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BOTANY

FOR

AGRICULTURAL STUDENTS

JOHN N. MARTIN

Professor of Botany at the Iowa State College of Agriculture
and Mechanic Arts

FIRST EDITION

NEW YORK

JOHN WILEY & SONS, INC.

LONDON: CHAPMAN & HALL, LIMITED
1919

BY
JOHN N. MARTIN

Stanbopc jprcss

F H. GILSON COMPANY
BOSTON, U.S.A.

PREFACE

Although students vary widely in their reasons for studying
Botany, the fundamental facts or principles of the subject are
not thereby altered. One has considerable freedom, however, in
the presentation of the subject to adapt the subject matter to
special aims of different classes of students, and especially is
this true in courses for agricultural students, since much of the
work in Agriculture is based upon the principles of Botany. In
the choice of material to illustrate principles and in the presen-
tation of the applications of principles, there is special oppor-
tunity to relate courses in Botany to courses in Agriculture.

In any elementary course in Botany, regardless of the kind of
education the student desires to obtain, the primary aim should
be to give the student a notion of the fundamental principles of
Botany. This aim should be the guiding one in both recitation
and laboratory, determining the trend of discussions in recita-
tion, and the nature of the material and procedure in the lab-
oratory. The primary aim should be accompanied by a secondary
aim to relate the subject to the student's major line of work.
When the relation of the subject to major lines of work is obvious,
the student is more likely to appreciate the subject and is thereby
put in a favorable mood to study the subject. Even for students
who take Botany merely as a part of a general education, it in no
way detracts from the course or makes botanical training less
efficient to present the practical aspects of the subject.

This book is intended for elementary courses in Botany in
colleges and universities. In its preparation the aim has been to
present the fundamental principles of Botany with emphasis
upon the practical application of these principles. The subject
matter is presented in two parts, part I being devoted to the
study of the structures and functions chiefly of Flowering Plants,
and Part II, to the study of the kinds of plants, relationships,
Evolution, Heredity, and Plant Breeding.

In the preparation of the book, I had the following objects in
view: (1) to present the structures and functions of Flowering

iii

iv PREFACE

Plants and relate them to such agricultural subjects as Farm
Crops, Forestry, and Horticulture, and to the more advanced
courses in Botany; (2) to present the kinds of plants with emphasis
upon their evolutionary relationships and their economic im-
portance; and (3) to present Evolution, Heredity, and Plant
Breeding as related to the improvement of plants.

The topics are arranged in the book in the order in which I
usually present them. The presentation of the reproductive
structures and processes of Flowering Plants, followed by
that of the vegetative organs, has fitted in at Iowa State
College with the time of year at which the agricultural students
begin the study of Botany and also with the courses in
Agriculture. In other schools where conditions are different,
other arrangements of the topics are more suitable. In recogni-
tion of this fact, most of the chapters have been written so as to
be separately understandable, the aim being to make the book
adaptable to any arrangement of topics that the teacher may
prefer.

In the discussion of a subject the presentation of the general
features precedes that of the particular features, and the latter
are presented in most cases by the study of type plants chosen on
account of their familiarity and economic importance.

The book is intended for an entire year's work in Botany and
to be accompanied by laboratory work. Where less time is de-
voted to the subject, the organization of the chapters so as to be
separately understandable permits a selection of topics according
to the requirements of the course.

The reproductive structures and processes in Flowering Plants
(Chapters III and IV) are dwelt upon more than is necessary
for students who have had a good course in Botany in a high
school. A large percentage of the students in my elementary
classes have had no Botany and have difficulty in understanding
sexual reproduction in Flowering Plants. In an effort to thor-
oughly acquaint the student with this subject, I have dwelt at
considerable length upon those phases of the subject that are in
my experience difficult for the student to understand. In case
students are familiar with this subject, parts of Chapters III and
IV can be omitted or read hastily in review.

Usually there are some students in the class that are especially
interested in certain topics and desire a more complete discussion

PREFACE V

of the topics than the text affords. In recognition of this fact,
I have added, chiefly as footnotes, many references. Most of the
references are bulletins on the special topics, and in addition to
giving further information on the special topics, these references
introduce the student to that vast source of information contained
in the bulletins published by the U. S. Department and the ex-
periment stations of the different states.

Many of the illustrations have been taken from the publica-
tions of various authors whose names or the names of their pub-
lications appear in connection with the illustrations. To these
authors I am much indebted. Most of the original illustrations
have been made by Mrs. Edith Martin, who has also given me
valuable assistance in other ways in the preparation of the book.
Also much credit is due Mr. H. S. Doty and Mr. L. E. Yocum,
my assistants, who have given me valuable suggestions. To
Dr. L. H. Pammel, who read some of the topics on Fungi and
offered valuable suggestions, I am also much indebted.

The book no doubt has many faults, but I hope it has some
particular value and that the criticisms which teachers offer will
make me a more efficient teacher.

J. N. MARTIN.
AMES, IOWA
Oct. 7, 1918

CONTENTS

INTRODUCTION

CHAPTER PAGB

I. THE NATURE AND SUBDIVISIONS OF BOTANY 1

II. A GENERAL VIEW OF PLANTS 5

PART I
PLANTS (CHIEFLY SEED PLANTS), AS TO STRUCTURES AND FUNCTIONS

III. FLOWERS 9

General characteristics and structure of flowers 9

Some particular forms of flowers 16

Arrangement of flowers or inflorescence 26

IV. PISTILS AND STAMENS 33

Structure and function of pistils and stamens 33

Pollination 46

V. SEEDS AND FRUITS 55

Nature and structure of seeds 55

Resting period, vitality, and longevity of seeds 67

Purity and analysis of seeds 74

Nature and types of fruits of Flowering Plants 77

Dissemination of seeds and fruits 82

VI. GERMINATION OF SEEDS; SEEDLINGS 89

Nature of germination and factors upon which it depends . 89

Germinative processes 93

Testing the germinative capacity of seeds 98

Seedlings 102

VII. CELLS AND TISSUES 112

Structure and function of cells ...» 112

Respiration 121

Cell multiplication 123

General view of tissues 126

vii

yiii CONTENTS

CHAPTER PAGE

VIII. ROOTS .'•'.-• 135

General features of roots 135

Root structure 143

Factors influencing the direction of growth in roots .... 150

The soil as the home of roots 152

Water, air, and parasitic roots * 162

Propagation by roots 163

IX. STEMS 166

Characteristic features and types of stems 166

General structure of stems 166

Structure of monocotyledonous stems 187

Structure of herbaceous dicotyledonous stems 192

Structure of woody stems 197

X. BUDS: GROWTH OF STEMS; PRUNING; PROPAGATION BY STEMS 204

Buds 204

Growth of stems 213

Pruning 221

Propagation by means of stems 225

XI. LEAVES 233

Characteristic features of leaves 233

Primary and secondary leaves 234

General structure of leaves 242

Cellular structure of leaves 246

The manufacture of food by leaves 252

Factors influencing photosynthesis . . 257

Transpiration from plants 260

Respiration 269

Special forms of leaves 270

Uses of the photosynthetic food 273

PART n

PLANTS AS TO KINDS, RELATIONSHIPS, EVOLUTION, AND HEREDITY

XII. INTRODUCTION 289

XIII. THALLOPHYTES 296

Algae (Thallophytes with a food-making pigment) .... 296

General characteristics 296

Blue-green Algae (Cyanophyceae) 297

Green Algae (Chlorophyceae) 301

Brown Algae (Phaeophyceae) 318

Red Algae (Rhodophyceae) 324

Some Alga-like Thallophytes not definitely classified . . 329

CONTENTS IX

CHAPTEB PAGE

XIV. THALLOPHYTES (continued) 336

Myxomycetes and Bacteria (Thallophytes lacking food-
making pigments) 336

Myxomycetes (Slime Molds) 336

Bacteria 341

XV. THALLOPHYTES (concluded) 351

Fungi (Thallophytes lacking food-making pigments) .... 351

Phycomycetes (Alga-like Fungi) 353

Ascomycetes (Sac Fungi) and Lichens 363

Basidiomycetes 382

Fungi Imperfecti (Imperfect Fungi) 404

XVI. BRYOPHYTES (Moss PLANTS) 405

Liverworts and Mosses 405

Liverworts 406

Mosses 417

XVII. PTERIDOPHYTES (FERN PLANTS) 425

Filicales 426

Equisetales (Horsetails) 435

Lycopodiales (Club Mosses) 438

XVIII. Spermatophytes (Seed Plants) 445

Gymnosperms (Seeds not enclosed) 445

Cycads (Cycadaies) 446

Pines (Pinaceae) 451

XIX. Spermatophytes (continued) 459

Angiosperms (seeds enclosed) 459

XX. CLASSIFICATION OF ANGIOSPERMS AND SOME OF THEIR FAMI-
LIES OF MOST ECONOMIC IMPORTANCE 471

Dicotyledons (Apetalae) . 473

Dicotyledons (Polypetalae) 481

Dicotyledons (Sympetalae) 489

Monocotyledons 495

XXI. Ecological classification of plants 500

Nature of Ecology 500

Ecological factors 501

Ecological societies 504

Plant succession 510

XXII. EVOLUTION 513

Meaning and Theories of Evolution 513

Experimental Evolution 524

XXIII. Heredity. 535

General features of Heredity 535

Experimental study of Heredity 537

X CONTENTS

CHAPTER PAGE

XXIV. PLANT BREEDING 557

Selection 557

Mass culture 558

Pedigree culture 560

Selection of Mutants 561

Hybridization 561

Crossing and vigor of offspring 564

BOTANY FOE AGKICULTUKAL STUDENTS
INTRODUCTION

Botany for Agricultural Students

CHAPTER I
THE NATURE OF BOTANY

Botany is a branch of Biology which includes all of the sciences
that deal with living things. Zoology, Bacteriology, Human
Anatomy and Physiology are some other biological sciences that
are familiar and closely related to Botany.

The word botany comes from a Greek word, bosko, meaning,
" I eat.' ' Botany was originally the science of things good to eat,
and in its naming the fact was recognized that plants are the
source of our food. Of course at the present time Botany studies
all kinds of plants which include besides the many useful for
food, many useful as medicine, and many that are poisonous.
Botany is commonly defined as that science which treats of
plants. This definition is not entirely satisfactory because it
does not separate Botany from such agricultural subjects as
Horticulture, Forestry, and Farm Crops which also treat of
plants.

Between Botany and those agricultural subjects which study
plants, there is no sharp division line. Much of the work in
these agricultural subjects is based upon the principles of Botany.
Such features as plant structures, plant functions, and relation
of functions to sunlight, air, soil, etc., which are studied in Botany,
are features of consideration in Horticulture, Forestry, and Farm
Crops. Although Botany and these agricultural subjects study
many plant features in common, the latter subjects differ from
Botany in studying only special groups of plants, and in limiting
the study to the practical and economic phases of plants.

A plant may be studied in a number of different ways. It may
be considered in reference to structure, functions, and in relation
to other plants. Botany is divided into a number of subjects
which consider different phases of plant life.

2 THE NATURE OF BOTANY

MORPHOLOGY considers the form and structure of plants. It
considers the forms of plant bodies and the organs and tissues
which compose them. Morphology studies the structure of
roots, stem, leaves, buds, and flowers, and establishes the rela-
tionships of organs. Morphology not only considers the more
complex plants but also the simpler ones, and traces the develop-
ment of plant structures through the different plant groups. The
phase of Morphology in which the development of the more
complex plants from the simpler ones is studied, is called Plant
Evolution. When Morphology is concerned with the micro-
scopical study of the finer structures of plants, then it is called
Anatomy, and if the study is mainly concerned with the structure
of the cell, then it is called Cytology. Anatomy and Cytology are
often spoken of as Histology. Another phase of Morphology is
Embryology which, as the term suggests, is the study of the
embryo, or the study of the plant during its formation in the seed.

PLANT PHYSIOLOGY studies the functions of plant structures and
the relation of these functions to light, temperature, air, soil, etc.
It treats of how the plant lives, respires, feeds, grows, and re-
produces. In the study of Plant Physiology we learn how plant
food is made and transported, and how plants grow. As a basis
for the study of Plant Physiology, one must have a knowledge
of the Morphology of plants and also a knowledge of Chemistry
and Physics.

PLANT PATHOLOGY treats of plant diseases. In this subject one
learns the disease producing plants and how they affect the plant
diseased. In the study of Plant Pathology, in order to know how
the diseased plant is injured, one must know the nature and
function of the tissues attacked. This means that one should
know Morphology and Plant Physiology. Furthermore, in order
to know how the disease producing form attacks other plants and
propagates itself, one needs to know its Morphology and Physi-
ology.

PLANT ECOLOGY considers plants in relation to the conditions
under which they live. Some plants can live on a dry hill top,
while others can live only in moist, shady places. Some can live
in colder regions than others. Some plants, like many of the
weeds, can thrive when crowded among other plants, while some
like the Corn plant can not. Marshes, bogs, forests, sandbars,
etc., all have their characteristic plants. One set of plants often

SUBJECTS TREATED IN THIS BOOK 3

prepares the way for others. On exposed rocks only very small
plants are able to grow at first, but due to their presence soil
accumulates and larger plants are able to follow. Such problems
as the above are studied in Ecology. Ecology studies plants in
relation to the effects of soil, climate, and friendly, or hostile
animals and plants. It also studies the effect of the different
conditions upon the form and structure of plants.

PLANT GEOGRAPHY is much like Ecology and treats of the dis-
tribution of the different kinds of plants over the earth's surface.

TAXONOMY, or SYSTEMATIC BOTANY, treats of the classification
of plants. As a result of this kind of study, plants have been
arranged in groups, such as Algae, Bacteria, Fungi, Mosses,
Ferns, and Seed Plants. These large groups are further sub-
divided into smaller groups. Keys have been arranged by which
plants unknown to the student may be identified. Through the
study of Systematic Botany one can learn the names and some
of the characteristics of the different kinds of Grasses, weeds,
shrubs, and trees that grow on the farm or in any other region.

ECONOMIC BOTANY treats of the uses of plants to man.

PALEOBOTANY is concerned with the history of plants as shown
by their preserved forms, known as fossils, which occur in the
different layers of rock composing the earth's crust. Paleobotany
is studied in connection with Geology. In the study of this
subject much has been learned about the plants which lived
millions of years ago, and this knowledge is very useful in under-
standing the evolution of the plants which now exist.

Subjects treated in this Book. — To become a master in any
one of the above subjects would require years of one's time. A
study of any of the special subjects of Botany requires a general
knowledge of the anatomy and the functions of plant struc-
tures This means that one must have a general course in
Botany before making a special study of Morphology, Plant
Physiology, or any of the special botanical subjects. • One
purpose of this book is to give a general knowledge of cultivated
plants, of plants not cultivated but like the Rusts and Smuts
related to Agriculture, and of those plants which one must
know in order to understand the evolution of plants. Another
purpose is to give such a general knowledge of plant anatomy
and the functions of plant structures, that one will have the
necessary knowledge for the study of such agricultural subjects

4 THE NATURE OF BOTANY

as Horticulture, Forestry, and Farm Crops, and also a basis for
the study of the special botanical subjects. These special
subjects of Botany are not only very important to one who
makes a special study of Botany, but some phases of Morphology,
Plant Physiology, Plant Pathology, Systematic Botany, and
Ecology are important studies for agricultural students in certain
agricultural courses. Part I, this book, deals mainly with the
parts of plants as to structure and function and, therefore,
emphasizes the Morphology and Physiology of plants. But
structure and function as well as other aspects of plants, accom-
pany and explain each other and can not well be separated in an
elementary study of Botany. So the different phases of the
plant are studied as they occur in relation to each other and
without any designation as to whether or not the fact belongs to
Morphology, Physiology, or any other special phase of Botany.
Part II is devoted chiefly to a study of plants as to kinds, rela-
tionships, evolution, and heredity.

CHAPTER II
A GENERAL VIEW OF PLANTS

Abundance and Distribution of Plants. — Plants are so abun-
dant and generally distributed that there are very few regions
that do not have plants. Plants occur in the water and in the
soil as well as on the surface of the earth. Some plants live in
the bodies of animals. Some are able to live where the tem-
perature is intensely cold, while others can live in hot springs
where the temperature is not far from the boiling point. Even
on rocks that look quite bare, a close examination will show that
some plant forms are present. Only in exceptional places, such
as volcanic regions, some hot springs, and regions of salt deposits,
are plants generally absent.

The abundance or scarcity of plants in a given region depends
upon how well the conditions of the region meet the requirements
for plant growth. If the soil is dry, as in desert regions, the
average number of plants per area is usually quite small, while in
regions where there is sufficient moisture and sufficient mineral
substances, more than 100,000 plants may occur on an area no
larger than an average garden. However, the number of plants
which can occur on a given area, is often very different from the
number that can do well on this same area. Many more grain
plants can be grown per acre than are grown, but agriculturists
have learned that only a limited number of plants per acre can
do well. Among plants, as among animals, there is competition.
Plants must compete with each other for moisture, mineral
substances, and sunlight, and when the competition is too great,
as occurs when plants are too much crowded, some or all of the
plants suffer and fail to produce good yields. By controlling
the amount of seed sown and by properly distributing the seed,
the farmer is able to raise the greatest number of plants per acre
with the least loss from competition among the plants.

Diversity of Plant Forms. — Plants are not only the smallest,
but also the largest of living organisms. Many plants are so

6 A GENERAL VIEW OF PLANTS

small that they can be seen on y with a microscope. Ranging
from these very small plants to the largest trees, plants of all
sizes and complexity occur about us. The different plant forms
differ very much in structure, methods of getting food, and
methods of reproduction. The plants which concern us most
are those which have flowers. They are known as the Flowering
Plants. Most of the cultivated plants and nearly all weeds
belong to this group. They are the plants which furnish nearly
all of our food and fibers and much of our lumber. Part I of
this book is devoted to the study of the Flowering Plants.

Although the Flowering Plants concern us most, it must not be
concluded that the simpler plants are of no importance. The
simpler plants, even the microscopic forms, not only help and
hinder in the cultivation of the Flowering Plants, but affect us in
other ways and must receive consideration. Much of Part II
is devoted to the study of them.

Parts of a Plant. — In plants, as in animals, there is a living
body consisting of parts each of which has a special work to
perform. The various parts of a plant having their own special
work are called organs, and the special work of an organ is its
function. Plants, like animals, being composed of organs, are
called organisms. In the Flowering Plants, the plant body con-
sists of roots, stem, leaves, buds, flowers, seeds, and fruit. All of
these structures are not present at all times, but unless a Flowering
Plant develops all of these organs during its life, its development
is considered incomplete. Through the special functions of its
organs, the plant is able to exist and reproduce itself. The roots
hold the plant to the soil and furnish water a'nd salts; the stem
supports the leaves, flowers, and fruit in the air and sunlight;
the leaves make food; the buds produce new leaves and flowers;
and the flowers, seed, and fruit have to do with the production
of new plants. But each organ is also composed of parts and to
understand an organ one must understand its special groups of
cells, known as tissues, of which the organ is composed.

Life Cycle of Flowering Plants. — A characteristic of living
organisms is their ability to use substances as food, grow, and
develop. Living organisms are also much influenced by their
surroundings. Plants are much influenced by the nature of the
soil, air, sunlight, and plants which grow about them.

To understand a plant one needs to study it in its various

LIFE CYCLE OF FLOWERING PLANTS 7

stages of development. The tiny Corn plant, called embryo or
germ, which we find in the Corn kernel, does not look much like
the plant that bears tassel and ears. From the embryo to the
flower and seed stage, many things take place. The series of
events which take place in the development of the embryo to a
mature plant constitutes the life cycle of a plant. Starting from

FIG. 1. — Life cycle as illustrated by the Corn plant, a, mature kernel;
6, germination; next, seedling; d, mature plant composed of roots, stems,
leaves, and flowers, all of which are composed of tissues having special func-
tions to perform; e, the two kinds of flowers with pollination indicated;
/, fertilization indicated by the two globular bodies, sperm and egg, on the
inside of the ovary or portion that develops into the kernel. After ferti-
lization the ovary develops into another kernel and thus the life cycle is
completed.

the seed, this series of events consists of germination, develop-
ment of seedling with its different organs and tissues, develop-
ment of root, stem bud, and leaf structures of the more mature
plant, development of flowers, pollination and fertilization, and
development of other kernels. The life cycle of any Flowering
Plant is similar to that of the Corn. Thus it is seen that the life

8 A GENERAL VIEW OF PLANTS

cycle of a plant returns us to the place of starting. The series of
events may be represented as shown in Figure 1, and in tracing
them one can begin at any point. The yield of the plant at
maturity depends upon how well it has done at the different
stages in its life cycle. The purpose of cultivation is to help the
plant to do well at all stages, and it is for this reason that we look
after the fertility of the soil, select seed, prepare a seed bed, sow
or plant a certain amount of seed and in a certain way, prevent
the growth of weeds, etc. But often methods of cultivation
must take into account the structure and function of plant
organs as they occur at the different stages in the life cycle of
the plant, and unless the peculiar features of the plant are
understood, the methods employed in cultivation may not be
adapted to secure the best results.

