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TRANSEAU. SAMPSON &TIFFANY

MARINE BIOLOGICAL LABORATORY.

Received Oct. 24. 19 4Q..

Accession No. 525o5

Given by Harper & B r o s • .
Place, New York City

***]^o book OP pamphlet is to be removed from the Iiab*-
ofatopy taithout the permission of the Trustees.

CO

TEXTBOOK OF BOTANY

Plate I

Above: Alpine tundra, Grand Teton Mountains, W'voming. Photo by L. R. Wilson, Coe
College. Beloic: "Red snow" and alpine tundra, Mt. Rainier, Washington. Photo by W. H.
Camp, New York Botanical Garden.

TEXTBOOK
OF

BOTANY

E. N. TRANSEAU H. C. SAMPSON

Professor of Botany Professor of Botany

Ohio State Uiiwersity Ohio State University

L. H. TIFFANY

Professor oj Botany
Northwestern University

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HARPER & BROTHERS PUBLISHERS

NEW YORK LONDON

TEXTBOOK OF BOTANY

Copyright, 1940, by Harper 6- Brothers

Printed in the United States of America

All rights in this hook are reserved.

No part of the hook may' be reproduced in any

manner ichatsoever without written permission

except in the case of brief quotations embodied

in critical articles and revietvs. For information

address Harper ir Brothers

FIRST EDITION

CONTENTS

<><x><>c><x><^<><^><><^><£K^<^><^^

Preface vii

I. Plant Science 1

II. The Parts of Plants 10

III. Learning to Name Plants 18

IV. Seasonal Aspects of Plants 26
V. Local Plant Communities 40

VI. Points of View in the Interpretation of Plant

Behavior 50

VII. Cells as Biological Units 59

VIII. The Tissue System of Leaves 68

IX. Environment and Leaf Development 79

X. Hereditary Differences in Leaves 84

XI. A Bit of Useful Chemistry 95

XII. The Food of Plants 101

XIII. Food Manufacture: The Synthesis of Sugar —

Photosynthesis 108

XIV. Food Manufacture: Factors Influencing the Rate

OF Photosynthesis 115

XV. Food Manufacture: Synthesis of Starches 129
XVI. Food Manufacture: Synthesis of Fats and

Proteins 138

XVII. Uses of Food in Plants: Respiration 149
XVIII. Uses of Food in Plants: Respiration and Plant

Development 160

XIX. Uses of Foods: Substances Made From Foods 168

XX. Some Biological Relations of Green Plants 180

XXI. Interrelations of the Parts of a Plant 188

XXII. Physical Processes Involved in the Movement of

Materials in Plants 195

XXIII. Plant Behavior Related to Osmosis 209

XXIV. The Loss of Water Vapor From Plants:

Transpiration 221

0^tJOt3

vi CONTENTS

XXV. Transpiration Affects Plant Development and

Distribution 233

XXVI. Forms and External Features of Stems 245

XXVII. General Regions and Processes in Stems 262

XXVIII. Tissues and Processes in Stems 278

XXIX. Roots: Development and Structures 294

XXX. Roots: Processes and Soil Relations 315

XXXI. Initiation of Fowers 333

XXXII. Flowers, Fruits, and Seeds 351

XXXIII. Sexual Reproduction in Flowering Plants 371

XXXIV. Growth, Dormancy, and Germination of Seeds 387
XXXV. Vegetative Multiplication of Flowering Plants 400

XXXVI. Origin of Plants Used by Man 423

XXXVII. Heredity in Plants 438

XXXVIII. Cross-Fertilization and Hybrid Segregation 458

XXXIX. Mutations 474

XL. Types of Variations and Diversity of Organisms 490

XLI. Non-Green Plants 504

XLII. The Biology of Bacteria 515

XLIII, Bacteria of the Soil 531

XLIV. The Fungi 541

XLV. Plant Diseases 567

XLVI. Under-Water Environments 587

XLVII. The Algae 606

XLVIII. Mosses and Liverworts 638

XLIX. Ferns, Club Mosses, and Equisetums 656

L. The Seed Plants 678

LI. Some Families of Flowering Plants 693

LII. Plants of the Past 719

LIII. The \'egetation of North America 740

Index 797

PREFACE

<><e><x><><x><x><><j><^<><x^

The accumulation of the tacts of science, as well as the interpretation,
evaluation, and application of these facts, is a continuing process. It
must follow, then, that the teaching of science cannot remain a static
procedure and that new texts of science must from time to time be
written.

This textbook of botany represents the authors' ideas of some of the
gradual changes in objectives, content, emphasis, sequence, and pro-
cedure in general botany that are necessary to incorporate effectively
appropriate new discoveries in science and their various applications
to human welfare. For anyone to assume that these features of general
botany are already standardized is to close one's eyes to the continual
increase of new material available for general botany courses as well
as to a scientific consideration of the problems of teaching.

Present-day botanists, moreover, cannot escape responsibility for
some share in a general educational program for students and laymen.
Many students of diverse interests elect general botany as their sole
requirement for a course in science; a smaller number will enroll
because of a desire for special training in the subject, still others merely
because it is one of the science requirements in the college program.
We cannot meet our obligations to these students merely by insisting
that they memorize what scientists ha\e disco\ered or by trying to tell
them what the scientific method is and what may be accomplished
by it. For the scientific method to become meaningful and a habit of
procedure, the students must experience it repeatedly and see it ex-
emplified as a nomial part of classroom procedure as well as a fait
accompli in the literature they are required to read.

As all botanists are fully aware, plants are complex systems influenced
by and in turn influencing other systems of their immediate environ-
ment, both animate and inanimate. There can be no clear appreciation
of the fundamental interrelations among these dissimilar systems with-
out some basic knowledge of the organization of the structures and the
nature of the processes inherent in them. In short, courses in botany
much of the time in the classroom must necessarily be devoted to acquir-

vii

viii PREFACE

ing this preliminary background. For that reason we have included in
the textbook numerous discussions of various kinds of interrelations
for further consideration outside of class.

Botany is primarily a concrete science and is most effectively ap-
proached through a first-hand study of plants. We have found class
discussions while observations are being made a most satisfactory
approach. A workbook is available as a convenient aid in some of the
class periods and as a means of further suggestions for work outside
class periods. The textbook has been written primarily to supplement
what is observed and discussed. It is intended to help students review,
organize, amplify, and correct their own observations, inferences, and
ideas, whether they obtained them first-hand or by hearsay. We have
tried to interfere as little as possible with the teacher who prefers to
have students observe and discuss plant phenomena before books are
consulted. Nearly every chapter has been written with the assumption
that it will not be read by the student until the instructor thinks that
the student's own observations should be supplemented by what is
written. The sequence of topics within a chapter is not always the
best one to follow in observation and oral discussion.

Since there is no fixed standard of knowledge and procedure that
one may rely upon indefinitelv and in all circumstances, we have tried
to present a cross section of what appears to us to be in keeping
with the immediate present. We believe that there is a body of basic
information, scientific inferences, and points of view about the plant
part of our environment that is as important a background for students
who intend to major in any phase of plant science as it is for that much
larger group of students who are taking the course in botany as part
of their general education. Our aim has been to help the students
organize that background and appreciate some of the numerous inter-
relations of plants and of plants and animals, particularly man. It is
of course impossible to incorporate in a book of this size all the informa-
tion demanded by students while they are comparing observations and
inferences. Furthermore, some of the information needed is best sup-
plied by the teacher, either directly or through the assignment of
specific observations or references.

The general course may begin at different seasons of the year, when
readily available plant materials may differ greatly. For that reason
the first five chapters contain discussions of materials which are useful
in approaching the study of plants under different conditions. As the

PREFACE ix

background of information increases in subsequent chapters, the inter-
relations discussed become more intricate and the student is expected
to participate more and more inteUigently in discussions involving a
knowledge of structural systems of cells or of whole plants, of sequences
of events in the development of a plant or of the whole life cycle, of
the intimate relations of chromosomes and hereditary factors, of the
interrelations of physiological processes and development, of physi-
ological processes and environment, or of a combination of all these
phenomena.

Material from all the artificial subdivisions of botany is included
not because we intended to survey the entire field, but because the
synthesis of material from all phases of botany is necessary to give a
student a general perspective of his relations to his plant environment.
In keeping with the scientific attitude, throughout all this diversitv
we have persistentlv tried to maintain a terminology and phraseologv
consistent with the outlook of special students in the various fields of
botany.

ACKNOWLEDGMENTS

The authors are deeply indebted to their associates at The Ohio
State University, to Dr. O. F. Curtis of Cornell Universit) , to Dr. C. L.
Wilson of Dartmouth College, and to Dr. W. F. Loehwing of the State
University of Iowa for their sympathetic and helpful criticism of the
manuscript.

The illustrations have been secured from many sources, and are
properly credited to those who have furnished the photographs, nega-
tives, and materials from which photographs were made. We are
particularly indebted to the World Book Company for pemiission to
use some of the drawings and illustrations in the General Botany pub-
lished by them. Through the courtesy of the U. S. Forest Service, the
U. S. Soil Conservation Service, the U. S. Department of Agriculture,
and the New York Botanical Garden, it has been possible to secure
photographs suitable for illustrating the various vegetation types of
North America and some of the featvues described in other chapters.
Miss Jane Roller, of Washington, D. C, has made many of the new
drawings. Mr. Eugene B. Wittlake has made some of the diagrams
and most of the photomicrographs. Mr. Gordon Growl, Mr. Fred
Norris, and Mr. Clyde Jones have made photographs of experiments,
cultures, and some landscapes especially for this book.

The greatest debt of all, of course, is to the thousands of fellow
scientists who cannot be mentioned individually but whose scientific
efforts have supplied humanit\^ with a fund of valuable data free for
all to use. A relativeh' few references appear at the end of certain
chapters, but it should not be assumed that we have always made
the best possible selections.

E. N. T.

H. C. S.

L. H. T.
Columbus, Ohio
Evanston, Illinois
September 1, 1940

TEXTBOOK OF BOTANY

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CHAPTER I

PLANT SCIENCE

Every animal is in some way dependent upon plants every moment of
its life. One may well begin the studv of botany by inquiring into the
ways in which man and other animals of his own community are de-
pendent upon plants and plant products, or are otherwise influenced by
plants. This local survey may be extended into other communities and
geographic regions. If the inquiry is also projected into historic and pre-
historic records one may learn how plants and a knowledge of plants
have played a significant and at times a decisive role in the advance and
decline of civilizations in many parts of the world.

Everywhere man and other animals use plants directly as food, or eat
other animals that feed on plants. Directly or indirectly all animals are
dependent upon plants as the source of food. During the growth of the
animal a part of this food is transformed by physiological processes into
the substances of which the body is composed. A larger part of it is
oxidized within the tissues of the animal, and energy is liberated. As a
result of this liberated energ\' the temperature of the bodv of the animal
is often maintained above that of its surroundings, and the animal is
also able to move about and to do other kinds of work.

Through the burning of such plant derivatives as wood, coal, petro-
leum and gas in furnaces or engines man secures light, heat, electricity,
and mechanical energv by which he has been able to modify his immedi-
ate environment, supplement his own ability to move about, and trans-
port materials and supplies wherever they are needed or desired. These
two products of green plants — food substances and chemically bound
energy — are indispensable to all living organisms.

There are also certain substances derived from plants which are essen-
tial to the health and well-being of all animals — for example, the vitamins
and mineral elements bound in organic compounds.

With the progress of civilization many other plant products have be-
come indispensable or at least highly desirable. From plants we obtain
the materials of which most of our houses are built, finished, and fur-

1

2 TEXTBOOK OF BOTANY

nished. Much of our clothing is made from plant fibers. Paper and rubber
made from plant products have increased our means of communication
and transportation. Many of our beverages and some of our important
medicines contain plant derivatives. A large part of the population earns
a living by supplying itself and the rest of society with plant parts or
plant products.

Certain of the larger plants are used to beautify our homes, to decorate
the landscape, and in other ways to add to the enjoyment of living. We
use trees to protect us from intense heat and light. Forested areas have
become the centers of many forms of sport and recreation. They are also
continuous sources of cover and food to countless animals both large and
small. Plants not only hold the soil against erosion by wind and water,
but they improve its texture and composition; and myriad microscopic
plants contribute to its fertility.

But the relation of plants to animals is not wholly beneficent. Weeds,
for example, may decrease the yield of crops and increase the labor of
cultivating them. Many plants contain toxic and irritating substances that
cause suflFering, illness, or the death of human beings and other animals.
Certain microscopic plants may invade the tissues of the animal body
and lead to disease and death. Others cause diseases and destruction of
the larger plants, and still others grow on commercial food products in
shipment and storage and lessen their value or destroy them. Such plants
test the ingenuity of every generation of human beings to devise means
of avoiding, controlling, or eradicating them.

The value of plant science to society has been increasingly appreciated
as knowledge of plants has advanced. The farmer who plants a field; the
orchardist who attempts to secure a crop of fruits; the florist who seeks
the production of ornamental plants and flowers; the forester who tries
to obtain a profitable yield of timber on a tract of land; the gardener who
supplies our markets with edible and succulent leaves, stems, and roots;
industriahsts who seek to find new sources and new forms of plant prod-
ucts to meet our social needs; and the conservationists who strive to
preserve our soils from wind and water erosion and to rescue our disap-
pearing fish and game animals — all these find themselves confronted by
an often bewildering array of problems that lie within the field of plant
science, or botany.

Many of these problems have been solved by a study of the structures,
the chemical composition, the physiological processes, the heredity, and

[Chap. I PLANT SCIENCE 3

the life histories of plants; or by a study of plants in relation to soil and
climatic conditions. Other problems have been solved through the dis-
covery of facts about the growth of bacteria and fungi, their various
effects on the growth of other organisms, and the means of controlling
their development.

Numerous unsoKed problems remain. That societv recognizes the
need of continued and far-reaching research with plants is attested by
its maintenance of forest and agricultural research stations in every
country and in nearly every state and province throughout the world.
Through the years these institutions have provided an increasinglv im-
portant addition to research by teachers and graduate students in col-
leges and universities. As a result of these investigations each generation
has an increased wealth of botanical knowledge available for both per-
sonal and social needs. Through instruction one may readilv learn how
to select and apply a portion of this accumulated knowledge.

Thus far we have stressed the importance of plants and of the science
of plants mainly in relation to human economics, occupations, health,
and the survival of all animals. The general desire of man to know and
interpret his environment and to speculate about himself and his environ-
ment must also be emphasized here. Plants are a conspicuous and impor-
tant part of that environment. In addition to the interrelations of plants
and animals there are many important relations among plants them-
selves, between plants and their phvsical environments, and between
the parts of each individual plant. These interrelations have been and
still are interpreted on the basis of several diverse assumptions ranging
from magic to science. Throughout the ages some of these interpreta-
tions, whether obtained in the classroom or elsewhere, whether correct
or incorrect, have become a part of the general outlook and culture of
everyone. A study of plants as part of a general education would be
quite inadequate unless it included interpretations of these interrelations
based on present scientific knowledge and method. In the chapters that
follow we shall find that an understanding of these interrelations on the
basis of evidence is not only a matter of personal interest, but one that
is both desirable and necessary for the welfare of society.

What is a plant? When we pull weeds in a garden, mow the grass on
a lawn, enjoy the shrubs and trees in parks and forests, we readily recog-
nize these living organisms as plants. Stale bread and fruit may be cov-
ered with mold ( Fig. 1 ) . We have all seen otlier fimgi ( Fig. 2 ) in the
form of mushrooms in fields and woods. The surfaces of ponds and moist

TEXTBOOK OF BOTANY

Fig. 1. A common mold on a bunch of grapes. Photo from U. S. Department of

Agriculture.

Fig. 2. One of the common edible mushrooms {Coprinus). Photo from W. G.

Stover.

[Chap. I PLANT SCIENCE

Fig. 3. Brown seaweeds (Algae) at low tide. New York Harbor. Photo from New

York Botanical Garden.

soil at times become green with "pond scums" — microscopic plants that
may be distinguished as al^ae ( Fig. 3 ) . Those who have visited the sea-
shore are familiar with "seaweeds," which are other and larger kinds of
algae. Tree trunks and rocks that have long been exposed to the weather
are often partially covered by gray-green or highly colored patches of
lichens ( Fig. 4 ) . Everyone who even occasionally strolls on forest trails
distinguishes some of the mosses (Fig. 5) and ferns (Fig. 6). Much of
the decay of plant and animal products, and many of the diseases of
both plants and animals are due to the presence of bacteria ( Fig. 7). All
these organisms are plants.

On first thought it may seem rather easy to answer the question: What
is a plant? But one may be thinking only of such plants as trees, shrubs,
and herbs. When we include in our survey mosses, algae, fungi, and
bacteria, the observable differences among plants are often far more strik-
ing than those between certain plants and certain animals. This fact
makes it surprisingly difficult to list any group of qualities that are char-
acteristic of plants alone.

Similarity of plants and animals. The larger plants differ from the

TEXTBOOK OF BOTANY

Fig. 4. Lichens growing on a vertical rock face. Photo by C. H. Jones.

larger animals in so many ways that they not only are readily distin-
guished but are commonly thought to be quite unrelated. To suggest that
horses and trees have certain characteristics in common may seem ab-
surd. But students of plant and animal life have found that many of the
processes in plants and animals are either identical or essentially similar
if the basis of comparison is limited to certain fundamental features.
Among the microscopic organisms are many that resemble animals quite
as much as they resemble plants, and it would be futile to try to classify
them strictly in either category.

The biological sciences. Knowledge that pertains to plants or animals
or to both is called biology. The range of biological information that has
accumulated through the centuries — but more especially during the last
hundred years — is now so extensive that no one person is entitled to be

[Chap. I

PLANT SCIENCE

Fig. 5. A mat of moss plants {Hijpnmn) on a decaying log. Several "mushrooms"
are also present. Photo by E. S. Thomas, Ohio State Museum.

called a biologist in the sense that he is a master of the whole field. The
facts and principles that have been or may be derived from the study of
plants are called botany, and the knowledge similarly gained from the
study of animals constitutes zoology.

Because of the great number and diversity of forms and life relations
among both plants and animals, botanists and zoologists have difficulty
in viewing their subjects as wholes. Hence most botanists have become
especially interested in some one or only a few of the many phases of
the subject, such as the classification of plants, their physiology, their
structures, life histories, diseases, heredity, geography, or geological his-
tory. This specialization of botanists has certain advantages in research
and in practice. However, the behavior of plants, in tlie broadest sense,
involves the interaction of all these phases of plant life. Hence in the
interpretation of plant phenomena botanists are forced to seek an ever-
increasing background of biological, physical, and chemical knowledge.

In a general study of plants we shall be concerned not with the special
phases of the subject, but with that general background which experience

TEXTBOOK OF BOTANY

Fig. 6. Polypody fern on a cliff edge. Photo by C. H. Jones.

Fig. 7. A, photograph of masses or colonies of bacteria that have grown on agar
in a petri dish; B, bacteria of fire blight from a similar culture when magnified
about 2000 times.

[Chap. 1 PLANT SCIENCE 9

seems to indicate is most worth while to anyone. We may begin our study
as indicated at the beginning of this chapter, or we may prefer to go
into the field and learn to distinguish plants from other objects in our
environment. The plants growing all about us are just as important ob-
jects of study where they grow as are the materials carried to the labora-
tory, and they may be studied much more frequently and leisurely. In
Chapters II to VI several other approaches to the study of plants are
suggested; any of tliem may be preferred to the ones suggested in this
chapter.

CHAPTER II

THE PARTS OF PLANTS

<x3><><e><><^c><><^e><><^<^^

The principal parts, or organs, of the more famihar seed-bearing plants
may be distinguished in a bean plant as it develops from the seed that is
planted to the seeds that are harvested. This life cycle from seed to seed
— one entire generation of the bean plant — may be completed in a few
weeks. From a study of the diagrams in Figs. 8 and 9 the complete life
cycle of a bean plant may be visualized.

Within the seed coat is an embryo consisting of a short stalk (the
hypocotyl) and a pair of very large, thick, leaf -like structures (the
cotyledons) which are at the first node. At the apex of the hypocotyl is
a terminal bud (the plumule), and at the other end is a root tip from
which the primary root develops.

Two or three days after the seed is planted, the primary root begins
to elongate. It breaks through the seed coat and penetrates into the soil.
From the primary root lateral roots grow more or less horizontally. The
young plant is now firmly anchored, and the rapid growth of the hy-
pocotvl lifts the cotyledons well above the soil surface.

As the cotyledons increase slightly in size the broken seed coats fall
away. From the plumule a stem bearing leaves begins to develop. A
lateral bud soon becomes visible in the axil of each leaf. Each bud con-
sists of a stem tip bearing embryonic leaves. The first true leaves that
develop at the second node of this stem are simple, opposite, and heart-
shaped. From the terminal bud another stem segment ( intemode -\- 3rd
node ) grows, and from this third node only one leaf forms — this time a
leaf of three leaflets. Growth in height continues by the development of
additional stem intemodes and new leaves at the nodes until 5 or 6
leaves are formed. In the meantime, leaf-bearing branches may also be
developing from the lower lateral buds. These three distinct parts — the
roots, the stems, and the leaves — constitute the vegetative organs of the
plant. Internally they are all interconnected by veins, or vascular bundles
(Fig. 10).

Usually after the formation of leaf-bearing branches, another type of

10

[Chap. II

THE PARTS OF PLANTS

11

Fig. 8. Embryo and early stages in the development of a bean plant.

branch appears bearing flowers. Within a few days a pod, or fruit, begins
to develop from the pistil of the flower. As the pod increases in size,
seeds develop within it and soon enlarge to the size of the seed that was
planted. When the seeds are mature the life cycle of the bean plant is
complete. The flowers, fruits, and seeds are often spoken of as the repro-
ductive organs of the plant in contrast to the vegetative body made up
of roots, stems, and leaves. The major part of the various processes in-
volved in food manufacture takes place in the vegetative organs. Other
processes in reproductive structures result in the formation of seeds. Each

12

TEXTBOOK OF BOTANY

FLOWER

PISTIL OF FLOWER

YOUNO FRUIT

OLDER FRUIT

LEAFLET OF TRIFOLIATE LEAF

FIRST PAIR OF LEAVES

STEM

COTYLEDON SCAR

Fig. 9. Stem of older bean plant with leaves and flowers, and with fruits (pods) of
various ages from the pistil. The seeds are borne within the fruit.

complete cycle from seed to seed may also be called a generation. Every
generation of a seed plant is in general a repetition of the same orderly
sequence of vegetative structures followed by reproductive structures.

From past experiences you probably have formed general ideas of
these several parts of plants. But if these ideas are vague they will not
be useful in either reading or discussion. To regard a leaf merely as the
part that is green, a root as the part in the ground, and a seed as what
one plants is quite inadequate. If enough plants are examined it will be
discovered that any of these organs may be green, any of them may be
found underground, and all of them are planted by man. A stroll on the

[Chap. II

THE PARTS OF PLANTS

13

TERMINAL BUD

LEAF BLADE
AXILLARY BUD
PETIOLE

ROOT HAIRS
ROOT CAP

GROWING POINT

Fig. 10. Some external and internal structures of plants.

campus or in nearby gardens and conservatories for the purpose of learn-
ing to distinguish the parts of plants will be much more profitable than a
search for technical book definitions.

Roots, for example, are usually thought of as cylindrical, more or less

14 TEXTBOOK OF BOTANY

tapering, underground structures. From observations of tropical plants in
conservatories, of mature vines grov^^ing on walls, and of mature corn
plants in a field one will learn that roots are not always underground.
Stems are still more variable in form. They usually are aerial cylindrical
structures, but some also develop below the soil surface and may become
greatlv thickened like the tuber of Irish potato. Among the lateral out-
growths of stems are the leaves. Only in a few plants will leaves be
found originating from other organs. There are myriad forms of leaves,
but generally they are flat organs with a very large surface compared
with their weight. Those above the surface of the soil are usually green
during at least a part of the year. Beginning with such general ideas, one
may readilv build more accurate and usable concepts of these organs
through further personal observations.

Flowers are of every size from a twentieth of an inch to thirty inches
in diameter, and of every color, often of superb and startling mixtures of
colors. Usually flowers are easily recognized, but careful observation is
needed to decide what is the flower in the flowering dogwood, Indian
paintbrush, calla lilly, jack-in-the-pulpit, snowball, poinsettia, and hy-
drangea. Brightly colored parts are not found exclusively in flowers. Nor
are flowers always brightly colored, as may be discovered by examining
flowers of grasses, sedges, and several kinds of trees.

Fruits usually develop from the pistils or from the pistils and adjoining
parts of flowers. The name fruit is applied to a great variety of structures,
such as the dry fruits of the grasses, and "sticktights," as well as the suc-
culent berries of the grape and tomato, the fleshy apples and pears, the
pumpkins and melons, the firm green fruits of the walnut, and of osage
orange. To obtain some notion of the remarkable variety of fruits it is
best to observe the transition from flowers to fruits on many kinds of
plants at any time such observations can be made in the field or
greenhouse.

Seeds develop from certain structures in the pistil of the flower and are
borne inside the fruit. They may be very minute; they rarely exceed a
few inches in length. Their most general characteristic is the presence of
a firm coat or shell surrounding an embryo plant from which an adult
plant may develop more or less like the plant on which the seed was
borne.

Length of vegetative period. The bean plant matures in a few weeks;
that is, the formation of reproductive organs takes place after a rela-
tively short vegetative period. The vegetative period of plants may be

[Chap. 11

THE PARTS OF PLANTS

15

either long or short; it depends both on the kind of plant and on the en-
vironment. Some of the autumn flowering plants start from seeds in the
spring. Some grasses, clovers, primroses, and carrots start from seeds in
late summer and remain in the vegetative condition until the second
summer. The so-called century plant grows vegetatively 20 to 30 years
before it bears flowers, fruits, and seeds. Woody plants grow vegetatively
from a few to many years, and then for many succeeding years reproduce
during each growing season. In later chapters it will be shown how the
vegetative period of a plant may be lengthened or shortened b\' such
external factors as light, temperature, moisture, and soil salts.

<3q 6^&j6

Fig. 11. The duckweeds are the smallest of the flowering plants: A, Wolffia
Columbiana; B, Lemna minor; C, L. trisulca, and D, Spirodela pohjrhiza, natural
size; E, Wolffia, enlarged; F, flowers of L. trisulca on floating plants, natural size;
G, a flower of L. minor, much enlarged. F and G drawn from photographs by
W. H. Camp and L. E. Hicks.

We have referred thus far only to the more familiar seed-bearing
plants, most of which have the six principal plant organs in one form or
another. Some seed plants lack one or more of these organs. Some of
the duckweeds have a small globose body with no distinction of root,
stem, or leaf; others have a flattened body with a well-formed simple
root; all of them have flowers, fruits and seeds (Fig. 11). The well-
known Spanish "moss" of the South lacks roots. Many cacti are notable
for the absence of leaves (Fig. 12). In some plants, such as the dandelion
and the common plantain, the stem is merely a small flattened cone at
the top of the root. The tropical parasitic plant, Rafflesia, has neither
roots nor leaves — just a short stem, a flower, a fruit, and seeds ( Fig. 13 ) .

Ferns have roots, stems, and leaves but no flowers, fruits, or seeds.

Fig. 12. The stems ot the giant cactus do not bear leaves. Some of the shrubs
in this desert scene have leaves only during rainy periods. Photo by U. S. Forest
Service.

Fig. 13. A flower and buds of Raffiesia, a root-parasite in the East Indies. The
species pictured above occurs in the Philippines. A species in Sumatra closely
resembles it in form and habit and has flowers three feet in diameter. Photo from
Phihppine Bureau of Science.

[Chap. II THE PARTS OF PLANTS 17

The pond scums and seaweeds (algae) have none of these organs, and
certainly there are no organs closely resembling them among the fungi.
It is evident that none of these organs is essential to plant life, but they
are generally present in seed-bearing plants. As we shall see later, cer-
tain fundamental processes are usually associated with each of them but
are not necessarily limited to them.

CHAPTER III
LEARNING TO NAME PLANTS

<><^<><J><><><3><c>C><>00<><i>^^

We shall not get very far in the study of plants without studying them
in the field and greenhouse. In order to observe and discuss plants in-
telligently, we must have appropriate names for them. Many common
plants have local names which may suffice for ordinary conversation.
But the names applied in one community may be different from those
in another. Furthemiore, the same name is often used for quite different
plants in other localities. Many other plants lack common names. To
overcome these difficulties scientific names have been given to all the
known species of plants, and by agreement these names are used and
understood by botanists everywhere.

Botanists have recognized and named more than a quarter of a million
species of plants. As a beginning it will be sufficient to know some of the
local seed-bearing plants. Many plants can be recognized readily by
their leaf and stem characters; recognition of others may require a
knowledge of the flowers and fruits in addition. Names of plants may be
obtained from others who know them or from published "keys." To make
the simplest keys or to use those made by others, it is necessary to ex-
amine the external features of leaves and stems and become acquainted
with the terms that are applied to their parts, forms, and arrangements.

The parts of a leaf. If one examines a leaf, such as that of Japanese
quince, it is evident that it consists of a broad, thin blade, a narrow
cylindrical petiole, and at the base of the petiole a pair of small ap-
pendages, the stipules. A leaf consisting of these three parts is frequently
called a complete leaf. The primary parts of a complete leaf, then, are
the blade, petiole, and stipules (Fig. 14).

The stipules are usually small structures; but some of them, such as
those of pansy, Japanese quince, and garden pea, are large and blade-
like. Those of buckwheat and smartweed are sheaths surrounding the
stem for some distance above the point of attachment. Stipules of the
greenbrier are tendrils, and those of the black locust are spines.

18

[Chap. Ill LEARNING TO NAME PLANTS 19

Some leaves have no apparent petioles and are described as sessile.
The leaves of many grasses, such as those of oats, wheat, and bluegrass,
have neither petioles nor stipules; the blades are attached to the stem by
a sheath which may be long or short. At the top of the sheath is a collar-
like extension called the ligule. The ligule of the bamboo and certain
other grasses may consist merely of several long bristles.

Fig. 14. Diagrams of some parts of leaves. A, blade, petiole, and stipules of
apple leaf; B, stem of pea bearing leaves composed of two large stipules, leaflets,
and a terminal tendril; C, sessile leaves of zinnia; D, blade, ligule, and sheath of a
grass leaf.

The needle leaves of such trees as pines and spruces appear super-
ficially to be quite unlike those of broad-leaved trees. The leaves of
pines and larches are in clusters, or fascicles, at the end of short dwarf
branches. Spruce, fir, and hemlock have solitary leaves. The leaves of
arbor vitae are small scales oppositely arranged on the stem.

The veins. The most conspicuous structures of an elm leaf are the
veins. The large vein near the middle of the blade is the inidrib. In a
maple leaf there are several prominent veins which are called the prin-
cipal veins. In general, the smaller veins form a network and unite with
larger veins, which in turn connect with the midrib or the principal

20

TEXTBOOK OF BOTANY

veins. These larger veins are smallest at the apex and margin of the leaf
and gradually become larger toward the middle and base of the blade.

Fig. 15. Arrangement of the larger veins. The leaves of magnolia (A) and tulip-
tree (B) exemplify pinnate venation; red bud (C) and black maple (D), palmate
venation. Veins may also be parallel as in the bamboo (F) or dichotomous as in the
ginkgo (E). From drawings by C. H. Otis, Mrs. A. E. Hoyle, and C. J. Cham-
berlain. Fig. E from Textbook of Botany (Coulter, Barnes, and Cowles), Amer.
Bk. Co.

Fig. 16. The leaves of bitternut hickory and clammy locust are pinnately com-
pound (A-B). Those of Aralia and horse chestnut are palmately compound (C-D).
From drawings by Mrs. A. E. Hoyle and E. B. Wittlake.

The arrangement of the veins of a leaf is termed venation. There are
four general arrangements of the principal veins in leaves that are easily

[Chap. Ill LEARNING TO NAME PLANTS 21

recognized (Fig. 15). Leaves of oats and other grasses have parallel
venation, the veins extending more or less parallel from base to apex.
Many ferns and the curious ginkgo tree have leaves with forked or
dichotomous venation; that is, each vein divides at intervals into two
smaller veins of equal size. When the secondary veins extend from the
midrib like the divisions of a feather, as in the leaves of elm, the venation
is said to be pinnate. In the maple leaf, however, the principal veins
extend from the petiole near the base of the blade, roughly simulating
the bones in our hands. This type of venation is palmate.

Simple and compound leaves. Every leaf consisting of one continuous
blade only is known as a simple leaf. A compoimd leaf such as that of
the rose consists of several leaflets. If the leaflets are joined to the end of
the petiole, as are those of the horse chestnut, the leaf is described as
palmately compound. When a compound leaf is composed of leaflets
joined to the sides of the central axis of the leaf (the rachis), it is termed
pinnately compound ( Fig. 16 ) . The leaflets in a compound leaf may be
odd or even in number.

Compound and simple leaves are usually readily distinguished, but
some divided simple leaves closely resemble those commonly described
as compound. Such leaves as those of tomato and potato which have
deeply divided blades can scarcely be distinguished from compound
leaves. In fact, there is apparently every gradation between simple undi-
vided leaves and distinctly compound leaves. A bud may be found in
the axil of each leaf of broad-leaved trees and shrubs, but there are no
buds in the axils of leaflets.

Leaf form. The shape of a leaf is usually rather characteristic of a
species, and the form of the apex and the base of the blade may be dis-
tinctive. Leaves may also have characteristic edges, or margins. Some of
the common teiTns used to describe leaves are illustrated in Fig. 17.

Arrangement of leaves on stems. Leaves usually have definite arrange-
ments on the stem. On willow twigs the leaves are alternately and
spirally arranged. There is a single leaf at each node, and a line drawn
through the successive points of attachment forms a spiral about the
stem (Fig. 18). The leaves of maple occur two at a node on opposite
sides of the stem and exemplify the opposite arrangement. The plane of
attachment of each successive pair of leaves is at right angles to that of
the leaves immediately above and below. The catalpa twigs have three
leaves at a node arranged radially. This arrangement is termed whorled
or cyclic.

22

TEXTBOOK OF BOTANY

Fig. 17. Forms of leaves. Upper row — forms of leaf blades: A, linear; B, lanceo-
late; C, spatulate; D, ovate; E, obovate; F, oblong; G, cordate; H, orbiculate;
I, peltate.

Second row — forms of leaf margins: A, entire; B, serrate; C, doubly serrate;
D, dentate; E, crenate; F, undulate; G, pinnately lobed; H, palmately lobed.

Third row — forms of apexes of blades: A, acute; B, acuminate; G, obtuse;
D, aristate; E, mucronate; F, refuse; G, truncate.

Lower row — forms of bases of blades: H, rounded; I, equally lobed; J, obliquely
lobed; K, acute; L, acuminate; M, sagittate; N, hastate.

There are several types of spiral arrangement of alternate leaves. If
the spiral runs halfway round the stem in passing from one node to the
next, as in elm or corn, the arrangement is described as 1/2 alternate.
This arrangement is called two-ranked because the points of attachment
of the leaves appear in two ranks, one on either side of the stem when
the stem is viewed endwise. If leaves are attached at angles of 120° —

[Chop, in

LEARNING TO NAME PLANTS

23

Fig. 18. Diagram of leaf arrangement on stems, as seen from above and from the
side. A, oppositely arranged leaves; B-D, alternately arranged leaves. B, the one-
half, or two-ranked, arrangement; C, the one-third, or three-ranked, arrangement;
D, the two-fifths, or five-ranked, arrangement.

that is, the spiral passes through 3 nodes before completing a cycle and
the fourth leaf -base is directly ov er the first — thev are said to have the
1/3 alternate arrangement. These leaves occur in three ranks on the
stem. This arrangement is best seen in the sedges.

Many trees have a more complicated leaf alignment. A spiral drawn
around the stem from one bud to another directly below it passes through
5 nodes and twice around the stem; the sixth bud is directly below the
first bud. This arrangement is called the 2/5 alternate type. Still other
leaf and scale arrangements may be found, such as 3/8, 5/13, 8/21. The

24 TEXTBOOK OF BOTANY

numerator of any fraction in the series is equal to the sum of the two
preceding numerators. The same relationship holds for the denominators.

These arrangements of leaves are not invariable. Owing to the influ-
ence of light on the growth of stems and leaves that are not equally illu-
minated on all sides, the stems become twisted and the arrangements are
altered.

Other stem characters. Certain other stem characters are often used to
identify trees and shrubs. Since a htid is present in the axil of each leaf of
most deciduous trees and shrubs, bud arrangement is a useful character
in the dormant season when there are no leaves on the stems. Buds differ
in size and form in different species. Their outer scales may be smooth
or hairy. Leaf scars, which are left when the leaves have fallen, also have
rather definite shapes and patterns. The outer covering or bark of a twig
may be green or gray or brown, and various other descriptive terms are
applied to its surface, such as smooth or rough, ridged or wartv. In a
few instances the color and form of the pith may be distinctive.

Keys to plants based on vegetative characters. If we examine a maple
tree we notice that its leaves are simple, lobed, palmatelv veined, and
oppositely arranged on the stem. No other trees in North America have
all four of these characters. The eastern hemlocks have short-petioled,
needle-like flattened leaves with white lines on the under surface. When
characters such as these are definitelv assorted and grouped so that by
careful reading and comparison with the specimen in hand we may deter-
mine the name of the plant, we have v/hat is known as a key.

Many keys have been published for the plants of local, state, and
national regions. Local keys are usually more convenient because the
number of plants included is smaller. For the beginner those based on
such external characters of leaves and stems as are outlined above are
most useful; moreover, thev are quite adequate for the identification of
most trees and shrubs.^ Keys are often based on the forms and detailed
structures of flowers, fruits, and seeds; such kevs are necessary to identify
many of the common herbs. Keys based on both vegetative and repro-
ductive characters are more difficult to use since they imply considerable
knowledge of structures and terminology.

Species and genus. No one expects anything but an oak tree to develop
from an acorn, or anything but a hickory tree from a hickory nut. To go

^ Every student should find the most convenient keys to the trees, shrubs, and herbs
of his region. There are many keys available that cover limited areas. These have been
published by agricultural experiment stations, museums, state academies of science, and
various other institutions.

[Chap. Ill LEARNING TO NAME PLANTS 25

a step farther, when white-oak acorns are planted the individual trees
that develop are normally all white oaks like the parent trees. These
individual white oaks all belong to a single species. Similarly black oaks
develop from the acorns of black-oak trees, and only yellow oaks grow
from yellow-oak acorns.

Species that have many fundamental characters in common are
grouped together as a genus (plural genera). We do this in common
speech when we speak of the "oaks," and we identify the species when
we say this is a white oak or a black oak. Botanists long ago began the
use of Latin names in order to avoid the confusion arising from the
fact that common names applied to the same plant in different localities
differ widely, and the impossibility of learning these common names in
several hundred languages. The Roman name of the oak is Quercus and
the Latin word for white is alba; hence the name chosen for white oak
is Quercus alba. All oaks belong to the genus Quercus, and for each
species a second name is chosen by the author who first describes the
species; by agreement subsequent authors use this same name. This is a
great convenience, for students of all countries can use the same lan-
guage when referring to the names of plants. These names have come
into such general acceptance in all scientific writings that they are often
spoken of as "scientific names" in contrast to the "common names" which
may vary in every locality and country.

REFERENCE

Hitchcock, A. S. Methods of Descriptive Systematic Botany. John Wiley &

Sons, Inc. 1925.

(Special references to plant floras adapted to particular regions are too
numerous to list. See footnote, p. 24, of this chapter.)

CHAPTER IV
SEASONAL ASPECTS OF PLANTS

The succession of the seasons and the changes in plants and landscapes
associated with them are familiar to everyone who has lived in the tem-
perate zone. Even in the tropics there are but few localities in which
seasonal changes in weather and in plants do not occur.

Changes in plants during the vear have always attracted attention,
even of primitive men. They have inspired much prose and poetry, as
well as scientific study and description. Explanations for their occurrence
were proposed so far back in the history of the human race that they
became an important part of mythology and folklore. Even today news-
papers and magazines frequently contain stories that are merely new
versions of these ancient myths which ascribed supernatural and mys-
terious powers of foresight to plants and animals alike.

The changes that occur in the form and appearance and in the relative
abundance of different species from season to season may be seen in the
lawns, pastures, cultivated fields, and forests of your own community.
An accurate account — a diary — of such observations over a period of a
few years would include many valuable botanical data. This is another
way in which the studv of plants may be approached.

Many seasonal phenomena may be studied best as individual prob-
lems during appropriate seasons when the material is abundant and
observations may be made over extended periods of time. As an aid to
such preliminary observations, certain facts about seasonal aspects of
plants have been brought together in this chapter. Data helpful in further
observations and interpretations are included in many of the subsequent
chapters.

There are such great differences in the behavior of individual species
that few generalizations apply equally to all kinds of plants. Some of
the questions and problems that are sure to arise from your own pre-
liminary observations may be solved by further study or by well-planned
simple experiments. Let us begin with plants in autumn when most
courses in botany begin, and, by following the cycle of the seasons, con-

26

[Chap. IV SEASONAL ASPECTS OF PLANTS 27

sider some of the most apparent changes that occur in plant processes
and plant organs.

The autumnal aspect. The summer with its long days, high tempera-
tures, and intense light is waning. The eflFects of cooler weather and of
the shorter period of daylight upon certain chemical processes in plants
soon become evident in the leaves of our deciduous trees and shrubs.
The green color of chlorophyll gradually disappears, and the yellow pig-
ments with which the green pigment had been associated in the cells of
the leaves now become conspicuous. In many leaves there is an addi-
tional formation of other pigments which range in color from red to
purple and which are called collectively the anthocyanins. As the season
advances both the yellow pigments and the anthocyanins break down;
and brown substances, especially tannins, increase and modify the leaf
color. When the autumn is characterized by bright sunshine and moder-
ately cool weather coloration is at its best. When frost occurs early or
when the weather is wet and cloudy, the anthocyanins are formed to
only a slight extent and the yellow and brown pigments are dominant
in the landscape (Plate 2).

These autumn color changes in the leaves of many deciduous trees
contrast sharply with the persistent green color of both the needle-leaved
and broad-leaved evergreens. Many herbs likewise remain green through
the winter months, at least in parts near the soil. Furthermore, the leaves
of some trees and shrubs remain green until after they have fallen to
the ground.

Colors and pigments. The color we ascribe to an object or substance
depends upon the kinds of light rays that pass from it to the retinas of
our eyes. When a beam of "white" light passes through a clear glass
prism the rays of different wave length are separated by refraction
and appear as bands of color. Similarly we see a rainbow when the
different rays of ordinary daylight are reflected to our eyes in separate
bands by drops of water in the atmosphere. On the one side of the bow
we perceive the longer waves of light as red, on the other side the short
waves as violet. Between these extremes are bands of orange, yellow,
green, blue, and indigo, each produced by rays of successively shorter
wave lengths that reach our eye.

The colors of objects are also partly dependent upon the relative sensi-
tiveness of our eyes to the different rays of light. They are especially
sensitive to the rays we perceive as green and yellow.

28 TEXTBOOK OF BOTANY

The various chemical compounds in plants absorb certain light rays
and reflect others. We receive our color impressions from the reflected
rays. As a matter of convenience we call these substances pigments and
we ascribe to them the colors perceived through the eye when light is
reflected from them. Thus we speak of green pigments, yellow pigments,
and various others from red to violet. Among the most important pig-
ments in plants are chlorophylls (leaf green); the carotinoids, varying
from pale yellow to orange-red; and the anthoctjanins, varying from red
through violet to blue.^

Pigments and the colors of leaves. The colors of certain leaves — for
example, the purple-leaved coleus and canna — may result from a com-
bination of all these pigments. The anthocyanins mav occur in the outer
cell layers of the leaf, the chlorophylls and carotinoids in the inner cell
layers. When a red leaf is placed in hot water the anthocyanins disap-
pear from the cells into the water and the green color of the chlorophyll
within becomes evident. The chlorophylls and carotinoids may be dis-
solved from the leaves by alcohol, and then separated from each other
by appropriate chemical means. Carotinoids are always present in green
leaves, but their presence is obscured by the chlorophvlls until the latter
disintegrate. Surprising as it may seem, the carotinoids may be present in
as large amounts in the leaves of midsummer as in the yellow autumn
leaves.

In many common trees and shrubs the anthocyanins are formed mainly
in the autumn; in others, such as certain varieties of maple, the young
leaves may be red also. The anthocyanin in purple varieties of beech,
plum, hazel, and barberry is evident throughout the growing season.

■ ^ Pigments Apparent Color Chemical Composition

Chlorophyll a Blue-green C56H7205X4Mg

Chlorophyll b Yellow-green C55H7o06X4Mg

Carotenes Y'ellow to red C40H56

Xanthophylls Yellow C40H56O2

Anthocyanins Red to blue Various combinations of C, H, and O

The anthocyanins are very complex substances composed of a few fundamental com-
pounds ( anthocyanidins ) combined with various sugars and benzene compounds. They are
soluble in water and occur in the solutions in the plant cells. Some of the anthocyanins,
such as those in certain varieties of apples and peaches, are formed only in light especially
of short wave length. Others, such as those in red beets and radishes, may be formed in
darkness. Chlorophylls and carotinoids are not soluble in water and are formed in definite
bodies in the protoplasm known as plastids. The carotinoids of plants when eaten by
animals reappear in yellow cream, yellow body fat, egg yolks, and butter. Carotene has
recently been shown to be the forerunner of \'itamin A. The molecule of the vitamin is
just one-half of the carotene molecule.

Plate II
Above: Autumn coloration under cool moist conditions. Cook Forest, Pennsylvania.
Photo by G. S. Growl. Middle: Autumn coloration under warm drought conditions, cen-
tral Ohio. The broad;leaved trees in both pictures are the same species. Photo by G. S.
Growl. Beloiv: The spectrum of sunlight.

[Chap. IV SEASONAL ASPECTS OF PLANTS 29

Many other examples of leaf pigmentation may be found in every
community.

Pigments not limited to leaves. Of course these pigments are not
limited to the leaves of plants. The presence of chlorophyll in most
herbaceous stems and the young stems and twigs of woody species is
familiar to all. Likewise, certain parts of flowers are usually green, and
the term "green fruits" has come to be a synonym of young fruits in
common speech. Many varieties of fruits remain green when ripe, while
in others the chlorophyll disintegrates and the associated yellow pig-
ments become more evident. In still others anthocyanins partially or
wholly mask the yellow. Some seeds, such as those of certain varieties
of peas and beans, are green. Chlorophyll is formed in the aerial roots
of many orchids and in the roots of many other plants when they are
exposed to light.

The carotinoids seem always to be present wherever chlorophyll
occurs. They may also occur in the absence of chlorophyll. Carotinoids
are the underlying cause of the yellow and orange colors of the flowers
of zinnias, sunflowers, and goldenrods; of the fruits of oranges, lemons,
and tomatoes; and of the seeds of corn, peas, and clover.

The anthocyanins are most conspicuous in red, purple, and blue
flowers and fruits; but purple cabbage, potatoes, popcorn, and beets
exemplify their common occurrence in other plant parts. Among the
fungi (molds, lichens, mushrooms) other pigments may be as brilliant
and varied as are those in our common green plants.

Pigment formation dependent upon both heredity and environment.
Among the common plants one may find some species that lack antho-
cyanins, while otliers are without chlorophyll and carotinoids. The
absence of a pigment from a plant, or a part of a plant, may be due to
its heredity, its stage of development, or some condition in the environ-
ment of the plant. Chlorophyll is not formed in the cells of toadstools
or in "Indian pipe" ( Fig. 19 ) in any environment. Neither is it formed
in some parts of variegated leaves under any circumstances. The other
pigments may also be absent from certain plants regardless of the con-
ditions under which they develop. The absence of pigments in these
plants must be due to the hereditary constitution of the plant.

Environment and chlorophyll. Any condition that is necessary for the
maintenance of the plant is indirectly essential to the formation of plant
pigments. There are, however, certain conditions that are more directly
related to it. In most plants light seems to be necessary for the making

30

TEXTBOOK OF BOTANY

Fig. 19. Indian pipe (left) and pinesap (right), two saprophytes common in
moist woods. The underground parts of the plants are penetrated throughout by-
fungous filaments, which enter from the humus in which the plants grow.

of chlorophyll. We are all familiar with blanched celery in which the
part deprived of hght lacks chlorophyll, or with the white and yellow
potato sprouts that develop in a dark cellar. During the warm nights
of early spring bluegrass grows rapidly, and in the early morning the
base of the blade that grew during the night is disclosed by its lack
of chlorophyll. But light is not necessary for the formation of the green
pigments in all plants. Green seedlings of spruce, pine, and other coni-
fers may develop in the dark. Grapefruit and lemon seedlings, certain
algae, and the sporelings of mosses and ferns may produce chlorophyll
in darkness if they are supplied with sugar.

Sugar is one of the substances from which the chlorophylls are made,
and from the formulas of chlorophyll (page 28) it is evident that com-
pounds of nitrogen and magnesium are also utilized. Experiments have
shown that manganese and iron are essential to the formation of chloro-
phyll, but they do not constitute a part of the chlorophyll molecule.
The seedlings of some green plants growing at temperatures below

[Chap. IV SEASONAL ASPECTS OF PLANTS 31

50^ F. may fail to form chlorophyll, and unless the temperature is raised
they ultimately die.

Environmental effects on carotinoids. The formation of the yellow pig-
ments is also dependent upon certain environmental conditions. The
seedlings of some varieties of com and other plants are conspicuously
yellow when growing in darkness; others, such as oats, are colorless
under the same conditions and become yellow only when exposed to
light. Perhaps there are certain factors that might be substituted for light
in these cases, but they have not been discovered. Seedlings in which
chlorophyll is made in the dark also contain carotinoids.

Anthocyanins and the environment. The roots of some plants, such as
beets and radish, become red or purple in darkness; but light is neces-
sary for the formation of anthocyanins in most leaves and fruits. In
some instances the blue and violet rays are necessary. In both leaves
and fruits intensity of color is increased by abundance of light, relatively
low temperature, and a low supply of nitrates in the soil. Apparently
these environmental conditions influence the formation of anthocyanins
partly through their influence on the sugar content of the cells. A high
sugar content seems to be one of the conditions within the cells neces-
sary to the formation of anthocyanins.

Leaves and fruits on the same tree or on the same variety of tree in
different local situations may differ widely in the amount of anthocyanin
they contain. Peaches in the top of the tree, and apples fully exposed to
the sunlight are redder than those inside the crown and shaded by the
outer foliage. The more intense coloring of fruits from the Northwestern
States as compared with those from the Eastern States exemplifies this
same principle. Low night temperatures are also very important in the
accumulation of sugar and the formation of bright colors in apples.

Low temperatures increase anthocyanin formation in many exergreen
plants; for example, the leaves of certain varieties of juniper and arbor
vitae become copper-colored in autumn, and similar changes occur in
many heaths, of which the cranberry is an example (Plate III).

The intensity of anthocyanin colors of flowers may vary with light
intensity. The red color in some flowers is associated with an acid
condition, and blue with an alkaline condition. If a red geranium petal
is crushed on a blotter and held alternately near ammonia and acetic
acid, one may see these color changes. Similar color changes may also
be seen in the uncrushed petals. This change in color does not occur in
all anthocyanins because of the presence of certain ions, such as potas-

32 TEXTBOOK OF BOTANY

slum. The flowers of the cobaea vine change from green to red, and
finally to violet, as they fade. Some rose flowers are pink when the buds
unfold and bluish when they fall. Flowers of the French hydrangea are
blue when the plants grow in acid soils containing salts of aluminum.
They are rose colored when the plants grow in alkaline limestone soils
where the aluminum salts are insoluble. It must be remembered that
the anthocyanins form a very large group of chemical compounds, and
the behavior of the pigments of any particular plant may be explained
only by the properties of the particular pigments present in that plant.

Deciduous and evergreen habits. Another striking autumn phenomenon
is the falling of the leaves from many species of plants. This has been
shown to be definitely related to the shortening daily period of light.
During the summer a specialized layer of cells, called the absciss layer,
forms at the base of petioles and leaflets. The subsequent disintegration
of this layer may be started by conditions within the leaf brought about
by a variety of external conditions such as drought, change in length
of day, low temperatures, or leaf injuries. Hence any one, or all, of these
conditions may bring about leaf fall. Trees and shrubs that lose all or
nearly all of their leaves annually are said to be deciduous.

In contrast to deciduous plants are those in which the life of any one
leaf extends through several years. These plants may have either broad
or needle leaves and their appearance varies but little from season to
season; the most familiar examples are the evergreen trees and shrubs.
Many herbaceous plants also have green leaves during the winter, as for
example, the common dandelion, evening primrose, teasel, and chick-
weed. These also might be classed as evergreens.

Dormancy and periodicity. The gradual lowering of the temperature
and decrease in the length of the daily light period lead not only to
pigment changes and leaf fall, but to the death of many plants that
started from seed the preceding spring. Some part of the plant, how-
ever, remains alive and dormant throughout the late summer, autumn,
or winter. The part or parts that remain alive and doniiant vary greatly
with the kind of plant. If the plants have completed one generation or a
complete life cycle — including vegetative development, flowering, fruit-
ing, seed production, and death — within a single season, they are called
annuals. The dormant organ of such plants is the seed. The seeds of
some annuals, however, may also germinate in the autumn; the plants
pass the winter in the vegetative condition and bear seed the fol-
lowing spring.

Plate III

Above: Tundra plants, including a 40-yeai-old nearly prostrate willow. White Horse
Pass, Alaska. Photo by W. H. Camp. Below (left): The snow plant (Sarcodes) parasitic
on fvnigi in the pine forests of the Sierra Mountains, California. The plant develops soon
after the snow melts. Photo by W. H. Camp. Below (right): A Florida lichen, in which
the fungus is parasitic on purple bacteria. Photo by G. S. Crowl.

[Chap. IV SEASONAL ASPECTS OF PLANTS 33

When the hfe cycle of a plant covers a part of two growing seasons
it is called a biennial. Shepherd's-purse may grow both as an annual and
as a biennial in regions with mild climates, and wheat is cultivated both
as "winter wheat" and as "spring wheat." Special varieties of wheat
have been selected for each of these growth regimes.

During the life cycle of many biennials the first season of growth
ends with the formation of a thickened root, a short stem, and a rosette
of leaves near the soil surface. These young plants remain dormant dur-
ing the winter and in the second growing season complete their life
cycle by the development of upright stems, flowers, fruits, and seeds.
Then the plant dies. Whether certain plants grow as annuals, biennials,
or perennials depends upon the temperature, length of day, and length
of the growing season to which they are exposed.

A B

Fig. 20. Diagram of winter rosette of leaves of teasel (A), and of evening

primrose ( B ) .

The perennial herbs are like annuals and biennials in having a vegeta-
tive stage previous to the formation of flowers and fruits, but differ
from them in having continuous vegetative growth and seed formation
for many successive seasons. The annual active period of growth of
perennial herbs is followed by a dormant one when most of the living
plant is underground, with no parts extending much above the soil sur-
face. Some of these plants have "winter rosettes" (Fig. 20) of leaves,
others have very short lateral branches with small leaves, and still
others have large buds above ground. Some of the perennials may con-
tinue living indefinitely. Perhaps some have been living in their present

34

TEXTBOOK OF BOTANY

localities for centuries; but since a part of the underground roots, or
stems, dies each year and new parts are added each year, the age of
the oldest part of the plant body is rarely more than 3 to 10 years.

Trees and shrubs are woody perennials; they may live 5 to 10 years
or longer before they begin to bear flowers, fruits, and seeds. Any horti-
culturist or forester will tell you that there is great variation in the
abundance of reproductive structures in trees from year to year and that
certain trees bear fruits and seeds only once in several years. This
periodicity is dependent in part on weather and soil conditions, and in
part on heredity.

Winter aspect. The most distinctive feature of plant life in winter is
the donnancy of most plant organs (Fig. 21). The internal causes of

Fig. 21. A forest in winter when all the aerial parts of plants are dormant. Photo

by C. H. Jones.

dormancy so characteristic of late summer and autumn usually disap-
pear during the winter months, but owing to low temperatures no growth
occurs. Many plants will start developing at this time if they are moved
into a greenhouse.

Nevertheless, some processes continue within plants that have every

[Chap. IV SEASONAL ASPECTS OF PLANTS 35

appearance of being doiTnant. Roots develop slowly in unfrozen soil, and
there may be some transfer of materials within the plant body. In plants
with green parts (winter wheat, bluegrass, and evergreen trees and
shrubs) food manufacture may occur when daytime temperatures are
above the freezing point. Witch hazel and alders may flower during
winter thaws, as well as during late autumn and early spring.

Winter and early spring are the best periods in which to study the
characteristic buds, twigs, and bark of the woody perennials. By means
of these characteristics one may readily learn to identify trees in winter.

The leaves of temperate evergreens vary in their endurance of freez-
ing temperatures. A sudden exposure to low winter temperatures during
midsummer kills them. Most of them, however, withstand temperatures
well below the freezing point after they have become "hardened" by
exposure to the gradual changes in temperature during autumn. But they
do have limits below which injury or death results from low tempera-
tures. In some species injury results only when the low temperatures
continue for several days. Others are killed by exposure to temperatures
below freezing for a few hours. Twigs and parts of larger stems may
also be killed by low temperature. In any part of the temperate zone
trees and shrubs may be "killed back" by extremely low temperatures.

The amount of "winter injury" varies from year to year, and many
interesting problems occur to the careful observer. Winter injuiy may
result from low temperature alone; but in many instances it results
from a drying up of the plant during sudden thaws in late winter or in
early spring while the soil is still frozen. The roots of winter wheat and
other grasses are often broken by the formation of layers of ice beneath
the surface of the soil. When subzero temperatures occur, the water in
the buds, twigs, and smaller branches is frozen. Still lower temperatures
may even freeze the water in the trunks of mature trees. A sudden drop
to a very low temperature may result in the splitting of the wood and
bark.

There is another effect of winter that is quite beneficial, or even neces-
sary, to many plants. Manv seeds and buds do not germinate readily
unless they have been exposed to temperatures near the freezing point
for several weeks. Many plants, such as blueberries, unless exposed to a
low temperature in the dormant period, do not grow well during the
following season. Neither of these cases of "winter conditioning" de-
pends upon actual freezing; indeed, many plants grow best after an
exposure to a temperature of 5° to 10" above the freezing point for a

36

TEXTBOOK OF BOTANY

few months. Tulip, hyacinth, and narcissus bulbs that are to be "forced"
into early blooming are planted out-of-doors; after several weeks of
winter temperatures they are brought indoors. Without this low-tempera-
ture treatment of the bulbs, the new plants will develop poorly and bear
malformed flowers or none. Inquiry among local nurserymen will prob-
ably disclose other practical problems connected with the winter season.

Fig. 22. Springtime: Winter dormancy is broken and the beech leaves are

unfolding.

Spring aspect. The lengthening of the daylight period and increase in
temperature bring to an end the dormant period of plants. Lawns and
fields begin to green through renewed growth of dormant leaves and
buds. Buds on many trees and shrubs enlarge and a new set of twigs
and leaves develops (Fig. 22). This is the best time of year to see that
the buds of woody plants are stem tips bearing either leaves or rudi-
mentary flowers, or both, usually with an outer covering of scales. Exami-
nation of trees and shrubs at this time will disclose not only these three
types of buds, but also several ways in which buds "open" and enlarge.
This is the time of year when you can learn by your own observations:

[Chap. IV SEASONAL ASPECTS OF PLANTS 37

which buds on different plants open first and from what buds the current
year's branches develop, whether the extension of the main stem or
branch always develops from the terminal bud, and if the branches from
all lateral buds grow equally. You can also ascertain whether leaves and
flowers develop on twigs from the same or from separate buds, which
of our common trees blossom before the development of leaves, and
whether these flowers are borne on twigs of the previous year or only on
new twigs. These are but a few of the questions that may be answered
by a study of woody plants in the field.

With the coming of spring countless millions of seedlings appear in
every unoccupied plot of ground. New branches develop on perennials
and biennials, and leaves are followed soon after by masses of flowers of
every imaginable hue. Spring is the period of most rapid development
in most plants, partly because the overwintering parts have been "condi-
tioned" by the low temperature and partly because of increased light,
increased length of day, and an abundance of available water in the soil.
After growth has started it may be retarded either by drought or by low
temperatures. On the other hand, elongation of the stems and expansion
of the leaves of most trees and shrubs stop within a few weeks, even if
temperature and moisture conditions continue unchanged. The growth
of new stem segments is definitely limited by internal conditions and
their elongation usually ceases by late May or June in northern latitudes.
Sprouts from stumps and pollarded trees continue to elongate for several
weeks more.

This is the period when many of the flowering plants of densely
shaded woods have their annual development, flower, fruit and return
to dormancy. In the open there are many plants in which the period of
development continues into the summer and autumn before flower, fruit,
and seed fomiation closes the annual cycle.

Spring, then, is the period of most active growth of roots and shoots
of woody plants. It is the period of most active utilization of food, and
the period of rapid respiration and food manufacture. The high rates of
these processes stand in sharp contrast to their much lower rates during
the winter season.

Summer aspect. With the coming of the longest days temperatures
also are high, and available soil water on the average begins to decline.
Growth of plants as a whole also declines, but many of the flowers of
springtime are now being followed by fruits and seeds.

38 TEXTBOOK OF BOTANY

The development of branches and the enlargement of trunks of trees
and shrubs are greatly reduced. Soil water declines to the point where
many plants with shallow root systems wilt, and summer leaf fall may
take place. Some plants or plant parts may become dormant as a result
of high temperature and the long daylight period.

There are fewer plants in bloom in midsummer than in spring or
autumn. The smaller grasses have attained maturity and completed their
life cycles. The larger grasses, such as corn, continue development and
bloom in summer. Summer is the period of greatest food accumulation
in the stems and roots of biennials and perennials. It is also the period
when fungi and bacteria cause numerous plant diseases and rapid decay
of organic matter. Insects also have reached maximal abundance, and
their injuries to leaves and stems and fruits become most apparent.

Some plants bloom onlv when the days are long and the nights are
short. As a result the summer season is the time of flowering of corn,
clover, mallows, cotton, and many other plants.

As the days become shorter at the close of summer and the tempera-
tures decline, increased water is available in many parts of the United
States because less of the rainfall is lost by evaporation. The water rela-
tions of plants are improved over those of the summer and the growth
of many herbaceous plants increases. Not infrequently a second wave of
flowering occurs in some plants, such as violets, that bloom abundantly
during the spring months.

But the further decline in temperature brings the conditions of au-
tumn, and we have completed our very brief view of the more noticeable
seasonal phenomena. We have observed that certain vegetational aspects
are rather characteristic of each season; that some phases of growth
belong to one season rather than to another; and that dormancy, growth,
maturity, and death of plants form a regularly recurring cycle.

The problems of plant science are all about us and each season
brings its own challenges to investigation and understanding. We need
confine our outdoor study of botany neither to a single environment, to
but one season of the year, nor to any one locality.

REFERENCES

(Chiefly on plant pigments)
Kuhn, R. Plant pigments. An/i. Rev. of Biochem. 4:479-496. 1935.
Mobius, M. Pigmentation in plants, exclusive of the algae. Bot. Rev. 3:351-
363. 1937.

[Chop. IV SEASONAL ASPECTS OF PLANTS 39

Onslow, M. W. TJie Anthocyanin Pigments of Plants. 2nd ed. Cambridge Univ.

Press. 1925.
Roberts, H. F. The causes of autumn coloration. Sci. Monthly. 45:427-435.

1937.
Sayre, J. D. The development of chlorophyll in seedlings in different ranges

of wave lengths of light. Plant Physiol. 3:71-77. 1928.
Smith, J. H. C. Plant pigments. Ann. Rev. of Biochem. 6:489-512. 1937.

CHAPTER V
LOCAL PLANT COMMUNITIES

Under natural conditions plants live in communities; no plant lives alone.
A potted fern at a window appears to be alone in an environment con-
sisting only of the physical factors of the soil, atmosphere, water, gravity,
and radiant energy from the sun. Its roots, however, are surrounded by
millions of microscopic plants and animals whose numerous activities
may influence the development of the fern. As we shall see later, some
of the activities of these minute organisms are beneficial to the larger
plant, others are detrimental. Some of them are known to cause diseases
of the roots or of the whole plant.

Plant communities. A lawn is a familiar plant communitv in which
the development of each individual plant may be influenced by the
plants that surround it. The intermingling of grasses and clover with
dandelion, plantain, and several other weeds is an illustration of the fact
that plant communities in nature are mixed populations composed of
several kinds of plants. Only in pure cultures, carefully prepared in
laboratories, may one expect to see plant communities composed of but
a single species of plant. In the practical consideration of certain com-
munities one may choose to disregard the microorganisms and look
upon a field of corn, of wheat, or of any cultivated crop as a community
of only one species if all visible weeds have been removed. The forester
may even disregard all the plants in a forest communitv but the trees,
and speak of a pure stand of beech or of hemlock. Usually in any mixed
population of plants a few species are much more abundant or modify
the habitat more than others; they are referred to as the dominant species
of the community. Plant communities are named according to their
dominant species; for example, a bluegrass lawn, a pigweed com-
munity, a tall-bluestem prairie, an oak forest, a beech-maple forest, an
elm-ash-silver maple forest.

Mutual effects of plants within a community. In lawns, pastures, or
golf courses in which Kentucky bluegrass is the most abundant and
dominant plant, the presence of the grass in some way interferes with

40

[Chap. V LOCAL PLANT COMMUNITIES 41

the growth of clover and weeds. If the bluegrass is carefully removed,
the weeds and clover rapidly increase in abundance. Similarly the pres-
ence of weeds interferes with the growth of bluegrass. When one re-
moves the leaves of a single large dandelion plant from a lawn for the
first time he is usually amazed by the area of soil from which bluegrass
was excluded by their presence. Trees with low branches may interfere
with the growth and dominance of bluegrass in a lawn unless the trees
are very small. Many people plant grass seed beneath such trees every
autumn. The grass grows well throughout autumn and during the follow-

er WIDTH of CROWN ^

Fig. 23. Bluegrass may grow beneath a tree if all the branches of the tree are
several feet above the ground. The diameter of the root system of a tree in a lawn
may be as much as 5 times the diameter of the crown of the tree.

ing spring, but by late June or early Jub' it becomes yellow and dies.
If all the branches of a tree are high above the ground, bluegrass may
continue to grow indefinitely beneath it (Fig. 23),

Succession. Since each plant in a lawn is influenced in some way by
the presence of other plants, the relative abundance of bluegrass or of
any other species of plant in the lawn may not remain the same from
year to year. During one growing season some species may increase
while others decrease in abundance. That is, the composition of a mixed
population of plants is not constant; it changes in time as a result of the
mutual eftects of the plants upon each other and of increase and decrease
in light, water, soil fertility, and other external factors. It is this in-
stability of a mixed population of a community that concerns the owner
of a beautiful lawn. Gradual changes in the relative abundance of the
constituent species may in time result in a community that is very

42 TEXTBOOK OF BOTANY

unlike the original one. Some of the species of the first community may
be entirely eliminated and other species may migrate into it from neigh-
boring communities. Through such changes one plant community may
be gradually replaced bv another, a process that is referred to as the
succession of plant communities.

The principles of dominance, changes in mixed populations, and suc-
cession referred to above are abundantly illustrated by local plant com-
munities in lawns, parks, vacant lots, eroding slopes, abandoned farms,
and forest remnants. All of these contain excellent field material that may
be studied first-hand. Lawns are especially valuable materials for study
because the\' mav be obser\'ed convenientlv and they probably attract
the interest and add to the pleasure of more people than most other
types of plant communities. Furthermore, they are similar to the com-
munities of cultivated grasses in pastures, golf links, parks, and college
campuses; consequentlv, many of the facts and principles discovered
about one of these communities may apply equally well to the others.
In this chapter we shall refer principallv to the more evident facts about
local plant communities that occur in the Central States. Explanations
of man\' of the observations will appear in later chapters.

Changes in mixed populations of plants. The results of a very exact
study of changes in the composition of a mixed population of plants in a
pasture in New York during a fi\'e-vear period are shown by the diagram
in Fig. 24.

The diagram shows the relative abundance of seven species of plants
during the year in which the seeds were sown, and for each of the next
five years. The change in composition of the plant population from year
to year is merely the result of a relati\'e increase in abundance of certain
species and a relative decrease of others. Red clover is entirely elimi-
nated at the end of the third year. Here then is an example of a change
in a mixed population of plants over a period of years that is due neither
to changes in the heredity of the plants nor to changes in climate or soil.
It is due to the characteristic differences in the growth of the several
species of plants and to their effects upon each other. One of the most
important changes is the gradual increase and dominance of bluegrass.

One may ask why lawns, pastures, and parks everywhere do not in the
course of time become dominated by Kentucky bluegrass. An analysis
of the areas not dominated by bluegrass reveals that they are either very
wet, very dry, very acid, sandy, shaded, or deficient in certain inorganic
salts, especially of nitrogen, or that they are areas where the soil tem-

[Chap. V

LOCAL PLANT COMMUNITIES

43

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Fig. 24. Diagram of changes that occurred in a mixed population of plants in a
pasture during a period of five years follo\\'ing the first year, when the seeds were
sown with those of a nurse crop, such as oats or rye. The comparative yield of each
kind of plant from year to year is given. Bluegrass is the only species that continued
to increase in amount every season. Data froin R. G. Wiggans.

peratures are high m summer. Evidently bluegrass can become the sole
dominant only in those areas in which each of these several environ-
mental factors does not exceed a certain intensity. Where one of these
factors is exceedingly high or low, a bluegrass lawn may be obtained
only if one is willing and able to adjust the environmental factors to an
intensity that is more favorable to the growth of bluegrass. Kentucky
bluegrass is most likely to become dominant in lawns with moderate
moisture and temperature, abundant light, and loamy soils that are
slightly acid but rich in salts of nitrogen, phosphorus, and potassium.
With persistent effort one can adjust the relative amounts of most of
these environmental factors.

Preference may be given, however, to another kind of grass that will
grow better than Kentucky bluegrass under the local conditions. There
are several varieties of bent and fescue grasses that grow well in soils of
greater acidity. Consequently in the acid soils along the Pacific Coast
and in the New England States bent grass lawns are more common than
they are in the Central States. Where the light is insufficient for the
growth of Kentucky bluegrass, certain varieties of bent and fescue

44 TEXTBOOK OF BOTANY

grasses may still grow well. In cool temperate climates other species of
bluegrass ( Poa trivialis and Poa annua ) are frequently planted in shaded
parts of lawns. These two species of bluegrass do not survive on hot dry
soils during the summer months. Kentucky bluegrass is also limited in
this respect. In the Southern States one may expect to find Kentucky
bluegrass as a dominant plant in lawns only at high elevations or where
the surface temperature of the soil is reduced by partial shade. The
characteristic lawn grasses of these states are Bermuda grass during the
summer months and Italian rye grass during the winter months. Near
the Gulf borders carpet grass, centipede grass, and St. Augustine grass
dominate the lawns.

The above facts about lawns and pastures sufficiently demonstrate
how the composition of a plant community is influenced by the climatic
and soil factors, and also by the effects of plants on each other. They
also illustrate that certain biological facts and principles may be ap-
plicable in theory and practice in any part of the world. On the other
hand, there are many facts and principles that cannot be generally
applied because of local and regional diversity in soil, climate, and avail-
able species.

Owing to the great diversity in cultivated plants and in environments,
at least one agricultural experiment station with several substations is
maintained in every state of the Union, and the effects of local differ-
ences are continuously investigated. Anyone who wishes to supplement
his own observations on lawns or other economic plant communities will
find valuable assistance in the weed manuals and other publications of
the nearest experiment station.

Bulletins on lawn-making frequently contain information about cer-
tain animals that destroy lawn plants or bring about changes in the
composition of plant communities. Recent publications are the most
desirable because some of the earlier recommendations for the care of
lawns have been revised as new facts were discovered. For instance, field
tests have shown that the traditional method of applying lime to all grass-
lands not only is unnecessary but may even bring about undesirable
results in certain types of soils. Many owners enjoy making their own
tests by applying various fertilizers to small strips in the lawn. Such tests
are reliable, inexpensive, and a source of valuable information from
year to year. Salts valuable as lawn fertilizers^ are often mixed with

^ In experiment station records the amounts and kinds of these salts tliat should be
added to the soil are usually indicated by a brief statement such as 1 lb. of 10-6-4 fer-
tihzer mixture per 100 sq. ft. The abbre\iation 10-6-4 refers to the percentages of
nitrogen, phosphoric acid, and potash respecti\ ely in a fertilizer mixture.

[Chap. V LOCAL PLANT COMMUNITIES 45

inert matter and sold on the market under the erroneous name of "plant
foods." This misleading use of a name is one of many examples in which
a scientific truth is ignored and denied for alleged commercial advantage.
We shall learn later that the food used by plants is the same as that used
by animals.

A change in the proportion of salts containing nitrogen, phosphorus,
and potassium may affect the composition of a plant community. For
example, soluble nitrogenous fertilizers may result in an increase in the
amount of bluegrass and a decrease in the amount of clover, while phos-
phate fertilizers may result in a marked increase in the amount of clover.
In the North Central States many pastures are dominated bv poverty
grass and weeds; bluegrass and clover are present but not abundant.
Hundreds of demonstration plots in these states have shown that the
addition of 300-400 pounds of acid phosphate per acre results in a
marked change in the composition of this pasture community. They also
showed that this change will occur without the addition of lime except
when the soils are very acid. At first there was a rapid increase in the
amount of clover, followed later by an increase of bluegrass. Within two
years these pastures had become dominated by clover and bluegrass,
and by the close of the fourth year they were bluegrass pastures. For the
farmer this procedure increases the value of the pasture from 200 to
300 per cent, and for others there is the pleasure of resting the eye on
luxuriant green pastures.

There is also an interesting biological background to this sequence.
The increase in clover followed the application of phosphate and re-
sulted in conditions becoming favorable to bluegrass. The practice of
using clover and other legumes as a means of enriching the soil is as
old as the history of agriculture, but it was not known until near the
close of the last century that the enrichment is due to the process of
nitrogen fixation by bacteria that inhabit the nodules of the roots of
legumes.

Animals, like plants, are dependent upon compounds containing cer-
tain kinds of mineral elements. They obtain nearly all of these com-
pounds either directly or indirectly from plants. We, for instance, obtain
them when we eat vegetables, meat, and dairy products. In diis way
some of the phosphate added to the pasture is gradually removed by
the grazing animals. Some of it also disappears from the land by erosion
and in drainage water. After several years the phosphate supply in the
pasture gets so low that clover and bluegrass begin to decrease in

46 TEXTBOOK OF BOTANY

abundance, and poverty grass and weeds again increase. When clippings
are continuously removed from lawns a similar loss in mineral salts
results and the growth of bluegrass and clover declines.

The succession of plant communities. One might refer to the demon-
stration cited above as an example of a poverty grass and weed com-
munity being succeeded by a bluegrass-clover community. The succes-
sion in this case, however, consists only of a change in the relative
abundance of species already present. Succession in which there is also
the added feature of species invading the community from neighboring
communities mav be observed locally by noting ( 1 ) the kinds of pioneer
plants that first occupy an eroding soil slope or some other bare soil area,
and (2) the kinds of plant communities that successively follow each
other on this same area through the years. One may shorten his period
of observation and obtain a similar story by noting the kinds of plant
communities that now occupy artificially made slopes of different known
ages. They may be found around any town where local construction
work involves the movement of large quantities of soil.

The pioneer plants that occupy such bare areas must of course start
from seeds that have been carried to the area by some means, and they
consist of annuals, biennials, and perennials that can survive in the
extreme environmental conditions that exist there. Most seedlings perish
in such situations. The presence of the pioneer plants brings about
changes in the habitat. They check erosion, increase the shade, and thus
decrease the temperature of the soil surface on hot summer days. Cer-
tain kinds of plants that were unable to survive in the pioneer condi-
tions may then develop on the area and increase in number. Their
abundance may produce conditions in which many of the pioneer
species are unable to survive. One of the most striking changes that
occur on such areas is the gradual disappearance of most of the pioneer
annuals and biennials, and the final dominance of perennials. Perennials
dominate the vegetation of the world, except in pioneer habitats.

This occupation of bare soil areas by a pioneer plant community, the
subsequent ehmination of the pioneers, and the succession of other plant
communities exemplify some of the fundamental processes involved in
the revegetation of areas on which the original communities were de-
stroyed by man, by fire, by wind, or by water.

As a nation we have recently become much concerned about the rapid
run-off of water from our agricultural lands and the amount of soil car-

[Chap. V

LOCAL PLANT COMMUNITIES

47

lied away from them (Fig. 25). Soil that has been thousands of years in
forming mider the influence of chmate, larger plants, and micro-
organisms is being washed from the land to the sea in some places at the
rate of more than 70 tons per acre annually. The subsoil that is exposed

fiiHynf-^rfifp^i

Fig. 25. During the century and a quarter that the phuits in this cemetery in
Ohio have checked erosion, sheet erosion on the adjacent gently sloping farm land
has removed the upper three feet of soil. Photo by C. H. Jones.

after a few years is a very poor habitat for most plants ( Fig. 26 ) . Where
the soil is completely covered with vegetation the amount of erosion
annuallv is so small that it can scarcely be measured; the run-off of
water after rains is also reduced to a minimum. One of the important
problems in any worth-while soil conservation program is to discover
better methods of speeding up the natural processes of revegetation
wherever practicable, and of preventing further unnecessary destruction
of established vegetation.

A first-hand study of local plant communities that occur on lawns,
campuses, eroding slopes, vacant lots, abandoned farms, or in forest
remnants introduces one to many botanical facts and principles. It also
raises numerous questions about the social behavior of plants, which
in turn depends upon the effects of the environment on the physiologi-
cal processes within these plants. These processes and their relations to
environment and plant behavior are subjects of discussion in later chap-
ters. The study of plant communities may lead not only to their biologi-

48

TEXTBOOK OF BOTANY

Fig. 26. One way of destroying a valuable heritage. The fertile top soil that
was formed during thousands of years has been eroded from the hill in the center
of the picture through bad farm practices. Photo by G. S. Growl.

cal analysis, but also to an understanding of their economic, esthetic, and
recreational possibilities. For the beginner those interests and appeals
are most valuable which come to him through his own observations.

Summary. The vegetation of any soil area is a mixed population of
several kinds of plants, some of which are readily visible, others micro-
scopic in size. The relative abundance of the different kinds of plants in
the mixed population, and also the kinds or species of plants present in
it, vary from place to place. These variations in the composition of plant
populations result in the formation of many different kinds of plant com-
munities, which we may recognize and name on the basis of the more
abundant or dominant species. The composition and distribution of these
communities are dependent upon the influence of the plants upon each
other, upon the animals that are present, and upon the factors of soil
and climate. Owing to all these factors, the composition of a plant com-
munity on a given area changes in time. This change may be rather
rapid or almost imperceptible, but it finally leads to one community
being succeeded by another. The change in composition may consist
only of changes in the relative abundance of the species already present
in the community, or it may be due in part to the invasion of the com-

[Chap. V LOCAL PLANT COMMUNITIES 49

munity by species from neighboring communities. These changes are not
due to an alteration in the heredity of the species involved. Changes in
heredity, however, must not be overlooked, for they are known to occur
at times, as we shall see in later chapters.

GENERAL REFERENCES

Bear, F. E. Theory and Practice in the Use of Fertilizers. John Wiley & Sons,
Inc. 1929.

Gustafson, A. F., ef al. Conservation in the United States. Comstock Publish-
ing Co., Inc. 1939.

Scott, John M. Permanent pastures for Florida. Florida Dept. of Agr. Bull. 27.
1929.

Sears, P. B. Deserts on the March. Univ. of Oklahoma Press. 1935.

Symposium: Erosion prevention capacity of plant cover. Iowa State College
Jour, of Science. 9:323-497. 1935.

U. S. D. A. Yearbook of Agriculture. Soils and Men. 1938.

U. S. Golf Association Green Section. Bulletins. Washington, D. C.

Welton, F. A., and V. H. Morris. Composition of grass from woodland and
from open pastures. Jour. Amer. Soc. Agron. 18:226-238. 1926.

SPECIAL READINGS

Barnes, E. E. Ohio's pasture program. Jour. Amer. Soc. Agron. 23:216-220.
1931.

Cook, I. S. West Virginia pastures. W. Va. Agr. Exper. Station Bull. 177. 1922.

Morgan, M. F. Lawn fertilization. Conn. Agr. Exper. Station Circidar 77. 1931.

Schuster, G. L. Pasture improvement. Delaware Agr. Exper. Station Bull.
164. 1930.

Sears, P. B. Deserts on the March. Univ. of Oklahoma Press. 1935. Chap. VHI.

Tehon, L. R. Rout the weeds! ///. Islat. Hist. "Survey Circular 34. 1939.

Westover, H. L., and C. R. Enlow. Planting and care of lawns. U. S. D. A.
Farmers Btdl. 1677. 1931 (Revised 1935).

Wiggans, R. G. Studies on various factors influencing the yield and the dura-
tion of life of meadow and pasture plants. Cornell Agr. Exper. Station Bull.
424. 1923.

CHAPTER VI

POINTS OF VIEW IN THE INTERPRETATION OF PLANT

BEHAVIOR

We have been concerned thus far mainly witli the acquisition of easily
observable facts and relations. We have seen how some of the resem-
blances and differences among plants may be used in distinguishing
and naming trees, in observing seasonal changes in plants, and in
studying the mixed plant populations of lawns and other local areas.
There is a further use of facts and observations that leads to a clearer
understanding and interpretation of the plant world.

The how and why of natural phenomena have always challenged
thoughtful persons, and out of their observations and experiments have
come a vast store of information and a great improvement in economic
and social welfare, as well as more adequate explanations and more
discriminating philosophies of life. Numerous individuals have directed
their efforts toward the discoverv of important new facts and new rela-
tions among natural phenomena. Their discoveries have raised step bv
step the level of explanations and clarified our understanding of the
sequence of natural processes. Still others have from time to time applied
previous discoveries to the needs and betterment of society. Both types
of contributions have given us a valuable social heritage which we
should try to understand, perpetuate, and increase.

When we attempt to interpret natural phenomena that lie within the
field of plant behavior we strive to use intelligently the contributions
of others. Considerable as our progress as individuals may be, we cannot
hope by our own observations to explain more than a small fraction of
the problems of plant behavior even though we devote a lifetime to it.
Our ability to utilize the discoveries of others, however, depends upon
our efforts to observe at first hand and to comprehend what we see.

Older points of view. As far back as human records go, man has at-
tempted to account for himself and his surroundings. Primitive man
lived in most intimate contact with nature. Wishing to survive, he was
compelled to give attention to other animals, to light and darkness, to

50

[CJiap. VI THE INTERPRETATION OF PLANT BEHAVIOR 51

Hood and drought, and to trees and herbs. He came to regard all these
objects and processes of his environment as separate entities. Before
he recognized their relation to one another, he had very definitely
related them to himself as either benevolent or malevolent. Lacking the
facts and the methods for testing the soundness of his ideas, he attempted
to explain why, without knowing how natural phenomena take place. To
him all natural objects appeared to be more or less human-like. Accord-
inglv he attributed to other animals, to plants, to the moon and other
inanimate objects the same abilities, motives, and emotions he recog-
nized within himself. In other words, he personijied them.

The cruder forms of these explanations of physical phenomena have
passed, but — if we mav judge by the accounts of biological phenomena
in popular literature — plants and animals are still looked upon as objects
that can control their behavior and growth, read the future better than
we can, and make plans accordingly. The authors of these accounts have
not really progressed much beyond certain inhabitants of the West
Indies who still fear the "duppy" that resides in the silk-cotton tree.
To some of these authors there is a duppy or mystic directing spirit in
everv plant, while to others the will of the plant itself directs the
behavior.

For example, who is not familiar with such explanatory statements as
these: that trees in a forest grow tall and straight because they are
trying to reach the light; that stems and petioles of plants growing at
a window bend toward the window in order to get more light; that
potato sprouts and stems of seedlings in a dark room grow longer in
search of light; that plants produce seeds in order to propagate their
kind, or to store food for future use or for the use of animals; that roots
grow deep into the soil or toward a cistern in search of water; finally,
that plants do this or do that particular thing in order to adapt them-
selves to their environment?

Do not all these statements imply that the plant is conscious of its
present and future needs and strives by various means to satisfy them?
Whatever happens, according to this point of view, the plant is always
forward-looking. In this type of explanation of natural phenomena every
act is assumed to be purposeful.

Alternative point of view. An alternative point of view is taken by
those who regard the end attained not as the purpose of the preceding
steps but merely as the consequence of the events or processes that
preceded it. Working from this point of view, the student tries to

52 TEXTBOOK OF BOTANY

recognize and understand the processes and conditions which bring
about the phenomenon he wishes to explain. From this point of view
natural events are assumed to follow each other in a cause and effect
sequence, and explanations are obtained only through the discovery of
the sequence of causes and effects.

All of us, therefore, are faced with deciding whether we shall accept,
as explanations of natural phenomena, those that are based on needs
and purposes or those that are based on preceding conditions and
processes.

Interpretations exemplified. Everyone is familiar with the evaporation
of water, with the formation of clouds, and with rain. Does water evapo-
rate from the earth's surface in order to form clouds, or is this evapora-
tion of water merely the consequence of a series of events that may be
traced to molecular motion? Do clouds form to produce rain, or are
clouds a consequence of a series of events that we may trace back to
the evaporation of water? Does the rain fall in order to moisten the soil
and keep the streams flowing, or because the force of gravity on larger
drops forming in the atmosphere exceeds the forces of air movement
and resistance? The origin of the large drops may be traced back through
smaller and smaller particles to water vapor and its relations to the
temperature of the atmosphere. This brief outline suggests merely a few
of the many facts and relations that must be apprehended before these
phenomena can be seen as causal sequences. Most persons, however,
prefer to seek such data rather than accept as an explanation for rainfall
the statement "that it is necessary to wet the soil and to make plants
grow."

Physical and chemical phenomena have for the most part passed out
of the stage of personification. It would be hard to find persons who
would consider the statement, "Sodium unites with chlorine in order to
form sodium chloride or common salt," a satisfactory explanation of the
union. No one would explain the flow of water downstream as a striving
of the water to get to the sea. Feature writers would never dream of
accounting for the movement of electrical energy from dynamos through
wires to motors by saying "the energy is needed to turn the motors."
Most readers would not be satisfied with the substitution of purpose,
necessity, or use, for cause in such examples.

But when biological phenomena are being described and explained
many writers feel no need for securing the results of experimental study.
On the contrary, they proceed with their discussion of natural processes

[Chap. VI THE INTERPRETATION OF PLANT BEHAVIOR 53

as if the processes were occurring to meet the needs and desires of the
plants and animals.

During the last quarter-century great advance has been made in
analyzing the chemical and physical phenomena that underlie plant
behavior and plant processes. Much remains to be discovered in this
field, but a broad foundation has certainly been built, mostly by those
who have applied the principles of chemistry and physics to biological
processes and have attacked problems from the point of view of causal
sequences.

A little better understanding of this point of view may be obtained
if we briefly sketch one of the examples of the effect of light on growth.

Fig. 27. Potato sprouts that have grown from tubers in diflFerent light intensities:
1, in a greenhouse; 2, in a darkroom. Photo by F. H. Norris.

54

TEXTBOOK OF BOTANY

By examining a vertical section of a growing stem tip ( Fig. 32, page 69 )
one may see that growth in length of a plant stem is a consequence of
( 1 ) the formation of new cells by cell division at the apex of the stem
and (2) the subsequent enlargement of these cells. As a result of these
two processes the stem elongates.

Fig. 27 shows the familiar fact that potato sprouts grow in length more
rapidly in darkness than in bright light. How shall we account for this?

Fig. 28. Differences in stems and leaves of bean seedlings that grew in different
light intensities: 100%, 50%, 0%. Photo by F. H. Norris.

The sprouts that grew in the dark have comparatively small leaves and
lack the familiar green color. Fig. 28 illustrates bean seedlings that grew
in bright light, in shade, and in darkness. The stems of the seedlings
that grew in shade and in darkness are longer than those that grew in
bright light. The seedling from the dark box has small leaves and lacks
chlorophyll, but the seedling in the shade is green and has larger leaves
than the one in bright light. Hence, the green color and the size of the
leaves do not account for the differences in the growth in length of the
stems.

Still more exact data have been obtained. An experimenter found that
certain tobacco plants grew in height 3 times more rapidly at night than

[Chap. VI THE INTERPRETATION OF PLANT BEHAVIOR 55

during the day, except on cloudy days. Another student found that the
stems of a scarlet runner bean that grew in the dark were 3.6 times as
long as those that grew in bright light. Upon further investigation he
discovered that one-third of the increased height in darkness was due to
the increase in the number of cells formed by cell division, and that two-
thirds of the increase was due to a greater elongation of cells. From these
facts we may conclude that the slower growth in height of stems in
bright light is a consequence of more restricted cell division and cell
enlargement.

From physicists we learn that light does not affect plants unless it is
absorbed bv them. Hence the sprouts in the dark are not affected by
light, they are not attracted by it, nor is their growth in length influ-
enced by it. Light apparently affects many plants in a way that results
in a decrease in both cell division and cell enlargement. We are familiar
with the fact that light initiates chemical changes in photographic films
and that it increases the rate of evaporation of water from wet surfaces.
Similarly, when light ra\'S strike the molecules in plant cells they start
physical and chemical changes which may initiate a series of events and
consequences that finally results in decreased cell division and cell en-
largement and thus in a reduced growth in length of the stem as a whole.

Evidently between the first action of light upon the cells and the
diminished growth in height of the stem, there may be a long series of
events and consequences. If we knew all the facts involved, we might
then be able to trace these events and consequences in the order of their
dependence upon each other. Some of the facts are known; but we must
be content for the present with the general statement that the slower
rate of cell division and enlargement during tlie day is often a conse-
quence of certain changes in cell processes related primarily to the
lower content of water in the cells.

The inhibiting effect of light on the elongation of stems may also be
due to its influence on certain chemical compounds called growth sub-
stances, or hormones. Cell enlargement in particular is affected by these
compounds through their influence on the growth of cell walls. When a
plant is placed at a window, growth in length of the brightly lighted
side of the stem is checked and the tip of the stem bends toward the
window. In such instances the inhibiting effect of light is due primarily
to its influence upon the translocation and the reaction of growth sub-
stances on the lighted side of the stem. The relation of these substances
to light and the growth of cells will be described further in later

56 TEXTBOOK OF BOTANY

chapters. At present it is more important to know that bright Hght often
checks the rate of growth in length of plant stems, and to understand
how this fact may be used to explain why stems and petioles bend when
the light to which they are exposed is not uniform on all sides.

The question of whether the result is good for the plant or plant part
does not arise in this type of explanation. Whether it be advantageous
or disadvantageous is another matter entirely unrelated to the cause of
the behavior of the plant. Results follow causes regardless of whether
they are good or bad for the organism. In fact, the result often is the
death of the plant.

The generalization that bright light has an inhibiting effect upon
growth in length of plant stems is true in numerous instances, but one
should not immediately infer that stems always grow more rapidly in
length in the shade than they do in bright light. The examples cited in
the discussion above were seedlings and sprouts. On the other hand,
young trees in heavily shaded forests grow in height very slowly for
many years. If they become exposed to increased light through the death
or removal of an older tree, their rate of growth in height increases
(Fig. 29). The differences in rates of growth of these young trees are
consequences of certain processes which the reader will appreciate when
he has become familiar with the facts in the chapters on plant nutrition
and water relations. They in no way contradict the explanation of the
example cited above, but they do emphasize the fact that cell division
and cell enlargement are influenced by many conditions within the cells,
and that under other circumstances another group of internal conditions
may become the important one.

Interpretation of hereditary differences. The previous paragraphs in
this chapter have centered on differences in plant behavior which are
due to a lack of uniformity in the environment. In Chapter I attention
was called to other differences, such as the presence of leaves, roots, and
seeds in some plants and their absence from others in all environments.
Many writers seem to think that such structures developed to meet a
need. The student of plant processes, however, is more likely to regard
the origin of these structures as consequences of changes that occurred
within the cells of remote ancestors without reference to the needs of
the plants of today. To him they are the result of heritable alterations in
the chemical and physical composition of the protoplasm. Purpose can
scarcely be thought of as the cause of these minute changes in the com-
position of the protoplasm, or of the far more striking differences in struc-

[Chap. VI THE INTERPRETATION OF PLANT BEHAVIOR

57

Fig. 29. The growth of young trees among the older trees in a forest is very slow.
Spruce and fir forest in New Hampshire. Photo by U. S. Forest Service.

58 TEXTBOOK OF BOTANY

ture and development that result from them. If these changes did not
result in the death of the organism, the plant survived and the altera-
tions v^^ere transmitted to the offspring. Such alterations in protoplasm
and their transmission from one generation to another are discussed
in the chapters on chromosomes, heredity, and variation.

Summary. Man seems always to have felt an urge to account for him-
self and for his surroundings. Perhaps this urge had its origin in the
fear of the unknown rather than in the intelligent curiosity that incites
the student of modern times. Until man began to apprehend the proc-
esses of nature and to perceive the consequences of one process in
relation to another, there could be no scientific approach to interpreta-
tion. The myths that had grown up about the objects and processes of
the physical world had to be replaced by facts and interpretations in
accord with the general principle that what happens is a consequence
of preceding events without reference to future needs or purposes. In
the interpretation of the biological world on the basis of the principle of
causality, progress has been comparatively slow. Personification of plants
and animals is still chosen by manv writers as the basis of interpreting
biological phenomena. Only the more thoughtful ones are skeptical of
interpretations that ascribe conscious effort to all sorts of living organ-
isms. Slowly during the last centurv, and much more rapidly during the
present one, biologists have paved the way for a more intelligent inter-
pretation of biological phenomena. The work of all our biological re-
search institutions is directed toward the discovery of the facts and inter-
dependence of biological processes. Perhaps the greatest need today is
that the average citizen should become as intelligently conscious of
advances that have been made in the scientific interpretation of his
biological environment as he is of those of his physical and chemical
world. This chapter emphasizes the importance of clearly recognizing
the different points of view from which interpretations of plant behavior
are made.

CHAPTER VII
CELLS AS BIOLOGICAL UNITS

<J><X><><><X><J><><><X><c><>£>^^

The development of every science is invigorated and its aims are
redirected from time to time as the result of some important invention.
A striking example in the field of biology is the invention of the micro-
scope in the seventeenth century. Superficial observation and study of
plants had been going on for untold centuries. The origin of many of our
most important cultivated and medicinal plants antedates the oldest
archeological discoveries. In Europe and Asia several thousand plants
had been described by the time that the invention of the microscope
made it possible to examine their more minute structure and to discover
that many plants consist only of single cells. ^

Cells. The early microscopists were so fascinated with the world of
minute plants and animals previouslv unseen and unsuspected that they
studied and described them in preference to the finer structures of the
larger plants. Cells were seen and recognized as structural parts of
plants, but for a hundred years observations were limited mainly to cell
walls. In the latter part of the 18th century various microscopists began
to study "cell contents." By the middle of the 19th century it became
evident to a number of eminent biologists- that the properties which we
associate with life are the properties of that part of the cell contents
which had come to be called protoplasm. Moreover, its organization into
cytoplasm and nucleus, and the enclosed vacuole had been recognized.
Many biologists had by this time accepted three general principles: ( 1 )
that the bodies of all living organisms are composed of cells, or products
of cells; (2) that in certain features the cells of plants and animals are
essentially alike; and (3) that protoplasm is the physical basis of life
phenomena. To these principles we may add (4) that when plants are

^ Perhaps the oldest book of plant descriptions extant is Shen Nung's Tree and Herb
Book, written during the 28th century b.c. In it 252 species of plants are classified accord-
ing to their alleged medicinal values.

- Attention may be called especially to Robert Brown, \on Mohl, Schleiden, Schwann,
Dujardin, Nageli, Payen, Cohn, and Schultze, whose contributions are admirably discussed
in L. W. Sharp's Introduction to Cytology, 3rd ed., McGraw-Hill Book Company, Inc.,
1934, pp. 422-447.

59

60

TEXTBOOK OF BOTANY

similar it is because their protoplasms are similar, ( 5 ) that when plants
develop or behave differently in the same environment it is because their
protoplasms are unlike, and (6) that life is a recognized property of a
very complex physical-chemical (colloidal) system.

Parts of the cell. As shown in Fig. 30, the essential parts of the cell
may be outlined as follows:

1. The protoplasm, differentiated into cytoplasm, plastids, and
nucleus,

2. The vacuole — a cavity within the cytoplasm filled with water con-
taining sugars, salts, acids, and other substances largely in solution.

3. The cell wall — a more or less complete covering around the proto-
plasm. Fine strands of protoplasm sometimes extend through the
wall from one cell to another.

cell wall

j^ 1^ r\ucleas '

cytoplasm

protoplasm

vacuole

Fig. 30. Plant cells as seen through a microscope: A, a meristematic cell com-
posed of a nucleus and cytoplasm surrounded by a cell wall; B, an older cell which
has become enlarged by growth of the cell wall and the formation of a large central
vacuole; C, cell B as seen in perspective; D, starch grains formed in plastids in a
cell of a potato tuber; E, chloroplasts in a cell of a moss leaf.

In order to appreciate protoplasm as the medium in which the numer-
ous chemical and physical processes of the cell occur, it will be neces-
sary to digress for a moment and consider some of the properties of
different states of matter. You have seen solid crystals of sugar and salt
disappear as they dissolved in water. You are also familiar with gelatin
and fruit jellies in which organic matter is dispersed in water without
being dissolved. Protoplasm seen through the microscope resembles a
jelly more than a solution.

The solid, liquid, and gaseous states of matter are familiar to every-

[Chap. VII CELLS AS BIOLOGICAL UNITS 61

one. But when matter is dissolved in water or when, as in jelly, it is
dispersed in water without dissolving, such strikingly new properties
become evident that we must recognize two additional states of matter:
solutions and colloids.

Solutions. When a substance dissolves in water its particles become
subdivided and separated as molecules. The molecules of salts, acids,
and bases are to some extent further separated into ions. The resulting
solutions are either colorless or colored homogeneous liquids, and the
particles remain equally dispersed throughout the solution because of
their constant motion. For example, if a gram of table salt (sodium
chloride ) dissolves in a glass of water the solution remains colorless, but
if a similar amount of copper sulfate dissolves in the water the solution
is blue.

When salts, acids, and bases have dispersed in water as ions, each ion
has a characteristic positive or negative electric charge. Many of the
reactions which occur in solution are the result of these electric charges :
particles with unlike charges attract each other and unite, and those
with similar charges repel each other. For instance, if one pours the
above solution of copper sulfate into the solution of sodium chloride,
the resultant solution will contain temporary molecules, or compounds,
of NaCl, Nai.S04, CuCL, and CUSO4; and also the free ions: Na+, CI",
Cu^^, S04~ ~ . . ., etc. The solution is a stable system in which one or
more substances become so finely divided in the form of molecules and
ions that it disperses among the molecules of the water throughout the
system.

Colloidal systems. When substances are almost completely insoluble
in water it is possible by various means to subdivide them into very
small particles which when dispersed in water continue to remain sepa-
rated and distributed throughout the water for a long time. If grains
of sand or pellets of clay are dropped into a beaker of water, they
immediately fall through the water to the bottom of the beaker. If the
pellets are crushed and the beaker is vigorously shaken for a moment,
the smaller particles of clay are held in suspension in the water and it
may remain turbid for an indefinite time. Streams and ponds that do not
become clear on long standing are excellent examples of turbid suspen-
sions. Some of the suspended particles in a drop of turbid water may
be seen through a microscope.

Turbid water due to fine clay therefore illustrates a suspension in
which solid particles are dispersed in water. Each particle is a cluster of

62 TEXTBOOK OF BOTANY

molecules and has a definite surface which is electrically charged. In very
fine suspensions the particles may not settle out for months or years,
because of the constant bombardment of the water molecules and be-
cause the particles have similar electric charges. Suspensions differ from
solutions in that the water and the particles dispersed in it form a two-
phase system. The one phase is the solid particle, the other is the water,
and between each particle and the surrounding water is a definite
surface of contact. When a mass is broken up into fine particles, the
aggregate surface of the particles is enormous. For example, a one-
centimeter cube has a surface of 6 sq. cm., but when disintegrated into
particles of colloidal size the combined surfaces of all the resulting par-
ticles are equal to about 1.5 acres.

Another tvpe of colloid is exemplified by milk, in which proteins and
fat-like compounds (lipoids) are dispersed in water which contains
other proteins, salts, and sugar in solution. Dispersed throughout this
liquid phase of milk are numerous visible globules composed largely of
fat. These globules slowly rise to the surface and form a layer of cream.
In butter the phases are reversed; fine droplets of water are dispersed in
fatty material. White of egg is a typical protein-in-water colloidal sys-
tem; the yolk is a colloidal system of many phases, with fats, proteins,
pigments, and other substances dispersed in water. When white of egg
is heated or treated with alcohol, vinegar, or strong salt solutions it
coagulates. When meat, which is largely protein, is boiled it also coagu-
lates. Gelatin desserts and fruit jellies are familiar examples of colloids.

When a gram of gelatin is dispersed in 50 grams of hot water a highly
fluid colloid, called a sol, is formed. When it cools and stands for a few
hours it becomes a semi-solid jelly, or gel. The gelatin becomes finely
divided and dispersed in the water but does not dissolve. Its particles
are much larger than molecules. Owing to the elasticity and certain
other properties of this type of colloid, the dispersed particles are
thought to be fiber-like and arranged like twigs in a loose brush heap.

When a gelatin-water gel is heated it becomes highly fluid, but after
it cools and stands for a time it again becomes organized into a more
or less rigid gel. Thus the gel and sol states of a colloid may be reversible,
and the change from the one to the other may be brought about by
changes in temperature, water content, acidity, or any one of several
other conditions. Drastic changes in the internal organization of a
colloid may result in coagulation.

Protoplasm is a very complex colloidal system with protein, carbo-

[Chap. VII CELLS AS BIOLOGICAL UNITS 63

hydrate, fat, and lipoid phases dispersed in water, in which are various
salts, sugars, acids, and other soluble compounds. Many of these sub-
stances are chemically unstable and react with extreme readiness. All
masses of protoplasm have a surface film rich in lipoids which prevents
unlimited dispersion and keeps the mass intact.

The highly reactive character of the protoplasmic system at ordinary
temperatures is one of its most important properties. Since it is a col-
loidal system there are verv large surfaces between the water and the
other phases, and here surface energy may bring about reactions not
possible in a solution. These surfaces also accumulate electrical charges
which may be significant in reactions. Because it is a manv-phase svs-
tem, a change of one phase may result in the alteration of other inter-
locking phases and the modification of the whole protoplasmic system.
Like gelatin and jelly, protoplasm is more or less elastic, and some of its
constituents may have a brush-heap structure.

Furthermore, protoplasm is a self-perpetuating system, in that it can
combine foods (carbohydrates, fats, and proteins) into the system and
enlarge its mass. It is evident, therefore, that the jellv-like part of the
cell which was named protoplasm is not a single chemical substance.
Life, like all the other distinctive properties of protoplasm, is a result of
the chemical components and their complex organization in a colloidal
system. When the system is disorganized the distinctive properties disap-
pear, for the materials or compounds are not alive.

Cytoplasm. The protoplasm in most plant cells is organized into
cytoplasm, plastids, and nucleus. In young cells these parts completely
occupy the space within the cell wall ( Fig. 31 ) . As the cell enlarges,
microscopic droplets of liquid become visible in the cytoplasm, the col-
loidal matrix is drawn together, and the droplets coalesce, forming larger
water-filled cavities called vacuoles. In most plant cells the coalescence
of small vacuoles results finallv in the formation of a single vacuole, and
the cvtoplasm with its embedded plastids and nucleus becomes relegated
to a thin layer lining the cell wall. In other cells the nucleus may retain
its central position, supported by cvtoplasmic strands which are the
remnants of the cytoplasm separating the several vacuoles before they
coalesced into the single large one.

Cytoplasm is usually granular in appearance because of the presence
of particles of various foods and other substances. Cvtoplasmic move-
ments (streaming) mav be seen in the cells of some plants.

Plastids are protoplasmic bodies in the cytoplasm. Special reactions

64

TEXTBOOK OF BOTANY

li-r^v^ ^<ji6■'

r •

■v^l

';^r

ism v'i^^-\

i!5-<ri'v-^/'-" - ^^

Fig. 31. Photomicrograph of a vertical section of a stem tip of coleus. The small
meristematic cells at the extreme tip are completely filled with protoplasm. The
older cells which have become much enlarged by vacuolation are similar in
structure to those in Fig. 30. Photo by E. B. Wittlake.

take place in them, often resulting in the accumulation of particular
substances, such as starch and chlorophyll. There are usually several to
many plastids in each cell; and as the cells multiply by division, the
plastids also increase in number by division. They are named on the
basis of certain substances that accumulate in them, as shown in
Table 1.

In very young cells there are small colorless plastids ( leucoplasts )
that apparently may develop into these more specialized types, depend-
ing upon environmental conditions.

The nucleus. The nucleus is usually more refractive to light than the
cytoplasm and under the microscope appears brighter. Often its proto-

[Chap. VII CELLS AS BIOLOGICAL UNITS

Table 1. Names of Plastids and Their Characteristic Contents

65

Names of Plastids

Characteristic Contents

Green plastids or chloroplasts.

Starch plastids or amyloplasts.
Fat plastids or elaioplasts.
Color plastids or chromoplasts.

Chlorophylls, carotenes, and xanthophyll,
sometimes minute starch grains.

Starch grains.

Oils and fats.

Pigments found in chloroplasts, also red,
yellow, and other pigments.

plasm is denser and forms a more rigid gel, but sometimes it is a fluid
sol surrounded by a membrane. Chemically it differs from the cytoplasm
in that the proportion of proteins is less and its proteins are far more
complex.' They are relatively high in phosphorus and possibly on this
account more reactive. There seems to be good reason to regard the
nucleus as the center of many cell activities; as we shall see in a later
chapter, it contains certain very minute bodies that are the carriers of
many of the hereditary factors of the plant.

The vacuole. At maturity the vacuole occupies most of the space in-
side the cell walls; it is filled with "cell sap," a solution of s,ugars, salts,
acids, and other soluble compounds. It mav also contain colloidally dis-
persed proteins, carbohvdrates, and other less soluble substances. Some
of the substances in solution readily pass from the vacuole into the sur-
rounding cytoplasm or from the cytoplasm to the vacuole; others do not.
Sometimes crystals of salts accumulate in the vacuole, or the cell sap may
become colored with pigments as in the cells of many flowers, fruits,
and red autumn leaves.

The cell wall. The outermost part of a plant cell is the cell wall,
formed at the surface of the enclosed protoplasm. During cell division
the first wall between the daughter cells is composed of pectic material,
and on the inside of this pectic wall ( middle lamella ) successive layers
composed of cellulose or of cellulose and pectic compounds are de-
posited. Sometimes other substances, such as cutin and lignin, accumu-
late in cell walls and thereby change their properties. Most cells on the
outer surface of plants contain cutin in the outer wall. The cells in wood

3 f Phosplioric acid

[ Nucleic acid < +

Nucleo-proteins \ + [ Sugars

Protein base

66 TEXTBOOK OF BOTANY

exemplify walls that started as pectic compounds and cellulose and
became lignified. Cell walls also possess colloidal properties and swell
in water. They are composed of small submicroscopic particles between
which molecules of water may penetrate and be held tenaciously.

The cell as a whole. From this description of the parts of the cell and
the study of cells with a microscope, it should be possible to picture
the cell as a structural unit, and at the same time as a dynamic unit, dis-
playing those properties and processes commonly associated with living
organisms. Some of these structures and processes may be briefly sum-
marized.

1. All plants consist of one or more cells.

2. Cells are formed by the division of previously existing cells, and
less frequently by the fusion of cells.

3. The properties associated with life reside in the protoplasm, which
is a complex colloidal system of proteins, carbohydrates, fats, and
lipoids permeated by a water solution of acids, bases, and salts,
many of which are highlv reactive. Owing to the enormous sur-
faces between the water and the colloidal particles the proto-
plasmic system contains surface energv and electrical energy not
characteristic of solutions.

4. The protoplasm therefore is a chemical and physical system, no
one constituent of which is living; the qualities that distinguish
protoplasm from non-living svstems result from the unique organi-
zation of atoms, ions, molecules, and colloidal particles with their
associated chemical, electrical, and surface energies.

5. The protoplasmic system sustains itself in the presence of food,
and can combine food substances into its own structure and
thereby increase its mass. In other words, it grows.

6. In the development of the plant cell, vacuolation occurs and the
cytoplasm at maturity becomes a sack-like layer surrounding a
solution of sugars, salts, acids, and very dilute colloidal dispersions
in the water of the vacuole.

7. During its development, the usual plant cell also becomes sur-
rounded by a cell wall — at first primarily of pectic material, later
of cellulose layers — which may be altered in several ways. The
cell wall has a high water content during the active life of the
cell.

8. The large proportion of water pervading all parts of the cell con-

[Chap. VII CELLS AS BIOLOGICAL UNITS 67

stitutes a medium in which the movement of materials and the
chemical and physical changes within the cell readily occur.
9. As a result of the chemical changes constantly going on in the
cell, electrical energy is released as in a battery, and one end
of the cell usually has a higher electric potential than the other.
Consequently most cells are polarized, and their opposite ends may
behave differently.
10. In cell masses of the larger plants electric potentials also develop,
and the apex of a plant organ may become electrically negative or
positive to the base.

With these principles in mind, one has a basis necessary for interpret-
ing the various extraordinarv reactions of living plants. Today we know
only a minor part of the energy and material relations within plant cells,
but our present knowledge is certainly sufficient to lead us to doubt the
necessity of assuming the presence of mysterious forces to account for
the behavior of organisms.

CHAPTER VIII
THE TISSUE SYSTEM OF LEAVES

In the preceding chapter, cells are described as unit structures embody-
ing several interrelated physical and chemical systems. Cells are con-
sidered as the physiological units of plants because they are the smallest
bits of protoplasm known to be capable both of independent existence
and of reproduction through division. In later chapters less highly organ-
ized cells of bacteria and certain algae are described. There are several
thousand species of plants that hve as single cells. In other thousands
the cells are aggregated in colonies in which the individuals cells are
more or less independent. In the larger and more familiar plants the mil-
lions and billions of cells of which they are composed remain not only
fii-mly attached but are to some extent mutually dependent. During the
development of these plants, systems of cells called tissues arise through
cell enlargement and differentiation.

Cells and tissues. The cells of a given tissue may have a common
origin; all of them may be similar in position, shape, texture, or color;
or several different kinds may form a distinct structural complex. Never-
theless, the tissues of many common plants may be distinguished readily.
In this chapter we shall try to picture the tissues of a leaf, emphasizing
the fact that the leaf, or foliage organ, like other organs of a plant, is a
system of tissues, the cells of which are differentiated, but intimately
related, structurally and physiologically.

Leaf bud development. Leaves develop from buds; they first become
visible through the microscope as small protuberances (primordia) in
the meristematic^ region near the growing apex of the stem (Fig. 32).
The cells of the primordium divide and continue to produce new cells —
all very similar in size and shape. The uniform brick-like cells of the leaf

^ A primordium ( pi. primordia ) is the beginning, original, or rudimentary state of an
organ — in most cases, a minute mound of similar cells. The term meristem (Greek
meris = a divider) refers to any part of a plant in which cell division, enlargement, and
differentiation are possible or taking place; it often is a synonym of growing region.
Meristematic cells, tissues, and regions are sometimes called embryonic cells, tissues, and
regions. After cells and tissues are mature they may again become meristematic through
changes in their immediate enxironment, and cell division begins again.

68

[Chap. VIIl

THE TISSUE SYSTEM OF LEAVES

69

p> YOUNG LEAVES

Fig. 32. Vertical section of the terminal bud of a coleus plant. Compare it with

Fig. 31.

primordium are completely filled with protoplasm and become stratified
in several (5-8) layers. As this protuberance expands farther — becoming
more leaf-like, but still very small — cells in the middle layers divide
irregularly and form groups of cells from which the vascular tissues of
veins develop (Fig. 33).

In the leaves of many of our trees, to which the above description
particularly applies, this entire development proceeds slowly through
the spring, summer, and autumn months within the buds. By the end of

70

TEXTBOOK OF BOTANY

J

STOMATE

UPPER EPIDERMIS

PALISADE 1

MESOPHYLL
SPONGY
LOWER EPIDERMIS

VEIN

Fig. 33. Cross sections of leaves: A-I, sections of embryonic leaves from the
terminal bud of a tobacco plant in the order of their ages. The oldest leaf ( I ) in the
series was less than one centimeter in length. J, cross section of a small portion of a
blade of a mature leaf of tobacco. After G. S. Avery, American Journal of Botany.

winter some of the leaves in the bud have attained their characteristic
pattern in miniature. The cells of the prospective future veins are slightly
differentiated, whereas the remaining ones are still similar. Thus far cell
division has been the dominating process of development. All cells re-
main in complete contact and there are no intercellular spaces.

Leaf development. With the warmer weather of the following spring,
growth is renewed. The stem tip and young leaves expand and press the
bud scales apart. During the rapid increase in size, cell enlargement and
cell differentiation dominate the growth process, leading to the forma-
tion of the distinctly different tissues as seen in microscopic sections.
Cell division stops first in the epidermis, next in the spongy mesophyll,
and last in the palisade mesophyll. Cell enlargement, however, con-
tinues longest in the epidermis; as a result, the spongy cells are pulled
into an open meshwork while the palisade cells are but slightly sepa-
rated. Meanwhile the vascular bundles of the veins have increased in
diameter. Expansion of the leaves follows rapidly after the opening of
the buds; they often double their size within twelve hours. In some
plants with small leaves the whole development from leaf primordium

[Chap. VIII THE TISSUE SYSTEM OF LEAVES 71

to mature leaf may occur at a rather uniform rate during ten days to
two weeks. In large-leaved plants, such as tobacco, the leaves grow for
a month or more before becoming mature.

Leaf tissues. When we look at the exterior of a leaf we find the upper
and lower surfaces composed of angular or interlocking cells of the epi-
dermis ( Fig. 34 ) . The outer cell wall contains a deposit of cutin, a fat-

Upper epidermis

■ Palisade layers

Intercellular
^ , y^ 5pace

Lower
epidermis

Fig. 34. Tissues of a leaf of the common periwinkle {Vinca). Courtesy of World

Book Co.

like substance. This cutinized layer is often called the cuticle. It may be
very thin, or it may be so thick that the outer wall is the most conspicu-
ous part of the epidemial cell.

In the lower epidennis, and sometimes among the upper epidermal
cells, paired specialized guard cells partially separate and form a pore
or stomate.- When open, the stomates are passages connecting the air
within the intercellular spaces of the leaf with the external atmosphere.

Between the upper and lower epidennis there are several layers of
cells constituting the mesophyll. The upper layers usually have elongated

- Some authors prefer stoma (Greek = mouth; pi. stomata); in either case the adjective
is stomctal.

72 TEXTBOOK OF BOTANY

vertical cells, the palisade layers; and the lower have irregular cells
more or less parallel to the leaf surface forming the spongy layers. Among
these cells the vascular bundles — the veins — form a network extending
to every part of the leaf. The midrib and larger veins increase in thick-
ness and may become several times as thick as the blade. Each vascular
bundle consists of xylem and phloem tissues surrounded by one or more
cell layers of bundle sheath. In the larger veins and petioles of leaves
the xylem and phloem may be surrounded by heavy-walled fibrous
sclerenchyma,^ or the sclerenchyma may be massed above and below the
vascular tissues, as in many grasses. In the smaller veins the bundle
sheath consists of thin-walled cells. So completely do the veins penetrate
all parts of the mesophyll that any one mesophyll cell is rarely more than
a few cells away from a vein.

The xylem consists of elongated cells, and of tube-like vessels that
originate by a disintegration of the cross walls in strands of several or
many successive cells. The xylem of a single vein usually contains sev-
eral of these tube-like vessels — hence the name vascular bundle. The
movement of water to the mesophvll occurs mostly in these vessels.
The spiral thickening of vessels in the leaves of many plants, such as
agapanthus or geranium, is readily seen when segments of the blade
are pulled apart. Petioles of nelumbo are also excellent material for this
demonstration.

The phloem tissue is composed of elongated small cells which differ
from those of the xylem in that they retain their protoplasmic content at
maturity. Foods move from the mesophyll cells through the phloem
tissue to other parts of the plant.

The petiole. The petiole connecting the blade of the leaf with the
stem consists essentially of one or more vascular bundles and associated
libers embedded in parenchyma tissues. The bundles are continuations
of the vein system of the blade, and at the basal end connect with the

^ The names of several common plant tissues are formed from the Greek word enchyma
preceded by a descripti\'e prefix. Early plant anatomists held that all tissues are deri\'ed
from a "fundamental" soft tissue, or parenchyma, a general term still used for thin-walled
tissues such as occur in many edible fruits, pith, and the soft parts of lea\'es, stems, and
roots. The term sclerenchyma is applied to hard tissues, such as the thick-walled fibers
of wood, flax, and hemp; and to the rounded grit cells such as occur in pear fruits and in
the bark of many trees. When mature, sclerenchyma cells die, and only the heavy walls
persist. Chlorenchyma is any tissue containing chlorophyll, whether in leaf, stem, root, or
other part of the plant. The term coUenchyma is applied to soft tissue in which the cell
walls are thickened irregularly, especially at the angles. These cells retain their protoplasm
and may be rounded or elongated. They occur in thick \'eins of leaves, in petioles, and in
herbaceous stems. Mesophyll (middle leaf) consists of parenchyma; it may be properly
called chlorenchyma only when it is green.

[Chap. VIII THE TISSUE SYSTEM OF LEAVES 73

bundles of the stem. Near the junction of the petiole and stem there is
a short region in which the sclerenchyma is either less or absent, and
in which parenchyma cells rich in cytoplasm fomi a disk-shaped layer
several cells thick across the petiole, except in the bundles. This is the
absciss layer (Fig. 35). It is formed during leaf development and may
be readily distinguished in longitudinal sections of the petiole from the
cell layers above and below it. Under various conditions — such as

Fig. 35. Vertical sections of the bases of two petioles of coleus leaves: A, in
which the absciss layer is almost fully developed, and B, in which abscission is
nearly completed. Photomicrographs by R. M. Myers.

drought, injury to the blade, low and high temperatures — chemical
changes are induced in these cells. The middle lamella and sometimes
other layers of the cell walls become jelly-like or are dissolved, and the
petiole is separated from the stem. The lea\ es of deciduous trees may be
supported for a time by the vascular strands, but these are ultimately
broken. The breakdown of the absciss layer in the petiole of some
herbaceous plants may take place and the leaf abscise within 48 hours.
Abscission is usually preceded or followed by changes in the cells at-
tached to the stem below the absciss layer. These changes result in
the closure of the vessels and the development of scar tissue evident in
the leaf scar.

74

TEXTBOOK OF BOTANY

The stomates. In describing the epidemiis the paired guard cells
which surround the stomate were mentioned. These are highly spe-
cialized cells formed by subdivision of epidermal cells; unlike the or-
dinary epidermal cells, they contain green plastids (Fig. 36). In tree
leaves the guard cells do not separate until the leaf has attained a fourth
or a third of its size. In many leaves they are found exclusively, or
mostly, in tlie lower epidermis. Less frequently they are most abundant
on the upper surface; they rarely occur in equal numbers on both sides

Fig. 36. Stomates in relation to guard cells, subsidiary cells, and ordinary epidermal
cells. Lower epidermis of (A) Boston fern, (B) Zehrina, and (C) corn.

of the leaf. Stomatal openings are so minute that the area of an average
pinhole may be equivalent to that of 2000 to 2500 stomates. On many
common leaves, however, there are from 100 to 600 stomates per square
millimeter of leaf surface, and when they are open their total area may
be equal to nearly 1 per cent of the lower leaf surface.

The most remarkable thing about stomates is that they are opened by
the swelling and arching of the guard cells, and closed by the shrinking
and straightening of these cells. When a plant is moved from darkness
to light, the guard cells are affected by light and their internal pressure
increases, causing them to separate and open the stomate. On a warm
day this opening may take place in about 15 to 30 minutes. Thus in the
summertime stomates open after sunrise and remain open for two or
more hours, depending on conditions discussed in Chapter XXIII. The
closing of the stomates depends on a number of factors, mostly internal.

[Chap. VIII THE TISSUE SYSTEM OF LEAVES 75

Here it need be emphasized only that external factors — light, water, and
a warm temperature — are the most important prerequisites for their
opening.

Summary, A leaf is a more or less flattened organ that develops from
a leaf primordium, which originates in the meristematic tissue at the
growing point of a stem. The blade may be sessile, or it may be sup-
ported at some distance from the stem by a petiole. The latter is essen-
tially a structure containing vascular bundles surrounded by other
tissues. The blade is a flattened body covered by an epidermis which is
perforated by stomates that are opened and closed by guard cells.
Within the blade, the mesophyll consists of several layers of parenchyma
cells which are variously arranged and partlv in contact with the inter-
cellular air spaces that ramify throughout the leaf. The air spaces are
continuous with the outside atmosphere when the stomates are open.
Within and throughout the mesophyll there extends an ever-branching
vascular system of water-conducting and food-conducting tissues that
are in contact with most of the mesophvll cells, and never more than a
few cells removed. Some or all of the mesophvll cells contain chloroplasts
and chlorophyll. The larger veins of the leax^es are often ensheathed
with hard sclerenchvma fibers. These fibers increase the rigidity of the
blade; but as shown bv wilted leaves, they are not sufficient to hold the
blade upright. Owing to the formation of layers of cutin, external cell
walls of the epidermis may be more or less impervious to water.

The mesophyll is a mechanism of living cells connected with the stem
by water- and food-conducting tissues, distributed in a labyrinth of air
passages through which water \'apor, oxygen, and carbon dioxide can
move freely. The whole is enclosed bv a cutinized epidennis, in which
are stomates that may be open in warm sunlight, thus connecting the
internal with the external atmosphere.

CHAPTER IX
ENVIRONMENT AND LEAF DEVELOPMENT

Development of a leaf begins with the terminal growth of a primordium.
This terminal growth in a fern leaf may continue long after the base of
the leaf is mature. The tender, young coiled tip of a growing Boston
fern leaf is familiar to all. In the broad leaves of common trees and
shrubs all parts of the blade continue to grow and mature about the
same time, though growth continues longest in the base. In the leaves of
grasses, iris, aloe, and pine the basal region remains meristematic much

Fig. 37. Growing regions of leaf blades indicated by stippling. The fern leaf
(A) continuous growth longest at the apex, and grass leaves (B) longest at the
base. The growth of leaves similar to those of the sunflower (C) is much less
localized.

76

[Chap. IX ENVIRONMENT AND LEAF DEVELOPMENT 77

longer than the apical portion. Hence these leaves continue to elongate
from the base after the other portions are mature. When growing plants
of grasses and aloe are placed in a dark box the leaves continue elongat-
ing; their white or yellow basal portions indicate the growing regions.
In early spring one may often see the same thing in bluegrass lawns
before sunrise. When bluegrass is clipped the clipped leaves continue
their elongation. Cutting the tip of a fern leaf removes the growing
region, and growth of the leaf stops. Appreciation of these three types
of leaf development (Fig. 37) may aid in the interpretation of many
leaf phenomena.

During development leaves may be subjected to drought, to intense
light, shade, or darkness, to high and low temperatures, and to numer-

LOQi

B

1

— "^

y\^ V i J R a

yrjo^~irzr~r

M^WVP]

^^&

S

^^S

-^Tn

^^^^

^^S

a

^^^

C

■^'-^pS^^^^i.

Fig. 38. Cross sections of leaves of sugar maple from different environments:
A, section of a very young leaf with the usual three layers of mesophyll cells which
do not increase in number in some environments; B, section of a leaf from the
base of a tree in a forest; C, section of a leaf from the middle of the crown of an
isolated tree; D, section of a leaf from the side of the crown exposed to the sun.
Drawings redrawn from G. H. Smith (A) and from H. C. Hanson (B-D), American
Journal of Botany.

ous other external conditions. The effects of these environmental factors
on the growth processes within the cells may bring about an increase or
decrease in the division, enlargement, and differentiation of cells. Conse-
quently any change in the intensity of these factors during the growth
period of a leaf may result in distinct differences in form and structure.
Light and leaf size. Leaves that have developed in full sunlight are
usually smaller and thicker than the corresponding leaves that have
developed in partial shade. The lobes of leaves, such as those of oak

78 TEXTBOOK OF BOTANY

and maple, are also thicker as well as narrower and longer when the leaf
grows in bright light. The thickness of leaves in full sunlight may be
as much as 2 or 3 times that of similar leaves that developed in shade.
Studies of leaf development have shown that in moderate shade the
number of cell la vers of mesophvll is the same as in the embryonic leaf,
while in full sunlight the number of these layers may be increased ( Fig.
38). Moreover, there is increased elongation of cells at right angles to
the leaf surface; that is, more of the mesophyll cells become palisade
cells.

In dense shade the leaf blades of many plants mature at an early stage
of development and remain smaller and thinner. In total darkness such
blades develop little or not at all. Corn and other parallel-veined leaves
are exceptions in that the blades of seedlings may enlarge in darkness
even more than in light. Petioles generally lengthen more in shade and
darkness than in light. Internally, the vascular tissues of the petiole
become differentiated more in full sunlight than in darkness.

Light and epidermal cells. What evidence there is seems to indicate
that in anv one species the larger size of shade leaves is due more to the
greater enlargement of the cells than to the increase in number of cells
(Fig. 39). Likewise, the total number of stomates in these leaves may
not increase, since the proportion of epidermal cells from which guard
cells develop is about the same in both sun and shade.

The mean number of stomates per 100 original epideiTnal cells is as
follows : ^

European beech Shade leaves 10.7 Sun leaves 10.4

Black elderberry Shade leaves 15.5 Sun leaves 16.1

Myrtle blueberry Shade leaves 12.2 Sun leaves 14,5

There are more epidermal cells and more stomates per square milli-
meter of surface in sun leaves than in shade leaves when the plants grow
out-of-doors. Under these conditions the leaves are heated more than in
shade, and evaporation of water from them reduces the water content
of their cells. Since, in growth,* enlargement of cells depends in part on
water pressures inside them, sun leaves have smaller epidermal cells,

iData from E. J. Salisbury, Philos. Trans. Roy. Soc. London, 216:1-65, 1927. The
mean nmnber of epidermal cells and pairs of guard cells, respectively, per square mm.
in sun and in shade are given as follows:

European beech Shade leaves 1157 and 145 Sun leaves 1905 and HI

Black elderberry Shade leaves 268 and 47 Sun leaves 581 and 112

Blueberry Shade leaves 545 and 90 Sun leaves 1049 and 132

[Chap. IX ENVIRONMENT AND LEAF DEVELOPMENT 79

and more per unit area; likewise their guard cells are smaller, and the
pairs of guard cells per unit area are more numerous. When moisture

B/

SHADE FORM

SUN FORM

Fig. 39. Relation of growth of leaf tissues of wandering jew (Tradescantia) to
light. Leaves (A) on plants shaded by two layers of cheesecloth are broader and
thinner, and have shorter pahsade cells, larger epidermal cells, and fewer stomates
per square millimeter than leaves (B) on plants exposed to full sunlight. From
F. H. Norris.

conditions are controlled so that evaporation from the leaves is about
the same in sun and in shade, the numbers are approximately the same
for equal surface areas. The effects of intense light upon leaf develop-
ment are similar to those of drought.

80 TEXTBOOK OF BOTANY

Following is a comparison of leaf structures- in a cross section of
Rhododendron catawbiense from a spruce-fir forest ( shade ) and from a
heath-bald (sun) on Mt. LeConte in the southern Appalachians, eleva-
tion 6600 ft. Evaporation rate in the heath-bald w^as about 5 times that
in the forest, whereas transpiration was only about 3.5 times as great.

Shade Sun Ratio

Shade

Average leaf dimensions 13.5 X 5.9 cm. 6.9 X 3 cm. 0.25

Mean thickness of leaves 0.358 mm. 0.525 mm. 1.5

Mean thickness, upper cuti-
cle 6.0 microns 8.9 microns 1.5

Mean thickness, upper epi-
dermis 11.0 " 13.5 " 1.2

Mean number layers of pal-
isade 2.5 " 4.5 " 1.8

Mean thickness, palisade

layers 70.0 " 144.0 " 2.1

Mean thickness, spongy

parenchyma 253.0 " 321.0 " 1.6

Mean thickness, lower epi-
dermis 10.5 " 16.2 " 1.5

Mean thickness, lower cuti-
cle 2.9 " 6.7 " 2.3

The development of the finer veins of the leaf is similarly affected,
and the meshes ( "vein-islets" ) enclosed by the veins are smaller in sun-
light than in shade. The area of vein-islets probably is determined not
directly by sunlight, but indirectly by the increased evaporation and
the decreased expansion of the leaf cells, as are leaf size, thickness,
number of stomates, and epidermal cells per square millimeter. In all
these phenomena growth hormones may play an appreciable part.

Light and leaf position. Plants placed at a window are exposed to only
a fourth or a fifth as much light as plants out-of-doors; moreover, most
of the light strikes onh' one side of the plant. This one-sided illumina-
tion of the young leaves results in unequal growth of the petioles. The
cells on the shaded side lengthen more than those on the lighted side.
The greater elongation of the shaded side of the petiole is evident in its
bending toward the window. Apparently minute quantities of growth

- Calculated from sections near the midrib. From S. A. Cain and J. D. Oli\er Miller,
Amer. Midland Naturalist, 14:69-82, 1933.

[Chap. IX ENVIRONMENT AND LEAF DEVELOPMENT 81

hormones are formed in the blades in hght, and their unequal distribu-
tion in the petiole results in a greater lengthening of the cells on the
shaded side.

When an environmental factor is changed, all the tissues of the leaf
may not be afiFected equally in their development. The enlargement or
the differentiation of one tissue may be increased or decreased more
than that of others. If the growth of the
epidermis is checked sooner than that of
the mesophyll, the mesophyll cells will
be closer together and the air spaces
smaller. If the growth of mesophyll cells
is checked sooner than that of the epi-
dermis, the leaf will have larger air
spaces. Air-space differences usually re-
sult in either an increase or a decrease of
the area of mesophyll wall surface ex-
posed to the air spaces.

If the edge of a blade matures before
growth stops in the middle, stresses de-
velop within the leaf and it may become
convex or curled. The rigidity of manv
large leaves and the curving and twisting
of petioles may be due to such internal
stresses and strains between tissues.

The lengthening, bending, and twist-
ing of petioles among adjacent leaves on
inclined stems, and on vines growing on
walls, often result in an arrangement and
spacing of blades which when viewed
from the direction of the most intense
light appear as "leaf mosaics." Rosettes
of plantain and dandelion have similar
leaf arrangements.

The leaves of prickly lettuce and other
"compass plants"^ growing in dry, in-
tensely lighted habitats are twisted to a
more or less vertical position in a north-

^ Lactuca scariola, L. saligna, Silphium lacinia-
tum, and S. terebinthinaceum.

Fig. 40. When the growing
stem tip of the mermaid weed
is submerged, the leaves that
develop are finely divided;
when it is in air, the leaves that
develop are not divided.

82 TEXTBOOK OF BOTANY

south plane. In shade, these leaves have the usual oblique or horizontal
position.

Some plants tliat grow partly submerged have finely divided leaves
under water and nearly entire leaves above water. The mermaid weed
( Proserpinaca ) , for example, grows well both on moist soil and in water.
It is what may be called a highly plastic species since its leaf types are
readily changed. As long as the leaf primordia are under water the leaves
that develop are divided. If the leaf primordia are raised above the

WARM MOIST

COOL MOIST

Fig. 41. EflFects of temperatiue and drought on leaf size and structure of the
sour dock. In the warm dry culture the plants wilted daily. The temperature of the
substrate in the "cool moist" culture was on the average 10 degrees below that of
the "warm moist" culture.

water, undivided serrate leaves result (Fig. 40). Moreover, if the stem
tip is held alternately above and below the water, successive tiers of
divided and undivided leaves may develop on the same stem. The
epidermis of submerged leaves may have paired guard cells which do
not separate and form stomates. Internally the submerged leaves have
considerably less differentiation of cells and tissues than do the aerial
leaves.

In a preceding chapter attention was called to the chemical effects of
light and temperature on the greening of leaves, on autumn coloration,
and on abscission of leaves. Some of the effects of drought and tempera-
ture on the development of leaves are illustrated in Fig. 41. Drought
often increases the spinescence of plants (Fig. 42).

[Chap. IX ENVIRONMENT AND LEAF DEVELOPMENT

83

Fig. 42. The English gorse growing in moist places has leafy shoots; in dry situa-
tions the shoots are largely spines and thorns. After Lothelier.

Some of the variations observable in leaves that grow out-of-doors are
the results of changes in such factors as light and water, similar to those
discussed in this chapter. When individuals of the same species grow in
very dissimilar habitats, environmental effects on leaf forai often make
it difficult to identify the plants. Kevs to plant species are based on
heritable characteristics as developed in a single natural habitat, prefer-
ably a favorable one.

CHAPTER X
HEREDITARY DIFFERENCES IN LEAVES

All leaves develop from a small mound of meristematic cells near the
tip of a stem, and all these primordia look much alike. From the leaf
primordia of elm trees simple pinnately veined leaves develop, but from
the leaf primordia of the sugar maple five-lobed palmately veined leaves
develop. The leaves of horse chestnut and buckeye become palmately
compound, and those of the ash become pinnately compound. Since
these characteristics regularly appear in anv, or all, of the environments
in which the trees grow, it is evident that these variations in leaf devel-
opment, foiTn, and structure are the result of hereditary differences.

Heritable variations in leaves have been occurring throughout the
millions of years that leaf -bearing plants have existed upon the earth. If
one were to begin studying these variations by noting all of the minute
heritable differences in the form and size of teeth and lobes on leaf
margins, or in the form, number, and arrangement of epidermal hairs on
leaves, he would undoubtedlv conclude that heritable variations in leaves
are legion. He would probably conclude also that for every one of these
variations tliat may be of some advantage to the plant there are hundreds
of others that are of no particular value or hami to it. Regardless of their
value or lack of value to the plant, all of them are the result of the same
general changes in the composition and arrangement of molecules in the
hereditary units of matter (genes) in plant cells. "Rain falls alike upon
the just and the unjust" because the factors underlying the formation of
clouds and rain bear no relation to justice. Similarly, the factors underly-
ing the origin of heritable variations bear no relation to their value. Dis-
similar variations occur in leaves of plants growing in similar habitats
and, conversely, similar variations occur in leaves of plants growing in
dissimilar habitats. A few kinds of variations have survived more abun-
dantly in some habitats than in others. Many of the heritable variations
in leaves are interesting as phenomena of nature. Man makes use of
them also as a means of classifying plants, for various sorts of decora-
tion, and as a source of certain economic products. We shall not at this

84

[Chap. X

HEREDITARY DIFFERENCES IN LEAVES

85

time consider how heritable variations occur, but Hmit our attention to
a few types of them in leaves.

Size and form. The smallest flowering plant in North America is a
little globular duckweed ( Wolffia ) about 2 millimeters in diameter, with-
out distinct root, stem, or leaf (Fig. 11). The opposite extreme is repre-
sented by palms with leaves 20 feet long. The native fern from which
the Boston fern was derived grows in the Everglades region of southern
Florida and has been found with leaves over 20 feet in length. Pines have
needle leaves which in some species may be more than a foot long;
structurally they are quite different from the leaves thus far described
( Fig. 324 ) . Iris is readily recognized by its sword-like vertical blades and
the tightly folded sheath enclosing the stem and younger leaf bases.
The cactus (Opuntia) has small temporary succulent leaves (Fig. 43).
Victoria regia of the Amazon River, the largest of the water lilies, may
have floating blades 5 feet in diameter with petioles 2 inches thick and

Fig. 43. The cactus {Opuntia) in which the younger stem segments and fruits bear
small, awl-shaped, succulent leaves. Photo by G. S. Crowl.

86

TEXTBOOK OF BOTANY

Fig. 44. The largest of water lilies, Victoria regia, in bloom. Photo from New York

Botanical Garden.

20 feet long (Figs. 44 and 45). The century plant (Agave) has a type
of hard leaf with a heavy base and thickened blade, often variously
curved because of unequal growth (Fig. 46). At the time of flowering
the upper leaves of poinsettia are bright red. The leaf ( spathe ) that sub-
tends the flower cluster in the calla lily becomes white before the flowers
mature.

Parts of leaves are sometimes so modified that it is difficult to identify
them as such. The stipules of the black locust tree develop as a pair of
spines at the base of the leaf. The stipules of pea vine are leaf-like, and
the terminal three or five "leaflets" are tendrils. On smart weeds the
stipules form a sheath about the stem, whereas on rose leaves they are
attached to the side of the petiole. The various species of asparagus have
only very small scale leaves; the leaf-like organs are branches of the
stem. All gradations between leaves and spines may be found on the
common barberry. Near the stem tip bud scales develop from primordia
similar to those of foliage leaves.

[Chap. X

HEREDITARY DIFFERExNCES IN LEAVES

87

Fig. 45. Petiole and under side of leaf of Victoria regia. Photo from New York

Botanical Garden.

Structures. The illustrations in this chapter show some of the striking
differences in the structures of leaves. Simple one-celled hairs, or multi-
cellular branched, stellate, and shield-like hairs are characteristic of
certain leaves. The pine needle has peculiar chlorenchyma cells, and
resin ducts. In the aloe leaf the chlorenchvma is a peripheral layer en-
closing a mass of large, thin-walled gelatinous cells that have a high
water-retaining capacity. The leaves of many aquatics that extend or
develop above the water level are noted for the large proportion of air
cavities extending not only throughout the leaf but downward into the
stem and roots. The relative numbers of stomates and their occurrence
on the two sides of a leaf are characteristic of a given species ( Table 2 ) .
The number of stomates per square millimeter varies from place to place
on a particular leaf, and the average of several counts must be taken
when comparisons are made.

88

TEXTBOOK OF BOTANY

Fig. 46. Agave in bloom. From U. S. Biological Survey.

The pulvinus. At the base of the leaves and leaflets of certain plants,
such as beans, clovers, honey locust, and red-bud, there is a thickened
portion of the petiole, termed the ptilvimis. It is composed primarily of
parenchyma cells; the veins are more nearly central in it than they are
in the rest of the petiole ( Fig. 47 ) . Water passes into or out of the cells
more freely on one side of the pulvinus than on the other. This unequal
movement of water causes unequal enlargement or shrinkage on oppo-
site sides of the pulvinus, and a consequent movement of the attached
petiole and blade. A more detailed account of the movement of water
into and out of cells is included in Chapters XXII and XXIII.

In a pulvinus of the sensitive plant (Mimosa pudica) such changes
in water content may be brought about on a warm day by simply touch-
ing the leaf. The observed effects are a hfting of the leaflets, a gradual
closure of the branch "petioles," and a downward movement of the main

[Chap. X HEREDITARY

DIFFEF

lENCES IN LEAVES

89

Table 2. Average Number of Stomates
and Lower Surfaces of

per Square Millimeter on
a Variety of Leaves^

the Upper

Tree Leaves

Herb Leaves

American beech

(Fagus grandifolia)

Up.

Lo.

0
100

Giant ragweed
[Ambrosia trifida)

Up.

Lo.

100
200

Yellow oak

(Qiiercus muhlenbergia)

Up.
Lo.

0
330

Hemp

(Cannabis indica)

Up.
Lo.

18
230

Black walnut
(Jiiglans nigra)

Up.

Lo.

0
460

Prickly lettuce
(Lactuca scariola)

Up.

Lo.

85
125

Tulip tree

(Liriodendron)

Up.
Lo.

0
240

Cattail

(Typha latifolia)

Up.
Lo.

225
235

Cottonwood
(Populus deltoid es)

Up.

Lo.

.55

85

Jack-in-the-pulpit
(Arisaema triphyllum)

Up.

Lo.

0
32

Scarlet oak
(Quercus coccinca)

Up.
Lo.

0
1038

Willow herb
(Dianfliera americana)

Up.

Lo.

80
80

Shrub Leaves

Water lily
(Castalia odorata)

Up.
Lo.

460
0

Sassafras

{Sassafras vari i folium)

Up.
Lo.

0
35

Cow vetch
iVicia cracca)

Up.
Lo.

130
44

Grape
{Vilis bicolor)

Up.

Lo.

0

72

Com
(Zea mays)

Up.

Lo.

70

88

Cranberry

( Oxy coccus mac r oca rp us)

Up.
Lo.

0
632

Fescue grass
(Festuca sylvatica)

Up.

Lo.

200

0

Hydrangea
(Hydrangea arborescens)

Up.
Lo.

0
25

Bl uegrass
iPoa praiensis)

Up.
Lo.

160
104

Maple-leaved vi})urnum
{Viburnum acerifolium)

Up.
Lo.

20
30

Water plantain
{Alisma plantago)

Up.

Lo.

50
36

"petiole. " This eollapse of the whole leaf is in no way due to a mere re-
laxing of the leaf. The petiole is actually forced downward by the change
in size of the cells of the pulvinus. Many of these leaves have charac-
teristic positions with reference to the sun on bright days, and some
"follow the sun" from morning to e\'ening. These movements occur
readily with changes in light intensity provided the temperature is high;
if it is low, they are slow or do not occur at all.

' Data from papers by G. VV. Blaydes, E. J. Salisbury, and L. E. Yocum.

90 TEXTBOOK OF BOTANY

Fig. 47. Pulvinus and section of pulvinus from leaf of sensitive plant, both en-
larged. When the leaf is touched, the water in the cells on side A passes outward
into the intercellular spaces, causing the cells partially to collapse. The pressure
of the cells on side B then forces the leaf downward. Courtesy World Book Co.

Rolling of leaves. Leaves of many grasses, especially those that live in
dry regions, have chloroplasts in part of the mesophvll tissue only.
There are other parenchyma cells, called reservoir cells, in v^hich water
noticeably accumulates. When the plants are subjected to drought these
cells lose water and the leaves become folded, or rolled. Fig. 48 illus-
trates sections of leaves with reservoir cells. The inward rolling of
leaves of corn and beach grass, and the folding of bluegrass leaves are
common examples. These leaf movements have often been cited as ex-
amples of purposeful behavior, on the assumption that the leaves always
roll so that the stomates will be on the inside. In nature, however, one
may find leaves rolled in such a way that the stomates are on the out-
side and more exposed to light and dry winds than before the rolling.
Bluegrass leaves which fold upward in drought have more of the
stomates on the upper surface, whereas corn leaves which roll up have
a greater number of stomates on the lower, exposed surface.

Aquatics. The leaves of submerged plants are noted for their thinness.
They are usually ribbon-like as in pondweeds, or branched and dissected
as in the water milfoil, water crowfoot, and water marigold. The leaf
blades of many aquatic plants float on the surface of the water. The
most striking features of floating leaves are the large air cavities and the
restriction of stomates to the upper surface. However, paired guard cells
often occur on the lower surfaces, and when the leaves develop above

[Chap. X

HEREDITARY DIFFERENCES IN LEAVES

91

Fig. 48. Cross sections of leaves of bluegrass (A) and beach grass (B). The
mechanism which unfolds and folds these leaves may be seen as enlarged upper
epidermal cells at the base of each sinus.

water stomates are formed. Air cavities make up a large proportion of
the volume of the leaves of water hyacinth, arrowhead, and cattail.

Insectivorous leaves. Most unusual leaf forms occur in the sundews,
pitcher plants, and the Venus's-flytrap. These are usually grouped to-
gether as insectivorous plants because small insects may be caught among
the sticky glandular hairs of the sundew leaves, and on the snap-trap
blades of the flytrap. Insects large and small fall into the "pitchers"
( Fig. 49 ) . In several of these plants it has been shown that soluble sub-

92 TEXTBOOK OF BOTANY

stances formed by the decay of the insects pass into the leaf tissues and
may be used as food. These plants, however, grow just as well in the
absence of insects.

Fig. 49. The tall southern pitcher plant {Sarracenia flava), commonly called
trumpet leaf. Photo by G. W. Blaydes.

Chemical differences. Some plants are noted for their anthocyanin
pigments. The patterns in which these pigments are arranged in the
leaves of coleus exemplify the fact that although the cells of the leaf
primordia are all alike, the cells derived from them differentiate not only
structurally but chemically. Some of the pigments in these leaf color
patterns may be in the epidermis, while others are in the mesophyll in
one part of the leaf but absent in other parts. Experiments have shown
that the inheritance of certain factors determines both pigment forma-
tion and patterns. Cells of certain areas behave diflFerently from corre-
sponding cells in other leaf areas. Evidently the tissue systems of the
whole leaf react on the individual cells. The leaf, therefore, is not just a
mass of independent units.

Another example of hereditary differences in the chemical compounds
formed in leaves is furnished by two closely related plants, spearmint
and peppermint, each producing a characteristic aromatic oil used in

[Chap. X HEREDITARY DIFFERENCES IN LEAVES 93

flavoring confections and in medicine. Other chemicals characteristic
of certain leaves are the nicotine of tobacco, cocaine of coca, theine of
tea, and atropine of the deadly nightshade.

Economic uses of leaves. The use of leaves as food for grazing and
browsing animals is of great economic importance. In connection w^ith
grazing animals it should be remembered that insects probably remove
as much or more plant materials from a pasture than the larger animals.

The several products made from tobacco leaves are derived from dis-
tinct and selected varieties of the plant. The aroma, color, size, and tex-
ture of these leaves may be further modified both by the conditions under
which the plants grow, and by the different methods of curing. Each
variety attains its highest quality only under certain climatic conditions
and on certain soils. For instance, the Mammoth variety cultivated in
Connecticut, Maryland, and Florida has more leaves than others. The
Cuban variety which is extensively used for cigar wrappers is sometimes
shaded with cloth to increase leaf size and decrease leaf thickness.
Burley, a coarse-textured variety, develops best on particular soils in
Kentucky; it is much used in plug and smoking tobacco. Certain varie-
ties cultivated on the sandy soils of Virginia and the Carolinas have been
preferred for cigarettes.

Grass leaves, and fibers from the agave and from a Philippine banana,
are extensively used for the manufacture of twine, ropes, and rugs.

A complete list of vegetables used as food would include a number of
tender succulent leaves that fonn an important item of the human dietary
in all parts of the world. Leafy vegetables are rich in vitamins; this is
possibly their greatest value, aside from their contribution to flavors
and odors.

A large and valuable literature deals with the effects of environmental
factors on heritable qualities of economic plants. In this and the preced-
ing chapter only a few examples have been cited. State and federal de-
partments of agriculture have published many bulletins dealing with
varieties of crop plants; with the particular temperature, light, moisture,
and soil conditions most favorable for their development; and with the
regions of the United States where they grow most successfully.

The references below contain additional information on hereditary
variations in leaves, as well as in other plant organs, as indicated by
differences in the substances made by plants from their foods. Further
discussion of some of these plant products occurs in Chapter XIX.

94 TEXTBOOK OF BOTANY

REFERENCES

Coulter, J. M., C. R. Barnes, and H. C. Cowles. Textbook of Botany. Vol. 3,
Ecology. American Book Company. 1930. ( For additional illustrations. )

Good, Ronald. Plants and Human Economics. Cambridge Univ. Press. 1933.
Pp. 69-161.

Hill, A. F. Economic Botany. McGraw-Hill Book Company, Inc. 1937.

Medsger, O. P. Edible Wild Plants. The Macmillan Company. 1939.

Muenscher, W. C. Poisonous Plants of the United States. The Macmillan
Company. 1939.

CHAPTER XI
A BIT OF USEFUL CHEMISTRY

Many gross features and properties of plants may be perceived by means
of our unaided senses. With the help of a microscope, tissues and cells
can be distinguished, and with the highest powers of this instrument
many minute and important structures within the cells can be studied.
These visible units of a plant are composed of invisible units, which
may be investigated and mentally visualized by the methods devised by
chemists and physicists for studying the composition and transforma-
tions of all matter. To understand the visible structures of a plant and
the processes by which these structures are built and broken down, cer-
tain definite ideas about the invisible units of matter are essential. Con-
sequently, it may be helpful to consider briefly here a few invisible units
of matter and some of the usual chemical processes by which these units
may be combined or changed. These general ideas and principles will be
amplified and applied in subsequent discussions of phvsiological proc-
esses of plants.

Molecules and atoms. Like all other objects of our environment, plants
are composed of molecules of definite chemical composition. The proper-
ties of the microscopically visible parts of a plant — the protoplasmic
structures and cell walls — are determined in part by the kinds of mole-
cules of which they are composed, and in part by the arrangement and
organization of these molecules into aggregates. Each molecule of a par-
ticular compound such as water, sugar, or cellulose is made of still
smaller units of matter that are definite in kind and in structural ar-
rangement. The characteristic properties and reactions of molecules de-
pend upon the presence and arrangement of these smaller units, the
atoms and ions.

Water is a familiar compound, and in its simplest fomi its molecule

consists of 2 atoms of hydrogen and 1 atom of oxygen — briefly designated

+
as H2O. Other designations are H — O — H and H — OH; the first of

these indicates that both atoms of hydrogen are directly combined with

95

96 TEXTBOOK OF BOTANY

the atom of oxygen, and the second indicates the electrically charged
ions into which the water molecule may be separated. A vessel of water
always contains some free H+ and OH~ ions. Likewise, every molecule
is made up of atoms held together by electrical forces.

In stable molecules the positive and negative charges balance each
other and the molecule is neutral; hence it is less active chemically than
the free charged ions. Moreover, the electrical charges are important
factors in determining the structural arrangement of the atoms and ions
within a molecule, and also the stability and constancy of the molecules.
Ions may separate under certain conditions and recombine under others
without losing their identity.

Oxygen and hydrogen are gases with specific properties. As gases
their molecules are widely dispersed. Each molecule of oxygen and of
hydrogen is composed of two atoms, expressed as O2 and H2. In fact,
every chemical element (hydrogen, oxygen, nitrogen, carbon, etc.) is
characterized by a particular kind of atom.

In turn, the properties of atoms are deteiTnined by still smaller units
of which they are composed. An atom is visualized no longer as a minute
solid ball, but as a system of electrons, neutrons, and protons in which
the negatively charged electrons are arranged about a central body com-
posed mostly of neutrons and the positively charged protons. The ar-
rangement and movement of these particles within the atom have been
likened to the solar system, in which several planetary bodies (the
planets) revolve about a central body, the sun. In an atomic system,
however, the planetary electrons may not all lie in a single plane and
their orbits of motion may not encircle the central body. Since the posi-
tive charges on the protons of an atom are balanced by the negative
charges on its electrons, an atom is electrically neutral.^

Some chemical reactions are dependent upon the movement of an
electron from one atomic system to another atomic svstem. Since the
electron is negatively charged, the atom from which it departs becomes
a positively charged ion and the atom to which it becomes attached be-
comes a negatively charged ion, both of which are chemically active and
readily unite with other oppositely charged ions. Many of the common
salts, such as Na+Cl~, are formed by the union of oppositely charged

^ For further information about the constitution of atoms see a modern textbook of
chemistry such as that by W. McPherson and W. E. Henderson, Course in General Chem-
istry, 4th ed., Ginn and Company, 1936, pp. 200-226; H. T. Briscoe, Introduction to
College Chemistry, Houghton Mifflin Company; W. H. Hatcher, Introduction to Chemi-
cal Science, John Wiley & Sons, Inc.

[Chap. XI A BIT OF USEFUL CHEMISTRY 97

ions. Chemical unions may also occur without a complete transfer of
electrons from one atomic system to another. When two atomic systems
approach each other, some of the outermost planetary electrons may be
mutually held (shared) by both systems. Electrostatic forces hold the
two atoms together as one molecule. Thousands of atoms may thus be
held together in the largest molecules of plants and animals.

This incessant activity of electrically charged units within the atom
makes its capacity for union with other atoms more easily understood.
All of this internal activity within a molecule, say of water, may well
cause us to wonder at its stability and uniformity of behavior. When
we further attempt to picture the complex molecules and unceasing
activities within every unit of a living cell, the regularity of development
and the stability of living organisms composed of millions of cells are
still more remarkable.

Our imagination is further taxed by fossil records which show that
certain species of both plants and animals are so stable that they have

Fig. .50. Navicula lijra, a diatom present in marine deposits formed 25 to 30
million years ago. This species still lives today on sea coasts throughout the world.
Photo by Spencer Lens Co.

remained apparently unchanged generation after generation through
millions of years (Fig. 50). The stability of the cell, of the whole or-
ganism, and of the species is in the last analysis dependent upon the
stability of physical-chemical units and upon the orderliness with which
cycles of processes are repeated untold numbers of times. Cellular ac-
tivity is far easier to comprehend than cellular stability; but this very
stability is evidence of the inherent orderliness of natural processes.

Chemical processes in plants. An astounding number of different sub-
stances is formed in plants. Yet the formations and transformations of
these substances involve only a few fundamental chemical processes.

98 TEXTBOOK OF BOTANY

An appreciation of four of these fundamental processes is an invaluable
aid to an understanding of many of the material and energy transforma-
tions in plants.

Oxidation and reduction. Originally the term oxidation referred to the
combining of oxvgen with other elements and the consequent release of
energy. For instance, when coal burns, free oxygen combines with the
carbon in the coal, carbon dioxide ( COi- ) is formed, and the chemically
bound energy in the coal is liberated as heat and light. The same process
occurs when wood or natural gas burns. The burning or oxidation of
marsh gas (methane) may be indicated by a simple equation:'

Methane , Free ^ Carbon ^Vater + ^^^^

(bound energy) "' oxygen dioxide energy

H

H-C-H + 2 O2 > 0=C=0 + 2 H-O-H + ^^l^^

H

Similar oxidations occur in living plants when the carbon of food sub-
stances combines with oxygen and the chemically bound energy in the
foods is liberated.

When sugar is made from CO2 and Hi-O in the green tissues of plants
the reverse of the burning process occurs. This reverse process is called
reduction. Some of the chemically bound oxygen in the CO 2 or H-O is
liberated, and free energy is chemically bound in the sugar that is
fonned. In this particular process it is the energy of sunlight that be-
comes chemically bound in the sugar.

This earlv concept of oxidation and reduction is adequate for many
of the problems we shall meet in general botany. When we recognize
that free oxygen is being chemically bound in the plant and chemically
bound energy is being liberated, we may infer that oxidation is occurring
in the plant. When the reverse occurs we may infer that reduction is
occurring.

For certain problems, however, we may want to think of these proc-
esses in a more fundamental way. When an atom is oxidized it may
lose one or more electrons to another atom; it thus gains in positive
charges or valence. The atom to which the electron becomes attached

O 111

(rains in negative charges or valence; i.e., it is reduced. By this concept,

2 In the equations in this book in which we wish to indicate energy transformations, the
name of the compound containing chemically bound energy will be followed by the words
"bound energy" in parentheses.

[Chap. XI A BIT OF USEFUL CHEMISTRY 99

oxidation and reduction are seen to be supplementary and simultaneous;
this fact is indicated by the term oxicJation-reduction.

This concept of oxidation and reduction is held to be true for many
cases, such as the oxidation of iron and other metals. In the oxidation of
methane, sugar, and similar carbon compounds, however, the electrons
do not actually pass from one atom to another but are mutually shared
by both atoms. Oxidation and reduction are involved in all physiological
processes in which energy is chemically bound or liberated.

Hydrolysis and condensation. Some seeds contain starch but no sugar.
When such seeds germinate, sugar appears in them and increases in
abundance while the starch gradually disappears. When an animal
eats a starchy food, the starch is converted to sugar in its alimentary
tract. We are accustomed to say that the starch has been digested to
sugar. The chemist refers to this process as hydrolysis because one mole-
cule of water is added to the starch molecule for every molecule of sugar
separated from it.

The reverse process also takes place in plant cells, sugar being changed
to starch. When several molecules of sugar are combined forming one
large molecule of starch, a molecule of water is separated from each
molecule of sugar added. Since several molecules are joined into one
larger molecule with a loss of H2O this process may be called condensa-
tion. Many similar changes in the fat and protein compounds in living
cells occur by hydrolysis and condensation. These changes do not in-
volve a transfer of electric charges and little or no energy is gained
or lost.

Attention is called to these two contrasting pairs of fundamental chem-
ical processes — oxidation versus reduction, and hydrolysis versus con-
densation— so that the discussion of food substances, their uses and
transformations may be understood more readily.

In addition to these four chemical processes, there is another in which
ions that have the same charge mav replace each other in certain chemi-
cal compounds. For example, in a solution Na+ and K+ may replace
each other. Likewise OH~ and NH7 may replace each other in certain
organic compounds in living cells. This is called chemical substitution.

The number of atoms in a molecule may be small, as in water ( H-O ) ,
or it may be very large, as in proteins where each molecule is composed
of more than a thousand atoms. Casein, one of the common proteins of
milk, has the formula C708Hn3o0224Ni8oS4P4. In nature, outside of living
cells, such complicated molecules are rarely formed. One of the reasons

100 TEXTBOOK OF BOTANY

why discovery of the chemistry of Hving organisms has progressed so
slowly is the presence of such large complex molecules. Another reason
is that within the colloidal system of protoplasm with its surface energy
and electrical energy there are many chemical transformations that are
dijBficult or impossible to duplicate in the laboratory. Furthermore, growth
hormones and other substances may affect life processes in a most strik-
ing way when present in amounts too small to be detected by the or-
dinary methods of chemical analysis. Owing to the complexity of the
problems of biochemistry, progress is slow; but students of this phase of
biology may well be proud of the advances made during the last half-
century.

Elements found in plants. About half of the known chemical elements
have been found in plants through chemical analyses of a great variety
of specimens. But this fact is of little significance. The roots of a plant
are in contact with the water in the soil, and any soluble substance in
the soil water is likely to pass into the water within the plant and thus
be reported in a chemical analysis of the plant. On the other hand cer-
tain of these elements are known to be in the compounds of which the
plant is made. First in abundance are carbon, hydrogen, and oxygen
which occur in most plant compounds. Nitrogen and sulfur occur in all
known plant proteins. Phosphorus occurs in some proteins and in various
lipoids (phosphatides). IMagnesium is one of the elements in the
chlorophyll molecule. Calcium forms salts with the acid substances, such
as pectic acid in the cell walls of plants. Any other metal may similarly
form salts with acid substances in plants, but its presence may not be
essential to the plant. Finally there are a few elements — namely, po-
tassium, iron, manganese, boron, and sometimes copper and zinc — which
are essential to the development of plants; but their definite relations to
plant processes are still inadequately known. Some of them are toxic to
the plant except when present in very minute amounts.

A list of the elements essential to the development of plants tells us
little about the chemical composition of plants. Thev do not occur in
plant tissues as free elements, but owe their importance to the part they
play in the formation of compounds and the processes they aflFect. With
the exception of oxygen they are not absorbed and used by green plants
as elements. Thev enter the plant either in the form of compounds or
as dissociated ions of compounds. A part of the oxygen used by plants
also enters the plant in the form of the compounds HiO and CO2.

CHAPTER XII
THE FOOD OF PLANTS

Three frequently used words will help us arrive at an understanding of
the nature of food. Thev are: starvation, desiccation, and suffocation.
Suffocation implies a lack of oxygen; desiccation, a deficient water con-
tent of the body; and starvation, a lack of food. More recently "mineral
deficiency" has become a common expression among physicians to denote
an inadequate supply of certain mineral salt ions. Botanists have found
these four expressions of bodily needs — mineral deficiency, suffocation,
desiccation, and starvation — equally pertinent in distinguishing the salt,
oxygen, water, and food relations of plants.

Sources of foods. The fact that cattle, dogs, birds, insects, and other
animals secure food either by eating plants, by eating animals that feed
on plants, or by eating commercial products obtained from plants, needs
no further evidence. Equally commonplace is the admonition that to
prevent the decay of food, the infection of wounds, and the spread of
certain diseases, it is necessarv to avoid, or to destroy by antiseptics,
certain non-green plants known as bacteria and fungi. Non-green plants,
like animals, obtain food from other plants or from animals, or from
plant and animal products. The green plants therefore appear to be
unique among organisms in not securing their food from other organisms
or from the products of other organisms. Evidently animals and non-
green plants depend upon an external source of food; green plants do not.

