be studied in the field.
Note the kinds of insects that visit a particular kind of flower.
Compare these insects with those that visit another kind of flower.
Is there a noticeable difference in the forms of the two flowers?
in the location of the pollen ? in the location of the nectar ? Does
this account for the difference in the kinds of insect visitors ?

11. Examine several varieties of flowers and note whether or
not the stamens shed their pollen when the stigma is ripe, before it
is ripe, or after. Does the relative time at which the stigma and
pollen mature favor self -pollination, insure it, make it difficult, or
prevent it?

196

CHAPTER SIXTEEN

REPRODUCTION IN FLOWERING PLANTS: FLOWERS, FRUITS,

AND SEEDS

FIG. IIQ. The plants in flower have reached the reproductive phase of
their life.

THE development of roots, stems, and leaves makes up the
first period of the life of a flowering plant. These parts form
the vegetative body of the plant, and are all primarily con-
cerned, as we have seen, with the production, distribution,
and accumulation of food materials. The period of growth of
the vegetative parts and of activity in connection with the
food supply is the nutritive phase of the plant's life.

The development of flowers, fruits, and seeds takes place
during the second phase of the plant's existence, the repro-
ductive phase. The foods that have previously accumulated
in various parts of the plant are, to a large extent, transferred ;
they are used in the building of the reproductive structure?

197

198 Science of Plant Life

and in supplying the seeds with the store of nourishment
needed for germination and for the early growth of the seed-
lings. The reproductive phase of the plant's life is essentially
a food-transferring and food-consuming one ; it begins with
the production of the flower and ends with the maturing of
the seeds. In perennials these phases are not distinct as they
are in annuals and biennials.

The flower. The flower fe a specialized shoot, whose end
is the production of seed. Commonly the word " flower "
is associated with the brightly colored parts that make many
of our garden and house plants so attractive. But here we
shall include under the term the simple structures associated
with seed production in plants like the grasses, poplars, and
birches, that have merely scalelike leaves and bracts inclosing
the reproductive parts. In the conifers the seeds are pro-
duced on scale leaves arranged spirally in cones. These cones
may be looked upon as a lower type of flower, structurally
very different from the flowers of the monocots and dicots.

Flower clusters. The arrangements of flowers on stems
are so varied in different plants that it is quite beyond the
scope of this book to describe the many kinds of flower clus-
ters. In many plants the flowers occur singly at the ends of
stems or lateral branches, as in the tulip and in some varieties
of roses. In other plants they are arranged in groups, as in
the spike of the common plantain and cat-tail ; the catkin of
the willow, alder, and oak; the umbel of the carrot, onion,
and milkweed ; the raceme of the snapdragon, spring beauty,
black locust, and larkspur; and the composite head of the
sunflower and chrysanthemum. The head of wheat and the
ear and tassel of corn are other forms of flower clusters. Plants
like the yucca, curly dock, rhubarb, broom corn, and hy-

Reproduction in Flowering Plants 199

drangea furnish examples of large and much-branched flower
clusters. A stem which bears a single flower or flower cluster
is the flower stalk or peduncle. The branches of the peduncle

FIG. 120. Flowers of the corn plant. The panicle of staminate flowers (tassel) is
shown above. Below are the pistillate flowers arranged in a spike (ear) inclosed by
sheathing leaves. The only part of the pistillate flowers exposed to the air is the long
style (silk).

200 Science of Plant Life

which bear the individual flowers in a flower cluster are called
pedicels.

The parts of the flower. The apex of the flower stalk is
called the receptacle. It is often enlarged and serves as a
place of attachment of the various floral organs. The outer
whorl of scales or leaflike organs is the calyx. It usually is
green in color, and in the bud stage it completely incloses the
flower. The individual parts of the calyx are called sepals.
Next inside the calyx is a whorl of white or brightly colored
leaves that make up the corolla. The several parts of the
corolla are called petals. The corolla is usually the attractively
colored part of the flower ; but in some flowers, as in the tulip
and clematis, the sepals have the same coloring as the petals.
The calyx and corolla are often spoken of as the floral envelopes,
because in the bud they form a wrapping, or envelope, for
the inner parts of the flower.

Inside the corolla is a group of stamens, each composed of
a stalklike filament and an anther that contains the pollen.
The center of the flower is occupied by one or more pistils,
each made up of an ovulary, style, and stigma. The ovulary
is the enlarged part of the pistil that contains the ovules,
which develop into the seeds. The style is the stalk above
the ovulary that bears at its summit the stigma. The stigma
is usually an enlarged surface, which secretes a sticky, sugary
solution in which the pollen grains are caught and germi-
nated. The pistils and stamens are called the " essential or-
gans " of the flower, because they produce the ovules and
pollen which are the two elements necessary for the production
of seed.

The variety of floral structures. The above is a descrip-
tion of a typical flower; but in the plant world we find an

Reproduction in Flowering Plants

201

almost endless variation in the number, form, size, color, and
arrangement of these parts. In some flowers the calyx or
the corolla may have their parts united into a tube, or one or
both may be wanting. Or the flowers may lack either pis-
tils or stamens. For example, the soft maples bear pistil-
late flowers on some trees and staminate flowers on others,
and the corn has staminate flowers in the tassel and pistil-
late flowers on the lower lateral branches or ears. It is not
our purpose to name and describe here the many different
variations in floral structure; a visit to a conservatory or
a tramp through the near-by fields and woods is a far more
effective way of securing an idea of the great diversity of
flowers.

Pollination. If the stamens of a lily or nasturtium are
examined, the pollen is found to be a fine yellow powder,
which under a microscope will be seen to be composed of a
multitude of small grains. For the production of seed it is
necessary that the pollen grains shall be carried to the stigma.
This transfer is called pollination. In some plants the pollen

FIG. 121. Flower spikes of the alder. The two clusters on the left are staminate
spikes ; on the right the mature pistillate spikes are shown.

202 Science of Plant Life

merely falls by gravity on the stigma. Wheat and oats are
examples of plants that are pollinated in this way. In other
plants, like the pines, elms, birches, oaks, rye, and corn, the
pollen is carried by the wind. It is an interesting fact that
the stigmas of wind-pollinated flowers are usually roughened
by hairs, which probably make them more effective in hold-
ing the pollen.

In the case of most plants with conspicuous flowers, the
pollen is carried by bees, flies, butterflies, and moths. As
the body of one of these insects is rough or hairy, pollen grains
become attached to it when the insect enters a flower. Then
when the insect passes to another flower, some of the pollen
from the first flower is brushed off on the stigma of the second.
Thus pollination is brought about by the insects in the course
of their visits to successive flowers. It is an advantage to
the plant to have its pollen carried by insects directly from
flower to flower instead of being blown about and reaching
a stigma by mere chance. If the amounts of pollen pro-
duced by the pine, corn, ragweed, and other wind-pollinated
plants are compared with the amounts produced by plants
that are pollinated by insects, it will be seen that insect-
pollinated plants generally produce less pollen.

Why insects visit flowers. Insects do not visit flowers to
carry pollen for the plants. They eat the pollen or feed their
young on it, and they also secure nectar from the flowers.
The nectar is a watery solution containing sugar, which is
secreted by glands called nectaries. One or more of these
nectaries is usually located near the base of the corolla, in-
side the flower. The insects visit the flowers, therefore, to
secure food for themselves, but as they make their visits they
brush against the stigmas and leave grains of pollen adhering

Reproduction in Flowering Plants 203

to the plants, and in this way perform a service for the plants.
The perfumes of flowers aid the insects in finding them, and
conspicuous white or brightly colored parts of flowers may
serve the same purpose. The massing of many small flowers
in clusters and heads certainly makes them more conspicuous.

Cross-pollination. When a flower is pollinated with its
own pollen or with that from another flower on the same
plant, it is said to be self -pollinated. If the pollen comes from
another plant, a flower is said to be cross-pollinated. In
many plants it makes no difference whether the pollen comes
from the stamens of the same plant or from those of another
plant. In the common tobacco plant the pollen may be
transferred to the stigma of the same flower, and seeds will
be produced. In some plants, however, it is only when the
flowers are cross-pollinated that seeds are formed. The
sunflower is a good example of this kind of plant. In still
other plants seeds that are formed after self-pollination are
less vigorous than those formed after cross-pollination.

From the above statements it will be seen that cross-pol-
lination is an advantage to some plants, and we find in flowers
many arrangements that help to bring this about and to pre-
vent self-pollination. Often the anthers do not shed their
pollen at the time when the adjoining stigma is in condition
to receive it. The pollen may be shed either before or after
the ripening of the stigma. In such plants there is little
possibility of the stigma's being pollinated from the stamens
of the same flower. So, as insects go from one flower to
another they transfer pollen from flowers in which the pollen
is ripe to flowers in which the stigmas are ripe. This favors
cross-pollination .

It is exceedingly interesting to study the various other

204 Science of Plant Life

mechanisms that favor cross-pollination, but it should be done
in the field or with the flowers in hand. In the white lily the
stigma is out of reach of the insects when the pollen is shed.
In other plants the pistillate and staminate flowers may
occur on different individuals, or on different branches of the
same plant. In primroses and bluets the stigmas and
stamens each have two different lengths ; the flowers on one
plant have long styles and short stamens, while the flowers
on another plant have short styles and long stamens.

The most remarkable cases of cross-pollination by insects
are those in which a particular species of insect is necessary
for the pollination of a plant. Such relations exist in the
yuccas and in some orchids. In the absence of the particular
insect, pollination and seed production fail. Yuccas may be
grown in our Northern states, but in certain localities they
fail to produce seeds because the moth (Pronuba) needed to
pollinate the flowers does not live there.

Formation and growth of the pollen tube. A second step
essential to the production of seed is the germination of the
pollen and the formation of the pollen tube. After a grain of
pollen is placed on the stigma, a microscopic tube develops
from its side and grows downward among the cells of the
stigma and style. At the time of shedding, the pollen grain
of most flowering plants contains three cells. One of the three
is active in the formation of the pollen tube ; the other two are
the sperms or male cells.

The stigma, as we have seen, secretes a sticky fluid con-
taining sugar, acids, and other substances. The pollen
germinates best in the fluid secreted by the stigmas of the
same kind of plant, and it usually germinates imperfectly or
not at all on the stigma of a different kind of plant. Perfect

Reproduction in Flowering Plants

L20S

pollen tubes sometimes develop, however, following pollination
from related species. The seeds produced by such crosses

FIG. 122.

Pollen grains and pollen tubes. S is the two sperms or male cells,
and T the tube nucleus.

often give new forms of plants that are different from both
parents.

After germination of the pollen, the pollen tube grows down-
ward through the stigma and style to the ovule, or young
seed. Usually this is but a short distance. In corn, how-
ever, the silk is the style and stigma, and the pollen tube must
grow several inches or a foot down the silk to the ovule below.
As the pollen tube lengthens, the sperms pass down the tube.

Fertilization. Inside the ovule there is an oval, saclike
body which contains several cells. One of these is the egg
cell, or female cell. At the beginning of fertilization the
pollen tube grows into the ovule and discharges the two
sperms into the sac in which the egg cell lies. One of the
sperms unites with the egg cell. This union of the sperm.

206

Science of Plant Life

or male cell, with the egg, or female cell, is the act oj
fertilization.

The fertilized egg immediately begins to develop ; that is, it
grows and divides into a large number of cells, and from it
eventually comes the embryo or young plant that is found
within the seed. Fertilization is the essential part of sexual
reproduction both in plants and animals, and it marks the
actual beginning of a new generation. After the egg is fer-
tilized, it is able to develop into a new plant or animal of the
same kind as its parents. It should be understood that the
sperms from one pollen tube fertilize the egg in only one ovule,
and that to fertilize all the eggs, as many pollen tubes must

FIG. 123. Diagram to illustrate stages in the reproduction of a seed plant. A shows
pollen grains (b) from anther (a) on top of stigma ; B shows a stage in the growth of
the pollen tube and the formation of the egg cell ; C is the stage when the pollen tube
has reached the egg and sperm and egg unite; D shows the formation of the embryo
plant within the seed; and E shows the further development of the embryo and the
maturing of the seed coats. The young plant thus formed and incased may remain
alive for years within the seed.

Reproduction in Flowering Plants 207

grow down through the style as there are ovules in the ovulary
below.

The seed. The seed is the final product of pollination and
fertilization. Its complete development ends the role of the
flower. The essential part of the seed is the embryo, for it
is the embryo that will later produce the seedling; and in
order to supply the embryo with nourishment during the
early stages of growth, food from the parent plant is ac-
cumulated in the seed. This may either be stored inside the
embryo itself, as in the bean ; or it may be stored in the tissue
surrounding the embryo, as in the castor bean and corn. The
food-containing tissue surrounding the embryo in many seeds
is called the endosperm. Since the seed carries the plant over
the winter or through an unfavorable season, the embryo with
its food supply is protected by the seed coats, one of which is
usually hard and resistant.

The three constituents of a seed, then, are (i) the embryo
or young plant, (2) the food supply, either inside the embryo
or in the endosperm, and (3) the seed coats or protective cover-
ing.

The structure of seeds. Although seeds vary as much in
form as do other plant organs, the different arrangements of
the three essential parts may be illustrated by a castor bean,
a bean, and a grain of corn.

In the castor bean the seed coats consist of a hard outer
layer and a thin inner membrane. These inclose an endo-
sperm, which is a mass of cells containing food in the form of
starch, oil, and protein. Within the endosperm lies the
embryo, ready to grow when favorable conditions for germi-
nation come. The embryo consists of the hypocotyl and two
cotyledons, with a small bud between the cotyledons, called

208

Science of Plant Life

the plumule. The cotyledons are the first leaflike organs of
the plant. The hypocotyl is the first stem, and the plumule

is the first bud. No root is
found in the embryo ; but
when the seed germinates the
hypocotyl elongates, and from
its end the primary root de-
velops. The cotyledons at
first absorb food from the en-
dosperm and later expand into
photosynthetic organs, which
become green when exposed
to the light. The plumule
grows upward to form the
stem. All these early changes
are made at the expense of the
food in the endosperm.

The bean seed consists
merely of the embryo with a
seed coat about it. The food
in this seed has already been

FIG. 124. Development of mangrove seed- .

lings. This small tree grows on soft mud absorbed into the embryo and
flats in the tropics and semi-tropics. The stored in the greatly thickened

seed (A and C) germinates while still at- .

tached to the tree and forms an embryo a Cotyledons ; that IS, the young

foot or more in length. The embryo embryo has Continued its

finally drops endwise like an arrow into , _,, -, , .1 •,

the mud and starts a seedling (D). growth m the Sf;ed Untl1 *

has all the food inside itself.

The parts of the embryo are the same as in the castor
bean, but the cotyledons are thick and contain a great
supply of food for the young plant. The bean is an ex-
ample of a large class of plants, including the pea, squash,

Reproduction in Flowering Plants

209

Bureau of Science, P. I.

FIG. 125. A mangrove swamp. Note the prop roots which support the trees,
and the young plants in the foreground which have dropped from the overhanging
branches.

and pumpkin, in the mature seeds of which the endosperm
is lacking.

A grain of corn is an example of a third kind of seed. In it
there is a large endosperm with a small embryo placed at one
end of it. The embryo differs from the embryos of the bean
and the castor bean in that it has only a single cotyledon,
wrapped more or less around the hypocotyl and plumule.
In germinating, the hypocotyl grows downward, and the
primary root develops from its tip. The plumule grows
upward to form the aerial shoot. As in the castor bean, the
cotyledon is the absorbing organ through which the foods in
the endosperm enter the young plant.

The flower and embryo in monocots and dicots. In dis-
cussing the subject of stems, attention was called to the fact

210

Science of Plant Life

Bureau of Agriculture, P. I.

FIG. 126. A pineapple field. The pineapple fruit is an enlarged fleshy
flower cluster.

that flowering plants are divided into two great groups, the
monocots and dicots. The monocots have parallel-veined
leaves ; the bundles of the stem are closed (have no cambium) ;
and the bundles are not arranged in a circle. The dicots in-
clude forms with net- veined leaves; the stem bundles are
open (have a cambium) ; and they are arranged in a circle.

The terms " monocot " and " dicot " (or, as they are fre-
quently written, " monocotyledon " and " dicotyledon ") are
based on the apparent number of cotyledons in the embryo,
whether there are one or two. Any one who has watched
plants beginning to grow in a garden will recall the two
cotyledons of the bean, pumpkin, sunflower, and radish, raised
above the soil. It will also be recalled that these plants
have net- veined leaves. The cotyledon of a monocot' is
usually an absorbing organ that remains below the ground in
contact with the endosperm.

Reproduction in Flowering Plants

211

The two groups differ in their
flowers also. In the monocots the
number of parts of the calyx and
corolla is usually three, and the
stamens and divisions of the pistil
are three or some multiple of three.
In the dicots the parts of the flower
are typically in fives or fours, or in
a multiple of these.

Thus the names "monocot" and
" dicot " relate to the form of the
embryo ; but the two groups are
further distinguished by differences
in leaf venation, bundle structure,
bundle arrangement, and flower
plan.

The gymnosperms and angio-
sperms. We have previously learned
that the conifers bear their seeds on
scale leaves arranged in cones. A
study of one of these cones shows
that the seeds are formed on the
upper surfaces of the scales and are
not inclosed in capsules. When the scales mature and be-
come dry, the cone opens and the seeds fall out. The word
" gymnosperm " means " naked seed," and this is the group
name for the conifers and all other plants whose seeds are
not inclosed.

The angiosperms are what we usually call the flowering
plants, although some of them, like the grasses and forest
trees, do not produce flowers with colored parts. The seeds

U. S. Dept. of Agriculture
FIG. 127. Fruit of mango.
This much-prized fruit has
hitherto been grown only in
the tropics, but recently it has
been introduced into southern
Florida.

212

Science of Plant Life

U. S. Dept. of Agriculture

FIG. 128. Avocado, or alligator
pear, a salad fruit now being grown,
in southern Florida and California.

U. S. Dept. of Agriculture

FIG. 1 29. Raspberries, a fruit
which is made up of a collection
of fleshy pistils.

of an angiosperm, in contrast to those of a gymnosperm, are
inclosed in an ovulary commonly called a pod or capsule, as
in the bean, horse-chestnut, hickory nut, and watermelon.
The term " angiosperm " means " hidden or covered seed,"
referring to the fact that the seeds are inclosed by a pod,
fleshy fruit, or other covering.

The fruit. The term "fruit" is commonly used to desig-
nate a great variety of organs that are developed as a result
of the flowering and pollination of plants. The direct result
of pollination and fertilization is the production of the seed.
The indirect effect of pollination is the further development of
adjacent structures. Primarily the fruit is the enlarged pistil
or ovulary ; but in many cases the calyx and the receptacle also
enlarge and form a part of the fruit — sometimes most of it.

Reproduction in Flowering Plants

2*3

In the cucumber, watermelon, yucca, and tomato the
fruit is the enlarged ovulary. In the okra, wild geranium,
maple, hickory, and elm the entire pistil is involved. In the
haw, apple, blueberry, and pear the calyx forms part of the
fruit. In the strawberry, lotus, rose, and fig the fleshy part
of the fruit is the enlarged receptacle. It is a remarkable fact
that the fertilization of the egg in an ovule not only causes
the egg to develop into a new plant (the embryo), but also
causes other parts of the flower to enlarge and become fruit.

The grains like wheat and corn are both seeds and fruits,
for the outer coat contains the ovulary wall as well as the
seed coats proper.

Economic importance of flowers, fruits, and seeds. The
economic value of flowers lies chiefly in their use for decorative
purposes, but certain flower clusters like the artichoke and
cauliflower are used as food. The fruit industry needs only
to be mentioned to call to mind the vast scale upon which
plants are grown for their fleshy edible fruits. It should be

U. S. Dept. of Agriculture
FIG. 130. Coffee berries, natural size. Each contains two seeds.

214

Science of Plant Life

noted that ripe fruits are made
up largely of water pleasantly
flavored with sugar, dilute
acids, and aromatic sub-
stances. The amount of food
actually present is usually
small. Fruits are valuable
chiefly for the variety which
they add to our diet. Through
canning, preserving, and dry-
ing they are made available at
all seasons of the year.

Seeds and grains supply the
most concentrated foods de-
rived from plants. They pro-
vide the larger part of the food
of all human beings. Seeds of
cotton, corn, and coconut fur-
nish enormous quantities of
oils used in the manufacture of
various fats and soap, and nuts of various kinds are coming
to be used more and more extensively as foods. Corn oil is
widely used in the making of rubber. Flaxseed is the source
of linseed oil, which is used in the manufacture of paints.
From the grains we derive also starch, glucose, alcohol,
ether, and many related organic substances. The seeds of
the coffee and cacao plants supply pleasant and mildly
stimulating drinks. The hairy covering of the cottonseed is
the most important fiber used in the manufacture of cloth.