PAET I

PLANTS (CHIEFLY SEED PLANTS) AS TO STRUC-
TURES AND FUNCTIONS

CHAPTER III

FLOWERS
General Characteristics and Structure of Flowers

On account of their colors and odors, flowers very much excel
other plant organs in attracting attention. Everybody is in-
terested in flowers on account of their aesthetic charm, if for no
other reason. The attractive colors and pleasant odors common
to flowers not only interest the scientist but also appeal to the
aesthetic sense of people in general. In fact many people would
define the flower as the showy part of the plant. However
showiness is not an essential feature, for there are many flowers
which have no attractive colors or odors and yet they are just as
genuine in function as are showy flowers. Most forest and shade
trees, the Grasses, and many weeds do not have showy flowers.
The flowers of such plants as the Oaks, Elms, Maples, and Pines
lack showy parts and are so inconspicuous that most people have
not noticed them, yet these flowers are just as genuine in function
as those of a Lily or Rose.

On account of their showiness and importance in reproduction,
flowers were first to receive careful study; and in the early
history of Botany, flowers were about the only plant structures
that received much attention. At the present time there are
some people who have the erroneous notion that the study of
Botany and flowers are still almost identical despite the fact that
the study of flowers is now of no more importance than many
other phases of plant life, as is well shown by the large amount
of space devoted by our present botanical texts to the study of
roots, stems, leaves, and other phases of plants.

In size, flowers may be almost microscopical as in some of the
small floating water plants, such as the Duckweeds, or they may
be of huge dimensions as some tropical flowers which are two or
more feet across. Even in the ordinary greenhouse, some flowers
are so small that they are not conspicuous except in large clusters,

10 FLOWERS

while those of Carnations and Roses are conspicuous when single.
In Chrysanthemums, Daisies, and Sunflowers the individual
flowers, although small, form a cluster so compact that it is often
erroneously considered a single flower.

As to color, which is the character most closely related to
securing pollination by insects, flowers are exceedingly various.
Some, especially those that depend upon the wind for pollination,
are green like leaves. Some are white, while among others nearly
every color imaginable can be found. It is claimed that by means
of colors flowers solicit the visitation of insects, which are im-
portant agents in pollination.

The odors of flowers, usually pleasant, but sometimes repul-
sive to us, as in case of the
Carrion-flower and Skunk
Cabbage, probably serve in
attracting insects. Further-
more, pleasant odors add

P il^iil Mj^Z''' to the value of plants for

ornamental purposes.

Flowers present various
•p forms. When well open,

FIG. 2. - Basswood flower with portions some are wheel-shaped,
removed from one side so that the interior some funnel-shaped, some
of the flower may be seen, a, calyx com- tubular, while others de-
posed of leaf-like portions or sepals; o, part from these forms with
corolla composed of leaf-like portions called varioug ; lariti ag in

petals; s, stamens; p, pistil; r, receptacle. .

Much enlarged. the Sweet Pea> where the

flower resembles a butter-
fly in shape, or in the Orchids where parts of the flower may be
so shaped as to resemble a slipper, as the Orchid known as the
Lady's-slipper illustrates. The shape of the flower in many cases
favors the visitation of only special insects, and, therefore, is
closely related to the problem of pollination.

To discover the essential features of a flower, it becomes
necessary to determine the function of the flower, and become
acquainted with its parts and the use of each part in relation to
the work of the flower.

Function of the Flower. — The flower is the plant's principal
organ of reproduction, being devoted to the production of seed
which is the plant's principal device for producing new plants.

PARTS OF THE FLOWER 11

Functionally, the flower may be defined as the organ which has
to do with seed production. Flowers which have been so modi-
fied through cultivation that they no longer produce seed are not
true flowers. However, the true function of the flower is often
not the important feature to the plant grower. Many flowers
are cultivated entirely for their aesthetic charm. In case of
fruit trees, Tomatoes, and many other plants, the structure
developing from the flower and
known as the fruit is more
important to the plant grower
than the seed. However, when
plants are grown for seed or
fruit, the amount of seed or fruit
harvested depends very much

upon the number of flowers pro- / ^ / llf^V^\
duced. For example, the gar- ^^""^^ II * ^^. \.
dener does not expect to gather
many Beans or Peas if the FIG. 3. -Apetalous flower of Buck-
, , P a wheat, c, calyx; s, stamens: p. pistil;

vines produce only a few flowers. r> receptac'le. Much enlarged. After
Likewise, good crops of Clover Marchand.
and Alfalfa seed depend upon a

good crop of flowers; and not much fruit is expected when the
flowers in the orchard are few. It is in connection with the
function of reproduction, that flowers have developed the various
colors, forms, and odors which assist in bringing about fertiliza-
tion, the central feature of sexual reproduction to which the
flower is devoted, and the process upon which the development
of seed usually depends.

Despite the multitudinous forms and colors which flowers
present; there is much unity and simplicity in structure, all parts
being organized to assist in performing the function of seed
production.

Parts of the Flower. — The parts of a flower are of two general
kinds; those which are directly concerned in the production of
seed; and those which act as protective and attractive organs.
The former are known as the essential organs, and consist of
stamens and pistils. The latter are known as floral envelopes
or perianth, and usually consist of two sets of organs, one called
calyx and the other, corolla. In Figure 2, the calyx is the lowest
whorl and consists of green leaf-like portions called sepals. The

FLOWERS

second whorl is the corolla and each separate portion is a petal.
The pistil occupies the central position and is surrounded by the
whorl of stamens. The end of the flower stem to which these

FIG.' 4. — A flower of Tobacco. c,the FIG. 5. — Flower of Red Clover,

funnel-shaped corolla made up of united c, corolla; 6, cup-like calyx. Much

petals; 6, calyx. The sepals are also enlarged. After Hayden.
united below. Reduced.

floral parts are attached is called torus or receptacle. The
receptacle may be flat, conical, or cup-shaped, and often forms

FIG. 6. — The two unisexual flowers of the Pumpkin with a portion of
the bell-shaped corollas torn away to show the interior of the flowers.

A, staminate flower; s, stamens fitting together, forming a column. B,
pistillate flower. Less than half natural size.

an important part of the fruit. The corolla is usually bright
colored, and, therefore, the conspicuous part of the flower. It is
also the fleeting part of the flower, usually lasting only a few days.

UNISEXUAL FLOWERS

P-

FIG. 7. — Section

Flowers having the four sets of organs, as shown in Figure 2,
are called complete flowers to distinguish them from incomplete
flowers, that is, flowers in which some of the
organs are lacking. The organs are gener-
ally arranged in a circular fashion around
the receptacle, and are characterized as be-
ing in cycles or whorls. In some flowers a
part or all of the perianth is lacking. In
the Buckwheat, as shown in Figure 3, only
one whorl surrounds the stamens and pistil,
and it is evident that this flower does not
have both calyx and corolla. In such cases,
the petals are considered missing and the
flower is said to be apetalous (" without

petals")- Often instead of being composed

c ,• -i , i / i - i \ through a flower of the

of entirely separate petals (polypetalous), Peac£ There ig but

the corolla is a tube or funnel-shaped struc- One pistil (p), but many
ture, which appears to be composed of united stamens (s) . Much en-
petals (gamopetalous) , separate only at the lar£ed-
top. (Fig. 4-) The flowers of the Tobacco Plant, .Pumpkins,

Squashes, and Water-
melons are examples of
gamopetalous flowers.
In some cases, as in the
Tobacco, Clover, and
some other plants, the
sepals seem to have
joined into one structure
(gamosepalous), forming
a tube- or cup-like calyx.
(Fig. 4 and 5.) Flowers
also differ in the essential
organs contained.
FIG. 8. — Section through an Apple flower Unisexual Flowers. —
showing the compound pistil composed of five Flowers having both sta-
carpels. The five carpels (a) are free above mens and pistils are
but joined below, c, corolla; s, stamens; i, known as perfect Or bisex-
calyx. Much enlarged. 7 n T

ual flowers. In some

plants, the stamens and pistils occur in different flowers, in which
case the flower having stamens only is called a staminate flower,

FLOWERS

while the other having pistils only is called a pistillate flower. Such
flowers are said to be unisexual. Pumpkins, Cucumbers, Corn,
Hemp, Willows, and Poplars are some of the familiar plants
which have unisexual flowers. In Figure 6 are shown the uni-
sexual flowers of the Pumpkin. In some cases, as in Corn,
Cucumbers, and Pumpkins, both staminate and pistillate flowers

are borne on the same plant.
Such plants are said to be monce-
cious (meaning " of one house-
hold "). In other cases, as in
Hemp, Willows, and Poplars,
the staminate and pistillate
flowers are borne on different in-
dividuals, that is, one plant has
FIG. 9.— Section through the flower OI^Y staminate while another has
of Cotton, s, stamens joined into a only pistillate flowers. Such
tube which surrounds the pistil; p, plants are said to be dioecious
pistil composed of carpels more united (meaning « of two households ") .
than those of the Apple. Smaller p.. ... , 0, .

i • * *r ™ MI Pistils and Stamens. — As

than natural size. After Baillon.

everyone knows, the pistils are

the organs in which fertilization occurs and seed is produced, while
the stamens furnish the pollen, which is essential for fertilization.
Flowers usually have more stamens than pistils, but the number

FIG. 10. — A flower of a Legume with petals removed to show the dia-r
delphous stamens, a, free stamen; 6, tube formed by the joining of the
other stamens.

of each varies much in the flowers of different plants. Some
flowers, as those of the Strawberry, have numerous stamens and
pistils, while in some flowers, as in the Peach or Plum, there is
only one pistil, but many stamens. (Fig. 7.) The Apple flower,
which has many stamens, really has five pistils, but the lower
parts of the pistils are joined, leaving only the upper parts free,

PISTILS AND STAMENS 15

A pistil like that of the Apple is called a compound pistil,
and the pistil-like structures which compose it, instead of being
called pistils, are called carpels. Thus in Figure 8, each of the
branches in the upper region of the pistil is the upper portion of
a carpel. If the enlarged bases of these were separated, then
each carpel would resemble the pistil of the Cherry or Plum
flower. Pistils like those of the Cherry and Plum consist of only
one carpel and are, therefore, called simple pistils. In flowers
having but one carpel,, pistil and carpel mean the same thing. The
flower of the Cotton Plant, shown in Figure 9, has a compound
pistil in which the carpels are more united than in the Apple.

In most flowers the stamens are separate from one another
(polyadelphous), but in some groups of plants they are more or

FIG. 11. — A, hypogynous flower of Pink; B, perigynous flower of Cherry;
C, epigynous flower of Wild Carrot. Modified from Warming.

less united (monadelphous) . In Cotton and other plants of this
group, the stamens are joined in such a way as to form a tube
around the pistil. (Fig. 9.) In Clover, Alfalfa, and some other
plants of this family, the ten stamens form two groups (diadel-
phous), nine being joined and one remaining free.

The relative positions of the different parts of the flower show
considerable variation. In some flowers, as those of the Dande-
lion or Sunflower illustrate, the calyx, corolla, and stamens arise
from the top of the ovary. (Fig. 2 4.) Such flowers are epigy-
nous, i.e., the floral structures are on the gynous the word
" gynous " referring to the ovary, which in this case is described
as inferior. In the Basswood flower, calyx, corolla, and stamens
are attached to the receptacle at the base of the ovary, which is

FLOWERS

described as superior. Such
flowers are hypogynous. In
some flowers, as in the Peach
shown in Figure 7, the calyx,
corolla, and stamens are at-
tached to the rim of a cup-
like structure surrounding the
ovary. In this case the flower
is perigynous, and the ovary
is described as half inferior. To
which of the above classes does
the Apple flower belong? In
Figure 11 the three positions
of the perianth and stamens in
reference to the ovary are shown
for comparison.

Some Particular Forms of
Flowers

That there are numerous
differences among flowers is
shown by the fact that largely
upon differences pertaining to
flowers, the Flowering Plants
have been divided into many
classes, such as orders, which in
turn are subdivided into fami-
lies, then into genera, and
finally into species of which
there are more than 100,000.
The differences are mainly struc-
tural, and between flowers of
FIG 12. -Corn _ plant t, tassel d;fferent famUies ^ are <rften

consisting of staminate flowers; e,

ears on which the pistillate flowers qulte prominent. For example,
are found. when such flowers as those of the

Grass, Bean, Sunflower, and

Orchid family are compared, that there are peculiar differences
in the character of flowers is obvious.

Grass Flowers. — One of the characteristic features of the
Grass flowers is, that there are no showy organs. Grass flowers

CORN FLOWERS

are usually green like leaves, and their stamens and pistils are
enclosed and protected by small leaf-like bodies called bracts,
which take the place of a calyx and corolla. Although quite
inconspicuous, yet in being characteristic of such Grasses
as Corn, Wheat, Oats, Barley, Rye, Rice,
and Timothy, Grass flowers are so im-
portant that they deserve some special
attention.

Corn Flowers. — As already stated (page
14) Corn flowers are unisexual. The stami-
nate flowers are produced in the tassel,
while the pistillate flowers occur on the ear.
(Fig. 12.)

The staminate flowers bear three stamens
and occur in groups of twos, called spikelets.
The branches of the tassel upon which the
spikelets are crowded are known as spikes.
In Figure 18 is shown a spike or branch of
the Corn tassel so drawn as to show the
spikelets.

The two flowers of each spikelet are in such
close contact, that in order to identify each Corn tassel, sp,
flower, the bracts must be spread apart as spikelets. Only three
shown in Figure 14- In the older flower, the of tne spikelets are
stamens have elongated and pushed out of
the bracts. The boat-shaped bracts are so
fitted together as to make a good enclosure for the stamens
during their development. The two outer bracts, situated
on opposite sides of the spikelet and facing each other, so as
to close together and enclose the flowers, are known as glumes.
Between each glume and set of stamens is the bract called lemma.
The bract on the opposite side of the stamens, with its concave
side turned toward that of the lemma, is known as the palea.
The palea and lemma, when closed against each other, enclose
the stamens. The small bodies at the base of the stamens are
called lodicules, and may, by their swelling, spread the bracts
apart, thus helping the stamens to escape from their enclosure.
The structure of the flower will be more easily understood by a
study of Figure 14. The glume is not considered a part of the
flower. The two glumes form a covering for the spikelet.

FIG. 13. — A branch
spike from the

Sligh%

FLOWERS

Other names are often applied to the glume and lemma. In
courses in Agriculture, the glume is often called outer or empty
glume and the lemma, the flowering glume.

The pistillate flowers are arranged on a cob and enclosed by
husks, so that only the outer ends or silks of the pistils are

FIG. 14. — A spikelet from the Corn tassel. Much enlarged to show the
two staminate flowers

The flowers are numbered (1} and (#), No. 1 being more mature, e, glumes;
/, lemma; p, palea; s, stamens; I, lodicules.

exposed. When the husks are removed, the flowers are seen
arranged on the cob just as the kernels are in the mature ear, for
each kernel develops from a flower. Explain what is shown in
Figure 15. The pistillate flowers occur in groups of two's or
spikelets, but only one flower of the » spikelet completes its
development. The flower which remains rudimentary develops
no silk and remains so inconspicuous that one needs a magnifier
to see it. Since it has no pistil, its presence is known only by its
bracts. In Figure 16, point out the rudimentary flower and the
one that develops.

CORN FLOWERS

-t

FIG. 15. — Lengthwise section
through the end of a young ear
of Corn, showing the spikelets
containing the pistillate flowers.
h, husk; s, silks of the pistils; 6,
enlarged bases of the pistils en-
closed by bracts; c, cob. Slightly
enlarged.

FIG. 16. — A spikelet from a young
ear of Corn to show the two pistil-
late flowers. I, the bracts of the
flower that develops no pistil. The
other bracts belong to the flower
having the pistil, r, ovary which
becomes the kernel; t, style of the
silk; s, the branched stigma; e,
glumes; /, lemmas; pa, paleas.
The lodicules are very small and are
not shown. Very much enlarged.

FLOWERS

A study of Figure 16 shows that the base of the pistil is sur-
rounded by bracts, corresponding to those surrounding the
stamens in the staminate flowers. The bracts of the pistillate
flowers are small, membranous, and form the chaff of the cob.

Oat Flower. — A head of
Oats, as shown in Figure 17, is
much branched and the spike-
lets occur at the ends of the
branches. Each spikelet con-
sists of two or more flowers,
which are well enclosed by the
two glumes. When the glumes
are spread apart as shown in
Figure 18, it is seen that the
flowers are attached, one above
another, to a small slender axis.
This axis is known as the ra-
chilla. Rachilla means small
rachis." Rachis is the name
applied to the main axis of
the Oat head from which the
branches arise. The small
branches bearing the spike-
lets at their ends are called
pedicels. Thus branches arise
from the rachis and end in
the rachilla to which the
flowers of the spikelets are
attached.

The spikelet shown in Figure 18 contains three flowers, but
the upper one is rudimentary and, therefore, produces no grain.
There is one very important difference between the flowers of
Oats and those of Corn. In Corn the pistils and stamens occur in
different flowers, but in Oats the stamens and pistils occur to-
gether in the same flower. The Oat flower is, therefore, a perfect
or bisexual flower. In each Oat flower there is one pistil and
three stamens enclosed by the lemma and palea. The lodicules,
which are two small scale-like bracts at the base of the pistil and
stamens, are not easily seen in the Oat flower. The two glumes
of the Oat spikelet are so large that when closed together they
ti

FIG. 17. — Head or panicle of the
Oat plant, s, spikelets; 6, branches;
r, rachis; p, pedicels*. About one-half
natural size.

OAT FLOWER

-I

FIG. 18. — Spikelet of the Oat head, with the bracts spread apart to show
the flowers. There are three flowers, only (1 ) and (2} of which develop and
produce kernels, e, glumes or empty glumes; /, lemma or flowering glume;
pa, palea; s, stamens; p, pistil; r, rachilla. The parts of flowers (2) and
(3) are not indicated. Many times enlarged.

FIG. 19. — Two views of a head of Wheat with some spikelets removed
to show the zig-zag rachis. An edge view of the spikelets is shown at the
left and a side view at the right, r, rachis; s, spikelets.

22 FLOWERS

almost completely enclose the flowers of the spikelet. In thresh-
ing most varieties of Oats, only the glumes are removed, the
kernel still remaining enclosed by the lemma and palea, which
form the covering known as the hull of the grain. A grain of
Oats, therefore consists of the kernel and its hull; and the
quality of Oats depends much upon the proportion of hull to
kernel. As indicated in Figure 18, the lower flower grows

FIG. 20. — Spikelet of Wheat much enlarged and shown with the bracts
spread apart, so that parts of the flower may be seen. The flowers are num-
bered and the parts of one flower are labelled, e, outer glumes; /, lemma;
pa, palea; p, pistil; s, stamens; I, lodicule; a, awn or beard; r, rachis.

more rapidly than the others and forms the larger kernel to
which the smaller one sometimes remains attached after
threshing.

Wheat Flowers. — In Wheat the head, usually called spike,
consists of many spikelets arranged in two rows along the zig-zag
axis of the head. (Fig. 19.) This zig-zag axis is the rachis of
the spike. The spikelets are not borne at the ends of branches

FLOWERS OF THE LEGUMES OR BEAN FAMILY

..a

as in Oats, but are directly attached to the rachis. This feature
distinguishes the spike from the branching head, called panicle,
of the Oats. In the varieties of common Wheat, each spikelet
contains three or more flowers arranged one above another on the
rachilla, and one or more of the upper flowers are rudimentary.
Each fully developed flower, just as in Oats, consists of three
stamens and a pistil enclosed by the lemma
and palea. The lodicules, like those of the
Oat flower, are small inconspicuous scales at
the base of pistil and stamens. In Wheat,
where the spikelets are broad, the spikelet is
only partly enclosed by the glumes. In thresh-
ing Wheat the kernel is separated from the
bracts — the latter being blown away as chaff.

A study of the
spikelet shown in
Figure 20 will aid
the student in un-
derstanding the
structure of
Wheat flo.wers
and their arrange-
ment in the spike-
let.