Aristotle's notions about plant foods. Twenty -three hundred years ago
the philosopher Aristotle, speculating on the ways of nature, concluded
that green plants obtain food from the soil. Reasoning by analogy from
his observations of animals, he concluded that the source of food of any
organism lies outside its own body. Since many green plants have parts
of their bodies in the soil, he thought that they must receive food from
that source. He had no basis for reaching a better conclusion regarding
the food of green plants because ver)' little was known about their
physiology. The chemistry of the inorganic world also was largely a
mystery at that time. Only a few of the metallic elements had been iden-
tified; and more than 20 centuries passed before oxygen, hydrogen,

101

102 TEXTBOOK OF BOTANY

nitrogen, carbon, and their simplest compounds were recognized and
their nature was understood. Another century had nearly passed before
the food of green plants was discovered.

Aristotle was further impressed by the absence of excretory organs in
plants, but concluded that the soil in some way acted as the stomach of
the plant and supplied it with only perfect food, from which no unused
products would accumulate and have to be eliminated. For centuries
these two notions of Aristotle prevailed and even now they have not
entirely disappeared.

The food of green plants not in the soil. During the latter half of the
last century students of plant physiology not only discarded the idea
that green plants get their food from the soil, but they discovered how
and where this food is made. In spite of all these well-publicized discov-
eries, the mistaken ideas of Aristotle are still current in conversation,
advertising propaganda, public addresses, and modern literature three-
fourths of a century after they were adequately disproved.

Even before the numerous discoveries of the 19th century, van Hel-
mont (1577-1644), a Flemish physician and chemist, showed by a sim-
ple experiment that Aristotle's ideas about the food of green plants were
erroneous. Van Helmont placed a willow branch in a tub of soil to
which thereafter only rain water was added for a period of five years.
At the conclusion of the experiment the willow branch had become a
small tree and had gained 2627 ounces in weight, not including the
leaves that had fallen each year. The weight of the soil in the tub de-
creased but two ounces.

In both animals and plants it is the protoplasm for which food is
necessary. We have already seen that the protoplasm of all organisms
is similar in its gross chemical composition. From these two facts alone
it appears that the food of green plants must be similar to that of ani-
mals regardless of how it is obtained.

A part of man's food is derived from seeds, bulbs, tubers, and roots.
If these same plant organs are placed in a vessel containing only pure
quartz sand and a little water, small plants presently grow from them.
The same results are often attained by merely placing the seeds in moist
air. Small plants will grow from detached bryophyllum leaves suspended
near a window.

During the growth of these young plants millions of new cells are
made, each composed of protoplasm and cell wall substances. The proc-
esses involved in forming new masses of cells require chemical energy
in addition to foods. One cannot avoid the conclusion that seeds, bulbs.

THE FOOD OF PLANTS

103

[Chap. XH

leaves, tubers, and roots already contain substances from which new
protoplasm and new cell walls can be made, and from which chemical
energy can be derived. What are these substances?

Foods in cells. Thin sections of seeds or tubers observed through a
microscope are seen to be composed of numerous cell walls surrounding
rather dense masses of granules and oil droplets ( Fig. 51 ) . Similar

C D

Fig. 51. Illustrations of various accumulated foods in plant cells: A, protein and
starch in cells of a wheat grain; B, oil droplets in a cell from a coconut seed;
C, small starch grains in chloroplasts; D, crystals of inulin in cells of salsify root;
E, starch grains and cubical crystals of protein in cells of a potato tuber.

granules and oil droplets may be found in most living plant cells, but
usually in much smaller quantities. The oil droplets may be distinguished
by their appearance or by their bright red color in the presence of dilute
solutions of certain dyes, such as Sudan III. The presence of fat-like sub-
stances in plant parts may also be detected by crushing a small piece of
the tissue on paper and warming it. The fat-like substance melts and
produces a translucent oil spot on the paper.

The remaining granules in the cells may be starch grains, protein
granules, or granules and crystals composed of certain organic com-
pounds combined with inorganic elements such as calcium and phos-
phorus. Organic substances when combined with phosphorus are fre-
quently referred to as phosphatides. Starch grains may be detected b\
applying a drop of a dilute solution of iodine and potassium iodide to a
section of the plant. When treated with this reagent, starch grains may
be recognized by their various shades of blue and purple; the other
substances in the cell remain colorless or are stained light brown.

104 TEXTBOOK OF BOTANY

Protein granules may be distinguished from fat droplets and starch
grains by a number of different stains; but perhaps a better concept of
proteins may be obtained by kneading a small amount of wheat flour
in water until all the starch grains have been washed from the dough.
Sugars also are present in plant cells, but they are usually in solution in
the cell and are invisible. Their presence may be detected by tasting
the plant, or by placing a section of the plant organ in a drop of 20 per
cent sodium hydroxide in which a few crystals of copper tartrate have
dissolved. If a reddish-yellow precipitate of cuprous oxide is obtained,
the presence of sugar is indicated. The reaction may be hastened by
heat (40° C). To detect cane sugar by this method the plant section
should first be treated for several minutes with the enzyme invertase or
with a dilute acid, for instance 5 per cent citric acid.

The cell walls are made principally of cellulose and pectic compounds,
though some of them may also contain a substance (lignin) character-
istic of wood, or a fat-like substance ( suberin ) characteristic of cork.

All the substances mentioned above have been extracted from plant
cells bv chemical methods, and most of them may be obtained on the
market in a relatively pure state. Thus one may readily buy several kinds
of sugar ( especially sucrose, glucose, and fructose ) , starch, several kinds
of plant oils, pectic compounds (Certo), and cellulose (filter paper);
and then apply to them the tests that were proposed above for detecting
the same substances in plant cells. Some of these substances we readily
recognize as constituents of our daily diet.

Sugars. Each of the sugars mentioned above mav be sold under any
one of three names. Glucose (CoHi^Oo), also known as grape sugar and
dextrose, is not very sweet, but it is one of the principal sugars in plants
and also in the blood stream of animals — in fact, it is sometimes called
"blood sugar." Fructose (CuHi^Ot;), also called fruit sugar and levulose,
is one of the sweetest of sugars and is common in plants.^ Sucrose

^ The apparent anomaly of two sugars ha\'ing identical formulas (CaH,.Oe), but differ-
ing in chemical properties, may be explained by the differences in the arrangement of the
atoms. Following are diagrams showing these differences in glucose and fructose molecules.

H H

H H H () H H H H H O H

I I I I I I I I I I

H— C— C — ( '— ( '— ( — C=<) H— C— C— ("— C— C— C— H

O 0 OHO O () O H O O

H II II II H II II H

Glucose Fructose

These diagrams of sugar molecules are introduced here merely to help visualize differences
in the arrangement of atoms in the simple sugars, not to memorize.

[Chap. XII THE FOOD OF PLANTS 105

(C1-H22O11), otherwise known as cane sugar and saccharose, is the com-
mon household sugar. It is present in most plants and is especially abun-
dant in sorghum, sugar cane, sugar beet, and sugar maple. Each molecule
of sucrose is composed of a molecule of glucose chemically bound with
a molecule of fructose minus one molecule of water. When treated with
dilute acids, it is hydrolyzed to these two simple sugars.

Classification of foods. For convenience we may think of all these
substances as belonging to a few large groups of chemical compounds:
the carbohydrates, fats, and proteins. In a detailed consideration of foods
this classification is incomplete unless numerous compounds that may be
derived from carbohydrates, fats, and proteins by chemical alteration
are assumed to be included. In any brief discussion of foods this assump-
tion is usually made. The chemical alterations of the carbohydrates, fats,
and proteins are brought about by such processes as partial oxidation,
reduction, and hydrolysis. These processes result in the fomiation of
alcohols, organic acids, organic bases, and other compounds which may
still be usable by organisms as food. The total number of such derived
compounds is undoubtedly very large, but aside from those formed in
digestion and fermentation their relative bulk is exceedingly small. Since
many of these derived compounds are acid or basic, they may form
numerous chemical combinations with each other and with the mineral
ions that pass from the soil into the plant.

Carbohydrates, The carbohydrates are composed of carbon, hydrogen,
and oxygen. The hydrogen and oxygen occur in the proportion of 2 to 1
as in water (H2O) and glucose (CoHi20fi). To this group belong all the
sugars, starches, inulin, and cellulose. The pectic compounds are closely
related to the carbohydrates. A molecule of pectic acid, for instance, is
composed of molecules of certain sugars chemically combined with acids
derived from sugars by partial oxidation. A molecule of starch, or of
cellulose, is composed of many molecules of glucose in chemical com-
bination, and a molecule of inulin is composed of many molecules of
fructose similarly combined.^ When acted upon by certain enzymes or
acids these complex carbohydrates are hydrolyzed to the simple sugars
out of which they were made. These complex carbohydrates are usually
inert, insoluble compounds of no food value to the organism unless they
are digested to the simple sugars which are soluble and chemically
active.

- For a more detailed statement of the formation of complex carbohydrates, see p. 135,
Chapter XV.

106 TEXTBOOK OF BOTANY

Fats, Fats and oils are likewise composed only of carbon, hydrogen,
and oxygen, but thev have comparatively little oxygen in proportion to
carbon and hydrogen. There is no general chemical distinction betw^een
fats and oils. Thev are distinguished by their melting points. At ordinary
room temperatures oils are liquids, while fats are solids. During digestion
fats and oils are hydrolvzed to glycerin and fatty acids.

Proteins. The proteins are composed of carbon, hydrogen, oxygen,
nitrogen, sulfur, and sometimes phosphorus also. During digestion the
large complex molecules of proteins are hydrolyzed to simpler sub-
stances known as amino acids.

When we consider all the substances that we have seen in the cells of
seeds and tubers, we readily recognize some of them as foods of man
and other animals. Four questions may now be considered. Which of
these substances are used as food in the animal body? When we buy a
pound of potatoes do we buy a pound of food? Is the food of green plants
and of animals identical? How does the green plant obtain food?

Human foods. For our own bodies we recognize as foods those com-
pounds which can be transformed into the substances of which cells are
composed and from which our body obtains energy by oxidation. These
compounds, we have already seen, are carbohydrates, fats, and proteins,
together with their partially oxidized, reduced, or hydrolyzed deriva-
tives which may or may not be combined with certain ions of the mineral
salts. But as foods these compounds are subject to one more limitation:
they must be either in a soluble state, or capable of being digested by
appropriate enzymes in the alimentary tract. The sugars and some of
the derived compounds are already dissolved in water and may pass
from the alimentary canal into the blood stream. Our bodies produce
enzymes that digest starches, fats, and proteins, but not enzymes that
digest cellulose, pectic compounds, wood, cork, and inulin. If we were
entirely dependent upon our own enzvmes even a pound of dried pota-
toes would not be a pound of food.

The non-green plants ( bacteria and fungi ) also produce enzymes that
digest starch, fats, and proteins; and a few kinds of non-green plants pro-
duce enzymes that digest cellulose, pectic compounds, inulin, and wood.
Thus the non-green plants as a group may utilize a wider range of
compounds as food than is used by animals. It is interesting to note that
some of the digestion that occurs in the alimentary tract of man and
other animals is due to the non-green plants that live there.

The food of green plants. The food of the green plant may now be

[Chap. XII THE FOOD OF PLANTS 107

considered. When a seed or a tuber is placed in a moist chamber and a
young plant grows from it, what substances in the seed or tuber dis-
appear as the cells of the new plant are made? They are exactly the
same kinds of carbohydrates, fats, proteins, and derived compounds that
the animal uses as food. Parts of the tuber, especially the cell walls,
remain but may be digested and used as food by certain bacteria and
fungi. From such facts as these, together with other facts that will be
discussed in later chapters, botanists regard the food of green plants to
be identical with that of animals.

The process by which the green plant obtains food, however, is unique.
The food is not obtained from the soil, from the air, from water, or
from other organisms. It is made within the plant from simple inorganic
compounds obtained from the soil, air, and water. In the next few chap-
ters we shall consider material and energy transformations involved in
the food-making processes of green plants, the influence of the environ-
ment upon these processes, the consequent behavior of the plant in cer-
tain environments, and the vital importance of these transformations to
the whole biological world and to industry.

Summary. In the foregoing discussion we have considered two very
different concepts of the food of living organisms, with special reference
to green plants. The older view proposed by Aristotle is based upon the
assumption that food is something that enters the organism from with-
out. A more recent concept of food is based upon a knowledge of the
physiology of organisms, that is, the ways in which substances are used
within the organism regardless of whether they are obtained from the
outer world or are made within the body. It limits food to such sub-
stances as carbohydrates, fats, proteins, and their derivatives which mav
be used in cell construction and as a source of energy within the
organism.

Such a concept of food emphasizes the fundamental similarity in the
nutritive processes of the protoplasm in all plants and animals. The older
concept denies this fundamental similarity in organisms, and excludes
all the modern concepts of the food of green plants.

CHAPTER XIII
FOOD MANUFACTURE

I. THE SYNTHESIS OF SUGAR— PHOTOSYNTHESIS

Some of the significant facts that finally enabled botanists to arrive at
an understanding of the food of green plants, and a realization that this
food is actually made within the plant were briefly mentioned in the
preceding chapter. Much time and exact experimentation were required
to discover the simpler material and energy transformations that occur in
food manufacture, and the various conditions both inside and outside
the plant that affect these transformations. It is now common knowledge
among botanists that sugar is the first kind of food made by green plants,
. and that all other kinds of food are made by chemical alterations of this
sugar. Furthemiore, the facts discussed in the next few chapters will
help us see that the material make-up of all living organisms is de-
pendent upon chemical derivatives of the sugar made by plants; that the
potential energy in sugar is the primarv source of the chemically bound
energy supply of all organisms; and that most of the energy that man
transfonns by various means into heat, light, electricity, and mechanical
energy may be traced back through various transformations to the
potential energv of sugar. In this chapter we are concerned primarily
with the making of sugar in green plants.

Carbon dioxide and oxygen in relation to green plants. Toward the
close of the 18th century oxygen and carbon dioxide were clearly recog-
nized and named bv the French chemist, Lavoisier. Following this ad-
vance in chemistry, a number of carefully conducted experiments by
different investigators finally led to the discovery of certain fundamental
relations between these two gases and living organisms. When the green
parts of plants were exposed to light, the amount of oxygen in the sur-
rounding air increased and the amount of carbon dioxide decreased.
During the hours of darkness the converse occurred. When the roots or
other non -green parts of a plant were substituted for the green tissues
in the experiments, the amount of oxygen in the surrounding air de-

108

[Chap. XIII SYNTHESIS OF SUGAR— PHOTOSYNTHESIS 109

creased and the amount of carbon dioxide increased, both in hght and
in darkness. The increase in oxygen and decrease in carbon dioxide in
the surrounding air occurred only when some green tissue of the plant
was exposed to light. When animals and non-green plants were used in
these experiments, the results were like those obtained for roots and
other non-green parts of the plant. How was one to account for this
unique effect of the green tissues of plants?

The phenomenon was not satisfactorily explained until after the
middle of the 19th century. So little was known about the substances
and processes within plants and about the factors in the environment of
plants, that numerous facts had to be discovered bv experimentation
before any of the various hypotheses proposed could be eliminated or
converted into a scientificallv established conclusion. The facts finallv
established mav be represented brieflv bv the following equation:^

Light energy + Water + Carbon dioxide > Sugar (bound energy) + Oxygen

Light energy + 6 H2O + 6 CO2 > CgHizOe + 6 O2

This equation indicates the more obvious material and energy trans-
formations that occur in the making of sugar. It represents the initial
substances used and the final products formed, but it does not indicate
the intermediate reactions that occur. Free energy of light is transformed
to potential energy in the sugar. For every 180 grams of sugar made, 674
Calories of light energy are transformed to chemicallv bound energy.
One gram of sugar contains about 3.75 Calories of chemically bound
energy.

Photosynthesis. This process of making sugar in the green parts of
plants is called photosynthesis (Greek, photos, light; and synthesis, put-
ting together), because light is necessary for this building of large mole-
cules (CiiHii'Oti) through the chemical union of smaller ones (COj
and H2O).-

^ One reservation to this statement should be made. The equation indicates that a
hexose sugar (the molecule being composed of 6 atoms of carbon, 12 atoms of hydrogen,
and 6 atoms of oxygen) is formed. This is generally considered to be glucose, though the
known facts are not sufficient to exclude fructose entirely, at least in some cases. These
two sugars are the only simple sugars found widely distributed in detectable amounts as
free sugars in plants. Either of them may be formed from the other one in plants. See
the structural formulas in the footnote on p. 104. Some in\ estigators ha\ e suggested sucrose
(C12H00O11) as the first sugar made, but the evidence is not con\incing.

^ The older term, carbon assimilation, is still used by some writers; but, as we shall see
later, the term assimilation is also used to designate the conversion of food into the
substances of which cells are composed, a process that occurs in all plants and animals,
whereas the process we are describing here occurs only in the green tissues of plants.
Furthermore, the term carbon assimilation is a heritage of the early misconceptions of plant

110 TEXTBOOK OF BOTANY

Chemical reactions that are brought about bv hght are often referred
to as photochemical reactions — reactions initiated by the collision of
photons (units of radiant energy) with some part of a molecule. There
are, of course, several kinds of photochemical reactions; but by tradition
in botany the term photosynthesis usually refers to the making of sugar
from COi- and Hi>0 in the chlorophyll-containing cells of plants.

Through the chemical union of 6 molecules of water with 6 molecules
of carbon dioxide 1 molecule of sugar is formed, and the excess 6 mole-
cules of free oxygen are set free. For every molecule of CO2 used, one
molecule of O- is released. From the principle first formulated bv
Avogadro — that equal volumes of gases under the same conditions of
temperature and pressure contain the same number of molecules — we
may infer that the volume of CO- consumed in photosynthesis is equal
to the volume of O2 liberated. This inference has been verified bv experi-
mentation. The comparative weights of the materials used and formed
during photosynthesis may also be indicated:

264 gm. CO2 -\- 108 gm. H2O > 180 gm. CeHiaOe + 192 gm. O2

Obviously if one knows the amount of COj used in photosynthesis, he
can compute the amount of sugar made and the amount of oxygen set
free. Similarly, if he knows the amount of oxygen set free or the amount
of sugar made, he can calculate the amount of any one of the other sub-
stances involved in the process. How can one calculate the amount of
energy that is chemically bound during photosynthesis?

It should be noted that tlie material and energy transformations that
occur in the making of sugar are the con\'erse of those that occur in the
burning of sugar. If we regard the burning of sugar as an oxidation
process, we may regard photosynthesis as a reduction process. It is the
primary energy-storing process of the organic world.

Oxidation processes are often regarded as exothermic ( heat-releasing ) ,
and reduction processes as endothermic ( heat-storing ) . These terms are
quite appropriate when transformations of heat energy only are involved.
They are quite confusing, however, when these processes of living cells
result in transformations to other forms of energy, as, for instance,
radiant energy to chemically bound energy, chemically bound energy to

nutrition. In order to avoid confusion of terms we shall refer to the above process as photo-
synthesis. The term carbon fixation is of course entirely ruled out, because carbon is
already "fixed" in CO„ before the process begins. If we must substitute another term for
photosynthesis in botany, it should be "sugar syntliesis."

[Chap. Xlll SYNTHESIS OF SUGAR— PHOTOSYNTHESIS 111

light, or to electrical energy. The first is exemplified by photosynthesis,
the second by the production of light by fireflies and luminous bacteria,
and the third by the electric discharge from an electric eel.

Before photosynthesis can occur in an illuminated green cell the raw
materials, CO- and H-O, must be available. Carbon dioxide is very solu-
ble in water. Hence, all the green cells of the simpler algae that live
submerged in water, and e\'en those in the leaves of the common water
weed (Elociea), are in direct contact with both water and CO2. Both of
these substances pass readily into the cells and then to the chlorophyll
in the chloroplasts.

In land plants the water moves up from the soil through the roots,
stems, and petioles into the \'einlets of the leaf ( Fig. 52 ) , and then passes

.^^

^^icii.'iiY"'-^^

Fig. 52. The vein system of a skeletonized sassafras leaf. The leaf was kept in
water until bacteria had digested the epidermis and mesophyll.

from cell to cell throughout the leaf. No appreciable amount of carbon
dioxide gets to the leaves from the soil by way of the roots. It enters the
leaf from the surrounding atmosphere above the soil. Some of it may
pass directly through the epidermal cells to the chlorenchyma beneath.
Most of it passes through the stomates into the intercellular spaces of the
mesophyll, where it comes into contact with the wet walls of the
mesophyll cells that are exposed to the internal atmosphere of the leaf.

112 TEXTBOOK OF BOTANY

It dissolves in the water of the cell and finally gets to the chlorophyll in
the chloroplasts (Fig. 34).

Most of the oxygen liberated during photosynthesis passes from the
cells into the intercellular spaces of the chlorenchyma, then through the
stomates into the outer atmosphere. This passage of CO2 into, and of
O2 out of, the chlorenchyma also occurs when photosynthesis takes place
in green stems and fruits or in any other green part of the plant. The
sugar that is not locally consumed or transformed within the chloroplasts
passes from them to all other parts of the plant. The manner of its transfer
will be discussed in later chapters.

Factors involved. Had the earlier investigators known that the increase
of Oi- and the decrease of COt- in the air surrounding green plant organs
exposed to light were the result of the synthesis of sugar, they could have
used their data to show that chlorophyll is necessary for this process.
The variations in the amount of CO2 and O2 in the air surrounding
leaves or other green plant tissues have been used in numerous experi-
ments in recent years not only to establish the fact of the necessity of
chlorophyll for photosynthesis, but also to study the effects of several
environmental factors upon the rate of photosynthesis.

From the facts discussed thus far, one would expect sugar to be made
in plants only when the following are present: chlorophyll-containing
cells, water, carbon dioxide, light, and a suitable temperature. If any
one of these factors is absent, no sugar will be made, and the manufac-
ture of other foods from sugar cannot long continue. If the plant is con-
tinuously deprived of either light or carbon dioxide it will starve to death
when all the food that has accumulated in it has been used. If deprived
of water, it will die of desiccation before the accumulated food is con-
sumed. The drying of freshly cut hay and fodder, which are later used
as a source of food for animals, is a good illustration of the death of plant
organs by desiccation before the food within them is consumed by the
plant.

In experiments it is possible to expose a plant to a temperature in
which it will consume sugar faster than it makes it. Plants in the field
are frequently exposed to such temperatures for short periods of time.
Various investigators have concluded that this condition accounts for the
limit of distribution of particular plants in certain geographic areas. As
we shall see later, it greatly affects the amount of growth of certain plant
organs. The death of plants exposed to extreme temperatures is more

[Chap. XIII SYNTHESIS OF SUGAR— PHOTOSYNTHESIS 113

often due to other causes, among which may be mentioned the coagula-
tion of protoplasm.

Sugar manufacture by bacteria. One outstanding exception to the mak-
ing of sugar b}' photosynthesis may be mentioned here, but it will be
discussed in more detail in a later chapter. A few kinds of bacteria are
known to make sugar from carbon dioxide and water in the absence of
light. The energy necessary for this process is obtained by the oxidation
of iron, sulfur, or nitrogen that has preyiously been reduced. A few other
kinds of bacteria ( purple bacteria ) are unique in containing a pigment
which, like chlorophyll, is effectiye in photosynthesis. The accompan\'ing
purple pigment (C4sH(!.i0.i) is similar to the carotenoids described in
Chapter IV.

The amount of sugar made. The amount of sugar made by plants ^'aries
so greatly in different plants and under dissimilar conditions that it is
difficult to make a general estimate of it. The results of many experiments
show that under fa\'orable conditions a square yard of leaf surface makes
on the ayerage about 0.5 gram of sugar per hour. At this rate about four
months would be required for a square \'ard of leaf surface to make
sugar equiyalent to the food a man consumes in one day. In Chapter
XVm an acre of corn is compared with an acre of young apple trees
with respect to seyeral plant processes. This acre of corn has about two
acres of leaf surface. During the growing season it makes about 10 tons
of sugar, an ayerage of about 2 pounds per plant. The acre of young
apple trees (10,000 leaves per tree) makes about 8.7 tons of sugar, an
average of about 44 pounds per tree. On the basis of data from many
sources it is estimated that the plants of the United States make nearh-
three-fifths of a cubic mile of sugar each year. For the plants of the
world, 9 cubic miles of sugar annualh' seems to be a conseryative
estimate.

Now let us recall the fact that for every 180 grams (almost 0.4 lb.) of
sugar made, 674 Calories of light energy are transformed to available
potential energy in this sugar. The potential energy in the sugar made
annually in the United States alone is estimated to be about 14 X 10''
Calories. Yet large as this figure is, it probably is below the one that
would represent the energy liberated annually on the earth today by
plants, man, and all other living organisms. Through the burning of
coal, oil, and gas alone we liberate about 5 X lO^*''* Calories of energy
each year. The energy was transformed from radiant energ\' to potential
energy by the plants of the distant past. The facts upon which these

114 TEXTBOOK OF BOTANY

statements are based will become clearer as we proceed with the study
of other processes that occur in living organisms.

What becomes of the sugar made by plants? The answer to this ques-
tion will become more evident as we proceed with the next six chapters.
For the present we may be brief. Some of the sugar is transformed to
other types of foods; some is transfomied directly into substances of
which cells are composed, particularly the substances in cell walls; some
of it is oxidized in the plant; and some of it accumulates within the plant.

The accumulated sugar of course merely represents the part that has
not been used within the plant or converted to other kinds of food
within it. Animals and non-green plants get some of it. The amount of
sugar that accumulates depends partly upon the rate of photosynthesis
and partlv upon a number of other processes and conditions. Some plants,
such as sugar cane and the sugar beet, are noted for the large amounts
of sugar that accumulate in them in a suitable environment. Under good
growing conditions, sugar is usually present in varying amounts, as
glucose, fructose, and sucrose, in the cells of all green plants.

In plant cells these three sugars are intraconvertible; i.e., one sugar
may be formed from another by certain rearrangements and combina-
tions of atoms:

glucose =F=^ fructose
glucose + fructose :^=^ sucrose + H2O

The rearrangements of atoms involved in the transformation of glucose
to fructose and vice versa may be inferred from the structural formulas
represented in the footnote on page 104. The formation of glucose and
fructose from sucrose is merely a process of hydrolysis; the converse is a
process of condensation.

Summary. The primary food of all living organisms is sugar. It is the
basis of all other food syntheses and is made in green plants by a process
of photosynthesis. In this process sugar is made from carbon dioxide and
water in the chloroplasts of green plants exposed to the radiant energy,
light. An insignificant amount of sugar is also made photosynthetically
by purple bacteria that contain a chlorophyll-like pigment, and chemo-
synthetically in the dark by certain colorless bacteria that obtain energy
by oxidizing reduced iron, sulfur, and nitrogen. In every case the raw
materials are carbon dioxide and water, and the end products are sugar j
and free oxygen. The intermediate steps in the process are not fulh'
known. j

CHAPTER XIV

FOOD MANUFACTURE

<><>O<X><>O<><S><><>0<><><><X><><^^^

II. FACTORS INFLUENCING THE RATE OF PHOTOSYNTHESIS

The basic facts about photosynthesis discussed in the preceding chapter
do not exhaust the subject; neither do they satisfy anyone who wants to
understand the fundamental processes of his natural environment. The
answers to many questions about plants involve facts concerning the
chain of chemical reactions that occur in photosynthesis, and the influ-
ence of the various factors of the environment. The value of certain prac-
tices in the handling of plants and the interpretation of plant phenomena
in nature depend also upon a knowledge of these relations. Many ques-
tions concerning these relations cannot be answered todav, but addi-
tional facts that are helpful in answering some of them will now be
discussed.

Light the energy of photosynthesis. Under natural conditions the en-
ergy of photosynthesis is sunlight. Heat cannot be substituted for light
in this process. No sugar is made from CO- and H-O bv green plants
deprived of light, regardless of the temperature to which they are
exposed. Radiant energy that we perceive as light is referred to as the

VISIBLE SPECTRUM

ULTRA-VIOLET

3>0 430 470 600

560 eoo

Fig. 53. The spectrum of radiant energy. One millimicron (m^) = one-millionth

of a millimeter.

visible spectrum (Fig. 53). This is the radiant energy that is effective in
photosynthesis. It can initiate chemical change by displacing or activat-
ing the outermost electrons of atoms. The effects of radiant energy of
longer wave lengths, such as infra-red, are apparently limited to the
movement of atoms and molecules. Photosynthesis is not known to occur
when the plant is exposed only to infra-red radiation.

115

116

TEXTBOOK OF BOTANY

Within the visible spectrum the radiant energy that we call red, orange,
yellow, and blue light is the most effective in photosynthesis. The color
of leaves is evidence that relatively more of the radiant energy we see as
green is reflected by chlorophyll. In land plants most of the photosyn-
thesis is brought about by the radiant energ\' of the red end of the
spectrum, but the radiant energy that penetrates the water to the depth
of deep-sea algae is mainly the blue end. The effectiveness of the blue
and the red rays is about equal when their intensities are equal ( Fig. 54 ) .

4O0 ASO 500 5SO 600 650 700 TSO

WAVE LENGTHS IN MILLIMICRONS

Fig. 54. Relative rates of photosynthesis in different rays of Hght of equal intensity.
After B. S. Meyer and D. B. Anderson.

Apparently only the radiant energy that is absorbed (transformed)
by chlorophyll is directly effective in photosynthesis. Any artificial light
that emits these particular rays may induce photosynthesis. Some plants
may start from seeds, grow to maturity, and produce flowers, fruits, and
seeds when exposed only to electric light at an intensity about one-tenth
that of full sunlight. Other plants will grow in this light intensity, but
without producing flowers and fruits.

The lowest intensity of light at which photosynthesis will occur has
not been discovered. It has been reported to occur in deep-sea algae at
light intensities lower than that of full moonlight.

A much more important fact to know is the light intensity from da>'
to day in which the plant can survive on the sugar made. For conven-
ience of reference we may call this the critical light intensity. This critical
light intensity varies when other external factors, such as temperature,
available moisture, and the concentration of carbon dioxide, are altered.
This fact will be demonstrated later. Among different plants there is a

[Chap. XIV THE RATE OF PHOTOSYNTHESIS 117

wide range of critical light intensity. Many fail to grow in the shade of
buildings or of other plants — for instance, in certain forests or beneath
some trees and shrubs on lawns — because the light intensity in such
situations is below their critical light intensity. Many of the small trees
one sees in the undergrowth of forests are merely surviving with a min-
imum of growth each \'ear; most of them finally die while still small.

Plants that endure the most shade may be those in which maximum
photosynthesis is attained at low light intensities. Many more measure-
ments are needed to test this idea, but a few facts may be cited by way
of illustration. The maximum rate of photosynthesis in certain deep-sea
algae may occur at depths of 50 feet or more where the light intensity
is very low. When these algae are brought nearer the surface where the
light intensity is higher, the rate of photosynthesis is much lower.

Similar data were obtained by a study of one of the shield ferns that
crrows in the dense shade of forests. Photosynthesis increased in this
fern with increase of light up to an intensity equal to one-fifth that of
full sunlight, but when the intensitv exceeded two-fifths that of full sun-
light the rate of photosvnthesis decreased. Photosynthesis in another
extreme shade fern (Trichomones) increased with increase of light up to
one-eighth of full sunlight and then decreased to none at one-half full
sunlisht. In the wood sorrel (OxaUs acetoseUa) , a small herbaceous
plant that grows well in the shade of northern forests, maximum photo-
synthesis was attained when the light intensity was only one-tenth that
of full sunlight, and this rate was maintained at higher intensities.

Such plants as corn, wheat, apple, and spinach do not survive in dense
shade. In them photos\'nthesis is reported to increase with increasing
light intensity, in some cases up to full sunlight. When the measurements
are made on simple leaves, however, maximum photosynthesis is usually
found at intensities considerabl)' below that of full sunlight. When the
whole plant is tested the results are different, because many of its leaves
are shaded by others and do not get the full effects of increased light.
The greatest increase in photosvnthesis with increase of light occurs
when the original light intensitv is verv low. Measurements of photosyn-
thesis in relation to light and other external factors are now being made
bv a rapidlv increasing number of investigators. Many of the problems
being attacked are too complicated to be mentioned here.

The owners of commercial o;ieenhouses are most interested in the
light intensitv that results in the best yields of flowers, fruits, and vege-
tables in different species of plants. During the cloudy days of winter,

118 TEXTBOOK OF BOTANY

photosynthesis is often much too low for best results, for light intensity
in greenhouses may range as low as 1/100 of full sunlight and be below
the critical light intensity of many plants.

Many investigations designed to determine the value of supplement-
ing natural light with electric light are in progress. This problem is fur-
ther complicated — and also rendered much more interesting — by the
fact that the length of dav to which a plant is exposed greatlv alters its
vegetative and reproductive processes. These length-of-day effects may
be brought about by light intensities much below those necessarv for
adequate photosynthesis. During the winter, exposure to lig;ht of an in-
tensity somewhere between 1/100 and 1/100,000 that of full sunlight
for a few additional hours each day greatly alters the behavior of certain
plants. Lengthening the day by this means, however, is not a substitute
for low light intensitv during dark days. These relations are discussed in
more detail in Chapter XXXI.

Water and photosynthesis. The degree of saturation of the tissues of
a plant ma\' modif \' the rate of photosvnthesis in two wavs : through the
opening and closing of the stomates, and through its influence on the
rate of chemical processes in the chlorenchyma cells. The effects are
apparent in the daily periodicity of photosynthesis. In the morning
hours the plant tissues are nearlv saturated, the stomates are open, and
photosynthesis soon attains a maximum rate. Later in the day the rate
declines because of water loss from the mesophyll cells. Still later
photosynthesis is further reduced by the gradual closing of the stomates.
Of course, the temperature of the leaf is another factor which influences
the daily increase and decrease in sugar manufacture. When plants
begin to wilt, photosynthesis is sharplv decreased, first because the water
content of the cells is lower, and second through the closure of the
stomates.

Structures and photosynthesis. Plant structures are the result of cer-
tain plant processes. These processes in turn are conditioned by both
heredity and environment. After a structure is formed it may in turn
influence certain processes in the plant. We may therefore expect the
rate of photosynthesis to be different in different kinds of plants, partly
because their structure-building processes and resultant structures are
different.

Even a cursory observation of plants in one's surroundings is suffi-
cient to show that chlorenchvma is not restricted to leaves. Green stems,
green fruits, green parts of flowers, and even certain green seeds (peas

[Chap. XIV THE RATE OF PHOTOSYNTHESIS 119

and lima beans) are objects of common obsei-vation. Many tropical
orchids and bromeliads have aerial roots that become green when ex-
posed to light ( Fig. 55 ) . Roots of trees and many other plants are often
exposed to light through soil erosion. An examination of these exposed

Fig. 55. The chlorenchyma of this Florida orchid is in the roots. It has no leaves
and very small stems. Photo by W. M. Buswell.

roots is an easy way of discovering that chlorophyll may be formed in
the roots of many kinds of plants. There are many microscopic green
plants in which all parts of the plant body are green. Most of the body
of a moss or liverwort is ereen. The green stems of cacti are the most
conspicuous part of the plant; leaves are either temporary or entirely
absent. Wherever chlorophyll is present in plants, photosynthesis will go
on if other necessary conditions are suitable.

120 TEXTBOOK OF BOTANY

Leaves and photosynthesis. In most land plants the bulk of photosyn-
thesis occurs in the leaves. We ha\'e already seen that leaves of various
species of plants may differ greatly in several ways, such as the amount
of cutin on the epidermis, the number of stomates per square centimeter
of surface, the thickness of the leaf, the compactness of the chlorenchyma
in relation to the number and size of intercellular spaces, and the conse-
quent exposure of the mesophyll cell walls to the internal atmosphere of
the leaf (Chapter X). All these structures may influence the entrance
of carbon dioxide into the cells of the chlorenchyma and thus indirectly
influence the rate of photosynthesis.

Moreover, the environment in which a leaf grows may greatly affect
the processes underlying the development of these structures (Chapter
IX). Hence plants of the same species that have grown in different
environments may have different rates of photosynthesis. Experiments
have shown that certain plants growing in a relativeh' dry atmosphere
may make more sugar during their lifetime than plants of the same
species growing in a very humid atmosphere. A part, or all, of this dif-
ference may depend upon differences in the development of the leaf
structure in dry and moist air, particularly the structure of the chloren-
chyma and stomates.

The mesophyll. The amount of mesophyll wall surface exposed to the
internal atmosphere of the leaf is considered to be one of the important
factors in photosynthesis. Through it most of the CO- enters the cells
of the chlorenchyma. Some CO2 may enter the leaf directly through the
epidermal cells, but entrance in this manner may be gready impeded
by the thickness of the outer walls of the epidermal cells and by the
presence of cutin. Moreover, the area of the exposed mesophyll surface
is usually many times greater than the total area of the epidermis. From
measurements made on several different kinds of leaves it was found
that the area of this inner exposed surface may be from 9 to 30 times
greater than that of the epidermis. The data from numerous tests indi-
cate that in leaves of land plants the COj enters the cells of the chloren-
chyma largely from the internal atmosphere of the leaf.

The epidermis. The chief structures in the epidermis that influence the
rate of photosynthesis are the guard cells enclosing the stomates through
which CO2 enters the intercellular spaces of the chlorenchyma ( Chapter
VIII ) . When one begins to consider stomates in relation to the entrance
of CO2, several complicated problems in physics soon arise. These prob-
lems will not be discussed here, but a few facts will be stated briefly.

[Chap. XIV THE RATE OF PHOTOSYNTHESIS 121

If the stomates are closed, the entrance of CO- is diminished. But
stomates that appear closed to the human eye may sometimes be open
wide enough to admit the passage of molecules of CO2, which are much
too small to be seen through the best microscopes. The length of the
shortest diameter of a fully opened stomate of average size is several
million times the diameter of a molecule of CO2.

When the stomates are open or partly open, the relative rates at which
COl' passes through them are proportional not to the respective areas of
the stomates, but to their respective boundaries (the distance around
each pore). Because of this fact and certain related ones, the amount of
COl' that passes through the stomates is about 50 times greater than
would be expected on the basis of their total area. When the stomates
are closely spaced, less CO- passes through each one than when they are
farther apart. From such facts as these it is obvious that the stomatal
mechanism of leaves may indirectly affect the rate of photos)aithesis.

Stomatal behavior. The opening and closing of stomates are largely
dependent upon alterations in the intensity of light, moisture, and tem-
perature. As we shall see in more detail in Chapter XXIII, these external
factors initiate a sequence of physical and chemical processes in the
guard cells. These processes ultimatelv result in changes in the volume
and shape of the guard cells, and consequentb' in the size of the stomate
between them.

Under excellent growing conditions the stomates of man\' plants are
closed at night and open during the greater part of the dav. But during
hot dry weather the stomates of many of these plants mav be open mainlv
at night and closed during the day. In other plants they may be closed
except for a few hours in the early morning. In certain cacti and other
succulents they may remain open twenty-four hours each day.

The entrance of CO- into the intercellular spaces of the chlorenchyma
of young stems, green fruits, flowers, and seeds may also occur through
stomates in the epidermis of these organs. Chlorenchvma in the stems
and exposed roots of woody plants, however, soon becomes covered by
a la)'er of cork, the presence of which is a partial obstruction to both light
and the entrance of COj. The chlorenchyma in the roots of orchids and
also in the leaves of certain plants is separated from the epidermis by
one to several layers of non-green cells. Stomates are absent in algae and
the so-called green leaves of mosses. The submerged leaves of aquatic
plants mav have rudimentarv stomates or none. Stomates may be present

122 TEXTBOOK OF BOTANY

in plants or parts of plants without chlorophyll. Rudimentary stomates
in all types of subterranean organs have also been reported.

Although the presence and behavior of the stomatal mechanism ma\'
greatlv influence the rate of photosynthesis, it is evident that one should
be cautious about drawing inferences which imply that stomates behave
as they do in order to facilitate photosvnthesis.

Chloroplasts and photosynthesis. In almost all green plants chlorophvll
is confined within the colloidal matrix of definite protoplasmic bodies,
the chloroplasts, which also contain carotene and xanthophyll. Certain
primitive algae do not have well-defined chloroplasts. In most of the
green plants, therefore, photosynthesis occurs only within the chloro-
plasts. As a result of changes in the viscositv and streaming of the proto-
plasm in which these plastids are located, their arrangement within the
cell mav vary from time to time. The number, distribution, and surface
area of chloroplasts constitute another structural mechanism that influ-
ences the relative rates of photosynthesis in different plants.

Many facts about plants are so variable that an exact determination
of them is phvsicallv impossible and of no great consequence. On the
other hand, a close approximation to the truth based upon a limited
number of facts is often very helpful in checking one's concepts. As an
example, a few estimates of the number and area of chloroplasts in
plants may be cited.

In the castor bean leaf there are about 495,000 chloroplasts in the
mesophvll beneath each square millimeter of leaf surface. About 82 per
cent of them are in the palisade cells, and 18 per cent in the cells of the
spongy mesophyll. In the sunflower, nasturtium, broad bean, and elm
the corresponding numbers of chloroplasts per square millimeter of leaf
surface are 465,000, 383,000, 283,000, and 400,000.

These figures mav be used as a basis for further estimates. Beneath
each square inch of elm leaf there are about 250,000,000 chloroplasts.
The combined surface of these plastids is approximately 20 square
inches, if we consider them as spheres 4 microns in diameter. An average
elm leaf (5 square inches) has 100 square inches of chloroplast surface.
A large elm tree with 1,000,000 leaves — estimates have been as high as
7,000,000— has a leaf surface of about 0.8 acre, and a chloroplast surface
of about 16 acres, or 1/40 square mile. To appreciate the importance of
this enonnous plastid surface in photosynthesis, one must remember that
most of the chlorophyll is concentrated near the surface of the chloro-
plast, and that CO-.- and H2O enter the chloroplast through this surface.

[Chap. XIV THE RATE OF PHOTOSYNTHESIS 123

This plastid surface is really the photosynthetic area of the leaf. Some
allowance should be made for the area occupied by the larger veins.

Enzymes and photosynthesis. In addition to chlorophyll and other
pigments, the chloroplasts also contain enzymes. Since enzymes are in-
volved in photosynthesis and in other biological processes discussed in
subsequent chapters, certain of their general features may be noted at
this time.

Enzymes are organic catalyzers which accelerate chemical reactions
that occur in physiological processes ( photosynthesis, digestion, and the
like) without becoming a permanent part of the final products of the
process, and without being destroyed by the reaction. An enzyme may
therefore continue to catalyze a particular chemical reaction until it is
destroyed or made inactive by other conditions in the cell. Ultra-violet
radiation and high temperatures may destroy enzymes. Synthesis of
enzymes undoubtedly occurs in all growing parts of a plant. Without
them the chemical changes in cells would be so slow that living organ-
isms could not long survive. Enzymes may both initiate and accelerate
reactions.

In order to speed up chemical reactions in laboratory experiments and
industrial processes, chemists often subject the materials to very high
temperatures. Eventually thev discovered that many of these processes
could be made to occur just as rapidly at lower temperatures by the addi-
tion of a small amount of some inorganic substance, called a catalyst. A
high temperature cannot be substituted for enzymes in living cells, but
a temperature increase that is not detrimental to protoplasm does ac-
celerate plant processes.

Some processes may be catalyzed by either organic or inorganic
catalysts. For example, if we boil cane sugar (sucrose) in pure water, it
hydrolyzes to glucose and fructose slowly.

Sucrose + Water > Glucose + Fructose

Ci2H220n+ H2O >C6H,206+ CeHiaOe

If a very small amount of acid is added, the reaction proceeds rapidly.
During the process of boiling, some of the sugar is converted into mucic
acid, which further accelerates the process. Even at much lower tem-
peratures the reaction is fairly rapid, but for every increase of 10° C.
the speed of the reaction increases from 2 to 3 times. Many other chemi-
cal processes have this same relation to increase in temperature.

124 TEXTBOOK OF BOTANY

In plant cells the hydrolysis of sucrose is catalyzed by a specific
enzyme known as invertase ( sucrase, or saccharase ) . The reverse proc-
ess, the synthesis of sucrose from glucose and fructose in a plant, is also
accelerated by invertase or other enzymes. Many enzymes have the
property of catalyzing a reaction in either direction, the direction being
dependent upon other conditions in the cells.

Invertase that has been extracted from plants may be obtained from
chemical supply companies. When sucrose is dissolved in water with a
small amount of this extracted invertase, it is hydrolyzed as indicated
above. If the solution is heated to 40-50'' C. the reaction is most rapid;
it decreases rapidly above this temperature. Temperatures of 60° C. or
above completely inactivate many enzymes when they are wet. Most of
them are destroyed at temperatures below the boiling point of water.

The activity of enzymes is influenced by many conditions within the
cells. Among these may be mentioned various concentrations of acids,
alkalies, salts, the substances acted upon, and the substances formed.

The potency of enzymes in accelerating chemical change in living
cells is apparently due to the ease with which they unite with certain
substances involved in the reaction and form unstable intermediate com-
pounds. Through reactions of these unstable compounds the enzyme is
again liberated, and only the stable products of the whole series of reac-
tions accumulate. For instance, sugar and free oxygen are the stable end
products of photosyntliesis, but between them and the initial raw ma-
terials (CO- and H^O) several temporary intermediate compounds are
formed. Chemically trained students in increasing numbers have tried to
discover what these intermediate compounds are and the order in which
they are formed. Some of the problems will be mentioned briefly.

Intermediate compounds in photosynthesis. Formaldehyde has often
been proposed as the first intermediate substance in photosynthesis, but
alwavs with inadequate proof. Recent in\'estigators are giving more at-
tention to enzymes and the part that chlorophyll may play in the process.
For many years it has been known that chlorophyll absorbs certain ra)'s
of radiant energy. This is only another way of saying that chlorophyll
transfomis certain radiant energy to some other kind of energy— or
better, that certain photons of light upon striking some part of the
chlorophyll molecule impart their energy to it. As a result, chlorophyll
becomes chemically reactive, and in this state it probably unites with CO-
and H-O, forming an intermediate unstable compound. Merely by way

[Chap. XIV

THE RATE OF PHOTOSYNTHESIS

125

of picturing what has just been said, one of the recent suggestions may
be outlined as follows:

Light energy + Chlorophyll + CO2 + H2O > Chlorophyll carbonate

Chlorophyll carbonate + Enzyme + HoO > Sugar + Free oxygen

In the process the chlorophyll and enzyme have been converted to their
original states.

The process is undoubtedly more complex than is indicated here. The
difficulty of arriving at scientifically established answers to many ques-
tions about biological processes lies partly in the complexity of the trans-
formations that proceed as a chain of reactions, and parth' in the com-

HC=CH,

H

H3C— C /

\/^\/X

- — ^ / "^

^ \ / \

iCH,) (HC=0l

/

)^^ JZ C2H5

HC

H3C— c^, \

>" \

// C CH3

I

CH.
I '
CH

I ^

c=o

COOCH

COOC20H39

Fig. 56. Diagram representing the arrangement and complexity of the molecule
of chlorophyll a. If the group, CHO, in the detached circle is substituted for the
CH3 group nearby, the diagram would represent the molecule of chlorophyll b.
From B. S. Meyer and D. B. Anderson, 1939.

plexity of the large molecules involved in the reactions. The empirical
formulas of the two chlorophylls, a and b, are given on page 28. Merely
to illustrate the complexity of these compounds, one of the recently pro-
posed structural formulas for chlorophyll is reproduced here ( Fig. 56 ) .
Before we can know exactly how photosynthesis occurs, it will be

126

TEXTBOOK OF BOTANY

necessary to find out how the CO2 and H2O are united, whether they
unite with the chlorophyll molecule, how the enzyme enters into the
process, and what products are formed first as the chlorophyll and
enzyme are again set free. It is impossible at present to say how many
intermediate products are formed, and how many enzymes catalyze
their formation.

Rate of photosynthesis. From the foregoing discussion it should be
evident that the rate of photosynthesis is influenced by many conditions
within the plant and by several environmental factors. Attention has
been called to some of the influences of the stomates, intercellular spaces,
distribution of chlorophyll, activity of enzymes, light, temperature, car-
bon dioxide, and water. Other conditions, such as the amount of
chlorophvll and possibly the acidity of the cells, also influence the rate of
photosynthesis. The effect of any one factor is probably always influenced
to some extent by the relative amounts of the other factors involved.
Some idea of this fact is best obtained by studying the results of experi-
mental data such as those represented bv the accompanying curves.

20 'yo 40'7o 60«7o 60«7o

UGHT INTENSITY IN PER.CENTS OF FULL SUNUGHT

IOO«To

Fig.
of

57. Rates of photosynthesis in relation to light intensity and the concentration
carbon dioxide. Data from W. H. Hoover, E. S. Johnson and F. S. Brackett.

The curves in Fig. 57 indicate the relative rates of photosynthesis in
wheat seedlings in relation to light intensity and the concentration of
carbon dioxide in the air, when the temperature is 22° C. and the rela-

[Chap. XIV

THE RATE OF PHOTOSYNTHESIS

127

tive humidity is 70 per cent. The lowest curve indicates that with only
0.01 of 1 per cent of CO2 in the air, an increase in the light intensity
beyond 40 per cent does not increase the rate of photosynthesis.

The percentage of CO2 in the atmosphere near the earth's surface is
usually given as about 0.03 ( 3 parts in 10,000 parts of air ) , though it may
vary considerably just above the surface of the soil, particularly if there
is much decaying organic matter in the soil. Several measurements made
in a field of sugar beets showed that the percentage of COl> in the air at
the tops of the plants varied from 0.04 to 0.06, and at the ground level
from 0.05 to 0.28.

The curves in Fig. 57 show that the rate of photosynthesis at a given
light intensity depends upon the available COi-, and vice versa. At low
light intensities the maximum rate of photosvnthesis is attained when the
available COl' is low, and at low concentrations of CO2 it is attained when
the available light is low.

ILI

f 50

z
to

2 40

o

5 35

^30 I-

(f) 25

liJ

^ 20

a.

15
UJ

> 10
b

Hi

CL 0

ith 1.227oC02

10° 15° 20° 25° 30° 35° 40'

TEMPERATURE IN DE6R.EES CENTIGRADE

Fig. 58. Rates of photosynthesis in potato leaves in relation to temperature and the
concentration of carbon dioxide. Calculated from data by H. G. Lundegardh.

The curves in Fig. 58 indicate the relative rates of photosynthesis in
leaves of a potato plant in relation to both temperature and the concen-
tration of CO2 in the air. The leaves were in full sunlight during each
experiment, which lasted only ten minutes. Longer exposures would
result in some changes in the curves, but without annulling the prin-

128 TEXTBOOK OF BOTANY

ciples represented. Here again certain interrelations between two dif-
ferent environmental factors and the rate of photosynthesis are evident.

Summary. The rate of photosynthesis is dependent upon the interrela-
tions of several external factors and certain conditions and processes
within the plant cells. Some of these factors affect photosynthesis di-
rectly, others less directlv through a series of other processes. The impor-
tant external factors that most directlv affect the rate of photosynthesis
are the intensity and kind of light, the concentration of carbon dioxide in
the air or water environment, the temperature, and water — though most
of the effects of water on photosvnthesis are indirect. Among the impor-
tant internal conditions are the amount and distribution of chlorophyll
and enzymes, and such structural features as stomates, intercellular
spaces, and veins, which influence the movement of materials into and
out of the chlorenchvma.

The importance of photosynthesis to all living organisms will be appre-
ciated better as it becomes more evident, from the facts in later chapters,
that all the organic compounds in the bodies of plants and animals and
all the commercial products obtained from them have their beginning in
the synthesis of sugar. When it is clear that the chemically bound energy
supply of the bodies of all living organisms, and most of the energy that
we use in our homes and industries can be traced back to the radiant
energy chemically bound in sugar, we can view photosynthesis in true
perspective.

REFERENCES

Meyer, B. S., and D. B. Anderson. Plant Physiology. D. Yan Nostrand Com-
pany, Inc. 1939.

Miller, E. C. Plant Phi/siology. 2nd ed. McGraw-Hill Book Company, Inc.
1938.

Spoehr, H. A. Photosynthesis. Chemical Catalog Company, Inc. 1926.

CHAPTER XV
FOOD MANUFACTURE

C><^«£><><X><><X><>C><>0<^C>^^

III. SYNTHESIS OF STARCHES

Some of the sugar in plants may be changed chemicallv to insokible
starches and other carbohydrates, to fats and oils, and to proteins.
In the body of animals sugar is changed to a starch-like compound
called glycogen, and also to fats and oils. While studying such trans-
formations we may also appropriately consider many facts about the
accumulation and digestion of these foods. In this chapter we shall be
interested mainly in the formation and digestion of starch and a few
other carbohydrates.

Starches, fats, and oils are made from sugar alone. Proteins are made
from sugar and certain salts of nitrogen, sulfur, and, in some cases, phos-
phorus also. These transformations of sugar to other kinds of food are
either condensation processes or a combination of oxidation-reduction
and condensation processes. The simplest transforaiations, such as the
formation of starch from glucose or of inulin from fructose, involve only
chemical condensation. We shall first consider the synthesis of starch
from sugar.

Synthesis of starch from sugar. Starch is formed in plant cells from
glucose by chemical condensation:

Glucose > Water + Starch

n CUnO, > (n - 1)H20 + (C6Hio05)„H20

The group of enzymes known as diastase apparently catalyzes the
process of starch synthesis. Some of the plants in which starch is not
formed have been reported to lack one of these several enzymes. Diastase
does catalyze the hydrolysis of starch back to sugar in plant cells. This
hydrolysis of starch is easily obtained in test tubes with diastase that has
been extracted from plants. But the synthesis of starch from sugar
is influenced by some condition present in the "starch-forming" plastids

129

130 TEXTBOOK OF BOTANY

of the protoplasm that man has not yet been able to duplicate in test
tubes.

A reaction that may proceed in either of two directions is said to be
reversible, and is often represented by two arrows. A longer arrow is
usually used to indicate the direction in which the process is proceeding
more rapidly.

(A) Glucose ^^^ H2O + Starch

(B) Glucose T— =^ H2O + Starch

Statement (A) above indicates that the condensation of glucose to
starch is proceeding more rapidly than the digestion of starch to glucose;
statement (B) indicates that digestion is the more rapid process.
Whether digestion will exceed condensation, or vice versa, at any par-
ticular time depends upon certain conditions in the plant cell, such as
acidity, water content, and the concentration of glucose. External factors,
such as temperature and light, affect these internal conditions and thus
indirectly affect the digestion and condensation processes. For instance,
when Irish potatoes are stored in bins at 10^-30° C. the sugar in them is
mostly condensed to starch; but when they are stored at temperatures
either above 30° C. or near freezing much of the starch is digested to
sugar. Potato chips prepared from potatoes in which sugar has accumu-
lated are dark in color.

When the chloroplasts are exposed to light more sugar is made in
them and some of it condenses to starch. On the other hand, when the
guard cells of leaves are exposed to light the conditions initiated in them
by the light result in a change of starch to sugar.

These changes of sugar to starch and of starch to sugar mav occur
repeatedly in the same cell, and at any time during the day or night.
Many efforts have been made to detect the amount of sugar that must
accumulate in a cell before some of it begins to condense to starch.
This amount of course is variable, depending both upon the kind of
plant and upon the conditions to which it is exposed. The formation of
starch is dependent upon the hereditary make-up of the plant. Closely
related plants may differ greatly in this respect, and in some plants
( onion, hyacinth, and many others ) starch is formed only under excep-
tional environmental conditions, or not at all.

The number of species of plants in which starch never forms under
any environmental condition is unknown. It is very simple to remove

[Chap. XV SYNTHESIS OF STARCHES 131

leaves from a plant with a knife and place the cut ends of the petioles
in beakers of water containing different concentrations of sugar and also
different kinds of sugar. They can then be readily exposed to different
environmental conditions. Even the leaves in which starch ordinarily
does not form may produce it under these experimental conditions either
in the light or in the dark. Best results are usually obtained with about a
10 per cent solution of sugar. Since starch is made from glucose, one
might expect to find starch in the leaves only when glucose is placed in
the beakers, unless he recalls that by the influence of the enzymes in
plant cells certain kinds of sugars may be transformed to others. In such
experiments starch is formed in the leaves whether the sugar in the
beakers is glucose, fructose, mannose, galactose, sucrose, or maltose.
Even certain alcohols may be converted to starch in the leaves of
plants.

Starch accumulation. Sugar is involved directly or indirectly in all
the major physiological processes in plants. Starch, on the contrary,
merely accumulates. In time it may be digested back to glucose, either
by the plant that makes it, or by some other organism that eats it. Starch
is formed in plastids (leucoplasts and chloroplasts ) and occurs in the
cells as small grains that are insoluble in water; therefore it does not pass
from cell to cell. When starch is found in a cell one may infer that it was
made in that cell. It may be formed in all kinds of living plant tissues,
in either hght or darkness if sugar is present. The fact that it accumu-
lates sooner or often er in certain cells or tissues indicates that the condi-
tions affecting its formation are not uniform throughout the plant. There
may be a lack of general uniformity in the types of plastids, the amounts
and kinds of enzymes, the concentration of sugar, and the acidity of
the cells.

This lack of uniformity among cells is easily demonstrated with green
and white variegated leaves. Since sugar is made only in the chloroplasts,
its concentration in the non-green parts of the leaves is comparatively
low, and starch is usuallv formed only in the chlorenchyma of these
leaves. This fact is demonstrated by the ordinary iodine test for starch
in leaves after the chlorophyll has been removed with hot alcohol.

If these variegated leaves are first detached from the plant and placed
for about one day in a beaker containing a sugar solution as described
above, starch may then be found in the non-green parts of the leaf.
Variegated leaves of geranium are excellent materials for these tests
(Fig. 59). It has been reported that even with this special treatment

132

TEXTBOOK OF BOTANY

Fig. 59. Photographs of white and green variegated leaves of geranium after
the leaves had been subjected to the iodine test for starch: A, leaves tested
directly after removal from the plant; B, comparable severed leaves tested after the
cut ends of their petioles had been immersed in a sugar solution for 15 hours.
Photo by A. G. Chapman and W. H. Camp.

starch does not form in the non-green parts of such plants if plastids are
absent.

Microscopic tests for starch are easily made, and the relations of
starch accumulation to certain conditions and processes in the plant are
fairly well known. As a consequence, starch synthesis, accumulation,
and digestion are often studied as an index to other processes in which
sugars are being made, consumed, or translocated.

Starch may accumulate temporarily in chloroplasts when sugar is
being made, but in some plants, as in onions, oil accumulates in
chloroplasts.

Commercial sources of starch. Conditions in certain tissues and organs
are especially suitable to starch accumulation. Certain roots, stems, fruits,
and seeds are of s;reat commercial value, mainlv because of the starch
they contain. Besides being valuable as a food, millions of tons are ex-
tracted from plants annually and used in the industries for making vari-
ous types of dusting powders, adhesives, pastes required in sizing cloth
and paper, and as a source of dextrin and glucose. In the manufacture of
alcohol a number of starch-rich cereals are used as a source of food for
yeast plants.

SYNTHESIS OF STARCHES

133

[Chap. XV

Starch grains. Starch grains in different kinds of plants may differ in
form, size, and internal structure (Fig. 60). This fact may be used as one
means of identifying certain commercially prepared drugs and foods,
and also certain impurities in them.

-"i

^Q^&

B

Oo^^

s^ ^

D E F

Fig. 60. Diagrams of starch grains from several kinds of plants: A, bean; B, corn;
C, oats; D, potato; E, rice; F, wheat. Adapted in part from Leffmann and
Beam.

Each grain is composed of at least two substances. The outer portion
is composed of amylopectin, which becomes gelatinous in boiling water,
but does not appear blue with iodine. The inner part of the grain is
largely amylose. It is colloidally dispersed in boiling water and becomes
blue with iodine.

Dextrins. Dextrins are generally considered to be interaiediate prod-
ucts between sugar and starch. The probable steps in the formation and
digestion of starch may be represented in brief outline, in which diastase
is represented by three enzymes:

Glucose ^^ Maltose ^=^ Dextrins ^=^ Starch

(Mallase)

(Dextrinases)

(Amylase)

In the young seeds of some plants (sweet corn, certain varieties of

134 TEXTBOOK OF BOTANY

sorghum) plastids containing dextrin can be found in all stages of
growth. Some of these dextrin-filled plastids may occur along with starch
grains in the ripe seeds.

When starch grains are heated for a short time thev are transformed
to grains of dextrin, and become red instead of blue when treated with
a solution of iodine. Commercial dextrin is prepared bv subjecting starch
to heat.

Glycogen and inulin. Glycogen, like starch, is a condensation
product of glucose. It is common in the animal kingdom, but among
plants it is known to occur onlv in certain fungi, bacteria and
algae.

Inulin is a condensation product of fructose, and occurs in several
kinds of plants. If roots of salsify or dandelion are placed in 70 per cent
alcohol for a time, most of the inulin in them crystallizes; but the clusters
of crystals, though they are large, are often difficult to see because of their
transparency. The accumulated food in Jerusalem artichoke and oyster
plant is largely inulin.

Sugars, starches, dextrins, glycogen, and inulin all belong to the gen-
eral class of compounds known as carbohydrates, i.e., compounds of
carbon, hydrogen, and oxygen in which the proportion of hydrogen to
oxygen is 2:1, as it is in water.

The transformation of carbohydrates by condensation and digestion.
The formation of starch from sugar by chemical condensation and the
subsequent digestion of starch to sugar are common processes in plants.
All complex carbohydrates, such as starches, glycogen, inulin, cellulose,
and hemicelluloses, are formed from simple sugars by chemical condensa-
tion. Moreover, during the formation of fats, oils, proteins, and many
other compounds in cells, chemical condensation usually occurs at one
or more stages in the process. Chemical condensation occurs in every
living cell of both plants and animals. It is the converse of all ordinary
processes of digestion. Evidently a clear concept of its fundamental
features is necessary to the understanding of many biological processes.
A few examples will be given.

When 2 molecules of a simple sugar, like glucose, unite bv condensa-
tion, the resultant sugar is called a disaccharide ( double sugar ) :

Simple sugar > Water + Disaccharide

2 C6Hi20e > H2O + C12H22O11

[Chap. XV SYNTHESIS OF STARCHES 135

When 3 molecules of simple sugars unite by condensation, the result-
ant sugar is called a trisaccharide:

Simple sugar > Water + Trisaccharide

3 CeHioOe > 2 H2O + CisHssOie

The formula Ci-Hi-Ou may also be written as (C<;HioO.-;)2H20. Like-
wise, the formula of the trisaccharide, CisHsl-Oi.;, may be written as
(C<>HioO.-,):^H20 to indicate the number of molecules of sugar utihzed in
the formation of the larger molecule. When 2 molecules of sugar unite, 1
molecule of water is released. When 3 molecules of sugar unite, 2 mole-
cules of water are released.

A general name for the product formed by the union of several mole-
cules of simple sugars by condensation is polysaccharide. All these prod-
ucts are carbohydrates. We may now write a generalized equation that
represents the formation of any polysaccharide from a simple hexose
sugar, such as glucose or fructose.

n CeHizOe > (n - 1)H20 + (C6Hio05)„H20

One important point to note is that for every molecule of sugar added,
one molecule of water is liberated. This fact may be briefly represented
in a diagram in which we let some symbol, such as <I>, represent the
main body of the glucose molecule.^

Glucose + Glucose • Water + A disacchoride

H0^(^^])^;H £Hbi-<(^^>-<)H . H,0 + HO-<^^^)^<K^3-OH, or (C.H,oO.),H.O

Dissccharide + Glucose > Water + A trisaccharide

HO-<^^><K^^]>0:H +;H0;<^^0H . H,0 i- H0<2^0x(^)mK;^0H, or (CH,oO.).H.O

Disacchaiide + Disaccharide > Water + A polysaccharide of 4 {C,H,oO.) units

^ The diagram in Chapter XVII indicating the arrangement of the atoms in a molecule
of sugar (glucose) shows the carbon atoms in a straight chain. The following arrange-
ment of the carbon and oxygen atoms is indicated by X-ray studies of polysaccharides.

HO OH HO OH HO OH
\ / \ / \ X
HC CH HC CH HC CH

/ \ / \ / \

HO— CH HC— OH HO— CH HC— O— CH HC— OH

\ / \ / \ /
HC O HC O HC O

CH2OH CH2OH CH2OH

Glucose A disaccharide

136 TEXTBOOK OF BOTANY

If this process were to continue, a polysaccharide consisting of a long
chain of many (CeHioOo) units would be formed. X-ray studies indicate
that the molecules of polysaccharides in plant cells do consist of long
chains of these smaller sugar units. In some substances, such as starch
and cellulose, the units are all (CcHioO.-,) from glucose. In other sub-
stances these units may be formed from another kind of sugar, or the
chain may be composed of units of different kinds of sugars, as in
hemicelluloses. The chain ma)' also consist of units of sugar and certain
acids derived from it, as in pectic compounds.

One other important biological fact about condensation should be
appreciated, namely, that the chemically bound energy in the sugar is
still in the units of sugar in the new products formed. It is not liberated
in the process of condensation.

Digestion, or hydrolysis, is the converse of condensation. As molecules
of water replace those lost during the condensation of simpler molecules
to more complex ones, the complex molecules are subdivided into the
simpler ones from which they were formed.

Summary. Starch is formed in plastids ( leucoplasts and chloroplasts )
in plant cells from glucose by chemical condensation — a process that is
just the converse of all ordinary processes of digestion. By condensation,
many molecules of sugar become joined together by a loss of one mole-
cule of water for every molecule of sugar added. Chemically bound
energy is not liberated b\' this process and the starch therefore contains
the potential energv that was in the sugar from which it was made.
In the same way, many other complex carbohydrates ( polysaccharides )
are fomied in plants.

Starch may be synthesized from sugar in both green and non-green
tissues, and in both light and darkness. It is especially dependent upon
the presence of plastids, certain enzymes, the concentration of sugar,
and the acidity of the cells. It is a reversible process. External factors,
particularly light and temperature, affect these internal conditions and
thereby indirectly affect the accumulation of starch in plants. These
internal conditions are not uniform throughout the plant, nor in differ-
ent kinds of plants. As a consequence, starch accumulation is more
abundant in some tissues, organs, and plants than in others. In some
plants it may occur onlv under exceptional conditions, or not at all.

Since starch is insoluble in water, it does not pass from cell to cell.
It is formed in the cells in which it is found, and becomes valuable
as a food only where it can be digested. Enzymes that digest starch are

[Chap. XV SYNTHESIS OF STARCHES 137

widely distributed in the plant and animal kingdoms. Starch is extracted
from plants and used commercially in several ways.

Dextrins may be formed by partial digestion of starch. They are also
made directly from sugar by chemical condensation. Glycogen is a
starch-like polysaccharide formed from glucose in the animal body and
in some of the bacteria, fungi, and algae. Inulin is a polysaccharide
formed by condensation from fructose.

CHAPTER XVI
FOOD MANUFACTURE

IV. SYNTHESIS OF FATS AND PROTEINS

At ordinary temperatures fats occur in plants as both solids and liquids.
The hquid fats are commonly called oils. The protoplasmic system of any
living cell seems to contain all the conditions necessary for the trans-
formation of sugar to fats or oils. This transformation may occur in light
or darkness, and in both plant and animal cells. You have probably
known for some time that fats may be formed from sugar in the human
body.

Fat Synthesis

First of all, sugar is transformed to glycerin (glycerol) and fattv
acids primarily by oxidation-reduction processes. Then 3 molecules of
fatty acid unite with 1 molecule of glycerin by condensation, and the
final result is a fat or oil. Many intermediate steps are involved, includ-
ing interactions with enzymes. The more obvious transformations may
be represented briefly:

Sugar > Glycerin^

y > Water -|- Fat or oil

Sugar > Fatty acids^ (condensation)

(oxidation-
reduction)

Many kinds of fatty acids are formed in cells. Among the more
abundant ones are palmitic (Ci.-.HsiCOOH), stearic (CitH^.-.COOH),
and oleic (Ci-Ha.iCOOH). When the proportion of carbon to oxygen in
sugar and in these fatty acids is compared, it is evident that the trans-
formation of sugar to fatty acids involves the liberation of chemically
bound oxygen. That is, it involves a reduction or energy-storing process;
a pound of fat has about 2/4 times as much potential energy as a pound
of sugar. Since this transformation may occur in plants in the dark and
also in the cells of animals, it is perhaps evident that the additional
energy in fats must come from the potential energy in sugar, that some

138

[Chap. XVI

SYNTHESIS OF FATS AND PROTEINS

139

sugar — or its derivatives — must be oxidized during the formation of
fatty acids. Sugar, therefore, is the primary source of both the material
and the energy in fat synthesis.

The final step in the process, the condensation of glycerin and fatty
acid to fats or oils, may be briefly represented:

Glycerin

+ Palmitic acid

(a fatty acid)

Tri-palmitin

(a tat)

+ Water

C3H5(OH)3 + 3 C15H31COOH > C.3H5(Ci,H3lC02)3 + 3 H2O

Certain important facts are much more evident when this process is
represented in a little more detail:

H O HO

HC^OjH + HOj— CH3,Ci.5
O

HC— OiH + HOl— CH31C15

O
HC— OjH + HOJ— CH31C15
H

HC— O— CH31C15

O

I!

HC— O— CH31C15 + 3H2O

O
HC— O— CH31C15

H

Glycerin + 3 Palmitic acid > Tri-palmitin + Water

The place of union of the 3 molecules of fatty acid with 1 molecule
of glycerin by loss of 3 molecules of water is indicated in the above
equation. During the digestion of fats just the converse occurs: the
3 molecules of water are added and the glycerin and fatt)' acid are
separated. When the digestion of fats occurs in the presence of mineral
salts, some of the free ions of calcium, potassium, sodium, etc., unite with
the fatty acids and form soaps.

Both the condensation and the digestion represented abo\'e are
catalyzed by an enzyme known as lipase. Here again is a reversible
reaction catalyzed by an enzyme (glycerin + fatty acids ^ fat or
oil + water). Whether condensation exceeds digestion or vice versa
depends upon several conditions in the cell. One of these conditions is
the water content of the cell. This fact may be demonstrated by placing
all these substances in test tubes in which the water content is varied.
If the amount of water in the test tube is increased, digestion is promi-

140 TEXTBOOK OF BOTANY

nent. When the water is decreased, condensation is prominent. A good
supply of sugars and a relatively low water content are two of the con-
ditions in plant cells that are conducive to fat synthesis. Fat accumulates
in seeds at a time when their water content is decreasing.

Fat digestion in germinating seeds occurs when the water content of
the cells is increasing. It is easy to see how certain factors in the environ-
ment may affect these two internal conditions and thereby affect the
formation of fats. The highest percentage of linseed oil is obtained from
flax plants that grow in relatively dry climates with an abundance of
clear days.

Fats not only are digested during the germination of seeds, but the
whole process of fat synthesis may be reversed, and the fat be recon-
verted to sugar. Both hydrolysis and oxidation-reduction occur in a com-
plete reversal of the process of fat synthesis. The transformation of fats
to simple sugars, of simple sugars to starch, of starch to simple sugars,
and of simple sugars to sucrose which moves down the stems to the
roots, may all be detected by microscopic studies of seedlings of soy-
beans.

Of the 3 molecules of fatty acids that may unite with 1 molecule of
glycerin in fat svnthesis, all 3 may be alike, 2 may be alike, or all 3 may
be unlike. Since there are many kinds of fatty acids, many different
kinds of fats and oils are made bv plants and animals.

This fact seems even more remarkable when we think of the trans-
formations that are characteristic of different species of plants and ani-
mals. A mouse eats corn. In its body some of the starch from the grain of
corn is digested to sugar which in turn is transfonned to fat, and some
of the corn oil is also digested and transformed to mouse fat. In the body
of the cat that eats the mouse, mouse fat is digested and transformed
to cat fat. These specific differences are of course due to different in-
herent systems of protoplasm and enzymes in the corn, mouse, and cat.
There are cases on record in which the cells of pigs failed to change all
the plant fat in their food (cottonseed meal) into pig fat, and some of
the cottonseed oil accumulated in the cells of the pigs. Similarly, turkeys
that were fed large quantities of cod-liver oil failed to convert all of it
into turkey fat.

Although fats are present in every living cell as a part of the proto-
plasm, they are especially abundant in the cells of many kinds of seeds.
In addition to their use as food, fats are extracted from plants and used
in making soaps, paints, lubricants, and many other products. Some of
the commonest commercial fats and oils derived from plants are corn,

[Chop. XVI SYNTHESIS OF FATS AND PROTEINS 141

coconut, cottonseed, linseed, castor, pea, peanut, tung, olive, soybean,
and cocoa.

Summary. Fats that melt at ordinary room temperatures are liquid
fats or oils. Fats are made from sugar in the cells of both plants and
animals. By chemical reduction some of the sugar is transformed to
glycerin and fattv acids. The energy required for this process comes from
the oxidation of sugar, or its derivatives, in the cells. Then 3 molecules
of fatty acid unite with 1 molecule of glycerin by condensation, and a
fat or oil is the result. A pound of fat, therefore, contains much more
potential energy than a pound of sugar — about 2}4 times as much.

When a fat is digested, glycerin and fattv acids are set free. These fatty
acids mav then recombine with the glycerin in a different order or they
may be altered and then recombine with glycerin, depending upon
enzymes and certain other conditions within the cells. As a consequence,
man}' different kinds of fats are present in both the plant and animal
kingdoms. Fats may also be retransfomied to sugar in li\'ing cells. The
protoplasm of all cells is composed in part of certain kinds of fats and
fat-like derivatives. Fats are extracted from plants and used commer-
cially in many ways.

Protein Synthesis

The proteins are a third class of foods. One sometimes hears that they
are the most important foods. But when three things are essential, no one
of them is more important than another. In a later chapter we shall see
that one may truthfull>' say that the cell walls of plants are constructed
mainlv from carbohvdrate foods, and that the larger part of the organic
matter in protoplasm is made from protein foods. But enzymes, pig-
ments, vitamins, hormones, and all three classes of foods are essential
in the construction of protoplasm. All of the foods contain chemically
bound energy that was bound first of all in sugar. This energy again
becomes free and actixe in the cells through oxidation, mostly of carbo-
hvdrate foods and least of all of proteins. Evidently no part of the plant
will grow much unless it is well supplied with all three of these foods,
or with all the conditions necessary for making them. By numerous ex-
periments this statement has been shown to be true particularly of
sugar and proteins, as we shall see in later chapters.

Protein molecules. The proteins, unlike the complex carbohydrates
and fats, are made onlv in part from sugar. A molecule of protein is
composed of nitrogen and sulfur in addition to the three elements

142 TEXTBOOK OF BOTANY

( carbon, hydrogen, and oxygen ) derived from sugar. The molecules of
some proteins also contain phosphorus. But 85 per cent or more of the
protein molecule is made from sugar. These facts and several others are
represented by the empirical formulas of a few common proteins:

Gliadin of wheat C685H1068O211N196S5

Zein of corn C736H1161O208N184S3

Egg albumin C696Hii25022nNi75S8

Casein of milk C708H1130O224N 18084^4

Among the facts exemplified by these fomiulas are the large size of the
molecules composed of more than 2000 atoms, the absence of phos-
phorus in some proteins, the relatively large amount of nitrogen in com-
parison to the amount of sulfur and phosphorus, and the fact that the
proportion of carbon to oxygen is higher in proteins than in sugar. This
last fact is evidence that oxidation-reduction processes occur in protein
synthesis. Lastly, one may infer that plants must obtain nitrogen, sulfur,
and phosphorus from external sources.

Sources of nitrogen, sulfur, and phosphorus. With the exception of the
nitrogen-fixing bacteria, plants are dependent upon nitrogen compounds,
especially nitrate and ammonium salts, as a source of nitrogen. In the
life of ail other plants the free nitrogen of the air is merely an inert
gas. Most plants also are dependent upon sulfate salts as a source
of sulfur, and upon phosphates as a source of phosphorus.

These salts of nitrogen, sulfur, and phosphorus are in the soil, and
they are all soluble in water. Those that are in solution in the water may
pass into the roots of land plants, and directly into all the cells of algae
and similar submerged plants. A summary of the major transformations
of nitrogen and sulfur compounds in the plant kingdom is reserved for
the chapters on non-green plants. For the present it is enough to recog-
nize that the relative amounts of salts of nitrogen, sulfur, and phosphorus
in the soil and in fertilizers may greatly affect the rate of growth of
plants, indirectly through their effects on the synthesis of proteins. Some
plants grow best when supplied with nitrate salts; others grow best when
supplied with ammonium salts. Some bacteria must have a supply of
organically bound nitrogen.

Intermediate steps. The inorganic salts and sugar are not transformed
into large protein molecules in one simple process. Intermediate prod-
ucts consisting of relatively small molecules are formed first. The best
known of these products are called amino acids because they contain the

[Chop. XVI SYNTHESIS OF FATS AND PROTEINS 143

amino group (NHir). Then by chemical condensation a large number
of these amino acids become joined together as one large molecule of
protein. The materials used and the more obvious products formed may
be represented briefly by the accompanying diagram.

+ XPO4

Sugar + XNO3 > ^JJJI"'' > Proteins + H2O

/
Sugar + XNO3 + XSO4 ^

(oxidation-reduction) (condensation)

In this diagram, X, as in XNO:{, represents some basic ion such as
potassium, calcium, or sodium. Sugar may be replaced in this process by
some of its derivatives, such as organic acids, but nevertheless sugar is
the primary substance used. Of the 20 known amino acids, only 2 con-
tain sulfur, and none contains phosphorus.

Synthesis of amino acids. The svnthesis of amino acids from sugar
and the inorganic salts is the most unique part of the whole process.
This synthesis may occur in practicalh all plants ( green and non-green ) ,
but apparentlv not in animals, at least not in adequate amounts. The
data of numerous experiments show that animals mav die, fail to grow,
fail to reproduce, or fail in other wavs merely because of the deficiency
of certain amino acids in their diet. These failures in development
begin to disappear immediately after the particular amino acids — or
the proteins containing them — are added to the diet. Some bacteria
also are unable to synthesize amino acids.

The svnthesis of amino acids ma\^ occur in roots or any part of a
green plant in which the cells are not too acid and all other conditions
are suitable. These conditions seem to be most suitable in the phloem
and in young tissues. Not onlv is there some reduction of carbon during
amino acid synthesis, but the nitrogen and sulfur are also chemically
reduced. The energy necessary for the process may be obtained entirely
from the oxidation of sugar or of some of the derivatives of sugar in
the cells. The process apparently depends upon a complex set of condi-
tions and enzymes that occur all together only in plant cells. It may go on
in the absence of light and chlorophyll if sugar is present, as for instance,
in roots and fungi. However, it has not yet been proved that all the
amino acids can be made in all fungi and in the roots of all plants.
There probably are exceptions to the general rule, as tliere are in most
biological processes.

144 TEXTBOOK OF BOTANY

Condensation of amino acids. The last step in the process of protein
synthesis, the condensation of amino acids to proteins, is similar to the
condensation processes in the formation of complex carbohydrates and
fats. More than 100 molecules of amino acids^ become joined together
in one large molecule of protein. Since the organic matter in protoplasm
is composed largely of proteins or of substances derived from them, the
condensation of amino acids to proteins must occur in every young cell
of both plants and animals. Animals, however, must obtain amino acids
from plants thev eat, or from other animals that have eaten plants.

Digestion of proteins. The digestion of proteins to amino acids is the
reverse of condensation. The process is catalyzed by enzymes in both
plants and animals.- The amino acids set free by digestion may be con-
^'erted back to sugar, or they may be broken down by oxidation-reduction
to such simple compounds as CO2, Hi-O, NH.., and H^S. That is, the
carbon is oxidized, but the reduced nitrogen and sulfur in amino acids
and proteins are not oxidized by living organisms, except by special
groups of bacteria.

Recondensation of amino acids. Instead of being oxidized, however,
the amino acids set free by the digestion of proteins are often recon-
densed to proteins again in both plants and animals. This time they may
be combined in a different order or in different proportions, or both,
and a different kind of protein is the result. When one considers that
there are at least 20 different kinds of amino acids, and that more than
100 molecules of amino acids are joined together in each protein mole-
cule, it is evident that the number of different kinds of proteins that
are mathematically possible is beyond comprehension.

1 Molecules of amino acids are small in comparison to those of proteins:

Glycocol ('2H5O2N Lysine C6H14O2N4

Tryptophane C11H12O2N2 Cystine C6H12O4N2S2

Amino acids may be converted temporarih to amides:

Amino acid (aspartic) + Ammonia ^ Amide (asparagine) + Water

HOOC— CHNH2— CH2-COOH + NH3 ^ H2X()( -CHNH2-CH2-COOH + H2O

- Perhaps it should be noted that in order to keep this story as simple as possible refer-
ence is made only to proteins and amino acids. Some of the other products formed may be
indicated briefly:

Amino a<-ids + NH3;=i Amides + H2O

Amino acids ;=i Intermediate products ;;=i Proteins + IW)

In both plants and animals amides may be formed temporarily by the union of amino
acids and ammonia. There are also partly digested products between proteins and amino
acids in solution in the cells.

[Chap. XVI SYNTHESIS OF FATS AND PROTEINS 145

This fact seems all the more remarkable when certain other aspects
of proteins are considered. First, since protoplasm is characterized
largely by proteins, there is a possibility of many different kinds of
protoplasms, even though the plants and animals do fall far short of
making all the different kinds of proteins that are mathematically pos-
sible. Second, the hereditary units of protoplasm, such as chromosomes
and genes, that determine different species and varieties of plants and
animals are composed mainly of proteins. The number of different kinds
of these units, therefore, may easily be great enough to account for many
different kinds of plants and animals, aside from the fact that they may
occur in countless different combinations.

Finally, these hereditary units in one animal are constructed from
proteins obtained either from plants or from another animal. But this
picture is not complete until we fully realize that in all the individuals of
a certain species of plant or animal the construction of the same kinds of
proteins and hereditary units proceeds generation after generation. To
this part of the story we shall return in the chapters on heredity.

Accumulation of proteins. The accumulation of proteins in plants, like
that of fats, is dependent upon the relative amount of sugar made.
Among other internal conditions upon which it is dependent are the
acidity and water content of the cells, and the presence of enzymes.
The more important environmental factors are light, temperature, mois-
ture, and certain salts. Protein accumulation is greatest in certain seeds,
though it may occur to some extent in all plant organs. The amount of
su2;ar made in one of the corn plants described in Chapter XVIII, and
the amount of "fats" and proteins that accumulated in it are as follows:

Amount of sugar made in the plant .32.0 ounces

" '' accumulated "fat" 0.7

" " " protein 2.2 "

The most expensive portion of the diet of human beings is the protein.
It may be seen from the accompanying diagram ( Fig. 61 ) , that soybeans
are the richest source of protein and why this bean is one of the most
important of foods in the Asiatic nations, where meat in the diet is
very limited. It is one of the least expensive sources of proteins. Even in
the United States, where meat is consumed in comparatively large
quantities, the principal source of protein in the diet is wheat.

Summary. Proteins are the most complex foods. They are made from
sugar and certain salts of nitrogen and sulfur, and in some cases of
phosphorus also. Some of the intermediate steps in the synthesis of pro-

146 TEXTBOOK OF BOTANY

BEEF STEAK

EGGS

I WHEAT FLOUR

J

CORN MEAL

I RICE
H H POTATOES
I ■ ■

28.3 ia.4 175 12.7 9.7 7.8 6.8 3.2 1.5

Fig. 61. Percentage of protein in various foods.

teins are not clearly understood. The best known of the intermediate
products are the amino acids. These are made by oxidation-reduction
processes from sugar and salts of nitrogen and in some cases of sulfur
also. In this process the carbon, nitrogen, and sulfur are chemically
reduced. The energy used in this process comes from the oxidation of
sugar or of some of its derivatives.

Amino acids may be made in both green and non-green cells
of the plant and in either light or darkness. Apparently all the condi-
tions necessary for the process are present only in the cells of plants.
Animals and some of the fungi and bacteria are as dependent upon cer-
tain plants for some amino acids as they are for sugar. Some of our
common plants, such as corn and wheat, do not contain all the amino
acids necessarv for the health and growth of the human body, at least
in adequate amounts. This is one of the reasons why mixed diets are
desirable and necessary.

After the amino acids are foraied, more than 100 amino acid molecules
become joined together by chemical condensation as a single large mole-
cule of protein. During this process phosphorus is sometimes joined in
the molecule. All protoplasmic proteins are insoluble in water and are
undoubtedly formed in the cells in which they occur. Hence this last step
in the chemical condensation of amino acids must occur in all cells of
all plants and animals.

When proteins are digested back to amino acids, these acids may
become rejoined in quite a different order and in different proportions,
depending upon the enzymes and other conditions within the cells. Pro-

[Chap. XVI SYNTHESIS OF FATS AND PROTEINS 147

teins and their derivatives appear to be the primary chemical bases of
different protoplasms, different hereditary miits, and different species
of plants and animals. The specificity of organic synthesis in plants and
animals and its constancy in repetition, cell after cell and generation
after generation, are among the most important facts of nature. Without
this specificity and constancy, organisms as we know them would not
exist.

What Becomes of the Food Made by Plants?

The answer to this question is the central theme of the next two
chapters. It may be briefly summarized here, however, as an introduc-
tion to what is to follow.

Some of the food is oxidized within the cells of the plant. The amount
of food that is consumed in plants by oxidation differs with the kind of
plant and also with the environment of the plant. Under excellent grow-
ing conditions about one-fourth of the food made by a corn plant is
oxidized in the plant during its lifetime. In a single growing season about
one-third of the food made by a young apple tree is oxidized within the
tree. However, plants may be placed in environments in which they
oxidize food faster than they make it.

Some of the food is converted into substances of which the cells of
the plant are composed. Here again, the amount so used varies with the
kind of plant and with the environment. In the com plant referred to
above, about one-half of the food is used in this manner.

The food that is made by the plant and not used by it in either of these
two ways of course accumulates within the plant. It is frequently re-
ferred to as "storage food." The amount of this accumulated food may
be great or small, depending upon the kind of plant and its environment.
It is the source of the food of most animals and numerous non-green
plants. Many of the non-green plants and some of the simpler animals,
because of their unique enzvmes, can also digest and use the cell walls
as food.

The relative proportion of the kinds of food that accumulate in a

plant is conditioned by the hereditary constitution of the plant. In Table

3 are shown the amounts of glucose, sucrose, starch, protein, and fat in

the leaves of several well-known plants. The analyses were made after a

period of rapid photosynthesis in detached leaves. Accumulated foods

are expressed in percentages of total dry weight of the leaves.^

^ Data from B. N. Singh, K. N. Lai, and K. Prasad, Proc. Indian Acad. ScL, VIII B,
301, 1938.

148 TEXTBOOK OF BOTANY

Table 3. Relative Composition of Leaves of Several Crop Plants the Stems,
Fruits, or Seeds of Which Are Sources of Commercial Sucrose, Starch, Pro-
tein, and Fat. The food that accumulates most abundantly in detached leaves
is the same as the one derived commercially from other organs of the plant.

Glucose Sucro.se • Starch Protein Fat

Sugar cane leaves 2.8 18. £ 14.4 13.0 6.5

Rice leaves 2.4 6.9 £2.7 13.1 5.6

Bean leaves 2.0 4.7 9.1 2J^.8 8.5

Castor bean leaves 1.6 4.0 8.0 10.2 U.4

CHAPTER XVII

USES OF FOOD IN PLANTS

<j><x><&<><>o<><x><><><><><>^^

I. RESPIRATION

In the chapter on photosynthesis attention was called to the early experi-
ments from which it was concluded that carbon dioxide increases and
oxygen decreases in the air surrounding all parts of plants in the dark,
and non-green parts of the plant in both light and dark. The results of
numerous later experiments showed this conclusion to be true both for
green plants and for many non-green plants. They also disclosed the
kinds of exceptions to this conclusion that may be found among the non-
green plants.

One may easily demonstrate some of these facts by enclosing a mass
of germinating seeds, opening flower buds, green leaves, or any other
actively growing parts of a plant in tightly stoppered bottles for several
hours, and then testing the enclosed air for carbon dioxide and oxygen.
When the air from one of these bottles is forced into limewater an
abundance of carbon dioxide is indicated by the rapid formation of a
precipitate of calcium carbonate. The decreased amount of oxygen in
another bottle may be detected by chemical tests, or by the failure of
an ignited match to continue burning in it. By pressing a finger into the
mass of seeds in one of the bottles one becomes aware of the higher
temperature of the germinating seeds. A smaller increase in temperature
may also be detected in the bottles containing opening flower buds and
leaves. This increase in temperature may be measured if the plant parts
are kept for a few hours in a thermos bottle into which a thermometer
extends.

All these changes in amounts of carbon dioxide and oxygen, and the
increase in temperature in the demonstrations are dependent upon the
activity of living plant cells. They do not occur in similar demonstrations
containing only sterilized dead plants or dead parts of plants.

These demonstrations may also be repeated with germinating seeds
that have been sterilized and placed in bottles containing air from
which all the oxygen has been removed. In the absence of free oxygen,

149

150 . TEXTBOOK OF BOTANY

growth soon ceases and there is little or no change in temperature in the
bottle. But if oxygen is allowed to enter the bottle before the seeds suf-
focate, the effects of respiration become evident and growth is renewed.

All these facts are in some way related to the chemically bound
energy of the food in the plant cells, and to its transformation into other
kinds of energy through the process of respiration. Yet none of the
phenomena mentioned above is respiration; they are merely the recog-
nizable external effects of respiration. These statements will become
much clearer as we consider the relations of certain facts, some of which
we have already met in previous chapters.

Thus far nothing has been said about the formation of water from
food during respiration. Water formed in this way is also an accessory
consequence of respiration, but of little importance except to clothes
moths and a few other animals which are dependent on water from
this source. These easily detectable consequences of respiration are often
referred to as external evidences of respiration.

Sugar the source of chemically bound energy in living organisms. All
plants and animals are dependent directly or indirectly upon the syn-
thesis of sugar as the sole source of their supply of chemically bound
energy. This is a broad generalization, and thus far we have encountered
only a few of the facts upon which it is based. But a clear understanding
of the energy transformations noted in the last two chapters is a necessary
first step toward a full appreciation of this generalization.

The available bound energy in a green plant is obtained by photo-
synthesis. Experiments have shown that sugar and other soluble organic
matter may pass from the environment into the cells of a green plant,
but the amount of available bound energy obtained in this way by
green plants growing in natural conditions is negligible. A possible ex-
ception is the absorption of soluble organic substances by algae that grow
where these substances are abundant.

Animals and non-green plants obtain this chemically bound energy
when food passes from some external source into the cells of their bodies.
No additional chemically bound energy is obtained in any other way
by these organisms, except by the few groups of bacteria that oxidize
reduced atoms of iron, nitrogen, and sulfur in certain compounds. But
even the reduction of nitrogen and sulfur in living organisms is depend-
ent upon the potential energy in sugar. Some of the evidence for this
last statement was encountered in the chapter on protein synthesis; addi-
tional evidence is presented under the discussion of nitrogen fixation.

[Chap. XVII RESPIRATION 151

External evidence of respiration. We have frequently referred to the
fact that this chemically bound energy is liberated in the cells of plants
and animals by the oxidation of foods. The biological name for this
process is respiration. Perhaps most of us have at some time assumed
respiration to be synonymous w^ith the movement of oxygen and carbon
dioxide into and out of plants and animals. Such an inference is contrarv
to fact, and it should not be allowed to distort our thinking. There is no
more reason for confusing respiration with breathing, or with the en-
trance and outgo of oxygen and carbon dioxide, than there is for con-
fusing the process of eating with the subsequent chemical conversion
of food into protoplasm and cell wall substances in each cell of the plant
or animal body.

The fundamental feature of respiration is the transformation of
chemically bound energy through the oxidation of foods within each
cell of the plant and animal body. The entrance of oxygen into a plant
and the escape of carbon dioxide from it are merely consequences of the
oxidation. In fact, we shall soon see that respiration ma\' occur without
the inward and outward movement of these two gases.

The belief that the movement of these two gases is the essence of
respiration has further led to the wholly erroneous conclusion that
respiration in plants is just the converse of that in animals, or that it
occurs in plants only in the daytime or only at night. It should be obvi-
ous that this conclusion arises from a failure to distinguish between the
results of photosynthesis and those of respiration. It also arises from a
failure to distinguish between respiration and its external signs, which
are often used to detect the occurrence and rate of respiration. For in-
stance, in man and certain other animals, breathing and bodily tempera-
ture are well-recognized external evidences of respiration within the cells
of the body. Since the entrance of oxvgen into the body and the escape
of carbon dioxide from it are also external evidences of respiration, both
the occurrence and the rate of respiration may be measured by placing
the animal in a special calorimeter. In this apparatus the increase in
temperature and also the increase in CO2 and decrease in O2 can be
measured over a period of time. The dependence of man and other
animals upon food as the sole source of their supply of chemically bound
energy was discovered by means of such calorimeter tests.

What external manifestations may be used to detect the occurrence
and rate of respiration in plants? Breathing, of course, need not be con-

152 TEXTBOOK OF BOTANY

sidered because it does not occur in plants. Body temperature ordinarily
cannot be used because the temperature of a plant is usually near that
of the surrounding air. The parts of a plant exposed to the sun and
very rapidly growing parts may be warmer than the surrounding air.
Manure heaps and other masses of decaying organic matter may become
hot as a result of the energy liberated by the respiration of the numerous
bacteria in them. Stacks of green hay and freshly harvested grain may
likewise become hot, partly as the result of the respiration in the living
cells of the hay or grain, but mainly because of the respiration of the
bacteria that are present. For a clear demonstration of the liberation of
heat energy by respiration in plants it is necessary only to enclose them
in a thermos bottle into which a thermometer extends.

Since the oxidation of food results in a loss in drv weight, the changes
in dry weight of comparable examples of plants during a given period
of time mav also be used to determine the amount of respiration that
occurs in them. From preceding facts and from the equation below
it should be obvious that this method can be used for green tissues only
in the dark, but isolated non-green tissues may be thus tested in either
light or darkness. The rate of respiration in different samples of germi-
nating seeds is easilv measured by this method. Since sugar is the most
frequently oxidized food in respiration, we may illustrate the loss of dry
weight by a simple equation :

Sugar (bound energy) + Oxygen > j- -j + Water -f

^>yA r> ^ ■ dioxide energy

674 Calories ^"^

180 gm. CeHizOc + 192 gm. O2 > 264 gm. C0« -f 108 gm. HoO + 674 Cal.

Note that while 192 grams of oxygen pass into the plant, 372 grams
of CO2 and H2O are liberated, a net loss of dry weight of 180 grams,
which is just the weight of the sugar oxidized.

This generalized equation is the converse of that of photosyntliesis,
except that the free energy in respiration is heat energy instead of the
radiant energy of light. It should always be remembered, however, that
such equations indicate only the initial substances and the ultimate prod-
ucts. In most biological processes there are complicated intermediate
steps involving enzymes, intennediate products, and intermediate trans-
formations of energy that may be the real essence of the process. The
intermediate processes and products in respiration and photosynthesis
are not exact opposites.

[Chap. XVII RESPIRATION 153

We shall see later that the heat energy liberated by respiration in green
plants is usually insignificant in the life of the plant. The intermediate
transformations of energy and substances must be the essential ones in
respiration.

Sometimes the free energy at the end of the process of respiration is
not entirely heat energy. For instance, at definite periods in the firefly
it is largely light. Likewise the phosphorescent light of certain bacteria,
fungi (Fig. 263), and deep-sea fishes is a result of energy transforma-
tions during respiration.

Both the liberation of energy and the decrease of dry weight have been
used in numerous experiments as a means of determining the occurrence
and rate of respiration in plants. It is well known among botanists that
respiration occurs continuously in all active living cells of plants just as
it does in the cells of animals, and that when deprived of a source of
oxygen green plants will eventuallv suffocate just as animals do. This
fact might have been deduced from the similarity of protoplasm in plants
and animals, for after all it is the processes in protoplasm that are de-
pendent upon respiration.

Perhaps dry dormant seeds and spores may remain alive for a time
without respiration — at least without detectable respiration. Experi-
ments have shown that dry seeds may remain alive in a vacuum for
months. It is often assumed that some respiration is occurring in these
seeds, but satisfactory evidence is lacking.

Respiration and the escape of carbon dioxide. The escape of carbon
dioxide from plants is also used as external evidence of respiration in
them. The amount that escapes within a given time may be measured
and considered to be a measure of the rate of respiration, though this
method has certain limitations.

The first of these limitations should be obvious from our knowledge
of the utilization of carbon dioxide in photosynthesis. During certain
periods of the day carbon dioxide may be utilized in photosynthesis
in green cells 15 to 20 times as rapidly as it is liberated by respiration.
Carbon dioxide that is released by respiration within or near chloroplasts
exposed to light is probably immediately used in the making of sugar,
and little, if any, escapes to the atmosphere. Even in the green cell as a
whole, carbon dioxide enters far more rapidly than it escapes; hence
its concentration in the surrounding air decreases and that of oxygen
increases even though respiration is occurring in the green tissues. This

154 TEXTBOOK OF BOTANY

apparent anomaly of course does not occur in the green tissues in the
dark.

Twice during the day, once in the morning and again in the evening,
the intensity of hght must be such that the rate of photosynthesis just
equals that of respiration. The rates of photosynthesis and respiration
may also be equal temporarily under certain other environmental condi-
tions such as dark cloudy weather and high temperatures. These condi-
tions are frequently present in greenhouses during the winter months;
during the darkest days photosynthesis may be less than respiration.

The relative amount of carbon dioxide formed during respiration is
related to the kind of food that is being oxidized. For instance, when
fats are being oxidized in plants the first steps in the process may result
in the oxidation of fats only to sugar, with little or no release of carbon
dioxide. The sugar may then be oxidized to CO2 and H2O. This fact may
be experimentally demonstrated during the germination of seeds con-
taining considerable fat. When fat is completely oxidized the volume of
oxygen consumed is much greater than the volume of carbon dioxide
released, whereas when only sugar is oxidized the volume of oxygen con-
sumed is just equal to the volume of carbon dioxide released. Conse-
quentlv the kind of food being oxidized in living organisms may often
be detected by placing them in air-tight enclosures and measuring the
amount of oxygen consumed in proportion to the amount of carbon
dioxide released. If the ratio is about one, the oxidation of sugar is indi-
cated; if it is much less than one, the oxidation of fats or even proteins
is indicated; however, proteins are seldom oxidized appreciably so long
as sugar or fat is available.^

The discovery that this ratio varies with the kind of food available in
the cells was one of the first definite contradictions of the oft-quoted
traditional belief that protoplasm is the substance that is oxidized in
respiration and must therefore be constantly repaired. There is no evi-
dence that protoplasm is destroyed by respiration or that it needs repair
in a healthy cell well supplied with food.

^ While the ratio described above may be used as a general rule, it should not be used
as an infallible one. Certain well-known exceptions to it occur in plants. These are mostly
cases in which the sugars and fats are not at once completely oxidized to CO, and H„0.
The lowest ratios are obtained when substances like the fatty acids, which have relati%ely
little oxygen in them, are oxidized in respiration; for example, the oxidation of palmitic
acid as shown in the following equation:

C16H32O2 + 23 O2 > 16 CO2 -h 16 H2O

[Chap. XVII RESPIRATION 155

A third limitation to the method of detecting the rate of respiration
by measuring the amount of carbon dioxide released is rehited to the
fact that sometimes the food is not completely oxidized to CO2 and
H2O. It may be only partially oxidized, with the result that such sub-
stances as alcohol and various organic acids are formed. The alcohol and
organic acids may accumulate in the plants. Respiration by incomplete
oxidation of food occurs to some extent in all plants. It is most easily
detected when the supply of oxygen is low. Certain types of plants and
plant organs, such as cacti and the large fruits of tomatoes, for instance,
usually contain a small percentage of alcohol.

This sort of respiration is frequently referred to as fermentation. It is
particularly conspicuous in many of the non-green plants such as yeast
and acetic acid bacteria, in which rather high percentages of the incom-
pletely oxidized products (alcohol and acetic acid) accumulate and
escape into the surrounding medium. In all such cases of respiration the
volume of carbon dioxide released is not likely to be the same as the
volume of oxygen consumed. It may be many times greater or many
times less. These facts may be illustrated briefly by equations:

Sugar — -^ Ethyl alcohol + Carbon dioxide + Free heat energy

674 Calories of
bound energy

enzyme of

180 gm. CeHizOe > 92 gm. C2H5OH + 88 gm. CO2 + 28 Calories

yeast

The facts indicated by the above equation may be demonstrated if a
mass of living yeast plants is placed in a small thermos bottle filled with
water containing some sugar in solution, and a tightly fitting stopper is
inserted. Within a short time, the stopper will be forcibly ejected by the
pressure of gas that develops within the bottle, the temperature of the
water will have increased a little, and the odor of alcohol may be de-
tected. If the gas that is formed is tested by the usual limewater test, it
will be found to be carbon dioxide.

If one end of a tube is put through the stopper of this bottle and the
other end beneath the surface of water in another vessel, the carbon
dioxide formed can escape, but at the same time air is excluded from the
yeast culture in the bottle. Under these conditions, if sufiicient sugar is
present, the yeast plants will live and produce alcohol until they are
checked by the toxic effect of the alcohol. Varieties of yeast that continue
to remain active when the amount of alcohol formed in the vat is equiva-

156 TEXTBOOK OF BOTANY

lent to 12 per cent or more are the most desirable for the production of
alcohol in commercial quantities.

If the air is not excluded, however, the alcohol will be oxidized rapidly
by acetic acid bacteria which were placed in the bottle with the mass of
yeast. Instead of having a solution of alcohol we will have a solution of
acetic acid (vinegar). The formation of acid cider and the souring of
wines are the result of respiration in acetic acid bacteria. These bacteria
are also used for the production of commercial vinegar. We may repre-
sent the process briefly as follows:

Alcohol + Oxygen > Acetic acid + Water + Free heat energy

Enzymes of acetic

C2H5OH + O2 — '- > CH3COOH + H2O + Free heat energy

acid bacteria

Lactic acid bacteria exemplify an interesting type of respiration in
which no free oxygen is used, no carbon dioxide escapes, and there is
no loss in dry weight.

Sugar > Lactic acid + Free heat energy

Enzymes of lactic

CgHiaOe — > 2 CH3 • CHOH • COOH + Free heat energy

acid bacteria

In many organisms respiration results in the formation of an incom-
pletely oxidized food, and is usually referred to as fermentation. The
different types of fermentation are recognized and named on the basis
of the incompletely oxidized product formed. The three examples cited
above are called, in order, alcoholic, acetic acid, and lactic acid fermen-
tation. The fundamental fact, however, is that this is the way respiration
occurs in these particular organisms. In each one food is oxidized in
living cells and some of the chemically bound energy in it is liberated.
In two of the examples no free oxygen enters into the process, and in
two of them no carbon dioxide is liberated.

Enzyme

Sugar > 2 Molecules of alcohol + 2CO2

HHHOHH HH HH 00

I I I I Enzyme | II II II

H— C— C— C— C— C— C = O > H— C— C— OH + H-C-C— OH + C + C

0 0 0
H H H

It may be noted that the oxidation-reduction process in alcohoHc and

HO HH HH 00
H H H H

Carbon reduced Carbon oxidized

more than in more than in

sugar sugar

[Chap. XVII RESPIRATION 157

lactic acid femientation involves only the transfer of oxygen and hy-
drogen atoms between the carbon atoms within the molecule of sugar.^
Oxidation-reduction processes of this sort are said to be intramolecular
(within the molecule). When respiration occurs in the absence of free
oxygen it is said to be anaerobic (without air), in contrast to aerobic
respiration.

Evidently respiration is not always accompanied by a movement of
oxygen into the plant, or of carbon dioxide from it. When such move-
ments of these gases do occur they are merely the result of oxidation-
reduction processes in living cells. Food is either partially or completely
oxidized, and chemically bound energy in it is released mainly as free
heat energy. These are the most easily detectable facts about respiration.

Heat energy from respiration in plants is accessory. We may now ask
whether the transformation of bound energy in the food to free heat
energy during respiration is of any value to the plant. In the human
body the release of free heat energy is certainly essential to the mainte-
nance of a fairly constant body temperature of about 98° F. But the
temperature of plants and of some small animals is primarily dependent
upon the temperature of the environment, in comparison with which
the amount of heat energy liberated by respiration is insignificant. Since
respiration is as essential to these organisms as it is to man, something
more generally essential than the liberation of heat energy is involved.

Accessory features of respiration in plants. As exemplified by this chap-
ter, discussions of respiration in plants usually deal mainly with the
accessory processes: the liberation of heat energy, and the foraiation
of carbon dioxide and water, or of other easily detectable end products
such as alcohol and acetic acid. The reasons for this apparently super-
ficial treatment of the subject are two. First, the accessory features are
valuable means bv which the occurrence and rate of respiration in plants
may be measured under various environmental conditions, at different
stages of development, and during dormancy. Second, respiration is a
very complicated series of chemical processes, and many of the inter-
mediate processes and products are unknown or only partially under-
stood.

Essential features of respiration in plants. It is now known that the
oxidation of sugar to CO- and H2O in living cells involves a long series

2 It may further be noted tliat some of the incompletely oxidized products of respiration
are less oxidized than the sugar from which they are formed. Alcohol, for instance, is
reduced as compared with sugar. The formation of other reduced compounds in plants is
also dependent upon the energy transformations in respiration.

158 TEXTBOOK OF BOTANY

of intermediate steps in which successive transformations of materials
and energy occur. The hberation of heat energy is just a consequence of
this series of energy transformations in which some of the intermediate
steps are the essential ones. For instance, the electrical potentials char-
acteristic of all living active cells are dependent upon continuous
oxidation-reduction processes in the cells. The formation of the proto-
plasmic fats and proteins, tlie pigments, enzymes, and hundreds of other
compounds in cells is likewise dependent upon the energy of respiration.
The intermediate steps by which these compounds are formed are much
too complicated to be discussed here.

All the organic compounds of plants in which some of the carbon
atoms are either more oxidized or less oxidized than the carbon atoms
in the molecule of sugar, have been formed from sugar or from some of
its derivatives by oxidation-reduction processes and are, to that extent,
products of respiration. With these we may also include all the organic
compounds of plants that contain reduced nitrogen or reduced sulfur.
All the processes and compounds we recognize in living cells are the
results of series of intermediate processes and unstable compounds. Some
of these intermediate compounds are highly unstable; and unless they
are continuously renewed by oxidation-reduction processes the other
processes that we recognize in living cells will cease. The essential
features of respiration appear to be in the intermediate transformations
of energy and materials by which these unstable compounds are formed.

Summary. Respiration is an oxidation-reduction process by which the
chemically bound energy in food is transformed to other kinds of
energy upon which certain processes (chemical transfomiations ) in all
living cells are dependent. The best known of these processes are those
in which some of the food is converted to the partially reduced and
partially oxidized compounds of which the protoplasm, parts of cell
walls, pigments, enzymes, and other fundamental cell substances are
composed. Some of the energy involved in the formation of these sub-
stances remains bound within them as long as they exist, but usually a
much larger amount of the energy that is released by respiration eventu-
ally escapes from the plant as free heat energy. This free heat energy,
together with the escape of carbon dioxide, the formation of water, and
the loss in dry weight, should be regarded as an accessory consequence
of respiration in plants. The escape of heat energy is the only one of the
three that is known to occur in all cases of respiration, and there is no
evidence that it is of anv essential value to the plant. The kinds of

[Chap. XVII RESPIRATION 159

energy that are temporarily released and actively associated with chemi-
cal transformations during respiration are the ones stressed in the state-
ment: the fundamental feature of respiration is the transfomiation of
energy through the oxidation of foods in living cells. Some of the partly
oxidized and partly reduced products formed during respiration are also
fundamental features.

Respiration occurs continuously in all living active cells of plants, and
food and free oxygen are necessary. The few exceptions known are all
found among certain groups of bacteria and fungi. The food most com-
monly oxidized in respiration is sugar. The process is catalyzed by several
enzymes and there are several intermediate steps. The sugar may be
completely oxidized to carbon dioxide and water, or it may be incom-
pletely oxidized, resulting in the formation of a large variety of products,
among which are alcohol and organic acids. All products that are di-
rectly fomied by the oxidation-reduction of foods in living cells may
be regarded as products of respiration; their number is very large.

When the oxidation of food is incomplete the process of respiration is
often referred to as fermentation. When it occurs without free oxygen
it is called anaerobic; with free oxvsen, aerobic. Both anaerobic and
aerobic respiration regularly occur in a plant. Some of the fungi (yeast)
and bacteria (lactic acid bacteria) are able to survive by anaerobic
respiration alone.

REFERENCES

Needham, Joseph, and David E. Green. Perspectives in Biochemistry. Cam-
bridge Univ. Press. 1938. Pp. 114-126.
Stiles, W. Respiration. Bot. Rev. 1:249-268. 1935.

CHAPTER XVIII
USES OF FOOD IN PLANTS

ChX>C><><>C><><><J><^<X><><J><J>0^

II. RESPIRATION AND PLANT DEVELOPMENT

There is often a close correlation between the rate of growth of plants
and the rate of respiration. If any one of the conditions necessary for
respiration is not present, growth soon stops. On the other hand, the
rate of respiration may be high or low after growth has ceased in mature
plant organs, such as leaves and fruits, because it is still affected by such
factors as temperature and the amount of soluble food present.

The amount of food that accumulates in a green plant is in part
dependent upon the rate of respiration. Sugar made by photosynthesis
is oxidized back to carbon dioxide and water during respiration, and
unless photosynthesis exceeds respiration no food will accumulate in the
plant. Several external factors affect the relative rates of these two op-
posed processes. Consequently, a knowledge of these processes in rela-
tion to environmental factors may help us understand many common
plant phenomena and also the bases of certain practices in the cultiva-
tion and handling of plants and plant products.

The food that accumulates in green plants may be used by them, but
it is also available to animals and to non-green plants. The amount of
food available to the human race depends, therefore, on the amount
made by green plants in excess of what they use in their development,
and on the amount consumed by non-green plants and by all the other
animals. Man may obtain much more food per acre by eating plants
directly than by eating animals that feed on plants, because most of the
food eaten by an animal is consumed in respiration. The above state-
ments will now be considered in more detail.

Factors that affect respiration. Respiration is dependent upon the pres-
ence of available food (especially sugar) and ordinarily upon an external
supply of free oxygen. Among other important internal conditions are
the presence of certain enzymes in an active state, the acidity and water
content of the cells, and the presence of the ions of certain salts that

160

[Chap. XVIII RESPIRATION AND PLANT DEVELOPMENT 161

probably act mainly as catalysts. Of the external factors, temperature,
oxygen, and sometimes water occupy a place of first importance under
natural conditions, and also in the cultivation and care of plants/ Under
experimental conditions many factors are known either to increase or
to decrease respiration in plants.

The relation of respiration to plant development and to certain prac-
tices in the care of plants and plant products. Plants often fail to survive,
or tlieir growth is retarded, merelv because of a lack of sufficient oxvgen.
The aerial parts above ground are exposed to an abundance of oxygen
in the surrounding atmosphere, but some parts are exposed onlv to the
oxygen that is in the air of the soil. The parts of land plants that are
underground (roots, underground stems, and also seeds and seedlings)
may be deprived of sufficient oxvgen when thev are flooded bv water, or
when the ground water level rises too near the surface of the soil. The
water per se is not harmful, but it excludes nearly all the free oxygen,
and death by suflFocation is the result. Less oxygen is likelv to be avail-
able to the plant when the water is stagnant than when it is flowing.

Perhaps everyone is familiar with the fact that manv common plants
grow only where their roots are in well-drained soils, while others may
grow where the soil is saturated or even submerged. This difference
may be due to the fact that some plants, like some animals, can sur\'i\e
with less free oxygen than others. The roots of the black willow, for
instance, will grow in soils almost wholly deprived of free oxvgen,
whereas the roots of sugar maple, scarlet oak, and man\' other trees
will die of suffocation. However, the roots of most land plants grow well
in water that is continuouslv aerated.

Some interesting ideas of the growth and distribution of plants in rela-
tion to free oxygen may be gained bv observing the zones of plants on
the wet shores of shallow lakes and in low places in fields and woods
(Figs. 267, 268). Pondweeds, cattails, and bulrushes grow with their
roots continuously submerged in water, and some have roots in muck
where there is almost no free oxygen. In some of these plants large in-
ternal air passages extend from the leaves and stems all the way to the
root tips. The submerged roots of such plants may not survive unless
the oxygen that diffuses downward from the air-exposed leaves is ade-
quate. The underwater parts of these plants drown, or suffocate, if the
tops are cut off below the water le\'el.

^ Light should also be recalled in this connection because of its indirect effects on
respiration through the amount of food made, the opening of stomates, and the raising of
the internal temperatures.

162 TEXTBOOK OF BOTANY

There are other species, however, that survive without access to the
atmosphere through large internal air passages. Willows, bitter pecan,
and overcup oak are examples. Algae and other continuously submerged
plants may grow where the amount of oxygen is very low. Of course,
during the day, when photosynthesis is occurring, they have an abun-
dance of oxvgen.

The agricultural practice of draining soils is a common means of
supplying the roots with an adequate amount of oxygen. Coarse soils
are much better aerated than those of extremely fine texture. This fact is
often exemplified bv the differences in plants on such soils. The greater
root growth in the small crevices of soils is undoubtedlv related to the
better aeration there. Soaking the soil of potted house plants about once
each week not only assures more adequate watering but also results in a
fresh supply of air to the roots as the water disappears from the soil.

The storage and shipping of fruits and bulbs introduce another situa-
tion in which a deficiency of oxygen may be detrimental. The cells in
these plant organs are still alive, and their well-being depends upon a
certain amount of respiration. If fruits are packed too closely, many of
the cells die of suffocation and become brown; the color and flavor of the
fruits that survive mav become less desirable. Dark and hollow centers
in potatoes are often the result of the death of cells bv suffocation,
because of high temperature and a lack of oxvgen in storage bins. The
need of further knowledge of the relation of respiration to the best
preservation of fruits and other plant organs in storage and shipping
accounts for some of the research that is being done on the best methods
of refrigeration and ventilation in storage houses and shipping cars.
Truckers commonly transport ripe fruits at night to avoid the su£Focation
of cells in the fruit that would result from the higher daytime tem-
peratures.

When plant tissues are cut or otherwise injured, the cut or injured
surfaces are exposed to an increased supply of oxygen. The exposed
surface often becomes dark colored because of the oxidation of tannin
or other substances in the cells. In the preparation of dried fruits this
darkening is undesirable; either methods are used to prevent it, or the
finished product is subjected to a bleaching process.

Water content effects. A deficiencv of water mav influence the rate
of respiration primarilv because cell processes are dependent upon the
reactive substances being in solution in water. This condition is most
common in dry seeds. For instance, the rate of respiration in wheat seeds

[Chap. XVIII RESPIRATION AND PLANT DEVELOPMENT 163

containing 16 per cent of water may be 2.5 times as rapid as it is when
the seeds contain 15 per cent of water; and when they contain 17 per
cent of water it may be 10 times as rapid. This fact illustrates the effect
water may have on the rate of respiration in seeds during germination
and also when they are in storage, where an injurious amount of the
released heat energy may accumulate.

Temperature effects. When no other factors are deficient, the rate of
respiration is doubled for every increase of 10° C. up to a temperature
that becomes detrimental to the further survival of the plant. This fact is
recognized in refrigeration, which preserves the food value of fruits in
two ways : ( 1 ) by decreasing the rate of oxidation of the food within the
cells of the fruit, and ( 2 ) by decreasing the respiration and rate of
growth of bacteria and fungi that may destroy the fruit.

The increase in rate of respiration with increase in temperature is
often an important factor when the plant is growing, because the maxi-
mum rate of photosynthesis occurs at much lower temperatures than the
maximum rate of respiration. During the hot days and nights of summer
the rate of respiration may exceed the rate at which food is being made.
This fact may be illustrated by curves showing the relative rates of
photosynthesis and respiration in leaves of potato during a 10-minute
exposure (Fig. 62).

14

12

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O

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PHOTOSYNTHESIS IN

FULL SUNLIGHT

/

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/

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v,^

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—

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/

/

A,

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/ PHOTOSYNTHESIS IN
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TEMPERATURE IN DEGREES FAHRENHEIT
Fig. 62. Relative rates of photosynthesis and respiration in potato leaves during
10-minute exposures to different temperatures in shade and in full sunlight. Recal-
culated from data by H. G. Lundegardh.

164 TEXTBOOK OF BOTANY

The rate of respiration in this experiment increased with the increase
of temperature until the heat became injurious. If the experiment
had continued for a whole day instead of ten minutes, the first evi-
dences of this injurious effect would have become visible somewhere
between 100°-110° F. The maximum rate of photosynthesis, however,
occurred at a much lower temperature, 68° F. In connection with these
curves it is interesting to know that the maximum )^ield of potatoes is
obtained in areas that have cool summers, where the mean temperature
during the growing season is less than 65° F. Under experimental con-
ditions where the temperature is kept constant both day and night
throughout the period of growth, maximum yields of potatoes are ob-
tained at about 62° F. If the plant is exposed continuously to a tempera-
ture of 85° F., no tubers are formed. The interrelations of the different
organs of a plant with respect to environments will be discussed in
Chapter XXI.

The compensation point. Another interesting point in curve No. 3 is
at 104° F. where the rate of respiration is equal to the rate of photo-
synthesis. For convenience of reference this point is often called the
compensation point. In curve No. 2, which represents the rate of photo-
synthesis in potato at 1/25 of full sunlight, the compensation point
occurs at 90° F. This fact is of interest even when the plant is in full
sunlight, for many of its leaves are in the shade of others. These facts
illustrate the general principle that the compensation point occurs at
lower temperatures when the light intensity is low. It also occurs at lower
temperatures when the concentration of carbon dioxide is decreased.

It is perhaps evident that a plant kept throughout the day at the
compensation point would not gain in dry weight, except for the very
small amount of mineral salts that may enter it from the soil; and that
during each night there would be a loss in dry weight due to respiration
in the absence of photosynthesis. A plant kept continuously under
such conditions would soon starve to death. If the nights were cool,
it would survive longer than it would if the nights were hot.

It must also be evident that the yield of plants depends upon the rela-
tive rates of photosynthesis and respiration. Under excellent growing
conditions photosynthesis may exceed respiration in com fourfold dur-
ing the growing season. This would not occur unless photosynthesis ex-
ceeded respiration on the average about eightfold during the day. In
plants that are merely surviving in the shade of others, growth is limited

[Chap. XVIII RESPIRATION AND PLANT DEVELOPMENT 165

because respiration consumes a much larger percentage of the food
made by them in such situations.

Equally illuminating is the fact that the compensation point varies
greatly with the kind of plant. In Scotch pine it is reported to occur at
18° -20° C. when the intensity of light is 1/25 that of the full sunlight
of summer. Comparing this report with the data obtained by several
other investigators, we find that the compensation point occurs in some
of our pines, arbor vitae, and tamarack at about this same light intensity.
For white pine and red oak it is nearer 1/50 to 1/70, and for beech and
sugar maple, 1/100 to 1/200. For a few of the herbs, ferns, and mosses
that grow in dense shade, it may be as low as 1/300; and for some algae,
1/10,000 or less. In general it is lower in "shade plants" than in "sun
plants." This difference is less pronounced at low than at high tem-
peratures.

There can be little doubt that the differences in respiration and
photosynthesis in plants indicated by the compensation point are an
important factor in the survival and distribution of plants in different
habitats in relation to temperature and hght. Beech and sugar maple
endure more shade than oaks, and many oaks endure more shade than
most pines. One kind of forest therefore may gradually replace another
by producing more shade than its seedlings can endure. On lawns and
campuses many examples of differences in shade endurance by plants
may be observed. For instance, during a period of excessively hot clear
days bluegrass is favored by a small amount of shade, but it endures
more shade in the cool days of early spring and late autumn than it
does in the hot days of midsummer.

Comparative rates of respiration. The rate of respiration is greatest
where there is rapid growth, as in germinating seeds and opening
flowers. In some plant organs respiration may be. more rapid on the basis
of weight than in animals. The amount of food oxidized during a day
( == 1500 Cal. ) by a man at rest is equal to about 4 per cent of his
dry weight. The amount of food oxidized in a corn plant each day is
equal to about 1 per cent of its dry weight. The corresponding figure for
an opening cluster of flowers is about 8 per cent; and for some kinds of
germinating seeds, about 20 per cent. The lowest rates of respiration
occur in dry seeds and other dormant structures, and there is compara-
tively little respiration in woody stems and other hard parts in which
there are only a few living cells.

The amount of carbon dioxide liberated by the respiration of the

166

TEXTBOOK OF BOTANY

plants in the world each year is probably in the neighborhood of 8-10
times the amount liberated by the animals in the world during the same
time. A corn plant, for instance, oxidizes about one-fourth of the food it
makes. One-half of the food it makes is converted into fibers (cellulose,
wood, cutin, etc.) which are indigestible by the enzymes of animals.
They may, however, be digested and used as food by certain special
groups of bacteria and fungi. The fibers that are not digested by these
non-green plants become the humus of the soil. Onlv about one-fourth of
the food made by the com plant accumulates, part of which becomes
available to animals."

Food per acre in terms of available energy. With a knowledge of respi-
ration as a background we may compare the amount of food harvested
per acre on the basis of Calories of energy. In Table 4 a summary is given
of the average yield per acre, its food value calculated in Calories, and
tlie number of men that can be fed for one day by the different crops
harvested from one acre, assuming that each man requires 3000 Calories
per day.

Table 4. Energy Content of Food Products from an Average Acre

Yield per Acre

Millions of

Number of Men

Food Products

Calories

That Might Be

Bushels

Pounds

Equivalent

Fed for One Day

Corn

35

1,960

3.1

1000

Sweet potatoes

110

5,940

2.8

900

Irish potatoes

100

6,000

1.9

600

Wheat

20

1,200

1.8

600

Rice

40

1,154

1.7

560

Soybeans

16

960

1.5

500

Beans

14

840

1.1

375

If the grain from an average acre of corn is fed to beef cattle, the
dressed beef produced amounts to only 125 pounds and yields energy
equivalent to the food of 43 men for one day, a loss of 95.7 per cent of

- There is a possible error in this comparison, namely, there are many bacteria in tlie
alimentary tracts of animals that may digest the cell walls of plants. Some of the products
of these digestions pass into the cells of the animal. Some of the wood-boring beetles
have fungi in their intestines that digest cellulose, and the beetle's food is the resulting
sugars and other end products.

[Chap. XVm RESPIRATION AND PLANT DEVELOPMENT 167

the energy of the corn. If fed to pigs, the yield of pork is 273 pounds,
or sufficient food for 220 men for one day. Dairy cattle are second only
to pigs as transformers of food in proportion to the energy lost. A great
loss of energy always results when the food in plants is converted into
meat before it reaches the human consumer. It is evident that as the
population of a country increases beyond a certain point, unless it can
afford to buy meat from foreign markets it must depend more and more
directly upon the food in plants.

There are, however, certain animals that feed, either directly or indi-
rectly, on plants in which the food is physically unavailable to man.
All our sea-food animals, such as fish, clams, and oysters, convert large
quantities of the food in aquatic plants into food that is available to
man. The sheep and cattle grazing on the open range and forest reserves
in the Western States and on similar areas in other countries may be
looked upon as gatherers and converters of the food in plants that is not
otherwise available to man.

CHAPTER XIX
USES OF FOOD IN PLANTS

III. SUBSTANCES MADE FROM FOOD

All plants contain a variety of substances made from foods. Some of
them are important constituents of cell structures; others, such as
chlorophyll and enzymes, are essential in certain plant processes. Several
others seem to be of no essential survival value or detriment to the plant.
Foods are the source of building materials as well as the source of
chemically bound energy. All the organic compounds of which cells are
composed are constructed directly or indirectly from foods.

The manner in which cellulose may be formed from sugar by chemical
condensation was described in Chapter XIV. It is briefly indicated as
follows:

Glucose > Cellulose + Water

n CeHioOe > (CeHioOa)^ H2O + (n - 1)H20

This is merely a condensation process in which n represents an un-
known number. The value of n may vary in the same cell and also in
diflFerent plants. It probably represents a number greater than 50, and
perhaps much greater. After condensation all the chemically bound
energy that was in the sugar is in the cellulose.

Cutin and suberin are fat-like substances that occur in the walls of cer-
tain cells. They are formed from sugar in the same manner in which fats
and oils are formed: oxidation-reduction processes precede condensation:

Sugar > Glycerin ^^

\ > Water + Suberin or cutin

Sugar > Fatty acids

(oxidation-reduction) (condensation)

The fatty acids in suberin and cutin are unlike those in ordinary fats
and oils. Wax is foimed in a similar manner, but both the fatty acids and
the alcohol ( glycerin ) are different.

These two examples show the ways in which constituents of plant

168

[Chap. XIX

SUBSTANCES MADE FROM FOOD

169

cells are formed from soluble foods, though there are a greater number
of inteiTnediate steps of oxidation-reduction and perhaps also of con-
densation in some cases. We may generalize them as follows:

A. Soluble food

B. Soluble food —
condensation

condensation —

> oxidation-reduction
cell constituent

cell constituent

^ intermediate products

The intermediate steps in the formation of all cell substances are not
fully known, and very little is known about the formation of protoplasm.
Several different enzymes catalyze the many processes that occur. By
omitting the intermediate steps we may represent in tabular form many
of the substances formed in cells and the basic foods from which they are
made (Table 5).

Some of these substances were described in previous chapters. A few
additional facts will be briefly summarized.

Table 5. Substances Made Within the Plant from Sugar

Food Used

Chemical Processes

Substances Made

Where Found in the Plant

Glucose

Condensation

Cellulose

Cell walls of all tissues

Sugars

Condensation

Hemicelluloses

Cell walls of many tissues

Sugars

Oxidation-reduction and
condensation

Pectic compounds

Cell walls of all tissues

Sugars

"

Lignin

Cell walls of wood tissues

Sugars

»

Suberin

Cell walls of cork

Sugars

»

Cut in

Epidermal cell walls

Sugars

»

Wax

Epidermal cell walls

Sugars

»

Carotenoids

Green and yellow tissues

Sugars

»

Anthocyanins

\'acuoles of many tissues

Sugars

»

Essential oils

General or localized

Sugars

»

Mucilages

Many tissues

Sugars

»

Resins

General or localized

Sugars

»

Gums

General or localized

Sugar and

proteins

»

Alkaloids

Generally distributed

Sugar and

proteins

»

Enzymes

All living cells

Sugar and

proteins

»

Hormones

Variously distributed

Sugar and

proteins

»

\ itamins

All living cells

All foods

Protoplasm

All living cells

Cellulose. During the formation of new cells in all growing parts of
plants, enormous quantities of food are annually converted into proto-
plasm, enzymes, pectic compounds, and cellulose. About one-half of
the food made by a green plant is converted into these substances. More

170 TEXTBOOK OF BOTANY

than one-third of it is converted into cellulose alone. The youngest cell
walls of nearly all plants are composed primarily of pectic compounds
and cellulose. If thin sections of a plant are treated with a strong solution
of chlorozinc iodide, or with a solution of iodine followed by 70 per cent
sulfuric acid, and then observed through a microscope the location
of cellulose in the cell walls is often indicated by a blue color. If pectic
compounds are abundant in the wall, a light brown color is produced.
In some cell walls it is necessary first to remove lignin, cutin, or other
substances that interfere with the test for cellulose.

Cellulose is formed in every cell of all plants, except in certain groups
of algae (diatoms) and perhaps in certain bacteria. It is also found in
the cell walls of a few species of animals. When formed it becomes a
part of the permanent framework of the cell, for there are very few
organisms that possess the enzyme celhdase, that digests cellulose. The
digestion of cellulose in the alimentary tracts of animals is due to the
presence of certain non-green plants and sometimes certain protozoa.
In contrast to cellulose, hemicelluloses may be formed as secondary
thickenings on the cell walls of a plant and later digested to sugar. They
are most conspicuous in certain kinds of hard seeds, such as the seeds
of dates, palms, coflFee, and iris. The manufacture of cloth, paper, cellu-
loid, collodion, acetic acid, artificial rubber, charcoal, rayon, cellophane,
and explosives is but a few of the hundreds of industrial uses made of
cellulose.

Pectic compounds. Apparently pectic compounds are present in the
walls of all plant cells. They consist of large molecules formed mainly
by the condensation of certain sugars and an acid derived from sugar.
Various amounts of pectic compounds are usually intemiingled with
cellulose in the walls of plants. The middle lamella, which holds the cells
of plants together, is composed of pectose or of calcium pectate. The
softening of fruits and vegetables when boiled in water or as they ripen
under natural conditions or in storage, is due to the breaking down of
this layer and the partial separation of the individual cells. A similar
process occurs in the absciss layer at the bases of the petioles of leaves,
pedicels of fruits, and at the bases of petals and stamens of flowers.
The pistils of flowers often abscise in the same way, and no fruits are
formed. In simple structures, such as algae and root hairs, the pectic
compounds are mainly in the outermost exposed layer of the cell walls.
Many plant mucilages and gums are also composed largely of pectic

[Chap. XIX SUBSTANCES MADE FROM FOOD 171

compounds. Changes that occur in pectic compounds in plants may be
represented briefly.

Pectose > Pectins > Pectic acid.

Pectic acid + Calcium salt > Calcium pectate.

Fungi that live parasitically within green plants are often found only
in the layer of pectic compounds between the cells. There they digest
and use these compounds as food. The isolated cells of the host die and
disintegrate. A similar process may result in overripened fruits and vege-
tables wherever the enzymes that digest pectic compounds are present.
The retting of flax fiber is merely a process of breaking down the pectic
compounds that hold the fibers together.

Large quantities of pectin are extracted annually from plants and
used in the preparation of jellies, salads, creams, and emulsions desired
as foods and drugs. Most of the commercial pectin, such as Certo, is
extracted from apple pomace or cull lemons. A smaller amount is ob-
tained from the pulp of oranges, sugar beets, carrots, and algae.

Other cell wall constituents. Suberin, the substance that is character-
istic of cork, apparently occurs in a definite layer in the cell walls of
cork tissue. It may also become deposited on or within cellulose walls,
a process called suberization. Several other substances, such as lignin,
cutin, wax, resins, and tannins, may become intermingled with cellulose
and pectic compounds as the walls increase in age. The cutinized layer
of the exposed surfaces of epidermal cells is called the cuticle. Suberin,
cutin, wax, and resin when present in or on cell walls make them less
permeable to water and restrict the entrance of parasites. These sub-
stances often constitute a relatively large part of the vegetable material
that ultimately becomes transformed to coal, for only a few kinds of
organisms can digest them.

Lignin is characteristic of woody cells and of hard plant tissues in
general. Its presence is easily demonstrated by treating plant tissue with
a dilute solution of phloroglucin followed b\' strong hydrochloric acid;
lignified walls become brilliantly red with this treatment. One of the
problems of obtaining cellulose from wood is the difficulty of removing
lignin and other substances that are associated with it in the cell walls
of wood.

Pigments. In addition to the chlorophylls, carotenoids, and antho-
cyanins discussed in Chapter IV, many other pigments are formed from
food in plants. Some of them are important as co-enzymes in oxidation-

172

TEXTBOOK OF BOTANY

reduction processes. Others appear to be of no particular value or detri-
ment to the species in which they are formed. Before chemists learned
how to make dyes in the laboratory man was dependent solely upon these
natural dyes. Among the more important are annatto, camwood, fustic,
henna, indigo, litmus, logwood ( haematoxylin ) , madder, red sandal-
wood, saffron, and tuiTneric.

Resins, gums, and mucilages. Resins are insoluble in water and render
cell walls impervious to it. They occur usually in definite glandular struc-
tures, or in tubes extending throughout the plant. Amber is a fossil resin.
Resins are soluble in alcohol and certain oils. Commercially they are used
in the preparation of a variety of varnishes, soaps, dyes and medicines.
Resin is one of the important products of the pine forests of the south-
eastern United States (Fig. 63).

Fig. 63. Method of tapping southern long leaf pine to collect resin.

Gums and mucilages are much alike. They are insoluble in alcohol
and become jelly-like in water. Upon drying, gums first become sticky
and then hard. Mucilages swell greatly in water. They may be im-
portant in holding water in the plant tissues in which they occur. Com-
mercially they are important bases of various mucilages and chewing
gum.

Latex. Many plants, such as the milkweeds, euphorbias, figs, and
rubber plants, have a milky colloidal "juice" called latex. This is a mix-

[Chap. XIX

SUBSTANCES MADE FROM FOOD

173

tuie of resins, gums and foods. It is the source of commercial rubber

(Fig. 64).

Fig. 64. Interior view of a forest on a rubber plantation in the East Indies. The
trees have been tapped to collect the latex from which rubber is made. Photo from
Field Museum of Natmal History.

Essential oils. The odors of flowers, and the odors and flavors of fruits
and vegetables are due in part to minute quantities of essential oils.
They are often associated with resins and gums and are very unlike the
liquid fats ( fatty oils ) . They are of no importance as food, but are highlv
prized as flavors and perfumes, and for a few medicinal and industrial
uses. You are probably familiar with menthol, camphor, turpentine, the
characteristic oils of mint, pine, juniper, wintergreen, geranium, lemon,
orange, ginger, anise, cloves, sage, hops, wormwood, lavender, berga-
mot, bitter almonds, and vanilla. Some of the essential oils contain
sulfur. These underlie the odor and flavor of onion, garlic, watercress,
radishes, and many kinds of mustard.

The edible parts of many plants, such as sweet corn, turnips, peas, and
others, when first harvested have a delicious flavor that is lost very
rapidly. Some of this loss in flavor is due to the disappearance of essen-
tial oils, but a part of it is the result of the oxidation of sugar and of

174 TEXTBOOK OF BOTANY

the rapid change of sugar to starch in the harvested product. The higher
the temperature the more quickly the change occurs.

Alkaloids. Under this general name may be grouped a large variety
of chemical substances apparently of little or no value to the plant in
which they are made, but which have been of great importance in
medicine. They are nitrogen-containing organic compounds, derived
from proteins, generally odorless but bitter to the taste. They have
marked physiological effects upon animals, and are extensively used as
stimulants and narcotics. The best known are nicotine, from tobacco;
atropin, from nightshade; strychnine, from strychnos; cocaine, from
coca leaves; quinine, from chinchona bark; morphine and codeine, from
the poppy; and caffeine, from coffee and cacao seeds.

Tannins. The bark of many trees, the galls occurring on oaks, and
certain unripe fruits, such as the persimmon, contain bitter astringent
substances known as tannins. These substances coagulate proteins to
insoluble compounds. The coagulation of proteins in raw hides is the
basic process in the tanning of leather. With iron salts, tannins become
black or green in color. Ink was formerly made in this way. The freshly
exposed surfaces of many fruits and vegetables become dark because of
the oxidation of tannic acid, especially in the presence of a trace of iron
from a knife.

Enzymes. The importance of enzymes in all metabolic processes in
plants has already been indicated. Their chemical composition is un-
known, but many of them seem to be protein-like. We know of their I
presence only through the effects they produce. Likewise, they are
named either on the basis of the kind of substance upon which they act
or the kind of action which they initiate or accelerate, or both. For
instance, sucrase acts upon sucrose, and oxidases accelerate the process
of oxidation. Many enzymes are very specific, acting only in one kind
of process. In all, there must be several hundred, perhaps thousands,
of them. They are primary factors in numerous biological processes. A
few of the common digestive enzymes in plants, the substrate ( substance
acted upon), and the end product are presented in Table 6.

Vitamins. These organic substances, which are formed in plants in
minute quantities, were first discovered through their influence on the
health of man and other animals. The chemical composition of some
of them has been discovered. Vitamins are like enzymes rather than
food; that is, they are necessary not as building material or as a source
of energy, but rather as catalysts of certain basic biological processes
underlying health, growth, and reproduction. Scurvy, beriberi, and

[Chap. XIX SUBSTANCES MADE FROM FOOD 175

Table 6. Enzymes, Subtrates, and End Products of Enzyme Action

Enzyme

Substrate

End Products

Diastase

Maltase

Sucrase

Inulase

Hemicellulases

Pectosinase

Pectase

Pectinase

Tannase

Lipase

Peptase (pepsin)

Ereptase (trypsin)

Starch

Maltose

Sucrose

Inulin

Hemicelluloses

Pectose

Pectin

Pectic acid

Tannin

Fats and oils

Proteins

Proteins and peptones

Glucose

Glucose

Glucose and fructose

Fructose

Simple sugars

Pectin

Pectic acid

Simple sugars and acids

Glucose and gallic acids

Glycerin and fatty acids

Soluble peptones

Amino acids

rickets were the first symptoms of vitamin deficiency to be clearly demon-
strated.

Vitamins, or their precursors, are manufactured mostly by plants. They
accumulate in certain animal tissues and in milk from the plants eaten by
the animals. Carotene is a precursor of vitamin A, and ergosterol of
vitamin D. The bacteria in the alimentary tracts of animals probably
manufacture certain vitamins.

Methods of detecting the manufacture of vitamins in plants and their
effects upon the development of plants are not well established. There is
evidence, however, that vitamins also affect plants in certain funda-
mental ways.

Hormones. These substances constitute another group of organic com-
pounds that produce profound physiological effects when present in
extremely minute amounts. Like the enzymes and vitamins, they influ-
ence certain basic biological processes. Their presence or absence is
recognized by certain symptoms of development. They were first defi-
nitely discovered in animals as secretions of certain glands. In 1928 the
presence of similar substances in plants was definitely established. Three
of them have been isolated from plants and their chemical constitution
is known. They were called auxins; others have been called calines; but
in a general textbook we shall refer to all of them as hormones.

Hormones influence the enlargement of cells, the formation of roots,
and the dominance of certain parts of plants over others. There are prob-
ably very few, if any, developmental processes in plants that are not
influenced by them, but many experiments will have to be performed

176 TEXTBOOK OF BOTANY

before summary statements can be made. They are soluble in water and
may pass from cell to cell; hence they may be made in one part of the
plant, such as leaves, and influence the development of cells in another
part of the plant. General effects resembling those due to natural hor-
mones may be brought about by closely related compounds sold by
chemical supply companies.

Such substances as enzymes, vitamins, and hormones are the modern
substitutes for the earlier assumptions of special vital forces and en-
telechies. Not only are development and health dependent upon the
presence of these substances, but certain differences in species are de-
pendent upon their systems of enzymes, vitamins, and hormones.

Assimilation. It is often convenient to have one word by which we
may refer to the transformation of food into the substances of which cells
are composed. The term assimilation has been used with that intent in
biological sciences for a long time. But it has also been given different
interpretations as the knowledge of plant and animal nutrition advanced.
One therefore finds it used today as it might have been used a century
ago, and also as it probably should be used on the basis of present
knowledge. A number of other expressions likewise became associated
with the concept of assimilation at different historical periods, and they
have also survived to the present time. Other biological terms, such as
food, respiration, osmosis, adaptation, natural selection, and many others
have a similar history. All of them, as well as the associated expressions,
need to be critically examined by each succeeding generation of students.

When protoplasm was first recognized it seemed so all-important,
especially in the animal body, that the term assimilation was, and still
is, sometimes restricted to the transformation of food into protoplasm
alone. This is a definite use of the term; but to adopt it we must seek
another term to refer to the transformations of food into cell walls and
other essential constituents of cells. It seems most convenient to use one
term to include all of these processes in both plant and animal bodies.

The greatest confusion arose before the discover\^ that the food of
green plants and of man is essentially the same, regardless of the manner
in which they obtain it. Assimilation is really one phase of growth that
is common to all plants and animals. It is quite different from the forma-
tion of sugar from CO2 and H-O or of amino acids and proteins from
sugar and certain inorganic salts. That is, food manufacture and the
subsequent use of food in assimilation are two different groups of bio-
logical processes.

[Chap. XIX

SUBSTANCES MADE FROM FOOD

177

We have a sufficient background now to see that the food of green
plants and man are the same; that food consists of organic substances
that may be used as a source of chemically bound energy and as material
that may be directly transformed into protoplasm, cell walls, or other
essential constituents of cells. Accumulated insoluble food must of
course first be digested in either case.

Water and inorganic salts (fertilizers) which are essential to plants
in many ways are not a source of energy to green plants. They may
become incorporated in cell structures more or less directly or through
their use as raw materials in food manufacture. Nitrates and sulfates,
for example, are utilized in the manufacture of amino acids and proteins
which in turn are assimilated in the making of protoplasm. Attempts to
include carbon dioxide, oxygen, water, and inorganic salts under the
term food result only in rendering the term meaningless and useless.
Some of the ways in which inorganic salts may affect plants have already
been noted; others will become evident later.

Composition of a plant. All reports of the chemical composition of
plants and of protoplasm have at least two defects: (1) no distinction
is made between accumulated food and assimilated materials, and ( 2 )
inorganic salts that have accumulated in crystals or are merely in solu-
tion in the water in the plant are not distinguished from those that
have become an integral part of the cell structures.

With these defects in mind, the elemental composition of a corn plant
that has attained full growth is presented.

Corn Plant

Water

79.7%

Organic matter 19.5%
Mineral elements 0.8%

Dry Matter
20.3%

' Carbon

44.58%

Oxygen

43.79%

Hydrogen

6.26%

Nitrogen

1.43%

Potassium

1.62%

Phosphorus

.25%

Calcium

.59%

Magnesium

.44%

Iron

10%

Sulfur

.05%

Chlorine

.20%

Sodium

.15%

Silicon

.54%

Organic matter as compounds:

Carbohydrates and fiber 17.2%
Fats 0.5%

Proteins 1 . 8%

178 TEXTBOOK OF BOTANY

From these figures and the facts in the last few chapters it is evident
that 94.63 per cent of the dry matter in the corn plant had its origin in
the sugar made from carbon dioxide and water. As the plant stood in the
field, only 0.8 of 1 per cent of its weight was due to inorganic salts that
had passed into it from the soil, and some of these were present in excess
amounts. But one should not infer from this fact that these salts are not
of much importance to the plant. It is merely another case in which a
relatively small amount of something is essential to the development of
the plant. A deficiency is soon reflected in slow growth or even death.

Such phenomena as the growth of seedlings rooted in pure sand, of a
large plant with roots confined in a small pot of soil, or aerial epiphytes
and free-floating algae, or, on the contrary, the failure of lawn grass to
grow in dense shade even when fertilizers and water are added, may all
be adequately explained on the basis of the facts in the last few chapters.

Summary. In these last four chapters we were concerned primarily
with the manufacture and uses of food in plants. The ways in which
man and other animals are dependent upon food manufactured in plants
were also noted. Both plants and animals are dependent upon food as
a source of chemically bound energy which is liberated by respiration,
and as a source of building material in assimilation. Owing to the gross
similarity of protoplasm in plants and animals their food is essentially
the same. It consists of carbohydrates, fats, and proteins, and any of their
derivatives that may be used in respiration or assimilation. The range of
these substances that can be used by an organism as a source of food
depends upon its system of enzymes. Some organisms, for instance, can
digest substances that are indigestible to others.

Assimilation may be considered as the transformation of food into
protoplasm, cell walls, and other essential constituents of cells, such as
enzymes, chlorophyll, vitamins, and honnones. During this transforma-
tion certain inorganic salts are variously combined with the organic com-
pounds. Some authors use the term to denote only the transformation
of food into protoplasm, thus leaving no general term to designate the
transformation of food into other essential parts of the cell. Assimilation
should be strictly distinguished from food manufacture and also from
digestion.

The substances in the cell walls of plants are constructed primarily
from sugar. An enormous quantity of food is annually converted into
cellulose. In the making of protoplasm all types of foods are assimilated,
and in addition water and the ions of inorganic salts are incorporated

[Chap. XIX SUBSTANCES MADE FROM FOOD 179

in the system. Chemically, assimilation of foods consists of condensation
processes or of a combination of oxidation-reduction and condensation
processes. Assimilation is one phase of growth. About one-half of the
food made by a plant is converted into its own tissues. Perhaps the
amount is often greater than one-half.

The known cases in which food, enzymes, vitamins, and hormones are
essential to certain basic processes exemplify an extremely intricate phase
of the chemistry of biological phenomena. Many of the substances that
affect the reactions in living cells are known, but relatively few of the
details of the many chains of reaction that occur are known. Enough
facts have been discovered, however, for one to learn ( 1 ) to relate cer-
tain symptoms of development to the presence or deficiency of particular
substances, and ( 2 ) to know many of the major links in numerous chains
of reaction. As this kind of knowledge increases, the bases of certain
types of superstitions and assumptions about living cells disappear.

REFERENCES

Booker, L. E., et al. Vitamin needs of man. Food and Life. Yearbook of Agri-
culture, U.S.D.A. 1939. Pp. 221-271.

Clute, W. N. Useful plants of the world. W. N. Cliite Co. 1932.

Daniel, E. P. Vitamin content of foods. Food and Life. Yearbook of Agricul-
ture,'U.S.D.A. 1939. Pp. 286-295.

Good, R. Plants and human economics. Cambridge Univ. Press. 1933.

Hitchcock, A. S. How plants are used by men. Pt. 1. Old and New Plant Lore.
Smithson. Inst. Series. 11:97-111. 1934.

Saunders, C. F. Useful wild plants of the United States and Canada. Robert
M. McBride & Company. 1930.

CHAPTER XX
SOME BIOLOGICAL RELATIONS OF GREEN PLANTS

<>C><X>0<X><><^C><><><><>^^

In the previous chapters many facts about the dependence of animals
and non-green plants upon green plants became evident from time to
time. A consideration of several of these facts in one chapter will help
us obtain a better perspective of their interrelations. Perhaps this can
best be done by reference to two concrete examples. For one example
we may choose the so-called balanced aquarium or microcosm; for the
other we may consider man on some populated island, or in some specific
country such as the United States.

To understand these examples we must be able to recall and relate
the facts about the following processes:

The manner in which food is obtained by plants and animals.

The making of sugar by photosynthesis in green plants.

The synthesis of proteins, especially the synthesis of amino acids.

The synthesis of vitamins in plants.

The assimilation of food to cell constituents in both plants and animals.

The oxidation of foods in respiration in both plants and animals.

The transformations of energy in foods and products of assimilation.

The loss of material and energy when plants are not eaten directly

bv man.
The ways in which man obtains and transforms energy.
The transformation of energy-containing materials to peat, coal, and

gas.

A microcosm. To set up a microcosm all that is necessary is to obtain
a 3- to 5-gallon bottle; fill it about one-fourth full of pond water; add a
few ounces of soil, a goldfish, and a quantity of algae that are edible by
fish; seal the bottle air-tight and set it in a well-lighted part of the room,
but not in bright sunlight and not where its temperature will often
exceed 70° F. The fish and algae will then continue to live and grow in
the bottle for months or even years (Fig. 65).

With the pond water and soil we incidentally place numerous bac-

180

[Chap. XX BIOLOGICAL RELATIONS OF GREEN PLANTS 181

teria and small animals in the bottle. We might avoid many of these or-
ganisms by using sterilized water and pure chemical salts, but some of
them are also in the gelatinous walls of the algae, upon the surface of
the fish, and especially in its alimentary canal.

Fig. 65. Diagrams of microcosms in sealed containers in which the organisms
indicated live for many months: A, algae; B, algae and a fish; C, Helxine
(Paddy's wool).

We may ignore these small organisms for the moment and begin with
the fish and algae. The statement that "the fish and algae each give off
what the other needs," is both false and misleading. Of the many things
necessary for the existence and growth of the algae in the bottle, only
one of them comes in part from the fish, but the algae are not dependent
upon the fish or any other animal for even that one. To explain the
microcosm it is necessary to account for (1) the oxygen used by both
the fish and the algae in the process of respiration, ( 2 ) the food used by
botli fish and algae in respiration and assimilation, (3) the carbon di-
oxide used by the algae in photosynthesis, (4) the energy used by the
plant in photosynthesis, ( 5 ) the chemically bound energy used by both
fish and algae, and (6) the conditions under which respiration and
assimilation in both fish and algae are compensated by photosynthesis in

182 TEXTBOOK OF BOTANY

the algae. Without this compensation the fish would soon starve to death.
It is assumed that the reader can account for all six of the conditions
listed above. Perhaps vitamins should also be considered.

Since an adequate supply of inorganic salts was included in the soil,
they may be ignored in this discussion. The presence of bacteria that
cause decay undoubtedly prolongs the duration of the microcosm. They
may digest and oxidize cell structures that are not acted upon by the
enzymes of the fish and algae, and by this means delay the time at
which the supply of carbon dioxide in the bottle becomes inadequate.

Perhaps it is evident that the algae and certain decay-producing bac-
teria could exist together in this bottle without any animal being present.
The animals are dependent upon the plants, but the plants are not de-
pendent upon the animals. They were placed in the bottle merely to
imitate natural conditions. Even if all the animals in nature were de-
stroyed, most of the plants would still survive indefinitely. Only those
that are dependent upon the cross-pollination effected by animals, and
those that are strictly parasitic upon animals would perish.

A human cosmos. Our second case, man in the United States, is some-
what more complicated because man uses plant products and the
chemically bound energy of plants in many ways in which the fish does
not. Food is used in the body of man in the same manner in which it is
used in the body of a fish: as a source of chemically bound energy, and
as a source of material that is transformed into the substances of which
cells are composed. Moreover, food is used in this same manner in the
bodies of the many domesticated animals which man employs in various
ways. The dependence of man and his domesticated animals upon green
plants for sugar, amino acids, vitamins, and a supply of chemically bound
energy necessary to body processes probably does not need further
emphasis.

Many of the uses which man makes of the material products of plants
are also familiar to all of us. That plants are the main source of all the
energy employed by man is not so well appreciated. The remainder of
this chapter, therefore, will be devoted primarily to his dependence upon
plants as a source of energy.

In addition to the energy that is transformed in the bodily processes
of man and his domesticated animals, man employs energy as heat, light,
electricity, and mechanical power in his home, in public buildings, on
the street, on the farm, in the factory, and on the highways. Less than
10 per cent of all this energy is obtained from wind and water power.

[Chap. XX BIOLOGICAL RELATIONS OF GREEN PLANTS 183

More than 90 per cent of it may be traced back through various energy
transformations to the hght energy that was chemically bound in sugar
during photosynthesis. Many questions relevant to these facts may be
considered.

How efficient are plants as transformers of light energy? How rapidly
is man releasing the chemically bound energy of plants? Is the present
civilization releasing it more rapidly than it is being bound by plants
today? Is all the energy bound by plants available to man? Has the re-
placement of the horse by the automobile and tractor — which use the
energy bound by the plants of the past — altered the value of the energy
bound by plants of the present? What of the future?

The answer to any one of these questions must be an approximate
one; but an estimate based upon available data is so much better than a
mere guess or no answer at all, that space will be given to a tabular
summary of a few data and certain general statements based upon data
too numerous to be presented here ( Table 7 ) .

The acre of corn referred to in the table was located in central Illinois.
The yield of 100 bushels of grain on this acre is three times the average.
The whole plant is considered in the calculations. The acre of apple
trees was located at Ithaca, New York.

Most all of the chemically bound energy that accumulates in the com
plant and apple tree would ultimately be available to non-green plants,
for as a group of organisms they may digest and oxidize most of the
substances made in the green plants. Only about one-fourth of the
energy that accumulates in a corn plant is available to man as a source
of bound energy in food, and still less of the energy that accumulates
in the young apple tree is similarly available. As a source of energy in
fuel, however, the amount that is available to man depends upon how
meticulously he harvests and uses the different parts of the plant. In
one country the roots may be left in the soil and the smaller branches
may be heaped in piles and burned, while in another where the supply
of chemically bound energy is scarce and expensive the smaller twigs,
roots, and even the leaves may be collected for fuel. In the early days
pioneers on our own prairies and plains sometimes used the stems and
leaves of native grasses as fuel. But regardless of how meticulously man
harvests green plants, other animals and the non-green plants will take
a certain toll. The insects in a pasture, for instance, may eat almost as
much grass as do grazing animals.

184

TEXTBOOK OF BOTANY

Table 7. A Comparative Summary of Energy Transformations in an Annual
Herb and in a Woody Perennial

Acre of Corn
(Illinois)

Acre of
Apple Trees
(New York)

Length of growing season

Number of plants per acre

Average leaf area is attained in

Total radiant energy available

Total energy bound in sugar

Percentage of light energy bound in
photosynthesis

Photosynthetic efficiency based on part
of spectrum that is effective in photo-
synthesis

Average amount of sugar made per day

Total sugar made per season

Chemically bound energy released by
respiration in the plant

Percentage of energy bound by photo-
synthesis released within the plants
by respiration

Of the sugar made each season there is
oxidized by respiration in the plant

The average rate of photosynthesis (l!2
hrs.) compared with respiration (24
hrs.)

Bound energy in the structures and ac-
cumulated food in the plants

Percentage of total energy of photo-
synthesis in accumulated foods

Percentage of total energy of photo-
synthesis accumulated in structures

100 days
10,000

50 days
2,043 million Cal.
33 million Cal.

1.6%

188 days
400

28 days
2,718 million Cal.

30 million Cal.

1%

8%
200 pounds

about 10 tons

5.5%
93 pounds

about 8.7 tons

8 million Cal.

10 million Cal.

23.4%

33.3%

about 3^

about 3^

about 8 times

about 6 times

25 million Cal.

20 million Cal.

about 25%

about 50%

Sources and amounts of energy. Several investigators have analyzed
plants and calculated the amount of carbon and bound energy in them
per acre. From all these data it seems that an average acre of corn
accumulated a little more than 8 million Calories of energy, or about
one-third as much as the acre of corn described above; and that in all
kinds of crops the average annual accumulation of energy per acre is
about 6 million Calories. In an acre of forest it is about 10 million

[Chap. XX BIOLOGICAL RELATIONS OF GREEN PLANTS 185

Calories.^ By using these figures for the country as a whole and adding a
httle extra for range and desert vegetation, we find that the green plants
of the United States accumulate about 10 X 10^' Calories of energy
annually. This is approximately 70 per cent of the energy chemically
bound by them in photosynthesis.

Through the use of plants as food for himself and his domesticated
animals, and in several other ways, man in the United States annually
destroys plants containing about 6 X 10^'' Calories of bound energy. In
addition to this the coal, oil, and gas removed from the earth each year in
this country have an energy value of more than 5 X 10^^ Calories.
From these calculations we seem to be responsible directly or indirectly
for the dissipation of chemically bound energy about as rapidly as it
accumulates in the green plants of today. To this we must add the amount
released by undomesticated animals and a part of that released by non-
green plants.- What of the future?

Energy transformations and conservation. In the early years of the
present century the chief energy transformers in industry and transpor-
tation were the steam engine and the horse. Coal was often used for the
supply of chemically bound energy in the steam engine, but the use of
wood obtained from nearby living forests was not uncommon. All the
chemically bound energy transformed into mechanical energy in the
horse came each year from the living plants of the farm. Horses were
used for practically all transportation except that upon railways and
the sea. The farmer not only harvested the chemically bound energy
which he used on his own farm, but in addition he sold enormous quan-
tities of it for horses employed in various ways in the city. Since the
horses also came from the farm, the farmer furnished both the energy
and the transformer. With the coming of the automobile and tractor,
conditions were reversed. The farmer now buys from industry not only
the transformer ( motor ) but also the chemically bound energy ( gas and
oil) that was stored in plants millions of years ago. Such drastic changes
in the sources of energy have been reflected in economic relations for
many years. Moreover, there is a limit to the natural deposits of chem-
ically bound energy in coal and related products.

^ If the reader wishes to calculate the equivalent of Calories of energy in terms of grams
of sugar, he can do so by multiplying by 180/674 or by dividing by 3.75. Grams of sugar
may be stated in terms of COo used in photosynthesis by multiplying by 264/180. See the
equation of relative weights in Chapter XIII.

- One may also consider this problem on the basis of whether plant residue ( humus in
the soil, peat, coal, etc.) is accumulating today as rapidly as it is being destroyed. It did
accumulate more rapidly than it was destroyed before man came upon the scene.

186 TEXTBOOK OF BOTANY

The major transformations of chemically bound energy among living
organisms may be summarized diagrammatically.

Released by respiration in
green plants, animals, non-
Radiant energy j Chemically bound ^^ \ green plants; and by fire

[ green plants ^--^^^ [ Retained in unoxidized plant

and animal residues, such as
humus in soil, peat and coal,
petroleum, natural gas

All animals eat large quantities of food in proportion to their gain in
weight. Evidently much more food is used in the animal body as a
source of energy than as a source of body material. Every time one
animal eats another it secures usually less than 10 per cent of the energy
the first animal obtained directly from plants. If a third animal eats the
second one, it derives less than 1 per cent of the energy in the plants
eaten by the first one. The animal population of a region cannot exceed
the supply of the chemically bound energy in the plants of that region
except in those instances v^here man is able to secure food from a foreign
market.

Through such procedures as plant breeding, conservation of the soil
and water, the use of fertilizers, and improved methods of forestry, the
annual accumulation of chemically bound energy in plants may be
greatly increased, perhaps doubled or even more. But these procedures
require intelligent effort and foresight, and the elimination of wasteful
exploitation of natural resources.

Summary. In the world of living organisms green plants have a unique
position. Except for their dependence upon certain groups of bacteria,^
and the relatively few plants requiring cross-pollination by animals, they
are independent of other organisms. From one point of view all animals
are parasites upon the plant kingdom. Animals are not essential to
plant life. But animals and non-green plants are dependent upon the
green plants for chemically bound energy and certain synthesized prod-
ucts. Man in particular is dependent upon green plants for sugar, amino
acids, vitamins, precursors of vitamins, and, up to the present, for more
than 90 per cent of the energy he utilizes. In addition, there are thou-
sands of uses made of plants and plant products for convenience, recrea-
tion, and art. We are passing through a stage of development in this

^ The importance of certain bacteria to green plants will become evident in the chapters
on non-green plants.

[Chap. XX BIOLOGICAL RELATIONS OF GREEN PLANTS 187

country in which ignorant, indifferent, uninformed, and ruthless exploi-
tation of plants and plant environments should speedily be replaced by
intelligent efforts at conservation based upon a knowledge of plants and
of their biological relations.

REFERENCES

Heinicke, A. J., and N. F. Childers. The daily rate of photosynthesis of a young
apple tree of bearing age. Cornell Univ. Agr. Exp. Sta. Memoir 201. 1937.

Schroeder, H. Die Naturwissenschaften. 70:8-12, 23-29. 1919.

Transeau, E. N. The accumulation of energy by plants. Ohio Jour. Sci. 26:1-10.
1926.

U. S. D. A., Various Yearbooks contain data on crop yields, animal popula-
tions, and industry. Since 1935 these data have been published annually in
a volume entitled Agricultural Statistics.

CHAPTER XXI
INTERRELATIONS OF THE PARTS OF A PLANT

One part of a plant may be dependent upon processes that occur in
another part. Roots and leaves are mutually dependent. Consequently
the effects of an environmental factor upon the processes in one part of
a plant may influence the development of other parts. We now have a
suflBcient background of facts to understand some of these interrelations;
others will be postponed to later chapters.

Interrelations involving movement of materials. Several of these inter-
relations are known to be dependent upon the movement of materials
from one part of the plant to another. Many of them depend upon the
movement of sugar from the green to the non-green parts of a plant. The
growth of seeds, roots, and underground stems depends upon sugar
made primarily in the leaves. Even green fruits and green stems, except
in such plants as cacti, are dependent in part upon sugar from the
leaves. The influence of any factor upon the making of sugar in the
leaves may be reflected in the growth of all these organs. The processes
in the leaves, in turn, are dependent upon the movement of water and
inorganic salts from the roots. The relative growth of the aerial stems,
leaves, and fruits also depends in part upon which ones get the major
supply of these substances from the roots, especially when the supply is
low or inadequate.

We may begin the analysis of interrelations by citing a number of
growth phenomena which are dependent in large part (1) upon the
manufacture of sugar in the chlorenchyma and its movement to other
parts of the plant, and ( 2 ) upon the manufacture of proteins from this
sugar. The correct interpretation of some of these examples may be evi-
dent as rapidly as they are read; others will be correctly interpreted
only if the reader pauses long enough to analyze the problems in con-
siderable detail.

Specific examples of interrelations. Under good growing conditions the
largest Jonathan apples are not obtained if there are fewer than 30
leaves per apple. The largest Delicious apples are not obtained if there

188

[Chap. XXI INTERRELATIONS OF THE PARTS OF A PLANT 189

are fewer than 50 leaves per apple. For the largest Elberta peaches
about 30 leaves per peach are necessary. For the best cluster of grapes
there should be not less than 12 to 15 leaves per cluster.

The roots and underground stems of troublesome weeds, such as
dandelion and thistle, will die of starvation if the green tops are removed
frequently and immediately after they begin to develop, or if they are
densely shaded by another plant such as alfalfa. In interpreting this
fact it should be remembered that the synthesis of starch, fats, and
proteins may occur in roots and underground stems, and that any of
these three substances may be transformed to sugar in the cells of these
organs. An adequate interpretation of this fact and of several of those
that follow involves, therefore, an explanation of why the roots and
underground stems cannot make starch, fats, or proteins and survive
upon them as food when the tops of the plants have been removed or
when the rate of photosynthesis is continuously low.

