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PRACTICAL PROBLEMS IN
BOTANY

ELEMENTS OF BOTANY

By RICHARD M. HOLMAN, Late Associate Pro-
fessor of Botany in the College of Letters and Science
of the University of California; and WILFRED W.
BOBBINS, Professor of Botany in the College of Agri-
culture of the University of California. Third Edition,
392 pages. 6 by 9 &. 273 figures. Cloth.

TEXTBOOK OF GENERAL BOTANY

For Colleges and Universities. By the late RICHARD
M. HOLMAN and WILFRED W. BOBBINS. Fourth
Edition, 664 pages. 6by9>. 482 figures. Cloth,

LABORATORY GUIDE FOR A COURSE
IN GENERAL BOTANY

By LEE BONAR, Associate Professor of Botany in
the University of California, LUCILB ROUSH, Formerly
of the Department of Biological Science, Mills College,
and the late RICHAKD M. HOLMAN, Fourth Edition,
110 pages. 6 by 9J4. Cloth.

PUBLISHED BT

JOHN WILEY & SONS, Inc.

PRACTICAL PROBLEMS IN
BOTANY

BY
WILFRED W. BOBBINS,

Professor of Botany in the College of Agriculture
of the University of California

AND

JEROME ISENBARGER

Lecturer, Natural Science
Loyola University , Chicago

Sixth Printing

NEW YORK

JOHN WILEY & SONS, INC.
LONDON: CHAPMAN & HALL, LIMITED

COPYRIGHT, 1936, BY

WILFRED W. ROBBINS

AND
JEROME ISENBARGER

All Rights Reserved

^his book or any part thereof must not
e reproduced in any form without
ke written permission oj the publisher,

8IXTH PRINTING, OCTOBER, 1948

PRINTED IN U. S. A.

PREFACE

It is the belief of the authors of this book that no subject con-
tributes more, when properly taught, to the attainment of the
cardinal principles of secondary education than does biological
science. It is the main province of this text to lay a foundation
of fundamental principles which will enable pupils to develop an
understanding of the significance of plant life which is such an
important part of their environment. Moreover, the work in
botany should be made practical in the sense that it should supply
a basis of fact necessary to an understanding of principles, so that
the student can use them in developing within himself a degree of
social, civic, ethical, and esthetic efficiency.

A practical course in botany should aid in developing an
appreciation of the possibilities of improvement of the home
environment through putting into practice a knowledge of the
principles of plant growth. Window-plant culture, landscaping
of home grounds, vegetable and flower gardening not only con-
tribute to the attractiveness of the home, but they also provide
pleasant and profitable avocations as worthy use of leisure time.
Also, knowledge of foods, bacteria, and the laws of sanitation, and
the life out-of-doors occasioned by engaging in vocations and
avocations along the lines of plant study and plant culture, both
tend toward personal efficiency by making the person a healthier
individual.

Certain aims and .objectives have been set up. The problems
and exercises are such as to be a direct aid in the attainment of
these aims and objectives. The teacher who administers the
course should not only have in mind the general objectives, but
he should also recognize locally adapted specific objectives which
should aid in determining points of special emphasis. Local con-
ditions which affect specific aims include: dominant interests of
pupils, community interests and needs, and availability of local
resources, as woods and streams, greenhouses, parks, farms,

v

vi PREFACE

health laboratories, landscaped homes, milk-pasteurizing plants,
canneries, and facilities for sewage disposal.

The problem involves learning activities which when properly
directed by the teacher and carried out by the pupil will lead to
the development of significant biological ideas and to the acquisi-
tion of the elements of scientific thinking.

The course is organized as a series of problems and sub-
problems. It is intended that each problem shall lead pupils
inductively to an understanding of important generalizations.
The introduction of the book is an over-view of the entire course,
and the introduction of each unit is an over-view of that unit.
Provision is made for meeting individual differences by including
suggested activities and additional exercises and problems which
may be done by pupils who are able to finish the required work
ahead of the majority of the class.

The arrangement of the units is logical as it stands, but it is
not intended as the best order under all conditions. Teachers
who prefer a seasonal arrangement will find it possible to change
the order of presentation of units to suit their requirements.
A course beginning at mid-year will require an arrangement of
units different from that which is best for a course beginning in
the fall.

Teachers are referred to The Teaching of Biology, by William
E. Cole, published by D. Appleton-Century Company, for helpful
suggestions regarding points of view in biology, laboratories,
equipment, bibliographies, and materials.

WILFRED W. ROBBINS

September 9, 1935. JEROME ISENBARGER

CONTENTS

PAGE

INTRODUCTION 2

UNIT I. ORGANIZATION AND COMPOSITION OF PLANTS

PREVIEW OF THE UNIT 9

PROBLEM 1. WHAT ARE THE DIFFERENT FORMS OF PLANT BODY?. . 10

PROBLEM 2. WHAT is THE STRUCTURE OF THE PLANT CELL? 24

PROBLEM 3. WHAT is THE NATURE OF PROTOPLASM THE LIVING

MATERIAL? 28

PROBLEM 4. How ARE CELLS GROUPED TO FORM TISSUES AND ORGANS? 30

PROBLEM 5. WHAT is THE RELATION OF STRUCTURE AND FUNCTION? 31

PROBLEM 6. WHAT ARE THE CHEMICAL SUBSTANCES FOUND IN PLANTS? 33

UNIT II. THE NUTRITION OF GREEN PLANTS

PREVIEW OF THE UNIT 39

PROBLEM 1. WHAT is THE NATURE OF PLANT FOODS? 40

PROBLEM 2. WHAT ARE THE RAW MATERIALS USED BY PLANTS IN THE

MANUFACTURE OF FOOD? 41

PROBLEM 3. How DO RAW MATERIALS ENTER THE PLANT? 43

PROBLEM 4. How DO RAW MATERIALS MOVE IN THE PLANT? 61

PROBLEM 5. WHAT ARE THE PROCESSES OF FOOD BUILDING? 66

PROBLEM 6. WHAT USE DOES THE PLANT MAKE OF THE FOOD MANU-
FACTURED IN GREEN TISSUE? 68

PROBLEM 7. WHAT is THE R6LE OF THE DIFFERENT ELEMENTS IN THE

NUTRITION OF GREEN PLANTS? 72

PROBLEM 8. WHERE DO FOODS MOVE IN THE PLANT? 75

PROBLEM 9. How DOES THE PLANT STORE AND DIGEST ITS FOOD? 76

PROBLEM 10. How DOES THE PLANT ASSIMILATE FOOD? 79

UNIT III. NUTRITION OF NON-GREEN PLANTS

PREVIEW OF THE UNIT 81

PROBLEM 1. WHAT ARE THE MAIN CHARACTERISTICS OF THE NON-
GREEN PLANTS? 83

PROBLEM 2. WHAT ARE THE NUTRITIVE RELATIONS OF THE SAPRO-
PHYTES? 88

PROBLEM 3. How DO PARASITIC PLANTS CAUSE DISEASE IN ANIMALS? 97
PROBLEM 4. How MAY BACTERIA AND MOLDS BE STUDIED IN THE

LABORATORY? 99

vii

Vlll

CONTENTS

PAGE

PROBLEM 5. How DO PARASITIC PLANTS CAUSE PLANT DISEASES?. . . 104

PROBLEM 6. WHAT ARE THE PRINCIPAL GROUPS OF FUNGI? 109

UNIT IV. THE GROWTH OF PLANTS

PREVIEW OF THE UNIT 116

PROBLEM 1. How DO EMBRYOS GROW? 118

PROBLEM 2. How DOES THE PLANT CELL GROW? 119

PROBLEM 3. WHAT is THE NATURE OF SEED GERMINATION? 121

PROBLEM 4. How DO STEMS GROW IN LENGTH? 135

PROBLEM 5. How DO STEMS GROW IN DIAMETER? 138

PROBLEM 6. How DO ROOTS GROW? 143

PROBLEM 7. How DO LEAVES GROW? 144

PROBLEM 8. How DO SEEDS AND FRUITS GROW? 145

UNIT V. REPRODUCTION OF PLANTS

PREVIEW OF THE UNIT 149

PROBLEM 1. How DO FLOWERING PLANTS REPRODUCE? 150

PROBLEM 2. How is POLLEN DISPERSED? 156

PROBLEM 3. WHAT ARE THE IMPORTANT DIFFERENT TYPES OF FLOWERS? 162
PROBLEM 4. WHAT ARE THE PRINCIPAL CAUSES OF THE FAILURE OF

BLOSSOMS TO SET FRUIT? 171

PROBLEM 5. How DO FERNS AND MOSSES REPRODUCE? 175

PROBLEM 6. How DO PLANTS REPRODUCE ASEXUALLY? 181

PROBLEM 7. How ARE PLANTS PROPAGATED ARTIFICIALLY? 184

PROBLEM 8. How DID REPRODUCTION BY MEANS OF SEX IN PLANTS

ORIGINATE? 199

UNIT VI. THE DEPENDENCE OF PLANTS ON THE
CONDITIONS OF THEIR SURROUNDINGS

PREVIEW OF THE UNIT 204

PROBLEM 1. WHAT is THE IMPORTANCE OF WATER TO PLANT LIFE?. 204

PROBLEM 2. WHAT is THE RELATION OF TEMPERATURE TO PLANT LIFE? 210

PROBLEM 3. WHAT is THE RELATION OF LIGHT TO PLANT LIFE? .... 215

PROBLEM 4. WHAT is THE RELATION OF PLANTS TO THE SOIL? 222

PROBLEM 5. WHAT is THE RELATION OF PLANTS TO THE AIR? 236

PROBLEM 6. WHAT is THE INTERRELATION OF PLANTS AND ANIMALS? . . . 238

UNIT VII. HOW PLANTS ARE FITTED TO THE
CONDITIONS OF THEIR SURROUNDINGS

PREVIEW OF THE UNIT 243

PROBLEM 1. To WHAT KINDS OF STIMULI DO PLANTS RESPOND?. . . . 245
PROBLEM 2. How ARE PLANTS RELATED BY STRUCTURE TO THE

WATER SUPPLY? 249

CONTENTS

IX

PAGE

PROBLEM 3. WHY ARE CERTAIN TYPES OF PLANTS FOUND LIVING

TOGETHER? 257

PROBLEM 4. How ARE PLANTS RELATED BY STRUCTURE TO THE PROCESS

OF POLLINATION? 271

PROBLEM 5. How ARE FRUITS AND SEEDS FITTED TO THE PROCESS OF

DISPERSAL OF PLANTS? 279

UNIT VIII. THE DEVELOPMENT AND IMPROVEMENT
OF PLANTS

PREVIEW OF THE UNIT 290

PROBLEM 1. IN WHAT WAYS HAVE PLANTS CHANGED? 291

PROBLEM 2. How DO WE KNOW THAT PLANTS HAVE CHANGED? 293

PROBLEM 3. WHAT ARE THE METHOD AND CAUSE OF CHANGE IN PLANTS? 301
PROBLEM 4. How DOES MAN DEVELOP NEW KINDS OF PLANTS?. ... 315

UNIT IX. THE CLASSIFICATION OF PLANTS

PREVIEW OF THE UNIT 323

PROBLEM 1. How ARE PLANTS CLASSIFIED? 324

PROBLEM 2. WHAT ARE THE P'OUR GREAT GROUPS OF PLANTS? 326

PROBLEM 3. How ARE THE SEED PLANTS CLASSIFIED? 327

PROBLEM 4. WHAT is A SCIENTIFIC NAME? 329

PROBLEM 5. WHAT DO WE MEAN WHEN WE SPEAK OF " SIMPLE PLANTS "

AND "COMPLEX PLANTS"? 330

UNIT X. THE ECONOMIC IMPORTANCE OF PLANTS

TO MAN

PREVIEW OF THE UNIT 334

PROBLEM 1. WHAT ARE THE PRINCIPAL FOOD PLANTS OF THE WORLD? 335

PROBLEM 2. WHAT ARE THE PRINCIPAL INDUSTRIAL PLANTS? 348

PROBLEM 3. WHAT ARE THE PRINCIPAL MEDICINAL PLANTS? 355

PROBLEM 4. WHAT ARE THE PRINCIPAL BY-PRODUCTS DERIVED FROM

PLANTS? 358

PROBLEM 5. How DO PLANTS INTERFERE WITH MAN? 361

INDEX 387

PRACTICAL PROBLEMS IN BOTANY

INTRODUCTION

Wherever man has gone on the earth he has found some kind
of plant life. Expeditions to arctic and antarctic regions, to the
tops of the highest mountains, into the sandy stretches of the
driest deserts, to all parts of the world, have always revealed some
form of plant life. In the ocean, in both fresh- and salt-water
lakes, in ponds and streams, in drinking water, in the waters of
hot springs, there is an abundance of plant life. Certain bacteria,
fungi, and algae occur in countless numbers in the soil, without
which organisms soil fertility would not be maintained; bacteria
and fungi are ever-present in the air, stealing rides on floating dust
particles; bacteria are also always present in the digestive tracts
of all kinds of animals; in fact, both plants and animals serve as
hosts to many different kinds of bacteria and fungi, some harm-
less, or even beneficial, others disease-causing. Name five diseases
of man caused by bacteria.

In Schimper's Plant Geography, a monumental work of 839
pages and 502 illustrations, published by the Clarendon Press,
Oxford, England, we find the following:

"As has been already shown, there is nowhere on earth a
place too cold for plant life, and only a few spots of very limited
area that are too hot. As regards light, there is no limitation;
it is nowhere too dark, nowhere too bright to exclude plant life of
some kind. In the depths of the ocean, where light is absolutely
absent, the decaying corpses of animals are decomposed by bac-
teria. ... In the well-known Guacharo Cave near Caribe in
Venezuela we found the ground covered with patches of dense
etiolated vegetation up to half a meter in height, which had sprung
up from the dung of the Guacharo birds, the only inhabitants of
the Cave."

2 INTRODUCTION

Further, Schimper says: " The perpetual snow and ice of the
polar zone and of mountains, here and there, exhibit conspicuous
coloring caused by microscopic algae. . . . The occurrence of algae
associated with red snow has been demonstrated on the most
distant points in the Arctic and Antarctic zones and on most
mountains with perpetual snow, so that the phenomenon may be
assumed to be of general distribution."

Schimper quotes from Volkens' description of the vegetation
on the highest peaks of Kilimanjara, a mountain of Africa which
attains an elevation of 6010 meters. Volkens says, " Finally, at
4500 meters we have reached the last outposts, all isolated plants,
forming little cushions under the shelter of stones. . . . Beyond
this, wherever the ground is dry, only lichens and mosses prevail."

Thus we see that plants invade all sorts of environments, that
is, they live under all kinds of conditions. To do so successfully,
they must be fitted to the conditions of their surroundings. Most
certainly land plants can not live in the water; and ordinary, thin-
leaved water plants would soon succumb if transplanted to a dry
hillside. Why? Plants accustomed to the shade of the forest
floor do not survive if the forest is cut or burned over; and plants
of the open can not thrive in the shade. The ability of plants
to live in all sorts of environments is possible only because of their
great variation in form and structure. For example, plants of the
desert must possess those characteristics which enable them to
survive where water is scarce. Some of these so-called drought-
resistant characteristics are: water-storage tissue, greatly reduced
leaf surface, and thick coverings on the leaves which cut down
water loss. Many plants of high mountains are low and mat-
forming, thus getting the warmth close to the soil and avoiding the
greater loss of water which would occur if they grew several feet
tall.

One has only to examine plants growing under varied condi-
tions to learn how well they are fitted to their environment. How
are lichens able to live upon bare rock surfaces? How can cer-
tain orchids manage to survive on the branches of trees without
any connection with the soil? How can cacti and many other
plants live in the desert where the rainfall is but a few inches a
year? What peculiarities do those plants have which can thrive

INTRODUCTION 3

in alkali flats? What structures enable the cypress to grow in
swamps where water always covers the roots? Why is it possible
to grow Durum wheat and certain sorghums on the dry plains of
western United States without irrigation, whereas many other
crops will not thrive there without irrigation? What character-
istics would you expect those plants to have which can grow suc-
cessfully in acid bogs? These are suggestive of the problems con-
fronting the student who would know the relation of plants to
their environment.

Plants are living things. True it is they do not move about
from place to place, as do most animals. But they manifest all
the essential characters that we associate with livingness. Plants
absorb materials from the outside world; they make food; they
digest food; they respire; they make plant substances out of
foods; they grow; they are sensitive to light, gravity, moisture,
heat, and other environmental factors; they reproduce. Plants
are indeed living things organisms.

The fundamental organization and composition of plants and
animals are quite similar. The unit of structure in both plants
and animals is the cell, a microscopic sac containing the living
stuff protoplasm. The cells are grouped to form tissues, such
as absorbing tissue, conducting tissue, protective tissue, storage
tissue, reproductive tissue, etc.; and the tissues are grouped to
form organs, such as roots, leaves, stems, flowers, fruit, and seed.

There are no chemical elements found in animals that do not
occur in plants. This is a rather remarkable fact. In both, the
principal elements which enter into the composition of the body
are oxygen, potassium, magnesium, hydrogen, nitrogen, sulphur,
phosphorus, and carbon. These are common elements which
occur in the air and soil. The foods of plants and animals are the
same. At first thought, we may question this statement. But
the foods of both plants and animals are carbohydrates (sugars,
starch, cellulose, etc.), fats, and proteins. What are the chemical
characteristics of these three groups of foods? The processes of
absorption, digestion, respiration, assimilation, growth, and repro-
duction are essentially alike in plants and animals. This state-
ment may also arouse doubt in the mind of the reader, but the
discussion of these processes which will come in succeeding units

4 INTRODUCTION

will assist in removing this doubt. Of course, there are marked dif-
ferences between plants and animals. For example, the great
majority of plants are not able to move from place to place,
whereas locomotion is characteristic of most animals; in the
majority of plants each of the cells is surrounded by a relatively
rigid wall, whereas the cells of animals are usually without such
surrounding walls; from the very simple substances such as
water, carbon dioxide, and mineral salts, obtained from the soil
and air, most plants can build the foods necessary to nourish
their bodies, whereas animals are unable to make their own foods;
and the growth in length of most plants takes place at or near the
ends of the organs, such growth generally continuing as long as the
plant is alive, whereas in animals, growth is not usually restricted
to the extremities, and ceases long before death.

It is of interest to note that the very simplest plants and the
very simplest animals, both groups of which are aquatic, have
much more in common than do higher plants and higher animals.
In fact, there is substantial evidence that the plant and animal
kingdoms had a common ancestor; that, in the process of devel-
opment of the races of plants and animals, the distinction between
these two great groups of living things has become greater and
greater. But they have retained, by virtue of their common
ancestry, many essential features. In other words, life on this
earth is much the same whether it expresses itself in plants or
animals.

The present assemblage of plants in the world varies greatly in
complexity. The ordinary trees, shrubs, and herbs are relatively
complex plants, by which we mean that they possess many different
organs and tissues for carrying on their life activities. For exam-
ple, they have roots, stems, leaves, flowers, fruit, and seed. There
are many plants, such. as the pond scums, seaweeds, bacteria,
molds, mushrooms, etc., which do not have roots, stems, leaves,
flowers, or seed. Such plants are simple in their bodily organiza-
tion. Moreover, their methods of reproduction are not as com-
plex and advanced as in seed plants. Then, there are such plants
as mosses, liverworts, and ferns, which have roots or root-like
structures, stems, and leaves, but no flowers or seeds. It is re-
garded by students of plant life, who have carefully examined and

INTRODUCTION 5

compared the structure of a great many different kinds of plants
and their methods of reproduction and coupled this with a study
of fossil plants (Fig. 173), that during the past hundreds of thou-
sands of years as life developed on the earth there has been great
change in the nature of plants. There is reliable evidence that
the first plants that appeared on the earth were water plants,
similar in many particulars to our present-day pond scums; that
from these primitive ancestors there developed more complex
plants, such as liverworts and mosses; that, as the thousands of
years passed by, there appeared ferns and their allies; and that
from certain fern-like plants were developed our present-day seed
plants. Seed plants are regarded as the most advanced and most
complex of plants, just as man is considered the most advanced
and complex of animals.

The plant world as we see it today is not as it always was in
the earth's history. For example, geological records show unmis-
takably that the vegetative covering of a large part of the earth
during the Carboniferous or Coal Age was composed chiefly of
giant ferns and closely related forms. Seed plants as we know
them today appeared much later. But the important point to
keep in mind is that plants of the ast are the ancestors of those
populating the earth now. There has been a gradual development
of the plant kingdom extending over a period of many hundreds
of thousands of years, and this process of development or unfolding
is still going on today.

Green plants are the great converters of solar energy. With-
out them, animal life on the earth would be impossible. Green
plants are the only organisms on the earth which have the power
to convert light energy into food energy. Green plants alone can
take materials from the soil and air, and with the aid of light,
change these materials into food the food of both plants and
animals. Animals and non-green plants must have their foods
prepared for them; they are dependent organisms. Green plants,
on the other hand, are independent in that they make their own
foods. Just like any other manufacturing process, that of food-
making by green plants requires energy; to build up foods from
simple materials derived from the soil and air, energy is needed.
This energy comes from light. And it is through the medium of

6 INTRODUCTION

green plants that light energy is transformed to food energy.

When wood is burned, energy is liberated in the form of heat.
In burning, the energy of the chemical compounds composing
wood is transformed to heat energy. But in the building of these
chemical compounds which compose food, light energy is necessary.
Hence the energy set free in burning wood is in reality solar
energy. When coal is burned there is liberated light energy which
came to the earth hundreds of thousands of years ago, which en-
ergy was utilized by the plants of that period, transformed into
plant tissue, which later formed coal.

When plants respire, there is a destruction of plant substance,
accompanied by the liberation of energy. To build plant substance
there is required, in the last analysis, solar energy. Hence, the
energy freed in respiration is transformed solar energy. Is energy
liberated when we, as humans, respire? Explain.

Whatever way one may consider it, the fact remains that all
life on this earth depends upon light energy and it is only green
plants that are capable of transforming this light energy into a
form which can be used by plants and animals alike as food.

Plants are of great economic importance. They furnish food,
clothing, and shelter. The great civilizations of the world have
developed where natural conditions favored the cultivation of
certain food plants, chiefly cereals. Consider the importance of
such products of plant origin as wood, coal, cork, fiber, resins and
turpentine, gums, plant dyes, fixed and volatile oils, and rubber.
Many plants yield valued drugs, such as morphine, quinine, digi-
talin, and atropine. Many species are of ornamental value, being
employed to beautify our homes, gardens, and parks. A large
number of plants are of economic importance because they are
harmful, or interfere with man's operations. Consider here the
plants which cause rusts, smuts, molds, mildews, and other plant
diseases; also poisonous plants, hay-fever plants, and weeds.

Botany, the science which deals with plants, is today an
extremely important field of study. A knowledge of plants
their structure, their behavior, their relation to the environment,
their classification and naming, their improvement by breeding
and selection, their relation to diseases of useful plants and of ani-
mals is as essential to a proper understanding of agriculture in

INTRODUCTION 7

all its many branches, and to certain phases of medicine, as is
mathematics to advancement in the field of engineering. For
the individual seeking a life work, there are innumerable opportu-
nities in the field of botany. In the educational institutions of
the country universities, colleges, and high schools there are
many technically trained botanists, specialists in some branch of
plant science. These individuals are either teachers or research
workers, or both. There are systematic botanists, plant mor-
phologists, plant cytologists, plant geneticists, etc. In the United
States Department of Agriculture, and in the agricultural experi-
ment stations, of which there is one or more in each of the states,
there are altogether several thousand workers, trained in some
special field of botany. In addition to these, individuals equipped
with a knowledge of some phase of plant science are found in the
employ of botanical gardens, of museums, of national parks, of
large companies which grow drug plants, sugar plants, nursery
stock, seeds, rubber, tobacco, fruits, vegetables, fibers, and other
industrial plants.

In addition to these botany specialists, a certain knowledge of
plants is usually required of those whose major interest may be in
such fields as zoology, entomology, geology, pharmacology, animal
husbandry, veterinary medicine, bacteriology, soil technology,
irrigation practice, etc. (NOTE: If the student does not know
what the above sciences treat of, he should attempt to find out.
Consult dictionary or encyclopedia, or special books.)

The student may gain some knowledge of the number of botan-
ists in the United States, and the character of the positions they
hold, from American Men of Science, a Biographical Directory,
edited by J. McKeen and Jaques Cattell, and published by the
Science Press, New York. The fifth edition has 1278 pages.

SELECTED REFERENCES

Ernest H. Wilson, Plant Hunter, by EDWARD I. FARRINGTON, is a well-
illustrated book of 187 pages, published by the Stratford Company, Boston.
1931. This book describes the colorful adventures of Ernest Wilson in his
search for plants in various parts of the world.

The Geography of Plants, by M. E. Hardy, published by the Clarendon
Press, Oxford, England. 1920. A brief description of the plant life char-
acteristic of different parts of the world. 327 pages, 114 illustrations.

8 INTRODUCTION

An Outline of Plant Geography, by DOUGLAS HOUGHTON CAMPBELL,
published by the Macmillan Company, New York. 1926. "For more than
thirty years the writer has made excursions into many parts of the world,
and the specimens, notes, sketches and photographs accumulated during
these journeys have served as the basis of the present volume." 392 pages,
153 illustrations.

America's Greatest Garden, the Arnold Arboretum, by E. H. WILSON,
published by the Stratford Company, Boston. 1925. This is "a note of
invitation to a banquet of flowers and fruit provided by an assemblage of
the World's best hardy trees and shrubs." 123 pages and 50 full-page illus-
trations of great beauty and interest.

Tree Ancestors, a Glimpse into the Past, by E. W. BERRY, published by
Williams and Wilkins Company, Baltimore. 1923. The sketches of the
book are "an attempt to interest the general public in the marvellous history
of some of our trees." It discusses geological principles, methods of preserva-
tion of fossil plants, geological time and methods of reckoning, the later
geological history of North America, the present forests of North America,
and the history of such trees as the sequoias, bald cypress, walnuts, beech,
magnolia, -maple, ash, and many others. 270 pages and 48 illustrations.

Plant Hunting on the Edge of the World, by F. KINGDOM WARD, published
by Victor Gollancz, Ltd., Co vent Garden, London. 1930. This is a travel
book with a strong botanical flavor. The author describes his journeys to
"collect seeds of beautiful hardy flowering plants for English gardens, to collect
dried specimens of interesting plants for study," and "to explore unknown
mountain ranges and find out something about their past history, the distri-
bution of their plants, and any other secrets they are willing to reveal."
383 pages and 15 illustrations.

Exploring for Plants, by DAVID FAIRCHILD, published by the Macmillan
Company, New York. 1930. A most interesting book by one who for many
years was in charge of the Office of Foreign Plant Introduction, of the U. S.
Department of Agriculture. 591 pages and 179 illustrations.

The Natural History of Plants, by ANTON KERNER and F. W. OLIVER.
Published by Blackie and Son, London. 1895-96. In two volumes, Vol. I,
777 pages; Vol. II, 983 pages, with about 2000 original woodcut illustrations
A classical work replete with interesting facts about plants.

UNIT I
THE ORGANIZATION AND COMPOSITION OF PLANTS

The human body is made up of a number of organs, each with a
work to do. There are organs of sight, of hearing, of digestion, of
circulation, etc. The plant body, too, is composed of a number of
organs, each having some definite work to do. For example,
among seed plants those plants with which we are most familiar
the roots are the absorbing and anchoring organs, the stems are
the supporting and conducting organs, the leaves are the chief
water-losing and food-making organs, and the flowers are the
reproductive organs. But there are many plants much different
from seed plants and in many respects simpler in their organiza-
tion. For example, ferns have bodies with roots, stems, and a pecu-
liar form of leaf which we call a " frond," (Fig. 177), but ferns do not
have flowers and seeds. Mosses are prostrate plants, much simpler
in their make-up than either seed plants or ferns; they have a
very poorly developed conducting system, weak stems, extremely
small leaves, no roots in the ordinary sense, and reproductive
structures which have no resemblance to flowers. Still lower in
the scale of plant life is that great group of plants which includes
pond scums and seaweeds, bacteria and yeasts, rusts and smuts,
molds and mildews, mushrooms and toadstools plants which
have no roots, stems, or leaves, and very simple reproductive
organs. There are even plants, bacteria and certain algae, the
entire body of which consists of a single cell. A plant body of one
cell is the simplest kind possible (Fig. 1, A, B, C, 3).

Not only is there great variation in the structure of plants
which compose the population of the world, but also there are
very considerable differences in their chemical composition. For
example, the sugar beet and sugar cane are richer in sugar than
most other plants; oranges and lemons contain a relatively large
amount of citric acid; in the seed of the castor-bean plant there is
an oil known as castor oil; a latex, the basis of commercial rubber,

9

10 ORGANIZATION AND COMPOSITION OF PLANTS

exists in rubber plants; the bark of Cinchona yields a chemical
known as quinine; the coffee berry produces an alkaloid caffein;
tannins are chemical compounds derived from the bark of certain
trees; and so on, there being literally thousands of chemicals
manufactured in different plants. Name other chemical com-
pounds derived from plants.

Thus we see that, among the vast assemblage of plants which
clothe the earth, there is great variation in their structure, that is,
their organization, and in their chemical composition.

Problem 1. What are the different forms of the plant body?

When we consider the microscopic animal life of water and
land, and the vast assemblage of insects, worms, Crustacea, rep-
tiles, birds, and mammals, it would appear that almost every
conceivable form of animal body is represented. Likewise in the
plant kingdom is there tremendous variation in the forms of the
plant body. The mention of a few plants will call to mind some
of the different forms of body: seaweeds, yeast, bread mold, wheat
smut, toadstools, liverworts, mosses, ferns, cycads, and the great
variety of herbs, shrubs, and trees.

The simplest plant body is a single cell. Among such simple
one-celled plants are the bacteria. A plant body in which cells
are joined end to end to form a thread represents the next stage
of advance in complexity over the one-celled forms. There are
many such thread-like plants among the pond scums, seaweeds,
and certain fungi. Then there is a grouping of cells to make up
such simple plant bodies as toadstools and mushrooms; these are
many-celled plants, but devoid of roots, stems, leaves, and flowers.
Still more complex plant bodies are those of flowering plants which
have many different tissues and organs with which to carry on
the work of the body.

Thallus plants. There is a large group of plants known as the
Thallophyta (thallus plants), to which belong the algae and the
fungi. They are primitive members of the plant kingdom.
The plant body is either a single cell or a simple grouping of cells
to form a body that has no leaves, stems, or roots. Moreover,
they do not have flowers, fruits, or seeds.

FORMS OF PLANT BODY

11

l~- Spore

FIG. 1. Different kinds of algae. A,
Gloeocapsa, a blue-green alga in which
the individuals are surrounded by a
gelatinous coating; B, Synechococcus,
a blue-green alga; C, Protococcus, a
simple green alga; D, Oscillat&ria t a
thread-like blue-green alga ; E, Nostoc,
another thread-like blue-green alga;
with germinating spore at left. (From
Holman and Robbins, in a Textbook
of General Botany.)

E

Algae. Algae are plants of simple structure which grow in the
water or in very moist situations. We are familiar with those that

12 ORGANIZATION -AND COMPOSITION OF PLANTS

form a greenish slime on the sides and bottom of watering troughs
and drinking fountains; and those that appear as a green coating
on the north sides of trees in moist forests; and those that form a
green, frothy, repulsive-looking scum on the water of ditches,
ponds, reservoirs, and stagnant pools. Algae are found in both
fresh and salt water. Many of the larger kinds in the ocean are
known as " seaweeds."

Four different groups of algae. As to color, there are four
groups of algae: blue-green, green, brown, and red.

The blue-green algae and green algae are chiefly of fresh waters,
whereas the brown algae and the red algae are principally of salt
waters. The brown algae, or brown seaweeds, are common along
the shores of all oceans. They are attached by specially modified
structures, holdfasts, to pilings, rocks, etc. The best-known brown
algae are the giant kelps, some of which may reach a length of
800 to 900 feet; the rockweeds, which are found on the rocks
between high-tide and low-tide marks; and Sargassum, which
becomes detached from its growing places along shores, and is
often carried far into the ocean. The " Sargasso Sea " in the
North Atlantic Ocean is a floating mass of the brown alga, Sargas-
sum, carried there by ocean currents from distant shores. It is
recorded that Columbus saw the Sargasso Sea on his memorable
voyage to the New World. William Beebe and Ruth Rose in
The Arcturus Adventure, a Putnam publication, describe in a most
interesting manner the " Sargasso weeds and waves."

The red algae, or red seaweeds, are quite beautiful, delicate
plants and are usually much smaller than the brown algae. The
plants are often very highly branched, the divisions being fine and
thread-like. The red algae are found in deeper waters than the
brown algae.

The simplest algae, such as Gloeocapsa, and Protococcus, are
one-celled plants. The whole plant consists of but one spherical
cell. What simpler plant could there be? But these microscopic
one-celled plants carry on all the life processes, such as absorption
of water and mineral nutrients which are taken in at all points on
the plant body, respiration, food manufacture, digestion, assimila-
tion, and reproduction.

Many of the algae are filamentous or thread-like forms. That

FORMS OF PLANT BODY 13

is, their bodies consist of a single chain of cells. Common examples
of filamentous algae are : Nostoc and Oscillatoria of the blue-green
algae, and Spirogyra and Ulothrix of the green algae.

Nostoc plants are often aggregated to form bluish-green balls,
which may be found on damp earth or in water. The threads or
filaments (the plants) are embedded in, and held together by,
a gelatinous material secreted by the different cells that make
up the colony. Chlorophyll is present, and with it is a blue-green
pigment which gives the blue-green color to the whole cell.

Spirogyra or common pond scum is one of the most widely dis-
tributed of the green algae. Each cell as shown in Fig. 2 is a
short cylinder, with well-defined walls of cellulose. Each cell has
one or more conspicuous spiral chlorophyll bands. The spiral band

FIG. 2. Drawing showing the structure of a cell of Spirogyra and its rela-
tion to other cells of a filament. The cell wall is lined by a thin layer of
cytoplasm which holds a spiral chloroplast. This bag of cytoplasm is filled
with cell sap within which is suspended a small mass of cytoplasm containing
the nucleus of the cell.

is a specialized mass of living material. In addition, there will be
found near the center of the cell a nucleus, and from it strands of
protoplasm radiating to and connecting with the protoplasm that
lines the wall.

A number of algae, such as Cladophora, are branching, filament-
ous forms. Some, like f/foa, consist of a single plate of cells. The
brown and red algae, the " seaweeds," however, possess the most
complex structure of all the algae. Their bodies may be large.
In one of the rockweeds (Ascophyllum) , for example, the plant pos-
sesses special holdfasts; it is highly branched, and the branches are
of two different kinds; there are two quite distinct systems of
tissues; and its reproductive organs are more complex than those
in blue-green and green algae.

14 ORGANIZATION AND COMPOSITION OF PLANTS

Thus we see that algae, as a group, vary considerably in struc-
ture. The blue-green algae include the simplest forms, many of
them being simple one-celled plants. Some of the blue-green
algae are filamentous. The green algae include one-celled, fila-
mentous, and plate forms. The brown and red algae (" sea-
weeds ") are often very large plants.

Exercise 1. Different kinds of algae. Make a microscopic study of the
different forms of algae which may be collected from ponds, streams, foun-
tains, and moist surfaces of trees and rocks. Observe principally the varia-
tions in the form of the plant body. Also, if possible, examine various kinds
of brown and red seaweeds.

Fungi. The fungi are forms of plant life which have no chloro-
phyll and hence must secure their food ready-made from living
organisms or from substances which were once a part of the bodies
of living organisms. The foods of fungi, as of all plants and
animals, are chiefly carbohydrates, fats, and proteins. As we have
learned, green plants have the power of manufacturing these foods
from carbon dioxide, water, and various mineral salts, that is, from
inorganic materials. But the fungi, lacking chlorophyll, do not
have this ability. They are dependent either directly or indirectly
upon green plants.

Those fungi which gain their foods from living plants or animals
are called parasites; those which take their foods from the dead
remains or products of living plants or animals are called sapro-
phytes. For example, the rusts and smuts which gain nourish-
ment from live tissues are parasites; and the molds of bread and
fruits, and the various fungi which grow on decayed logs are
saprophytes. The term host refers to the plant or animal from
which the parasite derives nourishment.

Different kinds of fungi. There are many thousand different
kinds of fungi, and they affect man's welfare in many ways. It is
difficult to overemphasize their importance. A number of bacteria
and other fungi bring about decomposition of organic material,
and are necessary to maintain soil fertility; thousands of them
cause diseases of plants and animals, including man; they are
essential in the making of cheese and of bread, in the retting of
flax, and in many other commercial processes.

We shall discuss briefly here a few of the most important

FORMS OF PLANT BODY

15

groups of fungi, namely, bacteria, molds, mildews, yeast, smuts,
rusts, mushrooms, and toadstools.

The bacteria (Fig. 3). Bacteria teem in countless millions in
the air, in the soil, and in the water; they are present upon the
surface of the human body, and that of other animals, and in the
intestinal tract; they abound in sewage, and in all decaying mate-
rial ; they occur in the surface of all objects about us. Bacteria are
the smallest known organisms. The average size is about 25
inch in diameter.

There are three principal types as
to shape (Fig. 3): the spherical (coccus),
the rod (bacillus), and the spiral (spiril-
lum).

Exercise 2. Bacteriological laboratory. If

possible the student, either alone, or with the
class, should visit a bacteriological laboratory.
In all cities of any size there is such a laboratory
connected with the city health department.
Observe here the equipment, methods, bacterial
cultures, etc. Take the opportunity of observ-
ing bacteria under an oil-immersion lens.

-Three forms of
bacteria. A, bacillus
forms; B, coccus forms;
C, spirillum forms.

The molds. These usually form a
cobwebby growth, and they occur on a

great variety of organic materials, such as stale bread, fruit and
vegetables, jellies, old leather, cheese, and moist paper. Fruit
and vegetables in the market, in storage, or during shipment
may be greatly damaged by these saprophytic fungi, especially if
the air is warm and moist. To prevent their molding, such prod-
ucts are shipped and stored at low temperatures, in refrigerator
cars.

There are many different kinds of molds varying as to the
color of the spores they produce: black, blue, brown, green, and
yellow.

The mildews. There are two different groups of mildews, the
downy mildews, and the powdery mildews. Both groups are
parasitic. The mildews of the first group form a downy white
growth upon the surface of leaves, and sometimes on that of
stems and fruits. The parasite is not confined to the surface, how-

16

ORGANIZATION AND COMPOSITION OF PLANTS

ever, but its threads enter the tissues and absorb food from them.
The spores are borne in abundance on the surface of the host, and
under suitable conditions will germinate immediately. Well-known
destructive downy mildews are those causing the late blight of
potato, and the mildew of grape, onion, lettuce, lima beans, cucum-
ber, pumpkin, and watermelon.

The rusts. The rust fungi constitute a very large group of
plants, all of which are parasites, and many of which are of great

/Conidiophores \

tL\

Conidiospores

FIG. 4. Blue and green molds. A and B, common blue mold; C, germinating

spore of blue mold; D, green mold. An hypha is a fungous thread; conid-

iophore is a spore-bearing branch; conidiospore is a special type of spore.

(From Holman and Robbins, in A Textbook of General Botany.)

economic importance on account of their destruction of crop
plants. Among the most important rusts are those of the cereals,
asparagus, apple, raspberry, and pine. The black stem rust of
wheat, oats, barley, rye, and other grasses has caused damage to
the cereal crops amounting to millions of dollars, and at times has
become epidemic. In the United States in 1916, the black stem

FORMS OF PLANT BODY 17

rust (Fig. 43) caused a loss of wheat amounting to 200,000,000
bushels. The white pine blister rust has threatened the destruc-
tion of the white pine forests, the timber of which is valued at
$411,000,000.

The smuts. All the smut fungi (Fig. 5) are parasites, occur-
ring chiefly on members of the grass family. The annual losses of
cereal crops due to the smuts frequently amount to 150,000,000
bushels. The smuts may be recognized by the black masses of

FIG. 5. Ear of corn showing a mild attack of corn smut. Corn smut is a
parasite, deriving its food from the tissues of the living corn plant.

spores. In cereal smuts these usually develop in the head and
mature at about the same time the head matures.

Mushrooms and toadstoools. These are fleshy fungi which
are found growing in fields, pastures, and woodlands, and also
upon decaying logs and tree trunks. There is a great variety of
" mushrooms " and " toadstools " (Fig. 6). Probably the best
known are those which bear gills and are known as the gill fungi
or " agarics." Others are the pore fungi, the tooth fungi, the
carrion fungi, and puffballs.

18

ORGANIZATION AND COMPOSITION OF PLANTS

Let us describe very briefly the common " meadow mushroom/'
the familiar edible one. The " mushroom " as we see it consists
of a stalk and an umbrella-shaped cap. On the under side of the
cap are thin gills. The spores are borne in enormous numbers on
the surfaces of these gills. Each spore is a microscopic spherical
body, light in weight, and capable of being carried long distances by
air currents. The cap with its stalk, constituting the mushroom,
forms in reality only a small part of the plant body. The mush-
room is a fruiting body and arises from a great mass of fungous

FIG. 6. Longitudinal section of a mush-
room (Tricholoma). The umbrella-shaped
cap, or pileus, from which the spore-bearing
gills hang, is supported by the stalk, or stipe.

FIG. 7. Puffballs, showing

some of the underground

hypha threads.

threads which are distributed in the soil, and which gain their food
from decayed organic matter.

The so-called mushroom " spawn " sold by seedsmen usually
consists of dried manure containing the fungous threads, all being
pressed together in brick form. When a mushroom bed is made,
the spawn is broken up, mixed with earth, and used to start the
beds. The mushroom originates from the fungous threads in the
ground.

In the tooth fungi there are teeth or spines on the under side
of the cap, and these bear the spores. In the pore fungi the spores

FORMS OF PLANT BODY 19

are borne in open tubes or pits on the under side of the cap. A
number of pore fungi cause the rotting of wood. The puffballs
(Fig. 7) rupture when mature, setting free black clouds of spores.
A number of the fleshy fungi are poisonous. Although there
is no botanical difference between " mushrooms " and " toad-
stools," the former name is commonly applied to those believed
to be edible, and the latter to those thought to be poisonous.

Exercise 3. The plant body of different fungi. Observe the plant bodies
of a variety of fungi such as molds of bread and fruit, yeast, mildews, smuts,
rusts, toadstools, and mushrooms. In what respects are they alike? In
what respects do they differ?

Mosses and Liverworts. The name " moss " is applied popu-
larly to a number of different kinds of plants. Some of the sea-
weeds are called " sea mosses," but the true mosses never occur
in saline waters. The "Spanish moss" (Tillandsia) which hangs
from the trees in our southern swamps is not a true moss but a
flowering plant. "Reindeer moss" is a lichen, as is also the
"moss" that hangs from the limbs of conifers in the northern
states and in the high mountains of the West.

The true mosses are low plants seldom more than a few inches
in height, with an erect stem, upon which very small leaves are
densely crowded. The leaves are usually but one cell in thickness,
except along the midrib and sometimes around the margin. There
are no true roots in mosses. They possess structures known as
rhizoids, which, although they have not the structure of roots, serve
the same purposes of absorption and anchorage. Identify differ-
ent moss structures shown in Fig. 104.

A distinctive feature of mosses is the " fruiting " or spore-
producing body. This is a spore-case or capsule (Fig. 104) at the
tip of a stalk. Numerous spores are borne within this capsule.

Mosses are found chiefly in moist woods and in swamps, but
some species occur on the bark of trees and in dry rock crevices.
In regions with a prolonged moist season, they may be seen grow-
ing on fences, and on the shingle roof of old buildings. They
are conspicuous on account of the " carpet " or mass of vegeta-
tion they form.

The mosses are divided into three distinct groups: (1) the peat
mosses, (2) the bkck mosses, and (3) the true mosses. They

20

ORGANIZATION AND COMPOSITION OF PLANTS

differ somewhat in their appearance, structure, habits, and life
history.

The liverworts (Fig. 107) are low-growing plants, chiefly found
in moist places. The plant body is thin, green, and flat against
the ground, being attached to the soil by slender root-like
structures, known as rhizoids. Marchantia is a well-known liver-
wort, the body of which is more or less lobed. There are
certain leafy liverworts, the body of which is composed of a
slender prostrate axis or "stem" bearing three crowded rows of

small leaf -like structures.
The body is attached to the
soil by rhizoids.

Exercise 4. The plant bodies
of mosses and liverworts. Ob-
serve in the field or greenhouse
the plant bodies of different
mosses and liverworts. Contrast
them with those of the thallus
plants, enumerating differences.

The ferns and their allies.

The ferns and their close re-
latives, the club mosses and
scouring rushes (horsetails)
(Fig. 8), constitute a large
group of plants. Like the
algae, fungi, and mosses,
they reproduce by means
of spores, but unlike these
groups, they possess woody
stems and roots, and a con-
ducting tissue, similar to
that in flowering plants.
The ferns and their allies

FIG. 8. Horsetail or scouring rush. A,
early spring stems arising from rootstock;
note the scale-like leaves at the joints and
the spore-bearing cones at the tip; B,
branching form which appears later in the
season than the preceding. (From Glover
and Robbins, in Colo. Agr. Exp. Station
Bulletin.)

do not produce flowers.
In the common cultivated ferns, the stem system is wholly
under ground. It persists from year to year, growing in length at
the tip, branching somewhat, and sending into the air each season
a number of leaves, the so-called fronds. After a time there appear

FORMS OF PLANT BODY

21

m

FIG. 9. The stag-horn fern.

on the under side of the frond brownish groups of spores which are

often mistaken for some disease or insect. They are, however, the

reproductive bodies. The

fern-lover should read the

article " Ferns as a Hobby "

by William R. Maxon, in

the National Geographic

Magazine, Vol. 47, pages

541-586, 1925.
The horsetails or scouring

rushes have harsh, jointed

stems which arise from a

rootstock. The leaves are

mere scales. The spores are

borne in a cone at the tip of certain branches. On account of

their harsh texture, the plants have been used for cleaning and

polishing utensils. They are re-
puted to be poisonous to live-
stock, chiefly horses. Sometimes
they behave as weeds.

The club mosses are usually
creeping or trailing plants, some-
times known as " ground pine " or
"running cypress." The spores are
borne in leafy cones at the tips of
branches. The spores are sold in
drug stores under the name " ly co-
podium powder," and are used as a
drying powder and to some extent
in the manufacture of fireworks.

FIG. 10. Ferns and club mosses in

the Garfield Park Conservatory,

Chicago.

Exercise 5. Plant bodies of ferns
and their allies. Observe in the field
or greenhouse the plant bodies of
different kinds of ferns, club mosses,

and horsetails. Enumerate the differences between the plant bodies of ferns

and their allies, and those of mosses and liverworts.

Seed plants. The seed plants possess the most complex body
of all plants. There are many different organs and tissues. There

22

ORGANIZATION AND COMPOSITION OF PLANTS

are roots, stems, leaves, flowers, fruit, and seed, except in one large
group, the Gymnosperms, which have no flowers in the ordinary

FIG. 11. Wheat plant showing the general habit of growth of grasses. (From
Robbins, in Botany of Crop Plants.)

FORMS OF PLANT BODY

23

sense, and there is tremendous variation in the form of these
organs. Moreover, there are many different kinds of tissues which
compose these various organs.

FIG. 12. The Deodar Cedar, a seed plant. In this and other conifers there
is a "leader" one main stem which throughout the life of the plant holds

this leadership.

Fia. 13. The Live Oak, a seed plant. Compare the branching habit of

this plant with that of the Deodar in Fig. 12. The form of the plant body is

largely determined by its branching habit.

Exercise 6. The seed-plant body. The student should take a field trip
and note the different forms of seed-plant body. The various species of trees
and shrubs have characteristic shapes; these may be shown well by quickly

24

ORGANIZATION AND COMPOSITION OF PLANTS

Cell Wall

Nucleus

Cytoplasm

sketching their outline. Observe not only erect forms of plants, but also
climbing and creeping forms. Also note the form of plant bodies growing
under different environmental conditions.

Problem 2. What is the structure of the plant cell?

If we study with the microscope the structure of plant tissues,
we find them to be made up of many small bags or sacs with walls

which are usually thin and trans-
parent. Each of these microscopic
sacs or compartments is called a cell
(Fig. 14). The term "cell" was
first used by Robert Hooke, an
Englishman who lived from 1636 to
1703. With his improved micro-
scope he examined all sorts of things,
among them ordinary bottle cork.
He observed this plant tissue to be
made up of numerous compartments
resembling the cells of honeycomb.
So Hooke named the compartments
cells.

Just as a brick house is made up
of separate units, the bricks, so is the
plant body made of separate units,
the cells. It is in the cells that all
the complex physical and chemical
changes of the living body go on.
Careful observation of plant cells
under the microscope reveals that
within each of the cells there is a
quantity of a jelly-like substance.
This is the living material and is called

protoplasm. What is the literal meaning of the term protoplasm?
The protoplasm of the cell is not of the same structure through-
out. A denser mass of living material, the nucleus, is usually
prominent in the cell. The nucleus is a very important part of the
cell, taking an active part when the cell divides. Most important of
all, the nucleus carries those determiners of characteristics which are

^illto

'' "' '''AT- <' - A" ^ n "j. -

fr~*&*.&f* s &ii&i ^ *:
^

FIG. 14. Two young cells
from the growing point of a
root. (From Holman and
Robbins, in A Textbook of
General Botany.)

STRUCTURE OF PLANT CELL 25

passed on from cell to cell, that is, from parent to offspring. In
addition to the nucleus, there may be other specialized masses of
protoplasm in the cell, known as plastids; chief of these are the
green plastids (chloroplastids) which are the centers of the process of
carbohydrate manufacture. All the protoplasm of the cell outside of
the nucleus is called cytoplasm. Protoplasm is a mixture of many
different chemical compounds, some of which are exceedingly
complex.

In addition to the living substance the cell contains much
material that is not alive. For example, every cell contains water
in which are dissolved various substances that have come from the
soil, and certain foods, such as sugar, which have been manufac-
tured in the leaves. Sap is the name we apply to the water of
the cells plus the various substances which are dissolved in it.
In other words, cell sap is a solution, in which water is the solvent.
The cell may also contain substances such as starch and protein
which are not soluble in the water of the cell. The wall about the
cell is not alive. It is made up of a material called cellulose, a sub-
stance closely related in its chemical composition to starch and
sugar. Cellulose, like starch and sugar, is made up of but three
elementary substances, carbon, oxygen, and hydrogen. Cellulose
is the most abundant plant substance in the world. It is of interest
to note here, in passing, that cotton, linen, hemp, and wood consist
of cellulose, and that it is used as a raw material in the manufacture
of such substances as artificial silk, paper, celluloid, cellophane, and
guncotton. The inquiring student will want to find out how these
materials, as well as many others, are manufactured from cellulose.

Exercise 7. The cells of soft tissue. Examine the soft tissues of broken
leaves, vegetables, fruits, stems, etc., with a binocular dissecting microscope.
It will be possible even with a magnification of 20 diameters to see that the
tissues are composed of many small compartments, varying considerably in
shape. These compartments are the cells. They are the units of structure.

Exercise 8. Cells in the leaf of Elodea, a water plant. The leaves of the
water plant Elodea are exceedingly thin, mostly one layer of cells thick.
Mount a single leaf flat in a drop of water on a slide, and cover with cover-
slip. Examine with compound microscope. Compare what you see with
Fig. 15. The cells are filled with green plastids (chloroplastids), which may
wholly or partly obscure the more transparent nucleus. The walls are of
cellulose and thin. Observe the different shapes of cells in different parts of
the leaf. In fresh, young leaves, the student will observe a movement or

26

ORGANIZATION AND COMPOSITION OF PLANTS

streaming of the protoplasm, the chloroplastids being carried along with the
stream, as chips of wood in flowing water. The chloroplastids themselves are
living bodies. It is in them that glucose, a sugar, is manufactured, with the
aid of light. In all living cells there is a movement of the protoplasm, but in
only a few plants is it rapid enough to be readily observed. What do you
believe to be the advantage of this protoplasmic movement?

Exercise 9. Storage cells of the potato. Cut very thin sections of the
inside white tissue of a potato tuber. Mount in water and examine with
compound microscope. Observe the large, thin-walled cells, rilled with starch

FIG. 15. View of a portion
of an Elodea leaf as seen
under the high power of the
microscope. The cell struc-
ture is similar to that of
onion skin (Fig. 16). How-
ever, as these cells are from
a green leaf, they contain
chloroplasts which float in
the cytoplasm. While the
nucleus can be shown to be
present by staining, yet it
is not easily seen in the
fresh material.

FIG. 16. View of onion skin
as it appears under the micro-
scope. Note the well-defined
cell wall. Within the cell wall
there is a thin layer of cyto-
plasm which, together with the
small disk-shaped nucleus, is
made up of protoplasm, the
living matter of the cell. Cell
sap fills the cavity within the
cytoplasm.

grains. Starch is a non-living substance a storage product. It is of interest
to know that potatoes which are mealy when cooked are those in which the
cells are well filled with starch; whereas in watery potatoes starch grains do
not fill the cells. Americans prefer mealy potatoes, but Frenchmen as a rule
prefer the more watery sorts. Name other plants that store large amounts of
starch in, some part of the plant.

Spiral

Cell Wall
Cell Cavity
Intercellular Space
K

FIG. 17. Different kinds of cells and tissues. A, fibers as seen in cross-
section; B, fibers as seen in lengthwise section; C, a single fiber; D, stone
cells from the shell of English walnut; E, cork cells; F, a food-manufacturing
cell from a leaf; G, thick-walled pitted cells from endosperm of asparagus
seed; H, starch-storing cell; I, different types of vessels; J, a simple pitted
tracheid from pine wood ; K, tissue made of thin-walled cells which fit loosely
together. (Except A, B, C, and G, from Holman and Robbins, in A Textbook

of General Botany.)
27

28 ORGANIZATION AND COMPOSITION OF PLANTS

Exercise 10. Different kinds of cells. The inquiring student will be
interested in examining the tissues of different organs of ordinary flowering
plants, and also those of lower plants, such as algae, mosses, liverworts, ferns,
etc. See Fig. 17. He will see cells differing in size, in shape, in the thickness
and markings of the walls, and in the nature of their contents.

Problem 3. What is the nature of protoplasm the living

material?

In 1590, two Dutch brothers, spectacle-makers, invented the
compound microscope, an instrument which was destined to
become the most important tool of biological science a tool
which has made possible much of the progress in our knowledge
of plants and animals, of medicine, of agriculture, of heredity.

As stated above, Robert Hooke greatly improved the micro-
scope, and examined with great curiosity all sorts of things, among
them bottle cork. Hooke saw in the cork tissue only the walls of
dead cells. He had no clear idea of the cell contents. It was not
until 1831 that another Englishman, Robert Brown, first recog-
nized the importance of the nucleus in the cell, and not until 1861
that Max Schultze, a German, established the close similarity of
the living substance of plants and of animals, and formulated what
is known as the protoplasmic doctrine which says that the essen-
tial part of a cell, the part which is responsible for its life, is the
protoplasm. The unit of structure and activity is really a highly
organized protoplasmic mass; the wall is merely a non-living,
enclosing shell. Protoplasm has well been called the physical basis
of life.

Physical properties of protoplasm. Protoplasm is a semi-
transparent, slime-like substance, much the consistency of the
white of an egg. However, it does not have the same appearance
throughout all parts of the cell; the nucleus is much denser and
is darker than the cytoplasm as seen in stained cells; also there
are small, dark granules embedded in the protoplasm, some
of which are living; and, as we have learned, there are larger,
living bodies, the plastids, floating in the mass of protoplasm.
Sometimes these plastids contain a green pigment (chlorophyll),
sometimes orange and red pigments (carotin and xanthophyll).
Is it true that the green color of the vegetation of the world is due

NATURE OF PROTOPLASM 29

to the pigment chlorophyll? When a cell dies the protoplasm loses
its liquid consistency and coagulates, that is, sets into a more or
less firm mass, like the white of an egg when it is boiled.

Exercise 11. The living protoplasm in the cells of squash and Elodea.
Mount in water the hairs found on the stems, near the tip, of the squash plant.
Under the high power of the microscope one will see, in certain cells of the
hairs, a grayish, semi-transparent substance, the protoplasm. The nucleus is
darker gray, and leading from it are strands or threads of cytoplasm. Cyto-
plasm also lines the wall of the cell. Mount fresh leaves of Elodea, as you did
in Exercise 8, Problem 2. The cells are filled with green plastids (chloro-
plastids), which are embedded in _a gray cytoplasm. Each plastid is a jelly-
like mass of protoplasm, and dissolved within it is a pigment known as
chlorophyll. See Fig. 15.

Chemical properties of protoplasm. Protoplasm appears to
be a mixture of a number of different chemical compounds. It is
not a single compound like sugar. We can write the chemical for-
mula of cane sugar as follows: Ci2H220n; but we cannot write a
chemical formula for protoplasm. The following chemical ele-
ments are always found when protoplasm is analyzed: carbon,
hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium,
magnesium, and calcium. There is no chemical element found in
protoplasm that does not occur in the soil and air. The compounds
occurring in the mixture of compounds composing protoplasm
include principally different proteins, fatty substances known as
lipoids, carbohydrates, and salts. Usually 80 per cent, by weight,
or more of protoplasm is water. Proteins rank second, by weight.
The proportion of the different compounds making up the proto-
plasm of any cell is constantly changing. Furthermore, there
appear to be significant differences between the chemical composi-
tion of the protoplasm of different kinds of plants, and of different
cells of the same plant. Generally speaking, protoplasm, the liv-
ing material, is a very complex mixture of chemical compounds,
including among them proteins, known to be in themselves the
most complex chemical compounds thus far analyzed. (Refer to
Chemical Phenomena in Life, by Frederick Czapek, 152 pages,
published by Harper and Brothers, New York.)

Physiological properties of protoplasm. Protoplasm, the living
material, possesses certain properties and powers that are peculiar
to it and that distinguish it from non-living material. For exam-

30 ORGANIZATION AND COMPOSITION OF PLANTS

pie, a cell grows and develops new cells; that is, it has the power
of growth and reproduction. In order to grow and reproduce,
the cell must secure material from outside of itself and make that
material a part of itself; that is, it must synthesize carbohydrates,
fats, proteins, and other substances, and then change these mate-
rials into living stuff. This change of non-living stuff to living is
known as assimilation, a change the nature of which is but poorly
understood. We have come to believe that all life comes from
pre-existing life; that is, new cells and new organisms are de-
rived only from existing ones. In the processes of growth and
reproduction, the protoplasm throws off waste products, that is,
it has the power of excretion. For the building processes occurring
in protoplasm, there must be a supply of energy. This the cell
secures through respiration, another one of its peculiar processes,
in which complex substances are broken down and energy liber-
ated. Still another physiological property of protoplasm is that of
irritability. By this we mean its ability to " sense " stimuli and
respond to them. Protoplasm " perceives " light, gravity, water,
and other external factors, and responds to them. Have you
observed that plants in a window grow toward the light? See
Fig. 128. That roots grow downward in response to gravity?
That roots grow into moist soil rather than into dry soil? Proto-
plasm is irritable.

Suggested activities, (a) Is there spontaneous origin of life? The student
should read a short account and prepare a report on how man was led, through
his discoveries, to overthrow the notion that life might originate spontaneously.
See one of the following: (1) Locy's Biology and Its Makers, (2) DeKruif's
Microbe Hunters. (6) Devise an experiment to show that plants are respon-
sive to light, (c) Devise another experiment demonstrating that roots
respond to gravity.

Problem 4. How are cells grouped to form tissues and organs?

The cells of the plant body vary a great deal in size, in shape, in
age, in the kind of material they contain, and in the nature of the
work they have to do. It would take about one thousand average-
sized sugar storage cells, from the root of a sugar beet, placed side
by side to make an inch. Many cells of the plant are box-shaped
with either square or rounded corners, others are spherical, and

RELATION OF STRUCTURE AND FUNCTION 31

still others are much longer than wide, and with pointed ends. In
the growing points of the plant, as in buds and at root tips, the
cells are much younger than those found farther removed from
these tips.

There are cells in the plant body adapted to carry on the dif-
ferent kinds of work it has to do. Some are fitted to absorb water
and mineral salts from the soil; others carry water, mineral salts,
and foods from one part of the plant to another; others are spe-
cially fitted to manufacture food; many cells act as storage
reservoirs of food; and still others are chiefly concerned with
reproduction.

The cells of the plant body having special kinds of work to do
are usually grouped together, forming tissues. These tissues are
given names describing the functions or kinds of work they per-
form. For example, there are absorptive tissue, conductive tissue,
strengthening tissue, food-making tissue, storage tissue, and repro-
ductive tissue. An organ may be composed of several kinds of
tissue. For example, a leaf possesses conductive and strengthen-
ing tissue in its veins, food-making tissue in the softer parts, and
a protective tissue (epidermis) which covers the entire surface.

Thus we see that the living plant body, like the human body,
is a complex structure, composed of innumerable units, the cells,
grouped together to form tissues, each with a special work to do,
and the tissues are in turn grouped to form the organs of the plant.
We may picture the healthy, living plant as a marvelously con-
structed body, in which there is a splendid division of labor, with
all cells, tissues, and organs working in harmony. We come to
realize that living things resemble each other not only in gross
structure and function, but also in microscopic structure.

Exercise 12. Different kinds of tissues. The student should examine
thin sections of different kinds of plant materials and observe how the cells
are grouped to form tissues and organs. How do cells differ in shape? Com-
pare different tissues as to hardness, compactness, and strength.

Suggested activity. Make plasticene models of different kinds of cells.

Problem 6. What is the relation of structure and function?

The living plant expends energy and does work. If it were
possible to take moving pictures of the plant at work, and to run

32 ORGANIZATION AND COMPOSITION OF PLANTS

the film at high speed, we would see the roots twisting and turning,
making their way about, and between the soil particles we would
observe the young sprout of the germinating seed straining to lift
the load of soil from its path ; we would see parts of opening buds
moving vigorously; in fact, there would be active movement
throughout the plant body; and if we examined the interior of the
cells, we would see the living material, the protoplasm, moving in
streams from one part of the cell to the other. Plants, like animals,
actually do work as long as they are alive, and this work requires a
supply of energy.

The various activities or kinds of work performed by plants are
spoken of as their functions. For example, absorption of water
and salts from the soil, movement of materials in the plant tissues,
manufacture of foods in green tissues, digestion of foods, loss of
water, respiration, and reproduction are all functions of the plant
body. Clearly, these functions are associated with certain struc-
tures. For instance, absorption of water and salts is associated
with roots, loss of water chiefly with leaves, manufacture of food
chiefly with leaves, etc. Furthermore, the organs are so con-
structed as to carry on well the particular functions associated
with them.

Let us consider the function, photosynthesis, the manufacture
of sugar in the leaf. Is the leaf structure as shown in Fig. 29
well fitted to carry on this process? In the process of photo-
synthesis carbon dioxide gas from the atmosphere and water from
the soil are brought together in green leaf cells, and there, with the
aid of light, built into a sugar, glucose. The manufacturing cells
of the leaf require water, carbon dioxide gas, and light. It is
obvious that a thin, flat, expanded structure, like a leaf, exposes
a large surface for the absorption of both light and carbon dioxide.
As concerns light absorption, the epidermis of the leaf is thin and
transparent, and thus allows light to penetrate the underlying
tissues; and the internal cells possess a pigment, chlorophyll, which
is effective in absorbing light energy. As concerns carbon dioxide
absorption, the epidermis of the leaf has numerous small openings
or pores (stomata) which permit the movement inward of gases;
furthermore, inside of the leaf the cells fit together very loosely,
leaving large air spaces which facilitate the movement of carbon

CHEMICAL SUBSTANCES IN PLANTS 33

dioxide in the leaf, allowing it to come in contact with the surfaces
of almost all cells, by which it is absorbed. There is a network of
fine veins in the leaf, which bring water to the food-making cells.
No food-making cell is far removed from a water-conducting vein.
Thus, it would appear that the leaf is a structure well adapted to
carry on the function of photosynthesis or manufacture of sugar.
It would be possible to show how many other organs and tissues
of the plant body are adapted for the specific work they perform.
The relationship between structure and function is a close one.
Thought on the part of the student will bring to mind many of
these relationships in the plant.

Exercise 13. Relation of structure and function. Discuss the relation-
ship between the following structures and functions: (a) waxy coating of leaf
surface and loss of water from the leaf; (6) hairs on leaf surface and loss of
water from leaf; (c) thin cellulose wall of root hair and absorption; (d) roots
and absorption of materials from the soil; (e) leaf and food-manufacture.

Problem 6. What are the chemical substances found in plants?

In his study of all sorts of food and forage and medicinal plants,
the scientist has made a great many chemical analyses. These
analyses have told him much of the economic value of plants, of
the relative importance of different parts of the same plant, and
also have thrown light upon the movement of materials in the
plant, and the influence of various environmental factors, such as
fertilizers, upon the composition of the plant. For example, he
knows from such chemical studies that some varieties of wheat
are richer in protein than others, that the most starchy and valu-
able part of the potato tuber is the cortex or that portion just be-
neath the skin, that the food made use of by the opening fruit
buds of orchard plants is stored in woody tissue not far removed
from these buds, that an excess of nitrates as compared with carbo-
hydrates in a tomato plant suppresses fruit development, that a
deficiency of iron in the plant causes it to be pale and sickly, that
sugar beets growing in a soil with a too liberal supply of nitrogen-
carrying fertilizers are usually low in sugar content, and many
other valuable and interesting facts about plants. There are sev-
eral hundred plant chemists in the laboratories of this country.

34 ORGANIZATION AND COMPOSITION OF PLANTS

Analyses have detected in plants no chemical elements except
those found in the soil or air. That is, there are no chemical ele-
ments peculiar to plants. The principal elements which compose
the living and non-living parts of plants are carbon, hydrogen,
oxygen, nitrogen, phosphorus, sulphur, potassium, calcium, mag-
nesium, iron, and silicon. These are common elements found in the
minerals of the soil. Of course, many more chemical elements
than those listed above occur in plants. The different materials
of the soil are not absorbed in equal amounts by different kinds of
plants. Plants have some " selective power/' We well know
that if two different kinds of plants, peas and tomatoes, for exam-
ple, are growing in the same soil, their mineral composition is not
the same. Doesn't this show that different plants absorb mate-
rials from the soil in different proportions?

On the basis of their nutrition, plants may be grouped into
(1) the green plants, arid (2) the non-green plants. The green
plants, which include all common crop plants, absorb mineral salts,
Water, and carbon dioxide, and from them make the plant foods.
The non-green plants, such as bacteria, yeast, molds, mildews,
rusts, smuts, and mushrooms, subsist on living or dead plants or
animals and derive foods from them directly. But the foods
the materials which nourish the plant body are the same for both
plants and animals.

Considering green plants the common plants of garden, field,
orchard, and forest we can say that the source of the chemical
elements which make up their bodies is derived from the air and
soil. From the air they derive carbon and oxygen; from the soil,
oxygen and all other chemical elements which compose the plant.

The principal substances found in plants belong to the chemical
groups known as carbohydrates, fats, and proteins. The principal
carbohydrates are starch, sugars, and cellulose. Starch is proba-
bly the most important storage product in all plants. It is found
in fruits, seeds, leaves, roots, and stems in fact, there is scarcely
a part of the plant that does not contain some starch. In most
leaves, starch accumulates during the daytime, and at night is
changed to sugar, r which moves in solution to various parts of the
plant where it is stored in some form. Starch occurs in the vas-
cular rays of woody stems and roots, and in other tissues of these

CHEMICAL SUBSTANCES IN PLANTS

35

organs. Starch is accumulated in large quantities in many seeds,
such as wheat, oats, barley, beans; in certain roots such as pars-
nips; and in certain special types of storage stems such as potato
tubers. Starch is used by the plant as a stored or reserve food.
When needed for growth, it is digested, and the digested products
move to growing points.

Make a list of the principal starch-storing food plants.
There are many dif-
ferent kinds of sugars
in plants. The princi-
pal ones are grape sugar
(glucose), cane sugar
(sucrose), and fruit sugar
(fructose). Glucose is
considered to be the
immediate product of
photosynthesis, that is,
of the process by which
water and carbon diox-
ide are converted into
this product. Glucose
is the usual form in
which carbohydrates
move from one part of
the plant to another
part. It is found in
the conducting tubes,

FIG. 18. Section through the tuber of Irish
potato. The flesh of the tuber consists chiefly
of large, thin-walled starch-storing cells, in
which are scattered bundles of conducting
tissue. A ring of vascular bundles is visible,
and at the right-lower corner is an "eye."

and in the sap of most
cells. It appears to be
the foundation material
used in the synthesis
of most other plant sub-

stances. Sucrose or cane

sugar is particularly abundant in the root of sugar beet and
in the stems of sugar cane. Fructose or fruit sugar is particularly
common in fruits.

Cellulose is a carbohydrate which enters into the structure of
cell walls. It is structural material.

36

ORGANIZATION AND COMPOSITION OF PLANTS

The fats or oils occur chiefly in seeds. For example, in the seed
of the cotton plant there is much reserve oil. It is this oil which

the seed uses as a food supply
during the germination process.
Make a list of plants which are
the source of commercial oils.

Proteins are reserve foods
found principally in seeds, such as
beans, peas, and cereals. Proto-
plasm, itself, is rich in proteins.
In addition to the carbo-
hydrates, fats, and proteins just
mentioned, numerous other sub-
stances are found in plants. For
example, there are the various
plant pigments, resins, gums,
alkaloids, organic acids, and es-
sential oils.

As regards the chemical composition of the plant, we may sum-
marize as follows : The framework of the plant that is, the walls

FIG. 19. Cross-section of a sugar
beet root. Observe the rings of
growth, all of which are produced in
one season. Most of the cells of the
root are rich in sucrose (cane sugar).

&& r ' ' --- n.- i; ^a&afa

pericarp

=^. i inner
=^ I integument
nvcetlus

FIG. 20. Microscopic section of wheat grain. The aleurone layer and the

starchy endosperm are rich in stored food; the other layers constitute the

"bran." (From Robbins, in Botany of Crop Plants.)

CHEMICAL SUBSTANCES IN PLAixTS 37

of all cells is chiefly cellulose; protoplasm itself is largely a mix-
ture of various proteins and fatty substances in water; the chief
products stored in cells are sugars, starch, and proteins; and, in
addition to these more common materials, there are gums, resins,
alkaloids, acids, various pigments, and essential oils. The sap
of a plant is chiefly water, carrying in solution salts derived from
the soil, and various other substances manufactured by the plant.
The use of plant materials by man and animals. It is well
recognized that all animal life on the earth, including man himself,
is dependent upon plants for its very existence. Green plants
are the only agencies by which the inexhaustible supply of solar
energy is caught and held, and transformed into foods, for plants
and animals alike. We are familiar with the fact that plants
furnish us with food, clothing, shelter, and numerous drugs; that
plants are used to beautify our homes and landscapes; that they
supply feed for livestock; and are used in the manufacture of
scores of commercial products. In Unit X there will be a more
extended discussion of plants of economic importance to man.

ADDITIONAL EXERCISES AND QUESTIONS

1. What do we mean by saying that the cell is the unit of structure of the
plant?

2. Enumerate the different things that protoplasm of plants can do.

3. Justify the statement that protoplasm is the most wonderful substance
in the world.

4. Why do complex plants need different kinds of tissues and organs?

5. Explain why plants which live on the land have more highly developed
tissues and organs than plants which live in the water.

6. Give the main characteristics of each of the four great groups of plants.

7. Upon which of the four great groups does man depend chiefly for his
food supply?

8. Name as many farm crops as you can in which the root is useful to man;
the leaf; the stem; the flower; the seed; the fruit.

9. Which of the four groups of plants is of least importance to man?
30. Of what importance to plants is the food stored in the seed or root?

REFERENCES

Plants Useful to Man, by W. W. ROBBINS and FRANCIS RAMALEY, pub-
lished by P. Blakiston's Son and Company, Philadelphia, 1933. A well-
illustrated book (241 figures) of 428 pages giving a discussion of the principal

38 ORGANIZATION AND COMPOSITION OF PLANTS

food plants of the world, spices, beverage plants, medicinal plants, and indus-
trial products of vegetable origin. "It furnishes a background of knowledge
of the world's commercial plant products, both for students of botany and for
those whose interests are in the fields of geography, economics, and agri-
culture."

The Growth of Biology, by WILLIAM A. LOCY, published by Henry Holt
and Company, 1925. "The growth of our knowledge of living organisms is
a part of the larger story of human progress; the struggles and triumphs of
the human spirit. In a history of any science it is not sufficient to give an
impersonal account of the discoveries as coming in a certain sequence the
human element is involved as an essential part of the story. " Among other
things, it includes an interesting account of the discovery of the plant cell,
the development of our knowledge of protoplasm, and the structure of plants.
481 pages and 140 illustrations.

Geography of the World's Agriculture, by V. C. FINCH and O. E. BAKER,
published by the United States Department of Agriculture, Washington, D. C.
1917. 149 pages, 206 figures. "The purpose of this study is to show the
geographic origin of the world's supply of food and of other important agri-
cultural products and to indicate briefly the climatic, soil and economic
conditions that account for the distribution of the crops and livestock of the
world/' There are numerous maps and figures,

UNIT II
THE NUTRITION OF GREEN PLANTS

If we classify all living things on a food basis, we will have
two groups. In one group there will be all green plants; in the
other group, all animals, including man, and all plants such as
bacteria, rusts, smuts, toadstools, etc., which lack a green color.
The first group is independent; the second absolutely dependent
upon the first for its food. By " independent " we mean possess-
ing the ability to manufacture food out of such materials as the
plant absorbs from the soil and air. We, as animals, just like the
non-green plants, do not possess organs which enable us to take
such simple materials as carbon dioxide, water, and mineral salts
and make them into foods that is, into carbohydrates, fats, and
proteins. Only green plants have this power; they alone make
the food, not only for themselves but also for every other living
creature. The important fact for us to understand, the fact which
is emphasized in this chapter, is that the food laboratory of the
world is to be found in the green plant.

Foods the carbohydrates, fats, and proteins possess energy.
When foods are oxidized, either in the bodies of plants or animals,
that is, broken down into simpler substances, they liberate energy,
and this energy is used by the living body to do work. In the
building of these foods by green plants, energy is stored up. What
is the source of this energy? We well know that energy can not
be created or destroyed. But it can be changed from one form
into another. For example, sunlight is a form of energy. We
call it solar energy. The green plant has the unique power of
converting solar energy into the energy represented in foods,
which is chemical energy. This means, in short, that all life on
the earth depends upon sunlight. The student will gain much by
reading from Chapter I of Spoehr's Photosynthesis, published by the
Chemical Catalog Company, New York. This chapter deals with

.39

40 THE NUTRITION OF GREEN PLANTS

the origin of organic matter and the cosmical function of green
plants.

Sunlight has been called the " prime-mover of civilization/'
Discuss the significance of this statement.

Some animals, like fleas, lice, and ticks, for example, gain all
their food from animals. Show how, in the last analysis, they are
dependent upon green plants for their food, and finally upon sun-
light for their energy.

Problem 1. What is the nature of plant foods?

We are accustomed to speak of the mineral salts containing
such essential elements as nitrogen, sulphur, phosporus, calcium
magnesium, potassium, and iron as the "food" of plants. This
statement is not strictly true. A " food " for plant or animal
is a substance that can be incorporated directly by the living cells
and used as a source of energy or in making new plant substances.
The mineral salts, as such, cannot nourish the living cells any more
than can nails and tacks, if taken into the human stomach. The
mineral salts are used by the plant not as a "food "but as a
raw material from which foods are made.

The foods the substances which are capable of furnishing
energy or of building tissue for all plants and animals are identi-
cal. The chief foods of plants and animals are substances known
chemically as carbohydrates, fats, and proteins. Well-known
carbohydrates are sugars and starches. Fats occur in the liquid
state (" oils ") or solid state (" fats "). Proteins are the most
complex, chemically, of all foods. Carbohydrates contain but
three chemical elements; carbon, hydrogen, and oxygen; fats also
contain the elements carbon, hydrogen, and oxygen, but in much
different proportion from that in which they occur in carbohy-
drates; and proteins possess, in addition to carbon, hydrogen, and
oxygen, such elements as nitrogen, phosphorus, sulphur, and
others.

Exercise 14. Lists of food-storing plants. Make lists of plants which
manufacture and store large amounts of each of the following foods : starch,
sugar, oils, proteins.

Exercise 16. How to test for starch in plants. Cut a thin section of the
potato tuber, place this on a microscope slide, and cover with a drop of iodine

RAW MATERIALS USED BY GREEN PLANTS 41

in potassium iodide (0.3 gram iodine, 1.5 grams potassium iodide, 100 cc.
water). Discuss results.

Exercise 16. How to test for sugars in plants. The juice of plant tissues
may be pressed out and tested for sugar as follows Prepare Fehling's solution
thus: In bottle A dissolve 6.9 grams of copper sulphate crystals in 100 cc.
water; in bottle B dissolve 34 grams of Rochelle salt (potassium sodium
tartrate) and 12 grams of sodium hydroxide in 100 cc. water. Keep the two
solutions A and B separate. To the plant juice to be tested add equal quan-
tities of A and B, and heat slowly in a test tube over a Bunsen burner to
boiling. A precipitation of red cuprous oxide crystals indicates the presence
of glucose or other reducing sugar. Cane sugar (sucrose) does not give the
Fehling's test. However, by treating cane sugar solution with a few drops ol
concentrated sulphuric acid, it is changed to glucose and fructose, both of
which give the characteristic Fehling's test.

Exercise 17. How to test for fats in plants. Fat bodies and membranes
containing fats give a characteristic red or orange color when treated with a
solution of Sudan III (1 gram Sudan III crystals in 200 cc. 70 per cent alcohol).

Exercise 18. How to test for proteins in plants. Grind a soaked bean
in a mortar. Place this in a test tube and add a little water. Add 5 cc. of
nitric acid and heat slowly to boiling. Allow the solution to cool, pour off
the acid, and add 10 cc. of ammonium hydroxide to the bean material in the
tube. A deep orange color indicates the presence of protein. Test other
plant materials for protein.

Is there any difference between the foods of animals and those of plants?
Discuss.

What is meant when we say that a plant is " independent " or " depend-
ent"? Name five independent plants and five dependent plants.

Suggested activity. Consult dietary charts and make a collection of foods
produced by plants which are rich in carbohydrates, another of those contain-
ing large amounts of fats, and a third of those having much protein.

Problem 2. What are the raw materials used by green plants
in the manufacture of food?

We have learned that the principal foods of plants are carbo-
hydrates, fats, and proteins. Also, we have learned that only
green plants are capable of manufacturing foods. It has been
found by careful experimentation that the building of foods by
green plants proceeds by rather definite stages. The initial
process appears to be the building of a simple sugar, known as
glucose. The fact is, this sugar is the foundation material upon
which the more complex foods, such as fats and proteins, are
built. Without this simple carbohydrate, manufactured in green
cells, as a " starter/ ' the more complex foods could not be formed.

42 THE NUTRITION OF GREEN PLANTS

The raw materials in the manufacture of glucose are carbon
dioxide and water. Carbon dioxide comes from the atmosphere,
water from the soil. The process of manufacturing glucose from
carbon dioxide and water, in the presence of light, is called
photosynthesis.

It was for a long time believed that humus (decomposed plant
and animal material) in the soil was the source of carbon for
plants. But if a plant is grown in pure sand, free from humus, it
will increase in the weight of carbon. Moreover, it has been found
by experiment that green plants placed in an atmosphere from
which every trace of carbon dioxide has been removed soon
cease to grow, and that green plants cultivated in a soil from
which every trace of compounds containing carbon is removed
thrive perfectly. It is concluded that carbon dioxide is essential
to a green plant and that the source of carbon in the plant is the
carbon dioxide gas of the atmosphere. It is useless to try " feed-
ing " plants carbon by applying fertilizers to the soil. Carbon
dioxide enters the plant through small pores in the leaf surfaces;
water enters through the root hairs. These two simple chemical
compounds or raw materials are brought together in those cells of
the plant, chiefly leaf cells, which contain a green coloring matter
(chlorophyll).