Flowers of the
Legumes or Bean
Family. — The

fl o w e r s of the

FIG. 22. — End view of an un-
tripped and tripped flower of Red
Clover.

b, flower untripped. a, stand- Peas, Clover, Al-
ard; w, wings; k, keel, d, flower falfa, and Vetch
tripped, in which case the keel and are faminar representatives have a
wings are bent down, exposing the h f npollijar features The

• i-i / \ j j. / \ iv/r i- IlUIIlUtJl Ul UcLU.llctI IcoiLliltJo. -L lie

pistil (p) and stamens (s). Much

enlarged. After C. M. King. one most prominent among the

cultivated ones of the family is the

irregularity in the shape of the parts of the perianth, as the
flowers of Peas or Red Clover illustrate. The calyx is a shallow
five-toothed cup. The corolla is composed of four pieces; the
large expanded portion at the back, known as the standard or

-ca

FIG. 21. — Flower
of Red Clover, ca,
calyx; co, corolla;
a, standard; w,

Bean Family of wings; k, keel.

which Beans,

Many

times en-
After C.
. King.

FLOWERS

banner; the two side pieces, known as wings; and the single
boat-shaped portion beneath the wings, known as the keel. In
the Red Clover flower shown in Figure 21 , these parts are pointed
out. The stamens and pistil are entirely enclosed by the keel,
and when pressure is exerted on the keel, the stamens and pistil
spring out of their enclosure with considerable force. (Fig. 22.)

B <-

FIG. 23. — Flowers of the Yarrow (Achillea
millefolium), a Composite. A, a head of
flowers sectioned, showing the strap-shaped
flowers around the margin and the tubular
flowers occupying the central region of the
head. B and C are tubular' and strap-
shaped flowers more enlarged

FIG. 24. — A, flower from
the head of Dandelion, a,
strap-shaped corolla; 6, calyx
made up of many slender hairs
known as pappus; p, base of
pistil; s, stamens forming a
tube around the upper part of
the pistil. B, tubular flower
and fruit of Beggar's Tick
showing tubular corolla (a) and
the calyx (6) consisting of two
spiny teeth which persist and
aid in scattering the fruit.

This process of releasing the stamens and pistil, known as
11 tripping the flower," is mainly the work of insects and is im-
portant, because in some of the Legumes the flowers will produce
no seed unless tripped.

Composite Flowers. — There is a large group of plants to
which Lettuce, Dandelions, Sunflowers, Beggar's Tick, Thistles,

COMPOSITE FLOWERS

and many other plants belong, that have their many small flowers
grouped in a compact head as shown at A in Figure 23. This
group of plants is called Composites, and includes some of our
useful plants as well as some of the most troublesome weeds.

FIG. 25. — A cluster of Lady's-slippers.

Both calyx and corolla are somewhat peculiar. In some cases,
as in the Sunflower, the flowers occupying the center of the head
have tube-like corollas and are called tubular flowers, while those
around the margin have strap-shaped and much more showy
corollas, and are called ligulate flowers. See A, B, and C of
Figure 23. In some of the Composites, as in the Dandelion, all
of the flowers of the head are ligulate, while in some, like the
Thistle, all the flowers are tubular. The calyx is often composed

FLOWERS

of hair-like structures called pappus, as shown in Figure 24. In
some, as the Dandelion illustrates, the pappus remains after the
seed is mature, forming a parachute-like arrangement which
assists in floating the seed about. In some of the Composites,
the calyx consists of a few teeth, which in the Spanish Needles

and Beggar's Tick, become spiny, and
thereby assist in seed distribution by
catching onto passing objects.

Orchid Flowers. — It is among
Orchid flowers, many of which are
spectacular, that the most notable
irregu'arities occur. Besides the dis-
tinguishing feature of having the
stamens and pistil joined into one
body, known as the column, Orchid
flowers often have pronounced varia-
tions in the shape and size of petals.
In some, as in the Lady's-slipper, one
of the petals is developed into a great
sac or " slipper," while the others
have no extraordinary features. These
peculiarities in flower structure, which
Turnip (Arisoema triphyllum). are apparently adjustments for insect
The flowers shown are pistil- pollination, sometimes so closely con-
late and are clustered at the r ,, , ji_i_-.r
base of the fleshy axis or form to the shaPe and hablt of cer-
spadix which is enclosed in the tain insects that only one or a few
large leaf-like bract or spathe. kinds of insects can pollinate a flower.
Reduced about one-half. guch highly modified flowers contrast

strikingly with the simple, inconspicuous flowers of such plants
as the Jack-in-the-pulpit or Indian Turnip and Skunk Cabbage,
in which a perianth is either lacking or inconspicuous and the
flowers are crowded on a fleshy spike, known as a spadix, which
is enclosed in or attended by a leaf, called spathe. The spathe,
by becoming colored, often aids like a corolla in attracting
insects. (Figs. 25 and 26.}

Arrangement of Flowers or Inflorescence

The arrangement of flowers on the stem is one of the floral
characters much used in the classification of the Flowering Plants.
In the arrangement of flowers, a number of things are considered,

FIG. 26. —The
ous flowers of

uiconspicu-
the Indian

ARRANGEMENT OF FLOWERS OR INFLORESCENCE 27

the principal ones being: (1) the position of the flower on the
stem, whether terminal or lateral; (2) whether the flowers are
single or in clusters; (3) whether the terminal or lateral flowers
of a cluster open first; and (4)
the character of the cluster in
regard to shape and compact-
ness, which depend upon the
elongation of the stem region
bearing the flowers and the length
of the individual flower stalks.
These features taken singly, to-
gether, and along with some
minor features form the basis
upon which floral arrangements
are classified.

Flowers develop from buds
and buds are either terminal or
•lateral on the stem. So as to
position, flowers are either ter-
minal or lateral on the flower axis. FlG- 27- — Solitary terminal flower
Flowers borne singly are called

of a Lily. After Andrews.

solitary flowers, and solitary flowers may be terminal, as in some

FIG. 28. — A portion of a Squash plant showing the axillary arrangement
of flowers. Much reduced.

Lilies of which the Tulip is an example, or lateral, as Squashes
illustrate. (Figs. 27 and 28.)

The flower cluster may be regarded as a modification of that
lateral arrangement, in which the flowers are scattered on a fully

FLOWERS

elongated stem bearing normally developed leaves in the axils
of which the flowers occur. Thus, if a Pumpkin or Gourd vine
should remain short, the flowers instead of being well separated
as they normally are, would be crowded, and, with the reduction
of leaves to bracts, a typical flower cluster would result. Most
small flowers are produced in clusters. For small flowers polli-
nated by insects, there is considerable advantage in the cluster

habit, since the cluster, being much
more conspicuous than the individual
flowers, serves well as an attractive
device.

Flower clusters are divided into
two main classes according to their
method of development. In the
corymbose or indeterminate cluster,
growth at the tip and the develop-
ment of new flowers just behind
continues throughout a considerable
period, thus producing a cluster in
which the older flowers are left
farther and farther behind. As the
term indeterminate suggests, such a
method of development permits a
rather indefinite expansion of the
cluster. In the cymose or determi-
nate cluster, the oldest flower is
formed at the tip, which is thereby
closed to further growth, and the
new flowers are formed from buds
developing lower down. Such a
cluster is much limited in its power
to expand. The flower clusters of Apples and Pears, known as
cymes, illustrate the determinate type of cluster.

The simplest form of the indeterminate cluster is the raceme,
an unbranched cluster in which the flowers are borne on short
stalks. The racemes of the Shepherd's-purse, Radish, Cabbage,
and others of the Mustard family, in which the flower cluster
may continue its expansion for a long period, producing new
flowers at the tip while pods are maturing at the base, well
illustrate the nature of the raceme. (Fig. 29.} The racemes of

FIG. 29. — Raceme of Com-
mon Cabbage (Brassica). From
Warming.

ARRANGEMENT OF FLOWERS OR INFLORESCENCE 29

the Snap-dragon, Sweet Clover, and Alfalfa are examples of
racemes with a short growth period. Racemes may be terminal
or lateral, as in case of Sweet Clover.

FIG. 30. — A, spike of Rye. B, panicle of Grass. C, flowers of the Hazel
with staminate flowers in catkins and the pistillate flowers borne singly.

FIG. 31. — A, head of Clover. B, close head of Yellow Daisy.

Raceme-like clusters in which the flowers have very snort
stalks or none at all are called spikes of which the heads of Wheat
and Timothy are familiar examples. A special form of the spike

30 FLOWERS

is the catkin in which the flowers, unisexual in typical cases,
usually have scaly bracts instead of a true perianth, and the
whole cluster falls after fruiting. Catkins are typical of Poplars,
Willows, Hickories, and Birches. When the raceme is so short
that the compact mass of flowers form a more or less rounded
cluster as in Red Clover, then a head is formed. In the Composites
there is the special kind of head which is the most highly organ-
ized of all flower clusters. The flowers besides often being differ-
entiated into two kinds are so compactly arranged as to form a
cluster resembling a single flower and the cluster is surrounded by
bracts, which form a structure known as the involucre. (Fig. 31.)

FIG. 32. — A, Corymb of one of the Cherries. B, umbel of a species

of Onion.

In contrast to the spike there are those raceme-like clusters
in which the flowers have long stalks, as in the typical panicle,
where the cluster is loosely branched. When the portion of stem
to which the flowers are attached is short and the stalks of all of
the flowers are so elongated as to bring all of the flowers to about
the same level then a corymb results. A further modification in
which the portion of stem to which the flowers are attached is so
short that the flower stalks appear to be of the same length and
attached in a circle around the stem results in the umbel, the form
of cluster characteristic of the Parsley Family, called Umbellif-
erce, on account of the character of the flower cluster. Of this
family the Parsnips, Carrots, and others are common. The um-
bel is also common among the Milkweeds. Umbels may be
simple or compound, that is, so branched as to be composed of
a number of small umbels. (Fig. 32.}

!* •

ARRANGEMENT OF FLOWERS OR INFLORESCENCE 31

FIG. 33. — A, cyme of the Apple. B, thyrse of the Lilac.

FLOWERS

In complex flower clusters combinations of the simpler types
of clusters often occur together. Thus, in the thyrse, the complex
cluster which is typical of the Lilac and Horse-Chestnut, and, in

FIG. 34. — Upper diagrams show types of indeterminate inflorescences,
o, raceme; 6, corymb; c, compound corymb; d, umbel; e, spike; /, panicle;
g, head.

Lower diagrams show types of determinate inflorescences; h, cyme half
developed (scorpioid); i, flat-topped or corymbose cyme; ,;', typical cyme.

the panicle of the Grasses, the characteristics of both racemes and
cymes are present. (Fig. 33.)

The diagrams in Figure 34 show the common types of flower
arrangements.

CHAPTER IV
PISTILS AND STAMENS

Structure and Function of Pistils and Stamens

The pistils and stamens are the organs upon which the pro-
duction of seed depends and for this reason are called the
essential parts of the flower. The calyx and corolla protect the
essential organs and often assist in seed production, but they
are not essential.

In unisexual flowers, seeds appear only in the flowers having
pistils. The staminate flowers in the Corn tassel produce no
kernels, and in dioecious plants,
such as Hemp, Willows, and the
Mulberry, seed and fruit are
limited to those individuals bearing
pistillate flowers. From this it
might appear that the stamens
take no part in the work of pro-
ducing the seed; but observations
show that unless stamens are close
at hand, the pistil will produce
no seed. A well isolated Corn
plant with tassel removed before FIG. 35. — Flower of the Cherry
the stamens are mature will pro- with Parts of fche Pistil indicated.

duce no kernels. Some varieties °> °7ary1; *' stigma; «' style*

f 0, , ,. , Much enlarged,

of Strawberries are dioecious, and

unless both kinds of plants are grown in the same bed, there will
be no seed or fruit.

To understand just how the essential organs function in seed
production, a careful study of their parts must be made.

Parts of the Pistil. — The pistil usually consists of three parts:
the enlarged base which is the ovary and the portion in which the
seeds develop; the flattened or expanded surface at the upper
extremity, known as the stigma; and the stalk-like part connect-

PISTILS AND STAMENS

ing the ovary and stigma, known as the style. In the pistil of the
Cherry shown in Figure 85 the parts are indicated. The ovary is
at o. The stigma is the expanded surface at st. The style is at s
and is a stalk-like structure projecting from
the ovary and supporting the stigma.

In the Corn the style is extremely long and
the stigma branched. (Fig. 36.) In Wheat,
Oats, Barley, and Rice there are two very
short styles and the stigmas are much
branched and plume-like. (Fig. 87.) Styles
and stigmas vary much among plants.

Ovary. — The ovary is the most impor-
tant part of the pistil because within it the
seeds are produced, and often it makes the
edible portion of fruits.

— st

FIG. 36. — Pistillate
flower of Corn, drawn
to show the parts of
the pistil. A portion
of the bracts have
been cut away to give
a view of the ovary,
o, ovary, the portion
that becomes the ker-
nel; s, style; st, stigma.
Much enlarged.

FIG. 37. — Pistil of
Wheat and the two
lodicules. o, ovary; st,
stigmas; s, styles; Z,
lodicules. Much en-
larged.

FIG. 38. — Cross section
of the ovary of a Tomato.
o, ovary wall; b, partition
walls of the ovary; c, locules
or cavities in the ovary; d,
ovules; p, placentas or parts
of the ovary to which the
ovules are attached. Much
enlarged.

When the 'ovary is sectioned so that its interior may be studied,
it is seen that it is not a solid body, but consists of a wall
enclosing one or more cavities, called locules. (Fig. 38.) In
these cavities or locules are the small bodies called ovules, each
of which is capable of developing into a seed. Point out the
parts of the ovary shown in Figure 38.

The ovary may contain one locule or many and the number of
ovules in a locule also varies in different ovaries. In Beans and

OVARY

FIG. 39. — Flower and pod of the Garden Pea. A, section through the
flower to show ovules, a, ovary; o, ovules; 6, stamens; t, stigma; s, style.
B, the matured ovary, called pod, opened to show the matured ovules or
seeds (e). Flower enlarged but pod less than natural size.

FIG. 40. — A, pistil of Red Clover with one side of ovary cut away so that
the ovules (o) may be seen, a, stigma; s, style. B, lengthwise section
through the ovary and ovules of Red Clover and very much enlarged to
show the parts of the young ovules, w, ovary wall; o, ovules; s, base of
style; st, stalk or funiculus of the ovules; n, nucellus; i, integuments.

PISTILS AND STAMENS

Peas, the ovary has one locule enclosing a number of ovules. In
A of Figure 39, showing a lengthwise section through the flower
of the Pea, one side of the ovary wall is removed to show the
locule with its ovules. In this particular flower of the Pea, there
are six ovules, but other flowers might have more or fewer. In
B of Figure 39 is shown the ovary after it becomes a mature pod.
The pod is opened to show the seeds. Each
/> seed is a developed ovule and the pod enclos-

ing the seeds is the ovary wall much enlarged.
Notice how the ovules and seeds compare in
number.

In Red Clover, shown in Figure 40, there is
one locule and two ovules. The ovaries of
Alfalfa have only one locule, but may have as
many as eighteen ovules.

In the ovary of Corn, Wheat, Oats, and
Grasses in general, there is one locule and a

FIG. 41. — Length-
wise section through
a young pistil of
Corn to show the
locule and ovule,
a, ovary; s, style;
o, ovule consisting
of nucellus (n) and
integuments (i); I,
locule or cavity in
which the ovule is
located. Much en-
larged.

•FiG. 42. — Lengthwise section through a Tomato
flower to show the interior of the ovary, a, ovary;
I, locules, represented by dark shading; o, ovules;
p, placentas. Much enlarged.

single large ovule. A lengthwise section through the pistil of
Corn is shown in Figure 41- Notice the ovule at o and that it
almost fills the locule.

Tomato ovaries have few or many locules which contain a large
number of ovules. Figure 4@ shows a lengthwise section of a
Tomato ovary showing two locules and many ovules. By count-

SIZE OF OVULES 37

ing the ovules shown in Figure J^2 and those shown in Figure 38
the number of ovules in a Tomato may be roughly estimated.

An examination of the ovaries of many plants would show
considerable variation in the number of locules and ovules, but
in general, all ovaries consist of an ovary wall enclosing one or
more locules which contain one or more ovules.

Ovule. — Since ovules develop into seeds, they have the most
to do with seed production and are, therefore, the most directly
related to the function of the flower. The process of fertilization,
one of the most important events in plant life, takes place in the
ovule and a good understanding of fertilization requires a knowl-
edge of the ovule.

Size of Ovules and how their Number Compares with the Num-
ber of Seeds. — Although ovules are the chief structures in per-
forming the function of seed-production, in size they are usually
very inconspicuous and not much can be learned
about them without the aid of the microscope. a

In many plants the ovules are barely visible to [

the unaided eye. When ovaries and ovules are @

shown in drawings, they are usually much en-
larged, so that much more is shown than could , , ,IG' ' ~

of the Tomato taken
be seen by cutting sections and studying the from the flower and

ovaries themselves, unless a microscope were drawn natural size.
used. In Figure 4$, the pistil of the Tomato is
shown natural size. By comparing it with the pistil shown in
Figure 4@, it will be seen that in order to show the structures of
the ovary, the pistil in the latter Figure is much enlarged.

Since ovules are small, it is difficult to count them in ovaries
where they are numerous. It is possible in many cases to make
a rough estimate of the number of ovules by counting the seeds
produced. Since each seed is a developed ovule, there must
occur in the young ovary as many ovules as there are seeds in the
mature ovary. From this it follows that those Tomatoes con-
taining two hundred or more seeds must have had as many ovules
in their young ovaries.

If all the ovules became seeds then a count of the seeds would
give the exact number of ovules ; but in many cases, due to a lack
of fertilization, space, or sufficient food supply, only a part of the
ovules complete their development and become seeds. In Red
Clover, as shown in Figure 40, there are two ovules, but when the

PISTILS AND STAMENS

mature pod is threshed, only one seed is found. In Alfalfa only
about one third of the ovules produce seed. In the Apple, Pear,
Tomato, and other fruits some of the ovules often fail to develop,
and in case of seedless fruits none of the ovules complete their
development. In most fruits the production of seed is not an
important feature to the plant grower, the seedless fruit in many
cases being more desirable; but in case of Clover, Alfalfa, Flax,
and other plants valuable for seed, the value of the plant as a seed
producer is directly related to the number of ovules which be-

Ji B

FIG. 44. — Surface view of an ovule
at two stages of development. A,
stage of development showing the
integuments (a, 6) growing up over
the nucellus (n). B, older stage in
which the integuments have closed
over the nucellus, leaving only a
small opening, the micropyle (ra) . s,
the funiculus. Much enlarged.

FIG. 45. — Section through
the ovule of Red Clover show-
ing the embryo sac. em, em-
bryo sac with the egg (e) and
the primary endosperm nucleus
(en) indicated; i, integuments;
m, micropyle. Many times
enlarged.

come seed. How much could the seed yield of Clover and Alfalfa
be increased if they could be made to develop all of their ovules
into seed? If clover seed were selling at $10 per bushel, what
would be the value of the increased yield on ten acres of average
Clover?

Parts of the Ovule. — The ovule consists of a main body and
a stalk known as the funiculus which connects to the ovary wall.
The main body consists of a central (usually rounded) portion
called nucellus, which is enclosed by one or more coverings called
integuments that grow up from the funiculus. In Figure 40,
showing the ovules of Clover, the stalk or funiculus is at st; the
central portion or nucellus of the main body is at n; the coverings
or integuments of the nucellus are at i. Turn to this Figure and
point out these parts. In the ovule of the Corn, shown in Figure
41, the funiculus is apparently absent. In Figure 44 is shown a

HOW THE PARTS OF AN OVULE ARE MADE UP 39

surface view of an ovule at two stages of development. Notice
how the nucellus is enclosed by the integuments, leaving only
a small opening at m known as the micropyle.

The pollen tube, a tube-like structure produced by the pollen
grain in connection with fertilization, often uses the micropyle as
an entrance to the ovule. Some ovules are straight but oftener
there is a curving to one side during growth as shown in Figure 44-
By curving the micropyle is brought near the base of the ovule, a
position more favorable for the entrance of the pollen tube.

How the Parts of an Ovule are made up. — The ovule, like all
other parts of the plant, is made up of many living units called
cells. A cell consists of a mass of
living matter called protoplasm,
which is generally enclosed by
walls. A very important part
of the living matter is the nu-
cleus, a globular body commonly
occupying a central position in
the cell. The ovule, although a
very small body, is composed of
many hundreds of cells, all of
which are in some way related to
seed formation.