A good growth of Irish potato tubers is not obtained if the potato
plants are continuously exposed to high temperatures ( Fig. 66 ) or dense

Fig. 66. Effects of different temperatures on the formation of potato tubers. Photo

by John Bushnell.

shade, or if the soil contains an abundance of nitrates. Under any of
these conditions the tops (aerial stems and leaves) may grow fairly
well; with a high nitrate supply there is usually a luxuriant growth of
tops. Under experimental conditions a low yield of tubers also results

190 TEXTBOOK OF BOTANY

when the potato plant is exposed to a very low concentration of carbon
dioxide.

As a rule there is a much greater growth of tops in proportion to the
growth of roots when plants in a moist climate are exposed to a high
temperature, dense shade, high nitrate supply in the soil, or to a low con-
centration of carbon dioxide in the air. On the contrary, there is a rela-
tively greater growth of roots in proportion to tops when the supply of
water or of salts of nitrogen in the soil is moderately low ( Fig. 67 ) .

Fig. 67. Relative growth of tops and roots of squash and wheat seedlings that
grew in culture solutions without and with added nitrogen (N). Photos from
Mary E. Reid, Boyce Thompson Institute.

The amount of food that accumulates in roots and underground stems
is influenced by the environmental conditions to which the tops of the
plants are exposed.

Bluegrass in the shade of trees in a lawn that is frequently watered,
treated with ammonium sulfate, and mowed during the warmest months
of summer may not survive as well under this treatment as bluegrass
that is neither watered nor fertilized during this part of the season.

The practice of pruning and pollarding trees checks the growth of
both tops and roots, but the growth of the roots is checked more than
that of the tops.

In dry climates alfalfa is mowed 3 to 5 times in a season, but if it is
mowed as frequently as this in moist climates most of the plants die
within a year or two.

[Chap. XXI INTERRELATIONS OF THE PARTS OF A PLANT 191

Sassafras may grow as a weed in pastures in certain parts of the coun-
try, but not when sheep are present in abundance. Almost pure forests
of hemlock, of pine, or of hickory have been known to develop in pas-
tures in which seeds of many other trees were present. In a forest the
kinds of seedling trees that grow to maturity may depend in part on the
presence of browsing animals, such as deer.

On the roots of the plants of the clover family there are nodules result-
ing from the presence and activities of nitrogen-fixing bacteria. When
the clover plant is exposed to dense shade, or when its roots are in soils
containing an abundance of salts of nitrogen, there are fewer nitrogen-
fixing bacteria and nodules than there are when the clover tops are ex-
posed to an abundance of light, or when the roots are in soils in which
the supply of nitrogen salts is low.

Many plants propagate vegetatively if one merely cuts off the leafy tip
of a stem ( a cutting ) and inserts its basal end into moist soil. In a short
time roots will grow from the base of the stem and the cutting will con-
tinue to grow and become a complete individual plant. Many persons
remove the leaves from cuttings, but the new roots will appear sooner,
be more abundant, and grow more rapidly if some of the leaves are not
removed. Exact observations may be made more readily if the cuttings
are placed in water in a beaker instead of in soil ( Fig. 68 ) .

Fig.

68. The initiation and the growth of roots from cuttings are dependent upon
leaves. Photo by F. H. Norris.

192 TEXTBOOK OF BOTANY

Interpretations exemplified. The reader may be satisfied with the in-
ferences he drew and the interpretations he made as he read each of the
above paragraphs; or he may wish some means of checking them. Only
one example will be discussed here, namely, the relative amomit of
growth of roots and shoots of plants supplied with different amounts of
nitrates in the soil. If the supply of nitrates is very low the root system
is large in proportion to the tops. If it is verv high the root system is
small in proportion to the tops. One might speedily dismiss the whole
matter by saying that in the first instance the roots grew long in search
of more nitrates for the tops, and in the second instance the tops had
plenty of nitrates and the roots did not have to grow long in search for
more. But would one be content to ascribe to roots either the intelli-
gence necessary to diagnose the needs of the tops, or the superhuman
ability to grow, or not to grow, at will?

Now, if we try to explain these growth phenomena on the basis of the
manufacture and uses of food in plants, it is necessary only to recall
and relate those groups of facts which showed that ( 1 ) foods are used in
the building of cells and that growth does not occur without them; (2)
the amount of sugar that enters the non-green parts of a plant depends
upon the extent to which photosynthesis exceeds respiration and other
uses of sugar in the chlorenchyma; (3) no part of the plant can grow
well unless it is adequatelv supplied with both sugar and proteins, or
with the conditions necessary for making them; (4) although rapid
growth depends upon a plentiful supply of proteins from which the new
protoplasm is largely made, considerably more sugar than protein is
consumed in the building of new cells; (5) particular parts of the plant
are dependent upon the movement of certain materials from other parts;
and ( 6 ) a definite set of conditions is necessary for the manufacture of
sugar and proteins in plants. Fats were omitted from this summary be-
cause they seem to be made readily enough in the cells when sugar is
present.

Since all the sugar is made in the tops of the plant, the roots will get
an ample supply of it only when it is made in the shoots much more
rapidly than it is used there. On the other hand, the shoots will have a
good supply of the nitrates needed in protein synthesis only when the
amount that passes into the roots from the soil greatly exceeds the use of
nitrates in protein synthesis in the roots.

Chemical analyses of plants show that if the supply of nitrates in the
soil is low the tops have larger amounts of carbohydrates in proportion

[Chap. XXI INTERRELATIONS OF THE PARTS OF A PLANT 193

to proteins. The tops do not grow well because there is a deficiency of
protein in them. Sugar and starch accumulate there and the sugar moves
downward and the roots become well supplied with it. The nitrates that
enter the roots are used there in protein synthesis, and as a consequence
the roots grow relatively large in proportion to the tops.

Chemical analyses also show that the tops have a larger proportion
of protein when the supply of nitrates in the soil is high. Under these
conditions the tops grow well. A relatively greater amount of the sugar
made in the chlorenchyma is used directly in the growth of tops, and
consequently the roots are less well supplied with sugar and their rela-
tive growth is less.^

Hormones and interrelations. Having seen how this one example of
growth can be interpreted upon the basis of foods without assuming any
purposeful acts by a part of the plant, we may now ask whether the
growth of roots is dependent upon the tops only for sugar. It is possible
that their growth may be dependent upon the tops also for minute
amounts of certain hormones (or auxins), vitamins, and amino acids,
or for certain precursors of these substances. This question was asked
many years ago, and it has been answered in the affirmative and the
negative many times. Recently, however, there has been a concerted
effort by many workers to decide it on the basis of critical experiments.

A brief account of one type of these experiments will indicate some
interesting features of the general problem. Several attempts have been
made to place small pieces of roots in solutions of certain materials in
which they would continue to grow entirely isolated from the rest of the
plant. These efforts were finally successful. One investigator put a small
piece (1 cm. long) of the tip of a tomato root in 50 cc. of water in a
sealed flask which also contained:

1,000.000 mg. of sucrose

17.300 mg. of essential inorganic salts
0.005 mg. of brewer's yeast extract.

The pieces of root grew in this culture. At the end of one week a small
tip of this root was transferred to a fresh solution, and so on week after
week. These pieces of roots continued to grow after each transfer with-
out any decline in the rate of growth with time. But if the brewer's yeast
was omitted the root ceased to grow after two or three weeks.

What is there in this minute amount of brewer's yeast that is so im-

^ The other examples listed are left for class discussion.

194 TEXTBOOK OF BOTANY

portant to the growth of roots? It is not in the ash (inorganic salts) of
the yeast. It is organic. Is it one substance or more than one? Can any-
thing else be substituted for it? Are some substances necessary for the
starting of roots, and others for root growth? Are the same substances
necessary for the roots of all plants? Are the formation and growth of
tubers also dependent upon such substances? These are some of the
questions being investigated at the present time. It has been found that
vitamin Bi ( thiamin ) is at least a partial substitute for yeast extract.

Other types of experiments indicate that the formation and growth
of roots and tubers are dependent upon the migration of hormones and
vitamin Bi in addition to sugar from the tops of the plant. These dis-
coveries are the basis of the attempts being made to discover what sub-
stances may be used to hasten root formation in numerous plant cut-
tings. Certain substances are now being advertised for this purpose.

Summary. The organs of a plant are mutually dependent because of
substances made in, or absorbed by, one part of the plant tliat are neces-
sary in the growth of other parts. Many common interrelations in plant
organs are dependent upon the manufacture and transfer of sugar and
of the substances used in protein synthesis. As a result of these interrela-
tions environmental factors may influence the development of one part
of a plant through their influence upon the processes in another part of
it. Recent discoveries indicate that minute amounts of hormones and
vitamins are involved in the way one plant organ affects. another. Other
interrelations between the parts of plants will be noted in later chapters.

CHAPTER XXII

PHYSICAL PROCESSES INVOLVED IN THE MOVEMENT OF
MATERIALS IN PLANTS

<><><><x><X><><>C><><><><><^^

The movement of materials into and out of plants, and from place to
place within the plant, was often mentioned in previous chapters with-
out reference to the manner of movement or to the energy necessary
for it.

The reason for this omission will soon be apparent. Energy is always
involved in the movement of material from one position to another. The
immediate source of this energy may be in the moving object, or it may
be in the environment of the object. For a clear concept of the move-
ment of materials into, out of, and within a plant it is necessary, there-
fore, that one distinguish whether the source of the energy which causes
the movement is within the moving material, within the plant, or in
some other external agency. It will be much easier to attempt this dis-
tinction now by viewing in retrospect some of the processes already
discussed and considering some additional ones. It is sometimes diffi-
cult, even impossible today, to make this distinction; but the major
source of energy in many plant processes is not difficult to recognize.

Intrinsic and extrinsic sources of energy. Perhaps a crude analogy may
clarify this problem of recognizing different sources of energy. The
movement of a floating log in a river is dependent upon a source of
energy obviously outside the log. The movement of a man swimming in
the river results from a combination of the energy that moves the log
plus a source of energy within the man. By utilizing this second source
of energy the man may move upstream. The movement of a boat in the
river may be traced to the same source of energy that accounts for the
movement of the log, plus another external source of energy that may be
traced to the motor attached to it, or to the man rowing it.

The movement of materials into, out of, and within plants, like that
of the objects in the river, depends upon several diverse sources of
energy. The plant may have no active part in the movement. We must
not be misled into thinking that the plant, being alive, can take in sub-

195

196 TEXTBOOK OF BOTANY

stances, move them where they are needed, and throw off those that
are not needed. The active agent may be the moving substance itself,
and the energy that impels it may be external to the plant. Obviously
such movements should be described not by terms implying that the
plant "takes up," "takes in," and "gives off" the materials, but by terms
implying that the materials are moving into, or out of, the plant. The
latter terms focus one's attention upon the really active agent, while the
former imply that the passive medium (the plant) is the active agent.
By common agreement the term absorption is used merely to indicate
that a substance enters the plant, without reference to the manner of
its entrance or the energy involved.

Movements of materials and the water medium within plants. In addi-
tion to recognizing the energy relations of moving substances, it is also
important to visualize the water within the plant, since this water is the
medium in which substances move. We have already seen that leaves
are composed of cells. In each living cell of the leaf a central vacuole
filled with water and dissolved substances is surrounded by a thin film
of protoplasm, which is in turn enclosed by a cell wall composed of
cellulose and pectic compounds and sometimes of other substances also.
Similarly, all other organs of a plant are composed of cells. Water not
only fills the vacuoles of the cells, but it also surrounds and is between
most of the minute particles of which the protoplasm and cell walls are
composed. Cell walls become much thinner when the water in them
disappears by evaporation, just as a wet cardboard becomes thinner as
it dries.

By weight, about 80 per cent of a growing corn plant in midsummer
consists of water. We may therefore think of a com plant as a branched
column of water held in place by exceedingly thin films of protoplasm
and cell walls. Substances that move into and within the plant are mov-
ing mainly in solution in this water. Substances that move out of the
plant are moving out of this water. The water also moves.

These movements may be inhibited or retarded by layers of cork and
by cell walls that are highly cutinized because the fat-like particles in
these walls may not be embedded in water. Movement of materials into
and out of cells may be impeded also by the film of protoplasm in each
cell. This fact may seem strange because of the large amount of water
in protoplasm, but it is probably due in part to a greater accumulation
of fat-like substances in the outer surface layer of the protoplasm. For
the present, however, we may accept and use the fact without account-

[Chap. XXII THE MOVEMENT OF MATERIALS IN PLANTS 197

ing for all the intricate physico-chemical conditions of which it is a
consequence.

For many years students of plants have been trying to explain the
movement of materials into, out of, and within plants. Not all the ques-
tions raised have been satisfactorily answered, but enough has been
learned for botanists to conclude that these movements are dependent
upon the same physical processes that occur in non-living systems. Most
of them have already been imitated in experiments by means of labora-
tory apparatus. These movements will now be considered in more detail
in relation to some of the physical processes involved.

Mass movements. The flow of water in a stream and the movement of
a current of air (wind) along the street are familiar examples of what
is meant by mass movement. Frequenth' it is a flowing movement or
current. Mass movements of materials may occur in plants. If you put
your finger in water and then withdraw it, a film of water adheres to it
because of the cohesion of the molecules of water, and the adhesion
of the molecules of the water and those of the skin. These cohesive and
adhesive forces between molecules at the surfaces of objects may result
in tensions, called surface tension. Surface tension may cause the move-
ment of water for short distances, and any substances dissolved or dis-
persed therein move with the water. For instance, if two microscope
slides are held closely together and one end is placed in a shallow dish
containing a dye dissolved in water, a small mass of the solution quickly
moves up between the two slides. This is a familiar phenomenon to
anyone who has frequently washed slides or dishes. Similarly, water
may move into the small spaces between the fibers of a blotter, a piece
of wood, or a plant, or into the pores of the soil. This movement of
water into small spaces as a result of surface tension is often referred
to as capillarity.

We have already seen that protoplasm may move about within a cell
by flowing movements (protoplasmic streaming). Substances, such as
food and salts, that are in the protoplasm are of course carried along
in the same manner that a log is carried in a stream. There may even
be some movement of materials in this way between cells when strands of
protoplasm extend from cell to cell through minute pores in the cell
walls. Surface tension is one of the forces which cause protoplasmic
streaming.

There is also a streaming, or mass movement, of water up the con-

198 TEXTBOOK OF BOTANY

ductive tissues of a plant from the roots to the leaves. This movement
is discussed in a later chapter.

In Chapter VII we mentioned the fact that one of the results of oxi-
dation-reduction processes in living cells is a small electrical difference
between the parts of a cell and between the tip and base of a plant
organ. These electrical differences are mentioned here merely because
they may affect the mass movements mentioned above and also some of
the diffusion phenomena described below. However, these electrical
influences are not well enough known to be discussed in a general
textbook.

Diffusion. All soluble materials that move into, out of, or within plants
move wholly or in part by diffusion. Plants may wither and become dry.
Water placed in an open dish slowly disappears. We cannot see the
water moving from the dish or from the plant, because it is moving in
the form of free molecules that have separated from the liquid mass.
It is diffusing from them into the air, and what we see is the result of
this diffusion. Later we shall see some of the results of the diffusion
of water into a plant, and from cell to cell within it.

Instead of water we may place some aromatic liquid, such as pepper-
mint oil, in the open dish. In a short time the presence of this oil in
all parts of the room may be detected by its odor. Neither the oil nor
the water has been destroyed, but the molecules of each have become
widely dispersed in the air of the room. That is, they are now scattered
among the molecules of oxygen, nitrogen, and other gases in the air.
Similarly, the odor of flowers and fruits is the result of the diffusion of
some substance from them that we detect by our sense of smell. From
these examples it is perhaps clear that we are using the term diffusion
to refer to the dispersion of material by molecular movement. Although
we can neither see nor smell oxygen and carbon dioxide, there are ways
of proving that they also move into, within, and out of plants by
diffusion.

If a colored salt ( potassium peiTnanganate, copper sulfate, etc. ) , that
is soluble in water is placed at the base of a column of water in a slender
glass tube, the result of the slow diffusion of the salt throughout the
entire column of water can be detected by its color. The molecules of the
salt move between the molecules of water. Even the force of gravity
does not prevent the upward movement of the molecules of a salt in
the water. Similarly, any substance that is soluble in the water of the
plant may move into the plant and from cell to cell within the plant by

[Chap. XXII THE MOVEMENT OF MATERIALS IN PLANTS 199

diffusion. As we shall see later, some substances do not pass through
the layer of protoplasm in the cells as readily as others.

The accumulation of water in the vacuole by the process of osmosis
and the swelling of cell walls or of a piece of wood by the process of
imbibition, are merely processes of diffusion under special conditions
with special results. Even the mass movement of a stream of water up
the conductive tissues of a plant is indirectly dependent upon the energy
of diffusion.

An understanding of all these processes evidently depends upon a fairly
clear concept of the process of diffusion. One may begin by thinking
of all solids, colloids, solutions, liquids, and gases as composed of mole-
cules always in motion. Anyone who has used a microscope has probably
observed that the smallest visible particles mounted in a drop of water —
for example, chalk dust — are constantly in motion as the result of being
bombarded from all sides by the molecules of the water. This is called
Brownian movement after Robert Brown, an English botanist who first
described it. The smaller the particles of a given substance the more
rapidly they move. While watching them one needs only to imagine
more and more rapid movement of smaller and still smaller particles
to form a mental image of molecular motion.

Diffusion is the result of molecular motion. Consequently the energy
involved in all diffusion processes can be traced to the energy of molec-
ular motion of the diffusing substances themselves. Each substance
diffuses independently of the diffusion of other substances, except for
the interference caused bv one molecule colliding with another. A sub-
stance will diffuse more rapidly into a vacuum than into air because of
the fewer collisions with other molecules. Likewise, a gas, such as
oxvgen or carbon dioxide, will diffuse more rapidly in air than in water.

The energy of molecular motion is correlated with heat and is directly
proportional to the absolute temperature. Apparently at minus 273° C.
there would be no molecular motion and no diffusion. One may therefore
think of the heat energy of the environment as being the source of
energy of the movement of materials into, within, and out of plants by
diffusion. The heat energy liberated in the plant by respiration would
be an additional source of energy in this process, but a rather insignifi-
cant one. The radiation of energy from the sun is the principal source
of the heat energy of the earth's surface, and this in turn becomes the
source of the energy of molecular motion in plants and all other objects
on the surface of the earth.

200 TEXTBOOK OF BOTANY

When a soluble dye is placed in the bottom of a slender glass tube
filled with water, the greater concentration of the dye in the bottom of
the tube at the beginning of the demonstration is evident by its deeper
color there. Gradually, however, the color of the dye becomes evident
farther and farther up the tube, and after many months the color is
uniform throughout the tube. The dye is now equally concentrated
throughout the entire column of water. That is, the number of molecules
of dye in comparison to the number of molecules of water would be
the same in a cubic centimeter of the solution taken from any part of
the tube.

Evidently for several months before the concentration of the dye
became uniform a greater number of molecules of dye moved upward
than downward in the tube. Since more molecules of dye are moving in
any direction in the region where its concentration is greatest, the dye
will diffuse from regions where it is more concentrated toward regions
where it is less concentrated. When the concentration of the dye becomes
uniform, as many of its molecules will be moving in any direction in
any part of the tube as in any other.

When the above facts are fully appreciated it will be easy to see why
carbon dioxide and oxygen diffuse into the green parts of a plant at one
time and out of them at another time. Each gas diffuses independently
of the other, and solely in relation to its own concentration inside and
outside the plant. The same principle holds for the diffusion of water,
of a salt, or of any other substance dissolved in the water. For instance,
if carbon dioxide is used in photosynthesis in a particular cell, its con-
centration within that cell becomes lower than it is outside the cell.
Then, according to the laws of diffusion, a greater number of molecules
of carbon dioxide will move into that cell than out of it, until its effective
concentration is again equalized.

If we gauge our language h\ our knowledge, we will no longer sa\'
that the plant "takes in" and "gives off" carbon dioxide; We will use
expressions that imply that the carbon dioxide passes into and out of
the plant. To be specific, we may say that it diffuses into and out of
the plant. The movement of other materials should be similarly analyzed.

As our knowledge increases, it becomes more and more evident that a
plant is a complex system of materials and processes surrounded by
another system of materials and processes that we call the environment;
and that the same natural laws are operating in both systems. Processes
follow one another as a consequence of their dependent relations in these

[Chop. XXII THE MOVEMENT OF MATERIALS IN PLANTS 201

systems. To explain why a particular process occurs we must be able to
show that it is a consequence of preceding processes.

Diffusion under special conditions. When the basic facts of diffusion
are clearly understood thev may be used in interpreting complicated
processes in which diffusion plays a part. Among these processes are
imbibition, osmosis, and transpiration.

Imbibition. We have already seen that a piece of wood is composed
of cell walls. Even more familiar is the fact that when a piece of dry
wood is placed in water or in a moist atmosphere it swells, or increases in
volume and weight. Before the discovery of explosives stone was quar-
ried by pouring water upon pieces of drv wood that had been wedged
into holes drilled in the rock. Even at the present time this method is
used in quarrying marble and granite for special purposes. The swelling
of wood is the result of the entrance of water between the particles of
which the cell walls are composed. The entering water pushes these
particles farther apart and increases the size of each cell wall. This fact
is easily demonstrated by placing a flat piece of dry gelatin, or agar, in
water for a few minutes. Any of the solid or colloidal parts of a plant
may increase in size in the same manner, but the swelling of cell walls,
protoplasm, and mucilages are the most conspicuous examples. The en-
trance of water into solids or colloids and the resultant swelling have
long been referred to as imbibition.

Evidently the energy of diffusion is involved in imbibition. The con-
centration of water is greater outside than inside the piece of dry
gelatin or the dry cell walls of the piece of wood. But the water also
adheres to the smaller particles of the wall and fomis thin films around
them. The cohesion of the wall particles is overcome by the pressure of
diffusion and by the adhesion of the water to them. They are forced
farther apart by the entering water and the walls swell. When a piece of
dry wood is placed in water the water first enters the intercellular spaces,
pores, and open vessels, forcing out the contained air. This movement is
the result of surface tension ( capillarity ) . It rapidly distributes the water
into some of the xylem tubes and intercellular spaces in the wood block,
but it does not result in increasing (swelling) the volume of the block.

Osmosis. The term osmosis is sometimes used synonymously with
diffusion. It has been used also to refer to the diffusion of anything
through any membrane. These uses are not acceptable, because there is
no need for the word if it is a synonym of diffusion, and because the

202 TEXTBOOK OF BOTANY

term is needed to designate diffusion through a particular kind of mem-
brane under special conditions with special results.

There are certain conditions in plant cells under which the number
of molecules of water moving from outside the cell through the film of
protoplasm into the vacuole is greater than the number moving outward
from the vacuole. As a result, the water in the vacuole increases in vol-
ume and thereby presses against the protoplasm and cell wall. These
structures become stretched and extended, and the whole cell becomes
larger and firmer, or turgid. When the enlarging cells occur in masses as
in plant organs and press against each other, the mutual pressure among
all the cells causes the whole plant to become rigid or turgid. When
these processes and conditions become reversed, the plant wilts. If there
is any advantage in adding the word osmosis to our consideration of
diflFusion in plants, it lies in limiting its use to the diffusion of water
through certain kinds of membranes, such as protoplasm, when certain
other conditions exist. What are these conditions, and what is peculiar
about these membranes?

The concentration of water in the vacuole is a factor in osmosis. When
the diffusion of water is inward through protoplasm into the vacuole,
one may infer from the general laws of diffusion that the water inside
the vacuole is less concentrated than the water outside the cell. This
condition actually occurs when the concentration of dissolved substances
(sugars, salts, organic acids) in the water enclosed in the vacuole ex-
ceeds the concentration of dissolved substances in the water surrounding
the cell.

At room temperature (20° C.) there is a definite number of molecules
of water in each cubic centimeter of pure water. If some sugar is added,
the water then appears to occupy more than a cubic centimeter of space.
The reader will undoubtedly explain this increase in volume by visualiz-
ing the molecules of sugar as diffusing or moving among the molecules
of water and jostling them farther apart. Molecules of sugar now occupy
spaces formerly occupied by some of the molecules of water, and there
are fewer molecules of water in a cubic centimeter of this solution than
there are in a cubic centimeter of pure water. The presence of the sugar
molecules decreases the concentration of the water. Obviously the more
sugar present the more dilute the water will be. In solutions of this sort
the water is said to be the solvent, and the sugar the solute.

Differentially permeable membranes are factors in osmosis. The nature
of the membrane, however, is as important in osmosis in plant cells as

[Chap. XXII THE MOVEMENT OF MATERIALS IiN PLANTS 203

is the concentration of water. This fact may be demonstrated with a
simple physical apparatus and also with living plant cells. Membranes
such as filter paper and cellulose cell walls are very permeable to both
water and the solutes in water. This is only another way of saying that
water and solutes can diffuse readily through these membranes. Such
membranes are of no importance in osmosis in plant cells.

There are, however, certain kinds of membranes through which water
can diffuse readily, but through which certain solutes in the water do
not diffuse so readily. These are the membranes that are important in
osmosis in plant cells. Protoplasm is an excellent example of this type of
membrane. It differs in its peiTneability to different substances. It is very
pemieable to water, but under most conditions it is practically im-
permeable to the anthocyanin and some of the other substances in solu-
tion in the water in the vacuole. Membranes of this type are said to be
differentially 'permeable}

There is a second type of differentially permeable membrane which
is more pemieable to certain solutes than to water. Sheets of rubber and
cutinized cell walls are more permeable to carbon dioxide than they are
to water. Such membranes are of no importance in osmosis in plant
cells, but they are useful in demonstrating certain principles of osmosis
and permeability.

Molecules of oxygen and nitrogen are smaller than those of carbon
dioxide, but sheets of rubber are much more permeable to carbon di-
oxide than to the other two gases. Consequently, if one ties a knot in the
neck of a rubber balloon filled with ordinary air and then places it in a
bottle of carbon dioxide," the balloon will gradually increase in size.
Outside the balloon there is pure carbon dioxide. In the air inside the
balloon there are about 3 parts of carbon dioxide in 10,000. At the be-
ginning of the demonstration, therefore, there must be about 3000
molecules of carbon dioxide entering the balloon for every one that
leaves it. Obviously both the swelling of the balloon and the pressure
inside that stretches it result from the entrance of carbon dioxide. The
energy of molecular motion is responsible for both the entrance of the
carbon dioxide and the pressure on the walls of the balloon.

For a demonstration of the osmosis of water in a non-living system one

^ The less appropriate terms, semipermeable and selecti\'ely permeable, are sometimes
used to indicate this type of membrane.

- Place the balloon in a bottle of water. After inxerting the bottle replace the water with
carbon dioxide.

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TEXTBOOK OF BOTANY

may use the apparatus represented in Fig. 69. Various kinds of mem-
branes and solutes may be tested by this means. The thistle tube with
attached membrane is first placed in a vessel of water as in Fig. 69A to
see if the membrane is permeable to water. The solute may then be added

Fig. 69A. Diagram to illus-
trate the passage of water
through a membrane: A, rep-
resents a molecule of inside
water; B, a molecule of outside
water; and C, the membrane.
Equal numbers of water mole-
cules are in contact with the
inside and outside of the
membrane, and the movement
of water molecules through
the membrane in both direc-
tions is the same. Hence the
level of the water in the tube
remains unchanged.

to the water in the thistle tube. If there is a definite continued rise of
water in the thistle tube the membrane is more permeable to water than
to the solute. A temporary rise that soon subsides, such as one obtained
with a membrane of filter paper, is due merely to the fact that the molec-
ular motion of water molecules is more rapid than that of the molecules
of the solute. One may also tie the membrane across the mouth of a

B

Fig. 69B. Diagram to illus-
trate osmosis: A, represents a
sugar molecule; B, a water
molecule; and C, a differen-
tially permeable membrane.
The sugar in solution dilutes
the water so that fewer water
molecules are in contact with
the inside than with the out-
side of the membrane. Hence
water passes in more rapidly
than it passes out, and the level
of the water in the tube rises.

[Chap. XXII THE MOVEMENT OF MATERIALS IN PLANTS 205

bottle filled with a solution and then immerse the bottle in water; or
one may convert the membrane into a small bag.

Demonstrations that are even more instructive may be made directly
with living plant cells. In a series of small glass dishes or hollow ground-
glass slides we may place distilled water containing different amounts
of sugar varying all the way from none at all on one end of the series to
as much as 20 per cent on the opposite end. We may now place living
plant cells in these different solutions and with a microscope watch what
happens. At one end of the series where there is little or no sugar in the
external water, the plant cells may become larger; a little farther along
in the series the cells appear to remain unchanged in size and shape; still
farther along they will decrease slightly in size. Toward the end of the
series the protoplasm is partly separated from the cell wall, and in the last
of the series the protoplasm has become a small mass in the cell, since
the vacuole has entirely disappeared. That is, the water that was formerly
in the vacuole has diffused out of it.

All these observations may be interpreted on the basis of the laws of
diffusion and the differential penueability of membranes. Perhaps it is
sufficient to add here that where the protoplasm just began to separate
from the wall the concentration of the water in the vacuole was prac-
tically the same as that of the water in the dish if the temperature was
the same; where the cells became larger the concentration of the water in
the vacuoles was originally lower than that in the dish; and where the
protoplasm was separated from the wall the concentration of the water
in the vacuole was originallv higher than that of the water in the dish.
Since the protoplasm separated from the cell wall, it is the differentially
permeable membrane of the cell. The bounding surfaces of plastids and
nuclei within the protoplasm are also differentially permeable. Instead
of single cells, pieces of plant tissue may be placed in the series of solu-
tions and the relative changes in turgidity and size of the tissue may be
observed.

When fresh-water plants are placed in salt lakes, water diffuses out of
them into the lake because the concentration of the water in the lake is
less than it is inside the plant cells. Such conditions are sometimes re-
ferred to as physiological drought. There is an abundance of water out-
side the plant but its concentration is too low. A whole lake full of salt
water may be as drv as a desert to a plant.

Plasmolysis and turgor. When the protoplasm of a cell separates from
the wall because water is diffusing out of the vacuole, the cell is said to

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be plasmolyzed, and the process is referred to as plasmolysis. The
shrunken cell is flaccid, and conversely the swollen cell is turgid. Cell
turgor may result from the entrance of water by imbibition or osmosis,
or both. Other conditions being equivalent, turgor becomes greatest in
the cells with the least extensible walls and in which the volume changes

least.

Permeability. Permeability is a property of
membranes that affects the movement of ma-
terials into and through them. At the present
time it appears to be due to a number of quali-
ties; we shall mention only the simplest ones
here. A differentially permeable membrane
should not always be visualized as a crude sort
of sieve through which particles can or cannot
pass according to their size. This fact is illus-
trated by the differential permeability of rub-
ber membranes, through which the larger
molecules of carbon dioxide pass more readily
than the smaller molecules of water, oxygen,
and nitrogen.

One of the features of permeability appears
to be illustrated by a simple demonstration in
which the membrane consists only of water
(Fig. 70). Ether diffuses through a water
membrane more rapidly than chloroform. Since
ether is much more soluble in water than
chloroform, this demonstration indicates that
solubility of the diffusing substance in the
membrane is one of the factors of permeabil-
ity. Solubility in turn seems to depend upon
the relative attraction between the molecules
of the substances involved. If the membrane
were equally permeable to these two substances, ether would still diffuse
through it the more rapidly because its diffusion rate is about 1.6 times
that of chloroform.

The most interesting features about the permeability of protoplasm
are (1) its variability, and (2) the fact that the continuation of life
depends upon its variation remaining within certain limits. A great num-
ber of conditions and substances increase or decrease the permeability

■EtKer

_Water
'Ynembraae""'

Chloroform

B

Fig. 70. Diagram of ap-
paratus used to demon-
strate the differential per-
meability of water to ether
and chloroform: A, when
first set up; B, several
days later.

[Chap. XXII THE MOVEMENT OF MATERIALS IN PLANTS 207

of protoplasm to water and solutes. Among them are variations of tem-
perature and light, or the presence of chloroform, ether, alcohol, saponin,
and the ions of various salts.

The influence of external factors upon the permeability of protoplasm
may be demonstrated easily by placing small pieces of the red garden
beet in water and exposing them to any one of a number of factors.
The anthocyanin is in solution in the vacuoles of the cells. If some factor
increases the permeability of protoplasm beyond a certain point, this
red pigment diffuses out of the cells. The influence of a factor that in-
creases the pemieability of protoplasm mav be counteracted by another
factor that decreases it. For instance, if sodium chloride is added, the
red pigment begins to diffuse out of the cells; but if a little calcium
chloride is also added the influence of sodium chloride is annulled. The
permeability of protoplasm at any instant in a living cell is apparently an
equilibrium dependent upon the interaction of many factors.

Summary. The movement of materials into, out of, and within plants
occurs by xarious combinations of phvsical processes. All investigations
indicate that these processes occur, not because plants exert some pe-
culiar vital force, but because these processes are universal properties
of matter. Their interactions in simple physical systems may be accurately
measured, but the plant is such a complex system of structures and
processes that it is difficult to detect these interactions in all their details.
We begin by recognizing the forces involved and then try to relate
them to our present knowledge of plants. Among these forces we must
recognize the mutual attractions among molecules of gases, liquids, and
solids; surface tension; molecular motion and diffusion; and electrical
forces. Certainly we shall no longer think that substances move into and
out of plants because they are good or bad for the plant, or because the
plant needs or does not need them. Also we shall be less inclined to
think and say that the plant "takes in" and "gives off" these substances.

The movement of materials into, out of, and within plants by diffusion
is much easier to perceive and is better understood than movement by
other means. Moreover, several of the common observable results of this
diffusion are not difficult to analyze. We have therefore given most atten-
tion to diffusion. A substance diffuses from a region where it is more
concentrated toward a region where it is less concentrated, regardless of
the concentration or the diffusion of any other substance with which it
may be mixed. This is the result merely of the relative number of
moving molecules. There are many more molecules of a substance mov-

208 TEXTBOOK OF BOTANY

ing in all directions where the substance is most concentrated. Water
may be diffusing out of a cell, while the solutes in the water are diffusing
into the cell or vice versa. The factors that influence diffusion most are
temperature in relation to the rate of molecular motion, the concentration
of the diffusing substance, and the nature of the medium into which
it is diffusing. Diffusion under certain special conditions may be dis-
tinguished as imbibition and osmosis. Imbibition results in the swelling
of colloidal membranes and cell walls. Osmosis results in the enlarge-
ment of vacuoles. In the following chapter we shall note how diffusion
that is called osmosis underlies certain plant behavior familiar to all of
us. In later chapters we shall see how the energy of diffusion is the force
underlying some of the mass movement of materials within a plant.

CHAPTER XXIII
PLANT BEHAVIOR RELATED TO OSMOSIS

Osmosis and the turgidity of cells are primary causal factors in certain
kinds of plant behavior that have attracted the attention of almost every-
one. We have already seen that when the concentration of water sur-
rounding a plant cell is higher than its concentration within the vacuole,
the water diffuses into the cell, thus causing an increase in cell turgor
and also in cell size if the cell wall is extensible. The converse occurs
when the higher concentration of water is within the vacuole: water
diffuses out of the cell, cell turgor decreases, the cell may become
smaller, and in extreme cases plasmolysis, wilting, and death may result.
When all the cells of some part of a plant (leaf, flower, stem, etc.) in-
crease in size they press against each other, with the result that the
whole plant structure becomes firm and rigid. On the contrary, when all
the cells of a plant shrink in size their pressure against each other di-
minishes and the plant becomes flaccid, or wilts. If the cells on one side
of an organ shrink or swell more than those on the opposite side, curva-
tures or movements result.

Movements and growth curvatures. In growing regions increase in cell
size is dependent both upon the entrance of water and upon an accom-
panying growth in area of cell walls. Increase in area of a cell wall,
therefore, may be the result of stretching or of growth. The former may
be reversible, the latter is non-reversible. Likewise, movements and
curvatures that are dependent solelv upon turgor and the elasticity of
cell walls may be reversible. Growth curvatures may be permanent or
temporary. For example, the growth curvature of a stem toward the more
intense light during the dav mav be annulled by the greater growth of
the cells on the concave side of the stem tip during the night.

Superficially these movements and curvatures of plant organs may
appear to be intelligent acts of the plant. Speculations as to their advan-
tages or disadvantages are not uncommon. Perhaps the reader will dis-
cover that they are all consequences of similar fundamental conditions
and processes, regardless of whether they have "survival value."

209

210 TEXTBOOK OF BOTANY

By means of the facts and inferences already encountered and the
brief summary of several examples in this chapter, the reader is offered
an opportunity ( 1 ) to check his own observations of the kinds of plant
behavior that are consequences of the conditions underlying osmosis and
cell turgor, and (2) to evaluate conflicting interpretations of the ob-
served behavior. Movements in some plants differ from those in other
plants because the plant structures are different. A consideration of how
these differences in structure originated must of course be postponed to
the chapters on heritable variations and evolution. They are, however,
the consequences of processes to which no one would ascribe intelligence
and purpose unless he is accustomed to ascribe these human attributes
to all physical and chemical processes in the universe.

Rigidity of plants. The rigidity of all herbaceous parts of a plant is de-
pendent upon the turgidity and mutual pressure of cells described above.
During dry weather leaves, young stems, flowers, and even fruits may
wilt during the day and become firm at night or following rains, as the
result of changes in the turgor of the individual cells. Spray systems
are now commonly installed over market stands to prevent the loss of
turgor in the cells of leafy vegetables on display. The rigidity of woody
tissues is dependent not upon osmosis and turgor but upon the thickness
of the walls of the wood cells.

Physiological drought. Owing to the relatively high concentration of
water in the vacuoles of their cells, many fresh-water plants cannot grow
in the ocean or in salt lakes and salt marshes where the concentration
of water is relatively low. The fact that water plants may wilt in such
habitats can be demonstrated by placing a plant in a vessel of salt water.
Similarly, most land plants do not grow on salt plains (Fig. 71). Man
sometimes makes miniature salt plains by adding salt to tlie soil to kill
weeds. One of the difficult problems encountered in applying irrigation
water to soils of arid regions is the necessity of preventing the accumu-
lation of inorganic salts in the soil surface as the added water evaporates.
The preservation of food in water of low concentration ( brine ) is a very
old custom. Sugar and salt are used as the means of diluting the water
to the point of physiological drought for the bacteria and molds that are
responsible for the spoiling of food.

Vinegar is also used to prevent the growth of these organisms. It is
primarily a solution of acetic acid in water. This acid lowers the con-
centration of water in vinegar and also coagulates the protoplasm of

[Chap. XXIII PLANT BEHAVIOR RELATED TO OSMOSIS

211

^«,w-^. , . y iig-l

lyx ..■■;»''*5sS&\' t. '

wtgsy-U* ■» 'J* ' '" '""g'f

Fig. 71. Hummocks of pickleweed (AUenrolfea) on salt-Hats in Utah. Plioto by

H. L. Shantz.

living cells. This coagulative effect of acetic acid may be demonstrated
by pouring a little vinegar on the "white" of an uncooked egg, or by
examining living cells mounted in vinegar. The killing effect of many
kinds of acids and salts should be attributed not to physiological drought,
but to some other condition such as the coagulation of protoplasm, altera-
tions in penneabilitv, or inactivation of enzymes. For example, the small
amount of copper sulfate or mercuric chloride necessary to kill plant
cells has little influence on the concentration of the water in which these
salts are dissolved.

Enlargement of cells during growth. In all growing regions of a plant
cell division is usually followed by an increase in size of the young cells.
The volume of these newly formed cells may increase many times ( Fig.
31, page 64).

Some new protoplasm is made, but the total enlargement is primarily
the result of the entrance of water into the cell under the conditions of
imbibition and osmosis. The enlargement of the cell depends, therefore,

212 TEXTBOOK OF BOTANY

upon the concentration of water inside and outside the cell, the conse-
quent osmosis and cell turgor, and the extension of the cell wall.

In some unknown way a hormone ( auxin ) formed in the young leaves
seems to be necessary to some process involved in the extension of cell
walls. If the hormone is absent little or no cell wall extension occurs.
If it varies in amount in different parts of the plant, the cells do not
enlarge uniformly. It is perhaps evident that turgor pressure will not
become so high in the cells with walls that are growing or are easily
extended. We may therefore have the anomalous condition of the en-
trance of water as a cause of increase in cell volume, and at the same
time the lowest turgor pressure in the enlarging cells. If the cell walls
are not growing or do not stretch readily, the entering water may result
in a high pressure within the cells.

Turgor and growth pressure. Similarly, if the enlargement of a plant
organ is restricted by mechanical means, the diffusion of water into each

ffavtgjac-

Fig. 72. Fern leaves pushing upward through a cement sidewalk. Growth pressure
may amount to hundreds of pounds to the square inch. After G. E. Stone.

of its cells exerts tremendous force against the obstruction (Fig. 72).
The combined pressure in all the cells of a root may lift stones weighing
several tons, displace stone curbings, or rupture concrete pavements.
Turgor pressure may be just as great in the cells of stems and other plant
organs. It is the force underlying the pushing of young stems of seed-
lings upward through the soil and of roots into the soil.

Since turgor pressure in cells is the result of the diffusion of water
into them, the energy of molecular motion is evidently the force back
of all these "marvelous powers" of plant growth. While the pressure
exerted by the plant organ as a whole may be equivalent to several
hundred pounds per square inch, a little calculation will show that the
pressure on a single cell wall is but a small fraction of an ounce. This
pressure is usually great enough to cause the walls of cells to bulge out-
ward and become convex except where they press against adjacent cells
and the surfaces become flattened or plane. The same phenomena may

[Chap. XXIII PLANT BEHAVIOR RELATED TO OSMOSIS 213

be seen in a mass of soap bubbles. The walls of many cells are not
uniformly thick and equally extensible in all parts, and some of the curi-
ous shapes of cells may be attributed to this fact.

It is important to remember that increase in size of a plant organ is
the result of the enlargement of its component cells. The mere division
of cells results only in increasing the number of cells that may enlarge.
A cell may become larger onlv bv the entrance of material from an ex-
ternal source. The great bulk of this material is water.

Plant movements and curvatures. Some of the smallest plants are
motile and can swim about in water just as freely as small animals. But
the movements of larger plants are limited to the bending, twisting, or
elongating of certain organs or restricted parts of organs.

Tropisms. Tropic movements, or tropisms, are exemplified by the fa-
miliar curving of plant organs toward, or away from, different inten-
sities of external factors, or the direction of the force of gravity or of an
electric current. For example, one-sided illumination subjects the cells
on opposite sides of a plant organ to different intensities of light. Simi-
larly the force of gravity or an electric current may induce curvatures.
If a plant organ curves toward the most intense light to which it is ex-
posed, its curvature is said to be positively phototropic. If it bends away
from intense light it is negatively phototropic. Similarly compounded
names are applied to curvatures related to other external factors, namely,
positively or negatively geotropic, hydrotropic, chemotropic, electro-
tropic, etc.; but the names are the least important features of the
processes.

Tropic curvatures may occur in any plant organ. They occur pri-
marily and are most prominent in growing regions, such as the tips of
stems and roots and the petioles of young leaves. Moreover, they are
primarily restricted to that part of the growing region in which the
cells are enlarging. They are the result of unequal enlargement of the
cells on opposite sides of the plant organ. Instead of personifying them,
therefore, we may base our interpretations upon our knowledge of the
dependent relations of the energy of molecular motion and diffusion,
permeability of protoplasm, osmosis, cell turgor, and the extension of
cell walls. If the initial influence of some external factor results in a
decrease of turgor and cell wall extension, the plant organ will bend
toward that side to which the factor is applied, and vice versa.

Hormones and cell enlargement. During the last decade the influence
of hormones upon cell enlargement and tropisms has become an inter-

214

TEXTBOOK OF BOTANY

esting subject of research. In spite of the fact that some of the conchi-
sions of today may have to be modified as additional facts are discovered,
some of them are too interesting to be omitted even from a general text-
book of botany.

Cell enlargement in stems appears to be dependent upon hormones
formed in young leaves exposed to light, though an excessive amount
of these hormones inhibits cell enlargement. In contrast to the cells of
stems, the enlargement of cells in the growing tips of roots is inhibited

Fig. 73. Geotropic growth in a seedling after it was clamped in a horizontal position.

by these hormones except when only the merest trace of them is present.
They seem to affect some process involved in the extension of cell walls.
Their movement down the stem or up the root may be deflected by such
external factors as light and gravity. In horizontal stems and roots they
are deflected toward the force of gravity. Consequently, if a seedling is
placed horizontally they accumulate in the cells in the lower side in
amounts that are favorable to the enlargement of cells of the stems but
inhibiting to the enlargement of cells of the roots. The stem tip therefore
curves upward and the root tip curves downward ( Fig. 73 ) .

These homiones are complex organic acids that move toward the posi-
tive pole in an electric current. It is inferred, therefore, that the slight
electrical difference on the two sides of a stem or root may be the cause

[Chap. XXIII PLANT BEHAVIOR RELATED TO OSMOSIS 215

of their deflection toward the lower side. Similarly, in many plants
they accumulate more abundantly on the shaded side of stems than on
the side exposed to the more intense light. As a result, the cells on the
shaded side enlarge more readily; this side of the stem becomes longer
than the lighted side and actually pushes the tip of the stem toward
the light.

These hormones may be extracted from plants and* dissolved in wool
tat, called lanolin. If a little of this lanolin containing the extracted hor-
mones is placed on one side of a root or stem tip, the expected curva-
tures are obtained. Similar curvatures may be obtained by placing any
of several closely related chemical compounds in the lanolin instead of
the naturally occurring hormones (Fig. 74).

Fig. 74. Curvatures of stems and petioles resulting from the effects of chemical
compounds similar to naturally occurring hormones: A, untreated plants; B, treated
plants. The compounds in low concentration (O.OI mg. in 100 mg. of lanolin)
were placed on the right side of the stems and on the upper surface of the two
petioles that have curved downward. Photos from P. W. Zimmerman and A. E.
Hitchcock.

Leaf mosaics. The formation of leaf mosaics (Fig. 75) as a result of
differences in elongation, bending, and twisting of petioles and stems is
a consequence partly of unequal illumination of petioles and stems and
partly of unequal illumination of the blades of the leaves. Bending and
twisting of petioles are the results of differences in cell enlargement in
opposite sides of the petiole. This cell enlargement is dependent in part
upon hormones that are formed in young leaf blades exposed to light
and pass down the veins to the petiole and stem. If the right and left
halves of a blade are unequallv illuminated, the amounts of hormones
that are formed and get to the right and left halves of the petiole will be
unequal. Consequently, there will be a difference in the amount of cell
enlargement on the two sides of the petiole and a curvature will result.

216

TEXTBOOK OF BOTANY

The petioles of leaves which develop in the shade of other leaves may
continue to elongate for a longer period and the blade finally reaches
the plane of the leaves above. Both cell division and elongation take
place. Many of the details of these processes are still unknown.

Fig. 75. Mosaics formed by the leaves of two species of maple {Acer maciophijl-
lum and Acer circinatiim) , Olympic Mountains, Washington. Photo by W. S.
Cooper.

Turgor movements and growth movements. Distinctions between these
two kinds of moxements are sometimes attempted. The difficulty in
making a distinction lies in the fact that all growth movements involve
turgor effects. The distinction becomes clear onlv when movements
occur in those tissues in which all growth has ceased. Movements that
are due entirely to changes in turgor are reversible. Those that are due
to differences in growth may become permanent.

There are also curvatures and movements in which the cell structure
determines the direction in which the plant organ curves or moves. These
curvatures occur in such plant organs as leaves and petals of flowers in
which the cells in the upper side differ from those in the lower. Such
plant organs are said to be bilateral, in contrast to organs that are
radially symmetrical. The movements may be purely turgor movements,
or a combination of turgor and growth. Among the more familiar exam-
ples are the daily opening and closing of certain flowers in relation to

[Chap. XXIII PLANT BEHAVIOR RELATED TO OSMOSIS 217

changes in temperature or light intensity; the opening of buds; the roll-
ing and folding of leaves; and pulvinal movements, such as the mo\e-
ment of leaves of clover and of "sensitive" plants ( Fig. 76 ) .

Fig. 76. Sensitive plants (Mimosa pudica). The one on the left was mildly jarred
just before the picture was taken. Photo by G. S. Growl.

If young flowers of tulip are exposed alternately to low and high tem-
peratures they may open and close several times in one hour. When the
temperature is lowered the rate of growth of the outer side of the
sepals and petals is greater than that of the inner side. As the tempera-
ture is raised tlie greater rate of growth of the inner side results in
opening the flower. White water lily flowers open in light and close in
weak light and darkness. A commonly cultivated purple water lily has
flowers that close in bright sunlight and are open only in dense shade or
at night.

The opening of buds is the result of growth of the bud scales and
young leaves. In many buds there is increased growth on the inner side
of the bud scales which results in spreading of the scales.

The folding and rolling of leaf blades of some species of plants are
dependent upon the relative turgidity in rows of special cells in the upper

218 TEXTBOOK OF BOTANY

epidermis ( Fig. 48, page 91 ) . Turgor pressure in these cells is opposed
to turgor pressure in the cells in the lower side of the leaf. When turgor
pressure is high in these special cells the leaf is expanded. Leaves of
many kinds of plants without such special cells also exhibit various
degrees of rolling during periods of drought.

The folding of leaves of clover, honey locust, and sensitive plant in
late afternoon or early evening is probably the result of unequal changes
in permeability or in digestion of starch to sugar accompanied by changes
in turgor and swelling of cells on opposite sides of the pulvinus (Fig.
47). These movements take place rapidly at higher temperatures, and
very slowly or not at all at lower temperatures or when the plant is
anaesthetized.

Changes in permeability, turgor, and growth accompanied by conse-
quent curvatures and movements may occur in some plants as the result
of mechanical contact. The folding of leaves of the sensitive plant and
the twining of tendrils are familiar examples. Somewhat less familiar are
similar movements of the stigmas and stamens of certain plants, the
closing of the leaves of the Venus's fly-trap Dionaea, and the bending
of the tentacles on the leaves of sundew, Drosera (Fig. 77).

Some of the proteins in the insects that are entrapped by such plants
are digested by enzymes from the plant or from bacteria. The resulting
compounds diffuse into the cells of the plant. The insectivorous plant,
however, is not necessarily dependent upon this external source of
nitrogen compounds.

Stomates. The opening and closing of stomates are dependent upon
the turgor and relative wall extensibility of the two guard cells. When
they are expanded by increased turgor, unlike ordinary cells, tliey do
not become spherical. The inner wall that bounds the pore is thicker and
less stretchable than the outer wall away from the pore. Hence when
turgid, the outer walls stretch and the guard cells become curved or bean-
shaped. Since the concave sides of the two guard cells are adjacent, an
opening appears between them. When these cells contract with de-
creased turgor the concave walls straighten and come together; the
stomate is closed. This straightening of the concave walls may be due
to the contraction of the walls, but the pressure of the surrounding epi-
dermal cells also pushes the non-turgid guard cells together. If these
epidermal cells were to shrink excessively they might also pull the guard
cells away from each other.

As a guard cell expands with increased turgor evidently the wall on

[Chap. XXIII PLANT BEHAVIOR RELATED TO OSMOSIS

219

Fig. 77. Five plants of Venus's fly-trap (Dionaea) two of which are in bloom, and
four plants of sundew (Dwsera). Photo by G. S. Growl.

the convex side is stretched more than the wall on the concave side.
The convex wall is the more extensible probably because it is thinner.

Increase in turgor of the guard cells is the result of the entrance of
water by osmosis. The energy back of the expansion of guard cells, there-
fore, may be traced through diffusion to the energy of molecular motion.
The concentration of the water in the guard cells is largely dependent
upon the transformation of starch to sugar and vice versa, which in turn
depends upon the acidity of these cells. In the guard cells that have been
studied, a slight decrease in acidity results in the digestion of starch to

220 TEXTBOOK OF BOTANY

sugar, while a slight increase in acidity results in a condensation of sugar
to starch. These changes in acidity and their consequences may be
brought about experimentally by immersing pieces of the epidermis in
chemical solutions differing in acidity. Under these experimental condi-
tions the opening and closing of the stomates may be brought about at
any time of day or night.

Under natural conditions the stomates of many plants are open during
the day and closed at night. Certain oxidation-reduction processes in-
itiated in the guard cells by light result in a decrease in their acidity;
the starch in them is soon hydrolyzed to sugar, which dilutes the water
in the guard cells below that of the surrounding cells. The consequent
osmosis, increase in cell turgor, and swelling of the guard cells result
in the opening of the stomate.

The stomates may also open in darkness if the temperature is high.
The effect of high temperature on the relative rates of processes in the
guard cells reduces the acidity sufficiently to initiate the chain of proc-
esses that results in the opening of stomates. The decreased turgor in
the guard cells of leaves with reduced water content may result in the
closing of stomates during the day. During droughts they may remain
open but an hour or two in the morning. The behavior of guard cells is
not the same in all species of plants. In a few plants the stomates are
open both day and night.

Summary. In many of the examples cited in this chapter the diffusion
of water may be correctly referred to as osmosis in contrast to the move-
ment of materials that should be referred to simply as diffusion. Several
familiar plant phenomena that are dependent largely upon osmosis and
cell turgor have been cited and briefly described so that the reader may
check his own observations and interpretations on the basis of the facts
involved.

Some of the structures, movements, and curvatures described are un-
doubtedly advantageous to the plant and may be important in its sur-
vival in nature. Others belong to that great group of plant phenomena
that evolve through the ages and survive through heredity without being
either essential or destructive to the species in which they occur. The
movements and curvatures, whether advantageous or not, are the con-
sequences of cell wall extension and of the energy of molecular motion
in diffusion, osmosis, and cell turgor. Since we do not personify these
processes and their consequences when they occur in non-living sys-
tems, there seems to be no good reason why we should personify them
when they occur in living systems.

CHAPTER XXIV
THE LOSS OF WATER VAPOR FROM PLANTS: TRANSPIRATION

Transpiration is essentially the evaporation of water from within plants.
It includes both the vaporization of water at all cell surfaces exposed to
the air and the subsequent diffusion of the vapor into the atmosphere
surrounding the plant. In the chapters immediately preceding it was
apparent that the diffusion of water molecules is a primary process in
imbibition and osmosis. It is also a primarv process in transpiration.

Transpiration is the most generally recognized of all processes in
plants. Among the common practices based on this recognition are the
daily watering of house plants; the elaborate systems of overhead and
underground irrigation of gardens; the wrapping of cut flowers and
freshly dug plants in waxed paper or other waterproof containers; the
packing of nursery stock in moist sphagnum moss for long shipments;
the enclosing of fruit and vegetables in oiled clotli or covered dishes in
electric refrigerators; the periodic sprinkling of the floor and crates in
the low-temperature storage of apples; and the placing of glass or paper
covers over newly transplanted seedlings in the garden. These and
many other practices are attempts to reduce the more harmful effects of
evaporation of water from plants or isolated parts of plants.

Transpiration not limited to leaves. Transpiration mav occur from the
surface of any plant organ. E\en the bark of trees does not prevent it
entirely. "Sun-scald" of trees that have been transplanted from nurseries
where their stems were shaded is the result of heating and drving bv
direct sunlight. Mature potato tubers, in spite of a cork covering, lose
noticeable amounts of water and shrivel in the course of time. The
evaporation of water from roots in dry soil, or from roots during the
process of transplanting may result in serious injury or death. Transpira-
tion from fruits in drought periods may result in shrinkage. The tips of
tomato fruits mav die as a result of excessive transpiration. The subse-
quent decay of these dead portions of the fruit is referred to as "blossom-
end rot." After removal from the plant such fruits as prunes and grapes
are often artificially exposed to warm dry air to hasten the loss of water

221

222 TEXTBOOK OF BOTANY

vapor from them, so that they may be safely stored for future use. The
harvesting of many seeds, grains, and hay involves the drying, or the
acceleration of transpiration, under natural or artificial conditions. High
transpiration may result in the death of young flowers and in the cessa-
tion of sexual reproduction.

Of the various plant organs the thin blades of the leaves of most plants
have the largest evaporation surface exposed to the atmosphere, and it
is in them that the greatest amount of transpiration occurs in the grow-
ing plant.

The bulk of the growing parts of plants is water. As we learned earlier,
all the living cells of a plant contain protoplasm and a vacuole filled
with water in which various substances are dissolved. Young tissues of
plants may contain as much as 95 per cent water by weight, and older
parts 60 to 75 per cent. When the water content of the active cells of
plants is gradually decreased they become less and less active until a
point is reached where injury or death results.

The leaves of most plants, therefore, with their high water content
(60-85 per cent), their relatively large areas, and their exposure to an
atmosphere that is usually only 10 to 75 per cent saturated, constitute
the most important surface of water loss from plants. Discussion in this
chapter, therefore, will be confined largely to the processes and factors
that influence transpiration as it occurs in leaves.

Circumstances of water loss from leaves. In Chapter VIII the organi-
zation of water-conducting tissues and their distribution among the
mesophyll cells are described. Attention was called to the air spaces
among these myriad cells and also to the epidermis which encloses the
mesophyll cells, air spaces, and veins.

The upper epidermis of the leaves in many species of plants is an
unbroken layer of cells. Its outer cellulose walls may be thick and have
a cutinized outer layer which is commonly called the cuticle. The lower
epidermis, and frequently the upper, have stomates distributed among
the epidermal cells. Hence when the stomates are open the epideiTnis of
the leaf contains many thousands of minute passages through which
gases may diffuse into the labyrinth of air passages among the mesophyll
cells, or diffuse out of them. Any gas may do this, and its diffusion is
quite independent of the diffusion of other gases or vapors.

Molecular motion and consequently evaporation are accelerated by a
rise in temperature. When the surface molecules of the water acquire
sufficient momentum to overcome the cohesion bv which thev are held
in the liquid state, they diffuse into the surrounding air. The same is

[Chap. XXIV

TRANSPIRATION

223

true of the evaporation of water that is dispersed among the particles
of a cell wall.

In the diagram (Fig. 78) of the diffusion of water from the vessels of
the stem to the veins and cells of the leaf, and finally into the atmos-

CUTICLE

UPPER EPIDERMIS

PALISADE

XYLEM-

•PHLOEK

SPONGY MESOPHYLL

AIR SPACE

LOWER EPIDERMIS

Fig. 78. Diagrammatic section of a stem and leaf. Arrows indicate paths of move-
ment of water molecules.

phere, the paths of the diffusion of liquid water and of water vapor are
indicated by arrows. From this diagram it should be clear that there
are two surfaces where evaporation of water occurs: first, the outer sur-
face of the epidermis, and, second, the cell wall surfaces of the mesoph\'ll.
The diffusion of water vapor from the outer surface is comparatively
simple and is discussed under "cuticular transpiration"; that from the
internal surfaces of the leaf is complicated bv certain structural and
physical factors and will be discussed under "stomatal transpiration."

Cuticular transpiration. Let us now have another look at a leaf — first
the outside. The epidermis of the leaf is composed of living cells all con-
taining water and having more or less saturated cell walls. Some of the
water molecules continuallv acquire sufficient energv to break the
mutual bonds of cohesion and also of adhesion of the water to the wall
molecules, and pass into the atmosphere. The cuticle, which is the outer
layer of the epidermal walls, reduces this evaporation to an extent which
depends upon its thickness and its fat or wax content. The cutin simply
decreases the number of water molecules that reach the epidermal
surface since water molecules do not pass readilv into fat-like and wax-
like substances. Nevertheless the cuticle is not wholly impervious and

224 TEXTBOOK OF BOTANY

some water reaches the surface, vaporizes, and diffuses into the air. This
water-vapor loss from the surface of the epidermis is termed cuticular
transpiration. It occurs at all times, but less rapidly from leaves coated
with cutin and wax than from leaves with little or no cutin. Cuticular
transpiration amounts to only 5 to 15 per cent of the total water-vapor
loss from the leaves of most plants.

The epidermis of manv plants has small unicellular or multicellular
outgrowths known as hairs and glands. Some of these remain alive as
long as the leaf or stem on which they grow; others die early and become
filled with air. All living hairs and glandular outgrowths increase the
cuticular surface of the leaf and also increase the cuticular transpiration.
Leaves of pumpkin and squash, nettles, tobacco, cultivated geraniums,
and petunias have long-lived hair-like epidermal appendages.

Dead hairs are common on leaves and stems of most plants. The
mullein has a dense covering of much-branched dead hairs on all ex-
posed surfaces, and on the leaves the felt-like layer on either side may
be thicker than the blade itself. Other examples of plants with hairv
leaves are velvet grass, silky willow, Labrador tea, Shepherdia, Spanish
moss, some species of sage, goldenrod, and aster.

Experiments have shown that when plants of equal leaf area are com-
pared, the rate of water-vapor loss from mullein is about the same as that
from tobacco, which has very similar leaves but lacks the dense hairy
coverings. Mullein leaves are often mentioned as examples of leaves that
conserve water allegedly because "the hairs cut down sunlight and wind."
Further experiments with mullein show that these effects are quite unim-
portant and that the hairs do not reduce stomatal transpiration at all.
They may decrease cuticular transpiration slightly in the dark and in
still air. Mullein plants with hairs removed from the upper leaf surfaces,
mullein plants with hairs removed from botli surfaces, and mullein plants
with hairs intact, under the same conditions lost water at rates so similar
as to be indistinguishable.

Whether the shield-shaped hairs of such plants as Shepherdia are
effective in reducing transpiration is unknown. But it is a safe assumption
that hairy coverings are of no importance in "protecting" the plant from
excessive transpiration in dry situations and in enabling them to survive
in dry habitats.

Anyone who digs down to the very end of the taproot of mullein will
be able to explain why this plant survives in dry habitats as well as in
moist ones.

Of the numerous differences among lea\'es, such as those mentioned

[Chap. XXIV TRANSPIRATION 225

in Chapters IX and X, a few may influence the rate of transpiration one
way or the other, but a casual inspection of leaf differences is not a
reliable means of discovering their influence on transpiration rates. Very
thick cuticles, waxy layers, and resinous layers on leaves and stems un-
doubtedly decrease cuticular transpiration; but their effectiveness in
curbing stomatal transpiration of any plant cannot be judged by appear-
ances. It can be determined only by carefully planned experiments.

Stomatal transpiration. Transpiration from mesophyll cells is similar
to that from epidermal cells, but there are a few important differences.
Let us first consider conditions on a spring morning when the soil is
moist and the whole plant is turgid with water. The sun is up and the
air is clear. Under these conditions the stomates are fully open.

The water-conducting tissues of the veins are filled with water slightly
diluted by inorganic salts or other substances. The vacuoles, the cyto-
plasm, and the walls of the mesophyll cells are nearly saturated with
water, and likewise the walls on the inner side of the epidermal cells.
This internal moist surface is 6 to 30 times that of the cuticular surface
and bounds the labyrinthine intercellular air passages. The energy from
the sun increases the molecular energy of the water molecules and their
rate of movement is speeded up. They leave the cell surfaces more rap-
idly and diffuse in all directions in the intercellular spaces, from which
they diffuse through the stomates into the atmosphere.

Outside the leaf, the atmosphere has a lower humidity and a lower
concentration of water molecules. Consequently the diffusion of water
vapor will be outward through the stomates. This loss of water vapor
from the mesophyll cells is far greater than the evaporation of water
from the epidermal cells. Quantitatively it amounts to 85 to 95 per cent
of the water that passes into the atmosphere from plants. Evapora-
tion of water from cell walls inside the leaf and the subsequent diffu-
sion of water vapor through the stomates is called stomatal transpiration.
When the leaf only is considered, it might also be called mesophyll
transpiration.

The number of stomates is so great, they are so evenly spaced among
the epidermal cells, and the individual pores are so small that diffusion
of gases and vapor molecules from the interior of the leaf may be almost
as great when the pores are open as if there were no epidermis on the
leaf. Later in the day as the stomates gradually close, the stomatal tran-
spiration is also reduced, but not much until the stomates are nearly
closed, because the rate of diffusion through the stomates is dependent
not upon their area, but upon the perimeter of the pore.

226 TEXTBOOK OF BOTANY

During the daylight period both stomatal and cuticular transpiration
occur. At night there is usually only cuticular transpiration. This differ-
ence plus the effects of the higher day temperatures are shown in the
following table in which the relative amounts of the total transpiration
from certain plants during daylight and darkness under field conditions
are stated in percentages.

Plant Daylight Darkness

Wheat 96% 4%

Oats 94% 6%

Alfalfa 97% 3%

Pigweed 97%, 3%

Opening and closing of stomates. The opening and closing of stomates
are obviously matters of first importance in modifying transpiration.
The relation of stomates to the movement of guard cells was discussed in
Chapter XXIII. The stomates on a plant are not opened or closed simul-
taneously, because conditions are not identical in all parts of a leaf or
in the different leaves on the same stem. Consequently, when we say
that the stomates are gradually closed, we mean that some are closed
quickly, some are closed slowly, and others scarcely at all. When we say
that the stomates are closed, we mean that almost all are closed and the
remainder nearly so.

We learned earlier that oxygen and carbon dioxide, gases important in
respiration and photosynthesis, diffuse into and out of the leaf largely
through the stomates. Here we are emphasizing water-vapor loss through
stomates. In this connection it is well to remember that stomates do not
open to promote photosynthesis and do not close to conserve water.
During droughts they may open most inopportunely and allow further
water losses. In late summer they may close early in the daytime and
restrict photosynthesis when all other conditions appear to be favorable.
The turgor movements of guard cells are conditioned neither by photo-
synthesis nor by transpiration, but by a series of changes in these cells.
These are slight changes in acidity which influence the activity of
enzymes and also the change from starch to sugar, or sugar to starch.
Increase in sugar content leads to diffusion of water into the guard cells,
and greater turgor. The stomates are opened. Change of sugar to starch
in the guard cells results in closing the stomates.

Movement of water from veins to mesophyll cells. Let us, however,
again turn our attention to the mesophyll cells. During the morning the
sugar content of these cells increases. They also contain salts, acids, and

[Chap. XXIV TRANSPIRATION

other soluble compounds that dilute the water.
As water molecules leave the walls and diffuse
into the intercellular spaces, other water mole-
cules are pulled into their places by the forces
of cohesion and surface tension. This cohesion
of the molecules of water in closed tubes is so
great that the pull extends all the way down
the veins and vessels of the leaves and stems.
For example, when the cut ends of stems in a
bouquet are placed in a vase of water, tran-
spiration from the leaves results in a movement
of water from the vase and up the veins of the
stems to the leaves and flowers exposed to the
air.

The removal of water from the cells of the
leaf increases the concentration of the sugar
and other solutes and decreases the concentra-
tion of the water. The water becomes less con-
centrated in the mesophyll cells than it is in
the xylem of the veins. Water molecules, there-
fore, diffuse osmoticallv from the vessels into
the adjoining mesophvll cells, and finally into
the epidei-mal cells, in both of which the con-
centration of water has been lowered by tran-
spiration.

The lifting power of evaporation may be
demonstrated by evaporation from a porous
porcelain cup suitably mounted on a long glass
tube, both of which have been completely filled
with freshlv boiled water ( Fig. 79 ) . The lower
end of the tube dips into a vessel of water and
mercury,^ When the porous cup is exposed to
the air, evaporation occurs at the surface of the
cup. This develops a tension which is trans-

^ Full details for setting up these experiments may be
obtained from papers by H. Thut, Ohio Jour. Science,
28:292, 298, 1928; Amei'. Jour. Bot., 19:358-364, 1932.

with a stopcock when the apparatus is not in use to pr
siphoned from the beaker.

227

X

T

\=

ill

*is=i>

A

Water

Mercury

Fig. 79. Diagram of ap-
paratus used to demon-
strate the lifting power of
evaporation. Tul^e C is a
meter or more in length.
This tube and the porous
cup, which is immersed in
water in beaker A, are
filled with air-free water.
The beaker is removed
onlv when the apparatus
is being used. The short
tube in bottle B is closed
event the water from being

228 TEXTBOOK OF BOTANY

mitted through the water column in the tube to the mercury, and the
mercury is pulled upward.

If the apparatus is carefully constructed and all air removed from
the water, the mercury column may be readily pulled to a height of 120
to 130 cm. When the cups were coated with gelatin and the size of the
pores was thus reduced, a height of 226.6 cm. was attained. Mercury is
13.6 times as heavy as water, and when evaporation of water lifts mer-
cury 226.6 centimeters it is equivalent to lifting water 100 feet. The
height to which the mercury is lifted in such experiments is limited by
the entrance of air into the cup or tube through minute pores. The
tensile strength of water enclosed in tubes is equivalent to at least 150 at-
mospheres, and the energy of molecular motion is sufficient to lift col-
umns of water many times the height of the tallest trees.

When twigs of arbor vitae, red cedar, or box elder are used in place
of the porous cup, transpiration may raise mercury columns 90 to 101
cm. in height before enough air enters the tube to terminate the demon-
stration. This is equivalent to a column of water 40 to 50 feet in height.
During the experiments from which the above figures were obtained
the height of the barometer varied from 73 to 75 cm. of mercury. Does
barometric pressure account for the rise of water in these experiments?

Water-vapor gradient. Next to the opening and closing of the stomates,
the most important factor influencing transpiration is the condition of
the water vapor inside and outside the leaf. If the concentration of
water vapor inside the intercellular spaces of the mesophyll is greater
than it is outside the leaf, there will be a gradient in the diffusion of
vapor from the cells of the leaf to the outer atmosphere. This gradient
is augmented by the relative rates of molecular motion whenever the
temperature of the leaf is higher than that of the atmosphere. The
gradient may be steep if the air outside is very dry and the air inside
nearly saturated, and the temperatures of the leaf and the air are the
same. Under these conditions the concentration of water molecules is
much greater in the air spaces of the leaf than in the atmosphere. If the
temperature of the leaf becomes higher than that of the atmosphere the
gradient will be increased because within the air spaces the number of
free molecules is increased as well as their rate of movement. If the
temperature of the leaf becomes lower than that of the atmosphere the
gradient will be reduced because the number of free molecules within
the air spaces and their rate of movement are decreased.

Let us assume that during the course of a clear warm morning in

[CJuip. XXIV TRANSPIRATION 229

summer the temperatures of both the atmosphere and the leaf are gradu-
ally raised from 68° F. to 86° F. The concentration of water vapor in
the atmosphere changes very Httle, because the volume of air is so
enormous and the air is continually moved by currents. Inside the leaf
the same rise in temperature results in rapid increase in water-vapor
content, because the volume of the intercellular spaces is very small
compared with the evaporation surface. Calculations indicate that the
vapor diffusion gradient between the inside of the leaf and the atmos-
phere has increased 2.5 times. If now the internal temperature of the
leaf rises an additional 9° above that of the air, the gradient is increased
to 4 times what it was in the early morning. Assuming that there is an
abundant supply of water within the plant and the stomates are open,
the rate of transpiration would be about 4 times as great. Guard cells
may open and close stomates and thus accelerate or stop stomatal tran-
spiration. But when the stomates are even partially open the most impor-
tant factor is the water-vapor gradient from inside the leaf to the air
outside.

Intensity of sunlight and transpiration. Under good growing conditions
light intensities naturally occurring in daytime are sufficient to result in
the opening of the stomates, except on extremely cloudy days and when
the plants are densely shaded. Sunlight thus indirectly accelerates tran-
spiration. Furthermore, the radiant energ)- of sunlight directly increases
transpiration in humid climates, because it raises the internal tempera-
tures of exposed leaves above that of the air, sometimes as much as 5° to
10° F. This in midsummer is enough to double or treble the vapor
diffusion gradient between the inside of the leaf and the atmosphere.

In the shade, leaf temperatures are about the same as those of the
air, or lower. When the atmosphere is very dry, leaf temperatures in
the shade may be several degrees cooler than air temperature.

Soil water and transpiration. Another factor that greatly influences
transpiration is the water content of the soil. Even though the stomates
are open and other conditions favor high transpiration, if the water
supply in the soil becomes low, transpiration may be markedly decreased.
If the soil water in immediate contact with the roots is soon exhausted,
water in the xylem vessels of the stem and roots fails to diffuse into the
leaf cells as rapidly as it evaporates from them. There is an increase of
tension in the water in the veins and vessels of the leaf, the stem, and
finally of the root; the mesophyll cell walls become less and less saturated
— hence fewer water molecules leave the cell surfaces.

230 TEXTBOOK OF BOTANY

The percentage of water in leaves during dry periods has been found
to be 5 to 10 per cent less in early afternoon than in similar leaves at
night. Under moist conditions the daily fluctuations may be only 1 to 5
per cent. Obviously even if the stomates are open the rate of transpira-
tion is reduced, since the mesophyll walls are dryer by midday than they
were in the earlv morning.

A dandelion plant rooted in a shaded ravine bottom may lose 5 to 10
times as much water as a similar plant living near the top of the south-
facing slope of the same ravine. The lower plant has a constant soil-
water supply; the upper plant has a very limited supply. A corn plant
in a moist region may lose more water in a season than a similar plant
in a dry region, simply because there is more water available. If the
corn plants in the dry region were growing in irrigated fields, they
would lose more water than the plants in moist regions.

If the soil is dry and little water passes into the plant, transpiration
from the leaf and stem surfaces may lead to wilting and finally to the
death of the cells of the leaves, the stem, and the roots. This is death by
desiccation. The lower or older leaves usually dry out first and the
younger leaves last.

Water-holding substances. Another eflFective factor that may retard
transpiration is the presence within the plant tissues of colloidal gels
such as pectic compounds, mucilages, and gums. When saturated, these
compounds, like saturated jelly, have little effect on the rate of evapora-
tion of water from them; but as their water content decreases, the force
by which the remaining water is held increases. Many succulents con-
tain these compounds. Cacti, which contain mucilages and are highly
cutinized, may retain sufficient water for life and renewal of growth for
a year or more. Even such thin leaves as those of tobacco, when re-
moved from the plant, may retain sufficient water to keep them pliable
for weeks in a room where most other leaves become desiccated and
brittle.

Wind. Moderate air currents may accelerate transpiration by the re-
moval of more or less saturated layers of air from the immediate sur-
faces of the plant. When these layers have been removed further increase
in wind velocity has little or no effect on the rate of transpiration.

The pull of transpiration. One of the consequences of transpiration
that extends throughout the plant is the movement of water to the leaves
from other organs. The net result of all the diffusions of water mole-
cules in leaves from vessels to mesophyll and epidermal cells is that

TRANSPIRATION

231

[Chap. XXIV

water, because of the cohesion of its molecules when confined in small
tubes, is pulled forward in the veins of the leaf and the vessels of the
stem. It is this pull upon the continuous water columns in the xylem
vessels of stems that accelerates the movement of water to the leaves at
the tops of trees.

If during rapid transpiration the upward movement of water is
stopped by compressing the stem or by severing the stem from the roots,
the water in the veins of the leaf and in the vessels of the stem is soon
depleted. No more water passes to the mesophyll cells and the epidermis.
Transpiration continues and leads to the loss of turgor of the mesophyll
and epidei-mal cells. The stomates are closed by the loss of turgor of the
guard cells, and the rigidity of the leaf declines — the leaf is "wilting."

On a clear warm day in midsummer the same sort of thing happens in
an intact plant, but it does not always proceed so far. If the rate of
transpiration is more rapid than the rate at which water enters from
the soil and is drawn up the stem into the leaf, turgor decreases and
stomates gradually close. Cuticular transpiration alone continues, and
the transpiration rate is greatly reduced. Water continues to move up
from the roots through the stem and into the leaf cells; and after a time
the water content of all cells is restored, and the cells become turgid.
Under the conditions assumed in this paragraph the lea\ es may not
regain their customary rigidity before evening. The stomates, how-
ever, may remain closed until after sunrise the following morning.

During dry periods in summer, water may move to the leaves from
nearby fleshy fruits on a tree, and as a result the fruits become shriveled.
The water is kept from the fruits by the pull of transpiration from the
leaves (Fig. 80).

a

hi

t-
liJ

MONDAY

NOON
i

4- 8 12 4- 6

TUESDAY

NOON
I

4- 8 12 4 6

WEDNESDAY

NOON
i

4. 8 12 4 6

THURSDAY

NOON

i

4 8 12 4 6

FRIDAY

NOON
\

4 8 12 4 6

SATURDAY

NOON

i

4 8 12 4 8

SUNDAY

NOON
i

4 8 12 4 6

4

O

/

^

->

\

/

y

\

/

y

\

/

^

>

\

/

/

■>

V

Z'

s

\

/

^

N

>

/

\

h

/

V

]

\

s

/

i

\

/

'

\

J

\

/

^

SI

/

V

J

^

/

\

j

J

bJ
(Y

Fig. 80. Diagram of daily increase and decrease in diameter of lemon fruits
attached to the tree. Transpiration from these fruits is negligible. The daily shrink-
age was due to movement of water from the fruit to adjoining stems and leaves.
During the night the water content of the fruit was restored. After E. T. Bar-
tholomew, 1926.

232 TEXTBOOK OF BOTANY

Summary. The principal internal factors that influence the rates of
evaporation and diffusion of water vapor from plants are: ( 1 ) the open-
ing and closing of the stomates; (2) the concentration of water vapor in
the internal air spaces of the leaf in comparison with that of the atmos-
phere; (3) the temperature of the leaf; (4) the water content of the
plant tissues, as affected by the rate of movement of water from the
soil; (5) the occurrence in the cells of colloidal gels which have a high
water-holding capacity, such as pectic compounds, mucilages, and
gums; and (6) the cutinized epidermal walls in some plants.

Wax and resinous coatings on the epidermis may reduce cuticular
transpiration, but their effect on stomatal transpiration is slight. Dead
hairs, even when forming felt-like coverings, decrease transpiration very
slightly or not at all; living hairs increase it to some extent. The value
of any superficial structures in conserving water cannot be estimated by
examining the plants. Their effects can be determined only by careful
experimentation .

The most important external factors that affect the rate of transpira-
tion are: (1) the energy of sunlight as it affects the internal tempera-
tures of leaves in relation to external temperature, and the opening and
closing of the stomates; (2) the temperature of the atmosphere and the
soil; (3) the concentration of water vapor in the atmosphere; (4) the
water conditions in the soil; and (5) the movement of air. Wind pre-
vents the accumulation of moist air about the plant surfaces; but when
the velocity is increased beyond that necessary to remove the layer of
moist air, there is little further increase in transpiration.

The evaporation and diffusion of water from the mesophyll cells are
followed by the osmotic movement of water from the veins into the
mesophyll cells. This movement in turn exerts a pull on the water col-
umns in the veins of the leaf, stem, and roots, and probably on the water
in all the other cells of the stems and roots. This is commonly called the
pull of transpiration.

REFERENCES

Meyer, B. S., and D. B. Anderson. Plant Physiology. D. Van Nostrand Com-
pany, Inc. 1939.

Sayre, J. D. Relation of hairy leaf coverings to the resistance of leaves to
transpiration. Ohio Jour. Sci. 20:55-86. 1920.

Thut, H. F. Relative humidity gradient of stomatal transpiration. Amer. Jour.
Bot. 26:315-319. 1939.

Turrell, F. M. Area of the internal exposed surface of leaves. Amer. Jour. Bot.
23:255-264. 1936.

CHAPTER XXV

TRANSPIRATION AFFECTS PLANT DEVELOPMENT
AND DISTRIBUTION

<><><j>(X><X><><><5><><>^^

Environmental factors that affect the rates of transpiration, photosyn-
thesis, or respiration often hmit the development and distribution of
plants. In green plants both photosynthesis and respiration are essential
processes. Without them these plants neither grow nor survive very
long. When respiration continuously exceeds photosynthesis green plants
die of starvation.

Although transpiration, unlike respiration and photosynthesis, is not
known to be essential to plants, its harmful effects become apparent
when water loss exceeds absorption and results in wilting, desiccation,
and even death. Excessive transpiration may profoundh' affect food
manufacture, growth, and the production of flowers, fruits, and seeds. It
is often an effective factor in limiting the geographic distribution of
individual plants and of plant communities. Because of its effects on the
qualities and yields of crop plants, transpiration has been an ever-present
influence in the regional allocation of crops in the United States.
Throughout human history it has been one of the primary factors that
limited the population of various geographic regions.

Tissue development in the growing regions of plants depends upon
an adequate water content in the cells. The enlargement of cells and
tissues ceases when transpiration exceeds the water supply. During hot
dry weather in summer a plant exposed to full sunlight may be actually
smaller in the evening that it was in the early morning. Tree trunks may
be measurably smaller in diameter in late afternoon than they were in
early morning.

Each spring we may observe the opening of buds and the grovvi:h of
new leaves on trees and shrubs. There is little cell division after the
leaves are one-fourth grown, and further expansion is the result of the
growth of cell walls and the osmotic absorption of water in the cells
of the leaf. During wann moist weather the volume of a young growing
leaf may become doubled during one night. When transpiration exceeds

233

234 TEXTBOOK OF BOTANY

absorption there will be no further enlargement. Likewise, any increase
in size of other parts of a plant occurs by cell division and enlargement
or by the enlargement of cells previously formed. Consequently, the
growth of all plant organs may be affected by transpiration. Further-
more, the rates of chemical processes (oxidation-reduction, hydrolysis,
and condensation ) may vary as the water content of the cells fluctuates,
and result in changing the chemical compounds formed in these cells.
Some of the effects of drought upon the growth of the epidermis and
mesophyll of leaves are illustrated in Chapter IX.

The effects of low water content in plants are most pronounced dur-
ing those occasional short or prolonged periods of drought, of high tem-
perature, or of low humidity, when transpiration greatly exceeds water
absorption in many species of plants. If a number of different species
are exposed to these extreme environmental conditions, the effects on
their individual rates of transpiration, their photosynthesis, their growth,
and their survival are usually different. The vegetative tissues of certain
fungi, mosses, ferns, and a few seed plants may become dry and
brittle without being killed. They may survive in this condition for
days or weeks, and resume growth when water becomes available.
The vegetative tissues of most plants, however, die from the effects of
excessive water loss long before they become air dry. A deficiency of
water within the cells of plants is sometimes referred to as "internal
drought."

Transpiration from seeds, bulbs, tubers, and other dormant organs is
slow, and these are the only organs of many plants that survive prolonged
periods of drought.

The very slow-growing and succulent species of desert regions and
other extremely dry habitats may survive, grow, and reproduce in sur-
roundings in which other species wither and die in a short time. Experi-
ments have shown that some of these succulents, when detached from
the soil and placed on a window ledge or table, not only survive for
months, but may even bloom and bear fruit several months later. The
extremely slow rate of transpiration from these plants in a hot desert
climate is almost incredible.

Some of the differences in rates of transpiration among a variety of
species of plants growing in diverse environments, are summarized in
Table 8.

On an average day in the growing season a mature corn plant may

[Chap. XXV TRANSPIRATION AFFECTS PLANT DEVELOPMENT 235

Table 8. Rates of Water Loss per Day in Midsummer:

A single plant of corn may lose 3 to 4 quarts

A single plant of giant ragweed 6 to 7 quarts

A single young 10-foot apple tree 10 to 20 quarts

A 12-foot columnar cactus 0.02 quart

A coconut palm in the moist tropics 70 to 80 quarts

A date palm in a Sahara desert oasis 400-500 quarts

lose the equivalent of 100 per cent of the water contained within it. On
the same basis a date palm may lose 90 per cent and a columnar cactus
.04 per cent.

Table 9. The Water Losses from Single Plants during a Growing Season
Estimated from Experimental Data

Length

of

Plant

Season
Days

in

Water Loss

Tomato

100

30 gallons

Corn

100

50 gallons

Sunflower

90

125 gallons

Giant ragweed

90

140 gallons

Mature apple tree

188

1,800 gallons

Coconut palm,

Phi

ilippines,

moist

tropics

365

4,200 gallons

Date palm, Sah

ara

desert oasis

365

35,000 gallons

Effects of excessive water loss. When rains are frequent and well spaced
throughout the growing season and the soil contains a favorable supply
of water, the enormous losses of water vapor by individual plants, or by
plant communities, are of little importance in the survival of the plants.
However, if transpiration in excess of water absorption leads to a
reduced water content in tissues, photosynthesis is decreased and growth
and reproduction are retarded. If this retardation of processes takes
place during the early life of many species of annual plants, the plants
may not fully recover from these effects when subsequently supplied
with an abundance of water.

Transpiration also affects the rate of loss of water from the soil. Water
may evaporate directly from the soil or it may pass from the soil into
the roots and then through the plant to the atmosphere. Every farm
boy knows that the soil under grass loses water more rapidly in the

236

TEXTBOOK OF BOTANY

Table 10. Estimates of the Water Losses by Transpiration from Crops and
Plant Communities Growing Under a Variety of Conditions

- ^

j Amount of Water Loss
[ per Acre

Kind of Plant i ;

in Gallons

in x\cre-
inches

Corn in eastern Kansas, 6,000 plants per acre, 100 325,000

days
Corn in central Illinois, 10,000 plants per acre, 100 400,000

days
Young apple orchard in central New York, 400 trees 240,000

per acre, 188 days
An acre of irrigated date palms in a southern Cali- 2,. 500, 000

fornia desert, 400 trees per acre, per year
An acre of 12-ft. columnar cacti in a southern Arizona 275

desert, 400 plants per acre, per year

11
15

9
90

0.01

spring of the year than land which has remained barren since the previ-
ous season. It may therefore be plowed earlier. In dry farming the pro-
cedure is to cultivate the fields during the seasons in which no crops are
planted to prevent the growth of weeds and the loss of water through
them from the deeper layers of soil.

Crop yields. Intimately associated with the effects of transpiration are
the yields of grain and other cultivated crop plants. The effects of exces-
sive transpiration probably result in greater reductions in the yield of
crop plants than all other factors combined, including diseases and
insects.

In dry regions where irrigation is practiced the yield of grain is greatly
influenced by the amount of water added to the soil. Following are
figures for corn ( Utah Experiment Station ) :

Acre-inches of

Yield in Bushels

Water Added

per Acre

0.00

26.00

15.00

53 . 00

20 . 00

63.00

37.00

76 00

[Chap. XXV TRANSPIRATION AFFECTS PLANT DEVELOPMENT 237

Similar results were obtained in experiments with wheat:

Acre-inches of Yield in Bushels

Water Added per Acre

4 6 4.5

8.9 11.8

17.5 15.8

30.0 26.6

In the eastern United States where irrigation is usually considered
unnecessary, market gardeners ha\'e found it quite profitable to add
water in addition to what enters the soil from rains. During prolonged
droughts this practice has often returned profitable crops when adjoin-
ing unirrigated gardens were a total loss.

Geographic allocation of crops. Crops are planted and cultivated for
economic reasons. The amount and quality of the \ield are accordingly
of the greatest importance. During pioneer days a far greater variety
of crops was planted on individual farms and in local regions. But as
transportation facilities increased, the commercial production of the
more important crops has gradually become allocated to those regions in
which the different varieties grow best.

Among the conditions which influence the yield of a particular crop
in various regions is the available water supply in comparison with the
rate of water loss. The center of broomcorn production, for example,
has been moved westward from the eastern seaboard to Illinois and
finally to Oklahoma. There broomcorn grows in company with other
species of sorghum. These plants ha^•e extensive root systems and smaller
leaf areas than corn and >'ield a larger profit than other grain crops in
dry regions. Corn is a more profitable crop in the corn belt states from
Ohio to Nebraska, where the soil is fertile, and, in addition, the tempera-
ture is high enough for maximum photosynthesis in the davtime and
not too low at night for continued growth, and the transpiration rate
is not so excessive. Macaroni or hard wheats attain their best quality in
the northern plains states, where drought is frequent but rarely exces-
sive for this kind of wheat. Tobacco that is culti\ ated for cigar wrappers
should have large thin leaves. Such leaves develop best where transpira-
tion is low, and the plants are set out either in moist or shaded valleys
or in the reduced light under cloth shades.

Irrigation is an ancient practice, but rapid means of transportation,
methods of preserving and shipping perishable crops, and scientific
methods of obtaining varieties of plants that mav be cultiA^ated in drier

238 TEXTBOOK OF BOTANY

climates are relatively modem. Previous to these discoveries every human
community was largely dependent upon local conditions. The discovery
of buried cities representing the former presence of large populations of
people in regions that are now deserts is historical evidence that trans-
piration is a primary factor in the restriction and migration of human
populations. At the present time more than 25 per cent of the land sur-
face of the earth is too deficient in moisture for crop plants without
irrigation, and only a very small portion of this land can be profitably
irrigated.

Distribution of local plant communities. The differences among the
native plant communities that develop in ponds, marshes, shrub swamps,
swamp forests, and the adjoining upland are so marked that almost
everyone can recall the distinctive appearance of both the plants and
the communities.

In the pond are wholly submerged "pond weeds," some rooted and
others not. On the surface may be floating algae or duckweeds, and
extending above the water surface are water plantain, arrowhead, water
smartweed, and many others. The marshes are characterized by bul-
rushes, cattails, sedges, and grasses; the shrub swamps bv certain dog-
woods, buttonbush, alders, and shrubby willows. The swamp forest is
dominated by species of sycamore, elm, ash, maple, hickory, and oak.
There are also scattered small trees and shrubs beneath the forest
canopy, and certain characteristic herbaceous plants form a ground
cover. An undisturbed upland forest in the same region consists of
another group of tree species such as the upland oaks, hickories, maples,
beech, walnut, and linden. It also has several layers of undergrowth —
small trees, shrubs, and herbs.

In the chapters on photosynthesis attention was called to the manner
by which certain kinds of plants may exclude others in a forest by
overshading them. Likewise the discussion of respiration included the
fact that only certain species of plants can grow submerged in water,
or where the soil is saturated with water and deficient in free oxygen.

Transpiration is a third factor in the elimination of species, and is
most effective in upland forests. Here high transpiration rates may cause
wilting and death during periods of drought. Again the plants most
affected are the seedlings with their limited root systems. But in ver\'
prolonged droughts certain species of trees in the swamp forest may be
injured to a greater extent than the upland trees because thev have
shallow root systems. In the complex forest communities the largest

[Chap. XXV TRANSPIRATION AFFECTS PLANT DEVELOPMENT 239

plants, while shading and reducing the transpiration of the plants
beneath them, also have far more extensive root systems. Consequently
they may remove so much water from large areas of the soil that the
herbaceous and woody plants beneath with small or shallow root systems
die.

Another example of the effectiveness of transpiration may be illus-
trated by the vegetation in gorges ( Fig. 81 ) . On the sides of the gorge

Most Intense Li^Kt"
V Higher Summer Temperatures
|v^\. HidKeat Evaporation Rate

Stream

;5^ Most Intense Shade

Lower Summer lemperaturcs
Lowest Evaporation Rate

2 feet above t^c sod

tli^hcst Ironspiratioa Rate
from low plants

Fig. 81. Diagram of contrast in vegetation on north- and south-facing slopes of a

gorge in Pennsylvania.

are rock cliffs above long talus slopes. A narrow flood plain borders the
stream. To picture the extreme conditions we may consider a section
of a valley extending in an east and west direction.

On top of the cliffs the upland hard pines are the only trees that have
survived. On the south-facing talus slope the mixed oak forest pre-
dominates, with scattered pines and hickories. On the steep north-facing
talus, hemlock, beech, birch, maple, and tulip are dominant. In the
upland surrounding this gorge the vegetation is oak, hickory, and pine.
Beech, hemlock, and birch have not survived above or on the south-
facing slope. Evaporation is several times as great there as on the
shaded, humid side of the gorge, and the water supply in the upper
layers of the soil is exhausted much sooner. Similar gorges where the
upland climate is too dry for trees such as beech, hemlock, tulip, and
birch may be found in many parts of the eastern United States. The

240 TEXTBOOK OF BOTANY

trees in the gorges are living in a humid "microchmate" where drought
rarely becomes sufficiently excessive to inhibit their development. The
undergrowth in such situations is just as different from that on the
south slope and the upland as are the dominant tree species.

Transpiration in relation to available soil water will be discussed fur-
ther in Chapter XXX.

Major vegetation types and transpiration. Transpiration when studied
in the laborator\' and greenhouse is often compared with evaporation
from a wet surface. Under the experimental conditions the curves for
evaporation and transpiration are frequently very similar. One may
therefore be led to the conclusion that where the rate of evaporation
of water is high in nature, the transpiration will also be high. This
reasoning overlooks the matter of water supply upon which all transpira-
tion is dependent. Forests, prairies, steppes, semi-deserts, and deserts
occur in regions of successively increasing drought and increasing
evaporation from water surfaces. The annual evaporation may be 50
times as great from an open pan of water in the desert as from a similar
pan in some wet forest region. The annual transpiration from desert
plants, however, is very slight when compared with that of plants in a
wet forested region.

Transpiration in nature decreases as one goes from forest to prairie,
to steppe, to semi-desert, and to desert for the obvious reason that the
plants in these formations have successively decreasing amounts of water
available. Their periods of greatest loss of water are more and more
limited to moist periods instead of throughout the year. The date palm
cited earlier in this chapter illustrates how great transpiration may
become in some plants when there is an unlimited water supply. The
cactus exemplifies how low it may be when the water supply is reduced
to a minimum. An acre of forest may lose 10 to 25 acre-inches of water
in moist temperate regions, while an acre of prairie grass in Nebraska
loses about 2 acre-inches. If transpiration of trees were as effectively
checked as in certain cacti without interfering with photosynthesis and
respiration, the whole landscape east of the Rockies would be domi-
nated by forests.

Is transpiration an essential process? Transpiration is an inescapable
physical process in most species of plants. Is it merely neutral, or harm-
ful, or is it an essential process comparable to photosynthesis and respira-
tion? This question has often been discussed pro and con in botanical
literature, and it is frequently answered without a clear analysis of the

[Chap. XXV TRANSPIRATION AFFECTS PLANT DEVELOPMENT 241

problem. In the case of a green alga or a flowering plant growing sub-
merged in water transpiration certainly is not an essential process. If the
plant is removed from the water and placed in the air, transpiration
occurs only as an unavoidable process and its effects soon cause death.
Transpiration goes on whether its effects are harmful or beneficial, just
as water evaporates from a wet towel or any other saturated object
exposed to the air. Consequently if transpiration is an essential process
in plants, it must be limited to those in which some part or the whole
plant is exposed to the atmosphere.

One should be clear whether the term "essential" is applied to the
survival of the plant, or merely to some particular quantitative difference
in its development. We have already seen that leaves are thicker, have
a thicker cuticle, more layers of palisade cells, and a greater amount
of woody cells when they develop in dry air where transpiration is
high than when they develop in moist air where transpiration is low.
Transpiration mav be essential to many such quantitative differences
among plants, without being essential to the survival of the plant.
These differences are merely the ultimate effects of transpiration on
development.

Among the claims that transpiration is an essential process are: (1)
that it is necessary to cool the plant on hot summer days, (2) that it is
necessary in the absorption and conduction of salts from the soil to the
leaves, (3) that it increases the water supply of the plant, (4) that it
regulates the water content of the plant, ( 5 ) that it maintains the water
supply of leaf cells, and (6) that it would not occur in plants if it were
not of some value to them.

The fallacies implied in the last four statements should be so obvious
from data discussed in previous chapters as to make further discussion
unnecessary here. Some additional data may help to evaluate the first
two statements.

The cooling effect of transpiration is most frequently cited as its
essential feature. It is claimed that if transpiration did not occur, the
temperature of leaves in full sunshine on a hot summer day would rise to
150° F. or more; this rise is sufficient to kill protoplasm. The well-known
fact that the evaporation of water lowers the temperature of the mass
from which it is evaporating is sufficient evidence that transpiration has
a cooling effect on the plant, but it is not sufficient evidence that it is
essential, or that it prevents the temperature of a leaf from rising to 150°
F. every day in summer.

242 TEXTBOOK OF BOTANY

When one compares the enormous differences in the rate of transpira-
tion in the date palm, coconut palm, and cactus in the tropics he might
expect them to differ greatly in their internal temperature. Yet measure-
ments have shown that the temperature of all plants corresponds fairly
closely to that of the atmosphere in which they are growing. The daily
evaporation of water from the columnar cactus cited earlier in this chap-
ter would lower its temperature less than 1/1000 of a degree, yet it
daily absorbs sufficient radiant energy from the sun to raise the tempera-
ture and kill the protoplasm if there were no other way by which heat
passed from the plant to the surrounding air. A hot stove loses heat by
radiation and convection. A plant warmer than the surrounding air also
loses heat by radiation and convection.

Measurements have shown that when the temperature of a plant is
more than a few degrees above that of the surrounding air the loss of
heat from the plant by radiation and convection usually exceeds the
amount lost by transpiration. The greater the difference between the
temperature of the plant and that of the surrounding air, the less impor-
tant is the loss of heat b)^^anspiration in comparison to that lost by
radiation and convection.

The relation of transpiration to salt absorption and conduction is an
indirect one. The date palm mentioned earlier absorbs and loses 40,000
times as much water as the cactus plant cited in the same table. The roots
of the coconut palm grow in both brackish and fresh water. If the
absorption of salts depended upon the absorption and transpiration of
water, one would expect to find corresponding differences in the salt con-
tent of these plants. No such differences occur. Salts enter the roots of
plants as molecules and ions by diffusion. This diffusion of salts is
undoubtedly influenced by electrical charges, but, as we have already
seen, it is independent of the diffusion of water into the cells. After the
salts have entered the plant, further diffusion accompanied by flow in
the xylem vessels and protoplasmic streaming accounts for their move-
ment within the plant.

Transpiration may indirectly influence the total amount of salts ab-
sorbed by plants through its effects upon several other processes. Two
plots of bean plants were placed under experimental conditions so
that the plants in one plot grew in a saturated atmosphere, the others in
an atmosphere having only 25 per cent humidity. Transpiration was
much greater in the plants exposed to the drier atmosphere. These plants
were not as tall as those that grew in the saturated atmosphere. Their

[Chap. XXV TRANSPIRATION AFFECTS PLANT DEVELOPMENT 243

root systems were larger in proportion to the leaves and stems above
ground, and both their dry weight and salt content were greater. That
is, there was a greater amount of photosynthesis in the plants in the
drier air, a greater amount of assimilation of foods, and a greater amount
of salts absorbed. Here again is evidence of quantitative variations in
plant development due in part to differences in transpiration, but such
facts are not evidence that transpiration is essential to the plant. One
may say, however, that it is essential to certain differences in develop-
ment, provided that these same differences cannot be obtained by chang-
ing some other process.

Most plant phvsiologists have come to the conclusion that wet cell
walls, exposed either directly to the atmosphere or to the intercellular
spaces necessarily lose water to the surrounding air by evaporation.
Through a long process of evolution stomates have become an hereditarv
structure in all the larger land plants. The opening and closing of these
structures have a marked effect upon the inward and outward diffusion
of COi', O2, and water vapor. When stomates are closed there is nothing
to prevent the slow diffusion of water through all plant surface walls
even though the walls contain cutin and suberin. When stomates are
open there is nothing to prevent the rapid diffusion of water vapor
from the mesophyll cell walls through the intercellular spaces and the
stomates to the atmosphere. This rate is greatlv increased in full sun-
light when leaf temperatures rise above that of the atmosphere.

If in the course of plant historv a cell wall substance permeable to
oxygen and carbon dioxide but impermeable to water vapor had
developed, the present daily waste of water in transpiration from plants
might have been eliminated.

Summary. Transpiration effects are best visualized when excessive
water-vapor loss decreases photosynthesis, growth, and the develop-
ment of flowers, fruits, and seeds. It is a menace in the life of every land
plant except those having roots in a permanent water supply.

Excessive transpiration is responsible for more crop failures than all
otlier factors combined. As new lands were opened in the Central States,
centers of production of various crops became stabilized where the crops
were most productive, had superior quality, and therefore where
they were most profitable. One of the factors involved in this move-
ment is transpiration. When agriculture moved during a few favorable
years into the short-grass lands, succeeding dry years resulted in exces-
sive transpiration, killing of the vegetation bv desiccation, wind erosion,

244 TEXTBOOK OF BOTANY

dust storms, and dunes. Transpiration is a factor both in the develop-
ment of local plant communities and the segregation of the larger vege-
tation types as forests, grasslands, and deserts.

REFERENCES

Freeland, R. O. Effect of transpiration upon the absorption of mineral salts.

Amer. Jour. Bot. 21:373-374. 1937.
Miller, E. C. Plant Pliysiologij, 2nd ed. McGraw-Hill Book Company, Inc.

1938. Pp. 236-240.
Muenscher, W. C. Effect of transpiration on the absorption of salts. Ainer.

Jour. Bot. 9:311-330. 1922.
Wright, K. E. Transpiration and the absorption of mineral salts. Flant Physiol.

14:171-174. 1939.

CHAPTER XXVI

FORMS AND EXTERNAL FEATURES OF STEMS

<><><><x><><>e<><^<><><x><^^

Several facts about stems have already been considered. We have seen
that leaves and axillary buds appear on the nodes of stems in a definite
arrangement, and that stems of shrubs branch at or near the surface of
the ground, in contrast to stems of trees. Foods may accumulate in any
stem, but a greater amount accumulates in some stems than in others.
The green stems of cacti and man)' herbs and the numerous young
twigs of both shrubs and trees contain chlorophyll, and hence photo-
synthesis may occur in them. In many processes previously discussed
it was evident that sugar made in the leaves moves through the stems
to the roots; that water and inorganic salts after diffusing into the roots
move to the leaves through the stems, and that processes characteristic
of all living cells occur in living cells of stems. Some stems are short-
lived; others remain alive and increase in both height and diameter
for centuries. We have already seen that the direction and rate of stem
growth are affected by gravity, light, moisture, and other environmental
factors. In many sections of the country where deciduous forests prevail,
the principal features of the forested landscape from late autumn until
the early springs are stems.

What is a stem? The cylindrical, erect, aerial stems of most plants are
readilv distinguishable from leaves and roots. Many stems, however,
are flattened leaf -like organs; others are succulent, and still others are
so short that they are nearly obscured by leafy scales or rosettes of leaves.
Many stems are entirely underground. What, then, are the distinguishing
characteristics of stems? A stem develops from the plumule (bud) of the
embryo, usually bears leaves and flowers, and has certain other charac-
teristic features, both external and internal. Examination of several kinds
of stems will help us recognize these features.

Woody stems. Stems 1 to 2 feet long of such plants as hickory, horse-
chestnut, or maple (Fig. 82) consist of several yearly increments of
growth. The most prominent features of woody stems in winter are the
dormant buds, leaf scars, terminal bud scars, and lenticels. Leaves and

245

Fig 82 External features of stems and composition of buds of horse chestnut
(Aescidus hippocastanum). A, twig bearing a terminal reproductive bud (1), and
a lateral branch bearing a terminal vegetative bud (2); nodes, internodes, lea
scars, vascular bundle scars, terminal bud scale scars, lenticels, and the lateral
axillary buds are also represented. B-C, longitudinal sections of reproductive buds:
B dormant in winter; C, unfolding in spring. D-E, longitudinal sections of vegeta-
tive buds: D, dormant in winter; E, unfolding in spring. F termmal bud scale
scars and other external features on a 7-year-old branch in which there was com-
paratively httle elongation of internodes each season.

246

[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS 247

branches usually occur only at certain places on the stem called nodes.
The part of the stem segment between two nodes is the inter node. The
length of a twig or branch and the height of the plant depend upon the
amount of elongation of the internodes.

We noted in an earlier chapter that deciduous trees lose their leaves
annually and that the scar on the stem left by the abscised leaf is called
the leaf scar. Within the leaf scar are smaller scars. These small scars
are the severed ends of vascular bundles that extended from the leaf
into the stem, and are known as bundle scars or vein scars. The age of
the twig may often be determined by counting the ring-like markings,
the terminal bud scale scars, found at intervals along the stem. These
rings may be used as a means of locating the position of a dormant
terminal bud of some previous year. The distance between two succes-
sive bud scale scars is a measure of the growth in length of the stem
during one year. This length may be used to compare the annual growth
of stems of different plants, as well as the variations in growth from
year to year on the same plant.

Scattered over the surface of the internodes are numerous small,
roundish or elongated structures, the lenticels. When young, these are
made up of loosely arranged cells through which gases pass into or out
of the bark of the twig. Stomates are often present in very young stems,
and the formation of a lenticel may be initiated by the division of cells
immediately beneath a stomate. Later cork develops beneath the
lenticel. Owing to the peculiar growth of the bark of such trees as
birch and cherry, the lenticels become greatly elongated and partly
encircle the stem.

Conspicuous external features of stems during dormant periods are
the buds. There may be only one bud at a node, as on most stems with
alternate leaves. Where the leaves are opposite there are two buds at
each node. On the stems of other species of plants there may be several
buds at each node either arranged in a whorl about the node or grouped
about the primary axillary bud. In species of soft maple and forsythia
these accessory buds are usually flower buds. The bud at the apical end
of the stem is the terminal bud; all others are lateral buds. The terminal
buds of some plants, such as willow, elm, hackberry, linden, and tree of
heaven, die and drop off in the spring of the year before the elongation
of the internodes of the new stem segment is completed. The last lateral
bud formed on twigs of these plants appears to be terminal in position,
if only a casual observation is made.

248 TEXTBOOK OF BOTANY

Bud scales. The buds of most trees and shrubs of the temperate
zones have external leaf -like scales, which are usually hard and fibrous
as are those in tlie buds of oak, elm, maple, and buckeye. Less frequently,
the buds are enclosed by stipules as are those of the tulip tree, magnolia,
and some viburnums. The scales are often hairy or sometimes covered
with wax and resins. These scales decrease transpiration from the tissues
in the bud but do not "protect" them from freezing. In some plants
there is a gradual transition from the bud scales on the outside to true
leaves on the inside of the bud.

Nearly everyone has witnessed the opening of buds on shrubs and
trees in spring. Sometimes the hard scales on the outside of the bud are
pushed off by the enlargement of the inner scales and young leaves ( Fig.
82 ) . In other buds the scales may elongate somewhat, but in a short time
abscise and fall off.

Composition of scaly buds. The composition of a scaly bud may be
recognized easily by removing the scales and other structures, or b\'
examining longitudinal sections of the bud, or, better still, by studying
unfolding buds in spring. Each bud contains the growing point of a
stem, the stem tip; in most buds leaves have begun to develop on the
stem tips. Such buds are vegetative buds (also called branch buds and
leaf buds) and from them develop the leaf-bearing segments of the
stems of the current season. A careful examination of perennial woody
stems will indicate that the number of young leaves in the bud cor-
responds with the number of leaf scars on each annual segment of stem
growth. What does that fact indicate? What exceptions have you seen?
Are all the leaves of a herbaceous plant, such as coleus, present in the
terminal bud at a given time?

A similar examination of some buds of elm, forsythia, cherry, peach,
or soft maple will reveal a stem tip bearing young flowers, but no leaves.
These are the reproductive buds, which are also called flower buds and
fruit buds. Subsequent growth of these structures results in flowers and
fruits. In other buds, such as those of horse-chestnut, lilac, apple, and
catalpa, both flower clusters and leaves are present. Such buds may be
termed mixed buds.

Composition of non-scaly buds. If we examine actively growing her-
baceous plants, we find that the buds are less conspicuous because of
their small size and the usual absence of bud scales. The leaves near the
growing stem tip are small; the youngest ones are folded about the
stem tip just as they are in scaly buds. A longitudinal section of a coleus

249

[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS

stem tip under the microscope is seen to have all the structures found in
scaly buds except the scales ( Fig. 32 ) . In addition to the stem tip there
are young leaves, with additional buds in their axils. These small axil-
lary buds within the larger bud may consist either of stem tips bearing

AXILLARY FLOWER PRIMOROIUM

•AXILLARV BRANCH PR1^K3RDIUW
BASE OF LEAF

eJ

-^^

r A 1

B

Fig. 83. Diagrams of longitudinal sections of terminal buds of coleus plants.
In A the axillary buds, except the lowest pair, consist of stem tips bearing flower
primordia of various ages. In B the axillary buds consist of stem tips or of stem
tips bearing leaf primordia.

leaf primordia or of stem tips bearing flower primordia, depending on
whether the plant is continuing vegetative growth or is changing to
flower formation ( Fig. 83 ) . This change to a reproductive state in coleus
is accompanied by a change in the shape of the temiinal bud and the
upper pair of leaves.

The youngest bud primordia consist only of stem tips. Buds of many
embryos and those in the "eyes" of potato tubers often consist only of
stem tips. Apical growth is particularly characteristic of stems and roots.

A bud then may be defined as a stem tip having extremely short
internodes and bearing young leaves or flowers, or both. It may or may
not be enclosed by bud scales. Stem segments bear leaves but once,

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and all the leaves of a tree or shrub may have been present in some
stage of development within the doraiant buds of the preceding season.
Adventitious buds. Branches usually develop from buds in the axils of
leaves and may be referred to as axillary branches. Branch buds may,
however, develop from internodes of young stems, from roots, and even
from leaves. These are adventitious buds. Their occurrence on the inter-
nodes of stems is often a result of some interference with the growth of
the tissues, as injurv or disease. New branches from trunks of trees

Fig. 84. Cross sections of stems of Virginia crab apple. A, two latent buds, the
traces of which extend inward to the first cylinder of xylem. They began as axillary
buds on the one-year-old stem and have survived for 13 years. B, a water-sprout
has developed from a latent lateral bud. Photo by V. T. Stoutemyer.

several years old, and sprouts from the base of stumps were formerly
thought to develop solely from adventitious buds, but recent investiga-
tions have shown that some of them develop from lateral buds that
survive in the bark of the tree for many years (Fig. 84). Others start
from adventitious buds. Pollarding (Fig. 85) results in the growth of
many lateral buds and a dense growth of new branches. A more thorough
study will have to be made before one can say whether these new
branches on pollarded trees start from latent lateral buds or from
adventitious buds.

Apical dominance. When the terminal bud of a twig is cut off, branches
develop from lateral buds that would otherwise have remained donnant
or died. Moreover, such branches usually grow from the uppermost
lateral buds. This inhibiting influence of the buds nearest the apex over

[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS

251

Fig. 85. Pollarding ot trees is followed by the growth ot numerous branches from
latent lateral buds in the decapitated stems. Pollarding is practiced where willow
shoots are used in making baskets and furniture. From sketch by Van Bosse.

the buds lower down the stem is known as apical dominance. From pres-
ent experimental data it appears to be the eflFect of at least two hormones,
one of which is made in the young leaves. If this hormone is artificially
applied to the cut ends of decapitated stems it checks the growth of the
lateral buds. Apical dominance, through its influence on the relative
growth of different parts of the plant, is one of the primary factors in
determining tlie form of a plant.

Buds and stem types. Many trees and shrubs may be recognized at
considerable distances by the form of the plant and the type of branch-
ing. Such features are often obscured by the effects of environmental
factors or by repeated pruning. Plants of the same species may be far
from identical in appearance when growing in dense forests and in the
open, or along water courses and on dry uplands. The appearance of
ornamental shrubs may be changed markedly by the removal of certain
dominant buds, thus allowing for the growth of other buds as noted in
the paragraph above.

By comparing a palm, a spruce, and an elm, three types of tree
crowns may be distinguished with reference to the mode of branching.

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Branches usually do not grow from the lateral buds of the palm, and the
growth of the terminal bud alone results in an unbranched stem or
trunk with a crown of leaves at its summit. Such a stem is said to be
columnar (Fig. 86).

Fig. 86. Columnar stems of the cabbage palmetto. Photo from U. S. Forest Service.

A dominant terminal bud is present in spruce. Each year the main
stem increases in length, and from the lateral buds branches grow out-
ward, forming a whorl at the apex of the previous year's growth. The
type of branching which results in a prominent main stem that extends
beyond the smaller lateral branches is referred to as excurrent branch-
ing(Fig.87).

In contrast to the spruce, the terminal buds of the elm are tempo-
rary. Since all branches of an elm develop from lateral buds, the main
stem appears to be repeatedly divided and subdivided until it is lost in
the crown of the tree. The tree trunk is ultimately terminated by in-
numerable branchlets, and this suggested the name, deliquescent branch-

[Chap. XXVI FORMS AiND EXTERNAL FEATURES OF STEMS 253

Fig. 87. Excurrent branching of the alpine fir, black hemlock, and lodgepole pine;
Bitter Root Mountains of Idaho. Photo from U. S. Forest Service.

Fig. 88. Deliquescent branching of post oak in an open field. Diameter of crown,
60 feet. Photo by C. H. Jones.

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TEXTBOOK OF BOTANY

ing ( Fig. 88 ) . Although these types of branching are readily distinguish-
able, all gradations among them occur.

The form of a plant is thus seen to depend on the relative development
of terminal and lateral buds. As indicated above, advantage is taken of
this knowledge by florists, gardeners, and orchardists. Various orna-
mental effects in lawn trees and shrubs may be obtained by removing the
terminal buds that inhibit the growth of the lateral buds. Fruit trees and
grape vines are "pruned" to regulate the shape of crowns as well as the
number and spacing of the branches.

Fruit spurs. It was indicated earlier that the age of a twig could be
determined by counting the number of scars left in successive years by
the terminal buds. The distance between two such scars may be indica-
tive of the favorable or unfavorable conditions under which the plant
grew in a particular year, or it may be indicative of local internal
conditions.

On fruit trees such as apple and pear, the flower buds often occur at
the ends of short stems known as "spurs." When the terminal bud is a

Fig. 89. Fruit spur of apple. Fruits were borne every other year, as indicated
by the large scars on the annual stem segments formed during odd-numbered
years. Courtesy of J. H. Gourley.

flower bud, subsequent elongation of the spur is dependent on a lateral
bud. Frequently the spurs bear flowers only in alternate years, and the
fruit scars are found only on alternate annual segments of the spur
(Fig. 89). The spurs usually continue to bear flowers and fruits for
many years and grow much more slowly than the vegetative branches.
Successive development of lateral buds results in a crooked spur. The
interval between successive terminal bud scale scars is exceedingly
short during the years that fruits are borne.

Saint John's shoots. Some trees, such as elms, hackberry, and certain
oaks, have two periods of stem elongation each growing season: one in
early spring and another in early summer. The twigs of citrus trees in

[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS 255

California have three such intra-seasonal cycles of growth. The twigs on
young trees of the rubber plant ( Hevea ) in Brazil may have as many as
eight cycles of growth in one year. In the latitude of Ohio, a second
elongation of shoots on elms is usually evident during the latter half of
June, and frequently a third extension appears in August. In Europe
these shoots have been called "St. John's shoots" and "Lammas shoots"
because of their coincidence with the festivals of St. John's Day and
Lammas Day.

Recognition of trees by twig characters. To those of us who are in the
habit of recognizing common trees by leaf characters, it is at first sur-
prising to find that identification can be just as certain with twig charac-

FiG. 90. Bud and twig characters of a few woody plants: A, tree of heaven
(Ailanthus) ; B, tulip tree (Liriodendron) ; C, hazel (Conjlits) ; D, red oak {Qiier-
cus); E, white heart hickory {Carya alba); F, bitternut hickory {Canja cordi-
jormis) .

ters (Fig. 90). Trees may be recognized by types of branching, which
are usually more obvious in the absence of leaves. A critical comparison
of the bark, buds, lenticels, and pith of our common trees soon enables
one to select a few simple distinctive characters by which these trees
may be readily identified during the winter. The prevailingly opposite

256 TEXTBOOK OF BOTANY

branching of the younger twigs of ash, maple, and buckeye; the in-
numerable and slender ultimate branches of the elm; the papery bark
and elongated lenticels of some birches and cherries; the resinous buds
of horse-chestnut; the star-shaped pith visible in cross sections of oak
twigs; and the chambered pith of walnut and hackberry are a few ex-
amples of distinguishing characters that may be selected. Such characters
may be used as a basis for making a key for the ready identification of
woody plants.

Herbaceous aerial stems. Stems of herbs are usually readily distin-
guished from those of trees and shrubs by the comparatively smaller
amounts of woody tissues. Some annual stems, such as those of sunflower,
giant ragweed, and a few asters, become woody at maturity in certain
habitats, but they are classed with herbaceous plants. It is really a mat-
ter of opinion whether certain semi- woody plants should be called herbs.
Manv herbs are annuals or biennials, in contrast to trees and shrubs
which are generally perennials. Many herbaceous plants have perennial
underground stems some of which are very hard and woody; but the
annual aerial shoots that develop from these stems are herbaceous.
Herbaceous stems are frequently green and have stomates.

Although many external features common to woody stems are absent,
herbaceous plants are nevertheless often identified by certain features of
their stems, such as shape, color, amount of hairy covering, and the
presence of certain types of prickles, thorns, and tendrils.

Rhizomes and runners. A common type of underground stem is repre-
sented by the rhizome of bluegrass, Johnson grass, cord grass, Canada
thistle, and Solomon's seal. Rhizomes grow horizontally at some depth
below the surface of the soil, have scale-like, non-green leaves, and
axillary lateral buds from which aerial branches develop at certain
seasons of the year. Roots generally appear on the ventral sides of a
rhizome at the nodes. The rhizome may be thick and fleshy ( Solomon's
seal. Fig. 91 ) , or slender and woody ( many grasses ) .

Slender, prostrate stems, such as occur in strawberry and some ferns,
are often termed runners or stolons. These prostrate stems and all under-
ground stems are excellent means of vegetative propagation, both natu-
rally and artificially. They are distinguished from roots by the presence
of nodes and internodes. The "turf" of lawns and meadows is a shallow
layer of soil held together by an interwoven mass of rhizomes and the
accompanying roots.

[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS 257

Fig. 91. Rhizome, roots, and base of an aerial stem of Solomon's seal collected
early in July. An erect aerial shoot develops each year from the growing apex ot
the rhizome. The round scar formed at the end of the season when the aerial
stem abscises is the so-called seal. Following the growth of the aerial shoot a new
rhizome segment develops annually. The older segments at the opposite end of
the rhizome gradually die and decay.

Tubers, corms, and bulbs. An underground stem familiar to everyone
is that of the Irish potato. The slender rhizomes of the potato enlarge
terminally into thickened structures known as tubers. Each eye of the
tuber consists of small scales and a cluster of rudimentary buds. When
the tuber is planted, an aerial branch develops from a bud in one or more
of the eyes, depending upon the degree of apical dominance. Large
quantities of starch accumulate in the potato tuber, and it has become an
important source of food, even replacing bread made from cereals in
some countries. Tuber-like thickening may occur also in aerial stems as
in kohlrabi. Some of the so-called short thick roots such as globe radish
and turnip, are mostly enlarged hypocotyls.

Another distinctive type of stem resembling the tuber is the corm,
characteristic of jack-in-tlie-pulpit, dasheen, and gladiolus (Fig. 92).
It is a short, upright, thick stem commonly covered by thin membranous
scale-like leaves. It bears both lateral and terminal buds.

A bulb consists of a short, upright stem bearing thick leaf bases,
axillary buds, and a prominent terminal bud. The underground bulbs of
hyacinth, tulip, onion, and garlic are familiar to all. A few bulbs are
aerial, such as the sets produced on the flower stalks of onion and

258 TEXTBOOK OF BOTANY

Fig. 92. Pictures of corms of gladiolus shortly after germination; at the right
is a vertical section cut from the center of a corm. Photos from P. W. Zimmerman
and A. E. Hitchcock, Boyce Thompson Institute.

the axillary structures of some lilies. Young bulbs which grow from
tlie axillary buds of the main bulb are sometimes termed bulblets.

Failure of the internodes of the stem to elongate may also result in a
head, as in cabbage; or a rosette, as in dandelion, evening primrose, and
mullein. Ultimately elongated stems bearing leaves and flowers develop
from the terminal bud in the center of the rosette.

Thorns, spines, and prickles. Thorns are small, sharp-pointed stems;
they may bear buds and leaves. Common examples are the thorns of
hawthorn and honey locust ( Fig. 93 ) . The term spine should probably
be used for leaf structures alone, such as the spines of barberry and the
stipular spines of black locust, in spite of the fact that spines and thorns
in common language often refer to the same structures. Pointed struc-
tures common on the stems of blackberry, rose, and smilax are prickles,
which are merely outgrowths of the epidermis and cortex.

Climbing and twining stems. Many plants, such as grape, Virginia
creeper, Boston ivy, and wild cucumber, may extend vertically some-
times for great distances by growing on or about other plants or objects.
Some stems have tendrils^ which encircle the support and anchor the
growing vine. The Virginia creeper and Boston ivy have branching
tendrils the ends of which are flattened disks or holdfasts. Tiny out-
growths from these holdfasts penetrate the crevices of the surface on

■^ These should not be confused with the leaf tendrils of the garden pea or the nasturtium.

[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS 259

Fig. 93. Thorns on honey locust.

which the plants are growing. In this way the vines can "cUmb" vertical
walls. In the morning glory, the hop, and some beans the stem itself
winds about the support and may be called a twiner. It is interesting to
note that some plants twine clockwise and others counterclockwise, when
viewed from above. No explanation of this difference in twining has been
established.

Cladodia. This type of stem is found among the cacti and some other
desert plants. Leaves are usually absent or ephemeral, and spines often
develop on the green stems. Such stems are referred to as cladodia
( singular cladodiiim ) .

Dormancy in buds. It is a well-known fact that mature buds of most
woody plants, those in the "eyes" of potatoes and in bulbs pass through
a dormant period after maturity. During this time there is no external
evidence of growth, regardless of the environment in which the buds
are placed. Such dormancy is the result of physiological conditions
within the buds, and does not disappear until these inner conditions
have changed. In temperate climates it may persist for several months.
It is often commercially important for this dormant period to be short-
ened as much as possible.

Most dormant buds will grow much sooner than usual if they are sub-
jected to various artificial treatments that accelerate certain physiologi-
cal changes in them. These treatments are referred to as methods of
"breaking dormancy." In temperate climates the internal conditions that
cause dormancy in buds disappear under the influence of the low

260 TEXTBOOK OF BOTANY

temperature of the winter months. These conditions disappear much
more slowly in some species than in others. As soon as they have
disappeared, both flower and leaf buds of many woody plants will grow
if the plant or stem cuttings are placed in a warm room and supplied
with water. Often root fonnation occurs at the base of the cuttings. If
the conditions causing dormancy have not disappeared, the plant may
slowly die.

In recent years the vapor of certain chemical compounds" has been
used to shorten the period of dormancy in the buds of many plants. The
dormant period of buds of Irish potato has been experimentally shortened
from one to four months, depending upon the variety tested. In the lati-
tude of New York some of the common shrubs, such as lilac, flowering
plums, crabs, and quinces, if exposed to vapor of ethylene chlorhydrin
in December will bloom within a month. The plants or twigs should
be placed within a tight box containing 1 ounce of ethylene chlorhydrin
for each 8 cubic feet of space. After two days they should be trans-
ferred to a warm room or greenhouse. If these artificial treatments are
applied early in autumn the results are unsuccessful. Treatment with
temperatures near freezing for three or four weeks in early autumn
before ethylene chlorhydrin is applied shortens the dormant period still
more. Methods for breaking dormancy in buds are at present in an
experimental stage of development but anyone may try them on some
particular plant. Storage at low and high temperatures, drying, wound-
ing, and treatment with special chemicals have all been used to
shorten the dormant period of buds. Diverse results have been obtained
with different plants.

This chapter contains a brief account of external similarities in stems
and of some of the usual differences in form and habit of growth that
may be found. Ordinarily stems may be recognized by the presence of
one or more characteristic external features, such as nodes, leaves, and
leaf scars. They may be herbaceous or woody, aerial or underground.
They may vary in form from the cylindrical stems of trees, shrubs, herbs,
and vines to tendrils, thorns, and the short thick stems characteristic of
succulents, tubers, corms, and bulbs. Most stems bear leaves, flowers,
and branches. Under certain conditions roots may also grow as lateral
organs on many kinds of stems. The fonii of the plant and its type of
branching are dependent on the relative development of branches from

^ Ethylene chlorhydrin, ethylene dichloride, edier, carbon bisulfide and many others.

[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS 261

terminal and lateral buds. Dormancy during summer and autumn is com-
mon in buds of most trees and shrubs in temperate climates. The period
of this type of dormancy may often be shortened by artificial changes
in temperature and by the application of certain chemical compounds.
Numerous buds on each plant are kept dormant by apical dominance.

CHAPTER XXVII
GENERAL REGIONS AND PROCESSES IN STEMS

The facts already learned about processes and structures in cells and
tissues of leaves may be applied at once to an understanding of stems.
Moreover, we should by this time more fully appreciate and apply four
generalizations concerning the relations of structures and processes : ( 1 )
that all structures are the consequences of certain processes; (2) that
these structure-forming processes are conditioned by heredity, and influ-
enced by environmental factors; (3) that differences in structure,
whether they are dependent upon differences in heredity or in environ-
ment, were preceded by differences in processes; and (4) that after
the structures are once formed they in turn influence all the processes
that continue to occur in them. In the light of these generalizations,
reference to any structure as having a "function" leads to a misinterpre-
tation of all plant phenomena.

The more minute differences in stem structure, like those of leaves, are
so numerous that one might study them for a lifetime without learning
all about them. Much useful and interesting information about processes
and structures in stems may be obtained, however, in a short time, and
much of it without even a microscope. All one needs at first is a sharp
knife and the stems of several kinds of plants. The familiar trees, shrubs,
and herbs of any locality are suitable material. In some of them certain
stem structures are more prominent than in others, and there are particu-
lar structures that are not present in the stems of all kinds of plants. A
hand lens will be useful in these preliminary observations, and a micro-
scope will be needed to obtain answers to some of the problems that
arise.

Woody stems. If small pieces of woody stems, such as those of apple,
ash, and grape, are cut crosswise and split lengthwise, the bark, the
wood ( xylem ) , and the pith are at once evident. The relatively greater
hardness of the wood cylinder may be detected by pressing on each tissue
with the edge of the thumb nail. These three regions are characteristic of
all woody stems.

262

[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 263

If blocks of older and larger woody stems are examined, it is evident
that the wood cylinder has increased in diameter, that the bark is
somewhat thicker than in younger stems, and that the diameter of the
pith has remained unchanged. Evidently growth in diameter of stems
consists of the formation of new xylem and, to a less extent, of new bark
tissues.

When the older wood in the central part of the stem becomes darker in
color it is referred to as heartwood. The younger, lighter-colored wood is
called sapwood. Almost all the older wood is dead; and when fungi that
can digest and oxidize the substances in the cell walls gain entrance into
the center of the stem through wounds, they may destroy all but a thin
shell of sapwood adjacent to the bark. Such hollow trees, however, may
continue to live and grow for many years (Figs. 94 and 101 ).

1 ■

Fig. 94. Photograph of a hollow log from a sycamore tree which lived and
grew for many years after the heartwood had been destroyed by fungi. See also
Fig. 101.

It is evident from the differences in color and texture that the bark is
composed of several kinds of tissues and that the cells of the wood
cylinder are not all alike. The annual growth rings (ends of annual
cylinders of xylem) may usuallv be counted in the wood cylinder,
though sometimes a hand lens or even a microscope is necessary to
distinguish them clearly. A careful examination of the ends of the

264 TEXTBOOK OF BOTANY

water-conducting tubes (vessels), which are visible through the hand
lens, will reveal one of the two reasons why annual rings are evident to
the eye. Wood cells must be seen through a microscope to discover
the second reason. Annual rings are clearly evident in stems of pine and
other conifers that have no vessels.

Usually radially arranged rays, the vascular rays, can be seen in the
split surfaces and ends of blocks of wood. The vascular rays in the
xylem may be called xylem rays. On the split surfaces they appear as
smooth, narrow, thin ribbons of different lengths extending crosswise
to the "grain" of the wood. The xylem rays may also be seen on pieces
of polished lumber one side of which was cut in a plane parallel to them.
We shall see presently, however, that much of the conspicuous grain of
polished wood is dependent upon differences in the wood cells in each
annual cylinder of xylem.

Bark tissues. From the bark of a living woody twig or small branch
one may scrape or pick off an outer layer of brownish cork tissue, be-
neath which there is a layer of parenchyma, which is usually green
because many of the cells contain chlorophyll. These two layers of the
bark are generally referred to as the coHex. The parenchyma is the
cortical parenchyma, and any part of it that contains chlorophyll may
also be called cortical chlorenchyma. The cortex is just within the epi-
dermis, which, being only one cell thick, may be difficult to detect with-
out a microscope. If one carefully scrapes the cortical chlorenchyma
away with the edge of a knife, a number of hard fibers extending
lengthwise in the bark become visible. These are pericycle fibers. If the
point of the knife is slipped under these fibers, they may be lifted a
slight distance from the stem before they break. Beneath the pericycle
fibers is a layer of soft, usually colorless, tissue. When this layer is
scraped off, the outer surface of the wood cylinder is visible. This inner-
most soft tissue of the bark is the phloem, containing the food-conducting
cells. In the stems of some species, such as basswood and pawpaw, the
phloem also contains fibers, phloem fibers, comparable to the pericycle
fibers already noted. The tenu bast fibers is sometimes used commer-
cially to refer to any of the fibers in the bark, whether in the cortex,
pericycle, or phloem. In some twigs the radially arranged vascular rays
in the phloem, phloem rays, are visible, but they can usually be seen
only through a microscope.

'The phloem and wood cylinder together constitute the vascular
cylinder, or stele, of the stem. Its outer boundary is the pericycle. The

[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 265

adjacent innermost layer of cells in the cortex is sometimes referred to as
an endodermis, but in numerous stems it does not differ in structure
from cortical cells immediately external to it. One may therefore visualize
a woody twig as consisting of the pith, the vascular cylinder ( stele ) , the
cortex, and the epidermis if it is still present. The vascular cylinder in
the youngest parts of stems is not completely closed but consists of
several separate vascular bundles with parenchyma between.

Between the bark and the wood cylinder is a cylinder of meristematic
cells, the vascular cambium. Since this cambium is but one cell, or at
most only a few cells, in thickness, it is too thin to be seen without a
microscope. Although in cross section it appears as a ring of very small,
thin-walled cells, it should be visualized as a cylinder of meristematic
cells immediately surrounding the wood cylinder and lining the inner-
most layer of the bark. As the cambium cells divide, new xylem cells de-
velop at the outer surface of the wood cylinder, and new phloem cells
develop at the inner surface of the bark. It is the formation of new cells in
the cambium that results in the annual increase in diameter of perennial
stems. In spring, when the cambium cells are dividing, the bark is
easily separated from the wood cylinder. Those who have made whistles
out of twigs or helped peel spruce logs for pulp mills are fullv aware
of this fact.

One should continue to whittle and observe until the relative arrange-
ment of all these general regions is distinctly visualized. With the excep-
tion of the pith, each general region of woody stems should be visualized
as a cylinder surrounding other cylinders of the stem. The wood cvlinder
surrounds the pith, and in turn is surrounded by a cylinder of cam-
bium; the cylinder of bark surrounds both the cambium and the wood
cylinder. Within the bark next to the cambium is a cylinder of phloem,
followed in order by cylinders of pericycle, cortical chlorenchvma, cork,
and epidermis, provided one is examining a twig or small branch.
Within a few years the epidermis, cortex (cortical cork and chloren-
chyma), pericycle, and other outer layers of the bark die and slough
off. Consequently, the bark surrounding the trunk of a large tree con-
sists only of the phloem and layers of cork that develop annually from
cork cambiums, which in turn develop each year from living phloem
cells. Most of these features of a woody stem are represented in Fig. 95.

It is well known that the age of any part of a branch may be ascer-
tained by noting its location in reference to the terminal bud scale scars.

266

TEXTBOOK OF BOTANY

Since an annual cylinder of wood is formed each year and the ends of
these cylinders seen in cross section appear as distinct rings of wood,

Pericycle

Phloem

^Cambium

^Xylem

Vascular ray
Pith

^// Epidermis and

Fig 95. General regions and tissues of a stem of moonseed vine. Courtesy of

World Book Co.

the age of the stem may also be ascertained by counting the rings. An
annual cylinder of phloem is also formed each year, but the rmgs ot

[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 267

phloem in cross section are not so evident. They may be seen in the
ends of a twig of basswood with the aid of a hand lens.

The terminal stem segment contains one cylinder of wood and one of
phloem. In the next stem segment below the first terminal bud scale
scars there are two cylinders of wood and two of phloem. The cylinder
of wood adjacent to the cambium is the one that extends through both
annual stem segments. Likewise, the cylinder of phloem adjacent to the
cambium is the one that extends through both stem segments.

In the stem segment below the second teiTninal bud scale scar there
are three cylinders of wood and three of phloem. Only the cylinder
of the wood and the cylinder of phloem adjacent to the cambium extend
through all three stem segments. Can you visualize the positions of
these cylinders all the way down the branch and trunk of a tree to its
base near the soil?

If the tree is a century old there should be 100 annual rings of wood
at its base. The innermost cylinder of wood adjacent to the pith is as
high as the tree was when it was one year old. The next cylinder is as
high as the tree was when it was two years old, and so on to the 100th
and youngest cylinder of wood adjacent to the cambium. This cylinder
extends all the way from the base of the tree to the tip of the terminal
twig. Each lateral branch also has a similar arrangement of annual
cylinders of wood.

Likewise, the innermost cylinder of phloem extends all the way from
the tip of the terminal twig to the base of the tree. Each branch contains
a cylinder of youngest phloem external to the cambium. The cylinders of
xylem and phloem in any stem are continuous with those of the same age
in each of its lateral branches.

Furthermore, these youngest cylinders of wood and phloem also ex-
tend all the way down the root to near its tip. The older cylinders of
wood and phloem in the root, like those in the stem, are successively
shorter and shorter until we come to the shortest ones, which were
formed during the first season of growth. The pith in the stem, on the
other hand, develops from the apical meristem and merely elongates
each year and is therefore continuous from the tips of the branches
to the base of the trunk ( Fig. 96 ) .

Since annual stem segments bear leaves but once, the youngest cylin-
der of wood is the only one that is continuous with the xylem in the
veins of the leaves of deciduous trees and shrubs. Likewise, the youngest
cylinder of phloem in the stem is the only one that is continuous with

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STEM

CORTEX

PHLOEM 1936
1939
r940

CAMBIUM

XYLEM 1940
1939
1938

ROOT

Fig. 96. Diagram of relative positions of xylem and phloem in a 3-year-old stem

and root.

tlie phloem in the vems of these leaves. It is in this youngest cylinder
of wood each year that most of the water is pulled up the stem by
transpiration. Food translocation occurs mostly in the youngest phloem.
Girdling. As the term is used by botanists, girdling consists in com-
pletely removing a band of bark and cambium all the way around the
stem. If the girdle is made on the trunk of the tree within a few feet of

[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 269

the ground and below all lateral branches, an adequate translocation of
sugar from leaves to roots no longer occurs in most species of plants, but
movement of water in the xylem from roots to leaves still continues. In
trees the roots and base of the trunk below the girdle may contain enough
food to survive for a year or two after the girdle is made. Death and
decay of the roots are ultimately followed by the death of the plant above
the girdle as a result of desiccation.

In most species of plants apical dominance of the tops is sufficient to
inhibit the growth of sprouts from the base of the trunk. When girdling
annuls this apical dominance sprouts may grow from the roots or base of
the trunk of girdled trees, as they usually do when a live tree is suddenlv
cut down. Likewise, if there are lateral branches below the girdle, or if
apical dominance fails to inhibit the growth of sprouts from the base of
the tree, the tree may survive for many years.

If the bark is not completely removed, or if the girdle is very narrow,
new phloem may develop from the cambium before accumulated food in
the roots is exhausted, and the tree may survive. When the girdled area
does not become drv and is otherwise protected by grafting wax, a
new cambium and phloem may develop across the girdle while the roots
are still alive. Advantage is sometimes taken of this fact in horticultural
practice. Branches, or even the main trunk, of fruit trees are girdled to
bring about internal conditions favorable to the formation of flowers
and fruits without killing the tree. Girdling prevents not only the down-
ward translocation of sugar, but also the upward translocation of the
amino acids and proteins made in the roots.

Girdling is often incorrectly done in that so much of the younger
xvlem is cut awav with the bark that the movement of water to the
leaves is greatlv diminished. In such cases the leaves wilt within a few
davs, and the top above the girdle dies of desiccation before the roots
starve to death. With the death of the top its apical dominance disap-
pears, and sprouts may grow from the roots or from the base of the
trunk. Apparent exceptions to these statements of course need further
analysis than given here.

Herbaceous steins. If stems of herbaceous plants, such as those of
common weeds, garden plants, and crop plants, are cut and dissected,
as directed above for woody stems, soft tissues and vascular bundles are
readily discernible, and in some species a hard outer rind is evident.
Here again differences in color and texture are indicative of different
kinds of tissues definitelv arranged. In stems of dicots the cortex and

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circle of bundles surround a relatively large pith ( Fig. 97 ) . Ridges of the
pith that extend between the bundles may be visible in cross sections as
pith rays. In old herbaceous stems, as those of sunflower, there may be a
conspicuous ring of xylem with narrow xylem rays. Chlorenchyma is
usually present in the cortex of herbaceous stems and sometimes in the
phloem and pith. In a monocot like corn, the bundles are scattered

Fig. 97. Photomicrograph of a cross section of a young sunflower stem.

throughout the stem (Fig. 98). In many species of both monocots and
dicots the cells of the pith disintegrate at an early stage of development,
with the result that the stem is hollow. If an herbaceous stem bearing
leaves is cut off near the surface of the soil and the cut end of the stem
is placed in a weak solution of eosin or other dye that stains the
xylem, the dye is pulled up the stem with the water, and the stained
bundles in both stems and leaves may be seen more readily.

The stems of most monocots differ from those of dicots also in the
absence of a vascular cambium. Hence, stems of most monocot plants,
even though they are perennial, do not increase in diameter from year
to year. For example, no slender bamboo stem ever becomes a big

[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 271

Node

Leaf sheath

Vascular
bundle

Vascular
bundle

Fig. 98. Arrangement of tissues in a solid monocot stem {Panicum) . Courtesy of

World Book Co.

bamboo stem, and no big bamboo stem was ever a slender bamboo stem
(Fig. 99). The diameter of monocot stems usually does not increase
from year to year. There are, however, some exceptions. In the stems
of some monocots such as palms and yuccas a temporary cambium de-

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TEXTBOOK OF BOTANY

Fig. 99. Interior and exterior view of bamboo forest at Savannah, Georgia. Photo
from United States Department of Agriculture.

velops periodically from cells in the cortex of the stem. The stem thus
increases in diameter from time to time (Fig. 100).

Place of growth in length. The place of growth in length of the stem
may be recognized by both its external and internal appearance. At the
top of the stem the leaves are relatively small and young, and several of
the internodes are not fully elongated. Internally, parenchyma is the

[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS

Fig.

100. The Joshvia tree, an arborescent yucca, in bloom. Mojave Desert near
Kramer, Calif. Photo bv U. S. Forest Service.

most prominent tissue. At the extreme tip only parenchyma is present
( Fig. 32, page 69 ) . We have aheady seen ( Chapter XXIII ) how such
external factors as light and gravity may influence the direction of growth
of an elongating stem tip. To see the growing tip of a grass stem one
must first carefully remove the ensheathing leaves. Unless the plant is
very young the stem tip will be bearing rudimentary flowers entirely
hidden within the leaf sheaths. As the grass stem grows, the bases of the
internodes are the last parts of the stem to stop elongating and become
mature.

Size of stems. The forerunner of the whole stem system of a plant is
the apical meristem in the bud (plumule) of the embryo (Fig. 8, page
11). Through the formation and enlargement of cells in this apical
meristem the stem elongates and axillary buds develop from cortical and
epidermal cells along its sides. From some of these axillary buds lateral
branches develop, each of which also has an apical meristem.

Although there are seasonal periods of growth and dormancy, it might

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appear that the stem of a perennial plant would have unlimited growth
in length. The vascular cambium of woody stems also is perennial, with
alternating periods of growth and dormancy. Trees and shrubs, therefore,
might be expected to increase in diameter indefinitely.

From observation, however, it is evident that there are limits to the
age of trees and also to both height and diameter growth of stems. Fur-
thei-more, among the different species there is a wide range in these
limits. The smallest species of trees never exceed a few feet in height
and a few inches in diameter. Among herbaceous plants there are
many species that are even more restricted in age, height, and diameter.
The tallest trees may exceed 300 feet, and the diameter of some trees
may become more than 30 feet. Vines in the tropics may become a thou-
sand feet or more in length.

Fig. 101. A giant sycamore 45 feet in circumference at breast height. White River
Valley, Indiana. Photo by G. W. Blaydes.

[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 275

Many of us are impressed with the superlative. Large trees, tall trees,
and old trees elicit considerable interest as shown by their frequent men-
tion in the press ( Fig. 101 ) . Some of the present-day trees are the largest
and have attained the greatest age of any plants that ever existed ( Fig.

Fig. 102. Big Tree or Sequoia in the Sierra National Forest, where trees as
large as those pictured above are common. They are hmited to unglaciated areas
in the canyons of the Sierra Nevada Mountains. Photo from the U. S. Forest
Service.

102). What determines the limits of age, size, and height? It would be
interesting to consider some of the factors correlated with size limits,
but space can be given here only to a few suggestions and to a summary
of reports of the largest trees (Table 11), What world events were in
progress when these trees were seedlings?

That the stem may have potentialities of unlimited growth may be
demonstrated with plants, such as English ivy, that propagate readily
from cuttings. To do this it is necessary only to use repeatedly the upper
part of the same stem as a cutting and to keep it in an environment that
is favorable to contmuous vegetative growth. That is, it must not be

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TEXTBOOK OF BOTANY

Table 11. Maximum Sizes and Ages Attained by Certain Trees

Name of Tree

Location

Diameter

Height

Age

8 ft.

230 ft.

500 yrs.

12 ft.

300 ft.

600 yrs.

25 ft.

417 ft.

700 yrs.

28 ft.

360 ft.

1000 yrs.

35 ft.

330 ft.

4000 vrs.

50 ft.

140 ft.

3000 vrs.

14 ft.

80 ft.

3000 vrs.

6 ft.

80 ft.

500 yrs.

4 ft.

120 ft.

8 ft.

170 ft.

10 ft.

200 ft.

16 ft.

1.50 ft.

Gft.

1.50 ft.

Western yellow pine

Sugar pine

Douglas fir

Redwood

Big tree (sequoia)

Big cypress of Tule

Western juniper

Norway maple

Sugar maple

Bur oak

Tulip

Sycamore

Black walnut

Western N. A.
California
British Columbia
California
California
Oaxaca, ^Mexico
California
Europe

Wabash Valley
Wabash ^'alley
Wabash Valley
Southern Indiana
Wabash Vallev

placed in an environment in which the terminal vegetative bud will
become a flower bud. Similarly, artificial cultures of root tips, described
in Chapter XXI, indicate that the apical meristem of a root tip has
potentialities of unlimited growth if it is repeatedly removed from the
remainder of the root svstem and kept in a suitable environment. What
then are the factors that limit the growth in length of stems and roots
when thev are attached to the whole plant?

Processes in stems. All the processes characteristic of a vegetative cell
occur in the cells of stems. In addition, there are certain processes that
occur in some vegetative cells but not in others. Photosynthesis does not
occur in all vegetative cells, but such processes as respiration, digestion,
and assimilation do. On the other hand, photosynthesis occurs in any
cell of a leaf, or a stem, or a root that contains chlorophyll and is
exposed to a suitable environment. But many leaves and stems do not
contain chlorophyll. All we can say is that in most ferns and seed plants
photosynthesis occurs more abundantly in leaves than in stems and roots.

When all kinds of leaves, stems, and roots are compared, there are very
few processes that can be said to be distinctly characteristic of any
one of these three types of vegetative organs. Perhaps there is no process
that occurs in all stems that does not occur in some roots or some leaves.
There are very few species of plants which have leaves that bear other
leaves; roots may never bear leaves, but there are also certain kinds of
stems that do not bear leaves. Flowers do not develop directly from
leaves or from roots; neither do they develop on all types of stems. When
water and salts move from the soil to the leaves of a dandelion plant, the

[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 277

distance they move through the roots is many times the distance they
move through a stem. There are certain species of plants in which photo-
synthesis occurs primarily in the stems. There are others in which the
absorption of water and inorganic salts occurs primarily in the leaves and
stems. The reader may find it both interesting and instructive to add to
these examples of similarities among plant organs.

On the basis of what has been learned so far, it should not be difficult
for the reader to formulate intelligent answers to questions that may
be asked about stem processes. Why may the girdling of a tree result in
its death? Why do people girdle trees? Why may they sometimes fail
to obtain the desired results? Why may the leaves soon wilt when the
girdle is made deep enough to include a few cylinders of sapwood?
If a tree is wounded, will it "bleed to death"? How does healing occur?
What is the gravest danger to which a tree is exposed when it is
wounded, and how may it be prevented? How do trunks of trees be-
come hollow? Why may hollow trees live for many years? If a nail is
driven in a tree will it continue to remain the same distance from the
base of the tree as the tree increases in height from year to year? Why
are there anastomosing longitudinal ridges and crevices in the bark of
a tree? What tissue of a tree is used as a source of lumber? Why do plants
bearing leaves absorb more water from the soil than they do when
leaves are removed? Why may a young tree have a longer growing
period during the summer than an older tree of the same species? Why
may the new branches of a pollarded tree grow se\'eral feet in length
and bear numerous leaves in a single season, while those of an unpol-
larded tree may grow but a few inches in length and bear relatively few
leaves? Can you add other questions, and answer them?

REFERENCES

Chamberlain, C. J. The age and size of plants. Sci. Monthly. 35:481-491. 1932.

Fry, Walter, and J. R. White. Big Trees. Stanford Univ. Press. 1938.

Clock, W. S. The" language of the tree rings. Sci. Monthly. 38:501-510. 1934.

CHAPTER XXVIII
TISSUES AND PROCESSES OF STEMS

Some of the general regions of stems discussed in the preceding chapter
are composed of several kinds of tissues. This is particularly true of the
cortex, the bark, and the stele. On the other hand, the pith, vascular
cambium, endodermis, cork cambium, cork, and the epidermis each
consist of a single tissue. Moreover, some of the tissues of stems are
complex tissues, being composed of a system of several kinds of cells.
This is particularly true of xylem and phloem, if we regard each of these
as but one tissue.

Stem tissues that develop from cells in the apical meristem are called
primary tissues. Those that develop from cells in the vascular cambium,
or cork cambium, are called secondary tissues. In stems of most mono-
cots, therefore, all the tissues are primary tissues. In stems of w^oody
plants nearly all the xylem and phloem and the cork are secondary
tissues; and the others, together with the first strands of xylem and
phloem adjacent to the pith, are primary tissues. The only primary
tissues that may still be present in the trunk of a large tree are the pith
and a small amount of primary xylem. On the other hand, stems of
herbaceous plants such as coleus consist mostly of primary tissues. Cer-
tain microscopic structures in these tissues will be given further attention.

Apical growth and the origin of primary tissues. At the extreme apex
of the stem there is a group of small, closely packed, isodiametric,
meristematic cells, the apical meristem. Immediately below it is a zone
of cells in various stages of enlargement, some of which are also dividing.
In the apical meristem food is being changed into new protoplasm, pectic
compounds, and cellulose, as the cells divide and the daughter cells
become as large as the mother cells. In the zone characterized by cell
enlargement, growth of the cell wall, vacuolation, and the osmotic
absorption of water are prominent processes. Apparently there is little,
if any, increase in protoplasm, and the protoplasm that is present is
being pushed out against the cell wall by the enlargement of the vacuole
formed by the coalescence of many smaller vacuoles accompanying the

278

[Chap. XXVIII TISSUES AND PROCESSES OF STEMS 279

entrance of water. In the zone of cell enlargement, and especially toward
its lower end, some of the cells are visibly different from others; that is,
the results of cell differentiation are becoming evident.

Various types of cell differentiation in stem tips may be noted; ( 1 )
some of the cells soon become the primordia of leaves and axillary
buds; (2) these axillary buds, and sometimes the terminal bud, while
still meristematic, may become either vegetative buds or flower buds
(Fig. 83 in Chap. XXVI); (3) some of the cells may retain their meri-
stematic condition and become the vascular cambium; (4) manv other
cells merely enlarge with or without evident change in form and com-
position, and, because of their position, origin, shape, and differences in
cell walls, may be recognized as belonging to the epidennis, cortex,
pericycle, phloem, xylem, or pith; and finally (5) the cross walls of
some of the xylem cells partly disintegrate, leaving long tubes, or vessels,
the lateral walls of which become thickened and lignified, with spiral
thickenings that are particularly conspicuous. Numbers (1) and (2)
above may be considered organ differentiations in meristematic tissue,
while numbers (4) and (5) are tissue differentiations in enlarging and
maturing cells. All the tissues formed by the differentiation of cells that
originated in the apical meristem are primary tissues.

The above description applies to both woody and herbaceous stems
of dicots. It applies also to growing stem tips of pines and other conifers,
except that xylem vessels do not develop in conifers. Secondary xylem
and phloem soon begin to develop from cambial cells. In stems of
monocots, such as grasses, in addition to the apical meristem there is a
temporary zone of meristematic cells at the base of each internode.
This is the softest and weakest part of a growing grass stem. In young
woody stems a cork cambium develops from cells in the cortex, and a
layer of cork may develop on the new stem segment before the close
of the first growing season. In some species of plants latent root pri-
mordia develop from parenchyma cells in the pericycle.

Just below the apical meristem short internodes, as well as nodes bear-
ing primordia of leaves and axillary buds, are recognizable. Ultimately,
flower primordia develop either from the apical meristem or from the
meristems of axillary buds. The cells in the center of the apical meristem
are forerunners of pith, which is surrounded by longitudinal strands of
primary xylem.

The pith. Since the pith develops directly from the central cells of
the apical meristem, it increases in length as the stem elongates; thus, it is

280 TEXTBOOK OF BOTANY

continuous from year to year in perennial stems. In time the cells of the
pith usually die, and the protoplasm disintegrates. All soluble substances
may diffuse out, and onlv the cell walls remain. In stems of some plants
(walnut, hackberry, etc.), some of the cell walls also disintegrate or
collapse, and the pith becomes chambered. In many species of herbs all
the cells of the pith soon disintegrate and the stem becomes hollow. The
stems of all these herbs are solid at the tip where the pith is young and
intact.

In perennial plants the pith, or certain cells in it, may live from one to
many years: one year or less in the European larch, 10 years in Scotch
pine, 17 years in horse chestnut, 27 years in gray birch, and more than a
century in a columnar cactus of the American desert. The pith cells in
this cactus not only remained alive from 100 to 150 years, but when the
pith was wounded some of the cells near the wound divided and fomied
new tissues.

The epidermis. The epidermis of the stem is continuous with that of
the leaves and is comparable to it in most respects. The outer wall is
usually thick and heavily cutinized. Glaucous stems, like glaucous leaves,
are covered with wax. Epidermal hairs are the rule rather than the excep-
tion. Stomates are present, though usually not as abundant as in leaves.
In many stems they ultimately become ruptured by lenticels that develop
from cortical cells beneath them. The first cork cambium that develops
in stems occasionally originates in epidermal cells.

The epidermis of a leaf lives as long as the leaf. This is also true of
some stems; but in perennial stems that increase in diameter by cambial
growth, the epidermis is soon ruptured, except in those plants where it
continues to grow for a few years.

The cortex. When all types of stems are compared, the cortex is found
to be a very diversified region. It consists primarily of parenchyma cells.
The outer layer of cells just beneath the epidermis is often referred to as
a hijpodermis; the inner layer of cells adjacent to the pericycle is the
endodermis. Each of these layers of cells may be parenchyma and differ
little if at all from neighboring cells, or they may be easily distinguished
by the shape of the cells and thickness of cell walls. The hypodemiis
may be composed of ordinary parenchyma cells, sclerenchyma cells,
collenchyma cells, or palisade cells similar to those in leaves.

The first cork cambium in a stem frequently develops from the hypo-
dermis. The development of cork from cork cambium is similar, whether
it occurs in herbaceous stems, woody stems, roots, or any other plant

[Chap. XXVIH TISSUES AND PROCESSES OF STEMS 281

organ ( Fig. 103 ) . Cork cells are short-lived; but the cell walls, which are
composed primarily of layers of suberin and lignified cellulose lamellae,
last indefinitely because very few kinds of non-green plants can digest
them. On stems that increase in diameter by cambial growth, however,
the cortex is ruptured and sloughed off.

CORK

CORTEX

Fig. 103. Photomicrograph of a portion of a cross section of geranium stem
in which the cork and cork cambium (CC) are evident. P. F., pericycle fibers;
V. C, vascular cambium.

In herbaceous plants the cortex may live as long as the stem does, but
in many woody stems it may die and be sloughed off while the stem is
comparatively young. New cork cambiums periodically develop in
deeper-lying tissues. When cork develops in the pericycle, it prevents
the diffusion of water from the xylem to the cortex, which soon dies. In
beech the cortex lives and grows as long as the tree is alive. It is also
long-lived in birch and certain other trees. Chlorenchyma is present in
the cortex of most young aerial stems.

Sclerenchyma and collenchyma are frequently present in the cortex.
In some species of plants there may be many other kinds of cells in the
cortex, such as isolated sclerenchyma cells (stone cells or grit cells),
groups of stone cells surrounded by a cambium, and isolated cells or
groups of cells containing anthocyanins, essential oils, tannins, or cal-

282 TEXTBOOK OF BOTANY

cium oxalate crystals. Various canals or ducts, such as resin ducts, essen-
tial oil ducts, and latex tubes, may also be found in the cortex of some
stems. No one of these features, however, is limited to the cortex or even
to the stem of a plant.

Latex, the milky juice found in many species of plants, is collected in
large quantities from a few kinds of plants and converted to rubber.
Latex tubes, which originate from cells in the embryo in some species,
grow through the parenchyma tissues of the plant as if they were para-
sites. In other species they are formed by the disintegration of scattered
parenchyma cells. Resin ducts are surrounded by special kinds of cells.

The pericycle. As noted earlier, the pericycle is the peripheral cylinder
of the stele ( vascular cylinder ) and in cross section appears as a ring of
cells. It varies from one to several cells in thickness. When it is composed
entirely of parenchyma it may be difficult to recognize. In fact, in some
stems cortical parenchyma appears to extend all the way from the epi-
dermis to the phloem; that is, there is no visible differentiation of either
endodermis or pericycle. When sclerenchyma ( pericycle fibers ) is pres-
ent, the pericycle is easily recognized. These fibers may occur as con-
tinuous cylinders or as separate strands, and because of their location
they often appear to be the outer part of a vascular bundle. Some of the
important fibers of commerce are pericycle fibers — for example, flax and
hemp. Those of flax are mainly cellulose, and are less lignified than hemp
fibers.

The pericycle is important in one other way. It is the region in which
the primordia of lateral roots, and frequently those of adventitious stems,
develop. It is from the pericycle that roots usually develop on stem cut-
tings. In some species, such as willow and cottonwood, root primordia
develop in twigs on the tree and remain dormant unless the twig is ex-
posed to conditions favorable to their further development.

The phloem. The phloem is a complex tissue in which several kinds
of cells may usually be recognized with the aid of the microscope. The
chief food-conducting cells are the sieve tubes, which are generally
elongate, short-lived, and interconnected through perforated diaphragms
called sieve plates (Fig. 104). Associated with the sieve tubes, perhaps
in all the vascular plants except ferns and gymnosperms, are long-lived,
slender parenchyma cells called companion cells. Various other paren-
chyma cells, not associated directly with the sieve tubes, may also be
present. All these cells together constitute the part of the phloem in

[Chap. XXVIII TISSUES AND PROCESSES OF STEMS 283

which conduction of food may take place and in which starch frequently
accumulates.

sieve plate

comparvion cell

phloem pareacKymacell

sieve tube

Fig. 104. Types of cells in the phloem of a gourd stem.

In the phloem of many vascular plants, except ferns, some gymno-
sperms, and most herbs, there are sclerenchyma cells, known as phloem
fibers, comparable to the fibers of the pericycle. These libers may occur
in bands, as hollow cylinders surrounding softer tissue, singly, or irregu-
larly. The commercially important fiber, jute, is obtained from the phloem
of the jute plant. The individual phloem fiber is a long, tapering cell,
usually with lignified walls. Short thick-walled cells, called stone cells,
may occur in the phloem eitlier associated with the fibers or alone. In a

284 TEXTBOOK OF BOTANY

few plants, such as sycamore and beech, the stone cells are the only
sclerenchyma cells in the phloem.

Vascular rays are present m most plants having a vascular cambium.
That part of the ray extending from the cambium into the phloem is the
phloem ray, and it varies considerably in size, shape, and appearance in
different species of plants.

In stems of most trees and shrubs the epidermis, cortex, and pericycle
are present for only a few years. The tissues outside the cork cambium
crradually die, and ultimately a new cork cambium develops from paren-
chyma cells in the phloem. The cylinder of cork formed from this cam-
bium prevents diffusion of water to tissues external to it. The whole
outer bark soon dies, cracks, and sloughs off.

In the birch and cork oak there are conspicuous cylinders of cork that
resemble annual rings of the xylem, but in most species such rings are
not evident. These cylinders are due to alternating layers of thin-walled
and thick- walled cells. The thin-walled cells separate easily, and this ac-
counts for the papery sheets of birch bark and the scaly bark of the syca-
more. External masses of cork in such trees as cork oak accumulate in
layers several centimeters in thickness and are of great commercial
importance.

Stems of pumpkin, squash, tomato, potato, and several other kinds of
plants differ from the usual by having phloem both external and internal
to the xylem.

The xylem. The xylem of a woody dicot is a complex tissue composed
chiefly of vessels, or water-conducting tubes; tracheids; ivood fibers;
xylem parenchyma cells, and xylem ray cells. A vessel may be several
millimeters, several feet, or even yards in length. The length of a vessel
depends upon the number of end walls that disintegrate in the row of
cells from which it is formed. Old vessels may be closed by tyloses, out-
growths from adjacent xylem parenchyma ( Fig. 105A ) .

In the xylem of conifers the vessels and sometimes the libers are ab-
sent; the chief cells are tracheids and xylem ray cells. The tracheids are
both water-conducting and wood cells. They are thick-walled and spin-
dle-shaped, and have thin areas or pits in the walls ( Fig. 105B ) .

Certain features of the xylem are more easily understood in relation
to the growth of the cambium.

The vascular cambium. In order to see the cambium of stems clearly,
one must have thin sections cut from growing stems of dicots or conifers.
Between the easily recognized xylem and phloem in cross sections of

[Chap. XXVIII TISSUES AND PROCESSES OF STEMS

285

Fig. 105. Radial longitudinal sections of wood of (A) white oak and (B) long
leaf pine. In the former wood fibers, xylem rays, and vessels filled with tyloses
may be seen. In the latter section tracheids with bordered pits, xylem rays, and a
resin duct are visible. Photos from Forest Products Lab.

these stems there is a region composed of several layers of thin-walled
cells. The cells near the middle of this region are very small and thin-
walled. These are the cambial cells from which additional xylem and
phloem develop by cell division, enlargement, and differentiation and
bring about an increase in the diameter of stems. Adjacent to and on
both the phloem and the xylem sides of this cambium there are a few
layers of cells which are thin-walled but inceasingly larger at greater
distances from the cambium. One may therefore see from one to several
layers of cells in the processes of enlargement and differentiation be-
tween the cambium and the mature phloem on the one side and the
cambium and the mature xylem on the other ( Fig. 106 ) .

In the larger stems the cambium is a closed c)'lindrical sheath between
the xylem and the phloem, that is, between the xylem and the bark; but
in the younger part of the stem it is present only in the individual vas-
cular bundles, which are separated from one another by broad rays of

286

TEXTBOOK OF BOTANY

pith-like parenchyma. As the young stem increases in age cambium
usually develops from the parenchyma cells between the bundles and
becomes a complete cylinder in the stem (Fig. 107).

Fig. 106. A, photomicrograph of a cross section of a stem of moonseed vine; B,
one of the vascular bundles, adjacent pericycle fibers, and cortex greatly enlarged
to show the small cambium cells in contrast to the enlarged and diflFerentiated cells
of the xylem and phloem. The outermost phloem cells have been crushed by the
enlargement of the inner younger cells formed by the division of cambial cells.

Since the xylem of woody perennials increases in diameter from year
to year, it presses against the encircling bark. Some of the cells of the
bark become crushed, and the outer dead parts of the bark are ruptured.
As a result, the outer bark of woody perennials is usually furrowed and
irregularly broken.

