Full text of "General botany; an introductory text for colleges and advanced classes in secondary schools"

See other formats

lIlillH

HL/ifH/^OAfT(}Ofi0^l?)^

! m

: ru

s:=g

m
m

—1

ffi^^S

□

^^^^^

" '"

=as^H

GENERAL BOTANY

NEW-WORLD SCIENCE SERIES

Edited by John W. Ritchie

Science for Beginners

By Delos Fall
Trees, Stars, and Birds

By Edivin Lincoln Moseley
Common Science

By Carleton W. Washburne

Gardening

By A. B. Stout
Human Physiology

By John W. Ritchie
Sanitation and Physiology

By John W. Ritchie
Laboratory Manual for Use with
"Human Physiology"

By Carl Hartman
General Science Syllabus

By /. C. Loevenguth

Exercise and Review Book in Biology

By /. G. Blaisdell
Personal Hygiene and Home Nursing

By Louisa C. Lippitt
Science of Plant Life

By Edgar Nelson Transeau

Zoology

By T. D. A. Cockerell
Experimental Organic Chemistry

By Augustus P. West
General Botany

By Edgar Nelson Transeau

N EW-WORLB SCIENCE SERIES

Edited by John W. Ritchie

GENERAL BOTANY

AN INTRODUCTORY TEXT FOR

COLLEGES AND ADVANCED CLASSES

IN SECONDARY SCHOOLS

Edgar Nelson Transeau

Professor of Botany, The Ohio State University
ILLUSTRATED

Tonkers-on-Hudson, New York

WORLD BOOK COMPANY

1924

WORLD BOOK COMPANY

THE HOUSE OF APPLIED KNOWLEDGE
Established 1905 by Caspar W. Hodgson

YONKERS-ON-HUDSON, NeW YORK

2126 Prairie Avenue, Chicago

The advance of botany has caused the de-
velopment of several distinct branches of
the subject — taxonomy, morphology, physi-
ology, ecology, plant geography, genetics.
As a consequence, the older first courses in
botany are no longer general but special,
covering only parts of the course. Editor
and publishers offer General Botany as a gen-
eral introductory text. They believe that this
is a general botany in fact as well as in name

NWSS : TGB-2

PRINTED IN U.S.A.

PREFACE

This book is a by-product of a rather extended experience in the
teaching of botany and especially of seven years of effort to de-
velop an introductory course that would give students a broad
view of the subject and enable them to see its problems and
appreciate the importance of the solution of these problems.

The first step in developing the course was the arranging of
laboratory and field work. The second was the working out of
the classroom discussions so that the class work and the actual
work with the plant materials would run parallel and supple-
ment each other. The laboratory directions have been used and
revised at intervals during the past five years, and the textbook
has been tested and revised and used in mimeographed form for
the past two years. During this period the work has been pre-
sented to upward of three thousand students and the course has
been improved in every detail through the experience and con-
structive criticisms of the eight instructors who have tried out
various methods and devices and different arrangements of the
material with their classes at The Ohio State University.

Suggestions and ideas for guidance in the selection of subject
matter have been derived from four sources. The first of these
is the traditional course in general botany. This embodies the
facts and principles which most botanists agree are essential for
a foundation in the subject, and these are to be retained unless
there is definite reason why they should be set aside.

The second source from which suggestions as to subject matter
have come is the large body of men engaged in the teaching or
practice of horticulture, agriculture, and forestry. These workers
in the appHed fields of botany .are the men who more than any
others use information concerning plants, and certainly a course
in general botany should afford a foundation for their courses and
their practice.

A third source of suggestions and criticism is the students to
whom the course has been given. The questions they have asked,

vi Preface

the relative interest they have manifested in different kinds of
subject matter, and their responses to various methods of pres-
entation have furnished a valuable basis for evaluating ideas and
suggestions from the first two sources.

A fourth set of suggestions has come from the questions asked
by the public. These inquiries are usually very practical. Often
they are unanswerable, but an introductory course should en-
able a student either to answer many of them or to find the
information that is called for.

Suggestions from the above four sources have been consciously
sought during the working out of the course. We have at-
tempted to avoid trespassing on allied fields, but we have not
hesitated to point out many important uses and applications of
botanical principles.

The selection of subject matter, however, is only one of two
important elements in presenting any science course. Equally
important is the efficient use of the time given to field and lab-
oratory exercises, which after all are the heart of the course.
Textbooks may furnish a fund of information that will conserve
time in the classroom, but it is the work in the field and the lab-
oratory that tests the ability and insight of the student, and
makes real (or sometimes unreal) classroom and textbook state-
ments.

The use of the field and laboratory time for the answering of
questions and the solving of problems, rather than for the making
of detailed drawings, has changed the attitude of our students
toward laboratory work. It has also made it possible to cover a
far greater range of materials and principles than formerly, and
to give the student with a scientific mind as good a chance as the
student with an artistic hand. We prefer to use the laboratory
and field periods not for drawing exercises but for study and
recitation in the presence of the materials.

Although the laboratory and field work were developed first,
the textbook is being published in advance of the laboratory out-

Preface vii

line. This is done because having an illustrated textbook in the
students' hands renders certain changes in the outline advisable.
These changes are now being made and the outline will be pub-
lished in the near future. In it a fuller explanation of how the
laboratory and field work has been conducted will be given.

Throughout the book the author has tried to avoid purposeful
explanations and words implying such explanations. Teleology
answers all questions by the easiest method, and closes the mind
of the student to the means by which scientific explanations may
be discovered. It is an inheritance from the dark ages and
should be eradicated from the laboratory and classroom. Stu-
dents should learn at the very beginning that plant phenomena,
so far as we now know, take place in the plant in accord with the
laws of physics and chemistry ; that they do not happen because
of some alleged purpose any more than hydrochloric acid unites
with soda in order to form table salt. If certain phenomena
and structures eventuate to the advantage of the plant, well and
good. There are many that do not ! And neither the advanta-
geous nor the disadvantageous should be cited as a cause.

E. N. T.

ACKNOWLEDGMENTS

The author is deeply indebted to his associates at The Ohio
State University. Professor H. C, Sampson has had direct
charge of this general course since it was first given and has
conducted various experiments in teaching methods that have
greatly improved the efficiency of the course. He has read and
criticized the various revisions of the manuscript, has suggested
numerous changes in the text, and has contributed much both
to the form and substance of the book. Professor A. E. Waller,
Professor W. G. Stover, Dr. L. H. Tiffany, Dr. E. L. Stover, and
Dr. J. D. Sayre have read a part or all of the manuscript and its
revision. Without their cooperation and constructive ideas
neither the course nor the book would have had its present form.

Professor H. N. Whitford, and Professor G. E. Nichols of Yale
University and Professor W. S. Cooper of the University of
Minnesota, made helpful suggestions regarding the chapters on
plant distribution. Dr. Cooper also generously supplied a num-
ber of photographs. The author's thanks are also due to various
men in the United States Department of Agriculture who have
furnished photographs of plants used in their investigations.
These are credited below the illustrations in the book.

CONTENTS

CHAPTER

I. Plants from Our Standpoint

II. Plants as Living Things

III. The Plant ant) Its Environment

IV. The Cellular Structure of Plants
V. Leaves and Their Structures

VI. The Manufacture of Food .
VII. The Release of Energy
VIII. Substances Made from Foods
IX. Leaves in Relation to Light
X. The Water Relations of Leaves
XL Physical Processes Involved in the Movement of

Materials in Plants

XII. The Water Balance in Plants .

XIII. The Growth and Fall of Leaves

XIV. The Stems of Plants

XV. The External Features of Stems

XVI. The Structure of Stems ....
XVII. Longevity of Herbaceous and Woody Stems
XVIII. The Growth of Stems

XIX. The Movement and Accumulation of Materials in

Stems

XX. Ecological Types of Stems .
XXI. The Forms and Structures of Roots
XXII. The Processes of Roots

XXIII. En\t:ronmental Factors Affecting Growth and Re-

production ....

XXIV. Vegetative Multiplication and Plant Propagation
XXV. Flowers and Flower Clusters

XXVI. Sexual Reproduction in Flowering Plants

XXVII. Fruits and Seeds

XXVIII. Dormancy and Germination of Seeds .

XXIX. Plant Breeding ......

XXX. Variations and Mutations

383CX

Contents

CHAPTER

XXXI.

XXXII.

XXXIII.

XXXIV.

XXXV.

XXXVI.

XXXVII.

XXXVIII.

XXXIX.

XL.

XLI.

XLII.

XLIII.

XLIV.

XLV.

XL VI.

XLVII.

XL VIII.

XLIX.

Index

Hybridization and Selection

The Distribution of Plants in Nature

The Vegetation of North America .

Relation of Plant Industries to Climatic Plant

Formations

Weeds and Their Control .

The Non-Green Plants

Bacteria and Their Relations to Life

Soil Bacteria and the Nitrogen Cycle

Fungi

Plant Diseases

The Classification of Plants .

The Alg^

Bryophytes: Liverworts and Mosses
The Pteridophytes ....

Fossil Plants

The Gymnosperms — Cycads

The Gymnosperms — Conifers

The Angiosperms or Flowering Plants

Some Families of Angiosperms .

Evolution of Plants ....

PAGE
286

302

310

375
378
384
396
402

423
437
441

465
477
495
503
508

517
524
545

553

GENERAL BOTANY

CHAPTER ONE

PLANTS FROM OUR STANDPOINT

One of the most pressing economic problems of the world today
is the securing of an adequate food supply. In the older and more
densely populated parts of Asia an unfavorable growing season
has for centuries meant famine and death for thousands of per-
sons, and as a result of the world war nearly every nation on the
globe has recently experienced inconvenience or suffering be-
cause of limited food resources. The fact that the population of
the earth is increasing far more rapidly than the food supply
should give us an increased interest in plants, the primary source
of all foods. When we realize further that our resources of lum-
ber, fuel, fibers, paper pulp, oils, resin, rubber, and numerous
other products com 3 from plants, our absolute dependence on
plant life for our necessities and comforts is apparent.

How plants affect our own lives. Before taking up the study
of plant hfe let us enumerate briefly some of the more important
ways in which plants contribute to pur welfare or detract
from it.

1 . Foods derived from plants. The principal foods of all nations
are derived directly from plants. Animal foods are of secondary
importance, and all animals live directly or indirectly upon plants.
Agriculture is, therefore, the most fundamental of human occu-
pations, for of all living beings green plants alone are able to
organize the simple materials found in the air, water, and soil
into the complex substances which all plants and animals must
have for food.

2. Fuel a plant product. A second necessity of all nations is a
fuel supply. Like food, fuel is primarily a plant product. Wood
is the most universal source of heat and light energy. Coal,

2 General Botany

petroleum, and natural gas, although obtained from the earth,
are the products of plants which lived in former geological times.
When wood is burned, the great store of energy which the tree has
accumulated from sunlight is released in the form of heat. When
coal, petroleum, or natural gas is burned, the energy stored by
plants from sunlight of former geological ages is liberated.

3. Plant fibers. Of almost equal importance is the production
of fibers for clothing and many other articles. Such plants as
cotton, flax, and hemp supply the bulk of these fibers, and the
fibers not directly derived from plants come from animals that
feed on plants. Artificial silk and vegetable wool are recent
substitutes for animal fibers that are being manufactured from
plant fibers. \

4. Wood products. Lumber is a primary necessity in the con-
struction of houses and buildings of all kinds. It is also the chief
material for furniture and countless other articles of household
use and ornament. Paper, the principal medium of communica-
tion and commercial exchange, is also essentially a wood product.

5. Oils, resins, and drugs. Plants have played an important
part in the progress of civilization by supplying oils and fats,
gums, resins, dyes, rubber, drugs, alcohols, cork, the materials for
explosives, and many other basic substances for the arts and in-
dustries.

6. Other uses of vegetation. Trees and grasses are the great
stabilizers of the soil on mountains and in valleys. They help to
retain flood water and prevent destructive erosion. They pro-
vide food and shelter for numerous wild animals that are of great
economic importance. The plants of our lakes, ponds, and
streams are the primary sources of food and shelter for fishes,
ducks, and other water animals.

7. Importance of bacteria and fungi. Certain small plants, the
bacteria and fungi, are beneficial in bringing about the decay of
the bodies of plants and animals. This results in the production
of substances that can be used again by green plants in the mak-

Plants from Our Standpoint 3

ing of foods and the construction of their bodies. Some bacteria
increase the fertility of the soil by building nitrates and other

J-hper
pulp

'Vegetable
oils

z/Tgm

(food

Jjamher

tJuel

Wood
disfillafes]

Tib

ers

lowers

'Uanning
^solutions]

PLANTS

are the sources

of our most essential

commercial products

and the agencies in many

important processes

Qutns

U\ubber

■^

esins

'uermen-
tation

mant

^ 1
animai

diseases J

3).

umtLs

ecay

rugs

:S)yes

pigments}

S)rying

oils

Fig. 2.

nitrogen compounds. Other bacteria and fungi cause most of the
diseases to which plants and animals are subject, and they are
being studied extensively in order that people may know how to
avoid, control, or destroy them.

8. Plants a source of pleasure. Aside from all these great
economic considerations plants afford us an aesthetic pleasure
that cannot be measured in money but which is nevertheless real

General Botany

Fig. 3. Primitive peoples in the tropics are depemknl upon i)lants even more than are
civilized races. Most of their foods are from plant sources, and their houses, mats, cloth,
boats, ropes, and household utensils are made almost wholly of plant materials.

and important. Cities, towns, and individuals are expending
millions of dollars every year in beautifying parks, boulevards,
and residences with artistic groupings of trees, shrubs, and flower-
ing plants.

Similarity of plants and animals. We have a further interest
in plants in that they form one of the two great divisions of Uving
things. They differ from animals in many particulars and to
such an extent superficially that the largest and more complex
forms are not only readily distinguished but are commonly
thought of as being quite unrelated. It is difficult for those who
have not been students of plant Hfe to realize how similar the Kfe
processes of plants and animals are.

Nevertheless, they have many points of similarity. Both
plants and animals use food as a source of building material and
energy, and the foods are essentially the same. Both plants and
animals use oxygen in respiration, and give off carbon dioxide.

Plants from Our Standpoint 5

Digestion, assimilation, growth, and reproduction are carried on
by similar chemical and physical processes. As will be brought
out more fully later, all the processes of both plants and animals
depend upon the properties and activities of cells which are very
much alike in the two groups.

Need for scientific study of plants. The profitable cultivation
of plants for food, fiber, timber, and ornamental purposes, and the
control of plant and animal diseases, depend primarily on our
knowledge of the structures, products, and processes of the par-
ticular plants involved. Only a beginning has been made in the
appKcation of science to the industry of plant production.

For centuries agricultural practices have depended almost en-
tirely on observation, experience, and tradition. Only recently
has it been possible to explain on a scientific basis many of the
principles underlying agricultural practice, and many problems of
plant growing still await solution. The production of increased
yields of crops per acre, the improvement in the quality and
variety of the products, and the prevention of the ever increasing
losses from diseases — in fact, the future development of agri-
culture and of all industries dependent upon plant products
— will be based on scientific experiments with plants. For
this development a better understanding of the laws of inherit-
ance is fundamental. In addition, we must have clear insight
into the effects of the en\ironment on plant processes and
structures.

The need for conserving plant resources. With the growth
of population the conservation of our natural plant resources and
the proper utilization of our lands for forestry and agricultural
purposes becomes increasingly important. The United States
started with a huge bank account of natural forest resources,
much of which has now been dissipated. Every year we are using
timber at a rate greatly in excess of the annual growth of all trees
on our forest land. The future outcome of this system of timber
destruction is clear. To formulate wise plans for the better use

6 General Botany

of forest lands requires a complete understanding of the relation
of forest growth to climate and soil.

These are a few outstanding products and problems of plant
life which have been enumerated in order to emphasize the funda-
mental importance of botany, which is the science of plant life.

PROBLEMS

1. Make a list of the uses you are making of plant products today.

2. What percentage of your diet is derived directly from plants?

3. How many acres of forest must be destroyed each week to furnish paper for our

weekly magazine with the largest circulation ?

REFERENCES

Smith, J. Russell. The World's Food Resources. Henry Holt & Co.

East, E. M. "Population in Relation to Agriculture," Eugenics in Race and

State, Vol. II, page 215. Williams and Wilkins Company.
Pearl, Raymond. The Nation's Food. W. B. Saunders Company.
Berry, James B. Farm Woodlands. World Book Company.

CHAPTER TWO

PLANTS AS LIVING THINGS

Thus far plants have been discussed in their relation to men ;
they have been considered as objects of interest, and as a part
of man's environment that may promote or interfere with his
welfare. But, of course, plants do not grow, or flower, or fruit
for the sake of animals or man. Their various organs grow and
their structures develop as a result of their own life processes.
A plant is successful in nature when it secures nourishment for its
complete development, and when it produces offspring and thus
insures the perpetuation of its kind.

It is important for the beginner in the study of botany to
realize that plants are Hving things. We are accustomed to
think of movement as a necessary evidence of life, and to one
who has given no thought to the subject a tree may seem more
akin to the stones among which it is rooted than to the animals
that live about it. But when we study living beings, we find
there are activities more fundamental than movement that are
regularly associated with all life. As we shall see later, these
more fundamental vital processes — such as respiration, growth,
and reproduction — take place in plants just as they do in
animals, and plants may therefore be considered as truly alive
as are animals.

Parts of a plant. The vegetative body of the ordinary flower-
ing plant is made up of three parts : root, stem, and leaves. The
root anchors the plant and absorbs water and mineral salts
from the soil. The leaf carries on a remarkable process in which
water and carbon dioxide are united by the energy of the sunhght
to form sugar, thus providing food for the plant. The stem sup-
ports the leaves and conducts water and mineral salts from the
roots to the leaves, and foods from the leaves to the roots and
other organs. The chief advantage of an erect stem is that it
can display a large number of leaves to the light. In the roots,

8 General Botany

stems, and leaves, all those processes which are related to the
nourishment of the plant are carried on.

In the course of the plant's life three other organs are developed.
These are the flower, fruit, and seed, which are the reproductive
parts. The flower is usually very different in structure, texture,
and color from the vegetative parts and also much shorter lived.
The fruit follows the flower and is usually developed by the con-
tinued growth of one or more of the parts of the flower. Within
the fruit are the seeds. They are commonly small bodies con-
taining within them a young undeveloped plant (embryo) and
a food supply. Seeds can withstand cold and drying; so, in
addition to multiplying the plant, they carry the species through
winters and periods of drought. Under favorable conditions
they germinate and reproduce the plant from which they sprang.
Flowers, fruits, and seeds are reproductive organs.

Interdependence of the parts of a plant. The roots, stems, and
leaves make up the plant's machinery of nutrition, and the nour-
ishment of the entire plant depends upon each part doing its
work. If the roots are broken, the water supply is cut off and the
leaves wither. If the leaves are removed, food manufacture
stops and all the parts die for lack of nourishment. If the con-
ducting vessels in the stem are cut, the water supply to the leaves
fails and the roots have no food. The farmer destroys bushes
by keeping them cut down so closely that they cannot expose
leaves to the light, and he knows how to kill trees by cutting a
ring about them through the bark, so that food cannot pass from
the leaves to the roots.

It will thus be seen that when we discuss the relation of any
particular part of a plant to the energy-supplying and nutritive
processes, we must ever keep in mind the interrelation and inter-
dependence of ah parts of the plant. Just as no part of the
human body lives an independent life but is dependent for its
welfare on the activities of all the other parts, so the Ufe of each
part of a plant is bound up with the life of the plant as a whole.

Plants as Living Things 9

Reproduction an essential process in plant life. To be success-
ful, plants must not only maintain themselves, but in addition
they must provide for the continuation of similar plants in the
future ; for plants maintain their kind from year to year and from
one century to the next by producing new plants like themselves.
Reproduction is sometimes accomplished by the separation and
further development of a part of the parent body, as the tuber of
a potato or the runner of a strawberry plant. In most plants,
however, it takes place also through the development of seeds,
and it is upon the growth of the young plant within the seed that
the production of another generation of that particular kind of
plant depends. A sunflower may develop a tall stem and a large
leaf area, but unless it flowers and produces seed, no young plants
will be grown from it. If it were the only sunflower in existence,
there could be no more sunflowers after its death. Reproduction
in plants must, therefore, be considered as an essential process,
for without it plant life would soon disappear from the earth.

Summary. Plants are not nearly so complicated as the higher
animals. Nevertheless, in the course of their long history on
the earth, the plant body has become differentiated into several
rather distinct organs which differ from each other in structure,
and each of which carries on quite a different group of physio-
logical processes. Roots, stems, and leaves are the chief organs
of nutrition. Flowers, fruits, and seeds are the organs concerned
with the reproduction of the plant. Each organ of a plant is
more or less dependent upon the other organs, and the plant
attains its greatest development only when all the organs are
working in harmony together. This is possible only when each
of the organs is placed in favorable conditions.

CHAPTER THREE

THE PLANT AND ITS ENVIRONMENT

The seasonal changes in plants and landscapes are so marked
that they have always attracted attention. Numerous expla-
nations as to why and how these changes are produced have been
given. You yourself have probably accounted for the opening of
buds in the spring, the blooming of certain flowers during certain
seasons, the autumn colorations, and the fall of leaves as the
effects of temperature, hght, and moisture conditions.

Any one who travels extensively will also be impressed by the
striking changes in the vegetation as he passes from one region
to another. These differences, too, are to be accounted for by
the differences in the conditions that surround the plants ; the
various types of forests, grasslands, and deserts are the results
of climates and soils. Even locally one notices how different
the plants are that grow in ponds and swamps, or on cliffs, from
those that occur in valley bottoms and on gentle slopes. The
plants differ not only in kind, but in size, form, and structure.
The plant is profoundly affected by its environment.

So firmly estabHshed is this fact in our minds that when atten-
tion is directed to a familiar plant we at once call to mind the
situation in which it grows and perhaps some of the conditions
surrounding it.

Definition of environment. By the environment of a plant is
meant the complex of all those influences outside the plant which
directly or indirectly ajffect its physiological processes, its structures,
and its development and propagation. These influences are
numerous and are usually spoken of as environmental factors.
The factors of the environment include the physical and chemical
properties of the soil and the air surrounding the plant ; also
light, gravity, and the influence of other plants and of animals
(Fig. 4).

The Plant and Its Environment

Development of plants influenced by the environment. Each
factor of the environment affects the growth of every plant. We

Fig. 4. Results of an experiment to show effects of environmental factors (light and mois-
ture) on the growth of potato shoots: A, light but no water; B, light and water; C, water
but no light.

are all familiar with the fact that light may determine the position
of leaves and stems ; that drought may reduce the size of a plant ;
that gravity has something to do with the upward growth of
stems and the downward growth of roots ; and that insect in-
juries and plant diseases may reduce the vitality of plants so that
they produce neither flowers nor fruits. So the texture and the
fertihty of the soil, the temperature of the soil and air, and all
the other environmental factors influence the plant's develop-
ment and its growth. Successful farmers know that they can-
not secure vigorous plants and profitable crops except under
favorable conditions of hght, temperature, moisture, and soil.

Limiting factors of plant growth. Wherever plants grow the
several environmental factors are not all equally favorable.
One or more conditions may be somewhat unfavorable, and when
this is the case these unfavorable factors interfere with certain
physiological processes, and the final form of the plants is greatly
modified. Just as the strength of a chain is determined by the

General Botany

weakest link, so the development of a plant is limited by the
factor which is least favorable.

Fig. 5. Differences in form of the water smartweed (Polygonum amphihium) due to the
environment: A, grown on moist soil, stem erect, covered with hairs; B, grown in water,
upper leaves floating, smooth throughout ; C, grown on dry soil, stems and leaves rough
hairy, and prostrate. {After Massart.)

If soil conditions prevent the entrance of an adequate supply
of water into the roots, the stem and leaves will be dwarfed, no
matter how rich the supply of minerals may be. If the leaves
are exposed to unfavorable conditions of light or temperature,
their work will be retarded and the nourishment of the whole
plant curtailed even though water in abundance be supphed.
Corn grown in sand under the most favorable conditions of light,
temperature, and moisture is small and may fail entirely to
produce seed, because sand does not supply the needed minerals.
Or, if during the winter months corn is grown in a greenhouse
in the richest of soil, it attains only a small size, because the inten-
sity, or the duration, of light is insufficient for normal development.

The practical farmer knows that the yield of a crop plant is
limited by the least favorable conditions of the environment.

The Plant and Its Environment 13

He does not, therefore, try to improve all the environmental
factors that affect his plants, but gives his attention to those that
require it most. He irrigates if the soil lacks water. He adds
fertilizer if the soil is deficient in certain minerals. Insect injuries
he tries to prevent by spraying the plants with substances that
kill the insects. These are all efforts directed toward removing
or taking care of the limiting factors. It is often difficult to
determine, in a particular case, what the limiting factor may be.
But the ever increasing study of effects of environmental factors
on plants is every year making the discovery, and the correction,
of the limiting factors in crop production easier and more efficient.

Distribution of plants determined by the environment. Every
plant has a somewhat definite form and structure. In order that
it may live, it must carry on certain chemical and physical
processes. Because of these processes and its particular structure
and composition, a plant also has certain indispensable require-
ments as to its environment. The requirements of some plants
are very definite, of others more or less indefinite ; and the re-
quirements of different plants vary greatly. If we survey any
extensive tract of land, we see that its surface is more or less
broken into elevations and depressions containing streams, lakes,
or ponds. Slopes extend in various directions and are thus
differently exposed to sunlight. The soil may be shallow on
some slopes, and deep on others ; it may vary in texture and
mineral components. Some areas may be wet or moist, and
others dry. In some places the soil is fertile, in others more or
less sterile. Each of these smaller divisions of the land affords
different opportunities and conditions for plant growth, and
under natural conditions we find very different plants growing
on them.

Habitats. The smaller areas into which every large land sur-
face is broken are characterized by various groups and combina-
tions of environmental factors. These areas from the stand-
point of plant distribution are called habitats. Lakes, ponds,

14 General Botany

swamps, bogs, cliffs, bottom lands, sand plains, and clay slopes
are examples.

Every habitat affords a particular complex of environmental
factors. In a particular habitat we shall find only those plants
growing whose requirements are satisfied by the factors of that
habitat. In similar habitats, therefore, we expect the same or
similar plants ; in different habitats, different kinds of plants
having different requirements. Plants whose requisites are
very definite may occur only in a single habitat, while those
whose requirements are rather indefinite may live in a variety
of habitats.

Summary. It is quite impossible to understand the life of a
plant without having constantly in mind the environment in
which the plant lives. The environment is made up of several,
or many, factors, among which are light, temperature, water,
gravity, and the various properties of the air and soil. Every
individual plant is modified in its development by these external
conditions. Plants growing wild, or as crops, are limited in their
development by the least favorable factors of the habitat. These
are called limiting factors. In nature, plants are not distributed
haphazard, but each occurs only in those habitats that afford
the conditions which are necessary for its development. Similar
habitats have the same or similar plants living in them. Habi-
tats that differ in character are occupied by dissimilar plants.

CHAPTER FOUR

THE CELLULAR STRUCTURE OF PLANTS

The body of every plant either is a single cell or is made up of
a mass of cells. Many one-celled plants are so small that they
can be seen only with a microscope. A large plant like a tree
is composed of so many cells that their number can scarcely be
conceived. A cubic inch of a potato, for example, contains at
least 600 million cells, and a cubic inch of pine wood more than a
billion.

The cellular structure of plants was first noticed about the
middle of the seventeenth century, soon after the invention of the
microscope. Cells were first seen in examining thin sections of
cork, which is composed of layers of minute rectangular box-like
structures. Observations were extended to other parts of
plants, and to other plants, and it was finally recognized that
cells are the units of which all plant structures are built.

The plant cell. Nearly two hundred years passed, however,
after the discovery of cells before it was known that the cell walls
which had been previously studied constitute merely the frame-
work of the plant ; that the most important part of the cell is a
transparent, jelly-like, living substance inclosed by the walls.
This hving matter is called protoplasm (Greek : protos, first, and
plasma, form). When we now speak of cells, we usually have
in mind both the protoplasm and the wall around it. A cell may
be defined as a unit mass of protoplasm, capable of exhibiting all
the phenomena usually associated with life, such as growth and
respiration.

We have omitted the cell wall from the definition because some
cells are without walls for a part or the whole of their existence.
It is through the activity of the protoplasm that the walls and
other structures that are found in different types of cells are
formed ; so it seems best to define the cell in terms of the funda-
mental material of all cells.

1 6 General Botany

In young plant cells the protoplasm occupies all the space
within the walls. As cells grow older and enlarge, small cavities
containing water appear in the protoplasm. As the cell takes up
more water, these cavities, or vacuoles, expand and unite, until
finally there is a single large vacuole within the protoplasm, con-
taining the cell-sap. There are, then, three primary divisions
of the plant cell : the protoplasm, the vacuole, and the cell wall
(Fig. 6). ^

Protoplasm. The living matter of the cell when active has
about the same consistency as the white of egg. Active proto-
plasm contains a large amount of water, while in dry seeds the
protoplasm contains less water and may be quite rigid. Like
gelatine, protoplasm is more or less liquid when it contains a
high percentage of water ; when it contains smaller amounts of
water it becomes more nearly soKd. The abihty to absorb
and hold large amounts of water is one of its most important
qualities.

In composition protoplasm is very complex and may vary
considerably, not only in different plants and in different parts
of a plant, but also in the same cell, as a result of a change of
environment or of increasing age. Analyses show that aside
from water about one half of the protoplasm consists of protein.
The remainder is made up of sugars and other carbohydrates,
fats, and smaller amounts of salts and other substances. In
some manner that is not fully understood at present, the com-
ponents of protoplasm maintain a continuous group of activities
which result in the phenomena known as life. Its most remark-
able property is its abihty to take up food and to construct from
it more protoplasm like itself.

Protoplasm can also use food substances as a source of energy,
and it carries on physical and chemical processes with the energy
thus obtained. These processes are regulated in one way or
another by the protoplasm itself. We may, therefore, look upon
protoplasm as a body of matter that can absorb food materials,

The Cellular Structure of Plants

Fig. 6. Plant cells : A is from a moss leaf ; B is from a squash-vine
hair ; C is a starch-filled cell from a potato tuber ; and Z> is a cell from
the palisade layer of a leaf. E shows a cell in cross-section.

that can liberate energy from a part of its food and with the
remainder construct more matter Like itself, and that regulates
its own activities. It is self-constructing and self-regulating,
the only truly automatic mechanism known.

Differentiation of the protoplasm. The living matter of the
cell is primarily differentiated into two parts, the cytoplasm and
the nucleus. The cytoplasm constitutes the bulk of the proto-
plasm and forms a definite layer within the cell wall and surround-
ing the central vacuole.

General Botany

Fig. 7. Pari of a moss leal that

Portions of the cytoplasm may
be organized into definite struc-
tures called plastids, in which food
substances or coloring matters ac-
cumulate. Starch is formed in plas-
tids, as is also the green pigment of
leaves, and some plastids accumulate
fats and oils (Fig. 7).

The nucleus is a small, round body
of greater density than the cyto-
plasm. It occupies the center of the
young cell, but as the vacuole en-
larges it is carried with the cyto-

composed of a single layer of cells. .^^ ^j^^^ ^^ ^j^^ ^^Ij ^^jj jj^^
The dark bodies shown m the cells ^ i • •

are the plastids which contain the UUclcUS SCCmS tO be the Startmg pomt

green coloring matter. q£ many of the activities of the cell.

It is beheved to control and determine the course of develop-
ment of the cell and the formation of new cells in growth and
reproduction.

Vacuoles. The cell sap inclosed by the cytoplasm is a small
drop of water containing sugars, salts, acids, and other soluble
substances. As will be seen later, the cell sap influences many
of the cell processes, especially those concerned with absorption
and growth. The vacuole is the reservoir of the cell into which
dissolved substances may pass from the cytoplasm and from which
substances may again move into the cytoplasm as they are being
utilized in cell activities.

The cell wall. The walls of plant cells are composed of non-
living materials deposited by the cytoplasm. They support the
soft protoplasm of the cells in somewhat the same way that the
wax of the honeycomb supports the honey. They also give
stiffness to all parts of the plants. Most cell walls are composed
in part of cellulose, a substance closely related to starch and sugar.
You have seen pure cellulose in the form of cotton fiber. Filter

The Cellular Structure of Plants 19

paper and most book papers are made of cellulose derived from
wood cells. The walls of cells may be modified in various ways
by changes in composition, and thickened by the deposit of
additional layers. Other substances may be added to the cellu-
lose which render the wall hard and rigid, as is found in the
shells of nuts, or impervious to water, as in the outer cells of
leaves.

Animals, as well as plants, are composed of cells ; but the
animal cell, instead of having a stiff cellulose wall like a plant
cell, has a soft wall, or it may lack a wall entirely, as in nerve cells
and white blood corpuscles. Consequently, the tissues of animals
(except the skeletal tissues) are usually softer and more pliable
than plant tissues. This makes it easy for an animal to bend
and to move about. The difference in cell walls and in the
phability of tissues is so general throughout the plant and animal
kingdoms that it is one of the important distinctions between
plants and animals.

Cell division and enlargement. Among simple one-celled
plants, new individuals are formed by the division of the cell into
two. The cell first enlarges ; then the nucleus divides and the
two newly formed nuclei separate. The cytoplasm then divides,
the division beginning at the outside and gradually extending
to the middle of the cell. As the cytoplasm divides, a new
division wall is formed between the two daughter cells. In one-
celled plants this wall sphts and the two cells separate. In the
more complex plants the same kind of cell division takes place
when growth occurs and when new parts are formed, but the
cells remain together.

Cell division is accompanied, or immediately followed, by an
increase in the amount of protoplasm and the taking in of addi-
tional water. These two processes lead to the enlargement of the
newly formed cells. Cell division and cell enlargement are first
steps in the growth of all plants, whether the plants be small and
simple or large and complex,

General Botany-

Cell differentiation. If we could trace the development of one
of the more complex plants, we should find that it begins as a

Fig. 8. Types of plant cells and tissues, resulting from the difJerentiation of cells: A, a
supporting and water-conducting cell (tracheid) from fern stem ; B, water-conducting vessels
with spiral thickenings on the walls and soft, thin-walled cells (parenchyma) between them,
from sunflower stem ; C, water-conducting vessel with ring-like thickenings ; D, giant stone
cell in parenchyma of camellia leaf; E, wood cells from sunflower stem; F, stone cells,
with greatly thickened walls, from stem of club moss ; G, stone cells from shell of pecan ;
H, wood cells from pine. D, E, F, and G are cross-sections. {After Sachs.)

single cell. This cell divides, forming two cells, each of which
divides again, forming four. Cell division continues until an
embryo composed of hundreds of cells, all very much alike, is
formed.

But as the embryo grows farther, some rows or groups of cells

The Cellular Structure of Plants 21

begin to differentiate, and they quickly change into the different
types of cells found in the mature plants. Some of them enlarge
but remain round ; others greatly elongate ; still others become
flattened, disk-shaped bodies. In some the walls remain thin,
while in others the walls are greatly thickened or are marked by
lines and pits and irregular thickenings. They may differ in size,
form, thickness of cell wall, content, or color, as will be made
evident by our later studies of different plant parts.

Tissues and organs. Cells of a given kind are usually con-
cerned with the carrying on of some particular process, and they
tend to be grouped together. Such a group of cells is called a
tissue. The epidermis of a leaf is a tissue covering the leaf ; the
soft green inner part of a leaf is a tissue that is concerned with the
manufacture of food ; the shell of a nut is a hard tissue inclosing
the kernel.

An actively working tissue needs supphes and a means of dis-
posing of its products. Hence, tissues are grouped together,
and by their cooperation carry on some general function of the
plant more efficiently. A number of tissues grouped and work-
ing together in this way form an organ. The leaf, for example,
is an organ especially concerned with the manufacture of foods.
It is made up commonly of five different tissues, each composed
of thousands of cells.

Summary. Before proceeding to the study of structures and
processes of plants it is important that we understand (i) that
all plant structures are either single cells or masses of cells,
(2) that the protoplasm is the active living part of the cells,
and (3) that the processes carried on by any plant organs are
the combined results of processes going on in the cells of which
it is composed. Further details of cell structures and activities
will be given as they are needed to understand the processes of
particular parts of plants.

CHAPTER FIVE

LEAVES AND THEIR STRUCTURES

The leaves of plants are generally their most conspicuous part.
The prominence of leaves is the natural result of their relation to
light. Leaves manufacture food, and sunlight is necessary in
this process. In this chapter we shall study the structure of a
leaf, and in subsequent chapters we shall discuss the work of the
leaves and the processes that take place within these organs of
the plant.

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

Fig. q. Leaves with prominent stipules : pea, black willow, red clover, Japanese quince, rose.

Leaves and Their Structures

the apex or outer end of the leaf and gradually become larger
toward the base of the blade. They continue down through the
petiole, or leafstalk, into the interior of the stem.

Fig. 10. Divided and compound leaves: A, buckeye; B, oxalis; C, avens; D, celandine;
E, cliff fern ; F, dandelion.

At the base of the petiole there is in many leaves a pair of small
appendages, the stipules (Fig. 9) . These are usually unimportant
structures, but occasionally, as in the pansy and garden pea,
they are large and blade-like. These enlarged stipules supple-
ment the blade, or in some plants may even take its place in food
manufacture. The primary divisions of the leaf are the blade, the
petiole, and the stipules.

The leaves of many grass-like plants have no petioles or stip-
ules. In such plants the blade is attached to the stem by a
sheath, which may be long or short. At the top of the sheath is a
short, collar-like extension called the ligule. In the bamboo the
ligule consists of several long bristles.

When the blade of a leaf is attached directly to the stem with-
out an intervening petiole, it is said to be sessile (Latin : sedere,
to sit).

Compound leaves. When several blades are attached to a
single petiole, as in clover, buckeye, walnut, ash, and hickory, the
leaf is called a compound leaf. The blades of the compound leaf
are called leaflets. There is usually a distinct joint between the

General Botany

leaflet and the petiole. The leaf of the orange may be said to
be compound because it has such a joint. The fact that some
species of oranges have three leaflets gives support to this view.
If the leaflets are joined to the end of the petiole, like the fingers
of the hands, the leaf is described as palmately compound; if
joined to the sides of the petiole, it is termed pinnately com-
pound (Fig. lo).

The leaf made up of tissues. The soft green tissue essential to
food production is found chiefly in the blade of the leaf. This
may be shown by dissecting a fleshy leaf like that of the common
houseleek or live-for-ever. Cutting across the blade of such a
leaf, we find that there is a skin covering it above and below.
The skin is readily stripped off, leaving the interior of the leaf as
a green, granular mass of cells with veins running through it in
all directions. The skin is called the epidermis or epidermal
tissue (Greek: epi, upon, and derma, skin). The soft tissue

'!-{ ^Uppcr epidermis

I \ Palisade layers

Water-
ducting tissue

Food-con- /Chloroplast
ducting ti55ue i ^

Bundle sheatky Guard cell

Lower
epidermis

Stoma

Fig. II. Model of a small portion ot a leaf from the common periwinkle {Vinca), showing

cells and tissues.

Leaves and Their Structures

between the upper and lower epidermis is the mesophyll tissue
(Greek: w^^o, middle, and ^/^j/Z, leaf). This tissue is green in

1/ rX\9

Figs. 12 and 13. Upper epidermis of "Wandering Jew" (Zebrina) leaf, on the left, and lower
epidermis, on the right. St is a stoma, G a guard cell, and Sc a subsidiary cell. The stomata
are found only on the lower surface of this leaf.

color and may also be called chlorench)mia (Greek : chlor, green,
and enchyma, tissue).

The veins consist of three tissues : the water-conducting, food-
conducting, and mechanical tissues. The blade, therefore, com-
monly contains five tissues : the epidermis, the mesophyll, and
three tissues of the veins (Fig. 11).

How cells are held together. The cells which form the tissues
of plants are held together by a layer known as the middle lamella.
This layer binds the cells together much as cement binds the
bricks in a wall. The middle lamella is usually composed of
calcium pectate. If it is dissolved out or changed by chemical
action, the cells fall apart, just as bricks fall apart when the
mortar between them is dissolved and removed by weathering.
In boiling, the pectate between the cells is dissolved, and it is
this action that causes fruits and vegetables to break up into a
soft mass when cooked. The cells of ripe fruits are also easily
pulled apart because of changes in the pectic compounds of the
cell wall during the process of ripening.

General Botany

The epidermis and the stomata.

flat (Fig. 14), irregularly shaped,

The cells of the epidermis are
closely united, and in most

Figs. 14 and 15. Upper and lower epidermis of leaf of common periwinkle (Vinca). Lobed,
interlocking epidermal cells are strikingly different from the regular type shown on the
preceding page.

plants colorless. The cell walls on the side of the epidermis
which is exposed to the air become thickened with a wax-like
material called cutin, which forms a layer over the surface of the
leaf. This layer is called the cuticle. It is useful to the plant
because water does not pass through it readily, and it reduces
the amount of water that would otherwise evaporate from the
epidermal cells. It may be compared to the enamel covering of

Fig. I

6. Illustrating terms used in describing the shapes of leaves : A, linear ; B, lanceolate ;
C, spatulate ; D, ovate ; E, obovate ; F, oblong ; G, cordate ; H, peltate.

Leaves and Their Structures

oilcloth, and when thick it is quite as impervious to water. The
cuticle is useful to the plant also because it serves as a first line of
defense against disease germs. The importance of the epidermis
as a protective covering for the delicate inner tissues of the plant
may be judged from the drying and decay that follow the breaking
of the thin epidermal coat of an apple or a pear.

Scattered among the colorless cells of the epidermis are pairs of
small, crescent-shaped green cells, the guard cells. Each pair
of these surrounds a small opening or pore, the stoma (Greek :
stoma, mouth ; plural, stomata) ,^ which is opened or closed by the
expansion or contraction of the guard cells. The stomata are
very important, for they connect the air spaces among the cells
inside the leaf with the external atmosphere. When open, they
allow the exchange of water vapor and other gases through the
epidermis ; and when closed, they complete the barrier to gas
movements in either direction (Fig. 11).

In most of our trees and in many other plants the stomata
occur only in the epidermis on the lower surfaces of the leaves.
In some plants, especially in those growing in shaded situations,
they are found in the epidermis on both the upper and lower leaf
surfaces. In such leaves the number of stomata is always greater
on the lower side.

^ Stomata are so small that 2500 of them have an area about equivalent to that
of an ordinary pinhole. They are so numerous, however, that they occupy about
Ywo of the area of the average leaf. On a square centimeter of the lower surface of
a sunflower leaf there are about 15,000 of them.

Illustrating terms used in describing leaf margins : A , entire ;
D, dentate ; E, crenate ; F, undulate ; G, pinnately lobed ;

B, serrate ; C, doubly
H, palmately lobed.

General Botany

The mesophyll tissue. The mesophyll tissue is composed of
the soft, thin-walled cells that He among the veins in the interior
of the leaf. In most leaves there are beneath the upper epidermis
one or more palisade layers, which are composed of elongated cells
standing close together, as is shown in Figure 1 1 . The remainder
of the mesophyll tissue is made up of ovoid or irregularly shaped
cells joined quite loosely, so that air spaces are left between them.
In fact, a much larger part of the surfaces of these cells than of
other cells is in contact with air spaces. The air spaces within
the leaf are continuous, and through them the oxygen and carbon
dioxide of the atmosphere can reach every cell in the leaf. We
shall see later that the differences in the epidermal and mesophyll
cells, and in the way they are arranged, are definitely related to
the different processes carried on by each of them.

The mesophyll in many plants contains other cells in addition
to chlorenchyma. These additional cells are filled with water,
and sometimes form a compact layer between the epidermis and
the chlorenchyma, as in the begonia; they may also occur in
long lines along the veins, as in the corn plant. It is the loss of
water from these colorless water-storage cells that causes leaves

Fig. 1 8. Illustrating terms used in describing the apexes and bases of leaves: A, acute;
B, acuminate; C, obtuse; D, truncate; E, aristate; F, mucronate; G, emarginate;
H, rounded ; /, cordate ; J, obliquely cordate ; K, acute ; L, acuminate ; M, sagittate ;
N, hastate

Leaves and Their Structures

of many grasses to curl up during a period of drought. Some-
times these colorless mesophyll cells have heavy walls and con-
tribute to the stiffening of the leaf.

The chloroplasts. Of the several structures found within the
mesophyll cells, the most important in the primary process of
food manufacture are the chloroplasts. These are round or lens-
shaped plastids which contain a green coloring matter called
chlorophyll They are living organized bodies" of protoplasm and
multiply by division as the cells grow or new cells are formed.
Cells may contain many or only a few chloroplasts, and these may
be located deep within the leaf or near its surface. Since the
chloroplasts are the special apparatus which manufactures food,
the amount of food produced by a plant under any given condi-
tions is roughly proportional to their number.

The chlorophyll. Chlorophyll is formed in the chloroplasts
and colors them green. It can be taken out of the chloroplasts
by puttmg the leaf in alcohol. After the chlorophyll is removed,
the chloroplasts remain in the cell, but they are then colorless and
the leaf is white or yellowish instead of green.

For the development of chlorophyll, light is usually necessary.
The white sprouts on potatoes in a dark cellar, the blanching of
celery when the lower part of the leaves is covered, and the whit-
ening of young growing grass under a board are familiar evidences
of this fact. In the inner tissues of plants and in the underground
parts the plastids are usually colorless, but in many plants these

Fig. 19. Illustrating terms used in describing the attachment of leaves to stems: A, with
margined petiole; B, petiole clasping; C, sessile; D, perfoliate; E, connate perfoliate.

General Botany

Fig. 20. The vein system of a skeletonized sassafras leaf. The leaf was prepared by placing
it in water and allowing bacteria to digest the epidermis and mesophyll.

parts become green if they are exposed to the hght. This is why
potatoes that grow at the surface of the soil are likely to be green.

Seedlings of pine, spruce, lemon, and lotus develop green color
even in the dark. One occasionally finds in lemons, for example,
green sprouted seeds that have developed inside the fruit in
complete absence of light. Evidently there are substances in
some plants which make possible the formation of chlorophyll
without light.

The veins. The veins in a leaf branch again and again, forming
a fine meshwork through all its parts. ^ Each vein is composed of
a bundle of water-conducting and food-conducting tissues sur-
rounded by a bundle sheath. The water-conducting tissues are
located in the upper side of the vein. These tissues are made up

^ The venation, or arrangement of the veins of leaves, is of three general types :
parallel, extending more or less parallel from the base to the apex ; dichotomous or
forked, when the veins divide at intervals into two smaller veins; and net-veined,
when the veins form an irregular network throughout the blade. The principal
veins may be arranged either palmately, as in maple leaves, or pinnately, as in oak
leaves

Leaves and Their Structures 31

of long, cylindrical cells placed end to end. Usually the inner
walls of these cells have spiral thickenings, and sometimes the end
walls of the cells are absorbed, leaving continuous tubes, or ves-
sels, several cells in length. After the growth of the cells is com-
pleted, the living protoplasm within them dies, and the dead
cases of the cells, with their porous walls, lie like bundles of very
fine pipes within the leaf. Through these vessels the water and
mineral salts that are absorbed by the roots pass into the leaf and
supply its living cells. The supplies of water and mineral salts
pass out through the walls of the water-conducting vessels into
the cells that adjoin them, and then from these they pass to other
cells of the leaf.

The food-conducting tissues or vessels he below the water-con-
ducting vessels within the leaf veins. They provide an elaborate
system of channels by which the surplus foods manufactured in
the leaf are distributed throughout the plant. The foods pass
from the mesophyll cells into these food-conducting tissues, and
then down through the petiole of the leaf to the living cells of the
stem and roots. The conductive tissues, or bundles, may be
readily studied in the petioles of celery leaves.

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

CHAPTER SIX

THE MANUFACTURE OF FOOD

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

Plants the source of all food. Mineral soils and the air do not
contain any of the substances that we class as foods. Yet green
plants may grow luxuriantly on mineral soils. It follows, there-
fore, that green plants are able to manufacture their own foods.
They can synthesize, or build together, simple substances that
they obtain from the soil and air into the complex foods that they
require. Animals lack this power. They must have foods that
have already been built up, rather than the simple materials of
which foods are made. These foods they secure either directly or
indirectly from plants. The abihty of plants to manufacture
complex foods from simple substances brings up several questions :
What is the method by which plants produce food ? Just what
parts of the plants do the work? What constitutes the machin-
ery? Out of what materials is the food manufactured? How
is the energy suppHed? And what are the conditions under
which the process goes on ?

Photosynthesis. The primary step in the making of food is
the building of simple carbohydrates through the process called
photosynthesis (Greek: photos, light, and synthesis, putting to-
gether). In this process carbon dioxide from the air and water

The Manufacture of Food 33

from the soil are brought together in the chloroplasts and united
to form carbohydrates. Sugar is the first abundant product,
but being soluble in the water of the cell, it is quite invisible.
In most plants a large part of the sugar is rapidly changed to
starch, and as the starch is insoluble in water, it accumulates
temporarily in the chloroplasts in the form of Uttle grains which
may be readily seen with a microscope. There is a very simple
test for the presence of starch. A solution of iodine stains most
substances yellow or brown, but it colors starch blue or purple.
So any object that contains starch — a cell, a leaf, or a piece of
cloth — will be colored purple if iodine is appHed to it.

Light and photosynthesis. If we take a leaf from a plant that
has been in the dark for two days, place the leaf in warm alcohol
to remove the chlorophyll, and then put it in a solution of iodine,
it is stained yellow. This proves the absence of starch. If the
plant is then put in the Hght for an hour, a leaf tested in the same
way will be colored purple, showing that starch is present. Evi-
dently light is necessary for photosynthesis.

It is not surprising to find that light is so effective in building
up compounds in the green parts of plants, for it is a powerful
agent in causing chemical change. You may be familiar with its
use in photography.. The film and the printing paper have on
them a layer of gelatin containing certain chemicals. Exposure
to the light for even a fraction of a second effects changes in these
which may be seen when the film or paper is developed. Many
chemical substances kept in drug stores must be protected from
the light; otherwise they soon change their composition and
become different substances.

The amount of hght required iox photosynthesis varies in dif-
ferent plants. Among trees, for example, the beech, sugar maple,
and hemlock do not require as much light as the willow, cotton-
wood, pine, and aspen. Usually a reduction in intensity to one
fifth of full sunlight does not decrease the rate of photosynthesis.
In some shade plants the rate does not fall off until the Hght is

34 General Botany

reduced to one twelfth. This is still several times the intensity
of light in an oak, maple, or spruce forest, where one finds herbs
on the forest floor that must be able slowly to manufacture suffi-
cient food with a fiftieth or a hundredth of full sunlight.

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

Effects of temperature on photosynthesis. The effects of
temperature on photosynthesis may be demonstrated by taking
plants that have been in the dark long enough for the starch to be
removed from the leaves, placing them in the light under different
conditions, and noting the time that it takes for starch to form.
Such tests show that the ordinary summer temperature is most
favorable for photosynthesis. When the temperature falls nearly
to the freezing point, photosynthesis slows up and finally ceases
entirely; and, on the other hand, when it rises above ioo° F.,
the process is slowed down rapidly.

Materials and products. Experiments have shown that the
materials used in photosynthesis are carbon dioxide and water.
Carbon dioxide is a gas that makes up from three to four out of
every 10,000 parts of the air. Its molecule contains one atom of
carbon and two atoms of oxygen (CO2). Water, which the plant
gets from the soil, has two atoms of hydrogen and one atom of
oxygen in every molecule (H2O). The simple sugars made in
photosynthesis from the carbon dioxide and water contain these
same elements (Fig. 23).

Carbohydrates include many substances commonly classified
as sugars, starches, and celluloses. The simple sugars, glucose
and fructose, have a formula CeHi-iOe- The double sugars like
sucrose (cane and beet sugar) and maltose (malt sugar) may be
built up by combining two simple sugars.

The Manufacture of Food 35

C6H12O6+C6H12O6 — ^ C12H00O11+ H2O,

glucose + fructose — >■ cane sugar + water,

one molecule of water being lost in the process. Cane sugar may
be split into glucose and fructose by heating it in dilute sulfuric
acid for a few minutes. This brings about the addition of a
molecule of water (by hydrolysis) and the subsequent splitting :

C12H22O11 + H2O — >■ C6H12O6 + C6H12O6

cane sugar + water — >- glucose + fructose

The starches and celluloses are formed by combining many mole-
cules of the simple sugars and removing a molecule of water for
each molecule that enters into the combination :

n(C6Hi206) — ^ (CeHioOs)^ + n(H20) ■
glucose starch + water

Consequently their formulas are (CeHioOs)^, in which n represents
a rather large number. The starches and celluloses may also be
split up into simple sugars by adding the required number of
molecules of water. This last process is the one by which corn
sirup (glucose) is made from corn starch. Corn starch is hydro-
lyzed in the same way as cane sugar, mentioned above, with the
result that it breaks down into glucose. The process may be
represented by the equation,

(C6Hio05)n + n(H20)— >n(C6Hi206)
starch water glucose

Those sugars like glucose, which are the first abundant products
of photosynthesis, contain six atoms of carbon, twelve atoms of
hydrogen, and six atoms of oxygen in each molecule. For every
molecule of glucose manufactured, therefore, it would require
six molecules of carbon dioxide to furnish the carbon and six
molecules of water to provide the hydrogen. These amounts of
water and carbon dioxide, however, contain eighteen atoms of
oxygen, twelve more than are needed for the making of glucose,
6CO2 + 6H2O — >■ C6H12O6 + 602

We should, therefore, expect oxygen to be given off from leaves

General Botany

during photosynthesis. That this actually happens may easily
be shown by inverting under water a bundle of the branches of
some water plants, like Elodea, with the cut ends placed under
the mouth of a test tube that is filled with water. When exposed
to the hght for a day, the tube will be partly filled with gas. By

testing with a glowing match or
splinter (Fig. 21), the gas may be
shown to be mostly oxygen.^

How the supplies are obtained.
Every industrial workshop must con-
stantly be provided with the raw
materials needed in the manufacture
of its product. Likewise the leaf
must be supplied with the substances
that it uses in the making of food.
These necessary supplies come to the
leaf through the veins and the
stomata. The water passes into the
leaf through the water-conducting
tissue of the veins. The supply of
carbon dioxide reaches the cells of
the mesophyll through the stomata
and the intercellular spaces. When
the stomata are closed, little or no
carbon dioxide can enter, and at such
times the process of photosynthesis is
of necessity greatly retarded or completely stopped.

That the carbon found in a plant does not come from the soil
was shown 300 years ago by one of the earliest students^ of plant

Fig. 21. Experiment to show the
giving off of oxygen from a water
plant {Elodea) during photosyn-
thesis.

^ Water containing a considerable amount of dissolved carbon dioxide should
be used in this experiment so that photosynthesis may go on rapidly. Pond
water is better than tap water.

2 Van Helmont (15 77-1644) grew the branch of a willow tree for 5 years. x\t
the beginning it weighed 5 pounds, at the end 164 pounds. The loss in weight
of the soil was 2 ounces.

The Manufacture of Food 37

physiology. He grew a plant for several years in an accurately
weighed body of soil. He then carefully removed the plant,
dried it, weighed it, and also reweighed the soil. He found that
the increase in dry weight of the plant was more than a thousand
times the loss in weight of the soil. This proved that the plant
must have obtained most of its materials from some other source
than the soil. The plant, of course, used vast quantities of water
during its growth, but since water contains no carbon, the only
other source of this material must therefore have been the carbon
dioxide of the air.

How the products and wastes are removed. The manufacture
of carbohydrates in the leaf goes on only during the hours of sun-
light ; the removal of food goes on at all times. The food-con-
ducting tissue of the veins furnishes the outlet for the product,
which is transferred in the form of sugar. During the day the
rate of manufacture is so much greater than the rate of removal
of food that starch and sugar accumulate. During the night
the movement of food into the stem nearly empties the chloren-
chyma. The waste product, oxygen, passes from the cells to the
intercellular spaces and out through the stomata to the atmos-
phere.

A leaf, then, is carrying on photosynthesis at its full capacity
only when there are sunlight, a favorable temperature, and an
abundant supply of water, and when the stomata are open. Even
under these conditions the work may be interfered with if more
than a certain amount of the products accumulates in the cells.

The amount of the product. The amount of carbohydrates
produced in photosynthesis varies so greatly in different plants
and under dissimilar conditions that it is very difficult to make a
general estimate of it. The result of many experiments shows that
under favorable conditi«dft a square meter of leaf surface makes
on an average about i gram of carbohydrates per hour. At this
rate a square meter of leaf surface in midsummer would require
2 months to produce food equivalent to that consumed by the

General Botany

Fig. 22. A maple leaf and the sugar and maple
sirup equivalent to the amount it could manu-
facture in a season. All drawn to the same scale.

average man in a day. This average rate of carbohydrate manu-
facture may also be expressed by saying that the leaf makes

enough sugar in a summer
to cover it with a layer i
millimeter thick. Because as
a whole the factors involved
in photosynthesis are most
favorable during the morning
hours, the greater part of
food manufacture occurs be-
fore noon.

An acre of corn exposes
about 2 acres of leaf surface
to the light. The total
weight of organic material in an acre of mature corn plants
having a yield of loo bushels of corn is about 7 tons. Of
this amount, about 3 tons is carbon. To secure such a large
quantity of carbon, not less than 11 tons of carbon dioxide were
taken in by the plants. Furthermore, as we shall see in connec-
tion with respiration, not all the carbon taken in and built into
organic compounds remains in the mature plants. It is estimated
that the plants of the United States manufacture nearly a cubic
mile of sugar each year.

Hindrances to photosynthesis. Aside from the lack of light,
water, and carbon dioxide, the process of photosynthesis may be
interfered with in several ways. In cities where there are much
dust and smoke, plants do not grow well because (i) the amount
of sunlight is greatly reduced ; (2) the dust forms a layer on
the upper surface of the leaf and reduces still further the amount
of light that actually reaches the chlorenchyma ; and (3) soot
and dust collect in the stomata and interfere with the entrance
of carbon dioxide. If the dust and smoke are very abundant,
the stomata may even become completely blocked and photo-
synthesis stopped altogether.

The Manufacture of Food 39

Insects and plant diseases are often serious hindrances to photo-
synthesis. When insects eat the leaves of plants, they decrease
the supply of carbohydrates in proportion to the amount of leaves
they destroy. If the plant happens to be a crop plant, the injury
done by insects may result in the failure of the plant to manu-
facture sufficient food for filling out the fruit, grain, or seed for
which it was grown. Diseases of plants caused by fungi or bac-
teria also greatly interfere with the power of the plant to manu-
facture carbohydrates.

Carbohydrates as storehouses for energy. When the carbon
dioxide and water are converted into carbohydrates by photo-
synthesis, the energy supplied by the sunlight in doing this work
is stored as potential energy in the new substances formed. Then,
when these carbohydrates are oxidized or burned (or in other
words, when they are changed back into carbon dioxide and
water), the exact amount of energy that was stored is set free.
Thus the plant acts as a storehouse from which we can draw energy
at any time.

The importance of photosynthesis as a life process. Photo-
synthesis is not only important to the plant itself, but, broadly
speaking, it is the most important of all life processes. The sun
pours a constant flood of energy on the earth, and this energy
warms the earth, causes the winds and rains, and in general
furnishes the power for the work that we see going on in nature
about us. From running water, winds, and direct sunKght man
obtains a certain amount of energy for his own use, but the great
source of the energy that we use for heating purposes and for
power is wood, coal, petroleum, or gas. The energy stored in
these was accumulated through photosynthesis. It came origi-
nally from the sun, and but for the plants would have radiated
off into space as heat waves from the earth. But through the
work of green plants it was locked up in the molecules of the wood
and coal, and by burning these fuels man can release the energy
that is stored in them and use it for his own purposes. We may,

General Botany

therefore, say that most of the work of the world, including that
done by men and animals, is accomplished by the use of energy
accumulated by plants in photosynthesis.

Fig. 23. Diagram of the process of photosynthesis, showing primary and secondary products.

The importance of photosynthesis as a source of wealth. The
value of all the plant products of field and forest for one year is
many times as great as the value of all the minerals dug from the
earth during the same amount of time. Furthermore, minerals
are limited in amount and are gradually being exhausted ; while
on the other hand the products of photosynthesis are being con-
stantly renewed, and we may continue to collect them indefinitely.

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

is the green tissue, especially that of the leaves.

are the cells.

is the chloroplasts and the chlorophyll.

is the sunlight.

are the carbon dioxide and water (CO2 and H2O) .

is the stomata and intercellular spaces, and
the water-conducting tissue.

are carbohydrates : sugars (CeHioOe) and
starches (CeHioOs)^.

is the food-conducting tissues, and it works
both day and night.

is oxygen, which escapes through the inter-
cellular spaces and the stomata.

are all the hours of sunHght.

The factory

The workrooms

The machinery

The energy

The raw materials

The supply department

The products

The forwarding department

The waste material

The working hours

The Manufacture of Food 41

The production of fats. In addition to carbohydrates, plants
make and use two other important classes of foods : fats and pro-
tein. The fats are quite similar to the carbohydrates in composi-
tion. They contain the same chemical elements : carbon, hydro-
gen, and oxygen. The proportion of the oxygen to carbon,
however, is smaller. At ordinary temperatures fats occur in
plants both as solids and liquids. The liquid fats are commonly
called oils. They are probably made directly from the carbo-
hydrates, for the plant has no special fat-producing apparatus
comparable with the carbohydrate-producing chloroplasts of the
leaves. The chemical changes are probably effected by the
protoplasm ; therefore fat can be formed in any living part of
the plant.

In some plants belonging to the hly family (the onion, for
example) small drops of oil appear in the cells of the leaf as the
first visible product of food manufacture. The primary product
of photosynthesis (probably glucose) is changed directly into oil
when it accumulates, instead of into starch as it is in the leaves of
most plants. Starch does not form in these leaves at any time,
but when the materials of which the fats are composed are trans-
ferred and accumulate in the underground bulbs of these plants,
they then assume the form of starch. This emphasizes the close
relationship existing between starch, glucose, and fat.

Fats and oils, like starch, are inactive storage substances ; that
is, before being used or transferred they must be converted into
substances soluble in water. Although fats are widely dis-
tributed in the plant body, they are especially abundant in seeds
and fruits. Some of the commonest fats and oils of commerce
derived from plants are corn, coconut, cottonseed, linseed, castor,
pea, peanut, and olive oils, and cocoa butter.

Fats are formed from carbohydrates by two different series of
chemical changes. In the first of these series the carbohydrates
are changed to fatty acids, the more important of which are oleic,
palmitic, and stearic. In the second series of chemical changes

General Botany

the carbohydrates are transformed into glycerin,
with the fatty acids and forms fats and oils.

This unites

Fig. 24. Diagram of fat synthesis

carbohydrate
carbohydrate

■>- glycerin
-^ fatty acid

fats and oils

When fats are broken down or digested, they are changed back
again to glycerin and fatty acid, and may be finally altered to
glucose. The digestion of fats consists in forcing water into the
oil molecule, thus breaking it up into two or more molecules.

C3H5(Ci8H3502)3 + 3H20

fat water

-^ C3H5(OH)3 + 3 HC18H35O2

glycerin stearic acid

In plant cells the glycerin and stearic acid may be further trans-
formed into sugar before leaving the cell. Compare the formula
of a fat with the formula of a sugar. Does it contain a larger
or smaller proportion of oxygen ? In changing from sugar to fat
is oxygen added or removed ?

Sunlight is the direct source of energy used in photosynthesis.
The energy used in the transformation of the simple sugars into
fats and many allied compounds is derived from the oxidation
of a part of the sugar formed, not directly from sunlight. This
is discussed more fully under respiration.

The making and use of proteins. The proteins are the third
class of foods. They too are constructed in large part from the
carbohydrates ; but their molecules are vastly more complex than
are the molecules of carbohydrates and fats, and they all con-
tain the elements nitrogen and sulfur and some of them contain

The Manufacture of Food 43

phosphorus, in addition to carbon, hydrogen, and oxygen. In
protein synthesis the amount of sulfur and phosphorus consumed
is small, but a very large amount of nitrogen is required. Further-
more, nitrogen in the gaseous condition in which it occurs in the
air does not readily unite with other substances ; so, although it
makes up four fifths of the atmosphere, green plants cannot take
it directly from the air. For the nitrogen needed for protein
making, plants must depend, therefore, on the supply which comes
from the soil in the form of nitrates. This is carried to the cells
with the water that is absorbed by the roots.

Protein synthesis, like the synthesis of fats, is probably
effected by the protoplasm. It may occur in nearly all parts of
a plant, but it takes place for the most part in the leaves where
the carbohydrates are being made and where their constituent
parts are in a condition to unite with the nitrogen, sulfur, and
phosphorus compounds. Light may be a factor in the process
when it takes place in the leaves, but it has been definitely proved
that it may also take place in the absence of light. Proteins, like
fats and starch, are mostly inert storage substances, and many of
them are insoluble in water. Because of their chemical composi-
tion they are especially used in building up protoplasm (Fig. 25).

Steps in protein synthesis. The various steps in the building
of proteins are not fully known. It is probable, however, that
the nitrates derived from the soil are transformed into ammonia
(NH3) within the plant and that this unites with certain acids,
derived from the carbohydrates, forming amino acids. They
are called amino acids because the amino group (NH2) forms a
part of the molecule.

The amino acids are comparatively simple substances, but, like
the simple sugars, they may be built together to form large and
very complex molecules. Just as many glucose molecules may
be joined together in the formation of starch, so amino acids may
be joined together to form protein. In fact, there is good evi-
dence that in some proteins the molecules are formed by the union

General Botany

of a hundred or more amino-acid molecules. Just as starch
yields many molecules of glucose when it is digested, or broken
down, so when proteins are digested they yield many amino-acid
molecules.

Fig. 25. Diagram of protein synthesis.

The proteins are transported from the leaves in the food-con-
ducting tissue of the bundles, usually after they have been broken
down into simpler and more soluble substances (amino acids and
amides) .

Importance of nitrogen in soil. Since the proteins make up
more than half of the living protoplasm, and since all of them
contain a considerable percentage of nitrogen, the need for abun-
dant nitrates in the soil is evident. Any kind of moist land would
furnish the raw materials for making carbohydrates and fats,
but to supply the necessary materials for protein manufacture,
the land must contain nitrogen, sulfur, and phosphorus. It is the
varying amounts of these three substances in the soil that make
the difference in agricultural land values when other conditions
are equally favorable.

Sources of protein in the human diet. The most expensive
portion of the diet of human beings is the proteins. Figure
26 shows that in soy beans we possess the richest source of
protein. It also shows why the soy bean is one of the most
important of foods in the Asiatic nations, where animal foods
are very limited. One dollar will buy several times as much
protein in soy beans as it will in any other plant or animal

The Manufacture of Food

food. However, recent experiments in animal feeding have
shown that for maintenance and growth some proteins are more

Soy beans

7-5 '2.7 9.7 78 6.8

Fig. 26. Percentage of protein in various foods.

valuable pound for pound than others. Curiously enough, the
protein of the soy bean is not only furnished in large amounts,
but in its abihty to be digested and assimilated it stands at the
very top of the vegetable proteins. It is of interest to know that
even in the United States, where meat is consumed in compar-
atively large quantities, the principal source of protein in our
diet is wheat.

Importance of understanding the food-making processes. A
knowledge of the essential facts of food manufacture by plants
Hes at the foundation of all agricultural, horticultural, and silvi-
cultural practices.

We have gone far enough now to be perfectly sure that plants
do not get their food from the soil any more than animals do. Both
plants and animals require varit)us salts. Plants get these salts
from the soil, but they constitute only from i to 3 per cent of the
plant body. Both plants and animals require carbohydrates , fats ,
and proteins as their principal food. Animals can obtain these
from plants, but green plants must manufacture them.

General Botany

It is therefore evident that to obtain the best crop yields it is
not only essential to have sufficient nutrient salts in the soil but
that the temperature and light conditions be favorable for photo-
synthesis. Water must always be available. It is possible to
increase crop yields by increasing the supply of carbon dioxide
and water, as well as by adding more mineral salts (fertilizers)
to the soil.

Amount of food produced per acre. Since the food supply of
all living beings depends primarily upon these synthetic processes
that are carried on in plants, it is of interest to inquire how much
food may be derived from an average acre of land when planted
to different crops. It must be remembered that the plants that
produce this food take a considerable part for their own main-
tenance, and that the part which the farmer harvests is the
plant's surplus. The following table shows the average yield per
acre, its food value calculated in Calories, and the number of
men that i acre planted to different crops might feed for i day,
assuming that each man required 3000 Calories per day :

Food Products

Yield per Acre

Millions
of Calories 1
Equivalent

No. of Men That

Might be Fed for

One Day

Bu. Lbs.

Corn . . -

35 i960
no 5940
100 6uou
20 1200
40 1 154
16 960
14 840

3.1
2.8
1.9
1.8
1-7
1-5
I.I

1000

Sweet potatoes
Irish potatoes
Wheat . .
Rice . .

900
600
600
560

Soy beans
Beans

500

375

If the plant products of an average acre are fed to cattle, the
dressed beef produced amounts to only 125 pounds, yielding an
energy equivalent to the food of 43 men for i day. If trans-
formed into pork, the yield is 273 pounds, or sufficient food for

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

The Manufacture of Food 47

220 men for i day.^ This shows the great loss of energy that
results when plant foods are converted into meat before they
reach the human consumer. It is evident that as the human
family becomes larger and food becomes scarcer, we shall have to
take more and more of our foods directly from plants.

There are, however, certain animals that feed, either directly
or indirectly, on plants that cannot be used for human food.
All of our sea-food animals, such as fish, clams, and oysters, are
able to convert otherwise unusable food into food that can be
used, thus adding much to our diet. Sheep and cattle grazing on
the open range and forest reserves in the Western states and on
the pampas of Argentina may be looked upon as gatherers and
converters into available forms of food not directly usable by
man.

PROBLEMS

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

2. Occasionally in a field of young corn a stalk that lacks chlorophyll will be found.

How long will it live ?

3. Geraniums with variegated leaves occasionally produce branches that are en-

tirely white. A noted horticultural firm offered $1000 to any one of its
gardeners who would root one of these branches and thus produce a white-
leafed geranium. What was the chance for success ? Why?

4. Why do trees in the open retain their lower as well as their upper branches,

while the same trees grown in a dense forest retain only their uppermost
branches?

5. Why are there comparatively few weeds in a cornfield in the autumn as com-

pared with an adjoining field in which wheat has been grown?

6. Bushbeans cannot be grown profitably between rows of corn in a cornfield,

but polebeans, if properly spaced in the field, will yield abundantly and not
interfere with the corn. Explain.

7. Why is it best to wait until celery is well grown before tying it up with paper,

or covering it with boards to blanch it?

8. In how many ways could you cause a plant to starve to death? Are any of the

methods used in controlhng weeds ?

1 United States Department of Agricillture, Farmers' Bulletin No. 877. The
table does not take into account the necessity for using a variety of food substances
in our diet. Milk cattle return a larger proportion of food for human consumption
than the above statistics indicate.

CHAPTER SEVEN

THE RELEASE OF ENERGY

In order to do work, every machine in a manufacturing estab-
lishment must be suppHed with energy, and every Hving cell in a
plant requires energy for carrying on its work of repair, growth,
and movement. In manufacturing estabhshments the energy is
usually generated at one place and is then transmitted by means
of shafts and belts or by wires and motors to all parts of the
factory. It has already been shown that the plant obtains en-
ergy from sunhght during photosynthesis, and that this energy
is stored as potential energy in the food. Since the food passes
from cell to cell, some of the stored or potential energy finally
reaches every living cell of the plant. Here the energy that is
in the food may be liberated, or changed to free energy, and used
in the hfe processes of the cell, such as the synthesis of fats,
proteins, and other compounds.

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

The substance most commonly oxidized in plants is glucose.
Other carbohydrates like starch are changed to glucose before
oxidation takes place. Fats occurring in seeds are first oxidized
to sugars, and the sugars may be used in building tissue or they
may be further oxidized to carbon dioxide and water in respira-
tion. Protein may be oxidized in respiration, but this does not
usually occur unless sugar is scarce or lacking entirely. The
leaves and stems of land plants obtain their oxygen from the

The Release of Energy 49

atmosphere ; the roots obtain theirs from the air that is in the
soil. Wet soils are unsuited to the growth of many plants, not
because of the water present, but because of the lack of a suffi-
cient oxygen supply for the roots. Drainage is a valuable agri-
cultural practice, not only because it removes excess water, but
also because it draws air (oxygen) into the soil. When the
farmer breaks the crust on the surface, he is making it possible
for more oxygen to reach the roots of his crop.

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

The power stations are every Hving cell of root, stem, and leaf.

The machinery is the protoplasm and enzymes.

The fuel is foods, especially carbohydrates.

The process is the combining of food and oxygen.

The product is energy.

The waste is carbon dioxide and water.

The working hours are twenty-four hours a day.

Respiration and photosynthesis contrasted. Respiration is the
reverse of photosynthesis. In photosynthesis, carbon dioxide
and water are combined, complex molecules of carbohydrates
are formed, and a large number of oxygen atoms are set free
in the process. In respiration, the complex carbohydrate mole-
cules are broken up, oxygen is again combined with them,
and simple molecules of carbon dioxide and water are formed.
In photosynthesis, the energy of the sunhght used in building up
the carbohydrates is stored in them. In respiration, this energy
is released when carbohydrates are oxidized and changed back to
the simple substances out of which they were made.

When we wind up a clock spring, we put energy into the
tightened coil. When the springis allowed to uncoil, this energy
is released and turns the wheels of the clock. So in photo-
synthesis the energy is stored in the carbohydrates, and in the
process of respiration this energy is released and used in the life
processes of the cell.

50 General Botany

In photosynthesis In respiration

Oxygen is released. Oxygen is' consumed.

Energy is accumulated. Energy is released.

Simple molecules are built up into Complex molecules are broken down

complex ones. into simple ones.

P'ants accumulate food and increase Plants consume food and decrease in

in weight. weight.

Comparative rates of respiration. The rate of respiration is
greatest where there is rapid growth, as in germinating seeds,
opening flowers, and ripening fruits. In some of these it is much
more rapid, bulk for bulk, than in animals. A man gives off in
respiration about 2.5 per cent of his dry body weight of carbon
dioxide every twenty- four hours. Actively growing parts of
plants, hke opening flower clusters, may give off 10 per cent of
their dry weight in the same time. Some kinds of germinating
seeds give off carbon dioxide equivalent to 30 per cent of their
dry weight in a day. The average growing herbaceous plant,
like corn, loses carbon dioxide at a rate not far from i per cent
of its dry weight per day. About one fourth of the food manu-
factured by an acre of corn is used in respiration. Thus a mature
plant contains only about three fourths of the carbon that was
absorbed in photosynthesis. Since photosynthesis takes place
only during sunlight, the average rate of photosynthesis in a
corn plant is how many times the rate of respiration?

The lowest rates of respiration occur in dry seeds and other
dormant structures, and there is comparatively httle respiration
in woody stems and other hard parts in which there are only a
few living cells.

Respiration of fruits and vegetables. How important is the
recognition of the respiratory requirement of living cells may be
illustrated by the difflculties that have been met with in storing
and shipping fruits and bulbs. Peaches, during shipment, some-
times develop brownish spots where they touch each other.
These spots were formerly thought to be due to jarring in trans-

The Release of Energy 51

portation, but they are now known to be caused by packing the
peaches so closely that the air does not have full access to all the
fruit. The respiration of the cells at the points of contact is in
consequence interfered with, and these cells are suffocated and
die.

One sometimes finds large potatoes that are hollow in the
center, the cells lining the interior colored brown or black.
Otherwise the potatoes are sound. This also is a respiration
injury. While the tuber was in the soil or after it had been placed
in storage, the outer layers of tissue used all the available oxygen,
and the innermost tissue died, leaving a hollow. Cellars, pits,
and storage houses for fruits and vegetables must be carefully
ventilated.

Ships with specially ventilated holds are used in importing
bulbs from Holland and fruits from the tropics. The building of
ventilated holds came as a result of the death through suffocation
of several men who attempted to unload a cargo of bulbs from an
unventilated ship bottom.

CHAPTER EIGHT

SUBSTANCES MADE FROM FOODS

All plants contain a variety of substances made from foods
that cannot properly be classed as foods. Some of them are of
great importance in plant processes ; others form the constituents
of cell structures. Some may be changed again into foods, and
others seem to be waste, or by-products, of cell activities. The
most important of these substances will be briefly described in
this chapter.

Colorless plant tissues. One occasionally finds on plants
leaves that are wholly, or partly, white. This is simply the
natural color of living plant tissues that lack chlorophyll or other
pigments. The protoplasm, cell sap, and cell walls are trans-
parent and colorless. The presence of air spaces among the cells
makes these tissues appear white, just as ice is white when it is
filled with minute air bubbles. White leaves and flowers merely
show the natural appearance of plant tissues in the absence of
chlorophyll and other pigments.

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

^ The coloring matter in a green leaf is composed of about 66 per cent green
pigment (chlorophyll) ; 2^ per cent yellow pigment (xanthophyll) ; and lo per cent
orange pigment (carotin ; so named because of its abundance in the carrot). The
green pigment is not a simple substance, however, but a mixture of two kinds of
chlorophyll, one of which is blue-green and the other yellow-green. The depth
of the green color in a leaf depends in part on the proportions in which these various
pigments are combined. Chlorophyll contains carbon, hydrogen, nitrogen, oxygen,
and magnesium. Carotin contains only carbon and hydrogen, while xanthophyll
contains in addition a small amount of oxygen.

Substances Made from Foods 53

In the chloroplasts all three are present at the same time, so
that we cannot distinguish them under the microscope. As the
three are soluble in alcohol, the presence of the yellow and orange
pigment does not become apparent when the coloring matter is
extracted from leaves by means of alcohol. The chlorophyll
within a leaf is constantly breaking down, and new chlorophyll
is being formed constantly in the chloroplast exposed to light.
Since it is the chlorophyll in the chloroplast that effects the union
of carbon dioxide and water in photosynthesis, it is scarcely an
exaggeration to say that chlorophyll is the most important pig-
ment in the world.

Conditions affecting the development of chlorophyll. Chlo-
rophyll is produced only in the presence of light, but the yellow
and orange pigments are developed in the dark as well as in the
light. When we lay a board on grass or shut out the hght to
blanch the leaves of celery, the green color disappears, exposing
the yellow or orange. Likewise seedhngs grown in the dark and
the inner leaves of head lettuce show a yellow but not a green hue.
These facts make it clear that the yellow pigments do not require
light to develop, while the green pigment does.

There are a number of conditions besides absence of hght that
result in the partial, or complete, disappearance of the green pig-
ment, but these affect various plants quite differently. Low
temperature, drought, injuries, and diseases of various kinds may
interfere with the nutrition of the leaf ; even a sHght decrease
in hght may do so. All these factors tend to affect the green
pigment more than the yellow and orange. Although these
same influences — low temperature, drought, reduced light,
injuries, and diseases — may be effective at other seasons, they
become generally operative in late summer and autumn. Hence
it is at this time of the year that the green pigment disappears
from the leaves of most deciduous plants and unmasks the yellow
pigments in the chloroplasts. There is every gradation in the
readiness with which the green pigment disappears from the

54 General Botany

leaves of different species of deciduous trees, from the cotton-
wood, in which the leaves become yellow during a midsummer
drought, to the peach, in which they may still be vivid green when
shed. In evergreens the chlorophyll is less sensitive, and external
conditions are not so effective in causing changes in the color of
the leaves.

Red pigment in plants. The red colors of autumn leaves are
not due to changes in the content of the chloroplasts, but to the
formation in the cell sap of a red pigment called anthocyan. This
pigment is present in the cells of many young leaves in early
spring. It occurs also in beets, in red cabbage, in the petioles
and veins of many different kinds of leaves, in the coleus and other
foHage plants, and in many flowers. The presence of anthocyan
in the cell sap makes the whole cell red, and any or all of the cells
may develop the pigment. The anthocyans are soluble in water,
as is shown by the red color of water in which beets have been
cooked.

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

The development of the most brilKant red coloring of autumn
is commonly ascribed to the action of frost. This explanation is
probably incorrect, for careful observation indicates that the
color is most intense when a moderately low temperature is
accompanied by bright sunshine. In warm, cloudy autumns the

Substances Made from Foods 55

colors are more likely to be dull, with the yellows predominant.
In other seasons, when cold weather is delayed, autumn coloration
may be brilhant and near its climax before the first frost occurs.
That sunlight is important in the development of the red pigment
in many plants may be shown also by an examination of a leaf
that has been closely shaded by another. The pigment stops so
abruptly where the shade begins that a perfect print of the upper-
most leaf results. An abundance of nitrogen in the soil prevents
anthocyan formation in some plants. This fact may explain in
part the greater brilUancy of colors seen on hillsides and river
bluffs than on adjoining floodplains.

Among different plants there is much variation in the amount
of light that is required for the development of anthocyan colors.
This accounts for the great variation in the brilhancy of autumn
coloration in different years. One autumn affords light condi-
tions which promote the formation of anthocyan in only a few
trees and shrubs ; another autumn furnishes conditions so favor-
able that many plants become brilliant.

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

The red, blue, and purple colors of many flowers are due to
anthocyans, which are red when acid, purple when neutral, and
blue when alkaline. The anthocyan pigment that occurs in some
vegetables like beets, radishes, and purple cabbage bears no
relation to light.

Other pigments. A number of other pigments occur widely
distributed among plants, particularly the yellow pigments of

56 General Botany

many flowers, the yellow bark of some trees, and certain yellow
fruits. Some of these pigments have a commercial value as dyes.
Indigo and htmus are blue dyes of vegetable origin. Madder is
one of a group of red dyes used in making artists' colors.

Cell-wall constituents. Cellulose is the best known of the sub-
stances found in cell walls. It belongs to the more complex of
the carbohydrates and is a strong, white, insoluble substance.
It forms the framework of most plants and is the important con-
stituent of all textile fibers, hke cotton, hemp, flax, and jute. It
is also the basis of a large number of manufactured products,
such as paper, celluloid, acetic acid, artificial rubber, lamp
black, charcoal, vegetable silk, and numerous explosives.

Pectic compounds, which closely resemble cellulose in chemical
composition, occur in most cell walls. The middle lamella, which
holds together the cells of the higher plants, is made of pectose
or of calcium pectate. It is this layer which breaks down in the
boiling of fruits and vegetables and allows them to soften and
separate. Pectic compounds occur in many fruits, and when
these fruits are boiled with sugar, jelly is formed. In the Kving
plant pectic compounds aid in holding water in the cell.

Lignin, suberin, cutin, and wax. Closely associated with cel-
lulose is a group of substances which modify the cell walls of
certain tissues as they increase in age. These substances form
mixtures, or chemical combinations, with the cellulose already
present. Lignin increases the hardness and rigidity of cellulose
walls and is present in most woody tissues. Suberin is the impor-
tant constituent of the walls of cork tissue. Cutin and wax are
usually present in the outer walls of the epidermis of land plants.
Suberin, cutin, and wax are all related chemically to the fats,
and when present in, or on, cellulose walls render them less per-
meable to water.

Resins, gums, and mucilages. Resins and gums are products
frequently formed in all parts of plants. Resins are insoluble
in water and render walls impervious. They occur usually in

Substances Made from Foods

Fig. 27. Tapping the Para rubber trees, in the Malay
States, to get the latex from which crude rubber is made.

definite glandular structures, or in tubes extending throughout
the plant. Gums are soluble in water, forming a sticky solution.
Gums and resins form the bases of a variety of varnishes and
paints. Mucilages frequently occur in plant cells. Like gums,
they are soluble in water and are often useful in holding water
in plant tissues. The drought fesistance of some plants is due
to the presence of mucilages.

Latex. Many plants like the milkweeds, euphorbias, figs, and
rubber plants have a milky juice, called latex. This is a mixture
of resins, gums, fats, and food substances. It is the source of

58 General Botany

commercial rubber. Whether it has a definite function in the
living plant is unknown.

Alkaloids. Under this general name may be grouped a large
variety of chemical substances that seem to be of little importance
in the economy of the plant, but which have been of great impor-
tance in medicine. They are nitrogen compounds, are generally
odorless, and have a bitter taste and marked physiological effects
upon animals. They 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 cinchona bark; morphine and codeine,
from the poppy ; and caft'eine from coffee, tea, and cacao seeds.

Essential oils. The odors of flowers and the taste of many
fruits and vegetables are due to minute quantities of these sub-
stances. Because of their medicinal uses their composition is
well known. You are familiar w^ith menthol, the characteristic
oil of mint, camphor, oils of lavender, bergamot, bitter almonds,
and vanilla. Some of the oils contain sulfur. These produce
the odor and taste of onions, garlic, water cress, radishes, and
many kinds of mustard.

Vitamins. These substances, which are essential in the nutri-
tion of animals, occur only in minute quantities. We cannot
test for them or find them in the cell, and we know of their occur-
rence only through feeding experiments with animals. If they
are destroyed by prolonged boiling before the food containing
them is fed to animals, the animals fail to grow properly and
gradually weaken and die. Scurvy, beriberi, and rickets are
diseases produced by lack of the essential vitamins. Vitamins
are manufactured mostly by plants, and accumulate in certain
animal tissues and in milk, from the plants eaten by the animals.

Tannins. The bark of many trees, the galls occurring on oaks,
and certain unripe fruits like the persimmon, contain bitter
astringent substances known as tannins. These substances
react with gelatin or raw hides, forming insoluble compounds,

Substances Made from Foods

and this reaction is the basic one in the tanning of leather. With
iron salts, tannins produce black or green colors. Ink was

Fig. 28. Structures and substances occurring in plant cells : A, cell from pulp of ripe tomato,
showing chromoplasts in which the red pigment arises ; B, vertical section of upper cells
of petal of yellow lupine — a yellow pigment forms in the chromoplasts ; C, cells from green
bark of grapevine, some of which contain needle crystals (r) and other crystal aggregates
of calcium oxalate ; D, cells from castor bean containing aleurone (protein) grains ; E, part
of a vertical section of leaf of rubber plant showing crystal aggregate of calcium carbonate ;
F, compound starch grains of oats ; G, sphaero-crystals of inulin in cells of dahlia roots after
storage in alcohol ; H, calciuni oxalate crystals in cells of spiderwort ; I, cells from seed of pea
containing starch grains and protein granules. {After Frank.)

formerly made in this way. Many fruits are discolored when
cut with a steel knife, because of black compounds formed by
tannic acid and the iron in the knife.

Enzymes. The enzymes make up another group of very im-
portant substances found in all living cells. Their composition
is unknown, and we know of their presence only through the
effects that they produce. They are usually soluble in water,
in dilute salt solutions, or in glycerin, and are insoluble in alcohol.

6o General Botany

They are important in speeding up all chemical reactions in cells ;
without them the chemical changes would be so slow that hfe
could not continue.

Chemists have known for a long time that many reactions can
be accelerated by the addition of small quantities of certain sub-
stances which do not appear to take any immediate part in the
reactions. For example, if we boil cane sugar in pure water,
glucose and fructose are formed very slowly.

C12H22O11 + H2O — ^ C6H12O6 + CeHioOe

cane sugar water glucose fructose

If a very small amount of a mineral acid is added, the reaction
takes place very rapidly. Substances which accelerate chemical
reactions are called catalysts, or catalytic agents.

All cell processes, including oxidation, take place at ordinary
temperatures, often indeed at very low temperatures ; and it
would be quite impossible for them to take place so rapidly in
the absence of catalytic agents. Enzymes are the catalytic agents
of the cell. Many enzymes have been isolated from plant tissues,
the best known of which are diastase, used in digesting starch ;
lipase, employed in breaking down fats ; and papain, used in
digesting proteins.

Enzymes will be more fully discussed later in connection with
digestion, but they are mentioned at this time because of their
occurrence in all cells and because they are concerned in all chemi-
cal processes that occur in cells. Enzymes not only aid in break-
ing down complex substances in cells, but under slightly different
conditions bring about the reverse process, the building up of
complex substances. They are concerned in photosynthesis,
fat synthesis, protein synthesis, and the formation of the many
substances described in this chapter.