U. S. Dept. of Agriculture
FIG. 131. Persimmon fruits. The
persimmon grows wild over a large
part of the Southeastern states, and
improved varieties are now cultivated.

Reproduction in Flowering Plants

215

Bureau of Agriculture, P. I .

FIG. 132. Drying copra, the fleshy portion (endosperm) of the coconut seed. The
coconuts are split and set in the sun, and the meats, after they shrink and become
loose, are removed from the shells and placed on mats to dry. Copra is the source of
coconut oil.

PROBLEMS

1. Why does a corn plant growing alone produce imperfect ears?

2. When cucumbers are grown in commercial greenhouses, how is pollination
accomplished ?

3. Why are large, heavy seeds of agricultural plants more desirable for plant-
ing than small, light seeds?

4. What common market "vegetables" are included in the botanical term
"fruit"?

Suggestions for Laboratory and Field Work to Precede
Chapter Seventeen

1. Review and as far as possible illustrate with specimens tne
various methods of propagating plants vegetatively. Here may
be included :

a. Bulbs, tubers, corms, and rootstocks.

b. Cuttings, as in the geranium and grape.

c. Runners and horizontal stems, as in strawberry and blue

grass.

d. Leaf cuttings, as in Begonia and Bryophyllum.

e. Small lateral branches (suckers), as in carnation, pine-

apple, and banana.

2. Make a field trip to a commercial greenhouse or nursery to
see how plants are propagated.

3. Report on the origins of some of the common garden vege-
tables and field crops. Information may be obtained in encyclo-
pedias and in agricultural and horticultural textbooks.

4. Study a few weeds in the field, trying in each case to deter-
mine why the plant is successful in growing where it is not wanted.
List the weeds, and put the results of your study in the form of a
table :

NAME OF WEED

PROPAGATED BY

REPRODUCED BY

SEEDS CARRIED BY

WHY DIFFICULT
TO EXTERMINATE

5. What are the five commonest weeds of lawns? of cornfields?
of pastures? of gardens? Why do different kinds of weeds occur
in these several habitats ?

216

CHAPTER SEVENTEEN

REPRODUCTION IN RELATION TO AGRICULTURE

THE reproduction of plants is of particular interest in agri-
culture because the reproductive structures themselves have
an economic value. In a former chapter the importance of
seeds and fruits as sources of food has been discussed. In this
chapter we shall consider the relation of reproduction to three
important phases of agriculture: plant propagation, plant
breeding, and weed control. Propagation and breeding fur-
nish better crop plants ; weed control provides more pro-
ductive fields.

Vegetative multiplication. In the discussion of stems
(page 159) attention was called to the fact that one of the
advantages in underground stems lies in the facility with
which the plant may be multiplied. From rootstocks arise
new terminal and lateral buds that later form new aerial
shoots, and through the death of the older parts of the under-
ground stems these branches become separate plants. Bulbs,

FIG. 133. Multiplication of the raspberry. The stems arch over and take root

where they touch the ground, thus starting new plants.

217

218 Science of Plant Life

corms, and tubers bring about vegetative propagation in a

similar way.

Plants may multiply from the aerial vegetative parts also.

The stems of the raspberry com-
monly bend over, and where they
touch the ground they form buds
from which adventitious roots and
new upright stems develop. A
grapevine will take root where a
node comes in contact with the
soil. In the walking fern (page

FIG. 134. Bryophyllum leaf, with 283) the tips of the leaves develop
young plants starting from the -

notches in the margin. buds, roots, and new plants when

in contact with the soil. The

strawberry is an example of certain plants, including many
grasses, which have horizontal branches (runners) on the soil
surface that take root at intervals and produce new plants.
In Bryophyllum, a weed in cultivated fields of the West
Indies, the leaves when they fall to the ground develop new
plants from the notches in their margins (Fig. 134).

These illustrations, which might be multiplied, show the im-
portance of vegetative propagation in the increase and spread
of plants. Among wild plants it is entirely possible that vege-
tative multiplication is as effective in spreading the plants as
is reproduction by seeds. By the former method the young
plant is able to start more vigorously than a seedling, because
it is able to draw water and food materials from the parent
plant until its own root and leaf systems are well developed.

Vegetative propagation of cultivated plants. In agriculture
and horticulture, vegetative multiplication is relied upon for
starting many cultivated plants. Potatoes, mint, horse-
radish, sugar cane, sweet potatoes, and certain varieties of

Reproduction in Relation to Agriculture 219

Bureau of Science, P. /.

FIG. 135. Sugar-cane cuttings. The plants are started in the fields
from cuttings and not from seeds.

onion are examples of crop plants started in this way. Gera-
niums, coleus, willows, currants, grapes, and most ornamental
shrubs are grown from cuttings. These cuttings are pieces
of stem containing two or more nodes. Propagation by
bulbs, conns, or rootstocks is the method commonly employed
in starting lilies, tulips, hyacinths, irises, cannas, Caladiums,
and chrysanthemums. Many of our fruit trees are multi-
plied by budding and grafting, which are specialized forms
of vegetative propagation (page 132).

Vegetative propagation has been found advantageous in
crop plants wherever its use is possible, (i) because the vari-
ability of the plants produced is much less than when they are
propagated by seeds, (2) because some plants, like the sugar
cane, banana, and horseradish, do not produce seed, and (3) be-
cause it saves time in securing the product, as a longer period
is required for the maturing of plants started from seeds. In
practical plant production, vegetative multiplication is as
important as reproduction by seeds.

220

Science of Plant Life

Plant breeding. Reproduction in plants is the
foundation upon which the important industry of plant
breeding is being built. Plant breeding is concerned
with the improvement of economic plants and with
the production of new plants of economic value. The
activities of plant breeders are
being directed toward four
principal objectives : (i) the
breeding of plants with more
desirable products, as flowers,
fruits, leaves, and fibers ; (2) the
breeding of new varieties which
will increase the yield per acre ;
(3) the securing of varieties
better fitted to particular cli-
mates and soils ; and (4) the
producing of varieties capable of
greater resistance to diseases.

Plant breeding with these pur-
poses is possible and profitable
because (i) variations naturally
occur among plants ; (2) some
variations are inherited and
may be preserved by selection;
and (3) different varieties and
species may be crossed to pro-
duce hybrids having a still larger
range of variations than the

number of seeds produced, and by parent plants Or possessing new

selecting seed from only the plants combinations of desirable quali-

with long spikes the yield of seed may

be increased several fold. tlCS.

FIG. 136. Timothy spikes. The heads
show great variation in length and

Reproduction in Relation to Agriculture 221

Bureau of A griculture, P. /.

FIG. 137. Four types of Indian corn. From left to right : Sweet corn, which has a
high sugar content and shrivels when dried ; dent corn, which has a depression in
the crown of each kernel due to the shrinking of the endosperm ; flint corn, which has
a hard endosperm and rounded kernels; and popcorn, which is a small, extremely
flinty variety.

The methods used by the plant breeder depend upon the
reproductive structures and habits of the particular plants
with which he is working. For example, the means by which
a plant is naturally pollinated will determine how it must be
handled at the time of flowering to secure self-pollinated seed
or cross-pollinated seed. Methods of vegetative propagation
may be used to multiply the plant after a desirable variety
has been produced. In this way the plant grower avoids
cross-pollination and the variations that appear when many
cultivated crops are grown from seed. The best ways of
propagating particular crops vegetatively may be found

222

Science of Plant Life

described in recent publications devoted to plant production
and plant breeding.

Variations. No two fruits, flowers, or other plant organs
are exactly alike. The variations may be small or large, and
there may be every gradation between the extremes of any
character. The several thousand sunflowers that might be
grown from a pound of seed would vary in height of stem,
amount of branching, and size of flowers. Not only may
there be variations in the structures of plants, but there may
also be variations in the composition of the plant organs.
For example, the great variety of colors, flavors, and other
qualities in apples is due to variations in the composition of
this fruit. The variation in each of these characters is quite
independent of variations in the others. It is possible, there-
fore, to get all kinds of
combinations of different
characters, and by careful
breeding and selection to
combine many desirable
qualities in a single plant.
The Shasta daisy, for ex-
ample, was made by breed-
ing together the English,
American, and Japanese
daisies, and combining in
one plant the pleasing foli-
age of the English species,

FIG. 138. Two plants of sweet corn of the the free-blooming habit of
same variety, one grown in poor soil and the American daisy, and the

waxy luster of the petals of

one in soil to which fertilizer was added.
The differences in the plants are due to the
environment.

the Japanese plant.

Reproduction in Relation to Agriculture 223

Bureau of A griculture, P. I.

FIG. 139. Five varieties of lima beans, all grown in the same soil. The differences
are due to differences in the plants.

Two kinds of variations. When a crop plant like corn is
grown in rich soil and in poor soil, there are great differences
in the size of the plants and in the yield per acre. These
variations in size and yield are due to the environment. Even
though the seed planted in each kind of soil is exactly the
same, there will be a wide difference in the plants.

On the other hand, there are variations that are due to in-
nate differences in the plants themselves. If dwarf sweet
corn and large field corn be planted in the same soil and
given the same treatment in every way, they will still be very
different. It is a natural quality of sweet corn, as compared
with field corn, to produce a smaller stalk and to have more
sugar in the grain; and these differences will appear even
when the environments are the same. We have, therefore,

224 Science of Plant Life

U. S. Dept. of Agriculture

FIG. 140. Tobacco plants of the same variety grown from large, medium, and small
seeds, showing the relation between the size of the seed and the size and vigor of the
seedling. Is the difference in size in the plants due to environment or to differences
in the plants themselves?

two kinds of variations in plants : variations due to environ-
ment and variations due to differences in the plants themselves.

All variations are of interest to the plant breeder ; but since
he is trying to produce new plants that may be perpetuated,
he is most interested in those variations which are inherited,
-that is, variations which may be carried over from one
generation to another. There is little or no evidence that
variations produced by environment may be inherited; a
variety of corn grown for many generations on rich soil would
develop no larger plants on poor soil than would corn of the
same variety that had always been grown on poor soil.

Mutation. ' Sometimes, among many thousands of individ-
uals, a single plant appears which is markedly different from
all the others. For example, a few years ago a sunflower was
discovered that had some red pigment near the base of the
otherwise yellow corollas. Among the millions of sunflowers

Reproduction in Relation to Agriculture 225

that have been seen, this was the first one in which a red
color was noticed. In some unknown way there was produced
in this plant a red pigment not formed in other sunflowers.
From the seeds of this plant there were developed other plants
having red pigment in their flowers. Evidently the new
character is inherited and these sunflowers have a chemical
constitution which enables red pigment as well as yellow to
be formed. The sudden appearance of the sunflower with
the red pigment is an example of mutation. Individuals that
first show new characters are called mutants (Latin : mutarej
to change).

What the plant breeders have long known as " sports " are
the rare mutants in which notable changes have occurred.
They show new characters, and these characters are in-
herited. Consequently, their discovery is of the greatest
importance.

The Concord grape was a mutant selected from among many
seedlings, the others of which showed only the usual variations.
The many modern varieties of tomato have been developed

FIG. 141. Varieties derived from the wild cabbage (F), a native plant of Europe.
A is kohl-rabi, B cabbage, C cauliflower, D kale, and E Brussels sprouts.

226 Science of Plant Life

from mutations that occurred among the currant tomatoes
or love apples grown for ornament in our great-grandmothers'
gardens. The original fruits resembled large red currants.
Today single tomato berries may weigh a pound. In color
they may be red, yellow, or pink, and in shape they may be
spherical, plum-shaped, pear-shaped, or flattened. They
exhibit at least three types of leaves and two types of stems.
The characteristics due to mutation are inherited, no matter
what the soil and climatic conditions may be.

Mutations occur not only among plants grown from
seed, but also among plants, or plant parts, developed from
buds. These are called bud mutations, or bud sports. On
fruit trees, one branch will occasionally produce fruit that
is of different quality from the fruit produced on other
branches. If the quality of the fruit is superior, these
branches may be used in budding and grafting to preserve
the new variety. Bud sports are comparatively rare, but it is
estimated that at least several hundred horticultural varieties
have originated from them. In this country the improved
varieties of navel oranges have been secured entirely by this
method. The Boston fern and its forty or more varieties
originated in bud mutations from a wild tropical fern. In the
potato and some other plants that are usually propagated
vegetatively, bud variations are known to occur; but they
are so rare and so difficult to discover in plants of this kind
that they have not been of much practical value.

Hybridization. The crossing of two species or varieties of
plants is known as hybridization. It is brought about by
transferring the pollen from one to the stigma of the other.
The plants grown from seed produced in this way are called
hybrids. Hybrids may resemble one of the parents, or they

Reproduction in Relation to Agriculture 227

U. S. Dept. of Agriculture

FIG. 142. Vegetable trial grounds of the Office of Seed and Plant Introduction,
United States Department of Agriculture, Washington, D. C.

•'••"j&tefe,

• . -.:•:•

••!. I

*«••

U. S. Dept. of Agriculture

FIG. 143. Bulb trial grounds of the Office of Seed and Plant Introduction, United
States Department of Agriculture, Washington, D. C.

228 Science of Plant Life

may show some characters of each. In the second generation
derived from crosses some show a wide range of variation, with
all possible combinations of the characteristics of the parent
plants. Successful hybridization, therefore, frequently in-
creases the number of variations available for selection by
the plant breeder.

In many plants hybridizing has a physiological effect
which is of importance, in that it increases the vigor of the
offspring. In sunflowers, for example, the hybrids secured
by crossing the American sunflower and the Russian sun-
flower, neither of which was over 10 feet in height, grew
under the same conditions to a height of 15 feet.

Selection. Variations, mutations, and the results of hy-
bridization furnish the material from which valuable new
varieties of animals and plants may be selected. The plant
breeder selects from among the hundreds or thousands of
plants grown in trial grounds those individuals that most
nearly approach the form or quality desired. The seeds of
these plants are kept separate and are planted the following
season. This process of selection may be repeated year after
year until a large part of the progeny or all of them show the
quality aimed at in the selection. Some variations can be
maintained only by continual selection of the best individuals.
Others may soon become stable in character, and the plants
are then said by plant breeders to breed true. In fruit trees,
desirable varieties may be maintained by grafting and
budding ; and in other plants that can be propagated vegeta-
tively, varieties may be perpetuated by cuttings, bulbs,
tubers, and corms.

Weeds. Another way in which agriculture is affected by
reproduction in plants is through the multiplication of un-

Reproduction in Relation to Agriculture 229

FIG. 144. Fiber from new varieties of long-fibered cotton at the right, obtained
by hybridizing and selecting progeny from the two forms producing the shorter
fibers at the left. The hybrid offspring excel both parents in the length of

fiber produced.

»

desirable plants. Weeds as a class are plants in which re-
production has reached the highest degree of efficiency. The
sequoia may stand for the culmination of vegetative efficiency,
the dandelion for efficiency in reproduction and dispersal.
The dandelion produces good seed without pollination ; if the
stem is cut, the plant develops numerous new sprouts; if
the root is cut into small pieces, each piece may sprout
from either end or from both ends at the same time. The
dandelion can thrive in a swamp, and it can withstand the
droughts of a sand plain. The sequoia still occupies the
comparatively small area in which it existed several thousand
years ago. It reproduces very slowly, and it is restricted to
a single habitat. The dandelion has in recent times spread
to all parts of the world, and it occurs in most habitats,
from the seashore to the alpine summits of mountains. In
other words, it becomes adjusted to many environments.

230 Science of Plant Life

In the ordinary sense the term " weed " applies to any un-
desirable plant that reproduces abundantly and that adjusts
itself to diverse habitats. In its broadest sense the term is
applied to any plant growing out of place. Rye may become
a weed in wheatfields. Red clover is very desirable in a field
on the farm, but it becomes a weed when it springs up in a
lawn. The most pernicious weeds, like the dandelion, cockle
bur, Canada thistle, bindweed, and sand bur, are not desir-
able plants anywhere.

Weeds interfere with crop production. Weeds are un-
desirable and harmful, (i) because they use soil water needed
by crop plants, (2) because they interfere with the growth of
crops by shading them and by occupying the land, (3) be-
cause some weeds promote the spread of diseases and injurious
insects, and (4) because some of the species found in pastures
and grazing lands are poisonous, or -harmful to cattle, or
spoil the quality of milk, butter, and cheese by their persistent
and unpleasant flavors. It is estimated that the average
American farmer loses annually one dollar on every acre of
his land through the damage done by weeds.

Weeds spread rapidly. How rapidly a weed may spread
is illustrated by the history of the Russian thistle. It was
introduced into South Dakota in imported flaxseed, in 1874,
and it had become a common weed by 1888. In 1894 it had
spread as far as Chicago. In 1900 it was reported in all the
states and provinces east of the Rockies, from the Gulf of
Mexico and Atlantic Ocean to Saskatchewan.

The control of weeds. In crops that are cultivated at
intervals, most weeds can be controlled by turning under the
seedlings as fast as they develop. In fields of wheat and
other small grains the problem of weed control is more dif-

Reproduction in Relation to Agriculture 231

licult. By changing the crop from year to year so as to fol-
low or precede the crop of small grain with a crop like corn,
that is cultivated at intervals during its development, the
weeds may to some extent be held in check. Grazing animals,
particularly sheep, may aid greatly in removing weeds from
pastures.

In grainfields, pasture lands, and meadows some success in
weed control has recently been attained by the use of solu-
tions of poisons, particularly copper sulfate and iron sulfate.
The fields are sprayed with the poisonous solution while the
plants are young. Grasses are not greatly injured by the
spray and recover quickly, while weeds like the dandelion,
wild mustard, and corn cockle are killed.

In order to control the weeds in sugar-cane fields, some fields
are now covered with paper. The strong, spear like shoots of
the sugar cane break through the paper, but the weed seed-
lings do not. The paper is made from the pulp left after the
sap has been pressed out from the cane. Formerly this pulp
was wasted, but now it has been put to good use in the form of
field paper.

Weeds should not be allowed to ripen seeds. The state
laws which require that weeds be cut along the roads aim not
so much at the removal of the unsightly plants from the
roadsides as at the protection of farmers from the seeds.

Seed that is to be planted on' any farm should be examined
for weed seeds. If they are present, the seed should be either
cleaned or rejected. Many a farmer plants his weeds with his
grain.

232 Science of Plant Life

PROBLEMS

1. How much of the potato must be planted with each eye in order to get normal
growth?

2. In what plants would it be profitable to select varieties for increased size of
the flower cluster?

3. Why should field corn and sweet corn be widely separated when grown on the
same farm?

4. What are the most important things to know about seed, when buying it
for the farm?

5. In a row of radishes a few will develop bulbous roots before the others.
Would seed from these produce an earlier maturing crop than seed taken
at random from the row?

6. In what plants would it be profitable to select for increased height of stem?

Suggestions for Laboratory and Field Work to Precede
Chapter Eighteen

1. Make a field study of Protococcus to determine (i) where it
grows, (2) its relation to light, moisture, gravity, and roughness
of the substratum, and (3) how it is disseminated.

2. Make a field study of pond scums in a pond or small stream.
In a jar, collect material for laboratory study ; cover the bottom
of the jar with mud from the bottom of a pond or stream. The
amount of algae should be very small compared with the volume of
water. If these algae are collected in late autumn, they may be
grown in winter at a north window.

3. The forms of the cells and the methods of reproduction may
be studied from fresh or preserved material. Particular attention
should be given to the differences between vegetative multiplica-
tion, sexual reproduction, and asexual reproduction.

4. Dried specimens of Fucus, Laminaria, Sargassum, Poly-
siphonia, and Dasya will give an idea of the Brown and Red Algae.

5. In early spring place some algae in an aquarium with snails
and young tadpoles. Note the rate at which the algae disappear.
If you can find frog or toad eggs, the experiment may be started
with them. The water should be changed occasionally.

233

CHAPTER EIGHTEEN

THE ALGJE

THE plants that we have discussed in the preceding chapters
are all seed plants, — plants with well-developed roots, stems,
leaves, flowers, and seeds. In these plants the physiological
processes — photosynthesis, digestion, absorption, conduction,
accumulation, and reproduction — are carried on in tissues
and organs that are specialized to varying degrees, and the
plant body is a complex structure in which all the organs are
mutually dependent. The seed plants make up by far the
largest and most conspicuous part of the earth's vegetation, and
it is with them that the word " plant " is ordinarily associated.