In most parts of the world cambial growth is limited to a few months
of each year. In woody perennials of temperate climates it begins in the
young twigs about the time the buds begin to open in spring, and some-
what later in the older branches and trunk of the tree. Two or three
months later the cambium becomes dormant in the trunk of the tree and
remains in that condition until the following spring. Each season cambial
growth begins first in the twigs and continues longest in them. Growth
of the cambium appears to be dependent upon hormones from the
leaves, but the evidence upon which this inference is based is too
limited for final conclusions at present.

The xylem cells and vessels that develop in spring are usually larger
than those that develop toward the close of the growth period in summer.

[Chap. XXVIII TISSUES AND PROCESSES OF STEMS

287

Fig. 107. Photomicrograph of a small portion of a cross section of a sunflower
stem in which a continuous cambium has developed between the original bundles
of the younger stem as pictured in Fig. 97.

Consequently the appearance of the inner portion of each annual ring
of xylem as seen in cross section is quite different from that of the outer
portion of the ring. Without these differences in spring wood and
summer wood the annual rings of growth would not be evident to
the eye.

The width of the annual rings in particular species varies with the
age of the individual plant and with the fluctuations in environmental
factors from year to }ear. In certain localities the effects of fluctuations
in water supply and humidity are more prominent than those of all other
factors combined. Tree rings mav therefore be used, along with other
data, as an index of periods of drought and of abundant moisture. Such
studies have shown that there are rather definite and predictable cycles
of periods of drought. Comparison of annual rings of living trees with
those of wooden beams used in dwellings in the southeastern part of the
United States has made it possible to date quite accurately certain events

288

TEXTBOOK OF BOTANY

in the lives of the diff dwellers and other ancient peoples as far back as a
diousand years or more/

Ring-porous, diffuse-porous, and non-porous wood. When the vessels

iff- \\'' mSf \i\

> t I

Fig. 108. Bing porous wood of white oak (A, above), diffuse porous wood of river
birch (B, below). Photos from Forest Products Laboratory.

and xylem cells that develop in spring are large and numerous (Fig.
108A), the annual rings are particularly conspicuous. Such wood is ring-

1 Papers by A. E. Douglass and Waldo S. Glock. Carnegie Institution of Washington,
D. C. Leonardo da Vinci, o\er 400 years ago, recorded tlie relation of tree-ring growth to
weather conditions. The matter did not receixe serious attention, howexer, until 1901 when
Douglass began to report his observations.

[Chap. XXVIII TISSUES AND PROCESSES OF STEMS

289

Fig. 109. Non-porous woods of arbor vitae (above) and of long leaf yellow
pine (below). The large openings in the latter are cross sections of resin ducts.
Photos from Forest Products Laboratory.

porous. If the vessels are of about the same size and are scattered uni-
fomily in the annual ring (Fig. 108B), the wood is said to be diffttse-
poroits. In conifer stems, where no vessels occur, tlie wood is non-porous
(Fig. 109). These facts are useful in the identification of woods of dif-
ferent species.

Grain of wood. When boards are cut from stems and polished, vari-
ous patterns of wood are often conspicuous. Such patterns are dependent
on the relative size of the cells and vessels of spring and summer wood,
on the width of the annual rings, and on the plane in which the saw
passes through the log with reference to the xylem rays (Figs. 110 and
111). "Quarter-sawed" boards are cut parallel to these rays. Tangential
or "slab" cuts at various angles to the rays result in a variety of patterns.
The summer wood is harder than the spring wood and takes a finer
polish; and this difference is the cause of the often attractive grain pat-
terns of furniture, panels, and cabinet work. In perfect radial sections
the grain appears as straight lines crossed by patches of xylem rays.

290

TEXTBOOK OF BOTANY

Annual rings
spring wood
Summer wooc

Xylem rays

Fig. 110. Diagram of a block of oak wood, magnified to show the arrangement
of tissues which underhes the patterns on pohshed wood surfaces. Courtesy of
World Book Co.

When the cut passes through several annual rings, as in slab-cut boards,
the pattern is most striking. Xylem rays in oaks and some other trees are
prominent, and in quarter-sawed boards from such trees the rays appear
as the light-colored areas.

If the annual rings are narrow, the wood is "fine-grained"; if they are
wide, "coarse-grained." Some patterns of wood have special names. The
"silver grain" of quarter-sawed oak refers to the large wood rays. If the
patterns are wavy in appearance, as they sometimes are in birch, cherry,
and chestnut, the wood is regarded as "curly grain." The best known is
the "bird's-eye grain" of certain maples. This is caused by the presence
of numerous partly dormant buds from which minute, embedded lateral
stems develop. These small "stems within a stem" are the "eyes" of the
wood seen in tangential section.

The monocot stem. Since the stems of monocots were discussed with
those of dicots and conifers, some of the major facts about them may be
summarized. The vascular bundles are scattered throughout the stem
(Fig. 98) . No cambium, except temporary vestiges, occurs in the bundles.

lili(i'iWIU'li/n,l<i

Fig. 111. Transverse, radial, and tangential sections ot wood ot sugai maple.

Photos from Forest Products Laboratory.

291

292 TEXTBOOK OF BOTANY

and hence secondary growth, so evident in dicots and conifers, does not
occur. The bundles are composed of xylem and phloem surrounded by a
bundle sheath of more or less sclerenchyma-like cells. Growth in length
occurs at the apex and also at the bases of several of the uppermost in-
ternodes. The epidermis is composed of hard, thick walls and may be
subtended by several rows of thick-walled cells, the htjpodermis. The
epidei-mis, together with the hypodermis, is sometimes called the rind;
this is evident in the corn stem.

The remainder of the stem consists of parenchyma. It is usually im-
possible to distinguish pericycle or cortex or pith. In some few monocots
the central region is devoid of bundles, and the central parenchyma
resembles a pith. Many monocots have hollow stems.

Because of the absence of a cambium, stems of most monocots do not
increase in diameter after the cells are mature. The bamboo and corn,
for example, grow to considerable heights but remain slender. Many
perennial monocots have underground stems that elongate each year and
enlarge the area occupied by the plant. Such aerial stems as those of
palms, dragon tree, and yucca have secondary thickening. Cells in the
pericvcle or parts of the undifferentiated parenchyma become meri-
stematic, and from them additional parenchyma and new vascular
bundles develop.

Healing of wounds. As a result of the injurious effects of winds, ice,
insects, and fungi, areas of living tissue on trees and branches may be
killed or completely removed. Girdling a tree is another way of destroy-
ing its living tissue. It is perhaps evident by now that "recovery" from
such "wounds" is possible only through the activity of the vascular cam-
bium. Expert "tree surgeons" are able to save valuable trees, apparently
ruined by mechanical injury, disease, and decay, by removing the in-
jured areas and covering the exposed tissue with wax, tar, or other sub-
stances that reduce water loss and prevent the entrance of destructive
organisms. If the area involved is not too large, the scar may in time be
covered completelv by tissue that develops from a cambium.

Grafting and budding. The importance of the cambium has long been
recognized in grafting and budding. Many species of plants, especially
fruit trees, do not "come true to seed" and must be propagated vege-
tatively. Sometimes it is commercially advisable to use vegetative mul-
tiplication, instead of seeds, for other reasons. One common practice used
in perpetuating plants without the use of seeds is grafting.

In grafting, a twig or stem ( the scion ) of one plant is attached to the

TISSUES AND PROCESSES OF STEMS

293

[Chap. XXVIII

stem or root of another plant ( the stock ) . If the cambiums of the stock
and the scion are in perfect contact and if proper precautions are taken,
union of the meristematic tissues takes place rapidly. There are many
types of grafting (Fig. 112), and reference to any manual on horticul-

FiG. 112. Methods of grafting and budding. At the left, whip grafting; in the
middle, cleft grafting; at the right, budding. A, represents the scion, and B, the
stock. C, the scion and stock are joined. In both grafting and budding, success
depends on bringing the cambium of the scion into contact with the cambium of
the stock. Courtesy of World Book Co.

tural practices will indicate the preferred methods for different kinds
of plants. Budding or bud grafting differs from twig grafting only in the
fact that a bud and some additional tissues are inserted in a T-shaped slit
in a branch. If the cambiums of the bud tissue and the branch are in
contact, the two pieces soon unite.

It is evident that neither twig grafting nor budding is possible in
stems without cambiums, as for example in corn, wheat, rye, and many
other grasses. A further discussion of grafting is gi\'en in Chapter XXXV.

REFERENCES

Bailey, L. H. Standard Cyclopedia of Horticulture. The Macmillan Company.

1915.
Fames, A. J., and L. H. McDaniels. An Introduction to Plant Anatomy.

McGraw-Hill Book Company, Inc. 1925.
Hayward, H. E. The Structure of Economic Plants. The xMacmillan Company.

1938.

LIBRARY

'^

^^^

y^.

<>*■

CHAPTER XXIX
ROOTS: DEVELOPMENT AND STRUCTURES

To complete the story of the water and salt relations of plants we shall
have to examine the underground parts of plants that have roots. The
soil is a part of the immediate environment of these plants. The water in
the upper layers of the soil, where most roots grow, is transitory. It enters
from rain and melting snow and moves out by seepage, by evaporation,
and through the plant. Only a portion of it is held in the interstices
among the innumerable particles of the soil. From these interstices the
water moves into the roots and upward into the stems and leaves and
then into the air by transpiration. We have seen that in many plants the
amount of water that passes through them during a growing season is
large in comparison with either the volume of the plant or the water
necessary for photosynthesis and other plant processes. How such large
amounts of water can enter the plant from the soil will be better compre-
hended when we see how roots develop and become distributed among
the soil particles.

Types of roots. When the seeds of most plants germinate, the first part
of the embryo to enlarge and push through the seed coats is the hypo-
cotyl, having at its lower end a root primordiiim. The hypocotyl is the
part of the embryo between the cotyledons and the primary root, and is
really the base, or the first-formed part of the stem of many plants
(Fig. 8). This initial expansion of the embryo is due to cell enlarge-
ment. The hypocotyl may ultimately elongate from a small fraction of
an inch to several inches shortly after germination. Following the emer-
gence of the hypocotyl from the seed coats, the cells at the lower end
of the root primordium begin to divide, and from this primordium the
primary root develops. The hypocotyl appears to be lacking in the
embryos of some plants, such as the grasses (Fig. 113). The plumule
appears to be separated from the primary root principally by a node. If
a hypocotyl is present in this type of embryo it is quite small and does
not elongate when the embryo germinates.^

^ The term radicle is variously used by different writers to refer either to the rudimentary
root of the embryo or to both the root and the hypocotyl.

294

[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES

295

Scuteilum

Coleoptile
Ist leaf

2nd leaf

Stem tip
3rd leaf

The primary root continues growth
vertically downward for several days
at least, and sometimes for weeks or
even months after germination. Usually
lateral roots grow from it and elongate
either horizontally or obliquely down-
ward. These are the secondary roots;
and when still smaller branches de-
velop from the secondary roots, they
are called tertiary roots.

During the germination of seeds like
those of corn and wheat, two or more
small roots may develop from the base
of the first internode of the embr\'0,
and still later others from the second
and third nodes of the stem. Bv the
time the corn plant is two months old
there is usually a whorl of roots at each
of the eight lower nodes. Each succes-
sive whorl appears about one week
later than the one below it. These and
all other roots that originate from stems
or from leaves are adventitious roots.
All the roots of a plant — primary, sec-
ondary, tertiary, and adventitious —
may be called collectively its root sys-
tem. The primary root with all its
branches may be called the primary
root system, and all the adventitious roots of a plant its adventitious
root system.

A mature com root system comprises both of these root systems, but
the primary root svstem is limited in growth. Hickory trees, English
plantain, and dandelion plants from seed have primary root systems
consisting of a large taproot and minor secondaiy roots. All plants
artificially propagated by cuttings have adventitious root systems. The
root systems of plants that grow from bulbs, tubers, and other under-
ground stems likewise are only adventitious root systems after the
first season from seed. The above terms are useful in classifying roots
according to their origin.

Another way of classifying roots is based on their forni and appear-

FiG. 113. Median section through
a wheat embryo; c, coleorhiza; p,
primary root; r, root cap. After M.
A. McCall, Journal of Agricultural
Research.

296

TEXTBOOK OF BOTANY

ance. When they are slender, elongate cylinders, such as those of grasses,
they are called fibrous roots. If the primary root continues its downward
course and becomes the principal large root of a plant, it is often called
a taproot. Sugar beet, mullein, sunflower, pigweed, walnut, hickory,
alfalfa, and red clover have taproots. Taproots are of two types, woodv
and herbaceous, with many gradations between. Those of hickory and
walnut are woodv. Those of beets, long conical radishes, dahlias, and
sweet potatoes are usually herbaceous and greatly thickened. Thev are
composed mainly of parenchyma cells with relatively few woody cells.
Such thickened herbaceous roots are often referred to as fleshy roots.

The beets and radishes of commerce are not entirely roots. The upper
part of each one consists of a thickened hypocotyl and the short stem to
which the rosette of leaves was attached (Fig. 114). The edible part
of some varieties of globe radishes is almost entirely thickened hypocotyl.

COTYLEDONS

Fig. 114. Globe radishes develop primarily from the hypocotyl of the seedling.

ROOTS: DEVELOPMENT AND STRUCTURES

297

[Chap. XXIX

Mature corn plants in the field usually have "prop roots" that have
developed from the lower nodes above the ground. Pandanus, mangro\e,
and other tropical trees like the banyan and fig, have adventitious roots
from the main trunk and also from the lateral branches of the crown
(Figs. 115-118). When these so-called "prop roots" and "drop roots"

Fig. 115. Prop roots of pandanus, a tropical shore plant, planted in Florida. Plroto

by W. Fifield.

have grown into the soil, they become additional supports and have the
same relation to the soil as primary root systems.

Many vines, such as the Virginia creeper, English ivy, poison ivy, and
trumpet creeper, have aerial roots which become attached to the trunks
of trees, walls of buildings, or rock cliffs, and anchor the slender shoots
far above the height to which the stem can support itself. Orchids,
bromelias, and many ferns in the tropics live as epiphytes, that is, perched
on the branches of trees and shrubs (Fig. 119). These plants also have
aerial roots which form masses below the leaves and not only attach the

298

TEXTBOOK OF BOTANY

Fig. 116. Prop roots of red mangrove (Rhizophora) in Bermuda. Photo by B. E.
Dahlgren, Field Museum of Natural History.

plants but also in some species become the only region of entrance of
water and mineral salts.

Finally, there are the root-like organs of parasites, such as dodder and
mistletoe, which grow into the tissues of the host plant and connect the
tissues of the parasite with those of the host. Such roots are also called
haustoria. Through them pass not only water and inorganic salts but
also foods made by the host plant.

When the seedling of the common dodder, which is a mere thread-
like stem two or three inches in length, comes in contact with a living
green plant, it coils about it and a row of haustoria develops wherever
the stem is in contact with the host. The haustoria penetrate between
the cells of the host plant to the water-conducting and food-conducting
tissues ( Fig. 120 ) . The dodder then grows rapidly, branches, and makes
new contacts; and its haustoria penetrate the tissues of other parts of

Fig. 117. Banyan tree with prop roots that resemble trunks. Hawaii. Photo by

U. S. Forest Service.

Fig. 118. The stranghng fig. The tree on the left started as an epiphyte and
its roots may be seen encircling the trunk on which it started; the specimen on
the right began as a seedling in the soil. Photo by G. W. Blaydes.

299

300

TEXTBOOK OF BOTANY

Fig. 119. Epiphytic bromelias on live oaks in southern Florida. Photo from New

York Botanical Garden.

the same green plant or those of other nearby plants. The haustoria em-
bedded in woody stems may survive from one season to another, and
from them the dodder plants propagate vegetatively each year.

Internal structure of roots. A limited observation of root systems is
sufficient for one to see that roots are usuallv largest at or near their
junction with the base of the stem and gradually taper in size to slender
and tender tips. From these facts one may infer that the root tip is the
zone of growth in length. Since the older portion of a root toward its
base is the largest, there must be some means by which roots increase in
diameter.

If a cross section of a young root of a tulip tree is compared with a

[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES

301

Fig. 120. Section of stem of dodder above and a cross section of petiole of host
plant below. A haustorium from the dodder has grown through the cortex to a
vascular bundle in the petiole.

cross section of a woody twig, the same kind of tissues in the same order
of arrangement may be seen, except that no pith will be found in the
root. From the center of this woody root outward therefore, there are, in
order of occurrence: xylem, vascular cambium, phloem, pericycle, en-
dodermis, cortical parenchyma, cork cambium, cork, and epidermis if it
has not already become broken and sloughed off (Fig. 121). In most
of the roots that have been studied the first cork cambium originated in
the pericycle and the cortex was soon sloughed off.

The larger roots of a tree, like the larger stems above ground, consist
only of the vascular cylinder (stele) and the cork that develops peri-
odically from temporary cork cambiums derived from pericvcle or from
phloem parenchyma. Obviously, water and mineral salts do not enter
roots where they are enclosed by a cylinder of cork. They enter only
the youngest zone of tree roots that have not become enclosed in cork.

The growing root tips of manv plants are sufficiently transparent so
that when they are examined with a hand lens or low-power microscope,
one may easily recognize the terminal root cap, the central vascular
cylinder surrounded by a cortical cylinder, and the numerous root hairs
which protrude from the epidermis a short distance back of the apex
( Fig. 122 ) . Root hairs are not present on the roots of all species of plants,
and their abundance on others varies in different environments.

302

TEXTBOOK OF BOTANY

Fig. 121. Photomicrographs of cross sections of roots of a tuhp tree {Lirioden-
dron) in which the arrangement of tissues is evident: A, a young root; B, a 2-
year-old root.

The terminal portion of the root, then, is the zone of origin of cer-
tain root tissues, the zone of growth in length and of growth curvature,
and the principal zone through which water and salts enter many kinds
of plants.

Tissues of roots. When longitudinal sections of the growing tips of
stems and roots are compared, certain similarities and differences are
easily recognized. The three zones characterized in general by cell
division, enlargement, and differentiation (maturation); and the three
longitudinal cylinders represented by the epidermis, cortex, and vascular
cylinder are common to both. In some roots these three cylinders are
distinguishable all the way to the so-called growing point in the center
of the zone of cell division, which lies just above the root cap ( Fig. 123 ) .
In other roots the differentiation of cells in these three regions is not
obvious so near the growing point.

The four regions (root cap, epideraiis, cortex, and primary vascular
axis) of the growing tip of the root all originate from cells formed in
this growing point. The root cap is a cup-shaped mass of cells renewed
by cell division and enlargement at the lower side of the growing point
and sloughed off on the outer surface by disintegration of the middle
lamellae of the cell walls, accompanied by abrasion when the roots are
growing in the soil.

In the zone characterized by cell enlargement there are also some cell

[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES

303

Region of
elongation

Region of _J^
cell division |^^ I j^^^ ^^p

Fig. 122. Enlarged view
of the end of a root, show-
ing root cap, growing re-
gion, and root hairs. Cour-
tesy of World Book Co.

Growing

point

Root cap

Fig. 123. Diagram of a root tip,
showing the tissues and their ar-
rangement. Courtesy of World
Book Co.

division and cell differentiation. Farther up the young root, differentiated
cells become more and more conspicuous. The epidermal cells elongate
parallel to the axis of the root. As elongation slows down, root hairs
begin to appear as tubular extensions of some of the epidermal cells, and
within a few hours they attain their complete elongation at right angles
to the surface of the root.

Near the center of the vascular cylinder rows of cells extending length-
wise in the root become conspicuously large. These cells are the fore-
runners of the primary xylem vessels. The cross walls disintegrate, and
spiral thickenings are formed along the longitudinal walls before the
protoplasm disintegrates. Other strands of cells in the vascular cylinder

304

TEXTBOOK OF BOTANY

become phloem. In cross sections the primary xylem is seen as the central
tissue of most roots and is often more or less star-shaped, although the
so-called radii ( ends of longitudinal ridges alternating with troughs ) may
vary in number from 2 to 5 or more in different kinds of plants. However,
the roots of some plants, especially monocots and herbaceous dicots,
may have a central pith. When the central xylem is star-shaped in cross
section, the primary phloem tissue usually alternates with the radii of

,PRIMARY PHLOEM

\ PRIMARY XYLEM'

-SECONDARY PHLOEM

rt--CAMBIUM-

SECONDARY XYLEM

Fig. 124. Diagram of relations of primary and secondary, xylem and phloem in a

young growing root.

the xylem tissue. A little later a vascular cambium develops from paren-
chyma cells between the xylem and phloem ( Fig. 124 ) .

The outermost cells of the vascular cylinder may be recognizable as
pericycle. This is a highly versatile tissue and of great importance, since
from it secondary roots and adventitious stems often develop. It mav,
through further growth, add to the thickening of the root. Cork cambiums
usually develop from the pericycle in young roots.

The inner layer of the cortex adjoining the pericycle usually becomes
more or less recognizable as an endodermis. The thickness of the cortex
varies greatly in roots of different kinds of plants. In very young roots
it is usually the most conspicuous part. The first cork cambium in the
young roots of some plants may develop from cortical parenchyma.

Among the last tissues to appear in a young root are the primordia
of lateral roots. Unlike the primordia of axillary branches of stems, these
root primordia develop not from the young epidermal and cortical cells
near the apex but from deeper lying tissues (usually the pericycle or,
in older roots, the phloem ) at some distance from the apex. Renewal of
cell division in scattered groups of cells in the pericycle results in root
primordia that grow outward through the cortex and emerge through
the epidermis as secondary roots. Lateral roots are said, therefore, to
have an endogenous origin, in contrast to the exogenous origin of the
stems that grow as axillary branches. Stem branches may also have an

[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 305

endogenous origin, especially the adventitious stems that develop from
roots.

Lateral roots often appear mainly in two or more rows up and down
the root, because most of the root primordia originated opposite the
radially arranged ridges of the primary xylem.

Some of the differences in the apical regions of stems and roots may
now be noted. In stem tips there is no structure comparable to the root
cap, even in the rhizomes that grow in the soil. The cells from the center
of the apical meristem are forerunners of pith which is surrounded by
the primary xylem, whereas in most roots thev are the forerunners of a
central xylem. At the root tip there are no lateral organs comparable to
the lateral primordia of axillary buds, foliage leaves, or the primordia of
flowers. Root hairs are present near the tips of roots of most plants.
They are usually short-lived, but in some species they may live for
months.

The hormone known as auxin a appears to be necessary for cell en-
largement in both stems and roots, but as it is increased in amount cell
enlargement is decreased. The concentration of hormone necessary to de-
crease cell enlargement occurs naturally in roots, but usually not in
stems. These facts help us explain the difference in geotropism of stem
and root tips, as shown in Chapter XXIII. The direction of growth of
young lateral roots, like that of lateral stems, is influenced bv apical
dominance. The results of recent experiments indicate that hormones
necessary for growth in root tips may be made in stem tips, and vice
versa.

Secondary growth of roots. Most of the root tissues mentioned above
are primary tissues that developed from the cells of the apical meristem.
Secondary growth in thickness results primarily from the development
of a vascular cambium as a cylindrical sheath between the xylem and
phloem. From the cells formed by this cambium, increase in xylem and
phloem occurs as in stems.

The secondary growth may result in breaking and loss of epidermis.
The cortical tissue may continue enlarging for a time, or it too may be
broken and sloughed off. The pericycle survives in roots of some kinds
of plants much longer than in others. The thickened herbaceous roots of
some plants, such as those in long conical radishes and sweet potatoes,
are the result of growth of the vascular cambium plus further division
of the parenchyma cells of xylem, phloem, or pericycle. The concentric
rings seen in cross sections of beets are the cut edges of a series of cone-

306 TEXTBOOK OF BOTANY

shaped layers of tissue ( Fig. 125 ) . The innermost cone is formed by the
growth of the primary vascular cambium. The others are formed bv the
growth of a series of anomalous secondary cambiums, which may origi-
nate from parenchyma cells in the phloem or in the pericycle. The thick-
ened portions of turnips and conical radishes are largely xylem; but in
parsnips the tissues external to the xylem are most prominent. Increase in
thickness of bark, especially of roots of trees and shrubs, is partly the
result of the de\'elopment of cork from cork cambiums.

Fig. 125. Concentric rings of tissues visible in cross sections of beet roots
formed by the development of a succession of secondary cambiums in pericycle
and phloem parenchyma. Photo by E. F. Artschwager, Journal of Agricultural
Research.

The size and extent of root systems. The primary root of a seedling
continues its downward extension into the soil. Root hairs develop
within a few hours, and lateral secondary roots within a few days. If
the plant has a taproot system, the primary root continues growth for
weeks or months, and under favorable conditions may penetrate the soil
to a depth of 5, 10, 20, and even 30 feet. The roots of most plants, how-
ever, develop in the upper 3 to 5 feet of soil. In regions of abundant
rainfall and high water table, root systems in general are nearer the
surface than in regions subjected to periodic drought (Fig. 126). In
deserts there are many species of cacti with widespreading root sys-
tems less than a foot below the surface, and species of yuccas and desert
shrubs nearby with roots which penetrate deeply.

[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES

307

Contrary to common be-
lief, roots of our common
plants which have been ex-
perimentally tested do not
grow through a layer of air-
dry soil to moist soil beneath
(Fig. 127). When a portion
of the root system of a plant
is exposed to moist soil and
the remainder of its system

:S^3?

■^jr-i^^/- '"^ ,

\ \^,mif^'

.t:"

Fig. 126. Roots of hemlock and spruce on
a rocky slope in the Smoky Mountains. The
layer of mosses and ferns under which they
first grew have been washed away. The
terminal roots are located deep in the crevices
of the rock. Photo by E. N. Doan.

to dry soil, the portion in
moist soil grows extensively,
while the portion in dry soil
grows but little or not at all,
depending upon the dryness
of the soil. Similar unequal
growth of portions of the root
system of a plant occurs in
relation to several other fac-
tors.

The horizontal distribution
of the root svstem of a plant
is not dependent upon either
the diameter or the height of
the part above ground. The
frequent statement in popu-

308

TEXTBOOK OF BOTANY

Fig. 127. Effects of dry soil (lighter color) on the growth of roots. Photo by

F. H. Norris.

lar literature that the horizontal spread of the root system of a tree is
the same as the diameter of the crown is erroneous. Isolated trees on a
lawn or campus may have a horizontal spread of roots as much as 4 to
5 times the diameter of the crown. A young pear tree was found to have
a horizontal spread of roots 9 times the diameter of its crown.

Plants closely crowded together do not have as extensive root systems
as isolated ones. The more extensive root system may or may not occur
with the larger crown. The root system of a corn plant at Lincoln, Ne-
braska, commonly has a horizontal spread of about 7 feet and a vertical
penetration of about 6 feet. At Wooster, Ohio, the horizontal spread of
the root system of a corn plant is generally about 3 feet, and the depth
about 2 feet. These figures represent differences in the growth of root
systems due to the combined effects of heredity, climate, and soil.

Many factors affect the development of root systems. The striking dif-
ferences in form of the root systems of different species of plants are
expressions of different heredities in similar environments. But expres-

[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 309

sion of heredity may vary greatly in different environments. The wild
ancestors of our cultivated varieties of beets and radishes had compara-
tively slender and woody roots. The thickened herbaceous characteris-
tics of these cultivated varieties are hereditary, but under certain con-
ditions of temperature and length of day the roots fail to thicken; that
is, this particular hereditary potentiality is not expressed under certain
environmental conditions. It will be helpful to visualize root systems as
we do the crowns of trees. Certain specific characteristics are often evi-
dent, but in different environments their development may be greatly
altered.

In considering the factors that affect the growth of root systems, one
should not forget that sugar, vitamins, and hormones from the leaves are
essential factors in root growth, and that any factor that influences the
supply of these substances to the roots indirectly affects their growth
(Chapter XXI). Gravity affects the direction of growth of roots but
cannot be classified as either an atmospheric or a soil factor. Among
the factors of the soil environment that affect physiological processes
and growth in root systems are compactness or texture of the soil, water,
oxygen, carbon dioxide, soil temperature, inorganic salts ( essential, non-
essential, and toxic), acidity and alkalinity, and soil fungi and bacteria.
Only a few of these factors will be mentioned further in this chapter.

The compactness of the soil indirectly affects the growth of roots
through its effects upon aeration and the rapidity of the movement of
water and mineral salts. Directly it affects the penetration of roots. Many
clay soils may become so hard during dry seasons that germinating seeds
at the surface die through failure of the roots to enter the soil. Root
systems in general are less extensive in compact soils, and root growth
occurs mainly in the crevices of such soils. Soils of coarse texture are
not good soils because, in spite of good aeration, the supply of water
and available salts is often deficient under natural conditions. Between
tliese two extremes of fineness and coarseness are the soils known as
loams, in which roots grow best.

When roots grow through alternating layers of coarse and fine soils,
far more branching and development of lateral roots occur in the layer
of loam. Trampling, as it occurs in paths in gardens and greenhouses or
on trails and roadways, makes the soil compact. Pioneer roads abandoned
for a century or more are still evident in forests because few plants can
grow in such compact soils. Excellent virgin soils that were thousands
of years in fomiing become more compact through careless farm prac-

310

TEXTBOOK OF BOTANY

tices, partly because of the oxidation of organic matter in them and
partly by the removal of the surface layers down to the more compact
subsoil. The failure of plants to grow well on many abandoned farms is
due not so much to lack of essential salts as to the compactness of the
soil. The social significance of these facts has but recently been recog-
nized by the general public.

Fig. 128. Development of "knees" from the roots of bald cypress when growing
in lowlands annually submerged by floods. Photo by U. S. Forest Service.

Available oxygen has an immediate effect on respiration, and conse-
quently on growth. The roots of some kinds of plants, such as willow,
Cottonwood, and cypress, can survive and grow in much lower concen-
trations of free oxygen than roots of many other kinds of plants (Fig.
128). Oxygen is not necessarily obtained from the soil air by some
plants, such as those growing in swamps or in partly submerged
areas. Many of these marsh plants have continuous air cavities
extending throughout the plant. The green shoots above may well have
an excess of oxygen in the daytime, and there is always some

[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 311

movement of oxygen from the atmosphere into the air cavities through
open stomates.

Windfalls frequently display the root systems of trees that have grown
on bogs and swamps (Fig. 129). Often none of the larger roots has

Fig. 129. Major root system of an arbor vitae tree overthrown by a wind storm.
The tree had been growing in a bog and the uppermost layers of peat were lifted
with the roots. Photo by G. W. Blaydes.

penetrated the substratum more than one foot. The roots that lived and
attained age were those just beneath the surface. Toward the close of
the last century, thousands of miles of fences were made of these flat
root bases of white pine set up side by side, enclosing pastures after
lumbering had denuded the land. Yet when white pine stumps were
pulled from sandy upland, they had several roots an inch or more in
thickness down to ten feet below the surface.

The effects of submergence in rapidly flowing water differ from those

312 TEXTBOOK OF BOTANY

of Stagnant water largely because of the different amounts of oxygen
available. When plants are being considered, the expression "good drain-
age" should be interpreted as good oxygen supply rather than water
movement.

The well-knov^m fact that the roots of willows, cottonwoods, and elms
will develop excessively in drain pipes and stop the flow of water is due
to the combined result of constantly available moisture and oxygen.

The more direct effects of temperature on root development are largely
expressions of the fact that higher soil temperatures accelerate and
lower ones retard development. In polar regions the soil may be con-
stantly frozen a few feet below the surface; only the upper layers thaw
in midsummer. Root systems there are usually very shallow.

The available inorganic salts in a soil may increase or decrease root
development. Low concentrations of nitrogen in the form of ammonium
compounds and nitrate may restrict it. If nitrogen is present in greater
concentration and the manufacture of proteins is not curtailed, average
root systems are formed. Excessive amounts of nitrogen may result in
larger root systems in some plants and in smaller ones in others, accom-
panied in both cases by excessive development of the tops. Conse-
quently, one frequently finds greater development of roots in proportion
to the shoots in poor soils than in soils rich in nitrogen.

Large amounts of salts, such as occur in salt and alkali flats in dry
regions, become limiting factors to most plants, and only a few species
survive these conditions. When salts accumulate to 2 per cent of the
dry weight of the soil, practically all flowering plants are killed.

With the effects of all these factors in mind, one should be able to
explain some of the examples of root distribution that he encounters in
the field, in road cuts, and in excavations, and the many diagrams of
root systems available in botanical literature.

Root surfaces compared with leaf surfaces. Many studies of the rela-
tion between the depth and extent of root penetration and the height
and size of the shoots of plants have been made. The relative weights
of roots and shoots of many economic plants have been studied at the
agricultural experiment stations, particularly following different methods
of cultivation, crop rotations, frequency of cutting, and the addition of
fertilizers. Few studies have been made of the areas of entire root sys-
tems, including the finest roots and root hairs, in relation to the areas of
leaves and stems.

The total areas of tops and roots of winter rye grass have been meas-

[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 313

ured in detail. Some of the results as reported are given here. The plant
grew for 4 months in a fertile soil contained in a wooden box a foot
square at the top and 22 inches deep; that is, somewhat less than 2 cubic
feet of soil. The plant produced 80 shoots with an average of 6 leaves
per shoot, and was about 20 inches tall at the end of the experiment. The
total exposed surface of the top of the plant was 51.4 square feet. The
number, length (if placed end to end), and area of the roots are best
seen from the accompanying table.

Roots

Number

Total Length

Total Surface

Main roots
Secondary
Tertiary
Quaternary

143
35 , 600
2.3 million
11.5 million

214 ft.
17,800 ft.
574,000 ft.
1 .4 million ft.

1.5 sq. ft.

45 . 0 sq. ft.

758.6 sq. ft.

1,748.9 sq. ft.

Total

14 million

2 million ft.

2,554 sq. ft.

Root hairs

14 billion

6,000 miles

4,321 sq. ft.

These figures show that the total surface of the roots is about 130
times that of the shoots. The presence of nearly 14 million roots and 14
billion root hairs with a total surface of 6800 sq. ft. in less than 2 cu. ft. of
soil shows how completely every part of the soil may be penetrated. The
roots are thus in contact with much of the water surrounding the soil
particles and with the salts and other compounds dissolved in the soil
water.

If we assume an average daily rate of growth during the four-month
experimental period, 3 miles of new roots and 50 miles of new root
hairs ( 100,000,000 ) were added each day. In nature the daily invasion
of new soil masses might be of great importance if the water content of
a soil were decreasing, or if the soil solution of inorganic salts were very
dilute.

In the chapter on transpiration it was shown that the principal evapo-
ration surface is the mesophyll when the stomates are open. The meso-
phyll surface of winter rye grass is perhaps 6 times that of the epidermis
of the shoots. Even when the stomates were open, the plant described
in this experiment had an absorbing surface 22 times that of the evapora-
tion surface.

314

TEXTBOOK OF BOTANY

When the root systems of winter rye grass were compared with those
of oats, and of bluegrass under field conditions, the following relative
results were obtained:

Winter rye grass

Oats

Bluegrass

Number
of Roots

4,700

6,400

84,500

Total
Length

150 ft.

210 ft.

1 , 250 ft.

Total
Surface

50 sq. in.

78 sq. in.

332 sq. in.

From these figures it is evident why bluegrass is so effective in form-
ing turf and in preventing soil erosion. Further evidence of its copious
root system is obtained when a comparison is made of the roots of three
grasses growing in the field under similar conditions. An examination
of sample volumes of soil, 3 inches in diameter and 6 inches deep (42
cu. in.) showed that for each cubic inch of soil:

Oats had 15 sq. in. of root surface and 150,000 root hairs.

Winter rye grass had 30 sq. in. of root surface and 300,000 root hairs.

Bluegrass had 65 sq. in. of root surface and 1,000,000 root hairs.
The roots of oats occupied 0.55 of 1 per cent of the soil volume; those of
winter rye grass 0.85 of 1 per cent; and those of bluegrass 2.8 per cent.

REFERENCES

Dittmer, H. J. A quantitative study of the roots of a winter rye plant. Amer.
Jour. Bot. 24:417-420. 1937.

Dittmer, H. J. The efficiency of monocot roots in soil conservation. Univ. of
Iowa Nat. Hist. Studies. 17:343-346. 1938.

Dittmer, H. J. Quantitative study of the subterranean members of three field
grasses. Amer. Jour. Bot. 25:654-657. 1938.

Weaver, J. E. Root Development of Field Crops. McGraw-Hill Book Com-
pany, Inc. 1926.

CHAPTER XXX
ROOTS: PROCESSES AND SOIL RELATIONS

If we include both subterranean and aerial roots in our purview there
are few processes in leaves and stems that do not also occur in roots.
Cell division, enlargement, and differentiation; respiration; transpiration;
sugar, fat, and protein syntheses; digestion; assimilation; absorption and
movement of water and mineral salts; and the translocation and accumu-
lation of substances are just as characteristic of some kinds of roots as
they are of some kinds of leaves and stems. Not all of these processes
occur in every type of leaf, stem, or root.

Photosynthesis in roots is limited to those roots which become green
when exposed to light, and is characteristic especially of the epiphytes,
such as tropical orchids, bromelias, and ferns. However, the roots of
many land plants exposed by current and wave action along lake shores
and stream banks may become green and add to the sugar supply of
the root. Transpiration occurs from the roots of epiphytes, and from
any other root surfaces exposed to air, either above or below the soil
surface.

The soil environment. Before discussing the other processes occurring
in roots we should consider a few of the conditions in the soil as an
environment in which roots grow. First of all the soil is a mass of larger
and smaller particles of minerals derived from underlying rocks, or car-
ried in from adjoining regions by water, ice, and wind. Through the cen-
turies this "parent material" of the soil is modified by weathering which
includes rainfall and drought, freezing and thawing, solution and pre-
cipitation, leaching and chemical reorganization.

Meanwhile plant stems and roots have thickly penetrated all the sur-
face layers every year, and the lower layers at least every few years. All
or parts of these organs have died each year, and their organic com-
pounds have been incorporated in the soil. These compounds become
the food supply of an enormous population of bacteria, fungi, and minute
animals that alter the organic residues and, according to conditions, in-

315

316

TEXTBOOK OF BOTANY

crease or decrease the available organic compounds and the water-
holding capacity of the soil, as well as its texture and penetrability.

The interstices among the irregular soil particles make up from a third
to a half of the soil volume. After a rain they are partly or entireh filled
with water (Fig. 130). The air in these interstices constitutes a "soil

Fig. 130. Arrangement of soil particles and interstices in a cultivated field. Photo

by L. D. Baver.

atmosphere." Water movement in these spaces is for the most part by
capillarity, and to a slight extent by gravity. The lateral movement is
almost as rapid as the downward movement.

Following a rain the water gradually approaches an equilibrium in
which similar parts of the soil mass have about the same water content.
Water vapor is also diffusing through the soil mass, and some of it may
diffuse out of the pores at the soil surface. Soluble salts are likewise
diffusing in the soil solution from regions of high concentration to
regions of low concentration. Thev may be carried about also during
mass movement of the water in tlie soil. Owing to electrical forces,
mineral ions may also become adsorbed (electrically held) on the sur-
faces of soil particles.

[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 317

Oxygen diffuses into soil from the atmosphere and is also carried into
it in solution in rain water. Within the soil, oxygen is combined with
various reduced compounds by soil organisms and is consumed in the
respiration of roots and countless numbers of small plants and animals
that inhabit the soil.

Carbon dioxide, because of its high solubility in water, enters the
soils in rain water especially if that water percolates through a layer of
leaf litter, duff, and humus at the soil surface. It also diffuses into the
soil air and soil solution from the respiration of roots and soil organisms.
Soil air may contain as much as 5 to 15 per cent of carbon dioxide.

In temperate regions soils may undergo rather rapid temperature
changes at the soil surface, where in the summer time fully exposed areas
may be heated as high as 120° to 140^ F., or even higher. Temperature
fluctuations gradually decrease from the surface downward until the
temperature is more or less stabilized at about the average annual tem-
perature of the locality.

Fig. 131. Profile of a cultivated soil (a) with the roots of a carrot in place;
the profile of a mature soil (b) that developed beneath a northern coniferous
forest; and (c) the profile under a northern mixed prairie. One-foot intervals are
indicated at the side of each picture. Photos, (a) from H. C. Thompson, Cornell
Univ.; (b) and (c) from C. E. Kellogg, United States Department of Agriculture.

318

TEXTBOOK OF BOTANY

.. . pa O.. • .• •: o... . .0 _ •i.-oo-lo ■„ ^^-0-..o -.0 ey-. . •.^■" o 6 •■ • I o/^"- ••••<;»^:-o" "^

-C»

6f*

".^.o-^ ■S~'~~a^:~.~.^.~^. ■ VimI "sahO

I ■ I I 1 L-

FiG. 132. Two diagrams illustrating the distribution of the roots of white pine
in soil profiles having layers of different texture, color, water content, and aeration.
After H. J. Lutz, Yale Forestry Bulletin 44, 1937.

Diagram of soil profile (A). Uppermost layers of white pine debris and fungous
mycelia. Horizon A, fine sandy loam, dark brown above, light brown below.
Horizon B, fine sandy loam, brown slightly mottled. Horizon C, loam above,
coarse sand below.

Diagram of soil profile (B). Loamy coarse sand in horizon A; color brownish
gray, motded. Horizon B, dark brown coarse sand. Horizon C, gray in color,
gravelly, coarse sand except layer of fine sand at 6 ft. below surface.

[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 319

After all these soil processes and activities have continued for hun-
dreds or thousands of years, a soil usually has attained maturity and cer-
tain characteristic chemical and physical properties and water-holding
capacities. Its constituents become arranged in layers or horizons of
varying thickness beginning at the soil surface and ending below in the
parent material. These layers mav differ greatly in composition, reac-
tion, and structure, but some parts of most root systems extend into each
of them (Figs. 131 and 132).

Roots growing in a soil, then, are developing in a highly dvnamic
medium, in which chemical, phvsical, and biological processes are con-
tinuous, and in which the available soil water, soil solutes, and soil air
may be as variable as those of the atmospheric environment in which the
foliage and stems develop.

Field capacity and wilting percentage of soils. With reference to the
actively growing plant there are two critical points in the water relations
of a soil. The first is the amount of water that is held in equilibrium
against further movement by gravity and capillaritv in a given soil under
field conditions. One may visualize this condition bv assuming that a
large mass of a uniform soil after a prolonged drought has become air
dry to a depth of 5 feet. This soil still contains some water — about 1 per
cent if composed of sand, and as much as 5 per cent if clav. Seeds do not
germinate and seedlings do not survive under these conditions.

Suppose now that an inch of rain falls on this soil and it all penetrates
the interstices between the soil particles. If the soil were sandy it might
moisten the upper layer to a depth of a foot. If the soil were a fine clay
only a few inches would become moist. After a day or two the water in
the moist layer of soil has become uniformly dispersed. Neither capil-
larity nor gravity causes it to move farther downward. The soil water is
now at equilibrium, and films of water surround the soil particles but
do not fill the larger spaces among them. The water held at equilibrium
is commonly called the "field capacity" of a soil. Sandy soils have a field
capacity of about 5 per cent, and clay loams about 35 per cent ( Fig. 133 ) .

If a second rain of one inch followed, capillary forces would cause
the water to move through the moist layer into the dry layer just be-
neath, and at equilibrium the fine soil would be moist to a depth of
several additional inches, and the sandv soil another foot.

Under these conditions, seeds can germinate and seedlings grow rap-
idly. The roots of seedlings have a continuous water supply as their

320 TEXTBOOK OF BOTANY

WILTING FIELD

PERCENTAGE CAPACITY

LITTLE OR NO MOVEMENT OF

FREE MOVEMENT OF

WATER IN SOIL ' WATER IN SOIL

WATER CONTENT

N^o^GRowTH POOR GROWTH BEST GROWTH poo" growth or death

Fig. 133. Diagram illustrating water relations of a clay loam.

innumerable branches and root hairs penetrate to all parts of the moist
layer. Oxygen is abundant in the soil air and respiration is not restricted.

Let us now assume that no more water is added. After a few weeks,
depending on the temperature, the young plants begin to wilt in the
middle of the day, but recover their turgidity at night. The rate of absorp-
tion of water is not equal to the rate of transpiration during midday, but
at night it is greater than the water loss. After several more days, how-
ever, the plants wilt and do not recover at night. In spite of the large
root surface, water no longer moves from the soil to the plants, and they
become permanently wilted. The wilting percentage is the amount of
water present in a soil under these conditions. It varies from 1 to 10 per
cent in sandy loams, from 15 to 20 per cent in fine clay loams.

Absorption of water by osmosis. Thus far we have visualized osmosis
as occurring in each individual cell, in which the water inside the vacuole
is separated from the more concentrated water outside the cell by the
differentially permeable cytoplasmic membrane. It is possible also to
visualize a whole root as one osmotic unit; that is, the water in the xylem
vessels in the central cylinder of the root is separated from the water in
the soil by the cambium and bark ( phloem, pericycle, cortex, epidermis ) ,
which may be considered as one complex differentially permeable
membrane.

These relations may be demonstrated by means of a thickened root,
such as that of carrot. To simplify the problem, one may remove most
of the xylem of the root with a cork borer and regard the cavity thus
formed as one large xylem vessel, in lieu of the many original vessels
removed by the cork borer. A glass tube may be securely attached to
the top of the root to represent the continuation of a xylem vessel into
the stem of the plant. If the cavity in the root made by the removal of

[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 321

natural xylem is filled with a sugar solution and the root is then immersed
in water, tlie sugar solution slowly rises in the attached glass tube.

The observed phenomena may be duplicated by substituting for the
carrot root a long porous clay cup in the walls of which there is a
differentially permeable membrane of copper ferrocyanide.^ In the carrot
root the differentially permeable membrane is considered to be all the
layers of living cells between the xylem vessels and the water surround-
ing the root.

This demonstration is probably analogous to the movement of water
from the soil through the cortex to the xylem vessels in a living root
under certain conditions. When the stems of some plants, growing with
an ample water supply, are cut off an inch or two abo\'e the soil, water
may exude from the cut surfaces. Under these circumstances large quan-
tities of sap exude from the stumps of a few kinds of plants, such as
birch and grape. The water may exude from these stumps against a pres-
sure of one or two atmospheres, which would be sufficient to push water
to the tops of small trees. Such pressures, however, are lacking at the
time of year when the water loss from leaves is greatest, and the results
of numerous experiments indicate that the rate of exudation is too slow
to account for the large volume of water that is lost in transpiration.

Guttation. The extremely wet grass one often sees on lawns at night
and early morning is not always a result of the formation of dew. When
the humidity of the air is high and the soil is moist, each blade of grass
has a glistening drop of water at its tip. This loss of water in liquid fomi
at the ends of veins of leaves is called guttation. Early-morning golfers
are often annoyed by the fact that this water contains sugar, because
when the water evaporates from their hands and from the handles of
golf clubs a sticky sirup remains.

Guttation may be demonstrated in the leaves of many kinds of plants
by placing bell jars over well-watered plants in pots or by attaching
leaves or branches to a water faucet by a hose connection. Guttation may
also occur in plants in greenhouses when excessive amounts of water have
been added ( Fig. 134 ) . The veins of the leaves are continuous with those
of the stems and roots, and where they end in the leaves there may be
an open meshwork of parenchyma cells covered by an epidermis con-
taining stomates or pores. Guttation is probably the result of conditions

^ The copper ferrocyanide membrane is formed in the walls of a porous clay cup pre-
viously soaked in water, if the cup is filled with a solution of potassium ferrocyanide and
immersed in a solution of copper sulfate.

322

TEXTBOOK OF BOTANY

exemplified by the carrot-root demonstration without any artificial man-
ipulation of the structures involved. In other words, guttation may be
the result of the processes by which pressure is built up in the xylem
vessels of the root system by the osmotic absorption of water across the
complex membrane of tissues external to the xylem. This pressure may
become great enough to push water up the vessels and out of the ends
of the veins of some of the leaves of a plant.

Fig. 134. Guttation in leaves of tomato in a moist atmosphere. Photo from J. H.

Gourley.

During early autumn, when the soil is still warm and moist and the
atmospheric humidity is near saturation, drops of water exude from the
points on the leaf margin of oaks, maples, cottonwoods, and other trees.
This is another example of guttation, often mistaken for dew.

Absorption of water by pull of transpiration and osmosis. The phe-
nomena of "bleeding" from cut stems and guttation have sometimes led
to the conclusion that the pressure developed in roots is the cause of the
upward movement of water in tall stems under all conditions.

However, if a hole is bored through the bark of a tree and an inch or

ROOTS: PROCESSES AND SOIL RELATIONS

323

[Chap. XXX

two into the wood and a glass tube filled with water is quickly attached
by a tight-fitting rubber stopper, one may readily determine whether
the water in the trunk of the tree is under pressure or under tension. If
the lower end of the tube dips into a vessel of water and there is internal
pressure, water will flow into the vessel; if there is tension, water will
flow into the tree. The best time to set up such an experiment is when
it is raining, or in the early morning before sunrise. Gauges that register
both positive and negative pressures have been used in similar experi-
ments.

Most of the experiments have shown that in late spring and early
summer, when transpiration is highest, there is tension, not pressure, and
that water may be drawn from the vessel into the tree. This would be
difficult to explain on the basis of root pressure.

Another type of experiment to show root pressure is mentioned in a
preceding section. If a plant is cut off when transpiration is rapid, and a

30|

-

/
/

t

1 1 1 1 1

1

0=25
|20

gio
a s

1 TRANSPIRATION */ /^

V ABSORPTION 1/

\\ZC^¥^S^ \ 1 1 1 1

Fig. 135. Curves of hourly rates of absorption of water, and the simultaneous
rates of transpiration. Internal water tension, or pressure, results. Data from P. J.
Kramer.

water-filled tube is immediately attached to the stump, the water level
in the tube falls. Under the most favorable conditions positive pressures
( shown by a rise in the water level ) do not occur until after a lapse of
one-half to two hours' time. This experiment seems to indicate that in
spite of the osmotic absorption by the roots, the root cells were not fully
turgid. Some of the water evidently had been drawn out of the roots
by transpiration from the mesophyll cells above ( Fig. 135 ) .

Additional evidence that the water content of tree trunks decreases
during periods of active transpiration is shown by the fact that tree
trunks have daily variations in diameter as indicated by exact measure-
ments. They are smallest in the afternoons of clear hot days, and largest
in the early morning hours ( Fig. 136 ) .

The flow of maple sap. The flow of maple sap from trees in early spring

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TEXTBOOK OF BOTANY

is conditioned not only by the effects of root pressure but also by the
presence of gases in the xylem tissues. The flow is greatest during the
period when freezing temperatures occur at night and davtime tempera-

i I L^L 14-L i ilii I Ll l±i f f ( J

N

M-<rSHrTKU--rWa-Hv.>+4 i-ULU^^ I

<a

VV \ \ \ v \ \ V U \ \T\ V \ \ \ \ n \ \ \ \ \ \ \

Fig. 136. Daily variations in the diameter of the trunk of a Monterey pine. The
contraction of the wood cells is a result of tension on the water columns due to
transpiration. Stems of plants shrink in diameter in the afternoons when the rate
of transpiration is relatively high. Data of D. T. MacDougal (1936).

tures are well above the freezing point. At this season the sugar content of
the tree is high. Transpiration is low because the buds have not yet

\ ■* I ^ u /

Fig. 137. Method of collecting "sap" from sugar maple.

opened and absorption of water by the roots is active. When the twigs
and smaller branches are warmed by the sunlight, the gases in the xylem
expand. The resulting pressure added to the pressure developed in the
roots causes the sap to flow from the tap-holes in the trunk (Fig. 137).

[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 325

When the leaves have begun to develop, the "sap pressure" becomes
negative, sap ceases to flow from the tap-holes, and (with experimental
equipment) water is drawn into the tree instead of being forced out.

During the maple sap season, most of the sap and sugar flows out
before noon of any one day. The total yield of sugar from different trees
is roughly proportional to their summer leaf areas. When three taps were
made in a tree, one at the top of the root, another at a height of four
feet, and a third just below the branches, one-half of the yield came from
t'he tap four feet from the ground, one-fifth from tlie root tap, and the
remainder from the upper tap. These are some of the more interesting
facts about the flow of maple sap. Experimental data seem to indicate
that a satisfactory explanation of all the phenomena will invoh'e not only
root pressure and gas pressure, but also other internal relations now
unknown.

Dead root systems. If water is pulled upward in stems through xvlem
cells that are dead, what will be the effect of killing the roots without
injuring the stem? An answer to this question mav be obtained bv killing
the roots of plants with hot water. Of course the living membranes of the
roots of these plants will be destroved and osmotic movement of water
ended. Experiments have indicated that not onlv mav the plants continue
to live for a week or two but the transpiration rate increases and in
some plants becomes several times as great if the soil is wet. Under
what conditions and in how many ways are living root systems superior
to dead root systems?

Absorption of inorganic salts. Not only does water enter plants from
the soil, but inorganic salts also enter from the soil solution. When plants
grow freely suspended in water, as the algae in the microcosm described
in Chapter XX, soluble salts in the water mav diffuse into any cell of the
plant. In rooted plants the salts enter mainly through the root svstem.
A smaller amount may enter through rhizomes, or even through leaves
and young stems that have been covered by a film of salt-containing
water. This is particularly true of leaves of Spanish moss, pineapple,
and other plants of the bromelia family. The peculiar leaves of the
pitcher plant exemplify another condition in which salts may diffuse
directly into the leaves. Perhaps many other leaves with cup-like bases
in which water collects during rains similarly absorb a small amount of
salts directly.

Salts in solution may diffuse directly into the root tip and root hairs, or
the outer layer of pectic compounds on the root hairs may be in such close

326 TEXTBOOK OF BOTANY

contact with soil particles that the salt ions adsorbed on their surfaces
may move from one to the other without actually being in solution.
Thus the entrance of a salt into roots is influenced not only by the con-
centration of the salt and the permeability of the protoplasm, but also
by the electrical charges on its ions and those on the surfaces of soil
particles and of root cells. The physical and chemical processes involved
in the absorption of salts other than simple diffusion are so complex that
we shall omit further consideration of them.

It may be noted, however, that any salt in the soil surrounding the
roots may enter the plant. Neither the kinds nor the proportions of salts
found in plants bear any necessary relation to "what the plant needs."
After the ions of a salt have entered a plant they may remain in solution,
become a part of organic molecules, form salts with organic acids and
bases, accumulate in crystalline form, or be adsorbed on the surfaces of
the colloidal particles in the protoplasm and cell walls. The ease with
which some of the ions of a salt may be removed from the protoplasm
without killing the cells may be demonstrated by placing leaves of
elodea in water at 90° F. for an hour or two. Some of the calcium in the
protoplasm is set free and forms beautiful crystals of calcium oxalate
with the oxalic acid that was already free in the vacuoles of the cells.

Relation of inorganic salts to plant development. Most people know
that the growth of a plant is often improved after certain salts have
been added to the soil, and that the practice of applying fertilizers con-
taining salts of nitrogen, phosphorus, and potassium to fields and gardens
would be discontinued if it were not profitable. On some kinds of soils
it is profitable to apply salts containing certain other elements.

Fig. 138 illustrates the relative development of a tomato plant when
nitrogen or some one of several mineral elements is not present in the
salts added to the sand in which the roots grow. It is customary to refer
to these plants as having grown in a sand culture; that is, the pots were
filled with pure quartz sand, and then a culture solution, made by placing
a known amount of certain chemically pure salts in distilled water, was
added from time to time. Perhaps no chemical element is absolutely lack-
ing in these experiments. Many kinds of chemical elements were in the
compounds in the seed from which each plant grew, and it is impossible
to obtain absolutely pure sand, water, or salts. It is quite evident, how-
ever, that any one of several elements may be so deficient that the plant
grows but little and fails to complete its entire life cycle.

The characteristic differences in growth and appearance of a plant.

[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS

327

'^^^ fCF':

Fig. 138. Relative growth of plants in relation to the concentration of various
ions of inorganic salts: Upper — tomato plants; lower — a diagrammatic representa-
tion of the effects of ions on the yield of hay in an unplowed meadow in southern
Greenland. The ions added to different parts of the meadow are indicated on the
left. Each haystack represents a yield of 1,000 pounds. The yields indicated are
averages per acre of 8 successive harvests. Data from Danish Experiment Station
of south Greenland.

328 TEXTBOOK OF BOTANY

when the supply of some element is inadequate, are referred to as de-
ficiency effects, or symptoms, in respect to that element; for example,
effects of nitrogen deficiency. The term, of course, refers to the available
nitrogen in ammonium or nitrate salts and not to the molecular nitrogen
in the air. When a cucumber plant is deficient in nitrogen, the apical
end of the fruit is small. When it is deficient in potassium, the basal end
of the fruit is small. When plants with thick roots, such as sweet potato,
are deficient in potassium or phosphorus, the roots are long and slender
because of a failure of cell division in the cambium. Continued cell
division in the meristem appears to be related to the fact that mobile
ions of potassium and phosphorus accumulate most abundantly in apical
meristems.

Several investigators have made summaries of deficiency symptoms
of each of several elements in different species of plants. One of these
summaries describing tobacco plants growing in culture solutions is
given in abbreviated form below as a basis for comparing symptoms in
other kinds of plants.

Nitrogen deficiency. Plant light green. Lower leaves become yellow, die,
and dry to a light-brown color. Stem slender and short. Roots long, with few
lateral branches.

Phosphorus deficiency. Plant dark green. Lower leaves may become yellow,
die, and dry to a greenish to black color. Stem slender and short. Roots long,
with few lateral branches, reddish brown in color.

Potassium deficiency. Lower leaves mottled, with dead spots at tips and
along margins, which curve downward. Stem slender. Roots long with few
branches; yellowish and slimy in appearance.

Magnesium deficiency. Lower leaves chlorotic (no chlorophyll), usually
without spots, and with tips and margins curved upward. Stem slender. Roots
long, with few lateral branches; slimy in appearance.

Iron deficiency. Young leaves chlorotic, usually without spots, principal
veins green. Stem slender and short. Root short, with abundant short laterals,
brown in color. Terminal bud of stem remains alive.

Manganese deficiency. Young leaves chlorotic witli scattered dead spots.
Leaf appears checkered because of the green color of the small veins. Stem
slender. Roots not abundant; brownish in color. Terminal bud of stem re-
mains alive.

Sulfur deficiency. Young leaves light green, no dead spots. Veins lighter
green than intervein tissue. Stem short and slender. Roots white, abundant
and much branched. Terminal bud of stem remains alive.

Calcium deficiency. Young leaves in bud curved and hook-like; die at tips

[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 329

and margins. Terminal bud dies. Roots short, much branched, dark brown in
color.

Boron deficiency. Young leaves in terminal bud at first light green at base;
later disintegration is evident, followed by twisted growth. Terminal bud dies.
Roots have many short laterals; brown in color.

If one were to attempt a complete explanation of each deficiency
effect, it would be necessary to know the first effect of the chemical ele-
ment on one or more processes in the cell and then to show that without
these first effects certain other processes would not occur. This is not
easy to do, and with our present knowledge it is usually impossible.
Perhaps the reader would be content to say that chlorosis in the absence
of magnesium is the result of the fact that magnesium occurs in the
chlorophyll molecule and no chlorophyll could be made without it. But
how shall we explain chlorosis in the absence of iron or of manganese?
We cannot, of course, pursue this problem in detail in a general textbook,
but we may emphasize certain useful perspectives.

In previous chapters each of the major plant processes (photosynthe-
sis, respiration, digestion, assimilation, etc.) was presented in a simple
manner. That is, the products used and the products formed were
named, and it was usually noted whether oxidation, reduction, conden-
sation, or hydrolysis was involved. We omitted the series of intermediate
steps in each of these major processes, partly because many of them
have not yet been discovered, and also because they involve many com-
plex chemical compounds. For example, in muscles of animals, glycogen
is changed to lactic acid, and vice versa. It is now fairly well established
that at least 10 diflFerent enzymes and as many kinds of intermediate
products are foiTued during these major changes. Furthermore, nearly
every one of these enzymes and intermediate products is adequately
active only when phosphorus is present and temporarily combines with
them; that is, phosphorus is a catalyst in these processes. It appears to
be a catalyst in most of the major processes in living cells, and for that
reason has been referred to as "the dynamite of living cells." Many other
elements, such as iron, manganese, magnesium, and potassium, are also
catalysts in some of the major processes that occur in cells.

From the above discussion it is obvious that one cannot understand
all the relations of the mineral elements to cell processes until all the
steps in each of these processes are fully known. We may, however, at
this time summarize the relations of the elements already discussed in
previous chapters. We have seen, for instance, that nitrogen, sulfur, and

330 TEXTBOOK OF BOTANY

phosphorus are constituents of protein molecules; that nitrogen and
magnesium are constituents of chlorophyll; and that calcium forms salts
with pectic acid in cell walls. Carbon, hydrogen, and oxygen are omitted
from this discussion, since, with the exception of oxygen in a few com-
pounds like carotene, they are constituents of the molecules of all the
substances of which cells are composed.

We have also seen that the ions of some of these elements are catalysts
in cell processes. Others affect osmosis through their influence upon the
permeability of protoplasm and to a minor extent upon the concentration
of water in the vacuole. It was noted that sodium, which is not essential
to most plants, increases the permeability of protoplasm and that cal-
cium annuls this effect. This is but one example of numerous balancing
effects of the different ions in cells. Almost any ion may influence the
permeabilitv, viscosity, and water content of protoplasm. Some of them
even cause death of the cells in one or the other of these ways. The
whole complex of organic acids, bases, and inorganic salts, especially of
phosphorus, is capable of so many interrelated reactions that fluctuations
in the acidity of the cell seldom become fatal.

Chemical elements essential to plants. If a botanist had been asked in
1920 to list the external conditions necessary to the complete develop-
ment of a green plant, his reply would have been about as follows: the
absence of destructive and toxic agencies; the presence of a suitable
temperature, length of day, and acidity; adequate light, water, free
oxygen, carbon dioxide, and certain soluble salts containing the elements
of nitrogen, phosphorus, potassium, sulfur, magnesium, calcium, and
iron. Today he would add that there must also be a trace of salts of
manganese, boron, copper, and zinc. He would also say that other ele-
ments mav be found essential to plants, at least to some plants. Perhaps
the last four elements named above are not essential to all plants. Even
calcium is not essential to certain algae and fungi.

During the 19th century, botanists and agriculturists got along fairly
well on the conclusion that only 10 elements are essential to plants. Later
it was observed that copper-containing spravs sometimes improved the
growth of plants, and that molds would not grow and reproduce re-
peatedly in the same glass vessels unless a trace of zinc was added. The
first cultures of molds had obtained sufficient zinc from the new glass.

These observations did not constitute a final proof that these elements
are essential. Final proof was obtained only when all the containers used
were of pyrex glass. The water used in the water cultures prepared for

[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 331

green plants and in the nutrient media prepared for non-green plants was
redistilled several times, until its total metal content was less than 0.0002
part per million (0.0002 ppm.). The salts used were recrystallized until
the metal impurities in them were less than 0.0001 ppm.

To cite but one example, it was discovered at the Universitv of Cali-
fornia that the symptoms of manganese, zinc, and copper deficiencies in
tomato plants disappeared when the culture solution contained 0.5 ppm.
of manganese, 0.05 ppm. of zinc, and 0.02 ppm. of copper. The copper
deficiency symptoms also disappeared when the leaves were merelv
spraved with a verv dilute solution of copper sulfate. Increased amounts
of these elements soon result in toxic effects; 2 ppm. of copper in the
culture solution was injurious to tomato plants.

Several botanists have proposed culture solutions in which many kinds
of green plants will grow well. The following is an example:

Water 1 , 000 . 00 cc.

KH2PO4 0.31 gm.

Ca(N03)2.4H20 1.04 gm.

MgS04 . 7H2O 0 . 54 gm.

(NH4)2S04 0.09 gm.
Very small quantities of FeS04, H3BO3J
MnS04, ZnS04, and CUSO4.

Water cultures in tanks. Botanists have studied the growth of plants
in water cultures in small laboratory vessels for nearlv a centurv. When
the size of the vessel was increased to that of a small tank, the process
attracted public attention. It is necessary to stretch "hardware cloth"
across the top of the tank and cover it with a thin layer of such substances
as peat or excelsior that will keep the seedlings upright. If all the factors
are properly adjusted, many kinds of plants will grow and reproduce in
these "tank cultures" as well as they do in highlv fertile soils. Tank cul-
ture has been commercialized under the name "hydroponics."

Anyone can play with the idea of tank culture; but the means of main-
taining a proper supply of salts and of testing its commercial value are
problems that can be solved only by research students. The most modern
tank-culture apparatus found in greenhouses consists of a tank filled
with cinders or coarse gravel, into which the culture solution is pumped
from another reservoir at stated intervals during the day and allowed to
drain out immediately. Various modifications of the apparatus may also
be used (Fig. 139). The apparatus can be constructed to run auto-
matically.

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TEXTBOOK OF BOTANY

Fig. 139. A convenient method of repeatedly adding a culture solution to plants
potted in gravel. Each bucket contains a prepared solution and is connected to a
pot by means of a rubber hose. At intervals the bucket is hung on the suspended
hook above the pot until the solution flows into the gravel. It is then returned to
the shelf below the pot and the solution drains from the gravel into the bucket.
Photo by J. D. Sayre, Ohio Agricultural Experiment Station.

REFERENCES

American Potash Institute. Potasli Deficiency Symptoms. 1937. In this publi-
cation potassium deficiency effects in many kinds of plants are illustrated
in color.

Anion, D. I., and P. R. Stout. The essentiality of certain elements in minute
quantity for plants, with special reference to copper. Plant Physiol. 14:371-
375. 1939.

Chilean Nitrate Educational Bureau. Bibliography of references to the litera-
ture on the minor elements. 1936.

Hoagland, D. R., and D. I. Arnon. The water-culture method for growing
plants without soil. Calif. Sta. Circ. 347. 1938.

Laurie, Alex. Soilless Culture Simplified. McGraw-Hill Book Company, Inc.
1940.

McMurtrey, J. E., Jr. Distinctive plant symptoms caused by deficiency of
any one of the chemical elements essential for normal development. Bot.
Rev. 4:183-203. 1938.

CHAPTER XXXI
INITIATION OF FLOWERS

Many plants bloom regularly only at certain seasons of the year. We all
know that some of them bloom much earlier during the growing season
than others. Without giving much thought to these familiar facts, we
might casually assume that a plant blooms when the temperature is just
right, or when it has reached the right age to bloom. But anyone who is
aware of the behavior of plants in greenhouses or in window boxes may
have noted that some of these plants likewise bloom only at certain sea-
sons of the year, or that they do not bloom at any season.

Some of the plants that bloom regularly only during the summer may
bloom in winter if the room is lighted during the first half of each night.
Likewise, some that regularly bloom only in autumn, or in greenhouses
during the winter, will bloom in midsummer if they are placed in a dark
room from the middle of the afternoon until the next morning for several
successive days.

Cocklebur plants that grow from seeds in late summer may be in
bloom two or three weeks later. Those that begin growth from seeds in
late spring may grow vegetatively for several months before the first
flowers appear.

If the germinating seeds or seedlings of certain plants are exposed to
continuous low temperature for a few weeks, they may bloom sooner
than those kept warm at all times. In the eastern United States certain
varieties of apple and pear bloom abundantly in alternate years. When
facts such as those listed above are considered, one may feel that casual
explanations of why plants bloom when the>^ do may be inadequate or
even erroneous.

All seed plants have a period of youth during which the growth of
roots, stems, and leaves precedes the fonnation of flowers and fruits. The
length of this purely vegetative period may vary from a few weeks to
many years, depending upon the species of plant and the habitat in
which it is growing.

The occurrence of flower primordia in buds is the first visible evidence

333

334 TEXTBOOK OF BOTANY

of flower formation. Their first appearance can be detected only by
examining thin sections of buds with a microscope. During the winter
many of the buds of trees and shrubs contain rudimentary flowers that
may be recognized without a microscope (Chapter XXVI). Evidently
the initiation of these dormant rudimentary flowers occurred sometime
during the previous growing season.

In the latitude of Ohio the initiation of flower buds on trees of apple,
peach, plum, and cherry usually occurs sometime during the last two
months of summer. The initiation of flower primordia in many shrubs,
such as currants, gooseberries, and cranberries, also occurs during these
months. In the early spring-flowering plants that grow from the terminal
bud in bulbs, the differentiation of flower primordia occurred sometime
during the preceding growing season.

The opening of flower buds, the relatively rapid enlargement of the
different parts of flowers, and the subsequent development of fruits the
following spring are familiar to all. In annuals, biennials, and many
herbaceous perennials, the initiation of flower primordia and the subse-
quent development of flowers and fruits occur during the same season.

Since the fruit and the parts of a flower are composed of cells, one
may reasonably infer that the processes of growth in them are similar to
those in leaves, stems, and roots. Here again the formation, enlargement,
and differentiation of cells are influenced by food, water, hormones, and
all the other factors that affect the growth of cells. As in leaves, an absciss
layer may develop at the base of each of these floral structures, and they
may abscise and fall off at maturity or in some earlier stage of develop-
ment. During certain years this abscission may be so excessive that the
ground beneath a tree is covered with a laver of abscised flowers and
young fruits. Biloxi soybeans which grow where they are exposed to
light 14 to 15 hours each day may bloom, but no fruits develop. The
initiation of flower primordia, therefore, may occur when conditions are
unfavorable to the further development of both flowers and fruits. It is
because of this fact that attention has been called to each of these three
processes: initiation of flower primordia, flowering, and fruiting. In the
cultivation of plants one strives to maintain a combination of conditions
that is favorable to all three of these processes.

The formation of seeds in fruits is dependent upon the formation of
pollen and gametes and is the result of a long series of processes that
will be considered in Chapter XXXIII. These, too, are influenced by ex-
ternal conditions.

[Chap. XXXI INITIATION OF FLOWERS 335

In tliis chapter the initiation of flower primordia will be given most
attention. The further development of these primordia to full-grown
flower structures and fruits will be mentioned occasionally.

We shall have to turn our attention once more to the growing stem
tip, for it is from this meristem that all flower primordia develop. They
may develop from the stem tip in the terminal buds of main stems or of
lateral branches, as in roses, zinnias, dahlias, petunias, and asters. In
these plants there is a change from a vegetative bud to a flower bud.

Flower primordia may also develop from the unelongated stem tips of
axillary buds, as in rose of Sharon, hibiscus, hollyhock, bindweed, mul-
lein, and coleus. In these latter plants the fully developed flowers are
later seen along the sides of the stem in the axils of leaves. The leaves
subtending the axillary flowers may be large, as in hibiscus, or they may
be small green bracts, as in coleus. The stem tips from which these flowers
developed never bore leaves.

Observable differentiation of flower primordia. In the longitudinal sec-
tion of a vegetative bud (Fig. 32) the apical meristem and the primordia
of the lateral foliage leaves and of axillary buds are evident. When such
a bud changes to a flower bud, no more primordia of foliage leaves and
axillary buds are formed; but a number of small mounds or ridges of
cells develop from the apical meristem in regular arrangement. In the
simplest cases the lowest and outermost mounds of these cells are the
primordia of the sepals of the flower, and they are usually the first to
appear (Fig. 140). Just above them are the primordia of the petals of
the flower, then the primordia of stamens; and finally the center of the
meristem becomes the primordium of the pistil. The order in which
these different primordia become visible is not the same in all species of
plants. This short stem tip, with the several mounds of meristematic
cells, is the beginning of a flower.

Physiological differentiation precedes the occurrence of flower pri-
mordia. Back of all visible changes in development there are, of course,
changes in phvsiological processes and conditions. What we see are
merely the consequences of the accumulated physiological conditions.
What are these conditions?

Among the conditions postulated to be important in flower formation,
two — namely, specific hormones called "florigens," and the relative
amounts of carbohydrates and proteins in the plants — have been given
the most attention in recent years. While an unqualified specific answer

336

TEXTBOOK OF BOTANY

microsporocytes

stamea
carpel

ovule containing
me^aspore mother cell

K

Fig. 140. Frimordia of the parts of a flower and the forerunners of fruit and
seeds of pepper. A, the vegetative stem tip previous to the development of flower
primordia; B-G, early stages in the development of flower primordia; H-K, stages
in the development of fruit and seeds. A-J, after H. L. Cochran, Journal of Agri-
cultural Research. K, drawn from a photograph by H. P. Stuckey and J. A.
McClintock.

[Chap. XXXI Initiation of flowers 337

to the above question is impossible today, some of the facts relating to it
may be of more interest than the answer.

The leaves of begonia may be used as a means of vegetative multipli-
cation; that is, if the base of a leaf which has been broken from a plant
is placed in moist soil, a complete individual plant will develop from
it. If the plant from which the leaf was taken is about to bloom, the new
individual that grows from the isolated leaf will grow but little and
bloom within a short time. On the other hand, if the original plant is
vigorously vegetative, the new individual from the isolated leaf will
grow much larger and for a long period of time before flowers develop
on it. From such experiments as these a German botanist (Julius von
Sachs ) suggested in 1893 that flower formation may be dependent upon
specific chemical substances made in the leaves. At that time, and for
several years later, the idea that a small amount of some chemical sub-
stance formed in the plant could have such a profound influence on its
development attracted little serious consideration.

A few years later, another German botanist (Georg Klebs) began a
series of experiments in which he was able either to keep plants in a
vegetative state for many years or to bring about flower formation and
sexual reproduction in a relatively short time. He thought that all his
experimental data could be explained on the basis of the influence of
light, inorganic salts, and other external factors upon the nutrition of
the plants, especially upon the synthesis and accumulation of carbohy-
drates in relation to other products. He concluded that the initiation of
flower primordia is dependent upon a previous accumulation of physio-
logical conditions.

In 1918, two American botanists (E. J. Kraus and H. R. Kraybill) de-
scribed their experiments with tomato plants in which the number of
flowers formed, flower development, and subsequent fruit development
varied with external conditions that influence photosvnthesis and protein
synthesis in plants. From the discussions in previous chapters (XIII-
XVI ) we are aware of the ways in which certain external factors could
be changed and thereby alter the relative rates of photosynthesis and
protein synthesis in plants. Following this report many investigators,
working with several kinds of plants, performed hundreds of experi-
ments in attempts to discover the relation of the relative amount of
carbohydrates and proteins to the development of all parts of plants.

These investigators showed that no organ of a plant grows well unless
it is well supplied with both carbohydrates and proteins, or with the con-

338 TEXTBOOK OF BOTANY

ditions by which it can make these two kinds of foods. Flowers and
fruits will grow no better than leaves, stems, or roots without these
foods. Nor will they grow well when conditions are favorable to the
synthesis of onlv one of them. Many facts of both practical and scientific
interest were discovered. Some of them have been included in previous
chapters.

Most of these investigators think that these foods are not a direct
cause of the initiation of flower primordia. When cultivated plants in
gardens, orchards, and fields are exposed to conditions favorable to both
photosynthesis and protein synthesis, they usually grow well and bear
an abundance of flowers and fruits. It is possible, however, that other
processes essential to the initiation of flower primordia also occur best
under these same conditions, and that the foods are important because
they are a necessary source of material and energy in the growing cells
of the primordia and of the expanding flowers and fruits.

Following the definite proof of hormones in plants about 1928, several
botanists became interested in testing Sachs' idea that the initiation of
flowers is dependent upon specific hormones made in the leaves. From
the results of experiments performed within the last five years it appears
reasonably certain that hormone-like substances are made in the leaves
which promote the initiation of flower primordia, and that probably
others inhibit it. The evidence back of this conclusion and the differences
discovered in the plants investigated cannot be presented in this short
chapter, but a few facts selected from two of the reports are too inter-
esting to be omitted.

No initiation of flower primordia occurs in plants of cocklebur (Xan-
thium pennsijlvanicum ) growing in an ordinary greenhouse if the plants
are exposed to continuous hght for 16 hours each day. Under these con-
ditions they remain vegetative indefinitely. If the daily light period is
shortened to 9 hours, microscopic flower primordia are evident within 5
days, and the plants are in bloom in about 2 weeks. If only one branch
bearing mature leaves is exposed to a daily light period of 9 hours, and
another branch of the same plant is exposed to the 16-hour daily light
period, flowers develop on both branches. Finally, if only one mature
leaf on the plant in the 16-hour day is exposed to a short day of 9 hours,
flowers develop on all branches of the plant.

Whatever it is that passes from the leaves exposed to the short days
and promotes the initiation of flower primordia on the part of the plant
exposed to the 16-hour day, it passes both up and down the stems. It

[Chap. XXXI INITIATION OF FLOWERS 339

also diffuses across a graft between two branches of the same or dif-
ferent plants, even if direct contact in the graft is prevented by the inser-
tion of lens paper.

Approach grafting was employed in these experiments; that is, por-
tions of the sides of two stems were shaved to the cambium, and the two
shaved surfaces were bound together without removing either stem from
the plant. The leaves above the graft on one stem were then exposed to
a 9-hour daily light period. Those on the other stem were exposed to the
16-hour dav. Flowers developed on both branches.

As in the cocklebur the initiation of flowers in soybean plants also
occurs only during relativelv short davs, but there is one striking differ-
ence. If the leaves on one part of a plant are exposed to short days and
those on the remainder of the plant to long days, flowers develop only
on the branches exposed to short days.

This localized effect has also been reported as occurring in other
plants, such as cosmos, chrysanthemum, tobacco, and poinsettia. But it
has been shown that in soybeans if the leaves are removed from the
branches exposed to long days when the experiment is started, flowers
develop on these branches also. If the leaves are removed from the
branches exposed to short days, no flowers develop. From these facts
one might infer that there are at least two substances that influence the
initiation of flowers in soybeans: one made in leaves exposed to short
days which induces the initiation of flowers, and the other one made in
leaves exposed to long days that inhibits it.

Environmental factors. By this time it is probably evident that en-
vironmental factors, through their effects upon processes and conditions
within the plant, may affect the initiation of flower primordia. Of these
factors light and temperature are often the most important and will be
discussed further.

Light. When light is mentioned as a factor, one should know whether
intensity, quality, or daily duration is being considered. From investi-
gations made to date it appears that in the initiation of flower primordia,
the so-called red rays of light are more effective than the blue, and
green is least effective of all.

Many plants have been known to grow and die in light of low inten-
sity without bearing flowers and fruits. Some of them may live in this
purely vegetative state for many years. Whether the failure of flower
formation in deep shade is the result of deficient photosynthesis, hor-
mone synthesis, or of some other internal condition is unknown. The in-

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TEXTBOOK OF BOTANY

tensity of light, through its influence upon photosynthesis, is a very
important factor in the growth of flowers and fruits after they are
initiated.

The daily duration of light to which plants mav be exposed is often the
most potent of all external conditions in the initiation of flower primordia.

Fig. 141. The efiect of long and short days on the evening primrose. Both
plants were brought into the greenhouse in November. The one at the left was
exposed to natural winter daylight and also to illumination from an electric light
from sunset to midnight for about two months. The one at the right was kept
under the same conditions, except that it was exposed only to the natural winter
daylight. This is a typical long-day plant, in nature flowering when the days are
long. W. W. Garner and H. A. Allard, U.S.D.A.

This fact was first clearly recognized in 1920 by two American botanists
(W. W. Gamer and H. A. Allard), of the U. S. Department of Agricul-
ture (Figs. 141 and 142). They suggested the term photoperiod for the
daily light period, or for any other period of continuous light to which
the plants were exposed; and the term photoperiodism to refer to the
recognized effects of the light period on the development of the plant.
The photoperiod. The daily light period to which plants may be ex-

[Chap. XXXI

INITIATION OF FLOWERS

341

^'■^

y

Fig. 142. Effects of length of day on tobacco plant. Both plants grew in a green-
house during the winter. The plant at the right was exposed to daylight and also
to electric light from sunset to midnight, while the plant at the left was exposed to
natural daylight only. This is a typical short-day plant. When exposed to long
days, this variety will grow 15 feet or more in height and produce upward of 100
leaves. W. W. Garner and H. A. Allard, U.S.D.A.

posed influences not only the initiation of flowers, but also physiological
processes in plants that affect the growth of all plant organs. Here only a
few of its relations to the initiation of flower primordia will be discussed.

Since the species and varieties of plants are inherently different, they
are not all affected in the same way when exposed to the same photo-
period. Garner and Allard recognized three different groups of plants
with reference to length of day and flower formation.

First, there are the long-day plants, which bloom only when the days
are relatively long, usually more than 12-14 hours. The shortest daily
photoperiod under which one of these species will bloom is called its
critical photoperiod. When exposed to continuous light many of these
species grow and reproduce; others fail to survive. These are the plants
that regularly bloom in the long days of summer. During the short days
of the growing season they remain vegetative, often as rosettes. They
will also bloom in greenhouses in winter if the daily photoperiod is
lengthened by means of electric lights. This supplemental electric light

342

TEXTBOOK OF BOTANY

need not be very intense. The main thing is that the daily photoperiod be
lengthened.

If the intensity of this supplemental light is between 1/500 and 1/2000
that of a bright summer day, it is sufficient for the initiation of flowers
in most of the plants that have been tested. Some species bloom when
the intensity of the supplemental light is only 2 to 5 times that of bright
moonlight. The longer rays of light are more effective than the shorter
ones.

Second, there are the plants that bloom only when the days are rela-
tively short, usually under 12-14 hours. The longest daily light period
under which one of these species will bloom is called its critical photo-
period. These are the plants that bloom in early spring and autumn.
That is, the initiation of flowers in them occurs during short days. The
young flowers may grow to maturity during the season they are initiated,
or thev may remain dormant until the following spring, as they usually
do in trees and shrubs.

Fig. 143. Effects of length of day on vegetative growth and Hower formation
in morning glory. C, control plant exposed to long days from time of germination.
Figures indicate the number of short-day periods to which the other plants were
exposed during the seedling stage. All plants completed their development in long
days. Photo by Victor A. Greulach.

[Chap. XXXI INITIATION OF FLOWERS 343

Herbaceous short-day plants that have been tested bloom during the
long days of summer if for a few successive davs thev are placed in a
dark room from about the middle of the afternoon until the next morning
(Fig. 143). The number of these short-day exposures that are eflFective
depends upon the species of plant and also upon the temperature. One
or two exposures are sufficient to initiate flower primordia in some plants.
Short days are necessary for the further growth of the flower primordia
after they are initiated in some species, whereas in other species the
flowers may continue to develop in either long or short photoperiods.

These short-dav plants will bloom even when exposed to the long
days of summer if the intervening dark period is made longer, for exam-
ple, from the evening of one day to the morning of the second day there-
after. The dark period must be continuous. If it is interrupted, even bv
very short light periods, flower primordia do not form. Such facts as these
have led some investigators to suggest that the short-day plants should
be called long-night plants. An idea of the importance of the long dark
period may be obtained by comparing the behavior of the plants under
difl^erent photoperiods as summarized in Table 12.

Table 12. Initiation of Flowers in Cocklebur in Relation to the Length of
Alternating Periods of Light and Darkness

Length of

Light Period

in Hours

Length of

Dark Period

in Hours

Condition

of Plant

After 15 Days

8

4

16

16

16
8
8

3'2

In bloom
Vegetative only
Vegetative only
In bloom

If short-day plants are exposed only to long daily light periods, they
remain vegetative for an indefinite time and grow much larger than
when thev are exposed to short days. Advantage is sometimes taken
of this fact. After the plants have grown to some desired size in long
days, they may be transferred to short days. In this way larger plants
with a greater number of flowers and fruits may be obtained. If the
seeds germinate in spring the plants are exposed to long days in earlv

344

TEXTBOOK OF BOTANY

summer and to short days in late summer and autumn without being
transferred. What is the explanation of the differences in the bloom-
ing of the cocklebur plants mentioned in the third paragraph of this
chapter?

Finally, there is a third group of plants, in which the initiation of
flowers occurs during either long or short photoperiods, although it may
occur more readily in one type of photoperiod than in the other. These
plants are called day-neutral plants. They may bloom any time during

Fig. 144. Photographs of a cabbage plant kept in a greenhouse at about 67° F.
At the end of two years plant (A) was more than 6 feet tall and had borne 4
compact heads of leaves at the points indicated by the figures 1, 2, 3, 4. At the
end^of 2.5 years it had borne 6 heads (B). When it was transferred to a cool
(55° F.) greenhouse, flower primordia developed and the plant bloomed within
a few months (B). Photo from H. C. Thompson.

INITIATION OF FLOWERS

345

[Chap. XXXI

the growing season. Hybrid roses, chickweed, dandelion in gardens and
lawns, and tomatoes and buckwheat in greenhouses are familiar examples
of day-neutral plants.

Temperature. Both the degree and the duration of any particular de-
gree of temperature are important factors in the development of plants.
Under certain conditions the initiation of flowers may be influenced more
bv temperature than by the length of day. Some species, such as beets,

Fig. 145. Effects of temperature on vegetative growth and the formation of
fiowers in celery. The four plants pictured on the left grew in a cool (60° F.)
greenhouse; at the same time those on the right grew in a warm (70° F.) green-
house. Photo from H. C. Thompson, Cornell University.

stocks, and celery, do not bloom when kept continuously at a relatively
high temperature in greenhouses. Cabbage, a biennial, may grow vege-
tatively as a perennial if kept continuously at 70° F. ( Figs. 144 and 145 ) .
Some plants of temperate climates when transported to the tropics
similarly remain vegetative. Low temperature delays the time of flower
formation in some species and shortens it in others. In many species the
initiation of flowers occurs sooner at a moderate temperature than at
either a low or a high temperature.

346

TEXTBOOK OF BOTANY

Much more striking, however, is the fact that the initiation of flowers
may occur much sooner in some plants if the germinating seeds or the
young seedhngs are exposed to a low temperature for a short time. For
example, wheat plants bloom sooner if the germinating seeds are ex-
posed to a low temperature. Celery and beets, which are regularly
biennials in the latitude of New York, grow as annuals if the seedlings

Fig. 146. Preconditioning effects of temperature on the development of flower
primordia in cabbage plants. The four plants pictured above grew in the field
during Summer. The two on the right were transferred directly from the field to a
warm greenhouse, while the two on the left were brought from the field and kept
at 40° F. for 2 months before they were placed in the warm greenhouse. Photo
from H. C. Thompson.

are exposed to a temperature of 40° F. for about one month. Some of the
effects of temperature at these early stages of development persist in
the plant as it continues to grow and finally result in an earlier initiation
of flowers (Figs. 146 and 147). This phenomenon may be referred to as
preconditioning. That is, the plants bloomed sooner because they were
preconditioned by a low temperature during some earlier stage of their
development. Likewise, a similar preconditioning occurs when short-day
plants are exposed to a long dark period. Our present knowledge of

[Chap. XXXI

INITIATION OF FLOWERS

34-;

Fig. 147. Preconditioning effects of temperature on the development of flower
primordia in celery plants. The plants pictured in the center row grew in a cold
frame (40-50° F.) for 35 days during their seedling stage before they were
transferred to the field. The plants in the next rows in the foreground were kept
in a warm greenhouse during their seedling stage. They have continued to grow
vegetatively, while those in the center row have borne flowers and seeds. Photo
from H. C. Thompson.

plant physiology is not sufficient to enable us to explain these precondi-
tioning effects either of temperature or of light.

Since the initiation of flowers may be either promoted or inhibited
by temperature or by the photoperiod, one of these external conditions
may either annul or accentuate the effects of the other. What happens
when the effects of the one are opposed to the effects of the other in a
particular plant? At the present time one may obtain an answer to
this question by only one of two ways. Someone may have already
discovered the answer and reported it. If the answer cannot be found
in botanical literature, the only way to obtain it is by a series of experi-
ments.

When a plant is exposed to a length of day near its critical photo-
period, it is more readily influenced one way or the other by changes in
temperature or some of the factors that affect photosynthesis and protein
synthesis. For example, barley is a long-day plant; when exposed to a
photoperiod near its critical day-length it will bloom or remain vegeta-
tive, depending upon the temperature and upon the factors that affect the
relative rates of photosynthesis and protein synthesis. What has just
been said may be made clearer by the following summary:

348

TEXTBOOK OF BOTANY

Photoperiod

i Intermediate Day
Long Day i (Length Near Critical
Photoperiod)

Short Day

Long-day plants that have been
tested by experiments

Usually bloom

\'ariable, dependent upon
several other factors

Usually remain
vegetative

Sliort-day plants that have been
tested by experiments

LIsually remain
vegetative

Variable, dependent upon
several other factors

Usually bloom

The greater variability in behavior of plants exposed to intermediate
day-lengths near their critical photoperiod includes not only the vari-
ability in the initiation of flowers, but also the variabilitv in the further
development of flowers and fruits and the processes involved in seed
formation. Changes in vegetative structures, such as growth in height

ism

Fig. 148.

Fig. 149.

Fig. 148, the plant on the right, an apple seedling that grew more rapidly with
10 hours' daily illumination than the control plant on the left with a full day's
illumination. In contrast, the maple seedlings {Acer negundo) on the left in Fig.
149 were dwarfed and forced into dormancy by shortening the illumination period
to 10 hours, while the plant exposed for the full length of day grew rapidly. The
photograph of the apples was made July 13 and that of the maples September 22
by W. W. Garner and H. A. Allard. Photo from U. S. Dept. Agric.

INITIATION OF FLOWERS

349

[Chap. XXXI

and the formation of tubers, are also more variable under these photo-
periods, because the influence of other environmental factors on them is
relatively more pronounced.

Plants may also be classified as long-day and short-day types with
respect to tuber and bulb formation, root growth, stem growth (Figs.
148 and 149 ) , deciduous and evergreen habit, kinds of flowers, and yield
of crop plants.

Tubers ordinarily develop at the ends of rhizomes of Jerusalem arti-
choke during the short days of autumn. They will also develop during

Fig. 150. Photographs of the roots, underground stems, and bases of the aerial
stems of Jerusalem artichokes (Helianthiis tuberosus) . The plants on the left were
exposed to the long days of summer, those in the center and on the right were
covered with a dark cloth from 4:30 to 9 a.m. each day, but only the tips of
those on the right were covered. The elongated rootstocks of the plants on the
left, and the thickened tubers of the others are evident. Photo by P. W. Zimmer-
man and A. E. Hitchcock, Boyce Thompson Institute.

midsummer if the photoperiod is artificially shortened each day by plac-
ing the plants in a dark room or by covering the leaves and stems above-
ground with a black cloth. Darkening of the stem tips alone is sufficient
to initiate tuber formation in artichokes ( Fig. 150 ) .

Girdling. In certain habitats young apple trees and other orchard trees
may remain vegetative for several years before they bear flowers and

350 TEXTBOOK OF BOTANY

fruits. What can one do that will result in the initiation of flowers in
such plants? It has been known for a long time that if a branch of one
of these trees is girdled (ringed), it will bear flowers and fruits sooner
than other branches on the tree. If the trunk of the tree is girdled, numer-
ous branches on the tree will bear flowers and fruits sooner. If one wishes
to avoid killing the tree in this way, the girdle should be very narrow,
about one-eighth of an inch. It should be so protected that no parasites
can enter the wound and that complete healing by the growth of a new
cambium over the wound can occur within a few months. Merelv cut-
ting through the bark by drawing a knife blade around the trunk of the
tree may be as effective in initiating flowers as making a wider girdle.
How may one explain the initiation of flowers that results from girdling?

REFERENCES

Berth wick, H. A., and M. W. Parker. Initiation of flowers in soybean. Bot.

Gaz. 100:374-387. 1938. Ibid. 651-689. 1939.
Garner, W. W. Recent work on photoperiodism. Bot. Rev. 3:259-275. 1937.
Gourley, J. H., and F. S. Hewlett. Ringing applied te the commercial orchard.

Ohio Agr. Exp. Sta. Bull. 410. 1927.
Hamner, K. C, and James Benner. Initiation of flowers in cocklebur. Bot.

Gaz. 100:388-431. 1938.
Loehwing, W. F. Initiation of flowers in soybean. Proc. Soc. Exp. Biol, and

Med. 37:631-634. 1938.
Thompson, H. C. Temperature in relation to vegetative and reproductive

development in plants. Proc. Amer. Soc. Hort. Sci. 37:672-679. 1939.
Numerous other publications are cited in the above publications.

CHAPTER XXXIl

FLOWERS, FRUITS, AND SEEDS

c><JKX><><x><><><^<^<^^^c><c^^

Flowers are the precursors of fruits and seeds, and within the seeds are
the embryos of a new generation of plants. The growth of the parts of a
flower from the primordia in a flower bud is but the first of a series of
processes and structures that ultimately result in sexual reproduction in
seed plants. As we proceed with further chapters some of the interrela-
tions of sexual reproduction and other plant phenomena will become
evident.

The rapid expansion of an opening flower is mainly the result of cell
enlargement by growth of cell walls accompanied by the osmotic absorp-
tion of water. Most of the cell division and a considerable amount of
cell differentiation, including the differentiation of some of the reproduc-
tive cells, occurred in the flower bud before it opened.

Interest in flowers, however, is not limited to sexual reproduction. The
widest general interest lies in the pleasure of seeing them on cultivated
plants and also on plants growing under natural conditions. The flowers
sold for decorative purposes are valued annually in millions of dollars.

Flowers are also sources of nectar and perfumes. But the direct eco-
nomic importance of flowers is small when compared with the value of
the fruits and seeds that develop from them. The accumulated food in
these organs is one of the most important commercial products of cul-
tivated plants.

The colors, shapes, and sizes of flowers; the forms and arrangement
of the parts of a flower; and the arrangement of flowers and fruits of a
plant are as definite as are the fonns and arrangements of leaves. Be-
cause of this inherent constancy of form and stmcture, the specific dif-
ferences in flowers and fruits, as well as in leaves and stems, are used in
classifying plants. Some appreciation of this use of flowers may be gained
first-hand by comparing the flowers of a few common plants such as lily,
bean, tomato, mint, morning glory, rose, carrot, and sunflower.

The flower. Numerous flowers are commonly recognized by their
form, color, texture, fragrance, or other distinguishing feature. In fact,

351

352

TEXTBOOK OF BOTANY

flowers are often regarded as something distinct from the rest of the
plant, as is shown by the very common advertisement, "Plants and
Flowers for Sale." Few people realize that about one-half of the species
of plants bear flowers.

The variety in floral organs is probably greater than that in other
organs of plants. Many flowers are conspicuous, but others are quite in-
conspicuous and appear as aggregates of scales and bracts, as in grasses,
alders, poplars, and birches. Technically, even a young pine cone may be
regarded as a type of flower. Such cones, however, lack both sepals and
petals. Flowers vary in size from the nearly microscopic flower of Wolffia
to the "fleshy" flower of Rafflesia, nearly three feet in diameter. Although
nearly all colors and almost all conceivable blends and mixtures of
colors may be found in flowers, the most common are green, white, and
yellow. In the flora of Ohio, for example, the percentages of different
flower colors are approximatelyly 36 green, 21 white, 20 yellow, 15 blue
to purple, and 7.5 red to pink. Locally and seasonally one or another of
these flower colors may predominate in the landscape.

The flower is a short stem bearing floral organs and its development
is similar in many respects to that of a branch from a vegetative bud.

Stigma"]

' Style I Pistil

OvularyJ

Sepal
Receptacle
Peduncle

Fig. 151. Vertical section through a complete flower (Flax). Courtesy of World

Book Co.

Mounds of meristematic tissue are formed by cell division at the growing
tip of a stem or floral axis. From them the parts of the flower develop as
lateral structures, in much the same way as ordinary leaves develop from
similar mounds of tissue ( Fig. 140 ) . There is, however, little or no elonga-
tion of the internodes between the floral organs; consequently flowers

[Chap. XXXII

FLOWERS, FRUITS, AND SEEDS

353

are usually compact structures, with the parts in close spirals or whorls
( cycles ) . In the more primitive types of flowers, floral organs are spirally
arranged; in the more complex types they are usually cyclic and also
fewer in number ( Fig. 151 ) . The best way to study flowers is first to
examine a few common ones critically and then to amplify these initial
studies by further observation of some of the hundreds of different kinds
available each season.

A simple flower. The flowers of tulip and sweet pea exemplifv two
common forms of simple flowers. The floral organs of the tulip (Fig.

Fig. 152. Flower, fruit, and seeds of tulip. A, vertical section through center
of flower; B, diagram of cyclic arrangement of the floral organs; C, enlarged
cross section of the pistil in which the three carpels and six of the ovules are
evident; D, cross section of a mature pistil in which the ovules have become
seeds; E, a mature fruit which has split open along the midrib of each carpel.

152) are arranged in cycles of three at the top of the flower stalk or
peduncle. This end of the peduncle is slightlv enlarged and is known as
the receptacle. The outermost cycle of leaf-like structures growing from
the receptacle is the calyx, composed of three sepals. In the enlarging
flower bud these sepals are green. They may remain green or become
variously colored as the flower matures, depending upon the varietv. Ex-
cept for position, the next cycle is very similar to the first; its three petals

354 TEXTBOOK OF BOTANY

constitute the corolla. The calyx and corolla together are often called the
floral envelope or perianth. The next two whorls of organs are less leaf-
like in appearance and each consists of three stamens. The stamens in
one cycle alternate with the sepals; those in the other cycle alternate
with the petals. Each stamen has a stalk or filament terminating in a
pollen-bearing anther. The pistil is centrally located in the flower and is
composed of three leaf-like parts, the carpels. The slightly thicker lower
portion of the pistil is the omilary,^ which tapers apically into a neck-like
structure, the style, at the summit of which is the three-lobed stigma.
Each lobe of the stigma is in line with the midrib of a carpel.

If the ovulary is cut or broken crosswise the ovules from which seeds
develop may be seen. If the carpels are pulled apart lengthwise, the six
vertical rows of ovules are easily seen. As the pistil enlarges and becomes
a fruit, the ovules become seeds.

The foregoing is a description of the usual tulip flower. Variations may
be found, some of which are inherent in the variety of tulip, while others
are the result of the conditions in which the bulbs are stored during their
dormant period.

The calyx of the pea flower (Fig. 153) like that of bean is green, and
the four or five sepals are united except at their tips which may appear
as large teeth or calyx lobes. The corolla is variously colored and is com-
posed of five petals, so unique in form that special terms have been ap-
plied to them. One, the "standard," is broad and encloses the others in
the bud; the two lateral petals are "wings"; and the remaining two are
more or less united ventrally, forming the "keel" and enclosing the
stamens and pistil. Of the ten stamens, one is free; but the filaments of
the other nine are united in a tube surrounding the pistil.

The pistil of the pea or bean consists of a single carpel but usually
contains several ovules attached to a longitudinal ridge at one side of
the ovulary. The region to which an ovule is attached is called a placenta.
The leaf-like nature of the carpel is clearer when a green pea pod is
opened along one side and spread out flat. If a cross section of the pod
is made, the ovules appear to be borne on the fused margins of a folded

leaf.

The stamens, petals, and sepals of such flowers as the tulip develop

1 The ovulary of the pistil is often referred to as an ovary. The term ovary is used to
designate a very different sort of structure in animals. Later on we shall see that the
origin of egg-bearing structures in plants is quite unlike that of the ovule-bearing struc-
tures. It seems preferable, therefore, to use the term ovulary to refer to the ovule-bearing
structure in plants.

[Chap. XXXII FLOWERS, FRUITS, AND SEEDS

355

Fig. 153. Flower, fruit, and seeds of the garden pea. A, the flower; B, the
five petals; C, Hower with petals removed so that the stamen tube which sur-
rounds the pistil may be seen; D, arrangement of ovules as seen in a longitudinal
section of the pistil; E, F, and G, stages in the development of the fruit from
the pistil and of the seeds from ovules; H, a young ovule enlarged; I, enlarged
cross section of a young fruit to show that it is composed of one carpel and that
the ovules are borne on the infolded margins of the carpel.

from separate primordia. The growing carpels, however, are united and
a compound pistil develops from them.

A pistil that develops from one carpel, as in beans and peas, is a
simple pistil. In the flowers of some plants, such as the buttercup, the
carpels do not unite, and manv simple pistils develop separately in the
same flower.

Still other plants have flowers in which the calyx and corolla are
tubular because the sepals and petals are united in a tube as they develop.

Such unions of the parts of a flower take place between flower pri-
mordia, and the tubular structure develops thereafter as a unit. The
calyx of the flower of peas, for example, begins as five separate sepal
primordia near the apex of the receptacle. Shortly, however, later growth
at the bases of these primordia results in a complete collar of tissue.
Continued growth of this collar and of the lobe of each sepal results in
a cup-shaped calyx tube with five lobes at its margin. The ten stamen

356

TEXTBOOK OF BOTANY

primordia likewise begin as separate mounds of tissue, but nine of them
soon become united at their bases, leaving the tenth stamen primordium
free. Fusions of floral parts are common and of great variety, as may be
seen if numerous flowers are examined.

In many flowers, such as those of apple, there is a floral cup composed
of the united bases of sepals, petals, and stamens. This floral cup may

calyx and stamens

tissue developed .
J from the floral cup
inner cartilaginous
tissue of- ttie o/ulary
outer soft tissue
of the ovulary

Fig. 154. Flower, fruit, and seeds of apple. A-D, vertical sections through
flowers of different ages. In C, which is about 2.5 natural size, all the parts of
the flower have been drawn. The tissues of the ovulary and the floral cup are
united. A and B are about 9 times natural size and were made before the flower
buds opened. C and D are about 2.5 natural size and were made soon after
the petals abscised. E, cross section of a young fruit in which the five carpels
and ten of the ovules are evident; F and G, cross and longitudinal sections of a
mature fruit, about one-half natural size.

partially or completely enclose the ovulary and become united with it.
The upper parts of the stamens and the lobes of the calyx and corolla
appear to be attached to the rim of the cup (Fig. 154). Furthermore,
while the ovulary develops as the core of the apple, the floral cup grows
and becomes the edible part of the fruit.

[Chap. XXXII

FLOWERS, FRUITS, AND SEEDS

357

The floral cup of the EngHsh walnut is still more complex. The pri-
mordia of bracts below the pistillate flower become united with those of
the floral organs, and by further development become the husk of the
mature fruit. The hard shell of the nut develops from the two-carpellate
ovulary; the "meat" within is an embryo surrounded by thin seed coats.

The floral cups of some flowers, such as those of the cultivated shrub
pearl bush, abscise when the flower is mature and do not form a part of
the fruits.

Flowers of grasses and sedges have nothing closely resembling a
calyx or a corolla; in them the stamens and pistils are enclosed by small

Fig. 155. Spike and flower of wheat (Triticum vulgare) . The two floral bracts,
or glumes (lemma on the right and palet on the left), enclose three stamens and
a pistil with two plumose stigmas. At the base of the flower inside the glumes
are two minute scales, the lodicules, which are not evident in the above figure.
Courtesy of World Book Co.

scales or bracts, which are usually green and are often referred to as
glumes (Fig. 155).

Flowers composed of calyx, corolla, stamens, and pistils are referred
to as complete. They are said to be perfect if they have both stamens and
pistils, regardless of the presence or absence of calyx or corolla, because
stamens and pistils are the essential parts of flowers. Not infrequently

I

Fig. 156. Sumatra's giant krubi (Amorphophalhis titanum) in bloom. This
plant has the largest flower cluster of the Arum family. The spadix shown
above was 8.5 feet tall and the circumference of the spathe was 12 feet 10 inches.
The corm B from which it grew weighed 113 pounds. In C the staminate flowers
(above) and the carpellate flowers (below) may be seen through the opening
cut in the spathe. D shows a leaf of a similar but smaller plant. Further details
have been published by W. H. Camp, Joitr. N. Y. Bot. Gard. 38:177. 1937. Photos
from New York Botanical Garden.

358

[Chap. XXXIl

FLOWERS, FRUITS, AND SEEDS

359

flowers have stamens but no pistils, and other flowers on the same plant
or in the same inflorescence have pistils but no stamens. These flowers
are termed staminate and pistillate, respectively. In the inflorescence of
jack-in-the-pulpit, Amorphophallus (Fig. 156), and other aroids stami-

FiG. 157. Flowers of the corn plant. The staminate flowers are borne in a
large open panicle (tassel). The pistillate flowers are arranged in a spike (ear),
enclosed by sheathing leaves. The onlv part of a pistillate flower exposed to the
outer air is the end of the long style (silk). Courtesy of \\'orld Book Co.

nate flowers occur on the upper part and pistillate flowers on the lower
part of the same floral axis. Maize, or Indian corn, has staminate flowers
on "tassels" and pistillate flowers on "ears" ( Fig. 157 ) , but the environ-
ment may greatlv affect this arrangement.

Many trees of Christmas holly, willow, and poplar have only staminate

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flowers; others have only pistillate flowers. Hemp plants likewise usually
bear only staminate or only pistillate flowers. Such plants are said to be
dioecious (Greek "two households"), in contrast to monoecious (Greek
"one household") plants on which both staminate and pistillate flowers,
or perfect flowers, occur. Some plants are monoecious in certain environ-
ments and dioecious in others.

Flower clusters. The flowers of some plants, such as those of tulip and
trillium, grow singly at the ends of solitary unbranched peduncles. But

Fig. 158. Diagrams illustrating terms applied to flower clusters: A, spike; B,
spadix; C, catkin; D, raceme; E, panicle; F, head; G, head with disk flowers and
ray flowers; H, corymb; I, umbel; J, compound umbel. See also illustrations in
Chapter LI.

in many species of plants the peduncle branches; and each of its ulti-
mate branches, the pedicels, terminates in a single flower. The flowers
are thus grouped in various types of clusters called inflorescences. (Fig.
158). Flowers mav also be clustered along the sides of the main floral
axis, or on numerous small lateral branches at the upper part of a plant.
The variety of flower clusters is so great that a special terminology is
necessary to describe them. The spike exemplified by the common plan-

[Chap. XXXII FLOWERS, FRUITS, AND SEEDS 361

tain, wheat, and timothy has numerous flowers which are sessile, or
nearly so, on an erect and elongate axis. If the axis is fleshy, as in calla
lily and jack-in-the-pulpit, it is called a spadix. A catkin is a spike-like
cluster of flowers, but drooping and scaly, as in willow, cottonwood, oak,
hickory, and birch. When the pedicels of the individual flowers are
longer, as in the inflorescence of yellow toad-flax, pepper-grass, and
snapdragon, the arrangement is designated a raceme. Rounded or flat-
topped clusters of milkweed and carrot are umbels, while those of the
hawthorn are corymbs. The pedicels of umbels all originate in the apex
of a stem, whereas those of corymbs are axillary outgrowths along the
sides of the floral axis. If the peduncle, or floral axis, is repeatedly
branched, as in the inflorescence of yucca and many grasses, the cluster
is called a panicle. The flowers of red clover, dandelion, and sunflower
are sessile and grouped in a compact head. The head of flowers of
dahlia, daisy, cosmos, and sunflower is often mistaken for a simple flower
because the ray flowers have the appearance of petals and beneath them
is an involucre of green bracts that resembles a calyx. There are other
types of inflorescences, but the foregoing are the common ones, and usu-
ally they are not difficult to recognize.

There are, however, many intergradations among the classified types
of flower clusters. Sometimes it is a matter of choice whether a particular
spike is short enough to be called a head, flesh v enough to be called a
spadix, or scaly and pendant enough to be called a catkin. In spikes,
racemes, and panicles the inflorescence continues to elongate at the tip
during the period of flowering, and it may even revert to a vegetative
condition as it ordinarily does in pineapple. Elongation usuallv does not
occur in heads, umbels, or in certain other types of inflorescences in
which the central terminal flower develops first.

Flowers as forerunners of fruits. One has but to compare the pistil with
the mature fruit of such plants as tulip or sweet pea to see many points
of similarity. If successively younger fruits are examined, it soon becomes
evident that the fruits of these plants develop solely from the pistil or
from the ovulary. Such simple fruits, therefore, are merelv the result of
further growth of the carpels after pollination has occurred. If the stigma
and style wither and die, only the basal part of the carpels, the ovulary,
becomes the fruit. The enlarged and mature ovulary may be thin and
membranous, hard and thick, or soft and juicy. It encloses the seeds and
is often referred to as a pericarp.

The simpler fruits of all flowering plants consist only of enlarged

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Fig. 159. Types of flowers in relation to fruits. A, flower of cranesbill {Gera-
nium) in which the floral organs develop separately on the receptacle and the
base of the pistil is above the bases of the stamens, petals, and sepals (hypogy-
nous). The pistil alone becomes the fruit. B, flowers and fruits of pearl bush
(Exachorda) in which the bases of the stamens, petals, and sepals develop as
a floral cup around the base of the pistil (perigynous) . The floral cup abscises
and the pistil alone becomes the fruit. C, vertical section of a flower of pearl
bush; D, vertical section of a perigynous flower of peach; E, vertical section of a
flower of pear in which the floral cup surrounds and is united with the ovulary
(epigynous), and becomes a part of the fruit.

[Chap. XXXII

FLOWERS, FRUITS, AND SEEDS

363

carpels, or pericarp. The more complex fruits of flowering plants are
composed in part of enlarged carpels and in part of other structures
which develop from the receptacle, floral cup, or other parts adjacent to
the flower. The edible portion of an apple and the rind of a banana, for
example, develop from the floral cup and surround the ovulary. In the
flowers of many plants, such as apple, rose, banana, gooseberry, blue-

FiG. 160. Types of fruits: A, legume (pea); B, drupe or stone fruit (peach);
C, berry (tomato); D, akene (buckwheat); E, pome (apple); F-G, several rip-
ened pistils on the receptacle of one flower, aggregate fruits (blackberry and
strawberry); H, several pistils enclosed in an enlarged urn-shaped floral cup
(rose); I, several pistils embedded in the enlarged apex of the floral axis (water
lotus); J, an enlarged stem tip with a central cavity containing many small flowers
(fig); K, a single flower of mulberry with thickened sepals and also the whole
flower cluster ripened as one compact fruit. J and K are types of multiple fruits.

berry, cucumber, and sunflower, the floral cup partly or wholly encloses
the ovularies of the pistil and becomes a part of the fruit (Fig. 159).
The cups of acorns and the husks enclosing hickory nuts apparently
develop from involucral bracts that grow below the flower. We may
therefore consider a fruit as a plant organ that develops from one or
more carpels, together with any other closely associated flower part that
may enlarge and ripen with the carpels ( Fig. 160 ) .

To many persons the term "fruit" refers only to certain edible struc-
tures, such as apples, pears, plums, and apricots, that grow on trees.
In certain practices the fruits of many herbaceous plants, such as tomato,
string bean, okra, pepper, cucumber, and squash, are popularly called

364

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"vegetables." Vegetable salads may even contain the seeds of beans and
peas. The term "vegetable" is not a technical term. Sometimes it is used
synonymously with the term "plant," as in the expressions "vegetable
kingdom" and "vegetarian." We shall soon see that in everyday language
the fruits of many plants are not ordinarily distinguished from seeds.

Types of fruits. The variety of fruits appears to be as great as that of
the flower from which they develop. In the course of a year everyone
eats or otherwise encounters many types of fruits, and it may be of
interest to be able to recognize the principal structures of the more
familiar ones. Botanists and horticulturists often try to distinguish two
general types of fruits: the shnple fruits, which develop solely from an
ovulary or pistil, and complex fruits, which develop from ovularies and
a variety of adjacent structures. Fine distinctions are not always made.
A tomato, for example, is usually regarded as a simple fruit, although, as
in the fruit of pepper (Fig. 140), the placenta probably originates mainly
from the apex of the receptacle.

Several types of simple fruits are easily recognized. If young bean
pods and young peach fruits are cut crosswise, the cut surfaces of

Fig. 161. Cross section of three young simple fruits each composed of a single
carpel. The ovules develop from the infolded margins of the carpel, best seen
in C, which is a cross section of a very young pistil of a plum flower. A, cross
section of a young bean pod in which one ovule is evident; B, cross section of a
young peach with two ovules; D, cross section of a plum in which one of the
two ovules has failed to become a seed. The cells in the innermost tissue of the
carpels of peach and plum ultimately become thick-walled and form the hard
"pit" or "stone" of the fruit.

these two types of fruits are seen to have certain features in common
(Fig. 161). Each consists of a single carpel with united margins. The
ovule or young seed visible in each section is attached by a short stalk
to one of the enfolded margins of the carpel. The bean pod may con-

FLOWERS, FRUITS, AND SEEDS

365

[Chap. XXXII

tain from one to many ovules or seeds, but usually only one of the two
ovules in a fruit of peach becomes a seed. Another significant similarity
in these two fruits is the two obvious layers of tissue in tlie carpel. The
inner of these two layers in the peach ripens dry and stony, whereas
the outer layer ripens soft and juicy. In the bean pod both of these
layers ripen dry and hard. The fruit of grape is a third type of simple
fruit in which the whole ovulary ripens soft and juicy.

The peach is an example of stone fruits, or drupes. Other examples of
stone fruits are plum, cherry, olive, apricot, mango, almond, and the
fruits of hackberry, poison ivy, basswood, and American holly. The
suture formed by the fusion of the margins of the single carpel of these
drupes is usually evident as a slight groove on one side of the fruit. The
almond differs from the other fruits in the list in that the entire fruit
becomes dry at maturity. The coconut likewise resembles a stone fruit,
but it consists of three carpels, both layers of which ripen dry and hard.

The grape exemplifies the berry type of fruit. Other examples are
persimmon, date, avocado, nightshade, lemon, orange, grapefruit, and
tomato.

Simple dry fruits may be one-seeded and indehiscent, as the akene of
buckwheat, and the grain, or caryopsis, of wheat, corn, and other grasses.
They are said to be indehiscent because they do not split open when
mature. In all grains the ovulary wall is united with the coats of the single

Fig. 162. Several types of simple fruits: A, maple; B, elm; C, ash; D, basswood;
E, dandelion; F, sunflower; G, clematis.

seed. The shelled nuts of hickory, pecan, walnut, oak (acorn), and chest-
nut as they appear on the market are similar to akenes. The individual
fruits of sunflower, dandelion, and other plants of the composite family
are also called akenes, but they are not simple fruits, for they consist of
an ovulary united with a floral cup (Fig. 162). The winged fruits of
maple, ash, and elm are similar to akenes.

366 TEXTBOOK OF BOTANY

A large number of simple dry fruits are many-seeded and dehiscent.
The legumes of the pea and the bean split along two sutures: the fol-
licles of milkweed and larkspur split along one suture; the capsules of
tulip, cotton, and okra consist of more than one carpel and split along
several sutures. The ptjxis of plantain, purslane, and pigweed splits
crosswise.

Representative examples of some of the more familiar complex fruits
are the apple and the cucumber, the raspberry and the strawberry, and
the mulberry and the pineapple.

The fruits of apple, pear, quince, hawthorn are called pomes. The core
resembles a drupe in that the cartilaginous part is the inner layer of the
ovulary and is surrounded by a soft juicy layer that develops from the
outer tissue of the ovulary. Most of the edible part, however, develops
from the floral cup.

The fruit of the cucumber likewise develops from the ovulary and
adherent floral cup, but does not have the inner cartilaginous tissue.
Other examples are gooseberry, currant, cranberry, blueberry, banana,
pomegranate, guava, coffee, gourd, pumpkin, squash, muskmelon, and
watermelon.

In each pistillate flower of a strawberry are many separate pistils on
an enlarged receptacle. As the receptacle enlarges and becomes the
major part of the fruit, the pistils ripen as separate akenes and are partly
embedded in the receptacle. Because of this assemblage of pistils on
one receptacle the strawberry is called an aggregate fruit. Other examples
are blackberrv and raspberrv, in each of which the separate pistils ripen
as small drupes, or drupelets. When a raspberry is ripe the closely
adhering aggregate of drupelets separates from the receptacle. In the
fruit of wild roses the group of ripened pistils is enclosed by a floral cup.

When fruits from several flowers are united in a compact mass, the
entire assemblage is a multiple fruit. The formation of multiple fruits
varies with the kind of plant. In the fruit of the fig the enlarged fleshy
receptacle is hollow, and on its inner surface there is a compact layer of
simple fruits that de\'eloped from the enclosed flowers. The flowers of
the pistillate catkin of the mulberry surround the floral axis in a compact
mass as the sepals become thickened and succulent. The multiple fruit
of pineapple also develops from a spike-like cluster of flowers in which
the floral axis, flowers, and subtending bracts all become enlarged and
united in a single mass (Fig. 163).

[Chap. XXXII

FLOWERS, FRUITS, AND SEEDS

367

Fig. 163. Pineapples in Hawaii. The entire flower cluster ot each plant becomes a
compact multiple fruit. Photo from U. S. Forest Service.

The seed. We have already seen that the ovulary contains ovules,
which ultimately become seeds. Seeds are unlike in size, foim, composi-
tion, and in several other wavs. The seeds of some orchids are almost
microscopic in size, while those of a few tropical legumes and of certain
nut-bearing trees mav be several inches in diameter. From a study of a
few kinds of seeds one mav learn to recognize their essential parts.

The seed coats of the castor bean, for example, consist of a hard outer
layer and a thin paper v inner membrane. The seed coats enclose the
endosperm and the embryo. The embryo is the young plant of another
generation of castor bean. It consists of a very short stalk {hypocotyl)
and two thin, colorless leaves, or cotijledons. Between the bases of the
two cotvledons is a bud — the plumule — from which the stems and
leaves of the seedling de\'elop. No root is visible in the embryo; but
when the seed germinates, the basal tip of the hypocotyl elongates, and
from its root primordium a primary root develops. The seedling that
develops from the embryo of a germinating seed is at first dependent
upon the food which accumulated in the cells of the embryo and endo-
sperm while thev were developing in the seed.

The seed of the kidney bean differs from that of the castor bean in the
absence of an endosperm. It consists only of an embryo enclosed by

368 TEXTBOOK OF BOTANY

seed coats. The embryo consists of a hypocotyl, a plumule, and two
large, greatly thickened cotyledons. All the accumulated food in a
bean seed is in the embryo. The seeds of many other plants, such as
species of legumes, mustards, hickory nuts, and pumpkins, also lack
endosperms.

The grain of corn, like that of wheat, consists of a grain coat, a large
endosperm, and an embryo, which is near the inner end of the seed.
There is a single large cotyledon (scutellum) more or less enclosing
the hypocotyl and the plumule (Fig. 113). Because of the fact that the
corn grain develops from both the ovule and the ovularv wall, it is really
a fruit. The grain coat is composed of seed coats plus the adhering
ovulary wall, or pericarp.

Monocots and dicots. From our early study of the vegetative organs
of a plant we learned that monocots and dicots may be recognized partly
bv the venation of the leaves and the arrangement of the vascular bun-
dles in the stem. The Hower parts of monocots are often in 3's or 6's,
while those of dicots are often in 4's or 5's or in some multiple of these
numbers. The terms refer, however, to the number of cotvledons" in the
embryo of the seed. The seeds of bean, squash, apple, and ash, along
with many others, have two cotyledons, and these plants are called
dicots; the seeds of wheat, corn, rye, and other grasses contain but one
cotyledon, and such plants are monocots.

Gymnosperms and angiosperms. Conifers, such as pines and spruces,
bear seeds on the upper sides of overlapping scales arranged in a cone.
The seeds are not enclosed by the scales but are exposed on their flat
open surfaces. Plants bearing seeds in this manner are termed gymno-
sperms ( "gymnosperm" means "naked seed"). Most seed plants have
carpels united in the form of a distinct pistil. The seeds enclosed in the
ovulary of the pistil are covered, or "hidden"; hence the name, angio-
sperms.

Dissemination of seeds. When seeds are mature they may become
separated from the fruit or the parent plant in several ways. The seeds
mav be expelled from fruits such as legumes and capsules when the
ovulary walls dry out, twist, or curl and shrink. The one-seeded fruits,
such as the akene and the grain, become free b\' abscission from the
receptacle. The seeds in many fruits, such as those in pomes, nuts,
drupes, and berries, are set free only by the decay of the fruit.

2 "Monocots" and "dicots" are abbre\iations now in rather general use for the longer
terms "monocotyledons" and "dicotyledons."

FLOWERS, FRUITS, AND SEEDS

369

[Chap. XXXII

Seeds may be scattered by wind, by water, by birds and other animals,
and by means of temporary lodgment on moving vehicles, such as auto-
mobiles, airplanes, trains, and ships. Perhaps the most important natural
agent in the dissemination of seeds is the wind. Many seeds are light in
weight, have relatively large surface areas, and may be carried many
miles by air currents. Larger and heavier seeds may be washed great
distances downstream, particularly during time of flood. Animals scatter
seeds in several ways: (1) the seeds may be eaten and survive the

Fig. 164. Fruits frequently transported by animals: A, beggar-ticks (Bidens);
B, Spanish needles (Bidens); C, sweet cicely (Washingtonia) ; D, tick trefoil
(Desmodiiim) ; E, cocklebur (Xanthium) ; and F, sand bur {Solanum) . Courtesy
of World Book Co.

digestive juices of the animal; (2) the walls of the fruit enclosing the
seeds may be spinous, prickly, or otherwise roughened and become en-
tangled in the fur and hair of animals; (3) the fruit coats may be sticky
and adhere to the feet of birds or other animals (Fig. 164). Man is one
of the main agents in the dispersing of seeds, especially when it comes to
carrying them across the continent or from one continent to another.
Dissemination of seed does not necessarily result in the wider distribu-
tion of the plant. Germination of the seed does not occur except when
the environment is favorable; and even then the seedling may fail to
develop or may be destroyed.

Economic value of flowers, fruits, and seeds. These three plant organs
are the primary economic products of floriculture, horticulture, and
agronomy. Corn in the United States alone is a billion-dollar crop. This
is its value to those who cultivate and harvest it. It may then be con-
verted to pork in the body of a pig, or to eggs in the body of a chicken;
or man, by means of industrial processes, may place it on the market as
meal, corn flakes, hominy grits, starch, glucose, syrup, alcohol, acetic
acid, dextrin, mucilage, sizing paste, corn oil, artgum, rubber, or any
one of a hundred other products. The economic value of a plant organ

370 TEXTBOOK OF BOTANY

is thus intimately related to the uses made of it and to the number of
people who make a living wholly or in part by cultivating the plants that
bear it or by preparing it for ultimate consumption. Some commercially
important seeds are those of wheat, oats, barley, rice, cotton, beans,
peas, coflFee, peanuts, coconuts, almonds, pecans, walnuts, filberts, and
pistachios. Economically the seeds of the grasses are the most important;
then come those of cotton and the legumes.

The cultivation of plants for their seeds is the most important eco-
nomic phase of agriculture. The secondary industries related to it include
shipping, marketing, feeding of domesticated animals, industrial process-
ing, scientific investigation of processes and of the uses of products,
and the dissemination of information about agricultural methods and
procedures.

The number of kinds of commercially important fruits and flowers is
large, but their total economic value is much less than that of seeds.
Among the commercially important fruits of the world are bananas,
breadfruit, dates, figs, olives, plantains, apples, grapes, plums, apricots,
peaches, tomatoes, pineapples, melons, and several varieties of citrous
fruits. The secondary industries related to the cultivation of plants for
their fruits include, in addition to those listed for seeds, greenhouse cul-
ture, extraction of juices, canning, drying, freezing, and cold storage
which make them available at all seasons of the year. Other industries
related to the cultivation of plants for their flowers include the numerous
flower shops, the keeping of bees for honey, and the manufacture of
perfumes.

The economic value of flowers, fruits, and seeds and of the products
derived from them thus amounts annually to many billions of dollars,
and a large percentage of the population finds a means of livelihood
either by cultivating the plants or by preparing and distributing the
harvested products for final consumption.

REFERENCES

Chamberlain, Charles J. Elements of Plant Science. McGraw-Hill Book Com-
pany, Inc. 1930.

Fairchild, David. The World Was My Garden. Charles Scribner's Sons. 1938.

Hayward, H. E. The Structure of Economic Plants. The Macmillan Company.
1938.

Hill, A. F. Economic Botany. McGraw-Hill Book Company, Inc. 1937.

Hitchcock, A. S., and Agnes Chase. Grass. Pt. IV. Old and New Plant Lore.
Smithsonian Sci. Series, 11:201-237. 1934.

CHAPTER XXXIII
SEXUAL REPRODUCTION IN FLOWERING PLANTS

Sexual reproduction occurs in nearly all kinds of plants. In angiosperms,
commonly called flowering plants, and in gymnosperms (pines, firs,
spruces, and other cone-bearing plants ) it is associated with the forma-
tion of seeds. With a few exceptions the processes of sexual reproduction
and seed formation in these two groups of plants are similar, but in this
chapter all statements about seeds and seed plants refer particularly to
angiosperms.

A complete account of the principal events in the sexual reproduction
of seed plants must include a description of how the embryo and endo-
sperm come to be in the seed, how pollen develops in the anther, and
how pollen is related to the formation of seeds and fruits.

Since seeds develop from ovules, and since the seed coats, embryo,
and endosperm are each composed of cells, one may infer at once that
each part of the seed grows from certain cells in the ovule. The origin of
the embryo of a seed plant, like the origin of the embryo of other organ-
isms, usually is a fertilized egg — a cell formed by the union of an egg
and a sperm.

From what cells, then, do the eggs and sperms develop in a seed
plant? What is the precursor, or forerunner, of the endosperm of a
seed? Of the pollen grain in the anther? Each of these several struc-
tures develops from others in a definite order or sequence; and to know
the story of sexual reproduction one must learn which processes and
structures are the precursors of others.

Deviations from the usual sequence may occur. The results of some
of these deviations are famihar objects. Seedless fruits, seeds without
endosperms, seeds with more than one embryo, identical twins, and
abscission of pistils are consequences of certain departures at some par-
ticular stage in the usual story of reproduction. One must be able to
visualize this usual story in order to appreciate such deviations as those
just mentioned, to compare sexual reproduction in seed plants with that
of other organisms, and to understand its relation to heredity. To do this

371

372 TEXTBOOK OF BOTANY

we need to know only what happens in the stamen and pistil of the
flower. Sepals and petals arc merely accessory structures. We may begin
with the stamen.

What takes place in the stamen? At certain seasons of the year the
stamens become mature, their anthers open or dehisce, and from them
millions of powderv grains known as pollen or pollen grains are released.
What is their origin and how have they developed? To answer this
question it will be necessary to begin with a young anther.

In a cross section of a verv young anther the cells look alike; but in
sections of slightly older anthers there may be seen four groups of
meristematic cells quite distinct from the remaining cells, and known as
the sporogenoiis (spore-bearing) tissue (Fig. 165). Further differentia-
tion in this tissue results in special spore-producing cells called micro-
spore mother cells, or microsporocytes. Each mother cell becomes sepa-
rated from the others and divides. The anther now contains numerous
cells attached in groups of two (dyads). When these cells divide, the
resulting daughter cells appear in groups of four (tetrads) and are
known as microspores. Each microspore mother cell, therefore, is really
the grandmother cell of four microspores. At maturity the microspores
fall apart and are seen as separate cells. Many microspores are ordinarily
formed in an anther at about the same time, and the parts of the anther
enclosing them are microsporangia (microspore cases).

Each microspore at first contains a single nucleus. When this nucleus
divides, the microspore technicallv becomes a 2-nucleate pollen grain
and the microsporangium may now be termed a pollen sac. One of the
two nuclei in the pollen is the forerunner of the sperms and is known
as the generative nucleus; the other one is referred to as the tube
nucleus because of its relation to the growth of the pollen tube from
the pollen grain. A subsequent division of the generative nucleus,^ either
in the pollen grain or in the pollen tube, results in two male gametes, or
sperms. Dehiscence of the anther and shedding of the pollen occur in
most plants before the sperms are fomied. In the anthers of many
plants the wall between certain pollen sacs may open so that b}' the time
of dehiscence of the anther the original four microsporangia have be-
come two pollen sacs.

We shall interrupt the story of what is happening in the stamen

^ These nuclei are surrounded by cytoplasm, but the individual cells are not always
enclosed by distinct cell walls. It is more convenient, therefore, to observe and refer to the
nuclei than to the cells of which they are a part.

[Chap. XXXIII SEXUAL REPRODUCTION IN FLOWERING PLANTS 373

D ■■'1/^^--

Fig. 165. Formation of microspores and pollen in the anther. A, cross section
of a young anther with four microsporangia containing microspore mother cells;
B, an enlarged cross section of one microsporangium containing cells in groups of
two formed by division of the microspore mother cells; C, a slightly older stage
in which further cell division has resulted in microspores adhering in groups of
four; D, the microspores have separated and some of them have become pollen
grains. Two of the microsporangia have become one pollen sac, which has split
open along one side.

at this point, and return to it after we have found out what is occurring
in the pistil.

What takes place in the pistil? The externally visible parts of a pistil
are the stigma, style, and ovulary. If a young ovularv is broken in two or
dissected, it will be seen to contain one or more small roundish bodies
known as ovules.

374 TEXTBOOK OF BOTANY

In a very young ovulary the ovules are seen as small mounds of cells
that have developed from a particular part of the ovulary termed the
placenta. In microscopic sections of these young ovules all the cells
look alike; but in sections of a slightly older ovule one cell near the tip is
seen to be larger than any of the others. This cell is a megaspore mother
cell, or megasporocijte. From this cell through successive cell divisions
there results a group of four (tetrad) megaspores. Three of the mega-
spores ordinarily disintegrate, and only one develops further.

By this time the ovule has enlarged and its three parts — the stalk,
nucellus, and integuments, or coats — can be recognized (Fig. 166).
Since the megaspores develop within it, the ovule may be called a
megasporangitim. The integuments do not quite enclose the ovule at
one end, thus leaving a small pore known as the micropyle. After the
first nuclear division the megaspore technicallv becomes a 2-nucleate
embryo sac. Through subsequent nuclear divisions the embryo sac finally
contains eight^ nuclei, one of which becomes the female gamete, or egg.
Two other nuclei unite and form the fusion nucleus. The embryo sac
thus becomes 7-nucleate. Of these the egg and fusion nucleus are the
important ones. The other five nuclei are usually transient and disinte-
grate, though in certain species of plants some of them divide and a
small group of cells develops from them; occasionally an embryo may
develop from one or more of them.

The pollen in the stamen now becomes important in our account
of sexual reproduction.

Pollination and growth of the pollen tube. Upon the dehiscence of the
anther and shedding of the pollen from the pollen sacs, some pollen
grains either fall upon or are carried by some means to the stigma of
the same or of another flower. The transfer of pollen from the anther to
the stigma is called pollination. This process may be brought about by
wind, water, insects, man, and gravity. On the stigma the pollen grain
germinates almost immediately, and a pollen tube develops from it. The
stigma is covered with a sticky fluid of sugars, acids, and other sub-
stances, and the pollen grain will usuallv germinate best on stigmas of
the same or closely related plants. Pollen grains of many kinds of plants
germinate when placed in solutions of sugar and water.

The pollen tube grows rapidly, penetrates the stigma, and extends

^ In the embryo sacs of some plants, such as triUium, the number of nuclei is four. One
becomes the egg, two form the fusion nucleus, and the fourth disintegrates. Several other
deviations in embryo sac development in other plants are known, but only the usual type
of embryo sac will be described here.

[Chap. XXXIII SEXUAL REPRODUCTION IN FLOWERING PLANTS 375

through the style into the ovulary and finally to the ovule (Fig. 167).
This distance in most plants is short; but in the style of corn, which is
the silk, the pollen tube must often elongate more than a foot before

Fig. 166. Stages in the development of an ovule and a young seed ot pepper.
A-D, formation of four megaspores (m) from a megaspore mother cell (mm) in
a young ovule; E, disintegration of three of the megaspores; F-I, development
of an 8-nucleate embryo sac from one of the megaspores, a fusion nucleus
is formed by the fusion of the two nuclei near the center of the sac, only a
little of the surrounding ovule tissue is represented; J, fertilization (f) and triple
fusion (tf); K, zygote (z) and triple-fusion nucleus (tfn); L, early stages in
the development of the embryo and endosperm of the seed. After H. L. Cochran,
Journal of Agricultural Research, with some modification.

it penetrates the ovule. The direction of its growth is influenced by
chemical conditions within the pistil. A part of the distance it is grow-
ing in channels and intercellular spaces. Resistance to its growth
between closely packed cells is decreased by enzymes which bring
about softening of the tissues through which the tube grows. During the
growth of the tube, the nuclei are near the growing end. The pollen

376

TEXTBOOK OF BOTANY

tube enters the

Fig. 167. Dia-
gram of a pistil
with germinating
pollen grains and
pollen tubes of
various lengths.
The embryo sac
is in the 7-celled
stage, with a cen-
tral fusion nu-
cleus and an egg
(below). Fertil-
ization occurs
when a
from a

tube unites with
the egg. After J.
T. B u c h h o 1 z.
Courtesy of
World Book Co.

sperm
pollen

embryo sac, often through the micropyle; the end of the
pollen tube swells and bursts, and the two sperms are
released. The tube nucleus then disintegrates. The em-
bryo sac now contains, in addition to the five transient
nuclei, an egg, a fusion nucleus, and two sperms.

Fertilization. The next event is the fusion of one
sperm with the egg, resulting in the fertilized egg, or
zygote. The union is called fertilization. The other
sperm unites with the fusion nucleus, and the resulting
3-nucleate structure is the triple-fusion nucleus, also
called the endosperm nucleus. The union of the three
nuclei is termed triple fusion.

The embryo and the endosperm. The fertilized egg
soon begins to grow, and manv cell divisions result in a
tissue known as the embryo ( Fig. 168 ) . The sugar used
in the growth of the embryo must have come from the
green parts of the plant on which the flowers were
borne. The embryo, when mature, is differentiated into
cotyledons, hypocotyl, and plumule, as noted in the
preceding chapter. It is a new plant.

While the embryo is developing in the ovule, an-
other tissue, known as the endosperm, may develop
from the triple-fusion nucleus (Fig. 169). The cells
of the embryo and endosperm often contain consider-
able food, chiefly oils, starch, and protein. The condi-
tions in these cells are particularly suitable to the con-
version of sugar to these more complex foods. In seeds
of many plants the endosperm is very small or absent,
because it failed to grow. In such seeds the cotyledons
of the embryo are usually large and contain most of
the food.

The embryo and endosperm of an angiosperm differ
little in their origin, but in their final outcome they are
quite unlike. The endosperm, when present, makes its
complete growth within the seed, and ordinarily no
other structures develop from it; the embryo, on the
other hand, is the young stage of a new generation of
plants.

The seed and the fruit. The growth and hardening

[Chap. XXXm SEXUAL REPRODUCTION IN FLOWERING PLANTS 377

Fig. 168. Stages in the development of the embryo of shepherd's pmse (Bursa)
from the fertiUzed egg to the matme seed. Drawings by Mabel Schaffner.

of the ovule coats result in the seed coats which enclose the endosperm
and embryo. These three structures — embryo, endosperm, and seed coat
— are the principal parts of the seed. The seeds of numerous plants, how-
ever, consist of an embrvo surrounded bv seed coats onlv. Growth and
various other changes in the ovulary result in the formation of a fruit.

378

TEXTBOOK OF BOTANY

GRAIN COAT -

NUCELLUS

Fig. 169. Stages in the growth of embryo and endosperm in a developing grain
of corn: A, 5 days after poUination; B, 10 days; C, 13 days; D, 15 days, and E,
20 days after pollination. Photos by Lois Lampe.

although none of its structures is the direct result of fertihzation. Some-
times structures adjacent to the ovulary also grow and form a part of
the fruit.

The life cycle of a flowering plant. We have now observed the series of
changes in the flower known as sexual reproduction, leading directly
or indirectly to the formation of fruits and the different parts of the
seeds. If a seed is planted, a seedling may develop from the embryo. At
first, food used in germination and in initial growth comes from the
cotyledons of the embryo or from the endosperm. As soon as chlorophyll
is formed in the seedling, however, photosynthesis may occur, and we
have the usual green plant with which we are quite familiar.

Upon further growth of the seedling and completion of the vegetative
stage, buds containing flower primordia develop. Through the series of
events we have just described, seeds are again formed. We started with
a seed and now another seed has been formed: this cycle may be
repeated indefinitely. The development from seed to seed is known as

[Chap. XXXIII SEXUAL REPRODUCTION IN FLOWERING PLANTS 379

the life cycle of the plant; and in the study of heredity it is generally
considered one generation.

The life cycle of a flowering plant may be analyzed from another point
of view which may be stated briefly now, and then reconsidered after
the life cycle of a fern has been discussed. A plant in which micro-
spores and megaspores'^ develop may be termed a sporophyte (spore-
bearing plant). The parts of the flower in which spores develop are
often referred to as sporophylls (spore-bearing leaves); hence, the
stamens may be thought of as microsporophylls and the carpels of the
pistil as megasporophylls.

Eggs and sperms, collectively called gametes,^ are also formed during
the life cycle of a seed plant. In ferns and mosses, gametes develop in
small multicellular plants that grow free upon the soil. Since these plants
bear gametes they are called gametophytes. If this terminology is applied
to seed plants, then the pollen grain plus the pollen tube in which the
sperms are formed is a male gametophyte (sperm-bearing plant), and
the embryo sac containing the egg is female gametophyte (egg-bearing
plant ) .

The gametophytes of flowering plants are almost microscopic in size,
non-green, and parasitic upon the tissues of the sporophyte. The gameto-
phyte phase may be thought of as alternating with the sporophyte
phase in the life cycle. The life cycle then is composed of two phases,
each bearing the particular reproductive cells from which the other
develops. The endosperm, however, is neither gametophyte nor sporo-
phyte; it is termed the xeniophyte.

Summary of the usual sequences of events in sexual reproduction. The
formation of fruits and seeds from flowers usually follows the sequence
of events described in the preceding pages. Summary statements of
these events are: (1) the formation of microsporocytes, microspores,
and pollen in regular sequence within the anther of the stamen; (2) the
development of two sperm nuclei from the generative nucleus within
the pollen grain or the pollen tube; (3) the formation of one or more
ovules in the ovulary of the pistil; (4) the formation in regular sequence
within each ovule of a megasporocyte, an active megaspore, and an

* It is customary to use the term "microspore" to refer to the precursor of a pollen grain,
and the term "megaspore" to refer to the precursor of the embryo sac, even though the
terms are sometimes literally inappropriate, since in many flowering plants there is little
difference in size and sometimes the microspore is the larger of the two.

* Eggs and sperms are called "megagametes" and "microgametes" respectively by some
authors.

380 TEXTBOOK OF BOTANY

embryo sac with an egg nucleus and a fusion nucleus; (5) pollination;
(6) the germination of the pollen grain and growth of the pollen tube
from the stigma to the ovule; ( 7 ) the fusion of one sperm with the egg —
fertilization — and the fusion of the other sperm with the fusion nucleus
— triple fusion; (8) the development of the embryo from the fertilized
egg; (9) the development of the endospenn from the triple-fusion
nucleus in seeds of some plants; (10) the formation of seed coats from
the ovule coats; and (11) the development of the fruit from the ovulary
and other parts of the flower. The fertilized egg and embryo are the early
stages of a new generation.

Deviations from the usual story of sexual reproduction. An examina-
tion of a seed of lima bean or garden pea reveals the absence of an
endosperm. Seeds of such plants as onion, grapefruit, and pine often
have more than one embryo, a condition known as poli/embryony.
Fruits of certain varieties of sunflower, orange, grapefruit, and grape are
seedless, a condition referred to as parthenocarpij . What deviations from
the usual story of sexual reproduction would account for these phe-
nomena?

Plants such as the common dandelion have seeds containing viable
embryos which developed from unfertilized eggs: a process termed
parthenogenesis.

In the seeds of some plants, such as corn cockle, coffee, water lily,
spinach, and pepper, the nucellus grows and becomes much enlarged,
and foods accumulate in it just as thev do in the embryo and endosperm.
When mature, this tissue resembles the endosperm in appearance and is
known as the perisperm. Sometimes it is merely a thin compressed
membrane within the seed coats.

The failure of pollination and fertilization often results in the abscis-
sion of the pistil and no fruit develops. The early fall of immature fruits
such as those of apple, peach, pear, and plum may of course be caused
also by the invasion of parasites and by other unfavorable conditions. In
spite of numerous exceptions, pollination and fertilization are generally
necessary for the development of fruits and seeds in the great majority
of flowering plants.

One of the most striking deviations occurs in species of Trillium. As
already noted, the embryo sac of trilliums ordinarily contains but four
nuclei; one of these becomes an egg and two others form the fusion
nucleus. No fertilization occurs. An abortive embrvo develops from the
unfertilized egg, and an endosperm develops from the fusion nucleus.

[Chap. XXXIII SEXUAL REPRODUCTION IN FLOWERING PLANTS 381

After the embryo from the egg aborts, an embryo develops from cells in
the endosperm. This is the embryo that perpetuates the species.

Ectogony. Pollination and fertilization directly affect the develop-
ment of the embryo and of the endosperm because of the union of the
sperms with the egg and with the fusion nucleus. They may also affect
the abscission of the pistil as well as the chemical composition, the color,
and the time of ripening of the fruit. Their influence on the development
of structures outside of the embryo and endospeiTn may be referred to
as ectogony.

Considerable study has been made of the effect of pollination and
fertilization on the fruit of the date palm. The pollen from staminate
flowers of several different varieties of palm was artificially transferred
to the pistillate flower of a single species of palm. The development of
the fruits varied with the source of the pollen. The fruits varied in size,
in shape, and in the time of ripening. Some fruits ripened as much as
10 days earlier than others.

Ectogony is undoubtedly the effect of hormones from the pollen tube
and the developing embryo. Experiments have shown that certain
ectogonous effects mav be obtained in plants such as tomato, squash,
tobacco, and petunia merely by the application of extracts of pollen to
the stigma, or to the cut end of a style from which the stigma has been
removed. By this means abscission of the pistils may be prevented, and
fully formed seedless fruits may develop from them.

Xenia. The effect of the sperm upon the development of the endo-
sperm may be visibly evident. If, for example, pollen from a certain corn
plant that had grown from a seed with blue endosperm is transferred
to the stigma of another corn plant that had grown from a seed with
white endospenn, and if triple fusion follows pollination, the resulting
endosperm may be blue. Similar effects may be seen in form, shape, and
chemical composition of the endosperm. The immediate effect of the
pollen parent on the endosperm of the ovule parent is known as xenia,
and will be described further in a later chapter.

Self- and cross-pollination. The transfer of pollen from anther to
stigma of the same flower or of another flower on the same plant may be
tenned self-pollination. The transfer of pollen from the anthers on one
plant to the stigmas on another plant is ordinarily referred to as cross-
pollination. In Chapter XXXVII, however, it will be shown that two
plants may be genetically identical, and what appears to be cross-
pollination is equivalent to self-pollination. For example, plants that

382 TEXTBOOK OF BOTANY

develop from cuttings, or by other vegetative means, from the same
individual are usually genetically identical. They are merely isolated
branches of that individual. This is an important distinction to make
in practice because many varieties of horticultural plants, such as certain
varieties of apple and cherry, bear little or no fruit unless they are cross-
pollinated. Cross-pollination is often referred to as crossing, and self-
pollination as selfing.^

Pollination in itself is important in sexual reproduction only when it
is followed by fertilization. Promiscuous cross-pollination may occur
among numerous kinds of plants that are in bloom at the same time,
but unless they are closely related no cross-fertilization occurs; that is,
they are cross-sterile.

Cross-pollination is often necessary to sexual reproduction, and in
some cultivated plants it is important in increasing yield and size of
fruit. Self-pollination may not be followed by fertilization in rye and
certain orchids because the pollen tube fails to grow on the stigma except
when the plants are crossed; and in avocado, plantain, red clover, and
lettuce because anthers and pistils do not mature simultaneously. Self-
pollination does not occur in willows, palms, hemp, and many other
plants because the stamens and pistils do not develop on the same
plant. If fertilization occurs in a plant only when it is crossed with an-
other one, the plant is said to be self-sterile. Self -sterility in Petunia
violacea appears to be due to a hormone formed in the placenta which
inhibits pollen germination and pollen-tube growth unless the pollen
comes from a different variety.

Self-pollination is, however, usual in oats, wheat, barley, tobacco, and
many other plants because the pollen is shed before the opening of the
flowers. A small amount of crossing may occur when insects eat parts
of the young flowers and move from one plant to another.

The avocado, or alligator pear, has complete flowers, but often no
fruit develops. This was puzzling to the owners of avocado orchards

^ The terms self- and cross-pollination are \ ariously used by different writers, depending
upon whether the flower, the "indixidual plant," the variety, or the species of plant is
chosen as the basis of comparison. Since the chief xalue of these terms lies in the use we
may make of them in discussing various horticultural procedures and heredity in seed
plants, the more appropriate uses of them will become clearer in the following six chapters.

The term self-pollination has also been used in an entirely different sense to refer to
pollination by direct contact of anther and stigma in contrast to the transfer of pollen by
wind, insects, or other external agents. For example, as the pistils of sunflower approach
maturity the styles curve downward and bring the stigmas in direct contact with the
anthers below them. In spite of this "marvelous adaptation" sunflowers are self -sterile! The
term "contact pollination" might well be used when self-pollination occurs in this way.

[Chap. XXXIU SEXUAL REPRODUCTION IN FLOWERING PLANTS 383

until careful observations showed certain peculiarities in the time of
flower opening. Each flower opens twice. In some varieties this may be
the morning of one day and the afternoon of the next day; in other
varieties, the afternoon of one day and the morning of the third day.
At the first opening the pistils are mature but the pollen has not been
shed. At the second opening the anthers dehisce, but the stigmas are
too mature for the pollen to germinate. Evidently self-pollination in
avocado does not result in fertilization and the production of fruit. If
these two varieties of avocado grow sufficiently close together, cross-
pollination and fertilization may take place, and good yields of fruit
result. The whole process is, however, complicated by the erratic opening
of the flowers, and by the insects that transfer the pollen.

Dissemination of pollen. In the flowers that remain closed during
ripening, as in oats and wheat, gravity plays an important part in pollina-
tion. A few plants, such as eel grass, certain pond weeds, and the water
buttercup, are pollinated by water-borne pollen. Often the pistillate
flowers develop just at the surface of the water, while the staminate
flowers may be formed below the surface. The staminate flowers rise to
the surface in large numbers, and the pollen eventually comes in contact
with the stigmas.

In the great majority of land plants, particularly where cross-pollina-
tion occurs, wind and insects are the most important agents of pollina-

,*!^

Fig. 170. Various forms of pollen grains. From W. Hamilton Gibson.

tion. Pollen grains vary widely in form (Fig. 170). Pollen may be car-
ried many miles by air currents, but the pollen of most plants under

384

TEXTBOOK OF BOTANY

natural conditions remains viable but a short time, usually not more than
a day or two. Under artificial conditions and proper drying it may be
kept alive for a few weeks or even months. It has been reported that
the pollen of date palms has been kept viable from two to eighteen years.
The number of pollen grains produced by a single plant is often
prodigious. A single corn plant, for example, may have as many as 50
million pollen grains. The number of seeds on an ear produced by this
same corn plant will rarely exceed 1000.

The importance of incidental pollination brought about by bees,
wasps, Hies, and other insects that use pollen and nectar as a source of
food has been known for years ( Fig. 171 ) . The current statements that

/

WASP BEARING POLLEN

GRAINS BRINGS THEM

IN CONTACT WITH

THE STIGMA

ABSCiSED /-
FUDRAL CUP ■•

THIRD DAY

FOURTH DAY

FRUIT

Fig. 171. Successive stages in the development of the flower, polhnation, and growth
of fruit in the common figwort. From W. Hamilton Gibson.

insects are more likely to visit flowers having a striking color, a special
appearance, or a large size have little if any foundation in fact. Such
matters cannot be discussed with certainty until more experimental data

[Chap. XXXIIl SEXUAL REPRODUCTION IN FLOWERING PLANTS 385

are available. Some of the facts and speculations about the relation of
flowers to pollination by insects may be found in the books cited at the
end of this chapter.

The most remarkable cases of pollination by insects are those in
which the plants have become dependent upon certain insects for the
transfer of pollen. Cross-pollination of red clover is brought about
primarily by bumblebees, and pollination of several of our horticultural
plants is accomplished mainly by honeybees. The pollination of the
widely cultivated yucca by the Pronuba moth is even more remarkable.
The female moth deposits her eggs in the ovulary of the yucca flower,
collects pollen from the anthers, carries it to the tip of the pistil, and
then pushes it down inside the tubular stigma. The young larval insects
which develop from the moth eggs inside the ovularv eat some of the
developing seeds, but not all of them. The mature moths do not live long
enough to see their ofi^spring, but the instinct to deposit eggs in the
pistil of the yucca flower and then pollinate it reappears through heredity
in each generation of moths. The relation of the fig to a parasitic fig
wasp is equally dependent, but much more complicated.

All such cases of specialization are, of course, the result of a long
series of heritable variations during the millions of vears that flowering
plants and insects have been in existence. An enormous \arietv of
heritable differences in floral structures has evolved and sur\ ived, but
attention is usuallv directed onh^ to those that appear to influence polli-
nation, and to those we find desirable for decoration or for the classifica-
tion of plants. Those of no survixal value to the plant, or of no direct
value to man, are usually ignored except by a few special students who
find in them numerous contradictions to the claims of those who have an
eye onlv for useful or allegedlv useful structures.

Pollen and hay fever. Many people suffer every vear from hay fever
and "seasonal asthma," caused by unusual sensitivity of the mucous
membranes of the eves, nose, throat, and bronchial tubes to pollen of
certain species of plants. The worst onslaught of hav fever is usually in
late summer and is caused by the pollen of plants such as ragweeds,
cocklebur, sagebrush, pigweeds, and thistles. Early-season hav fever
mav be caused by the pollen of such plants as maples, willows, birches,
tulips, and grasses. The number of plants to which one person or another
is allergic is large.

The sufferer is frequently affected seriously onlv bv the pollen of one
or a few species of plants, and these can be ascertained by proper skin

386 TEXTBOOK OF BOTANY

tests under the directions of specialists. When the pollen to which the
individual is allergic has been determined, the physician may inject
dosages of extracts specially prepared from the offending pollen. In this
way complete or partial immunity to the pollen may usually be acquired.
As an alternative the sufferer, once having found out what plants caused
his discomfort, may — during the tim.e of their pollen production — go to
regions where these particular plants do not grow.

REFERENCES

Coulter, J. M., C. R. Barnes, and H. C. Cowles. A Textbook of Botany. Amer-
ican Book Company. 1931. Vol. 3, pp. 344-393.
Gustafson, F. G. Parthenocarpy induced by pollen extracts. Amer. Jour. Bot.

24:102-107. 1937.
Gustafson, F. G. The cause of natural parthenocarpy. Amer. Jour. Bot. 26:

135-138. 1939.
Riley, C. V. The yucca moth and yucca pollination. Third Annual Report,

Missouri Botanical Garden, pp. 99-158. 1892.
Stout, A. B. The flower mechanism of avocados with reference to pollination

and the production of fruit. Jour. N. Y. Bot. Gard. 25:1-9. 1924.
Thompson, J. Arthur. Biology for Everyman. Vol. 2, pp. 1139-1151. E. P.

Button & Co., Inc. 1935.
Wodehouse, R. P. Pollen grains. McGraw-Hill Book Company, Inc. 1935.

Chapter VI.

CHAPTER XXXIV
GROWTH, DORMANCY, AND GERMINATION OF SEEDS

<xX><^<X><^C<><t><t><><><><X><><>«><t>^^

From the child who is thrilled by the story of Jack's miraculous beans
that grew to sublime heights in a single night, to the world tourists who
are perennially duped by the peddlers of Egypt with their living grains
of wheat allegedly taken from the tombs of the Pharaohs, most persons
are curious about how long seeds remain alive, why some seeds germi-
nate readily and why the seeds of other equally common plants fail to
germinate within a few weeks although the embryo appears to be in
perfect condition. These phenomena have been studied experimentally
in recent years and some of the results will be discussed in this chapter.

From zygote to mature embryo. The germination of the zygote is the
beginning of the development of the embryo of a new plant similar in
most respects to that in which the zygote was formed. Through cell
division and some enlargement the embryo soon consists of a small body
of meristem cells. In seeds of some plants, such as those of orchids, de-
velopment of the embryo ceases at this early stage. In seeds of most
plants, however, cell division, enlargement, and differentiation continue,
and an embryo with well-defined cotyledons, hypocotyl, and plumule
results. This development of the embryo of a seed plant occurs inside
the ovule. Meanwhile the ovule coats grow and become seed coats. The
nucellus also grows in the seeds of a few kinds of plants, but in most
seeds it either becomes a negligible structure, or is entirely destroyed by
the growth of the other seed structures.

The growth of the embryo is thus definitely limited by the tissues that
enclose it. As its supply of water and oxygen decreases, and the dead
seed coats prevent further expansion, the embryo ceases to grow and
becomes dormant.

If the young embryos are removed from the ovules and placed in a
sterile culture medium containing inorganic salts, water, and sugar, with
oxygen available, they do not become dormant but continue to grow as
seedlings. When the immature embryos of certain varieties of peach are
planted under these conditions the plants are better than those that

387

388 TEXTBOOK OF BOTANY

develop from the embryos of seeds that matured within the fruit. Some-
times during excessively wet weather the embryos of corn, wheat, and
other grains germinate almost immediately after maturity. In these wet
seasons one frequently finds embryos continuing growth inside the ears
of standing corn in September, and wheat and oats sprouting in the
shocks on the field during midsummer.

Extreme cases of embryo development are found in seeds of such
plants as Christmas holly, orchids, and mangrove. The embryo of the
holly is merely a minute spherical body of cells when the fruit turns red
in December. This clump of cells grows very slowly during a period of
8 to 12 months. The germination of the seed is further complicated be-,
cause the developing embrvo cannot break out of the hard fruit coat
until the coat is partially decayed.

Orchid seeds also consist of comparatively few cells and do not germi-
nate unless thev are artificiallv supplied with sugar or unless fungi digest
the insoluble food in the surrounding substrate. The balloon-shaped
green seedlings become readily visible onlv after a lapse of 6 months, and
at the end of two years seedlings in many species are less than an inch in
height.

The seeds of mangrove, on the other hand, germinate while the fruit
containing them is still attached to the tree. The hypocotyl emerges from
the seed and fruit and becomes more than a foot in length and a half
inch in diameter. Then this great hypocotyl, with a tiny plumule at its
apex and a root primordium at its heavier basal end, drops from the tree
into the mud below, and rapid development of plumule and roots fol-
lows (Fig. 172).

The embryos of most plants, however, are fully developed within the
ovule, but the seed will not germinate immediately. Seeds of these plants
germinate only after a longer or shorter period of time has elapsed, even
though placed in environments with water, oxygen, and temperature
conditions ordinarily suitable for growth.

From embryo to seedling. Some of the processes in embryos during
germination have been described in Chapter II and Chapter XXIX. If
several seedlings are watched as they emerge from the soil, certain strik-
ing differences in their growth are evident.

When the castor bean germinates, the lengthening of the hypocotyl
raises the cotyledons, endosperm, and broken seed coats above the soil
surface. The cotyledons rapidly expand in area and slough off the remain-
ing endospenn tissues and seed coats. The cotyledons are thin blade-like

[Chap. XXXIV GROWTH, DORMANCY, GERMINATIOiN OF SEEDS 389

Fig. 172. Development of mangrove seedlings. The tree {Rhizophora mangle)
grows on soft mud flats in the tropics and semi-tropics. The seed in the fruit
pictured on the left germinates while the fruit is still attached to the tree. The
embryo grows a foot or more in length before it finally drops endwise like an
arrow into the mud below and becomes a seedling. Photo from Field Museum of
Natural History.

Structures, and most of the food used during germination comes from the
enclosing endosperm. By the time the endosperm has dropped, the
cotyledons have doubled or trebled in area. Meanwhile they have
become green, and photosynthesis has begun. Further growth is de-
pendent on sugar and other foods made within the green seedling.

The cotyledons of the common bean and lima bean are likewise raised
above the soil surface. There is no endospeiTn, and the initial growth is
dependent solely upon the food contained within the embryo. As the
growth of the plumule proceeds, the cotyledons shrivel and abscise
within two or three weeks. Bv this time a shoot containing two or
three leaves has developed from the plumule.

The growth of roots during the germination of seeds of grasses was de-
scribed in Chapter XXIX. The hypocotyl of grasses does not elongate,
and the cotyledon remains in the soil. The plumule, which is enclosed
in a sheath (the coleoptile), grows upward through the soil, and by its
enlargement breaks through the sheath. The plumule of a grass seed is
thus the forerunner of all the plant that appears above the surface of the
soil.

390 TEXTBOOK OF BOTANY

During the germination of peas, scarlet runner beans, corn, sorghum,
and acorns the cotyledons remain in the soil because the hypocotyl does
not elongate. In peas, scarlet runner beans, and acorns the initial food of
the seedling comes from the cotyledons of the embryo; in corn and
sorghum, from both the embryo and the endosperm.

From our previous study of physiological processes in the vegetative
organs of plants, v^e have much of the information needed to under-
stand the changes that take place during gennination and the early
seedling stages. The v^ater content of "dry" seeds, such as com and
wheat, is insufficient for growth, and the seeds remain dormant until
water is added. The absorption of water results in a swelling of the tissues
and a renewal of the many cell processes that had been active during
the initial growth of the embryo. Water is a medium in which enzyme
activities proceed, and it also combines with various insoluble foods
during their digestion, or hydrolysis, to soluble compounds. It is like-
wise the medium in which soluble foods and soluble inorganic salts move
or are moved about in the developing seedling. The greater part of the
enlargement of the seedling is the result of the osmotic absorption of
water and the increase in size of the vacuoles of the enlarging cells.

Oxygen is essential during germination because of the many oxidation-
reduction processes that occur in the complex protoplasmic system, espe-
cially in the transformation of foods into more permanent cell structures.

"Favorable" temperatures obviously refer to those temperatures in
which the complex colloidal system of the protoplasm and physiological
activity are best maintained, since upon this condition permeability and
the coordination of many essential physical and chemical processes de-
pend. But temperatures ordinarily best for the rapid growth of plants
are not best for the geiTnination of all kinds of seeds.

The upward growth of the shoot and the downward growth of roots
of seedlings are primarily the result of the distribution and influence of
hormones described in Chapter XXIII.

Changes in foods during germination. The chemical changes that occur
in germinating seeds have been studied by means of gross chemical
analyses at frequent intervals during the germination process. The
changes have also been followed by observation of small seeds, or thin
sections of seeds, through the microscope with appropriate chemical
tests. It is not difficult to follow the transformation of starch, for example,
as it is hydrolyzed to dextrin and sugar. The determination of the dif-
ferent sugars is somewhat more difficult, as described in Chapter XII.

[Chap. XXXIV GROWTH, DORMANCY, GERMINATION OF SEEDS 391

The disappearance of fats and proteins through digestion is not difficult
to observe, but methods of detecting the intermediate and end products
are somewhat compHcated (Fig. 173).

6 9

TIME IN DAYS

Fig. 173. Changes in amounts of foods in the cotyledons of germinating sun-
flower seeds. The sugar moves out of the cotyledons into the hypocotyl about as
rapidly as it is formed from the disappearing fats and proteins; some of it is tem-
porarily condensed to starch. Data from E. C. Miller.

A part of the sugar is oxidized in respiration, and during germination
the temperature of seeds is raised slightly by the heat energy liberated.
Another part of the energy is used in the chemical transformations in-
volved in the synthesis of pectic compounds, the constituents of proto-
plasm and certain other parts of the cell. As a result of respiration, seeds
and seedlings progressively decrease in weight if germination takes place
in the dark.

Delayed germination. The seeds of most wild plants do not germinate
immediately after they fall from the plant, even when placed in condi-
tions ordinarily most suitable for rapid growth. Even under experi-
mental conditions the seeds of many plants germinate only after the lapse
of several weeks or months, or even years. The length of the delay in
germination of seeds of different plants is often a specific characteristic.
The seeds of hundreds of species, both wild and cultivated, have been
tested and the conditions most favorable for their germination have been
discovered. Gennination of these seeds does not occur previous to cer-
tain changes in the composition and structure of the embryos or of seed

392 TEXTBOOK OF BOTANY

coats, even after the seeds are apparently mature. These changes are
referred to as after-ripening. Delayed gennination may result from a
number of factors, which may be considered in three groups:

1. Delayed germination, the result of environmental conditions out-
side the seed.

2. Delaved gennination, the result of conditions within the seed, but
outside the embryo.

3. Delayed germination, the result of conditions within the embryo
itself.

The delayed germination in the last two groups is the result of true
dormancv in seeds, since the cause of dormancy is within the seed.

Germination delayed by the environment. Environmental conditions
profoundly affect the growth of the embryo, both before and after the
seed appears to be mature. In temperate regions winter temperatures are
too low for the germination of most seeds, and in our southwestern
deserts summer temperatures are too high for the germination of many
seeds, even though moisture conditions are favorable. Temperatures
that facilitate germination vary with the plant species, but comparatively
few seeds germinate below 50° F. Temperatures either unfavorably high
or low for a particular species result in no germination or in stunted
seedlings. At temperatures above 70° F. the root systems of some plants
develop very slowly, and vigorous plants rarely form. The embryos of
two water plants, wild rice and eel grass, die soon after maturity unless
stored at temperatures just above the freezing point in moist situations.