Protoplasm. The hving substance of the cell is the most
important product made from food. Carbohydrates, fats, and
proteins are in some way by the aid of enzymes built into proto-
plasm. This process can only be carried on by previously exist-

Substances Made from Foods 6i

ing protoplasm. How non-living materials are transformed into
living protoplasm is one of the greatest problems in biology.

Assimilation. Assimilation may be defined as the process by
which protoplasm, cell walls, and other essential constituents of
cells are made from foods. Protoplasm must be made in the forma-
tion of new cells, and it must be constantly renewed in cells
already formed. Of all the foods the proteins most nearly ap-
proach protoplasm in composition and are most used in the build-
ing of the living matter. However, carbohydrates and fats also
enter into the construction of both protoplasm and cell walls.
Before being assimilated, the complex foods are broken up into
simpler and more active compounds. Assimilation takes place
in all living cells, but it is most active in growing parts. Respira-
tion is also most active in these parts, and some of the energy
liberated by respiration is used in forming other compounds.

Summary. The many substances of which plants are composed
are made from foods. Some of these substances, hke chlorophyll
and enzymes, are of vital importance ; others, like tannins, alka-
loids, and essential oils, may be merely by-products of the nutri-
tive processes. Protoplasm is an organization of many sub-
stances possessing various properties. Cell walls are composed
primarily of cellulose, which is modified by the addition of other
substances. Vitamins, which are formed mostly by plants, are
essential additions to the food of animals, and there can be little
doubt that the presence of enzymes in the vegetable food of
animals aids in digestion. The building of new tissues is known
as assimilation, and is considered the culmination of all the chemi-
cal processes occurring in cells. Of the sugar made in photo-
synthesis by a corn plant, about one fourth is used in respiration,
about one half is assimilated in the construction of the plant, and
the remaining one fourth is accumulated in various forms of food
within the plant.

REFERENCES

Thatcher, R. W. The Chemistry of Plant Life. McGraw-Hill Book Co.
Haas and Hill. Chemistry of Plant Products. Longmans, Green & Co.

CHAPTER NINE

LEAVES IN RELATION TO LIGHT

Fig. 29. Vertical branch of
magnolia. Note the alternate
arrangement of the leaves.

Leaves grow from points variously arranged on stems that
have all sorts of positions. If these leaves grew out in random

directions, many of them would re-
ceive httle light. But an examination
of a plant shows its leaves arranged
in positions which display them ad-
vantageously to the hght. The raised
leaves of the pumpkin, the mosaics
formed on the sides of buildings by the
leaves of vines, and the successive tiers
of leaves on a beech, maple, or dog-
wood illustrate diilerent arrangements
by which large numbers of leaves are
efficiently displayed to light. Evidently
something controls the positions of leaves on a plant.

Growth influenced by light. Light affects growth in all organs
of the plant, including the leaf. The amount of light received
by a leaf blade not only affects the growth of the blade, but also
the petiole, and in some plants the adjoining stem. The influence
of the hght, during the growth of leaves, is such that when they
are mature most leaves are favorably placed with respect to hght.

The arrangement
of leaves on stems.
Leaves develop from
somewhat thickened
places on the stems,
called the nodes.
Each node may bear
one, two, or several

leaves. AcCOrdmg ^^^ ^^^ Horizontal branch of magnolia. Compare leaf

to the number of positions with those of Figure 29.

Leaves in Relation to Light

leaves that the node bears, the leaf arrangement is designated

as alternate, opposite, or whorled.

In the alternate arrangement each node

bears one leaf. This is also called the spiral

arrangement, because a line drawn through

successive leaf bases forms a spiral about the

stem. Sometimes, as in the corn plant, the

spiral passes half around the stem in going

from one node to the next. In other plants,

hke the sedges, the spiral passes but a third

around the stem between nodes. In several of

our common fruit trees, as, for example, the

apple and the peach, the spiral between

nodes passes two fifths around the stem.

These variations of the spiral arrangement

of leaves on stems are called the two-ranked

(Fig. 37), three-ranked (Fig. 31), and five-
ranked arrangements (Fig. 29).

In the opposite
arrangement two
leaves occur at each
node (Fig. ^^). The leaves at succes-
sive nodes, however, are at right
angles to each other, giving four ranks
of leaves. The maple, ash, dogwood,
and lilac furnish examples of the op-
posite arrangement. In the whorled
arrangement the leaves are in a circle
about the node (Fig. 32). The Indian
cucumber root (Medeola) and the
wood Hly furnish excellent examples
of the whorled arrangement.

However, it is only on upright
stems which receive the light equally

Fig. 31. A sedge {Duli-
chiuni), showing three-
ranked arrangement of
the leaves.

Fig. 32. Indian cucumber root,
showing the whorled arrangement
of the leaves.

General Botany

on all sides that the blades take their normal positions directly
out from the nodes. If an erect shoot be placed in an incHned

position, it is easy to see that the
leaves are no longer well displayed to
the hght. As may be readily seen by
examining the branches of trees and
the stems of traihng plants, horizontal
or inclined stems become twisted dur-
ing development through the influence
of unequal illumination upon the rela-
tive growth of different sectors of the
stem. The twisting of the stems
brings the leaves into better-illumi-
nated positions, but it often obscures
the normal arrangement of the leaves.
The positions of leaves with ref-
erence to light. If leaves are moder-
ately sensitive to hght, their posi-
tions when mature are approximately
at right angles to the Hne along which
the greatest amount of light reaches them. Consequently the
leaves on most of our common trees, shrubs, and herbs have an
approximately horizontal position (Figs. 33, 34). The sugar
maple and the magnolia are examples of trees whose leaves
are displayed in this manner (Figs. 30, 35). In the cottonwood

Fig. 33. Vertical branch of dog-
wood, showing the opposite ar-
rangement of the leaves.

Fig. 34. Horizontal branch of dogwood. Compare with Figure $3-

Leaves in Relation to Light

S^O]

*-« . p^^^t"^ >■• ^^^^^

Wl^s w.^mS^mSM^^mm^i

^'^;i**j<'*||^ ' :i^<*:l^^TB*«-* *^' ** ■■■ ;'^''^^

PF. 5. Cooper

Fig. 35. Leaf mosaics formed by maple leaves {Acer macro phy II urn and Acer circinatum),
Olympic Mountains, Washington. The light affects the growth of the petioles and branches
in such a manner that the leaves are fitted together side by side.

and tulip trees the leaves are less sensitive to light, and the re-
sult is that their leaves assume a great variety of positions. If
leaves are extremely sensitive to hght, the blades may turn
toward the sun in the early morning and follow it throughout
the day, always keeping the broad face of the leaf to the light.
The leaves of the common mallow move in this way.

Leaf mosaics. The leaves of many plants, hke the Boston ivy,
sugar maple, and beech, are so arranged that if we look at them
from the direction from which they receive the most hght they
seem to be fitted together like the stones in a mosaic. In this
way each leaf is exposed to the most possible hght.

Many small herbs, like the dandelion, moth mullein, common
plantain, and evening primrose, form rosettes of leaves near the
soil. An examination of these rosettes will show that each leaf
is arranged so that it fihs a space in the circle. Further exam-
ination will show that the leaves that would otherwise be more
or less shaded have changed their positions and occupy the
spaces between the leaves above them.

General Botany

Compass plants. There is another class of plants which are
sensitive to light, but which respond to it in a very different

manner. These are the so-called
compass plants, of which the
wild prickly lettuce is a widely
distributed example. In sunny
situations the leaves of these
plants tend to take positions
edgewise to the direction of the
most intense light. As the sun-
light is most intense at noon, it is
only in the morning and late
afternoon that the flat sides of
the leaves are perpendicular to
the sun's rays. This response
to the hght also places most of
the leaves in a vertical north-
and-south plane and suggests the
name '' compass plant." When
grown in partial shade, the leaves
of these same plants are hori-
zontal. Hence it is clear that the
Fig. 36. Prickly lettuce, which is called positiou of their Icavcs in suuny

;'compassplant''because its leaves stand situations is the result of hght

in a north-south plane: A, viewed from ...

west; B, viewed from south. Drawn COuditionS (Fig. 36).

from a specimen grown under exposure to HoW the blade attains ItS pO-

bright sunlight.

sition with reference to the light.

The position of the leaf blade is partly attained, as has been noted,
by the bending and twisting of the plant stem during its develop-
ment. To a much greater extent the blade owes its position to
the bending, twisting, and elongating of the petiole. Indeed,
its ability to place the leaf in an advantageous position toward
the hght is the particular advantage of the petiole. Its length
and direction of growth are for the most part determined by the

Leaves in Relation to Light

way in which the Hght falls on the blade during the period of
development.

Strong light retards growth in length of the petiole. If the
blade is shaded, the petiole elongates more than usual ; if shaded
on one side, the petiole grows unequally on its two sides until
the blade is about equally illuminated. The position of the leaf,
when it has stopped growing, is usually fixed, and shading will
no longer affect the growth of the petiole. When a leaf that has
attained its full growth is overshadowed, it loses its chlorophyll,
turns yellow, and dies. You can find examples of such leaves
under the green leaves of rosettes, or on the lower branches of
trees that form mosaics.

Vertical leaves. In a number of common plants, including the
iris, cat-tail, calamus, and many grasses, the leaves are vertical
because they are held in this position by their sheathing bases
rather than because of a response to hght. These plants usually
occur in dense growths (Fig. $S), and the vertical position of
the leaves permits the light to penetrate to their bases. This
has the advantage of allowing photosynthesis to go on through-
out the entire length of the leaves.

Differences in vertical and horizontal
leaves. The structure of vertical leaves
differs from that of horizontal leaves in
several particulars :

In vertical leaves the mesophyll may be
composed of spongy tissue, or it may be
composed entirely of palisade cells. Mdre
rarely there are pahsade layers on both
sides, with a spongy layer between. In
contrast, a horizontal leaf usually has a
pahsade layer beneath the upper epider- ^ ,

mis, and the lower portion of the meso- ♦./O'^y^iv-^-^-'
phyll is composed of loosely arranged fr/^.lirjrp::;^^

cells. In vertical leaves stoma ta usually by their sheathing bases.

General Botany

Fig. 38. Guinea grass, a plant grown in the tropics for fodder. Vertical leaves expose a
large surface to the sunlight in spite of the crowding.

occur on both surfaces, while in most horizontal leaves the
stomata are confined to the lower surface (page 27). Vertical
leaves are hkely to be of the same color on both surfaces, while
horizontal leaves are generally of a darker green on the upper
surface.

The difference in the color of the two sides of a horizontal leaf
is due in part to the presence of a larger amount of chlorophyll
in the compact palisade layers of the mesophyll than in the loose
spongy layer beneath. In vertical leaves the similarity of struc-
ture in the mesophyll on each side, and the fact that both surfaces
of the leaf are equally illuminated, account for the sameness of
color of the two surfaces. The color of leaves is sometimes modi-
fied by the presence of hairs, wax, or drops of resin.

Motile leaves. The leaves of which we have been speaking
have their positions rather definitely fixed when they reach ma-
turity. There is another class of leaves, however, in which the

Leaves in Relation to Light

positions of the blades are not fixed, but are changed according to
the intensity and direction of light. A familiar example is the
roadside sweet clover. At night the three leaflets of the com-
pound leaf droop downward from the petiole; in the medium
light of a cloudy day they are held perpendicular to the light ;
in the most intense sunhght the blades are raised above the petiole
until they are edgewise and point toward the light. Some ob-
servation of lima bean seedhngs (Fig. 41), which may readily be
grown in the laboratory, will be instructive in this connection.
Other examples of motile leaves may be seen in the honey locust,
the leaflets of which fold upward at night, and in white clover,
oxalis, and the red-bud tree.
The leaflets of the sensitive
plant vary their positions ac-
cording to light intensity, and
also when touched or injured
in any way (Fig. 39).

The changes of position in
motile leaves is brought about
by changes in the water con-
tent of the cells on opposite
sides of a special organ called
the pulvinus (Fig. 40), which
is located at the base of the
leaves and the leaflets. This
device may readily be studied
in the leaf of the bean.

The leaves of shade plants.
As may be observed by a trip
to the woods, the leaves of

plants growing in the shade Fig. 39- Sensitive plant. The leaves on the

„ , , J left side are in normal positions ; those on the

are usually darker and more j-ight side have been touched and the leaflets

bluish-green than the leaves have folded together wholly or in part, and

^ • r n ^^^ petioles have folded toward the stem.

of plants growing m full sun- p is the pulvinus.

General Botany

light. This difference in color is accounted for in part by the
amount of chlorophyll near the surface and in part by a
slight difference in the color of the green pigment in the chloro-
plasts. In a few shade plants the depth of the green color is
increased by the presence of chloroplasts in the epidermal cells.
Shade plants are not subjected to drying, as are plants growing in
exposed situations, and, generally speaking, their leaves are broad
and thin. The leaves of these plants differ further from the or-
dinary leaf in that the cuticle is less developed, the mesophyll
is composed almost entirely of spongy tissue, and usually stomata
are present on both surfaces of the leaf.

Submerged leaves. Every one who has gone fishing or rowing
knows that a great deal of sunhght is reflected from the surface
of water. A smooth water surface reflects about one fourth, and
a rough water surface about one half, of the hght that falls on it.

This means that the amount of
hght that passes into the water
is reduced by the amount that is
reflected. The penetration of
the water by the sun's rays is
further interfered with by the
fine sediment that clouds our
ponds and lakes. Every one
who has dived and opened his
eyes under water knows that it
is dark at a comparatively shght
depth. Measurements have
shown that one half to three
fourths of all the light that en-
ters the water may be stopped in
the first three feet, depending
upon the amount of suspended
particles present. Hence sub-
merged plants always grow in

Fig. 40. Pulvinus and section of pulvinus
from leaf of sensitive plant, both enlarged.
When the leaf is touched, the water in
the cells on the side A passes outward
into the intercellular spaces, causing the
cells partially to collapse. The pressure
of the cells on the side B then forces the
leaf downward.

Leaves in Relation to Light 71

light of reduced intensity. They receive an amount of light
comparable to that received by the shade plants found in

Fig. 41. Various positions taken by leaflets of lima bean: A, position in
intense light ; B, position in diffuse light ; C, position in darkness.

forested ravines. Submerged leaves, too, are of very soft tex-
ture, and are quite without mechanical tissue in the veins, so
that they are unable to support themselves when hfted from
the water. They are kept upright in the water by their buoyancy.
Summary. Light has marked effects upon the positions, the
color, and the structure of leaves. Leaves tend to be placed
directly outward from the nodes to which they are attached, but
light affects them during their development, and most leaves
come to occupy positions that have more relation to the light
than to the stem which bears them. The position of leaves
and the movements of leaves are determined by differences in
water content and in the rate of growth on opposite sides of the
stems and petioles that support them.

PROBLEMS

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

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

3. Why do gardeners shade lettuce plants in midsummer?

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

5. Why is tobacco that is intended to be used for cigar wrappers usually grown

under canvas or beneath lattice frames?

CHAPTER TEN

THE WATER RELATIONS OF LEAVES

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

Why water is necessary to a plant. The active protoplasm of
all plant cells is in a semiliquid condition. More than 90 per
cent of its weight is made up of water, and in consistency it closely
resembles white of egg. The several parts of the protoplasm —
the cytoplasm, the nucleus, and the plastids — differ somewhat
in their water content, but all of them must be nearly saturated
with water to carry on the hfe processes. When the amount of
water in the cell falls much below this point, the protoplasm
becomes rigid and all its activities are retarded. The curled-up
leaves of corn during a summer drought illustrate this effect.
The corn manufactures little food, and consequently growth is
retarded. In many plants the protoplasm may even die if the
water content is greatly reduced. For example, the seeds of the
soft or silver maple which are shed in late spring and germinate
soon afterward die if the water content is reduced below 30 per
cent. Water is necessary for the life of the protoplasm of plant

The Water Relations of Leaves 73

cells. We have previously shown that water enters into the com-
position of all carbohydrates ; therefore water is necessary for
photosynthesis.

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

Growth and reproduction result from a series of many physical
and chemical changes within the cells. These changes can take
place only in the presence of water. Water is necessary for all
physical and chemical changes within the plant and consequently
for all plant activities.

Transpiration. If we expose a wet cloth to the air, the water
evaporates ; that is, it changes from a hquid to a vapor and
passes off into the atmosphere. The same thing happens when
a plant is exposed to the air. The mesophyll cells of the leaf are
continually losing water vapor to the intercellular spaces, from
which, if the stoma ta are open, this vapor passes out into the
atmosphere. The epidermis of the leaf also allows some water
to pass through it, but in land plants this is a relatively small
amount, because the cuticle hinders the process. The loss of
water vapor from plants is called transpiration.

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

74 General Botany

Importance of transpiration. Just how important transpira-
tion is to the plant may be easily seen by a study of the energy
changes that take place in a leaf. First of all, we must understand
that light energy is very readily changed to heat energy, and that
when heat energy accumulates in a body it raises its temperature.
When a body loses heat energy, it is cooled. When the sun shines
on a leaf, it is estimated that about lo per cent of the light is
reflected by the leaf surface and about 25 per cent goes through
the leaf. Sixty-five per cent is retained by the leaf. This is
sufficient energy to raise in a few minutes the temperature of the
leaf from air temperature to the danger point for protoplasm.

As soon as the temperature of a leaf rises through exposure to
sunlight, the water particles become more active, and as they
leave the surface and fly off into the air the excess heat energy is
reduced. In this way the leaf is kept at, or within a few degrees
of, the air temperature. Transpiration cools the leaf just as
water evaporation from your hand cools the skin. Transpiration
is important to plants because it helps to regulate the temperature and
prevent overheating. As will he shown later, it is also an important
factor in raising water and mineral salts from the roots to the leaves.

It is estimated that nearly one half the energy of sunhght that
falls on a cornfield in Illinois is used in transpiration.

Transpiration and stomata. Most of the water vapor that
leaves the plant in transpiration (80-97 per cent) is derived from
the mesophyll cells and passes through the stomata from the
intercellular spaces. Comparatively httle (3-20 per cent) is
lost through the epidermis directly into the air. It is evident,
then, that the movements of the guard cells, as they result in
opening or closing the stomata, modify the rate of transpiration.

In most plants the stomata are closed at night, and as there is
little or no heat energy added to the leaves, the rate of transpira-
tion is very low. The stomata open slowly after sunrise, but as
soon as the light strikes the leaf its temperature rises. Trans-
piration increases rapidly. Toward the middle of the afternoon

The Water Relations of Leaves 75

the stomata begin to close, and before sundown are completely
closed. The rate of transpiration begins to decrease about
2 o'clock and reaches the slow night rate before, or soon after,
sunset. The stomata, therefore, modify the rate of transpiration
greatly.

It must not be supposed, however, that they act as safety
mechanisms to conserve water in the plant. They may open in
the hght, whether the plant has an adequate water supply or not.
Likewise they may close when the plant has an abundance of
water. Usually the stomata close when the leaves wilt, but there
are exceptions even to this rule. The opening of the stomata in
hght not only allows the outward diffusion of water vapor, but
also the inward passage of carbon dioxide used in photosynthesis
and the escape of oxygen liberated in this process.

The amount of water transpired by plants. The amount of
water lost in transpiration is surprisingly large. During its hfe-
time, a well-watered corn plant may lose 40 gallons of water.
The water lost by a field of wheat during its entire period of
development would cover the field to a depth of 4 or 5 inches.
A medium-sized date palm growing in the Sahara Desert under
irrigation is estimated to require from 100 to 190 gallons of water
per day during at least four months of the year. For the best
growth of plants, therefore, there must be available in the soil
enough water to replace all that is lost by transpiration and
the smaller amount used in the growth of new parts.

When we consider that the quantity of water transpired by
wheat in cultivation is one fifth to one eighth of the rainfall of
the central United States, we begin to realize how large a fraction
of all the water that falls on the soil is actually used by the plants.
In all rainfall, some water runs off the soil without penetrating
the surface, some evaporates from the soil surface itself, and some
sinks below the level of the plant roots. Consequently, it is
only when there are abundant rains, distributed throughout the
growing season, that the amount of water needed by the plants

General Botany

for transpiration and their best development is available in the
upper layers of the soil. It has been shown by experiment that
for production of every pound of solid matter in the above-ground
parts of crop plants, from 300 to 500 pounds of water are required
in the central United States, and that from 400 to 1000 pounds
are needed on the plains of Colorado. The amount of water
used in transpiration is, therefore, many times the amount used
in the manufacture of food. It is estimated that an acre of corn
uses 1700 tons of water in transpiration and 4 J tons in photo-
synthesis.

Substances and structures modify transpiration. Most leaves
possess certain structures that reduce the rate of transpiration.
The possession of these structures enables the plants to hve under
somewhat drier conditions than they otherwise could.

(i) Thickened cuticle and " hloom.^^ The cuticle of a leaf checks
transpiration, and in plants of dry cli-
mates the cuticle may be so thick as to
reduce transpiration through the epi-
dermis to almost nothing. There are
many plants which secrete, in addition
to the cuticle, particles of wax on their
leaves or other parts. This is the so-
called ^' bloom " which may be seen on the
leaves of the houseleek and cabbage and
on the fruits of the grape, plum, and
blueberry. The bloom consists of a
layer of wax particles scattered thickly
over the surface of a leaf or fruit. It
forms a layer that is nearly impervious
_ . , . , to water and helps to reduce water loss

Fig. 42. Vertical sections of ^ ^ ^

leaves of Mertensia, showing thrOUgh the Cpidcrmis.

gJo:r;S Z£^:iZ I^ transpiration, however, most of the
ation (above), and when water is lost through the stomata. So we

growing in dry, intensely i . -n r,

lighted situation (below). may have a heavy cuticle and stiil nave a

E. S. Clements

The Water Relations of Leaves

W. S. Cooper

Fig. 43. A xerophytic morning-glory and a succulent-leafed Mesembryanthetnum (above at
right) growing on the dunes near Coronado, California. The thick cuticle of the morning-
glory leaf reduces the transpiration rate. The other plant has a relatively small leaf area
and holds its water tenaciously because of substances within its cells.

high transpiration rate if the stomata are open. In leaves with
a heavy cuticle the stomata are usually small and do not open so
freely as in leaves with a very thin cuticle ; consequently trans-
piration is generally less from hard, thick, and heavily cutinized
leaves, even though the cuticle prevents evaporation only from
the outer leaf surfaces.

(2) Compact leaves. A plant may become adjusted to an inade-
quate water supply by the development of leaves with compact
tissues, as a result of exposure to drought or bright sunshine. In
such leaves the intercellular spaces are much reduced, and evapo-
ration from the mesophyll cells is greatly lessened. In extreme
cases the mesophyll cehs are all of the compact pahsade type,
which leaves the minimum of air -space within the leaf. Compact
tissues reduce the rate of transpiration through the stomata.
The tissues are compact under these circumstances simply because
drought prevented the further expansion of the leaf, leaving the
cells close together (Fig. 42).

(3) Small leaf area. A third way in which plants become ad-

yS General Botany

justed to dry conditions is by a decrease in the total leaf area.
When a plant is brought into the house in autumn, some of its
leaves usually fall off. The air inside houses being much drier
than the air outside, transpiration is greatly increased. As the
water supply remains about the same, the loss of a few leaves
restores the water balance of the plant. Some trees, hke the
Cottonwood, lose part of their leaves during a summer drought.
If a wet period follows, more leaves may be added, and in this
way a nearly uniform water balance is maintained.

(4) Hairs. Hairs are frequently described as structures that
greatly reduce transpiration. Some of the very broad scale-
hairs may reduce the rate slightly but experiments show that the
hairy covering of the common mullein, which is exceedingly
dense, has little or no effect on its rate of transpiration either in
still air or in wind.

(5) Resin, wax, and mucilage. Some plants produce resin,
wax, or mucilage, which retard transpiration. For example, the
horsechestnut has a coating of resin on its buds. The bayberry
has wax covering its fruits. The tissues of cactus contain muci-
laginous substances that have a great water-holding capacity.

External factors that modify transpiration. That plants grow-
ing under moist conditions have larger leaves and more leaves
than the same kinds of plants growing under dry conditions has
been noted by every one. Experiments show that their rates of
transpiration are far greater than when grown under dry condi-
tions. Humidity of the air directly affects transpiration, because
when the air is nearly saturated the difference between the
humidity inside the leaf and outside is so small that water vapor
passes out through the stomata into the air very slowly.

The amount of available water in the soil and the humidity of
the air are important because they determine the amount of water
in the cells of the plant. The condition of the mesophyll cells
in turn regulates the rate of water loss to the intercellular spaces.
A dearth of water in the plant may also prevent the opening of the

The Water Relations of Leaves

E. S. Clements

Fig. 44. Vertical sections of
leaves of Hippuris, a water
plant. The upper figure
shows an aerial leaf, the lower
figure a submerged leaf. The
aerial leaf is much thinner
and the tissues more compact.

stomata. Light affects the opening of stomata and raises the
temperature of the leaf, and consequently increases the rate of
transpiration.

Intense light and drought decrease the
size and number of leaves and increase
the compactness of the mesophyll, the
amount of cutin, mucilages, hairiness, and
mechanical tissue. Incidentally, these
changes retard transpiration. Hence
these external factors, by producing
changes in the physical and chemical
processes within the plant, indirectly
modify transpiration.

The higher the temperature is, the
greater the rate of transpiration, not only ^^ ^^^ ^^^^^^^^^ ^^^^ ^^^
because the water in the mesophyll cells guard ceils form but the sto-
changes to vapor more rapidly, but be- mata do not open,
cause the vapor particles move out of the leaf faster.

Submerged and floating leaves. An examination of a sub-
merged leaf on any pondweed shows that it has no stomatal open-
ings. Sometimes the guard cells are formed but do not separate
(Fig. 44). The floating leaves of water hlies and other pond
plants have stomata only on the upper surfaces. Being com-
pletely surrounded by water, submerged plants have no transpira-
tion. It is also certain that they get their carbon dioxide directly
from the water through the epidermis, for carbon dioxide is found
dissolved in pond waters, often in larger proportion than in the air.

In water-h'ly leaves the upper surface is covered by a cuticle
that is not readily made wet, and it has stomata that do not open
until the leaf is above water, li the leaves are raised entirely
above the surface of the water, as sometimes happens when the
plants are crowded, both surfaces develop stomata.

Desert plants and water storage. In the desert, where the air
is very dry and the scanty rainfall is confined to one or two periods

General Botany

in the year, plants have great difficulty in securing water. The
perennial plants have various ways of conserving water from one

Fig. 45. Resurrection plant (Selaginella) of Texas and New Mexico. During the rainy
season the plant spreads out and grows as a rosette. When drought comes, it dries out and
curls up into a ball, as shown at the right.

rainy period to the next. The barrel cactus has no leaves at all,
and the stem is a thick cyhnder composed largely of water-storage
tissue ; it may hve without additional water for two years or
longer. Some of the desert shrubs have leaves during the rainy
periods only, and these fall as soon as the drought comes. Still
others, like the agaves, have very thick, leathery leaves with much
internal water-storage tissue and a very low transpiration rate
(Fig. 48).

Adjustment to desert conditions by ability to withstand drying.
Another group of plants is adjusted to desert conditions by being
able to withstand complete drying. The resurrection plant of
Texas is an example of this group (Fig. 45). During the rainy
season it is green and has its many scale-leafed branches spread
out, making possible food manufacture and growth. When
drought comes, the plant dries out and its branches curl upward
until it is in the form of a ball. In this condition it may be blown
about by the wind and remain dormant for weeks or months,

The Water Relations of Leaves 8i

all its physiological processes having been reduced to a minimum.
When the plant becomes wet it unfolds, and its processes become
active again. In the eastern United States we find plants of
this same type in the hchens, mosses, and a few small ferns that
grow on the bark of trees and on bare, dry rocks.

CHAPTER ELEVEN

PHYSICAL PROCESSES INVOLVED IN THE MOVEMENT OF
MATERIALS IN PLANTS

Since the earliest times students of plants have been trying to
find out just how gases, water, and minerals get into plant cells
and how they pass from one cell to another ; also how the soluble
foods move from one organ to another. Not all of these ques-
tions have been satisfactorily answered, but there are certain
physical processes that at least help to explain all of them. We
must not be misled into thinking that the plant does all these
things because, being ahve, it can take in substances, move them
where they are needed, and throw off those that are not needed.
All investigations indicate that these processes take place, not
because plants exert some peculiar vital force, but because plant
cells possess those particular physical and chemical properties
and structural arrangements which, even in a non-living piece of
physical apparatus, induce these same processes. Although we
cannot now imitate perfectly all the processes concerned in the
movement of materials within the plant, it is fair to predict that
we shall be able to do so in the not distant future.

Solution. All substances, whether gaseous, liquid, or sohd,
must be dissolved in water before they can pass into, or out of, a
plant cell. By solution is meant the dividing of a substance into
invisible particles that distribute themselves throughout a liquid.
Carbon dioxide, for example, occurs in the air as a gas. Its
particles strike the water surface of the cell and go into solution.
Mineral substances coming in contact with the water in the soil
do the same thing, and it is only after they are dissolved that they
enter the root. Sugar, likewise, can move out of or into a cell
only when it is dissolved in the water of the cell. Solution is the
first of four physical processes which are important in the move-
ment of substances in plants.

Diffusion. If a small dish of ether is exposed in a room, in a
few minutes the odor of the ether may be noticed in all parts of

Processes in Movement of Materials S;^

the room. Even if there were no air currents, the ether would
evaporate ; that is, particles of ether would rise from the surface
of the liquid, pass out of the dish, and move through the room in
every direction. This is an example of the diffusion of a vapor.
The vapor is concentrated in the dish and the particles move
outward into the room where there is none ; that is, the particles
move from the place where their concentration is greater to
where it is less. After the ether has evaporated, the vapor tends
to become evenly distributed throughout the room. SoUds like
camphor and naphthalene might be used in place of the ether.

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

Diffusion of a gas or vapor is very rapid. Diffusion of a dis-r
solved substance in a hquid is slow, but the distances that sub-
stances must travel in plant cells are very small. Oxygen and
carbon dioxide, when once dissolved in the water of cells, move
about partly by diffusion. The soluble foods in plants move
from one part to another by diffusion. Soil salts enter the root
by diffusion and are not carried into it by water. Diffusion may
occur under special conditions, and it is then conveniently spoken
of as imbibition and osmosis.

Imbibition. The process of imbibition may be illustrated by
placing a sheet of gelatin in water. Dry gelatin is a hard, brittle,
partly transparent solid. After it has been in water for a few
minutes, it will be found to have increased in weight and in
length, breadth, and thickness. The gelatin, instead of being

General Botany

W. S. Cooper

Fig. 46. The barrel cactus {Echinocactus cylindraceus) of the Colorado desert, California.
These plants are highly resistant to water loss because of the presence of mucilaginous
carbohydrates which imbibe and hold water.

brittle, is now soft and pliable; it is also more transparent than
it was.

The increase in size and weight is explained by the fact that
particles of water have diffused into the gelatin and have forced
the particles apart. Since the gelatin particles have been forced
farther apart, the gelatin is more phable and the particles chng to
one another less firmly. The cell walls of a plant take up water
in the same way. Hence when a piece of dry wood is put in
water, it imbibes water and swells. When dry seeds are placed
in water, they imbibe water and increase in size. Indeed, most
organic substances have the property of imbibing water and
swelling. Imbibition is a form of diffusion that results in swelling.
Compare the size of a sponge when dry with its size after it has
been soaked in water and squeezed as dry as possible.

When a piece of wood becomes saturated, it stops taking up
water. If, however, the water were being removed from the
inside, more would continue to pass into the wood. This is
exactly what happens in the root of a hving plant. The external

Processes in Movement of Materials 85

cells of the root are in contact with the water of the soil. Inside
the root the water is being used and removed by being drawn up
through the stem to the leaves. More water then passes into the
cell walls and protoplasm to take the place of that which is drawn
away, and this tends to keep the amount of water in the plant
nearly constant. Imbibition becomes very powerful in plants
that have mucilage, gums, and pectic compounds in their tissues,
both in absorbing water and in holding it against evaporation.

Osmosis. A third form of diffusion that aids in the absorp-
tion of water is osmosis. If an animal membrane, as a piece of
bladder, is tied over the broad end of a thistle tube and the bulb
of the tube is immersed in water, the water will gradually pass
through the membrane. The membrane is permeable to water ;
that is, it allows water to pass through it. The water continues
to move through until its level is the same inside and outside
(Fig. 47)-

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

If we put a little sugar into the thistle tube, something dif-
ferent happens, as is shown by the fact that the liquid in the tube
begins to rise. Evidently, more water is passing through the
membrane into the tube than is passing out, and this change has
been brought about by the presence of the sugar. Perhaps we
can get a mental picture of what causes this difference from the
diagram in Figure 48. The membrane (C) allows water mole-
cules to pass through it freely, but it permits scarcely any of the
sugar molecules to pass. The outer side of the membrane is
completely covered with water molecules, tending to diffuse
through the membrane. The inner side is only partly covered
with water molecules, since part of the area is occupied by sugar

General Botany

molecules. Consequently there are fewer water particles on the
inside tending to diffuse outward. Sugar dilutes the water in the

■ W II

— —/^

"^r ZI~

^Z — ((~ WAT

'^-^i—

— ~^^^w"^X~

^fe^-

WATER ^^— j

Fig. 47. Diagram to illustrate
the passage of water through a
membrane : A represents a mole-
cule of inside water, B a molecule
of outside water, and C the mem-
brane. Equal numbers of water
molecules are in contact with the
inside and outside of the mem-
brane, and the rate of movement
through the membrane in both
directions is the same. Hence
the level of the water in the tube
remains unchanged.

Fig. 48. Diagram to illustrate
osmosis: A represents a sugar
molecule, B a water molecule,
and C a differentially permeable
membrane. The sugar in solu-
tion dilutes the water so that
fewer water molecules are in
contact with the inside than with
the outside of the membrane.
Hence water passes in more
rapidly than it passes out, and
the level of the water in the tube
rises.

tube. Consequently the water is more concentrated outside the
tube than inside, and in keeping with a general law of diffusion
the water passes from the place of greater concentration to the
place of less concentration.

Moreover, sugar is a highly soluble substance ; that is, it has a
great affinity for water, and the sugar particles tend to hold the
water particles in contact with them inside the thistle tube. The
sugar, hke the water, tends to pass from the place of greatest

Processes in iVIovement of Materials 87

concentration but is restrained by the membrane from moving
outward.

Differentially permeable membranes necessary for osmosis.

When a membrane permits water or other substances to pass
through, it is said to be permeable to that substance. For example,
animal membranes are permeable to water and to various dyes.
A membrane that allows one substance to pass through it, but
retards the passage of another substance, is said to be differen-
tially permeable. The membrane on the thistle tube is differ-
entially permeable, because it allows the passage of water but
restrains the sugar that is dissolved in the water. The diffusion
of water through a differentially permeable membrane to the side
where it is less concentrated is called osmosis.

Osmotic pressure. If we close the upper end of the thistle
tube, the water will continue to rise and compress the inclosed
air. If a large amount of sugar is put inside the tube, the water
will rise rapidly and exert great pressure. If only a small amount
of sugar is present inside the tube, the water will rise slowly and
exert but Httle pressure. The pressure which is developed by
diffusion under these conditions is called osmotic pressure.^

The plant cell as an osmotic apparatus. In hving plant cells
the cytoplasm hning the cell walls is the differentially permeable
membrane. The cell walls of some tissues are also differentially
permeable.

The cells contain sugars, salts, acids, and other substances
dissolved in the water of the vacuole, just as the sugar is contained
within the thistle tube in the experiment described above. The

1 Experiments to show osmosis and osmotic pressure are best performed with
paper, or collodion, diffusion shells, or with specially prepared porcelain cups. The
thistle tube and animal membrane are used in this discussion because of the sim-
plicity of the apparatus.

There is no agreement among scientists as to the complete explanation of osmotic
pressure. The explanation given above leaves out of account some of the factors,
principally electrical, involved in the process. This simple explanation is intro-
duced merely to help the student to form a mental picture of the mechanics of
osmosis as it occurs in plant cells.

88 General Botany

cells of the plant are in contact either with water or with other
cells containing water. Cells may, therefore, take up water
either from adjacent cells or from their surroundings.

The conditions for osmosis as it occurs in plants, then, include
a differentially permeable membrane between two masses of
water, one of which is capable of developing a higher osmotic
pressure than the other. The water passes from the mass of
water or adjoining cell in which it is most concentrated to the
cell in which dissolved substances are most concentrated and the
water is least concentrated.

Turgor. When cells contain dissolved substances, or sub-
stances that swell greatly in water, osmosis and imbibition lead
to the taking up of water and the stretching of the cell walls.
A cell that is thus distended is said to be turgid. Cells with an
inadequate water supply may be only partly filled, and the cell
walls are consequently not stretched. Cells in this condition are
flaccid. A condition of turgor is present in actively growing
tissues. Cells are flaccid when a plant shows the familiar signs
of wilting.

The movement of the guard cells of stomata (page 74) is
brought about by turgor pressure, being open when the pressure
is high and closed when the pressure is low. The changes in the
position of leaves possessing a pulvinus are also due to changes
in the turgidity of a part of the cells of the pulvinus.

Materials move by various combinations of physical processes.
The physiological processes involved in the movement of materials
in plants are various combinations of the four physical processes
briefly outlined. Oxygen and carbon dioxide move into a cell
by solution and diffusion. They pass out of a cell by diffusion
when the cell contains a greater concentration than the surround-
ing air or the adjacent cells. Water passes from one cell to
another by diffusion, particularly the forms of diffusion called
imbibition and osmosis. As a result of imbibition and osmosis,
pressure develops in the cell, stretching the walls and resulting

Processes in Movement of Materials 89

in turgidity. Water is also held in cells against evaporation by
the conditions that give rise to osmosis and imbibition ; that is,
water loss from cells is retarded by the same internal conditions
that facilitate water absorption. Salts and sugar pass from cell
to cell by simple diffusion. We can account for the rate of move-
ment of soluble food substances only in part, but diffusion is
certainly one of the physical processes involved.

PROBLEMS

1. Apply the principles of diffusion discussed in this chapter to :

a. The opening and closing of stomata ;

b. The movements of leaves having pulvini ;

c. The wilting and recovery of plant tissues ;

d. The interchange of gases between a land plant and the surrounding air ;

e. The interchange of gases between a water plant and the surrounding water.

2. Why do red beets retain their red color when placed in cold water, but lose it

when placed in boiling water?

CHAPTER TWELVE

THE WATER BALANCE IN PLANTS

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

The balance between transpiration and absorption. The
amount of water in the cells of the plant as a whole is determined
largely by two processes : (i) the rate of absorption — taking of
water from the soil ; and (2) the rate of transpiration. The rela-
tion between these two rates determines the water balance of the
plant. If the transpiration is rapid and absorption is slow, in-
ternal drought results and the plant wilts. If the transpiration
is slow and the water intake is rapid, the cells will be filled to their
utmost capacity.

In summer, when the soil is dry and the air is hot, transpiration
may cause the leaves to lose water so rapidly that they droop, and
we say that the plant is wilted. Water has passed out of the
cells of the leaf faster than the water-conducting tissue has
brought in water to replace it, and the cells are no longer dis-
tended and firm. They are like a football that is only partly
inflated. After a heavy shower the plants quickly recover,
because the water available in the soil has been increased and
more water is taken into the plant. The shower has also covered
the leaves with a film of water and made the air moist around

The Water Balance in Plants 91

them, and this reduces the water loss. Under these conditions,
the cells of the plant quickly become turgid and the leaves recover
their firmness. The leaves of many plants hke lettuce, pumpkin,
and ragweed, that have little or no woody tissue in them, depend
for their firmness almost entirely upon the turgidity of the leaf
cells. The balance between the rate of water supply and the rate
of water loss is the most important water relation of the plant.

The water balance illustrated. The presence of a water bal-
ance may be clearly seen by cutting the stem of a potted plant in
two and then connecting the two pieces by a T-tube filled with
water. To the side arm of the T-tube connect a U-shaped tube
containing mercury and water. Then allow the U-tube to dip
into a beaker of water. If transpiration is more rapid than ab-
sorption, the mercury will be drawn toward the plant. If ab-
sorption is more rapid than water loss, the mercury will move
up the outer end of the tube. Figure 50 shows a sketch of the
experiment. Adding water to the soil, placing it in full sunlight
or in darkness, or moving it from a high to a low temperature will
soon change the position of the mercury in the tube.

The water balance can be further illustrated by using the ap-
paratus shown in Figure 49. This consists of a porous cup at
the top, connected by a glass tube, with two bulbs, to a porous
cup filled with a solution of sugar. The middle of the tube is
further connected with a U-tube containing mercury. The
whole interior is filled with water. When the lower cup is placed
in a beaker of water, absorption begins and evaporation takes
place from the upper cup. By placing the apparatus in different
conditions, the changes in internal water balance will be shown
by movement of the mercury in the tube.

Transplanting and the watef balance. When the skillful
gardener transplants a tree, he cuts off a number of branches to
reduce the number of leaves, in order that the plant may not dry
out before new water-absorbing roots are developed. Before
lettuce, tomato, and cabbages are Hfted for transplanting, the

General Botany

plants should be watered and allowed to become turgid ; water
should be poured into the holes in which they are placed before

the soil is closed in around the
plants. It is customary also to
cover the plants for a day or two
with boards or paper covers so as to
reduce the transpiration. Main-
taining the water balance in trans-
planted plants may prevent the loss
of many of them and may save
weeks of delay in the maturing of
the crop. When herbaceous plants
are propagated by cuttings, pieces
of the stem a few inches in length,
with one or two of the uppermost
leaves, are taken and the lower
half of the cutting is put in wet
sand. In a few days, or weeks, de-
pending on the kind of plants, roots
are developed and a new plant is
estabhshed. The leaves are left
on the cutting so that some photo-
synthesis may go on. Most of the
leaves are removed, so that the
transpiration will not be sufficient
to dry out the stem. The cuttings
are kept moist, so that the absorp-
tion will be sufficient to keep all
the tissues turgid ; that is, to main-
tain the water balance. Cuttings of
woody plants that root with diffi-
culty are successfully started by painting the parts exposed to
the air with melted paraffine in order to keep them from
drying out.

Fig. 49. Apparatus set up to per-
form an experiment to show the prin-
ciples involved in the maintenance
of the water balance in a plant. The
entire apparatus is filled with water,
and A and C are immersed in water.
The water is evaporated from the
porous cup B, and other water to
take its place is absorbed by osmosis
into the porous cup A. If the rate
of evaporation is faster than the rate
of absorption, the mercury falls in
the outer end of the tube C, as is
shown in the illustration.

The Water Balance in Plants

The water balance and plant habitats. The place where a
plant grows naturally is called its habitat. The willow grows
beside a stream and the cactus
grows in the desert, each in its
natural habitat. If we put the
willow in the desert and the cactus
on a wet stream bank, both die.
This means that the conditions that
make up each habitat are favorable
to one kind of plant and not to an-
other. The conditions include not
only the kind of soil and the amount
of soil water, but also the evaporative
power of the air. In selecting plants
that may hve in a particular habitat,
the great importance of the dryness
or the moistness of the air is to be
kept in mind. Plants whose leaves
are soft and transpire water rapidly ^^^ ^^ ^ ^^^^, ^.,^ .,^ ^,^^ ^^,

can succeed only in moist air, while in two and connected again with a

those that have a low transpiration '"^e similar to that shown in Figure

^ 49- If the roots absorb water more

rate maintain a suitable water bal- rapidly than the leaves transpire it,

ance even in a dry atmosphere, the mercury at D is pushed away from

the plant, as m the illustration. If

This is one of the reasons why on a the plant is set in bright sunshine,

southern slope we find a set of plants '^^ transpiration win be increased

and the mercury will then almost im-
that are different from those on the mediately be drawn toward the plant.

northern slopes.

Recent studies have shown that the leaves of plants growing
near the bottom of a ravine transpire water ten to fifty times as
fast as do those of plants growing higher up on an adjoining
southern slope. Doubtless, each year seeds of plants that grow
in the low ground germinate on the upper part of the slope ; but
each year the plants that spring from those seeds are eliminated
through their inability to get the water needed by their higher

94 General Botany

rate of transpiration. There are plants like the dandehon that
become adjusted to both these conditions. Most plants, how-
ever, are not so readily modified, and those with a high transpira-
tion rate die off on a dry hillside, while those with a low transpira-
tion rate survive. This indicates only one of the factors which
must be taken into account in attempting to explain the distri-
bution of plants in nature and in the selection of plants for par-
ticular habitats. Other habitat factors will be considered in
later chapters.

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

Irrigation is a method of artificially maintaining a constant
supply of soil water for crop plants in dry regions. It prevents
the slowing down of the plant processes during the growing sea-
son, thus enabhng the plant to work at its highest efficiency and
produce its greatest yield. For example, at the Utah Experi-
ment Station an acre of corn without irrigation produced 26
bushels; with 15 inches of irrigation water added, 52 bushels;
with 7,S inches, 82 bushels. An acre of wheat produced 4J
bushels without irrigation and 26 bushels when 30 inches of irriga-
tion water was added. One of the highest recorded yields of
corn on a small plot (225 bushels an acre) was obtained in
Colorado with irrigation.

Plants classified according to their water relations. In pre-
ceding chapters we have pointed out the importance of water in

The Water Balance in Plants 95

the physiological processes of plants, and how plants are modified
in size and structure by growing under different conditions of
water supply.

Most plants cannot be grown to maturity in all kinds of wet
and dry conditions. Each kind of plant has a rather definite
water requirement for its best development. Hence in nature
plants live only in those situations in which they are able to main-
tain a suitable water balance. Three great classes of plants are
distinguished on this basis :

(i) The plants that naturally live where the evaporative
power of the air is intense and the available water is hmited are
called xerophytes (Greek : xeros, dry, and phyton, plant). These
are the plants that are adjusted to a nearly continuous dearth of
water ; the cacti, agaves, yuccas, and sagebrush of our Western
plains and deserts are striking representatives. In the eastern
United States there are less marked examples of xerophytes in
the plants that live on dry cliffs and sand beaches, and in the
mosses and Hchens that grow on trees and rocks.

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

(3) Between these extremes are the mesophytes (Greek : meso,
middle, and phyton, plant), by far the largest class of seed plants.
They have a medium rate of transpiration and grow best with a
moderate water supply. In this group are included the plants
that yield most of our garden, -field, and meadow crops; also
most of the forms that are found in the maple, beech, and elm
forests of the Eastern states and in the fir and spruce forests of
the canyons and bottom lands of the Western states.

Xerophytes, hydrophytes, and mesophytes are readily dis-

96 General Botany

tinguished as groups because of their great difference of habitat
and appearance. But it is not always easy to decide whether a
particular plant is a xerophyte, hydrophyte, or mesophyte, be-
cause we find all gradations of form among plants of the three
classes. Nevertheless, these terms are useful in describing the
water relations of most plants.

PROBLEMS

1. Why do plants that are wilted in the late afternoon of a hot summer day recover

their firmness during the night, even though there is no rain ?

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

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

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

5. In what parts of the United States are hydrophytes common? W^hat states

have very few hydrophytes ?

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

7. Are there any economic plants that are hydrophytes?

c-''"

CHAPTER THIRTEEN \^\

THE GROWTH AND FALL OF LEAVES ^^/.u — V V.

We are all familiar with the fact that when a live seed is
planted in the soil it germinates, and that from it there devel-
ops a seedhng which continues to enlarge for a longer or shorter
time, depending on the plant and the conditions for growth. The
period of growth may be a month, as in the radish in midsummer,
or it may be hundreds of years, as in some trees. In the process
of growth vast quantities of food are consumed. During the
early stages of a plant's development most of the food it manu-
factures is used in this way. In order to grow, a plant must
make new protoplasm, develop new cell walls, and thicken and
strengthen old cell walls. Growth requires not only food but
energy as well. Indeed, a part of the energy derived from respira-
tion is used in certain chemical changes involved in growth. We
might expect assimilation, respiration, and food consumption to
be most active in young growing parts, and that this is the case
has many times been verified by experiment. Grow^th takes
place through the enlargement of cells already present in the
plants, through cell division, and through modification of cells
without enlargement.

Conditions for growth. The conditions most favorable for
growth are abundant water and oxygen supphes and warm tem-
peratures, such as normally occur in summer. For the growth
of the plant as a whole, moderately strong hght is favorable
because it increases the supply of food. For the growth of leaves
in particular, medium light is generally most favorable. In
darkness the blades of many plants do not expand, and in very
intense hght they do not expand fully because of the retarding
effect of the hght itself and the excessive w^ater loss.

The growing regions of leaves. By watching the development
of leaves on any common herb, or on the trees in spring, we can
see that growth takes place rapidly; also, that growth ceases

General Botany

when the leaves have developed to a certain rather definite size.

The leaf starts as a small protuberance on the side of the apex

of the stem. The mass of cells
that make up this protuberance
are all similar. As growth pro-
ceeds, cell division, cell enlarge-
ment, and cell differentiation take
place and the five tissues of the
leaf are formed. At first, then,
all parts of the leaf are growing.
After the leaf is mature, further
enlargement will not take place,
no matter how favorable to
growth the external conditions
may be. The question arises, do
all parts of the leaf mature at
the same time, or does growth
continue in some parts longer
than in others? There is one

sunflower (C). The fern leaf grows at the characteristic of grOwing tisSUe

apex; the leaf of the grass, as is common . •^^ ^ i

in paraUel-veined leaves, grows at the that Will help US m anSWCHUg

base ; the sunflower leaf, like other net- this inquiry : yOUng tisSUC is VCry

veined leaves, grows in all its parts. . -, i -i i_ i um

tender and easily broken, while
old tissue is stronger and firmer (Figs. 51 and 72).

Fern leaves grow at the apex. The fern leaf is one that
may be studied in this connection, for the growing portion
is not only tender but coiled up, and its unfolding may be
noted from day to day by marking with India ink the suc-
cessive positions of the coil. In the Boston fern, which is
so commonly cultivated as a window plant, the leaf may con-
tinue to unfold for weeks, if the water supply is adequate and
other conditions are favorable. Evidently in the ferns the last
growing region is at the apex and the older part of the leaf is
the base. If the tip of a fern leaf is injured, further growth
is stopped.

Fig. 51. Growing regions (shaded por-
tions) of leaf of fern {A), grass (B), and

The Growth and Fall of Leaves 99

Growth of leaves of seed plants. The flowering plants have
either parallel-veined leaves or net-veined leaves, and the place
of growth in these two types of leaves is different. In parallel-
veined leaves, like those of the members of the grass family, the
growth continues longest at the base. If you have pulled a grow-
ing leaf from any of the taller grasses like wheat or timothy, you
will recall that it broke near the base, and if you put the broken
end in your mouth that it had a sweet taste.

The breaking near the base, and the sugar there, indicate that
the final growing region of the grass leaf is at the base. A more
exact determination of the growing region may be made by mark-
ing a young grass leaf into equal spaces with India ink. This
will show that as the leaf develops, it is continually pushed up-
ward and outward from the node where it is attached. This
mode of growth is characteristic not only in grasses but in many
other plants having parallel-veined leaves (monocots, page 130).
It is a great advantage in maintaining pastures, but on the other
hand we are obliged to mow our lawns more frequently because
of it.

In plants with net-veined leaves (dicots, page 131) development
is different from that in either ferns or grasses. The growth
of a young leaf of this type — for example, a geranium or
nasturtium leaf — may be studied by marking it off into equal
squares by means of two series of parallel lines at right
angles to each other. After several days it will be seen that
the only change has been an increase in size of the squares.
The lines in each direction are still roughly parallel. This in-
dicates that all parts of the blade, are growing equally. Note
that all parts of the blade seem equally firm, which indicates
that they are all of the same age. All parts of needle leaves of
pine develop at the same time.

These facts regarding the growth of leaves may be summarized
in a somewhat different way. In the ferns the last part of the
leaf to mature is the apex. In parallel-veined leaves a region

lOO

General Botany

Fjg. 52. Longitudinal sec-
tion of base of a petiole,
showing abscission layer.
The dropping of the leaf is
due to the softening of the
cell walls in this layer.

near the base is still in growing condition after the other parts
are mature. In net-veined leaves all parts of the blade mature
at the same time.

Leaf fall, or abscission. In the hfe of a
leaf the final stage is abscission, or the fall
of the leaf. On many temperate plants the
leaves remain only 5 to 8 months. On
evergreen shrubs and trees the leaves are
attached from 3 to 8 years. The first part
of the process of leaf fall is a phase of
growth, and we shall see that the tissue
which makes abscission possible is con-
structed long before the leaf falls (Fig. 52).
The causes of leaf fall. There are two
distinct stages in the process by which
plants drop their leaves : (i) the formation
at the base of the petiole of two or more
plates of thin-walled cells, known as the abscission layer:
this takes place during the development of the leaves and
may require weeks or months for completion ; and (2) the
actual separation of the cells of the abscission layer, which is
brought about by the softening or dissolving of pectic com-
pounds in the middle layer of the walls of the abscission
cells : this stage of the process may take place within a few
hours, or at most within a few days.

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

Conditions promoting leaf fall. After an abscission layer has
developed, there are many climatic and soil conditions that may
accelerate the falhng of the leaves. Among these are low tem-
perature, reduced light intensity, and any disturbance of the

The Growth and Fall of Leaves

lOI

water relations of the plant which results in internal drought.
Disease and insect injuries to the blade frequently bring about
abscission.

Leaves contain food materials when
they fall. The materials used in
building the cell walls in a leaf are
lost to the tree when the leaf falls,
and the fallen leaves still retain con-
siderable amounts of starch, sugar,
protein, and other nutrient material
which leach back to the soil. In the
autumn, however, photosynthesis de-
clines, and the amount of food lost
by a deciduous tree through leaf fall
is small in comparison with the quan-
tity that has accumulated in other
parts of the plant. Even the part
that falls to the ground is not entirely
lost to the plant. It is used by other
plants and animals, which in turn
produce substances that are of great
importance to the original plants.
Forest trees are in this way benefited
by the leaves that fall to the ground.
In agriculture the leaves, stems, and
roots of one crop are frequently
plowed under to improve the soil for
succeeding crops.

Abscission in compound leaves. In
many compound leaves, hke- the
horsechestnut, ash, and hickory, ab-
scission first takes place at the base
of each leaflet. Later the petiole is
cut off from the stem in the same

Fig. 53. Shagbark hickory twig.
A is the bud scales of the terminal
bud of the previous year; B,
several petioles remaining at-
tached after leaf fall; and C, the
terminal bud that will develop
the following spring. Drawn
from a specimen collected in De-
cember.

I02

General Botany

way. Consequently the leaflets fall first and the petioles later.

In the king-nut hickory and occasionally in the shagbark also,

the petiole remains attached to the
tree through the following year

(Fig. 53)-

Self-pruning. A large number of
our common trees, like the cotton-
wood, maple, and elm, develop ab-
scission layers which cut off twigs
and sometimes branches an inch in
thickness. In these trees we have
twig fall as well as leaf fall. The
faUing of flowers, and of fruits hke
apples and nuts, is also due to the
softening or dissolving of abscission
layers formed across the stem at the
point of attachment. Sometimes
abscission is of advantage to the
plant, sometimes it is quite disad-
vantageous.
Evergreen and deciduous trees. In the Northern states many
persons have come to think that the evergreen habit is associated
only with needle leaves, because in the North the evergreens are
mostly of the needle-leafed type. But in the Southern states
there are many broad-leafed trees, hke the magnoha, rhododen-
dron, and holly, that are also evergreen. Moreover, the tama-
racks of the North and the bald cypress of the South furnish
examples of needle-leafed trees that are deciduous. If we include
the shrubs, there are many broad-leafed plants, both in the North
and in the South, that have the evergreen habit. In the tropics
most of the trees are evergreen, and almost all have broad leaves.
It must be noted that even in the case of evergreens individual
leaves remain on the trees for only a hmited number of years.
The leaves of the evergreens of temperate regions are quite dif-

FiG. 54. Abscission of branches of
the Cottonwood. Twigs and small
branches, as well as leaves and fruits,
are cut off by the formation of abscis-
sion layers.

The Growth and Fall of Leaves 103

ferent structurally and physiologically from the leaves of decidu-
ous trees. The evergreens of temperate regions must be able
to withstand freezing and thawing, and also the dry winds of
winter, which cause water loss even when the ground is frozen.
Their usual transpiration rate is very low in comparison with
that of deciduous trees, and in the autumn they undergo changes
which reduce the water loss nearly to that of deciduous trees that
have dropped their leaves.

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

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

I04 General Botany

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

PROBLEMS

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

2. What trees of your vicinity develop leaves earliest in the spring? What trees

develop their leaves last ?

3. What trees develop flowers before the leaves?

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

last in autumn ?

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

6. During how many months does each of these trees carry on food manufacture ?

7. During how many months do the evergreen trees of your locality manufacture

food?

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

on photosynthesis ?
9 Do all specimens of the same species of trees put out new leaves at the same

time? How much variation is there?
10. Do those which develop leaves first drop them earliest in the autumn? This
information is important in selecting trees for street planting and general
landscape effects.

CHAPTER FOURTEEN

THE STEMS OF PLANTS

The stem forms the axis of the plant and bears the leaves,
flowers, and fruits. Plants showing all degrees of stem branch-
ing are found, from the unbranched palm and corn to the finely
divided asparagus and elm. In most plants the stems are upright,
aerial structures ; but in some plants they he on the surface of
the soil, in other plants the main stem is underground and only
the branches rise above the surface, and in still other plants the
entire stem is underground. The upright stem is the common
type and has many advantages over a horizontal stem.

Upright stems. The photographer uses light to effect chemical
changes in photographic papers and plates. Light also brings
about chemical changes in the green tissues of the plant. The
photographer who uses sunlight for his work usually locates his
studio at the top of a tall building, because there he avoids the
shadows of near-by buildings and secures a more constant ex-
posure to hght. The same advantages come to the plant that
has its leaves raised well above surrounding plants; the leaves
are in less danger of being shaded, and each day they are exposed
to the sunshine during a longer period (Fig, 55).

The tall plant has an additional advantage in being able to
expose to the light a greater leaf area over a given space of ground,
because it can display several or many layers of leaves one above
the other. The rosette of leaves formed by the burdock illus-
trates the possibilities of leaf display near the soil. A large sun-
flower plant covers no greater soil area than a burdock, but it is
able to expose to the sunhght several times as great a leaf area
because the sunflower leaves are placed at several different levels.
Trees have the greatest stem development and the greatest leaf
display. Rosette plants, hke the dandelions and plantains, repre-
sent the opposite extreme of slight stem development, small leaf
area, and a relatively poor leaf display. One advantage in a tall,

105

io6

General Botany

upright stem is that it holds the leaves up to the light and thereby
makes possible a greater leaf display. Under certain conditions
upright stems may facihtate pollination and seed dispersal

(Fig. 55)-

The advantages of the upright stem are all dependent on its
capacity to support other organs. The stem must be strong
enough to support leaves, flowers, and fruits. The city sky-
scraper needs first of all a strong framework about which the
building is constructed. The stems of tall, erect plants must be
correspondingly strengthened by a mechanical structure. The
base of a tree is much smaller in proportion to its height than that
of the tallest and narrowest building, and it is possible for trees
to reach great heights only because their stems are composed in
large part of supporting tissues of great strength and pliabiHty.
The largest upright stems. Stems attain their best develop-
ment under medium conditions of
moisture, light, and temperature.
Such conditions are found in the
eastern United States, in the can-
yons of the western mountains,
and along the Northern Pacific
slope, and there the plants are
characterized by large leaf area.
In the East the vegetation cul-
minates in the forests of the rich,
well-watered soils of the river val-
leys. Here may be found oaks,
walnuts, elms, maples, sycamores,
and magnolias which reach
heights up to ico or i8o feet and
have trunks attaining diameters

Fig. 55. Sunflower and burdock, showing of from 4 tO 14 feet.

advantage of the tall, upright stem. The j^ ^^^^^ CanyOnS of the

sunflower covers no greater soil area but -^

it displays more leaves to the light. Sierras of California the giant

The Stems of Plants

107

sequoia reaches heights of
from 250 to 320 feet above
the ground, with extreme
trunk diameters of 35 feet.
A cypress tree near Oaxaca,
Mexico, has a trunk 50 feet
in diameter. These trees
are the largest and prob-
ably the oldest of all living
things. The redwood, a
near relative of the sequoia,
grows in the fog-abound-
ing ravines of the Coast
Ranges north of San Fran-
cisco. Its trunk diameter
may be not over 28 feet,
but it surpasses the giant
sequoia in height. You
can better appreciate the
size of these trees if you
will pace off from an ordi-
nary tree a distance equal to
the diameter of a sequoia
trunk and will calculate

how many times the height j,^^^, Group oi Bi, Trees (Sequoia smntea) on
of the tallest tree in your western slope of the Sierra Mountains, California.

locality a giant sequoia is, ^/^ '^'\''! '^f' '''' "^^^ \^ ^f f ^ ^^ comparing

•^ \ ^ them with the man at the left, rormerly se-

and then try to imagine quoias were widely distributed over the northern

how a sequoia would look hemisphere, but now they are practically restricted

. to a fe^w localities in California.

growing beside it.

Climbing stems. Among mesophytes are many vines with
exceedingly long, slender stems. The Virginia creeper, wild
cucumber, poison ivy, and grape have stems 50 to 300 feet long,
and usually only a fraction of an inch, or at most a few inches,

io8

General Botany

in diameter. These long, slender stems grow rapidly and enable
the plants to spread their leaves quickly over the tops of large
trees. With a proper support these stems have all the advantages
of upright stems, without having to use so mucn material in
building woody supporting tissue.

Some climbers — for example,
the morning-glory — gain sup-
port by twining. Others, like the
grape, have tendrils, specialized
organs developed in place of
branches or leaves. A tendril
responds to contact with a sup-
port by coiHng tightly about it
(Fig. 57). When attached, me-
chanical tissues develop within,
greatly increasing their strength.
Then, due to unequal growth on
the two sides, the tendril twists
spirally and draws the plant
close to the support.

In some vines, hke the Boston
ivy (Fig. 59) the tendrils have at
their tips sensitive disks which, when rubbed against a support,
secrete a sticky substance and become cemented to it. This

Fig. 57. Tendrils of wild cucumber. Note
the coiling of the tendril by which the plant
is drawn nearer the support and the rever-
sal of the spiral in different parts of the
tendril. Is there always such a reversal?

The Stems of Plants

109

Fig. 58. Climbing stems on a tree trunk in a tropical forest. This type of stem does not
require the use of so much material in building woody supporting tissue, and if it comes
in contact with a support, it has all the advantages of an upright stem. Its disadvantage
is that unless a support is found, the leaves are poorly displayed.

General Botany

type of tendril is especially effective in taking hold of the bark
of trees, rock chffs, and walls. Still other climbers, like the
trumpet creeper and poison ivy, have aerial roots that become
fastened to a support by growing into cracks and crevices ; or, by
the cementing action of the outer pectic layer of the epidermal
cell walls, they may become attached to quite smooth surfaces. In
the moist tropics, chmbing stems may attain a length of more than
looo feet. Thus the water transpired by terminal leaves has been
carried about a fifth of a mile within the plant. A cross-section of
such a stem will show not only that most of the stem is occupied
by water-conducting tissue, but that the individual tracheae are
very large and long when compared with those of upright stems.
Horizontal stems. Horizontal stems have httle woody tissue,

and they display leaves to the
light advantageously only when
they grow in the open. There
are advantages in stems of this
type, however, because by grow-
ing horizontally on the soil or
beneath the surface of the soil
they spread the plant; because
they are in contact with the soil
and may take root at frequent
intervals; and because they are
better protected than upright
stems during the winter and
other unfavorable seasons.

Underground stems. Many
plants, both herbaceous and
woody, possess underground
stems. They are particularly
useful as places of food accu-
FiG. 59. Young and old stems of Boston mulation and in vcgctativcly
ivy. T:_e young stems are held by spreading and multiplying the

means oi tendrils, the older stems by ^ x ^ <->

means of adventitious aerial roots. plant. Their position renders

The Stems of Plants iii

Fig. 6o. Sand-reed grass (Ammophila) planted on dunes near Casmalia, California, to
prevent further movement of the sand. The bare areas are dunes formed since the planting.
The underground stems of the grass bind the sand and aid in preventing its movement by
the wind.

them nearly free from transpiration, from injury by fires (an im-
portant matter on the prairies) , and from the destructive effects of
winds. Yet when plants having underground stems only come into
competition with those having erect stems, they are quite likely to
be overshaded ; at any rate they cannot compare with erect stems
in leaf display. In competition with annuals, however, they are
highly successful by occupying all of the space and thus prevent-
ing the young seedling from getting a start. One need only look
at the plants in old meadows, pastures, prairies, and swamps to
see the result of such competition.

The commonest type of underground stem is the rootstock or
rhizome. Rootstocks are horizontally growing stems, from which
the aerial stems arise. They may be slender, or thick and fleshy.
Usually they have small scale leaves and buds at the nodes, as
well as roots that arise from the nodes or from the entire under
surface. The presence of nodes is the external feature of under-
ground stems that distinguishes them from roots (Fig. 62).

112

General Botany

In many of the grasses and grass-like plants, rootstocks develop
rapidly in all directions, sending up erect branches at short in-
tervals. The rootstocks and their accompanying roots soon
become mixed with those of adjoining plants, finally forming a
closely interwoven mat which is the '' turf " of lawns and mead-
ows. Turf-forming grasses are often of great value for holding
in place the soil of embankments, dikes, and levees. In these
plants the rootstocks are mainly useful in spreading or extending
the plant. Bermuda grass and Johnson grass are troublesome
weeds in the Southern states because of their extensive rootstock
systems. On the other hand, this same feature makes some
plants of great use to man. The sand-reed grass (Ammophila)
has been planted extensively in Europe and in America to hold

drifting sand in place and to pre-
vent the sand from invading towns
and cultivated fields. This grass
may also be used as a soil binder in
starting forests in sandy places.

In plants like the May apple,
Solomon's seal, and yellow water
lily, the rootstock
not only causes
the plant to
spread, but it
also accumulates
a part of the food
manufactured
each season and
thus serves as a
storage organ.
j.E.wea.er The uudergrouud

Fig. 6i. Vertical section of a gravel slide, showing dogbane Stem 01 aspara-

(Apocynum) with underground stems (rootstocks) connecting „^^ jg ^ StOrage
the several shoots, and the much-branched root system. Section

divided into one-foot squares. Organ, and aS it

The Stems of Plants

113

increases in thickness each year, the upright branches used as
food become larger and more succulent. It is this store of food

J. E. Weaver

Fig. 62. A common grass {Redfiddia) in the sand hills of Nebraska, showing parts of the
extensive system of rootstocks and roots. Sections divided into one-foot squares. The
underground parts of the plant were carefully dug out, and their horizontal extent and
depth in the soil were found to be as shown in the illustration.

and the readiness with which the rootstock sends up shoots, that
make the bindweed and perennial morning-glories so difficult to
eradicate from cultivated fields.

A short, upright, fleshy rootstock, like that of the jack-in-the-
pulpit, caladium (elephant's ear), or gladiolus, is called a corm.
Corms contain large amounts of food, and by the development
of their lateral buds may serve to reproduce the plant as well as
to carry it over the winter. The dasheen, a tropical plant which
resembles the caladium, and which has recently been introduced
into the United States, has an edible corm that is an important
source of food (Fig. 63).

114

General Botany

A hulh is a fleshy underground bud, made up of a short stem
covered with several layers of thick scales in which food is stored.
Tulips, hyacinths, and onions are commonly propagated by means
of bulbs. Some kinds of onions also produce small bulbs
(" sets ") in place of flowers, and some hhes develop them in
the axils of their leaves.

By planting bulbs of the tulip in autumn, we can have flowers
early in the following spring, whereas if we planted the seeds, we
should have to wait several years for flowers. It requires two or
three seasons of photosynthesis to accumulate sufficient food for
flower production. Furthermore, tulips do not grow well except
in a very moist climate, and the development of large, vigorous
bulbs is impossible in most parts of the United States. For this

reason nearly all our tulip
bulbs are brought from Hol-
land. The importation of
bulbs from countries where
they grow particularly well
is an important industry and
enables us to have many
flowers which cannot be as
successfully propagated in
our climate.

Tubers are the enormously
thickened portions of short
underground stems. The
potato and the Jerusalem
artichoke are the most fa-
miliar plants forming tubers.
The scale leaves of the or-
dinary rootstock are in tu-
bers reduced to ridges, and
the buds themselves to mere
points. The scales and buds

Fig. 63. Dasheen and edible corms produced by
it. The dasheen is related to the common "ele-
phant's ear" or Caladium, and is extensively
grown in the tropics for food. In the states
along the Gulf Coast it is being introduced as a
food plant.

The Stems of Plants

115

together form the eyes of tubers. Tubers, like other fleshy un-
derground stems, accumulate surplus food and multiply the
plant. The potato tuber has become
one of the most important sources of
food for man.

Summary. Stems vary greatly in
structure, size, and position. Each
type of stem has certain advantages
and gives the plant characteristic
habits of growth. Each of these stem
types also fits into certain environ-
ments better than into others ; con-
sequently there are great differences
to be observed in the kinds of plant
stems in different habitats and dif-

ferent regions.

Fig. 64. Amaryllis bulb. A bulb
is a fleshy underground bud made
up of a short stem covered with
several layers of thick scales in
which food is stored.

PROBLEMS

What advantage in resisting wind have tall, columnar tree trunks over equally

tall smoke stacks or monuments ? What disadvantage ?
What are the best trees for street planting in your locality? What trees now

planted there are objectionable ? Why ?
Compare a tree growing in an open field with one of the same species growing in

the woods. Account for the differences in arrangement of branches and leaves.
Which will furnish the better lumber, a tree grown in the open, or one grown in

the forest? Why?
W'hat commercial products are derived from each of the several types of stems

described in this chapter?

CHAPTER FIFTEEN

THE EXTERNAL FEATURES OF STEMS

On a woody stem, nodes, leaf scars, buds, and lenticels may be
seen. The nodes are the places where the leaves arise, and they
are usually the most prominent external feature of stems. The
arrangement of leaves at the nodes has already been discussed
(page 62). In addition to the leaf, the node gives rise to one or
more buds, just above the place of leaf attachment, in the so-
called axil (Latin : axilla, armpit) of the leaf. The part of a stem
between two nodes is called an internode. The leaf scars are
markings on the stem where leaves have fallen. At intervals
along the stem ring-like markings {hud scars) may be found.
These show where a terminal bud was formed at some previous
time. The lenticels are small, dot-like elevations scattered over
the surfaces of the internodes.

Buds. Stems and branches produce leaves only once. We
are accustomed to speak of deciduous trees clothing themselves
with a new set of leaves each spring, as though the branches of the
previous year put forth a new set of leaves to replace those lost
the preceding autumn. As a matter of fact, when we look at a
deciduous tree in winter, we see branches and twigs, all of which
have borne leaves and none of which will ever bear leaves again.
The possibility of producing new foKage lies in the development
of new branches and twigs. This is the function of the buds ;
from them the new growth of each year arises (Fig. 65).

The buds of many tropical plants are like those we see at the
tops of the stems of garden vegetables. A bud of this kind con-
sists of the stem's growing point and the undeveloped leaves,
with no special covering of any kind. These naked buds occur
also on the underground stems of some of our herbaceous plants.
A simple sort of bud covering, which is common in the tropics,
is made by the folding together of the stipules. This type of bud
covering may be seen in the tulip tree and the magnolias of tem-

116

The External Features of Stems

117

perate climates. The buds of most temperate perennials are
covered with specialized scale leaves. Frequently the outer or the
exposed parts of scales die with the approach of winter. Not
infrequently the scales are further covered with matted hairs and
secretions of wax and resin. These all tend to make the bud
coverings impervious to water.
By these coverings the tender
growing parts are protected
from excessive loss of water
during the winter and during
the still more critical stage in
early spring when the buds are
opening. Bud scales do not
protect the growing point of
the stem from low tempera-
tures. During zero weather all
the tissues of buds and twigs
are frozen soHd.

We are likely to think of
buds as being formed at about
the time when the leaves fall
from the trees. A good ob-
server, however, will have
noted that the buds begin to
develop when the leaves un-
fold in spring, and that they
grow all summer long. Be-
cause of the prominence of
the leaves, the buds are ob-
scured somewhat during the
summer months and become ^^^ ^^ ^^.^^ ^^ ^^.^^^ ^^^^ ^^^^i^^^^ ^s),

conspicuous only after the and tree-of-heaven {Ailanthus) (C). The

leaves have fallen from the 'T'\'^^^'^^'\''T''''''^,\VlZi''Tl

buds; c IS a leaf scar, a a bundle scar, e, a
trees. lenticel, / a terminal bud scar, and g a tendril.

ii8

General Botany

The opening of
buds. When the
warm weather of
springtime comes, the
innermost bud scales
begin to grow and ex-
pand. Sometimes the
outer scales are pushed
off ; sometimes they
elongate and grow Hke
the inner ones. But
the scales quickly
reach their full
growth, and soon they
are cut off by the for-
mation of an abscis-
sion layer at the base
of each. In the buds
of a few plants all the
scales are dead and
are pushed off by the
growth of the stem
and leaves inside.
The expansion of bud
scales and leaves
takes place partly
through cell multipli-
cation and partly through the enlargement of cells already
formed the preceding year. Within the bud the minute leaf
cells absorb water and develop large vacuoles. The expansion
of these cells results in the enlargement and spreading of the
leaves. Material for the study of the different habits of bud
expansion may be secured in winter by bringing branches of
different kinds of trees into a warm room and placing them in
water until the leaves expand.

Fig. 66. Date palms in fruit, on an oasis in the Algerian
desert. The strong terminal bud, and the failure of the
lateral buds to develop, leads to an unbranched stem.
An unbranched stem is more common among monocots
than among dicots. {From photo U. S. Dept. of Agricid-
ture.)

The External Features of Stems 119

Contents of buds. Every bud contains the growing point of a
stem. In addition, most buds contain the beginnings of foliage
leaves ; that is, the leaves have already begun to develop on the
sides of the young stem within the bud. These are called branch
buds, because when they grow they produce a new leaf-bearing
branch. Some buds, as for example many of those on the maples
and elms, contain the beginnings of flowers (flower buds or fruit
buds). Other buds, like some of those of the catalpa and the
horse-chestnut, contain both leaves and a flower cluster (mixed
buds). Bulbs are really a special underground form of bud, and
they are similar in structure to other buds.

Bud development and plant form. Buds which occur at the
ends of stems are called terminal buds; those which occur at the
nodes are called lateral buds. This classification is useful because
only a part of the buds on a stem ever develop and because the
form of a plant depends on which set of buds develops more freely
and grows more rapidly. In most plants the terminal bud simply
extends a stem or branch ; the lateral buds produce new branches.
Plants with very strong terminal buds tend to become columnar
in form, like the large, unbranched sunflowers of the garden or
like the spruce and palm (Fig. 87) among trees. Plants with
strong lateral buds usually branch continually and become bushy
in form, like the lilac and hydrangea. There are all gradations
between these extremes, in the development of the terminal and
lateral buds and in the resulting plant forms.

In many roses the shoots from the base of the stem develop
only through their terminal buds the first year. The shoot is
thus extended to great length by the season's growth. The fol-
lowing year the lateral buds develop, and the long shoot becomes
highly branched. As these lateral branches bear the flowers and
produce them abundantly only once, we can promote flowering
in these roses by trimming away each year all but the long,
unbranched shoots. In many other shrubs, as spirea, barberry,
and privet, a few strong lateral buds at the surface of the soil

I20

General Botany

W. S. Cooper

Fig. 67. Fir and spruce forest on slope opposite Mt. Aberdeen, Alberta.
The excurrent stems and spire-like form of the trees result from the con-
tinued growth of the terminal buds and the slow development of lateral
branches.

The External Features of Stems

121

,;|l'W

i- jt£

o\^

ii^'

V" ^^^^^^M^

^^ XAri

%^M-

•V^^

t^uW*

V^^™

y^y

m^-2

^ i^- M^ -

--"^

Fig. 68. A hackberry (Celtis occidciiUdis}. -huviiiL; k liquescent
stem. This tree is growing on the open prairie in Illinois. Through
the development of the lateral buds, the central stem has been
lost in the branches.

develop each year. This accounts for the basal branching of
these plants.

Excurrent and deliquescent stems. When trees have strong
terminal buds, the main stem extends to the top and is called
excurrent (Latin: excurrens, running out). The spruce has a
strong terminal bud, and just beneath it a whorl of several smaller
lateral buds (Fig. 71). The terininal bud grows upward, and
the lateral buds grow outward, forming a whorl of branches at
the base of the season's growth. This is repeated each year, the
terminal shoot lengthening the stem and the lateral buds adding
a new whorl of branches. Consequently each year's growth is

122

General Botany

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

The terminal buds of the elm tree seldom survive the winter.
The lateral buds develop, and the main stem divides and sub-

FiG. 6g. An American elm {Ulmus americana) . The terminal
buds of the elm seldom survive the winter, and the development of
the lateral buds causes the main stem to divide and subdivide until
it dissolves into the branchlets that form the crown. This tree
is growing in the Berkshire Hills, Massachusetts. It probably de-
veloped in a forest which was afterward cut down.

The External Features of Stems 123

W. S. Cooper
Fig. 70. A deliquescent monocot (the tree yucca, Yucca arborescens), photographed at
Cajon Pass, California. The dehquescent type of stem is unusual among the monocots.

divides until it is lost in the crown of the tree. The gradual
dissolving of the trunk into a spray of terminal branchlets sug-
gested the name deliquescent (Latin : deliquescens, dissolving)
for this type of stem (Fig. 69) .

We see, therefore, that the excurrent type of stem depends on
the continual development of terminal buds, while the deliques-
cent type depends on the growth of lateral buds. The form of
plants in cultivation may be modified by trimming them, and so
forcing the growth of certain buds. Lawn trees and shrubs are
grown either for shade or for ornamental effects. We secure
shade by trimming off the terminal buds and so causing many
of the lateral buds to develop into branches and thus form a
denser crown. Ornamental effects are secured by trimming
plants so that they will be in artistic harmony with their
surroundings.

Fruit trees and grapes have been found to produce more fruit,
and fruit of a better quality, when the number of branches is
limited. A smaller number of branches on a tree secures an
open crown and permits the sunlight to penetrate to every leaf,

124 General Botany

and the removal of some of the branches forces the development
of flower buds which might remain dormant if the terminal and
branch buds were allowed to grow uninterruptedly. In grape
culture, only four or five branches are allowed to remain on a
vine each year, and these branches are shortened. This insures
full development for a few of the lateral flowering branches and
the production of the best quality of fruit.

Black raspberry bushes produce fruit only at the ends of
branches. Hence the object in pruning is to develop the maxi-
mum number of short branches. Each year new shoots develop
from the base of the stem. If the tips are cut off when they are
1 8 inches high, five or six lateral buds immediately start growth
and by the end of the season have formed branches. If these
lateral branches are also trimmed back to a length of eight inches
the following spring, each will develop several side branches
which will be terminated later by flower clusters and fruits.
After fruiting, the old much-branched ^' canes " should be re-
moved and new shoots should be pruned for the next year's
fruit production.

Leaf scars and bud scars. The leaf scars on some plants are
round ; on others they are narrow lines ; on most plants they
are crescent-shaped. Usually they are smooth, except for small,
dot-like markings. These markings are bundle scars; they show
where the bundles of conductive and mechanical tissue extended
outward from the stem into the petiole and thus into the veins of
the blade. The shape of the leaf scar and the arrangement of
the bundle scars are so characteristic for many kinds of trees
that they may serve to identify the tree in winter.

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

The External Features of Stems

125

Fig. 71. Plantation of Norway spruce, showing whorls of branches at base of each year's

growth.

Determining annual growth of shoots from terminal-bud scars.

Since the terminal bud marks the end of each year's growth,
the terminal-bud scars mark off a perennial stem into segments,
each of which represents the growth of a single year (Fig. 65).
Often an interesting life history is suggested by the varying
length of the intervals between the bud scars on a particular
stem. By a study of these intervals we can determine the
seasons that were favorable and those that were unfavorable
because of drought, excessive rain, attacks of insects, or some
other cause.

In the pines and spruces the year's growth is marked off not
only by the bud scars, but also by whorls of branches. Dif-
ferences in the color of the bark and in its texture will also help
to distinguish successive annual stem segment in most trees.

126 General Botany

On many varieties of apple and pear trees, flower buds are
usually borne on the ends of " spurs " or short, slow-growing
twigs. When the spur ends in a flower, the further growth of the
shoot depends upon the development of a lateral bud. Com-
monly the spurs produce flowers in alternate years. When once
a branch produces flowers, it continues to do so and its growth
rate is usually much slower than that of the vegetative branches.
Because of the successive development of lateral buds, spur
branches are crooked and the intervals between terminal bud
scars are short.

Lenticels. AH living cells require energy. This is mostly
obtained from respiration. Therefore, in addition to a constant
food supply, the cells of the stem must have access to oxygen.
As in the leaves the oxygen is supplied through the intercellular
spaces, so in stems there must be sufficient intercellular spaces
to permit oxygen to diffuse inward and carbon dioxide to diffuse
outward. There must also be openings through the epidermis
or bark to connect these intercellular spaces with the outside
atmosphere.

The young green stems of all plants have stomata. Perennial
stems, however, soon develop a corky layer beneath the epidermis,
which cuts the cells in the interior of the stem off from the
stomata. While this layer is developing, masses of round, loose
cells form beneath some of the stomata, pushing out and tearing
the epidermis above them. These open places are the lenticels.
They permit gas exchanges, and in older stems take the place of
the stomata. The lenticels of most twigs of trees and shrubs are
closed in the late autumn by the growth of a thin layer of cork
beneath them. The following spring loose cefls are again formed
at the same point, the cork is burst open, and the lenticels again
permit gas exchanges. Apparently water influences the develop-
ment of open lenticels. If a willow twig is placed in water, the
submerged lenticels enlarge greatly. Perhaps a similar condition
effects the opening of lenticels in the spring.

The External Features of Stems 127

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

PROBLEMS

1. Find out how your local gardeners trim their grapevines, berry bushes, and fruit

trees. Secure definite information for five of these plants, and determine
the reasons underlying the practices.

2. Remove the scales from various buds in December and determine how long the

unprotected buds live. What weather conditions are most unfavorable
to uncovered buds?

CHAPTER SIXTEEN

THE STRUCTURE OF STEMS

If we study the development of a stem from a bud, we find
that the growing point is made up of very minute cells, all of
which are practically alike. These cehs divide, making other
cells like themselves, and the lower ones begin to enlarge. In
this way the growing point is pushed forward and the diameter
of the stem increased. Then certain groups of cells begin to take
on special forms. The cells that are to form the bundles elongate
and the cross-walls between some of the cells disappear, forming
the water -conducting tissue. Some of the cells develop thick
woody walls {wood tissue). Others elongate but remain thin-
walled, and these form Xh^ food-conducting tissue. Just outside
the food-conducting tissue very slender elongated cells with
thick walls develop (the hast). The other tissues of the stem are
composed of cells which have enlarged and have become rounded
or variously angled through mutual pressure and which have their
walls more or less thickened. These cells form the pith or soft
inner part of the stem, and the cortex or outer portion. In this
way the various tissues of stems composed of a variety of cells
arise from the small uniform cells of the growing point.

This tissue composing the growing points of leaves, stems, and
other organs is called meristematic tissue, or simply meristem.
The tissues that are formed from the meristem are composed of a
variety of cells, which have been classified into several types to
facihtate the description of plant organs.

Parenchyma and prosenchyma. The cells that make up the
epidermis and mesophyll of leaves, the softer parts of herbaceous
stems, and the fleshy fruits are either spherical or spheres that
have been compressed. Their length and breadth are not very
different and they are arranged in rows and layers, with their walls
touching each other. Cells of this type make up parenchyma
tissue. In the veins of leaves and the bundles of stems there are

128

The Structure of Stems

129

Vf >-;.

Fig. 72. Photograph of a stem tip of coleus, an opposite-leafed herb. The
growing point is in the center above, surrounded by two young leaves. Just
below are two shoulders representing the second node, the leaves of which are at
right angles to the first pair and do not show. The leaves of the third node
(partly shown in photograph) are quite large and have growing points in their
axils. Note change in form and size of cells as growth proceeds.

cells which are greatly elongated — the length is many times the
breadth. These cells touch the adjoining cells along their sides,
but their ends are pointed or wedge-shaped and fit in between the
cells above and below them. A tissue composed of this type of
cells is called prosenchyma.

I30

General Botany

Sometimes the cell walls of both parenchyma and prosenchyma
become thickened by the deposition of additional layers of cellu-
lose, and the cellulose may be hardened by the addition of Jignin.
Cells with thick, hard walls are said to be sclerotic (Greek : scleras,
hard) . Sclerotic parenchyma and prosenchyma are often grouped
together under the term sclerenchyma. Thus the stone cells found
in pear fruits, and in the shells of nuts, may be called sclerotic
parenchyma. Bast fibers and wood cells are sclerotic prosen-
chyma. If we wish merely to call attention to them as strong,
hard tissues, we may call them sclerenchyma. When scleren-
chyma cells are mature, they are usually devoid of protoplasm
and are filled with either air or water.

Collenchyma. Another kind of tissue widely distributed in
plants and closely related to parenchyma is known as collenchyma.
This tissue differs from parenchyma in having the corners, or
edges, thickened where three or more cells come together. These
thickened edges give rigidity to the tissue, and for this reason
collenchyma is often placed among the mechanical tissues.
Stem structures and plant groups. There are three groups of

seed plants that we wish to
distinguish at this time, be-
cause the stems of the plants
that belong to these groups
differ fundamentally. These
groups are: (i) the conifers,
or cone-bearing trees, like
pines, spruces, firs, and cedars,
that have scale or needle
leaves and are for the most
part evergreen ; (2) themono-
cotyledonous plants (mono-

FiG. 73. Photograph of a cross-section of cots) , OX plants with parallcl-
the outer part of calamus rootstock. The .^^^ ^^ y^^^ ^^^ graSSeS,

tissue formmg the background is collenchyma, ' _ °

in which starch accumulates. lifies, cannas, orchids, and

The Structure of Stems

131

Fig. 74. Photograph of a cross-section of a young sunflower stem,
showing arrangement of the bundles. Locate the several tissues.

palms; and (3) dicotyledonous plants (dicots), or plants with
net-veined leaves, like oaks, maples, sunflowers, asters, and
clovers.

The stems of the plants belonging to these three groups differ
in (i) the kinds of tissues and cells making up the bundles, and
(2) the arrangement of the bundles in the stem. We shall first
study the bundles and the arrangement in a dicot stem, and then
we shall learn how the stems of monocots and conifers differ from
those of dicots.

The structure of a dicot stem. When a young dicot stem is cut
across, the bundles are seen to be arranged in a ring. The core
of tissue lying inside the bundle cylinder is the pith ; outside the
bundles is the cortex; and covering the cortex is an epidermis
very similar to that of leaves. In older and harder stems the
epidermis disappears and the outer cortical cells may be replaced

132

General Botany

Ba5t

Food conductiiTj^

Cambium

Water conducting

Wood fibers

Pith ray
Pith

•Vascular bundles

Cortex

Epidermis and

/ cuticle

Lenticel

Fig. 75. Stem of moonseed vine, showing tissues and their arrangement.
This stem is typical of a herbaceous dicot.

The Structure of Stems 133

by soft layers of cork cells or by layers of sclerenchyma. The
pith and the inner part of the cortex are made up of parenchyma.
In annuals and young perennials the cortical parenchyma con-
tains chlorophyll and resembles the mesophyll of the leaf in
appearance and function. It is this tissue that forms the inner
'' green bark " of twigs and gives the green color to the stems and
branches of herbaceous plants.

There are, then, four distinct layers in dicot stems : (i) on the
outside is the epidermis ; (2) from the epidermis to the bundles
is the cortex; (3) inside the cortex is the bundle-cylinder;
(4) the pith forms the axis of the stem, filling the space inside
the cylinder of bundles (Fig. 75).