There are thousands of other plants, however, that are
far simpler in structure. They usually pass unnoticed among
the larger plants that crowd the landscape; but this does
not imply that they are unimportant in the world of plants
and animals. Indeed, quite the reverse is true. They play
a very definite role in nature : they modify the earth's surface,
supply food, and produce other effects which are of great con-
sequence to the seed plants, to animals, and to man. In this
and the succeeding chapters we shall make a brief study of
these simple plants in order that our conception of the plant
kingdom as a whole may be more complete, and also that we
may gain some appreciation of the relations of man, of the
animals, and of the complex plants to the lower forms of
plant life. The first group that we shall study are the alga,
small green plants that are very common in ponds, brooks,
and pools, but which are found in other habitats also. The
algae occur in all parts of the earth.

The simple structure of the algae. The algae are simple
plants that consist of single cells, groups of cells, or masses of

234

The Algae 235

cells. In the lowest algae the cells are quite independent of
one another, and in most of them dependence among the
cells of the plant body is very slight. That is, in most forms,
organs corresponding to the roots, stems, leaves, conducting
systems, and reproductive parts of higher plants are not
found, and all the cells in the plant body are much alike.
Under these conditions each cell must carry on for itself the
processes of absorption, photosynthesis, digestion, assimi-
lation, and respiration. So, when we speak of the algae as
simple plants, we mean only that they are simple in structure ;
in their physiological processes they may be very complex.
They all contain chlorophyll and are therefore capable of
manufacturing food. Some of them, however, grow better
in water that contains organic matter, and these forms doubt-
less get a part of their food directly from the organic sub-
stances in solution in the water. A good type with which to
begin a study of algae is Protococcus, a very common and ex-
tremely simple form.

Protococcus. On the partly shaded, moist sides of trees,
rocks, buildings, and fences everywhere, there occur patches
that look as if they had been stained green. If a little of
this stain is scraped off and examined under a microscope, it
is seen to be made up of little rounded green cells. Each cell
has a cell wall, cytoplasm, and nucleus. In the cytoplasm is
a large green plastid which almost fills the cell.

When the cells are examined, certain of them will be found
to be elongated ; some of these may be dividing into two.
Sometimes there are two or more cells still clinging together,
showing clearly that they have just been formed by division.
These groups separate readily when the cover glass is tapped,
and each single cell may go on living quite independently of

236

Science of Plant Life

FIG. 145. Freshwater algae. The upright filaments are, from left to right:
(Edogonium, producing swimming spores, eggs, and sperms; Microspora, form-
ing resting spores and swimming spofes; and Ulothrix, forming swimming
spores and gametes. The horizontal filaments are Spirogyra (left) and
Vaucheria (right).

The Algae 237

the others. The plant, therefore, consists of a single cell
which carries on all the essential processes of life and is able
to reproduce itself. Moreover, it is a highly successful plant,
for Protococcus occurs in all parts of the world, from the
tropics to the polar regions, in habitats of many different
kinds.

The pond scums. If examined in the spring or fall, al-
most every pond and little stream will be found to contain
many kinds of algae. Some of these are merely masses of
rounded cells like the cells of Protococcus. Others have the
cells arranged in rows, forming simple filaments. In still
others the filaments are highly branched and the plant body
may be several feet in length. Some of the forms are im-
bedded in a gelatinous matrix. All these various kinds of
algae taken together are called the " pond scums." They are
the algae that a person who has not studied botany is most
likely to know.

Many of the pond scums are at first attached to under-
water objects, but during warm weather they break loose and
come to the surface. All cells carrying on photosynthesis
give off oxygen, and the bubbles of oxygen that come from
the filaments cling to them and help to buoy them up. Fur-
thermore, bubbles of air which are given off from the water
when it becomes warm collect under the masses of algae and
cause them to come to the top of the water, where they form
a green or yellowish-green surface layer. The pond scums
are generally considered unsightly, and not a few persons
think them poisonous. In reality, they are quite as harm-
less as lettuce. The danger in drinking from ponds lies not
in the green scums, but in the presence of certain disease-
producing bacteria that may have been carried into the ponds

238 Science of Plant Life

by surface water. Several thousand different species of algae
are concerned in the formation of pond scums. Microspora
may be studied as an example of the more simple filamentous
forms.

Microspora. The Microspora plant is a filament made up
of cylindrical or barrel-shaped, cells placed end to end. Each
cell carries on all its own food- and energy-producing pro-
cesses. During early spring, as food is manufactured, the
cells enlarge and divide. The division is always in the same
direction, however, and the cells remain attached to each
other, so that the growth and division of the cells causes the
filament to increase in length. This long, slender line of
cells is easily broken, and the plant may be multiplied by the
breaking of the filaments into parts.

Spores in Microspora. Microspora produces swimming
spores and resting spores. These are special cells that re-
produce the plant. A swimming spore is formed by the
contents of a cell in the filament contracting into an ovoid
body. At one end of this body two cilia, which are small,
hairlike propellers, are developed. The wall of the original
cell then breaks and the swimming spore is set free. After
swimming about in the water for a short time, it becomes
attached to some object under water, loses its cilia, and
grows into a cylindrical vegetative cell. This cell then con-
tinues to grow and divide until a new filament is formed. The
advantages of swimming spores are that they multiply the
plant, and by their ability to swim they enable the plant to
spread to new locations that it might not reach without these
motile cells.

The resting spores are usually formed in the spring after
the active period of vegetative growth has passed. At this

The Algae 239

season the cells in the filament stop dividing and food accumu-
lates in the form of starch and protein granules. The proto-
plasm in each cell then contracts into a spherical form and
secretes a heavy cell wall about itself inside the original cell
wall. In this way the cells of a filament form a row of ovoid
or spherical heavy- walled resting spores. Usually the walls
of these spores become yellow or brownish. The resting spore
remains dormant until the late fall or early spring. Then,
like a seed, it germinates, or begins to grow. The wall that
incloses the spore breaks and the protoplasm pushes out and
forms a cylindrical vegetative cell which continues to grow
and divide until a new filament is formed.

Microspora, then, in addition to the vegetative multipli-
cation of the cells shown by Protococcus, has specialized
swimming spores that multiply and spread the plant, and
resting spores that undergo a dormant period, after which,
when favorable conditions for growth appear, they produce
a new plant. Its life cycle and that of other similar algae
includes (i) an active chlorophyll- working period, during
which the plant grows and enlarges its body and accumulates
food ; (2) a reproductive phase, which closes with the pro-
duction of resting spores ; and (3) a period of dormancy, dur-
ing which only the resting spores are alive. The length of
the dormant period for a particular alga is practically the same,
whether it lives in a permanent pond or in a pool that dries
up in summer.

The living conditions of the pond algae. Curiously, the
ponds in which the algae are most abundant are the ones that
dry up in the summer. Yet these plants are among the most
delicate of all living things ; you may readily discover, by
putting the green masses in a dish and letting the water

240 Science of Plant Life

•

evaporate, that the vegetative plants are killed by drying.
How, then, do they withstand the dry period ?

It is resting spores that enable the plant to be carried over
the dry season. These spores are produced in large numbers
and, being heavier than water, sink and become a part of the
mud bottom. When the pond dries up, the spores are in-
cased in the dried mud. At intervals they are watered by
rains, but they do not germinate for weeks or months after
their formation. No doubt a large part of the spores that were
formed perish during this period, but enough survive a drought
to start the plant the following season.

Ulothrix. Another green alga occurring on the margins of
lakes, in running streams, and in clear springs is Ulothrix.
It has a filamentous body similar in many respects to Micro-
spora, and like that form it is attached to rocks and other
objects. Its methods of reproduction, however, are more
numerous and more complex than those of Microspora, and
they will serve to exemplify the reproductive processes of
many other forms of algae. When the filaments are mature,
the protoplasm within some of the cells divides into two,
four, or eight parts, each of which contains nucleus, cytoplasm,
chloroplast, and vacuole. Each of these parts becomes oval
in shape and develops into a swimming spore with four cilia.
An opening appears at one side of the original cell wall, and
a few minutes later the swimming spores pass out from the
cell cavity and swim away. Sometimes all the cells in a
filament produce swimming spores at about the same time,
and hundreds of these small green bodies may be found
moving about in the water. At the end of from 15 to 30
minutes the swimming spores settle down on some object
and become attached. By the end of a day the cell formed

The Algae 241

from each spore has divided and produced the first two cells
of a new filament.

The protoplasm of other cells of the same or other filaments
continues to divide until 16, 32, 64, or more bodies have been
formed. These are called gametes. They are similar to the
swimming spores but much smaller, and each possesses two
cilia for swimming. Like a swimming spore, each of them
leaves the old cell through an opening in the wall. The
gametes swim about for some minutes and then unite in pairs.
They are attached at first only by the ciliated ends, but later
the two gametes fuse. The body thus formed may grow
directly into a new filament, or it may produce swimming
spores from each of which a new filament is formed.

Asexual and sexual spores. Spores of Microspora and the
swimming spores of Ulothrix are formed directly from vegeta-
tive cells or by the division of the contents of these cells. The
spore which is formed by the -two gametes of Ulothrix is pro-
duced by the union of two cells. Since cell division takes
place in all parts of plants while cell union occurs only in the
sexual process, it is plain that we have in Ulothrix a simple
type of sexual reproduction. The spore that is formed by
the union of the gametes is, therefore, a sexual spore. The
swimming spores and the resting spores are formed without
the union of cells, and these are called asexual spores. The
gametes of Ulothrix look alike, but physiologically they are
probably different. One of them corresponds to the male
reproductive cell of the higher plants, and the other to the
female reproductive cell (page 205).

(Edogonium. (Edogonium is another filamentous alga
that flourishes in ponds and streams. In early life the fila-
ments are attached, but large masses of them will often be

242 Science of Plant Life

iound free in ponds and stagnant pools. From the cylindrical
vegetative cells, large swimming spores are formed. Gametes
also are produced. These are of two distinct forms, male
and female. Plants belonging to the (Edogonium group may
be used to exemplify reproduction in many other algae, whose
gametes are essentially like those of more complex plants.

At the time of production of the gametes, some of the cells
in the filament enlarge, become rounded, and accumulate
starch and other food material ; also, a small opening is formed
in the cell wall. The content of this cell is the female gamete
or egg, which like other eggs has in it a store of food.

Other cells of the filament are cut up into very short cells
by the formation of transverse walls. In each of these short
cells there are formed two small gametes, which escape from
the filament and swim out into the water. These are the
male gametes, or sperms. Fertilization takes place when one
of the sperms enters through the opening in the cell wall that
surrounds an egg and unites with the egg. The egg and the
sperm may be of the same filament or of different filaments.
The product is a sexual spore, usually called an oospore (egg
spore). After a dormant period this produces swimming
spores that start new filaments.

In (Edogonium, therefore, the sex cells are of two kinds
quite distinct in structure and function. The egg is a large,
stationary cell filled with food. The sperm is a small, swim-
ming cell that moves to the egg and accomplishes fertilization
by uniting with it. The product is an oospore which may
start a new generation of plants like that which produced it.
This condition is essentially like that in the seed plant, where
the egg is produced inside a complex structure called the
ovule and the sperm reaches it through a pollen tube (page

The Algae 243

205). When the two unite in fertilization, the product in a
seed plant also is an oospore, or fertilized egg, which im-
mediately divides and forms the embryo within the seed.

Reproduction among the algae. The methods of repro-
duction among the algae that we have studied are representa-
tive of those found in the entire group. The three general
types are :

(1) Vegetative multiplication. By means of cell division
cell masses, filaments, or highly branched plant bodies are
produced. If the individual cells separate from each other
after division, as in Protococcus, many new individual plants
are produced ; and when filaments and branched forms are
broken, as in Microspora, a new individual plant is produced
by each part.

(2) Reproduction by asexual spores. Vegetative cells form
thick-walled resting spores which carry the plant over to the
next season. Another kind of asexual spore is the swimming
spore, by means of which the plant secures immediate repro-
duction and spreads to other parts of the pond or stream.
These spores are formed directly from vegetative cells or by
the division of vegetative cells. There is no union of cells as
there is when sexual spores are formed.

(3) Sexual reproduction. A sexual spore, or oospore, is
formed by the union of two gametes. The gametes may be
similar in size and appearance, as in Ulothrix, or they may be
unlike, as in (Edogonium, where one gamete accumulates a
large food supply and the other is small and motile. In either
case, the one gamete corresponds to the sperm and the othd
to the egg that is found in higher plants. The union is the
process of fertilization. The oospore may germinate imme-
diately, but more often it remains dormant for a period of

244

Science of Plant Life

FIG. 146. Food relations of aquatic life.

weeks or months. Essentially this same process occurs in
the sexual reproduction of all plants and animals.

Other algae found among pond scums. Great numbers of
different kinds of algae are found growing in fresh- water ponds
and pools, and when these are examined under the microscope
they are seen to be composed of the most beautiful and the
most varied cells found in the whole plant kingdom. Some
have intricate star-shaped and latticelike chloroplasts ; in
other forms the chloroplasts wind about within the cells like
spiral green ribbons. Some forms have no cell walls in the
filament, but all the living matter of the plant, with hun-
dreds of nuclei in it, lies together in one mass. A great group
of one-celled forms (Diatoms) have siliceous cell walls that
are like delicate cases of glass and that are sculptured and
marked with lines in a highly complex way. Some of these
forms are stationary, while others move slowly through the
water by invisible means. Still other forms swim about

The Algae 245

rapidly by means of little hair like appendages, seeming more
like animals than plants.

The pond scums include many blue-green algae, which are
much simpler in structure than the green plants that we have
studied. These contain, in addition to chlorophyll, a blue-
green pigment which gives them the% color from which the
group takes its name. The cells are generally smaller than
those in the green algae, and distinct nucleii and chloroplasts
are absent. These small blue-green forms are common in all
kinds of water habitats and also in the upper layers of the
soil. Sometimes during wet weather they become sufficiently
abundant in fields to give a blue-green color to the soil.

The importance of the pond scums. Both green and blue-
green algae are generally considered a nuisance in ponds and
streams, and they are commonly thought to have no economic
importance ; but the fact is that these despised pond scums
are the primary food supply of all the water animals. They
bear the same relation to aquatic animal life that the her-
baceous plants bear to animal life on the land. Nearly all the
water animals, from minute insects and crustaceans to the
largest fishes, ultimately depend upon them for their supply
of food. For, like the land plants, these small water plants
manufacture food, and the animals that live in the water
must feed either on them or on other animals that get their
living from the plants. Without the pond scums the fish
would soon disappear from our waters, because their food
supply would be cut off. A decrease in the number of fish in
a lake frequently follows the draining of its swampy margins,
for the algae thrive best in shallow water, and it is from the
algae that the small animals on which the fish feed secure
their food. The time is not far distant when fish will be

246 Science of Plant Life

more highly prized as food than they are now. When that
time comes, the cultivation of algae will probably be under-
taken as a first step toward greater fish production.

But while the pond scums are a source of food for water
animals, they are also a source of annoyance in reservoirs in
which drinking water is stored. When they accumulate in
large quantities and die, they cause the so-called " fishy
taste " of water. This trouble has been to some extent con-
trolled during recent years by the exclusion of light from
small reservoirs, and by the addition of small amounts of
copper sulfate to the water in large reservoirs. Copper sul-
fate is very poisonous to algae, even in quantities of one part
to a million parts of water. Since animals are not injured by
such small amounts, the water may be used without harm for
drinking purposes.

The seaweeds. The brown and red algae, commonly known
as seaweeds, grow attached to rocks in the shallow water
along our coasts. They are generally from a few inches to
several feet in length. Some of the brown kelps grow in
deep water and attain lengths of 100 feet or more. In tex-
ture the seaweeds vary from the most delicate filaments to
broad, leathery expanses, with stalks so tough that they are
used for making ropes.

In China and Japan, seaweeds have long been used for food.
They are cooked with fish and take the place of vegetables.
Along our northern coasts the " Irish moss " and " dulse "
are collected in considerable quantities for food purposes. The
agar used in laboratories for growing bacteria and fungi is
made from certain kinds of red algae that grow abundantly on
the coasts of Asia.

The brown kelps are an important source of iodin, and also

The Algge

247

FIG. 147. Seaweeds on the rocky coast of Nova Scotia.

of the potassium salts which are required in large quantities
for the manufacture of commercial fertilizers. The great kelp
beds of the Pacific coast are being harvested on a large scale
in order to secure these products. The kelps are dried and
burned, and the potassium and iodin are obtained from the
ashes.

Suggestions for Laboratory Work to Precede Chapter Nineteen

1. Bacteria and molds may be grown for study by placing bread
and slices of boiled potato in moist chambers (tumblers inverted
in saucers).

2. Place cultures under different temperature conditions and
note the effect of temperature on the rate of growth. Dry a part
of the bread and potato slices somewhat, and compare the growth
of bacteria and fungi on the dry slices with the growth of those on
the moist and wet slices. What do these experiments suggest re-
garding methods of avoiding bacteria and molds ?

3. In test tubes, yeast may be grown in sugar solutions from
commercial yeast cakes. The formation of carbon dioxid and
alcohol should be noted. The cells, buds, and filaments may be
seen with a microscope.

4. From grainfields in the summer, specimens of rusts and
smuts can be obtained, and they may be preserved by drying.
The cluster-cup stage of rusts is common on elder, crowfoot, bar-
berry, and Indian turnip. In the laboratory the spores may be
examined and injuries to the leaves noted.

5. Mushrooms and toadstools may be collected in the woods at
all seasons. Study the relation of the fruiting body to the vegeta-
tive part of the plant. Dried material and specimens preserved
in strong alcohol will serve to show the variety of these plants.

248

CHAPTER NINETEEN

BACTERIA AND FUNGI

THE seed plants and algae which have been described manu-
facture their own food. Both of them are independent of
other plants and both are self-supporting. They are called
autophytes, since they make their own food and can live
without the assistance of other plants. There are many
other plants, including a few of the seed plants, which lack
chlorophyll and are unable to manufacture their own food.
There are two groups of these dependent plants, saprophytes
and parasites.

Saprophytes live on organic substances which have been
manufactured by other plants or animals. The mold that
develops on bread is a good example of a plant of this kind.
The starch, sugar, and protein in the bread were made orig-
inally by wheat plants ; and the mold, therefore, derives its
nourishment from substances previously manufactured by a
green plant. It grows as well in the dark as in the light, be-
cause its food is already prepared. It needs merely to digest
the food and absorb it into its own body. The mushrooms
are mostly saprophytes, living on fallen leaves, tree trunks,
and humus. The Indian pipe is an example of a flowering
plant that secures its food in the same way.

Parasites take their food directly from other living plants,
or from animals. The mistletoe is a common parasite on
various trees in the Southern states. Its seeds are sticky and
adhere to the bark of trees. When a seed germinates, the
hypocotyl penetrates the bark and grows inward to the
cambium and the food-conducting tissue. There it reaches
the food made by the tree and appropriates some of it to
its own use. The plant on which a parasite lives is called its

249

250 Science of Plant Life

host. The yellow dodder, which lacks chlorophyll altogether,
is a common parasite related to the morning-glory. It
twines about green plants and sends small roots into them
to obtain its food. Some parasites may take all their food
from the host plant, as does the dodder ; others, like the
mistletoe, which contain some chlorophyll, may secure only
a part of their nourishment from the host.

The bacteria and fungi form a great group of simple plants
which resemble the algae in structure, but which differ from
the algae in having no chlorophyll. They must, therefore,
live as saprophytes or parasites. Either they grow on or in
living plants or animals and draw their food directly from them,
or they feed on organic matter.

The bacteria. The best known and the most discussed of
all the simple plants are the bacteria. They are so intimately
related to human welfare that most persons, even though they
have never seen bacteria, know something about them. Bac-
teria constitute a group of one-celled plants, at once the
smallest in size, the simplest in structure, and the most
abundant of all plants. They live in immense numbers in
the water and in the upper layers of the soil, and they are
blown about in dust in the air. Some are too small to be seen
except with the highest powers of the microscope. Others
may be seen with an ordinary laboratory microscope. They
make up for the small size of the individual by their rapid
multiplication, and by the formation of colonies containing
countless numbers of individuals. Bacteria are responsible
for many of the diseases of men, animals, and plants, and
bacteria affect our lives in almost countless other ways. All
our modern methods of sanitation, quarantine, surgery, water
supply and sewage disposal, and much of our personal

Bacteria and Fungi

251

hygiene, are primarily based on our knowledge of the be-
havior of this group of plants.

FIG. 148. Various forms of bacteria.

Economically the bacteria are of the greatest importance.
Together with the fungi they are the principal cause of decay.
Bacteria bring about the ripening of milk in butter and
cheese making, and they produce both the pleasant flavors
in these products and the unpleasant flavors that develop in
them with age. The bacteria are also the source of much of
the available nitrogen in agricultural soils (page 32). The
drying of hay, vegetables, and fruits, the canning and pickling
of vegetables, fruits, and meats, and refrigeration and cold
storage, are methods of avoiding or making impossible the
growth of bacteria. Thus a knowledge of these plants is
fundamental to our understanding of thousands of details
of our daily life.

