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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. 



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. 




Professor of Botany Professor of Botany 

Ohio State Uiiwersity Ohio State University 


Professor oj Botany 
Northwestern University 





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 




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 



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 



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 

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- 



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 


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 

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 


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 

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 





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- 



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 


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 


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


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



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 


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 


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 


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. 


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 




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 


[Chap. II 



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 











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 







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 


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 

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 

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 



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 

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. 


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. 



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. 



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 



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 


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. 



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 



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 


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 

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. 


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. 


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.) 


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 

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- 



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. 


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 

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. 


Many other examples of leaf pigmentation may be found in every 

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 



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 


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- 


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. 


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 



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 


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 

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 



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 


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: 


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. 


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. 


(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. 


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. 

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. 


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 



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 


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 




1 1 

1 1_ 





" V 


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a^iYr : 


;■:;:! ORCH. GR, 




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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 


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. 


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 

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 


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 



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 


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- 



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- 


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. 


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. 

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. 


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

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. 




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 



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 

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 


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 

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 

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 


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 

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. 



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 

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


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 

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 


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- 



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. 


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. 



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. 




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 

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 ' 




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- 


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 

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 


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- 


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 



li-r^v^ ^<ji6■' 

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- 


Table 1. Names of Plastids and Their Characteristic Contents 


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 


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- 

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 

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


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. 


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. 


[Chap. VIIl 




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 









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 


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 

^ , y^ 5pace 


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. 


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 

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. 


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. 



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. 


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

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. 


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 



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- 




— "^ 

y\^ V i J R a 













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 


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 

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 


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 




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. 


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 


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. 


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. 


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

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 



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 

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). 



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. 


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 


[Chap. X 



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. 



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 

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 



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. 



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 





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) 




Giant ragweed 
[Ambrosia trifida) 




Yellow oak 

(Qiiercus muhlenbergia) 




(Cannabis indica) 



Black walnut 
(Jiiglans nigra) 




Prickly lettuce 
(Lactuca scariola) 




Tulip tree 





(Typha latifolia) 



(Populus deltoid es) 





(Arisaema triphyllum) 




Scarlet oak 
(Quercus coccinca) 



Willow herb 
(Dianfliera americana) 




Shrub Leaves 

Water lily 
(Castalia odorata) 




{Sassafras vari i folium) 



Cow vetch 
iVicia cracca) 



{Vilis bicolor) 




(Zea mays) 






( Oxy coccus mac r oca rp us) 



Fescue grass 
(Festuca sylvatica) 




(Hydrangea arborescens) 



Bl uegrass 
iPoa praiensis) 



Maple-leaved vi})urnum 
{Viburnum acerifolium) 



Water plantain 
{Alisma plantago) 




"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. 


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 



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- 


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 


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. 



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. 


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 



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. 


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. 


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


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 

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. 


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 


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- 

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. 


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, 



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. 



[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. 


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 OHO O () O H O O 


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. 


(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 

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


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 


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 

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. 



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- 



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 

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 


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." 


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 


^ ^ic ii . ' 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. 


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 

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 


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 

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 


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 





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 

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 



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. 




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 


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 


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, 


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 

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 

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 


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. 


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. 


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 


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. 


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 

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. 


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 



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- 



H3C— C / 


- — ^ / "^ 

^ \ / \ 

iCH,) (HC=0l 


)^^ JZ C2H5 


H3C— c^, \ 

>" \ 

// C CH3 


I ' 

I ^ 




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 



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 




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 



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. 


f 50 


2 40 


5 35 

^30 I- 

(f) 25 


^ 20 



> 10 



ith 1.227oC02 

10° 15° 20° 25° 30° 35° 40' 


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- 


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 


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. 

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




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 



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

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 


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 

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 



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 

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 

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. 



[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. 





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 

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 




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


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 

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 


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. 

\ / \ / \ X 

/ \ / \ / \ 


\ / \ / \ / 


Glucose A disaccharide 


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 


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. 