The cells of the funiculus, in-
teguments, and most of those of

the nucellus furnish food and de- thr°ugh the ovary °.f Corn sh<ring

. embryo sac. o, ovule; em, embryo

velop a covering for the inner and sac; 6) egg; eri) the two nuclei which

more vital parts of the seed. In fuse to form the primary endosperm
form and structure they are nucleus; i, integuments; w, ovary
similar to cells composing other wall; s, base of style or silk. Much
parts of the plant. The cells enlar^ed-

peculiar to the ovule are those forming a special group, usually
seven or eight in number and occupying a central position in the
nucellus One peculiar feature of these cells is that they usually
are not separated by cell walls and their masses of protoplasm lie
in contact or closely join with each other. The region which
these cells occupy is known as the embryo sac, so named because
within it the embryo develops. The embryo sac, being deeply
buried in the nucellus wh ch is in turn enclosed by the integu-
ments, is well protected and to study it the ovule must be sec-

FIG. 46. — Lengthwise section

PISTILS AND STAMENS

tioned. In some ovules the embryo sac may be seen without
the microscope, but in most ovules it is microscopic. There is
only one cell and one nucleus in the embryo sac, which have an
important function in the formation of the seed. The important
cell is the egg. The egg is at the micropylar end and after
fertilization produces the embryo of the seed. The important
nucleus, referred to as nucleus because it has no definite
amount of protoplasm, is the primary endosperm nucleus. It
is near the center of the embryo sac and is important because
upon it the development of the stored food or endosperm of
the seed depends. The remaining cells and nuclei of the

FIG. 47. — A vertical section through an Oat ovary to show the parts of
the ovule. Parts of the lemma, palea, and two stamens are shown, and one
style and stigma remains. Label the parts of the ovule. Much enlarged.

embryo sac are absorbed and disappear soon after the egg is
fertilized. In the ovules of Clover and many other plants, the
cells at the inner end (chalazal end) of the embryo sac disappear
even before the egg is fertilized.

A section through an ovule of Red Clover is shown in Figure
45. Point out the embryo sac. Notice the egg at e and the
endosperm nucleus at en. Point out the embryo sac of Corn in
Figure 46. Notice that instead of a single primary endosperm
nucleus, there are two nuclei lying in contact. These nuclei fuse
and form the primary endosperm nucleus. A section through
an ovule of Oats is shown n Figure Jtf. Point out the embryo

THE POLLEN GRAIN AND ITS WORK 41

sac, egg, and primary endosperm nucleus. Redraw this figure
on a sheet of paper and label the parts.

Although pistils vary much in number of carpels, length of
styles, and in number of locules and ovules, there is uniformity
in organization and adaptation of parts to special functions.
The stigma is especially adapted for receiving pollen, the style
supports the stigma in a position suitable for receiving the pollen,
and the ovary protects the delicate ovules in which is the embryo
sac containing the egg and primary
endosperm nucleus, which are the
chief structures of the pistil.

The Stamen. — The stamen usu-
ally consists of two parts; the en-
larged terminal portion, or anther;
and the stalk, or filament. The
filament is often so short as to seem
to be absent. Point out the parts

of the stamen in A of Figure 48.

a, an-

ch en-
an anther,

called locule, which contains many showing the locules and pollen
globular bodies known as pollen or grains. The two locules at the
pollen grains. When the pollen is lef^ ka™es°pened' allowing the
mature, the walls of the anther *

open and allow the pollen to escape. Notice the cross sec-
tion of an anther shown in B of Figure 48- Point out the
locules and pollen grains. Notice that two of the locules have
opened.

The Pollen Grain and its Work. — The pollen grain is a cell
with its living matter enclosed in a heavy protective wall. It
needs to be well protected, for during its journey to the pistil,
destructive agencies such as cold, heat, and drying are encoun-
tered. The transference of the pollen to the stigma is called
pollination. Pollination is a very important event, for the pollen
cannot perform its function except on the stigma.

On the stigma the pollen grain grows a tube which traverses
the stigma and style, pierces the ovule, and reaches the embryo
sac. Pollen grains, when first formed in the anther, have only
one nucleus, but in preparation for the work of fertilization, there
is nuclear division and as a result there are three nuclei in a well

r^\^ j.i_ 11 £ 111 . . — ,

The anther is usually four lobed ther; ^ ^^^ ^

and within each lobe is a cavity, larged cross section of

PISTILS AND STAMENS

developed pollen tube. This feature is shown in Figure 49. The
nucleus at the end of the tube and known as tube nucleus directs
the growth of the tube and disappears
soon after reaching the embryo sac.
The two nuclei following closely be-
hind the tube nucleus are the sperms
or male nuclei, the structures which
join with the egg and primary endo-
sperm nucleus in fertilization. The
pollen tube is a passage way through
which the sperms pass to the embryo
sac.

Fertilization. — After the two sperms
reach the embryo sac, one approaches
the egg and fuses with its nucleus, while

FIG. 49. — Pollen grains in different stages
preparatory to fertilization. A, surface view
of a pollen grain; B, section through pollen
grain in uni-nucleate stage; C, section through
pollen grain showing the nucleus divided into
the generative (g) and tube nucleus (<); /),
pollen tube forming into which the two nuclei
have passed; E, tube more developed and
generative nucleus divided into two sperms
(g). Much enlarged.

FIG. 50. — A diagram of a length-
wise section through the pistil of Red
Clover, showing pollen tubes trav-
ersing the stigma and style. Two
pollen tubes have reached the em-
bryo sacs, p, pollen grains develop-
ing tubes; st, stigma; p.t, pollen
tubes; o, ovules; e, egg; en, en-
dosperm nucleus; s, sperms. Much
enlarged.

the other approaches the primary endosperm nucleus and fuses with
it. This process of fusion is called fertilization. Since there are two

THE DEVELOPMENT OF THE OVULE INTO A SEED 43

fusions, there are two fertilizations, and the two fertilizations
are called " double fertilization." Both egg and primary endo-
sperm nucleus are now said to be fertilized, and the pollen grain
has performed its function, which is an important one, for with-
out fertilization the ovule would not develop into a seed.
Pollination, the growth of the pollen tube to the embryo sac,
and the formation of the two sperms are simply preliminary
acts to fertilization, which is the final achievement of the pollen

grain. Study the pollen grains
shown in Figure 49. Notice that
the tube has broken through the

FIG. 51. — Stigma of Corn show-
ing how the pollen grains grow
their tubes into the stigma, p,
pollen grains; t, pollen tube. Much
enlarged.

FIG. 52. — A, diagrammatic section
of an ovule of the Tomato in which
the egg (6) and primary endosperm
nucleus (d) have been fertilized, o,
portion of ovule surrounding"1 and en-
closing the embryo sac. B, diagram-
matic section of the seed of the
Tomato, e, embryo; c, endosperm;
t, seed coat. The lines drawn from
the ovule to the seed indicate the
parts of the ovule from which the
different parts of the seed have de-
veloped. Both are enlarged but the
ovule is enlarged much more than the
seed.

pollen wall. How have the two sperms been formed? In Figure
50 trace the pollen tubes to the embryo sac. How do the pollen
tubes make their way through the style? Where do they obtain
their food for growth? Notice how the pollen tubes enter the
branched stigma of Corn in Figure 51.

The Development of the Ovule into a Seed. — After the egg and
primary endosperm nucleus have been fertilized, the ovule begins
its development, which results in the production of a seed. There

PISTILS AND STAMENS

are three main structures involved in this development: (1) the
fertilized egg; (2) the fertilized primary endosperm nucleus;
and (3) the parts of the ovule surrounding the embryo sac.
The development of each of these parts into their respective seed
parts takes place simultaneously. The fertilized egg becomes
the embryo, the endosperm nucleus has to do with the forming
of the endosperm, and a part of the surrounding portion of the
ovule becomes the seed coat. Figure 52 shows a Tomato ovule

-w

FIG. 53. — A young ovary of Corn just after fertilization and a mature
ovary or kernel, both of which are sectioned lengthwise and the relation of
parts indicated. A, lengthwise section of the young ovary showing nucellus
(n), egg (e), endosperm nucleus (en), integuments (i), ovary wall (w), and
base of style (6). B, the lengthwise section through the kernel showing the
embryo (em), endosperm (end), seed coat (c), ovary wall (w), and the base
of the style (6) . The dotted lines indicate the parts of the ovule from which
the different parts of the kernel have developed.

in which the egg and endosperm nucleus have just been fertilized
and also shows the seed which develops from the ovule. The lines
indicate the parts of the ovule from which the different parts of
the seed have come. Study Figure 53 showing the development
of the ovule of Corn into a seed. Point out the different parts of
the kernel and the part of the ovule from which they came.
Notice that the heavy outer covering of the kernel is the ovary
wall, and does not come from the ovule. A kernel of Corn is a
seed closely jacketed by the ovary wall. Copy on a sheet of

THE DEVELOPMENT OF THE OVULE INTO A SEED 45

FIG. 54. — A, a vertical section through an Oat ovary showing one style
and stigma, the ovary wall, and the parts of the ovule. B, a vertical section
through an Oat kernel showing its parts. After comparing with Figure 53
label the parts of A and B and with lines indicate the parts of A from which
the parts of B have developed.

FIG. 55. — A diagram showing the relation of the parts of the ovule to
those of the seed in Red Clover. A, ovule just after fertilization showing
the egg (e) and the endosperm nucleus (d). B, seed with half of the seed
coat (s) removed to show the large embryo (em). The dotted lines indicate
the relation of the parts of the ovule to those of the seed.

46 PISTILS AND STAMENS

paper the drawings in Figure 54 and with lines indicate the parts
in A from which the different parts shown in B have come.

In many plants the endosperm does not remain outside of the
embryo as it does in Corn and other grains. If one removes the
thin rind-like testa from a soaked Bean, all that remains is the
large embryo. The endosperm is stored in the embryo and as a
result the embryo is much enlarged and fills the space within the
testa. Clover, Alfalfa seed, and many other seeds have the endo-
sperm stored in the embryo. Study the Clover seed in Figure 55.
Notice that there is apparently no endosperm, and that the .much
enlarged embryo occupies nearly all the space within the testa.

In some seeds a stored food known as perisperm occurs.
Usually as the ovule develops into the seed, the nucellus is de-
stroyed and replaced by the developing endosperm, leaving only
the integuments from which the seed coat is formed. However,
in the formation of a few seeds, some of the nucellus remains, and
a portion of its outer region becomes filled with stored food, thus
forming the layer of stored food known as perisperm, which sur-
rounds the endosperm and embryo.

Pollination

Nature of Pollination. — Pollination is the transference of
pollen to the stigma. After the pollen is on the stigma, it may
produce a tube reaching to an ovule and effect fertilization, or
it may lie dormant; but in either case the stigma is considered
pollinated. Much pollination occurs in nature that does not
result in fertilization. Corn pollen, for example, as it is blown
about may fall on the stigmas of various other species of plants,
but since no fertilization results, the pollination is not effective.
Pollen is usually effective only on stigmas of plants similar to
the plant which produced the pollen. Thus Apple pollen is
effective only on Apple stigmas, Corn pollen only on Corn
stigmas, etc.

Pollinating Agents. — The most important pollinating agents
are gravity, wind, insects, and man. In some cases, as in Rice,
Wheat, and Oats, where the pollen falls from the anthers to the
stigma, pollination depends upon gravity. Even in orchards
some pollination may be accomplished by pollen falling from the
higher branches. x In early spring, before there are many insects,
many of our trees, such as Willows, Poplars, Oaks, and Pines,

KINDS OF POLLINATION 47

depend upon the wind for pollination. The wind is also an
important agent in the pollination of Corn and aids some in
orchard pollination. Plants having showy flowers depend upon
insects for pollination and it is among these plants that attractive
colors, secretions of nectar, and various structural arrangements,
which are interpreted as adaptations to secure pollination, occur.
The pollination of Fruit trees, Clovers, and Alfalfa is done chiefly
by insects. (Fig. 56.) In experimental work, such as crossing

FIG. 56. — Bumble bee pollinating Red Clover.

Tomatoes, Corn, and Fruit trees, man himself often does the
pollinating so as to have it under control.

Kinds of Pollination. — On the basis of the relation of the
stamen furnishing the pollen to the pistil pollinated, there can
be different kinds of pollination. The transfer of pollen from
the stamen to the pistil of the same flower is self-pollination,
while the transfer to the pistil of another flower is cross-pollina-
tion. Various relationships may occur in pollination. Thus the

48 PISTILS AND STAMENS

pistil of a Ben Davis Apple blossom may be pollinated: (1) with
pollen from the same flower; (2) with pollen from another flower
in the same cluster; (3) with pollen from a flower on another
branch; (4) with pollen from another Ben Davis tree located in
the same or a neighboring orchard; or (5) with pollen from a
Jonathan or some other different variety. In case of fruit trees
horticulturists sometimes consider the pistil of a blossom self-
pollinated if the pollen comes from the same flower, from another
flower on the same tree, or from another tree of the same kind,
and consider the pistil cross-pollinated only when the pollen
comes from another variety of fruit tree. Corn breeders speak
of self-, close-, and cross-pollination. Pollination resulting from
the pollen falling from the tassel to the silks of the same plant is
called self-pollination. Pollination in which the pollen from one
plant falls on the silks of another plant is called close-pollination
if both of these plants came from kernels taken from the same
ear, but cross-pollination if these plants came from kernels taken
from different ears. In case of cross-pollination, the plants may
be of the same variety or of different varieties.

The Amount of Pollen Required for Good Pollination. — One
pollen grain is required to fertilize each ovule, and, therefore, a
pistil with many ovules requires many pollen grains for good
pollination. In Corn, Wheat, and Oats where there is only one
ovule, one good pollen grain on the stigma is sufficient, although a
large number is usually present. Due to the great waste of pol-
len during transportation, much more is produced than is really
needed. A medium-sized plant of Indian Corn produces about
50,000,000 pollen grains or about 7000 for each silk. Many of
these never reach a silk, and of the many that do all, except the
one that reaches the ovule first with its tube, accomplish nothing.
On the stigma of the Red Clover, although each pistil has only
two ovules, there are often as many as 25 pollen grains, 23 of
which are wasted.

On the other hand, in flowers where the ovaries contain numer-
ous ovules, as in Tomatoes and Melons, it often happens that
not enough pollen reaches the stigma to effect fertilization in all
the ovules. In the Tomato, for example, an ovary may contain
as many as 200 ovules, in some of which fertilization may not
occur because of insufficient pollination. Even in Beans, Apples,
and Pears, where the ovules are not numerous, one often finds in

HOW POLLEN IS AFFECTED BY EXTERNAL FACTORS 49

the mature fruit some undeveloped ovules, which due to the lack
of fertilization did not become seeds. Although much of the vari-
ation that occurs in the number of seeds in many of the fruits is
due to the failure of the pollen to function properly on the stigma
or to the insufficient nourishment of the ovules, much of the vari-
ation can be attributed to insufficient pollination.

There is good evidence that the imperfect development of
fruit is due in some cases to insufficient pollination. By polli-
nating the stigmas of Tomatoes in such a way that portions of
the stigmas received no pollen, one 1 investigator found that no
fertilization occurred in some locules, and that the portion of the
ovary surrounding these locules developed much less than those
portions of the ovary surrounding those locules in which fertili-
zation occurred, thus causing one-sided fruits.

How Pollen is Affected by External Factors. — Pollen is not
so specially prepared as seeds are to endure extreme conditions
during transportation. During transportation and while on the
stigma, pollen may be either killed or rendered functionless
by extremes of temperature and moisture. The pollen of most
plants is so sensitive to dryness that an exposure to the ordinary
dryness of the air cannot be endured more than a few days and
in many cases only a few hours.

In the storage of pollen, which is sometimes necessary in experi-
mental work, the main caution is to store the pollen where it
will not be dried out too much by evaporation, although the pol-
len must be kept dry enough that it will not mold. It has been
found that Plum and Apple pollen can be kept alive much longer
when stored in closed chambers where there is less drying than
in laboratory air. One investigator has reported that Corn pol-
len will die in two or three hours when exposed to the air of the
laboratory or living room, but will live two days when stored
in a moist chamber. Some investigators think that hot dry
weather during the pollination of fruit trees may affect the setting
of fruit by destroying some of the pollen.

The pollen of some plants, as in case of Red Clover and Alfalfa,
absorbs water so rapidly that it is destroyed by bursting when
immersed in water or stored in a saturated air. Consequently
these plants are not successfully pollinated when they are wet

1 Pollination and Reproduction of Lycopersicum esculentum (Tomato).
Minnesota Botanical Studies, p. 636, Nov. 30, 1896.

50 PISTILS AND STAMENS

with dew or rain. Apple pollen and the pollen of many other
fruit trees, although not destroyed when immersed in water, will
not function nearly so well and for this reason rain or dew on
a stigma may hinder the pollen in its work.

The pollen of many plants is quite sensitive to a low tempera-
ture, showing a decrease in vitality when exposed for a few hours
to a temperature only a little below freezing. Pollen, if not in-
jured by cold, will not germinate while the temperature is low.
In the Apple, Pear, Plum, Peach, and Cherry l a temperature of
— 1°C. has been found to interfere with the proper functioning
of the pollen by injuring the stigmas and preventing the ger-
mination of the pollen. Cold during the blooming period may
be responsible for much failure in fruit-setting.

The Results of Pollination. — The most immediate as well as
the most important result of pollination is the fertilization of the
egg cell and primary endosperm nucleus. Through the process
of fertilization the pollen stimulates the ovule and other struc-
tures to develop, and transmits factors by means of which the
embryo and the endosperm of the seed inherit the characters of
the pollen parent.

The importance of the stimulative effect of fertilization in
the development of a seed is obvious, for unless fertilization
occurs, the egg, endosperm nucleus, and other parts of the ovule
rarely develop into their respective seed structures, and con-
sequently the ovule either disappears or remains as a small
withered body as often seen in fruits. Furthermore, the devel-
opment of fruit depends upon the stimulative effect of fertiliza-
tion, as shown in case of fruit trees, Melons, Alfalfa, etc., in which
the flowers wither and fall from the plant unless fertilization
occurs in some of the ovules. There are, however, a few instances
in which the stimulative effect of fertilization is not necessary,
as in seedless Oranges, seedless Persimmons, Bananas, and a
few other fruits known as parthenocarpic fruits, which develop,
although no fertilization occurs. There are a few plants, the
Dandelion being a common one, in which ovules develop into
seeds parthenogenetically, that is, without fertilization, but such
plants as well as those that develop seedless fruits are exceptional.
In most cases our harvest of seed and fruit depends upon the
stimulative effect of fertilization.

1 Research Bulletin 4, Wisconsin Agr. Exp. Sta., 1909.

THE RESULTS OF POLLINATION

The effect of fertilization in reference to the influence which
the sperms have upon the character of the endosperm of the seed
and upon the character of the plant which
the embryo of the seed will produce is a sub-
ject receiving much attention in plant-breed-
ing. The endosperm nucleus consists of a
sperm and a primary endosperm nucleus,
each of which is capable of determining the
character of the endosperm. Likewise, in
the fertilized egg, the contents of both sperm
and egg are capable of determining the
characteristics of the plant developing from
the fertilized egg. But the influence of
the sperm is toward the production of both
endosperm and plants having the features
which are characteristic of the pollen parent,
while the egg and primary endosperm
nucleus tend to reproduce in the offspring
those features characteristic of the mother
plant. Thus it follows that if the pollen
parent is very different from the mother
plant, as is the case when the parents be-
long to different varieties or species, there
will be opposing tendencies in the fertilized
egg and endosperm nucleus. Such a fertil-
ized egg develops into a plant known as a
hybrid. The hybrid character of the endo-
sperm in most seeds is either lost through Sweet Corn showing
the absorption of the endosperm by the the effect of the pollen

embryo or obscured by coverings. It is in of, Yellow Dent Co™-
,, /; £ , . ~ , The plump kernels

the Grass type of seeds, as in Corn where have endosperm like

the endosperm remains outside of the em- the Yellow Dent Corn,
bryo and can be seen through the pericarp, due to the influence of
that the influence of the sperm on the the sperm which fused
endosperm and known as xenia is often with the primary endo-
..ul AT ,. ,,- £ ^ , sperm nucleus. After

noticeable. Notice the ear of Corn shown H j ^ebber

in Figure 57. This was an ear of Sweet
Corn which was partly pollinated with pollen from hard Field
Corn. Notice the kernels which have the hard plump endo-
sperm and resemble the kernels of Field Corn. In the de-

FIG. 57. — An ear of

PISTILS AND STAMENS

velopment of these kernels, the sperm portion of the endo-
sperm nucleus dominated, and thus the endosperm is like the
endosperm of the pollen parent. The sperm may even deter-
mine the color and fat content of the endosperm. On the other
hand, if Field Corn is pollinated with pollen from Sweet Corn,
then usually the primary endosperm nucleus dominates and one
sees no effect of the sperm. Thus it is seen that the character
of the endosperm of a seed may be determined by either of the
members which fused in forming the endosperm nucleus.