After the plant has manufactured glucose sugar in its green
tissue, this sugar now becomes building material or basis for other
foods. Part of it is converted into cellulose for the formation of
the walls of new cells, and for the thickening of old walls of living
cells; a portion of it is used in the production of oil; part of it is
employed, together with nitrogen, sulphur, phosphorus, and other
compounds of simple character, to form proteins, and part of it is
oxidized in the process of respiration. The chemical elements
nitrogen, sulphur, phosphorus, potassium, magnesium, calcium, and
iron, all of which are indispensable, either directly or indirectly, in
the building of certain plant foods, all come from the soil, occurring
in the soil and entering the plant in the form of a salt. The chief
salts supplying these elements are the nitrates, the phosphates,
and sulphates.

Suggested activity. Prepare a paper on " mineral fertilizers." Name the
chief chemical substances found in those offered by local dealers, and show

INTAKE OF RAW MATERIALS 43

what results may be expected from the use of each, as large top growth, a
green lawn, or large yield of grain.

Summarizing: the raw materials used by green plants in the
manufacture of food are (1) carbon dioxide, (2) water, and (3)
mineral salts. It will be noticed that all these substances belong
to the chemical group known as inorganic. In other words, the
taw materials out of which green plants make food are inorganic
compounds. The foods belong to a group of chemicals known
as organic. These are compounds which for the most part com-
pose the bodies of plants and animals. So, the green plant con"
verts inorganic materials into organic a process peculiar to green
plants alone. Also, note that, of the raw materials, water and
mineral salts come from the soil and carbon dioxide from the air.

Before describing in more detail the processes in the building
of foods by plants, let us find out how the raw materials carbon
dioxide, water and mineral salts enter the plant, and how they
move to the parts of the plant where they are used.

Problem 3. How do raw materials enter the plant?

Water and mineral salts are raw materials which come from
the soil and enter the plant through the roots. First of all, let us
find out the important facts about the different kinds of roots and
their functions.

Kinds of root systems. There is much variation in the form,
the spread, and the depth of the root systems of plants. Two
common types of root systems are recognized. The taproot
system is well illustrated in such plants as the beet (Fig. 21), rad-
ish, turnip, parsnip, dandelion, maple, and pine. In this there is
one main root, which grows almost directly downward, giving off
numerous branch roots. The fibrous root system is seen in its
typical form in such plants as wheat, oats, corn, and other cereals.
In this, one can not distinguish a main root, but there is a great
number of relatively small roots of about the same size which
form a network. In still another form of root system, com-
mon to many of our fruit trees, there are several large roots of
about equal size which anchor the plant in the ground and which
give off numerous finer branch roots.

44

THE NUTRITION OF GREEN PLANTS

If soil conditions are favorable, the taproot by its direct down-
ward growth is able to penetrate the deeper layers of soil; for this
reason it is adapted to dry regions. The taproot of alfalfa, for
example, may extend to a depth of 10 to 12 feet, or even more.

The fibrous root system, on the other hand, is usually shallow.
Fibrous-rooted plants are employed as soil binders on ditch banks.

^ ~f ; ' x -,- ,.,,.

' l- /r *"..:> ',-*

f fvi:

FIG. 21. Tap-root system of young sugar beet. (From Robbing, in Botany

of Crop Plants.)

steep hillsides, and in other situations where the soil is likely to be
moved away by rain or wind. In the reclamation of eroded soils,
fibrous-rooted plants are used extensively.

Exercise 19. Field study of root systems. Study in the field and report
on the root systems of a number of common plants. Along the steep banks
of streams or where erosion is rapid, one may be able to trace the root systems

INTAKE OF RAW MATERIALS 45

of trees and shrubs. (Refer to Root Development of Field Crops by John E.
Weaver, McGraw-Hill Book Company, New York.)

Factors which influence the growth and character of the root
system. The depth and spread of the roots, although characteris-
tic of the kind of plant, are nevertheless influenced by environ-
mental conditions, chiefly the water content of the soil, the air
supply, and the available mineral nutrients. These in turn are
modified by tillage, fertilizers, crop rotation, irrigation, and
drainage.

In most of our fruit plants the root systems extend as deeply
in the soil as the air supply of the soil will permit, providing there
is sufficient moisture. It is undoubtedly true that air supply of the
soil often limits root development. It should be repeated here
that all living cells must have a supply of oxygen in order to live;
the root hairs and other active cells of the roots must secure most
of their oxygen from the soil air which immediately surrounds
them; sufficient oxygen is probably not conducted long distances
through the tissues of the plant from above to below ground. The
absence or scarcity of root hairs in very wet soil, and in water, can
probably be attributed to poor oxygen supply. In a water-soaked
soil, the air spaces are filled with water.

Excessive irrigation may produce an actual decrease in the
yield of a crop, chiefly because it produces a soil condition in
which aeration is faulty. It has been said that the best irrigation
practice involves the most effective compromise between too
much water and too little air. Why do florists use porous earthen-
ware pots with a hole in the bottom?

Soil fertility influences the root development of a plant. It has
been found that " crops grown in soil of high fertility have roots
that are shorter, more branched, and more compact than those in
similar but less fertile soil." Moreover, it has been shown that,
where roots in their growth come in contact with a fertilized layer
of soil, they not only are more branched, but they also are slow in
penetrating the soil below.

Transplanting and root pruning encourage the development
of branch roots. In transplanting, many root tips are injured
and, as a result, branch roots are stimulated to develop. Thus, a
compact root system is formed.

46

THE NUTRITION OF GREEN PLANTS

The extent of root systems. The extent of roots is often much
greater than that of the top growth. This is true of many native

species of plants and also of a
number of cultivated plants. For
example, in the sugar beet, the
main root may extend to a depth
of 5, 6, or 7 feet, and give rise to
numerous branches which spread
laterally several feet. Corn has a
root system which may occupy as
much as 200 cubic feet of soil.
Weaver points out that a corn
plant in the eight-leaf stage has
from 8,000 to 10,000 lateral roots
from the 15 to 23 main roots. A
corn plant 5 weeks old, grown in
a fertile, moist soil, developed a
root system the absorbing area of
which (excluding the root hairs)
was 1.2 times as great as the area
of the top, and in a dry soil 2.2
as great as the area of the top.
Weaver also found that the root
area of Turkey Red winter wheat
exceeded that of the top by 10 to 35 per cent. The roots of com-

FIG. 22. Seedling of corn. The
primary or temporary roots (C),
adventitious or permanent roots
(B), and the stem structure (A)
which may be long or short de-
pending upon the depth at which
the seed was planted.

FIG. 23. The sweet potato is a fleshy storage root.

mon asparagus may extend to a depth of 8 or 9 feet and as far
laterally, occupying the soil very completely. The roots of a

INTAKE OF RAW MATERIALS 47

7-year-old apple tree were known to spread horizontally more
than 12 feet and to a depth of 9 feet.

Kinds of roots and their functions.

Exercise 20. Primary and secondary roots. Examine the roots of
radish and wheat seedlings of various ages. Note that in each one there is,
first of all, a primary root which develops from the radicle or rudimentary root
of the embryo of the seed from which the plant grew. And in each there are
small secondary roots, branches of the primary root. Compare older seed-
lings of radish and wheat. Does the primary root of the radish become the
principal root throughout the life of the plant? In older seedlings of wheat,
observe that numerous fibrous roots arise from the base of the young stem.
These come to form the permanent root system of the wheat plant, whereas
the primary roots (those from the embryo) are but temporary. The perma-
nent roots of wheat are said to be adventitious (Fig. 22).

Exercise 21. Adventitious roots. Make stem cuttings of willow, gera-
nium, or begonia; place in moist sand, and observe the position and nature of
the roots which arise at the cut surface. These are adventitious roots.

Exercise 22. Prop roots. Observe the prop roots of corn, the clinging
aerial roots of ivy, and the aerial roots of orchids.

Exercise 23. Root crops. Make a list of plants which are grown pri-
marily for their roots.

QUESTIONS

By way of review of the studies thus far made of roots, write out answers
to the following:

1. Name two kinds of roots as to origin, giving examples of each. 2. Name
three kinds of roots as to the medium in which they grow, with examples of
each. 3. Name four kinds of roots as to form, with examples of each.
4. Give the functions of roots, and after each function name a plant whose root
performs that function. 5. From what part of the embryo does the primary
root grow? 6. What is the usual direction of growth of primary root? Of
secondary roots? Give advantage of each direction to the plant. 7. Name
plants whose roots live but one year, others whose roots live two years, and
still others whose roots live many years.

Structure of roots, and their adaptation to absorption of raw
materials. Now that we have in mind the principal kinds of roots
and root systems, let us inquire into their structure and how they
are adapted to the intake of raw materials from the soil. We
have learned that anchorage is one of the most important func-
tions of the roots; also that many roots store food, and that a few
may be used to propagate the plant. A primary root function is
absorption.

48

THE NUTRITION OF GREEN PLANTS

Exercise 24. Root structure external features. Germinate seeds of
red top or blue grass by dropping the dry seeds on the surface of water in a
large flat dish and keeping at room temperature for three to five days. Study
the primary roots in water in a watch glass when they are % to % inch long.
Place the watch glass on the stage of a compound microscope and examine with
low power to see the principal structures. Note the root cap, the central
conducting cylinder surrounded by a sheath, the cortex, and the root hairs.
Make a diagrammatic sketch of the root, and label all parts.

Exercise 25. Root structure in-
ternal features. Mount young roots
as above in water on a microscope
slide, and cover with cover-slip. Ex-
amine with low power of compound
microscope. Identify the parts of the
root as observed in the preceding ex-
ercise. While watching, gently press
on the cover-slip so as to crush the

FIG. 24. A young root tip cut
lengthwise. Observe the root
cap, the central conducting cyl-
inder, and the cortex.

FIG. 25. A portion of a root showing

stages in the development of a root

hair. A root hair is an outgrowth of

an epidermal cell.

root. In the region of the central cylinder one should be able to see the
vessels. These are the structures which conduct water and mineral salts
up through the roots.

Exercise 26. Root structure as seen in cross-section. Examine prepared
cross-sections of young roots. Observe the following zones or regions: (a) the
epidermis with its root hairs; (&) the cortex, a sheath surrounding the (c) cen-
tral cylinder, and within the central cylinder, strands of phloem and xylem.

INTAKE OF RAW MATERIALS 49

Phloem possesses the structural elements which conduct foods, whereas xylem
has the structural elements which conduct water and mineral salts.

Root hairs (Fig. 25). We are familiar with the fact that, in
ordinary land plants, the very important work of absorbing water
and mineral nutrients from the soil is confined to the roots. But
not all root surfaces can absorb. On the slender, thread-like roots
are innumerable outgrowths, the root hairs, and these are the prin-
cipal absorbing organs. In some plants it is almost impossible
to see them without a magnifying lens. However, they may stand
so thickly on the fine roots as to form a white fuzzy growth.
The younger parts of the very fine rootlets which have not become
corked over absorb to a limited extent; but in the majority of
plants by far the greatest amount of absorption is carried on by
the root hairs. Even the fine rootlets soon become covered with a
corky layer which is impervious to water.

Transplanting destroys root hairs. If a plant is transplanted
or disturbed in cultivation or at thinning, it wilts. We may see
no apparent injury to the roots. However, on closer examination
we observe that the root hairs have had their connection with the
soil particles broken, and that many of the finest rootlets bearing
root hairs are destroyed. The plant does not recover until new
root hairs are developed.

It is well known that it is dangerous to transplant young trees,
shrubs, or vines when they are clothed with leaves and are actively
growing. In this condition the plant demands more water than
when in a dormant state. If the fine rootlets and root hairs are
destroyed, as is largely unavoidable during transplanting, the
demands for water are greater than the roots can supply. Exces-
sive watering will not make up for the loss of roots and root hairs
by trees, shrubs, and vines transplanted during the growing sea-
son. The roots may be surrounded by nearly saturated soil, but
in the absence of root hairs, they can not absorb adequate amounts
of water. In transplanting it is the usual practice to reduce the
water-losing area (leaf surface) by pruning.

Exercise 27. Relation of root hairs to soil particles. Carefully remove
seedlings growing in fine sand. Observe the relation of root hairs to soil
particles. (Fig. 26.)

50

THE NUTRITION OF GREEN PLANTS

The soft, delicate root hairs, in growing, wrap themselves about
and spread over the soil particles coming in very close contact
with them. Thus, they expose a maximum surface to the parti-
cles, from which they absorb both water and mineral salts. In a
soil that is well drained and contains the
proper amount of moisture, the water occurs
as very thin films and as wedge-shaped
masses at the points of contact of the soil
particles, and in this water the various min-
eral salts are dissolved. Consequently, by
growing about the particles the roots place
themselves in the best position for getting
water and salts.

Root hairs increase the absorbing sur-
face of a plant. A plant with hundreds of
thousands of root hairs on its fine rootlets has
a tremendous total absorbing surface. The
fine rootlets alone, without the root hairs,
would have a relatively small total surface.
It has been computed that a corn plant with
root hairs has an absorbing surface of about
6 times that of one from which the root
hairs have been removed.

Duration of root hairs. Root hairs have
a very short life. As the rootlet upon which
they grow pushes its way through the soil by
growth at the tip, new root hairs are contin-
ually formed just behind the tip, and the
oldest root hairs farther back are constantly
dying off. Any one individual root hair lives
only a day or so. Thus there is a constant
new supply of rapidly growing root hairs.
Moreover, the roots are continually exploring new soil
regions.

Exercise 28. The structure of the root hairs. Study the structure of
root hairs with a compound microscope. Note that each root hair is a tube-
shaped body, growing out from an epidermal cell of the rootlet. In fact,
each root hair is a single plant cell with the power to carry on the various life

FIG. 26. Wheat
seedling showing soil
particles clinging to
root hairs ; note that
the root cap is free
of root hairs.
(From Robbins, in
Botany of Crop
Plants.)

INTAKE OF RAW MATERIALS 51

processes. It is a living structure, The living material and the cell sap are
surrounded by a non-living wall. The wall is lined on the inside with a very
thin membrane of living material, the protoplasmic membrane. This mem-
brane is an extremely important part of the cell, as we shall see later. Make a
drawing of a root hair and several bordering cells.

Substances from the soil enter the plant chiefly through the
root hairs. It should be emphasized that almost all materials
which enter the plant from the soil must pass through the wall
and living membrane-lining of the root hair. From the root hair,
the absorbed materials are passed on to the other root cells, and
from them, into conducting tubes and thence to various parts of
the plant.

It used to be thought that plants " suck up " tiny particles
of soil, and " feed " upon them. It is now known that the plant
is incapable of absorbing solid particles. All materials taken in
by the plant from the soil must be in a liquid state, that is, they
must be in solution. Otherwise, they cannot pass, or diffuse,
through the root-hair wall and the living membrane which lines it.

Give the structural features of a root which fit it to the process
of absorption. What is the function of the root cap? Compare
soil roots with roots of water plants, and also with the air roots of
an orchid. Explain the differences. How do root hairs differ from
rootlets in structure and in place of origin? Do root hairs become
rootlets?

The process of absorption. The principal point to kedp in
mind in connection with absorption by roots is that all substances
which enter the root hairs from the soil must be in solution.
Water is the solvent. The second important point is that all
substances which pass from the soil to the inside of the root hair
must pass through the cell wall of the root hair and also the living,
protoplasmic membrane which lines the wall. Several simple
experiments should first be performed by the student so that he
will better understand the process of absorption.

Exercise 29. Diffusion. In the bottom of a beaker or glass tumbler of
water place a few crystals of copper sulphate (blue vitriol). If the vessel is not
disturbed, the copper sulphate goes gradually into solution. At first the
solution is a deeper blue in the neighborhood of the crystals, the color being
less and less blue as the distance from the crystals increases. This indicates

52 THE NUTRITION OF GREEN PLANTS

that particles (molecules) of copper sulphate are gradually moving out into
the water. After a long time, the whole solution becomes a uniform blue
color, indicating the equal distribution of copper sulphate molecules through-
out. In the solution there are two components, water and copper sulphate.
Water is the solvent, copper sulphate the solute. In this experiment the
movement of copper sulphate molecules into the water from a region where
they are in greater concentration to one where they are in less concentra-
tion, and the movement of water molecules into the copper sulphate from a
region where they aie in greater concentration to one where they are in less
concentration, illustrate diffusion. Keep in mind that diffusion plays a most
important part in the absorption of materials by the roots.

Exercise 30. Diffusion through membranes. In the preceding experi-
ment, the two substances, water and copper sulphate, were not separated
by any kind of a membrane. Now let us devise an experiment which resem-
bled, in part at least, the situation represented in the root hair. Recall that the
sap in the root hair is separated from the soil solution by two membranes:
(1) the cell wall, composed of non-living matter called cellulose; and (2) the
living protoplasmic membrane. The two membranes have very significant
differences in their behavior.

With a carpenter's 1-inch bit, drill a smooth hole down into the middle of a
fresh carrot, being careful that the bit does not cut through the surface at any
point. Connect a one-hole No. 6 rubber stopper to the end of a 5-foot length
of glass tubing of small diameter. Pour corn syrup into the carrot, and fit the
stopper into the hole so that the syrup extends into the glass tubing to a point
above the stopper. Support the carrot in a jar of water so that the surface of
the water is on a level with the surface of the syrup in the tube. After a time
you will observe that the liquid in the tube has risen. We are forced to the
conclusion that more material passed into the sugar solution than passed out
of it. Both kinds of membranes are present in the shell of carrot surrounding
the sugar solution. The living protoplasmic membranes of the carrot are
semi-permeable. That is, they allow some substances to pass through them
more readily than others. In this case the sugar molecules pass through the
membrane only slowly, if at, all, and since there is a relatively greater concen-
tration of water molecules on the outside than in the syrup on the inside the
more rapid diffusion of water is toward the inside.

The protoplasmic membrane of a root hair is a semi-permeable
membrane. The cell wall is wholly permeable, that is, does not
inhibit the movements of any kind of molecules through it. Con-
sequently, in the movement of materials into the root hair, the cell
wall can be disregarded. The root-hair sap is a solution containing
not only sugar and other organic compounds, but also a number of
different salts, all in solution in water. The soil solution contains
many different mineral salts in aqueous solution. These two

INTAKE OF RAW MATERIALS 53

solutions that of the root-hair sap and that of the soil are sepa-
rated by a semi-permeable membrane, the protoplasmic membrane,
which does not allow all substances to move through it with equal
ease and speed.

Under natural conditions, the cell sap of the root hair is
always more concentrated than the soil solution; that is, the water
molecules are in less concentration inside the root hair than out-
side. We learned from the foregoing experiment that water
passed from the region where the water molecules are more con-
centrated into a region where they are less concentrated. Apply
this to the movement of water from the soil into the root hair.

Exercise 31. Plasmolysis. This experiment throws further light on the
movement of substances through semi-permeable membranes. Immerse
whole fresh leaves of the water plant Elodea into a strong solution of ordinary
table salt or sugar. The solution can be added to the leaf on a microscope
slide, covered with cover-slip, and observed with the high-power compound
microscope. After a while it will be noticed that the contents of the leaf
cells are shrinking, and pulling away from the wall. This shrinking, known
as plasmolysis, is due to the withdrawal of water from the cells by the solution
outside. Water continues to move out of the cell just as long as the solution
outside is more concentrated than that in the cell. In the plasmolyzed cell
what solution is between the cell wall and the protoplasmic membrane?

Exercise 32. Turgidity of cells. The swollen state of cells is called tur-
gidity. We know that the crispness of the leaves and the rigidity of the young
parts of plants is due to the turgid condition of the many hundreds of
thousands of individual cells which compose it. We can understand this
behavior better if we consider the following exercise. When slices of potato
are placed in water, they remain rigid. All cells are well rilled with water, and
as a result are swollen. The combined effect is a turgid slice. If, on the other
hand, the slices of potato are placed in a strong salt or sugar solution, they
soon become limp and flexible; all cells lose water; it passes from them into
the strong solution outside where water molecules are in less concentration.
As a result, each cell is like a partially deflated automobile tire, and the whole
slice is of the same character. It is not so much the nature of the substance as
its concentration which determines the movement of water in or out of the cell.
For example if the slices of potato are put in a salt or sugar solution, the strength
(concentration) of which is less than that of the sap in the potato cells, they
will remain fresh* instead of losing water they will absorb it. The cells of the
slices will lose wacer only when the strength of the solution in which they are
immersed is greater than that of the solution in them, that is, when the water
molecules are in greater concentration inside the cells. Water tends to move
from a solution of low concentration to one of higher concentration. In most

54 THE NUTRITION OF GREEN PLANTS

agricultural lands, the soil solution has a lower concentration than has the
solutions in the plant. Consequently, the movement of water is from the
soil to the plant.

The normal, active root hair is swollen and distended like an
automobile tire inflated with air. It is filled with water, various
mineral salts in solution, and other substances, all of which render
it swollen. It does its best work only in this distended condition.
If, through lack of water, the plant wilts, the root hairs, as well
as other cells of the plants, collapse to a certain extent, and in this
state, they do not perform their work properly. Any part of a
plant in a wilted condition is incapable of making growth. It may
remain alive and be able to recover if given water, but it will not
grow. Withholding water from a plant to the extent of causing
prolonged wilting cannot fail to retard its growth.

Conditions which influence the rate of absorption. Several
conditions have a marked influence upon the rate of water absorp-
tion by root hairs. In the first place, when the amount of moisture
in the soil reaches a low point, the rate at which it is absorbed by
the roots is diminished. There are forces in the soil which hold the
water, and unless the forces of absorption exceed these soil forces,
water will not be taken in by the roots.

The temperature of the soil is another important factor deter-
mining the rate of water intake. The rate is decreased by lowering
the temperature, and most plants cease to absorb at or slightly
above freezing. Place a small potted plant in a dish of ice water,
and another in water of room temperature. After a time note
results. Explain.

The air supply of the soil also influences the absorption rate.
Root hairs do their best work only when they are well supplied
with air. In soil that is compacted, or so wet that air is excluded
from the soil spaces, absorption is slowed down.

The strength or concentration of the soil solution is a very
important condition determining the rate at which water from
the soil enters the plant. Normally, the solution in the root hair is
of greater concentration than that in the soil. Under this con-
dition, water moves into the hair through the living membrane
which separates the two solutions. If, on the other hand, the soil
solution should become more highly concentrated than that in the

INTAKE OF RAW MATERIALS 55

root hair, water would move from the root hair to the soil, and a
wilting of the root and entire plant would follow. In an alkali
soil, the soil solution surrounding t! e root hairs is often quite con-
centrated ; at times its strength may approach that of the sap of the
root hairs; under these conditions, water movement inward is slow,
and the plant finds it difficult to take in as much water from the
soil as it loses through the leaves to the air. Water continues to
move inward as long as the concentration of the solution inside
of the root hair exceeds that of the solution in the soil on the out-
side. In other words, if the cell sap be denser than the soil solu-
tion, that is, poorer in water, water will be absorbed from the soil
solution.

It is well known that grass, weeds, and other plants can be
killed by drenching the soil in which they are growing with a
strong salt solution. In this case the plant is killed by a failure to
absorb sufficient water or by an actual loss of water, rather than
poisoned by the chemical. The root hairs, being surrounded by a
soil solution which has a greater concentration than that of the
root-hair sap, die because water is withdrawn from them. Soon
the entire plant dies, for it continues to lose water through its
leaves, while at the same time absorption of water is stopped.

The rate of water absorption is also increased if the rate of
water loss from the leaves is increased, providing there is an ample
supply of soil water to draw upon.

The movement inward of any particular salt appears to depend
chiefly upon the available supply in the soil and the rate at which
it is used in the plant. The essential elements which are absorbed
by the plant are soon changed into new substances, and so their
concentration in the plant is kept constantly less than that in the
soil solution.

Enumerate the factors which influence the rate of water absorp-
tion by plants.

Most substances in the root are not lost to the soil. It should
be pointed out that, whereas substances pass from the soil through
the root-hair membrane into the root, most substances in the
root hairs are unable to pass back into the soil. Carbon dioxide,
in the ordinary process of respiration of root cells, continually
passes from the roots to the soil; and, under certain

56 THE NUTRITION OF GREEN PLANTS

water also may be lost to the soil. It has been thought by some
that under certain circumstances sugar in the sugar-beet root
passed from the root into the soil. This is not the case. The
percentage of sugar in the root may go up and down as a result
of different amounts of water in the beet, but the actual ounces
of sugar in the root do not become less, except under very unusual
circumstances when the stored supply is called upon to make an
abundance of new leaf growth.

Summary. We may summarize our discussion of the nutrition
of green plants thus far as follows: (1) The foods of plants, as well
as those of animals, are organic, and chiefly carbohydrates, fats,
and proteins. (2) These foods are built in the plant body, utilizing
as raw materials carbon dioxide (from the air) and water and
mineral salts (from the soil). (3) Water and mineral salts are
absorbed from the soil by the roots, which are structures well
adapted to the process of absorption.

QUESTIONS

1. Define solute, solvent, solution.

2. Why is it necessary to understand diffusion in order to study the process
of absorption by roots?

3. Explain why plants growing in cold bogs have difficulty in absorbing
water.

4. In what ways are alkali soils injurious to plants?

5. What is meant by the "selective power" of root hairs?

6. When slices of red garden beets are washed, and then placed in fresh
cold water, the water does not become red. But when slices of such beets are
boiled, the water soon becomes red. Explain.

7. Why do young plants become limp when they wilt?

8. Why does a dried prune swell when placed in water?

9. Why is it possible to kill Canada thistles or other perennial weeds by
the use of a liberal application of salt to the soil?

10. Is it possible for a grower to injure or kill plants by applying to them
too much mineral fertilizer? Explain.

11. Explain why corn on land which has been flooded loses its green color
and turns yellow.

Types of leaves. We now have to consider the intake of carbon
dioxide, and the plant structures (leaves) chiefly concerned in this
process.

INTAKE OF RAW MATERIALS

57

Exercise 33. Types of leaves. The student should collect and bring into
the laboratory a considerable variety of leaves and have them before him
while reading the following paragraphs. A suggested list is as follows: oak,
maple, rose, clover, pea, cherry, ash, locust, horse-chestnut, strawberry, corn,
iris, lily-of-the-valley.

All common leaves have a broad, thin blade, which is covered
above and below by a thin, transparent epidermis (Fig. 28). The
blade has two portions, the veins, and the soft green tissue known
as mesophyll supported by the veins.
Look up the literal meaning of the
word mesophyll. Most leaves have
a stalk or petiole which attaches the
blade to the stem, and through
which materials are conducted to
and from the blade. Some leaves,
such as those of grasses and lilies,
have no petioles, and are said to
be sessile (meaning, sitting). Some
leaves also have a pair of leaf-like
outgrowths at the leaf base where
it joins the stem. These are known
as stipules.

The arrangement of veins in a
leaf is spoken of as venation. This
varies in different kinds of leaves.
For example, in grasses, lilies, iris,
etc., the veins that are easily seen
run parallel to each other the full
length of the blade. This is known
as parallel venation. In most broad-leaved trees and shrubs, also
in many herbs, the veins that are easily seen branch frequently
and join again, so that they form a network. This is known as
net venation. In many leaves with net venation there is a single
midrib or primary vein, from which the smaller veins extend,
somewhat like the divisions of a feather. Such leaves are said
to be pinnately veined. In other plants there may be several
principal veins spreading out from the upper end of the petiole.
Such leaves are palmately veined.

FIG. 27. Drawing of the blade
and a portion of the petiole of a
geranium leaf. Note the pal-
mately-arranged veins which ra-
diate out from the point of attach-
ment of the petiole. Explain the
roles of the petiole, the veins,
and of the thin, expanded blade.

58

THE NUTRITION OF GREEN PLANTS

Leaves vary greatly in form. A simple leaf (Fig. 27) has a
single blade, either with or without a petiole, or stipules. A
compound leaf has a leaf blade consisting of a number of separate
leaf-like parts, called leaflets. How can you tell a leaf from a
leaflet?

Leaves also vary in size, in shape, in nature of the margin,
in kind of lobing, in number of leaflets, and in many other charac-
ters. In fact, there are hundreds of terms descriptive of leaves.

Exercise 34. Terms describing leaves. Using the following form, check
off in the proper columns the terms descriptive of some 20 different kinds of
leaves collected by you.

Name
of
plant

Simple

Com-
pound

Petio-
late

Sessile

Paiallel
vena-
tion

Netted
vena-
tion

Pin-

nately
veined

Pal-
mately
veined

Exercise 35. Types of leaves. Make accurate sketches of the following
and label the parts:

1. Simple pinnate leaf.

2. Simple palmate leaf.

3. Compound pinnate leaf.

4. Compound palmate leaf.

5. Parallel-veined leaf.

Structure of leaves and their adaptation to absorption of carbon
dioxide. We have seen that the leaf blade is a thin, flat structure
which exposes a large amount of tissue to the gases of the atmos-
phere. In this particular it would appear to be adapted to absorp-
tion of carbon dioxide. But let us examine the microscopic struc-
ture of the leaf and see if in any other ways it is adapted to this
purpose.

Exercise 36. Leaf structure. With binocular dissecting microscope,
using 20 to 50 magnifications and strong illumination, both transmitted and
reflected, examine both surfaces of a number of different kinds of leaves,

INTAKE OF RAW MATERIALS

59

noting form of epidermal cells, stomata, epidermal hairs, and other gross
characters. The leaves of Tradescantia (Wandering Jew) have very large
stomata.

Exercise 37. Epidermis of leaves. Strip pieces of the epidermis from
the lower and upper surfaces of some leaf, such as Tradescantia. Mount
these with outer surface up, and examine with compound microscope. Are
the stomata equally nu-
merous on both surfaces?
Are epidermal cells trans-
parent? Make a drawing
of several stomata and 4
to 8 epidermal cells ad-
joining. Label. See Fig.
28.

Thus we sec that,
in addition to the great
surface exposed by
leaves, there are open-
ings in the epidermis
through which carbon
dioxide and other
gases may pass in.
Now, lot us exam-
ine the internal struc-
ture of the leaf. For
this purpose we need

FIG. 28. At right, the upper epidermis of a
geranium leaf is shown. A portion of the lower
epidermis of the same leaf is shown at left. Both
are made up of irregularly shaped cells. Bean-
shaped cells, the guard cells, which fit together
in such a way as to leave a slit, a stoma, be-
tween each pair are seen at left. Why are
stomata usually found only on the under side
of the leaf?

thin cross-sections of the leaf.

Exercise 38. Anatomy of leaf. Examine with compound microscope
prepared cross-section of somo loaf. Compare with Fig. 29. Note the (1) epi-
dermal coverings, upper and lower; (2) the stomata with their guard cells;

(3) the mass of green tissue between the epidermal layers, known as mesophyll ;

(4) the veins; and (5) large air spaces between the mesophyll cells. Make a
diagram of the leaf as seen in cross-section, labeling all parts. From your obser-
vation of the slide, answer the following questions: (1) How do guard cells
differ from other epidermal cells? (2) Is it possible for carbon dioxide which
may come through stomata to diffuse throughout the leaf and come in contact
with even the innermost cells? (3) Do you think that the leaf structure is such
as to adapt it to absorption of carbon dioxide? Why?

From our study of the leaf, we see that it has three kinds of
tissue: (1) the epidermis, (2) the mesophyll, and (3) the veins.
In the epidermis are stomata, each with two guard cells. These

60

THE NUTRITION OF GREEN PLANTS

*7 Upper
^Epidermis

Palisade
Laijer

Vein

Sponqq
Tissue

tuith Air
Spaces

Guard Cell
Stoma

Louuer
Elpidermis

FIG. 29. Cross-section of a typical leaf, showing its internal structure.

change their shape, so that the stoma may be closed or open.
Further on in our study of water loss from the plant we shall have
more to say about stomatal movement. The mesophyll is com-
posed of two kinds of tissue
(Fig. 29): (1) the palisade
parenchyma, and (2) the
spongy parenchyma. Pali-
sade parenchyma cells have
their long axes at right angles
to the leaf surface. There
may be one to several layers.
Spongy parenchyma usually
forms somewhat more than
half of the mesophyll. The
cells are not elongated. Cer-
tain parenchyma cells may
form a single-layered sheath
FIG. 30. This leaf has been treated with surrounding the veins. All
a caustic soda solution to destroy the soft cells of the pa li sa( Je par-
leaf tissue, thus making readily visible , ,, f ,,
the framework of veins. These veins <*<*&**> a11 <*lls of the
not only support the soft food-making spongy parenchyma, except
tissue, but they carry materials. in certain plants those which

MOVEMENT OF RAW MATERIALS 61

surround the veins, and guard cells are abundantly supplied with
chloroplasts. These possess a green pigment known as chloro-
phyll. The chloroplasts, by virtue of their chlorophyll, play a most
important part in the manufacture of food by the plant.

The process of carbon dioxide intake. Only about 3 to 4 parts
in 10,000 of the atmosphere is carbon dioxide. There appears to be
nothing to prevent the diffusion of the gases of the atmosphere,
including carbon dioxide, through the stomata, and into the inter-
cellular spaces of the mesophyll tissue. We know that carbon
dioxide gas is soluble in water, and that the walls of mesophyll
cells are wet. Therefore, the surface film of water on the outside
of mesophyll cells contains a quantity of carbon dioxide in solu-
tion. In the diffusion of carbon dioxide, just like that of copper
sulphate described in a previous exercise, the molecules move
from a point where there are many of them to a point where there
are fewer of them. Therefore, if carbon dioxide is being used up in
the mesophyll cells, that which is in solution on the surface film
of water readily diffuses through the wall and protoplasmic mem-
brane, and comes into contact with the cell contents. As a matter
of fact, carbon, dioxide gas diffuses into the chloroplasts.

Problem 4. How do raw materials move in the plant?

We have discussed the movement of the raw materials used by
the green plant in the manufacture of food into the plant from the
outside world how water and mineral salts enter the roots, and
how carbon dioxide enters the leaves. These raw materials are
made into food chiefly in cells that are green, that is, contain chlo-
rophyll. The leaves are the chief food-making organs of the plant.
Consequently, it will be necessary that we explain how water and
mineral salts, once they are in the root hairs, move from the root
hairs to the leaves. In order to explain this movement, we should
know something of the structure of stems as well as of roots, for it
is in them that the materials move to the leaves.

Types of stems. We are familiar with the different kinds of
common stems. For example, there are stems of the woody type,
represented by the trees and shrubs, and stems of the herbaceous
type, represented by the many thousands of different kinds of

62 THE NUTRITION OF GREN PLANTS

annual plants. Herbs have comparatively little hard, woody
tissue.

Exercise 39. Types of stems. Make a list of ten plants of woody type
and another of ten of the herbaceous type.

As to form, we recognize three common types of stems, namely,
erect, prostrate, and climbing. The majority of plants have erect
steins. The student can readily name many among herbs as well
as among trees. Such plants as cucumber, squash, field morning-
glory, and strawberry have prostrate stems. Examples of plants
which climb upon other plants and depend upon them for mechan-
ical support are grape, woodbine, hop, and scarlet runner bean.

It is obvious that one of the chief functions of all stems, in
addition to that of conduction of materials, is to support the leaves,
raising them into the light and air. But many stems are also
food-storage organs. For example, in the stems of all trees and
shrubs, large amounts of food are stored. Also, many plants have
sterns underground, as well as above ground, and such stems are
usually storehouses for food. Examples of such are the tubers
of Irish potato, the bulbs of onion and Narcissus, the corm of
gladiolus, and the rhizome of Solomon's seal, of Canada thistle,
of field morning-glory, and many other plants. Of these we will
speak in another unit.

Structure of stems. Simple water plants have little need for
conducting tissue, for the plants are surrounded on all sides by
water, containing in solution the various mineral salts necessary
for plant life. Likewise those plants which grow close to moist
soil, such as mosses and liverworts, do not require an extensive con-
ducting tissue. But in land plants, with their leaves raised into
the air, and with a great amount of tissue far distant from the
source of water and mineral salts, there is need for an efficient
means of conducting these materials throughout the plant.

In our study of the structure of stems, let us take three differ-
ent cases: (1) the herbaceous stem of sunflower, (2) the woody
stem of box elder, or cottonwood, or other common broad-leaved
tree, (3) the stem of corn.

Exercise 40. The structure of the herbaceous stem of sunflower. Examine
prepared cross-sections and lengthwise sections. Observe the following parts:

MOVEMENT OF RAW MATERIALS 63

(1) epidermis, a single layer of cells which protect the underlying tissues
from drying out and from mechanical injury; (2) the cortex, composed chiefly
of parenchyma tissue, also fiber groups, and just beneath the epidermis, a
tissue with thickenings in the cell corners, known as collenchyma; (3) the
vascular bundles, each with an outer region, the phloem, an inner more
woody region, the xylem, and between them a growing tissue, the cambium;
(4) central pith; and (5) broad vascular rays extending between the vascular
bundles. Make a large diagram of the cross-section showing the relationship
of stem regions, without showing cellular detail. Label. Compare the
appearance of the different tissues as seen in cross- and lengthwise sections.

Exercise 41. Structure of a woody stem. Study cross- and lengthwise
sections of a one-year-old woody stem (Fig. 31). Note three distinct regions
the bark, the wood, and the pith. The bark may be peeled away from the
wood. It separates from the wood along a region known as the cambiunic
The outer part of the bark is a layer of cork, beneath which are several layers of
cells containing chlorophyll. The inner part of the bark is the phloem. In the
woody part of the stem note the vascular rays, radiating from the pith. Make a
diagram of the cross-section, labeling all parts.

Exercise 42. Structure of the corn stem. Study prepared cross-sections.
Observe the epidermis, the cortex, and a large number of vascular bundle?
scattered in parenchyma tissue. Make a diagram, labeling all parts.

Before studying in greater detail the structure of the stem, let us perform
an experiment which will demonstrate in which tissues of the stem water moves.

Exercise 43. To find out through what region water rises in stems. Place
a fresh, leafy twig with the cut end in a solution of eosin. After several hours,
cut sections at intervals along the stem and observe the course the liquid has
taken. Repeat the experiment, using some herbaceous stem. Are the veins
stained red? In another experiment remove a ring of bark from a leafy twig,
and place the cut end, below the girdle, in water. Do the leaves wilt? From
these experiments, what is your conclusion as to the tissues in which water is
carried?

Exercise 44. Structure of vascular bundles. Study once more the stem
slides of Exercises 40, 41, and 42, paying particular attention to the vascular
bundles. Observe that the bundles of sunflower and the woody stem have a
cambium between the phloem and xylcm, whereas there is no cambium in the
vascular bundles of corn. The principal structural elements of the xylem are
the vessels in which water and mineral salts are conducted, wood fibers which
give strength and support, and wood parenchyma, a tissue which stores food.
The principal structural elements of the phloem are the sieve tubes in which
foods move, companion cells adjoining the sieve tubes, phloem fibers, and
phloem parenchyma. Make a drawing of a single vascular bundle of corn or
sunflower as seen under the high power of the compound microscope. Label.

You will remember from your study of root structure that there
is a central cylinder in which vascular tissue is found, a cortex, sur-

Sec-Han of Second Year Ste

VAaqnified

FIG. 31. Growth in diameter of a stem of box elder. Sections made June 25,
1934, at different points of the same branch. After studying the diagram
in connection with the description in the context of how growth in diameter
of stems takes place, you should be able to answer the following questions:
At what place in the stem are new elements of wood and new elements of
bark added? How can a cambium cell form a xylem vessel in the wood?
How can a cambium cell form a phloem (sieve) tube in the bark? What
makes it possible to tell from the section where growth ended in the wood
at the end of the growing season of 1933? How can you tell where growth
in diameter began in 1934? Why is the 1933 layer of wood thicker than
the 1934 layer? Why is the bark in a two-year-old stem thicker than that
of a one-year-old stem?
64

MOVEMENT OF RAW MATERIALS 65

rounding the central cylinder, and an epidermis, including root hairs.
These structures of the root connect corresponding structures in
the stem. After water has moved into the root hairs, it proceeds by
diffusion from root-hair cells to adjacent cortex cells, and by the
same process from one cortex cell to another until it reaches the
vessels in the xylem of the vascular region. In the vessels it
moves rapidly upward through the root into the stem, and thence
on upward in the vessels of the stem to the leaves. The veins
of the leaves are vascular bundles, and they are directly con-
tinuous with the vascular bundles of the stem. So that water
and mineral salts move out through the petiole of the leaf into
all the veins. From the veins, these raw materials diffuse outward
into mesophyll cells.

Exercise 45. Leaf veins. (Fig. 30.) Treat small thin leaves with warm
8d per cent alcohol until the chlorophyll is gone. Place in a shallow dish
of water, and examine with high magnification of binocular dissecting micro-
scope. Observe the network of veins, and the vein endings. Picture to
yourself the movement of raw materials outward in these small veins, and
their diffusion into adjoining mesophyll cells.

The process of movement of raw materials in the plant. In

the last paragraph we have given the path of movement of water
and mineral salts from the soil to the mesophyll cells of the leaf.
This path briefly is as follows; soil root hair cortex vessels
of root vessels of stem vessels of leaf petiole vessels and
tracheids of leaf veins mesophyll cells of leaf.

What are the forces concerned in the movement of a body of
water upward in the plant? When we consider the height of our
tallest trees, we realize that it must require an enormous force to
bring water to their leaves. We should say at the start that there
is still much controversy as to the cause of the rise of sap in plants.
We will give briefly what seems to be the most plausible and widely
accepted explanation.

Exercise 46. The pull of water in stems due to loss of water from the
leaves. Arrange a demonstration of the lifting power of transpiring leaves.
Secure a shoot with a long stem, and cut it off to a proper length under water.
Fill a tube with water, and insert the twig, previously pushed through a
rubber stopper. Make sure that the stopper fits very tightly into the tube,
and that the twig fits tightly into the stopper. Sealing wax may be used
to insure a water-tight connection. Dip the lower end of the tube into a

66 THE NUTRITION OF GREEN PLANTS

dish of mercury. As the shoot loses water, it is withdrawn from the water
of the tube, and mercury is drawn upward. Observe and record results.

From this experiment, it will be seen that when the leaves of a
plant lose water to the atmosphere, they exert a pull on the water
in the conducting system of the plant. It appears that the water
column in the plant is continuous and unbroken from the leaves
to the roots. The pull exerted in the leaves is transferred all along
the conducting system to the roots. If a leaf cell loses water to
the atmosphere, the sap in that cell becomes more concentrated:
by virtue of the greater concentration of the sap, water passes from
adjoining cells, the sap of which in turn becomes more concen-
trated, and so on to the conducting vessels in the leaves. Hence
there is exerted, at the top of the continuous water column, a pull
which is transmitted downward throughout the whole plant. It
used to be thought that the water in a stem was " pushed "
upward by what is called a " root pressure. " Although root pres-
sure may play a small part in the rise of sap, there is more sub-
stantial proof that the rise is due to a pulling force. The initial
pulling force is created by the loss of water from leaf cells.

Problem 5. What are the processes of food building?

In the preceding paragraphs we have discussed the movement
of the water and mineral salts from the soil to the mesophyll cells
of the leaf, and also the movement of carbon dioxide from the
atmosphere to these same mesophyll cells. Here, the raw mate-
rials used in food-making meet, and here the process of food man-
ufacture goes on.

The role of light in food-building. In any manufacturing
process, energy is required. In the primary food-making process,
light is the energy used ; it is absorbed by the green coloring mate-
rials. The living substances make use of the energy in building a
food from the raw materials, carbon dioxide and water. The
manufactured product is a simple sugar. The elements carbon
and oxygen, of carbon dioxide, have been united to the elements
hydrogen and oxygen, of water, in such a way as to form sugar.
Some oxygen is left over from this process, and most of this
passes out into the air.

PROCESSES OF FOOD BUILDING 67

Exercise 47. To find out whether leaves make starch only when exposed
to light. It can easily be determined whether light is necessary for the manu-
facture of carbohydrates in leaves. We have learned that sugar is probably
the first carbohydrate manufactured ; as a rule, some of it is changed to starch
and accumulates during the day in the leaf. The presence of starch may be
detected by treating the leaf with iodine, which turns starch grains bluish-
purple. A simple experiment consists in covering a portion of a leaf attached
to growing plant, which has been in the dark a day, with tinfoil to exclude the
light. Then expose the plant to the light for several hours. At the end of
that period, remove the leaf from the plant, extract the chlorophyll by warming
the leaf in alcohol, and then treat the leaf with iodine. What is the action of
the iodine on the portion of the leaf which was uncovered? What does this
indicate? What is the effect of the iodine on the part which was covered?
Explain.

Exercise 48. The rdle of chlorophyll in food-building. To find out whether
it is only in the green parts of leaves that starch is made, extract the clorophyll
from a variegated leaf of Coleus, or other plant which has white areas in it,
which has been exposed to light for several hours, and treat with iodine. Note
the effect of the starch on the parts of the leaf which were white and on the
portions of the leaf which were green. Write a paragraph explaining the results
of this experiment.

The end-products and by-products. In most plants the first
visible product of photosynthesis is starch. In many of the
Musaceae (banana family), starch is absent from the leaves, oil
being the first visible product of photosynthesis. In many other
monocotyledons there is no visible product of photosynthesis to
be found within the green cells, for the sugar produced remains in
solution.

The sugar which is the immediate product of photosynthesis
is generally assumed to be glucose (C(;Hi 2 6 ). There is some
evidence, however, that it maybe sucrose (cane sugar) (Ci2H 22 Ou).
Both of these sugars are present in considerable quantities in
plants.

The equation which has been used to express in simple form
the chemical changes which take place during photosynthesis
(6CO 2 + 6H 2 O - C 6 Hi 2 O 6 + 6O 2 ) shows that oxygen is freed
during the process. This gas is a by-product of photosynthesis.

Exercise 49. To determine whether oxygen is given off in photosynthesis.

Tn a gallon battery jar place a bunch of fresh, active Elodea plants. Keep in
sunlight. Over them invert a wide funnel, and over the tube of the funnel

68 THE JSfCJTRITION OF GREEN PLANTS

place a test tube filled with water, so arranged as to catch the gas bubbles which
are given off. Apply the oxygen test to the gas.

The process summarized. We see that, in the process of car-
bohydrate manufacture or photosynthesis carried on only by
green plants, and only in those tissues of green plants which possess
chlorophyll, carbon dioxide and water are the raw materials;
sunlight, the energy; special living green bodies chiefly in leaf
cells, the factory or laboratory; sugar, the final product; and
oxygen, the by-product. Carbohydrate manufacture is a process
carried on only in cells containing a green coloring matter and
only in the presence of light.

Careful measurements have been made of the amount of light
energy which is absorbed by the green leaf, and of the quantity of
this actually used in food-making. It has been found that about
10 per cent of the total light energy which falls upon the leaf is
absorbed by the green coloring matter, and that of this amount
only about 35 per cent is used in food-making. This means that
approximately 3.5 per cent of the energy falling on the leaf is
utilized in carbohydrate building.

Problem 6. What use does the plant make of the food manufac-
tured in green tissue?

The sugar, glucose, which is thought to be the immediate
product of the photosynthetic process, is the foundation material
used in the synthesis of most other plant substances. Part of
this simple carbohydrate is respired, liberating energy to be used
by the plant in doing work. Some of it is stored as such, to be
changed later into various other substances.

Respiration. Respiration is one of the vital processes in
plants. In all essential particulars the process is the same in
plants as in animals. It is true that plants do not have organs
in any way resembling lungs, which serve to facilitate the exchange
of gases between the atmosphere and the cells of the body, but the
essential features of respiration are the same in both plants and
animals.

Respiration is a process which goes on only in the living cells.
Every cell of the plant body must respire if it is to maintain its life.

USE OF PLANT FOODS 69

We have often been led to believe that leaves are the respiring
organs of the plant. It is true that respiration is quite rapid in
leaves, but they are no more the respiring organs of the plant than
are the stems, the roots, or other living parts. Moreover, respira-
tion is not going on any more rapidly in leaves than in many other
living organs of the plant. It is in all living cells, no matter in
what organs or tissues they may be found, that respiration takes
place.

Respiration is a process in which substances of the plants, such
as sugars, are broken down by the aid of oxygen into simpler
products, the principal ones of which are carbon dioxide and water.
It is a destructive process. In this breaking-down process, energy
is liberated. Some of the energy of respiration is used directly by
the living cell for the processes which are essential to its life; the
remainder is lost as heat. In respiration, plant foods are used up,
being oxidized or " burned " by means of oxygen. In many
respects respiration is similar to combustion. As far as each living
cell or the entire plant body is concerned, the exchange of gases
involves an intake of oxygen and an outgo of carbon dioxide.

There may be some confusion regarding two of the important
processes of the green plant, namely, carbohydrate manufacture
(photosynthesis) and respiration. In preceding pages the carbo-
hydrate-manufacturing process of plants was described. This
process, too, goes on in the living cells, but only in those cells
which are exposed to light and contain a green coloring matter
(chlorophyll). And, in the carbohydrate-manufacturing process,
carbon dioxide is taken in, and oxygen given off. This is just the
reverse of the gas exchange in respiration. Carbohydrate manu-
facture takes place in a relatively few cells of the plant, and only
during the day. Respiration, on the other hand, proceeds day
and night in all living cells, whether they contain chlorophyll or
not. Again, we should recognize that whereas respiration is a
food-destroying or energy-releasing process, that process peculiar
to green cells is a food-building or energy-storing process.

It is very probable that there are times during the day wheil
respiration and carbohydrate manufacture go on at about the
same rate in the green tissues of the plant. At such times, the
oxygen set free in the latter process is not lost to the atmosphere but

70 THE NUTRITION OF GREEN PLANTS

is immediately utilized by the cells in respiration, and the carbon
dioxide eliminated in respiration is taken by the green cells and
used in the process of carbohydrate manufacture. But during
the night, when the utilization of carbon dioxide in food-building
ceases, this gas escapes from the plant to the atmosphere. In an
actively growing green plant, the amount of oxygen liberated to
the atmosphere by the carbohydrate-manufacturing process during
the twenty-four hours exceeds that absorbed in respiration, and
the carbon dioxide contributed to the atmosphere by the respira-
tion of such a plant is much less than that absorbed during carbo-
hydrate manufacture. Thus, we see that green plants play a
great part in the scheme of nature, in that they maintain a proper
ratio of the important gases of the atmosphere, by removing carbon
dioxide from it and adding oxygen. It has been computed that
approximately 280 square feet of green leaf surface will give out,
during a moderately warm and sunny day, the quantity of oxygen
used by a man for respiration during the same period.

Materials which compose the plant skeleton. Glucose may
be changed into cellulose, a carbohydrate which enters into the
structure of cell walls. Cellulose is the most abundant carbo-
hydrate in the plant kingdom. Cellulose is the basis of a large
number of commercial products such as paper, explosives, cello-
phane, celluloid, rayon, etc. Glucose may be changed into pectic
substances. Pectic compounds occur in the cell walls of many
fruits, breaking down in boiling to form a jelly.

Reserve foods. Glucose may be changed into other sugars
such as fructose and sucrose. Fructose is common in the fruits
of plants. Sucrose is a reserve food, particularly abundant in the
root of sugar beet and in the stems of sugar cane.

Glucose, or other carbohydrates, may be converted into fats
or oils. Fats and oils are especially abundant in seeds and fruits.
Common oils of commerce are castor, linseed, cottonseed, olive,
coconut, and peanut. State the commercial uses of these different
kinds of plant oils.

The proteins, substances of importance in such seed as beans,
peas, and cereals, are also in large part built from carbohydrates,
chiefly glucose. In addition to the elements carbon, hydrogen,

USE OF PLANT FOODS 71

and oxygen, which occur in all carbohydrates, proteins also contain
nitrogen, sulphur, and some of them phosphorus. These three
elements are derived from mineral salts which come from the soil.
Frequently we have mentioned the mineral salts as raw materials
in the food-manufacturing process. They do not form any part
of glucose, but they do enter into the make-up of all plant proteins
and many other important plant substances.

Various secretions and other substances. But these foods
carbohydrates, fats, and proteins are not the only plant sub-
stances of economic importance. Consider the various plant pig-
ments, the resins and gums, the milky latex of many plants, some
of which yield the latex from which rubber is made, and the
alkaloids nitrogenous substances such as quinine (from bark of
cinchona), caffeine (from coffee), thein (from tea), morphine
(from the poppy), nicotine (from tobacco), and atropin (from
nightshade). Also a large number of organic acids are found in
plants, such as citric acid (from citrus fruits) ; there are innumer-
able essential plant oils, such as lemon oil, cedar oil, clove oil, va-
nilla, camphor, etc. Also, consider the great commercial impor-
tance of tannin, a bitter substance found in the bark of many
trees and employed in the tanning of leather.

Thus we see that the sugar, glucose, manufactured only in
green cells, forms the foundation of many other plant foods, and
other plant substances that probably cannot be classed as foods.
Glucose is manufactured in the cells only when they are exposed
to light. However, all the other chemical changes in the plant,
including the synthesis of fats, of proteins, of alkaloids, of acids, or
essential oils, and other substances, are independent of direct sun-
light. Sunlight is directly necessary for only one process, namely,
photosynthesis, or the building of glucose out of carbon dioxide
and water. We have seen that glucose manufacture utilizes the
energy absorbed from light. In other words, in glucose manufac-
ture radiant energy is transformed into the chemical or potential
energy of the glucose molecule. In all other chemical changes in
the plant, for example, the synthesis of proteins, the energy for
doing the work is derived from respiration.

72 THE NUTRITION OF GREEN PLANTS

Problem 7. Whaf is the role of the different elements in the
nutrition of green plants?

In the preceding sections it was emphasized that only green
plants have the power of manufacturing, from the simple com-
pounds derived from the soil and atmosphere, the foods which
are used in nourishing the plant body.

We have also seen that this food-manufacturing process is of
great significance, in that the foods constructed furnish the mate-
rial out of which the bodies of both plants and animals are built;
and moreover, in the making of these foods by green plants,
energy from the sun, which is the world's great and only source of
energy, is stored.

It has long been realized by agriculturists that a proper fer-
tilizer practice was dependent upon a knowledge of the influence
which the different chemical elements exert upon the plant's
growth. If, for example, we would apply nitrates to the soil, we
should know how this salt is going to affect the crop. Moreover,
we should be able to tell when a crop is suffering from a deficiency
of any essential chemical element, and what the effects are of
an excess.

Principal substances used by green plants. It was also
pointed out that there were certain substances which the plant
must have in order to maintain life. The principal substances
which are taken into the green plant, and in some way made use
of, are as follows:

From the soil. (1) Water, and (2) salts containing principally
nitrogen, phosphorus, sulphur, potassium, calcium, magnesium,
and iron. From the atmosphere. (1) Carbon dioxide and (2)
oxygen.

Let us briefly discuss the part that each of these substances
plays in the life of the plant.

Water. The living material (protoplasm) of the plant is 80
to 90 per cent water. We have learned that water is an essential
raw material for the manufacture of sugar. Water is the solvent
of the gases, oxygen and carbon dioxide, and also of all mineral
salts. None of these substances can enter the plant unless they
are in solution. We have seen that raw materials and foods move

ROLE OF DIFFERENT ELEMENTS 73

from one part of the plant to another in watery solution. The
cells of the plant function normally only when distended with
water.

Nitrogen. Nitrogen is a constituent of all proteins, which are
essential components of protoplasm. Protein contains 15 to 19
per cent of nitrogen. It is well known that an abundance of nitro-
gen tends to produce a rank growth of leaves, stems, and roots
and to retard the date of maturing of the plant. Crops grown for
their leaves only are improved by applications of nitrate. How-
ever, an excess of nitrogen in such a crop as cabbage may result
in a softness and tenderness which make it difficult to ship and
keep well. Cereal crops produce, as a rule, too much straw, and
" lodge " badly if there is an excessive supply of nitrogen. An
excess of nitrogen applied to potatoes stimulates a leafy growth,
but not a proportionate weight of tuber; applied to tomatoes, it
produces too much leaf and too little fruit; applied to sugar beets,
it results in high tonnage, but reduced sugar content. Heavy ap-
plications of nitrogenous fertilizers to fruit-bearing plants may
cause increased vegetative growth, which is usually associated
with decreased fruit production. Nitrates are by far the most
available source of nitrogen for crop plants.

Phosphorus. Like nitrogen, phosphorus is a constituent of
proteins. It is essential to a rapid multiplication of cells. It is
known that such insoluble carbohydrates as starch are not changed
into the soluble form (sugar) unless phosphorus is present. In
the early stages of the plant's growth, phosphorus promotes devel-
opment. The fact that applications of phosphorus to the soil
hasten the maturity of plants is probably due to the stimulation
of rapid early growth. On heavy soils, where roots do not form
well, phosphorus stimulates root development. Plants secure
their phosphorus from the soil phosphates.

Sulphur. This is an indispensable element in plant growth.
It is essential to the formation of proteins. A deficiency of sulphur
results in a failure of the cells to divide at a normal rate, and in a
suppression of fruit development. The characteristic flavor of
onions and garlic is due to certain sulphur compounds. Plants
secure sulphur from the sulphates of potassium, calcium, magne-
sium, and iron.

74 THE NUTRITION OF GREEN PLANTS

Potassium. Potassium is essential to the manufacture and
movement of carbohydrates. Such plants as sugar beets, pota-
toes, and others which manufacture and store large quantities of
carbohydrates are particularly responsive to the available supply of
potassium. This element has a marked effect on the weight of
grain. Potash starvation shows in the dull, yellowish color of the
foliage, in a loss of vigor, and a lessened resistance to disease.

Calcium. This element seems to stimulate root growth. A
deficiency retards the movement of carbohydrates in the plant
and their utilization by the plant. Calcium aids in neutralizing
acids, both without and within the plant, which might limit the
growth. Calcium enters into the composition of the middle mem-
brane of cell walls.

Magnesium. It is now known that magnesium is necessary
for the formation of the green coloring matter (chlorophyll) of
plants. In fact, it is a component of the green coloring matter.
A deficiency of magnesium results in pale, colorless foliage. Mag-
nesium also appears to aid in the movement of phosphorus in
the plant.

Iron. Although iron does not enter into the composition of
chlorophyll, it is absolutely essential to its formation. Even in
the light, plants become pale when grown without iron. Very
small amounts of iron salts in the soil are sufficient.

Carbon. As has been stated, green plants derive all their
carbon from the air in the form of carbon dioxide. Carbon enters
into the composition of all carbohydrates, such as sugars, starches,
cell walls, and is also an essential component of fats, of proteins,
and of living material itself. Carbon makes up from 40 to 50
per cent of the dry weight of all plants.

Oxygen. This element enters into many chemical compounds,
but in its elemental form is essential in the process of respiration.
This important process will be discussed later.

It should be understood that the plant is taking in a great
many more chemical elements than those mentioned in the preced-
ing paragraphs. The fact is that an analysis of plant ash reveals
the presence of most of the elements which occur in the soil. How-
ever, it is not known what part, if any, many of the rarer elements
play in the plant's life. It may be that some of them, like iron,

PATH OF FOOD MOVEMENT 75

even in small traces, are indispensable to normal plant growth, or
at least influence the plant's development. Experiments seem
to bear out the truth of this statement. It should also be pointed
out that the salts of the soil are ordinarily in very dilute solutions,
and are taken in by the plant in small quantities.

Problem 8. Where do foods move in the plant?

Although all living cells of the plant contain sap, not all of them
are concerned in its rapid movement throughout the plant. In all
stems and roots, there is an upward-moving sap stream and a
downward-moving sap stream, and these differ in their chemical
composition. The upward-moving stream is mainly water and
mineral salts from the soil, and food substances which have been
stored in roots and stems; the downward-moving stream carries
food substances, dissolved, of course, in water.

We learned that the conductive tissues of the plant are grouped
into bundles called vascular bundles. Each bundle is composed
of three groups of structural elements, the xylem, the phloem, and,
in most plants, a cambium between the xylem and phloem. We
learned that water and mineral salts moved in the vessels and
tracheids of the xylem. That is, the upward-moving sap stream
is in the xylem or woody portion of the stem. Now recall that
the conducting elements in the phloem are sieve tubes. Gird-
ling experiments with stems show that foods, chiefly sugars, are
transported in the sieve tubes of the phloem. That is, the down-
ward-moving sap stream is in the phloem or bark portion of the stern.
And bear in mind that all foods which are moving from one part
of the plant to another are in solution. Starch grains or protein
granules cannot move as such throughout the plant. Why?

In girdling, the bark is cut completely around the stem down
to the wood. That this operation does not stop the upward flow
of water is evidenced by the fact that the leaves do not wilt. But
foods do not move past the girdle. This indicates that their con-
duction is in the bark. If the main trunk of a tree is girdled, the
roots are starved for want of food, and the tree finally dies. The
girdling of stems often results in increased growth above the girdle.
This condition also seems to show that there is a downward move-

76 THE NUTRITION OF GREEN PLANTS

ment of foods in the bark, and that they have a tendency to
accumulate above the girdle, thus supplying material for addi-
tional growth.

The path of movement of the food manufactured in the meso-
phyll cells of the leaf is probably as follows: It diffuses into cells
joining the vein endings in the leaves, from one cell to another,
until it comes to sieve tubes; once in the sieve tubes it is free to
move rather rapidly to all parts of the plant, passing down into the
petiole, thence to the stem and roots.

Problem 9. How does the plant store and digest its food?

As a rule, the food that is manufactured by a plant accumulates
faster than it is needed. Accordingly, there is some provision for
the storage of food. The amount of food stored and the place
of storage depend somewhat upon the length of life of the plant.
For example, in annual plants, those that live but one year, the
supply of stored food is confined to the seeds. In biennials, plants
that live two years, producing seed at the end of the second year,
not only is food stored in seeds, but also large amounts occur in
roots. Examples of such plants are carrot, parsnip, turnip, sugar
beet, etc. In perennials, plants that live from year to year, food
is stored not only in seeds, but also in large quantities in roots and
stems. For example, the dandelion root is perennial, and at all
times has a supply of food in reserve. Well do we know this, for
if the dandelion is cut off, new shoots promptly arise, making their
growth at the expense of food stored in the roots. Many perennial
weeds have underground stems; examples are wild morning-glory
or bindweed, Canada thistle, and Russian knapweed; and many
common economic plants, such as Solomon's seal, Trillium, certain
larkspurs, Irish potato, Jerusalem artichoke, asparagus, and
others also have underground stems. In all these plants, the
underground stems are food-storage organs. In woody plants, the
trees and shrubs, food is stored in twigs and branches of all sizes,
in the main trunk, and in the roots. In fact, a tree lays up for the
dormant season an enormous reserve of food which, in the spring,
moves into all the buds, furnishing nourishment for their early
growth. In the woody stems and roots, foods may be stored in

FOOD STORAGE AND DIGESTION 77

the vascular x rays, in wood parenchyma, in phloem parenchyma, in
cortex, and in pith.

The kinds of stored foods. The principal stored foods are
starch, sugars, proteins, and oils. Probably the most common
food stored in plants is starch, as for example in the seeds of corn,
wheat, oats, and rice, the tuber of potato, and in the roots and
stems of woody plants. Starch occurs in the form of grains, the
shape, markings, and structure of which are characteristic of each
species.

Exercise 60. Starch test. Apply the starch test to sections of a number of
different kinds of structures including seeds, roots, and stems. Write up your
observations.

When photosynthesis is actively going on, starch usually accu-
mulates in the leaf, and can be detected by applying the iodine test
described on page 40. This accumulation means that glucose is
being made more rapidly than it can be carried aw T ay, and that it
is changed to starch and stored as such temporarily. That this is
so is borne out by the following simple experiment. Small portions
of a leaf tested for starch in the evening after a day of photo-
synthetic activity show starch. If pieces of the leaf are taken in
the morning before it is light, or if the leaf is covered with opaque
paper in the evening, so that light does not strike it in the morning
before the sample for testing is made, a test for starch is negative.
Evidently during the night stored food has moved out of the leaf
to various parts of the plant. Thus starch may be a temporary
storage product of leaves.

Sugars, chiefly glucose, fructose, and sucrose, are very common
stored foods. Usually they may be detected in the sap of cells
in almost any part of the plant. They are stored in large quanti-
ties in certain fruits and in some vegetative structures. Notable
examples are the roots of sugar beet and the stems of sugar cane,
which may contain from 15 to 20 per cent of sucrose.

Proteins are also an extremely common storage product, espe-
cially in seeds, particularly such seeds as beans and peas. Oil is a
reserve food in such seeds as flax, cotton, olive, and peanut.

The process of food digestion. Plants digest their foods, and
essentially in much the same manner as animals. As an example,

78 THE NUTRITION OF GREEN PLANTS

let us consider starch. We have learned that substances which
move from place to place in the plant must be in solution. Starch
grains can not move through cell walls and protoplasmic mem-
branes. Consequently, starch stored at points in the plant far
removed from growing points, where it is most needed, must first
be changed into some form which will diffuse through cell walls and
protoplasmic membranes. This change of starch, a substance
which is not soluble in the cell sap, to a material which is soluble
in the cell sap, is a process called digestion.

Digestion is brought about by various complex substances
known as enzymes. They are protein in character. They are
usually present in very small amounts in cells, but it ap-
pears that even a small quantity may be sufficient to bring
about the digestion of a relatively large amount of material.
Moreover, there is no appreciable decrease in the amount of
enzyme as a result of its action. There is a specific enzyme,
known as diastase, through the action of which starch is changed
to sugar, glucose. Diastase has no other digestive function; its
action is specific.

Let us illustrate the processes of digestion, movement of food,
and storage of food as they occur in the potato plant. The cells
of the potato plant which contain chlorophyll manufacture sugar
from carbon dioxide and water. Some of this sugar, as such, is
immediately conveyed from the leaf and goes to various parts
of the plant where it is used in respiration and to nourish the
tissues. Some of it reaches the developing tubers underground.
Some of it may form the basis for fats and proteins. And a large
proportion of it is converted to starch and temporarily stored in
leaf cells. At night, when the sugar-making process has stopped
on account of the lack of light, the temporarily stored starch is
digested, that is, converted to a soluble form, which in this case
is sugar. The agency causing this change is the enzyme diastase.
The sugar moves out of the leaf, through the leaf stalk into the
stem, down through conducting tubes of the vascular bundles, and
into the tubers. This sugar is converted back into starch and
stored as such in the tuber. The potato tuber is a starch-storing
organ. When the tuber is planted and begins to sprout, it becomes
sweet, indicating that starch is being converted to sugar. Further-

FOOD ASSIMILATION 79

more, actual test shows sugar on the increase and in transport to
the developing sprouts.

As another illustration, let us start with starch stored in the
vascular rays of a peach twig. Vascular ray cells are living; in
fact, food storage occurs only in living cells. When temperature
and other conditions are favorable in the spring, these vascular ray
cells begin the secretion of diastase, and stored starch is changed
to sugar, that is, starch is digested. The sugar diffuses from vas-
cular ray cells into adjoining tissues and finally reaches the growing
cells in the buds.

There are many different kinds of enzymes, each having a
rather specific function. Lipase is an enzyme which facilitates the
breaking down of fats into glycerin and fatty acids; pepsin digests
proteins, converting them into water-soluble peptones and pro-
teoses; trypsin digests peptones and proteoscs, changing them
into amino acids.

Enzymes may be secreted by any living cells of the plant, or
wherever digestion is necessary.

Exercise 61. Starch digestion. Place some starch in a small, shallow
dish and cover with a solution of diastase, which may be obtained as a com-
mercial product. Keep at a temperature of about 75 to 80 F. for 12 to 24
hours. Examine the starch grains with the high power of a compound micro-
scope, and observe that they are " eaten " and corroded. If facilities permit,
test some of the solution for sugar.

Problem 10. How does the plant assimilate food?

Up to this point in our discussion of the nutrition of green
plants we have discussed the processes of food manufacture, its
storage, its digestion, its use, and its movement within the plant.
Digested food within the plant cell is not a part of the living
protoplasm. One more step is necessary that of making it a
part of the living protoplasm itself. It must be changed from
lifeless food to living protoplasm. This process is called assimila-
tion. The nature of this transformation is not understood. But
we may be assured that everywhere throughout the plant, in
living cells, there is going on this marvelous change of non-living
foods to living stuff. But the change is brought about only by
the action of other living matter already existing.

80 THE NUTRITION OF GREEN PLANTS

ADDITIONAL QUESTIONS AND EXERCISES

1. Define organic substances and inorganic substances, and cite examples
of each.

2. Which classes of substances nourish the bodies of plants and animals?

3. What is the great role that green plants play in the world's economy?

4. What inorganic compounds do living plants give off?

5. What elements do carbohydrates contain?

6. Why is a sprouted potato sweeter than an unsprouted one?

7. Sweet-corn kernels contain much more sugar than field-corn kernels.
Do you see any relation between this fact and the wrinkledness of dry sweet
corn?

8. What is meant by an independent plant?

9. Define assimilation.

10. Discuss digestion in relation to seed germination.

11. When slices of red beet are placed in water what prevents the coloring
matter from diffusing out? Why does the coloring material come out when the
beet is boiled?

12. What is the difference between the food of plants and that of animals?

13. What gases enter a green leaf in sunlight?

14. What is an enzyme?

15. Explain why an apple tree dies eventually when a ring of bark is removed
from the main stem.

16. Explain why a tree girdled in summer may live and remain green during
the remainder of the season but fail to leaf out the following spring.

REFERENCE

The Green Leaf, by D. T. MACDOUGAL, published by D. Appleton and
Company, 1930, 142 pages, 22 figures. Mention of chapter headings shows
how much there is of interest in this little volume : living matter from rocks,
water and air; place in the sun; models of sun-screens and our utilization of
their products; the grass blade; pine needles; tree records of climate; the
oak leaf; movements of sap and autumnal colors; green stems; a visit to
green leaf mills; green mills and their grist; protoplasm, how it started and
how it goes; growth; ghost and other dwellers in darkness; leaf-products
and human populations.

UNIT III
NUTRITION OF NON-GREEN PLANTS

We learned in Unit II that green plants make their own food.
For this reason they are called independent plants, or autophytes.
Plants without chlorophyll and the animal life of the earth are
dependent, since they have not the power to make food, but must
get their food from supplies furnished, directly or indirectly, by
green plants.

Almost all the non-green plants are included in the groups
which we know as bacteria, yeasts, molds, mildews, rusts, smuts,
and mushrooms. Certain of these plants are related in an impor-
tant way to man's welfare. They get into his foods which are
unprotected and cause them to spoil. They enter his body and
the bodies of animals and produce poisonous substances that
cause disease. In a similar way they affect plants, causing serious
damage to cereals, fruits, and other crops. Through unceasing
effort scientists have learned much about the nature of these plants
and about how they live, and this knowledge has been a help to
mankind in protecting against the injurious forms.

Although certain of the bacteria, yeasts, and molds are man's
enemies, we should know, also, that representatives of these plant
groups are absolutely essential to the existence of other life on
the earth. Without the help of the microscopic forms of life
which are working quietly within the soil, the continued existence
of man and his civilization would be impossible. Certain of the
soil bacteria and molds cause the decay of dead plant and animal
bodies. Others use the nitrogen of the air in making new chemical
compounds which are essential to the growth of other plants. In
this way the soil is kept fertile, making possible the production of
plants year after year.

In general, the non-green plants are simple forms without
roots, stems, or leaves; yet there are a few flowering plants which

81

82 NUTRITION OF NON-GREEN PLANTS

lack chlorophyll and so must depend upon green plants. Among
the dependent seed plants are the Indian pipe and pinesap, which
derive food from dead plant materials in the soil, and beech drops
and squaw root which live as parasites on the roots of certain
trees.

It will be interesting to study the relationships between the
independent life of the earth and the dependent life. Green
plants, in the process x of food-making, use carbon dioxide in large
amounts and give off to the atmosphere an equal volume of oxygen;
in respiration, they use oxygen and give off carbon dioxide in small
quantities. The ultimate effect of green plants on the air is to
decrease the amount of carbon dioxide and increase the amount of
oxygen. Animals and non-green plants affect the atmosphere by
increasing the amount of carbon dioxide and decreasing the amount
of oxygen through the process of respiration. Thus there is a

balance in nature between
green plants and living things
without chlorophyll. The
plants with chlorophyll fur-
nish oxygen which all living
things require and use carbon
dioxide which is a waste prod-
uct of all life.

F7a. 32.-A balanced aquarium. A balanced aquarium (Fig.

Denoyer Geppert Co. 32) is set up by adding animals

and plants to the water in pro-
portion so that the plants will furnish the animals the required
amounts of food and oxygen and the animals will furnish supplies
of carbon dioxide and other raw materials needed by the plants.
The balanced aquarium is a miniature world with a definite
balance between the plant life and the animal life which it
contains.

We can consider green plants independent only in the sense
that they are able to build foods from raw materials; they are
dependent, in a way, since a large part of the raw materials neces-
sary for the process of food-making is furnished by other forms of
life.

CHARACTERISTICS OF NON-GREEN PLANTS 83

Problem 1. What are the main characteristics of the
non-green plants?

The outstanding characteristic of the non-green plants is the
absence of chlorophyll in their tissues. In a field planted to corn
an occasional plant appears with white leaves. No one has seen
a full-grown stalk of corn having only white leaves. It is signifi-
cant that these young albino corn plants live only as long as the
food stored in the seed lasts. The parent plant had chlorophyll,
so food could be produced and stored in the seed. The albino
seedling, having no chlorophyll, must die as soon as it has used
this original store.

There are no chlorophyll-bearing forms among that simple
group of plants known as fungi. The plant begins development
in contact with organic material, and all its needs for producing
the plant body and reproductive structures are supplied in the
form of ready-made food.

All living organisms need energy for life processes. Whether
plant or animal, every living thing uses oxygen and gives off
carbon dioxide. Substances are oxidized in the cells of the living
body, and energy is released. If the living thing can not store
energy for itself in the form of food it must get its supply of energy
from the store provided by other living things which can make food.

We have considered non-green plants dependent on green
plants, but certain bacteria are exceptions. Anyone who has seen
a mineral spring has noted the foul smell and the whitish or yel-
lowish coating of objects in the stream of water running away from
the spring. The same may be noted in a sluggish stream contain-
ing sewage. The smell is due to a gas known as hydrogen sulphide
which escapes from solution in the water. The coating of objects
is due to sulphur bacteria. These bacteria are primitive forms
which can live and secure energy from the oxidation of hydrogen
sulphide to sulphur and the oxidation of sulphur to sulphuric acid.
These or similar forms must have represented the first life on the
earth when very few of the types of animals or plants found today
could have survived the severe conditions.

Saprophytes. Dependent plants which derive their food from
non-living organic material are known as saprophytes. The

84

NUTRITION OF NON-GREEN PLANTS

organic matter may be either plant or animal. There are a great
many different kinds of saprophytic fungi. Mushrooms are a
common example. They may be grown in beds prepared from
partially decayed stable manure mixed with rich loam. The

decayed manure and loam
furnish the organic material
for the use of the mushroom.
Enzymes are secreted by the
part of the plant consisting of
a network of tiny underground
threads. These enzymes
change the complex food sub-
stances into simpler materials
which can pass through the
membranes of the plant in
absorption. Inside the plant
the food materials may be oxi-
dized, or they may be assimi-
lated, forming protoplasm
which is used in the building
of new plant structures. A
saprophyte is similar to an
animal in that it requires food
which has been derived directly
or indirectly from a green
plant.

Why are mushrooms fre-
quently found growing in the
woods around dead trees or
stumps? Explain why mush-
rooms are found growing in
soils that have humus (decay-
ing plant material).

During the hundreds of
thousands of years that life
has been in existence on the earth enormous quantities of material
have been produced in the form of animal and plant bodies. It is
to be noted that these materials have not accumulated on the

FIG. 33. Shaggy-mane mushroom
growing in a city backyard. Why is
this mushroom growing here among
the green plants? Does it need the
sunlight? Are the materials which it
takes from the soil the same as those
taken by the green plants? Could
plants without chlorophyll develop
without the aid of green plants? Could
green plants succeed without the aid
of the non-green plants?

CHARACTERISTICS OF NON-GREEN PLANTS

85

surface of the earth. Without the great variety of saprophytes
which dispose of the dead organic substances, life on the earth
would have become impossible long ago because of the debris
resulting from the accumulation of these materials.

tfiG. 34. These large mushrooms are saprophytes, getting food materials
from the humus on which they are growing.

The annual herbaceous plants and the leaves of trees and
shrubs fall to the ground each year, and myriads of non-green
plants begin the process of
transformation. The com-
plex plant materials are at-
tacked in the process of
decay, and this and other
processes which follow change
these substances into simple
raw materials suitable for use
of other plants. In the same
way, the parts of crop plants
which are not removed from
the fields by man are returned
to the soil where bacteria and molds transform them into materials
that can be used by other crops. What conditions would follow
if all the saprophytes were suddenly destroyed?

FIG. 35. Certain fleshy fungi live as
saprophytes on fallen logs and on stumps.

86 NUTRITION OF NON-GREEN PLANTS

Saprophytes are not always beneficial to man. Good food
for man is also suitable food for other organisms which may get in
and spoil the food for man. It has been necessary for man to
devise methods of food preservation such as drying, canning, use of
chemicals, and refrigeration in order to make it impossible for
injurious saprophytes to enter or grow in the food and thus
destroy it.

Exercise 52. How does a saprophyte secure food? Mount in water on a
clean slide a small portion of rotten apple or orange, cover with cover-glass, and
examine under the low power of the microscope. What evidence do you see
of plant growth within the apple? What is the source of food of the structures
which you see? How does the food material of the apple get into the plant
body of the fungus?

Exercise 53. How may saprophytes be spread? Examine an apple or
orange in the advanced stages of rot for any evidence of the fungus on the
outside of the fruit. Mount on a slide in water under a cover-glass a small
amount of any blue or green powdery fungus material that you may find.
What is the shape of the bodies as they appear under the microscope? What
seems to be the relation between the structures of the plant which you find
on the surfaces of the fruit to those which you found within? Why are the
thread-like structures produced within the fruit rather than on its surface?
Why were the resistant spherical structures produced on the surface of the fruit
rather than within? Show how the part of the plant within the fruit and the
part of the plant on the outside is in each case fitted to live where it is produced.
How could the small spherical bodies on the outside known as spores be spread
to other fruits?

Exercise 54. How may saprophytes enter food materials? Place in a dish
a sound apple in contact with the rotting portion of an affected apple. On the
opposite side of the sound apple prick the skin with the point of a knife and
introduce some of the spore material found on the surface of decayed fruit.
Cover the dish and examine each day for a few days. Describe fully what
happens to both sides of the apple which was formerly sound. What rules
would you give for preventing decay in sound fruits, such as apples and
oranges?

Parasites. Many of the non-green plants, though dependent
like the saprophytes, derive their food from living organic matter,
that is, from the bodies of living plants or animals. These are
parasites. Some of the greatest discoveries of medical science are
concerned with methods of protecting against disease-producing
bacteria which are parasitic plants. Many cultivated plants and
domestic animals are attacked by plant parasites, and huge sums

CHARACTERISTICS OF NON-GREEN PLANTS 87

of money are expended annually by public agencies and private
individuals in fighting these parasitic plant pests.

The main point of difference between the saprophytes and para-
sites is the fact that the parasites derive their food from living
plant and animal bodies whereas the saprophytes thrive on dead
organic materials. They are alike in the fact that both require
organic material for their nutrition, that is, neither has the power
to make foods from raw materials.

Since bacteria and other fungi use the living material of plants
and animals as food, they naturally are injurious in their relation
to the host (the living thing in which they grow). They do injury
by destroying the living material of the host. Also, many of them
are injurious because they produce substances which are poisonous
to the host. These substances are known as toxins. Both plants
and animals are affected by toxins, but since the animal has a
circulatory system and a plant has nothing to compare with this,
the toxic substances can not affect the plant in the same way they
affect animals.

Many of the most serious diseases which attack man, such
as typhoid fever and diphtheria, are due to the effects of bacteria.
The non-green plants which produce disease conditions in plants
or animals are called pathogenic forms. The science which deals
vith plant diseases is plant pathology.

Following the work of Louis Pasteur, a Frenchman, and Robert
Koch, a German, showing that bacteria are the cause of many
diseases in animals, science has built up a system of preventive
medicine. It has been found possible in many cases to prevent
invasion of the body by bacteria, and in others, to prevent the
most injurious effects of those that have made a start. The
medical doctor has come more and more to be an educator, showing
people how to keep well.