The effects of environmental conditions on the growth of embryos
and seedlings are not always limited to this period of growth. They may
continue to influence the subsequent development of the plant, even if
the environment is changed. Rapid germination of seeds and growth of
seedlings are not necessarily beneficial to the subsequent growth of the
plants. The relative growth of shoots and roots and the length of the
vegetative period may be influenced by the temperature at germination
and early growth of seedlings ( Chapter XXXI ) .

Prolonged droughts obviously restrict germination, and are often a
source of great agricultural losses when they follow the planting of
seeds of crop plants. Gennination may be stopped at any stage from
the swelling of the seed to the emergence of the seedling above the soil
surface.

The seeds of such plants as willow, cottonwood, and elm will germi-

[Chap. XXXIV GROWTH, DORMANCY, GERMINATION OF SEEDS 393

nate immediately after being shed, but are soon killed by desiccation/
The chances of growth of such seedlings and their survival are there-
fore greater on wet banks, stream and pond margins, and in unoccupied
wet lands. The seeds of silver maple rarely germinate if their water
content falls below 30 per cent.

Oxygen deficiencv most frequently results from the burial of seeas
too far below the soil surface, especially of tight clay loams. Flooding
of soils by continuous rains mav so compact the soil and fill the spaces
between particles that aeration becomes almost zero. Oxygen deficiency
is a constant condition in the ooze at the bottom of lakes, ponds, and per-
manent marshes and swamps. Here the diffusion of oxygen from above
not only is very slow, but the mud and ooze have an enormous popu-
lation of bacteria and other organisms that use it as fast as it diffuses
from the water. These bottom organisms also release CO- in large quan-
tities; and when the CO- accumulates about seeds it may of itself be-
come a cause of delayed germination.

Most seeds will germinate in either light or darkness, but a long list
of species has been published in which better germination was obtained
in light. Among these plants are bluegrass, certain varieties of tobacco,
mullein, carrot, mistletoe, evening primrose, loostrife, and willow herb.
On the other hand, light interferes with the germination of some seeds,
among which are species of Phacelia, 'Ni^ella, waterleaf, and onion.
After a period of suitable storage some of these seeds germinate in either
light or darkness. There is evidence that light may alter seed coats in
some seeds, and in certain others may affect the embryos or the endo-
sperms. Some of these seeds will germinate when alternating periods of
higher and lower temperatures are substituted for light. Of the light
rays in the visible spectrum, in general the red to yellow rays seem to
facilitate germination, while the blue to violet rays retard it.

Germination delayed by seed and fruit coats. There are many seeds
with embryos enclosed by very hard and tough coats: either seed coats
or fruit coats. The common pigweed, the water plantain, Christmas
holly, and the bloodroot have coats so resistant to mechanical pressure
that expansion of the embryo cannot occur until the coats are partially
destroyed by soil organisms or by some other means. These coats may

^ The seeds of some of these plants can be dried and kept ahve for a time under special
experimental conditions. Willow seeds, for example, may sur\'ive for several months in a
refrigerator if they are placed over 50 per cent H^SO, in a small vessel. The relative
humidity in the vessel would be almost 13 per cent. Similarly sugar cane seeds live longer
in an atmosphere of carbon dioxide kept dry with calcium chloride.

394 TEXTBOOK OF BOTANY

delay germination in nature for months or years. The embryos of Christ-
mas holly do not become dormant, and most of them perish before the
surrounding coats are sufficiently weakened by decay. Under natural
conditions only about one holly embryo in ten million germinates. In
horticultural practice the best germination of holly seeds has been ob-
tained by keeping the seed bed above 70° F.

Seed and fruit coats may often exclude water from the endosperm
and embryo. These coats may be impervious to water because of the
presence of wax, suberin, resin, and certain structural features. Embryos
thus entombed germinate in nature only when the coats have been partly
digested by bacteria and other organisms. Seeds of red clover and
numerous other legumes have seed coats impervious to water. In lotus
and certain stone fruits, it is the fruit coat that is impermeable.

In horticultural practice such seeds may be treated for a few minutes,
or for longer periods, with strong sulfuric acid until the impervious layer
of the coat is altered or removed. These seeds may also be scarified by
means of sandpaper, sharp sand, and special scarifying machines until
the coats are partially removed. Often only the two outermost layers of
cells need to be broken. Legumes are noted for the great variation in
the "hardness" of the coats of the seeds from a single harvest.

Oxvgen likewise may be excluded from embryos by impermeable seed
and fruit coats. An example of delayed germination due to oxygen defi-
ciency has been discovered in cocklebur. There are two seeds in each
bur, the lower of which is near the surface and germinates freely after
the fruit is shed. The upper embryo, however, is surrounded by tissue
that is rather impervious to oxygen, and it does not germinate when the
concentration of oxygen is low, even if the coats are destroyed. Both
these conditions delay germination for additional months or years, as
compared with the lower embryo. Certain grasses and some of the plants
of the sunflower family also have seed and fruit coats that are not very
permeable to oxygen.

Intermittent germination of seeds of a species over a period of years
is probably an advantage to a plant in nature. In horticulture, forestry,
and agriculture it is uneconomical because it results in uneven-aged
stands of crop plants, and delays the starting of nursery stock.

Germination delayed by conditions within the embryo. When the seed
of the ginkgo tree falls to the ground its embryo may consist of only a
few cells, and sometimes fertilization has not yet occurred. During ensu-

[Chap. XXXIV GROWTH, DORMANCY, GERMINATION OF SEEDS 395

ing months the embryo grows and its three parts become diflFerentiated.
Germination of the seed occurs after the embryo has fully developed in
the seed. To this class of seeds with only rudimentary embryos belong
those of buttercup, anemone, adder's tongue, holly, columbine, and
hepatica. Some of these seeds germinate more rapidly in dilute sugar
solutions than in water. The dormant period of these seeds is a time of
active chemical and morphological development of the embryo. The
embryo itself is not dormant.

A second group of plants with fully formed embryos have seeds that
are also truly dormant when shed. These seeds undergo a process of
after-ripening in which certain chemical changes in the embryo slowly
occur. They do not germinate at once when planted under external con-
ditions ordinarily suitable for growth; but if the seed coats are removed
and the embryos are treated with a dilute acid, some of them germinate
comparatively soon. The dormancy of these embryos is conditioned by
chemical processes.

Little is known about the processes that occur during dormancy, but
it is wrong to assume that all dormant seeds contain a "resting embryo."
Experiments have shown that during after-ripening of this last class of
seeds the embryos become more acid, enzymes are more abundant, com-
plex carbohydrates and proteins are hydrolyzed, and respiration is in-
creased. These are merely the superficial, easily tested indicators of
more important, but unknown, steps in the reorganization of the proto-
plasmic system.

To this group of seeds with embrvos in which an increase in acidity
appears to be a primary part of the process of after-ripening belong
those of apple, rose, cherry, sugar maple, giant ragweed, basswood,
cotoneaster, peach, and plum.

After-ripening may be hastened in many of these seeds by storing them
at a temperature of 40° F. Other seeds after-ripen soonest at a tempera-
ture nearer 50° F., and still others after-ripen best with daily fluctuations
in temperature as low as 40° F. and as high as 70° F. Moist acid peat
probably is the best medium in which they may be placed during the
dormant period. Table 13 is a summary of some of the results of after-
ripening tests made at the Boyce Thompson Institute for Plant Research.
Most of these seeds were stored dry until February, and were then
stratified, or mixed with moist granular peat, and placed at a variety of
temperatures.

396 TEXTBOOK OF BOTANY

Table 13. Temperature and Length of Stratification Periods for Seeds to
Obtain the Best Yield of SeedHngs.

^, , Most Effective Effective Time Required for

r'lant rpi . lemperature t> ^ t> ix

lemperature _,^ Best Results

Range

Jack pine 32° F.

32-41° F.

60 days

Sugar pine 50° F.

32-50° F.

90 '

Pitch pine 41° F.

32-50° F.

30 '

Eastern white pine 50° F.

32-50° F.

60 '

White spruce 32° F.

32-41° F.

60 '

Sitka spruce 32° F.

32-50° F.

30-60 '

Arizona fir 32° F.

32-41° F.

30 '

Bald cypress 41° F.

32-50° F.

30 '

Eastern red cedar 50° F.

32-50° F.

65 '

Lily of the valley 41° F.

32-50° F.

150 '

Blackberry lily " 41° F.

41-50° F.

120 '

Cotoneaster — best results in covered cold frames out

of doors

280 '

Apple 41° F.

41-46° F.

60-70 '

French pear (stored dry 6 months) 41° F.

41-50° F.

90 '

Carolina rose 41° F.

32-50° F.

90 '

Apricot, carpel intact 50° F.

32-50° F.

45 '

Apricot, carpel removed 41° F.

32-50° F.

25 '

Red Japanese maple 41° F.

32-50° F.

100-120 '

Tropical pawpaw 50° F.

32-50° F.

100 '

Flowering dogwood 41° F.

32-50° F.

120 "

These results were secured under carefully controlled conditions. One
can hardly avoid wondering how long the dormant period of these seeds
may be in nature, and, if prolonged, what fraction of the embryos escapes
destruction by bacteria, fungi, and the innumerable animals near the soil
surface. Is it one in a million or one in a billion?

Longevity of seeds. Present knowledge of how to shorten the period
of delayed germination of seeds gives some insight into the factors
involved in the presei'vation of seeds through a long period of time,
alive but ungerminated. Some seeds have greater longevity when rap-
idly dried in a desiccator and stored in sealed vessels or in an atmos-
phere of carbon dioxide. This is true even for seeds that survive but a
short time in nature where oxygen is abundant and the seeds are alter-
nately wet and dry.

It has been clear for many years that lowering the temperature to near
the freezing point lengthens the period of vitality of most seeds. Sealed
storage of dry seeds at refrigerator temperatures apparently prolongs
the life of embryos still further and is now used by commercial seedmen.
A comparison of the results obtained with delphinium seeds emphasizes

[Chap. XXXIV GROWTH, DORMANCY, GERMINATION OF SEEDS 397

the effectiveness of low oxygen and low temperature storage conditions
on the retention of viability.

Table 14. Percentage Germination of Air-dried Seeds of Delphinium Stored
in Sealed and Open Containers at About 70° F. and 46° F. (Data from L. V.
Barton. )

Storage 11 mos. 22 mos. 3 vrs. 10 mos. 5 vrs. 9 mos. 9 yrs. 3 mos.

Condition

Open, 70° F.

57%

44%

00%

00%

00%

Sealed, 70° F.

75

80

50

15

00

Open, 46° F.

50

41

31

5

00

Sealed, 46° F.

70

4.5

66

80

76

The longest known period of survival of any seed is that of the lotus.
Seeds germinated after being kept in dry storage at the British Museum
for 150 years, but seeds from the same lot tested after 215 years failed
to grow. Seeds of this same species, Neltimbo nucifera, were viable after
burial in an ancient lake bed for at least 160 years, and probably for
more than 250 years, before their discovery in a road cut in Manchuria.
The hard seeds of the legumes are well known for their longevity; there
is a record of 158 years for a species of Cassia. Most agricultural seeds
live for 10 or 20 years, but the hard seeds of many crop legumes are
viable after 40 to 60 years. It should be noted, however, that the per-
centage of germination at the end of the longer periods is usually very
low. A number of seeds of wild plants with hard coats have records of
longevity up to 90 years.

We still lack authentic records of the longevity of seeds stored in
hermetically sealed containers. The longest records are those derived
from "hard seeds" which represent seeds with embryos sealed indi-
vidually. Putting seeds in sealed containers is merely duplicating the con-
ditions within hard seed coats, for it likewise results in excluding oxygen
and water from embryos.

A diagrammatic record of the famous experiments by Beal at the
Michigan Agricultural College is given in Fig. 174. Beal buried the seeds
in the soil in inverted open bottles in 1879. Bottles containing the seed
mixture have been dug up at five-year intervals and the number of living
seeds tested, with the results shown in the diagram.

Why do dormant seeds die? This question has been asked for many
years and many answers have been proposed, but none of them has been

398

TEXTBOOK OF BOTANY

10 20 30 40 50 YEARS

I 1 1 1 1 1 1 1 1 1 1

BLACK MUSTARD
EVENING PRIMROSE
CURLr DOCK
COMMON PLANTAIN
PIGWEED

ROMAN RAGWEED
PUMSLANE
SHEPHERD'S PURSE
COMMON MULLEIN
CHICKWEED
FOXTAIL-GRASS
DOC-FENNEL
IWUND-LEAVED MAUOW
WHITE CLOVER
CORN COCKLE
SPOTTED SPURGE

1 1 1 1 ' 1 ■ -■

> NO CERMINATIOn

Fig. 174. Longevity of embryos of seeds in Beal's experiment.

completely satisfactory. The earlier suggestions that the change from
viable to non-viable seeds is the result of the oxidation of the food supply
and the destruction of the enzymes seem to have been adequately dis-
proved. Foods in seeds have been carefully analyzed and the chemical
changes are slight. The enzymes, on the other hand, seem to be just
as active as before death. Evidence against the idea that the food has
been completely oxidized is the dry storage of minute one-celled algal
spores in sealed soil samples for 30 to 40 years and their germination
w^hen the soils were moistened.

Later it was suggested that death is due to the coagulation of the
proteins in the cells. This had in its favor the fact that there is a simi-
larity in the curve of longevity of seeds at high, medium, and low tem-
peratures and the curve of coagulation of proteins at corresponding
temperatures. Coagulation of proteins in cells might disrupt the organi-
zation of the protoplasm. Changes in the protoplasmic lipoids might like-
wise destroy the organization.

A more recent answer is suggested by the discovery that the seedlings
derived from old seeds, from X-rayed seeds, and from heated seeds are
similar in survival, in growth, and in variability. Moreover, similar ir-
regularities in nuclear structures have been observed in microscopic

[Chap. XXXIV GROWTH, DORMANCY, GERMINATION OF SEEDS 399

studies of the nuclei of the cells after these three different treatments.
These facts seem to indicate that prolonged storage of seeds results in the
breakdown of nuclear organization and the mechanism of heredity. This
answer has more experimental results to commend it than the preceding
ones, and it seems to be the best one at the moment.

Caution. Anyone interested in the preparation, storage, or germina-
tion of particular kinds of seeds should not depend wholly on the
generalized statements concerning the best methods. There is now a con-
siderable body of data available regarding these techniques, and it
should be searched before attempting to prepare, store, or plant the
seeds of valuable plants.

Another precaution may also save time and disappointment. One
should first of all be sure that the seeds have embrvos inside them,
since there are seeds that look perfectlv normal on the outside but
lack the most essential part of a seed. One should also make sure that
the parts inside the seed coat are in a healthy condition, that they have
not become infected bv bacteria and fungi, or infested by insect larvae.

The references below will be helpful in seeking further details about
seeds, their preservation, and their germination.

REFERENCES

Crocker, William. Mechanics of dormancy in seeds. Amer. Jour. Bot. 3:99-120.

1916.
Crocker, William. Life-span of seeds. Bot. Rev. 4:235-274. 1938.
Davis, W. E. The development of dormancy in seeds of cocklebur (Xan-

thiitm). Amer. Jow: Bot. 17:77-87. 1930.
Ives, S. A. Maturation and germination of seeds of Ilex opaca. Bot. Gaz. 76:

60-77. 1933.
Knudson, Lewis. Nonsymbiotic germination of orchid seeds. Bot. Gaz. 73:1-25.

1922.
Meyer, B. S., and D. B. Anderson. Plant Physiology. D. Van Nostrand Com-
pany, Inc. 1939.
Miller, E. C. Plant Physiology. 2nd ed. McGraw-Hill Book Company, Inc.

1938.
Shaw, Margaret F. A microchemical study of the fruit coat of Nelumbo lutea.

Amer. Jour. Bot. 16:259-276. Plates XIX and XX. 1929.
Contributions from the Boyce Thompson Institute, 1926 to the present.

CHAPTER XXXV
VEGETATIVE MULTIPLICATION OF FLOWERING PLANTS

Most flowering plants may increase in number by either of two methods :
( 1 ) the formation of special reproductive cells and seeds — sexual repro-
duction, or (2) the development of a separate individual from one or
more vegetative cells of a plant — vegetative multiplication or vegetative
propagation. The vegetative organ, or any fragment of it, from which the
separate individual develops may be referred to as a vegetative propa-
gule, in contrast to a seed.

If two or more separate individuals develop from single vegetative
cells or from branches of the same plant, they are merely isolated parts
of it. They are "chips off the same block," not a new generation of plants.
These facts are most readily appreciated when vegetative multiplication
by cuttings is considered.

We mav, for example, cut a thousand twigs from a willow tree in early
spring and place the basal end of each twig in moist aerated soil. These
severed twigs are cuttings. Within a few weeks roots develop from the
lower nodes of each cutting, and branches grow from the buds above
the soil. Thus within a short period of time we can secure a thousand
young willow trees, which may in turn become large trees.

These thousand separate trees, however, are as much alike inherently
as are the unsevered branches of a single tree; and in all matters per-
taining to pollination, sex, and hereditv they should be considered as
branches of the tree from which the cuttings were taken. Branches of
each of these thousand trees mav in turn be used as cuttings, and so on
ad infinitum. In this manner a single generation of a willow tree may be
perpetuated and multiplied indefinitely. It is often convenient to have a
name to refer to all the individuals which through repeated vegetative
multiplication have a common origin. Such individuals are collectively
called a clone. Their common origin is the embryo of the first individual
of the clone (Fig. 175).

All the individuals of the Concord grape constitute one clone, for this
variety of grape has been perpetuated by vegetative multiplication since

400

[Chap. XXXV VEGETATIVE MULTIPLICATION

401

Fig. 175. A miniature clone of redwood trees which originated vegetatively as
a group of sprouts from the roots of an older tree which has been cut down.
Photo from U. S. Forest Service.

it was first selected from the progeny of a hybrid in 1853. Many other
varieties of cultivated plants are clonal varieties. Among them are the
several varieties of Irish potato, horse-radish, pineapple, rhubarb, coleus,
and raspberry, v^hich are perpetuated in cultivation by vegetative propa-
gation. The Carolina poplar trees too frequently seen along the streets
of cities are usually staminate trees which developed from cuttings
taken from other staminate trees. Most ornamental plants are perpetu-
ated by vegetative propagation.

Vegetative multiplication may occur by either of two ways: by the
natural separation and further development of vegetative parts of a plant,
or by the further development of segments cut from a plant by man and
placed under various suitable conditions. Man promotes vegetative

402

TEXTBOOK OF BOTANY

propagation among cultivated plants by collecting, storing, and trans-
planting naturally occurring vegetative propagules; and by cuttings.

Among the vegetative parts of a plant that may become separated
naturally and develop as separate individuals are the branches that grow
from runners, rhizomes, and roots; oflFsets or sprouts that grow from the
bases of stems; plantlets that develop from leaves; and the familiar
bulbs, corms, and tubers with terminal and lateral buds from which new
shoots and roots develop (Chapter XXVI). Methods by which plants
multiply vegetatively from these naturally occurring vegetative propa-
gules will be described first.

Abscised leaves and stem segments. Under natural conditions the

abscised stem segments of some plants and
abscised leaves of others are vegetative
propagules. Embryos develop from vege-
tative cells in the notches of the leaves of
bryophyllum and kalanchoe (Fig. 176).
From these embryos roots develop first in
leaves of Bryophyllum calyciniim, and
shoots develop first in leaves of Bryophyl-
lum crenatum ( Fig. 194 ) . The plantlets on
the leaves of some varieties of kalanchoe
abscise before the leaves do. On falling to
the ground they continue growth, forming
a clone beneath the larger plant.

Abscised lateral buds of some species of
sedum and lily fall to the ground and new
individual plants develop from them. Simi-
larly abscised stem segments and fruits of
certain cacti become vegetative propagules.
In pastures where these cacti are weeds the
stem segments may be scattered rather
rapidly by grazing animals. The abscised stem segments of such plants
as willow and cottonwood appear to be of no importance in propaga-
tion. Vegetative multiplication of some water plants, such as elodea,
occurs frequently from broken fragments of leafy stems. The elodea in
the canals and rivers of central Europe is said to have been dispersed
vegetatively from clones introduced from America about 1840.

Runners and other "creeping" stems. Runners are common means of
vegetative multiplication of strawberry, of some ferns and grasses, of
water hyacinth, and of numerous other plants ( Fig. 177 ) . As many as a

Fig. 176. Kalanchoe leaves.
Young plants have originated
from cells near the margin of
the older leaf on the left.
Photo from P. W. Zimmerman.

[Chap. XXXV

VEGETATIVE MULTIPLICATION

403

dozen runners from 3 to 10 feet in length may grow from a single
strawberry plant during one season. Roots and shoots of new strawberry
plants develop at every other node of each runner. Secondary runners
develop from the alternate nodes and also from the new strawberry

Fig. 177. Clones of strawberry plants develop from runners. Note the regular
occurrence of new shoots and secondary runners at alternate nodes.

plants. By this means a clone of a score or more strawberry plants may
develop from a single plant during one summer. Under cultivation, the
new plants may be lifted from the soil and reset in rows. In nature they
become separated by the death of the connecting runners.

Fig. 178. Tip-layering of black raspberry. In late summer when the ends of
the branches bear very small leaves and have a rat-tail appearance, they are
buried vertically in holes dug in the soil (A). After adventitious roots have de-
veloped from the buried nodes the stem tip begins to grow upward (B). The
rooted tips with a portion of the old canes attached are dug up and reset early
the following spring before the tip has emerged from the soil.

404

TEXTBOOK OF BOTANY

Fig. 179. A, aerial shoots of beach grass from the rhizomes of a single plant;
B, aerial shoots from a rhizome of bamboo (Arundinaria) 10 inches below the
soil surface.

[Chap. XXXV

VEGETATIVE MULTIPLICATION

405

Vegetative multiplication may also occur at the nodes of leafy prostrate
stems, and at the nodes of elongated stems of such plants as raspberry,
grape, and honeysuckle, which bend over and come in contact with moist
soil. In horticultural practice the stems of such plants are bent over and
some of the nodes or whole stem tips are covered with soil: a procedure
called layering (Fig. 178). The terminal buds of the buried stem grow
upward and adventitious roots grow from the oldest buried nodes. The
rooted branches may then be severed from the parent plant and trans-
planted.

In bog forests propagation of trees by layering occurs naturallv when
the lower branches of spruce and arbor vitae lie on the ground or are
pressed down by snow. This method of propagation may be more fre-
quent among bog plants than propagation bv seeds. Lavering by leaf
tips occurs in a few species of plants, such as the walking fern.

Rhizomes. Many perennial grasses, fems, mints, and a host of other
plants have rhizomes from the nodes of which roots and aerial shoots
develop ( Fig. 179 ) . The rhizomes may elongate each vear bv the growth
of temiinal buds. When the older parts of these rhizomes die, groups of
individuals of the clone become separated and are new centers of dis-
persal by growth of rhizomes.

As a result of this method of propagation, certain perennial grasses
and other perennial herbs become the dominant plants of the vegetation
of lawns and meadows, and of the natural prairies and plains in various

Fig. ISO. Erosion stopped by the kudzu vine. The ditch is now being filled by the
accumulation of silt. Photo horn the U. S. Soil Conservation Service.

406

TEXTBOOK OF BOTANY

parts of the world. Since their roots and rhizomes are perennial, shoots
from buds on rhizomes begin to grow rapidly in spring and soon overtop
the annual plants, which start from seeds each year. Similarly cattails,
rushes, sedges, and water lilies frequently exclude many other plants from
certain habitats. Their rhizomes may grow several feet in length each
year, and the plants occupy new areas rapidly.

Abandoned farans and denuded areas about cities are first occupied by
a mixed population of annual, biennial, and perennial weeds; but the
perennials increase their area year by year through vegetative multipli-
cation and finally exclude nearly all the annuals and biennials.

For the prevention of soil erosion in gullies and on freshly made em-

FiG. 181. A, uncontrolled wind erosion accompanied by crop failures and the
formation of deserts in Sherman County, Texas; B, same site after the soil had
been stabilized by grasses planted according to methods devised to control
this type of erosion. Photo from U. S. Conservation Service.

[Chap. XXXV

VEGETATIVE MULTIPLICATION

40";

bankments quick-growing perennials that multiply by means of rhizomes,
creeping stems, and sprouts from roots should be planted first. For a
pemianent vegetation on such areas one may prefer a bluegrass turf or
trees; but these plants usually become established too slowly to be used
as pioneers unless some means of temporarily preventing soil erosion is
employed. A few trees and shrubs, such as black locust and the creeping
honeysuckle, which multiply vegetatively by branches from roots, or
from creeping stems, soon become established on such areas. Kudzu vine
has been used to prevent gullying in the Southern States ( Fig. 180 ) , In

Fig. 182. Depth of rhizomes below soil surface: A, a, bur-reed (Sparganium) ; b,
broad-leaved arrowhead (Sagittaria) ; c, swamp persicaria (Polygonum) .
B, a, yellow water crowfoot {Ranunculus); b, large yellow pond lily {Nijm-
phaea); c, water parsnip (Sium) ; d, cat-tail (Typha); mild water smartweed
{Polygonum). Sketches by E. E. Sherft".

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TEXTBOOK OF BOTANY

the West Central States grasses ha\'e been employed in stabilizing wind-
blown farm lands ( Fig. 181 ) .

Pieces of rhizomes are often planted in preference to seeds. Rhizomes
of each species grow at a fairly definite depth below the surface of a
particular soil ( Fig. 182 ) . If they are planted below or above this depth
they grow up or down to the specific soil le\'el.

When weeds, such as couch grass, bindweed, flowering spurge, and
nettle, grow among crop plants, their rhizomes may be broken and scat-
tered when the crops are being cultivated, and the weeds become still
more abundant.

Fig. 183. The groundnut {Apios tuberosa) has many edible underground tuber.s
from which aerial shoots develop.

Tubers. The tubers or thick terminal portions of rhizomes of Irish
potato are a means of vegetative multiplication common to many herba-
ceous plants (Figs. 183 and 184). The wild species of potato are native

[Chap. XXXV VEGETATIVE MULTIPLICATION 409

to the highlands of tropical America, where they grow naturally as peren-
nials. In the latitude of central Ohio the tubers left in the field are killed
by low temperature in winter. But farther north, where the temperature
of the soil beneath deep snow may remain above freezing, the cultivated
potato may in some local areas live indefinitely as a perennial.

Fig. 184. Vegetative multiplication of potato from a bud on a piece of tuber.

Photo by J. Bushnell.

The tubers of many kinds of plants survive the temperature of winter
even if snow is not deep or present throughout the cold season. Such
plants are classified as perennials; but the vegetative propagules, the
tubers, develop annually and live only through parts of two growing sea-
sons. Each year leafv shoots with roots at their bases develop from buds
in the so-called eyes of the tuber. As these shoots grow to mature
plants, another crop of tuber-bearing rhizomes develops.

The buds of tubers, like those of many other stems, have a definite
dormant period; and when they grow, apical dominance is also evident
(Chapter XXVI). If the temperature of the storage bin is unsuitable
during this period, or if artificial treatments of the buds with thiourea
to break their dormancy are excessive, apical dominance may be an-
nulled, and inferior plants result. If the tubers are planted before the

410

TEXTBOOK OF BOTANY

dormant period is completed, the tops of the young sprouts may enlarge
and become tubers instead of growing into upright green shoots. The
same eflFect is produced if the sprouts are repeatedly remo\'ed from
tubers stored for several months ( Fig. 185 ) .

Fig. 185. Growth of potato sprouts. A, apical dominance; B, apical dominance
is annulled when the tuber is cut into several pieces which are planted separately;
C, apical dominance partially annulled and vigor of sprouts decreased by storage
at high temperatures (33-34° C.) for a year; D, repeated removal of sprouts on
tubers in storage results in the growth of sprout tubers. A-C from C. O. Apple-
man; D from J. Bushnell.

Corms and bulbs. The corms of gladiolus and crocus, and the bulbs
of onion and tulip are examples of other types of vegetative propagules
common to many herbaceous plants. These organs develop annually from
lateral buds, and when mature they become separated from the parent
plant (Fig. 186A). Like tubers, thev usuallv live through parts of two

Fig. 186. Vegetative multiplication of wild garlic by bulbs: A, roots and base
of flowering shoot from old bulb and three new bulbs which have developed from
lateral buds; B, clusters of small bulbs, or "sets," in the flowering heads of aerial
shoots. Vegetative multiplication by these small bulbs is so effective that garlic
often becomes abundant in pastures. Courtesy of World Book Co.

[Chap. XXXV

VEGETATIVE MULTIPLICATION

411

growing seasons. Bulbs or bulblets may develop from buds on aerial
stems also, as in certain varieties of lily and onion (Fig. 186B). Some
water plants have bulblike compact winter buds which become sepa-
rated by the death of the older parts of the stem. These buds float about
in the water or settle to the bottom of the lake or pond. In the following
spring roots and shoots develop from them.

Gardeners do not usually transplant bulbs and corms until after the
tops of the plants have died. These propagules may also be stored
and shipped, but the temperatures to which they are exposed may greatly
affect the subsequent growth of plants from them. Improper storage
temperatures are sometimes the cause of worthless plants. Diseases
caused by viruses which spread throughout the plants may also become
a menace to the bulb industry. Intelligent inspection of the growing
plants and of the bulbs placed on the market, accompanied by reason-
able laws and their enforcement, is the only means of protecting the
purchaser of bulbs.

The buds of bulbs and of corms have definite periods of domiancy, and
the best plants are not obtained unless the donuant period is passed

Fig. 187. Rhizome of Eryngiiim with distinct annual segments and branches, seen
from the side, and from below. From H. S. Jurica.

under suitable conditions. Tubers, corms, and bulbs, like rhizomes, have
fairly specific levels of growth in the soil. If they are planted below or
above this level, the new ones are formed a little nearer to it each suc-
ceeding year. This phenomenon appears to be related primarily to light.
Offsets and basal sprouts. Many plants, both herbaceous and woody,
multiply vegetatively by offsets, sprouts, and tillers, all of which have a
similar origin from lateral buds near the base of the stem (Figs. 187 and
188). This process occurs annually in some species of plants, such as
rose, sedum, aster, and goldenrod; but in shrubs and trees the lateral

412

TEXTBOOK OF BOTANY

buds may have been dormant for many years ( Fig. 84 ) . Under natural
conditions the plants become separated by breaking away naturally or
through the death of tlie parent plant. In practice these vegetative
propagules may be severed from the parent plant and transplanted.

Fig. 188. Owing to its rapid vegetative multiplication by runners, the water
hyacinth, which is a floating plant, may completely cover the surface of slow-
flowing streams in the southern states and the American tropics. Photo by G. W.
Blaydes.

Apical dominance at the tops of many plants prevents the growth of
shoots from the base of the stem. When the stems of such plants are cut
off, sprouts often grow profusely from the stumps. If regeneration b}
sprouts from the stumps is allowed to occur naturally after the trees of a
forest have been cut down, the next forest is largely a sprout forest, or
coppice. Since the sprouts on the stumps have a root system ahead}'
established in the soil, they may grow more rapidly than seedlings with
a comparatively small root system. Unfortunately, as the sprouts from
the larger stumps increase in age, their dead heartwood may be de-
stroyed by the same fungi that cause decay of the stumps.

If the forest is repeatedly cut, such trees as chestnut and linden, which

[Chap. XXXV

VEGETATIVE MULTIPLICATION

413

sprout profusely, become increasingly abundant in subsequent forests.
Forests composed almost entirely of chestnut have resulted from this
practice. When the sprouts originate from parts of the plant below
(Ground, and especially from roots, they are frequently called "suckers."
There are many plants, such as lilac, plum, cherry, sumac, black locust,
silver poplar, and bindweed, that multiply vegetatively by the growth of
shoots, or sprouts, from adventitious buds in the roots (Fig. 189).

4?t^

Fig. 189. The field bindweed (Convolvulus arvensis) multiplies rapidly by
the growth of aerial shoots from buds on vertical rhizomes which develop at
intervals from the many lateral roots. The plant pictured above had been grow-
ing for 14 weeks. The scale is in feet and inches. The larger shoot near the
center grew from the embryo in the seed. The lateral roots grow horizontally
for a foot or more and then curve downward. Near the bend a second crop of
laterals develop; when they curve downward a third crop develops from them,
and so on throughout the' season. Photo by J. C. Frazier, Kansas Agricultural
College.

Thickets of such plants are often formed in this manner. Ordinarily
shoots grow from the roots of many other plants only after the tops have
been removed — for example, dandelion or cottonwood. New sweet
potato plants are obtained from old roots each season by moving them
From storage bins to warm moist propagating beds for a short time.
Dozens of new plants may be obtained from each root. Leaves of the
sweet potato plant may also be used as vegetative propagules.

414

TEXTBOOK OF BOTANY

Vegetative multiplication not limited to leaves, stems, and roots. Vege-
tative multiplication occurs more frequently in stems and roots than in
other organs of seed plants. Experiments have shown, however, that it
may occur also in bulb scales, fruits, cotyledons, hopocotyls of embryos,
zygotes, endosperms, and in the coat and nucellus of an ovule. Later on
it will become evident that the development of embryos from unfertilized

' i.

'•^'>^

'J

Fig. 190. Stages in the development of an embryo-like bud from the epidermis of
Crassula. From Ilda McVeigh.

eggs in dandelion is genetically equivalent to vegetative multiplication.
Whether the new individual develops from a single cell or a group of
cells, both root and stem primordia resembling those of an embryo soon
become differentiated ( Fig. 190 ) . Among the bacteria, fungi, and algae,
vegetative multiplication by single cells is a regular phenomenon.

Cuttings. Man, however, is not limited to naturally occurring vege-
tative propagules, because he has learned how to utilize cuttings from
various parts of the plant. Among ornamental plants propagation by cut-
tings alone greatly exceeds that by all other methods. Trees and shrubs
cultivated for their fruits are usually started from cuttings used as grafts.
A few of them, such as black raspberry and hazel, are started by layering.
Red raspberry, banana, and pineapple are started by offsets and sprouts
from the base of the stem or from roots. Sugar cane is started from short
stem segments bearing several lateral buds. Cuttings may be made from
stems, leaves, or roots and are valuable means of insuring the propaga-
tion of varieties that do not reproduce true from seeds.

Stem cuttings. Shoot primordia are already present in the terminal
and axillary buds of a stem cutting. Preformed root primordia may also
be present, as in branches of willow, cottonwood, and flowering currant.
Adventitious root primordia may develop from parenchvma cells in the

VEGETATIVE MULTIPLICATION

415

[Chap. XXXV

pericycle or from cells in the phloem or cambium of the stem (Fig. 191 ) .
They do not develop readily in stem cuttings of some plants, and not at
all in others.

'7l-\ f-'i

- 't*.

Fig. 191. Cross section of a root of Cissm in which lateral root primordia have
developed from cells in the pericycle at points directly external to the vascular
bundles of the main root. Photomicrograph from N. E. Pfeiffer, Boyce Thompson
Institute.

Stem cuttings may be placed directly in soil, moist sand, peat, and
other media. They may also be grafted on the stem or roots of another
plant.

When a stem cutting is placed in a suitable "rooting medium," roots
grow from the morphologically basal end, and new shoots develop from
the apical buds. The development of these apical shoots prevents the
growth of shoots from buds further down the stem. Likewise the growth
of basal roots prevents the growth of roots farther up the stem, even
though preformed root primordia are present. This apical dominance is
so pronounced in both roots and stems that it is seldom reversed, except
by drastic changes in the physiological condition of the cuttings.

The origin and growth of the roots depend upon sugar and hormones
and perhaps vitamins from the leaves (Chapter XX). Consequently
when cuttings are made of herbaceous plants ("slips"), or of the young
stem segments of woody plants ( soft-wood cuttings ) , the origin of roots
and their rate of growth are influenced by the leaves left on the cuttings

416

TEXTBOOK OF BOTANY

( Fig. 68 ) . They are also influenced by such external factors as water,
oxygen, temperature, and light.

Perhaps with further research, stems of nearly any plant may be used
successfully as cuttings. In addition to the factors mentioned above, the
origin of roots in stem cuttings varies with the age of the plant, with con-
ditions under which it grew, with the part of the stem taken as a cutting,
with the season of the year, and with the presence of special chemical
substances.

Experiments have shown that some thirty different chemical com-
pounds, when applied artificially, accelerate and increase the initiation
of root primordia in parenchyma cells in almost any part of the plant
( Fig. 192 ) . Such compounds are of value in shortening the time of root

Fig. 192. A-B, effect of treatment of African marigold plants with 1 per cent
carbon monoxide for 10 days: A, untreated; B, treated. Numerous roots have
developed from the stems and petioles. C, adventitious roots on stem and petioles
of African marigold 6 days after a 3-day treatment with 0.5 per cent acetylene.
Photo by P. W. Zimmerman and A. E. Hitchcock, Boyce Thompson Institute.

[Chap. XXXV VEGETATIVE MULTIPLICATION 417

formation in cuttings of certain varieties in which the initiation of roots
is very slow.^

The relative amount of shoot and root growth from a cutting is affected
by the presence of carbohydrates and proteins within it. Both grow well
if there is a good initial supply of each of these foods. But if carbo-
hydrates are abundant and proteins are scarce, root growth is much bet-
ter than shoot growth.

The physiological condition of cuttings greatly influences their be-
havior. If the plant is dormant, the cuttings do not live unless they are
kept in cool moist conditions until adventitious roots have begun to
develop. If the plant is just beginning to bear flowers, the cutting may
grow but little and then bear flowers.

Grafting. Grafting is an artificial method of promoting vegetative
propagation when cuttings do not root readily, and when the plants do
not reproduce true from seeds. Most varieties of trees cultivated for their
fruits are perpetuated by some method of grafting.

Grafting consists in attaching a cutting, the scion, to the root, stem
base, or branches of another plant, the stock. The scion ma\^ be a twig
bearing several buds, or it may be a single bud attached to a small piece
of bark. If two plants are growing close together the bark may be re-
moved from convenient points on branches or main stems, which mav
then be bound together at these points. This is called approach grafting.
When the union between the stems is complete, the parts of either that
are not desired may be cut away.

Valuable trees that have been girdled by animals or by extremes of
temperature may be saved by bridge grafting, in which opposite ends of
each of several twigs are inserted as grafts beneath the bark on the upper
and lower edges of the girdle.

The parts of plants used in grafting are illustrated by diag^ams in
Fig. 193. The cambiums of the stock and scion must be brought into
close contact. The grafted sections are then bound together with a string,
and the junction is covered with waxed tape to prevent drying and the
entrance of destructive organisms.

The cambium and other meristematic cells at the cut surfaces grow

^ Three of these compounds most \'aluable in promoting plant propagation on a com-
mercial scale are indole butyric acid, naphthalene acetic acid, and indole acetic acid.
Others important scientifically are indole propionic acid, ethylene, propylene, acetylene, and
carbon monoxide. Directions for applying these compounds to cuttings of different vari-
eties of plants may be found in publications from the Boyce Thompson Institute of Plant
Research, and also in circulars from chemical supply companies that market them under
such trade names as "hormidin," "auxillin," and "rootone."

418

TEXTBOOK OF BOTANY

Fig. 193. Parts ot plants used in grafting: A, stump or crown grafting; B, piece-
root grafting; C, C budding; D, bridge grafting; E, approach grafting.

and form wound tissue, or callus. In successful grafts, new cambium cells
develop from some of the cells in the callus and unite the cambiums of
stock and scion. The new xylem and phloem tissues formed become con-
tinuous; and water, salts, sugar, hormones, and other soluble substances
move across the junction.

If this union of tissues fails to occur, the scion dies. If the union is
not well formed, the movement of water and salts to the scion and of
sugar to the stock is restricted and growth of the whole plant is slow.

The fundamental features of bud grafting are similar to those of twig
grafting. If the bud of a peach tree is grafted in place in late summer,
healing occurs within a few weeks, but the bud remains donnant until
the following spring when a branch develops from it in the usual way. To
insure the growth of the grafted bud, the stem is cut off just above it to
remove the source of apical dominance. Branches will develop from the
buds the same year the graft is made, if the buds formed the previous
year are collected in winter and kept on ice until the graft is made in
June.

Established grafts are commonly obtained onlv among related plants,
as among \'arieties of peach, apricot, and plum, or among varieties of

[Chap. XXXV VEGETATIVE MULTIPLICATION 419

apple, pear, and quince. Intergrafting among genera is possible in plants
of the potato family, and in plants of the sunflower family, but inter-
grafting between families of plants is rare. The idea that horticulturists
can make a kind of "table d'hote" plant through multiple grafting of
tomatoes, cucumbers, potatoes, apples, beans, and other plants is not
based upon reliable data.

Effects of scion and stock upon each other. Removing a cutting from a
plant and placing it in soil, or grafting it on to the stem or root of another
plant, does not change its heredity. Both the stock and the scion retain
their inherent qualities, but do not acquire new ones from each other.
Each of them, however, is a part of the environment of the other. They
become physically united as parts of the same individual, and as such
they are subject to the interrelation of the parts of an individual as
described in Chapter XXI.

The scion is dependent upon certain processes in the roots of the stock,
and the stock is dependent upon certain processes in the leaves of the
scion. Sugar, hormones, and other soluble substances pass from one
to the other. Nicotine, for example, may pass from a tobacco scion into
the roots and tubers of a potato stock. It is reported that when potato and
tomato are intergrafted with jimson weed, an alkaloid, atropin, from the
jimson weed accumulates in the tuber of the one and the fruit of the
other. As a result of these interrelations, the growth of roots and tops, the
size and flavor of fruits, and the time of flowering may be altered.

Graft chimeras. A stem primordium mav develop bv the division and
enlargement of cells at the area of contact of scion and stock. If some of
the cells of this primordium develop from one or more cells in the base
of the scion and some of them develop from adjoining cells in the stock,
the young stem tip will be composed of the two kinds of cells, which may
be referred to briefly as "scion cells" and "stock cells." Since this stem
tip is the forerunner of a leafy branch, these cells are the remote fore-
runners of all the tissues that develop in it (Fig. 32). Consequenth
whole tissues in the leaves and stems of this branch may be composed
entirely of the one or the other kind of cell. The epidermis, for example,
may be composed entirely of scion cells, and the other tissues entirely
of stock cells. Such compound structures are called chimeras; and since
they develop from the junction of a graft, they may be called graft
chimeras to distinguish them from mutant chimeras, which originate
within cells by processes described in Chapter XXXIX.

In time flower primordia may develop on stem tips of this branch.
These flower primordia may develop entirely from scion cells, entirely

420 TEXTBOOK OF BOTANY

from stock cells, or from both kinds of cells. If the primordium of the
pistil develops from both kinds of cells, some of the tissues in the
resultant fruit will resemble those in the fruit of the scion, and others
will resemble tissues in the fruit of the stock.

When the tissues characteristic of one kind of plant surround those
of the other, the chimera is said to be periclinal; when neither tissue sur-
rounds that of the other, but each appears in distinct sectors, the chimera
is sectorial. Incomplete periclinal chimeras appear superficially to be
sectorial.

Experimental attempts to increase the number of chimeras include
wounding or cutting across the stem at the junction of stock and scion
after they have become united.

Advantages of grafting. Grafting is a valuable means of perpetuating
desired varieties of plants that do not multiply readily by other methods
of vegetative propagation or reproduce true from seeds. There are also
certain other advantages, three of which will be mentioned briefly.

The range of distribution of a variety may be extended if it can be
grafted on root svstems that grow better than its own roots in certain
habitats. For example, peach trees may grow better on the roots of plum
in poorly drained soils, and plum trees may grow better on roots of peach
in light or sandy soils. The root systems of some varieties of pear grow
well in light, well-drained soils, others in heavy, poorly drained soils.

The destructive effects of insects, fungi, and bacteria on roots may be
avoided by grafting the desired variety on root systems that are immune
to these parasites. A classical example is the avoidance of the destruc-
tive effects of the root louse. Phylloxera, on roots of the wine grape, Vitis
vinifera, of France by grafting the European grape on an immune species
of American grape, Vitis lahnisca. Similarly, certain susceptible varieties
of apple are grafted on the roots of other varieties immune to the woolly
aphis. English walnut is protected from the fungus Armillaria by being
grafted on black walnut, which is immune to it.

Plants, such as apple, have a long vegetative period before they bear
flowers and fruits. The shoots of apple seedlings will bear fruit in two
or three years if they are grafted on the branches of a tree that has
begun to bear flowers and fruits. Advantage is sometimes taken of this
fact in testing the quality of apple seedlings that would otherwise not
bear fruit for ten years.

Root cuttings. Root cuttings are made from several kinds of plants,
such as blackberry, red raspberry, plum, cherry, horse-radish, apple,
Japanese quince, sumac, phlox, and anemone. The cuttings, which are

[Chap. XXXV

VEGETATIVE MULTIPLICATION

421

usually 2 to 3 inches long, are covered with about one-half inch of soil.
They differ from stem cuttings in the absence of preformed shoot buds.
Owing to the polarity of the roots, adventitious shoot buds usually de-
velop from the upper, or basal, end of the cutting and roots develop from

Fig. 194. Vegetative multiplication of plants from leaf cuttings: A, Bryophtjllum
calycinum; B, Crassula; C, Sanseveria; D, English ivy; E, bud scale of lily; F,
Bn/ophi/lhim crenatum; G, yam; H, Echeveria.

422 TEXTBOOK OF BOTANY

the tip. Some plants, such as apple, which fail to propagate from stem
cuttings taken from the crown of the tree, propagate readily from root
cuttings. Moreover, sprouts from the roots of apple may be used as stem
cuttings, in contrast to branches from the crown.

Leaf cuttings. For many years floriculturists have used the leaves of
certain species of plants as vegetative propagules. Among those most
frequently used are the leaves of the African violet, begonia, pepperomia,
sedum, echeveria, lily (bulb scales), bryophyllum, bowstring hemp, and
kalanchoe (Fig. 194).

Vegetati\'e multiplication fails completely in some leaves, and is in-
complete in others, because adventitious stem and root primordia do
not develop in them. When the lower end of the petiole of a leaf of
English ivy is inserted in soil, roots develop from the petiole and the
blade enlarges and lives for many years, but no shoots develop. If a leaf
of sweet potato is treated in the same manner, roots develop and then
aerial shoots develop from the roots.

One investigator, after testing the probable use of leaves of more than
600 species of plants as vegetative propagules, reported the following
results : no vegetative multiplication in 32 per cent of the species, growth
of roots but no shoots in 42 per cent, growth of shoots but no roots in 2
per cent, and growth of both roots and shoots in 24 per cent.

The behavior of vegetative propagules is much more variable than that
of seeds. Only the merest outline of information about them can be
given in a single chapter. Detailed information desired about specific
varieties and methods may be found in encyclopedias of horticulture
and in special research papers.

REFERENCES

Bailey, L. H. Hortus. The Macmillan Company. 1935.

Chandler, W. H. Fruit Growing. Houghton Mifflin Company. 1925.

Denny, F. E., P. W. Zimmerman, and A. E. Hitchcock. Contributions from
the Boijce Thompson Institute. 1929-40.

Priestley, J. H., and C. F. Swingle. Vegetative propagation from the stand-
point of plant anatomy. U. S. D. A. Tech. Bull. No. 151. 1929.

Salisbury, E. J. The Living Garden. The Macmillan Company. 1936.

Stoutemyer, V. T. Regeneration in various types of apple wood. Iowa Agr.
Exp. Sta. Research Bull. 220. 1937.

Yerkes, G. E. Propagation of trees and shrubs. U. S. D. A. Farmers Bull.
1567. 1929.

Zimmerman, P. W. Responses of plants to hormonelike substances. Ohio Jour.
Sci. 37:333-348. 1937.

CHAPTER XXXVI

ORIGIN OF PLANTS USED BY MAN

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All about us in forests, fields, orchards, gardens, lawns, and even in
test tubes in commercial and research laboratories are numerous plants
which man values for the various uses he makes of them. Some of these

Fig. 195. Iroquois Indians harvesting and preparing corn for storage. Photo from
American Museum of Natural History, New York.

plants are the uncultivated, or wild, species which are either harvested
where they are found, or planted where we want them to grow. During
the period when the early settlers were invading the American wilder-
ness and converting it to famis, some of their domesticated animals sub-
sisted mainly on the native wild plants. The pioneers brought seeds of
domesticated plants from Europe and also secured seeds of corn and
other domesticated plants from the American Indian ( Fig. 195 ) , but for
many years wild animals and the fruits and seeds of wild plants were
important sources of their food.

The native vegetation on the farms was destroyed to make room for
domesticated plants; but in certain practices, especially those in which

423

424 TEXTBOOK OF BOTANY

the plants of the forest are needed, we still use many of the native wild
species. The origin of these wild species will be considered in a later
chapter. Many of the plants we use, and especially our crop plants, have
been obtained from wild species by repeated selection of more desirable
variations, and in recent times by selection accompanied by controlled
pollination. Even forest trees are now being selected and artificially
pollinated to obtain varieties with certain desired qualities.

When man transfers plants from the wild state and provides artificial
environments favorable to their growth and propagation, he observes
many of the variations that naturally occur in them and eventually selects
and maintains certain of the new varieties. These cultivated varieties are
often referred to as agronomic, horticultural, and garden varieties, or
briefly as domesticated plants.

Many domesticated plants are unable to survive in the natural condi-
tions that prevail over most of the area in which they are cultivated.
Domesticated annuals, for instance, may perish because their seeds do
not survive from one growing season to the next unless they are preserved
in artificial conditions. Some domesticated varieties, such as special
hybrids of corn, are reobtained each season only by carefully controlled
pollination. On the other hand some domesticated plants, through seed
dispersal, may become distributed and continue to persist as weeds in
fields and along roadsides, but they usually do not survive in areas occu-
pied by native plant communities.

A heritage from prehistoric man. Nearly all of our more important
species of cultivated plants were domesticated by prehistoric man. Nu-
merous new varieties have been obtained from these domesticated plants
within historic times, but the domestication of additional wild species
has been limited largely to plants chosen for decorative purposes or for
their fruits and their forage value.

On the basis of our present knowledge of plants, we may, with reason-
able certainty, enumerate the major steps by which prehistoric man
secured domesticated plants. First, there was a recognition of certain
valuable parts or properties in the wild ancestors. For a time these parts
were collected from the plants wherever they were found in a wild state.
Later particular species were intentionally cultivated. Variations in the
heredity of the cultivated individuals continued to occur by the same
process that brings them about in plants in the wild state. Under the
conditions of cultivation, seeds or vegetative propagules of some of the
variants were either consciously or unconsciously selected and planted

[Chap. XXXVI ORIGIN OF PLANTS USED BY MAN 425

more abundantly than others. Continued variation accompanied by selec-
tion through many thousands of years resulted in cultivated varieties
that differ in many ways from their wild ancestors.

There is good evidence that long before historic times man began
to notice and select desirable variants of the plants on which his ex-
istence depended. The primitive tribes in the interior of New Guinea,
whose civilization todav is considered by ethnologists to correspond to
the late Stone Age, distinguish and name the numerous varieties of sugar
cane they are cultivating.

Man may also unconsciouslv select and promote the propagation of
certain varieties of cultivated plants. Owing to the occurrence of natural
variations, a field of crop plants usuallv consists of a mixture of se\ eral
varieties just as the population of a citv is composed of manv different
kinds of people all of whom are members of the same species. Experi-
ments have shown that in a mixed population of crop plants some vari-
eties may produce more seeds than others, have a greater number of
ripe seeds at harvest time, or have seeds that germinate more readih-
when planted. Consequently in a mixed population of plants that propa-
gate by seeds, certain varieties mav gradually increase and others de-
crease in abundance without anv conscious selection on the part of
man (Table 15).

Table 15. Changes in a Mixed Crop of Wheat During a Five-year Period
Without Anv Conscious Selection bv the Farmer. Percentages of the vari-
eties and species in the crop during the first season of the test are given in
column A; at the end of five years in column B.

Wheat Varieties A B

Triticum vulgare lutescens
" " ferrugineum
" " erythrospermum
" " milturuni
" durum 1
" compactum J

72.0%
10.9

9.3

6.1

1.7

7.6%
82.4
5.7
4.3

0.0

100.0%

100.0%

The evolution of cultivated plants has occurred over such a long period
of time and resulted in such marked changes in the plants that botanists
have had great difficulty in discovering the wild ancestors of some of

426

TEXTBOOK OF BOTANY

them. Compare, for instance, our present varieties of cabbage, kohlrabi,
cauHflower, kale, and Brussels sprouts with the mustard-like wild an-
cestor from which these cultivated varieties were derived (Fig. 196).

Fig. 196. Domesticated varieties derived from the wild cabbage, Brassica oleracea
(A), a native of Europe. B is kale, Brassica oleracea acephala; C, kohlrabi, B.
oleracea caulo-rapa; D, Brussels sprouts, B. oleracea gemmifera; E, pointed-head
cabbage, B. oleracea capitata; F, round-head cabbage, B. oleracea capitata; G, cauli-
flower, B. oleracea botnjtis.

The wild ancestors of the domesticated plants of Eurasia are better
known than those of America. Com is the only important cereal that
originated in America. Its wild ancestor was a grass; but in spite of a
prolonged search for this grass, it is still unknown today.

When Columbus arrived in America, the Indians of the New World
were cultivating several distinct varieties of corn, potatoes, sweet po-
tatoes, kidney and lima beans, peanuts, pumpkins, squash, tomatoes,
pineapple, pepper, arrowroot, sunflower, Jerusalem artichoke, beach
strawberry, tobacco, cotton, and many other plants (Fig. 197). These
plants were domesticated by prehistoric man in the highlands of Mexico,
Central America, and the northwestern part of South America: the
regions in which the Incas, Mayas, and Aztecs later developed their
remarkable civilizations. Previous to the voyage of Columbus none of
these domesticated plants of America was known in Europe, and none
of the domesticated plants of Europe was known in America. Within
each hemisphere, however, several important crop species had become
widely distributed. The Indians were cultivating corn in many local

[Chap. XXXVI ORIGIN OF PLANTS USED BY MAN

427

Fig. 197. Woodland Indians cultivating corn, snnHower, squash, and tobacco.
Photo from American Museum of Natural History, New York.

areas between what is now Canada and Argentina, while in Eurasia
wheat and rice were just as extensively cultivated. Some of the impor-
tance which the early American Indians attached to their domesticated
plants is reflected in their art (Fig. 198).

Fig. 198. Ancient Peruvian pottery. Photo from American Museum of Natural

History.

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TEXTBOOK OF BOTANY

In the eastern hemisphere the principal centers in which the basic
crop plants originated are certain parts of central Asia, Asia Minor,
central and southern China, northern India and perhaps Ethiopia: the
regions in which the earliest civilizations of Eurasia later became most
highly developed. Among the cultivated plants on which these early
civilizations of Eurasia depended were the cereals (wheat, barley, rye,
rice, millet, oats, sorghum), soybeans, and several common vegetables
and edible fruits. Most of the cultivated forage crops (clo\'ers and
grasses) also originated in the eastern hemisphere (Fig. 199).

Fig. 199. Forage crop — red clover and timothy.

Civilization in the past depended upon an unfailing supply of plants
as the primary source of food just as it does today. During his long
existence upon the earth as a primitive nomad, early man depended
upon plants in the wild state. Furthennore, his migrations were condi-
tioned by the abundance, or the scarcity, of wild plants. The kinds of
wild plants used by prehistoric tribes in all parts of the globe are num-
bered in the thousands. The Indians who lived in what is now the
United States and Canada used as a source of food more than 1000
species of plants, only a few of which were ever cultivated ( Fig. 200 ) .

[Chap. XXXVI ORIGIN OF PLANTS USED BY MAN 429

Many others were used as sources of material for weaving, medicine,
and personal equipment (Figs. 201 and 202).

Fig. 200. Woodland Indians collecting and boiling maple "sap" as a source of
sugar. Photo from American Museum of Natural History, New York.

Fig. 201. Birch bark industries of North American Indians. Photo from American

Museum of Natural History.

430

TEXTBOOK OF BOTANY

Fig. 202. Basswood bark industries of eastern woodland Indians. Photo from
American Museum of Natural History.

No civilization of any note was possible prior to the development of a
primitive agriculture. Among the more valuable accomplishments of pre-
historic man we must include his discovery of the methods of plant
propagation. With this knowledge the tribes were no longer forced to live
as nomads and raid other tribes when the local supply of wild plants and
animals became depleted. They could establish stable abodes in moist
fertile areas, and through a division of labor obtain food sufficient not
only for those engaged in agriculture but also for other members of
the community otherwise employed.

Following the discovery of methods of plant propagation, primitive
man, by transporting and planting seeds and vegetative propagules,
could permanently occupy new areas. Without this knowledge the early
civilization that developed in the Nile Valley would have been impos-
sible, for Egyptian agriculture was accomplished with plants introduced
from Eurasia. In the United States today we are largely dependent upon
plants that were first domesticated in other parts of the world. The most
outstanding exceptions are our forest trees.

The evolution of cultivated plants has gradually progressed since pre-
historic times to the present. Today this progress is accelerated by a more

[Chap. XXXVI ORIGIN OF PLANTS USED BY MAN 431

rapid introduction of foreign species from all parts of the world, accom-
panied by intelligent control of plant breeding and a clear understanding
of the different types of variations that occur in plants.

The introduction of foreign species is merely an attempt to secure
plants with other heredities. The ancient caravan trade routes were
important means of distributing plants in the early days. Botanic gardens,
and more recently experiment stations, became centers of receiving,
testing, and distributing these foreign plants. Plant scientists at the U. S.
Department of Agriculture annually obtain numerous varieties of prob-
ably desirable plants and distribute them to appropriate testing stations
throughout the country. They have secured and tested a total of more
than 8000 varieties of wheat collected from more than 50 different
countries.

Plants are now critically selected with reference to a number of quali-
tative and quantitative characteristics, such as yield and quality of cer-
tain organs and tissues, chemical composition, relative development in
different soils and climates, and immunity to causal agents of disease.

New varieties obtained by control of plant breeding. Man no longer
depends entirely upon the selection of chance or fortuitous variations as
a means of securing new varieties of plants. He has learned how to
control plant breeding and obtain more desirable \arieties ( 1 ) by com-
bining the desirable heredities of two or more kinds of plants, (2) by
eliminating or preventing the expression of undesirable heredity in
otherwise desirable plants, and (3) by first eliminating undesirable
hereditary qualities in two or more kinds of plants and then combining
the desirable heredities.

Plant breeding is controlled by means of pollination. Plants, such as
wheat, peas, and beans, in which close pollination and self-fertilization
naturally occur, have to be manipulated differently than plants, such as
corn, in which open pollination and both cross-fertilization and self-
fertilization naturally occur. In open pollinated plants the pollen of the
stamens of a flower may come in contact with the stigma of the pistil of
the same flower, of flowers of the same plant, or of flowers of other plants.

The term close pollination is used here to refer to the fact that in some
species of plants pollination usually occurs before the flower buds open.
A very small amount of cross-pollination and cross-fertilization occurs in
these close pollinated plants. In some varieties of wheat it may be as
much as 4 per cent. Insects that break through the floral envelope may
become agents of cross-pollination in closed flowers.

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TEXTBOOK OF BOTANY

When the poHination of plants which are naturally open pollinated is
controlled so that only selfing can occur, several different kinds of
progeny are obtained from the same parent. That is, the plant does not
"breed true." It is a hybrid, and its mixed parentage becomes evident in
its progeny (Fig. 203). But if each individual in its progeny is selfed,
and if this process of selfing is repeated in all individuals for several
successive generations, several different kinds of plants are eventually ob-

FiG. 203. Hybrid segregation in corn. Photo by G. W. Blaydes.

tained, each of whose progeny appears uniform when growing in a
similar environment. In this way several inbred lines of plants are
obtained from the original hybrid parent.^ Further inbreeding results
only in a continuation of uniformity in the progeny of each inbred line.
Inbred plants are pure-line plants with respect to many of their char-
acters. In contrast to hybrids, pure-line plants continue to breed true.
Plants that naturally have only close pollination are natural inbred and
pure-line plants.

Pure-line plants of some species, such as those of corn, may be less
desirable than the hvbrid varieties obtained by cross-pollination because
they are smaller, less vigorous, or more susceptible to parasites, or have
fewer seeds and fruits than the hybrids. More desirable and uniform
hybrids may be obtained by restricting cross-pollination to certain se-

^ Probably more than 2000 inbred hnes of corn liave been obtained by this method,
and sexeral hundred of them have been used as a basis of securing better N-arieties of
hybrid corn. Some of the inbred hnes of corn are albinos and perish in the seedling stage.
Certain others also fail to grow to maturity and reproduce.

[Chap. XXXVI ORIGIN OF PLANTS USED BY MAN 433

lected pure-line plants ( Fig. 204 ) . Environment affects the development
of a pure-line plant just as it does that of any plant; but changing the
environment does not change its characteristic of breeding true to tvpe.
Compared with plant selection, the controlling of plant breeding is a
very modern occupation. Prehistoric man, by the introduction and culti-
vation of closelv related species and \'arieties of plants in the same

Fig. 204. Uniformity of height and maturity in hybrid corn of similar genetic com-
position. Photo by G. H. Stringfield, Ohio Agricultural Experiment Station.

locality, was unconsciously responsible for cross-pollination and the con-
sequent hybrid varieties that developed as a result of cross-fertilization.
On the other hand, the ancient practice of collecting pollen-bearing
branches from staminate trees of dates and tying them among the flowers
of pistillate trees to insure the development of fruit resulted in some
wholly unintended limitation of cross-pollination. In the wild state, the
staminate trees of dates are as abundant as the pistillate trees, and the
pollen is distributed by wind. More than 5000 varieties of dates are now
under cultivation.

Intelligent efforts to control plant breeding through pollination de-
pended upon the discovery of sex in plants and of the dependence of
sexual reproduction in seed plants upon pollination. The experiments of
Camerarius from 1691 to 1694, by which he showed that embryos do not
develop in seeds in the absence of pollination, were the first definite

434 TEXTBOOK OF BOTANY

proof of sex in plants." The experiments of several other investigators
during the 18th century were necessary to show that sex occurs in
all seed plants and that pollen is essential to the sexual process in
them.

The actual process of fertilization as the union of a sperm cell with an
egg cell was unknown prior to the middle of the 19th centurv, and was
first discovered in the algae. The union of the egg in the embrvo sac with
a sperm from the pollen tube was first noted in 1884. The fact that the
endosperm of seeds develops from the triple-fusion nucleus formed by
the union of one sperm from the pollen tube with the fusion nucleus of
the embryo sac was discovered in 1898. The first careful experimental
study of the hybridization of plants is accredited to Koelreuter, 1761-66.

Nevertheless, a considerable amount of controlled plant breeding was
accomplished by the method of trial and error previous to the 20th
century. Many new varieties of cultivated plants were obtained by
crossincr individuals of domesticated varieties with each other and

o

with wild species. Some new varieties were also obtained by crossing
wild plants with each other. The Concord grape, for instance, was ob-
tained by Ephraim Bull in Massachusetts by crossing two wild species.
This variety, which is now cultivated in most temperate regions of the
earth, was selected in 1853 from among 22,000 progeny of the two wild
parents.

Toward the close of the 19th century new varieties of domesticated
plants were being obtained by foreign introductions, by cross-pollination
and inbreeding, by mass selection, and by single-line selection.

As already indicated, a crop of cultivated plants is likely to be a
mixed population of several different varieties. In mass selection, seeds
from the more desirable plants in the field are collected and then planted
the following season. By this method the more undesirable varieties in
a mixed population are supposed to be eliminated and the more desirable
ones perpetuated. In a later chapter we shall see that the appearance of
an individual in the field may be verv deceptive with respect to the
kind of heredity it transmits to its progeny. In spite of this weakness of
mass selection it has been very effective in increasing the average yield
of many crops and is still considered a valuable practice.

The method of single-line selection has recently become preferable to

^ Camerarius was fortunate in choosing for experimental study plants in which par-
thenocarpy occurred, but in which parthenogenesis did not occur. Had he chosen dandelion
to study, his conclusions would have been different.

[Chap. XXXVI ORIGIN OF PLANTS USED BY MAN 435

mass selection in the attempted improvement of many cultivated plants.
This method may be illustrated by one example. David Fife of Canada
secured a small sample of hard spring wheat from a friend in Scotland.
When he planted the wheat in a small plot in Canada in the spring of
the year, he discovered that this new wheat was a winter wheat and
should have been planted in autumn. A single plant in the plot, however,
developed as spring wheat. Seeds from this single plant were saved and
planted. This one plant is the ancestor of all the Red Fife wheat now
cultivated in Canada and the United States.

Red Fife, in turn, when crossed with Hard Red Calcutta wheat from
Turkey, became one of the parents of the famous Marquis wheat, which
for 20 years was regarded as the king of hard red spring wheats. In time
it became surpassed by some of its more desirable progeny.

In contrast to corn, crops of close-pollinated plants, such as wheat,
oats, and barley, are composed of inbred lines. The story of the search
for new varieties of plants, as summarized in the 1936 and 1937 Year
Books of Agriculture, portravs modern methods of securing new varieties
of domesticated plants, and also the magnitude of this enterprise.

The work of botanists during the second half of the 19th century is
remarkable for the number of important facts discovered about plants.
In 1866 Gregor Johann Mendel, an abbot of Briuin, Austria, after eight
years of experiments formulated certain general principles about hybrid
variations in the garden pea that were later found to have general appli-
cation to both plants and animals. In 1900 the principles fonnulated by
Mendel and also his publication, which had been forgotten, were re-
discovered independently by de Vries of Holland, Correns of Germany,
and Tschennak of Austria.'* These principles have been thoroughly-
tested bv means of numerous experiments and they are now widely used
as a scientific basis for further investigation and interpretation of
heredity.

About the same time, de Vries began to emphasize the importance
of another type of heritable variation in plants that is not dependent upon
hybridization; it is known today as a mutation. Mutations had been
noted earlier and referred to by various names, but their importance had
not been recognized. The change of yellow to red sweet potatoes illus-
trated in Plate 4 is one example of a mutation. Another example on the

^ After eiglit vears of experiments with the progeny oljtained by cross- and self-
fertilization in the garden pea, Mendel published his data and conclusions in the Proceed-
ings of the Briinn Natural History Society in 1866.

436 TEXTBOOK OF BOTANY

same plate is the red-flowered hibiscus plant, some branches of which
regularly bear white flowers.

It thus became evident that three general types of variations should
be recognized in plants. There are, first, those variations that are the
direct results of the effect of environmental factors upon the conditions
and rates of processes within the plant, without anv changes in the
hereditary make-up of the plant or its progeny. We have referred to
many examples of this type of variation in previous chapters. Such
variations are non-heritable and are referred to as fluctuations. In con-
trast to fluctuations are the two types of heritable variations: the hybrid
variations resulting from cross-fertilization, and the mutations which are
changes in heredity not dependent upon cross-fertilization, though thev
may be increased by it. Their rate of occurrence is also influenced b\
factors in the external environment, as we shall see later. The scientific
recognition of the causes and consequences of these three different kinds
of variations is largely a product of the research during the present
century.

During all of this time, certain other botanists were interested in
studying the detailed structures of cells. The cytoplasm and nucleus
were clearly recognized and referred to as protoplasm about the
middle of the 19th century. During the latter half of the century the
chromosomes of the nucleus were seen, and their gross behavior
during cell division in both xegetative and reproductive cells was
recognized. During the present century it was discovered that nearlv
all of the known hereditar>^ factors are definitely associated with the
chromosomes.

While the 19th century is noted for the discovery of many important
facts about plants, the final verification of many of those facts and of
their several dependent relations was not accomplished until the present
century. To those interested in plant and animal breeding, all these dis-
coveries are of prime importance because thev are the basis of a new sci-
ence known as genetics. To us in general botany they will help explain
many of our everyday observations; and as we proceed to amplif\' and
clarify them in the next few chapters it will become evident that they
constitute a basis of fact essential to an>' critical analysis of ideas about
evolution, inheritance of acquired characters, relative importance of
heredity and environment, adaptation, natural selection, and related
phenomena.

Plate IV

Above (left): Result of mutation in vegetative cells in stem of red-flowered hibiscus.
Above (right): Pigments in different varieties of coleus. When the ordinary yellow green
coleus is selfed, % of the progeny are like the parent; the remainder are like the plant
pictured to the left of the yellow green plant. Beloic: Result of mutation in vegetative
cells of a yellow rooted variety of sweet potato.

[Chap. XXXVI ORIGIN OF PLANTS USED BY MAN 437

REFERENCES

DeCandolle, A. The Origin of Cultivated Plants. D. Appleton Co. 1902.
Fairchild, David. The World Was My Garden. Charles Scribner's Sons. 1938.
Freeman, G. F. A mechanical explanation of progressive changes in the pro-
portion of hard and soft kernels of wheat. Jour. Amer. Soc. Agronomy.

10:23-28. 1918.
Hawks, Ellison. Pioneers of Plant Study. Sheldon Press. London. 1928.
Kempton, J. H. Maize, the plant-breeding achievement of the North American

Indian. Old and New Plant Lore, Pt. VII. Smithsonian Scientific Series.

Vol. 11, pp. 319-349. 1934.
Kempton, |. H. Maize — our heritage from the Indian. Smithsonian Report,

pp. 385-408. 1937.
Merrill, E. D. Domesticated plants in relation to the diffusion of culture. Bot.

Rev. 4:1-20. 1938.
Merrill, E. D. Scuttling Atlantis and Mu. Amer. Scholar. 5:142-148. 1938.
U. S. D. A. Yearbook of Agriculture. 1936 and 1937.
Yanovsky, Elias. Food plants of the North American Indians. U. S. D. A. Misc.

Pub. No. 237:1-83. 1936.

CHAPTER XXXVII
HEREDITY IN PLANTS

Many hereditary resemblances and differences among plants were men-
tioned in previous chapters, but the conditions and processes of which
they are the results were not analyzed. In the study of plants and ani-
mals the terms "heredity," "inheritable," or "heritable" are used to indi-
cate that something inherent in the parent is transmitted to its progeny,
and influences, or conditions, individual development and behavior of
the progeny. The distinctive characters of species, of varieties, and of
either general or specific races reappear by development in succeeding
generations over long periods of time. The modern species of elms and
maples, for instance, still resemble their ancient ancestors that lived dur-
ing the Cretaceous period many millions of years ago. The human race
as a whole, or anv one of the specific races of man, likewise exemplifies
inheritance.

General Aspects of Heredity

The term hereditv is quite properly used in several ways : ( 1 ) to refer
to the resemblances of plants and animals to their progenitors, or ( 2 ) to
designate what is obtained in the fertilized egg from the gametes, or
(3) to indicate the whole range of biological processes underlying in-
heritance. The sequences of events in heredity that have been discovered
and shown by experiment to recur in successive generations are often
referred to as the laws of hereditv. Manv of the more fundamental laws
of heredity in plants and animals are similar, but the related processes
and structures involved in their operation may be quite different. Sev-
eral examples of conditions in plants not met with in the study of heredity
in animals are: the frequent occurrence of self-fertilization, the preva-
lence of vegetative multiplication, the differentiation of reproductive
tissue from vegetative (somatic) cells throughout the life of the plant,
the two-phase life cvcle, and the development of endosperms in some
seeds (Chapters XXXI, XXXIII, and XXXIV).

Hereditv is not limited to parental resemblances. Among the progen\^

438

[Chap. XXXVII HEREDITY IN PLANTS 439

of the same parents, or even of one plant, some of the individuals may
differ greatly from others. All these variants among the progeny that
develop in the same environment are the results of heredity, and they
are referred to as heritable variations. Hereditary resemblances and dis-
similarities reappear in succeeding generations. Maple leaves continue
to reappear in opposite arrangement on maple trees growing in very
different habitats; peach fruits develop from the flowers of peaches but
not from apple blossoms; and white oaks grow only from the acorns of
white oaks. Some of the progeny, however, may resemble a grandparent,
or even a remote ancestor, more closely than the immediate parent.

On the other hand, if the individuals of the progeny differ from each
other solely as a consequence of having developed in different kinds of
environments, these differences will not reappear in subsequent genera-
tions unless equivalent environmental conditions are repeated. These
differences are non-heritable, in contrast to those that are heritable. We
have already learned to refer to non-heritable variations as fluctuations.
They should also be recognized from now on as differences in degree of
expression of certain hereditary properties, or potentialities, of the plant.

The expression of hereditary potentialities during the development
of a plant is, of course, dependent upon external conditions. The po-
tentiality of producing chlorophyll, for instance, is not expressed in most
"green plants" growing in the dark. Many other examples may be re-
called. Every plant has a great many hereditary potentialities, some of
which are expressed more than others in a particular environment
(Chapter XXXI). What any plant does during its lifetime depends,
therefore, upon its environment as well as upon its heredity.

Now that we are familiar with the nature and causes of non-heritable
variations and with the sequences of processes and structures in sexual
reproduction and also in vegetative multiplication of plants, we may con-
sider some of the fundamental processes of heredity and heritable varia-
tions. What, for instance, is actually transmitted from parent to progeny?
Is it transmitted through cell division to all the cells of the roots, stems,
leaves, and other organs of the plant? May it be transmitted by any cell
of the plant? How is it transmitted? Is it a mysterious vital force, or is it
a material mechanism composed of discrete units of matter? How does
it increase and reproduce? What determines its constancy' from genera-
tion to generation over long periods of time? Is it alterable?

These questions will appear even more significant if one begins to
recall the numerous ways in which all the plants he knows differ from

440 TEXTBOOK OF BOTANY

one particular kind of plant. These thousands of differences are heritable
and are determined by something that is transmitted from parent to
offspring in each kind of plant. Then one may try to visualize some of
the thousands of successive events that must occur in approximately the
same manner and order every time an oak tree develops from an acorn.
These repeated similarities also are dependent upon something that is
transmitted from parent to offspring.

If we can first secure acceptable scientific answers to such questions
as those listed above, we shall then have a much surer background of
fact by which to explain why the progeny resembles the parents in some
respects and differs from them in others, and why these resemblances
and differences appear in the definite proportions first described bv
Mendel. The answers will also help us to understand why there are
fewer variations when plants propagate only by vegetative means, why
plants do not always "come true from seed," and why inbreeding does
not always result in less desirable plants.

In addition we may wish to apply our knowledge to a consideration
of questions of more general significance. What, for instance, is the ex-
planation of the occurrence of new kinds of plants in successive geologic
periods? How do new kinds of plants come to be? What are the causes
of evolution, and in what ways is it limited?

In this and the next three chapters you will find a discussion of some
of the more basic facts necessary to arrive at reasonable answers to the
questions suggested above. For lack of space many interesting facts will
have to be omitted. If the reader is interested in knowing more about
some particular phenomenon, the books listed at the end of the chapter
will be helpful as sources of data and further references.

Heredity in vegetative multiplication and cross-fertilization contrasted.
We may begin with the familiar fact that when a desirable plant is ob-
tained by means of cross-pollination it does not always reproduce true
from seeds, but it may multiply unaltered by vegetative propagation for
an indefinite period of time. In practice a large number of plants having
the same heredity may be obtained by means of continued vegetative
propagation, starting, of course, with cuttings all of which were taken
from the same plant. The cuttings, however, may be taken from the
roots, stems, leaves, or from any two or three of these organs. The indi-
viduals of a clone of plants obtained in this manner all have the same
heredity ( Chapter XXXV ) . This fact is evidence that all the cells of the

[Chap. XXXVII HEREDITY IN PLANTS 441

leaves, stems, and roots of a plant usually have the same hereditary
properties, or potentialities.

Properties, or potentialities, are merely the qualities or attributes of
material objects and of their special arrangements in organized systems,
or mechanisms. We may infer therefore that the individual cells of the
several vegetative organs of a plant have within them exacth' the same
structural mechanism of inheritance. Furthemiore, since cells multiph'
bv cell division (two cells being formed from one), this hereditary
mechanism must be exactly dupHcated by some means every time cells
divide.

This intrinsic mechanism by which the hereditary potentialities of the
cells of the vegetative organs of a plant are preserved throughout its
lifetime is surprisingly stable. In long-lived plants such as the redwood,
and in plants that propagate vegetatively this mechanism may be per-
petuated unaltered for centuries. Occasionally it becomes altered in
some of the branches of a plant. Stable alterations in this mechanism are
heritable, and are the means by which new kinds of plants are deri\'ed
from preexisting ones. Seedless oranges and the red-rooted sweet potato,
Plate 4, are examples of heritable variations that occurred in certain
l:)ranches of the plant. Heritable variations that occur in this manner are
known as mutations.

By cross-fertilization certain parts of the hereditary mechanism of the
two parent plants are brought together in the fertilized egg. That is, the
fertilized egg contains the hereditary mechanism that was in the sperm
and also the one that was in the egg. The plant that dexelops from this
fertilized egg thus possesses a hereditary mechanism partly like and
partlv unlike that of either parent. In certain characters it may resemble
one or the other parent, while in other characters it may differ from
both parents. Such individuals are called hybrids. When hybrids repro-
duce by seeds, their progeny usually are variable in appearance.

For example, when cross-fertilization occurs between red-flowered
and white-flowered plants of snapdragon, the resultant progeny are
hybrids and all of them are pink-flowered. This difference between
parents and progeny is a hereditary difference. Cross-fertilization be-
tween the pink-flowered plants, or self-fertilization within any one of
them, results in progeny in which approximately one-fourth of the
plants are red-flowered like the original red-flowered plant, two-fourths
are pink-flowered like the immediate hybrid parent, and one-fourth are
white-flowered (Fig. 205).*

442

TEXTBOOK OF BOTANY

FLOWERS

OF TWO

PARENTS

FLOWERS ON ALL
HYBRID PLANTS
F, GENERATION

r ^

FLOWERS OF HYBRID PROGENY: F^ GENERATION

Fig. 205. A diagrammatic representation of the color of flowers on plants of
snapdragon of the F^ and F2 generations following a cross between a red-Howered
and a white-flowered variety.

If cross-fertilization occurs between the red-flowered and pink-flow-
ered plants, one-half of the progeny are red-flowered, the others are pink-
flowered. If cross-fertilization occurs between the white-flowered and
pink-flowered plants, one-half of the progeny are white-flowered, the
others are pink-flowered.

Self-fertilization in the red-flowered plants results only in red-flowered
plants. Consequently all the red-flowered plants of this species of snap-
dragon are said to be a pure line with respect to flower color. Note,
however, that our attention is only on flower color. The plants may or
may not be a pure line with respect to certain other characters such as
height, shape of leaf, and immunity to rust. Self-fertilization in the white-
flowered plants results in only white-flowered plants. They constitute a
pure line of white-flowered plants. The pink-flowered plants are hybrids
with respect to flower color, but they may be pure-line plants with re-
spect to other characters that we are ignoring at the present.

Red- and white-colored flowers are hereditary traits or characters in
snapdragon, but the flowers and the pigments are not actually inherited.
They develop as a result of something that is inherited. Anything that is
directly inherited must be in the egg or in the sperm, since these two

[Chap. XXXVII HEREDITY IN PLANTS 443

gametes by their union form the fertiHzed egg, which is the first cell of
each plant of the new generation. The gametes may be said to constitute
the bridge by which the hereditary factors pass from one generation to
the next one.

Since the microspore is the forerunner of the pollen, and the mega-
spore is the forerunner of the embryo sac (Figs. 165 and 166), these
spores also constitute another single-celled bridge through which the
hereditary factors must pass during the life cycle of a seed plant.

What is transmitted from parent to progeny? Hereditary potentialities
are merely the properties of certain units of matter. Hence certain units
of matter that pass from one generation to another in the sperm and egg
must be responsible for the hereditary resemblances and differences that
may be seen in plants. Any stable alterations of these units of matter
that change their properties would result in the development of heritable
variations, known as mutations. The differences in grouping of these units
of matter that occur in sexual reproduction account for the kinds of
hybrid variations exemplified by the flowers of snapdragon described
above.

We may now consider the question whether there are anv visible
units of matter in cells that possess all the qualities necessary to account
for organic inheritance and also for heritable variations. From what has
already been said, it is evident that such units of matter must possess
certain definite qualities : namely, ( 1 ) they must be small enough that
several thousand of them may occur in a single cell, ( 2 ) their chemical
composition and organization must be sufficiently stable to account for
the fact that the distinctive characteristics of species and races of plants
have reappeared bv development in numerous succeeding generations
throughout centuries of time, ( 3 ) thev must reproduce during each cell
division in every growing region of the plant without losing their indi-
viduality, and ( 4 ) they must survive slight alterations in composition or
internal arrangement that account for occasional changes in their prop-
erties, or potentialities.

Microscopic studies of cells for more than half a century have shown
that chromosomes may fulfill the last three of these conditions, but they
are too large and too few in number in the cells to fulfill the first condi-
tion. The cells of the common horsetail {Eqtiisetum arvense) have 272
chromosomes, the largest number known in plant cells. In some of the
fungi there are but 4 chromosomes per cell. The cells of some seed plants
have as few as 6. The majority of plants probably have less than 30

444

TEXTBOOK OF BOTANY

chromosomes per cell. Each vegetative cell of the plant, however, con-
tains all the hereditary potentialities of any other vegetative cell. It is
necessary to find, or to infer, smaller hereditary units of matter such as
the genes, which consist of one or, at most, of only a few molecules.
Numerous small bodies of matter are visible in the chromosomes, but
no one is certain that he has ever seen a gene; hence for the present
they are inferred units of matter. The best-known hereditary potentiali-
ties are dependent upon genes located in the chromosomes which in
turn are located in the nucleus of the cell.^

The genes are an-anged in a definite series from one end of the
chromosome to the other. The number of genes in a chromosome ma\' be
very large, perhaps a few thousand in some. The egg and the sperm of
corn each contains 10 chromosomes. The geneticists have alreadv recog-
nized hereditary potentialities of more than 350 pairs of corn genes all
of which are located in these 20 chromosomes. Moreover, thev can tell
us which genes are in the different chromosomes and the approximate
location of many of the genes within the chromosome. The total number
of genes in these 20 chromosomes of com is unknown, but there are
probably many thousand.

Chromosomes are known to reproduce by dividing ( splitting ) length-
wise into two identical halves (Fig. 206). Hence when a chromosome

f ' 1

€>

^<^

V '•' J

f#?

~\

^:^

Fig. 206. A diagrammatic representation of certain stages of the duplication of
chromosomes by longitudinal division, or "splitting," during the division of
vegetative cells.

divides, each gene within it must also become duplicated by some means.
Plastids in the cytoplasm of plant cells ma\^ also pass bodih' from the
dividing cell to the two new cells formed. Thev are thus bodilv in-
herited and continue to multiply in the newly formed cells by simple
division. Any properties they possess are thus transmitted with them.
Plastids in the egg become the first plastids in the fertilized egg. Not
much is known about cytoplasmic inheritance or about the influence of

^ Exceptions found in the cells of bacteria and blue-green algae will be noted in later
chapters.

[Chap. XXXVn HEREDITY IN PLANTS 445

cytoplasm upon heredity. It is generally inferred, however, that cyto-
plasm contributes much less than the nucleus and is involved mainly in
the physiological effects of the genes upon cell processes.

It is important to remember that the whole hereditary mechanism of a
plant is in its protoplasm, and that at one stage in the life cycle of a plant
this mechanism is all contained in the fertilized egg. Differences in
heredity must be the result of differences in this mechanism.

We have become accustomed to thinking of protoplasm as a living sys-
tem composed of molecules of many kinds of substances which by them-
selves do not have the property of aliveness. It now becomes necessary
to recognize chromosomes and genes as distinct components of this liv-
ing system. In some way not clearly understood, they influence various
cell processes and thus impart certain properties to the cells which be-
come evident as the plant develops. If some of these components of
protoplasm are removed, the living system may survive, but its proper-
ties will be different. Even a change in the arrangement of the genes
within a chromosome may be the cause of a change in the development
of a plant. The relative location of the genes within a chromosome seems
to be fairly constant.

The exact manner in which a gene may influence the development of
a plant is as far from being understood today as is the manner in which
small amounts of vitamins and hormones influence the development of
an organism. All these compounds influence certain complicated chemi-
cal processes within the cells.

Later on we shall see that certain reproductive cells (spores, sperms,
eggs) and also the cells of one whole phase of the life cycle of plants
survive and grow with only one-half the number of chromosomes that
are present in the cells of the other phase of the life cycle.

Heredity and Chromosome Behavior

Since each chromosome contains many genes and accordingly many
hereditary potentialities, a knowledge of the behavior of chromosomes
during the life cycle of a plant is essential to the understanding of the
processes and consequences of heredity. Every cell that contains a par-
ticular chromosome also contains all its genes and their potentialities.
If a chromosome becomes fragmented and a part of it is lost, then certain
of its genes and potentialities are also lost. For the present we shall con-
sider onlv the usual behavior of chromosomes. In a later chapter we shall

446

TEXTBOOK OF BOTANY

L.LAtiPE.

Fig. 207. A diagrammatic representation of chromosome behavior during vegeta-
tive cell division. Cytoplasm omitted.

A, non-dividing cell in which the thread-like structures ( chromonemata ) of
the chromosomes appear as a network (reticulum) in the colloidal medium of the
nucleus. B, beginning of nuclear division is evident. Some of the small connecting
strands of the reticulum have disappeared and the chromosomes are becoming
individually distinct. C, the chromosomes appear double because the longitudinal
division, or "splitting," of the chromosomes has already begun. D, a spindle of
"fibers" begins to form from the nucleus, and the nuclear membrane begins to
disappear. E, the chromosomes become arranged in a circle in the equatorial plane
of the spindle. F-G, each chromosome becomes completely divided longitudinally
into identical halves, or daughter chromosomes, which separate and migrate to
opposite poles of the cell. H, chromosomes at the poles where they become
reticulate and surrounded by a nuclear membrane. A cell wall develops between
the daughter nuclei. I, the two new cells, identical with each other and with the
parent cell in chromosome complement.

Names that are often applied to different stages of cell division are: prophase,
B-D; metaphase, E; anaphase, F-G; telophase, H-I.

[Chap. XXXVII HEREDITY IN PLANTS 447

consider certain irregularities that are known to occur, and also the way
in which they influence the development of plants.

Cell division in vegetative cells: mitosis. In Fig. 207 certain stages in
the behavior of chromosomes in each cell division that occurs in all
growing regions of plants are depicted. It should be easy to visualize the
continuous behavior of chromosomes between the stages depicted. The

Fig. 208. Cell division and enlargement in a root tip of onion. A series of stages
in the behavior of chromosomes during vegetative cell division is represented
and labeled a to g. Courtesy of World Book Co.

whole process may be completed within tliirty minutes or it may extend
over a period of several hours. Cells in various stages of division may be
seen in microscopic sections cut from some growing region of the plant
( Fig. 208 ) . Many facts about vegetative cell division ( "somatic mitosis" )
are known; but for the present purpose of seeing the relation of chromo-
some behavior to the transmission of hereditary factors, it is necessary
only to study the diagrams carefully. Then it will be seen that every
time vegetative cells divide, each chromosome of a cell splits ( divides )

448 TEXTBOOK OF BOTANY

longitudinally into two identical halves, or daughter chromosomes, which
separate and migrate to opposite poles of the cell, resulting finally in
two new cells identical with each other and with the parent cell in
chromosome number and composition (chromosome complement). In
other words, the important things to visualize as cells divide in the
growing regions of a plant are ( 1 ) the exact duplication of each chromo-
some and of its included genes, and (2) the definite manner of the
chromosome migration which results in the same chromosome comple-
ment in all the vegetative cells of the plant. Consequently all cells of
the vegetative tissues of a plant have the same chromosome comple-
ment and hereditary potentialities that were present in the fertilized egg
from which the plant developed.

These facts help us to understand why heredity remains unaltered
when plants multiply vegetatively in nature or in cultivation. The cells
of cuttings, for instance, all have the same chromosome complement,
and it continues to be the same in the separate individuals that develop
from the cuttings. If the chromosomes do not behave regularly as de-
scribed above, mutations may occur in the vegetative tissues of the plant.
For example, the occasional white branch that is found in plants must
be the result of something that happened either in the plastids or in the
part of a chromosome containing the genes that condition some of the
processes involved in the synthesis of chlorophyll.

Since all the cells of the vegetative parts of a plant ha\^e the same
chromosome complement and the same hereditary potentialities, one
may well wonder why all the cells of the plant are not exactly alike in
appearance. Their differentiation into the various tissues, such as epi-
dermis, cambium, xylem, and phloem, must be dependent partly upon
the influence of neighboring cells and partly upon an inheritance of
tissue patterns about which little is known at present. Furthermore,
certain hereditary potentialities are not expressed unless the plants at-
tain certain stages of development. Hereditary potentialities that influ-
ence flower color and form, for instance, are expressed only when flowers
develop; and potentialities that affect the color of endosperm of seeds
are expressed only when the endosperm tissue de\ elops from the triple-
fusion nucleus.

Reduction division: meiosis. Cell division in the growing regions of
plants is often referred to as ordinary cell division in contrast to a
notable exception known as reduction division, or meiosis, which occurs
in the life cycle of a seed plant only when the microspore mother cells

[Chap. XXXVII HEREDITY IN PLANTS 449

and the megaspore mother cells divide. This particular cell division is
called reduction division because the two daughter cells formed have
only one-half as many chromosomes as the mother cell. The cell and
nucleus divide, but the chromosomes do not "split." This very unique
cell division plays an equally unique part in the transmission of heredi-
tary factors. It accounts for the principle of heredity known as the
"purity of gametes" or the "law of segregation of hereditary factors" with
which we shall be concerned presently.

Some of the features of reduction division are depicted in Fig. 209.
Obviously the facts depicted here were difficult to discover. Fortunately
certain plants and animals have only a few chromosomes in each cell,
and their chromosomes differ sufficiently in form that they may be recog-
nized readily and watched during cell division. Here again many de-
tailed facts are known; but for our present purposes it will be sufficient
if we clearly understand and apply certain basic facts. In contrast to
ordinary cell division, the chromosomes during reduction division do not
separate longitudinally but assemble at the center of the spore mother
cell in pairs, each paternal chromosome pairing with its homologous
maternal chromosome. Later the mates of each pair of chromosomes
separate and migrate to opposite poles of the cell, resulting finally in
two new cells, each having only one-half as many chromosomes as the
spore mother cell.

Usually these two new cells divide immediately by ordinary mitosis,
forming a tetrad of spores: a tetrad of microspores if the mother cell was
a microsporocyte in the anther; a tetrad of megaspores if the mother
cell was a megasporocyte in the ovule.

Chromosomes in the life cycle of a seed plant. Since the microspore is
the forerunner of the pollen grain, and since the generative nucleus in
the pollen grain is the forerunner of the two sperms in the pollen tube,
the chromosome complement of these two speiTns is identical, and it is
also identical with that of the microspore from which the pollen grain
developed. Hence in solving problems in heredity that involve the trans-
mission of chromosomes by spenus, we can predict the chromosome com-
plement of the spenns from that of the microspores.

Similarly a megaspore is the forerunner of an embryo sac; hence the
chromosome complement of each of the eight nuclei in the embryo sac
is the same, and it is identical with that of the megaspore. The fusion
nucleus formed by the union of two of these nuclei has a double set of

450

TEXTBOOK OF BOTANY

Fig. 209. A diagrammatic representation of chromosome behavior during reduc-
tion division and the formation of microspores and megaspores.

I, reduction division ending in cells represented by J or K in which the nuclei
contain but one-half as many chromosomes as cell C. The early stages in reduction
division are essentially like those of vegetative cell division illustrated in A and B
in Fig. 207. Hence we may begin with C above, a spore mother cell in which the
chromosomes have become distinct; D-H, the chromosomes assemble at the
equator of the spindle in pairs, each paternal chromosome pairing with its
homologous maternal chiomosome; I, the mates of each pair of homologous
chromosomes separate and begin to migrate to opposite poles of the spindle; J,
chromosomes at the poles of the cell. Each daughter nucleus contains but one-half
as many chromosomes as the parent nucleus in cell C.

II, K-N, the two cells formed by reduction division usually divide immediately
by ordinary cell division resulting in a tetrad of spores ( microspores or megaspores ) .
The longitudinal splitting of the chromosomes seen in the division of each of these
two cells usually begins before reduction division is completed as shown by the
doubleness of the chromosomes in E-K. Drawn by Lois Lampe.

these chromosomes. When it unites with a sperm the resulting triple-
fusion nucleus has three sets of chromosomes.

If we represent the number of chromosomes in the megaspore and in
each of the eight nuclei of the embryo sac bv n, then the number of
chromosomes in the fusion nucleus is 2n; but in the triple-fusion nucleus
and in every cell of the endospenn of the seed it is 3n. Since the fer-
tilized egg has a set of chromosomes from the egg and another from the

[Chap. XXXVII HEREDITY IN PLANTS 451

sperm, it has the 2n number of chromosomes. Starting then with the
f ertihzed egg, vegetative cell division ( mitosis ) results in an exact dupli-
cation of tlie 2n number of chromosomes in every cell of the embryo,
root, stem, leaf, sepal, petal, stamen, anther (including microsporo-
cytes ) , pistil, and ovule ( including the megasporocyte ) . Thus the cycle
is completed. Reduction division in the microsporocyte and megasporo-
cyte initiates another cycle.

Unless some irregularity occurs, this chromosome cycle is repeated
annually in all plants that reproduce once each season. The 2n, or dip-
loid, number of chromosomes occurs in all cells of the plant with two
exceptions: (1) the microspores, pollen grains, pollen tubes, mega-
spores, and embryo sacs ( previous to nuclear fusions ) in all of which the
nuclei have the n, or monoploid,^ number of chromosomes; and (2) the
cells of the endosperms of seeds which have the Sn, or triploid, number
of chromosomes.^ Irregularities in the occurrence of the n and 2n num-
ber of chromosomes are described in Chapter XXXIX.

The pairing and segregation of chromosomes during reduction division.
The pairing and subsequent distribution (separation and segregation)
of chromosomes during reduction division are so important in the trans-
mission of hereditary factors in sexual reproduction that they are singled
out for further emphasis. The two chromosomes of each pair on the
spindle of the cell are often referred to as homologs and as synaptic
mates, and the pairing is called synapsis. One of these two chromosomes
came from the sperm of the pollen parent, the other from the egg of the
ovule parent. They are usually similar in appearance and contain many
genes that influence the development of the same kinds of traits such as
flower color and height.

These homologous chromosomes are all present in the fertilized egg,
in each cell of the vegetative tissues of the plant, and in every micro-
sporocyte and megasporocyte. They become segregated in separate cells
following their migration to the opposite poles of the cell spindle during
reduction division, and do not get together in the same cell again until
fertilization occurs. Hence each microspore and sperm, and each mega-
spore and egg contains only one of the homologs of each pair.

The pairing of chromosomes is remarkably constant. That is, the
same homologs pair during every reduction division in a plant, and in

2 Also called haploid by many authors.

^ In those plants in which the fusion nucleus is formed by the union of more than two
nuclei, the number of chromosomes is correspondingly increased in it and in the subsequent
triple-fusion nucleus and endosperm cells.

452

TEXTBOOK OF BOTANY

A

B

each subsequent generation. They seldom change mates. Irregularities
in their mating usually result in recognizable mutations. In hybrids re-
sulting from cross-fertilization of remotely related plants, the chromo-
somes brought together in the same cells may never have occurred to-
gether before. Sometimes they fail to mate or to mate regularly during

reduction division, and very unu-
sual sorts of progeny may be ob-
tained, or the plant may fail to re-
produce at all.

While the same chromosomes
usually pair in all microsporocytes
and in all megasporocytes of each
generation of a plant, their orienta-
tion in the cell with respect to the
poles of the spindle is a matter of
chance. The two possible chances
of orientation when only two pairs
of unlike chromosomes are present
are illustrated in Fig. 210. Evidently
all the paternal chromosomes may
go to one pole of the cell and all
the maternal chromosomes to the
other pole; or some maternal and
some paternal chromosomes may go
to the same pole.

This chance or random orienta-
tion of pairs of unlike chromosomes
during reduction division becomes
more interesting as the number of
pairs of unlike chromosomes that
are considered is increased. Thus if
the megasporocyte or microsporo-
cyte has three pairs of unlike
chromosomes, there are four different ways in which the pairs may be-
come oriented on the spindle, and consequently eight different kinds of
megaspores or microspores with respect to chromosome complement
may be formed (Fig. 211). On the other hand, if there are only one pair

* The term "like chromosomes" is somewhat of a misnomer in that the chromosomes are
usually not alike with respect to all genes, but are alike with respect to those genes which
are being considered.

< J

\

^

CD

•

J

>

1

CD

•

CD

-J

--it}-

V J

^

'

f

^

v

•

C3
J

\

,

•

CD

/

CD

J

c

Fig. 210. a diagrammatic representa-
tion of the behavior of chromosomes
during the formation of microspores
from microsporocytes, or of megaspores
from megasporocytes in a plant having
two pairs of unlike chromosomes.

The sperms formed in the pollen
from this plant have the same chromo-
some complement as the microspores
from which the pollen grains develop.
Likewise, the eggs have the same
chromosome complement as the mega-
spores from which the embryo sacs
develop.

HEREDITY IN PLANTS

453

[Chap. XXXVII

of like chromosomes^ and two pairs of unlike chromosomes, there are
only two different ways in which the chromosomes may be oriented, and
four different kinds of spores, eggs, or sperms may be formed. If there
are two pairs of like chromosomes and only one pair of unlike chromo-
somes, there is but one possible orientation and two kinds of spores,
sperms, or eggs. If there are 24 pairs of unlike chromosomes, there are

B

^^

C

o

^^.

•.

CD

•.

■■■o»-'

OR

\

1

O

•

•

J

\

t

' o

•

•

/

( --'»■■■■■ 1

I •o- J

OR

\

1

•

O

\

'

•

o

\

o

/

\

1

•

o

)

1

'

•

o

Fig. 211. a diagrammatic representation of the behavior of chromosomes during
the formation of microspores from microsporocytes, or of megaspores from mega-
sporocytes in a plant having three pairs of unhke chromosomes.

What will be the chromosome complement of the sperms and eggs formed in
this plant?

8,388,608 different ways in which the chromosomes may become ori-
ented, and a possible 16,777,216 different kinds of spores, sperms, or
eggs. The number of different kinds of microspores or megaspores and
consequently of different kinds of sperms or eggs that can be produced
by a plant may be represented by 2", where n is the number of pairs of
unlike chromosomes in the cells of the spore-bearing plant.

Chromosomes in cross-fertilization. Let us see if the facts we have
learned about hereditary factors and chromosomes in relation to the life
history of a plant are sufficient to explain the relative numbers of the
different kinds of plants obtained by crossing and selling in snapdragon,
as described earlier in this chapter (Fig. 205). One of the eight chromo-
somes in the gametes of the red-flowered plant contained at least one
gene with the potentiality of conditioning the formation of red pigment

454 TEXTBOOK OF BOTANY

in the flower; and its homolog, or synaptic mate, in the gametes of the
white-flowered plant lacked this gene or contained a corresponding gene
that lacked this particular potentiality. The zygotes formed by cross-
fertilization contained both of these chromosomes and the plants that
grew from them were pink-flowered.

We may for the present ignore all the other genes in these two
chromosomes and also all the other chromosomes in these two kinds of
plants, and fix our attention on these two chromosomes through two
complete life cycles of snapdragon. This we may do by means of a
diagram. We shall use the letter R as a symbol for the gene, or the po-
tentiality, in the chromosome that conditions the formation of red pig-
ment in the flower; and the letter r to indicate the potentiality in the
homologous chromosome that conditions colorless flowers (Fig. 212).'^

Owing to the behavior of chromosomes in reduction division, two
kinds of sperms and two kinds of eggs with respect to flower color will
be formed in the pink-flowered hybrid as shown in the diagram. The
plants of the F2 generation that obtain the chromosome containing the
gene represented by R from both sperm and egg have red flowers. If
they obtain this chromosome only from the egg or from the sperm and
obtain its synaptic mate from the other gamete, the flowers are pink as
in the Fi generation. If the chromosomes from both sperm and egg lack
the gene represented by R, the flowers are white. The different kinds of
sperms and eggs unite at random. That is, their union is influenced not
by the kinds of chromosomes and hereditary potentialities they contain,
but by the chance of a sperm coming in contact with the egg.^ Hence, if
a large number of them unite, the progeny ( F2 generation ) would con-
sist of plants of which approximately 1/4 are red-flowered, 2/4 pink-
flowered, and 1/4 white-flowered. Both the red- and the white-flowered
plants, if selfed, would continue to breed true.

In order to check some of the inferences one might draw from this
example, it will be necessary to consider another example of cross-fertili-
zation in which one of the chromosomes in the sperm from a tall, smooth
plant has a gene (T) that conditions height of plant and another gene
(S) that conditions smooth epidermis; while one of the chromosomes in

^ In preference to letters, small dots may be used to represent the genes in the chromo-
somes.

" In seed plants the kind of sperm that may come in contact with an egg depends upon
the kinds of pollen grains that become attached to the pistil, and upon the subsequent
growth of pollen tubes.

[Chap. XXXVII

HEREDITY IN PLANTS

45.'

One of the chromosomes

in a sperm from
the red -flowered plant

One of the chromosomes

in an egg from

the white -flowered plant

generation

Two of the chromosomes in

the fertilized egg and

hence in the vegetative

cells, microsporocytes, and

megasporocytes of the

plant that develops from

this fertilized egg

r

Possible kinds of sperms and eggs with respect to
these two chromosomes that can be produced in this plant

A

Sperms

and

Eggs

Possible chances of

fertili:ation if only

self-fertiliiation occurs

Possible kinds of

chromosome complements

in the fertilized eggs

and subsequent cells of

the various plants of

the F2 generation

Fig. 212. A diagrammatic representation of the relation of chromosomes and
factors of flower color in snapdragon.

the egg from a dwarf hairy plant has a gene (d) that conditions height
and another gene (h) that conditions growth of hairs on the epidermis.
If we use the suggested letters as symbols for the genes that condition
the development of these characters, we may represent the distribution
of these two kinds of chromosomes in the plants of the Fi and F2 genera-
tions by a diagram (Fig. 213).

All the plants of the Fi generation were tall and smooth like one of the
parents, showing that the genes that condition the development of tall,

456

TEXTBOOK OF BOTANY

One chromosome in a sperm from
the tall smooth plant

One chromosome in an egg from
the dwarf hairy plant

Chromosome complement in the Fj generation

Chromosomes in the gametes of the Fi generation
Sperms and Eggs

T

S

h

Chromosome complements in the

various plants of the

F2 generation

Fig. 213. A diagrammatic representation of the transmission of two hereditary
factors located in the same chromosome.

smooth plants completely dominated the effects of the genes that condi-
tion dwarfness and hairiness.' In the F2 generation 1/4 of the plants
were pure-line tall, smooth plants; 2/4 were hybrid tall, smooth plants;
and 1/4 were pure-hne dwarf, hairy plants. All the genes in one chromo-
some go with the chromosome. They appear, therefore, to be linked
together. In this example all the dominant genes considered are in the
same chromosome, but this condition should not be construed to be the
general rule. The reader may be interested in solving a similar problem
when the genes represented by T and h are in one chromosome, and
those represented by d and S are in its synaptic mate.^

" Because of this fact we followed the convention of capitalizing the symbols for the
dominant genes, hut not those for the recessive genes. As symbols for the genes condition-
ing height we might also have chosen T and t, or D and d. The latter is preferred by many
geneticists.

* It is important to remember that chromosomes and genes are in the cells of the plant,
and that the letters used as symbols for the genes are only on the printed page. Progress
in understanding is made only when there is an effort to \isualize the chromosomes and
genes. Moreo\er, when each factor in which one is interested is in a different chromosome,
as in Fig. 212, the factors may be represented in diagrams as letters without indicating
their relation to chromosomes. But when Uvo or more of these factors are in the same
chromosome, the fact must be indicated in some way, as suggested in Fig. 213. That is,
the letter symbols for the genes in any one chromosome should be kept together in the
diagrams.

[Chap. XXXVII HEREDITY IN PLANTS 457

In the chapter on fat and protein synthesis one other amazing fact
about chromosomes and genes was mentioned. Since the properties of
chromosomes and genes remain constant through bilhons of cell di-
visions in each plant and from generation to generation, their chemical
composition and internal organization must also remain constant. They
are composed largely of protein substances. Since the chromosomes and
genes are duplicated in number every time cells divide, the maintenance
of their size must depend upon growth by the synthesis of new chromo-
somal substances from foods in the cell. The constancy of chromosomal
synthesis peculiar to each species of plant and animal, and the constancy
of chromosome behavior in cell division constitute an important part of
the hereditary mechanism of cells. The persistence of species and races
is dependent upon this constancy. A high degree of constancy accom-
panied by some irregularity results in a gradual evolution of varieties
and of species of plants.

REFERENCES

Schrader, F. The present status of mitosis. Biological Symposia. Vol. 1, pp.

87-95. The Jacques Cattail Press. 1940.
Sharp, L. W. Introduction to Cytology. McCraw-Hill Book Company, Inc.

1934.
Sinnott, E. W., and L. C. Dunn. Principles of Genetics. 3rd ed. McGraw-Hill

Book Company, Inc. 1939.
Snyder, L. H. The Principles of Heredity. 2nd ed. D. C. Heath and Company.

1940.

CHAPTER XXXVIII
CROSS-FERTILIZATION AND HYBRID SEGREGATION

<^0<><X><J><><>C><£><><^OC><c>^^

From the facts presented in the preceding chapter it must be evident
that heredity is definitely related to several basic processes in cell divi-
sion. The characters we see developing in a plant as it grows are condi-
tioned by the hereditary factors that are transmitted from one generation
to the next in the sperms and eggs. Many of these hereditary factors
we now know to be the genes located in the different chromosomes. We
have seen how the chromosomes with their particular hereditary factors
assemble in pairs and then become separated during reduction division,
and are still separated in the spenns and eggs; and how they are brought
together again in different combinations in the fertilized egg.

As a consequence of this definite pairing, segregation, and recombina-
tion of chromosomes during sexual reproduction, the progeny of a hybrid
plant are not all alike. When our attention is centered upon only one
character, such as height of plant or flower color, the different kinds of
hybrid progeny may appear in simple proportions such as 1:2:1, 3:1,
and 1:1 as described in the preceding chapter. Since Mendel was the
first to discover these definite ratios of the different progeny of hybrids
they are often referred to as Mendelian ratios.

Owing to the fact that during reduction division the orientation of
the pairs of chromosomes on the spindle of the dividing cell occurs at
random, and the subsequent union of the different kinds of sperms and
eggs also occurs at random, these ideal mathematical ratios are seldom
olDtained in experiments. One seldom obtains an exact 1:1 ratio of heads
and tails when a penny is tossed several times, or an exact 1:2:1 ratio
when two penines are tossed simultaneously a certain number of times.
The ideal ratio, 5:5, is occasionally obtained when a penny is tossed 10
times, but sometimes deviations as great as 9:1 or 1:9 occur. As the
number of tosses is increased, the magnitude of deviation from the ideal
ratio decreases.

In the study of heredity we must recognize the role of chance during
reduction division and fertilization. Many other processes in chromo-

458

[Chap. XXXVIII HYBRID SEGREGATION 459

somes and heredity, such as the dupHcation of chromosomes and genes
in ordinary cell division and pairing of synaptic mates in reduction
division, are much too definite to be regarded as the results of chance.

The basic facts of segregation and recombination of hereditary factors
as distinct units in the sperms and eggs, and the consequent ratios of
hybrid progeny were discovered by Mendel before chromosomes had
been clearly recognized. The subsequent discovery of chromosomes and
of their definite behavior, however, furnished a basis of fact that enabled
biologists of this century to explain the principles of hybrid segregation
formulated by Mendel, and to extend and clarify our knowledge of
hvbridization. It was difficult, for instance, to understand why certain
factors seemed to be "linked together" until it was discovered that they
were in the same chromosome, and would be present wherever that
particular chromosome was present. Two or more factors that always
occur together are referred to today as linkage groups. All the factors in
a single chromosome constitute one linkage group. Thus through a slow
accumulation of facts and the discovery of their dependent relations,
the subject of hvbridization has been removed from the realm of mystery
and speculation and has become an understandable science.

Mendel's experiments. Mendel investigated the results of cross-fertili-
zation in several kinds of plants, and also in mice and bees. From several
seedsmen he obtained seeds of more than thirty varieties of peas, mostly
of one species ( Pisum sativum ) , and planted them in a small garden at
his monastery. He noted that certain distinctive characters of each va-
riety remained constant from year to year. Since some of these characters
were easy to detect, he decided that peas would be admirable plants
with which to test experimentally the various ideas about hybrids that
were current at the time. His success in discovering fundamental laws
of heredity when all others had failed depended in part on his choice of
suitable experimental material, and in part on his persistence in keeping
exact records of all the progeny of his plants through a series of several
generations.

To appreciate Mendel's experiments one should imagine him at work
in his garden among peas some of which were slightly taller than he was;
others were dwarfs less than knee-high. Some were white flowered;
others were violet-red flowered. On some plants the flowers and pods
developed only in terminal clusters at the top of the stem; on others
they appeared in the axils of the leaves along the stem. The pods on
some plants were yellow; on others they were green. Some pods were

460 TEXTBOOK OF BOTANY

inflated; others were constricted between the seeds. The seeds within
the pods were either all green or all yellow, depending upon the pig-
ments in the cotyledons of the embryo. In some pods the seeds were
rough; in others they were smooth.

Peas are naturally close-pollinated, and unless they have been arti-
ficially cross-pollinated they may be considered pure-line plants with
respect to all of their characters. After studying his plants carefully for
two years, Mendel became assured that each of the fourteen varieties
named above bred true to type, but he continued to test them for purity
throughout the period of experimentation.

By means of cross-pollination Mendel began to secure hybrids among
these plants. We can imagine him with a pair of forceps removing the
young anthers from some of the flowers, placing pollen upon the stigmas
by hand, and carefully labeling each one with a description of the ovule
and pollen parent. Later he collected the ripened pods and seeds, kept
them separate, and planted the seeds the following season.

During this second season he kept a record of the appearance of all
the hybrid plants, i.e., the Fi generation. No matter which way the
crosses were made between any two of the pure-line plants described
above, the influence of certain hereditary factors was found to dominate
the influence of others completelv. In these peas the factors that severall)'
conditioned the development of tall plants, red flowers, axillary flowers,
green pods, inflated pods, smooth seeds, and yellow seeds completely
dominated the effect of the factors that conditioned the contrasting
characters of dwarfness, white flowers, terminal flowers, yellow pods,
constricted pods, rough seeds, and green seeds. Mendel, however, was
not the first person to note the complete dominance of some factors in
hybrids, and the lack of complete dominance of others.

When the Fi plants bloomed during this second season, Mendel al-
lowed them to self-pollinate naturally. Again he kept careful records,
and planted the seeds the following season. From the embryos of these
seeds the mature plants of the F2 generation developed. During this
third season he obtained an accurate record of more than 5000 progeny
of the Fi generation. He continued the experiments for eight years, test-
ing and retesting his data and conclusions. Aided by the young men at
the monastery he obtained data about many thousands of progeny.

Mendel had several reasons for choosing the garden pea for his ex-
periments. The plants are easily cultivated, they have a short period of

[Cliap. XXXVIII HYBRID SEGREGATION 461

growth, all types of hybrids are fertile, the plants do not have to be pre-
vented from cross-pollinating, the characters chosen are easily recog-
nizable and may be combined and separated in different plants as if each
depended upon a single hereditary factor. Because of this simplicity
some phases of the experiment were relatively easy and the results were
not difficult to analyze. For the same reason most students in elementar)'
botany learn something about Mendel and his hybrid peas. They also
learn something about Hugo de Vries and mutation, but very little about
his evening primroses, for the heredity of primroses is pecuharly complex
and not the sort of thing one enjoys during his first approach to the
study of heredity.

Some explanations. We now know that the garden pea has seven pairs
of chromosomes. Consequently each sperm and egg has but seven
chromosomes, while each cell of the vegetative body of the plant has
fourteen chromosomes. The genes that specifically condition the four-
teen characters listed earlier occur separately in these fourteen chromo-
somes. Thus they appear to be unit factors, and the characters they con-
dition appear to be independent unit characters. In one chromosome is
the factor of tallness; in its homolog, or synaptic mate, is the factor of
dwarfness. During reduction division these two chromosomes become
separated. Consequently each speiTn or egg of a hybrid pea will have
but one of these two height factors — a fact often referred to as the purity
of gametes, or as Mendel's law of the segregation of hereditary factors.
Half of the total number of microspores and resultant sperms will have
one of these height factors, and half will have the other height factor.
A similar distribution of height factors also occurs in the megaspores and
in the resultant eggs and fusion nuclei in the embryo sac.

Similarly in another pair of homologous chromosomes are the factors
that condition flower color; in another pair are the factors that condition
seed form, and so on for each of the seven pairs of homologous chromo-
somes and seven pairs of contrasting factors. Furthermore, one factor of
each pair of contrasting factors completely dominates the influence of the
other.

Evidentlv, if either height of plant, flower color, flower position, seed
form, seed color, pod form, or pod color is considered separately without
regard to the other characters, the results of hybridization in the garden
pea are as simple and as easy to decipher as were the two examples
described in the previous chapter.

462

TEXTBOOK OF BOTANY

Anyone who is familiar with the facts discussed in the section on the
pairing and segregation of chromosomes in the previous chapter can use
the foregoing facts about peas and predict what Mendel must have ob-
tained in each of his experiments. Before going further, the reader will
find it profitable to predict what Mendel must have obtained in the Fi,
F2, and F3 generations after crossing (1) a pure-line tall plant with a
pure-line dwarf plant, ( 2 ) a pure-line red-flowered plant with a pure-line
white-flowered plant, (3) a pure-line yellow-seeded plant with a pure-
line green-seeded plant; and so on for each of the other pairs of con-
trasting factors.

Such predictions may be shovni diagrammatically by means of con-
ventional symbols for the different hereditary factors as described in the
previous chapter. Any kind of symbols may be chosen for the factors if
one indicates what factors they represent.^ It is important to remember
that these symbols indicate only the genes, or hereditary factors, in the
chromosomes. Words should always be used to describe the visible char-
acters which develop as a result of the presence of these genes. Hence
the symbols TT and Td each represent a pair of genes either of which,
when present in the cells of a plant, influences its growth in height.
They represent the hereditary make-up, or genotype, of the plants with
respect to height. If these plants are peas, both of them will be tall plants.

^ For the hereditary factors in the garden pea discussed in this chapter we ha\ e chosen
the following letter symbols. The dominant factors are indicated by capital letters.

Visible Characters
in the Plant

Symbols Used for

Hereditary Factors

in the Cells

Visible Characters
in the Plant

Symbols Used for

Hereditary Factors

in the Cells

Height of plant

T and d

Form of pods

I and c

Tall

TT and Td

Inflated

n and Ic

Dwarf

(id

Constricted

cc

Color of flowers

R and w

Surface of seeds

S and r

Red

RR and Rw

Smooth

SS and Sr

White

WW

Rough

rr

Position of flowers

\ and t

Color of seeds

Y and g

Axillary

A A and At

Yellow

YY and Yg

Terminal

tt

Green

gg

Color of pods

G and y

Green

GG and Gy

Yellow

.v.V

[Chap. XXXVIII

HYBRID SEGREGATION

463

The visible character that develops is referred to as the phenotijpe. For
example, if 400 progeny of a hybrid plant of genotype Td are found to
differ from each other in the proportion of 100TT:200Td:100dd, the
genotypic ratio is 1:2:1; but the phenotypic ratio — what one sees in the
field or garden — is 3:1. These ratios are often called monohybrid ratios
because they represent the progeny with respect to a single character.

All the plants that have the factors TT are pure-line tall plants; the
eggs and sperms produced in the pollen and embryo sacs in these plants
each contain the factor T. If only self-fertilization occurs, each individ-
ual in the progeny will have the factors TT, since one height factor is
obtained from the egg and the other from the sperm. Similarly, the
sperms and eggs produced in the pure-line dwarf plants of the genotype
dd each contain the factor d, and all the progeny obtained by selfing
have the factors dd. When plants having the factors TT and dd are
crossed, each individual in the progeny has the factors Td, because the
fertilized egg from which it develops has the factor T from one gamete
and the factor d from the other one.

For convenience in representing the proportions of the dififerent kinds
of progeny of hybrids, the checkerboard diagram is often used. If the
height factors in a plant are represented by Td, then half of the sperms
will have the factor T, the other half will have the factor d. Half of the
eggs also will have the factor T, the others will have the factor d. Simi-
larly, in a plant having the factors of flower color, Rw, half of the sperms
will have the factor R; the other half the factor w. Half of the eggs will
have the factor R; the other half will have the factor w. Now if we indi-
cate the factors in the eggs on one side of a checkerboard diagram and
the factors in the sperms along a side at right angles, we can indicate
quickly ( 1 ) the possible chances of union of the different kinds of sperms
and eggs, and (2) the proportion of the different kinds of progeny.

R

w

Genotypes
of F2 gener-
ation

R

RR

Rw

Rw

WW

Genotypes
of F2 gener-
ation

Within the checkerboard diagrams are represented the genotypes of
all the different kinds of progeny of each of the hybrid parents having
the genotypes Td and Rw, and also the ideal proportion in which they
may be expected to occur, especially if the total number of progeny in

464

TEXTBOOK OF BOTANY

the field is very large. In predicting the Fs generation no new problems

arise. ^

Two characters considered simultaneously. If we wish to understand
the inheritance of factors when two characters are considered simul-
taneously, we have only to apply the same principles previously used
for a single character and the facts we know about the behavior of
chromosomes during reduction division. The ratios that result from a
cross when two independent characters are considered are called dihy-
brid ratios.

Suppose we cross a plant whose genotype is TTRR with a plant whose
genotype is ddww. All individuals of the Fi hybrids will have the geno-
type TdRw, for they will have obtained one height factor and one flower-
color factor from each of the two pure-line parents. Since the factors T
and d are in separate homologous chromosomes (one from the pollen
parent, the other from the ovule parent) and the factors R and w are
likewise in another pair of homologous chromosomes, the hybrid with
the genotype TdRw may produce four kinds of sperms and four kinds of
eggs with respect to height and flower color (Fig. 210). We may repre-
sent the factors in these sperms and eggs as follows: TR, Tw, dR, and
dw.'^ If the homologous chromosomes that pair during reduction division
always separate and migrate to opposite poles of the spindle, then the
factors T and d would not occur in the same sperm or in the same egg;

2 Mendel, of course, seldom obtained ideal ratios of hybrid progeny in the F^ and F,
generations! Some phenotypic ratios actually obtained by him from the F, hybrids are
given below.

Number of Parents. All were Fi Hybrids.
The genotype is indicated in parentheses.

1064 tall plants (Td)
929 red-flowered plants (Rw)
258 plants from yellow seeds (Yg)
253 plants from smooth seeds (Ss)
1181 plants with inflated pods (Ic)

580 plants with green pods (Gy)

858 plants with axillary flowers (At)

Number of Progeny of the Fi Hybrids.
The F2 generation.

787 tall and 277 dwarf plants. 2.84:1.
705 red- and 222 white-flowered plants. 3.15:1.
6002 yellow and 2001 green seeds. 3.01:1.
5474 smooth and 1850 rough seeds. 2.96:1.
882 plants with inflated pods and 299 plants

with constricted pods. 2.95:1.
428 plants with green pods and 152 plants with

yellow pods. 2.82:1.
657 plants with axillary flowers and 201 plants

with terminal flowers. 3.14:1.

Hereditary factors that conditioned red flowers were "linked" with those that conditioned
grayish-brown seed coats, and those that conditioned white flowers and white seed coats
were "linked."

^ See footnote, p. 456.

[Chap. XXXVIII HYBRID SEGREGATION 465

neither would the factors R and w. We may now represent all these facts
briefly, including also the union of the different eggs and sperms, and the
resultant genotypes of the progeny.

Genotypes of
Parents

Factors in
Gametes

Genotype of All
Fi Hybrids

Factors in
Gametes of
Hybrids

TTRR

TR

ddww

dw

TdR^

TR, Tw, dR, dw

(Factors in Sperms)

TR, Tw, dR, dw

(Factors in Eggs)

TR

(Selfing)
Tw i dR

dw

TR
Tw
dR

dw

TTRR

TTRw

TdRR

TdRw

TTRw

TTww

TdRw

Tdww

TdRR

TdRw

ddRR

ddRw

TdRw

Tdww

ddRw

ddww

Genotypes of
F2 generation

Within the diagram are represented the genotypes of all the diflFerent
kinds of progeny with respect to height and flower color that may be
obtained from the hybrid pea having the genotvpe TdRw, and also the
ideal proportion in which they may be expected to occur in the field if
the total number of plants is very large. Note that many of the plants
that would appear to be similar in the field differ from each other
in their hereditary constitution.

By using the same principles, one may determine the probable progeny
of crosses between other pure lines of peas, such as ( TTYY ) X ( ddgg )
and (TTgg) X (ddYY). Mendel experimentallv proved that each of
the fourteen factors which he studied in peas segregated in the same
manner.

Three characters considered simultaneously. No new problems arise
when three characters, such as Mendel studied in peas, are considered
simultaneously. Thus Fi hybrids of the pure lines that have the geno-
types TTRRgg and ddwwYY would all have the factors TdRwYg, and
considering these three characters alone there would be eight different

466

TEXTBOOK OF BOTANY

kinds of sperms and eggs. One merely needs to visualize the chances
of orientation of the three pairs of chromosomes (Fig. 211) and their
contained factors during the process of reduction division to see that the
factors in the sperms and eggs of this hybrid would be distributed as
follow^s: TRY, TRg, TwY, dRy, Twg, dRg, dwY, and dwg. In this case
the checkerboard diagram is very large.

Other problems with hybrids. It is, of course, possible to consider two
or more characters simultaneously even though the parents are pure
lines for some of them. For example, if the genotype of the parent were
TTRwgg the checkerboard would be very small, for there would be
only two different kinds of sperms and eggs, with respect to these par-
ticular hereditary factors. The paired chromosomes containing height
factors contain the same height factor, and the paired chromosomes
containing seed-color factors have the same seed-color factor. But the
paired chromosomes containing flower-color factors have unlike flower-
color factors, and these are the only ones that differ in the sperms and

eggs.

What kind of progeny would be obtained by crossing a plant of
genotype TTRw with a plant of genotype Tdww? Since it does not mat-
ter which way the cross is made, one merely decides for convenience
that one of these plants is the pollen parent; the other one the ovule
parent. If we decide that the plant of genotype TTRw is the pollen
parent, the sperms will have the factors TR and Tw, and the eggs from
the ovule parent will have the factors Tw and dw. Hence the progeny
have the genotypes TTRw, TTww, TdRw, and Tdww, as shown in the
diagram:

TR

Tw

To predict the proportion of the resultant kinds of progeny after
either selfing or crossing, one merely applies the foregoing principles
to decide (1) what kinds of sperms and eggs can be produced in the
parents, and (2) the possible chances of fertilization. It will be excellent
practice for the student similarly to predict the nature of the progeny
that would be obtained by selfing any one, or by crossing any two, of
the types of plants represented in the preceding diagrams.

[Chap. XXXVIII HYBRID SEGREGATION 467

Interpreting the progeny represented by the checkerboard diagrams.
There is no value in being able to construct checkerboard diagrams that
represent the progeny of a plant, or of two kinds of plants, unless one
can read them and visualize what they represent, after they are con-
structed. Since they are made to be read, one should always follow cer-
tain accepted conventions in making them, so that he may see almost
at a glance both the phenotypic and the genotypic ratios of the progeny.

In the diagrams ( page 463 ) representing the progeny of the genotypes
Td and Rw it is obvious in the one that 3/4 of the plants are tall and
1/4 are dwarf; in the other, 3/4 of the plants are red-flowered and 1/4
are white-flowered. The same fact may be obtained from the diagram
( page 465 ) showing dihybrid ratios where 12/16 of the plants are tall and
4/16 are dwarf, while 12/16 are red-flowered and 4/16 are white-
flowered. From this same diagram one may also learn that 9/16 of the
plants in the field are tall red-flowered, 3/16 are tall white-flowered,
3/16 are dwarf red-flowered, and 1/16 are dwarf white-flowered. All
these ratios refer to phenotypes, not to genotypes. It should always be
remembered that the checkerboard diagram represents the proportion of
the different kinds of plants in a field and not the actual number of plants
in the field, and these diagrams should be read accordingly.

Much more can be learned by looking at the genotypic ratios, and
also the genotypes of particular individuals in the progeny. For instance
in the diagram illustrating the progeny of the genotype Td, it is obvious
that 1/3 of the tall plants in the garden are pure tall plants, and 2/3
of them are hybrid tall plants; also that all the dwarf plants are pure
dwarf plants. The term homozygous is often used to refer to any char-
acter for which the plant is a pure line, that is, contains a pair of like
factors, of which one factor was obtained from the sperm and the other
from the egg. The plants of genotypes TT and dd are homozygous with
respect to height, while the plants of genotype Td are all heterozygous
with respect to height. An absolutely pure-line plant would be homo-
zygous with respect to all its factors.

In the diagram representing the progeny of the plant (TdRw) 1/4
of the plants are homozygous with respect to both height and flower
color; another 1/4 are heterozygous with respect to both height and
flower color. Still another 1/4 are homozygous with respect to height
but heterozygous with respect to flower color, while the remaining 1/4
are just the opposite.

In the field one can see only four kinds of plants: tall red-flowered,

468 TEXTBOOK OF BOTANY

tall white-flowered, dwarf red-flowered, and dwarf white-flowered. But
from the diagram we may learn that of the tall red-flowered plants in the
field only 1/9 are pure tall red-flowered, 4/9 are hybrid tall red-flowered,
2/9 are homozygous with respect to height and heterozygous with
respect to flower color, and the remaining 2/9 are just the converse of
the preceding. Of the tall white-flowered plants, 1/3 are pure tall white-
flowered. One third of the dwarf red-flowered plants are homozygous
with respect to height and flower color. All the dwarf white-flowered
plants in the field are pure dwarf white-flowered.

Ascertaining the genotype of parents. If one were told that a dwarf
white-flowered pea plant was crossed with a tall red-flowered one, and
that all the progeny were tall but some of them were white-flowered, he
should be able to state at once the genotypes of the two parents.

Disregarding the progeny for a moment, we know that the genotype
of the dwarf white-flowered parent was ddww, and that the tall red-
flowered parent contained the factors T and R. Since all the progeny
are tall, we are sure that the other height factor in it was also T. Since
some of the progeny are white-flowered, we can be sure that the other
flower-color factor was not R, and must have been w. Hence the geno-
types of the two parents were ddww and TTRw.

Inbreeding. Attention has already been called to the fact that con-
tinued inbreeding of hybrids and their progeny results in a number of
inbred lines that are homozygous with respect to many of their heredi-
tary factors. A hint of this fact is evident in the checkerboard diagram
of the progeny of the plant of genotype TdRw, since 1/4 of the plants
of the F2 generation are homozygous with respect to height and flower
color. A much better idea of the effect of inbreeding may be obtained
by working out a diagram for several successive generations, beginning
with three characters in a hybrid. Of course, the making of these dia-
grams becomes increasingly tedious as the number of characters repre-
sented increases. If all the fourteen characters discussed in this chapter
were considered, the number of different eggs and sperms would be
equal to the seventh power of 2, or 128.

Obviously, inbreeding may be undesirable when it increases the
homozygosity of undesirable factors, or when it decreases the number
of desirable factors one wishes to have in one plant. Following inbreed-
ing, these desirable factors may be brought into one plant by crossing
inbred lines from which the undesirable factors have been eliminated.
On the other hand, inbred lines that are homozygous for several desired

[Chap. XXXVIII HYBRID SEGREGATION 469

factors may be very important as economic plants. Many of our best
varieties of close pollinated plants are inbred lines, namely, wheat, oats,
barley, peas, and beans.

Inbreeding without selection will not entirely eliminate hybrids. This
fact may be discovered by starting with a plant of genotype Td and
noting the percentage of pure-line plants and hybrids in each successive
generation obtained by selfing all individuals. In the first generation the
hybrids constitute 1/2 of the progeny, 1/4 of the next, 1/8 in the next,
and so on. In the n*^'' generation the proportion of hybrids to pure lines
may be represented by the equation (2" — 1 ) TT : 2Td : ( 2" — 1 ) dd.
Thus in the 8th generation it would be 255TT : 2Td : 255dd.

Other problems. The foregoing discussion has been limited to simple
cases in which the several factors that specifically influence the develop-
ment of the characters are in different chromosomes. If one always thinks
of hereditary factors in association with chromosomes, problems in which
two or more of the factors are in the same chromosome are no more diffi-
cult than the cases discussed. A more difficult type of problem is met
with when the development of a particular character specifically depends
upon the presence and interaction of several hereditary factors. Such
characters are referred to as multiple-factor characters. A discussion of
these factors and of other complications of Mendelian inheritance lies
beyond the scope of this book.

Hybrid endosperms. Inheritance in the endosperm is a unique feature
limited to certain seed plants because the endosperm is a unique tissue
that develops from the triple-fusion nucleus only in the seeds of some
plants (Chapter XXXIII). Hybrid characters in the endosperm are
always evident the same season that cross-fertilization occurs, for the
endosperm grows to maturity during that season. We have already seen
that hybrid characters of the embryo, such as the green and yellow
cotyledons of pea seeds, are also evident the same season that cross-
fertilization occurs; but they are dependent upon factors inherited
through the fertilized egg, whereas endosperm characters are dependent
upon factors inherited through the triple-fusion nucleus (Fig. 214).

To understand hybridization in endosperms it is necessary only to
remember (1) that the two sperms in the pollen tube have the same
complement of chromosomes and the same hereditary factors, and (2)
that the egg and the fusion nucleus in the embryo sac have the same
complement of chromosomes and hereditary factors, except that the

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SPERMSi

Fig. 214. Diagram illustrating the derivation of the chromosome complement of the
triple fusion nucleus and the endosperm cells resulting in xenia.

fusion nucleus has a double set of these chromosomes. With this one
exception, the fertilized egg and triple-fusion nucleus in the same embryo
sac have the same chromosome complement.

Some of the factors in these chromosomes specifically affect the
processes in the endosperms, such as the formation of starch, dextrin.

Fig. 215. Ear of Stowell's Evergreen sweet com from an open pollinated plant.
The development of the endosperm of some of the grains was influenced by
sperms from waxy corn, and black Mexican sweet corn growing nearby. Photo
by G. W. Blaydes.

protein, and pigment. It is often observed that when brown-, blue-, or
yellow-grained corn is growing near white-grained com, a few colored
grains of corn appear on the ears of white com, but white grains do not
appear upon the ears of colored corn this same season (Fig. 215).
Similarly grains of starchy corn appear on the ears of sweet corn, and
grains of field com appear on the ears of pop corn.

[Chap. XXXVm HYBRID SEGREGATION 471

Long before the processes of fertilization and triple fusion were known
in plants, this phenomenon was called xenia, a term that means hospi-
tality. It was thought that the white corn, sweet corn, and pop corn
were being hospitable to the other kinds of corn — unreciprocated hos-
pitality. The Indians who wanted to keep their ceremonial com "pure"
looked upon it as contamination. We now know that it is the result of
cross-pollination, triple fusion, and the dominance of certain hereditary
factors in the endosperm, as indicated in Fig. 214. The pollen grain
came from a variety of blue-grained corn; the ovule grew in a variety
of white-grained com.

Fig. 216. Diagram illustrating the results of inheritance in the endosperm follow-
ing selfing of a hybrid plant from a seed like seed C in Fig. 214.

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I- ^

m

A

B

D

D

Fig. 217. C represents the grains on an ear of corn the same season when plants
from A and B are crossed. D represents ears of corn on plants from C when selfed
the following season. Photos by G. W. Blaydes.

[Chap. XXXVIII HYBRID SEGREGATION 473

Xenia is the evident result of the influence of the pollen parent on the
endosperm of the ovule parent the same season that cross-pollination
occurs. Hence, it will occur only in seeds in which an endosperm de-
velops from the triple-fusion nucleus. Furthermore, the sperm must con-
tain a dominant endosperm factor, and the fusion nucleus the contrasting
recessive endosperm factor.

If seed "C" in Fig. 214 is planted and only self-fertilization occurs, in
the mature plant that develops from the embryo there will be two kinds
of pollen grains, two kinds of embryo sacs, four kinds of triple-fusion
nuclei, and an ear of corn on which 3/4 of the grains will be blue and
1/4 will be white, as shown in Figs. 216 and 217.

The fact that the ovule in a grain of corn is enclosed in the closelv
adhering ovularv is not shown in the diagram. Some of the colors in
grains of corn are due to pigments in the grain coats. Xenia, however,
is strictly an endosperm phenomenon.

We have described here a simple example of hybrid endosperms.
Other examples, some of which are complicated because of partial domi-
nance, linkage, or multiple factors, have been analyzed and explained
in the literature.

GENERAL REFERENCES

Altenburg, Edward. How We Inherit. Henry Holt & Company, Inc. 1928.
Castle, W. E. Genetics and Eugenics. Harvard Univ. Press. 1922. Translation

of Mendel's paper in appendix,
litis, Hugo. Life of Mendel. W. W. Norton & Company, Inc. New York. 1932.
Weatherwax, Paul. The Story of the Maize Plant. Univ. of Chicago Press. 1923.
Zirkle, Conway. The Beginnings of Plant Htjbridization. Univ. of Pennsylvania

Press. 1935.

SELECTED REFERENCES

Rabcock, E. R. Recent progress in plant breeding. Sci. Monthly. 40:313-322.

1935.
Harbou, D. J. Gregor Johann Mendel. Sci. Monthly. 49:393-400. 1939.
Waller, A. E. Xenia and otlier influences following fertilization. Ohio. Jour.

Sci. 17:273-284. 1917.

CHAPTER XXXIX
MUTATIONS

We have been emphasizing the individuaHty, stabiUty, and regularity of
chromosomes and the smaller hereditary units of matter in the proto-
plasm, particularly the genes. It is the stability and orderly behavior of
these hereditary units of matter that maintain the constancy of the dif-
ferent kinds of plants during their lifetime, and during succeeding
generations. This constancy in organization and processes is, however,
only relative. Changes in both chromosomes and genes are known to
occur, and departures from the orderly splitting, pairing, and migration
of chromosomes have been observed many times. When these changes
in the hereditary units of matter in the cell are relatively stable, they are
initial steps in the evolution of new kinds of plants. Their eflFects may
become evident in the appearance of new characters or in the modifica-
tion of preexisting ones, and in the consequent formation of new varieties
of plants.

We have already learned that all that is inherited is within the proto-
plasm of the sperm and the egg, and hence in the resultant fertihzed
egg. The hereditary units of matter in these cells, and the physiological
processes they condition are the precursors of all the characters we see
in the growing plant. The inherent differences in plants we see about us,
therefore, are dependent upon the composition and arrangement of the
microscopic and submicroscopic units of matter of which protoplasm is
composed. Likewise the heritable changes that occur in the visible char-
acters, through time, must be dependent upon certain alterations in these
small heritable units of matter. In them may be found the origin of all
heritable differences and of all the initial steps of evolution. If we wish
to understand the gross phenomena of living organisms we must first try
to understand some of the microscopic and submicroscopic structures
and processes of which these gross features are the consequences.

A change in a single gene may result in a very striking change in the
development of some visible character. In such cases the relation of the
character to the gene is easily detected by the study of Mendelian ratios

474

[Chap. XXXIX MUTATIONS 475

as described in the preceding chapter. Among the characters that appear
to be results of changes in single genes are red-flowered snapdragons
and fuzzless peaches, or nectarines. The relationship is not so easily de-
tected when the development of the character depends specifically upon
the interaction of several genes.

We should not be misled into attributing the development of a par-
ticular character solely to the influence of a single gene, even though it
appears, or fails to appear, when the particular gene is present or absent.
The total development of a plant, or of any part of it, is dependent upon
the collective influences of numerous genes. When a plant having all of
its genes is compared with a plant in which one gene is lacking, we may
be able to detect one or more effects of this one gene on the development
of the plant. The number of different kinds of influences a particular
gene may have has never been fully discovered.

Thus far we have emphasized the orderliness of the numerous proc-
esses that occur every time cells divide, and when plants reproduce
sexually. It is time now to turn our attention to some of the irregularities,
or aberrations, that sometimes occur, and to try to visualize their rela-
tion to the changes that are occurring in plants today and have been
occurring in plants for a billion years or more.

Mutations in the vegetative cells of plants. We have seen that the exact
duplication of the hereditary mechanism when a cell divides in the
growing regions of the vegetative body of a plant depends upon an
exact duplication of all its chromosomes and genes, and also upon a very
definite distribution and assemblage of these daughter chromosomes in
the nuclei of the two new cells.

When we are able to visualize the usual orderly procedure of events
in cell division, we can begin to appreciate some of the irregularities
which occur occasionally, and with a little scientific imagination we can
even predict what some of them will be. For example, the chromosomes
may divide without further division of the nucleus or of the cell, resulting
in a cell with a tetraploid (4n) number of chromosomes. Some or all of
the chromosomes on their way to the poles of a cell might divide a
second time before a new cell wall is formed, resulting in cells with an
increase in number of chromosomes. Some of the chromosomes might
fail to divide, and one of the new cells would lack one or more chromo-
somes. The two halves of a divided chromosome might go to the same
pole of the spindle, resulting in two new cells, one of which has 2n +
1, the other 2n — 1 chromosomes. A chromosome might become

476 TEXTBOOK OF BOTANY

"stranded" on the spindle and not be included in the new nucleus. A
chromosome might break in two during the process of splitting, and the
remaining fragment might adhere to the other daughter chromosome or
be lost completely.

Having predicted that such irregularities might occur in chromosome
behavior during ordinary mitosis in vegetative cells, one has only to turn
to the literature of cytology to learn that all of them actually do occur
in the growing regions of plants. Specialists who devote their time to the
study of cells either have seen them occurring, or the\' have made
observations from which no other logical inferences could be drawn.

If the altered cell survives, it may become the forerunner of a sector
of tissue, a whole branch, a leaf, a root, or a part of a flower that differs
from other corresponding organs of the same plant. Some of the colors
and patterns of variegated leaves, the white branches of plants, and the
red roots of sweet potatoes (Plate 4) are easily observed results of such
changes in the vegetative cells of plants. In similar fashion the branches
bearing seedless oranges originate. The nectarine and the Starke delicious
apple are the results of mutations in vegetative cells. Many varieties of
sugar cane, potatoes, and other plants that propagate mainly by vege-
tative means are known to have originated by aberrations in ordinary
cell division. If the aberration occurs in the fertilized egg, the whole
plant will be different, since the fertilized egg is the forerunner of all
the other cells of a plant.

When such a change occurs in the body cells of the higher animals
after they have developed bevond a very early embryonic stage, it
perishes with the individual in which it occurs. The reproductive tissues
in these animals soon become differentiated from the body (somatic)
tissues. In plants the situation is quite different. If the aberration occurs
in the growing tip of a stem, flowers may later develop from cells con-
taining the aberrant chromosomes. Or if it occurs in some organ of a
plant which propagates vegetatively, a whole plant containing the aber-
rant chromosomes may be obtained by this means, and all its flowers
would develop from altered cells. In either case, if the alteration in
chromosomes is a stable one, it is perpetuated in plants by sexual repro-
duction. The white flower of hibiscus shown in Plate 4 is borne on a
branch that grew from the stem base of a red-flowered plant. White-
flowered branches have developed from this side of the stem base an-
nually for the past 9 years. This aberration in the hereditary units must

[Chap. XXXIX MUTATIONS 477

have occurred in the meristem of the seedling, and has been perpetuated
in all branches from that part of the meristem.

We may consider a case in which the division of the chromosomes was
not followed by further cell division and wall formation, resulting in a
cell with the tetraploid (4n) number of chromosomes. This aberration
is known to occur under natural conditions. The frequency of its occur-
rence has been increased experimentallv by exposing certain plants to
high or low temperature, by wounding them, or by treating them with
various chemicals, especially anesthetics and alkaloids. Examples are
the increased frequency of occurrence of 4n number of chromosomes in
the first cells formed from the zygotes of corn, wheat, barley, and sweet
clover when the flowers have been exposed to 40° -45° C. for a half hour
while the zygotes are dividing. Similar results were obtained with low
temperatures in Jimson weeds, and in some of the adventitious branches
that developed from wound surfaces in tomato stems. A weak solution
of chloral hydrate was one of the first chemical agents used to increase
the rate of chromosome doubling. In cells treated with this reagent
some or all of the chromosomes divided a second time while they were
migrating toward the poles of the spindle in the dividing cells.

All these external factors are known to alter the polarity of cells and
the viscosity, permeabilitv, surface tension, and streaming of protoplasm.
It is probably through such indirect means that they influence the regu-
lar behavior of chromosomes. Spindle formation and the migration of
chromosomes are certainlv influenced by all of these internal conditions
in the protoplasm, except permeability.

Recently it was discovered that an alkaloid, colchicine, from the
autumn crocus is very effective in preventing spindle formation and
subsequent cell wall formation in dividing cells, thus stopping the
process with the splitting of the chromosomes. These treated cells con-
tain the tetraploid (4n) number of chromosomes. The plant, or some
part of it, may be soaked in a very dilute solution of colchicine, or the
solution may be sprayed upon the plant. How far botanists may go in
increasing the number of chromosomes in plant cells by this method will
have to be decided by future experiments.

Even before the discovery of colchicine and other means of increasing
the number of chromosomes in a cell, it was known that tetraploid ( 4n )
cells would survive and grow, and that whole plants bearing flowers
could be obtained from them as described above. They had been found
in the wild state. In such plants the gametes contain the diploid (2n)

478 TEXTBOOK OF BOTANY

number of chromosomes when compared with gametes of the ancestor
of the tetraploid plant. If they unite they form a zygote with the 4n
number of chromosomes and thus the tetraploid condition is perpetuated.
But if a diploid (2n) gamete unites with a monoploid (n) gamete, a
triploid (3n) zygote is obtained. Certain irregularities, however, may
occur during reduction division, and some of the progeny may not have
exactly 4n or 3n number of chromosomes.

Tetraploid plants that have been obtained by experimental methods
usually have larger cells, stems, leaves, flowers, and seeds than their
diploid ancestors, though this increase in size does not always occur. A
slower growth and a longer vegetative period are more generally char-
acteristic of them than increase in size. Many irregularities and various
degrees of sterility occur.

Monoploid tomato plants and Jimson weeds that develop from mono-
ploid unfertilized eggs (parthenogenesis) are smaller and less vigorous
than the parent diploid plant. Those that have been studied are sterile
unless they produce gametes without reduction division. All such
gametes have the same chromosome complement and it is identical with
that of the vegetative cells of the plant. If self-fertilization were to occur,
absolutely homozygous individuals would be obtained and perpetuated
until some change occurred in a gene of one or more of the chromosomes.
In later chapters we shall see that the formation of absolutely homo-
zygous individuals is a common occurrence in some species of mosses
and ferns.

Other aberrations of chromosomes noted above, such as a gain or a
loss of a chromosome or of a fragment of a chromosome, are, of course,
accompanied by changes in development of the plant, depending upon
whether the cells gained or lost certain genes and their potentialities.
These changes may be more striking than those that are merely the result
of the doubling of the number of similar chromosomes.

We shall see presently that tetraploid (4n) plants, octoploid (8n)
plants, and 16-ploid plants may originate in another way; but certain
other points may be considered first. It is now known that species of
many genera of plants differ from each other by multiples of some basic
number of chromosomes. For instance, the different species of wheat
have either 7, 14, or 21 pairs of chromosomes. Similar series of multiples
of seven also occur in the different species of oats, cinquefoil, tall
meadow rue, and certain other genera of plants. These facts are suffi-
cient evidence that the number of chromosomes alone is not enough to

[Chap. XXXIX MUTATIONS 479

account for tliese different species and genera. The chromosomes must
also be different. Species of other genera differ by multiples of other
basic numbers, such as 6, 8, 9, and several others. The different species
of chrysanthemum have 9, 18, 27, 36, and 45 pairs of chromosomes.

Apparently these species originated in part by a doubling of chromo-
somes; but there have been certain changes within the chromosomes
themselves, such as fragmentation or even changes in the genes. If a
gene consists of a group of a few (perhaps a dozen or less) molecules,
then changes in a gene might occur by ( 1 ) a gain or a loss in molecules,
or (2) a rearrangement, a gain or a loss of atoms within the molecules.
On the other hand, if a gene consists of only one molecule, then altera-
tions within it would be limited to rearrangement, gain or loss of atoms
or groups of atoms.

What are mutations? Any of the irregularities in the composition and
arrangement of chromosomes and genes and in the consequent changes
of development that cannot be ascribed to the direct effects of cross-
fertilization may be called mutations. The term mutation was first used
to refer to the occurrence of observable differences in hereditary char-
acters that were considered not to be the direct result of cross-fertiliza-
tion. It was later discovered that these recognized mutations were the
results of differences in the composition, behavior, and arrangement of
both chromosomes and genes. Some authors use the expressions "chromo-
somal aberrations" and "gene mutation," preferring to limit the term
mutation to changes that occur only in the genes, whenever such a dis-
tinction can be made. For convenience we shall use the term mutation in
these chapters to include an aberrant condition in either chromosomes
or genes, or any hereditary variation that is not the direct result of
cross-fertilization.

Mutations during reduction division. We have seen that the pairing
and segregation of chromosomes during reduction division are very regu-
lar processes, and that the constancy of pure lines, as well as the
Mendelian ratios of hybrids, are the consequences of this regularity.
But here again, with a little scientific imagination we may visualize the
possible occurrence of certain irregularities. Reduction division might
fail to occur, with the result that the spores and gametes formed would
have the diploid ( 2n ) number of chromosomes, just like the cells of the
vegetative body of tlie plant. The synaptic mates, instead of separating
and moving to opposite poles of the cell, might continue to adhere to
each other and move to the same pole of the cell. This might happen in

480 TEXTBOOK OF BOTANY

one or in several pairs of chromosomes. Or the chromosomes might con-
tinue to adhere in certain parts, and separate and break in other parts,
with the result that each chromosome would contain one or more pieces
of its synaptic mate. Sometimes when chromosomes of two different
species are brought together in the cells of a hybrid by means of cross-
fertilization thev fail to mate regularly in reduction division, and many
irregularities occur. As shown by the literature of cytology, these sus-
pected irregularities actually do occur. Each of them will be considered
briefly.

When the chromosomes fail to undergo reduction division, the result-
ing spores, sperms, and eggs not only have the diploid ( 2n ) number of
chromosomes but they also have the same chromosome complement,
which is identical with that of the cells of the vegetative body of the
plant. This is the usual process in dandelion. Since fertihzation does not
occur in dandelion, the parthenogenic embryo develops directly from
diploid (2?i) eggs. Thus there is no change in chromosome number in
the cells during the entire life cycle of the dandelion. If self-fertilization
were to occur among these gametes of dandelion, absolutely homozygous
plants would be obtained until some mutation occurred in one or more
of the chromosomes.

When fertilization does occur in such plants, a union of diploid ( 2n )
gametes results in tetraploid (4n ) plants. This is the second way in which
tetraploid plants or plants with some higher multiple number of chromo-
somes may originate.

The type of irregularity by which the members of one or more pairs
of homologous chromosomes fail to separate during reduction division
results in a change in chromosome number and complement. If the
chromosomes of one pair fail to separate, both of them migrate to the
same pole of the spindle. As a result, some of the spores, sperms, and
eggs then have n + 1 chromosomes; others have n — 1 chromosomes. If
two of the pairs of chromosomes fail to separate, the gametes have n +
2 and m — 2 chromosomes. Hence some fertilized eggs lack one or more
chromosomes, and others have one or more extra chromosomes with all
their genes and potentialities.

Such changes in chromosome behavior may be the initial steps in the
formation of new varieties and species. Within certain genera of plants
one may arrange several of the species in a series, such that each species
in the series differs from the preceding one and the next one following
it bv but one or two chromosomes. A similar situation may be found

[Chap. XXXIX MUTATIONS 481

among the varieties of a species. The usual diploid (2n) number of
chromosomes in corn is 20. The number of chromosomes that have been
found in different varieties of corn are 21, 22, 23, 24, 25, 26, 27, and 28.
A third type of irregularity has been variously called "translocation,"
"segmental interchange," and "crossing over." That is, when the pair of
homologous chromosomes are separating, they may be so interwoven
that a piece of one homolog remains attached to the other one. Many
such irresularities are known to occur, and thev have been studied with
interest as a means of discovering the relative locations of the genes
within the chromosome. The displaced piece of chromosome may have
certain genes whose presence can be detected by the appearance or
absence of certain characters in the plant. By comparing the effects of
different pieces of the same chromosome in different individual plants,
the relative position of the different genes in the chromosome may be
closely estimated. For the solution of such problems the combined efforts
of both cytologists and geneticists are needed.

Irreeularities in reduction division in hvbrids mav be obtained (1)
by crossing remotely related plants having the same number of chromo-
somes, but chromosomes which previously have not been together in the
same cell, or (2) by crossing plants differing in chromosome number.
The irregularities that may occur in the pairing, or lack of pairing, of
the chromosomes in such hvbrids are rather numerous, and only a few
examples will be mentioned here. These irregularities of chromosomes,
though indirectly the result of cross-fertilization, should be distinguished
from the hybrid effects of cross-fertilization that are directly dependent
upon the mixing of chromosomes of two kinds of plants as described in
the preceding chapter.

Of the eighteen recognized species of wheat, three have 7 pairs of
chromosomes, nine have 14 pairs, and six have 21 pairs. Self-fertile
hybrids are readily obtained from crosses between species having the
same number of chromosomes. Hybrids are less readily obtained by
crosses between species with different numbers of chromosomes, and
they are often self-sterile. Hybrids obtained by crossing species with 14
and 21 pairs of chromosomes are sometimes self-fertile. Hybrids between
species with 7 and 14 pairs of chromosomes or between those with 7
and 21 pairs are self-sterile; but they may be successfully back-crossed
with parents, or crossed with certain other species. Obviously several
new kinds of chromosome combinations may be obtained by such means.

Hvbrids have also been obtained between species of different genera

482 TEXTBOOK OF BOTANY

of plants, such as wheat and rye, wheat and couch grass, radish and
cabbage. Here again there is often a high degree of sterihty and many
pecuhar irregularities are obtained. Radish and cabbage each have 9
pairs of chromosomes and their Fi hybrid has 9 cabbage chromosomes
and 9 radish chromosomes. Among the F2 hybrid segregates was a hybrid
containing 18 cabbage chromosomes and 18 radish chromosomes, which
must have been obtained from gametes formed without reduction divi-
sion. Subsequent generations of this particular hybrid also contained
9 pairs of radish chromosomes and 9 pairs of cabbage chromosomes.
Apparently the cabbage and radish chromosomes did not pair with each
other during reduction division. This new hybrid has all the character-
istics of a distinct species. Perhaps species are sometimes formed in this
way in the wild state.

Influence of environment. The above summary of some of the better-
known types of aberrations in chromosomes and genes is barely more
than an introduction to this particular phase of natural phenomena.
But perhaps it is sufficient to indicate some of the research that is being
done and to supply a basis of understanding as to how hereditary dif-
ferences originate. All such diflFerences begin as changes in the composi-
tion and arrangement of units of matter in the protoplasm of cells. For
example, the molecules of which genes are composed may undergo
change in composition. We have already seen that changes in the com-
position of molecules may be the result of a change in the arrangement
of atoms, or of a loss or gain of certain atoms. In an earlier chapter it
was shown that a rearrangement of a few atoms in a molecule of glucose
resulted in the formation of fructose, and vice versa; and that the com-
position of molecules is altered by oxidation-reduction processes.

Next in order are changes in the arrangement of genes within the
chromosomes; and among the larger units are changes in the arrange-
ment of chromosomes themselves. Changes in the composition of the
molecules of the genes are often regarded as the most fundamental
ones. They are chemical changes, while all the others are physical
changes. Each of these changes is the first step in a type of heritable
variation known as a mutation. In brief, a mutation may occur among
the chromosomes as a whole, among the genes within a chromosome, or
among the molecules and atoms within a gene.

Moreover, mutations may occur in an apparently uniform environ-
ment, or the same kinds of changes may occur in different kinds of
environments, and their frequency of occurrence may be increased or

[Chap. XXXIX MUTATIONS 483

decreased by a change in environment. To illustrate these points we shall
have to begin with a simple analogy.

Glucose, fructose, and mannose are all distinct sugars of the formula
CuHiiiOe. If one of them is placed in a bottle of water, the other two
will be slowly fomied from it even if the bottle is kept in a uniform
environment. But the rate of change from one kind of sugar to another
varies with the intensity of certain factors in the environment, such as
temperature and the presence or absence of barium hydroxide or other
chemicals. Geochemists calculate the age of rocks on the basis of the
rate at which uranium changes to lead and helium. Even the rate of
change of this process is unaffected by ordinary changes in the environ-
ment that occur on the surface of the earth.

These simple examples are cited as a reminder that within a system
(molecule, gene, chromosome, and cell) there may be intrinsic condi-
tions that result in change, and that the eflFects of the environment on
these intrinsic conditions may result only in altering the rate of change.
That is, the effects of the environment may not result in any new com-
pound or structure.

By special laboratory methods chemists are able to make 16 kinds of
sugars of the formula C(iHi20(5. Only about one-third of these have ever
been found in plants. This fact should keep us from inferring that just
anything may happen in a living organism. The number of kinds of
changes that may occur in any system is dependent upon conditions
within the system. The number that does occur may depend in part upon
the environment. Of those that do occur, only certain ones are sufficiently
stable to persist. Obviously if all heritable changes in living cells de-
pended upon the conversion of one hexose sugar to another, only a few
kinds of heritable differences could occur.

The changes that occur in the composition of the molecules of the
genes are not changes in sugar molecules, but changes in the much
more complex molecules of proteins, or protein-like substances. The
kinds of changes in protein molecules, however, are probably as inde-
pendent of changes in the external environment as are those in the mole-
cules of sugar. Likewise, the rate of change may be influenced by
changes in certain factors of the environment. We should not, therefore,
overemphasize environment and underemphasize the intrinsic nature of
change in hereditary units of matter. If differences in environment are
important in causing heritable change, one would expect that artificial

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TEXTBOOK OF BOTANY

environments, such as X-rays, would cause new kinds of variations,
whereas they merely increase the frequency of those that occur in nature.
This point is further illustrated by the mutations that have occurred
in the Boston fern and its varieties growing in greenhouses where the
environment is comparatively uniform, though not as uniform as that in
a tropical rain forest. The sword fem {Nephrolepis exaltata), a native
of Florida and the American tropics, has been cultivated in greenhouses
and as a house plant for more than a centurv. In 1895 a florist near Bos-
ton discovered among his sword ferns a mutant which became known

Fig. 218. The cultivated Boston fern (upper left) and five of its many mutant
varieties, all of which originated from mutations in vegetative cells.

as the Boston fern {Nephrolepis exaltata bostoniensis) , and also as the
best fem among house plants. Since 1895 more than 200 mutant varieties
of the Boston fern are known to have come into existence. Only one of
these new varieties has spores that germinate. One variety reproduces
by viviparous budding of the leaves. Some of the varieties survive when
exposed continuously to the dry air of our homes, which in winter is
usually drier than desert air; others can be kept alive only in a saturated
atmosphere. The leaves differ greatlv in size and fomi (Fig. 218). Some
varieties have leaves several feet long; the smallest varietv has leaves
scarcely more than an inch in length. The simplest leaves are once-
pinnate and plain. The more complex leaves are 5- to 6-pinnate with vari-

[Chap. XXXIX MUTATIONS 485

ously crested leaflets. These plants also exemplify the fact that mutations
occur in vegetative organs, for these mutations occur when the ferns
propagate vegetatively from rhizomes and runners.

Although the kinds of mutations are apparently independent of
changes in the environment, the rate at which they occur is influenced
by the environment. Very little is known about the comparative increase
in frequency of the several kinds of mutations. If the rate of certain
mutations is consistently increased more than that of others in a given
environment, one might expect that certain variations would accumulate
more abundantly than others in certain habitats, provided, of course,
that they are not eliminated more rapidly.

Experimental means of speeding up mutations by decreasing or in-
creasing the temperature, by treating the plant with chemicals, or with
ultra-violet lisht, X-ravs, and radium emanations have enabled investi-
gators to obtain data about mutations much more readily. The idea that
mutations under natural conditions may depend upon cosmic rays has
already been discredited. It is interesting to note that plants from old
seeds, like those from seeds treated with X-rays, have more mutations
than plants that grow from freshlv harvested seeds. Apparently X-rays
speed up deleterious changes in seeds similar to those that occur natu-
rally with age. The effects of X-rays are the result of chromosomal aber-
rations and of the destruction of hereditary units of matter.

Some genes are much more stable than others. When the colors of
variegated leaves, flowers, and grain coats are the visible effects of local
mutation, the various sizes of the pigmented areas indicate the amount
of tissue that developed from the cells in which the mutation occurred.
The occurrence, shape, and location of the white and pigmented areas in
the leaves of coleus, however, are seldom due to local mutations in the
leaves. These patterns are as definitely heritable through genes in the
gametes as are the pigments. There are pure lines of coleus with respect
to these patterns; and when they are crossed with each other, the pat-
terns recur in the usual Mendelian ratios in the progeny of the hybrids.

The idea that only the best or "fittest" mutations survive has been
abandoned bv most students of plant physiology and heredity. So, also,
have the notions that a heritable variation must have some adaptive
value to survive, that the variation occurred to meet some need, and
that the variation was selected by nature. Numerous heritable variations
that neither interfere with nor contribute to the life of the plant occur
and survive, and their mode of origin is precisely the same as those

486 TEXTBOOK OF BOTANY

that are beneficial or those that are lethal. Only a few of the numerous
heritable differences that we see in plants are life and death differences.

To one who tries to classify the plants of the world, the most funda-
mental heritable differences among them appear to be those that dif-
ferentiate the major groups, such as bacteria, fungi, algae, mosses, ferns,
and seed plants. Members of all these different groups of plants may be
found growing near each other in any natural forest, grassland and
desert, or even in water. Since they occur in most environments, the
great groups of plants became differentiated by heritable mutations that
have no survival value. Within each of these great groups, however,
species differ from each other by gene complements that have survival
value in some habitats. These points will be amplified in the next
chapter.

Many mutations are of such a nature that the mutants cannot survive
in any environment. These are called lethal mutations. Because of them
some of the several fundamental processes, such as chlorophyll synthesis
and photosynthesis, fail to occur. If the lethal mutation is dominant, the
plant perishes. But if it is recessive, it may survive in the race indefi-
nitely, if there is sufficient cross-fertilization to keep it associated in
hybrids with the alternative dominant factor. Individuals that become
homozygous for the lethal mutation perish.

Mutations that occur when plants are exposed to X-rays, extreme
temperatures, or other special conditions are reported to be of the same
kind as those that ordinarily occur in fairly uniform environments. It
appears, therefore, that the influence of environment is limited to
changes in rate of mutation and to the survival and distribution of the
new types of plants formed.

Importance of scientific evidence. In view of the facts discussed in the
last four chapters it should be evident that the present generation is
aware of numerous botanical facts and principles that were unknown in
earlier times, and many of them were unknown or but vaguely glimpsed
before the beginning of the present century. These chapters could not
have been written today except for numerous recent discoveries in the
fields of physics, chemistry, and geology; and in the specialized fields of
botany: physiology, morphology, cytology, genetics, and ecology. The
present points of view are the results of more knowledge about the
fundamental nature of chemical stability and chemical change, a better
understanding of plant processes and development in relation to environ-
ment, and the integration of the facts of sexual reproduction, chromo-

[Chap. XXXIX MUTATIONS 487

some behavior, and the transmission of hereditary factors during the
hfe cycles of plants.

For all these advances in knowledge civilization is indebted to the
scientific method of procedure in experimentation and observation. It is
by this procedure that man has learned what to believe about his en-
vironment, and how to manipulate a portion of it to his own advantage.
In the absence of data, he observes the obvious phenomena about him
and speculates about their origin. Dependable decisions come only
through intelhgent experimentation and observation, and by repeatedly
subjecting the inferences drawn to the test of new situations and addi-
tional data. The discovery of basic principles ("laws") of nature enable
us to infer with greater precision what has occurred in the past and
what is most likely to occur in the future. We can be reasonably sure, for
instance, that every person in the world older than we are, was once an
infant who developed from a fertilized egg. We can be equally certain
that if there are new species of oak trees a million years from now, they
will have been derived from oak species now living. The number of such
inferences one may draw about living organisms depends upon the num-
ber and kind of scientific principles which he fully appreciates.