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

General structure of the dicot bundle. The bundles in a plant
stem are continued above in the veins of the leaves, and below
in the bundles of the roots. In the dicot stem these bundles
contain four tissues; (i) the water-conducting tissue, (2) the
food-conducting tissue, (3) the cambium, and (4) the mechanical
tissue. The cambium is a layer of thin-walled cells that hes
lengthwise in the bundles and separates the water-conducting
tissue from the food-conducting tissue.

The water-conducting tissue contains long, tube-hke vessels
made up of cylindrical cells joined end to end, often for consider-
able distances without end-walls between them. These tubes
(tracheae) usually have heavy walls marked by spiral and lattice-
form thickenings. When mature they are empty of protoplasm.
In other words, they are the coverings of dead cells joined to-
gether, forming tubes usually several inches, more rarely several
feet, in length. Mixed with them are smaller and shorter tubes

134 General Botany

and cylindrical living cells. All together these tissues form the
passageway for the movement of water and mineral salts to all
parts of the plant. The general direction of the water move-
ment in this tissue is upward, because the lifting of the water is
brought about principally by transpiration from the leaves.

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

The food-conducting tissue differs from the water-conducting
tissue in being composed of smaller, thin-walled cells, all of which
retain their living protoplasm. The largest of these cells are set
end to end, and the end-walls have holes in them like the top of a
salt shaker. These rows of cells, therefore, form tubes with
sieve-like cross-walls in them, and on this account they are called
sieve tubes. Through the openings in the sieve plate the proto-
plasm is continuous from cell to cell, and through these tubes
the foods pass from one part of the plant to another. Surround-
ing the sieve tubes are smaller living cells called companion cells.
Because the cells of the stem and root are supplied with food
manufactured in the leaves, it is often said that the movement of
foods is downward in a plant. In reality, the direction of the
food current is not so fixed as is that of the water current. Food
moves toward any part of the plant where it is being used or
being accumulated.

The roots and stems require a continuous supply of food for
nourishing old cells and for building new ones. Since the foods

The Structure of Stems

135

Node

Leaf sheath

Vascular
bundle

Fig. 76. A solid grass stem (Panicum), showing arrangement of
the tissues in a typical monocot stem.

136 General Botany

are manufactured primarily in the leaves, there must be food-
conducting tissues that are adequate to carry them to all parts
of the stem and roots. The food-conducting tissues also transfer
food from the leaves to the seeds and growing parts, and when
food has accumulated in the stem or roots it may pass up through
the conductive tissues of the stem to other parts of the plant.

The mechanical tissue is made up of cyHndrical or spindle-
shaped cells with very heavy walls. Indeed, the walls at ma-
turity may be so thick as to render the cells almost solid. Ordi-
nary cellulose is not very hard, but the walls of the mechanical
tissue are hardened and thickened by a deposit of lignin, a sub-
stance composed of cellulose and certain aromatic compounds.
The difference between hard and soft woods is for the most part
due to the thickening of the walls of the mechanical cells ; sec-
ondarily it is due to chemical changes in the walls themselves
(lignification) .

Mechanical tissue is found on both the water-conducting and
food-conducting sides of the bundles. On the food-conducting
side it lies outside the food-conducting tissue, and is made up of
long, exceedingly slender, nearly solid, spindle-shaped cells.
These cells are called hast fibers, and the tissue that is made up
of them is called the hast. Bast may be seen in the stringy fibers
on a grapevine or in the bark of trees. It is the bast fibers from
flax, hemp, jute, and other dicotyledonous plants that are used
in the manufacture of thread and cordage.

The cells of the mechanical tissue on the water-conducting
side of the bundle are somewhat shorter and thicker than the
bast fibers. They are known as wood fibers, and make up what
is properly called the wood. In most dicots the wood fibers
(Fig. 91) are mixed with the water-conducting vessels and Hving
thin-walled cells called wood parenchyma, and the whole inner part
of the bundles is known as wood. In woody dicots this mechanical
tissue is present in abundance and forms the bulk of the stem.
The lumber that is obtained from dicotyledonous trees is derived

The Structure of Stems

137

Fig. 77. Photograph of a cross-section of
corn stem, showing arrangement of fibro-
vascular bundles.

from the inner parts of the
bundles and is made up of wood
fibers and water-conducting tis-
sues. Examine a smooth piece
of oak and you can readily see
the small wood fibers and the
larger water tubes. You can
also see the pith rays that ex-
tend radially in thin layers at
right angles to the wood fibers.

The cambium is a layer of
soft tissue between the two
sides of the bundles. It is the growing tissue which results in the
increase in thickness of the dicot stem. Growth takes place by the
longitudinal division of the cells. New cells formed on the inner
side by the division of the cambium layer change into water-con-
ducting cells or wood fibers ; on the outer side they change into
food-conducting cells or bast fibers. In this way the bundles of
perennial dicots enlarge from year to year, and this causes the
stem to increase in thickness. In a tree, cambium cells form a
continuous layer between the wood and the bark, and the di-
ameter is increased by the addition of successive layers of tissues
built by these cells. These layers are the annual rings that one
sees at the ends of logs. At the apex of the stem the cambium
terminates in the growing region. At the lower end of the stem
it connects with a similar tissue in the root.

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

As the trunks and branches of trees age, secondary cambiums
arise in the cortex that develop secondary layers of cork, or hard
cells, or even bast fibers. These cambiums are usually irregular

138

General Botany

in their position in the bark, and in their extent. Secondary
cambiums are in part responsible for the plate-like peeling of

the sycamore and shagbark hickory,
and the knobs and ridges that oc-
cur on cork oak, cork elm, hack-
berry, and sweet gum.

The monocot stem. The monocot
stem, like dicot and conifer stems, is
bounded externally by an epidermis
which closely resembles that of the
leaf. The groundwork of the stem
is made up of parenchyma, which is
commonly called the pith. The
parenchyma is usually composed of
thin-walled cells, and is the princi-
pal tissue in which the temporary
accumulation of foods occurs ; from
it the sugar solution is obtained
when the stems of sorghum and
sugar cane are crushed. In a mono-
cot stem the bundles are scattered,
instead of being arranged in a
cylinder as they are in a dicot stem.
In stems of grasses that are hollow
(Fig. 79) they are scattered through
the cylinder of parenchyma tissue ;
in a cornstalk, a shoot of asparagus,
or the trunk of a palm they are dis-
tributed through the whole stem.
As in the dicot and conifer bundles,
the water-conducting tissue is on
the side next the center of the
stem, and the food-conducting tis-
sue is on the side toward the epi-

FiG. 78. Diagram showing the path
of the fibro-vascular bundles in stems
of the pahn type. The bundles of
each leaf arise by the growth outward
of the innermost and largest bundles
of the stem at that point. Lower
down, these bundles connect with
the outer bundles of the stem. {From
"Plants and Their Uses," by Frederick
Leroy Sargent; Henry Holt &° Co.)

The Structure of Stems

139

Fig. 79. Photograph of part of a cross-
section of a bamboo stem, showing thick-
walled mechanical tissues massed in the
outer layers of the stem. The large open-
ings in each bundle are water-conducting
tubes.

Fig. 80. Photograph of part of a cross-sec-
tion of a rattan stem, showing bundles.
The dark ring surrounding each bundle
is the mechanical tissue. The scattered
dark cells contain crystals of calcium
oxalate.

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

The monocot bundle. The monocot bundle differs from the
dicot bundle in that it lacks a cambium layer. It is frequently
called a closed bundle because, in the absence of cambium tissue,
the bundle cannot increase in size and there can be no growth in
diameter of the monocot stem through the multiplication of
cambium cells. The dicot bundle, on the other hand, is spoken
of as open, because there is a canibium layer between its water-
conducting and food-conducting tissues and the bundles can
increase in thickness. The monocot bundle differs further from
the dicot bundle in that its mechanical tissues form a complete
sheath about the food- and water-conducting parts. It is as
though the bast of the outer part of the dicot bundle and the wood
of the inner part were joined at the sides of the bundle, forming a
sheath about the conducting tissues.

The fibers like sisal and Manila hemp (Figs. 81, 235) that are

I40

General Botany

derived from the monocots are usually coarser than the fibers
derived from dicots, because the monocot fibers are entire
bundles, while the dicot fibers are made up of only the strands of
bast cells from the food-conducting side. The bundle sheaths
are usually thicker in the bundles near the outer part of the mono-
cot stem. In fact, in some monocots, like rattan and bamboo,
the sheaths of adjacent outer bundles may join each other and

l^ /V:: _ _ _ \_l^k^tii

Tig. t)i. PlaiiULiuii ui abaca, a species of banana, from the petioles of
which Manila fiber is obtained. Abaca flourishes only in the Philippines.
The fibers are used chiefly in the manufacture of ropes.

The Structure of Stems

141

tiureaii of A.griciUlure, P. I

Fig. 82. Stripping abaca for fiber. The long petioles are pulled under toothed knives
which scrape the soft tissues from the bundles. Abaca is a monocot, and the fiber is com-
posed of an entire bundle.

thus form a hard layer beneath the epidermis (Fig. 79). Some
monocot stems, hke the palms and dragon tree, increase in thick-
ness a number of years during their early life. This is accom-
plished by the development of secondary cambiums in the pith
between the bundles. The cells of the secondary cambiums
divide, forming new bundles between the older ones. In this way
the stems increase in diameter, without forming annual rings.

Dicot stems are enlarged by the development of new layers of
cells between the wood and the food-conducting tissue. It follows
that there will be annual rings in such stems.

The structure of conifer stems. The conifers, like the dicots,
have their bundles arranged in a cylinder. In structure these
bundles are somewhat similar to those of dicots, except that the
wood and water-conducting tissues are not distinct. The wood
cells form the water-conducting tissue as well as the mechanical
tissue. In keeping with their double function, these cells (tra-

142 General Botany

cheids) are thick-walled and spindle-shaped, with numerous thin
places, or pits, in two of the walls. Because of this structure, the
stem retains its rigidity and still permits the ready passage of
water and mineral salts.

The stems of some conifers, such as pine, spruce, and fir, have
resin ducts distributed more or less irregularly in the wood. In
cedar, hemlock, sequoia, cypress, and arbor vitae, resin ducts are
absent. Resin 'ducts are not tubes Hke the tracheae of dicot stems,
but are intercellular spaces in which resin accumulates.

CHAPTER SEVENTEEN

LONGEVITY OF HERBACEOUS AND WOODY STEMS
Every one who has occasion to grow plants needs to know
something about the length of life of the plants he is concerned
with, and he must know also whether they have herbaceous or
woody stems. For example, suppose a farmer wishes to deter-
mine whether it will be more profitable to grow sweet clover or
alfalfa in a certain field. Before planting either of these crops,
he should know that one of them is a biennial and the other
perennial, because all his plans for handling the crop will depend
on this information. Or suppose that another man wishes to
have a permanent border of flowering plants about his lawn to
obstruct the view of some unattractive fields or buildings. He
can choose wisely from among the hundreds of plants listed in
nursery catalogues only when he has definite information about
the longevity of the plants and as to whether they are herbs,
shrubs, or trees. A clear understanding of the classification of
plants on the basis of their length of life, their woodiness, and
their habit of forming single large trunks or a number of smaller
stems is helpful ; also in any study of the structure and processes
of stems. Plants differ greatly in their length of life. To indi-
cate the length of the natural life periods, the terms annual, bien-
nial, and perennial are commonly applied to plants.

Annuals. Most of our common garden vegetables and field
crops are started from seeds in early spring. The seeds germinate ;
roots and shoots develop ; and by midsummer or autumn,
flowers and fruits are produced and new seeds, which contain the
beginning of another generation of plants, are formed. Then the
plants die. The period from seed^germination to seed production
and death is called the life period. If it is completed within a
single growing season, the plant is called an annual (Latin : annus,
year). Corn, lettuce, radishes, beans, pumpkins, morning-
glories, and ragweeds are familiar annual plants.

143

144

General Botany

Biennials. During the first season some plants develop only
leaves and roots and a very short stem. The root is usually large

and accumulates a considerable
amount of food. In the second
season growth is renewed, and
there is developed an elongated
stem with leaves, flowers, fruits,
and seeds. These plants which
pass a winter season during their
vegetative development, and
whose life period includes two
different growing seasons, are
called biennials (Latin : hien-
nium, space of two years). The
seeds of some common weeds,
like the shepherd's purse, even-
ing primrose, and wild lettuce,
germinate in August or Septem-
ber, and a little rosette of leaves
is formed close to the ground.
Food accumulates in the root
until winter comes. The follow-
ing spring the plants make rapid
growth, and by midsummer they
have blossomed, produced seed,
and died. In spite of the fact
that their whole life is passed
within a twelve-month period,
these plants are called biennials,
because their life period covers
parts of two growing seasons.

The term annual or biennial as
applied to plants, therefore, does
not imply any definite length of

Fig. 83. Wild carrot {Daucus carota),
showing the plant as a seedling, at the end
of the first growing season, and as a ma-
ture plant during the second growing year.
The life history shown above is typical of
biennials,

Longevity of Herbaceous and Woody Stems 145

life in months. Wheat may be grown either as an annual or
as a biennial, depending upon whether it is planted in the
spring or in the fall. Shepherd's purse and wild lettuce not in-
frequently live as annuals in nature. The commonest biennials
of the garden are beets, carrots, parsnips, turnips, and cabbage.
In the first four, large amounts of food are accumulated in the
roots; in the cabbage the food is stored in the
enormous terminal bud, the " head." These 1?

stores of food are used in the production of seeds cj^

the following year. Usually biennials and annuals i^^*^

are herbs. Biennials, like annuals, are compara-
tively small in size, and die after flowers and ^^
seeds have been produced.

Perennials. Perennials (Latin : perennis, last-
ing through the year) are plants that live for a
number of years. Some of them, as for example
certain grasses, produce seed during the first and
succeeding years. Other perennials, like alfalfa,
form seed at the end of the
second and succeeding seasons.
Trees and shrubs usually require
several seasons' growth before
seeds are produced. The century
plant of our Southwestern deserts
develops vegetatively for 25 or 30

1 r .^ 1 r\ Fig. 84. Moth mullein, a biennial : first-

years before it produces a flower- ^^ason rosettes (in foreground) and the

ing stem and seeds. Then it be- mature plant,
haves like an annual or a biennial, for as soon as the seeds are
mature the whole plant dies. This calls our attention to the
interesting fact that in annuals, biennials, and a few perennials
there is no well-marked period of senility or old age. They die
suddenly at maturity, immediately after their period of greatest
vigor. Trees and shrubs, on the contrary, have a distinct
period of old age in which the physiological processes are slowed

146 General Botany

down gradually until the plants succumb to diseases and unfa-
vorable conditions which they could have withstood in youth.

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

Evergreen trees and shrubs are perennial in all parts of the
plant body. Deciduous trees and shrubs are perennial in their
stems and roots. Many herbaceous perennials, like the cat-
tails, grasses, mints, peonies, trilliums, and bananas, have annual
above-ground stems but perennial underground stems and roots.
Dahlias and sweet potatoes have perennial roots. Potatoes and
the Jerusalem artichoke (a kind of sunflower) have perennial
thickened underground stems (tubers). Tulips and hyacinths
have perennial underground stems and buds (bulbs). These
examples show that perennial plants have many different ways
of living over unfavorable seasons like periods of cold or drought.

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

Herbs, shrubs, and trees. Shrubs and trees have woody
stems. The stems of herbs have comparatively little woody
tissue. Our garden and field crops are all herbaceous plants.
Their stems contain little woody tissue, and in temperate climates
the above-ground parts live only during a single growing season.

The principal difference between shrubs and trees lies in the
fact that shrubs develop numerous slender above-ground stems
from a single base, while trees develop a single stem or trunk.

Longevity of Herbaceous and Woody Stems 147

This distinction may be expressed in another way by saying that
shrubs branch underground, while trees branch only above
ground. Most shrubs are less
than 10 feet in height, but some,
like the staghorn sumac, may reach
a height of 20 feet, or, like the
bamboo, 40 feet. Most trees are
between 25 and 200 feet in height.
However, the distinction be-
tween herbs, shrubs, and trees is
not one of size. Herbaceous plants,
like the sunflower, may reach a
height of 20 feet, and in the tropics
corn and bananas a height of 30
feet, while some shrubs are only a
few inches in height and some of
the dwarf trees of Japan that are ^ . . a ^ ■ c

-' ^ riG. 85. Japanese dwarf pine, borne

a century old are less than 5 feet of these small potted trees are a cen-

in height. ^""'y ''^'^■

Some of the oldest trees known were seedlings 3000 years ago.
Many trees now standing are over a thousand years old. The
average age of the older trees in our Eastern forests, however,
is much less than this, ranging from one to three hundred
years; in some of the Western forests, three to five hundred
years.

Plant characteristics and the plant-producing arts. The dif-
ferences in the habits of growth, longevity, and materials stored
by plants has led to specialization among those who grow plants.
For many evident reasons the most important art of growing
plants is agriculture. The farmer deals entirely with herbs and
largely with annuals, though biennials and perennials may be
grown for forage crops. He is for the most part concerned with
plants that accumulate foods in a highly concentrated form in
seeds. He transforms some of this food into meat and dairy

148 General Botany

products by feeding it to animals. But the basis of all animal
industry is the growing of plants.

Horticulture embraces a wider range of plants, but in actual
practice a horticulturist usually specializes on plants having
somewhat similar habits. The growing of food-producing shrubs
and trees represents one division of horticulture. The object
sought is the production of fruits containing pleasantly flavored
substances stored in cells with the thinnest possible cell walls.
The truck gardener specializes on annuals and biennial herbs that
accumulate both food and flavors, and to a less extent on peren-
nials, like asparagus, strawberries, rhubarb, and berries of various
kinds. Floriculture deals with all classes of plants and has for
its object the production of attractive flowers and foliage. It
reaches its highest development in landscape architecture, in which
masses of vegetation are arranged to beautify a landscape with
effective arrangements of foliage, and with varied texture and
color effects at different seasons of the year.

Silviculture is the art of growing trees to create forests. The
silviculturist specializes on growing trees of many different types
for a great variety of uses, such as lumber, pulp wood, bark,
rubber, cork, and fuel.

CHAPTER EIGHTEEN

THE GROWTH OF STEMS

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

Growth in length. The growth in length takes place at the
apex of a stem, the growing point being located in the terminal
bud. The growing region extends back from the tip, sometimes
for only a fraction of an inch, more rarely, as in rapidly growing
vines, a foot or two. If we mark the upper portion of a growing
stem into equal spaces, we may observe on the following day that
the uppermost spaces have elongated the most. The adjoining
spaces below are less and less elongated. This indicates that the
greater part of the cell division takes place near the tip (the grow-
ing point), but that some cell division and most of the enlarge-
ment of cells occurs in the adjoining part of the stem (the elongat-
ing region). During enlargement the minute cells of the growing
point absorb water and increase their volume from one hundred
to two thousand times. In the growing point the nuclei and the
surrounding cytoplasm completely fill the cell walls. In the
elongating region the cytoplasm forms merely a thin layer lining
the inside of the cell wall, most of the internal space being
occupied by cell sap.

The above description holds for most stems. In grasses and
some other monocots, however, the process is slightly modified.
In these plants the tissue at or just above each node continues
to grow for some time after the tissue of the upper part of the

149

150 General Botany

internode is mature. In these plants the growing point (primary
meristem) develops nodes, and short internodes that continue
growing independently. Instead of a continuous growing region
extending back from the growing point, there is a series of shorter
and shorter growing regions at the base of each internode. It
is this fact which explains why growing corn breaks easily
just above the nodes, and why it grows into an upright position
again when blown over during its period of development.

Diameter growth of annuals. Annual stems increase in thick-
ness until the plant matures. This increase in size is brought
about by the enlargement of cells and by the formation of addi-
tional cells by the cambium. In many annuals, like mustard,
zinnia, onion, squash, and corn, the stem thickens by increase
of the size of cells. In the sunflower the cambium continues to
form woody tissue and bast for a considerable part of the growing
season, so that very large plants have stems i to 2 inches in
diameter near the base.

Growth in diameter of trees and shrubs. Shrubs and trees
increase in thickness each growing season. This is often called
secondary growth; as we have seen, it is brought about by the
continued growth of the cambium. This layer of cells produces
new water-conducting tissue and wood fibers on its inner side,
and it produces food-conducting tissue and bast fibers on its outer
side. As growth proceeds from year to year, annual rings mark
the successive additions to the wood. The bark also develops
annual layers, but in most woody plants these are much thinner
and less conspicuous than the annual layers of the wood. Further,
since growth takes place inside the cortex, the cortex is con-
tinually being split and broken. The outer layers may die and
after a few years will be gradually weathered oflf. Secondary
cambiums in the cortex may develop additional layers of cork.
The ridges and grooves of the bark show how much too small
the outer bark is to cover the more recently formed wood.
Smooth, thin-barked trees lose their bark very rapidly. Trees

The Growth of Stems

151

Fig.

Bureau of Agriculture, P. 1.

Rafting large bamboo stems to market in the Philippines.

with bark that is thick and has large ridges are the ones that
hold their bark more tenaciously. But in all large trees the bark
contains only a part of the layers formed by the outer side of the
cambium ; much material has scaled off and fallen away. It
should be noted that as a tree gains in diameter, the annual rings
of wood in the stem are each year farther removed from the corre-
sponding annual layers of the bark. That is, the wood rings near-
est the pith are nearest in age to the outermost rings of the bark.
Diameter growth of perennial monocot stems. Most perennial
monocots, like the bamboo and asparagus, have horizontal under-
ground stems to which new and thicker stem segments with a
larger number of bundles are added each year. The aerial, erect
branches never increase in size after they are once mature ; but
the erect branches from old underground stems are from the
beginning much thicker than those from young plants. Con-
sequently, no little bamboo rod could ever grow into a bamboo
beam. No large bamboo beam was ever a slender rod. These
aerial branches come out of the ground nearly as thick as they
will be when mature. Asparagus plants are several years old

152 General Botany

before the underground stems become large enough to send up
thick, upright branches suitable for marketing.

The increase in thickness of stems of the palm type has already
been described. Usually the growth in thickness ceases after a
time, and the further growth of the stem takes place only in the
terminal bud. Such stems taper for a short distance at the base,
but above they are quite cylindrical.

Annual rings. The wood derived from conifers and dicots have
certain characteristics which are due to the variation in growth
during spring and summer. It is the difference in size of cells
and thickness of walls laid down at different times of the year
that make the annual rings visible. In such woods as the oak,
ash, and yellow pine the annual rings are conspicuous because of
the difference in texture of the spring and summer wood. Beech,
birch, and redwood have quite inconspicuous annual rings because
growth is quite uniform throughout the growing season. In all
trees, however, there is a perceptible slowing down toward the
end of summer and the wood cells are smaller near the outer edge
of a ring.

The width of rings is primarily dependent upon the amount of
carbohydrate furnished by the leaves, and secondarily on the
water supply. In wet seasons the rings are wider, in dry seasons
narrower. Indeed, the width of annual rings shows such perfect
correlation with wet and dry years, that the rings of our oldest
trees are being measured and studied to determine periods of
excessive rainfall and drought during prehistoric times, and to
estimate changes in climate.

Classification of woods. The structure of the wood of any
species of tree is so characteristic that any piece of wood may
be identified by a careful examination. Woods are primarily
classified as ring porous, diffuse porous, and non-porous. The
" pores " refer to the openings among the wood cells made by the
tracheae or water tubes. Since these are not present in conifers,
conifer woods all belong to the third class.

The Growth of Stems

153

Fig. 87. The "bottle" palm along a river in Cuba. An unusual type of palm stem, in
which secondary thickening occurs above the base but not throughout the entire length
of the stem.

154

The Growth of Stems 155

(i) Ring-porous woods have larger and more numerous tra-
cheae in the spring wood, and denser summer wood. To
this class belong ash, catalpa, locust, elm, chestnut, oak,
and hickory.

(2) Diffuse-porous woods have the pores about equally dis-

tributed through both spring and summer wood, and
annual rings inconspicuous. Here belong walnut,
cherry, cottonwood, beech, maple, holly, birch, gum,
and basswood.

(3) Non-porous woods may have conspicuous rings when the

wood cells, or tracheids, are large and thin walled in the
spring wood, and smaller and heavier walled in the
summer wood. Yellow pine and hemlock exemplify this
type. Red cedar, spruce, and arbor vitae have incon-
spicuous rings, marked only by the slight decrease in
size of cells near the outer edge of the ring. Some of the
non-porous woods have prominent resin ducts ; in
others ducts are wanting.^

Structure determines usefulness. For manufacturing pur-
poses the differences in wood structure just outlined are of the
greatest importance, because the quality of the products is largely
a matter of wood structure. Spruce, because of its soft texture
and freedom from resin, is used for paper pulp. Its uniform
grain makes it desirable for sounding boards of musical instru-
ments and in the manufacture of airplanes. Walnut, because of
its color and the ease with which it may be polished, is prized for
gunstocks and furniture. Hickory is the best wood for tool
handles and the spokes and rims of wheels. Oak is valuable for
flooring, interior finish, and furniture. Cypress and redwood are
especially noted for their durability in the soil or under conditions
where other woods decay. Ash is notably strong, elastic, and

^ Further information on the identification of woods may be obtained from
Guidebook for the Identification of Woods Used for Ties and Timbers (United States
Forest Service).

IS6

General Botany

not very heavy, and consequently is extensively used for imple-
ment handles, for wagons, automobile bodies, and railway-car
frames. The best baseball bats are made of second-growth ash,
because the layers of dense summer wood are thicker in more
rapidly growing shoots. These are but a few of many interesting
relations that exist between the structure and use of wood.

Attention must also be called to the effects of the environment
on the woody tissue. The quality of any kind of wood is modified
by the conditions in which the tree lived. Oak lumber from rich,
well-drained, moist uplands is very different from oak that grew
in relatively sterile, poorly drained lowlands. Consequently,
lumber from certain localities is far more valuable than the same
kind of lumber from others.

Annual rin^s
Spring wood
Summer wood^-^^^

Porea (vessels)

Medullary rays

Fig. qi. Diagram of a block of oak wood, magnified to show the arrangement of the various
tissues that produce the patterns on polished wood surfaces.

mrrr

The Growth of Stems

^ n ^1 Itr

157

Southern Lumbertnan

Figs. 92 and 93, Patterns formed by annual rings and medullary rays. The board at
the left is longleaf yellow pine, and the one at the right "quarter-sawed" white oak. In
the pine the markings are due to the rings. In the oak the edges of the rings appear as
longitudinal lines and the pith rays as irregular cross markings. Can you explain exactly
how each board was cut to show these markings ?

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

Grafting and budding. In the propagation of many varieties of
fruit trees it has been found that seeds are not satisfactory.
Most of our cultivated fruit trees are so highly variable that their
seedlings are not like the parent plants in quality of fruit. Horti-

iS8

General Botany

culturists long ago learned to overcome this difficulty by grafting
a twig from the desired variety of tree on a seedling of a similar

Cambi

Fig. 94. Methods of grafting and budding. At the left, whip grafting ; in the middle,
cleft grafting ; at the right, budding. A is the cion, and B the stock. C shows the cion
and stock joined. In both grafting and budding, success depends on bringing the cambium
of the cion into contact with the cambium of the stock.

tree. The graf ted-in branch then becomes the top of the tree, and
the fruit it bears is like that of the tree from which it came.

In grafting, the plant that furnishes the root is called the stock.
The twig that is attached to it is called the cion. In cleft graft-
ing, the top of the stock is cut off. The stock is then split and
two cions with chisel-shaped ends are placed in the cleft, one on
either side, so that the cambium of the cion is in close contact with
the cambium of the stock. The wound is covered with wax to
prevent the drying out of the tissues. If the cambium tissues
are in perfect contact, they will soon unite. New tissue will
grow under the wax and finally cover the wound. If both cions
grow, the weaker one is removed. In grafting nut trees the cions,
after being set, are painted with melted paraffine to protect them
from drying while the union between stock and cion is taking
place. This practice will increase the number of '' takes " in
fruit-tree grafting also.

Whip grafting is the common method of uniting cions to small

The Growth of Stems 159

seedlings. Usually this is done at or below the surface of the soil.
Both cion and stock are cut obhquely, and each is spHt. The
upper half of the oblique end of the cion is pushed into the cleft
of the stock and is bound firmly in place with raffia or twine.
Again, the success of the graft depends upon the contact between the
cambium of the cion and the cambium of the stock.

In budding, a T-shaped cut is made on the side of the stock,
through the cortex, down to the cambium. A bud from a tree
of the desired variety, with a small oval piece of wood and bark
attached, is slipped down inside the cortex of the stock and tied
firmly in place. This places the two cambium layers in contact
and the two pieces unite. The stock is then trimmed and the bud
develops into a branch. When the branch is well started, the
original stem is cut off just above the base of the branch. ^

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

The essential features of budding and grafting are relatively
simple, but in practice there are details and refinements which
are of the greatest importance. The selection of stock and cion
and the best method of operation vary not only with the species
but with climate and soil. The best results can be obtained only
through profiting by the experience of others and the results
attained by scientific experiment.-

^ In budding some hardwoods like hickory and walnut, better results are ob-
tained by cutting about a small patch of the bark and allowing the formation of
wound callus about the cut edges. As soon as the callus forms, the patch is
removed, and a patch of the same size bearing a bud is fitted accurately into its
place. The method is called "patch budding."

CHAPTER NINETEEN

THE MOVEMENT AND ACCUMULATION OF MATERIALS
IN STEMS

Aside from growth the most important processes going on in
stems are those connected with the transfer of water, foods, and
other materials. The Hving cells of the stem secure energy for
chemical processes through respiration. They also assimilate
foods, and may temporarily, or permanently, accumulate food
and other substances.

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

There can be no doubt that one of the principal factors in the
rise of sap is the evaporation from the cells of the mesophyll in
transpiration. The water thus lost is replaced by more water
passing into these cells from the adjoining water-conducting
tissue of the veins by osmosis. This is brought about by the
sugar and other substances in solution in these cells, as we learned
in Chapter XI.

Water inclosed in tubes has a high cohesive power ; that is,
it holds together like a solid. If a pull is exerted on the upper end
of a column of water in the vessels of a tree, the column holds
together like a cord or wire, and the whole column is pulled
upward. As the water at the upper end of the water-conducting
tissue moves into the mesophyll cells, additional water is pulled
upward into the vessels of the blades, petioles, and stems.

Transpiration is greatest and the largest amounts of water are
being lifted in trees during the summer. If at this season a hole
is bored into the trunk of a tree and an air-tight connection made
between this hole and a tube that has its lower end in a vessel of
water, the water is drawn into the stem, not forced out. This
indicates that there is more pull on the water from above than

1 60

Movement of Materials in Stems

i6i

there is pressure from below. It is known also that there may

be currents moving downward in one layer of the wood and

upward in another, although the

general direction of water transport

is upward to the leaves. The

movement of water in the tracheae

and tracheids is a mass movement,

similar to the flow of water in a

pipe, in spite of the fact that it

frequently must pass through the

cross-walls which divide the vessels

at intervals. It is certain that the

roots alone do not force water up

into the tops of trees.

The primary factor, then, in the
rise of sap is transpiration ; the
second factor is the movement of
water from the water-conducting
tissue to the mesophyll cells, re-
placing that lost through transpi-
ration ; the third factor is the cohe-
sion of water columns in the long
strands of water-conducting tissue,
which makes it transmit the pull
from the mesophyll cells all the
way down to the roots. In Chap-
ter XXI we shall learn how the
water passes from the soil into the
roots, and to what extent the roots aid in the lifting of water.
(See Fig. 95.)

The pulling up of water by transpiration is exempHfied when
cut flowers are placed in a vase containing water. That water is
drawn into the flowers maybe shown by placing the stems of white
flowers in water colored with red ink. Try this experiment with

Fig. 95. Experiment to show the lift-
ing power of transpiration and evapo-
ration. Both tubes were filled with
boiled water and placed in a dish of
mercury. In C the mercury has been
drawn up by transpiration from a
branch of arbor- vitae (A) ; in D, by
evaporation from a porous cup (B).

i62 General Botany

a white carnation or a chrysanthemum. Repeat, using dilute
ammonia instead of red ink.

Accumulation of water. The stems of many plants are suc-
culent ; that is, they accumulate water. Numerous examples
might be cited among desert plants of the cactus type, but they
also occur among mesophytes ; for example, purslane, begonia,
and certain orchids. Water retention depends in some instances
upon the presence in the cells of substances like pectic compounds
and mucilage, in other cases upon high osmotic pressures ; but for
still other cases no explanation can be given at the present time.

The flow of sap. The water in stems may contain a small
amount of sugar in addition to mineral salts. In the maple,
during early spring when the days are warm but at night freezing
still occurs, quantities of sugar pass into the water-conducting
tissue. This sugar comes from the medullary rays and other
tissues where it accumulated in the form of starch during the
preceding growing season. With the coming of warmer weather
the starch is changed to sugar and diffuses into the water-con-
ducting vessels.

The earlier sap is the richer and apparently comes largely from
the upper parts of the trunk. The last sap is more dilute and
probably comes from the roots. The positive pressure that pro-
duces the flow occurs usually during the day but may occur during
warm nights. When the temperature falls to the freezing point
at night, the pressure becomes negative and the sap flow ceases.
The causes of the pressure are only partly known. A portion of
it is due to the expansion of gas bubbles within the tree, but this
gas expansion accounts for only a small part of the pressure.

Whether the flow shall continue for weeks or stop after a few
days is determined by weather conditions ; . but just how the
several weather factors (like changes in temperature or rainfall)
bring about the increase and decrease of pressure, is unknown.
The flow continues longest when the night temperatures are below
the freezing point, and the day temperatures above. Even under

Movement of Materials in Stems 163

the most favorable conditions it is not possible to draw out of a
tree more than 5 per cent of the sugar that it contains.

A flow of sap somewhat similar to that in the maples occurs
in the spring in some other species of trees, as in the birch, butter-
nut, and hornbeam. In the birch the flow is more regular and
continues until May ; but the rate of flow and sugar content are
less than in the maple.

The movement of sugar in the water-conducting tissues of
stems is rather exceptional ; its usual path hes in the food-con-
ducting tissues.

Movement of foods. In previous chapters we learned that the
vascular bundles of stems contain food-conducting tissue. This
tissue is composed of thin-walled elongated cells and extends
from the veins of the leaves, through the stem, into the extrem-
ities of the roots. The larger of the vessels are the sieve tubes.
We also learned that dissolved substances alone can move from
one cell to another, and that the movement is by diffusion from
regions of greater concentration to regions of less concentration.

Applying this information to the movement of food in stems,
it is evident that in annuals the general direction is from the leaves
to the stems and roots during the vegetative period. When
flowering and the development of seeds begin, a considerable
part of the excess food moves into the reproductive structures.

In trees food moves into the branches and trunk during the
summer and autumn, and accumulates in the food-conducting
tissues, in the pith rays, and in young stems in the pith also.
In the spring there is a great increase in the amount of soluble
foods, and these move both into the roots and into the twigs.
At this time new roots are developing, and buds are growing into
new leafy shoots. When growth has stopped and photosynthesis
is active, the movement is again from the twigs toward the trunk.

Digestion. Before insoluble foods may be moved from one
part of a plant to another, they must be changed to soluble sub-
stances. This process may be illustrated by the changes that

164 General Botany

take place in starch within the plant. Starch is insoluble in
water. It does not dissolve in the cell sap, and the starch
within the cells is not divided into particles small enough to
pass through the cell walls. The process of changing starch
into a soluble substance has been carefully studied ; and we
know that it is first converted into the sugar, maltose, and that
the maltose is further spht into the simple sugar, glucose. Both
of these sugars are readily soluble in water and consequently
can pass from cell to cell and thus to any living part of the
plant. The changing of insoluble substances like starch into
simpler soluble substances like glucose is called digestion. Unlike
animals, plants have no special organs of digestion. All their
living cells are capable of digesting the insoluble substances that
are required for their nutrition.

Digestion brought about by enzymes. Digestion is a chemical
process and is brought about in cells by the catalyzers called
enzymes. These are produced by the living protoplasm of the
cells. A large number of different kinds of enzymes have been
recognized in plants ; each enzyme usually digests only one
particular kind of food, and there must be a different enzyme to
digest each kind of food within the cell. The enzyme which
changes starch to maltose is called amylase, and the enzyme
which changes maltose to glucose is called maltase. A mixture of
these two enzymes is called diastase.

The digestion of fats and proteins. The enzyme which digests
fats is called lipase. Many seeds are rich in fats. More than
one third of the weight of a peanut seed, for example, is fat.
When the seed germinates, this fat is digested and changed to
fatty acids and glycerin, which are soluble in the cell sap and
which may move to other cells. Furthermore, the fats are chemi-
cally stable substances, while the fatty acids are active substances
that may enter into a great variety of chemical processes and are,
therefore, more readily used in the life processes that go on
within the cells. There are other enzymes, proteases, which

Movement of Materials in Stems 165

act upon the insoluble forms of proteins and render them soluble.
Proteins are mostly inactive storage materials and bear much the
same relation to the simpler and more active nitrogen compounds
(that is, amides and amino acids, produced by protein digestion)
that starch bears to the simple sugars, or fats to the fatty acids.

It seems probable that enzymes are concerned in the principal
activities of all Hving cells. Without them there could be none
of the rapid chemical changes in foods that are necessary for the
transfer of foods within the plant and for carrying on the other
processes. Both the building up of the complex food molecules
from simpler ones and the spHtting of these large molecules again
is brought about by enzymes that are within the cells.

Properties of enzymes. It is interesting to know that if an
enzyme is put into a test tube with the appropriate food substance,
under favorable conditions it will bring about digestion the same
as if it were in the living cell. This proves that digestion is not
directly carried on by the Hving protoplasm, and that to be
digested, foods do not need to be in contact with living matter.
It requires but a very minute quantity of enzyme to digest a
large amount of the particular food upon which it acts ; for
example, a preparation of an enzyme extracted from the pancreas
of an animal was found to digest 2,000,000 times its weight of
starch. The amount of diastase in a mesophyll cell necessary to
transform the starch in that particular cell to sugar is so small
that it cannot be measured. Its presence is inferred from the
observed changes in the starch.

In most forms of digestion water chemically is added to the
original substances. This seems to weaken the bonds between
the different parts of the molecules and to bring about a spHtting
into simpler compounds. For example :

and

zCCeHioOe)^

n(H20) — ^

n(Ci2H220n)

starch

water — >•

maltose

C12H22O11

H2O -^

C6H12O6 +

C6H12O6

maltose

water — >■

glucose +

glucose

1 66 General Botany

Accumulation of food. A healthy plant usually manufactures
more food than it uses immediately, and this food may accumu-
late in various parts of plants. In the potato, surplus food passes
to underground stems, the tubers, where it accumulates. Tur-
nips and beets are examples of plants that accumulate excess
food in their roots. In the maple, the food accumulates in the
branches, trunk, and roots. In cabbage, food is stored in the
cluster of leaves at the top of the stem. In corn and cereals,
most of the food finally accumulates in the grain. In the century
plant, a considerable part of the excess food is stored in the thick,
fleshy leaves ; the process of accumulation may go on from 20
to 30 years, and the total quantity of food stored may amount
to many pounds. In nature such accumulated foods may be
used during the next season's growth of the plant or in starting
the growth of the offspring.

When soluble substances pass into and accumulate in the cells
of the stem, they are largely transformed again into an insoluble
form. This makes possible the continued entrance of the soluble
material. For example, starch formed in potato leaves is trans-
ferred through the plant to the underground tubers in the form
of glucose and maltose, and there it accumulates in the cells in
the form of starch. It is beheved that the same enzyme which
changes the starch to maltose, under suitable conditions changes
the maltose back again to starch, and that in general the enzymes
that digest foods are the agents that under shghtly different con-
ditions build them up again into the more complex insoluble
forms. Enzyme activities may be reversed by changes of tem-
perature, acidity, and water content of the cell.

Kinds of food accumulated. In any given plant in which food
is accumulated, protein, carbohydrate, and fat are all present.
Depending on the plant, however, the amount of any one of these
may be very great or it may be so small as to be practically negli-
gible. In the sugar cane and sugar beet the excess food occurs
mainly in the form of cane sugar (sucrose) . In the potato it is

Movement of Materials in Stems 167

almost wholly starch. The grains of wheat, oats, and rice contain
mostly starch, but also some protein. In sweet corn there are
both sugar ^ and starch ; in field corn the excess food is mostly
starch. In both sweet and field corn there are considerable
quantities of protein and oil. In the soy bean and peanut there
are large quantities of both protein and oil. In the seeds of the
coconut, flax, and cotton there is a large proportion of oil.

Summary. The upward movement of water from roots to
leaves through the water-conducting tissue is due mainly to pull
of transpiration. Water moves from the water-conducting
tissues to other tissues of the stem by imbibition and osmosis.

The movement of foods takes place mainly through the sieve
tubes and companion cells. Diffusion is known to be important
in this process, but it is inadequate to account for the rapid
transfer which occurs in many plants.

Before insoluble foods move out of or into a cell, they must be
digested. This is done by enzymes, and there are probably as
many kinds of enzymes as there are classes of food substances.
The products of digestion are soluble substances.

Both soluble and insoluble foods accumulate in stems and other
organs of plants. Insoluble foods are built up out of the soluble
foods that enter the cell by the same enzymes concerned in their
digestion. The reversal of the process is accounted for by certain
changes of conditions in the cell.

PROBLEMS

1. Why can a hollow tree continue to live for many years?

2. Why do sprouts not develop from stumps of trees that were girdled a year before

being cut down?

1 Unless sweet com is cooked almost immediately after its removal from the
plant, it rapidly loses its sweetness. This is because the enzymes in the grains
constantly convert the sugar into starch. Peas and some other vegetables lose
their sweetness after being gathered, for the same reason. The enzymes work more
slowly at a low temperature, and the vegetables will lose their sweetness less rapidly
if kept in a refrigerator.

CHAPTER TWENTY

ECOLOGICAL TYPES OF STEMS

Attention has already been called to the variety of stems,
and the advantages of upright, horizontal, climbing, and under-
ground stems. In a sense these are ecological types of stems,
since each of them bears a slightly different relation to the envi-
ronment. All the kinds of stems discussed, however, occur among
mesophytes, and the descriptions of stem structures that have
been given were also based on the stems of plants that live under
medium moisture conditions. Since the great bulk of plants live
as mesophytes, we may look upon these structural arrange-
ments as typical of the plants living under the most favorable
circumstances.

In this chapter the peculiar features of the stems of water
plants (hydrophytes) and of desert plants (xerophytes) are de-
scribed. Only a comparatively small number of flowering plants
are included in these groups.

How drought modifies mesophytes. When mesophytes are
grown under very dry conditions, the stems are reduced in size,
the stem tissues are more compact, and the cell walls are heavier.
Some plants also develop thorns, spines, and hairy coverings
under these conditions. Plants that have a tendency toward
succulence become thicker and more succulent when subjected to
drought. The water-holding mucilages which they contain are
increased, and their water-holding capacity is enlarged. The
leaf area of mesophytes is greatly reduced by drought ; conse-
quently, photosynthesis and transpiration are also reduced.
Under the same conditions the mechanical tissues of woody plants
are increased, their stems become more rigid, and the bark be-
comes thicker and more impermeable to water. If we keep all
these facts in mind in studying the stems of xerophytes, we may
be able to understand better the causes of the pecuHar features of
this ecological group of plants.

1 68

Ecological Types of Stems

169

Fig. 96. A group of xerophytes, including species of Cereus, Opuntia,
Yucca, Aloe, Euphorbia, and Agave. Such plants are characterized by
compact form, little or no leaf area, thick cuticle, and water absorbing
and retaining substances in the cells.

lyo General Botany

Xerophytes. The xerophytes are the characteristic plants of
deserts and dry plains, but they are by no means confined to these
regions. They occupy sand dunes and sand plains along the
Atlantic coast and on the shores of the Great Lakes. They may
even be found locally on rock cliffs and on dry, exposed hilltops.
In fact, they may occur in any situation in which a reduced water
supply in the soil is accompanied by atmospheric conditions that
promote rapid transpiration, or in which the plants are periodi-
cally, or continuously, subjected to drought. The stems of plants
that thrive in these habitats are reduced both in size and in the
amount of branching. The leaf area is reduced, or temporary,
or leaves may be entirely wanting.

The cactus type. The cactuses represent the extreme type of
drought-resistant, succulent plants. Leaves are wanting except
during the early stages of growth, and then they occur only as
small scales at the nodes. The stems are columnar, often ridged
and fluted, and always thick and fleshy. The photosynthetic
work in cactuses is done by the chlorenchyma of the cortex. As
the green surface is small compared with the green surface in
mesophytic plants, food manufacture is slower and growth is
correspondingly less. Yet some of the cactuses of Mexico attain
heights of 50 feet (Fig. 97). The cactus form points clearly to
one of the most characteristic features of desert plants ; namely,
water storage. A single large plant may contain from i to 25
tons of water. As the plant loses moisture so slowly, it may
continue to live for several years without an additional supply
of water. At every node there are usually clusters of spines
and spicules. A heavy cuticle and one or several layers of epider-
mal cells prevent rapid transpiration.

The shrub type. The mesquite, greasewood, and sagebrush
represent a second type of extreme xerophyte with much-
branched, hard woody stems. These plants are characteristic of
semi-deserts and of those parts of deserts in which soil moisture
is more constant. As a rule these plants are deeper rooted than

Ecological Types of Stems

171

C. J. Chamberlain

Fig. 97. Giant cereus of south-central Mexico in bloom. This specimen is 35 feet in height ;
occasional plants attain a height of 50 feet. In the pulpy interior of a cactus plant of this
size IS to 25 tons of water may accumulate.

those of the cactus type. The stems are covered with cork,
heavy cuticle, and sometimes wax and hairs. In the " palo
verde " leaves are absent, and the cortex contains chlorophyll.
Many of these shrubs have spines and thorns.

Short-stemmed type. These plants are often called stemless
because the leaves occur seemingly at the top of the root. In
reality the stem is a disk, or a rounded cone of nodes topping a
fleshy root. Internodes fail to develop, and the result is a rosette
of leaves, either flattened against the ground as in the evening
primrose, or raised and forming a hemispherical group of radiating
fleshy or bayonet-like leaves, as in the yucca and agave. Some

172

General Botany

yuccas develop very rapidly, and in spite of the short internodes
the stem rises a few feet above the ground. In one species the
stem branches and forms a tree with ro-
settes at the tips of the branches (Fig. 70) .
Annual-stemmed type. These include
many herbaceous perennials with under-
ground rootstocks, tubers, corms, bulbs,
and fleshy roots. The aerial stems are
developed during the moist periods, and
after supporting leaves and flowers die
down to the ground. To this group be-
long the conspicuous grasses and peren-
nial flowering herbs. Many of the aerial
stems show the characteristic structures
which reduce transpiration.

Summary of xerophytes. Stems of
xerophytes, then, are either (i) succulent
with a high water-holding capacity, or

(2) hard, water-proofed, and woody, or

(3) short underground supports for ever-
green or temporary rosettes of leaves,
or (4) temporary stems arising from un-
derground structures. The stems of the
leafless forms possess chlorenchyma.
Many of these stems have coverings of
hairs, spines, and thorns.

A word of caution concerning the origin of xerophytes may not
be out of place in this summary. It is not to be assumed that
xerophytes may be formed from mesophytes by the direct action
of the environment, though- some plants are temporarily modified
in this way when grown under conditions of drought. The
characteristics of the more pronounced xerophytes are hereditary
quahties which may have arisen as variations entirely aside from
the influence of a desert environment. Likewise, the pecuhar

Fig. 98. Century plant {Agave),
showing the rosette of fleshy
leaves and the flowering stalk.
It is a perennial, but, like an
annual or a biennial, it dies
when it flowers and fruits. The
century plant is an example of
the short-stemmed type of xero-
phyte.

Ecological Types of Stems

173

features of the hydrophytes, about to be described, must not be
considered as necessarily due to the direct influence of the envi-
ronmental factors to which they are exposed.

Effect of submergence on mesophytes. When mesophytes are
grown under very moist conditions, one of the first changes ob-
served is the great increase in air spaces among the cells. Fur-
thermore, when stems of mesophytes are submerged they may
develop large air cavities by the breaking down and separation
of certain groups of cells in the cortex. While the stems conse-
quently increase in diameter, there is a decrease in the amount
of wood tissue. Certain terrestrial plants with heavy cuticle and
hairy coats are smooth and without cuticle when grown under
water (Fig. 5).

Stems of hydrophytes. The most distinctive feature of sub-
merged stems of hydrophytes is the presence of large air chambers
extending throughout their length. When the stems are broken
open, the tissues are seen to occupy much less space than the air
cavities. We may properly speak of '' intercellular spaces " in
mesophytic stems ; but in describing hydrophytes, the term '' air
cavities " is more appropriate. These air cavities buoy up the
plant and provide an internal
atmosphere in which gas ex-
changes between the leaves and
roots take place. Frequently
the cells are so distinctive in form
and arrangement that we have
as a result a special tissue which
is called aerenchyma. Living
cells of plants are slightly heavier
than water, and the ability of any
plant to float is due to the air con- p^^ ^^ ^.^ ^^^.^^^ ^^^ ^.^^^ ^^^^ ,^

tained in the intercellular spaces, petiole of yellow water Uly {Nymphaa).

Floating type. Many hydro- The most characteristic feature of the stems

*=• "^ ^ ./ -' of hydrophytes is the presence of large air

phytes are free-floating. Among spaces in the tissues. {After Frank.)

174

General Botany

these free-floating forms
are the duckweeds, the
Salvinia (a fern), and the
water hyacinth. These
plants float because of the
internal air cavities. In
the duckweeds stem and
leaf are not differentiated
— the plant consisting of
a flattened globular green
mass of cells with a pen-
dant root. In other float-
ing plants the stem extends
the plant by branching
and by forming new plants
at intervals. The duck-
weed plants that form in
cold water in late autumn
are constructed with more
compact tissues than those
formed earlier in the sea-
son. These later plants, being heavier than water, drop to the
bottom of the pond and remain there during the winter. Only
the late fall plants survive ; the earher ones that are Hghter re-
main afloat and are frozen.

Submerged-rooted type. Other hydrophytes, like the pond-
weeds and water lihes, are rooted in the soil, and their stems bear
submerged or floating leaves. The stems have little or no me-
chanical tissue. As compared with land plants, the conductive
system is much reduced. Most hydrophytes develop horizontal
underground rootstocks and tubers. For this reason the plants
commonly grow in masses. Contrary to the usual opinion, even
whoUy submerged seed plants obtain their water and mineral
salts from the soil, and not from the water surrounding the upper

Fig. ioo. Submerged-rooted plants. From left to
right: eelgrass {Vallisneria), naiad (Najas), water
weed (Elodea), and pond weed {Potamogcton). In
such plants the mechanical tissue and the conduc-
tive system are poorly developed.

Ecological Types of Stems

175

part of the plants. Thus plants growing on sandy bottoms make
poor growth when compared with the same plants growing on
rich humus-covered bottoms in the same lake. In submerged
plants there is a definite movement of water through the water-
conducting tissue just as in land plants. At the upper ends of the
leaves the water is given off through water pores.

Emersed hydrophytes. A third group of hydrophytes, like
the cat-tails, rushes, bulrushes, and sedges, may have their roots,
stem bases, and underground stems under water, while the upper
parts are exposed to the air. These plants have both the con-
ductive and mechanical tissues well developed and are therefore
able to grow erect without being supported by the water. The
stems and leaves are exposed to the action of wind and wave and
to the conditions that bring about normal transpiration.

The importance of the aerial portions of these plants and the

Fig. ioi. Water lilies. In the foreground, species of Castalia; near the middle, the giant
water lily of the Amazon {Victoria regia). Stomata are found on the upper surfaces of the
leaves, and, as in land plants, there is a definite upward movement of water through the
plant,

176 General Botany

internal air cavities is shown by the fact that the plants may
be exterminated by cutting off all the rootstocks at the water's
edge and cutting the erect stems below the water level. Even
water plants may be drowned.

Summary of hydrophytes. Among hydrophytes, then, there
are plants with upright stems, some with floating or buoyant
stems, and others with underground stems. Hydrophytes
resemble the mesophytes in most respects but differ in having
aerenchyma, or air cavities, as well as in having a somewhat
reduced water vascular system and less mechanical tissue.
Hydrophytes spread rapidly by growth and branching of hori-
zontal stems. Many of them survive the winter, through the
development during the lower-temperature period of autumn of
short, heavier-than-water shoots and buds.

CHAPTER TWENTY-ONE

THE FORMS AND STRUCTURES OF ROOTS

In preceding chapters we have learned that leaves manufac-
ture food in the presence of light ; that their exposure to air facili-
tates the entrance and exit of carbon dioxide and oxygen ; and
that their efficiency is increased by being raised and displayed on
erect stems.

Tall, erect stems with sufficient strength for the maximum
display of leaves are made possible by the development of
mechanical tissue in the stems.
The display of leaves high
above the water supply of the
soil requires a conductive sys-
tem capable of raising water
and mineral salts from the
roots and of permitting the
movement of food away from
the leaves.

Stems, in turn, must be
firmly anchored, and they
must be supplied with water
and mineral substances from
the soil. Anchorage and ab-
sorption are the particular
functions of roots, though they
carry on other processes also,
such as conduction of water,
transfer of food materials, ac-
cumulation of food, respira-
tion, assimilation, and growth.

Primary and secondary Fig. 102. stages in the development of a com

roots. The root system of a ^^^^^i^g- -^ is the primary root, 5 a secondary

'' root, and A an adventitious root from the base

well-developed bean seedling of the stem.

177

178

General Botany

will show the essential features of roots. The primary root
extends downward from the base of the stem. On its sides are
numerous secondary roots which extend
at right angles, or grow obliquely
downward (Fig. 102). Unlike stems,
roots possess no definite nodes from
which branches arise. A secondary
root may originate at any point on the
primary root. Some common plants,
like parsnip and carrot, have primary
roots that thicken above ,^,,^_^.^^,,^,__
and taper gradually, ex- 1§i|ft|*
tending deeply in the soil.
These are called tap roots
(Fig. 104).
Adventitious roots. In

many seedlings there are Fig. 103. stages in the growth of an onion

, i . 1 . 1 1 r seedUng, showing the lifting and shedding of

also roots that develop from ^^e seed coats and the development of the

the first node of the stem. primary and secondary roots.

All roots arising from stems and
leaves are called adventitious roots.
The '' prop roots " that develop
from the lower nodes of corn stems
and the roots that grow from
^' cuttings " are familiar examples.
The adventitious roots of corn,
sorghum, wheat, and many other
grasses are far more important than
the primary and secondary roots.
In many instances only the adven-
titious roots remain at maturity
(Fig. 108).

Fig. X04. Dandelion plant showing the AdveutitioUS rOOtS dcvelop alsO

primary tap root and its branches, the ^

secondary roots. from the stems of many plants like

The Forms and Structures of Roots

179

- '\-'- ''.y#'kr fM

'■•>'--'^^.'^:'^U^i

1 . ..

-•V, :?:'.- '• '-fSf/

4'*

Fig. 105. Prop roots of a rubber tree in Florida. As the trees age, these roots thicken and
become secondary trunks.

i8o

General Botany

Bureau of Science, P. I.

Fig. io6.

A mangrove swamp along the seashore of an island in the PhiUppines. The
prop roots support the plants in the soft mud.

the poison ivy and trumpet creeper (Fig. iii) and act as hold-
fasts in supporting these climbers on trees and walls. Adventi-
tious roots may arise also at any point on a primary or secondary
root, following injuries. For example, when we plant pieces of
horse-radish or dandelion roots, adventitious roots develop.
During the dry season in deserts the younger parts of the root
system of some plants like the cactuses are dried out and killed,
and when the next wet season comes, many adventitious roots
develop from the parts of the root system still alive. In desert
plants these new adventitious roots do the absorbing work during
the moist period.

In plants that propagate by " runners " and by underground
stems, all the roots that have developed from the horizontal
branches are adventitious. All plants that are commonly started
by bulbs, like the tulip, hyacinth, crocus, onion, and Hly, have
only adventitious roots. Probably all the plants of the potato,
sweet potato, yam, sugar cane, banana, dahlia, and peony that

The Forms and Structures of Roots

i8i

you have ever seen had only adventitious roots, because seedlings
of these plants are either unknown or are rarely grown except by
plant breeders.

Deep and shallow root systems. The root systems of plants
are distributed in the soil in a variety of ways. In a soil that is
deep, readily penetrated, and sufficiently moist to permit growth,
some plants have roots widely distributed just beneath the sur-
face, while others have roots penetrating to great depths. There
are also plants that combine these two habits ; that is, their
roots are spread in a surface group and a deep group (Fig. 107).

Each of these habits has its advantages. The shallow roots
are in contact with water after every rain, and in dry regions
where the rainfall of summer showers penetrates but a few inches
these plants may be better supplied than others. Deep roots
have a distinct advantage in anchoring the stem firmly. Their
absorbing surfaces are in contact with the ground water that
accumulates from all the rains of the region. This supply
is usually a more permanent one in moist regions, but in dry
seasons it may fail for longer periods than the supply near the

J . E. Weaver

Fig. 107. The principal absorbing root system of a prickly-pear cactus, Opuntia caman-
chica, as seen from above. Beneath the stem is a group of vertical anchorage roots not
shown in the figure. The diagram is divided into one-foot squares.

l82

General Botany

surface. Plants whose root systems are a combination of the
two obviously have the best possible arrangement with reference
to soil water.

J. E. Weaver

Fig. io8. Vertical section showing the extent of the root system of mature corn grown at
Peru, Nebraska. The section is divided into one-foot squares.

Corn, cabbage, mock orange, blue-grass, and buffalo grass are
examples of shallow-rooted plants. Cottonwood, oak, hickory,
alfalfa, sugar beet, and California poppy have long tap roots which
may penetrate to the water table. Many of our tall grasses, par-
ticularly the '' bunch grasses," have great masses of long, fibrous
roots — the underground part of the plant being far greater than
that above ground. However, the form of the root system of
any plant may be greatly modified by soil conditions.

The Forms and Structures of Roots

183

Structure of roots. In the root, the xylem, made up of water-
conducting and wood tissues, forms the central core, or axis. In
seedhngs and in a few exceptional plants the root may possess a
central pith like the stem. The water-conducting tissue of the
root, Hke that of the stem, is composed of tracheae, and tracheids
which die at maturity. Surrounding the xylem is the phloem,
which is composed of the food-conducting and bast tissue. In
perennial roots a cam-
bium tissue lies between
the central axis and the
food-conducting tissue.
Outside the phloem there
are usually many layers
of parenchyma cells, mak-
ing up the cortex. The
innermost layer of the
cortex is often composed
of smaller or thick-walled
cells and is called the en-
dodermis. The outermost
layer or layers may like-
wise be modified in vari-
ous ways, forming cork or
coUenchyma. All young
roots are bounded out-
wardly by an epidermis.
In roots like the radish
that thicken rapidly this
is soon broken. The strips
of epidermis remain at-
tached to the growing root
for some time after it is
broken and may be readily ^ t^- . •. • , •

■^ I'IG. 109. Diagram of a root tip, showing the tissues
seen on the radish. and their arrangement.

Water-con-,
ducting

Growing
point

184 General Botany

In fleshy roots like the beet, which form in a single growing
season, the thickening takes place through the continued forma-
tion of new cells by the cambium. As the cambium produces
conductive tissue, alternating rhythmically with parenchyma
tissue, the mature root appears to be composed of concentric
layers.

Perennial roots. The perennial roots of shrubs and trees
increase in length from year to year, and the older roots increase
in thickness by the formation of annual rings. These roots soon
lose their epidermis, and later the cortex also dies, and a dead
bark similar to that of tree trunks is formed. In some instances
new cambiums arise in the cortex, which produce a layer of cork
enveloping the roots.

Growth of roots. Roots develop from growing points near the
tip. The growing point, however, does not lie at the surface, as
in stems, but some distance below, being covered by a root cap
(Fig. 109). As it moves, or forces its way between the particles
of the soil, it is in this way protected from abrasion. The cells
of the growing point are alike, but at a very short distance
back from the growing point they become difTerentiated into the
vascular axis, with elongated cells, cortex, and epidermis. The
growing region of soil roots is usually very short. Lateral roots
arise from growing points that develop on the vascular axis and
push outward through the cortex, breaking it as they elongate.

Root hairs. The young roots of land plants generally bear root
hairs. These are delicate elongations of the epidermal cells of
the root. They are especially concerned with absorption of
water and mineral salts, and their presence increases the absorb-
ing surface of the root from two to one hundred times. Since
the rate of absorption depends in part upon the surface area in
contact with the soil water, the advantage of root hairs is evident
(Fig. no).

Root hairs are usually short-lived structures, their duration
being best measured in days. Their walls are not only thin, but

The Forms and Structures of Roots

i8s

they are composed of pectic material which causes them to adhere
to the soil particles and brings them in intimate contact with the
water films on the soil particles. They
begin to develop at a short distance
from the tip of the root. Farther back
they have attained full length, and be-
yond this they are in a dying or dead
condition. Thus from day to day the
zone of root hairs moves forward with
the growth in length of the root, by
the continual production of new root
hairs just above the elongating region
of the root. This brings new root hairs
continually into contact with new sup-
plies of water and minerals in the soil.
As a plant enlarges, its root system
becomes more complete through re-
peated branching and the elongation
of the branches. Most of the absorp-
tion occurs in the root-hair zone, and
this is continually moved farther and
farther from the base of the stem. In
large trees this zone may be many
feet from the base of the trunk. The
fact that the outside of root hairs is

. Fig. iio. Enlarged view of the

composed of pectic substances is of end of a root, showing root cap,

great interest, because these substances g^^^'i^g region, and root hairs.
have so strong an attraction for water. It may be of great im-
portance, on this account, in absorbing the last particles of
available water.

Root contraction. As roots mature, they may contract in
length and so draw the base of the stem a slight distance into the
soil. In this way crevice plants on cliffs are continually held
firmly in place, in spite of the wearing away of the chff face by

, Region, of
elongation

Growing
point

1 86 General Botany

erosion. In the same way the crowns of clover and plantain roots
that have been lifted up by frosts may be drawn into the soil, and
small bulbs and tubers, many of which are formed at higher
levels than the parent bulbs, may be pulled deeper into the soil
by root contraction.

Root duration. Roots are annual, biennial, or perennial.
Perennial plants may have either annual or perennial roots, just as
they may have either annual or perennial aerial stems. Plants
with bulbs, tubers, or corms grow a new set of roots each year.
Plants with rootstocks, like the May apple and Solomon's seal?
generally have roots that last for several years. Shrubs and trees
also have perennial roots. We must be sure to understand, how-
ever, that even in perennial roots the work of absorption is for the
most part done by the youngest portions of the new roots which
are added each year. Most biennials, like the common evening
primrose and wild carrot, have fleshy roots in which food accumu-
lated during the first year. This food is used in the rapid develop-
ment of the plant during the second season.

Roots of hydrophytes. Most of the root characteristics thus
far described are those of the roots of mesophytes. In hydro-
phytes the roots are notably smaller and less branched than in
mesophytes. They absorb water and mineral substances from
the soil, even when the plants are totally submerged. The roots
of hydrophytes, like the leaves and stems, are remarkable for the
presence of internal air cavities.

When the roots of land plants (mesophytes) extend into well-
aerated water, they develop innumerable branches, differing in
this respect very markedly from the roots of hydrophytes. On
account of this fact, roots of trees, especially those of willow and
Cottonwood, that enter drainpipes and tiles often develop masses
of fine branches that obstruct the flow of the water even when
the entering root is not thicker than the lead in a pencil. The
banks of streams are often protected from erosion by the mat of
roots developed along the water's edge. This is why willows

The Forms and Structures of Roots

187

are planted on levees. When roots of mesophytes grow in
water, they also develop air cavities in the cortex.

Holdfast roots. CHmb-
ing plants, Hke the Vir- .^,
ginia creeper, poison ivy, '
Boston ivy, and trumpet I
creeper, develop holdfast ('
roots which help to support >
the vines on trees, walls, • •
and rocks. By forcing their
way into minute pores and
crevices, they hold the
plant firmly in place. Usu-
ally the roots die at the
end of the first season, V
but in the trumpet creeper
they are perennial. In the z
tropics some of the large —'
climbing plants have hold-
fast roots by which they at-
tach themselves, and long,

Fig. III. Holdfast roots of trumpet creeper, de-
COrd-llke roots that extend veloped from the nodes. These roots are perennial
downward through the air ^^^ ^^y lengthen and branch for several years.

until they strike the soil and become absorbent roots.

Epiphytes. A plant that lives perched on another plant is an
epiphyte (Greek: epi, upon, and phyton, plant). Mosses and
lichens are the most common epiphytes in temperate regions,
but in the rainy tropics and along our own Southern coast many
flowering plants five attached to the branches of trees. They
usually have leathery leaves and a Jow transpiration rate. Many
have water-storage tissue in fleshy stems or in thickened leaves.
Others are caUed tank epiphytes because they catch water in the
axils of the leaves or in pitcher-like leaves. Ephiphytes cling to
the supporting tree by means of roots that act both as holdfasts

1 88 General Botany

and water-absorbing organs. They do not take their nourish-
ment from the plants on which they grow. They carry on food
syntheses as other green plants do, but depend for their water
upon the evenly distributed rainfall and for their mineral sub-
stances upon dust and the decay of the bark on which they live.

Epiphytes are pronounced xerophytes, for there is probably no
habitat in which it is more difficult to maintain a water balance
than the one in which they live. It is not surprising, therefore,
to find that among the epiphytic plants of the West Indies there
are several species of cactus. Among epiphytes there are many
species of ferns, and many species belong to two families of
flowering plants, the bromelias and orchids. The bromehas are
related to the pineapple and have leaves of the same form (Fig.
162). The orchids have flowers remarkable for their shapes and
colors, and have the distinction of being the highest-priced of all
flowering plants. The Spanish moss of Florida, a flowering plant,
is perhaps the best known of American epiphytes. It is an
extreme form and is devoid of roots. Spanish moss and some
other members of the bromelia family have peculiar scale hairs
through which water is absorbed directly by the leaves and stems.
The roots of many epiphytes contain chlorophyll and assist in the
manufacture of food.

Roots in relation to bacteria. The roots of many plants have
bacteria or fungi growing about them or inside them. The best-
known crop plants belonging to this group are the clover , cow-
pea, and alfalfa ; their roots develop small nodules in which
certain kinds of bacteria change nitrogen of the air into nitrogen
compounds which may be used by the plants. More information
about these bacteria will be found in Chapter XXXVIII, on
" Bacteria and the Nitrogen Cycle " (page 396).

Mycorhiza. A root which has a fungus regularly associated
with it is called a mycorhiza (Greek : myco, fungus, and rhiza, root) .
Many of our trees, like the oaks, maples, poplars, and conifers,
have fungi surrounding their roots. The beech tree, for example,

The Forms and Structures of Roots

189

Bureau of Science, P. I.

Fig. 112. Epiphytes on the branches of trees in wet mountain forests of the tropics.
The plants in the picture are mostly ferns and orchids. {Photo by H. N. Whitjord.)

igo General Botany

flourishes only when it grows under such conditions. The diffi-
culty in transplanting azaleas, laurels, and rhododendrons from
the woods to our lawns Hes largely in supplying conditions favor-
able to the fungi that infest the roots. It is easy to supply the
proper shade and water conditions for the shrubs, but it is diffi-
cult to furnish soil conditions favorable to the life of the fungi.
The transplanting of these shrubs is therefore most frequently
successful when they are planted in large bodies of soil brought
with them from their natural habitat. Such soils may be kept
in their natural acid condition by the use of tan-bark extract and
alum, but the addition of lime is harmful. In orchids and some
ferns the fungi live inside the cortical cells of the roots. Just
how the fungi aid the plant is not fully understood ; that they
are essential is very clear. A few of these fungi are known to
furnish nitrogen to the roots, and they may also aid in the absorp-
tion of water and minerals from the soil.

PROBLEMS

1. Make a classification of roots and cite examples, on the basis of: (i) their

origin, (2) their form, (3) their environment, and (4) their function.

2. Why cannot a dead root system absorb as much water as it did when alive ?

CHAPTER TWENTY-TWO

THE PROCESSES OF ROOTS

The absorption of water and mineral salts is the process most
generally associated with roots. The three physical processes
involved in absorption were defined in Chapter XI, and are here
briefly summarized.

Diffusion is the movement of molecules and atoms from places
of greater concentration to places of less concentration. When
the diffusion of water into a substance, or body, results in swell-
ing, the process is called imbibition. The diffusion of water
through a differentially permeable membrane that separates a
mass of water, or a dilute solution, from another is called osmosis.
In osmosis water moves from the place of its greater concentration
through the membrane to the place of its less concentration. If
the solution is inclosed by walls, the movement of water into
the solution produces a pressure known as osmotic pressure.

Absorbing mechanism of roots. The epidermis is the primary
absorbing tissue of the root. We have seen that it consists of
delicate walled cells, some of them prolonged outwardly as root
hairs. The wall is composed inwardly of cellulose and outwardly
of pectic material, the latter having a powerful imbibing capacity
for water. The wall is permeable to both water and dissolved
salts.

The cytoplasm of the epidermal cell is the differentially per-
meable membrane which separates the cell sap from the water in
the soil. It prevents the outward diffusion of sugar and other sol-
uble organic substances, and permits the inward diffusion of water
and mineral salts. The epidermal cell, then, has a wall with a
great capacity for imbibition of water ; it forms an efficient
osmotic cell for the taking up of water ; and it affords a permeable
medium for the inward diffusion of salts. Because of the presence
of sugars and other substances made by the plant, the concentra-
tion of soluble substances in the cell sap is necessarily greater
than in the soil solution in which a plant grows.

igi

192

General Botany

The cortical cells are essentially like those of the epidermis,
and the concentration of their cell sap is progressively higher

I Cuticular transpiration [
itAttnospherc

I (^evaporation j
epidermal cells -^ cuticle

Stomatal transpiration [
tjltmosphere

I (yaseous diffusion) 1 5,^^ t^^^pi^ation |

Stomata ' ; '

* (Atmosphere

I ^^'"'"'"' ^'■ff^''^^ \ (gaseous diffi^ion)

('Tecretion oj

iDead
cells

Stomata or Icnticcls

T (gaseous diffusion )
intercellular spaces

I (evaporation )

Jeod-conducfing '¥■
living cortical f issues
I (osmosis)

TVood parenchyma
pith rags

Fig. 113.

Sntercellular spaces

(evaporation)
J\jiesophyll cells

t (osmosis )
I fi^er- conducting tissue
of veins I of leaf

T (mass movement
I . orjloiv)
Heater-conducting tissue
qf\ petiole

T (mass movement
or flow)
'Water-conducting tissues

ofstem^ (osmosis)
(mass movement
or/low)
Titter-conducting tissue of
, root, tracheae'* tracheids

\ (?)

Celts of root cortex

T (osmosis)
^ioot hairs - epidermis
I (osmosis -
imbibition)-

Soil water

Diagram of the path of water through a plant. The most important process
involved in each step is indicated.

from the epidermis inward to the water-conducting tissue. Con-
sequently water may pass by osmosis from cell to cell across the
root to the tracheae. Mineral salts may diffuse across the cortex
into the tracheae.

At this point the fourth factor in absorption enters. This is
the tension, or pull, on the water in the tracheae of the root caused
by the evaporation of water from the above-ground shoot. ^

^ Large trees have been kept alive for days by placing the cut-oflf trunks in water.
This shows that sufficient water to maintain the water balance of the plant for at
least several days may be lifted in a plant by the pull of transpiration without the
aid of roots. It is of practical interest to know that cut flowers will last much
longer if the ends of the stems are bent over into a vessel and cut under the water.

The Processes of Roots

193

The soil as a water-delivering mechanism. Soil is composed of
various-sized solid particles massed together with small spaces
between them. A root in its develop-

t/ill other tissues

ment pushes in among the particles and ( (diffusion)

pushes some aside. The very small root '^"^iZti^aZj"'"'^
hairs grow outward between the soil par- t f^nass movemene—

-^ I dissolved in xuater)

tides, and press against them on all sides. i/Water-condx-xHng tissues
Due to rains, water enters the soil and ^"'i (diffusion)

spreads rapidly downward between the Corticai ceiis
soil particles. Some of the minerals ^oot hairs
among the soil particles dissolve, and ( ('^'^'^^'°'^)

the result is a dilute solution, usually _ "* *^ ". ""

Fig. 114. Diagram to show the

called soil water." The water sinks, processes involved in the move-
partly because of its weight ; that is, it "^^^^^ of mineral salts into the
,, , , , . ^ . - tissues of a plant.

IS pulled down by gravity. It is also

pulled in all directions by capillarity, just as water is pulled into

small tubes, or as it is pulled into blotting paper.

If a flower pot having a perforated bottom is filled with soil
and placed in a pail of water, the air in the spaces between the soil
particles will be gradually driven out and water will take its
place. The soil is then saturated with water. When the pot
is lifted out of the pail, a part of the water drains out and it will
continue to drip for some hours. This is the water that is pulled
down by gravity. The water that remains is held by capillarity
and by the attraction of the soil particles. As water percolates
out of the mass of soil, air is again drawn into a part of the soil
spaces, but films of water surround every soil particle.

If a small plant is growing in the soil, its roots and root hairs

If cut in the air, air bubbles get into the ^water-conducting tubes and prevent the
subsequent movement of water into them. Air bubbles already in stems that have
been cut in the air may sometimes be removed by placing the lower ends of the
stems in warm water and cutting off an inch or two. After standing in water for a
day, the tracheae may become clogged with bacteria and the rise of water prevented.
Florists avoid this possibility by cutting off an inch or two from the stems of cut
flowers each day.

194 General Botany

are in contact with only a small part of the soil solution. Let us
suppose that transpiration is active and that all the water in
immediate contact with the roots passes into the plants. When
this occurs, water moves from adjoining spaces to replace it and
all the spaces again have about the same relative amount of water
in them. This movement of water to the spaces near the roots
continues in the soil as long as the capillary columns of water
among the particles near the plant do not break. If this happens,
the movement of water in the soil toward the root is stopped and
the soil no longer delivers water to the root. If transpiration
continues, the plant wilts.

The freedom with which water moves by capillarity varies
greatly in different soils. Since it is the lateral and upward
movement of water that determines the continued supply to the
root, plants wilt in some soil sooner than in others. Wilting is
determined by the availabiHty of water, and this in turn depends
upon the conditions in the soil that maintain continuous water
movement toward the root. Consequently all kinds of plants
show wilting when the water content of a given soil is reduced to
a certain point.

Summary of absorption. The roots and root hairs form a
mechanism into which water and mineral salts move readily by
diffusion, or by those forms of diffusion known as imbibition and
osmosis. Outside the root is another mechanism, the soil, in
which water moves freely or with difficulty, according to its
texture. If the water moves freely by capillarity, a large part of
the water in a given body of soil will pass to the water-absorbing
root, even though the root is in contact with only a small part
of the soil water. If the soil spaces are very minute, the water
is held more tightly by the soil particles, movement is impeded,
the continuous capillary columns of water break, and the water
ceases to move toward the root. Under these conditions absorp-
tion is stopped and the plant wilts, although there may still be a
considerable amount of water in the body of soil as a whole.

The Processes of Roots 195

Plants wilted in the daytime sometimes recover at night, because
of the reestablishment of the capillary water columns in the soil
surrounding the root.

Before we leave the subject of absorption, attention must be
called to the fact that the permeability of the epidermal cells
determines what dissolved salts in the soil water will pass into
the plant. These cells are more permeable to some salts than to
others. Hence, some salts diffuse in more rapidly than others.
But the root does not in any sense select the salts it needs and
retard the salts it does not need. Neither does it prevent the
entrance of poisonous substances. Salts of zinc, lead, copper,
and arsenic readily pass into and accumulate in plants growing in
the vicinity of smelters, and ultimately kill them.

Root pressure and sap pressure. If a number of well- watered
plants are cut off just above the soil, some of them will exude
water for a day or two. Experiments have shown that the sap
may be forced out with pressure sufhcient to raise water 30 to
40 feet. In most plants, however, a rise of only a few inches is
obtained. This pressure is called root pressure. When such
pressures exist in plants, they probably aid in the lifting of water
in stems. Extensive experiments have shown, however, that
root pressure is intermittent. It may exist at one time and not
at another, and when transpiration is most active and the largest
volumes of water are being raised in a plant, root pressure is
wanting entirely. Because of all these facts, it is generally be-
lieved that root pressure is not a necessary, or important, factor
in the raising of water in tall stems.

Imbibition and osmosis sometimes lead to the development of
high sap pressure, and they are partly responsible for the flow of
maple sap. Grapevines pruned in the spring exude water or
" bleed " for days afterward. On a small scale the same thing
may be seen when well-watered geraniums, begonias, and fuchsias,
are cut off near the soil. There is evidence, however, that in
" bleeding " only the cells near the cut surface are involved.

196 General Botany

Roots and transplanting. Only a few years ago it was thought
impossible to transplant large trees or even medium sized conifers.
Today trees of large size are dug up, transported many miles,
and replanted successfully. Even whole hedgerows several feet
in height are transplanted without injury. This advance in the
art of tree moving is a fine example of the appKcation of a knowl-
edge of root physiology to practical problems.

We have learned that the absorbing part of the roots is mostly
in the root-hair zone near the root tips and that the older roots
are largely organs of conduction. Formerly, when a tree was
dug up for transplanting, all the roots were cut off 3 or 4 feet from
the base of the stem. This operation destroyed practically all
the absorbing organs, and the tree could not absorb water from
the soil until a new set of roots had developed. Meanwhile
transpiration went on and the plant cells lost so much water that
they were injured and not infrequently killed.

Success in transplanting is attained by gradually trimming the
roots months before the tree is moved, and by loosening the soil
near the tree so as to develop a mass of absorbing roots near the
base of the stem. When the tree is Hfted, the roots are not cut
off, but as many as possible of them are carefully removed from
the soil. The small roots of trees are killed by drying, and for
this reason they are protected from wilting by being bound up in
wet moss. Sometimes the trees are loosened somewhat in the
autumn and moved during the winter, together with much of
the frozen soil surrounding the roots. Successful transplanting
depends upon reducing temporarily the loss of water by trimming
the top, preserving the absorbing roots, and exercising care in
handling both roots and stems so that they may not be injured.
In transplanting smaller plants the greatest care should be
taken to prevent the drying out of the youngest roots. Many of
the roots are sure to be injured in the digging and resetting, and
efficient absorption is thereby reduced. This reduced absorption
may be balanced by trimming off a part of the stem or leaves.

The Processes of Roots 197

Respiration in roots. Respiration goes on in the hving cells
of the roots, and this process requires a constant supply of oxygen.
In obtaining oxygen, the division of the roots into numerous fine
branches is an advantage, because a large surface is exposed to
the soil air. Some plants are easily injured by the lack of oxygen
in the soil ; if water stands on the soil and excludes the air, the
roots gradually suffocate. Suffocation of a part of the roots inter-
feres with absorption and other root processes besides respiration,
and the whole plant suffers. For example, corn becomes yellow
and sickly in low fields where water has stood for some time.
Such plants may recover if the soil is drained as soon as these
symptoms appear ; if delayed, the plants will never completely
recover. Water plants and swamp plants can grow in poorly
aerated soils either because the roots secure sufficient oxygen
through the internal air spaces of the plants or because they have
a low oxygen requirement.

The energy liberated in respiration is used in the chemical
processes associated with food transformations in the cells of the
root. If the aeration of the soil is poor, respiration is slowed
down, and instead of carbon dioxide, poisonous substances, such
as alcohols and organic acids, are formed. Hence the oxygen
content of a soil is one of its important properties.

A soil is in its most favorable condition for plant growth when
there is enough water to balance the loss by transpiration but not
enough to interfere with the access of oxygen to the roots.
Gardeners determine when it is in this condition by squeezing a
handful of the soil. If it barely cHngs together in a ball when
the hand is opened, it is about right. In this condition soil has
its largest volume, best aeration, is mellowest, and is most easily
penetrated by roots.

Carbon dioxide. A slight increase in the amount of carbon
dioxide in the atmosphere increases the rate of photosynthesis
and indirectly the growth of many plants.

When carbon dioxide accumulates in the soil, it becomes toxic

198

General Botany

J.E. Weave/'

Fig. 115. Section showing part of the root system of the bush morning-glory {Ipomoea lep-
tophylla), a widely distributed plant of the Great Plains region. The lateral and vertical
branches start from a perennial fleshy root, one foot in diameter, and extend into more
than 5000 cubic feet of soil. The section is divided into one-foot squares.

to some plants. Buckwheat, for example, can withstand a low
content of oxygen in soils, but is killed by an accumulation of
carbon dioxide. The black willow is indifferent to both low
oxygen and high carbon dioxide content of soils.

The distribution of roots in the soil. In addition to the in-
herited root habits of plants, an important factor that determines
the distribution of roots in soil is the oxygen supply. Various
plants have different requirements, but all roots require oxygen
for growth. Those who have seen stumps pulled from the land
know that the roots go deep in upland sandy soils ; that they
do not go so deep in heavy clay soils ; and that they are just
beneath the surface in swamp and bog land. The principal
reason why one finds the roots near the surface in swamps is that
these roots were the only ones that continued to live and grow.
The roots that in times of drought penetrated to greater depths
were killed off (suffocated) when the water stood at higher levels.
The distribution of roots in the soil, therefore, is determined
principally by the combined influences of gravity, water, and
oxygen. Water and gravity control the direction of growth, and

The Processes of Roots

199

the oxygen supply determines whether or not growth can take
place or the roots survive.

J . E. Weaver

Fig. 116. Root systems of oats {A) and wheat (5) at time of blossoming:
Lincoln, Nebraska. Section divided into one-foot squares.

200 General Botany

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

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

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

Temperature has a marked effect on root development. Many
common annual weeds develop extensive root systems only at
temperatures below 68° or 70° F. Poor root development means
retarded vegetative growth and small plants. Wheat is an ex-
ample of a plant whose root system develops best below 60° F.
Its principal absorbing roots are those formed from the first node
of the stem. These are formed down near the seed in cool soil.
In warm soil the first node may be at, or above, the surface of the
soil and the roots are then not advantageously placed in the soil.

The pressure of growth. In the growth of plant organs hun-
dreds, or thousands, of cells are expanding simultaneously through
the taking up of water. The pressure which these cells develop
is called growth pressure. The pressure exerted by roots in pene-
trating the soil may be very great, amounting to hundreds of

The Processes of Roots 201

pounds to the square inch. This is readily appreciated when one
sees cement sidewalks broken and large rocks moved by the

/?2Vc3=aB-

FiG. 117. Fern leaves pushing upward through a cement sidewalk.
Growth pressure may amount to hundreds of pounds to the square
inch. {After G. E. Stone.)

growth of roots under them. Growth pressure is just as powerful
in stems and other growing parts. Fleshy roots like those of
the radish and turnip sometimes force themselves partly out of
the ground by the thickening of the upper portion. How is it
possible for the cells of plants to withstand internal pressure of a
thousand pounds to the square inch without bursting? They
must have pressure equal to this amount or they could not move
rocks or break cement walks, which they do. The explanation
lies in the fact that they are so small that the pressure exerted
by a single cell is trifling compared with the strength of its cellu-.
lose wall. In a cell mass expanding against a pressure of 1000
pounds to the square inch, if the cells are of average size, say .03
mm. in diameter, the wall of each cell will not have to resist a
pressure of more than .13 of an ounce.

Food conduction. The transfer of food takes place in the food-
conducting tissue of roots in the same way as in stems and leaves.
Substances that are transferred are in a soluble form, and they
are usually in a comparatively simple form.

The movement of a substance in^o or out of a cell depends upon
the permeability of the cell protoplasm to that particular sub-
stance ; if the cytoplasm is impermeable to a substance, it can-
not enter or leave a cell. That the direction of the movement of
foods may change from time to time is shown by the fact that

202 General Botany

sugar and soluble proteins may move down into the root during
one season and up out of the root at another season. For ex-
ample, in the turnip or beet the excess food made by the leaves
during the first summer passes downward into the roots ; the
next year, food passes upward from the roots to the developing
stems and leaves. This may be due to changes in the permeabil-
ity of the cells, or to changes in the foods stored in the cells.

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

Accumulation of food in roots. Food accumulates in the roots
of many plants, notably in those of biennials like the beet, carrot,
turnip, and salsify. The sweet potato and the dahlia are ex-
amples of perennials with large storage roots. The most common
forms in which carbohydrates accumulate in roots are starch and
sugar. Starch as a storage material has the advantages of being
insoluble and more concentrated than sugar. When growth
begins anew, starch is readily converted (digested) into sugar.
Most roots also accumulate proteins to the extent of 2 or 3 per
cent. When these are digested, they change to soluble amides.
Fats occur in still smaller amounts. They break up when digested
into fatty acids and glycerin, both of which may be further modi-
fied by enzymes into simple sugars. In the sugar beet sucrose is
formed in the leaves and accumulates to a large extent in the
same form in the root, although some of it is changed to starch.

CHAPTER TWENTY-THREE

ENVIRONMENTAL FACTORS AFFECTING GROWTH AND
REPRODUCTION

The environment of a plant is made up of many factors.
Moreover, the factors are more or less interdependent, and it
is very difficult, and often impossible, to change one factor with-
out altering related factors. Consequently, it is often difficult
to determine the underlying cause of a change in the form of a
plant that is undoubtedly produced by something in the envi-
ronment.

Changes in the water content of a soil are complicated by the
effects of decreased oxygen content. The addition of lime to a
soil changes the permeability of the roots to water and mineral
salts, modifies the rate of transpiration, as well as alters the
chemical and physical qualities of the soil itself. The final effect
upon the plant is the combined result of all these changes.

In the following paragraphs the more important factors of the
environment and some of their effects upon plant growth and
reproduction are discussed.

Light an important environmental factor. The amount of light
available to a plant depends primarily upon the intensity of the
sunshine. This is greatest in the tropics and least at the poles.
The total amount of light is influenced also by the length of the
day. At the equator the daylight lasts 1 2 hours ; at the poles the
light continues all summer. So tropical plants have intense light
during half of each day, while arctic plants have weaker light
continuously through the growing season.

Orange growers at the northern end of the Central Valley of
California are able to ripen their fruit for market 3 to 6 weeks
earlier than their competitors 400 miles farther south, due to the
longer daily period of sunlight and the protection from cool night
winds afforded by the surrounding high mountains.

As much as 1000 bushels of potatoes have been grown on an
acre of land along the Mackenzie River, at the arctic circle.

203

204

General Botany

Long day Short day

Fig. ii8. The effect of long and short days on the evening
primrose. Both plants were brought into the greenhouse in No-
vember. The one at the left received, in addition to daylight,
illumination from an electric light from sunset to midnight for
about two months. The one at the right was kept under; ex-
actly the same conditions, except that it received only the nat-
ural winter daylight. This is a typical long-day plant, in nature
flowering when the days are long. (Garner and Allard. U.S.D.A.)

Wheat has been ripened at 57° north latitude, and immense yields
of hay are produced at 60° north latitude along the coast of Alaska.
These large crops are made possible by the continuous or long
daily period of sunhght during the growing season.

Medium light favors the growth of vegetative structures.
Attention has already been called to the fact that leaves and many
kinds of stems attain their largest size in partial shade.

Factors Affecting Growth and Reproduction 205

Exposure to full sunlight increases the rate of transpiration to a
point where the water content of the plant tissues is reduced
below that necessary for the greatest amount of growth. Vege-
tative organs, in general, do not require intense illumination for
their greatest development, because only a small fraction of the
sunHght is used in photosynthesis, and their growth is favored
by moisture both in the soil and the air. Just how much the sun-
light should be reduced to promote growth varies with different
plants and in different geographic regions. A reduction of 20
to 50 per cent is favorable to vegetative growth in many plants.
In moist regions, as for example along northern coasts, the

Short day

Long day

Fig. 119. Effects of length of day on tobacco plant. Both plants were grown in a green-
house during the winter. The plant at the right received, in addition to daylight, electric
light from sunset to midnight, while the plant at the left received the 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. (Garner and A Hard, u.s.d.a.)