Suggested activites. You will find interesting accounts of the life and
work of Robert Koch and of Louis Pasteur in Science in the Service of Health,
by Downing (Longmans). Prepare a report to be read to the class on the
life and work of Robert Koch. Prepare a paper on the service to mankind of
the work of Louis Pasteur.

88 NUTRITION OF NON-GREEN PLANTS

Problem 2. What are the nutritive relations of the saprophytes?

The principal saprophytic plants of economic importance are
the bacteria, yeasts, and molds. All these groups are widely dis-
tributed on the earth. Organic materials either of plant or animal
origin when exposed under suitable conditions of moisture and
temperature are soon alive with representatives of one or more
of these forms. The material in which they grow becomes
changed. Through the life processes of saprophytes, sweet milk
becomes sour, alcohol is formed in fruit juices, and the alcohol
solution may be changed to vinegar.

Exercise 65. How are organic materials changed by saprophytes? Place
in a fruit jar various dampened plant materials such as pieces of banana,
apple, and bread; and other organic materials as cheese and leather. Put on
the cover without the rubber, and leave in a dark place at living-room tem-
perature. Examine each day and note changes in the appearance of the mate-
rials. Describe and account for the changes from day to day. Why is apple
or cheese a suitable material for the growth of saprophytes? Why is it pos-
sible for these plants to grow in the dark? Why are they classed as sapro-
phytes? Under what conditions are these forms beneficial? Under what
conditions are they injurious to, man? Explain the part (if any) that each of
the following processes has played in the life of these saprophytes: photo-
synthesis, diffusion, osmosis, respiration.

The use of saprophytes in the preparation of food. The yeast
plant is a very small single-celled plant which reproduces by send-
ing out buds which gradually are pinched off and become ne\\
yeast plants. Yeast cells grow in a sugar solution, giving off
carbon dioxide gas and producing alcohol. This property of yeast
cells is taken advantage of in bread-making. Dough containing
yeast plants is kept at a temperature suitable for their growth.
As the yeast cells grow, bubbles of carbon dioxide are formed
throughout the dough. The bubbles cause the dough to rise.
When this is baked, the alcohol escapes and the bubbles remain
as holes in the bread, making it light. What is the source of the
gas that forms the bubbles in bread dough?

Bacteria and molds are used in the manufacture of dairy prod-
ucts. Butter may be made either from cream as it is separated
from the milk or from cream that has been allowed to sour. Sweet
Bream gives butter which lacks the desired butter flavor and the

NUTRITIVE RELATIONS OF SAPROPHYTES 89

butter soon becomes rancid. Sour-cream butter is the common
type. The cream is allowed to sour from the action of lactic-acid
bacteria. Country butter was formerly made from cream which
soured naturally. Now most of the butter is produced in cream-
eries where the cream is pasteurized to destroy wild bacteria
present and then pure cultures of the lactic-acid bacteria are
added. Under these conditions the flavor of the butter can be
controlled.

In the ripening of cheese, the desired flavor is produced by
bacteria and molds, the particular flavor depending upon the type
of organism present.

Bacteria are also involved in the production of vinegar and in
the making of sauerkraut. In a similar way the farmer uses a
silo to preserve, by means of the acid formed, large quantities of
food (silage) to be fed to cattle in the winter.

Exercise 56. How can we test for the presence of carbon dioxide? Fit

into a bottle a two-hole rubber stopper with funnel tube and with delivery tube
extending into a test tube of lime water. Dissolve a little cream of tartar in
water and pour the solution into the bottle through the funnel tube. Add a
solution of baking soda. Carbon dioxide is released from the baking soda by
action of the cream of tartar. It changes the appearance of the lime water
from clear to milky as the gas bubbles through it. This is a test for carbon
dioxide.

Exercise 67. What is the effect of yeasts on a sugar solution? Using the
apparatus of Exercise 56, put a solution of one part molasses in nine parts of
water into the bottle. Pulverize a small portion of a dry yeast cake and add to
the molasses solution in the bottle. Let stand in a warm place for twenty-four
hours with the delivery tube extending into the tube of lime water. Is there
any evidence of the evolution of a gas in the bottle? Is the gas carbon dioxide?
Note the odor of the solution in the bottle. The yeast cells produce an enzyme
known as zymase. Sugar is taken into the yeast cell where the zymase causes a
decomposition of the sugar with the production of alcohol and carbon dioxide.

Suggested activity. How is yeast used in bread-making? Find out at
home or from a baker how yeast is used in bread-making. What are some of
the conditions necessary for success in making bread with yeast?

Exercise 68. What is the nature of yeast cells? Place a drop of the liquid
from the bottle used in Exercise 57 on a slide. Place over it a cover-glass, and
examine under the low power of the microscope. The numerous small bodies
are yeast cells. Using the high power, note the shape and structure. The
knobs on some of the cells are buds which develop and finally separate, pro-
ducing new cells. This process, which is the ordinary method of reproduc-
tion of yeast, is known as budding.

90 NUTRITION OF NON-GREEN PLANTS

Food in which saprophytic organisms are growing may or may
not be poisonous, but the presence of molds on preserved food is
always a danger sign indicating that injurious organisms may
be present.

There are two types of poisoning which may result from eating
contaminated foods: (1) the so-called ptomaine poisoning, and
(2) botulinus poisoning. In the decomposition by bacteria of fish,
meats, etc., which are mainly protein, poisonous substances are
often formed which are very toxic when taken into the human
digestive tract. These, together with the living bacteria present,
may cause serious illness. In the second type of poisoning, an
organism which is hard to kill because it forms spores may be sealed
up in a can with food, and if the can is not sterilized by heat the
bacteria may grow and reproduce in the food. This organism grows
readily in the absence of air, and it produces a substance which is
very poisonous but is easily destroyed by heating. Between the
years 1919 to 1924 there was an outbreak of food poisoning in vari-
ous parts of the United States. Canned ripe olives caused most of
the trouble. As ripe olives are eaten without being cooked, the
toxin was taken into the digestive tract. It is well to remember
that clean, fresh, sound food will not cause botulism, and preserved
foods freshly heated to the boiling point will not cause botulism.
Ordinarily there is no danger in eating factory-canned foods as
they are subjected in the can to high temperatures sufficient to kill
spores as well as all vegetative bacteria.

Food showing any signs of decomposition evident by appear-
ance, odor, or formation of gas should be destroyed. It is unsafe
to taste food which shows any of these signs of spoilage.

How may disease be spread by milk? Milk, a balanced food
for man, is also a suitable food for bacteria. The souring of milk
is one of the first evidences that bacteria are growing in it. Disease
bacteria from the cow herself or from infected persons may be
present as well as the lactic-acid organism which causes the souring.
Diseases which may be spread by milk are tuberculosis (to which
the cow is subject as well as human beings) and diseases of human
origin as typhoid fever, scarlet fever, septic sore throat, and dysen-
tery. In what ways may harmful bacteria get into milk?

Boards of health of our larger cities set standards for sanitary

NUTRITIVE RELATIONS OF SAPROPHYTES

91

production of the supplies of milk which go into the cities. They
also maintain an inspection service to see that the standards are
met by the dairymen. One of the first requirements is healthy
cows. They must pass the tuberculin test a test to determine
the absence of any tuberculosis infection. Stables must be well
lighted and kept clean.

Pasteurization of milk. Heat not only reduces the bacterial
content, but it may also cause changes in the protein food sub-
stances in the milk. Pasteurization is a process in which the milk
is kept at a temperature of
65 C. for 20 minutes. This
heat is sufficient to kill 95 to
99 per cent of the micro-
organisms, and the changes in
the proteins caused by heat
are reduced to a minimum.
Give two reasons why city milk
supplies should be pasteurized.

Certified milk is produced
under the strictest require-
ments for cleanliness and is
thus kept relatively free from
bacteria. The cows are kept
clean; they are washed before
being milked ; the milker must
wash his hands before milking
each cow, and the milk must
be cooled quickly. It is not
heated but is sold as raw milk.

It costs money to observe all the extra precautions required so
that when ordinary pasteurized milk may sell for ten cents a
quart, certified milk may sell for twenty cents a quart.

The preservation of foods. During certain seasons of the
year foods are plentiful in the fresh state. With our modern
methods of packing and transportation we really have a wide
choice of fresh fruits and vegetables the year round. However,
during the winter, certain fresh foods are high priced and man
has learned how to preserve foods when they are plentiful so that

FIG. 36. A milk pasteurizer. The
steam and refrigerator pipes with
which it is connected are shown. (From
California Agricultural Experiment
Station Circular 319.)

92 NUTRITION OF NON-GREEN PLANTS

an abundance of a wide variety of foods is available throughout
the year at moderate prices.

The bacteria, yeasts, and molds thrive on our foods; they can
be considered our rivals as they will surely spoil foods for our use
if they can get to them first. The methods of preservation some-
times change the flavor of foods, but we have learned to enjoy
them and usually the nutritive value is not impaired.

Methods of coping with the saprophytic menace to our food
supply are concerned with keeping the micro-organisms out of the
food by setting up conditions which make it impossible for them
to grow in the food materials.

One of the oldest and simplest of the methods of food preserva-
tion is drying. Bacteria, yeasts, and molds require water for
growth, and it is impossible for them even to begin growth in
dried meats, fruits, and vegetables so long as they are kept dry.
Drying reduces the amount of water in the food substances; if
bacteria or other saprophytes come in contact with them they can
not live, since the water molecules are in greater Concentration
in the cells of the plant than in the food, and water passes from
the plant cell and causes its death.

The use of salt and sugar in the preservation of foods is a drying
process, and the destruction of bacteria and other plants in this
type of food preservation is really the result of osmosis. Water
passes from the cells of the invading organism into the food, and
the plants are killed by the loss of water.

Sometimes chemicals, such as benzoate of soda and salycilic
acid, are used in food preservation, but these are considered
injurious to man to a greater or less degree. Meat and fish hung
near a smoldering wood fire absorb certain acids from the smoke
which preserve the foods without being markedly injurious.
Smoking has long been used as a method of preservation of meat
and fish.

Methods of food preservation by canning were introduced
early in the nineteenth century. These have proved far superior
to the older methods. Bacteria do not thrive in the acid juices
of fruits. Boiling the fruit for a few minutes kills the yeasts and
molds, and if the heated materials are sealed in the can while they
are hot, they will usually keep indefinitely in perfect condition

NUTRITIVE RELATIONS OF SAPROPHYTES 93

This type of canning is easily done at home. For peas, beans, corn,
and meats, the problem becomes more complex, as the spores of
bacteria which may be present in these non-acid substances are not
killed by simple boiling and, sealed in the can, the spores germinate
and the bacteria reproduce rapidly in the abundant food supply
with the result that the food is spoiled for human use. The com-
mercial method of canning these substances consists in placing
the hot materials in cans, sealing, and heating them under steam
pressure to a temperature of 240 F. for 40 to 60 minutes. This
treatment kills the very resistant spores of bacteria on the food
and the food can not spoil.

In home canning the cold-pack method is usually followed.
The cans of food are sterilized either by steaming in an ordinary
steam cooker for 3 hours or by heating in a pressure cooker under
a pressure of 5 to 10 pounds of steam for one hour.

One of the commonest methods of keeping the food from
spoiling is by keeping it cold. Although the growth of molds and
bacteria is not entirely stopped by low temperatures, foods can be
kept very much longer if they are kept cold. Ordinary low tem-
peratures used in refrigeration can not be depended upon to pre-
vent food spoilage indefinitely. Mechanical refrigeration has
added much to the convenience and safety of preserving foods
temporarily by keeping them cold. Foods, especially fish and
meats, should be used promptly after removal from cold storage.

Exercise 59. Putrefaction of food materials. Place in a series of test
tubes with a little water small bits of food substances, as meat, potato, bread,
sugar, starch, flour, beans, and corn meal. Plug with a cotton stopper and
leave in a warm place for a few days. What evidence is there that putrefaction
has taken place? Is the result the same in all the tubes? What foods, if any,
have not putrefied?

Exercise 60. Will dry foods putrefy? Place in a series of test tubes dry
food substances as beans, flour, corn meal, rolled oats, dried beef. Plug with
a cotton stopper and examine after they have remained in a warm place for
several days. Determine whether there has been any putrefaction. What is
the relation of moisture to putrefaction?

Exercise 61. What is the effect of heat on putrefaction? Put bits of
meat, boiled beans, bread, milk, etc., into a series of test tubes with a little
water. Plug with cotton stoppers and heat in a pressure cooker at 10 pounds
pressure for 15 minutes, or in steam in a closed vessel for one hour. Set the
tubes aside in a warm place and examine after a week. Answer the question
of the exercise.

94 NUTRITION OF NON-GREEN PLANTS

Saprophytes in the soil. Under suitable conditions of moisture
and temperature, a fertile soil will continue to support a vigorous
growth of plant life year after year, and this has been continuing
for hundreds of years. Even large trees have reached maturity
and fallen, the wood gradually decomposing and dropping to
pieces to become a part of the soil. Leaves, as they fall from the
trees each autumn, do not continue to accumulate but gradually
disappear, their partly decomposed structures forming humus in
the upper layers of soil. We might expect that, while the plants
are taking raw materials from the soil season after season, as time
goes on it would gradually become exhausted so that it would be
less able to support plant growth. This, however, is not the case.
In fact, the opposite is true ; the soil actually increases in fertility.

Bacteria and molds cause decay. The succession of plant
growth on soil is made possible by the action of microscopic plant
life. Many different micro-organisms in the soil bring about the
decay of plant and animal materials.

Some forms of saprophytes destroy the cellulose which makes
up the walls of the plant cell. These are among the most impor-
tant destroyers of plant material. It is interesting to note that
bacteria of this class are present in the intestinal tract of herbivo-
rous animals, such as cattle and sheep, and make digestion of coarse
Jhay and fodder possible. What different things must happen to
-a piece of wood before the substances of which it is composed can
be used by other plants?

The cycle of nitrogen in nature (Fig. 37). One of the most
important of the elements used by plants is nitrogen. All protein
substances contain this element along with carbon, hydrogen,
oxygen, and small amounts of other elements. Although nitrogen
makes up four-fifths of the atmosphere by volume, its compounds
are the most expensive of the commercial fertilizers. Green
plants can not make direct use of any of this necessary element in
the form in which it occurs in the air. Nitrogen is set free and
lost to plants in decaying protein materials, and large quantities
of its compounds are lost as sewage. It was formerly thought that
when the nitrate beds of Chile became exhausted there would be
famine on the earth for the want of raw materials containing nitro-
gen for plant growth. It has been found that certain micro-

NUTRITIVE RELATIONS OF SAPROPHYTES

95

organisms in the soil have the power to bring about a combination
of nitrogen of the air with elements in the soil, forming compounds

Living things use food
produced by green plants,
use oxygen and give off
carbon -dioxide.

Oxygen

Carbon Dioxide

Plant and
Animal Remains

Food of Animals

Green plants use
carbon-dioxide and give off
oxygen in food-making.
Nitrogen salts are used
in making proteins.

Nitrogen of
S* the Air "\

Animal Wastes

5acteria of decay act
on the dead bodies of
plants and animals and
on animal uuastes forming
ammonia.

Bacteria on legumes and
other nitrogen -fixing bacteria
take nitrogen from the air and
form compounds used by plants.

V,

Denitrifying bacteria break
doom useful nitrogen compounds
setting nitrogen free.

Nitrates

Ammonia

Nitrifying bacteria act
on ammonia , forming
nitrates vjuhich green
plants can use.

FIG. 37. Diagram showing the relation of living things in nature. In what
ways do other living things depend on green plants? How are green plants
dependent on bacteria? What part do the nitrogen-fixing bacteria play in
nature? Trace the cycle of carbon in the diagram. Trace the cycle of
oxygen as it appears in the diagram. Trace the cycle of nitrogen from the
time it is a part of the food of animals, through the different processes until
it is again a part of the food of animals.

96 NUTRITION OF NON-GREEN PLANTS

which plants can use. In this way the supply of nitrogen in the
soil may be kept practically constant.

There are two principal groups of nitrogen-fixing bacteria:
(1) those which live free in the soil, and (2) those which are asso-
ciated with the roots of legumes, as red clover and alfalfa. The
latter are examples of symbionts. At the site of their invasion
of the root tissues, galls or tubercles form. The legume absorbs
the raw materials, salts and water from the soil, and the bac-
teria make use of certain of these materials. The bacteria combine
nitrogen of the air with materials furnished by the legume to form
proteins for themselves; and the legume also makes direct or
indirect use of these. In this relationship the two organisms living
together are mutually beneficial. So dependent are the legumes
on the tubercle bacilli that, if the necessary bacteria are not present
in the soil, the seed of alfalfa or the soil in which the seed is planted
must be inoculated; that is, the necessary form of bacteria must
be introduced before the alfalfa can be grown successfully.

It has long been known that the same crop can not be grown
indefinitely year after year on the same soil. The farmer may
grow corn followed by wheat, and with the wheat he may sow
clover. The clover crop may be used for hay, but roots and por-
tions of the stem rich in nitrogen compounds are left in the soil.
When these materials decay, nitrogen is added to the soil. The
system in which different crops are grown successively is known
as crop rotation, and every well-planned crop rotation scheme should
include a legume every two or three years. Give two reasons why
a certain crop should not be grown on the same soil year after year.

The nitrogen-fixing bacteria require well-drained soil which
contains organic matter. If it is highly acid, lime must be added
to reduce the acidity as these bacteria do not grow well in an acid
medium.

In our account of the nitrogen cycle we may start with the
complex plant and animal proteins. These are broken down by
many different soil organisms into simpler and simpler substances
until the final products can be absorbed by green plants as raw
materials out of which more proteins are made.

The bacteria of decay cause the decomposition of complex
proteins in plant and animal remains and the formation of ammo-

PARASITIC PLANTS AND ANIMAL DISEASES 97

nia. Resulting ammonia compounds are changed by other bac-
teria into nitrites. Nitrities are changed by still other forms into
nitrates. Nitrates in solution may be absorbed by the green
plant, where they unite with carbohydrates to form amino acids.
Amino acids are combined by plants to form plant proteins.
Plant proteins are eaten by animals, and animal proteins are
formed. The dead bodies of animals and plants are attacked by
bacteria, and a new cycle is begun.

It should be noted that not all the bacteria involved in the proc-
esses of the nitrogen cycle are saprophytes. The ammonia-
formers derive energy from the decomposition of dead plant and
animal materials; thus, they are true saprophytes. The nitrate-
and nitrite-formers are as truly independent as green plants since
their sources of energy are chemical substances, the ammonium
compounds. The nitrogen-fixing forms which live free in the soil
require carbohydrate foods produced by green plants for their
source of energy, and therefore they are saprophytes. The nitro-
gen-fixing forms which live in tubercles on legume roots derive
their energy from the carbohydrates and other food supplies of the
cells of the living host plants, and hence they are parasites.

Exercise 62. Root tubercles. Examine roots of a clover plant for evi-
dences of the presence of bacteria. Crush one of the tubercles, and note the
milky contents; this material contains the nitrogen-fixing bacteria. Where
do they get their food? What do they do for the plant? What does the
legume plant do for them? Are they saprophytes or parasites? Show that
the legume plant and its nitrogen-fixing bacteria are symbionts. Of what
importance are these forms in nature? Of what importance are they in
agriculture? In what way does a clover crop add to soil fertility? Write a
paragraph showing what would be the result if all the nitrogen-fixing bacteria
should cease to function.

Problem 3. How do parasitic plants cause disease in
animals?

Comparatively few of the very large number of different kinds of
fungi that have been identified have been found to cause disease
in animals. Although a number of molds cause disease, such as
ring- worm and athlete's foot, yet most of the pathogenic fungi are
bacteria.

98 NUTRITION OF NON-GREEN PLANTS

Leeuwenhoek first discovered bacteria late in the seventeenth
century. It was not until two hundred years later that they were
shown to be the cause of disease. In a series of epoch-making
experiments and in the face of much opposition, Robert Koch,
around 1876, showed that anthrax in sheep is caused by a rod-
shaped bacterium. He later showed that tuberculosis is caused
by another rod-shaped form. Louis Pasteur entered into the
study of anthrax in France with a view to finding some method
of preventing or curing the disease. His work resulted in a method
of preventing anthrax in animals by vaccination. This early
work was important as it not only met the immediate need of a
method of preventing anthrax but it also laid the foundation for
the development of our later knowledge along the lines of disease
prevention.

Many pathogenic bacteria produce poisons in the body of the
animal which is their host. These poisons, known as toxins, cause
the symptoms of the disease. It has been found that substances
which counteract the toxins are developed in the body of the
infected animal. These are known as antitoxins.

The resistance of a body to disease bacteria is known as immu-
nity. For some unknown reason, some individuals have a natural
immunity to a certain disease. They do not easily contract the
disease. A person who has had diphtheria is not likely to have
the disease a second time. He is protected by substances in the
blood which are formed as a reaction to the effects of the toxins.
This immunity is known as acquired immunity. Investigators
have learned that diphtheria antitoxins, developed in the blood of
a horse, can be used safely in protecting children against diph-
theria by providing an immunity. Vaccination is practiced by
physicians to protect against certain diseases by causing the
patient to have a mild form of the disease, with development of an
immunity which protects against the more serious form of the
disease.

The first successful vaccination was performed by Louis
Pasteur in the prevention of fowl cholera in chickens. He later
successfully vaccinated sheep against anthrax. Pasteur's name
has gone down in history as one of the greatest benefactors of the
race. He was able to show from his knowledge gained in the

LABORATORY STUDY OF BACTERIA AND MOLDS 99

study of dangerous bacteria how bacteria could be kept out of
wounds. Keeping a wound free from bacteria is known as asepsis
(without disease). Lister had already shown how bacteria could
be destroyed by the use of chemicals. The term applied to this
method of preventing infection is antisepsis (against disease).
Name five diseases of man which are caused by bacteria.

Problem 4, How may bacteria and molds be studied in the

laboratory?

In growing bacteria and molds for study, it is necessary to
furnish food materials for the plants and keep the temperature
and moisture conditions suitable. It is also necessary to be cer-
tain in most experiments that the food materials and glassware
used are free from bacteria and mold life at the beginning of the
experiment and that they be kept so throughout its progress.
Glass bottles or test tubes with stoppers made of wads of cotton
can be sterilized by heating in an oven until the cotton is slightly
browned. Culture media containing the food for the plants can
be sterilized by heating the cotton-stoppered bottles or flasks of
the material in a pressure cooker with steam at a pressure of
15 pounds. An ordinary kitchen steam cooker may be used if a
pressure cooker is not available. Heating in steam in such a
vessel for 30 minutes may not destroy all the spores that may be
in the culture material, but this method is sufficiently efficient
for use in experiments in high-school classes. Conn's Bacteria,
Yeasts, and Molds in the Home gives detailed directions for the
preparation of media and the doing of many interesting and instruc-
tive experiments^ suit able for the work of high-school classes.

Exercise 63. Bacteria in water. Crowd leaves and stems of water plants
into a jar of water and set aside to let the materials decay. Note changes in
the appearance of the liquid as decomposition proceeds. Mount on a glass
slide under a cover-glass some of the cloudy liquid as it develops. Examine
under the high power of the microscope with most of the light shut off. What
evidence of life do you see? Does decay give rise to the bacteria, or do the
bacteria cause the process of decay? What happens to the solid materials as
decay goes on? What is in the water that was not there before decay started?
Why is pure water less likely to have living bacteria in it than water polluted
with sewage? Why is water from a deep well less likely to contain large
numbers of bacteria than water from a river?

100 NUTRITION OF NON-GREEN PLANTS

Suggested activity. Make a study of the water supply of your community:
is it surface water, as that from a stream, pond, or lake; or is it ground water,
as that from a deep or a shallow well? Is it likely to have sewage in it and thus
carry dangerous as well as other forms of bacteria? Present a report of your
study to the class.

Write a report on methods used in your community to make the water
supply safe for drinking purposes.

Exercise 64. Bacteria in milk. Put in each of six large-mouth, half-pint
bottles one-third of a pint of raw milk that has not been heated. Close each
bottle with a wad of cotton. Heat three of the bottles in a steam cooker or
other closed vessel with water for half an hour. Label the bottles to identify
them, and set aside with the unheated bottles in a moderately warm place.
Note the appearance of each of the bottles daily for a week. At the end of
that time remove the stoppers and note the odor of the milk in each bottle.
Account for any differences. Why is milk a good medium for bacteria?
Under what conditions will milk sour? What is the danger In washing
bottles, cans, and other utensils used in handling milk in water contaminated
by sewage wastes? What is the danger in allowing a person harboring disease
germs, as a carrier or himself having the disease, to handle the milk?

Suggested activities. Write an account of the methods used to keep your
milk supply free from dangerous bacteria.
Justify the expense of city milk inspection.

Bacteria in the air. Wherever there is dust in the air there are
bacteria. Bacteria ride on particles of solid material of which dust is
composed. It is true, not every speck of dust has its passenger;
but wherever there is dust you can be certain there are bacteria.
Most of them are harmless ; others are helpful, falling upon organic
waste materials and causing them to decay; still others get into
exposed foods and cause them to spoil, and we breathe some into
our throat and lungs which may cause inflammation. On a
mountain top there are few bacteria, but in crowded centers of
population they are everywhere in great numbers.

Exercise 65. Bacteria in the air. Wash and dry thoroughly petri dishes
with covers. Place them in an oven and gradually raise the temperature to
moderate heat. Turn off the heat after half an hour, and when they are cool
wrap each dish without removing the cover in paper for protection from dust.
Prepare nutrient Bacto agar as directed, and while it is still hot remove care-
fully the cover of a dish and pour into it enough of the liquid to cover the
bottom of the glass. Replace the cover immediately and repeat the process
until the required number of dishes has been prepared. When cool, the
nutrient material which contains the food for bacteria should be a jelly.

LABORATORY STUDY OF BACTERIA AND MOLDS 101

Place a dish on a table in the laboratory and remove the cover, exposing the
jelly for five minutes. Replace the cover and set the dish in a warm place.
Examine daily. The spots which appear on the agar are colonies of bacteria,
each having started with a single bacterium which fell upon the jelly at that
point while the surface was exposed in the room. Test for the presence of
bacteria in the air of the assembly hall, that of the corridor when classes
are passing, of the living room at home, and of a park. Which is most effect-
ive in dusting, from a sanitary point of view a feather duster, a dry
cloth, or a damp cloth? Why are outdoor diversions to be preferred, in
general, to spending leisure time indoors?

Suggested activity.
Devise an experiment us-
ing petri dishes of nutrient
agar to show that it is not
only bad form to sneeze or
cough in public without
covering the nose and
mouth with a handker-
chief but that the prac-
tice is, besides, decidedly
insanitary.

Exercise 66. To collect
bread mold. Moisten a
thin slice of rye bread
with prune juice and fit it
into a petri dish. Leave
uncovered in the labor- FIG. 38. Drawing of a portion of a mycelium of

bread mold (Rhizopus nigricans). The mold
spreads over the surface of the food material by
means of hyphae called stolons. Other hyphae,
known as rhizoids, are special absorbing organs
of the plant. The black knobs (white when
young) are sporangia which bear asexual spores.
They are supported by hyphae called sporangio-
phores (spore-sac-bearers).

atory for fifteen minutes,
then cover and set away
in a moderately warm
place for two or three days.
Molds of various kinds,
including bread mold, will
probably be found growing
on the bread from spores

collected from the air of

the room. Dark portions of the molds collected in this way are usually growths
of bread mold. These can be picked out with a pair of fine-pointed forceps
for use in starting a pure culture of bread mold in the following exercise.

Exercise 67. What is the nature of bread mold? Fit thin slices of rye
bread into petri dishes. Cut a IJ^-inch square of the bread out of the center
of each dish, leaving a window through which the growing mold can be studied.
Moisten the bread in the petri dish with prune juice, and transfer some of the
mold of the previous exercise to the bread on two opposite sides of the window.
Replace the cover and set the dishes away in a moderately warm place. Examine

102 NUTRITION OF NON-GREEN PLANTS

frequently after the first twenty-four hours without removing the cover. As
the mold grows out from the bread and across the window, it can be studied
with a lens or by placing the dish bottom side up under the microscope, using
the low power. Placing a few drops of prune juice on the portion of mold in
the window to be studied, covering this with a cover-slip will permit a better
view of the growing mold. Note that the separate threads (hyphae) are with-
out cross walls and that they branch repeatedly. The whole mass of hyphae is
a mycelium. The hyphae that grow along the surface of the bread are known
as stolons. The knobs, at first white and later black, on the ends of hyphae
extending out from the stolons are sporangia. These bear spores which ripen
and are blown about by the wind, some of them reaching new sources of food
where they may germinate and produce other mold plants. At the base of the
sporangium-bearing hyphae are root-like hyphae (rhizoids) fitted by structure
for going down into the medium (the bread) on which the mold is growing and
from which it is absorbing food.

Write a summary giving the r61e of each of the three types of hyphae of
bread mold which you have seen. In what two ways does bread mold spread
to new sources of food? In what respects are spores better for distributing the
plant than fragments of the mycelium?

Gametic reproduction. Under certain conditions a fourth
type of hypha is produced by bread mold. It has been demon-
strated that there are different, distinct strains of the plant. If a
stolon of one strain grows near the stolon of another strain of the
plant, special hyphae are sent out from the two stolons. These
approach each other. When they come in contact, end to end,
a special cell is formed in each. These cells fuse, making a single
cell which forms about itself a heavy wall which protects the living
material inside, and carries it through extended periods during
which conditions are not favorable for growth of bread mold.
The two cells which unite are called gametes, and the single cell
formed from their union is a second kind of spore, a zygospore.
Under favorable conditions, after a dormant period, the zygospore
germinates and produces another plant. The type of reproduc-
tion involving gametes is known as gametic or sexual reproduction.
Note the four types of hyphae shown in Fig. 39.

Exercise 68. Gametic reproduction in bread mold. To grow zygospores
of bread mold. It is necessary to have a culture of a " plus " strain of
Rhizopus nigricans and a culture of a " minus " strain of the mold. These
can be bought from dealers in biological supplies. Prepare a petri dish with
rye bread and prune juice as in Exercise 67. Using fine-pointed forceps,
inoculate, with a streak of the " plus " strain and a streak of the " minus "

LABORATORY STUDY OF BACTERIA AND MOLDS 103

When asexual spores from
two strains of bread mold
qermmate near each other
qametes are formed

When food is plentiful
asexual spores man reproduce
the plants aver and over aqain
This is the usual method of
reproduction

Each plant sends out a
peculiar Kind of hvjpha

Gametes develop where
the hi|phae meet

Asexual spores
qermmate and form two
strains of mold plants

The zqqote qerminates
and two kinds of asexual
spores are formed

Two qametes umte to
form a z.t|qote, a restmq spore

FIG. 39. Life cycle of bread mold (Rhizopux m^ncans). The usual method
of reproduction of bread mold is by asexual spores. Under what conditions
in nature may zygotes be formed? How could bread mold be distributed
to new sources of food? By what two methods can a piece of bread be com-
pletely covered with mold as the result of the germination of a single asexual
spore? What is a possible r61e of zygotes in the life cycle of bread mold?
The union of two similar gametes resulting in the production of a zygote
as it occurs in bread mold is a simple type of gametic reproduction known

as conjugation.

104 NUTRITION OF NON-GREEN PLANTS

strain, the surface of the bread on opposite sides of the window. Allow growth
to continue for a week. Look for stages in the forming of zygospores in the
window between the two streaks. Bits of this material may be easily lifted out
and studied under the microscope.

How can bread mold use the nutrient materials on which it
grows? From the action of enzymes produced by the rhizoids,
complex food materials in the bread are broken down into simpler
substances. In these simpler forms the nutrient materials can
diffuse through the membranes of the rhizoids. Inside the plant
these foods pass along the hyphae by diffusion. By oxidation of
nutrient materials, energy is released. Thus the plant can carry
on the necessary life processes. By assimilation, some of the
nutrient materials become a part of the protoplasm, making it
possible for the plant to grow.

Problem 5. How do parasitic plants cause plant diseases?

We have found that bread mold may cause the gradual break-
ing down of the chemical compounds which are found in bread or
other nutrient substances which are non-living organic materials.
Rhizopus nigricans and related molds may also become parasitic,
invading living plant tissues and causing plant diseases, among
which are certain rots of sweet potatoes, strawberries, apples,
raspberries, and other plant products. Spores of Rhizopus are
widely distributed, and every precaution must be taken by growers
to avoid conditions which would be favorable for their invasion
and growth in the food materials. As an example, sweet potatoes
must not be bruised in handling, and they must be carefully dried
and aired during the time that they are apt to heat and collect
moisture in storage. Wrapping apples in paper tends to keep
them dry and protects against invasion by Rhizopus, thus prevent-
ing apple rot from this cause.

Plant diseases caused by bacteria. The plant juices form
favorable nutritive material for many bacteria. An acid condition
of the sap may keep them out, as bacteria require a slightly
alkaline or neutral medium. The juices of most plants, however,
are suitable for the growth of these organisms. Bacteria may
enter the host through wounds, or through openings, as stomata

PARASITIC PLANTS AND PLANT DISEASES

105

and lenticels. The bacteria may cause injury to the host by invad-
ing the water-conducting vessels of the plant and cutting off the
water supply, or by destroying soft tissues of the plant and pro-
ducing blight or rot, or by stimulating the cells of the host to pro-
duce abnormal growths as knots or galls.

Fire blight. Where pears, apples, and quinces are grown it is
quite common for branches of trees in full leaf to turn brown and

FIG. 40. The large galls or warts are the result of infection by bacteria.
The disease is known as crown-gall.

die. The disease causing this symptom is fire blight, a serious
bacterial disease. It may attack the blossoms, causing blossom
blight, or the leaves, causing leaf blight. It may also affect the
twigs, larger branches, or main trunk of the tree. It affects peat
trees more than any other and is sometimes called pear blight,

106

NUTRITION OF NON-GREEN PLANTS

The cells of the soft tissues are invaded and killed by the bacteria.
Measures for dealing with fire blight include control of insects, such
as plant lice and other insects which carry the bacteria; cutting
out infected parts of trees; and selection of varieties of fruit which
are resistant to attack.

Diseases due to rust fungi. The common name, rust, of this
group is suggested by the colored spores which are conspicuous
in certain stages of the development of the parasite. The rusts

cause disease in nearly all groups of
economic plants: grains, grasses, or-
chard trees, garden vegetables, and
forest trees. During the World War,
when methods of greater food produc-
tion and conservation were studied
and carried out, a systematic attempt
was made to eradicate wheat rust by
ridding the country of its alternate
host, the common barberry.

Stem rust of wheat. This is some-
times called black rust because of
the black blotches which are found
on the stem of wheat in certain stages
of the disease. When the Chicago
Board of Trade gets reports of extensive
invasion of the wheat fields by black
stem rust, there is usually an immediate
rise of price of wheat. Why? The
effect of rust may be slight, or almost a
complete failure may result, depending

upon the severity and time of attack. The damage to the host is
caused by the loss of the food appropriated by the parasite and
by the loss of water at the points where the epidermis is ruptured
by the rust plant.

Life cycle of stem rust. A convenient place from which to
start is the germination of the teliospore, the two-celled resting
spore of the parasite, by means of which it goes through the winter.
In the spring, under favorable conditions, each of the two cells of
the teliospore gives rise to a short hypha from which four sporidia

FIG. 41. Germinating rust

spores shown penetrating the

tissues of a leaf by way of

the stoma.

PARASITIC PLANTS AND PLANT DISEASES

107

(early spring spores) are loosed and blown away by the wind.
These sporidia can not infect the wheat, and are lost unless they
fall upon a leaf of the common barberry. The infection of the
barberry leaf results in the production of aeciospores (late spring
spores) which can not germinate on the barberry, but if blown to
susceptible wheat, germinate and produce an infection. Reddish
oblong spots, uredinia, soon appear with the production of large

FIG. 42. A portion of a wheat stem
infested with the black stem rust.
Observe the hyphae (threads) among
the cells of the host, and the cluster
of spores. These are the one-celled
summer spores (urediniospores).

FIG. 43. A portion of a wheat stem
infested with the black stem rust.
Observe the hyphae (threads)
among the cells of the stem, and
the cluster of spores. These are
the two-celled winter or resting
spores (teliospores).

numbers of the oval one-celled red summer spores of the rust,
urediniospores. These spores are scattered by the wind to other
plants of wheat, and thus the fungus is spread extensively during
the growing season. Later, the urediniospores may be replaced
on the same mycelium by the black or chestnut-brown two-celled
winter spores, the teliospores. These are not capable of germinat-
ing at once but must pass through a dormant period. Thus they

108

NUTRITION OF NON-GREEN PLANTS

are the resting cell of the fungus. Of what advantage is it to
wheat rust to produce teliospores?

It is seen that the barberry is the intermediate host in the life
cycle of stem rust of wheat when barberries are present, and the
presence of the barberry is probably responsible for severe destruc-
tive attacks of stem rust. However, urediniospores may over-
winter under certain mild winter conditions and be spread directly
to the new crop of wheat plants.

The main methods of control of stem rust are (1) eradication
of the common barberry, and (2) the selection and breeding of
immune varieties of wheat. Denmark has eradicated the bar-
berry, and black rust has
almost disappeared. In
Wales, where the barberry
is still allowed to grow,
there are serious losses of
wheat each year from rust.
Among resistant varieties
of wheat, Kanred, a variety
of winter wheat, and Black
Persian, a spring strain,
were found from extensive
experiments, to be most
promising.

Apple rust may be men-
tioned as another rust

which normally requires an intermediate host for the completion
of the life cycle. The infection of the apple leaves and fruit results
in the production of aeciospores. These are blown to cedar trees,
where they produce infection which results in the so-called cedar
apples. The cedar apples develop spore horns which produce
teliospores. These germinate, forming sporidia which are blown
away and infect the leaves and fruit of the apple with the rust.
In regions where there are cedar trees to be protected, cedar rust
can be prevented by eradication of apple trees. In regions where
apple growing is profitable, apple rust can be prevented by de-
struction of the cedar trees.

Stem rust of wheat and apple rust are examples of diseases

I'iG. 44. A large uush ot common barberry

an alternate host for the black stem rust of

wheat and other cereals.

GROUPS OF FUNGI 109

caused by fungi which have more than one host. This habit of
life makes their control by man very difficult. Consequently they
have become the worst enemies of certain crop plants.

Plant diseases due to smut fungi. Many plants among the
cultivated cereals are affected by smuts. The parts most fre-
quently destroyed are the grains or the flowers. As a result, every
year there are heavy losses of cereal crops due to smut.

One of the most destructive diseases of wheat is stinking smut
or bunt. Infection takes place in the young seedling stage, from
spores carried on the seed. As a result of germination, infection
threads are produced which penetrate the seedling and reach the
growing point of the host. The fungus remains inside the host
and gives little or no indication of its presence until the head of
wheat emerges. Infected plants produce smut balls instead of
grain. Since the disease is caused by an internal parasite, the
smut plant is living at the expense of the host, consuming food,
destroying seed, and retarding the normal life processes of the host.

Among other smuts which affect cereals are loose smut of
wheat, loose smut of oats, and common corn smut. In the loose
smuts of wheat and oats, the grains are destroyed. The smut
which is formed in their place is loose and easily blown away. In
corn smut, the usual symptom is the formation of small or large
tumors on various aerial parts of the corn plant. At first, these are
whitish, but later they become black owing to the development of
spores. The membrane covering the mass breaks, and the mil-
lions of spores are released. The estimated annual loss in reduction
of yield of the corn crop due to smut is 21 per cent. (See Fig. 5.)

Suggested activity. Make a collection of dry plant parts showing effects
of plant parasites.

Problem 6. What are the principal groups of fungi?

As we have seen, fungi may be classified on the basis of nutri-
tion into two main groups: saprophytes, forms which use non-
living organic food; and parasites, which obtain their energy and
materials for growth from living hosts. We have also learned that
some of the bacteria are autophytes, securing their energy from the
oxidation of inorganic substances.

110 NUTRITION OF NON-GREEN PLANTS

Bacteria. The bacteria represent the simplest fungi, being very
minute, strictly single-celled forms.

Phycomycetes. The Phycomycetes or alga-like fungi usually
have a mycelium composed of hyphae which are tubes and not
made up of cells arranged end to end in filaments. In this respect
they resemble certain algae, as Vaucheria. This group includes
bread mold, certain blights, and many related forms. The late
blight-fungus of the potato, a phycomycete, was responsible for the
loss of the potato crop which resulted in the great Irish famine
of 1845.

The white mold appearing on injured gold fish and frequently

FIG. 45. The reindeer lichen thrives in the tundra of the far north;

on young fish in hatcheries is a water mold, one of the alga-like
fungi. ' The hyphae grow into the flesh of the fish, usually resulting
in its death.

Ascomycetes. The Ascomycetes, or sac fungi, differ from the
alga-like fungi in that they are made up of hyphae composed of
separate cells. The common blue and green molds often seen
growing on fruits are examples of this group. The blue and green
molds are the most widely known of the molds which are destroyers
of foods. Fruits, fresh and canned, bread, and ever* smoked
meats are attacked. Molds of this group are used ir> the manufac-

GROUPS OF FUNGI

111

ture of certain types of cheese to give them the desired flavor.
These molds illustrate the characteristic of molds in general in
being able to secure food and water from concentrated sugar
solutions such as jellies and preserves. This property of the molds

FIG. 46. Mushrooms growing from an old stump of a tree.

results from the fact that the solution in the vacuole of the mold
cell is comparatively highly concentrated.

Lichens. The greenish or grayish patches growing on rocks,
trunks of trees, and sometimes on the ground, which are known as
lichens, really represent two different plants living together. The
two plants, one an alga made
up of green spherical cells,
and the other a fungus, live
together in a symbiotic rela-
tionship. They are mutually
beneficial. The alga makes
food for itself and for the
fungus. The fungus holds
moisture which is used by the
alga in the manufacture of

food. Most lichen fungi are ascomycetes, bearing spores in sacs
(asci) formed in brown cups or fruiting bodies appearing on the
upper surface of the flat body of the lichen. Lichens are usually
distributed by fragments of the lichen body.

FIG. 47. Indian pipe, a saprophytic
non-green seed plant.

112

NUTRITION OF NON-GREEN PLANTS

A number of serious plant diseases are caused by parasitic
Ascomycetes. Among these are peach leaf curl and brown rot of
stone fruits.

Basidiomycetes or basidium fungi. The character which gives
this group its name is the production of club-shaped hyphae or
basidia which bear the spores. The group includes important

parasitic and saprophytic
forms. The rusts and smuts
which affect grains, and the
fungi which rot timber, are
among the parasitic forms.
Familiar saprophytes of
the group are mushrooms,

FIG. 48. Dodder, a parasitic non-green
seed plant. Growing on goldenrod stems.

FIG. 49. Cross-section of alfalfa
stem and lengthwise section of a
portion of dodder stem which is
attached to the alfalfa stem by
two haustoria. Dodder is a para-
sitic seed plant.

puffballs, earth stars, and stinkhorns. The part of the plant which
attracts attention is really the fruiting body; the structures related
directly to nutrition are hidden away in the substratum.

Aside from the rusts and smuts, most of the Basidiomycetes
which are of economic importance are wood-destroying forms.
Some of these are purely saprophytic, destroying posts, piling,

GROUPS OF FUNGI 113

timbers, etc., which are continually moist and in contact with air.
Others enter living trees through wounds, as those made in pruning
or by hail or lightning. Infected trees may be greatly weakened
by root rot or heart rot. The parasites are unable to advance into
the active sapwood, and a tree may seem healthy from outward
appearance even though its trunk may be hollow from the dis-
integration of the heartwood by wood-destroying organisms.

How do non-green seed plants obtain food? A number of
seed plants without chlorophyll attach themselves to the roots of
green plants and thus secure food which takes the plant through
the complete cycle of flower-bearing and seed-producing above
ground. The broom rape family includes the Indian pipe, pine-
sap, beech drops, and squaw root. Some of these plants are
parasites growing in contact with roots of trees. Indian pipe and
pinesap are saprophytic, non-green seed plants, getting their food
from humus.

One of the most familiar of the parasitic seed plants without
chlorophyll is the common dodder. The dodders are very close
relatives of the morning-glory and sweet potato, although their
resemblance is not evident to the casual observer. They have
undergone marked changes due to their parasitic life. The seed
germinates in the soil as does that of other seed plants. If the
young seedling comes in contact with a suitable host, which may
be a weed or a crop plant, it twines about the adopted plant, send-
ing absorptive organs called haustoria into the tissues of the host.
Manufactured foods are thus taken from the tissues of the host
plant directly into the tissues of the parasite. Dodders are of
considerable importance in various parts of the country as destruc-
tive parasites on clovers, alfalfa, sugar beets, and other cultivated
plants.

ADDITIONAL QUESTIONS AND EXERCISES

1. What is the explanation for the fact that among seed plants we find an
occasional white seedling?

2. Explain why a variegated plant is more likely to go down in the struggle
in competition with all-green plants.

3. One use of food in animals is to supply a source of energy for physical
exercise. Why do plants need an energy supply in the form of food?

4. We find large trees and small trees growing together in a forest. Explain

114 NUTRITION OF NON-GREEN PLANTS

why the age of the forest can not be determined by counting the annual rings
on the stumps of the largest trees.

5. Are the bacteria of decay the farmer's friends or his enemies?

6. Why are bruised fruits more likely to rot than sound fruits?

7. Why is it that bread dough, while rising, must be neither hot nor cold?

8. Why is the flavor of butter made from pasteurized cream more constant
than that made from raw cream?

9. Why should canned fruit or vegetables be discarded without tasting if
the ends of the tins are bulged outward before opening?

10. Explain why it is not safe to eat canned fruits which have mold on the
top of the fruit, even though the molds are usually not poisonous.

11. How does smoke preserve meats?

12. Why is it that clover and other legumes do not grow well on acid soils?

13. In what way does a clover crop add to soil fertility?

14. In what respects may the widely distributed sweet clover be considered
a beneficial weed?

15. What is meant by the nitrogen cycle in nature?

16. What is the carbon cycle?

17. Show the importance of the wide distribution of bacteria on the earth.

18. Explain why a deep well is usually better as a source of drinking water
than a shallow well.

19. How can mold spores which are on the surface of fruit be prevented from
germinating and causing rot?

20. Knowing what you do about molds, how would you store apples to pre-
vent loss by rotting?

21. Make a spore print of a mushroom by placing the cap of the mushroom
on paper, gill side down. Why is it necessary that such large numbers of spores
are produced?

22. What are the disadvantages of the parasitic habit to the plant parasite
itself?

REFERENCES

Bacteria, Yeasts, and Molds in the Home, by H. W. CONN, revised edition
by H. J. Conn, 1932. 320 p. Published by Ginn and Company. Should be
available for reference throughout the study of the unit. Useful and inter-
esting facts about non-green plants that every student should know. Appendix
with many easy experiments that may be performed by students who are able
to do extra work.

Microbe Hunters, by PAUL DEKRUIF. Revised edition. Published by
Harcourt, Brace, 1926. Biographies. Interesting account in story form of
the lives and work of Koch, Leeuwenhoek, Pasteur and others who were
pioneers in the field of the relation of bacteria to disease.

Science in the Service of Health, by E. R. DOWNING. 320 p. Published
by Longmans, Green and Company, 1930. Easy reading in the history of
the discoveries concerning the relation of bacteria to disease.

GROUPS OF FUNGI 115

Man and Microbes, by S. BAYNE-JONES. Published by Williams and
Wilkins Company, Baltimore, 1932. 128 p. An interesting written picture
of Bacteriology.

Soil and the Microbe, by S. A. WAKSMAN and R. L. STACKEY. Published
by John Wiley and Sons, New York, 1931.

Civilization and the Microbe, by ARTHUR I. KENDALL. Published by
Houghton Mifflin Company, Boston, 1923. 231 pages. This book tells the
story of the microbe in simple language understandable by the student of
high-school age. It tells in a most interesting way what bacteria are, of the
various forms of bacteria, of the temperature range of microbic life, of the
nutrition of bacteria, of bacteria and the industries, of disintegration of animal
and plant remains by bacteria, of diseases caused by bacteria.

Who's Who Among the Microbes, by WM. H. and ANNA W. WILLIAMS.
Published by the Century Company, New York, 1929. 302 pages, illustrated.
The sketches in this book grew out of a series of radio talks on communicable
diseases and their microbes. The authors have endeavored to describe
"simply and accurately the most important facts known that help us determine
how and why some microbes are harmful to man, others harmless and still
others helpful." They also tell how man can use available knowledge to
protect himself against harmful bacteria and utilize more fully the activities
of the useful bacteria.

Bacteria in Relation to Man, by JEAN BROADHURST, published by J. B.
Lippincott Company, Philadelphia, 1925. 306 pages, 144 figures. A study-
text in general microbiology, dealing with molds, bacteria, air, water, milk,
soils, and human disease. At the end of chapters are excellent reference lists.

UNIT IV
GROWTH OF PLANTS

When a plant or animal grows it becomes not only larger, but
also different in the structure and relationship of its organs, and
in its behavior. That is, growth involves more than simple
enlargement. When an acorn grows into the mighty oak, the
full-grown tree is not merely the enlargement of a miniature oak
concealed within the acorn. The mature tree possesses tissues and
organs that did not exist in the acorn. Moreover, it has certain
activities not carried on by the young plant. And, too, growth
usually involves a change in the chemical composition of the plant.
Growth then has two phases: (1) increase in size, due to increase in
number and size of cells, and (2) "a becoming-different," more tech-
nically called differentiation. This latter phase usually means an
increase in complexity. The marvelous feature of growth is this
differentiation the development of a complex structure such as
the oak tree with its roots, leaves, stems, flowers, fruits, and seeds,
and the many different kinds of tissues which compose these vari-
ous organs, from a few simple tissues in the acorn. But still
more wonderful is the development of the miniature plant hidden
within the seed from a single cell. The fundamental fact to keep
in mind is that the adult plant is derived from a single cell. The
beginning of every plant is a single cell. In seed plants this
single cell is in a part of the flower called the pistil.

Not all parts of the plant grow at the same rate. For example,
early in the germination of a bean seed the young root grows more
rapidly than the stem. When a flower bud opens there is, for a
short period, a very rapid growth of the flower organs.

Growth of a plant is the resultant of all its other activities.
Every activity must progress normally if the plant is to grow
properly. There must be absorption of mineral salts from the soil
and of oxygen and carbon dioxide from the atmosphere ; food man-

116

GROWTH OF PLANTS

117

ufacture and respiration there must be; there must be adequate
transportation of substances throughout the plant; and there must
be food digestion and assimilation. Then we know that all these
functions are influenced by certain environmental conditions such
as temperature, moisture, light, the supply of raw materials for
food manufacture, etc. The growth of a plant is, in many respects,
like that of a boy or girl. Unless all functions of the body

^Cotyledons

Hypocotyl

First
Foliage Leaves

FIG. 50. Successive stages in the germination of the seed of the bean. (From
Holman and Robbins in Elements of Botany.)

respiration, digestion, assimilation, excretion, etc. are normal,
and unless the environmental conditions which control these func-
tions are suitable, the boy or girl does not grow as he or she should.
Not all plants grow at the same rate. We recognize that it is
the nature of some kinds of plants to grow more rapidly than
others. For example, certain poplars attain a height of 15 to 20
feet within a few years, whereas slow-growing oaks may require

118 GROWTH OF PLANTS

three or four times as many years to attain this height. Ordi-
nary field corn grows to a height of 6 or 8 feet during the growing
season, whereas certain dwarf varieties of corn under similar
environmental conditions may attain a maximum height of not
over 2 or 3 feet. We have learned that food is made in the chloro-
phyll-bearing cells. Leaves are the principal food-manufacturing
organs. Do you think that the total leaf area of a plant has an
influence on the rate of growth of a plant? Do you think that
temperature, light, moisture, fertility of the soil, and other environ-
mental conditions influence the rate of growth? Explain.

Problem 1. How do embryos grow?

The young, undeveloped plant as it exists in the seed is called
an embryo. It is a simple structure with relatively few organs.
For that reason, we say it is undifferentiated.

Exercise 69. Structure of embryo. Soak the seeds of beans, peas, squash,
etc., in water. After they are softened, carefully remove the seed coats and
separate the embryo. Each kind of embryo has a characteristic form, and
certain organs. For example, in the bean embryo, notice the two large fleshy
cotyledons, the very short, young root, the miniature stem, bearing two dimin-
utive leaves. Compare the embryos of several different kinds of plants.

Development of the embryo following fertilization. We are
familiar with the fact that seeds are formed hi flowers. Seeds
develop within a certain structure of the flower called the pistil.
Long before the flower is open, there develops within its pistil one
or more small masses of tissue, each of which is destined to become
a seed. Among the cells of each mass of such tissue there is one cell,
the so-called egg cell, which is destined to become the embryo
but only after it is fertilized. Fertilization consists in the union
of the egg cell with a cell which is formed within the pollen grain.
In other words, fertilization involves the union of two living cells
one developed in the pistil, the other developed in the anther.
The embryo begins its life as a single cell the result of a union.

This single cell a fertilized egg cell immediately divides and
redivides. Soon protuberances, composed of groups of cells which
are to become leaves, stem, and root, appear in the solid mass of
cells. Then the seed coats harden, and the embryonic plant

GROWTH OF PLANT CELL 119

ceases to grow, awaiting favorable conditions for resuming growth.
The whole structure has now become a seed. Its essential struc-
ture is the embryo. Consider the oak tree again. We are accus-
tomed to think that the plant begins its life at the time the seed
germinates. This is not true. The embryo oak may have been
resting for months or even years within the seed. The very begin-
ning of the individual oak tree is the fertilized egg cell.

Problem 2. How does the plant cell grow?

We have learned that the plant body is composed of innumer-
able units called cells. All many-celled living things grow by the
multiplication and enlargement of their cells. Each cell con-
sists of a minute mass of living substance, or protoplasm, enclosed
by a more or less rigid, non-living wall. The living substance has
two main parts: an outer region, the cytoplasm, which probing
with an extremely fine needle proves to be of about the consistency
of glycerin; and an inner slightly less fluid region, the nucleus,
which seems to control the activity of the whole. Both are sur-
rounded by delicate membranes. Throughout the cytoplasm are
small, clear, watery areas called vacuoles, which contain cell sap
(water plus other substances in solution).

There are conspicuous differences between a young cell and
an old cell. Near the growing point of any stem or root we can
observe cells of different ages, and thus note the changes which
they go through as they grow.

Exercise 70. Growth of cells in a root tip. Examine prepared, specially
stained, lengthwise sections of a root tip. Observe the youngest cells at the
growing point, and cells of increasing age farther and farther back from the
growing point. Compare the cells observed with Fig. 51.

From the observations in this exercise we note the following very obvious
changes in a growing cell: (1) enlargement, which is usually not equal in all
directions; (2) increase in the size of the vacuoles, such that in an old cell the
cytoplasm and nucleus merely line the inner surface of the cell wall; (3) in
addition to these more obvious changes, there is often, in a growing cell,
increase in thickness of the cell wall, and changes in the wall's structure and
chemical composition.

Exercise 71. Differentiation. Examine the prepared slides of Exercise
70, and observe the different kinds of tissues distant from the growing point,
that is, in the zone of differentiation. Here we see cells of many different sizes

120

GROWTH OF PLANTS

and shapes, with differences in the thickness of walls and markings on the
walls. Clearly, these cells are fitted to carry on different functions. A point
worthy of special note is that all the different kinds of cells back of the growing
point have been derived from the same kind of cells those found at the grow-
ing point. In the development of growing-point cells, some have taken one
course, some another; some have become conducting elements, others storage
elements, etc. This is a process called differentiation.

One Larqe.;
Vacuole

Cqtoplasm^

and
Nucleus

at
Cell uuall

EJonqated
Cells

Elonqatinq
Cells

No
Vacuoles

Cells
Loose,
Irreqular

Cell Division
Req ion

Root Cap Cel Is

Tqpical Cell Characteristics in Root Tip

FIG. 51. The root tip at right, cut in lengthwise section. At left are cells

(enlarged) taken from different zones of the root. Note that the cells become

older the farther they are from the region of cell division. Furthermore,

observe the changes in the cells as they mature.

NATURE OF SEED GERMINATION 121

At the very growing point, in the region of active cell division, growth of a
cell is soon followed by division of the cell. That is, a cell grows to a certain
size, then divides into two similar cells, each resulting cell then doubling in size
and dividing in two, and so on. The process of cell division is very complicated;
it will be discussed in another chapter. Thus growth of this region involves
both an increase in the size of cells and in the number of cells.

Problem 3. What is the nature of seed germination?

We have learned that the essential part of a seed is the young
plant the embryo. The plant in the embryo stage of its develop-
ment may remain alive for years. That is, some seeds have a very
great longevity. Under proper conditions of moisture, tempera-
ture, and supply of oxygen, the embryo starts to grow. This
growth of the embryo, with the accompanying bursting of the seed
coats, is called germination. Seed germination is essentially the
resumption of embryo growth. Germination is completed when
the young plant has developed far enough to lead an independent
life, that is, does not derive nourishment from food stored within
the seed. The food stored within the seed was obtained from
the parent plant during the growing season.

Exercise 72. Water and germination. Place a number (100 to 200) of
dry seeds of corn, radish, wheat, or other different kinds of seeds in glass
tumblers or wide-mouthed bottles with a substratum of cloth, sand, paper, or
sawdust that is barely moistened; in a second series, use soaked seeds, placing
them on substrata saturated with water; in a third series, cover the dry seeds
with water. Keep all at same temperature. Record your conclusion.

Water and germination. Water softens the seed coats and
makes it possible for the young sprout to break through them more
easily. Water also facilitates the entrance of oxygen into the seed,
for when the seed coats are wet, oxygen will diffuse through them
more readily than when they are dry, and too, carbon dioxide
which is given off in the respiration of the living cells of the embryo
can diffuse outward more easily. The secretion of digestive fluids,
the digestion of stored foods, the movement of foods, in fact, all
activities of the cells of the seed proceed only when they are well
filled with water. Seeds which are old will not stand as much
water as vigorous, fresh seeds. In handling old seeds, care must
be taken to apply the water to them gradually and uniformly;

122

GROWTH OF PLANTS

variations in, the amount of water are injurious. Seeds will
endure greater extremes of temperature when they are dry than
when moist. Why?

Temperature and germination. No less essential than water
to seed germination is a proper temperature. The temperature
which is the most favorable to germination is not the same for the
different kinds of seeds. It is quite well known that cucumbers
and melons require a higher temperature to germinate properly
than wheat, barley, and certain other small cereals. The seeds
of " cool-season crops " such as peas, lettuce, radish, and small
cereals, will germinate readily at 50 to 60 F., but corn, pumpkin,
cucumber, eggplant, and other " warm-season crops " require a
temperature of 70 to 80 F. to give fairly rapid germination.
It requires from 10 to 12 days for corn grains to germinate at a
temperature of 49 F., whereas at 80 F. they will germinate in
2 days.

The lowest temperature at which seeds can germinate is called
the minimum temperature; the temperature at which they germi-
nate quickest, the optimum; and the highest temperature at which
they can germinate is the maximum. These three temperatures
for a number of different kinds of seeds are shown in the following
table:

Minimum F.

Optimum F.

Maximum F.

Barley . .

32-41

77-88

88^99

Clover (red)

32-41

88-99

99-111

Cucumber . . . .

60-65

88-99

111-112

Corn

41-51

99-111

111-122

Flax

31-41

77-88

88-99

Alfalfa

32-41

88-99

99-111

Melon

60-65

88-99

111-122

Oats

32-41

77-88

88-99

Pea

32-41

77-88

88-99

Rye

32-41

77-88

88-99

Wheat

32-41

77-88

88-108

From 60 to 80 F. is satisfactory for the germination of most
seeds of temperate regions. The temperature of the soil is referred

NATURE OF SEED GERMINATION

123

to here, rather than the temperature of the air. When the soil
is cold and moist, seeds should not be planted as deep as when it is
warm and moderately dry. Why?

Exercise 73. Temperature and germination. Use petri dishes, or other
dishes with cover, as germinators, and in the bottom of each place several

brush .

FIG. 52. Three germinating stages in wheat. (From Robbins, in Botany

of Crop Plants.)

thicknesses of filter paper or moist cloth. In each, place 100 lettuce seeds.
Subject these lots to different temperatures ranging from to 30 C. Keep a
record of rate and percentage of germination. This exercise may be repeated
using different kinds of seeds as desired. Draw conclusions.

Suggested activity. Place a half pint of soaked peas in a thermos bottle.
Insert a thermometer down into the peas, and pack cotton into the neck of the
bottle around the thermometer. Observe the reading of the thermometer
as sprouting of the peas proceeds. Explain any increase in temperature.

124 GROWTH OF PLANTS

Oxygen and germination. It must be kept in mind that all
living cells of the seed must respire in order to maintain life, and

that some oxygen is necessary in this process. In the resting stage
the seed requires very small amounts of oxygen, but when germi-
nation starts it demands a greater amount. That germinating
seeds respire actively is shown by the large quantities of carbon
dioxide they give off, and also by the heat liberated in the process.
A mass of germinating seeds of barley, or other seeds which gerini-

FIQ. 53. Germination of pumpkin (Big Tom) seeds, showing the pegs func-
tioning in the removal of the coats. (After Crocker, Knight and Roberts,
from Robbins, in Botany of Crop Plants.)

nate rapidly, may actually become heated until they feel warm to
the hand. Even though there is sufficient water and warmth,
unless seeds are planted so that free oxygen can reach them, they
will not germinate. If seeds are planted too deep in a heavy clay
soil, or in a soil that is too wet, they are quite likely to have a poor
supply of oxygen and to germinate slowly. Explain why seeds
should not be stored in air-tight containers.

Exercise 74. Oxygen and germination. Some seeds, those of rice for
example, will germinate with a very small amount of oxygen, even with that
which occurs in water. Place rice seeds in a beaker of ordinary tap water.

NATURE OF SEED GERMINATION 125

In a second tumbler place some boiled water, allow to cool, place rice seeds in
the bottom, and cover the surface of the water with a thin film of oil to prevent
the absorption of oxygen by the water. Record results. Explain.

Exercise 75. Process of germination. Observe stages in the germination
of beans, peas, corn, squash, castor beans, or other seeds. Make comparisons.
Examine each with regard to the cotyledons, the root, the hypocotyl, and the
plumule. Which part first appears above the ground? Compare the growth of
seedlings from which the cotyledons are removed soon after they come above
ground with that of seedlings with the cotyledons intact. Test the cotyledons
for starch. What is the function of the cotyledons? What is their fate?

FIG. 54. Germinating seeds of mustard. The white cottony masses are
root hairs on the primary roots.

Write descriptions of the stages of germination, comparing the different
seedlings.

The process of germination. The first stage in the process of
germination of the seed is the absorption of water. The rate at
which seeds absorb water from soil depends chiefly upon the
water content of the soil, the compactness of the soil, its tempera-
ture, and the character of the seed coats. A soil may be so dry
that the seed does not absorb enough water to germinate, but re-
mains in a dormant condition. Although a seed may absorb water
very rapidly from a very wet soil, it will not necessarily grow so
rapidly in such a soil. If a soil is excessively wet the oxygen supply

126

GROWTH OF PLANTS

in it is low, and oxygen is as essential to the growth of the embryo as
is water.

It is common practice to compact the soil over the seed after
it is planted. This brings the moist particles close about the seeds
and increases the points of contact between them. This object is
attained by the use of the press wheel on planters.

Seeds absorb water more rapidly from a warm soil than from a
cold one.

" Hard seeds." Although most seeds have coats which per-
mit the ready intake of water, in some seeds, such as those of

alfalfa, sweet clover, and other legumes,
the coats may be almost impermeable
to water. Such seeds are called
" hard seeds." They will not grow
readily, even when placed under per-
fect conditions for germination. Hard
seeds are not necessarily poor seeds.
Some of them will germinate in several
weeks; some will remain in the ground
for an indefinite period without germi-
nating. Unhulled sweet clover seed
may often have as much as 85 per cent
hard seed. Hulled sweet clover has
a lower percentage of hard seed than
the unhulled. Why?

The permeability of seeds of le-
gumes can be increased by " scarifying/'
that is, passing them through a
machine that scratches the surface.
The ordinary alfalfa huller is effective
for this purpose, as is shown by experiments; alfalfa grown under
a variety of soil and climatic conditions had about 90 per cent of
hard seeds if hulled by hand, and only about 20 per cent if hulled
by machine.

Digestion of stored food in seeds and its transfer. The second
stage in the germination of the seed is the digestion of stored foods,
and its transfer to the growing points of the young plant (embryo).
Certain cells of the seed secrete digestive juices which act upon

FIG. 55. Stages in the devel-
opment of a cactus plant. At
the left, the cotyledons of the
seedling are just breaking
loose from the seed coats.

NATURE OF SEED GERMINATION

127

the stored foods, render them soluble, and make possible their
movement to the growing points of the roots and stem. As the
young plant grows, the stored food in the seed is used up. It is

FIG. 56. Germinating seeds between the folds of Canton flannel. Use two

dinner plates, one inverted over the other. (From Robbins and Egginton,

in Colo. Agr. College Extension Bulletin.)

important to keep in mind that all the early growth of the young
plant is made wholly at the expense of this stored food. However,
not all the reserve food enters into the plant substance of the

FIG. 57. The seeds of bluegrass should be germinated on top of a blotter

and kept in the light; cover the germinating dish with a plate of glass. (From

Robbins and Egginton, in Colo. Agr. College Extension Bulletin.)

seedling, but a portion of it is lost in respiration. It is not until
the roots are established in the soil, and the leaves are green, that

128

GROWTH OF PLANTS

the plant is capable of making its own food and leading an inde-
pendent life.

Depth of planting seed. The depth at which a seed can be
planted safely depends somewhat upon the amount of food stored
within it. Many small seeds, such as those of tobacco and certain
grasses, are planted on the soil surface; large ones may be planted
more deeply. If a seed is planted so deeply that its reserve food
supply is consumed before the plant reaches the light, the plant
will die from starvation. Seeds should not be planted deeper than
is necessary to insure a proper amount of moisture.

Growth of embryo. The swelling of the seed caused by the.
absorption of water, and the growth of the embryo, break open the

FIG. 58. The dinner plate seed tester, (a) One hundred seeds are scattered
on one-half of the blotter. The other half of the blotter is folded over the
seeds, (b) Cover with another dinner plate, thus making a moist chamber,
(c) The seeds have germinated, and the sprouts are ready to be counted.
(From Robbins and Egginton, in Colo. Agr. College Extension Bulletin.)

seed coat. The young root is usually the first part of the embryo
to protrude. The cotyledons may remain in the soil or may be
brought above the soil. The single cotyledon in all grains remains
in the soil. The cotyledons of peas also remain underground.
But in such seeds as bean and squash, the cotyledons are brought
above ground and become the temporary leaves. Being exposed
to the light, they may become green and aid in the food-making
process. After a time, however, all the food that has been stored
within them is absorbed by the growing seedling and they shortly
wither and fall off.

W&"^ ^

'**

FIG. ay. Tne soil Hat or box tester, used in making individual ear tests of
corn, (a) Number the squares on the cloth and ears to correspond; (b) place
the kernels from individual ears on the squares; (c) cover the seeds with a
second layer of Canton flannel, moisten, and cover with moist soil or sand; (d)
at proper time remove the top cloth carefully, count and record the sprouts.
(From Robbins and Egginton, in Colo. Agr. College Extension Bulletin.)

129

130

GROWTH OF PLANTS

FIG. 60. The rag doll tester, used in making individual ear tests of corn.
(From Robbins and Egginton, in Colo. Agr. College Extension Bulletin.)

NATURE OF SEED GERMINATION

131

Conditions affecting the vitality of seeds. It is a common
observation that when a lot of seeds is planted under the most
favorable conditions for germination a number of them fail to
germinate. Some seeds are quick to germinate and form strong,
vigorous seedlings. Others sprout but slowly, and the young plants
are weak and sickly. The grower wants seeds which have a power
to germinate readily and produce vigorous sprouts. In other
words, he wants seeds of high vitality.

Many conditions have an influence on the vitality of seeds;
they are discussed in the following paragraphs:

1. Vigor of the parent plant. Seeds from strong, vigorous
parent plants usually have larger embryos and a greater amount
of food reserve than those from weakly parent plants. In the

FIG. 61. Making the purity test. (From Robbins and Egginton, in \joio.
Agr. College Extension Bulletin.)

selection of seed for planting, strong healthy mothers should be
considered.

2. Conditions to which the seeds are exposed while developing.
The vitality of the seed is influenced by the temperature of the air
and the amount of moisture in the air at the time the seed is
maturing. Most seed mature best under dry atmospheric condi-
tions, and with moderately high temperature. Low temperatures
early in the autumn may injure the partly mature seed. Corn,
for example, sufl'ers from freezing before the grain is thoroughly

132 GROWTH OF PLANTS

dry. The tissue of the grain is broken down by the freezing
of the water in it. If the grain becomes thoroughly dried, it will
withstand very low temperatures. Corn containing 13 per cent
moisture may be stored with safety in bins exposed to tempera-
tures much below freezing.

3. Maturity of the seed. Although seeds will often germinate
when they are not fully ripe, the plants from such seeds are fre-
quently weak. Lack of maturity or low vitality in corn is usually
indicated by soft ears, by any discoloration of the grain, especially

FIG. 62. Showing the method of dividing the original sample in order to

obtain the proper amount for purity analysis. (From Robbins and Egginton,

in Colo. Agr. College Extension Bulletin.)

at the tips, and by blisters on the skin. Immature corn quickly
loses its germinating power.

4. Conditions under which seeds are stored. Seeds should be
stored under conditions that are uniformly dry and cool. If the
atmosphere is moist and warm, germination may be started; if
it is, the respiration rate of the live cells is increased and the seed
uses up a certain amount of its stored food. Its energy is thereby
diminished. The seed may not have sufficient moisture and heat
to germinate fully, but it will be kept in a greater state of activity
than when in a completely dormant condition. Hence, its vitality
is gradually being reduced. Moreover, in certain instances, seeds
in bulk stored under moist, warm conditions may " heat " to such
an extent that the embryos are actually killed by the high tem-
perature.

NATURE OF SEED GERMINATION 133

5. Age of seeds. It is well known that seeds gradually lose
their vitality as they grow older. The rate at which they lose their
vitality depends upon the kind of seed and upon the conditions
of storage. Seeds containing oil, such as corn and flax, lose their
vitality much quicker than starch-bearing seeds, such as those of
legumes. The seeds of legumes are noted for their great longevity.
Some have been known to retain their vitality for 150 to 200 years.
We hear the claim that wheat grains taken from the ancient tombs
of Egypt will germinate. What is your opinion of this?

FIG. 63. The purity test. With a knife and with the aid of a tripod lens

the sample of seed is separated into three piles; (a) pure seed, (b) weeds

and other foreign seeds, and (c) inert matter. (From Robbins and Egginton,

in Colo. Agr. College Extension Bulletin.)

Delay in the germination of seeds. The seeds of many plants
have a rest or dormant period. That is, they will germinate better
after a period of rest than they will when first mature. This
dormancy is more common among wild plants than among domes-
ticated ones. For example, wild oat seeds experience a delay in
their germination, seldom germinating the year they are formed.
The seeds of a number of weeds will lie in the ground for years in a
dormant state. It has been shown that some seeds are still viable
after 30 years 7 burial in the soil. Among such are the seeds of

134

GROWTH OF PLANTS

pigweed, black mustard, shepherd's purse, common dock, green
foxtail, and evening primrose.

There is an old saying that one year of seeds means seven years
of weeds. A crop of seeds is borne ; some of them may germinate
immediately if the conditions are favorable; others may remain
dormant for a year or two, and still others may remain dormant
for five or six or more years. In cultivation, the seeds may be
buried to such a depth that they do not get enough oxygen to

germinate. Consequently,
they lie dormant in the
soil. Later, perchance,
they may be turned to
the surface in plowing and
brought under conditions
favorable to their germi-
nation.

The delay in the ger-
mination of seeds may be
due to several causes.
Probably the most com-
mon cause is an impervious
seed coat which prevents
or retards the absorption
of water. This topic was
discussed on page 126.
Another cause of dormancy
is the inability of the
embryo to break the seed
coat. This is true of the
common pigweed seed.
As the seed lies in the ground, freezing and thawing and the
action of soil organisms gradually soften the coats and make
germination possible. Still another cause of seed dormancy is
the inability of the embryo to germinate until it has gone through
a series of changes known as " after-ripening. " This process
may be hastened in some instances by exposure of the seeds to
low temperatures.

" Hard seeds " (see page 126), whose coats are impervious to

FIG. 64. A common commercial type of
seed germinator. The cloths or blotters
holding the seeds are placed on the sliding
trays. The germinator is heated by an
electric plate and the temperature is con-
trolled by a thermostat.

GROWTH IN LENGTH OF STEMS 135

water, may be hastened in their germination by scratching the
surface. The delay in the germination of olive seeds, which have
a stony covering, may be overcome in part by soaking them in
warm water, soaking in alkaline or acid solutions, or clipping the
ends. Germination of some seeds may be hastened by soaking
them in water before planting. For example, asparagus seed
soaked a period of three to five days at a temperature of 75 to
85 F. will germinate more quickly than unsoaked seeds. The
seeds of beets and lettuce have their germination hastened by
soaking for a period of six hours in water.

Such hard-coated seeds as those of the peach, cherry, and
walnut are often stratified in the early winter and permitted to
freeze and thaw in order to break the seed coats. In stratifying
seeds, alternate layers of sand and seeds are put in a box. They
are then placed in a well-drained place and allowed to freeze.
In mild climates where winter freezes seldom occur, the germina-
tion of many seeds is improved by stratifying them in a moist
place; the moisture and the temperature fluctuations are probably
responsible.

Suggested activity. Make rag doll seed testers according to methods
given in farm bulletins and test corn grains from different ears obtained from
various sources. What is the practical value of seed testing?

QUESTIONS

1. In what states has the vegetable and flower seed industry been developed
extensively? Why?

2. Why have seeds of legumes, as a class, relatively great longevity?

3. What is the probable explanation of the belief that " wheat changes to
cheat"?

4. Do you believe that all farmers should test, or have tested, their seed
before planting? Give reasons.

Problem 4. How do stems grow in length?

In order to understand the manner in which stems grow in
length, it will be necessary for us to be familiar with their external
characters, and with the structure of buds.

Exercise 76. Twig characteristics. Examine a leafy twig of some woody
plant, the cottonwood, for example. Observe that it is divided into sections

136

GROWTH OF PLANTS

(internodes), and that at the enlarged joints (nodes) the leaves arise. Buds
develop in the axils of the leaves, and also at the tip of the branch. The bud

at the tip of the twig is called
the terminal bud. Those along
the side of the stem at regular
intervals are lateral buds. The
terminal bud develops the
following spring into a branch,
which, in turn, bears leaves.
A lateral bud may be a leaf
bud or a flower bud. If a
leaf bud, it will develop into
a side branch bearing leaves;
if a flower bud, it will bear
flowers.

Exercise 77. The structure
of buds. Make cross- and length-
wise sections of a large terminal
bud. Note that it consists of
a short axis, bearing small
leaves. The outer leaves of the
bud are called scales; they pro-

latent bud*-

\terminal bud- scar

terminal leaf-bud

[ flower-bud*
\stipule-scar

\tenticels

/ la 'feral leaf-buds

. flower-bud-scars

one year
old branch \

FIG. 65. Cotton wood twig two years

old. (After Longyear from Robbins,

in Botany of Crop Plants.)

FIG. 66. Section of stem
showing a shedding leaf; also
bark, wood, and pith are seen
in cross- and longitudinal sec-
tions. (After Longyear, from
Robbins, in Botany of Crop
Plants.)

GROWTH IN LENGTH OF STEMS

137

tect the soft, tender tissue within from drying out and from mechanical
injury. The inner leaves are undeveloped foliage leaves.

Examine lengthwise sections of a terminal bud, made thin enough so that
the structures may be studied with a compound microscope. At the very tip
of the stem is a region made up of cells which are capable of division; back of
this is a region in which the cells are rapidly elongating; then, farther back, is a
region in which various stem tissues are differentiating; and still farther back
is the mature part of the stem. It will be observed that within this bud there
are very short internodes, and that the leaves come off at regular intervals,
following identically the same arrangement as in the adult twig.

Exercise 78. Examine preserved specimens of twigs showing buds in the
process of swelling and opening in early spring. What is happening to the
young shoot which was hidden and protected by the scales of the bud during

FIG. 67. Growth as shown in opening buds of hickory. Left, opening bud;
center, section of opening bud; right, growing shoot from a recently opened bud.

the winter? Explain why it is possible for a shoot to make such rapid develop-
ment in early spring.

It is apparent from the above studies that a bud is simply an
undeveloped stem. A bud is a very short, young stem in which
the internodes are exceedingly short. The growing point of
a stem, then, consists of a number of very much shortened inter-
nodes; growth in length of the shoot consists in the lengthen-
ing of these internodes by increase in the number and the size of
cells that compose internode tissue. When a twig has made its
year's growth, the internodes do not lengthen during subsequent
years. Increase in length of that shoot is due to the addition of

138

GROWTH OF PLANTS

other " joints " at the end. The fixed length of old internodes is
well proved by the common observation that nails driven into the
trunk of a tree, or a branch, are not elevated above the ground as
the tree grows. If, when you were a young boy, you carved your
initials deep in the bark of the old tree that grew by the swimming
hole, those initials, although probably partly obliterated by the
growth of bark, ate today at the same distance above the ground
as they were the day you carved them. A common impression
prevails that, in pruning, the branches of a young tree should

FIG. 68. The beauty of our landscapes is being marred by tree-butchery,
such as is shown here.

be started low to the ground, so that they will be at about the
proper elevation above the ground when the tree reaches maturity.
The errroneous supposition here is that the limbs are raised by the
growth of the tree.

Problem 5. How do stems grow in diameter?

Exercise 79. Structure of the woody stem. With a safety razor blade
cut thin cross-sections of a one-year old woody stem, such as a twig of the
cottonwood, box elder, cherry, or apple. Note the three principal regions : the

GROWTH IN DIAMETER OF STEMS

139

bark, the wood, and the pith. The bark can be separated from the wood.
It separates from the wood along a region known as the cambium. The
cambium is composed of thin-walled, tender cells, capable of rapid division
and growth. The cambium is the growing layer of the stem. (See p. 64).

The bark is covered with a corky layer which successfully pre-
vents the rapid loss of water from the stem. Beneath the corky
layer of the bark are several layers of cells containing chlorophyll,
and hence capable of manufacturing sugar. The inner part of the
bark is the phloem. The phloem is that portion of the stem which

Conducting Tube

Baric

-Heart Wood

Vascular
~-~ Rays

Spring Wood
Summer Wood

Annual Ring

FIG. 69. Portion of a four-year-old stem of the pine, shown in transverse,
radial, and tangential views. (Redrawn from Strasburger.)

is largely concerned in the conduction downward of foods manu-
factured in the leaves (probably mineral substances and foods
upward, also). Large tubes, known as sieve tubes, in the phloem
are the conducting elements. In addition to cork, chlorophyll-
bearing tissue, and phloem or food-conducting tissue, the bark
may have fibers and other cells which give strength.

The wood of the stem is made up chiefly of large conducting
tubes or vessels, fibers, and storage cells. It is in the vessels that
water (and probably salts and foods, also) is carried. The fibers

140 GROWTH OF PLANTS

?ive strength to the stem. The storage cells store water and
foods, and may also conduct these substances short distances.

The pith of the stem consists of a group of large, thin-walled
cells which store food to some extent. The amount of pith in
stems varies greatly.

Radiating from the pith, and extending through the wood and
the phloem part of the bark, are rows of cells which constitute the

FIG. 70. Cross-section of six-year-old woody stem. Note the dark-colored
heartwood, light-colored sapwood, bark (dotted) and vascular rays.

vascular rays. Water, salts, and foods are carried radially in these
ray cells; they also serve as places of food storage. If a cross-
section of a twig is treated with iodine, starch, which is stained
blue, is seen to occur chiefly in the vascular rays and in the outei
cells of the pith.

Exercise 80. Structure of woody stem two years old. Cut sections as
in Exercise 79, but of two-year-old twigs. Compare with the one-year-olc

GROWTH IN DIAMETER OF STEMS

141

stem. With a hand lens determine the number of rings of growth of wood.
Note the " pores/' the vessels as seen in cross-section. (Fig. 31).