The kernels in Figure 57 which have the endosperm features
of Field Corn also have embryos with opposing tendencies.

FIG. 58. — Pears showing a difference between the results of self- and cross-
pollination, a, fruit resulting from self-pollination; b, fruit resulting from
cross-pollination. After Waite.

These embryos received from the egg tendencies to develop into
plants having all of the features of Sweet Corn. They also re-
ceived from the sperm tendencies to develop plants having all
of the features of Field Corn. In the hybrid offspring it is likely
that some of the characters of both parents will be present.

The Kind of Pollination Giving the Best Results. — Plants in
general seem to favor cross-pollination and often have their
flowers so constructed as to prevent self-pollination. In some
plants, however, as in the small grains, Beans, Peas, and some
other plants, self-pollination is the usual method and gives good
results. Red clover, many fruit trees, and many other plants
require cross-pollination and will develop very little seed or fruit

THE KIND OF POLLINATION GIVING BEST RESULTS 53

when self-pollinated. Many of our Pears, such as the Anjou,
Bartlett, Pound, Lawrence, Jones, Howell, Sheldon, Wilder, and
some others will not produce much fruit unless pollinated with
pollen from other varieties, while the Kiefer, Buffum, Seckel, and
some others known as self-fertile varieties set fruit well when
self -pollinated. Moreover, some trees which are self-fertile
develop larger and better fruit when cross-pollinated. (Fig. 58.)
Many of our Apple trees and Cherry trees are known to require
cross-pollination .
Furthermore, some varieties of fruit trees l which require cross-

FIG. 59. — Results of cross-pollination with different varieties in the
Sweet Cherry. A, fruit obtained by pollinating a cluster of flowers of the
Bing with pollen from the Black Republican. B, fruit obtained by polli-
nating a cluster of flowers of the Bing with pollen from the Knight. After
V. R. Gardner.

pollination will not do equally well when crossed with all varieties.
In Apples, Pears, and Cherries better results have been obtained

1 The pollination of pear flowers. Bulletin 5, Div. of Veg. Path., U. S.
Dept. of Agr., 1894.

Pollination of the apple. Bulletin 104, Oregon Agr. College Exp. Sta., 1909.

Pollination of the Sweet Cherry. Bulletin 116, Oregon Agr. College Exp.
Sta., 1913.

Read Pollination in Orchards. Bulletin 187, Cornell University Exp. Sta.,
1909. Also Pollination of Bartlett and Kiefer Pear. Ann. Report, Virginia
Agr. Exp. Sta., 1911.

54 PISTILS AND STAMENS

by crossing with some varieties than with others. (Fig. 59.) In
case of Sweet Cherries, when flowers of the Bing, a variety requir-
ing cross-pollination, were pollinated with pollen from the varietj^
called the Knight, only a few fruits developed; while flowers
pollinated with pollen from the Black Republican produced
fruit abundantly. Obviously much of the success in orcharding
has to do with securing for each variety of fruits the best kind of
pollination.

CHAPTER V

SEEDS AND FRUITS

Nature and Structure of Seeds

The seed is the principal structure by which plants increase
in number. The chief function of a seed is to produce a plant
like the one that bore it. For plants to increase in number and
at the same time thrive well, they must spread to new areas.
Seeds are thus so constructed that they can separate from the
parent plant and be carried to regions where there is opportunity
for new plants to develop. Seeds, being able in a dormant state
to live long and endure adverse conditions, are the means by
which those plants living only one season are able to perpetuate
themselves. As to origin the seed is sometimes defined as a
matured ovule, that is, it is an ovule in which three things have
taken place: (1) the fertilized egg has developed into an embryo,
the miniature plant of the seed; (2) the fertilized primary endo-
sperm nucleus with some adjacent protoplasm has produced a
mass of stored food or endosperm; and (3) the outer portions of
the ovule have been modified into a testa or seed coat. Despite
a wide variation in size, shape, color, and other external features,
seeds possess in common an embryo, stored food, and seed coat.
In many cases these three parts are not separate, for the endo-
sperm may be absorbed by the embryo during the development
of the seed. This is true in the Bean, Pumpkin, and a number
of other families, where the seeds consequently have only two
distinct parts, embryo and testa.

Each part of the seed has a distinct function to perform. The
embryo develops into a new plant, the reserve food nourishes the
young plant until roots and leaves are established, and the seed
coat protects the embryo and endosperm during the resting stage
of the seed. It is due to the embryo that seeds are valuable
in the production of new plants, while the stored food makes
many seeds valuable food for animals.

The embryo, which is the chief structure of the seed, is the

SEEDS AND FRUITS

young plant, which after reaching a certain stage of development,
varying in different plants, passes into a dormant stage from which
it may awake if conditions are favorable and continue its devel-
opment until it becomes a mature plant. In the development of
the embryo from the fertilization of the egg to the dormant stage,
certain structures which function in the further development of
the young plant are usually more or less developed. In a well
formed embryo like that of the Bean, there are four parts, hyocotyl,
plumule, cotyledons, and radicle. In Figure 60 of the Bean, h is
hypocotyl, p, plumule, and c, cotyledons. The radicle (r) is at
the lower end of the hypocotyl and is so closely joined with the

hypocotyl that it does not appear as
a separate structure. The cotyledons
of the Bean have absorbed the endo-
sperm and consequently are so much
enlarged that they form the bulk of
the embryo. The special functions
performed by the different parts of the
embryo are quite noticeable in the
germination of the seed. The cotyle-
dons supply food; the plumule develops
stem and leaves; the radicle develops
a root; and the hypocotyl in many
cases pulls the cotyledons and plumule
out of the seed coat and raises them
above ground.

The stored food and seed coat are temporary structures. They
nourish and protect the young plant in its early stage of develop-
ment and then disappear. The stored food, consisting chiefly of
starch, proteins, and oils, the proportion varying in different
seeds, develops in close contact with the embryo and when not
absorbed as rapidly as it develops, it forms the storage tissue or
endosperm in which the embryo becomes imbedded. The testa,
the protective structure of the seed and usually formed from
the integuments of the ovule, generally consists of a single
covering so much thickened and hardened that it protects the
embryo against injuries. Often there is a thin inner covering
and in exceptional seeds, like those of the Water Lily, an extra
outer covering called the aril develops later than the integuments
and forms a loose covering about the seed. (Fig. 62.)

FIG. 60. — Bean with testa
removed and cotyledons
spread apart, c, cotyledons;
h, hypocotyl; p, plumule; r,
radicle.

NATURE AND STRUCTURE OF SEEDS

On the surface of seeds occur certain structures which suggest
the structural relation of the seed to the ovule. The micropyle,
the small opening through which the pollen tube entered the
ovule, persists as a tiny pit on the seed coat. Usually near the
micropyle there is a much larger scar, called the hilum, left where
the seed broke away from the funiculus, the stalk-like structure
which attached the ovule to the ovary and through which the
seed received food and water during its development. (Fig. 61.)
In case an ovule turns over on its elongated stalk and grows fast
to it, the stalk persists on the
seed coat as a distinct ridge,
called the raphe. (Fig. 62.)
In some seeds, like those of

FIG. 61. — Beans showing the
hylum at h and the micropyle
at m.

A B <<'

FIG. 62. — A, seed of Pansy showing
raphe (r) . B, seed of Castor Bean show-
ing caruncle (c). C, seed of White
Water Lily showing the aril or loose
jacket around the seed.

the Castor Bean, an enlargement known as the caruncle develops
near the micropyle.

Structures such as hairs, plumes, hooks, and other appendages
which do not occur on ovules, are direct outgrowths of the seed
coat and function chiefly in dissemination. Similar appendages
occur often on one-seeded ovaries in which case one can tell only
by dissection whether the structure is a seed or one-seeded fruit.

Many of the small one-seeded fruits are commonly called seeds.
In addition to a seed, they contain the ovary wall which persists
as an outer covering over the seed. The so-called seeds of Let-
tuce, Buckwheat, Ragweed, and the grains such as Corn, Wheat,
Barley, Rye, and Oats are familiar examples of one-seeded fruits
which are commonly called seeds. While they are not identical
with true seeds in structure, they are in function and therefore
may be appropriately discussed with seeds. In these one-seeded
fruits, the seed is protected by .the hardened ovary wall, and
consequently, the seed coat is poorly developed, forming only a

58 SEEDS AND FRUITS

thin covering, which is usually .tightly pressed against the inner
side of the ovary wall.

In general structure seeds are similar, all having an embryo,
stored food, and seed coat, but in size, shape, and in features
which pertain to the structure of the embryo, composition of the
stored food, and character of the seed coat, seeds vary widely
and can be used in many ways by man. The number of coty-
ledons developed by the embryo is used as a basis upon which to
classify the Flowering Plants into two classes, Monocotyledons
and Dicotyledons. From the stored food, whether stored as
endosperm or in the embryo, various valuable products, such as
starch, protein, fats and oils, are obtained; and from the hair-
like outgrowth of the seed coat, as in case of Cotton, various fiber
products are made. Although seeds may be divided into many
types on the basis of their structure and external features, only
those types which include the most common seeds will be studied
in this presentation.

Bean Type of Seeds. — Of this type of seeds, those of the
Bean, Pea, Peanut, Clover, Vetch, Alfalfa, Cotton, Pumpkin,
Squash, Melon, Apple, Peach, Oak, Hickory, and Walnut are ex-
amples. The type is so named because it is characteristic of the
Bean family (Leguminosae) , a family notable for its many valu-
able cultivated forms among which are Clover, Alfalfa, Beans,
Peas, Vetch, and Peanuts. The type is also characteristic of the
Rose family (Rosaceae) , the family to which most fruits, such as
the Apple, Peach, Pear, Cherry, etc. belong. In this family,
however, it is the fruit (rarely the seed) that is important.
The seeds of the Bean type are common to a number of
plant families and to species and varieties of plants so numer-
ous that a list naming them all would require a page or more.
Although many are valuable commercial seeds, some are borne
by weeds and hence of interest because of their undesirable
features.

These seeds differ from other types in having little or no endo-
sperm. As the seed develops, all or almost all of the food tis-
sue formed by the endosperm nucleus and adjacent cytoplasm
is absorbed by the embryo where it is stored in the cotyledons,
which, consequently, are so much enlarged that they are much
the largest part of the embryo. (Fig. 63.) For this reason
these seeds are called exalhuminous seeds, that is, seeds without

SEEDS OF THE BUCKWHEAT AND FLAX TYPE

-e-

FIG. 63. —4, Squash

endosperm. Another feature to be noted is that the embryo
has two cotyledons.

In external characters they vary so much that their type in
most cases can be determined only by an examination of their
structure. In size, those most commonly
grown in our region vary from the smallest
of the Clover Seeds up to the largest of
the Beans. They are kidney-shaped, glob-
ular, oval, or flattened. Among them vari-
ous colors such as red, purple, brown,
yellow, green, mottled, and black occur.
In identifying the different seeds of this
type, especially those of the Bean family,
size, shape, and color are important aids.

In importance, the seeds of this type
rank next to those of the Grass family. In
Beans, Peas, and Peanuts, which are used
directly as food, the value depends upon

the protein, fats, and starches stored in the seed sectioned longitudi-

embryo. In the nally. B, Apple seed
Cotton seed sectioned longitudinally.

1,1 i e, embryo. B much more

both embryo \ *, A.

J enlarged than A.

and seed coat

are valuable structures. The embryo
is rich in oil from which many useful
products are made, while the hairs of
the seed coat are the Cotton fibers
of commerce. (Fig. 64-) The seeds
of Clover and Alfalfa are important
because the plants which bear them
FIG. 64. — Section through increase the soil fertility and are valu-
a Cotton seed showing the able for hay and forage,
embryo with its much folded Seeds of the Buckwheat and Tomato
cotyledons, and the seed coat — , ~ , - ,,.

with the seed hairs. Enlarged Type. — Some common seeds of this
about twice. type are those of the Buckwheat, Beet,

Tomato, Potato, Tobacco, Red Pep-
per, Coffee, Flax, and Castor-oil plant. The type is common to
a number of families, which contain some useful plants and many
weeds, such as Nightshades, Spurges, Morning Glories, Bind-
weeds, Dodders, Milkweeds, Docks, Smartweeds, and Corn Cockle.

60 . SEEDS AND FRUITS

In structure these seeds differ from those of the Bean type in
that they have three distinct parts, an embryo, endosperm, and
seed coat, but in number of cotyledons, which is two, the two
types are identical. Since endosperm is present, the seeds of
this type are known as albuminous seeds. (Fig. 65.) Although
some endosperm is always present, sometimes, however, much
of it is absorbed by the embryo during the development of the
seed, and in this case the cotyledons, which are comparatively

free from stored food in
many of these seeds, as-
sume some importance as
yfe^:S?S$^v storage organs, though not
so much as in the Bean
tyPe- I*1 tne Buckwheat
family, represented by
Buckwheat, Rhubarb,

A /} Docks, and Smartweeds,

FIG. 65. — A, section through a Potato and also in some plants
seed, c, embryo; e, endosperm; t, testa, of the Goosefoot family,
B, section through an achene of Buckwheat. the hardened ovary wall,
em embryo; e endosperm; o, ovary wall which when mature re_
and testa. Enlarged.

sembles a seed coat, per-
sists as an outer covering over the seed, thus forming with the
seed a fruit-like structure known as an achene, a term which is
applied to many hard, usually one-seeded fruits, that do not dehisce
or, in other words, that do not open to allow the seed to escape.

In external characters, seeds of this type present various differ-
ences by means of which one can usually identify the family and
often the species to which the seed belongs. Those most com-
mon in our region range in size from the smallest of the Dodder
seeds, which are almost dust fine, to the size of the Castor
Bean. The shape, which in many cases is the chief character by
which the family and often the species to which the seed belongs
is identified, may be globular, oval, flat, or angled. Such colors
as red, yellow, brown, and black are common and serve along
with shape and size as a means of identifying different seeds.
Sometimes the seed coat is much roughened, as in the Cockle,
and in some cases, as in the Milkweeds, the seed coat develops
hair-like appendages.

In case of Flax, Buckwheat, Coffee, and the Castor-oil plant,

GRASS TYPE OF SEEDS 61

the seeds themselves are valuable on account of the oil, protein,
starch, or alkaloid-like substances which they contain. From the
endosperm and embryo of the Flax seed, linseed oil, the chief sol-
vent for paints, is obtained. After the oil is pressed out of the
flax seed, there remains the cake, which has considerable value as
a feed for stock. The Castor Bean yields castor-oil which is
much used as a medicine and sometimes as a lubricant and illu-
minant. Buckwheat, which contains much starch and some fat
and protein, is much used for food when ground into flour. Often,
as in case of the Tomato, Potato, Beet, and Tobacco, the value
of the plants depends upon the fruit, tubers, roots, or leaves, and
not upon the seed, which in these cases has no value except for
growing new plants. Of the weed seeds of this type, some com-
monly occur as impurities among the seeds of Clover, Alfalfa,
Flax, and the small grains and, when present in considerable quan-
tities, they either lower the price or prevent the sale of these agri-
cultural seeds, thus bringing loss to the farmer. In case of Cow
Cockle and Corn Cockle, the seeds, which are frequently found
among the small grains, are poisonous and when ground with
Wheat make the flour unwholesome and when fed with grain to
stock often cause injury. Other weed seeds of this type, as those
of Dodder, Morning Glories, Black Bindweed, Sheep Sorrel, and
others are objectionable because the plants themselves hinder
the cultivation and growth of useful plants. Sometimes, as in
case of the Black Nightshade and Jimson Weed, the plants are
poisonous.

Grass Type of Seeds. — As the name suggests, these are the
seeds of the Grass family, the family to which Corn, Wheat, Oats,
Rye, Barley, and Rice belong and hence the family most depended
upon for food. Many of the Grass seeds, as in case of Timothy,
Red Top, Blue Grass, etc., though not used for food, are valuable
because the plants themselves are useful for pasture and hay.
Some of the Grasses, however, are regarded as weeds and their
seeds are often troublesome impurities among agricultural seeds.

As previously noted, in structure the seeds of the Grass type
are not true seeds. Besides a seed, they contain the ovary wall,
called the pericarp, which remains about the seed as a closely
fitting jacket. They are one-seeded ovaries and hence struc-
turally they are fruits rather than seeds. Although popularly
known as a seed, this fruit-like structure of the Grasses is scien-

62 SEEDS AND FRUITS

tifically called a cariopsis, a term which refers to its nut-like char-
acter.

The seed itself contains three distinct parts, embryo, endo-
sperm, and seed coat. The seed coat, however, since it is covered
by the ovary wall which performs the protective function of an
ordinary seed coat, is poorly developed and so closely joined
with the ovary wall that it appears to be a part of its structure.
In containing three distinct parts, embryo, endosperm, and seed
coat, it is seen that the seeds of the Grass type are identical with
those of the Flax and Buckwheat type: but in possessing only
one cotyledon instead of two, they are clearly distinguished
from both of the other types.

In external features, the seeds of the Grasses present many
variations, though probably not so many as occur among some
other types of seeds. Most of them are small, but various sizes,
ranging from that of a Timothy seed and even smaller up to
that of a Corn kernel occur. In most cases they are elongated,
and have a groove on one side. In most varieties of Oats and
Barley, and in many of the Grasses having very small seeds, the
cariopsis remains enveloped by the palea and flowering glume, in
which case the entire structure may have the appearance of a
seed, especially when the barbed awns and other structures devel-
oped by the flowering glume function in dissemination. The
seeds of most Grasses are white, gray, yellow or brown, but in
Corn such colors as red, blue, purple, and black often occur.

The seeds of those Grasses known as the grains are our chief
source of food. Although all of the grains contain practically
the same food elements, they differ in the proportion of the differ-
ent elements and consequently are fitted for different uses. Even
within a seed, various structures differ so much in composition
that they are adapted to special uses as is well shown in the
milling of Wheat. Likewise in case of Corn, the oil and protein
content are so closely related to structure that one can judge the
relative proportion of these substances by observing the relative
sizes of certain structures of the kernel.

Corn Kernel. — A study of a section through a kernel of corn,
as shown in Figure 66, will give a notion of the general structure
of the Grass type of seeds. Notice that within the covering (a)
there are two distinct regions, that to the right and below being
the embryo, and that to the left and above being the endosperm.

CORN KERNEL

c°t

The location of the embryo at one side of the endosperm, instead
of being centrally located and surrounded by the endosperm, is a
peculiar feature of the Grass type of seeds.

The embryo consists of two main parts : the large scutellum or
cotyledon (cot) which lies in con-
tact with the endosperm, and the
embryonic axis which upon germi-
nation produces the stem at its
upper and roots at its lower end.
The axis is attached along its
central region to the cotyledon,
which supplies it food during
growth. At the upper end of the
axis is the plumule, a small bud-
like structure consisting of a grow-
ing point (gr) and some small
leaves (I). The plumule is en-
closed in a sheath (ct) called col-

eoptile. Between the plumule

, , , , £ , -f FIG. 66. — Section through a ker-

and the attachment of the coty- nel of Corn. cot) cotyledon; ej)) epi.

ledon is a short stem (st), which thelial layer of cotyledon; ct, coleop-
with the plumule is often called tile; gr, growing point of plumule;
epicotyl (the portion above the z> young leaves; st, epicotyl; r, radi-

cotyledon). The portion of the cle; rc' root caP> cr> coleorhiza; ".

. , . , ,, ,11 • . soft endosperm; h, hard endosperm;

axis below the cotyledon consists 0> covering called pericarp Much

chiefly of the radicle (r), the struc- enlarged.

ture which develops the first root.

The radicle bears at its tip the root cap (re) and is enclosed by

the coleorhiza (cr).

The hypocotyl, which is all or only a part of the axis between the
plumule and radicle (a point in dispute among botanists), is the
portion of the axis developing least when the embryo resumes
growth. In the Grasses there is very little elongation of the hypo-
cotyl and, consequently, the establishment of the young plant in
the soil and light depends mainly upon the growth of the radicle
and plumule. The fact that the hypocotyl remains small while
the radicle, since it forms the first root, becomes a prominent
structure, accounts for the general application of the term rad-
icle to all of the lower portion of the axis, and the rare use of the
term hypocotyl in connection with grass embryos.