You may remember that Camerarius and other investigators during
the 18th century fully confirmed the fact that sexual reproduction occurs
in plants. But the discovery that fertilization is the actual union of sperm
and egg, and that the embryo develops from a fertilized egg, is the
product of the experiments of the second half of the 19th century. All
sorts of fantastic ideas about sex had prevailed before that time. Each of
the other major ideas discussed in these chapters passed through a simi-
lar history. We need, therefore, to distinguish clearly these early guesses,
which have been passed on to most of us by tradition, from the conclu-
sions that may be drawn today from more extensive and more critical
data.

Darwin did not know how to evaluate the various ideas current in his
time concerning evolution. He therefore began to collect facts that might
help him reach some reasonable conclusions. At the end of 20 years of
investigation he concluded that species are mutable, that by variation
new species originate from preexisting ones, and that by innumerable
slight variations the plants and animals of the present have descended
from only a few primordial progenitors of early geologic time. The nu-
merous discoveries since Darwin's time have reasonably confirmed his
conclusions that species are mutable, and that the plants and animals of

488 TEXTBOOK OF BOTANY

today are the modified descendants of preexisting ones, and so on back
through milhons of years. Darwin, of course, tried to explain how all
these changes in living organisms may have occurred. We may not agree
with all his explanations today, for at that time ( middle of 19th century )
nothing was known about chromosomes or their relations to hereditary
factors, and there was no clear distinction between heritable and non-
heritable variations.

Mendel did not know what to believe about the various ideas con-
cerning hybrids that were current in his time. Two years before Darwin
published his book, the Origin of Species, Mendel began his famous
experiments with peas. We are already familiar with the definite ratios
he discovered through his experiments. Even more important, however,
is his inference that hereditary factors must be discrete units that may be
combined in cells and later separated without loss of their individuality.
At the turn of the century, Bateson of England and de Vries of Holland
came to the conclusion, on the basis of experiments, that all hereditary
variations occur as if they were the result of abrupt (discontinuous)
changes in the cells. The clear recognition of chromosomes in 1884, and
the subsequent discoverv of their regular behavior, of their relation to
the transmission of hereditary factors, and of their occasional aberra-
tions furnished a background of fact adequate to confirm the conclu-
sion of Bateson and de Vries.

This conclusion is quite distinct from Darwin's earlier idea that
changes in organisms come about bv the accumulation of slight continu-
ous changes occurring in the same direction, and surviving because they
are of some "advantage," or of "adaptive value." We have already shown
that mutations may be beneficial, harmful, or inconsequential, and that
their survival does not depend upon their being beneficial to the plant.

In the quest for knowledge and understanding of living organisms, if
we cannot make pertinent observations and perform critical experiments
ourselves, we can at least base our thinking upon the well-attested data
discovered by others.

REFERENCES

Aase, Hannah C. Cytology of cereals. Bot. Rev. 1:467-496. 1935.

Blakeslee, A. F. The present and potential service of chemistry to plant breed-
ing. Amer. Jour. Bot. 26:163-172. 1939.

Blakeslee, A. F., and A. G. Avery. Methods of inducing doubling of chromo-
somes in plants. Jour. Heredity. 28:393-411. 1937.

[Chap. XXXIX MUTATIONS 489

Cooper, D. C. Artificial induction of polyploidy in alfalfa. Amer. Jour. Bot.
26:65-67. 1939.

Crane, M. B., and W. J. C. Lawrence. The Genetics of Garden Plants. 2nd
ed. The Macmillan Company. 1938.

Demerec, M. Unstable genes. Bot. Rev. 1:233-248. 1935.

Goodspeed, T. H., and Fred M. Uber. Radiation and plant cytogenetics. Bot.
Rev. 5:1-48. 1939.

Jones, W. N. Chimeras: a summary of some special aspects. Bot. Rev. 3:545-
562. 1937.

Lindstrom, F. W. Genetics and polyploidy. Bot. Rev. 2:197-215. 1936.

Muller, H. J. Why polyploid\' is rarer in animals than in plants. Amer. Natu-
ralist. 59:346-353. 1925.

CHAPTER XL

TYPES OF VARIATIONS AND DIVERSITY OF ORGANISMS

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In all his attempts to arrive at a scientific basis of improving plants and
animals by selection and controlled breeding, and in his efforts to inter-
pret the diversity of living organisms in the world about him, man has
been confronted v^ith the problem of understanding the nature of organic
variation. His various ansv^ers to the problem are voluminously illus-
trated throughout recorded history, as are also the various social impli-
cations he attached to any particular answ^er. For a long time the prob-
lem was attacked by casual observations accompanied by philosophical
and metaphysical speculation. As a consequence we have socially in-
herited all sorts of contradictory explanations and their associated
phrases and slogans.

It is most important for us today, therefore, to try to recognize estab-
lished facts and see what inferences may reasonably be drawn from
them. The numerous experimentally established facts included in previ-
ous chapters may be used as a basis for ( 1 ) distinguishing types of varia-
tions, ( 2 ) considering the causes and consequences of the different types,
and ( 3 ) understanding the uses one may make of a knowledge of these
variations. Other data will be included in the chapters on the great
groups of plants; but we shall have a better appreciation of certain
features of the different types of plants in the world today if we can
approach a study of them with certain basic ideas about how they came
to be.

Solely on the basis of heredity, and without any reference to causes,
the variations in living organisms may be classified as either heritable or
non-heritable. Having made such a classification of variations, one may
then attempt to distinguish their underlying causes and consequences,
as we have tried to do in many of the foregoing chapters.

The first part of this chapter is primarily a review and summary of
several previously described facts about variations. Some of the uses
that may be made of these facts in evaluating various ideas about evolu-
tion and adaptation are briefly outlined in the remainder of the chapter.

490

[Chap. XL VARIATIONS AND DIVERSITY OF ORGANISMS 491

Non-heritable variations. Non-heritable variations, or fluctuations, are
merely differences in the expression of hereditary potentialities in indi-
viduals of similar heredity growing in different kinds of environments.
They are the kinds of variations in the development of a plant that man
is trying to obtain when he adds water and fertilizers to the soil, or other-
wise tries to change the environment in a way that will be most favor-
able to the growth of a particular kind of plant. Any one of several
environmental factors may greatly alter the development and appearance
of a plant by affecting the rate of such processes as photosynthesis,
respiration, and transpiration, or by affecting the permeability or other
physical conditions of the protoplasm. In fluctuations, therefore, the
hereditary mechanism of the plant remains unchanged, and the heredity
of the progeny is not influenced by the environment of the parent plant.

This fact is most easily tested experimentally with pure-line plants.
One example will be cited. In Norway a pure line of oats was cultivated
in experimental plots in several different types of soil and climate in
different parts of the country. When the growth and development of the
plants in the various plots were compared, several differences in yield
and other characters were evident. A careful record of all these diflFer-
ences was made in an experiment which was continued for 7 years. At
the end of that time seeds from five of the plots in different parts of the
country were planted in one plot in the same conditions of soil and
climate. The resulting plants were all alike. That is, there were no last-
ing effects of the variations in development that occurred in the plants of
the preceding seven generations in the diverse environments to which
they had been exposed. In all, seven different pure lines of oats and two
pure lines of barley were similarly tested and no changes occurred in the
heredity of the progeny.

Experiments of this sort do not prove that a change in environment
never causes a heritable change in a plant. But they do show that the
variations we have called fluctuations are non -heritable, that they have
no influence on subsequent progeny, and that they are not the means by
which improved varieties were obtained through domestication, or by
which evolution occurs. Incidentally, these experiments also show some
of the fallacies in the popular belief in the inheritance of acquired
characters.

Fluctuations are, however, of great economic importance. It is to se-
cure the best possible expressions of the hereditary potentialities in our
present crop plants that so much labor is annually expended in cultivat-

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TEXTBOOK OF BOTANY

ing them, and that concerted eflForts are now being made to check the
rapid deterioration of one important part of their environment, the soil.
New kinds of plants are obtained only by changes in hereditary factors,
not through fluctuations.

Heritable variations. Heritable variations may be further distinguished
as hybrid variations and mutant variations. Further illustrations of mu-
tants are given in Plates 5, 6, and 7.

Plate 5. Double and crested flowers of cosmos (right) from mutations in the

parent stock (left).

The cause of hybrid variations is easily recognized. Cross-fertilization
results in combining a part of the hereditary mechanism and potentiali-
ties of two genetically diflFerent plants. The chromosomes and their con-
tained genes are brought together in the fertilized egg during cross-
fertilization and become segregated in subsequent generations of the
hybrid.

The causes of mutations are not as easy to recognize as are the causes
of fluctuations and hybrid variations. In spite of the prevalent popular

[Chap. XI. VARIATIONS AND DIVERSITY OF ORGANISMS

493

Plate 6. Tnllium grandifloriim above and a mutant variety {Trillium grandijlonim

plenum album.) below.

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TEXTBOOK OF BOTANY

Plate 7. The red sunflower, a mutant from the common yellow-flowered ancestor

(Helianthus anninis) .

idea that environment is responsible for about everything a hving or-
ganism does, it appears, on the basis of experimental evidence, that the
causes of mutations are wholly or mainly intrinsic. The effects of the
environmental factors appear to be limited largely to changes in the rates
of mutation rather than changes in the kinds of mutation.

Perhaps the popular belief in the omnipotence of the environment is

[Chap. XL VARIATIONS AND DIVERSITY OF ORGANISMS 495

due to the acceptance and retention of a too literal version of the fiction
with which we were entertained in childhood. The fantastic but enter-
taining Just So Stories in which Kipling tells us how the elephant got its
trunk and the leopard got its spots; or the equally fantastic stories in sup-
posedly more serious literature that tell us how the giraflFe got its long
neck and why certain reptiles have no legs, should be read and remem-
bered as interesting flights of fancy, but not allowed to interfere with
scientific thinking. They admirably illustrate the traditional belief in the
inheritance of acquired characters.

If environment does change the heredity of a plant it must cause an
alteration in the composition or in the arrangement of the hereditary
units of matter. Furthermore, any change that is inherited through sexual
reproduction must occur either in the reproductive cells, or in cells that
are the forerunners of reproductive cells.

Changes in the forerunners of reproductive cells which later become
inherited are much more likely to occur in plants than in animals be-
cause the reproductive tissue becomes differentiated from bodv tissue
much earlier in the development of an animal. Neither the reproductive
cells nor any forerunners of them are in the neck of the giraffe, in the
legs of a reptile, or in the trunk of an elephant. In a seed plant, however,
cells in the growing stem tips are remote forerunners of the reproductive
cells, which develop in the stamens and pistils of flowers after the plant
has passed through a vegetative phase of growth. Beginning with a
fertilized egg, or with a vegetative propagule, cells follow each other in
regular sequence by billions of cell divisions up to the immediate fore-
runners of the reproductive cells. Hereditary changes that occur in these
vegetative (body) cells previous to or during their division may be
transmitted by subsequent cell divisions to the reproductive cells. Even
this condition does not exemplify the traditional belief in the inheritance
of acquired characters, for that belief is based upon the assumption
that changes — including fluctuations — in mature body cells that are not
forerunners of reproductive cells may be transmitted to the offspring.

The origin of new kinds of plants by evolution. There are two very
different methods of approaching a study of evolution. One method is
descriptive; the other is interpretative. The interpretative method will be
noted first. The fundamental processes in living organisms may be studied
until those that inevitably result in evolution are recognized. This is the
method of approach we have been using throughout most of this book.

496 TEXTBOOK OF BOTANY

Not much more need be said to emphasize this point of view or to review
the initial processes of evolution enumerated in the preceding chapter.

Heredity and heritable variations in living organisms must depend
upon small heritable units of matter, such as chromosomes and genes, in
the protoplasm. These units of matter are in the fertilized egg, and are
transmitted from cell to cell during cell division throughout the life cycle
of a plant. The smallest plants, such as the blue-green algae and bacteria,
have no sex, no organized nuclei and chromosomes, and yet there are
distinct species of them just as there are of other plants. In these smaller
plants the hereditary units of matter appear to be distributed throughout
the protoplasm of the cell. In other plants these units of matter, so far
as known, are mostly concentrated in the chromosomes within the nuclei.

Changes in one or more of these units of matter may result in the for-
mation of new varieties. Further changes may result in what are called
new species. New species, then, are primarily the old species with cer-
tain changes in chromosome and gene complements and their consequent
effects on visible characters. Whenever the accumulation of aberrant
changes in chromosomes and genes in a particular variety of plant
reaches the point where the individuals of that variety in nature no
longer cross freely with individuals of the ancestral stock, or with indi-
viduals of other varieties, it is well on the way to being a new species:
the kind of species we recognize when we speak of red oaks, black oaks,
and white oaks. So long as the individuals of a variety cross freely with
those of other varieties of the ancestral stock, the variety fails to acquire
that individuality we recognize in most species, and we refer to it merely
as a hybrid variety. It is not always easy to decide when a variety has
enough individuality to be called a species.

The origin of a new species from an old one may come about by a
series of changes over a long period of time, but perhaps it may occa-
sionallv occur rather abruptly following a rare kind of cross-fertilization
exemplified by the new radish-cabbage species mentioned in the pre-
ceding chapter. The doubling of chromosomes may also be a rather
abrupt beginning of a new species, particularly if the progeny with the
increased number of chromosomes grow in a wider range of habitats. If
there is no crossing with the parent stock, a few more changes in the
chromosomes and genes may make the variety with the double number
of chromosomes sufficiently different to be considered a new species.

It is important to remember that a new species is primarily the old one
with a few differences in chromosomes and genes. These hereditary
changes in plants and animals on the earth have been occurring for a

[Chap. XL VARIATIONS AND DIVERSITY OF ORGANISMS 497

billion years or more, and they are the fundamental underlying steps of
the great diversity of plants and animals on the earth today. Geologically,
the most recent kind of plants are the seed plants, and the most recent
animals are the mammals. Since they were formed by changes in a
comparatively few of the hereditary units of their ancestors, we may
expect to find them fundamentally much more like than unlike their
ancestors. In fact, we may expect to find in them certain basic features
that were in their remote ancestors many hundreds of millions of years
ago. These basic features are, of course, the fundamental physiological
processes that are essential to all living organisms. Variations in heredi-
tary factors that inhibit, or unbalance, any one of these fundamental
processes are lethal, and they result in the extinction of organisms when
not annulled by alternative dominant factors.

The initial steps in the formation of a new species, then, may be
visualized somewhat as follows: origin of heritable differences by such
changes in chromosomes and genes as those described in the preceding
chapter; segregation of these differences in certain individuals; a de-
crease in cross-fertility, or a decrease in cross-fertihzation because of
geographic isolation; and an increase in homozygosity and distinct in-
dividuality.

The descriptive method of approach to the study of evolution was the
one followed by our forefathers before chromosomes and Mendel's law
of hybrid segregation were knovm. This method includes the study of
the visible characters of organisms; their methods of development and
reproduction; and their fundamental similarities, differences, and rela-
tionships, including those of fossils. Some of these facts will be evident
in subsequent chapters.

The study of evolution from this point of view led to the discovery of
several fundamental principles about living organisms. A few of those
upon which there is general agreement are briefly stated below. They
will be of greater interest if each one is considered with reference ( 1 ) to
the probability of its being explained on the basis of the facts you know
about chromosomes and genes, and ( 2 ) to the extent to which any pre-
conceived ideas you have held about the subject are incorrect.

The kinships of organism, both living and fossil, are interpretable only
on the basis of descent by modification (evolution) through a long period
of time.

Diversity of organisms is the result of evolution. That is, species are
not specially created, unchangeable units.

498 TEXTBOOK OF BOTANY

Within a group of related organisms, such as the species in a genus,
the different ones may be arranged in series such that some particular
character appears in ascending or descending order of magnitude. This
fact does not imply that the different species necessarily evolved in this
order, but it does imply that intrinsic conditions limit evolution.

Evolution of organisms is limited in direction. That is, it does not pro-
ceed in all conceivable directions.

The same general trends of evolution occur in organisms living in
very diverse habitats — for instance, in water and on land.

Similar trends of evolution occur in different taxonomic groups of
organisms.

Divergent trends of evolution occur in organisms living in similar
environments.

Evolution may continue in a definite direction in a race of organisms
until it results in processes and structures that are handicaps to the race
and bring about its extinction. That is, "adaptation" is not the end of
evolution.

Evolution may result in the development of useful organs and in the
subsequent deterioration of such organs to vestigial structures.

Species formation is increased by geographic isolation, and by other
means that eliminate cross-fertilization.

Adaptation. In an earlier chapter attention was called to the desir-
ability of periodically considering the appropriateness of terms used in
biology. The term adaptation was in the list cited at that time. It is often
stated that plants, and also animals, may become "acclimated" or
"adapted" to new conditions. One even hears that they adapt, or accli-
mate, themselves to new conditions. If this latter statement is true, there
is no need to discuss the matter, for there is no evidence that plants can
plan and direct changes within themselves to meet new conditions. To
say that they become acclimated, or adapted, states a condition that can
be experimentally investigated. We may arrive at an answer as to how
they become adapted, if they do.

It may be untrue, however, to say that a plant becomes adapted to a
new condition. When we say that a plant is adapted to a certain condi-
tion we are merely saying that it can live, grow, and reproduce in that
condition. If it cannot live there — is not adapted — and we place it there,
it dies. It neither adapts itself nor becomes adapted. To do so, it would
suddenly have to become another kind of plant by an abrupt change in
its heredity.

[Chap. XL VARIATIONS AND DI\'ERSITY OF ORGANISMS 499

One often hears that plants are well adapted to the environments in
which the)' grow. Any serious ecological study of a plant will most
probably show that it may be found in some of the habitats in which it
grows well — is well adapted — that it also occurs in other habitats in
which it barely survi\'es, but that it does not occur in habitats in which
it cannot survive. All this merely means that some plants grow better
in some environments than others do because they are different kinds
of plants. It will become clearer later on that a distinction should be
made between the popular idea that an individual plant becomes
adapted, and the botanist's idea that a species of plant becomes adapted.

In one sense, however, it is true that an individual plant becomes
adapted, or better adapted, to a certain environmental condition. This
fact may be easily illustrated with seedlings, say of cabbage. If seeds of
cabbage are planted in three different plots, one of which is kept warm
and moist, another wami and comparatively dry, and the third cool and
either moist or drv, the seedlings will grow in all three plots. A few weeks
later, if the seedlings are exposed to a freezing temperature for a few
hours, those in the warm plot will be killed, while those in the dry and
cool plots may survive. They have become physiologically different.
The gardener says they have become "hardened." This physiological
conditioning of a plant, however, is not what is usuallv meant by the
terms acclimated and adapted. It is merely a fluctuation that is not trans-
mitted to the progeny.

One usually refers to the acclimation and adaptation of a mixed popu-
lation of plants rather than to a single kind of plant. A few examples will
be discussed briefly because of the principles they exemplify. A popula-
tion composed entirely of the same kind of homozygous close pollinated
plants 'SA'ould behave like a single indixidual, provided, of course, that
no mutations occurred in any of the indi\'iduals. If there were mutations
in some of the individuals then we would no longer have a homogeneous
population of plants, but a mixed one of different kinds of plants. In a
population composed of different kinds of plants, some plants behave
differently than others when the environment changes.

Examples of mixed populations of plants are: a bluegrass lawn in
which there are also other kinds of grasses, clover, and weeds; a forest;
or a field of crop plants composed of closely related species and varieties
of plants.

Several kinds of changes may occur in a mixed population, but it is a
question whether any of them is correctly referred to as becoming ac-

500 TEXTBOOK OF BOTANY

climated, or adapted. From your own experience with lawns, you know
that new kinds of plants may come in through seed dispersal, some plants
present may die during a hot dry summer, and following the proper
application of water and certain fertilizers bluegrass may grow so luxuri-
antly that many other plants become overshaded and die.

Experimental studies of the so-called acclimation of crop plants that
have been introduced from one country into another show that some of
the species and varieties in the mixed crop simply grew better than
others in the new countrv. These varieties produced more seed than
those that did not grow so well. Hence as the farmer harvested the seeds
each year he obtained a greater and greater percentage of seeds of those
plants that were already well adapted to the conditions in the new
country.

The above example refers to experimental studies of a mixed popula-
tion of close pollinated plants such as wheat, oats, and barley. If, how-
ever, the mixed crop consists of hybrids, or of open pollinated plants
that are cross-fertile, various combinations and segregations of factors
would occur each year. Here again man, by selecting seeds, would be an
additional factor in the final outcome.

The influence of man is exemplified by our great array of domesti-
cated plants, most of which would become extinct within a few years if
he were suddenly to disappear from the face of the earth. Exclusive of
modern methods of controlling plant breeding, he has been a factor in
the evolution of domesticated plants for many thousands of years ( 1 ) by
placing the plants close together in his fields and gardens, thus facilitat-
ing cross-fertilization, ( 2 ) by introducing plants from other parts of the
world that would interbreed, ( 3 ) by cultivating in a way that eliminated
ecological influences of one plant upon another ("competition"), (4) by
preserving the plant during seasons of the year in which it might other-
wise perish, and (5) by selecting certain varieties and eliminating
others.

In the wild state all plants grow in mixed populations. In what ways
do these mixed populations, when not interfered with by man, differ
from those of crop plants under the conditions of cultivation? In the
first place, all their interrelations are dependent solely upon the "laws"
of nature. No conscious selecting, preserving, protecting, and distributing
takes place. Instead, random (chance) survival, random natural elimi-
nation (destruction) by physical factors and by other organisms, and
random dispersal checked by geographic barriers are paramount.

Ever since Darwin adopted the terms "natural selection" and "sur-

[Chap. XL VARIATIONS AND DIVERSITY OF ORGANISMS 501

vival of the fittest," they have been accepted and used freely with a
variety of imphcations by some biologists, adversely criticized or re-
jected by others. Those who object to the terms regard them as slogans
that are used without an adequate analysis of the conditions from which
they are inferred or to which they are applied. Such scientists think the
terms "random survival" and "random elimination" are more in accord
with the established facts.

For example, in a natural climax forest undisturbed by man, the area
is fully occupied by all the plants that can grow there. Countless num-
bers of seedlings may be found in such forests, but the survival and
growth of any one of them to maturity depend upon the death of some
established plant. Even these innumerable seedlings are but a small per-
centage of the seeds that began to develop, because many of them are
eaten by animals and others are destroved by fungi and bacteria, or by
other means.

Since the embryo in the seed develops from a fertilized egg, there are
hazards that precede the formation of embrvos. Out of the cloud of
pollen grains containing sperms, or the generative nucleus from which
sperms are formed, perhaps not more than one potential sperm in a
trillion, or several trillions, ever unites with an egg. If any one of the rare
speiTns that unites with an egg, or an\' one of the relatively few seeds
that germinate, or of the seedlings that grow to maturitv happens to be
the "fittest" of the lot, would it be a case of "natural selection" or of
random survival?

Before the advent of man, biological conditions in the natural vegeta-
tion throughout the world were comparable to those in the forest de-
scribed above. Erosion, flooding, fires started by lightning, volcanic erup-
tions, aridity, and glaciation occasionallv destroved some of the vegeta-
tion locally and increased the areas in which seedlings might develop to
maturitv.

Furthermore, much that has survived is of no particular value to any
plant. You have learned to distinguish several different kinds of trees by
distinctive hereditary characters, such as leaf form, leaf arrangement,
type of venation, and kind of leaf margin. Not one of these characters
is of any survival value to the plant, yet each of them is an external
expression of hereditary changes that occurred and have in some in-
stances survived for millions of years.

When we distinguish families of plants on the basis of such differences
as the number of carpels in the pistil, the arrangement of the floral parts
in spirals or cycles, and the number of stamens and petals, we are again

502 TEXTBOOK OF BOTANY

recognizing hereditary differences that have occurred and survived, yet
they cannot possibly be of any consequence in the survival of the species.
In view of the fact that innumerable hereditary characters that are of
no survival value to the plant have originated by exactly the same kind
of processes that initiate valuable characters, "survival of the fittest," if
it does occur, must be limited to very special cases. The teiTn "valuable"
in the above sentence refers to those characteristics that are essential to
existence, or to those that enable plants to grow in different kinds of
environments.

Finally, a species, as generally conceived, is a mixed population of
very similar individuals, differing slightly from each other in gene com-
plement. If different individuals of a species become separated, or iso-
lated, in places where they no longer interbreed, they will in time be-
come sufRcientlv different to be regarded as different races or varieties.
Such race differences as have been intensively studied seem to bear no
relation whatever to survival, for they occurred in various local areas
having similar environments. Yet this differentiation of races is undoubt-
edly an initial step in the evolution of new species. The decisive fact is
that the indivduals became isolated from each other, and the different
kinds of mutations that occurred remained segregated in the local areas
because of the absence of cross-fertilization.

Some of these racial differences do influence fundamental processes
such as photosynthesis and respiration; and if the environment changes,
some of the races may survive better than others, and some may be com-
pletely eliminated. One mav refer to the races that survive as examples
of the "survival of the fittest." Which of the races will be the fittest, how-
ever, depends upon the nature of the changes in the environment. The
races that are destro\'ed bv one kind of change in the environment misiht
be the fittest of the population if some other kind of change had oc-
curred. There seems to be no serious objection to the use of the expres-
sion "survival of the fittest" in this specific sense. One may rightly object,
however, to the idea that the heritable changes by which the different
races originated "occurred to meet the change in environment," or that
they were actually "selected by nature." To accept this last idea, one
would have to assume that the environment changed in order to favor
certain races and exterminate others.

Another interesting point about the development of these isolated races
is the fact that some of the heritable changes that occur in them are
similar, or parallel, while others are di\ ergent. Since the individuals that
became isolated in different regional areas belonged to the same species.

[Chap. XL VARIATIONS AND DIVERSITY OF ORGANISMS 503

their gene complements were largely alike but somewhat unlike. If herit-
able variations are dependent primarily upon intrinsic conditions, there
should be both parallel and divergent evolution, regardless of whether
the environments in the isolated areas are similar or dissimilar.

In the chapter on plants of the past, we shall see that various kinds of
plants evolved, flourished for a time, and then became extinct. The
redwoods, ginkgo, and stinking cedar as well as numerous other species
of plants that were once widely distributed became extinct everywhere
except in a few local areas. Yet they grow well today when planted in
other parts of the world. They do not reproduce naturally in all these
new environments. Within the first third of the present centurv the
American chestnut was almost exterminated by a parasitic fungus. Other
species of plants are being diminished by insects, bacteria, viruses, and
fungi. By destroying natural habitats, man is also exterminating species.
Similar phenomena also occur in the animal kingdom. Toward the close
of the glacial period several kinds of animals became extinct on the
American continent. Among them were species of elephant, horse, camel,
rhinoceros, and tiger. The causes of their failure to survive are not
known, but among probable eliminating factors are diseases, changes in
physical environment, failure to reproduce, overadaptation, and perhaps
the accumulation of lethal genes.

REFERENCES

Berry, E. W. Tree Ancestors. Williams & Wilkins Company. 1923.

Darwin, Charles. The Origin of Species. A. L. Burt Company, Inc. 1859.

De\^ries, Hugo. Species and Varieties: Their Origin by Mutation. Open Court
Publishing Company. 1912.

Dunn, L. C. Hereditij and Variation. University Society. 1931.

Gruenberg, B. C. The Story of Evohition. D. Van Nostrand Company. 1929.

Hiesey, W. H. Environmental influences and transplant experiments. Bot. Rev.
6:181-203. 1940.

Hurst, C. C. The Mechanism of Creative Evolution. The Macmillan Company.
1932. ( For additional illustrations )

McAtee, W. L. Survival of the ordinary. Quar. Rev. Biol. 12:47-64. 1937.

Newman, H. H. Evolution Yesterday and Today. Williams & Wilkins Com-
pany. 1932.

Oparin, A. I. TJie Origin of Life. The Macmillan Company. 1938.

Schaffner, J. H. Color in various plant structures and the so-called principle
of selective adaptation. Ohio Jour. Sci. 23:182-191. 1933.

Smith, H. W. Kamongo. The Viking Press. 1932.

CHAPTER XLl
NON-GREEN PLANTS

<><><>0<X><£><><>C><XX><><>^^

The term "non-green plants" is ordinarily used by botanists to refer to
all species of plants that lack the inherent potentiality of synthesizing
chlorophyll. It is a hereditary characteristic of species. For example, no
chlorophyll is synthesized in any individual plant of the meadow mush-
room, or of the common bread mold, under any environmental condi-
tions. On the other hand, chlorophyll is synthesized in most corn plants
in the light, but not in the dark. The meadow mushroom is a species of
the non-green plants. Corn is a species of the green plants, even though
some individual corn plants are albinos. These albinos cannot survive
and reproduce in nature as non-green plants. Likewise coleus is a species
of the green plants, though certain varieties of it contain so much
anthocyanin that the plants appear pui-ple. Green-colored molds are not
species of green plants because the pigment in them is not chlorophyll.
The green color of frogs, snakes, insects, and birds is due to other kinds
of pigments or to the refraction of light. Some of the flatworms and
smaller animals are green because green algae live within their bodies.
The terms chlorophyllous plants and non-chlorophyllous plants may be
used in preference to green plants and non-green plants.

Nearly 80 per cent of the known species of plants are green, but there
are fewer individual green plants on the earth than non-green ones. Most
non-green plants are small and inconspicuous. Many of them are one-
celled plants which cannot be seen unless they are magnified by means
of a microscope. Among the larger fungi are the familiar molds, puffballs,
mushrooms, toadstools, and bracket fungi. Most non-green plants are
either fungi or bacteria, but there are also a few species of non-green
algae and several dozen species of non-green seed plants. Among them
are Indian pipe, dodder (Fig. 219), beechdrop, snow plant (Plate 3),
squaw-root (Fig. 220), "pine sap" and broom-rape (Fig. 221).

Certain species of algae can live in the light as green plants, and also
in the dark as non-green plants if an external supply of sugar or of
some of its derivatives is available.

504

[Chap. XLI

NON-GREEN PLANTS

505

Fig. 219. Species ot dodder {Cusciita) grow as parasites on numerous green
plants. The slender yellow stems of the one pictured here form a loose net among
the broad leaves of the host (Abronia). Photo by W. S. Cooper.

Fig. 220. Squaw-root or cancer-root (CouoplioU.s) , parasitic on roots of oak.
Flowers are present on the plants pictured on the right. Photo by G. S. Growl.

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TEXTBOOK OF BOTANY

Fig. 221. Two parasitic plants of Louisiana broom-rape (Orobanche) attached by
haustoria to roots of a tobacco plant. Photo by A. L. Pierstorff.

Non-green plants live in all sorts of habitats in water and soil, and also
within and on the bodies of plants and animals. Those that grow within
or on the living tissues of plants and animals and obtain food from them
are parasites. Those that obtain food from the dead bodies of plants and
animals, from their products, or from their non-living parts, such as the
dead bark and heartwood of trees, are saprophytes. If thev can grow onlv
within living tissues they are called obligate parasites. If they cannot
grow within living tissues, they are said to be obligate saprophytes. But
if they can grow either as parasites or as saprophytes, they are referred
to as facultative species. There are all sorts of gradations between sapro-
phytic and parasitic plants.

With the exception of a few special groups of bacteria, non-green
plants cannot svnthesize sugar. Some of them, such as veast, when sup-
plied with an external source of sugar, inorganic salts, and water, can
make the rest of the foods necessary for growth and reproduction.
Others are dependent upon an external source of amino acids, or of pro-
teins also. Plants that can svnthesize su2;ar are sometimes called auto-
phytes in contrast to parasites and saprophvtes. In addition to food, non-
green plants may also be dependent upon an external supply of certain
vitamins and hormones.

[Chop. XLI NON-GREEN PLANTS 507

We mav applv at once all that we know about the physiology and
heredity of green plants toward an understanding of non-green plants,
for the living part of a non-green plant is protoplasm and there are
definite species of them just as there are of green plants. The principal
differences between these two types of plants lie in the processes by
which thev obtain food and in the consequences of these processes.
Some of these consequences are beneficial to other plants and to animals,
and some of them are harmful and destructive.

Parasitic plants. When a plant grows within or on, and also subsists
upon food from the living parts of another organism, it is a parasite. The
parasitized organism is the host. It may or mav not be injured by the
parasite. If it is injured, it is said to be diseased. Some parasitic plants
are beneficial to the host. The nitrogen-fixing bacteria in the roots of
clover, for example, are beneficial to
their host, since upon their death
their chemically bound nitrogen be-
comes available to the clover plant.

All sorts of gradations may be
found from extreme parasitism to a
total lack of it. Some parasites grow
only within certain organs of one
kind of host. Others grow within or
on numerous kinds of hosts. Still
others are but partial parasites. Some-
times both host and parasite are
benefited rather than harmed by
their relationship.

Some plants are merelv perched
upon or attached to others, but se-
cure no food from them (Fig. 222).
These plants are known as epiphytes,
in contrast to parasites. Examples of
epiphytes are Spanish moss hanging on the branches of a tree (Fig.
223 ) , and the mosses and lichens attached to the bark of trees wherever
trees grow. In deciding whether a plant is a parasite, it is necessary to
consider both its position and its source of food. It may be even more
difficult to decide when an animal is a parasite. A protozoan may live
within the cells of a plant. Roundworms burrow into roots and survive
on the food within the root. Aphids obtain food by sucking the "juice"

Fig. 222. An epiphytic orchid the
roots of which merely hold the plant
on the branch of the tree. After Kerner.

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TEXTBOOK OF BOTANY

Fig. 223. Festoons of Spanish moss {Tillandsia usneoides) pendant on the
branches of a hve oak in Florida. Spanish moss is a member of the pineapple
family. Photo by G. W. Blaydes.

from living plants. A grasshopper climbs upon a plant and eats its young
leaves, while a cow roams over the pasture and eats the live leaves of
grasses.

A mistletoe which grows on a variety of trees in the southern half of
the United States is a good example of a partial parasite ( Fig. 224 ) . Its
sticky seeds adhere to the branches of trees; and when one of them
germinates, a root-like haustorium grows through the bark to the con-
ductive tissues of the host. The stems and leaves of this mistletoe con-
tain chlorophyll, and some sugar is synthesized in them. Water and
salts are of course obtained from the soil through the host. No one knows
at present what percentage of the food of the mistletoe is obtained from
the host, or whether it can develop under experimental conditions in
the absence of a host. The relation of such plants as the mistletoe to
their hosts is similar to that of a grafted scion to the stock.

An analogous partial parasitism occurs when the root tips of green
plants penetrate the roots of other green plants and become haustoria.

[Chap. XLI

NON-GREEN PLANTS

509

Fig. 224. The evergreen mistletoe is conspicuous on deciduous trees during the

winter months.

How frequently this process occurs is unknown. The roots of some plants,
such as those of bastard toad flax of the eastern states, and of the desert
shrub, Krameria, of the southwestern states, habitually form haustorial
unions with the roots of man^' other plants. It is well known that unions
occur among the roots of adjoining forest trees. Whether they are of
any yalue to the trees is not known.

Some non-green seed plants, such as the small purple beechdrops at-
tached to the roots of beech trees and the slender climbing dodder which
becomes attached by haustoria to the stems of a great yariety of hosts,
obtain food directly from green plants. The Indian pipe, when casually
obseryed, appears to be a saprophyte liying on the litter of decaying
leayes on the forest floor, but it apparenth obtains all its food from
fungi which digest the fallen leayes, and also liye in and on its roots.
Seyeral layers of fungal filaments completely invest each of its many
short roots, and numerous branching filaments of the fungus peiTneate
the surrounding layer of decayino; leaves and humus. Owino^ to the sur-
rounding sheath of fungal filaments, the cells of the roots of Indian pipe
are not in direct contact with the soil.

Fungi that live in such intimate relation with the roots of plants are

510

TEXTBOOK OF BOTANY

called mycorhizal fungi. They may live wholly external to the root, merely
forming a sheath around it; the fungal filaments may grow into the cells
of the root; or the entire fungus may grow within the cells of the root
( Fig. 225 ) . Such mycorhizal fungi are habitually present on the roots of

Fig. 225. External and internal mycorhizal fungi. After Frank and Magnus.

most green plants in forests. Some of them haye been reported as harmful
to certain trees, others as beneficial, or even essential to them. Under
natural conditions, pines (Fig. 226), heath plants, and beeches are ap-

FiG. 226. Eastern white pine mycorhiza (A), and root (B) not infected by fungi.
Drawn from photo in Black Rock Forest Papers, 1:66, 1937.

parently dependent upon mycorhizal fungi, for their seedlings live but a
few years if deprived of them under experimental conditions.

Diversity among the non-green plants. There are all sorts of gradations

[Chap. XLI NON-GREEN PLANTS 511

between saprophytes and parasites, and also between these two groups
and the autophytes. Many terms have been proposed to designate these
intergradations.^ Some of this diversity is related to ( 1 ) the kinds of
substances the organism can digest and utilize as food, (2) the method
of securing food, and (3) the kind of substance oxidized during
respiration.

In the cells of all animals, of all green plants, and of most non-green
plants also, respiration involves the oxidation of reduced carbon atoms
in sugar, or in organic compounds derived from sugar. When this oxi-
dation is complete, one of the end products is carbon dioxide. A few
groups of bacteria, however, are unique in that they oxidize reduced
nitrogen, reduced sulfur, or reduced iron. Consequently nitrates, sulfates,
and oxidized iron in the soil are among the end products of respiration
of these bacteria. The oxidized compounds are essential to green plants,
which are therefore partly dependent upon the activities of these bac-
teria. The bacteria in turn are partly dependent upon the activities of
green plants for reduced nitrogen, and upon both green plants and other
kinds of bacteria for reduced sulfur and iron.

These special groups of bacteria are unique in one other way. Al-
though they are non-green and are underground, they synthesize sugar
from carbon dioxide and water. The energy necessary for this synthesis
is obtained from the oxidation of reduced nitrogen, sulfur, and iron.
They are as truly autophvtes as the green plants. The synthesis of sugar
in these bacteria in the absence of light is often referred to as chemosyn-
thesis in contrast to photosynthesis. Similarly, certain bacteria may oxi-
dize reduced manganese and synthesize sugar.

The green and some purple bacteria ( Plate 3 ) contain pigments simi-
lar to chlorophyll; and when they are exposed to light, photosynthesis
occurs within them.

Autophytes. All plants, whether green or non-green, in which sugar is
synthesized from carbon dioxide and water are called autophytes in con-
trast to parasites and saprophytes. Perhaps the term should be limited to
those in which the synthesis of amino acids and proteins also occurs.
Most autophytes, however, are not independent plants. They could not
continue to live in a world devoid of all non-green plants. The non-green
plants are dependent upon certain reduced compounds made in the

^ J. M. Coulter, C. R. Barnes, and H. C. Cowles, A Textbook of Botany, American
Book Company, 1931, Vol. 3.

512

TEXTBOOK OF BOTANY

green plants.- The green plants in turn are dependent upon certain oxi-
dized compounds from the non-green plants. Dependent relations be-
tween green and non-green plants briefly outlined above are indicated in

Fig. 227. Diagram of some dependent relations among green and non-green plants.

Figs. 227, 228, and 233. Others will be indicated in later chapters.
In the absence of adequate data we have assumed that the first food
made from carbon dioxide by autophytic bacteria is some kind of sugar,
as it is in green plants.

Saprophytic plants. These are the organisms that cause most of the
decay of organic matter. They live upon and obtain food from the dead
bodies of plants and animals, and also from the products of all living
organisms. Some of them have enzymes that digest cellulose, pectic com-
pounds, and even kerosene and coal. Digestion of these substances occurs
outside the body of the plant. Their enzymes, therefore, must diffuse
from the living cells where they are synthesized into the surrounding
media.

- Probable exceptions are the sulfur bacteria. Although they utilize the sulfur reduced by
other plants, it is possible that sulfofying bacteria could survive independently by utilizing
the sulfur and sulfur compounds (SO2 and H„S) emitted by volcanoes. The iron bacteria
also have possibilities of living independently of other plants.

[Chap. XLl NON-GREEN PLANTS 513

External digestion and the digestion of substances that cannot be
digested by most other organisms are outstanding features of this group
of plants. Among the animals there are a few protozoans, such as those
in the alimentary tract of termites, that can digest cellulose. External
digestion, however, is a common characteristic of animals, for the food
within the alimentary tract is still outside the cells of the body. The proc-
esses that occur in the digestion of bread in the alimentary tract of an
animal are no different fundamentally from those that occur when bread
is decomposed by mold. The plant may digest certain substances the
animal cannot, but saprophytic plants are present in the alimentary tracts
of animals and they may digest certain substances there. Some wood-
boring beetles can chew the wood, but their internal fungi digest it.

Without the saprophytic plants the surface of the earth would soon
become encumbered with the undigested remains of plants and animals.

'^DISSOLVINGN

GREEN
PLANTS

SUGAR

\NIMAL$

AND

PARASITIC
PLANTS

'/?

PLANT

AND

ANIMAL
.RESIDUES/

LIMESTONE

/ORGANISMSN

sDther Rocksy

Y %

:arbon dioxide

/OCEAN WATER\

AND k^

VOTHER NATURALS
WATERS

CARBON
DIOXIDE

OF THE

V ATMOSPHERE;
CO2

t)EPOSITION^

OF

ARBONATESl

I as Shells. Marl,
Vand Coral Rsefa

,v<^

r^Oli,

■-£2!^SUST10N

DECAY

iacteria, FunpV

COAL,
fPETROLEUM'l

AND

GAS

Fig. 228. The caibon cycle in nature.

Except in bogs where the growth of saprophytes is comparatively slow,
only a thin surface layer of humus has accumulated in all of geological

514 TEXTBOOK OF BOTANY

time. Moreover, without a comparatively rapid digestion of cellulose and
the subsequent oxidation of the products of digestion, the supply of
carbon dioxide in the air would gradually become depleted ( Fig. 228 ) .

During decay not only is carbon dioxide formed and liberated to the
air, but inorganic ions are liberated from their organic union and are
again available to green plants.

Some saprophytic plants are valued in industry because they do not
completely oxidize certain compounds, but leave a residue which man
values for certain purposes. These residues are often referred to as prod-
ucts of fermentation. The complete oxidation of a substance in sapro-
phytic plants usually consists of a series of steps, each of which is the
result of the activity of successively different organisms. Sugar is oxidized
to alcohol by yeast. Acetic acid bacteria oxidize the alcohol to acetic
acid. Other organisms then oxidize the acetic acid to simpler compounds
and finally all are oxidized to carbon dioxide and water. After insoluble
substances are digested, many intemiediate and successively simpler
compounds may be formed before oxidation is complete. Moreover, the
kinds of intermediate products formed vary with the species of organism
that oxidizes each of them. The industries producing beer, wine, vinegar,
glycerin, alcohol, and cheese all depend upon the fermentation products
of carefully cultivated species of saprophytes. The quantity and quality
of the products vary with the kinds of fungi and bacteria present.

The unpleasant odors of decaying matter likewise are due to the in-
termediate products of oxidation-reduction which are formed during the
growth of certain bacteria and fungi.

Saprophvtes are ever-present agents of destruction, and are the or-
ganisms that make cold storage and refrigeration necessary in our modern
civilization. The freezing, canning, drying, and preserving industries are
based on methods of eliminating saprophytes. Buildings and other wood
structures are protected from them by means of metal, paint, tar, and
creosote. On the other hand their very destructiveness is valued as a
means of removing sewage and other undesired organic products. Sapro-
phytes, then, are as intimately associated with our daily lives as are the
green plants and parasites.

CHAPTER XLII
THE BIOLOGY OF BACTERIA

<><><x^<t>0<>0<><cXX><c><><><^<X>^^

Of all the groups of non-green plants, the ever-present bacteria and
their effects are most often discussed among educated people every-
where. Although the great majoritv of the human race has never seen
bacteria, every individual has had both direct and indirect contacts with
them. The forms of some of the common species of bacteria are shown
in Fig. 229. They are the causes of many diseases and of much of the
decay of organic substances. Certain pleasant and unpleasant flavors of
foods such as cheese, milk, butter, and eggs result from bacterial action,
and bacteria influence our lives in many other ways. As a matter of
fact, household sanitation, sewage disposal, refrigeration, quarantine,
vaccination, aseptic surgery, personal hygiene, and all of our splendidly
organized and vitally necessary methods of guarding and purifying water
have developed as we have learned how and where bacteria live and
how they affect plants and animals, including ourselves.

Some of the factors affecting growth of bacteria are water, tempera-
ture, certain gases, mineral salts, light, and food. Such factors also
influence the growth of green plants, but not necessarfly in the same way
or to the same degree. A particular species of bacteria does not grow
in all environments, but the diversity of species is so great that there
are few habitats in which some kinds of bacteria do not live. Certain
species of bacteria are widely distributed over the earth. Others live
only in very specific habitats. For example, some grow only in certain
species of plants and animals, and sometimes only within certain tissues
of the host.

Some bacteria manufacture their own food; others secure it from liv-
ing or non-living sources. A few species are unique in that they can
synthesize sugar and other foods from water, carbon dioxide, and in-
organic salts. Most species, however, depend upon an external source of
sugar, and sometimes of protein also. At one end of the scale are the
bacteria that derive energy from light, or from compounds containing
reduced nitrogen, iron or sulfur, and synthesize sugars and other foods.

515

516

TEXTBOOK OF BOTANY

,»• .O- %» ^J»

•."»

8 -^ • . «•

Staphylococcus aureus Diplococcus pneumoniae Staphylococcus pyogenes

^A\

Bacillus subtilis

Corynebacterium diphtheriae

Eberthella typhi

Vibrio comma Spirillum volutans Treponema pallidum

Fig. 229. Various types of bacteria. Copyright by General Biological Supply House.

Except for the sources of the reduced nitrogen and sulfur, such organ-
isms are independent of other Hving plants. At the other extreme is the
obligate parasite that grows only in the living tissues of another organ-
ism. Between these two extremes are the various saprophytes that live
upon dead tissues and products of other organisms.

The autophytes among bacteria include the nitrifying bacteria which
oxidize ammonia to nitrites and nitrates; the iron bacteria which oxidize
ferrous to ferric salts; and some sulfur bacteria which oxidize reduced
sulfur. These are extremely important in soil development and will be

[Chap. XLII THE BIOLOGY OF BACTERIA 517

dealt with in more detail in the following chapter. The energy released
by these oxidations is utilized in the synthesis of sugar and other foods.
The other autophytes are the green and purple sulfur bacteria, the first
of which utilize the energy of light and the second the energy of reduced
sulfur in making sugar. ^

Most species of bacteria are obligate saprophytes or parasites, but
some of them may grow as parasites or as saprophytes.

External factors and bacteria. The environments in which bacteria and
their allied organisms live and reproduce are so different from those of
larger plants that some of the effects of environmental factors need to be
considered, especially those of light, water, oxygen, and temperature.

Light. Most bacteria, because of their location in soil, in foods, in
decaying matter, and within other living organisms are rarely exposed
to the direct rays of light. Most species live only a few hours when
exposed to full sunlight. Sunlight, therefore, is a destructive factor in
the development of bacteria and it is of great importance in the purifica-
tion of rivers polluted with sewage, and in the elimination of bacteria
from all exposed surfaces.

The rapidity with which certain bacteria are killed bv direct light
depends upon the intensity and quality of the light. Although the evi-
dence is extremely difficult to obtain and the results are often conflicting,
it appears that the rays of the blue and violet end of the visible spectrum
are most injurious to bacteria, particularly the pathogenic species. The
intensity of the light used must always be kept in mind in comparing
the destructive effects of various radiations. Near infra-red rays through
their heating and drying effects may be germicidal. Ultra-violet rays, if
not absorbed too quickly by the medium, are very destructive. Bacteria
are destroyed by short rays of light apparently through the coagulation
of the protoplasm, or the formation of some toxic substance in the
medium.

Water. Bacteria cannot grow and multiply in the absence of water,
although some may remain alive for months or years in an arid environ-
ment.

About 85 per cent of the weight of active bacteria is water. Spores are

^ In the green sulfur bacteria hydrogen sulfide replaces water in the process of photo-
synthesis and sugar, sulfur and water are the end products. The purple sulfur bacteria
utilize the energy from oxidation of sulfur to sulfates in the synthesis of sugar. Generalized
equations representing these chemical changes are:

6CO2 + I2H2S » CeHisOe + 6H2O + 12s

.SH2S + 6CO2 + I2H2O > CeH.sOe + 6H2O + 3H2SO4

518 TEXTBOOK OF BOTANY

supposed to contain much less water than the vegetative cells. Diffusion
of raw materials and foods into the bacterial cell is impossible unless
the cell is surrounded by at least a film of water. Current statements
that living bacteria have persisted inside rocks since the time of their
formation a million years ago have not been generally accepted, because
of the almost insurmountable difficulties in proving that these micro-
organisms may not have been carried into the minute pores of the rocks
with the movements of underground water and gases.

The solution or substrate in which bacteria grow is commonly called
the medium (pi., media). The properties of a medium are determined
by the substances it contains. For example, sugar and salts may be in
solution in the medium, thus determining its concentration. When the
concentration of water in the medium is less than in the bacterial cells,
water diffuses out of the cells and the bacteria may become inactive.
Consequently jellies keep more readily than preserves, preserves more
readily than canned fruits, and canned fruits more readily than fruit
juices to which no sugar has been added. Jellies have a high concentra-
tion of sugar and a comparatively low concentration of water. In the
fruit juices the converse is true.

In the laboratory bacteria are often placed in gelatin or agar media.
When the water content of gelatin falls below 50 per cent, bacteria de-
velop very slowly. When the gelatin dries out, the vegetative cells be-
come inactive and eventually die. As noted above, however, many
bacteria, especially those found in the soil, may live — but not grow — for
months or years in a desiccated state. Since most of the bacteria that
are causes of disease cannot survive desiccation, there is little danger
of living ones being carried long distances by air currents.

Oxygen. Molecular oxygen is essential to nearly all living organisms.
A few kinds of bacteria, however, can live without it. Most organisms
use free oxvgen in the respiratory processes and are known as aerobes.
Some bacteria, such as those that are the causative agents of lockjaw
and those that cause butter to become rancid, grow only when the free
oxygen content of the medium is extremely low, and when organic
substances containing combined oxygen are available. These bacteria
are known as anaerobes. Anaerobic bacteria occur especially in poorly
drained soils, in deep waters of lakes and seas, and in all sorts of media
from which oxygen has been removed or excluded.

Bacteria are exposed only to the oxygen that is dissolved in the medium
surrounding them. At room temperature this is ordinarily a very dilute

[Chap. XLII THE BIOLOGY OF BACTERIA 519

solution equivalent to dissolving 1 cc. of free oxygen in 100 liters of
w^ater. If the oxygen content of the medium is increased to 30 times this
amount, most bacteria die. In other words, oxygen at such concentrations
has the same effect on bacteria as a solution of formaldehyde, or bi-
chloride of mercury, two of our commonly used antiseptics. Even pro-
nouncedly aerobic bacteria cannot withstand very high concentrations
of oxygen. If the oxygen content of the air above the nutrient medium is
increased to 4 times the usual amount, the growth of some strains of
Streptococcus is stopped. Even the same strain of bacteria differs in its
susceptibility to oxygen according to the medium in which it is growing.
Anaerobic bacteria grow better within liquid media or below the surface
of agar media because of the decreased oxygen content. Among the bac-
teria there is a marked diversity in their endurance of molecular oxygen.

Hydrogen peroxide (H2O2) is toxic to bacteria because it changes
readily to water and atomic oxygen, which is very active chemically.
In the presence of free oxygen, hydrogen peroxide may be formed in the
medium in which the bacteria are growing. The hydrogen peroxide may
in turn be toxic to the bacteria.

Temperature. When compared with other plants, bacteria as a group
can live under a very wide range of temperature conditions. In this
respect they are perhaps equaled only by the blue-green algae. The
temperature of the bacteria is of course the same as that of the media in
which they grow. Within a limited range, high temperatures accelerate
life processes and low temperatures retard them. Food is used less
rapidly at lower temperatures, and bacteria with a limited food supply
live longer under such conditions.

Some of the bacteria that thrive in hay infusions multiply at tempera-
tures as low as 40° F. or as high as 110° F. The minimum temperature
at which some species can grow may be higher than the maximum
temperature at which others can sur\'ive. The optimum temperature for
most bacteria lies between 70° and 100° F. Few species continue vegeta-
tive development at temperatures higher than 115° F. Some bacteria,
however, that bring about the rapid decay of organic matter, such as
silage, may live at temperatures as high as 175° F. Living bacteria have
also been found in hot springs at temperatures but little below this
figure. This endurance of high temperature is remarkable in view of the
fact that man\' proteins begin to coagulate and some fats begin to liquefy
and separate at 145° F.

Although bacteria are often distinguished as aerobes and anaerobes as

520 TEXTBOOK OF BOTANY

noted above, certain species may grow as aerobes at one temperature and
as anaerobes at another. Many of the thermophihc bacteria, for ex-
ample, will not grow at temperatures below 110° F. when exposed to
air, but will grow under anaerobic conditions at 95° F.

At temperatures near the freezing point bacteria grow very slowly,
but may survive for weeks or even months. When freezing occurs and
the medium becomes solid, diffusion is extremely slow and life processes
are reduced to a minimum. It is interesting to note, however, that
typhoid bacteria for die most part are destroyed immediately upon the
freezing of water and few individuals live longer than three or four
weeks. Certain pathogenic bacteria and some of the common micro-
organisms of the soil are known to survive for a few days when exposed
to the temperature of liquid air (about —310° F.). Cultures of certain
bacteria have been exposed to the temperature of liquid hydrogen
(about —425° F. or about 35° above absolute zero) for short intervals
with no apparent harm to the organisms.

What are bacteria? Bacteria are at once the simplest in structure, the
smallest in size, the most abundant, and the most generally distributed
of all plants, both green and non-green. Only the blue-green algae are
comparable in these respects. Bacteria are one-celled organisms, but
the individuals often cohere and fomi aggregates of visible size. The
cells have gelatinous sheaths; and when many of the sheaths coalesce,
the bacterial scums often seen on water and on damp objects are foraied.

The cells of bacteria are so small that their exact structure is difficult
to ascertain. Definite cell walls exist in most bacteria as is shown by
their rigidity, by their various shapes, and by the effects of plasmolysis.
The composition of the cell walls is not definitely known, but chitin and
waxes have been reported in some species. The protoplasm is apparently
undifferentiated as cytoplasm and nucleus, but both nucleic acids and
nucleo-proteins have been obtained from some bacteria by chemical
analysis. The cell evidently carries on all the processes usually associated
with more differentiated cells, and it is unnecessary to assume any unique
hereditary mechanism in this great group of plants. The most important
constituents of the cell are proteins; carbohydrates, including starch,
glycogen, gums, and simple sugars; fats, waxes, and phospholipoids;
enzymes; and various organic salts. It has not been established experi-
mentally which salts are essential in the growth of bacteria. Some bac-
teria apparently can synthesize vitamin B, but neither A nor C.

A few bacteria move about by a gliding movement not understood

[Chap. XLII THE BIOLOGY OF BACTERIA 521

at present. Many forms are not motile at all; but in others protoplasmic
threads known as flagella (sing., fagellum) extend through the cell wall
and are organs of locomotion. A single Hagellum may be several times
the length of the cell. Locomotion results from a rotary or whip-like
movement of the flagella. The flagellate forms are active for a time but
later become stationary and lose their flagella. Brownian movement of
both living and dead bacteria is a common phenomenon.

Bacterial cells are slightly heavier than water (sp. gr. 1.038 to 1.065).
The largest bacillus studied is from 3 to 6 microns in diameter and
40-60 M in length. The coccus forms may vary from 0.15 ^ to more than 1
micron in diameter. It requires several billions of such cells to weigh a
milligram.

The ultramicroscopic: viruses and bacteriophages. We have just seen
that some bacteria are so small as to be scarcely visible even with the
highest magnification of the best microscopes. There is some evidence of
the existence of even smaller bodies with the properties of micro-
organisms. One group of these' bodies includes the filterable viruses, so
called because they pass through filters through which bacteria do not
pass.- They are causes of infectious diseases in both plants and animals,
such as tobacco mosaic, infantile paralysis, and mumps.

Other ultramicroscopic filterable bodies encountered in bacterial cul-
tures are called bacteriophages. They appear in bacterial cultures and
destroy the bacteria, but it is not definitely known just how this destruc-
tion takes place. The bacteria may be digested. Some investigators think
that the 'phages are enzymes that increase in the presence of bacteria.
Other investigators have thought that they are living cells of submicro-
scopic size in the life cycle of bacteria. Viruses and 'phages are extremely
small but they are large enough to contain many protein molecules the
size of those of egg albumen ( Fig. 230 ) . There is no evidence that they
increase in number except in living cells of plants and animals.

The virus causing the tobacco mosaic disease has been isolated by
chemical means, crystalhzed, and found to be protein-like. Its injection
into another susceptible tobacco plant results in the production of other
molecules like itself in a comparatively short time.

Cell division and reproduction. Bacteria multiply by simple cell di-
vision, and the resulting halves fomi two new individuals. Under favor-
able conditions these individuals in turn divide, and there are four

- Such filters are usually made from diatomaceous earth, unglazed porcelain, or com-
pressed asbestos.

522

TEXTBOOK OF BOTANY

CIRCUMFERENCE OF

BACILLUS PRODIGIOSIS

DIAMETER 750 -mfi-

INFLUENZA VIRUS 1 20 wn

VIRUS OF TOBACCO MOSAIC 430X12.3

TOBACCO RING SPOT VIRUS 180 x 10.4,

STAPHYLOCOCCUS
BACTERIOPHAGE 90 irif,.

O

TOBACCO BUSHY STUNT
VIRUS 28mfi.

o

VIRUS OF YELLOW FEVER
22t„u.

HAEMOGLOBIN MOLECULE FOOT AND MOUTH DISEASE

2.8x0.6m|«. VIRUS 10 -mfi.

EGG ALBUMEN MOLECULE
1.8X0.6 my.

Fig. 230. Diagram illustrating the sizes of several viruses compared with those
of certain bacteria, bacteriophage, and protein molecules. On the same scale the
diameter of a human red blood corpuscle would be 3.5 feet. Data from W. M.
Stanley.

individuals. This process may continue indefinitely. At suitable tempera-
tures divisions may occur as often as every 20 minutes, which is the
same as saying that an individual may exist but 20 minutes. A little com-
putation will show that within a day the number o£ such rapidly dividing
bacteria becomes enormous. If division occurred every 30 minutes,
starting with one bacterium, we would have: At the end of 24 hours,
281 million million bacteria = 1 pint in volume. At the end of 48 hours,
281 million million pints = about 32 cubic miles of bacteria.

Such rapid increases of bacteria are limited to very short periods of

[Chap. XLII THE BIOLOGY OF BACTERIA 523

time. They often develop toxic substances in the medium that inhibit
further growth. Their food, water, oxygen, and salt supply is limited to
the film immediately surrounding them. Colonies of bacteria cannot
become very large because the movements of substances through the
mass is limited by the rate of diffusion. In nature other environmental
factors such as light, moisture, and temperature rarely remain favorable
for long periods. Bacteria are also consumed by many microscopic
animals.

The protoplasm of a bacterial cell may contract into a rounded mass
at one end, or in the middle, with the formation of a secondary wall;
this mass is termed a spore. It contains less water, is less likely to be
injured by drying, and will endure greater extremes of temperature and
greater concentrations of poisons than the ordinary bacterial cell. It is
because the spores of certain bacteria can withstand a temperature of
boiling water that steam pressure is used in sterilizing cans of com,
beans, peas, and other vegetables. Most of the bacteria that cause dis-
eases do not form spores. Those that cause botulism and tetanus are
exceptions.

Bacteria and sanitation. We often go to considerable trouble to prexent
decay; but we owe our continued existence on the earth to the fact that
bacteria (and fungi) remove the dead bodies of plants and animals that
would otherwise have accumulated on the earth. Together with the fungi
they secrete enzymes which digest the highly complex organic sub-
stances composing the bodies of plants and animals into simpler sub-
stances. By repeated oxidations these substances are ultimately changed
into carbon dioxide, water, and inorganic salts. These are again available
to green plants.

The sewage that is drained into rivers is digested and oxidized by
bacteria to simple harmless compounds. Our great cities, where thou-
sands and sometimes millions of people are crowded into a small area,
must have enormous sewage-disposal works in which aeration and other
conditions are made favorable for a more rapid growth of the bacteria of
decay, and the quick destruction of the sewage. This prevents the pollu-
tion of streams and lakes into which the sewage would otherwise be
carried.

The modern processes of filtering and sterilizing the water supplies
of cities not only remove sediment, but also eliminate the bacteria of
disease. The processes include adding minute quantities of alum and
chloride of lime to the water, and then filtering it through sand. For

524 TEXTBOOK OF BOTANY

many years the water-borne organism causing typhoid fever took a
large annual toll in human lives. Cities are now practically free of this
disease; most typhoid cases originate from polluted well water in rural
communities. To eliminate the contamination of surface wells requires
merely careful location and construction of the wells with reference
to surface drainage which often contains large numbers of bacteria.

Sanitary practices, such as quarantine, disinfection, admitting direct
sunlight into living rooms, cleanliness, cooking foods, pasteurizing milk,
and keeping foods in refrigerators, are all related to the elimination or
the reduction of the number and kinds of bacteria with which we come
into contact.

Bacteria and disease. While most bacteria are not causes of disease,
some are, and these are known as pathogenic bacteria. The invasion of
the bodv tissues bv bacteria is known as infection. The severitv of the
disease depends upon the ability of the organisms to invade the tissues,
multiply rapidly, and cause injurv. The injurv to the bodv is due to the
destruction of tissues and the formation by the bacteria of certain poison-
ous substances called toxins. When the bodv is invaded, antitoxins, or
antibodies, are formed that neutralize the effects of the toxins either by
combining with them chemically, or otherwise rendering the cells im-
mune. In this way the body is protected until the bacteria are destroyed
by the colorless blood cells ( leucocvtes ) , or until the bacteria are made
harmless bv other means.

The term immunity denotes the qualities of a plant or animal by
which the invasion or growth of pathogenic organisms is prevented, or
their products are rendered hamiless. A plant or animal is susceptible to
an infectious disease when such qualities are absent, or inadequately
developed. Not all persons are equallv susceptible to certain diseases,
and susceptibility may vary in the same individual from time to time.
A person is usuallv immune to a disease if his blood contains the
corresponding antibodv, or is able to produce it. Some of the more
common bacterial diseases of the human bodv are tuberculosis, pneu-
monia, diphtheria, tvphoid fever, and tetanus.

Diseases of plants. Although most of the diseases of plants are caused
bv fungi, the destruction of plants by bacteria is often sufficient to
present a serious economic problem. Bacteria may be present in the
healthy tissues of some plants. They mav, however, penetrate the host
through wounds or natural openings and then spread throughout the
plant or remain localized within certain tissues. They are carried from

[Chap. XLIl THE BIOLOGY OF BACTERIA 525

plant to plant by insects, by man, and by other animals. Bacteria are less
frequently dispersed by rain and wind.

Bacteria in milk. Among the natural media in which bacteria grow,
milk is ideal for most bacteria of decay and some pathogenic organisms
( pathogens ) . Milk, then, must be handled carefully if it is to be used as
food. The cows, the stable, the vessels in which the milk is placed, the
persons who handle it are all possible sources of bacterial contamination.
It is of paramount importance that attention be paid to cleanliness and
to the retardation of bacterial growth by immediate refrigeration.

In spite of all precautions, bacteria do occur in milk. To check their
multiplication, particularly during shipment from the dairy fami to
the cities, the milk is heated to 140° or 145° F. for 30 minutes.^ This
process, which is known as pasteurization, destroys nearly all the active
pathogens. Such heating, however, does not kill spores; but if the milk
is kept cool, their subsequent germination and growth are to a large
extent prevented.

The bacteria of natural waters are discussed in Chapter XL VI.

The science of bacteriology. The studv of bacteria has made great
strides since Leeuwenhoek in 1676 first saw through his crude magnifier
the "animalcules" from his teeth. Many years elapsed and many investi-
gations were made before the biological significance of these micro-
organisms was established. Even now a large portion of the world's
population has never heard of bacteria, a larger number is indifferent
to their importance, and too many people still like to believe that disease
results from mvsterious influences rather than from bacterial infection.

Spontaneous generation. Since earliest times man has probably been
interested in the question of the origin of living organisms on the earth,
and has tried to fonnulate answers on the basis of what he imagined
or what he observed. Some of these observations, and also the inferences
drawn from them, we now know were erroneous. It was believed, for
example, that mice came from rags and meal, that mud produced frogs,
that putrid meat changed to bees and flies, and that water in some man-
ner gave rise suddenly to all sorts of fully formed aquatic organisms.
Although many people ardently subscribed to a special creation of each
kind of plant and animal, they at the same time accepted the idea of
the spontaneous production of such organisms from inorganic matter.
It required, of course, onlv simple experiments to show that screened

^ Another method is to heat the milk for two minutes over a steam coil to a temperature
of 176° F. and cool it rapidly.

526 TEXTBOOK OF BOTANY

meats produce no maggots, that rags and meal inside stoppered bottles
never give rise to mice, and that frogs develop from frog eggs and not
from mud. Although a few individuals as earlv as the 17th centurv defi-
nitely expressed the opinion that organisms arise only from preexisting
organisms, it was not until after the middle of the 19th century that the
theory of spontaneous generation was shown to be utterly false as an
explanation of the origin of fullv fomied plants and animals.

Speculation. Early Greek philosophers, including Aristotle (384-322
B.C. ) , all subscribed in their writings to the idea of spontaneous develop-
ment of living organisms. Their statements were copied and enlarged
upon by nearly all the medieval scholars of Europe. For example, there
was then a widespread belief in the "goose tree" and the "vegetable
lamb." Geese and ducks were thought to be formed either directly from
the fruit of certain trees, or from sea shells borne by these trees. Current
also were the so-called observations that certain trees bore melon-like
fruits containing fully formed lambs. Such incredible accounts as these
were widely accepted until early in the 18th century.

Early experimentation. During the 16th century a few investigators
with a desire to experiment for themselves showed that spontaneous
generation of some organisms was clearly erroneous. Van Helmont
( 1577-1644), although he thought his experiment proved that mice came
from wheat grains, was troubled because these mice were just like those
borne bv a female mouse.

The Italian phvsician Redi (1626-1697) proved experimentally that
"woiTns" in meat are fly larvae. He further showed that if the flies are
kept from depositing eggs on meat, no larvae develop in it. Redi did not,
however, draw the inference that this experiment disproved spontaneous
generation. Indeed, he suspected that "worms" in plant galls arose spon-
taneously from plant juices. Vallisneri (1661-1730) soon afterward
proved that these "worms" likewise developed from the eggs of insects.

Leeuwenhoek and his microscopes. The Dutch inventor of the micro-
scope, Leeuwenhoek (1632-1723), looking through his simple magni-
fiers, saw organisms invisible to the unaided eye. He examined all sorts
of infusions containing microorganisms, and left accurate descriptions,
and many remarkable figures of bacteria, yeasts, algae, and many other
organisms. Leeuwenhoek did not subscribe to the theory of spontaneous
generation, but insisted that the tiny organisms he saw grew from sim-
ilar organisms that got into his infusions from the air.

[Chap. XLII THE BIOLOGY OF BACTERIA 527

Sterilization by heat and chemicals. In 1776 Spallanzani ( 1729-1799 ) ,
an Italian, proved that if meat broth is heated for an hour or two in
hermetically sealed flasks, no microorganisms develop in the broth. The
methods employed in this simple experiment were used by Appert in
1810 to exclude bacteria and molds from foodstuffs, and are the basis
of our canning industry. The objection was immediately made, in the
case of Spallanzani's experiment, that the heating had spoiled the air
inside the flask so that no organism could live there, and therefore that
spontaneous generation had not been disproved. Upon exposure to air
the broth was soon teeming with organisms. For this objection Spal-
lanzani's experiments provided no explanation.

Schultze in 1836 obtained similar results, although he admitted air
to his previously heated flasks of broth through tubes containing strong
acids and other chemical compounds. Schwann (1810-1882) the same
year allowed the air to enter the flasks through intensely heated tubes.
It was argued by opponents that both the heat and the chemicals had
altered the air. Von Schroeder and von Dusch in 1854 filtered air en-
tering the flasks through cotton or wool plugs, and no organisms de^'el-
oped in the flasks of broth. These experiments are the forerunners of
the modern use of cotton plugs in culture flasks in all pathology and
bacteriology laboratories.

The chemical theory of fermentation. Owing to poor techniques, other
investigators often failed to confirm the results of these experiments or
to secure consistent results, and the belief in spontaneous generation
continued. About this time the so-called chemical theory of fermenta-
tion was current, and this indirectly gave comfort to the proponents of
spontaneous generation. This theory, supported by Liebig ( 1803-1873 )
and others, assumed that in decay, or in fermentation, large molecules
simph disintegrated into smaller ones, and that fresh meat spoiled be-
cause it came into contact with other spoiled meat which initiated the
breaking up of the molecules. In other words, this theory would imply
that microorganisms, though thev may accompany decay and fermenta-
tion, are not the causes of these putrefacti\ e processes.

Louis Pasteur. A few people had suggested as early as the 18th cen-
turv that bacteria, or certain microscopic agents, are the causes of fer-
mentation and disease. But it was not until the latter half of the 19th
centurv that Louis Pasteur (1822-1895) with a series of brilliantly
planned and expertly conducted experiments finally' eliminated the idea
of spontaneous generation as a serious explanation for these phenomena.

528 TEXTBOOK OF BOTANY

Pasteur's first conviction that microorganisms are the cause and not
the result of fermentation came when he was investigating the troubles
in tlie beet-sugar distilleries at Lille, France. Besides the normal fer-
mentation resulting in alcohol, the vats frequently contained lactic acid,
a product for which there was no market. Pasteur found that when
yeasts were present alone in the vats, alcohol was formed. When certain
rod-shaped bacteria were present, however, lactic acid appeared also.
B\ properly transferring the two organisms to sugar solutions he was
able to secure either alcohol or lactic acid at will, depending upon
whether the veast or the bacterium was present in the solutions.

The proponents of spontaneous generation were still not convinced
and demanded more proof. Pasteur felt certain that bacteria are present
in the air and set about to verify his conviction. He prepared numerous
flasks of various sterilized media, sealed the sterilized flasks, and later
opened them at various places, such as in rural districts, on busv street
corners, and on mountain tops. The results indicated clearlv that bac-
teria are much more abundant in some areas, such as dustv streets, than
in quiet countrysides or mountain tops. Even then the skeptics were
unconvinced: they insisted that the sealing of the flasks had brought
about an unnatural condition.

Pasteur soon afterward utilized an experimental idea that did away
with the necessitv of sealing the flask. Sterile media were placed in
sterile flasks the necks of which had been drawn out into long, narrow
S-shaped tubes having verv small openings at the outer ends. In this
experiment the flasks were not sealed, and the culture media were given
free access to the air. No matter where the flasks were placed or how
long thev were kept, bacteria did not develop in the media. But when
the flasks were opened so that the media became exposed to air in the
usual wav, thev soon became contaminated. The results of these ex-
periments were clear, but there was a lack of unanimitv in the infer-
ences drawn from them. Pasteur was so sure of the completeness of his
demonstration that in a public lecture at the Sorbonne in 1864 he closed
his address with these prophetic words: "Never will the doctrine of
spontaneous generation recover from the mortal blow of this simple ex-
periment." Nearly all thinking people now accept the fact that organisms
arise only from preexisting organisms and not by spontaneous genera-
tion. Some of the flasks prepared bv Pasteur have been kept to the
present time, and no living organisms have appeared in them.

Two centuries earlier, Redi had proved experimentally that worms

[Chap. XLII THE BIOLOGY OF BACTERIA 529

do not arise spontaneously from meat. From the time of Redi to that
of Pasteur many critical experiments were performed. One result of
these experiments was the gradual abandonment of the idea that plants
and animals like those on the earth today arise by spontaneous gen-
eration. Progress in understanding the origin of living organisms, and
in following certain practices, such as those of sanitation, personal hy-
giene, aseptic surgery, and the preservation of food, was seen to depend
upon other ideas and upon further discoveries.

Even more impressive than all these earlv experiments are the facts
known todav about the composition, organization, and correlations of
the parts of living cells, and of multicellular organisms. That such
highly organized structures could arise suddenly from either inorganic
or organic masses of matter appears quite untenable. Even if non-living
colloidal masses of organic matter ever attain tlie state of being alive,
escape destruction, and multiply, a very long geological period of time
would elapse before highly organized cells capable of exact duplication
by reproduction could evolve. After considering pertinent data from all
fields of science for 25 vears, Oparin came to certain conclusions about
the probable origin of living organisms on the earth, and clearly sum-
marized the data and conclusions in a small book which is cited at the
end of this chapter.

Pasteur's experiments were not limited to the question of spontaneous
generation. He studied vinegar-making, silkworm diseases, rabies, and
diseases of sheep and cattle. He not only showed that bacteria and
other microorganisms cause diseases, but he developed methods of
vaccination that made man and other animals immune or at least less
susceptible to certain diseases. Perhaps one of the most touching stories
in all biologv is that of Pasteur's treatment of the little boy who was
bitten by a mad dog.

Modern bacteriology. Out of the experiments of Pasteur and equally
significant ones by Koch (1843-1910), Lister (1827-1912), and many
others, the modern techniques used in the control of pathogenic or-
ganisms have developed. Bacteriology is now an important biological
science with many specialized phases of research in medicine, den-
tistry, veterinary medicine, agriculture, dairy technique, plant pathology,
and manv industrial processes.

Methods of killing and controlling bacteria. It was known long before
bacteria were discovered that certain practices are valuable in the
preservation of foods, caring for wounds, and the prevention of disease.

530 TEXTBOOK OF BOTANY

Most of these methods were found out accidentally because the true
nature of putrefaction and diseases was unknown for man\' centuries,
and these methods are consequenth crude and often not dissociated
from magic. There is now a great body of experimentally established
facts about the control of bacteria. In the references below, further
information mav be secured about the effectiveness and uses of the
following means of controlling bacteria: cleanliness, ventilation, sun-
light, drving, refrigeration, antiseptics, brine and sugar solutions, steri-
lization, fumigation, pasteurization, canning, precipitation by chemicals,
isolation, vaccination, antitoxins, and serum treatment.

REFERENCES

Gay, F. P., and associates. Agents of Disease and Host Resistance. Charles C.

Thomas, Publisher. 1935.
Henrici, A. T. The Biology of Bacteria. 2nd ed. D. C. Heath & Company.

1939.
Oparin, A. I. The Origin of Life. The Macmillan Company. 1938. Chaps. ITII.
Parker, G. H. Anthony van Leeuwenhoek and his microscopes. Sci. Monthly.

37:434-441. 1933.
Salle, A. J. Fundamental Principles of Bacteriology. McGraw-Hill Book Com-
pany, Inc. 1939.
Stanley, W. M. Recent advances in the study of viruses. Science in Progress

(G. A. Baitsell, ed.). Yale Univ. Press. 1939. Chap. III.
Vallery-Radot, R. The Life of Pasteur. Doubleday, Doran & Company, Inc.

1928.
Watkeys, C. W., and associates. An Orientation in Science. McGraw-Hill Book

Company, Inc. 1938. Chap. X.

CHAPTER XLIII

BACTERIA OF THE SOIL

<^<5><><><><><><>CKX><^>c><x>^^

Bacteria of many kinds — both harmful and beneficial — live and grow
in the soil. They are often incredibly numerous, sometimes numbering
as high as a billion individuals in a gram of fertile garden soil. Some
of the relations of bacteria to decay and disease were described in the
preceding chapter. We shall now consider the importance of soil bac-
teria to cultivated plants and soil formation. Many soil bacteria are
important in the maintenance and improvement of fertility; some have
quite the opposite effect.

Distribution. Most soil bacteria occur in the upper two to ten inches
of the soil where the supply of organic compounds. is usually greater.
In the surface inch of soil, bacteria are not so numerous because of the
destructive effects of light and of frequent desiccation. Relatively few
bacteria are found below a depth of two or three feet in humid regions.
In well-aerated soils, such as those of irrigated semi-deserts, bacteria
grow at greater depths. They are probably always present as far down
as the roots of plants extend.

Nitrogen sources of green plants. Since nitrogen makes up such a large
part of the atmosphere of the earth, it was early assumed that green
plants obtain nitrogen from the air. The noted chemist, Liebig, was
convinced that this must be so, for he beliex ed that the ammonia of
the air was a sufficient source of nitrogen for plants. It required a long
time and many experiments to show that nitrates and ammonium salts
in the soil are the usual sources of nitrogen in plants. Although this fact
was surmised in 1838 by Boussingault of France, and in 1847 by Lawes
and Gilbert of the Rothamsted Experimental Farms in England, nearly
a half century elapsed before final proof was secured. The various
methods by which atmospheric nitrogen is combined in compounds
which green plants can utilize began to be understood just before the
beginning of the present century.^

^ For a discussion of the experiments of some of the most important contributors to our
knowledge of how green plants obtain nitrogen, consult any good text on soil microbiology.

531

532 TEXTBOOK OF BOTANY

The occurrence of nitrogen in a form usable by green plants is due
almost entirely to the action of certain bacteria. Such bacteria may be
referred to as nitrogen bacteria, and several groups are known to play
an important part in the nitrogen cycle in nature. The importance of
such microorganisms is recognized when we recall that every time a
crop is harvested, the nitrogen within the plant is removed from the
field. In the course of a few years this practice, along with the natural
leaching by water, would seriously deplete the nitrogen content of the
soil. To maintain soil fertility the nitrogen compounds lost in this way
must be constantly renewed. The nitrogen bacteria are very important
in this renewal.

The three groups of bacteria that form nitrogen compounds usable
by green plants are the ammonifi/ing bacteria, the nitrifying bacteria,
and the nitrogen-fixing bacteria. A fourth group, the denitrifying bac-
teria, by releasing nitrogen from these compounds, have the opposite
effect and decrease the nitrogen content of soils.

Ammonifying and nitrifying bacteria. Although millions of dollars are
spent every year in adding commercial fertilizers containing nitrogen
compounds to soils, our immediate interest is how the combined ni-
trogen in natural organic compounds is again made usable to green
plants. These compounds include the undecaved remains of plants and
animals. The ammonifying and nitrifying bacteria bring about a chain
of chemical and physical processes by which the nitrogenous substances
are transformed into simpler and more soluble compounds. These are
really processes of decay, and the nitrogen bacteria are among the
saprophytes that bring this about.

Ammonification. The disintegration and digestion of the large protein
molecules of organic bodies in the soil are activated by 'proteolytic
(protein-dissolving) enzymes. The resulting products, largely urea,
peptones, and amino acids, undergo additional chemical changes by
which ammonia (NH.s) is released. In so far as nitrogen is involved,
this is a reduction process known as ammonification, and it takes place
only through the agency of microorganisms. Some of the chemical re-
actions may be represented as follows:

Urea + Water > Ammonium carbonate

CO(NH2)2+2H20 > (NH4)2C03

Ammonium carbonate > Carbon dioxide + Ammonia -f- Water

(NH4)2C03 > COo -f^ 2NH3 + H2O

In addition to the investigators named above, Berthelot, Hellriegel, Wilfarth, Beijerinck,
and Winogradski should be mentioned.

[Chap. XLIII BACTERIA OF THE SOIL 533

Nitrification. Ammonia in turn becomes the compound transformed
in nitrate formation. Only a few species of soil bacteria activate this
process; they may be classified into two groups. The first group oxi-
dizes ammonia to nitrous acid.

Ammonia + Oxygen > Nitrous acid + Water

2NH3 + 3O2 > 2HNO2 + 2H2O

The nitrous acid may form salts with basic ions in the soil.

Nitrous + Potassium > Potassium + Carbon + Water

acid carbonate nitrite dioxide

2HNO2+ K2CO3 > 2KNO2 + CO2 + H2O

The formation of nitrous acid and nitrites from ammonia occurs in
soils in America and Australia primarily in the presence of species of
Nitrosococciis; in the soils of Europe, Asia and Africa species of Nitro-
somonas are the nitrite formers.

The last stage in nitrification is the oxidation of nitrous acid and
nitrites to nitric acid and nitrates.

Nitrous acid + Oxygen > Nitric acid

2HNO2 + O2 > 2HNO3

The nitric acid may form salts with basic ions in the soil.

Nitric + Potassium > Potassium + Carbon + Water

acid carbonate nitrate dioxide

2HNO3+ K2CO3 > 2KN0, + CO2 + H2O

Nitrous acid and nitrites seldom accumulate in soils, for as rapidly as
they are formed other nitrifying bacteria of the genus Nitrobacter
oxidize them to nitrates.

Thus the process of nitrification includes the oxidation of ammonia
to nitrites, and the further oxidation of nitrites to nitrates. Each step in
the process from proteins to ammonia, from ammonia to nitrites, nitrites
to nitrates, is dependent upon the presence of appropriate bacteria.
Nitrification is an oxidation process and the energy released is used by
the bacteria in the synthesis of sugar from carbon dioxide and water,
and also in the further elaboration of sugar and certain inorganic salts
into proteins and other cell compounds of the bacteria. These micro-
organisms, then, are autophytes.

The conditions necessary for natural nitrification include moderate
moisture, wami temperatures of the soil, good aeration, and fairly

534 TEXTBOOK OF BOTANY

neutral reaction, in addition to the raw materials necessary for certain
food syntheses in the bacteria. The bacteria involved in ammonification
are common and widespread in most soils, but the nitrifying bacteria
are largely restricted to well-aerated, moist, nearly neutral soils.

Denitrification. The nitrates accumulated in soils through the agency
of bacteria may disappear in several ways. They may be used by green
plants, carried away by water, or changed into insoluble substances.
They may also be reduced by other organisms to nitrous acid and am-
monia, or to molecular nitrogen. This reduction of nitrates to gaseous
nitrogen is referred to as denitrification. It occurs under certain condi-
tions and decreases the nitrogen content of the soil. Some of the chemi-
cal reactions involved in the reduction of nitrates may be indicated as
follows :

Nitric acid > Nitrous acid + Oxygen

2HNO3 > 2HNO2 + O2

Nitrous acid > Water + Nitrogen + Oxygen

4HNO2 >^2H20 + 2N2 + 8O2

The organisms that bring about the reduction of nitrates are prin-
cipally anaerobic bacteria, known collectively as denitrifying bacteria.
Since denitrification is a reduction process and may occur in the absence
of light, the organisms must obtain energy by oxidizing carbohydrates
and other organic compounds.

Denitrification is likely to occur in any soil containing nitrates under
anaerobic conditions. It is characteristic of poorly drained soils, and of
soils periodically flooded, as in rice fields. In certain regions denitrifica-
tion may so deplete the soil of available nitrogen compounds that plant
growth is restricted.

Nitrogen-fixation. Nitrogen constitutes nearly 80 per cent of the earth's
atmosphere; the combined nitrogen in all plants is relatively so small
that if plants could directly use the free nitrogen of the air the supply
would be inexhaustible. Over every acre of land surface the air contains
nearly 300 million pounds of nitrogen. It has been calculated that the
earth's atmosphere contains the prodigious total of over 5000 million mil-
lion tons of nitrogen. In spite of this, nitrogen is the most expensive
component of fertilizers.

Certain bacteria together with a few other organisms directly utilize
the free nitrogen of the air. These bacteria transform this free nitrogen

[Chap. XLIII BACTERIA OF THE SOIL 535

into compounds usable by all plants. The chain of processes in this
transfonnation is teniied nitrogen-fixation.

It has been known for more than 20 centuries that certain legumes,
such as clover, beans, and peas, enrich the soil. For this reason legumes
have long been used in crop rotations because farmers have known that
other crop plants grow better and yield more following the plowing
under of legumes.

It was not definitely known that nitrogen compounds accumulate in
legumes through the agency of nitrogen-fixing bacteria until 1886-1888.-
Since that time the life cycles of these organisms have been thoroughly
studied. These bacteria occur as microscopic motile rods free in the
soil. When legumes are planted and roots form, the bacteria invade the
cells of the roots by wav of the root hairs. The root becomes infected
and enlarges locally in the form of nodules ( Fig. 231 ) . These nodules

Fig. 231. Nodules containing nitrogen-fixing bacteria on the roots of soybean.
Photo from Agricultural Extension Department, Ohio State University.

- First shown by Hellriegel and Wilfarth. Pure cultures of these bacteria were first iso-
lated by Beijerinck in 1888.

536

TEXTBOOK OF BOTANY

contain millions of bacteria, which become somewhat enlarged and
branched (Fig. 232).

^.1>^'^'

^., ^

Fig. 232. Nitrogen-fixing bacteria: A, section of a nodule of bean root in which
bacteria were present in many cells; B, one of the cells containing bacteria much
enlarged; C-E, forms of nitrogen-fixing bacteria from nodules of different host
plants: C, from soybean, D, from alfalfa, E, from common bean. From E. B.
Fred, I. L. Baldwin, and E. McCoy, Univ. of Wisconsin, 1932.

The bacteria are dependent on the carbohydrates in the legume roots.
Unless there is an abundance of sugar in the roots of the host plant, few
or no nodules develop, because the bacteria do not invade the root hairs.

Although se\eral species of bacteria infect legumes, there mav be
considerable specificity in their relations to the host. Some inxestigators
have classified legumes on the basis of the bacteria to which the^' are
most susceptible. The red, white, and alsike clovers, for example, consti-
tute one group; alfalfa, sweet clover, and yellow trefoil another group;
certain peas a third group; and cowpeas, peanuts, lima beans, and Japan
clover still another group. Recent experiments indicate that the specificity
of the nitrogen-fixing bacteria may not be as pronounced as earlier ex-
periments seemed to show.

Although the series of events in "fixing" nitrogen are not well known,
the important process is the combining of free nitrogen with other

[Chap. XLlll BACTERIA OF THE SOIL 537

elements, resulting in a nitrogenous compound, perhaps an amino com-
pound. The formation of these complex compounds is a reduction proc-
ess and requires the energy made available through the oxidation of
suc^ars made by the leguminous plant. The bacteria multiply, mature,
and die within the nodules, and the nitrogen compounds are used or
accumulate within the legume. At the death of any part of the roots of
the legume these compounds enter the soil.

Still another group of bacteria fix nitrogen without being associated
with higher green plants. These organisms are saprophytes and make
possible a series of processes sometimes called non-symbiotic nitrogen-
fixation, in contrast to the nitrogen-fixation in legumes, which is called
si/mbiotic. They live especially in humous soils and derive their energy
by the oxidation of such organic compounds as carbohydrates, organic
acids, and other products of fermentation. In addition to energy, the
fixation of nitrogen by the bacteria is affected by many other conditions
in the soil. Dark prairie soils are ideal conditions for the development
of such bacteria.

Non-symbiotic nitrogen-fixing bacteria may be classified into two
groups: those using free oxygen (aerobic), largely species of the genus
Azotobacter; and anaerobic species belonging to the genus Clostridium.
The species of Azotobacter are coccus forms and non-motile; those of
Clostridium are rod-shaped and spore-fonning.

Summary: the nitrogen cvcle. We have now discussed the origin of
nitrogenous compounds in soils through the agency of bacteria. Neither
nitrification nor nitrogen-fixation in soils occurs in the absence of certain
microorganisms, chiefly bacteria. We may now summarize the complete
cycle of events in the origin and transformation of nitrogen compounds.
In the green plants, amino acids are made from carbohydrates and nitro-
gen salts and are used by both plants and animals in the synthesis of
proteins. The death of plants and animals results in a residue of sub-
stances, some of which are proteins. Through the agency of bacteria the
proteins are digested to simpler compounds such as amino acids. Then
through further chemical change ammonia is foraied from water and
the NH7 ions. Ammonification is the name applied to this series of
processes.

Ammonia is then oxidized to nitrites and to nitrates through the
agencv of nitrifving bacteria. These processes are nitrification. The
nitrates mav be used by green plants and by most non-green plants in
the production of proteins, and we are back where we started. Certain

538

TEXTBOOK OF BOTANY

other forms of bacteria may reduce the nitrates and nitrites to gaseous
nitrogen. This is denitrification.

Other groups of bacteria bring about the transformation of free nitro-
gen of the air to amino compounds which accumulate in the organisms.
This is known as nitrogen- fixation. Nitrogen-fixing bacteria are of two

PLANT

AMINO

ACIDS.

SULPHATES

"~~~~YplantV-

(phosphates

iTOTElNSr

^-^

I PROTEINS/ \

7 ANIMAL^
. I PROTEINS-

.^

w

<

^

death

FREE
rNlTRATEs\2t^ilIM:l^£QWN ITR06EN\

RN03 / i °^ ^^^

TLANT^

ANIMAL^

vRESIDUESy

f AMMONIA!
NH 3

(^Bacteria)

Fig. 233. Diagram of the nitrogen cycle in nature. Broken arrows indicate escape
of ammonia and other volatile nitrogen compounds to the air.

kinds: those living symbiotically in legume roots, and those living free
in the soil. Upon their death the nitrogen compounds become available.

All the processes having to do with nitrogen in its relation to green
plants may be grouped together into what has generally been knoMoi as
the nitrogen cycle (Fig. 233).

Sulfur bacteria. We have seen earlier that sulfur is a part of the mole-
cule of some amino acids and proteins. Green plants in general secure

[Chap. XLIII BACTERIA OF THE SOIL 539

sulfur from sulfates of the soil. Upon the death of the plant the proteins
containing sulfur are acted upon by bacteria, with the production of
hydrogen sulfide. The hydrogen sulfide is oxidized through the agency
of sulfur bacteria to elemental sulfur and sulfur dioxide. The latter is
further oxidized to sulfuric acid which may react with a base, such as
calcium carbonate, and form calcium sulfate. This formation of sulfates
is known as sulfofication, and takes place through the agency of the
bacteria. The sulfates are then available to green plants and the cycle
is complete.

Sulfates in the soil may be depleted through the action of another
group of bacteria, forming hydrogen sulfide. Reduction of elemental
sulfur may also result in the formation of sulfides. This reduction process
is known as desulfofication.

Sulfofication is an oxidation process through which the sulfur bacteria
secure energy that is used by them in the synthesis of sugar and other
foods. Such microorganisms are thus autophytes, and in this respect
resemble the nitrifying bacteria.

Bacteria and phosphates. Organic compounds of phosphorus are pres-
ent in plant and animal residues. Before the phosphorus is usable by
green plants, these complex compounds must be resolved by soil micro-
organisms. Such decomposition involves a number of reactions; phos-
phoric acid is finally formed, it accumulates in the soil as phosphates of
calcium, magnesium, iron, and aluminum.

Iron bacteria. Bacteria are associated with the transformation of iron
compounds in the soil through oxidation-reduction reactions. Certain
iron bacteria are able to bring about the oxidation of ferrous to ferric
iron, therebv securing energv bv which thev synthesize their sugars and
other foods; thus they are autophytes. Under anaerobic conditions ferric
iron may be reduced to ferrous iron by other iron bacteria. Although
it is rarely necessary to add iron to soils, iron deficiencies sometimes
occur in alkaline regions owing to the formation of insoluble compounds
of iron both by colloidal aggregation and by bacterial action.

If we could review all the activities of the many kinds of bacteria we
would appreciate still more their fundamental importance in our bio-
logical world. Parasitic bacteria may cause the death of living organisms.
Numerous saprophytic bacteria in turn digest the organic compounds in
the dead bodies, and oxidize the carbon compounds in respiration.
Through these disintegrative processes the inorganic ions, such as those
of phosphorus, iron, and magnesium, that are chemically bound in the

540 TEXTBOOK OF BOTANY

compounds of living cells are liberated; and ammonia and carbon dioxide
are formed. A few special groups of bacteria through these unique
respiratory processes oxidize the reduced nitrogen and sulfur of body
compounds to nitrates and sulfates, which may again be reduced by bac-
teria or by green plants. Thus by numerous activities of groups of bac-
teria following each other in definite succession the body compounds
indigestible by animals and green plants become converted into simpler
compounds that are usable by green plants. In the grand "passing show"
of nature, green plants and the larger animals play the easily visible
roles; but their action would soon cease were it not for the bacteria
and their associates, the fungi, most of whose activities are off-stage, un-
seen, and even unsuspected a century ago (Fig. 227).

REFERENCES

Waksman, S. A. Principles of Soil Biology. Williams & Wilkins Company. 1932.
Waksman, S. A., and R. L. Starkey. The Soil and the Microbe. John Wiley &
Sons. 1931.

CHAPTER XLIV

THE FUNGI

<^<^<><><^o<><>c><x><^c>C'<><><^o<^^

Slightly moist bread in a warm room soon becomes covered with fila-
mentous molds. Clothing, leather, and books left in moist rooms are soon
"musty" because of the development of other kinds of molds. The flavors
of some kinds of cheese are due to the growth of blue and green molds.
Around old stumps in meadows, and within forests where organic mat-
ter is abundant, puff balls, mushrooms, and toadstools may be found
(Fig. 234). From decaying logs and even from standing trees bracket
mushrooms often protrude ( Fig. 247 ) . Field and garden crops are often
infected with rnildews, rusts, and smuts. In the process of bread-making
yeast causes the dough to "rise." Many animals, including man, suffer
discomfort through skin invasions of molds which cause, among other
diseases, "ringworm" and "athlete's foot." Some of these plants are
saprophvtes, some are parasites, and some live as both saprophytes and
parasites. All these plants collectively are called fungi (sing, fungus).

How are fungi recognized? We have already learned that non-green
plants include fungi, bacteria, an occasional alga, and some seed plants.
The name fungus is the old Latin word for mushroom. Just as it is diffi-
cult to tell whether some organisms are plants or animals, the great
diversity among fungi makes it equallv difficult to distinguish clearly
certain fungi from certain bacteria and algae. The most common charac-
teristic of fungi is a vegetative body of either a loose web or a compact
mass of filaments none of which contains chlorophyll (Fig. 235). The
fruiting and reproductive bodies of numerous fungi are readily recog-
nized, but the reproductive bodies of many common fungi are seldom
found; hence some fungi must be recognized without them.

Vegetative parts of a fungus. A toadstool among the leaves on the forest
floor, or a bracket fungus on a fallen tree trunk is but a small part of the
fungous plant. The part not readilv seen consists of a widely dispersed
mass of filaments (Fig. 235). In describing fungi these filaments are
called htjphae (sing, hijpha). Hyphae are either continuous tubular
structures, or divided by cross walls into cell-like segments containing

541

542

TEXTBOOK OF BOTANY

Fig. 234. Reproductive bodies of some common fungi: A, bear's-head fungus
(Htjdniun); B, sponge mushroom or morel (Morchella) ; C, a poisonous toadstool
(Amanita); D, a puff ball (Calvatia) .

one to many nuclei. Usually they are highly branched and form loose
cottony masses, such as mav be readilv seen in the bread mold. The
common meadow mushroom has colorless hyphae that spread in all
directions through a large mass of soil and are usually unseen unless one
digs carefully around the fruiting bodies that appear above the soil.
The "mushroom" is formed by the growing together and coalescence of
numerous hyphae into a compact characteristic structure. The whole

[Chap. XLIV THE FUNGI 543

aggregate of hyphae growing on, or within, the substrate is called the
mycelhim (Greek for fungus). The underground hyphae of some species
grow in small compact masses, forming tuber-like bodies. Those of some
other species form compact hard strands of many hyphae that grow as a
collective unit and push their way through firm substrates.

Fig. 235. Stages in the development of the common edible pink-gilled mushroom
(P.salliota campestris) . Note the underground vegetative body of the plant.

The cell walls of hyphae are composed of such substances as cellulose,
pectose, callose, and chitin. The foods found in fungi are carbohydrates
(including sugars and glycogen), fats, and proteins. Various kinds of
enzymes are produced by fungi, and these are important in transfomiing
substances not only within the fungous cells, but outside in the host or in
the substrate. Fungous hyphae may grow into the tissues of the host or
other substrate bv mechanical pressure similar to that of growing roots,
or the substrate may be digested by excreted enzymes just ahead of the
lengthening hypha.

Fungi mav be found wherever the environment is not inimical to the
establishment of a mvcelium, and where there are substances they can
digest and use as food. They are found on land and in water, growing
free as saprophvtes or as parasites within, or upon, many different hosts.
Aquatic fungi are not as numerous as terrestrial ones, and are far less
abundant than the algae and bacteria. The aquatic species live as para-
sites on fresh- water and marine organisms, or as saprophytes on the dead
bodies and residues of such organisms. They are most numerous in well-

544 TEXTBOOK OF BOTANY

lighted and well-aerated water where there is the largest number of host
plants and animals. Any aquatic organism may be invaded, injured, or
destroyed by parasitic fungi at some stage of its life cycle. The growth
of aquatic fungi may be checked by epidemics of bacteria.

Terrestrial fungi occur everywhere on, or in, most kinds of plants and
many animals, as well as on their dead bodies. It would be difficult to
find a twig Iving on the ground in a forest that is not being invaded
and disintegrated by some fungus, and one may easily discover the cob-
webby fungous hyphae among the fallen leaves on the forest floor. Soil
fungi are most abundant in the upper foot of soil. Fungi digest the woody
tissue bv enzyme action. They injure the living hosts largelv through the
destruction of tissues, through the production of toxic substances, through
the consumption of food, and through interference with phvsiological
processes.

There are about 75,000 species of fungi and thousands of chemically
different substrates; hence some fungi grow where others cannot. Fungi
are most numerous and grow best in moist warm situations, but they
mav be found also on the desert, in refrigerators, and on the arctic
tundra.

Millions of bushels of fruits and vegetables in storage are destroyed
each year by fungi. Most of the apples, for example, that spoil in storage
are destroved by a single fungus, a blue mold which causes a soft rot.
In northern states where snow covers the ground throughout the winter,
much of the grass on golf greens is killed at the soil surface by another
fungus, the so-called "snow mold." The spores and other domiant struc-
tures of fungi often survive the natural extremes of temperature and
drought for long periods.

We shall now consider in more detail a few of the common species
of fungi, in order to secure a better understanding of their structures,
growth, and reproduction, and also of their biological significance.

Bread mold. If a moist piece of bread is placed under a bell jar for a
few davs, a tangle of colorless hyphae may cover the bread and even fill
the jar. This is the mycelium of the bread mold, Rhizoptis nigricans.
Its hyphae are much branched and have no cross walls; some of them
(rhizoids — root-like hyphae) penetrate the bread for short distances,
while others form the visible web. Bread mold, of course, grows on many
other substrates. The fungus secures food and water from the bread, and
as a result of enzyme action the bread is digested. Decay is nothing

THE FUNGI

545

[Chap. XLIV

more than the digestion and oxidation of organic substances. Growth of
the fungus may continue until the food is exhausted.

The mycehum starts from a spore as a single hypha and spreads over
the bread by terminal growth of its multitudinous branches. At node-like
points on the spreading hyphae upright branches and several rhizoids
develop. These nodes also become new centers from which radiating
horizontal hyphae extend and increase the complexity and density of
the mycelium.

The tips of the upright hyphae enlarge and foim globular spore cases,
sporangia, which eventually contain many small round spores (Fig. 236).

Fig. 236. Bread mold {Rhizopus): A, general habit of growth and reproduction
by asexually formed spores; B, C, D, E, and F, stages in the fusion of hyphae of
male and female mycelia and the formation of a zygote.

Many sporangia are usually produced about the same time, and upon
their rupture millions of black spores are liberated from them. The
spores are so light that they float in the air for long periods; hence it is
easy to see whv moist bread when exposed to the air for even a short
time and then placed in a closed container soon becomes "moldy." When
spores come to rest on a moist organic substrate, they germinate almost
immediately, and from them new mycelia originate. When bread is

546 TEXTBOOK OF BOTANY

wrapped in waxed paper while still warm from the oven, mold spores
are more likely to be kept out, and the bread keeps in good condition
for a much longer period.

Sexual reproduction can be observed between mycelia of bread mold
started from different spores on the same culture plate. Where hyphae of
the two mycelia come in contact, the adjoining walls dissolve and the
contents of the two "cells" fuse. The union of the two protoplasts, or
gametes, results in a heavy-walled black zygote. Upon gemiination of a
zygote, a short hypha develops which terminates in a globular spor-
angium containing spores. From these spores branching hyphae grow,
and the development of a new mvcelium begins.

Fig. 237. Yeast (Saccharomyces) : cells and branching filaments. Above are three
cells, each containing four resting spores. Courtesy of World Book Co.

Yeasts. In the discussion of respiration it was noted that the fomia-
tion of alcohol and carbon dioxide from a sugar solution may be brought
about in oxygen-free containers if certain yeasts are present. When
yeasts are thoroughlv mixed with flour and water, their activities bring
about the "rising" of bread dough. The carbon dioxide resulting from
the oxidation of sugar accumulates in bubbles throughout the dough and
makes it porous. While the bread is baking, the alcohol vaporizes and
with the carbon dioxide passes from the bread into the air.

The yeasts have little resemblance to the bread mold described above,
for they are generallv microscopic one-celled plants more or less ellip-
soidal in form. Some species of yeast are so small that thev resemble the
largest bacteria. Although a single yeast cell is microscopic, the enomious
numbers that develop in a sugar solution soon make it cloudy, and
eventuallv sediment collects at the bottom of the vessel. The yeast cakes
sold bv commercial firms consist of starch grains and yeast cells pressed
together into a compact mass.

THE FUNGI

547

[Chap. XLIV

During growth and multiplication, a small outgrowth ( "bud" ) appears
at the side or end of a cell. This increases in size and finally separates
from the parent cell as a new yeast plant. This process is repeated in-
definitely in favorable moisture, food, and temperature conditions. This

Fig. 238. Diagrammatic representation of zygotes formed in tlie water mold,
Achlija, when liyphae of a female mycelium (left) come in contact with those of
a male mycelium (right). The numerous small branching hyphae around the
periphery of the male mycelium are antheridial branches on which the male
organs (antheridia) containing male nuclei develop. The scattered small darkened
spheres on the female mycelium represent immature egg cases (oogonial initials).
The darkened awl-shaped hyphae and spherical masses of spores at the apexes of
certain hyphae in both mycelia are vegetative propagules. For further details of
the sexual processes, see Fig. 239. From ]. R. Raper.

mode of vegetative multiplication is known as "budding." Some yeasts
have simple cell division only. Under some conditions the cells do not
separate immediately, but form fragile bead-like chains that are more
or less branched (Fig. 237). Under other conditions the contents of the
yeast cell may divide internally into 2, 4, or 8 separate protoplasts, and
these round up as spores, capable of a relatively long period of dormancy.
Water molds. In temperate latitudes in early spring dead fish and
other aquatic animals may be found in ponds and streams, completely
covered with a colorless slimy mass of fungous hyphae. The same

548

TEXTBOOK OF BOTANY

Fig. 239. Diagrammatic representation of the sex organs of the water mold,
Achlija, and of their hormonal relations as indicated by arrows; an elaboration
of Fig. 238. Hormone A from the female mycelium (right) is essential to the
formation of antheridial branches on the male mycelium (left). Hormone B from
the antheridial branches is essential to the formation of oogonial initials on the
female mycelium. Hormone C from the oogonial initials is essential to the forma-
tion of antheridia on the antheridial branches. The direction of growth of the
antheridial branches is also influenced by this hormone. Hormone D from the
antheridia is essential to the further development of the oogonia and the forma-
tion of eggs within them. Finally sexual fusions result in the formation of zygotes.
From J. R. Raper.

fungus, or other species of water molds, may also grow on living animals,
and on decaying plant and animal remains. They are at times very de-
structive to fish eggs and young fish. Water molds were so named before
it was generally known that many species of them grow in moist soils.
The vegetative phase consists of tubular branching hyphae within the
host or the substrate.

The slimv mass of filaments on the outside of the animal is largely
made up of hyphae that terminate in club-shaped sporangia, within
which the protoplasts divide into many motile spores. After liberation
from the sporangium, the motile spores may undergo various changes in
appearance; from them new hvphae develop. Sexual reproduction begins
by the formation of egg cases, or oogonia, each containing from one to

THE FUNGI

549

[Chap. XLIV

twenty egg cells, and by the development of antheridia containing sperm
nuclei. The union of a sperm nucleus and an egg nucleus results in a
zygote. Upon germination the zygote divides internally and forms motile
spores. From these motile spores new hyphae may develop under favor-
able conditions. Hyphae may develop, however, from unfertilized eggs —
another example of parthenogenesis in plants. It has recently been dis-
covered that four hormones are necessary to sexual reproduction in one
of the water molds (Figs. 238 and 239).

Fig. 240. A, blue mold {Penicillium) ; B, green mold {Aspergillus). The spores
(conidia) are formed by successive abstrictions of the tips of upright hyphae.

Blue and green molds. This group of molds was named on the basis
of the color of the spores of some of the more common species. Certain
of the blue molds are important because of the flavor and color they give
to some of our highly esteemed varieties of cheese, such as Camembert
and Roquefort. Besides cheese, they can digest and subsist upon a large
number of other substances. These fungi and others are the causes of
the decay of vegetables, fruits, and meats, both in storage and during
transportation, and they occasion enormous monetary losses. They are
aerobic and may grow on the surface of moist jelly, and on the surface
of wine in a bottle when the stopper is imperfect.

The hyphae of these fungi have cross walls. The mycelium is a felt-
like meshwork on the substrate. Upright hyphae develop, and from
their brush-like branching tips numerous one-celled spores are formed

550

TEXTBOOK OF BOTANY

Fig. 241. Reproductive structures of a powdery mildew: A, conidia formed
during the summer months; B, an ascocarp, which hves through the winter and
within which the asci and ascospores develop. They can be forced out of the
ascocarp by a slight pressure as indicated in the drawing. B from C. J. Chamber-
lain, Elements of Plant Science, McGraw-Hill Book Company, Inc.

in chains by successive abstrictions (Fig. 240). Species of Pemcillium
have blue or green spores. The related mold, Aspergillus, differs from
the blue mold in producing globular masses of green, yellow, or black
spores at the enlarged ends of upright hyphae. Spores formed by con-
strictions at the ends of hyphae are called conidia ( sing, conidium ) , and
the hyphae producing them are conidiophores. Conidia are readily car-
ried by air currents or by rain water; and since they are produced in
such enormous numbers they are present everywhere, especially during
the summer months.

Powdery mildews. During summer and early autumn cobwebby
mycelia may be seen on the leaves of such plants as ragweed, knotweed,
red clover, lilac, crimson rambler, sunflowers, and willows. The hyphae
of these fungi also have cross walls. When in contact with a leaf, some
of the branches of the hyphae become haustoria or rhizoids which pene-
trate the epidermal cells of the host tissue. The mycelia of these fungi
are on the surface of the host. Other hvphae, growing erect from the sur-
face of the leaf, form at their tips chains of colorless conidia in a manner
similar to that of the blue molds noted above. The conidia, or summer
spores, are carried about by air currents from leaf to leaf, and many
plants thus become infected during the growing season. These fungi are
called powdery mildews because of the appearance of the conidia.

[Chap. XLIV

THE FUNGI

551

Conidmm

Oogonium

Arxthfindium

Fig. 242. One of the tube fungi, Cystopiis, a parasite in leaves of many green
plants: A, section of a leaf of a host plant in which a pustule has been made by
the formation and growth of conidia on hyphae beneath the epidermis, zygotes
have formed from intercellular hyphae within the mesophyll of the leaf; B, part
of the fungus illustrated in A dissected out and enlarged; C, mesophyll cell sur-
rounded and penetrated by hyphae. Motile spores are formed when the conidia
and zygotes germinate. Courtesy of World Book Co.

In late summer small black dots may appear among the hyphae on
the infected leaves. The dots are reproductive bodies and under the
microscope are seen to be spherical and thick-walled, with curiously
branched transparent appendages. These fruiting bodies, or ascocarps,
are the result of the growth of coalescent hyphae, following the fusion
of the protoplasts of an oogonium and an antheridium. The ascocarps
contain several sac-like structures termed asci (sing, asciis) (Fig. 241).
Within each ascus are usually 8 ascospores, although the number may
vary from 2 to many. The germination of an ascospore the following
spring results in a hypha which may grow upon other plants and start a
new life cycle of the fungus.

Downy mildews. The downy mildews are internal obligate parasites
that are verv destructive to many plants, such as potato, onion, tobacco,
and grape. They are not classified with the powdery mildews because of
differences in structure and reproduction (Fig. 242). The downy mil-
dews have tubular, non-septate hyphae similar to those of bread mold
and water molds. More than 1000 species of molds are characterized by

552

TEXTBOOK OF BOTANY

non-septate hyphae. As a group they were named Fhij corny cetes (Gr.
phykos, alga; mykes, fungus) because they appeared to resemble algae
more closely than other fungi. One group of phycomy cetes, the Ento-

FiG. 243. Enlarged drawings of the reproductive structures of a downy mildew
parasitic within tobacco, tomato, eggphxnt, and pepper. The myceHum is inter-
cellular. A, a branched conidia-bearing hypha protruding from a leaf through one
of the stomates; B, germination of conidia; C, stages in the development of a
conidium at the tip of a hypha; D, the hypha from a germinating conidium has
grown into a leaf through a stomate; E-I, stages in the development of a zygote
by sexual reproduction. From North Carolina Agricultural Extension Service.

mophthorales, is of interest because many of the species live as parasites
in insects.

The powdery mildews, on the other hand, have septate hyphae. Their
reproductive structures (ascocarp, ascus, and ascospore) also differ
from those of downy mildew. More than 25,000 species of fungi produce
ascospores, and as a group thev are called Ascomy cetes. Other examples
of them are yeast, blue and green molds, cup fungi, morels, and truffles.

[Chop. XLIV

THE FUNGI

558

Some of the ascomycetes produce numerous conidia during the growing
season.

The mycehum of the downy mildew parasitic on potatoes may over-
winter in the tuber and the next spring grow into the tissues of the new
potato plant developing from that tuber. Branched conidiophores grow
outward through the stomates, and conidia are formed from the terminal
branches. The conidia may be carried by wind or water to other parts of
the plant or to other plants. Motile spores develop within each conidium.
When the leaves are wet, the motile spores emerge from the conidia and
germinate almost immediately. The resulting hvphae penetrate the host,
and new internal mycelia develop. Sexual reproduction ma\' take place
when the fungus is growing in culture, but apparently does not occur
when the fungus is growing in the potato plant. ^ The reproductive struc-
tures of the downy mildew of tobacco are illustrated in Fig. 243.

Cup fungi, morels, and truffles. The cup fungi are usually found grow-
ing on soil and on decaying wood. The vegetative bodv, or mvcelium.

Fig. 244. Photomicrographs of reproductixe structures of a cup fungus (Peziza) :
A, section through the cup which is lined with a layer (hymenium) of asci and
intermingled sterile hyphae; B, a portion of the hymenium further magnified to
show asci and ascospores in more detail.

is largely underground or near the surface. The part usually visible, the
fruiting body, is cup-shaped and lined with a layer of parallel asci and
sterile hyphae. Each ascus generally contains 8 ascospores (Fig. 244).
The morel, prized for its flavor, is related to the cup fungi, but has a
hollow reproductive body covered above with a layer of asci. The fungus
is not edible after the spores have matured and should be eaten only
when immature.

^ It is of historical interest to recall that this so-called "potato blight" caused a serious
famine in Ireland in 1843-46 and led to tlie repeal of the Corn Laws. These laws had
imposed severe restrictions on the importation of wheat and other small grains. Pre\'ious
to this date the annual consumption of potatoes in Ireland was 25 bushels per person.

554

TEXTBOOK OF BOTANY

A related group of fungi are the truffles, which are widespread in
Europe; in the United States they have been found in CaHfornia. Truffles
are considered a dehcacy and gathering them is an industry in France.
The plants, which generally grow from 3 to 12 inches below the surface
of the soil, have a peculiar odor which is not evident to most people.
Pigs and dogs have been trained to locate the plants. The fruiting body
of the truffle is a globose, warty structure enclosing the spore-bearing
tissues. This fungus is spread by rodents that eat the truffles and scat-
ter the undigested spores.

Fig. 245. Spore prints may be used to identify spore color and the arrangement
of spore-bearing surfaces of mushrooms. C is a spore print of a pore fungus repre-
sented by A. D is a spore print of a gill fungus represented by B. The method of
obtaining spore prints is represented by E. The inverted vessel is used to prevent
air currents from scattering the spores as they fall on the paper from the cap of
the mushroom. The cap should be carefully lifted as soon as enough spores have
fallen to form a distinct print. Permanent mounts can be made if the spores fall
on a paper covered with a thin coating of mucilage; this may then be moistened
enough to hold the spores in place by carefully placing the paper on a wet
towel until the moisture has penetrated through the paper to the mucilage. From
W. Hamilton Gibson.

[Chap. XLIV

THE FUNGI

555

Mushrooms, toadstools, and puffballs. These are popular terms and do
not correspond to any technical classification of fungi. To many people
the term "mushroom" means an edible fungus while "toadstool" refers
to a poisonous or inedible one. No such distinctions can be made on the
basis of the form, color, structure, or place where they grow. A few
species may be safely eaten by some persons, but not by others. Beside
the edible species, others are woody or unpalatable, and a number of
species are deadly poisonous to everyone. The temi "puffball" refers to
a fungus that emits clouds of spores when stepped on. Puffballs are
inedible when mature, but many of them are edible when young. Per-
haps the best advice to would-be mushroom hunters is to learn to identifv
positively a few of the common edible species and avoid all others.

The vegetative parts of all these fungi are in the soil, wood, or other
substrate on which the fruiting bodies appear. The fruiting bodies are
compact masses of hyphae, varying from a fraction of an inch to two
feet or more in height, or diameter. When mature, the reproductive
structures may contain countless numbers of spores. The spores of toad-
stools develop in groups of four from the ends of short club-shaped
hyphae, called basidia, which in turn develop on the sides of gills, tubes,
or spine-like projections on the lower side of the cap of the toadstools
(Figs. 245 and 246).

h^^' ■' "C.'^:"'.C^£&-- ■ f K

Fig. 246. Arrangement of spores on the gills of the cultivated mushroom: at
the left a vertical section of one gill as it appears when magnified enough for one
to see the continuous external layer of basidia; on the right a highly magnified sec-
tion of a small portion of a gill including several basidia, three of which have
2 basidiospores. In the wild species of this mushroom each basidium bears 4
basidiospores. Modified from J. Sachs.

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TEXTBOOK OF BOTANY

Fig. 247. Reproductive structures of three species of fungi: A, the sulfur mush-
room (Polyponis sulphur ens) ; B, a coral fungus (Clavaria) ; C, stink -horn
(Dictijophora) . Photos from W. G. Stover and R. B. Gordon.

The common meadow mushroom may produce 2 biHion spores, a
polypore 10 bilHon, and a good-sized puffball may have 7000 bilHon
spores. It has been estimated that only about one spore out of each
trilhon spores ever develops into a new plant. The appearance of fruiting
bodies is illustrated in Figs. 234 and 247. Owing to the club-shaped
hypha on which their spores are borne, such fungi as toadstools, puff-
balls, smuts, and rusts are called Basidiomycetes. There are more than
20,000 species of basidiomycetes. Some mushrooms are basidiomycetes,
but others are ascomycetes.

The culture of the common meadow mushroom is an important indus-
try in scattered localities, where it is carried on in specially constructed
buildings with insulated walls, in caves, or in old mine tunnels where
the temperature and humidity can be controlled. Light is not harmful,
but it is not necessary; and it increases the difficulty of controlling other
conditions. After the beds of soil rich in organic matter are prepared and
sterilized, they are inoculated with pieces of mycelia of pure races. These

[Chap. XLIV THE FUNGI 557

mycelia develop from spores sown on boiled grain in sterilized culture
bottles.

Fairy rings. When growth conditions are favorable in lawns and pas-
tures, the fruiting bodies of the common meadow mushroom often occur
in so-called "fairy rings" (Fig. 248). The rings were thought in ancient

Fig. 248. A fairy ring of a common mushroom {Lepiota) on a ranch in Colorado.
Photo from the New York Botanical Garden.

times to represent the paths traversed b\' dancing fairies. The circular
appearance is due to the outward growth of the underground mycelium
from the original center. The hyphae are perennial, and their radial ex-
tens'ion is accompanied each year by the death of the older portions of
the hyphae in the central areas. The fruiting bodies thus appear in circles
which increase in diameter from year to year. Nearly perfect rings of
this mushroom 160 feet in diameter have been reported, but the diameter
is usuallv less than 20 feet. Similar rings are developed by many^ other
fungi, including morels and puff balls. Some haxe attained diameters of
nearh^ a quarter of a mile and could have resulted only after hundreds
of years of outward extension of the mycelium from the original center
of growth. These largest rings are usually irregular, and occur in incom-
plete circles.

An eruptive skin disease of man and other mammals, known as "ring-
worm," is caused by a fungus. Pustules appear in irregular circles about
the point of infection because of the radial growth of the hyphae in the
skin. The lesions are caused by the fruiting bodies of the fungus as they
break the skin, and thus appear as miniature fairy rings.

558

TEXTBOOK OF BOTANY

Smuts and rusts. Seed plants of many kinds, particularly the grasses,
are infected by parasitic fungi that are unlike those just described.
Owing to their appearance, some of them are referred to as rusts; others
as smuts. The importance of these fungi in decreasing crop yields will be
discussed in the next chapter. Here we shall describe the growth and
reproduction of a few characteristic species. The vegetative and floral
parts of oats, wheat, rye, corn, rice, and onions sometimes contain black

Fig. 249. Effects of smuts of cereals: A and B, loose smut of oats; C, covered
smut of oats; D, enlarged and distorted "corn grains" filled with smut. Photos
from U. S. Department of Agriculture.

or dark brown masses of fungous spores (Fig. 249). These masses are
generally regarded as the smut plants, but thev are merely the reproduc-
tive structures of an extensive mycelium within the host. Several types
of smuts may be exemplified by the loose and covered smuts of oats, the
loose and stinking smuts of wheat, the loose and covered smuts of barley,
and bv the corn smut.

Bunt or stinking smut of wheat. At threshing time mature spores of the
fungus may stick to the surface of the wheat grains and remain there
until the grains are planted.- At that time the spores gemiinate in the soil

- The spores may also fall to the ground, germinate, and grow saprophytically in the soil
for a short time only. Each spore produces a short hypha that forms se\eral "sporidia."

[Chap. XLIV THE FUNGI 559

and produce short hyphae that bear "sporidia." These fuse m pairs and
then germinate. The resulting hyphae penetrate the young wheat seed-
hng. The growing internal mvcelium reaches the stem tip of the seedling,
keeps pace with the growing wheat plant, and finally enters the ovularies
of the inflorescence. The floral parts are greatly distended by harvest
time. Masses of smut spores have replaced the embryo and endosperm
of the seeds. During the threshing of the wheat these spores are released
and scattered among the other grains of wheat.

Loose smut of wheat and barley. The life cycle of the loose smut of
wheat is in one important respect very diflFerent from the bunt of wheat
just described. The spores produced in the dark brown or black masses
in the inflorescences of wheat at flowering time may be blown to the
stigmas of other wheat plants. There thev mav germinate, producing
hvphae that penetrate the ovularies. The mycelium remains dormant
within the embryo until the seed is planted and starts to grow. It is not
possible bv superficial examination of the seed to ascertain the presence
or absence of the internal mvcelium. Following the germination of these
seeds the mycelium grows within the developing host plant, and masses
of spores appear just before flowering time, replacing the destroved
flower parts. Thus the life cycle is complete. The methods of control of
this fundus must evidentlv be different from those used in the elimina-
tion of the stinking smut of wheat. The loose smut of barley is similar
to that of wheat.

Smut of corn. Smut masses mav be seen on anv part of an infected
mature corn plant. The infection, however, alwavs takes place when the
tissue is young. The corn smut fungus does not spread throughout the
host from the point of infection but remains more or less localized. The
smut galls and their masses of spores appear one to three weeks after
infection. The spores may remain alive in the soil or in plant litter for
many months, and consequentlv the fungus is difficult to eliminate.

Stem rust of wheat. One of the most complex life histories in the plant
kingdom is that of the black stem rust of cereals and grasses.^ The stem
rust of wheat is important commercialh' because during some years it
decreases the yield of the American wheat crop bv an estimated 120

^ There are at least 8 varieties of the black stem rust fungus, Puccinia graminis, but only
3 infect cereals: P. graminis tritici, infecting wheat, barley, and many wild grasses; P.
graminis avenae, on oats and some wild grasses; and P. graminis secalis, on rye, barley,
and wild grasses.

560

TEXTBOOK OF BOTANY

million bushels. The life history of this rust illustrates so well how diffi-
cult may be the problems encountered in attempting to control the fungi
that cause diseases of economic plants, that it will be described in more
detail than would otherwise be justified in a book on general botany
(Fig. 250).

Fig. 250. Photomicrographs of the reproductive structures of stem rust (Pucciniu
graminis) of wheat: A, cross section of a leaf of common barberry with aecia
containing aeciospores (b) on the lower side and pycnia (a) on the upper; B,
a pustule of red summer spores (uredospores) that has broken through the
epidermis of a wheat stem; C, a pustule of teliospores in a wheat stem a little
later in the season; D, the following spring, a basidium bearing 4 basidiospores
has developed from each of the two cells of a teliospore. Hyphae from the basidio-
spores infect leaves of the common barberry and the cycle is repeated.

During the summer the fungus is first apparent on the surface of
stems and leaf sheaths as patches of innumerable red spores (uredo-
spores). These spores are blown about by the wind and the hvphae from
them infect other wheat plants. New mycelia develop, and the spores
from them may be blown to other wheat plants. This process may be
repeated every week or ten days during the spring, thus spreading the
fungus rapidly over large areas.

These same internal mycelia a little later produce many two-celled,
thick-walled black spores {teliospores) . These second spores in the life
cycle begin to appear before the wheat plant is mature, and they live

[Chap. XLIV THE FUNGI 561

over the winter on the stubble and straw. After winter dormancy they
germinate, and a short chib-shaped hypha (basiditim) develops from
each of the two cells. The basidium is four-celled, and from each of
its cells a third kind of spore ( basidiospore ) is produced.

During the remainder of the life cycle of this rust certain remarkable
phenomena occur. The basidiospores are blown about by the wind.
Hvphae from these spores do not infect wheat plants, but thev do infect
the common l^arberry plant and a few of its relatives.^ The hyphae pene-
trate the leaves of barberr>'. In a short time there appear on the upper
surfaces of the leaves pustules of flask-shaped masses of hvphae, sperma-
gonia, which produce a fourth type of reproductive body, spennatiiim.
The spermatia may unite with receptive hyphae of the internal mycelium
and result in binucleate hyphae. Since the different spermatia in a leaf
mav have originated from various races of the rust, such a union makes
possible hybrids between different races. Such hybrids still further com-
plicate measures of control.

Further growth of these binucleate hvphae within the barberry leaf
results in a mycelium and pustules of cluster cups (aecia, sing, aecium)
on the lower surface of the barberry leaf. Within these cups, a fifth re-
productive body, the cluster cup spore, or aeciospore, is formed. Aecio-
spores in turn continue the life cycle onl\' when the\^ lodge and germinate
on young wheat plants and a limited number of other grasses. The
mycelium from them grows within the tissues of the wheat plant, and
first the red spores, and later the black spores, develop — thus completing
the life cycle.

Here, then, is a parasitic rust that lives successively on two totallv
unrelated host plants during a complete life cycle. It has several kinds
of spores, which are morphologicallv and physiologically different from
each other. Curious as it may seem, the first definite proof that diseases
may be caused by parasites was obtained when Anton de Barv discov-
ered many of the facts of the life history of wheat rust in 1865.

It is interesting to note the possible variations in the life cycle of the
stem rust of wheat, as given above. Red spores cannot survive the winters
of the North Central States. They may, however, be blown many miles
southward and there cause infection of growing wheat or other grasses
in the autumn. The red spores produced in the South cannot survive the

* Berberis vulgaris is very susceptible to this rust. The green and red \'arieties of the
cultixated Japanese barberry, Berberis thntibergii, are immune to it. Certain other species,
such as the American barberry (B. canadensis) and the species of MaJionia. are susceptible.

562 TEXTBOOK OF BOTANY

hot dry summers and so do not cause infection of the young wheat plants
there in the following fall. In early spring the red spores from southern
wheat are blown northward and successively lead to infection of the
growing wheat of higher and higher latitudes. This cycle of infection by
means of red spores alone, makes possible widespread epidemics of
wheat rust in the absence of barberry plants. Such a condition can be
controlled only by procuring immune races of wheat. These general
epidemics, however, occur more rarely than the local epidemics in
regions in which the common barberry is growing.

The problem of crossing and selecting varieties of wheat immune to
the stem rust is further comphcated by the fact that there are about 180
races of the fungus which pass one stage of their life cycle in barberry.
A variety of wheat may be immune to most of these races, but a few
races may infect and injure it so severely that it is unprofitable to culti-
vate except in the limited areas where these particular races do not
occur.

Other interesting examples of two-host rusts are the white pine blister
rust ( white pine stem, and currant or gooseberry leaves ) , and apple rust
(apple leaves and fruits and red cedar shoots). However, not all rusts
involve two host plants, and many of them do not infect crop plants.

Slime molds. A rather curious group of non-green organisms are the
slime molds. They are frequently classified as plants, sometimes as ani-
mals, and may be discussed in connection with the fungi. The vegetative
part of the plant is a multinucleate mass of naked protoplasm with about
the consistency of mayonnaise, called a plasmodiwn. The plasmodium
lives in moist places and streams through and over the substrate and
finally comes to rest on the surface of plants and other objects. The
different species of slime molds have characteristically colored plasmodia
and variously shaped sporangia bearing a large number of spores.

Lichens. Certain fungi and certain algae live together, forming the
compound plant structures called lichens (Figs. 251-254, and Plate 3).
These plants grow in a great diversity of habitats, such as on exposed
rocks, on tree trunks, and on the ground. They grow and survive in the
most extreme habitats on the earth, from the mountains of the antarctic
where the temperatures are rarely above freezing, to the deserts of
southern California where midsummer temperatures of rocks on which
they grow mav be as high as 175° F. Some kinds of lichens appear almost
structureless, forming a thin coating on the rock surface and on soil
(crustose); others are somewhat leaf -like in appearance (foliose); and

[Chap. XLIV

THE FUNGI

563

Fig. 251. Photograph of a fohose hchen (above) and a crustose hchen (below)
on a rock substrate. Several cup-shaped ascocarps of the fungus in the foliose
lichen are evident. Photo by G. S. Growl.

Fig. 252. Two species of reindeer lichen on a moss substrate. Photo by C. H. Jones.

564

TEXTBOOK OF BOTAiNY

Fig. 253. Rock tripe (Gijrophora) a foliose lichen on a sandstone cliflF. The
smaller lichen (lower left) is the toad-skin lichen {Umbilicaria) . Photo by R. T.
Wareham.

Fig. 254. A plant community of lichens, mosses, and other small plants; pioneer
vegetation with consequent soil formation and accumulation on rock substrates.
Photo by C. H. Jones.

[Chap. XLIV THE FUNGI 565

still others are much branched and cushion-like (fruticose). The latter
are represented by the so-called "reindeer moss" (Cladonia) of north
temperate and tundra regions. Lichens are usually grayish-green in color,
but many species are bright yellow, red, brown, or black.

A lichen is sometimes considered to be merely a fungus that is para-
sitic on certain algae, just as fungi are parasites on wheat, corn, and
potatoes (Fig. 255). Experiments have shown that the fungus of the

Fig. 255. Early stages in the development of a lichen. The hyphae surround and
penetrate the cells of the alga. After Bonnier.

lichen, when growing on culture media, usually does not develop a body
structure resembling that of the lichen; nor does the alga in pure culture.
Nevertheless, the form of the lichen is heritable and reproduced from
one generation to another. For this reason it seems desirable to regard
the lichens as distinct compound plants and to classify them as a separate
group. There can be no doubt that the fungus derives most of its food
from the algae enclosed by it. The alga in turn may have a more constant
water supplv because of its covering of fungous hyphae. The algae have
a much longer photosynthetic season inside the lichen than if they grew
directly on the same substrate.

The species of fungi of the lichens usually belong to the group pro-
ducing asci and ascospores. A few species of basidiomycetes are also
known to form a part of the hchen body. The alga is either a green or a
blue-green; one of the commonest is Pleurococctis ( see Chapter XLVII ) .
Vegetative propagation is exceedingly common among lichens, and takes

566 TEXTBOOK OF BOTANY

place through the dissemination of minute propagules containing both
the fungous hyphae and the alga.

Fungi Imperfecti. These are fungi in which zygotes, ascospores, or
basidiospores either are not formed or have not yet been discovered.
Nearly 24,000 species are now referred to this group. Here belong, for
example, the fungi that cause the leaf spot of beets, "athlete's foot," and
early blight of potato.

GENERAL REFERENCES

Brown, W. H. The Plant Kingdom. Ginn and Company. 1935.

Dodge, C. W. Medical Mycology. C. V. Mosby Company. 1935.

Fink, B. The Lichen Flora of the United States. Univ. of Mich. Press. 1935.

Gwynne-Vaughan, H. C. I., and B. Barnes. The Structure and Development
of the Fungi. 2d ed. Cambridge Univ. Press. 1937.

Krieger, L. C. C. The Mushroom Handbook. The Macmillan Company. 1935.

Macbride, T. H., and G. W. Martin. The Myxomycetes. The Macmillan Com-
pany. 1934.

Raper, J. R. Sexual hormones in Achlya. Amer. Jour. Bot. 26:639-650, 1939;
Amer. Jour. Bot. 27:162-173. 1940; Science. 89:321-322. 1939.

Smith, A. L. Lichens. Cambridge Univ. Press. 1921.

Smith, G. M. Cryptogamic Botany. Vol. I. McGraw-Hill Book Company, Inc.
1938.

SELECTED READINGS

Colley, R. H. The telephone pole and the mushroom. Sci. Monthly. 38:378-

383. 1934.
Lambert, E. B. Principles and problems of mushroom culture. Bot. Rev.

4:397-426. 1938.
McCubbin, W. A. Fungi and Human Affairs. World Book Company. 1924.
Pray, L. L. Common mushrooms. Botany Leaflet No. 18. Field Museum of

Natural History. 1936.
Stakman, E. C, W. L. Popham, and R. C. Cassell. Observations on stem rust

epidemiology in Mexico. Arner. Jour. Bot. 27:90-99. 1940.

CHAPTER XLV
PLANT DISEASES

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Among the earliest records of cultivated plants occur references to plant
diseases, but for centuries the causes were thought to be mysterious and
supernatural. The ancient Hebrew, Greek, and Roman writers frequently
mentioned smuts and rusts, and diseases of the olive, the vine, and the
fig. From the fall of the Roman empire ( 476 a.d. ) until the 19th century,
however, little was added to the knowledge of plant diseases. Even in
modem times explanations of the causes of diseases and plants were
impossible as long as people subscribed to the doctrine of spontaneous
generation of living organisms. As late as the first quarter of the 19th
century many fungi were regarded even by scientific men as merely
transformations of the cellular structure of the plant upon which they
grew. Plant diseases were thought to be due to internal disturbances
resulting in the degeneration of the tissues themselves.

About the middle of the 19th century the recognition of fungi as causes
of plant diseases came about through the researches of certain mycolo-
gists, the most famous of whom was Anton de Bary, who discovered the
true nature of rusts in 1853 and published an account of tlie life history
of stem rust of wheat in 1865.^ The idea that some diseases are caused
by parasites had often been expressed, but de Bary proved it beyond any
doubt.

About this time the researches of Louis Pasteur and others were form-
ing the foundations of the science of bacteriology. These investigations
soon led to the rejection of the belief that fungi and bacteria, associated
with diseased parts of plants, were produced in the lesions by the tissues
of the diseased plants. That bacteria may cause diseases of plants was
first proved definitely in 1880 when Burrill of Illinois discovered that
bacteria cause fire blight of pears and apples (Fig. 256). Proof of in-

1 Farmers had long believed the barberry to be associated in some mysterious way with
rust epidemics. As early as 1660 planting the barberry was prohibited in certain sections
of France. Massachusetts in 1760 decreed that all barberry plants in the colony must be
removed.

567

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TEXTBOOK OF BOTANY

fectious diseases in both plants and animals was established at about the
same time.

Fig. 256. Fire blight of apple. Gelatinization of the cell walls results in a slime
which oozes to the surface of the host, and in the above pictures appears as dark
blotches on the leaf blades, petioles, and twig. At right a photomicrograph of
bacteria in ooze as seen when magnified about 2000 times. Photos from U. S.
Department of Agriculture.

While the early contributions to our knowledge of plant diseases were
mostly by European workers, the study of plant diseases and their con-
trol is now progressing most rapidlv in America. Research in plant path-
ology is being carried forward not only by the universities and colleges
but also by the United States Department of Agriculture and by every
state agricultural experiment station, as well as by several privatelv en-
dowed institutions and industrial laboratories.

Plant diseases are so diversified that no single statement satisfactorih
defines them as a distinct group of phenomena. When parasites partially
or wholly invade and infect a host, the resulting injuries or alterations
in development, behavior, and well-being may be referred to as "in-
fectious diseases." Agents, such as fungi, bacteria, viruses, nematodes,
and certain insects, which cause infectious diseases are called pathogens.
Examples of easily observable symptoms of plant diseases are galls,

/

[Chap. XLV PLANT DISEASES 569

blights, leaf spots, and wilts. These symptoms are just as "normal" to an
infected plant as their absence is to an uninfected plant. Indeed, a well-
trained pathologist can often tell which pathogen is present in a host by
the characteristic appearance of the symptom.

One mav wish to use the term "plant disease" to in-
clude physiological conditions that result from a de-
ficiency of food, water, oxygen, light, or inorganic
salts. Such diseases have been called "deficiency dis-
eases." The observable symptoms when such deficien-
cies exist are a characteristic, or "normal," development
of the plant under such conditions. They are often suf-
ficiently specific that by studying their appearance one
may learn to decide correctly which particular condi-
tion is deficient ( Chapter XXX ) .

Likewise, wounding results in plant development
peculiar to wounding but unusual in an unwounded
plant. Yet the effects of many kinds of wounds are not
considered to be diseases. Gnawing insects and graz-
ing animals, as well as lawn mowers and pruning
knives used bv man, injure plant tissues, but such
wounds in themselves are not diseases.

Economic aspects of plant diseases. One needs onh
to examine the records of modern history to realize
the extent of human misery and distress, as well as the
starvation of millions of other animals, brought about
bv plant diseases. When rye is infected by the ergot
fungus, large purple to black bodies replace many of
the rye grains (Fig. 257). These structures are poison-
ous to man and other animals. From the 17th to the
19th centurv there were about 45 epidemics of ergot
poisoning in Germanv and al:)out 20 in France and
Spain. The Irish famine was brought about by the
destruction of most of the potato crop by a blight fungus during the
vears 1843 to 1846. This blight resulted in the death of a quarter of a
million people and the migration of a million and a half persons from
Ireland to America.

The increasing occurrence of plant diseases is undoubtedly the result
of increased concentration of crops, continuitv of areas devoted to single
crops, more speedv svstems of transportation, and greater transfer of

Fig. 257. Head
of rye in which
two large black
ergot sclerotia
have developed.
From Luerssen.

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TEXTBOOK OF BOTANY

plants and plant products from one part of the world to another. When
a new plant is introduced nito a country great care must be exercised to
prevent the simultaneous introduction of its associated parasitic organ-
isms. When certain plants of a given area are badly infected with patho-
genic organisms, the government sometimes prohibits the transpor-
tation of such plants, or plant parts, into other sections of the country.
This is known as "plant quarantine."

In spite of our ever-increasing knowledge about plant diseases, they
continue to cause enormous losses every year throughout the world
(Table 16). Farmers, orchardists, and nurserymen frequently cultivate a
crop at a loss instead of a profit because of diseases or unfavorable
weather conditions.

Table 16. Average Annual Reduction in Yield Due to Diseases, in the
United States for the Years 1933 to 1937 Inclusive. Estimates by U. S. Dept.
of Agriculture.

Crop

Percentage

Reduction

in Yield

Actual Reduction
in Yield

Field corn

14.66

266,947,000 bu.

Wheat

12.64

89,227,000 bu.

Irish potato

18.12

58,031,000 bu.

Peach

10.82

7,639,000 bu.

Apple

13.62

18,331,000 bu.

Tomato (manufacture)

14.86

186.000 tons

Tomato (market)

19.64

2,687,000 bu.

Tobacco"

23.20

379,249,000 lb.

Cotton

15.46

1,881,000 bales

^Estimates reported For UVM only.

Environmental conditions and plant diseases. We are aware that the
existence of an\' plant depends upon its environment. Some of us have
seen whole crops destroyed bv unusual environmental conditions, such
as flood, drought, tornadoes, and fires. As might be expected, plant dis-
eases are influenced bv the environmental factors of the soil and the
atmosphere. Whate^'er affects the host, such as temperature, light, oxygen
supplv, and the water content of the soil, will also directly or indirectly
influence the organism causing the disease. Considerable information

PLANT DISEASES

571

[Chap. XLV

has been accumulated in recent years regarding the direct relationship
between the prevalence or scarcity of some diseases and certain environ-
mental factors. A few illustrations will be cited. It should be noted also
that environment alone may cause injuries, some of which, such as
bitter pit of apples and blossom-end rot of tomatoes, have been re-
ferred to as "diseases" (Fig. 258).

Fig. 258. Injuries to fruit caused by environmental conditions such as tem-
perature and drought. A, apple soft scald; B, apple brown core; C, tomato blossom
end rot; D, freezing injury of orange. Photos from U. S. Department of Agriculture.

Potato scab, an infectious disease, is likely to be more severe in an
alkaline soil, whereas the growth of the organism causing clubroot of
cabbage is favored by an acid soil. The organism causing tobacco
root rot is practically eliminated from soils of very high acidity.

The weather influences the abundance or scarcity of plant diseases
largely through the effects of temperature and moisture either on the
hosts or on the parasitic organisms.

572 TEXTBOOK OF BOTANY

Peach leaf curl is generally more widespread in years having cold wet
spring seasons. Potato scab has been found to be most severe at a tem-
perature of about 70^ F. The fusarium-wilt organism which infects
tomatoes growing in greenhouses makes little progress at soil tempera-
tures of 63° F. and 95° F., but develops rapidly at temperatures midway
between these extremes. When growing in soils of either low or high
water content, the tomato plant is not affected much bv the wilt organ-
ism. A medium water supply results in a vigorous and succulent plant
which is quite susceptible to wilt. Potato scab appears to be more severe
when the host plant grows in dry rather than in moist soil.

Apple scab develops rapidly in cool, moist weather. Rainy periods be-
tween the time of opening buds and petal fall increase the prevalence of
the disease. Discharge of ascospores from the asci in fallen leaves occurs
only when they are thoroughly wet. The new leaves on the tree will
become infected only when they are continuously wet for several hours.

Variations in hosts, parasites, and plant diseases. It has been known
for many years that the extent of infection and the destructiveness of
plant diseases vary from season to season, from host to host, and even
among individuals of the same species of host. Many of these phenomena
cannot be accounted for by the effects of the environmental factors
enumerated above. A better understanding is possible today because of
increased knowledge of the genetic complexitv both of host plants and
of plant pathogens.

Varying degrees of infection and injury may occur even under ap-
parently similar field environments. A species of fungus causing disease
may be composed of several races, each differing from the others in its
virulence on specific hosts. Owing to population shifts, some of these
races may be more abundant or less abundant than others at various
times on different hosts. For example, it has been reported that the
species of fungus ( Puccinia graminis ) which causes stem rust of wheat
is a mixed population of at least 180 races, each of which can be dis-
tinguished by its relative virulence in different varieties of wheat.
Through a period of time the relative abundance of the different races
fluctuates, with corresponding changes in infection and destructiveness
of the disease.

Both mutation and hybridization must be considered. They may occur
in the host, or in the pathogen when it is growing either in pure culture
or inside the host. From a highly mutating pathogen several new mutant
races, differing in virulence in various hosts, may appear within a few

[Chap. XLV PLANT DISEASES 573

generations. These new races may remain constant or continue the vari-
ability of the species by further mutations.

In those fungi where nuclear fusions occur, recombinations of heri-
table factors due to hybridization of different races may result in hybrids
varying in degree of virulence. For example, investigators at the Univer-
sity of Minnesota crossed two monoploid races of the corn smut species
and from the resulting hybrid obtained nearly 40 segregates, no two of
which were alike.

How is an organism proved to be the cause of a disease? When lesions
are produced in a host plant by a parasite, other parasites may enter
and also become established there. The presence of more than one or-
ganism sometimes makes it difficult to decide which one caused the
initial lesion. When a new disease is discovered, it is important to deter-
mine the causal organism. These decisions are made by following a
standard procedure known as "Koch's rules of proof."

1. The organism must be shown to be present wherever the disease
occurs.

2. The organism must be isolated in pure culture from the lesions in
the host.

3. Healthy tissues when inoculated with the pathogen from these cul-
tures must have the usual svmptoms of the disease.

4. The organism should be reisolated from the second host and identi-
fied with the organism originallv isolated.

Symptoms of plant diseases. The external symptoms of plant diseases
are of many kinds, and onlv a few of the commoner ones will be sum-
marized here.

1. Pti.stiilcs are fruiting bodies or spore masses of parasitic fungi which
have ruptured the outer tissues of the host. Rusts and smuts are common
examples.

2. Scabs and blotches are surface lesions caused by local erowth of
fungi. Apple scab is a common example.

3. Mycelia may be prominent, such as the external mvcelia of powdery
mildew, the compact masses of mycelia of ergot of rve, and the black
"tar-spots" on leaves of willow and maple.

4. Overgrowths of host tissues result in malformations of leaves, stems,
and roots. Crown gall of apple, raspberry, and grape; black wart of
potato; black knot of cherry and plum; clubroot of cabbage; root nodules
of legumes; nematode galls on roots of tomato and cotton; insect galls on

574

TEXTBOOK OF BOTANY

leaves and stems of various plants; "witches' brooms" on hackberry; and
peach leaf curl are among the commoner examples.

5. Leaf spots are dead spots caused by the local growth of parasites in
the leaf tissues of the host. The)' occur frequently on leaves of cherry,
tomato, rose, and many other plants.

6. Wilts include various diseases that are first noticeable by the sudden
wilting of leaves or of the whole plant. Bacterial wilts of cucumber and
cantaloupe are rather common. Damping-off of seedlings and cuttings is
a type of wilt disease caused by local infection and weakening of stems
near the surface of the soil.

Fig. 259. Apple infected by the fungus which causes bitter rot of apples. Photo
from Ohio Agricultural Extension Service.

7. Rots are the result of the decomposition of the cell walls of aflFected
tissues, such as the soft rot of sweet potatoes, bitter rot of apples (Fig.
259), and brown rot of stone fruits.

8. Blight is the term applied to the sudden dying of leaves, shoots,
and blossoms. The fire blight of apple and pear is common wherever
these trees grow in America.

9. Cankers are sunken dead areas on stems in which the bark is

[Chap. XLV PLANT DISEASES 575

killed. Fire-blight cankers are caused by bacteria. The nailhead canker
on stems of apple trees is caused by a fungus. Similar dead areas may be
caused by low temperatures and by sun scald.

10. Yellowing or chlorosis of leaves is caused by viruses, by some
fungi and bacteria, by low temperatures, or by a deficiency of certain
salts.

Control of plant diseases. In recent years it has become increasingly
evident that really efficient measures for controlling plant diseases must
depend on an accurate and detailed knowledge of the life history of the
pathogen. The acquisition of this knowledge is sometimes a laborious
and technical procedure. Special training and well-equipped laboratories
are essential to progress in this field.

When the life cycle of the causal organism is known, experience has
shown that there is usually some point in the cycle that is subject to
attack by methods of control. Spores or young hyphae outside the plant
are often easily destroyed by chemicals or heat; but it may be impossible
to eliminate an internal mycelium that develops from them without in-
juring the host. For example, a spore or hypha of the apple scab fungus
may be destroyed by a suitable fungicide on an apple leaf before the
first hypha has grown into the leaf. After the hvpha is once inside the
leaf it is difficult to kill it without destroying the leaf. Similarly it is much
more economical to try to eradicate the black stem rust of wheat by re-
moving barberry plants than by attempting to sprav the wheat.

Numerous plant diseases are known, and many control measures have
been devised and applied with varying success. Even when a partially
efiicient control measure has been worked out, it has been found by
bitter experience that the application is not a rule-of -thumb matter. No
single procedure will give the same results in all kinds of weather, with
all varieties of plants, or at all times of the year. In other words, the suc-
cessful application of such treatments as sprays, dusts, and hot water
depends on the condition both of the host and of the parasite; and such
measures must be used with keen discrimination if the results are to be
eflFective.

With these general considerations in mind, representative diseases
caused by fungi, bacteria, slime molds, nematodes, and viruses will now
be considered.

Apple scab. In the United States as a whole, scab is the most destruc-
tive of apple diseases. The olive-brov^ni areas of "scab" are quite notice-

576

TEXTBOOK OF BOTANY

able on the leaves, fruits, and flower pedicels (Fig. 260). These spots
not only lessen the commercial value of the fruit, but also decrease photo-
synthesis, cause early abscission of fruits and leaves, and check the
growth of the remaining apples. The mature ascospores in the old dead
leaves on the ground are projected into the air from the bursting asci
during rainv weather in the spring and are blown about by the wind to

j&itk * ,^ll

Fig. 260. Apple seal): A, on the fruit; B, on a leaf in which the fungus lives
throughout the winter; C, magnified section of an ascocarp with asci and asco-
spores from an over-wintered leaf; D, ascocarps visible in an old leaf after it has
been cleared in hot potassium hydroxide. Adapted in part from A. L. Pierstorff
and Ohio Agricultural Extension Sendee.

the apple trees. The spores germinate almost immediately on the living
tissues of opening buds, young leaves, and flowers if a film of water is
present. Sprays of lime sulfur, or wettable sulfur, must be applied before
extended periods of rain to prevent infection by the hyphae from the
germinated ascospores. After infection and growth of an internal my-
celium, conidia develop and are distributed to other parts of the tree
by rain water.

It is extremely desirable that all pre-blossom infections of apple scab
be prevented, since conidia may be produced all summer long from
hyphae of mycelia if the early infection by ascospores is allowed to

[Chap. XLV PLANT DISEASES 577

occur. Sprays are applied often enough to protect all voung growing
parts from invasion. This protection is generally secured by some such
program as the following: (1) a delayed dormant spray is applied as
soon as the leaves which surround the young flowers have emerged
about 1/4 inch; (2) a pre-pink spray before the slightly emerged
petals have become colored; (3) the pink spray before the flowers are
in full bloom; (4) the calyx spray just after petal fall and before the
blossom end of the fruit has been covered by the calyx; (5) another
spray ten days after the calyx spray; and ( 6 ) later sprays applied when
necessarv.

A well-informed plant pathologist, with the aid of weather forecasts
and infoiTnation from orchardists concerning the condition of the buds,
can intelligently modify the above spray program according to weather
conditions. In some states pertinent information concerning sprav pro-
grams is broadcast daily by radio for all sections of the state during
the spring months.

Damping-ofl. Seedlings of plants often are infected at the surface of
the ground in such a way as to cause the killing of the stem and conse-
quent death of the whole plant. Gardeners and nurser\'men are es-
pecially troubled by this damping-off disease in seedbeds. Sexeral
species of fungi cause damping-oft, and they have been reported from
all parts of the world. Practically all species of plants are susceptible
to these fungi. Sometimes the plant is destroyed before the seedling
emerges from the ground; in other cases the disease may not be appar-
ent until after the seedlings or cuttings are transplanted. The fungi
may live for long periods of time in the soil as saprophytes. It is doubtful
if highly organic soils are ever free from such organisms. Abundant
moisture on the plant or in the soil, and a fairly high temperature are
fa\'orable to the growth of these fungi and their inxasion of green
plants.

Control measures for damping-off lie largely in soil disinfection and
the proper regulation of temperature and humidity. Hot water, steam,
sulfuric acid, and formaldehyde have all been found effective means
of disinfecting soils. Dusting seeds with copper or mercury compounds
is also an efficient control.

Downy mildew of grapes. This disease is probably native to North
America, having been reported in the United States in 1834. It was not
known in Europe until 1875. The presence of the downv mildew on
grapes in France led more or less accidentally to the discovery about

578 TEXTBOOK OF BOTANY

1881 of an important fungicide known as Bordeaux mixture. A disease
caused by the root louse ( Phylloxera ) had become a serious menace to
the vineyards of France, and in order to combat this disease the French
imported some American grapes which are immune to the root louse.
The American stock, however, was infected with the downy mildew
fungus which then spread all over Europe. Apparently at least one
vineyard-keeper customarily sprinkled a copper sulfate-lime mixture on
the vines along the highways to discourage theft of the grapes. The
French pathologist, Millardet, observed that where such a mixture was
sprinkled on the vines, the ravages of mildew were reduced. He began
experiments which led to the general use of Bordeaux mixture as a
spray for the successful control of this and other diseases.

The fungus attacks all green parts of the grape plant and appears as
whitish patches of mildew. The fungous hyphae enter the plant largely
through the stomates. The presence of the mycelium within the plant
may result in loss of leaves, dwarfing of young twigs, and destruction of
fruit. Since the fungus winters over as spores in fallen leaves, the disease
may be combated by getting rid of the leaves (sanitation), and by
spraying the plants with Bordeaux mixture before the young hyphae
from the spores invade the young leaves and flowers.

The powdery mildews. The powdery mildews occur everywhere in
temperate and tropical regions and on a large variety of hosts. These
fungi are visible on the surface of the affected plant, and sometimes the
parasitized tissues of the host become malfoiTned. They are either
whitish to grayish, powdery or mealy depending upon whether the
visible stiTicture is the mycelium, the summer spores, or the blackish
fruiting bodies. The powdery mildews are always superficial parasites
and in this way differ sharply from the downy mildews considered
above. The methods recommended for the control of powdery mildews
include removing the infected parts, or using an appropriate spray
or a sulfur dust.

Brown rot of stone fruits. This disease is important in the United
States and Europe because it destroys the fruit of such orchard crops as
peaches, plums, cherries, and apricots (Fig 261). Among the pome
fruits the apple may also be affected. The disease is usually recognized
by the rotting of fruits, blasting of flowers, and the killing of young
stems. A brown spot appears on the fruit and may enlarge until the
whole fruit is decayed. In later stages the fruit dries and shrivels to a

[Chap. XLV

PLANT DISEASES

579

condition that has given rise to the term "mummified fruit." When the
flowers are infected, "blossom bhght" occurs and results in the death
of the flowers. The fungus may grow from the blossoms into the twigs,
and there cause cankers and death of the twig.

Fig. 261. Brown rot of peaches. Photo by J- A. McChutock, Georgia Experiment

Station.

New infections may occur from the time of early flowering until the
fruit is mature. Insects and winds are the principal means of spore
dispersal. The fungus may overwinter in mummified fruits and in twig
cankers. Control of the disease may be accomplished by thorough prun-
ing accompanied by removal of mummies and diseased twigs and the
application of the appropriate sulfur sprays or dusts throughout the
growing season.

Peach leaf curl. Typical symptoms of peach leaf curl are pinkish, puck-
ered, and thickened leaves or parts of leaves. Infection of peach leaves
is most frequent when a cold wet period follows the parting of the
bud scales in spring. The mycelium invades the tissues and in a few
weeks a layer of asci fonns on the upper surface of the leaves. This
disease causes the early abscission of the leaves. Young shoots and
flowers are also infected and killed. The disease is easily controlled by
one application of a lime-sulfur or Bordeaux spray during the dormant
period before the buds begin to swell.

Wood rots. Trees, fence posts, telephone poles, railroad ties, and
bridge timbers sooner or later decay, as the result of the activities of
certain wood-rotting fungi. These fungi nearly all belong to the groups

580 TEXTBOOK OF BOTANY

of "fleshy " fungi. They may destroy the heartwood, the sapwood, or both
( Fig. 262 ) . Changes in the color and texture of the wood usually accom-
pany the growth of the fungi. The internal mycelium is of course the
destructive agent through its fonnation of wood-digesting enzymes.

Fig. 262. Cross section of a stem of white ash in which a saprophytic fungus
has destroyed a part of the heartwood. The obvious mycehum pictured on the cut
surface grew after the section was removed from the tree and placed in a moist
chamber. From U. S. Department of Agriculture.

The external fruiting bodies (Fig. 247), popularly known as "conks,"
produce numerous spores, which are carried to other trees primarily by
wind. Living trees are infected through wounds, as well as by the
direct growth of the fungus from infected stumps into the heartwood
of trees in sprout forests.

It is practically impossible to control wood rot in forests. The infec-
tion of orchard trees mav be avoided by careful treatment of wounded
trees, and by the eradication of all infected ones. Telephone poles,
railroad ties, and other timbers in contact with soil may be protected
for long periods of time by appropriate treatment with coal-tar creosote,
tar, crude oil, or certain salts of zinc.

[Chap. XLV PLANT DISEASES 581

When fox-fire is traced to its source it is usually found coming from
saprophytic fungi (Fig. 263) living in partially decayed wood or bark.
Some of the energy set free in respiration in these fungi is light energy.

The smuts. The smuts described in the preceding chapter are de-
structive to cereals, but effective measures of controlling some species
have been discovered. The importance of knowing exact life histories
of fungi that cause disease before one attempts to formulate methods of

Fig. 263. Fox-fire. Fungous mycelium in a piece of wet bark as seen in light and
in darkness. After W. Hamilton Gibson.

control is shown by a study of three kinds of smuts: corn smut, loose
smut of oats, and loose smut of wheat and barley ( see Chapter XLIV ) .

The spores from the black masses or lesions on the corn plant fall on
the ground, remain there over the winter, and may be the source of
infection of young plants if carried to wounds during the following
spring. The most effective methods of control are selection of seed
from healthy plants, selection of hybrids immune to the disease, and
crop rotation.

The mycelium of the fungus which causes the disease known as loose
smut of oats grows within the oat plant and produces millions of spores
in the young flower panicle. These spores are blown about by the wind
and some of them lodge in the flowers of healthy oat plants. There
they germinate; and the mycelium may grow among the glumes, or
beneath the epidermis of the grain coat. Some spores may not germinate
but remain attached to the outer portion of the glumes. The fungus
overwinters in either of these three places, and when the oats are sowed
may cause infection of the seedling and eventually a smutted head at or
before harvest time. The fungus is killed by treating the seed with
formaldehyde or with ethyl mercury phosphate before planting.

The fungi causing loose smut of wheat and barley live through the

582 TEXTBOOK OF BOTANY

winter as mycelia inside the grain. Spores may also survive the winter
on the seed. The spores on the surface are easily killed, but it is difficult
to kill the hyphae inside the seed without injuring the embryo. The
fungous hyphae in the wheat seed are killed when the seeds are im-
mersed in water at a temperature of 129° F. for ten minutes; the hyphae
in barley seeds may be killed by similar treatment at 127° F. Neither
the wheat nor the barley embryos are injured if these directions are
accurately followed.

Plant diseases caused by bacteria. It is sometimes difficult to discover
whether a disease is caused by bacteria or by fungi since both are
often present in the same lesion. It is of course necessary that this dis-
covery be made before effective remedial measures can be worked out.
Most bacterial diseases of plants are very difficult to control. Among
the better measures of control are: rotation of crops, use of disinfected
seed, prompt and complete destruction of diseased plants or parts of
plants, proper care of wounds made by storms and pruning, seed
sterilization, and the planting of less susceptible varieties. The fire blight
of pear is probably the best known of these diseases, and the methods
suggested for its control will be discussed.

Fire blight. Fire blight is one of the most destructive of the diseases
of pome fruits. Its most frequent symptoms appear as blighted twigs,
flowers, and leaves. Infection takes place in the flowers, young shoots,
and leaves. Any diseased part of the plant may exude a sticky fluid con-
taining the bacteria. Rain may distribute the bacteria to other parts of
the plant. Insects, however, are the most important carriers of the
bacteria from one plant to others.

Fire blight is difficult to control. To decrease the possibilities of in-
fection of apple and pear trees, tender sprouts should be kept removed
from the trunk and larger branches during the early part of the grow-
ing season. In addition, all blighted twigs should be removed from
the pear orchard at least twice weekly during the active blight season.
Blighted branches found in the fall of the year should also be removed.
Other suggested methods of control include the application of sprays
at blossoming time and the planting of varieties of pears and apples
less susceptible to the disease.

Clubroot of cabbage. The organism which causes this disease in cer-
tain members of the mustard family, such as cabbage, turnips, and
radishes, is a simple mold. Enlarged, deformed, and club-shaped roots
are evidence of the progress of the disease. The organism enters through
root hairs or tlirough wounds, and digests the tissues of the host as the

PLANT DISEASES

583

[Chap. XLV

Plasmodium grows. If the cambium is invaded, further development of
the tissues of the root is very irregular ( Fig. 264A ) . The disease is most
prevalent in warm, wet, acid soils. Control methods recommended are:
use of disease-free plants, application of hydrated lime (from one to
two tons per acre), planting of less susceptible varieties, avoidance of
planting cabbage in infected fields.

/

Fig. 264. A, clubroot of cabbage; B, nematode galls on roots of tomato. Photos
from U. S. Department of Agriculture.

Galls. The development of galls on leaves, stems, and other parts of
plants is induced by insects," bacteria, and fungi. The greater variety of
galls are caused by insects. Galls mav be formed on many kinds of
plants and are readily seen on oaks, hackberries, willows, goldenrods,
asters, and roses. In some way, perhaps primarily by means of hormones,
the organisms living in these plants initiate peculiar overgrowths of
certain local tissues, with the ultimate formation of a gall having a
specific form and pattern. Leaves and young twigs are the parts usually
affected. The different forms and patterns of galls are correlated with
the causal insects, rather than with the plants on which they occur.
In many instances the insects can be identified from the characteristics
of the galls (Fig. 265).

Nematode galls. The nematodes, sometimes called roundworms, may
live either as parasites or free in moist soil containing organic matter.

- These insects include gall-wasps, gnats, aphids, and mites.

584

TEXTBOOK OF BOTANY

Fig. 265. Fifteen diflFerent galls on hickory leaves caused by as many different
insects. Drawings by B. W. Wells.

They eat their way into the roots of many plants, feeding upon the juices,
and cause the overgrowth of the infested organs (Fig. 264B). They
infest the roots of tomato, tobacco, cucumber, lettuce, peony, straw-
berry, cotton, and many other cultivated and wild plants. The so-called
nematode disease of wheat affects the flowering parts, transforming
the grains into galls. Stems and leaves of rye, clover, alfalfa, begonia,
and many bulbous plants become distorted, swollen, and variously
colored because of infestation by leaf and stem nematodes. An ex-
cessive development of fibrous roots of sugar beet is also caused by
nematodes. The chief methods of control are soil disinfestation and
crop rotation.

Virus-diseases of plants. Among the diseases recognized in relativelv
recent times as being caused by viruses are tobacco mosaic, curlv top
of sugar beet, peach yellows, yellow dwarfing of onions, tomato streak,
leaf roll of potatoes, virus gall of sugar cane, and witches' broom of
sandalwood tree. Viruses also affect animals and cause such diseases
as infantile paralysis, smallpox, influenza, common colds, measles, and
rabies. There are many viruses; each has characteristic effects on the host
plant (Fig. 266). Some viiTises are limited to a single host, whereas
others occur in a wide variety of unrelated plants. Viruses have been
detected in more than a thousand species of plants growing in all

PLANT DISEASES

585

[Chap. XLV

parts of the world. The tobacco mosaic has been shown to be caused
by a protein of high molecular weight, which may be purified by re-
peated recrystalhzation without losing its property of causing the dis-
ease. This causal agent increases in number in contact with the proto-
plasm of the host plant. It cannot be cultivated in non-living culture
media.

Fig. 266. Symptoms of mosaic disease on leaves of tomato and bean. Photos: A,
by J. D. Wilson; B, by U. S. Department of Agriculture.

Plants seldom recover from virus infections. The plants may survive
infection for long periods of time, but the virus causes decreased
photosynthesis, decreased yields of crop plants, small and distorted root
systems, and increased susceptibility to other parasites. Viruses are
usually not localized in certain tissues and organs, but permeate all living
parts of the plant. Herbaceous plants are apparently more susceptible
to viruses than woody species, although lethal virus diseases of elm,
plum, and peach are well known. Vimses are transmitted from dis-
eased to healthy plants by insects, such as leaf hoppers, aphids, and flea
beetles, and sometimes on the tools used in cultivating, pruning, and
grafting. Certain viruses are transmitted through the seed, while others
are not. Some viruses may be carried over from one season to the next
in other hosts such as weeds and wild plants which grow nearby. Con-
trol has been accomplished by the removal of such additional hosts.