2o6

General Botany

"^^.^^-^'aM

^^^m

'^m^.

n,'-j^»i " , . «|J- V ^ * .. ■ -i

w. ^^^-^

J^^^-^'^m

r-w'-f^rs^y^r

Short day

Long day

Short day

Figs. 120 and 121. Like the evening primrose and in contrast to the tobacco shown in the
preceding illustration, red clover (Fig. 120) is a long-day plant. The plants growing in the
two pails were photographed June 28 and had each received the same treatment, except
that those on the left were illuminated for only 10 hours daily, while those on the right re-
ceived the light during the whole day. The ones exposed for the shorter period grew but 7
inches high and produced no flowers, while those illuminated during the whole day flowered
abundantly and the tallest plants grew to a height of 33 inches.

On the other hand, the dahlia, shown in Figure 121, is, like the tobacco, a short-day plant.
The plant on the right, beginning May 12, was exposed to 10 hours of light daily and flowered
July 8, when the photograph was made. On the control plant, under the natural length of
day, the first blossoms appeared September 27. {Gamer and Aliard, u. S. D.A.)

light intensity is often reduced locally by fogs and clouds. In
the northern Lake states cloudy days may form a considerable
part of the growing season, and the total light that reaches plants
is much less than on the plains and deserts. Germany, France,
Great Britain, and our own Northeastern states are noted for
their high yields of potatoes, turnips, carrots, beets, and other
vegetative crops.

The slope of the land, especially in mountain regions, may
increase or decrease the intensity and the length of dayhght.
Finally, plants may have their light reduced or cut off by trees
or other objects. Commercial growers of ginseng cover their

Factors Affecting Growth and Reproduction 207

gardens with slat frames so as to approximate the intensity of
hght found in the woods where ginseng grows wild. Tea, a
leaf product, attains its best quahty and yield in the shade
of taller trees purposely planted in alternate rows with the
tea plants.

The influence of sunhght and moisture may also be seen in the
geographic distribution of the flax crop. Flax is grown for two
distinct products: one, the bast fibers (a vegetative part), used
in making hnen thread ; and the other, the seed (a reproductive
structure), used in manufacturing Hnseed oil. The leading
centers of fiber production are in northwestern Russia, northern

..u

^ ■ ^*A\r^

Short day

Long day

Figs. 122 and 123. Figure 122 shows, 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 negimdo), shown on the left in
Figure 123, 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.

These photographs show clearly that light has effects other than those of photosynthesis
on plants, and that its effects are different on different plants (Garner and Allard. u. S. D. A.)

2o8 General Botany

France, Belgium, and Ireland, in regions of low light intensity
and high humidity. The northern plains from Minnesota to
Alberta, northern Argentina, Japan, Italy, and India are the
centers of seed production, all of these being regions of low
humidity and high Kght intensity during the season when the
crop is grown.

Intense light favors the development of reproductive struc-
tures. The production of flowers, fruits, and seeds is promoted
by bright sunshine, provided there is sufficient soil moisture to
permit normal growth of the plant. Our greatest grain and fruit
producing areas are in regions where these conditions prevail :
the Middle Western states, Washington, California, and Colorado.
In some of the areas the water supply is maintained by irrigation,
and the intensity of the hght is but slightly reduced by clouds or
atmospheric moisture during the growing season.

It is a common observation that partial shade reduces flower
production, and one of the difficulties in producing flowers in
greenhouses in the winter time is the low hght intensity. Keeping
greenhouse glass clean at this season is as important as providing
favorable temperatures.

In regions of intense light (Colorado, for example) many fruit
trees blossom and set fruit every year, while the same varieties,
in regions of less light, fruit only once in two or three years. In
wet seasons in the Southeastern states the yield of cotton fiber and
cotton seed per acre is greatly reduced, although the plants grow
to more than normal size.

The most brilliant wild flowers occur in alpine meadows, where
the light is intense and the moisture always sufficient. The
flowers also are larger, although the plants are smaller than those
of the same species growing at low altitudes.

Length of day. The length of the day is an important factor
in determining the flowering and reproduction of some plants.
For example, ragweeds given 7 hours of light daily blossom 2
months earher than similar plants exposed for 14 hours daily.

Factors Affecting Growth and Reproduction 209

The plants with a restricted period of sunHght grow to a height of
4 to 5 feet ; the other plants grow to be 7 or 8 feet tall. Evi-
dently long days favor vegetative growth in this plant ; short
days favor reproduction.

Under similar conditions radishes respond very differently.
They continue to develop a thickened root throughout the grow-
ing season and do not form flowers when the daily period of
illumination is shortened to 7 hours. With twice that amount
of sunlight these plants bloom in about i month.

These two plants are each typical of many species whose
vegetative development and reproduction are determined by the
length of day. It is also probable that the length of day is the
important factor that makes the beet a biennial in the latitude of
Kansas and an annual in the latitude of Alaska.

Quality of light. The quality of light is also an important
factor in growth. You have seen that when a beam of light is
separated into its constituent rays, as in a rainbow, they form a
series of colors running in order through red, orange, yellow,
green, blue, indigo, and violet. The red rays have the longest
wave lengths, while the violet rays have the shortest wave lengths.
Ultra-violet light has still shorter wave lengths. Under natural
conditions the longer light waves of the red end of the spectrum
are most important in photosynthesis. The shorter wave
lengths of the violet and ultra-violet rays are most important in
inhibiting vegetative growth. Ultra-violet rays are sometimes
used to kill bacteria, and they no doubt have similar dele-
terious effects upon the protoplasm of green plants. They
are rapidly absorbed by the atmosphere and by clouds, and
this probably has something to do with the difference of vege-
tative growth at low and high attitudes and in clear and foggy
climates.

Indirect effects of light. The various effects of light upon
growth and reproduction are a result of the physical and chemical
effects of light upon the numerous physiological processes in the

210 General Botany

plant. The synthesis of carbohydrates and other organic com-
pounds in the plant depends directly or indirectly upon sunlight.
As we shall see later, the relative amount of carbohydrates pro-
duced in the plant determines in the main whether the plant
continues vegetative growth or carries on reproduction. Light
also greatly affects transpiration, directly through raising the
temperature of the plant and indirectly by causing the stomata
to open.

Atmospheric water. The water in the air affects plants directly
in several ways. The moistness or dryness of the air determines
whether less water or more is required for transpiration, and the
amount of water precipitated from the air in the form of rain
determines to a large extent the amount of water available in the
soil. Atmospheric water condensed in the form of fog and cloud
reduces transpiration and also lessens the amount of light that
reaches the plant. Under conditions of high humidity and
favorable temperature vegetative activity reaches its maximum.
Drought greatly decreases vegetative growth and shortens the
vegetative period of plants. A high rate of transpiration may
not only prevent any increase in the size of a plant during the
daytime but may actually bring about a decrease.

Distribution of rainfall. The distrihution of rainfall through
the year is of the greatest importance to vegetation. When the
period of heaviest rainfall coincides with the hottest part of the
year, the conditions are best for the rapid growth of plants. If
the rainfall is scanty during the time of highest temperatures,
plants are hindered in their growth, and only xerophytes may be
able to withstand the conditions. In these regions irrigation is
absolutely necessary for the growth of mesophytes. The greater
amount of available sunlight in summer-dry regions accounts in
part for the unusually large crops that can be raised on the
irrigated lands of the Western states. This is one of the principal
reasons why California has become an important center of the
production of flower and vegetable seeds.

Factors Affecting Growth and Reproduction 211

Methods of conserving soil water. — In dry regions there are two
methods by which the soil water is conserved. By cultivating

Fig. 124. Cross-sections of kernels of hard or macaroni wheat. This wheat is grown in
dry regions and is valued because of its large content of protein. In the figures the flinty
or high-protein parts are shaded and the soft or starchy parts are white. When the wheat is
grown under the conditions of dry farming, the protein content is highest (.4) ; when regu-
larly irrigated, the same wheat produces soft, starchy grains (C). An intermediate con-
dition is shown by B. This exemplifies the effect of the water balance on the composition
of a grain.

the soil several times during the growing season, the soil struc-
ture is broken a few inches below the surface, where evapora-
tion takes place. Consequently the capillary water columns are
broken and water rises only to the top of the undisturbed soil
layer. The cultivated layer soon dries out and forms a blanket
that reduces water loss by evaporation. The rough, loose sur-
face is of further advantage when it rains, in that the water settles
into the soil quickly and there is little run-off. This method of
conserving water is called the "dust mulch.."

A second method of making land suitable for crop production
in regions of sKght rainfall is to plant crops only in alternate
years. By plowing the land so that it will take up water as fast
as it falls, and especially by destroying weeds which would other-
wise remove large amounts of water, each crop has a large part of
two years' rainfall available. These two methods of conserving
water form the basis of the so-called '' dry farming."

Effect of temperature. As one goes north or south from the
equator, the temperatures of the soil and the air decrease. In-
creasing altitude in mountains brings about the same effects.
Temperature directly influences the rate of all plant processes, and

212 General Botany

most plants grow best under certain rather fixed temperature
conditions. For tropical plants, air temperatures above 90° F.
are most favorable. Temperate plants develop best at between
60° and 90° F. Arctic and alpine plants grow at temperatures
but little above the freezing point.

The time during which the temperature remains above the
freezing point is the growing season. In the tropics this extends
throughout the year. In arctic and alpine regions it may be
reduced to 2 or 3 months. The temperature of the air and the
length of the growing season are important factors in determining
the amount of food a plant may manufacture, and consequently
the amount of growth. Rice and peanuts, for example, require
high temperatures for their best growth, while cotton must have
a long season in which to mature its seeds. None of these crops,
consequently, is profitable north of Tennessee.

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

Soil temperatures also are important. Dark-colored soils are
warmer than hght-colored soils of the same texture, because they
absorb the sun's rays more readily. Well-drained soils are
warmer than wet soils, (i) because less heat is required to raise
their temperatures, and (2) because the temperature of a wet
soil is lowered by the constant evaporation of water. The most
valuable farm lands are those with dark-colored, well-drained
soils. On north slopes, soils do not warm up so rapidly in the
spring, and plants growing there start their growth later than do
those on the south slopes of the same hills. Peach growers prefer
not only uplands but north slopes.

Factors Affecting Growth and Reproduction 213

Recent investigations indicate that much of importance may
be learned from a more careful study of the effect of soil tempera-
tures upon plant form and behavior. Very low soil temperatures
have been reported to shorten the vegetative period of beets, with
the result that only slender roots are obtained, while very high
temperatures shorten the vegetative period of kohl-rabi and pre-
vent the formation of the thickened stems for which the plant is
cultivated. Wheat germinated at low temxperature produces its
adventitious root system at a favorable depth beneath the soil
surface. If germinated at higher temperature the adventitious
root system is produced at or above the surface of the soil, with
the result that a very weak plant results even if the conditions
after germination are the best obtainable.

Disease-producing organisms are much more destructive at
one soil temperature than at another. If the best temperature
for the growth of the organisms differs from that of the host
plant, the disease may often be avoided by planting when the
soil temperatures are favorable to the crop and unfavorable to
the disease-producing organism.

Freezing. When plant tissues freeze, the formation of ice
takes place in the intercellular spaces. As the ice forms, water
is withdrawn from the cells, just as it is when a plant wilts. The
result of this withdrawal of water is a greater concentration of
the salts inside the cell and a higher osmotic pressure. Water-
imbibing substances and the osmotic pressure resist the outward
movement of water. In general it has been found that those
plants and tissues that have the highest water-holding power
are also most resistant to freezing injuries. They are likewise
least affected by drought. One of the sources of injury to the cell
when a large proportion of its water is removed is the precipitation
of proteins. Young growing tissues usually have a high water
content and the cells contain but Kttle of the water-holding sub-
stances ; consequently they are very susceptible to freezing in-
juries.

214 General Botany

Hardening of plants. " Hardening " is a term applied by
gardeners to the practice of rendering young plants immune to
drought and frost injuries. Seedhngs grown in hotbeds and
greenhouses in early spring, if set out directly into the open
ground, are very susceptible to drought and freezing tempera-
tures. If kept, however, in cold-frames for a few days at tem-
peratures several degrees above the freezing point, they increase
in hardiness, and if set out will withstand frost.

Investigation shows that hardened plants differ from tender
plants in having (i) more water-imbibing substance in the cells
and (2) in having more soluble proteins. The former prevents
the withdrawal of too much water from the cells when freezing or
drought occurs. The latter prevents the precipitation of the
proteins when the cell sap becomes more concentrated by the
partial withdrawal of water. Some plants may be hardened by
subjecting them to drought before freezing weather, and the
changes in the cells are quite similar to those that occur when
hardening is brought about by the low temperature of cold-
frames.

The hardiness of certain varieties of peach is due to the slowness
with which they take up water in early spring. The cell sap in
the buds is consequently very concentrated, and hght frosts are
not sufficient to freeze the water in the tissues.

If cultivated perennials are kept rather dry in the autumn,
they are much less likely to be winter killed than if they are kept
wet and green up to the time of kiUing frosts.

Plants differ greatly in their ability to synthesize the water-
holding substances that produce hardiness. Wheat, for example,
hardens readily and can withstand drought and extremely low
temperatures. Oats, on the contrary, seems to lack these water-
holding substances and is resistant neither to drought nor to low
temperature.

Winds. Winds and air currents are of importance, as they
affect the rate of transpiration or modify the temperature. Pre-

Factors Affecting Growth and Reproduction 215

W. S. Cooper
Fig. 125. Limber pine {Pinus flexilis) at timber line on Long's Peak, Colorado, showing
effects of environmental factors on growth. When exposed to the wind, snow, and low
temperature of the mountain peak, the tree has the scraggy, much-branched form shown
in the illustration. At lower altitudes it is a single-stemmed timber tree,

vailing winds increase transpiration and slow down growth on the
windward side of trees to such an extent that a larger part of the
crown of any tree standing in the open is on the leeward side of
the trunk. This is so generally true that one can tell the direction
of the prevailing winds of a region by a careful examination of the
trees. Occasionally violent winds may destroy large areas of
timber and crops, and along exposed coasts and mountain tops
bring about the development of stunted and gnarled trees.

Gravity. The direction of growth of many plant organs is
determined by gravity. The downward growth of primary roots,
the upward growth of stems, and the direction of growth of
lateral branches are responses to gravity acting as a stimulus.

The peculiar shapes of York Irtiperial apples are the results of
gravity stimulating growth in a vertical direction, no matter
what the position of the axis, or core, of the apple. If the apple
hangs vertically downward during growth, the mature fruit will
have a long, sheep-nosed form. If it extends horizontally from

2i6 General Botany

the branch, the apple will be short, flat, and vertically elliptical
instead of round in cross-section. If it hangs obliquely from the
branch, the fruit will be obliquely elongated. In all cases the
apple is longest in the direction of the pull of gravity.

The annual rings in the horizontal branches of trees are often
thicker on the lower side than on the upper. When corn and
other grasses are blown down by wind, they again become up-
right, because gravity not only stimulates growth of the nodes,
but causes the lower half to grow faster than the upper half, until
the stem is brought again to a vertical position.

Chemical elements essential to plants. We have already
learned that in order to have a plant grow the soil must furnish
it with sufficient water for transpiration and for the manufacture
of food. At the same time, the soil must not be so filled with
water as to exclude oxygen from the roots. Carbon dioxide and
water supply the plant with the three elements, carbon, hydrogen,
and oxygen, needed in building carbohydrates.

From the soil solution plants obtain other essential chemical
elements used directly, or indirectly, in the manufacture of food
and in the development of their tissues. These elements are
potassium, calcium, magnesium, nitrogen, phosphorus, sulfur, iron,
and possibly manganese. The growth of plants is hindered, and
certain plants are excluded from soils that contain insufficient
amounts of any of these substances. It should be borne in mind,
however, that from 60 to 95 per cent of a plant is water and that
most of the remainder is organic matter. When plants are
burned, the water and organic matter pass into the air and only
the mineral matter remains as ash. The ash seldom amounts to
more than 3 per cent of the green weight, and sometimes it is as
low as .3 of I per cent. It is evident, then, that each of these
essential elements has one, or several, special uses in the plant,
and no other can be successfully substituted for it. There are
other elements, like silicon, aluminium, and sodium, that accumu-
late in plants but take no necessary part in either their processes

Factors Affecting Growth and Reproduction 217

or structures. These non-essential elements, however, may
greatly affect the growth of plants. When present in small
amounts they may be favorable to the plant, but when present
in larger amounts they may be injurious.

Nitrogen enters into the composition of all proteins and of
many related but less complex compounds, like chlorophyll,
amino acids, and alkaloids. Carbohydrates furnish the basis of
these compounds, and both carbohydrates and proteins enter
largely into the making of protoplasm. Hence, when there is an
abundance of carbohydrates and nitrogen, vegetative growth is
greatly increased. This condition may be seen especially in
potatoes. If too much nitrogen is available, the plants develop
enormous tops but produce almost no tubers, because as fast as
carbohydrates are made nitrogen is available for the production of
proteins and protoplasm and further growth of the shoots ensues.
If the amount of nitrogen is just sufficient for the growth of an
average potato plant, there will be an excess of carbohydrates
formed, and these will accumulate in the tuber as starch. This
example illustrates what is meant by the proper balance of
carbohydrates and nitrogen.

Another example of the carbohydrate-nitrogen balance may
be seen in wheat. If the proportion of nitrogen is too large, the
wheat grows tall and the straw is so weak that it falls over, and
the grains fail to accumulate the usual amount of starch. Carbo-
hydrates form the cell walls, and if they are all consumed in
extending the stems and leaves, there are none left for thickening
the cell walls, upon which the stiffness of stems depends, and none
for filling the grain. Too much nitrogen added to orchard soils
leads to great vegetative growth and very little fruit. In-
sufficient nitrogen leads to poor growth and few blossoms, and the
fruit is small and woody, because the excess carbohydrates
accumulate as starch and cellulose.

Calcium occurs in many plant cells in the form of calcium
oxalate crystals. These may be large, rounded masses occupying

2i8 General Botany

most of the cell, cr bundles of microscopic needle crystals. If
you happen to have bitten into the corm of the common Jack-
in-the-pulpit and felt the stinging sensation in your mouth, you
have come in contact with the needle crystals, even if you have
not seen them. The crystals pierce the soft tissues of the mouth
and continue to irritate until they are dissolved. Oxalic acid
is produced in plant cells under certain conditions, and in the
presence of calcium it is precipitated as calcium oxalate (Fig. 28).

The middle lamella, which holds plant tissues together, is com-
posed of calcium pectate, a compound of calcium and pectic acid.
The use of calcium in the formation of this compound is probably
the most important role of calcium in the plant, since no other
element can be successfully substituted for it in the building of
the cell wall. The presence of calcium in the soil also affects
the permeability of the cell membranes and thus facilitates the
absorption and retention of other salts by the roots.

Calcium is also important in soils because it neutralizes acidity.
Red clover, alfalfa, and blue grass, for example, cannot withstand
acid soil conditions. This explains why lime and wood ashes are
recommended for improving lawns. Lime improves the texture
of many soils, and this in turn improves its drainage, water-
supplying power, and its aeration.

Potassium is essential to the growth of plants, although we
know of few potassium compounds in plant tissues. Cell division
does not occur in its absence, and it plays an important role in
the chemical transformations that are continually being made in
the living cells among carbohydrates, organic acids, fats, pro-
teins, and other less familiar substances. Weak-stemmed plants
that occur in the absence of sufficient potassium appear to be due
to the need of potassium in the synthesis and translocation of
carbohydrates necessary for the formation of thick cell walls.

Magnesium forms a part of the chlorophyll molecule and is
therefore indispensable to all green plants. It is also necessary
to the growth of non-green plants.

Factors Affecting Growth and Reproduction 219

Sulfur forms a part of all plant proteins. It also occurs in
certain compounds common in the mustard family, that give

<■

GRAVITY

4' V V V V V V

Fig. 126. Diagram showing the principal factors in the environment of land

plants.

them their pungent odor and taste. Onions, garlic, and leeks,
members of the Hly family, owe their flavors to sulfur compounds.

Iron is essential for the development of chlorophyll, although
it forms no part of the chlorophyll molecule. It appears to
function chiefly as a catalyzer in the plant.

Phosphorus is a necessary element in certain compounds found

220 General Botany

in the nuclei of cells. It is essential for cell division and many-
enzyme activities.

Manganese, like iron, is a catalyzer and is said to be associated
with iron in the formation of chlorophyll.

Fertilizers. Fertilizers are added to agricultural soils, (i) to
increase the supply of the essential mineral elements, (2) to
improve the texture of the soil and its water-supplying quahties,
(3) to liberate other mineral elements by breaking up insoluble
compounds in the soil, and (4) to correct acidity.

Of all the elements needed for plant growth, phosphorus,
potassium, and nitrogen are most frequently found in quan-
tities insufficient for the best yields of agricultural products.
Phosphorus may be added in the form of crushed phosphate rock,
or as '' acid phosphate " (rock phosphate treated with sul-
furic acid), which is more soluble and contains sulfur in addition.
Potassium may be suppHed by the use of potassium chloride.

Nitrogen may be added to soils in the form of sodium nitrate or
ammonium sulfate, but these salts are very expensive and can
only be profitably used on truck gardens where the value of the
crops amounts to hundreds of dollars per acre. In agricultural
soils generally, the nitrates are best secured and maintained by
making the soil favorable for the growth of the nitrogen-fixing
bacteria. Calcium is usually added to soils in the form of
crushed, or burned, limestone, not only to furnish this element to
the plants, but also to improve the physical condition of the soil.
Lime also helps to liberate potassium from insoluble compounds.
Acidity of soils. Most plants grow best in soils that are nearly
neutral. The use of Hme in neutrahzing acid soils has been men-
tioned previously. There are some plants, however, that are
favored by acid soils. Cranberries, blueberries, azaleas, laurels,
and rhododendrons flourish only under these conditions. In
growing these plants in cultivation the acidity of the soil is some-
times maintained by adding ammonium sulfate, by watering
occasionally with tanbark extract, and by adding alum.

Factors Affecting Growth and Reproduction 221

Bog soils, which are naturally acid, must be neutralized when
reclaimed for the growing of celery, onions, cabbage, and mint.
These soils are also deficient in potassium, and this element must
be supplied in some form to obtain the best yields.

Alkalinity of soils. In arid regions, the evaporation of water
may cause salts to accumulate in the surface layers of the soil to
such an extent that most or all plants are excluded. About many
of the lakes in the Great Basin region are alkali lands of this
kind. Various salts have been leached from the rocks and
minerals of the mountains, and washed down into these lake
basins, which have no outlets. This has been going on for
thousands of years and the water that carried them has evapo-
rated, leaving the salts behind. Some of these salts, like sodium
chloride (common table salt), sodium carbonate (washing soda),
sodium sulfate (black alkali), and borax, are poisonous to plants.
Others are not poisonous, but when present in considerable
amounts interfere with the absorption of water by roots. When
the concentration of salt is slight and relatively pure water is
available, these lands may be irrigated and drained and a part
of the alkah removed. These lands then become valuable for
agriculture. The cultivated sugar-beet and alfalfa lands near
Salt Lake City are of this character.

Humus. Another soil factor of great importance is humus.
This material, which gives the brown and black colors to rich
agricultural land, is composed of the partially decayed remains
of plants. Leaves and other plant organs that fall to the ground
are slowly changed and broken up by bacteria, fungi, and other
agencies until only the brown, powdery humus remains. Moist
or wet grasslands accumulate more humus than forested lands,
because so large a part of the plant is underground where the
decay is slower, and because these lands are covered with water
during a part of the year so that there is less oxygen available
for completely oxidizing the plant remains.

Humus favors plant growth by increasing the water-holding

2 22 General Botany

capacity of the soil and so rendering the water supply more
uniform throughout the growing season. It improves the
physical properties of the soil by making it mellow. Humus also
makes it possible for bacteria and other organisms that increase
fertiHty to hve within the soil.

Loam. Soils containing a large percentage of humus are called

loams. Some of the prairies of IlHnois, Iowa, and southern

Minnesota were originally poorly drained areas largely covered

with water during late winter and spring. During the summer

they dried off and were covered with tall grasses that died down

to the ground in the late autumn. During the winter they

became matted together, forming a thick layer of plant materials.

In time these partially decayed, and each year a new layer was

added, until after hundreds and thousands of years humus

accumulated to a depth of from i to 5 feet. Later, when the

settlers broke the prairie-grass turf and the land^was drained by

tiles and ditches, these areas became the most productive lands in

the United States and the center of production of corn and wheat.

Animals as a factor in plant environment. Leaf-eating insects,

such as the potato beetle, injure the plant by destroying the

chlorenchyma and thus preventing food manufacture. It is

estimated that grasshoppers and other insects often eat as much

of the grass in a pasture as do the farm animals. Plant lice,

leaf hoppers, and scale insects remove the sap from the cells of the

tender growing parts and may kill the entire plants. Plant lice

and leaf hoppers may also carry disease-producing organisms

from one plant to another. Other animals, like the earthworm,

favor the growth of plants by loosening the soil and promote the

formation of humus by eating and by puUing bits of leaves into

their burrows. Herbivorous (Latin: herha, herb, and vorare,

to eat) wild animals, like the rabbits, squirrels, and deer, markedly

affect natural vegetation, while the domesticated cattle, sheep,

and hogs to a large extent determine what plants can survive in

pastures and grazing lands.

Factors Affecting Growth and Reproduction 223

W. A.Orton. U.S. D. A.

Figs. 127 and 128. A healthy potato plant and one showing the mosaic disease. The
organisms that produce disease are a most important factor in the environment of both
wild and cultivated plants.

Man, more than all other animals put together, has modified
the natural vegetation of the earth. In some cases he has de-
stroyed it ; in other cases he has encouraged and protected it.
Most of all, he has selected certain plants and made of them the
food supply of the world. If he understands the interrelations
of the processes occurring in plants and how these processes are
affected by the various factors of the environment, he may secure
desirable modifications of both the vegetative and reproductive
structures of the plant.

Other plants as an environmental factor. Other plants, such
as weeds growing among cultivated crops, may modify the en-
vironment of plants by shading them and by removing water and
soluble salts from the soil. Or a plant may directly affect another
plant by growing on it and taking its nourishment from it. For
example, the mistletoe grows on trees and injures them. Corn
smut and wheat rust live on corn and wheat, and decrease or
prevent the production of grain. These are only two out of

2 24 General Botany

many disease-producing organisms that injure and destroy wild
and cultivated plants.

Importance of further study of the environment. From this
brief survey of the more important factors it must be evident
that the plant lives in a highly complex environment, that these
factors vary from one season to another, and that they are closely
interrelated. For these reasons it is difficult fully to explain the
effects produced by changing one of these factors. Certainly
the day has passed when offhand answers can be given to the
many questions arising from intelligent observation of plants in
nature or in cultivation. These questions can only be answered
correctly by experiments carried on by men who have made a
special study of plants in relation to environmental factors.
Furthermore, only well-trained men can make investigations that
will advance our knowledge in this important field. Yet this
more than any other is the field of botany that will contribute
information of fundamental importance to the farmer, the
gardener, and the forester. Every advance in our knowledge
of the relation between a plant process and a definite environ-
mental factor can be advantageously applied to improve cultural
practices.

CHAPTER TWENTY-FOUR

VEGETATIVE MULTIPLICATION AND PLANT
PROPAGATION

The development of new plants from roots, stems, and leaves
is called vegetative multiplication in distinction from reproduc-
tion by seeds. Growers of plants make use of these natural
methods of multiplying plants, and in addition have devised
methods of artificially multiplying them by grafting, budding,
and the growing of cuttings. In the plant-growing arts these
methods are grouped under plant propagation.

Vegetative multiplication. In the discussion of stems attention
was called to the fact that one of the advantages in underground
stems lies in the facility with which the plant may be multiplied.
From rootstocks arise new terminal and lateral buds that later
form new aerial shoots, and through the death of the older parts
of the underground stems these branches become separate plants.
Bulbs, corms, and tubers bring about vegetative propagation in
a similar way.

Plants may multiply from the aerial vegetative parts also.
The stems of the black raspberry commonly bend over, and where
they touch the ground they form buds from which adventitious
roots and new upright stems develop. A grapevine will take
root where a node comes in contact with the soil. In the walk-
ing fern the tips of the leaves (Fig. 291) develop buds, roots,
and new plants when in contact with the soil. The strawberry
is an example of certain plants, in-
cluding many grasses, which have
horizontal branches (runners) on the
soil surface that take root at inter-
vals and produce new plants. In
Bryophyllum, a persistent weed m
cultivated fields of the West Indies,
the leaves when they fall to the '^'-'
ground develop new plants from the ^^"- "?• bryophyllum leaf, with

° ^ ^ \ young plants starting from the notches

notches in their margins. in the margin.

225

226

General Botany

These illustrations, which might be indefinitely multiplied,
show the importance of vegetative propagation in the increase
and spread of plants. In nature it is probable that vegetative
multiplication is as effective in spreading plants as is reproduction
by seeds. By the former method the young plant is able to start
more vigorously than a seedhng, because it is able to draw water
and food materials from the parent plant until its own root and
leaf systems are well developed.

On this account, among wild herbaceous plants vegetative
multiplication frequently determines which species shall dominate

/a habitat. Vegetative multipHcation,
for example, gives blue-grass the ad-
vantage over other plants in our
lawns. Cat-tails and water liHes fre-
quently exclude all other plants from
their habitats by this means. Grass-
lands the world over are dominated
by perennial grasses with underground
stems. Denuded soil areas near cities
are at first populated by annual weeds,
but in a few years are occupied by per-
ennials, which have gradually crowded
out the annuals.

Vegetative propagation of cultivated
plants. In agriculture and horticul-
ture, vegetative propagation is rehed
upon for starting many cultivated
plants. Especially with plants that
do not usually produce seed, and de-
sirable hybrids and horticultural va-
rieties that do not come true from
seeds, is this method of propagation
used. Potatoes, mint, horse-radish,
sugar cane, sweet potatoes, and car-

Ofice of Farm Management {J. S. Gates)

Fig. 130. Underground bulb of
wild garlic, showing vegetative
multiplication by the formation of
three bulbs from the one planted.
The terminal bud of the original
bulb developed a flowering shoot,
and three of its lateral buds
formed the new bulbs.

Vegetative Multiplication and Plant Propagation 227

Office of Farm Management {J. S. Gates)

Fig. 131. "Sets" of wild garlic, showing the flowering heads, in some instances entirely-
made up of small bulbs. These small bulbs are very effective in spreading the plant, which
often becomes a serious weed in pastures.

tain varieties of onion are examples of crop plants started in
this way. Propagation by bulbs, corms, or rootstocks is the
method commonly employed in starting hlies, tulips, hya-
cinths, irises, cannas, caladiums, and chrysanthemums. Most
of our fruit trees are multiplied by budding and grafting, which
are specialized methods of vegetative propagation. Geraniums,
coleus, willows, currants, grapes, and most ornamental shrubs
are grown from cuttings. These cuttings are pieces of a stem
usually containing several nodes. Cuttings with a single node
may be used when it is desired to propagate from a very Hmited
supply of stock. It is obvious that a cutting must either have
a small leaf surface or sufficient stored food to carry on growth
until a leaf surface is developed.

Hardwood cuttings. Cuttings from woody plants are usually
made when the wood is dormant in the fall or early winter. They
are immediately tied in bundles of twenty-five or more, and buried
in a trench with the uppermost buds turned downward. Sand
or fight soil is then added until the basal ends are covered 2 or 3
inches. This method of storage lessens the freezing and thawing

2 28 General Botany

of the upper buds, and in the spring the basal ends are warmed
first and start developing roots. Cuttings may also be kept over
winter in cool cellars in sand. In the spring the bundles are
taken up and the cuttings set about 3 to 4 inches apart in trenches,
with only the topmost end above the soil. The object of the
winter treatment is to allow the formation of a callus at the lower
end of the cutting. From this callus the first roots develop.

Softwood cuttings. Cuttings or roses, geraniums, chrysanthe-
mums, coleus, and begonias are commonly termed ^ ' slips. ' ' These
are propagated in greenhouses and hotbeds under glass. Most
of the leaves are removed to prevent excessive transpiration and
wilting, and the cuttings are placed in rows in sand kept con-
stantly moist. Leaf cuttings are often used in propagating
begonias. Parts of leaves, or complete leaves, may be used, and
are simply laid on the surface of clean, moist sand. New plants
develop at the base of the leaf, or at the lower end of the principal
vein of a leaf segment. When the cuttings are transplanted, they
should be placed immediately in moist soil before the root hairs
are killed by drying.

Grafting and budding. As pointed out in Chapter XVIII,
there are methods of propagating desirable varieties of a plant by
growing them on the roots of a less desirable variety. SeedHng
apple and pear trees occur everywhere and are almost invariably
worthless, but can be made to bear choice fruit by grafting twigs
from desirable trees on them when young. This is so easily
accompHshed that it seems a pity to find hundreds of these trees
along roadsides and fence rows bearing worthless fruit.

By grafting standard apple, pear, cherry, peach, and apricot
cions on certain slow-growing stocks, with small root systems,
dwarf trees are produced. Apples, for example, may be dwarfed
by grafting them on " Paradise " stocks, pears on quince roots,
and standard cherries on native shrubby plums. The root
systems of these stocks, being small, decrease the water content
of the cion and use a smaller part of the food manufactured in

Vegetative Multiplication and Plant Propagation 229

Fig. 132. The banana, a perennial herb propagated by planting the "suckers" that de-
velop from the thick underground stem. The aerial stem, because of the many layers of
overlapping leaf sheaths, appears to be nearly a foot in diameter, but in reality it is only
about as thick as that seen on a bunch of bananas.

230 General Botany

growth. Consequently the plants accumulate food more rapidly
and come into bearing earlier.

Plants may be grown in a region where they would otherwise
perish by grafting them on other stocks. For example, the
vineyards of France were threatened with destruction by the
ravages of a root louse {Phylloxera) . American grape roots were
found to be immune to attacks of this insect, and the grape in-
dustry of France was saved by grafting the French vines on roots
of American grapes. In Florida it was found possible to extend
the cultivation of oranges farther north by growing the edible
orange on the roots of the Japanese bitter orange, which is quite
hardy.

In testing apple seedlings for their possibilities as new varieties,
plant breeders take i -year-old stems and graft them into the
branches of a large, thrifty tree. As the tree has a large store of
carbohydrate food at hand, fruit may be developed and its value
determined on these cions the second or third year. To test the
seedHngs on their own roots would require perhaps from 10 to 15
years. In this instance grafting is used to hasten reproduction.

When two varieties of apple are grafted together and the cion
does not make a perfect union with the stock, food may not pass
freely from the cion to the stock. This results in accumulation
of food above the point where the cion was set. The accumulated
food leads to increased growth and the formation of a thicker
trunk above than below the union.

Sprout forests. With the exception of the California redwood,
cypress, and pitch pine, most conifers reproduce only by seed.
Redwood, poplars, oaks, chestnut, and many other broad-leafed
trees develop sprouts from stumps. Sprout forests, or coppice,
as foresters call them, grow more quickly because the sprouts
have a root system already established in the soil, while a seedhng
must first manufacture food and grow one. Chestnut coppice will
grow large enough to furnish railroad ties in 25 to 35 years, or
in about half the time required by seedlings. Sprout forests

Vegetative Multiplication and Plant Propagation 231

Bureau of Science, P. I.

Fig. 133. Cuttings of sugar cane. This plant is not propagated by seeds,
but by pieces of the stalk placed in furrows in the field and partially covered
with earth.

do not grow as tall as forests developed from seed, and they are
more subject to disease, because the trees become infected
through the decay of the stump. Nevertheless, coppice is a
rapid and efficient method of growing small timbers, posts, and
pulp wood.

Advantages of vegetative propagation. Vegetative propaga-
tion has been found advantageous in crop plants wherever its
use is possible, (i) because desirable varieties which do not come
true from seeds may be perpetuated, (2) because some plants,
like the sugar cane, banana, and horse-radish, do not produce
seeds, (3) because it saves time in securing the product, as a
longer period is required for the maturing of plants started from
seeds, (4) because by grafting and budding plants may be grown
in regions where they could not survive, and standard plants may
be dwarfed to fit special conditions. y^C \C A/^^

;^y

CHAPTER TWENTY-FIVE

FLOWERS AND FLOWER CLUSTERS

The flower is a specialized shoot in which the reproductive
processes, pollination and fertilization, lead to the production of

A IfB i C ifD " E n F

Fig. 134. Diagrams illustrating terms applied to flower clusters: A, corymb; B, head;
C, compound umbel; D, head with disk and ray flowers; E, umbel; and F, spadix.

seed. Commonly the word " flower " is associated with the
brightly colored parts that make many of our garden and house
plants so attractive. But here we shall include under the term
the simple structures associated with seed production in plants
like the grasses, poplars, and birches, that have merely scale-like
leaves and bracts inclosing the reproductive parts. In the conifers

Fig. 135. Diagrams illustrating terms applied to flower clusters: A, spike;

B, catkin; C, raceme; D, panicle.

232

Flowers and Flower Clusters 233

the seeds are produced on scale leaves arranged spirally in cones.
These cones may be looked upon as a lower type of flower, struc-
turally very different from the flowers of the monocots and dicots.
Flower clusters. The arrangements of flowers on stems are
very varied in different plants, and many descriptive terms have

Fig. 136. Flowers of the corn plant. The panicle of staminate flowers (tassel)
is shown above. Below are the pistillate flowers arranged in a spike (ear), in-
closed by sheathing leaves. The only part of the pistillate flowers exposed to
the air is the long style (silk).

General Botany

Fig. 137. Fan palm with panicles of flowers. The photograph was made in Cuba.

Flowers and Flower Clusters

23s

Fig. 13S. Catkins of staminate flowers of red oak {Quercus rubra).

been invented to describe them. In many plants the flowers
occur singly at the ends of stems or lateral branches, as in the
tulip and in some varieties of roses. The stem which bears a
flower or flower cluster is called a peduncle. In flower clusters
the small branches which bear the individual flowers are called
pedicels.

In the spike of the common plantain, cat- tail, and timothy, the
flowers are arranged along the sides of the upper part of the pe-
duncle. The catkin of the willow, poplar, alder, and oak differs
only in that it droops. The raceme of the garden currant differs
from the spike in the fact that the^ flowers are borne on long pedi-
cels at some distance from the peduncle. In the umhel of the
onion, milkweed, carrot, and cherry, the pedicels all arise at the
top of the peduncle and are of about the same length, so that the
cluster is more or less flat topped. The corymb of the hawthorn

236

General Botany

Fig. 139. Staminate inflorescence and opening bud of the white
ash {Fraxinus americana).

is a flat-topped cluster in which the pedicels vary in length, the
outer being the larger, and arise from different nodes of the
peduncle. In a panicle the peduncle is repeatedly branched and
the branches are wide-spreading. Yucca, hydrangea, the ''corn
tassel," and many other large grasses will exemplify the panicle.
The head is a flower cluster in which the flowers are all crowded
together at the end of a peduncle as in the red and white clover.
The head of flowers of daisies, asters, sunflowers, and chrysanthe-
mums are often mistaken for simple flowers, because the larger
ray flowers have the appearance of petals and beneath them is a
cycle of bracts that might be mistaken for the parts of a calyx.

Flowers and Flower Clusters

237

140

Flower spikes of the alder. The two clusters on the left are staminate
spikes ; on the right the mature pistillate spikes are shown.

The parts of the flower. The apex of the flower stalk is called
the receptacle. It is often enlarged and serves as a place of attach-

FiG. 141. Spike and flower of wheat {Triticum vulgare), showing two bracts,
lemma (at right) and palea (at left), inclosing three stamens and a pistil with
two plumose stigmas. At the base of the flower inside the lemma are twg
minute scales, the lodicules.

238

General Botany

ment of the various floral organs. The outer whorl of scales or
leaf-like organs is the calyx. It usually is green in color, and in

Starnen

Fig. 142.

otigmal
Style pistil

Filament — ^ ^'^HJICI)!!^

5epal
Receptacle
Peduncle

Flower of flax sectioned to show the several parts of a
typical flower.

the bud stage it completely incloses the flower. The individual
parts of the calyx are called sepals. Next inside the calyx is a
• • • •

Fig. 143. Floral plans of several families of plants. The large dot above each figure
represents the position of the axis ; small dots represent missing members of a cycle.
Unshaded stamens indicate presence of stamens without anthers. A, lily family;
B, orchid family ; C, most grasses ; D, bamboo ; E, mustard family ; F, legume
family; G, heath family; and H, composites. {After Frank.)

Flowers and Flower Clusters

239

Fig. 144. Terms used in describing flowers. When the stamens,
petals, and sepals are inserted on the receptacle below the pistil (A),
they are said to be hypogynous; when united around the ovulary
{B), they are perigynous; and when united above the ovulary (C),
they are described as epigynous. The position of the ovulary in A
is superior — that is, above the insertion of the stamens and perianth ;
in C the ovulary is infer i or —thsit is, below the insertion of the perianth.

Fig. 145. Terms used in describing ovularies : A, one-celled, with
ovules on three parietal placentas ; B, three-celled, with ovules on
three central placentas ; C, one-celled ovulary with free central pla-
centa. The surface to which the ovules are attached is the placenta.

A ^'^ B 1^ C

Fig. 146. Pistils formed of one, three, and five carpels, respectively.

240

General Botany

Fig. 147. A tropical orchid {Lcelia). The perianth consists of three sepals and three petals,
one of which is greatly modified.

whorl of white or brightly colored leaves that make up the corolla.
The several parts of the corolla are called petals. The corolla is
usually the attactively colored part of the flower, but in some
flowers, as in the tulip and clematis, the sepals have the same
coloring as the petals. The calyx and corolla are often spoken
of as the floral envelopes, because in the bud they form a wrapping,
or envelope, for the inner parts of the flower.

Inside the corolla is a group of stamens, each composed of a
stalk-like filament and an anther, that contains the pollen. The
center of the flower is occupied by one or more pistils, each made
up of an ovulary, style, and stigma. The ovulary is the enlarged
part of the pistil that contains the ovules, which develop into the
seeds. The style is the stalk above the ovulary that bears at its
summit the stigma. The stigma is usually an enlarged surface,
which secretes a sticky, sugary solution in which the pollen grains

Flowers and Flower Clusters

241

are caught and in which they germinate. The pistils and stamens

are called the '' essential organs " of the flower, because they

produce the ovules and pollen

which are the two elements

necessary in the production of

seed.

Fig. 148. Epidermis from the petal of a
geranium. The velvety appearance of many
leaves and flowers is due to similar projec-
tions of the epidermal cells.

Pistils are variously con-
structed out of one or more
leaf-like parts called carpels.
For example, the pod of the
bean or pea is composed of a
single carpel. It may be com-
pared to a simple leaf folded at
the midrib, with the margin united. The fruit of the yucca,
tulip, and lily is composed of three carpels. The apple, pear,
and quince pistils are made up of five carpels, which constitute
the hard papery walls of the seed cavities in the '' core."

The variety of floral structures. The above is a description of a
typical flower ; but in the plant world we find an almost endless
variation in the number, form, size, color, and arrangement of

r-m

jC> ■ .

■ f**

.,;.£*

.m-'

--..,-

"^^^H.

Figs. 149 and 150. Cross-sections of the "essential organs" of a flower. At the left, an-
ther of a lily, showing the four microsporangia and the contained pollen (microspores) ;
at the right, ovulary of a lily, showing six of the ovules, arranged in pairs within the three
carpels.

242 General Botany

these parts. In some flowers the calyx, or the corolla, may have
its parts united into a tube, or one or both may be wanting. Or
the flowers may lack either pistils or stamens. For example,
the red maples and the cottonwood bear pistillate flowers on
some trees and staminate flowers on others, and the corn has
staminate flowers on the tassel and pistillate flowers on the lateral
branches or ears. It is not our purpose to name and describe here
the many different variations in floral structure ; a visit to a
conservatory or a tramp through the near-by fields and woods is
the most effective way of securing an idea of the great diversity
of flowers.

CHAPTER TWENTY-SIX

SEXUAL REPRODUCTION IN FLOWERING PLANTS

The most important fact that should be associated with the
flower is that in it occurs the sexual reproduction of the plants.
Sexual reproduction takes place by the union of two specialized
cells, the gametes. One of these cells is called the male gamete, or
sperm, and the other the female gamete, or egg. When these cells
unite they form a single cell called a zygote, or fertilized egg. This
process of sexual union, or fertilization, is the first step in the
development of nearly all plants, and every individual plant
normally starts as a single cell, no matter how complicated it
may be at maturity. The flower is a complicated structure in
which the development of the gametes, fertilization, the forma-
tion of the zygote, and its further development takes place.
These several steps ending with the seed are described in the
following paragraphs.

Pollination. If the anthers of a lily or nasturtium are ex-
amined, a fine yellow powder will be found which under a micro-
scope appears as a multitude of small grains. This is pollen. In
the production of seed it is necessary that the pollen grains be
carried to the stigma, and this transfer is called pollination. In
some plants the pollen merely falls by gravity on the stigma.
Wheat and oats are examples of plants that are pollinated in this
way. In other plants, like the pines, elms, birches, oaks, rye,
and corn, the pollen is carried by the wind. It is an interesting
fact that the stigma of wind-pollinated flowers are usually
roughened by hairs, which probably make them more effective
in holding the pollen.

The pollen of most plants with conspicuous flowers is carried
by bees, flies, butterflies, or moths. As the body of one of these
insects is rough or hairy, pollen grains become attached to it when
the insect enters a flower. Then when the insect passes to an-
other flower, some of the pollen from the first flower is brushed

243

244

General Botany

off on the stigma of the second. Thus pollination is brought
about by the insects in the course of their visits to successive
flowers. It is an advantage to the plant to
have its pollen carried by insects directly from
flower to flower instead of having it blown
about and reaching a stigma by mere chance.
If the amounts of pollen produced by the
pine, corn, ragweed, and other wind-polli-
nated plants are compared with the amounts
produced by plants that are pollinated by
insects, it will be seen that insect-pollinated
plants generally produce less pollen and are
no less effectively pollinated.

Why insects visit flowers. Insects do not
visit flowers to carry pollen for the plants.
They eat the pollen or feed their young on it,
and they also secure nectar from the flowers.
The nectar is a watery solution containing
sugar, which is secreted by glands called
nectaries. One or more of these nectaries is
usually located near the base of the corolla,
inside the flower. The insects visit the flow-
ers and secure food for themselves, but as
they make their visits they brush against the
anthers and become covered with pollen.
Later they come in contact with the stigmas
of other flowers and leave pollen adhering to
the stigmatic surface. In this way they per-
form a service for the plants. The perfumes
of flowers assist the insects in finding them,
and conspicuous white or brightly colored
parts of flov/ers may aid in the same way. The
massing of many small flowers in clusters and
heads certainly makes them more conspicuous.

Fig. 151. Diagram of
a pistil with germinat-
ing pollen grains and
pollen tubes of various
lengths. The embryo
sac is in the seven-
celled stage, with a
central fusion nucleus
and an egg (below)-
Fertilization occurs
when one of these pol-
len tubes reaches the
egg. {After Buchholz.)

Sexual Reproduction in Flowering Plants 245

Fig. 152.

Pollen grains and pollen tubes. 5 is the two sperms or male cells,
and T the tube nucleus.

Cross-pollination. When a flower is pollinated with its own
pollen or with that from another flower on the same plant, it is
said to be self- pollinated. If the pollen comes from another plant,
a flower is said to be cross-pollinated. In many plants it makes
no difference whether the pollen comes from the stamens of the
same plant or from those of another plant. In the common
tobacco plant the pollen may be transferred to the stigma of the
same flower, and seeds will be produced. In some plants, how-
ever, it is only when the flowers are cross-pollinated that seeds
are formed. The sunflower is a good example of this kind
of plant. In still other plants, seeds that are formed after
self-polKnation are less vigorous than those formed after cross-
pollination.

In some species of plants that are self-sterile, the pollen from
plants started by cuttings will not fertilize the egg ceUs on other
plants derived from cuttings of the same plant. This is a matter
of practical importance in cultivating blueberries, which are
propagated by cuttings. Unless fertilization takes place, perfect
fruits are not formed. Hence, cuttings from different sources

246 General Botany

must be alternated in the field in order to secure abundant
production.

Certain varieties of strawberries, which are usually propagated
by runners, must be alternated in culture in order to secure fruit,
because they are either self-sterile or produce no pollen.

From the above statements it will be seen that cross-pollination
is an advantage to some plants. Many flowers have arrange-
ments that make self-pollination impossible. Often the anthers
do not shed their pollen at the time when the adjoining stigma
is in condition to receive it. The pollen may be shed either
before or after the ripening of the stigma. In such plants there
is httle possibiHty of the stigma's being pollinated from the
stamens of the same flower. So, as insects go from one flower
to another they transfer pollen from flowers in which the pollen
is ripe to flowers in which the stigmas are ripe. This favors
cross-pollination.

It is exceedingly interesting to study the various other mech-
anisms that favor cross-pollination, but it should be done in the
field or with the flowers in hand. In the white lily the stigma
is out of reach of the insects when the pollen is shed. In other
plants the pistillate and staminate flowers may occur on different
individuals, or on different branches of the same plant. In
primroses and bluets the stigmas and stamens each have two
different lengths ; the flowers on one plant have long styles and
short stamens, while the flowers on another plant have short
styles and long stamens.

The most remarkable cases of cross-pollination by insects are
those in which a particular species of insect is necessary for the
pollination of a plant. Such relations exist in the yuccas and in
some orchids. In the absence of the particular insect, polKnation
and seed production fail. Yuccas may be grown in our Northern
states, but in certain locaKties they fail to produce seeds because
the moth (Pronuba) needed to poUinate the flowers does not live
there. The Pronuba moth coflects pollen from the anthers of the

Sexual Reproduction in Flowering Plants 247

yucca flower, and carries it to the top of the pistil and pushes it
down into the tubular stigma. The eggs of the moths are then
laid in the ovulary by piercing the pistil wall. As a result of
pollination the ovules develop and furnish food for the young
Pronuba larvae. But the larvae eat only a small percentage of
the ovules. So the larvae of the moth are fed on the ovules that
resulted from pollination. The yucca matures many undisturbed
seeds in every pod where none are produced in the absence of the
moth. How the relation became established we do not know,
for the Pronuba moth never sees her offspring and they never see
her.

Germination of the pollen. Further steps in the production of
seed are the germination of the pollen, the formation of the
pollen tube, and the fertilization of the egg. The details of these
processes vary in different plant groups, but the account here
given is representative of what is found in many flowering plants.
At the time of shedding, the pollen grains of many plants contain
three cells. One of the three cells is active in the formation
of the pollen tube ; the other two are the sperms, or male gam-
etes. The stigma, as we have learned, secretes a sticky fluid
containing sugars, acids, and other substances. Pollen ger-
minates best in fluid secreted by the stigmas of the same kind
of plant, and it usually germinates imperfectly, or not at all, on
the stigmas of other kinds of plants. Germination results in the
formation of a microscopic tube that grows downward among the
cells of the stigma and style into the ovulary and into an ovule.
Usually this is but a short distance. In corn, however, the silk
is the style and stigma, and the pollen tube must grow several
inches, or a foot, down the silk before reaching the ovules below.
As it grows downward, the two sperms move along near the end
of the tube.

To summarize the steps in the formation and movement of the
male gametes preceding fertilization, there is (i) formation of
pollen in the pollen sacs, (2) opening of the pollen sacs and shed-

248

General Botany

Figs. 153 and 154. Cross-sections of ovules. At the left, megaspore of lily ; note the
surrounding nucellus and two integuments making up the ovule. At the right, the first
division in the megaspore (embryo sac), resulting in two nuclei.

ding of the pollen, (3) pollination, or transfer of pollen to the
stigma, (4) germination and formation of the pollen tube,
(5) growth of the pollen tube into the ovule, and (6) movement
of the sperms to the end of the pollen tube.

Development of the egg in the ovule. Inside the ovule a
parallel series of cell activities is going on which results in the
production of the embryo sac and the egg, or female gamete.

\ V

.*;5>iN.,tl\j

Figs. 155 and 156. Cross-sections of ovules. At the left the second division in the
embryo sac, resulting in four nuclei. At the right, the eight-celled stage of the em-
bryo sac ; two nuclei are about to unite to form the fusion nucleus.

Sexual Reproduction in Flowering Plants 249

At an early stage the center of the ovule is occupied by a single
large cell. This cell continues to enlarge as the ovule grows, and
its nucleus divides, forming two nuclei, each of which divides a
second and third time, forming all together eight nuclei. The
large cell we will now call the embryo sac. There are four nuclei
near each end of it. Three nuclei out of each group move nearer
the ends of the sac and form cells. The two remaining move to
the center of the sac and fuse, forming one large fusion nucleus.
One of the cells at the outer end of the embryo sac is the egg.
This completes the development of the female gamete.

.#'-^

^ .

Figs. 157 and 158. Cross-sections of ovules. At the left, the seven-celled stage of
the embryo sac just before fertilization ; note the large fusion nucleus and just above it
the egg. At the ight, the embryo sac after enlargement through the development of the
embryo and the surrounding endosperm.

The series of events inside the ovule preceding fertilization
begins with (i) the formation of a large cell, (2) three successive
divisions of the nucleus, forming the eight-celled embryo sac,
(3) the formation of three cells in each end of the embryo
sac, (4) the fusion of two of the nuclei at the center of the sac,
and (5) the changes connected with the development of one of
the three cells at the outer end of the sac into the egg.

Fertilization. At the beginning of fertilization the pojlen tube
grows into the embryo sac and the two sperms are liberated. One

250 General Botany

of the sperms moves to the egg and fuses with it, forming the
fertihzed egg. In more general terms, the male gamete unites
with the female gamete and forms a zygote. This is the beginning,
or first cell, of the embryo. Fertilization is the essential part of
sexual reproduction both in plants and animals, and it marks the
actual beginning of a new individual. After fertilization the
zygote may develop into a new plant or animal of the same kind
as its parents. It should be clear that the sperms from one pollen
tube fertilize the egg in only one ovule, and that to fertilize all
the eggs in a pistil, as many pollen tubes must grow down through
the style as there are ovules in the ovulary below.

The endosperm. At the same time that the egg is fertilized,
the second sperm from the pollen tube unites with the fusion
nucleus at the center of the embryo sac and forms the endosperm
nucleus. The endosperm nucleus is therefore a nucleus made up
of three nuclei, and on this account the process by which it is
formed is called triple fusion. This fusion is followed by rapid
cell division and the formation of a soft tissue filling the rapidly
enlarging embryo sac. The tissue thus formed is the endosperm,
and into it pass large amounts of food from the plant. In the
grains and some other kinds of seeds the endosperm occupies
most of the space within the seed coat.

The embryo. The zygote, or fertilized egg, starts growth and
cell division at once. As the mass of cells enlarges, it grows
farther and farther into the endosperm, from which its food
materials are derived. It finally takes on the form of the embryo,
or young plant, that we find inside the seed. As development
proceeds, the growth is slowed down and finally ceases until the
seed germinates. Sometimes, as in the bean, all the contents
of the endosperm are consumed and all that remains of it in
the mature seed is a thin layer of cells around the embryo.
In other seeds the endosperm partly or wholly surrounds the
embryo with a thick layer of cells, as in the castor bean, corn,
and lotus.

Sexual Reproduction in Flowering Plants 251

The perisperm. In some seeds the tissue immediately sur-
rounding the embryo sac becomes much enlarged and accumu-

FiG. i5g. Floral organs and development of fruit of bean : ^, side view of floral envelopes;
B, petals; C, stamens in two groups; D, stamen with anther; E, young pistil; F, young
pistil enlarged to show ovules ; G, H, I, and /, stages in the development of the fruit.

lates food materials. In the mature seed this tissue resembles
the endosperm. Physiologically, endosperm and perisperm are
alike, in that they supply food to the growing embryo during the
germination of the seed. Corn cockle, spinach, and pepper seeds
have the food supply in the perisperm tissue.

The seed. The final product of pollination and fertilization
is the seed. Its complete development ends the role of the flower.
During the development of the endosperm and embryo the wall
of the ovule, commonly called the integument, enlarges and may
change in various ways, sometimes forming a hard outer coat of
stone cells and an inner soft coat. The primary parts of a seed,
then, are (i) the seed coats, (2) the embryo, and (3) the endo-
sperm (or perisperm). When we plant a seed, we are placing a
small, partly developed plant, with a hmited supply of food,
under conditions in which it may continue its growth. Seeds
are discussed in more detail in the next chapter.

CHAPTER TWENTY-SEVEN

FRUITS AND SEEDS

The term " fruit " is commonly used to designate a great
variety of organs that are developed as a result of the flowering

Fig. i6o. Types of fruits: A, enlarged receptacle with imbedded nut-like pistils (water
lotus) ; B, fleshy stem tip with a central cavity containing many minute flowers (fig) ;
C, enlarged fleshy receptacle with pistils attached to surface (strawberry) ; D, fleshy urn-
shaped calyx-tube with pistils inserted on the inner surface (rose).

of plants. The direct result of pollination and fertilization is the
production of the seed. The indirect effect of pollination is the
further development of adjacent structures. The pistil, or at
least the ovulary wall, enlarges and sometimes becomes greatly
thickened. Primarily the fruit is the enlarged pistil or ovulary,
but in many cases the calyx and the receptacle also enlarge and
form a part of the fruit, sometimes most of it. The pineapple is
a fruit in which an entire flower cluster has become fleshy, and
this fruit is formed without fertilization. Like the common
banana, it is seedless.

In some fruits the enlarged pistil forms a thin wall inclosing
the seeds. At maturity the pistil wall dries out, forming a dry
fruit. In others the pistil wall, or some of the adjacent structures,
become enormously enlarged by the formation of soft parenchyma
tissue in which sugars, fats, acids, and other substances accumu-
late. These are distinguished as fleshy fruits. Among the dry
fruits the most familiar are the grains, illustrated by wheat,
barley, and corn. The outer coat is the ovulary wall ; the embryo
is small, and most of the seed is made up of the starchy endosperm.

252

Fruits and Seeds

253

Very similar is the akene, a small dry fruit inclosing a single seed,
as in the buckwheat, buttercup, and sunflower. The pod, or
legume, is a dry fruit of one carpel which splits down the sides
when mature, freeing the several seeds, as in the bean, pea,
clover, peanut, and alfalfa.

Among fleshy fruits the commonest is the drupe, or stone fruit,
illustrated by the plum, cherry, olive, and peach, in which a single
seed is surrounded by an inner stony layer and an outer fleshy
layer. The pome is a fleshy fruit in which the receptacle enlarges
and surrounds the pistil (core), which is composed of five carpels
each containing several seeds. Pomes include apples, pears, and
quinces. The fruits of tomatoes, potatoes, currants, grapes, cran-
berries, and blueberries are true berries having a fleshy wall inclos-
ing several seeds. The pepo, or gourd fruit, is exemplified by the
cucumber, watermelon, and cantaloupe. It is a greatly enlarged
and fleshy ovulary containing numerous seeds.

There are many other kinds of fruit distinguished, but they are
too numerous to describe here. In the strawberry the fruit is the
greatly enlarged receptacle bearing numerous little akenes on its
surface. The rose fruit is similarly an enlarged cup-like recep-

FiG. 161. Types of fruits: A, legume (pea); B, pome (apple); C, berry (tomato):
D, drupe, or stone fruit (peach) ; E, samara, or key fruit (maple) ; F, akene (sunflower) :
Ct aggregate fruit (blackberry),

2 54

General Botany

Fig. 162. Pineapples growing in Porto Rico. The entire flower cluster becomes fleshy
and forms the fruit. Like most of the members of the Bromelia family, the leaves are
leathery, rigid, and arranged in a rosette. Pineapples are propagated by planting cuttings
of small lateral branches.

tacle, with small akenes on the inner surface. The fruit of the
fig is a greatly enlarged and hollow peduncle, with numerous
flowers lining the inside. Blackberries and dewberries are
clusters of fleshy pistils held together by the inclosed receptacle.
Raspberries differ in that the cluster of fleshy pistils separate
from the receptacle when ripe.

The development and ripening of fleshy fruits. The process
of development and ripening may be illustrated by the changes
that occur in an apple. As soon as fertilization occurs, the tissues
that finally make up the fruit begin to enlarge by cell division.
Food materials from the stem pass into this tissue and accumulate
as starch, acids, fats, and proteins. In the young green apple the
cells are very dense and gorged with starch. The sourness is due

Fruits and Seeds

255

to malic (Latin : malum, an apple)
acid. During the process of ripening,
great chemical changes occur. The
starch is changed to sugar, — sucrose,
glucose, and fructose. The water
content increases, and the acid gradu-
ally becomes less and less. The middle
lamella of the cell walls is partly dis-
solved and the cells separate more
or less, thus producing intercellular
spaces, and making the fruit softer
and more '' mealy." The ripening
process begins at the core and gradu-
ally extends outward, until all the
tissues are affected.

The middle lamella, composed of
calcium pectate and pectose, makes

up part of the cell walls. The chang- u.s.Depi. of Agriculture

ing of these substances to pectin and ^ig. 163. Fruit of mango, now

. , . 11 ,., 1 , being successfully grown in south-

pectic acid, jelly-like substances, con- ^^^ Florida.
tributes to the softening of the

fruit. The pectic compounds are important in jelly mak-
ing, and those fruits that contain large amounts of them
form jellies readily when they are mixed with sugar, boiled, and
allowed to cool. Fruits like the quince, apple, and currant are
plentifully supplied with pectic compounds. In elderberries and
grapes the pectic compounds are less abundant, and juices of
apple or quince are commonly added to them in jelly making to
make them jell more readily. When fruit juices and sugar are
boiled too long, they may not jell. This is because the pectins
have been chemically changed to mother substances which do not
have this property.

Recently it has been found possible to remove pectin from
carrot roots, which contain large amounts, and the pectin may be
added to fruit juices to insure jelHng.

256

General Botany

Economic importance of flowers
and fruits. The economic value
of flowers lies chiefly in their use
for decorative purposes, but cer-
tain flower clusters like the ar-
tichoke, pineapple, and cauli-
flower are used as fruits or vege-
tables. The fruit industry needs
only to be mentioned to call to
mind the vast scale upon which
plants are grown for their fleshy
edible fruits. It should be noted
that ripe fruits are made up largely
of water pleasantly flavored with
sugar, dilute acids, and aromatic
substances. The amount of food
actually present is not large.
Most fruits contain from 10 to 15
per cent of carbohydrates, i to 2
per cent of proteins and fat. Per-
simmons and bananas run somewhat higher in carbohydrates ;
olives and avocados may contain as much as 10 per cent of oil.
Fruits are valuable chiefly for the variety which they add to
our diet. Through canning, preserving, and drying they are
made available at all seasons of the year.

The coconut fruit when mature consists of a thick, fibrous husk
surrounding the seed. This fruit is an important source of coarse
fibers, both in the tropics and in temperate regions. Several
million pounds are annually imported into America and manu-
factured into door mats, floor mats, and coarse brushes.

The olive is the source of olive oil, which is extensively produced
in Spain, Italy, and California. In recent years it has been
partly replaced as a salad ofl by cottonseed and corn ofls, or
mixtures of them.

U. S. Dept. of Agriculture

Fig. 164. Persimmon fruits. The per-
simmon grows wild over a large part
of the Southeastern states, and im-
proved varieties are now cultivated.

Fruits and Seeds

257

Fig.

The structure of seeds. Although
seeds vary as much in form as do
other plant organs, the different ar-
rangements of the three essential
parts may be illustrated by a castor
bean, a lima bean, and a grain of
corn.

In the castor bean the seed coats
consist of a hard outer layer and a
thin inner membrane. These inclose
an endosperm, which is a mass of
cells containing food in the form of
starch, oil, and protein. Within
the endosperm lies the embryo,
ready to grow when favorable ex-
ternal conditions for germination
come. The embryo consists of the
hypocotyl and two very thin coty-
ledons,with a small bud between the cotyledons, called the plumule.
The cotyledons are the first leaf-like organs of the plant. The
hypocotyl is the first stem, and the plumule is the first bud. No
root is found in the embryo ; but when the seed germinates the
hypocotyl elongates, and from its basal tip the primary root de-
velops. The cotyledons at first absorb food from the endosperm,
expand, and when exposed to the light turn green and carry on
photosynthesis. The plumule grows upward and forms the stem.
These early stages of growth use up most of the food in the
endosperm.

The lima-bean seed consists merely of the embryo, with a seed
coat inclosing it. The food in this seed has already been absorbed
into the embryo and stored in the greatly thickened cotyledons ;
that is, the young embryo continued its growth in the ovule and
absorbed all the food from the endosperm. The parts of the
embryo are the same as in the castor bear , but the cotyledons are

U.S. Dept. of Agriculture

165. Avocado, or alligator
pear, a salad fruit now being grown
in southern Florida and California.

2S8

General Botany

U. S. Dept. oj Agriculture

Fig. i66. Coffee berries, natural size. Each contains two seeds.

thick and contain a great supply of food. The bean is an example
of a large group of plants, including the pea, squash, apple, and
pumpkin, in the mature seeds of which the endosperm is lacking.

A grain of corn is an example of a third kind of seed. In it
there is a large endosperm, with a small embryo near one end of
the seed. The embryo differs from the embryos of the bean and
the castor bean in that it has only a single cotyledon, wrapped
more or less around the hypocotyl and plumule. The plumule
grows upward and forms the aerial shoot. As in the castor
bean, the cotyledon is the absorbing organ through which the
foods in the endosperm enter the young plant, but in the corn
the cotyledon is not forced out of the soil by the elongation of the
hypocotyl.

The flower and embryo in monocots and dicots. In discussing
the subject of stems, attention was called to the fact that flowering
plants are divided into two great groups, the monocots and dicots.
The monocots have parallel-veined leaves ; the bundles of the
stem are closed (have no cambium) and are not arranged in a
circle.

The terms '^monocot" and "dicot" (or, as they are frequently
written, ''monocotyledon" and ''dicotyledon") are based on

Fruits and Seeds 259

the apparent number of cotyledons in the embryo, whether there
are one or two. Any one who has watched plants beginning to
grow in a garden will recall the two cotyledons of the bean,
pumpkin, sunflower, and radish, raised above the soil. Seeds,
of fruits, of our broad-leafed forest trees — maple, ash, tulip,
linden — may be readily secured and germinated, and they too
will be seen to be dicots. It will also be recalled that these plants
have net-veined leaves.

The cotyledon of a monocot is usually an absorbing organ that
remains below the ground in contact with the endosperm, and in
wheat, corn, and other grasses it is the first leaf that appears
above the ground — not the cotyledon. In other monocots, like
the onion, the cotyledon is leaf-like and rises above ground.

The two groups differ in their flowers also. In the monocots
the number of parts of the calyx and corolla is usually three, and
the stamens and divisions of the pistil are three or some multiple
of three. In the dicots the parts of the flower are typically in
fives or fours, or in a multiple of these.

Thus the names ''monocot" and "dicot" relate to the form
of the embryo ; but the two groups are further distinguished by
differences in leaf venation, bundle structure, bundle arrange-
ment, and flower plan.

The gymnosperms and angiosperms. We have previously
learned that the conifers bear their seeds on scale leaves arranged
in cones (page 232). A study of one of these cones shows that the
seeds are formed on the upper surface of the scales and are not
inclosed in capsules. When the scales mature and become dry,
the cone opens and the seeds fall out. The word " gymnosperm "
means "naked seed," and this is the group name for the conifers
and all other plants whose seeds are not inclosed in an ovulary.

The angiosperms are what we usually call the flowering plants,
although some of them, like the grasses and many forest trees,
produce small, inconspicuous flowers without colored parts.
The seeds of an angiosperm, in contrast to those of the gymno-

26o

General Botany

sperm, are inclosed in an ovulary commonly called a pod or cap-
sule, as in the bean, horsechestnut, hickory nut, and watermelon.

Fig. 167. Fruits frequently transported by animals: A, beggar-ticks {Bidens) ; B, Spanish
needles {Bidens) ; C, sweet cicely {W ashingtonia) ; D, tick trefoil {Desmodium) ; E, cockle-
bur {Xanthium) ; and F, sand bur (Solanum).

The term "angiosperm" means "hidden or covered seed."
The gymnosperm seed consists of an embryo, surrounded by
an endosperm (rich in carbohydrate, fat, and protein material),
and two seed coats. The embryo has several distinct cotyledons.
Separation of the seed. Seeds become free of the fruit or the
parent plant in various ways. Fruits of the akene type (sun-
flower, Spanish needle) are dry, one-seeded fruits, and are set
free at maturity by abscission from the receptacle. In the case
of some legumes, like the bean, the pods split open and the two
halves curl and twist, forcibly expelling the seeds. In the witch-
hazel the pod dries out at maturity and the outer wall shrinks
more than the inner, thus producing a tension. When a certain
tension is reached, the four-seeded capsule suddenly springs
apart, throwing the seeds several feet. In the walnut, coconut,
and many fleshy fruits (e.g., the apple) the seeds are set free only
by the decay of the fruit. The hard, resistant fruit wall of the
coconut is of advantage, since this palm is a common seashore
plant. The seeds may be carried for weeks by ocean currents
without injury. Orange and lemon seeds sometimes germinate
inside the ripe fruits.

The dissemination of seeds. The wind is probably the most
important agent in transporting seeds. How far a seed may be

Fruits and Seeds

261

carried by the air depends on the amount of surface it exposes in
proportion to its weight. The greater the surface in proportion

Fig. 168. Fruits frequently transported by wind: A, maple; B, elm; C, ash.

D. bass-

wood ; E, dandelion ; and F, clematis.

to the weight, the more the resistance it offers to falHng through
the air and the farther it may be carried by the wind. The
plumes of the milkweed, thistle, dandehon, willow, and cotton-
wood increase the surface tens, hundreds, or thousands of times
without materially increasing the weight. Consequently, these
seeds may be carried many miles if they get well up into the air
at the start. Maple, elm, ash, and catalpa have relatively large
surfaces (wings) for their mass and they are easily blown about.
The chief disadvantage of wind as a disseminator of seeds is that
so many seeds are carried to habitats where germination will not
take place.

Many seeds are transported by streams and lake currents.
During spring freshets enormous numbers of seeds are picked up
from the overflowed lands and transported downstream. After
floods in the Mississippi and other large rivers one may find nu-
merous rows of seedlings extending along the sides of the valley at
definite levels, marking the height of a rather prolonged stage of
high water, during which the seeds were washed ashore. Similar
lines of seedings are not uncommon along the shores of the
Great Lakes at certain times of the year. Here they are soon
destroyed by storm waves. The seeds most commonly trans-
ported by water are those of water, shore, and bottom-land plants.

262

General Botany

Seeds are transported by animals in several ways. They may be
inclosed in fruits like the burdock, cocklebur, and Spanish needle,

and become entangled in the
fur coat of the animal. They
may be eaten and, due to im-
pervious seed coats, survive the
digestive juices of the animals.
They may be carried and buried
by squirrels, ground squirrels,
and gophers. The walnut, but-
ternut, and hickory nut have no
other means of being carried
away from the parent plants.
Small seeds that have fallen on
the muddy banks of ponds and
streams may be carried by water
birds in the mud that clings to
their feet. The mistletoe pro-
duces seeds with an outside
sticky coat. These also are said
to be spread to other trees by
adhering to the feet of birds.
Finally, the greatest of all trans-
porters of seed are human be-
ings. Wherever man goes there
follows shortly in his trail a host
of weeds. His ships carry them
across the oceans, and his railroad trains scatter them over
the land. The continual shipment of agricultural and horti-
cultural products of necessity leads to the spread of seeds of
various other plants that grew with them. More than half of
our weeds have been introduced from Europe in this way.

Economic importance of seeds. Seeds and grains supply the
most concentrated foods derived from plants. They provide

Fig. 169. Development of mangrove
seedlings. This small tree grows on soft
mud flats in the tropics and semi-tropics.
The seed {A and C) germinates while
still attached to the tree and forms an
embryo a foot or more in length. The
embryo finally drops endwise like an
arrow into the mud below and starts a seed-
ling (D).

Fruits and Seeds 263

the larger part of the food of all human beings. Seeds of cotton,
corn, and coconut furnish enormous quantities of oils used in the
manufacture of various fats and soap, and nuts of various kinds
are coming to be used more and more extensively as foods. Corn
oil is widely used in the making of a rubber substitute. Flax-
seed is the source of linseed oil, which is used in the manufac-
ture of paints. From the grains we derive also starch, glucose,
alcohol, ether, and many related organic substances. The seeds
of the coffee and cacao plants supply pleasant and mildly stimu-
lating drinks. The hairy covering of the cottonseed is the most
important fiber used in the manufacture of cloth.

PROBLEMS

1. WTiy does a corn plant growing alone produce imperfect ears?

2. When cucumbers are grown in commercial greenhouses, how is pollination

accomplished ?

3. Why are large, heavy seeds of agricultural plants more desirable for planting

than small, light seeds?

4. What common market " vegetables " are included in the botanical term " fruit " ?

CHAPTER TWENTY-EIGHT
DORMANCY AND GERMINATION OF SEEDS

Although the embryo is made up almost wholly of meriste-
matic tissue, seeds do not usually germinate as soon as they are
mature. In nature only a very small percentage of embryos
germinate and grow to maturity ; most of them either fail to
germinate, or they perish in the seedling stage.

Dormancy. When a seed does not germinate immediately
after leaving the parent plant, it is spoken of as being in a state
of dormancy. The seeds of most wild plants do not germinate
immediately after ripening, even though external conditions are
favorable. The seeds of many cultivated plants, on the other
hand, have little or no dormancy and will germinate during wet
weather even before they leave the parent plant. Corn germi-
nating in the ear, and wheat and oats sprouting in the shock, are
common occurrences in extremely wet weather. The lack of
dormancy in seeds of cultivated plants is due, in part, to the fact
that man has unconsciously selected those seeds that germinate
readily. Obviously, only those seeds that germinate soon after
sowing produce mature seeds at harvest time. In the long his-
tory of agriculture those individuals and races of plants whose
seeds did not germinate readily were largely eliminated.

An interesting example of the difference in dormancy be-
tween wild and cultivated species may be seen in the Dakotas.
The common oats and wild European oats both occur and produce
seeds in summer. The cultivated oats, however, germinate in
early autumn and the seedKngs freeze and die during the cold
winter. The seeds of wild oats do not germinate until spring,
and consequently they become troublesome weeds in grain fields.

Sometimes the period of dormancy is very short, as in the seeds
of willow, elm, cottonwood, and other spring-fruiting trees.
Seeds of this type do not withstand drying and consequently
never germinate unless they fall in a moist habitat. The soft

264

Dormancy and Germination of Seeds 265

maple is another seed of this kind. When its water content falls
below 30 per cent, it dies.

Dormancy is generally more pronounced in seeds produced in
late summer and autumn. Even when the seeds are kept in
conditions usually favorable for germination, they will not germi-
nate for several months — sometimes not for several years.

External causes of dormancy. Dormancy may be due to
various causes. Some of these causes are environmental, others
He within the seed itself. In late autumn and winter the tem-
perature may be too low for germination. This condition often
prevails when the seeds of many late-flowering plants are mature.
On the other hand, the temperature may be too high, as it often
is when the seeds of cool temperature species of the desert mature.

The seeds may fall on ground that is too dry, or they may fall
to the bottom of a pond where the oxygen content is very low or
insufflcient. They may be covered with earth, under conditions
that exclude oxygen entirely. Some seeds will not germinate in
the absence of Hght. Unfavorable temperature, too Kttle water,
the insufflciency of oxygen, are the most common external causes
that prevent germination in nature.

Internal causes of dormancy. The failure to germinate, even
when external conditions are favorable, often depends on certain
characteristics of the seed coats or of the embryos. It is usually
possible to break dormancy when the cause is known, a fact that
is of great importance to horticulturists in their work of growing
seedlings. Many of these specific causes have been determined ;
some of the most important are given below.

I. Seed coats impermeable to water. One of the commonest
causes of dormancy is the exclusion of water by hard seed coats.
The seeds of many water plants, and also of plants belonging to
the legume family (clover, alfalfa; and lupine), have coats of this
character. When placed in water no absorption takes place, and
germination is therefore impossible. In some species all the
seeds are hard and impervious to water ; in others, only some of

266 General Botany

them. In either case the germination of a given crop of seeds,
under natural conditions, would take place over a series of years.
Red clover seeds, for example, when placed in water do not all
swell at once, but only a few at a time ; some may remain hard,
yet alive, at the end of ten years. It has been found that all these
seeds will germinate at once if the seed coats are scratched, broken,
or removed by being shaken together with sharp sand. Ninety
per cent of these clover seeds may be germinated immediately
also by immersing them in boiling water for 50 to 60 seconds, or
by pouring strong sulfuric acid over them and then after 30 to
50 minutes carefully washing them with a 5 per cent solution of
sodium bicarbonate (baking soda) and water. In this kind of
dormancy the embryo is in a condition to grow when mature,
but it cannot do so until the seed coat allows water to enter. In
nature, freezing and thawing, bacteria, and fungi are important
agencies in breaking the coats of this class of seeds.

2. Seed coats mechanically resistant. In many seeds the seed
coat is a membrane strong enough to prevent the expansion of
the embryo by the absorption of water. Although such seed
coats are permeable, when put in water only a small amount
enters. The seeds of the common pigweed, water plantain,
black mustard, shepherd's-purse, and peppergrass furnish ex-
amples of this kind of dormancy. In nature the dormancy of
such seeds is broken by chemical changes which weaken the
seed coats as they lie on the soil. Dry storage promotes changes
of the same kind, but if the seeds are stored in water or wet soil
they remain dormant for many years. This explains why they
are such persistent weeds in gardens, even in gardens that are
kept clean for a number of years. So long as the seeds lie deep
in the soil they will not germinate ; but when brought to the sur-
face by cultivation, the coat dries out, and as soon as they are
again moistened germination follows.

3. Seed coats impermeable to oxygen. In a third class of seeds
dormancy is brought about by the exclusion of oxygen. The

Dormancy and Germination of Seeds 267

seed coats or fruit coats inclosing these seeds are either imperme-
able to oxygen or at least retard the entrance of oxygen to such
an extent that germination cannot take place. The best-known
example of this class of seeds is the cocklebur, a weed very diffi-
cult to eradicate in low grounds. The dry fruit of the cocklebur
contains two seeds, an upper and a lower. In nature the lower
seed usually germinates in the spring following maturity, while
the upper seed does not grow until later, or not until the second
spring. Both seeds require considerable oxygen to germinate,
but the upper requires about one third more than the lower. So
long as the seed coats remain intact, no germination is possible ;
but if the embryos are removed, they germinate at once. In
nature, freezing and thawing and the action of the bacteria and
fungi of the soil alter the seed, so that after a few months suffi-
cient oxygen penetrates the lower seed to start growth in the
embryo ; but it requires more than a year for the seed coats of
the upper seed to decay, or to become altered sufficiently for
enough oxygen to reach the embryo to induce growth. Many
grasses — for example, the Johnson grass — and many com-
posites have seeds whose behavior is similar to that of the cockle-
bur. Seeds of this kind have been found to germinate rather
readily when kept for a time at a temperature above 120° F.

4. Embryos requiring acidity. The seeds of the peach, haw-
thorn, red cedar, hard maple, basswood, ragweed, and to a less
extent apple seeds, will not germinate when first matured, even
if the seed coats are removed. In these seeds dormancy is due
to the condition of the embryo. Experiments have shown that
the stem-part (hypocotyl) of the embryo is neutral or alkaline in
reaction in the mature seed, and that dormancy lasts as long as
this condition lasts. As soon as the whole embryo becomes acid
in reaction, the seed will germinate.

Seeds of this class may be germinated at once by removing
the seed coat and immersing the embryos in weak acids. It has
long been the practice of horticulturists to '' layer " such seeds;

268 General Botany

that is, place them under a thin layer of soil over winter. In
the spring they germinate readily. The same seeds kept at
room temperature and then planted will not germinate for a
much longer period. A low temperature (about 40° F.) is most
favorable for the development of acidity in the growing embryo.
Acidity favors growth because it increases the ability of the cells
to take up water, and it favors the production and activities of
enzymes that are necessary for digestion, assimilation, and
respiration.

5. Imperfectly developed embryos. A fifth form of dormancy
occurs in many seeds in which the embryo is only partly developed
when the seed matures. Sometimes it is little more than a
fertilized egg; but one may find all gradations between these
few-celled embryos and those that are nearly or completely
grown. They occur among a wide range of plants, but most of
these plants are not commonly known, except the ginkgo, holly,
buttercup, dogtooth violet, and many orchids. Orchid seed,
formerly thought to germinate only in the presence of certain
fungi, may be germinated in a sugar solution, which furnishes
both the water and the food material necessary to complete the
growth of the embryo.

Longevity of seeds. How long do seeds live in a dormant con-
dition? This question is frequently asked, and in connection
with it there are many stories told that are either based on wild
guesses or are merely fiction. Unscrupulous individuals have
at times taken advantage of people by these fictitious stories and
made large profits by selling small vials of ordinary seeds at high
prices. '' Miracle " wheat, purporting to have been taken from
mummy cases in Egypt, had a wide sale among unsuspecting
farmers until investigation showed the true source and the
worthlessness of the seeds.

There are few unquestionable records of seeds germinating
after 100 years of dormancy ; even those germinating after
periods much shorter than 100 years are rare. The seeds of a

Dormancy and Germination of Seeds 269

few legumes have germinated after storage for 80 years. Experi-
ments have shown that the seeds of a number of our common
weeds will withstand burial deep in the soil for more than 30
years. The seeds of water plants will remain alive under aerated
water for the same length of time. The seeds of several land
plants have remained alive under water for periods of from 4 to
12 years. It is therefore safe to say that the seeds of most culti-
vated plants deteriorate rapidly after 2 years; that the seeds
of many wild plants remain alive for 5 to 10 years ; and that a
few may live under favorable conditions 25 to 50 years. No
seeds are known to have remained alive 200 years.

Storage of seeds. One of the important conditions for the
storage of most seeds is that they be kept dry. When seeds are
stored for long periods in soil, the absence of germination seems
to depend on lack of sufficient oxygen. The same is probably
true for storage under water. Seeds like those of the soft maple
live longest when kept cool and moist.

In general, seeds may be kept longer and show greater vitality
if they are thoroughly mature when harvested. Corn and wheat
seeds lose their vitahty rapidly when not mature and well dried
out. The seeds produced in wet seasons usually show poorer
germination than those produced in dry seasons. Moreover,
corn matures late in the autumn, and unless its water content
falls below 20 per cent before killing frosts come it is sure to be
injured. Corn that is to be used for seed the following year should
be gathered as soon as mature and placed in racks, so that it will
dry out rapidly.

The changes that take place during storage that lead to the
death of the embryo have been much studied. One important
fact discovered is that during prolonged storage the proteins are
gradually coagulated or changed into insoluble forms, so that
when the seeds are planted the proteins do not become soluble
and the protoplasm dies. Seeds will not remain alive, therefore,
after their proteins have coagulated. Since this takes place

270 General Botany

more rapidly at a high temperature than at a low, with other
conditions favorable, seeds will keep longer when the temperature
is low.

External conditions necessary for germination. We have
already discussed some of the internal conditions necessary for
the germination of seeds. The seed coats must be permeable to
water and oxygen, and they must allow complete swelling of the
embryo. The embryo must be fully grown and in some cases in
an acid condition.

The first external condition necessary for germination is abun-
dant moisture, but there should not be enough moisture to inter-
fere with the access of oxygen. Water is needed to bring about
the swelling of the cells and tissues ; to dissolve various salts,
sugars, and other organic substances in the cells ; and to facihtate
chemical changes in the cells.

Oxygen is needed for respiration. Oxidation liberates energy
for chemical changes in the cells. The respiration of germinating
seeds goes on at a very high rate ; when compared with that of
human beings the rate is several times as great. Human beings
give off carbon dioxide equivalent to 2.5 per cent of their dry
body weight in 24 hours ; germinating seeds may give off from 5
to 20 per cent of their weight of carbon dioxide in a day.

Seeds are planted as near the surface of the soil as possible to
insure an adequate oxygen supply. They are planted below the
surface to insure a sufficient supply of water. As the relation of
water and oxygen to soil particles varies greatly in different soils,
it is evident that to obtain a sufficient amount of both oxygen and
water, seeds must be planted deeper in some soils than in others.
We may plant them deeper in loose sandy soil, for example, than
in tight clay.

The third important external factor for germination is tempera-
ture. Temperatures favorable for germination are usually lower
than those for the subsequent development of the plant. But as
few seeds germinate much below 50° F., the temperature of the

Dormancy and Germination of Seeds 271

soil should be at least as high as this when seeds are planted.
If the temperature is much lower, the vitahty of the seedling is
reduced, and the plant is then more readily attacked and injured
or destroyed by bacteria and fungi. On the other hand, when
germination occurs in soil that is higher than 70° F., many plants
form very poor root systems ; consequently the growth of the
plant is retarded.

The germination of most seeds takes place equally well in light
or in darkness. Light retards the germination of some seeds,
while others, like those of bluegrass, certain varieties of tobacco,
mullein, and mistletoe, germinate better in the light.

Seedlings. Recent experiments have definitely proved that
large, vigorous plants develop only from vigorous seedlings. In
many plants large seeds produce better seedlings than small
seeds. Therefore, in order to produce the best plants we must
start with seeds of good quahty and we must make sure that the
seedlings are not interfered with during their early development.
Planting many seeds in a row and then removing all but the most
vigorous of the seedlings is, therefore, good agricultural practice.
Removing weeds and keeping the soil porous and fairly moist
keep up the water and oxygen supphes for the roots and prevent
any interference with the hght that the seedlings should receive
for food manufacture.

CHAPTER TWENTY-NINE

PLANT BREEDING

The origin of our most important cultivated plants is in most
instances shrouded in mystery, for they were brought into culti-
vation by prehistoric peoples. When Columbus discovered
America, the Indians of the New World were cultivating corn,
potatoes, cotton, kidney and lima beans, arrowroot, peppers,
peanuts, pineapples, tomatoes, tobacco, sweet potatoes, squash,
pumpkin, and a number of tropical food and fiber plants. The
other important food plants, like wheat, rice, barley, rye, and
oats, were mostly selected from wild species by the prehistoric
races of Asia and Africa. It is a singular fact that within historic
times no important additions have been made to the food plants
of the world except through borrowing from the so-called primi-
tive races. During historic times, however, these food plants
have been greatly modified and innumerable superior varieties
have been developed. Among plants that produce edible fruits,
berries, and nuts, some additional species have been brought into
cultivation during recent centuries — notably grapes, cran-
berries, raspberries, dewberries, cherries, plums, and pecans.

Objectives in plant breeding. Plant breeding is concerned
with the improvement of economic plants, with the discovery of
new varieties, and with the production of new plants of economic

A. B. Stout

Fig. 170. An ear of white sweet corn partly pollinated by pollen from black Mexican sweet
corn. The color is in the endosperm (xeniophyte) and was produced by the factor for color
carried by the sperm nucleus which furnishes one of the three sets of chromosomes in the
endosperm nucleus.

272

Plant Breeding 273

New Jersey Expt. Sta.

Fig. 171. Three new varieties of squashes produced by crossing a white scallop summer
squash {P, at the left side of picture) with a warty, yellow-colored summer crookneck {P,
at right side). The photograph shows three new varieties that have been produced. The
upper row shows a type of short-necked "jug" fruit of medium size with a smooth, cream-
colored surface. The middle row shows a longer-necked type of "jug" fruit, somewhat
like the crookneck in shape, but greens iriped and not warty. In the lower row the fruits
are very thin-fleshed, nearly spherical, cream-colored, and not warty. After the first cross-
ing, the plants were selected and self-bred for five generations, after which some of the new
kinds would breed true enough to make new varieties.

value. Plant breeding is actively carried on at the State Agri-
cultural Experiment Stations, by the United States Department
of Agriculture, by experimenters at several of the larger univer-
sities, by seedsmen, and by breeders of nursery stock. The activi-
ties of plant breeders are being directed toward four principal
objectives : (i) the breeding of plants with more desirable prod-
ucts, as flowers, fruits, leaves, and fibers ; (2) the breeding of new
varieties which will increase the yield per acre ; (3) the securing
of varieties better fitted to particular cHmates and soils; and
(4) the producing of varieties capable of greater resistance to
diseases.