In stems of the type to which our common orchard trees belong,
there is a continuous
cambium layer be-
tween the bark and
the wood. The cam-
bium cells divide and
redivide, adding to
the bark cells on the
outside and to the
wood cells on the in-
side. Hence, by a di-
vision of cambium
cells, new phloem is
laid down on the inside
of old phloem, and
new wood is laid down
on the outside of the
old wood. A layer of
phloem and a layer of
wood are formed each
year. The phloem
rings are less distinct
than those of the wood,
and as the stem grows
older the older phloem
may peel off with other
bark tissue.

Annual rings of
growth. An annual
ring, as generally un-
derstood, is one year's
growth of wood. The
ring varies in width,
depending upon the
time in the life of the plant it was formed, and upon seasonal
and climatic conditions. Furthermore, it is known that some

FIG. 71. Cross-section of a portion of pine wood.
One complete annual ring (center), and parts of
two other annual rings (above and below) are
shown. The narrow, dark part of the annual
ring is " summer wood/' the broad, light part,
"spring wood."

142

GROWTH OF PLANTS

' ^'-' ; * '

II

I m

trees grow rapidly, producing wide annual rings, whereas it is
a specific character of others to grow slowly, i.e., produce nar-
row rings. The amount of carbohydrates supplied by the leaves
and the water supply are two chief factors determining the width
of rings.

There is usually a marked difference in the wood formed in the
spring and early summer and that produced in late summer and

fall. In early or so-called
spring wood, conducting
tubes are large and quite
numerous; in late or
summer wood, conducting
tubes are smaller and fewer,
and wood fibers are rela-
tively more abundant.
Hence, summer wood has
more strength than spring
wood. The summer wood
of one year (say 1925) is
adjacent to the spring
wood of the following year
(1926).

Soft wood is usually
one from a tree which
grows rapidly. The con-
ducting tubes are rather
small and uniform in size
and evenly distributed
throughout the year's
growth. Hard wood is
usually a comparatively
slow-growing wood. The
conducting tubes of the

spring and early summer are large and numerous, but the autumn
wood is solid as a consequence of the greater abundance of strength-
ening elements.

. .

Is

E" '"''TV*'- ' 7v,, ;;,:.,,,,
: '**ai!iiis?* s

FIG. 72. A portion of pine wood cut length-
wise, showing the conspicuous, circular bor-
dered pits in the walls of tracheids.

Exercise 81. Determining the age of trees,
or log, attempt to determine the age of a tree.

Using a freshly cut stump
If the rings of growth are

GROWTH OF ROOTS 143

counted on a stump three feet high, does this represent the true age of the tree?
Why? Can we always rely absolutely upon the number of rings in determining
the age of a tree? Why?

Cork. Usually when we speak about the growth in diameter
of a woody stem, we refer to the annual rings of wood, that is, of
tissue inside the cambium. The bark also develops annual layers
but they are much thinner than those in the wood and generally
are broken and split off. Also, woody plants develop a cambium,
known as cork cambium, which usually originates in the cortex.
This cambium forms cork to the outside and cortex to the inside.
In some plants, notably the cork oak of commerce, the layers of
cork, formed year after year, adhere to the tree, and we observe
them as definite annual layers of growth. Cork cells have walls
which are impregnated with a fatty substance known as suberin,
which is impervious to water.

Summarizing, the growth in diameter of a woody stem is due
to the activity of two cambiums: (1) the vascular cambium which
lies between the wood and bark; this adds new rings of growth of
wood to the outside of the old wood, arid new bark tissue to the
inside of the old bark ; (2) the cork cambium, situated in the bark ;
this adds layers of cork to the inside of old cork, and cortex tissue
to the outside of old cortex. In most woody plants there is a
gradual peeling off of the bark, which includes tissues arising from
both cambiums.

Suggested activities, (a) Find out by inquiry and from books on horticulture
how fruit trees are grafted, and prepare a report to be read to the class. Ex-
plain why it is necessary to bring the cambiums of stock and scion into contact.
Include in your report a description of " budding " as done by fruit-growers.

Problem 6. How do roots grow?

In Problem 2, Exercises 70 and 71, we examined young roots
and found that the growth in length of a root is near the tip. This
may be ascertained by the following simple experiment.

Exercise 82. Method of growth in length of a root. Germinate horse
beans or lima beans on moist cloth or filter paper in a covered dish. When the
first root is 1 or 2 inches long, carefully mark with lines of India ink, 1 mm.
apart, beginning at the tip and extending backwards 2 or 3 cm. After 24

144 GROWTH OF PLANTS

hours, observe. Which marks are the farthest apart? Draw conclusions as to
the regions of growth.

The growing point, which includes cells capable of division and
growth, is some distance from the root tip, being covered and pro-
tected by a root cap. Immediately back of the growing point, the-
cells are elongating, and still farther back the cells are differentiat-
ing. As a matter of fact, the growth in length of a root is confined
to the growing point and region of elongation, these two regions
together usually being not more than \ to \ inch long. A root is
not pushed through the ground by growth of cells far removed
from the tip. Rather, by the addition of new cells immediately
behind the protective root cap, and their elongation, the root tip
finds its way between the particles of soil. There are no joints in
the root as there are in the stem. Why is it practically impossible
for a root to pursue a straight course through the soil?

The growth in diameter of the roots of perennial plants is simi-
lar to that in the stems. An old root of our common trees and
shrubs has annual rings, and very much the same structure and
appearance as an old stem. Can you cite an example of the lifting
power of roots?

Problem 7. How do leaves grow?

We know that leaves grow very rapidly in the spring. After a
few warm days, the entire tree appears green, and in two or three
weeks leaves have attained their maximum size for the season.
It must be that leaves are fairly well formed in the bud. This fact
is well demonstrated by the following exercise.

Exercise 83. The growth of leaves. Remove the scales from winter leaf
buds of several different kinds of deciduous plants and carefully dissect out
the young foliage leaves, observing whether they are rolled, folded, or plaited.
Spread these young leaves out flat, examine with binoculars, and observe that
even in the bud the leaves have veins, and very much the form they will have
when fully grown.

From these observations we are led to conclude that the leaves
of our common temperate-climate deciduous plants are formed the
season before their expansion. When the bud breaks open in the

GROWTH OF SEEDS AND FRUITS

145

spring, the leaves grow very rapidly, attaining full size within two
or three weeks.

We have learned that stems and roots grow in length chiefly at
or very near the tip. There is a very unequal rate of growth in
different parts of these organs. Not so
with leaves, as is shown by the following
exercise:

Exercise 84. The growth of leaves. With a
leaf-marker (rubber stamp marked into millimeter
squares) stamp a young leaf, y% to 1 inch in width.
After the leaf has attained full size, observe the size
and shape of the squares. If throughout the leaf the
squares have maintained their shape, it is an indi-
cation that growth has been at an equal rate
throughout all portions of the leaf. As a matter of
fact, growth of the leaf after it breaks from the
bud is simple enlargement of cells already formed;
additional cells are not developed.

Thus far we have not accounted for the
growth and development of leaves in the
bud. From Exercise 77, we learned that
the stem growing point consists of a very
short axis with nodes and extremely short
internodes. The nodes are the places on
the stem where the leaves arise. In the
lengthwise sections studied in the above
exercise, we observed slight protuberances
near the growing point, which consisted of
groups of cells, each destined to become a leaf. Each group of
cells finally enlarges and takes on the form of a leaf, which rests
in the bud stage until the spring of the following year.

FIG. 73. A single fern
leaf unrolling.

Problem 8. How do seeds and fruits grow?

In the^ discussion of Problem 1, it was pointed out that seeds
develop within a certain structure of the flower known as the
pistil, and that long before the flower opens there develops within
the pistil one or more small masses of tissue, each of which is de&-

146 GROWTH OF PLANTS

tined to become a seed. These masses of tissue are called ovules.
The ovule is a small spherical or egg-shaped structure in the ovary
of the pistil. It is attached to the ovary by a short stalk, which
becomes the stalk of the seed. The mature ovule, just before fer-
tilization, consists of a central mass of tissue, surrounded by one
or two coats which become the protective coats about the mature
seed. These fit closely about the ovule, except at one point, where
there is a very small opening, the micropyle. See p. 154.

Within the central mass of tissue is the embryo sac, the struc-
ture in which the embryo or young plant develops. The mature
embryo sac commonly has eight nuclei, one of which, after fertili-
zation, develops into the embryo plant; two others unite with a
second nucleus from the pollen tube, and the resulting body devel-
ops into endosperm, which is a food supply surrounding the
embryo. The remaining five nuclei usually soon disappear, being
absorbed or disintegrating.

Pollen grains play a part in the formation of fruit. They are a
product of the anthers. At maturity, the anthers split open and
the pollen grains are distributed. The pollen grains of plants vary
widely in form, size, color, and particularly in surface markings.
The wall of the grain usually consists of two coats, an outer thick
one and an inner thin one. The wall encloses a mass of proto-
plasm, the essential parts of which are three nuclei. One of these,
the tube nucleus, plays a part in the growth of the pollen tube;
the other two, sperm nuclei, fertilize certain nuclei in the ovule.

Ferilization. The pollen grain is usually brought to the stigma
by wind or insects. It absorbs water and nutrient materials from
the surface of the stigma, and grows by sending out a tube, known
as the pollen tube. The pollen tube grows downward through the
stigma and style and finally reaches the ovule. It goes through
the micropyle and penetrates the ovule tissue. After the dissolv-
ing of the wall at the tip of the pollen tube, the three nuclei are
discharged into the embryo sac. The tube nucleus is absorbed.
One sperm nucleus unites with the egg or female nucleus to form
the fertilized egg. Thus, this nuclear mass contains determiners
for characters from the plant furnishing the pollen (paternal char-
acters) and also those from the plant fertilized (maternal charac-
ters). The union of the sperm nucleus of the pollen tube with the

GROWTH OF SEEDS AND FRUITS 147

egg nucleus of the embryo sac is fertilization. The fertilized egg
nucleus now develops into a young plant (embryo).

In cereals and lilies and a number of other plants, so-called
double fertilization has been observed. One sperm nucleus has
been accounted for as uniting with the embryo nucleus. The other
unites with the two so-called polar nuclei of the embryo sac. The
body resulting from this union also carries determiners for both
maternal and paternal characters. It develops into the endo-
sperm of the seed.

Immediately following fertilization, there is a series of changes
not only in the ovule, resulting in a seed, but in the ovary wall as
well. Normally, if the egg nucleus is not fertilized the ovule does
not develop, but withers and dies.

Just one pollen tube penetrates the embryo sac to bring about
fertilization. Many pollen tubes, even hundreds, may grow down
the style, although comparatively few may function. Those which
do not, wither and die. We may be sure that every ovule that
develops into a seed has been visited by at least one pollen tube,
and that only one pollen tube has functioned there.

Summarizing : The seed develops from the ovule in the ovary,
but ordinarily only after fertilization. After fertilization, the
embryo or young plant develops from the egg nucleus, the endo-
sperm develops from other nuclei in the embryo sac, the ovule
coats harden to form the seed coats, certain tissues disintegrate,
and the whole resulting structure we call a seed. The fruit is the
matured ovary, with its seeds, and any other part of the flower
which may be closely associated with it. The fruit contains ..the
seed or seeds. For example, the entire bean pod is a fruit; the
beans within are the seeds. It is often difficult to realize that a
large fleshy fruit, such as a tomato, is derived from the ovary.
The walls and partitions of the ovary enlarge greatly to form the
mature fruit. But, throughout all the changes which occur during
the development of the tomato fruit from a small structure much
less than \ inch in diameter to the large tomato, there is very little
increase in the number of cells; rather, simple enlargement of cells
already formed, coupled with chemical and physical changes which
affect texture, color, flavor, and edibility.

148 GROWTH OF PLANTS

QUESTIONS

1. What is the force which pushes young roots through the soil?

2. How do cells of the young root change to vessels and tubes as the root
grows older?

3. Why is it that moist soil should be packed about seeds that are planted?

4. Explain why plants in clay soil should never be cultivated when the soil
is wet.

5. Explain why corn is cultivated by digging the soil deeply at first,
whereas shallow cultivation is used around older plants.

6. Why should seeds never be planted in soil which is either very dry or
very wet?

7. Give two reasons why oats should be planted earlier in the season than
corn.

8. Explain why only the tips of asparagus shoots are tender.

9. Why is the bark of tree trunks usually ridged?

10. Why is the surface of twigs of a tree more smooth than that of the
trunk?

11. If it requires 30 feet of rope to make a swing by tying the two ends to a
branch of a tree, what length of rope will be required for a swing attached to
the same branch 30 years later, the tree having increased in height 20 feet
during the period?

12. In what two ways can you tell the age of a twig?

13. Why is the bark of a tree thinner than the wood?

14. Explain the appearance of the grain of lumber.

15. Explain why it is possible for rabbits to kill young trees by gnawing the
bark from a ring around the base.

UNIT V
REPRODUCTION OF PLANTS

Reproduction is one of the fundamental characteristics of life.

Reproduction is race preservation. It is a process which occurs not
only in animals, but also in all plants in trees and shrubs and
herbs, in toadstools and ferns, bacteria and seaweeds in short,
in every kind of plant existing on the surface of the earth. It is a
process which, in the broadest sense, involves the production of
new individuals, by any method whatsoever.

An individual plant is designed to function not only for itself,
but also for the race to which it belongs, to sacrifice all or a part
of itself in propagating the species.

All life comes from pre-existing life. The new organism is
nothing more or less than a piece of living material separated from
its parents. Living things as we know them originate only by
reproduction. Even the tiniest germ visible with the most power-
ful microscope must have ancestors. Fossil remains bear witness
that millions of years ago, in the waters of ancient seas, life first
appeared. From that day to this there seems to have been an
unbroken continuity in the chain of living things.

There are two general methods of reproduction of plants,
namely, asexual reproduction and sexual reproduction. When
you divide dahlia roots, or start " slips " and cuttings from roses
and other woody plants, you are reproducing these plants asexu-
ally, that is, without sex. Many of the primitive plants, such as
the bacteria, reproduce by asexual means alone. With them,
sexual reproduction is unknown. Sexual reproduction in plants
involves the union of parts of two parents the egg of the female
plant and the sperm of the male plant to form a new individual.
All eggs and sperms are cells minute units of living substance or
protoplasm. The cell resulting from the union of the egg and
sperm grows into a mature plant. The cell is the unit of reproduc-
tion.

149

150

REPRODUCTION OF PLANTS

Stamen

Petal

V- Pistil

Problem 1. How do flowering plants reproduce?

In the higher plants, including cultivated plants of all kinds,
the flower is the organ of sexual reproduction. It is in the flower
that the seed is developed. Primarily the flowers are organs of
seed production. Many flowering plants can be multiplied by
means of vegetative organs, such as stems, roots, and sometimes

leaves, but the princi-
pal way in which they
multiply is by means
of seeds, which are a
product of the flower.

Flowers are exceed-
ingly various. There
are flowers so small
that their organs are
scarcely visible to the
unaided eye; such are
the flowers of the duck-
weed or duckmeat
(Lemna), which aro
free, floating plants
common in ponds
throughout the world.
Then there are the
flowers of a tropical
plant (Rafflesia), grow-
ing on the floor of dark
forests, which are as
much as a yard in
diameter. A great host

of flowers like those of grasses, cotton-woods, and oaks are adapted
to wind-pollination, whereas others, like those of orchids, snap-
dragons, and mints, are so peculiarly constructed that the pollen
is distributed only by certain insects whose bodily form enables
them to enter the flower. The flowers of grasses, cottonwoods,
birches, and many other wind-pollinated plants have no showy
bright-colored parts; insect-pollinated flowers, on the other hand,

Sepal

: -> Receptacle

-Pedicel

FIG. 74. Diagram of a flower from which all
but one of each whorl of flower parts have been
removed. (Modified after Hall. From Hoi-
man and Robbing, in a Textbook of General
Botany.)

REPRODUCTION OF FLOWERING PLANTS

151

are usually gaudy and conspicuous. We might go on enumerat-
ing the great number of variations in the size, color, structure,
and form of flowers, but lack of space prohibits.

Let us now familiarize ourselves with the structure of some
typical flower one which has all parts present.

Exercise 85. The parts of a flower. Examine the flowers of some plant,
such as cherry, sweet pea, radish, or lily. The following principal parts will
be observed:

1. The sepals, green
structures; taken together
they form the calyx. The
calyx covers the other
flower parts in the bud.

2. The petals, showy,
colored structures; taken
together they form the
corolla.

3. The stamens, slen-
der structures, each with
a thread-like stalk or fila-
ment at the end of which
is an anther. The anther
produces a yellow powder
called pollen.

4. The pistil, the cen-
tral structure of the flower.
The parts of the pistil
are the ovary, the swollen

FIG. 75. A lengthwise section of an apricot
flower bud, long before it is ready to open.
Observe the immature ovary in the center, the
stamens, the petals and sepals, and the over-
lapping bud scales. (Photograph furnished by
Division of Pomology, California College of
Agriculture.)

base which contains the
ovules, that is, the struc-
tures which later develop
into seeds; the stigma, the
topmost part of the pistil
which steals the pollen
from insects or wind or

other agency which trans-
ports it; and the style, the slender part of the pistil which connects the
stigma with the ovary.

Within recent years scientists have found that all flowering
plants bear microscopic sexual plants as parasites in their flowers.
Just so is the human embryo a parasite upon its mother. The
yellow pollen grains are nothing more or less than male plants;

152

REPRODUCTION OF PLANTS

and hidden within the young seeds (ovules) are the parasitic female
plants. The germ cells, that is, the eggs and sperms, are not
produced directly by the flowers. Instead, flowers develop these
email sexual plants which in turn bear the eggs and sperms.

Within each anther
there are developed a
number of spores, a
peculiar type of cell
which, unlike eggs and
sperms, is capable of
growing into a plant
without entering into
the mysterious process
of fertilization. Each
of the spores in the
anther grows into a
minute male plant, a
pollen grain. When
the anther dries up
and splits open, pow-
dery masses of yellow
male plants are carried
by insects or wind to
the pistils, inside of
which the female plants
are waiting.

Exercise 86. The pollen
grain. With the compound
microscope examine the
pollen grains of some flow-
ering plant. In specially
stained pollen grains will
be seen the protective coat
enclosing two cells. The
nuclei of these cells are
visible. Thus, it is seen

that the pollen grain is not a single cell, but in reality a small sexual plant

consisting of but two cells.

Exercise 87. Germination of pollen grains. The pollen grains of many

plants will germinate in a 10 per cent solution of cane sugar. Prepare hanging

FIG. 76. An apricot flower bud just before
opening. The ovary, covered with hair, is
seen in the center, and above are the anthers.
(Photograph furnished by Division of Pomol-
ogy, California College of Agriculture.)

REPRODUCTION OF FLOWERING PLANTS 153

drop cultures of a number of different kinds of pollen in the above solution.
Germination of the pollen grain, like that of the seed, is resumption of growth.
Under favorable conditions the two-celled male plant (pollen grain) germinates,
germination consisting of the growth of a long tube the pollen tube. One of
the cells divides to form two sperms or male elements. These may be seen in
properly stained material, usually occupying a position near the end of the
tube. In the mature pollen tube may thus be seen three nuclei, a so-called
tube-nucleus and two sperm nuclei. These nuclei are accompanied by some
cytoplasm.

The pistil is the young seed pod. Inside of each potential
seed, which in the early stages is called the ovule, there is a single
female plant. This is a minute, swollen bag and is called the
embryo sac. At the end of the female plant or embryo sac, near-
est an opening which is always left in the seed coats, there lies the
cell which is to be fertilized. This is
the egg cell, the female element. Such a
cell, wherever found, whose sole function
is union with a male cell, is called an egg.

Exercise 88. Structure of the ovule. Split
open the pistil of a flower and observe the one or
more ovules. The internal structure of these can
be studied only by appropriate microscopic sec- piQ ?7 __ Cross . section of
tions. The central part of the ovule consists of ft matufe anther gh
a mass of tissue called the nucellus Embedded ^ f(mr chambers

within it is the embryo sac, the female plant. ... n

" . . : * A . r containing pollen grams.

Entirely surrounding the nucellus, excepting for

one small opening, the micropyle, is a protective

layer, consisting of one or two coats, the seed coats. Within the embryo sac
are a number of cells, one of which, the egg cell, after union with a sperm
from the pollen grain, grows into a new plant.

Fertilization. When the embryo sac or female plant in the
ovule is mature, the stigma is usually moist and somewhat sticky
and conditions upon its surface are such as to cause the young male
plant, the pollen grain, to resume its growth. In its growth it
becomes, as we have seen, a microscopic, hair-like tube, the pollen
tube. This tube grows down inside the pistil, through the micro-
pyle, and into the female plant. The end of the tube bursts,
emptying into the female plant the two sperm cells of the male.
A sperm slowly dissolves itself in the egg. The two become one.
This union of a cell from the male plant with a cell from the female

The qamete plant staqe
(qametophL|te) beqins vMh
the formation of microspore
and meqaspore and end&
with the fertilization of
the eqq;

The spore plant staqe
(sporopht|fe) bcqins with
fertilization in a f \ower and
ends in tne forminq of
>pores in a flower of
the off-sprinq

Endoopenm

coa-Vs

Fertiliz.afion

FIG. 78. Life cycle of a seed plant with an enclosed ovule (Angiosperm).
The seed (s) germinates and develops into the mature spore plant (s.p.).
In the flower of the plant the sex organs, pistil (1) and the stamens (2) appear.
The flower produces two different kinds of asexual spores. Within the ovule
(3) the megaspore (4) develops. This megaspore germinates and goes through
the stages, 5, 6, 7, in developing into the female gamete plant (f.g.) which
produces an egg (8) and a fusion nucleus (9). The pollen grains germinate
and develop (11, 12) into the male gamete plant (m.g.) with a pollen tube (p)
containing a tube nucleus (n) and two sperm cells (a, b). In developing, the
pollen tube grows down through the style of the pistil and around the ovule
to the micropyle where it enters the ovule. The end of the pollen tube enters
the female gamete plant where its wall dissolves, setting free the cells, a and b.
The union of a male gamete with the fusion nucleus results in the development
of the endosperm (stored food) of the seed. The union of b with the egg (fer-
tilization) results in the forming of the embryo spore plant in the seed. The
seed coats result from the development of the outer coats (integument) of the
ovule. The seed usually goes through a dormant period before germinating.

154

REPRODUCTION OF FLOWERING PLANTS

155

plant is fertilization. The process of fertilization, wherever it
occurs in the plant and animal kingdoms, is really the same, in
that it is the union of two masses of
living material, a sperm and an egg.

The fertilized egg immediately di- .
vides and redivides. Soon, it changes
from a shapeless mass to one showing
the beginnings of leaves, stem, and
root. Then the seed coats harden and
the embryonic plant ceases to grow,
awaiting favorable conditions for re- B
suming growth. The whole structure
has now become a seed. Its essential
structure is the embryo the result
of fertilization of an egg by a sperm.
The embryo is a new plant, borne for s-<
a while by the mother plant. Inas-
much as one mother plant may produce
thousands of seeds, there is a great
multiplication of individuals. This is
reproduction. j}

Parthenogenesis. Normally, as
stated, the egg or female gamete will
not start on the train of changes which
result in the embryo plant unless a
sperm or male gamete fuses with it.
Rarely, however, the embryo develops
from an unfertilized egg nucleus. This
phenomenon is called parthenogenesis.
It is a rare occurrence among plants.
The phenomenon has been observed in
the dandelion, in hawkweeds, meadow
rue, and several other groups of flower-
ing plants. Also it has been observed
in certain of the lower plants, chiefly
fungi.

Parthenocarpy. As a general rule,
lack of fertilization of the ovules is

FIG. 79. The earliest stages
in the life of a plant. This
shows how a plant starts out
in life. The sperm nucleus
moves into position beside
the egg nucleus, as shown in
A and B. The two divide
side by side as shown in C
and D so that there are two
resulting groups of six chro-
mosomes, instead of four
groups of three. Each of the
two groups then forms a sin-
gle nucleus, and a cell wall
forms between them. (From
Robbins and Pearson, in Sex
in the Plant World.)

156

REPRODUCTION OF PLANTS

followed by the shedding of the blossoms; the fruit fails to
develop completely if a good number of the ovules are not fer-
tilized. However, development of the ovary does sometimes occur
although fertilization fails. Such an unusual development is
called parthenocarpy. With certain sorts of both apples and
pears, fruits have been developed without fertilization. Of course,
parthenocarpic fruit is seedless. There are among cultivated
plants many which bear seedless fruit. Seedless tomatoes, egg-

FIG. 80. The peculiar cell divisions by which eggs and sperms are formed.
(Redrawn from Robbins and Pearson in Sex in the Plant World.)

plants, English forcing cucumbers, oranges, grapes, and bananas
are quite common examples.

Problem 2. How is pollen dispersed?

We have learned that the pollen grain, when shed by the
anther, usually consists of a two-celled male plant, enclosed by
a thick, protective wall. The pollen grains of flowering plants
differ greatly in size, shape, and surface markings. Inasmuch
as plant pollens are responsible for much of the hay fever, there has
been much interest in them, and some investigators have become
proficient in identifying them under the microscope.

Exercise 89. Different kinds of pollen grains. Examine, under the
compound microscope, pollen from a variety of plants, including such common
hay-fever plants as ragweeds, Russian thistle, pigweeds, grasses, oak, black
walnut, poplars, and elms. Also observe the winged pollen grains of pine. For

DISPERSAL OF POLLEN

157

examination of pollen grains, mount them dry on a slide, or in a mineral oil and
cover with a glass slip. When mounted in a watery and most other liquid
media, the grains either shrink or swell, or become distorted.

Quantity of pollen. The amount of pollen given off by plants
is enormous. One worker counted 243,000 pollen grains, the out-
put of a single dandelion blossom. This same worker estimated
that an entire rhododendron plant produced approximately

FIG. 80. The pollen-bearing catkins of walnut.
The catkins are easily swayed by the wind, and
the pollen is light in weight and produced in
abundance. These characters make the plant
well adapted to wind pollination. (Photograph
furnished by Division of Pomology, California
College of Agriculture.)

FIG. 82. The cleis-
togamous flowers of
closed gentian. The
flowers never open;
hence only self polli-
nation can occur.

72,620,000 pollen grains. It is said that a medium-sized Indian
corn plant will produce as many as 50,000,000 pollen grains.
Gager says: " It was calculated that between 8 A.M. and 1 P.M.
on a certain day there were given off from a single plant of Am-
brosia trifida (ragweed) the amazing number of eight thousand
million (8,000,000,000) pollen grains."

Agents which disperse pollen. Pollen is carried chiefly by
wind and insects. Even when male and female organs are borne

158

REPRODUCTION OF PLANTS

in the same flower, as they usually are, outside agencies are most
always depended upon for pollen transportation. In fact, there
are only about 150 species of flowering plants which do not need
pollinating agents. These are the cleistogamous flowers (from
cleisto-closed + gamos-marriage). Their flowers never open, and
their pollen tubes grow directly from the stamens into the pistils.
Certain violets are an example of such flowers.

Such inconspicuous flowers as those of grasses, cottonwoods,
alders, birches, oaks, hickories, and pines are notable among those

which have their pollen
dispersed by the wind.
With the exception of
the nut fruits, the com-
mon tree fruits are largely
dependent upon insects
for the dispersal of their
pollen.

In grasses the flowers
are inconspicuous, they
lack odor and nectar,
and hence are unat-
tractive to insects;
furthermore, the pollen
is light and dry, and
easily blown; the stigmas
are feathery and expose

FIG. 83. Staminate flowers of cottonwood.

Wind carries the pollen from the cottonwood

tree bearing staminate flowers to the pistils

of the tree bearing pistillate flowers.

a large surface to fly-
ing pollen; and pollen
is often produced in

great quantities. For example, in corn, it is estimated that
each staminate flower group (tassel) produces 20,000,000 to
50,000,000 grains of pollen. There are in the neighborhood of
45,000 pollen grains produced for each ovule. The styles, the corn
" silks," are long and plumose, and are receptive throughout their
entire length. Pollen grains of wind-pollinated flowers are often
much roughened. What is the advantage of this to the plant?

In cottonwoods, alders, birches, oaks, and hickories, the flowers
are in catkins. The staminate catkins are pendulous and move

DISPERSAL OF POLLEN

159

easily in the wind, and the light pollen is shaken from the anthers
and readily carried away by the breezes. In many catkin-bearing
trees the flowers open before the leaves unfold so that pollen move-
ment is unhampered.

In pines, the flowers are also borne in short catkins; and, in
addition to this feature which favors wind dispersal of pollen, the
pollen grains themselves are provided with two wings which
assist in their distribution by the
wind. In pines, pollen is produced
in tremendous quantities. At the
proper season, showers of pollen
may be witnessed in the pine
forest; one's clothing may become
yellow with the pollen grains.

The principal pollinating in-
sects are bees, the most efficient
of which are the honeybee and the
bumblebee. It is known that
French and sugar prunes in Cal-
ifornia and Napoleon and black
Tartarian cherries set a very light
crop unless a large number of bees
are present in the orchards at
the time of blooming. In fact,
insects are necessary for the pol-
lination of most deciduous fruit
trees except certain nuts.

The flowers of red clover must
be cross-pollinated in order to set
seed on a commercial basis, and
the bumblebee is chiefly respon-
sible for carrying the pollen. This insect is capable of pollinating
30 to 35 clover flowers a minute. Have you noticed that bees in
their work confine themselves, for the most part, to visitation of
the flowers of one species? What is the advantage of this to the
plant?

In many types of figs, including Smyrnas, but excepting the
common black fig, all or at least one of the crops require the visi-

FIQ. 84. Timothy in bloom. The
grasses have flowers that are not
showy or fragrant. They are fitted
by structure and position to wind-
pollination.

160 REPRODUCTION OF PLANTS

tation of the fig wasp, bringing with it pollen, for the fruit to form
properly.

The moths and butterflies are also important pollinating agents.
They are particularly adapted with their long mouth-parts to secur-
ing nectar from flowers with long tube-shaped corollas, such as
larkspurs, columbine, and nasturtium.

Insects are attracted to flowers chiefly by their odor and color.
Odor appears to be the more important influence. Many flowers

FIG. 85. Hives of honey bees in an orchard. The insects carry pollen from

flower to flower, thus bringing about a better setting of fruit. (From Division

of Pomology, College of Agriculture, University of California.)

have special nectar-secreting structures known as nectaries or
nectar glands. Sugar is the main secretion of these glands. In-
sects also visit flowers in search of pollen, which is used as a food
mainly for the larvae.

In general, insect-pollinated flowers have both stamens and
pistils in the same flower; the stamens usually have short fila-
ments, the flower groups are quite inflexible, the pollen is often

DISPERSAL OF POLLEN 161

sticky and produced in relatively small quantities, and the flowers
are attractive because of their showiness or odor.

Enumerate the features which are favorable to insect pollina-
tion. To wind pollination.

Compare insect- and wind-pollinated plants as to waste of
pollen.

How do you account for the fact that house plants often set
less seed than plants growing out-of-doors?

Longevity and viability of pollen. Pollen varies considerably
in the length of time it will remain viable (capable of germination),
depending upon the moisture and temperature conditions sur-
rounding the grains, and upon the kind of pollen.

Corn pollen does not remain viable much longer than 24 hours
after shedding. That of Hibiscus trionum lives no longer than
3 days. Pollen of the date palm will retain its viability for several
months, if kept dry. The longevity of apple pollen has been
variously reported by different investigators. One worker
records germinations of 12, 10, 5, and 8 per cents for different lots
after 7 months of storage in the laboratory, with a temperature
ranging from 50 to 65 F. The pollen of apple and plum remains
alive much longer if stored in closed vessels which prevent drying
out than when stored in the open. The pollen of some plants,
such as sugar beet, alfalfa, and red clover, absorb water rapidly
and burst in water or in a saturated atmosphere. Such pollen
loses its viability rapidly in an atmosphere of high relative
humidity.

Dry pollen will withstand greater temperature extremes than
moist pollen. However, resistance to low temperature is also a
specific character. For example, pollen of apple, pear, and plum
will withstand temperatures ranging from 33 to 34 F., whereas
about 50 per cent of peach and apricot pollen grains are killed by
this temperature.

Immediate effect of pollen. It is noticed that shortly after
pollination the stigma withers. This is the immediate effect of
pollination. After a time the petals also wither and drop off. If
flowers are bagged and pollination prevented, the petals remain
fresh for a much longer time than they do in pollinated flowers.

162

REPRODUCTION OF PLANTS

Problem 3. What are the important different types of flowers?

In Exercise 85 we learned the principal parts of a typical
flower. These are as follows: the sepals, the petals, the stamens,
the pistil. Such a flower is said to be a complete flower. But
not all flowers have these four sets of floral organs; one or more of

V

FIG. 86. Garden asparagus (Asparagus officinalis). A, pistillate flower;

B, staminate flower; C, mature fruit; D, section of fruit; E and F, portions

of the plant showing method of branching, position of flowers and leaves.

(From Robbins, in Botany of Crop Plants.)

these sets may be lacking, in which case the flower is said to be
incomplete.

Incomplete flowers. In the buckwheat flower, for example,
the petals are absent. In the flowers of willows and cottonwoods,
both sepals and petals are lacking. In both of the foregoing cases

TYPES OF FLOWERS 163

the essential organs (stamens and pistil) are present. However,
some flowers have but one set of essential organs, either stamens
or a pistil. A flower with stamens only, and no pistil, is said to
be staminate (male). On the other hand, a flower with a pistil
but no stamens is said to be pistillate (female). Staminate plants
do not bear fruit and seed; only pistillate plants perform this
function. Staminate and pistillate flowers may be on the same
individual plant; this is true of corn, in which the " tassel " is a
group of staminate flowers and the " ear " a group of pistillate
flowers. The squashes, pumpkins, and melons are other examples
of plants which bear staminate and pistillate flowers on the same
plant. Or staminate and pistillate flowers may be on different
individual plants; examples of such plants are asparagus, spinach,
hops, willows, and date palm. In these plants the flowers on any
one plant are either all staminate or all pistillate. Thus we may
speak of staminate (male) plants and pistillate (female) plants.

In certain cultivated species which have male and female indi-
viduals, one of the two kinds of plants may be more desirable
from the grower's standpoint than the other. For example, in the
date palm it is desirable that most of the individuals be pistillate
since these alone can bear the edible fruit. In the hop plant, it is
only from the pistillate plants that the " hops " are obtained. In
asparagus, it has been found that the yield of edible spears from
staminate plants exceeds that from pistillate. Staminate cotton-
woods are preferred to pistillate ones because of the " litter "
caused by the cotton-covered seeds.

Exercise 90. Incomplete flowers. Study the incomplete flowers of such
plants as pumpkin, spinach, asparagus, willow, cottonwood, and begonia.
What is a staminate flower? A pistillate? Why are staminate cottonwood
trees better as street trees than pistillate ones? How can you propagate
staminate individuals? Will a solitary asparagus plant produce fruit? A
solitary cottonwood? A solitary hop-vine? Explain why.

Lily type of flower. The lily family includes such well-known
plants as the lily, yucca, hyacinth, tulip, onion, and asparagus.
In this family, the parts of the flowers are in threes. The non-
essential organs consist of six separate parts, in two circles of three
each, which are generally very similar in size, shape, and color.
The anthers are usually large and conspicuous. The ovary is

164

REPRODUCTION OF PLANTS

FIG. 87. Lilium graridiflorum, a
monocotyledon.

FIG. 88. The inflorescence
(umbel) of leek, a plant closely
related to onion. At the top
and right is a flower-group still
enclosed.

2(J glume

/if glume

Fia. 89. Spikelet of common panicle oats,

of Crop Plants.)

(From Robbins, in Botany

TYPES OF FLOWERS

165

divided into three chambers, each of which commonly has several
seeds. Flowers of the lily type are chiefly insect-pollinated
Name ten plants of economic importance belonging to the lily
family.

Grass type of flower. The flower of the grass family
(Gramineae) is peculiar. It may be studied to advantage in such
common grasses as wheat, oats, barley, and rye. In all grasses,
the flowers are in groups, each group being called a spikelet. A
typical spikelet, such as
that of oats or wheat, con-
sists of a short axis, bear-
ing a number of chaff-like
bracts. The two lowermost
bracts, called glumes, are
empty, that is, do not bear
flowers in their axils.
Above the two glumes are
one or more bracts called
lemmas, and usually there is
a flower in the axil of each.
Each flower consists of three
stamens and a single pistil.
The ovary contains a single
ovule and has two feathery
stigmas. The awns or
beards of a grass are brittle
structures usually attached
to the lemmas. Do you
think that inconspicuous
flowers of the grass type,
with their lack of showy parts and nectar glands, are wind-
pollinated or insect-pollinated? Explain.

-anther

loJicules

Fia. 90. Wheat flower with lemma re-
moved; considerably magnified. (From
Robbins, in Botany of Crop Plants.)

Exercise 91. Dissect the spikelets of oats, wheat, or barley, and find the
parts described in the previous paragraph. Write a short paper on the topic
" Grasses and Man."

Mustard type of flower. The mustard family (Cruciferae)
includes a number of familiar plants such as cabbage, turnip,

166

REPRODUCTION OF PLANTS

rutabaga, rape, mustard, radish, watercress, and horseradish, and
a number of pernicious weeds such as pennycress, wild mustard or
charlock, shepherd's purse, false flax, and tansy mustard. The
mustard flower is characteristic. It has four sepals, four petals,

six stamens (two short and
four long), and a two-
celled ovary. The four
petals are so arranged that
when one looks at the face
of the flower it has the ap-
pearance of a Greek cross,
hence the name Cruciferae
(Latin, crux, cross, +fero
bear). The pistil has a
single style with a more or
less two-lobed stigma.
Insects are the principal
agents in the pollination
of mustard flowers. Why?
Name ten plants of eco-
nomic importance belong-
ing to this family.

Rose type of flower.
The family Rosaceae in-
cludes such plants as the
raspberry, blackberry,
dewberry, strawberry,
spiraea, and rose. The
flowers are generally com-
plete, except in some culti-
vated varieties of straw-
berries. There 'are usually
five sepals and five petals.
In most cultivated roses,
which have double flowers,

there are numerous petals which have developed from young
tissue that normally becomes stamens. Except in these double
sorts there are numerous stamens, and as a rule, a number

FIG. 91. Corn (Zea mays). Young pistil-
late inflorescence ("ear"), showing the long
styles ("silks"). (From Robbins, in Botany
of Crop Plants.)

TYPES OF FLOWERS

167

of separate pistils. The rose type of flower is chiefly insect-
pollinated.

Apple type of flower. The apple, pear, quince, loquat, and
service berry are members of the apple family (Pomaceae). This
family bears flowers which are complete and usually have a concave
or cup-shaped receptacle, to which are attached a five-lobed or
five-toothed calyx, five separate petals, numerous distinct stamens,
and a one- to five-celled ovary.
Pollination of the apple type of
flower is brought about by insects.

Plum type of flower. The
plum family (Drupaceae) includes
the plum, cherry, almond, peach,
and apricot. This is a group com-
monly known as the stone fruits.
The flowers are complete. The
corolla and calyx each have five
distinct parts. There are nume-
rous stamens. In a longitudinal
section of the drupaceous flower
it is seen that the ovary is placed
down within a cup commonly
called the " calyx tube." There
is one pistil situated at bottom of
the hollow receptacle, and a one-
celled ovary, usually maturing
one seed. Pollination of the plum
type of flower is chiefly by in-
sects.

Legume type of flower. The
pea family (Leguminosae) is one

of wide geographical distribution and possesses a great many
species. Well-known representatives are common garden pea,
vetch, sweet pea, clovers, sweet clovers, alfalfa, bean, cow-pea, soy
bean, and peanut. The flowers are irregular in form; they have a
butterfly-like shape. The calyx is usually four- or five-toothed.
The petals are normally five in number, a broad upper one
(standard), two side ones (wings), and two lower ones more or less

FIG. 92. Flower of mustard.
Diagram of flower above, and
flower in median lengthwise sec-
tion below. (From Robbins, in
Botany of Crop Plants.)

nm of receptacle/

FIG. 93. Median longitudinal section of apple flower. (From Robbing, in
Botany of Crop Plants.)

FIG. 94. Median lengthwise sec-
tion of the flower of sour cherry.
(From Robbins, in Botany of Crop
Plants.)

TYPES OF FLOWERS

169

united along one edge, forming the keel; this keel incloses the
stamens and pistil. Stamens are usually ten in number; com-
monly nine are united and one is free. There is a single pistil,
with one cell. Some of the legumes, such as the garden pea, are
self-pollinated; many others are pollinated by insects.

Composite type of flower. The thistle or composite family
(Compositae) possesses a number of well-known plants, among
which are common lettuce, Jerusalem artichoke, endive, salsify,
dandelion, yarrow, sage, chrysanthemum, sunflower, golden rod,

FIG. 95. The choke cherry

has its flowers in long

racemes.

FIG. 96. Flower structures of the
pea or legume family. Left, the
calyx and corolla have been re-
moved, exposing the stamens (10)
and the style. Nine filaments are
united at base to form a tube
which surrounds the ovary; 1
stamen is free.

sow thistle, dahlia, aster, marigold, fleabane, everlasting, Spanish
needles, and thistle. In this family the individual flowers are
grouped to form a flowerhead. A " sunflower " is not a single
flower, but a group of individual flowers, mounted on a common
receptacle. As a rule, in the flower head, there are two kinds of
flowers: (1) those about the margin, called ray flowers; and (2)
those in the center, known as disk flowers. In such composites
as lettuce, however, all the flowers of a head are alike. The disk

170

REPRODUCTION OF PLANTS

flowers have a calyx made up of bristles or scales. These are
attached at the top of the ovary. The corolla is tube-like, and on
its side are attached the five stamens. There is a single pistil,
which has a one-seeded ovary, and a single style. The ray

unfnppecf

tripped

FIG. 97. Pollination of alfalfa. A, flower untripped with calyx and stan-
dard removed; B, same tripped; C, position of staminal tube untripped
and tripped. (After U. S. Dept. Agr. from Robbins, in Botany of Crop

Plants.)

flowers are usually imperfect. Insects are the principal agents in
the pollination of composite flowers.

Double flowers. Many cultivated plants tend to develop
double flowers. Well-known examples are forms of dahlias, chrys-
anthemums, pinks, roses, and hollyhocks. Doubling may arise

FAILURE OF FRUIT SETTING

171

through the change of stamens or pistils to petals, or through the
origin of extra petals in the circle of petals.

ay
Jrowgrs

^ fkWrj
^mvolucr.al

bracts

ovary wall
ovule

FIG. 98. Jerusalem artichoke, a member of the composite family. A,
lengthwise section of the flowering head, Xl; B, ray flower, X6; C, disk flower,
cut lengthwise, X6. (From Robbins, in Botany of Crop Plants. A after

Baillon.)

Exercise 92. A study of flower types. Study the following types of
flowers: mustard, rose, apple, plum, legume, composite, and double. These
should be dissected. Prove to your own satisfaction that each of the flowers
studied illustrates the features of the type to which it belongs.

Problem 4. What are the principal causes of the failure of
blossoms to set fruit?

There are often reproductive failures in plants. They may
bear an abundance of blossoms, but owing to one or more causes,

172 REPRODUCTION OF PLANTS

fail to set fruit. Among these causes may be mentioned the fol-
lowing:

1. Pollen is not shed at the time when the stigmas are recep-
tive. The pollen may be shed before, or after, the stigmas are
receptive. In some American plums, particularly during periods
of cold weather, the stigma may pass the receptive condition before
the pollen is mature. What visible evidence is there that a stigma
is ready to recieve pollen?

2. Pollen is not viable. Some cultivated varieties of grapes
bear impotent pollen. Certain varieties of strawberries (Glen
Mary and Crescent) produce impotent pollen and hence are self-
sterile. Several commercial varieties of peaches, notably Chinese
Cling and J. H. Hale, produce no good pollen. They are male
sterile. Of course, these varieties will not set fruit unless inter-
planted with varieties which will furnish pollen.

3. Imperfect flowers. There is a commercial sterility problem
with strawberries involving chiefly the impotence of the pistils of
the perfect flowers. Certain varieties of strawberries develop only
perfect flowers, and all flowers are fertile. Other varieties have
more or less female sterile perfect flowers, and still others bear
only pistillate flowers. If such a variety as the last mentioned
is planted by itself, there will be no pollen. In planting varieties
with pistillate flowers only, it is necessary to have rows nearby
planted to pollen-bearing individuals.

4. Self-sterility. Many varieties of orchard fruits are not
capable of setting fruit unless pollen from another variety is used.
That is, they are self-sterile. For example, the Montmorency
cherry is self-sterile but may be cross-pollinated by Early Rich-
mond or English Morello. In some localities the Spitzenburg
apple is self-sterile, but can be fertilized with pollen from a number
of other varieties, such as Yellow Newton, Arkansas Black, Jona-
than, and Baldwin. Evidently, the mutual affinities of varieties
must be considered in setting out an orchard. It would not be
well to plant solid blocks of Spitzenburg apple, for example. There
should be, here and there in the orchard, trees of some one of the
other varieties, the pollen of which is capable of fertilizing it.

Self -sterility in pears is the reason for the barrenness of many
pear orchards. It has been frequently observed in many parts of

FAILURE OF FRUIT SETTING

173

Copyright of Journal of Heredity

FIG. 99. Certain varieties of strawberry bear only hermaphroditic flowers

(above). Other varieties of strawberry bear only pistillate flowers (below).

(After Darrow, from Robbins and Pearson, in Sex in the Plant Woild.)

174 REPRODUCTION OF PLANTS

the country that when a certain variety of pear is planted in a
solid block, a pronounced failure to set fruit often results. This is
particularly true, it seems, of Bartlett and Kieffer pears. These
varieties give much better results when they are planted with such
varieties as Lawrence, Duchess, and Anjou.

5. Excessive production of flowers. Many plants initiate
development of far more flowers than they can perfect; and often
plants cannot mature all the fruits that set from those flowers
which are perfected. The " June drop " of the immature fruits
of certain fruit trees is due to the abortion of embryos which the
trees have not reserve food enough to mature.

6. Unfavorable weather conditions. Fruit-setting may some-
times fail because of frost or because of cold, rainy weather which
interferes with the movement of insects or delays the growth of
the pollen tube; or hard rains which come immediately after the
pollen is brought to the stigma may wash the grains from the
stigmas. In the case of corn, hot dry winds may wither the
" silks," making it impossible for the pollen to stick to them and
germinate; as a result there is an incomplete " filling " of ears.
What are the " silks " of corn? The tassels? Is corn wind- or
insect-pollinated ?

7. Lack of pollinating insects. In most of our orchard trees
the pollen is carried by insects, chiefly bees. It has been shown
that under certain conditions the percentage of flowers setting fruit
can be increased by placing beehives in the orchard. It is usually
considered that one hive of bees to one or two acres of orchard is
sufficient.

There are other types of sterility among flowering plants.
Abortion is a rather common occurrence in the plant kingdom.
For example, in the coconut normally there are three sections in the
ovary, but, while it is maturing, two of the sections abort ; likewise
in the date palm, which also has three sections in the ovary, only
one section becomes a fruit; in oaks there are three sections in the
ovary, each with two ovules, but the mature acorn has but one
seed; in plums, cherries, almonds, and other stone fruits, the
ovary normally has two ovules, but only rarely do both oi these
develop into seeds. In fact, when two seeds are found in a cherry
it is occasion for special remark.

REPRODUCTION OF FERNS AND MOSSES 175

Several cultivated plants develop but a small number of flowers
or only poorly developed ones. For example, some Irish potato
varieties produce very few flowers; the same is true of the sweet
potato, in which seeds may be almost lacking. In France, garlic
seldom flowers. Sugar cane is a notable example of a plant which
seldom produces seeds. How is a plant like sugar cane propa-
gated? What is the advantage of reproducing potatoes by means
of tubers rather than seeds?

Problem 5. How do ferns and mosses reproduce?

Among ferns, as among seed plants, the sexual function is
performed by minute, very peculiar sexual plants. But in ferns
there is no embryo sac in an ovule of the ovary, no masses of yellow
pollen to be carried about by bees or blown in clouds by the winds.
There are, instead, flat green growths hidden under the forest moss
and mold. The largest kinds of sexual plants in ferns are seldom
half an inch across; the smallest of them grow to maturity and
produce their eggs and sperms all within the protective coat of the
single spore from which they grew.

Exercise 93. How do ferns produce spores? Ferns have no flowers or
stamens or pistils. Look on the under side of almost any fern frond in late
summer or autumn and notice there the small, regularly arranged brown
warts. Each of these consists of a group of spore cases (sporangia), in many
instances protected by a covering of thin tissue (the indusium) . Each group of
spore cases is called a sorus (plural, sori). Mount some of the spore cases in
water, and examine with the compound microscope. The spore case or sporan-
gium is watch-shaped, with a thick wall about one edge, thin side walls, and a
stalk. Inside are the spores. Give important differences between a spore
and a seed.

In moist, protected soil, the spores germinate, each growing into
a miniature green plant of one sex or the other, or even into plants
bearing the organs of both sexes. Most fern sexual plants are
peculiar flat growth like bits of leaves, heart-shaped and about
J- to -- inch across. The sexual plants of ferns are called 'prothallia
(singular, prothallium).

Exercise 94. How do ferns produce sex cells or gametes? On the moist
soil beneath ferns, look for the prothallia of ferns. An experienced greenhouse-
man will always be able to find them for you. Study them with the dissecting

176

REPRODUCTION OF PLANTS

microscope. Observe that they have root-like threads (rhizoids) fastening
them to the soil. Examine prepared slides showing the sexual organs.

The male organs the structures producing sperms are among the
rhizoids; the female organs the structures producing eggs are clustered near
the notch of the heart-shaped plant. A male organ is a minute spherical mass
of cells protruding below the under surface of the prothallium. The inside

Archegonium

fertilization
of egg in

Archegonium
by sperm

Antheridium

r

Gametophyte
(Protha Ilium)

Germinated
Spores Develop

5pore

Sporangium
Breaks open
-Spores are
Released
after Tetrads
Break up

Sporophyte

Tetrads of 5pores
are Formed unthin
Sporangium

Old

Gametophyte
Withers
and Dies
as Young
Sporophyte
Grouus

Sporangium

Developed Sporophyte
or Pern Plant

Gametophyte Generation * 5porophyte Generation

FIG. 100. Life history of a fern. Note that the cycle is made up of a sporo-
phyte generation which includes the showy fern plant, and a gametophyte
generation which includes structures that are small and not frequently seen

in nature.

REPRODUCTION OF FERNS AND MOSSES

177

cells of the male organ grow into long, coiled sperms, out of which curious long
arms of protoplasm grow. With these arms, the sperms swim agitatedly
about in the moisture under the prothallium until they reach the chimney-like
structure of the female organs. A female organ is a flask-shaped structure,

FIG. 101. Fern fronds showing peculiar " warts " on the under sides. These
are spore clusters. (From Robbins and Pearson, in Sex in the Plant World.)

178

REPRODUCTION OF PLANTS

the base of which is a chamber in which there is a single egg. The " neck "
of the flask-shaped female organ is a chimney-like structure. The sperms swim

FIG. 102. Two of the " warts " of Figure 101 highly magnified. These
" warts" are essentially clusters of minute balls on stems. The balls are full
of spores. (From Robbins and Pearson, in Sex in the Plant World.)

through these chimneys and fertilize the egg. Fertilized eggs grow out of the
female plant, developing directly into the beautiful fern fronds we know.

Fia. 103. Fern prothallia in the laboratory. Fern spores may be germinated

in the laboratory if sown on damp inverted flower pots standing in a solution

of small quantities of the salts required for their germination and growth.

Exercise 95. What is the nature of the leafy moss plant? Examine indi-
vidual moss plants taken from a mossy turf. Each plant consists of a slender

REPRODUCTION OF FERNS AND MOSSES

179

stem or axis, bearing many
tiny overlapping leaves.
The plant is anchored to
the soil by rhizoids which
not only serve as hold-
fasts, but also absorb mois-
ture. If moss leaves are
mounted flat in water and
examined with the com-
pound microscope, it will
be seen that they are very
thin, often no more than
one cell thick.

The asexual moss
plant. The familiar
mossy turf is the sex-
ual generation of the
moss plant; and in-
stead of plants with
leaves and stems and
roots producing spores,
like seed plants, the
spore-producing moss
plant has only a small
brown capsule on a
stalk growing directly
out of the female sex
organs and feeds as a
parasite on the sexual
plant. In the green-
house or woods one can
find these small cap-
sule plants growing out
of the mats of moss.

Exercise 96. How do
mosses produce spores?

Study moss plants bearing
the capsules (sporangia)
each on a long stalk. The
capsules or sporangia are

FIG. 104. Moss. A, portion of leafy moss plant
and spore-bearing structure; B, single spore
showing germ tube; C, algal growth which de-
velops from germinating spore; D, female sex
organ; E, male sex organ. (From Holman and
Robbins, in A Textbook of General Botany.)

180

REPRODUCTION OF PLANTS

Archaqoruum
bears the eqq

Anthendium
produces sperms

Sporophi^fe VH

Spore plant develops
from the fertilized eqq,
qrows from the leaf q
plant and produces
spore "tissue

Leafif plants develop
from buds and develop
sex orqans

\

Spore qermi nates and
produces a protonema
with buds

Spores escape
from capsule of
the spore plan*

FIG. 105. Life cycle of a moss plant, showing alternation of generations.
Note that the spore plant results from the union of gametes and that the
gamete plant results from the germination of a spore. How does the gameto-
phyte of the moss get its food? What is the source of food of the sporophyte?
Of what importance is chlorophyll in the life cycle of the moss plant? In what
way is the gametic reproduction of moss different from the gametic reproduc-
tion of bread mold. What is the name given to the union of two unlike
gametes resulting in the formation of a zygote?

ASEXUAL REPRODUCTION 181

nothing more than powder boxes of spores. Break open a mature capsule in a
drop of water and examine the spores. When these spores germinate they
develop into green moss plants. The early stages in the growth of moss plants
from spores are green branching threads, growing close to the moist soil, and
often giving to it a greenish color.

The sex organs of mosses look much like the sex organs of
ferns. However, they are not embedded in the leafy sexual plant,
and the male organs are larger and stalked and the female organs
have longer necks. They are borne in clusters in the hearts of the
buds at the tips of the moss branches. Their sperms can swim to
the eggs in the slightest bit of moisture perhaps a drop of dew.

Suggested activities. What is the importance of mosses in nature?
Write a report on the importance of sphagnum and other mosses to man.

Problem 6. How do plants reproduce asexually?

The starfish, which is such a pest because it feeds on oysters,
and the quackgrass, which chokes out crop plants and which the
farmers of the country annually spend thousands of dollars to
control, can not be killed by chopping up. Each dissevered arm
of the starfish grows a whole new starfish; each stalk of quack-
grass grows a whole new plant. Only a few simple animals can be
multiplied by breaking them into pieces, but think of the many
complex plants which can be reproduced in this asexual " vegeta-
tive " manner. Cuttings of stems of many roses, willows, apples,
and a host of other plants will develop roots and grow if kept in
moist soil. The strawberry sends out runners stems with buds
at the end which root and form new strawberry plants in much
the same fashion as the interesting " walking fern," except that
in the fern it is the tip of a leaf from which the new plant grows.
Begonia leaves will send out roots and stems and leaves if pinned
closely to moist soil. Potato tubers and lily bulbs are also special
asexual reproductive structures.

Exercise 97. The gemmae of Uvei worts. The liverworts, common in
greenhouses and moist woods, are simple flat green growths, with distinct
under and upper surfaces. Some kinds of them are much larger than most
mosses and look like green snakeskin. They reproduce both sexually and
asexually. Observe on the upper surface of certain plants minute cups full of
very small green pieces of liverwort tissue no larger than the head of a pin;

182

REPRODUCTION OF PLANTS

these are called " gemmae." Currents of air scatter these on the moist soil
and they grow directly into new liverwort plants. The plant has reproduced
asexually.

Reproduction by fission. When a bacterium reproduces it
splits across the middle into two bacteria. Reproduction here is
nothing more than cell division. The cell in this case is a minute
system which can grow and split into two systems, each capable

of growing and splitting across
the center into two more systems,
and so on ad infinitum. This
simple type of reproduction is
called fission. It occurs among
more complex one-celled organ-
isms than bacteria.

The blue-green algae also re-
produce by fission. Many kinds
grow in hot springs, lining them
with bright-colored crusts. Others
grow in soil. Some are very
troublesome in reservoirs, impart-
ing a disagreeable flavor to the
water. Also certain green algae
reproduce by fission.

Exercise 98. Asexual reproduction
in Protococcus. This is a one-celled
plant which may be found growing on
the north side of trees, on old damp
boards, and on foundations of buildings.
It forms a growth which resembles
green paint. Scrape off some of the
material with a knife and mount in
water on a slide. Each cell is a sepa-

FIG. 106. Walking fern showing
vegetative reproduction, and migra-
tion of the plant.

rate plant. It will be observed that reproduction is accomplished by the
division of the whole body of the parent.

Reproduction by spores. Everyone is familiar with the white
threads of the " bread mold/ 7 which grows, as the name implies,
on stale bread. The small black spherical bodies on stalks which it
sometimes produces contain the asexual spores. These are in the

ASEXUAL REPRODUCTION

183

air, even in the best-regulated kitchen, and when in abundance
produce the well-known musty odor.

Exercise 99. Asexual reproduction in bread mold. Moisten stale bread
and place under a bell jar or other cover. Within a few days the surface of the
bread will be covered with a cottony growth, and after a time there will appear
numerous black bodies. These are spore cases or sporangia. Each sporangium
contains thousands of spores. Examine with high-power dissecting micro-
scope. Also mount spore cases in water, and study with the compound
microscope. The significant feature of the asexual method of reproduction in
bread mold is that the spores are capable of growing directly into new bread-

Fia. 107. Cupfuls of tiny liverworts grow on the upper sides of old liverworts,
odd relatives of the mosses. (From Robbins and Pearson, in Sex in the

Plant World.)

mold plants. There is, however, sexual reproduction in bread mold, but the
asexual method is by far the more important. (See pp. 101, 103.)

Slime molds are simple plants which creep about, streaming
masses of slime, in rotting logs and stumps. Single slime-mold
cells creep about and reproduce freely by fission. But besides
fission these slimy masses have another method of asexual repro-
duction; this method is spore formation.

A spore is merely a living cell which seems to find it necessary

184 REPRODUCTION OF PLANTS

to escape from its mother that it may grow into a new plant.
When slime molds initiate reproduction by spores, erect growths
with solid walls appear on the slimy mass, and grow into small odd-
shaped powder puffs of threads and heavy-walled spores. If
these spores eventually settle in a sufficiently moist place, they
open and release more spores. This time the spores can swim.
They swim about for a time in the wet leaf mold or soggy wood,
then again flow about.

In the seaweeds, in mosses, ferns, and seed plants, there are
asexual spores, that is, spores which are capable of growing directly
into a new plant without union with any other mass of living
material. For example, in mosses asexual spores are borne in a
capsule (sporangium), and these spores grow into the moss plant,
which in turn develops eggs and sperms. In ferns, asexual spores
are borne in cases which, grouped together, form the brown
" warts " on the back of fern fronds. These spores germinate,
developing a small heart-shaped structure, which in turn is the
bearer of male and female organs. In seed plants there are also
asexual spores, as well as sexual bodies. There are small asexual
spores in the anthers which develop into pollen grains (male
plants) ; and large asexual spores in the ovules which develop into
embryo sacs (female plants).

Problem 7. How are plants propagated artificially?

The multiplication of plants artificially is commonly known as
propagation. Among our common seed plants there are two dis-
tinct methods of propagation: (1) by the use of seeds, and (2)
by the use of some vegetative organ, such as stem, or root, or leaf.

We have learned that in the seed there is an embryo, or young
plant, and that it is formed as a result of the process of fertilization.
The embryo of the seed is a result of the union of two sex elements.
One of these elements, the pollen grain, may or may not have come
from the plant which bears the seed; if it did not, then the embryo
in the seed has characters of two parents. This means that, when
a plant is cross-fertilized, the resulting seeds will produce plants
in many particulars unlike the parent which bears them. Propaga-
tion by seeds is a sexual method.

ARTIFICIAL PROPAGATION

185

On the other hand, when a plant is propagated by means of
stems or roots or leaves, we may be certain that the parts used
will produce plants closely resembling those from which they were
taken. This method of propagation is vegetative.

Let us cite an example which further distinguishes between the
methods of propagation by seeds and by vegetative organs. The
pollen of strawberries is carried chiefly by bees; hence there are
great chances that the ovules of any particular flower will be fer-

FIG. 108. Modern method of propagating the tropical orchid. Formerly

propagated in cultivation only by vegetative means, orchids are now grown

from seed germinated on specially prepared media.

tilized by pollen from another plant. If they are so fertilized, the
embryo which results will possess characters of two parents.
This accounts for the fact that strawberries when grown from seed
seldom come true to type ; that is, they seldom are like the parent
plant from which the seed was taken. If we wish to propagate a
desirable variety of strawberry and keep it " true," we use the
" runners " or vegetative parts. Runners are merely " chips off
the old block."

Propagation by the use of vegetative organs is practiced if the

186

REPRODUCTION OF PLANTS

plants do not come true from seed. This is the case with hybrids
and many horticultural varieties. Also a number of plants such as
sugar cane, banana, sweet potatoes, and Irish potatoes seldom
produce seed and, of course, must be propagated vegetatively.

In propagating plants by vegetative means, use is made of
steins, of roots, and of leaves. Many more plants can be vegeta-
tively propagated by stems than by roots or leaves. (A. stem struc-
ture usually may be readily distinguished from a root as follows:
The stem is divided into definite joints and the buds, hence, the
branches arise in regular order; the buds occur in the axils of the
leaves. Roots, on the other hand, do not have definite joints, and

usually bear no buds or leaves.
Relatively few plants can be
propagated by means of their
roots. Among them may be
mentioned the sweet potato,
dahlia, raspberry, and certain
blackberries. Rarely, the leaf
may be employed as a prop-
agating organ. A striking
example is seen in begonia
and Bryophyllum.

Propagation by separation.
In this process of propagation
use is made of such vegetative
parts of the plants as become
detached naturally. These in-
clude such structures as bulbs, bulblets, bulbels, corms, and tubers.
Each of these possesses one or more buds which, under proper
conditions, are capable of growth.

Exercise 100. Propagation by separation. Study bulbs, bulbels, bulblets,
corms, and tubers. These should be brought to the laboratory, their structure
studied, and some set out under conditions favorable for growth. Talk with a
plant propagator in a greenhouse concerning methods of growing them and
their use in multiplying plants commercially by separation.

Bulb. A splendid example of the bulb is the onion. Examina-
tion of the mature bulb of the common onion shows it to be made

FIG. 109. Vegetative reproduction of
Tolmiea menziesii from a leaf.

ARTIFICIAL PROPAGATION 187

up of the much-thickened bases of leaves, attached to a small cone-
shaped stem. These scale leaves are quite rich in food material.
The bulb possesses a terminal bud and occasionally lateral buds in
the axils of the leaves.

Bulbel. Often a number of small bulbs will develop about a
large mother bulb. These are called bulbels. For example, in
the white lily (Lilium candidum) a group of bulbels is formed at the
top of the mother bulb, and each produces a number of roots.
These bulbels may be separated from the mother bulb, and each
used to propagate a new plant.

Bulblet. These are small bulbs which are developed above
ground, usually in the axils of leaves or in the flower group. They
do not differ essentially from bulbels except in their position on the
plant. In the tiger lily (Lilium tigrinum) small bulbs occur in
the axils of the leaves; they may be removed from the parent plant
and used for the purpose of propagation. In " top/' " tree/' or
Egyptian onions, clusters of bulblets are developed at the top of
the flower-bearing stalk. They will grow while still attached to
the stem, but, of course, are detached for the purposes of prop-
agation.

Corm. This type of stem is well exemplified in the crocus and
gladiolus. It is a short, solid underground stem, differing from the
bulb in the absence of scale leaves. The corm is usually flattened
from top to bottom and bears a cluster of thick fibrous roots at the
lower side and a tuft of leaves on the upper side. A number of
small corms, called cormels, may develop on the mother corm.
Both the large corm and the cormels may be used in propagation.

Tuber. The best example of the tuber is the common Irish
potato. It is a simple enlargement of the tip of a slender under-
ground stem. The buds of the potato tuber usually occur in
groups, each group being called an "eye." The potato plant may
be propagated by planting the whole tuber, or by cutting it into
pieces, each piece having one or more " eyes."

Propagation by division. In this method of propagation, living
vegetative organs are artificially broken or cut into sections, or
pieces, and these placed under conditions suitable for growth. For
example, as cited previously, the tuber may be divided into a
number of pieces, each having one or more eyes (buds), and these

188 REPRODUCTION OF PLANTS

may be employed to propagate the plant. Plants such as canna,
iris, asparagus, rhubarb, and ferns which produce horizontal
underground stems (rhizomes or rootstocks) may be readily propa-
gated by dividing these into a number of sections, each of which
bears several buds. Most of our perennial herbs may be propa-
gated by division of the " crown/' The name " crown " usually
applies to that part of a perennial plant which is just below or at

f / Stolon

Scale Leaf

FIG. 110. -A strawberry plant showing propagation by means of "runners" or
stolons. (From Holman and Robbins, in A Textbook of General Botany.)

the ground surface, and which is essentially a clump of shortened
stems bearing buds and roots.

Exercise 101. Propagation by division. Propagate certain of the follow-
ing plants by division: potato, canna, rhubarb, ferns, columbine, larkspur.

Propagation by cuttings. In this method, some vegetative part
of the plant is detached and placed under conditions favorable for
growth. Cuttings may be made from stem, root, or leaf. Of
course, the cutting must contain living tissue, have a certain
amount of stored food for growth, and be capable of developing
growing points. The growth of the missing organs from a cutting
is regeneration. It is believed that any vegetative part of a plant
(stem for example) which possesses active, growing cells has the
power of regenerating the missing organs (roots, leaves, and

ARTIFICIAL PROPAGATION

189

flowers) if placed under proper environmental conditions. Failures
are probably due to our inability to create satisfactory growth
conditions. We have learned by experience that some plants can
be propagated easily by stem cuttings, others by root cuttings, and
a relatively few by leaf cuttings. We can not always tell by obser-
vation of the plant whether or not these organs can be used to
propagate its kind. We are still unable to offer a satisfactory ex-

e

\

FIG. 111. Root and stem cuttings. From left to right: root cutting, hard-
wood cutting, soft-wood cutting, and soft-wood cutting showing proper and
improper age of stem from which cutting is to be made; the stem should snap in
two when bent, as shown at the left of the leafy stem, rather than crush, as shown
at the right. (Figure at right redrawn from Kains, in Plant Propagation.)

planation why root cuttings of such plants as the blackberry and
red raspberry will readily produce buds, whereas those of many
other plants under the same conditions lack this power.

Cuttings should be placed under conditions favorable for the
rapid healing of the cut surface. When a stem is cut in such
a way as to involve the growing layer or cambium, the cambium
is stimulated to active growth and forms a mass of large, thin-

190 REPRODUCTION OF PLANTS

walled cells known as callus. It is generally believed that callus
formation must precede the development of roots, although it is
known that roots do not arise directly from callus tissue. A new
cambium arises by differentiation of inner callus cells, and as a
rule it is from this cambium that the growing points of roots and
stems originate. However, best results are usually obtained
from callused cuttings. At any rate, the conditions which favor
the development of callus also favor root formation.
Stem cuttings. These may be made from:

1. Mature or dormant growth, that is, from hard or ripened
wood, or from

2. Immature growth, that is, from soft wood or from herba-
ceous stems which are vegetatively active.

As a rule, in cuttings of mature growth, wood of one season's
growth is employed, although sometimes wood two or more years
old is used. Cuttings are usually made from 6 to 10 inches long,
although they may be longer or shorter. Each cutting should
possess at least one but preferably two or three buds. It is
advisable to cut the lower end just below a bud. The cuttings
are usually made in the fall or early winter. They are then
stored in a place that is warm and moist enough to allow callus
formation and some root growth but not warm enough to permit
bud growth. Root growth will proceed at temperatures too low
for bud growth. It is often the practice to bury the cuttings, in
bundles, with the uppermost buds downward. Thus the cut end
of each stem is near the soil surface, which, being warmed first
in the spring, stimulates the development of roots. Cuttings
should be set out in the spring before the buds open.

It is possible to propagate most plants from immature growth
although greater care is often required to get them to root than
is necessary for hard-wood cuttings. Stems should be used that
are neither too hard nor too soft, but break with a snap when
bent. Such plants as begonias, coleus, geraniums, and chrysan-
themums are propagated by green cuttings, or so-called slips.
They are usually grown in hotbeds or greenhouses. It must be
kept in mind in the growing of cuttings that the tissues above
ground are subject to the loss of water for a period before there
is root development and sufficient water absorption. This makes

ARTIFICIAL PROPAGATION

191

it necessary that the water-losing surface be reduced by removing
most of the leaves, and that the cuttings be protected in some
way from drying out. The small leaf surface that is left on the
plant manufactures food that is used in the formation of roots.

Root cuttings. Many plants can be propagated by means of
root cuttings. The sections of the plants used in this case do

FIG. 112. A stem cutting, showing the independence of root and callus for-
mation. (From Bioletti, in Calif. Agr. Exp. Sta. Bulletin.)

not possess buds. Buds develop during growth of the cutting.
Root cuttings are planted shallowly, about J to f inch deep.
Among plants that can be propagated by root cuttings may be
mentioned the blackberry, red raspberry, horseradish, willows,
poplars, osage orange, plums, cherries, and juneberry. Generally

192

REPRODUCTION OF PLANTS

those plants which have a tendency to produce suckers from the
roots can be propagated readily by means of root cuttings.

Leaf cuttings. A number of plants, chiefly ones with thick,
fleshy leaves, can be propagated by means of leaf cuttings. Among
such are begonias, bryophyllum, gesneria, and gloxinias. The
leaves may be cut into a number of pieces, each including a part
of a main vein. Under proper growth conditions the ends of the
veins callus, and roots and buds are developed. In bryophyllum,
the whole leaf may be used, without cutting the veins; when this
is placed in contact with moist sand, roots and buds are formed
at the notches. (See Fig. 109.)

Exercise 102. Cuttings.

Each student should propagate a number of
different plants by cuttings, including
stem, root, and leaf cuttings. Suggested
material is as follows: stem cuttings:
chrysanthemum, geranium, carnation,
coleus, begonia; root cuttings: horse-
radish, willows, cherries, blackberry,
dandelion; leaf cuttings: begonias, bryo-
phyllum, gloxinias.

FIG. 113. Propagation by layer-
ing. A branch is bent down and
partly buried. It is here shown
wounded, which seems to hasten
root development. (Redrawn from
Kains, in Plant Propagation.)

Propagation by layering. This
is a method by which the plant
is propagated on its own roots.
It consists of bending over a por-
tion of a branch and covering it
with soil to keep it moist. Roots
develop at the nodes that are

covered, and buds develop at these rooted nodes. The section
of the stem thus rooted may be severed from the parent plant
and lead an independent life. In many plants, rooting of the
buried stem may be hastened by injuring it. This appears to
limit the movement of food and brings about its accumulation
above the point of cutting from which roots develop. Plants
which root readily at the nodes, when their branches come in
contact with moist earth, are easily propagated by layering.
As contrasted with cuttings, the layer is attached to the parent
plant, and thus derives water and food from it until it becomes
established. It is a more certain mode of propagation than that

ARTIFICIAL PROPAGATION

193

of cuttings. Among plants which are commonly layered are
wisterias, honeysuckles, grape, passiflora, raspberry, and hy-
drangea.

Exercise 103. Layering. Propagate by layering some plant such as
grape, raspberry, hydrangea, English ivy, or honeysuckle.

Propagation by suckers. In the strict sense a sucker is a stem
arising from an adventitious bud which develops on a root. The
term is sometimes wrongly extended to include branch stems from

FIG. 114. Cleft grafting. Left, making cleft; right, cleft being held open for

inserting scion. (From photograph by Division of Pomology, College of

Agriculture, University of California.)

the base of the plant. Among plants which develop suckers may
be named the silver-leaf poplar, black locust, blackberry, red
raspberry, and plum. Propagation by suckers involves cutting
off a portion of the root which bears the sucker, and trans-
planting.

Propagation by stolons or runners. A stolon or runner is a
stem that grows more or less horizontally along the surface of the
ground. The best common example is the strawberry. This
plant naturally propagates itself by means of runners. A runner

194

REPRODUCTION OF PLANTS

sent out from the parent plant produces both roots and new
shoots after which the runner may die, thus severing the daughter
plant from the parent. The young plants which form at the
rooting nodes of the runner may be cut off and set out. Stolons
form roots naturally, but rooting may be hastened by covering
them with soil. It will be readily observed that the layer is in
reality an artificial stolon. (See Fig. 110.)

Exercise 104. Suckering and propagation by runners. Observe in the
field the suckers of such plants as mentioned in the foregoing paragraph. Cut

FIG. 115. Cleft grafting. At right, FIG. 116. Steps in tongue or whip
two views of the scion, and at left, the grafting,

scions in position in the cleft of the
stock.

off a portion of a root which bears a sucker, and transplant. Also observe in
a strawberry bed how the plants naturally propagate themselves by runners.

Propagation by grafting. This is a very old horticultural
practice, and is in common use in propagating fruit trees. The
fruit grower, in order that he may be certain as to the variety

ARTIFICIAL PROPAGATION

195

he will have, propagates them vegetatively, one of the chief ways
being by grafting. The operation may also be carried on in order
to change the size of the tree. For example, when pears are
grafted on the more slowly growing roots of the quince, the stock
retards the growth of the pear, and dwarfing results. Another
object of grafting is to grow desirable varieties on disease-resistant
roots, or on roots which will adapt the plant to various soil condi-
tions. For example, the northern California black walnut is
resistant to a soil fungus (mushroom) known as Armillaria, whereas
the English walnut is not so resistant to the organism; this is one

Fia. 117. A and B, side grafting; C and D, saddle grafting. (Redrawn from

Newsham.)

reason for the practice of grafting the English walnut on black
walnut. The Northern Spy variety of apple is resistant to the
woolly aphis which attacks the roots. This variety is used as a
stock upon which to graft less-resistant kinds. The common
peach is sometimes grafted on Davidiana root because of the
latter's resistance to alkali.

196

REPRODUCTION OF PLANTS

Three kinds of grafting are recognized: scion-grafting, bud-
grafting, and approach-grafting. In scion-grafting, a stem called
the scion, containing several buds, is attached to another rooted
stem or root, called the stock, in such a way as to bring the growing
layers (cambiums) of each together. After a time there is a union
of the stock and scion. In budding, the scion is a single bud
together with a small strip of bark. This is attached to the cut
surface of the stock so as to bring the growing layers of stock

FIG. 118. Method of bridge
grafting in a girdled trunk.
Scions are inserted under the
bark, thus bringing about a
bridging of the girdled zone.
(Redrawn from Solotaroff, in
Shade Trees in Towns and Cities.)

i/r

FIG. 119. Steps in patch budding, often
used with walnuts.

and scion together. Approach-grafting, also sometimes known as
inarching, consists in uniting two plants while they are still growing
on their own roots. It is obvious that this is possible only between
plants which are standing close together, or between different
parts of the same plant. As a rule, approach grafting is executed
by removing a piece of the bark, down to the cambium, of the
two stems to be united, and binding these two cut surfaces to-
gether. After the two have grown together, the scion is cut off
below the union, and the stock above the union. Care should

ARTIFICIAL PROPAGATION

197

be taken to sever the scions gradually in order that growth will
not be retarded.

It is well known from experience that certain plants can be
grafted upon one another, whereas others are united with diffi-
culty. For example, peach can be grafted on the plum, but it can
not be grafted on the apple. As a general rule, the more closely
two plants are related botanically, the better are the chances of

FIG. 120. Steps in shield budding. A, the T-cut in the stock; B, inserting
bud; C, bud in place; D, bud tied; E, bud stick, showing bud ready for

inserting in stock.

a union by grafting. Plants belonging to different families can
not be grafted. For example, it is impossible to graft the peach,
which is a member of the rose family, upon the walnut, a member
of the hickory family. Plants belonging to different genera
within the same family may or may not unite readily. For exam-
ple, in the rose family, the peach will unite well with the plum, but

198

REPRODUCTION OF PLANTS

not so satisfactorily with the apricot. The tomato and the potato
may be intergrafted; they represent different genera in the family
Solanaceae. Plants belonging to different species within the
same genus, as for example, crabapples and common apples, usually
form a satisfactory graft union. And different varieties of the

1^'IG. 121. A budded apricot tree. (.Photograph furnished by JJi vision ol
Pomology, California College of Agriculture.)

same species, as for example, the different varieties of peaches, may
be grafted upon one another. In short, as a general rule plants
belonging to varieties of the same species graft upon one another
with greater certainty than plants of different species of the same
genus, these in turn with greater certainty than those belonging to

ORIGIN OF SEX IN PLANTS

199

different genera of the same family; and plants belonging to
different families will not unite at all by grafting.

It should be kept in mind that, when two plants are united
by grafting, each keeps its
individual characters. If the
crabapple (scion) is grafted on
the common apple (stock), the
branches which arise from the
scion will bear crabapples, and
those from the stock common
apples.

In grafting, the stock may
be the root, the crown, the
main stem, the main branches,
or the tips of the branches.
There are many methods of
joining the stock and scion,
among which are the follow-
ing: whip or tongue, cleft,
bark, kerf, veneer, splice,
saddle, and bridge. In all
these methods, care is taken
to bring the growing layers
together and have them touch
at as many points as possible.

What are the purposes of
grafting? Navel oranges are

FIG. 122. Approach grafting, also
known as inarching. The stock and
scion are bound together, cuts being
made in each (two stems at left), ex-
posing the cambiums, to hasten
union. (Redrawn from Kains, in Plant
Propagation.)

seedless; how is the variety propagated?
tree girdled by gophers be saved?

How can the life of a

Suggested activity. Prepare a demonstration of different kinds of grafts
using a large square of heavy cardboard to mount the prepared material.

Problem 8. How did reproduction by means of sex in plants

originate?

There is strong evidence that millions of years ago the only
plant life of the world was slimy masses of one-celled organism,
that the type of plant life or prehistoric waters and muddy shores

200 REPRODUCTION OF PLANTS

was composed exclusively of sexless species. In our modern world,
plants which probably most resemble these organisms are the
blue-green algae, many of which grow in hot springs, withstanding
temperatures which would kill most other modern plants. Flow-
ering plants as we known them today, and pines and spruces, even
ferns and mosses, formed no part of the scanty vegetation of those
early-day landscapes. After millions of generations of asexually
reproducing organisms, sexual reproduction came into existence.
Scientists believe that it arose independently in several different
kinds of plants and animals.

Even today there are some plants in which sexual reproduction
is so primitive that it seems that it can not have changed very
much through all the ages since sex first appeared. One such plant
is Ulothrix. This is an alga, whose fine green, almost microscopic
threads grow anchored by a holdfast cell to sticks and stones in
moving water. The Ulothrix cell is able to organize its proto-
plasmic contents into several masses. These peculiar bodies break
through the hard outer shell of the cell and swim away. The orig-
inal cell is thus divided into several naked cells, each propelled by
microscopic arms of protoplasm. Most of them swim and find
favorable surfaces upon which to anchor and gow into new indi-
vidual filaments. These cells are spores. They are asexual
bodies in that they are capable of growing into plants, just as
are those in moss capsules and those on the under sides of fern
fronds.

The spores of Ulothrix are not all alike. This is significant.
They vary in size. The number of spores from a single cell may
vary from one to thirty-two. If one spore is produced, this
means that the entire protoplasmic body underwent no division,
but was freed from the cell. If thirty-two spores are formed in a
single cell, this means that the mother mass of protoplasm under-
went five successive divisions. These spores are, of course, very
small and contain very little stored food. Associated with these
size differences are amazing differences in behavior. Large spores
and those of intermediate size behave as asexual spores, that is,
develop directly into a new individual Ulothrix plant. The small-
est spores, however, are apparently incapable of developing into a
new plant. If a drop of water containing these smallest bodies,

ORIGIN OF SEX IN PLANTS

201

swimming or squirming about, is examined with a high-power
microscope, they are seen behaving in a most extraordinary fashion.
They are coming together in pairs, a mating of the most primitive
sort, and each pair is fusing to form a single cell. This new cell,
formed by the union of two, now has the power of growing into
a Ulothrix plant. Separated, the smallest spores perish; united
they are strong enough to carry on the race.