Other control measures include the eradication of diseased plants,
control of the insects that transmit viruses, use of disease-free seeds,
the avoidance of potentially infected parts of plants in grafting and
vegetative propagation, and the selection of less susceptible varieties.

586 TEXTBOOK OF BOTANY

REFERENCES

Boyce, J. S. Forest Pathology. McGraw-Hill Book Company, Inc. 1938.
Chupp, C. Manual of Vegetable-Garden Diseases. The Macmillan Company.

1925.
Heald, F. D. Introduction to Plant Pathology. McGraw-Hill Book Company,

Inc. 1937.
Hesler, L. R., and H. H. Whetzel. Manual of Fruit Diseases. The Macmillan

Company. 1920.
Melhus, I. E., and G. C. Kent. Elements of Plant Pathology. The Macmillan

Company. 1939.
Owens, C. E. Principles of Plant Pathology. John Wiley & Sons, Inc. 1928.
Rivers, T. M. Filterable Viruses. Williams & Wilkins Company. 1928.
Smith, K. M. Plant Virus Diseases. P. Blakiston's Son & Co., Inc. 1937.
Reports and Bulletins from the State Agricultural Experiment Stations and

from the United States Department of Agriculture.

CHAPTER XLVI

UNDER-WATER ENVIRONMENTS

<x><x^c><xxc>e><x><x>o<x>o<^^

Numerous kinds of plants live continuously under water. Many epi-
phytes, on the other hand, grow in a medium of air and are in contact
with water only during rains or when covered with dew. Even a rooted
plant in dry uplands grows almost entirely in the atmosphere which
surrounds its tops and fills the spaces between the particles of the soil.
Except for short intervals after rains, its root system in summer is in
contact only with thin films of water held by the soil particles, and much
of it is exposed to the soil air.

Fig. 267. An abrupt transition in habitats and vegetation.

Other species of plants live partially submerged in the shallow water
of marshes and swamps between uplands and near the borders of
oceans, lakes, and streams (Fig. 267). Some of them are limited to areas
in which the water level fluctuates with the rains and tides. Many plants
can endure partial submergence for limited periods of time, and many

587

588

TEXTBOOK OF BOTANY

others thrive with their roots and rhizomes in soil continuously below
the water surface. The erect stems of such plants extend above the
water surface, or because of long petioles their leaf blades are exposed
to the air above the water. Among the plants of the world there appear
to be all gradations between those limited to land and those limited
to water habitats (Fig. 268). Those on land can endure varying degrees

Fig. 268. Plants in various degrees ol submergence. Many plants not evident
in the pictvire are wholly submerged. Water milfoils, hornwort, elodeas, pond-
weeds, and many others may be wholly submerged or with their tips slightly
above the surface (foreground). Leaf blades of water lily and pond lily may be
floating or raised above the surface. The leaves of cat-tail and bulrush are mostly
aerial, and all but the root systems or parts of root systems of dogwood, alder, and
willow are above the water. Photo by E. S. Thomas, Ohio State Museum.

of exposure to the air and to water loss bv exaporation. Those below
the water surface can endure low concentrations of oxygen.

Some submerged plants live wholly suspended in water. Others
have roots in the underlying substrate. The general processes of nutri-
tion and growth in submerged plants are the same as those in land
plants, and they are dependent upon the same external factors. These
factors, however, are not similarly distributed or equally available in
air and in water media. A few of the conditions in land habitats and
in water habitats are compared in the accompan}dng summary.

[Chap. XLVI UNDER-WATER ENVIRONMENTS

The Land Habitat

589

The Water Habitat

Plants and animals with high
water content living in a medinm of
air.

The medium: the atmosphere
above soil always in motion, and its
molecnles always in rapid motion;
rarelv saturated with water \'apor;
compressible, hence rarely destruc-
tive when in motion; air and water
movement in soil restricted by soil
particles.

Carbon dioxide: always available
above and below soil surface.

Oxygen: always available above
soil surface, not alwavs available
below soil surface.

Temperature: large daily and sea-
sonal fluctuations of air temperature;
smaller variations of soil tempera-
ture, except where relatively dry soil
surface is exposed to the sun, where
the daily and seasonal extremes com-
monly exceed tliose of the air.

So/7; stability, penetrability, poros-
ity, gas content, and water content
important in root development. In-
organic salts in soil and dust avail-

o

able when in solution or in direct
contact; concentration sometimes
very low; in arid regions more con-
centrated and sometimes toxic.

Light: usually abundant at upper
vegetation surface; decreased at
lower levels by taller plants.

Plants and animals with high
water content living in a medium of
water.

The medium: the hydrosphere rel-
ativelv quiescent, except surface
lavers of large bodies of water and
rapid streams; molecular motion
comparativelv slow; incompressible,
hence offers great resistance to move-
ment within it, and when in rapid
motion very destructive.

Carbon dioxide: in solution, both
free and bound in bicarbonates; con-
centration variable, often high in
water containing organic matter and
in soil beneath water.

0x1/ gen: in solution; concentration
highest in surface layer, often de-
ficient in deep water and in soil be-
neath water.

Temperature: smaller daily and
seasonal fluctuations even in upper
layers; temperature very constant in
deeper water; temperatures of water
and underlying soil not very dif-
ferent.

Soil: stability, porosity important.
Inorganic salts obtained from me-
dium by suspended plants, from soil
by rooted plants; salt concentration
sometimes very low; usually higher
in oceans than in moist soils; in
closed basins salts may accumulate
to toxic concentrations.

Light: abundant at water surface
except for clouds, fogs, and shade of
plants above surface; rapidly de-
creases below surface. Rough sur-
faces and turbidity increase reflec-
tion, decrease light penetration.

590 TEXTBOOK OF BOTANY

It is evident that an aquatic environment includes most of the factors
of a land environment, but the intensity or quantity of any factor is
very different. Some raw materials essential to green plants, such as
light, carbon dioxide, and oxygen, are more often at a minimum in
water than in air. Rates of diffusion also are so much slower in water
that, in the absence of currents, the movement of dissolved gases to
plants may be so slow that both photosynthesis and respiration are
limited. On land transpiration often becomes destructive. Because of
these effects of the medium, some kinds of plants survive only in certain
places on land, others only in certain places in water. Plants that can
live both in water and on land are either microscopic plants or those
larger plants which have emergent leaves when growing in water. The
vertical spread of plants on the earth is diagrammatically represented in
Fig. 269.

Under-water habitats are roughly of two kinds : fresh water, including
lakes and ponds, rivers and streams; and marine, such as oceans, seas,
gulfs, and bays. There are also inland bodies of water of high salt
content in the plains, semi-deserts, and deserts.

The Fresh-water Environment

The fresh-water environment includes all non-saline lakes and ponds,
rivers and smaller streams, swamps, marshes, and bogs. Each of these
types of habitat is considered separately.

LAKES AND PONDS

Lakes and ponds differ from each other chiefly in area, depth of water,
permanence, effects of winds and temperature changes, and depth of
light penetration. They represent the so-called standing water, in con-
trast to the flowing water of streams, although there may be surface
movements due to winds, changes in temperature, and deep springs.

Light. The intensity of light at the surface of a lake is dependent pri-
marily on the latitude and the time of day. It may be modified locally
by clouds, fog, smoke, dust, and marginal shade. It is not, however,
the intensity of the light at the surface that is of importance in the
growth of submerged plants, but the light that actually penetrates the
water to the depth at which they live.

The amount of direct sunlight that is reflected from a smooth water
surface ranges from 2 per cent when the sun is directly overhead to
nearly 100 per cent when the sun is near the horizon. Of the total

[Chap. XLVl

UNDER-WATER ENVIRONMENTS

591

BACTERIA AND FUNGUS

SPORES COLLECTED

AT 36,O0O FEET

MT EVEREST, 29,141 FEET

y. yJMT. MCKINLEY. 20,300 FEET

:j7///iu\\%_

DW ALGAE %\ "^

Fig. 269. The vertical distribution of living plants on the earth.

592 TEXTBOOK OF BOTANY

radiation from the sun and sky during a day, 7 per cent is reflected from
a smooth water surface; but when the water surface is rough, as much
as 25 per cent may be reflected. The hght below the water surface is
thus always less than that above.

Natural bodies of water are usually colored or turbid with suspended
colloidal particles and microorganisms. These minute objects reflect
and refract the light in all directions. They also absorb it and are highly
effective in reducing light intensity in the water below them. About 40
per cent of the light that enters the water is usually absorbed in the
first meter. Not more than 5 to 10 per cent of the visible rays penetrate
to a depth of 10 meters; and at depths of 50 meters the light is negligi-
ble and consists onlv of certain green, blue, and violet rays.

As a result of the reflection and absorption of light at the surface of
the water, the length of the daylight period in deep water is relatively
short and is limited to the middle of the day, when the sun's rays are
near the vertical. Radiation from the sky is insufiicient to illuminate the
depths.

Photosynthesis. In clear- water lakes the layer of water in which effec-
tive photosynthesis occurs rarely exceeds a depth of 10 meters. In tur-
bid or colored water it is much less. It is tlierefore readily seen why
the bulk of the suspended and rooted plant populations of lakes is
confined to the upper layers of open water and shallow margins. Only
in the extremely clear water of high mountain lakes is photosynthesis
adequate at greater depths. There are a few very slow-growing green
plants that exist at depths of 15 and even 30 meters, where photosyn-
thesis in them is evidently just above the compensation point. The
maximum depth of most rooted aquatics in the lakes of the United States
is 6 to 7 meters. At this depth the plants receive about 2 per cent of
total sunlight.

Oxvgen. Oxygen enters lake and pond water primarily from the at-
mosphere. It diffuses into the water more rapidly when the surface is
agitated and increased by wind. Additional dissolved oxygen is brought
into lakes from the atmosphere bv rain water and by rapid tributary
streams, particularly if they flow over falls and riffles. Oxygen is also
added during the daylight period by photosynthesis in submerged
green plants.

The oxvgen concentration is decreased through the respiration of
both plants and animals. It is also dissipated by high temperature of
the water. Indeed, in shallow ponds containing an abundance of both

[Chap. XLVI UNDER- WATER ENVIRONMENTS 593

plants and animals, the oxvgen content mav become so reduced during
hot weather that fish and other submerged animals suffocate. The death
of submerged plants in shallow water may likewise occur from suffo-
cation.

Carbon dioxide. The sources of CO2 in the water of ponds and lakes
are diffusion from the atmosphere, oxidation of organic matter by bac-
teria and also the respiration of all other organisms, inflowing streams,
and the release of CO- when the soluble bicarbonates of calcium and
magnesium are changed to insoluble carbonates and accumulate as marl
(Fig. 228).

When carbon dioxide dissolves in natural bodies of water, more than
half of it combines with the water, forming carbonic acid: COi- + H-O
-^ HlCO:^. The remainder exists as free carbon dioxide. Submerged
green plants utilize in photosynthesis not only the free CO- but also
the CO- in carbonic acid and the so-called "bound" CO- in bicarbonates,
such as those of calcium, magnesium, sodium, and iron.^

There is about one-third of a cubic centimeter of CO- in a liter of air,
and it is constantly available at the surface of land plants because of
continuous air currents. In lakes the COl> concentration is highly varia-
ble. It may be unavailable at times, particularlv in bog lakes, and at
other times it may accumulate to the equivalent of 20 cubic centi-
meters per liter of water. Apparently the death of submerged vegetation
in warm shallow water in midsummer mav result from starvation, from
suffocation, or from some other indirect effect of high temperature. In
spring and autumn carbon dioxide is quite unifomily distributed from
the top to the bottom of deep lakes, but in summer and winter it may
be either abundant or nearlv absent at different depths.

Nitrogen. The nitrogen (N2) in solution in water is no more usable
by aquatic plants than is the nitrogen of the atmosphere by land plants.
However, as on land, nitrogen may be combined by microorganisms
into such usable substances as ammonia, nitrates, and organic nitrogen

.HCO3 /HCO3 yHCOj

^ The bicarbonates are metal hydrogen carbonates: Ca\ - Mg<( - Fe<( >

' ^HCOa \HCO3 ^HCOa

and NaHCOa. When carbon dioxide is released from these bicarbonates, the carbonates
CaCOa, MgCOri, and FeCOs are formed. These carbonates, with the exception of sodium,
are relatixely insoluble and accumulate on plant surfaces; eventually they form layers of
marl on lake bottoms in limestone regions. Some of the limestones of the Middle Western
States originated in fresh-water lakes.

594 TEXTBOOK OF BOTANY

compounds. Since so small a portion of it is transformed in biological
processes in lakes, its concentration is rather constant and uniform.

Inorganic salts. The chemical elements essential to fresh-water plants
are the same as those essential to land plants. Rooted aquatics obtain
the salts containing these elements primarily from tlie underlying soil.
A much smaller amount enters the plants from the water surrounding
their green shoots. Suspended and floating plants are dependent on
the salts in solution in the water. The concentration of these inorganic
salts in lake water is lower on the average than in soil water. Extensive
sand areas in lakes may be as barren of rooted vegetation as similar
areas on land, even though the lake water contains sufficient salts for
the growth of large numbers of suspended and floating plants.

Seasonal stratification of water. For a period of time in spring and
autumn the temperature of the water at difterent depths in lakes be-
comes nearlv uniform (about 4° C). This is the temperature at which
water reaches its greatest density. It becomes lighter whether cooled
below or warmed above 4° C. As these uniform temperatures develop,
convection currents are formed by the lighter water moving upward and
the heavier water moving downward. Wind storms at these seasons
result in mixing and stirring the water to great depths. These are the
spring and autumn "overturns."

During the summer the surface water becomes warmer and lighter,
with the result that layers of warm water float on the colder and denser
water beneath. Winds then usually disturb only these upper layers of
warm water, and there is little vertical mixing of water. This condition
prevails until autumn. As the water at the surface is cooled to 4° C.
(39.2° F. ) convection currents carrv the water downward until the lake
becomes uniformly dense and the autumnal overturn occurs.

With the coming of winter the density of the surface water is de-
creased again as it is cooled below 4° C. These upper layers of colder
and lighter water float on the denser water below. If ice is foiTned it
also floats, because its density is about nine-tenths that of water. Under
these conditions the water is quite stagnant, and there is little or no
mixing.

During both summer and winter the water in deep lakes may become
stratified. In summer, when there are the greatest differences in tempera-
ture between the upper and lower layers of water, there is an inter-
mediate layer in which the records of temperature on a thermometer
change as much as 3° to 5° C. per foot as the thermometer is raised or

UNDER-WATER ENVIRONMENTS

595

[Chap. XLVI

lowered through the water. This layer in which differences in tempera-
ture are comparatively abrupt is known as the thermocline.

Since many organisms, both plant and animal, move about only within
certain temperature limits, and since their growth is also limited by the
available CO2 and O2, it must be evident that these organisms in deep
lakes are distributed in strata in summer, and to a less extent in winter.
During an exceptionally warm winter in a mild temperate climate the
surface water may not be cooled below A" C; no spring overturn oc-
curs, and there is no renewal of oxygen in the lower layers of water.
During the following summer those fish that can live only in layers of
cold water may suffocate. Similar consequences may also follow when-
ever there is insufficient wind to cause the spring overturn.

Z|0

-21°

21°

THERMOCLINE
6°

> ..

SPRING
OVERTURN

SUMMER

AUTUMN
OVERTURN

WINTER

Fig. 270. Diagram of seasonal changes in temperature and stratification in cer-
tain deep lakes. Also usual effects on distribution of oxygen and carbon dioxide.
Data from Paul Welch.

The data given in Fig. 270 should help one to understand ( 1 ) why in
some lakes the thermocline in summer is a potent barrier between the
many organisms that live above and below it; (2) why the greatest pro-
duction of plankton, both plant and animal, occurs in the spring and
autumn; (3) why certain fishes in the summer time live only in deep
water and others only in shallow water.

Violent winds may at any time cause the mixing of water to con-
siderable depths. When the mixing results in carrying oxygen to greater
depths, it may be followed by an enormous increase of certain species.
Violent winds may also increase wave action and destroy the plants
in shallow water or wash them up on land. Wave action may also stir

596 TEXTBOOK OF BOTANY

up mud; and with the increased turbidity of the water resulting in
decreased hght penetration, the algal population may decrease.

Plant population. Most flowering plants cannot survive complete sub-
mergence, and only a limited number can thrive with their roots and
lower stems permanently covered with water. Besides the rooted-seed
plants along the shore, the aquatic plant population consists of the
countless numbers of minute floating and submerged algae which are
described in the next chapter.

All bodies of fresh water contain bacteria and lesser numbers of
parasitic and saprophytic fungi. Bacteria are known to occur in the
open water of lakes and ponds at all depths, and are often exceedingly
abundant on the bottom deposits. They are usually not harmful to man.
There are bacteria of decay, of nitrification, and of nitrogen-fixation as
well as those truly parasitic within other plants and animals. Their
abundance is influenced by light, by sudden changes in temperature,
by the amount of sedimentation, and by certain animals, such as the
protozoa and rotifers, that feed on them. Direct counts have shown that
there may be from a few thousand to several million bacteria in a cubic
centimeter of lake water. By way of comparison, a cubic centimeter of
a moist, fertile soil may contain several billion bacteria.

RIVERS AND STREAMS

The temperature of the water in rivers and streams is fairly uniform
from top to bottom. The greatest extremes of turbidity occur, varying
from the muddy Missouri and Yangtze rivers to the nearly transparent
water of "spring-fed" mountain brooks. Light penetration is diminished
greatly in muddy waters. Suspended silt may decrease the light in the
first inch of water to 10 per cent of that at the surface. Oxygen deficiency
does not usually limit the growth of aquatic plants in unobstructed
streams. The oxygen content varies between day and night and may
at times even exceed the saturation point in streams with abundant
submerged plants.

Stream water is usually moderately alkaline or neutral; only rarely
is it as acid as the water in bog lakes, unless there is seepage from coal
mines or other mineral deposits. Free carbon dioxide does not accumu-
late in running water but may increase in deep quiet stream pools.

In swift streams seed plants are not conspicuous, except for a few
well-rooted species on the margins. Sluggish waters may, however,
become completely choked by the luxuriant growth of submerged and

[Chap. XLVI UNDER-WATER ENVIRONMENTS 597

floating plants, such as elodea, eel grass, pond weeds, water hyacinth,
water cress, and willow herb. The margins of rivers rarely have perma-
nent plant communities because of deposition or erosion and ever-
changing water levels. The flora and fauna most characteristic of streams
and rivers are the suspended microscopic or near-microscopic organ-
isms known as plankton. The plant constituents of the plankton are
largely diatoms and other minute algae. The plankton "harvest" of
rivers and streams is surprisingly large. The total mass of plankton
organisms of the Illinois River at Havana, Illinois — where it flows
slowly — amounted in one year to 200,000 tons live weight, or about
10,000 tons of dry matter. In other parts of the same river the tonnage
may be greater or much less.

Plankton develops in the more sluggish and deeper pools of streams,
and these areas become the feeding grounds of innumerable animals.
Plankton is not to be looked upon as a mass of living organisms floating
downstream as fast as the stream flows. It is decidedly localized.
Neither is it true that plankton increases in variety and abundance
downsteam. Flash floods in small tributaries may increase the plankton
in the master stream. Great floods temporarily decrease the plankton in
the whole stream system. Compared with lakes, running water is a dis-
tinctly different environmental complex. Temperatures are more uni-
form, and no periods of winter and summer stagnation occur. In
general the concentration of inorganic salts is higher in streams than
in most lakes. Effects of pollution with manufacturing and municipal
wastes may be much more evident and destructive. Spores and seeds are
less likely to lodge on stream bottoms; hence the establishment of new
plants by this means is less frequent in streams than in still water.

MARSHES, SWAMPS, AND BOGS

As lakes and ponds are gradually filled by partially decayed organic
matter and by silt washed from the uplands, rooted vegetation en-
croaches upon the area. This filling gradually brings about changes
in the nature of the environment. The effects of winds become less, and
there is no overturn of water caused by winds in spring and fall. Shading
by rooted aquatics becomes important, and organic matter accumulates
rapidly. In fact, all stages of transition from a typically acquatic habitat
to a land environment may occur as lakes and ponds become bogs,
marshes, or swamps through increased drought, through drainage, or
through the accumulation of organic matter and soil.

598 TEXTBOOK OF BOTANY

A marsh is dominated by cattails, grasses, sedges, and rushes, and
may be found in either temperate or tropical regions. A swamp is
usually regarded as an area where the water never covers the soil deeply
but is never far below the surface, and where the vegetation is domi-
nated by shrubs and trees, such as buttonbush, low willows, alders,
and swamp trees.

Bogs differ from marshes and swamps in that mosses form an im-
portant part of the vegetation; they and the other aquatic plants become
enmeshed in a floating substrate which rises and falls with the water
level and upon which sedges, grasses, low and tall shrubs, and even
trees may subsequently grow. Many northern lakes have an open area
of water surrounded by floating marginal mats of herbs, bog mosses, and
shrubs. As peat accumulates below the mat the bog may become more
or less solid and support a forest of conifers, such as black spruce, tama-
rack, and arbor vitae.

Bogs may be either acid or alkaline. If alkaline, the water is gen-
erally clear, and shrubby cinquefoil occurs among the shrubs; if acid,
the water may be brown in color, and sphagnum moss is usually
present in the substrate. Bog water is usually lower in dissolved oxygen,
free carbon dioxide, and the salts of potassium and nitrogen than that
of the more open type of lake. Likewise, the plant and animal popula-
tion in bog lakes is comparativelv low, and there are fewer species.
In the Great Lakes region and northward, a peat substrate may accumu-
late on valley bottoms from bog plants that grew there.

The term "bog" is frequentlv applied in the Southern States to wet,
acid sand flats on which manv characteristic bog plants grow, as well as
to areas having a peat and muck substrate. Some typical bog plants
associated with the grasses, sedges, and mosses of acid bogs are cran-
berry, blueberry, snowberry, leatherleaf, pitcher plant, sundew, and
dwarf birch.

The Salt-water Environment

The oceans, bays, gulfs, and a few inland bodies of salt water are also
habitats of aquatic plants and animals. These marine habitats differ
from the fresh- water environment principally in being saline. The salin-
ity is due largely to chlorides and sulfates with smaller percentages of
carbonates. Few species of plants can grow both in the sea and in fresh
water. The salinitx' is essential to the marine species and toxic to the
fresh- water species.

[Chap. XLVI UNDER-WATER ENVIRONMENTS 599

Our largest bodies of water are the oceans, with a combined surface
area of nearly 140 million square miles, or about 70 per cent of the
earth's surface. In contrast to this, the surface of all bodies of fresh
water taken together is scarcely one million square miles. The vast ex-
panse and diverse conditions of oceans constitute enormous possibilities
of development of marine plant and animal populations. Ocean water
is practicallv a continuous medium, in sharp contrast to the more isolated
bodies of fresh water.

The oceans vary in depth from a few feet of water along the coasts
to nearly 6 miles in the so-called "deeps." On the ocean floor are plains,
hills, valleys, and mountain ranges.

Pressure varies from 15 pounds to the square inch at the surface to
nearly 8 tons at the greatest depths. Such enormous pressures would
seem to preclude the possibility of living organims, but the effects of
this pressure are annulled by an equivalent pressure inside the body.
Equalization of pressure within the body, however, does not take place
rapidly; consequently, vertical migration of living plants and animals
under these conditions is somewhat restricted. Except along the coast,
the influence of the land on the ocean is much less than it is on bodies
of fresh water.

The specific gravity of sea water with a salinity of 3.5 per cent is about
1.028 at 0° C. The density of the aerial environment of land plants is
much less than that of their protoplasm, while the density of the living
parts of marine organisms is about the same as that of the medium in
which they live. Most marine plants are slightly heavier than the water,
but their small size and proportionally large surface areas, as well as
occasional gas bladders and fat globules, contribute to their floating
capacity. Dead bodies of marine plants and animals eventually sink to
the bottom, unless devoured or dissolved, although the rate of sinking
is extremely low. Below depths of 5000 meters none of the parts of
most of the smaller plants and animals can be found, for the whole body
has gone into solution.

The solar energy actually available to plants is decreased in the ocean
by the same factors that decrease it in fresh water: absorption, reflec-
tion, suspended matter, wave action. Where suspended matter and
minute organisms are abundant, the water is apparently green because
of tlie scattering of the short blue and violet rays and the absorption of
the red and yellow. Blue water contains little or no suspended matter.

Photosynthesis in plants of the ocean generally occurs above a depth

600 TEXTBOOK OF BOTANY

of 150 meters, although a few Hving green plants have been found at
300 meters. Beebe reports the disappearance of all but the blue and
violet rays at depths of 250 meters in the clear water near the Bermuda
Islands. Below 500 meters almost complete darkness prevails, except
for the feeble light emitted by certain deep-sea fishes and other animals.

The gases of the air are absorbed directly from the atmosphere, al-
though carbon dioxide and oxygen are subject to local variation because
of photosynthesis and various oxidations. Gases are carried to depths
far below the surface through the action of storm waves and ocean
currents.

All chemical elements are probably present in sea water, although
only about 32 have been reported. Some occur in marine organisms
but have not as yet been recognized in the water itself because of their
extremely low concentrations. The relative amount of an element may
be far greater in a plant or an animal than in the medium in which it
lives. Large quantities of potassium and small amounts of sodium, for
example, accumulate in some marine plants, even though sodium salts
are very abundant in the ocean and potassium salts relatively scarce.
The relative amount of any element that accumulates in the different
species of plants may vary greatly even though the plants are growing
in the same environment.

The relative proportions of salts in the main body of the ocean are
fairly constant at all latitudes. The relative percentages of the different
elements in the salt after the water has evaporated do not vary much
from the following: chlorine, 54 per cent; sodium, 31 per cent; mag-
nesium, 4 per cent; calcium, 1 per cent; bromine, 0.2 of 1 per cent;
sulfate radicals, 8 per cent; and carbonate radicals, 0.2 of 1 per cent.

Surface temperatures of the ocean vary greatly with latitude and
with the time of year. However, surface variations between winter and
summer are much less than those encountered on land. Sea water be-
comes heavier as it is cooled until the freezing point is reached. As a
result, temperatures below freezing may occur, in contrast to the rather
constant 4° C. of most deep-lake bottoms. The bottom temperatures
of the oceans in temperate regions are around 2° C. Seasonal variations
in temperature are unusual below 500 feet. At depths of a mile or more
the temperature is near the freezing point at all times.

The effects of temperature are so closely tied up with those of other
factors that its influence has been frequently exaggerated. Temperature
apparently often determines the range of plant distribution both verti-
cally and latitudinally. Some species of plants grow and reproduce at or

[Chap. XLVI

UNDER-WATER ENVIRONMENTS

601

below the freezing point, such for example as the marine algae of arctic
and antarctic regions. The destructive effects of temperature, however,
are much less in the ocean than on land because of the absence of
extremes and rapid changes. Indirectly temperature is important in
another way since carbon dioxide is much more soluble in cold than
in warm water; in the tropics the warm water may become almost
depleted of this gas. Likewise the production of plant and animal plank-
ton is less in the warmer water of tropical oceans than in the cooler
water of temperate regions. Other factors that reduce plankton produc-
tion in the tropics are the increased rate of respiration, and deficiency
in compounds of nitrogen and phosphorus.

PLANTS AND ANIMALS IN THE OCEAN

Water environments, both fresh and marine, are populated with great
numbers and many kinds of plants and animals. The largest as well as
the smallest known organisms live in the ocean. In the sea are whales,
large fishes, crustaceans, and squids, as well as myriads of microscopic
species. The largest plants in the sea are the brown kelps (Fig. 271)

Fig. 271. One of the brown algae (Nereoci/stis) along the western coast of
North America. Only a part of the plant is visible at the surface of the water at
low tide. See Fig. 287 F. Photo from Lois Lampe.

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TEXTBOOK OF BOTANY

some of which are 70 meters long. Among the smaller plants are the
one-celled diatoms ( Fig. 272 ) and bacteria. Animal life probably exists
at all depths and over the entire ocean floor; but living green plants
are rare below 300 meters because of inadequate light. The plant popu-
lation consists of both suspended and bottom species. It reaches its

Fig. 272. Diatoms. The names of these species and others may be found in
papers by P. S. Conger and Albert Mann pubhshed by the Smithsonian Institution,
Washington. Photos by P. S. Conger, except the middle upper from Bausch &
Lomb Optical Co.

greatest density and has the greatest number of species in shallower
water near shore where inorganic salts are more abundant and there is
more adequate light from top to bottom. Mud bottoms fairly teem with
organisms, and many plants and animals are attached to rocky shores,
whereas sand is a comparatively barren habitat.

The bacteria of the sea vary greatly in abundance depending upon
depth of water, distance from shore, availability of suspended or dis-
solved organic matter, and the presence of other plants as well as ani-
mals. The sea water itself is not an especially good medium for the
growth of most bacteria unless considerable organic matter is present.
As might be expected, bacteria are much more numerous near shore,
and especially where the water is polluted by large centers of population.
The number of surface bacteria, living and dead, may range from a
few or none up to some 300 million individuals in one cubic centimeter
of water.

In bodies of water less than 200 meters deep the bacteria are much
more numerous in the mud bottoms than in the water above. These

[Chap. XLVI UNDER-WATER ENVIRONMENTS 603

bacteria are largely anaerobic and spore-forming, as compared with the
aerobic and non-spore-forming bacteria floating in tlie water. The bot-
tom bacteria include the nitrifying, the nitrogen-fixing, and the humus-
decomposing types. Certain bacteria of the sea are of interest because
they secure energy by the oxidation of such substances as hydrogen
sulfide, sulfur, ammonia, nitrites, and methane, and use the energy in
the synthesis of cell substances.

An accurate quantitative comparison between life on land and life
in the ocean is difiicult to make. Krogh has estimated that the entire
life zone on land is rarely more than 30 meters thick, from tree tops to
root tips inclusive, in contrast to the 4000 meters of ocean in which
animals live, and the 300 meters populated by green plants. For every
tree in a forest there may be nearly a half million animals large enough
to be seen by the naked eye. When the microscopic animals, bacteria,
and algae are added to this, the total number is prodigious. Under a
square meter of ocean water near the equator, down to a depth of 200
meters, there may be a billion or more scarcely visible or microscopic
plants and a million animals of various sizes, mostly microscopic. But
the mass of these organisms is surprisingly small, perhaps aggregating
only about 1/100 of a corresponding volume in a forest. Forest trees
stand for years, but many of the ocean plants are renewed several times
a year. The total biological productivity in the ocean, however, probably
never equals that in the forest, though locally it may be of comparable
magnitude.

In sharp contrast to the land, no seed plants, no ferns, no mosses, no
liverworts ever grow in the open sea. The plant life characteristic of the
ocean is confined to algae, bacteria, and a few fungi. The species of
plants characteristic of the ocean rarely occur in fresh water. On the
land the larger plants are the chief sources of food of the animal popu-
lations. In the water this is not the case, for the microscopic plants are
the initial links in the food chains of nearly all the animals. Enormous
numbers of these plants grow every year, but directly thev constitute
only a small part of the food of the largest animals ( Fig. 273 ) .

The food chain from green plants to the larger animals ("producer"
to "consumer" ) may become greatly extended because the smallest ani-
mals, having consumed the plankton, may become the food of crus-
taceans and small fishes. The latter in turn may be eaten by still larger
fishes ( Fig. 274 ) . It is evident that much of the energy value of the food
originally made in green plants is never realized by the "ultimate con-

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TEXTBOOK OF BOTANY

r'^

;s

^/VX-,

Algae

Blue-greens

Greens

Desmids

Diatoms

/ etc

Water fleasA, Larger /^>X ^.l^ZZ'

/°"Mother water :>^^;
animals

Fig. 273. Food relations of aquatic lite. No matter how long the chain
is from Algae to fish, the fundamental food organisms are the algae that
inorganic materials into foods. Courtesy of World Book Co.

of animals
transform

Fig. 274. A diagrammatic representation of the dependence of Esquimaux on
diatoms. Assume that each organism obtains only 10 per cent of the food consumed
by the preceding one in the chain, and compute the amount of diatoms necessary
for a gain in weight of 1 pound in an Esquimau boy.

[Chap. XLVI UNDER-WATER ENVIRONMENTS 605

sumer" because of the necessary respiratory and assimilatory processes
of the often numerous "middlemen." Each "middleman" may use up
in these processes as much as 90 per cent of the initial energy value of
the food.

In spite of the small size of most marine plants and animals, a rapidly
moving feeder can secure large amounts of food in the ocean. This is
illustrated by the growth of the calf of the blue whale. At birth it is
7 meters long and weighs 2 tons, but may grow to be 23 meters long
and weigh as much as 60 tons at the end of two years. The food of the
blue whale consists entirely of plankton which includes small Crustacea.
The plankton passes into the mammal through the so-called "whale-
bone sieve." As much as two tons of plankton have been found in the
stomach of a mature blue whale at one time. If there are only three
links in the food chain of this animal, the gain in weight would implv
the consumption of food equivalent to 58,000 tons of algae in a period
of two years. It is interesting to note that whalers often locate the
places where whales may be found bv the presence of large quantities
of plankton at the surface of the water. Biologically algae and bacteria
are the most important of aquatic plants. The algae are described in
more detail in the following chapter.

REFERENCES

Beebe, William. The depth of the sea. Nat. Geog. Mag. 51:65-88. 1932.
Beebe, William. Half Mile Down. Harcoiirt, Brace & Company, Inc. 1934.
Birge, E. A., and C. Juday. The inland lakes of Wisconsin. Wise. Geol. and

Nat. Hist. Survey. Bull. 64. 1922.
Coker, R. E. Life in the Sea. Sci. Monthly for April and May. 1938.
Hesse, R., W. C. Allee, and K. P. Schmidt. Eeological Animal Geographi/.

John Wiley & Sons. 1937. Chaps. X and XI.
Krogh, A. Conditions of life in the ocean. Ecological Monographs. 4:421-429.

1934.
Symposium: Problems of Lake Biology. A. A. A. S. Pub. No. 10. The Science

Press. 1939.
Tiffany, L. H. Algae, the Grass of Many Waters. C. C. Thomas, Publisher.

1938.
Welch, P. S. Limnology. McGraw-Hill Book Company, Inc. 1935.

CHAPTER XLVII
THE ALGAE

Of all the plants in the biological communities of both fresh and salt
water the algae (sing, alga) are the most important synthesizers of
food. They are commonly called pond scums and seaweeds. Algae, how-
ever, are not confined to water habitats. In the rainy tropics and sub-
tropics they grow on moist surfaces everywhere, and even within the
leaves of many plants. Some of them may live in snow as shown by
the pink color of polar and high alpine snowfields ( Plate I ) . The trunks
of many woody plants of the moist temperate zone have green patches
of algae, usually located on the shaded side. The algae of the soil are
so numerous that one gram of earth from a heavily fertilized field may
contain as many as a million individuals. A few algae grow in intimate
association with certain fungi, forming compound plants known as
lichens. Algae grow within the aerial roots of cycads and in the hair
follicles of the three-toed sloth. They are attached to the appendages
of crustaceans and to the backs of turtles. They live within the bodies
of many minute animals, and even in the intestines of some mammals,
including man.

The algae, moreover, exhibit as much variation in color, in size, in
form, and in method of reproduction as any of the other groups of
plants. Thev may be red, orange, brown, yellow, yellow-green, green,
blue-green, purple, and violet. Rarely are they colorless. Some algae are
so minute that their fomi is just discernible with a microscope. The
giant kelps of the Pacific, on the other hand, attain lengths of more
than 200 feet. In form algae mav be globular, disk-shaped, thread-like,
sheet-like, leaf-like, or large and paddle-shaped; branched or un-
branched; attached or free-floating.

The structures associated with sexual reproduction in some species
are comparatively simple; in others, they are remarkably complex.
Among the various species of algae, vegetative propagation may result
from the division and separation of cells, from the breaking of fila-
ments, from specialized thick-walled donnant cells, or from motile spores

606

[Chap. XLVII THE ALGAE 607

that develop directly from vegetative cells. In one large group of algae
no sexual reproduction occurs.

Algae are economically important both directly and indirectly. Com-
mercial transactions involving the collection, processing, and sale of
algae amount annually to several million dollars. The animals of the
oceans, lakes, and streams are for the most part dependent upon the
algae.

What are algae? First of all, algae are plants. Some consist of onlv a
single cell, a single filament, or branched filaments. Even the more
complex ones have no organs exactly comparable to those of seed
plants. They contain chlorophyll, although other pigments may parth'
or entirely mask the green color. The presence of these pigments usualh
enables one to distinguish algae from fungi, bacteria, and small animals.
Many of them are easily distinguished from other green plants by the
number, size, and form of their chloroplasts. Many species are capable
of self-locomotion by twisting, bending, gliding, and swimming; and
others produce motile spores and gametes.

The occurrence of motile algal plants and cells may require us to
change one of our notions about plants if we have been accustomed to
associate independent movement only with animals. It should be re-
membered that the names "plants" and "animals" are very old and
were used long before microscopes were made. It is easy to separate
plants from animals when we are thinking of oaks and horses or corn
and mice. Now that the microscope has enabled us to see minute living
organisms not even suspected before, it is not surprising to find that
the criteria we are accustomed to use in separating plants and animals
will not apply to many of these smaller organisms.

Among the many organisms having flagella some are more plant-like
than others. When exposed to light, some of them are green and syn-
thesize both sugar and amino acids; others remain colorless and obtain
food only from external sources. Even within a single genus, such as
Euglena, some species are more plant-like than others. For example,
many species of Euglena are green and synthesize sugar when exposed
to light. Some of these green species can live in darkness as colorless
saprophytes if supplied with sugar and protein foods, but others have
failed to grow in continuous darkness. They differ also in their ability
to utilize nitrogen compounds. Some may utilize ammonia or nitrate
salts, which means that they can synthesize amino acids from sugar
and either ammonia or nitrate salts. Still others cannot utilize either of

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TEXTBOOK OF BOTANY

these salts; consequently they cannot synthesize amino acids. Some
can live only where peptones or other protein compounds are present
in the external medium.

The kinds of algae. For many years algae were grouped on the basis
of tlieir color as red algae, brown algae, green algae, and blue-green
algae. This classification is still adequate for a general consideration of
algae, though anyone unaware of other bases of classification would
be confused on being told that some algae are colorless, that the red
color of arctic snow is due to the presence of a red-colored green alga,
and that the Red Sea was so named because of the abundance of a red-
colored blue-green alga in it.

Most students of the algae now recognize 9 or more classes (Table
17). This more critical classification is based in part on color, but also
on other distinctive characters of (1) the structure of the vegetative
cells and tissues, (2) the reproductive structures, (3) the successive
stages in the life cycles, (4) the kinds of accumulated food, and (5)
the forms of the chloroplasts or chromatophores.

Table 17. Classes of Algae and Their Characteristic Pigments

,__

_

Classes of Algae

Common Names

Apparent Color

"5,
o

6

ii

1

a
o

be
C
ci

C
0)

1

"S
>-

1

.s

Ph

2

!£
c

cS

8

3

Accessory Pigments
Brown, Red, etc.,
not Well Known

Myxophyceae

Blue-greens

Blue-green, green, yellow,
brown, purple

X

X

X

X

Chlorophyceae

Greens

Pale yellow to dark green

X

X

X

Xanthophyceae

Yellow-greens

Yellow to yellowish green

X

X

X

Bacillariophyceae

Diatoms

Yellow to brown

X

X

X

X

Chrysophyceae

Golden-browns

Golden-brown

X

X

X

X

Dinophyceae

Dinoflagellates

Yellow-brown

X

X

X

Euglenophyceae

Euglenas, etc.

Green, red

X

X

X

Pliaeophyceae

Browns

Greenish yellow to dark
brown

X

X

X

X

Rhodophyceae

Reds

Red, purple, blue

X

X

X

X

X

The various colors of algae are due largely to the relative abundance
of certain pigments in the cells. Chlorophyll is present in all algae except
the colorless ones. Carotene and xanthophyll occur in most of them. The
pigment fucoxanthin ( C4(.H.-,(iO<; ) is characteristic of the brown algae,

[Chap. XLVII THE ALGAE 609

and phvcoeivthrin and phycocyanin occur in the reds and blue-greens.

Economic aspects of the algae. To most people the algae of ponds and
lakes, as well as those of the ocean deposited along shore after storms,
are nuisances. Algae may accumulate in sufficient quantities to become
an annoyance to iDathers. Occasionally stock die after using water heav-
ily populated with certain blue-green algae. Bad odors and tastes of
drinking water are often attributed to the presence of decaying algae in
reservoirs. Tea and other subtropical plants are severely injured by
algae which grow either on the surface of the leaves, interfering with
photosynthesis, or within the leaves as parasites. Small fishes often be-
come so entangled in algal mats that they die either there or subse-
quently from injuries received in extricating themselves. Algae in fish
hatchery ponds may so deplete the oxygen content of the water at
night as to cause the death of the young fish through suffocation.

Algae are rather easily destroyed in small bodies of water by means
of certain chemical compounds, particularly arsenites and copper sul-
fate. The latter is so effective that one part of it in a million parts of
water is sufficient to kill most algae. Copper sulfate in such low con-
centrations is not harmful to fishes, nor is the water unfit for human
consumption. Although there are on the market numerous commercial
products containing copper sulfate with adequate instructions for their
use, a few crystals thrown into small pools will usuallv suffice. For
larger ponds a quantity of the chemical may be placed in a sack, tied
to the rear of a boat, and allowed gradual! \' to dissolve in the water
as the boat is moved over the surface.

Among other practical measures used for eradicating algae from
fish ponds are ( 1 ) the removal of the accumulating organic matter bv
draining the pond from the bottom instead of the surface, and ( 2 ) the
maintenance of high turbidity. Young cra\'fish are often added to the
ponds because they increase the turbiditv of the water, preventing light
from penetrating; the algae are unable to survive. Reservoirs mav be
kept free of algae by stirring up the muddy bottom with large propellers
on boats.

Suppose we now examine the credit side of the ledger. As has been
noted before, the algae are the ultimate sources of food and enersv of
all strictly aquatic animals. The food chains of animals are numerous
and varied (Fig. 275). Some fishes feed directly on the algae, while
others secure the energy bound up in the plants by eating animals that
feed on algae. Some blue-greens, a few greens, and manv species of

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TEXTBOOK OF BOTANY

red and browii algae are used directly as food by human beings in many
parts of the world, particularly in the Orient. Among algae, as among
other plants, some species are much more important than others as
sources of food, whether the consumer be man, fish, insect, or whale.

Fig. 275. Diagram of representative food chains from algae to man.

The giant kelps and other seaweeds are sources of iodine and potash.
They have been used for many years by farmers along the coast as
fertilizers because of their potassium content. Dried and treated in
various ways, they are used as stabilizers in making ice cream, candy,
shaving cream, various other creams, jellies, salads, and emulsions; and
also as the source of agar-agar, so important in the culture of bacteria
and fungi.

The algae of the soil are very numerous, but the extent to which they
contribute to soil fertility and aeration is only partly known. Soils have
definite algal floras. Those living on the surface grow and multiply
rapidly whenever the soil is moist. The algae below the surface grow
very slowly; and since they live in darkness tliey are obviously living as
saprophytes, using carbohydrates from the soil solution. Algae cer-
tainly contribute organic matter to soils, and it has been proved that cer-
tain nitrogen-fixing bacteria are much more eftective when they grow
in association with soil algae. The fungi and algae of lichens are the

[Chap. XLVII THE ALGAE 611

pioneer plants on certain kinds of exposed rock surfaces and are primary
factors in the formation of the first layers of soil on such surfaces.

Algal periodicity. Many fresh-water and marine algae have seasonal
periods of germination of spores, vegetative growth, reproduction, and
dormancy similar to those of flowering plants. Many species in tem-
perate climates attain their greatest development in winter, in spring,
in summer, or in the autumn; produce reproductive structures, and sub-
sequently disappear. These algae are comparable to annual flowering
plants. Their dormant period is passed as spores (Fig. 276).

Fig. 276. Estimated relative abundance of several seasonal assemblages of
green algae in central Illinois. Seasonal assemblages would be very different
fartfier north and farther south. Ephemerals include plankton species in which
a life cycle is completed in a few days.

There are also many perennial filamentous, or thick and leathery,
species in which at least a part, usually the base of the vegetative plant,
survives long periods of drought and cold. Each year new vegetative
fronds develop. This group includes many seaweeds, certain red algae
in fresh-water streams, and green algae such as Cladophora and
Vaucheria.

Plankton algae and some soil algae are very short-lived. A new popu-
lation of these extremely abundant and important species may develop
every few days. They are the plants that form the so-called "water
bloom" so common on ponds and lakes from midsummer to early
autumn. Obviously, enormous numbers result from the accumulative
reproduction of several generations in a few weeks.

Algae are more numerous in seasons when the water levels are high.
They also reproduce most abundantly under these conditions. Their

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TEXTBOOK OF BOTANY

individual periodicity determines what species will be found associated
at any time of the year. Ponds that regularly dry up in late summer ap-
pear to have a greater variety of species than permanent ponds.

The situation in large lakes is somewhat unlike that in ponds because
great bodies of water neither warm up quickly in the spring nor cool
rapidly in the autumn. In Lake Erie, for example, the diatoms are likely
to be abundant in May and June, the green algae in July and early
August, the blue-greens in late August and September, and the diatoms
again in October and November (Fig. 277). A few species of diatoms
are more abundant in winter than at anv other season.

MAY JUNE JULY AUGUST SEPTEMBER

Fig. 277. Seasonal abundance of some plankton algae in Lake Erie.

Water blooms are just as characteristic of large lakes as of ponds.
At certain seasons of the year diatoms, blue-greens, or greens may
within a few days become so abundant as literally to cover acres of
water to the depth of an inch or two. This colored soup-like layer may
be objectionable to bathers, and it may be the source of offensive odors;
but it contains an abundance of food available to the smallest aquatic
animals. The life cycles of the algae composing the bloom are soon
completed, and after several days the plants may disappear as sud-
denly as they appeared. Most plankton algae are found within a few
inches of the surface of the water, although one may usually find them
in collections made at depths of 10 to 50 feet.

[Chap. XLVII THE ALGAE 613

We have discussed so far some of the general characteristics, distri-
bution, and economic aspects of algae as a whole. They constitute such
a large and diversified assemblage of plants, however, that a better
understanding of their growth and reproduction depends on a study
of some of the commoner ones in more detail. We shall discuss in the
next few pages some representatives of each of the important classes
of algae.

The Green Algae: Chlorophyceae

About 5000 species of green algae have been described and named.
As a group of plants they are quite varied in structure, in appearance,
and in several other ways. They are usually predominantly green in
color during the vegetative phase, containing chlorophvll, xanthophyll,
and carotene in about the proportion found in the seed plants. They
grow chiefly in water, both fresh and marine, but occur also on and in
the soil and on many other kinds of moist substrates. Biologically they
influence both the abundance and the distribution of certain other
plants and of certain animals. Green algae are by far the most common
ones in lichens. They may be parasitized by animals and occur inside
animals, such as hydras and rotifers. In size thev varv from the micro-
scopic forms to the filamentous, highly branched cladophoras, which
may be several feet in length and are attached to rocks and stones of
streams and lakes. A few common species are described below.

Pleurococcus. On the bark of trees and shrubs throughout the world
there occur green-colored areas of Pleurococcus, sometimes several
square feet in extent. This alga grows most frequently on the shaded
parts of the tree. In drier regions it may be restricted to the lee side.
Although most abundant near the ground, it mav be found on the bark
all the way to the tops of trees in moist open forests. No spores are
formed and no sexual reproduction occurs in this alga.

Under the microscope the green areas of Pleurococcus are seen to
consist of masses of rounded solitary cells or of groups of two to several
cells ( Fig. 278 ) . The chloroplast occupies most of the cell, and the cell
walls are comparatively thick. The plant may survive prolonged droughts
and remain dormant during long periods of low temperature, in spite
of the fact that there are no apparent structural modifications that might
prevent desiccation of the cells. When periods of rain and higher tem-
peratures return, the cells again become active. Propagation takes place
only through the division and subsequent separation of the cells. No

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TEXTBOOK OF BOTANY

reduction division occurs; and, except for mutations, all the cells of
Pleurococctis have the same chromosome complement. It is a non-
motile one-celled plant, though several plants may remain attached
temporarily as an unorganized colony.

Fig. 278. Forms of cells of a Pleurococcus. Species of this genus of green algae
are non-motile and mostly aerial. The individuals are usually one-celled, though
they may remain united in colonies temporarily as indicated in the drawing.
Species of this group are the algae most frequently found in lichens.

Chlamydomonas. In contrast to the one-celled non-motile Pleuro-
coccus is the one-celled motile Chla7mjdomonas (Fig. 279). It is often
found as a part of the plankton, sometimes occurring in large numbers in
small pools or even in indoor aquaria. The cell is spherical, ovoid, or
elongate, and often flattened. Two flagella project from the anterior end.
Both spore formation and sexual reproduction occur.

Fig. 279. Forms of vegetative and reproductive cells of a Chlamydomonas.
Species of this genus of green algae are one-celled and motile. They reproduce
vegetatively by motile spores and sexually by the union of gametes as indicated
in the drawing.

Cells with flagella similar to Chlamijdomonas may remain attached
and thus form definite colonies of several to many cells. Examples of such
colonial algae are Pandorina, Eudorina, and Volvox (Fig. 280). All of
these algae are termed flagellate organisms.

Ulothrix. In the spring one may see bright green slimy filamentous
growths of Ulothrix on the stones and rocks of the shores of streams,
lakes, and ponds. Since the cylindrical cells of this alga are joined end

[Chap. XLVll

THE ALGAE

615

25 26

Fig. 280. Forms of
1 and 5 Gloeocystis,

some miicellular

2 Kirchneriella,

8 Qiiadrigula, 9 Vo/uox, 10 Aiferococcu

30

ae. Genera represented:

and colonial al^

3-4 Oocijstis, 6 Carteria, 7 Sphaerella,
,11 Chlamydomonas, 12 Selenastmm, 13
Micractinium, 14 Pleodorina, 15 Euglena, 16-17 Phacus, 18 Coelastrum, 19 Goniiim.
20 Ankistrodesmus, 21 Gloeotaenium, 22 Actinastrum, 23 Pediastrum, 24 Pandorina,
25-26 Scenedesmus, 27 Tetraedron, 28 Ophiocytitnn, 29 Pediastrum, 30 Sorastrum.

616

TEXTBOOK OF BOTANY

to end and always divide in the same plane, the plant is a single un-
branched filament. Each plant is attached to rocks or other objects by
a special holdfast cell (Fig. 281). The chloroplast is a green iDand of

t

^

V

^'

Fig. 281. Fresh-water algae. The upright filaments are, from left to right:
Oedogonium, producing motile spores, eggs, and sperms; Microspora, forming rest-
ing spores and motile spores; and Ulothrix, forming motile spores and gametes.
The horizontal filaments are Spirogyra (left) and Vaucheria (right). Highly
magnified. Courtesy of World Book Co.

[Chap. XLVII THE ALGAE 617

cytoplasm open on one side. It appears to be pressed against the cell wall
and is frequently confined to the middle section of the cylindrical cell.
Its form in cross section resembles that of a horseshoe. Starches and
proteins accumulate in vegetative cells, but in the spores one finds mainly
oils. The life history of a Ulothrix is far more complex than that of
Pleurococciis.

After vegetative development the protoplast in each of many cells may
become subdivided by a succession of mitotic divisions into 2, 4, or 8
small protoplasts, each of which escapes through an opening in the wall
of the parent cell and becomes a motile spore^ with four flagella. Both
big and little motile spores may be formed in different cells of the same
filament. The smaller ones are formed in groups of 16 or 32. After swim-
ming about for a short period, a motile spore may become attached to
some object and germinate by elongating and then dividing transversely.
Subsequent cell divisions all in the same plane result in a new filament.

In still other filaments, or occasionally in some cells of the same fila-
ment, the protoplast may become subdivided by successive mitotic
divisions into 8, 16, 32, 64, 128, or rarely more cells. Each of these cells
may pass through the wall of the parent cell in the same manner as the
motile spore, which it resembles superficially except for its smaller size
and its single pair of flagella. Such motile structures do not germinate,
and new filaments do not develop from them. They are termed gametes.
One gamete unites with another gamete forming a zvgote, which eventu-
ally becomes thick-walled and sinks into the mud at the bottom of the
pond where it remains dormant for several months. The union of the
two gametes is termed conjugation. When the zvgote germinates, its
protoplast becomes subdivided into 4 to 16 parts, each of which becomes
a motile spore from which a new filament grows. The two gametes that
unite appear to differ chemically and are referred to as + and —
gametes. Since they are similar in appearance they are regarded as the
simplest kind of sex cells.

The special method of vegetative multiplication by means of either
motile or non-motile spores which are formed without a previous union
of gametes is often termed asexual reproduction. The related series of
processes including the fomiation of gametes, their subsequent union,
and the development of the resulting zygote into motile spores from
which new filaments develop are referred to as the sexual reproduction

^ Motile spores have flagella and are frequently termed "zoospores" ( animal spores ) be-
cause they swim about in the water.

618 TEXTBOOK OF BOTANY

of the alga. Reduction division occurs during the first nuclear division in
the zygote. Hence each motile spore of Ulothrix and each cell of the fila-
ment has the monoploid number of chromosomes. The zygote alone has
the diploid number of chromosomes.

Oedogonium. In fresh water throughout the world, attached to vari-
ous objects during at least a part of their lifetime are several hundred
species of Oedogonium. All of them are filamentous and unbranched,
and each cell has a hollow cylindrical meshwork chloroplast. The method
of reproduction is in some respects similar to, but in others quite unlike,
that of Ulothrix.

Motile spores develop singly in the vegetative cells ( Fig. 281 ) . Each
spore has a ring of flagella at one end and escapes from the cell through
a circular break in the wall. Upon coming to rest, it germinates directlv,
and the new plant elongates by subsequent division of certain cells
scattered at intervals in the filament. The first cell is the holdfast cell.

The two kinds of gametes are quite distinct in appearance. The
male gametes, or sperms, formed singly or in pairs from the protoplast
in special short cells known as antheridia (sing, antheridium, a sperm
case), are quite similar in appearance to small motile spores. The female
gamete, or egg, develops singly within a swollen vegetative cell, the
oogonium (pi. oogonia, egg cases). The egg is much larger than the
sperm, never escapes from the egg case, is incapable of locomotion, and
contains much food, chiefly oils.

The sperms and eggs may develop in cells of the same filament or of
separate filaments. That is, some species are bisexual, or monoecious;
others consist of two kinds of unisexual filaments, one of which is a male
filament, the other female. They may or may not differ in size. In some
species the male filaments are very small and grow as epiphytes on the
female filaments near the oogonium. These dwarf males develop from
motile spores which germinate only when they become attached to cells
adjacent to an oogonium. They usually consist of a holdfast cell and one
or more antheridia.

The sperm after liberation from the antheridium may swim to and
enter an oogonium through a pore or slit, and fuse with the egg. This
fusion appears to occur mostly late at night or early morning. The re-
sulting oospore^ is thick-walled, remains dormant for some months or

^ The term oospore is often used to indicate the cell resulting from a union of two
gametes that may be distinctly recognized as an egg and a sperm; zygote is used when
the two gametes are similar in appearance. The term zygote is often used also as a general
term for any cell formed by the union of gametes, regardless of their appearance.

[Chap. XLVII

THE ALGAE

619

Fig. 282. Common filamentous algae. Genera represented: A, Stigeocloniiim;
B, Ulothrix; C, Spirogijra; D, Tribonema; E, Chaetophora; F and G, Zijgnema; H,
Mougeotia; I, Vaucheria; J and K, Oedogonium.

620

TEXTBOOK OF BOTANY

even years, and upon germination usually produces four motile spores,
each of which has the reduced number of chromosomes. A new filament
develops directly from each motile spore, and each of its cells has the
reduced number of chromosomes.

Other common filamentous genera. Among the most widely distrib-
uted genera (Fig. 282) of pond algae is Spirogyra, with its usually

KL L M

Fig. 283. Reproduction in Spirogyra and inheritance of the chloroplast.

spirally arranged, ribbon-like chloroplasts. Zygnema with stellate chloro-
plasts, and Motigeotia with a straight, ribbon-shaped chloroplast are al-
most as common. In the large group of algae represented by these three
genera, the cells are always cylindrical and there is a complete absence
of flagellate spores. Sexual reproduction is effected by the union of
gametes, one ( or both ) of which moves ameba-like to the other through
a conjugation tube ( Fig. 283 ) . The zygotes resulting from the union of
the gametes may remain dormant for several months or years. When they
germinate, new filaments develop from them. Reduction division occurs
at the first division of the zygote nucleus.

The green felt-like Vaiicheria is very common in small streams and

[Chap. XLVII THE ALGAE 621

ditches. Cell division in it is unaccompanied by the formation of cross
walls and thus the filaments are tubular and multinucleate. The motile
spores of Vaticheria are unusual in that a number of nuclei are present
in a so-called "compound zoospore," from which a new filament develops
almost immediately. Most species of Vaucheria have an oogonium with a
single egg, and an antheridium containing several sperms. These sex
organs are formed at the ends of short lateral branches (Figs. 281 and
282 ) . The oospore resulting from fertilization remains dormant for some
time before a new filament develops from it. Reduction division probablv
occurs during the formation of gametes as it does in animals. If it does,
the nuclei of the zygote, filament, and all of the motile spores have the
diploid number of chromosomes.

The algae described above and manv others are usuallv available in
their natural habitats, and the details of their life histories ma\' be studied
from living or preserved specimens. The desmids are either unicellular
or filamentous forms, a few of which are illustrated in Fig. 284.

The stoneworts. These plants are mentioned here largelv because of
their importance in the formation of marl deposits. The stoneworts
(Charales) are rather widely distributed in both fresh and salt waters; a
few species inhabit both. They are attached to the soil beneath the water
and often fomi extensive meadows several feet below the water surface.
The plants become incrusted with calcium carbonate in limestone re-
gions, and as the plants die, marl accumulates. About one hundred
species have been described.

The plant body consists of a cylindrical axis bearing a whorl of
branches at its several "nodes." The plants are nearly alwavs erect and
may vary in height from an inch to 3 or 4 feet. The stoneworts constitute
a group verv distinct from all other algae.

The Blue-green Algae: Myxophyceae

The blue-greens differ from all the great groups of algae in that the
nuclear substances of the cell are not organized in a well-defined struc-
ture in the cytoplasm as in other green plants. Hereditarv units of
matter comparable to genes must be present, however, for the species
are hereditarib' different. Chlorophvll and other pigments are dispersed
throughout the cell and not confined to plastids. No motile spores are
formed and no sexual reproduction occurs. Onh^ about half the blue-
green algae are really blue-green in color. The others vary greatly in
color owing to different amounts of blue, red, green, and vellow pig-

622

TEXTBOOK OF BOTANY

!jiiiaii--^-u-.i'-i-LiJ-jii'-^'j-'^'i'j^-L'i-Cjj.

Fig. 284. Various forms of desmids, and conjugation in Desmidium (14).
Genera represented: 1-2 Micrasterias, 3 and 8 Euastrum, 4 and 7 Staurastnim,
5, 6 and 13 Cosmarium, 9 Pleitrotaenium, 10 Triploceras, 11 Closteriiim, 12
Penium, 14-15 Desmidium. Figures from Fr. Irene-Marie, H. Skuja.

[Chap. XLVII THE ALGAE 623

ments. Common to all blue-greens is the presence of a mucilaginous
outer wall which may be so thick that the filaments appear to be em-
bedded in a mass of mucilage (Fig. 285).

Fig. 285. Forms of blue-green algae. Genera represented: 1 Microcystis, 2
Aphanocapsa, 3 Coelosphaerium, 4 Chroococciis, 5 Merismopedia, 6 and 13 Lijng-
btja, 7 and 10 Anahaena, 8 Spindina, 9 Chroococcus, 11 and 14 Oscillatoria, 12
Phormidiiim, 15 Scijtonema, 16 Tolypothrix, 17 Glocotiichia, 18 Cylindrospermitm.
Figures from G. M. Smith, J. E. Tilden.

Approximately 2500 species of blue-greens have been described and
named. Most of them grow in fresh water, but marine and other salt-
water species are not uncommon. They are able to grow in almost every
conceivable habitat. They are important constituents of fresh-water
plankton, and some of the water blooms of summer are due to their
abundance. The Red Sea was probably so named because of the presence
in large numbers of one of these plants (Trichodesmium) , which is pre-
dominantly red in color. Blue-greens that grow in hot springs live

624 TEXTBOOK OF BOTANY

at temperatures between 150° and 170° F. Other species may remain
frozen in ice for many months without injury. Blue-greens may become
serious pests on soil in greenhouses by forming a gelatinous layer that
prevents aeration of the soil beneath. They are very common in ponds
in the far north.

The blue-greens constitute an important part of the algal population
of soils, and their spores can withstand complete desiccation for many
years. They may grow as parasites within the bodies of man and other
animals; as epiphytes on other algae and on the vegetative parts, particu-
larly leaves, of many seed plants; and as hosts of fungi in a few kinds
of lichens. Species of blue-green algae regularly occur in the leaves of a
water fern (Azolla), and on the scales of some liverworts. One blue-
green species grows in the aerial roots of some cycads, and many others
may be found within the gelatinous envelopes of other algae.

In general, this group of algae may be considered as the characteristic
plants in bodies of water high in organic matter, but not necessarily so.
When fresh-water streams are polluted by sewage, by poisonous wastes
from various manufacturing processes, and by drainage from coal mines
and oil wells, the blue-greens are the last to disappear. In fact, in such
habitats certain bacteria are about their only living associates.

Blue-green algae are either unicellular, colonial, or multicellular. Cells
of the colonial species may be loosely or definitely aggregated into glob-
ular, saccate, plate-like, or irregular colonies with a few to many cells
held together by mucilage. The cells of some other species are joined
end to end in definite regular or irregular filaments; the filaments may
be branched or not. Division of the cells in only one plane results in a
chain of cells; division in two planes, a plate-like colony; and division
in three planes, a solid and often nearly cubical aggregate.

The granules of food that accumulate in the vegetative cells are not
definitely known chemically. They undoubtedly are conversion products
from sugar formed in photosynthesis; they are not starch. Glycogen, or
possibly glycoproteins, and droplets of oil may accumulate, especially in
the spores.

The plants increase in number by vegetative multiplication. Any vege-
tative cell in some plants, or only certain vegetative cells in other plants,
may develop thick walls and become dormant. After a period of time,
new plants grow from these thick-walled cells. New filaments also result
from the fragmentation of old filaments into small groups of cells, fol-
lowed by repeated cell division.

[Chap. XLVII THE ALGAE 625

The Diatoms: Bacillariophyceae

The vegetative cell of a diatom has highly silicified walls (i.e., glass
walls ) composed of two overlapping parts ( valves ) that fit together like
the halves of a petri dish. The wall is usually beautifully sculptured and
ornamented (Fig, 286). The plant may be unicellular, filamentous, or
joined in irregular colonies by gelatinous sheaths. Pectic compounds are
present, but there is no cellulose. The chloroplasts are yellowish to
brownish, and contain chlorophyll, xanthophyll, and carotene together
with accessory pigments of unknown chemical composition. Some 12,000
species have been described.

Diatoms are distributed widely in both fresh and salt water. They are
the important constituents of the plankton of the cooler waters of the
sea, and at times are the principal algae of lakes and ponds, especially in
early spring and in the autumn. Owing to the mucilage from their cells,
diatoms may become attached to other plants and to all sorts of under-
water objects. Depending upon the amount and place of major forma-
tion of mucilage, the colonial species may form ribbon, chain, zigzag, or
branching aggregates. Although not as important as either greens or
blue-greens in the soil, some species of diatoms are distinctly terrestrial
and capable of withstanding desiccation for many years. In regions of
frequent rainfall diatoms may grow subaerially on tree trunks, on fences,
and on rocky cliffs.

As noted above, diatoms may occur in prodigious numbers in both sea
and inland waters at certain times of the year. They are probably the
most important constituent of the ocean plankton. When the plants die,
the cell walls remain unaltered after the death and decay of the rest of
the cell. The empty "glass cases" may thus accumulate over a period of
years in great quantities on the bottom of seas, bays, and lakes. Such
deposits of fossil diatoms are known as diatomaceous earth and are found
in many parts of the world. One of the largest deposits is near Lompoc,
California; it covers about 12 square miles with a stratum of commer-
cially pure diatoms some 1400 feet thick, and in addition 3000 feet of
diatoms mixed with silt, sand, and gravel.

Diatomaceous earth is quarried or mined, the annual output approach-
ing 100,000 tons. Its largest use is in the filtration of liquids, especially
those of sugar refineries. The powdered earth is added to the solution,
collects on the cloth screens of the filter press, and prevents the sus-
pended matter from passing through them. Diatomaceous earth in the

626

TEXTBOOK OF BOTANY

Fig. 286. Eight species of diatoms differing in structure and symmetry. Genera
represented: A, Amphipleura; B, Sitrirella; C, Pinniilaria; D, Trihrachia; E,
Arachnoidiscus; F, Eimotia; G, Stictodiscus; H, Coscinodiscus. Photos from Paul
Conger, Smithsonian Institution, except A from Bausch & Lomb Optical Co., and
E from Spencer Lens Co.

[Chap. XLVIl THE ALGAE 627

form of bricks is used in the insulation of boilers and smelting furnaces.
At high temperatures it is more effective than asbestos or magnesia. Metal
polishes, abrasive soaps, and toothpaste often contain diatom shells.

Living diatoms are of major importance as the food of fishes, oysters,
and clams. Certain marine fishes feed almost entirely on them, or on the
small animals that have consumed diatoms. The commercially important
hake, for example, feeds upon herring; the herring subsist on copepods;
and the copepods are dependent upon the diatoms for their food
(Fig. 275).

The accumulation and decay of diatoms in water resenoirs and ponds,
on the other hand, may result in compounds that are extremely obnoxious
to our senses of smell and taste. Some species are able to grow in waters
highly polluted with organic matter. Diatom abundance is often asso-
ciated with a plentiful supply of nitrates and silicates. Paucitv of diatoms
is sometimes directly due to the absence of soluble silicates in the pond.

Cell division in the diatoms is an almost unique phenomenon anions;
plants. The new cell wall in the center of the dixiding cell consists of
two new valves. Each of the two resulting daughter cells then has one
new valve, and one older valve that belonged to the parent cell. As a
result, any given diatom cell may have a valve that has been a part of
many ancestral cells. Spores are fonned in the diatoms both asexually and
sexually; and from them, after a period of dormancy, new plants develop
directly. The spores of diatoms have been indiscriminately called auxo-
spores regardless of whether or not they are the result of sexual union.
The prefix "auxo" refers to the increase in size of this cell. The cell
formed from the auxospore is usually much larger than the cell, or cells,
from which the auxospore originated. Subsequent divisions of the cells,
in vegetative multiplication of some species, lead to decrease in size of
cells until auxospores form.

The diatom cell is diploid, the monoploid condition being represented
only in the nuclei of the gametes, a condition not common among algae,
but usual in animals.

The Yellow-green Algae: Xanthophyceae

This comparatively small group of algae is almost entirely confined to
fresh water and consists of about 400 species. The yellow-greens are
similar to the greens"^ in many respects, but are characterized by chloro-

^ Since many of the green algae may often appear yellowish green during certain stages
of development, it is sometimes difficult to separate species of the two groups with

628 TEXTBOOK OF BOTANY

plasts in which yellow pigments predominate over the green; by motile
spores having two flagella of unequal length; by the presence of oils, but
never starch; and by the rare occurrence of sexual reproduction.

Most of the yellow-greens occur free-floating in temporary ponds and
pools, or as epiphytes upon other algae and upon the stems and leaves
of submerged plants. A few species grow on small crustaceans. The aerial
yellow-greens may be found on tree trunks or damp walls, or growing
with liverworts and mosses. A common yellow-green alga on prairie and
bottom-land soils is the balloon-shaped Botrydium.

The cells of the yellow-greens may be solitary, or united into colonies
and filaments. The cell walls are chiefly pectic compounds (pectose or
pectic acid) often impregnated with silica, with carbonates, and some-
times with small amounts of cellulose. The cells of several genera have
walls made up of two overlapping halves that fit together like the two
parts of a gelatine capsule used by druggists. Some of these characters
of the yellow-greens resemble those of the diatoms.

Reproduction occurs through the formation and subsequent growth of
motile and non-motile spores. Sexual reproduction has been observed
only a few times, but seems to involve the union of motile gametes.

The Brown Algae: Phaeophyceae

The predominant color of brown algae is due to the presence of
fucoxanthin, although green and yellow pigments are also present. Brown
algae are almost entirely marine, and the larger ones are often referred
to as seaweeds (Fig. 287). The largest ones, which are said to exceed
200 feet in length, are in the colder waters of temperate continental
shores. The motile spores and gametes have two laterally placed flagella,
one of which extends forward and the other backward. The cells are
uninucleate and have from one to many chloroplasts. Oil, sugar, and
complex polysaccharides accumulate in them. Some 1100 species are
recognized.

Most species of brown algae are found on rocky shores and do not
ordinarily grow at much greater depths than 50 to 75 feet, although
some of the kelps are attached to rocks at depths of 200 feet. Many of
the plants are completely submerged only at high tide, being exposed to

vegetative material alone. In hot concentrated HCl the chloroplasts of the yellow-greens
become blue-green, whereas those of the greens remain green or become yellow-green
when similarly treated. This may be taken as evidence of a difference in chemical com-
position of the two groups.

[Chap. XLVII

THE ALGAE

629

Fig 287 Forms of brown algae (figures not drawn to scale). Genera repre-
sented: (A) Fuciis, (B) Postelsia, (C) Ectocarpm, (D and G) Laminaria, (E)
Chorda, and (F) Nereocystis.

630

TEXTBOOK OF BOTANY

the air at low tide. Waves, especially during storms, rupture the blades
of the larger plants, and it is often difficult to find a perfect specimen.

The simplest brown algae structurally are filamentous and super-
ficially resemble some of the greens. The majority, however, are large

Fig. 288. PustelsUi, a brown alga, on the rocky coast ot Santa Cruz County,
Cahfornia. The plants are covered by water at high tide. Below, near the water,
are other brown algae. Photo by W. S. Cooper.

and parenchymatous with a rather high degree of differentiation in the
plant body ( Fig. 288 ) . The leathery kelps, abundant in the colder tem-
perate waters, have internal tissues that resemble the sieve tubes of seed
plants. These algae consist of branching root-like holdfasts attached to
rocks or other substrates, a long submerged stalk, and one or more con-
spicuously large, ribbon-like, or variously shaped floating blades. Some

[Chap. XLVII THE ALGAE 631

of the kelps are perennial, and the stalk increases in diameter bv the
formation of new rings of peripheral cells.

In the upper littoral zone along our coasts there grows in great
abundance a branched, filamentous brown alga, known as Ectocarpiis.
It is practically world-wide in distribution. Along the Atlantic coast it
is common as an epiphyte on the rockweeds (Fiiciis and Ascophyllum),
and less common along the Pacific coast on certain kelps. Motile spores
produced from special cells may germinate and from them new plants
develop. Other filaments produce flagellate gametes which upon fusion
form zygotes. The germination of the zygote results in a plant that is
able to produce only motile spores. In general, one cannot distinguish
the filaments that produce motile spores from those that produce gametes
until the reproductive structures actually appear. There occurs then in
Ectocarpiis an alternation of asexual and sexual phases similar to those of
seed plants, ferns, and mosses. Reduction division takes place during the
formation of the motile spores.

Kelps. The body of a kelp consists of a holdfast, a stipe, and a very
large blade. Kelps grow only in the colder waters of the oceans; they
are absent from the warm waters of tropical and semitropical regions.
Most species of kelp grow below low tide and are thus permanently in
water. The life cycle of the kelps may be indicated by a study of
Laminaria, sometimes called devil's apron, found along both coasts of
this continent. On the surface of the blade occur patches of sporangia,
from which motile spores escape (Fig. 289). From these spores minute
gamete-bearing filaments develop. These tiny plants may be few-celled
or many-celled, and each one is unisexual, producing either only male
gametes or only female gametes. The union of two gametes results in a
zygote, from which the large plant we know as the kelp de\'elops. We
thus have a pair of microscopic gametophytes ( gamete-bearing plants )
alternating with a large sporophyte ( spore-bearing plant ) .

The large blade-like kelp is the diploid plant. Reduction division
occurs in it during the formation of the motile spores, which are the fore-
runners of the small unisexual gametophytes.

It is interesting to recall that in nearly all green algae the conspicuous
plant is the one bearing gametes. In Laminaria, and a few other algae as
well, the gamete-producing plant is microscopic; the spore-bearing plant
is the conspicuous structure, as it is in the seed plants. Moreover, the
conspicuous phase of Laminaria is diploid, and the microscopic game-
tophytes are monoploid. The conspicuous phase of green algae usually

632

TEXTBOOK OF BOTANY

has the reduced number of chromosomes, and spore formation (except
zygotes ) results only in vegetative multiplication of the gamete-bearing
phase of the life cycle of the plant.

Fig. 289. Diagram illustrating the life history of a Laminaria. On the mature
thallus (A) an enlongated patch of sporogenous tissue develops, an enlarged
portion of which is represented in (B) with three sporangia from which motile
spores (C) later escape. Reduction division occurs during the formation of these
spores, and from one half of them male gametophytes (D) develop, and from
the other half female gametophytes. Sperms are liberated from the male gameto-
phytes and after swimming in the water unite with the egg cells in the female ga-
metophyte. From the zygote a young sporophyte grows at first attached to the
oogonium wall ( F ) , but later it is set free and ultimately becomes attached to some
other substrate (G).

Covering the rocks in the intertidal zones are the widely distributed
"rockweeds" or "bladder wracks," represented on our coasts by Fiiciis
(Fig. 3). They occur in tropical, temperate, and arctic seas, but the
species or genera are usually characteristic in each zone. Most of the
genera grow permanently attached to the rocks, although certain species
of Sargassum are free-floating. Fucits is a dichotomously branched plant,
attached by holdfasts and kept afloat at high tide by bladder-like struc-
tures filled with air. It has no asexual propagation except by fragmenta-
tion, and has no alternation of spore-bearing and gamete-bearing phases.
The reproductive cycle is similar to that of animals. Spemis and eggs
are formed by the first cell division immediately following reduction

[Chap. XLVII THE ALGAE 633

division, and all other cells of the plant have the diploid number of
chromosomes.

The reproductive organs are produced in special branches. Swollen
conical structures (receptacles) at the tips of the branches contain
rounded cavities (conceptacles) in which antheridia and oogonia de-
velop. Usually 8 eggs are formed in a single oogonium, and 64 sperms in
a single antheridium. Upon release from the oogonium and conceptacle,
the non-flagellate egg may be fertilized by a motile sperm. A new plant
grows directly from the zygote. Reduction division occurs in the mother
cell ( oocyte ) of the 8 eggs and in the mother cell ( spermatocyte ) of the
64 sperms.

The Red Algae: Rhodophyceae

The red algae are by far the most beautiful of all macroscopic sub-
merged plants (Fig. 290). They are most abundant in salt water, but
are not infrequent in fresh water. Of the approximately 3500 known
species, 200 grow in fresh water. They attain their greatest development
in subtropical seas. Five different pigments (Table 17) are known to
occur in the cells, with the red usually predominating, although various
shades of blue and purple are common. The red algae have definite
nuclei and chloroplasts, and the chief accumulated food is a starch-like
carbohydrate. They attain greater size on the whole than either the
greens, blue-greens, diatoms, or yellow-greens, but none attains the size
of the brown kelps.

The red algae grow attached to rocks near shore, but considerable
numbers may be found at depths of 50 to 100 feet, and a few live 200
feet or more below the surface. Some species are epiphytes upon other
algae. The smallest species are little more than one-celled structures, but
the majority are colonial aggregates, or filamentous and much branched,
or blade-like and leatherv. The larger species branch profusely, mav at-
tain lengths of several feet, and often have considerable differentiation
of tissues. A single filament, for example, mav have a row of axial cells
ensheathed by a cortical layer several cells in thickness. The red algae
superficially resemble other modern algae, but their genealogical rela-
tions are very remote.

Some of the red algae in tropical waters take part in the formation of
coral reefs, and are known as coralline algae (Fig. 291). Corals are
colonial animals; but always associated with them are red algae that also
accumulate calcium carbonate and likewise form massive reefs. Fossil

634

TEXTBOOK OF BOTANY

Fig. 290. Forms of red algae (figures not drawn to scale). Genera represented
(A) Chondrits, (B) Dasya, (C) Nenmlion, (D) Grinnellia, and (E) Corallina.

[Chap. XLVU THE ALGAE 635

algal and coral reefs are found in the limestone rocks formed during the
Paleozoic Era.

Fig. 291. Photographs of two coralline algae: A, Goniolithon; B, LithopJu/Uiiiu.
Courtesy of the N. Y. Botanical Garden.

Asexual reproduction among the red algae occurs through fragmenta-
tion, or the formation of non-motile spores. Strange as it may seem, no
flagellate, motile cells of any kind are produced b\' the plants of this
group. Sexual reproduction is so unlike that known in any other algae
that a special terminology is necessary to describe it. The unique features
of these algae are of interest mainly as examples of peculiar outcomes of
eyolution. A few examples are cited below.

Nemalion. When the tide is out, one may see the gelatinous strands
of the marine summer annual, Nemalion, attached to the rocks in the
midlittoral zone along coasts in temperate regions. It is a gelatinous
short-branched cylindrical plant with a reddish-brown color. Compared
with some other red algae its life history is simple, but it is more com-
plex than the life histories of other algae described in this chapter. It
will be described briefly.

The female sex organs are apical cells of special short branches. The
apical cell (carpogonium), which contains the egg protoplast and re-
sembles an oogonium, terminates in a slender outgrowth (trichogyne).
There are also numerous small antheridia, each of which contains a non-
flagellate male cell (sperrrmtium) which is borne by water currents to
the trichogyne. After the male cell becomes attached to the trichogyne,

636

TEXTBOOK OF BOTANY

Summary of Life Cycles of Algae Described

Name of Alga

or of Special

Groups of Algae

Sporophyte Phase,
Diploid

Monoploid Re-
productive Cells
Produced by
Sporophyte

Gametophyte Phase,
Monoploid

L'lothrix

Zygote

Flagellate spores
4-16

Unbranched filaments; flagellate
spores and gametes

Oedogonium

Zygote

4 flagellate spores

Unbranched filaments; flagellate
spores and sperms, non-motile
egg

Spirogyra

Zygote

4 nucleated zy-
gote; 3 nuclei
disintegrate

Unbranched filaments; non-
flagellate gametes

Vaucheria

Zygote and probably
the branched coeno-
cytic filaments anfl
flagellate spores

Sperms and non-
motile eggs

Gametes

Diatoms

Zygote » vegetative

cells and colonies;
non-flagellate spores

Gametes

Xon-motile gametes

Yellow-green algae

Zygote

Spores

Vegetative cells, colonies and fila-
ments; non-fiagellate and flagel-
late spores and gametes

Ectocarpus

Zygote > branched

filaments

Flagellate spores

Flagellate spores > branched

filaments and flagellate gametes

Laminaria

Zygote > large

blade-like kelp

Flagellate spores

Flagellate spores > small fihi-

ments; flagellate sperms, non-
motile eggs

Fucus

Zygote > large

branched plant

Gametes

Flagellate sperms, non-motile eggs

Nemalion

Zygote > filaments

of fruiting body (cys-
tocarp) , carposporo-
phyte

Carpospore

Carpospores > branched fila-
mentous plant; non-flagellate
egg (carpogoniumj. non-
flagellate sperm (spermatium)

Pleurococcus

Vegetative cells and temporary colonies; no spores and no gametes.

Blue-green algae

Vegetative cells, colonies, and filaments; non-flagellate spores, but no gametes

its nucleus divides and one of the resulting nuclei fuses with the egg.
After the egg is fertilized, numerous short branches develop from it. Each
of these branches is terminated by a spore ( carpospore ) . Branches may
also develop from cells adjacent to the fertilized egg. The whole mass

[Chap. XLVII THE ALGAE 637

of branches and carpospores, which superficially resembles a fruiting
body, is called a cystocarp. When a carpospore becomes free and
germinates, a new Nemalion plant develops from it. Reduction division
occurs during the formation of the carpospores.

The ordinary Nemalion plant, therefore, is the gametophyte phase
with the monoploid number of chromosomes. The short branches of the
cystocarp that developed from the fertilized egg have the diploid num-
ber of chromosomes and constitute the sporophvte { carposporophyte )
phase in the life cycle of Nemalion.

Batrachospermum and Lemanea. The fresh-water Batrachospermum
grows on the rocks in riffles and waterfalls in streams and in the wave-
disturbed margins of lakes of the temperate zone. It has whorls of short
branches at the nodes of a central axis, all enclosed in a heavv mucilasi-
nous sheath. The color varies from bluish green to violet. Its life cvcle
is similar to that of Nemalion. Another common relative in rapidlv flowing
streams is Lemanea. The filaments of some species when magnified look
like bamboo rods. The life cycles of these plants are more complicated
than that of Nemalion, but they are not as complex as those of many
other red algae.

REFERENCES

Brown, William H. Tltc Plant Kingdom. Ginn & Company. 1935.

Conger, Paul S. Origin and utilization of diatomaceous peat deposits. Sci.

Monthly 49:509-523. 1939.
Fritsch, F. E. The Structure and Reproduction of the Algae. \'ol. I. Macmillan

Company. 1935.
Hall, R. P. The trophic nature of the plant-like flagellates. Quart. Rev. Riol.

14:1-12. 1939.
Kudo, R. R. Protozoology. C. C. Thomas, Publisher. 1939.
Smith, G. M. Cryptogamic Rotamj. Vol. I. Algae and Fungi. McGraw-Hill

Book Company, Inc. 1938.
Smith, G. M. Freshwater Algae of the United States. McGraw-Hill Book Com-
pany, Inc. 1933.
Taylor, W. R. Marine Algae of the Northeastern Coast of North America.

Univ. of Michigan Press. 1937.
Tiffany, L. H. Algae, the Grass of Mam/ Waters. C. C. Thomas, Publisher.

1938.
Tilden, J. E. The Algae and Their Life Relationships. Univ. of Minnesota

Press. 1935.
Tressler, D. K. Marine Products of Commerce. Chemical Catalogue Companv.

1923.

CHAPTER XLVin

MOSSES AND LIVERWORTS
(BRYOPHYTES)

Mosses and liverworts grow throughout the world, from the high moun-
tains of Antarctica to northernmost Greenland. Species of moss occur in
nearly every habitat, from 180 feet under water in a Swiss lake to the
driest deserts of Arizona. Liverworts grow in less extreme situations but
in a surprising variety of them. Neither the mosses nor the liverworts live
in sea water.

Bryophytes are common in the vegetation on the arctic and alpine
tundra (Fig. 292). They are most abundant in moist and wet situations.
Bryophytes are also abundant in the north woods and in the moist
forested regions of the Pacific coast (Fig. 293). There they and the
lichens not only live on the soil and on the trunks and branches of living
trees, but they soon cover the fallen trees with a living green spongy
mantle, beneath which the relentless processes of disintegration proceed
( Fig. 294 ) . Mosses form the primary mats of the floating and semi-solid
substrates of bogs and "muskegs." Held together by the tangled fibrous
roots of sedges, ferns, and shrubs, they grow above and die below, thus
contributing to the gradual filling of depressions and lake basins with
peat.

Clearing of the forests and cultivation of the land by man have de-
stroyed numerous habitats of many kinds of bryophytes in agricultural
regions. But these species may still be found in rock gorges, swamp
woods, and other forest remnants where they have survived and are
often locally abundant. On the other hand, some species are more
abundant in partially cleared forests, and a few may be found in culti-
vated fields.

The grasses of old shaded lawns are often replaced by several species
of mosses and liverworts. Examination of clover fields and pastures will
reveal a surprising carpet of these little plants growing with the soil
algae. When farms are abandoned, bryophytes are among the pioneer
invaders of "worn-out" soils. Years later, when young woodland has occu-

638

[Chap. XLVIII

MOSSES AND LIVERWORTS

639

Fig. 292. Earlier and later stages in the development of alpine tundra on Mt.
Robson, British Columbia. Vegetation starts in the crevices of the glaciated rock
surface (A); ultimately rock and plant debris covers the entire surface; and sedges,
grasses, creeping willows, heath plants, and Drxjas (in flower) form a continuous
association (B). Photos by W. S. Cooper.

pied the farm site, it is often amazing to see the extent to which the
species of bryophytes that foiTnerly hved in the forests of the region
have reappeared.

In tropical and subtropical forests, where rainfall is ample, mosses
and liverworts grow profusely in glades and on stream banks. Many
grow as epiphytes, forming spongy cushions on tree trunks and branches
in the forest canopy. Some even grow on the surfaces of leaves.

Certain aquatic species live attached to rocks in swift clear streams,
and even in waterfalls. Most species of bryophytes are land plants, in
contrast to the algae, most of which are aquatics.

640

TEXTBOOK OF BOTANY

Fig. 293. Moss-covered terrain in a hemlock ioicst on a terminal moraine at the
mouth of Glacier Bay, Alaska. The moraine was deposited 150-200 years ago and
the maximum age of the trees is 110 years. Photo by W. S. Cooper.

Fig. 294. A mat of the common fern moss (Thuidium) . Photo by E. S. Thomas,

Ohio State Museum.

[Chap. XLVIII

MOSSES AND LIVERWORTS

641

Locally, the occurrence of some species is limited by the acidity or
alkalinity of the habitat. The pigeon-wheat moss (Polytrichum), the
wind-blown moss (Dicranum), and the white-cushion moss (Letico-
hrijum ) grow almost wholly in acid situations. The cord moss (Funaria)
and the silvery Bryiim are largely limited to alkaline or neutral areas.
Other mosses, such as Climachim and Mniwn, grow equally well in both
acid and alkaline habitats. Some species of bryophytes survive in very
acid, and others in extremely alkaline, substrates.

Many mosses withstand freezing temperatures at any stage of develop-
ment, and in subarctic lands live under snow and ice for the longer part
of every year. Whenever the snow disappears, food-making, growth, and
reproduction proceed from the stage at which they stopped the previous
season. In rock crevices on the mountains of the Antarctic the Bvrd Ex-
pedition found living mosses and lichens, where temperatures are seldom
above freezing, and in winter often 75"^ F. below zero. Mosses also occur
in the water of hot springs.

Their endurance of cold and drought is not dependent upon any spe-
cial anatomical structures. Owing to their habit of growth as compact

Fig. 295. A tutt ot llcdwigia on a rock substrate where it may become dry and
brittle during droughts, but will grow again when it becomes moist. Photo by
E. S. Thomas, Ohio State Museum.

642 TEXTBOOK OF BOTANY

cushions or as sponge-like mats, water accumulates during rains and fogs
by capillarity and remains available for much longer periods. When the
plants become dry, life processes are largely suspended; but in nature
recovery is possible even after prolonged droughts (Fig. 295). Moss
plants dried for several days in the laboratory fail to recover, but the
same species endure droughts of several weeks out-of-doors. Apparently
they secure enough water from the atmosphere at night to survive. The
spores of certain mosses, however, have germinated after being kept for
14 years in the dry air of a herbarium.

Adequate light, a moist atmosphere, a rather constant water supply,
and suitable temperatures are all essential to the abundant growth of
most bryophvtes. Some grow in the entrances to caves and below rock
ledges, where the light is as low as 1/500 of full sunlight. Moss plants
have been found living in complete darkness. Obviously these plants
were living as saprophytes similar to the subterranean green algae. The
species of moss that lives at a depth of 180 feet in the water of Lake
Geneva, Switzerland, survives at an extremely low light intensity, and
must have a very low compensation point. The so-called "luminous
moss" (Schistostega) found under rock ledges and in dark recesses in
woods is probably the material basis of the fairy tales of "goblin gold."
The cobwebby threads of the green plant are easily overlooked among
the rock particles. From certain angles, however, the refracted and re-
flected light rays from the lens-like cells of the plant are seen as a golden-
green glow.

None of the bryophytes is more than a few inches tall. They are
simple in structure, although many of them have organs that super-
ficially resemble leaves, stems, and rootlets. No conducting system is
present and the cell walls are not lignified. Mosses imbibe water rapidly,
even from moist air. The cells of the green parts of the plants have well-
developed round chloroplasts. No true parasites are known among bryo-
phytes, but many species are probably partial saprophytes. Moss com-
munities frequently become the substrates of lichens and die in their
shade. Their remains accumulate as humus on bare rock surfaces, and
become the substrate in which the roots of ferns and other plants may
grow. The remains of these plants in turn contribute further to the
humus, and the roots of the living plants bind the material together and
anchor it still more securelv. These are the pioneer stages of soil develop-
ment, and of vegetation on exposed rocks.

Owing to their rapid and compact horizontal spreading, bryophytes

[Chap. XLVIII MOSSES AND LIVERWORTS 643

soon form patch-like communities in many kinds of situations. On the
basis of their habitat relations, they are conveniently classified ( 1 ) as
suspended, attached, and floating species in quiet water, (2) as species
anchored in running water, (3) as plants on wet or dry fallen logs,
soils, and rocks, (4) as epiphytes on bark and leaves, (5) as bog species,
and (6) as pioneers on calcareous and non-calcareous rock surfaces.

Bryophytes are either annuals or perennials. It is difficult to estimate
the age of the perennials, since they have no annual rings or bud scars.

Fig. 296. The juniper hair-cap moss {Fohjtrichuin) . The male branches may
be recognized by their terminal rosettes of broad scales. Photo by E. S. Thomas,
Ohio State Museum.

Annual stem segments are recognizable, however, in some moss plants,
such as Polytrichiim ( Fig. 296 ) , in which a characteristic rosette of per-
manent leaves surrounds the terminal male reproductive structures. Each
year a new stem segment grows from near the center of the terminal
rosette of the previous stem segment (Fig. 297). Polytrichum plants
have been found in bogs, with 1 to 3 upper live segments and 4 to 8 dead

644

TEXTBOOK OF BOTANY

ones below. In most species the lower and older parts disintegrate about
as fast as new growths appear terminally.

Fig. 297. Polijtrichum. Annual stem segments and terms applied to parts of
the capsule. A and B, gametophyte and sporophyte phases; C and D, male
gametophytes.

The bog mosses grow in poorly aerated habitats, and as they die below
they accmiiulate as peat deposits, several to many feet in thickness.
Mosses that grow in springs heavily charged with calcium bicarbonate
become incrusted below with lime and build up layers of porous hard
tufa many feet thick. The tufa deposits later recrystallize and form solid
masses of limestone. The moss Gt/mnostomum, important in tufa forma-
tion, is known to elongate a little over an inch in four years. In a thou-
sand years these processes would form a layer of tufa and rock some 20
feet in thickness. Living cells occur onh' in the uppermost segments.

[Chap. XLVIII MOSSES AND LIVERWORTS 645

In moist, shaded habitats various small salamanders, insects, snails,
worms, fungi, bacteria, and algae live with the bryophytes as a miniature
microcosm. From the plants, both living and dead, the animals secure
food, cover, and a more continuous water supply. The saprophytic fungi
simply grow through the mass of liverworts and mosses and become in-
terlaced with them. Some of the fungi, however, are wholly or partially
parasitic, and live on and within the bryophyte cells. Certain species of
aerial and terrestrial algae as well as various soil bacteria are also sure
to be found in the community. The commonest example of an alga living
within liverworts is the blue-green Nostoc in cavities of the two liver-
worts, Anthoceros and Blasia.

The economic and biological importance of the bryophytes is much
less than that of either the algae or the fungi. They have been con-
tributory factors in the formation of peat, muck, and brown coal through
the centuries, and thev contribute annually to the organic matter of the
soil. Thev hold the soil against wind and water erosion, and decrease
run-off of water by their spongy texture. They are the food of many
small animals. Some species are important pioneers, sharing with the
lichens the initial development of soil on bare rocks, volcanic deposits,
and newly exposed land. During the World War certain bog mosses of
the genus Sphci<imim were collected, sterilized, and used as wound
dressings because of their superior absorptixe quality. Peat formed by
Spha^^miiih CaUiergon, and other mosses is prized by gardeners as a
means of increasing the organic content and porosity of soils in gardens
and in lawns. A bale, or about 8 bushels, of peat moss absorbs nearly
200 gallons of water. Florists and nurserymen use these mosses as pack-
ing around the roots of shrubs and trees when they transport and replant
them.

Approximately 23,000 species of bryophytes have been described and
classified. Of these, nearly 9000 are liverworts (Hepaticae), and 14,000
are mosses (Musci).

The moss plant. Some of the mosses and liverworts look much alike,
but most of the common mosses are easily recognized. They usually
have leafy erect or creeping stems, which develop from a much-branched
filamentous or thallose green structure growing on the surface of the soil,
and known as the protonema (Gr. first thread). A clone of numerous
upright leafv stems mav grow from each protonema. A leafless stalk ma}'
eventually develop from the apex or from the side of the leafy stem,

646 TEXTBOOK OF BOTANY

ending tenninally in a spore-bearing capsule. At the basal end of the
leafy stem, many-celled branching rhizoids, seldom more than a few
millimeters long, penetrate tlie soil or other substrate.

Mosses multiply freely by vegetative propagation. On culture media
new individuals may develop from nearly any fragment of the plant. In
many species small buds occur on stems, and when they become de-
tached new indi\'iduals develop from each of them. More speciahzed
propagative buds, called "brood bodies," occur abundantly on leaves,
stems, and even rhizoids of some species. \^egetative multiplication by
fragmentation of the protonema appears almost limitless, especially if
the protonema is perennial. Adventitious protonemata may develop from
almost any part of the plant.

The filaments of the protonema, except for their small roundish chloro-
plasts, are difficult to distinguish from branching filamentous algae. They
continue to branch, and soon fonn a thin green mat over the surface of
the soil. Numerous buds, or bulbils, form on the branches of the pro-
tonema, and rhizoids and leafy branches develop from them. If the buds
are numerous, the upright branches develop close together and form
dense clusters or cushions. Some species have an extensive perennial
felt-like protonema that completely covers the soil surface, but the up-
risht shoots are small and scattered.

When the spores formed in the capsule of the leafless stalk fall upon
soil or other moist substrates, they geiTninate, and from each of them a
new protonema develops. These spores constitute a definite stage in the
life cycle of a moss plant, which consists of two distinct phases. One
phase ends with the development of spores; the other, with the develop-
ment of gametes, and fertilization.

The gametophvte phase and sexual reproduction. The commonly ob-
served moss plant composed of protonema, leafy branches, and rhizoids
is the gamete-bearing phase, or gametophyte. At the apex of the green
leafy stems there appear tufted heads of leaves enclosing groups of
slender flask-shaped egg cases, called archegonia (sing, archegonium) .
An archegonium ordinarily contains one egg cell and is analogous to the
oogonium of the algae, but differs from it in being a multicellular struc-
ture (Fig. 298). Antheridia also develop at the apex of leafy stems,
sometimes in the same head with the archegonia, and sometimes alone
on other stem tips. They may also develop singly or in pairs in the axils
of leaves. They are multicellular, and many spenns develop in each of

[Chap. XLVIIl

MOSSES AND LIVERWORTS

647

them. Scattered among the antheridia and archegonia may be numerous
sterile filaments.

Fig. 298. A moss plant (Mniitm). E represents a vegetative branch, B a female
branch, and A a male branch. After fertilization, an upright stalk bearing a spore
case (C) develops from the zygote. A' represents a longitudinal section of the tip
of a female branch bearing three archegonia each of which contains an egg; B'
represents a section of the tip of a male branch bearing three antheridia each of
which contains manv sperms. Courtesv of World Book Co.

When a mature antheridium absorbs so much water that it swells and
bursts, the sperms are set free. Since they are motile the^■ swim about in
the film of water on top of the plant or mass of plants, and some of them
eventually swim near the tips of archegonia. As the egg matures, the
interior row of cells (neck-canal cells) of each archegonium disinte-
grates into a mucilaginous mass from which sugars diffuse into the sm-
rounding water. When the sperms come in contact with the diffusing
sugar they may swim toward the archegonium where the sugar is most
concentrated. A few eventually pass into the neck of the archegonium,
and one may finally move all the way to the egg and fuse with it. The
fertilized egg, or zvgote, is the beginning of the sporophvte phase of the
moss plant.

The sporophvte and asexual reproduction. The oospore germinates
while still within the archegonium at the apex of the leafy stem, and
from it develops a slender, stalk-like, leafless structure tenninating in a
capsule, or sporangium. In some mosses the stalk, or seta, is very short;
in others two to four inches in length. The capsule is usually simple, but
in some of the common mosses it is quite complex. In most species the
main body or urn of the sporangium is surmounted bv an easilv separable

648 TEXTBOOK OF BOTANY

lid, or operculum, and the whole structure, or at least a part of it, is
covered hx a hood or calyptra ( Fig. 297 ) . Extending partly across the
mouth of the urn is a peristome composed of from 4 to 64 variously
ornamented and radially disposed teeth. Large numbers of spores de-
velop in the sporangium; and when they are released, new protonemata
develop from them.

Thus the life cvcle of a moss from spore to spore consists of two
alternating phases: the gamete-bearing phase (the gametophyte) and
the spore-bearing phase (the sporophyte). Reduction division takes
place at the time of the formation of the spores within the sporangium.
The spores and each of the cells of the gametophyte ordinarily have the
monoploid number of chromosomes, and those of the sporophyte the
diploid number.

The leafless stalk with its sporangium is the sporophyte. Since it is
not a continuation of the stem of the gametophyte usually onl)' a slight
pull is necessarv to separate them. The sporophyte is green when young;
at most it is onlv a partial parasite on the green leafy gametophyte.
Owing to the location of the archegonium and the germination of the
oospore within it, the position of the sporophyte in some mosses is apical
on the main stem, and in others on minute lateral branches.

The sporophytes of most species de^^elop at a definite season of the
year. This fact mav often be helpful in distinguishing one species from
another. In a few mosses, notably Tetraphis, the sporophyte persists for
a year or more. In others the sporangia mature in autumn and remain
until spring. In still other species the sporophytes mature in the spring
and early summer and soon disappear. There are a few species of mosses
on which sporophytes have ne\'er been found.

The gametophyte phase of the moss begins with the formation of
spores in the sporangium of the sporophyte, and ends with the develop-
ment of gametes, and fertilization. The sporophyte phase l:)egins with
the fertilized egg, or zvgote, and ends with the formation of spores.

Among flowering plants the plant that bears the flowers is the sporo-
phyte phase. The gametophvte phase consists of the embryo sac in the
ovule, and of the pollen grain and pollen tube: a female and a male
gametophvte. Among the mosses, the gametophyte (protonema and
leafy branches) ma)- be male or female, or it may be bisexual (monoe-
cious), bearing both antheridia and archegonia with their sperms and
eggs. Of all the plants in the world, the bryophytes have the largest
gametophytes, and the seed plants have the largest sporophytes.

[Chap. XLVIII MOSSES AND LIVERWORTS 649

Chromosomes and the Hfe cycle of mosses. Since reduction division
occurs during the formation of spores in the sporophyte, these spores and
all cells of the gametophvte have the n, or monoploid, number of chromo-
somes; while the cells of the sporoph\ te, which develops from the fer-
tilized egg, have the 2m, or diploid number. Moreover, all the cells of
the gametoph\ te, including the protonema with all of its leafv branches
and all of their sperms and eggs, have the same chromosome comple-
ment. Consequentlv if self-fertilization occurs, each of the fertilized eggs
and resulting sporophytes will have two sets of like chromosomes. They
will be completelv homozvgous. If reduction division is regular, all the
spores fomied in these sporophytes will have the same chromosome com-
plement, and so will all of the subsequent gametophytes that develop
from them. Similar chromosome phenomena mav occur in bisexual liver-
worts and ferns also.

Classification of mosses. The foregoing description of a moss life cycle
refers especiallv to the group sometimes called the "true mosses"
(Brijales). Two other groups or orders of mosses are generally recog-
nized: the peat or bog mosses {Sphagnales) and the rock or granite
mosses (Andreaeales). These differ from the Bnjales in various struc-
tural details of both gametophvtes and sporophytes. Descriptions of these
orders mav be readilv found in books on the mosses, some of which are
listed at the end of this chapter.

The peat mosses, Sphagnales, differ from other mosses in several char-
acteristics. The protonema is a flat thallus similar in appearance to a
small liverwort or the prothallus of a fern. It begins as a filament, but
soon becomes a flat plate one-cell thick and irregularly lobed ( Fig. 299 ) .
The visible stalk ( pseudopodium ) upon which the capsule is borne is an
outgrowth of the gametophvte and is not the lower part of the spor-
ophyte. The sporophyte consists of a globular capsule, a very short and
slender stalk, and a foot. The spore-bearing part of the capsule is dome-
shaped, overarching a region of sterile tissue. The leaves are a single
cell laver in thickness, have no midrib, and contain both living and dead
cells. In \ouno; leaves all the cells look alike; but later, differentiation
results in slender living cells, which possess chloroplasts and fonn a
network enclosing larger, short-lived, non-green cells.

The peat mosses are of more commercial value than any others. These
plants often form floating masses on the surface of lakes and ponds.
Continued growth of the plants upward forces lower layers downward,
and the dead and decaying parts accumulate, disintegrate but little in

650

TEXTBOOK OF BOTANY

Fig. 299. Three species of Sphagnum (A-C), reproductive structures (D-G),
and thalloid protonema with young leafy shoot ( H ) . G represents the sporophyte
stage, which consists of a sporangium and a very short stalk. The small urn-shaped
bodies on A, B, and C are sporophytes.

the absence of oxygen, and become peat ( Fig. 300 ) . These mosses may
also become estabhshed on the shore of a lake and gradually encroach on
the water, forming a quaking bog. Creeping bogs may develop in very
moist climates. The sphagnum mat grows out of the depression, invades
forested land, and kills the trees by raising the water table and excluding
oxygen from the tree roots. The characteristic genus of peat mosses is
Sphagnum, more than 300 species of which have been described.

Mature peat mosses have no rhizoids, and absorption of water is
direct. The upward movement of water is largely effected bv capillarity
through the closely adhering lateral branches and the compactness of
the plant mass.

The rock mosses, Andreaeales, grow principally in arctic and alpine
regions upon granite or slate. They are dark or almost black in appear-
ance, have very brittle leaves densely aggregated, and are rarely more
than an inch or two long. The gametophytes resemble the true mosses,
while the sporophyte is supported by a pseudopodium as in the peat
mosses. The capsule is unique among mosses in its longitudinal de-
hiscence by four valves when mature. There are approximately 120
species of rock mosses.

\Chup. XLVIII

MOSSES AND LIVERWORTS

651

Fig. 300. Method of drying peat bricks cut from tfie black vertical wall in the
foreground. Degermoor, Southern Bavaria.

The true mosses, or Bryales, consist of about 13,500 species classified
among 80 families and 655 genera. This is the largest group of the mosses.

The liverwort plant. Two groups of liverworts are easily recognizable:
the thalloid and the leafy forms. The thalloid species are easily distin-
guished from mosses by their irregular, leaf-like, much-branched vegeta-
tive body (Fig. 301). The thalli (sing, thallus) may be from one to
several cell layers in thickness, and are sometimes dichotomously
branched at short intervals. The middle line of the thallus is a groove in
some species and has the appearance of a midrib, but no veins are
present.

The leafy liverworts have flat shoots and the conspicuous leaves grow
in two ranks. There is a third row of smaller leaves on the under side of
the stem in many species. The leaves are entire, lobed, folded, or
dissected.

Many of the thalloid liverworts, such as Marchantia (Fig. 302), have
distinctly differentiated tissues. Above is an epidermis with open pores
beneath which are cavities, or air chambers, containing tufts of erect
green filaments. Photosynthesis occurs in these upper cell layers. Below
are large, compactly arranged cells containing water; and extending from
the lower epidermis are scales and one-celled rhizoids.

652

TEXTBOOK OF BOTANY

Fig. 301. Some widely distributed liverworts: A, Pellia thallus with antheridia
(dots on surface) and a sporophyte; B and C, land and water forms of Riccia;
D, archegonial thallus of Anthocews with sporophytes; E, a portion of the thallus
of Porella, a leafy liverwort. Courtesy of World Book Co.

Fig. 302. Marchantia plants with archegonial and antheridial branches; cupules and
gemmae also present. Photo by R. B. Gordon.

[Chap. XLVIII

MOSSES AND LIVERWORTS

653

As an example of the foliose liverworts, Porella may be studied. It is
common on tree trunks, on rocks, and even on walls of buildings. The
novice often mistakes it for a moss. Its curved reclining stems bear three
rows of flattened leaves, two on the dorsal side and one on the ventral
side.

Vegetative propagation occurs in liverworts by fragmentation, by
"brood bodies," and by gemmae formed in variously shaped gemmae-
cups. These are small cup-shaped outgrowths from the upper epidermis
of the thallus. From the bottom of the cup numerous upright fiddle-
shaped green thalli develop. They germinate immediately when trans-
ported to the soil bv rain or wind.

Fig. 303. \^egetative and reproductive structures of Marchantia: A, vegetative
propagules, or gemmae, in cupules on a portion of a thallus; B, part of a male
thallus with upright antheridial branches terminating in disk-like structures in
which antheridia and sperms develop; C, a female thallus with upright archegonial
branches terminating in radially branched structures in which archegonia and
eggs develop; D, diagram of a vertical section of the apex of an archegonial
branch to show location of the pendant archegonia; E, diagram of a vertical sec-
tion of the apex of an antheridial branch to show location of antheridia; F, form
of sperms; G, isolated sporophytes that have developed from fertilized eggs within
archegonia represented in D. Elongation of the sporophyte stalk pushes the
sporangium out of the archegonium, but the base of the stalk remains attached to
the gametophyte as it does in mosses.

654

TEXTBOOK OF BOTANY

The parts of Marchantia and Porella just described are the game-
tophyte phases of these two Hverworts. They bear archegonia and
antheridia in which eggs and sperms are formed much as in the mosses.
The sporophyte phase develops from the fertihzed egg and is similar to
the sporophvtes of certain mosses (Figs. 303 and 304).

Fig. 304. Antheridial and archegonial thalli of Pallavicinia (A-B); stages in the
development of the sporophyte (C-I). Courtesy of World Book Co.

Reduction division occurs when the spores are formed in the spo-
rangium. When a spore germinates, a new thallus develops from it.
Elongated, spindle-shaped, and often spirally thickened cells known as
elaters may develop within the sporangia along with the spores. This is
one of the characteristics of liverworts.

Some of the features by which mosses can be distinguished from
liverworts may be summarized as follows:

Mosses Liverworts

Stems leafy, never thalloid Stems leafy or thalloid

Protonema present No protonema

Many-celled branching rhizoids One-celled rhizoids

Elaters absent from sporangia Elaters present in sporangia

REFERENCES

Dixon, H. N. Student's Handbook of British Mosses. 3rd ed. Wheldon and

Wesley, London. 1924.
Frye, T. C, and Lois Clark. Hepaticae of North America. Univ. of Washing,-

ton Publications in Biology. 6:1-161. 1937.

[Chap. XLVHI MOSSES AND LIVERWORTS 655

Grout, A. y. Mosses with Hand-lens and Microscope. Published by the author

1903.
Grout, A. J. Mosses with a Hand-lens. 3rd ed. with Hepatics. Published bv the

author. 1924.

Grout, A. J. Moss Flora of North America. Published by the author. 1928 .

Jennings, O. E. Manual of the Mosses of Western Pennsylvania. Published by

the author. 1913.
MacVicar, S. M. The Student's Handbook of British Hepatics. 2nd ed. Whel-

don and Wesley, London. 1926.
Verdoorn, Fr. and others. Manual of Bryology. Nijhoft. The Hague. 1932.

CHAPTER XLIX

FERNS, CLUB MOSSES AND EQUISETUMS
(PTERIDOPHYTES)

Superficially the ferns, club mosses, equisetums, and quill worts may ap-
pear much too diverse in form to be classified within one group, known
as the Pteridophytes (Gr. pteris, fern, and phyton, plant). The funda-
mental features of their life cycles, however, are quite similar, and are
sufiiciently unlike those of other groups of plants to warrant the con-
sideration of them as a single group. Altogether about 8000 species of
Pteridophvtes have been described and named, of which more than 7000
are ferns.

During the Carboniferous Period, some 300 million years ago, the
forests were dominated by hundreds of tree species of Pteridophytes
( Chap. LII ) . The coal deposits of Pennsylvania were formed primarily

Fig. 305. Ferns (Polystichum miinitiim) often 4 feet in height in the moist forest
region of the northwestern states. Photo by W. S. Cooper.

656

[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS

657

from the wood of these trees. These species have all become extmct,
and only a few of the modern ferns are trees. Most of the modern
Pteridophvtes are small and comparatively inconspicuous except when
they grow in masses. In the forests of today they are understory plants
( Fig. 305 ) , and are usually most abundant along stream banks, in open
glades, in clearings made by man, or where the forests have been de-
stroyed by storm and fire. In such areas some of the species reproduce
so rapidly and grow in such dense masses that reforestation is greatlv
delayed. Some of these species also become troublesome weeds in pas-
tures in moist climates. A few species are prized as decorative plants,
and others are used in medicine.

The Ferns

Ferns grow in a great diversity of habitats. We usually associate them
with shade, ample water supply, and warm temperatures. While in gen-

FiG. 306. A fern (Asplenium) widely distributed on rocks. Photo by C. H. Jones.

eral this is true, particularly for those of very rapid growth, ferns are
not necessarily so restricted. Some species thrive in open fields and on
dry hillsides, others on dry exposed rocks (Fig. 306), still others in
marshes not especially shaded, and many in the forests of the tropics and
semi-tropics as epiphytes (Fig. 307); a few are strictly aquatic. They are

658

TEXTBOOK OF BOTANY

Fig. 307. The largest known specimen of the staghorn fern, an epiphyte on
trees in the tropics. Photosynthesis occurs chiefly in the upright leaves. The
rounded leaves pressed against the tree cover masses of roots. Reproductive struc-
tures develop on the pendant leaves. The photograph was made on a small island
near Brisbane, Australia, by C. J. Chamberlain. Courtesy of World Book Co.

[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS 659

reported from the frigid zone; but the number of species there, as well
as of individuals, is small. As one goes from cool temperate regions
toward the equator the number of species increases rapidly. Ferns attain
their greatest size and variety in the moist tropics, where some of the
species are trees. In warm temperate regions the largest and most striking
ferns occur in swamps and in rich soil along streams. Most species of
ferns grow in acid soils, but many of them grow on both acid and alka-
line soils.

Geologically ferns are very old. Fossil remains of ferns are reported
from the Devonian rocks, but their greatest abundance occurred during
Carboniferous times. Some of these early ferns were trees, and some of
them reproduced by seeds (Figs. 356A and 360).

Aside from their use as decorative plants, the commercial value of
ferns is almost negligible. Certain species that grow in masses and in-
vade bare soil areas contribute to soil development and the protection
of soil from wind and water erosion. The young rhizomes of a few spe-
cies are sometimes used as a source of food by animals and man; and
the leaves have been used in the making of tea and liquors. The aquatic
species are eaten by ducks, some fish, and by other water animals. The
greatest common interest in ferns, however, is undoubtedly esthetic.
They are cultivated for ornamental purposes, and the supply and sale
of cut ferns have become an important industry.

The fern plant. Ferns are generally recognized by their leaves. Thev
are the most conspicuous and characteristic parts of the plant and bear
characteristic reproductive structures. They vary in length from an inch,
or less, in the smallest species to 100 feet in species of climbing ferns.
The spirally twisting petiole ( rachis ) of the climbing fern of the Eastern
States seldom exceeds 3 feet in length. Some of the common ferns of
temperate zones have a blade a yard or more in length supported by a
prominent petiole often mistaken for a stem by the novice. Sessile leaves
are rare among ferns.

The leaves may be simple, but they are more often compound. The
leaflets of compound leaves may be further divided and dissected, re-
sulting in a very complex frond. ^ The leaf develops from a primordium
near the apex of the stem and, owing to slightly more rapid enlargements
on the outer side, becomes coiled. As development proceeds, growth on
the inner side catches up with the outer, and the apex uncoils ( Fig. 308 ) .

^ Some writers use an entirely different set of terms to describe the parts of a fern
plant: the leaves are called "fronds," the leaflets "pinnae," and the divisions of the
leaflets "pinnules."

660

TEXTBOOK OF BOTANY

Lea\ es usualh' mature in one season, though in some species they grow
for a period of three or more years.

Fig. 308. Uncoiling of young leaves of cinnamon fern (Osmiinda cinnamomea) .

The characteristic \'enation of ferns is dichotomous, but there are all
gradations from dichotom\' to a very complex net arrangement that is
better tenned reticulate. The lea\'es of a few species are somewhat suc-
culent and the venation is not easih' seen. Most of our common ferns
may be readilv recognized, howe\'er, b\^ their dichotomous venation and
the uncoiling of their voung leaves, known as circinate vernation.

The fern leaf is similar in structure to that of the seed plant. The
epidemiis is a definite layer with stomates usualh' on the under side of
the leaf. Green plastids occur in the guard cells. The mesophyll is often
not differentiated into palisade and spongv tissues. The leaflets of some
of the film\' ferns are but a single cell la^er in thickness. Structurally,
the veins are quite like those of seed plants, and thev are branches of
the vein tissues of the stem and roots.

The stems of most ferns are horizontal branched rhizomes. Thev mav

[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS

661

Fig. 309. The tree fern pictured here is in Hawaii and is 30 feet high and 3 feet
in diameter. Photo from the U. S. Forest Service.

662 TEXTBOOK OF BOTANY

grow on the surface of the soil or below. The life duration of any part
of a horizontal stem is relatively short, for the rhizome continues to
elongate at the growing tip and die at the other end. A few of our
common ferns, such as the cinnamon fern, have erect stems, sometimes
extending a foot or more above the soil in swamps. The upright stems
of tree ferns in the tropics may be 20 to 30 feet high. Some ferns are
epiphytes (Fig. 309).

The ferns, in marked contrast to all bryophytes and thallophytes, have
highly differentiated conducting systems with both phloem and xylem
tissues. Special elongated cells known as tracheids are the fundamental
structures of the xylem. Xylem tubes, or vessels, occur rarely. The
phloem is composed of sieve tubes and phloem parenchyma. Cambium,
though common in the fossil ancestors, is very rare in living ferns, hav-
ing been found in only two genera. The "bundles" of xylem and phloem
mav be few and scattered, or sometimes numerous and arranged in a
cylinder. Pith is absent in stems of some species and conspicuous in that
of others. The cortex is usually prominent and may have several outer
la vers of thick-walled cells.

The roots of ferns in comparison to leaf surface are smaller, shorter,
and less branched than the roots of seed plants. In the herbaceous ferns
they originate adventitiously at the nodes and along the internodes of
the rhizome. In the tree ferns the root systems are more complex, but
they do not attain the size and spread of the root systems of seed plants.
Their internal structure is quite similar to that of the roots of seed plants.
Roots may be lacking in some aquatic species, and in the filmy ferns
only rhizoids develop.

Ferns multiply vegetatively by means of their branching rhizomes,
from which a succession of new leaves develops each season. New in-
dividuals may also develop vegetatively from adventitious buds (bul-
bils) on the surface or in the axils of the leaves of certain species. New
plants of the walking fern develop from buds formed at the tips of the
leaves in contact with the soil (Fig. 310). Some tropical species multiply
vegetativelv from the swollen leaf bases.

The sporophyte. The conspicuous fern plants described abo\'e are all
sporophytes. They are much larger and far more complex than the
sporophvtes of the mosses and liverworts. Among the bryophytes the
conspicuous phase in the life history is the gametophyte; among the
ferns it is the sporophyte phase. The sporophyte of ferns has definite
leaves, stems, and roots with a well-developed vascular system and other
tissues comparable to those of seed plants.

[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS

663

Fig. 310. The walking fern (Cainpfosorus rhizophyUus) . New plants develop
\ egetati\'ely from buds at the apex of the leaf blades. Photo by R. T. Wareham.

Sporangia may develop on the under side of a green fern leaf (Fig.
311). They may occur in irregular lines or areas, or co\er nearly the

Fig. 311. The life history of a fern. The gametophyte (A) bears egg cells and
sperms in archegonia and antheridia. When one of the sperms from an antheridium
(B) unites with an egg cell in an archegonium (C), a zygote is formed. From
this zygote the leafy fern plant (D) develops and bears spores in sporangia (F
and G) on the lower side of the leaves. By the bursting of the sporangium (H)
the spores are set free. When these spores germinate on moist substrates a new
gametophyte, like the one represented by A, develops from each of them.

664

TEXTBOOK OF BOTANY

whole under surface of the leaf. The sporangia are often grouped in dis-
tinct clusters known as sort ( sing, sorus ) . The sori may be located along
the veins, at the ends of veins, or along the margin of the leaf. The
shape, arrangement, and general appearance of the sori are commonly
used in distinguishing different genera and species of ferns. They are
usually covered by a delicate shield-like structure known as the in-
diisitim (Fig. 312).

Fig. 312. Reproductive structures of a fern: A, section through part of a leaf
and a sorus composed of a stalked shield, or indusium, which covers a cluster
of sporangia in which numerous spores have developed; B, ventral surface of a
bisexual gametophyte; the antheridia are near the bases of the rhizoids, the
archegonia are just below the central sinus. B drawn by C. J. Chamberlain. Text-
book of Botany (Coulter, Barnes, and Cowles) , Amer. Bk. Co.

Species such as the cinnamon fem and sensitive fern have two kinds
of leaves: the usual foliage leaf, and a smaller one on which sporangia

[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS 665

and spores develop. Such special spore-bearing leaves have been called
sporophylls; and this term is applied to any spore-bearing leaf, whether
of fern or of seed plant. Stamens and carpels of ordinary flowers and
the scales of pine cones are often referred to as sporophylls. The cinna-
mon fern has cinnamon-colored sporophylls.

Fern sporangia develop at the ends of short stalks. The epideiTnis of
the sporangium is composed of two kinds of cells: a single row of cells
with walls greatly thickened inwardly ( the annulus ) , and several larger
thin-walled cells forming the remainder of the sporangium wall. Upon
losing water the cells of the annulus contract, and the maturing sporan-
gium splits open at the side. The number of spores in a sporangium
ordinarily ranges from 32 to 64, but it may be higher in a few genera:
400-500 in Osinwida, the royal ferns; 1500-2000 in Botn/chium, the
grape ferns; and 12,500-15,000 in Ophioglossum, the adder's-tongue
ferns. Many millions of spores may develop on one fern plant in a
single season. The spores develop in tetrads following reduction division
of the spore mother cells.

Familiar ferns include, besides the ones referred to above, the walking
fern, Camptosortis; the common polvpodv, Polijpodiwn; the maidenhair
fern, Adianttim; the bracken fern, Pteris; the Christmas fern, Polys-
tichum; and the sensitive fern, Onoclea.

The gametophyte. Under appropriate conditions the spores germinate
either directly or after a short period of dormancv, and from each spore
there develops a small heart-shaped thallus that superficially resembles
a very simple liverwort. The first few cells fomi a filament that resembles
an alga or a moss protonema; in a few species of feni the filamentous
form is permanent. In most species, however, subsequent cell divisions
result in a thallose structure. This structure, unnecessarily termed a
prothalltis, is the gametophyte of the fern. Thalli of ferns may be found
growing naturally on moist rocks, on soil, and on decaying logs near
the sporophytes. They grow readily from spores sown on culture media
or on the surface of flower pots after the pots have been filled with
sphagnum moss and inverted in a pan of water.

In two genera of ferns (the grape fern, Botrychiurn and the adder's-
tongue, Ophioglossum) the gametophyte is quite unique. In Botrychiurn
it is entirely subterranean and without chlorophyll; in Ophioglossum it
is subterranean witli green parts extending above the soil surface. In
both, a fungus lives in the non-green part of the gametophyte and ex-
tends into the surrounding humus.

666 TEXTBOOK OF BOTANY

As the flat expanse of cells forming the fern gametophyte grows,
rhizoids appear on the lower side; and soon afterward antheridia de-
velop near them (Fig. 312). The antheridia are comparatively simple
structures, with an external layer of several cells enclosing the sperm
mother cells, in each of which one sperm develops. The sperms have a
spirallv twisted bodv with 40 or 50 long cilia. The archegonia develop
as the gametophyte matures, and are also located on the under side
near the notch or youngest part of the thallus.

Fertilization. The sperms are released by the swelling and bursting of
the mature antheridium when water collects under the gametophyte.
Under similar conditions the archegonium opens, and the products of
the disintegration of the neck canal cells diffuse into the water. The
sperms swim toward the regions of greatest concentration of these
diffusing substances, and one of the spenns after entering the arche-
gonium fuses with the egg cell, forming a zygote. When fertilization
has taken place in one or two of the archegonia, the further develop-
ment of the remaining archegonia of the gametophyte ceases. Conse-
quently one sporophyte ordinarily develops from a zygote in each fern
gametophyte, rarely two of them. If the young embryos of the sporo-
phytes are removed as rapidly as they begin to develop, the feni game-
tophytes continue to grow vegetatively for years. Apparently a hormone-
like substance formed in the young sporophyte inhibits not only the
further development of other embryos, but also the further growth of
the prothallus, or gametophyte.

Embryo of the sporophyte. The zvgote germinates directly after fer-
tilization. Cell division soon results in an embryo with four distinct
regions : ( 1 ) the foot, or holdfast, by which the embryo is attached for
a short time to the gametophyte; (2) a root tip, which rapidly elongates
and penetrates the soil; (3) a leaf primordium, from which the first leaf
of the sporophyte develops; and (4) a stem tip, which elongates slowly,
and from which successive leaves and adventitious roots develop. The
sporophvte is thus at first dependent on the gametophyte, but it soon
becomes green and develops rapidly; and the thallus disintegrates. From
the embryo the mature sporophvte grows.

Chromosome numbers. The cells of the sporophyte all have the dip-
loid number of chromosomes. Reduction division occurs in the spore
mother cells during the formation of spores in the sporangia on the
leaves. These spores, the cells of the subsequent gametophytes, and the
sperms and eggs have the monoploid number of chromosomes.

[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS 667

All the eggs and sperms produced in the same gametophyte have the
same chromosome complement. If self-fertilization occurs, the resultant
zygote and sporophyte will be completely homozygous. If reduction
division is regular, all of the spores formed in this sporophyte will have
the same chromosome complement; and so will all of the cells of all of
the gametophytes that develop from these spores.

Mutation, hybridization, and parthenogenesis occur in ferns as in
flowering plants. Changes in chromosome number in ferns, however,
may also occur in another peculiar way. Gametophytes sometimes origi-
nate vegetatively from the diploid cells of the sporophyte; and sporo-
phytes sometimes originate vegetatively from monoploid cells of the
gametophyte.

Alternating phases in the life cycle of plants. In the life cycle of many
algae, and of all seed plants, bryophytes, and pteridophytes, a diploid
spore-bearing phase, the sporophyte, ordinarily alternates with a mon-
oploid gamete-bearing phase, the gametophyte. In the ferns and brown
algae either phase may continue to multiply vegetatively. In the bryo-
phytes and such algae as Oedogonmm, vegetative multiplication is lim-
ited largely to the gametophyte, while in the seed plants it usually occurs
only in the sporophyte. There are no alternating phases in the blue-
green algae.

The sporophyte phase of seed plants, pteridophytes, and brown
algae is the conspicuous one. The gametophyte phase (pollen and
embryo sac) of the seed plants and of some of the pteridophytes is
very small. The gametophyte phase of the bryophytes is the conspicuous
one, and the smaller sporophyte phase is wholly or partially dependent
upon it as a source of food.

The gametophyte phase of the seed plants develops within the meg-
asporangium (ovule), whereas that of non-seed plants develops on the
soil or in water. The first step in the formation of seeds is the germina-
tion of the spores within the sporangia of the sporophyte. If these spores
fall upon the soil, no seeds are formed. Some of the ancient ferns had
seeds, and some of the modern pteridophytes occasionally have seeds or
structures closely resembling seeds.

The aquatic ferns. The water ferns constitute a small group consisting
of four genera that are considerably unlike the plants so far described
in this chapter. Salviriia and Azolla are floating, while Marsilia and
Piliilaria are rooted at the bottom of small ponds or sluggish streams.
Salvinia has a whorl of three leaves at each node, two of which are

668

TEXTBOOK OF BOTANY

floating, hairy, entire leaves; the third leaf looks much like a branched
root. Stems and roots are more conspicuous in Azolla. The other two
genera are larger plants with prominent leaves from rhizomes rooted in
the mud below the water. Superficially the leaves of Marsilia resemble
a four-leaved clover; those of Pilularia resemble leaves of grasses.

Two kinds of spores develop in the water ferns: microspores and
megaspores. Male gametophytes develop from the microspores, and
female gametophytes from the megaspores. Plants that bear both micro-
spores and megaspores are said to be heterosporotis ( different spores ) ,
in contrast to homosporoiis plants in which the spores are all alike and
the gametophytes that develop from them are bisexual.

The Lycopods or Club Mosses

The club mosses are relatively small in size, frequently evergreen,
and flourish especiallv in the tropics. The common name refers to their

Fig. 313. One of the common club mosses {Lycopodium complanatiim) . The
plant pictured here is the sporophyte phase. The sporangia are borne in the terminal
cones. Photo by E. S. Thomas, Ohio State Museum.

moss-like leaves and club-shaped cones. The upright appearance of
some species is that of a small pine tree; hence the name "ground pine.
Species of the two genera, Lycopodium (Fig. 313) and Selaginella

\Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS 669

(Fig. 314), also grow in temperate climates, and as far north as New-
foundland and Labrador. They are much more abundant in the northern
conifer forests than in the deciduous forest of the Central States. The
tropical forms are frequently epiphytic. Many of the species have long

Fig. 314. A club moss {SelagincUa apm) widely distributed on dry rocks.
The half-decayed branch in the background is 3 inches in diameter. Photo by
E. S. Thomas.

creeping stems; in others the stems are erect. The stems usually branch
dichotomously. Most of the species are perennials, but a few of the
smaller ones are annuals.

Some species of Selaginella grow in dry climates. During the rainy
seasons the numerous stems are spread out in the form of a dense
rosette. During dry weather the stems curye upward and inroll toward
the center of the plant, with the result that the whole plant has the
form of a ball and may be blown about (Fig. 315). When moisture is
again available, the stems unroll and new roots grow. Such plants are the
so-called "resurrection plants." The plant rolls and unrolls even when
dead.

670

TEXTBOOK OF BOTANY

ImM

Fig. 315. Resurrection plant (Selaginella) of Texas and New Mexico. During
the rainy season the plant spreads out and grows as a rosette. When drought
comes, it dries out and curls up into a ball, as indicated at the right. Courtesy
World Book Co.

Fossil imprints of the ancestors of the lycopods are very abundant
in Carboniferous rocks. Fossil remains indicate that during this period
hundreds of species of club mosses were dichotomously branched trees
up to several feet in diameter and a hundred or more feet in height.
The leaves were somewhat scale-like, arranged in spirals or in whorls,
and often a half-foot long. The number of species of present-day lyco-
pods, all of which are small plants, is estimated at about 600.

The sporophyte. Both Lycopodiiim and Selaginella consist of a much-
branched, creeping or erect stem, practically covered by small scale-like
leaves attached by a broad base. The adventitious roots are filiform and
develop from the under side of the stem in contact with the soil or in
moist air. Both the stems and leaves have a fundamental structure
similar to that of seed plants. The more primitive species have rather
large sporangia in the axils of leaves; in the more specialized types the
sporophylls are small, often non-green, and arranged in compact cones
at the ends of upright branches (Fig. 316).

In Lycopodium four spores are produced from each mother cell in
the sporangium, and all the spores are alike morphologically and physio-
logically. Consequently Lycopodium is said to be homosporous. In
Selaginella the spores are of two kinds formed in different sporangia.
The small spores ( microspores ) develop by the hundreds in small spor-
angia ( microsporangia ) , and four large spores (megaspores) develop
in each large sporangium ( megasporangium ) . Selaginella is thus het-
erosporous.

[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS

671

Fig. 316. Three species of Lijcopodiian: A, L. complanatiim with gametophyte
(after Bruchmann); B, L. lucidulum with sporangium (bi) and vegetative
propagules (b.); C, L. clavatum with gametophyte (Ci).

The relative size of the two kinds of spores is perhaps insignificant,
for in equisetums the spores are indistinguishable, and in some seed
plants the microspores are larger than the megaspores. The significant
fact is that in Selaginella the gametophvtes resulting from the germina-
tion of the spores bear either archegonia or antheridia, but not both. A
thallus producing only one kind of gamete is unisexual: one producing
both kinds is bisexual.

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TEXTBOOK OF BOTANY

The gametophyte. From the germinating spore of Lycopodium cylin-
drical, tuberous, or fleshy thalloid gametophytes develop, either under-
ground and without chlorophyll, or partly aboveground and green ( Fig.
316). They are thus partly saprophytic or parasitic on fungi. These
gametophytes are very minute, rarely more than a few millimeter in
length. The antheridia and archegonia are borne at the apex of the
gametophytes. The sperms are straight, uncoiled, and biciliate. Fertiliza-
tion of the egg by the sperm occurs in the same manner as in ferns.

As noted above, two kinds of gametophytes develop from the spores
of Sela^inella. Male gametophytes develop within microspores, and

Fig. 317. Selaginella: A, vegetative branch with terminal cone; B, longitudinal
section of cone with microsporangia on one side and megasporangia on the other;
C, female gametophyte protruding from the megaspore wall with several arche-
gonial openings located near the evident rhizoids; D, male gametophyte within
the microspore wall; E, male gametophyte with sperms formed in the cells; F,
section of female gametophyte and two embryos following fertilization.

female gametophytes develop within the megaspores. Both the male and
the female gametophytes are small and are largely contained within the
old spore walls (Fig. 317). The male gametophyte consists of a vegeta-
tive cell and a single antheridium. The sperms formed are few in number
and are similar to those of Lycopodium. The female gametophvte is
multicellular and at first is contained within the megaspore. Subsequent

[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS 673

growth bursts the spore wall, and part of the gametophyte protrudes.
The archegonia develop on the apical segments that extend beyond the
spore wall. Fertilization takes place as in ferns.

Germination of the zygote begins at once. The early stages of the
developing embryo occur within the walls of the megaspore. Later the
partly formed embryo continues growth on the soil or other substrate.
These gametophytes are among the smallest found in pteridophytes, and
are somewhat similar in size to those of seed plants.

Seeds, Occasionally a megaspore of Selaginella germinates within the
megasporangium, in contrast to others that fall out of the sporangium
and germinate on the soil. Consequently the female gametophyte with
its archegonium and egg is inside the megaspore wall within the mega-
sporangium. Fertilization may take place, followed by the development
of an embryo sporophyte within these structures. These phenomena
occur rarely in Selaginella, but when they do occur a seed is the result.
The seeds of pine and all other gymnosperms are formed in this same
manner. A seed is merely the result of the germination of the megaspore
within the megasporangium or ovule, and the formation of the gameto-
phyte and of a new sporophyte within the megasporangium. If the
spores fall out of the sporangium and germinate on the soil as they ordi-
narily do in mosses and ferns, no seeds are formed.

The quillworts. This group of unique plants is quite unlike any other
pteridophvtes. Approximatelv 60 species of quillworts have been de-
scribed and named, and all of them belong to the genus Isoetes. They
have the general appearance of a small tufted rush and grow on muddy
flats or in the water. They are perennial, and native to north temperate
regions. The plant has a bulb-like axis consisting of an upper leaf-
bearing part and a lower part from which lateral roots grow. The leaves
are narrow and grass-like, with a bulbous clasping base, rarely more
than a few inches long. Isoetes and the grape fern, Botrychium, are the
only modern pteridophvtes in which the stem uniformly has a cambium.
Each leaf of Isoetes is a potential sporophyll. The plants are heterospo-
rous, and as many as 300 megaspores or 300,000 microspores may be
formed within a single sporangium. The spemis are multiciliate, but the
other reproductive structures are similar to those of Selaginella.

The Equisetums

The equisetums constitute another small group of about 25 living
species that superficially bear little resemblance to the ferns. Their life

674 TEXTBOOK OF BOTANY

histories, however, are very similar. Like the ferns and lycopods, they
are the modern descendants of an ancient phyhmi of plants. During the
Carboniferous period equisetum-like plants with trunks 90 feet in height
and 3 feet in diameter formed extensive forests. Fossil equisetum-
relatives have been found also in rocks of the Devonian age. The mod-
ern species are usually less than 3 feet high, although there are two
tropical species that may grow to a height of 10 or 15 feet, and a South
American species that is said to attain a height of 40 feet when partly
supported by trees.

Equisetums occur today in the tropics and semi-tropics, and in the
temperate and arctic zones. None has as yet been reported from
Australia or New Zealand, from the islands of the Indian Ocean, or from
Antarctica. The plants grow best in mildly acid habitats; in the north
they are likely to be found in peat bogs. Elsewhere they are found gen-
erally in sandy areas, particularly along streams and rivers, or where
there is an underground source of water. One species, Equisetum kansa-
num, is found in prairie patches and the more moist situations of the
plains.

The horsetails — or scouring rushes, as they are often popularly called
— have columnar, upright, jointed stems, externally fluted and internally
characterized by long, tubular air cavities. The central axis of the young
stem is pith, but this soon disappears, and the older stem is hollow. Lat-
erally fused colorless scales occur in whorls at the nodes (Fig. 318).
Photosynthesis takes place in the chlorenchyma of the stem. Some of
the ancient fossil species had true foliage leaves. Stomates occur regu-
larly in the furrows of the stems, and their arrangement is used in the
classification of the various species.

The name horsetail is probably suggested by the brush-like character
of the numerous slender whorled branches occurring on the upright
stems of a few species. The name scouring rush refers to the accumula-
tion of silica in the walls of the stem tissues. In pioneer days these were
gathered and used for scouring metal utensils. The roots are small and
adventitious, and arise along the perennial rhizomes, mostly at the nodes.

The sporophyte. The plant described above is the sporophyte phase of
Equisetum. It is perennial, although the erect aerial branches may be
annual. The peculiar shield-shaped sporophylls are arranged in whorls
within a terminal cone and each bears five to ten sporangia.

Within the sporangia are numerous spores which are peculiar in hav-
ing four long appendages that coil around the spore when moist and

[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS

675

Fig. 318. Three widely distributed species of Equisetum: A, silvaticum; B,
hiemale; and C, arvense; D and E, underground tubers; F, spores with appendages;
G, prothaUus.

676 TEXTBOOK OF BOTANY

uncoil when dry. The spores contain chlorophyll, are short-lived, and
cannot withstand desiccation. Consequently, they survive longest and
germinate best on shaded moist banks.

The gametophyte. The gametophytes of equisetums are irregularly
lobed, thalloid structures growing on moist substrates near streams or
in bogs. The spores from the sporangia are essentially alike in appear-
ance, and the gametophytes of most species, if not all, are potentially
bisexual. But the expression of sex in bisexual gametophytes is influenced
by environmental factors, and may be controlled by experimental condi-
tions. Some species such as E. arvense, which may be bisexual under
certain conditions, are usually unisexual. Archegonia usually develop on
the larger gametophytes, and antheridia on the smaller ones. Fertiliza-
tion takes place when a motile sperm fuses with an egg. The resulting
zygote germinates at once, and an embryo sporophyte is formed.

Summary. The ferns, club mosses, quillworts, and equisetums are
classified in one group, the pteridophytes. The sporophyte of these
plants, in sharp contrast to liverworts and mosses, has a vascular system
and is much more conspicuous than the gametophyte. The vegetative
phase of the sporophyte generally consists of definite leaves, stems, and
roots; but modern pteridophytes rarely bear seeds. The gametophyte is
generally thalloid; it may be wholly green and grow on the soil or in
other moist situations; it may be subterranean, devoid of chlorophyll,
and hence saprophytic; or it may be partly green and partly non-green.
The terrestrial ferns and the lycopods produce one kind of spore only
(homospory); aquatic ferns and Selaginella produce microspores and
megaspores ( heterospory ) . The spores usually gemiinate on the ground
or in the water, resulting in the formation of a gametophyte separated
from the sporophvte. The megaspore of Selaginella, however, sometimes
germinates in the megasporangium. This is a step in a series of events
that leads to the formation of a seed. Geologically the pteridophytes are
very ancient.

REFERENCES

Blomquist, H. L. Ferns of North Carolina. Duke Univ. Press. 1934.
Brown, W. H. The Plant Kingdom. Ginn and Company. 1935.
Clute, W. N. The Fern Allies. Willard N. Clute and Company. 1928.
Fames, A. J. Morphologij of Vascular Plants. McCraw-Hill Book Company,

Inc. 1936.
Frye, T. C. Ferns of the Northwest. Metropolitan Press. 1934.
Schaffner, J. H. Papers on Equisetums in American Fern Journal. 1930-36.

[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS 677

Small, J. K. Ferns of Tropical Florida. Published by the author. 1918.
Smith, G. M. Cryptogamic Botany. Vol. 2. McGraw-Hill Book Company, Inc.

1938.
Wherry, E. T. Guide to Eastern Ferns. The Science Press Printing Company.

1937.

CHAPTER L

THE SEED PLANTS
( SPERMATOPHYTES )

<£><><><><><c><X><^<><J><><J><XX

In most places throughout the world the seed plants constitute the
conspicuous part of the land flora. They may be magnificent trees
over 300 feet high with trunk diameters of 25 to 50 feet. They may
be vines that climb to the tops of the tallest trees, or that extend hori-
zontally for distances approaching a quarter of a mile. The majority

Fig. 319. The longest diameter of this largest cypress (Taxodium mucronatuni)
is more than 50 feet. This tree is in a churchyard in the village of Santa Maria del
Tule, Oaxaca, Mexico. Photo by Don Classman. Courtesy of New York Botanical
Carden.

678

[Chap. L THE SEED PLANTS 679

of the seed plants are herbs, commonly less than a few feet in height and
rarely over 25 feet. The smallest seed plant known is a floating duck-
weed (Wolffia) scarcely more than l/20th of an inch in diameter.

Many seed plants complete their life cycles in a single season, others
in 2 to 30 seasons; the remainder flower annually over a long period of

Fig. 320. A large American elm at Lancaster, Massachusetts.

years. The oldest living things on earth are trees. The Big Cypress
Tree of Tule ( Fig. 319 ) and the giant sequoias of California have lived
for 3000 to 4000 years. Elms (Fig. 320) and maples rarely live longer
than 300 years, oaks 500 years, and Douglas fir and western arbor vitae
700 to 800 years.

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TEXTBOOK OF BOTANY

Seed plants are by no means restricted to land habitats; but if aquatic
they are generally confined to shallow waters of swamps and marshes
and along shores of lakes and rivers. They never grow in the open
waters of oceans or deep parts of lakes. Many flowering plants cannot
survive in deserts, but a few species grow in the most arid regions on
the continent. They are absent from polar areas and alpine heights
that are covered by ice and snow most of the year (Fig. 321).

Fig. 321. Timberline on Mt. Edgecumbe, near Sitka, Alaska. Note valleys and
avalanche tracks. Below timberline is the hemlock-Sitka spruce forest; above is
the tundra. Altitude of mountain, 3,465 ft. Alaskan Aerial Survey Expedition, 1929.
Photo from U. S. Forest Service.

The spermatophytes may be distinguished from all other plants by
the production of pollen tubes and seeds. They are either gymnospemis
or angiosperms. The seed of the gymnosperm is usually not enclosed by
any tissue corresponding to the carpel, and is said to be exposed or
naked. The seed of the angiosperm on the other hand is surrounded by
carpels.^

The gymnospemis are represented by the palm-like cycads ( Fig. 322 ) ,
the Ginkgo (Fig. 323) with its dichotomously veined leaf, the shrubby
Ephedra, the shrubby or vine-like Gnetum, and the more common

^ The so-called berries of cedars, yews, and arbor \'itae which are gymnosperms super-
ficially resemble fruits. Certain buttercups which are angiosperms have seeds incompletely
enclosed by carpels.

Fig. 322. Cycads {Cijcas revoluta) in a temple garden in Chimizu, Japan. From
Wieland's American Fossil Cycads, photo by Field Museum of Natural History.

Fig. 323. Ginkgo tree, now native only in China. In former geological periods there
were several species of ginkgo widely distributed in North America.

681

682

TEXTBOOK OF BOTANY

conifers, such as the pine, spruce, hemlock, cedar, sequoia, and aura-
caria. No herbaceous gymnosperms are known. The angiosperms may-
be herbs, shrubs, or trees, and are classified as monocots and dicots. The
gymnosperms will be described first.

The Gymnosperms: Conifers

The gymnosperms are woody perennials, and nearly all of them are
evergreen. Of the approximately 500 species, about 350 are conifers,
among which are the oldest and largest of living plants. They are
widely distributed from the tropics to the tundra border and the tree
line on mountains. In contrast to the pteridophytes they bear seeds, have
deep and well-developed root systems, much-branched stems, and ex-
tensive leaf surfaces. The buds of conifers are scale-covered; they may
be dormant or grow similarly to those of angiosperms. The pine may
be used to exemplify the life cycle of the group.

The pine sporophyte. The pines are generally recognized by their ex-
current branching, needle-like leaves usually in clusters of two to five

Fig. 324. Cross and longitudinal sections of an Austrian pine needle. An
endodermal sheath one cell thick surrounds the centrally located veins. The
chlorenchyma, in which several resin ducts occur, is composed of peculiarly lobed
cells. The stomates are in pits, which are bordered by sclerenchyma, located just
beneath the epidermis. Section B was cut in the plane a-b of section A.

on dwarf branches, and their evergreen habit. Leaves may remain on
the tree from 3 to 15 years, but their photosynthetic value decreases
rapidly with age. The linear leaves have a thick cuticle, sunken stomates.

THE SEED PLANTS

683

[Chap. L

and two or three rows of sclerenchymatous cells beneath the epidermis
(Fig. 324).

The stem of the pine is quite similar to a woody dicot (Chapter
XXVII) except for the absence of xylem vessels. The pine tree with
its leaves, branched stem, and root is the sporophyte.

The sporophylls of the pine are spirally arranged in cones or strobili
(sing, strobihis). The cones are of two kinds: staminate and ovulate,
both occurring on the same plant (Fig. 325).

Fig. 325. A branch of Austrian pine in June. At the tip of the youngest stem
segment are two small ovulate cones. At the tip of the next stem segment are two
one-year-old ovulate cones, and below them is a two-year-old ovulate cone. On
the branch to the right is a cluster of staminate cones.

The staminate cone. The staminate cones are comparatively small and
develop in clusters near the base of the new stem soon after the be-
ginning of the growing season. They live for a few weeks only, and
wither and fall from the tree after the pollen is shed. The cone con-
sists of an axis on which membranous cone scales, or "microsporo-
phylls," are arranged spirally. Each microsporophyll contains on the
under side of its scale-like portion two microsporangia (pollen sacs).
Within the microsporangia are the microspores which later become
pollen grains.

Microspore mother cells are formed in the scarcely differentiated

684 TEXTBOOK OF BOTANY

cones in the terminal buds, sometimes as early as the autumn preceding
the spring in which the cones mature. Early in the spring the tetrad
of microspores is formed from the mother cell, and the number of
chromosomes becomes monoploid. At first the microspore is a cell with
a single nucleus. It germinates about a month before the pollen is shed,
and nuclear and cell division results in a generative cell and a tube
cell." The outer spore wall of the pollen grain has by this time enlarged
and become separated from the inner wall, forming two balloon-like
sacs. The cells within the pollen constitute the male gametophyte of the
pine.

Pollination. At this stage the pollen sacs dehisce and the released
pollen grains are blown about by the wind. Pollen grains are produced
in prodigious numbers, and their yellow color gives the soil and objects
nearby the appearance of having been sprinkled with powdered sulfur.
A few of these pollen grains fall upon the ovulate cones in which the
cone scales are at this time somewhat separated, and some of them
come in contact with the megasporangia.

The ovulate cone. The larger, or ovulate, cones develop singly or,
more often, two or three close together near the upper end of the new
stem segment. Each cone consists of an axis upon which the cone
scales, or "megasporophylls," are spirally arranged. The sporophylls of
the young cone are small green scales that enlarge and become woody
at maturity. The ovulate cones remain on the trees for nearly two years
and on the trees of some species indefinitely.

Each megasporophyll has on the upper side of its scale-like portion
two megasporangia or ovules. The ovule consists of an oval body of
tissue (nucellus) enclosed by a single integument containing an open-
ing (micropyle) near the base of the sporophyll.

In each ovule of the very young ovulate cone a megaspore mother
cell becomes difl^erentiated, and upon two successive divisions there re-
sult four megaspores arranged in a row. Reduction division occurs
when the spore mother cell divides. Each megaspore has the monoploid
number of chromosomes. The megaspore most distant from the micro-
pyle enlarges as the other three disintegrate, and later the female game-
tophyte develops from it within the ovule.

Gametophytes and fertilization. Pollination occurs in May or June

' The nucleus of the microspore first divides into two vegetative nuclei, one of which
degenerates. The other daughter nucleus then dixides, and one of the resulting nuclei
together with its cytoplasm becomes the antheridial cell; the other nucleus degenerates.
It is from this antheridial cell that the generative and tube cells are formed.

[Chap. L THE SEED PLANTS 685

about the time the megaspore is developing within the nucellus. Soon
after poHination, the scales of the ovulate cone close and the whole
cone becomes inverted. Sometimes the outer surface of the cone is prac-
ticallv sealed by exuded resin.

There is little development of the male gametophyte during the rest
of the year, except the formation of a very short pollen tube from each
pollen grain. The tube does not grow through the nucellus until the
following spring, although the distance is extremely short. In the mean-
time the generative cell has divided into two cells, one of which divides
and forms the two sperms.

During this first year the megaspore has undergone considerable de-
velopment, having formed a body of tissue often called "endosperm."'
This body of tissue is in reality the female gametophyte. At the end of
the gametophyte nearest the micropvle two or three archegonia develop

Fig. 326. Diagram of a vertical section of a pine ovule and the scale to which
it is attached and of the male and female gametophvtes at the time of fertilization:
pr represents the prothallus with two archegonia; //) represents the integument, rut
the nucellus, m the micropyle, and p pollen tubes, two of which have reached
the neck cells of the archegonia. Redrawn from Strasburger.

(Fig. 326). When mature, each archegonium consists of eight small
neck cells, a ventral-canal cell, and an egg. When the pollen tube grows
between the neck cells to the egg, it swells and bursts and the two
sperms are discharged. One spenn disintegrates and the other fuses
with the egg.

The fertilized egg or zygote with its diploid chromosome comple-
ment is the beginning of a new sporophyte. Its development is de-

^ Not to be confused with the real endosperm of flowering plants. The cells of the pine
"endosperm" are monoploid; tliose of the corn endosperm are triploid.

686

TEXTBOOK OF BOTANY

pendent upon the food that comes through the female gametophyte
and the nucellus. Pine embryos multiply vegetatively during their early
stages of development horn, the zygote; a young pine ovule may con-
tain four or more embryos (Fig. 327), but usually only one of them is

Fig. 327. The formation of the embryo of a pine from the fertihzed egg (A)
to the development of four rudimentary embryos. Only one of the embryos may
survive in the mature seed. After Buchholz. Courtesy of the World Book Co.

fully developed in the mature seed. It is usually about a year from
fertilization to the fullv formed embrvo.

The seed. Two years have thus elapsed since pollination. In the mean-
time the integument has grown and become the seed coat, the nucellus
has been reduced to a membranous layer, and the female gametophyte
("endosperm") has continued to grow (Fig. 328). The embryo consists

FEMALE GAMETOPHYTE
EGG IN ARCHEGONIUM
POLLEN TUBE
EMBRYO

WING

EMBRYO

FEMALE GAMETOPHYTE
(ENDOSPERM)

Fig. 328. Diagrammatic representation of the forerunners of the various parts

of a pine seed.

[Chap. L THE SEED PLANTS 687

of the plumule, many cotyledons, and the hypocotyl, at the end of which
is the root primodium. The seed thus consists of the embryo, female
gametophvte, perisperm (nucellus), and integument.

In late autumn or early winter, the tissues of the ovulate cone become
dry, the sporophylls curl outward, and the seeds are liberated. The
seed has a thin membranous wing derived from the upper part of the
scale on which it develops.

Summary. The principal steps in the life history of the pine are: (1)
the formation of microspores and pollen in microsporangia of the stami-
nate cone; (2) the development of two sperms in the pollen grain; (3)
dehiscence of the microsporangia and liberation of the pollen grains;
(4) the formation of megaspores in megasporangia (ovules) of the
ovulate cone; (5) development of the female gametophyte from the
megaspore within the ovule; (6) formation of archegonia, each con-
taining an egg cell; (7) pollination and growth of the pollen tube into
the archegonium; (8) union of a sperm with the egg, resulting in a
zygote; (9) development of embryo from the zvgote, further growth of
the "endosperm," and formation of seed coats from ovule coats.

The pine tree is a sporophyte. The male gametophyte is a simple
structure composed of the cells within the pollen grain. The female
gametophyte is a small mass of tissue within the nucellus of the ovule.
The gametophytes of the pine, in sharp contrast to those of liverworts,
mosses, and some ferns, are never green and are parasitic upon the
sporophyte. Flagellate sperms occur in C)cads, but not in the other
gymnosperms (Fig. 329).

Classification of gymnosperms. The living gymnosperms include four
orders:

Cycadales ( cycads ) : Zamia, Dioon, Macrozamia
Ginkgoales: Ginkgo (maidenhair tree)
Gnetales: Titmboa, Gnetum, Ephedra

Coniferales: Pintis, Larix, Taxus, Picea, Thuja, Sequoia, Juniperus,
Podocarpus, Auracaria

The Angiosperms: Flowering Plants

About 150,000 flowering plants have been described. This is about
half of all the plants known. They have the greatest diversity of vege-
tative and reproductive structures. They may be found in all land and
shallow water habitats, except the permanent snow and ice fields and

688

TEXTBOOK OF BOTANY

Fig. 329. Longitudinal section of a part of an ovule of a cycad (Diodn) at time
of fertilization. The reproducti\e structures represented are the pollen tubes in
the nucellus; the spirally ciliated sperms: and two archegonia. one (right) with
egg, the other (left) after union of sperm and egg nucleus. Surrounding the
archegonia is the female prothallus. Drawing by C. J. Chamberlain.

the driest deserts. The vegetation of prairies, plains, deserts, most tropi-
cal forests, and the deciduous forests is dominated by angiosperms.
Furthermore, the undergrowth of most conifer forests contains flower-
ing plants. Our most valuable crop and forage plants and our most
annoying weeds are angiosperms.

The seeds of angiosperms are enclosed in ovularies. Variously colored
bracts and floral lea\'es appear in whorls around the spore-bearing

[Chap. L THE SEED PLANTS 689

stamens and carpels. The floral organs are of many forms, colors, and
arrangements ( Chapter XXXII ) .

The life cycle. The earlier chapters of this book include discussions of
the physiological processes, the structures, reproduction, and heredity
of flowering plants. The life cycle of angiosperms may now be briefly
compared with that of other great groups of green plants. As would be
expected, the details of the life cvcle are not the same in all plants.

The sporophvte. The familiar plant of angiosperms, such as tulip,
wheat, or maple, is the sporophyte. Leaves, stems, roots, and flowers
are conspicuous in most species. A perianth, usually composed of calyx
and corolla, surrounds the microsporophylls (stamens) and the mega-
sporophylls ( carpels ) .

The microspores form in the anther and become pollen grains upon
nuclear division. The dehiscence of the pollen sacs liberates the pollen,
and pollination may be effected in various ways. Just prior to being
shed, or sometimes shortly after, the pollen grain contains a tube nucleus
and Hvo sperms formed from a generative nucleus. The pollen with
the pollen tube is the male gametophyte.

The megaspores form in the megasporangia (ovules), which are en-
closed in ovularies. Each ovule ordinarily has one megaspore which upon
nuclear division becomes the embryo sac. Subsequent nuclear divisions
result in an embryo sac with eight nuclei, one of which becomes the
egg; two unite and form a fusion nucleus, and the remainder usually
disintegrate. The embryo sac is the female gametophyte, but no struc-
tures comparable to archegonia occur.

Fertilization. After pollination and germination of the pollen grain on
the stigma, the pollen tube penetrates the tissues of the style and enters
the ovule, often through the micropvle. When the pollen tube enters the
embryo sac and bursts, the two sperms are released and two fusions
occur: one sperm unites with the egg, resulting in a fertilized egg or
zvgote; the other unites with the fusion nucleus, forming the triple-
fusion nucleus.

The embryo. The zygote germinates, and from it an embryo or voung
sporophyte develops. In some seeds an endosperm develops around the
embryo from the triple-fusion nucleus. The growth and hardening of
the ovule coats (integuments) complete the formation of the seed.
The embryo consists of a hypocotyl, a plumule, and cotyledons.

Chromosome constitution. Reduction division in most angiosperms
occurs during the formation of megaspores and microspores. Each of

690 TEXTBOOK OF BOTANY

these structures as well as the egg and sperm contains the monoploid
number of chromosomes. The zygote and every subsequent cell of the
sporophyte have the diploid number of chromosomes. The endosperm
has developed from a nucleus that resulted from the union of three
nuclei, and hence each cell of the endosperm has the triploid iiumber
of chromosomes. The endosperm is neither gametophyte nor sporo-
phyte: it is termed the xeniophyte.

Comparative summary of life cycles of the great groups of green plants.
Several life cycles in each of the great groups of green plants have been
considered. Many of the main features of the structure, growth, and
reproduction of algae, mosses, liverworts, ferns, and their near relatives,
and of seed plants are familiar. If now the fundamental similarities and
differences among the life cycles of the great groups of green plants
can be visualized, we shall have made a start toward understanding
this diverse assemblage of plants as a whole, rather than as individuals
or groups. In addition, such an analysis should form a broader basis
for an appreciation of the evolution of plants.

The following statements are true for a majority of the plants belong-
ing to the groups indicated. Exceptions may be made, however, to all
of them, because simple generalizations can scarcely be formulated for
groups containing thousands of species having the greatest diversity of
form and habitat. Finding and listing exceptions to these generaliza-
tions may prove to be as interesting and profitable as accepting them
as universally true.

1. There are two reproductive phases in the life cycles of most green
plants: a gametophyte, in which gametes develop; and a sporophyte, in
which spores develop.

2. In a complete life cycle (or generation) of the plant the gameto-
phyte regularly alternates with the sporophyte. Zygotes germinate and
sporophytes develop; spores formed in the sporophytes germinate and
gametophytes develop.

3. The gametophyte begins with the spore and ends with the forma-
tion of gametes.

4. The sporophyte begins with the zygote and ends with the forma-
tion of spores.

5. Reduction division occurs during the formation of spores in the
sporophyte.

6. The cells of the gametophyte are characterized by the monoploid

[Chap. L THE SEED PLANTS 691

number of chromosomes in each nucleus, whereas each cell of the sporo-
phyte has the diploid number of chromosomes.

7. Monoploid and diploid numbers of chromosomes are not causes
of the two phases in the life cycle, but are merely associated with them.
Doubling or other multiplication of the number of chromosomes neither
changes the phase nor necessarily prevents the alternation of phases.

8. The conspicuous phase of many algae, the liverworts, and the
mosses is the gametophyte; the conspicuous phase of brown algae, the
ferns, and seed plants is the sporophyte.

9. The sporophytes of the liverworts and of the mosses are partial
parasites on the gametophytes.

10. Both the gametophytes and sporophytes of ferns develop as sep-
arate green plants.

11. The gametophytes of seed plants are parasitic on the sporophytes.

12. Some ferns, some club mosses, and all seed plants have two kinds
of spores: microspores and megaspores.

13. Two distinct gametophytes develop from these spores. From the
germination of microspores there result gametophytes in which sperms
only are formed; and from the germination of megaspores there result
gametophytes in which egg cells only are formed. The size of the spore,
however, is not a cause of the difference in development of the two
gametophytes.

14. In the equisetums the spores are physiologically unlike but sim-
ilar in appearance. From half of them antheridial gametophytes de-
velop, and from the other half archegonial gametophytes result in cer-
tain habitats.

15. In ferns, club mosses, and equisetums the two kinds of spores
germinate on the soil after falling from the parent sporophyte.

16. A megaspore of Selaginella occasionally germinates in the mega-
sporangium, resulting in an enclosed gametophyte with archegonia and
egg cells. The germination of the fertilized egg may result in the de-
velopment of an embryo within this sporangium, and the final result is
a seed. If the megaspore falls to the ground and germinates there, no
seed results.

17. In seed plants the germination of the megaspore occurs within
the ovule ( megasporangium ) , and this is a first step in a series of proc-
esses that results in the formation of a seed.

18. Seeds of flowering plants consist of seed coats, endosperm, and
embryo; or only of seed coats and embryo.

692 TEXTBOOK OF BOTANY

19. The embryo is a voiing stage in the development of any indi-
vidual. Embryos ma\' develop from fertilized eggs or from vegetative
cells. The endosperm is a tissue that develops from a triple-fusion
nucleus. Seed coats develop from the coats of the ovule.

20. The so-called "endosperm" of the gymnosperms is gametophytic
tissue and is not analogous to the endosperm of angiosperms. Its cells
have the monoploid number of chromosomes.

21. In the angiosperms, each cell of the embryo has the diploid num-
ber of chromosomes, and each cell of the endosperm has the triploid
number of chromosomes.

22. The endosperm of flowering plants is neither gametophyte nor
sporophyte, but is an accessorv phase termed the xeniophvte.

REFERENCES

Chamberlain, C. J. The Livififi Ci/cads. Univ. of Chicago Press. 1919.
Chamberlain, C. J. Gymnosperms, Structure and Evolution. Univ. of Chicago

Press. 1935.
Eames, A. J. Morphologi/ of Vascular Plants. McGraw-Hill Book Company,

Inc. 1936.

CHAPTER LI
SOME FAMILIES OF FLOWERING PLANTS

The flowering plants are very numerous both in species and in indi-
viduals, extremely variable in structure, and of great economic impor-
tance. It is not possible in one chapter to discuss more than a few of
the 300 families that compose this great assemblage of plants. The few
that are discussed here are selected on the basis of their abundance,
economic value, or some other feature that makes them of peculiar
interest (Fig. 330).

Fig. 330. The large-leaved magnolia in flower and in fruit. Photo by R. B. Gordon.

The grass family (Gramineae or Poaceae). The progress of man from
the dawn of civilization to the present has been dependent upon the
grasses more than upon other groups of plants. The fact that primitive
man changed from a nomad to a settler was due in large measure to
his discovery that the seeds and fruits of grasses, along with those of
some other plants, could be obtained in greater amounts bv means of
cultivation and after harvest be stored for his use at some future time.
Even the calendar mav have been suggested bv the necessit\' of know-
ing recurring periods connected with cereal agriculture: time of plant-
ing, of cultivating, of ripening, and of harvesting.

693

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TEXTBOOK OF BOTANY

^ I

Fig. 331. Ornamental grasses: on the border, eulalia (Miscanthiis .sinensis); the
taller grass pictured in the background is the giant reed (Arundo donax) .

The human race is dependent upon bread, made usually from grasses.
Meats, such as beef, mutton, pork, and poultry, as well as other animal
products, such as eggs, butter, cheese, and milk, are formed largely
from grasses the animals have eaten. Much of the commercial sugar
supply is a product of another grass, sugar cane.

In addition to their use as food by man and by the other animals,
grasses are important as sources of building materials, fiber and paper
pulp, as stabilizers of soil, and as ornaments ( Fig. 331 ) . In parts of
the Orient the bamboo alone is used to make houses and boats, buckets

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS

695

Fig. 332. Two important grasses of the western plains: blue grama (left) and
buffalo grass (right) in bloom. Photos from U. S. Soil Conservation Service and
F. H. Norris.

and pans, household utensils and agricultural implements, musical
instruments and umbrellas, paper and mattresses, and as food and
medicine.

The plants of the grass famil)- are mostly herbs, although woody bam-
boos may be trees of considerable size. Wild grasses are distributed
throughout the world, and are the dominant plants on the steppes,
plains (Fig. 332), and prairies of all the continents. They occur from
sea level to alpine heights, in shallow water and in the desert, in the
open as well as in the shade, and on both acid and alkaline soils. The
cultivated grasses — corn, wheat, rye, barley, oats, rice, sorghum (Fig.
333), sugar cane — also cover an enormous acreage throughout the
world. There are some 5000 described species of grasses, and the num-
ber of individuals doubtless surpasses that of all other cultivated plants
put together.