Breeding for more desirable products. The first object of
plant breeding may be illustrated by recent improvements of the
pecan. Among the hundreds of thousands of pecan trees scat-
tered through the Southern states, a few trees have been dis-
covered that produce nuts of large size and good flavor and with
thin shells. Breeders have found that these types may be pre-
served by budding pecan seedlings with buds from the most
desirable trees. The best paper-shell pecans on the market are

274

General Botany

now grown in orchards started in this way from perhaps a single
tree.

The most valuable fiber plant known is a new variety of " long-
staple " upland cotton. It was produced by hybridizing or
breeding together two well-known varieties of Egyptian cotton.
Among the numerous varieties obtained from this cross was one
whose seeds were covered with hairs an inch to an inch and
a half in length. This variety has been propagated by the
United States Department of Agriculture and is now widely
grown.

The Concord grape is now grown in most temperate regions
of the earth. It was produced by Ephraim Bull in New York
by crossing two wild species, and was one plant selected in 1853
from among 22,000 seedlings tested.

Breeding for increased yield. The way in which the yield per
acre may be increased is strikingly illustrated by a tobacco dis-
covered and distributed by the Connecticut Experiment Station.
The usual varieties of tobacco develop about twenty leaves and

Fig. 172. Fiber from new varieties of long-fibered cotton at the right, obtained by hybridiz-
ing and selecting progeny from the two forms producing the shorter fibers at the left. The
hybrid offspring excel both parents in the length of fiber produced.

Plant Breeding

275

then produce a flower cluster. The new varieties found by the

plant breeder occurred as a few scattered plants, among the

hundreds of acres grown in the state.

These plants had indeterminate growth ;

that is, the stem continued to produce

leaves until the end of the growing season.

Seeds were secured by transferring the

plant to the greenhouse. The average

number of leaves on these plants is

seventy, and the yield of tobacco per acre

has been increased 90 per cent. Since

the cost of growing a crop is nearly the

same in both cases, the increased yield is

largely added profit. (See also page 205.)

By the selection and propagation of
timothy plants of large size for a few
years, the Cornell Experiment Station was
able to furnish seed to growers of hay
which increased the yield 36 per cent
over ordinary timothy.

At the Maine Experiment Station ex-
periments in the breeding of oats led to
the separation of varieties w^hich gave a
yield of 80 bushels per acre, where the best commercial varieties
produced but 75 bushels.

Better varieties for particular climates. The Florida Velvet
Bean was formerly confined to Florida and the Gulf Coast. By
the selection of early varieties which suddenly appeared at several
different places, the plant can now be grown throughout the cotton
belt, and there are in our Southern states more than six million
acres devoted to this crop.

Plums suited to the Northern Plains region have been produced
by crossing the Japanese and European plums with the wild plum
of the region, and selecting the best of the resulting hybrids.

Fig. 173. Variations in
length of timothy spike.

276

General Botany

Wheat, barley, oats, and rye have all had their areas of culti-
vation extended by the discovery of, or production of, new-
varieties with qualities which enabled them to be grown in other
climatic regions.

Greater resistance to diseases. A striking example of breeding
for resistance to a plant disease is the successful production of
watermelons resistant to " wilt." All the edible varieties were
highly susceptible to this disease. By crossing the Eden variety
of watermelon with an inedible citron which was highly resistant,
hybrids of great vigor and productiveness were produced. After
8 years of selection and trial a uniform edible variety was isolated
which possessed the good qualities of the Eden watermelon and
the wilt resistance of the citron. Curiously enough, this resist-
ance is maintained throughout the eastern United States, but
in Cahfornia the new melon is susceptible. This emphasizes the
importance of breeding plants for particular regions.

\gric . Expt. Sta.

Fig. 174. In a field of cabbage that was almost entirely destroyed by yellows, a plant that
had formed a good head was found. This plant was slaved for seed.

Plant Breeding

277

Univ. of Wis. Agrtc. Expt. Sta.

Fig. 175. The rows of cabbage at the right were grown from seed from resistant stock.
They have inherited the power of the parent plants to resist the disease. The plants on
the left are from ordinary seed.

Successful disease-resistant plants have been discovered and
bred among cabbage, tomatoes, asparagus, potatoes, cowpeas,
flax, wheat, and cotton.

The basis of breeding. Plant breeding with these purposes in
view is possible and profitable because (i) variations naturally
occur among plants ; (2) some variations are inherited and may
be preserved by selection and propagation ; and (3) different
varieties and species may be crossed, producing hybrids having
a still larger range of variations than the parent plants, or possess-
ing new combinations of desirable qualities which may be selected
and preserved.

The methods used by the plant breeder depend upon the repro-
ductive structures and habits of the particular plants with which
he is working. For example, the means by which a plant is
naturally pollinated will determine how it must be handled at
the time of flowering to secure self-pollinated seed or cross-
pollinated seed.

Plants like wheat self-polHnate naturally. A particular variety
may, therefore, be readily kept pure; and the seeds produced come

278

General Botany

true when planted. Seeds from self-pollinated corn do not pro-
duce the most vigorous plants and the largest yields. The best
corn seed is obtained by planting in rows and removing the
tassels (the staminate flowers) from alternate rows. The seed
for the next year is then collected only from the detasseled plants.
In this way the breeder is assured that the pollen came from another
plant and that the plants grown from these seeds will have the
increased vigor that is characteristic of cross-polhnated corn.
Sunflowers are not self-fertile. That is, even though pollen does
fall on a stigma of the same flower cluster, fertilization does not
occur and no embryo is formed. In plants of this type pollen
must be carried from one plant to another. These three examples
illustrate some of the details of reproduction with which the plant
breeder must be famihar before he can intelHgently engage in
plant-breeding work.

Methods of vegetative propagation may be used to multiply

Bureau of Agriculture, P.I.

Fig. 176. Varieties of lima beans, showing differences in size, shape, and color.

Plant Breeding

279

Bureau of Agriculture, P.I.

Fig. 177. Four types of corn {Zea mays) : sweet, dent, flint, and pop — mutant varieties
that have been selected, each for a particular quality, since prehistoric times.

perennial plants after a desirable variety has been produced.
In this way the plant grower avoids cross-polKnation and the
variations that appear when many cultivated crops are grown
from seed. The best ways of discovering, selecting, hybridizing,
and propagating particular crop plants may be found described
in recent pubhcations devoted to plant breeding. Much Htera-
ture on the subject may be secured from your state experiment
station and from the United States Department of Agriculture.

Babcock and Clausen.
Company.

REFERENCE

Genetics in. Relation to Agriculture.

McGraw-Hill Book

CHAPTER THIRTY

VARIATIONS AND MUTATIONS

No two fruits, flowers, or other plant organs are exactly alike.
The variations may be small or large, and there may be every
gradation between the extremes of any character. The several
thousand sunflowers that might be grown from a pound of seed
would vary in height of stem, amount of branching, and size
of flowers. Not only may there be variations in the structures
of plants, but there may also be variations in the composition of
the plant organs. For example, the great variety of colors, flavors,
and other qualities of apples is due to variations in the chemical
composition of this fruit. The variation in each of these char-
acters is quite independent of variations in other characters. A
thorough knowledge of the possible kinds of variations that
occur in plants is a necessary preliminary to progress in dis-
covering their causes and in utilizing them in plant breeding.

Variations due to environ-
ment. Many variations are
due to the environment. When
a crop plant like corn is grown
in rich soil and in poor soil,
there are great differences in
the size of the plants and in
the yield per acre. These
variations in size and yield are
due to the environment. Even
though the seed planted in
each kind of soil is exactly the
same, there will be wide dif-
ferences in the plants. Ex-
amples of these variations
were discussed in the chapters
on ecological variations of
280

Fig. 178. Two plants of sweet corn of the
same variety, one grown in poor soil and
one in soil to which fertilizer was added.
The differences in the plants are due to the
environment.

Variations and Mutations

281

stems and leaves and in the chapter dealing with environ-
mental factors.

Fig. 179. Varieties derived from the wild cabbage (F), a native plant of Europe.
A is kohl-rabi, B cabbage, C cauliflower, D kale, and E Brussels sprouts.

Temperature, moisture, light, mineral salts of the soil, all have
effects upon the plant. Consequently, when plants having the
same hereditary qualities are grown in dissimilar habitats, the
different environment brings about marked variances in the ex-
pression of those qualities. Variations of this kind are not
inherited, although when the vigor is decreased by the conditions
in which the plant is grown, the plants of the next generation
may get a poor start by less vigorous seedlings and show at matu-.
rity some of the effects of unfavorable conditions to which the
previous generation was exposed.

Fluctuations. Variations due to differences of the environment
are often called fluctuations, and we can now associate many of
them with the particular external factors which produce them.
Another class of fluctuations are those which appear to be due to
unknown internal causes. The leaves that occur on a mulberry
tree, for example, may vary from leaves that are almost perfectly
heart-shaped to those with severahlobes. The number of leaflets
that makes up a compound leaf of the horse chestnut, walnut,
ailanthus, and sumac varies somewhat widely. In a California
privet hedge one finds branches usually with leaves opposite,

282 General Botany

U . S. Oepl. oj Agriculture

Fig. 180. Tobacco plants of the same variety grown from large, medium, and small seeds,
showing the relation between the size of the seed and the size and vigor of the seedling. Is
the difference in size in the plants due to environment or to differences in the plants them-
selves?

but an occasional branch has three leaves at a node, and another
has only one leaf at each node. Variations of this kind are not
heritable. Perhaps they are mere accidents of development.

Heritable variations. These variations are the result of he-
reditary differences in the nature of the plant. For example,
dwarf nasturtiums were variations that occurred among the com-
mon tall forms. The tall nasturtiums showed fluctuations in
height, but all the fluctuations were near a certain size. The
dwarfs were very different in size at the start, and when bred
among themselves it was found that the small size is heritable.
Similar variations have resulted in the production of dwarf peas
from tall peas, bush beans from the pole beans, bush squashes
from squashes with long, trailing stems. The differences between
variants are not always so great as in the examples just mentioned.
Indeed, the amount of variation may be very small. The several
varieties of hma beans, for example, show only sHght differences
in shape and size. The varieties of mock orange show small
differences in the form of the flower and leaf. The important
point about these variations is that the particular characteristics

Variations and Mutations 2S3

of each are inherited and appears in successive generations.
Heritable variations are the result either of mutation or of
hybridization.

It is possible to get all kinds of combinations of different char-
acters, and by careful hybridizing and selection to combine many
desirable qualities in a single plant. The Shasta daisy, for ex-
ample, was made by breeding together the English, American,
and Japanese daisies, and combining in one plant the pleasing
foliage of the English species, the free-blooming habit of the Amer-
ican daisy, and the waxy luster of the petals of the Japanese plant.

Variations of all kinds are of interest to the plant breeder,
because he must learn to distinguish between the two kinds. His
attempts to develop a new variety from plants having certain
qualities will be futile unless he is dealing with heritable varia-
tions. It is not always a simple matter to discover the nature
of a particular variation. It may require careful breeding ex-
periments carried through several generations to determine
whether a variation is due to environment, to heritable causes,
or to internal non-heritable causes.

Mutations. Sometimes, among many thousands of individuals
a single plant appears which is markedly different from all the
others. For example, a few years ago a sunflower was discovered
that had some red pigment near the base of the otherwise yellow
corollas. Among the millions of sunflowers that have been seen,
this was the first one in which a red color was noticed. In some
unknown way there was produced in this plant a red pigment not
formed in other sunflowers. From the seeds of this plant there
were developed other plants having red pigment in their flowers.
Evidently the new character is inherited and these sunflowers
have a chemical constitution which enables red pigment as well
as yellow to be formed. The sudden appearance of the sun-
flower with the red pigment is an example of mutation. Indi-
viduals that first show new characters are called mutants (Latin :
mutare, to change).

284

General Botany

T. D. A. Cockerell

Fig. 181. ihe red sunflower, a color mutant from the common sunflower of the

plains {Helianthus annuus).

What the plant breeders have long known as " sports " are the
rare mutants in which notable changes have occurred. They
show new characters, and these characters are inherited. Con-
sequently, their discovery is of the greatest importance. How-
ever, mutations are not necessarily large, and the term mutant
is applied to any variant showing a distinct heritable character.

The many modern varieties of tomato have been developed
from mutations that occurred among the currant tomatoes or
love apples grown for ornament in our great-grandmothers'
gardens. The original fruits resembled large red currants.

Variations and Mutations 285

Today single tomato berries may weigh a pound. In color they
may be red, yellow, or pink, and in shape they may be spherical,
plum-shaped, or flattened. They exhibit at least three types
of leaves and two types of stems. The characteristics due to
mutation are inherited, no matter what the soil and climatic
conditions may be.

Bud mutations. Mutations occur not only among plants
grown from seed but also among plants, or plant parts, developed
from buds. These are called hud mutations or hud sports. On
fruit trees one branch will occasionally produce fruit that is of
different quahty from the fruit produced on other branches. If
the quality of the fruit is superior, these branches may be used
in budding and grafting to preserve the variety. Known bud
sports are comparatively rare, but it is estimated that at least
several hundred horticultural varieties have originated from
them. In this country the improved varieties of seedless or
navel oranges have been secured entirely by this method. The
Boston fern and its forty or more varieties originated in bud
mutations from a wild tropical fern. In the potato and some
other plants that are usually propagated vegetatively, bud
variations are known to occur ; but they are so difflcult to dis-
cover in plants of this kind that they have not been of much
practical value.

CHAPTER THIRTY-ONE

HYBRIDIZATION AND SELECTION

The crossing of two species or varieties of plants is known as
hybridization. It is brought about by transferring the pollen
from one plant to the stigma of the other plant, which ultimately
results in cross-fertilization. The plants grown from seed pro-
duced in this way are called hybrids. Hybrids may resemble
one of the parents, or they may have some characters of both
parents. In the second generation derived from crosses some
plants show a wide range of variations, with all possible combina-
tions of the characteristics of the parent plants. Successful hy-
bridization, therefore, increases the number of variations avail-
able for selection by the plant breeder.

Hybrid vigor. Hybridizing often has a physiological effect
which is of importance, for in many plants it increases the vigor
of the offspring. This may result in increased yield of grain, or
in greater resistance to disease organisms, to drought, and to the
effects of high or low temperature. Sometimes the vigor is ex-
pressed merely in the size attained by the hybrids. For example,
the hybrids secured by crossing the American sunflower and the
Russian sunflower, neither of which is over lo feet in height,
grow to a height of 15 feet. In a series of experiments with corn,
hybrids gave an average yield more than 50 per cent greater
than the average of their parents.

Mendel's experiments. To see more definitely what may hap-
pen in hybridization, we may review an experiment performed by
Gregor Mendel about the middle of the last century. Mendel
crossed a tall variety of the common garden pea with a dwarf
variety. In this way he secured hybrids which had received some
characters from each of two different parent plants. One of
the most important discoveries he made was that in plant breed-
ing every feature of a plant must be studied separately. Consid-
ering height growth alone, he found that when he planted the
seeds secured from the cross all the plants grew tall. This first

286

Hybridization and Selection

287

hybrid generation is called among breeders the " first filial ''
(Latin : filialis, related as a child), or Fi generation.

Mendel concluded from this and many similar experiments
that in any pair of contrasting characters one usually appears
to be unmodified by the other. In other words, one of the two
characters shows in the Fi plants, and the other does not. In the
experiment with peas cited above, tallness dominates over dwarf-
ness. Mendel, therefore, called that character which appeared in
the Fi generation dominant, and the contrasting character which
did not appear recessive. Working further with peas, Mendel
discovered that purple flowers are dominant over white flowers,
smooth seeds over wrinkled seeds, and yellow seeds over green.

The second important fact brought out by Mendel is that when
the Fi plants are self-pollinated and the resulting seeds planted,
an F2 generation is obtained in which both tall and dwarf plants
occur. Furthermore, there is a definite ratio of 3 to i between the
number of tall and dwarf plants. The results of these experi-
ments may be expressed in the following diagram :

(Pollen parent)
TaU

(Ovule parent)
Dwarf

Fi generation =
Self-pollinating

F2 generation =

Self-pollinating
F3 generation =

All Tall
(Tall X Tall)

/ 1 \ \

(A)
Tall

Pure Tall

Always breeds

true

(B)
Tall

\ \ \

3 Tall and 3 Tall and Pure Dwarf
I Dwarf I Dwarf Always breed

true

Evidently the factor for dwarfness is carried along with the
factor for tallness in the Fi generation, but the tallness dominates
it and the dwarfness does not show. In the F2 generation there

288 General Botany

are two kinds of tall plants: (A) those that breed true and
(B) those that, like the hybrid tails of the Fi generation, have the
dwarfness latent in them. These latter plants produce three
tall and one dwarf in the F3 generation. The recessive dwarfs
that come out in the F2 generation always breed true. Hence,
in the F2 generation one fourth of the plants are pure tails, two
fourths are hybrid tails, and one fourth are pure dwarfs.

A plant that breeds true for any character when self-pollinated
is said by breeders to be homozygous with respect to that character.
This implies that it carries only one kind of factor out of any pair
of contrasting characters. In the A-group of tall plants above,
both the pollen and the ovule (or more definitely the sperm and
the egg) carry only the factor for tallness. Hence all the off-
spring are tall. The other two tall plants, those of the B-group,
that appear among every four in the F2 generation, carry two
different factors and are called heterozygous. Half of the number
of their eggs and sperms will carry the factor for tallness, the
other half carry the factor for dwarfness. They produce, when
self -pollinated, plants with the dominant character and plants
with the recessive character in the proportion of 3 to i.

With these facts in mind we can rewrite the above diagram,
using T for the dominant character, tallness, and d for the reces-
sive character, dwarfness :

Sperm (or egg) Egg (or sperm)
T X d

Fi generation = Td Td

Selfing Td X Td

/ / \ \

F2 generation = TT Td — Td dd

Selfing TT X TT d X Td dd X dd

I I Y

F3 gensration = AU TT TT— Td— Td— dd All dd

Hybridization and Selection 289

Mendel concluded from these and other experiments that most
characters that make up a plant are each inherited independently
of others. He also concluded that each egg and sperm carried
but one of two contrasting characters, while the zygote which
is formed by the union of the sperm and egg will, therefore, be
either homozygous (containing two similar factors) or hetero-
zygous (containing two contrasting factors). In the latter case
the plant that develops will show the dominant character but
not the recessive. Its offspring, however, will show both char-
acters, three fourths being like the dominant and one fourth
being pure recessives.

The combinations of characters that will appear in the progeny
in the F2 generation above may be shown more clearly by a dia-
gram.

Eggs
T

perm? t

Application of Mendel's laws. Just how Mendel's laws may
be applied to a particular breeding problem will be apparent if we
follow the steps in the making of a hybrid with two pairs of con-
trasting characters. Suppose we have a tall pea with smooth
seeds and a dwarf pea with wrinkled seeds. Tall peas require a
longer time to develop and produce more peas per plant. Dwarf
peas are, therefore, earlier. Wrinkled peas are generally sweeter
than smooth peas because they contain more sugar. For these
reasons it is decided that a tall pea with wrinkled seeds would
be a desirable variety to produce. How can the plant breeder
secure this combination from the two plants in his possession ?
Knowing that tallness (T) is dominant over dwarfness (d) and
that smoothness (S) is dominant over wrinkledness (w), the
problem is a rather simple one.

290 General Botany

The plant breeder starts by cross-pollinating the two plants,
a process which may be represented as follows :

T S X d w
He wishes to secure a plant that will have the composition T w.

Sperm Egg
T S X d w
All plants have TdSw in Fi generation

When the plants of the Fi generation are mature, they will all be
tall and will have smooth seeds.

He now allows these hybrids to self-pollinate and plants the
resulting seeds. Only one of each pair of contrasting characters
will be contained in each sperm and each egg from these plants.
It will be either the factor for tallness or for dwarfness, but not
both. It will be either the factor for smoothness or wrinkledness,
but not both. Hence, there will be four kinds of sperms and four
kinds of eggs, whose cornposition may be indicated as follows :

Sperms Eggs

TS TS

Tw Tw

dS dS

dw dw

Any one of these sperms may unite with any one of the eggs, and
if hundreds of zygotes are formed the several kinds of sperms
will unite with each of the several kinds of eggs in about equal
numbers, and there will be sixteen possible combinations, which
may be seen by writing out the following checkerboard diagram.
Write first the four kinds of eggs above, then the four kinds of
sperms on the left side. Then put in each square the combina-
tion of the factors carried by the egg (indicated above) and the
sperm (indicated to the left). At the right of this diagram we
may make a second diagram giving the characteristics of each
of the plants represented by the sixteen squares :

Hybridization and Selection

291

Eggs
TS Tw dS

A
TS

Tw
Sperm

dw
D

TSi;s

2
TSTw

3
TSdS

T^w

TwTS

\6
TvvlRw

jidS

8
Twdw

dSTS

dSTw

dS^

12
dSdw

14
dwTw

dwdS

\6
d\v^w

Note that in square 6 there is represented a plant that has the
factors desired and no others. It is homozygous for both the
desired characters, tall plants and wrinkled seeds. There will
be twelve seeds out of every sixteen that will produce tall plants
in the third generation, and four that will produce dwarfs (ratio
3:1). Which one of the twelve will produce only tall plants with
wrinkled seeds can be determined only by planting the seeds of the
F2 generation and watching the characteristics and proportion
of tall and dwarf and smooth and wrinkled seeds developed
from each individual plant. If this is accurately done, the
plant breeder will be able to select the plants having the
composition represented by square 6 and his problem will be
solved.

Referring now to the diagram at the right, all the plants repre-
sented by the squares along the axis AB are homozygous and will
breed true if self-pollinated. All the plants represented by the
squares on the axis CD are exact duplicates of the hybrid or Fi
generation and will produce all the sixteen possible combinations
when self-pollinated. These four squares represent plants that
are heterozygous for both pairs of characters. The remaining
eight squares represent plants that are homozygous for one
character but heterozygous for the other. By means of similar
diagrams it is possible to calculate the chances of securing a

292

General Botany

fm^

ii^:

Fig. 182.

Diagram to illustrate the behavior of the chromosomes during
vegetative cell division. {After Strasburger.)

certain combination and to plan a definite set of crosses that will
bring the desired result in a few generations.

Importance of Mendel's laws. With the above example in
mind, it is not difficult to see that the use of Mendel's methods
has greatly simplified the problems of the plant breeder and made
possible the early attainment of desirable combinations of charac-
ters by hybridizing difTerent varieties and closely related species
of plants. It has made it possible to breed for definite combina-
tions and to determine when the combinations have been secured.

Before the discovery of Mendel's laws of inheritance, all breed-
ing was merely a matter of chance, and much of it is still being
carried on in that way because we are still ignorant of the factors
involved in many characters of numerous plants and animals.
On the other hand, many important food and ornamental plants

Hybridization and Selection 293

have been extensively studied and their hereditary behavior dis-
covered. With these studies as a foundation further progress in
improving, modifying, and combining qualities in new ways is
rapidly being made.

Cell structures and Mendel's laws. Since Mendel's time
much has been learned concerning the physiological explanation
of these laws. It is now well established that the explanation
rests on the behavior of certain nuclear structures, the chromo-
somes, during cell division, during the formation of pollen and
embryo sac, and when fertilization occurs.

Fig. 183. Root tip of onion, showing cell division and enlargement. A series of stages
showing behavior of the chromosomes during vegetative cell division is labeled a to g.

194

General Botany

Chromosomes and vegetative cell division. In ordinary cell
division in a growing tissue a cell near the growing point is more

Fig. 184. Diagram showing behavior of chromosomes in division of a vegetative eel
The daughter cells have the same number and kind of chromosomes as the mother cell.
{After Sharp.)

or less cubical. Cytoplasm and nucleus occupy all the space
within the cell wall. The nucleus is very large in proportion to
the volume of the cell. Inside the nucleus when properly stained
may be seen a tangled network of material called chromatin
(Greek : chroma, color) because it may be stained deeply by cer-
tain dyes. When the cell is about to divide, the chromatin
becomes aggregated into a single much-twisted thread. The
thread then shortens and thickens and becomes arranged in a
number of loops, and a little later it divides into a definite number
of segments, the chromosomes (Greek : chroma, color ; soma,hody).
The chromosomes are U-shaped and collect in the equatorial re-
gion of the nucleus. By this time lines have appeared in the
nucleus radiating from the two opposite poles of the nucleus,
forming the so-called spindle. Each chromosome splits longitudi-
nally, and a half moves toward each of the two poles. A little
later the chromosomes become merged at each pole, granules
appear at the equator of the spindle, and the first layer of a
transverse wall is laid down. At the former poles of the spindle
two new nuclei are organized about the chromatin. The divid-
ing wall between the cells is thickened and we have two daughter
cells in place of the original or mother cell. This process is re-
peated as often as new vegetative cells are formed in roots, stems,
leaves, or other organs.

Hybridization and Selection 295

The number of chromosomes in the vegetative cells of any
species of plant is definite, and in the cell divisions that occur

Fig. 185. Diagram showing behavior of chromosomes in the reduction division. The
daughter cells have half as many chromosomes as the mother cell. {Afkr Sharp.)

during vegetative growth each chromosome splits longitudinally
and one half goes to each of the daughter cells. So every cell has
a set of chromosomes similar to that of every other cell. Each
chromosome is the bearer of certain hereditary factors. In
vegetative multiplication the chromosome complement of cut-
tings, cions, and vegetative offshoots is the same as that of the
parent plant. Hence, the hereditary qualities of vegetative prop-
agating shoots are similar to those of the parent plant.

Behavior of chromosomes in sexual reproduction. In Chapter
XXVI (page 249) the process of fertilization is discussed and at-
tention is called to the fact that fertilization consists of the union
of a sperm and egg, forming a zygote. This fusion would lead to a
doubling of the number of chromosomes in each successive gener-
ation if it were not for the fact that in the formation of the pollen
and the embryo sac which precedes fertilization a cell division
takes place in which the behavior of the chromosomes is differ-
ent from that described for vegetative growth.

In all the complex plants the mother cells, which give rise to
the pollen and embryo sac, and ultimately the sperms and eggs
(gametes) divide by a method called the reduction division. The
word ''reduction" refers to the fact that in this division the num-
ber of chromosomes is reduced to half the number that occur in
the vegetative cells.

296 General Botany

Reduction division. In the reduction division the chromo-
somes become arranged in pairs in the early stages of nuclear
division and group themselves at the equator of the spindle,
as double chromosomes. Then, instead of splitting, as in vege-
tative cell division, the two chromosomes of each pair separate
and migrate to the opposite poles of the spindle, forming the chro-
mosomes of the new daughter nuclei. As a result each daughter
nucleus contains one half the number of chromosomes contained
in the vegetative cells of the plant.

For example, the vegetative cells of the white lily contain
twenty-four chromosomes. When the mother cells of the pollen
and the embryo sac divide, the daughter cells each contain twelve
chromosomes. These daughter cells divide again and again,
until the sperms and eggs are finally formed and each nucleus
contains twelve, the reduced number of chromosomes. When
fertilization takes place and the sperm and egg unite, each carries
twelve chromosomes. The resulting zygote nucleus contains
twenty-four. The zygote is the first cell of the plant of the
next generation, and in the further development of the embryo
and plant all the cells carry twenty-four chromosomes.

Explanation of Mendel's laws. There are many reasons for
concluding that the chromosomes are the principal carriers of the
factors of heredity, and that each individual chromosome carries
certain particular factors. We can explain the Mendelian be-
havior of hybrids if we assume that in the reduction division
each of the two chromosomes that are paired in the early stages
contains one of two contrasting factors. Then, when they
separate, one of these factors is carried to one of the daughter
nuclei and the contrasting factor to the other daughter nucleus.
In this way each sperm and each egg will contain only one mem-
ber of each pair of contrasting factors, and the plant that
develops from each zygote has its characteristics determined by
the factors contained in the chromosomes of the sperm and tgg
that united to produce it.

Hybridization and Selection

297

Fig. 186. Vegetable trial grounds of the Office of Seed and Plant Introduction, United
States Department of Agriculture, Washington, D.C.

Mode of inheritance the same in plants and animals. One of

the most remarkable facts of heredity is the similarity of behavior
of hereditary factors in both plants and animals. Not only can
we predict from the behavior of one plant what will occur in
another plant, but we can predict in many instances what will
happen in animals. This may be taken as further evidence of
the essential similarity and close relationship of plants and
animals.

Selection. The heritable variations or mutations and the
products of hybridization furnish the materials from which valu-
able new varieties of animals and plants may be isolated. The
plant breeder grows large numbers of plants derived in these
several ways in gardens which h^e calls " plots," in order to
determine what the plants do or how they will behave under a
certain set of conditions. Then, from among the hundreds or
thousands of individuals he selects those most nearly approach-

General Botany

C . S. DepL of Agriculture

Fig. 187. A field of upland cotton, in South Carolina, attacked by the wilt disease.
The fungus that causes the wilt remains in the soil for many years.

ing the ideal or standard he has in mind for further study
and testing. This process may be repeated year after year until
he has secured the desired qualities.

Mass selection. For many years selection was carried on,
particularly in the case of cereals, by what is termed mass selec-
tion. This method consists in selecting seed from all those plants
that most nearly approach the breeder's ideal. The next year
these seeds are planted and the process repeated. In this way
yields of many crop plants were improved. But the method is
one that may require a long time to accomplish results, and the
results are usually uncertain, because the end product is still a
mixture of plants with a great variety of hereditary qualities
and, therefore, lacks uniformity.

Selection of individual plants. In recent years, since the gen-
eral recognition of the importance of mutations and the Men-
delian behavior of hybrids, mass selection has been superseded by
the selection of individual plants. Instead of taking seeds from

Hybridization and Selection

299

Uim^

' . '^■''%h*' ''"'*""?^l*''f '^'

i^OJft.:^

. i^?;

Fig. 188.

U. S. Dept. of Agriculture

The same field shown hi Figure 187, planted with seeds from plants that sur-
vived the attack of the disease.

the best plants found in a field and sowing them together the fol-
lowing year, the seeds from each of the best plants are kept sepa-
rate. These are then planted separately in short rows and a
record is kept of the performance of the progeny of each of the
selected plants, and the most desirable varieties are speedily
isolated.

Since these trial rows are short, the amount of seed produced is
small ; and when a desirable strain or variety is discovered it
must be grown in larger plots C increase plots ") until sufficient
seed is obtained for distributing or marketing.

Marquis wheat. As an example of what may be accomplished
in plant breeding by individual plant selection attention may be
called to Marquis wheat, which is perhaps the most valuable
variety of wheat, and perhaps the most valuable variety of a
food plant, thus far discovered. In 1903 it was found by
Charles Saunders as a single plant derived from a cross made
several years previously between two varieties of wheat, known

300 General Botany

as " Red Fife " and " Calcutta." In 1904 there were just
twelve plants. In 1909 sufficient seed had been grown to dis-
tribute four hundred samples to farmers in various parts of
Canada. So successful did it prove to be that its cultivation
spread rapidly, and in 191 8 North Dakota and Minnesota alone
produced nearly 150 million bushels of Marquis wheat. It is
now grown in all the states from Ohio to Nebraska and Wash-
ington.

Marquis wheat has short straw, a rather short spike, and short,
broad kernels. Its straw is stiff and remains erect under un-
favorable weather conditions. It ripens on an average about
115 days after sowing, and has repeatedly won the International
Prize as the best spring wheat.

Summary. The discoveries of Mendel were announced in
1865, but their importance was not at first appreciated. In 1900
his laws of heredity were rediscovered, and this marked a new
epoch in the study of heredity and the principles underlying
hybridization. Since that time investigation has shown that
Mendel's laws apply equally well to plants and animals. The
explanation of these laws involves the behavior of the chromo-
somes in vegetative cell division and in fertilization. It is now
possible to plan breeding experiments and attain the desired
result in a fraction of the time formerly required, when breed-
ing work was merely a process of crossing, planting, selecting,
and trusting to luck for results. It is fair to say that the work
of Mendel has revolutionized plant and animal breeding.

Hybridization and Selection 301

PROBLEMS

Whj

at kind of variations are brought about by cultivating and by adding
fertilizers to the soil?

2. What kind of variations appear to have been most important in the pro-

duction of the different kinds of plants that occur on the earth?

3. How would you attempt to secure a new mutant?

4. Why are there fewer variations when plants are propagated vegetatively

than when reproduced b}^ seed ?

5. Corn is propagated by seeds. Plow would you plant and care for a corn

seed plot in order to secure loo per cent hybrid seed?

6. If a plant has three pairs of different chromosomes, how many different

kinds of sperms and eggs can it produce?

7. If you were to cross a tall white-flowered pea with a dwarf purple-flowered

pea, what would you obtain in the hybrid or Fi generation ? What per
cent or fraction of the Fo generation would be dwarf white-flowered peas>
What fraction of the Fo generation would be tall purple-flowered peas?
What fraction of these tall purple-flowered peas would be homozygous
for both characters? What per cent of the dwarf white-flowered peas
would be homozygous for both characters?
I. If you were to cross a pure tall purple-flowered pea with smooth seeds with a
pure dwarf white-flowered pea with wrinkled seeds, what chance would
you have of obtaining a pure tall white-flowered pea with wrinkled seeds
in the F2 generation?

CHAPTER THIRTY-TWO

THE DISTRIBUTION OF PLANTS IN NATURE

The distribution of plants in nature is determined by the
hereditary qualities of the plant on the one hand and by the
characteristics of the environment on the other. Some plants,
like the dandelion and sheep sorrel, have such indefinite require-
ments that they can thrive in, or at least endure, the conditions
in many different habitats. Most plants, however, have a far
more definite set of requirements, and if any one of these require-
ments is not met by a habitat, the plant is excluded from the
habitat. Orange trees, for example, cannot withstand freezing
temperatures; the low creeping Arctic willows will not grow
where the summer season is hot; cacti are excluded from soil
that is wet and poorly aerated for even a few weeks each year ;
cat-tails die on land that is not submerged at least a part of each
year; sycamore, poplar, and willow seedlings will not thrive in
dense shade ; alfalfa and certain species of clover thrive only in
well-drained soils that are neutral or slightly alkaline (many
limestone soils furnish both these conditions) ; rhododendrons,
azaleas, blueberries, and cranberries grow well only on soils that
are acid — they soon die on limestone soils.

In general, it may be said that plants requiring similar environ-
mental conditions are restricted to certain regions of a continent
and to certain habitats within these regions, because there only
are environmental conditions suited to their hereditary struc-
tures and qualities and favorable to their complete development.

Vegetation. By vegetation is meant the plant covering of the
earth or of its subdivisions. The plant covering of any region
is a great organization of hundreds, perhaps millions, of individ-
uals. Some of these are dependent only upon the conditions
determined by climate and soil. Some are also dependent upon
nitrate supply, water supply, or the presence of certain other

302

Distribution of Plants in Nature

303

U. S. Forest Service

Fig. 189. A view in a mixed mesophytic forest in eastern Tennessee. The prominent
trees in the picture are cucumber tree {Magnolia acuminata) and shagbark hickory {Carya
ovata). The deciduous forest formation approximates its best development in this region.

plants from which they derive a food supply or from which they
obtain shade.

One of the most familiar and important .t}^es of vegetation
is a forest. A mature forest consists of several stories or layers
of plants. The tallest trees form the canopy, and their leaves
are exposed to full sunlight. Below these trees there are usually
low trees and young trees that endure the shade, and are bene-
fited at least during the seedling stage by the more even temper-
ature and moisture conditions within the forest. Then there are
tall and low shrubs, some of which thrive in the forest because
of the accumulation of humus and the more constant water con-
ditions that go with it. On the floor of the forest are many her-
baceous plants and mushrooms of various kinds. Collectively,
all the low trees, the tall and low shrubs, and the herbs make up

General Botany

U. S. Forest Service

Fig. I go. Mature hemlock forest on a mountain slope in Pennsylvania. Note the
sparsity of the undergrowth.

what is termed undergrowth. Besides these plants there are mi-
croscopic plants innumerable — some on the bark of trees, some

Distribution of Plants in Nature 305

on the surface of the ground, and others below the surface. All
these plants are in one way or another affected by the other plants,
and so we may very properly speak of the forest as an organization.

When we study forests still further, we find that there are many
kinds in North America, and that when a particular kind of tree
is dominant a definite group of low trees, shrubs, and herbs usu-
ally grow with these dominant trees, and that when another kind
is dominant, a different set of plants make up the undergrowth.

Plant associations. Even a brief study of a mature forest,
such as we have just described, brings out clearly the fact that
plants in nature do not live as isolated individuals, but in com-
munities, more or less definitely organized. The organization,
to be sure, develops gradually through the carrying of seeds into
the area, and through the elimination of those species of plants
that cannot endure the environment. Each year some new plants
are starting and others are dying ; the population is continually
changing, but in an orderly w^ay which is determined by the
conditions in the community and the kinds of plants whose
seed is carried into it. Such communities of plants are called
plant associations.

All the plants in one association have somewhat similar re-
quirements, but usually each plant differs from the others in
some one or more requirements. For example, some are shallow-
rooted, others are deep-rooted ; some complete their growth,
flower, and produce seed in the spring ; others require a longer
growing season and flower in the summer or autumn ; some re-
quire full sunlight, while others need partial shade. Plants that
are associated, then, are species that have certain water, soil,
fight, or temperature requirements in common, and are enough
different in other respects not to interfere materially with one an-
other. In the naming of associations, we use the name of the
plants that are most prominent and that dominate the community
{dominant species) . The other, less prominent plants are spoken
of as secondary species of the association.

3o6 General Botany

The water lily, bulrush, and cat-tail associations of ponds are
small communities of plants dominated by these particular plants.
The hemlock forest, the redwood forest, the yellow-pine forest,
and the oak-hickory forest are all associations of large and
small plants of numerous species that thrive under certain
conditions.

Climatic plant formations. In any particular part of the
country there are usually many different plant associations.
Local differences of elevation, topography, soil, drainage, and
slope exposure result in a diversity of local environments.
Some plants fit into each of these environments better than
others ; consequently in each there arises a local community, or
association, of plants. In general it has been found that through-
out a region having similar climatic conditions, the group of
plant associations is essentially similar. When we extend our
study into regions with very different climates, we find a very
different series of plant associations.

The plant associations of Indiana are very similar to those of
Ohio because the climate in the two states is very similar. For
the same reason the plant associations of Kansas and Nebraska
are very similar. However, there is a vast difference between
the plant associations of the Ohio-Indiana region and those of
the Kansas-Nebraska region. This difference is primarily the
result of difference in climate.

Different groups of plant associations, then, are character-
istic of different climates. For this reason it is customary to
group associations into larger units called climatic plant forma-
tions. The terms ^' evergreen forest," '' deciduous forest,"
" prairie," "plains," and " desert " show the general recognition
of these larger groupings of vegetation that are primarily deter-
mined by the. light, moisture, and temperature conditions that
make up climate.

Plant associations not permanent. Students of physiography
are familiar with the fact that land forms are constantly changing.

Distribution of Plants in Nature 307

Hills are eroded by wind and water, and their surface materials
are being constantly carried to lower levels. Ponds and lakes
are continually being filled by material that is carried into them
by the wind and water, or by the material that accumulates
through the death of the plants and animals living in the water.
Streams enlarge their valleys, eroding here and depositing there,
but constantly changing and wearing away the slopes and the
valley bottoms. The plant associations that exist in these vari-
ous physiographic situations are affected by all these changes ;
consequently the character of the association also changes.

In addition to the physiographic changes, the vegetation
itself, through shading, brings about* changes in light, tempera-
ture, and moisture. Humus accumulates in the soil, increasing
the constancy of the water supply and affording better conditions
for the growth of the bacteria and fungi which improve the
available supply of soil salts. Animals, particularly earthworms
and insects, also aid in these processes.

Habitats, then, are constantly changing; and in the course
of years, decades, or centuries the conditions may be so altered
that the kinds of plants now living in the habitat cannot survive
and other kinds will have taken their places. This process of
change in vegetation is called succession. Examples of suc-
cession may be seen in fields that have been abandoned and al-
lowed to return to a wild condition.

In the forested regions of New England it is not uncommon to
see areas embracing several hundred acres, once highly cultivated,
but now, through abandonment, completely reverted to forest
again. Other examples of succession may be seen along railroad
cuts and fills, on sand dunes, and on sand bars and islands in
streams. Here the newly exposed areas are occupied by asso-
ciations of plants very different from the plants on areas that
have been covered with vegetation for 10, 20, or more years.
The youngest areas may contain mostly a great variety of an-
nual weeds; the older areas are covered with perennial herbs

3o8 General Botany

and shrubs or with young trees. The order of succession is usu-
ally quite definite in a given region. By noting the seedling
trees in a forest one can often predict what the composition of
the forest will be 50 years from now, if it is left undisturbed.

The vegetation of a continent like North America, then, is
made up of several great climatic plant formations, each of which
is composed of many local plant associations. The plant asso-
ciations are not permanent, but change as the habitats change,
and are succeeded by other plant associations.

Plant realms. Taking the world as a whole, geographers dis-
tinguish three great realms that differ in their vegetation, mainly
because of differences in tehiperature. These are (i) the torrid
realm, where the temperatures are uniformly warm and frosts
are unknown ; (2) the temperate realm, where a warm growing sea-
son alternates with a cold period during which plant processes
are slowed down, or stopped, each year ; and (3) the frigid realm,
where the cold is either continuous or alternates with a short,
cool summer having almost uninterrupted light of low intensity.
Vegetation is markedly different in these three realms. Under
the most favorable conditions vegetation is densest and the
number and variety of plants are greatest in the torrid realm
and least in the frigid realm. Each of these realms, of course, is
occupied by several or by many climatic plant formations, de-
pending upon differences in climate. There are torrid forests,
grasslands, and deserts, just as there are temperate forests, grass-
lands, and deserts.

Plant formations on mountains. High mountains occur in
all parts of the world. The vegetation of the summits differs
very materially from the vegetation at their bases. In polar
regions the summits may be continually hidden in ice and snow,
and the only plants that can grow there are microscopic ones
that live on the surface and cause the so-called " red snow."
Within the tropics the base of the mountain may be surrounded
by tropical forest ; higher up, temperate forests occur ; and then

Distribution of Plants in Nature

309

comes a " timber line/' beyond which only low-growing plants
related to those of the frigid realm occur ; the summits may be
clothed in snow and ice. Increasing altitudes bring about the
development of vegetation similar to, or corresponding to, the
vegetation of higher latitudes.

Summary. A study of vegetation shows that the plants are
naturally grouped into plant associations. The plant associa-
tions of any uniform climatic region are essentially similar and
may be grouped into climatic plant formations. Climatic plant
formations in turn are conveniently grouped into plant realms
characterized by torrid, temperate, and frigid climates.

CHAPTER THIRTY-THREE

THE VEGETATION OF NORTH AMERICA

North America extends from the North Polar Sea nearly to
the equator, and consequently its vegetation includes climatic
plant formations belonging to the frigid, temperate, and tropical
realms. In this chapter the more important of these plant
formations, and the factors which determine or limit their dis-
tribution, will be discussed. There are at least nine of these
natural divisions of the vegetation of North America: (i) Tun-
dra, (2) Northern evergreen forest, (3) Deciduous forest, (4) South-
eastern evergreen forest, (5) Prairie, (6) Plains grassland, (7) West-
ern evergreen forest, (8) Desert, and (9) Tropical hroadleafed
evergreen forest.

Climate — especially moisture, temperature, and light —
determines the particular part of North America where each of
these several types of plants may live. The habitats within the
climatic formations determine the number and location of the
plant associations. In the paragraphs that follow, the vegeta-
tion is described as it was before it was modified or destroyed
by man. In the next chapter attention is called to the close
correlation that exists between the climatic plant formations and
the distribution of the industries directly dependent upon plant
life.

The tundra formation. There is no more distinctive type of
vegetation on the earth than the low-growing vegetation of the
frigid realm, to which the name tundra has come to be generally
applied. Originally used to designate the vast stretches of low,
swampy, and rocky plains of northern Russia, this term is now
applied also to the vegetation that covers the " barren grounds "
from northwestern Alaska to Hudson Bay and eastern Labrador.

The tundra is a region of shallow, poorly drained soils, where
the winters are long and the average temperature so low that the
ground thaws only a few inches, or at most a few feet, during the

310

The Vegetation of North America

311

Fig. igi. The forest formations of North America. North of the northern evergreen
forest is the tundra formation. On the unshaded -areas south of it. are the prairie, plains,
and desert formations.

312

General Botany

The Vegetation of North America 313

two or three months of summer. Consequently the only plants
that thrive are the low plants that have shallow roots, like grasses
and sedges, or that grow entirely on the soil surface, like the
mosses and lichens. Northward, the tundra is limited by the
polar seas and the areas of perpetual ice and snow ; southward,
by the northern evergreen forest. Even farther south, among
the forests, are patches of tundra-like vegetation that remain in
bogs and on bare rock outcrops. On the higher mountain
summits everywhere are alpine areas covered with vegetation
that is closely related to that of the tundra.

The winter season is characterized by intense cold, violent
dry winds, and very light snows. The short growing period
during the summer, the low soil temperature, poor drainage, and
consequent scanty aeration of the soil are important factors in
excluding most plants, particularly trees, from this region. The
better-drained stony uplands are covered with grasses ; the low
places, with mosses, sedges, shrubs, and flowering herbs.

Most of the flowering plants are only a few inches in height.
Many have leathery leaves and creeping or reclining stems, and
are typical xerophytes. Cranberries, crowberries, and snow-
berries are examples of common low shrubs. Many of these
plants are evergreen, and many of them are small compared with
those of temperate regions. The willows are represented by
several dwarf species that rise only a few inches above the soil.

The northern evergreen forest formation. Stretching from
Newfoundland and Labrador to Alaska by way of the St. Law-
rence Valley and the lower Hudson Bay region is the northern
evergreen forest. This forest is composed of the white spruce,
black spruce, paper birch, aspen, balsam poplar, tamarack,
balsam fir, white pine, red pine, jack pine, arbor vitae, and hem-
lock. The last five of these trees^ are found mostly east of
Winnipeg. All the trees attain their greatest size in the region
between northern Minnesota, Maine, and eastern Quebec. On
the western plains of Canada, where the rainfall is reduced to

314

General Botany

U . S. Forest Service

Fig. 193. Peat bog in northern Minnesota, with mature tamarack {Larix laricina) and
black spruce {Picea mariana). In the foreground are alder {Alniis), sedges (Carex), and
bulrushes {Scirpus). Throughout the northern evergreen forest region, thousands of
square miles are covered by such bogs.

The Vegetation of North America

315

.i«?S^j. »'i<af

Fig. 194. Black spruce trees on the cliffs near Yarmouth, Nova Scotia, showing effects of
winds from the Bay of Fundy.

15 inches, they are more or less confined to the stream margins,
and in Alaska to the river valleys. On the best soils the white
spruce, balsam fir, and paper birch grow in a dense mixture,
forming the finest of northern forest types. The jack pine and
white pine in some localities occupy sterile soils and form ex-
tensive forests. On mountain slopes and along the shores of the
Great Lakes and the sea the black spruce is a common forest
tree. In the numerous bogs and poorly drained areas the tama-
rack and black spruce dominate. On limestone outcrops and in
the better-drained swamps the arbor vitag is common. Where
the original forest has been cut or burned over, there are ex-
tensive areas covered by birch and aspen poplar. These form
only temporary forests that are later succeeded by pine and
spruce.

From northern Minnesota to Nova Scotia the southern portion
of the evergreen forest is mixed with the trees of the deciduous
forest formation, especially on the best soils and at low elevations.
There is an extension of the northern evergreen forest, made up
of white pine, hemlock, yellow birch, and spruce, on the Appala-

3i6

General Botany

(

U.S. Forest Service

Fig. 195. Red spruce forest on the high mountains of western Virginia, an extension
southward of the northern evergreen forest.

The Vegetation of North America 317

chian Mountains southward through Pennsylvania to northern
Alabama, where it is confined to the mountain summits.

The northern evergreen forest region is characterized by long
winters with deep snows and a short, warm growing period of
about 3 to 4 months' duration. The total rainfall varies from
40 inches in the east to 1 5 inches in the west, and the evaporation
from a free water surface is equivalent to about one half to two
thirds of the rainfall. The humidity is high, varying from 70
to 80 per cent of saturation. The snowfall begins before the
ground is frozen, and where the snow is heaviest the ground
remains unfrozen throughout the winter and the slow melting
of the snow keeps the soil moist far into the summer. This is
important, for it insures the trees an adequate water supply at
all times. On the tundra to the north the soil is permanently
frozen at a comparatively slight depth. South of the evergreen
forest the ground freezes from 2 to 4 feet every winter, and this
is possibly one of the factors which limits the southern extension
of some of the evergreen trees. Another important factor is the
competition of the deciduous hardwood trees. On good soil the
hardwoods soon shade out the evergreens, with the exception of
hemlock. On poor soils, sand plains, sand dunes, and sandstone
cliffs the evergreens are more successful in maintaining a foothold.

In general, the soils of the evergreen forest region are shallow,
and the drainage is poor except in the highlands. The soils are
shallow because of the glaciers that once covered all of this part
of North America to a thickness of i to 2 miles. The ice of these
glaciers flowed toward the south, smoothing off the land surface
at the north, and carrying away whatever soil there was in pre-
glacial times. As the glaciers disappeared only 20 to 30 thousand
years ago, there has been comparatively little time for soil to
accumulate. From these facts we^may infer that during glacial
times the northern evergreen forest occurred farther south, from
southern New Jersey to Kentucky and Nebraska, and has moved
into its present region in geologically recent times (Fig. 311).

3i8

General Botany

Fig. 196. White pine forest in northern Michigan, in which are scattered maples, birches,
and aspens. A typical view in the northern evergreen forest.

The Vegetation of North America 319

The deciduous forest formation. Limited on the north by the
northern evergreen forest and on the south by the Gulf coastal
plain, a great forest of broadleafed deciduous trees extends from
the Atlantic coast westward to the great plains of central Ne-
braska, Kansas, and Texas. This is the oldest forest on the
continent and has occupied much of this region for several million
years. Sometimes it was far more extensive, sometimes it was
more restricted ; but it has been practically continuous since the
Cretaceous period of the earth's history.

This forest is dominated by oaks, hickories, elms, ashes, maples,
chestnut, beech, sycamore, cottonwood, and tulip. It attains
its best development on the mountain slopes in North Carolina
and Tennessee and the lower Ohio River Valley. Under the
most favorable conditions this forest attains a height of 150 feet,
and some of its trees develop trunks 6 to 14 feet in diameter.
In summer they spread an enormous area of green foliage; in
winter the above-ground shoots consist only of cork-covered
trunks and branches. On uplands and on the poorer soils the
oak, chestnut, and oak-hickory forest types dominate. On the
richer uplands sugar maple, beech, and tulip trees, with various
other mesophytic species, occur. In the river valleys elm, ash,
soft maple, birch, and sycamore make up the forest covering.
Under the larger trees dogwood, redbud, sourwood, and numerous
other shrubs decorate the second levels. On the ground are
flowering plants that bloom before the trees have set their leaves.
The autumn coloration is a notable feature each year at the close
of the vegetative season.

The deciduous forest region is characterized by short, cold
winters, with some snow, usually averaging less than 2 feet, and
a frostless season of from 5 months at the north to 8 or 9 months
at the south. The rainfall varies f-rom 40 to 50 inches eastward
and diminishes westward. Generally it exceeds the annual rate
of evaporation ; in the mountains it may be twice as great. The
average relative humidity is less than in the northern evergreen

320

General Botany

Fig. 197. Oak-hickory forest in central Illinois. In this region the deciduous forest
formation and the prairies meet, the forests occupying the slopes and stream valleys, and
the prairies the flat uplands.

forest, varying from 70 per cent in the east to 50 per cent at the
western edge, where the forest extends along the rivers into the
prairies and plains.

Toward the north and on mountain slopes, northern conifers
like the white pine and hemlock occupy considerable areas, or
they may be mixed with the broadleafed species. Southward,
on cliffs, sandy plains, and shallow soils, many trees, like the
shortleafed pine, pitch pine, and scrub pine, occupy pioneer
habitats. On the Piedmont plateau region the shortleafed and
longleafed pine are mixed with oak-hickory forests. The uplands
immediately west of the Mississippi River were originally covered
by oak and hickory forests, shading into walnut, elm, and beech
of magnificent proportions on the more fertile soils. Here the
shortleafed pine was mixed with the oaks and hickories. In

The Vegetation of North America 32!

many places the red cedar covered extensive areas of shallow,
rocky upland.

Toward the west the deciduous forest occupies long, finger-like
extensions covering the valleys in the prairie region, which finally
become narrowed down to mere strips of elm, ash, poplar, and
willows along the margins of the streams in the plains country.
Toward the south the hardwoods compete successfully with the
southern pines and occupy the better lands.

The southeastern evergreen forest formation. This forest
centers on the Coastal Plain from eastern Virginia to eastern
Texas. On the sandy uplands it is the home of the longleafed
and shortleafed pine. In the swamps there are extensive areas
of cypress ; and along the streams and bayous near the coast,
tupelos (sour gum), water oaks, pecans, sweet bay magnolias,
and live oaks flourish.

The cHmate of this region is marked by average summer tem-
peratures of 70 to 80 degrees and winter temperatures of 40 to 68

U. S. Forest Service

Fig. 198. Longleaf yellow pine encroaching on grassland in Florida. An example of

succession.

322

General Botany

Fig.

U.S. Forest Service

[99. Bald cypress swamp near Memphis, Tennessee. Note cypress knees at the

left. During the wet season the water covers the area nearly to the tops of the knees.

degrees. The relative humidity is high. The rainfall varies
from 60 inches along the coast to 44 inches inland. At the coast
this amounts to about i .3 times the evaporation ; at the inner
edge of the Coastal Plain it is i.i times the evaporation. Snow-
falls occur at rare intervals, but killing frosts occur over most
of the area every year. For the most part, the soils are loose
and arranged in belts parallel to the coast, except the alluvial
deposits which extend up the Mississippi and other large rivers.
The climate is favorable for a broadleaf evergreen forest, but
the poor soils of the Coastal Plain are better suited to the conifers

The Vegetation of North America 323

and consequently the most extensive forest is dominated by the
longleaf and other pines. The pine barrens are comparatively
open woods on the dry or moist sandy plains ; on better and on
moist soils the forest is more dense and has an undergrowth of
small oaks and other trees, some of which are evergreen. Among
the undergrowth is the low-growing palmetto, which suggests
an approach to subtropical conditions.

Along the eastern coasts white cedar swamps occur as far north
as southern Maine. The cypress and tupelo swamps are common
along the lower courses of the rivers from Chesapeake Bay south-
ward, and extend as far inland as southern Illinois.

One of the remarkable plant associations of the South is that
of the canebrakes, our only native representatives of the bamboos,
which are so abundant in Asia. The canebrakes were formerly
extensively developed on the low hills bordering both sides of the
Mississippi flood plain, and in central Alabama as undergrowth
on the oak-covered black-soil areas.

Just as the deciduous forest trees like the maple occur in the
best habitats in many places as far north as Nova Scotia, so dense
growths of oaks, beech, hickories, and magnolias occupy the most
mesophytic habitats, called '* hammocks," as far south as central
Florida. As we go southward from the northern evergreen forest, •
the rate of evaporation gradually increases, and an increasing
amount of rainfall becomes necessary to permit the growth of
forests.

The tropical evergreen forest formation. The southern third
of the peninsula of Florida, the West Indies, the lowlands of
Mexico, and the eastern slopes of Central America are occupied
by tropical forests. Where the rainfall is more than 50 inches,
these forests attain magnificent proportions and great density.
Where the rainfall is less, one finds tropical scrub and desert.
As the trade winds of the tropics blow from the east, the greatest
rainfall occurs on eastern slopes, and there the conditions for
forest growth are at their best. On western slopes the rainfall

324

General Botany

Fig. 200.

Subtropical vegetation in southern Florida. Live oak covered with epiphytic
bromelias. In the background, cabbage pahnetto.

The Vegetation of North America

325

U.S. Forest Service

Fig. 201. Buttressed trunks of mahogany trees in southern Mexico.

is reduced sHghtly by low mountains and greatly by high moun-
tains, and the vegetation changes accordingly.

In Florida the tropical forest is poorly developed, but its
relationship to the tropical forest is shown by the presence of
palmettos, palms, and other tropical trees. Along the coast
are mangrove swamps, very similar to those found on all muddy
coasts in the tropics. The Everglades constitute a vast area of
shallow water largely occupied by saw grass, with narrow open-
water channels forming a labyrinth of passages. Interspersed
are many small islands covered with tropical trees, which support
numerous epiphytic bromelias and orchids on their branches.

In the West Indies and in Central America the original tropical
evergreen forest has been destroyed by centuries of migratory
agriculture. This term is appHed to a general practice in tropical
countries, of clearing a piece of forest land and growing crops on
it for a few years, while the returns are large and the weeds are
easily controlled. When crop growing becomes more difficult,

326

General Botany

U.S. Forest Service

Fig. 202. Tropical jungle in British Honduras. In the West Indies and Central America
the original forest has been destroyed by migratory agriculture and the jungle has taken
its place. The pictiure shows a large mahogany tree near the center.

The Vegetation of North America 327

the native moves on and clears another area. The sequel of
migratory agriculture is the tropical jungle, with its dense,
tangled, and almost impenetrable masses of vegetation.

In Dominica, Trinidad, Venezuela, and other northern states
of South America, are remnants of the original broadleafed ever-
green tropical forest. These forests are noted for their great
variety of tree species and their freedom from dense undergrowth.

The prairie formation. Extending from North Dakota to
Texas and eastward to Indiana is a roughly triangular region
in which vast areas of level and rolling uplands formerly were
covered with tall grasses from 3 to 10 feet in height, while decidu-
ous forests dominated the river valleys. These are the true
prairies. Toward the western margin the prairies were well
drained, or even overdrained, but to the eastward they were
interspersed with sloughs and temporary ponds which were also
dominated by grasses.

During the summer the prairies were studded with the brightly
colored flowers of scattered perennial herbs. In the fall the
prairies were a vast sea of highly inflammable grasses, and often
they were swept by fires that destroyed everything in their path.
In winter they were bleak and exposed to the full sweep of the
wind and drifting snow.

The dominance of the prairie grasses over this great area and
the absence of forests was made possible by the climate. The
most important climatic factors influencing plant growth are
rainfall, temperature, humidity of the air, and wind velocity.
The first — rainfall — represents the source of the water supply
in the soil. The other three factors determine the rate of evapo-
ration from a water surface. In the prairie region the rainfall is
less than the amount of evaporation ; it is about six tenths as
great on the western side and eight tenths on the eastern border.
The prairie region is characterized by high summer temperatures
and summer droughts. Another characteristic feature of the
prairie climate is the uneven distribution of the rainfall during

328 General Botany

the growing season. During one season the heavy rains occur
in the spring, during others in midsummer or autumn. This
leads to annual droughts either preceding or following the rains.

The soils are for the most part clays and sandy loams upon
which there has accumulated since glacial times, through the
comparatively slow decay of the prairie vegetation, several inches
to several feet of black humus. But the nature of the soil was of
less importance in the maintenance of the prairie than the cli-
matic factors which controlled the moisture content of the soil.
The prairies that were low and poorly drained were unfavorable
to the growth of trees because of too much water in the spring
and early summer. The more western prairies were subjected
to too intense droughts in summer to favor the growth of trees.
In the years that have passed since the prairies were first settled,
thousands of miles of tile drains and ditches have drained the
ponds and sloughs, and the ground-water table is today several
feet lower than it was originally. This has made possible the
growth of trees in the wet prairies, where formerly they were
absent. The absence of fires is also favorable to the extension
of the forests.

Although there were several species of grasses common on the
prairies, by far the most important is the '' big bluestem." This
grass formed an almost pure growth over the large areas, and in
late summer was so tall and dense that cattle were lost to sight
in it and their position could be told only by the swaying of the
grass tops as they moved about.

On the sandy and more exposed dry prairies " bunch grass,"
or " little bluestem," 2 to 3 feet high, was most abundant.-^ As
humus accumulated and the soil moisture was increased, these
areas were invaded and often occupied by the " big bluestem."

In the wet areas of the prairie the '' slough grass," 6 to 10 feet
in height, was dominant. This grass was used frequently by the
pioneers to thatch the roofs of their smaller farm buildings.

^Big bluestem is Andropogon furcatus ; little bluestem is Andropogon scoparius.

The Vegetation of North America 329

Scattered throughout the prairies were large and small her-
baceous plants, including milkweeds, sunflowers, rosin weeds, cone-
flowers, asters, and goldenrods. These plants gave color to the
prairies at certain seasons. They never made up a large part of
the original prairie covering, however, and they were most
numerous on the borders between the prairie and the forest and
on eroding slopes.

Among the explanations sometimes given for the treelessness
of the prairies are the fires set by the Indians and by hghtning.
That these fires occurred in the autumn there is no doubt, and
that they killed young trees on the forest edge and acted as a
check to tree invasion there can be no doubt also. The prairies,
however, preceded the prairie fires, and the fires could at best
only delay forest invasion — not prevent it over such vast areas.
Deciduous forests occurred throughout the prairies along the
streams, on river bluffs, on valley slopes, and on the flood plains.
The highly fertile character of the prairie soil caused them to be
occupied by farms as rapidly as they could be broken and properly
drained. Today patches of original prairie are far more difficult
to find than patches of original forest.

In Ilhnois and Iowa the prairies occupy for the most part
upland areas between the stream valleys. In Kansas and
Nebraska, where the region that was dominated by the big
bluestem reaches its western Kmit, the prairies were confined
to the river valleys and lowlands.

The plains grassland formation. Between the Rocky Moun-
tains and the prairies and from Saskatchewan to Texas is a vast
rolHng plain more or less dissected by streams and covered with
grasses. West of the Rockies, from Montana to Washington
and California, are similar areas of grassland bordering the
forests.

The annual rainfall varies between 10 and 20 inches and is
distributed irregularly in showers and occasional heavy down-
pours. As the depth of evaporation from a water surface is

330 General Botany

between 30 and 50 inches during this same period, the extent of
the rainfall is only from two tenths to six tenths of the evapora-
tion. Furthermore, in late summer this region is subject to
prolonged hot dry winds from the southwest. At such times the
temperature rises above 100° F., the humidity falls, soil moisture
becomes very low, and all vegetation suffers through excessive
transpiration. These " hot winds " were the source of great
losses to the early settlers when they occurred before the crops
were mature. On the high plains of western Kansas and eastern
Colorado the soil is generally dry below a depth of 6 to 15 feet.
The plains region is the home of occasional violent winds, tor-
nadoes in summer and blizzards in winter. The snowfall is
generally light but is subject to drifting and may become deep
in the depressions.

The most characteristic grasses of the Great Plains are the
buffalo grass {Bulhilis and Boutelona), the bunch grass {Andro-
pogon), and the wire grass (Aristida). The buffalo grass is a
turf-forming grass, a few inches in height, which affords highly
nutritious forage. The bunch grasses received their name from
the habit of growing in scattered dense tufts, especially in lands
that have been disturbed by streams and wind erosion. The
wire grass is a coarse grass, 2 feet in height, which also grows in
tufts, usually mixed with other grasses.

Just as the deciduous forests stretch westward, occupying the
valleys in the prairie region and the stream margins in the plains
country, so the tall-grass prairies extend westward in the valleys,
forming finger-like extensions between the tree-bordered rivers
and the short-grass uplands.

At their western margin the plains are invaded on rocky slopes
by the western yellow pine, and at the southwest by semi-desert
scrub, consisting of mesquite, red cedar, and scrub oaks. The
grasslands also grade into sagebrush, which occupies extensive
areas from Colorado and Montana to the Great Basin and Sierra
Nevada Mountains. Scattered over the plains are many small,

The Vegetation of North America

33^

W.S. Cooper

FlG.^ 203. Forests on a high mountain (Mt. Robson, British Columbia), showing timber
on ridges and absent from valleys where the snow accumulates to great depths. On the
talus cone are numerous avalanche tracks where the trees have been destroyed. The white
line crossing the talus cone near the base is a trail.

332

General Botany

iV. 6. Cooper

Fig. 204. Sitka spruce forest at Glacier Bay, Alaska. This forest has grown up since
the retreat of the glacier about 100 years ago.

xerophytic, flowering herbs, as well as cacti, yuccas, legumes, and
composites that reheve the gray-green monotony of the grasses
by their vari-colored flowers.

We have seen that the treelessness of the prairies is due, for
the most part, to an excessive transpiration rate in proportion
to the soil-water supply ; locally toward the east, to unfavorable
soil drainage also. The treelessness of the plains is due to in-
adequate water supply; intense summer and winter droughts
make it very difficult for tree seedlings to become established.

Western evergreen forest formation. The western Cordillera,
extending from Alaska through the Rockies, the Sierra Nevada,
and the Coast Ranges to Mexico, are clothed with conifer forests
of pines, firs, spruces, hemlocks, and cedars. This forest roughly
has three divisions : (i) the northern coastal forest, extending
from Washington to southern Alaska, (2) the Coast Range and
Sierra forest, extending southward to southern California, and
(3) the Rocky Mountain forest, stretching from northern British
Columbia to Arizona and Mexico. The region from Washington

The Vegetation of North America

333

to western Montana is a meeting ground for species from all
these divisions.

U.S. tore^t -it'r:ic<;

Fig. 205. Redwood {Sequoia sempenirens) forest in the mountains of northwestern Cali-
fornia. The tree nearest the man is 45^ feet in girth at the level of his hat. The redwood
is found in the moist valleys of the Coast Range and is the tallest of all conifers.

334

General Botany

U.S. barest Service

Fig. 206. An alpine meadow in Cascade Forest Reserve. Fir and hemlock forests cover
the rocky slopes. The alpine meadows occupy depressions at elevations above 9000 feet.
The snow lies on them 8 to 10 months of the year; the soils are composed in large part of
wet muck, and many of the plants are the same as those found on the tundra.

The Puget Sound region, the lower altitudes of southern British
Columbia, and the coastal mountains of Oregon are the home of
the most magnificent conifer forests in the world. Not only are
the trees of great height (200 to 250 feet), but they have trunks
8 to 15 feet in diameter and they stand very close together. This
great forest is the natural outcome of a moist climate with a rain-
fall of about 100 inches, together with mild winters due to the
proximity of the Pacific Ocean. It is dominated by the Douglas
fir. Western hemlock, and Western arbor vitae. In spite of the
thick growth of trees, there is a dense undergrowth of ferns,
shrubs, and low-growing trees.

From Washington north to Alaska the forests on the western
slopes are dominated by the Sitka spruce. Southward from
Oregon, in the fog-laden valleys of the Coast Ranges to San Fran-

The Vegetation of North America

335

cisco Bay, the forest is composed of redwoods, the tallest of all
conifers.

From this point southward the Coast Ranges are dominated by
vegetation consisting of scrub oaks, hardleafed shrubs, and xero-
phytic grasses — collectively known as chaparral. The chapar-
ral also forms a belt surrounding the central valley of California
and the lower elevations of the southern California mountains.
This is a region of winter rainfall and hot, dry summers.

Inland from southern Oregon and south along the Sierra
Nevadas is an extensive forest of Western yellow pine, with
Douglas fir, incense cedar, and sugar pine intermingled. In
California this forest is restricted to the moist slopes above 1500
feet in the north and above 3000 feet in the south. Above the
pine forest is a belt of firs and hemlock, and at the timber line
the white-barked pine occurs. Between the pine belt and the
desert is a belt of oak and digger pine, and at lower levels an
extensive growth of chaparral. On rolling uplands between the
large canyons on the western slope of the Sierras occur groves of
the celebrated '' Big Trees." At timber line here and elsewhere

W. S. Cooper

Fig. 207. Foothills and valley land in Arizona covered with grass and oak brush, affording

grazing range for goats.

33^

General Botany

U.S. Forest Service
Fig. 208. Pinon forest in northern Arizona, with sagebrush and grassland in the foreground,

throughout the Western mountains, on flat areas are alpine
meadows, where the snow accumulates to great depths in the
winter. During July and August these meadows are covered
with the most briUiantly colored flowers.

In general, alpine vegetation occurs at lower and lower levels
as we go north, but much depends upon the local exposure to
moisture-laden winds and whether the slopes face north or south.
North-facing slopes are moister and cooler and the growing season
is shorter than on slopes facing south.

The forest trees of the Rocky Mountains are closely related
to those of the California-Puget Sound region. The upper limit
of tree growth is about 9000 feet in Montana and 12,000 feet in
southern Colorado. Since these mountains rise above an arid
plateau region, there is also a lower limit to tree growth ; this
limit is between 4000 and 6000 feet. In the Canadian Rockies
the forests are continuous in the broad valleys and mountain
slopes. Southward, beginning at Montana, the broad inter-
mountain valley is occupied by sagebrush semi-desert, and farther
south by desert vegetation. The most characteristic tree of the
entire region is the Western yellow pine. A close second is the

The Vegetation of North America

337

widely distributed lodgepole pine. In Colorado the limber pine,
and farther north in Montana the mountain pine, are locally
abundant, as is also the Douglas fir.

In southern Colorado the yellow pine gives way to the nut pines
and junipers in semi-arid places. The forests on the plateau of
Arizona, and those above 5000 feet on the mountains of Arizona,
New Mexico, and western Texas, are dominated by yellow pine
bordered by belts of nut pine, juniper, and scrub oak at the semi-
arid lower levels. Usually there are belts of grassland and sage-
brush in the transition to the desert.

At higher altitudes and in the moist canyons of Colorado,
Engelmann spruce and subalpine fir are abundant ; farther north,
firs. Western hemlock, and larch constitute important forest
types. Grasses, composites, and legumes furnish the bulk of

t/ . 6 . 7* orest bervtce

Fig. 209. Desert scrub near the east end of the San Bernardino Mountains, CaUfornia.

33^

General Botany

Fig. 2IO.

W.S.Cooper

Small-leafed desert shrub vegetation on dunes, Monterey Bay, California.

the small flowering plants. Throughout the Rockies the streams
are bordered by alders, willows, and poplars.

The southwestern desert formation. From the plateau of
Mexico, extending northward into California, Arizona, Nevada,
Utah, and Idaho, and eastward to New Mexico and western
Texas, is the desert. This great region is, for the most part, a
more or less broken plateau, with a rainfall of from 3 to 20 inches,
and with an evaporation rate five to thirty times as great.
Temperatures as high as 120° F. occur in the summer, and frosts
are not unknown even in southern Arizona. Farther north
the winters are severe, and, due to the great intensity of the sun-
light, the summers are very hot. Toward the south there are
two rainy periods, one in July and another in January. Follow-
ing these rains the desert is green with a covering of summer or
winter annuals that spring up quickly between the perennials
and, within a few weeks, flower, fruit, and die.

At other seasons the vegetation is scattered, of a gray-green
color, and consists of thorny and spiny shrubs, large and small
cacti, fleshy-leafed agaves, yuccas, and other small succulent

The Vegetation of North America

339

^m.sk.

■'^miM

Fig. 211.

[^ 5 Cooper

Desert vegetation on Tonto Platform, Grand Canon, Arizona, consisting of
prickly-pear cactus, yucca, and low shrubs.

and woody perennials. Desert plants are either extreme xero-
phytes, or they are mesophytic short-lived annuals that com-
plete their life cycle during a single moist period. The xerophytes
include those with thick stems that accumulate enough water
during the rains to carry them over dry periods, like the shallow-

S.-r^ *A;* i^*i

^^i;

tr. 5. Cooper
Fig. 212. A desert shrub, Fouquieria splendens, in leaf, near Tucson, Arizona. This plant
is found over wide areas in the Southwestern desert.

340

General Botany

Fig. 213. Giant cactus and desert shrubs near Tucson, Arizona.

W. S. Cooper

rooted cacti. The agaves accumulate water in their fleshy
leaves, and the yuccas are deep-rooted. The shrubs for the
most part are deep-rooted and live in soils where water flows
or seeps from the better-watered mountains and elevations.
Northward the succulent desert gives way to the sagebrush
semi-desert ; eastward and westward it passes into small-leafed
desert scrub.

All the perennial plants show reduced leaf surfaces. Some
have leaves only during the rainy season, and others, like the
cacti, are quite devoid of foliage leaves. Heavy cuticles, bloom,
and thickened epidermal cells are found on the agaves and yuccas.

In southern Mexico the desert gives way to semi-desert tropical
scrub, which in turn merges into the tropical jungle that occupies
more and more of the land through Central America to Panama.
On the higher mountains subtropical oak and other hardwood
forests pass into pine forests at still higher elevations. The
highest peaks reach above timber line and have small areas of
alpine vegetation.

CHAPTER THIRTY-FOUR

RELATION OF PLANT INDUSTRIES TO CLIMATIC PLANT
FORMATIONS

The climatic conditions that restrict each of the great plant
formations to a definite region of North America also determine
to a large extent the location of many industries dependent upon
plants or plant products. It is self-evident that most industries
that directly utilize wild plants are located near those plants.
And for the same reason industries directly dependent upon crop
plants are usually located in regions where the particular crop
plants grow best.

Climate and the production of crops. Cultivated plants are
affected in their development by climatic factors in much the
same way as wild plants. Temperature, moisture, and Hght
conditions must be favorable ; and each of the important crop
plants has its own requirements in this matter.

Crop plants must not only be able to grow in a particular cli-
mate, as wild plants do, but they must yield a profit to the grower.
Peanuts, for example, can be grown in the Northern states, but
the yield is so small that they are unprofitable ; they produce the
largest yields in the Southern states, where the temperature is
high. A crop grown in the region whose climate is most favor-
able to that crop will produce a better quality and a greater re-
turn to the grower than the same crop grown in a less favorable
locality. The distribution of crop plants is determined, then, by
the same factors that limit wild plants, and in addition, by cer-
tain economic factors.

Soil factors and crop production. Within each climatic region
the various crops, like wild plants, are further limited by the
great variety of soil conditions. Some soils are poorly drained,
others are over-drained, and still others show every gradation
between. Soils may be lacking in some of the necessary min-
eral salts, or they may have some salts in excess. They may have
a high humus content, or be nearly lacking in humus. Some

341

342

General Botany

HU^Mi'V

«^tt' -' V ''V''^ll^^^^HI

' ^ A-^M

f\,

r\wKft

■■^^^ ^- ■' '• IM*'^^^.

^^^H|ra|l>^ .9

^Hp|^^Q\/*i^

^^^^^^^^^^H^L^^^ft^^i ^B w^^ ' 'W^Pf^i^. ^v '■'J^^^H

^H!iitiK H

■It

tu O

Plant Industries and Climatic Plant Formations 343

soils are acid, others neutral, and still others are alkaline. The
slope of the soil may be so great, or the rocks so near the surface,
that cultivation is impossible.

In other words, within each climatic region there are many
plant habitats, some of which may be used for one crop, some for
another, and some that are best left to produce pasturage or crops
of trees. Consequently, any one crop usually occupies only a
part of the climatic region in which it might be grown if the
conditions in all localities were favorable. For example, tobacco
is one of the most profitable crops of the deciduous forest region ;
but since its quality is greatly influenced by soil conditions, its
cultivation on a large scale is limited to certain definite soil
areas. Moreover, each of these soil areas is given over to the
growing of some particular type of tobacco.

Crop centers. A study of the geography of crop plants will
show that each crop has a region in which it is so profitable that
a considerable proportion of the suitable land is given over to
it. These regions are called crop centers. Moreover, the centers
of production of most crops coincide in large measure with the
climatic plant formations. Each plant formation has, therefore,
become a center of production of a certain group of crop plants.

In colonial days all the possible crops were grown in every lo-
cality. As the West became settled and transportation facilities
increased, the several crops were gradually moved into the most
favorable regions, and farming in any one locality became more
specialized. This movement is still going on and is of great
importance for the future supplies of agricultural products. Dis-
regarding market factors, those crops are most profitable which
best fit both the climate and the soil. As a result of competition
among farmers in different sections of the country, the pro-
duction of a particular crop at time^ increases in certain localities
and decreases in others.

Crop plants less restricted than wild plants. Crops may be
so valuable that the grower can afford to make artificial habitats

344 General Botany

for the plants. He may irrigate the land with water from the
mountains, or he may modify the climate by growing the crops
under glass, or under shades and screens. Such devices lead to
wide extension of the areas of crop production beyond their nat-
ural areas. The hundreds of tracts of irrigated lands in the
Western states ; the growing of tobacco under shade in Connecti-
cut, Florida, and the West Indies ; the growing of vegetables
under palms in desert oases ; and the growing of tropical plants
and summer vegetables in greenhouses in winter are familiar
examples of the extension of crop production into regions nat-
urally unfavorable.

Extending areas of crop plants through plant breeding. There
is still another reason why crop plants are less restricted than
wild plants. This is the production of new plant forms through
the activities of plant breeders. Ever since plants were first
cultivated, men have tried to find better or more suitable varie-
ties for cultivating in particular localities. As a result we now
have hundreds of varieties of crop plants, some of which grow
better in one climatic region and others in another. By produc-
ing or discovering new varieties, the areas of all the familiar crop
plants have been greatly extended.

The factor of transportation. Some plant products — like
the potato, for example — are bulky and the cost of transpor-
tation correspondingly great. Although potatoes grow best
on sandy loam in the cool Northern states, they can be produced
at a profit elsewhere, in spite of lower yields, when sold locally
and transportation charges avoided, or when they can reach the
market earlier. A map of potato production shows that potatoes
are grown in quantity near all the large cities of the country.

Other plant products — like mahogany, oils, resins, rubber, and
spices of tropical forests — are so valuable that they may be
transported long distances before they are made into commercial
products. Consequently, the industries dependent upon plants
of this type may be far removed from the source of supply.

Plant Industries and Climatic Plant Formations 345

In spite of these exceptions to the rule, it is generally true that
the great climatic plant formations are each characterized by
certain groups of plant products and plant industries. In the
following paragraphs the more important products and indus-
tries of each of the natural divisions of the vegetation of North
America are discussed.

The tundra. This inhospitable region, lying far to the north,
has few inhabitants except the Eskimos along the northern
coasts. They derive most of their food from the seals and shore
birds, though they do invade the tundra on hunting expeditions
for caribou, musk oxen, and smaller animals.

Here more than anywhere else on the continent the vegetation
of the sea is important to men. On this vegetation, especially
the microscopic plants, the fish are dependent, and they in turn
are fed on by seals, walrus, and shore birds — the primary food
of the inhabitants. On land the lichens, grasses, and other plants
furnish the food of the arctic hares, caribou, and musk oxen —
the secondary food of the Eskimos. Direct use of plants is very
limited, and plant industries are entirely wanting.

The northern evergreen forest. The excellent quality of the
wood that is derived from the white pine, spruce, red pine, and
arbor vitae, and its value for building houses and ships, led to
the early invasion of the forests of Canada, New England, and
the Great Lakes region by lumbermen ; and until 1900 the north-
ern evergreen forest was the most important center of lumber
production.

During the past 30 years the spruce has become a valuable
wood for the production of paper pulp. Its freedom from resin,
its white color, and its soft, smooth, and uniform grain make it
the best source of white book and print paper. Hemlock is
second in importance in the making- of pulp for newspaper, wrap-
ping paper, and other cheap grades of paper. Consequently, at
the present time the paper-pulp industry centers in the north-
ern forest.

346

General Botany

U . S. Forest Servue

Fig. 215. White pine {Pinus strobus) about 120 years old, with understory of balsam fir
{Abies balsamea), in northern Minnesota.

Plant Industries and Climatic Plant Formations 347

The invention of new processes is gradually making it possible
to use a variety of other woods, and also certain herbaceous plants,
for the manufacture of paper ; consequently the industry is now
spreading to the Southern and the Northwestern coast states.
All together, 600 million cubic feet of wood are used annually in
pulp manufacture.

Another important product of the northern evergreen forest
is tannin, which is used in the manufacture of leather. For-
merly the bark of the northern hemlock furnished the bulk of
this material and the nearness to hemlock forests determined
the location of many of the large tanneries. This industry has
been forced to move to other regions and find other sources of
tanning materials. The bark of the western hemlock, the oak,
and the chestnut are now used for this purpose.

The northern arbor vitae has been one of the principal sources
of telephone and telegraph poles because of its durability in the
soil, its light weight, and its comparative strength.

Agriculture in the northern evergreen forest region is more or
less limited to the production of hay and forage crops, and much
of the remaining land is given over to permanent pasture. Rye
and buckwheat are produced to some extent. This is the natural
region for the production of spring wheat, but owing to the shal-
lowness and poor quality of the soils it cannot be grown on a
large scale profitably in competition with the northern prairie
region. Potatoes thrive best in a cool, moist climate and are
mostly produced in the states along the Canadian border and the
east coast from Virginia northward. Cranberry production
centers in Massachusetts, and the total crop amounts to upward
of a half-million barrels. Large areas of pasturage lead to the
production of dairy cattle and determine the location of great
numbers of creameries and cheese factories in the Lake states
and southern Canada.

The deciduous forest region. This great area is a region of
plentiful coal, oil, and gas, and it has abundant water power for

348

General Botany

Plant Industries and Climatic Plant Formations 349

manufacturing purposes. Furthermore, there are large areas
of fertile soil that make agriculture profitable. Consequently
it has become the region of the densest population of the United
States.

The trees of the deciduous forest region are commonly known
as '' hardwoods." Oaks, maples, hickories, elms, ashes, chest-
nut, beech, sycamore, cherry, walnut, birch, basswood, and tu-
lip constitute the important economic species. In consequence
of the variety of products derivable from these hardwoods, these
forests have been largely cut over, except on the more remote
mountain slopes of the Appalachian system.

The oaks have furnished railroad ties and heavy beams for
wooden structures. Oak is also used in large quantities, together
with maple, birch, and w^alnut, in the manufacture of furniture.
Nearness of supply led to the establishment of the center of the
furniture industry in Michigan, New York, and Pennsylvania.
Chestnut wood and bark and chestnut-oak bark have been most
important sources of tannin in this region. Hickory, because of
its great strength, is used for the handles of tools ; and ash is
important in the manufacture of vehicles.

Elm, beech, maple, chestnut, and birch furnish much of the
material for staves in the manufacture of slack barrels for the
shipment of cement, flour, sugar, apples, vegetables, and many
other commodities. Elm also is the best wood for making the
hoops of these barrels, because of its toughness and tensile strength.
For tight cooperage — that is, barrels for the storage and ship-
ment of liquids — white oak is the wood most desired. Cotton-
wood is one of the leading sources of excelsior and " wood wool "
used for packing and for filling for mattresses and upholstery.

The making of syrup and sugar from the sugar maple was prac-
ticed by the Indians long before the advent of European settlers.
The early settlers quickly took up the process and improved it.
Today more than 2 million gallons of syrup and upward of 5
million pounds of sugar are produced. The sugar maple grows

350

General Botany

U. S. Depl. of Agriculture

Fig. 217. Map showing the acreage of sorghums, buckwheat, and velvet beans in the

United States.

best in the states from Wisconsin to Maryland and Maine.
The sap flows longest and the yield is greatest during a gradual
northern spring, when there is freezing at night, thawing in the
daytime, and a slow thawing of the ground. Such conditions
are most perfectly attained in Vermont, New York, and northern
Ohio, and the industry centers in these three states.

The distillation of hardwoods for the production of wood al-
cohol, acetate of lime, and charcoal is another industry that
centers in Wisconsin, Michigan, New York, and Pennsylvania
because of the large available supply of beech, birch, and maple,
and the nearness to blast furnaces, which are the chief users of
the charcoal. Much charcoal has been made in the past by simply
driving out the volatile matter in the wood by slow combustion
in pits. But in this way all the volatile matter was lost. Now the
wood is heated in great retorts, and the by-products are far more
valuable than the charcoal. Ash, oak, and hickory are also being
used for distillation as the more desirable species become scarcer.

Plant Industries and Climatic Plant Formations 351

Fig.

U. S. Dept. of Agriculture

218. Map showing the acreage of sugar crops in the United States.

Agriculturally, the deciduous forest region is best suited for
the production of winter wheat and corn ; but due to the fact
that the soils are far better in the prairies, the center of corn pro-
duction lies in Illinois and Iowa.

Because of the numerous cities and industrial towns scattered
from the Atlantic to the Mississippi River, market gardening
and the production of cut flowers, ornamental plants, and nur-
sery stock have been highly developed in this region.

The lands along the eastern and southern shores of Lakes
Michigan and Erie are favorable localities for the growth of
grapes, because killing frosts in the autumn are delayed by the
warming effects of the lake. These areas are also favorable for
the growth of peaches, because the lakes warm up more slowly
than the land in the spring of the year ; this retards the opening
of the buds until danger of late spring frosts is past.

Apple, pear, peach, and cherry orchards are scattered over this
region. These orchards produce a large part of the fruits of
this t3rpe that are marketed in the eastern United States.

352 General Botany

^^'^^■'■>->^ ''?™^^'l?r,a'' EACH OCT «P««EHTS

l^ TTT"*"^***.^ acreage. 1919 10,000 ACRES

A nff

/O— JV ~ \ ''~>/^ ^

/"~~--L i~ r^ iAiikk ] r oC^ ■ ■ T

) / ^~~" — 1 '^^^mS^w^^^W^ \—r^'''\ " t^

\ . / / • — 7~~— — — L-_ -H^lTOMBSwI^P^^^^^a. ]yr_^<=^^^v

S . ' • \ / / -WMnBlpJl^/E^J^^^^^^JJIfBTg^' "SiFTsShjjC / Ji ^^S

\ *■ \ h-—^^ ■ y^^ S, fef^# V^^'l.ji--*^

^ \r 1 p ^ — i, ■ . ■ _^y''w*'^"w''^ - ^

— '-' \ \ / -^-^Sii^

\ JJ^^^\\

i ^f -^

V. S. Dept. of Agriculture

Fig. 219. Acreage of corn grown for grain in 1919. This crop is raised over most of the
deciduous forest region, but the greatest production is in the prairie region because of the
fertility of the prairie soils.

The northern deciduous forest region was formerly the center
of production of sugar from the sugar beet. These plants re-
quire the richest of agricultural land. Michigan was at one time
the leading beet-sugar state ; but there are now scattered fac-
tories from California and Washington to Ohio, and irrigation
has recently made Colorado the leader in the production of beet
sugar.

Certain soils of the deciduous forest region, from Kentucky and
Pennsylvania to North Carolina and Virginia, are much utilized
for the growing of tobacco. This is a highly profitable crop and
gives a large return for each acre planted. In growing leaves
for cigar wrappers it is important that they be large, thin, and
have small bundles in the veins. Such leaves are secured, par-
ticularly in the Connecticut Valley, by growing the plants under
canvas ; this reduces the transpiration rate of the leaves, increases
the size, and insures all the desired qualities. Burley tobacco

Plant Industries and Climatic Plant Formations 353

U. S. Dept. of Agriculture

Fig. 220. Tobacco, rice, flax, and hemp acreage, igig. The regions in which tobacco is
grown are determined by soil characters; each particular tobacco-producing area grows
only certain kinds that develop on its soil the desired flavors. Flax is produced in the
northern Great Plains region, because the drought and the prolonged illumination favor the
production of seed. The necessity for hot growing seasons and inundation of the land for
rice production explains why the growing of the crop centers in the areas shown on the map.

is mostly grown on the limestone soils of Kentucky and Ohio.
Cigarette and light smoking tobaccos are grown on rather in-
fertile sandy loams on the eastern Coastal Plain.

Sorghum, used for the manufacture of table syrup and sorghum
molasses, is grown in the southern half of the deciduous forest
region. It resembles corn in appearance, and the syrup is ob-
tained by crushing it and evaporating the juice. In the North its
sugar content is lower and its cultivation is not so profitable. Its
cultivation is also excluded from soils rich in nitrates, because of
the increased amount of bitter substances present under these
conditions. As much as 50 million gallons has been produced
in one year.

i The southeastern conifer forest. The greatest plant industry
of the Southeast has been the lumbering of the longleaf pine, the

354 General Botany

shortleaf pine, cypress, white cedar, and gum. Cypress is an
especially valuable wood for use in building greenhouses, for
it withstands without decay warm and moist conditions.