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 

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) 


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 


[Chap. XVI 



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: 


+ Palmitic acid 

(a fatty acid) 


(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: 


HC^OjH + HOj— CH3,Ci.5 

HC— OiH + HOl— CH31C15 

HC— OjH + HOJ— CH31C15 

HC— O— CH31C15 



HC— O— CH31C15 + 3H2O 

HC— O— CH31C15 


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- 


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- 

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, 


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 


( 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 


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. 


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 


- 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. 


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- 







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- 


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 

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. 


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 • 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 





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 

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, 



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. 


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- 


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. 


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 

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 


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

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 


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 


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- 


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. 


Sugar > 2 Molecules of alcohol + 2CO2 


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 

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 


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 

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- 

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. 


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 


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. 


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. 




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 



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. 


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- 

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 


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). 




§ 10 
O ^ 

< ® 


_l 4 


2 - 





























/ / 



/ / 1 

"'■~— — - 


//^ RESPiR.ATION,,,-^^''''\^ 



1 1 1 1 

1 1 \ 

1 N 1 



59 « 






122 < 

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. 


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 


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- 

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 



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 


That Might Be 




Fed for One Day 






Sweet potatoes 





Irish potatoes 

























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. 


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. 



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 

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 


[Chap. XIX 



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 — 

> 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 




Cell walls of all tissues 




Cell walls of many tissues 


Oxidation-reduction and 

Pectic compounds 

Cell walls of all tissues 




Cell walls of wood tissues 




Cell walls of cork 



Cut in 

Epidermal cell walls 




Epidermal cell walls 




Green and yellow tissues 




\'acuoles of many tissues 



Essential oils 

General or localized 




Many tissues 




General or localized 




General or localized 

Sugar and 




Generally distributed 

Sugar and 




All living cells 

Sugar and 




Variously distributed 

Sugar and 



\ itamins 

All living cells 

All foods 


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 


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 

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 


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- 



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 

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 



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 


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 


Table 6. Enzymes, Subtrates, and End Products of Enzyme Action 



End Products 











Peptase (pepsin) 

Ereptase (trypsin) 








Pectic acid 


Fats and oils 


Proteins and peptones 



Glucose and fructose 


Simple sugars 


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- 

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 


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 



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 



Organic matter 19.5% 
Mineral elements 0.8% 

Dry Matter 

' Carbon 


























Organic matter as compounds: 

Carbohydrates and fiber 17.2% 
Fats 0.5% 

Proteins 1 . 8% 


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 

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 


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. 


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. 



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 


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- 



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 


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 

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. 


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 

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. 



Table 7. A Comparative Summary of Energy Transformations in an Annual 
Herb and in a Woody Perennial 

Acre of Corn 

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 

Photosynthetic efficiency based on part 
of spectrum that is effective in photo- 

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 

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 

50 days 
2,043 million Cal. 
33 million Cal. 


188 days 

28 days 
2,718 million Cal. 

30 million Cal. 


200 pounds 

about 10 tons 

93 pounds 

about 8.7 tons 

8 million Cal. 

10 million Cal. 



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 


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. 


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 

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. 


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. 


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. 

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. 


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 



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 


when the potato plant is exposed to a very low concentration of carbon 

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. 


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 ) . 


68. The initiation and the growth of roots from cuttings are dependent upon 
leaves. Photo by F. H. Norris. 


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 

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 


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. 


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. 




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- 



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- 


ing for all the intricate physico-chemical conditions of which it is a 

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 

There is also a streaming, or mass movement, of water up the con- 


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 

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 

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 


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. 


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 


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 


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 


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. 



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 


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. 


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 

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 



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 


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 





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. 


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- 


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. 


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." 



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 



^«,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, 


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 

ffav tgj ac- 

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 


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 

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- 



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 


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. 

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. 



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. 

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 


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 


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 

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 



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 


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 

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. 


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 



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 



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- 









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 


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 


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 

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. 


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 


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 

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- 

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. 










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 


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 


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 

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. 


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 

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 



[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). 






4- 8 12 4- 6 



4- 8 12 4 6 



4. 8 12 4 6 




4 8 12 4 6 



4 8 12 4 6 




4 8 12 4 8 



4 8 12 4 6 
























































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. 