Fia. 123. From one to thirty-two naked cells may swim out of a singie Ulo-
thrix cell. The larger, four-armed ones are able to make a go of it by them-
selves, but the smaller ones must first undergo the simplest sexual process
known. Several are shown engaged in mating. (From Robbins and Pearson,
in Sex in the Plant World.)

The mysteriously rejuvenating fusion of spore-like bodies
which we call gametes in Ulothrix is the most primitive form of
sex act known. In the development of the plant kingdom, when a
spore first behaved as a gamete, sex originated. Thus sex origi-
nated in the plant kingdom merely as a modification of an asexual
method. It is significant that, once sexual reproduction appeared
among primitive plants, it apparently gave them and their offspring

202 REPRODUCTION OF PLANTS

which inherited sexuality such an advantage in the struggle to
populate the world that they did not die out. On the contrary,
sexually reproducing plants have gone a long way towards " in-
heriting the earth."

In a primitive plant like UlothriXj sex probably originated.
In this plant the gametes are to all appearances similar. But in
sex development in the plant kingdom, one gamete becomes a small,
motile sperm, the other gamete becomes a large motionless egg.
In all plants, in seaweeds, in mosses and liverworts, in ferns
and lycopods, sperms have fundamental likenesses; they are
small, motile, and have very little stored food. Eggs on the other
hand are relatively large, inactive, and rich in stored food. The
fusion of an egg with a sperm is the process of fertilization. Ordi-
narily a sperm alone, or an egg alone, is incapable of developing
into a new individual, but once an egg has united with a sperm the
fertilized egg is able to develop into a new individual.

ADDITIONAL QUESTIONS

1. Why do insects visit flowers?

2. Of what use are the bright colors of flowers.

3. Name five plants which produce more than one kind of flower.

4. What is the difference between pollination and fertilization?

5. What is meant by the expression: " The flower is not a sex organ? "

6. It has been observed that apples, even in a good season, set no more than
5 per cent of fruit. Explain.

7. Why does a strawberry bed sometimes fail to fruit well, although
flowers are produced in abundance?

8. Are berries found on all sassafras trees? Explain.

9. Describe the course of the pollen tube.

10. Why do the seeds of fruit trees so seldom produce offspring true to the
stock?

11. In a " double " rose, what is the origin of the extra petals? How does
such a plant reproduce?

12. Why are several varieties of pears often planted together in an orchard?

13. Which are the most important hay-fever plants, those which are wind-
pollinated or those which are insect-pollinated?

14. What is the general relation between the height of a plant and its method
of pollination?

15. Flowers are frequently clustered. What is the advantage of this?

ORIGIN OF SEX IN PLANTS 203

SELECTED REFERENCES

Practical Plant Propagation, by ALFRED C. HOTTFS, published by A. T.
De La Mare Company, New York, 1922. An exposition of the art and
science of increasing plants as practiced by the nurseryman, florist, and
gardener. 244 pages, 108 figures.

Plant Propagation, by M. G. KAINS, published by Orange Judd Company,
1920. A discussion of greenhouse and nursery practice. 322 pages, 213
illustrations.

The Modern Nursery, by A. LAURIE and L. C. CHADWICK, published by
the Macmillan Company, New York, 1931. A guide to plant propagation,
culture, and handling. 494 pages, 107 illustrations.

The Nursery Book, by L. H. BAILEY, published by the Macmillan Com-
pany, New York, 1912. A complete guide to the multiplication of plants.
365 pages, 152 illustrations.

Sex in the Plant World, by W. W. ROBBINS and HELEN M. PEARSON.
Published by D. Appleton-Century Company, 1933. 193 pages, 66 illustra-
tions. The subject matter treated in this small volume may be judged from
the following list of chapter headings: All Life from Life; Sex in Flowers; Sex in
Ferns and Mosses; The Origin of Sex; Primitive Sex; Sex in the Scavengers
and Parasites; Begetting without Sex; Parthenogenesis; The Discovery of
Sex in Plants; Plant Courtships; Males, Females and Otherwise; Sex Traits;
Sterility; How Plants Prevent Inbreeding; Hybrid Sterility; What Deter-
mines Sex; Sex Chromosomes; The Secret of Heredity.

UNIT VI

THE DEPENDENCE OF PLANTS ON THE CONDITIONS
OF THEIR SURROUNDINGS

As humans, our daily life, our health, our capacity for doing
work, our happiness all are influenced by the conditions about
us. The food we eat, the temperature and humidity of the room
in which we work, the light by which we read, the people with
whom we come in contact all have an effect upon our life. The
sum total of our surroundings we call the environment. We can
not escape it. Everything we experience with our senses see,
feel, touch, hear, or smell is a part of our environment and has
its influence upon us.

So it is with plants. Their growth and development, and all
the associated activities, are influenced by the various factors of
the environment. The principal factors influencing plants are:
(1) water, (2) heat, (3) light, (4) soil, (5) air, and (6) organisms,
both plants and animals. The environment is a complex set of
factors. All these factors are operating at once upon the plant.
How the plant behaves, and what its structure is, are determined
not by water alone, or light alone, or any other single factor alone,
but by all of them exerting their influence at the same time.

Problem 1, What is the importance of water to plant life?

Life without water is impossible. Water is just as essential to
plants as it is to animals. Water is the principal part of all living
tissue. It constitutes about 80 to 90 per cent of the weight of
protoplasm (living material). Protoplasm becomes less and less
active as water is removed, until a state of dryness is reached
which causes death.

Water in the living cells maintains their turgor, which condi-
tion is necessary for their proper functioning. A plant does not

204

WATER AND PLANT LIFE 205

manufacture food, and grow, unless its tissues are well filled with
water. A wilted plant is inactive. Moreover, the combined
turgor of all cells maintains erectness in plants. The brittleness
of young plants is partly due to water pressure within the cells.

All materials which move from one part of the plant to another
must be in a watery solution. All mineral salts must be dissolved
in water before they can enter the plant through the root hairs.
Also, oxygen and carbon dioxide can not enter or leave the plant
cell except in solution.

As we have seen, water is an essential raw material in the man-
ufacture of food. That is, many of the organic compounds which
occur in plants are formed by the chemical combination of water
with certain inorganic compounds which are absorbed by the plant
from the soil and air. But, as we shall see later, the amount of
water actually found in a plant is a very small fraction of the total
amount which is absorbed by the roots; a large portion passes
through the plant and out through pores in the leaves to the atmos-
phere.

Water is the chief limiting factor in the growth of most crops.
The farmer, except in the most rainy sections of the country, is
usually confronted at some time during the season with a shortage
of water. This is particularly true in arid and semi-arid regions.

Water is a most important factor determining the character
of plants upon the earth's surface. The striking differences in the
vegetation of the high mountains, the dry plains, the prairies,
the eastern deciduous belt; in the character of the plant life of
tropical rain forests, deserts, and tundra; in the vegetation of hill-
side, brook-bank, gravel slope, bog, meadow, and open water are
largely due to differences in the available water supply.

Amount of water in plants. The amount of water in different
plants varies widely. As a rule, plants growing under dry condi-
tions contain less water in proportion to their total weight than
plants growing in wet situations. However, many desert plants,
such as cacti, may possess large amounts of stored waters. The
percentage of water is usually greater in young, growing parts than
in older portions of the same plant. Seeds and woody tissues con-
tain less water than the leaves or young roots and stems. Tough-
ness of tissue is usually associated with a low water content of the

206 DEPENDENCE OF PLANTS ON SURROUNDINGS

tissue. Succulency and tenderness of tissue usually signify a high
water content.

Exercise 105. Determination of water content of plant parts. Determine
the water content of the following plant parts: (a) fruit of apple, (6) potato
tuber, (c) grain of corn, (d) green leaves of any plant, (e) twigs of any woody
plant, keeping bark and wood separate. (1) Weigh and record weight of
vessels to be employed. (2) Cut or break up the material finely, place in
container, weigh immediately, and record. (3) Place in constant-temperature
dry-oven, and dry at a temperature of 90 C. until a constant weight has been
attained. (4) Record dry weight. (5) Compute percentage of water. Why
must care be taken not to subject the tissue to too high a temperature? What
is the difference between dry matter and ash of a plant?

The water problem of the plant. It is quite clear that one of the
chief problems of a plant is to take in at least as much water as it
gives off. Of course, the intake must exceed the outgo, for some
of the water is used in the plant. However, absorption must
at least equal transpiration (the water-losing process in plants)
if the plant is to maintain life. The great dangers that confront
most plants, particularly dry-land plants or those subject to dry
periods, are too little absorption or too much transpiration. A
plant dies when the rate of water loss exceeds the rate of water
intake for any length of time.

There is a stream of water through the plant from root hairs
to leaves. Its rate of flow may be limited or restricted at two
points: (1) at the point of entry (root hairs) ; and (2) at the point
of exit (leaves). Plants withstand dryness in two general ways:

(1) by increasing the amount of water taken in through the roots;

(2) by limiting or retarding the amount of water lost from the
plant.

The wilting of plants. The importance of water in the plant's
life is well shown by the phenomenon of wilting. A plant tissue
is made up of a mass of cells. When a cell is filled with water and
various materials in solution, its walls are bulged outward and
we say that the cell is turgid. If the water is removed from the
cell, it becomes flaccid and wilts. If all the cells of a herbaceous
plant are filled full of water, each cell wall being stretched out
because of the pressure from within, the plant as a whole stands
erect. Of course, it must be understood that most plants possess

WATER AND PLANT LIFE 207

strengthening woody tissue and are not entirely dependent upon
turgor to hold them in an erect position. But the leaves of all
plants and the young stems of herbaceous plants are largely
dependent upon the turgor of the cells for their rigidity.

When a plant wilts, water is not being absorbed as rapidly as it
is being used or lost. Its cells are not full. The freshness and
crispness of lettuce, for example, are associated with turgidity of
the leaves. Any condition by which transpiration can be checked,
such as by a cool atmosphere laden with moisture, will prevent
wilting to a large extent.

Exercise 106. Loss of water from leaves. Place a handful of fresh, green
leaves, free from water on the surface, under a bell jar. Set up at the same
time and in the same way a bell jar which has no leaves. Place both in a window.
After 30 or more minutes examine for presence of moisture on the inner sur-
face of the jars. How do you account for what you see? Does the water come
from the leaves? In what form does moisture escape from the leaves? Why
do leaves become limp when they lose water?

Exercise 107. Loss of water from a growing plant. Secure a small,
vigorous, potted plant. Cover the entire pot with a piece of oiled paper or
rubber cloth, fitting it up closely about the stem ot the plant and over the soil.
This is to make sure that no water escapes from the walls of the pot or surface
of soil. Now place a bell jar over the plant and set in a place suitable for
growth. From where does the water come that collects on the inside of the jar?

The transpiration stream. All exposed surfaces of the plant
are losing water, but the leaves are the principal transpiring
organs. The water passes off from plants in the form of water
vapor.

In many respects transpiration resembles evaporation, such as
takes place from a free water surface. It is different from evapo-
ration, however, in that it is controlled in part by the plant itself.
For example, the rate of water loss, by transpiration, from a living
plant is less than the water loss, by evaporation, from a dead plant.
And the amount of water lost from a given area of leaf is less than
that lost from an equal area of free water surface. Why? For
example, it was found, for one plant, that a given area of free water
surface lost about ten times as much water in an hour as an equal
area of leaf surface. As we learned in Unit II, the living plant has
numerous pores (stomata) in the epidermis of the leaves which
open and close with changes in the conditions of the surroundings.

208 DEPENDENCE OF PLANTS ON SURROUNDINGS

Although water loss is a constant danger to the plant, the
process plays an important role in that it maintains a stream of
water (the transpiration stream) from the roots to the leaves, and
throughout the entire plant body. There has been a general
impression that, the greater the rate of transpiration, the greater
the rate of intake of mineral salts by the root hairs. It has been
shown by careful experiments, however, that an increase in the
rate of transpiration does not proportionately increase the quan-
tity of mineral nutrients absorbed. For example, if, by being
placed in a dry atmosphere, a plant is caused to absorb and trans-
pire water at double its former rate, the mineral salts absorbed are
increased in amount, but are by no means doubled.

How does water get out of the leaf? We have learned that
surfaces of leaves are covered with an epidermis or skin composed
of box-shaped cells, the outer walls of which are thicker than the
inner and side walls, and often waxy in nature, such that they
effectively prevent the loss of water through them. Here and
there in the epidermal layers of the leaves are small openings or
pores, known as stomata (Figs. 28, 29). Each pore or stoma is
bordered or guarded by two cells, which differ from all other
adjoining cells in their shape, in the possession of green coloring
matter, and in their behavior. In most plants these two guard
cells of each stoma or opening are capable of changing their
shape, and by so doing bringing about the opening or closing
of the pore. In a wilted plant the stomata are usually in a fairly
closed condition, and hence the opportunity for water loss is
reduced.

There are usually more stomata on the under surface of a leaf
than on the upper, and in some plants there are none at all on the
upper surface. For example, in the apple leaf there are no stomata
on the upper surface, whereas on the under surface there are
approximately 161,000 per square inch. What is the advantage
of this? In corn there are about 60,000 per square inch on the
upper surface and 102,000 per square inch on the under surface.
It has been computed that in a single corn plant of average size
there are approximately 104,057,850 stomata in the epidermis or
skin of its leaves.

There is some slight loss of water through the waxy cuticle of

WATER AND PLANT LIFE 209

a leaf, but most of the water loss is through the stomata. This is
well demonstrated in the following exercise.

Exercise 108. The loss of water from leaves is chiefly through stomata.
Take three fresh green leaves of the common rubber plant (Ficus elastica) of
conservatories. In this plant the stomata are confined to the lower surface.
Smear vaseline on the upper surface of one leaf and on the lower surface of a
second; and keep the third leaf free from vaseline on both surfaces. Hang
them up by the leaf-stalks. Observe the results at various intervals of one,
two, three, five, and ten days. Discuss.

The amount of water lost by plants. The amount of water
that is absorbed by plants, passed through their body, and out
through the stomata to the air is enormous, and really much
greater than most people realize. It was computed that a single
corn plant during one growing season lost 54 gallons of water. A
perfect stand of corn would be about 6000 plants per acre, so the
total amount of water that would be evaporated from the leaves
and sheaths of an acre of corn during the growing season would
be 324,000 gallons, or 1296 tons. Disregarding all other losses of
water from the soil, how many inches of rainfall would it require
to supply the foregoing acre of corn?

It has been calculated that an average-sized oak may have
700,000 leaves; that about 244,695 pounds of water will pass from
its surface in the five months, June to October. In a single year
there will pass through the oak tree an amount of water equal to
226 times its own weight.

From which is the most water lost : an area covered with vege-
tation, or one devoid of vegetation? From the water standpoint,
why are weeds injurious in a field of crops?

Exercise 109. Measuring the amount of water lost by leaves. Fill a jar
with a measured amount of water. Cover the top with a rubber cloth.
Secure a freshly cut healthy peach twig containing approximately 20 to 30
leaves. Cut in the rubber cover a slit just large enough to allow the twig to
enter. Allow the cut end of the stem to reach well down into the water. Ob-
serve for a period of three or four days. If it is necessary to add more water,
measure the amount. Determine for any time interval the amount of water
lost from the jar. Does this represent that lost by transpiration through the
leaves? Find out from the instructor a simple method for securing the com-
bined area represented by all the leaves of the twig. Estimate the number of
leaves on an average-sized peach tree, and from your computation make a

210 DEPENDENCE OF PLANTS ON SURROUNDINGS

rough estimation of the amount of water lost from the entire tree during a
24-hour period.

The water requirement of plants. Plants differ greatly in the
total amount of water which is expended in producing a unit of
dry matter, that is, in their water-requirement. Some plants,
like millet, are economical; others, like alfalfa, are comparatively
uneconomical. The water requirements of a number of plants
have been determined experimentally. In the following table is
given the water requirement of a number of plants under certain
conditions as they existed at Akron, Colorado. For different cli-
matic conditions these values will be somewhat different.

ACTUAL WATER

REQUIREMENT

(pounds)

Alfalfa, Grimm 659

Rye, Spring 496

Oats, Swedish Select 423

Barley, Beardless 403

Wheat, Kubanka 394

Wheat, Kharkov 365

Corn, China White 315

Wheat, Turkey 364

Sudan grass 359

Milo, Dwarf 273

Kaoliang, Brown 223

Millet, German 248

It will be observed from this table that alfalfa, for example,
uses more than twice as much water to produce a unit of dry
matter as does German millet. In general, a plant with a low
water requirement is relatively drought-resistant.

Problem 2. What is the relation of temperature to plant life?

It is well known that the amount of heat a plant receives
greatly influences its growth. The germination of seeds, the
growth of roots, of stems, and of leaves, the opening of buds and
flowers, and the development of seeds and fruits all are dependent
upon certain temperature conditions. Every process of the plant,
including such important functions as respiration, food manufac-

TEMPERATURE AND PLANT LIFE 211

ture, absorption, digestion, and reproduction, is influenced by the
temperature. These different functions of the plant, and the
growth of various organs, may have different temperature rela-
tions. For example, absorption of water and salts from the soil
will go on at lower temperatures than will the development of
flower structures, and the seeds of a plant will usually germinate
at a temperature lower than that which is necessary for the matur-
ing of the seeds of that same plant.

The temperature plays a great part in determining the distri-
bution of plants over the surface of the earth. There is a decrease
in the temperature as we go from the equator to the poles, and
from low to high altitudes. We recognize the broad zones of vege-
tation peculiar to the tropics, the subtropics, the temperate
zones, and the arctic regions. Plants vary in their resistance to
low and to high temperatures. And the yields and quality of
orchard, garden, and field crops depend greatly upon the tem-
perature.

Effect of temperature upon growth. When a plant grows,
there is an increase in the number of cells and in the size of the
cells. Two new cells result from the division of an old one. The
two newly formed daughter cells, at first small, increase in size.
Of course, the growth of the plant body as a whole is the result of
the combined growth activity of the many cells which make up the
body. Not all cells of a plant at any one time are growing at the
same rate; in fact, they are not all exposed to the same environ-
mental conditions.

Every plant carries on its life processes between certain tem-
perature limits. There is a certain low temperature below which,
and a certain high temperature above which, a given plant can not
grow. We call these temperatures the minimum and the maxi-
mum, respectively. Somewhere between these two, there is a
temperature at which the plant grows to the best advantage.
This we call the optimum. These three important temperature
points are called cardinal temperatures.

The germination temperature influences the development of
the adventitious roots in wheat. In this plant, the first whorl of
adventitious roots forms much nearer the soil surface at high tem-
peratures than it does at low temperatures. The plants in the

212 DEPENDENCE OF PLANTS ON SURROUNDINGS

former case are relatively weak. Seedlings of wheat, germinated
at temperatures just above freezing, develop a root system two to
three times as large as those grown at higher soil temperatures.

Exercise 110. At what temperature do seeds lose their ability to germi-
nate? (a) Secure seeds of wheat, corn, beans, radish, buckwheat, cherry
(" stones "), etc. Place several hundred of each kind of seed in separate
beakers or large test tubes and cover with water. Bring each to a temperature
of 65 C. and retain at that point 15 minutes. Take out 25 of each kind of
seed and place under conditions suitable for their germination. Raise the
temperature of the remaining seeds to 80 C. and after 15 minutes arrange
test for germination of each kind of seed. Raise the temperature to 95 C.,
and again after 15 minutes arrange test for germination. Raise the tempera-
ture to boiling (100 C.) and repeat the procedure given above. (6) Repeat
the above experiment, but instead of heating the seeds in water, subject them
to a dry heat. A dry-oven may be employed, or a double-boiler may be
improvised using two beakers of different size. Compare the results, (c) Soak
lots of the above seeds in water for 1 or 2 hours. Expose lots of each, both
dry and soaked, to different low temperatures. Different low temperatures
may be obtained by preparing freezing mixtures of salt and ice. After exposure
of seeds to the low temperatures for 1 hour, test their germination. Arrange
the results of your tests in the form of a table and give your conclusions.

Exercise 111. What is the effect of temperature on root growth? Place
seeds of lettuce, radish, beans, corn, or other plants in germinators and germi-
nate at room temperature. When the roots are about J4 inch long, place a
number of sprouting seeds of each kind at different temperatures. Select
certain sprouting seeds, marking their position in the germinating dish, and
measure the length of the main root at intervals of 12 hours. Discuss results.

Resistance of plants to low temperatures. It is well known
that some plants will withstand a much lower temperature than
others. For example, the date palm is usually injured by tem-
peratures below 20 F., whereas most varieties of apples will
endure temperatures much below zero, if the tissues are mature
and in a dormant condition. It is also recognized that different
tissues of the same plant vary in their resistance to low tempera-
tures. In our common woody plants the tissue least resistant to
freezing is the pith; the next least resistant is the sapwood; then
the bark; and the most resistant, in well-matured and well-hard-
ened stems, is the cambium. However, in actively growing stems,
the cambium is not so resistant to freezing as other tissues.

The more water plant tissues contain, the more readily are
they killed by freezing. Active, growing tissues have more water

FIG. 124. Heaters in a California orchard. In certain parts of the country
late frosts threaten the blossoms of fruit trees. The United States Weather
Bureau issues warnings of approaching low temperatures. Orchardists light
the heaters, which may develop enough heat to prevent freezing of the tender

blossoms.
213

214 DEPENDENCE OF PLANTS ON SURROUNDINGS

than dormant tissues, and consequently are more easily frozen to
death. Seeds that are well dried will stand much lower tempera-
tures than seeds filled with moisture. For example, corn often
suffers from freezing before the grain is quite dry. If the grain
becomes thoroughly dried, it will withstand very low temperatures.
Corn containing 10 to 14 per cent moisture may be stored with
safety in bins exposed to temperatures considerably below F.
A frozen grain of corn may have the appearance of being healthy,
but the germ (embryo) may be killed or its vitality considerably
reduced. It is very essential that corn seed be given a careful
test for germination before planting.

The maturing or " hardening " of plant tissues that takes place
in late summer, autumn, or early winter seems to influence the
resistance to winter temperatures. Well-matured or " hardened "
tissues are more resistant than those not completely matured or
hardened. For example, wood that has had late growth and gone
into the winter incompletely matured is comparatively susceptible
to winter injury.

Gardeners commonly practice the process of " hardening off "
their transplants. If a tomato plant is removed suddenly from a
warm greenhouse in the spring to the garden out-of-doors, it has
little resistance to low temperatures, and the death-point is rela-
tively high. The usual procedure is to move the plants from the
greenhouse to a cold frame where the temperature extremes are
not so great as in the open; here the plants become adjusted to the
lower temperatures and after a period may be planted with safety
in the open. After the hardening-off process, the plant is able
finally to withstand a lower temperature than if suddenly removed
from a warm to a cool situation. Hardened plants differ from
tender plants in having in the cells more soluble proteins and more
water-imbibing substances.

QUESTIONS

1. The minimum temperature for the growth of rye ranges from to
4.8 C., whereas that of tobacco ranges from 10.5 to 15.6 C. What is the-
relation of these temperatures to the distribution of these crops in the United
States?

2. Discuss the meaning of the expression: " An annual plant may be

LIGHT AND PLANT LIFE 215

said to belong to no country in particular, because it completes its existence
during the summer months."

3. What is there about the habit of cereals which makes it possible to grow
them in a wide range of climates?

4. Why are partly open buds more easily frozen than dormant buds?

5. What is the purpose of placing straw mulches over such low-growing
plants as strawberries?

6. A wet soil is usually a cool soil, whereas a dry soil is usually a warm soil.
Why?

7. Cite various ways by which man controls the temperatures about plants.

8. What are the chief factors which account for the difference in the type
of vegetation as one proceeds from low to high altitudes, and from low to high
latitudes?

9. Cite plants that will withstand very high temperatures.

10. Why is it necessary that seed be thoroughly dry before it is stored for
the winter?

11. What are the dangers of an "early spring"?

Problem 3. What is the relation of light to plant life?

We have learned that light is the sole source of the energy
without which no single living thing, whether plant or animal,
could continue to exist. The enormous amount of energy which
comes from the sun, and upon which the life activities of plants
and animals depend, is absorbed by the
green leaves and other green parts of
plants. In the green plant this energy
is converted into plant food. The de-
velopment of a green color and the
amount of food manufactured by a
plant strictly depend upon the dura-
tion and intensity of light.

Light also affects the movement
and position of plant organs. We are
all familiar with the movement of the Fia 125. Rosette arrange-
leaves of a house plant toward the light ment of the leaves of the
when placed in a window. purple star-thistle, a weed.

The size and form and structure of

plants are also influenced by light. For example, plants growing
in intense light often have a dwarf form, with very short stems,
whereas plants in light of low intensity usually develop long stems.

216 DEPENDENCE OF PLANTS ON SURROUNDINGS

The movement and position of plant organs are influenced by
light. As stated, the direction of light and its intensity have an
effect upon the movement and position of plant organs. Leaves
are especially sensitive to light. Many leaves assume a position
which will expose a flat surface to the direct rays of light.

A

FIG. 126. Leaf mosaic of ground ivy, show-
ing a minimum of shading.

Exercise 112. Light and
leaf position. Observe in the
open or greenhouse the rela-
tion of shoots and leaves of
any plant to light. Observe
how the leaves are so arranged
that there is little shading of
one leaf by another. In fact,
the leaves of ten form a mosaic.
See Figs. 125 and 126. This is
particularly evident in plants
that form a rosette, or in
climbing plants like ivy which
adhere to a wall. Also ob-
serve the position of the
leaves of the wild lettuce,
which on account of intense
illumination take a position
which exposes their edges to
the sun during the hottest
part of the day. (Fig. 127.)

Exercise 113. Response
of leaves to light. Grow
seedlings of lettuce, peas, or
beans in a pot which stands
in a window, and observe the
direction of growth of the
plants. Discuss results.

The size, form, and structure of plants are influenced by light.

Intense light seems to have a stunting effect. This is shown by
the low stature of alpine plants, although other factors, chiefly low
temperatures and excessive transpiration, may play a part. Plants
in the dark grow long and spindling, which tendency is noticeable,
but less so, in the shade. Examine potato sprouts that have de-
veloped in a dark cellar. Contrast with those which develop in the
light. Why do house plants sometimes grow long and spindling?

LIGHT AND PLANT LIFE

217

There is a marked difference between sun and shade plants
of the same species. Trees in the open branch and spread pro-
fusely, whereas the same species in the forest, where the light
on all sides is cut off, grows taller and produces fewer side branches.
How do you account for the splendid form of trees which grow
in the open? The leaves of shade plants are thinner and broader
than those of sun individuals. What is the advantage of this?

FIG. 127. Wild lettuce, a compass plant. Left, as seen from south; right;
as seen from west. Of what advantage is the habit here illustrated?

As has been stated, in the total absence of light, plants do not
develop chlorophyll. The leaves of shade plants, however, are
often a deeper green than those of sun plants. This is due to
the fact that the epidermal layer of shade plants is thinner than
that of sun plants, and consequently the underlying green tissue
of the shade leaf shows through. Moreover, sun leaves are

218 DEPENDENCE OF PLANTS ON SURROUNDINGS

frequently covered with hairs, scales, or a waxy coat which
prevent the chlorophyll tissue beneath showing through. Such
plants often have a grayish color.

Duration of light. In a discussion of light and its influence
upon plant growth, we must take into consideration its duration,
its intensity, and its quality. In the arctic regions the summers
have long days, but the light intensity is low; at the equator, the
daylight period is shorter, but the light is very intense. The
long daily period of sunlight at high latitudes, even though the
light is of low intensity, makes possible the maturing of splendid
grain and vegetable crops. It has been found that the accumula-
tion of carbohydrates in plants, and their rate of growth, are in

Fig. 128. Response to light. Two views of the same geranium plant. What

was the effect of light on the length and position of the stem? How did the

leaf petioles respond to light? The leaf blades took a position perpendicular

to the incident rays of light.

proportion to the number of hours the plant is exposed to the
light, rather than to the intensity of light.

Light intensity. Plants vary greatly as to the intensity of
light .which they can withstand. Most plants do best in diffuse
light; high light intensity is injurious in that it destroys the
chlorophyll and thus retards the food-manufacturing process.
Many plants, especially in the seedling stage, can not withstand
direct sunlight for any considerable period. In full sunlight,
the date palm leaf ceases to grow. Normal growth of this organ
is made chiefly in the time between sunset and sunrise, but also
to a slight extent in daylight when direct sunlight is cut off by
clouds.

LIGHT AND PLANT LIFE 219

It has been found by experiment that the Norway maple is
capable of carrying on the manufacture of sugar when the light
intensity is only ^V of the total daylight, whereas cherry is incap-
able of performing this function if the intensity falls below of
the total sunlight. In other words, Norway maple has the
ability to endure shade.

If a plant is grown under conditions in which light is insuffi-
cient, it shows certain distinctive characters. For example, the
color of the foliage is pale green and often sickly, the number of
leaves is decreased, there is a scanty development of roots, the
growth is more succulent, that is, less woody tissue is developed,

FIG. 129. Morning (right), noon (center), and evening (left) positions of the
same sunflower plant, showing response to the stimulus of light.

the stems are long and spindling, and the plant may fail to bloom
and produce fruit. If nursery trees are planted too close, so
that there is not sufficient light, they tend to grow long and
slender and to have a weak root system. It must be remembered
that the vigor of a root system depends primarily upon the food
brought to it from the leaves. A decrease in the leaf surface
brought about by too close planting is equivalent to a decrease
in the food-manufacturing surface.

Sometimes advantage is taken of the response of the plant to
decreased light brought on by close planting. For example, flax
grown for fiber is planted in a closer stand than when grown for
seed. The close stand induces the development of long, slender

DEPENDENCE OF PLANTS ON SURROUNDINGS

stems, which yield fiber of high quality. Also, in growing sor-
ghums and corn for fodder, or for ensilage, where a large amount
of succulent growth is desired, the plants are grown close together.
Tobacco plants are often grown in the shade of tents, which condi-
tion makes a larger and thinner leaf with less vascular tissue.
The leaf is thus improved for wrapper purposes.

FIG. 130. The evening-primrose opens its flowers in late afternoon or early
evening. The illustration shows three stages in the opening of the flowers.

In general, light of medium intensity promotes vegetative
growth, whereas intense light favors the development of repro-
ductive structures. In the northeastern states, where cloudy
days are frequent during the growing season, there are splendid
yields of potatoes, carrots, turnips, and other crops which are
grown for their vegetative structures. On the other hand, the

LIGHT AND PLANT LIFE

221

principal seed-producing regions are found in the western and
middlewestern states where the percentage of sunshine during
the year is high and the light intensity is relatively great.

Blanching is .a process in which the plant is prevented from
becoming green by growing it in the dark. To produce blanched
(white) asparagus, for example, the plants are banked or ridged
up with soil, so that the " spears "
must make an additional growth
of 4 to 10 inches before they
come to light. The shoots that
develop in the soil are, of course,
whitish. The blanching of celery
is accomplished by placing boards,
paper, or earth about the stalks
to exclude light. The heads of
cauliflower are blanched by bring-
ing the outer leaves up over the
head and tying them, thus exclud-
ing light.

The intensity of light to
which a plant is exposed may be
increased by pruning and by a
thin stand. One of the objects in
pruning trees is to allow light to
reach the center of the tree.

The use of artificial light to
supplement natural daylight and
thus bring about the forcing of
plants has been the object of much
experimentation. Vegetables such as lettuce and radishes,
kept under a strong arc light during a part of the night, become
ready for the market from 10 to 14 days earlier than those exposed
to normal light duration. Other kinds of artificial light have been
used in forcing plants, among them the ordinary carbon incan-
descent electric light, acetylene light, that of the Welsbach burner.
Although artificial light is effective in forcing certain vegetables
and flowers, its use is not usually attended with commercial gain,
on account of the cost of the light.

FIG. 131. Above, celery before
blanching is green and not at all
like the celery we see in the mar-
ket. Below, celery in the process
of blanching, soil is packed around
the leafstalks to exclude the light.
The portions of the leaves that
are covered lose their chlorophyll
and the later leaves develop with-
out chlorophyll.

222 DEPENDENCE OF PLANTS ON SURROUNDINGS

Exercise 114. snaae and sun plants. Make a list of shade-demanding
plants and of sun-demanding plants. Refer to the catalogs of nurserymen.
Exercise 115. Extraction of the chlorophyll from leaves, and the effect of
light on chlorophyll. Place the leaves from which chlorophyll is to be extracted
in a flask and add water. Boil for a minute. Replace the water with 80 per
cent alcohol and continue to heat on a water bath. Keep the alcohol vapors
out of range of the flame. When the chlorophyll is extracted, filter the solution.
Place a portion of the solution in the direct sunlight, and an equal portion in a

dark cupboard. After 30 minutes com-
pare the color of the two solutions,
What is the influence of light upon chlo-
rophyll?

The quality of light. The

white light that shines upon the
leaf is composed of a number of
different rays, which vary in their
effect upon plant growth. The
visible spectrum, so beautifully
shown in the rainbow, is com-
posed of red, orange, yellow, green,
blue, and violet light. Beyond

FIG. 132. A cauliflower head. In the visible red are invisible rays
the process of blanching cauli- known ag infra . red; and b ond
flower, the broad, long leaves are . , . ., , . , , f

tied about the head, excluding the the V1Slble Vlolet are ra ^ s of
light and thus preventing the head

from becoming discolored.

invisible to the eye, known as
ultra-violet. It has been demon-
strated that the red rays of light

are the most effective in sugar manufacture, and that the green,
blue, and violet rays are the least useful of all in this process.
Ultra-violet rays of light have an injurious effect upon plants.

Problem 4. What is the relation of plants to the soil?

The soil is the environment of roots. It is in the soil that
plants are anchored in fact, frequently half or more of the
ordinary plant body develops within the soil; it is from the soil
that a plant absorbs water and mineral nutrients; most perennial
plants store considerable quantities of reserve food in organs
(roots or rootstocks) which are in the soil.

SOIL RELATIONS 223

The soil environment of a plant is very complex. It is more
complex than the air. In its effects upon plants we must consider
the soil from the three different standpoints; physical, chemical,
biological.

The chief physical properties of a soil are texture and structure.
Soil texture refers to the size of the particles which compose a
soil mass. As a rule, we distinguish three general kinds of soils
as to texture, namely, sandy soils (coarse), loam soils (medium),
and clay soils (fine).

Exercise 116. Kinds of soil. Secure three kinds of soil: sandy, loam,
and clay. Shake an equal quantity of each in an equal amount of water, using
the same kind of receptacle for each mixture. Set in a place where they will
not be disturbed. Compare as to the time required for the soil particles to
settle out, and for the liquid above the soil to become clear. What is your
conclusion regarding the relative sizes of the particles in the different kinds of
soil?

Soil structure refers to the arrangement or grouping of the soil
particles. A sandy soil is usually of simple structure, in that
the separate particles are much alike, and function separately.
On the other hand, a clay soil may be very complex in its struc-
ture, in that it may consist of soil granules of many different sizes,
held together by glue-like colloidal material. Loam soils are
usually regarded as having excellent structure. By this we mean
that loam soils are not only porous, but they also hold moisture.

As far as the plant is concerned, the soil is the source of most
of the many chemical elements which enter into the plant's com-
position. Throughout the ages, the rocks of the earth have slowly
become fragmented and decomposed to form, along with decaying
plant and animal material, the soil. Hence, most soils contain
a mixture of both mineral matter and organic matter. We apply
the term humus to the organic portion of the soil. Humus
improves the physical condition of the soil, making of it a better
medium for plant growth. We have learned that all substances
which enter the roots must be in solution. Examination of the
liquid portion of a soil shows that it consists almost entirely of
water, carrying in solution many different kinds of mineral salts
such as nitrates, phosphates, sulphates, etc. The mineral
salts in the soil solution, together with carbon dioxide and

224 DEPENDENCE OF PLANTS ON SURROUNDINGS

water, are the raw materials from which the plant manufactures
foods.

The soil as a medium is not wholly inert and lifeless. It is the
home of countless micro-organisms, including bacteria, fungi, and
protozoa. Bacteria and fungi, particularly, are indispensable in
that they are responsible for the processes of decay of organic
matter in the soil. Earthworms also play an important role in
certain soils by aiding in maintaining its tilth.

Water in the soil. All the water taken in by ordinary land
plants is obtained from the soil and is absorbed by the roots.
All substances which enter the plant must do so in solution, and
the solvent is water.

What are the chief conditions which influence the intake of
water from a soil? These are as follows: (1) available water
in the soil; (2) power of soil to deliver water; (3) extent of the
root system; (4) temperature of the soil water; and (5) concen-
tration of the soil solution.

It is a well-known fact that, of the total amount of water in
the soil, not all is available for plant growth. If we allow a plant
to grow in a soil until it undergoes wilting, to the extent that it
will not revive until water is added to the soil, we find that con-
siderable water is still left in the soil. This is water that the
plant can not get readily, and hence the plant shows distress.
There is water in the soil, but the plant is unable to remove it
and utilize it readily for growth. Hence, as far as the plant is
concerned, the soil is dry. The percentage of water left in a soil
at the time the plant undergoes permanent wilting is spoken of
as permanent wilting percentage. This permanent wilting is not
the same as temporary wilting which frequently takes place when
the air is very dry.

The amount of water available for growth varies with the
soil. A plant can reduce the water content of a sandy soil to a
lower point than it can reduce that of a clay soil. That is, when
a plant growing in a sandy loam soil has used all the water it can
for growth purposes, the percentage left in the soil is smaller than
that left in a clay soil under similar conditions.

For example, after a plant growing in a sandy loam soil has
used all the water it can, without permanently wilting, there is

SOIL RELATIONS 225

left in that soil but 8.3 per cent water. On the other hand, the
same plant growing in a clay loam wilts at a moisture content of
13.6 per cent. Looking at this in another way, a sandy loam
having 15 per cent total water would be much " wetter/ 7 as far
as the plant is concerned, than a clay loam with 16 per cent total
water. For the plant growing in a sandy loam with 15 per cent
total water can reduce it to 7.8 or 9 per cent; the same plant
growing in a clay loam would wilt when the water content was
reduced to only 13.6 per cent.

The above facts emphasize the need of knowing not only
how much the total water in a soil is, but also how much of it is
available for the growth of the plant. Most soils whose moisture
content corresponds to the permanent wilting percentage are in a
perceptibly dry condition and would be judged by anyone to be
in need of water.

But there are other considerations. For a long while it was
thought that the greater drought resistance of one plant as com-
pared with another was due to the greater ability of that plant to
absorb water from the soil. It has been demonstrated that dif-
ferent plants growing in a similar soil and under similar conditions
have approximately the same permanent wilting percentage, in
other words, that they reduce the percentage of water to about
the same figures. The ability of a plant to resist drought appar-
ently does not depend upon its power to extract water from a
soil.

Power of soil to deliver water. If moisture is absorbed by
root hairs from the adjacent soil particles at a very rapid rate, as
on hot, dry days, it may not move from remote soil layers rapidly
enough to supply that lost. It is clear that under this circum-
stance the soil immediately surrounding the root hairs will become
too dry to give up more moisture. Water moves from soil par-
ticle to soil particle more rapidly in some soils than in others.
The finer the soil, the slower are all water movements through it,
but the extent of the movement may be greater.

Extent of the root system. The character of the root systems
of plants varies widely. There are root systems (1) that penetrate
deeply in the soil; and (2) those that are confined to the surface
layers. Some plants do not suffer from drought, because of their

226 DEPENDENCE OF PLANTS ON SURROUNDINGS

ability to send their roots into the deeper and moister layers of soil.
Such a plant is alfalfa. If, on the other hand, the soil is shallow
and the rainfall slight, the plants with a shallow root system may
be somewhat more successful than deep-rooted sorts, on account
of their ability to take advantage of the water that comes to the
soil in the form of occasional light showers. It must be remem-
bered that the depth of the root system is an inherited character
of the plant, and is independent, to some extent at least, of external
conditions. Root development, however, will not take place in a
dry soil. Name five plants that have a shallow root system, and
five that have a deep root system.

Temperature of the soil water. The rate of absorption is low-
ered by a decrease in the soil temperature. A plant may wilt in a
soil saturated with water if the temperature of the soil sinks below
a certain degree. In cold, dry climates winter killing may be the
result of a cold soil, which slows up absorption, accompanied by a
high transpiration rate. It is believed that in winter killing the
plant is as frequently killed by direct drying as by actual
freezing.

Concentration of the soil solution. Water passes from the soil
through the living membrane of the root-hair cells into the plant.
This process of water intake goes on as long as the total concentra-
tion of the cell sap is greater than the total concentration of the
soil solution surrounding the root hairs. Other things being
equal, the greater the concentration of the cell sap as compared
with that of the soil solution, the more rapid the water intake.
As the concentration of the soil solution approaches that of the
cell sap, the rate of absorption slows down. Plants growing in
an " alkali " soil are exposed to a soil solution of high concentra-
tion. Hence absorption is retarded. There may be plenty of
water present in the soil, but the plant gets it with difficulty, on
account of the high concentration of the soil solution. Name five
alkali plants.

Likewise, bog plants are growing in a medium which retards
water intake. This may be due sometimes to the high concentra-
tion of bog waters, but more often to toxic substances in the soil,
which hinder root development. Name five bog plants.

The temperature of the soil. We just pointed out that the soil

SOIL RELATIONS 227

temperature influences the rate of absorption by the roots; absorp-
tion is retarded or inhibited at low temperatures. Soil temperature
also affects the growth of roots, the germination of seeds, and the
various activities of soil organisms.

The soil temperature is by no means always the same as the
temperature of the air above it. It may be lower or higher than the
air temperature. Numerous factors affect the temperature of a
soil; chief of these are as follows:

1. Air temperatures. Changes in the air temperature above
a soil result in changes of the soil temperature. The fluctuations
near the surface are almost parallel to those of the air, but at
deeper layers the variations correspond to a lesser degree. The
daily temperature change in bare, fallow soil extends to between
12 and 24 inches from the surface.

2. Exposure. By exposure is meant direction of slope. A
north exposure, for example, faces north. The effect of exposure
is much more marked at high altitudes than at low elevations.
This greater effect is a direct result of the increased rate of radia-
tion at high altitudes. The intensity of sunlight is distinctly
affected by exposure and also by degree of slope. A given area of
soil or plant surface that is at right angles to the direction of the
rays of light will receive much more heat than one upon which the
sun's rays fall obliquely, for under the latter condition the rays
are spread out over a larger area than when they fall perpendicu-
larly. If we assume the intensity of sunlight to be 100 when it
strikes a surface at right angles, its intensity when striking that
surface at an angle of 70 will be approximately 98.5; at an angle
of 60, 96.5; and at an angle of 10, 33.4. Light intensity has its
effect upon both air and surface temperatures, which indirectly
affect the amount of moisture in the soil and the relative humidity
over the soil. The differences between the native vegetation on
adjacent north and south exposures is so conspicuous in the moun-
tainous sections as to attract the attention of the most unobservant
person. In a valley that trends east and west the slope exposed
to the south has a much greater total effective heat during the
year than the northerly exposure across the valley. The greater
light intensity on the south exposure results in not only a warmer,
but also a drier, habitat than that on the neighboring north

228 DEPENDENCE OF PLANTS ON SURROUNDINGS

exposure. A south exposure receives the greatest total heat during
the day, the east the next greatest, then the west, and the north
exposure least of all. On which exposure will plants bloom earliest
in the spring?

3. Living cover. A crop shades the ground and tends to pre-
vent the soil from warming up. A bare soil warms up more quickly
and cools off more rapidly than one covered with vegetation.

4. Non-living cover (snow and mulch). It is well known that
a snow covering prevents rapid changes in the temperature of the
soil. The temperature of soil under snow is higher than that of
soil unprotected.

The temperature of a cultivated soil fluctuates to a less degree
than that of an uncultivated soil. This is probably due to the
poor heat-conducting power of the mulch formed on the surface
of the cultivated soil.

A non-living vegetative cover, such as a straw mulch, prevents
rapid changes in the soil temperature. It has a cooling effect in
the summer and a warming effect in the winter. In the winter
the dead vegetative covering acts as a poor heat-conducting
medium, which prevents a rapid loss of heat from the soil; and it
tends to keep the cold air currents from coming in contact with
the soil. It is common practice to place straw mulches over such
low-growing plants as strawberries. Why?

5. Moisture. A wet soil is usually a cool soil, whereas a dry
soil is usually a warm one. Some of the heat absorbed by a soil
is used in evaporating the water in it. A wet soil will absorb more
heat than a dry one. Even a light shower will lower the tempera-
ture of the surface soil to a considerable degree. Not only does it
directly cool the soil by its addition, but, as stated, evaporation
also lowers the temperature.

6. Color. Dark soils absorb heat more readily than light-
colored ones, and consequently heat up more rapidly.

7. Physical nature of the soil. A coarse soil does not retain
water readily, consequently it warms up rapidly. On the other
hand, a fine-grained soil, like clay or loam, holds water well and as
a result warms up slowly. It is customary to speak of coarse
soils as " warm or early ," and of the fine-grained soils as " cold or
iate." Compact soils conduct heat more rapidly than loose ones.

SOIL RELATIONS 229

This means that a compact soil will heat up quickly and cool off
just as readily.

8. Manures. The general effect of applying manures to a soil
is to raise its temperature. In one experiment it was noted that
20 tons of manure applied to an acre increased its soil tempera-
ture 5 F.

The air of the soil. The soil is porous. The pore spaces are
filled with air and water. A moderately dry soil contains much
air in its pores. A very wet soil contains less air than the same
soil in a drier condition, for part of the space which would be occu-
pied by water in the wet soil is occupied by air in the drier soil.
In a water-soaked soil there is practically no air save that which
is dissolved in water. It is well known that most ordinary plants
can not thrive for long in a water-soaked soil, for there is an inade-
quate supply of oxygen.

The composition of the soil air is quite different from that of
the ordinary atmosphere. Soil air is richer in carbon dioxide than
that of the atmosphere. This is due to the fact that the roots
and micro-organisms are absorbing oxygen and giving off carbon
dioxide.

What is the role of air hi the soil? The living cells of the roots
must have oxygen in order to respire. A scarcity of oxygen retards
root-hair development and the absorption of water and of mineral
salts. Seeds must have oxygen in order to germinate. Moreover,
most of the bacteria and fungi and other living things of the soil
require oxygen, and these have an indirect effect upon green
plants growing in the soil.

Mineral nutrients of the soil. The water of the soil carries in a
dissolved form many different mineral salts, the so-called mineral
nutrients. Many of these constitute raw materials used in the
manufacture of food. They furnish to the plant such essential
elements as nitrogen, potassium, phosphorus, sulphur, calcium,
and iron. The soil solution also carries various gases, chiefly car-
bon dioxide and oxygen, in addition to the mineral salts.

The nutrient relations of plants are as different as are their
bodily form and structure. There are plants such as blueberries
which are intolerant of calcareous soils. We speak of plants
which are " heavy feeders " and make great demands upon the

230 DEPENDENCE OF PLANTS ON SURROUNDINGS

soil. Tobacco is such a plant. Other plants are " light feeders "
and do not draw heavily upon the chemicals in the soil.

When plants are analyzed chemically we see readily that they
have taken varying quantities of the different mineral nutrients
from the soil, even when growing on the same soil. They make
different demands upon the mineral elements in the soil. It must
not be thought, however, that wheat, for example, growing in dif-
ferent kinds of soils and under varied climatic conditions, would
take the same amounts or relative proportions of the different ele-
ments. We must consider averages based upon the chemical
analyses of crop plants made in many laboratories. For example,
a wheat crop yielding 30 bushels of grain and 1.6 tons of straw
contains on the average 51.6 pounds of nitrogen, 8.6 pounds of
phosphorus, 27.5 pounds of potassium, and 5 pounds of calcium.
A 200-bushel yield of Irish potatoes removes, on the average, from
the soil 42 pounds of nitrogen, 6.3 pounds of phosphorus, 53 pounds
of potassium, and 55 pounds of calcium. A 15-ton crop of sugar
beets takes from the soil 78 pounds of nitrogen, 10.5 pounds of
phosphorus, 79.5 pounds of potassium, and 8 pounds of calcium.
Thus we see that crops vary in their demands upon the different
principal elements in the soil. Note in the figures above, for
example, that a crop of wheat requires very much less potassium
than a crop of potatoes or of sugar beets, but it requires more
nitrogen than potatoes.

A harvest of fruit from an orchard removes a certain amount
of mineral elements, to which must be added those used in the
making of leaves, stems, and roots. For example, the fruit only
of a 100-barrel apple crop will remove from the soil on the average
about 13.8 pounds of nitrogen, 2 pounds of phosphorus, 14.5
pounds of potassium, and 1 pound of calcium.

In the growth of plants for special purposes, man has attempted
to find the nutrient conditions which will give him maximum
production. He has learned that an abundance of water and of
nitrates in proportion to potash makes for succulency in the plant,
vegetative growth, and scant fruit production. On the other hand,
if the supply of nitrogen is withheld to a degree and potash in the
soil is relatively more available, fruit production is stimulated.
It is well known that tomatoes on a soil excessively rich in nitrogen

SOIL RELATIONS 231

" go to vine " and produce little fruit. In certain wheat-growing
sections it has been shown that an excess of available nitrogen
over potash in the soil gives a flinty, hard grain; and that a
starchy, mealy, and soft grain results if there is a lack of nitrogen
and a relatively good supply of potash. Too much nitrogen, on
the other hand, produces a tall plant, with a weak stem, which
lodges easily.

Fertile and infertile soils. In common understanding a " fer-
tile soil " is one which will produce. A soil to be productive or
fertile must have, of course, (1) the proper amount of water;
(2) a supply of free oxygen; (3) a supply of available mineral ele-
ments; (4) no harmful agents such as fungous diseases, weeds,
insect pests, alkalies, acids, and toxins; (5) certain beneficial bac-
teria and other fungi; and (6) a physical condition which is favor-
able to seed germination and root development.

In a more restricted sense, " fertility " or " infertility " has
reference to the mineral nutrients, the so-called plant foods of the
soil.

Man's control of the nutrient relation is largely concerned with
making up a deficiency of some mineral nutrient brought about
by the growth of plants. When a soil becomes infertile, that is,
incapable of producing a normal yield, the infertility is usually
due to a lack of either nitrogen, potassium, or phosphorus. These
are the elements most commonly deficient in soils. The other
3ssential elements are seldom lacking. Thus it is that most arti-
ficial fertilizers contain one or more of these elements. Barnyard
manure contains all the elements necessary to increase a soiPs pro-
ductive power. Of course, it is true that manures of different
farm animals differ in their chemical composition. Name several
common commercial fertilizers.

However, there is reason to believe that infertility of soil is not
always due to a lack of essential mineral nutrients, although this
is probably the most important cause. It appears that some soils
with an abundance of mineral nutrients are non-productive because
of toxic substances in them. There is evidence that, if a given
crop is grown year after year on the same piece of soil, there accu-
mulate in that soil toxic substances which are deleterious to the
growth of that plant. Thus it would seem that one of the advan-

232 DEPENDENCE OF PLANTS ON SURROUNDINGS

tages of crop rotation is the counteraction of these toxic substances
by the new crop. Further, it is believed that the value of adding
manure in a case of this kind is in counteracting the. toxic sub-
stances in the soil rather than in adding mineral nutrients which
are deficient.

In some instances, an unproductive soil may be due to organ-
isms in the soil which attack the roots of the plant and cause
disease. If one kind of crop is grown year after year on the same
land, the soil fungi which prey upon that plant accumulate and the
soil becomes non-productive, even though mineral nutrients are
abundant.

Living organisms in the soil. The upper layers of the soil
teem with living organisms, chiefly bacteria. In addition to bac-
teria there are various fungi, algae, protozoa, and worms. It is
difficult to overemphasize the tremendous importance of bacteria
in the economy of nature. The groups of soil bacteria particu-
larly beneficial are those which bring about the decay of organic
matter and those which fix nitrogen. At this point the student
should review the discussion of soil organisms on pages 94-97,
and throughout the following section reference should be made to
Fig. 37.

Bacteria in relation to soil fertility. Contrary to popular
opinion, not all bacteria are harmful. In fact, many of them are
absolutely essential in maintaining the life of the earth. Chief of
these indispensable bacteria are those which bring about decay,
breaking down complex organic substances such as proteins, fats,
and carbohydrates into simpler substances that can be used again
as raw materials in the manufacture of foods by green plants.
Their presence and activity in the soil are necessary to maintain
soil fertility.

Ammonifying and nitrifying bacteria. Nitrogen is an essential
element in plant growth. It is a constituent of the living material
(protoplasm) itself. It is one of the principal components of both
plant and animal proteins, and of many other chemical compounds
in living bodies. It is well known that soil infertility is often due
to a scarcity of available nitrogen, and that one of the principal
ingredients of fertilizers is nitrogen in some form.

Nitrogen occurs in the atmosphere as a gas. About 80 per

SOIL RELATIONS 233

cent of the air is nitrogen. However, green plants are not able
to absorb the atmospheric nitrogen and use it in the building of
foods. Although nitrogen gas, along with carbon dioxide and
oxygen, passes through the pores (stomata) of the leaf, it is not
utilized by the plant in the free, gaseous form.

Nitrogen occurs in combination with many other chemical
elements. For example, ammonia (NHs) is a combination of
nitrogen and hydrogen, 1 part of nitrogen to 3 parts of hydrogen.
Ammonia is a chemical compound of nitrogen and hydrogen. The
nitrogen in this compound is not free, but is bound to hydrogen.
In other words, the nitrogen is fixed. Another very common
chemical compound is sodium nitrate or Chile saltpeter. This
compound contains sodium, nitrogen, and oxygen in the propor-
tion of 1 part of sodium, 1 part of nitrogen, and 3 parts of oxygen
(NaNO 3 ).

A great many mineral salts contain nitrogen, but of these,
sodium nitrate, and its close relative, potassium nitrate, are the
most important as sources of nitrogen for green plants. The nitro-
gen in nitrates is spoken of as nitrate nitrogen.

As has been stated, nitrogen is one of the most important ele-
ments in plant and animal proteins. Manures are rich in nitro-
gen, for they contain plant and animal products. But the nitro-
gen in manures, or in any plant and animal refuse, is chiefly in a
protein compound. It is protein nitrogen. It is significant that
green plants can not use directly the nitrogen of proteins. It is
necessary that the relatively complex protein compounds be
broken down into simpler compounds of nitrogen, and that finally
nitrates be formed. In other words, protein nitrogen must be
changed to nitrate nitrogen. In all soils, under proper conditions,
the nitrogen-containing compounds of manure are being changed
to nitrates. This change is dependent upon the activity of three
different kinds of bacteria. If these soil bacteria are not present,
or if conditions are unsuitable for their growth and multiplication,
manure does not decompose, and nitrate nitrogen is not formed.

In the first place, the proteins of manure are decomposed
through the activity of a group of bacteria known as ammonifying
bacteria, and among the various decomposition products is ammo-
nia, which of course contains nitrogen. The first step then is the

234 DEPENDENCE OF PLANTS ON SURROUNDINGS

change of protein nitrogen to ammonia nitrogen. Following this,
another distinct group of bacteria changes ammonia nitrogen to
nitrite nitrogen, and still another group of bacteria changes the
nitrites to nitrates. The two groups of bacteria which change
ammonia to nitrates are called nitrifying bacteria. In the three
chemical changes brought about through the activity of soil bac-
teria, the unavailable protein nitrogen has been changed to the
available nitrate nitrogen. In this last form the nitrogen is
readily absorbed by green plants, and utilized by them in the
building of the proteins of their own bodies.

It is seen that the soil teems with bacteria which are extremely
beneficial and essential. It is clear that conditions in the soil
must be such as to promote their growth and development. These
organisms require a good supply of oxygen, a certain amount of
water and warmth, and usually the presence of calcium or mag-
nesium compounds.

Nitrogen Fixation. It was stated in a preceding paragraph
that green plants can not use free nitrogen gas of the air. The
same is true of most plants and of all animals. However, a very
few bacteria and other fungi are able to take free nitrogen and to
build it into the nitrogenous compounds of their bodies. Such
organisms have the power of nitrogen fixation.

There are two principal groups of nitrogen-fixing organisms:
(1) those which live on the roots of other plants, chiefly legumes,
and (2) those which live without any association with the roots of
higher plants.

Legume bacteria cause the development of tubercles or nodules
on roots. These tubercles or nodules vary considerably in size.
Examination of a tubercle shows it to be composed of the swollen
tissue of the host, in which are millions of the nitrogen-fixing bac-
teria. Examine the roots of a number of different legumes for the
presence of bacterial nodules.

How the growing of legumes improves soils. It has been
found that a clover plant, for example, secures about two-thirds of
its nitrogen from the bacteria in the nodules, and one-third from
the soil. Further, it is known that about two-thirds of the total
nitrogen in the clover plant is in the tops (hay) and that the
remainder is in the roots. Thus, it is seen that, when a crop of

SOIL RELATIONS 235

hay is taken from the land, there is removed an amount of nitrogen
about equal to that coming from the air, and fixed by nodule bac-
teria. The roots remain in the soil and in time decay, the nitro-
gen they contain being returned to the soil. It will be clear, from
the figures given above, that if a clover crop is to enrich the soil in
nitrogen, it must either be plowed under, or fed to animals whose
manure, which of course contains nitrogen, is returned to the soil.
If the hay if sold off the farm, the growth of the legume has not
enriched the soil in nitrogen.

Denitrification. In addition to the ammonifying, nitrifying,
and nitrogen-fixing bacteria of soils, still another group plays a
part in influencing soil fertility. This is the denitrifying bacteria
which change ammonia nitrogen to free atmospheric nitrogen.
Such bacteria are undesirable from the soil fertility standpoint, for
they take nitrogen from the soil. Denitrifying bacteria are most
active in soils that are poorly drained and hence not well aerated,
and in soils which contain large quantities of unfermented organic
matters.

The nitrogen cycle in nature. As has been stated, nitrogen
occurs in many different forms in nature: in the free gaseous form;
as a part of inorganic compounds, such as ammonia, nitrites, and
nitrates; and as a part of organic matter, either in the non-living
or living form. In the processes of nature, nitrogen is constantly
being changed from one form to another. Through the activity of
denitrifying bacteria, and in electric discharges, nitrogen com-
pounds are being broken down and nitrogen set free. The free
nitrogen of the air in turn is taken by certain bacteria and changed
into proteins and other compounds of plants containing nitrogen.
The nitrogenous compounds of plants are changed to ammonia,
the ammonia to nitrites, and the nitrites to nitrates. The nitrates
are then used to rebuild plant proteins. Or plants are consumed
by animals and the nitrogen of plants is used in the making of the
nitrogenous compounds of animals. The organic nitrogenous sub-
stances excreted by animals and the dead bodies of animals undergo
decomposition, as a result of which nitrogenous compounds break
down, liberating ammonia, which is in turn changed to nitrites,
and nitrites in turn to nitrates. It is worthy of repetition to say
that the processes of decomposition, of nitrification, of nitrogen

236 DEPENDENCE OF PLANTS ON SURROUNDINGS

fixation, and of denitrification are brought about through the
action of bacteria.

QUESTIONS

1. Why is there so little humus in the soils of arid and semi-arid regions?

2. Compare the three common types of soil as to their ability to hold water.

3. In canyons, gulches, or ravines which run east and west observe the
differences in the plant life on north and south exposures. Explain.

4. How does man artificially increase the temperature of the air about
plants?

5. Describe the way to construct a hot bed; a cold frame.

6. Give the physical characters of a warm or early soil; a cold or late soil.

7. Mention three ways of raising the temperature of a cold or late soil.

8. Why are crusted soils injurious to plants growing therein?

9. Give five reasons for crop rotation.

Problem 5. What is the relation of plants to the air?

The entire plant is surrounded by the gases of the atmosphere.
This includes the roots, which are surrounded by the air of the
soil. The air that surrounds the plant supplies it with oxygen,
necessary for respiration. All parts of the green plant absorb
oxygen rather directly from the air which is about them; and in
the presence of light, green plant tissue also absorbs carbon dioxide.
Even the roots obtain oxygen from the soil air immediately sur-
rounding them.

In animals, the oxygen used in respiration, and the carbon
dioxide eliminated in this process, are conveyed to and from the
cells by the blood. In plants there is nothing corresponding to
the blood stream. The sap of a plant does not carry appreciable
quantities of these gases. In plants there is, however, an extensive
system of air spaces between the cells, which communicate directly
with the exterior through the stomata in the leaves, and through
loose, open groups of cells (lenticels) in the bark. Consequently,
the cells are surrounded by the gases of the atmosphere, and these
can move inward and outward through the moist cell walls. Cer-
tain types of cell walls will permit oxygen and carbon dioxide to
diffuse through them, even when they are dry.

In most crop plants the system of air spaces in the plant is not
extensive or continuous enough to permit ready movement of air

AIR AND PLANT LIFE

237

through the leaves and stems to the roots. Roots and root hairs
absorb oxygen from the air which is present in the soil between the
soil particles. In other words, the rapidly growing and active root
hairs on a root which is several feet beneath the soil surface obtain
oxygen for respiration chiefly from the soil air immediately sur-
rounding them. There is no system of ventilating tubes which is
adequate to convey sufficient oxygen from above ground to these
subterranean structures except in aquatic and semi-aquatic plants.
The normal growth and functioning of the root hairs, and conse-
quently the absorption of water and mineral salts from the soil

Fia. 133. Wind timber on the slopes of Long's Peak, Colorado. On wind-
swept slopes of high mountains, near timber line, where the winds are prevail-
ingly from one direction, the trees are often prostrate, and with twisted
branches. (Photograph furnished by R. J. Pool.)

and in fact the health of the entire plant body all depend upon
an adequate supply of oxygen to the soil. It is known that the
oxygen requirement of roots varies with the temperature of the
soil. At a high soil temperature, the amount of oxygen necessary
to give a normal rate of growth is greater than that required at a
low soil temperature. Be that as it may, the fact remains that in
all our treatments of the soil we should keep in mind the require-
ment an ample and constant supply of oxygen in reach of the
root hairs. The soil needs ventilation. It must be porous enough

238 DEPENDENCE OF PLANTS ON SURROUNDINGS

to permit the free movement of air through it. If it is crusted on
top, owing to irrigation or rain, the free movement of air is inter-
fered with, and the roots of plants are likely to suffer from a lack
of oxygen. If the soil is extremely wet, the air supply is also
diminished.

Air Movement, We are familiar with the fact that clothes on
the line dry much more quickly on a windy day than on a quiet
day. The rate of evaporation, or loss of water vapor, is increased
by wind movement. Wind is one of the several important factors
which influence the loss of water vapor from the surfaces of plants.
It does this by constantly removing from the surface of the plant
the film or extremely thin layer of moist air which accumulates
there, thus making way for the more free diffusion of water vapor
outward from the air spaces within the leaf.

Wind also influences the form of trees. Witness trees that
grow at timber line on wind-swept mountain slopes, or along the
sea coast. Here, as a result of winds which blow prevailingly in
one direction, the trees often have most of the branches coming
off the leeward side. This is due to the drying effect of the wind?
which prevent buds from developing on the windward side. More-
over, as a result of the continued mechanical strain, such trees fre-
quently lean strongly leeward.

Wind and Reproduction. Wind is of service in the reproduc-
tion of plants in that it transports pollen and disperses seeds.

Problem 6. What is the interrelation of plants and animals?

There is an intimate interrelation between animal life and
plant life. We have already called attention to the fact that the
soil is traversed by many species of animals: earthworms, insect
larvae, ants, etc. These animals are dependent upon plants for
their food, and the animals in turn, especially earthworms, grind
up plant remains into small pieces, mix their food with mineral
particles, bury soil fragments in the soil, and by burrowing in the
soil render it more porous. Thus, these animals render physical
and chemical changes in the soil which are beneficial.

One of the most intimate interrelations between animals and
plants is seen in the insects which carry pollen. The flowers

PLANT AND ANIMAL RELATIONSHIPS 239

provide the insects with nectar and pollen; the insects carry the
pollen and thus contribute to the plant's reproduction. Thus,
the insect and the plants are mutually helpful.

So completely dependent are some plants upon some one
species of insect for pollination that they can not exist without
that insect. Some time ago Australians decided that red clover
would be a splendid crop plant for their soil and climate, so they
imported some. But they could not get it to set any seed until
they imported the red clover's special pollinator, the bumblebee.

Fia. 134. The anthers of the meadow sage form a lever which the insect works
himself. The anther lever is illustrated in the two upper flowers. After the
anthers have withered the stigma grows down into a position where the insect
must rub his back, covered with pollen from visits to younger sage flowers,
against it, as shown in the lower left flower. (From Robbins and Pearson,
in Sex in the Plant World.)

Orchids are usually pollinated by moths. There is a famous
cacc of an orchid, the Madagascar orchid, which produces its
necLar in a spur nearly a foot long. On the strength of the exist-
ence of this flower, a naturalist predicted that a moth would be
discovered with a proboscis that long, although none was known
at the time. Surely enough, such a moth was soon found fre-
quenting the habitat of the Madagascar orchid.

240 DEPENDENCE OP PLANTS ON SURROUNDINGS

Very rarely, the insect seeks the flower, not to sip the nectar,
but for a place to lay its egg. A remarkable instance occurs in the

flowers of Yucca, a common desert
plant. Reproduction in Yucca depends
upon the assistance of the Pronuba moth.
In turn the larvae of the moth gain their
livelihood from the Yucca. The female
moth with the aid of her special tentacles
collects from a number of flowers a
mass of pollen. Then, while still cling-
ing to her cargo, she pierces the ovary
of a Yucca flower with her long egg-
depositor and lays an egg within the
ovary tissue. This duty having been
performed, she proceeds promptly to the
stigma of the flower and presses the
pollen ball into the stigma. This process
of collecting pollen, of depositing an
egg in the ovary of a flower, and of
pounding the pollen into its stigma is
repeated time and time again by the
insect. Yucca ovules are fertilized; thus
does the plant profit. Moth larvae
hatch within the ovary and live upon
some of the developing seeds; thus does
the insect profit. It is no exaggeration
to say that the very existence of Yucca
plants depends upon the strange habits
of Pronuba moths, and likewise the con-
tinuity of the moth upon this earth is
dependent upon the Yucca.

The wasp "psen" which Aristotle saw
fly out of a fig thousands of years ago,
was born and raised there, and was on
its way to crawl into another fig to lay
its eggs and die, like all the females of
its kind ever since that day and long
before. A fig is a peculiar sort of flower-

FIQ. 135. The Pronuba
moth "hand pollinates"
the Yucca flower. Each
time she lays an egg in the
yucca ovary, she makes
sure that yucca seeds will
be growing to feed her
larvae when they hatch.
In the upper flower a moth
is collecting pollen; in the
lower flower a moth is lay-
ing an egg; in the central
flower a moth is placing a
ball of pollen on the stig-
ma. The pod shows holes
through which the Pro-
nuba offspring have es-
caped. (From Robbins
and Pearson, in Sex in the
Plant World.)

PLANT AND ANIMAL RELATIONSHIPS

241

bearing stem which has grown long and fleshy at the periph-
ery until it completely encloses the whole cluster of very
small staminate and pistillate flowers. The Blastophaga wasp
goes through its entire life history from grub to adult within
the fig. The males never see the outside world; they fertilize the
females and die. But the females crawl out of the fig. By the
time they get out they are well dusted with pollen from the male
fig flowers they have had to crawl over. Now they enter other
figs in quest of female flowers in which to lay their eggs. The
wasps do not know it, but they can not get their eggs into the
ovaries of ordinary female flowers. However, the fig also bears

FIG. 136. Pollination of the fig. A, median lengthwise section of a fig show-
ing fertile female flowers; note the female fig wasp near the opening, also
another one inside. B, similar section of a fig showing gall flowers. (From
Robbins, in Botany of Crop Plants.)

a peculiar deformed type of female flower in which they can lay
their eggs, and they finally do. But in the meantime, while they
have been trying out the good female flowers, they have scattered
pollen plentifully over the stigmas, and the fig seeds will set and
the fig race be perpetuated.

The seeds and fruits of many plants are a source of food for
animals. Unwittingly, often the seeds are scattered by animals,
and thus does the plant profit. Many birds eat fleshy fruits, the
seeds either being regurgitated or passing through their alimentary
tracts uninjured. Such seeds are quite likely to be deposited

242 DEPENDENCE OF PLANTS ON SURROUNDINGS

under conditions suitable to their germination. The seeds or
fruits of some plants are provided with devices which enable them
to adhere to the hairs of animals and thus be disseminated to new
soil areas. As examples we cite the beards of certain grasses,
the spines of the sand-bur, and the hooks or barbs of the cocklebur.
Squirrels carry away and hide nuts, some of which may find
favorable conditions for their germination.

A few plants have means for securing animal food. The
leaves of Venus' fly-trap, sundew, pitcher plants, and bladderwort
are so constructed as to capture insects, which are finally used
as food by the plant.

The character and distribution of plants in the world have
been profoundly influenced by man. He has domesticated many
wild plants; he has hybridized them and carried them to all parts
of the world, far from their original homes. He has removed
forests, plowed the plains and prairies, and introduced grazing
animals, thus greatly modifying the natural plant covering.

REFERENCE

Productive Soils, by W. W. WEIR, published by J. B. Lippincott Company,
Philadelphia, 1920. 395 pages, 234 illustrations. Definite and practical
information concerning soils and profitable crop production. It treats of soil
and its origin, soils from a chemical point of view, soil and plant relations, crop
production, factors determining soil fertility and principles of soil fertility and
soil management.

UNIT VH

HOW PLANTS ARE FITTED TO THE CONDITIONS OF
THEIR SURROUNDINGS

We speak of the surroundings of plants as their environment.
The study of plants in relation to the environment in which they
are living is known as plant ecology. Different factors of the
environment which affect plants are water, light, air, soil, tempera-
ture, and living things, both plants and animals. Any condition
in the surroundings which causes a change in a plant is known
as a stimulus. The effect that is caused by a stimulus is a
response. Only living things can be affected by a stimulus.
There can be no response in a lifeless object.

Most seed plants have both a soil environment and an air
environment. That is, they have their " feet " in the soil, and
their " heads " in the air. The roots grow in the soil and they
are related by structure in a way that fits them to the conditions
of the soil environment. The factors of the soil environment are
water, temperature, soil salts in solution, solid mineral substances,
soil air, and soil organisms. In a way, we may also consider
gravity as a factor of the soil environment. It differs from the
other factors, however, in that it is nearly constant all over the
surface of the earth, while the others are extremely variable.
The roots are fitted to absorb water and salts from the soil solution
and to anchor the plant in the solid earth materials.

There may be substances in the soil which are injurious to
plants. Such substances as acids and alkali may be present in
the soil solution in concentrations which make it impossible for
plant processes, as absorption, to go on in a normal way. Plant
species differ in their ability to maintain life and grow under
severe conditions. When a plant is resistant to an unfavorable
factor of the environment, we say the plant is tolerant of that

243

244 HOW PLANTS ARE FITTED

particular condition. Thus we have acid-tolerant plants, drought-
tolerant plants, and alkali-tolerant plants.

Plant breeders have produced, by hybridization and selection,
varieties of cultivated plants which are tolerant of conditions
under which other varieties of the same genus would fail. Most
varieties of alfalfa winter-kill in the severe winters of our northern
states. Grimm alfalfa, developed from a cross between common
alfalfa and yellow-flowered alfalfa, is tolerant of the low tem-
peratures of the northern winters and is grown successfully in these
states.

As we look about and see plants growing in nature, we note
that some species of plants are found flourishing under conditions
of extreme dryness, as lichens and mosses on the face of a rock
cliff. We see others wholly or partially submersed in the water
of ponds and lakes, as Elodea and water lily. Other plants with
peculiar structural features can survive the conditions of the
extremely dry habitat (we speak of the place in which a plant is
growing as its habitat); likewise, only plants fitted to such an
environment could live in water. In each type of situation we
are apt to find a number of species living together, each fitted,
by structure, to the particular habitat. Plants which are not
tolerant of the conditions of a habitat are not found in the group.
Thus we find plants of the same species and those of similar
species of plants regularly living together in plant societies as,
for example, the pond society and the desert society.

When we study the fitness of plants in any situation, we are
impressed with the fact that the two roles of plants are nutrition
and reproduction. A plant must be able to provide food for itself,
but it must also provide for the future of the species by some
means of reproduction. Many plants have the ability to repro-
duce rapidly by vegetative means, as the strawberry by stolons
or runners, the horseradish by roots, the quack grass by rhizomes,
and the Jerusalem artichoke by tubers. Seed plants, in general,
must be fitted by structure for the processes of pollination, for-
mation of gametes, fertilization, and subsequent development of
the seed, in which is tucked away the young plant, together with
a food supply sufficient to give it the necessary start in life when
planted. Various structures in connection with fruits, as wings,

PLANT RESPONSES 245

hooks, and barbs, provide aid in dispersal of seeds by wind or
animals.

Problem 1. To what kinds of stimuli do plants respond?

When we consider the relation of plants to their surroundings
we must realize that they are living things, their active parts
being composed of the living material which we call protoplasm.
We can study the properties of protoplasm, but we know very
little about the reasons for its power to move, to grow, or to be
affected by stimuli. Scientists have learned that living things
behave as they do because of two major factors, both depending
upon the nature of the living stuff, protoplasm. First, they
inherit certain characteristics from their parents; and second,
they are influenced by their surroundings.

Tomato plants always have the same general characteristics
of stem, leaf, and fruit. We can always recognize the plant as
tomato. However, when plants of tomato grown in the sun are
compared with plants grown in the shade, marked differences
are found. The shade plants are apt to be tall, have slender
stems and thin leaves, and bear little or no fruit; sun plants of
tomato on the same type of soil will be strong and sturdy and
set an abundance of fine fruit. The characteristics which enable
one to recognize the plants as tomato are inherited; those which
vary with change in habitat are the result of reactions to conditions
in the surroundings.

How do green plants respond to light? If you have tried to
grow a plant by a window of your room, you have noted that the
stem gradually became curved toward the light. You probably
noted also that the petioles of the leaves twisted in such a manner
as to bring the leaf into a plane transverse to the direction of the
rays of light. A maple tree growing in an open field where light
comes from all sides develops a low, symmetrical, and dense top
with branches extending from the trunk almost to the base,
whereas a tree of the same species in the thick forest grows a
tall and straight stem having branches only at the top where light
penetrates. See Figs. 197, 152.

Exercise 117. What effect does light have upon the form of the plant?
Grow in darkness a pot of bean plants for two weeks or longer. Compare

246 HOW PLANTS ARE FITTED

general shape of plants, size of stem, size of leaves, color, etc., with the same
features of plants growing in normal light for the same length of time. Answer
the question of the exercise by means of diagrams.

Exercise 118. What effect does gravity have upon the primary root of
the plant? Plant corn grains which have been soaked over night, with tip
of grain down, slightly below the surface in moist sawdust or sand. Remove
the grains when the root is )4 inch in length and transfer to a moist chamber
prepared as follows: Fasten four cork stoppers to the bottom of a dish by
means of melted paraffin. Pack into the bottom of the dish about the stoppers
absorbent material, such as moss, or bits of newspaper, filling up and leveling
off after wetting the material until it is even with the tops of the attached
corks. Place sheets of blotting paper over all, cutting to extend slightly up
along the edges of the dish. Fasten four sprouted grains of corn through the
blotting paper into the corks by means of pins so that the roots point in dif-
ferent directions. Cut a piece of glass to fit as a cover and fasten in place with
adhesive tape. Support on edge in a shallow dish holding water to supply
moisture which will rise in the absorbent material by capillarity. Make a
drawing of the moist chamber as set up, showing definitely the position of
the roots at first. In what direction did gravity pull on the roots while the
seeds were germinating in the sawdust? In how many different directions
is gravity pulling on the four roots now? In so far as the seedlings are con-
cerned, you have changed the direction of the pull of gravity. Examine after
a day to determine the direction taken by the root tips in the meantime.
Make a drawing showing the seedlings as they appear now and compare
with the drawing made when the moist chamber was set up. What has
been the response to gravity in this experiment?

Exercise 119. What effect has gravity upon the stem of the plant? Sup-
port a vigorously growing potted plant in a horizontal position in a dark room
and examine after 24 hours. What change has taken place in the stems of the
plant? By changing the position of the stem you have changed the direction in
which the pull of gravity is applied to it. What is the response in this case?
What is the stimulus? Compare the stimulus of Exercise 1 18 with the stimulus
in this experiment. Compare the response of Exercise 118 with the response
in this experiment. In a single connected statement, answer the question at
the beginning of this exercise.

It is evident from the results of our experiments that the root and the stem
are affected by gravity in quite different ways. In some way which has not
been explained, gravity causes the taproot of a plant to grow downward and
the main stem to grow upward. These reactions are beneficial to the plant as
they take the root system down into the soil where there is water together with
nutrient soil salts. They take the shoot up into the air where the leaves are
exposed to the necessary sunlight and where the flowers and fruits are in a
position which favors the processes of pollination and seed dispersal. Give
two reasons why the main stem of a tree usually grows upward. Give two
reasons why the branches of a tree usually grow outward from the main stem.

PLANT RESPONSES 247

Tropisms. Curvatures in plant organs, such as those caused
by light and gravity, are known as tropisms, and the organ of the
plant which is affected is said to be tropic. Tropisms due to the
stimulus of light are phototropisms, and those due to gravity are
geo tropisms. The primary root is positively geotropic and nega-
tively phototropic, while the main stem is negatively geotropic
and positively phototropic. Other responses of plants are those
caused by the stimuli of chemicals, chemotropism; those caused
by water, hydrotropism; and those caused by contact, thigmotrop-

FIG. 137. Explain what you see in this picture. What response determined
the behavior of the roots of the tree?

isin. Roots grow towards available soil salts and water and
around solid objects with which they come in contact. It is
sometimes necessary to cut down willow and poplar trees which
are near tile drains because of the tendency of the roots of those
trees to enter the drains through the cracks between the separate
tiles and fill up the tiles with masses of fibrous roots, completely
stopping the flow of water. It is a well-known fact that an

248 HOW PLANTS ARE FITTED

occasional thorough soaking of a lawn will produce a deeper root
system of the grass than frequent light sprinkling. How can you
explain these facts in terms of the foregoing discussion?

It should be remembered that the whole plant, above and
below the surface of the soil, is in a complex environment, subject
to the application of a number of forces. The form and nature
of the plant, including root, main stem, branches, and leaves, is
governed by all these forces acting at the same time. On the side
of a cliff the taproot of a young seedling may take a horizontal
direction into a crevice which holds moisture, reaction to moisture
being greater in this case than reaction to gravity. Secondary
roots are not affected by gravity in the same way as primary roots.
Their course extends out from the main root approximately
perpendicular to the line of pull of gravity. Although the reasons
for this are not understood, it is recognized as a definite advantage
to the plant as it enables the root system to reach all parts of the
soil within the radius represented by the length of the longest
lateral roots. The farmer, in cultivating his growing corn, digs
the soil deeply at first to prepare loose soil for the spreading second-
ary roots; then later he uses shallow cultivation to avoid injury
to the lateral roots while he prevents the growth of weeds and
provides a mulch of fine soil to hold the moisture.

The leaves of most plants are dia-phototropic. By this we
mean that the petiole twists in such a manner as to place the blade
in a plane at right angles to the incident light rays. At the same
time the leaves are placed in such a position with reference to
each other that a minimum of shading is secured, so that in looking
down upon the plant one sees a mosaic of leaves. The mosaic is so
complete in some trees, as the Norway maple, that scarcely a
speck of sunlight can be seen in the shade of the tree. The
mosaic is well shown also in the rosette of mullein, dandelion, or
evening primrose. It is also shown in an interesting way by
Boston ivy on a wall where the light reaches the plant from one
side only. Mosaics are shown in Figs. 125, 126.

Give one case in which an animal reacts to light and two cases
in which a plant reacts to light. Compare the reaction of an
animal to light with the way a plant reacts to the same stimulus.
Why does a plant react differently from an animal?