The southeastern conifer forest region has also been the center
of production of turpentine and rosin, which are obtained from
the longleaf yellow pine. A V-shaped cut is made through the
sapwood of these big trees ; the resin from the wood flows out
slowly and collects in a large cup placed at the lower end of the V.
The cups are allowed to remain for several months and then
the accumulated resin is collected and distilled. The volatile
oil, turpentine, passes over into a condenser and the rosin is left
behind in the retort. The trees are tapped for three or four years
and then cut for timber. The annual output amounts to more
than 29 million gallons of turpentine and 3 million barrels of rosin.

Next to oak, the yellow pine is the biggest source of railroad
ties. It is also used for poles and fence posts and for soft-wood
distillation. The products of distillation in this case are char-
coal, turpentine, pitch, and tar. Red gum is the principal wood
used in the manufacture of cheap barrel staves, and pine is used
for barrel heads. Arkansas is the leading state in the produc-
tion of " slack " cooperage, and in the production of red gum,
yellow pine, and cottonwood veneers, now so extensively used
in packing crates, door panels, drawer bottoms, and chair seats.
Sumac leaves are gathered in large quantities in the southern
coastal plain, dried, ground up, and used as one of the sources of
tannin. About a fifth of the paper pulp now comes from the
southern yellow pine, gum, and cottonwood. Osage orange wood
is a promising source of dyes for wool, leather, wood, and paper ; the
shades of these dyes varies from orange-yellow to olive and brown.

The greatest crop plant of the southeastern forest region is
cotton. This plant belongs to the mallow family and is a native
of the tropics. It requires high temperatures and can be grown
only where the frostless season exceeds 6 months. Cotton lint
is made up of the hairs that thickly surround the cotton seeds ;

Plant Industries and Climatic Plant Formations 355

-5,11. .*c:^
U. ^. toresl Service

Fig. 221. Rcbins are formed by many plants and are produced in abundance by the pines.
This illustration shows the collecting of resin in a forest of Southern longleaf pines.

356

General Botany

these hairs vary in length from a half-inch to ij inches. For-
merly, only the fiber was marketed ; but today the hulls of
the seeds are used in the manufacture of hard paper board, the
^' meats " of the seeds are pressed for cottonseed oil, and the
" oil cake " that remains after pressing is used for stock feed.
The United States produces more than ii million bales (500
pounds each) of cotton. Seven and a half million bales were
consumed in this country in 1919; and at the same time 175
million gallons of oil and 2 million tons of oil cake were pro-
duced. Most of the oil is used in making soap, lard sub-
stitutes, and for salad oil.

Another coastal plain crop that has recently assumed great
economic importance is the peanut. This legume has the pe-
culiar habit of burying its developing ovularies in the soil by the

Fig. 222. A sugar-cane field in Porto Rico. The plant accumulates

stems.

Bruce Fink

sucrose in its solid

Plant Industries and Climatic Plant Formations 357

downward elongation of the flower stalk after pollination. The
fruit does not develop normally unless buried. For this reason
sandy soils are preferred for growing them. About one half of
the peanut is composed of oil. The bulk of the crop is utilized
in making confections and peanut butter.

Most of the rice produced in the United States is grown on
low alluvial lands and on delta soils with heavy clay subsoils
that can be readily flooded. Louisiana, Texas, Arkansas, and
California produce more than 35 million bushels. The land
is prepared and the seed planted as in growing other grain crops.
After planting, however, the land is flooded ; but before harvest
time the land is again dried out to permit the use of machinery
in gathering the crop.

The southeastern evergreen forest region produces most of the
sweet potatoes. They are largely consumed locally and partly
take the place of the Irish potatoes used in the Northern states.

About one third of the 2 J billion pounds of sugar produced
in the United States comes from sugar cane grown in Louisiana
and Texas. Sugar cane is a large perennial grass, 8 to 15 feet in
height, that accumulates sugar in its stems. After the leaves have
been removed, the stems are crushed and the juice is evaporated.
The refuse stalks are used as fuel and for making coarse paper.

Hay and forage crops of cowpeas and other legumes are impor-
tant throughout this region, usually ranking third or fourth in
importance, following cotton, corn, and sometimes oats. Be-
cause of its mild winters, this region is able to supply the East-
ern city markets with the first berries, melons, and fruits of the
season. Tea has been successfully grown in South Carolina,
but the great amount of hand labor required in picking and pre-
paring the leaves has prevented production on a large scale.

The prairie grass region. The northern prairies have become
the leading region of spring wheat production. The unusually
fertile soils and the bright sunshine in this region give ideal con-
ditions for abundant growth, and the dryness of the climate in-

358

General Botany

WINTER WHEAT ACREAGE

U. S. Dept. of Agriculture

Fig. 223. Map showing the winter wheat acreage in the United States.

creases the hardness and amount of gluten in the grains, thus mak-
ing the flour obtained from this wheat more valuable. Because
of these facts and because there is easily available water power
in this region, the greatest flour mills in the world are located
at Minneapolis. Barley and flaxseed are also produced in large
quantities on the northern prairies.

The bulk of the corn in the United States is grown on the rich
black soil of the central prairies, from eastern Kansas and Ne-
braska to Ohio.

Because of the abundance of feed, the great cattle markets
and the packing industries center in this same region. The va-
rious substances manufactured from corn and known as " corn
products " — starch, oil, alcohol, glucose, and meal — are pro-
duced here on a large scale. The secondary crops of this re-
gion are winter wheat, oats, hay, and sweet corn. The areas in
which sweet corn is grown abundantly determine the location
of large canning establishments and also factories for the manu-
facture of cans.

Plant Industries and Climatic Plant Formations 359

{H*r"~--^

SPRING WHEAT ACREAGE

A^^^V^^

1919

trTr

^^Si

\-\J~~~~r~

i^ \ Xr

'C^

EACH DOT REPRESENTS
10,000 ACRES

^^ii^

f. .S'. Dept. of Agriculture

Fig. 224. Map showing the spring wheat acreage in the United States.

The Great Plains. Originally the plains were the grazing lands
of the continent, and as the buffaloes were killed off cattle took
their place. At first the cattle were driven from one locality to
another, whenever the grass gave out or water became scarce;
and they were generally driven southward in the winter and north-
ward in the summer. But as the land came into private owner-
ship, ranches were established, and in addition to grazing at-
tempts were made to grow crops. Durum or " hard " wheat
was introduced from the steppes of Russia ; thus wheat growing
became possible 300 miles farther west than previously. From
Africa came a grain-producing sorghum known as " kafir
corn " and the millet called " milo," both of which thrive under
plains conditions. The growing of broomcorn, another variety
of sorghum, has become centralized in Texas, Oklahoma, and
Kansas. The flowering branches of this plant furnish the straws
for brooms.

The greatest forage crop of the central plains is alfalfa. This
is a perennial, deep-rooted clover that thrives on well-drained

360

General Botany

Cereal lHvesti(idt.un., L .S.D.A.

Fig. 225. A field of kafir corn in western Oklahoma ; a good dry-land crop.

soils. On the plains uplands it may produce two or three crops
of hay in a season ; on the lowlands it may produce three or
four, and in exceptional seasons, toward the south, five. Alfalfa,
like all clovers, has bacterial nodules on its roots that accumulate
organic nitrogen compounds. Thus by growing alfalfa in fields
for several years, the fields not only yield a good return, but if
the final crop is plowed under before planting other crops, the
soil will be improved in its nitrogen and humus content. The
growing of alfalfa has spread from the plains country into all
the Western irrigated districts and to the prairie and deciduous
forest regions. The drought-resistant millet grasses are of sec-
ondary importance as hay and forage crops on the plains and
prairie border.

The western evergreen forest region. The plant industries
associated with the western evergreen forest are, for the most
part, lumbering and the manufacture of lumber products. As
the timber of the Eastern states became scarcer and poorer in
quality, the exploitation of these great Western forests began;
and now products from them, such as rough lumber, shingles,

Plant Industries and Climatic Plant Formations 361

U . S. Forest Service

Fig. 226. Characteristic dense forest ot W estern hiemiock and Douglas fir in Washington.
The trees are from 5 to 8 feet in diameter.

362

General Botany

25 BILLION BD.FT.

■ FOREST LANDS

(PLOTTED TO SCALE OF MAP)

U . S. Dept. of Agriculture

Figs. 227 and 228. The upper map shows the relative amounts of standing timber in the
various states in 1919. The lower map indicates the areas of land in each state which
should be in forest. Note that the largest areas, suitable only for forests, are located east
of the Mississippi River near the greatest lumber markets and timber-consuming industries.
Our tax laws should be revised in such a manner as to encourage the reforesting of these lands.

Plant Industries and Climatic Plant Formations 363

I .\

^ e-i

i.%

^ X t

iri

■t.

f/. 5. Forest Service

Fig. 229, Western yellow pine plantation, ii years old ; Lolo National Forest, Montana.

pulp woods, finishing woods, excelsior, and tannin, are rapidly
taking a leading place, even in the Eastern markets. At the same
time more and more of the wood-consuming industries are be-
coming established in the West coast states.

The numerous mines scattered throughout this region have
consumed large quantities of wood for fuel and for mine props.
The railroads, traversing great stretches of plains and desert as
well as forest lands, have required a vast amount of timber for
poles and crossties.

The Douglas fir, found in such abundance in Oregon and Wash-
ington, has proved to be adapted to a great variety of uses. The
Western yellow pine, the lodgepole pine, and the sugar pine have
come into the market for rough timber, fuel, and construction.

3^4

General Botany

The giant cedar is the leading shingle wood of the United States,
and the Sitka spruce is the leading source of paper pulp in the
Western states.

Forest reserves and their uses. When the attention of lum-
bermen was turning from the Eastern forests to the timber on
the Western mountains, the United States government still owned
vast tracts of these forested lands. During the administration
of President Roosevelt, a definite policy was decided upon by
which many large areas of these forests were turned over to the
management of the United States Forest Service. By this plan
the forests are to be harvested as the trees come to maturity,
and the methods of harvesting are those which will insure a con-
stant supply of timber. Many of the forests in the drier regions
have an undergrowth of grass. It is highly desirable that these
grasses be utilized, and at the same time it is important that the
development of tree seedlings shall not be permanently prevented.

Fig.

U. S. Forest Setvtce
230. Typical section of a mountain slope in western North Carolina, after removal

of forest. The binding effects of the roots have been removed, and the erosion of the soil is
so rapid that it is difficult for seedlings to take hold. When the forest was cut, enough young
trees should have been left to hold the soil and start a new lumber crop.

Plant Industries and Climatic Plant Formations 365

James B. Berry

Fig. 231. The pinon pine "protection" forest of the desert mountains of the Great Basin
region. The stand is very open but is of great value in preventing wind and water erosion.
In addition, it suppUes the ranchers of the adjacent valleys with fuel wood.

Under proper supervision these grasslands are periodically grazed
by sheep, and the Forest Service receives a fee for the grazing
privilege. A third use made of forest reserves is the maintain-
ing of water supplies for irrigating purposes. In many of the
irrigated areas adjoining the mountains the rainfall on the moun-
tains is insufficient to maintain the streams throughout the grow-
ing season. The snowfall, however, is very considerable in the
mountains, and if the snow melts gradually it adds largely to
the available water at lower levels. When the forests are re-
moved, the snow melts rapidly and causes spring floods, which
not only waste the water but also cause erosion on the steep slopes
and destruction along the rivers at the bases of the mountains.
Where forests remain, the run-off is slower and the water more
evenly distributed.

Forest reserves, therefore, are areas set aside by the govern-
ment to maintain a timber supply, to provide grazing lands, to

366 General Botany

control water supplies for irrigation, and to prevent destructive
floods. They are not intended to prevent the cutting of ma-
ture timber or to keep suitable land from being settled and turned
over to agriculture. These lands are being accurately surveyed,
and whenever areas are found that are fitted to become farm-
lands they are sold to settlers to use for farming.

The total amount of land in the national reserves is 160 million
acres. In addition, many of the state governments have taken
over smaller forest tracts within their borders for the same purpose.
Out of the original 822 million acres in the United States only 137
milhons of virgin timber remain. Only 6 billion cubic feet of tim-
ber are added by growth on all forested lands each year, while
25 billion cubic feet are being consumed. This is the same as
saying that in the United States we are using the timber four times
as fast as it is being replaced by growth.

Reforestation. Many of the lands owned by the United States
and by the state governments have been partly or wholly de-
nuded of forests, and are worthless for agriculture. After being
lumbered, the tops and branches of the trees were not removed
and destructive fires swept away all that the lumbermen at the
time considered worthless. To make this land valuable, the for-
esters either replant the area with small trees grown in nurser-
ies, or plant seeds which will develop into trees where they ger-
minate. In areas where the forests have been only partly de-
stroyed, the species that remain are usually worthless for timber
purposes. In such places " improvement cuttings " are made ;
that is, the worthless trees are either cut for fuel purposes or
cut and burned to provide room for the young trees that will
make valuable timber in the course of time. Some trees, like
the chestnut of the East and redwood of the West, sprout from
the stumps when the trees are cut. Thus for several years these
trees make use of the roots of the parent tree and the food
materials stored in them. They possess a great advantage in
reforestation over the trees that have to start from seed.

Plant Industries and Climatic Plant Formations 367

U . S. Forest Service

Fig. 232. Fighting a forest fire. During a recent 5-year period, fire destroyed 56 million
acres of forest in the United States.

Fire patrol. Most of the Western forest reserves and of the
smaller state forests are regularly patrolled during the summer
and autumn to prevent the spread of fires started by careless
campers or by lightning. Airplanes and. permanent lookout
stations on mountain peaks aid in this work.

In recent years almost as much forest has been destroyed by
fire as has been cut into merchantable timber, and the fires have
consumed not only the trees but also the forest humus or '' duff,"
and made the areas unfit to establish valuable forests for genera-
tions to come.

Irrigation. Most of the large irrigation projects of the United
States are located in the semi-arid regions adjoining the lower
forest borders. Through the construction of dams and canals
the water from the mountains is made available throughout the
growing season. The soils of these areas are highly fertile, and

368

General Botany

Fig.

U . S. Dept. of Agriculture

233. Irrigated and unirrigated sugar cane, showing the value of suflScient water in the

growing of this crop.

because the supply of irrigation water is certain, these lands have
become unusually valuable and produce a great variety of crops.
The alfalfa, melons, and fruits of the Rocky Ford district of
Colorado ; the wheat of Idaho, Washington, and Oregon ; the
fruit orchards of Oregon and Washington; and the millions of
acres of oranges, lemons, citrus fruits, grapes, English walnuts,
almonds, figs, prunes, peaches, and apricots of southern and cen-
tral California are made possible by irrigating systems that use
the water from the adjoining mountains. The products of these
irrigated lands are one of the principal sources of wealth of these
Western states, as well as an important source of food for the
country as a whole.

The southwestern desert region. The border lands of the
desert, where they adjoin plateaus and mountain ranges, afford
large areas of xerophytic grasses, sagebrush, and chaparral for
grazing sheep, goats, and cattle.

The true desert regions also produce some vegetable products

Plant Industries and Climatic Plant Formations 369

'' "^^-r/ ■ •

>^\. '

■ ^1

^;%.:j-^'p^^'%}

4*i»^',:

; w ■'^^^^

fe*-

%ft?^ ":■■•' ■:.. - '

PF. 5. Cooper

Fig. 234. Mixed forest of oak {Quercus chrysolepis) and fir {Pseudotsuga mucronata) ;
Santa Cruz Mountains near Palo x\lto, California.

of economic importance. The various species of cactus {Opuntia)j
especially those forms with but few spines, are valuable as forage
for cattle. These plants contain at all times a considerable
amount of water, and in a region of such little rainfall this is an
important factor. When they are used as cattle food, it is cus-
tomary to burn off the spines with torches before feeding.

Guayule is one of the minor sources of rubber. It is a small
shrub which accumulates resinous material in the cortex of the
stem. Most of the supply has heretofore been obtained from
wild plants, but now its cultivation has been begun in Arizona
and California. The agaves of northern Yucatan and the pla-
teau of Mexico furnish the sisal and hennequin fibers used in
making binder twine. Other agaves furnish fibers of less im-
portance. The cultivation of agaves has spread to the West
Indies and other semi-arid parts of the tropics. " Pulpue,"
the Mexican national alcoholic drink, is made by fermenting and
distilling the juice of an agave.

370

General Botany

U. S. Dept. of Agriculture

Fig. 235. Cutting leaves from the sisal, an agave, in the semi-desert of Yucatan, for making
the fiber used in the manufacture of binder twine.

Fig. 230. Drying sisal fiber in Yucatan.

U. S. Dept. of Agriculture

Plant Industries and Climatic Plant Formations 371

""^-■^rir-^''' •' "

Fig. 2.37. The coconut palm in fruit. A tree under the
best conditions will yield a nut each day in the year.

The desert region produces abundantly when irrigated. The
Salt River Valley, in Arizona, is a good example of what can be
done under these conditions. As the valley is located in southern
Arizona, however, it has a subtropical climate, and consequently
many tropical crops can be grown. Alfalfa and cotton are the
most important crops at present ; the date palm has been suc-
cessfully transplanted from northern Africa and is now being
grown commercially. A great variety of other crops, such as
olives, figs, avocados, and tropical pawpaws, are also cultivated,
on a smaller scale.

373

General Botany

U. S. Forest Service
Fig. 238. Semi-tropical aspect of the vegetation of southern Florida. Live oak festooned
with Spanish moss {TiUandsia); the cabbage palmetto; and, in the foreground, young
orange trees.

The tropical forest region. Southern Florida, Mexico, Central
America, and the West Indies present a variety of forest and cul-
tivated plant products unequaled by any temperate forest re-

Plant Industries and Climatic Plant Formations 373

gion. Yet it may be safely stated that only a beginning has been
made in the utilization of tropical plant products ; certainly the
possible forest products are largely unknown.

Among woods of this region the most important is the so-
called '' mahogany " of commerce. Wood from not less than
forty different species of trees are imported into the United
States under this name. This gives some idea of the large number
of hard, fine-grained woods, suitable for cabinet work and veneers,
that occur in tropical forests. There are also many lighter woods
in the tropical forests which are suited to general construction
purposes, and eventually these will be exported to the United
States.

Formerly much wild rubber came from the American tropics,
but the yield is now so small as compared with the demand
that most of the rubber used in the United States comes from the
rubber plantations in the East Indies. Logwood and other
dyewoods are of growing commercial importance. Tobacco
is extensively cultivated. Sugar cane and cotton furnish the most
valuable products of herbaceous plants. Cinnamon is derived
from the inner bark of a tree now cultivated in the West Indies.
The bark is carefully removed, piled in heaps, and allowed to
ferment. When fermentation has reached a certain point, the
bark is dried and prepared for the market. Coffee is obtained
from the fruit of the coffee tree. The outer husk is removed and
the two seeds or '^ beans " in each fruit dried and sent to market.
Coffee is of some commercial importance in this region, but most
of our supply comes from Brazil. Nutmegs are also seeds of a
tree fruit. The fleshy part of the nutmeg fruit is discarded when
fully ripe, and the seeds dried. The outer coat is then broken
and removed and sold under the name of '' mace." The inner
part is the familiar nutmeg of commerce.

Tapioca is prepared from the starch of the cassava, which is
related to our American milkweeds. Cocoa is derived from the
seeds of the cocoa tree. Oranges, lemons, grapefruit, guavas,

374 General Botany

avocados, pineapples, and bananas are grown in large quantities
for export. Along the coasts the coconut palm is extensively
planted. It is valuable for its nutritious seeds and for the fibers
in the husks that surround the seeds. Chicle, the coagulated sap
of the naseberry tree, is a forest product used in the manufacture of
chewing gum. Vanilla is obtained from the dried fruits of a
climbing orchid native to America. Cloves are the unopened
flower buds of a small tree cultivated in the West Indies.

At higher elevations on the mountains of the tropical forest
region, wheat, corn, and beans are local sources of food for the
natives. Deciduous forests furnish valuable timber, and the
clearings afford rich pasture. The mountain slopes still higher
up are covered with pine forests.

CHAPTER THIRTY-FIVE

WEEDS AND THEIR CONTROL

The term " weed " is commonly applied to any undesirable
plant, and to any plant growing out of place. Rye may become
a weed in wheat fields. Red clover is very desirable in a field on
the farm, but it becomes a weed when it springs up in a lawn.
The most pernicious weeds, like the dandelion, cockle bur,
Canada thistle, poison ivy, bindweed, plantain, and sand bur,
are not desirable plants anywhere.

Weeds decrease the yield of crop plants, reduce the value of
grain and seed crops, interfere with the growth and use of forage
crops, and greatly increase the cost of agricultural production.
Many weeds are conspicuous and unsightly on farms and lawns
and thus depreciate the value of land. Some weeds are harmful
or poisonous to stock, and others impart unpleasant tastes to
farm and dairy products. Weeds may also harbor injurious
insects and the bacteria and fungi that produce disease. One
of the commonest sources of hay fever and asthma is the wind-
borne pollen of ragweed, horseweed, and other weeds.

High reproductive capacity. Weeds are plants in which re-
production has reached the highest degree of efficiency. The
sequoia may stand for the culmination of vegetative efficiency,
the dandelion for efficiency in reproduction and dispersal. The
dandelion produces good seed without pollination ; if the stem
is cut, the plant develops numerous new sprouts ; if the root is
cut into small pieces, each piece may sprout from either end or
from both ends at the same time. The dandelion can thrive in
a swamp, and it can withstand the droughts of a sand plain.
The sequoia still occupies the comparatively small area to which
it was restricted during the glacial-period. The dandelion has
in recent times spread to all parts of the world, and it occurs in
most habitats, from the seashore to the alpine summits of moun-
tains.

375

376 General Botany

How rapidly a weed may spread is illustrated by the history
of the Russian thistle. It was introduced into South Dakota
in 1874 with imported flaxseed. By 1888 there were enough
plants in the Dakotas to have it reported as a weed. In 1893
it was abundant around Chicago. In 1898 it was reported in all
the states and provinces east of the Rockies, from the Gulf of
Mexico to Saskatchewan.

The control of weeds. The measures taken to control weeds
depend first of all upon whether the weed is (i) an annual, like
crabgrass, smartweed, ragweed, or foxtail grass; (2) a biennial,
like blueweed, bull thistle, or wild carrot; or (3) a perennial,
like Johnson grass, Canada thistle, wild onion, or milkweed.

The first principle to be observed in controlling weeds is to
avoid bringing weed seeds to the farm or lawn. All seeds planted
should be inspected for weed seeds, and if they are present the
seed should be either cleaned or discarded.

The second rule is that no weeds should be allowed to produce
seed. Since annuals and biennials are propagated only by seeds,
the strict observance of this rule will ultimately rid an area of
these two classes of weeds.

The third principle of weed control is the prevention of the
growth of shoots. Depriving a plant of its photosynthetic tissues
leads to starvation of the underground parts. This principle
is particularly applicable to perennial weeds with underground
stems. The shoots may be destroyed by cutting, by spraying
with poisons such as salt, copper sulfate, and petroleum, or by
covering the area when small with roofing paper.

Preventing weeds from producing seed. A single plant of
many common weeds will produce hundreds or thousands of
seeds. Moreover, not all these seeds may germinate the first
year, and seedlings may continue to appear for several years.
Harrowing and cultivating farm lands not only improve soil
conditions for the growing crop, but they also destroy countless
numbers of weed seedlings, which in good soil is far more im-

Weeds and Their Control 377

portant. Mowing pastures and fencerows or pasturing off the
weeds with sheep and cattle are efficient means of destroying
weeds if practiced before they come into bloom.

Preventing the introduction of weed seeds. Weed seeds are
introduced not only by the purchase and planting of uncleaned
seeds but through other common practices. The use of fresh
manure is a common source of weed introduction. Stock feeds
made from screenings are likely to contain a large percentage of
weed seeds. Finally, the seeds that may be spread by the wind
from neighboring lawns and farms make the problem of weed
control a community affair. One careless neighbor is a menace
to the entire community, and he should be treated as we treat
any one who maintains a nuisance. Cooperation through com-
munity organizations is essential to efficient weed control.

Poisonous weeds. Not only may weeds reduce crop yields,
but some weeds are poisonous to human beings and to animals.
Poison ivy is poisonous to many persons who come in contact
with it. White snakeroot produces " trembles " in cattle and
" milk sickness " among human beings who use milk from such
cattle. Wild cherry leaves, especially when in a wilted con-
dition, are poisonous to cattle. Larkspur and loco weed are
poisonous to cattle and a great source of loss in Western pastures.
Wild onions, garlic, and other less-well-known aromatic herbs
produce unpleasant odors and tastes in dairy products. All
these plants can be destroyed by persistent and intelligent effort,
and the results are of such far-reaching importance that their
eradication will in the end be profitable.

CHAPTER THIRTY-SIX

THE NON-GREEN PLANTS

The green plants are called autophytes (Greek : autos, self, and
phyton, plant), because they are independent or self-supporting.
Given sunlight, they can make their own food from water, carbon
dioxide, and mineral salts.

There are, however, great numbers of plants that lack chloro-
phyll and hence are not able to make their own food. Many
of them, like the bacteria and yeasts, are microscopic in size;
others, like the molds and mildews, are small but visible to the
unaided eye; still others, like the puffballs, mushrooms, the
bracket fungi that are seen on trees and logs, and the Indian
pipe of the forest, reach a size comparable with that of many green
plants.

Energy necessary for life. In the study of biology it is well
to have in mind always that a perpetual-motion machine is no
more possible in the living than in the non-living physical world.
A living organism must have energy to carry on its activities
and life processes. Green plants, non-green plants, and animals
are alike in requiring an energy income for their life activities.

As we have seen, the green plant secures its energy from the
sunlight. The energy of the light is not used directly in the
operation of the vital mechanism, but it starts synthetic processes
within the plant that end in the complex compounds we call
" foods." These are then oxidized, and the energy required by
the plant is released in the breaking-down process.

Lacking chlorophyll, non-green plants cannot use the sunlight
in synthetic processes. They must, therefore, secure their energy
from materials already built up so that it can be oxidized.
This the great majority of them do either by living directly
on other living organisms or by feeding on dead organic matter
originally synthesized by green plants.

378

The Non-Green Plants

379

W. S. Cooper
Fig. 239. The dodder (Cuscuta), a yellow parasite belonging to the morning-glory family,
grows on other plants not only in moist regions, but also in the arid coastal region of Cali-
fornia. It is here shown growing on Abronia.

Some small non-green plants, however, secure their energy,
not from organic substances, but by oxidizing inorganic salts.
The most important of these are the nitrifying bacteria in the
soil that oxidize ammonia and nitrites to nitrates in their respira-
tory processes. With the energy thus secured they construct
carbohydrates, fats, proteins, and organic compounds. These
plants are autophytes as truly as the green plants. They live
in the soil independent of other plants, and they can grow without
organic compounds. Another group of bacteria common in
sewage-polluted water is able to secure energy through the
oxidation of inorganic sulfur compounds.

Vital syntheses. In this connection it may be noted that
living organisms differ greatly in their synthetic powers. The
green plant using the energy of the light in the first processes
can build everything that it requires. Colorless plants, if given
sugar or other carbohydrates that they can use, can construct
fats and proteins, and, as we have seen, some of the bacteria

380 General Botany

can even synthesize their own carbohydrates. An animal can
transform carbohydrates into fat, but it apparently lacks the
power to make certain vitamins and several of the amino acids
needed in protein synthesis.

Parasites. An organism that derives its food directly from
another living organism is called a parasite. A parasitic plant
may live inside the host, from which it secures food, as is the case
with many bacteria and fungi. Or a parasitic plant may be
merely attached to the host plant at one or more points. Beech
drops are small, purple, flowering plants attached to the roots
of beech trees. The dodder, or " gold thread," is a slender,
yellow, climbing plant, related to the morning-glory, that becomes
attached to the stems of a great variety of hosts by means of
small, root-like structures {haustoria; singular, haustorium) that
penetrate the cortex of the host and finally reach the conductive
tissues (Fig. 239).

True parasites among the flowering plants are generally small ;
their leaves are mere scales, and the most prominent parts are the
flowers and the reproductive structures. In color they vary
from yellow to red and purple. Some apparently do not injure
the host plant ; others may injure and eventually kill the plant
on which they grow.

Partial parasites. Some parasites contain chlorophyll and
are able to manufacture at least a part of their foods. For
example, the mistletoes occur on a great variety of trees from the
Atlantic to the Pacific, and are very common particularly in the
subtropics. The sticky seeds adhere to the bark of branches,
and a root-like haustorium dissolves its way into the bark and
forms a connection with the conductive tissues of the host
trees. Mistletoes sometimes form much-branched masses of
stems and foliage 2 or 3 feet in diameter. These plants have
lost the power of growing on soils, and apparently are dependent
on their hosts for their water supply and a part of their foods.
They may be called partial parasites.

The Non-Green Plants

381

U . S. Forest :iervice

Fig. 240. A winter view in Texas, showing the mistletoe, an ever-
green parasite, growing on the deciduous mesquite.

Saprophytes. A saprophyte is a plant that depends for its
food on dead organic material. These plants live in the soil, on
the dead bark and heartwood of trees, and on a great variety of
plant products. They are exemplified among flowering plants
by the Indian pipe, a common plant of moist woods. The body
of the plant consists of a rather large root-like base from which
colorless branches bearing flowers arise. From the humus in
which it grows it secures water and organic compounds sufficient
to furnish the materials and energy used in building its tissues.
Like many of the flowering plants the root-like base of the
Indian pipe is penetrated by fungi, which seem to be essential
to its growth. Perhaps the fungi aid in transforming a part of

;82

General Botany

Fig. 241. 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.

the humus into substances that are readily assimilated by the
plant.

Among the bacteria and fungi there are thousands of sap-
rophytes. They occur everywhere, and the amount of change
that they bring about in the world is so great that it is impossible
to overestimate their importance. Saprophytes are the direct
causes of all decay and fermentation. They are present in the
alimentary canals of the higher animals, and aid in the digestion
of food. They are ever-present agents of destruction, and are
the organisms that make cold-storage houses and refrigerators

The Non-Green Plants 383

necessary. The canning, drying, and preserving industries are
based on methods of ehminating saprophytes. The beer, wine,
vinegar, and cheese industries depend upon the fermentations
induced by carefully cultivated saprophytes. The tarring and
creosoting of telegraph poles and railroad ties are made necessary
by the universal presence in the soil of these destructive plants.

Among the one-celled plants there are some that can live either
as green autophytes or as colorless saprophytes. There are many
that may live either as saprophytes or as parasites. It is often
very difficult, therefore, to classify plants among these three
groups or to determine the exact sources of their food and energy.

The non-green plants, then, include a very large number and
variety of plants. Autophytes are world-wide in their distri-
bution ; the occurrence of a species is limited only by climatic
and habitat conditions. Parasites are widely distributed, but
any species is limited by the occurrence of its particular plant
or animal hosts. Saprophytes occur everywhere where organic
matter exists. The non-green plants are not so conspicuous as
the green plants, but they are of overwhelming importance to
plants, to animals, and to man.

REFERENCE

Marshall, C. E. Microbiology (3d edition). P. Blakiston's Son & Co., Phila-
delphia; 192 1.

CHAPTER THIRTY-SEVEN

BACTERIA AND THEIR RELATIONS TO LIFE

The best-known and the most discussed of all the non-green
plants are the bacteria. They are so intimately related to human
welfare that most persons, even though they have never seen
bacteria, know something about them. They are one-celled
plants, at once the smallest in size, the simplest in structure, and
the most abundant of all plants. They live in immense numbers
in the water and in the upper layers of the soil, and they are
blown about in dust in the air. Some are too small to be seen
except with the highest powers of the microscope. Others may
be seen with an ordinary laboratory microscope. They make up
for the small size of the individual by their rapid multiplication
and by the formation of colonies containing countless numbers
of individuals. Bacteria are responsible for many of the diseases
of men, animals, and plants, and bacteria affect our lives in
almost countless other ways. All our modern methods of sanita-
tion, quarantine, surgery, water supply, and sewage disposal,
and much of our personal hygiene, are primarily based on our
knowledge of the behavior of this group of plants.

Economic importance of bacteria. Economically the bacteria
are of the greatest importance. Together with the fungi they
are the principal cause of disease, decay, and the formation of
humus. Bacteria bring about the ripening of milk in butter and
cheese making, and they produce both the pleasant flavors in
these products and the unpleasant flavors that develop in them
with age. The bacteria are also the source of much of the
available nitrogen in agricultural soils. The drying of hay,
vegetables, and fruits, the canning and pickling of vegetables,
fruits, and meats, and refrigeration and cold storage are methods
of avoiding or making impossible the growth of bacteria. Thus a
knowledge of these plants is fundamental to our understanding
of thousands of details of our daily life.

384

Bacteria and Their Relations to Life

385

Environmental conditions affecting bacteria. Like the higher
and more complex plants the bacteria have certain rather definite

Fig. 242. Various forms of bacteria.

% %

water, temperature, light, and nutritive requirements for growth
and reproduction. The different species vary greatly in these
requirements ; consequently some kinds of bacteria are able to
live almost everywhere in nature.

Moisture. Since water makes up about 85 per cent of the
bacterial cells, water is essential to their activities. Furthermore,
since all of their nutrient materials are absorbed by diffusion, they '
must be surrounded by at least a film of water. The water or
solution in which bacteria live is commonly called the medium
(plural, media), and its properties are determined by the sub-
stances it contains.

For example, sugar and salts may be dissolved in the medium,
thus determining its concentration. In dilute solutions the water
and nutrient materials diffuse readily into the cells. In con-
centrated solutions (15 to 40 per cent) the concentration of water
is less in the media than inside the cells, and water either does not
pass in or diffuses out of the cells and the bacteria are unable to
grow. They are affected in the same way as if they were dried.
This explains why jellies keep more readily than preserves, pre-

386

General Botany

serves more readily than canned fruits, and canned fruits more
readily than fruit juices to which no sugar has been added. The

Fig. 243. The carbon cycle in nature. Bacteria and other saprophytes play
role opposite to that of the green plants.

first has a high concentration of sugar, the last a low concentra-
tion. Bacteria develop very slowly in the first medium and very
rapidly in the last. In the laboratory, bacteria are cultivated
on gelatine or on agar (seaweed jelly) plates. Many bacteria
grow very slowly when the water content of the gelatine falls
below 50 per cent. If the medium on which they live dries out,
all the vegetative cells become inactive and death gradually

Bacteria and Their Relations to Life 387

follows. However, some bacteria, especially those found in
soils, may be dried for days, months, and even years and remain
alive. Most disease-producing bacteria cannot withstand desic-
cation, so that there is little danger of their being spread by dust.

Temperature. Bacteria have the temperature of the medium
in which they live. Low temperatures retard the life processes,
and high temperatures accelerate them. Likewise at low tem-
peratures less amounts of food are consumed ; hence they may
live longer on a limited supply.

Between the highest and lowest temperatures at which an
organism can live is a point at which it develops most rapidly,
called the optimum, or best, temperature. Most bacteria grow
best in temperatures between 70° and 100° F.

Very few bacteria grow well above 115° F. There are some,
however, that live in rapidly decaying organic matter (e.g., in
silos and self-heating hay) and in hot springs at temperatures as
high as 175° F. — a most remarkable fact, when we consider that
proteins which make up so much of the protoplasm commonly
begin to coagulate at 145° F.

At the freezing point most bacteria grow very slowly. When
freezing occurs and the medium becomes solid, diffusion of nu-
trients no longer takes place and all activities are checked. The ■
bacteria may remain alive, however, for weeks and months in
this condition.

Light. Bacteria living in nature in the soil, in decaying matter,
in foods, and inside plants and animals are only temporarily
exposed to the light. Most of them cannot withstand exposure
to full sunlight for even a few hours. This action of sunlight is
of great importance in the purification of rivers and in the destruc-
tion of bacteria on streets and sidewalks. Death is brought about
either by chemical processes initiated by light within the cells
or in the medium.

Oxygen. Bacteria are very sensitive to oxygen. Although it
makes up 20 per cent of the atmosphere, bacteria are exposed only

^8S General Botany

to the oxygen that dissolves in the water surrounding them. At
room temperature this forms an extremely dilute solution (0.0009
per cent). If the oxygen content is increased artificially to
thirty times this amount (0.027 P^^ cent), practically all bacteria
die. In other words, oxygen is about as poisonous to bacteria
as formaldehyde and corrosive sublimate, two of the commonly
used disinfectants.

Nevertheless, small amounts of oxygen favor the growth of
most bacteria. On this account they are called aerobes (Greek :
aer, air, and bios, life). Some bacteria, like the germ of lockjaw
and the bacteria that produce the rancid taste of butter, can
grow only when the oxygen content of the medium is extremely
low and when there are organic substances available containing
combined oxygen. These bacteria are called anaerobes (Greek :
an, without). Anaerobic bacteria occur in poorly drained soils,
in the bottoms of lakes, and in the deep waters of the ocean.

The effectiveness of hydrogen peroxide in dressing wounds
and cleaning teeth depends upon the fact that it releases oxygen
readily.^

Food supply. Almost all bacteria require organic foods, and
live usually as saprophytes or parasites. They all depend upon
the oxidation of a part of these foods for their energy. They
differ widely in their food requirements and in their effects upon
the medium in which they live.

The most important exceptions to this general rule of requiring
organic foods are found in the nitrifying bacteria of soils. These
resemble green plants in the fact that they can synthesize organic
compounds from CO2, H2O, and mineral salts, but differ in that
they cannot utilize sunlight.

Structure and reproduction. Bacteria consist of one-celled
individuals, that occur usually in masses on or in the food-

^ Hydrogen peroxide changes to water and oxygen on exposure to the air :

2H2O2 ^2H20+02

Hydrogen peroxide — >■ water + oxygen

Bacteria and Their Relations to Life 389

containing medium. They are so small that the details of cell
structure are not well known. The protoplasm is surrounded
by a cell wall probably composed of cellulose and chitin. In
some forms protoplasmic threads, called flagella (singular,
flagellum) extend through the cell wall and provide organs of
locomotion. The flagellate forms are active individuals, that
become stationary later and lose the flagella.

Many bacteria have each cell further surrounded by a gelat-
inous sheath. Sometimes the sheaths of many individuals
coalesce, forming slimy scums on stagnant water and on objects
in the water.

When all the conditions are favorable, bacteria may multiply
very rapidly. This is accomplished by cell division, the cell
simply pinching in at the middle and separating, forming two
new individuals. As the daughter cells quickly grow to the size
of the original, this process may be repeated in 20 minutes to an
hour. A little calculating will show that if this process continued
for 24 hours there would be hundreds of million-millions of
individuals.^ Of course, long before any such number can
accumulate, the water and food supplies are consumed and the
products of their activities accumulate and cell division is
stopped. If this were not true, the whole organic world would be
turned to bacteria over night.

Spores are formed by many bacteria, by the contraction of the

^ Starting with one bacterium and counting a generation every half hour,
the number at the end of a day would be 281 million-millions, or about one
pint of bacteria. Starting the second day with one pint of individuals all
multiplying at the same rate, at the end of 48 hours there would be 281 million-
million pints of bacteria, or about 32 cubic miles. At the end of the third
day there would be enough to fill the ocean basins 3 milHon times, or sufficient
to make 33,000 bodies the size of the earth.

Why do not bacteria capture the earth? First, because they produce
acids and other harmful substances in the medium, that stop their develop-
ment ; second, because they can obtain only the food that diffuses to them from
infinitely small distances beyond their own cell walls ; third, because they soon
meet unfavorable temperature, moisture, or light conditions ; and fourth,
because they are eaten by microscopic animals in large numbers. A short
life is the rule among bacteria.

390 General Botany

protoplasm into a rounded mass at one end, or near the middle
of the cell, and by the secretion of a secondary spore wall. In
this condition the protoplasm contains less water and is highly
resistant to drying, to high and low temperatures, and to poisons
which readily kill the ordinary bacterial cells. It is because the
spores of certain forms withstand the temperature of boiling
water that steam pressure is used in sterilizing cans of corn,
beans, peas, and other vegetables. Most of the common disease-
producing bacteria, however, do not produce spores.

Forms of bacteria. Some of the largest bacteria form long
rows, or filaments of cells. These may be found commonly in
stagnant water or in streams that carry sewage. Among the
small forms it is customary to call the rod-shaped cells Bacillus
(plural, bacilli) the round ones. Coccus (plural, cocci), and the
spiral forms Spirillum. Some of these type-forms are shown in
Figure 242.

Bacteria and sanitation. The bacteria of decay help to keep
the surface of the earth clean. They change the highly complex
organic substances that form the bodies of plants and animals
into simple substances that may be used again by other plants
in building foods. When plants and animals die, their bodies are
gradually transformed by the bacteria into carbon dioxide, water,
and mineral salts. The sewage that is turned into our rivers is
chemically changed and disposed of in the same way by these
minute plants. The great increase in the number and size of
our cities has made it necessary to build large sewage-disposal
plants where the bacteria can act rapidly and efficiently. This
prevents the pollution of streams and keeps the water suitable
for city water supplies.

The modern processes of filtering and sterilizing the water
supplies of cities are carried on partly to remove sediment and
partly to remove disease-producing bacteria. Adding minute
quantities of alum and chloride of lime to the water and then
filtering it through sand not only renders the water clear but

Bacteria and Their Relations to Life 391

removes from it disease-producing bacteria. The most dreaded
of all the water-borne diseases is typhoid fever, and the cities
are now much freer from this disease than are the country
districts where people depend upon well water. Surveys in some
of the Middle Western states showed that from one fifth to one
third of the wells examined contained large numbers of bacteria
derived from surface drainage. In such wells there is always
danger that the surface waters may bring in disease-producing
bacteria, especially typhoid germs derived from human sources.

Other sanitary practices, such as quarantine, disinfection,
admitting plenty of sunshine into living rooms, cleaning walls
and floors, removing dust, cooking food, washing and scald-
ing dishes, pasteurizing milk, and keeping food supplies in
refrigerators, are all related to the control or elimination of
bacteria.

Bacteria and disease. When certain bacteria grow in the
body, they produce poisonous substances called toxins. These
interfere with the normal working of the bodily processes and
cause illness. The body under these circumstances produces
substances called antitoxins. These are substances formed by
the cells of the body, which neutralize the effects of the toxins,
either by combining with them chemically, or by rendering the
cells insensitive to the toxins. In this way they protect the
tissues until the bacteria are destroyed by leucocytes (color-
less blood corpuscles) or in other ways. Not all persons are
equally susceptible to infectious diseases. A person may be
immune to a disease because his blood contains the corresponding
antitoxin or is able to produce it, or because his body is insensitive
to the bacterial toxins. Some of the commoner bacterial diseases
are tuberculosis, pneumonia, grippe, diphtheria, typhoid fever,
colds, lockjaw, and " blood poisoning."

A fundamental fact that should be learned in this connection
is that no one can contract a bacterial disease unless he comes in
contact with the particular bacterium which causes that disease.

39^ General Botany

Furthermore, persons rarely contract bacterial diseases unless
they come in contact with another person carrying the disease.
With the exception of lockjaw and wound infections, diseases
are rarely spread by clothing, dust, or other objects. Apparent
exceptions to this statement are typhoid and diphtheria, carried
by water, milk, and other foods when handled and contaminated
by a diseased person. Typhoid may also be carried by flies
that have visited infected matter.

Natural barriers to disease. The natural means of defense
against disease are somewhat similar in the higher plants and in
animals. The plant, in addition to protective chemical sub-
stances within its cells, has an epidermis which renders the
entrance of bacteria diihcult. Bacteria are able to enter, how-
ever, if the epidermis is bruised or broken. Plants probably
suffer from bacterial diseases as much as do animals. Most of
the well-known plant diseases, however, are produced by fungi.
Of the bacterial diseases of plants, the twig blight of pear and
apple, the cucumber wilt, and the crown gall of various plants
are perhaps best known. Some of these diseases are transported
from one plant to another by insects.

Bacteria in the dairy. Milk is an ideal medium for the growth
of bacteria. This makes necessary the most careful handhng of
milk, especially when it is used directly as food. The bacteria
get into the milk from the cow, from the stable, from the vessels
into which the milk is put, and from the persons who handle it.
Evidently the cows should be kept clean, and the stable should
be as clean and free from dust as possible. The vessels with
which the milk comes in contact should be sterile. The dairy-
men should have clean hands and clothes, and above all they
should be free from infectious diseases. Because bacteria multiply
very rapidly at high temperatures, the milk should be chilled
at once and kept on ice. To make butter and cheese of fine
flavor, pure cultures of the proper bacteria are added to the milk
and allowed to develop for a time.

«.

Bacteria and Their Relations to Life 393

In order to avoid the danger that Kes in the use of milk con-
taminated with disease germs, milk that is shipped into the
large cities is usually pasteurized before being sold. This treat-
ment kills most of the bacteria, destroying all the kinds that
produce disease in human beings. By " pasteurization '* is
meant the heating of the liquid to 150° or 160° F. for from 10 to
30 minutes. This does not kill the spores, but they are to a
large extent prevented from developing by the subsequent
cooling that the milk receives.

The preservation of foods. The greatest losses that occur in
the utilization of crops are connected with the distribution of the
products to the consumer. Much of the food produced never
reaches the consumer, because bacteria and molds render it
unfit for use before it can be distributed through the markets.
There are four methods of preventing this loss : (i) cold-storage
warehouses and refrigerator cars are used to keep foods below
the temperature at which bacteria grow appreciably; (2) fruits,
vegetables, or other foods are packed in cans, and the cans are
then sterilized by heat and are sealed so that they are bacteria-
tight ; (3) food products are dried to make it impossible for
bacteria to grow in them ; and (4) foods like meat and fish are
treated with salt or with some other chemical that will prevent
the growth of bacteria. Refrigeration enables us to preserve
foods for weeks and months. Canning and drying make foods
available after months and years of storage.

Soil bacteria and humus. In the process by which the bac-
teria of decay destroy animal and vegetable bodies, the humus
represents the products of partial decomposition, particularly
of cellulose. Carbohydrates, fats, proteins, and related com-
pounds are all subjected to bacterial action. Some are oxidized,
and some are split into less complex substances. Among the
many products of decay are hydrogen, marsh gas, organic acids
(e.g., acetic, butyric), ammonia, hydrogen sulfide, carbon dioxide,
and water. Usually the production of the final products CO2,

394 General Botany

H2O, and nitrogen are delayed by the formation of rather stable
intermediate products. These form the humus of soils.

Some of the bacteria of decay are of importance in industrial
processes, as in the retting of flax and hemp fibers and in the prep-
aration of hides for the making of leather.

Spontaneous generation and bacteriology. Not many years
ago it was thought, even by the most learned persons, that the
minute plants and animals that occur in stagnant water and that
cause decay and fermentation arose spontaneously in the water.
It was the experiments of Pasteur (1862) and Tyndall (1869) that
finally proved that the organisms get into liquid media from the
air. It was these studies that led to the discovery of the relation
between bacteria and disease. The experiments of Lister (i860)
led to the use of antiseptics (Latin : anti, against, and septicus,
putrid) in surgery. Modern methods of sanitation, the control
of diseases, and antiseptic surgery have all been developed since
i860. It is quite impossible for us to realize to what extent the
dangers to life have been removed through the development of
the science of bacteriology. This science has also made it possible
to make use of bacteria in many important industries.

Methods of killing and controlling bacteria. Long before the
discovery of the importance of bacteria, many methods of pre-
serving foods, of caring for wounds, and of avoiding disease
had been tried. They were very crude when compared with
those that have been perfected since bacteria have been carefully
studied. In the following table some of the methods of control
are listed, and opposite them are a few domestic and industrial
applications. Can you add to the list ?

1. Cleanliness Washing, keeping down dust, certified milk,

disposal of garbage, sewage disposal

2. Ventilation Sleeping porches, open-air schools

3. Sunlight Purification of water supplies

4. Drying Hay, fruits, vegetables, milk, eggs, pem-

mican

Bacteria and Their Relations to Life

395

5. Refrigeration

6. Antiseptics :

Common salt

Acetic acid
Hydrogen peroxide
Chloride of lime
Formaldehyde
Corrosive sublimate
Iodine

7. High osmotic pressure

by salt and sugar

8. Sterilization by heating

to boiling point

Pasteurization
Sealing

Precipitation by alum
Vaccination
Antitoxins

Avoiding contact with
infected persons

INIeats, fruits, vegetables, dairy products

Meat packing, cleansing mucous mem-
branes, surgery
Packing and pickling
Cleansing wounds, preservation of milk
Purifying water supplies
Fumigation, seed treatment
Sterilizing wounds, surgery
Sterilizing wounds, surgery

Curing meats, preserves, jellies

Canning and cooking, seed treatment,

sterilizing surgeons' instruments
Milk, beer, wine

Canning, sterile bandages, and dressings
Water supplies

Typhoid, bubonic plague, "colds"
Diphtheria, tetanus

'' Colds," influenza, and other diseases

W. B. Saunders Company,

REFERENCES

DucLAUX, E. Pasteur: The History of a Mind.

Philadelphia; 1920.
Smith, E. F. Introduction to Bacterial Diseases of Plants. W. B. Saunders

Company, Philadelphia ; 1920.
Vallery-Radot. The Life of Pasteur. Doubleday, Page & Co., Garden City,

New York.

CHAPTER THIRTY-EIGHT

SOIL BACTERIA AND THE NITROGEN CYCLE

Next to carbon, hydrogen, and oxygen the most important
element used by plants is nitrogen. Agricultural crops on
mineral soils are very frequently limited by an insufficiency of
this element. As we shall see, the occurrence of nitrates in soils
is due almost entirely to the action of bacteria and fungi. Owing
to differences in their modes of life, several groups of nitrogen
bacteria are distinguished, all of which play an important role
in the nitrogen cycle in nature.

Nitrifying bacteria. In order to manufacture proteins, seed
plants must have a supply of nitrogen, usually in the form of
nitrates. There may be other nitrogen compounds in the soil,
but they are for the most part unavailable until certain nitrifying
bacteria change them to nitrates. Ammonia is one of the nitro-
gen compounds produced in the process of humus formation.
If the soil is moist, the temperatures high, and the drainage
sufficient to provide an adequate air supply, ammonia will be
acted upon by certain bacteria and changed to nitrites, which in
turn are changed by other bacteria into nitrates. These are
oxidizing processes, and the energy liberated is used by the nitri-
fying bacteria in the various chemical syntheses necessary to
transform CO2 and H2O and the soil salts into food and into
tissue substances. These plants are as truly autophytic as the
complex green plants. They require nothing but inorganic
substances to maintain themselves. Wherever ammonia occurs,
the nitrifying bacteria soon make it available for green plants.

Saprophytic nitrogen-fixing bacteria. Still other bacteria bring
about a process known as nitrogen fixation, by which nitrogen is
actually taken from the air and built into compounds which
are added to the soil. The nitrogen-fixing bacteria are, with a
few exceptions, the only plants that can take nitrogen from the
air and combine it to form nitrogen compounds. They flourish

396

Soil Bacteria and the Nitrogen Cycle

397

F. Lohnis, U.S.D.A.

Fig. 244. Nodules containing nitrogen-fixing bacteria on the roots of legumes: A, red
clover ; B, sweet pea ; C, soy bean.

only in rich, well-drained soil. They are of great importance in
agriculture because nitrogen is the most expensive of all the ele-
ments that are bought for fertilizers. Their relation to the
humus IS very different from that of the nitrifying bacteria just
described. To fix nitrogen (N2) — that is, break up the mole-
cules— requires much energy. The nitrogen-fixing bacteria
secure this energy by oxidizing the carbon compounds (especially
carbohydrates) found in the humus. It is estimated that 100
pounds of humus must be oxidized for every pound of nitrate
formed in the soil. The bacteria that carry on this process are
true saprophytes.

Bacteria and legumes. Clover, alfalfa, beans, soy beans, and
peas belong to a family of plants called legumes. They increase
the nitrogen in soils on which they are grown, and for many
years they have been used in crop rotations, following wheat or

398

General Botany

^., "^^.- 'jT ■^ "•"

4. -^

^^4i^^^

F. Lohnis, U.S. D. A.

Fig. 245. A, cross-section of root nodule of a legume; B, a single root cell showing nitro-
gen-fixing bacteria within it ; C, branched bacteria from a nodule.

corn. The practice of using legumes in crop rotations was fol-
lowed long before the real cause of the increase in soil nitrogen
was discovered, and even before it was understood how the dif-
ferent elements in the soil contribute to its fertility. By expe-
rience it was learned that other plants flourish on land after
leguminous plants have been grown on it, and for this reason
the farmer included legumes in his scheme of crop rotation.

It is now clearly understood that nitrogen compounds accumu-
late in leguminous plants only because of the presence of certain
nitrogen-fixing bacteria. These bacteria occur in many soils,
and when the legume is planted and develops roots, they invade
the cells of the root. This causes the infected parts of the root
to enlarge, forming nodules. If a nodule from a clover or alfalfa
root is crushed and examined under a microscope, it will be found
to be filled with bacteria. These bacteria are parasites and take
their food from the legume. A part of it they use in building
their tissues ; the remainder is oxidized and the energy used in
changing nitrogen from the soil air into nitrogen compounds,
just as the other nitrogen-fixing bacteria mentioned in the pre-

Soil Bacteria and the Nitrogen Cycle

399

ceding section do. The nitrogen compounds thus formed are
used by the host plant, and when the latter is plowed under and
decays, the nitrogen compounds are made available for a succeed-
ing crop of wheat or corn.

The nitrogen cycle. It may be well at this point to call to
mind all the facts we have learned concerning the uses of nitro-
gen and the transformations that it and its compounds undergo in
nature, involving the nitrogen of the air and the nitrogen com-
pounds of soils, of plants, and of animals (Fig. 246).

If we start with green plants, carbohydrates (see diagram) com-
bine with (i) nitrates and form (2) amino acids. These are built
up into (3) proteins of plants. Plant proteins are the sources of
amino acids used by animals in building (4) animal proteins.

Carbo- \

tMmino \

^ ^y (3) \

J SPlant \

hydrate j_

A acids j

\ proteins I

( (Animal
\l proteins

[g)eath ]

[ J\(ttrates

qyenitrijyincf
l,acterio

lj^itro^en\

[mant \
{^animal

\ 'NO J J

\ ^' /^---'-^

, \residues J

\ ... — ^~\

Y <^) \

f(6>\

\ J\At rites \

iftAmmonia)

v!™->

^itrijyin9\ ^Hj )
bacterid ^>^.__^^

Fig. 246. Diagram of the nitrogen cycle in nati

400 General Botany

Animal and plant proteins are a part of the residues (5) left by
death of the plants and animals.

Bacteria now become active agents of disintegration. By
hydrolysis, reduction, and oxidation the complex substances of
the cells are broken up into simpler compounds. Among these is
(6) ammonia. This compound, if liberated in soil in the presence
of water and carbon dioxide, forms ammonium carbonate
((NHJ2CO3).

A second group of bacteria, the nitrifying forms, use the am-
monia and oxidize it to a nitrite (— NO2). The nitrites are in
turn oxidized by other nitrifying bacteria to nitrates (— NO3).
The cycle is complete and we are back where the process started.

Notice, however, that we have only used nitrogen that occurred
in organic matter, and that some has been lost by the way, by
going into the atmosphere as nitrogen gas. Furthermore, the
above cycle is not the only possibility. The soil may not be
well drained and aerated, and the (i) nitrates are then attacked
by denitrifying bacteria and broken down to (8) nitrogen gas.
Thus it is released and becomes unavailable for the higher green
plants. So if there were not some means of securing an additional
nitrogen supply, the land would become poorer and poorer as
time went on.

The nitrogen-fixing bacteria provide this additional supply.
In well-drained neutral soil the saprophytic varieties that ob-
tain energy by oxidizing carbon compounds in the humus fix
enough atmospheric nitrogen to form the compounds used in
building their own cells. At their death these compounds be-
come available to other bacteria and the first cycle is repeated
with the addition of nitrates from the air to the soil. The bac-
teria that live in the nodules of legumes also build the free ni-
trogen of the air into organic compounds, and when the bac-
teria die the nitrogen compounds become directly available to
the legume plant. If legumes are plowed under, the cycle
starts over again with plant residues (Fig. 247) and in a few
months it has come around to the nitrates.

Soil Bacteria and the Nitrogen Cycle

401

Bureau of Agriculture, P. I.
Fig. 247. Cowpeas, which, like other legumes, accumulate nitrogen from the air and
build it into organic compounds. The advantage of growing a legume in any system of
crop rotation is that these compounds may be added to the soil by plowmg the plants under.

So the ever changing nitrogen compounds pass from the soil to
higher plants, to animals, to a succession of bacteria, and by the
changes that they undergo they increase or decrease the fertility
of the soil. If we understand each of these stages in the cycle
and the conditions that favor the changes from one stage to
another, and the injection of atmospheric nitrogen into the
cycle, we can improve the nitrate content of soils and greatly
increase crop yields.

REFERENCE

Marshall, C. E, Microbiology (3d edition),
delphia; 192 1.

P. Blakiston's Son & Co., Phila-

CHAPTER THIRTY-NINE

FUNGI

Or the plants without chlorophyll the most conspicuous are
the fungi. They form an exceedingly large and diversified group,
ranging in size from microscopic one-celled forms almost as small
as the larger bacteria to the large, fleshy mushrooms and to the
massive bracket fungi found on tree trunks and logs, which
may weigh 20 or 30 pounds. Among the most important fungi
are the yeasts, molds, mildews, smuts, rusts, and mushrooms.

All the fungi derive their food either from living plants,
from animals, or from dead plant and animal tissues and their
products. The yeasts, molds, and most of the mushrooms are
saprophytes. These and their bacterial associates are the chief
agents of fermentation and decay. The smuts, rusts, and some
of the mildews are parasites on the seed plants. They produce
injurious effects (diseases) on the host plant, which result in
serious losses to the farmer and gardener and in reducing the
supply of plant products for every one.

Some of the mushrooms are edible and furnish small quantities
of pleasantly flavored food for man and animals ; some of the
fungi found on roots undoubtedly aid in the nutrition of the plant
on which they grow ; and others produce diseases among annoy-
ing insects and help to destroy or keep them in check.

The vegetative body of a fungus. The vegetative bodies of
most fungi are composed of branching filaments called hyphce
(singular, hypha). In the molds and mildews these fine threads
are readily visible with a magnifier, and under favorable condi-
tions for growth form a soft, cottony layer in or on the substrate
where the fungus is growing. The whole mass of hyphae which
make up the vegetative body of a fungus is called a mycelium.
Sometimes the hyphae, as in the yeasts, are very short ; they are
composed of cells that separate readily, and the filaments are
rarely composed of more than a few cells. In the fleshy fungi

402

Fungi

403

Fig. 248. Stages in the development of the common edible pink-gilled mush-
room. Note the underground vegetative body of the plant.

the hyphae are massed together, although each grows more or less
independently of the others.

In decaying logs or in masses of fallen leaves one often finds
cord-like strands of a white, brown, or black color. Not infre-
quently these may be traced for considerable distances and found
to be connected with a puffball or other mushroom. They are
parts of the mycelium, and absorb and conduct food to the fruiting
bodies and growing parts. Sometimes these underground strands
of the mycelia accumulate food and become greatly enlarged and
act as storage organs. In parasitic forms that grow in contact
with roots (mycorhiza) they probably absorb and transfer food
into the host plant.

Food supply. When the fungus lives on the soil, its food is
derived from the soluble organic materials like sugars, soluble
proteins, and amino acids from the plant and animal matter
occurring there. Moreover, the fungus may give off enzymes,
that act on organic matter and bring about processes that change
much of it to soluble substances. This is nothing less than a kind
of external digestion, which makes the organic substances capable
of diffusing into the cells of the fungus and provides material for

404

General Botany

Fig. 248 a. A fungus {Cordyceps)
parasitic on the pupa of the to-
bacco worm.

building tissue and supplying energy.
The large surface exposed by the in-
numerable hyphae is obviously ad-
vantageous in making contact with
the food substances.

When the fungus lives within or on
the tissues of another plant, a part
of its hyphae extends among or into
the living cells of the host, and the
food accumulated there becomes a
source of food to the fungus. In this
case also, by the secretion of enzymes,
external digestion may occur before
the food passes into the fungus
hyphae. The growth of the hyphae
through the cell walls of the host
plant comes about in some cases by
the liberation of enzymes that dis-
solve the walls ahead of the growing tip of the hypha, in others
by the mechanical breaking of the tissues. Fungi, like most
plants without chlorophyll, must have access to complex carbon
compounds for food and energy, and in some cases they must
have nitrogen compounds also.

Conditions for growth. Many fungi grow best in partial shade.
Since they require a certain amount of moisture, fungi are usually
most abundant in damp places. The most rapid destruction of
organic matter also occurs in such situations. The decay of
wooden beams under porches and in mines, the rotting of fruits
and vegetables in cellars, and the disintegration of partially buried
railroad ties are familiar examples of the results of favorable
conditions for the growth of fungi. Bacteria also are present in
such situations, and their growth and activities may go along
with those of the fungi and hasten the final destruction. In the
desert, where drought and intense light hinder the growth of these
organisms, timbers may withstand exposure for centuries.

Fungi 405

Reproduction. The development of the vegetative body
culminates in the production of numerous fruiting bodies.
Among the simpler forms of fungi the reproductive structures,
or fruiting bodies, may consist merely of specialized reproduc-
tive cells cut off from the ends of hyphae, or of ends of hyphae
that become enlarged and form several or many reproductive
cells inside them. The reproductive cells are called spores, and
the cells in which they are formed are called sporangia (singular,
sporangium). Probably no other group of plants compares
with the fungi in the variety of its reproductive bodies. Cer-
tainly no other plants produce such enormous numbers of
spores in comparison with the size of the plants. As a conse-
quence of the small size and great number of these spores, they are
carried long distances by the wind and scattered everywhere.
Furthermore, when the spores germinate and develop a new myce-
lium, a second crop of spores may be produced within a few days.
In this way, under favorable conditions, as many as 120 crops of
bread mold may be grown in a year.

Among the more complex and fleshy fungi the mycelium may
grow for weeks and months before reproductive structures begin
to develop. There may be large structures formed by the con-
solidation of large numbers of hyphae, part of which later produce
spores. The fruiting bodies may take the form of small disks,
saucers, cups, hollow capsules, solid balls, toadstools, woody
brackets, or irregular coralline masses.

Germination of spores. The spores of many of the parasitic
fungi germinate readily when placed in water. However, un-
less the sporeling is in contact with its proper host plant, it fails
to develop further and dies. Of course, many of them have
been grown on nutrient media.

It is difficult to germinate the spores of many of the saprophytic
fungi, like the puffballs and toadstools, because of their very
exacting requirements. Such spores may be germinated only
in a nutrient solution containing certain sugars, proteins, and

4o6 General Botany-

organic acids. The germination of all fungus spores takes place
only in the presence of moisture and oxygen and at suitable tem-
peratures.

Distribution of fungi. Many fungi are world-wide in their
distribution. Others, however, have rather definite temperature
requirements which limit their development to certain regions
and to certain seasons of the year. If the spores cannot with-
stand freezing temperatures, the fungus will not thrive in cold
temperate and arctic regions. If the vegetative development
requires warm temperatures, the fungus may be suppressed by low
temperature even when the spores are present in abundance.
The efficiency of cold-storage houses and refrigerators in con-
serving foods depends in large part upon the temperature require-
ments of the fungi and bacteria.

Groups of fungi. Of the nearly 50,000 described species of
fungi, there are three conspicuous groups, commonly known as
the Tube fungi, the Sac fungi, and the Basidium fungi.

The Tube fungi (Phycomycetes) receive their name from the fact
that the hyphae are for the most part tubular and without cross-
walls. They include the molds, water molds, and downy mildews.

The name of the Sac fungi {Ascomycetes) is derived from the
peculiar way in which the spores are produced. There are
usually two, four, or eight spores formed inside a sac-like body.
To this group belong the yeasts, the powdery mildews, most of
the lichens, and certain of the fleshy fungi.

In the Basidium fungi (Basidiomycetes) spores are produced
usually four or two in number on the end of a club-shaped hypha,
which is called a basidium from the Latin word for club. In
this division of the fungi are the smuts, rusts, and most of the
fleshy fungi, such as puffballs and toadstools.

THE TUBE FUNGI OR PHYCOMYCETES

Among the commonest examples of the tube fungi are the bread
molds and water molds. These molds are usually white, filamen-

Fungi

407

tous plants that are of great economic importance because of the
damage that they do to foods during storage or shipment. Like

Oo^oTiiiim ii

ArLtheridium

Conidium

Hyplia

Haustorium
Plant cell

Fig. 249. One of the tube fungi, Cystopus, a parasite on leaves of green plants: A, section
of pustule developed on a leaf showing fungus producing conidia (under the broken epidermis)
and zygotes (below, in the mesophyll of the leaf) ; B, part of fungus dissected out of the
mycelium shown in A ; C, mesophyll cell surrounded and penetrated by absorbing hyphae.
Cystopus produces swimming spores when the conidia and zygotes germinate.

bacteria, the spores of molds are in the air and in the dust every-
where, and foods of all kinds are thus continually exposed to them.
If the temperature is warm and the food is moist, they germinate
and, together with bacteria, soon destroy the food. The same
measures that will prevent the growth of bacteria in foods will
prevent the growth of the molds, which are usually associated
with them.

Bread mold. When a spore of bread mold germinates, a tube-
like hypha develops. This hypha soon branches profusely.
Some of the branches penetrate the bread and become absorbing
organs, others spread over the surface of the bread like the
runners of a strawberry plant and at intervals develop clusters of

4o8

General Botany

upright hjrphae which terminate above in sporangia. Just be-
neath each cluster of sporangial hyphae a much-branched

Columella -

Fig. 250. Bread mold {Rhizo pus) : ^ , showing general habit of growth ; 5, sporangium
enlarged ; C, D, E, and F, stages in the formation of a zygospore.

rhizoid grows downward into the bread, affording anchorage and
a food- and water-absorbing surface.

Under favorable moisture and temperature conditions the my-
celium developed from a single spore may entirely surround
a slice of bread with a white, fluffy, cottony growth, dotted with
countless sporangia filled with black spores.

The bread mold may also reproduce in another way. When
filaments from different strains of the fungus grow in contact,
they may each form small, lateral, club-shaped branches.
Then each of the branches is cut off by a wall near the
inner end, forming a cell, the contents of which act as a
gamete. Finally, the cell wall between the two gametes unite,
forming a zygote or sexual spore. A heavy, rough black wall
forms around it and it becomes a resting spore. Under favorable
conditions, after a period of time, it germinates and produces a
new mycelium with sporangia and asexual spores. The bread
mold derives its water and food from the moist bread through

Fungi 409

the hyphae and rhizoids. The rhizoids secrete enzymes in the
bread, which change the starch, fat, and protein into various
soluble substances that diffuse into the fungus hyphae and pass to
all parts of the mycelium. Try an experiment with a small
piece of moist bread in a covered tumbler and see how long it
takes for the bread to be consumed. These molds are for the
most part saprophytes.

There are molds, however, that are both parasites and sapro-
phytes. Perhaps you have seen goldfish growing in an aquarium,
that were given to turning sidewise somersaults in the water and
rubbing their sides on the gravel in the bottom of the jar. You
may also have noticed cobwebby filaments attached to their
scales; perhaps there were enough of them to make the sides
appear white. This growth is one of the water molds (Sapro-
legnia) common in ponds and streams and often a cause of great
losses at fish hatcheries. There are other related molds that
attack flies and other insects and cause their death.

THE SAC FUNGI OR ASCOMYCETES

The sac fungi differ from the tube fungi in being composed of
hyphae made up of short cells. They exhibit a great variety of
forms, from those like the green molds to fleshy forms like the
morels. This is the largest division of the fungi and includes more
than 30,000 species.

The outstanding feature of the sac fungi is the method of pro-
ducing spores inside sac-like sporangia called asci (singular, a^cz^^),
which are the terminal cells of upright hyphae. Usually the sacs
are grouped in clusters ; sometimes they stand upright side by
side and form a layer over a part of the plant body. Many
species also produce spores that are pinched off at the ends of
short hyphae. These spores are called conidia, and are impor-
tant in spreading the fungus during the growing season. They
may be produced in such abundance that an infected leaf has
the appearance of being covered with white powder.

4IO General Botany

The best known of the sac fungi are the yeasts, the green molds
{Penicillium and Aspergillus), the powdery mildews, the cup fungi

{Peziza) and morels (Morchella),
and the lichens.

The yeasts. The yeasts are
small, one-celled plants that mul-
tiply very rapidly. In the mak-
ing of bread, they are of primary
importance. When properly
mixed with flour and water they

Fig. 251. Yeast (Saccharomyces): cells develop in all partS of the doUgh.
and branching filaments. Above are three The ycastS have withiu them
cells, each containing four resting spores. , . , . ,. • . .

enzymes which oxidize part of
the sugar that is present into carbon dioxide and alcohol.
In this way the yeast obtains its energy. The carbon
dioxide accumulates in bubbles and causes the dough to
rise and become " light." When the dough is put into a hot
oven, the alcohol is vaporized, and together with the carbon
dioxide it is driven off into the air. The high temperature kills the
yeast, bakes the dough, and changes some of the starch into its
soluble form, dextrin, which makes it more readily digestible.
Sour bread is produced when the yeast that is added contains
acid-forming bacteria which change part of the alcohol into
acetic acid.

Yeasts and bacteria are the organisms that change fruit juice
into " hard " cider and vinegar. Yeast first changes the sugar
in apple juice to carbon dioxide and alcohol, and bacteria further
oxidize the alcohol to acetic acid, thus forming vinegar. Yeasts
are also used in the manufacture of beer and wines.

Yeast fungi may readily be grown by adding a bit of yeast
to a 5 per cent sugar solution in a test tube. The branching
groups of cells may then be examined under the microscope.
The manner of forming new cells among yeasts is unique, in that
the new cells start as small protuberances (buds) from the older

Fungi

411

cells. These buds gradually enlarge until they attain their
complete growth and separate, forming new individuals. The

Fig. 252. A, blue mold (Penicillium) ; B, green mold {Aspergillus). Both show the
hyphce, upright branches, and conidia. {After Frank.)

alcohol formed in the test tube by the yeast may easily be de-
tected by its odor.

The green and blue molds. Among the most widely known of
the fungi are the green and blue molds. They are conspicuous
destroyers of food. Fruits, vegetables, bread, and other starchy
materials; canned fruits, preserves, and jellies; and even,
smoked meats, are all subject to the attack of the fungi under
favorable moisture and temperature conditions. Some of the
blue molds, on the other hand, are used in the manufacture of
cheese. The flavors of Roquefort and Camembert cheese, for
example, are largely due to the blue mold present.

The blue and green molds are so common and so widely dis-
tributed that their spores are present in the dust and air every-
where. When they fall on moist food they germinate, forming a
hypha which soon branches profusely and forms a disk-like myce-
lium. After two or three days the upright hyphae toward the
center begin to produce spores, and the green color is due to the
colored spores.

412

General Botany

The method of spore formation is quite simple. The upright
hyphae of the blue mold branch several times near the end. In

this way a broom-like tuft is formed,
the branches of which terminate in
rows of spores (conidia). In the green
mold numerous branches arise at the
enlarged ends of the upright hyphae,
forming a globular mass of spores.

Under certain conditions these fungi
may form round, capsule-like fruiting
bodies in the material on which they
are growing. Within a capsule is a
group of sacs (sporangia), each of
which contains eight spores. It is the
presence of these sacs and sac spores
that show the relationship of the blue
and green molds to the sac fungi rather
than to the bread molds.

The powdery mildews. Closely
related to the green mold are the
powdery mildews which form con-
spicuous white cobwebby patches

Fig. 253. A, vertical section of OU the IcaVCS of rOSCS, lilaCS, willoWS,

an ascomycete Pcsfca; B, en- dandelions, and many other plants in

larged view of fruiting layer,

showing asci- and ascospores. late summcr. They too produce m-
{After Frank.) numerable spores from simple upright

hyphae. These form the dust-like powder which suggested their
common name. Late in the season, after the union of two
hyphae, larger fruiting bodies may be formed, and within them
groups of sacs and sac spores. These appear on the leaves as
black dots and usually have a number of appendages. The
powdery mildews are external parasites and derive their food
from the epidermal cells of the leaves or stems of the host
plant.

Fungi

413

Cup fungi and morels. These are fleshy forms found in open
forests on the soil and on decaying wood. In the cup fungi

Fig. 254. A, B, C, Craterellus, showing habit of growth, a cross-section of the wall, and a
basidium with two spores ; D, E, F, Cantharellus, showing stipe and pileus, a cross-section
of the pileus showing gills, and a part of the gill showing basidia with four spores.

the ascospores are produced in a layer covering the inside of the
cup. In the morels the spore-bearing layer covers the elongated
and much-wrinkled top of the hollow fruiting body. Morels
are edible when fresh and by many persons are much esteemed for.
their peculiar flavor. Truffles also belong to this group. They
resemble puffballs in form, but grow underground. They are
much sought after in France and are collected by the use of
pigs, and of dogs trained to locate them.

Lichens. Among the parasitic fungi are some that live on
such one-celled green algae as Protococcus. The fungus forms
the plant body and completely envelops the algal cells. These
forms constitute the lichens, which are gray-green, irregular-
shaped plants that are common -on the bark of trees, on rock
surfaces, and occasionally on the soil (Fig. 255). Like other
fungi they produce fruiting bodies, small disk-like or cup-shaped
elevations, in which sac spores are produced in great numbers.

414

General Botany

Fig. 255. A group of lichens : Parmelia {on tree) ; in the middle foreground, P£;///|:6'm;
the renaaining forms are species of Cladonia.

Lichens also multiply vegetatively by the breaking away of small
bits of the thalli.

BASIDIUM FUNGI OR BASIDIOMYCETES

The second largest division of the fungi includes the smuts,
rusts, and toadstools, numbering more than 20,000 species.
The distinctive feature of the Basidiomycetes is the formation of
one, two, or four spores at the end of a short, club-shaped hypha
called a basidium. In the smuts and rusts the basidium and
basidiospores make up a separate plant ; in the toadstools the
basidium and its spores are formed on or in the fleshy fruiting
body of the fungus.

The smuts. The smut fungi of the small cereals have a myce-
lium extending throughout the tissues of the host plant, which was
infected during its seedling stage. When the plant " heads out,"
the smut causes the grains to enlarge, and the smut hyphae con-
sume the food usually stored in the grain and then produce
black spores in such abundance as completely to occupy or
replace the grain. These spores have heavy walls and are capable
of living over to the next season. In some cases this involves

Fungi

415

W. S. Cooper
Fig. 256. Lichens covering the branches of a Monterey cypress, Point
Lobos, California.

4i6 General Botany

passing the winter in the granary and in the field. When the
grain is harvested and threshed, the smut is spread to the good

Fig. 257. Early stages in the development of a lichen, showing relation of the hyphae to
the algal cells. {After Bonnier.)

grain. Unless the grain that is used for seed the following year
is suitably treated with disinfectants, the smut spores will be
planted with the grain the following year. The spores of the
commoner smuts germinate readily in water and produce a
filament of four cells, each of which then produces a small,
thin-walled spring spore (basidiospore) which may infect the
young seedling. The smuts of the small cereals usually are carried
over from one season to the next in the form of spores on the
grain or of hyphae inside the grain.

Corn smut behaves somewhat differently, in that the entire
plant is not usually traversed or invaded by the mycelium.
The spores last over from one season to the next in the soil and

Fungi

417

germinate there, producing the
basidia and basidiospores which
are blown about and infect the
new crop of corn. The infected
region of the corn plant usually
swells, forming a large, glistening
white ball. This later turns black
and disintegrates, liberating myr-
iads of spores to be further scat-
tered by the wind. When the
soil in a field becomes greatly
infected with corn-smut spores,
the best way to avoid further
trouble is to plant another kind
of crop for a year or two. Seed
treatment is of no value.

The rusts. Among the most
serious diseases affecting wheat,
rye, barley, and oats are those
produced by the fungi known as
the rusts. These fungi are called
rusts because plants that are in-
fected with them develop yellow
and brown spots that have the
appearance of iron rust. The
rusts occur wherever grains are
grown, and they cause millions
of dollars' worth of damage to
crops every year.

The rusts are parasites that
live inside the host plants and in-
jure or destroy the tissues that
are concerned in food manufac-
ture. Their life history is pe-
culiar in that the fungus usually

Office of Cereal Investigations, U. S.D.A.

Fig. 258. Corn smut. The large white
masses of tissue protruding from the stem
contain the black spores. The mycelium
of this smut does not extend far from the
point of infection.

4i8

General Botany

produces disease on two different kinds of host plants. The stem
rust of wheat, for example, produces patches of red spores (ure-

FiG. 259. Life history of the stem rust of wheat. In fields where wheat has been
grown, the stubble (A) carries over the winter black spores {B), that germinate in early
spring, producing smaller spores (C) . These infect the leaves of the common barberry (D) .
In the leaves of the barberry the fungus grows and produces cup-shaped cavities
filled with spores (£) that are carried by the wind to wheat fields and infect the wheat
plants. After growing in the wheat a short time, the fungus produces first the red
spores (G) that spread the disease to other wheat plants, and later the two-celled black
spores that carry the disease over the winter again.

diniospores) which will infect other wheat plants. It pro-
duces also black spores (teliospores) which live over winter on the
stubble and straw, and which germinate the following spring and
produce spring spores (basidiospores) that infect the barberry.
On the barberry leaves the fungus produces a cup-like depression
within which a fourth kind of spores (aeciospores) are formed.
These spores will not germinate on the barberry, but they will
infect wheat. Thus the stem rust of wheat spreads from one
wheat plant to another by means of red spores. The following
season it may spread from wheat stubble to the barberry by
basidiospores produced by the teliospores, and from the barberry
back to the wheat by still another kind of spore (Fig. 259).
In the Northern states, from the Dakotas to New England, the

Fungi

419

barberry stage is of special importance in the life history of the
rust. In the southwestern United States, where winter wheat

Fig. 260. The white-pine blister rust. The fruiting bodies on the white
pine {A) produce spores that infect the leaves of the gooseberry {B and C). On
the gooseberry leaves the fungus produces at first yellow spores that will infect
other gooseberry plants, and later brown spores that carry the disease back to
the pine. When a pine (D) is infected by the disease, the younger parts soon
die (£).

is grown, the red spores produced during the summer may be
carried from one field to another by the winds and infect the wheat
over wide areas. In the Northern states the destruction of all
barberry plants has been undertaken, and this work has already
reduced the amount of infection. The hope of entirely control-
ling wheat rust, however, probably lies in breeding new varie-
ties that are immune to the disease. This has been accomplished
in Kansas producing a variety known as " Kanred wheat."

Apple rust. Other rusts also live on two host plants, and be-
cause of this double life and the fact that the fungus grows on
the inside of its host, they are very difhcult to control. The rust
on the red cedar produces the so-called " cedar apples." In the
spring these swell and protrude masses of teliospores that germi-
nate at once and form basidiospores, which infect the leaves of the
apple tree and may do great damage to them.

420

General Botany

Fig. 261. Group of fleshy fungi. Beginning at the top, are species of Hydnum,
puffball, bracket fungus (Fomes), coral fungus {Clavaria), poisonous Amanita,
cornucopia fungus {Craterellus), Russula, and earthstars (Geasier).

Fungi 421

Pine blister rust. Recently the blister rust of the white pine
has been brought to America, and it threatens to destroy what
remains of our white-pine forests. The alternate host plants of
this fungus are the wild and cultivated gooseberries and currants.
Attempts are being made to prevent its spread westward, both
by cutting the diseased white pine and by systematic destruction
of the wild and cultivated currants and gooseberry bushes in
newly infected regions.

Another common rust is frequently seen on raspberry and black-
berry bushes along roadsides; it colors the under sides of the
leaves with its bright, orange-red spores. In this instance no
alternate host is known.

Mushrooms and toadstools. The largest and most complex
of the fungi are the mushrooms and toadstools. They are com-
mon in fields and woods and for the most part live on decaying
wood and organic matter in the soil. There is no real distinction
between mushrooms and toadstools. Some of them are edible,
others are indigestible, and some are deadly poisonous. Edible
forms are cultivated on a large scale in caves and abandoned
mines, and on a smaller scale in cellars. Wild forms should not
be eaten unless they are gathered by persons competent to distin-
guish the different species, many of which are similar in appear-
ance but very different in their effects when eaten.

The mushrooms as they are gathered are only the fruiting
bodies of the fungi. The vegetative part of the plant consists
of bundles of hyphae extending in all directions throughout a
large mass of soil on which the fruiting bodies appear. It may
take several years for the underground vegetative part of the
fungus to develop, while the fruiting bodies may develop in a
few days. It is the enlargement of the fruiting bodies that per-
sons have in mind when they speak of " mushroom growth."
This expression leaves out of account the months or years of
growth during which the materials were accumulated that led
to the sudden production of the fruiting body.

42 2 General Botany

The spores of mushrooms are produced in unthinkable num-
bers either inside the fruiting body (puffballs) or on the under
side of the umbrella-shaped cap (mushrooms). A large puffball
has been estimated to contain 7000 bilHon spores ; the shaggy-
mane mushroom {Coprinus) about 5 billion spores on each pileus ;
and one of the bracket fungi produces about 100 bilhon spores
each year for several years.

REFERENCES

DuGGAR, B. M. Fungous Diseases of Plants. Ginn & Co., Boston; 1909.
Harshberger, J. W. Mycology and Plant Pathology. P. Blakiston's Son & Co.,

Philadelphia.
Hesler, L. R., and Whetzel, H. H. Manual of Fruit Diseases. The Macmillan

Company, New York; 1917.
McIlvane, C, and Macadam, R. K. One Thousand American Fungi. Bobbs-

Merrill Company, Indianapolis; 191 2.
Stevens, F. L. Fungi Which Cause Plant Disease. The Macmillan Company,

New York; 1913.
Stevens, F. L., and Hall, J. G. Diseases of Economic Plants. The Macmillan

Company, New York; 192 1.

CHAPTER FORTY

PLANT DISEASES

It is difficult to define a plant disease, although it is usually
not difficult to distinguish in a particular example between a
healthy and a diseased plant. The difficulty of definition arises
from the fact that diseases or abnormal conditions are produced
by so great a variety of causes, and the effects or symptoms are so
diverse, that it is impossible to include all examples of diseases
and at the same time exclude others that are merely effects of
unfavorable environmental conditions.

Plant diseases are usually defined as derangements of the nor-
mal structures and physiological processes of plants or parts of
plants. The formation of galls and tumors, the development of
gray, brown, and black spots on leaves, the sudden wilting and
drying of shoots, and the rotting of seedlings are familiar external
signs of disease.

Prevalence. Plant diseases have been recognized since the
earliest historic times. They have been extensively studied,
however, with a view to their control only during the last half
century. They could not be well understood until the life
histories of the bacteria and fungi were discovered.

The increasing prevalence of destructive diseases in America is
the natural result of the more extensive cultivation of crop plants.
The present world-wide exchange of plants and plant products
has resulted in the accidental widespread transference of the
bacteria, fungi, insects, and other organisms that injure plants.

Losses from plant diseases. The present losses from plant
diseases are enormous when we view them for a state or for the
United States as a whole. Sometimes on a single farm a disease
may reduce the yield so greatly that the crop was grown at a
loss instead of a profit. Some idea of the extent of the injuries
inflicted may be gained from the following table of estimates
by the United States Department of Agriculture for 191 7 :

423

424

General Botany

Loss BY Plant

Loss m

Loss in

Crop

Yield

Diseases

Dollars

Wheat . . .

650,828,000 bu.

64,227,000 bu.

128,000,000

Oats . . .

1,587,286,000 bu.

154,120,000 bu.

8.8

103,000,000

Corn . . .

3,159,494,000 bu.

175,368,000 bu.

5-2

224,000,000

Potatoes . .

442,536,000 bu.

117,167,000 bu.

20.9

143,000,000

Sweet potatoes

87,141,000 bu.

41,706,000 bu.

32.4

46,000,000

Cotton . . .

10,949,000 bales

1,866,000 bales

14-5

256,000,000

Peaches . .

45,066,000 bu.

14,459,000 bu.

24-3

21,000,000

These figures are still further increased by the losses caused
by bacteria and fungi during storage and shipment. When any
of these products reach the market, their price is increased in
proportion to these losses.

Control of plant diseases. The control of plant diseases
may be said to have begun with the discovery of the efficiency
of the Bordeaux spray mixture in 1862. In 1896 the formalde-
hyde treatment of oats seed for smut was introduced, and more

R. F. Poole, N. J. Expt. Sta.
Fig. 262. Bean anthracnose. The disease is due to a fungus which
attacks all parts of the plant. The fungus is carried over winter in
the seeds.

Plant Diseases

425

■flSN?^'

/«. F. Pooie. A?^. J. Expt. Sta.

Fig. 263. Bean seeds affected by anthracnose. The principal means
of controlling the disease is to reject all seeds for planting that show
the characteristic reddish spots.

recently the lime-sulfur sprays have been found an efficient pre-
ventive of certain diseases. All sprays are directed toward
killing of the parasites before they invade the host tissue.

Another method of disease control is exemplified by the removal
of all common barberry bushes, now being extensively carried on
in the Northern wheat-growing states to decrease the losses from
stem rust of wheat. In the Northern states also attempts are
being made to check the ravages of the white-pine blister rust
by removing the wild currants and gooseberries which form the
alternate host of the pine-rust fungus. The removal of red-
cedar trees from the vicinity of an orchard will prevent apple rust.

The development of disease-resistant varieties of crop plants
is a third method of control that has been successfully used. The
Kanred wheat is highly resistant to rust. A strain of cabbage
has been selected that is resistant to cabbage yellows. New
immune varieties of watermelon, tomato, and cotton have in
certain sections replaced the older varieties which were suscep-
tible to " wilt " diseases.

426

General Botany

Rotating crops on farms so that there is small chance of the
disease-producing organisms being carried in the soil from one
season to the next has been found an efficient deterrent of certain
diseases. The root rot of tobacco, wilt of potatoes, and smut of
corn are some examples of soil-borne diseases.

Sterilizing soils and treating seed with fungicides before plant-
ing are used to control diseases that attack seedlings. Finally,
diseases of one state or country may be excluded from another by
the enforcing of a strict quarantine. At the present time nursery
stock grown in Europe is excluded from the United States.
Nursery stock that is to be shipped is also carefully inspected by
government experts for symptoms of certain diseases, and only
healthy stock may be shipped from one state to another.

Symptoms of disease. The external signs of diseases are of
many kinds, among which the following classes may be noted :

Erwin F. Smith, U.S.D.A.
Fig. 264. Tumor on a leaf of Bryophyllum inoculated with Bacterium iumifaciens. Roots
are developing from the tumor at r. This disease is commonly known as "crown gall."

Plant Diseases

42.7

ErwinF. Smith, U.S.D.A.

Fig. 265. Tumors developed on leaf of tobacco by inoculating with Bacterium tumijaciens.
From the tumors, leafy shoots are growing.

(i) Malformations of leaves, stems, roots, and fruits. Galls,
knots, " witches' brooms," and curled and wrinkled leaves are
common examples.

(2) Cankers, or rough sunken spots on stems and on branches
of trees. They may be caused by frost injuries. Sometimes
they are produced by a fungus, as in the brown rot of the peach
tree and the black rot of apple .

(3) Blight is the term applied to the sudden dying of leaves,
shoots, and blossoms. The fire blight of apple and pear are
known wherever these trees are grown in America.

(4) Leaf and fruit spots are the result of local injuries to leaves
and fruit through the growth of a parasite in or under the surface.

(5) Wilts include various diseases that are first noticeable by
a sudden wilting of the leaves or of the complete plant. Cucum-
bers, cotton, cowpeas, and watermelons are particularly subject
to diseases of this type.

(6) Yellowing of foliage or development of yellow spots and
blotches characterizes a disease of peaches and the mosaic disease
of tobacco.

428

General Botany

Causes of diseases. Bacteria and fungi are the most common
causes of plant diseases. Certain insects are almost equally
injurious, and not infrequently act as carriers of disease-producing
bacteria and fungi. Insects also are the commonest cause of
galls. Nematode worms are sources of injury to the roots of
many plants, particularly in greenhouses and in tropical and sub-
tropical countries.