64 SEEDS AND FRUITS

Concerning the cotyledon of the Grass embryo, there is some
dispute. Some morphologists regard the scutellum as the coty-
ledon, while others think that the cotyledon includes both the scu-
tellum and coleoptile. Although the cotyledon may include
other structures, the scutellum, in absorbing and supplying food
to the growing parts of the embryo, performs the function of a
cotyledon. The scutellum is a boat-shaped structure with its
keel-like portion imbedded in the endosperm. Its broad side
bearing the axis of the embryo is visible through the testa and
ovary wall. The keel-like portion is covered with specialized
cells formed into a layer called the epithelium. The epithelium
secretes soluble substances called enzymes, which after diffusing
to the endosperm change the foods stored there into soluble
forms, which are then absorbed by the cotyledon and carried to
the plumule and radicle where they are used for growth.

The principal food substances,
stored in the endosperm are starch,
fat, and protein. Although occur-
ring together in most parts of the
A B endosperm, each substance is

FIG. 67. — Kernels of Corn with present in a greater proportion
high and with low percentages of in gome iong than in otherg>
protein. A, kernel with high per- ,-,, „ , ., , , ,.

centage of protein. B, kernel with The Ce"S arOUnd the b°rder °f
low percentage of protein, a, horny *he endosperm and forming the
endosperm; b, white starchy endo- aleuron layer are especially rich in
sperm ;e, embryo. After Bulletin 87, protein, which is present in the
University of Illinois Agricultural form of granules and go abundant
Experiment Station. ,1^1 n i

that the cells appear as dense

granular masses. The remaining endosperm, which is especially
rich in starch, consists of two regions. The outer region (more
deeply shaded) is the horny endosperm (h) and contains much
protein in addition to starch. The inner region (n) (with lighter
shading) is the starchy endosperm, which is not only much softer
and more granular than the horny endosperm but also contains
less protein. The richness1 of the kernel in protein depends so
much upon the amount of horny endosperm that by cutting across
a kernel as shown in Figure 67, one may judge the richness of the
kernel in protein by observing the relative amount of the two
kinds of the endosperm. Likewise, since the embryo of the ker-

1 See Bulletins 44) 82, and 87, University of Illinois Agricultural Experiment
Station.

tl ' •

GRAIN OF WHEAT

nel contains most of the oil, the oil content depends largely upon
the size of the embryo. (Fig. 68.) Sometimes, however, much
of the starch of the endosperm is replaced by sugar, as in case
of Sweet Corn, which is much used as a -vegetable on account of
its soft sweet endosperm.

Grain of Wheat. — In structure, a grain of Wheat is similar to
a kernel of Corn. In the section through a Wheat grain, shown
in Figure 69, though the parts are not labelled, they can be deter-
mined by referring to the section of the Corn kernel shown in

FIG. 68. — Kernels of Corn
with high and with low per-
centage of oil. A, kernel with
large embryo and hence rich
in oil. B, kernel with small
embryo and low percentage of
oil. C and D, face views of
two kernels differing in size of
embryos and therefore in oil
content, e, embryo. After
Bulletin 87, University of Illi-
nois Agricultural Experiment
Station.

FIG. 69. — Lengthwise section
through a Wheat kernel. The
embryo is to be compared with
the embryo of the Corn kernel
(Fig. 66} and parts labelled.

Figure 66. In milling1 a grain of Wheat, a number of special
products are obtained. The woody pericarp and seed coat
with the aleuron layer and some of the outermost starch cells
constitute the bran. When bran is finely ground, it is known as
shorts. Middlings differ from shorts only in containing a larger
percentage of starchy endosperm. In making the best grades of
flour, only the starchy endosperm is used and the quality of the

1 On Bread. Bulletin 4, Ohio Agricultural College. Bread and Bread
Making. Farmers' Bulletin 389, U. S. Dept. of Agriculture.

SEEDS AND FRUITS

flour depends much upon the amount and quality of protein
(gluten) which the endosperm contains. When there is a good
quality of gluten present, the flour is characterized as being strong
and is the kind which.bakers prefer. Graham flour is the entire
grain finely ground. In making entire wheat flour, after the
grain is finely ground, some of the bran is removed. The em-
bryo, which constitutes about eight per cent of the grain, contains
much fat and oil, and, if the embryo is ground up with the flour,
the oil is apt to become rancid and impair the flavor of the flour.
For this reason, the embryo is removed in making high grade

flour and sold with the middlings
or used in making breakfast foods.
(Fig. 70.)

Oat Kernel. — In general form
and structure the Oat kernel is
similar to the grain of Wheat, ex-
cepting that it is more elongated
and the ovary wall is hairy.
The kernel usually remains en-
closed in the lemma and palea,
and the quality of Oats depends

upon the proportion of hull to
FIG. 70. — Section through the , , ^i -, , .

- TTrL kernel. I he endosperm contains

outer portion of a Wheat gram, w,

ovary wall, often called pericarp; t, Protein, starch, and fat and is a
testa or seed coat; a, aleuron layer valuable food for both man and
with cells filled with grains of protein; live stock.

Comparison of Seed Types. —

The three types of seeds differ
fundamentally in the number of
cotyledons and in the location
of the stored food. The difference in the number of cotyledons
is probably the more important one because all Flowering Plants
have been divided into two classes on the basis of whether or not
one or two cotyledons are present. Plants with seeds having
but one cotyledon are called Monocotyledons (one cotyledon).
Not only the grains and all other Grasses, but also Palms, Lilies,
Asparagus, Onions, and many other plants are Monocotyledons.
Plants with seeds having two cotyledons, as in case of the Bean,
and Buckwheat type, are called Dicotyledons (two cotyledons).
Besides Beans and Buckwheat, many other common plants,

g, starchy endosperm with cells large
and filled mainly with starch grains.
Some protein grains are present in the
starch cells, n, nucleus.

RESTING PERIOD

such as Peas, Clover, Alfalfa, Tomatoes, Melons, Cotton, Fruit
trees, and many forest trees are Dicotyledons. Each of these
classes includes a large number of important cultivated plants as
well as many that are regarded as weeds.

Since the classification into Monocotyledons and Dicotyledons
applies only to the Flowering Plants, such plants as the Larch,
Pine, Spruce, Fir, Hemlock, which belong to the
Gymnosperms where there are no true flowers,
are omitted in this classification. The seeds of
a number of the Gymnosperms commonly have
more than two and those of the Pine and Cypress
commonly have many cotyledons. (Fig. 71.)
They are polycotyledonous seeds and the plants
may be described as Poly cotyledons.

The difference in the location of the stored
food in seeds serves in distinguishing them but
does not affect their function or commercial
value. In all types of seeds, the endosperm
must be absorbed by the cotyledons before it is
available for the growth of the embryo. This
absorption occurs before germination in the ex-
albuminous seeds but during germination in albu-
minous seeds. Among Monocotyledons albumi- pine seed
nous seeds prevail, while both types are about tioned lensthwise
equally common among Dicotyledons. ° ? °w p° ^° '

Resting Period, Vitality, and Longevity of Seeds !?ed germimf "g>

the many cotyle-

The function of a seed as the plant's organ of dons becoming
dissemination depends upon a number of physi- free from the seed

FIG.

71. — a,
see-

ological features, of which the chief one is the

c°? ' , c

ability of the seed to remain alive with the ordi-
nary life processes so slowed that they seem not to be taking
place at all.

Resting Period. — The resting condition is a valuable physio-
logical feature to the seed, because in this condition the embryo
can endure cold, heat, dryness, intense light, and various other
conditions to which the seed is exposed during dissemination.
How the resting condition is brought about and how it is main-
tained, often many years, are not thoroughly understood. The
resting condition is associated with dryness, a condition obtained

68 SEEDS AND FRUITS

in the seed by allowing the greater part of the water to escape
during the process of maturing. Since the life processes depend
upon water for dissolving and transporting the necessary sub-
stances, they are naturally slowed down when water is with-
drawn and apparently without injury even when so checked
that no action can be detected by ordinary laboratory methods.
Some investigators have maintained that these life processes
actually stop, but the evidence sustains the view that these
processes never stop so long as the seed remains capable of ger-
minating. There are various factors involved in maintaining the
rest period, but chiefly they have to do with keeping water and
oxygen from the embryo.

The ability of seeds to endure extreme conditions while in the
resting stage is well shown in the case of temperature. In liquid
air, seeds of Alfalfa, Mustard, and Wheat have been kept at a
temperature of —250° C. for three days and afterwards success-
fully germinated, though their embryos when active are quickly
killed by a temperature a little below freezing. The ability of
dry seeds to endure heat is also surprising. Some in the resting
stage, if kept dry, can endure a temperature of 100° C., the tem-
perature of boiling water, without having their vitality impaired,
while their embryos, if active, would perish at 60° C.

The length of the resting period varies much for different kinds
of seeds and for seeds of the same kind. In a sample of Clover
seed, for example, many of the seeds may germinate in two or
three days, and s"ome may not germinate for a month or a year.
Although the seeds of some wild plants will germinate as soon as
mature, if given favorable conditions of moisture and warmth,
most of them, however, have a rest period which extends over
days, weeks, months, or even years, and often saves the young
plants from getting started at a time when they would soon be
caught by unfavorable conditions. Excepting some seeds like
those of the Clovers and Alfalfa, the seeds of cultivated plants will
usually germinate about as soon as mature. Although a desirable
feature, it sometimes results in loss, in that Corn, Wheat, Oats,
and other crops germinate in the field if the weather following
harvest is warm and wet. The resting period, which is retained
by Wild Oats and some other wild plants kindred to cultivated
ones, has been lost from our cultivated plants through many
years of selection.

VITALITY AND VIGOR OF SEEDS 69

In preventing the absorption of water and oxygen, which are
the elements upon which germination in most cases depends, the
seed coat and other protective structures are important factors.

Seed coats that prevent the escape of water and thus protect
the embryo against excessive drying also prevent the entrance of
water, and, if the seed coat is too impervious to water and air,
the germination of the seed is delayed. Seeds which have very
hard coats, unless they are treated artificially, must be exposed
to the weather until the seed coat is decayed sufficiently to allow
the entrance of water and air to the embryo before germination
can take place. In a sample of Red Clover, Sweet Clover, and
Alfalfa seed, often there are many seeds, known as hard seeds, with
coats so hard that germination is delayed or prevented. When
sown, they either lie in the ground too long before germination or
do not germinate at all. By scratching or pricking their seed coats,
so that water and air can enter more readily, they germinate more
promptly. Experiments1 have shown that Clover seed which
has been thrashed through a huller where it is scratched by the
spikes germinates much better than seed hulled by hand. This
principle is so well recognized that machines especially devised
for scratching or pricking the coats of Clover seed have been in-
vented. The opening of the seed coats of the Sweet Pea and
Canna with a file and Peach pits with a hammer are other in-
stances in which the rest period is broken by artificial means.

In some cases, as in the Hawthorne, delayed germination de-
pends upon the embryo, which must undergo a process known as
" after-ripening " in which acids, enzymes, or other essential sub-
stances are formed. In some weed seeds, delayed germination
has been found to depend upon the toughness of the seed coat,
which allows water and air to enter, but is so resistant to pressure
that it will not allow the embryo to expand until its resistance is
weakened by decay.

Vitality and Vigor of Seeds. — Seeds are worthless for planting
unless they have life, or vitality. Not only the vitality, but also
the amount of life or vigor the seed has is an important feature.
If the embryo of a seed is dead, the seed will not germinate. If
the embryo is lacking in energy, though it may germinate, the
plant which it produces will be weak. Only seeds with vigorous
embryos are fit for planting.

1 See, Bulletin 177, Vermont Agricultural Experiment Station.

SEEDS AND FRUITS

The vitality and vigor of seeds depend upon the following
factors: (1) the vigor of the plant which produced the seeds; (2)
external conditions which affect seeds during their development;
(3) maturity of seeds; (4) weight and size of seeds; (5) methods
of storing; and (6) age of seeds.

The seeds of vigorous plants are preferable to those of weak
plants, for the sperms and eggs of vigorous plants are likely to be
more vigorous than those of weak plants, and, therefore, more
capable of producing vigorous embryos. Furthermore, seeds
of vigorous plants may have more stored food for the embryo to
feed upon during germination and the seedling stage. Plants
having a stunted growth, due to drought, lack of food, or attacks
of enemies, are likely to produce small and often shriveled seeds
which are lacking in stored food and usually have weak embryos.

Seeds are often injured by frosts occurring while the seeds are
immature and full of water. The embryos of Corn and other
seeds are sometimes killed by early frosts. Even seeds which
have reached maturity cannot endure hard freezing unless they
are dry. For this reason most seeds should be collected from
the field before they have been exposed to a hard freeze.

Abnormal seeds have a low vitality or will not germinate at all.
Kernels of Corn produced on the tassel usually give a low per-
centage of germination. Sometimes, as in case of Sweet Clover
and Alfalfa, when the conditions are unfavorable, seeds are pro-
duced with imperfect embryos which are not capable of developing
plants. There are some plants in which seeds sometimes develop
without embryos and of course will not germinate at all. This
sometimes occurs in the Apple and Pear. When seeds are muti-
lated their vitality is usually impaired. Larbaletrier asserts that
15 per cent of the Wheat crop in France is injured by the thresh-
ing machine. He cut the kernels with a knife so as to represent
the injury from the machine and compared their germinative
power with that of sound kernels, obtaining a much lower per-
centage of germination as the results given in the table below

Sound kernels, per
cent of germination.

Cut kernels, per cent
of germination.

68
74
99

34
3
38

LONGEVITY 71

show. Sturtevant mutilated the kernels of a Flint Corn and the
seeds of Beans and found the percentage of germination much
reduced in each case.

Seeds collected while immature usually show a low percentage
of germination and their embryos grow slowly. In the case of
Rye, seeds have been harvested at different stages of their devel-
opment and, after similar treatment in respect to drying and
storage, the percentage of germination and vigor of embryos de-
termined. In the milk stage five per cent germinated, while in
the dry ripe stage eighty-four per cent germinated. The embryos
of the dry ripe seeds were much more vigorous in growth than
those of the immature seeds. Tomato seeds, while still green and
not more than two-thirds the weight of mature seeds, may be
germinated, if properly cured, but the plants produced are likely
to be weak. The germination of unripe seeds has been given
considerable attention by Sturtevant, Arthur, and Golf.1

Experiments 2 with seeds of the Radish, Sweet Pea, Cane, Rye,
Oats, and Cotton have shown that better stands in the field and
more vigorous and better yielding plants are secured by using
only the heavier seeds.

The vitality and vigor of seeds depend very much upon the
methods of storing. Seeds are more easily killed by extremes of
temperature when wet. Seeds stored where there is considerable
moisture may start to germinate, and then die. Seeds, massed
together before they are well dried, become moist and often so
warm that the embryos are injured. On the other hand, when
stored in rooms where the air is warm and extremely dry, seeds
may lose moisture so rapidly that the embryos are killed. A
storage room should be cool but above freezing, and dry, although
not excessively dry. Until the seeds are well dried, they should
not be massed together, but so arranged that the air can circulate
about them. Thus methods of storing seed Corn and other seeds
must reckon with a number of factors which affect the vitality
and vigor of seeds under storage conditions.

Longevity. — The vitality and vigor of seeds depend much
upon their age. Seeds in excellent condition and stored by the best
methods finally lose their vitality, due to the coagulation of their
protoplasms, too much drying, or some other factor not under-

1 American Naturalist, pp. 806 and 904. 1895.

2 Farmers' Bulletin 676, U. S. Dept. of Agriculture.

72 SEEDS AND FRUITS

stood. Some seeds may retain their vitality for centuries, but
most seeds lose it in a few years. The length of time during which
seeds retain their vitality is called their longevity. Most agri-
cultural seeds can be stored two or three years without much loss
of vitality, and some, when stored a much longer period, may
contain a large number of live seeds. One investigator found
that 50 per cent of samples of Red Clover seeds germinated after
being stored in bottles for 12 years; and in samples of the seeds
of Pigweed, Sheep Sorrel, Black Mustard, and Pepper Grass,
stored in the same way, a large percentage germinated after a
storage of 25 years. In samples of White Sweet Clover seeds,
which have well modified seed coats, 18 per cent have germinated
after a storage of 50 years. There is good evidence that some
of the leguminous seeds may retain their vitality for more than
a century. Many of the weed seeds when buried in the soil can
retain their vitality for many years and then germinate when
conditions become favorable.

The longevity of seeds depends so much upon the conditions
under which the seeds were grown, maturity when collected, and
methods of storing, that statements as to how old any kind of
seeds may be and still be safe for planting are not reliable. Old
seeds are often preferable to new ones grown under unfavorable
conditions. Seeds from poorly developed plants, although sim-
ilar in appearance to those produced under favorable conditions
and giving a high percentage of germination soon after harvest,
decline rapidly in vitality, often being worthless at the next plant-
ing season. For example, Cabbage seeds eight years old may
germinate 70 or 80 per cent, while some only three years of age
but grown in an unfavorable year may germinate less than 40
per cent. Seeds collected green may germinate well after proper
curing but they have a short longevity.

The longevity of seeds depends probably more upon dryness
than any other factor. For this reason the place of storage
should be dry and the seeds should be cured before they are
stored by placing them in a dry airy place. Experiments show
that Corn collected soon after maturity and properly cured and
stored gives a much higher percentage of germination the next
season than Corn allowed to stand in the shock, or taken from the
crib. Comparative L germinative tests of seeds stored in different
1 Bulletin 58, Bureau of Plant Industry, U. S. Dept. of Agriculture.

LONGEVITY

parts of the United States have shown that seeds do not live as
long in the warm moist air of the Southern states as they do in
the cool dry air of the Northern states.

In the following table compiled from various sources is given
the time "beyond which it is not advisable to use the seeds men-
tioned unless the contrary is shown by germinative tests.

Corn

Wheat

Oats

Barley .

Rye

Buckwheat

Beans (common) 4 to 5

Peas 4 to 5

Clovers 2 to 3

Alfalfa 3 to 4

Onion 1

Years.
2
2
2
2
2
2

Years.

Mustard 3 to 4

Cabbage 3 to 4

Turnips 3 to 4

Swede 3 to 4

Pumpkin 5

Melon (musk) 5

Melon (water) 5

Squash 3

Tomato 6

Timothy 1 to 2

Celery 1

In some cases perfect seeds well stored may have more than
double the longevity given in the above table. Thus Sturtevant
obtained 100 per cent germination of various varieties of Corn
after being stored 5 years. Tomato seeds
14 years old have been known to give a
high percentage of germination. On the
other hand, using the same seeds as an
example, both Corn and Tomato seeds
are sometimes unfit for use when only
1 year of age. These varying results em-
phasize the importance of testing the
germinative power of seeds before use.

The variation in the longevity of the
seeds of a given lot is obvious when the
percentages of germination for different
periods of storage are compared. The
decrease in the percentage of germination
as the length of the storage period in-
creases shows that some seeds die early magnifier from above,
and others later until finally all are dead.

In the following table are given the results of an experiment
to determine the rate at which vitality is lost as indicated by the
percentage of germination obtained in each of the 6 years of
storage.

FIG. 72. — A cheap mag-
nifier well adapted for use
in analyzing seeds. The
magnifier is set over the
seeds, leaving the hands
free to separate the seeds
as one looks through the

SEEDS AND FRUITS

PER CENT OF GERMINATION FOR EACH OF 6 YEARS
OF STORAGE

Seed.

lyr.

2yr.

3yr.

4yr.

5 yr.

6yr.

Wheat

82.3

77.3

Oats

90 2

78.2

29.5

Barley

78.5

19.5

7.5

Peas

Flax

Purity and the Analysis of Seeds

The impurities of seeds consist of seeds of other species and of
dirt, such as soil particles, chaff, hulls, and other plant fragments.
In sowing impure seeds one can not estimate the amount of
desirable seeds sown unless the percentage of impurities is pre-
viously determined so that allowance can be made. Besides one
is likely to sow the seeds of undesirable plants, which choke the
crop and cause much trouble and expense in eradicating them.
A small per cent of weed seeds is often a serious matter. For
example, in sowing Grass seeds which contain only 1 per cent of
weed seeds there is the possibility of 20 or more weeds to the
square yard. Nobbe found enough weed seeds in a certain
sample of Timothy seeds, if sown at the ordinary rate, to supply
24 weeds to every square foot of land. Furthermore, in purchas-
ing impure seeds, unless a deduction from the price is made for
the impurities, one pays more than he should for the desirable
seeds obtained.

More impurities occur among the smaller agricultural seeds,
as Grass, Clover, and Alfalfa seeds, than among the grains, al-
though a few very bad weed seeds, such as those of Quack Grass
(Agropyron repens), Cow Cockle (Saponaria Vaccaria), Corn
Cockle (Lychnis Githago), and English Charlock (Brassica
Sinapistrum) , are common among the small grains.