STRUCTURE AND WATER SUPPLY 249

Problem 2. How are plants related by structure to the water supply?

With respect to water there are many different types of plant
habitats in nature. They range from those types in which there
is an abundance of water, as in ponds and marshes, to those in
which there is a scarcity of water, as on the bark of trees, on the
surface of rocks, or in desert sands. In chese different situations
we find plants related to the water supply in different ways
according to the conditions in the individual habitat. Plants of
the same species are not found in all these situations. A plant
species has inherited qualities which fit it, in general, to live in a
certain type of habitat. There is a certain condition, as to water
supply, in which the plants of a species are most likely to succeed.
There is, however, frequently a rather wide range of habitats in
which the plants of the same species will grow. Most plants have
the ability to react immediately to changed conditions in a way
that is an advantage to the plant. Plants growing where water is
scarce are usually structurally different from plants of the same
species growing where the water supply is sufficient. Likewise,
plants growing in bright sunlight are different from plants growing
in the shade. It is a well-known fact that lettuce grown early in
the season while the water supply is ample and the days are
neither too hot nor too long is apt to be crisp and delicious, while
that from a later sowing which reaches development during the
hot, dry weather of mid-summer will be tough and bitter.

How are plants fitted for absorption? As we have seen, the
younger parts of the roots are covered with a fuzzy growth of root
hairs. These make possible rapid absorption of water by increas-
ing the area of suitable absorbing surface. Water roots are nor-
mally without root hairs, but most land plants have the root hairs
exceptionally well developed.

The roots of tropical orchids are peculiarly fitted for water
absorption. These plants are known as epiphytes, since they live
upon other plants from which they derive no nutrient materials.
Their roots do not touch soil, and the whole plant is suspended in
an air habitat. The air surrounding the roots is usually quite
moist, but this has only the indirect effect upon the water supply
of reducing the evaporation from the plant. The only water avail-

250

HOW PLANTS ARE FITTED

able to epiphytes is that which wets the absorbing surface in the
form of rain or dew. A spongy outer layer of roots of orchids,

known as the velamen, takes up
the water by capillarity in the
same way that blotting paper takes
up water. From the velamen the
water passes inward from cell to
cell by diffusion until it enters the
conductive system of the root.

The seed plant, Spanish moss
(Tillandsid) , which hangs from
the branches of trees in great
festoons in the southern states,
has no roots. Absorption is ac-
complished by special structures
on the leaves which take in water
that wets the plants as rain or dew.
Carnivorous plants. The word
carnivorous comes from the two
Latin words, carnis (flesh), and
vorare (to devour). We are not
accustomed to think of anything
but animals as using flesh for food.
A few plants, however, have the
power of digesting and absorbing
animal substances. Examples of
this group of plants are, the
pitcher plants, sundew, and
Venus' fly-trap, all living in
swamps and bogs. Plants living
in these situations have poorly
developed root systems, and as
a result water absorption is re-
stricted. This condition also
hinders the absorption of nitrogen
salts in sufficient amount to supply
the needs of the plant for raw ma-
terials. Besides, there is very

FIG. 138. Drawings made from
cross-sections of three types of
leaves. A shows the internal struc-
ture of a leaf of a plant immersed
in water. Account for the presence
of thin epidermis and large air
spaces, and the absence of sto-
mata. Note in B the heavy cuti-
cle, thick epidermis, compact in-
ternal structure, and location of
the stomata in depression and pro-
tection by hairs. Explain how
these features fit the plant to living
under desert conditions. C shows
the section of a leaf grown under
conditions of moisture intermedi-
ate between those of A and B.
Explain presence of the thin epi-
dermis, moderately compact inter-
nal structure, and exposed stoma.

STRUCTURE AND WATER SUPPLY 251

little mineral material in the waters of the peat bog, and this
places an additional difficulty in the way of the plant's securing
mineral salts. In some way, which is not well understood, these
plants, in the course of their evolution, have developed the car-
nivorous habit.

In pitcher plants the modified leaves form pitcher-shaped
vessels which catch rain water as it falls and hold it indefinitely.
The leaf pitcher is smooth inside and is provided with a lip which
has hairs over its surface pointing downward toward the interior of
the vessel. The pitcher, partly filled with rain water, forms a
trap for beetles and other insects that happen to get into it. Be-
cause of the peculiar structure of the vessel, the hapless insect is

FIG. 139. The leaves of Venus' fly trap are adapted to the capture and

digestion of insects.

unable to crawl up the sides, and as a result, it finally dies. In the
case of the pitcher plant shown in Fig. 140, substances resulting
from the decomposition of the insects are absorbed by the cells of
the pitcher as organic food for the plant. Like other green
plants, the pitcher plant is able to make food for its own use,
but it also seems able to supplement this supply with that which it
secures through the carnivorous habit.

The sundews grow in situations similar to those in which pitcher
plants are found. Each leaf has a definite petiole which holds a
circular leaf blade. Numerous hair-like projections extend ver-
tically upward from the upper surface of the blade, the ones at the

252

HOW PLANTS ARE FITTED

edge being longer than those at the center. At the end of each
projection there is a glistening droplet of a viscid substance which
resembles dew in appearance. Insects, attracted by the sparkling
droplets and wine-red color of the leaves, alight and stick fast to
the hairs. This contact stimulates the leaf, which reacts by rolling
the edges inward, thus bringing other hairs in contact with the
insect and entangling it still more. The trapped insect dies, its
body is digested by enzymes secreted by the hairs, and the organic
food thus made available serves to supplement the supply which the
sundew is able to make.

Partial parasites. Some plants which are dependent upon
other plants contain chlorophyll and can make a part or all of their
foods. Mistletoe is a plant with chlorophyll which is a widely
.^..^^^.^^^ distributed parasite on trees.

The sticky seeds adhere to the
branches of trees where they
germinate. The young plants
send their complex absorptive
structures down into the tissues
of the branch until they reach
the conductive structures. Here
they derive from the host raw
materials, including water and
mineral salts, and also a part
of their food supply. Since
-these plants are somewhat de-
pendent but do not obtain all their foods from the host, they
have been called partial parasites.

Ability to withstand drying. You have probably noticed, as
you walked in the woods, a slight coating on the north side of many
trees, grayish green in dry weather and bright green during rainy
periods. This is an alga known as Protococcus, the Indian
compass plant, so called because the presence of masses of this
plant on trees could always be relied upon by Indians in the woods
as a guide to direction. In dry weather the cells are inactive.
When rain comes they rapidly absorb water, begin to grow and
multiply, and because of the large number of active cells they
appear as a light-green coating on the bark. Many of the lichens

FIG. 140. The pitcher plant, a car-
nivorous plant.

STRUCTURE AND WATER SUPPLY 253

and mosses show this same ability to retain life during periods of
extreme drying, to revive quickly when wet, and resume their
normal life activities. The resurrection fern (Poly podium poly-
podioides) may grow from the crevices of rocks or as an epiphyte on
trees. It has the usual fern characteristics when moisture is suffi-
cient, but as the drought approaches, the leaves wither and curl
and the plant appears to be without life. When water again be-
comes available, the leaves uncurl and immediately take on a
bright green, and the plant begins a new period of activity. The
Mexican resurrection plant (Selaginella) , sold in stores as a
novelty, may remain dormant for months in a dried condition and
still retain life. (See Fig. 147.)

Exercise 120. Collect pieces of bark to which dried lichens are attached
and pieces of bark with a coating of Protococcus. Moisten, place in a shallow
dish with a little water, and cover with a bell jar or inverted battery jar.
What change do you note in the appearance of the plants? Place in the light
and observe for several days. What do you conclude as to the ability of the
plants to revive after drought?

Water loss in plants. Under ordinary conditions, a green plant
is continuously losing water. In fact, water loss is one of the most
serious of the problems connected with the growth of plants.
Water plants which are submersed are not subject to danger from
loss of water. In most swamp plants which have their roots in a
soil saturated with water, excessive loss by evaporation is readily
made good by increased absorption. In bog plants, however, and
the plants of salt marshes, which have water in abundance, absorp-
tion is difficult, and hence excessive water loss is destructive. The
question is one of water balance. The amount of water lost by the
plant through evaporation together with that used in food manufac-
ture must not exceed the amount of water taken into the plant by
absorption. In habitats where the water supply is deficient or
where absorption is difficult, specialized structures are necessary to
prevent excessive water loss.

Why do plants lose water? Since water loss incurs danger to
the plant, why should there be water loss? Suppose the plant were
so well protected that there would be little or no loss of water to
the outside. The deciduous tree in winter condition is almost
completely impervious to water. At this time, however, the tree is

254 HOW PLANTS ARE FITTED

comparatively inactive. The different plant processes are practi-
cally at a standstill. The plant could not continue to live in that
condition indefinitely. If you will recall the features of leaf
structure which make food synthesis possible, you will remember
that exchange of gases with the outside is made possible by open-
ings, usually in the lower epidermis, the stomata. See Fig. 28.
These holes in the epidermis of the leaf are an absolute necessity
since carbon dioxide must enter the leaf as raw material for food
synthesis, and oxygen must escape as a by-product of the same
process. Besides, some oxygen must be available inside the leaf
at all times to be used by the plant in the process of respiration.
When the leaf is active the stomata are open and the water which
evaporates from the moist cell surfaces into the intercellular spaces
in the spongy tissue constantly diffuses into the atmosphere sur-
rounding the leaf as long as the humidity of the air outside the leaf
is less than that of the air inside. If the air surrounding the leaf is
dry, evaporation and diffusion from the active leaf take place
rapidly and water loss is great. If the air outside the leaf is
saturated with water vapor, as it frequently is in the tropics, there
is little water loss from the leaf. Thus, while water loss may be a
menace to the plant under dry conditions, it can not be prevented
if the plant is to function. The success of plants in many situa-
tions depends upon their ability to develop protective structures
which guard against excessive loss of water. The loss of water
from plants by evaporation is called transpiration.

How is the water supply of plants conserved? There is nothing
in the environment of plants more variable than the water supply.
It is an exceptional season if we do not have to sprinkle our lawns
many times to keep the grass from drying up. At times, some
means of conserving water is necessary to save the life of the plant.
It may be necessary, even, for a plant to drop its leaves in mid-
summer during a severe drought. In general, plants or parts of
plants which are exposed to dangers of excessive drying develop
protective structures which tend to prevent conditions that might
be fatal to the plant. Sun plants are apt to be protected more than
shade plants, and even in the same tree, the exposed upper leaves
are more protected than the shaded lower ones. In the older leaf
the outer walls are usually thickened by a deposit of a fatty sub-

STRUCTURE AND WATER SUPPLY 255

stance known as cutin. This aids in protecting the leaf against
excessive water loss. Cutinization of the epidermis of the apple
and other fruits tends to hold the water inside. Broad-leaved
evergreens, as holly and magnolia, show especially heavy cutin de-
posits which protect the plants during the colder seasons when ab-
sorption of water from the soil is more difficult and excessive loss
from leaves might be fatal. The leaves of the sedums (live-
forever) and those of many other plants are coated with a layer of
wax, known as " bloom/ 7 which prevents the escape of water. In
spraying cabbage plants for protection against insect pests, soap
must be added to the spray solution to dissolve the wax on the
leaves. Otherwise, the spray solution rolls off the waxy leaf in
large drops without wetting it and the insects are able to eat the
leaf without getting any of the poison.

Leaves of mullein, Shepherdia, and other plants are covered with
hairs which prevent free movement of air currents and thus reduce
evaporation. This condition seems also to reduce the absorption
of heat by the leaf and in this way indirectly prevents water loss.
There is a decided tendency toward leaf reduction in plants
exposed to dangers of excessive transpiration. In the cactus, food
synthesis occurs exclusively in the stem; the only leaves of the
plant are small structures which appear on very young stems and
are soon lost. In some of the euphorbias, natives of Africa, which
are occasionally seen in our conservatories, there are true func-
tioning leaves which are dropped at the approach of drought, a new
crop appearing when rains bring about conditions which are more
favorable. Frequently, in conditions of severe drought, our decid-
uous trees lose many of their leaves, even in mid-summer. This is
a protection against more severe injury which might result if trans-
piration were not checked.

Many plants which live in regions subject to drought conditions,
as the aloes and cacti of the arid regions of the southwestern part
of the United States, are fitted to these conditions by having thick,
succulent leaves or stems. A large amount of water is stored in
these structures when water is available. The plant is able to
draw upon this store for use in maintaining the normal plant proc-
esses when there is a scarcity of water in the soil. Purslane, a
common weed of field and garden, has a succulent stem and succu-

256 HOW PLANTS ARE FITTED

lent leaves which may hold water sufficient to keep the plant alive
for days, even when uprooted. You may dig a purslane plant and
cut it into pieces, and if these pieces lie on moist soil the various
fragments will send out adventitious roots and produce a large
number of separate purslane plants. This characteristic of water-
retention is taken advantage of by florists in propagating certain
varieties of begonia. Triangular pieces of the leaf, each containing
a portion of a prominent vein, are set in sand. In this position
they remain succulent and fresh until they have had time to de-
velop roots and a bud, drawing upon the supply of food and water
stored in the succulent leaf portion. After the adventitious roots
and young shoot are well developed, the plants are transferred
to pots of good soil where they soon become established and de-
velop into sturdy begonia plants.

In some plants, as in Aloe, the water-storage tissue is below the
green tissues; in others, as in begonia, the water-storage tissue con-
sists of the lower cells of a thickened epidermis. Stonecrop, which
belongs to the former class, thrives on the scant water supply
afforded by the thin layers of soil in the depressions of rock out-
crops. The different sedums (live-forever) are used extensively
in rock gardens and at other places for carpeting very dry, sandy,
or rocky places in the open sun.

Water absorption and water retention are especially difficult in
plants living in salt marshes and on alkali soils where the salts in
the soil solution are highly concentrated. Plants in these situa-
tions must have a sap with a total concentration of solutes (solids
dissolved) greater than that of the soil solution. In other words,
the concentration of water particles in the soil must be greater
than the concentration of water particles in the plant sap, other-
wise water can not enter the root hairs of the plant by diffusion.
This same principle seems, also, to be of importance in explaining
the resistance of many plants to drought conditions. Protective
structures of leaf and stem are important in preventing excessive
water loss by transpiration, but the condition of concentration of
cell sap which makes possible the absorption of water from com-
paratively dry soil is probably of no less importance.

What type of roots are plants likely to have if they are growing
in a region where there are occasional light rains? What type of

PLANT COMMUNITIES 257

stem are these plants likely to have? What types of roots and
stem is a plant likely to have in a region where there is regularly
a period of fairly heavy rainfall alternating with a long rainless
season?

Problem 3. Why are certain types of plants found living together?

What are plant communities? It is a source of interest to be
able to study the vegetation from what we may call a bird's-eye
view of the landscape. From such a study one of the first things
that we discover is that vegetation is grouped according to habi-

FIG. 141. Lily pond in a backyard. What different types of plant surround-
ings are represented in this rock garden and lily pond?

tats. There are ponds, swamps, flood-plains, sand hills, uplands,
deserts, each habitat having a particular type of vegetation. A
closer study reveals the fact that not only are certain species of
plants found regularly in a certain type of habitat, but, in general,
all the species found in a given type of habitat have many charac-
teristics in common. Botanists have found that the most con-
venient way to classify plants on the basis of habitat is according
to their water relations. Plants found regularly in pond or swamp
habitats are called hydrophytes; those found regularly in dry
situations are known as xerophytes. By far the greatest number

258

HOW PLANTS. ARE FITTED

of different species are found growing regularly under conditions
which may be considered intermediate between those of hydro-
phytes and xerophytes. Plants which grow best under conditions
of moderate water supply are called mesophytes. All the plants

FIG. 142. Mesophytic woods of beech and maple trees carpeted by spring

flowers. The soil is rich in humus. It is not wet, but the supply of moisture

is usually sufficient during the entire growing season.

of a habitat make up a plant community, and, classed on basis of
water supply, communities are hydrophytic, xerophytic, or meso-
phytic.

PLANT COMMUNITIES

259

Where oaks are found growing, one is likely to find hickories
along with them. Then there are apt to be shrubs of witch hazel
and spice bush, and such herbs as meadow rue and false Solomon's
seal. These plants and many others are found growing together
regularly in definite plant communities, and in habitats that are
similar. Different grasses, rosin weeds, and blazing star are plants
of the prairie community; Sphagnum moss, pitcher plant, sundew,
and cranberry are plants of the bog community; and beech, maple,
tulip tree, blood root, and dog-tooth violet are found regularly

FIG. 143. The prickly pear, a xerophyte. The thick flattened structures are
green stems with much water storage tissue.

together in the mesophytic forest. The pioneer settlers of parts
of our country were able to select lands which promised to be suit-
able for certain crops which they desired to grow, by noting the
type of plants which the land supported in the uncultivated state.
Soil in the eastern part of our country on which was found a good
growth of maple, beech, black walnut, and tulip tree was consid-
ered ideal, when cleared of trees, for growing crops of corn, wheat,
oats, and clover.

260

HOW PLANTS ARE FITTED

Hydrophytic plant communities. Plants growing in ponds and
lakes are subject to fewer and less abrupt changes than plants in

any other habitat. Temperature
is more nearly uniform in water
than in air, and the water require-
ments of the plant are satisfied
at all times without any necessity
for special provisions for absorp-
tion or the prevention of water
loss. The plants may be partly
floating on the surface, or sub-

. . _ , _ _ merged and rooted, or floating free.

FIG. 144. Explain why there are no . . , , , , , , ,,

Btomata on the under side of a Amon S the P lants that are USUa11 ^
water-lily leaf. Account for the waxy submersed are the pond weeds
condition of the upper epidermis. (Potamogeiori) and water weed

(Elodea) . Some of these frequently

become a nuisance in park ponds because of their rapid and profuse
growth. Many of the pond plants, as yellow pond lily, reproduce
vegetatively by rhizomes in the
mud. The exposed surface of
floating leaves, as those of water
lily, is usually coated with wax
which prevents wetting of the
surface and filling of the stomata
with water. Submersed leaves
are finely cut or ribbon-form
and for that reason are not
easily injured by water currents.
Stems of water plants have
little mechanical tissue. The
plants do not need to support
themselves to any great degree
since they are held up by the
buoyant force of the water. Air

FIG. 145. The "knees" of cypress.
The cypress is an conebearing tree
growing in the swamps of southeastern
United States. The roots are in very
wet soil, and consequently there is
not an ample supply of oxygen. The
"knees" are growths of spongy tissue
sent up from the roots into the air,
and through them oxygen passes
down to the roots.

passes readily through the vege-
tative structures of the plant

within a system of air chambers and open spaces in the tissues,
giving the plant buoyancy and facilitating the exchange of gases

PLANT COMMUNITIES 261

in the food-making and other tissues of the plant. Plants of
the pond community succeed in their particular habitat because
of their special fitness to live submersed, wholly or partly, in water.
Young growing wheat or corn plants, if covered by a pond of water
after a heavy rain, even if for only a short time, are killed. The
wheat and corn plants are not fitted by structure to the conditions
of a water habitat. Year after year plants migrate to the water's
edge and hundreds of seeds are blown into the water, but still the
pond community is limited in species to the comparatively small
number which are fitted to the conditions which the pond affords.

Swamp plants are similar to pond plants in some respects, hav-
ing a reduced root system and prominent air chambers; however,
in general characteristics they are more like mesophytes, particu-
larly in leaf thickness, distribution of stomata, in amount and
character of green tissue, and in protective structures.

The peat bog is a peculiar type of swamp in which there are
deposits of varying depth of partly decayed plant matter upon
which vegetation is growing. The usual type is the Sphagnum bog
in which the substratum is mainly dead Sphagnum moss. In the
early stages the substratum is always saturated with water.
Although some plants migrate into the bog from the outside and
grow fairly well, in the main the plants of this community are
peculiar to the bog, consisting of such species as Sphagnum, sun-
dew, cranberry, pitcher plant, dwarf birch, poison sumac; and tam-
arack. The plants of the bog are characterized by small leathery
leaves and sparse root systems. Tamarack trees in the bog have
roots only at or near the surface of the soil, but these trees planted
in high ground become deep-rooted and show no tendency to
remain at the surface. The saturated peat soil is unusual in many
respects. The Sphagnum deposits are sour, and this has a ten-
dency to prevent the growth of soil bacteria. This condition, along
with the absence of oxygen, tends to prevent decay and other
necessary soil reactions. Under these circumstances, acids and
other toxic substances form in the soil, and there is a scarcity of
soil salts in the bog waters. These conditions inhibit root growth
and restrict the process of absorption.

It seems odd that plants growing in soil saturated with water
should have leaf structures similar to those of plants of dry regions

262

HOW PLANTS ARE FITTED

which serve in preventing excessive water loss. In the light of the
foregoing discussion, the need of protective structures of plants
growing in bogs is evident. It has been found that tamarack trees
and other plants of the bog will grow even better in situations out-
side the bog under suitable ordinary conditions. Why, then, are
they characteristic bog plants? The answer is found in the fact
that the bog is highly selective. These plants, being tolerant of
bog conditions, grow here because there is less competition. So we
have in the bog a plant community made up of species which are

FIG. 146. Lichens and mosses are among the first plants which are active in
the process of transforming rocks into soil.

Xerophytic plant communities. The deserts and dry plains of
the southwestern part of the United States offer examples of habi-
tats where water is scarce and plant growth is difficult. Plants
must possess special structures to be able to withstand the severe
conditions of these habitats. The root system is generally exten-
sive and deep, the sparse, shallow root system of the cactus being
a notable exception. The plants contain much water-storage tis-

PLANT COMMUNITIES

263

sue, or a highly developed cuticle, of a wax covering; or a plant
may possess more than one or even all of these features which tend
to prevent an excess of water consumption and water loss over
water intake. Some of these plants either have no leaves, food
synthesis being accomplished by modified stem structures, or there
may be small leaves, or leaves which are easily dropped during
extended drought periods.

FIG. 147. Plants that can withstand drying. The lichens growing on this

rock face and ferns clinging to the crevice are active during rainy periods.

During times of drought they remain in a semi-dormant condition.

Xerophytic plants have had an important part in the making
of soil out of solid rock. Lichens and mosses are able to grow on
rock surfaces when the rock is moist from rains, sending their
absorbing structures (rhizoids) out into contact with the surface.
These structures give off carbon dioxide, which with water forms
carbonic acid. This slowly dissolves the rock. A part of the
dissolved rock becomes raw materials for plants and a part goes
back to rock in the form of very fine particles which, with the
decaying plant bodies, form soil. As this process goes on, together

264

HOW PLANTS ARE FITTED

with the action of water and frost, more and more soil is added
until other plants which migrate into the community can get estab-
lished, and finally the rock is covered with a layer of soil sufficient
to support a rich mesophytic vegetation. As the habitat changed,
migrants came into competition with the xerophytes and gradually
crowded them out. So, as the plants change the habitat, they
bring about conditions suitable for other plants which are better
fitted to the new conditions, and the pioneers are eliminated.

Here is an example of a plant suc-
cession starting in a xerophytic
habitat. Under mesophytic climatic
conditions this type of succession
goes through the various stages
from xerophyte to mesophyte.

Mesophytic plant communities.
In a mesophytic climate such as the
deciduous forest region of the
United States, succession is always
toward the condition in which the
habitat is occupied by mesophytic
plants. In the ordinary swamp and
bog, development begins in a hydro-
phytic habitat. As the succession
continues through the swamp stage,
there is a gradual development of
a mesophytic community in which
the plants require only a mod-
erate water supply. On dry sand
or rock surface where plants sur-
vive only with difficulty, plant
deposits, as leaves and other vege-
tative parts, are added year after year, and these organic sub-
stances, together with fine sand or clay, form a humus which
has the property of retaining moisture. After long periods of
time, the habitat, formerly extremely dry, has developed a soil
which holds sufficient water to support a mesophytic vegetation
consisting of such plants as beech, maple, jack-in-the-pulpit, blood
root, and spring beauty. Thus, the types of plant communities

FIG. 148. A tree Yucca in the

Mohave Desert, California. A

typical xerophytic tree.

PLANT COMMUNITIES

265

FIG. 149. Halophytes (salt plants) m
the dune succession, Oceano, Calif.

we see today in parts of our landscape which have not been altered
by man are the results of changes which have been going on for
many thousands of years.
Plants fitted to the environ-
ment have come, and as the
environment changed these
have gone and their places
have been taken by others in
a succession which has cul-
minated in what we see now.
Plant succession. Plants
growing in a pond or lake
tend to leave deposits which
gradually fill up the depression
and produce conditions which are unfavorable for the plants them-
selves. There are usually swamp zones about the pond, and many

of the plants in the zone next to the shore
push out to the water's edge, and even
into the water, by means of the promi-
nent rhizomes which they possess. The
swamp plants encroach upon the pond
and by their deposits aid in eliminating it.
Thus the pond with its typical commu-
nity of plants gradually disappears and
is followed in turn by the different stages
of the swamp and finally by a mesophytic
forest. As conditions change, plants of
the habitat are displaced by migrants
which are more suited to the new con-
ditions, and so become established.
Thus there is a series of more or less
definite stages in the development of a
region. The succession of plant com-
munities which results from successive
changes in plant habitats we call plant
succession.

How has man affected plant succession? When man becomes
a pioneer in a new region he cuts down the trees of the forest or he

FIG. 150. Following lich-
ens and mosses on rock ma-
terials, first herbs appear,
then trees and shrubs. Scene
in Southern Illinois.

266

HOW PLANTS ARE FITTED

breaks up the sod of the prairie or drains swamps to prepare the
soil for growing his crops. He builds dams which cause streams to
overflow their natural banks, causing the flooding of many acres
of dry soil. These changes in the habitat of plants result in
sudden changes in the different plant communities.

A result of rapid water run-off sometimes brought on by
forest destruction is the washing of much of the fertile humus
from the hills to the lower grounds and into the streams. The
fact that flood-plains and deltas are built up from the humus

FIG. 151. Trees get a foothold in crevices of the rock and thrive in this

difficult habitat.

washed from higher lands accounts for the great fertility of the
river lowlands and the increasing infertility of the hills. Thousands
of farms in our country have been abandoned because the cultiva-
tion of the soil on the lands is no longer profitable.

Burned-over forests and abandoned farm lands represent
habitats in which few plants but xerophytes can grow. Plants
fitted to these severe conditions gradually come in and there is
the beginning of a new succession. Herbs, shrubs, and trees
come into the habitat, and these add humus to the soil and hold

PLANT COMMUNITIES

267

it in place. After many generations, if man allows the succession
to go on, the final result will be a condition similar to that which
man found when he arrived as a pioneer. Alabama, Mississippi,
Michigan, Wisconsin, and other states have thousands of square
miles of land which is being reforested in this natural manner.

FIG. 152. A magnificent pine forest in Michigan. Forests similar to this one
formerly covered extensive areas in the north central states.

Man is the most destructive of the forces that affect plant suc-
cession. Frequently through carelessness he starts a forest fire
which sweeps over thousands of acres of territory, leaving destruc-
tion of the work of centuries in its wake and changing a mesophytic
plant habitat into one in which only xerophytes can dominate.

268

HOW PLANTS ARE FITTED

Statistics for the year 1931 show that forest fires swept over
52,000,000 acres in the United States in that year alone, with a
money loss of $65,968,350. It has been estimated that approx-
imately 50 per cent of forest fires are caused by locomotives, 8
per cent by smokers, 0.1 per cent by camp-fires, and 11 per cent
by boys. By proper education and care, man could prevent
forest fires, or greatly reduce their number, except those caused by
lightning. What factors of the native environment is it sometimes

FIG. 153. A good stand of hardwood trees at the University of Illinois.
Practical forestry is becoming more and more of a necessity.

possible for man to change in a way to increase productivity when
he begins to cultivate the soil?

Exercise 121. Field study. How are the plants of a pond fitted to the
conditions of the water habitat? If possible, study the conditions and plant
life of a pond or lake; if this is not possible, answer as many of the questions
as you can by a study of a well-stocked aquarium or lily pond. Note the
characteristics of the plants which are submersed, paying attention to charac-
ter of leaves as to texture, thickness, size, form, and shade of green; to the
character of the stem as to size, amount of mechanical tissue, and whether
covered by a protective coat of cutin, wax, or cork ; and to the roots, if
present, noting whether they are water roots or soil roots. If both kinds of

PLANT COMMUNITIES 269

roots are present account for the presence or absence of root hairs on either
water or soil roots. How do submersed plants' secure carbon dioxide and
oxygen? Explain whether cutinization of leaves would be a benefit or hin-
drance to submersed plants. Explain presence or absence of much mechanical
tissue in submersed plants. Try to determine why the plants do not sink to
the bottom. Describe features of any plants which you find floating on the
surface of the water, as Riccia (a liverwort), duckweed, or water hyacinth.
What advantage has a floating plant over a submersed plant? What disad-
vantages to the plant are there in the floating habit? Study rooted plants
with floating leaves, as water lily, noting wax coating of the exposed surface.

FIG. 154. Forest fire destruction. Man's carelessness caused the destruction
of a fine growth of young timber.

Splash some water over these leaves and try to determine the r61e of the wax.
Describe the light exposure of the water lily. Take water plants in closed
cans back to the laboratory with you for further study. Write a full account
of your field trip, answering the question at the head of the exercise.

Exercise 122. Field study. How are swamp plants fitted to the conditions
of the habitat? Visit a swamp at the edge of a pond, if possible. Study emersed
plants, as bulrushes, arrow head, and water plantain. Examine the interior
of portions of the plants, as stem and leaf petiole. What characteristics have
these swamp plants in common with emersed pond plants? Explain. Study
the plants of the zones of the swamp seen as one goes out from the pond. These
represent stages in the development of the habitat from the hydrophytic to

270

HOW PLANTS ARE FITTED

the mesophytic condition. Note the large number of plants with vertical
leaves, as cat tails, reeds, and sedges. What is the advantage of this type of
leaf in places where vegetation is dense? Remembering that the soil of the
swamp in the earlier stages is saturated with water, how do you account for the
fact that the roots of the plants extend in a horizontal direction near the
surface, and some even extend upward? In so far as you are able to make this
study of the swamp, try to answer the question at the head of this exercise in a
clear and concise statement of the results of your investigations.

Exercise 123. Field study. How are plants fitted to the conditions of a
xerophytic habitat? Study the plants of any dry habitat, as a dry prairie, sandy
hill slope, or railroad embankment. Note the character of the leaves of the plants,
as to thickness, texture, color, and size. Break some of the stems. Are they

FIG. 155. Why are our wild flowers disappearing? Here is one answer.

succulent and brittle, or are they hard and tough? Determine the character
of the roots by digging up some of the plants. Describe any tendency to the
development of thorns and spines, or hairy leaves, or the rosette habit? Do
you find any compass plants, as wild lettuce or rosinweeds? Write a detailed
account of your study, giving your opinion as to why the plants which you
found were able to become established in this xerophytic habitat.

Exercise 124. Laboratory study of a mesophyte. In our study of plants in
previous units, we have considered the mesophyte as our typical plant. It
remains for us here only to consider the characteristics of mesophytes which
distinguish them from hydrophytes and xerophytes. Select any available
mesophytic plant of the greenhouse or garden, as geranium, bean, or four
o'clock. Note the character of the leaves as to color, size, thickness, texture,
and cutinization. Study Fig. 138, showing sections of the leaves of a hydro-

POLLINATION 271

phyte, of a xerophyte, and of a mesophyte. Compare the structure of the
three types of leaves, paying special attention to cuticle, upper and lower
epidermis, palisade tissue, spongy tissue, and occurrence of stomata. How do
mechanical tissues of the stem of mesophytes compare with those of hydro-
phytes and with those of xerophytes? Write a summary comparing the fea-
tures of mesophytes with those of hydrophytes and with those of xerophytes.

Problem 4. How are plants related by structure to the process

of pollination?

Pollination has been defined as the transfer of pollen from the
anther of a flower to a stigma. The stigma receiving the pollen
may be in the same flower with the anther producing the pollen
or it may be in another flower. When pollen is transferred from
the anther to the stigma of the same flower, the process is called
close pollination. When close pollination is effected by contact
of stigma and anther, it is called self-pollination. When the trans-
fer of pollen is from the anther of a flower to the stigma of a flower
on another plant, the process is termed cross-pollination. A con-
dition intermediate between close-pollination and cross-pollination,
in which pollen is carried from a flower to another flower on the
same plant, is sometimes classed with cross-pollination but, in
reality, it is more nearly related to close pollination.

Most flowers seem to be fitted to the process of cross-polli-
nation. It has been noted that some flowers are especially
suited to cross-pollination by certain insects, as red clover by
bumblebees. Many other examples of pollination by certain
animals may be cited. In connection with this, the student should
review the interrelations of plants and animals discussed on
pages 238-242. The long bill of the humming-bird easily reaches
the nectar at the bottom of the long spurs of the columbine, which
is out of reach of the mouth-parts of bees, so the humming-bird is
the principal agent in cross-pollination of the columbine. The
long proboscis of the hawk moth, hovering before the Nicotiana
flower, is uncoiled and thrust down into the tube of the corolla,
at the base of which there is an abundance of nectar which is used
by the moths as food. The relationship between Nicotiana and
the hawk moth is seen further in the fact that the moths are active
only in the evening or at night and they are aided in finding the

272

HOW PLANTS ARE FITTED

flowers by the light color of the corolla and the marked fragrance
of the blossoms. On the body and legs of insects are hairs and
bristles to which pollen may stick and be carried from flower to
flower. Indeed, the relation between the structures of flowers
and the structures of insects is so noticeable that biologists believe
that flowers and certain insects developed together in their changes
in form.

Why do insects visit flowers? It is a well-known fact that
many insects secure nectar from flowers, using it for food on the
spot, as butterflies moths, and certain flies, while others, as bees,

FIG. 156. Columbine flower.
Only humming birds or but-
terflies and moths can reach
the nectar in the tip of the
spurs.

FIG. 157. The hind legs of the honey
bee are well constructed for the collec-
tion of pollen. (From California Agri-
cultural Experiment Station Bui. 517.)

lay by a store for future use. Bees collect nectar, not honey,
from flowers. This substance, which contains only from 15
to 40 per cent of sugar, is lapped up by the mouth-parts
of the bee and transferred to a honey sac near the stomach, in
which it is held until the bee reaches the hive. Here the nectar is
placed in cells and left until evaporation of water changes it into
honey with a very high percentage of sugar. Bees also gather
pollen and store it temporarily in cells, to be used later as food,
chiefly by the larvae or young bees. Bumblebees and honeybees

POLLINATION

273

are the most efficient pollinators among the insects. This is due
partly to their unceasing activity, and partly to their habit of con-
fining their visits on the same trip and on many succeeding trips to
flowers of the same species. " White clover " honey as offered by
apiarists is produced in the white clover season and may really

FIG. 158. Nectar glands sometimes occur on
the leaves of certain acacias. These glands ap-
pear as small protuberances. (From California
Agricultural Experiment Station Circular 62.)

FIG. 159. Pelican
flower (Aristo-
lochia), a carrion-
scented flower, at-
tracts flies as pol-
linating agents.

have been made almost exclusively from nectar secured from white
clover blossoms.

Nectar is secreted by special structures of the flower known as
nectaries which are exposed in some flowers and hidden in others.
In flowers having exposed nectaries the nectar is usually accessible
to flies and other insects without specialized mouth-parts. In

274

HOW PLANTS ARE FITTED

specialized flowers having concealed nectaries, the nectar can be
obtained only by insects with mouth-parts modified to form some
type of proboscis.

In some flowers, as those of the poppy and nightshade families,
there is little or no nectar. Insects visit these flowers only for
pollen, which is usually produced in great abundance. In some
cases pollinating insects visit flowers for sap, and in others, for
protection. Certain flowers, as pelican flower (Aristolochia
grandiflora) of conservatories, and our native carrion flower
(Smilax herbacea) attract different species of scavenger flies by an

odor resembling that of decaying
flesh. These flies may get nothing
from the flowers, but serve as efficient
pollinators as they are apt to visit
only flowers which are carrion-scented,
and these are likely to be of the same
species. (See Fig. 159.)

How is cross-pollination brought
about? There are two main types of
flowers with regard to pollination,
wind-pollinated flowers and animal-
pollinated flowers. Wind-pollinated
flowers usually have neither showy
structures nor marked fragrance.
Light, dry pollen is produced in great
abundance, and it may be carried many
The receptive structure of the female
flower, the stigma, is feathery in form and, when ready for pollina-
tion > is covered by a sticky secretion which catches and holds any ,
pollen grains that happen to reach it. In a species having pollen
and ovules borne on different plants, as in the cottonwood, only
cross-pollination is possible. In other cases the pollen which
reaches the stigma may be from a flower on the same plant.

It is significant that the showy and fragrant flowers are animal-
pollinated. Differences in flower color and flower structure seem to
have a direct relation to the process of pollination, usually to the
process of cross-pollination. In some flowers, Iris, for example, and
the orchid known as lady's slipper, the flower parts are so arranged

FIG. 160. Orchid flowers are
peculiarly fitted to cross-
pollination by insects.

miles by a strong breeze.

POLLINATION

275

that the insect bearing pollen on its body enters the flower in such a
manner as to rub against the stigma, and upon leaving, it rubs against
the anther. Thus, pollen deposited on the stigma is from another
flower.

A very common method of preventing close pollination is
by successive maturing of the stigma and anther of the same
flower; the pollen may be shed before the stigma is mature, or
the stigma may mature before the pollen grains. Insects, in going
from flower to flower, carry mature
pollen from a flower with an immature
stigma to another flower having a
mature stigma. Here close pollination
is impossible and cross-pollination is
likely to occur.

In some plants, as flax and prim-
rose, there are different types of flowers.
Some have long styles and short
stamens, while other plants of the
same species have short styles and
long stamens. An insect visiting the
former type of flower receives pollen
on the front part of the body and
leaves pollen from another flower pre-
viously visited on the stigma from
the back part of the body. When
this insect visits the latter type of
flower, the front part of the body,
which is covered with pollen, comes in

contact with the stigma and the back part of the body receives
more pollen from the long stamens.

Sometimes pollen will not germinate on the stigma of the
same flower or even on the stigma of any other flower on the same
plant; or pollen may not germinate as readily on a stigma of the
plant which produced it as on the stigma of another plant. In
the former case, close pollination is impossible; in the latter,
cross-pollination is more likely to occur.

One of the simplest features of flowers which tends to favor
cross-pollination and prevent close pollination is the long style

FIG. 161. The moccasin
flower, an orchid, a flower
peculiarly fitted to cross-pol-
lination by insects.

276

HOW PLANTS ARE FITTED

of many flowers which brings the stigma out above the stamens.
The visiting insect, as it enters the flower, is apt to brush over
the stigma, in its exposed position, and leave pollen which it has
brought from another flower. Also, pollen from the anther of
the same flower can not fall upon the stigma.

Many examples of highly specialized relationships between
flowers and insects might be given. The pollination of the Smyrna
fig is the most remarkable of the known examples of cross-pollina-
tion. The details of this process have been noted. (See page 240.)

Pollination between anther
and stigma on the same plant.
Although we have been accus-
tomed to thinking that there
must be marked advantages in
cross-pollination and judging
from the large number of struc-
tural features that fit flowers to
this process, and from the results
of investigation, we have good
grounds for this belief yet pol-
lination between anther and
stigma on the same plant is
quite common. Indeed, some
flowers possessing features which
favor this type of pollination
are about as specialized as those

FIG. 162. Insect pollination. This
butterfly is procuring nectar from the
milkweed blossoms, but it is also
carrying pollen from flower to flower.

which we have noted as favor-
ing cross-pollination. In the
composite family, each so-called
flower is really a head of a large number of flowers, usually of two
kinds, ray flowers around the edge and disk flowers in the center
of the head. As a rule, in the sunflowers, only the disk flowers
produce seeds. Some composites, as the dandelion, mature all
the flowers of the head at the same time, but in the greater
number, the flowers mature and are ready for pollination from
the outer edge of the disk inward, and pollination of all the
flowers of a head may require a week or longer. The anthers
of a given flower mature before its stigma. As the stigma ma-

POLLINATION 277

tures, it elongates and comes in contact with anthers of other
flowers of the head. Thus, pollination between different flowers
on the same plant is accomplished, and judging from the vigor
of the plants and from the very large number of seeds produced
by such composites as wild lettuce, common thistle, dandelion,
and sunflower, their method of pollination is very efficient. The
pollination of Yucca by the Pronuba moth furnishes an example
of highly specialized close pollination. (See page 240.) Certain
flowers (cleistogamous flowers), as some of the flowers of violets,
never open. In these flowers seeds are produced regularly in
abundance as a result of close pollination. In flowers of this type
in which close pollination results from contact of anther with
stigma, we have examples of true self-pollination. (See Fig 82.)

Exercise 125. What are the essential parts of a flower? Review your
study of a flower made in Unit V. Identify in several flowers of different
species the anther and filament of the stamen, and the stigma, style, and ovary
of the pistil. What is the role of each of these parts in pollination and fertili-
zation? Make sketches to show the relative size and arrangement of the pistil
and stamen in one flower. Note the position in the flower of the anther in
relation to that of the stigma.

Exercise 126. Where in the flower is pollen produced? Study under a
microscope (binocular, if available), without water or cover-slip, anthers
from different flowers of the same species, one from a flower in the bud,
another from a flower just opened, and a third from an older flower. How
does pollen get out of the anther? Does the pollen seem sticky and suitable
for being carried away by animals, or is it light and dry and suited to wind
dispersal? Does there seem to be any certain time when the pollen is ripe
and ready for pollination? Can pollen be carried away from a flower before
it is ripe? Make careful notes, explaining your answers to the questions of
the exercise, and make a sketch to show how pollen is set free from the pollen
sacs of the anther.

Exercise 127. How do flowers receive pollen? Study several stigmas
under the microscope (binocular preferred) of flowers fully opened. Find
pollen which has been transferred to the stigma. What makes it stick to
the stigma? How is the stigma different from the style? Find stigmas which
seem to be too young to receive pollen. Find others which are too old. What
would happen if ripe pollen grains were to touch a stigma either too young or
too old. What are two r61es of the sweet, sticky liquid on the surface of a ripe
stigma? Explain whether close pollination can occur if the stigma of a flower
gets ripe before its pollen, or if the pollen of a flower gets ripe before its stigma.
In what way is it possible for close pollination to take place in these two types
of flowers?

278 HOW PLANTS ARE FITTED

Exercise 128. Types of flowers with reference to pollination. The
flowers of this study will need to be chosen from suitable specimens, represent-
ing the different types, which are available at the time the study is made.

Open type flower, as buttercup or nasturtium. Note the relation of anther

FIG. 163. Maize, or Indian corn. At left, pistillate inflorescence, or "ear";
at right, staminate inflorescence, or "tassel." (From Robbins, in Botany of

Crop Plants.)

and stigma in position. Is the plant monoclinous (stamens and pistil in the
same flower) or diclinous (stamens and pistils in separate flowers)? Describe
the flower with reference to odor, color, whether nectar is present, and whether
it is regular (floral leaves similar) or irregular (floral leaves unlike). Describe

DISPERSAL OF FRUITS AND SEEDS 279

the features of the flower which fit it for pollination by wind, insects, contact,
gravity, or water. Try to determine whether close pollination or cross-
pollination is more likely to occur. Make a drawing of the flower to bring out
pollination features.

Specialized flower. Choose for this study a sympetalous (petals united)
flower, as butter-and-eggs or snapdragon. Make a drawing of the flower with
the floral parts in their natural position. Cut away enough of the flower to
show the position of the pollination structures. Is the flower fitted to polli-
nation by wind or by insects? Give reasons for your answer. What struc-
tures are present which would attract insects? If insect-pollinated, what
insects would be suitable as pollinators? Would cross-pollination or close
pollination be more likely to occur? Write a summary of the features of the
flower which show specialization, and explain how this specialization is an
advantage to the plant.

Composite flowers. Use any available composite, as yarrow, Coreopsis
or sunflower. Note that the so-called flower is in reality not a single flower,
but a head of a large number of flowers set upon a flat receptacle and sur-
rounded by green leaf -like bracts. Separate the head vertically by breaking it
open through the middle of the disk. Make a drawing of the exposed flowers
to show their relation to each other in natural position. The flowers at the
outer margin of the disk mature first, then the other flowers mature gradually
from outside to center. The anther matures and sheds its ripe pollen before
the stigma of the same flower is mature and ready to receive pollen. The
pollen is pulled out of the corolla tube by the hairy style as it pushes out,
bearing the immature stigma. The flowers being so near together, mature
stigmas of other flowers come in contact with this ripe pollen, bringing about
pollination. Insects visiting the head of flowers aid in the transfer of pollen.
Pick out suitable flowers of the head, and try to make out the different pollina-
tion features outlined above. Sketch under a dissecting lens or binocular
microscope disk flowers in different stages of development, one before the
stamens appear, another at the time the pollen is ripe, and a third at the time
the stigma is mature.

Problem 5. How are fruits and seeds fitted to the process of
dispersal of plants?

According to popular usage, the term fruit clearly includes
such plant structures as peaches, apples, blackberries, and pine-
apples; tomatoes, peppers, and cantaloupes, on the other hand,
are ordinarily classified as " vegetables. " To a botanist, the
term fruit has a much wider meaning. In botany, a fruit is a
ripened ovary, together with any other structures that may have
developed with it. Pollination and fertilization usually result ia

280 HOW PLANTS ARE FITTED

the development of the seed, together with the development of
the other structures of the fruit. A few fruits, as the banana and
certain oranges and grapes, develop without seeds. Fruits may
be either fleshy, as grapefruit, or dry, as a grain of corn. In both
kinds, however, the principal roles of the structures which enclose
the seed are protection and dispersal.

How are the fleshy fruits fitted to dispersal? The apple, pear,
crabapple, and quince belong to the group known as pome fruits.
The ovary is the core, the ovary walls being the hard plates sur-
rounding the seeds. The fleshy part of the apple is the specialized
receptacle which has grown up and around the ovary. During

FIG. 164. The fruit (drupe) of peach with the single seed surrounded by the
ovary wall in two layers, the inner one forming the " stone " and the outer

layer the pulp.

the time of development, the fleshy part contains starch and is
quite sour, because of the presence of malic acid and the absence
of sugar at this time. The green fruits are hard, the cells being
held together by a substance which is mainly calcium pectate.
As the seeds become mature, starch is changed to sugar and the
quantity of malic acid may be reduced somewhat. At the same
time, the pectic compounds are being broken down into other
substances with the result that the cells are separated to some
extent and the fruit becomes mealy. The changes in the fruit
which make it edible as the seeds develop suggest the role of seed
dispersal by animals that use the fruit as food. Man has been

DISPERSAL OF FRUITS AND SEEDS 281

especially active in propagation and distribution of these fruits
because he has found them desirable as valuable food supplies.

One of the most common of the fleshy fruits is the drupe, or
stone fruit, which includes the peach, plum, cherry, apricot, and
olive. The seed is enclosed in a stony layer which we ordinarily
consider a part of the seed structure, but which, in reality, is not
a part of it. Around this stony layer is a second layer which is
fleshy. In all these fruits the stony covering of the seed serves
as a protection. The smaller fruits, as cherries, may be swal-
lowed, stone and all, by the larger birds and pass through the
alimentary tract without injury to the seed. The chances are
favorable that many of the seeds will be dropped in a suitable

FIG. 165. The tomato, a berry. The seeds are enclosed in the fleshy ovary

wall.

place for germination and growth at some distance from the
parent tree.

The berry has a fleshy wall enclosing seeds. Berries include the
fruits of such common plants as the tomato, currant, grape, blue-
berry, and cranberry. Seeds of these fruits are small, and many
of them are distributed uninjured by the animals that use them
as food. The pepo is a berry with a hard rind, for example,
squash and cucumber. The hesperidium is a berry with a leathery
rind, as the lemon and orange.

The blackberry is not a real berry in the botanical sense, but
is a body formed by a large number of separate ovaries, each a

282

HOW PLANTS ARE FITTED

tiny drupe, attached to a single receptacle. The raspberry is
similar to the blackberry, but the mass of fruits becomes detached
from the receptacle when ripe. The strawberry is really not a
fruit at all, but a fleshy receptacle bearing numerous tiny achenes,
containing the seeds, on its surface. Each of these achenes is,
in reality, a tiny fruit. Fruits of this type are known as aggregate
fruits.

The mulberry and pineapple are formed from many individual

flowers all fastened tightly
together, and for this reason
are called multiple fruits.
The Smyrna fig is a syco-
nium, consisting of a fleshy,
hollow receptacle, the one-
celled ovaries developing into
nutlets which are embedded
in the inside wall.

The fruits mentioned in
the foregoing discussion either
have fleshy ovary walls or
are developed in connection
with other fleshy flower parts
which are edible. Most of
them are characterized, in
addition, by being attrac-
tively colored, shades of blue,
red, and yellow being espe-
cially prominent. Practically
all of them are green in color and inconspicuous before maturity,
becoming edible and showy as the seeds ripen. Because of the
possession of these features, animals have a prominent role in
the dispersal of many of the plants which produce these attractive,
fleshy fruits.

How are dry fruits fitted to dispersal? The dry fruits are of
two types, dehiscent (splitting open when ripe) and indehiscent
(not splitting open).

Dehiscent fruits. Among the dry, dehiscent fruits, the legume
is a common example. The pod of the garden pea or bean shows

FIG. 166. The aggregate fruit of the

raspberry, made up of many separate

fruits massed on a single receptacle and

developed from a single flower.

DISPERSAL OF FRUITS AND SEEDS

283

the characteristics of this type of fruit. You can not help noting,
in shelling peas, the resemblance of the opened pod to a leaf, the
outcurved edge of the pod being the midrib. You have probably
noted that the seeds are fastened to the pod at the incurved suture.
The opened pod shows that the legume is, in reality, a modified
leaf, in this case a single carpel bearing seeds at the edges. When
the legume dries, it usually splits open at the two sutures, the two
halves of the carpel curling in
such a way as to expel the seeds.

The follicle is a dry fruit
developed from a single carpel
which opens along one suture.
You may have noted in your
garden the opening of the
follicles of larkspur, or those
of columbine, at the top and
along the inner edge in such a
manner that the dry seeds are
thrown some distance as the
tall stem is swayed about by
the wind. The fruit of the
poppy or that of the violet is
an example of a capsule. Cap-
sules open in different ways,
allowing the seeds to drop or
be thrown out by movements
caused by the wind. The silique
is made up of two carpels which
open at maturity, the two valves
curling upward leaving a parti-
tion from which the dry seeds

become detached. Most of the members of the mustard family
have this type of fruit.

Dry indehiscent fruits. A very common example of a dry
indehiscent fruit is the achene, represented by the fruit of the
sunflower and by that of the buttercup. The single seed is
attached to the ovary at one point only. A grain of corn, typical
of the fruit of the grasses, is a caryopsis. Corn meal, as you buy it,

FIG. 167. Multiple fruit of the pine
apple. The fruit is developed fron*
the ovaries of many separate flowers.

284

HOW PLANTS ARE FITTED

consists of granular bits of the endosperm and embryo of the
seed, the tough fragments of ovary wall and the testa having been
sifted out in the process of milling. The samara or key fruit has

the ovary expanded in such

as

Examples of plants pro-
ducing this type of fruit
are
hop

significance of the fact
that winged fruits are
common among trees?
The nut, as the acorn,
chestnut and hazelnut, is
similar to an achene, but
has a hard outer wall.
Though nuts are eaten in

large numbers by animals, many that are buried by squirrels
are never found, and so are in a suitable place for germination.

FIG. 168. The

fruits (pods) of Lima
beans.

FIG. 169. Fruits (pods) of the black locust.

DISPERSAL OF FRUITS AND SEEDS

285

It is possible, also, that many fruits of oak, hickory, and walnut
are carried away in time of flood and deposited by water on a
bank where germination and growth of the seeds may take place.

By what means are fruits and seeds dispersed? Dispersal by
propulsion. It has been noted in a previous section that legumes
disperse their seeds by a curling of the two halves of the dried pod
in such a way as to throw the seeds some distance. The well-
known garden balsam or touch-me-not has a pod which suddenly
explodes upon being touched
and a violent curling of the
carpels throws the seeds out
with considerable force.
When one walks among jewel
weeds (Impatiens) in the
woods in late summer, one can
hear seeds falling all about as
a result of the bursting of the
fruits.

Dispersal by water. Many
seeds have walls that are
not readily penetrated by
water. These may retain
viability for a long period
while being -carried great dis-
tances by water. As the water
recedes, many seeds are left
in the silt of flood-plains and on
banks where they have suitable
conditions for germination.

Dispersal by animals. The farmers of the country suffer huge
losses every year on account of burs which get into the wool of
sheep and reduce its value. Fruits with reflexed spines and barbs,
as burs of cocklebur and burdock, beggar ticks, and Spanish
needles, cling to the furry or woolly coats of animals and to the
clothing of man and may be carried miles from the plant which
produced them. Our worst weeds have been brought into the
United States from abroad by man on his clothing, in his luggage,
and in farm and garden seeds that he has imported, and they have

FIG. 170. The fruits of burdock are

provided with hooks which fasten the

fruits to passing animals.

286

HOW PLANTS ARE FITTED

FIG. 171. Dispersal by the wind. Fruit (samara) of white ash.

been scattered throughout the country over man's highways and his
railroads. We have already noted the dispersal of seeds by birds that
eat fleshy fruits and drop the seeds in places suitable for growth.

Dispersal by wind. If you
live near cottonwood trees, you
have noticed, in late spring,
" cotton 7 ' flying about. If you
have examined some of the cotton,
you have noticed that tiny seeds
are attached to the small tufts.
In this way seeds are carried by
the wind. Dispersal by the wind
is wasteful, but it is effective.
Seeds of some of our most trouble-
some weeds, as wild lettuce and
Canada thistle, are scattered far
and wide by means of wind which
FIG. 172. Fruit dispersal. Fruit lifts the parachute attached to
heads of the composite, goat's tne see( j an( j carries parachute
baud. The opened head at the d h h ^ ^ d

left shows the fruits, each with

its parachute, ready to be lifted possibly for miles before they are
and carried away by the wind, dropped. Various states have at-

DISPERSAL OF FRUITS AND SEEDS 287

tempted to cope with the Canada thistle menace by enacting laws
requiring owners of land to keep all thistles mowed before they
blossom in order to prevent any production of seeds. The
tumbleweeds, as tumble mustard, winged pigweed, and Russian
thistle, branch in such a way as to form a large spherical mass
which, when mature, breaks off near the ground and goes tumbling
before the wind, scattering ripe seeds as it goes. When we
realize the possibilities of wind dispersal of plants, we are re-
minded that^ weed prevention on our premises is not only profit-
able for ourselves, but is also a civic duty, since weeds are no
respecters of fences and, if allowed to grow in our own fields
and gardens, are sure to spread to those of our neighbors.

Exercise 129. What is a fruit? Examine fruits of the following list and
make sketches, labeling the parts of the flower, as ovary, style, receptacle,
stigma, represented in the developed fruit: bean, grain of corn, sunflower,
prune (soaked for study), maple.

Explain whether the definition of a fruit as " a ripened ovary " holds for
the fruits mentioned above.

Examine and sketch in the same way the following fruits: apple, pineapple,
strawberry.

Explain whether the definition of a fruit suggested above holds for these
fruits. If it does not, use your labeled sketches in revising the definition to
include all fruits.

Exercise 130. What are the different types of fruits? Using your
sketches made in the previous exercise, together with your text, classify the
fruits in the lists of Exercise 129 and others, and give the characteristics of each
type of fruit, as: Bean, legume or true pod dry, dehiscent; one carpel,
splitting along two sutures.

Exercise 131. How are fruits and seeds dispersed? Study available
fruits to determine probable means of dispersal and describe structural features
which aid, using sketches where desirable. Make lists under the headings as
follows:

1. Dispersal by propulsion, as sweet pea, witch hazel.

2. Dispersal by attachment to animals, as cocklebur, burdock.

3. Dispersal by means of indigestible seeds of fleshy fruits that are eaten
by animals, as raspberry, black haw.

4. Dispersal by wind, as dandelion, ash, tumbleweed.

5. Fruits and seeds without obvious means of dispersal, as acorns.

Suggested activities, (a) Make a collection of fruits found in the market
and classify on basis of type; as apple a pome fruit.

(6) Make collections of dry fruits and classify on basis of means of dis-
persal

288 HOW PLANTS ARE FITTED

ADDITIONAL QUESTIONS AND EXERCISES

1. Explain why the stem of a plant placed in a horizontal position in
darkness will grow upward at the tip.

2. What will happen if onion sets are planted upside down in the soil?

3. Explain why gladiolus corms will send out roots and shoots more
quickly if the dry scales and other dead parts are removed from the corms be-
fore planting.

4. Why do celery stalks (leaf petioles) grow taller if the soil is banked up
around the plants?

5. Why is celery more crisp and tender when banked with earth than when
allowed to grow without banking?

6. In selecting strawberry plants, why is it safer to get plants from a
neighbor who has a successful strawberry patch than to send away to another
part of the country for plants of an advertised, fancy variety?

7. If a farmer is moving to a distant part of the country in about the same
latitude, he might take farm seeds along with him, or he might wait and get
seeds from farmers in the new location. What would be your advice? Ex-
plain.

8. What is the advantage of the deciduous habit?

9. Why is it necessary for the leaves of the evergreens of our colder
regions to have xerophytic structures?

10. Why is it a good plan to give the soil about the roots of ornamental
evergreens a good soaking with water on the approach of cold weather?

11. In growing plants in a region new to you, what use could you make of
information concerning the native wild plants growing in the vicinity?

12. Should a farmer who raises red clover permit the boys to destroy
bumblebees' nests? Explain.

13. Why do gardeners have a hive of bees in a greenhouse where cucumbers
are grown?

14. Explain reasons for the practice of spraying fruit trees with poison,
for destroying codling moth, once just before the blossoms open, and a second
time just after the petals have fallen, but avoiding spraying while the trees are
in full bloom.

15. What do honeybees gain from buckwheat blossoms, and what does
buckwheat gain from visits of honeybees?

16. Of what advantage is it to the carrion flower to possess an odor similar
to that of decaying flesh?

17. What kinds of flowers cannot be close pollinated?

18. What kinds of plants cannot be cross-pollinated under natural conditions?

19. What advantage is it to the species to possess flowers in which both
cross- and close pollination are possible?

20. In what way might wading-birds carry seeds and place them in a loca-
tion suitable for germination?

21. Why are some of our most persistent weed pests found among the com-
posites?

DISPERSAL OF FRUITS AND SEEDS 289

22. Explain why saturating the soil about the roots of Canada thistle with
strong salt solution will kill the plants.

23. Explain why ragweeds appear in the stubble of grain fields, although
few weeds were noticed while the grain was standing.

24. Why is it necessary to dig weeds from a newly seeded lawn although
little weeding is needed in an established lawn?

REFERENCES

House Plants, by PARKER T. BARNES, published by Doubleday, Doran and
Company, New York, 1909. 236 pages, 31 illustrations. This gives direc-
tions for the growing of house plants, including their selection, soil preparation,
seed sowing, potting, propagation, and other operations.

Plant Ecology, by JOHN E. WEAVER and FREDERIC E. CLEMENTS. Pub-
lished by McGraw-Hill Book Company, New York, 1929. 520 pages, 262
illustrations.

Familiar Flowers of Field and Garden, by F. SCHULER MATHEWS, pub-
lished by D. Appleton-Century Company, New York, 1903. 308 pages,
numerous illustrations.

Insectivorous Plants, by CHARLES DARWIN, published by D. Appleton-
Century Company, New York, 1899. 462 pages, 30 figures. This book is a
classic on insectivorous plants. It describes in detail the characteristics and
behavior of the sundew, the bladderwort, Pinguicula, and other insectivorous
plants.

Plant Ecology, by W. B. McDouGALL, published by Lea and Febiger,
Philadelphia, 1927. 326 pages, 114 figures. This discusses the environmental
factors, plant communities, plant succession, phenology, symbiosis, pollination,
and the ecology of roots, stems, and leaves.

Manual of Weeds, by ADA GEORGIA, published by the Macmillan Com-
pany, New York, 1914. 593 pages, 386 illustrations. A description of all the
most pernicious and troublesome plants in the United States and Canada,
their habits of growth and distribution, with methods of control.

Weeds, by W. C. MUENSCHER, published by the Macmillan Company,
New York, 1935. 577 pages, 123 illustrations. Discusses dissemination and
importance of weed, weeds of special habitats, weed control, and describes
the important weeds of the United States.

UNIT VIII
THE DEVELOPMENT AND IMPROVEMENT OF PLANTS

One of the fundamental laws of nature is that life comes from
life. Man has been able to do wonderful things in the chemical
laboratory, but he has not been able to produce any substance
with the properties of living material. Geology teaches that the
earth has gone through a series of changes in its development,
and that life has existed on the earth during at least half of its
geologic history.

Much is known of the early plant forms from studies of their
fossils. These records also reveal something of the story of the
development and disappearance from the earth of great plant
groups as well as the rise and development of those which have
become the dominant plant groups of today.

Primitive man was able to use plants in many different ways
in his daily life. They were food, shelter, medicine to him, and
they beautified his landscapes. The same laws which were in
operation in nature changing plants in the wild were also changing
the plants which man had brought under some degree of cultiva-
tion and control. Man early learned to take advantage of the
changes in plants which made them more suitable for his uses,
and he became a plant breeder. It is true the methods he used
were haphazard at first, but in the end they were effective in
securing better plants to meet his needs.

It had long been known that plants tend to be similar to their
parents, that is, that certain characteristics are inherited by
offspring. It is also known that no two plants are exactly alike,
that offspring tend to be different in certain respects from their
parents. In other words, plants show variation. It was not
until the middle of the nineteenth century that it was shown that
living things inherit characters from their parents in a certain
way and that inheritance in nature obeys fixed laws. Discovery

290

PLANT CHANGES 291

of the laws of heredity by Gregor Mendel opened the way to the
explanation of what man had been able to accomplish in the
development of improved strains of plants. It has also sim-
plified the processes of plant breeding and introduced the new
science of genetics.

The economic importance of plant improvement to the people
of the world may be illustrated by improvement in wheat. As
an example, Roberts of the Kansas State Agricultural College
made collections of wheat from all parts of the world and especially
from the wheat-growing regions where conditions are similar to
those of Kansas. An early winter with little snow killed most of
the wheat in Kansas. There was one exception. A plot which
Roberts had planted with seed imported from southern Russia
passed through the winter without serious harm. The seeds
were carefully saved and planted, and from this beginning was
developed the Kanred variety which not only was frost resistant,
but also ripened earlier than other Kansas wheats, was more
resistant to stem rust, and the flour of which proved excellent for
bread-making.

Plant improvement has been of untold benefit to the plant
grower, who has been able to produce larger and better crops,
but it has been of even greater benefit to the consumer, who can
get cereals, fruits, and vegetables of higher quality and at less
cost than would be possible with unimproved varieties of plants.

Problem 1. In what ways have plants changed?

At least 250,000 different species of plants are living on the
earth at the present time. Rocks are found which contain fossil
remains of simple plants which lived in the remote past it is
judged around one thousand million years ago. It is impossible
to conceive of the time expressed in that figure. The time cov-
ered by the average life span of a human being is only a fraction
over a second as compared with the time covered by the history
of plants as found in the rocks.

Botanists believe that simple plant life came into existence
at a much earlier period than that represented by the record of the
rocks, and that from the beginning many plant forms have come

292 DEVELOPMENT AND IMPROVEMENT OF PLANTS

and gone, others have remained and become the ancestors of the
plant life that we see on the earth today. We can ask many
questions concerning the nature of the first forms of life on the
earth, but no one has been able to answer definitely any of them.
They were certainly very simple, probably small bits of jelly-
like protoplasm possessing the powers of assimilating food, respir-
ing, growing, excreting wastes, responding to stimuli, and repro-
ducing. These are the outstanding properties of living matter,
protoplasm, the most wonderful of the different forms of matter
of which man has any knowledge. From the very beginning of
life, there has extended a line or lines without a break living
material giving rise to more living material. We can trace back
in our imagination this line from every living thing now on the
earth through all the countless years to these simple sources.

The simplest forms of plant life must have lived in the presence
of water, probably in the sea. They must have been so delicate
that even the slightest amount of drying would have destroyed
them. Among the simplest forms of plant life which we know
at present are the bacteria. It is thought that bacteria in the
past had an important part in the separation of calcium carbonate
from the sea-water and in the resultant formation of the great
deposits of limestone which we find in various parts of the earth.
It is believed, also, that bacteria have been responsible for the
laying down of certain deposits of iron ore and of the graphite
from which the lead in pencils is made. Scientists have estimated
the time which must have elapsed during the formation of these
deposits, and from the estimates have been able to guess concern-
ing the antiquity of such simple plants as bacteria. Whatever the
forms of early life were, there must have been changes which
tended to fit these forms to the changing habitats in which they
lived. As time went on, some appeared which could live on
land in the moist air of those early periods. From these early
land plants have developed our mosses and ferns and finally the
seed plants. As plants progressed, those changes which were
advantageous tended to add to the chances of survival. Changes
were not all in a direction which proved an advantage. On this
account great numbers of plants which once lived on the earth
have gone out of existence. We have abundant evidence of this

FOSSILS 293

fact in the fossil remains of plants which are found in the sedi-
mentary rocks such as limestone, sandstone, and shale.

We are led, in our discussion of the changes in plants, to the
two statements: first, that plants of today are descendants of
plants that preceded them, and these in turn came from other
plants; second, in time certain forms came to be different from
their ancestors. We are led to believe that new plants will appear
on the earth in the future in the form of modifications of plants
that are now living.

Problem 2. How do we know that plants have changed?

Botanists who have made a study of the evidences of change in
plants in the progress of their development on the earth have
offered facts to prove to their own satisfaction that the plants of
the present are modified descendants of the plants which preceded
them. First, a great variety of plant forms are found as fossils in
the rocks and coal deposits. Second, the geographical distribution
of plants as they are found today indicates that many families had
their origin in early forms in some particular part of the earth's
surface from which their descendants gradually spread to other
regions. Third, plant forms of today have a remarkable similarity
to each other in structure and function of parts. Fourth, the
plants of today are remarkably similar in their cycle of life.
Fifth, plant breeders have developed new forms which could
easily be mistaken for distinct species. Sixth, man has found
forms in nature (mutants) which are distinctly different from their
parents.

What are fossils? Have you ever gone fossil hunting? If you
have, you know the thrill one gets upon releasing, from its rock
prison, a record made millions of years ago when your plant was shut
off from the light of day by a deposit of clay or sand which subse-
quently turned to stone deep down in the earth. A blow of your
hammer brings to light again after all these years what is left to
show of the plant life which existed in times which are now remote
geologic history.

Ancient plants have been preserved as impressions. Some
part of the plant was covered by clay, sand, or mud, and this

294 DEVELOPMENT AND IMPROVEMENT OF PLANTS

left a permanent impression in the material which, subjected to
great pressure and age, turned to stone.

Much of our knowledge of plants of the past has been gained
from a study of impressions found in the layers of rock just above
beds of coal. Fig. 173 shows such an impression of a fern leaf in a
piece of shale taken from a coal mine in Illinois. It was removed
from a position in the earth 60 feet below the present surface.

FIG. 173. Fossil of a part of a frond of a seed-fern. These ferns formed a
part of the plant life of the Coal Age forests.

What we know as petrified wood is not in reality wood at all.
Fig. 174 shows a piece of so-called petrified wood. It has the
grain and even the cell structure of wood. The original wood
or plant part was covered with water which contained in solution
a large amount of mineral matter. This material penetrated the
wood, and as the wood decayed the mineral matter took its place;
when the process was complete the rock material had taken the
form of the original plant material.

FOSSILS

295

In the earliest rocks few fossils of
plants are found. There may have
been many more plants than the
fossil remains indicate, but during the
millions of years which followed, the
rocks were subjected to the effects
of running water, high temperatures,
and enormous pressures, any one of
which conditions could have de-
stroyed the record.

Fossils of the different geologic
periods show that simple plant struc-
tures were succeeded by structures
more complex as plant groups suc-
ceeded one another in the long period
of their development. The records
show that great groups of plants de-
clined and entirely faded from the
picture. The first seed-bearing plants
were fern-like. The flowering plants
as we know them today are a com-
paratively recent development. In general, the simplest
plants are found in the oldest fossil-bearing rocks. The

FIG. 174. Note the resem-
blance of this "petrified wood"
to real wood.

FIG. 175. Ferns and cycads. It is thought that the vegetation in many parts of
wha tis now temperate America was something similarto this a hundred million years
ago. (Photograph furnished by the Field Museum of Natural History, Chicago.)

296 DEVELOPMENT AND IMPROVEMENT OF PLANTS

FIG. 176. Cycas revoluta, the "sago
palm " of our conservatories.

fossils of more complex plants are found in the rocks of later
times.

The first seed plants to attain prominence in the earth's flora
were all gymnosperms, that is, forms with seeds not enclosed in an

ovary such as our own living
cone-bearing trees, like pine and
spruce. Many of these plants
became extinct at about the
time the angiosperms (flowering
plants with enclosed seeds) were
becoming established. The few
forms of these early plants
which remain are the fern-like
cycads of the tropical regions
of both the eastern and western
hemispheres, and the maiden-
hair tree (Ginkgo biloba of China) which is the lone survivor of
what was a large order of trees at about the time the flowering
plants were becoming numerous.

Evidences from geographic distribution. Geologists tell us
that in the course of geologic time the earth has undergone many
changes which affected plant
life. During the millions of
years in which the plant ma-
terial that later became coal was
being laid down, the nature of
the plant life over the earth
was extremely uniform. During
this time and, indeed, over
much of geologic time, the cli-
mate must have been uniformly
warm and moist over most of
the earth's surface. Under
these conditions plants of the
same group could have almost world-wide distribution. Be-
cause of the more uniform conditions in water than on land,
algae and other water plants of today are more widely distributed
than species of land plants. It has been noted in a previous unit

FIG. 177. Dioon, a cycad. Note

the resemblance to the ferns from

which the cycads came.

FOSSILS 297

that, through changes in such plant structures as leaves or flowers,
plants are able to meet the conditions of a changed environment.
Plant groups that cannot change are eventually crowded out by
others more suited to the changed conditions. We probably
have the explanation in these facts for the entire destruction or
reduction to a mere vestige of great groups of plants in different
periods of geologic time.

Barriers, as oceans, mountain chains, or deserts, have the
effect of isolating plant groups. It is believed that at different
times in the history of the earth the land masses were very different

FIG. 178. Zamia, showing carpellate cones. The cycads, to which this plant
belongs, are the most primitive of the living gymnosperms.

in form and extent from the continents as they are at present.
North America and Asia were once connected by a land bridge.
There is also evidence that Europe and America were connected
by land in the north Atlantic, and that, in the south Pacific,
South America and Australia were more or less connected at a
very early period.

At about the time the flowering plants were becoming estab-
lished as a future dominant group, the Gulf of Mexico extended to
the Arctic Ocean, forming a barrier between the eastern and west-

298 DEVELOPMENT AND IMPROVEMENT OF PLANTS

ern parts of the United States. With this invasion by the sea
here and in other parts of the earth, climatic conditions being
mild and moist, there was rapid development in plant life.
In the rocks of this period are found such modern trees as
oaks and willows, along with many other genera of plants now
living.

Following this era there was a period of mountain building,
especially in the western part of the United States, and of land
elevation, and North America came to have about its present
form and elevation. With these changes in land contour came
changes in climate until it was probably about as it is at present.
The Pacific slope was a region of dry summers and mild wet

winters, and the eastern part
of the United States had moist
summers and severe winters.
These regions were separated
by two great barriers, the
mountains of the west and the
dry plains to the east of the
mountains.

With these changes of con-
ditions came a sorting out of
the plants. It is fair to assume
that the trees of the west slope
and those of the east slope had
the same or similar ancestors.
But because of the differences
in climate, the vegetation de-
veloped in different ways. On
the west slope the trees are mainly conebearing; on the east
they are mainly deciduous. It is stated that of the nearly one
hundred trees native to California only two kinds are found east
of the Sierra Nevadas.

In general, closely related species of plants are found living in
the same geographic region, as if the family had its origin in some
particular center and gradually moved out from that center.
The cactus family seems to have had its origin in the Mexican
plateau, whence it spread widely into regions to which it was

JbiG. JL?y. .Leaf ol the maiden-hair

tree, or ginkgo, which is one of the

most ancient of living trees.

FOSSILS 299

suited. It has been diversified until there are now about 1500
separate species. No other continent has native cacti.

Evidence of change from similarity of structures and roles.

In studying plant cells, it makes little difference what cells we study
inasmuch as the structure is in general the same. We recognize
in each a nucleus, cytoplasm, and a wall of cellulose. Some cells
are more easily studied in the living state than others, so we select
our living cell from the tissues that are not too complex, or we
select a single-celled plant. True, some cells are simpler than
others. A 'bacterium does not have a clearly defined nucleus.
The nuclear granules are diffused throughout the cytoplasm.
The bacterium is considered a primitive type of cell. Another
primitive cell is that of the blue-green algae.

Any organ or system in the more complex plants might be
chosen for comparison, and we would find a remarkably similarity
in structure. Leaves of plants under similar, conditions of light,
temperature, and moisture are remarkably similar. Likewise, all
plants which make food make use of sunlight and raw materials
similarly. Absorption and digestion are accomplished in the
same general way. Thus comparative anatomy and physiology
offer many facts which give us reasons for believing that plants
of the present came from pre-existing forms.

Evidence from the results of experiments. If we could carry
on a series of experiments and actually see a new species of plants
come into existence, the riddle of life would be very much simpli-
fied. In one way or another, forms of plants in the wild have been
improved by man for his own use. Some plants, such as wheat
and apple, have been under cultivation for so long a time that we
do not know for certain what their wild ancestors were or just
where the improved forms were developed. The Indians were
growing maize long before the white man landed in America.
The potato is also a product of our country, but the form we
grow is quite different from any other member of its group, the
nightshade family, which is found growing wild today.

Numerous varieties of plants have been developed as a result
of the cultural practices of the practical farmer, gardener, and
fruit-grower, but not a single entirely new species. Many other
varieties have been developed through experiments by trained

300 DEVELOPMENT AND IMPROVEMENT OF PLANTS

biologists, but even trained biologists have not been able to
produce a new species.

When we examine the parents of the hundreds of different
varieties of the cultivated Chrysanthemum and compare the
parents with their descendants we are struck by the wonderful
changes that have been brought about in developing, from a
plant with yellow flowers less than an inch across, to plants more
than six feet tall and with flowers of many colors and shades eight
or even ten inches in diameter. Numerous other examples could
be cited from the abundance of evidence of changes in plants

FIG. 180. At left, Chrysanthemum indicum, with flowers an inch in diameter,
one of the ancestors of our showy cultivated chrysanthemum; right, an im-
proved variety, a descendant of Chrysanthemum indicum.

brought about in connection with plant propagation and improve-
ment.

Exercise 132. Study any available rocks showing animal or plant remains.
Find out from your instructor the kind of rock in which your specimen is em-
bedded. Is your fossil an impression, plant or animal remains, or an infiltra-
tion of mineral matter taking the place of the specimen? How do geologists
tell the probable age of fossils? Under what conditions was your fossil formed?
Try to determine its probable age. Make a drawing of the specimen in your
notebook.

Exercise 133. Copy a map in your notebook showing the condition of the
continent of North America at the time of the ice age. What changes in
climate brought about the ice age? What changes caused the ice to disappear?

CAUSE OF PLANT CHANGES 301

What changes in the plant life of the continent were brought about by the
ice age?

Exercise 134. Using tree books, make a list of trees which are found only
west of the Cascade Mountains and another list of trees found in the eastern
part of the United States. Are the same species of trees found in both regions?
Account for the likenesses or differences in the character of the vegetation in
the two regions.

Problem 3. What are the method and cause of change
in plants?

The young or progeny of plants tend to resemble their parents.
We have reason to expect nasturtiums to grow from nasturtium
seeds and sweet peas from the seeds of sweet peas. Besides, if we
desire dwarf nasturtiums, we plant seeds from plant parents which
are dwarf in habit, and if we want pink sweet peas, we select the
seed from plants which bear pink flowers. The tendency of
offspring to resemble their parents is known as heredity.

A plant which results from vegetative reproduction shows a
remarkable similarity to the plant from which it came. A straw-
berry sends out runners upon which appear buds, and from these
buds grow adventitious roots which fasten them to the soil.
After awhile the bud with roots is able to lead an independent
life, and the connecting runner from the parent plant withers and
dies. The young plant, under suitable conditions, will grow into
a plant similar to its parent in character of leaves, size and flavor
of fruit, and in fact, similar in every other respect. This is an
example of vegetative reproduction. In general, the results in
every other example of vegetative reproduction are similar to
those related for the strawberry.

Why are plants which are produced by vegetative means
similar to the plant which produces them? Every plant starts as
a single cell. This cell divides and forms two similar cells by the
process known as simple cell division. In this process there is a
division of the cytoplasm, and across the dividing cell is formed a
partition consisting of two walls so that after division each of the
daughter cells is completely surrounded by a cell wall. But most
important of all, in cell division there is a division of the nucleus
of the cell into two exactly similar parts. This division of the

302 DEVELOPMENT AND IMPROVEMENT OF PLANTS

nucleus has been carefully studied under the high power of the
microscope. The details of nuclear division can not be studied
in living cells. Growing structures, as the tip of a young onion
root, are killed so that the processes of cell division stop imme-
diately. Then they are prepared for examination under the
microscope. Careful staining brings out the structures so that
they can be studied.

Cell division. Scattered through the nuclear protoplasm and
arranged in a sort of net are granules of material which stain more
deeply than any other parts of the nucleus. The material which

makes up these granules is
known as chromatin. When a
stained lengthwise section of a
young root tip is examined,
most of the cells are seen to
be in the resting condition with
cell wall, cytoplasm, and the
nucleus in which are the
chromatin granules plainly
showing. A careful study of
the section reveals some cells
in which the chromatin gran-
ules are no longer in the net
arrangement, but have taken
the form of a dense coiled ribbon
the nucleus. It has been

FIG. 181. Cells near the tip of an
onion root, showing in three cells the
darkly stained chromosomes. These
three cells are in a state of division,
whereas the remainder of the cells are
resting.

in

found that this is not a single
ribbon but rather one with a
division along the middle
throughout its length. This double ribbon is recognized as a be-
ginning stage in simple cell division and is known as the spireme.
Looking further among the cells of our preparation we see that
in certain cells the spireme has broken up into a number of sections
and that each section has divided into two exactly similar parts.
The sections are of various shapes and are arranged in a plane
across the cell midway between two sides.

Examining still other cells, we find a peculiar behavior. It is
just as if an elastic thread were attached to each section and with

CAUSE OF PLANT CHANGES

FIG. 182. Stages in cell division. (The nuclei redrawn from a figure in Sharp's
Introduction to Cytology, 3rd Edition. McGraw-Hill Book Co.)

304 DEVELOPMENT AND IMPROVEMENT OF PLANTS

the other end of the thread attached near the ends of the cell (the
poles of the cell), half of the threads attached to one pole and half
to the other. Cytologists (persons who study cells) have found
that one section of a pair seems to be attracted or drawn to one
pole and the other section of the pair to the other pole. Some
force causes the two members of each pair of sections to separate
and move towards the opposite ends of the cell. Some cells of the
preparation will show the sections in an aggregation at the two
poles of the cell, and since each group contains a section from each
pair the two groups will be exactly similar in number and kinds of
sections. These take a deep stain and for this reason have been
named chromosomes.

Other cells of our preparation will show cell walls forming
across the equator of the mother cell, the chromosomes breaking
up into chromatin granules and the cell division nearing comple-
tion, after which the two daughter cells may be called resting cells.
We now have two similar cells which have resulted from a division
of material contained in the mother cell.

What are chromosomes? Biologists who have made a study
of heredity are of the opinion that characters are handed down
from parent to offspring by means of the chromosomes of cells.
Small bodies of material are contained within the chromosomes,
and these carry something which determines the characters of the
new individual. These bodies are known as genes or determiners.
Each chromosome seems to be made up of a large number of genes
or determiners, and when a chromosome divides in cell division
the process includes a division of the determiners so that the two
daughter cells will have exactly similar determiners. That is
why the daughter cells are similar to each other and also similar
to the mother cell from which they came.