Just how these various parasites affect the plants is not thor-
oughly known. Some of them certainly withdraw sufficient
food from the host plant to cause injury. Others seem to produce
poisonous substances that injure the tissues or kill them. Still
others in some way stimulate the tissues and cause abnormal
growths.

Weather influences. The temperature and moisture con-
ditions frequently influence the prevalence of diseases. Water
is necessary for the germination of spores. Severe epidemics of
potato blight, brown rot of stone fruits, and rots of grapes and

A . B. StoiU

Fig. 266. Cucumber plants attacked by bacterial wilt. The plants wilt because the vessels
are plugged by the bacteria that cause the disease.

Plant Diseases

429

Ue of Forest Pathology, U.S.D.A.

Fig. 267. Effects of chestnut bark disease. A familiar sight everywhere within 200 miles of
New York City, where the disease was first seen in 1905.

apples occur usually only in wet seasons. Dry weather is
favorable to the spread of the spores of the loose smuts of cereals
from an infected plant to the blossoms of another. When the
seed is planted, untreated, the following year an abundance of
smut results.

Low temperatures at the time of planting favor certain diseases,
and high temperatures favor others. For this reason the preva-
lent diseases of one season may be very different from those of the
following season.

Soil influences. The most usual effect of soil is in harboring
bacteria and fungi from one season to another and in this way
transferring the infective agent from one crop to the next. This
is particularly important in causing infection with the fungi that
cause the damping off of seedlings ^nd the various forms of root
rot.

The character of the soil, whether acid or alkaline, may also
influence the growth of the fungi or the host in such a way as to

430 General Botany

increase or decrease the amount of the infection. The scab of
potatoes is increased by liming.

Plant pathology. The foregoing paragraphs are sufficient to
indicate the complexity of the problems of plant pathology, which
is the science that treats of plant diseases and their control. They
may also serve to show the importance of studying and under-
standing the life histories of fungi and bacteria, of their effects
on their host plants, and of discovering new and better methods
of eradicating them. One of the most important functions of the
federal and state agricultural experiment stations is the pro-
motion of research for the control of plant diseases. In view of
the enormous annual losses to growers of plants, and the im-
proved yields already obtained through the discovery of control
measures, the expenditure of large sums of money for the agri-
cultural experiment stations is more than justified. Every one
profits by these investigations through more abundant and
cheaper food supplies.

In the following paragraphs a few common diseases are de-
scribed. More specific information concerning them may always
be obtained by writing to your State Experiment Station.

Fire blight is one of the commonest diseases of pear, apple,
and quince trees. It becomes noticeable in early summer
through the turning brown of the leaves of twigs on these trees
as though they had been scorched. The infective agent is a
bacterium which lives in the interior of the affected branches,
and unless it is checked as soon as it appears, it may extend into
other tissues of the plant. The bacteria are apparently spread
by insects. After a rain the infected branches bear numerous
drops of a gummy nature containing countless numbers of bacteria,
and insect visitors carry the bacteria from infected branches to
blossoms and other twigs. The bacteria pass the winter in the
living tissues at the edge of the cankers on the larger branches,
and in the following spring these may become a source of further
infection to near-by trees. The only known raethod of control

Plant Diseases

431

is the cutting and immediate burning of all infected branches.
These branches should be cut well below the darkened portions,
because the bacteria usually extend some distance down the water-
conducting tissues. Care must also be exercised in keeping the
knife or saw from coming in contact with the diseased portion of
the branches. Cankers on the older branches should be cut out
during the fall or winter to prevent infection the following season.
Damping off of seedlings. Gardeners and nurserymen are
much troubled by the damping off of seedlings in seedbeds.
The fungi concerned in this process occur in the soil, and some
are related to the common bread mold. The seedlings when
first attacked become transparent, fall to the ground, die, and
are finally consumed by the fungus. These fungi reproduce by
spores, some of which are thick-walled and carry the plant over
winter in the soil. Damping off may be controlled by sterilizing
the soil of seedbeds. This may be accomplished by treating the
soil with a 10 per cent solution of formalin and covering the bed
with a piece of oilcloth for 24 hours.

A. B. Stout

Fig. 268. Damping off of seedling lettuce. This disease is due to molds and other fungi
that attack the stems at the surface of the soil and cause the seedlings to rot.

432 General Botany

In greenhouses and small seedbeds the sterilization is
frequently accomplished by forcing live steam from a boiler
through the soil by means of an inverted galvanized iron pan.
The sterilization of soil not only kills the damping-off fungi but
many other disease-producing organisms, both plant and animal.
Moreover, it kills weeds and has a beneficial effect upon the soil.

Clubroot of cabbage. Many plants belonging to the mustard
family, particularly cabbage, turnip, and cauliflower, are subject
to a disease that causes swellings on the roots and impairs the
efficiency of the roots. When young seedlings are infected
many die, and those that continue to live never produce normal
plants that can be marketed. The fungus belongs to one of the
lowest groups of fungi, the slime molds, and produces enormous
numbers of spores that winter over in the soil. In Europe the
disease is a constant menace to the cabbage industry. In
America it is not so prevalent. The disease may be partly con-
trolled by liming the soil and by changing the location of the

Erwin F. Smith, U.S.D.A.

Fig. 269. Clubroot of cabbage, caused by infection with a slime mold, Plasmodia phor a.

Plant Diseases 433

cabbage field from year to year, alternating with crops that are
not affected by this fungus.

Black knot. This disease is frequently seen in plum and
cherry orchards and is made conspicuous by the presence of
greatly thickened portions of twigs and branches. The spores
of the fungus causing it seem to be carried by the wind, and
infection takes place through the bark. As soon as the hyphae
of the fungus penetrate the inner tissues, the twigs begin to en-
large and the outer bark is broken. On the swollen surface in
the spring and early summer one may see with a magnifying
glass hyphae producing spores.

During late summer another type of small, rounded fruiting
body develops on the knots, which last over the winter. In the
spring these bodies contain numerous sacs, each with eight spores
inside it. These constitute a second means of spreading the
disease. Where the disease is very prevalent, as in the Atlantic
coastal states, it is difficult to control because it affects the wild
as well as the cultivated cherries. The removal and destruction
of all knots by burning has been found effective, and may best
be done during the winter when the trees are leafless and before
the winter spores are ripe.

The smuts. The smuts, described in the last chapter, are ,
very destructive to cereals, but effective methods of control have
been found. Corn smut differs from the other smuts in that its
spores are shed from the lesions on the plant and remain over the
winter on the soil. Hence the only methods of control available
are the rotation of crops and the removal and burning of infected
plants from the field as soon as they are noticeable.

Among the small cereals there are two types of smut, one of
which results in the total breaking down of the grain and glumes
into fine powder and spores. These are the so-called " loose-
smuts." Other species of smut do not produce as complete a
destruction of the seed coats and glumes. These are known as
the " covered smuts." Experiments have shown that the covered

434

General Botany

//. B. Humphrey, U.S.D.A.

Fig. 270. Smuts of small grains: A, loose smut of wheat; B, loose smut of barley;
C, covered smut of barley. In each figure one of the heads is free from disease.

smut of oats, wheat, and barley and the loose smut of oats may
be successfully prevented by treating the seed with formaldehyde
before planting. This is possible because the plant is carried
over the winter by spores on the grain or by masses of spores
(smut balls) among the grain.

The loose smuts of wheat and barley cannot be successfully pre-
vented by formaldehyde treatment, because the fungus is carried
over from one year to the next by means of living hyphae inside
the grain. These fungus hyphas, however, cannot withstand a
temperature of 125° to 130° F., while the seeds of wheat and
barley are uninjured by a lo-minute exposure to this temperature.
Consequently a method of control has been devised by which the
grain is dipped for 10 minutes into water carefully maintained
at 129° F. Detailed directions for both the formaldehyde treat-
ment and the hot-water treatment may be obtained from your
State Agricultural Experiment Station. The hot-water treat-

Plant Diseases

435

ment and the subsequent drying of the grain are so difficult to
perform that this method of prevention is rarely used for the
general crop. It is used, however, for treating grain that is to
be planted for the production of seed for the following year.

Galls. Among the most striking examples of abnormal develop-
ment of tissues and organs are the galls produced on a great
variety of plants by insects, and more rarely by fungi and bac-
teria. Almost every one has seen the large papery galls of oak
leaves, the velvety gall of the rose, the cone-like shoots of the
pussy willow, and the swellings in the stems of goldenrod. These
are all brought about in some unknown way by insects living in
the plant tissues.

Downy mildew. A downy mildew commonly occurs on leaves
and stems of grape leaves and may cause a reduction of the grape
crop by injuring the leaves and causing them to drop. In some
cases the fungus may attack the green fruit, causing it to wither
and drop to the ground.

Brown rot of stone fruits. One of the most destructive dis-
eases of cherries, plums, and peaches is the brown rot. The
fungus causing this disease is carried over the winter on the
mummied fruits hanging on the trees or lying on the ground be-
neath. Beginning in June, spores are carried to the developing
fruits where they germinate, and decay follows the growth of
hyphae, resulting in brown spots and finally the withering of
the entire fruit. When infection occurs late, at the time of
gathering, this disease may cause serious losses during the mar-
keting of the fruit.

Brown rot may be controlled by carefully removing all mum-
mied fruits at the end of the year, and by spraying with a mixture
of lime and sulfur at proper intervals during the season.

Mosaic disease. This disease has been especially injurious
to tobacco, but it also affects tomatoes and to a less extent a
great variety of wild and cultivated herbs. The external signs
are a light-green mottling of the leaves or distorted and stringy

436 General Botany

leaf development. The cause of the disease is not known. It
may be transferred to healthy plants by injecting juice from a
diseased plant. In the field it is apparently spread by insects.
Until the parasite is known and the exact manner of its transfer
from one plant to another has been discovered, control will be
impossible.

Fig. 271. Mottled and crinkled leaf of potato afifected by mosaic disease.
(See also Figure 128, page 223.)

CHAPTER FORTY-ONE

THE CLASSIFICATION OF PLANTS

Whenever an attempt is made to describe the plants that
occur on the earth, it becomes necessary at once to adopt some
scheme of classification. About 250,000 plants have been
distinguished, and they vary so greatly in size, structure, physio-
logical requirements, and life histories that it is obviously im-
possible to describe them as a whole.

Since the earliest times students of plants have proposed
schemes of classification which would group together plants hav-
ing somewhat similar structures and life histories. At first
these attempts were very artificial and unsatisfactory because so
little was known about the plants themselves. During the past
century and a half, great progress has been made in studying the
plants that are now living and the plant forms of past geological
ages now found as fossils in the rocks. The large amount of
data thus accumulated has made it possible to build classifi-
cations that more nearly approach actual or natural relation-
ships. Back of all modern classifications is the idea that the
plants of the present have been derived through modification
from the plants of the past.

Terminology. Since the time of Linnaeus it has been agreed
among botanists that all the individual plants which are essen-
tially identical in structure and life history shall be grouped
together as a species and given a two- word name. Thus all
the millions of corn plants are grouped together as one species,
Zea mays. Species have long been recognized and many of them
have been given common names, such as Kentucky bluegrass,
black mustard, cottonwood, black walnut, and white pine.

Groups of closely related plants having many characters in
common have also been recognized, such as oak, willow, hickory,
and pine. These larger groups are called genera (singular, genus) .
In each of these groups several or many species are distinguished.
For example, the oaks are commonly separated into white oak,

437

438

General Botany

Fig. 272. Four species of oak: A, white oak {Quercus alba) ; B, bur oak {Quercus macro-
car pa) ; C, red oak {Quercus rubra); and D, pin oak {Quercus palustris). Note that the
species differ in shape of leaves and in size and form of acorns, but that they have many
characters in common. {After E. L. Moseley.)

black oak, blue oak, red oak, live oak, all differing from each
other and from other oaks.

Common names are quite satisfactory for ordinary purposes

The Classification of Plants 439

and when used to name plants of a given locality, but they are
very unsatisfactory when used to name plants in widely separated
locahties. For example, the white and black oaks of California
are not the same trees as the white and black oaks of Pennsyl-
vania. The white pine of New England is not the same as the
white pine of Colorado, and both differ from the white pine of
Idaho. Consequently taxonomists, or students of classification,
have been forced to give each kind of plant a scientific name.
This name consists of two words having a Latin or Greek form :
(i) a genus name and (2) a species name. The name for the oak
genus is the old Latin word for oak, Quercus; for the species
called '' white oak " in the eastern United States, Quercus alba
(Latin for '' white ") ; for the California white oak, Quercus lohata.

Larger groups. Similar individuals, then, are grouped into
species, and species having certain fundamental characters in
common are placed together in a genus. In a similar way genera
are grouped into families, and families are grouped into orders.
Several orders taken together form a class, and a group of classes
forms a phylum (or division) of the plant kingdom. In some of
the largest phyla it may be convenient to recognize secondary
divisions in each of these groups, such as subclass, suborder,
subfamilies, and subgenera. Highly variable species are some-
times subdivided into varieties.

The following diagram showing the higher groups to which
^ the oaks belong will help to make these groupings clear :

THE PLANT KINGDOM
Phylum — Angiospermae

I. Class — Dicotyledoneae (Includes more than 25 orders of plants
whose embryos have 2 cotyledons)
a. Order — ■ Fagales (Includes Birch and Beech families)

(i) Family — Fagaceae (Beeckfamily) (Includes genera of Beech
Oak, Chestnut, etc.)

(a) Genus — Quercus (Oak) (Includes more than 200 species
mostly in North America and Asia)

440 General Botany

The great plant groups. We have already described two of
the major plant groups or phyla : the Bacteria {Schizomycetes)
and the Fungi (Eumycetes). In succeeding chapters the more
important characteristics of other major plant groups will be
discussed. These groups are the Algce (including several distinct
phyla), the Bryophytes (mosses and liverworts), the Pterido-
phytes (ferns, horsetails, and club-mosses), the Gymnosperms
(conifers and cycads), and the Angios perms (flowering plants).

Because of the great diversity of the plant kingdom, the
number of groups in it is very large. For this reason only brief
generalized descriptions of the major groups can be given in a
text like this, and all consideration of some phyla must be omitted.
Furthermore, it may be necessary to group together other phyla
under series names that do not necessarily imply relationships.
For example, Thallophyta is often used to designate all plants
below the mosses and liverworts — all plants with a vegetative
body undifferentiated into leaf-like, stem-like, or root-like organs.
The algae constitute the series of chlorophyll-bearing phyla
placed among the Thallophyta, and the fungi include the non-
green phyla. Yet in grouping algae and fungi together we do not
mean to imply relationships between them.

CHAPTER FORTY-TWO

THE ALG^

There is a large assemblage of chlorophyll-bearing plants
usually small and comparatively simple in structure, known as
algcB (singular, alga). Some species are unicellular and micro-
scopic, with cells so simple in structure that they are comparable
to those of bacteria ; other species are multicellular, filamentous
colonies, often branched and attaining lengths of several inches ;
a few species have thick, leathery, vegetative bodies, composed
of several distinct tissues and varying from a foot to many feet
in length. The algae are of peculiar interest in showing various
methods by which complex plants may be derived from simpler
forms.

The algae include many diverse types of plants which are
grouped together because their vegetative and reproductive
structures are simple when compared with other groups of
chlorophyll-bearing plants. All algae reproduce by cell division,
and usually also by spores.

Classification of algae. For convenience of description the
more important algae may be divided into five groups, four of
whose common names are suggested by the characteristic color
of the plants in each group. The classification, however, is
based upon more fundamental characters than color; namely,
(i) the structure of cells, or vegetative body; (2) the reproduc-
tive structures; and (3) the life history, or the series of events
that occur in the life of the plant, beginning with the germination
of a spore and ending with the formation of similar spores.

THE BLUE- GREEN ALG^ OR MYXOPHYCE^

These are the simplest known autophytes. A majority of the
plants contain a water-soluble blue pigment in addition to the
chlorophyll. Both this pigment and the chlorophyll are dis-
persed in the protoplasm and are not in definitely organized

441

442 General Botany

bodies like the chloroplasts of higher plants. The cells lack
true nuclei. One of the striking characteristics of the group
is the abundant formation of mucilage by the cells, leading in
many instances to the production of gelatinous colonies of cells
and to simple and branched filaments. About 1200 species
have been described.

Occurrence. The blue-green algae occur in abundance in all
parts of the earth. They are the prominent algae of the tropics
and the polar regions, and they may impart their color to the
landscape. Most of these algae are in fresh water ; a few are
found in salt and brackish water along coasts. Many of the
forms are violet, red, gray, or brown in color. The Red Sea
owes its name and color to a red species of '' blue-green " algae.
The so-called " water bloom " is frequently a sudden develop-
ment and accumulation of certain blue-greens near the surfaces
of lakes and ponds. Pond waters at such times have a distinct
greenish color and have been known to poison cattle and horses.

Blue-greens occur on all moist soils, and in some parts of the
tropics as epiphytes also. Some of these algae form papery layers
on the soil surface which are very hygroscopic and aid in re-
taining the soil water. The soil algae are commonly associated
with the nitrogen-fixing bacteria, and as they die they set free
carbon compounds useful to the bacteria. The bacteria, in
turn, at their death liberate nitrogen compounds useful to the
algae and the higher plants.

Blue-greens often become troublesome weeds on soils in green-
houses — our artificial tropics.

Resistance to unfavorable conditions. Some blue-greens
thrive in conditions where no other autophytes can live. This
resistance is partly due to the high water-retaining capacity
of their gelatinous walls, and perhaps partly to the nature of
their proteins and their simplicity of organization.

Blue-green algae have been known to remain alive in dry soil
samples for a period of 50 to 70 years. They withstand being
frozen for several months.

The Algae

443

Certain blue-greens constitute the principal vegetation of
hot springs, and are known to live in temperatures between 1 50°

Fig. 273. Blue-green algae: A, Oscillator ia; B, Nostoc; C, Merismopedia; D, Ccelo-
sphcBrium. {B, D, after G. M. Smith; A, C, after J. E. Tilden.)

and 185° F. These algae become encrusted with minerals from
the water, and in this way the brightly colored rock basins are
formed about such springs — as, for example, the Hot Springs
terraces in the Yellowstone National Park. When fresh-water
streams are polluted by sewage, by poisonous wastes from manu-
facturing processes, and by drainage from coal mines and oil
wells, the blue-green algae survive long after all other plants
except certain bacteria are killed. Many blue-greens grow more
luxuriantly when they have access to organic matter ; that is,
they are partial saprophytes. These forms aid in destroying
sewage in streams and are of great economic importance ; for
they, with the bacteria, constitute a first step in transforming such
waste materials into food for fishes and other aquatic animals.

Blue-green forms in lichens. Many of the genera of lichens
(page 413) have blue-green algae as the food-manufacturing part
of the fungus-alga complex. When this host happens to be one
of the highly gelatinous algae like Nostoc, the lichen also forms a
jelly-like mass.

Common genera. Some of the ^commonest genera among the
blue-greens are : Oscillatoria, filamentous forms with short
cylindrical or disk-shaped cells ; Glceocapsa, gelatinous unicellu-
lar forms ; Nostoc, gelatinous forms in which the cells are formed

444 General Botany

in chains held together in large, irregular masses by the gelati-
nous sheaths; Anabcena, with short, curved chains of cells, very
frequent in ''water blooms"; and Tolypothrix, filamentous
forms that are often highly branched.

THE DIATOMS OR BACILLARIACE^

The diatoms include about 12,000 species of one-celled algae,
remarkable for their abundance in moist places everywhere,
their small size, and their beautifully sculptured cell walls. These
algae differ from the other algae described in this book in having
siliceous cell walls. The cell wall consists of two overlapping
valves, like the two halves of a pill box. In cell division the
two valves are moved outward, and following the division of
the protoplasm, two new valves are formed between the two
halves of the original cell. Diatoms have distinct nuclei and
chloroplasts. Many of them are motile, but some have a stage
in which they are fixed by chitinous stalks and gelatinous sheaths
to under-water objects.

Attention is directed to the diatoms because of their great
economic importance. They are one of the most important
sources of food, directly or indirectly, of marine and fresh-water
fishes. The gizzard shad, for example, consumes directly
enormous numbers of diatoms, while the hake feeds upon a
series of aquatic animals all of which directly or indirectly have
diatoms as their ultimate food supply. No matter how long the
chain of animals, up to the fish, the fundamental organism is the
diatom which changes inorganic compounds into food.

Diatoms occur in vast numbers in the upper layers of the
ocean and in all kinds of streams and ponds. They sometimes
multiply rapidly in reservoirs, and when they subsequently die
become a source of annoyance by producing bad odors and tastes
in drinking water. Extensive deposits of diatom shells occur
in many parts of the world, and these shells are the basis of
powders and soaps for polishing metals. Diatomaceous earth

The Algae

445

is also used as an absorbent of nitro-glycerine in making dyna-
mite, and in the manufacture of fireproof linings and walls.

Fig. 274. Chlamydomonas, a simple free-swimming form of green alga.
Four stages in the reproduction of the plant are shown above. {After Dill.)

THE GREEN ALG^ OR CHLOROPHYCE^

Probably no other group of plants exhibits as wide a range of
forms, structures, and life histories as the green algae. So great
is this diversity that it is exceedingly difficult to describe the
group as a whole. More than 5000 species have been described,
and these are scattered among many families, the interrelation-
ships of which are far from clear.

The green algae include unicellular forms, simple and branched
filaments, plates or extensive sheets, and small gelatinous masses.
All possess chlorophyll inclosed in definite chloroplasts, which
also have an almost endless variety of form. The cell walls are
variously formed of cellulose and pectic compounds. Some of
the forms have an outer layer of chitin on the wall. Most of the
green algae produce motile cells at some stage in their develop-
ment. In the simplest forms the motile stage is the most promi-
nent one in the life history.

The green algae are of peculiar interest, because among them
are not only the simplest plants containing a definite nucleus
and chlorophyll-bearing structure in the protoplasm, but also
the types of plants from which the more complex land plants are

446 General Botany

thought to have been derived. The product of photosynthesis
which accumulates most frequently in the ChlorophycecB is starch,
supplemented in many instances with oil. Both the starch and
the oil occur in greatest amounts in cells and parts of plants that
become reproductive structures.

Multiplication occurs by cell division in the unicellular forms
and by the fragmentation of filaments in more complex forms.
Thick-walled cells are frequently formed from vegetative cells,
and these are highly resistant to drought and cold. Rounded
spores may also be formed by the contraction of the contents of
vegetative cells and the subsequent secretion of a new cell wall.
These spores usually remain dormant for weeks or months before
germinating and starting growth anew. In this way the plants
live through drought and winter conditions.

Reproduction may also take place by swimming spores. These
are naked protoplasts, with two or more cilia, or flagella, that
vibrate and propel the cell through the water. A swimming
spore may consist of the entire content of a vegetative cell, or
several or many spores may be formed by several successive in-
ternal divisions of a vegetative cell. In any event they pass
out of the original cell wall and swim about for a few minutes or
a half hour, and then become attached to some object. They
then begin to divide and form new cells.

If certain green algae are kept in the dark 12 to 24 hours, and
are then brought into the light, swimming spores will appear
in profusion in a half hour or an hour. Transferring algas from
cold water to warm water will frequently produce the same
effect. Swimming spores reproduce the plants rapidly and help
to spread them during favorable conditions.

Sexual reproduction. The green algae present a remarkable
series of forms, with every gradation of sexuality. In sexual
reproduction the essential fact is the union of two gametes to
form a zygote. The gametes in the simplest forms are similar
free-swimming protoplasts, resembling swimming spores but

The Algae 447

smaller in size. In the more specialized forms the male gamete
is a small, free-swimming cell (sperm), and the female gamete
(egg) is a large, non-motile cell containing a large amount of food
materials. The cell in which the sperms are formed is called an
antheridium (plural, antheridia), while the cell that forms an egg
is called an oogonium (plural, oogonia). The spore formed by
the union of a sperm and an egg is called an oospore.

In the peculiar group to which Spirogyra and Zygnema belong,
the gametes unite through a tube that forms between the two
gamete-producing cells. There are neither swimming spores
nor free-swimming gametes.

Distribution. Green algae are comparatively abundant in
tropical seas and decrease rapidly in number along the northern
coasts. On land they are most numerous in fresh-water ponds,
pools, and streams of the temperate zone. The small forms occur
in every conceivable habitat. Some of them cause the red
patches on snow banks in the arctic regions and on high moun-
tain tops.

Protococcus. On the partly shaded, moist sides of trees, rocks,
buildings, and fences everywhere, there occur patches that look
as if they had been stained green. If a little of this stain is
scraped off and examined under a microscope, it is seen to be
made up of little, rounded green cells. Each cell consists of a
cell wall, cytoplasm, and nucleus. In the cytoplasm is a large
green plastid which almost fills the cell.

When the cells are examined, certain of them will be found to
be elongated ; some of these may be dividing into two. Some-
times there are two or more cells still clinging together, showing
clearly that they have just been formed by division. These
groups separate readily when the cover glass is tapped, and each
single cell may go on living quite independently of the others.
The plant, therefore, consists of a single cell which carries on
all the essential processes of life and is able to reproduce itself.
Moreover, it is a highly successful plant, for Protococcus occurs

448 General Botany

in moist places in all parts of the world, from the tropics to the
polar regions, in habitats of many different kinds.

Chlamydomonas. This alga is one of the simplest unicellular
plants and differs from Protococcus in being free-swimming
during its vegetative phase. Chlamydomonas occurs in fresh-
water ponds, ditches, and roadside pools. Reproduction occurs
both by forming resting cells, by swimming spores, and by
gametes. This genus is of interest because it seems to present
better than any other the characteristics of the primitive plants
from which all the green algse and possibly the higher plants
may have been derived.

The pond scums. If examined in the spring or fall, almost
every pond and little stream will be found to contain many kinds
of algae. Some of these are merely masses of rounded cells like
the cells of Protococcus. Others have the cells arranged in rows,
forming simple filaments. In still others the filaments are
highly branched and the plant body may be several feet in length.
Some of the forms are embedded in a gelatinous matrix. All
these various kinds of algae taken together are popularly called
the " pond scums." They are forms of algae most commonly
observed. As pond scums they are most unattractive, but seen
through a microscope they present varied and beautiful examples
of cell architecture.

Many of the pond scums are at first attached to under-water
objects, but during warm weather they break loose and come to
the surface, where they form a green or yellowish-green surface
layer. All cells carrying on photosynthesis give off oxygen,
and the bubbles of oxygen that come from the filaments cling
to them and help to buoy them up. Furthermore, bubbles of air
from the water when it becomes warm also collect in the masses
of algae and help to support them at the top of the water. The
pond scums are generally considered unsightly, and not a few
persons think them poisonous. In reality, they are quite as
harmless as lettuce. The danger in drinking from ponds lies,

The x\lgae 449

not in the green scums, but in the presence of certain disease-
producing bacteria that may have been carried into the ponds
by surface water. Several thousand different species of algae
are concerned in the formation of pond scums. Microspora may
be studied as an example of the more simple filamentous forms.

Microspora. The Microspora plant is a filament made up of
cylindrical or barrel-shaped cells placed end to end. Each cell
carries on all its own food-producing and energy-producing
processes. During early spring, as food is manufactured, the
cells enlarge and divide. The division is always in the same
direction, however, and the cells remain attached to each other,
so that the growth and division of the cells cause the filament to
increase in length. This long, slender line of cells is easily broken,
and the plant may be multiplied by the breaking of the filaments
into parts.

Spores in Microspora. Microspora produces swimming spores
and resting spores. These are special cells that reproduce the
plant. A swimming spore is formed by the contents of a cell
in the filament contracting into an ovoid body. At one end of
this body two cilia, which are small, hair-like propellers, are
developed. The wall of the original cell then breaks and the
swimming spore is set free. After swimming about in the water,
for a short time, it becomes attached to some object under
water, loses its cilia, and grows into a cylindrical vegetative
cell. This cell then continues to grow and divide until a new
filament is formed. The advantages of swimming spores are
that they multiply the plant, and by their ability to swim they
enable the plant to spread to new locations that it might not
reach without these motile cells.

The resting spores are usually formed in the spring after the
active period of vegetative growth has passed. At this season
the cells in the filament stop dividing and food accumulates in
the form of starch and protein granules. The protoplasm in
each cell then contracts into a spherical form and secretes a heavy

450 General Botany

cell wall about itself inside the original cell wall. In this way
the cells of a filament form a row of ovoid or spherical heavy-
walled resting spores. Usually the walls of these spores become
yellow or brownish. The resting spore remains dormant until
the late fall or early spring. Then it germinates; the outer
wall that incloses the spore breaks and the protoplasm and
delicate inner wall push out and form a cylindrical vegetative
cell, which continues to grow and divide, producing a new fila-
ment.

Microspora, then, in addition to the vegetative multiplication
of the cells shown by Protococcus, has swimming spores that
multiply and spread the plant, a stage that recalls Chlamydomo-
nas. It also has resting spores that undergo a dormant period,
after which, when favorable conditions for growth appear, they
produce a new plant. Its life cycle and that of other similar
algas include (i) an active chlorophyll-working period, during
which the plant grows and enlarges its body and accumulates
food ; (2) a reproductive phase, which closes with the production
of resting spores; and (3) a period of dormancy, during which
only the resting spores are alive. The length of the dormant
period for a particular alga is practically the same, whether it
lives in a permanent pond or in a pool that dries up in summer.

Ulothrix. Another green alga occurring on the margins of
lakes, in running streams, and in clear springs is Ulothrix. It
has a filamentous body similar in many respects to Microspora,
and like that form it is attached to rocks and other objects.
Its methods of reproduction, however, are more numerous and
more complex than those of Microspora, and they will serve to
exemplify the reproductive processes of many other forms of
algae. When the filaments are mature, the protoplasm within
some of the cells divides into two, four, or eight parts, each of
which contains nucleus, cytoplasm, chloroplast, and vacuole.
Each of these parts becomes oval in shape and develops into a
swimming spore with four cilia. An opening appears at one

The Algae

451

Fig . 27 s- Fresh-water algae. The upright filaments are, from left to right : CEdogonium,
producing swimming spores, eggs, and sperms ; Microspora, forming resting spores and
swimming spores ; and Ulothrix, forming swimming spores and gametes. The hori-
zontal filaments are Spirogyra (left) and Vaucheria (right). Highly magnified.

452 General Botany

side of the original cell wall, and a few minutes later the swimming
spores pass out from the cell cavity and swim away. Sometimes
all the cells in a filament produce swimming spores at about the
same time, and hundreds of these small green bodies may be
found moving about in the water. At the end of from 15 to 30
minutes the swimming spores settle down on some object and
become attached. By the end of a day the cell formed from
each spore has divided and produced the first two cells of a new
filament.

The protoplasm of other cells, of the same or other filaments,
continues to divide until 16, 32, 64, or more bodies have been
formed. These are called gametes. They are similar to the
swimming spores but much smaller, and each possesses two cilia
for swimming. Like a swimming spore, each of them leaves the
old cell through an opening in the wall. The gametes swim
about for some minutes and then unite in pairs. They are
attached at first only by the ciliated ends, but later the two
gametes fuse. The body thus formed may grow directly into a
new filament, or it may produce swimming spores from each of
which a new filament is formed.

(Edogonium. (Edogonium is another filamentous alga that
flourishes in ponds and streams. In early life the filaments
are attached, but large masses of them will often be found free in
ponds and stagnant pools. From the cylindrical vegetative
cells, large swimming spores are formed. Gametes also are
produced. These are of two distinct forms, male and female.
Plants belonging to the (Edogonium group may be used to
exemplify reproduction in many other algae, whose gametes are
essentially like those of more complex plants.

At the time of production of the gametes, some of the cells in
the filament enlarge, become rounded, and accumulate starch
and other food material ; also, a small opening is formed in the
cell wall. The content of this cell is the female gamete or egg,
which like other eggs has in it a store of food.

The Algae 453

Other cells of the filament are cut up into very short cells by
the formation of transverse walls. In each of these short cells
there are formed two small gametes, which escape from the
filament and swim out into the water. These are the male
gametes, or sperms. Fertilization takes place when one of the
sperms enters through the opening in the oogonium and unites
with the egg. The egg and the sperm may be of the same
filament or of different filaments. The product is an oospore
(egg spore). After a dormant period this produces four swim-
ming spores that start new filaments.

In (Edogonium, therefore, the sex cells are of two kinds quite
distinct in structure and function. The egg is a large, stationary
cell filled with food. The sperm is a small, swimming cell that
moves to the egg and accomplishes fertilization by uniting with
it. The product is an oospore which germinates and produces
four swimming spores. These start a new generation of the
filamentous plants.

Reproduction among the algae. The methods of reproduction
among the algae that we have studied are representative of those
found in the entire group. The three general types are :

(i) Vegetative multiplication. By means of cell division all
masses, filaments, or highly branched plant bodies are produced.
If the individual cells separate from each other after division,'
as in Protococcus, many new individual plants are produced ;
and when filaments and branched forms are broken, as in Mi-
crospora, a new individual plant is produced by each part.

(2) Reproduction by spores. Vegetative cells form thick-
walled resting spores which carry the plant over to the next
season. Another kind of spore is the swimming spore, by means
of which the plant secures immediate reproduction and spreads
to other parts of the pond or stream. These spores are formed
directly from vegetative cells, or by the division of vegetative
cells. There is no union of cells as there is when sexual spores
are formed.

454 General Botany

(3) Sexual reproduction. A sexual spore, or oospore, is
formed by the union of two gametes. The gametes may be
similar in size and appearance, as in Ulothrix, or they may be
unlike, as in (Edogonium, where one gamete accumulates a large
food supply and the other is small and motile. In either case,
the one gamete corresponds to the sperm and the other to the
egg that is found in higher plants. The union is the process
of fertilization. The oospore may germinate immediately,
but more often it remains dormant for a period of weeks or
months.

Other genera of green algae. The most beautiful of the
larger green algae are the Draparnaldias, having a main filament
with little plumose tufts of lateral branches. Closely related
are the Stigeocloniums. Both are frequently found in springs
and small temporary streams.

Among the most readily recognized forms are the species of
Spirogyra, with their spiral chloroplasts — sometimes as many
as sixteen bands in each cell ; and Zygnema, with cells marked by
two large, radially branched chloroplasts.

Vaucheria is a common group found in pools, ditches, and
streams and on moist soil. These algae are remarkable in having
no cross-walls in the long and much-branched vegetative fila-
ments. In the warmer seas are a number of genera related to
Vaucheria, that attain considerable size.

Species of Cladophora are highly branched. They are coarse
forms, found attached to rocks in lakes and swift-flowing streams
everywhere.

Plankton algae. The microscopic plants and animals that
Qoat or swim in all bodies of water make up what is known as the
plankton (Greek : planktos, wandering). It includes hundreds of
species of algae, that multiply rapidly and go through their life
cycles in a few days. These algae are so minute that they can
be collected only by passing the water through silk bolting cloth,
or filter paper. Nevertheless, they are quite as important as

The Algae

455

the filamentous algae as a source of food for small fish and minute
water animals.

G. M. Smith

Fig. 276. Plankton algae: A, Chlamydomonas ; B, Paniorina; C, Ccelastrum; D, Pedi-
aslrum; E, Pleodorina ; F, GlceotcBnium ; G, Selcnastrwn ,
J, Sorastrum; K, Crucigenia; and Z, Nephrocytium.

H, Scenedesmus ; I, Trochiscia;

The importance of the algae. Both green and blue-green algae
are generally considered a nuisance in ponds and streams, and
they are commonly thought to have no economic importance;
but the fact is that these pond scums are the primary food supply
of all the water animals. They bear the same relation to aquatic
animal life that the herbaceous plants bear to animal life on the
land. Nearly all the water animals, from minute insects and
crustaceans to the largest fishes, ultimately depend upon them
for their supply of food. For, like the land plants, these small
water plants manufacture food, and the animals that live in the
water must feed either on them or on other animals that get their
living from the plants. Without the algae the fish would soon
disappear from our waters, because their primary food supply
would be cut off. A decrease in the number of fish in a lake
frequently follows the draining of its swampy margins, for the

456

General Botany

Miih^^r^

Fig. 277. Food relations of aquatic life. No matter how long the chain of animals is
up to the fish, the fundamental food organisms are the algae that transform inorganic
materials into foods.

algae thrive best in shallow water, and it is from the algae that the
small animals on which the fish feed secure their food.

But while the algae are a source of food for water animals, they
are also a source of annoyance in reservoirs in which drinking
water is stored. When they accumulate in large quantities and
die, they cause the so-called '' fishy taste " of water. This
trouble has been to some extent controlled during recent years
by the exclusion of light from small reservoirs, and by the addi-
tion of small amounts of copper sulfate to the water in large
reservoirs. Copper sulfate is very poisonous to algae, even in
quantities of one part to a million parts of water. Since animals
are not injured by such small amounts, the water may be used
without harm for drinking purposes.

Periodicity of algae. The fresh-water algae show somewhat the
same periodicity of development, reproduction, and dormancy
that is shown by the more familiar land plants. There are six
general seasonal classes that may be distinguished. There are
winter annuals, whose spores germinate in the autumn and
which increase during winter thaws by cell division and swimming

The Algae 457

spores, and whose Kfe cycles culminate in the production of
oospores and resting spores in March and April. During the
summer the vegetative plants disappear and only the spores
live over in the mud.

The spring annuals constitute by far the largest wave of
algas. The spores germinate in autumn, winter, and early spring,
and reproduction reaches its maximum in May and early June.
Most of these plants disappear by July.

The summer and autumn annuals germinate in spring and have
longer vegetative periods before fruiting.

There are also perennials, like Cladophora, that live over
from one year to the next and produce spores at various times
of the year. These form the long, green streamers that one sees
in swift streams, on dams and waterfalls, and attached to objects
in lakes.

Finally, there are the ephemerals — short-lived unicellular or
colonial forms of the plankton and wet soils. Here a new genera-
tion may arise every few days. They reach their greatest
abundance in late summer.

Algae are more numerous in seasons when the water levels
are high. They also fruit most abundantly under these con-
ditions. The periodicity determines what species will be found
associated at any time of the year. Ponds that dry up in early
spring obviously can have only winter annuals, while ponds
that last until June will have both the winter and spring annuals.

THE BROWN ALG^ OR PH^OPHYCE^

The brown algae are with few exceptions marine plants. They
possess in addition to chlorophyll a brown pigment which masks
the green color. They attain their greatest dimensions along
rocky coasts where the temperatures are low. The vegetative
body, or thallus, is in many species larger and is far more complex
in structure than in the green algae. Some of the plants attain

458

General Botany

lengths of several or many feet, and internally the plants show
distinct tissue systems. There are three distinct lines of develop-

FiG. 278. A filamentous brown alga, Ectocarpus. On the tips of three of the branches
are many-celled sporangia, which develop zoospores. This alga is common along the
Atlantic coast, growing as an epiphyte on the coarser rockweeds.

ment in this group: the filamentous forms {Ectocarpus), the
highly branched rockweeds (Fucus), and the large stalked
forms with flat blades (Laminaria). About 1000 species are
known.

The filamentous forms. There are a large number of branch-
ing, filamentous forms that are not very different from some of
the green algae. These reproduce by swimming spores, and by
zygotes formed from swimming gametes. The swimming spores
and the gametes of the brown algae differ from those of all other
groups in having two cilia laterally placed.

The rockweeds. A second group are the rockweeds or bladder
wracks (Fucus), which cover the rocks between tide levels.
These are thick, leathery, highly branched plants with internal
air sacs at intervals and with reproductive structures in the

The Algae

459

W. S. Cooper
Fig. 279. Postelsia, a brown alga, on the rocky coast of Santa Cruz County,
California. The plants are covered by water at high tide. Below, near
the water, are other brown algs.

460

General Botany

Antheridium

Fig. 280. Rockweed {Fucus evanescens) : A, sketch of plant showing dichoto-
mous branching, air bladders, and fruiting bodies ; B, growing point ; C, cross-
section of thallus ; D, enlarged view of a part of the cross-section, showing tissues ;
E, section of a conceptacle from a fruiting body, in which antheridia and oogonia
are forming; F, oogonium with eight eggs; G, branch with three antheridia;
H, egg with attached sperms, only one of which unites with the egg.

The Algae

461

Fig. 281. Brown seaweeds, principally species of Fucus, Ascophyllum, and Laminaria, on
the coast of Nova Scotia at low tide.

swollen ends of the branches. The reproductive organs, oogonia
and antheridia, are contained within hollow depressions (con-
ceptacles). The eight egg cells formed within each oogonium
are discharged into the sea and are there fertilized by the sperms
set free from the antheridia. The oospores germinate at once,
and from them the leathery plants develop. The rockweeds do
not produce swimming spores.

In tropical waters species of Sargassum or gulf weed that are
related to Fucus are abundant. These forms are remarkable
for their resemblance to seed plants with leaves and berries. The
berry-like bodies are filled with air and aid in flotation. When
torn from their native rocks in the Caribbean, these alg£e drift
to all parts of the North Atlantic.

The kelps. The third line of development is represented by
the kelps, which vary from forms a few feet in length {Laminaria),
with a root-like holdfast, a stalk,^ and a large, leaf-like blade,
to forms in which the stalk is terminated above by a float and
several branches each with one or more large blades. Here
belongs the Nereocystis of our own northwest coast, and the

462

General Botany

Macrocystis, which is best developed on the west coast of South
America. The former attains lengths of 10 to 30 feet, and the

Apical cell
1

Fig. 282. Thallus of Dictyota, a brown alga. At the right is a growing point sectioned
parallel and perpendicular to the flat surface to show the regularity of the cell division.

latter, growing in 200 feet of water, may reach a total length of
500 feet. These large, leathery plants produce swimming spores
which germinate and produce a small filamentous, or single-
celled, generation. These in turn produce antheridia and
oogonia, and sperms and eggs. After fertilization the resulting
oospore develops into the large, leathery generation.

Here, then, are two distinct generations — one a large food-
manufacturing plant which is called a sporophyte (spore-plant),
because it develops spores ; the other a small, or microscopic,
generation, the gametophyte (gamete-plant), which ends in the
production of gametes.

Economic importance of the brown algse. In China, Japan,
and along the northern coasts various brown algae are cooked with
fish and used as food. Japan exports many tons of dried kelps
to China.

The kelp beds of our own western coast have during recent

The Algae

463

years been used as a source of potassium salts and of other chemi-
cals. It has been estimated that they are capable of furnishing

Fig. 283. Parts of a red alga, Polysiphonia, showing vegetative branch {A, B) and re-
productive structures ; C, antheridium ; D, cystocarp and carpospores ; E, branch forming
tetraspores.

all the potassium needed for agricultural fertilizers in the United
States, and more iodine than we now annually use. In Europe
and Asia the kelps were formerly the chief source of iodine.

RED ALG^ OR RHODOPHYCE^

The red algae, noted for their beautiful colorings and graceful
forms, reach their greatest development in the warm temperate
and tropical seas. Many species occur in shallow water, but
some likewise grow at great depths. The red pigment found in
the cells with the chlorophyll aids in photosynthesis in deep
water. A few genera occur in fresh water. More than 3000
species are known.

The red algae are usually filamentous and highly branched;
sometimes they are irregular blades, or have slender stalks with
leaf-like branches. Among the red algae are a number of families
that deposit calcium carbonate about them. These are the

464 General Botany

Corallines, which are often associated with the true corals on the
coral reefs of the tropics, and a few species of which extend into
cold waters.

The spores and sperms of the red algae are without cilia and are
not motile. The egg cell is inclosed and stationary. Their
methods of reproduction and their life histories are highly com-
plicated and cannot be detailed here.

Economic importance. The common " dulse " and Irish
moss of northern coasts are used in the production of blanc-
mange and jellies. In Asia several species of red algas, together
with a few species of brown algas, are used in the making of agar,
which is in composition a complex of gelatinous carbohydrates.
In Japan the red algae are not only collected, dried, and used
as food in enormous quantities, but the algae are actually culti-
vated in shallow arms of the sea. The edible bird's-nest of the
Orient is constructed of seaweeds.

REFERENCES

Collins, F. S. Green Algce of North America (3 Supplements). Tufts College

Library, Medford, Massachusetts; 1918.
TiLDEN, J. E. MyxophycecB of North America. Minnesota Biological Survey;

1910.
Ward, H. B., and Whipple, G. C. Freshwater Biology. John Wiley & Sons,

Inc., New York; 1918.
West, G. S. Alga. G. P. Putnam's Sons, New York; 1916.
Treatise on British Freshwater Algce. G. P. Putnam's Sons, New York; 1904.

CHAPTER FORTY-THREE

BRYOPHYTES: LIVERWORTS AND MOSSES

The phylum Bryophyta includes two diversified kinds of
plants commonly known as the liverworts and mosses. Their
structures and life histories are somewhat more complicated than
those of the algae. All together they comprise some 16,000
species, three fourths of which are mosses.

The largest of the mosses and liverworts never attain a height
or length of more than a few inches, and they are of very simple
structure in comparison with the flowering plants. In contrast
with the algas, which on the whole are water plants, mosses and
liverworts for the most part live on land. The passing of plants
from a water to a land habitat is one of the notable steps in the
evolution of the plant kingdom, and in connection with the study
of this group we shall contrast the environments of land and water
plants and consider the modifications in structure that accompany
the passing of simple plants from the water to a land habitat.

Living conditions of land and water plants contrasted. In the
preceding chapter attention was called to the conditions under
which the algas grow. The water environment is most favorable
for the growth of simple plants, because of (i) the avoidance of
heating and drying effects of intense sunshine, (2) abundant
supply of carbon dioxide, oxygen, and mineral salts, (3) more
uniform temperature, and (4) longer growing season.

The environment of the land plant, on the other hand, furnishes
through wet cell walls a supply of carbon dioxide and oxygen from
the atmosphere, and mineral salts may be secured only from the
soil water with which the plants are in contact. If the plant grows
in full sunlight, it is subjected to much more intense illumination
and heating than are water plants, and it must withstand the
drying effects of the air. A study of the amphibious liverworts
shows that they have become adjusted only to a medium light
and a moderate amount of drying. These plants, therefOxC,

465

466 General Botany

grow in moist, shaded situations. During wet periods many in-
dividuals start in other places, only to be killed off later by the
light and its secondary temperature and drought effects. The
shaded situation where the water is near the surface of the soil
is evidently the habitat where these plants suffer the least, and
this explains why liverworts persist in moist situations and not
in the open.

Responses of plants to the aerial environment. In contrast
to the algae the land liverworts show several changes in structure
that are of advantage to plants in an aerial habitat. The more
important of them are :

(i) Firmer, and in some cases thicker ^ cell walls and water -
storage tissue. The firmer cell walls are less permeable to water
and reduce the rate of water loss. Furthermore, the plants grow
flat on the soil in contact with the water supply, and some of the
forms develop layers of water-storage cells and mucilage pockets
on the side in contact with the soil . This enables them to with-
stand short dry periods better than do those forms that have
only the usual aquatic type of cell wall.

(2) The development of rhizoids. Land plants are favored by
being anchored, and by having structures that will bring them
into contact with the soil-water supply. In the liverworts,
rhizoids anchor the plant and to some extent absorb water and
mineral salts from the soil. Rhizoids are elongated cells that
develop on the lower side of the plant body and penetrate the soil.
They resemble root hairs in form.

(3) The development of an epidermis. The land liverworts
are covered by an epidermis which decreases the rate of water
loss. The liverworts with thicker bodies have pores in the
epidermis which afford a ready access to the carbon dioxide and
oxygen necessary for photosynthesis and respiration. In the
more complex liverworts the epidermis is raised like a transparent
roof on ridges of supporting tissues, leaving beneath it a series of
small air chambers in which the chlorophyll-bearing cells stand

Bryophytes : Liverworts and Mosses

467

up in short chains. Each chamber is connected with the air by
epidermis, but the epidermal pores in them are chimney-like

Fife. 284. Diagram of a small portion of a Marchantia thallus showing, above, the upper
epidermis with chimney-like openings and the air cavities containing the chlorenchyma.
Below are the water-containing tissue and the lower epidermis with a single rhizoid.

openings and are incapable of closing as do the stomata of the
higher plants. The presence of a distinct epidermis having pores
is a third feature of plants which improves their chances of living
on land.

(4) The ability to withstand drying. When the vegetative
cells of water plants are dried, the protoplasm dies at once ; but
a few of the liverworts, like many mosses and like Protococcus and
a few other algae, do not die when water is lost from the cells..
Just what quality the protoplasm possesses that enables it to
withstand drying, it is at present impossible to say ; but some of
the liverworts that grow on trees and rocks possess this quality,
and certain mosses have to a remarkable degree the ability
to withstand drying. A fourth factor which enables some
plants to live in the land environment is the ability to with-
stand drying.

(5) The production of light spores. At some point in the life
cycle of most land plants, spores- of small size and light weight
are produced. The food contained in these spores is largely oil
— the lightest form in which a given amount of energy may be
stored. Spores of this kind are readily carried scores of miles by

468

General Botany

the wind. The development of light-weight spores is a fifth im-
portant characteristic of plants fitted to the land environment.

Fig. 285. Figures showing the life history of a liverwort {Pallamcinia) : A, B, archegonial
and antheridial thalli; C, cross-section of thallus, showing antheridium ; D, cross-section
of thallus, showing archegonium ; E, F, G, stages in development of archegonium;
H, embryonic sporophyte within the greatly enlarged archegonium wall (calyptra) ; I, mature
sporophyte with spores.

LIVERWORTS

The body of many liverworts is fiat and leaf -like, and is called
a thallus (plural, thalli). It may be from one to several cell layers
in thickness. Growth takes place by repeated divisions of a
single cell at the tip. The thalli branch at intervals by forking.
Liverworts do not stand erect, but usually have their thalli in
close contact with the substrata on which they grow. In most
forms the thallus is a continuous plate of cells, but some forms have
prostrate stems with small leaves on either side. Even the
thalloid forms like Marchantia have scales on the under surface.
All the forms have small, hair-like rhizoids that anchor the plant
and absorb water and minerals. They reproduce by spores,
produced either directly on the thallus or on special reproduc-
tive branches. In some liverworts there are produced also
special bodies called gemmce (singular, gemma), which propagate
the plants vegetatively.

The 4000 species of liverworts are widely distributed but are

Bryophytes : Liverworts and Mosses

469

most numerous in the tropics. Liverworts may be found along
streams, on overhanging rocks, on shaded moist soil, and on
trunks of trees. Li the tropics they often occur as epiphytes on
the stems and leaves of trees.

The liverworts are probably descended from plants like the
green algae; for it is thought that the simplest plants existed
first and that plant life (as well as animal life) had its origin in the
water. The liverworts may be considered, therefore, as a group of
simple plants that exhibit some of the evolutionary stages through
which plants passed in taking up life upon the land. In this re-
spect they can be compared to the amphibious (Greek : amphi,
double, and bios, life) frogs and salamanders of the animal world.

Life history of a liverwort. The most common of the aquatic
liverworts is Ricciocarpus, a small, heart-shaped thallus which
floats on the surface of ponds and lakes. On its lower side are
hair-like rhizoids and scales that aid in absorption. On its

Fig. 286. Some widely distributed liverworts: A, Pellia thallus with antheridia (dots
on surface) and a sporophyte; B, archegonial thallus of Anthoceros with sporophytes; C,
antheridial thallus of the same ; D and E, land and water forms of Riccia. {After Vdenovsky.)

470 General Botany

upper side are two divergent grooves in which antheridia and
archegonia (singular, archegonium ^) are formed. The antheridia
produce the sperms. The archegonium is a flask-shaped organ
which contains the egg. Fertilization is effected by the small
sperm swimming to the archegonium when the thallus is wet,
passing down the neck of the archegonium and fusing with the
egg. The oospore, or fertilized egg, germinates directly, pro-
ducing a rounded body of cells. The inner cells of this body
divide, each forming four spores, while the outer layer of cells
forms the sporangium wall. At maturity the sporangium wall
breaks, liberating the spores.

As will become more evident when we study the ferns, this life
history is made up of two distinct phases, or generations. The
one producing the gametes is called the gametophyte; the one
ending with the production of spores is the sporophyte. The
gametophyte of all liverworts and mosses is a food-manufacturing
phase, the sporophyte is a parasitic phase.

Other liverworts. Marchantia is a common thallose liverwort
found on moist rocks and in swamps. It differs from Riccio-
carpus mainly in having specialized branches (Fig. 287), arising
from the thallus, on which the antheridia and archegonia are pro-
duced. The sporophyte also has a short stalk below the sporan-
gium, the base (foot) of which grows downward into the tissue of
the gametophyte, thus becoming a distinct absorbing organ.

Anthoceros is another thallose form, in which the sporophyte
is greatly elongated, growing upward from the thallus in which
the archegonium is embedded. The anthoceros sporophyte is
of further interest because the sporangium wall is several cell
layers in thickness and the cells contain chlorophyll. Moreover,
the epidermis has guard cells and stomata.

There are many genera of leafy liverworts, and about 3000
species have been described. These forms are very abundant in

^ The archegonium is found in the mosses, liverworts, ferns, and in gym-
nosperms. It is analogous to the oogonium of the algae.

Bryophytes : Liverworts and Mosses

471

Fig. 287. The liverwort, Marchantia, showing capsules and various stagts in ihe dc\ tlop-
ment of the antheridial and archegonial branches.

the tropics. Porella is a rather common example found at the
bases of trees, or on rocks in moist ravines. The life histories
of the leafy liverworts are very similar to that of the thallose
forms.

MOSSES

The mosses form a very large group found in all parts of the-
world. Like the liverworts, they are most abundant in. moist,
partly shaded habitats. A few, however, grow on rocks and trees
where they are exposed to intense light and periodic drought.
When dry, they are dormant; and when wet, they carry on
the usual life processes.

As a result of their methods of vegetative multiplication, mosses
have the habit of growing in compact clusters. This gives
them an external means of conserving water and maintaining
the water balance. The dense masses of plants take up water
from rains and hold it for some time like a sponge.

The plant body. Mosses usually have upright and radially
symmetrical stems, though many live close to the substratum and

472

General Botany

have only horizontal or inclined stems. They possess very simple
leaves, frequently only one cell layer in thickness, sometimes
thicker toward the midrib. Like the liverworts, mosses have
rhizoids. But the rhizoids of the liverworts are one-celled struc-
tures, while those of the mosses are branching, many-celled struc-
tures which penetrate the soil. These afford a firm anchorage for
the plant and absorb a part of the water used by it. The stems
of the largest mosses have elongated cells forming the central
axis. These cells probably form a primitive conducting tissue.

ii.

K>.

rfi^^r^^S

Fig. 288. Habitat sketch of three common mosses: Climacium
(at left), Polytrichum (above at right), and Mnium.

Bryophytes : Liverworts and Mosses

473

Mosses, therefore, show some advances over the liverworts
in their upright radial stems and branching rhizoids, in the regular

Fig. 289. Mosses: A, Bryum, showing leafy gametophyte with attached sporophyte;
B, sporangium enlarged, showing the peristome teeth; C, details of peristome teeth.
D, Andrcea, showing leafy gametophyte and sporophyte with two sporangia; the one at the
right has shed its spores. E, germinating spore and protonema of a moss, showing bud from
which a leafy gametophyte develops. {After Frank.)

occurrence of simple leaves, and in their ability to grow in drier
habitats.

Life history of the moss. Mosses reproduce freely by vegeta-
tive propagation and by spores. A study of each of these meth-
ods will make clear the somewhat complicated life history of the
moss plant.

Vegetative multiplication. When a moss spore germinates on
the soil, it produces a branching, filamentous body, the protonema
(Greek : protos, first, and nema, thread), which resembles some of
the branching forms among the green algae. The protonema
spreads over the soil for some distance and then develops numer-
ous buds (Fig. 289). The buds give rise to the upright leafy

474

General Botany

branches which we commonly call the moss plant. Because of the
numerous buds developed on the protonema, the moss plants
stand in thick clusters or masses.

Fig. 2go. A moss plant {Mnium). £ is a vegetative branch, B a branch that
produces eggs, and A a branch that produces sperms. After fertilization, an
upright stalk bearing a spore case (C) develops from the egg. ^' is a longitudinal
section of a female branch, showing three egg cells in the archegonia in which
they are produced : B' is a section of a male branch, showing three of the
antheridia that produce the sperms.

The upright leafy stems of the moss also have the power of
producing protonema-like branches which spread still farther over
the soil, thus serving to multiply the plants and to make the plant
mass denser and larger. In some mosses with horizontal or in-
clined stems, the stem tips when in contact with the soil develop
rhizoids and give rise to new branches, much as the stems of the
raspberry develop new plants. These methods of vegetative
propagation are common among the mosses, and some mosses
are not known to multiply in any other way. Some mosses
also produce gemmae.

Gametophyte and sexual reproduction. Archegonia and an-
theridia are produced on the mature upright stems of most
mosses. The antheridia are many-celled structures, each of
the smaller interior cells of which produces a sperm. The

Bryophytes : Liverworts and Mosses

475

archegonium is a multicellular, flask-shaped body in which a
single large egg is formed at the base of the neck. These

Fig. 2gi. Sphagnum moss: A, upright shoot, with antheridial branches above and two
archegonial branches below ; B, prothallus with young sporophyte ; C, archegonial branch
with mature sporophyte ; D, archegonia within the scales of the archegonial branch ; E, tip
of archegonial branch and the attached sporophyte seen in section. The old archegonium
wall still surrounds the sporangium. {After Frank.)

organs may be on the same branch tip or on different branches.
The sperms are discharged from the antheridium by the absorp-
tion of water and consequent bursting when the moss is wet.
The sperms swim about in the film of water on top of the plants.
Some reach the archegonia and fertilization follows. The in-
terior row of cells (neck-canal cells) of the archegonium (Fig. 290)
disintegrate as the egg matures and form a mucilaginous mass
from which sugars diffuse into the water. When this diffusing
sugar reaches the swimming sperms, their direction of swimming
is changed toward the diffusing sugar and in this way they swim
into the archegonium and one finally reaches the egg and fuses
with it. When the sperm unites with the egg, it forms an oospore.
The protonema and the leafy branches that arise from the
spore make up the gametophyte generation of the moss.
Generally the gametophyte is perennial and gametes are produced
each year from new branches.

476 General Botany

Sporophyte and asexual reproduction. The oospore germi-
nates while still within the archegonium on top of the stem,
and produces a slender, stalk-like body. The base of this body
grows downward into the parent stem and draws water and
nourishment from it. At the top of the stalk a sporangium,
or capsule, develops which contains spores. The stalk and
sporangium live parasitically on the green, leafy moss plant and
constitute the sporophyte generation.

Summary. The Bryophytes probably present some of the fea-
tures that characterized the first land plants. They are com-
paratively simple in structure, but they are more differentiated
than the green algae. They show (i) tendencies toward the de-
velopment of distinct absorptive and photosynthetic tissues;
(2) the presence of chloroplasts similar to those of the seed plants
in both gametophyte and sporophyte ; (3) the development of
intercellular spaces, air pores, and (in the sporophyte) guard cells
and stomata ; and (4) a life cycle of two distinct phases, each
producing a spore that develops the alternate generation.

The vegetative plant is the gametophyte, and it is among the
Bryophytes that the gametophyte attains its greatest size and
differentiation among land plants.

REFERENCES

Campbell, D. H. Structure and Development of Mosses and Ferns. The Mac-

millan Company, New York; 1918.
Grout, A. J. Mosses with a Hand-lens. Published by the Author, Brooklyn,

New York; 1905.
Mosses with Hand-lens and Microscope. Published by the Author, Brooklyn,

New York.
Jennings, O. E. Manual of the Mosses of Western Pennsylvania. Published by

the Author, Pittsburgh, Pennsylvania; 1913.

CHAPTER FORTY-FOUR

THE PTERIDOPHYTES

The Pteridophytes (Greek: pteris, fern,
phyton, plant) include a series of several phyla
that have formed a conspicuous part of the
earth's vegetation since Paleozoic times. The
prevailing modern representatives of these
ancient groups are the ferns, the equisetums,
and the lycopods. About 8000 species have
been described, of which 7000 belong to the
ferns.

Like the Bryophytes, the Pteridophytes re-
produce by spores, but in contrast to them the
two generations of the life cycle are distinct
plants, both living on the soil, and the con-
spicuous generation is the sporophyte. Fur-
thermore, the sporophyte is differentiated into
leaves, stems, and roots. Some of these plants
attain large size, with stems 10 to 50 feet in
height and leaves 10 to 30 feet in length.
This remarkable differentiation is made pos-
sible by the presence of vascular bundles
and mechanical tissues that are not very dif-
ferent from those of the seed plants.

477

Fig. 292. The walk-
ing fern (Camptosorus
rhyzophyllus). New
plants are developed
from buds at the ends
of the leaves.

478

General Botany

H. N. Whilford

Fig. 293. A large tropical fern {Maratlia), with leaves 15 feet in length.

The appearance of a vascular system in the evolution of the
plant kingdom may be compared with the coming in of a back-
bone in the evolution of animals. There could be no large land
plants raised far above the soil without efhcient conductive
tissues through which water and food may move rapidly. The
vascular conductive system is therefore a most important ad-
justment to land conditions.

The Pteridophytes are at once the simplest of the land plants,
with true roots, stems, and leaves, and the most highly organized
plants without seeds. The origin of all these phyla is unknown,
and although there have been many evolutionary developments
since Paleozoic times, the distinctive features of each of the phyla
are found in the oldest known fossil forms.

THE FERNS (fILICALES)

The ferns attain their greatest size, number, and variety
in the moist tropical and subtropical regions. Some of the smaller
epiphytic forms, the filmy ferns, have leaf blades only a few cell
layers in thickness and are confined to the dripping forests of the

The Pteridophytes

479

Fig. 294. A large tree fern in the Philippines.

48o

General Botany

-Ml-.-

«^f^Y

g-i^M^ ^jt,M

Fig. 295. A roadside group of the cinnamon fern {Osmunda cinnamomea) growing in the
shade of red maples, eastern Pennsylvania.

The Pteridophytes

481

Fig. 2g6. The shield fern {Aspidium marginale).

rainy tropics. Most ferns are mesophytic and attain their
best development in partial shade and in rich humus soils.

The ferns are readily distinguished by their divided and com-
pound leaves. The leaves arise near the apex of the stems and
uncoil as they develop. The youngest part of the leaf is
the apex. The venation is also characteristic, being forked or
dichotomous.

The stems of most ferns are horizontal and branched, extend-
ing either at or below the surface of the soil. The cinnamon
fern of the United States has an erect stem, sometimes rising a

482

General Botany

^ ,j WJ

-« 9f

^^%

r *>* f

^^M

■p% r -<; k

^::

>-■->

f ^s^ '*

W'-

''?'

^ ... f

Fig. 297. The sensitive fern {Onoclea sensibilis), showing foliage
leaf and sporophylls.

foot above the soil. In the tropics woody, erect stems give rise
to the tree ferns, which attain an extreme height of 60 feet.

The roots of most ferns are comparatively small and less
branched than the roots of seed plants. In the herbaceous ferns
they arise irregularly from the sides and under surface of the
rhizome. In the tree ferns the root systems are more complex,
but they do not attain the size and spread of the root systems of
the seed plants. A restricted water-absorbing system is one of
the reasons why ferns are uncommon in dry habitats.

Ferns multiply vegetatively by their branching rootstocks.
Some, like the walking fern, develop new plants at the tips of
the leaves when in contact with the substratum. In some trop-

The Pteridophytes

483

L J Lhd>,iberlain

Fig. 298. One of the largest known specimens of the staghorn fern, an epiphyte on trees in
the tropics. The upright leaves are the photosynthetic organs ; the rounded leaves pressed
against the tree cover masses of roots ; the pendant leaves produce the reproductive bodies.
The photograph was made on a small island near Brisbane, Australia.

484

General Botany

ical species new plants develop vegetatively from the swollen
leaf bases.

The sporophyte. The familiar fern plant is the sporophyte.
In many species the foliage leaves develop groups of fruiting
bodies called sori (singular, sorus), on the under surfaces of the
later leaves. Each sorus consists of several or many sporangia,
and within each sporangium from 32 to 64 spores develop. In
the cinnamon fern the fertile leaves differ from the foliage leaves,

being reduced in size, without
chlorophyll, and with the leaf-
lets (pinnae) acting merely as
supports of sporangia. Special
spore-bearing leaves are called
sporophylls. An average fern
plant in this way produces
several to many million spores
each season.

The gametophyte. The
spores germinate either im-
mediately, or after a dormant
period. From them there de-
velops a small, heart-shaped
thallus that superficially re-
sembles a liverwort. This is
called the prothallus, and con-
stitutes the gametophyte gen-
eration of the fern. Prothalli
may be found commonly on
moist rocks, or on the soil
near fern plants.

As the flat expanse of cells

forming the prothallus de-

r, ^ ^ , velops, rhizoids appear on the

Underground stem, roots, and

leaves of a fern. lower sidc ; and soon af ter-

FlG. 299.

The Pteridophytes

485

ward, in the vicinity of the rhizoids, antheridia develop. The
antheridia are comparatively simple structures, with a wall
composed of several cells, inclos-
ing the sperm mother cells, each
of which produces a sperm. The
sperms have a spirally twisted
body and a beak with forty or
fifty long cilia. The archegonia
appear as the gametophyte ma-
tures, and like the antheridia
are located on the under side
near the notch, or growing re-
gion, of the prothallus. They
are simpler in structure than
those of Bryophytes; the neck
is curved, and the egg cell is
embedded in the prothallus.

Fertilization. The sperms are
released by the swelling and bursting of the antheridium, when
water stands under the prothallus. Under similar conditions
the archegonium opens and the products of the disintegration
of the neck canal cells diffuse into the water. The sperms are
directed in their swimming by these substances, and one of the
sperms after entering the archegonium fuses with the egg cell,
forming an oospore. When fertilization has taken place in one
of the archegonia, the further development of the remaining
immature archegonia ceases. For this reason fern prothalli
usually produce but a single sporophyte. The same general
statement might also be made for the Bryophytes.

Embryo of sporophyte. The oospore germinates directly after
fertilization. Cell division takes place rapidly, and an embryo
is soon formed that shows four general regions: (i) the foot, a
holdfast and absorbing region by which the embryo is attached
for a short time to the prothallus ; (2) a root, which rapidly elon-

FiG. 300. Under side of a fern prothal-
lus, showing egg-producing organs (arche-
gonia) {A), the sperm-producing orgf.ns
(antheridia) (B), and the rhizoids (C).

486

General Botany

gates and pushes into the soil ; (3) a leaf of very simple structure,
which soon rises above the prothallus and forms the first photo-

FiG. 301. The life history of a fern. The prothalkis (.-I) produces egg cells and sperms
in organs on the lower surface. One of the sperms set free from B unites with an egg
cell (shown in C) and produces an oospore. This germinates and produces the leafy
fern plant (D), which in turn produces spores in sporangia {F and G) on the lower
side of the leaves. By the bursting of the walls of the sporangium {H) the spores are
set free. They then germinate on the soil (in some species on rocks or trees) and
produce a new generation of prothalli like the one shown in A . The prothallus is here
shown about four times its natural size.

synthetic organ of the sporophyte; and (4) a stem tip, which
extends more slowly and gives rise to the successive leaves and
adventitious roots. The sporophyte is thus at first parasitic on
the gametophyte, but it soon becomes independent and the pro-
thallus dies and disappears.

The embryo develops into the mature sporophyte, which has
already been sufficiently described.

Alternation of generations. Among the algae both the oo-
spores and the spores formed from vegetative cells usually re-
produce the plant directly. In CEdogonium the oospore, when
it germinates, produces an enlarged cell {sporangium), in which
four swimming spores are formed, and these reproduce the fila-
mentous plant.

Among the Bryophytes the spores, formed asexually, develop

The Pteridophytes 487

a fiat thallus, or a protonema and leafy branched gametophyte.
The gametophyte in turn produces oospores which germinate in
situ, and a simple parasitic or partially parasitic sporophyte,
consisting of a sporangium, foot, and stalk, ensues. Its life ter-
minates with the production of spores.

Among the ferns the asexually formed spores germinate on
the soil and produce a prothallus. This is the gametophyte,
and it in turn produces an oospore, from which the large, leafy
fern plant develops. Among all plant groups, beginning with the
Bryophytes, a corresponding alternation of generations may be
discerned.

Chromosome numbers. In Chapter XXXI attention was
called to the importance of chromosomes as carriers of heritable
qualities. The statement was made that the number of chromo-
somes is usually definite, and that at one step (formation of
pollen and embryo sac) in the life cycle of complex plants there
is a reduction division. Following the reduction division the
cells have just one half the number of chromosomes. In the
life cycle the last of the cells, with the reduced number, are
the sperm and egg. When they unite, forming a zygote or
oospore, the number of chromosomes is restored.

The reduction division occurs in the mother cells that produce
the spores. The spores, the cells of the subsequent gametophyte,^
and the sperms and eggs have the reduced number of chromosomes
(usually written n number of chromosomes). The oospore and
the sporophyte generation up to and including the spore mother
cells have the 2 n number of chromosomes.^

^ It should be mentioned in this connection that the form of neither the
gametophyte nor the sporophyte is determined by the number of chromosomes.
These alternate generations are sometimes produced vegetatively in both the
mosses and ferns. A gametophyte developed vegetatively from a sporophyte
has 2 n chromosomes ; likewise a sporopbyte developed vegetatively from a
gametophyte has n chromosomes.

2 In the brown alga Dictyota the chromosome numbers are i6 and 32;
in the red alga Polysiphonia, 20 and 40; in the liverwort Pellia, 8 and 16;
in Anthoceros, 4 and 8; in the moss Bryum, 10 and 20.

488 General Botany

THE EQUISETUMS

The equisetums constitute a small group of about twenty-
five living species that superficially bear little resemblance to
the ferns. Nevertheless, their life histories are quite similar.
Like the ferns they are representative of a very ancient phylum.
During the Carboniferous period there were allied plants that
formed extensive forests, with trunks 90 feet in height and 3
feet in diameter. The modern species are usually less than 3 feet
in height, although there are two tropical species that reach a
height of 10 to 15 feet and a South American species that attains
a height of 40 feet when partly supported by trees.

The equisetums usually have columnar, upright, jointed
stems, externally fluted and internally characterized by long,
tubular air cavities. The upright stems arise as branches from
underground horizontal rhizomes. The leaves are scales ar-
ranged in whorls at the nodes. As the leaves are without chloro-
phyll, the photosynthetic work is carried on by the chlorenchyma
of the stems. In several species the upright stems bear a multi-
tude of slender whorled branches, whose brush-like character sug-
gests one of the common names, " horsetail." Another common
name, " scouring rush," is suggested by the fact that the cell
walls contain large amounts of silica and that in pioneer days
the plants were used to scour metal utensils.

The roots are small and arise along the rhizomes mostly at the
nodes. The plants are essentially hydrophytes and are found
commonly on stream margins, swamps, and lake shores. A few
of the species occur in dry situations, but are there much dwarfed.

The sporophyte. The sporophyte generation is the plant
we have just described. It is a perennial. The reproductive
structures consist of whorls of peculiar shield-shaped sporophylls,
each bearing five to ten sporangia, that together form a terminal
cone.

Within the sporangia spores arise which are peculiar in

The Pteridophytes

489

having four long appendages, that coil around
moist and uncoil when dry. The spores contain
do not withstand drying, and they
die unless germinated within a
month.

The gametophytes. In most
species of equisetum the gameto-
phytes are irregularly lobed, thal-
loid structures which grow on
moist banks of streams. They
are bisexual, producing both an-
theridia and archegonia. In the
most-specialized species {E. ar-
vense) the gametophytes are usu-
ally unisexual ; that is, about one
half produce antheridia only and
the other half archegonia. Fer-
tilization takes place when a swim-
ming sperm fuses with an Qgg cell.
An oospore results, which germi-
nates at once as in the ferns,
and from it the sporophyte de-
velops (Fig. 304, C, D).

the spore when
chlorophyll and

Fig. 302. The common field equisetum
{Equisetum arvense). Rootstock with
sterile branches {B), spore-bearing
branches {A), and tubers (C). D shows
the spores with their appendages.

THE LYCOPODS (LYCOPODIALES)

Another, and perhaps the most
ancient, group of Pteridophytes
includes the lycopods or club
mosses. Only two genera remain,
Lycopodium (100 species) and Selaginella (500 species). Both
consist of scale-leafed creeping plants, often several feet in
length, with upright or inclined branches. The stems branch
by repeated forking, and this is true even of the fossil tree forms.
In all cases the leaves are scale-like. In the more primitive spe-

^go General Botany

cies the sporangia develop in the axils of every leaf ; in the more
specialized types the sporangia occur in cones of modified scales,
or sporophylls.

In Lycopodium the spores are all alike, and when they ger-
minate produce green, fleshy thalloid gametophytes, or thick
underground tuberous gametophytes. The subterranean ga-
metophytes are saprophytes, and in the other species they are
partial saprophytes. Each gametophyte produces both sperms

Fig. 303. Two species of club mosses {Lycopodium). In the species on
the left {Lycopodium lucidulum) the sporangia are borne in the axils of
the upper leaves ; in the other {Lycopodium clavatum) they are borne
in the terminal cones.

The Pteridophytes

491

and eggs. The sperms of lycopods differ from those of other
pteridophytes in being very small and in having only two cilia.

Fig. 304. Gametophy tes : A , Lyco podium complanatum, longitudinal section showing
antheridia, archegonia, and one embryo; B, Lycopodium annotinum, with three young
sporophytes; C, Equisetum Icevigatum, with four young sporophytes; D, Equisetum
debile; E, the fleshy prothallus of a fernwort, Ophioglossum vulgatum. {A, B, after
Bruchmann ; C, after Walker; D, after Kashyap; E, after Frank.)

In Selaginella the spores are of two kinds, produced in two dif- .
ferent kinds of sporangia. The small spores (microspores)
are produced by hundreds in small sporangia (microsporangia) ;
and four large spores (megaspores) develop in each large sporan-
gium (megasporangium) . These spores have special interest
because from the microspores only antheridial gametophytes
develop, and from the megaspores only archegonial gametophytes
are formed (Fig. 306).

Heterospory. The occurrence of two kinds of asexually
formed spores is known as heterospory, in contrast to homospory,
the formation of only one kind of spores. While we have de-
scribed heterospory only in the case of Selaginella, it should be
mentioned that heterospory occurs among the ferns as well as

492 General Botany

among the lycopods, and has occurred among the equisetums
that are now extinct.

Gametophytes. Both the male and female gametophytes
are small or microscopic, being formed partly or wholly inside
the spore wall. Here, then, are the most-reduced gametophytes
among the Pteridophytes.

Seeds. Before leaving the Pteridophytes, attention should
be called to the fact that occasionally the large spores of Selagi-
nella germinate within the sporangium and produce a prothallus.
Fertilization may take place, followed by the development of an
embryo sporophyte before the megaspore leaves the megasporan-
gium. These are rare and accidental happenings in Selaginella,
but when they occur we have the same arrangement of struc-
tures that regularly occurs in the formation of the seeds in sper-
matophytes. A seed is the result of telescoping a gametophyte
and a new sporophyte within a sporangium :

Fig. 3C5. Selaginella martensii, showing leaf -like shoots with cones at the ends of the

smaller branches.

The Pteridophytes

493

Megasporangium + female prothallus + embryo

Part of ist sporophyte + (gametophyte) -|- (2d sporophyte)

Seed coat -f endosperm (as

in conifers) -|- embryo

> = a seed

Fig. 306. Selaginella: A, vegetative branch with terminal cone; B, longitudinal section
of cone, showing microsporangia on one side, megasporangia on the other ; C, female gameto-
phyte protruding from the megaspore wall with several archegonial openings among the
rhizoids; D, male gametophyte within the microspore wall; E, male gametophyte with'
sperms formed in the cells ; F, section of female gametophyte, or prothallus, after fertiliza-
tion, showing two embryos. {After Frank.)

Summary. The occurrence of conductive tissues in the sporo-
phyte of the Pteridophytes not only made possible the develop-
ment of large land plants with roots, stems, and leaves, but it
gave the sporophyte generation possibilities of evolution far
beyond that of the gametophyte.

The three phyla, ferns, equisetums, and lycopods, all have inde-
pendent thalloid gametophytes. . In the ferns they are auto-
phytic and bisexual ; in the equisetums, autophytic and usually
bisexual ; and in the lycopods, partially saprophytic, sometimes
bisexual (Lycopodium) and sometimes unisexual (Selaginella).

494 General Botany

In all three phyla the production of two kinds of spores (heter-
ospory) occurs either in the modern representatives or in the fossil
forms.

Among the ferns the leaves are often extremely large and are
characterized by forked venation. During development the
leaves uncoil, as the growing points are in the tips of the leaves and
leaflets. The leaves of the lycopods are poorly developed, being
only scales, and those of the equisetums are scales devoid of
chlorophyll.

The roots of Pteridophytes are usually small and scattered
along the horizontal stems. In the large, upright tree types
they are generally basal and sometimes of considerable size, but
even then they do not compare with those of the seed plants in
relative absorbing area.

All of the phyla have forms in which the spores are not pro-
duced on foliage leaves. The special spore-bearing leaves are
reduced in size and in extreme forms lack chlorophyll. These
leaves are termed sporophylls. In the equisetums and lycopods
the sporophylls are arranged in cones. The extreme forms also
have two kinds of sporangia and spores : microsporangia and
microspores, and megasporangia and megaspores.

REFERENCES

Atkinson, G. F. Biology of Ferns. The Macmillan Company, New York.

Campbell, D. H. Structure and Development of Mosses and Ferns. The Mac-
millan Company, New York; 1918.

Clute, W. N. Our Ferns in Their Haunts. F. A. Stokes Company, New York;
1901.

Waters, C. E. Ferns. Henry Holt & Co., New York ; 1903.

CHAPTER FORTY-FIVE

FOSSIL PLANTS

When a leaf falls on soft mud, it may become imbedded
in it. Later the mud may be covered by other layers of sedi-
ment. When the mud dries, a perfect imprint of the outline
and veins may be left. As time goes on and the mud becomes
more deeply buried, it may harden into rock and retain the im-
print of the leaf as a record of a plant that lived when the rock
was merely soft mud. In this way leaves, fruits, seeds, stems,
and roots have left their imprints to testify, thousands and mil-
lions of years afterward, to their former existence.

Plant remains also accumulate in deep water or in water con-
taining large amounts of mineral matter in solution. In such
places they may decay very slowly, and the material of which
they are composed may be gradually replaced by the mineral
substances in the water. Under these favorable conditions the

T. D. A. Cockerell

Fig. 307. Fossil oak leaf from the Tertiary shales at Florissant, Colorado,
and a modern oak leaf from the same region.

495

496

General Botany

T. D. A . Cockerell

Fig. 308. Fossil flower from the Tertiary shales at Florissant,
Colorado.

internal structures of the plant are preserved. As animal re-
mains are preserved in the same way, we have in the rocks a
record of the plants and animals of the past. These petrified
plant and animal remains and the plant and animal imprints
from former geological ages are cdM^d fossils.

Fragmentary nature of fossil record. Present-day observa-
tions on the fate of fallen leaves and of other plant organs show
that they usually decay and disappear within a few months.
Fossils are being formed at the present time only in lakes, in
bogs, in muddy estuaries, and in a few other exceptional situa-
tions. It is only by rare chance that upland plants leave a
record. We should therefore expect to find the geological record
of plants very fragmentary.

The Pteridophytes and their near relatives, being plants of low
grounds, swamps, and bogs, were situated in the most favorable
habitats for preservation and their record is more complete,
perhaps, than that of any of the groups of land plants.

The record of plant groups below the Pteridophytes is very
scant for two evident reasons. The simple thalloid plants

Fossil Plants

497

lacked hard tissues which would resist bacterial action until
prints and casts were made. Furthermore, the rocks of the early
Paleozoic and preceding periods have been metamorphosed by
being subjected to great pressure by overlying rocks and by heat
due to crushing, faulting, and warping of the earth's crust.
Even though there had been a fossil record in them, it wauld
have been erased by the changes that have occurred during the
millions of years that have elapsed since the rocks were deposited.

Importance of fossils in tracing relationships. In spite of the
fragmentary character of the record, hundreds of species have
been found and they have been of great importance in establish-
ing the relationships between some of the phyla of plants. Con-
ditions during the Carboniferous period were such that plant
remains are very abundant in coal seams and in the shales
associated with coal deposits. The rocks of the Carboniferous
and succeeding periods have not been so greatly modified, and
they have accordingly yielded many fossils. Nevertheless,
there is as yet very little geo-
logical evidence concerning the
origin of the conifers and flow-
ering plants. Until fossils of
the ancestors of these groups
are discovered, there is no
satisfactory basis for explain-
ing their origin from the seed-
less plants, or their relation-
ships to the known plants of
earlier ages.

The fossil record. The dia-
gram on the next page shows in
a general way the occurrence
of the larger plant groups and
some of the probable relation- ^ ^ ., . . . . ,, ,

^ riG. 309. Fossil imprints of tern-like leaves

ships. The diagram shows very in a rock of the Carboniferous period.

498

General Botany

Fossil Plants 499

clearly that the origin of most of the groups is unknown. It also
shows that most of the great groups have had a very long his-
tory on the earth. Furthermore, there is evidence of progressive
changes in structure as we follow the plants in any one group from
the earliest records to the present time. One of the large plant
groups of the Carboniferous, the cordaites, became extinct.
This also has been the fate of many smaller groups not shown on
the diagram. These groups disappeared because their structures
were not suited to the changed environments of later times.

The dominant animal groups of the several periods are in-
dicated and also the time roughly estimated to have been neces-
sary for the deposition and consolidation of the rocks belonging
to each period.

Later Paleozoic forests. During the latter half of the Paleo-
zoic there were five great groups of plants that dominated the
vegetation. The lycopods were represented by large tree forms
with stems that showed secondary thickening, with scale, or
lance-shaped leaves, and spores produced on large sporophylls
arranged in cones. The ancestral forms of the equisetums are
the calamites (Calamariales), with tall, straight, hollow-jointed
stems with whorls of branches bearing slender simple or forked
leaves, and cones. While abundant in the coal measures, they
contributed little material to the coal itself. The calamites seem"
to have attained in some instances a height of 90 feet, but most
of the forms were smaller and with the ferns formed a conspic-
uous part of the vegetation of open places in the lycopod forests.

The seed-ferns {Pteridos pernio phyta or Cycadofilicales) include
several families of plants with fern-like leaves and stems but
which produced a simple type of seed. Many of the leaf im-
prints of the coal measures that were formerly classed as ferns
belong to this group. From the early Paleozoic to Permian
there is a gradual increase in the complexity of the stem structure,
in the direction of the cycads. This group became extinct about
the close of the Paleozoic.

500 General Botany

A fifth group of plants that contributed to the forests of the
Carboniferous is the family of cordaites (Cordaitales). These
were much-branched trees, sometimes a hundred feet in height,
with dense foliage of parallel-veined narrow, simple leaves.
They had cones of two types in which the small and large spores
were developed, much as they are formed in the cycads. In
the large cones nut-like seeds were produced. The wood of
cordaites bears a striking resemblance to the wood of some of
the living conifers.

In the vegetation of the later Paleozoic, then, there were forms
that combined in various ways the special characteristics of the
ferns, the lycopods, the equisetums, and such seed plants as the
cycads and conifers. The development of complex vascular
systems, of stems with wood and cambiums, and the develop-
ment of seeds were the great advances made during the Paleo-
zoic.

The Paleozoic closed with an uplift of the continents, and con-
sequent increase of land areas, and increased drought. There
is also evidence of glaciation during the Permian. These were
doubtless important factors in the extinction of many forms of
Paleozoic plants.

The vegetation of the Mesozoic. The Mesozoic era was
marked by the extinction of the cordaites and seed ferns and the
reduction of the lycopods and equisetums to herbaceous rem-
nants that are of slight importance in the vegetation. The
ferns continued their existence as forest undergrowth, but were
early forced to compete with a new group of seed plants, the
" fossil cycads " (Bennettitales). These plants had compound,
fern-like leaves and usually short, thick, woody trunks like those
of modern cycads. In most forms the reproductive structures
consist of a whorl of microsporophylls surrounding a central
cone-shaped body bearing the megasporangia. In the extreme
forms the inflorescence is highly suggestive of certain angiosperm
flowers. The forests of the Mesozoic were dominated by the

Fossil Plants

501

Fig. 311. Map showing the glaciation of the Wisconsin epoch, and the probable distri-
bution of the forests when the ice extended farthest south.

ancestors of our modern conifers. The Cretaceous period is
second only to the Carboniferous as a coal-making period.

The great event of the later Mesozoic era was the appearance

^02 General Botany

of many types of angiosperms. The rocks of the Upper Creta-
ceous contain an abundance of fossils of broad-leafed plants such
as oak, willow, beech, maple, tulip, sassafras, and palm. The
sudden appearance of so great a diversity of forms shows that
as a group they must have diverged from the other fossil groups
a long time previously. Thus far very few of the pre-Cretaceous
ancestors of the angiosperms have been discovered.

The Tertiary vegetation. During the Tertiary the forests were
dominated by angiosperms and conifers, much like the forests
of today. It was during the Tertiary that there came a gradual
lowering of temperature on the earth and the differentiation of
distinct torrid, temperate, and frigid zones, replacing the pre-
vious uniformly mild temperatures of the Cretaceous. This
lowering of temperatures culminated in the Glacial period, which
closed the Tertiary.

REFERENCE

Seward, A. C. Fossil Plants (4 vols.). G. P. Putnam's Sons, New York; 1917.

CHAPTER FORTY-SIX

GYMNOSPERMS: THE CYCADS

The term gymnosperm (naked seed) is applied to those plants
whose seeds are attached to a sporophyll, but are not inclosed
in an ovulary. Attention, was called in an earlier chapter (page
492) to the fact that a Selaginella may occasionally form a struc-
ture which could not be excluded from any definition of a seed.
Furthermore, it is definitely known that some of the ancestral
lycopods attained the seed habit in Paleozoic times. The
cordaites formed a second group of Paleozoic seed plants. At
the same time there were plants, Pteridosperms, so closely re-
sembling ferns that, until the rather recent discovery of seeds
attached to their leaves, they were classified as ferns. From
the seed ferns came the Mesozoic cycads (Bennettitales), having
thick tuberous stems with a crown of foliage leaves superficially
like some of the modern cycads. All these forms were gymno-
sperms, and attention is directed to them again merely to em-
phasize the fact that the record of the transition from Pterido-
phytes to gymnosperms is remarkably complete, and that seeds
arose in several quite independent phyla of plants.

The cycads. Of the living gymnosperms the most primitive
are the cycads. This interesting group of seed plants with fern-
like leaves and stems and many other characteristics reminiscent
of their fern-like ancestors is practically confined to tropical and
subtropical regions. There are about 100 species belonging to
nine genera, of which five occur only in the eastern hemisphere
and four only in the western. Specimens of several species
are common in conservatories, among them the " sago palm "
(Cycas revoluta). The graceful, rigid leaves of this species are
frequently seen in floral decorations and on Palm Sunday.

The cycad sporophyte. The cycad stem is either an under-
ground erect tuberous body, or a columnar trunk 5 to 60 feet in
height. The columnar stems are covered with an armor of old
leaf bases like that of certain tree ferns and of the fossil cycads.

503

504

General Botany

C.J. Chamberlain

Fig. 312. South African cycads (Encephalartos), showing characteristic leaning trunks.
These specimens are probably 500 years old.

The root system consists of a long tap root and usually some
basal adventitious roots. This root system is a distinct advance
over that of the ferns. The stem is surmounted by a crown of
leaves that is renewed by the growth of the terminal bud at
intervals of from i to 3 years. The pinnate leaves, like those
of the ferns, uncoil during their development.

Cycads produce the two kinds of spores on separate plants.
The sporangia are borne on spirally arranged sporophylls that
are aggregated into cones. The microspores or, as we are accus-
tomed to call them in the seed plants, pollen grains are produced
in large numbers, in sporangia scattered over the under surface
of the microsporophylls or stamens. We may therefore call
this aggregate of microsporophylls the staminate cone.