The grass flower and its associated parts are highly specialized ( Figs.
334 and 155). The unit flower cluster is the spikelet, made up of two
or more flowers enclosed by bracts, called empty glumes. Each flower
is also enclosed in two flowering glumes. Only remnants of a perianth

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TEXTBOOK OF BOTANY

Fig. 333. A field of milo in Texas. Courtesy U. S. Soil Conservation Service.

Fig. 334. Flower spikes of timothy.

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS

697

are present. The flowers may have three stamens and a pistil, or the
stamens and pistils may occur in different flowers. The fruit of the
grasses is a grain.

Closely related and often confused with the grasses are the sedges,
Cyperaceae (Fig. 335), and rushes, Jiincaceae. The leaves of sedges are

Fig. 335. A large sedge (Carex macrocephala) used to stabilize wind-blown
sand. The plant propagates vegetatively from rhizomes a foot or more below the
soil surface. Comtesy U. S. Soil Conservation Service.

three-ranked on usually solid stems, as contrasted with the two-ranked
leaves and hollow stems of grasses. The flowers of rushes have a six-
parted green perianth. The fruit of a sedge is an akene; that of a rush,
a capsule.

The palm family ( Palmaceae ) . Conspicuous plants of both the tropics
and the subtropics are the palms (Fig. 336). Because of their striking
appearance, they have been termed "the princes of the vegetable king-
dom." Fossil remains indicate that at one time these plants were widely
distributed, even beyond the arctic circle. The two chief centers of
distribution of modern palms are tropical America and tropical Asia,
with a lesser third center in tropical Africa. The present distribution
of wild species of palms in the United States is limited to the south-

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TEXTBOOK OF BOTANY

eastern coastal plain and California. Of the 1200 described species,
some 17 grow naturally in the United States.

Fig. 336. Coconut palm in fruit in Florida. Photo by W. M. Buswell.

The palms are important sources of food, supplying dates, coconut
"meat" (endosperm), and "cabbage" (tenninal buds), as well as other
materials such as thatch, fans, timber, fiber, oils, liquor, vegetable ivory, ^
sago, raffia, and rattan. Some of the palms rival the bamboos in the
multiplicity of uses man makes of them.

The stems of palms are usualh' unbranched. The columnar trunk
surmounted only by a terminal crown of leaves has a striking appear-
ance in such trees as the royal palm. The leaves are either pinnately or
palmately veined, or have some combination of these venations. If the

^ Horny endosperm from the seeds of the elephant palm is known as vegetable ivory.

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS 699

absciss layer occurs at the base of the petiole, the trunk of the tree
is quite smooth. If it occurs at some distance from the base of the petiole,
the leaf bases remaining give to the trunk a rough, shaggy appearance.

The lily family (Liliaceae). The yuccas and aloes of the desert; the
edible onions, garlic, and asparagus; and the well-known ornamental
tulips, hyacinths, lilies, and trilliums are members of the lily family.
All except a few are perennial or biennial herbs. One of the chief char-
acteristics of the plants is the presence of an underground bulb or
corm, or sometimes a fleshy rootstock.

The lilies are widely distributed naturally, and many species are cul-
tivated in enormous numbers. Few families of plants have more at-
tractive flowers. The flowers may be single or variouslv grouped into
racemes, panicles (Fig. 337), or umbels. Thev usuallv have three petal-
like sepals, three petals, six stamens, and a single three-carpellate pistil.
The fruit is a capsule" containing few to many seeds. Some 200 genera
and 2500 species have been described.

Closely related to the lilies is the amaryllis family {Amari/llidaceae),
which includes amaryllis, narcissus, and agave.

The pineapple family (Bromeliaceae) . The plants of the pineapple
family are largely epiphytes or air plants and are confined to the tropics
and subtropics. The pineapple and a few others grow on soil ( Fig. 163 ) .
The "Spanish moss" is a rootless plant which forms festoon-like masses
on all sorts of plants, such as oaks, pines, palms, and orange trees. This
plant and others, more like the pineapple in appearance, may grow not
only as epiphytes, but on wires and fences as well. There are about
40 genera and 1000 species.

The commercial "pineapple" is a flesh v multiple fruit. The terminal
floral axis above the flowers develops into a leafv stem, and this is used
as a cutting in propagating pineapples. The floral parts, bracts and axis
together constitute the "pineapple."

The banana family {Musaceae). This familv includes large-leaved
herbs 15 to 20 feet high. The sheathing leaf petioles overlap, forming
a cylinder of concentric layers that looks like a stem. The real stem is
a tuberous rhizome from which the flower stalks develop and extend
upward through the hollow cylinder fonned by the sheathing petioles.
The erect shoot bears one bunch of bananas and dies. New plants de-
velop from suckers. The banana of commerce never contains fully

- Asparagus, lily-of-the-\alley, and false Solomon's seal are often placed in a separate
family (Convallariaceae); in such plants the fruit is a many-seeded berry.

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TEXTBOOK OF BOTANY

»^ .

Fig. 337. An arborescent yucca. Photo from U. S. Forest Service.

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS

701

developed seeds. The family is composed of 6 genera and some 60
species.

The banana is a plant of the tropics and warmer subtropics and
takes the place of bread and potatoes in the diet. It is also a source of
medicine, of a fermented drink, and of fiber, such as the manila hemp
of commerce (Fig. 338). The \'ield of bananas is surprisingl\- large,

Fig. 338. One of the cultivated varieties of banana.

often as much as 100 tons to the acre as compared with 2 tons of
potatoes. A single bunch of ripe bananas may contain over 200 fruits
and sometimes weighs 175 to 200 pounds. Bananas often grow in green-
houses in northern latitudes, but the fruits of these plants are not as
large or as palatable as those of plants in the tropics.

The orchid family (Orchidaceae). The orchids constitute the most
highly specialized family of the monocots. They are perennial herbs
largely terrestrial in temperate regions, but predominantly epiphytic,
with aerial roots, in the tropics; some are saprophytic, or parasitic on
the roots of other plants. They may be erect or occasionally trailing,
and in most cases have fleshy roots, rootstocks, corms, or bulbs. In
saprophytic and parasitic species the leaves are mere scales; in all
others the leaves are conspicuous, green, and usually alternate.

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TEXTBOOK OF BOTANY

The flowers of orchids are remarkable as to both form and attractive-
ness (Fig, 339). The three sepals are either green or the same color
as the corolla. Two of the three petals are "wings." The third petal is
below the other two and may be inflated, spurred, or variously shaped;
it is known as the "lip." The masses of pollen and the lobes of the stigma

Fig. 339. Unique flowers of a cultivated orchid (Cypripedium) .

are often viscid, and this along with structural modifications facilitates
pollination by certain insects.

The fruit is a capsule, and the tiny seeds have an undifferentiated
embryo, but no endosperm. Some 700 genera and nearly 15,000 species
of orchids have been described. The plants are cultivated for their mag-
nificent flowers ( Fig, 340 ) .

The willow family (Salicaceae) . The willows and poplars constitute
this family; although widely distributed throughout the world, they are

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS

703

Fig. 340. A tropical epiphytic orchid (Dendrobiiim) in cultivation.

definitely plants of northern latitudes. Both willows and poplars are
ancient plants, as is evidenced by the finding of fossils in the rocks of
the Cretaceous period. They have hvbridized freely and some present-
day species are difficult to classify. There are nearly 200 species which
vary in size from low creeping shrubs to large trees.

The poplars are all trees, but the willows are both trees and shrubs.
The arctic willows may be only a few inches in height. The simple
naked flowers are borne in catkins, and the staminate and pistillate
flowers occur on separate plants. The plants are commercially valuable
chiefly as sources of paper pulp, baskets, and some kinds of furniture.
They are often planted where their quick growth may prevent erosion
or supply shade until more desirable trees can develop.

The walnut family {Juglandaceae) . The chief representatives of the
walnut family are walnuts, butternuts, hickories, pecans. There are 6
genera and about 35 species. The species of walnut were more numerous
during the Cretaceous period than now. The plants are trees, some in-
dividuals reaching heights of 150 feet and diameters of 5 to 6 feet. The
wood of the walnut family is especially valuable for furniture, ax han-

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TEXTBOOK OF BOTANY

dies, and wagon parts. Various species of the walnut family are culti-
vated for their nuts and as ornamentals (Fig. 341).

Fig. 341. Catkins of staminate flowers on a branch of the bhick walnut

(Jiiglans nigra).

The birch family ( Betulaceae ) . This well-known family has 6 genera
and some 100 species. The commonest representatives are birches, al-
ders, iron wood, hornbeam, hazelnuts, and filberts; and some of these
occur as fossils in the Cretaceous rocks. The plants are either trees or
shrubs varying from a few inches in height in some arctic birches to
occasional specimens 120 feet high and 3 or 4 feet in diameter. The
bark is often thin and mav be removed in papery sheets.

The birch familv is especiallv valued for its superior wood (furni-
ture, interior finishing, floors, athletic goods), its bark (canoes, baskets),
its nuts, and its ornamental uses. The inner bark is quite nutritious and
important in the food of certain wild animals.

The beech or oak family (Fagaceae). In many localities within the
deciduous forest areas the beech, oak (Fig. 342), and chestnuf^ are
some of our best-known trees. The trees are often of magnificent pro-
portions, well over 100 feet in height and with trunk diameters of 6

^ Because of the ravages caused by the chestnut bhght this statement is no longer true,
since the chestnut has been practically destroyed by the disease.

SOME FAMILIES OF FLOWERING PLANTS

705

[Chap. LI

to 12 feet. There are 5 genera and about 600 species. Some of the
oaks have hybridized freely and are extremely difficult to classify. The
members of this family also apparently had their origin during Cre-
taceous times.

Fig. 342. Catkins of staminate flowers on a branch of white oak {Qiiercu.s alba).

Economically the family is probably the most valuable of all woody
flowering plants. The wood is particularly durable and is important in
making furniture, floors, ships, posts, railroad ties. The plants yield
nuts and cork, and many are used for ornamental planting.

The elm family ( Ulmaceae ) . The family includes some 13 genera and
140 species, of which the elms and hackberries are the best known.
The plants are either shrubs or trees, common in the subtropics and
in temperate regions. The elm is especially valued as a shade tree, al-
though its existence is at present threatened by parasitic fungi and
viruses. The trees are not particularly valuable for their wood.

Related plants, and by some of the early authors included in this
family, are nettles, mulberries, figs, hemp and hops.

The mistletoe family ( Loranthaceae ) . This curious family consists of
about 20 genera and 500 species, chiefly tropical and subtropical but
occasionally found in temperate regions. The plants are either com-
pletely or partly parasitic on other woody plants. They are usually less
than a foot in height and often appear to be dichotomously branched.
The root is a modified haustorium and through it part or all of the

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TEXTBOOK OF BOTANY

food supply is secured from the host. The plants are often evergreen
and may be found on elms, poplars, oaks, mesquite, pines, larches, and
many other genera. A few bear scales without chlorophyll, and hence
are strictly parasitic. The mistletoes are of little commercial value except
as curiosities and as the cause of sUght damage to the host plant (Fig.
343).

Fig. 343. A branch of mistletoe (Phoradendion) with fruit.

The crowfoot family (Ranunculaceae) . The buttercups, anemone,
peony, larkspur, and columbine are plants of the crowfoot family. They
are nearly all herbs and generally perennial. These plants are not of
great economic importance, being desired by man chiefly for their
flowers. Cattle are poisoned by larkspur; a few useful drugs, such as
aconite and cimicifuga, are obtained from some species. There are
about 30 genera and 1200 species.

The mustard family {Brassicaceae or Cruciferae). The plants of this
important family are widely distributed in temperate climates. Here
belong the cultivated cabbage, cauliflower, turnip, radish, and cress;
the ornamental wallflowers, candytuft, and stocks; and numerous com-
mon weeds, such as wild mustard, shepherd's purse, pepper grass. The
cultivated species and weeds are of old-world origin.

The mustards are generally recognized by the arrangement of the
four petals into a Maltese cross, the pungent taste, and the four long
and two short stamens. From a simple, small, rosette-like ancestor the
common cabbage, broccoli, kale, kohlrabi, brussels sprouts, and cauli-

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS 707

flower have been derived. It has recently been found that members
of the cabbage group are richer in vitamin C than citrus fruits. There
are about 2000 species distributed among 200 genera.

The rose family (Rosaceae). The members of the rose family are
world-wide in distribution, and economically valuable for both their

Fig. 344. Floral structures of a rose {Rosa moschata) .

flowers and their fruits. They are perennial herbs, shrubs, or trees. The
flowers are usually bisexual, with five sepals, five petals, and often
numerous stamens and carpels ( Fig. 344 ) . There are many horticultural
varieties in which the number of petals is increased many times, as in

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TEXTBOOK OF BOTANY

roses, cherries, and spiraeas. The fruit is variable: it may be akene,
pome, drupe, or some compound fleshy structure.

The rose family is important not only because of its beautiful flowers
but on account of its fruits. Apples,^ cherries,^ plums,' peaches,^ straw-
berries, blackberries, raspberries, loganberries are much-prized fruits;
and their cultivation, preservation, and marketing yield an annual in-
come of many millions of dollars in the United States alone. There are
about 100 genera and some 1900 species in the family.

The pea family ( Papilionaceae ) . The members of this family belong
to the great group of dicots known as legumes, and are sometimes re-
ferred to a single family, the Leguminosae, with about 550 genera and
over 12,000 species. The legumes rank second to the grasses as sources
of food. They are cosmopolitan in distribution and are represented by
such herbs as clovers (Fig. 345), alfalfa, peas, beans, lupine, peanut

Fig. 345. White clover in bloom.
* Apples, hawthorn, mountain ash are often included in the apple family.
5 Cherries, plums, peaches, almonds may be likewise separated from the Rosaceae, form-
ing the peach family ( Drupaceae ) .

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS

709

(Fig. 346), and by such trees as black locust, honey locust, Kentucky
coffee tree. About two-thirds of the legumes belong in this family, the
others to the nearly related mimosa and senna families.

Fig. 346. The subterranean fruits on a peanut plant. Note also the root nodules.

Nitrogen-fixation occurs in the roots of legumes through symbiotic
relationships with certain bacteria (Chapter XLIII). The pea flower
has been discussed in Chapter XXXII. Legumes contain much protein
and thus are important food of animals, supplementing the largely
carbohydrate seeds and fruits of the grasses. Legumes are yery yalu-
able as both forage and hay crops. They are important sources of oils,
honey, drugs, and timber, and some species such as sweet peas have
decorative value.

The recent rise in importance of the soybean in the United States
deserves special mention. In 30 years the acreage devoted to soybeans
has increased from about 50,000 to nearly 6 million. These plants have
been extensively cultivated in China for centuries, but were not intro-
duced into this country until 1804. The uses of the soybean are astound-
ingly varied: pasture, silage, soil improvement; celluloid substitutes.

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TEXTBOOK OF BOTANY

fertilizers, glue, stock feed, water paints; candles, disinfectants, glycerin,
enamels, soaps, rubber substitutes from soybean oil; human foods in-
clude breakfast foods, flour, vegetable milk, butter substitutes, ice cream
cones, coffee substitutes, candies, chocolate, soy sauce, and seasoning
powders.

The cactus family (Cactaceae). The cactus family comprises about
100 genera and 1500 species. The plants appear to have originated in
Mexico and Central America and to have migrated both north and south
into dry subtropical and temperate regions. Only one genus of cactus

Fig. 347. A cactus (Opimtia) in bloom.

(Rhipsalis) is native to Asia and Africa; it is a tropical epiphyte. Most
cacti are succulents and are to be found in semi-arid areas. They include
such familiar plants as the prickly pear (Fig. 347), night-blooming
cereus, sahuaro, cholla, barrel cactus, and organ-pipe cactus.

The stems are thick and succulent, reaching a maximum height of
some 40 or 50 feet in the sahauro; thev are invariably spiny, and often
leafless. The roots are generally shallow, but widely extended in the
upper few inches of soil. The stems are green and contain large water-
storage tissues. The amount of water that accumulates in a large-sized
cactus may be as much as 30 tons, and the water is frequently 70 to

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS 711

90 per cent of the weight of the entire plant. The cattlemen and the
Indians of the southwest have long used cacti for stock feed, first burn-
ing off the spines and prickles.

The giant sahuaro (Fig. 348) lives 150 to 175 years, although the
absence of annual rings makes it impossible to ascertain the age with

Fig. 348. Giant sahuaro (Carnegia) in the vicinity of Tucson, Arizona. Old
specimens are favorite nesting sites of woodpeckers. Photos by A. E. Waller and
W. S. Cooper.

accuracy. It grows only about a foot the first 20 or 25 years, but after
that it may increase in height as much as three or four inches each
year. Its large flower is white and occurs near the top of the trunk and
at the tip of each branch. The fruit is quite succulent and is eagerly
devoured by animals. Consequently reproduction of the plant from
seed in nature is more nearly the exception rather than the rule.

The maple family (Aceraceae). The maples (Fig. 349) are favorite
shade trees among many people who live in the deciduous forest area.
They are especially common in the eastern half of the United States,
and like so many of our trees they are known as far back as the Cre-
taceous period. Most members of the family are trees, but some are

712

TEXTBOOK OF BOTANY

shrubs. Some species are magnificent specimens more than 100 feet in
height and with trunks up to 4 feet in diameter. The lumber of the
hard maple is strong, close grained, and very valuable. Some \'arieties
develop the so-called "bird's-eye" grain, so much sought after for furni-
ture and musical instruments. The wood is also prized as fuel, and from
the sap of the hard maple are obtained maple sugar and s\ rup.

Fig. 349. Leaves, flowers, and fruits of silver maple.

The carrot family ( Umbellifeme or Ammiaceae ) . The plants of this
large family are predominantly herbaceous perennials, though biennials
and annuals occur. The 275 genera and 3000 species are widely dis-
tributed. Common examples are carrot, parsnip, celery, water hemlock,
caraway, anise, golden alexander. The roots are often fleshv, and weeds
of this group are difficult to eradicate. The older name of the family —
Umbelliferae — describes the type of inflorescence: the umbel (Fig.
350). The plants are important sources of drugs and oils, as well as

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS 713

food. Hemlock, poison parsnip, conium, and a few others are v^ery
poisonous to livestock and sometimes fatal to man.

Fig. 350. Stages in the growth of the flower clusters (umbels) of the wild carrot
(Daiiciis) from buds to mature fruits.

The heath family {Ericaceae). The plants of this family are world-
wide in distribution, except in deserts and the moist tropics. They most
frequently grow on acid soils and are often found in bogs. The heath
family proper is represented by erica (Fig. 351), laurel, rhododendron,
sourwood, azalea, bearberry, and trailing arbutus. The plants are usually
shrubby; but herbs, trailing vines, and a few trees are included. The
leaves are often evergreen, and the flowers of some species are scarcely
exceeded in the plant kingdom for fragrance, luster, and general beauty.
The sourwood is an important source of honev in the southeastern
United States, but except for the esthetic value of some species the
group is commerciallv rather unimportant.

Closely related to and sometimes included in the heaths are the In-
dian pipe familv (Monotropaceae), the wintergreen family {Pyrola-
ceae ) , and the cranberry-blueberry-huckleberry family ( Vacciniaceae ) .
The Indian pipe, the pinesap, and pine drops are herbaceous annuals
without chlorophvll, and thus represent the relatively few non-green

714

TEXTBOOK OF BOTANY

Fig. 351. A heath plant (Erica), several species of which are common on the
moors and heaths of Europe. Often cultivated in America.

seed plants. They are parasites on other plants. The huckleberries, blue-
berries, and cranberries are well known for their delicious fruits.

The olive family ( Oleaceae ) . The members of the olive family are all
trees or shrubs. The principal commercial species are the olive, culti-
vated for its fruits; lilac, mock orange, golden bell, and privet, prized as
ornamentals; and species of ash, valued as timber trees of the deciduous
forests.

The madder family (Riibiaceae). This family comprises some 400
genera and over 5000 species, widely distributed. Common examples
of the wild representatives are buttonbush (Fig. 352), the bedstraws,
and the partridgeberry. Commercially valuable plants are coffee, the
ornamental gardenia, and cinchona, the source of the alkaloid quinine.

The morning-glory family (Convolvulaceae) . The members of the
morning-glory family are largely trailing and climbing herbaceous vines.
They are distributed throughout the world, but are most abundant in
the tropics. Familiar examples are the bindweed, morning glory, moon
flower, and sweet potato. The sweet potato with its thickened roots is
commercially the most important plant of the family. The more watery
sweet potatoes are often called "yams," but the true yam is a monocot.
There are some 50 genera and about 1000 species.

The mint family (Labiatae). The appearance and fragrance of most

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS

715

Fig. 352. Flower clusters of button-bush (Cephalanthiis occidcntalis) .

mints make them unusually well-recognized plants (Fig. 353). Some
200 genera and 3000 species are known. Mints are distributed very gen-
erally throughout the world. Common representatives are catnip, hoar-
hound, peppermint, thyme, sage, lavender, coleus. The stems of the
herbaceous species are commonly square in cross section, and many of
the flowers are irregular and "two-lipped." The mints are commercially
valuable for their volatile oils used in flavoring extracts, perfumes, and
medicine.

The potato family (Solanaceae). This family is generally distributed,
but especially abundant in the tropics and subtropics. It consists of
about 80 genera and 2000 species. They are usually herbs, but shrubby
and even tree-like species grow in the tropics. The presence of under-
ground tubers, as in the potato, is unusual in plants of the family.

Some plants are deadly poisonous (nightshade), troublesome weeds
(jimson weed), attractive ornamentals (petunia, jessamine, matrimony
vine), or sources of food (potato, tomato, eggplant, peppers), of to-
bacco, and of powerful drugs, such as atropin and belladonna secured
from the deadly nightshade. The potato in many countries ranks second
only to wheat as an agricultural source of food (Fig. 354).

Fig. 353. Flower spikes of the rouad-leaved mint {Mentha wtimdifoUa) .

Fig. 354. Potato plants in bloom. The flowers usually abscise, and no fruits develop.
When formed, the fruits resemble small green tomatoes.

716

[Chap. LI SOME FAMILIES OF FLOWERING PLANTS 717

The gourd family ( Cucurhitaceae ) . The cucurbits are largely tropical
and subtropical, and are represented by about 100 genera and 800
species. Thev are herbaceous vines, generally tendril-bearing, succulent,
and often hollow-stemmed. Common examples are pumpkin, squash,
gourd, cucumber, cantaloupe, watermelon, and balsam apple. Gourds
have been used for centuries as receptacles for water and foods. The
cucumbers, melons, squashes, and pumpkins are valued for their flavors,
their vitamin content, and their use in pies and preserves.

The sunflower family ( Compositae ) . The sunflower familv, including
the ragweed and chicorv groups, is the largest among flowering plants,
numbering nearlv 1000 genera and 20,000 species. Species of this famib
are found nearlv everywhere that land plants grow. They are mostly
herbs, though a few tropical species are shrubs and trees. The flowers
are generallv small with tubular or strap-shaped corollas, and are ar-
ranged in heads surrounded by circles of green bracts. The fruit is gen-
erallv an akene without an endosperm. Structuralh the plants are the
most specialized of dicots.

A few species of the family are valued as food (sunflower, artichoke,
lettuce, salsify, and dandelion); and as ornamentals (dahlia, aster,
chrysanthemum, daisy). Some are the causes of hav fever (sagebrush
and ragweed), and others are poisonous to livestock (white snakeroot,
the cause of trembles and milk sickness ) . Many of them are extremely
troublesome weeds in pastures and areas under cultivation (cocklebur,
Canada thistle, ragweed, varrow, Spanish needle, dandelion, ironweed ) .

REFERENCES

Castetter, E. F., and W. H. Bell. Ethnobiological studies in the American
Southwest. IV. The aboriginal utilization of the tall cacti in the American
Southwest. Univ. of Neiv Mexico Bull. No. 307. 1937.

Geske, E. J., and W. J. Showalter. Familiar grasses and their Howers. National
Geog. Mag. 39:625-636. 1921.

House, H. D. Wild Flowers. The Macniillan Companv. 1934.

Hylander, C. J. The World of Plant Life. The Macmillan Company. Part 4.
1939.

Pool, R. J. Flowers and Floivering Plants. McGraw-Hill Book Company, Inc.
1929.

Sargent, F. L. Plants and Their Uses. Henry Holt & Company, Inc. 1913.

Saunders, C. F. Useful Wild Plants of the United States and Canada. Robert
M. McBride & Company. 1934.

718 TEXTBOOK OF BOTANY

Shreve, Forrest. The Cactus and Its Home. Williams & Wilkins Company.
1931.

Small, J. K. Palms of the continental United States. Sci. Monthly. 32:240-255.
1931.

Swingle, D. B. A Textbook of Systematic Botany. McGraw-Hill Book Com-
pany, Inc. 1934.

U.S.D.A. Yearbook of Agriculture. 1937.

Numerous manuals containing descriptions of both wild and cultivated plants
are available.

CHAPTER LII
PLANTS OF THE PAST

The enormous numbers of plant species, genera, and families that are
now living on the earth; the almost unbelievable diversity of structural
patterns among the major and minor groups of plants; and the apparent
lack of relationship among the major, and even some of the minor,
groups that we attempt to discover in order to classify plants, should
be evident even from the brief review of the plant world in the pre-
ceding chapters.

At the same time there is an astonishing similarity in the reproductive
processes and structures within any one of the major groups. Among
the more complex plants the very regular occurrence of the two phases
in the hfe cycles indicates a unity of origin, and often definite relation-
ships. In each phase of the life cycle, moreover, there are so many
fundamental similarities among the structural features and the sequences
of stages in development that we can rather truthfully picture and
describe the most important phenomena in the lives of many species,
genera, and families of plants on a few printed pages. Even more im-
pressive are the similarities, in both plants and animals, of such basic
phenomena as reduction division, sex, mitosis, nutrition, general compo-
sition of protoplasm, and a number of other cellular phenomena.

The first paragraph of this chapter emphasizes the heterogeneous
character of the great assemblages of plants and the problems they
present when we attempt to discover relationships and classify them.
The second emphasizes the remarkable homogeneity of these same as-
semblages when we study their most fundamental characteristics such
as cell phenomena, reproduction, heredity, life cycles, and the sequences
of events during development. The solution of the problem of relation-
ships and origins would be almost insuperable if we were limited in
our search to the plants now living on the earth.

Fortunately, some of the plants that lived a thousand, a million, and
even a billion years ago have left a record — though a meager one —
among the sedimentary deposits in ponds, lakes and shallow seas. Some

719

720

TEXTBOOK OF BOTANY

of the later deposits are still unconsolidated, and we can study the
plants or parts of plants in peat, marl, and silt deposits.

The plants that grew a million years ago shed their spores and seeds,
their leaves and branches just as modern plants do. Swamp forests were
overwhelmed bv catastrophes, and buried in unaerated mud and water
and in the delta deposits at the mouths of great rivers just as they are
today. Fine "ashes" from violent volcanic eruptions have at various
times quicklv surrounded and buried living vegetation. Owing to the
poisonous gases and vapors emitted during such eruptions, all living
organisms were killed and the plants were often buried under sterile
debris. Some of the plants and animals that lived in or fell into bog
lakes, where decay is comparativelv slow, have been preserved in peat
and coal for centuries. Some of the peat bogs of Siberia have remained
frozen since glacial times, and in these natural refrigerators organisms
such as mammoths and rhinoceroses have been preserved intact to the
present time.

Both the remains and the various traces of organisms are teraied
fossils. Sometimes the material of the plant was carbonized by oxidation-
reduction processes which removed most of the oxygen and hydrogen
and left beautiful outlines of parts of the plant in carbon (Fig. 355).
Molds, casts, and, less commonly, petrified fossils were formed. When
ooze surrounds an organism and solidifies, the organism may slowly
disintegrate, but a mold of its form is left. If the mold is filled with

Fig. 355. Carbonized fossils of fern-like leaves in a rock of the carboniferous period.

Courtesy World Book Co.

[Chap. LII

PLANTS OF THE PAST

721

Fig. 356. Side and end view of cast of a seed of Upper Carbonifeions age (A);
cast of a secjuoia cone from the Upper Cretaceous shales of Alaska. Photos b)'
W. Berry and G. C. Martin.

mineral matter, the result is a cast (Fig. 356). If the substances of the
cell walls of plants are similarly replaced by mineral matter in suffi-
cient detail for one to recognize the cellular structure, the plant is said
to be petrified ( Fig. 357 ) . Some fossils are merely imprints ( Fig. 358 ) ,

yiro^

'\\'?>^'-^"#,^yi

Fig. 357. Petrified trunks of trees that were living in this locality during the Triassic
period. Fossil Forest Park, Arizona. Photo by U. S. National Museum.

121 TEXTBOOK OF BOTANY

similar to those formed when leaves fall upon freshlv laid cement and
a detailed outline of their form and venation is preserved as the cement
solidifies.

Fig. 358. Positive and negative imprints of a seed fern {Neuropteri.s) leaflet.
Photo from Field Museum of Natural History.

Fossils may be found in consolidated rocks, sand dunes, cave de-
posits, peat beds, coal, river deltas and varves, ooze at the bottom of
lakes and oceans, glacial drift, and in soil formed from the weathering
of rocks. Studies of the rocks and fossils within them have enabled
paleontologists to reconstruct many phenomena and organisms of the
different geological periods (Fig. 359).

Various means are used to estimate the relative and approximate
ages of fossils. For long-range estimates, the determination of the ex-
tent of disintegration of uranium to lead and helium appears to be far
more reliable than any of the methods formerly de^'ised. The rate of
this chemical process is not affected by the ordinary changes in tem-
perature and pressure at the surface of the earth. Since the rate of
disintegration of uranium is known, it is necessary only to determine
the amount of "uranium-lead" in a rock to estimate its age.

The plants of a billion years ago were less complex than those of
today, and apparently were neither very large nor woody. They too fell,
and some of them were deposited in situations where oxidation and dis-
integration were delayed for at least sufficient time for casts and imprints

[Chap. LII

PLANTS OF THE PAST

GEOLOGIC

ERAS

xO

c

^

p

(1>

o^

<c?

^i^

^■

^

#

.O

PERIODS

PLEISTOCENE

PLIOCENE

OLIGOCENE

EOCENE

CRETACEOUS

LIFE

AGE OF MAN

AGE OF

FLOWERING

SEED PLANTS

AND

MAMMALS

<^^^

o^

c

i^

r.<^

<^

.&^

^

PENNSYLVANIAN

MISSISSIPPIAN

DEVONIAN

ORDOVICIAN

CAMBRIAN

AGE OF
GYMNOSPERMS

AND

REPTILES

MOUNTAIN MAKING
EPISODES

CASCADES AND
2nd SIERRA NEVAOAS
2rvd UPLIFT OF
APPALACHIANS

HIMALAYAS

ROCKIES

1st SIERRA NEVADAS

1st APPALACHIANS

AGE OF

FERNS

SEED FERNS

CLUB MOSSES

AMPHIBIANS
FISHES

FIRST FOSSIL
RECORDS

(glaciation)

GREAT

INCREASE OF

HERBACEOUS

PLANTS

8,

FLOWERING

PLANTS

WILLOW, BEECH

MAGNOLIA

DIATOMS

CONIFERS

CYCADS

^e

o^

.o

THALLOPHYTES

INVERTEBRATES

GRAND CANYON
UPLIFT

LATER
LAURENTIAN

EARLIER
LAURENTIAN

YEARS AGO

15,000
1.500,000

30,000,000
65,000,000

(GLACIATION)

MOSSES AND
LIVERWORTS

FOSSIL FORESTS

GYMNOSPERMS
FERNS

FUNGI

SEA WEEDS

240,000,000

(glaciation)

algae and

bacteria

700,000,000

1,000.000,000

Fig. 359. Diagram showing some of the divisions of geologic time, together
with some of the important episodes, and types of plant life. The time scale is
based on the disintegration of uranium to helium and lead.

724 TEXTBOOK OF BOTANY

to be formed. Unfortunately, after the sediments had become rocks,
the rocks were subjected to such enormous pressure, to such high
temperatures, to the forces of crystalhzation, and were so altered that
little remains to mark the fossils that undoubtedly occurred in these
rocks when the)' were young.

In such metamorphosed rocks we can infer the existence of plants
from certain structural features of the rocks or from certain mineral
deposits that occur among them. For example, in the regions around
the west end of Lake Superior are great deposits of iron in pre-Cambrian
rocks. In view of what we know about the deposition of comparable
deposits in recent times we have a right to infer that iron bacteria were
living in the water where these deposits accumulated. Nearby are de-
posits of graphite, a compact form of carbon, which is the end product
when crushing pressures, heat, and recrystallization have acted upon
coal which in turn is a derivative of peat and other plant debris. Small
amounts of carbon in metamorphic rocks might possibly have a dif-
ferent origin, but large deposits represent much larger masses of plant
remains.

If one examines the natural processes of the present, he can roughly
estimate how many of the plants of today are falling into situations
where they, or their imprints, may be preserved. He will then more
fully realize how few are the land plants that will leave in the earth a
record that might be found a hundred years from now. He will also
understand more clearly whv the fossil plant record is so meager and
so scattered. We have no reason to assume that on the continent as a
whole the conditions favoring the preservation of plants were very
different from what they are now, except for the interference of man.

Of course there were long periods of time when shallow seas and
lakes were more extensive. For example, during the Cretaceous period
there was an open waterway between the Gulf of Mexico and Alaska
where now are the high plains. As the land rose toward the close of
the Cretaceous period, this waterway became a series of fresh-water
lakes and swamps. At the present time the northern states and Canada
are also marked by hundreds of thousands of lakes in depressions left
by the vast continental glaciers, the last of which disappeared only
about 10,000 years ago.

Extensive forested swamps existed during the Carboniferous time
(Fig. 360), and in them the trees and other plants of those periods ac-
cumulated. From the consolidated and altered plant remains the eastern

[Chap. LII PLANTS OF THE PAST 725

coal has resulted: bituminous in the plateaus, and anthracite in the
folded mountains of eastern Pennsylvania. The bituminous coals of the
eastern mountain region were converted into anthracite by the intense
heat of the folding rocks. Not as much metamorphism occurred in the
plateau and the plant debris was changed only to bituminous coal.

Fig. 360. Group restoration of trees ol the CJarbomleruus penotl. Copyright by
Field Museum of Natural History.

There were also long periods when the continent was far drier than
it is today and the desert areas were much larger, for example, during
the Silurian and Permian periods. When the Rockies were uplifted at
the close of the Cretaceous, the deciduous and broad-leaved evergreen
forests that existed in the interior of North America at that time were
graduallv killed by the increasing drought and became replaced by
grasslands. The eastern deciduous forest of today is the remnant of that
far more extensive forest of the early Tertiary. The grasslands of the
Central States are the continuation of the grasslands that became estab-
lished there during Tertiary times.

Many changes in the area and shape of the land surface of North
America have occurred. During the Pliocene and prior to the Glacial
period (Pleistocene) the North American continent was elevated more
than 1000 feet above the present level. At that time more of the con-

726 TEXTBOOK OF BOTANY

tinent was land above sea level than at present. During the early
Paleozoic w^hen there was widespread submergence of the continent
only a small part of the present continent was above water. The Pale-
ozoic was the period during which the thick and extensive beds of
limestone in the eastern states were deposited.

Obviously, all these changes — the elevation of the continent, the up-
lift of mountains, the periods of great volcanic activity and long interven-
ing periods of erosion and deposition, and the accompanying changes
in climate — were favorable to the growth and survival of some kinds
of plants but led to the death of others. These same phenomena oc-
curred on other continents, and they are occurring at the present per-
haps as rapidlv on the average as they did in the past. The processes
by which plants change and the manner in which environment may
affect their survival and rate of change are discussed in Chapters XXXIX
and XL,

At some time North America was broadly connected with Asia
through the Alaskan peninsula. Likewise, land connected North and
South America during certain millions of years, and during certain
other millions of years the ocean covered parts of Central America.
Land connections for long periods of time make possible the migration
of species from one continent to another. The severing of these con-
nections leads to the isolation of the plants of one continent from those
of another. Isolation bv restricting hybridization makes possible the
independent evolution of species, genera, and families, and the conse-
quent diversity of continental floras.

These facts help to account for the peculiarly distinct floras of Aus-
tralia and New Zealand, which have been isolated from the other con-
tinents since the first appearance of the mammals. Eastern China and
Japan have forests of the same genera of trees as those of the eastern
United States. They are both remnants of a forest that was once con-
tinuous through Alaska and Siberia.

Facts gleaned from the fossil record prove beyond a doubt how long
plants have lived on the earth. Studies of fossils have definitely proved
that the vegetation of the present is derived from that of the past. The
present flora is much more like that of the Pliocene than like that of
the upper Cretaceous, and it has still less in common with that of the
lower Cretaceous horizons. New kinds of plants evolve and older ones
become extinct.

\CJuip. LU PLANTS OF THE PAST 727

Pre-Cambrian and Paleozoic floras. The fossil record has also shown
that the simplest and least organized plants were the first on the earth.
Studies of the Archeozoic and Froterozoic rocks have up to this time
yielded no evidence of organisms more complex than the bacteria and
blue-green algae. But early in the Cambrian there is evidence that the
blue-green, the green, the red, and the brown algae — all lime-depositing
species — were present, and it is probable that they had been present
also in pre-Cambrian waters. So meager are the facts about the algae
of the Cambrian and Ordovician that any description must await the
disco verv of better-preserved fossils.

Silurian rocks contain fossils of vascular land plants which indicate
a long ancestry of terrestrial plants. Their dichotomous thalloid vege-
tative body bore apical sporangia containing four spores in a tetrad. In
plants of today, the formation of spores in tetrads is associated with
reduction division and sexual reproduction. These plants have been
distinguished as Psilophvtes ( Fig. 361 ) .

The middle Devonian Psilophytes included plants with upright di-
chotomouslv branched stems, having stomates, scale-like appendages,
and terminal sporangia. Moreover, the growing stem tips uncoiled like
those of fern leaves. Some species had spirally arranged scales, others
were naked. The stems consisted of distinct tissues: epidermis, cortex,
phloem, and xylem. The spores occurred in tetrads in terminal sporangia.
The upright stems developed from rhizomes, and some were several
inches in thickness. These are a few of the characteristics of early land
plants known from only a few scattered localities in Australia, North
America, and Europe.

Primitive lycopods were also present in the middle Devonian, and
the tree species became the dominant plants of Carboniferous time.
Lepidodendrons and Sigillarias are the best known of these plants ( Fig.
360). They had lance-shaped leaves, sporophylls arranged in terminal
cones on branches, and their stems possessed cambiums by which sec-
ondary thickening occurred. The leaf scars and their imprints are among
the commonest and most beautiful coal shale fossils.

The ancestral plants of present-day equisetums, the Calamites, were
present also. Some of them were trees with jointed stems up to 100
feet tall. Apparently they were common in the Devonian and abundant
in the Carboniferous. Their branches occurred in whorls. Leaves were
simple and the sporophylls were arranged in cones. Some had large
underground rhizomes.

728

TEXTBOOK OF BOTANY

Fig. 361. Two Devonian land plants (A-B) and a modern one (C) with some-
what similar characteristics. A, Rhijnia; B, Psilophijton; and C, Psilotum a fern-
like plant of tropics often cultivated in conservatories. Restorations by Kidson
and Lang (A), and by Dawson (B).

Ancestral forms of the ferns appeared in the later Paleozoic. Not all
the representatives were large, but again the woody species seem to
have been most abundant. The leaves bear a close resemblance to those
of modern ferns, but several differences in stem structure and in repro-
ductive organs are evident. Some of these ferns were heterosporous, and
others were homosporous.

For many years all the fossil leaf imprints and carbonized leaves that
resembled fern leaves were thought to be ferns; but evidence is con-
tinually appearing that a number, perhaps a majority, of the fern-like
leaves belonged to plants that bore true seeds: the Pteridosperms (Fig.

[Chap. Lll

PLANTS OF THE PAST

729

362 ) . These were the first seed plants and they seem to have developed
from the same ancestral stock as the true ferns. Thev reached their

Fig. 362. A seed tern (Eospermatopteris) of the Devonian period. Restoration by
W. Goldring. Courtesy New York State Museum, Albany.

greatest development in the late Paleozoic, and along with the primitive
lycopods soon after became extinct. From the Devonian onwards primi-
tive gymnosperms were associated with them, for example, Cordaites
(Fig. 363).

The luxuriant vegetation of the Carboniferous seems to indicate a
warm temperate, moist climate with a very narrow range of variation
of temperature. Trees and woody plants seem to have been predomi-
nant; the properties of coal strata are due to the woody character of the
parent materials. These coal strata, however, contain occasional layers
rich in spores, and layers that are possibly of algal origin.

The Permian rocks indicate a period of extreme aridity, and also of
glaciation, which can be accounted for by the great crustal changes
that occurred at the close of the Paleozoic Era. Both of these climatic
changes probably contributed to the destruction of the Carboniferous
floras.

No fossil flowering plants ( angiospemis ) have been found in Pa-
leozoic rocks. The evolution of sepals, petals, pistils, fruits, fusion nuclei,

730

TEXTBOOK OF BOTANY

1

Fig. 363. Restorations of Carboniferous plants: A, Lepidodendrou, a tree re-
lated to the club mosses; B, Cordaites, an ancient gymnosperm. Copyright by
Field Museum of Natural History.

triple-fusion nuclei, and endo.speims is comparatively recent. Some of
the features found in flowering plants, however, had been evolving
since pre-Cambrian times. The reader may find it interesting to specu-
late about the relative ages of some of these features. How old, for
example, are the processes of seed formation, formation of pollen,
cambial growth, development of leaves, organization of cells in a multi-
cellular individual, sex, mitosis, spore formation, chlorophyll synthesis,
vegetative multiplication, and respiration? Have these inherent proc-
esses remained unaltered since the time of their first appearance?

Mesozoic floras. Among the fossfls in Triassic and Jurassic rocks are
those of two major groups: the Bennettitales and the primitive cycads.
The former are gymnosperms without obvious internodes, and with an
apical crown of large compound leaves, in the axils of which are cones
of sporophylls having terminal pollen sacs and ovules. The stems and
leaves of the cycads were similar to those of the Bennettitales, but the
pollen sacs and ovules occurred on leaves arranged in a terminal cone.

PLANTS OF THE PAST

■31

[Chap. LII

The cycad plants were either staminate or carpellate, while the Ben-
nettitales had both staminate and carpellate, or even mixed cones on
the same plant. Both of these plant assemblages seem to have been
derived from the seed ferns, and to have been the forerunners of the
modern gymnosperms and angiosperms.

The land plants of the early Cretaceous period in America were
dominated by conifers, ferns, and cycads. During this period the land
surface became lowered and much of it was submerged for long inter-
vals. The sedimentary rocks formed at this time contain a large array

Fig. 364. Leaf prints in rocks of Upper Cretaceous age in Alaska. Represented
here are (a) dogwood, (b) alder, (c-d) two species of Gingko, (e) redwood, and
(f) oak. From Professional Paper No. 159. United States Geological Survey.

of fossil angiospemis with genera similar or identical to those of present-
day vegetation (Fig. 364). Apparently angiosperms had been evolving
on the earth for many thousands of years previous to this time. Ferns
and cycads declined rapidly; but the conifers remained abundant, with
species belonging to genera now extinct, but also including species of
pine and Sequoia.

The amazing part of the upper Cretaceous fossil record is the rapid
diversification of the angiosperms and their spread over all the northern
hemisphere, some even reaching Africa and South America. In the
fossil floras studied there is abundant evidence of the presence of
familiar genera such as Magnolia (10 species), Liriodendron (5 sp.),

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TEXTBOOK OF BOTANY

Aralia (10 sp.), Eucalijptiis (5 sp.), Sassafras (4 sp.), Querciis (12
sp. ), Ficus (8 sp.), and Salix (2 sp. ). Monocots were present in very
small numbers, but included palms, grasses, sedges and cattails. Another
remarkable feature of this widespread flora was the uniformity in
composition from such widely separated areas as Greenland and Texas.
This is the last of the widespread and uniform floras.

Cenozoic floras. Tertiary floras became more and more differentiated
as the climate of the polar regions became colder, and mountain-making
and volcanic activities in the western mountains broke up the uniform
temperate region into areas with different rainfall regimes, various
amounts of precipitation, increased or decreased humidity, and zonation
of environments on mountain slopes.

During the early Tertiary there was a migration of angiosperms to
Australia, New Zealand, South Africa, and South America; and also a
rapid development and differentiation of modern genera and species.
Plants and plant communities very similar to those of modern temperate
climates occurred in polar regions in both America and Eurasia. Warm
temperate plants were characteristic of the middle latitudes, and sub-
tropical plants were abundant in the southern half of the United States.
Throughout the Tertiary there was a progressive cooling and these
three zones of plants were gradually shifted southward; the arctic floras
emerged in the north. Redwoods that earlier grew in circumpolar re-
gions ( Fig. 356 ) are now represented bv a meager remnant in Oregon
and California. Some of the subtropical plants have survived only in

Fig. 365. Animals and plants in western Nebraska during the middle of the
Pliocene. Restoration by C. R. Knight, American Museum of Natmal History,
New York.

[Chap. LI I PLANTS OF THE PAST 733

Central and South America, the tip of Florida, and the islands of the
Caribbean Sea.

From the Miocene through the Pliocene and the glaciations of the
Pleistocene the outstanding changes in the vegetation have been the
extinction of manv species and numerous genera, and the reduction of
some genera to single species which survived in but one or only a few
comparatively small regions. Much of this extinction of species and
restriction in distribution of others had apparently taken place before

Fig. 366. Animals of the Mississippi \'alley at the close of the Glacial epoch.
Restoration by C. R. Knight. Copyright by American Museum of Natural History,
New York.

the end of the Pliocene ( Fig. 365 ) . The grasslands of the Central States
began their extension from the southwest in Eocene times as the Rockies
were elevated. Many other species became extinct after the glacial
period. This is strikingly illustrated bv the large mammals (Fig. 366).
The four great ice invasions from Canada into the Northern States
were separated by long time intervals during a period of at least a mil-
lion years (Fig. 367). At the beginning of the Pleistocene period the
continent as a whole seems to have been considerably higher and more
extensive than at present. The elevation of the land may have been a
factor in the decrease in temperature associated with the development
of continental ice sheets. The destruction of vegetation outside the ice-
covered areas was brought about partly by the small reduction (5° to
10° F. ) in average temperature and partly by the reversal of northward-
flowing streams, floods, and pondine^ where ice lobes pushed across
valleys. The apparent survival of preglacial species in protected coves
and in rock gorges just south of the ice indicates that the maximum
lobes were being pushed into territory where the summer temperatures
were not greatly below those of the present time.

734

TEXTBOOK OF BOTANY

Fig. 367. Centers of ice accumulation in North America during the Pleistocene.

There seems to have been no permanent ice on many of the islands,
peninsulas, and headlands along the northern coasts, and much of
Alaska was unglaciated. Moreover, the ice of each glacial invasion was
not continuous from the Pacific to the Atlantic. Ice developed inde-
pendently in the western mountains (Cordilleran Center), on the land
west of Hudson's Bay ( Keewatin Center ) , and on the Laurentian high-
lands (Laurentian Center). One of these glaciers was retreating when
another was advancing; consequently there were occasional local areas
between the ice fields where the flora spread following the retreat of

[Chap. Lll PLANTS OF THE PAST 735

the ice. Later the flora spread in the opposite direction when the second
glacier disappeared. Thus even in Canada some plants survived the
glacial epochs, and the present vegetation of Canada is not wholly com-
posed of species that migrated northward from the area south of the
last glacial invasion.

Moreover, the unglaciated area in southwestern Wisconsin, so often
shown on glacial maps as a small triangular area surrounded by ice,
was never completely surrounded by any one of the glaciers. This area
was always larger than the State of Illinois and most of the time it was
fully open to the southwest. This was consequently a nearly continuous
source of plant migrants to the Lake Superior shores and region of the
headwaters of the Mississippi. There were thus local sources of plants
scattered about the northern part of the continent. Except in the moun-
tains the vegetation was disturbed very little 50 to 100 miles south of
the ice margin.

South of the glacial boundaries, forests extended across the sagebrush
lands between the Sierra and the Rocky Mountains and eastward over
what is now plains and prairie grasslands. Farther east the drainage
of the Great Lakes through the St. Lawrence River was at times
blocked by the ice, and much of the adjacent lowland was covered by
extensive lakes. The beaches of these lakes are still recognizable, and
many of them have been accurately mapped. As the ice-fields slowly
disappeared, numerous smaller lakes were invaded by bog vegetation,
so common about the lakes farther north today. Bog plants occupied
many of these lake sites until the present century, when most of them
were destroyed by agricultural practices.

As the lakes slowly filled with peat, pollen carried by the wind from
the plants of the upland as well as those of the bog fell into them and
were preserved. Studies of the distribution of pollen in the successive
layers of the peat have yielded a fairly accurate record of the successive
associations of plants that lived on the surrounding uplands from gla-
cial times to the present (Figs. 368 and 369).

Summary. On the basis of the fossil record, it appears that plants have
been living upon the earth for a billion years or more. The first plants
were exceedingly small and simple in structure. Their physiology proba-
bly was complex, for no living systems are known to have a simple
physiology. Through a slow process of evolution, which appears to be
dependent solely upon changes in hereditary units of matter in living
cells ( Chapters XXXIX and XL ) , new kinds of plants have, at all times

736

TEXTBOOK OF BOTANY

Oaks predomioaril: oa upland, wLtK hickories D
and otKer deciduous trees
Tamarack la bo^s

White pine predominant
with hemlock on upland
Black spnace in boi^s

Northern oaks and pines with spruce

X^on Upland X
Black spruce in bo^s

White spruce with black spruce
predominant on upland

/ \

White pine and Jack pine
on sand anddravel terraces

'Oack pine and white spruce
predominant

30th fQQi-

foot

20^'' foot

15^^ foot

foot

foot

Base of peat

Fig. 368. A diagrammatic representation of the relative abundance of tree
pollens in successive layers of peat in a bog in central Ohio together with the
forest types inferred to be living near or in the bog at the time the layer of peat
was deposited. The diagram should be read from the bottom upward. Data from
L. R. Wilson and P. B. Sears.

Fig. 369. Four stages of the filling of a bog basin and the appearance of the
adjoining landscapes in central Ohio since the disappearance of the continental ice
sheet. The composition and succession of the forests are inferred from observations
made of present northern forests as well as the relative abundance of fossil pollens
in the peat bog referred to in Fig. 368.

737

738 TEXTBOOK OF BOTANY

since their origin on the earth, gradually developed from preexisting
ones.

Diverse groups appeared, became abundant and widespread over the
earth, waned, and either became extinct or are represented in our
present flora by isolated remnants. Early evolution of land plants seems
to have resulted in the development and dominance of woody plants,
often of tree size. Herbs are apparently more recent, for they constitute
a far larger proportion of the fossil flora of the Cenozoic than of pre-
vious eras.

Evolution of plants previous to the close of the Paleozoic had re-
sulted in the formation of most of the fundamental features found in
plants today, excepting those found only in the flowering plants, or
angiosperms. The early evolution of the angiosperms in the Mesozoic
seems to have been both rapid and widespread in the temperate lands
of the northern hemisphere. From there they spread across the equator
to the southern continents.

The vegetation of the present is composed of remnants of past vegeta-
tions, together with new combinations of old and new species. The
distribution of modern species and plant communities is dependent
partly upon the present physical and biological environments, and partly
upon the historical background, such as the elevation and submergence
of the land, the formation and erosion of mountains, land connections
with other continents, changes in climate and local diversification of
the climatic factors, prolonged intense drought, and glaciation. Some
appreciation of all these biological and physical factors is essential to
an understanding of the vegetation of a continent, such as that of North
America.

REFERENCES

Antevs, E. The last glaciation. Amer. Geog. Soc. Research Series No. 17. Amer-
ican Geog. Soc. 1928.

Berry, E. W. Tree Ancestors. Williams & Wilkins Company. 1923.

Chaney, Ralph. Ancient forests of Oregon. Carnegie Institution of Washing-
ton, Pub. 501, pp. 631-648. 1938.

Chaney, Ralph. Paleoecological interpretations of Cenozoic plants in Western
North America. Bot. Rev. 4:371-396. 1938.

Dahlgren, B. E. A forest of the coal age. Geology Leaflet 14. Field Museum,
Chicago. 1933.

Daly, R. A. The Changing World of the Ice Age. Yale Univ. Press. 1934.

[Chap. LIl PLANTS OF THE PAST 739

Darrah, W. C. Principles of Paleobotany. Chronica Botanica Company.

Leiden. 1939.
Goldring, Winifred. The oldest known forest. Sci. Monthly. 23:514-529. 1927.
Hirmer, Max. Handbuch der Palaobotanik. R. Oldenbourg. Berlin. 1927.
Roy, S. K. How old are fossils? Geology Leaflet 9. Field Museum, Chicago.

1927.
Schuchert, C. Outlines of Historical Geology. 2nd ed. J. Wiley & Sons, Inc.

1931.

CHAPTER LIII

THE VEGETATION OF NORTH AMERICA

<£><><><><><x><^e<><^<><><><^^

Recent improvements in facilities for land and air travel have afforded
millions of people the opportunity of visiting most parts of the North
American continent. The outposts of civilization in Canada, Mexico, and
Central America have rather suddenly become fairly accessible to anyone
interested in going there. This has awakened a new interest not only in
unfamiliar human populations, their customs, industries, and foods, but
also in the native vegetation, which has always been the chief source of
biological supplies of isolated peoples and pioneer settlements.

Such trips may now be even more enjovable and enlightening, for
enough has been learned about the biological and economic relations of
plants and their heredity and phvsiology that observations may be made
with increased understanding. Never again will plants be just something
green along the route, put there merely to be enjoyed, ignored, or de-
stroyed at will, for we know that they are the basis both of our existence
and of much of our individual and national wealth. The development of
the plants, on the other hand, is dependent upon their physical environ-
ment, a part of which we may alter, either to the advantage of both the
plants and ourselves or to the detriment of both.

This chapter on the vegetation of North America is a brief sketch of
the vegetation of a continent ( Fig. 370 ) . Only a few of the many biologi-
cal and economic relations of this vegetation are mentioned; but many
others, together with interpretations of the phenomena described, can be
inferred from the facts presented in preceding chapters.

The differences in climate from place to place on the larger continents
are so numerous that areas of similar climates and equivalent types of
vegetation occur on each of them. Most of the species, many of the
genera, and a few of the families, however, are different.

Climatic contrasts of the continent. From north to south the greatest
contrasts result from differences in light and temperature conditions.
North America extends southward from a region of nearly continuous
ice and snow, where three-fourths of the year is winter and one-fourth

740

[Chap. LIU THE VEGETATION OF NORTH AMERICA

741

[X]

mm

123
MB

TUNDRA

BOREAL FOREST

HEMLOCK-HARDWOOD F.

DECIDUOUS FOREST

S.E. EVERGREEN FOREST

GRASSLANDS

DESERT GRASS AND SCRUB

DESERT

COASTAL FOREST

ROCKY MT. FOREST

WET AND DRY TROPICAL F

Fig. 370. Principal vegetation types of North America.

742 TEXTBOOK OF BOTANY

spring, where light intensity is never high, and the daily light period is
24 hours long during the short growing season. The southern extremity
of the continent lies within the tropics, where, on the basis of tempera-
ture, there is but one season, summer, which may be wet or dry or pe-
riodically wet and dry, where light intensity is high, and the daily light
period is 12 hours long throughout the year.

A west-to-east belt across the continent from Oregon to Massachusetts
is the region of prevailing westerly winds and is marked by great differ-
ences in total precipitation, in snowfall and rainfall, in the prevalence of
drizzling rains or sudden downpours, and in seasonal distribution of pre-
cipitation. These variations affect the atmospheric humidity and the
available soil water. Starting at the west coast, with a hundred inches of
rain principally during the colder months of the year, and persistent
fogs, the high coastal mountains lead to a reduction in rainfall on their
eastern slopes, so that grassland and semi-desert conditions prevail on
the high plateau. Precipitation again increases in the Rockies, but the}'
in turn cast a rain shadow eastward on the high plains, and 10 to 15
inches of precipitation is general. The annual precipitation in the eastern
half of the United States varies from 30 to 60 inches and comes from the
moisture in the air masses moving in from the Gulf of Mexico. Central
America extends into the trade winds, or prevailing easterlies, and rains
are abundant east of the mountain crests and meager on the western
slopes.

As a result of the many possible combinations of light intensity, length
of day, temperature extremes, length of growing season, rainfall, snow-
fall, frequency and length of drought periods, fogs, and hot dry winds,
the continent as a whole has many climatic regions, large and small, ex-
tending in various directions; and the nature of the vegetation reflects
the diversity of conditions. Indeed, students of climates have long recog-
nized that the larger patterns of vegetation are among the best indicators
of climatic patterns and boundaries. High mountain slopes and crests
may approximate conditions of high latitudes, while protected rivers
with warm seacoasts in the north may simulate conditions of the south.

Prehistoric factors. In addition to the amount and intensity of present-
day factors, the distribution of many plants has been limited by prehis-
toric or geologic factors. Among these factors, slightly lower temperature
and glaciation, with their attendant effects on precipitation, drainage,
and soils, have left their imprint on the vegetation of many parts of the
continent. Climatic shifts involving prolonged periods of drought or of

[Chap. LIU THE VEGETATION OF NORTH AMERICA 743

increased rainfall in postglacial time account for many of the irregulari-
ties of plant distribution. On the margins of the continent the position of
the seashore has varied, in part because of the uplift or sinking of the
land and in part because of changes in the height of sea level itself.

Plant Formations

Tundra (Figs. 371-373). This name has come into general use
throughout the world for the low, sometimes scanty vegetation which

Fig. 371. Mature tundra at edge of Harriman Glacier, Prince William Sound,
Alaska. One of the heaths (Cassiope stelleriana) is the most abundant plant. A tree
one-half mile from the edge of this glacier is more than 300 years old. Photo by
W. S. Cooper.

encircles the polar ice caps and in America covers the "barren grounds."
At its southern border is what the Indians called "the land of little
sticks" and the boreal forest. Here and there the barren grounds are
broken by rivers flowing into the Arctic Ocean through deep valleys
where the snow melts somewhat earlier and the growing season is
longer. Valley slopes are also less exposed to the dry winter winds, and
on them the northernmost outliers of the forest occur.

The land in general is poorly drained and wet from the melting snows,
the fogs, and the drizzling rains of the warmer season. The drier parts
are covered with grasses, sedges, and rushes interspersed with lichens,

744

TEXTBOOK OF BOTANY

Fig. 372. Alder thicket on glacial drift two to three feet thick overlying stagnant
ice of the Allen Glacier, Copper River, Alaska. The man in the center of the
picture is facing the exposed ice front. Photo by W. S. Cooper.

Fig. 373. Remnants of a western hemlock-Sitka spruce forest buried under 200
feet of gravel about 1000 years ago. They have been uncovered recently by flood
water from a rapidly melting glacier farther up the valley. These tree trunks and
the plants of the forest Hoor are in a perfect state of preservation. Photo and data
from W. S. Cooper, Fourth Expedition to Glacier Bay, Alaska, 1935.

[Chap. Llll THE VEGETATION OF NORTH AMERICA

745

mosses, and a variety of small herbs with relatively large brilliant flowers
(Plates 1 and 3). There are also areas of such evergreen heaths as
bearberry, crowberry, and leatherleaf, together with creeping willows
and low birches. Alder, birch, and willow thickets occupy the warmer
slopes and stream banks. The myriad depressions and pools become
moss-covered bogs with sphagnum, polytrichum, and other bog mosses,
and are called "muskegs."

The substrate is wet and poorly aerated, and peat accumulates in the
surface layer. A few feet below the soil surface is the "ground ice" —
fossil ice, it might be called, since some of it was formed at the close of
the period of glaciation. In our latitude plants are affected by the depth
to which soils freeze in winter, while in the tundra they are affected by
the depth to which soils thaw in the warmer season. In general, the
tundra is a region of light snowfall; but where snow accumulates and
covers the vegetation it prevents destructive erosion by ice crystals car-
ried by the violent and desiccating winds of winter.

Plants of the tundra survive freezing night temperatures during the
growing season and grow at low average temperatures. Moreover, the
plants may freeze at any stage of development, including the flowering
period, survive under the snow, and continue growth the following sea-
son (Figs. 374 and 375). The vegetation of the tundra, then, is composed

Fig. 374. Adder 's-tongue (Erythroniiim montantim) that grew through a 3-inch
layer of snow and bloomed. Photo taken by W. H. Camp at Mt. Ranier, Wash-
ington.

746

TEXTBOOK OF BOTANY

Fig. 375. Marsh-marigold {Caltha leptosepala) , Mount Ranier National Park,
in bloom at the edge of a snow-bank. The flower buds were fully developed under
the snow and opened within a few hours after the snow melted. Photo by W. H.
Camp.

of plants that pass most of the year in low-temperature dormancy, and
grow but two to three months.

The survival of plants in different situations in the tundra is deter-
mined far more by the direct effects of environment than by the inter-
ference of other plants. Consequently, apparent successions are more
in the nature of fluctuations in the proportion of different species than
of the progressive development of more highly organized communities.

The southernmost isolated communities of tundra vegetation occur in
bogs, on sand plains, on cliff edges, and on the summits of high moun-
tains above the timber line. The conditions in alpine tundra are some-
what different from those in the polar tundra, especially the light inten-
sity, the length of day, and the quality of light. The length of the growing
season, however, and the soil conditions may be much the same. The
absence of trees is not due to the intense cold. The Siberian fir forest
grows in the coldest locality in the northern hemisphere.

Boreal forest (Figs. 376-377). This is the most extensive forest on the
continent and extends south of the tundra from western Alaska to New-
foundland. The most characteristic tree is the white spruce, which

[Chap. LIU THE VEGETATION OF NORTH AMERICA 747

occurs throughout the forest. Black spruce and tamarack (a deciduous
conifer) also occupy large areas, especially the poorly drained ones, as
well as wet bogs, and "muskeg." Both species sometimes occur on the
upland, and the black spruce is frequent on rock outcrops, cliffs, and
mountain tops. Other trees of importance are balsam fir, which is a
common associate of the white spruce east of the Rockies; jack pine, on

Fig. 376. Black spruce forest on a bog near Big Falls, Minnesota. Photo from U. S.

Forest Service.

sand plains and on sand and gravel terraces; balsam poplar, on flood
plains; and trembling aspen and canoe birch, now occupying extensive
burned-over areas. The most characteristic shrubs are thickets of alder
and birch, low junipers, with blueberries, huckleberries, and other
heaths, and sweet gale on acid soils. This formation is also characterized
by an abundance of mosses, liverworts, and lichens.

The northern boundary is an intricate pattern of interlacing sites.
Those with deeper soils and a longer growing season are occupied by
forest peninsulas and isolated groves; the tundra occupies the shallow
soils and short-season areas. The southern boundary in the west merges

748

TEXTBOOK OF BOTANY

with the northern boundary of the western evergreen forest. Across the
plains it is bordered by grasslands and aspen-grass savannas. Eastward
it intermingles with the hemlock-hardwood forest of the Great Lakes
region and the northern Appalachian Mountains.

The climate of the region is exceedingly rigorous, but the winter
temperature rarely falls to —40° F., and the snowfall is deeper than in

Fig. 377. A spruce-fir forest in the White Mountain National Forest of New Hamp-
shire. Photo from U. S. Forest Service.

the tundra. In the plains region the soil may freeze to a depth of 10 feet;
where the snowfall is greater the depth of freezing is much less. The
growing season is a month or two longer than in the tundra. There is a
distinct spring and summer period, but the autumn is short and winter
follows closely. Frosts may occur during any month of the year.

The boreal forest has occupied much of its present area only a com-
paratively short time, for this is the region which was covered ten to
twenty thousand years ago by continental glaciers, similar to the great

[Chap. LIIl THE VEGETATION OF NORTH AMERICA 749

glacier that now covers Greenland. The trees of the forest survived about
the edges of the several successive ice masses; and as the masses melted,
the forest spread into its present transcontinental region.

The tundra plants survived the glacial periods on the northern islands,
on transitory areas not occupied by ice, and near the coast, just as they
today live on the fringes of land encircling the Greenland ice. The poor
drainage, shallow soils, and frequent ponds and lakes are also the results
of glacial erosion and deposition. Irregularities result from the unequal
deposition of rock material in the ice at the final melting, and from the
meltine of buried, detached ice masses after the retreat of the main
masses of the glaciers.

Tundra and boreal forest are examples of climatic plant formations.
Each is a large area of nearly homogeneous vegetation limited in its
occurrence by a group of characteristic climatic conditions. The tundra
is covered by a mixed population of perennial herbs and low shrubs,
which vary in proportion from one locality to another; but organized
communities are indistinct. The boreal forest, on the other hand, has
well-defined communities of specific tree, shrub, and herbaceous layers,
with mosses and lichens in all layers. There is also more diversity of
communities on sand, on clay, and on rock exposures; on alkaline and
acid soils; and on well-drained soils in contrast to wet, poorly aerated
soils.

On burned-over lands there is abundant evidence of recent succession
from bare soil to annual and perennial weeds, to aspen thickets, or to
alder thickets on wetter soils, followed by jack pine or balsam poplar,
and finallv to white spruce and balsam fir. Bogs and muskeg, first cov-
ered by sphagnum and other mosses, are invaded by black spruce and
tamarack and in less acid situations by arbor vitae; and in the most
mature communities white spruce eventually becomes dominant. The
youngest flood plains have sedge, rush, and grass communities, soon fol-
lowed bv willow and alder thickets. These in turn may be succeeded by
balsam poplar communities, and followed by mixtures of balsam poplar
and white spruce; eventually white spruce becomes dominant.

White spruce may thus predominate in the final stage in the develop-
ment of vegetation which started with a variety of different communities
on such diverse habitats as flood plains, sand plains, bogs, and muskeg.
For this reason white spruce is regarded as a climax tree; and since a
comparable sequence occurs in similar habitats from Alaska to Labrador,

750

TEXTBOOK OF BOTANY

Fjg. 378. Exterior view of a forest of hemlock, white pine, beech, birch, and maple
on the plateau of northwestern Pennsyhania.

Fig. 379. Interior view of a hemlock-beech-birch-maple forest in northern Penn-
sylvania.

[Chap. LIU THE VEGETATION OF NORTH AMERICA

751

the white spruce forest is regarded as the chmax community or associa-
tion of the boreal forest formation.

Hemlock-hardwood forest (Figs. 378-380). From Winnipeg through
the Great Lakes region to Nova Scotia, and at gradually increasing eleva-
tions southward in the Appalachians, is a far more complex formation

Fig. 380. Interior view of a mixed spruce-hemlock -hardwood forest in the Mononga-
hela National Forest of West Virginia. Photo from U. S. Forest Service.

composed of many more species of plants and consequently of a greater
number of distinct communities.

This plant formation was originally characterized by the abundance
of hemlock, eastern white pine, red pine, yellow birch, red spruce, sugar
maple, red maple, beech, and black cherrv. The hemlock-hardwood for-
mation extends between the boreal forest formation and the deciduous
forest formation on the south. Since general climatic regions are never
separated by sharp lines because topograph)^ modifies climatic factors
locally, many small areas containing boreal communities ha^'e persisted

752 TEXTBOOK OF BOTANY

from early postglacial times, when the boreal forest dominated this
whole region. Likewise, there are areas of valley slopes and lake shores
where deciduous forest plants and communities have moved in and be-
come established. The hemlock-hardwood forest on its southern borders
has been most frequently replaced by deciduous forest communities
on the better soils.

The climatic conditions within this region are less rigorous than in the
boreal forest. The growing season is usuallv four to five and a half
months. Increased rainfall and deeper snows limit or prevent the freez-
ing of the soil in forested areas. Frosts seldom occur between June and
August, although night temperatures may frequently approach 40° F.
Winter temperatures of 20-25° below zero are not uncommon, and at
rare intervals temperatures 40° -50° below zero are recorded.

The period of dormano' thus is somewhat longer than the period of
growth, and the extreme winter temperatures limit the establishment
of many species common southward. For example, in some valleys of
the western Adirondacks during the winter of 1935-36 beech trees more
than 200 years old were killed bv extreme low temperatures, and of
course many of the younger trees died also. This is an example of the
fact that elimination of species from climatic formations may be effective
even if the extreme of a limiting factor occurs but once in many years.
The beech and maple trees near their northern limits are scrub-like trees
with numerous dead branches,

Agriculturalh' the area occupied by the hemlock-hardwood forest is a
region of hay crops, interspersed locally with potato and root crops and
certain northern varieties of apples. Vegetables, grapes, and other fruits
are cultivated near the larger lakes where in autumn killing frosts are
delayed by the heat liberated from the water.

Plant associations. As one studies the local diversity of plants and
plant communities within a climatic plant formation, he finds certain
communities with a similar appearance repeated again and again,
much the same group of species, and a similar internal organization.
They may occupy large or small areas, depending on physiography and
soils. These communities within a fonnation are called plant associations.
They are the smaller units of the vegetation of a great climatic region
which includes many topographic situations with difl^erent exposures to
light, wind, temperature, and precipitation. They are also limited by soils
of different depth, chemical composition, water content, aeration, and
texture.

[Chap. LIU THE VEGETATION OF NORTH AMERICA 753

Plant associations have a rather definite organization. This may be
illustrated by a forest such as the white pine association or the hemlock-
hardwood association. Both are named for their dominant species, but
both have accessory and subdominant species of trees, and in the under-
growth characteristic shrubs, herbs, and smaller plants. The dominant
trees that form the canopy, or top layer, of the association are exposed
to more sunlight, more wind, and a drier atmosphere than are the suc-
cessive lower layers of small trees, shrubs, herbs, and ground cover
beneath. Atmospheric moisture and carbon dioxide concentrations are
highest near the soil. After long periods of occupancy the accumulated
duff and underlying soils become strikingly different. These different
layers of plants in a forest and the relations of the plants in each layer to
those above and below constitute the ecological structure and organiza-
tion of an association.

Succession. Land forms are continually undergoing change. Uplands
are eroded by water and wind, and lowlands and flood plains are built
up by deposition of the products of erosion. Lakes and ponds are filled
up gradually; hill and mountain slopes are modified by slumping. All
these physiographic processes result in an orderly series of changes in the
environments of the various plant associations of these different sites,
and consequently most associations are undergoing changes in com-
position.

When new land areas are exposed as when lakes are drained, cut-over
forest areas are subjected to intense fires, or sand dunes and other wind-
blown deposits cover an older landscape, a comparatively rapid succes-
sion of plant associations follows one another until a final stabilized asso-
ciation forms a climax. These stages, beginning with weeds and passing
through pioneer shrubs and tree stages to the climax forests, have been
described for many parts of the country. Similar studies have been made
in the grasslands.

Succession may be readilv studied wherever there is abandoned faim
land; the age of the community can often be definitely ascertained from
historic records. On manv of these sites the primary cause of succession
is the changing of the soil and atmospheric factors by the plants them-
selves: shading, increasing or decreasing the water supply, adding
humus, increasing the stability and porosity of the soil. Pioneer plants
may make the habitat suitable to other species, which in turn may alter
it in ways that result in the death of the pioneers. Still other species may
follow these in time and have a similar effect on them. This influence of

754 TEXTBOOK OF BOTANY

plants upon each other indirectly through their effects upon the environ-
ment results in progressive changes in the communities that occupy an
area. It is sometimes referred to as "competition" among species and
communities.

The processes of vegetation change, however, are best studied in your
own locality and in the field. Here it is possible only to call attention
to their occurrence. Many of the recent projects in forest planting, soil
erosion control, irrigation, and building of dams are excellent sites for
such studies.

Vegetation, then, consists of larger and smaller groups of plants each
having a characteristic structure and appearance. The occurrence of the
larger groups ( formations ) , such as the boreal forest, eastern deciduous
forest, and prairie, is rather definitely related to climate. The distribu-
tion of the smaller communities ( associations ) within a climatic forma-
tion is correlated with local modifications of climate and diversity of soil
conditions. Because of gradual changes in climate, physiographic modifi-
cation of land forms, changes in available water, and the modification
of the environment by the plants themselves, rather definite successions
of plant associations occur within each of the formations.

Succession includes changes in composition, in organization, and in
the dominant and accessory species. If continued for long periods of
time, it leads ultimately to plant associations in which the component
species are in equilibrium with the local climate, with the soil, and with
each other. These are called climax associations and may cover larger or
smaller parts of a climatic formation. Man has frequently altered these
natural changes in vegetation.

Deciduous forest (Figs. 381-384). The most widely distributed forest
communities of the eastern United States belong to the deciduous forest
formation. The most characteristic trees of these various communities
are a score of oaks, several species of hickory, elm, ash, maple, linden,
poplar, and birch; also the beech, tulip, sycamore, and, until recently, the
chestnut. In the primeval forest many of these trees attained heights of
125 to 175 feet and trunk diameters of 5 to 14 feet. Many of the largest
trees of which we have records occurred in the lower Ohio River bottoms
and in the moist coves and valleys of the Appalachian Mountains and
plateaus. Probably the most important single species was the white oak,
which excelled in both abundance and dominance in a great variety of
habitats, partly because of its height, longevity, and freedom from
disease. The region of the deciduous forest was glaciated in the northern

[Chap. LIU THE VEGETATION OF NORTH AMERICA

755

and northwestern part, and there the topography is comparatively
smooth or rolHng. Most of the area, however, consists of stream-dissected
high and low plateaus and mountains, with a great diversity of under-
lying rocks and residual soils adjoined by low plains.

F^ic;. 381. General view of the mixed deciduous forest of the Pisguh National Forest
of North Garolina. Photo from U. S. Forest Service.

The most extensive communities of the deciduous forest on the up-
lands and slopes are the mixed hardwood, mixed oak, oak-hickory, white
oak, oak-maple-linden, beech-maple, and southward the beech-magnolia.
Interspersed among these upland types are the lowland forests of willow-
cottonwood-sycamore, elm-ash-maple, and swamp oak-hickory. On ridge
tops, cliff edges, gravel hills, and sand plains there are pioneer com-
munities of scrub, pitch, and short-leaf pines, either singly or in combi-
nation. Usually associated with them are scrub oaks as undergrowth,
and various tree oaks that eventually replace them on the better sites.

756

TEXTBOOK OF BOTANY

Fig. 382. A beech-sugar maple forest in West Virginia. Photo from U. S. Forest

Service.

[Chap. Llll THE VEGETATION OF NORTH AMERICA 757

Because these areas are subject to frequent droughts they are more sub-
ject to fires than other forest sites, and the resinous leaves that accumu-
late beneath the pines add to the fire hazard. Recurring fires that destroy
the ground cover and young trees restore the pioneer character of the
habitat; hence succession to oak and other broad-leaved deciduous
trees may be delayed or prevented. Pine communities if undisturbed by

Fig. 383. An oak-hickory forest in Ohio.

fire are relatively short-lived and are succeeded by oak and other com-
munities.

Tovv^ard the west the deciduous forest is limited by the decreased
precipitation — 35 inches annuallv in Texas to 25 inches in Minnesota —
and by the frequency and longer duration of droughts. The dry condi-
tion of the atmosphere during the summer is often accentuated by
periods of hot winds from the southwest. The upland forest border is
characterized by open groves of small trees with a ground cover of
grasses, or by scattered clumps of dwarfed oaks. Where the soils have
more water in them than that derived from direct rainfall, as in river
valleys, the deciduous forest extends farther into the grasslands.

Northward the deciduous forest species are limited by extremely low

758

TEXTBOOK OF BOTANY

winter temperatures, by the shortness of the growing season which
prevents the maturing of the vegetative branches and seeds, and by the
presence of species of the hemlock-hardwood and white spruce forests.
Southward the characteristic species of the deciduous forest are re-
stricted by the absence of a period of low-temperature dormancy, neces-
sarv both for the renewal of growth of the buds and cambiums of the
perennial plants and for the after-ripening of seeds. Leached sandy soils
and tlie absence of humus due to recurrent fires on the coastal plain

Fig. 384. Eastern white pine. An original stand at Cook Forest. Pennsyhania.

and mountain tops make conditions more favorable to the southern pines
and to other coastal plain species. Far south on the coastal plain, sur-
rounded by pine forests, are outlying communities of the deciduous
forest called "hammocks." The soils where they occur are better than
those of the general region, and the sites have been protected from fires.
These communities consist of either mixed oak and hickory or beech-
magnolia, with an undergrowth containing many typical deciduous forest
shrubs and herbs.

In many parts of the deciduous forest region are local communities
which are relicts of communities that lived under various former cli-
mates. Scattered bogs dating back to early postglacial time are still cov-
ered with mosses and flowering plants characteristic of the boreal and

[Chap. LIU THE VEGETATION OF NORTH AMERICA 759

hemlock-hardwood formation. These plants became established when
the climate was cooler and the uplands adjoining the bogs were clothed
with spruce, fir, pine, hemlock, and northern deciduous trees.

In protected coves and rock gorges there are localities with interesting
mixtures of trees, shrubs, and herbs, some of which antedate the last
glaciation, while others are relicts of postglacial time, when the hemlock-
hardwood forest extended much farther westward into Kentucky and
Indiana.

A third type of relict community was represented in the prairie open-
ings that the pioneers found in the deciduous forests from western Penn-
sylvania and New York to Illinois and western Tennessee. These were
remnants of a period drier and warmer than the present, during which
prairies extended much farther eastward than at the time of settlement.

On the Atlantic coastal plain are both relicts of southern plants that
extended far northward in preglacial times and remnants of northern
vegetation that extended farther south when moraines were being built
on Long Island bv the continental glaciers.

Southeastern evergreen forest (Figs. 385-387). The most conspicuous
feature of the Atlantic and Gulf coastal plains is the prevalence of pine
barrens. There is no abrupt change eastward from the Piedmont region,
or southward from the Cumberland and Ozark plateaus toward the Gulf.
One merely passes from regions predominantly oak, with pines on over-
drained and infertile sites and on abandoned farms, to regions where
pines were the dominant forest trees and oaks either formed the under-
growth or occupied the scattered areas of good soil and sites protected
from fires.

Long-leaf, loblolly, slash, and short-leaf pines are the most important
species. Long-leaf was the best source of lumber, as well as of resin and
turpentine. Loblolly predominated northward and is the weed tree of
old fields and cut-over lands of the inner coastal plain, just as slash pine
is the weed tree near the coast and on the northern half of the Florida
peninsula. The short-leaf pine occurs not onlv on the inner coastal plain
but also far inland, from Pennsylvania to Missouri, either scattered
among the oaks or in pine forests on shallow, poor soil.

On the overdrained, sandy, and leached soils are the dry pine barrens,
with open stands of pine either alone or with an understory of various
oaks. Where trees are scattered and oaks infrequent because of repeated
fires, there is often a ground cover of grasses. Near the south coast are
the moist and wet pine barrens, in which slash and lon^-leaf occurred

760

TEXTBOOK OF BOTANY

either separately or associated in various proportions. These resemble
the dry pine barrens except that oaks are less frequent or represented
only bv shrubby species. Other shrubs, including several species of holly,
various heaths, and a large number of herbs with bright-colored flowers

Fig. 385. Short-leaved pine {Pinus echinata) in New Jersey. Photo from U. S.

Forest Service.

are also present. A characteristic plant here is the creeping saw palmetto.
Coastal salt marshes of tall cord grass, adjoined inland by saw grass,
salt grass, or tall reed grass, occupy large areas of the Mississippi delta.
Still farther inland they merge with the canebrakes and the live-oak-cane
savannas. Westward they grade into the coastal prairies.

[Chap. LIU THE VEGETATION OF NORTH AMERICA

761

Fig. 386. A young stand of longleaf yellow pine, Talladega National Forest,
Alabama. Photo from U. S. Forest Service.

Fig. 387. Bald cypress in a submerged area in Arkansas. Photo by G. W. Blaydes.

762

TEXTBOOK OF BOTANY

Canebrakes were very extensive in pioneer days, not only near the
coast but also far up the slow^-flowing rivers and streams. The cane,
Arundinaria, is the only native American bamboo ( Fig. 388 ) . At or just
below the soil surface it has extensive branching rootstocks, from which
upright leafy poles grow to heights of 10 to 20 feet. These hard, rigid
stems, which develop from the numerous nodes of the rootstocks, form
almost impenetrable tangles in low ground. Canebrakes are often in-
vaded by evergreen "live oaks" on slightly higher land. Live oaks, with
their wide-spreading branches hung with the rootless flowering plant

Fig. 388. A canebrake on the edge of a cypress pond near Baton Rouge, Louisiana.

"Spanish moss," form an unforgettable feature of the coastal region.
Canebrakes also occurred on the bluffs of the Mississippi embayment as
far north as Illinois as undergrowth in the beech-magnolia and oak
forests. These brakes now have been largely destroyed by burning and
by grazing in early spring when the voung shoots appear above ground.
A second unique feature of the muddy rivers that cross the coastal
plain is the cypress swamps. This deciduous conifer grows in wet, low
grounds subject to flooding, and under these conditions upright conical
"knees" develop from bends in the shallow roots. When the tree is
planted on the upland such outgrowths do not occur. Associated fre-
quently with cypress is the tupelo, or black gum, in which the base of
the trunk is enlarged when growing in similar situations subject to pro-
longed flooding.

[Chap. LIU THE VEGETATION OF NORTH AMERICA 763

The southeastern pine forest has been the source of much kimber and
many resin products. More recently processes have been discovered by
which the wood of the hard pines may be used in the manufacture of
paper and wall board. This region is the "cotton belt," the principal
source not only of cotton fiber but also of many secondary cellulose
products, as well as cottonseed oil and its by-products. Other important
crops are sugar cane, certain varieties of tobacco, sovbeans, sweet
potatoes, peanuts, rice, early vegetables and fruits, and pecans.

Many of both the wild and the cultivated species are limited north-
ward by the occurrence of extreme winter temperatures, by the length
of the growing season, and by the prevailing temperatures during the
period of growth. Many of these plants when planted farther north fail
to grow well or to mature their fruits and seeds. Some trees continue to
live northward as shrubs, the shoots of which freeze to the ground almost
every winter. Other wild species have been cultivated successfullv more
than a hundred miles north of their natural boundary.

Grasslands (Figs. 389-394). Between the eastern forests and the Rocky
Mountains from Alberta to the Gulf of Mexico is a \'ast area of rolling
or flat land originally dominated by grasses, among which numerous
species of legumes, composites, and other herbs were conspicuous at the
time of their flowering. The rivers and streams are more or less en-
trenched in the plains which rise from an elevation of less than 500 feet
near the Mississippi to four or five thousand feet near the mountain
front. In presettlement days this was the grazing land of several million
buffaloes and antelopes; of rabbits, jack rabbits, prairie dogs, pocket
gophers, ground squirrels, and smaller rodents; as well as of several
hundred species of grasshoppers and other plant-consuming insects.

The eastern prairie border extended as a wedge into the deciduous
forest in the region now called the "corn belt." Here the average annual
precipitation is about 35 to 40 inches; northwestward it drops to about
20 inches, and southwestward to 30 inches in Texas. In general, the
precipitation in the grasslands is more irregular than in the forested
regions eastward and northward. Westward the precipitation gradually
declines to 10 to 12 inches near the mountains. Rains are most frequent
in the spring months and are usually in the form of showers. Drizzling
rains are rare. Droughts are characteristic of late summer, and the aver-
age winter snowfall is light. Blizzards and deep snows, however, do occur
northward during occasional winters.

Late summer droughts are often intensified by the hot winds from

764

TEXTBOOK OF BOTANY

the southwest. During such periods temperatures near 100^ F. may pre-
vail day and night for a week or for several weeks, and upland vegeta-
tion is soon desiccated. The underground parts of many grasses are far
larger than the tops, and they are less injured than are trees. But when

Fig. 389. Tall bluestem (Andropogon fiiicatiis) , a dominant grass in areas in-
termediate in moisture (the present corn belt) on the natural prairies of central
North America. In the eastern part of the prairie it grew in dense stands as pic-
tured above; the pioneer settlers had great diflRculty in plowing the sod formed
by the mesh work of roots and rhizomes of this grass. It grows as a bunch grass in
the dryer western part of the prairie region. Photo by C. H. Jones.

severe droughts occur during several successive years there is a high mor-
tality even among the native grasses.

The central grasslands mav be convenientlv subdi^'ided into three
types characterized by tall grass, mixed grass, and short grass. The boun-
daries between these grasslands were neither permanent nor regular, but
in general thev ranged north and south paralleling the mountain front.

The tall grass prairie, with grasses 5 to 10 feet in height, occupied both

[Chap. LIU THE VEGETATION OF NORTH AMERICA 765

wet and drier areas of the corn belt and extended northward to Manitoba
and southward to Texas. The deep, dark soils that developed under the
grasses are among the most fertile in the world; and the vast population
of big bluestem, Indian grass, little bluestem and slough grass have long
since been destroved and replaced by crop plants. The Gulf coastal

Fig. 390. Little bluestem {Andropogon scopariii.s) , a dominant grass of the
dry prairie areas. Like the tall bluestem, it grows in dense stands and also as a
bunch grass. Photo by H. L. Shantz.

prairies have become rice fields. Farther inland cotton is cultivated and
to the northward sorghums and winter wheat. Corn is the prevailing crop
in the middle region, while spring wheat, barley, Hax, and hay grasses
are the important crops northward. Alfalfa and other legumes are widely
planted and thrive without irrigation.

The mixed prairie is composed of grasses up to 4 feet in height in-
cluding needle grass, dropseed, little bluestem, June grass, wheat grass,
side oat grama and several shorter ones. Most of these grow as "bunch"
grasses, but some are sod formers in moist situations. Among the grasses

766

TEXTBOOK OF BOTANY

a great variety of flowering herbs, notably legumes and composites, add
color to the landscape when in bloom. Just as the tall grass prairie was
invaded by deciduous associations along the stream courses, the mixed
prairie was invaded along river bottoms and terraces by finger-like ex-
tensions of the tall grasses. Trees are more restricted to stream margins,
seepage areas, and ravines.

i^;*-.

tvM

M^J'M

ym

Fig. 391. A dense sod prairie of needle grass {Stipa spartea) and slender wheat
grass {Agropyron tenerum) in the central northern grasslands. Photo by H. L.
Shantz.

In the mixed prairie the several divisions of crop plants mentioned
under the tall grass prairie reach their western limits. Beginning in the
south and going northward one passes by fields of cotton, kaffir com and
milo, hard winter wheats, corn and alfalfa, and finally hard spring wheat
and hay in Canada.

The western high plains are characterized by short grasses, especially
grama and buffalo grass, with local areas of taller species common to the
mixed prairies. This is a region of restricted rainfall and only the upper
two or three feet of soil contains water. Here water evaporates both at
the upper soil surface and at the under surface of the moist horizon.

[Chap. LIU THE VEGETATION OF NORTH AMERICA

767

Fig. 392. Plains grassland near Akron, Colorado, dominated by grama and liuffali
grasses. See also Fig. 332. Photo by H. L. Shantz.

Fig. 393. An excellent stand of blue grama grass in New Mexico. Photo from U. S.

Soil Conservation Service.

768 TEXTBOOK OF BOTANY

At this lower surface calcium carbonate and other less soluble salts ac-
cumulate and form a hard layer. The water in the moist layer is lost by
evaporation from soil and plants each growing season, and there is
rarely a surplus from one year to the next. The deeper soil is perma-
nently dry. During a period of wet years taller grasses appear on the
short grass sod, and during dry years the short grasses form an inter-
rupted cover. The buffalo grass is most abundant from Colorado south-
ward, but the grama grass continues far to the north. Toward the
mountains sagebrush (Artemisia) intermingles with the grasses and in
Wyoming and Montana it dominates in scattered areas.

This is grazing land and farming is perilous. During a succession of wet
years, however, profitable crops of hard wheat have been harvested; but
in succeeding dry years there have been crop failures, accompanied by
wind erosion, dust storms, and poverty.

The desert o;rasslands extend still farther west and south from Texas
to Arizona and far into Mexico. These grasslands are much like the short
grass of the high plains, but the plants are farther apart and areas occu-
pied by grasses alone become smaller and more scattered. More often
the grasses form the ground cover under scattered desert shrubs such as
mesquite, creosote bush, vuccas, cacti, and scrub oaks. The rainy period
occurs in summer and evaporation is excessive. The growth period there-
fore is short and does not extend much beyond the rainv season. Curly
mesquite grass, black and crowfoot grama, tobosa, and "winter-fat"
grasses are characteristic, together with other species common to the
high plateaus.

Desert shrubland (Figs. 395-396). The evergreen creosote bush char-
acterizes the southern part, and sagebrush the northern part, of the vast
arid plateaus from Idaho to central Mexico and from eastern California
to western Colorado. Here precipitation is alwavs insufficient for long
periods of growth, and the perennials that form the conspicuous vegeta-
tion are either those that can dry out without injurv between rainy
periods, or those that accumulate water in thickened stems or roots. In
southern Arizona and California there are two rainy periods, one in win-
ter which is followed by a profusion of flowering winter annuals, the
other in summer which is also accompanied by an abundance of summer
annuals. Of course in the desert there are seepage areas and rather con-
stant underground water in many valle\'S, where water covers the river
bottoms onlv after torrential rains. In such areas trees mav grow, and
some of the characteristic shrubs may become tree-like in size.

[Chap. LIU THE VEGETATION OF NORTH AMERICA

769

Fig. 394. Winter fat {Eiirotia lanata) , one of the most valuable forage plants
of the Great Basin region, where it covers hundreds of square miles, especially
in Nevada and Utah. This shallow-rooted grass grows on mildly alkali soils, dryer
than those occupied by sagebrush. Photo by H. L. Shantz.

Fig. 395. Sagebrush vegetation on Trout Creek, Oregon. Photo by W. H. Camp.

770

TEXTBOOK OF BOTANY

In all semi-arid and arid regions there are undrained basins where
continued evaporation of water that flows into them from adjacent up-
lands leads to the accumulation of salts. These accumulations var\" all
the wav from common salt, basic carbonates, sulfates, to borax. These
salts are variously toxic, and when concentrated exclude all plants, but
when dilute max not interfere with the growth of a limited number of
plants such as grease wood, several species of salt bush, salt grass and
the succulents: seepweed, pickleweed, and samphire.

Desert (Figs. 397-400). The driest areas of North America are in
Lower California, adjacent parts of California, and parts of the Sonora

Fig. 396. A sagebrush desert near Nephi, Utah. The plants pictured here are
about 4 feet high and range in age from 30 to 50 years. The associated species
are mostly annuals which grow only during the rainy periods. Sagebrush is the
characteristic natural vegetation on the better soils of the Great Basin region.
Photo bv H. L. Shantz.

and Chihuahua deserts. The region as a whole has about 10 inches of
rainfall, but local areas ha\e as low as 3 inches and in some vears no

[Chap. Lin THE VEGETATION OF NORTH AMERICA

'71

Fig. 397. Giant cactus and desert shrubs near Tucson, Arizona. Photo bv A. E.

Waller.

k i

W".

}\L' :!; M asdiltv- k

Fig. 398. Desert vegetation on sand along the shore of the Gulf of California,
Libertad, Sonora. The cactus in the foreground is Opuntia bigelovii; in the back-
ground, Pachycereus pringlei. Note the osprey soaring in the background. Photo
by W. S. Cooper.

772

TEXTBOOK OF BOTANY

rain at all. At the same time it has the highest evaporation rate. Winter
temperatures are mild, and freezing temperatures are of short duration.
Here is the most bizarre assemblage of plants on the continent. The
largest and greatest variet\' of the columnar cacti are found in this
region. Some are much-branched and up to 40 feet in height. They all
have shallow wide-spreading root systems. All have exceedingly low
transpiration rates, because of impervious outer layers of cortical tissue.

Fig. 399. The native "desert" palm of southern Cahfornia (Washingtonia) in
Palm Canyon, where undergroimd water is always available. Photo by U. S. Forest
Service.

Some have closed stomates during the daytime. A year's supply of water
has accumulated in many of these plants, and this may amount to as
much as 25 to 30 tons in a single plant.

In Arizona there are two rainy periods, winter and summer. The rains
are typical downpours of short duration. During the winter rains the
desert "blossoms" with various flowering plants, some of which are com-
mon farther north. During the summer rains another assemblage of
annuals and perennials with more tropical affinities becomes conspicu-
ous. Between the rainy periods the perennial scattered woody plants and
succulents give character to the landscape. Mesquite, creosote bush.

[Chap. LIII THE VEGETATION OF NORTH AMERICA 773

acacias, yuccas, and the various species of flat-jointed and cylindrical
cacti are most conspicuous. These plants either have a low rate of tran-
spiration, accumulate water during rainy periods, or are uninjured by
prolonged droughts. The yuccas are deep-rooted, the cacti shallow. The
annuals live as long as the soil water lasts.

Fit;. 4UU. Colorado Desert, in soutiierii California. The "barrel cactus" is Fcnocactiis
ci/Iindraccus. Photo by W. S. Cooper.

Forests of the western mountains. From southern Alaska to southern
Mexico is a series of high mountain ranges, and between the Sierras and
Rockies a series of high plateaus. The plateaus also have small north
and south ridses on them, manv of which are of sufficient height to
intercept rainfall in excess of that of the arid plateau. The vegetation of
all these mountains is dominated by coniferous forests as far south as
central Mexico. To be sure, there are fringes of deciduous and evergreen
oaks, poplars, willows, and, locally, other broad-leaved trees. Oaks are
proportionately more numerous and widespread on the mountains of
southern California and Mexico. In Central America they are gradually
replaced by tropical scrub and jungle. Considering the great extent of
the forests from the tundra to the tropics, and the frequency of local
differences in altitude of 3000 to 15,000 feet, the diversity of climatic
and soil conditions may be inferred.

To describe the vegetation adequately would require consideration of

TEXTBOOK OF BOTANY

F'iG. 401. Sitka spruce and western hemlock forest near the Alaskan coast. Alaskan
Aerial Survey Expedition, 1929. Photo from U. S. Forest Service.

Fig. 402. The Sitka spruce and western hemlock cover vast areas in the \'alleys
and slopes of southern Alaska. Alaskan Aerial Survey Expedition, 1929. Photo
from U. S. Forest Service.

[Chap. Llll THE VEGETATION OF NORTH AMERICA 775

each of numerous subdivisions. Here we can sketch briefly only a few
characteristic forests.

Pacific coastal forest (Figs. 401-409). The most magnificent forest in
temperate regions occupied the western slopes of the coastal moun-

Fk:. 403. Interior \ lew ot mature lorest of western hemlock (Tsuga hetewphylla)
on a mountain slope in Washington. Photo hom U. S. Forest Service.

tains between Vancouver and San Francisco. Rainfall up to 125 inches
occurs along the Oregon coast, with a gradual decrease northward
to Alaska and southward to San Francisco. Moreover, this is a belt
of fos^s and low clouds. Rainfall is least during summer. These
conditions rarely extend more than 30 miles inland except in north-
ern Oregon and Washington where there is a break in the coast
ranges.

Beginning in Alaska the Sitka spruce is most abundant in the shore

776

TEXTBOOK OF BOTANY

Fig. 404. Western arbor vitae (Thuja plicata) in the Columbia National Forest,
Washington. The larger trees pictured here are from 4 to 6 feet in diameter. Photo
from U. S. Forest Service.

[Chap. LIU THE VEGETATION OF NORTH AMERICA

777

Fig. 405. A mature stand of Douglas fir {Pseudotsuga taxifolia) in Washington.
Photo from U. S. Forest Service.

778

TEXTBOOK OF BOTANY

Fig. 406. Margin of a redwood (Sequoia sempervirens) forest in California.
Three large trees near the horse in the picture are 7 feet in diameter and 260 feet
high. Photo from U. S. Forest Service.

[Chap. LUI THE VEGETATION OF NORTH AMERICA

779

Fig 407 Interior view ot a redwood iorest on Bull Creek flat, Trinity National
Forest, California. Photo by U. S. Forest Service.

780

TEXTBOOK OF BOTANY

forest southward for nearly 2000 miles. Because of this distribution it is
often called tidewater spruce. At higher elevations and in the more
mature forest western hemlock is dominant in the Puget Sound region;
both these trees attain heights of over 200 feet. The spruce has trunks
up to 15 feet in diameter, but the hemlock is more slender and the
maximum diameter is about 8 feet. In Washington and Oregon, Douglas
fir is by far the most abundant species. Its greatest recorded height
is near 400 feet. It grows rapidlv and overtops other species, is long-

FiG. 408. The Monterey cypress is the characteristic tree on the coast of
Monterey Bay, California. Many of the trees are distorted by wind and salt spray.
Photo by U. S. Forest Service.

lived and but little injured by fire. Other important species in these
forests are the western arbor vitae, a cedar, and several firs — all large
trees.

From southwestern Oregon the redwood becomes the dominant tree
of the moist, fog-laden slopes. It grows to a height of 340 feet, and the
trunk is usually 10 to 15 feet, rarely 20 feet in diameter. The coastal
forest not only contains these giant trees, but is noted for the luxuriance

[Chap. LIII THE VEGETATION OF NORTH AMERICA

781

of the undergrowth which includes tall ferns, and shrubs among which
are the devil's club, salmonberry, rhododendron, and dogwood. One of
the most remarkable features of this forest is the short distance between
trees. It is quite evident that the root systems are small compared with
the tops. Such enormous top development is possible onlv where trans-
piration is low and soil water alwavs available.

Still farther south the coastal ranges are clothed with evergreen oaks
and chaparral, a community of hard and leathery-leaved evergreen
shrubs and trees.

Fig. 409. Chaparral, Santa Lucia Mountains, Caliiornia. Manzanita (Arcto-
staphylos glaiica) , grayish-colored trees in shallow valley on left; coast live oak
{Quercus agrifolia), dark-colored trees in deep valley (lower right); chamiso
{Adenostoma fasciculata) , the apparentlv smooth vegetation cover on the crests
and the more exposed slopes. Photo by W. S. Cooper.

Other western mountain forests (Figs. 410-416). In the region of
Washington, Idaho, and Montana are extensive areas dominated by
western white pine, which is accompanied by the western larch, western
hemlock, white fir, and red cedar. These latter species form an eastward
extension of the coastal forest and here reach their eastern limit as
understorv trees among the western white pine.

Of all the Cordilleran forest trees, the western yellow pine and

782

TEXTBOOK OF BOTANY

Douglas fir cover by far the largest areas. The western yellow pine
occurs in open stands alone, or associated with other conifers, from
the eastern slopes of the Cascades, the Sierras, the Columbia basin,
the lower elevations of British Columbia southward in the Rockies
to the Arizona Plateau and the mountains of Texas. It occupies
rather drv sites in regions with 20 to 30 inches of rainfall adjoin-
ing grasslands, chaparral, oak scrub, or the dry pifion-juniper wood-
lands.

Fig. 410. Mature western yellow pine {Pimis ponderosa) , Eldorado National Forest,
California. Photo bv U. S. Forest Service.

At its northern limit the \ ellow pine occurs at elevations up to 3000
ft., and in Arizona and New Mexico, 6000 to 8000 ft. In the Sierras on
cool and moist slopes it is associated with Douglas fir, sugar pine, and
incense cedar. The mixed forest of pine and fir occurs in the Sierras up
to 5500 feet and at from 6000 to 9000 feet in the central Rockies. Forests
predominantly of Douglas fir occur at somewhat higher elevations. In the
northeni Rockies the western larch, the western white pine, and the
lodgepole pine may occur with Douglas fir in varying proportions. The
lodgepole pine is a pioneer tree that invades burned-over lands at all
levels from the dry yellow pine belt at low elevations through the belts

[Chap. LIU THE VEGETATION OF NORTH AMERICA

783

Fig. 411. Forest of western arbor vitae and Douglas fir on a mountain slope in
Idaho. Photo from U. S. Forest Service.

784

TEXTBOOK OF BOTANY

Fig. 412. A mature stand of western white pine, larch, arbor vitae, and hemlock
in the Kaniksu National Forest, Idaho. Photo from U. S. Forest Service.

[Chap. LIU THE VEGETATION OF NORTH AMERICA 785

of Douglas fir and Englemann spruce. This spruce forms the highest
belt of widespread forests in the Cordillera. The lodgepole pine grows
best in cool moist regions where the Douglas fir belt and Englemann
spruce overlap.

Englemann spruce dominates on cool, moist slopes where there is
abundant snow and a long dormant period. In British Columbia these

Fig. 413. A mature stand of western larch (Larix occidentalis) in the Lolo National
Forest, Montana. Photo from U. S. Forest Service.

slopes are only a few thousand feet above sea level while in Arizona
they lie between ten and twelve thousand feet. Associated with the
Englemann spruce in various parts of its range are several species of
fir, the alpine larch, mountain hemlock, white-bark and bristle-cone
pines. Above this belt lie the Alpine meadows, or high mountain tundra,
dominated by sedges, grasses, heaths, and low-growing willows, birches,
and alders.

786

TEXTBOOK OF BOTANY

Fig. 414. Englemann spruce {Picea englemannii) in a valley of the Sawtooth
Ranges, Arapaho National Forest, Colorado. Photo from U. S. Forest Service.

Fit;. 415. Subalpine forest of alpine fir and Englemann spruce adjoining alpine
meadow, Chelan National Forest, Washington. Photo by U. S. Forest Service.

[Chap. LIU THE VEGETATION OF NORTH AMERICA

787

Fig. 416. Lodgepole pine {Pinus contoita

from U. S. Forest Service

Cache National Forest, Utah. Photo

788

TEXTBOOK OF BOTANY

Pinon-juniper woodlands (Fig. 417). Usually below the western yel-
low pine belt where the rainfall is somewhat less or the evaporation
greater, there are extensive open woodlands particularly in the Great
Basin region between the Sierras and the Rockies and on the Arizona

Fig. 417. Juniper and pinon pine form the characteristic woodland on the plateaus
and lower slopes of mountains in the Southwestern States. Photo by U. S. Forest
Service.

plateau. Two small species of pine and several species of juniper occur
in various combinations over vast areas. Collectively they may be called
the pinon-juniper woodland. South of the Arizona plateau it is mixed
with and finally replaced by scrub oak woodlands (encinal). Below it
are the still drier grasslands and semi-desert scrub.

The higher levels of the Mexican plateau are also covered by pine
forests, which contain at least 6 species in addition to the western yellow
pine. Douglas fir is again present on mountains above 9000 feet, with
lodgepole pine, trembling aspen, and a Mexican fir. At levels below the
pine forests are areas of the pinon-juniper, or the scrub oak encinal,
together with shrubs and low trees of the dry tropics.

[Chap. LIU THE VEGETATION OF NORTH AMERICA 789

Tropical forests (Figs. 418-420). The tropical forests of southern Mex-
ico and Central America are made up of a large number of broad-leaved
evergreens, including palms, tree ferns, figs, rubber trees, various kinds
of mahogany, silk-cotton trees, Spanish cedar, logv^ood, and hundreds
of other species. The rainy tropical forest when undisturbed attains
heights of 200 feet, and there may be several tree layers beneath the

Fig. 418. Natural live oak forest near Miami, Florida. Photo from U. S.

Forest Service.

highest canopy, but the forest is open below. Stands of this kind still
occur on certain of the Caribbean islands, and in Guiana and Venezuela.
Mostly the present-day rainy tropical forests are jungles: tangled growths
of trees, vines, shrubs, and tall herbs difficult to penetrate, and containing
so many species that they can scarcely be described. The rainfall is as
much as 200 inches a year and the dry season is very short. Temperatures
are always high.

Jungles result from repeated cutting, burning, and clearing of the
land. The lateritic soils of the rain forest are not rich in mineral salts
and are soon leached and eroded under native methods of cultivation.
Furthermore, weeds from seeds and from underground roots and

790 TEXTBOOK OF BOTANY

rhizomes are difficult to suppress. Consequently the area is soon aban-
doned and the native moves to a new area. This kind of "migratory
agriculture" has been going on for several thousand years, and few are
the areas in which secondary forests have actually reached a completely
stable condition. One has only to travel through our southeastern states
to see comparable effects of migratory agriculture in this country.

Fig. 419. Tropical jungle on a mountain slope in western Cuba. The palm-like
trees are cycads (Micwcijcas) . Photo by O. W. Caldwell.

Tropical muddy coasts are characterized by mangrove swamps. These
small trees have extensive root systems which interfere with wave action
and increase the deposition of sand and mud by the shore currents.
Certain species also have embryos that genninate and grow more than
a foot in length before they drop from the tree into the mud below.
Mangrove swamps consequently cause the extension of land seaward. It
is said that this process has added 2500 square miles to the west side
of the Florida peninsula. Succession of plant associations occurs; and the
mangroves may be followed by grasslands as in Florida, or by scrub or
rain forest as along the Central American coasts.

The western slopes of Central America have decreased rainfall be-

[Chap. LIU THE VEGETATION OF NORTH AMERICA

f91

Fig 420 A tropical jungle in Hawaii: tree ferns, the climbing rattan palm, and a
banana plant in the foreground. Photo by U. S. Forest Service.

792 TEXTBOOK OF BOTANY

cause of the mountains, and the vegetation varies from deciduous dry
forest to tree savannas, scrub, and desert.

The products of the tropical forest region inchide bananas, cotton,
sugar cane, cacao, coffee, and many kinds of fruits. The banana planta-
tions are at lower elevations, the coffee at elevations of two or three
thousand feet.

One must not forget that on the mountains of tropical regions there is
a temperate belt and even cold temperate summits where frosts occur
regularly, and, on a few of the highest peaks, permanent snow. Unlike
the continental temperate regions the climate is more uniform, the day-
light period constant; the most variable factor is the length of the rainy
and dry seasons.

The wettest forests in the rainy tropics are those on mountain slopes
in the cloud belt. The trees are usually of low stature, and variously
gnarled and twisted. They are shrouded with epiphytic orchids, bro-
mehas, filmy ferns, mosses, lichens, fungi, and algae which accumulate
and continuously destrov the plants beneath. Constant moisture, mod-
erate light, abundant mineral salts in the plant debris, and nearly con-
stant temperatures foiTn an environment so nearly "ideal" that hundreds
of plant species compose an ever-changing living mass of vegetation.

Two centuries of destruction. Except along the sea coasts, most of the
United States was a wilderness less than two hundred years ago, and

Fig. 421. Effects of sheet erosion in a forest in Oklahoma started by overgrazing
of the grasslands higher up the slope by sheep. Photo from U. S. Soil Conserva-
tion Service.

[Chap. LIU THE VEGETATION OF NORTH AMERICA

793

only a little more than a century has passed since the lands away from
the larger rivers and lakes were occupied and brought under agricultural
and industrial control. During this short period most of the original vege-
tation has been destroyed. The accumulated fertility of the upper soil
horizons has been exploited by wasteful methods of cultivation and
destructive grazing, often accompanied by excessive drainage, erosion,
and the floating of the top soil to the river bottoms and the ocean.

Conservation (Figs. 421-424, 25-26, and 180-181). Only recently has
there been official recognition of and real concern about our decadent

Fig. 422. Ungrazed and overgrazed grama grassland in New Mexico. Photo from
U. S. Soil Conservation Service.

794

TEXTBOOK OF BOTANY

Fig. 423. Picture taken during a sand storm in Dallas County, Texas. The farm has
been ruined by wind erosion of soil. Photo by U. S. Soil Conservation Service.

Fig. 424. Dust storm in Prowers County, Colorado; wind velocity, 30 miles per
hour; duration of storm, about 3 hours. Photo by U. S. Soil Conservation Service.

[Chap. LIU THE VEGETATION OF NORTH AMERICA 795

woodlands and die depletion of our agricultural, game, and recreational
resources. This belated awakening has resulted in the initiation of wide-
spread research in land classification and utilization. Sincere attempts
are now being made to apply the results of scientific research to the
complex problems of reforestation, range improvement, soil conservation,
game management, stream control, and recreational facilities.

Detailed knowledge of the original vegetation of America as it was
when the explorers and first settlers saw it is a valuable index to climatic
conditions in relation to the growth of different kinds of forests, crop
plants, and grass. Natural vegetation is a reliable index of the possibili-
ties of plant growth, because it portrays the results of the operation of
both climatic and soil factors on plants over a long period of time. The
original soils no longer exist; and many years of natural processes and
intelligent procedure on the part of man will be necessary to restore
the qualities that made possible the best growth of trees and cultivated
plants. We can decrease the rate of present wastage and follow pro-
cedures that will speed up the redevelopment of top soil where we are
now farming the lowest soil horizon, or its "parent material."

REFERENCES

Bennett, H. H. Soil Conservation. McGraw-Hill Book Company, Inc. 1939.

Lord, R. Forest Outings. Papers by 30 foresters on the use of national forests
as recreation. Edited by Russel Lord. 4to. Washington, D. C. Superin-
tendent of Documents. 1940.

Shantz, H. L., and R. Zon. Natural Vegetation. Part I. Section E. Atlas of
American Agriculture. 1924. The Agricultural Atlas also contains informa-
tion on climate and soils.

Westveld, R, H. Applied Silviculture in the United States. John Wiley & Sons,
Inc. 1939.

Many articles on the vegetation of the world may be found in the journals:
Ecology, Ecological Monographs, and the British Journal of Ecology.

INDEX

Numbers in bold lace indicate pages with illustrations

Aberrations, chromosomal, 475-485
Abscission, 73, 170, 402
Absciss-layer, 32, 73

Absorption, 320, 322, 323, 325
Acclimation (see Adaptation)
Accumulation ot food, 51. 114, 131, 140,

145, 147; of water, 222, 230, 710
Aceraceae, 71 1
Acetic acid, 155
Achlya, 548
Acidity, 31, 43, 219
Acids, amino, 143, 175: fatty, 138, 168,

oleic,
138

palmitic,

175; lactic, 156;
138, 139; stearic.

Adaptation, 498

Adder's-tongue, 745

Aecia, 560, 561

y\eration, 310

Aerobes, 518

After-ripening. 395

Agave, 85, 88

Air spaces in leaves, 70, 71, 81, 87. 228

Akene, 363

Albumin. 142

Alcohol. 155

Algae, 5. 17. 565, 606-637: blue-green,
608, 621, 623, 636; brown, 601. 608,
628, 629, 630, 632; colonial 615; coral-
line, 634, 635; economic aspects of,
609; filamentous, 616, 619, 620; fossil.
625. 727; green, 608, 613-621; life
cycle, simimary of, 636; periodicity of,
611, 612: plankton, 611; red. 608, 633.
634; soil, 610: imiccUular, 615; yellow-
brown, 608; yellow-green, 608, 627,
636

Alkalinity, 31

Alkaloids, 169, 174

Amanita, 542

Amides, 144

Amino acids. 142-144, 146. 175

Aminiaceae, 712

Ammonia, 144, 532

Ammonification. 532, 537

Amorphophallus, 358

Amylase, 133

Amylopectin, 133

Amyloplasts, 65

Amylose, 133

Anaerobes, 518

Anaphase, 446

Andreaeales, 650

Angiosperms, 368, 687-690, 693-717

Animals, vs. plants, 1, 2, 5, 45, 107, 607;
fossil restorations of. 732, 733

Annuals, 32, 46

Anther, 354, 373

Antheridia, 547, 618. 647

Anthoceros, 652

Anthocyanins, 28. 31. 169

Antibody, 524

Antitoxin, 524

Apical dominance, 250. 410; growth. 278

i\ppert. 527

Apple, diseases of, 571, 574; energy trans-
formations in, 184; flower and fruit,
356; scab, 572, 576; seedlings. 348;
spur, 254

Aquariiun, balanced, 180

Aquatics, 90, 587, 601, 606. 667

Arbor vitae, 311; western, 776, 783, 784

Archegonium, 646, 666, 685

Archeozoic, 723, 727

Aristotle, 101, 526

Armillaria, 420

Arsenites and algae. 609

Arundinaria, 762

Ascocarp, 550, 551

Ascomycetes, 552

Ascospores, 550, 551

Ascus, 550. 551

797

798

1 iN D E X

Asexual reproduction, 545, 549, 617, 647

Aspergillus, 548, 550

Aspletjium, 657

Assimilation, 109, 147, 176

Associations, climax, 754

Atoms, 95, 98

Autophytes, 506, 511, 516

Autumnal aspect, 27; coloration, 27

Auxin, 175, 212, 305

Auxospore, 627

Avocado, 382

Azolla, 624, 667

Azotobacter, 537

Bacillariophyceae, 608, 625-627

Bacteria, 5, 8, 515-530, 531-540, 596, 602;
acetic acid, 156; aerobic, 537; am-
monifying, 532; anaerobic, 537; and
sanitation, 523; and sugar synthesis,
113; biology of, 515; characteristics of,
520; control of, 529; denitrifying, 534;
factors affecting, 517-520; green sulfur,
517; iron, 512, 539, 724; lactic acid,
156; multiplication of, 521; nitrifying,
512, 532; nitrogen-fixing, 191, 535,
536; of fireblight, 8, 568; of soil (see
Soil); pathogenic, 524; purple, 113; re-
production in, 521; size of, 521, 522;
sulfur, 512, 538; types of, 516

Bacteriology, 525, 529

Bacteriophage, 521, 522

Bamboo, 271, 272, 404, 762

Banana, 701; family, 699

Banyan tree, 299

Barberry, 560-561. 567

Bark, 262

Basidia, 555, 560

Basidiomycetes, 556

Basidiospore. 555, 561

Bateson, 488

Bairachosperimnn , 637

Bays, 598

Beach grass, 404; leaf, 91

Beal, 397

Bean, 10; development of, 11, 12. 54

Beech, 756; family, 704

Bennetittales. 730

Bent grass, 43

Bermuda grass. 44

Berry. 363. 365

Betulaceac, 704

Biennials, 33, 46

Bindweed, 413

Biologist, 7

Biology, 5

Birch family, 704

Bisexual, 671

Blade, leaf, 13, 14, 76

Blasia, 645

Blight, 574; chestnut, 704; fire, 568, 582;
potato, 553

Blotches, 573

Bluegrass, 40, 43; leaf, 91

Bluestem, little, 765; tall, 764

Bogs, 597, 650

Bordeaux mixture, 578

Boron, 329

Botany, 2, 7

Botrychium, 665

Botrydium, 628

Boussingault, 531

Brassicaceae, 706

Bread mold, 543

Breathing, 151

Bromeliaceae, 300. 699

Broom-rape, 506

Brownian mcjvement. 199

Brown-rot, 578, 579

Bryales, 649

Bryophyllum, 402: leaf, 421

Bryophytes, 638-654

Budding, 292, 546

Buds, 24, 36, 247; accessory, 247: ad-
ventitious, 250; axillary, 13; branch,
248; composition of, 248; develop-
ment of, 246; flower, 248, 335; fruit,
248; latent, 250; lateral, 246, 247, 249;
leaf, 246, 248; of potatoes, 409; repro-
ductive, 246; scale scars of, 246; scales
of, 24, 246, 248; terminal, 10, 13, 69,
246. 247, 249; vegetative, 246, 248, 335
Buffalo grass, 695. 767
Bulblets. 258
Bulbs. 257, 410
Bull, Ephraim, 434

Bundle scar, 246; sheath, 71. 72; vascu-
lar. 13, 71, 72, 265, 266. 271
Bunt, 558
Burrill, 567,
Button bush flowers. 715

INDEX

799

Cabbage, 344, 346, 426, 482, 582; do-
mesticated varieties of, 426; palmetto,
252

Cactaceae, 710

Cactus, 16, 85, 711, 771, 773; family, 710;
flowers, 710

Calamites, 727

Calcium, 177, 328; pectate, 171

Callose, 543

Callus, 418

Calyptra, 644, 648

Calyx, 353

Cambium, 266; cork, 280, 281; secon-
dary, 306; vascular, 265, 281, 284, 304

Cambrian period, 727

Camerarius, 433, 487

Camptosorus, 663

Cane brake, 762

Cankers, 574

Canopy, 753

Capillarity, 197, 201

Capsule, 366

Carbohydrates, 105, 134

Carbon, 177; "assimilation." 109; diox-
ide, 98, 108, 152, 589, 593; "fixation,"
1 10; monoxide, 416

Carboniferous plants, 725, 730

Carotene, 28, 175, 608

Carotenoids, 28, 31, 169

Carpel, 336, 354

Carpogonium, 635

Carpospore, 636

Carposporophyte, 637

Carrot family, 712; flowers and fruits,
713

Caryopsis, 365

Casein, 99, 142

Cast, 702

Catkin, 360, 361

Cause and effect, 50-58 [see also Preface)

Celery, 345, 347

Cell, 59, 67, 169; a biological unit, 59-67;
differentiation, 70, 279, 302; divisioji,
55, 70, 302. 446, 447, 523; enlarge-
ment, 55, 70, 211, 213, 302; wall, 60,
65

Cells and tissues, 68; companion, 282,
283; epidermal, 74. 78; guard, 74, 78;
meristematic, 68, 84; neck, 685; parts
of, 60; phloem, 283; subsidiary, 74;
ventral-canal, 685

Cellulase, 170

Cellulose, 65, 169, 543; synthesis of, 168;

test for, 170
Cenozoic floras, 732
Chaparral, 781
Chara, 621
Charales, 621
Checkerboards, 467
Chemical elements, 100, 600; processes,

97; substitution. 99
Chemistry, useful, 95-100
Chemosynthesis. 113, 114, 511
Chestnut, 704; blight, 704
Chimera, 419; graft, 419; mutant, 419;

periclinal, 420; sectorial, 420
Chitin, 543
Chlainydomonas. 614
Chlorcnchyma, 72, 118; cortical, 264; in

roots, 119
Chlorine, 177

Chlorophyceae, 608, 613-621
Chlorophyll, 28-30. 608; a and b, 28;

carbonate, 125; molecule, 125
Chloroplasts. 60, 65, 122; ninnbers and

areas of. 122
Chlorosis, 575
Chromoplasts, 65
Chromosome, 443; aberrations. 475-485;

complements of cells, 446, 450, 649.

667, 684, 689; behavior, 444, 446, 452.

475; numbers, 443
Chromosomes in cross-fertilization, 453:

in life cycles, 450; in reduction divi-
sion, 451
Chrysophyceae, 608
Cladodia,'259
Cladonia, 563
Cladophora, 61 1

Climate, 725, 729, 740, 757, 763
Climax associations, 754
Clone, 400, 401, 403,
Clostridium, 537
Clover. 43, 45; flowers, 708
Club fungi, 556; mosses, 668-673; root,

582, 58.3
Coagulation, 62, 211
Coal deposits, 656
Colchicine, 477
Coleorhiza, 295
Coleus buds, 69; leaves, 73, 435; stem

tip, 64

800

INDEX

Collenchyma, 72, 281

Colloids, 61

Color, 27; leaf, 28; spectrum, 27, 115

Communities, plant. 40-49

Companion cells, 282, 283

Compass plant, 81

Compensation point, 164

Compositae, 717

Conceptacle, 633

Concord grape, 400, 434

Condensation, 99. 129, 138, 143, 168

Conidia, 548, 550. 551

Conidiophore, 548

Conifers, 682-687

Conjugation. 667, 620, 622; tu!)e, 620

Conservation. 185. 405, 406, 793

Convolvuhiccae. 714

C:opper, 330; sulfate, 578, 609

Copy inns, 4

Cordaites. 730

Cordillcran center, 734

Cork, 280, 281

Corms, 257, 258. 358. 410

Corn, energy transformations in, 184;

flowers, 359; harvesting liy Indians,

423, 427; hybrids. 470-472; plant, 177
Corolla, 354
Correns, 435

Cortex, 13, 223. 264. 266, 280, 281, 303
Corymb. 360, 361
Cosmos, human, 182
Cotyledons, 10, 11, 13, 294, 296, 367
Creosote bush, 768
Cretaceous flora, 731
Cross-fertilization, 382, 442, 458-473
Crossing-over, 481
Crowfoot family, 706
Crown of trees, 41
Cruciferae, 706
Cucurbitaceae, 717
Culture media, 518; solutions, 193, 328,

331; tank, 331, 332
Cup fungi, 553
Cuticle, 71

Cutin, 65, 71, 168, 169
Cuttings, 191, 400, 414; leaf, 421, 422;

root, 420; stem, 414
Cycads, 681, 688, 790
Cylinder, vascular, 264
Cyperaceae, 697

Cypress, 678; bald, 761; "knees," 310,

762; Monterey, 780
Cystine, 144
Cystocarp, 637
Cytoplasm, 59, 60, 63

Damping-ott, 577

Darkness, growth in, 53, 54 (see also
Light)

Darwin, Charles, 487, 500

Day-neutral plants, 344

de Bary, 567

Deciduous habits, 32

Dehiscence, 372

Denitrification, 534, 538

Desert, 770; plants, 771, 772, 773; shrub-
land, 741, 768, 771

Desiccation, 101

Desmids. 622

Desulfofication, 539

Devonian floras, 727

de Vries, 435, 488

Dew, 321

Dextrin, 133

Dextrinase, 133

Diastase, 129, 175

Diatomaceous earth. 625

Diatoms, 97, 602, 608, (i25. 626, 636

Dicots, 368

Differentiation (see Cells and Tissues)

Diffusion, 198

Digestion, 99, 129. 139, 170

Dinoflagellates. 608

Dinophyceae. 608

Dioecious. 360

Diploid, 451, 636

Disaccharides, 134, 135

Diseases of plants, 2. 567. 572. 584; and
environment, 570. 571; control of,
575-585; "deficiency," 575; economics,
569; nature of, 568; symptoms of, 573;
virulence of, 572

Dissemination, pollen. 383: seed, 368

Distribution, plant. 165. 591 (see also
Vegetation of North America; Plants
of the past)

Dodder, 298, 301, 505

Domestication, 423 et seq.

Dominance, 456, 459

Dormancy, 32, 34, 259, 397

Douglas fir. 777, 783

INDEX

801

Drought and leaf development, 82; and
spinescence, 83; physiological, 205,
210

Drugs, 174

Drupe, 363, 365

Duckweeds, 15

Dust storm, 794

Dwarf branch, 19; male, 618

Dyad, 372

Dyes, natural, 172

Economic aspects, of algae, 609; of bac-
teria, 523-525, 582; of ferns, 659; of
flowers, 369; of fruits, 369; of fungi,
559, 567-572; of leaves, 93; of liver-
worts, 645; (jf mosses, 645; of seed
plants, 693-717; of seeds, 369

Ectocarpus, 629, 631, 636

Ectogony, 381

Egg," 374, 465, 466. 618, 647, 666; fer-
tilized, 376; parthenogenetic, 380

Elaioplasts, 65

Elaters, 654

Electrons, 96

Elements, chemical, 100, 330

Elm, 679; family, 705

Elodea, 1 1 1

Embryo, 10, 11, 295, 367, 376, 378, 665,
689; development of, 377, 686; sac,
374, 380; to seedling, 388

Endodermis, 265, 280, 304

Endogenous origin, 304

Endosperm. 367, 376, 378, 380; inheri-
tance, 469, 471, 472; nucleus, 376

Energy, bound, 98, 109, 150, 152, 155;
free, 98, 109, 152, 155; radiant, 115;
sources of, 1, 115, 150, 186, 195, 199,
212; transformations of, 110, 183-186

Englemann spruce, 786

Environment and leaf develojMuent, 76-
83; and pigments, 29-31; and muta-
tions, 482; and photosynthesis, 115-
118, 125; and respiration, 160-165;
fresh water, 590; land, 589; salt water,
598; under-water, 587-605

Enzymes, 106, 123, 129, 139, 155, 156,
169, 174, 175. 532

Eocene period, 723
Ephedra, 680

Epidermis, 70, 71, 120, 265, 280, 303;
and stomates, 74

Epiphytes, 297, 299, 300, 507, 508

Equiseluni, 675

Equisetums, 673-676, 727

Eras, geologic, 723

Ereptase, 175

Ergosterol, 175

Ergot, 569

Ericaceae, 713

Erosion, 46, 47, 48, 405, 406, 792, 793,

794
Essential oils, 173
Ei(glena, 607, 615
Eiiglenophyceae, 608
Evaporation, 80; lifting power of. 227
Evergreen habit. 32
Evolution and plant origins, 495, 738
Exogenous origin, 304

F, generation, 442, 460
F. generation. 442, 460
F;, generation. 442. 462
Factors, climatic {sec Climate): heredi-
tary, 462-466; prehistoric, 742; soil
(see Soil)
Fagaceae, 704
Fairy rings, 557
Fascicles, 19

Fats, 103, 106, 138, 148. 175; accumula-
tion of. HO; digestion of, 139-110;
synthesis of, 138-141
Fatty acids, 138, 168
Fermentation, 155-157, 527
F"ern, adder's-tongue, 745; Boston, 136.
484; cinnamon, 660; grape, 665; leaf,
76, 212; polypody. 8; royal, 665; seed,
729; staghorn, 658; sword, 484; tree.
661; walking. 663
Ferns. 5, 8, 15, 657-668; and chromo-
somes, 665; aquatic. 667; fossil, 659:
reproduction of, 662-666
Fertilization, 376, 665, 684, 689; cross.

440; self, 442
Fertilizers, 44, 610
Fescue grasses, 43
Fibers, pericycle, 264, 281; phloem, 264,

283
Fife. David. 435
Fig fruit, 363; strangling, 299
Filament, 354. 616, 619
Fireblight, 568, 582 X\>

Fishes, 601, 604, 610 /Cr><:ros7>s. <

>

LIBRARY

.A^i

802

INDEX

Flaccidity, 206

Flagella, 607, 614

Floral cup, 356, 357, 362; envelope, 354

Floras, Cambrian, 727; Carboniferous,
729; Cenozoic, 731; Devonian, 727;
fossil, 719-738; Mesozoic, 730; Paleo-
zoic, 727; pre-Cambrian, 727; present,
740-795

Flower, 12, 14, 351, 353; bud, 248, 335;
clusters, 360; color, 352; complete,
357; perfect, 357; pistillate, 359; pri-
mordial, 249, 333, 336; section of, 352;
simple, 353; staminate, 359

Flowering plants, 687-690, 693-717; time
of, 340-348

Flowers, development of, 354; economic
value of, 369; initiation of, 333-350

Fluctuations, 436, 491 {see also Light
effects; Temperature effects, etc.)

Follicle, 366

Food, 1, 101, 106, 107, 147, 177; chains,
603, 604, 610; classes of, 105; digestion
of, 99, 129, 139; energy content of,
166; in cells. 103; manufacture of, 107,
108-148, 176; of green plants, 102,
106; oxidation of, 98, 110, 155; sources
of, 101

Forage crops, 428

Forcing, 36, 260, 416

Forests, bamboo, 272; beech-sugar maple,
756; black spruce, 747; boreal, 741,
745; coastal, 741; deciduous, 741, 754,
755; hemlock, 640, 775; hemlock-
hardwood, 741, 750, 751; hemlock-
Sitka spruce, 680, 744, 774; live oak,
789; oak-hickory, 757; Pacific coastal,
775; pinon-juniper, 788; redwood,
778, 779; Rocky Mountain, 741;
southeastern evergreen, 741, 759;
spruce-fir, 57, 747; spruce-hemlock-
hardwood, 754; subalpine, 786; tropi-
cal, 789; western mountain, 773

Formaldehyde, 581

Fossils, 97, 719-738; ages of, 722, 723; of
fern leaves, 720; of leaf prints, 731;
of seeds, 721; of sequoia cone, 721;
of tree trunks, 721

Fox fire, 581

Fructose, 104, 109, 123, 175
Fruit bud, 248; spur, 254

Fruits, 11, 12, 14, 336, 353, 361, 369,
376; aggregate, 363, 366; complex,
363, 366; definition of, 363; dissemina-
tion of, 369; economic value of, 369;
formation of, 353, 355, 356, 362; mul-
tiple, 363, 366, 367; simple, 364, 365;
subterranean, 709; types of, 363, 364

Fucoxanthin, 608, 628

Fucus, 629, 632, 636

Fungi, 3, 17, 541-566; bear's-head, 542;
club, 556; coral, 556; cup, 553; dis-
eases due to, 575-582; economic as-
pects of, 559, 567-572; gill, 543; Im-
perfecti, 566; sac, 552; stinkhorn, 556;
sulfur mushroom, 556; tube, 551

Fusion, 545; nucleus, 376; triple, 376

Galls, 583, 584

Gametes, 372, 374, 617, 636

Gametophytes, 379; algal, 636; club

moss, 671, 672; equisetum, 675, 676:

fern, 663, 664, 665; flowering plant,

689: liverwort, 652, 653; moss, 644,

646; pine, 684
Garden pea, 355, 459, 462
Garner and Allard, 340
Gel, 62

Gemmae, 652, 653
Gemmae-cups, 653
Genes, 84, 444, 474, 485
Genotype, 462, 468
Genus, 24
Geologic eras, 723; life, 723; periods,

723
Geotropism, 214
Geranium, flowers and fruits, 362;

leaves, 132; stem, 281
Germination, delayed, 391-395; food

changes in, 390, 391; pollen, 374, 376;

seed, 10, 387-395
Ginkgo, 681
Girdling, 268, 349
Glaciation, 733, 734
Gliadin, 142
Globe radish, 296
Glucose, 104, 109, 123, 129, 148, 168,

169, 175
Glycerin, 129, 168, 175
Glycocol, 144
Glycogen, 129, 134, 624
Gnetiim, 680

INDEX

803

Gourd, 717

Grafting, 292, 3.^9, 417-420; types ol.
293, 418

Grain, 365; bird's-eye, 712: wood, 289

Gramineae, 693

Grass family, 693; flower, 357, 696; leal.
19, 76

Grasses, beach, 91; bent, 43; Bermuda,
44; blue grama, 695, 767; bluegrass,
40. 43, 91; buffalo, 695, 767; eulalia.
694; fescue, 43; giant reed, 694; lawn
and ]5asture. 42-46; needle, 766; orna-
mental, 694; poverty, 45; slender
wheat, 766, wheat, 766; winter fat.
769

Grasslands, 741, 763

Grazing, 793

Green plants, 101, 1«(), (i90: \s. noii-
green, 512

Greenland center, 734

Groundnut, 408

Growth, 36. 43, 161; and carbon mon-
oxide, 416; and cell enlargement, 211:
and hormones, 55, 81, 215; and in-
organic salts, 325, 327; and light, 53-
56, 215; and mineral deficiencies, 328:
and moisture {see Water): and nitro-
gen, 190; and rigidity, 210; and tem-
perature, 189; curvatures. 209, 215; in
leaves, 76; of lemon Irints. 231: pres-
sure, 212; root-shoot, 192; substances,
55, 81, 416

Guard cells, 74. 78

Gulfs, 598

Gums, 169, 172

Guttation, 321, 322

Gymnosperms, 368, 682-687

Gyrnnostomuin . 644

Hairs, epidermal, 87, 224
Hammocks, 758
Haploid, 451
Haustoria, 298. 506, 551
Hay fever, 385
Head. 258, 360. 361
Heartwood. 263
Heath family. 713; plant, 714
Hedivigia, 641
Hemicellulase, 175
Hemicelluloses. 169. 170, 175
Hemlock, 750; ^vestern. 784

Hepaticae, 645

Hereditary differences in leaves, 84-94:

factors, 455; potentialities, 443
Heredity, 29, 56, 84, 97, 147, 438-457
Heterospory, 668, 676
Heterozygosity, 467
Hibiscus, 435
Holdfasts, 258

Hollow log, 263; plants, 263
Homospory, 668, 676
Homozygosity, 467
Hormones, 55. 81, 169, 175, 193, 212,

213, 305, 335
Horse chestnut, stems and buds of, 246
Host, 507, 572
Human cosmos, 182
Human foods. 106
Hiunmocks, 211
Hybrid, 432, 481, 482; segregation, 432.

458-473
Hybridization, 459, 572
Hydrogen, 96, 177
Hydrolysis, 99, 130, 139
Hydroponics, 331
Hypha, 541

Hypocotyl, 10, 11, 294, 296. 367
Hypodermis, 280, 292

Imbibition, 199, 201

Immunity, 524

Inbreeding, 432, 468

Indian pipe, 30, 504. 509

Indians and agriculture, 423. 427; and

industries, 429, 430
Indusium, 664
Infection, 524, 572
Inflorescences, 360
Inheritance, 438; chromosomal, 445;

plastid, 444
Inorganic salts, 325, 594
Insectivorous leaves, 91
Internode, 10, 246, 247
Interpretations of plant behavior, 50-

58, 192
Interrelations of plant parts, 188-194
Inulase, 175
Inulin, 103, 134, 175
Invertase, 124
Iodine test, 103, 132
Ions, 61, 95
Iris, 85

804

INDEX

Iron, 177, 328; bacteria, 539, 724
Irrigation, 236
Isoetes, 673

Jerusalem artichoke and length of day,

349
Joshua tree, 273
Juglandaceae, 703
Juncaceae, 697
Jurassic period, 723

Kalanchoe leat, 402

Keewatin center, 734

Kelps, 631

Keys to plant species, 24, 83

Klebs, 337

Koch, 529; "rules of proof," 573

Koelreuter, 434

Kraus and Kraybill, 337

Kudzu vine, 405

Labiatae, 714

Labrador center, 734

Lactic acid, 156

Lakes, 590

Laminaria, 629, 632, 636

Lammas shoots, 255

Land habitat, 589

Lanolin, 215

Larch, western, 784, 785

Latex, 172, 282

Laurentian, 723

Lavoisier, 108

Lawes and Gilbert, 531

Lawn grasses, 40, 43

Lawns and pastures, 40-48

Layering, 403, 405

Leaf, 10, 12, 18-21; arrangement, 21, 23;
blade, 18, 246, 248; bud, 68; complete,
18; compound, 20, 21; cuttings, 421,
422; development, 70, 76; insectivo-
rous, 91; keys, 18, 24; margins, 21, 22;
mosaics, 81, 216; needle, 19, 682;
primordia, 68, 82, 84; prints, 731;
scales, 19, 86; scars, 24, 73, 246; sec-
tions, 70, 71, 77, 223, 682; sessile. 19;
sheath, 18, 86, 271; simple, 21; skele-
tonized, 111; spots, 574; surface, 122;
tendrils, 18, 86; tissues, 68-75, 71
Leaves, form and shape of, 19, 20, 21,
22, 85, 90; growing regions of, 76;

hereditary differences in, 84-94; of
cacti, 15, 85; rolling of, 90, 91; uses
of, 93; variegated, 132, 435
Leeuwenhoek, 525, 526
Legume, 363, 366
Leguminosae, 708
Lemanea, 357
Lemma, 357
Lemna, 15

Length of day, 118, 339-344, 349
Lenticel, 246, 247, 266
Lepidodeudron, 730
Leucoplasts, 64

Lichens, 5, 6, 31, 562, 563, 564, 565, 606
Liebig, 527, 531
Life cycles, 10, 667 {see also each of the

plant groups); summary of, 636, 690
Light, and abscission, 32; and autumn
coloration, 28-32; and bacteria, 517:
and bluegrass, 41; and epidermal cells,
78; and flower initiation, 339, 342,
343; and leaf color, 54; and leaf posi-
tion, 80; and leaf size, 53, 54, 77;
and leaf tissues, 77, 79; and photo-
synthesis, 109, 115, 163, 592; and plant
distribution, 165; and plant habitats,
589; and stem height, 53, 54; and
transpiration, 226, 229; and young
trees, 57; effects, 24, 31, 55, 216;
energy, 109, 110; in lakes and ponds,
590; in oceans, 599; spectrum, 27
Lignin, 65, 169, 171
Ligule, 19, 76
Liliaceae, 699
Lily, 700; family, 699
Lime, 44

Linkage, 459, 464
Lipase, 139, 175
Lipoids, 63
Lister, 529
Live oak, 508, 789
Liverworts. 651-654; leafy, 651, 652;

thallose, 651, 652
Long-day plants, 340, 341, 348
Longevity, 396, 679
Loranthaceae, 705
Lycopodium, 668, 671
Lycopods, 668-673, 727
Lysine, 144

INDEX

805

Madder family, 714

Magnesium, 177, 328

Magnolia, 693

Maltase, 133, 175

Maltose, 133, 175

Manganese, 328

Mangrove, 298; seedlings, 389

Maple, 756; family, 711; flowers and
fruits, 712; leaf, 77, 216; sap, 324

Marchantia, 652, 653

Marl, 593

Marshes, 597, 760

Marsh-marigold, 746

Marsilia, 667

Mass movement, 197

Media, 518, 589

Megasporangium, 374

Megaspore, 374; mother cell, 374

Megasporocyte, 374

Megasporophylls, 379, 684

Meiosis, 448

Membranes, artificial, 203; differentially
permeable, 202, 204; plant, 205

Mendel, 435, 459

Mendelian principles, 461; ratios, 458

Meristems, 68, 69, 278

Mermaid weed, 81, 82

Mesophyll, 71, 72. 81, 120; palisade, 70,
71; spongy, 70, 71

Mesozoic era, 723; floras, 730

Mesquite, 772

Metaphase, 446

Microcosms, 180, 181

Micropyle, 374, 684

Microsporangium, 372

Microspore, 372; mother cell, 372

Microsporocyte, 336, 372

Microsporophylls, 379

Middle lamella, 170

Midrib, 19

Mildews, downy, 550, 552, 557; pow-
dery, 549, 558

Millardet, 578

Milo, 696

Mineral deficiency. 101

Mint family, 714; flowers, 716; oils, 173

Miocene period, 723

Mistletoe, 508. 509, 706; family, 705

Mitosis, 447

Molds, 3, 4, 720; blue, 547; bread, 543;
green, 547; slime, 562; snow, 543;
water, 546, 548

Molecular motion, 199, 212

Molecules, 61, 95, 139, 142, 156; of
chlorophyll a, 125; of sugars, 104, 135

Monocots, 368

Monoecious, 360

Monoploid, 451, 636

Monotropaceae, 713

Monterey cypress, 780

Moonseed vine, stem of, 266, 286

Morel, 542, 553

Morning-glory family, 714; plants, 342

Mosiac, 81, 215, 216; disease, 585

Mosses, 5, 7, 564, 640, 641-651; and
chromosomes, 649; bog, 650; classes of,
649; club, 668-673; fern, 640; haircap.
643, 644; luminous, 642; peat, 649,
650; reindeer, 565; reproduction of,
646; rock, 650; true. 649

Movement, mass, 197; molecular, 199,
223; of plant parts, 209, 213, 216

Mucilages, 169, 172

Musaceae, 699

Musci, 645

Mushrooms, 4, 7. 542, 543, 555

Muskeg, 638, 745

Mustard family. 706

Mutations, 474-488; and environment.
481; in Boston fern, 436, 484; in
cosmos, 492; in hibiscus, 435; in
Piiccinia, 572; in radish-cabbage, 482;
in reduction division, 479: in sun-
flower, 494; in sweet potato, 435; in
trilliuni. 493; in vegetative cells, 475;
in wheat, 481

Mycelium, 543, 573

Mycorhizal fungi. 510

Myxophyceae, 608, 621-624

Names, common. 18, 25; plant, 18, 24;

scientific, 18, 25
Natural selection, 500
A^avicula, 97
Needle grass, 766
Ne^nalion, 634, 635. 636
Nematodes, 583
Nereocystis, 601, 629
Neutrons, 96
Nitrates, 45, 143, 192, 533

806

INDEX

Nitrification, 533, 537

Nitrites, 533

Nitrobacter, 533

Nitrogen, 142, 177, 190, 328, 531, 593;
cycle, 537, 538

Nitrogen-fixation, 534. 538, 709; non-
symbiotic, 537; symbiotic, 537

Nitrosococcus, 533

Nitrosomonas, 533

Node, 246, 247, 271

Nodules, 535, 709

Non-green cells, 132; plants, 101, 504-
514

Nucellus, 684

Nucleus, 59, 60, 64; composition of, 65:
endosperm, 376; fusion, 374, 376, 450;
generative, 372; triple-fusion, 376,
450; tube, 372

Oak, 508, 754, 757. 789; family, 704;

flowers, 705
Oceans, 598; life in. 601
Octoploid, 478
Oedogonium, 616, 618, 636
Offsets, 411
Oils {see Fats)
Oleaceae, 714
Olive family, 714
Onion root tip, 447; "sets," 410
Oogonium, 547, 618
Oospore, 618
Oparin, 529
Operculums, 644, 648
Ophioglossum, 665
Opuntia, 85
Orchid, 119, 507; family, 701; flowers,

702, 703
Orchidaceae, 701
Ordovician period, 723
Osmosis, 199, 201, 204, 320; and plant

behavior, 209-220
Osmiinda, 665
Overgrowths, 573
Overturns, 594, 595
Ovulary, 354, 373
Ovulate cones, 683, 684
Ovule, 336, 353, 373, 375, 688
Oxidation, 98, 110, 155
Oxidation-reduction, 99, 138, 143, 152,

156, 168

Oxygen, 96, 108, 152, 177, 310, 518,
589, 592; deficiency, 161

Paleozoic floras, 727

Palisade layers, 72

Fallavicinia, 654

Palm, 85, 698, 772; family, 697

Palmaceae, 697

Pandaniis, 297

Panicle, 360, 361

Papilionaceae, 708

Parasites, 31, 505, 506, 507, 551, 572;
facultative, 506; obligate, 506

Parenchyma, 72, 272; cortical, 264;
phloem, 283; xylem, 284

Parthenocarpy, 380

Parthenogenesis, 380

Pasteur, 527

Pasteurization, 525

Pathogen, 568

Patrician center, 734

Pea family, 708; flower, 354, 355; gar-
den, 355, 459, 462

Peach, brown-rot of, 578

Peach-leaf curl, 572, 579

Peanuts, 709

Pearl bush flowers and fruits, 362

Peat, 651; moss, 645, 649

Pectase, 175

Pectic acid, 171, 175; compounds, 65,
169, 170

Pectin, 171, 175

Pectinase, 175

Pectose, 170, 175

Pectosinase, 175

Peduncle, 353

Penicillium, 548, 550

Pepper flower development, 336

Peptase, 175

Peptones, 175

Perennials, 33, 46

Perianth, 354

Pericarp, 361

Pericycle, 264, 266, 282, 304; fibers, 264,
281

Periodicity, 32, 611, 612

Perisperm, 380

Peristome, 644, 648

Periwinkle, leaf section of, 71

Permeability, 205

Personification, 51

INDEX

807

Petal, 336, 353
Petiole, 13, 18, 72
Petrified logs, 721
Phaeophyceae, 608, 628-633
Phenotype, 463

Phloem, 71, 72, 223, 264, 266, 282, 303;
fibers, 264, 283; parenchyma, 283; pri-
mary, 304; rays, 264, 284; secondary,
304

Phosphates, 45, 143, 539

Phosphorus, 142, 177, 328

Photoperiod, 340

Photoperiodism, 340

Photosynthesis, 109-114, 315; and chloro-
plasts, 122; and CO„ concentration.
126, 127; and C0„-0„ ratio, 110; and
epidermis, 120; and evergreens, 123;
and leaves, 120; and light intensity,
116, 117, 126; and light rays, 116; and
mesophyll, 120; and plant structures,
118; and respiration, 152, 154, 163;
and temperature, 127, 163; and water.
118, 120; chemical equation of, 109;
factors involved in, 112, 115; inter-
mediate compounds, 124; rates of,
115-128

Phycocyanin, 608

Phycoerythrin. 608

Phycomycetes, 552

Phylloxera, 420. 578

Physiological drought, 205, 210

Pickleweed, 211

Pigments, 27-32, 171, 511

Pihilarin, 667

Pine, Austrian, 683; cones. 683. 684
eastern white. 758; embryo, 686
"endosperm," 685; lodgepolc, 787
long-leaved, 761; needle, 87, 682
pinon, 788; seed. 686; short-leaved
760; western white, 784; western vel
low, 782

Pineapple, 367, 699: familv. 699

Pinesap, 30, 504

Pistil, 12, 354, 373, 376

Pistillate cones, 683; flowers. 359

Pitcher plant. 92

Pith, 13, 24, 223. 265, 266. 279

Placenta, 336

Plankton, 596. 597, 602, 611

Plant associations. 752; breeding. 431;
communities. 40-49, 236. 238. 643:

composition. 177; curvatures. 209, 213,
215; diseases, 567-585; foods, 101-107;
formations, 743-792; introductions,
431; names, 18, 25; organs, 10-17;
parts, 10-17; populations, 42, 425, 499,
596; restorations, 725, 728, 729,/ 730;
science, 1-9; selection, 432; sucg^ssion,
41, 46, 737, 753
Plants, and animals, 1, 2. 5, 45, 604,
610; day-neutral, 344; difficulty in
definition of, 3; distribution of, 165,
719-738, 740-795; domesticated, 423.
500; long-day, 340, 341, 348; non-
green. 504-514; of the past, 719-738;
origin of. 423-436; pioneer, 46; sea-
sonal aspects of. 26-38; short-day, 341.
342, 348; uses of, 1-3; varieties of. 131

Plasmodium, 562

Plasmolysis, 205

Plastid, 60, 63, 65. 129; inheritance, 444

Pleistocene period, 723

Pleiirococciis, 565, 613, 614, 636

Pliocene period, 723

Plumule, 10, 11, 367

Poaceae, 693

Pollarding, 251

Pollen, 372, .383, .385; fossil, 736, 737;
sac, 372; tube, 374, 376

Pollination, 374, 431, 684; close, 431;
cross, 381, 440; insect, 384; self, .381

Polyembryony. 380

Polyploidy, 476-478. 480

Polysaccharides, 135. 628

Polystichxini, 656

Polytridnim, 643, 644

Pome, 356, 363, 366

Ponds. 590

Populations, changes in plant. 43, 425

Porella, 652. 653

Postelsia, 629, 630

Potassium, 45, 177, 328

Potato family, 715; flowers. 716; sprouts.
53; tubers, 189, 409. 410

Pottery, 427

Poverty grass, 45

Prairie, 764

Pre-Cambrian floras, 727

Preconditioning. 346, 347

Prickle, 258

Primordium, 68 (see also plant organs)

Pronuba moth. 385

808

INDEX

Propagule, vegetative, 400

Prophase, 446

Protein, 103, 106, 141, 146, 169, 175;
accumulation, 145, 148; digestion, 144;
molecules, 141, 144, 522; synthesis,
141-145

Proterozoic, 723, 727

Prothallus, 664, 665, 675, 685

Protonmea, 645, 649

Protons, 96

Protoplasm, 59, 60, 62, 145. 169; stream-
ing of, 197

Pseudopodium. 649, 650

Psilophytes, 727

Psilophyton, 728

Psilotum, 728

Pteridophytes, 656-676

Puccinia, 559, 560

Puffballs, 542, 555

Pulvinus, 88, 90

Pure-line plants, 432

"Purposeful behavior," 50-58

Pustules, 573

Pycnia, 560

Pyxis, 366

Quillworts, 673

Raceme. 360, 361

Rachis, 21, 659

Radicle, 294

Rafflesia, 15, 16

Ranunculaceae, 706

Ray, light, 115, 116; phloem, 264, 268;

vascular, 264; xylem, 264
Receptacle, 353, 633
Recessiveness, 456, 459
Red snow. Frontispiece
Redi, 526, 528
Reduction, 98. 110, 138, 156; division,

448, 450
Redwoods. 401, 778, 779
Reindeer lichen, 563
Reproduction, asexual, 545, 549, 617,

647; sexual, 371-386, 400, 617, 646
Reproductive organs of plants. 11
Research, plant, 3
Resin, 169, 172; ducts, 282. 289; method

of tapping, 172
Respiration. 149-159; accessory features,

157; aerobic. 157; anaerobic, 157; and

CO2, 153, 165; and photosynthesis,
152, 154, 163; and plant development.
160-166; comparative rates of, 165;
demonstration of, 149, 155; essential
features of, 157; external evidence for,
151; factors affecting, 160
Respiratory ratios, 154
Resurrection plant, 670
Reversible reactions, 130, 133, 144
Rhizoids, 545, 651
Rhizomes, 187, 256, 257, 404, 405, 407,

411
Rhizopus, 545
Rhodophyceae, 608
Rhynia, 728
Riccia, 652
Rind, 292
Ringworm, 557
Rivers and streams, 596
Rock tripe, 564

Root cap, 13, 295, 303; hairs, 13, 303,
313; pressure, 322; primordium, 294,
415; sections, 302, 304; structure, 300;
surfaces, 312; systems, 295, 300, 306,
311, 313, 325; tip, 10, 303
Root-parasite, 16
Root-shoot growth, 192
Roots, 10, 294-314, 315-332; adventitious,
295; aerial, 119, 297, 299; dependence
on leaves, 191; fibrous, 296; growth of,
304, 305, 307, 308; herbaceous, 296;
in dry soil, 308; lateral, 13, 295; of
arbor vitae, 311; of bald cypress, 310;
of beet, 306; of carrot, 317, 320; of
Cissus, 415; of hemlock and spruce,
307; of white pine, 318; primary, 10,
11, 294; processes in, 315; prop, 297,
298, 299; secondary, 11, 295, 313; tap,
13, 296; tertiary, 295, 313; tissues of,
302, 303; types of, 294
Rosaceae, 707

Rose family, 707; flower, 707
Rosettes, 33, 81, 258, 670; winter, 33
Rots, 574, 578, 580
Rubber, 173; plantation, 173
Rubiaceae, 714
Runners, 256, 402
Rushes, 697; scouring, -673
Rust, 558; apple, 562; cedar, 562; wheat
stem, 559. 560; white pine blister, 562

INDEX

809

Sachs, 337

Sagebrush, 768, 769, 770

Sahauro, 711 {see also Cactus)

Saint John's shoots, 254

Salicaceae, 702

Salvinia, 667

Sand storm, 794

Sanitation, 523, 575

Sap, maple, 324; tapping for, 324

Saprophytes, 30, 506, 512; economic as-
pects of, 514; facultative, 506; obli-
gate, 506

Sapwood, 263

Sargassum, 632

Sassafras leaf, 111

Scales, leaf, 19

Scar, bundle, 246, 247; leaf, 24, 73, 246;
terminal bud scale, 247; vein, 247

Schistostega, 642

Schultze, 527

Schwann, 527

Scientific evidence, 486

Scion, 293, 419

Sclerenchyma, 72, 281

Sclerotia, 569

Scutellum, 295, 367

Seasonal aspects, 26-38; autumnal. 27;
spring, 36, 36; summer, 37; winter,
34, 34

Seaweeds, 5, 601, 629, 631

Sedge, 697; family, 697

Seed coat, 10, 367; fern, 729; plants,
678-692

Seedlings, 296, 388; mangrove, 389

Seeds, 11, 14, 336, 367, 376, 673, 686;
death of, 398; dissemination of, 368;
economic value of, 369; formation of,
375, 377; germination of, 387-395;
longevity of, 396-399

Selaginella, 669, 670, 672

Selling, 382

Sensitive plant, 88, 217

Sepal, 336, 353

Sequoia, 275, 276, 679

Sex, 434, 487

Sexual reproduction, 371-386, 400, 687;
in angiosperms, 687: in bread mold,
545; in ferns, 666; in gymnosperms,
684; in Laminaria, 632; in liverworts,
653; in mosses, 646; in NemaUo7i, 635;
in Oedogonium. 618; in Spirogyra,

620; in Ulothrix. 617: in water mold,
547, 548, 549
Shade {see Light); leaf, 77, 78-80; plants,

117
Sheath, 18, 86

Short-day plants, 341, 342, 348
Sieve plates, 282, 283; tubes, 282, 283
Silicon, 177
Silurian period, 723
Slime molds, 562
Smuts, 558, 581; of barley, 559; of corn,

558, 559; of oats, 558; of wheat, 558
Snow plant, 31, 504; mold, 544
Sodium, 177

Soil, 309, 315, 589; algae, 610; atmos-
phere, 315; bacteria, 531-540; clay
loam, 320; distribution of roots in,
307, 308, 318; field capacity of, 319;
particles of, 316; profiles, 317, 318
Sol, 62

Solanaceae, 715
Solutes, 202
Solutions, 61, 202
Solvent, 202
Sorus, 664
Soybean, 709
Spadix, 358, 360, 361
Spallanzani, 528
Spanish "moss," 15, 508
Spathe, 358

Species, 18, 24; dominant, 40, 753: new,
431, 495; plastic, 82; subdominant,
753
Spectrum, 27, 115
Spermagonia, 561
Spermatium, 561, 635
Spermatophytes, 678-692
Sperms, 372, 465, 466, 647
Sphagnales, 649
Sphagnum, 649, 650
Spike, 357, 360, 696
Spines, 18, 83, 86, 258
Spirodela, 15

Spirogyra, 616, 619, 620, 636
Spontaneous generation, 525
Sporangia, 545, 631, 632, 644, 665
Spore, 545, 553, 616, 632, 636, 665;

print, 554
Sporophylls, 379, 665

810

INDEX

Sporophytes, 379; algal, 636; club moss,
668, 670, 671; equisetum, 674, 675
fern, 662, 663; flowering plant, 690
liverwort, 654; moss, 644, 647, 650
pine, 682

Sprays, 575, 577

Spring aspect, 36

Spring wheat, 435

Sprouts, 410, 411

Spruce [see Forests)

Squaw-root, 505

Stamen, 336, 354, 372

Staminate cones, 683; flowers, 359

Starch, 129, 175; accumulation, 131, 148;
grains, 60, 103, 133; sources, 132; syn-
thesis, 129-137

Starvation, 101, 164

Stele, 264

Stem, 10, 12, 24, 245; of moonseed vine,
266; of panicum, 271; sections, 223,
266, 270, 285, 286, 287, 288, 291; seg-
ments, 267; tip, 10, 64; types, 251

Stems, aerial, 256; age of, 265, 268;
climbing, 258; columnar, 252; creep-
ing, 402; deliquescent, 252, 253; dicot,
266, 270; excurrent, 252, 253; external
features of, 245-261, 246, 254, 255.
257, 258, 259; growth of, 267, 270.
272, 278; herbaceous, 256, 269; mono-
cot, 270, 271, 290; processes in, 276;
regions of, 262; size of, 273-276; tissues
of, 264-277, 278-293; twining, 258;
woody, 245, 262

Sterility, 382

Sterilization, 527

Stigma, 354, 373

Stipules, 18, 86

Stock, 293, 419

Stolons, 256

Stomates. 71, 74, 78, 79, 87, 89, 121:
opening and closing of, 74, 121, 218,
226

Stone cells, 281, 283; fruits, 365

Stoneworts, 621

Storage [see Accumulation)

Stratification, 396; in lakes, 594

Strawberry fruit, 363; runners. 403

Streptococcus, 519

Strobilus, 683

Style, 354, 373

Suberin, 168. 169. 171

Submergence, 589-590; and differentia-
tion, 82; and leaf form, 81, 90; and
plant species, 588; and stomates, 82,
90

Succession of plant communities, 41, 46,
737, 753

Sucrase, 124, 175

Sucrose, 104, 109, 123, 148, 175

Suffocation, 101, 161

Sugars, 104, 108, 129, 138, 150, 152, 169,
175, 483; amounts made, 113; fate
of, in plants, 114; synthesis of. 110

Sulfates, 143, 539

Sulfofication, 539

Sulfur, 142. 177, 328, 539; bacteria, 512.
538

Summer aspect, 37

Sun leaf, 77, 78, 79, 80

Sundew, 219

Sunflower, 494; family, 717; leaf, 76;
stem, 270, 287

Surface tension, 197

Survival of the fittest, 500-502

Susceptibility. 524

Suspensions, 61

Swamps, 597

Sword fern, 484

Sycamore, 274

Symptoms of plant diseases. 573

Tank culture, 331, 332

Tannase, 175

Tannins, 162, 171, 174, 175

Tassel, 359

Taxodium, 678

Teliospores, 560

Telophase, 446

Temperature, and abscission, 32; and
autumn coloration, 31; and bacteria.
619; and flower initiation, 345. 346,
347; and leaf development, 82; and
photcisyn thesis, 112, 127; and plant
distribution, 165, 600; and plant
habitats, 589; and respiration. 163;
and root development, 312; and seed
germination, 396; and tuber forma-
tion, 189; effects, 217; evaporation,
222

Tendrils, 18, 86, 258

Tertiary period. 723; roots, 295, 313

Tests for foods. 103. 104

INDEX

811

Tetrad, 372

Te trap his, 648

Tetraploids, 475

Thallus, 651, 652, 653

Thermocline, 595

Thiamin, 194

Thorns, 83, 258, 259

Timberline, 680

Timothy spikes, 696

Tissues, complex, 278; food-conducting,

75; meristematic, 68; primary, 278;

secondary, 278; sporogcnous, 372,

373; water-conducting, 75 (see also

appropriate organs of plant)
Toadstools, 542, 555
Tobacco, 54; leaf, 70; plants, 341
Tomato, fruit. 363; leaf, 322; plants,

327
Top-root ratios, 190
Toxins, 524
Tracheids, 284
Tradescantia, 79
Transpiration, 221-232,

absorption, 234, 242,

crop yields, 236; and

240,
322,
plant

324

323

dis

and

and

Uribu-

tion, 237, 239; and soil water, 229;

cooling effect of, 241; cuticular. 223;

effects on plants, 80. 233-244; lifting

power of, 227; pull. 230; rates of,

226. 232, 235, 240; stomatal, 225
Triassic period, 723
Trichodesmiiim , 623
Trichogyne. 635
Trillium. 380, 493
Tri-palmitin, 139
Triple fusion, 376
Triploid, 451
Trisaccharides. 1 35
Tropical jungle. 791
Tropisms. 213. 214
Truffles, 553
Tryptophane, 144
Tschermak, 435

Tubers, 189, 257, 408, 409. 410
Tufa, 644
Tulip, flower, fruit and seed. 353; root.

302
Tundra, frontispiece, 31. 6.39, 741. 743.

744, 745, 746
Turgor, 205. 212. 216
Tyloses, 285

Ulmaceae, 705
Uloihrix, 614, 616, 636
Umbel, 360, 361, 713
Umbelliferae, 712
Unisexual, 671
Uredospores, 560

Vacciniaceae, 713

Vacuole, 60, 63, 65

Vallisneri, 526

Valves of diatoms, 625

van Helmont, 102, 526

Variations, 436, 490; heritable, 84-93,
4.36, 492; hybrid, 436. 492; mutant.
492; non-heritable, 436, 491

Vaucheria, 616, 619, 620, 636

Vegetable, 364; ivory, 698 ^

Vegetation, contrast on slopes, 239; of
North America, 740-795; types, 240,
741

Vegetative bud, 335; multiplication, 191,
337, 400-422, 440, 646, 653, 661; or-
gans, 10; period, 14; propagules, 400

Vein, 19, 71, 72; scar. 247; systems. 20,
111

Venation, 20; dichotomous. 20, 21. 660;
palmate, 20, 21; parallel. 20. 21: pin-
nate, 20, 21

Venus'.s-flytrap, 91, 219

Vernation, 660

Vessels, 264, 284

Victoria regia, 85, 86; leaf, 87

Vinca, leaf section of, 71

Vinegar, 156

Viruses. 521, 522, 584; diseases caused
by, 584, 585

Vitamins, 28, 169. 174. 194. 520

Volvox, 615

von Dusch. 527

von Schroeder, 527

Walnut family, 703; flowers. 704
Water, 109, 123, 129, 139, 152, 168. 196,

222; and bacteria, 517; bloom, 611;

conducting cells {see Xylem); habitat,

587, 589; hyacinth, 412; lily, 85, 86;

loss from plants, 235, 236; lotus fruit.

363; mold, 546; molecule, 95, 223
Water-holding substances, 230

812

INDEX

Water-sprout, 250

Water-vapor gradient, 228

Wax, 169

Weeds, 2, 41, 45, 46, 717

Whales and plankton, 604, 605

Wheat, 425; embryo, 295; flower, 357;
grass, 766; rust, 559, 560; seedlings,
190; smut, 558; spring, 435; winter,
435

Willow, 102; family, 702

Wilting, 231; percentage, 320

Wilts, 574

Winter, aspect, 34; conditioning, 35;
fat grass. 769; injury, 35; view in
forest, 34; wheat, 435

Wolffia, 15, 85

Wood, 262; block. 290; cylinder, 262;
diffuse-porous, 288; fibers. 284, 285;
grain, 264, 289; non-porous, 289; ring-
porous. 288; rots, 579, 580; sections,
285, 288, 289, 291; spring. 287; sum-
mer, 287

Wound, 292; surfaces. 477; tissue, 292,
418

Xanthophyceae, 608, 627

Xanthophylls, 28, 608

Xenia, 381, 470

Xeniophyte, 379

X-rays, 398, 485

Xylem, 71, 72, 223, 262, 266, 284, 303;

parenchyma, 284; primary, 304; ray,

284; secondary, 304

Yam, 714

Yeasts, 155. 546, 547

Yucca, 700

Zein. 142
Zinc, 330
Zoology, 7
Zoospore. 617

Zygote, 376, 545, 547, 548, 636; to
embrvo. 387