Growth and reproduction of bacteria. Bacteria grow best
and reproduce most rapidly in the presence of organic food
materials, in the absence of light, in the presence of moisture,
and at moderately high temperatures like those of the mid-

252 Science of Plant Life

summer months and of our own bodies. They reproduce by
simple cell division, the cell simply pinching in two to form
new plants. Many forms produce spores, similar to the rest-
ing spores of the algae, by the contraction of the protoplasm
in the cell and the secretion of a new cell wall. Spores are
very resistant to drying, to high and low temperatures, and
to poisons which readily kill the ordinary bacterial cells. It
is because the spores of certain forms withstand the temper-
ature of boiling water that steam pressure is used in sterilizing
cans of corn, beans, peas, and other vegetables. Most of
the common disease-producing bacteria, however, do not pro-
duce spores.

Bacteria and sanitation. The bacteria of decay help to
keep the surface of the earth clean. They change the highly
complex organic substances that form the bodies of plants
and animals into simple substances that may be used again
by other plants in building foods. When plants and animals
die, their bodies are gradually transformed by the bacteria
into carbon dioxid, water, and mineral salts. The sewage
that is turned into our rivers is chemically changed and dis-
posed of in the same way by these minute plants. The great
increase in the number and size of our cities has made it
necessary to build large sewage-disposal plants where the
bacteria can act more rapidly and more efficiently. This
prevents the pollution of streams and keeps the water suitable
for city water supplies.

The modern processes of filtering and sterilizing the water
supplies of cities are carried on partly to remove sediment
and partly to remove disease-producing bacteria. Adding
minute quantities of alum and chloride of lime to the water
and then filtering it through sand not only renders the water

Bacteria and Fungi 253

clear, but removes from it disease-producing bacteria. The
most dreaded of all the water-borne diseases is typhoid fever,
and the cities are now much freer from this disease than are
the country districts where people depend upon well water.
Surveys in some of the Middle Western states showed that
from one fifth to one third of the wells examined contained
large numbers of bacteria derived from surface drainage. In
such wells there is always danger that the surface waters may
bring in disease-producing bacteria, especially typhoid germs
derived from human sources.

Other sanitary practices, such as quarantine, disinfection,
admitting plenty of sunshine into living rooms, cleaning walls
and floors, removing dust, cooking food, washing and scald-
ing dishes, pasteurizing milk, and keeping food supplies in
refrigerators, are all related to the control or elimination of
bacteria.

Bacteria and disease. When certain bacteria grow in the
body, they produce poisonous substances called toxins. These
toxins interfere with the normal working of the bodily processes
and cause illness. The body under these circumstances pro-
duces substances called antitoxins, which protect the tissues
until the bacteria are destroyed by leucocytes (colorless blood
corpuscles) or in other ways. Not all persons are equally
susceptible to infectious diseases. A person may be immune
to a disease because his blood contains the corresponding
antitoxin or is able to produce it or because his blood contains
substances that kill the germs. Habits of personal cleanli-
ness, sleeping in the open air, and careful diet help to increase
immunity against many of the common diseases to which we
are subject.

Some of the commoner bacterial diseases are tuberculosis,

254

Science of Plant Life

U. S. Dept. of Agriculture

FIG. 149. A field of upland cotton, in South Carolina, attacked by the wilt disease.
The fungus that causes the wilt remains in the soil for many years,

pneumonia, grip, diphtheria, typhoid fever, colds, lockjaw,
and blood poisoning. With the exception of lockjaw and
other infections through wounds, these diseases are largely
communicated by one person to another. Typhoid is carried
also by means of water and milk supplies which have been
contaminated, and by flies that have visited infected matter.

A fundamental fact that should be learned in this connec-
tion is that no one can contract a bacterial disease unless he
comes in contact with the particular bacterium which causes
that disease.

The natural means of defense against disease are some-
what similar in the higher plants and in animals. The plant,
in addition to protective chemical substances within its cells,
has an epidermis which renders the entrance of bacteria dif-
ficult. Bacteria are able to enter, however, if the epidermis
is bruised or broken. Plants probably suffer from bacterial
diseases as much as do animals. Most of the well-known

Bacteria and Fungi

255

U. S. Dept. of Agriculture

FIG. 150. The same field shown in Figure 140 planted with seeds from plants that
survived the attack of the disease.

plant diseases, however, are produced by fungi. Of the
bacterial diseases of plants, the twig blight of pear and apple,
the cucumber wilt, and the crown gall of various plants are
perhaps best known.

In crop plants subject to disease, it has been found possible
in some instances to breed varieties that are immune to a
particular disease. By selecting seed from plants that were
least affected, or not at all injured, in a diseased field, the
characteristic which gave that plant immunity may be pre-
served. In this way wilt-resistant cotton was bred in the
South. In fields where the wilt disease had killed nearly all
the cotton, it was noticed that an occasional plant suffered
scarcely at all. Seedlings grown from these plants were
found to be highly resistant.

By hybridizing with related species or varieties that are
immune, and then selecting the desired offspring for producing
seed, it is possible to transfer the immunity to the crop plant

256 Science of Plant Life

which possessed the other desired qualities but which was not
immune. Such desirable combinations are possible because
most of the qualities of plants are inherited independently of
one another. For example, the watermelon wilt threatened
to make the profitable growing of watermelons in certain
regions along the Atlantic coast impossible. It was dis-
covered, however, that a closely related plant, a small African
citron, was immune to the disease. The problem then was
to cross the watermelon with the citron and out of the hybrid
progeny to select those plants that possessed all the good
qualities of the watermelon and the immunity of the citron.
This was accomplished, and hybrid watermelons could then
be grown in the infected fields.

Bacteria in the dairy. Milk is an ideal medium for the
growth of bacteria. This makes necessary the most careful
handling of milk, especially when it is used directly as food.
The bacteria get into the milk from the cow, from the stable,
from the vessels into which the milk is put, and from the persons
who handle it. Evidently the cows should be kept clean, and
the stable should be as clean and free from dust as possible.
The vessels with which the milk comes in contact should be
sterile. The dairymen should have clean hands and clothes,
and above all they should be free from infectious diseases.
Because bacteria multiply very rapidly at high temperatures,
the milk should be chilled at once and kept on ice. To make
butter and cheese of fine flavor, pure cultures of desirable bac-
teria are added to the milk and allowed to develop for a time.

In order to avoid the danger that lies in the use of milk
contaminated with disease germs, milk that is shipped into
the larger cities is usually pasteurized before being sold. This
treatment kills most of the bacteria, destroying all the kinds

Bacteria and Fungi 257

that produce disease in human beings. By " pasteurization "
is meant the heating of the liquid to 150 degrees or 160 degrees
F. for from 10 to 30 minutes. This does not kill the spores,
but they are to a large extent prevented from developing by
the subsequent cooling that the milk receives.

The preservation of foods. The greatest losses that occur
in the utilization of crops are connected with the distribution
of the products to the consumer. Much of the food pro-
duced never reaches the consumer, because bacteria and
molds render it unfit for use before it can be distributed
through the markets. There are four methods of prevent-
ing this loss : (i) cold-storage warehouses and refrigerator
cars are used to keep foods below the temperature at which
bacteria grow appreciably; (2) fruits, vegetables, or other
foods are packed in cans, and the cans are then sterilized by
heat and are sealed so that they are bacteria-tight ; (3) food
products are dried to make it impossible for bacteria to grow
in them ; and (4) foods like meat and fish are treated with
salt or with some other chemical that will prevent the growth
of bacteria. Refrigeration enables us to preserve foods for
weeks and months. Canning and drying make foods avail-
able after months and years.

Soil bacteria and humus. In the process by which the
bacteria of decay destroy animal and vegetable bodies, the
brown organic matter represents the products of partial
decomposition. This matter is called humus, and it is one
of the important soil factors in crop production. Humus is
of importance in light soils because it helps to hold the water
and so to keep the soils moist. In heavy clay it mellows
and loosens the soil and promotes aeration and drainage.
Humus may also contribute something directly to the nu-

258 Science of Plant Life

trition of the crop, for among the multitude of substances
it contains are simple carbon and nitrogen compounds which
the higher plants may absorb and use in small amounts.

Soil bacteria and nitrogen. In order to manufacture pro-
teins, seed plants must have a supply of nitrogen. This they
secure principally in the form of nitrates. There may be
other nitrogen compounds in the soil, but they are unavail-
able until certain nitrifying bacteria change them to nitrates.
Ammonia is one of the nitrogen compounds produced in the
process of humus formation. The ammonia may be acted
upon by certain bacteria and changed to nitrites, which in
turn are changed by other bacteria into nitrates. These
nitrates are used by the green plant. The nitrifying bacteria
are of great importance to the seed plants, because they break
down the nitrogen compounds in humus and other soils, and
convert the nitrogen into an available form.

Still other soil bacteria bring about a process known as
nitrogen fixation, by which nitrogen is actually taken from the
air and built into compounds which are added to the soil.
The nitrogen-fixing bacteria are, with a few exceptions, the
only plants that can take nitrogen from the air and combine
it to form nitrogen compounds. They require rich, well-
drained soil in order to flourish. They are of great importance
in agriculture because nitrogen is one of the elements that is
most commonly lacking in soils, and because it is the most
expensive of all the elements that are purchased for fertilizers.

Bacteria and legumes. Clover, alfalfa, beans, soy beans,
and peas belong to a family of plants called legumes. They
increase the nitrogen in soils on which they are grown, and
for many years they have been used in crop rotations, fol-
lowing wheat or corn. The practice of using legumes in

Bacteria and Fungi

259

Bureau of Agriculture, P. /.

FIG. 151. A field of cowpeas. Like other legumes, cowpeas accumulate nitrogen
compounds, and when plowed under they add nitrogen and humus to the soil.

crop rotations existed long before the real cause of the increase
in soil nitrogen was understood, or even before it was under-
stood how different elements in the soil contribute to its
fertility. By experience it was learned that other plants
flourish in land after leguminous plants have been grown in
it, and for this reason the farmer included legumes in his
scheme of crop rotation.

It is now clearly understood that nitrogen compounds ac-
cumulate in leguminous plants only because of the presence
of certain nitrogen-fixing bacteria. These bacteria occur in
many soils, and when the legume is planted and develops roots,
they invade the cells of the root. This causes the infected
parts of the root to enlarge, forming nodules. If a nodule
from a clover or alfalfa root is crushed and examined under

260

Science of Plant Life

Bureau of Agriculture, P. I.

FIG. 152. A field of young bananas, with cowpeas planted between the rows to
enrich the soil.

a microscope, it will be found to be filled with bacteria. These
bacteria are parasites and take their food from the legume,
but by the processes which they carry on in the nodules, they
change nitrogen from the soil air into nitrogen compounds,
just as the soil bacteria mentioned in the preceding section
do. The nitrogen compounds thus formed are used by the
host plant, and when the latter is plowed under and decays,
the nitrogen compounds are made available for a succeeding
crop of wheat or corn.

The fungi. The fungi form an exceedingly large and di-
versified group, ranging in size from microscopic forms of a
single cell to the large, fleshy mushrooms and to the bracket
fungi found on tree trunks and logs. Among the most impor-
tant fungi are yeasts, molds, mildews, smuts, rusts, and
mushrooms. We cannot here describe the many interesting
forms and give their life histories. They all lack chlorophyll,

Bacteria and Fungi

261

however, and get their food either from organic matter or
from living plants. The plant body is always made up of
filaments like those of the algae. In the higher forms, like the
mushrooms, the filaments are massed together into a com-
pact, solid structure.

The yeasts and molds, and most of the mushrooms, are
saprophytes ; the smuts, rusts, and some of the mildews are
parasites upon the higher plants, and they cause serious losses
to the farmer and gardener. A few of the mushrooms are edible
and furnish small quantities of food for animals and man.

The yeasts. In the making of bread, yeasts are of primary
importance. They are small, one-celled plants that multi-
ply very rapidly, and when properly mixed with flour and
water they develop in all parts of the dough. The yeasts
have within them enzymes which change part of the sugar
that is present into carbon dioxid and alcohol. The carbon
dioxid accumulates in bubbles and makes the dough light.
When the dough is put into a hot oven the alcohol is vaporized,
and together with the carbon dioxid it is driven off into the
air. The high temperature kills the yeast, bakes the dough,
and changes some of the starch
into its soluble form, dextrin,
which makes it more readily
digestible. Sour bread is pro-
duced when the yeast that is
added contains acid-forming
bacteria which change part of
the alcohol into acetic 'acid. (j-^~& & ^®

Yeasts and bacteria are the ® ^

Organisms that Change fruit ^ I53' Yeast cells and chains of cells.

Above are three cells, each containing four

juice into cider and vinegar, resting spores.

262

Science of Plant Life

Yeast first changes the sugar in apple juice to carbon dioxid
and alcohol, and bacteria further oxidize the alcohol to acetic
acid or vinegar. Yeasts are also used in the fermenting of
beer and wines.

Yeast may readily be grown by adding a bit of yeast to a
5 per cent sugar solution in a test tube. The branching groups
of cells may then be examined under the microscope. The
manner of forming new cells among yeasts is unique, in that the
new cells start as small protuberances (buds) from the older
cells. These buds gradually enlarge until they attain their
complete growth and separate. The alcohol formed in the
test tube by the yeast may easily be detected by its odor.

The molds. The molds are usually white, filamentous
plants that are of great economic importance because of the
damage that they do to foods during storage or shipment.
Like bacteria, the spores of molds are in the air and in the
dust everywhere. If the temperature is warm and the
food is moist, they germinate and, together with bacteria, soon

FIG. 154. Bread mold, showing the rhizoids which penetrate the material on which
the mold grows and which absorb food, the horizontal filaments by which it spreads,
and the vertical filaments which bear the sporangia and spores.

Bacteria and Fungi 263

destroy food. The same measures that will prevent the
growth of bacteria in foods will prevent the growth of the
molds, which are usually associated with them.

The molds exemplify one of the fundamental characteristics
of the fungi ; namely, their capacity for producing enormous
numbers of spores. In some cases, as in the bread mold, these
spores are produced in rounded sacs called sporangia; in other
cases upright filaments of the mold develop spores by cutting
off chains of little rounded cells from the ends of the filaments.
These spores are of various colors, — brown, black, blue, green,
or yellow, — and they give the characteristic color to the
mold. In most molds the spores begin to develop at ordinary
temperatures within 2 or 3 days after the parent spore germi-
nates.

The rusts. Among the most serious diseases affecting
wheat, rye, barley, and oats are those produced by the fungi
known as the rusts. These fungi are called rusts because
plants that are infected with them develop yellow and brown
spots that have the appearance of iron rust. The rusts occur
wherever grains are grown, and they cause millions of dol-
lars' worth of damage to crops every year.

The rusts are parasites that live inside the host plants and
injure or destroy the tissues which are concerned in food manu-
facture. Their life history is peculiar in that the fungus usually
produces diseases on two different kinds of host plants. The
stem rust of wheat, for example, produces patches of red
spores which will infect other wheat plants. It produces also
black spores which live over winter on the stubble, and
which germinate the following spring and produce a third kind
of spore that infects the barberry. On the barberry leaves
the fungus produces a cuplike depression within which a fourth

264

Science of Plant Life

kind of spore is formed. This spore will not germinate on
the barberry, but it will infect wheat. Thus the stem rust

FIG. 155. Life history of the stem rust of wheat. In fields where wheat has been
grown, the stubble (A) carries over the winter black spores (B), that germinate in early
spring, producing smaller spores (C). These infect the leaves of the common bar-
berry (D). In the leaves of the barberry the fungus grows and produces cup-shaped
cavities filled with spores (£) that are carried by the wind to wheatfields and infect
the wheat plants. After growing in the wheat a short time, the fungus produces first
the red spores (G) that spread the disease to other wheat plants, and later the two-celled
black spores that carry the disease over the winter again.

of wheat spreads from one wheat plant to another by means
of red spores, from wheat to the barberry by spores that are
produced the next spring, and from the barberry back to the
wheat by still another kind of spore.

In the Northern states, from the Dakotas to New England,
the barberry stage is of special importance in the life history
of the rust. In the central United States where winter
wheat is grown, the red spores produced during the summer
drop to the ground and infect the wheat planted in the au-
tumn. In this way the rust may be perpetuated from year

Bacteria and Fungi

265

to year without the intervening barberry stage. In the
Northern states the destruction of all barberry plants has

FIG. 156. The white pine blister rust. The fruiting bodies on the white pine (A)
produce spores that infect the leaves of the gooseberry (B and C). On the gooseberry
leaves the fungus produces at first yellow spores that will infect other gooseberry plants,
and later brown spores that carry the disease back to the pine. When a pine (D) is
infected by the disease, the younger parts soon die (£).

been undertaken, and this work will doubtless reduce the
amount of infection. The hope of effectively controlling
wheat rust, however, probably lies in breeding new varieties
of wheat that are immune to the disease.

Other rusts also live on two host plants, and because of
this double life and the fact that the fungus grows on the
inside of its host, they are very difficult to control. The rust
on the red cedar produces the so-called " cedar apples," the
spores from which infect the leaves of the apple tree and may
do great damage to them. Recently the blister rust of the
white pine has been brought to America, and it threatens to
destroy what remains of our white-pine forests. In this case

266

Science of Plant Life

the alternate host plants are the wild and cultivated goose-
berries and currants. Another common rust is frequently seen
on raspberry and blackberry bushes along
roadsides ; it colors the under sides of the
leaves with its bright, orange-red spores.
The smuts. The smuts of oats, wheat,
barley, and corn often greatly reduce the
yield of these plants. But since their
life histories are known, it is compara-
tively easy to control them. The smut
fungi generally are carried over from one
year to the next, on or in the grain, and
they may live over winter in the soil.
When the grain is planted, the smut
spores germinate and infect the young
seedlings. The smut plant lives inside
the host, and its presence becomes ap-
parent only when the black spores of
the fungus appear, usually in the grain.
FIG. 157. A normal and In some cases the walls of the grain are
smutted flower cluster of destroyed and the spores are scattered

oats.

by the wind.

All smuts of the small grains may be prevented from
germinating by soaking the seed in hot water for a short time
before planting. It is easier, however, to wash the grain with
a weak solution of formalin, and this treatment is effective
in preventing the growth of those smuts whose spores are
carried over the winter on seed. Detailed information con-
cerning seed treatment for the prevention of the several
kinds of smuts may be obtained from State Agricultural Ex-
periment Stations. The corn smut is controlled by removing

Bacteria and Fungi

267

FIG. 158. Various forms of mushrooms. At top : puffballs in center, bracket
fungus (Fames) at right, Hydnums to left. Center : coral fungus (Clavaria)
on left and poisonous Amanita on right. At bottom, left to right: cornu-
copia fungus (Craterelhis),Russula,znd earthstars (Geaster).

268

Science of Plant Life

and burning the plants as soon as evidence of the disease
appears.

Mushrooms and toadstools. The largest and most com-
plex of the fungi are the mushrooms and toadstools. They
are common in fields and woods and for the most part live on
decaying wood and on humus in the soil. There is no real
distinction between mushrooms and toadstools. Some of
them are edible, others are indigestible, and some are deadly
poisonous. Edible forms are cultivated on a large scale in
caves and abandoned mines, and on a smaller scale in cellars.
Wild forms should not be eaten unless they are gathered by
persons competent to distinguish the different species, many
of which are similar in appearance but very different in their
effects when eaten.

The mushrooms as they are gathered are only the fruiting
bodies of the fungi. The real plant consists of bundles of
filaments extending in all directions throughout a large mass

FIG. 159. Stages in the development of the common edible pink-gilled mush-
room. Note the underground vegetative body of the plant.

Bacteria and Fungi

269

of soil on which the fruiting bodies appear. It may take
several years for the underground vegetative part of the fungus
to develop, while the fruiting bodies may develop in a few
days. It is the enlargement of the fruiting bodies that per-
sons have in mind when they speak of " mushroom growth."
This expression leaves out of account the months or years of
growth during which the materials were accumulated for the
sudden production of the fruiting body. The spores of mush-
rooms are produced in unthinkable numbers either inside the
fruiting body (puffballs), on the upper surface (morels), or
on the under side of the umbrella-shaped cap (mushrooms)

(Fig- 159)-'

Lichens. Among the parasitic fungi are some that live on
such one-celled algae as Protococcus. The fungus forms the
plant body, and the algal cells are completely enveloped.
These forms constitute the lichens, which are gray-green, ir-
regular-shaped plants that are common on the bark of trees,

FIG. 160. Several common lichens. The form on the tree is Parmelia ; the one in
the middle foreground is Peltigera; the other forms are species of Cladonia.

270 Science of Plant Life

on rock surfaces, and occasionally on the soil (Fig. 160). Like
other fungi they produce fruiting bodies, — small cup-shaped
or disklike elevations, — in which asexual spores are pro-
duced in great numbers.