The ovulate cones consist of aggregates of megasporophylls,
each of which bears from two to eight megasporangia (ovules)
on its lower margins. In the more primitive species {Cycas)

Gymnosperms : The Cycads

505

the ovulate sporophylls are divided, resembhng greatly reduced
leaves ; in the most specialized genus (Zamia) the sporophylls
are scale-like. The ovulate cone thus varies from loosely aggre-
gated, leaf-like sporophylls each with several ovules, to tight
cones of scale leaves each bearing two ovules.

The gametophyte generation. The mature microspore, or
pollen grain, consists of three cells, one of which forms the
sperms. The pollen is carried by the wind, and by chance
some reaches the open end and pollen chamber of the ovule.
There the pollen germinates, developing pollen tubes that grow
into the nucellus and absorb food from the adjacent cells
(Fig. 3 14) . After several months the pollen chamber has been en-
larged by the breaking down of the cells of the nucellus, and the

C. J. Chamberlain

Fig. 313. Large cycads (Diodn) in southern Mexico, showing staminate cones on plants
at left and carpellate cone at right.

5o6

General Botany

pollen tubes have enlarged downward into this cavity. Finally
two free-swimming sperms are liberated from each tube into the

C J . Chamherlait

Fig. 314. Fertilization in a cycad {Diodn), showing pollen tubes in the nucellus ;
the spirally ciliated sperms ; and two archegonia, one (right) with egg, the other
(left) after union of sperm and egg nucleus. Surrounding the archegonia is
the female prothallus.

pollen chamber. The sperms are relatively very large (.2 to
.3 mm.) and are propelled by a much-coiled spiral line of cilia
(Fig. 314). The pollen chamber at this time contains the liquid
discharged from the pollen tubes, and in this liquid the sperms
swim about.

Gymnosperms : The Cycads 507

In the cycads, then, the male gametophyte is a pollen tube.
As in the flowering plants, it is parasitic. It is unique, however,
in its manner of growth and in producing motile sperms — a
habit that has been carried along through all the Bryophytes
and Pteridophytes from the swimming sperms of the algae.

The female gametophyte. A single megaspore is formed
within each ovule or megasporangium. The megaspore germi-
nates inside the ovule, and utilizing the food in the inner soft
tissue (nucellus) of the ovule ultimately fills most of the space
inside the hard wall of the ovule. This is the female gameto-
phyte. Like the female gametophyte of Selaginella, it develops
within the megasporangium, but in this case the gametophyte is
entirely shut away from the light. Like the pollen tube, it is
wholly parasitic.

The female gametophyte at maturity organizes several arche-
gonia that consist merely of two small neck cells and the very
large egg cell. The neck cells open into the pollen chamber at
the time of the liberation of the sperms from the pollen tube.

Fertilization. A sperm moves down between the neck cells
and enters the egg. The sperm nucleus slips out of its covering
of cytoplasm and cilia and unites with the egg nucleus. The
fertilized egg is the first cell of the new sporophyte generation.
It soon begins to divide and ultimately forms the embryo. The
embryo pushes back into the gametophyte tissue until it occupies
the whole longitudinal axis of the seed. The embryo has two
cotyledons and develops the growing points of the stem and root.

The seed. The seed at maturity has an outer soft fleshy
layer surrounding a stony layer. Within these seed coats is a
membranous tissue, the remains of the nucellus and inner fleshy
wall of the sporangium. Next inside is food-containing tissue
(usually termed endosperm), the remnant of the female ga-
metophyte. The gametophyte incloses the embryo or young
sporophyte.

REFERENCE

Chamberlain, C. J. The Living Cycads. University of Chicago Press, Chicago

CHAPTER FORTY-SEVEN

GYMNOSPERMS : THE CONIFERS

The most important of living gymnosperms are the conifers.
They comprise about 350 species generally distributed from the
subtropics to the polar limits of tree growth. In North America
and Eurasia, pine, spruce, fir, hemlock, cypress, larch, juniper,
cedar, and sequoia cover the larger part of the forested areas.
In the southern hemisphere the araucarians and podocarps also
form extensive conifer forests.

Among the conifers are the largest and oldest of living plants.
They have a deep and wide-spreading root system, an efficient
water- and food-conductive system, much-branched stems, and a
larger leaf display than the Pteridophytes and cycads. Con-
sequently they grow far more rapidly, and are less restricted to
particular habitats. Many of the conifers are traversed through-
out by resin ducts.

The conifers have scale-covered buds, and are able to with-
stand droughts and the low temperatures of winter. With the
exception of the larch and bald cypress, the leaves remain on the
trees from 3 to 10 years. Because of the strong terminal
buds, they usually form a large excurrent trunk with many small
horizontal branches, and the trees become conical in form. Some
species, however, after attaining their height growth, become
ovoid through the lengthening of their upper lateral branches.

5. Forest Service

Fig. 315. Long branch of Western larch {Larix occidentalis) , showing lateral dwarf
branches with clusters of leaves and mature cones.

508

Gymnosperms : 1 he Conifers

509

There are two distinct types of branches : those which increase
from year to year, the long branches, and those which grow only a

s:.

■^..

^-^N

U.S. Forest Service

Fig. 316. Branch of Douglas fir, showing ovulate cone and seeds.

fraction of an inch, the dwarf branches. From the latter the
foliage leaves develop. A young growing stem of the pine is at
first clothed with spirally arranged brown scales. These fall
off as the stem elongates, and from the axil of each scale a spur
branch develops crowned by one or several needle leaves.

The leaves of most conifers are needle-shaped, but in some
families the leaves are reduced to scales and in others they are
quite broad. The life history of the pine will be used to exemplify
that of the group.

5IO General Botany

The pine sporophyte. The vegetative sporophyte consists of
the root system, the stem and its branches, and the scales and
needle leaves. The stem, in cross-section, is quite similar to
that of a woody dicot. It differs chiefly in the absence of true
vessels or tracheae.

Two kinds of cones, the staminate and ovulate, are produced on
the same tree. In them are formed the microspores and mega-
spores, within which the male and female gametophytes develop.

The staminate cone. A cluster of staminate cones develops
in the spring at the base of the new stem segment. These cones
are small, short-lived structures, falling from the tree as soon as
the pollen is shed. Each cone is made up of yellow, membra-
nous sporophylls, each bearing two microsporangia (pollen sacs)
on its lower face. The pollen, or microspore, at first consists of
a single cell, but before it is shed cell division occurs and the
mature pollen grain consists of four cells. Two of these cells
soon degenerate. The third cell is called the generative cell;
and the fourth, which occupies most of the pollen grain, is called
the tube cell. The outer wall of the pollen grain also enlarges
and separates from the inner wall, forming on either side of the
living cells two miniature balloons which help support the
grain in the air. The four cells within the pollen represent the
remnants of a male gametophyte.

In late spring the pollen sacs break open and the pollen is
blown about by the wind. The amount of pollen produced by
a pine forest is enormous, and when scattered may give the soil
and all near-by objects a yellow tinge as though powdered sulfur
had been sprinkled about.

The ovulate cone. The megasporophylls are at first small,
green, fleshy scales, but ultimately they enlarge and become
woody. They develop on small lateral branches near the upper
end of the year's growth segment. There are usually two or
three of these ovulate cones formed near each other.

Each sporophyll has two megasporangia or ovules on its upper

Gymnosperms : The Conifers

511

Fig. 317. Shoot of Pitius densiflora, with one-, two-, and three-year carpellate cones and a
few of the staminate cones.

surface (Fig. 321). The ovule consists of an outer integument
(sporangium wall), inclosing an oval body of tissue, the nucellus.
At the inner end there is an opening in the integument, the mi-

512

General Botany

^^aa^^^^^Si*-'***"

^^fe^

fm f^ '

.«&; F^?^^^^H^^^^H

M
'/,/ /

•j

Figs. 318 and 319. At the left, staminate cones of Pinus rigida clustered about the
bases of the new shoots. At the right, one-, two-, and three-year-old cones of Pinus pungens.

cropyle. Within the nucellus four megaspores are formed.
Three of these degenerate as the fourth enlarges. The production
of the megaspore ends the sporophyte generation.

The megaspore germinates the following spring and forms
within the nucellus a mass of tissue, the female gametophyte,
at the expense of the food contained in the surrounding cells.
The female gametophyte grows during the spring and by June
has organized several archegonia just beneath the micropyle.
Each archegonium consists of a large egg cell and several very
small neck cells.

Pollination. At the time the pollen is shed the axis of the
ovulate cone elongates, separating the sporophylls. Some of the
pollen grains drift in between the sporophylls and become
lodged near the micropyle. These are filled with a sticky fluid
at the time, and the pollen grains adhere to it. As the fluid sub-
sequently dries, the pollen grains are drawn within the micropyle.

Pollination occurs in May or June, about the same time that
the megaspore is being organized within the nucellus. The

Gymnosperms : The Conifers

513

pollen grains, now in contact with the nucellus, begin developing
pollen tubes into the nucellus. Elongation is very slow, and it
is not until the following June, or early July, that the tubes
pass entirely through the nucellus. Meanwhile two sperm
nuclei have been formed from the generative cell. This occurs
about the time that the archegonia are formed by the female
gametophyte.

Fertilization and growth of the embryo. When a pollen tube
passes between the neck cells and reaches the egg, the sperm

Fig. 320. Spray of Austrian pine. At the left (above) is a one-year-old
ovulate cone and (below) a two-year-old ovulate cone. On the right is a cluster
of staminate cones.

514

General Botany

nuclei are discharged into the egg. One of the nuclei unites
with the egg nucleus; the other breaks down and disappears.

Fig. 32 j. Sketch of a vertical section of a pine ovule and the scale to which
it is attached, showing male and female gametophytes at the time of fertilization :
pr is the prothallus with two archegonia ; in is the integument ; nu is the nucellus ;
m is the micropyle ; and ^ is a pollen tube, two of which have reached the neck
cells of the archegonia. (Redrawn from Strashurger.)

The fertilized egg is the beginning of a new sporophyte genera-
tion, and its growth and development take place within and at
the expense of the food accumulated in the female gametophyte.
Two months after fertilization the young sporophyte, or embryo,
is fully formed and occupies the axis of the ovule.

The seed. During the two years following pollination the
whole ovulate cone has been enlarging, and the ovules have
greatly increased in size. The integument has hardened into
a seed coat and the nucellus has been reduced to a membranous
layer inside it. Food has accumulated within the remaining
portion of the female gametophyte, usually called the endosperm.
The embryo has several cotyledons and a stem with growing
points at either end. The growing point which will ultimately
form the root is inclosed in a long sheath.

In late autumn, or winter, the ovulate cone dies and its
tissues dry out; the sporophylls curl outward and the seeds
are liberated. As the seed separates from the sporophyll, a

Gymnosperms : The Conifers

515

thin blade of sporophyll tissue goes with it, formmg the wing
of the seed.

Summary of the conifers. In comparison with the Pterido-
phytes the conifers show a greatly enlarged root system, more
massive trunk, with numerous branches, and an internal struc-
ture approaching that of the dicots. The leaves have the form
of needles or scales which are sometimes broad, and they are
peculiar in being produced, not on the elongating branches, but
on small dwarf branches. The rate of growth of the conifers
far exceeds that of the cycads and ferns. The conifers likewise
are able to grow in almost all land habitats.

The conifers show another step in the simplification of the
gametophyte generation. The cycad male gametophyte retains
the habit of producing swimming sperms. In the conifers this
habit is gone and the male nuclei pass directly from the pollen

Fig. 322. The formation of the embryo of a pine from the fertiUzed egg (A) to the de-
velopment of four rudimentary embryos. Only one of the embryos will survive in the
mature seed. (After Buchholz.)

5i6

General Botany

tube to the egg. The female gametophyte retains its habit of
producing a prothallus and archegonia, though the latter are
greatly simplified and the prothallus is strictly a parasite.

REFERENCE

Coulter, J. M., and Chamberlain, C. J. Morphology of Gymnos perms. D. Ap-
pleton & Co., New York.

>^'

pollen sac Of

or ovule

t:%

^^%

SEED

Fig. 323. Diagram ot life cycle of a conifer.

CHAPTER FORTY-EIGHT

THE ANGIOSPERMS OR FLOWERING PLANTS

The flowering plants, or angiosperms, have the shortest geo-
logical history of all the vascular plants, though they include the
largest number of species and the greatest diversity of vegetative
forms and reproductive structures. Since Cretaceous times they
have been gradually replacing the gymnosperms, until today
they are the dominating plants of the earth.

There are at least 140,000 angiosperms, or about 40,000 more
than of all other known species of plants combined. They include
herbs, shrubs, vines, and trees, and they vary in size from the
duckweed, Wolffia (half the size of a pin head), to the Australian
Eucalyptus, 340 feet high. To this variety of forms is added
structural, physiological, and chemical diversity that enables
the angiosperms to live in almost every habitat on the earth.
Consequently we find the angiosperms in control of most tropical
forests, the temperate deciduous forests, the prairies, the plains,
and the deserts, and forming the undergrowth of nearly all
conifer forests.

With the angiosperms came not only the habit of forming seeds
in closed ovularies, but also the production of variously colored
bracts and floral leaves surrounding the spore-bearing stamens
and carpels. The primitive angiosperm .flowers, represented
today by the magnohas and the tulip tree (" yellow poplar "),
had the sporophylls spirally arranged like the scales on the
cones of conifers. Another primitive type, represented by wil-
lows, have the very simple flowers spirally arranged in spikes
and catkins.

Among the floral organs nectaries appeared, usually near the
base of the sporophylls. The Paleozoic landscapes may have
rivaled our own, with their varieci textures and shades of green,
but they were devoid of that color interest which flowers added
to the Tertiary and more recent landscapes.

517

5i8

General Botany

Insects and flowers. The increasing importance of the angio-
sperms among plants of the Tertiary was paralleled among

U.S. Forest Service

Fig. 324. Black willow {Salix nigra): C, vegetative branch; A, branch with spike of
pistillate flowers ; B, spike of staminate flowers.

animals by the increasing number and diversity of insects.
This is of botanical interest because flowers and insects seem to
have reacted on each other ; and there have come to be numbers
of insects that are dependent upon certain plants, and likewise
many plants whose pollination is effected only by certain insects.
Like the gymnosperms, the early angiosperms seem to have been
mostly wind pollinated, while the later and more specialized
angiosperms are largely pollinated by insects.

The life history. The details of the life history of the angio-
sperms are somewhat variable in different orders and families.
We have already given a general account of floral structures,
pollination, fertflization, and seeds in Chapters XXV to XXVIII
(pages 232-271), and the first half of the book is concerned with
the vegetative structures and processes of the angiosperms. It
may be well, however, to repeat the life history in the same terms
that we have used to describe those of the preceding plant

The Angiosperms or Flowering Plants 519

groups, in order that the student may see that the sequence of
events in the life cycle of an angiosperm is very similar to that
of a fern, a cycad, or a conifer.

The sporophyte. In addition to the root, stem, scale leaves,
foliage leaves, and sporophylls displayed by the gymnosperms,
the angiosperms have bracts and floral leaves.

The bracts are small leaves that occur at the base of flowers
and flower clusters. Usually they are green, but in the flowering
dogwood they are white or pink and in poinsettia are scarlet.

The floral leaves form a perianth about the microsporophylls
(stamens) and the megasporophylls (carpels). In the simplest
flowers they are green; in more advanced flower types the
inner cycle forming the corolla is white, or variously colored.

Fig. 325. Flowers of Magnolia conspicua, showing cone-like arrangement of stamens and
carpels. The magnolia is a representative of the order Ranales, believed to be the most
primitive group of the angiosperms.

520 General Botany

The members of the outer cycle, or calyx, are usually green, but
in the lily family they are often similar to the corolla in form and
color.

The microspores are produced in four sporangia which make
up the anther. At maturity the sporangia open in pairs so that
they appear as two pollen sacs. The young pollen grain, or
microspore, consists of a single cell ; when shed, the microspore
has divided internally and the mature pollen contains two or
three cells.

The megaspores arise within megasporangia, which are usually
called ovules. These are variously arranged inside the carpels
(megasporophylls). There may be one or many hundred ovules
within each ovulary. Each ovule consists of a nucellus inclosed
by two integuments, except for one small opening, the micropyle.
Within the nucellus four megaspores form, but only one of them
matures.

The female gametophyte. The megaspore germinates, or con-
tinues to enlarge within the nucellus, and ultimately forms
the embryo sac or female gametophyte. This consists of seven
cells, one of which is the egg nucleus, and near the center of the
embryo sac is the fusion nucleus. The female gametophyte,
then, has been greatly simplified in comparison with that of
gymnosperms.

The male gametophyte. Previous to the time of pollination
the microspore has divided internally, forming two nuclei — one,
the tube nucleus ; the other, the generative nucleus. The latter
may have divided a second time, forming the two sperms. The
male gametophyte is, therefore, reduced to its simplest form.

Fertilization. After the pollen has reached the stigma and
germinated, the pollen tube penetrates the tissues of the style
and enters the nucellus of the ovule through the micropyle. As
the pollen tube elongates, the tube nucleus maintains a position
near the tip and the two sperms follow just behind it.

When the pollen tube reaches the embryo sac, it discharges

The Angiosperms or Flowering Plants

521

the sperms into it. One sperm unites with the egg, forming the
zygote or fertilized egg ; the other unites with the fusion nucleus,
forming the endosperm nucleus.

(polUn grain

^^ J^gasjjore-

J^ JLC ^^^-^ OvuXe wall

^EED

Fig. 326. The life history of an angiosperm.

The embryo. The zygote germinates at once and by cell
division forms the embryo, which pushes backward into the
developing endosperm. The embryo may consist of only a few
cells, as in orchids, or it may grow to considerable size, using
up the entire contents of the endosperm, the nucellus, and in
rare instances, as in the skunk cabbage, even the integuments.

522 General Botany

The embryo consists of a hypocotyl, a plumule, and on-e
(monocots) or two (dicots) cotyledons. This is the new sporo-
phyte generation. Since the sperm and the egg each contain n
chromosomes, the cells of the sporophyte contain the double
number (2 n) of chromosomes.

The xeniophyte. The germination and growth of the endo-
sperm nucleus leads to the formation of a tissue surrounding the
embryo, called the endosperm. In some seeds (e.g., castor bean
and corn), this tissue persists and becomes very large, and
accumulates starch and other nutritive materials. In other
seeds (e.g., common bean) the endosperm is a temporary tissue,
that is consumed by the growing embryo.

The endosperm of the angiosperms is formed by successive
divisions of a nucleus that resulted from the union of three
nuclei. Its cells, therefore, contain three sets (3 n) of chromo-
somes. To distinguish this kind of an endosperm from the very
different one of the gymnosperms (female gametophy te) , it is
sometimes called the xeniophyte.

The seed. In the seed of an angiosperm, therefore, there are
(or for a time were) three distinct generations represented :
(i) the integuments, a part of the first sporophyte producing the
seeds; (2) the endosperm, or xeniophyte; and (3) the embryo,
or second sporophyte.

The seed coats. Strictly speaking, the seed coats are the
mature integuments of the ovule. In many plants having
simple pistils that contain a single ovule, the seed coat at maturity
includes the pistil wall. This is the case in grasses and in the
buttercup, rose, carrot, and sunflower families. Such a seed is
termed an akene, or, in the grasses, a grain.

Divisions of the angiosperms. As noted earlier in the book,
the angiosperms consist of two great classes of plants, the dicoty-
ledons and the monocotyledons. The former seem to be the
older and to have given rise to the monocots in comparatively
recent geological time.

The Angiosperms or Flowering Plants 523

Distinguishing features of the dicotyledons. The embryo has
two cotyledons, except in certain parasites and saprophytes.
The stems are either herbaceous or woody, with open vascular
bundles arranged in a circle, and usually with a cambium inter-
secting the bundles between xylem and phloem tissues. The
stems of most dicots are profusely branched. The primary root
is retained until maturity, and not infrequently develops into a
long tap root. The leaves are usually net-veined, simple or com-
pound, entire or variously lobed and divided. The flowers are
usually made up of five cycles of floral parts : the sepals, petals,
two whorls of stamens, and the carpels. The number of members
in each cycle varies from two to six, but is usually five or four.
In some of the simplest flowers the corolla is wanting, and in
others there is but a single whorl of stamens or pistils. In some
families the number of stamens and pistils is very large.

Distinguishing features of the monocotyledons. The mono-
cots are herbs and woody plants, with closed and scattered bun-
dles. The embryo has usually only one cotyledon, sometimes
one large one and a very small one. A cambium is usually
absent, but when secondary cambiums are present they arise
outside the vascular bundles. The stems of monocots are not
as highly branched as those of the dicots. The primary root is
usually short-lived, and is replaced by adventitious roots, which
are in turn succeeded by new adventitious roots that develop
from points higher and higher up the stem.

The leaves are devoid of stipules and commonly lack a petiole,
though they frequently have a sheathing leaf base. The flowers
consist of five cycles of floral organs, as in the dicots, but the
second whorl of stamens is not infrequently wanting. The
number of members in each cycle is usually three, but in the
grasses this may be reduced to two.

REFERENCE

Coulter, J. M., and Chamberlain, C. J. Morphology of the Angiosperms.
D. Appleton & Co., New York ; 1903.

CHAPTER FORTY-NINE

SOME FAMILIES OF ANGIOSPERMS

Certain families of the angiosperms are of special interest
because of their economic importance, their common occurrence,
or their peculiarities. The following families have been selected
from among several hundred to exemplify the variety of plants
included among the angiosperms, and to show some further
characteristics of the monocots and dicots.

MONOCOTYLEDONS

The grass family (Gramineae) . The grass family includes the
most important food plants in the world. Wheat, corn, rice,
barley, rye, oats, and sugar cane furnish the bulk of human food.
Corn, wheat, oats, and a variety of wild and cultivated grasses
furnish most of the food of grazing animals.

Fig. 327. Flower cluster and spikelet of orchard grass {Dactylis), showing
glumes, pistils, and stamens.

524

Some Families of Angiosperms

525

Fig. 328. Bamboo thicket on an island in the Caribbean. The bamboo is the largest of
the grass family and one of the most useful plants in the tropics and subtropics.

526

General Botany

There are 3 50 genera and about 5000 species of grasses. Mostly
they are mesophytes, but there are grasses in nearly every habi-
tat where plants grow. In size they vary from grasses of 2 or
3 inches high to the woody bamboos 60 to 100 feet in height
and a foot in diameter. The larger grasses are an important
source of building materials, fiber, paper pulp, and in the Oriental
tropics of innumerable household articles.

The spikelet is the unit flower cluster of the grasses. Spikelets
may be variously arranged on a single axis, or on highly branched
axes, forming spikes, racemes, or panicles. Each spikelet consists
of two or more flowers inclosed by bracts, called empty glumes.
Each flower is also enveloped by two flowering glumes. Next
above are two (rarely three) very small bracts called lodicules,
which represent the remnants of a perianth. Next above are
three stamens ; and in the center of the flower is the pistfl, made
up of three carpels forming a single ovulary with one ovule.
It is very evident that the grass flower and inflorescence are
highly specialized structures.

Bureau of Agriculture, P. I.
Fig. 329. Japanese cane, a near relative of the sugar cane. It is grown as a fodder crop
in the Philippines, and is one of the many useful members of the grass family.

Some Families of Angiosperms

527

Grass stems are usually round, and the internodes are hollow.
The leaves are arranged alternately in two ranks.

Fig. 330. Panicle and flower of bent grass (Agrosiis), the latter showing two glumes
and the much shorter lemma adjoining the pistil and stamens.

Closely related to the grasses are the sedges {Cyperacece), with
triangular, solid, unjointed stems, and leaves in three ranks. The
sedges are common in swamps, marshes, and low grounds. The
papyrus, a sedge of the Nile Valley, was one of the first plants
used in paper making. In fact, our word " paper " comes from
the name of this sedge.

The aroids {Araceae). The calla lily, jack-in-the-pulpit,
skunk cabbage, and caladiums are examples of a large family of
tropical and temperate herbs, embracing not less than 1000 species.
The flower cluster consists of a spike (spadix) of very simple
flowers, inclosed by a large, sometimes highly colored bract (spat he) .
Many of these flowers are noted for their disagreeable odors.

The rootstocks of some species of caladium, colocasia, and arum
accumulate starch and are a source of food in the tropics.

528

General Botany

Fig. 331. A tropical climbing aroid {Monstera) in bloom. Note the white spathe which
incloses the spike (spadix) of flowers. Behind the flower can be seen a ripe fruit. Most
members of this family are tropical, but jack-in-the-pulpit and skunk cabbage are familiar
representatives.

The palm family (Palmaceae). The most familiar mark of a
tropical landscape is the presence of palms, with their tall un-
branched stems, topped with a rosette of large divided leaves.
Some palms attain a height of 150 feet, others have only a short,
upright rootstock, while a few, like the rattans, are climbing vines
several hundred feet in length. They are for the most part
intolerant of shade, and consequently occur along streams, in
clearings, on forest borders, and in oases in deserts. In addition
to their edible fruits — dates, coconuts, palm nuts — they are im-
portant sources of fibers, oils, wax, starch, sugar, and alcohol.
The leaves are used in thatching, in basket making, and in weav-
ing mats and hats. Vegetable ivory is derived from a palm nut.
The woody stems do not yield plank timber, because of the
scattered bundles. The palms include 130 genera and about 1200
species.

Some Families of Angiosperms

529

W. S. Cooper

Fig. 332. A yucca {Yucca whipplci) in bloom at Cajon Pass, California.

^3o General Botany

The lily family {Liliaceae). Tulips, hyacinths, lilies, lily-of-
the-valley, and trillium are familiar plants of this family.
Here also belong the edible onions, leeks, garlic, and asparagus.
The yuccas and aloes are the desert representatives of the family.
Some of these plants form woody stems and attain the size of trees.

The flowers are characterized by a perianth of six white or
colored parts surrounding six stamens, and a three-carpel pistil
containing numerous ovules. Two hundred genera and about
2700 species have been described.

The amaryllis family (Amaryllidaceae). Closely related to the
lily family, and likewise noted for its large number of ornamental

Fig. 3oo- Adder's tongue {Erylhronium amcricanum), an early
spring member of the lily family.

Some Families of Angiosperms

531

Fig. 334. Flowers of amaryllis.

plants, is the amaryllis family. They are characteristic of
dry climates and many have leaves only during the rains.

Here belong the amaryllis, narcissus, and tuberose. In the
American desert the agaves are widely distributed, and some of
the species are important sources of fibers for binder twine and
other coarse cordage. Seventy-five genera and 700 species are
known, mostly from the tropics and sub tropics.

The pineapple family {Bromeliaceae) . Found only in the
American tropics is the pineapple family, with 40 genera and 1000
species. Some of the species, like the pineapple, are terrestrial
plants, but most are epiphytes and form a characteristic feature
of the jungle growths from southern Florida and Mexico to south-
ern Brazil. Many of the epiphytes have rosettes of leaves with
the bases pressed tightly together, forming water pockets from
which the plants secure most of their water. The " Spanish
moss " is very different and consists of festoons of leafy branches

532

General Botany

without even holdfast roots. It is an extreme xerophyte, living
not only in the moist sub tropics but some species occurring even
in semi-desert regions.

The orchid family (Orchidaceae) . The orchids form the
culminating family of the monocots. They are all perennial
herbs, noted for their beautiful, highly specialized and diversi-
fied flowers. In temperate regions the species are mostly ter-
restrial, but in the tropics they are largely epiphytes. During
the dry season they drop their leaves, and only the thickened
stems, tubers, or fleshy roots pass the dormant period. The
fruit is a capsule containing a vast number of minute seeds. In
many of the tropical epiphytes the outer layers of the root con-
sist of dead perforated cells, which form a water-holding tissue

(the velamen). The great vari-
ety of orchids is indicated by the
occurrence of 750 genera and
7500 species.

Other monocot families. The
banana family includes many
gigantic herbs of the tropics,
noted not only for their fruit,
but in some species for the fibers
obtained from their leaf stalks
(Manila fiber).

The yam family includes
many climbing herbaceous
plants, with thick underground
tubers that in tropical countries
are eaten like potatoes. The
leaves of this family resemble
those of dicots.

To the monocots also belong
many of the families which in-

FiG. ^^S- A tropicsd orchid {Cypripedium „4-^^

caiiosum). cludc our commoucst water

Some Families of Angiosperms

533

Fig. 336. Leaves and flower of the tulip
tree {Liriodendron tidipifera). This mag-
nificent tree of the deciduous forest is
closely related to the magnolias.

plants, like pondweeds, pickerel
weed, cat-tails, rushes, eel grass,
and water hyacinths.

Without further examples it
is evident that most of the
monocot families attain their
best development in the tropical
and subtropical countries.

DICOTYLEDONS

Willow family {Salicaceae) .

Among the most widely distrib-
uted trees and shrubs of the
northern hemisphere are the wil-
low^s and poplars. Both have sim-
ple naked flowers in catkins, and the staminate flowers and car-
peUate flowers occur on different individuals. Both willows
and poplars reproduce freely from cuttings. The poplars,
especially the cottonwood, is used for paper pulp. Some species
of willow are grown for their sprouts, which are used in making
baskets and furniture.

Beech family (Fagaceae). The beech family includes the.
beeches, oaks, and chestnuts. The fruit consists of a cup-like
structure inclosing one, two, or three nuts. They are chiefly
valuable for their timber products.

Closely related to the beech family is another family of trees
and shrubs, the birch family {BetulacecE), which includes the
birches, hornbeams, hazels, and alders. All these plants have
very simple flowers in spikes or catkins, and most of them are
wind-pollinated.

Buttercup family (Ranunculace^) . This family is typical of
a large order, known as the Ranales, which includes many
common herbs, trees, and shrubs : the buttercups, water lilies,
anemones, columbine, May apple, larkspur, sassafras, tulip tree.

534

General Botany

Fig. 337. Flowers of the Japanese anemone {Anemone japonica). It belongs to tha

buttercup family.

and magnolias. The flowers are solitary and conspicuous. The
receptacle is usually elongated, and the parts of the flower
are arranged spirally about it. The calyx and corolla are not dis-
tinct in shape or color, and the sporophylls are indefinite in
number.

The flowers of this order are generally regarded as primitive,
and it has been suggested that the monocots were derived from
the ancestral forms of the order Ranales.

The mustard family (Cruciferae). The scientific name of the
family is derived from the cross-like arrangement of the four
petals. In this family the four sepals are green and quite dis-
tinct from the petals. The stamens are six in number, four
long and two short. The single ovulary is divided by a membrane
into two compartments, each of which contains a row of ovules.

To the family belong many troublesome weeds ; a variety of
edible herbs like cabbage, cauliflower, turnip, radish, caper, and
cress ; and the ornamental wallflowers and stocks.

The pitcher-plant family (Sarraceniaceae) . This small family
is one of three families belonging to the order Sarraceniales.

Some Families of Angiosperms

535

They are mentioned here
merely because of the fact that
all three families are made up
of insectivorous plants. In
the pitcher plants the insects
die by drowning in the water
contained in the pitcher-like
leaves. In the closely related
sundews, the insects are caught
by the sticky secretion from
glandular hairs on the upper
surface of the leaves. In the
Venus' flytrap the blade of the
leaf consists of two halves which
fold together. On the upper
surface of each half are three
hairs and numerous small red-
dish glands. The margins of
the blades have tooth-like pro-
jections. When the hairs are
touched the two halves of the
blade suddenly close, the mar-
ginal teeth interlock, and small
insects may be caught. In all
these plants the insects are sub-
sequently digested and the prod-
ucts absorbed by the plants.
It is rather remarkable that
three such unusual habits should
have arisen within a single order
of plants. It should be stated
that all these plants may be
grown in conservatories with-
out feeding them insects.

Fig.

of Nepenthes, one of a group of tropical
epiphytes. These pitchers contain water
and are provided with glands that secrete
enzymes and absorb the products resulting
from the digestion of insects that drown
in them.

536

General Botany

Fig. 339. A wild rose {Rosa lucida) of the Northeastern state's, that occurs on swamp
margins and rocky shores.

Fig. 340. The flowering raspberry {Rubus odoratus), a member of the rose family.

Some Families of Angiosperms

537

Rose family {Rosacex). This is a cosmopolitan family of loo
genera and more than 2000 species. It is notable because of the
large number of useful plants that are cultivated for their flow-
ers or fruits. Here belong the roses, spiraeas, cinquefoils, straw-
berries, raspberries, blackberries, pears, apples, cherries, and
plums. The receptacle of the flower is usually hollowed, so that
the five sepals and petals surround a cup in which the numer-
ous stamens and the five to many carpels are borne. In the
strawberry the fruit is the enlarged fleshy receptacle.

The legume family (Leguminosae) . This is the second
largest family of flowering plants, and in the importance of its
food products is second only to the grasses. It includes 500
genera and not less that 12,000 species, some of which grow in
every climate and habitat. Most of the plants have tubercles
on their roots and are hosts to nitrogen-fixing bacteria. Here
are included the sensitive plants (Mimosa), the acacias, red buds,
locusts, peanuts, lupines, clovers, beans, peas, and soy beans.

^i. i-orlion (,f a plaiU uf hairy vetch (Vicia). The papilionaceous llowers
identify it as a member of the legume family. The outer branches of the compound leaves
are tendrils, as in the pea.

538

General Botany

The acacias and mimosas are largely tropical and subtropical
trees and shrubs, with regular flowers. The genera common to

Fig. 342. Flowering branch of Acacia Sene-
gal, one of the many leguminous plants with
radial flowers widely distributed in the
tropics. In the southern United States
there are several common species of Acacia
and the closely related Mimosa. Acacia
Senegal is the source of gum arable. {After
Strashurger.)

Fig.

343. Peppermint {Mentha pi-

perita). The square stems and op-
posite leaves are] characteristic of
the mint family.

All

temperate regions have irregular flowers like the sweet pea
produce the pod, opening by two sutures called a legume.

The cactus family {Cactaceae). This group of succulent
desert plants seems to have originated in tropical America and
to have spread sparingly into the dry temperate regions both
north and south. There are about 25 genera and 1500 species.
For the most part they lack leaves and the stems are covered

Some Families of Angiosperms

539

Fig. 344. The cranberry {Vaccinium macrocarpon), one of the low heaths that grows
naturally in bogs. This plant was brought into cultivation many years ago. The size of
the berries has been doubled by selection of large-berried mutants.

Fig. 345. The great laurel {Rhododendron maximum), an evergreen shrub of the AUe-
ghenies. Like the mountain laurel {Kalmia) and the azalea, it belongs to the heath family.

540

General Botany

with spines. They vary from small perennial herbs to large,
much-branched, tree-like forms.

The carrot family (Umbelliferae) . The scientific name of
the family comes from the umbrella-shaped inflorescence. They
are mostly herbs with stout stems, hollow internodes, and divided
leaves. The carrot, parsnip, celery, fennel, coriander, and water
hemlock are familiar examples of the family. Some of these
plants are poisonous when eaten, and many are noted for their
peculiar flavors. They are chiefly found in the north temperate
zone and include 200 genera and 2700 species.

The heath family (Ericacex). The family is distributed
throughout the world, except in deserts and the moist tropics.
Most of the plants have simple, evergreen, entire leaves which

Fig. 346. Andromeda florihimda, one of the heaths common on moist hillsides
in the southern Alleghenies.

Some Families of Angiosperms

541

tend to be grouped at the ends of the branches. They are con-
fined to acid soils, and many are found in bogs. The azaleas,

U.S. Forest Servicz

Fig. 347. Mesquite {Prosopis julifiora), a widely dis-
tributed shrub in the Southwestern states, belonging
to the legume family. (See Figure 240.)

rhododendrons, laurels, arbutus, heather, huckleberries, blue-
berries, cranberries, and wintergreen are examples of both the
ornamental and fruit-producing members of this family.

The flowers usually have five parts in each whorl, and the
corolla differs from that of all the preceding families in showing a
tendency to have the petals united. In the succeeding famihes
this tendency culminates in the production of tube-like corollas.

The mint family (Labiatae). The corolla of the mints is tu-
bular, and frequently two-lipped, which suggested the technical
name. They are world-wide in their distribution, and they num-

General Botany

Fig. 348. Salvia, a large-flowered member of the mint family, with the characteristic
square stem and two-Upped tubular corolla and calyx.

Some Families of Angiosperms 543

ber 200 genera and 3000 species. They are mostly herbs, with
square stems and simple leaves, and with epidermal glands se-
creting volatile oils that give the
characteristic odors to many of the
species. The floral whorls, except
the pistil, each consist of five mem-
bers. The pistil is composed of
two carpels, each of which is two-

lobed, so that the fruit consists of Fig. 349. Climbing nightshade iSola-
four nutlets. The oils of pepper- ««w<^«^cawam), one of the wild species
• , • . . 1 1 1 belonging to the potato family.

mmt, spearmmt, thyme, lavender,

rosemary, and horehound are of commercial importance in the

manufacture of flavoring extracts, perfumes, and medicines.

The potato family (Solanaceae). This group of 75 genera
and 1500 tropical and temperate species is best developed in
Central and South America. It includes herbs, shrubs, and
small trees with petals united into a disk, or forming a tube with
flaring end. The fruit is a berry or a capsule. Many of the
species are poisonous. The potatoes, tomatoes, and peppers are
familiar garden species. Equally important commercially is
the tobacco plant. The " deadly nightshade " is the source of
the drugs atropine and belladonna.

The sunflower family (Compositae). This is the largest fam-
ily of flowering plants, comprising about 900 genera and more
than 13,000 species. Most of the species are herbs, though
in the tropics there are a few shrubs and trees. The flowers are
usually small, with tubular or strap-shaped corollas. The
flowers, however, are grouped in heads, with an outer circle of
green bracts so that the flower cluster is frequently mistaken
for a single flower. The fruits are achenes, and in many species
the fruits have a ring of bristles which lead to their distribu-
tion by the wind. In this, the culminating family of the dicoty-
ledons, the production of a multitude of seeds is accomplished
by the occurrence of many small flowers in heads. This is in

544

General Botany-

striking contrast to the orchids, which represent the most spe-
cialized family of the monocotyledons, in which the flowers are

Fig. 350. Flower clusters of dahlia, sunflower, and thistle, members of the
composite family. The small flowers are collected in heads, surrounded by
bracts.

few and the seeds are produced in enormous numbers in each
capsule.

The composites include the various species of chicory, dande-
lion, lettuce, ragweed, cocklebur, aster, sunflower, ironweed,
goldenrod, fieabane, everlasting, rosinweed, conefiower, Spanish
needle, chrysanthemum, and thistle. Many of these plants are
weeds, some are cultivated as ornamentals, and a few are of eco-
nomic importance as food.

REFERENCES

In addition to the well-known manuals :

Harshberger, J. W. Pastoral and Agricultural Botany. P. Blakiston's Son & Co.,
Philadelphia ; 1920.

RoBBiNS, W. W. The Botany of Crop Plants. P. Blakiston's Son & Co., Phila-
delphia; 1917-

Saunders, C. F. Useful Wild Plants of the United States and Canada. Robert M.
McBride & Co., New York; 1920.

Willis, J. C. Manual and Dictionary of the Flowering Plants and Ferns. The
Macmillan Company, New York; 1919.

CHAPTER FIFTY

EVOLUTION OF PLANTS

Those who have studied plants most have been led to the
conclusion that simple plants lived first on the earth, and that
from these simple forms all the varied and highly complex plants
of today have been derived ; that is, that the present-day plants
were evolved from simpler plants that existed on the earth in
former times. Some of the simple plants of the past still persist,
and many plants of intermediate degrees of complexity survive ;
but during the long period of geological time, new and increas-
ingly complex plant forms have been produced, and these higher
forms now dominate the vegetation of the earth. The process
by which the plants of today have come from the plants of the
past is called evolution (Latin : evolutio, an unrolling) . Evolu-
tion, with regard to plants, implies (i) that the plants of today
are the modified descendants of earlier forms, (2) that modifica-
tions are going on now as in the past, and (3) that there will be
new plants in the future, evolved from plants now living through
modification of present plant forms.

The proofs of evolution in plants have been gathered from
many sources by many different students. These proofs include
the evidence furnished (i) by plant remains found in rocks and
coal, (2) by the distribution of plants on the earth's surface,
(3) by the remarkable similarity of organs, tissues, and cells
among the thousands of plants now in existence, (4) by the
similarity in the life histories of all plants, (5) by intergrad-
ing species, (6) by the experience of plant breeders and the his-
tory of our cultivated plants, and (7) by the discovery of new
mutants from time to time.

The geological record. The earliest rocks (Precambrian)
contain few recognizable plant fossils, not because plants were
rare when these rocks were laid down, but because the rocks dur-
ing the long subsequent history of the earth were acted upon by

545

546

General Botany

water, high temperature, and the enormous pressure of overlying
later strata. The occurrence of carbon in these rocks is pre-
sumptive evidence that plant re-
mains were present when they were
originally laid down.

The known fossil record shows
that during successive periods of
the earth's history plant groups
succeeded one another and that
there was a gradual increase in the
diversity of plant forms, accom-
panied by progressive changes in
both the vegetative and repro-
ductive structures of the plants.
Modern plant structures are
clearly derived by further devel-
opment and modification of the
structures of plants of former geo-
logical periods.

That the geological history of
each of the plant phyla provides positive evidence for the
evolving of new and more complex forms from previously existing
forms is clear and unmistakable. In the phyla Cordaites and
Pteridosperms, we have the record of the evolution, the world-
wide dispersal, and the decline and extinction of two great
plant groups.

The trend of evolution. Not only does the geological record
furnish abundant proofs of evolution, but it shows the course of
the evolution of plants. The series of reproductive structures,
for example, beginning with simple sporangia on foliage leaves,
may be traced upward through the development of sporophylls
and finally to the production of flowers and seeds. The vascular
systems of plant stems show a progressive series of changes from
the primitive ferns to the modern flowering plants. The de-

FiG. 351. Fossil imprint of a leaf of
a species of sassafras in rock of the
Cretaceous period. But few fossil angio-
sperms have as yet been found in rocks
formed earlier than the Cretaceous
period.

Evolution of Plants 547

velopment of large and effective root systems may be traced in
the same way.

It is quite impossible to account for these gradual and pro-
gressive changes in plants except on the basis of evolution. When
we understand that the geological record of evolution covers
a period of an estimated length of several hundred million years,
we should not become impatient at failing to see new genera and
families of plants arising during our own brief period of observa-
tion. The time that has elapsed since critical observations upon
the evolution of living plants have been made, when compared
with the time represented by the geological record, is like one
second for the observation of the events of a year.

Plant geography. Closely related to the fossil record is the evi-
dence of evolution that is derived from the present distribution of
plant groups. Closely related species of plants are not scattered
haphazard over the earth. Many families bear evidence of having
originated on some particular continent, or part of a continent,
and of having spread from the center of origin as new species
appeared. Some species have not spread far from their point
of origin, while others have moved far from the place of their
first appearance because of characteristics which enabled them to
live in a variety of conditions. Families which, because of their
structures and the absence of a fossil record, are believed to be
very modern are usually restricted in their distribution. Ancient
families, on the other hand, often have species scattered over
several of the continents.

The cactus family, represented by about 1500 species, is native
in North and South America only. In North America the
family is best developed in Mexico, but it has spread north-
ward and eastward into the United States and to the islands of
the West Indies. The geographic distribution of all the North
American species points to a common origin in the Mexican
plateau. The yucca family and the agave family also appear to
have originated there and to have spread in a similar way to the

548 General Botany

United States and the West Indies. All these families are com-
paratively modern.

The laurel family {Lauracece) , to which the European laurel,
sassafras, cinnamon, and spice bush belong, is a very ancient
family. Its fossil record extends back to the lower Cretaceous.
Today its members are scattered widely over the earth, with
numerous species in Brazil and southeastern Asia.

Hundreds of examples of this kind might be cited, and all would
afford evidence that related plants are distributed over the earth's
surface as though they had originated in some one locality and
had then spread to other regions. Sometimes they became di-
versified chiefly at their center of origin ; sometimes as they spread
they formed secondary centers of diversification. But in all
cases the species that occur along a given line of migration are
closely related.

The geography of plants, therefore, furnishes a second line of
evidence that existing plant species have been derived from
preexisting species.

Comparative anatomy and physiology. One of the most strik-
ing proofs of evolution is the remarkable similarity of the cells,
tissues, and organs that make up plants belonging to diverse
groups. However much they may differ in superficial appearance
and in detail, they all have a common plan and organization.
The diversity has been brought about through modification in one
direction or another. Even more remarkable is the similarity
of the physiological processes underlying life, not only in all
plants, but in animals also.

Life histories. Except on the basis of evolution it would be
impossible to account for the fact that throughout the whole
plant kingdom the life histories are so strikingly similar. As we
pass from simple plants to the flowering plants, the life histories
become more and more complex. Attention has already been
called to these facts, and it is only necessary here to repeat that
the changes in life history have been made by comparatively

Evolution of Plants 549

small steps. There has been an occasional addition of a new
tissue or organ to the life cycle, or the replacement of one struc-
ture by another.

The life histories of related groups are similar in essentials and
differ only in details. This repetition of the stages in the life
cycles of the plants of diff'erent groups, when viewed along with
other facts of evolution, indicate that the plants with the more
complex life histories have evolved from those with less complex
life histories. Increase in complexity is one of the general tend-
encies of evolution. The order in which we should arrange
plants on the basis of the geological record is the same as the
order suggested by their life histories and structures.

Intergrading species. All who have attempted to classify
plants — that is, to determine the species to which individual
specimens belong — have been impressed by the intergrading
of related species. The existence of individuals intermediate
between species long ago suggested that one species may have
arisen from another. For example, the common asters, violets,
hawthorns, evening primroses, and willows are highly variable ;
and in any of these genera it is frequently impossible definitely
to classify a particular specimen and to say that it belongs to
this or that species. If forms intermediate between species
were rare, they would only suggest the possibility of evolution ;
but they are numerous, occurring in hundreds of genera through-
out the plant kingdom. These intergrades make it impossible
for us to think of the plant kingdom as being made up of distinct
and unrelated species, and so they must be regarded as evidences
of evolution.

Plant breeding and evolution. Our cultivated plants are the
modified descendants of wild species. Many of them, perhaps
most of them, were brought into cultivation by wild tribes of
men long before the dawn of written history. In many instances
the plants have been so greatly modified that it is difficult or
impossible to trace their origin to any known wild species. Thus

550 General Botany

corn was cultivated by the earliest races of men on the American
continents. When first found by the early explorers the Indians
not only had corn, but they were growing all of the subspecies
that we now distinguish as starchy, sweet, soft, waxy, flint, pod,
and pop corns. The wild species from which corn was derived
is unknown. Since the discovery of America some of these sub-
species have been greatly improved by crossing and selection.

All the modern varieties of cultivated plants which supply
our fruits, flowers, roots, tubers, and fibers have resulted from
the activities of plant breeders. They are mutants selected
either from former wild plants or from previously grown varieties.
In many plants which cross-pollinate readily, the selection was
preceded by hybridization, which often produces plants with
new combinations of desirable characters.

The experience of plant breeders furnishes abundant evidence
that plants produce mutants which differ in one or more char-
acters from their parents, and that these new characters are
heritable. These are the starting points of new varieties and
species.

Plant breeding, then, has afforded us an opportunity to see
new varieties of plants evolve from older ones. The evolution
of many cultivated plants is a matter of historic record. There
is evidence that wild plants also produce mutants, and there is
every reason to believe that they have evolved in the same way.

The fact of evolution conceded. The time has long since
passed when botanists have asked for further proofs of evolution.
Nevertheless, new evidences of evolution are appearing from day to
day in every field of botany, for the discovery of new facts about
the structure, physiology, or chemistry of plants frequently fur-
nishes important new proofs of evolution. We may say, then,
that the evolution of plants is a fact, not a theory.

The method of evolution. While evolution may be considered
a fact, the methods and causes of evolution are still problems
upon which field observations, intensive laboratory study, and

Evolution of Plants 551

extensive field experimentation are going on. The experimental
study of evolution is a comparatively new field, and the accumu-
lated data are not yet sufficiently numerous to do more than
suggest some of the factors which cause evolution to occur.
Among these are variation, heredity, and natural selection.

Variation. Mutants seem to be the chief sources of new varieties
of plants. These have been discussed in Chapter XXX. There
is another type of variation which is also important in evolution,
and that is the variations that result from hybridization. Mu-
tants result apparently from some change in the germ cells, due
to unknown causes. Hybrid variants are due to new combina-
tions of characters, derived in part from the pollen parent and
in part from the ovule parent. Among evening primroses, oaks,
and haw^thorns, hybrid variants are very common. Mutants
are known among evening primroses, sunflowers, grasses, hemp,
flax, and many other wild and cultivated species.

Heredity. The tendency of heredity is to make the offspring
like the parent. When a mutation has occurred, heredity be-
comes an important factor in evolution, since only through
heredity can the new variety be maintained. In hybrid vari-
ants heredity can maintain a new variety only when both the
pollen and ovule parents have the same constitution.

Natural selection. Most plants produce offspring by the
hundreds, thousands, or even millions, and there is room for
only a smafl part of the offsprmg to live. It is said that those
plants survive that are more vigorous, that are better adjusted
to their environments, or that happen to start in favorable places ;
the weak and the unfortunate perish. Certain variations or
mutations may fit plants the better to survive, and the persistence
of the forms showing these changes may lead to the formation
of new varieties and species. The wholesale destruction of
individual plants in nature, with the survival of a few, is called
natural selection, and it has been thought to resemble in some
respects the selection made, by the plant breeder. It is un-

552 General Botany

questionably true that most of the plants that start life in nature
die before reaching maturity ; but there are differences of opinion
as to whether or not the plants that do survive can through re-
peated selections in nature develop into new species. Man can
pick out new forms that originate among the plants that he culti-
vates and by breeding from them secure new varieties, and it is
believed by some that in nature certain advantageous mutations
are selected or preserved in a similar way.

The agencies that are supposed to do the selecting in nature
are the factors of the environment. Almost any environmental
factor may become a limiting factor for the growth of some par-
ticular variety of plant. If mutants occur which are not limited
in the same way or to the same degree, such mutants survive.
During geological time the great changes in the elevation of conti-
nents, in connections between continents and islands, in the
climate, and in the habitats available have been major factors
in determining the changes in the kinds of plants that survived.

Summary. In this final chapter an attempt is made to define
evolution, to show the sources from which the proofs of evolution
are obtained, and to distinguish between the fact of evolution
and the tentative explanations which have been offered to ac-
count for evolution. Botanists are now generally agreed (i) that
variations are the possible sources of evolution, (2) that those vari-
ations which are inherited, particularly mutants, are the only
ones which lead to new varieties and species, and (3) that from
among these mutants some survive and some perish, according as
they fit into or fail to fit into the environment.

INDEX

Abaca, 140.

Abscission, 100, 260.

Absorption, 191; and transpiration, 90.

Acacia, 538.

Accumulation, of food, 163, 166, 202; of
water, 79, 162.

Acidity of soils, 218, 220.

Acre, food products from an, 46.

Aerial roots, no, 187.

Agave, 80, 169, 369, 529.

Agriculture, 147.

Akene, 253, 522.

Alcohol, 410.

Alder, 237.

Aleurone, 59.

Alfalfa, 359, 397-

Alga:, 441; blue-green, 441; brown, 457;
classification of, 441 ; green, 445 ; impor-
tance of, 455, 4(52, 464; periodicity of,
456; red, 463; reproduction in, 453.

Alkali, 221.

Alkaloids, 58.

Alternation of generations, 486.

Amanita, 420.

Amaryllis, 115; family, 530.

Ammophila, in.

Amylase, 164.

Angiosperms, 259, 517; divisions of, 522;
families of, 524; life history, 518, 521,

Animals, 222.

Annual rings, 137, 150, 152.

Annuals, 143; as weeds, 376.

Anther, 241.

Antitoxins, 391.

Apple, rust of, 419; seedlings of, 207.

Aroids, 527.

Ascomycetes, 409.

x\ssimilation, 61.

Associations, plant, 305.

Autophyte, 378.

Autumn coloration, 54.

Avocado, 257.

Axil, 116.

BaciUariacece, 444.

Bacteria, 384; and disease, 391;

legumes, 397 ; and sanitation, 390 ;

soils, 393, 396; and nitrogen cycle,

control of, 394.
Bamboo, 139, 151; thicket, 525.
Banana, 140, 229.
Basidiomycetes, 414.

and
and
396;

velvet, 275.

basis of, 277.
economic importance of,

Bast, 136.

Bean, flower of, 251 ;
Beech family, 533.
Beets, sugar, 351.
Bennettitales, 500.
Berry, 253.
Biennials, 144.
Blackberry, 254.
Black knot, 433.
Bloom, 76.
Blue-green algae, 441.
Blue mold, 411.
Boston ivy, 108, no.
Box elder, 207.
Bracts, 236.
Bread-making, 410.
Breeding, plant, 272;
Bromeliacece, 531 •
Brown algae, 457

462.
Brown rot, 435.
Bryophyllum, 225.
Bryophytes, 465.
Budding, 157, 228.
Bud mutations, 285.
Buds, 116; and plant form, 119.
Bulbs, 114.
Bundles, 31; dicot, 131; leaf, 31

139; root, 183; stem, 131.
Burdock, 106.
Buttercup family, 533.

Cabbage, 281.

Cactus, 169, 171; family, 538, 547.

Calcium, 216, 217.

Calcium oxalate, cr>'stals of, 59, 139, 217.

Callus, 228.

Calorie, 46.

Calyx, 238.

Cambium, 134, 137; and grafting, 15S;

secondary, 141, 150.
Cantharellus. 413.

Carbohydrates, 34; synthesis of, 32, 40.
Carbon cycle, 386.
Carbon dioxide, effects of, 197.
Carboniferous plants, 497.
Carotin, 52.
Carpels, 239.
Carrot, 144; family, 540.
Catalyst, 60.
Catkin, 232.
Cauliflower, 281.

monocot,

553

554

Index

Cell division, 19; and chromosomes, 292-

296; and osmosis, 87; and walls, 56.
Cells, 15; guard, 26.
Cells, tissues, and organs, 21.
Cellulose, 56.
Cell walls, 18.
Century plant, 172.
Cereus, 169, 171.
Chemical elements, 216.
Chicle, 374.

Chlamydomonas, 445, 448, 455.
Chlorenchyma, 25.
Chlorophycecs, 445.
Chlorophyll, 29, 52, 69.
Chloroplasts, 29.
Chromosomes, 294; behavior of, 292-296;

numbers of, 487.
Cion, 158.
Cladonia, 414.

Classification, 437; of woods, 152.
Clavaria, 420.
Climate and crops, 340.
Climatic formations, 306.
Climbers, 107.

Clover, red, 206; root nodules of, 397.
Club mosses, 489.
Club root, 432.
Cocklebur, 267.
Coconut, 371.
Coffee, 258.
Collenchyma, 130.
Colors of leaves, 52.
Companion cells, 134.
Compass plants, 66.
Compositce, 543-
Concord grape, 274.

Conduction, of water, 160; of food, 163.
Conductive tissues, of leaves, 30; of roots,

183; of stems, 133-
Conifer forest, northern, 312; southeastern,

320; western, 330.
Conifers, 508 ; leaves of, 509 ; life history of,

510; seeds of, 514; stem structures, 141.
Coppice, 230.
Cork, s6.
Corm, 113.
Com, 177, 233; acre of, 38, 50, 76, 941 stem

of, 137-
Corolla, 238.
Cortex, 131. .
Corymb, 232.
Cotton, fiber of, 274; production of, 354;

wilt disease of, 298.
Cotyledons, 257.

Cowpeas, 401.

Cranberry, 539.

Craterellus, 413, 420.

Crop centers, 343.

Crops and water balance, 94, 211.

Cruciferm, 534-

Cup fungi, 413.

Cutin, 26, 56, 76.

Cuttings, 227.

Cycads, 503.

Cypress, 256; bald, 321.

Cytoplasm, 17.

Dahlia, 206.

Damping off, 431.

Dandelion, 178.

Dasheen, 113.

Date palm, 118; transpiration of, 75.

Deciduous forest, 319; industries of, 347.

Deciduous habit, 102.

Deliquescent stems, 121.

Desert, plants of, 79, 168; southwestern,

337-
Diastase, 164.
Diatoms, 444.

Dicots, stem structure of, 131.
Dicotyledons, 523; families of, 524.
Dictyota, 462.
Diffusion, 82.
Digestion, 163.
Diseases, bacterial, 391, 423; fungous, 423;

resistance to, 276.
Distribution of plants, 302.
Dodder, 380.
Dogbane, 112.
Dogwood, 64, 519.
Dormancy, 264.
Douglas fir, 361, 369, 509.
Drainage, 49.

Drought, effects of, 168; resistance to, 80.
Drupe, 253.

Ecological factors, 10, 203.

Egg, fertilization of, 249.

Elements, mineral, 216.

Elodea, 36.

Embryo, 250, 257; of angiosperm, 257,

521 ; of conifer, 513 ; of cycad, 507 ; of di-

cot, 258; of monocot, 258; sac, 248, 249;

of selaginella, 492.
Endosperm, 249, 250; of angiosperms, 522;

of gymnosperms, 514.
Energy and life, 378.
Energy release, 48.

Index

555

Environment, lo; and distribution, 13,302;
and growth, 10; changes in, 306; fac-
tors of, 203; responses to, 203-224.

Enzymes, 59, 164.

Epidermis, of leaves, 24; of petals, 28, 241 ;
of roots, 183; of stems, 132.

Epigynous, 239.

Epiphytes, 187, 189.

Equisetums, 488.

EricacecB, 540.

Eucalyptus, 517.

Euphorbia, 169.

Evening primrose, 204.

Evergreens, 102.

Evolution, 545.

Excurrent stems, 121.

Factors, limiting, 11, 14; of crop distribu-
tion, 340; of environment, 11, 203-224;
of evolution, 550.

Fall of leaves, 100.

Fat synthesis, 41.

Fermentation, 410.

Fern, cinnamon, 480; sensitive, 482;
shield, 481; staghorn, 483; tree, 479-

Ferns, 478; growth of leaves, 98; life his-
tory of, 486.

Fertilization, 243 ; in algae, 446 ; in conifers,
508; in cycads, 506; in ferns, 489; in
mosses, 474.

Fertilizers, 220.

Fibers, 56; Manila, 140.

Fig, 252.

Filicales, 478.

Fire blight, 430.

Flax, 136.

Floating plants, 173.

Floral, envelope, 240; plans, 238.

Floriculture, 148.

Flower, 232; clusters, 232; of dicot, 258;
economic importance of, 256; of monocot,
258.

Flowering plants, 517.

Flowers and insects, 243, 518.

Fluctuations, 281.

Fomes, 420.

Food, 32; movement of, 163, 201; per acre,
46.

Food accumulation, in fruits, 256; in roots,
202 ; in stems, 166.

Food-conducting tissues, 31, 132, 134.

Forest, deciduous, 318; fire patrol in,
367; lands, 362; northern evergreen,
312; reserves, 364; southeastern ever-

green, 320; tropical evergreen, 322;

western evergreen, 330.
Formations, climatic, 306.
Fossil, plants, 495; record, 497, 545.
Freezing, 213.
Fruits, 252; economic importance of, 256;

ripening of, 255 ; transportation of, 260.
Fuciis, 458, 461.
Fungi, 402 ; distribution of, 406 ; groups

of, 406; reproduction in, 405.

Galls, 435-

Gametes, 243.

Garlic, wild, 226, 227.

Geaster, 420.

Gemmae, 468.

Geographic distribution of vegetation, in
North America, 310; in United States,
312.

Geological record, 545.

Germination of seeds, 270.

Glaciation map, 501.

Glucose, 34.

Gourd, 253.

Grafting, 157, 228.

Grain, 252.

Grass family, 524.

Gravity effects, 215.

Green algae, 445.

Green mold, 411.

Green pigments, 52.

Growing region, of leaves, 98; of roots, 183 ;
of stems, 128, 149.

Growing season, 212.

Growth, 97, 148 ; conditions for, 97 ; diam-
eter, 150; pressure, 200.

Guard cells, 26.

Guayule, 369.

Guinea grass, 68.

Gums, 57.

Gymnosperms, 259, 503 ; leaves of, 509 ;
life history of, 503, 510; wood of, 152;
stems of, 141.

Habitats, 13; and water balance, 93.

Hairs, 78.

Hardening, 214.

Haustoria, 380.

Head, 232, 236.

Heartwood, 157.

Heath family, 540.

Hemp, Manila, 140.

Herbs, 146.

Heredity, 551.

5S6

Index

Heterospory, 491.
Heterozygous, 288.
Homozygous, 288.
Horsetails, 488.
Horticulture, 148.
Host, 380.
Humidity, 210.
Humus, 221.
Hyacinth, water, 174.
Hybridization, 276.
Hybrids, 286; vigor of, 286.
Hydathodes (water pores), 175.
Hydnutn, 420.
Hydrophytes, 95, 173, 176.
Hypocotyl, 257.
Hypogynous, 239.

Imbibition, 83.

Indian cucumber root, 63.

Indian pipe, 382.

Insect pollination, 243.

Intemodes, 116.

Inulin, 59.

Iris, 67.

Irish moss, 464.

Iron, 219.

Irrigation, 94, 367 ; eflfects on wheat, 211.

Jack-in-the-pulpit, 527.
Japanese cane, 526.
Jelly-making, 255.

Kelps, 458, 461.
Kohl-rabi, 281.

Labiates, 541.

Land and water environments contrasted,

465-
Landscape architecture, 148.
Larch, 508.
Latex, 58.

Laurel, 539; family, 548.
Leaf, arrangement, 62; coloration, 52;

fall, 100; mosaics, 65; parts of, 22;

pigments, 52; position, 62; scars, 116,

124; tissues, 24.
Leaves, 22, 55; effects of light on, 62;

floating, 79; horizontal, 64; motile, 68;
ertical, 67 ; water

submerged, 79

relations of, 72.
Legume, 253.
Legume family, 537-
Legumes and bacteria, 397.
Length of day effects, 204-207

Lenticels, 116, 126.

Lichens, 413, 416, 443.

Lifting of water, 160.

Light, 203 ; and bacteria, 387 ; and growth,
62, 203, 204; and photosynthesis, S3',
and reproduction, 208; quality of, 209.

Lignin, 56.

Lily family, 530.

Limiting factors, 11, 14.

Lipase, 164.

Liverworts, 465, 468.

Loam, 222.

Longevity, of plants, 143 ; of seeds, 268.

Lotus (Nehimbo), 252.

Lycopodiiim, 489.

Magnesium, 216, 218.

Magnolia, 303, 5i9-

Mahogany, 373.

Maltase, 164.

Manganese, 220.

Mango, 255.

Mangrove, 180; seedlings of, 262.

Manila fiber, 139.

Maple, flow of sap in, 162 ; sugar, 349 ; wood
of, 154-

Maratlia, 478.

Marchantia, 471.

Marquis wheat, 299.

Mass selection, 298.

Mechanical tissue, 136; of leaves, 30;
of roots, 183.

Medullary rays (pith rays), 133, 156, 157.

Membranes, 87.

Mendel, experiments of, 286; law of, 292;
and chromosomes, 292.

Meristem, 128.

Mesophyll, 25, 27.

Mesophytes, 95.

Mesozoic vegetation, 500.

Mesquite, 381.

Microspora, 449.

Middle lamella, 25, 218, 255.

Mildews, downy, 435 ; powdery, 412.

Mimosa, 538.

Mineral elements, 216.

Mint family, 541.

Mistletoe, 380.

Mnium, 474.

Mold, 406; bread, 407; blue, 411; green,
411.

Monocots, 130; families of, 524; stem struc-
ture of, 13s, 138.

Monocotyledons, 130.

Index

557

Morels, 413.

Morning-glory, root of, 198.
Mosaic disease, 435.
Mosaics, leaf, 65.
Mosses, 465, 471.
Moth mullein, 145.
Motile leaves, 68.
Movement of materials, 160.
Mucilage, 57, 78, 162.
Multiplication, vegetative, 225.
Mushrooms, 403, 421.
Mustard family, 534-
Mutation, 283; bud, 285.
Mycorhiza, 188.
Myxophyceae, 441.

Natural selection, 551.
Nectar, 244.
Nitrifying bacteria, 396.
Nitrogen, uses of, 44.
Nitrogen cycle, 399.
Nitrogen-fixing bacteria, 396.
Nodes, 62, 116.
Nodxiles, root, 188, 398.
Nucleus, 17.

Oak wood, 154, 156.

Oaks, 438.

Oats, 199.

(Edogonium, 452.

Oils, 41.

Orchid, 240.

Orchid family, 532.

Organ, 21; essential, 58.

Osmosis, 85.

Osmotic pressure, 87.

Ovulary, 238, 239, 241.

Ovule, development of, 248.

Oxidation, 48.

Oxygen and bacteria, 387.

Paleozoic forests, 499.

Palm, bottle, 153; coconut, 371; date, iii

family, 528; fan, 234; stem, 138.
Panicle, 232, 236.
Panicum stem, 135.
Paper pulp, 345, 354> 364-
Parasites, 380.
Parenchyma, 128.
Parmelia, 414.
Pasteurization, 393.
Peanut, 356.

Pectic compounds, 56, 254.
Pedicel, 235.

Peduncle, 235.

Perennials, 145.

Perigynous, 239.

Perisperm, 250.

Permeability, 86.

Persimmon, 256.

Pcziza, 412.

PhcBOphycecB, 457-

Phloem (food-conducting and bast tissues),

134-

Phosphorus, 219.

Photosynthesis, 32 ; amount of product, 37 ;
and quaUty of light, 209; contrasted with
respiration, 49; hindrances to, 38; im-
portance of, 40 ; summary of, 40.

Phycomycetes, 406.

Pigments, 52; green, 52; red, 54; yellow,
52, 55, 56.

Pine, Japanese dwarf, 145 ; life history of,
510; wood of, 154.

Pineapple, 254; family, 531.

Pine-blister rust, 419, 421.

Pinesap, 382.

Pinon, 336, 365.

Pitcher-plant family, 534.

Pith, 134, 138; rays, 132.

Placenta, 239.

Plains, grassland, 328.

Plankton, 454.

Plant, associations, 305; breeding, 272,
344; breeding and evolution, 549; dis-
eases, 423 ; distribution, 302 ; formations,
306; industries, 340; pathology, 430;
realms, 308.

Plant industries, 340; of deciduous forest
region, 347 ; of desert region, 368 ; . of
northern conifer forest region, 345 ; of
plains region, 359; of prairie region, 357;
of southern conifer forest region, 353 ; of
tundra, 345; of western conifer forest
region, 360.

Plants, classification of, 437 ; desert, 79,
168-173; floating, 79, 173; products of,
3; submerged, 79, 95, 173, 186.

Plastids, 18.

Plums, 275.

Plumule, 257.

Pollen, 241, 243; germination of, 244, 247;
tube, 244, 245.

Pollination, 243; cross, 245; self, 245.

Pome, 253.

Pond scums, 448.

Potamogeton, 174.

Potassium, 216, 218.

558

Index

Potato family, 543.

Potatoes, 344.

Prairie formation, 326.

Pressure, of growth, 200 ; osmotic, 87 ; root,

ig5; turgor, 88.
Prickly pear, 169, 181.
Propagation, vegetative, 226.
Prosenchyma, 128.
Protein synthesis, 42.
Prothallus, 485.
Prolococcus, 447.
Protoplasm, 15, 16, 61.
Pteridophytes, 477.
Puff balls, 422.
Pulp wood, 345, 354, 364-
Pulvinus, 6g.

Raceme, 242.

Rainfall, seasonal distribution of, 210.

RanunculacecB, 533-

Rattan, 139.

Receptacle, 237.

Red algae, 463.

Red pigment in plants, 54.

Redfieldia, 113.

Reduction division, 295, 296, 487.

Redwood forest, 332.

Reforestation, 366.

Reproduction, 9; in algce, 446; in angio-
sperms, 243, 518; in conifers, 510; in
cycads, 504 ; in ferns, 484 ; in fungi, 405 ;
in liverworts, 469 ; in mosses, 473.

Reservoirs, cleaning of, 456.

Resin ducts, 142, 154.

Resins, 57, 354-

Respiration, 48; and shipping, 50; con-
trasted with photosynthesis, 49; injuries,
50; in roots, 197; in stems, 126; rate of,
50.

Response, to aerial environment, 466 ; to
gravity, 215; to light, 203; to water,
173, 210.

Rhizoids, 466.

Rhizome, in.

Rhodophycece, 463.

Rice, 357.

Rockweeds, 458.

Root, absorption, 185, 191 ; contraction,
185; hairs, 184; pressure, 195; pro-
cesses, 1.77, 191, 197; systems, 181, 198;
tip, 293.

Roots, 177; adventitious, 178; and bac-
teria, 188, 397; and fungi, 188; and
transplanting, 196; classification of, 177;

growth of, 184; holdfast, 180, 187; of
climbers, 178; of conifers, 508; of epi-
phytes, 187; of ferns, 482; of hydro-
phytes, 186; of mesophytes, 181-186;
structures of, 183.

Rootstocks, III.

Rose family, 537.

Rosette plants, 105.

Rosin, 57, 354.

Rubber, 58, 373-

Rubber tree, 179.

Russian thistle, 376.

Russula, 420.

Rust, 417; of apple, 419; black-stem, 418;
pine-blister, 419, 421.

SalicacecB, 533.

Sand-reed grass, in, 112.

Sap, flow of, 162; pressure of, 195; wood,
157-

Saprophyte, 381.

Sarraceniacece, 534.

Sassafras, 30 ; fossil leaf of, 546,

Sclerenchyma, 130.

Seed, coats, 521; dormancy due to, 265;
structure of, 251.

Seedlings of corn, 177.

Seeds, 257; abscission of, 260; of angio-
sperms, 259; dissemination of, 260; dor-
mancy of, 264; germination of, 270; of
gymnosperms, 259; longevity of, 268;
storage of, 269.

Selaginella, 80, 489.

Selection, 297; natural, 551.

Self-pruning, 102.

Sensitive plant, 69.

Sequoia, 107, 2,53, 335-

Sexual reproduction, 243.

Shasta daisy, 283.

Shrubs, 146.

Sieve tubes, 134.

Silviculture, 148.

Sisal, 139, 369.

Smuts, 414, 416, 433.

Soil, acidity, 218, 220; bacteria, 393, 396;
erosion, 364; temperatures, 212; water-
delivering mechanism of, 193.

Soils, alkali, 221 ; and crop production, 340.

Solanacese, 543.

Solution, 82.

Sorghum, 351, 353.

Spadix, 232, 527.

Sperms, 243.

Sphagnum, 475-

Index

559

Spike, 232.

Spirogyra, 451. 454-

Spontaneous generation, 394.

Spores, 389.

Sporophylls, 484.

Sports, 283.

Sprout forests, 230.

Spruce, 194, 316, 332.

Squashes, 273.

Stamen, 238.

Starch, 35.

Stem, dicot, 132; monocot, 135; palm, 138;
processes, 160; structures, 130.

Stems, climbing, 107 ; conifer, 141 ; hori-
zontal, no; of hydrophytes, 173; of
mesophytes, 105-115, 168, 173; of xero-
phytes, 170; underground, no; upright,
105, 130.

Stipules, 23.

Stock, 158.

Stomata, 27.

Storage, of energy, 39; of seeds, 269; of
food, 393.

Strawberry, 225; fruit of, 252; pollination
of, 246.

Suberin, 56.

Submerged leaves, 70.

Submerged roots, 186.

Submerged stems, 173.

Subsidiary cells, 25.

Succession, 307.

Sugar, 34; beet, 3S1; cane, 357; maple,

349-
Sulfur, 219.
Sunflower, red, 283.
Sunflower family, 543.
Synthesis, of carbohydrates, 34; of fats,

41 ; of proteins, 42.

Tamarack, 314.

Tank epiphytes, 187.

Tannins, 59, 347.

Tapioca, 373.

Temperature, and bacteria, 387; and

photosynthesis, 34; effects of, 211.
Tendrils, 108.
Tertiary vegetation, 502.
Thallus, 468.
Thistle, 544.
Timothy, 275.
Tissues, 21; of leaf, 24; of root, 183; of

stem, 131.
Toadstools, 421.
Tobacco, 205, 274, 282; regions, 352.

Toxins, 391.

Tracheae, 20, 133.

Tracheids, 20, 141.

Transfer of food, 163.

Transpiration, 73; and absorption, 90;

and lifting of water, 160; and stomata,

74; external factors of, 78; internal

factors of, 76.
Transplanting, 91; and roots, 196; and

water balance, 91.
Trees, 146; age of, 147; evergreen and

deciduous, 102; size of, 106.
Tube fungi, 406.
Tubers, 114.
Tulip tree, 533-

Tundra, 310; plant industries of, 345.
Turgor, 88.
Turpentine, 354.

Ulothrix, 450.
Umbel, 232.
UmbellifercB, 540.
Underground stems, no.

Vacuoles, 16, 18.

VaniUa, 374.

Variations, 280; heritable, 282, 551.

Vascular bundles, 132, 133, 135.

Vaucheria, 451, 454.

Vegetation, 302; of North America, 310;
of United States, 312; Mesozoic, 500;
Paleozoic, 499.

Vegetative multiplication, 225.

Vegetative propagation, 226.

Venation, 30.

Vertical leaves, 67.

Vessels, 31; food-conducting, 31; water-
conducting, 31.

Victoria regia, 175.

Vinca, 24.

Vinegar making, 410.

Vital syntheses, 32, 379.

Vitamins, 58.

Water, accumulation of, 79, 162; balance,
90; conducting tissue, 30, 133; lifting of,
160; plants, 79; pores, 175; supply and
crop yields, 94, 211; supply, effects of,
on wheat, 211 ; uses of, 72.

Water lily, 175.

Watermelon wilt, 276.

Wax, 56.

Weeds, 375.

q6o

Index

Wheat, 199, 217, 237; black rust of, 418;

macaroni, 211; production, 357.
White pine, 318, 346; blister rust of, 419.
Wild garlic, 226, 227.
Willow family, 533.

Wilt disease, of cotton, 298 ; of cucumber, 428.
Wind, effects of, 214; pollination, 243.
Wood, 136; distillates, 350; sections, 154,

156; structure and use, 155.
Woods, classification of, 152.

Xeniophyte (endosperm), 250, 272, 522.
Xerophytes, 95 ; various types of, 170.

Yeast, 410.

Yellow pigments, 52, 55, 56.
Yellow pine, wood of, 154.
Yucca, 169; and pronuba, 246; in bloom,
529-

Zygote, 243, 250, 446, 521.

^iiiiliiiiiiiiiiiiiiiiiiiimiimiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii mil n iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiimiiiiiiiiiiiiiiiiiiiiiimiiiiiiiiiiiiiiiiimii:

I New- World Science Series |

I Edited by John W. Ritchie |

I The publication of "books that apply the world's know!- I

I edge to the world's needs," the ideal of this house, is |

I concretely carried out by the volumes of this series that |

I is planned to provide texts for all the sciences taught in |

I junior and senior high schools and colleges. |

I Common Science, by Carleton W. Wasbburne. A text that |

I explains the facts and phenomena with which the pupils come i

I into contact. Fundamentals are presented so as to arouse inter- i

I est in science. $1.68. Manual for teachers use, 20 cents. |

I Exercise and Review Book in Biology, by J. G. Blaisdell. A |

I combined laboratory guide, notebook, and review book to accom- |

I pany any text in general biology for high school work. $1.30. |

I Gardening, by A. B. Stout. A well ordered, practical course in |

I gardening, which emphasizes both the educational and practical |

I aspects of the subject. $1.60. |

I Human Physiology, by John W. Ritchie. A text that teaches the |

I essentials of physiology, hygiene and sanitation, $1.40. |

I Laboratory Manual for Human Physiology, by Carl Hartman. |

I A Manual giving brief, definite directions for laboratory work |

I in physiology chemistry, and bacteriology. $1.00. |

I Personal Hygiene and Home Nursing, by Louisa C. Lippitt. |

I A practical, non-technical text for girls in high schools, normal |

I schools and colleges. $1.68. |

I Sanitation and Physiology, by John W. Ritchie. A text that |

= combines teaching of public health and physiological hygiene. =

I $1.72. I

I Science for Beginners, by Delos Fall. An introductory text in |

I science that approaches the subject thru the experiences of |

I the child. Teaches the scientific method. $1.60. |

I Science of Plant Life, by E. N. Transeau. A botany text for a |

s half-year course in the high school. Emphasizes the structural as well i

I as the dynamic aspects of the subject. SI. 68. |

I Trees, Stars and Birds, by E. L. Moseley.. An outdoor science |

I book containing colored cuts of 58 common birds, 9 star maps, |

I and 69 drawings. $1.80. I

I College Texts |

I Experimental Organic Chemistry, by Augustus P. West. A text |

I containing full laboratory directions for the college student. $3.20. I

I Zoology, by T.D.A. Cockerell. A book that fully sets forth the |

= biological principles upon which much of our philosophical and |

= social thought is based. Possesses rare literary charm. $3.60. |

I WORLD BOOK COMPANY I

i YOXKERS-ON-HUDSON, NeW YoRK I

I 212G Prairie Avenue, Chicago |
^iiiiniiiuiMiinininiiiiiniiiiiniiiiiMiiiiiiiiiiiiiiiiiNiiiiiiiiiiiiiiiiiiiniiiiiiiiiiiimiiniiiiiiiiiiiiniiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiinin

Siiiiiiiiiiiiriitiiiiiiiiiiiiiiiiriiiiiiiiiiiiiiiiiiriiiiiiiiiiriiitiiiiiiiiiiriirniiiiiiiitiriiiiiiiiiiiiiiiriiriiiriiiiiiiiitiiii iiiiiiiiiiiiiiiiiiiiiiiiiriiiu

I NEJV -WORLD SCIENCE SERIES j

I Edited by John IF. Ritchie I

I SCIENCE of PLANT LIFE I

I A HIGH SCHOOL BOTANY treating of the I

I plant in its relation to its environment i

I ^^^ E. N. TRANSEAU |

I Professor of Botany, Ohio State University i

I 'TpHE fundamental aim of this text is to give the student |

I J. an understanding of the plant as it lives. Environ- |

I mental factors are considered in such a way that they I

I help to make clear the reasons which underlie many agri- |

I cultural practices. The author so arranges and presents I

I his facts that the idea that the plant is alive and that it |

I has a definite relation to every other living being is |

I unescapable. I

I The teacher of botany will' find an ally in this book. I

I Every page carries its answer to that old query of pupils, |

I "What is the use in studying botany?" Almost all the I

I chapters are preceded by suggestions for laboratory and |

I field work and are followed by practical problems. The |

I exercises are varied enough to provide work for any i

I class ftnd they are adapted to the succession of seasons. I

I The illustrations, the exercises, and the subjects that re- =

I ceive distinctive treatment are not embroidered on the |

I text; they are made a part of its warp and woof. |

I In^ bringing out Science of Plant Life, publisher, |

I printer, and artist have collaborated to produce a volume |

I which in appearance and in every detail of manufacture I

I is worthy of its content. The book is admirably adapted |

I to meet the needs of beginning students of botany every- I

I where, and it will prove helpful to all who wish to be |

I intelligent about plants. |

I viii + 344 pages. Price $1.68 |

I WORLD BOOK COMPANY |

I YONKERS-ON-HUDSON, NeW YoRK I

I 2126 Prairie Avenue, Chicago I

Siiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiriiiiiiiiiii I iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiriiriiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiis

u, iiiiiiiiiiiiiiiiiiiiiniiiiiiiiiiiiiiiiiiiiiiini iiniiiiiiiiMiiiiiniiMiiiiiiiiiiiiii iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii i iini niii i iiiii iiiiiiiiim|

WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NeW YoRK

2126 Prairie Avenue, Chicago

I NEW-WORLD SCIENCE SERIES

I Edited by John W. Ritchie

I A Textbook for Colleges and Universities

I ZOOLOGY

I By T. D. A. COCKERELL

I Professor of Zoology y Uni'versity of Colorado

I Zoology for the average individual who will not specialize in the subject

I should not be too rich in detailed facts of structure and classification. =

i Experience shows that much minutiae are not remembered, and do not |

I necessarily leave as a residue any broad and useful conceptions. |

1 Zoology, a Textbook for Colleges and Universities, by Professor Cocker- |

I ell, is based on these views. The book grew out of a course that was de- |

i veioped in actual and successful teaching experience. It is calculated |

i to convey to students a point of view that will be of social value. |

i =

I The first chapter is entitled The Physical Universe, and it considers the

I fundamental ideas of physical science , taking nothing for granted . The

I treatment of the animal kingdom is at once broad and systematic.

I The great phyla and some of the sub-groups are discussed from the

I structural angle. More space than usual is given to a description of

i animals. The geographic origin and the migrations of the world's fauna

I are ably considered. The Florissant Shales of Colorado and the sub-

1 jects of genetics and entomology are treated with unusual authority.

I The topics throughout are developed in a way that makes delightful

i as well as highly instructive reading. The volume is illustrated from

I photographs, many of which were selected from hundreds that were

1 taken at the New York Zoological Garden. It contains numerous

I drawings that were made under the direction of the author, expressly

I to illustrate his text. While Cockerell's Zoology is primarily designed

I for use in classes that are now being taught zoology, it presents ideas

I that make it available for use in a course in general biology. It is a

I workable text of college grade.

I An important critic has pronounced it the "first zoology since Huxley

I written in good English."

I Illustrated with maps, charts, drawings, and photographs.

I Cloth xii +558 pages. Price ^SM.

fniiiiiiiiiiiiiiiiiii iiiiiiiiiii iiiiiiiiiiiiiiiiiiiiiiiiiii iiiiiiiiiii

mill iiiiiiiiiiiiiiiiiiiiii iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii iiiHii|U|iiuimiinmi|i(i

j„„„„„„„i,iii in Ill iMiMiiiiiiiiiiiimiliiiitiiimiiiiiiiiiiininniiMiiiininininiininiiiiiiiiiiiiiiMiiniiiiiiiMiiiMmiiiiiiiiiiiiiimiMiiniiniiiniimiiiim^^

= =

1 Studies in JMet/iods of Teac/iiiig in the College |

1 1

I COLLEGE TEACHING ■

I Edited by Paul Klapper |

^HE

introduction to this first book on this subject is the |

work of President Nicholas Murray Butler of Columbia j

I University. Thirty-one leading American authorities are |

[ the authors of the chapters which deal with every subject in |

i the college curriculum. These were written on the same plan, |

I which makes the volume a unified production. The writers |

I were selected for their scholarship, interest in the teaching |

I phase of the subject, and reputation in the academic world. |

i The book is divided into six parts as follows: |

i The Introductory Studies |

I The Sciences |

I The Social Sciences |

I The Languages and Literatures |

I The Arts |

I Vocational Subjects I

I A treasure of wisdom is stored in the colleges of America. The |

I teachers in them are the custodians of the knowledge. Impar- |

I ting this knowledge is the function of these institutions of j

I higher learning. To do this most effectively is the plan of every |

I instructor. This book aims to make the college teacher effi- i

I cient in handing down this heritage of knowledge, rich and |

I vital, that will develop in youth the power of right thinking |

I and the courage of right living |

I Dr. Butler writes that "a careful reading of the book is com- |

I mended not only to the great army of college teachers and |

i college students, but to that still greater army of those who, |

i whether as alumni or parents or as citizens, are deeply con- |

I cerned with the preservation of the influence and character |

I of the American college for its effect upon our national Stan- |

I dards of thought and action " |

I Cloth, xvi +583 pages. Price U-OO. |

i !

I WORLD BOOK COMPANY |

i YONKERS-ON-IIUDSON, NeW YoRK |

I 2126 Pkairie Avenue, Chicago i

liiiiiiiiiiiiiiiiiiniiiiiiiiuiiiiiiiimiiiiiiiiiiimiiiin i iiiiii mill iiiiiiiiiiiniiiiiiiii i iiiiitiniiiiiniiiiiiMiiiniiiiini iiiiiiiiniiiiiiiiiiini iimiip

tinmiiiliiniiiiiiiiiiMiiiiiiiiiiiiiiiMiiiiiiiiMtritiiiiriiiiiiiiriiiiiiiriiiiriiiiiiiiiiiiiiiiiiiitiiiiiiiMMiiiiiiiiniiiiiiinitiiiiiiiiiiiiiiiiiriiiiiiiiim

I NEW-WORLD SCIENCE SERIES |

I Edited by John W. Ritchie I

I EXERCISE AND REVIEW
I BOOK IN BIOLOGY

I By J. G. BLAISDELL

A Combined Laboratory Guide, Notebook
and Review Book jor Students' Use

N submitting the manuscript of this manual to the
publisher, the author wrote:

i "As a teacher of biology, I have for several years felt the

= need of a laboratory guide and notebook that would lighten the

I labor and economize the time of both teacher and pupil in con-

1 nection with their laboratory work. In this age of cheap printing

I it seems a needless waste of time and strength to compel teachers

I to prepare laboratory outlines and to mimeograph or copy them

I on the blackboard, and it ought to be possible to permit the

5 pupil, when he enters the laboratory, to begin work at once with

I his laboratory directions, questions, needed outline drawings, and

i space for his notes and sketches all on one neatly printed note-

I book page. I have found the loose-leaf, blank-page, ring-cover

I notebook, and separate laboratory directions method both waste- 3

I ful of time and unsatisfactory in other respects, a.nd have used |

I careful thought and my experience as a teacher in planning a |

I tetter way." |

§ =

I Mr. Blaisdell has solved the problem with this book. His manual 5

I bears '.he same relation to laboratory study as a well-ordered 5

5 text bears to the recitation, and cannot fail to conserve the time |

i of both teacher and pupil. |

i It contains 100 laboratory exercises, with space for more, cover- |

I jng a year's work in general biology, planned to meet the re- |

5 quirements of the syllabus issued by the Regents of the Uni- =

I versity of the State of New York, and to accompany any high- |

I school text in general biology in common use. |

I Review exercises are provided, and a series of Regents' ques- =

I tions are so arranged as to give a review of the year's work. |

I viii -f 152 pages. Price $1.20 1

I I

I WORLD BOOK COMPANY 1

I YONKERS^ON-HUDSON, NeW YoRK |

i 2126 Prairie Avenue, Chicago i

i i

7iiiniiuiiiiiiiiiiiiiiiiMiiiniin»iiiiuuiuiiiiiiMiiiniuiiiuiiumiiiiMiiuuiimuummiiiuiiuuiiiiiiiiuuiiiiiiHiitiiiiiiii''}!^iffiTiiiii;ai

^iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiitiiiiiiiiiiiniiiiiiiiiiiiiiiiiiiiiiiiiiiiiiniiiifiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiniiiiiiiiiiiiiiiii^

' THE NEW WORLD i

PROBLEMS IN POLITICAL GEOGRAPHY |

% ISAIAH BOWMAN, Ph. D. |

Director of the American Geographical Society |

Presents in convenient form the facts bearing upon the |

new territorial, racial, religious, commercial, and political |

alignments in Europe, Asia, Africa, and the Pacific. |

Many highly significant racts contained in this book can be |

obtained from na other source. Problems are presented in |

their relation to world politics and in their historical and geo- |

graphical setting. The latest reliable statistics are given when- |

ever they serve to make a point clear. |

There are 21^ hand-drawn maps, all specific and clear. |

They are designed to bring out particular economic^ |

political, or geographical features treated in the text, |

A reading of The New World will enable the student of geog- |

raphy, history, and economics to see the significance of the |

changes that have been made in the map of the world, to un- |

derstand the influences that have brought them about, to esti- |

mate their bearing upon the course of political events, and to |

think in a more comprehensive way about the many vital prob- |

lems in the field of international relations. |

Cloth, vii + 6t,2 pages. Price i^b-oo |

WORLD BOOK COMPANY |

I YONKERS-ON-HUDSON, NeW YoRK |

I 2126 Prairie Avenue, Chicago |
iinMiimnimiiiiiiiiiiiiiiiiiiniiiiiniiiiiniiniiiniiniiiiiHiiiiiiiiiiniiiiiiiiiiiiiiiiniiiiiiiiiniiiiiiiiniiniiiiiiiiiiiniiiiiiiiiiiiniiiiiiiiiiniiiiiiiiiiii^

Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Books

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

Full text of "General botany; an introductory text for colleges and advanced classes in secondary schools"

See other formats