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. 


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. 




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 



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 

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 


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 






Water Loss 



30 gallons 



50 gallons 



125 gallons 

Giant ragweed 


140 gallons 

Mature apple tree 


1,800 gallons 

Coconut palm, 






4,200 gallons 

Date palm, Sah 


desert oasis 


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 



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- 

Corn in eastern Kansas, 6,000 plants per acre, 100 325,000 

Corn in central Illinois, 10,000 plants per acre, 100 400,000 

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 




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 

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 




53 . 00 

20 . 00 



76 00 


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 


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 

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 


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 

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 


;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 


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 


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 

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. 


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 


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, 


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. 


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. 




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 


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. 



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. 


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 



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 





r A 1 


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, 



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 



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. 



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- 

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- 


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. 



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 

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 


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 


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. 


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 


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. 


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 

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 


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. 


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. 


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 

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. 



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 

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 


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 


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. 



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, 





Vascular ray 

^// 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 


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 





PHLOEM 1936 


XYLEM 1940 


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 


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 



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 



Leaf sheath 



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- 



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 



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 

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 



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. 


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 

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 



Table 11. Maximum Sizes and Ages Attained by Certain Trees 

Name of Tree 





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. 


1.50 ft. 

Western yellow pine 

Sugar pine 

Douglas fir 


Big tree (sequoia) 

Big cypress of Tule 

Western juniper 

Norway maple 

Sugar maple 

Bur oak 



Black walnut 

Western N. A. 
British Columbia 
Oaxaca, ^Mexico 

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 


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? 


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. 


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 



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 


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 

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 


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. 



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- 


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 

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 


which conduction of food may take place and in which starch frequently 

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 


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 

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 



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 



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. 



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 



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. 



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. 



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. 


Fig. 111. Transverse, radial, and tangential sections ot wood ot sugai maple. 

Photos from Forest Products Laboratory. 



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 



[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. 


Bailey, L. H. Standard Cyclopedia of Horticulture. The Macmillan Company. 

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. 








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. 





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 



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. 


Fig. 114. Globe radishes develop primarily from the hypocotyl of the seedling. 



[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 



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. 




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 

If a cross section of a young root of a tulip tree is compared with a 



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. 



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 



Region of 

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. 



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 



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 






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 


endogenous origin, especially the adventitious stems that develop from 

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 

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 

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- 


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 

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. 



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 


■^jr-i^^/- '"^ , 

\ \^,mif^' 


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- 

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- 



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- 


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 

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- 



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 


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 


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 

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- 


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. 



Total Length 

Total Surface 

Main roots 

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 . sq. ft. 

758.6 sq. ft. 

1,748.9 sq. ft. 


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 

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 

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. 



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 



of Roots 





150 ft. 

210 ft. 

1 , 250 ft. 


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. 


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. 


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 

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- 




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. 


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. 



.. . pa O.. • .• •: o... . .0 _ •i.-oo-lo ■„ ^^-0-..o -.0 ey-. . •.^■" o 6 •■ • I o/^"- ••••<;»^:-o" "^ 



".^.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. 


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 








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 

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 


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. 



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. 


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 



[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- 

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 





1 1 1 1 1 



a s 



\\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. 

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 



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 


M-<rSHrTKU--rWa-Hv.>+4 i-ULU^^ I 


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). 


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 

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 


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. 



'^^^ 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. 


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 

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 


and margins. Terminal bud dies. Roots short, much branched, dark brown in 

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 


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 


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 . 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- 



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. 


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. 

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. 


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 



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. 


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 





ovule containing 
me^aspore mother cell 


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. 

[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- 


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 

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 


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- 



tensity of light, through its influence upon photosynthesis, is a very 
important factor in the growth of flowers and fruits after they are 

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 





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 



need not be very intense. The main thing is that the daily photoperiod be 

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 

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. 


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 


of Plant 

After 15 Days 







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 



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 

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. 



[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. 