When a new plant develops from a portion of stem, leaf, or
root of another plant by vegetative reproduction, the determiners
in the cells of the new plant are exactly similar to those of the
parent plant. Though the surroundings may have the effect of
modifying the form and size of the offspring, in general the
young plants tend to be similar to the plant from which they
came. They are really a part of that plant set over into a new
environment.

CAUSE OF PLANT CHANGES 305

Exercise 135. Why do the two daughter cells resulting from simple cell
division have similar characteristics? In this exercise you will see cells in the
resting condition and different cells showing various stages in simple cell divi-
sion (mitosis). It is not easy to find and study the progress of cell division
under the high power of the microscope, but it is necessary to know what takes
place in a dividing cell in order to understand inheritance. So, with the help
of your teacher and by referring to models of dividing cells or to the drawings
in your book, you will be able to identify dividing cells in the prepared slides.

Examine, under the low power of the microscope and then under the high
power, a lengthwise section of an onion root tip stained to show cell structure.
Look for resting cells and cells in the process of division. In general, how are
the dividing cells different from the resting cells? Why is the root tip a good
place to find dividing cells? What is the result of cell division in the root tip?

Read the description of cell division given in the preceding pages, and
compare the description with what you see under the microscope.

Make a series of six enlarged drawings or diagrams of cells as follows:

1. Resting cell, showing:

a. Cell wall.

b. Nucleus.

c. Chromatin network.

2. A cell showing a nucleus in which the chromatin network has become
arranged in a continuous looped band or spireme.

3. A cell showing separation of the spireme into definite pieces or chromo-
somes.

4. A cell showing the chromosomes arranged in a plate across the middle.

5. A cell showing the half -chromosomes moving to opposite poles of the cell.

6. Formation of cell wall between two daughter nuclei resulting in two new
cells.

The cell of the onion root tip has been found to have sixteen chromosomes.
How many chromosomes has each of the daughter cells? What is the source
of the chromatin material in the daughter cells? Remembering that chromo-
somes are thought to be bearers of determiners of characters, explain:

1 . Why the daughter cells are similar to the cell from which they came.

2. Why the daughter cells are similar to each other.

3. Why the vegetative cells of a plant that started from a single cell (the
fertilized egg) bear the same hereditary characters.

4. Why the vegetative cells of plants which are started from cuttings bear
the same hereditary characters.

What is the inheritance of plants that have two parents?

In a plant that is the product of sexual or gametic reproduction
two lines of heredity are represented. You will remember that
in gametic reproduction of flowering plants the sperm or male
gamete, produced in the germinating pollen, unites with the egg

306 DEVELOPMENT AND IMPROVEMENT OF PLANTS

or female gamete, which develops within the ovule, to form the
zygote. Thus the chromosomes from the sperm cell together
with the chromosomes from the egg cell are the chromosomes of
the zygote. You can see that the zygote, then, carries chromo-
somes with their genes which came from the plant which produced
the sperm and also chromosomes with their genes which came
from the plant which produced the egg. The young plant which
develops from the zygote will be made up of cells each of which
has chromosomes with their genes which came from both of the
parents.

The vegetative cells of plants of any species all contain the
same number of chromosomes. Each sex cell or gamete contains
half as many chromosomes as are found in a vegetative cell or
zygote. The chromosome number is halved in a peculiar kind of
division in the process of formation of the gametes, and moreover,
there is an assortment of the hereditary determiners.

When a zygote is formed from fertilization of an egg of one
plant by a sperm of a plant of another strain or species, the plant
which results is called a hybrid. In cross-fertilization there is a
combination of determiners. As a result many different kinds of
hybrid plants may come from the same parents. Thus another
way in which plants change results from cross-fertilization. This
is known as hybridization.

The combination of characters which a plant inherits from its
parents may be such as will make the plant less fit to meet the
conditions of the surroundings than either of the parents. In
this case the plant will not survive in competition with other
plants. On the other hand, if a plant inherits the strong char-
acters instead of weak characters from both parents, then the
plant will succeed and leave offspring which may crowd out
plants of other strains which are not so well fitted to the environ-
ment. In this way new strains of plants are coming into existence
and old strains are disappearing.

Why do offspring from hybrid parent plants differ? About
the middle of the last century, Gregor Mendel, an Austrian
monk, learned much about heredity by experimenting with com-
mon plants in his garden. In one experiment he planted wrinkled
peas and smooth peas and was careful to give both the same kind

CAUSE OF PLANT CHANGES

307

of growing conditions. When they were in bloom he removed the
anthers from the flowers of the one set of plants, say the ones from
round seeds, before any pollen was ripe. At the proper time he
transferred pollen from the plants from wrinkled seeds to the
stigma of the flowers without anthers. This was artificial
cross-pollination. The result was cross-fertilization. The sperm

FIG. 183. Diagrams illustrating Mendel's laws of heredity, (w) indicates
presence of the determiner for the recessive character, wrinkled; and (R)
indicates the presence of the determiner for the dominant character, round.
In the Fi generation one of the parents is pure for the character, wrinkled,
that is, it carries no determiners for pea shape except those for wrinkled; in
like manner, the other parent is pure for the character, round, as it carries
only determiners for the round shape. The results of the cross between these
two plants is a plant which is a hybrid, that is, it carries determiners for both
wrinkledness and roundness. Study the diagrams for the F 2 and Fg genera-
tions as you read the discussion of Mendel's Laws in the context.

308 DEVELOPMENT AND IMPROVEMENT OF PLANTS

carried the determiner for wrinkled to the egg which possessed the
determiner for round, and the seed which developed carried the
two determiners for the two characters wrinkled (w) and round (R).
The character of the seed resulting from the cross may be repre-
sented by wR (hybrid), which indicates that both determiners are
present.

Mendel found that the hybrid offspring of pure wrinkled (ww)
and pure round (RR) parents were all round. He planted these
round peas and found that in this second (F%) generation both
wrinkled and round peas were produced, and further that three-
fourths of the peas were round and one-fourth were wrinkled.
He planted some of the F% wrinkled peas and self-pollinated the
flowers. Only wrinkled peas developed on these plants. He also
planted Fz round peas and self-pollinated the flowers. In this
case one-third of the plants produced only round peas and two-
thirds of the plants produced both wrinkled and round peas as in
the Fz generation. These results may be illustrated by diagrams:

Mendel's laws. As a result of this experiment and many
similar ones, Mendel came to the conclusion that characters are
handed down from parents to offspring in a perfectly definite way.

1. Unit characters. Mendel thought of the plant as made up
of distinct and separate characters, as dwarf ness, white petals,
and smooth seed coat. He called these unit characters. In the
F2 generation when wrinkled peas are crossed with round peas,
according to our modern way of thinking, cells of the wrinkled
pea plants contain a double set of determiners of the wrinkled
unit characters or (ww) and their gametes would contain only
one or (w). When the wrinkled-round (wR) peas of this same F%
generation (see Fig. 183) are planted, the hybrid plants produce
two kinds of sperm cells, one having determiners of the wrinkled
(w) unit character and the other the round (R) unit character.
In like manner, corresponding types of eggs would be produced.
The different combinations of eggs and sperms are shown in Fig. 183.

In this case one of the zygotes (ww) will produce a plant with
only the determiners for wrinkled unit characters in the cells, a
second zygote (RR) with only those for the round unit characters,
and the other two zygotefc (wR) will have determiners for both
the wrinkled (w) and rouncl (R) unit characters.

CAUSE OF PLANT CHANGES 309

2. Segregation. In the F^ generation shown in Fig. 183 the
two (wR) peas are round, but when the plants from these peas
produce gametes, two kinds (w) and (JR) are produced, that is,
the unit characters are segregated in the production of gametes.

3. Dominance. Referring again to Fig. 183 it is seen that in
the F% generation three-fourths of the peas are round, although
the determiners for the character wrinkled are present in two-
thirds of these round peas. For some reason the wrinkled char-
acter, although its determiners are present, does not show in the

FIG. 184. Corn plants showing the Mendelian three- to-one ratio. The seed
from which these corn plants grew produced green plants and albino plants
without any green color in the Mendelian ratio, albinism being a recessive

character.

outward appearance of the seed when the round character is
present. Mendel called the kind of character represented by
round, dominant, and the wrinkled character, recessive. Other
dominant characters with their recessives in garden peas are
yellow seeds green seeds, colored flowers white flowers, and
tall vines dwarf vines.

Exercise 136. How do plants inherit the characters of their parents?
Corn is suitable material for use in experiments in heredity similar to those of

310 DEVELOPMENT AND IMPROVEMENT OF PLANTS

Mendel. Material for these experiments can be procured from Mr. George S.
Carter, Clinton, Conn., and from other dealers in biological supplies.

A. Inheritance of starchiness and sweetness in corn

1. Examine a hybrid FI ear of corn resulting from a cross between pure sweet
corn and pure starchy corn.

2. Also note the appearance of the ears of the parent stocks. Which is
dominant starchiness or sweetness in corn?

3. Examine an ear of corn grown from seed of an FI hybrid plant. Count the
starchy grains and the sweet grains. What is the ratio of starchy to sweet?

4. Make a diagram to show the FI generation and another to show the
F 2 generation. (See Fig. 183.)

5. If you planted the sweet grains, how many different kinds of grains
would you get?

6. If you planted the starchy grains, how many kinds of grains would you
get?

7. Which would be easier to pick out in pure state, starchy grains or sweet
grains?

8. How does this corn illustrate Mendel's Law of Dominance? The Law of
Unit Characters? The Law of Segregation?

B. A strain of corn has been developed which carries the albino character
as a recessive. That is, in plants from the seed of this strain a part of the
plants will appear without chlorophyll. Can a plant without chlorophyll
grow to maturity and produce pure albino seed? Explain. Representing the
green character by G and the albino character by w, the so-called hybrid plant
carrying the determiner for albino would be represented by Gw.

1. How many different kinds of sperms and how many different kinds of
eggs does this plant produce?

2. Make a diagram similar to that in Fig. 183 for this corn, showing
what occurs when it is self-fertilized. How many different kinds of seed are
produced when the zygotes mature?

3. Plant 16 grains of the Gw seed in a pot and determine the ratio of green
plants to white plants.

Exercise 137. Why is the Mendelian ratio 1 :2 :1? It was noted above that
in Fz of the cross between wrinkled peas and round peas the ratio of the dif-
ferent forms of the offspring was 1 RR: 2 Rw: I ww. This results from the
chance meeting in fertilization of the sperms, R and w, with the eggs, R and w.
This may be demonstrated by a simple experiment. Mix in a can 100 red
beans and 100 white beans of about the same size. Without looking, remove two
beans at a time and place in separate piles the pairs of red beans, the pairs of
white beans, and the pairs of red and white beans. When you have removed all
the beans from the can, count the number of pairs in each pile. What is the
ratio of numbers in each pile? This represents the chance combinations of
determiners which result from cross-fertilization. By the law of chance the
determiner for red, say of a sperm, is combined with the determiner for red in
an egg; the determiner for red in a sperm is combined with the determiner for

CAUSE OF PLANT CHANGES

311

white in an egg; and the determiner for white in a sperm will be combined
with the determiner for white in an egg. If we let R stand for red and w for
white, then the character of the resulting zygotes will be indicated by RR,
Rw, and ww.

Construct a diagram to show
the chance combinations of this
experiment. (Fig. 183.)

How do new forms of
plants originate? In study-
ing heredity in hybrid plants
and their offspring we found
that determiners for char-
acters are handed down to
offspring in a perfectly defi-
nite way. In all our dis-
cussion with regard to
change in plants due to
hybridization we have not
accounted for the appear-
ance of new characters
which were not present in

the ancestry in any visi- FlG - !85. Different types of leaves on the
U1 f T j.u i*/m same branch of scarlet oak. The lower

bleform. In the year 1699 shaded leaves are mesophy tic; those above
there was introduced into in the sun are xerophytic. These are modi-
Holland and England from fications due to environment and not to
Sicily a sweet pea species, heredity.

Lathyrus odoratus, the flowers of which were purple and blue
in color. In the descendants of this plant there appeared, in
1718, a plant which produced white flowers and in 1731 another
I which produced flowers which were pink and white. In the
generations which followed, the seeds from the white-flowering
plants continued to produce plants which bore only white flowers
and the seeds from the pink-and- white flowering plants, produced
plants with only pink-and-white flowers. From this original plant
species there have been produced hundreds of varieties of sweet
peas bearing flowers differing in size, form, and color, and in
each case the offspring bears flowers like those of its parent; that
is, the plants breed true.

312 DEVELOPMENT AND IMPROVEMENT OF PLANTS

Any variation in plants, such as those in color, size, and form
in sweet peas, which passes from parent to offspring unchanged is
known as a heritable variation. Dwarfness in the midget sweet
pea and in the low-growing nasturtiums is a heritable variation.
Variations due to differences in the conditions of the environment,
as differences in size and shape of oak leaves on the same tree,
are known as fluctuations. These variations are not inherited.

, FIG. 186. The weeping birch is a mutation.

The white sweet pea which appeared from purple-and-blue
stock in 1718 and the pink-and-white one that appeared in 1731
from the same parent stock are examples of mutants or mutations.
Mutation has furnished many plants of value to man. From the
small currant tomato or love apple of a hundred years ago have
developed by mutation many of the varieties of tomato which we
know today, some of which produce fruit weighing more than a
pound.

Sometimes a branch has appeared which produced fruit or
foliage differing in marked degree from that of the main plant.
In this way the Boston fern and its many varieties have appeared.

CAUSE OF PLANT CHANGES

313

All the improved varieties of the seedless orange have originated

from buds. These variations are known as bud gutatipns.^

There are reasons for believ-
ing that many mutations are tak-
ing place in nature. Most of these
changes are probably of no ad-
vantage to the plant, or they may
even prove a disadvantage. These
new forms may not be able to
compete successfully with other
plants of the environment and as
a result soon go out of existence.
On the other hand, a mutation
may be better fitted than its
parents to meet the conditions of
the environment so that in the
struggle for existence it succeeds
and may eventually crowd out
other plants. This principle is
known as natural selection. Mu-
tations appear, and they succeed
or fail according to whether or
not they are able to compete with
other plants in their habitat.

FIG. 187. -The Foray thia branch
on the right shows the regular
Forsythia habit of bearing two
opposite leaves at a node. The
branch on the left is a bud muta-
tion from the same plant bearing
three leaves at each node.

Exercise 138. How do fluctuating characters differ from hereditary
characters? 1. Compare leaves of mulberry which have grown in the sun with
leaves from the same tree which were shaded. Note especially size and shape
of leaves and their thickness and texture. Make drawings to show differences
in size and shape. How do the sun leaves of the mulberry differ from shade
leaves? How do you account for the differences?

2. Examine pine needles and leaves of the rubber plant, a xerophytic
grass, and Sedum or live-forever. Note especially the thickness, leaf surface,
and texture of a number of leaves of each kind. Does there seem to be any
marked variation between the different pine needles, between the different
rubber-plant leaves, between the different grass leaves, or between the different
succulent leaves of Sedum? Characters like the ones here noted that are
handed down from parent to offspring are known as heritable characters.
Any marked change which is heritable is known as a mutation.

Exercise 139. How do new forms of plants originate? A. The common
cabbage and a number of other varieties of plants have developed from the

314 DEVELOPMENT AND IMPROVEMENT OF PLANTS

FIG. 188. Two heads of the red sunflower (at sides) and a head of the ordinary

yellow sunflower (center). The red sunflower is a mutation. (Photograph

furnished by T. D. A. Cockerell.)

wild cabbage, a native of Europe. From seed catalogues select a list of
plants as Brussels sprouts, cauliflower, etc., which have come, or seem likely

to have come, from the wild form of cabbage.

What part of the plant, as stem, root, flower,

is useful to man in each one of the varieties

listed?

B. Make a comparison of characteristics
of different varieties of chrysanthemums listed
in a flower catalogue of some prominent seeds-
man. In what ways do these vary from the
Chrysanthemum indicum, one of the original
parents. See Fig. 180.

C. Make a list of vegetables and another
of flowers designated as hybrids in the seed
catalogues.

D. Make lists of plants which are prop-
agated vegetatively. Which are more apt
to come true, plants from seeds or plants
from some vegetative part as a tuber or
bulb?

FIG. 189. Chrysanthemum
plant showing bud muta-
tion. This plant of a white-
flowering variety produced
a branch which bore pink
flowers.

NEW KINDS OF PLANTS 315

Problem 4. How does man develop new kinds of plants?

Many of our most widely cultivated plants have been in the
process of development for so many centuries that we know little
of their wild ancestors. A small-grained variety of wheat was
found among the remains of the lake dwellers in Switzerland which
dates from the early stone age.

How have common crop plants originated? Indian corn or
maize is a New World product. When the white man first visited
America he found the Indians making use of maize as a crop plant.

FIG. 190. The wild potato of southwest United States (Solanum jamesii).
(After Fitch, Colo. Agr. Exp. Sta., from Robbins, in Botany of Crop Plants.)

We know of no wild form which can be recognized as the immediate
ancestor of this important crop plant.

The cultivation of the apple dates back at least as far as that
of wheat. The varieties which were used in very early days were
small, measuring an inch to an inch and a quarter in diameter.
This wonderful fruit seems to be a native of southeastern Europe.
The apple was widely distributed in Europe in its uncultivated
state, but the fruit was much inferior to that of the cultivated
varieties of the present. There are at least five native crabapples
in America. These wild apples are fit only for jelly-making and
for cooking. The Indians made some use of the wild apple, and
when the white man introduced the common apple, they were

316 DEVELOPMENT AND IMPROVEMENT OF PLANTS

quick to take advantage of it. Remains of old Indian orchards
still exist in different parts of the country.

You have all enjoyed the luscious fruit of the peach. Have
you tasted the fruit of the nectarine? An interesting riddle is con-
nected with the history of these two' fruits. The peach is readily
recognized by its well-known characteristics, including a downy
covering. A peach tree may be producing crops of peaches year
after year and all of a sudden it may begin to produce nectarines,
similar to the peach but without the downy covering. Afterwards
the tree or a part of it may produce either peaches or nectarines.
The nectarine is known as a bud sport or bud mutation of the
peach. Bud mutations have been responsible for the appearance
of some valuable varieties of plants which have been retained by
man and propagated by vegetative methods.

The orange belongs to the group of citrus fruits. The wild
variety produces fruits which are bitter. The sweet strains have
been long in cultivation. The seedless orange arose as a bud muta-
tion; it has been maintained by vegetative propagation and im-
proved by that method and by selection.

The grape of our western coast came from Old World stock;
that of eastern United States originated from native species of
wild grapes. The more than fifteen hundred varieties of European
grapes have all descended from a single species, Vitis vinifera,
supposed to be native of Asia. The early settlers of America
made persistent attempts to establish vineyards of the European
grapes in the new country, but without success. The failures
were due to an insect pest, Phylloxera, a plant louse which infests
the roots and to which the European grape was especially sus-
ceptible. This pest was later introduced on some nursery stock
into Europe where it played havoc with the vineyards. The prob-
lem was finally solved. Someone noticed that certain native
American species were almost immune to the attack of the phyllox-
era. Grape vines were started from pieces of vine of these
immune plants, then the European Vitis vinifera was grafted on
these plants. The grapes grown on our Pacific coast are varieties
developed from Vitis vinifera. Is this method of producing im-
mune grapes vegetative or sexual?

The grapes of the eastern part of the United States have all

NEW KINDS OF PLANTS

317

been developed from native American species. The original
Concord grape vine is still growing in Concord, Massachusetts.
It developed as a chance seedling from the native Labrusca species,
the northern fox grape. Was the method used in producing the
Concord grape vegetative or sexual?

How are desirable characters of parent plants combined in
offspring? In our discussion of Mendel's law only one set of con-
trasting characters was considered in the example taken from the

Parents

Sperms

V)

(Yw)

(gw)

FIG. 191. Diagram showing the results of self -pollinating a pea plant which
carries the determiners for two sets of contrasting characters, in this case,
yellow and green, and round and wrifikled (YgRw)} or of crossing two of the
plants. Zygotes 1-9 will produce yellow, round peas, since determiners for the
dominant characters, yellow and round, are present in each; 10-12 will pro-
duce yellow wrinkled peas, as the determiner for dominant yellow is present,
that for dominant round is not present, and that for recessive, wrinkled is
present. Why will 13-15 produce green, round peas, and 16 green, wrinkled

peas?

318 DEVELOPMENT AND IMPROVEMENT OF PLANTS

experiment with peas. These characters were round and wrinkled.
If now we take a second pair of contrasting characters into our
experiment the situation becomes much more complex.

The characters actually chosen by Mendel in one set of experi-
ments with peas were yellow-round and green-wrinkled; that is,
one of the parent plants selected produced peas which were yellow
and round, and the other plant produced peas all of which were green
and wrinkled (Fig. 191.) In this case when the plants were cross-
pollinated, one set of gametes, say the sperms, carried determiners
for yellow and round and the other set of gametes, the eggs, car-
ried the determiners for green and wrinkled. The zygote which
resulted from the union of these two gametes contained the de-
terminers for yellow, round, green, and wrinkled. Since yellow is
dominant over green, and round over wrinkled, the peas of this FI
generation were all yellow and round in character.

In Mendel's experiment the seeds of this FI generation were
planted and the cells of the hybrid plants which resulted all con-
tained the determiners for the characters yellow, round, green,
and wrinkled, and when these plants produced gametes, combina-
tions of determiners present were as follows : YR, Yw, gR, and gw.
The table (Fig. 191) shows the different possible zygotes. Since
yellow and round are dominant over green and wrinkled, these
characters, when their determiners are present, will show in the
peas produced regardless of whether or not the determiners for
green and wrinkled are present.

It is seen, then, that in general, out of every sixteen dihybrids
resulting from this cross, nine were yellow and round, three green
and round, three yellow and wrinkled, and one green and wrinkled.
The characters of the parents for color and nature of seed coat
were combined in different ways in different individuals. The
keen eye of the practical plant breeder is able to select from thou-
sands of hybrids the individual plant which may have the desired
combination of characters.

If the selected specimen is a potato or fruit or other plant which
may be propagated by vegetative means, the offspring will all
have the desired combination of characters. If, on the other
hand, the plant can be propagated only from seed, certain offspring
of the dihybrid will show the desired combination of characters,

NEW KINDS OF PLANTS

319

but other combinations of characters will also appear in others of
the offspring.

Suppose we desire the combination, yellow-wrinkled in our
plant. We select only yellow wrinkled seeds. Seeds YwYw, of
course, will produce only plants which bear yellow wrinkled peas.

(Yw;

(Yw)

(Yw)

FIG. 192. Results of self-pollination of a plant from the YwYw zygote. All
of the offspring are pure yellow wrinkled like the parent.

But some of the yellow wrinkled seeds are of the Ywgw kind.

If the Ywgw seeds are planted, some of the peas which result
will be green-wrinkled. However, by continued selection and
isolation that is, by pedigree culture it is often possible to
secure dihybrids which will breed true with respect to the desired
combination of characters.

Sperms-

19W)

(Yw)

(gw)

Zygolres

FIG. 193. Results of self-pollination of a plant or crossing two plants from

the Ywgw zygotes. J of the offspring are pure yellow wrinkled (YwYw);

\ are pure green wrinkled (gwgw); and ^ are yellow wrinkled (Ywgu), but,

like the parent, are not pure.

In the same way trihybrids may have the combination of three
desired characters from parents which differ from one another in
three respects.

The highly developed technique of Burbank made it possible
for him to produce a trihybrid, Burbank's shasta daisy, in which
he secured a combination of characters belonging to three distinct

320 DEVELOPMENT AND IMPROVEMENT OF PLANTS

parent stocks, American, English, and Japanese. The American
daisy, or marguerite, is a very free bloomer, but it has a sprawling
habit and unattractive leaves. The English plant is attractive
and sturdy and with handsome leaves. The Japanese type has
attractive lustrous flowers. Burbank's attempt to combine in
one plant all the desirable features of the three resulted in the
shasta daisy.

What methods has man used in the improvement of plants?
The most common method of plant improvement is the continuous
selection of the best for seed. This can be illustrated by the
practice of any practical corn-grower who goes into the field of
mature corn and picks seed for the next crop. He selects for
certain characters, as desirable stalk growth, long kernels, ears
well filled out to the tip, and earliness of maturity. In this way
an artificial selection of the best is made generation after genera-
tion. This method, known as mass selection, is effective in
improvement, but not for originating new forms.

The more recent method of plant improvement was that
which was introduced by Gregor Mendel in 1865 and which came
into general use in the study of heredity and plant improvement
thirty years later as the science of genetics. This method is known
as pedigree culture. Whereas mass selection secures a good
average plant, pedigree culture is concerned with the selection
of the individual plant with a view to the development of a
superior plant strain. This is the method' used in the develop-
ment of strains of plants which are drought-resistant or disease-
resistant or resistant to damage by frost.

Thirty-five years ago, a very destructive chestnut bark disease
was introduced into America on nursery stock. The American
chestnut is threatened with extinction from the ravages of this
disease, as it is especially susceptible. It has been found that a
Japanese species and also a Chinese species of chestnut are resistant
to the attack of the disease. Since the European as well as the
American species of chestnut is susceptible, the future of the
chestnut in the world rests with the use and improvement of the
resistant strains by hydridization. In fact, a resistant strain
has been produced already by crossing the American bush Chin-
quapin with the Japanese variety of chestnut.

NEW KINDS OF PLANTS

321

Pedigree culture is illustrated also by the development of a
watermelon resistant to the destructive wilt disease. The citron
Citrullus vulgaris is immune to wilt but inedible. The Eden
watermelon is edible, but susceptible to wilt. The two were
crossed, producing in the first generation hybrids of wonderful
vigor and productiveness. The second generation was extremely
variable, the citron characters appearing to be dominant. From
4000 plants, ten fruits were selected. Seeds of these were planted,

FIG. 194. Artificial pollination. The flowers to be cross-pollinated are emas-
culated, and then kept covered with a bag until the pistils are pollinated and
the stigmas have passed the receptive stage. (From Division of Pomology,
College of Agriculture, University of California.)

and after five years of selection the desired melon, combining wilt
resistance and edibility, was secured.

The characteristic noted in the F\ hybrid melons is known as
hybrid vigor. Frequently the hybrids resulting from a cross
between two strains show marked increase in vigor and pro-
ductiveness over those of either parent. Burbank made use of
the phenomenon of hybrid vigor in the production of a new walnut

322 DEVELOPMENT AND IMPROVEMENT OF PLANTS

tree by crossing the English walnut with the California black
walnut. The hybrid which resulted grew in 14 years to be 75 feet
tall and to have a trunk 2 feet in diameter while a tree of the
black walnut parent type grew a trunk only 6 or 8 inches in
diameter and attained a height of only 20 feet in 31 years.

Suggested activities. 1. Write a summary of the history of some fruit, as
the Delicious or Jonathan apple.

2. Write a revitw of a book describing the work of Luther Burbank.

3. Write a paper on the importance of the work of Gregor Mendel.

ADDITIONAL QUESTIONS AND EXERCISES

1. What reasons have we for thinking that the earliest forms of life on the
earth were plants?

2. Why is it more likely that the earliest plants lived in water than that
they lived on land?

3. In what ways did plants change in form in changing from a water habitat
to a land habitat?

4. What is a fossil plant?

5. In what different ways are fossils formed?

6. It is known that limestone was formed at the bottom of the sea. How
can you explain the fact that great limestone deposits are found at present far
inland?

7. If you have visited a " petrified forest " prepare a report to be read to
the class describing what you saw.

8. Under what conditions could mineral deposits take the place of the wood
and form the so-called petrified wood?

9. How do you explain the fact that we find fossils of plants which are
different from any plants now living?

10. How can you explain the fact that fossils of tropical plants are found
in regions which now have a temperate climate?

11. Explain the fact that vast coal deposits are found in Alaska.

12. Which plant species are more likely to change with a changing environ-
ment, those which reproduce only by vegetative methods or those which
reproduce sexually? Explain.

13. Explain why the two daughter cells resulting from simple cell division
are similar and also similar to the mother cell.

14. How does a knowledge of Mendel's Laws of Heredity Jielp man in pro-
ducing improved varieties of plants?

15. How does a farmer use mass selection hi selecting seed corn from his
fields?

UNIT IX
THE CLASSIFICATION OF PLANTS

We learned in the last unit that during past geologic times
the character of the plant life of the earth has changed; that in
the course of time certain species of plants have become extinct;
that new species have come into existence; that there has been
constant change. Scientists are convinced that the present is
the offspring of the past; that the plants which populate the
earth today have characteristics like those of the past, because
they are offspring of plants of the past. But it appears that,
although there has been increasing change for centuries and cen-
turies, certain plants in existence today resemble very remarkably
those of early geologic times. We refer particularly to such
aquatic plants as the algae (pond scums, seaweeds). Undoubt-
edly, the reason for this lack of change of such plants is that water
is a uniform environment, subject to very little change in tempera-
ture, in oxygen and carbon dioxide content, and in its mineral
composition. Water of the earth has been very much the same
throughout geologic history. Consequently, the plants of water
have not undergone much change during that time. Land environ-
ments, on the other hand, are extremely variable. Different types
of soil, varying in texture, in chemical composition, in exposure
to the sun's rays, in temperature, in water-holding capacity, pre-
sent different sets of conditions under which plants must grow.
And we find that the kinds of plants which grow under these dif-
ferent conditions are quite dissimilar. We are forced to the con-
clusion that differences in environmental conditions have been
responsible for inducing the changes which have occurred in
plants. So today we find on the earth's surface a great assemblage
of plants differing greatly in their form and structure and habits
of growth, populating all types of environments salt water, fresh

323

324 THE CLASSIFICATION OF PLANTS

water, cold water, hot water, sandy soils, clay soils, lime soils,
humus soils, wet soils, dry soils, rocky exposures, arctic and alpine
regions, tropics, and so on. And these plants grow under the
particular set of conditions they do because they have structures
and habits of growth which adapt them to these conditions.

There is strong evidence that life as we know it on this earth
originated in the water. The first forms of animals and plants
were water forms. Water plants very similar, probably, to cer-
tain simple algae found in the waters of the earth today were the
ancestors of our present-day plants, both of the water and of the
land. As the surface of the earth changed, as climates changed,
new kinds of plants came into being. But these new kinds re-
sembled their parents; and, too, they differed from their parents.
Slight modifications in form or behavior may have enabled certain
offspring to live and reproduce under slightly different conditions
from those to which their parents were accustomed. After many,
many generations the individuals may come to be very unlike
their ancestors of ages past.

All plants have much in common. This is one reason we
regard them as related. Pond scums, molds, mildews, mosses,
ferns, pines, roses, potatoes, oaks, and wheat all have certain
very definite characteristics which indicate a common ancestry.

Their cell structure is the same; the living stuff, protoplasm,
is essentially similar; they respire, nourish their bodies with the
same kinds of food, digest food, and reproduce in very much the
same manner. Living things which have so many likenesses in
their fundamental structures and processes, even though they may
look unlike, must certainly be related.

Problem 1. How are plants classified?

There are some 250,000 different kinds of plants in the world.
We call these different kinds species. We recognize different
degrees of relationship among them. For example, we recognize
that different kinds of oaks are more closely related to each other
than are oaks to maples. It is likely, we would assume, that
apples and pears are closer " kinfolks " than are apples and wheat.
And it would be safe to say that the different kinds of flowering

PLANT CLASSIFICATION 325

plants are more closely allied than are flowering plants and mush-
rooms.

From the earliest times man has attempted to classify plants,
just as he has attempted to classify all sorts of things, even his
ideas. One of the early attempts to classify the common plants
about us, before the days of the microscope and the detailed
knowledge of plant structure, was based upon habit of growth.
This was a classification into three large groups, namely trees,
shrubs, and herbs. This was a sort of artificial or arbitrary
system of classification. Study revealed the fact that bamboo
(a tree) and corn (an herb) were really more closely related than
bamboo and cotton wood, both trees; and, as another example,
the strawberry (an herb) is more closely allied to the rose (a shrub)
than are roses and sagebrush (both shrubs). The reader will
readily think of other examples. An attempt has been made in
grouping plants to place together those with fundamental like-
nesses; with similarities in structure which represent true rela-
tionships. The student readily understands that frogs, grass-
hoppers, and kangaroos are not placed in the same natural group
just because hopping about is common to them. The body form,
methods of reproduction, and all life habits are very greatly
different. But frogs and toads are closely related; kangaroos
and opossums are closely allied; and grasshoppers and locusts
are very much alike.

What then are the characters, among plants, which express
true affinities? How do we tell that one plant is near in its rela-
tionship to another? Is it the kind of root system they have?
Is it the sort of stem they have? Is it the kind of environment in
which they live? None of these. Scientists have learned to
place reliance in reproductive structures and behavior as marks
of true relationship. For example, beans, peas, clover, alfalfa,
and peanuts are naturally grouped together because the flowers
(reproductive structures) are built on the same general plan;
goldenrods, chrysanthemums, daisies, sunflowers, and thistles are
grouped together also because of very similar floral structure.

Space will not permit a discussion of the various systems of
classification of plants which have been proposed during the last
several centuries. Suffice it to say that one system has replaced

326 THE CLASSIFICATION OF PLANTS

another as new facts about the plants of the world have come to
light; and that system is considered most perfect which most
accurately expresses the true affinities of plants.

Problem 2. What are the four great groups of plants?

One modern system of classification which has been widely
accepted would throw all plants of the world into four large groups,
called phyla or grand divisions. These four large groups are as
follows:

1. Thallus plants (thallophytes), such as the pond scums,
.seaweeds, bacteria, molds, mildews, yeasts, rusts, smuts, mush-
rooms, and toadstools. The plants belonging to this large group
have no roots, stems, or leaves in the ordinary sense; they have
no flowers, the reproductive organs and methods of reproduction
being very simple. Certain thallus plants are regarded as simple
and primitive, that is, the first kind of plants to appear on the
earth; in fact, they are believed to be the progenitors of more
complex plants.

2. Mosses and liverworts (bryophytes). These are well-
known plants growing in moist places. Usually, they are close to
the soil, that is, in contact with a source of water. No part of the
plant is very far away from the soil, and hence there is very little
need in the plant for water-conductive tissue. However, moss
leaves have strands of long cells which may serve as channels of
water movement, but such channels never reach the degree of
specialization that they do in trees and shrubs. Mosses and
liverworts are regarded as being more advanced than thallus
plants. Not only have they more complex vegetative structures,
but also the methods of reproduction are more Jn'ghly specialized.

3. Ferns, horsetails, and club mosses (pteridophytes). This
is a large assemblage of plants which have structures enabling
them to live on the land, and bring their leaves up in the air,
distant from the immediate source of water the soil. In order
to do this there must be in the stems and leaves a certain amount
of strengthening tissue, and also special conductive tissue for the
rapid movement of water, mineral salts, and foods. This group
of plants the pteridophytes includes the first vascular plants,

CLASSIFICATION OF SEED PLANTS 327

that is, plants with vascular bundles. As stated, mosses and
liverworts do not have vascular bundles. In this particular, as in
methods of reproduction, they are more primitive than pterido-
phytes. None of the thallophytes, bryophytes, or pteridophytes
have seeds.

4. Seed plants (spermatophytes). Of the four large groups of
plants, this one has the greatest number of species, and has most
successfully occupied the surface of the earth. The one outstand-
ing characteristic of the group is the seed-bearing habit. The
seed is essentially a young embryo plant, in a dormant state, sur-
rounded by protective coats, and accompanied by a reserve of
food. Thus, the young plant in the seed may live for years.
Moreover, seeds often have devices, such as wings, barbs, prickles,
etc., which facilitate their spread over the earth's surface. In
addition to the seed-bearing habit, members of this group have
developed extensive vascular systems, and strengthening tissues,
enabling them to attain great heights, as witness the tall trees of
the forests.

Problem 3. How are the seed plants classified?

Each of the four large groups of plants, briefly described
above, is divided into subgroups, and these in turn into smaller
groups, and so on. As an illustration, let us consider a seed plant,
such as common wheat. Wheat belongs to the grand division
of the plant kingdom known as the spermatophytes. It is a seed
plant. Among seed plants there are two very distinct sub-
groups, which we will call classes. There are those seed plants,
such as pines, spruces, firs, cedars, etc., which do not bear flowers,
as we ordinarily understand that term, and which have naked
seeds, that is, seeds without any surrounding covering except
the seed coats. This class is called gymnosperms. And there
are those seed plants which have flowers, and seeds borne in a
case, such that at some part of its life the seed or seeds have a
surrounding covering in addition to the seed coat. This class is
called angiosperms. Common wheat falls into this second class.
It is an angiosperm. But the angiosperms is a very large group
of plants. Study of large numbers of plants belonging to the

328

THE CLASSIFICATION OF PLANTS

group has shown that all of them fall naturally within two sub-
classes. In one subclass, the so-called monocotyledons, there is
but one seed-leaf or cotyledon in the embryo and seedling; in the
other subclass, the so-called dicotyledons, there are two seed-
leaves or cotyledons in the embryo and seedling. Common
wheat is a monocotyledon. This subclass of plants, as all other
subclasses, is subdivided into smaller groups, called orders.
For example, wheat belongs to the order Graminales y one including

FIG. 195. First-year and second-year carpellate cones of pine (left), and
staminate cones (right).

all .grasses and sedges. The plants belonging to this order have
certain outstanding characteristics, such as a large supply of food
stored in the seed, and inconspicuous flowers with surrounding
scales, and no petals or sepals. All orders of plants are subdivided
into families. An order may possess few or many families. For
example, the order Graminales has but two families, Grarnineae
(grasses), and Cyperaceae (sedges, rushes, etc.). Wheat is a
grass (family Gramineae). All plant families are subdivided into
genera (singular, genus). For example, in the grass family there
are such common genera as Triticum (wheat), Avena (oats),

THE SCIENTIFIC NAME 329

Hordeum (barley), Zea (corn), etc. As a rule, in any genus of
plants there are a number of different species. Common bread
wheat is Triticum aestivum] Polish wheat is Triticum polonicum;
durum wheat is Triticum durum] etc. So, in the system of
classifying plants now in use, the common bread wheat is a plant
which has characteristics such as to place it in the genus Triticum,
which in turn is a member of the grass family (Gramineae) ; this
belongs to the grass order (Graminales) ; this, one of the orders
of the larger groups, belongs to the subclass, monocotyledons; this,
in turn, belongs to one of the subdivisions of the class angiosperms,
or flowering plants; and this class belongs to the grand division,
spermatophytes.

The system of classifying as applied to common wheat may be
shown in diagram form as follows :

Grand Division Spermatophytes
Class Angiosperms

Subclass Monocotyledons
Order Graminales
Family Gramineae
Genus Triticum
Species aestivum

Problem 4. What is a scientific name?

In giving the scientific name of bread wheat, for example, we
use both the generic and specific names: Triticum aestivum.
Triticum aestivum is a binomial; that is, it consists of two names,
Triticum and aestivum. Triticum, alone, refers to all kinds of
wheat. Triticum is the genus to which all wheats belong. A
certain kind or species of wheat, the common bread wheat, having
certain well-defined characteristics, is called Triticum aestivum,
aestivum being the specific name. However, aestivum alone
means nothing; it must be joined with the name of a genus in
order to refer definitely to a certain kind of plant.

Every known different kind of plant in the world has been
given a scientific name which is the same in all languages. Com-
mon names vary from country to country. For example, whereas

330 THE CLASSIFICATION OF PLANTS

the English know the great bread cereal as wheat, the Germans
call it Weizen, the French ble, etc. ; but the world over in scientific
language it is Triticum aestivum. From the scientific standpoint
the advantage of this is apparent. If one looks through any
nursery or seed catalogue, he will note that reference is made to
many plants, not only by their common names, but by their
scientific names as well.

Whenever a new species of plant is discovered somewhere in
the world, some botanist (systematic botanist), usually a specialist
in the group to which the plant belongs, writes a description of it,
and gives it a name. This description and name are published
in some one of the many scientific journals of the world, and the
specimen from which the description was made constitutes a type
specimen and is filed in some herbarium. It should be stated
that, of the hundreds of thousands of different species of plants
known to man, there exists somewhere in published form a
description of each. The individual who describes a new species
places after it his name or an abbreviation of his name. For
example, when we see the scientific name of common oats written
Avena saliva L., we know that the " L." is the abbreviation for
Linnaeus, an early Swedish botanist, who first described common
oats.

Suggested activity. Using a plant manual or nursery catalogue, record the
scientific names of 20 common plants. Find out from an unabridged dictionary
the derivation and meaning of the specific names. Are they descriptive of
some character of the plant, or of its habitat, or of its distribution?

Problem 5. What do we mean when we speak of " simple
plants " and " complex plants "?

Bacteria and blue-green algae are among the simplest of
plants because their whole body consists merely of a single cell,
or groups of similar cells. There are no special organs to perform
this and that function. All the activities of the plant body are
carried on in the single cell. Moreover, the cells themselves are
simple in structure, in that they have no definite nucleus, no
plastids, and but few special and definite cellular structures.
We would regard the plant body of Spirogyra, for example, as

SIMPLE AND COMPLEX PLANTS

331

more complex than that of a blue-green alga or a bacterium. In
each Spirogyra cell there is a definite nucleus and a well-defined
plastid or plastids, and the cells that are joined end to end to
make up the plant body do not all behave alike, for some of
them form reproductive bodies, whereas others do not. Thus,
there is within the Spirogyra plant body some differentiation,
which leads us to believe that Spirogyra is a more complex plant
than any of the blue-green algae. In still so-called higher, that is,
more complex, algae, there are special cells which act as hold-
fasts, others which produce male reproductive organs, others
which may act as protective organs. Thus there is further
differentiation or increase
in complexity.

As another illustration
of the difference in the com-
plexity and degree of ad-
vancement of plants, let us
consider flowering plants.
Now, there is no evidence
that all the known kinds
of flowering plants in the
world today came into ex-
istence at any one time.
Quite the reverse is true.
There wereflowering plants
with characters that we
regard as " primitive/'
and geologic record lends

evidence that such plants appeared on the surface of the
earth earlier in time than the more " advanced " kinds. More-
over, these " primitive " flowering plants are the ancestors of
those which came after them. Primitive flowering plants, for
the most part, had flowers with separate carpels with superior
ovary, with numerous stamens, with regular symmetry, and with
separate petals. The ordinary buttercup is a flower of this type.
As time passed, and development among flowering plants took
place, it is evident from a many-angled study that there were
certain tendencies in the development of flowering plants; the

Fia. 196. The orchid is one of the most
advanced of the flowering plants.

332 THE CLASSIFICATION OF PLANTS

direction of development was along rather definite lines. For
example, there was a tendency for development from separate
carpels to united carpels; from separate petals to united petals;
from regular flowers to irregular flowers; from numerous stamens
to a definite number of stamens (usually less than ten) ; and from a
superior ovary to an inferior ovary. The harebell has all the
" advanced " characteristics just noted, and for those reasons
would be considered a type of flower which has progressed farther
in its development and degree of complexity than the buttercup.
There are many other tendencies besides those mentioned which
enable us to judge of the relative complexity or advancement of
flowering plants. For example, wind pollination is usually asso-
ciated with more primitive flower types than is insect pollination.
So, as botanists have studied the great array of different flower-
ing plants which populate the earth, they have attempted, after
thorough study of all their characteristics, to place them in groups
and subgroups which show their actual affinities, or relationships,
or degree of advancement. In other words, botanists have
attempted the construction of natural systems of classification.
For example, the arrangement of different flowering plants into
certain classes, orders, families, genera, and species is by no means
an arbitrary one, made for man's convenience, but the particular
arrangement adopted is one which conforms to the natural rela-
tionships of the plants considered.

Suggested activities. 1. Grow different kinds of algae in the laboratory or
at home. Much can be learned of the nature of these simple plants by setting
up suitable conditions and growing the plants indoors. Collect pieces of
bark showing a green coating of Protococcus from the north side of trees in
the woods. Place the pieces of bark, green side up, in a soup dish, moisten,
and cover with a pane of glass. The light conditions of a north window are
suitable. Moderate temperature and light conditions and moist air are neces-
sary for the growth of the Protococcus on the pieces of bark. Scrape a small
amount of the green material and mount in water under a cover-glass. Exam-
ine with the low and high power of the microscope. What characteristics place
Protococcus in the thallophyte group?

Collect green masses of plant material found floating free or attached to
sticks and stones in streams or ponds. Arrange jars in moderate light in
the laboratory, and place in separate jars a small quantity of each specimen of
alga with water from the pond or stream in which it was found growing. Cover
each jar with a pane of glass and let it stand for observation. What character-

SIMPLE AND COMPLEX PLANTS 333

istics do you observe without or with the aid of the microscope which place
these algae in the thallophyte phylum?

2. Make a collection of dry fungi and arrange as a laboratory demonstration.

3. Grow mosses in the laboratory or at home. Make a collection of various
kinds of mosses, and arrange growing conditions for them as follows: Place in a
glass aquarium 2 inches of rich woods soil and over this arrange the different
mosses which you have collected. Moisten the soil and mosses well and cover
with a pane of glass. A north window affords suitable light for most mosses.
Note the characteristics which place mosses in the Bryophyta.

4. Make a collection of small fern plants and establish a fernery in the
manner described above for the mosses.

5. Prepare a laboratory or class demonstration of an entire fern plant
showing spore-bearing and other fronds, underground stem, and roots. Why
does the fern belong to the pteridophyte group?

6. Make a collection of seeds of dicotyledons, as bean, pumpkin, and
cucumber.

7. Make a collection of seeds of grasses (monocotyledons), as corn, oats,
and wheat.

SELECTED REFERENCES

Flower Families and Ancestors, by F. E. and E. S. CLEMENTS, published
by H. W. Wilson Company, New York, 1928. 156 pages, 64 illustrations.
This includes a full-page colored flower chart. "The present book has been
written in the hope of making the study of flowering plants both simple and
attractive to beginners of all ages." Among the interesting topics are the
following: The family tree, the work of flowers, standardized methods in
pollination, efficiency in flowers, evolution and relationship of flowers.

Flowers and Flowering Plants, by RAYMOND J. POOL, published by
McGraw-Hill Book Company, New York, 1929. 378 pages, 191 illustrations.
An introduction to the nature and work of flowers and the classification of
flowering plants.

UNIT X
THE ECONOMIC IMPORTANCE OF PLANTS TO MAN

We have learned that all animal life on the earth, including
man, is dependent upon green plants. Green plants are the only
organisms on this earth which possess the power of converting
the energy of light into food. All non-green plants and all animals
derive their food directly or indirectly from green plants. Thus,

FIG. 197. Trees and other plants help to make this country place a real farm

home.

the very life of man on the earth depends upon the activity of green
plants.

The great civilizations of the world have developed where
natural conditions favored the cultivation of certain food plants,
chiefly cereals. Rice, wheat, corn these have been the three
most important food plants which have made possible the develop-
ment of three great civilizations : (1) that which spread over eastern
Asia, Japan, the Indian Archipelago, the Malay Peninsula, and

334

FOOD PLANTS OF THE WORLD 335

the Philippine Islands was dominated by rice; (2) that which
developed in western Asia, northern Africa, and Europe had wheat
and related cereals as its chief food plants; and (3) the physical,
social and religious life of the Mayas, Aztecs, Incas, Guatemalans,
Peruvians, and other aboriginal American peoples was based on
maize or Indian corn.

The plants of economic importance to man fall into two large
groups, namely, (1) those that are useful, and (2) those that are
harmful, or interfere with man's operations. The number of
products of plant origin is enormous ; those useful to man in one
way or another may be grouped as follows: foods; industrial
plants including wood, coal, cork, fiber, resins, and turpentine,
gums, plant dyes, fixed and volatile oils, and rubber; medicinal
plants; and ornamentals. Those plants which interfere with
man's operations include weeds, poisonous plants, hay-fever plants,
and those which cause plant and animal diseases.

Problem 1. What are the principal food plants of the world?

The principal food plants of the world include the cereals,
fruits, nuts, vegetables, beverage plants, sugar plants, and spices.

Cereals. The principal cereals are wheat, oats, barley, rye, rice,
corn (maize), sorghums, millets, and buckwheat. All the cereals,
with the exception of buckwheat, belong to the grass family
(Gramineae). A cereal is a grass grown for its grain. The great
importance of cereals is due to the fact that a large reserve of food
is stored in the seed. Starch is the chief food reserve of such seeds.

By far the largest proportion of the world's supply of flour is
made from wheat. There are a number of economic types of
wheat, chief of which are common bread wheat, durum, club, and
emmer. Durum wheats are used extensively in the manufacture
of macaroni, spaghetti, and semolina. Emmer is used as a feed
for livestock, and to some extent in the manufacture of breakfast
food. Common bread wheat and club wheat are the ones ordi-
narily used for flour. Oats are consumed in large amounts in the
form of rolled oats or oatmeal. It is also valued as horse feed.
Barley has a great variety of uses: preparation of malt, flour,
cereal breakfast foods, stock feed, and hay. Rye flour is made into

336 ECONOMIC IMPORTANCE OF PLANTS TO MAN

FIG. 198. The six principal types of corn. From left to right, pod corn, soft

corn, flint corn, dent corn, sweet corn, and pop corn. (After Montgomery, from

Robbing, in Botany of Crop Plants.)

bread; the grain is fed to stock; the straw finds considerable use
in the manufacture of paper strawboard, hats, and other coarse

FIG. 199. Grain of corn. A, median lengthwise section, cut parallel to broad

surface, of grain of dent corn; B, cross-section of the same through the embryo;

C, section as in A of flint corn. (From Robbins, in Botany of Crop Plants.)

FOOD PLANTS OF THE WORLD

337

straw articles. No other cereal is put to such a variety of uses as
is corn. The grain and fodder are both valued stock feed; in
addition, there are such products as corn meal, cornflakes, starch,
glucose, etc. There are two types of sorghum, the sweet sorghums

FIG. 200. Panicle of rice (Oryza saliva). (From Robbins, in Botany of

Crop Plants.)

from which a syrup is made, and the non-saccharine or grain sorg-
hums, some of which are raised for the grain and others from which
brooms are made, utilizing the flower stalks. Rice is a food for
more human beings than any other grain. The millets are grown

338 ECONOMIC IMPORTANCE OF PLANTS TO MAN

chiefly as a hay crop, for pasturage purposes, and for the seeds.
The principal use of buckwheat is in the manufacture of pancake
flour.

Suggested activities.

1. Locate on an outline map of the United States the principal corn-growing
regions and the principal wheat-growing regions.

2. Prepare a paper on the manufacture of brooms,

3. Prepare a report on the milling of rice.

QUESTIONS

1. What are the differences between " soft wheat " and " hard wheat "?

2. What is the " gluten " of wheat?

3. What is " bran " of wheat?

4. What are the differences between graham, entire wheat, and patent or
straight bread flour?

5. What is meant by spring wheat? Winter wheat?

6. What is the relation between " wild oats " and ordinary cultivated oats?

7. What is the corn silk? The corn tassel?

8. How do you explain the occurrence of different colored grains on an ear
of corn?

FIG. 201. A very old grape vine, in Carpenteria, California. (Photograph
furnished by Division of Pomology, California College of Agriculture.)

FOOD PLANTS OF THE WORLD

339

9. What are the differences between popcorn and other types of corn?
Account for the popping qualities of the former.

10. What are the qualifications of a good malt barley?

11. Name five important breakfast foods and the cereals from which they
are made.

Fruits. In a popular sense a " fruit " is a juicy structure eaten
chiefly for its sweet or acid juice. This is the sense in which it is
used here. Strictly speaking, a fruit is the matured ovary of a

FIG. 202. Northern fox grape, Vitis labrusca; leafy flowering stem. (From
Bobbins, in Botany of Crop Plants.)

340

ECONOMIC IMPORTANCE OF PLANTS TO MAN

flower and, in some instances, other flower parts. Used in this
sense, it would include grains, nuts, and such common " vege-
tables " as peas, beans, squash, etc.

In temperate climates the more common fruits are found in
the following plant families: palm family (date); mulberry
family (mulberry, fig); gooseberry family (gooseberry, currant);
rose family (raspberry, blackberry, dewberry, strawberry) ; apple

FIG. 203. Meserve Avocado growu at Long Beach, California. (Photograph,
courtesy of Professor Ira J. Condit, University of California Branch of the
College of Agriculture at Los Angeles. (From Robbins and Ramaley, in Plants

Useful to Man.)

family (apple, pear, quince) ; plum family (plum, cherry, apricot,
peach, nectarine); citrus family (kumquat, orange, lemon, grape-
fruit, lime); grape family (grape); potato family (tomato);
cucurbit family (watermelon, muskmelon); olive family (olive).

The tropics produce a great many edible fruits. Chief of
them are the banana, pineapple, mango, avocado, and papaya.

FOOD PLANTS OF THE WORLD

341

Others of less importance, at least to us in temperate climates,
are the cherimoya, sugar apple, soursop, loquat, guava, Japanese
persimmon, and mangosteen.

QUESTIONS

1. In southwestern United States, where the date palm is grown, it is usu-
ally propagated by the offshoots rather than the seeds. Explain why this is
the practice.

2. What is the relation of the mulberry tree to the silk industry?

FIG. 204. Date palms, Phoenix dactylifera. Garden of Deglet Noor dates in
full bearing, southern California. (After Nixon, from Robbins, in Botany of

Crop Plants.)

3. In order to grow Smyrna figs it is necessary to introduce into the or-
chard the fig wasp. Explain.

4. What are the differences between currants and gooseberries?

5. How does the blackberry fruit differ from that of the raspberry?

6. What is the loganberry?

7. How is vinegar made from apples?

8. What is a prune?

9. What is the relation of the nectarine to the peach?

10. What is the difference between a peach and a nectarine?

11. What is commercial " citron "?

342 ECONOMIC IMPORTANCE OF PLANTS TO MAN

Nuts, The nut is a fruit, botanically speaking. It is eaten
for the edible kernel which is usually protected by a hard shell.
Nuts are rich in protein and oil. The principal nut-bearing
families are as follows: walnut family (walnut, butternut, pecan
and hickory nut); birch family (filbert or hazelnut); beech or
oak family (chestnut); plum family (almond); pea family

FIG. 205. Pineapple, Ananas sativus (Bromeliaceae), growing in a Florida
garden. (From Gager's General Botany.)

(peanut); palm family (coconut); myrtle family (Brazil nut);
and cashew family (pistachio).

Vegetables. In a popular sense a " vegetable " may either be
a vegetative structure of the plant, such as roots, stems, or leaves,
or a reproductive structure, that is a true fruit, botanically. For
example, the part of the lettuce plant used for food is the leaves,
and of the potato plant, the tubers. In both these vegetables,

FOOD PLANTS OF THE WORLD 343

vegetative parts of the plant are used. On the other hand, the
squash, commonly called a " vegetable/ 1 is in reality a true fruit,
being derived from the flower, a reproductive structure.

The common families producing " vegetables " which are dug
from the soil are as follows: potato family (Irish potato) ; morning-
glory family (sweet potato) ; lily family (onion, garlic, leek, chive) ;
goosefoot family (garden beet); carrot family (carrot, parsnip);
mustard family (turnip, rutabaga); composite family (Jerusalem
artichoke).

FIG. 206. Peanut, Arachis hypogaea] entire plant, reduced. The flower stalks

after pollination grow downward and the fruit is ripened underground. (After

Jones, from Robbins, in Botany of Crop Plants.)

The leafy vegetables include those known as salad plants or
pot herbs. Chief of these are: lily family (asparagus); mustard
family (cabbage, kohlrabi, kale, borecole, Brussels sprouts, water-
cress); goosefoot family (spinach); carrot family (celery, parsley);
composite family (lettuce).

The so-called fruit vegetables include representatives of the
following: cucurbit family (squash, pumpkin); potato family
(tomato, eggplant).

344 ECONOMIC IMPORTANCE OF PLANTS TO MAN

QUESTIONS

1. Why are potatoes grown from the true seeds not true to type?

2. The Burbank variety of potato was developed from the true seed.
How is the variety kept true to type?

3. What is the native home of the potato?

4. What is the principal food stored in the potato?

5. How do ordinary sweet potatoes differ from " yams "?

6. What gives the red color to the common garden beet?

7. Find out what you can about the manufacture of sugar from the
Jerusalem artichoke.

Beverage plants. Each of the three ancient centers of agri-
culture has furnished to the world a valuable non-alcoholic
beverage. Tea (Thea sinensis) is a native of the orient; coffee
(Coffea arabica) came originally from the Mediterranean region;
and the cacoa tree (Theobroma cacao), the source of chocolate,
belongs to the American tropics. Commercial tea is the leaves of
the plant, the coffee of commerce is the seeds, and commercial
chocolate is also the seeds. What is the difference between
" green tea " and " black tea "? What are the chief coffee-
producing countries?

Sugar plants. The world's supply of sugar is derived chiefly
from two plants, sugar cane, and sugar beet. Sugar cane
(Saccharum officinarum) is a member of the grass family, and a
native of the tropics. The sugar beet (Beta vulgaris) is a member
of the goosefoot family, and is grown in temperate climates. The
juice of sugar cane is derived from the stalks, that of the sugar
beet from the roots. The sugar extracted from the juice of these
two plants is identical chemically. It is sucrose or cane sugar
(Ci2H220n). Do you think there is any difference between
sugar from the beet and that from the cane in sweetening power,
or in its behavior when used in making candy, ice-cream, canned
fruit, jellies, and preserves?

The sap of the sugar maple tree also supplies a considerable
amount of sucrose sugar. A species of palm (Phoenix sylvestris)
has long been a source of sugar in India. What is the average
sugar percentage of cane? Of sugar beet? What is the food
value of sugar? What are the principal sugar-beet-producing
countries? Where is cane grown chiefly?

FOOD PLANTS OF THE WORLD

345

Spices and flavoring substances. A great number of plants
yield spices. Cinnamon is derived from a number of different
Asiatic trees of the laurel family. Black pepper is derived from

FIG. 207. Coconut palm, Cocos nucifera, growing in central Siam. (Photo-
graph by courtesy of Dr. Gordon Alexander. From Robbins and Ramaley, in
Plants Useful to Man.)

the outer part of the unripe fruit of a woody vine (Piper nigrum),
cultivated throughout the old world tropics. The inner stony
part of the fruit of this same plant is the source of white pepper.

346 ECONOMIC IMPORTANCE OF PLANTS TO MAN

Cloves are a product of an evergreen tree, Eugenia aromatica,
of the myrtle family. Nutmeg is the seed and mace the branched
fibrous outer coat of the seed belonging to a tree, Myristica fra-
grans, which grows wild in the Molucca Islands and New Guinea.
Ginger is the rhizome of a tall herbaceous canna-like plant,
Zinziber officinale, a native of Asia south of the Himalayas.
Cayenne pepper is derived from the tropical American plant,

FIG. 208. Coffee, Coffea arabica (Rubiaceae) ; young trees in the Dutch Gov-
ernment Agricultural Department plantation, Buitenzorg, Java. (From Rob-
bins and Ramaley, in Plants Useful to Man.)

Capsicum. The principal flavoring substances are peppermint,
wintergreen, lemon, and vanilla. Peppermint is extracted from
the whole plant of a member (Mentha) of the mint family; winter-
green from the leaves of a heath-like plant (Gaultheria)', lemon
from the rind of the fruit; and vanilla from the vanilla bean,
a tropical plant (Vanilla planifolia) belonging to the orchid
family.

FOOD PLANTS OF THE WORLD

347

Food for livestock. The land animals employed by man as
food and as beasts of burden are herbivorous, that is, they live
directly upon plants. The plains, prairies, mountain lands,
pampas, and other grass lands of the world support enormous
numbers of livestock. Man has cultivated many plants for the

FIG. 209. Pepper, Piper nigrum (Piperaceae); climbing upon a tree in a
tropical garden. (From Robbins and Ramaley, in Plants Useful to Man.)

use of domesticated animals. Chief of these are the cereals,
timothy, Sudan grass, and many other grasses, various clovers
(Trifolium), alfalfas (Medicago), and certain root crops, such as
mangel-wurzels, rutabagas, swede turnips, etc.

348 ECONOMIC IMPORTANCE OF PLANTS TO MAN

Problem 2. What are the principal industrial plants?

In addition to the common plants which yield food for man
and beasts, we may consider also the following industrial plant
products: (1) wood; (2) coal; (3) cork; (4) fibers, straws, and
twigs; (5) resins and turpentine; (6) gums; (7) vegetable dyes;
(8) fixed and volatile oils; and (9) rubber.

FIG. 210. Upland cotton, a fiber plant of great economic importance,

Fiber plants. The most important fiber plants in the world
are cotton (Gossypium), flax (Linum), hemp (Cannabis), sisal
(Agave species). Fibers are used for a great variety of purposes:
fabrics of all kinds, cordage, brushes, matting, paper, filling,
plaiting, etc.

There are more than 40 species of Gossypium. The mature

INDUSTRIAL PLANTS

349

fruit called the " boll " is filled with seeds, which are covered with
hairs, the cotton fibers. The cotton fiber is thus classed as a
" surface fiber. " As a rule, there are two kinds of hairs on the
seed: (a) the long hairs, the so-called lint or commercial fiber;
and (6) short hairs or fuzz. The principal varieties of cotton are
the Sea Island cottons, Egyptian cottons, and American Upland
cottons. The finer threads
are made from Sea Island
cotton; ordinary threads and
yarns are from Upland cotton.
What is a cotton gin?

The flax plant (Linum-
usitatissimum) is a slender an-
nual plant which has been used
for its fiber as far back as the
Swiss dwellers of the stone age.
The fiber of flax is in the bark.
It is known as "bast fiber."
From the fiber is made the
linen of commerce. Our
finest linens are from foreign-
grown flax, the best known
of which are the Flemish,
which is grown in Belgium,
and , the Irish linen. Flax
fiber is also employed for mak-
ing thread, fishing lines, seine
twines, canvas, and duck.

Hemp (Cannabis) is a rep-
resentative of the mulberry
family. The stem yields a
fiber which is the strongest and
most durable of soft fibers with
the single exception of flax.
Like flax, hemp is a " bast
fiber." Hemp fibers are used

in the manufacture of sail cloth, yacht cordage, binder twine,
sacking, bagging, rope, etc.

FIG. 211. Flax. A, floral diagram c,
calyx; co, corolla; s, stamens; p, pistil.

B, Median lengthwise section of flower.

C, calyx and corolla removed. D, fruit,
external view. E, cross-section of fruit.
(From Robbins, in Botany of Crop

Plants.)

FIG. 212. Hemp.

Fiber hung up to dry. Coconut palm in background.
(From Gager's General Botany.)

FIG. 213. Manila fiber plants, Musa textilis, growing in an experimental
garden at Buitenzorg, Java. (From Robbins and Ramaley, in Plants Useful

to Man.)
350

INDUSTRIAL PLANTS

351

The chief fiber competing with hemp is jute. Jute is produced
in India from two species of plants (Corchorus species) of the
linden family. It is used extensively for the manufacture of
sugar sacks, gunny sacks, burlaps, grain sacks, and wool sacking.

The husk of the coconut, a tropical tree of the palm family,
yields an inferior fiber which is employed in making coir rope and
matting.

Manila fiber, sometimes called " Manila hemp," is derived
from Musa textilis, of the Philippine Islands, a plant closely
related to the common banana. The fiber, known as a " hard
fiber," is obtained from the flower stalk
and leaf bases. Older and coarser fiber is
used for cordage (Manila rope and twine) ;
the younger and softer material is used
for fine fabrics.

Sisal hemp is derived from species of
Agave, growing in tropical and subtropical
America. The so-called century plant of
parks and gardens is Agave americana.

Exercise 140. Microscopic examination of
fibers. Examine microscopically the following
fibers, and learn to recognize the important
differences: wool, silk, cotton, flax, hemp, and
jute. After familiarizing yourself with the fiber
characteristics, examine different kinds of cloth,
identifying the kind of materials of which it is
made.

FIG. 214. Whip made
from bast fibers. The
abundance, flexibility, and
strength of the bast fibers
of some stems is illus-
trated by the fact that a
Filipino lad was able to
make this whip using the
fibers of the bark of a na-
tive Philippine shrub after
the wood and parts of the
bark were removed.

Wood. The uses of wood are so well
known that they need not be described in
detail; it will be sufficient to mention a
few of its uses as follows : fuel, building
materials, furniture, vehicles, musical

instruments, cooperage, boxes, watercraft, fences, poles, posts,
and wood pulp for paper-making. What are the structural
differences between " soft wood " and " hard wood? "

Coal. Botanically, coal is ancient vegetation variously modi-
fied through decay, pressure, and heat. What are the differences
between soft coal and hard coal?

352 ECONOMIC IMPORTANCE OF PLANTS TO MAN

Cork. Cork is derived chiefly from the cork oak (Quercus
suber), a tree of the Mediterranean region. What are the various
uses of cork? Why is cork waterproof?

Resins. The resins of commerce are exudations of trees and
shrubs, chiefly of the pine family. The best known resin is the

FIG. 215. A large bamboo, Bambusa polymorpha. Peradeniya, Ceylon. (From
Robbins and Ramaley, in Plants Useful to Man.)

common rosin derived from one or more species of pine in our
southern states. Rosin has a number of applications; it is an
ingredient of varnishes, low-grade sealing wax, and cheap soaps,
and it is used in various electrical instruments.

Turpentine. Turpentine is a volatile substance obtained by
distillation of the exudate of pines; it is the substance in which the
resin proper was dissolved as it occurred in the tree. What are
the chief uses of turpentine?

INDUSTRIAL PLANTS

353

Vegetable gums. The vegetable gums are exudations from
the stems of many trees and shrubs. Gum arable is derived from
a shrub, Acacia Senegal. Gum tragacanth comes from Astragalus
gummifer of southwestern Asia. Dextrin is produced artificially
from starch. It is used in place of the more expensive natural gums
to make mucilage. Gamboge is the dried juice from the bark of
an evergreen tree, Garcinia hariburyi] it is made into a bright
yellow paint and also has me-
dicinal uses.

Plant dyes. Many plants
have bright-colored juices in the
roots or sometimes in other parts.
One of the chief plant dyes is
logwood, Haematoxylin campe-
chianum, a Brazilian plant. From
itis prepared haematoxylin, a stain
used by microscopists. Indigo,
from the leaves of certain shrubby
plants of the pea family, is of
little commercial importance to-
day, having been replaced by
synthetic coal-tar products.

Oils. There are two different
kinds of vegetable oils : (a) fixed
oils, which make a permanent
stain or " spot "; and (6) volatile
oils, which do not make a perma-
nent stain or spot. The principal

fixed oils, and their chief uses, are as follows: cottonseed oil
(Gossypium), food; cacao butter (Theobroma cacao), pharmacy;
coconut oil (Cocos meet/era), food, soap; olive oil (Olea europoea),
food, soap; peanut oil (Arachis hypogoed), food, soap; ape oil
(Brassica napus), lubricant, food; linseed oil (Linum usitatissi-
mum), paint, varnish; castor oil (Ricinus communis) medicine,
lubricant; maize oil (Zea mays), food, paint; palm oil (Elaeis
guineensis), soap, lubricant. The most important volatile oils,
which are employed for flavoring, as medicine, and for perfumery,
are as follows: clove oil (Eugenia aromatica), cedar oil (Sabina

FIG. 216. A pine tree in Alabama,
tapped for collection of turpentine.

354

ECONOMIC IMPORTANCE OF PLANTS TO MAN

virginiana), nutmeg oil (Myristica fragrans), anise oil (Pumpi-
nella aniswri), thyme oil (Thymus vulgar is), wintergreen oil
(Gaultheria procwnbens), peppermint oil (Mentha piperita), and
lemon oil (Citrus limonid).

Rubber. Rubber is obtained from the milky juice of many
kinds of trees and shrubs but at the present time the Para rubber
tree, Hevea brasiliensis, furnishes most of the world's supply.
Crude rubber is prepared from the thick plant juice which exudes

FIG. 217. India rubber tree, Ficus elastica (Moraceae), in the botanic gardens
at Buitenzorg, Java. (From Robbing and Ramaley, in Plants Useful to Man.)

from the bark after it is slashed. Somewhat like rubber is the
substance gutta-percha derived from the milky juice of Palaquium
oblongifolia, native of the East Indies and the Malay Peninsula.
It is used chiefly to insulate and waterproof submarine and under-
ground electric cables. A member of the composite family, known
as " Guayula " (Parthenium argentatum) , has been cultivated to
some extent in the arid sections of the United States as a source of
rubber.

Suggested activity. Find out what you can about the discovery of vul-
canization, a process which revolutionized the rubber industry.

MEDICINAL PLANTS 355

QUESTIONS

1. Describe the method of wood formation in a tree.

2. What is quarter-sawed wood?

3. What is responsible for the grain of wood?

4. Do you know wood that is heavier than water?

5. Describe the method of bark formation in a tree.

6. What is the chemical composition of cotton fibers?

7. What is the source of the mucilage used on postage stamps?

8. Has man been successful in making a synthetic rubber?

9. What is the chemical composition of the milky juice or latex from which
rubber is made?

Problem 3. What are the principal medicinal plants?

The early history of botany is closely associated with the
development of medicine. The botanists of primitive peoples
were also the physicians, priests, and sorcerers. In the healing of
disease some plants or plant parts are employed as charms, some
as fetiches, some as true medicines used for supposed physiological
effect. From the earliest time the Chinese have been active in
the use of " herbs " for medicinal purposes. They have collected
roots, berries, and barks, and made from them various extracts,
decoctions, and infusions.

It is customary in modern medicine to judge of the value of
drugs by their physiological action upon lower animals. The
so-called " active principle " in a plant or plant part may be an
alkaloid, a glucoside, a fixed or volatile oil, a resin, or some other
specific chemical substance.

Examination of the Official United States Pharmacopoeia,
which is an authoritative book containing the formulas and
methods of preparation of medicines, etc., for the use of druggists,
will reveal the fact that many hundreds of plants yield drugs.

However, there are now many more different kinds of drugs
than are really necessary or desirable, for numerous drugs of plant
origin have essentially the same physiological action. The most
important drug plants in the world are as follows :

1. Poppy (Papaver somniferum). The dried juice, known as
opium, from the capsule, is a great reliever of pain. Opium is
obtained from the unripe fruits which are cut with a knife to allow

356 ECONOMIC IMPORTANCE OF PLANTS TO MAN

the milky juice to exude. This juice when dried forms the opium
of commerce and contains about 10 per cent of the alkaloid
morphine.

2. Cinchona (Cinchona spp.), a member of the madder family.
This medium-sized evergreen tree yields a bark known as Peruvian
bark, which furnishes quinine, a specific cure and preventive of
malaria. Quinine is a white amorphous or crystalline powder,
very bitter to the taste, which exists in cinchona bark to the
amount of 5 to 10 per cent. It is one of the very few specific

Fia. 218. Cinchona in fruit, photographed from a herbarium specimen.
(From Robbing and Ramaley, in Plants Useful to Man.)

medicines. Today the Dutch have made their Javanese planta-
tions " the throne of the quinine trade." The Dutch are prac-
tically the only exporters of cinchona in the world. They have
about 25,000 acres under plantation.

3. Digitalis (Digitalis purpurea). This plant is a member of
the figwort family. The leaves contain various glucosides, chief
of which is digitalin, a drug which slows up heart action, and hence
is used in the treatment of certain cases of heart disease.

MEDICINAL PLANTS

357

4. Belladonna (Atropa belladonna). This plant, a member of
the nightshade family, to which also belong potato, tomato,
tobacco, petunia, etc., yields a drug known as atropine, an alkaloid
employed by oculists to paralyze those muscles of the eye which
cause accommodation.

FIG. 219. Chaulmoogra; young tree on the grounds of the leper hospital at

Chiengmai, Siam. (Photograph by Dr. Gordon Alexander. From Robbing

and Ramaley, in Plants Useful to Man )

5. Chaulmoogra tree (Taraktogenos kurzii). This plant, also
known as " Kalaw," is a tall jungle tree of north Burma, which
yields Chaulmoogra oil, employed in the treatment of leprosy.

Other plant drugs of importance are as follows : gum arable, a
mucilaginous substance which is an exudation from the trunk and

358 ECONOMIC IMPORTANCE OF PLANTS TO MAN

branches of the gum arable tree (Acacia Senegal)] castor-oil,
obtained from the seeds of the castor-oil plant (Ricinus corn-
munis)] calamus, from the underground stem of sweet-flag
(Acorns calamus) ; camphor, a gum-like drug obtained by distil-
lation from the wood of the camphor tree (Cinnamomum cam-
phora}] menthol, from the volatile oil of peppermint; strychnine,
an alkaloid derived from the seeds of the nux vomica tree (Strych-
nos nux-vomica).

Problem 4. What are the principal by-products derived
from plants?

Innumerable by-products are derived from plants. The chief
product of the cotton plant is the long fiber; but there are numer-
ous important by-products. The short lint or fuzz, known as
" linters," which is not removed in ginning, is taken from the seeds
and made into poor-quality twine, carpets, and batting. Now,
" linters," 85 per cent of which is cellulose, are the basis of numer-
ous products: leather substitutes, toilet articles of all kinds,
kodak and movie films, rayon, fine paper, and collodion. Cotton-
seed hulls are used in the manufacture of paper and fiber board
from which are made gear wheels, trunks, etc.; and the hulls are
also utilized as fuel and fertilizer, and as a cattle food. Cotton-
seed oil is one of the most valuable products of the cotton plant.
The oil is in the embryo of the seed. This oil is now produced in
very large quantities, this country having exported 33,673,000
gallons in 1921. It appears on the market as " table oil,"
" sweet nut oil," and " salad oil." Some of it is utilized in the
manufacture of soap, also of " oleomargarine " and other butter
and lard substitutes. Guncotton is an explosive made by treating
cotton or some other form of cellulose with nitric or sulphuric
acid. Those kinds of guncotton soluble in alcohol or ether are
used in the manufacture of rayon, celluloid, etc.

The flax plant yields both fiber and oil (linseed oil). Neither
can be regarded as a by-product. In the manufacture of linseed
oil from the seed of flax, the residue from the crushed seeds gives
a cake or meal which is a valued stock food. Flax seeds are used
whole for various medical purposes. The threshed straw of the

BY-PRODUCTS FROM PLANTS 359

northwestern seed flax is employed to some extent for upholstering,
for insulating cold-storage plants, refrigerator cars, and ice boxes.
The principal use of hemp (Cannabis) is as a fiber plant. The
seeds are often fed to poultry and cage birds; the leaves and
flowers yield a drug known as Cannabis indica; the seeds give an
oil which is used for making soft soaps and as a paint oil, and low
grades are utilized for certain varnishes; hemp stems make a fair
grade of paper.

The sugar beet (Beta vulgaris) furnishes a large proportion of
the world's sugar supply. The by-products of the industry have
enormous value. The tops, molasses, and pulp are valued stock
feed, and the waste water from the manufacturing process is
boiled down, yielding fertilizer. It has been demonstrated that it
is possible to manufacture fusel oil, alcohol, rum, and vinegar from
the refuse beet molasses.

No other cereal is put to such a variety of uses as is corn.
Some economical use has been found for nearly every part of the
plant. It is a food for man and beast. Corn oil is obtained from
the embryo; it is used for salads, in cooking, in the manufacture
of soap and paints, and sometimes it is vulcanized into a cheap
grade of rubber substitutes. The manufacture of corn starch
consumes about 50,000,000 bushels of corn annually in the United
States. Commercial " glucose " is a thick syrup derived by the
partial hydrolysis of starch, and is employed as the basis of many
manufactured jellies and preserved fruits. Artificial gums,
known as dextrin and British gums, are made from corn starch;
they are used on envelopes and postage stamps, and also in many
of the textile industries. The pith from the stalks is made into
explosives and also employed as a packing material. The stalks
as a whole have served as a source of raw cellulosic material, from
which numerous products can be made. Corn cobs are still in
demand for pipes. A fine grade of charcoal is manufactured from
corn cobs. Corn cobs also have a practical value for the produc-
tion of furfural, paper stock, organic solvents, artificial silk, etc.
Paper is made from the stalks, and packing for mattresses from
the husks. Corn cake, left when oil is pressed from the embryos,
is a stock food. And corn is the most economical source of starch
for alcohol manufacture in the United States.

360 ECONOMIC IMPORTANCE OF PLANTS TO MAN

The sugar-cane plant also yields many valued by-products.
The molasses is used for baking purposes and as a table syrup;
poorer grades are made into rum and alcohol, and used as stock
food. The refuse has value as a fertilizer. Sugar-cane bagasse
was formerly used only as fuel, but now it is made into wall
board.

The soy bean (Soja max) is the most important legume in
Asiatic countries. The chief product of the bean is the oil which
is expressed from the seeds, and the plant is grown principally for
that purpose. The plant has a number of less important uses.
For example, after the oil is expressed from the seed, the " cake,' '
either unground or ground into a meal, is used as stock feed or as
a fertilizer. The seeds of soy beans are sometimes used as a
substitute for coffee.

Peanut seeds yield an oil, a nearly colorless product, employed
as a salad oil, and to a limited extent in the manufacture of soap
and oleomargarine. Peanut butter has become a standard food.
Peanut meal, the product left after pressing the oil from the seeds,
is a high-grade stock feed.

Almond seeds yield an oil used as an ingredient of flavoring
extracts, and the seeds are a source of prussic acid.

Cider is the juice of apples. In the transformation of cider
to vinegar, two fermentation processes take place; alcoholic
fermentation, and acetic acid fermentation. The characteristic
properties of vinegar are due to acetic acid.

The juice of quince is sometimes employed to flavor manufac-
tured food products.

The buckwheat plant (Fagopyrum vulgare) is grown chiefly for
the " grain " which is made into buckwheat flour. The " mid-
dlings " (hulls, mixed with bran) are employed as a bedding for
stock; and, not to be disregarded, are the flowers which produce
an excellent grade of honey.

Agave species are grown largely for their fiber (sisals), but
the juice of the plant is fermented to give a drink, pulque.

In addition to its supply of fruit, the date palm furnishes
material for building, for ropes, baskets, and numerous other
articles.

Rice hulls are used as a stock food; rice straw as a food for

HARMFUL PLANTS 361

stock, and also in the manufacture of paper, straw hats, straw
board, etc.

The orange gives an important by-product in the form of an
oil, which is employed in the manufacture of perfumes, soaps, and
flavoring extracts. Waste oranges may be used for this purpose.
One of the chief by-products of the lemon industry is lemon oil,
which ranks second to vanilla extract in the quantity consumed.

The potato plant is cultivated for its tubers, which are used
chiefly as a food for man. But it is an important source of com-
mercial starch and of alcohol.

In the canning of tomatoes large amounts of refuse accumulate.
The oil expressed from the seeds is used as a soap oil, which may
be made into a drying oil for paint ; the meal is used as a stock food.

Carrot roots, grown mostly for a table vegetable, contain a
yellow pigment, carotin, which is sometimes extracted and used for
coloring butter.

The grape plant has a number of by-products. Brandy, feed,
fertilizers, and acetic acid are made from the pomace. Tartaric
acid is manufactured from the steins, shells, and the " lees "
of wine. The seeds are used as a food for stock and as a source
of tannin and grape oil.

Sweet potatoes are used chiefly as a human food. Flour,
starch, glucose, and alcohol are minor products of the root.

Problem 5. How do plants interfere with man?

Not all plants are useful. Many are of economic importance
because they interfere with man's farming operations, or they in-
jure his health or that of domestic animals. The principal groups
of harmful plants are as follows: (1) weeds, (2) fungi which cause
animal and plant diseases, (3) plants directly poisonous to man and
livestock, (4) hay-fever plants.

WEEDS

We have come to consider as " weeds " those plants which
tend to grow where they are not desired; plants which tend to
resist man's efforts to subdue them; plants which will grow in
almost any kind of soil and under all conditions; plants which pro-

362 ECONOMIC IMPORTANCE OF PLANTS TO MAN

duce seeds in enormous numbers and have other rapid methods of
propagation; plants which in themselves sometimes are truly
beautiful, but which for us have lost their charm; plants useless
and troublesome.

Losses caused by weeds. The losses caused by weeds fall into
two chief classes: (1) losses brought about by a decrease in yield
or quality of crop, (2) losses brought about by an increase in the
labor cost of growing the crop.