Seed Analysis. — A bag of seeds may be analyzed for two
reasons: (1) to determine the percentage of the desirable seeds
contained or to determine the percentage of impurities regardless
of their kinds; and (2) to determine the kinds of impurities and
the percentage of each present. In either case the determination
is based upon the analysis of only a small sample, which is usually
prepared by mixing well a handful or more of seeds taken from

SEED ANALYSIS

different parts of the bag or container, usually from the top,
middle, and bottom. From the sample from 2 to 5 grams are
weighed out, and the impurities and desirable seeds are then sepa-
rated, usually by means of a lens like the one in Figure 72. By

JN'f

"a'w ^^ *\jk

UCanad^cx 15WAA ^

FIG. 73. — Some weed seeds and fruits commonly found among Red Clover
seeds. Enlarged and about natural size. From Farmers' Bulletin 455,
U. S. Dept. of Agriculture.

dividing the weight of the desirable seeds and the weight of the
impurities by the number of grams analyzed, the percentage of
each is obtained. Thus, if 5 grams are analyzed and the weight

of the desirable seeds found is 4.8 grams, then -^- = 96 per cent,

which is the percentage of purity. In determining the kinds of

SEEDS AND FRUITS

impurities and their percentages it is not enough to separate the
impurities and desirable seeds, but the kinds of impurities must
be identified, separated, and the weight necessary for finding the
percentage of each must be separately determined. In this kind

1 Jl/fe/fa 2 Yd/owfafoil 3 "* Burcfonr

<!> $

7 'Ufa 'mustard 8 Cttr/e</</oc/c

9 ftussiarr thisf/e

10 Lamb's quarters

FIG. 74. — Some weed seeds commonly found among Alfalfa seeds. En-
larged and natural size. Adapted from Farmers' Bulletin 495, U. S. Dept.
of Agriculture.

of analysis, the operator, unless he is well acquainted with the
various kinds of seeds, should have at hand for comparison
samples or figures of the seeds of weeds and other plants likely
to occur among the seeds which are being analyzed. Samples
are better, but figures as shown in Figures 73 and 74 may serve
quite well.

TOMATO OR BERRY TYPE

Nature and Types of Fruits of Flowering Plants

A fruit is difficult to define because not all fruits involve the
same structures in their formation. Some fruits are only much
enlarged ovaries; but there are others which involve other struc-

-5

-a

FIG. 75. — A, cross section of a Tomato. B, cross section of an Orange.
w, ovary wall; p, placentas; s, seeds; a, partition walls; I, locules.

tures closely related to the ovary. Since fruits involve a number
of structures in their formation, it will be best to study some
types and then formulate a definition.

Tomato or Berry Type. — The fruit of
the Tomato consists of the ovary which
has enlarged and become fleshy and juicy.
The most edible portion consists of the
fleshy enlargements which develop from
the inner angle of the locules and almost
fill them. These enlargements bear the
seeds and hence are the placentas much
enlarged. Also the citrus fruits, such as
Oranges, Lemons, etc., are of the berry
type. However, they have no fleshy
placentas. The seeds are attached to the
small central core, and the juicy tissues
developing from other parts of the ovary
and filling the locules constitute the flesh
of these fruits. The fleshy and juicy
features are characteristics of the berry; and a berry is often
defined as a fleshened juicy ovary. (Fig. 75.)

FIG. 76. — Lengthwise
section through a Plum,
s, seed; p, wall of pit;
/, fleshy portion of ovary.

SEEDS AND FRUITS

Plum or Stone Type. — The Plum, Peach, Cherry, and Apri-
cot, commonly called drupes, are fleshy ovaries, but differ from

-r

s B

FIG. 77. — Section through flower and fruit of the Apple. A, section
through the flower, a, receptacle; b, ovaries; d, ovules; t, floral organs,
calyx, corolla, stamens, styles and stigmas. B, section through the fruit,
a, receptacle; c, core; s, seeds; r, remains of floral parts; I, the flesh around
the core, bounded on the outside by the conductive vessels, indicated by the
lines. The inner portion of this band of flesh is the outer portion of the ovaries,
the remainder of it being the inner portion of the receptacle.

the berry type in that the portion of the ovary immediately sur-
rounding the locule hardens into the stone or pit. In Figure 76,
point out the seed, the pit, and the fleshy
portion of the ovary.

Apple or Pome Type. — The Apple,
Pear, and Quince are examples of pome
fruits, and their structure can best be un-
derstood by studying Figure 77. . The
receptacle of the flower is not flat as it
is in many flowers, but is hollow or urn-
shaped; and the five ovaries are located
in the hollow of the receptacle and are
grown fast to its sides. The calyx, petals,
and stamens are located on the rim of
the receptacle and thus above the ova-
ries. As the fruit develops, the receptacle
surrounding the ovaries thickens and
forms the greater part of the fruit, while the ovaries form the
portion known as the core.

FIG. 78. — Cross section
of a Cucumber, r, rind
consisting of receptacle
and ovary wall closely
joined; I, locules; p, pla-
centas; s, seeds.

BLACKBERRY TYPE

Melon or Pepo Type. — In the Melons, Cucumbers, Pump-
kins, and Squashes, which illustrate well the pepo type, the
ovaries are inclosed in the receptacle, and with the receptacle to

! FIG. 79. — Flower and fruit of Strawberry. A, section through flower,
showing the fleshy receptacle (r) and the many pistils (p) on its surface.
B, fruit consisting of enlarged receptacle (r), bearing the small hard ovaries (o).

which they are closely joined form the rind. (Fig. 78.) The
placentas are more or less fleshy and in case of the Watermelon,
where they form large juicy lobes, they constitute the bulk of
the edible portion. In most cases, however, as Muskmelons and
Pumpkins illustrate, the placentas break
loose from the ovary wall and are removed
with the seeds. In what way does the
Melon resemble the Apple in structure?
How does it differ from the Apple?

Strawberry Type. — In the Strawberry
the ovaries develop into hard one-seeded
fruits (akenes) which appear as small hard
bodies over the surface of the much flesh-
ened receptacle. (Fig. 79.) In the Straw-
berry, although the ovaries are included
when the fruit is used, the edible portion
is the receptacle.

Blackberry Type. — In this type the

u £ f, £j_ FIG. 80. — Fruit of the

ovaries develop as small stone fruits, often m , u

* ' Blackberry, r, recepta-

called drupelets (miniature drupes), and cle; /, fleshened ovaries,
with the fleshened receptacle form the

fruit. (Fig. 80.) Very similar to the Blackberry is the Rasp-
berry, in which the drupelets collectively separate from the re-
ceptacle and thus alone form the fruit.

SEEDS AND FRUITS

In developing from a single flower but involving a number of
pistils, the fruits of the Strawberry and Blackberry are similar
and are classed as aggregate fruits.

Pineapple Type. — In the formation of the Pineapple a num-
ber of flowers are involved, each of which consists of a small
pistil surrounded by large scales and is borne in the axil of a modi-
fied leaf. Each ovary with
its scales and modified leaf
becomes fleshy to form a
single fruit. The entire fruit
of the Pineapple consists of a
number of these single fruits
closely packed together on
an axis which forms the core
of the Pineapple. Since a
number of flowers are in-
volved, fruits of this type
are known as multiple fruits.
(Fig. 81.)

Nut Type. — In the nut
type of fruit, the ovary is
hard and is generally partly
or entirely covered by a
husk formed by the perianth
or by bracts which grow up
from the receptacle. (Fig.
82.) Notice the develop-
ment of the Acorn shown in
Figure 83.

Some Other Familiar Types of Fruits. — In many small fruits
the ovaries become dry and often hard as the fruit matures.
They are the kind which when small and one-seeded are often
called seeds. It has been mentioned that the akenes of the Buck-
wheat and the cariopsis of the Grasses are fruits with hard ovary
walls. In the Clovers, Alfalfa, and Beans the ovary wall becomes
dry and hard when mature, forming the structure known as the
pod or legume. (Fig. 84-) Many of the so-called weed seeds are
dry ovaries. In many cases, however, other structures are joined
with the hardened ovary in the formation of the fruit. In the
Dandelion and many other plants of the Composite type, the

FIG. 81. — Pineapple. After Koch.

DEFINITION OF A FRUIT

pappus, consisting of hair-like structures which correspond to
the calyx of the ordinary type of flower, remains as a part of the
fruit, forming a parachute-like arrangement which enables the

FIG. 82. — Pistillate flower and fruit of a Hickory (Gary a). A and B, ex-
terior and interior views of the flower. C, the nut. 6, bracts surrounding the
pistil (p)', o, ovary. Flower much enlarged but fruit reduced.

fruit to float in the air. Sometimes, as in the Spanish Needles,
the calyx remains on the fruit as spiny appendages. In the case
of the Birch, Elm, Ash, and Maple, the fruit known as a samara
or key-fruit has wing-like structures which are outgrowths from
the ovary wall.

FIG. 83. — Flower and fruit of an Oak (Quercus). A, pistillate flower,
showing the bracts (6) which surround the ovary. B, section of the flower,
showing the ovary (o) and the bracts (6). C, acorn, showing the ovary and
cup. s, stigmas. Flower much enlarged but fruit nearly natural size.

Definition of a Fruit. — From an examination of the above
types of fruits, it follows that a fruit may consist of: (1) simply
the ovary either dry or fleshy; (2) ovary or ovaries and recep-

SEEDS AND FRUITS

tacle; (3) ovary with perianth or bracts forming a husk; (4) ovary
with calyx forming hairs or spines; and (5) a number of single
fruits with the modified leaves and floral
axis of the flower group. A fruit may
be defined as one or more ripened ovaries
either with or without closely related
parts.

Dissemination of Seeds and Fruits

Dissemination has to do with the
scattering of seeds from the parent plant.
Sometimes the seed is transported naked,
but often it is transported enclosed in
the fruit or with some larger part of the
plant.

The necessity for dissemination is ob-
vious, for if the seeds of a plant were to
FIG. 84. — The dry coiled germinate where formed or on the ground
fruits (pods) of Alfalfa directly beneath, the resultant conges-
(Medieago sativa). From tion would prevent the normal develop-
Farmers' Bulletin 895, U.S. ment of any of the plantg> Green

Dept. of Agriculture. , ^• ^ , , •

plants must have sunlight and air, and

this means that they must have room.

Of course seeds and fruits are not the only means by which
plants spread. Many Seed Plants have an additional means in
either spreading stems or roots which give rise to new plants as
they spread farther and farther from the parent. The Straw-
berry depends mainly upon its runners, and the Quack Grass
much upon its underground stem as a means of spreading. Pop-
lars, some fruit trees, and Canada Thistle are well known to
spread by means of sprouts arising from their roots. Most plants
which do not have seeds spread by means of spores which in some
cases seem to be a more efficient means than seeds are. For
example, Wheat Rust, a disease which spreads very rapidly, is
spread by spores.

In the dissemination of seeds and fruits, wind, water, and ani-
mals are the chief agents. In a few plants there are explosive or
spring-like mechanisms which throw the seeds.

Seeds and Fruits Carried by Wind. — The wind is one of the
most important agents in the distribution of fruits and seeds. In

SEEDS AND FRUITS CARRIED BY WIND

the Thistle, Dandelion, Wild Lettuce, Fireweed, Iron weed, White
Weed, Fleabane, and others, the tufts of downy hairs on the small
dry fruits in which the seeds are enclosed enable the fruits with the
seeds to be lifted and carried many miles by the wind. In the
Milkweeds, the seeds bear long hairs which make them easily
carried by the wind. In some plants, as in the Curled and Smooth
Dock, Ash, Elm, and Maple, the fruits are winged and easily
borne away by a passing breeze. The fruits of some of the

FIG. 85. — Some fruits and seeds disseminated by the wind, a, fruits of the
Basswood (T ilia Americana) and the leaf -like bract which floats in the air and
thereby scatters the fruits. 6, samara or winged fruit of a Maple, c, fruit of
a Wild Lettuce (Lactuca Floridana). d, winged fruit of an Elm. e, pods of a
Milkweed (Asclepias syriaca) allowing the seeds to escape to be scattered by
the wind, a, c, and e from Hayden.

Grasses are enclosed in chaff bearing long hairs and are easily
blown about. The fruits and seeds of Ragweeds, Velvet-leaf,
Docks, Pigweeds, Chickweeds, and some plants of the Grass fam-
ily are blown long distances over the surface of snow, ice, or
frozen ground. (Fig. 85.)

Some plants break off near the ground after ripening their seeds
and are rolled over and over by the wind, dropping their seeds as
they go. These are known as the "tumble-weeds" and include
the Russian Thistle, Tumbling Mustard, Tumbling Pigweed,
Buffalo Bur, Old Witch Grass, and a number of others. (Fig. 86.)

SEEDS AND FRUITS

Seeds and Fruits Carried by Water. — Plants, such as the
Great Ragweed, Smartweeds, Bindweeds, Willows, Poplars, and
Walnuts, which grow along streams, have their seeds and fruits
floated away during overflows. Sometimes, when the banks of

FIG. 86. — Plants of the tumble weed (Amaranthus albus} tumbling over
the ground and scattering seeds as they go. After Bergen.

streams cave off, plants with ripened seeds fall into the current
bodily and are carried for miles down the stream, finally lodging
in fields where their seeds grow. The seeds of plants growing on
the upland are washed to the lowlands during rains and seed the
bottom fields. Some fruits, as in case of the Coconut, are so
resistant to salt water that they can be carried long distances by
ocean currents.

Seeds and Fruits Carried by Animals. — Birds eat the fruits
of some plants for the outer pulp, and the hard seeds pass undi-
gested. In this way .the seeds of the Nightshades, Poison Ivy,
Poke weed, Blackberry, Pepper Grass, and others are distributed.
Even the seeds and fruits of Thistles, Dandelion, Ragweeds, and
Knotgrass may be eaten in such large quantities that many pass
undigested and start new plants wherever they fall. Birds often
carry sprigs of plants to places where the seeds may be eaten
without molestation and in this way distribute seeds. (Fig. 87.)
Birds that wade in the edge of ponds, lakes, and streams
often carry away on their feet and legs mud containing seeds.

SEEDS AND FRUITS CARRIED BY ANIMALS

Darwin took 3 tablespoonfuls of mud from beneath the water at
the edge of a pond and kept it in his study until the seeds con-
tained developed into plants. From this small amount of mud,
he obtained 537 plants which represented a number of species.
From this it is evident that the
rnud, carried on the feet and legs
of water birds, may be the means
of distributing many seeds.

The fruits and seeds of many
plants have spines, or small hooks
by which they become attached
to passing animals and are carried
far and wide. Some familiar ex-
amples are the burs of Burdock,
Cockle Bur, and Sand Bur, and
the hooked and spiny fruits of the
Buttercups, Wild Carrot, Beggar's
Lice, Tick-trefoils, Beggar-ticks,
and Spanish Needles. They catch

• • 1 i T«T/» J- J.\JC. <^J I . J-i V-/J.a.iVJ.

in the wool, manes, and tails of fruit_ From Bulletin
stock and in the clothing of man, logical Survey,
and are carried from one pasture

to another or from one farm to another. Live stock are impor-
tant agents in distributing plants on the farm. The seeds of
the Mustards are mucilaginous when wet and, by sticking to the
feet of animals or the shoes of man, are carried to new situations.
(Fig. 88.)

Many plants owe their distribution to man more than to any
other agent. The railways, connecting all of the states and
reaching from ocean to ocean where they connect with steamship
lines from across the seas, are responsible for the wide distribution
of many plants. For example, the seeds of a number of weeds
are shipped across the country with grain and other farm seeds,
and also in hay, bedding, packing, in shipments of fruit, and
in the coats of live stock. They fall from the cars as the train
travels, and seed the right-of-way where the plants first appear
and then later spread to the surrounding fields. The railways
are responsible for the wide distribution of Russian Thistle,
Prickly Lettuce, Canada Thistle, and Texas Nettle, which first
appear along the railway and later spread to the surrounding

FIG. 87. — A Chickadee carrying
Iowa Geo-

SEEDS AND FRUITS

farms. Buckhorn, Ox-eye Daisy, and many other weeds are
often first found along the railway. Seeds of various kinds are
often carried in the packing around nursery stock. Quack Grass
Canada Thistle, Ox-eye Daisy, and other weeds are often spread

FIG. 88. — Some spiny weed fruits which catch to the coats of animals,
a, cow with tail loaded with weed fruits. 6, fruits of Beggar-ticks (Bidens).
c, spiny fruit of Burdock (Arctium Lappa). d, fruit of Comfrey (Symphytum} .
e, fruit of another Beggar-tick. Adapted from Bailey and from Hayden.

in this way. Quack Grass is often carried in straw, and may
be introduced on a farm by using straw for covering Grapes and
Strawberries. Manure hauled from livery stables is a very im-
portant means of introducing plants on the farms where the
manure is used. In hauling hay along the highways, seeds of
various kinds are dropped and from the highways the plants
spread to the fields. Those weeds, such as Quack Grass, White
Top, Field Sorrel, and others which are common in meadows,
are often spread in this way. When the fields are wet, seeds

SEEDS SCATTERED BY EXPLOSIVE MECHANISMS 87

collect on the wagon wheels and are carried to the highways or
to other fields. Threshing machines are important agents in
scattering seeds, for in their traveling through the country seeds
of various kinds are jostled
from them and seed the
fields and highways.

Man scatters many
weeds by sowing unclean
seed. Clover seed, Alfalfa
seed, Grass seed, Wheat,
Oats, etc., are often ob-
tained from distant states
or even from foreign coun-
tries for seeding. Weed
seeds are usually present FIG. 89. — The three-valved pod of the
in agricultural seeds, and Violet throwing its seeds. Much enlarged,
sometimes they are pres- e

ent in large quantities. In tracing weeds, it has been found
that many of the most troublesome ones have come from Europe,
Asia, or some other foreign country. Man has carried the seeds
and fruits of these weeds across the seas, and most of them have

been imported and sown with agri-
cultural seeds.

Seeds Scattered by Explosive
or Spring-like Mechanisms. — In
this kind of dissemination the
plant itself is the agent which,
either by sudden ruptures due to
strains or by explosions due to
the swelling of certain tissues, is
able to throw the seeds often a
considerable distance. In the
pods of some plants, as in the
Vetches, Witch-hazel, Castor
FIG. 90. — The Squirting Cu- Bean, and Field Sorrel, bands of
cumber (Ecbalium Elaierium) tisg which ri under tension,
squirting its seeds from the pod. . ,, , ,, -,

exert such a strain that the pods

suddenly rupture with so much violence that the seeds are thrown
in every direction. In the Violets the carpels, as thdy ripen and
dry, press harder and harder upon the seeds, which suddenly

88 SEEDS AND FRUITS

shoot out as a Watermelon seed may shoot out from between
one's pressed fingers. (Fig. 89.) In case of the Impatiens called
" Touch-me-not " and the " Squirting Cucumber/' tissues within
the pod take up water and swell so much that the pod finally
explodes and scatters the seeds as shown in Figure 90.

CHAPTER VI

GERMINATION OF SEEDS: SEEDLINGS
Nature of Germination and Factors upon which it Depends

Although the resting condition is very essential to the preser-
vation of the life of the seed during transportation and while
awaiting favorable conditions for germina ion, it must be aban-
doned at some time in order that the embryo may develop into
the plant, the . production of which is the seed's chief function.
By germination of a seed is meant that awakening from the rest-
ing condition in which the young plant shows practically no
signs of life to a state of active growth. The term germination
is used in different ways, being used to designate the beginning
growth of such structures as a pollen tube, fertilized egg, and
spore, but in each case, however, it refers to the initial growth.
Seeds are considered germinated when the radicle and plumule
have broken through and project beyond the seed coverings,
although germination is not complete until the little plant is able
to live independently of the stored food of the seed.

Conditions Necessary for Germination. — The awakening of
the seed into active growth depends upon the presence of
warmth, moisture, and oxygen. Germination is so dependent
upon these three external factors that, if either is lacking though
the other two are properly supplied, there will be very little or no
germination. Among different seeds, the degree of temperature
and the amount of moisture and oxygen required for the best
germination vary.