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 



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- 

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: 




i Intermediate Day 
Long Day i (Length Near Critical 

Short Day 

Long-day plants that have been 
tested by experiments 

Usually bloom 

\'ariable, dependent upon 
several other factors 

Usually remain 

Sliort-day plants that have been 
tested by experiments 

LIsually remain 

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 


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. 



[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 


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? 


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. 




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 

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, 




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. 


' Style I Pistil 



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 



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 


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 


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. 



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 



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 



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 


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. 


[Chap. XXXIl 



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 



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- 


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 



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 



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 



"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- 



[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 

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. 


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 

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 

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 



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 


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 

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- 

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." 



[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 


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 

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 

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. 


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. 

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. 


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 

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 



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. 


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. 


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. 


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 



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. 


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 

The seed and the fruit. The growth and hardening 


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. 





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 


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 


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- 

Plants such as the common dandelion have seeds containing viable 
embryos which developed from unfertilized eggs: a process termed 

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. 


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 


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. 


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 



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 










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 


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 


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. 


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. 



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 



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 

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 


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 


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. 


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 


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 


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 

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 

The seeds of such plants as willow, cottonwood, and elm will germi- 


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. 


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- 


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 


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 


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 


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. 


Open, 70° F. 






Sealed, 70° F. 






Open, 46° F. 






Sealed, 46° F. 






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 



10 20 30 40 50 YEARS 

I 1 1 1 1 1 1 1 1 1 1 



1 1 1 1 ' 1 ■ -■ 


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 


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. 


Crocker, William. Mechanics of dormancy in seeds. Amer. Jour. Bot. 3:99-120. 

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. 

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. 

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. 


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 




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 



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 



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. 



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 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- 

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. 



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 



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". 



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 


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 



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 



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 



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. 

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 



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). 


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 

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. 



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. 



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 



[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 

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 

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 



( 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 

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. 


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." 



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 

Established grafts are commonly obtained onlv among related plants, 
as among \'arieties of peach, apricot, and plum, or among varieties of 


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 


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 

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 



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. 


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. 


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. 




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 



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 


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 









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 



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 



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 




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 ) . 


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. 



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 


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 

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. 



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. 


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 


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 

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 


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 

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. 


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 

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. 


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 

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 

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 

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. 



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. 


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 

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\^ 



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 


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 


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 

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).* 







r ^ 


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 


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 

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 



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 




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 


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 




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. 


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 ) 


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 


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 



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 


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. 





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 













V J 
















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 



[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 



















' o 




( --'»■■■■■ 1 

I •o- J 






















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 


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- 

" 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 



One of the chromosomes 

in a sperm from 
the red -flowered plant 

One of the chromosomes 

in an egg from 

the white -flowered plant 


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 


Possible kinds of sperms and eggs with respect to 
these two chromosomes that can be produced in this plant 





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, 



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 




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 

* 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 


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. 


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. 

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. 




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- 



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 

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 


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 


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 

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. 



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 


TT and Td 


n and Ic 





Color of flowers 

R and w 

Surface of seeds 

S and r 


RR and Rw 


SS and Sr 





Position of flowers 

\ and t 

Color of seeds 

Y and g 


A A and At 


YY and Yg 





Color of pods 

G and y 


GG and Gy 






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. 



of F2 gener- 






of F2 gener- 

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 



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. 


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 

Factors in 

Genotype of All 
Fi Hybrids 

Factors in 
Gametes of 






TR, Tw, dR, dw 

(Factors in Sperms) 

TR, Tw, dR, dw 

(Factors in Eggs) 


Tw i dR 




















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 

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 



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 


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 



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. 


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, 


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 


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 




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. 


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. 



I- ^ 






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. 


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. 


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. 


Rabcock, E. R. Recent progress in plant breeding. Sci. Monthly. 40:313-322. 

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. 


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 



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 


"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 


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) 


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 


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 

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 


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 

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 


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 


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 


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 



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- 


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 


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 

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- 


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 

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 


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. 


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. 


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. 




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. 



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 

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- 



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 

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 



Plate 6. Tnllium grandifloriim above and a mutant variety {Trillium grandijlonim 

plenum album.) below. 



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 


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 o