Weeds rob cultivated plants of water. Weeds do great injury
in using up moisture. It is said that a large weed will use a barrel
of water during the season. The sunflower plant requires almost
twice as much water as corn to produce the same amount of dry
matter; the water requirement of ragweed is about three times
that of millet. These figures show the injury that weeds do to
our crops through their great demand upon soil moisture. In fact,
the main benefit derived from cultivating corn and other crops
is in the removal of weeds which compete with^ them for soil
moisture.

Weeds crowd out and shade crop plants. By shading, weeds
may retard in crop plants the process of food-making (photo-
synthesis). They may even prevent seedlings from getting a
start. Moreover, certain fungus pests develop better in the
shade than in direct light.

Weeds harbor insects and fungus pests. Insects and fungi
often spread from weeds to neighboring cultivated plants. Clean
culture about roadsides, fence rows and ditch banks is strongly
recommended to prevent the spread of such pests. Insects
deposit their eggs upon the weeds, and when the larvae hatch
they migrate into the fields. Insects often go into hibernation
somewhere near their native food plants, many of which are weeds,
and from them they scatter to adjoining fields of cultivated plants.
Insect pests gradually become more and more numerous until,
native plants being insufficient for their food supply, they move
to adjacent fields of cultivated plants. For example, the beet
webworm prefers lamb's quarters, Russian thistle, and Atriplex
rather than the sugar beet as plants upon which to deposit their
eggs. Fields infested with or bordered by these weeds attract
the webworm moths, and when these plants are exhausted by the

HARMFUL PLANTS 363

larvae, they move to nearby beets. Grasshoppers are always
worse next to ditchbanks and roadsides, fence rows, and other
waste land, overgrown with weeds and grass. Grasshoppers rarely
lay their eggs in cultivated fields, but select the native haunts in
preference. Potato bugs flourish on greenberries, nightshade, and
buffalo bur. False chinchbug, which does great injury to seed
crops, breeds and feeds during the early part of the season on
shepherd's purse and other wild mustards.

It is now well established that the fungus causing stem rust of
wheat is harbored by certain grasses such as wild barley and that
it may spread from wild barley and other grasses to wheat. Cer-
tain wild mustards may serve as a host for the fungus causing
" club-root " of cabbage.

Weeds retard the work of harvesting grain. Weeds increase
the pull for the horses and cause an extra wear and tear on machin-
ery. Dodder may so mat alfalfa plants together as to make
harvesting extremely difficult. Weeds increase the labor of
threshing, and make an added cost in cleaning the seed.

Weeds and dockage. One of the most serious losses occa-
sioned by weeds in fields results from the infestation of the grain or
seed crop. Farmers annually haul thousands of tons of weed
seeds, chaff, and other inert matter to the mill in their wheat.
And, of course, they are docked for this unclean wheat, and rightly
so. Moreover, unclean seed means that the fields which produced
the seed were weedy. And, what is worse, it means that the next
crop grown from such seed will be weedy.

Some weeds injure stock. The beards of downy brome grass,
wild barley, and certain other grasses may work into the gums of
animals, causing ulcers and the loss of teeth. Some weeds, such
as cocklebur and sandbur, injure wool and disfigure the tails and
manes of horses. A number of weeds are poisonous to stock.
Not only do weeds decrease the crop yield, but when they are
eaten they may also cause the death of stock.

Weeds retard the drying of grain and hay. Many weeds are
succulent and hold moisture, thus retarding the drying of crop
plants with which they are mixed.

Some weed seeds, such as cockle, damage the quality of
dairy products. Weeds such as common ragweed, wild onion,

364 ECONOMIC IMPORTANCE OF PLANTS TO MAN

and wild garlic, when eaten by cows, impart disagreeable flavors
to milk, butter, and cheese.

Why weeds are successful plants. Seed production of weeds.
Many weeds produce an enormous number of seeds. A large
purslane plant will produce as many as 1,250,000 seeds; a single
Russian thistle plant will ripen 100,000 to 200,000 seeds; tumbling
mustard as many as 1,500,000 seeds.

The seeds of many weeds are very small and escape notice.
A pound of clover dodder has 1,841,360 seeds; common plantain,
1,841,360 seeds; lamb's quarters, 604,786 seeds; Russian thistle,
266,817 seeds; wild mustard, 215,995 seeds; wild oats, 25,943
seeds. If 60 pounds of wheat are planted to the acre, and this
wheat has 2 per cent of wild mustard seed, there will be dis-
tributed over that acre 388,791 mustard seeds, or 9 seeds in
every square foot.

Vital weed seeds at different depths in the soil. Not only do
weeds produce seeds in tremendous numbers, but also many weeds
produce seeds with an ability to live a long time. The seeds of
some weeds, when buried in the soil, may retain their power of
germination for 15 to 30 years. This is true of the seeds of tall
pigweed, black mustard, shepherd's purse, dock, yellow foxtail,
chickweed, and others.

Some weeds seeds exhibit dormancy. Not all the seeds of
a given crop of seed may germinate the first year; some may
remain alive in the ground for a time. This has been given popu-
lar expression in the following statement: " One year of seed gives
seven years of weeds."

Weeds as a class are hardy. Weeds as a class are resistant
to insect and fungus pests. They also have the ability to with-
stand shading, excessive drought, temperature extremes, and other
unfavorable conditions. Of all weedy plants, the worst are those
with underground stems or rootstocks, which live over from year to
year in the soil, and enable the plant to spread rapidly in all
directions underground. These underground stems store food,
and, although the plant is cut off above ground, new stems are
sent up directly from below. Weeds with rootstocks are particu-
larly difficult to eradicate. Well-known examples of such weeds
are quack grass, poverty weed, and Canada thistle.

HARMFUL PLANTS 365

Weeds spread rapidly. It has been the history of nearly all
agricultural communities that weeds increase in abundance and
variety, unless concerted action is taken to combat them. Almost
every year sees the first appearance of some weed in a community,
and usually in a few years it is prevalent. In a few decades the
Russian thistle has spread throughout the agricultural sections of
the West, and in some localities is now a menace. In fact, some
sections are being abandoned on account of the Russian thistle.
Russian thistle seeds are now a common impurity of crop seeds.
The entire plant may break off at the ground line, become a
" tumble weed," and be blown for miles across the open country,
distributing its seeds as it tumbles along.

Weeds have excellent means of seed dispersal. Some seeds,
like those of the thistle, milkweed, sow-thistle, wild lettuce, and
dandelion, have cottony or feather-like attachments which enable
them to take long aerial journeys. Most seeds will float on water
and, consequently, are carried by streams and irrigation waters.
A number of seeds are provided with hooked prickles or barbs
by which they attach themselves to the clothing of man or the
hair of animals, and are thus carried from place to place.

Impure commercial seeds. Probably no other means of intro-
ducing weeds is so effective as the sale and distribution of impure
commercial seeds.

Seeds are carried in screenings, baled hay, the packing about
trees, and in feedstuffs. Some seeds are uninjured in passing
through the digestive tracts of animals and consequently are
spread on the field in manure. The use of feeding stuffs containing
live seeds may result in the spread of noxious weeds. A threshing
machine may carry weed seed from farm to farm. Some weeds
are dragged by plows, cultivators, and harrows from one part of
the field to another and even to adjacent farms. This is true of
those perennial weeds with underground stems which are cut up
into pieces by cultivating implements.

Wind and water are important agents in weed dissemination,
Wind carries seeds, and in some instances whole plants, long
distances. In the irrgated sections, water is one of the chief means
of spreading seeds. Ditch banks are densely overgrown with
weeds, which shed their seeds in the water; the seeds are carried

366 ECONOMIC IMPORTANCE OF PLANTS TO MAN

down stream, given a good soaking in transit, and planted
on a well-soaked soil all conditions being ideal for germi-
nation.

Birds may be responsible for the distribution of weed seeds.
However, birds probably do more good in eating weed seeds than
harm in distributing them.

Underground spreading of weeds. Perennial weeds, such as
Canada thistle, travel considerable distances each year under-
ground. A small patch in one corner of a field may appear harm-
less enough, but it may soon spread over a whole field by means
of underground growth alone.

Classes of weeds. Weeds fall into three classes according to
their length of life. It is necessary to know to which class a weed
belongs before one can wisely proceed to eradicate it. These
weed classes are :

1. Annuals. Those that live one year, such as Russian thistle,
pigweed, wild oats, shepherd's purse, pepper-grass, foxtail, and
ragweed.

2. Biennials. Those that live two years, producing seed at
the end of the second year, such as wild carrot, wild parsnip,
mullein, and bull-thistle.

3. Perennials. Those that live from year to year by means of
underground parts. They are our worst weeds, and when once
established are difficult to eradicate. Some common perennial
weeds are wild morning-glory, or bindweed, poverty weed, Ber-
muda grass, dandelion, sow-thistle, and Canada thistle.

Annual and biennial weeds. Annual and biennial weeds pro-
duce seed but once and then die down entirely, root and all. They
propagate themselves by seed alone. Consequently, all methods
of controlling weeds of these two classes have for their object the
prevention of seeding. Clearly, if they are kept from seeding, and
pains are taken to prevent seeds from being introduced to the land
in the many ways that are possible, annuals and biennials are kept
in check on the farm.

Annuals and biennials are easily killed by cultivation. The
seeds of some weeds of these classes retain their vitality in the soil
for several years, consequently several years of cultivation may be
necessary. The principle of eradication of annuals and biennials

HARMFUL PLANTS

367

is to prevent seeding, and to cause the seeds that are shed to ger-
minate and then destroy the seedlings before they mature.

There are two kinds of annuals : summer annuals and winter
annuals. Summer annuals germinate their seeds in the spring,
produce a crop of seed in the late summer or fall, and die down.
The seedlings are not capable of living through the winter season.
Russian thistle, foxtail grass, frenchweed or fanweed, barnyard

FIG. 220. Canada thistle, a perennial weed that propagates both by seeds

and underground stems. The illustration shows three different ages of shoots

that have arisen from the underground stem and reached the surface, and

several others which have not yet reached the surface.

grass, witch-grass, pigweed, and lamb's quarters are common sum-
mer annuals. Winter annuals come up from seed in the fall anc
live over the winter in the seedling stage, producing flowers and
fruit the following spring or early summer. Shepherd's purse,
pepper-grass, and prickly lettuce belong to this class. The
seeds of winter annuals germinate after fall rains, and the young
seedlings are easily killed at this time by cultivation. Winter
annuals frequent stubble fields.

368 ECONOMIC IMPORTANCE OF PLANTS TO MAN

Perennial weeds. As has been stated, perennial weeds live
from year to year without renewal from seed. They propagate
themselves by means of both seed and vegetative growth (roots and
underground stems). It is not sufficient to prevent seeding alone,
although this should be done. All methods of holding in check or
eradicating perennial weeds have for their object the starving or
smothering of the underground growth. It is well known that, if
the top growth of a perennial weed is cut off, new shoots are sent
up from the stems or roots beneath ground. This means, then,
that the underground parts possess reserve food material which is
called upon to produce new shoots. As soon as leaves are pro-
duced on the new shoots, the manufacture of food is begun by
them, and some of this food, made in leaves, finds its way back
into the roots or rootstocks which thereby gain in strength.
A rootstock or perennial root will increase in size each year just
as does a perennial stem above the ground; and the larger it gets,
the more difficult it is to starve out. Old, well-established peren-
nial weeds are very difficult to eradicate, for the reason that they
have a large store of reserve food to draw upon in the production
of one crop of leafy shoots after another. It is a common experi-
ence that one may, by a thorough cultivation, kill all the top
growth of a perennial weed, and find that the plant soon comes
up thicker than before. A second thorough cultivation may be
followed by like results. Each time that top growth was removed
and new shoots sent forth, reserve food in the roots and rootstocks
was used up. What is needed in the eradication of a well-estab-
lished perennial weed is persistence and patience, cultivation fre-
quent enough to keep down all leafy shoots, and no stopping
until the object is attained. This may mean a season or two
of bare fallow, followed by 'a crop which will permit of clean
cultivation. Some farmers place in a cultivated crop like corn,
potatoes, beets, or beans but do not cultivate often enough
or thoroughly enough to prevent perennial weeds from making
headway.

Small patches of perennial weeds can be eradicated by spading
up the soil, raking out and burning the underground parts, and
hoeing off every sprout as soon as it appears, during at least two
seasons.

HARMFUL PLANTS 369

Methods of weed control. Use of clean seed. The first
principle in weed control is the use of clean seed. A lot of "clean
seed" is one which contains only the larger and plumper seeds
of the crop desired, and is free from sticks, stones, gravel, dirt,
chaff, weed seeds, the seeds of other crops, smut balls, and small,
shrunken seeds of the crop desired.

Preventing weeds from seeding. The second principle in
weed control is the prevention of seeding. This means the use
of a hoed crop or meadow crop. Weeds in small grain cannot be
prevented from seeding; as a result, continuous cropping to grain
leads to foul fields. It is the inevitable result of a one-crop system.

Cutting the weeds along roadsides and fences. Throughout
our entire country, cultivated fields are surrounded or bordered
by areas infested with weeds. Roadsides are the chief sources of
weed-seed supply. It is true that states, counties, and cities have
laws which require the mowing of weeds along roadways prior to
their seeding. But in very few instances is the regulation ade-
quately enforced. As a matter of fact, weeds come to maturity
and stand man-high along our roadsides, contaminating the adja-
cent cultivated fields and gardens.

Giving special attention to manure and screenings. The seeds
of many weeds are not injured by passage through the digestive
tract of an animal. Consequently, weedy hay, weedy bedding,
grain-carrying weed seeds, or unground screening are a source
of contamination. Many stock foods contain unground screenings
in which may be found many small weed seeds, such as the plan-
tain. Cases are known in which the weed seeds from this source,
wild oats, for example, have been carried to the field in the manure.
Well-composted manure contains no visible seeds. All screenings
containing weed seeds should be thoroughly ground or steamed
before they are fed. It should be added here that weed seeds
are destroyed by the fermentation processes which ensilage
undergoes.

Use of cultivated or cleaning crops. Crops such as beets,
potatoes, beans, and corn which permit frequent cultivation are
rightly called cleaning crops. Practically all annual and biennial
weeds readily succumb to cultivation, and perennials are effectively
held in check. Continuous cropping, particlarly to small grain,

370 ECONOMIC IMPORTANCE OF PLANTS TO MAN

which does not allow cultivation, inevitably leads to weedy fields.
Cultivated crops are a necessity in any scheme to eradicate
weeds.

Rotation of crops. " Crop rotation " is practically a synonym
of " good farming." In fact, the control of weeds is one of the
principal reasons for crop rotating. There are farms on which
weeds are of little consideration, either in increasing the labor
expended or in decreasing crop yields. And these farms are ones
on which a definite plan of crop rotation is systematically adhered
to. One cannot expect to follow a continuous system of cropping
without trouble with weeds. Even old stands of alfalfa frequently
become weedy, and must be plowed up and placed in a cultivated
or cleaning crop.

Exercise 141. Field trip. Visit a weed-infested vacant lot or field.
Find out by asking other people, or by the use of weed manuals, the common
names of the different weeds which you do not already know. Is the habitat
mesophytic, xerophytic, or hydrophytic? What weeds seem to be best fitted
to the surroundings? Which of the weed characters mentioned in the text
are possessed by the most successful of these plants? What plants of the
habitat seem to be losing in the struggle for possession? Name qualities from
the list in the text which the losing plants seem to lack.

Exercise 142. Field trip. If you can find a weed-infested garden or field,
compare the condition of the crop plants which are struggling against the
effects of the weeds with the condition of crop plants in a well-kept garden or
field. List the reasons for the difference in the condition of the crops in the
two situations. When left to themselves, why do weeds win over crop plants
in competition for possession?

Exercise 143. Weeds which propagate vegetatively. Make a list of weeds
which reproduce readily by vegetative means, and classify on the basis of the
part of the plant which is important in vegetative reproduction, as quack
grass, by rhizomes; purslane, by fragments of the stem.

Suggested activity. Make a collection of fruits and seeds of weeds, classi-
fying on the basis of means of dispersal, that is, by propulsion, by wind, and
by animals.

Exercise 144. Laboratory. Make a study of at least ten common weeds
and record your data either in a table or as follows:

1. Common name.

2. Habitat:

a. Where found growing.

b. Mesophyte xerophyte hydrophyte .

HARMFUL PLANTS 371

3. Habit:

a. Mesophyte xerophyte hydrophyte

b. Stem: height erect branching

climbing prostrate rhizome

succulent woody

c. Leaves: large small medium

simple compound succulent

rosette mosaic

d. Roots: primary lateral fibrous

deep surface fleshy

4. Reproduction:

a. Pollination: by wind by insects by

gravity .'

b. Number and size of seeds

c. Methods of fruit and seed dispersal

d. Vegetative reproduction

5. Length of life: annual biennial perennial

6. Protection against animals: inedible spines offensive

odor

7. Native or introduced

8. Means of control

PLANT DISEASES

One may express surprise that plants, as well as animals,
become diseased, that they get " sick," and that they may need
care and treatment to prevent them from succumbing to various
maladies. But such is the case. The " plant doctor," or rather
the plant pathologist, studies the characteristic symptoms of plant
diseases, he determines the causes and searches for preventive
measures and remedies. The science of plant pathology has
made very rapid strides in recent years, and the plant pathologist
is a very necessary person in our agricultural development.
Scores of trained plant pathologists are now connected with the
United States Department of Agriculture and with the various
agricultural experiment stations and colleges.

It is estimated that the annual loss in the United States from
potato blight amounts to $36,000,000; from wheat bunt, $11,000,-
000; and from oat smut, $6,500,000. Cereal rust caused a loss
of about 200,000,000 bushels of wheat alone in the United States
in 1916.

372 ECONOMIC IMPORTANCE OF PLANTS TO MAN

The causes of plant diseases. The different plant diseases may
be classified as follows, according to their causes:

1. Those caused by the activity of living organisms.

a. Caused by animals, such as worms and insects.

b. Caused by plants.

1. Parasitic bacteria, and other fungi, and slime

molds.

2. Parasitic seed plants.

2. Those due to adverse non-living environmental factors such
as the chemical and physical condition of the soil, light, heat,
precipitation, wind, lightning, smoke, soot, gases and smelter
fumes, which may result in nutritive disturbances in the plant.

Diseases due to attacks of bacteria. Some of the chief bac-
terial diseases of cultivated plants are bacterial blight of alfalfa,
blight of apple and pear, bacteriosis of bean, crown gall of a num-
ber of different plants, black rot of cabbage, and wilt of cucurbits.
Bacterial diseases are often very destructive and spread rapidly.
The bacteria gain entrance to the plant through wounds or through
the pores (stomata) in the leaves. In the tissues of the host the
bacteria find a suitable food supply, and there grow and reproduce
rapidly. Bacteria are dependent organisms and draw their food
from the host, causing in it nutritional disturbances and struc-
tural modifications which have characteristic symptoms.

Diseases caused by parasitic fungi. The large majority of
plant diseases caused by plants are due to the activity of parasitic
fungi. Well-known diseases brought on by these organisms are
the mildews, the spots, rots, scabs, wilts, smuts, rusts, and cankers.

Diseases caused by parasitic seed plants. There is one very
harmful seed plant which lives a parasitic life on a number of dif-
ferent plants, but is most harmful to alfalfa and clovers. This
plant is dodder or love- vine. Dodder plants have slender, thread-
like stems of a yellowish or orange color which twine and coil
about the alfalfa plants. The dodder seeds germinate in the
soil about the same time as alfalfa seeds. Later, the plants
lose all connection with the soil. As the alfalfa plants grow,
dodder keeps pace, spreading and branching extensively. Soon
the dodder in a field may be detected by the dense growth of

HARMFUL PLANTS 373

yellow, tangled stems, or by the presence of patches of stunted
alfalfa plants, and in severe cases, by a mat of dodder and alfalfa.
The dodder plant sends small absorbing organs into the tissue
of the host, and takes foods from it.

Diseases due to adverse environmental conditions. It is well
known that hail will injure plants. Too much " alkali " in the
soil may also cause serious injury to plants. Plants vary consid-
erably in their ability to withstand alkali. The sugar beet, for
example, can withstand much more alkali than corn or wheat or
potatoes. Blueberries flourish in an acid soil, but become sick
and die under other conditions. In certain parts of the irrigated
sections of the West, nitrates have accumulated in the soil in
such large quantities as to injure the vegetation and make it impos-
sible to grow crops profitably upon them. This injury to plants
is called " niter " injury. The accumulation of niter in the soil
gives it a chocolate-brown color, and the accumulation is the
result of soil conditions which favor the very great activity of
certain nitrogen-fixing bacteria.

Too much water or too little water in the soil will cause the
plants to be sickly and to be seriously reduced in their growth.
" Tip burn " of lettuce is thought to be due to a fluctuation in the
temperature and moisture supply, particularly in the presence of
readily available potassium and nitrogen. Intense light may
cause sunscorch, " bronzing/' or sunscald. On the other hand,
dense shade may cause plants to become weakly; for example,
lawn grass in deep shade may languish and die. The injury of
potato and cotton plants by lightning has been reported. In the
neighborhood of cities where much smoke and fumes from manu-
facturing establishments are present, plants are injured; sulphur
dioxide and other gases act as toxins. It is known that shade
trees are harmed by small quantities of illuminating gases which
may escape into the soil from leaking pipes.

Some plant diseases which can not be ascribed to any of the
above causes are usually considered to be due to disturbances in
plant nutrition. The causes of such diseases are often hard to
determine. Any marked deficiency in one or more of the chemical
elements essential to growth will result in an unhealthy develop-
ment of the plant. The " pale " color of plants, a disease known

374 ECONOMIC IMPORTANCE OF PLANTS TO MAN

as chlorosis, may be due to a deficiency or unavailability of either
magnesium or iron, both of which are essential to the formation
of chlorophyll. " Die-back " of lemons has been ascribed to the
poor nitrifying power of the soil.

Diseases caused by insects. So many of our crop plants are
infected with insects that we are almost forced to the conclusion
that there is not one without its insect enemy. In dealing with
insect pests, two main types are recognized: that which includes
insects with biting mouth-parts which feed on plant tissues, and
that which includes insects with piercing and sucking mouth-parts
which pierce the plant tissues and suck the plant juices. These
small animals live upon the juices of the plant and reduce its
vitality by destroying plant structures, or they feed upon the plant
tissues, or steal the nourishment which is needed to make the
plant grow well. Some of the principal insect enemies of crops are
grasshoppers, cutworms, chinch bugs, alfalfa weevil, plant lice,
webworms, woolly aphis, codling moth, scales, mites, borers, and
leaf rollers.

The principles of disease control. In the preceding pages we
have discussed the various causes of diseases in plants. Let us
briefly outline the principles of disease control.

Determination of the cause. Of course, the first step in the
control of a particular disease is to determine its exact cause.
This may be difficult, at times, but a line of successful action can
not be followed unless the cause is first ascertained.

Knowledge of the life history. If the diseased condition is
due to an organism, it is essential to know the life history of this
organism, so that it can be attacked at its most vulnerable period.
For example, in those smuts, such as bunt of wheat, which infect the
host only in the seedling stage, a knowledge of the mode of infec-
tion has led to a method of control which involves the destruction
of spores on the seed and in the soil about the seed. Again, a
knowledge of the life history of black stem rust of wheat has led
to the eradication of the common barberry.

Cultural methods. It is becoming well recognized that many
plants succumb to diseases because of a weakened condition
brought about by poor cultural practices. A poorly nourished
and weak plant, like a poorly nourished and weak animal, is often

HARMFUL PLANTS 375

more subject to the attacks of fungi than are strong vigorous
plants. Hence, plants that are well cared for, that have ample
water and mineral nutrients, and favorable soil, light, and tempera-
ture conditions, such that growth is uninterrupted, have greater
powers of throwing off diseases than plants which suffer from a
lack of these factors. However, very vigorous plants may be
attacked by diseases.

Crop rotation. Our different crop plants have fungus diseases
peculiar to them. For example, corn smut is known only on one
common host, and that is maize. The smut spores survive in the
soil and may infect the succeeding crop. By growing corn con-
tinuously on a given area, spores accumulate and increase the
chance for infection. However, if the area is planted to a crop
other than corn, the spores lose their vitality or germinate after a
time. Then corn may be planted again in the area without fear
of infection from the soil.

Consider another example, potato scab. It has been demon-
strated experimentally that the fungus may persist in the soil for
several years. Continuous cropping to potatoes only increases the
soil infestation. But, if a crop not subject to the attacks of that
particular organism is grown on the infested soil, potatoes may be
grown there again after several years.

Disease-free seed. Great emphasis in the control of potato
disease has been placed upon the necessity of using disease-free
tubers ("seed"). Scab, Rhizoctonia, mosaic, dry rot, late blight,
and other potato diseases may be carried into the soil by the tubers.
Corn may carry within the kernel one or more disease-forming
fungi. A number of the smuts are seed-borne. Anthracnose of
beans is carried over from crop to crop largely in the seeds. Two
general methods are adopted to secure disease-free seed, as follows:
(1) seed selection, and (2) seed treatment.

Disease resistance. Much progress in the control of plant
diseases has been made through the breeding of disease-resistant
strains. In fact, this is one of the most hopeful lines of attack in
combating plant diseases. Resistance may be due to a structure
which prevents entrance of the organism, or to a nutritional con-
dition that does not supply the proper kind of food, or to other
causes not well understood.

376 ECONOMIC IMPORTANCE OF PLANTS TO MAN

No two plants are alike. They show variation. They may
vary not only in such particulars as height, color, leaf shape, charac-
ter of fruit, etc., but also in resistance to disease, or performance in
some other direction.

The wilt disease of cotton at one time threatened the cotton
industry in the southern states. Progress in its control has been
due to the development of resistant varieties. Tests have shown
that some varieties of cotton are several hundred times more
resistant to the disease than others. Kanred wheat is a variety
relatively resistant to black stem rust. Kieffer and Mclntosh
pears are relatively more resistant to bacterial blight (fire blight)
than the Bartlett variety. Early Crawford and Elberta peaches
are more resistant to brown rot of stone fruits than are such varie-
ties as Triumph and Alexandra.

Sanitation. This includes destruction or removal of diseased
tissues, the burning of refuse, and soil treatment.

Practical control of apple and pear blight, a bacterial disease,
is brought about by pruning out infected twigs and smaller
branches, and by scraping off all diseased tissue of the large
branches and main trunk. The instruments employed are also
sterilized.

In the case of black knot of plums and cherries, the developing
knots should be pruned out as soon as they appear in the spring,
and thus prevent the spread of spores which would develop on
these swollen areas. Several prunings a season may be necessary
to remove the diseased tissue.

The large, swollen, smut masses that develop on corn are well
known as the source of clouds of spores which are readily blown
by the wind and may infect other plants. Stalks affected with
smut should be cut out and burned before the spores mature.

Crop residues and refuse often carry a disease from one season
to the next. For example, if smutted corn is thrown on the
manure heap, the spores may be carried to the field and become a
source of infection.

As a measure of control in onion mildew, the tops of diseased
plants should be destroyed. If they are left on the land or returned
to it in the manure, infestation of the new crop will occur.

The mummied fruits characteristic of the brown rot of stone

HARMFUL PLANTS

377

fruits should be knocked from the trees, and together with those
on the ground raked together and burned. These mummied
fruits are the principal source of infection the following year.

Many other plant diseases are effectually controlled or held in
check by the destruction of crop residue or refuse.

A number of disease-forming organisms live over in soil, either
in the vegetative stage or spore stage. For example, damping-
off fungi, which are often prevalent in potting beds, in greenhouses,
in seedling nurseries, and sometimes even in fields, live from year
to year in the soil, attacking the plants in the seedling stage.
Fungi in the soil may be destroyed by steam sterilization of the
soil and by drenching it with formalin. Club root of cabbage,
a disease which attacks
a number of cruciferous
plants such as cauli-
flower, turnip, rutabaga,
Brussels sprouts, radish,
and other mustards, may
be prevented or checked
by applying lime to the
soil, at the rate of about
100 bushels per acre.

Application of fungi-
cides. The spraying or
dusting of plants with
chemicals is now a com-
mon method of

FIG. 221. Distributing sulphur by means o*
the aeroplane, in the control of fungous dis-
eases. The aeroplane is now used to distrib-
ute various insecticides and fungicides. (From
California Agricultural Experiment Station
Bulletin 511.)

con-
trolling many fungus diseases. Twenty-five years ago, however,
the practice was little used. In the use of most fungicides the object
is to cover the surface of the plant with a chemical which will pre-
vent the germination and growth of spores that may already be on
the surface or light on it later. The spore itself may absorb suffi-
cient of the poison on the plant surface to kill it; or when the spore
germinates, and sends out a slender tube, if there is a poisonous
chemical in its path, and this is absorbed, growth is prevented.
Thus sprays and dusts are preventives rather than cures.

Of course, the fungicide employed must not be injurious to the
host. Fortunately, the thick cuticle which covers the surfaces

378 ECONOMIC IMPORTANCE OF PLANTS TO MAN

of twigs, leaves, and fruits usually prevents the absorption of
sufficient poison to injure the host, providing the fungicide is
properly made.

The common fungicides in use today are Bordeaux mixture,
copper sulphate, ammoniacal copper carbonate, lime sulphur,
flowers of sulphur, corrosive sublimate, and formalin.

Application of insecticides. Insect pests with biting mouth-
parts, as cabbage caterpillars, may usually be destroyed by spray-

4 ing or dusting on a so-

called stomach poison
which is eaten by the in-
sect as it feeds on the tis-
sues of the plant. Insects
with piercing mouth-
parts, as plant lice and
scale bugs, are not affect-
ed by ordinary poisons
placed on the surface of
plants. These pests are
destroyed by spraying with
contact poisons. Examples
of stomach poisons are Pa-
ris green and lead arsen-
ate; examples of contact
poisons are tobacco infu-
sion, lime sulphur wash,
and kerosene emulsion.

Tree surgery. The
work of various wood-
destroying fungi, and me-

FIG. 222. Improper removal of a limb
may result in decay that is carried far into
the tree. In the above, a stub was left,
which does not heal over; the stub finally
decays, and falls out. The dark-colored
portion represents decayed tissue. (Re-
drawn from Solotaroff, in Shade Trees in
Towns and Cities.)

chanical injuries of differ-
ent sorts, may necessitate special surgical methods to prevent the
destruction of trees. The underlying principles in these methods
are (1) the removal of all diseased or dead tissue, so as to secure a
fresh surface, thus permitting the development of wound cork; (2)
the sterilization of the exposed surface; (3) the covering of the sur-
face with some material which will not allow the entrance of fungi.
If fungi invade the tissue and cause decay, it is necessary to

HARMFUL PLANTS 379

remove some live tissue beyond that which is clearly dead. Com-
mon materials used to sterilize and cover the exposed surface are
commercial creosote, asphaltum, tar, and Bordeaux paste.

POISONOUS PLANTS

Many hundreds of plants are known to be poisonous to man
and domestic animals, and many more hundreds are under sus-
picion. The annual toll of human lives in the United States due

FIG. 223. Edible mushrooms. It is often difficult to tell whether mushrooms
are edible or poisonous.

to eating " toadstools " and the roots and berries of various seed
plants is considerable; and the yearly losses of livestock, particu-
larly on the western ranges, due to poisonous plants amounts to
several millions of dollars.

From an early day the different Indian tribes have been skillful
in preparing arrow poisons. A considerable number of different
species of plants have furnished poison for arrow tips. The
Egyptians were familiar with such poisonous plants as hyoscyamus,
aconite, and conium. They also knew prussic acid, which they
extracted from peach pits.

380 ECONOMIC IMPORTANCE OF PLANTS TO MAN

Forage poisoning in livestock is thought to be caused by various
fungi. Ergotism is a disease of livestock caused from eating
grasses which contain ergot, a fungus.

A number of the fleshy fungi are poisonous. Although there is
no botanical difference between " mushrooms " and " toadstools/'

FIG. 224. Deadly A manita,
a poisonous mushroom.
Note the swollen base, and
the ring on the stem. The
gills bear white spores.

FIG. 225. Among our poisonous plants
is the white snakeroot of our pastures.
It not only poisons domestic animals
that eat it, but it has been found to
cause milk sickness in man.

the former name is commonly applied to those believed to be
edible, and the latter to those thought to be poisonous.

Nearly all the deadly poisonous fleshy fungi are species of the
genus Amanita. This is a group of mushrooms with gills. Some of
the species of Amanita have white caps, others have bright orange,
red, or yellow caps; but in all the gills are white. This deadly

HARMFUL PLANTS

381

poisonous group is distinguished by the following combination of
characters.

1. White gills.

2. A " ring " on the stem.

3. A cup at the base the so-called " death cup."

When a mushroom shows these three features, it should be
avoided. One should not de-
pend upon the various rules-of-
thumb for detecting poisonous
mushrooms.

A large number of plants
are known to be poisonous
from the presence of prussic
or hydrocyanic acid. This
acid is known to be one of the
most deadly poisons, and it
results from the presence in
the plant of what is known as
a glucoside, which must be
acted upon by a ferment. A
large number and a great
variety of plants contain a
hydrocyanic-acid-producing
glucoside. Plants conspicuous
in this class are cherry, peach,
and other stone fruits, sor- FIG. 226.The whorled milkweed is an
ghum, kafir corn, and Johnson important stock poisoning plant.

Plants chiefly responsible for injury and losses of livestock on
the ranges of the western United States are as follows : larkspurs
(Delphinium), aconite (Aconitum), death camas (Zygadenus),
lupine (Lupinus), loco weed (Aragallus and Astragalus), water
hemlock (Cicuta), milkweed (Asclepias), horsetail (Equisetum).

A number of plants are poisonous to the touch, causing skin
diseases. Chief of these is the poison oak or poison ivy. No
plant of the United States is more popularly recognized as harmful
to man than this.

382 ECONOMIC IMPORTANCE OF PLANTS TO MAN

One of the most deadly poisonous plants is the water hemlock
(Cicuta). Its poisonous principle is in an aromatic, oily fluid
which is found chiefly in the roots; not infrequently children eat
the roots, mistaking them for radish, parsnip, and other edible
roots. The poison hemlock (Conium maculatum), closely related
to water hemlock, is also a deadly poisonous plant, which has

FIG. 227. Water hemlock, an extremely poi-
sonous plant.

FIG. 228. A portion of the roc
stock and aerial stem of wal
hemlock, a very poisonous plai
cut in lengthwise section. Obser
the narrow parallel compa;
ments, a feature which enables o
to identify this plant.

become naturalized in the United States. It is a native of Europe.
It is the plant a decoction of which was administered to Socrates
and caused his death.

HAY-FEVER PLANTS /

It is now known that pollen is one of the chief causes of hay
fever or bronchial asthma. The plants which give the most

HARMFUL PLANTS

383

trouble are principally weeds, and certain wind-pollinated trees
which produce an abundance of pollen. Listed among the worst
hay-fever plants are the following: ragweeds, wormwoods, pig-
weeds, Russian thistle, many grasses, including corn, oats, timothy,
and wheat, and such trees as oak,
black walnut, cedar, elms, and
poplars.

The amount of pollen given off
by certain plants is enormous. It
has been computed that a single
plant of ragweed (Ambrosia trifida)
gave off between 8 A.M. and 1 P.M.
in one day approximately 8,000,000,-
000 pollen grains. These are carried
by the air currents, and reach the
membranes of the respiratory tract.
If the individual receiving them is
" sensitive " to the particular pollen,
he develops hay fever.

SELECTED REFERENCES

FIG. 229. One of the rag-
weeds, a plant which produces
an abundance of pollen. It is a
well-known hay fever plant.

Truck Crop Plants, by HENRY ALBERT
JONES and JOSEPH TOOKER ROSA, published
by the McGraw-Hill Book Company, New
York, 1928. 538 pages and 98 illustrations.
The authors express the hope that the "book
will satisfy more than the purely economic
side. There should be happiness and enjoy-
ment in the growing and handling of vegetable crops. Moreover, the gar-
dener working with various crops or plants and knowing something about their
behavior, structure, manner of growth, and relation to their environment is
usually more completely satisfied than one working with the same crops and
knowing nothing of the secrets which they hold."

The Tropical Crops, by O. W. BARRETT, published by the Macmillan
Company, New York, 1928. 445 pages, 24 plates. Forty per cent of the
earth's surface is in the tropics. The crops of the tropics now " supply a very
prominent part of the international trade in foodstuffs, fibers, and industrial
oil materials." This is a very readable account of such topics as tropical
field practices and conditions, living conditions for the tropical planter, and
the most important tropical crops.

The Standard Cyclopedia of Horticulture, by L. H. BAILEY, published by

384 ECONOMIC IMPORTANCE OF PLANTS TO MAN

the Macmillan Company, New York, 1922. In six large volumes, a total of
3639 pages and 4056 illustrations. It is described as a "discussion, for the
amateur, and the professional and commercial grower, of the kinds, char-
acteristics, and methods of cultivation of the species of plants grown in the
regions of the United States and Canada for food, for ornament, for fancy,
for fruit, and for vegetables; with keys to the natural families and genera,
descriptions of the horticultural capabilities of the states and provinces and
dependent islands, and sketches of eminent horticulturists.

Manual of Poisonous Plants of the United States, by L. H. PAMMEL,
published by the Torch Press, Cedar Rapids, Iowa, 1911. A large book of 977

FIG. 230. The golden rod is a hay fever plant. However, as its pollen is
sticky, it is not a serious offender.

pages and 458 illustrations. It contains chapters on bacterial poisons, derma-
titis, forage poisoning, poisoning from fungi, poisoning from various flowering
plants, fish and arrow poisons, classification of poisons, symptoms and anti-
dotes, the production of poisons in plants, chemistry of alkaloids, glucosides,
etc., and a catalogue of the most important poisonous plants of the United
States and Canada, and also a complete bibliography of poisonous plants.

Manual of Plant Diseases, by F. D. HEALD, published by the McGraw-
Hill Book Company, New York, 1926. 891 pages, 272 illustrations. This
book presents a view of the whole field of plant pathology. It deals with
symptoms of disease in plants, diseases due to deficiencies of food material in
the soil, diseases due to excesses of soluble salts in the soil, diseases due to

HARMFUL PLANTS 385

unfavorable water relations, diseases due to improper air relations, diseases
due to high and to low temperatures, diseases due to unfavorable light rela-
tions, diseases due to manufacturing or industrial processes, diseases due to
control practices, virus diseases, and the great number of diseases due to
parasitic organisms such as bacteria, slime molds, rust fungi, smut fungi, etc.

One Thousand American Fungi, by CHARLES MC!LVAINE, published by
the Bobbs-Merrill Company, Indianapolis, 1912. 749 pages, and 216 illus-
trations, many of which are full-page colored plates. It treats of toadstools
and mushrooms, edible and poisonous, and tells how to select and cook the
edible, and how to distinguish and avoid the poisonous.

Plants Useful to Man, by W. W. ROBBINS and FRANCIS RAMALEY, pub-
lished by Blakiston, Philadelphia, 1933. 428 pages, 241 illustrations. This
book furnishes a background of knowledge of the world's commercial plant
products both for students of botany and for those whose interests are in the
fields of geography, economics, and agriculture. It includes a discussion of
common crop plants of orchard, garden, and field, of the more usual orna-
mentals, and also of plants in tropical and subtropical countries which yield
such materials as tea, coffee, spices, drugs, fibers, and tropical fruits.

The Microscopy of Vegetable Foods, by A. L. WINTON, published by John
Wiley and Sons, New York, 1916. 701 pages, 589 illustrations. In addition
to a description of the microscopic structure of all important food products
and the organs from which they are derived, there is special mention of methods
to be employed in the detection of adulteration and the diagnosis of mixture.

Our Edible Toadstools and Mushrooms, by W. HAMILTON GIBSON,
published by Harper Brothers, New York, 1895. 337 pages, with 30 colored
plates and 57 other illustrations. "A selection of thirty native food varieties
easily recognizable by their marked individualities, with simple rules for the
identification of poisonous species/'

Botany of Crop Plants, by W. W. ROBBINS, third edition, published by
Blakiston, Philadelphia, 1931. 639 pages, 269 illustrations.

Shade Trees in Towns and Cities, by WILLIAM SOLOTAROFF. John Wiley
and Sons, New York, 1911. 287 pages, 45 plates and 35 figures. This book
deals with shade trees, their selection, planting, and care as applied to the art of
street decoration; their diseases and remedies; their municipal control and
supervision.

Economic Plants, by ERNEST E. STANFORD. D. Appleton-Century
Company, New York, 1934. 571 pages, 376 figures. "A brief survey of
several of the more important groups of plants and plant products utilized by
the human race."

INDEX

Numbers in bold-face indicate pages carrying illustrations.

Absorption, 3; conditions which in-
fluence rate of, 54; how plants are
fitted for, 249; of roots, 47; of
water, 125; process of, 51

Acacia Senegal, 353, 358

Achene, 282, 283

Aconite, 381

Aconitum, 381

Acorus calamus, 358

Acquired immunity, 98

Adventitious roots, 46, 47

Aeciospores of stem rust, 107

After-ripening, 134

Agarics, 17

Agave, 348, 351

Age: of seeds, 133; of trees, deter-
mining, 142

Agents which disperse pollen, 157

Aggregate fruits, 282

Air: bacteria in, 100; in the soil, 229;
movement of, 238; relation of
plants to, 236; temperature of, 227

Albinism in corn, 310

Aleurone layer of wheat grain, 36

Alfalfa, 347; flowers of, 170; pol-
lination of, 170

Algae, 11; different groups of, 12;
different kinds of, 11, 14

Alga-like fungi, 110

Alkaloids, 71

Almond, 342

Amanita, 380

Ambrosia trifida. 383

Ammonifying bacteria, 232, 233

Ananas sativus, 342

Anatomy of leaf, 59

Anchorage of roots, 47

Angiosperms, 327, 329

Animals: dispersal of fruits and seeds

by, 285; interrelation of to plants,

238; pollination by, 274
Anise oil, 354

Annual rings of growth, 139, 141
Annuals, 76, 336
Anther, 160, 151; cross-section of

mature, 153
Antheridium: of fern, 176; of moss,

180

Antitoxins, 98
Apple: flower of, 167, 168; rust of,

108

Approach grafting, 196, 199
Apricot, 340
Aquarium, balanced, 82
Arachis hypogoea, 343, 353
Aragallus, 381
Archegonium: of fern, 176; of moss,

180

Aristolochia, 273
Artificial cross-pollination, 306
Asclepias, 381
Ascomycetes, 110
Ascophyllum, 13

Asexual reproduction, 181, 182
Asparagus, 343; flowers and fruit of,

162

Assimilation, 3, 30; of food, 79
Astragalus, 381,353
Atropa belladonna, 357
Atropine, 71, 357

287

388

INDEX

Autophytes, 81
Avena, 329
Avocado, 340

B

Bacteria, 15, 110; ammonifying, 232,
233; diseases due to, 372; forms of,
16; in air, 100; in milk, 100; in
relation to soil fertility, 232; in
water, 99; nitrifying, 232, 234;
nitrogen fixing, 96; plant diseases
caused by, 104

Bacteria and molds: in decay, 94;
laboratory study of, 99

Balanced aquarium, 82

Bamboo, 352

Bambusa polymorpha, 362

Banana, 340

Barberry, 108

Bark, 63, 64, 139

Barley, 335

Barriers, 297

Basidiomycetes, 112

Basidium fungi, 112

Bean: fruit of Lima, 284; seeds,
stages in germination of, 117

Bees: as pollinating agents, 159; in
orchards, 160

Belladonna, 357

Berry, 281

Beta vulgaris, 344, 359

Beverage plants, 344

Biennials, 76, 366

Binomial, 329

Birds, agents in weed dissemination,
366

Blackberry, 340

Black locust, fruit of, 284

Black mosses, 19

Black pepper, 345

Blade of leaf, 57

Blanching: of cauliflower, 222; of
celery, 221

Bluegrass, germination of seeds of,
127

Blue-green algae, 11, 12

Blue and green molds, 16, 110

Borecole, 343

Botany, a definition, 6

Botulinus poisoning, 90

Botulism, 90

Box elder stem, growth in diameter
of, 64

Brassica napus, 353

Brazil nut, 342

Bread making, use of yeast in, 89

Bread mold, 101; asexual reproduc-
tion in, 183; collection of, 101;
gametic reproduction in, 102; life
cycle of, 103; nature of, 101;
sporangia of, 183; spore formation
in, 183

Bridge grafting, 196

Brown, Robert, 28

Brown algae, 12

Brussels sprouts, 343

Bryophytes, 326

Buckwheat, 335

Bud: definition, 137; flower, 136;
flower of apricot, 161; grafting,
196; lateral, 136; mutations, 314;
opening of, 137; structure of, 136;
terminal, 136

Budding, 196, 197, 198

Bulb, 62, 186

Bulbel, 187

Bulblet, 187

Burdock, fruit of, 286

Butternut, 342

By-products: derived from plants,
358; of food building, 67

Cabbage, 343

Cacao, 344, 353

Cactus, stages in development of, 126

Caffein, 10, 71

Calamus, 358

Calcium, 42; r61e of in plants, 74

Callus, 190, 191

Calyx, 151

INDEX

389

Cambium, 63, 64, 139, 141

Camphor, 358

Canada thistle, 367; rhizome of, 62

Cane sugar, 35, 67

Cannabis, 348, 349, 359

Cap of mushroom, 18

Capsicum, 346

Capsule, 283; of moss, 19

Carbohydrates, 34, 40; manufacture

of, 69
Carbon: r61e of in plants, 74; source

of, 42
Carbon dioxide: process of intake, 61 ;

test for presence of, 89
Carboniferous Age, 5
Carnivorous plants, 250, 252
Carotin, 28
Carrion fungi, 17
Carrot, 343
Caryopsis, 283
Castor oil, 353, 358
Catkins of walnut, 167
Cauliflower, blanching of, 222
Cayenne pepper, 346
Cedar oil, 353

Celery, 343; blanching of, 221
Cell, 3; division of, 302, 303; division

of in formation of eggs and sperms,

166; sap, 25; structure of, 24;

wall, 24, 119
Cells: and tissues, different kinds of,

27; different kinds of, 28; growth

of in root tip, 119; structure of, 24
Cellulose, 25, 35, 37, 70
Century plant, 351
Cereals, 335
Certified milk, 91
Chaulmoogra tree, 367
Chemical substances found in plants,

33

Chemotropism, 247
Cherimoya l 341
Cherry, 340
Chestnut, 342
Chives, 343

Chlorophyll, 28, 42, 61; effect of light
on, 223; extraction of from leaves,
222; role of in food building, 67

Chloroplasts, 26, 61

Chlorosis, 374

Chocolate, 344

Chokecherry, flower of, 169

Chromosomes, 302, 303, 304

Chrysanthemum, mutation in, 316

Chrysanthemum indicum, 300

Cicuta, 381, 382

Cinchona, 10, 71, 356

Cinnamomum camphora, 358

Cinnamon, 345

Citric acid, 71

Citrus limonia, 354

Cladophora, 13

Classes of plants, 327

Classification of plants, 321, 323, 333

Cleft grafting, 193, 194

Cleistogamous flowers, 157, 158

Climbing stems, 62

Close pollination, 271

Clove oil, 353

Cloves, 346

Club mosses, 21, 326

Club wheats, 335

Coal, 351

Coal Age, 5

Coconut, 342, 351; oil, 353; palm,
345

Cocos nucifera, 345, 353

Coffea arabica, 344, 346

Coffee, 344, 346

Coffee berry, 10

Coleus, variegated leaf of, 67

Collenchyma of stems, 63

Columbine flower, 272

Common barberry in life history of
wheat rust, 107, 108

Communities, plant, 257, 260, 262

Companion cells, 63

Compass plant, 217

Complete flower, 162

Compositae, 169

390

INDEX

Composite flower, 169, 279

Compound leaf, 58

Conidiophore, 16

Conidiospore, 16

Coniwn maculatum, 382

Conjugation in bread mold, 103

CorchoruSj 351

Cork, 63, 142, 352; cambium, 142;

cells, 27
Corm, 62, 187
Corn, 334, 335, 359; grain of, 336;

pistillate inflorescence of, 166;

seedling of, 46; smut, 17; stem,

structure of, 63; types of, 336
Cornstarch, 359
Corolla, 151

Cortex of roots, 48; of stems, 63, 64
Cotton, 348; principal varieties of,

349

Cottonseed oil, 353, 358
Cottonwood: staminate flowers of,

158; twig of, 136
Cotyledons, 117, 118, 125, 328
Crop rotation, 96, 375
Cross-fertilization, 306
Cross-pollination, 271, 274
Crown gall, 105
Cruciferae, 165, 166
Currant, 340
Cutin, 255
Cuttings: leaf, 192; propagation by,

188; root, 189, 191; stem, 189, 190,

191

Cycad, 296
Cyas revoluta, 296
Cycle of nitrogen in nature, 94
Cyperaceae, 328
Cypress, knees of, 260
Cytoplasm, 24, 25, 28, 119

D

Date, 340; paim, 341
Death camas, 381

Decay caused by bacteria and molds,
94

Dehiscent fruits, 282

Delay in the germination of seeds, 133

Delphinium, 381

Denitrification, 235

Deodar, 23

Dependent plants, 81

Determiners, 304

Dewberry, 340

Dextrin, 353, 359

Diastase, 78

Diclinous flowers, 278

Dicotyledons, 328

Differentiation, 116, 119, 331

Diffusion, 51, 52

Digestion, 3; definition of, 78; of
foods, 76; of starch, 79; of stored
foods in seeds, 126

Digitalin, 356

Digitalis, 356

Dihybrids, 319

Dioon, 296

Disease: principles of control, 374;
resistance, 375

Diseases due to rust fungi, 106

Disk flowers, 169, 171

Dispersal: by animals, 285; by pro-
pulsion, 285; by water, 285; of dry
fruits, 282; of fleshy fruits, 280;
of fruits and seeds, 285, 286; of
pollen, 156

Division, propagation by, 187

Dockage caused by weeds, 363

Dodder, 112, 113

Dominance, 311

Dominant characters, 311

Dormancy of weed seeds, 364

Double fertilization, 147

Double flowers, 170

Downy mildews, 15

Drupaceae, 167

Drupe, 280, 281

Duck meat, 150

Duck weed, 150

Dunes, succession in, 265

Durum wheats, 335

INDEX

391 .

E

Ear of corn, 163
Ecology, 243
Economic importance of plants to

man, 334-385

Egg, 153; cell, 118; nucleus, 155
Eggplant, 343
Elaeis guineensis, 353
Elements, role of in plant nutrition, 72
Elodea, 29, 53; leaf of, 25, 26; living

protoplasm in cells of, 29
Embryo, 119, 121, 155; growth of,

118, 128; sac, 146, 153; structure

of, 118
Emmer, 335
Elndosperm, 146, 336; of wheat grain,

36

End-products of food building, 67
Environment, 204, 243
Enzymes, 78
Epidermis: of geranium leaf, 59; of

stems, 63, 65
Epiphytes, 249
Equisetum, 381
Erect stems, 62
Ergotism, 380
Essential plant oils, 71
Eugenia aromatica, 346, 353
Evening primrose, opening of flowers

of, 220
Excretion, 30

Fagopyrum vulgar e, 360

Fats, 34, 36, 40; and oils, 70; test for,

41

Fehling's solution, 41
Fern: fronds of, 177; leaf of, 145;

life history of, 176; prothallia of,

178; sori of, 178; stag-horn, 21;

walking, 182
Ferns, 326; and allies, 20; and cy-

cads, 295; gamete production in,

175; how they reproduce, 175;

spore production in, 175

Fertilization, 118, 146, 153, 165, 202;
double, 147

Fiber plants, 348

Fibers, 27; bast, 351; microscopic
examination of, 351

Fibrous root system, 43

Ficus elastica, 354

Fig, 340; pollination of, 241

Filament, 150, 151

Filbert, 342

Fire blight, 105

Fission, reproduction by, 182

Flax, 348, 349; seed of, 358

Flower, 150; apple type of, 167;
bud, 136; bud of apricot, , 151;
complete, 162; composite type of,
169, 279; diagram of, 150; dicli-
nous, 278; essential parts of, 277;
grass type of, 165; incomplete, 162,
163; irregular, 278; legume type,
167; lily type of, 163; mustard
type of, 165, 167; of apple, 168;
of mustard, 167; of pea, 169; of
sour cherry, 168; of wheat, 165;
parts of, 151; plum type of, 167;
regular, 278; rose type of, 166

Flowering plants, how they repro-
duce, 160

Flowers: and insect visitation, 272;
animal pollinated, 274; cleistoga-
mous, 157, 158; different types
of, 162; disk, 169; double, 170;
hermaphroditic of strawberry, 173;
imperfect, 172; monoclinous, 278;
of chokecherry, 169; of Jerusalem
artichoke, 171; of maize, 278;
open types of, 278; pollen recep-
tion of, 277; ray, 169; specialized,
279; types of with reference to
pollination, 278; wind pollinated,
274

Fluctuations, 313

Follicle, 283

Food: assimilation of, 79: building
processes of, 66; energy, 6; for

392

INDEX

livestock, 347; materials, putrefac-
f action of, 93; plants, 335; use
made of by plant, 68

Foods, 339; kinds of stored, 77;
movement of in plants, 75; nature
of in plants, 40; preservation of,
91; process of digestion of, 77;
reserve, 70; storage and digestion
of, 76

Forest fire destruction, 369

Forms of the plant body, 10

Forsythia, 314

Fossils, 293, 294

Frond, 20; of fern, 177

Fructose, 35, 70

Fruit, 147; causes of failure to set,
171; definition of, 279; dispersal
of, 286; of black locust, 284; of
burdock, 286; of Lima bean, 284;
of pineapple, 283; of raspberry,
282; of white ash, 286

Fruits, 339; aggregate, 282; defini-
tion of, 287; dehiscent, 282; dif-
ferent types of, 287; dispersal, 279;
dry, how fitted to dispersal, 282;
fleshy, how fitted to dispersal, 280;
indehiscent, 282; means of dis-
persal, 285; multiple, 282; pome,
280; stone, 281

Functions of roots, 47

Fungi, 14; diseases caused by, 372;
principal groups of, 109

Fungicides, application of, 377

G

Gamboge, 353

Gametes of bread mold, 102

Gametophyte: of fern, 176; of moss,

180

Garcinia handuryu, 353
Garden beet, 343
Garlic, 343

Gaultheria, 346; procurnbens, 354
Gemmae of liverworts, 181, 183
Genes, 304

Gentian, 157

Genus, 328

Geotropism, 247

Geranium leaf, 57; epidermis of, 69;
response to light, 218

Germinating: seeds of mustard, 126;
stages of wheat, 123

Germination: as influenced by oxy-
gen, 124; as influenced by tem-
perature, 122; conditions neces-
sary for, 121; nature of seed, 121;
of bean seed, 117; of pollen grain,
152; of pumpkin seed, 124; proc-
ess of, 125; of seeds, delay in, 133

Gill fungi, 17

Ginger, 346

Ginkgo, 298

Girdling of stems, 75

Gladiolus, corm of, 62

Gloeocapsa, 11, 12

Glucose, 35, 67, 70, 359; manufac-
ture, 41

Glume, 165

Goldenrod, 384

Gooseberry, 340

Gossypium, 348, 353

Grafting: approach, 199; bridge,
196; cleft, 193, 194; kinds of, 196;
propagation by, 194; saddle, 196;
side, 196; tongue or whip, 194

Grain: of corn, 336; of wheat, in
section, 36

GraminaleSy 329

Gramineae, 328, 329

Grape, 338, 339, 340, 361; sugar, 35

Grapefruit, 340

Grass type of flower, 165

Grasses, habit of growth of, 22

Gravity, effect of: upon primary
root, 246; upon stem, 246

Green: algae, 12; plants, 39; plants,
converters of solar energy, 5

Growing point, 144

Growth, 3, 30; in diameter of stems,
64, 138; in length of stems, 135;

INDEX

393

of cells in a root tip, 119; of
embryo, 118, 128; of leaves, 144,
145; of plant cell, 119; of plants,
116-148; of roots, 143; of seeds
and fruits, 145

Guard cells, 59, 60, 208

Guava, 341

Guayula, 354

Gum: arabic, 353, 357; tragacanth,
353

Gums, 71; vegetable, 353

Guncotton, 358

Guttapercha, 354

Gymnosperms, 22, 327

H

Habitat, 244

Haematoxylin campechianum, 353

Halophytes, 365

Hard: seeds, 126, 134; wood, 142

Haustoria, 113

Hay fever plants, 382

Hazelnut, 342

Heartwood, 139, 140

Hemp, 348, 349, 350, 359

Herbaceous: stem of sunflower, struc-
ture of, 62; type of stem, 61

Herbs, 62, 325

Heredity, a definition, 301

Hermaphroditic flowers of straw-
berry, 173

Hesperidium, 281

Hevea brasiliensis, 354

Hickory nut, 342

Honey bee, legs of, 272

Hooke, Robert, 24, 28

Hordeum, 329

Horsetail, 20, 21, 326, 381

Host, 14

Humus, 223

Hybrid, 306; vigor, 321

Hybridization, 306

Hydrophytes, 267

Hydrotropism, 247

Hypha, 16

Hyphae, 102; of bread mold, 101
Hypocotyl, 125; of bean, 117

Immunity, 98
Inarching, 196, 199
Incomplete flower, 162
Indehiscent fruits, 282
Independent plants, 81
India rubber tree, 354
Indian compass plant, 252
Indian pipe, 111, 113
Indusium of ferns, 175
Industrial plants, 348
Insect pollination, 276
Insecticides, application of, 378
Insects, diseases caused by, 374
Internode, 136

Iron, 42; role of in plants, 74
Irregular flowers, 278

Jerusalem artichoke, 343; flowers of,

171
Jute, 351

K

Kalaw, 357
Kale, 343
Keel, 169
Kelp, 12
Key fruit, 284
Knees of cypress, 260
Koch, Robert, 87, 98
Kohlrabi, 343
Kumquat, 340

Larkspur, 381

Lateral buds, 136

Latex, 71

Layering, propagation by, 192

Leaf: anatomy of, 59; blade of
wheat, 22; compound, 58; cross-
section of, 60; cuttings, 192; loss
of water from, 208, 209; mosaic,

394

INDEX

216; of Elodea, 26; of fern, 145;
of geranium, 57,; position and light,
216; sheath of wheat, 22; simple,
58; veins, 60, 65

Leaflets, 58

Leaves: different types of, 312; epi-
dermis of, 59; how they grow, 144,
145; loss of water from, 207; of
Venus' flytrap, 251; structure of,
58; terms describing, 58; types
of, 56, 57, 58, 250

Leek, 343; inflorescence of, 164

Leeuwenhoek, 98

Legume, 282; type of flower, 167

Leguminosae, 167

Lemma, 165

Lemna, 150

Lemon, 340, 346; oil, 354

Lenticels, 236

Lettuce, 343

Leucoplasts, 27

Lichen, 19; reindeer, 110

Lichens, 111, 262, 263

Life cycle of bread mold, 103

Light: and leaf position, 216; as
influencing the movement and
position of plant organs, 216;
duration of, 218; effect upon form
of plant, 245; energy, 6; intensity
of, 218; its influence on size, form
and structure, 216; quality of, 218,
223; relation of to plant life, 215;
response of leaves to, 216; response
of plants to, 218, 219, 245; r61e of
in food building, 66

Lilium: candidum, bulbel in, 187;
grandiflorum, 164; tigrinum, bulb-
lets in, 187

Lily type of flower, 163

Lime, 340

Linnaeus, 330

Linseed oil, 353

Linum, 348; iwitatissimum, 349, 353

Lipase, 79

Lister, 99

Live oak, 23

Liverworts, 19, 20, 326; gemmae of,

181, 183
Loco weed, 381
Lodging of cereals, 73
Loquat, 340
Lupine, 381
Lupinus, 381

M

Mace, 346

Magnesium, 42; role of in plants, 74

Maidenhair tree, 298

Maize oil, 353; types of, 336

Mangosteen, 341

Mangelwurzels, 347

Mango, 340

Manila fiber, 350, 351; hemp, 351

Marchantia, 20

Mass selection, 320

Maximum temperature, 211; for
germination, 122

Meadow sage, pollination of, 239

Medicago, 347

Medicinal plants, 355

Mendel, Gregor, 306

Mendel's laws, 309; of heredity, dia-
grams illustrating, 307

Mentha, 346; piperitdj 354

Menthol, 358

Mesophyll of leaf, 57, 59

Mesophyte, laboratory study of, 270

Mesophytes, 258

Micropyle, 146, 153

Mildews, 15

Milk: bacteria in, 100; certified, 91;
how diseases may be spread by, 90;
pasteurization of, 91; pasteurizer,
91

Milkweed, 381

Millet 335

Mineral nutrients of the soil, 229

Minimum temperature, 211; for ger-
mination, 122

Moccasin flower, 275

INDEX

395

Molds, 15; blue and green, 16, 110

Monoclinous flowers, 278

Monocotyledons, 328, 329

Morning-glory, rhizome of, 62

Morphine, 71

Mosaic, 216, 248

Moss, 179; antheridium of, 180;
archegonium of, 180; asexual plant
of, 179; gametophyte of, 180; life
cycle of, 180; spore production in,
179; sporophyte of, 180

Mosses, 19, 262; and liverworts, 326;
groups of, 19; how they repro-
duce, 175

Movement and position of plant or-
gans as influenced by light, 216

Mulberry, 340

Multiple fruits, 282

Musa textilis, 350, 351

Mushroom: longitudinal section of,
18; poisonous, 380; shaggy-mane,
84

Mushrooms, 17, 18, 84, 111, 380;
edible, 379

Musk melon, 340

Mustard: flower of, 167; germinat-
ing seeds of, 125; type of flower,
165

Mutants, 314

Mutation, 313; bud, 314; in sun-
flower, 315

Mycelium, 102; of bread mold, 101

Myristica fragrans, 346, 354

N

Narcissus, bulb of, 62
Natural: immunity, 98; selection,

314

Nature of seed germination, 121
Nectar glands, 160, 273
Nectaries, 160, 273
Nectarine, 340
Net venation, 57
Nicotine, 71
Nitrates, 42, 73

Nitrifying bacteria, 232, 234

Nitrogen, 42; cycle in nature, 94, 96,
235; fixation, 233, 234; fixing
bacteria, 96; r61e of in plant, 73

Node, 136

Non-green plants: main characteris-
tics of, 83; nutrition of, 81-115

Nostoc, 11, 13

Nucellus, 153; of wheat grain, 36

Nuclei, sperm, 146

Nucleus, 24, 28, 119; tube, 146

Nut, 284, 342

Nutmeg, 346; oil, 354

Nutrition: of green plants, 39-80;
of non-green plants, 81-115

Nux-vomica, 358

O

Oats, 335; spikelet of, 164

Oil, first visible product of photo-
synthesis, 67

Oils, 36, 353

Olea europea, 353

Olive, 340; oil, 353

Onion, 343; skin of , 26

Opium, 355

Optimum temperature, 211; for ger-
mination, 122

Orange, 340, 361

Orchard heating, 213

Orchid: flower, 274, 331; method of
propagating, 185

Orders, 327

Organic acids, 71

Organization and composition of
plants, 9-38

Organs, 3, 9, 30, 31

Oryza sativa, 337

Oscillatoria, 13

Ovary, 150, 151

Ovule, 153; structure of, 153

Ovules, 146, 151

Oxygen: and germination, 124; given
off in photosynthesis, 67; r61e of in
plants, 74

396

INDEX

Palaquium oblongifolia, 354
Palisade: layer, 60; parenchyma,

60

Palm, 344; oil, 353
Palmately veined leaves, 57
Papaver somniferum, 355
Papaya, 340
Parallel venation, 57
Parasite, 14
Parasites, 86, 252
Parasitic plants which cause disease,

97, 104
Parsley, 343
Parsnip, 343

Parthemum argentatum, 354
Partheiiocarpy, 155
Parthenogenesis, 155
Pasteur, Louis, 87, 98
Pasteurization, 89; of milk, 91
Pasteurizer for milk, 91
Pathology of plants, 87
Pea, flower of, 169
Peach, 340; fruit of, 280
Peanut, 342, 343; oil, 353, 360
Pear, 340

Peat bog, 261; mosses, 19
Pecan, 342
Pedicel, 160

Pedigree culture, 319, 320
Pelican flower, 273
Pepo, 281
Pepper, 345, 347
Peppermint, 346; oil, 354
Pepsin, 79
Perennials, 76, 366
Pericarp of wheat grain, 36
Permanent wilting percentage, 224
Persimmon, 341
Petal, 160
Petals, 151, 162
Petiole of leaf, 57
Petrified wood, 294, 296
Phloem, 64, 139; fibers, 63; of

stems, 63; parenchyma, 63

Phoenix: dactylifera, 341; sylvestris,
344

Phosphates, 42

Phosphorus, 42 ; role of in plants, 73

Photosynthesis, 32, 42, 67, 69;
process summarized, 68

Phototropism, 247

Phy corny cetes, 110

Pileus of mushroom, 18

Pine: cones of, 328; four-year-old
stem of, 139; wood, 141, 142

Pineapple, 340, 342; fruit of, 283

Pinnately veined leaves, 57

Piper nigrum, 345, 347

Pistachio, 342

Pistil, 150, 151, 162

Pistillate: flowers, 163; flowers of
asparagus, 162; inflorescence of
corn, 166

Pitcher plant, 250, 262

Pith, 63, 139, 140

Pits, bordered, 27

Plant: body of, 9; forms of, 10; cell,
how it grows, 119; communities,
257, 260, 262; communities, meso-
phytic, 264; diseases, 371; dis-
eases, causes of, 372; diseases
caused by bacteria, 104; diseases
caused by parasitic plants, 104;
diseases due to smut fungi, 109;
dyes, 353; ecology, 243; foods,
nature of, 40; pathology, 87;
skeleton, materials which compose,
70; succession, 265

Plants: and animals, interrelation of,
238; carnivorous, 250; classifica-
tion of, 321, 323, 333; dependent,
81; development and improve-
ment of, 290-322; economic im-
portance of, 334; fiber, 348; four
great groups of, 326; growth of,
116-148; hay fever, 382; how
they interfere with man, 361; in-
dependent, 81; industrial 348; in
what ways they have changed, 291;

INDEX

397

medicinal, 355; nutrition of non-
green, 81-115; poisonous, 379;
reproduction of, 149-203; shade and
sun, 222

Plasmolysis, 53

Plastids, 25, 28

Plum, 340; type of flower, 167

Plumule, 117; 125

Pod, 284

Poison: hemlock, 382; oak, 381

Poisonous plants, 379

Pollen, 151; agents which disperse,
157; dispersal of, 156; grain, 152;
different kinds of, 156; germina-
tion of, 152; longevity and viabil-
ity of, 161; quantity of, 157; tube,
146; where produced in the flower,
277

Pollination: artificial, 321; close,
271; cross, 271; how plants are
related by structure to the process
of, 271; immediate effect of, 161;
of fig, 241; of meadow sage, 239;
self, 271, 277; types of flowers with
reference to, 278

Polypodium, 253

Pomaceae, 167

Pome fruits, 280

Pond scum, 13

Poppy, 355

Pore fungi, 17, 18

Potassium, 42 ; role of in plants, 74

Potato, 343, 361; bulb of, 62; stor-
age cells of, 26; tuber of, 35;
wild, 317

Powdery mildews, 15

Prickly pear, 259

Primary roots, 47

Pronuba moth, 240

Prop roots, 47

Propagation: by cuttings, 188; by
division, 187; by grafting, 194;
by layering, 192; by layers, 192;
by separation, 186; by stolons or
runners, 193; by suckers, 193; of

orchids, 186; of plants artificially,
184; of strawberry, 188

Propulsion, dispersal of fruits and
seeds by, 285

Prostrate stems, 62

Prothallium of ferns, 175, 176, 178

Proteins, 34, 36, 40, 70; test for, 41

Protococcus, 11, 12, 252; asexual re-
production in, 182

Protoplasm, 3, 24, 119; chemical
properties of, 29; nature of, 28;
physical properties of, 28; physi-
ological properties of, 29

Protoplasmic membrane, 52

Pteridophytes, 326

Ptomaine poisoning, 90

Puff balls, 17, 18, 19

Pulque, 360

Pumpinella anisum, 354

Pumpkin, 343; seed, germination of,
124

Purity test, method of making, 131,
132, 133

Putrefaction of food materials, 93

Q

Quercus suber, 352
Quince, 340
Quinine, 10, 71, 356

R

Rafflesia, 150

Ragweed, 382, 383

Rape oil, 353

Raspberry, 340; fruit of, 282

Raw materials: movement in plants,
61; used by green plant, 41, 43

Ray flowers, 169; of Jerusalem arti-
choke, 171

Receptacle, 160

Recessive characters, 311

Red: algae, 12; snow, 2

Regeneration, 188

Regular flowers, 278

Reindeer: lichen, 110; moss, 19

398

INDEX

Reproduction, 3, 30; and wind, 238;
asexual, 149; by fission, 182; by
means of sex, origin of, 199; by
spores, 182; in bread mold, 183;
in ferns and mosses, 175; in flow-
ering plants, 150; in plants, 149;
sexual, 149

Resins, 71, 352

Respiration, 3, 30, 68

Resurrection fern, 253

Rhizoids, 19; of bread mold, 101, 102;
of ferns, 176; of moss, 179

Rhizomes, 62, 188

Rhizopus nigricans, 101, 102, 104;
life cycle of, 103

Rice, 334, 335, 337, 360

Ricinus communis, 353, 358

Rockweed, 12, 13

Root: crops, 47; cuttings, 189, 191;
destruction of by transplanting, 49;
duration of, 50; growth and temper-
ature, 212; hairs, 48; in lengthwise
section, 48; of sugar beet, 36; of
wheat seedling, 50; pruning, 45; re-
lation to soil particles, 49; storage
of sweet potato, 46; structure, 48,
50; tip, 120; tubercles, 97

Root cap, 48

Roots: adventitious, 47; field study of,
44; factors which influence growth
and character of, 45; growth in
length of, 143; how they grow, 143;
kinds of, 43; kinds of and func-
tions, 47; primary and secondary,
47; prop, 47; systems, extent of,
46, 225

Rootstock, 188, 364

Rosaceae, 166

Rose type of flower, 166

Rosette, 216, 248

Rosin, 352

Rotation of crops, 370

Rubber, 354

Runners, 185; of strawberry, 188;
propagation by, 193

Rust fungi: diseases due to, 106;
germinating spores of, 106; of
apple, 108; of wheat, aeciospores
of, 107; teliospores of, 107

Rusts, 16

Rutabaga, 343, 347

Rye, 335

S

Sabina virginiana, 353

Sac fungi, 110

Saccharum offitinarium, 344

Sago palm, 296

Samara, 284, 286

Sanitation, 376

Sap, 25, 37

Saprophytes, 14, 83, 86; how they
secure food, 86; how they spread,
86; in the soil, 94; nutritive rela-
tions of, 88; use of in preparation
of food, 88

Sap wood, 140

Sargasso Sea, 1 2

Sargassum, 12

Scarifying, 126

Schimper, 1

Schultze, Max, 28

Scientific names, 329

Scion grafting, 196

Scouring rush, 20, 21

Sea mosses, 19

Sea weeds, 12

Secondary roots, 47

Secretions, 71

Seed, 147, 155; depth of planting,
128; disease-free, 375; dispersal of
weed, 365; fern, fossil of, 294;
germination and temperature, 212;
germinator, 134; nature of, 121;
plant, life cycle of, 164; plant
body, 23; plants, 9, 21, 327;
plants, diseases caused by para-
sitic, 372

Seedling: of corn, 46; of wheat, 50

Seeds: conditions affecting vitality
of, 131; delay in germination of

INDEX

399

133; dispersal of, 279; germina-
tion of, 124, 126, 127; hard, 126;
how dispersed, 287; impure com-
mercial, 365; means of dispersal of,
285; of bluegrass, germination of,
127

Seeds and fruits, how they grow,
145

Segregation, 310

Selaginella, 253

Selection, mass, 302

Self: pollination, 271, 277; sterility,
172

Semi-permeable membranes, 52

Sepals, 150, 151, 162

Separation, propagation by, 186

Sex in plants, origin of, 199

Sexual reproduction, 149

Shaggy-mane mushroom, 84

Shield budding, 197

Shrubs, 325

Sieve tubes, 63, 139

Silique, 283

Simple leaf, 58

Sisal, 348

Smut: fungi, plant diseases due to,
109; of corn, 17

Smuts, 17

Smyrna fig, pollination of, 276

Soft wood, 142

Soil: air of, 229; factors influencing
temperature of, 227; fertile and
infertile, 231; fertility, bacteria and
relation to, 232; kinds of, 223;
living organisms in, 232; mineral
nutrients of, 229; physical proper-
ties of, 223; relation of plants to,
222; saprophytes in, 94; solution,
concentration of, 226; structure of,
223; texture of, 223; temperature
of, 226

Sofa max, 360

Solanum jamesii, 317

Solar energy, 6, 39

Solomon seal, rhizome of, 62

Solute, 52

Solvent, 52

Sorghums, 335

Sorus of fern, 175, 176, 178

Sour cherry, flower of, 168

Soursop, 341

Soy bean, 360

Spanish moss, 19, 250

Spawn of mushroom, 18

Species, 324

Sperm: nuclei, 146, 153; nucleus, 165

Spermatophytes, 327, 329

Sphagnum, 261

Spices, 345

Spike of wheat, 22

Spikelet, 165; of oats, 164

Spinach, 343

Spireme, 302

Spirogyra, 13

Spongy parenchyma, 60

Sporangia: of bread mold, 102, 183;

of ferns, 175; of moss, 179
Sporangium of fern, 176
Spore case of moss, 19
Spores: of moss, 179; reproduction

by, 182
Sporidia, 106
Sporophyte: of fern, 176; of moss,

180

Spring wood, 139, 142
Squash, 343: living protoplasm in

cells of, 29
Stag-horn fern, 21
Stalk of leaf, 57
Stamen, 150
Stamens, 151, 162
Staminate flowers, 163; of asparagus,

162; of cotton wood, 168
Standard, 167
Starch, 34; digestion, 79; first visible

product of photosynthesis, 67;

formation in leaves, 67; grains, 27;

storing cells, 27; test for, 40, 77
Stem: cross-section of six-year-old

woody, 140; cuttings, 189, 190,

400

INDEX

191; growth in diameter of, 64;

of pine, 139; section of, showing

shedding leaf, 136; structure of

corn, 63; structure of woody, 63
Stem rust: life cycle of, 106; of

wheat, 106
Stems: growth in diameter of, 138;

how they grow in length, 135;

structure of, 62; structure of

woody, 138; types of, 61, 62
Stigma, 160, 151
Stimuli, kinds of, 245
Stipules of leaf, 57
Stock, 196
Stolons: of bread mold, 102; of

strawberry, 188; propagation by,

193

Stoma, 208
Stomata, 59
Stone cells, 27
Stone fruits, 281
Storage: cells, 26; of foods, 76
Stored foods in seeds, digestion of, 126
Strawberry, 340; hermaphroditic

flowers of, 173; propagation by

means of runners, 188
Structure and function, relation of,

31

Strychnine, 358
Strychnos nux-vomica, 358
Style, 160, 151
Succession, plant, 265
Suckering, 194
Suckers, propagation by, 193
Sucrose, 35, 67, 70
Sudan grass, 347
Sugar: beet, 344, 359; beet root, 36;

cane, 344, 360; plants, 344; test

for, 41
Sugars, 35
Sulphates, 42

Sulphur, 42; r61e of in plants, 73
Summer wood, 139, 141, 142
Sundew, 250
Sunflower, mutation in, 316

Swamp plants, 261, 269

Sweet potato, 343, 361; storage root

of, 46

Sweet turnip, 347
Syconium, 282
Symbiosis, 96
Synechocystis, 11

Tannin, 10, 71

Tap root system, 43; of sugar beet,
44

Taraktogenos kurzii, 357

Tassel of corn, 163

Tea, 344

Teliospore, 106, 107; of stem rust,
107

Temperature: air, 227; and germi-
nation, 122, 123, 212; and root
growth, 212; cardinal, 211; its
effect upon growth, 211; of the
soil water, 226; relation to plant
life, 210

Temperatures, resistance of plants to
low, 212

Terminal bud, 135

Thallophyta, 10

Thallophytes, 326

Thallus plants, 10, 326

Thea sinensis, 344

Thein, 71

Theobroma cacao, 344, 353

Thygmotropism, 247

Thyme oil, 354

Thymus vulgaris, 354

Tillandsia, 19, 250

Timothy, 347; in bloom, 159

Tissues: and cells, different kinds,
27, 31; and organs, 30

Toadstools, 17, 380

Tolmiea, vegetative reproduction of,
186

Tomato, 340, 343, 361; fruit of, 281

Tooth fungi, 17, 18

Toxins, 87

INDEX

401

Tracheid, 27, 142

Tradescantia, 59

Transfer of stored foods in seeds, 126

Transpiration, 254; stream, 207

Tree surgery, 378

Trees, 325; determining age of, 142

Tricholoma, longitudinal section of,

18

Trifolium, 347
Triticum, 329
Tropisms, 247
True mosses, 19
Trypsin, 79

Tube: nucleus, 146, 153; pollen, 146
Tuber, 62, 187; of Irish potato, 36
Tubercles, root, 97
Turgidity, 53
Turnip, 343
Turpentine tree, 353
Twig, characteristics, 134

U

Ulothrix, 13; origin of sex in, 200;
reproduction in, 201; spores of, 200
Ulm, 13

Umbel of leek, 164
Unit characters, 309
Uredinia, 107
Urediniospore, 107

Vaccination, 98

Vacuole, 119

Vanilla planifolia, 346

Variation: heritable, 313; in plants,

290
Vascular bundles, 63; structure of, 63;

rays of, 63, 139, 140
Vegetable gums, 353
Vegetables, 342

Vegetative propagation, 184, 185
Veins of leaf, 57, 59, 60, 65
Velamen, 250
Venation of leaves, 57
Venus' flytrap, 250, 251

Vessels, 63; different kinds of, 27
Vitality of seeds, conditions affecting,

131
Vitis labrusca, 339

W

Walking fern, 182

Walnut, 342

Wandering Jew, 59

Water: agent in weed dissemination,
365; amount lost by plant, 209;
amount of in plants, 205; and ger-
mination, 121; bacteria in, 99;
conditions influencing intake of
from soil, 224; content of plant
parts, determination of, 206; dis-
persal of fruits and seeds by, 285;
importance in plant life, 204; in
the soil, 224; limiting factor in
plant growth, 205; loss in plants,
253; measuring amount lost by
leaves, 209; movement of in stems,
65; power of soils to deliver, 225;
problem of the plant, 206; require-
ment of plant, 210; role of in plant,
72; supply, how plants are related
by structure to, 249

Water cress, 343

Water hemlock, 381, 382

Water lily, 260

Watermelon, 340

Water plants, 5

Weeds, 361; and dockage, 363; an-
nual, 366; biennial, 366; classes of,
366; dissemination of, 365; insects
and fungus pests harbored by, 362;
losses caused by, 362; methods of
control, 4, 369; perennial, 76, 368;
seed dispersal of, 365; seed pro-
duction of, 364; seeds, dormancy
of, 364

Weeping birch, 313

Wheat, 334; economic types of, 335;
flower of, 165; nominating stages
of, 123; grain, microscopic section

402

INDEX

of, 36; plant, 22; seedling, 50;
stem rust of, 106

White ash, fruit of, 286

White pepper, 345

White snakeroot, 380

Whorled milkweed, 381

Wilting of plants, 206

Wind: agent in seed dissemination,
365; and reproduction, 238; dis-
persal of fruits and seeds by, 286;
pollination, 274; timber, 237

Wings, 167

Wintergreen, 346; oil, 354

Wood, 63, 139, 351; fibers, 63; hard,
142; of pine, 141; parenchyma, 63 ;
petrified, 294; soft, 142; spring,
142; summer, 142

Woody stems: structure of, 63; two
year-old structure of, 140

X

Xanthophyll, 28
Xerophytes, 257, 259
Xylem of stems, 63

Yeast: cells, nature of, 89; effect of
on sugar solution, 89; use in bread-
making

Yucca, 264; pollination of, 240, 277

Zamia, 297

Zea mays, 329, 353; pistillate inflor-
escence of, 166
Zinziber officinale, 346
Zygadenus, 381
Zygospore of bread mold, 102
Zygote, 306; of bread mold, 103