Summary 6f the simple plants. The simplest forms of
plant life include three great groups that are of the highest
importance to man :

(1) The algae constitute the primary food of fishes, and
they will become increasingly important as the cultivation
of ponds, lakes, and streams (aquaculture) for the production
of fish becomes more necessary to augment our food supplies.

(2) The bacteria have a most important and intimate bear-
ing upon the lives of all of us. They have made necessary
our various food-preserving industries. Their existence every-
where is the chief factor that must be considered in personal
hygiene and in the making of our sanitary laws. Some kinds
of bacteria aid other forms of life by destroying the bodies of
dead organisms and improving the fertility of soils ; other
bacteria are responsible for many diseases of both plants and
animals.

(3) The fungi also are destructive agents, causing injury
to our crop plants, the loss of much valuable food, and the
destruction of timber. Forms like the yeasts are valuable
aids in certain food industries. A very few of the fungi are
themselves sources of food. The fungi are notable for the
enormous number of spores which they produce.

Algae are autophytes, while the bacteria and fungi are
saprophytes or parasites. The algae are of world- wide dis-
tribution, but they are chiefly confined to aquatic and marine
habitats. The bacteria and fungi are also world- wide in their
distribution, but they are most numerous on land.

Suggestions for Laboratory and Field Work to Precede
Chapter Twenty

1. Make a field study of liverworts and mosses, noting especially
conditions most favorable to them. Examine underground parts
as well as above-ground parts.

In winter a visit to a conservatory or greenhouse will enable
students to see these forms in a growing condition.

2. Specimens of liverworts may be secured in spring from moist
rocks along streams, from the soil in clover fields, and from trees
in moist woods. In Marchantia, study particularly the thallus,
rhizoids, growing region, cupules, gemmae, and reproductive
branches.

3. Of the mosses, Mnium and Polytrichum are particularly
good to show protomena, rhizoids, stem, leaves, and reproductive
structures. Compare the vegetative plant body of a moss with
that of a liverwort in its relation to water, light, and air.

271

CHAPTER TWENTY

LIVERWORTS AND MOSSES

FIG. 161. A common liverwort (M archantid) , showing thallus, cupules, and
reproductive branches.

THE largest of the mosses and liverworts never attain a
height or length of more than a few inches, and they are of
very simple structure in comparison with the flowering plants.
In contrast with the algae, which on the whole are water
plants, mosses and liverworts for the most part live on land.
The passing of plants from a water to a land habitat is one of
the notable steps in the development of the plant kingdom,
and in connection with the study of this group we shall con-
trast the environments of land and water plants and consider
the modifications in structure that accompany the passing of
plants from the water to a land habitat.

The liverworts. The body df a liverwort is flat and leaflike
and is called a thallus (plural, thalli). It may be from one to
several cell layers in thickness. It grows at the tip, and
usually branches by forking at intervals. Liverworts do not

272

Liverworts and Mosses 273

stand erect, but usually have their thalli in close contact
with the substrata on which they grow. In most forms the
thallus is a continuous plate of cells, but some forms have
prostrate stems with small leaves on either side. All of them
have chlorophyll and manufacture their own food. All the
forms have on their under surfaces small, hair like rhizoids
that anchor the plant and absorb water and minerals.

Liverworts are widely distributed but are most numerous
in the tropics. A few of them are found on trees and rocks
and a few are found floating on water, but as a whole they
live on moist soil and in shaded situations.

Reproduction among the liverworts takes place by means
of spores, produced either directly on the thallus or on special
branches. In many liverworts there are produced also special
bodies called gemma, (singular, gemma), which propagate the
plants vegetatively.

The liverworts are supposed to be descended from plants
like the green algae, for it is thought that the simplest plants
existed first and that plant life as well as animal life had its
origin in the water. The liverworts may be considered, there-
fore, as a group of simple plants that exhibit some of the stages
through which plants pass in going from the water and taking
up their life upon the land. In this respect they can be com-
pared to the amphibious (Greek : amphi, double, and bios,
life ; i.e., life both on land and in water) frogs and salamanders
of the animal world. Just how a plant becomes adjusted to
its new environment, when it is forced to live on land because
of the drying up of the pool in which it grew, is a most inter-
esting question to consider.

Living conditions of water and land plants contrasted.
The algae probably represent the remnants and derivatives

274 Science of Plant Life

of the first plants that grew on the earth, and the shallow
water in which the fresh-water algae live is the most favorable
of all habitats for plants. Because the plants are of about
the same weight as water or are lighter, they are supported by
the water and do not need to use their foods for building
mechanical tissues. In the shallow water there is sufficient
light for photosynthesis, while the plants are protected from
the heating and drying effects of the intense sunshine to which
land plants in many situations are exposed. An abundant
supply of the carbon dioxid and oxygen needed for photo-
synthesis and respiration is in solution in the water, and
mineral salts sufficient for the needs of the plants are washed
in with every rain. Furthermore, the temperature is more
uniform than on land, and this permits the processes of growth
and reproduction to go on almost uninterruptedly throughout
the entire twenty-four hours. The length of the growing
season in temperate and cold climates is longer under the
water than on land, because the water protects the plants from
the sudden changes of temperature to which land plants are
subjected in the spring and fall. The shallow-water plants,
therefore, live under the conditions most favorable for plant
life. Even the deep-water plants like some of the marine algae,
although they are under the great disadvantage of having
little light, gain many advantages through their water en-
vironment.

The environment of the land plant provides a supply of
carbon dioxid and oxygen directly from the atmosphere, and
mineral salts may be secured from the soil water with which
the plants are in contact. But if the plant grows in full
sunlight, it is subjected to much more intense illumination and
heating than are water plants, and it must withstand the

Liverworts and Mosses

275

drying effects of the air. In aquatic plants the cells are never
without an adequate supply of water, while in land plants

FIG. 162. Cross section of Marchantia thallus, showing rhizoid (below), water-
storage tissue, the air chambers containing the principal photosynthetic cells, and
the epidermis which forms a transparent roof over the air chambers. Starch
grains occur in many of the cells of the water-storage tissue.

transpiration may reduce the water content of the cells to
such an extent that they may be injured or even die. A
study of the amphibious liverworts shows that they have
become adjusted only to a medium light and a moderate
amount of drying. These plants, therefore, grow in shaded
and moist situations. During wet periods many individuals
start in intensely illuminated places, only to be killed off
by the light and its secondary temperature and drought ef-
fects. The shaded situation where the water is near the
surface of the soil is evidently the habitat where these plants
suffer the least, and this explains why liverworts persist in
moist situations and not in the open.

Responses of the plant to the aerial environment. The
land liverworts show several changes in structure that are
of advantage to the plants in an aerial life. The more im-
portant of them are :

276 Science of Plant Life

(1) Firmer, and in some cases thicker, cell walls and water-
storage tissue. The firmer cell walls are less permeable to
water and reduce the rate of water loss. Furthermore, the
plants grow flat on the soil in contact with the water supply,
and some of the forms develop layers of water-storage cells
on the side in contact with the soil (Fig. 162). This enables
them to withstand short dry periods better than do those
forms that have only the ordinary green cells. The develop-
ment of firmer cell walls and water-storage tissue is the first ad-
justment to land conditions.

(2) The development of rhizoids. Land plants are liable
to be washed away by rain and surface water, and on this
account they need some anchorage ; also, it is necessary for
them to have structures that will bring them into contact with
the soil-water supply. In the liverworts, rhizoids anchor
the plant and to some extent absorb water and mineral salts
from the soil. Rhizoids are elongated cells that develop on
the lower side of the plant body and penetrate the soil. They
resemble root hairs in form. The development of rhizoids,
therefore, represents a second important adjustment of plants to
the land environment.

(3) The development of an epidermis. The land liver-
worts are covered by an epidermis which helps to protect
them against water losses. Since a ready access to carbon
dioxid and oxygen is necessary for photosynthesis and res-
piration, the liverworts with thicker bodies have openings or
pores in the epidermis. Through these pores the gas ex-
changes between the air and the cells within the body can
take place. In the more complex liverworts, the epidermis
is raised like a transparent roof on ridges of supporting tissues,
leaving beneath it a series of small air chambers in which the

Liverworts and Mosses 277

chlorophyll-bearing cells stand up in short chains (Fig. 162).
Each chamber is connected with the air by a pore in the
center of its roof. In these rather simple plants, therefore,
protection against water loss is accomplished in much the
same way as in seed plants, but the epidermal pores in them
are chimney-like openings and are incapable of closing as do
the stomata of the higher plants. The development of an
epidermis and of pores is a third adjustment of plants to the
land environment.

(4) The ability to withstand drying. When the cells of
water plants are dried, the protoplasm dies at once; but a
few of the liverworts, like many mosses and like Protococcus
and a few other algae, do not die when water is lost from the
cells. Just what quality the protoplasm possesses that en-
ables it to withstand drying, it is impossible to say ; but some
of the liverworts that grow on trees and rocks possess this
quality, and certain mosses have to a remarkable degree the
ability to withstand drying. A fourth adjustment of plants
to the land environment is the development of the ability to with-
stand drying.

The mosses. The mosses form a very large group found
in all parts of the world. They usually have upright stems,
though many live close to the substratum and have only
horizontal or inclined stems. They possess very simple
leaves, frequently only one cell layer in thickness, sometimes
thicker toward the midrib.

The mosses, like the liverworts, are most abundant in moist,
partly shaded habitats. A few, however, grow on rocks and
trees where they are exposed to periodic drought. These
latter forms, like Protococcus, have the power to with-
stand complete drying. When dry, they are in a dormant

278

Science of Plant Life

FIG. 163. Mosses, showing the compact grouping of the plants.

condition; and when wet, they go on with their normal
processes of photosynthesis, respiration, growth, and repro-
duction.

Mosses also possess means of anchorage by rhizoids. The
rhizoids of the liverworts are one-celled structures. Those of
the mosses are branching, many-celled structures which pene-
trate the soil, affording a firm hold and absorbing a part of
the water used by the plant. The habit of growing in com-
pact clusters gives the mosses a method of conserving water
and maintaining the water balance, other than the methods
spoken of in connection with the liverworts ; the dense masses
of plants take up water from rains and hold it for some time
like a sponge.

Mosses, therefore, show some advances over the liverworts
in their upright stems and branching rhizoids, in the regular
occurrence of simple leaves, and in their ability to grow in
drier habitats. The liverworts and mosses together show the

Liverworts and Mosses 279

successful adjustment of simple green plants to the land
environment.

Life history of the moss. Mosses reproduce by vegetative
propagation and by both
sexual and asexual spores.
A study of each of these
methods will make clear the
somewhat complicated life
history of the moss plant.

Vegetative multiplication.

When a mOSS Spore germi- FIG. 164. Moss spores (A) and protonema

nates on the soil, it produces (B)' with f bud (C) from which an upright

stem develops.

a branching, filamentous

body, the protonema, which resembles some of the branching
forms among the green algae. The protonema spreads over
the soil for some distance and then develops numerous buds
(Fig. 164). The buds give rise to the upright leafy branches
which we commonly call the moss plant. Because of the
numerous buds developed on the protonema, the moss plants
stand in thick clusters or masses.

The upright leafy stems of the moss also have the power
of producing protonema-like branches which spread still
farther over the soil, thus serving to multiply the plants and
to make the plant mass denser and larger. In some mosses
with horizontal or inclined stems, the stem tips when in con-
tact with the soil develop rhizoids and give rise to new branches,
much as the stems of the raspberry develop new plants (page
217). These methods of vegetative propagation are com-
mon among the mosses, and some mosses are not known to
multiply in any other way.

Sexual reproduction. The upright stems of most mosses

280

Science of Plant Life

when mature produce gametes — egg and sperm cells — in
special organs at the stem tips. The sperms are swimming

FIG. 165. A moss plant (Mnium). £ is a vegetative branch, B a branch that produces
eggs, and A a branch that produces sperms. After fertilization, an upright stalk bear-
ing a spore case (C) develops from the egg. A ' is a longitudinal section of a female
branch, showing three egg cells in the cases in which they are produced ; B' is a section
of a male branch, showing three of the organs that produce the sperms.

cells much like those of algae. As they cannot reach the egg
cells except by swimming, fertilization takes place only when
the moss is wet. When the sperm unites with the egg, it
forms an oospore.

Asexual reproduction. The oospore germinates while still
on top of the parent stem, and produces a long, stalklike body.
The base of this body grows downward into the parent stem
and draws water and nourishment from it. At the top of
the stalk a sporangium or capsule develops which contains
asexual spores. The stalk and sporangium live parasitically
on the green, leafy moss plant and are a distinct stage in the
life of the plant.

Liverworts and Mosses . 281

The first stage in the life history of the plant ends with
the production of oospores. The second stage ends with the
formation of asexual spores. When the asexual spores germi-
nate, they again produce the protonema and the leafy moss
plants. The asexual spores are small, firm-walled, rounded
bodies that are able to withstand the drying effects of the
air. They are fitted to be blown about, and so to start the
growth of the plant in new areas. Most moss plants are
perennials, and they produce both oospores and asexual spores
on new branches for several years.

Summary. Liverworts and mosses constitute a group of
rather simple land and amphibious plants. They are of par-
ticular interest because they show some of the steps by which
simple water plants become adjusted to conditions on land.
This group represents the most complex of the land plants
that lack a conductive system. In the next group, the ferns,
the plant has a well-developed water-conducting and food-
conducting system, and correlated with this it has greater
size and more complex tissue systems.

Suggestions for Laboratory and Field Work to Precede
Chapter Twenty-one

1. Make a field trip to study ferns and their allies. Ferns are
common in rich woods and on moist cliffs. Equise turns may be
found along streams and railway embankments, and in swamps.
The club mosses occur in low grounds, in rich woods, and in moist,
sandy depressions along lake and sea beaches. Tropical forms
may be seen in conservatories and greenhouses. Note especially
the relative development of leaves, stems, and roots. Compare
them with the seed plants growing in the same habitats. Have
they any advantages over the mosses, liverworts, and algae that
live near them ?

2. Examine growing plants and note how fern leaves unfold.
Note the sporangia and spores on the under sides of the leaves.
Plant some of the spores on moist soil in a flower pot and cover with
a glass plate. Examine the prothalli that develop from the spores.

3. If field work is impossible, examine a collection of pressed
specimens of ferns and allied plants, in order to get an idea of the
great variety of forms belonging to these groups.

282

CHAPTER TWENTY-ONE

THE FERNS AND THEIR ALLIES

THE ferns and their allies are a large assem-
blage of plants. Like the simple plants that we
have studied, they reproduce by spores instead
of seeds ; but, like the seed plants, they possess
water-conducting and food-conducting tissues.
They appeared on the earth earlier than the
seed plants, and were more abundant during the
carboniferous or coal-making period of the
earth's history than they are now. Indeed,
judging by the fossils in coal, ferns and fernlike
plants dominated the vegetation of that time,
and the seed plants were much less prominent
than now. Ever since that period the seed
plants have been gaining in importance; and
now the ferns have been to a large extent
displaced, especially in the temperate and
colder parts of the earth. Three groups of

these plants that are sufficiently distinct to be

* * . FIG. 166. The walk-

readily recognized occur rather commonly in ingfern. New plants

North America. They are the ferns, equisetums, are developed from
and club moSSes.

283

284

Science of Plant Life

FIG. 167. A tree fern.

The Ferns and Their Allies

285

FIG. 168. A large tropical fern (Marratia), with leaves over 30 feet in length.
Philippine Islands.

The ferns. The ferns are noted for the large size of their
foliage leaves. In temperate regions these leaves are much
branched and in some species attain lengths of from 3 to 6
feet. They usually rise from underground stems, which may
be long, slender, and horizontal or short, thick, and upright.
The venation of the leaves differs from that of seed plants
in being forked ; that is, each vein divides into two smaller
ones, and each of these subdivides in the same manner. The
root systems of the ferns are all small and scantily branched as
compared with those of the seed plants. This is probably
the reason why ferns as a rule are confined to moist habitats.

In the moist tropics there are many species of tree ferns
with upright columnar stems bearing a rosette of leaves at
the summit. The stems may attain heights of from 10 to
50 feet, and they bear leaves 5 to 15 feet in length.

The equisetums, or horsetails. In swamps, on the banks
of streams, and on railroad embankments one can find slender,

286

Science of Plant Life

columnar plants with scale leaves and spore-bearing cones on
the ends of the upright branches. These are the remnants of
a group that made up a large part of the vegetation during
the carboniferous period. Many of the species are evergreen.

The equisetums are for the
most part stream-margin and
swamp plants, and, as might be
expected, the stems have air
cavities extending throughout.
In some species the stems de-
velop at each node whorls of
slender branches, which to-
gether form a brush, suggesting
the common name " horse-tail "
for the group.

Like the ferns, most of the
species develop an extensive
system of underground stems
from which the upright branches
rise. In the absence of foliage
leaves, the stems do all the
photosynthetic work. The
roots are small, like those of
the ferns. Spores are pro-
duced in the cones which termi-
nate the upright stems.
The club mosses. These

FIG. 169. The common field equisetum. are creeping and trailing ever-
Rootstock with sterile branches (#), green plants found in rich woods
^t±s^t^tr^ "^ bogs, particularly in the

pendages.

northern United States. They

The Ferns and Their Allies 287

are much used for decorative purposes at Christmas time
and are sold under various names such as " ground pine "

FIG. 1 70. Two species of club mosses (Lycopodium) . In one species (left) the sporangia
are borne in the axils of the upper leaves, in the other (right) they are borne in terminal

and " running cypress." The main stems are prostrate on
the soil and may be many feet long. They are anchored at
intervals by short roots. Numerous upright branches rise

288 Science of Plant Life

from the prostrate stems. The stems bear large numbers of
scale leaves, and the spores are produced in terminal cones.
The spores are sold in drug stores under the name " lyco-
podium powder." They are used as a drying powder and
in the manufacture of fireworks.

The club mosses were of large size and very abundant
during the time when the carboniferous rocks were deposited,
and with the ferns and equisetums they formed the chief part
of the forests of that geological period. The coal which we
now burn is the last remnant of the carbon compounds formed
by plants from the carbon dioxid then present in the atmos-
phere. Certain of the club mosses, like some of the ferns,
show stages in the transition from reproduction by spores
to reproduction by seeds. The club mosses and equisetums
have highly branched stems and many small, scalelike leaves.
In the ferns there is little branching of the stem and the leaves
are large and often much divided.

The two generations of the ferns. One of the most inter-
esting features of the life history of the fernlike plants is the
fact that there are two very different plants produced during
one life cycle. Asexual spores are produced on the under
surfaces of fern leaves, and when these are planted on soil
and allowed to germinate, we find that in about 6 weeks the
soil is covered with small, heart-shaped thalli which look very
much like liverworts. These thalli represent one generation
of the fern plant. Each thallus consists of an irregular plate
of cells, a single cell layer in thickness except toward the
middle, where it may consist of several layers (Fig. 172).
The prothallus, as the plant is called in this stage, is anchored
to the soil by one-celled rhizoids. It manufactures its own
food and has an existence similar to that of a liverwort.

The Ferns and Their Allies

289

When mature, the pro-
thallus produces egg cells
and sperms in small organs
on its under surface. The
sperms are small and swim
actively, and they reach the
egg cells when the thallus
is wet. The sperm unites
with the egg and forms an
oospore. The formation of
the oospore completes the
life of this, the sexual, gen-
eration of the fern.

The oospore germinates
wrhile still attached to the
little prothallus, and from
it develops the large fern
plant with which we are
familiar. This second gen-
eration has true roots,
stems, and leaves, and on
the lower side of the leaves
great numbers of sporangia
are produced. These lie in
groups which may be seen
with the naked eye as
brown dots. Within the sporangia asexual spores are formed,
and because asexual spores are produced by this generation,
it may be called the asexual generation of the plant.

The ferns, therefore, do not reproduce themselves directly.
The plants with the large, feathery leaves that we know as

FIG. 171. Underground stem, roots, and
leaves of fern.

290

Science of Plant Life

ferns produce asexual spores from which small, flat prothalli
develop. The prothalli in turn produce gametes and sexual

spores, which then germinate
to form the fern plants proper.
This alternation of two unlike
generations in the life history
of the fern is most interesting.
The alternation of an asexual
generation with a small sexual
generation occurs in mosses
(page 279) as well as in ferns,
and it is found also in most
of the other great plant
groups. However, the alter-
nation of generations is not

FIG. 172. Under side of fern prothallus, ,

showing egg-producing organs (archegonia) SO easy to Study in groups,

(A), the sperm-producing organs (anther- other than the ferns, Ul which

idia) OB), and the rhizoids (C). , ,

the two generations do not

grow independently of each other. In some of the ferns and
also in some of the club mosses, the sporangia and the sexual
generations show stages in the development of these struc-
tures into the seeds of the higher plants ; but the origin of
seeds is too complex a question to be discussed here.

The significance of the conductive system in plants. The
algae, being immersed in water, have a constant water supply.
The fungi have an adequate water supply because they live
inside other plants that have water-conducting tissue, or
because they grow in moist soil or within stumps and logs.
A water balance is maintained among the mosses and liver-
worts by their growing in spongelike masses and in close
contact with the soil water ; but within these plants water

The Ferns and Their Allies

291

can be transmitted from one part to another only slowly by
diffusion from cell to cell. They have no water-conducting

H

FIG. 173. The life history of a fern. The prothallus (A) produces egg cells and sperms
in organs on the lower surface. One of the sperms set free from B unites with an egg
cell (shown in C), and produces a sexual spore. This germinates and produces the leafy
fern plant (D), which in turn produces asexual spores in sporangia (F and G) on the
lower side of the leaves. By the bursting of the walls of the sporangium (H), the
spores are set free. They then germinate on the soil (in some species on rocks or
trees) and produce a new generation of prothalli like the one shown in A . The pro-
thallus is here shown about 4 times its natural size.

or food-conducting tissues, and for this reason they are all of
small size.

The development of a conductive system is, therefore, an
additional step in the complete adjustment of plants to a land
environment. With a conductive system, the water may be
carried rapidly from one part of a plant to another. Con-
sequently stems and leaves may be raised far above the ground
level and yet receive sufficient water from the roots to replace
that lost through transpiration. Likewise an adequate supply
of food may be transferred to the roots, which makes it pos-
sible for them to live in the soil, where they are unable to

292 Science of Plant Life

manufacture food for themselves. This permits a great in-
crease in the size of the plant body, and makes possible a
high degree of specialization in plant organs and tissues.
As far as is known, the ferns and their relatives were the
first plants to develop a conductive system. We do not
know how the conductive system originated, but its coming
into existence marked perhaps the most important step in
the progress of the plant kingdom.

Summary. The ferns and their allies constitute the rem-
nant of a very ancient and important group of spore plants.
In the study of the evolution of seed plants they are of in-
terest (i) because they have two entirely separate generations,
a small, sexual one which produces gametes, and the large,
leafy plant which produces asexual spores ; (2) because they
show some of the stages in the transition from plants which
reproduce only by spores to plants that produce seeds ; and
(3) because they were the first plants to develop a conductive
system. The presence of a conductive system enables these
plants to raise leaves and stems well above the soil and to
expose their photosynthetic tissues to the light far better
than any of the preceding groups. The ferns do not have
extensive root systems, and consequently are more closely
confined to moist habitats than are the seed plants.

Suggestions for Laboratory and Field Work to Precede
Chapter Twenty-two

1. Field work or a trip to a greenhouse will help to make clear
some of the characteristics of seed plants, and the differences in
the plants belonging to the several divisions of the group.

2. Examine vegetative shoots of pine, hemlock, spruce,, cypress,
arbor vitae, or other available conifers, and compare their leaves
and stems with those of the ferns and angiosperms.

3. Material collected from one of the conifers in the spring,
showing the staminate and carpellate cones, should be studied in
the laboratory. Mature cones may be obtained at any time, and
they will show the relation of seeds to scales.

293

CHAPTER TWENTY-TWO

SEED PLANTS: GYMNOSPERMS

SEED plants form the most conspicuous part of the earth's
vegetation, and they include the majority of the plants that
are of interest as sources of food, lumber, and fibers. They
are the plants with which we are most familiar, and for this
reason the first seventeen chapters of this book have been de-
voted to a description of their structures, processes, and en-
vironmental relations. Here the great groups into which the
seed plants are divided will be described, and some of the
more important families in these larger groups will be briefly
discussed. The two most important groups of living seed
plants are the Gymno sperms and the Anglos perms.

The gymnosperms. The familiar representatives of the
gymnosperm group are the conifers, or cone-bearing trees.
The name " gymnosperm " (Greek : gymnos, naked, and
sperm, seed) suggests the most distinctive feature of the
group. The seeds are borne exposed on the upper surface of
scales, which make up the cones. They are not inclosed by
a pistil wall as are the seeds of the next group.

The conifers. The conifers have scale and needle leaves.
The red cedar and arbor vitae are of the scale-leaf type ; the
pines, spruces, and hemlocks are of the needle-leaf type. The
stems are woody, much branched, and of large size. Indeed,
the largest trees in the world, the giant sequoias, or Big Trees,
of California, belong to this group. The distinctive feature
of gymnosperm wood is the absence of water-conducting
tubes. The wood cells perform the double function of
supporting the tree and conducting the water. Many of
the conifers have large resin tubes extending throughout
the plant. Just what advantage comes to a plant through

294

Seed Plants : Gymnosperms

295

FIG. 174. Pine forest, with an occasional maple, birch, or aspen.

296 Science of Plant Life

the formation of resin is not known. Resin is a valuable
commercial product, and in the Southern states the long-
leaf pine furnishes the crude rosin and the turpentine of
commerce.

Leaves of conifers. The leaves of most conifers are ever-
green and remain on the trees for a period of from 2 to 10
years, depending somewhat upon climatic conditions. In
general they last longer in moist habitats. The Northern
larch or tamarack and the Southern cypress are deciduous
trees with soft needle leaves that contrast strongly with the
hard needle leaves of the evergreens ; the hardness of the
evergreen needles makes it possible for them to withstand the
winter droughts.

Roots of conifers. The roots of conifers are far better
developed than are those of the ferns. Like deciduous trees,
they have root systems that gradually taper from the stem
base and spread over a wide area. The great root system
makes possible the development of a large crown and the
exposure of a greater leaf surface than in the ferns.

Production of seeds. The production of seeds in conifers
may be illustrated by the pine. Two kinds of cones are pro-
duced : the stamina te cones that produce the pollen, and the
ovulate cones that bear the seeds (Fig. 175). The staminate
cones are short-lived structures of early spring. Each scale
bears two pollen sacs, which contain the pollen grains. The
pollen is blown about by the wind, and each grain has on its
sides two little air sacs that cause it to be carried through
the air easily. It is produced in enormous quantities ; and
when it is shed, it oftentimes colors the ground yellow in the
vicinity of pine woods. The staminate cone of the conifers
corresponds to a staminate flower in the flowering plants ;

Seed Plants : Gymnosperms

297

it lacks a calyx and a corolla, but its scales produce pollen and
therefore correspond to the stamens of a lily or rose.

Each scale of the
ovulate cone has two
ovules, or young
seeds, on its upper
surface. At the time
when the pollen is
shed these scales
stand open, or are
separated from one
another, so that the
pollen falls on them
and slides down to
the bases of the
scales, where it comes
into contact with the
young ovules. Then
the scales of the cone

close Up tightly, and FlG- L75- Spray of Austrian pine. At the left (above)
i u . j is a i -year-old and (below) a 2 -year-old ovulate cone.

the pollen grains de- On the right is a cluster of staminate cones.
velop pollen tubes.

These grow down into the ovules and produce sperms which
finally fertilize the eggs. As a result of fertilization, an
embryo is produced, and the ovule walls become the seed
coats. The growth of the pollen tube is very slow in coni-
fers, and it is only at the close of one or two years' growth
that the seeds mature and the ovulate cones die. Then the
drying out and spreading of the scales permits the winged
seeds to fall out or to be blown out and carried to the ground.
In many cases the seeds of conifers have hard waterproof

298 Science of Plant Life

coats which may cause them to remain dormant for long
periods. When planted, some of the seeds germinate the
first season, others the second, and some not until the third
or fourth year. This tends to insure some of the seedlings
favorable conditions for growth. All the seeds will germinate
the first season if the waterproof outer coat is ground off by
shaking the seeds with sharp sand. In tree nurseries seeds
are usually treated in this way, so as to insure the rapid and
uniform development of the seedlings.

The conifer forests of North America. The greater part
of the forested area of this continent is occupied by conifers.
One of the striking characteristics of conifer forests is the fact
that extensive areas are often occupied by a single species.
In the Northern Evergreen Forest, which stretches from the
lower St. Lawrence basin to Alaska and extends southward in
the Alleghanies to northern Alabama, the principal trees are
the white and black spruce, white pine, hemlock, jack pine,
balsam fir, tamarack, and arbor vitae. The spruces are the
principal source of the wood pulp used in the manufacture of
paper. The white pine formerly furnished the most durable
and most desirable lumber in the Eastern states, but the
supply of this lumber is now almost exhausted.

In the Southeastern Evergreen Forest, which extends along
the coastal plain from New Jersey to Texas, the long-leafed
and short-leafed pines, the loblolly pine, the cypress, and the
red cedar are the most important timber trees. The long-
leafed pine is the source of rosin and turpentine, and with
other pines and the cypress it supplies much of the lumber
for interior finishings and other building purposes. Cypress
lumber is noted for its durability in moist situations where
other lumber soon decays through the attacks of fungi.

Seed Plants : Gymnosperms

299

FIG. 176. Map showing the distribution of the forests of North America.

The Western Evergreen Forest covers the Rockies, Sel-
kirks, and Sierra Nevada and Coast ranges, from southern
Alaska to Mexico. Throughout this vast area the dominant

300 Science of Plant Life

trees are the Western yellow pine, Douglas fir, lodgepole pine,
redwood, pinon, and Sitka spruce; and there are numerous
less important cedars, spruces, firs, and pines. With the in-
creasing demand for lumber and the gradual destruction of
the Eastern forests, more and more of the wood from the
Western forests is reaching Eastern lumber markets. The
seeds of the pinon are large and pleasantly flavored, and
formerly they furnished much of the food for the Indians of
California and the lower Great Basin region.

The significance of reproduction by seeds. In a former
chapter (page 207) some details of the manner in which a seed
is produced were given, and it was pointed out that the seed
is a structure containing a young plant (embryo) developed
from a fertilized egg. The seed is better fitted to undergo
a period of dormancy and to carry a plant over an unfavor-
able period than are the small spores of ferns, mosses, and
fungi, or even the resting spores that are produced by certain
algal forms. This is because the seed contains (i) a plant
already partly developed, (2) a larger supply of nourish-
ment to start the young plant when development begins, and
(3) seed coats that afford better protection during the dor-
mant period. So efficient, indeed, is the seed in passing over
periods unfavorable for growth, that nearly all seeds will
retain their vitality for from one to several years ; and the seeds
of certain plants have been found to be alive after 25 years.

Summary of the development of the plant kingdom. At
this point it may be well to review the most notable steps in
the development of plant life on the earth. These steps are :

(i) The development from previously existing water
plants of land forms with epidermis, rhizoids, and other ad-
justments to a land environment.

Seed Plants : Gymnosperms 301

(2) The development of a conductive system which made
possible the growth of aerial parts situated at a distance
from the water supply, and which also made possible the
transportation of food to roots that live in the dark and there-
fore are not able to make their own food.

(3) The development of seeds as a means of dispersing the
plant and of preserving it during seasons unfavorable for
vegetative life.

(4) The development of an extensive root system by which
larger amounts of water and mineral salts are made acces-
sible. Correlated with this is the development of large,
much-branched tree forms.

(5) The. development of true flowers made up of floral leaves,
stamens, and pistils. With this step came insect pollination
and the production of seeds inclosed in an ovulary. In some
cases the ovulary is fleshy and at maturity becomes an edible
fruit.

Suggestions for Laboratory Work to Precede Chapter
Twenty-three

1. A lily, tulip, or wild onion may be used in studying the
flowers of the monocotyledons. Attention should also be given
to the leaf and stem characters of this group. Narcissus bulbs
grown in water will furnish excellent material.

2. Any convenient dicotyledon may be used to show the type
of flower and the associated leaf and stem characteristics. Ge-
raniums, oxalis, and impatiens may be obtained at all seasons of
the year. In autumn and spring many plants are available.

3. A collection of the more important grain plants, fibers, and
fiber plants is easily made. These will be helpful in studying the
economic uses of plants.

4. The examination of pressed specimens representing the more
important families of angiosperms will be an aid in learning family
characteristics. Fresh specimens are always to be preferred, if
they can be obtained.

302

CHAPTER TWENTY-THREE

SEED PLANTS: ANGIOSPERMS

FIG. 177. Roadside clump of bamboo, Philippine Islands. The bamboo is a very
important tropical monocot.

ANGIOSPERMS (Greek : angio, receptacle, and sperm, seed)
form the second division of seed plants. The seeds are in-
closed in a pistil which later ripens into a pod or fruit. The
Angiosperms make up by far the largest part of the present
vegetation of the earth. There are more than 130,000 species,
as contrasted with 500 species of Gymnosperms and about
4500 species belonging to the fern group. In many forms the
stamens and pistils are surrounded by brightly colored floral
leaves. As compared with other groups of seed plants, a
most striking diversity of vegetative and reproductive struc-
tures is shown within the group. This diversity of form en-
ables Angiosperms to live in all land and water habitats, from
the margin of the ocean to alpine summits, and from the tropics
to the polar deserts.

303

304 Science of Plant Life

Diversification among the Angiosperms. The great dif-
ferences in the stems of Angiosperms may be realized by call-
ing to mind the elm, palm, cactus, dandelion, grape, morning-
glory, bamboo, tumbleweed, pondweed, water lily, dodder,
and duckweed. The leaves may be simple or divided into
leaflets ; and in size they range from the minute scales of the
heather to the leaves of the palm and banana, which may be
from 20 to 30 feet long. The form, color, and venation of
the leaves show a corresponding diversity, so that all shapes
and sizes of leaves may be found within the group. The
Angiosperms have extensively developed root systems that
are capable of anchoring the plants and absorbing water under
the greatest variety of soil conditions. The reproductive
structures are very diverse and are more complex than in the
Gymnosperms. In addition to a great number of methods
of vegetative multiplication by leaves, stems, and roots, there
is the production of seeds from flowers. In the higher
Angiosperms there are brightly colored floral parts which are
helpful in securing the transfer of pollen by insects. The
seeds are inclosed in fruits which protect the developing ovules
and frequently aid in the scattering of the seeds.

Two great groups of Angiosperms. The Angiosperms
naturally fall into two great groups : the monocotyledons and
the dicotyledons. These groups have already been discussed
(page 209), and only a general summary of their character-
istics will be presented here.

Monocotyledons. The Monocotyledons have their floral
parts usually in groups of three (rarely in four) ; the bundles
are closed and scattered throughout the pith of the stem;
in most forms the veins of the leaves are parallel ; and the
embryo has but one well-developed cotyledon. With the

Seed Plants : Angiosperms

305

FIG. 178. A deciduous forest In Illinois. The trees are oak, hickory, and elm,
important and typical dicot trees.

exception of the palms, bamboos, and a few other species, the
Monocotyledons are herbs ; and the tissues of even the woody
Monocotyledons are generally less woody than are those of
the conifers and the dicotyledonous trees and shrubs. This
great assemblage of flowering plants includes not fewer than
25,000 species.

Dicotyledons. The floral parts of Dicotyledons are usually
in groups of five or four ; the vascular bundles are arranged
in a circle about a central pith ; the bundles are open, which
makes possible an increase in stem diameter and permits forms
like the trees to reach a great size ; the leaves, with few
exceptions, are net-veined ; and the embryo has two coty-
ledons. In size the Dicotyledons range from the smallest
herbs to the largest of our hardwood trees.

306

Science of Plant Life

FIG. 179. Coconut palm in fruit. The palm family is very important in
the tropics.

Seed Plants : Angiosperms

3°7

FIG. 1 80. Transplanting lowland rice in the Philippines. The seed is germinated
in beds, and at the opening of the rainy season the seedlings are transplanted to the
flooded fields. The rice plant and other members of the grass family furnish most
of the food supply of animals and men.

The important families of the Monocotyledons. The

Monocotyledons are usually grouped in about forty families,
four of which are of surpassing interest. These four families
are the palms, grasses, lilies, and orchids.

The palm family. The palms form a large group abundant
throughout the tropics. They resemble the tree ferns and
cycads in having columnar, unbranched stems. The stem
increases in thickness during the first few years and then grows
only at the top. The leaves are often branched and of gigantic
size. In the fan palms they are rounded and split radially.

Palms are very useful to the natives of the tropics, furnish-
ing much of the material for thatching roofs and for the weav-
ing of mats, hats, and bags. They also produce fruits and
seeds, like the date and coconut, that are most important

3o8

Science of Plant Life

U. S. Dept. of Agriculture
FIG. 181. Harvesting rice in California.

sources of food. The rattan is a climbing palm, for the
" fiber " of which there is a world- wide market. This fiber,
which consists of flat or cylindrical strips of the stem, is used
in the making of chairs, tables, and baskets.

The grass family. The grasses furnish the bulk of the food of
animals and men; and with the possible exception of the
trees, no other group of plants compares with them in use-
fulness. They form the natural covering of a large part of the
earth's surface. The prairies and plains of North America,
the steppes of Russia, and the pampas of Argentina are vast
areas that were originally dominated by grasses. These sup-
ported great herds of grazing animals before the coming of
man, and since that time they have been the centers for cattle,
horse, and sheep production. Grasses are cultivated on most
farms to furnish pasturage for the live stock.

The stems of grasses are cylindrical and usually hollow,
except at the nodes. The leaves are usually two-ranked,
the basal portion forming a sheath about the stem, as may be

Seed Plants : Angiosperms

3°9

seen on a stalk of corn. The flowers are borne on spikelets
surrounded by green bracts.

The grasses reach their greatest size in the bamboos. These
have long been one of the chief sources of lumber in eastern
Asia and the East Indies. From the bamboos come the
materials for the native houses, furniture, and a large number
of articles of household use. In subtropical America these
plants are being used as windbreaks for the pineapple fields
and citrus orchards.

Sugar cane and sorghum are members of the grass family
that are cultivated for their sugar. The former is the source
of the sugar that is produced in our Gulf Coast region and in
the tropics. Sorghum is grown farther north and is used
for the manufacture of molasses. Broom corn is closely re-
lated to sorghum, and is grown for the stiff branches of the
flower cluster, which are used in the manufacture of brooms

Bureau of Agriculture, P. I.

FIG. 182. Japanese cane, a near relative of the sugar cane, grown as a fodder crop
in the Philippines.

310 Science of Plant Life

and brushes. Kafir corn and milo are other relatives of the
sorghum that have become important forage crops in parts

of our country that are too dry
for the successful production of
corn.

But the most important of all
the grasses are the grains : wheat,
rye, barley, oats, rice, millet, and
corn. Most of the world's food is
derived from these plants ; and,
depending upon the yield of these
several crops, human beings are
well supplied with food or famine
prevails. All other foods are of
secondary importance to the
grains.

The lily family. This is one of
the important families of the
monocots because of the large
number of species, their wide
distribution, and the beautiful
flowers that are borne by many
of them. The flowers usually
have six colored parts forming
the floral envelope. There are
FIG. 183. The wood lily. Many sjx stamens, and the pistil is made

members of the lily family are cul- ., . . r~^

tivated for their flowers. up of three divisions. The onion

and asparagus are members of the

lily family that are used for food. The Mexican century
plants and the New Zealand flax are fiber-producing plants
that belong to this group. Many of the bulbous forms like

Seed Plants : Angiosperms

the Easter lily, tulip, hyacinth, and narcissus are of com-
mercial importance because of their flowers.

The orchid family. This is
a large family most abundant
in the tropics, and it includes
about a fifth of all the mono-
cots. In temperate America
there are many species grow-
ing in bogs, swamps, and
shaded woods, some of which
are noted for their beautiful
flowers. The pink moccasin
flower and the yellow lady's
slipper are examples. In the
tropics most of the species
found are epiphytes (page
189). These species are
world-renowned for their
marvelous shapes and for
the coloring of their flowers.
Among flower fanciers the FlG- l84- A tr°Pical epiphytic orchid.

.. . The orchids are noted for thefr varied and

orchids are the most sought- beautifui flowers.

after plants, and prices of

more than $1000 have been paid for single plants of the rarer

species. They hybridize freely, and there are innumerable

hybrid varieties in cultivation.

Important families of the dicotyledons. This division of
the flowering plants contains over 200 families and more than
100,000 species. Consequently only a few conspicuous fami-
lies can be discussed here, and they will be dealt with briefly.
In this group belong our broad-leafed trees and shrubs, and

3I2

Science of Plant Life

the vast majority of our herbs. The poplars, oaks, hickories,
maples, and elms, and the chestnut, beech, and ash, largely
make up the Deciduous Forest of the eastern United States,
which dominates the area east of the Great Plains between the
upper Lake region and the Gulf coastal plain.

The mustard family. This family includes the mustards,
cabbages, radishes, turnips, and cresses. It is of great
economic importance. The flower is characterized by four
petals placed at right angles in the form of a cross. The
development of cultivated plants from wild species is nowhere
better exemplified than in the case of the cabbage. The small

wild plant from which it was
derived produced neither
heads nor swollen stems.
From it, by the selection of
mutations, have been devel-
oped several distinct forms of
cabbage and also cauliflower,
Brussels sprouts, kohl-rabi,
kale, and collards. These
differ so much in appearance
that one would scarcely guess
that they had a common wild
origin. The histories of cul-
tivated plants afford many ex-
amples of far-reaching changes
in the forms of plants, brought
about in the same way.
The rose family. This

FIG. 185. Wild rose, and a vertical section famjly js characterized by

of the flower. Most of the important tree .

and shrub fruits belong to the rose family, flowers With tlVC petals, tlVC

Seed Plants: Angiosperms

sepals, and many stamens. The fruits are fleshy, and in
many species they are edible. The group is of particular
interest to the horticulturist,
as it includes most of our im-
portant tree and shrub fruits,
as well as many beautiful
wild and cultivated flowering
plants. Among the fruits in-
cluded are strawberries, rasp-
berries, dewberries, and black-
berries; apples, pears, and
quinces ; and peaches, plums,
cherries, prunes, and almonds.
All these cultivated fruits, as
compared with their wild an-
cestors, have been improved
in flavor and greatly increased
in size and productiveness.

The legume family. The
legumes received their name

from the peculiar fruit or pod which contains the seeds.
Typical legumes are the pea and the bean. In the best-known
members of the family the flower has one large upper petal
which stands erect, two lateral petals forming wings, and two
lower petals united and inclosing the pistil and stamens.
The sweet pea shows the typical flower form.

There are several reasons for the great importance of the
legume family : (i) beans, soy beans, and peas furnish food
material that contains a high percentage of protein; (2)
all legumes, through the aid of bacteria in their root nodules,
accumulate nitrogen compounds (page 258) ; (3) clover,

FIG. 1 86. Sweet pea, and a vertical
section of the flower.

Science of Plant Life

FIG. 187. Peppermint. The square
stems and opposite leaves are char-
acteristic of the mint family.

alfalfa, and soy beans supply large
quantities of rich hay for horses,
cattle, and sheep. The sweet peas
are among the most beautiful of
cultivated flowering plants. The
locust, which furnishes a very en-
during wood, also is a legume ; and
many of the valuable woods of
tropical forests belong to this group.
In the tropics and subtropics one
of the divisions of this family is rep-
resented by the acacias, in which
the flowers are radial and have
numerous long stamens that give
the flower clusters the appearance
of balls of coarse hair. The acacias
are frequently cultivated as or-
namental trees, and they are a
source of commercial gums. Re-
lated to the acacias are the sensi-
tive plants (Mimosa) , one of which
is shown on page 44. In South
Africa flat-topped Mimosa trees
are common and form one of the
characteristic features of the land-

scape.

The mint family. This family
is marked by square stems, peculiar
two-lipped flowers, opposite leaves,
FIG. 188. Climbing nightshade, an(^ jn manv species an aromatic

one of the wild species belonging . .

to the potato family. odor. Peppermint and spearmint

Seed Plants: Angiosperms

315

are now cultivated extensively on
bog soils in the northern United
States, for the production of pep-
permint oil and menthol. Catnip,
scarlet sage, water horehound, and
hyssop are common plants belong-
ing to this family.

The potato family. This family is
composed of herbs and small woody
plants with expanded or tubular
flowers. The fruits are capsules or
berries containing numerous seeds.
Among the useful representatives
are the potato and the tomato,
eggplant, and red pepper. Several
plants containing narcotics, of
which the best-known example is
tobacco, belong here. Common
weeds that belong to the potato
family are the nightshade, horse

, ,1 * , i T- FIG. 189. Wild carrot. The flat-

nettle, ground cherry, and Jimson topped flower duster characterizes

Weed. the family to which the carrot

The carrot family. This family belongs'

is readily recognized by the flat-topped, much-branched
flower cluster (umbel). The parsnips, carrots, celery, and
sweet cicely are familiar examples. In addition to the food-
yielding forms, the water hemlocks should be known because
of their poisonous character.

The heath family. Small, bell-shaped flowers and leathery
leaves are characteristics of the heath family. To it belong
the cranberries, blueberries, huckleberries, low-growing tea-

316

Science of Plant Life

berry, trailing arbutus, laurel, azalea, and rhododendron.
The cranberry has long been cultivated on sandy bog lands.

Blueberries are just coming
into cultivation, and much
may be expected of them in
the future. They grow best
on acid soils, and are capable
of great improvement in size
and flavor through hybridiza-
tion and selection.

The composite family. The
composite family is the cul-
minating family of the dicots,
including more than 12,000
species, or nearly one tenth
of all seed plants. The dis-
tinguishing character of the
family is that the small flow-
ers are closely grouped in
heads, so that the entire
flower clusters, in forms like
the sunflowers, asters, chrys-
anthemums, and dandelions,
are often mistaken for single flowers. The calyx-like outside
covering is merely several rows of small leaves (bracts) . The
whole inner part of the head is made up of many individual
flowers, each with its own stamens, pistils, and corolla. Some-
times the flowers are all alike, as in the dandelion. Sometimes
the outer flowers are petaloid, as in the sunflower, while the
inner flowers are tubular and less conspicuous. The floricul-
turist has modified the chrysanthemum, aster, and dahlia to

FIG. 190.

Mountain laurel, a member of
the heath family.

Seed Plants : Angiosperms

317

such an extent that there are now hundreds of forms in culti-
vation that bear little resemblance to the wild stock from
which they were derived.

FIG. IQI. Flower clusters of dahlia, sunflower, and thistle, members of the composite
family. The small flowers are collected in heads, which are characteristic of the family.

In the United States the composites bloom chiefly in summer
and autumn, and often conspicuously color the landscape.
The daisies, goldenrods, and asters of the Eastern states and
the sunflowers of the prairie states are well known. The rag-
weeds, which are troublesome as weeds, are one of the chief
causes of hay fever. Their pollen is highly irritating to the
mucous membrane of the nose, and it is advisable that city and
village streets be kept clear of them.

Many other families of the dicots are of interest and im-
portance, but descriptions of them must be sought in more
specialized botanical texts.

CHAPTER TWENTY-FOUR

THE EVOLUTION OF PLANTS

THOSE who have studied plants most have been led to the
conclusion that simple plants lived first on the earth, and
that from these simple forms all the varied and highly com-
plex plants of today have been derived ; that is, they believe
that the present-day plants were evolved from those that
existed on the earth in former times. Some of the simple
plants of the past still persist, and many plants of inter-
mediate degrees of complexity survive ; but during the de-
velopment of the plant world, new and increasingly complex
forms have been produced, and these higher forms now
dominate the vegetation of the earth. The process by which
the plants of today have come from the plants of the past
is called evolution (Latin: evolutio, an unrolling). Evolu-
tion, with regard to plants, implies (i) that the plants of to-
day are the modified descendants of earlier forms, (2) that
modifications are going on now as in the past, and (3) that
the new plants of the future will be evolved from plants now
living through modification of present plant forms.

The proofs of evolution in plants have been gathered from
many sources by many different students. These proofs
include the evidence furnished (i) by plant remains found in
rocks and coal, (2) by the distribution of plants on the earth's
surface, (3) by the remarkable similarity of organs, tissues,
and cells among the thousands of plants now in existence,
(4) by the similarity in the life histories of all plants, (5) by
intergrading species, (6) by the experience of plant breeders
and the history of our cultivated plants.

The record of plants of the past. When a leaf falls from a
tree on soft mud, it may become imbedded in it. Later the

318

The Evolution of Plants

319

mud may be covered by other layers of sediment. When the
mud dries, a perfect imprint of the outline and veins may be
left. As time goes on and
the mud becomes more
deeply buried, it may harden
into rock and retain the im-
print of the leaf as a record
of a plant that lived when
the rock was merely soft
mud. In this way leaves,
fruits, seeds, stems, and
roots have left their im-
prints to testify, thousands
and millions of years after-
ward, to their former ex-
istence.

Plant remains also ac-
cumulate in deep water or
in water containing large
amounts of mineral matter in solution. In such places they
may not decay, but the material of which they are composed
may gradually be oxidized and replaced by the mineral
substances in the water. Under the most favorable condi-
tions the internal structures of the plant are preserved. As
animal remains are preserved in the same way, we have in the
rocks a record of the plants and animals of the past. These
petrified plant and animal remains and the plant and animal
imprints from former geological ages are called fossils.

When large collections of fossils are studied, we find in the
oldest fossil-bearing rocks few, if any, plant remains. This
is as we should expect, for plants like algae and liverworts are

FIG. 192. Fossil imprints of fern leaves in
a rock of the Carboniferous period.

320 Science of Plant Life

so minute and delicate that we cannot hope to find their fossil
remains. Yet we know that plants of some kind did exist
on the earth at that time, for the fossils of corals and of shell
animals are abundant in these rocks, and without plants to
manufacture food for them, life would have been impossible
for these animals. In rocks that were formed later than
these first fossil-bearing strata, remains of fernlike plants are
found. Later there were seed-bearing fernlike plants and
several groups of plants intermediate between the ferns and
modern gymnosperms. Still later, gymnosperms resembling
in many ways our coniferous trees became prominent, and
then came Angiosperms with very simple flowers of the wind-
pollinated type. Finally, Angiosperms with conspicuous
insect-pollinated flowers appeared. There is reason to be-
lieve that the first seed plants and the first flowering plants
were woody shrubs and trees, and that the flowering herbs
came last.

The meaning of the record. The fossil record of plants
reaches many millions of years into the past, and it shows
that changes in plants came about only very slowly. The
plants of each geological period resembled in some respects
those of the previous period ; in each period certain new
characters were added to the old ones, or they replaced the
old ones. To reproduction by spores was added reproduction
by seeds. The first seeds were exposed on the sides of foliage
leaves ; later came seeds borne on scales arranged in cones ;
and finally came seeds inclosed in pistils.

Leaves and stems also show progressive changes in form
and structure. Although the fossil record is very fragmen-
tary, being made up of chance imprints and petrified remains,
it is possible in some instances to discover when a group of

The Evolution of Plants

321

plants appeared and how the group became more widely dis-
tributed and more highly diversified in form and structure.
Some of these ancient plant
groups that at one time formed
a considerable part of the earth's
vegetation have entirely disap-
peared; others are now repre-
sented by only a few species.

These facts all lead to the
conclusion that existing plants
have been derived from those
of the past. The flowering
plants are the culmination of
a long series of constructive
changes in plants, which en- FlG I03. Fossii imprint of a leaf of a

abled them to live in a greater species of sassafras in rock of the Cre-
. . T taceous period. The Cretaceous rocks

variety of environments. In- were forraed much later than the Car.

Creasingly Complex and efficient boniferous rocks, and it is only in these
£ .• i«r j later rocks that the fossil remains of

structures for vegetative life and Angiosperms are found.
reproduction were developed,

and these structures made the plants better able to endure
unfavorable conditions. The progressive changes in plant
life have occurred during a geological history extending through
many millions of years, and it now seems impossible to account
for the geological record except on the basis of evolution.
Certainly no one has suggested any other plausible way to
explain the long series of gradually changing fossil forms that
begins with simple fernlike plants and ends in plants like those
found on the earth today.

Geographic distribution and evolution. When the geo-
graphic distribution of plant families is studied, it becomes

322

Science of Plant Life

The Evolution of Plants 323

apparent that the species belonging to any particular group
are not scattered haphazard over the earth. Many families
bear evidence of having originated on some particular con-
tinent or part of a continent, and of then having spread
from the center of origin. Commonly, in the center of origin
there is or was the greatest variety of species, and away from
the center the species are fewer and less varied. For example,
the Cactus family, represented by more than a thousand
species, is native in North and South America only. In
North America the family is best developed in Mexico, but
it has spread northward and eastward into the United States
and to the islands of the West Indies. The geographic dis-
tribution of all the North American species points to a com-
mon origin in the Mexican plateau. The Yucca family and
the Agave (century plant) family also appear to have orig-
inated there and to have spread in a similar way to the United
States and the West Indies.

Sometimes the record is incomplete because the families
are very ancient and most of the species have become extinct.
In other families the record is so complete that it is possible
from the distribution to trace the origin and relationship of
many of the species. The geography of plants, therefore,
furnishes a second line of evidence that existing plant species
have been derived from preexisting forms.

Similarity of structures of plants belonging to diverse
groups. One of the most striking proofs of evolution is found
in the remarkable similarity of the cells, tissues, and organs
that make up plants belonging to very different groups. The
conductive system, for example, is not very different in the
ferns and in the Angiosperms. The bundles are arranged dif-
ferently in these groups, and their arrangement becomes in-

324 Science of Plant Life

creasingly more complex from the ferns to the flowering plants ;
but the cells of which the bundles are composed are very
similar. The reproductive structures also show a gradual
modification in plants. They are comparatively simple in
the ferns and very complex in the flowering plants, and be-
tween the two extremes there are many gradations. These
gradations occur in such an order that they point to the grad-
ual coming into existence of complex reproductive structures,
culminating in those of insect-pollinated flowers.

Life histories of plants. Again, when we compare the de-
tailed life histories of plants belonging to the different groups,
the close resemblances among them point to their origin by
gradual evolution from common ancestors. As one passes
from the algae through the mosses and ferns to the Gymno-
sperms and Angiosperms, the life histories of the plants, like
their structures, become more and more complicated. This
increase in complexity of the life history is brought about by
gradual modification and by the addition of new steps and
new structures. The life histories of related groups are
similar in essentials and differ only in details. This repetition
of the stages in the life cycles of the plants of different groups
can be explained only by assuming that the plants with the
more complex life histories have evolved from those with less
complex life histories. Increase in complexity is one of the
general tendencies of evolution. The order in which we should
arrange plants on the basis of the geological records is the same
as the order suggested by their life histories and structures.

Intergrading species. All who have attempted to classify
plants — that is, to determine the species to which individual
specimens belong — have been impressed by the intergrading
of related species. The existence of individuals intermediate

The Evolution of Plants 325

between species long ago suggested that the one species may
have arisen from the other. For example, the common asters,
violets, hawthorns, evening primroses, and willows are highly
variable, and it is frequently impossible definitely to classify
a particular specimen and to say that it belongs to this or
that species. If such intermediate forms were rare, they
would only suggest the possibility of evolution ; but they are
numerous, occurring in hundreds of families throughout the
plant kingdom. These intergrades make it impossible for
us to think of the plant kingdom as being composed of dis-
tinct and unrelated species, and so they must be regarded as
evidences of evolution.

Plant breeding and evolution. If we study the histories of
various cultivated plants, — for example, the many varieties
of cabbage, tomatoes, corn, wheat, apples, peaches, chrysan-
themums, and asters, — we find abundant evidence that
they have been derived from wild plants. The plant breeder
has simply selected and preserved desirable forms that have
arisen by mutation and through hybridization.

When we realize the extent to which these plants have been
modified from the types of their wild ancestors in the com-
paratively short time that plants have been cultivated, it is
less difficult to understand the great changes in plant life
that may have occurred during the long period since plants
first grew on the earth.

Plant breeding furnishes us with experimental evidence that
plants may change in various ways, so that not all the plants
of succeeding generations are like their ancestors. Here and
there an individual arises that is different from its parents,
and such new individuals are the beginnings of new races,
varieties, and perhaps species. The experience of plant

326 Science of Plant Life

breeders is in harmony with the theory of evolution, and
plant breeding has furnished us suggestions as to the methods
by which evolution has taken place.

Evolution proved; methods of evolution not proved.
Among the thousands of students of modern and ancient
plants, there are few, if any, who do not regard evolution itself
as a fact. The evidence points so clearly toward evolution,
and all other explanations for the origin of the plant species
now on the earth have proved so inadequate, that botanists
consider evolution as proved. At the same time, we know
comparatively little today concerning the agencies that have
brought about evolution. Evolution as a process has been
studied only recently, and merely a beginning has been made
in unraveling the tangled skein of causes and methods by
which modern plants have come to be what they are.

Factors of evolution. When we seek to learn the reasons
for the changes in plant life during geological times, we find
that many factors were probably involved. We can get an
idea of some of the possible factors from suggestions that have
been made by students of evolution, but we must keep in
mind the fact that there is today little real evidence concern-
ing the factors of evolution and that the suggestions merely
indicate how evolution may have come about. It seems
probable, however, that in the production of new kinds of
plants variation and selection play a part. Certainly there
may be among the offspring of a plant individuals that differ
from the parent, and some variations are known to be inher-
itable. In order to explain evolution, it is necessary to assume
that new characters are produced in plants by variation, and
that the changes in the plants are transmitted from genera-
tion to generation.

The Evolution of Plants 327

Natural selection. Most plants produce offspring by the
hundreds, thousands, or even millions, and there is room for
only a small part of the offspring to live. It is said that those
plants survive that are more vigorous, that are better ad-
justed to their environments, or that happen to start in favor-
able places; the weak and the unfortunate perish. Certain
variations, or mutations, may fit plants the better to survive,
and the persistence of the forms showing these changes may
lead to the formation of new varieties and species. The
wholesale destruction of individual plants in nature, with the
survival of a few, is called Natural Selection, and it has been
thought to resemble in some respects the selection made by
the plant breeder. It is unquestionably true that most of
the plants that start life in nature die before reaching ma-
turity ; but there are great differences of opinion as to
whether or not the plants that do survive can through re-
peated selections in nature develop into new species. Man
can pick out new forms that originate in the plants that he
cultivates and by breeding from them secure new varieties,
but it is believed by some that in nature the variations
would be lost by interbreeding with the parent forms.

Changes in environment. Another factor in Natural
Selection is the fact that climates and environments are ever
changing. We know, for example, that for a long period
of time tropical plants grew in polar regions, for the fossils
of tropical plants are now found there in the rocks. At a
later period the climate of the earth was much colder than
it is now, and all of northern North America was covered
with ice that reached as far south as the Ohio and Missouri
rivers. As climates change, plants either become adjusted
to the changes or perish. These changes in climate afford

328 Science of Plant Life

opportunities for new sets of plants to take possession of the
land. They are perhaps also the cause of mutations in plants
that give rise to new forms which replace the species from
which they were derived.

The land areas have changed many times in the earth's
history. Sometimes large parts of the continents have been
under water ; at other times the continents have been more
elevated than at present. These changes in the land areas
have resulted in the extinction of many species of plants.
Forms derived from them may have survived on the land not
submerged. So the great changes in climate and in land
surfaces, and the less pronounced changes in plant habitats,
act as selective agencies, determining the kinds of plants that
survive.

Isolation. If a continuous land area, having a number of
elevations, were submerged so that the elevations became
islands, a plant that had been originally distributed over the
whole area might persist on a number of the islands. The
plants on the several islands would then be isolated from one
another. As time went on these isolated individuals might
give rise to new forms that were quite distinct in their char-
acteristics. Different new forms might arise on different
islands, and since the selective agencies on the various islands
would not be the same, the varieties that persisted on one
island might come to be quite distinct from those on other
islands. The production of the new forms depends on vari-
'ation, but isolation tends to preserve new forms by preventing
their spreading and interbreeding.

The deciduous forest of eastern North America was for-
merly continuous with the forests of China through Alaska.
Since then the connecting land has been depressed, and the

The Evolution of Plants 329

two continents, Asia and North America, have become
separated and their climates have changed. This separa-
tion resulted in the isolation of the deciduous forest plants
of eastern Asia from those of eastern America. Many of
the same species are still found in both areas. But in the
two widely separated regions there are many other quite
distinct species that have arisen by evolution from a com-
mon ancestor. The American sycamore, or example, is
distinct from the Chinese sycamore ; but the resemblance
between the two is sufficient to suggest that they have a com-
mon ancestry.

Variations; the elimination of many individuals through
lack of room in which to grow ; changes in climates and in land
areas ; and isolation are believed to be important factors in
the evolution of plants.

INDEX

Abscission, 67.

Absorption, 170; and rise of sap, 135, 176;

and transpiration, 176.
Accumulation, of food, 74, 75, 77, 105, 125,

159, !79; °f water, 58, 106, 158.
Aconite, 195.

Acre, food products from an, 34.
Advantages, of deciduous habit, 63 ; of

horizontal stems, 104; of spreading

roots, 169; of upright stems, 102.
Aerial roots, 189.
Agar, 246.

Agave, 58, 90, 91, 97-
Agriculture, 4, 99 ; and food accumulation,

78.

Alcohol, 261.
Alder, 198, 201.
Alfalfa, 194.
Algae, 234 ; blue-green, 245 ; brown and

red, 246; living conditions of, 239;

reproduction among, 243; structure of,

234-

Alligator pear, 212.
Amanita, 267.
Amaryllis, 161.
Angiosperms, 211, 303; divisions of, 304;

seeds of, 211.
Animals, 149, 231.

Annual growth, 129; rings, 123, 129.
Annuals, 94.
Anther, 200.
Anthocyan, 65.
Antitoxins, 253.
Apple, 76, 79; May, 98.
Artichoke, 214; Jerusalem, 97, 161, 163.
Arts, plant-producing, 99.
Asparagus, 163.
Assimilation, 82, 85.
Autophytes, 249.
Autumn coloration, 65.
Avens leaf, 15.
Avocado, 212.
Axil, 106.

Bacteria, 2, 250; and disease, 253; and

legumes, 194, 258; and sanitation, 252;

and soils, 257; and soil nitrogen, 258;

in the dairy, 256.
Bamboo, 164; leaf, 14; stem, 124, 130,

303-

Banana, 79, 97, 219.
Bark, 129, 170.
Bast, 119; fibers, 123.
Bean, 43, 207, 223; soy, 33, 78.
Beech, 194.
Beets, 96, 194.
Begonia leaf, 14.

Benedict's solution, 73.
Bermuda grass, 159.
Biennials, 94, 95, 96.
Black locust, 198.

Blade, 13, 14; position and light, 41.
Bloom, 54.
Blue-green algae, 245.
Boston fern, 226; ivy, 182, 189.
Botany, 3, 4; subdivisions, 151.
Bread making, 261.
Breeding, plant, 220.
Bromelias, 189, 191.
Brown color of leaves, 66.
Bryophyllum, 218.
Buckeye leaf, 15; twig, 107.
Buds, 1 06 ; and plant form, 109 ; kinds of,
109, 160; opening of, 108; scars, 107,

113, 114-
Budding, 132.
Bulbs, 97, 160, 219.
Bulrushes, 157.

Bundle, dicot, 121; monocot, 126.
Bundles, leaf, 20; root, 169; stem, 120,

121, 125, 126, 128.
Burdock, 102.

Cabbage, 96, 225.

Cactus, 56, 158, 190; spineless, 6.

Caladium, 160, 219.

Calorie, 34.

Calyx, 200.

Cambium, 119, 124; and grafting, 132.

Camphor, 163.

Canna, 219.

Capsule, 212.

Carbohydrates, 27, 135.

Carboniferous plants, 283.

Carotin, 63.

Carrot, 96, 186, 198; family, 315; wild,

316.

Castor bean, 207.
Catkin, 198.
Cat-tail, 157, 198.
Cauliflower, 214, 225.
Celandine leaf, 15.
Celeriac, 194.
Cells, 15, 1 6, 21 ; guard, 18; tissues and

organs, 21, 178.
Cell sap, 1 6.
Cellulose, 16, 86, 100.
Cell walls, 1 6, 276; making of, 86.
Century plant, 58, 76, 96, 97.
Cereus, 58.
Chlorophyll, 22, 63.
Chloroplasts, 22.
Cion, 132.
Cladonia, 269.

331

332

Index

Clavaria, 267.

Cleft grafting, 132.

Climbers, 155.

Closed bundle, 126.

Clover leaf, 16, 44; root, 194.

Club mosses, 286.

Coconut, 214, 215, 306.

Color of leaves, 43, 45.

Commercial products, from leaves, 91 ;

from roots, 194; from stems, 161.
Compass plants, 40.
Composite family, 315.
Concord grape, 225.

Conduction, food, 105, 121; water, 121.
Conductive system, 121; significance of,

290.
Conductive tissues, leaves, 20; roots, 169;

stems, 105, 121.
Conifer forests, 298.
Conifers, 118, 294; leaves of, 296; seeds

of, 296; stem structure in, 128.
Copra, 214.
Cork, 163.
Corm, 160, 219.
Corn, 8, 50, 78, 79, 142, 164, 167, 179, 199,

202, 205, 207, 209, 215, 221, 222.

Corolla, 200.

Cortex, 119, 1 20.

Cotton fiber, 215.

Cotyledon, 207.

Cowpeas, 194.

Craterellus, 267.

Crop yields, 34, 230 ; and water supply, 51.

Cucumber, wild, 155.

Cuticle, 1 8, 54.

Cutin, 17.

Cuttings, 167.

Cypress, 296.

Cytoplasm, 15, 175.

Dahlias, 97, 317.

Dandelion, 103, 166, 229; leaf, 15; root,

167.

, Dasheen, 160, 163.
Date palm, no.

Deciduous forest, 305, 312; habit, 63.
Deliquescent stems, in.
Desert plants, 58, 59, 154, 157.
Development of plant kingdom, 300.
Diastase, 75.
Diatoms, 244.

Dicots, 118, 209; stem structure, 120.
Dicotyledons, families of, 311.
Diffusion, 171.
Digestion, 74.
Diseases, bacterial, 253.
Dodder, 250.

Dogwood, 39.
Dulse, 246.

Duration, of leaves, 69 ; of stems, 94 ; of
roots, 185.

Ecological factors, 140, 151; types of
leaves, 42, 43, 44, 56, 57 ; types of roots,
1 86 ; types of stems, 154.

Ecology, 151.

Economic importance, of algae, 245 ; of
bacteria, 251; of flowers, 214; of fruits,
214; of pond scums, 245; of seeds,
214.

Eel grass, 57, 91.

Egg cells, 205.

Elements of soil, 141.

Elodea, 28, 29, 57.

Embryo, 206.

Endosperm, 207.

Energy, 82.

Environment, 8, 139; changes in, 327;
complexity of, 150; factors of, 139;
responses to, 130, 131, 275.

Enzymes, 74.

Epidermis, 15, 17, 50, 276; importance of,
18; stem, 120; root, 169.

Epiphytes, 1 88, 189.

Equisetums, 285.

Eucalyptus, 99.

Euphorbia, 58.

Evening primrose, 186.

Evergreen habit, 69, 71.

Evergreens, 64, 69.

Evolution, 300, 318; and geographic dis-
tribution, 321 ; and intergrading species,
324; and life histories, 324; and plant
breeding, 325 ; and plant structures,
323 ; factors of, 326.

Excurrent stems, in.

Fabrics from plants, i.

Factors, of evolution, 326; of the environ-
ment, 139.

Fats, production of, 31.

Fermentation, 261.

Ferns, 59, 98, 298; leaves of, 15, 88, 185,
283; life history of, 288; number of,
303 ; walking, 283.

Fertilization, 205; in conifers, 296; in
ferns, 289; in mosses, 280.

Fiber, i, 91, 215, 228; Manila, 91.

Filament, 200.

Flax, 163.

Floral envelope, 200.

Floriculture, 100.

Flower, 198, 200; clusters, 198; dicot,
209; monocot, 209.

Index

333

Flowers, variety of, 200; economic im-
portance of, 214.

Fog, 148.

Fomes, 245.

Food, 25 ; per acre, 33.

Food accumulation, and agriculture, 78;
in roots, 77, 179; in leaves, 75; in
stems, 105, 159, 161.

Food-conducting tissue, 20.

Food conduction, 21, 178.

Foods, produced per acre, 33 ; utilization
of, 82.

Forests of North America, 299, 312.

Fossils, 319.

Fructose, 28.

Fruit, 212; economic importance of, 214.

Fuel, 2.

Fungi, 260; and roots, 194.

Gametes, 241.

Geaster, 267.

Gemmae, 273.

Geographic distribution, 321.

Geotropism, 131, 182.

Ginseng, 195.

Glucose, 27, 28.

Grafting, 132.

Grape, 225.

Grass family, 308.

Gravity, 140, 149, 182.

Ground pine, 287.

Growing point, 129, 180.

Growing region, 88, 1 29, 180 ; of leaves, 87 ;
of roots, 180; of stems, 129.

Growth, 82, 85; conditions for, 87; de-
termination of annual, 114; of roots,
179; of stems, 128; pressure, 182, 184;
stages of, 1 80.

Guard cells, 18.

Guayule, 163.

Guinea grass, 42.

Gymnosperms, 211, 294; number of, 303;
seeds of, 211 ; wood of, 294.

Habitat, 56.

Heartwood, 130.

Heath family, 315.

Hemp, 163.

Herbs, 94 ; shrubs and trees, 98.

Hippuris, 56.

Horseradish, 167, 219.

Horsetails, 286.

Horticulture, 4, 100.

Host, 250.

Humus, 143, 257.

Hyacinth, 97, 160.

Hybridization, 226, 255.

Hybrids, 220, 226.

Hydnum, 267.

Hydrophytes, 56, 57, 60, 156, 179, 273.

Hydrotropism, 131, 183.

Hypocotyl, 207.

Imbibition, 172.

Indian cucumber root, 38.

Indian pipe, 249.

Insect pollination, 104, 202.

Internodes, '106.

lodin, 246.

Ipecac, 195.

Iris, 41.

Irish moss, 246.

Irrigation, 141, 144, 145.

Isolation, 328.

Jack-in-the-pulpit, 160.
Jerusalem artichoke, 97, 161, 163.
Johnson grass, 159.
Jute, 163.

Kelps, 246.
Kohl-rabi, 163.

Landscape architecture, 100.

Lantana leaf, 14.

Laurel, 316.

Leaf, arrangement, 37 ; coloration, 63 ;
fall, 67 ; parts of, 13 ; pigments, 63, 65 ;
position and light, 37, 46; scars, 113;
structure, 13, 42, 54, 56; tissues, 14.

Leaves, 13 ; commercial products from, 91 ;
effects of light on, 42 ; floating, 57 ; hori-
zontal, 43 ; motile, 43 ; of conifers, 296 ;
of shade plants, 44 ; relation to light, 37,
46 ; submerged, 45, 56 ; vertical, 42 ;
water relations of, 48 ; white, 66.

Legume family, 313.

Legumes and bacteria, 194, 258.

Lenticels, 106, 115.

Lichens, 59, 269.

Lifting of water, 134.

Light, 148.

Lignification, 123.

Lignin, 122.

Lily family, 310.

Lipase, 75.

Liverworts, 272.

Longevity of plants, 94.

Lumbering, 161.

Lycopodium, 287.

Magnolia, 37.
Mango, 211.

334

Index

Mangrove, seedlings, 208; swamp, 209.
Maple, 30, 38, 201; flow of sap, 135;

sugar, 30, 163.
Marchantia, 272, 275.
May apple, 98, 159.
Mechanical tissue, 21.
Membranes, 173.
Mertensia, 54.
Mesophyll, 15, 19.
Mesophytes, 60.
Microspora, 236, 238.
Midrib, 13.
Milkweed, 198.
Mimosa, 44, 314.
Mint family, 314.
Mistletoe, 249.
Mm'um, 280.
Molds, 262.
Monocots, 1 1 8, 209; families of, 307;

stem' structure, 125.
Morphology, 151.

Mosses, 23, 59, 277 ; life history of, 279.
Moth mullein, 95.
Motile leaves, 43.

Multiplication, vegetative, 217, 243, 279.
Mushrooms, 249, 267, 268.
Mustard family, 312.
Mutants, 225.
Mutation, 224, 327; bud, 226.

Naiad, 57.

Nasturtium leaf, 14.

Natural selection, 327.

Nectar, 202. ^

Nectaries, 202.

Net- veined leaves, 88, 118.

Nightshade, 315.

Nitrifying bacteria, 258.

Nitrogen, 32, 141 ; fixation, 258.

Nodes, 106.

Nucleus, 15.

Nutritive phase of plants, 197.

Oak, 161, 198.

Oats, 147.

Oedogonium, 236, 241.

Oils, 31, 215.

Onion, 79, 160, 198.

Oospore, 242.

Open bundle, 127.

Orange leaf, 14.

Orchid family, 311.

Orchids, 190, 204, 311.

Organs, 21.

Osier willow, 163.

Osmosis, 173; in roots, 175.

Ovulary, 200.

Ovule, 200.
Oxalis leaf, 15, 44.
Oxidation, 82.

Palisade cell, 17; layers, 19.

Palm, coconut, 214, 306; date, no;
family, 307; hat, 91.

Parallel- veined leaves, 88, 118; growing
region of, 87.

Parasites, 249.

Parenchyma, 120.

Parmelia, 269.

Parsnip, 79, 96.

Pasteurization, 256.

Pea leaf, 16.

Peach leaf, 14.

Peanut, 78, 79.

Pedicel, 200.

Peduncle, 199.

Peppermint, 314.

Pepsin, 75.

Perennials, 96.

Permeability, 173.

Persimmon, 213.

Petal, 200.

Petiole, 14.

Phloem = food conducting and bast tis-
sues, 119.

Photosynthesis, 26; summary of, 31.

Phototropism, 131, 183.

Physiology, 151.

Pigments, green, 63 ; red, 65 ; yellow, 63.

Pine, 202; Japanese dwarf , 99 ; trees, in.

Pineapple, 210.

Pistil, 200.

Pith, 120, 125; rays, 120.

Plant, breeding, 220; characteristics, 99;
fibers, i, 91, 215, 228; kingdom, de-
velopment of, 300; producing arts, 99.

P'ants, desert, 58, 157; importance of, i;
living things, 7, 8; mutual dependence
of oarts, 8, 9; of the past, 318; relation
tc environment, 8, 140; source of all
food, i 2.5.

Plastids, 22.

Plumule, 208.

Pod, 212.

Pollen, 200, 205 ; tube, 204.

Pollination, 201 ; cross, 203 ; in conifers,
202 ; self, 203.

Pond scums, 237, 245 ; living conditions of,
239-

Pondweed, 57.

Poplar leaf, 14.

Potassium, source of, 247.

Potato, 10, 76, 79, 87; sweet, 97, 194;
tuber cell, 17; family, 315.

Index

335

Preservation of foods, 257.

Pressure, of growth, 184; osmotic, 174;

root, 177.

Prickly lettuce, 40 ; pear, 58.
Proteins, making of, 32; use of, 32, 33.
Prothallus, 288.
Protococcus, 235.
Protonema, 279.
Protoplasm, 151.
Puffballs, 267.
Pulvinus, 45.

Raceme, 198.

Rainfall, distribution of, 51, 144; effect
on yield of corn, 48, 51.

Raspberry, 213, 217.

Rattan, 125, 163.

Receptacle, 200.

Red-bud tree, 44.

Red pigment, 65.

Reproduction, 10, 197, 206; and agri-
culture, 217; asexual, 243; by seeds,
207, 300; sexual, 205, 243.

Reproductive phase of plant life, 197.

Reservoirs, cleaning, 246.

Respiration, 82; and shipping, 84; con-
trasted with photosynthesis, 83; in
roots, 82, 179; rate of, 84.

Response, 131 ; to aerial environment,
2755 to gravity, 130, 149, 182; to light,
131, 149, 183 ; to water, 131, 157, 183.

Rhizoids, 273, 276 ; moss, 278.

Rhododendrons, 194.

Rhubarb, 195, 198.

Rice, 77, 145, 307, 308.

Root, contraction, 185; hairs, 168; pres-
sure, 177; processes, 166.

Roots, and bacteria, 194, 258; and fungi,
194; and transplanting, 190; classifica-
tion of, 1 66; commercial uses of, 194;
distribution of, 183 ; of climbers, 182,
187; of conifers, 296; of ferns, 285; of
hydrophytes, 187 ; of mesophytes, 187 ;
response to gravity, 182; response to
light, 183 ; tissues of, 169.

Rootstock, 159, 219; calamus, 122.

Rose family, 312.

Rosin, 161, 162.

Rubber, 163, 215.

Running cypress, 287.

Rushes, 157.

Russula, 267.

Rusts, 263.

Salsify, 194.
Sand-reed grass, 159.
Sanitation, 252.

Saprophytes, 249.

Sapwood, 130.

Sassafras, 195.

Scale leaves, 160, 161.

Science, defined, 3.

Seaweeds, 246.

Sedges, 38, 157.

Seeds, 198, 205, 207 ; economic impor-
tance of, 214; of conifers, 198, 211.

Selaginella, 59.

Selection, 220, 228; natural, 327.

Self-pruning, 69.

Sensitive plant, 44.

Sepal, 200.

Sequoia, 98, 154, 229.

Shasta daisy, 222.

Shrubs, 94.

Sieve tubes, 122.

Silviculture, 100.

Simple plants, summary of, 270.

Sirup, 163.

Sisal, 90, 91.

Smilax, 107.

Smuts, 266.

Snapdragon, 198.

Soil, 141 ; bacteria, 257 ; cultivafton of,
143 ; elements, 141 ; erosion, 186, 187 ;
temperature, 146.

Solomon's seal, 98, 159, 185.

Sorghum, 163.

Soy bean, 33, 78, 194.

Sperms, 205.

Spike, 198.

Spirogyra, 236.

Spores, asexual, 241 ; resting, 238; sexual,
241 ; swimming, 238.

Sports, 225; bud, 226.

Spruce, 114.

Stamen, 200.

Starch, 28, 74, 77.

Stem, processes, 118; structure, 118, 119,

!54-

Stems, advantages of horizontal, 104; as
photosynthetic organs, 106 ; commercial
products from, 161 ; excurrent and deli-
quescent, 1 1 1 ; external features of, 102 ;
growth of, 128 ; in relation to light, 131 ;
of climbing plants, 155 ; of hydrophytes,
156; of xerophytes, 157 ; underground,
97, 130; upright, 102.

Stigma, 200.

Stipules, 14.

Stock, 132.

Stomata, 19, 115.

Storage, food, 74, 77, 105, 125, 159, 160,
161 ; water, 58, 106, 158, 189.

Strawberry, 10.

336

Index

Structure, conifer stems, 128 ; dicot bundle,

121 ; dicot stems, 120; monocot bundle,

126; monocot stem, 125.
Style, 200.
Subsidiary cells, 19.
Sugar, 28, 202; beet, 77, 79, 195; cane,

77, 163, 219.

Sugar cane, 77, 78, 141, 163, 231.
Sunflower, 10, 50, 97, 102, 198, 203, 222,

228, 317.
Synthesis, of carbohydrates, 26, 27, 31 ;

of fats, 31 ; of proteins. 32.

Tamarack, 296.

Tank epiphytes, 189.

Tannic acid, 161.

Temperature, 146.

Tendrils, 107, 155.

Test, for oil, 73 ; for proteins, 73 ; for

starch, 26, 73 ; for sugar, 73.
Thallus, 272.
Thistle, 317.
Timothy, 220.
Tissues, 21 ; epidermal, 15, 16, 50, 119,

169; food-conducting, 15, 121, 169;

mechanical, 15, 122; storage, 125.
Toadstools, 268.
Tobacco, 89, 203, 224.
Tomato, 225.
Toxins, 253.
Tracheae, 121, 123.
Transfer of food, 76.
Transpiration, 49, 149 ; and absorption,

176; and lifting of water, 134; and

water balance, 52 ; rate of, 50.
Transplanting, 192; and roots, 190; and

water balance, 55.
Tree-of-heaven, 107.
Trees, 94; size of, 99, 154.
Tropisms, 131.
Tubers, 97, 161.
Tulip, 1 60, 200; tree leaf, 13.
Turf, _ 1 59.
Turnips, 96, 194.
Turpentine, 161.
Typhoid fever, 253.

Ulothrix, 236, 240.
Umbel, 198.

Underground stems, 75, 97, 130, 159, 160,

161.
Utilization of foods, 83.

Vacuoles, 16, 49.

Variations, 220, 222, 327; kinds of, 223.

Vaucheria, 236.

Vegetative multiplication, 217, 243, 279;
propagation, 218.

Veins, 13, 20, 88; principal, 13.

Vertical leaves, 41, 42.

Vessels, food-conducting, 121 ; water-con-
ducting, 121, 123.

Vinca, 14, 18, 21.

Vinegar making, 261.

Walnut, 79.

Wandering Jew, 19.

Water, balance, 52, 54; balance and plant
habitats, 56 ; balance and transplanting,
55 ; balance illustrated, 53 ; conducting
tissue, 1 8, 20, 105, 119, 121, 169; im-
portance of, 48, 51, 52; plants, 56, 57,
60, 156, 244, 273; storage, 58, 106, 158,
189; supply and crop yields, 51; why
necessary, 48.

Water lily, 57; yellow, 159.

Weeds, 229; and crop production, 230.

Wheat, 5, 79, 96, 144; rust, 263.

White clover, 44.

White leaves, 66.

White pine blister rust, 265.

Willow, 16, 56, 163, 198.

Wilt disease, 254.

Wind, 148, 149; pollination, 103.

Wintergreen oil, 163.

Wood, 123; fibers, 124; gymnosperm,
294; pulp, 161.

Xanthophyll, 63.
Xerophyte, 58, 59, 60, 157.
Xylem = water conducting and wood
tissues, 119. •

Yam, 194.
Yeast, 261.
Yucca, 58, 204, 213, 322.

Zebrina, 19.

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