Temperature Requirement. — Seeds vary more in the temper-
ature required for germination than in any other factor. Through
experience we have learned that among farm and garden seeds
there are different temperature requirements for germination,
and that the time of season at which different seeds should be
planted must be chosen accordingly. Thus Oats, Wheat, and Red
Clover seeds, which have a low temperature requirement, can be
planted in the early spring or late fall when the weather and soil are

GERMINATION OF SEEDS: SEEDLINGS

cool, but if Corn or Melons, which have a high temperature re-
quirement, are planted before the weather and ground are warm
they will decay and have to be replanted. In considering tempera-
ture in relation to germination, three temperatures are usually
noted; the minimum, the lowest temperature at which germina-
tion will occur; the optimum, the temperature most favorable
for germination; and the maximum, or highest temperature per-
mitting germination. As the following table shows, these tem-
peratures are very different for different seeds, sometimes differ-
ing as much as 25° or 30° (Fahrenheit).

GERMINATION TEMPERATURES (FAHRENHEIT)

Kind of seeds.

Minimum.

Optimum.

Maximum.

Oats

Deg.

32-41

Deg.

77- 88

Deg.

88- 99

Wheat, Rye
Indian Corn

32-41
41-51

77- 88
99-111

88-108
111-122

Red Clover
Peas

32-41
32-41

77- 88
77- 88

99-112
88- 98

Sunflower

41-51

93-111

111-122

Pumpkin

51-61

93-111

111-122

Musk Melon

60-65

88- 99

111-122

Cucumber

60-65

88- 89

111-122

Germination, which proceeds most rapidly at the optimum
temperature, decreases in rate as the temperature approaches
the minimum or maximum as the following table shows in
case of Corn, in which the time required for the radicle to break
through, though only 2 days at the optimum temperature, was
10 days in a temperature near the minimum. In the majority of
cases, the temperature of the soil in which seeds are planted is
somewhat below the optimum and, consequently, if the soil tem-

EFFECT OF TEMPERATURE ON RATE OF GERMINATION

Germinating Period in Hours.

Temperature ° F.

Indian Corn.

Red Clover.

240

180

144

102.6

111.2

MOISTURE REQUIREMENT 91

perature is lowered as it often is by heavy rains which fill the soil
with water or by days of cool cloudy weather, germination is
either very slow or prevented as is well known to every farmer
and gardener.

Moisture Requirement. — The amount of moisture required
for germination is, in general, that which will completely saturate
and soften the seeds. The water absorbed saturates the cell
walls and starch grains, and fills the living cells of the embryo
and all empty spaces that exist in the seed. Although the amount
of water required to saturate different seeds varies, it is always a
large per cent, sometimes more than 100 per cent of the dry
weight of the seed, as shown in the table below. Reckoning in
pounds from the percentages given in the table, 100 Ibs. of Corn
after being soaked for germination may weigh 144 Ibs. and 100
Ibs. of White Clover seeds after soaking may weigh 226.7 Ibs.

WATER ABSORBED BY GERMINATING SEEDS

Seeds.

Per cent of water
absorbed in
germination.

Indian Corn.

Wheat

45 5

Buckwheat . .

46 9

Rye .

57 7

Oats
White Beans

59.8
92 1

Peas

106 8

Red Clover .

117 5

Sugar Beet

120 5

White Clover .

126 7

Most seeds, though not all, swell as water is absorbed, some-
times more than doubling their dry size. In fact, the per cent
of increase in volume is often greater than the per cent of water
absorbed, as in case of the Pea which may increase in volume
167 per cent while absorbing only enough water to increase its
weight about 100 per cent.

If seeds are confined in a space which they fill when dry, their
swelling may exert a force of several hundred pounds and often
sufficient to break strong containers. This force is sometimes
used in opening skulls in anatomical laboratories, in which case
the skulls are filled with dry Peas, which after being moistened
swell and force the bones apart.

GERMINATION OF SEEDS: SEEDLINGS

Oxygen Requirement. — Although seeds are in the optimum
temperature and properly supplied with moisture, they will usu-
ally not germinate unless oxygen is supplied, as is often demon-
strated in the laboratory by the use of some substance to absorb
the oxygen in the germinator or by replacing the air in the ger-
minator with hydrogen, nitrogen, or some other substance, so that
oxygen is excluded. (Fig. 91 .) However, since the air is about
one-fifth oxygen, seeds receive enough oxygen to germinate well
if only air is supplied, although germination is often hastened

--S- -

FIG. 91. — The two U-shaped tubes, which contain soaked seeds (s) on
moist blotting paper at their stoppered ends, are alike except that in B the
open end of the tube is in pyrogallate of potash, which absorbs the oxygen
from the air in the tube, while in A the open end of the tube is in pure water,
in which case the oxygen still remains in the air of the tube. The seeds ger-
minate well in A but not in B.

when the amount of oxygen is increased artificially. For exam-
ple, in an experiment Wheat, requiring 4 to 5 days to germinate
in the air, germinated in 3 days in pure oxygen. There are a
few seeds, however, which begin to germinate without oxygen, but
they soon die unless oxygen is supplied.

For lack of oxygen seeds germinate poorly when planted in the
soil so deeply that not enough air is accessible, or when planted
in soils with their pores so full of water that the circulation of the
air is prevented.

CHANGES IN THE STORED FOOD 93

Germinative Processes

Seeds need water, oxygen, and warmth in germination because
upon these external factors the internal germinative processes
depend. For dissolving and transporting foods water is indis-
pensable; the occurrence of certain chemical processes depends
upon oxygen; and in order for both chemical and physical proc-
esses to be suitably active, as previously shown (page 90),
warmth is required.

Changes in the Stored Food. — The first of the germinative
processes has to do with the digestion and translocation of the
stored foods. Whether stored outside of the embryo or in the
cotyledons, the stored foods, until brought nearer, are beyond
the absorptive reach of the cells of the plumule and radicle where
they are most needed. But unless foods are in solution, which is
the only form in which they can pass through the walls and proto-
plasm of cells, they can not move from one region of a plant to
another. Therefore, since starch, fat, and protein, which are the
chief storage foods of seeds, are not readily soluble in water, they
must be changed to sugar, fatty acids, peptones, or other soluble
forms before being transported. However, this digestive process
occurs not only in seeds but also in all plant regions where foods
are transported, and also in animals it has its likeness in the diges-
tive process by which foods are made soluble, so that they can
-pass through the walls of the alimentary canal to the blood, which
carries them in solution throughout the body. Both the digestion
and transportation of the stored foods are quite noticeable during
the germination of some large seeds, as in case of Corn in which
the endosperm becomes watery and disappears as germination
proceeds, or in case of Beans where the cotyledons in which the
food is stored gradually shrink as the young plant develops.

The digestive process in plants as well as in animals is per-
formed by special substances known as enzymes, which in case of
the seed are secretions of the embryo. Enzymes occur in solu-
tion, either dissolved in water or in protoplasm, in all parts of the
plant where they either initiate or hasten chemical changes.
They are exceedingly important substances because upon them
the majority of chemical changes in plants depend. They are
specific in their action, that is, as a rule, each enzyme acts on only
one kind of a substance, and is concerned with only one or two

94 GERMINATION OF SEEDS: SEEDLINGS

chemical changes. Consequently, the kinds of enzymes are
almost as numerous in the plant as the kinds of substances to be
acted upon. Thus for changing starch into sugar there is the
enzyme known as diastase which is especially active in seeds, but
common in other plant organs and in animal saliva. An enzyme
secreted by the Yeast Plant and called zymase acts on sugar,
forming besides alcohol, carbon dioxide which puffs up the dough
when Yeast is used in bread-making. This enzyme also occurs
in seeds, fruits, and other plant organs. Lipase converts fats into
soluble fatty acids, and pepsin changes insoluble proteins into
peptones and other soluble forms. Then there are oxidases, en-
zymes which oxidize substances as the name suggests, and perox-
idases which take oxygen away from compounds, and many other
enzymes which play an important role in the chemical activities
of the plant. The exact chemical nature of enzymes has never
been determined because of the difficulty in separating them from
other protoplasmic substances which enter into and thus compli-
cate the analysis. Nevertheless, there is much evidence that
enzymes are protein-like substances. One striking feature of an
enzyme is that it does not enter into the chemical action which it
causes, and, therefore, a small quantity of an enzyme can keep
a chemical action going until a large quantity of a substance is
changed.

Although all living cells, whether in the embryo or elsewhere,
produce enzymes, sometimes, however, certain cells have the
secretion of enzymes as their special function, as in Corn, Wheat,
and other seeds of the Grass type, where the epithelial layer of the
scutellum has for its special function the secretion of the diastase
and other enzymes which are necessary for converting the endo-
sperm into soluble forms.

Transportation of Soluble Foods. — After the foods are made
into soluble forms and dissolved in the water present, they pass
from one region of the plant to another by the physical processes
known as diffusion and osmosis. Diffusion is probably better
known among gases where the spread of odors through a house,
the fragrance of flowers through gardens, and smoke through the
air are everyday illustrations of it. The spread of indigo, ink,
or any substance like salt and sugar through the water in
which they are dissolving illustrates it. By diffusion substances,
whether dissolved in a gas or a liquid, spread farther and farther

-I

ELABORATION OF FOODS INTO PLANT STRUCTURES 95

from the place where they entered the dissolving medium, and
thus toward those regions where they are less concentrated. In
case a number of substances are in solution at the same time, each
diffuses independently of the others. When, for example, sugar,
salt, and ink are dissolved in a vessel of water at the same time,
each diffuses to all parts of the vessel independently of the others
and, consequently, the substances become thoroughly mixed just
as the oxygen, nitrogen, carbon dioxide, and other gases of the
air by diffusion tend to thoroughly mix. It is apparent then in
case of the seed that foods in a concentrated solution in the endo-
sperm or cotyledons will diffuse to the radicle and plumule, where
the food, by being constantly removed from the 'solution to be
built into plant structures, is kept less concentrated.

Osmosis mentioned as another process involved in the trans-
portation of foods is also a diffusion, but differs from the ordinary
diffusion just described in that it takes place through a membrane
which alters the rate of the diffusion of different substances by
allowing some to pass through it more readily than others. It is
by this kind of diffusion that substances pass into and out of liv-
ing cells, in which case the membrane through which the sub-
stances must diffuse is the modified border of the protoplasm.
Thus, although foods depend much upon ordinary diffusion for
transportation when not passing through membranes, in entering
or leaving living cells they must also depend upon osmosis, the
nature and principles of which are more thoroughly discussed in
connection with the cell (Chapter VII).

The Elaboration of Foods into Plant Structures. — In the early
stages of germination the radicle and plumule elongate by the elon-
gation of the cells already present, but soon, however, in certain
regions, mainly at or near the tip of the radicle and plumule, there
begins cell division followed by elongation, growth, and forma-
tion of tissues - — the processes upon which the continued develop-
ment of the young plant depends. Throughout these processes
foods are elaborated: (1) into materials to thicken the cell walls
as they become thinner in stretching; (2) into protoplasm which
must increase as cells grow and divide; (3) into woody and other
elements for strength and conduction; (4) into fatty and waxy
substances and cell thickenings for protection; and (5) into the
various materials which are peculiar to food-making, reproduc-
tive, absorbing, secreting, and other structures which plants form

96 GERMINATION OF SEEDS: SEEDLINGS

during their development. But the transformation of foods into
the various structural elements of the plant involves chemical
reactions which take place only when there is energy supplied.
This brings us to another process called respiration by which the
energy required for the chemical changes involved in changing
foods into cell walls, protoplasm, and other structures is secured.

Respiration in plants, just as in animals, is an oxidation process
in which some food or other elements are burned, as we commonly
say, with the result that oxygen is required and energy, carbon
dioxide, and water vapor are produced. Respiration occurs only
within the cell in connection with which it will be more fully dis-
cussed. But since there is no place where respiration is more in
evidence than in germination where the cells are extremely ac-
tive, some of its features should be noted in connection with that
process. Furthermore, much about germination can not be un-
derstood until something is known about respiration.

Cells, like an electric motor, steam engine, etc., can not do work
unless they have energy. Some cells, like the green cells of leaves,
are able to utilize the sun's energy for some kinds of work; but
when cells are not specially provided with pigments for utilizing
the sunlight, they have to depend entirely upon the energy
which they produce within themselves. In the sugar and other
foods of the seed there is much latent energy which can be
released as active energy by oxidizing these substances, which
are thereby broken into simpler compounds of which carbon diox-
ide and water are the simplest and most noticeable ones. It is
this oxidizing of substances, so that their stored energy is re-
leased, that constitutes respiration, which necessarily must be
accompanied by a consumption of oxygen and the production
of simpler compounds. It is now clear why seeds do not ger-
minate well when oxygen is excluded as the experiment in Figure
91 demonstrates. Although most of the energy released is used
in carrying on the work of the cell, some, however, escapes as
heat, which, like the liberation of carbon dioxide and water vapor,
indicates that respiration is going on.

Respiration in seeds is easily demonstrated by germinating
seeds in a closed jar, in which the production of heat and carbon
dioxide with the accompanying loss of oxygen, and the accumu-
lation of moisture can be demonstrated. By germinating seeds,
such as Peas or Beans, in a closed vessel in which a thermometer

ELABORATION OF FOODS INTO PLANT STRUCTURES 97

is inserted, the temperature of the enclosed air may be raised 10° C.
and sometimes 20° C. by the heat of res-
piration; and the oxygen of the enclosed
air will usually be so nearly used up that
the flame of a burning match or splinter
is extinguished when inserted into the
jar. (Fig. 92.) To demonstrate the ac-
cumulation of carbon dioxide, one may
pour lime water into the jar where the
seeds are germinating, in which case the
calcium hydroxide of the lime water
unites with the carbon dioxide of the
enclosed air, forming calcium carbonate
which is insoluble and when abundant
gives the solution a milky appearance.
Since the amount of carbon dioxide in
ordinary air is not sufficient to give a
perceptible precipitate, the milky ap-
pearance, therefore, indicates that much
carbon dioxide has been added to the
enclosed air. Again, the carbon dioxide
liberated in germination can be quite
accurately measured by drawing the air
from over germinat ng seeds through a
solution of potassium hydroxide, where
the carbon dioxide is caught and its
weight calculated from the increased
weight of the solution. However, this
involves careful weighing as well as see-
ing to it that the carbon dioxide already
present in the air is removed before the
air enters the germinator, and that the
increased weight of the potassium hy-
droxide is not partly due to added mois-
ture. This method discloses that many
cubic centimeters of carbon dioxide may
be liberated by a small quantity of ger-
minating seeds, as shown by the experi-
ment in which 3 Beans with a dry weight
of only 1 gram produced 9^ cubic centimeters of carbon dioxide

A B

FIG. 92. — A simple ex-
periment to demonstrate
that heat is produced by
germinating seeds. The
bottle A contains germi-
nating seeds, while the
bottle B contains only
moist cotton. The higher
temperature, commonly
shown by the thermometer
in bottle A, demonstrates
that germination is ac-
companied by the pro-
duction of heat. If the
bottles are protected
against the loss of heat, or
if bottles like " Thermos"
bottles, which have double
walls with air-space be-
tween, are used, the re-
sults are much better.

98 GERMINATION OF SEEDS: SEEDLINGS

during a germinative period of only 48 hours. That moisture
is liberated during germination is obvious, for the air in a closed
germinator often becomes so saturated that moisture precipitates
on the walls of the germinator.

When green seeds, green hay, or any plant portions in which
the cells are quite active are massed together, so that the heat and
moisture are retained, they often become very warm and moist
due partly to their own respiration and partly to that of the micro-
organisms present. The so-called " sweating" of grains in the
stack or bin and the heating in the bin when the grain becomes
damp due to leaks are phenomena connected with respiration.

Summary. — In germination of seeds the following things take
place: (1) the absorption of water which softens the seed cover-
ings and acts as a dissolving and transporting medium of foods;
(2) the secretion of enzymes which digest the foods and assist in
other processes; (3) the transference of foods by diffusion and
osmosis; (4) respiration which supplies energy for the elabora-
tion of foods into plant structures and is accompanied by the ab-
sorption of oxygen and the production of carbon dioxide, water
vapor, and some heat; and (5) the growth of the radicle and
plumule, resulting in the breaking of the seed coverings and the
establishment of the young plant in the soil and sunlight.

Testing the Germinative Capacity of Seeds

The loss in crop and labor when poor seed is used may be so
serious that no one can afford to plant seeds with a doubtful ger-
minative capacity. It is not enough for seeds to germinate, but
they should have vigorous embryos, so that they will germinate
quickly and thus rapidly pass through the delicate stage in which
the young plant is likely to be destroyed by insects, Fungi, bad
weather, and unfavorable soil conditions.

In testing the germinative capacity, as in determining the im-
purities of a quantity of seeds, decision is based upon the results
obtained with a comparatively small number of the seeds as a
sample. In case of small seeds, such as Oats, Wheat, Barley, and
Clover, Alfalfa, and Grass seeds, tests are ordinarily made with
two lots consisting of 200 seeds each and free from impurities.
In Corn it is customary to use 6 kernels, 2 from near the tip,
2 from the butt, and 2 from the middle of the ear, with the
kernels of each pair selected from rows as far apart as possible.

TESTING THE GERMINATIVE CAPACITY OF SEEDS 99

There are a number of germinators on the market, but, if one
is not available, a box of moist soil or sand, or moist rags which
are rolled up with the seeds within are good germinators when
properly handled. (Fig. 93.) A very good germinator is made
with two dinner plates and blotting paper as shown in Figure 94-

During the test a temperature suitable for the germination of
the kind of seeds involved must be maintained. Some prefer to
keep the temperature near that of the soil, so as to more nearly

FIG. 93. — Doll rag testers, consisting of moist rags properly labeled and
rolled up with the seeds within. After H. D. Hughes.

imitate the soil conditions under which most seeds do not germi-
nate so well as they do in germinators. The germinator should
be opened each day to note the germinated seeds and to allow the
entrance of fresh air, if ventilation is not otherwise provided. At
the end of the germinative period, the results are usually ex-
pressed in percentages found by dividing the number of germinated
seeds by the number in the lot and multiplying by 100. Thus if

190 of a lot of 200 germinated,

19° * 10°

= 95 per cent. The

percentage of germination will vary for different lots and the

100

GERMINATION OF SEEDS: SEEDLINGS

greater the number of lots tested, the more the results will be
checked and, accordingly, the safer will be the conclusions.

In estimating the germinative capacity of seeds, the time
allowed for germination must be considered; for seeds having
weak embryos and, therefore, unfit for planting may give a high
percentage of germination if allowed enough time. It is, there-
fore, necessary to fix a time
limit, and in doing so the ger-
minative speed characteristic
of the type of seeds involved
and the temperature of the
germinator must be consid-
ered; for some seeds naturally
germinate more slowly than
others, and the effects of low
and high temperatures on ger-
mination are already known
to the student (page 90).
Furthermore, kinds of seeds
differ so much in germinative
capacity that a percentage of
germination considered good for one kind of seeds would be con-
sidered poor for another. Thus 70 per cent germination is good
for Parsnip seeds but very poor for Wheat or Corn. In the
following table1 the number of days in which the seeds should
germinate enough to show their germinative capacity, and the
percentages of germination considered good for first-class fresh
seeds, one year with another, are given.

FIG. 94. — Simple germinator. A,
closed. B, open. After F. H. Hillman.

Seed.

Germination
period, days.

Good
germination,
per cent.

Red Clover

Alsike Clover ... ...

White Clover
Alfalfa

6
6

90
90

Timothy

Bluegrass (Kentucky)

Millet

Wheat

Oats ....

Barley .

Flax

Corn

1 Testing Farm Seeds in the Home and in the Rural Schools. Farmers'
Bulletin £28, U. S. Dept. of Agriculture.

SEEDLINGS

101

Seedlings

After the radicle and plumule have escaped from the seed cov-
erings, the young plant passes into the seedling stage, which lasts
until the young plant becomes entirely self-supporting, that is,

FIG. 95. — Early stages in the development of the Corn seedling. A,
section through kernel, showing cotyledon (c), radicle (r), and plumule (p).
B, after germination with radicle or primary root (r) and plumule (p) much
elongated. C, radicle (r) and plumule (p) much further developed; s, sec-
ondary roots; I, leaves; t, coleoptile.

until it no longer receives any of its food supply from the seed.
From the seedling stage the plant passes into the adult stage, ex-
cept in trees where a sapling stage occurs. However, the division

102

GERMINATION OF SEEDS: SEEDLINGS

of a plant's life-cycle into successive stages is somewhat artificial,
for the stages so overlap that they can not be separated. In this
presentation we are chiefly concerned with the seedling stage —

the stage in which plants
present differences that
sometimes must be reck-
oned with in choosing
proper methods of plant-
ing and cultivating, and
that often explain the
peculiar features of the
plant in the adult stage.
Among our cultivated
plants there are four rather
distinct types of seedlings
as those of the Grasses,
Onion, Beans, and Peas
illustrate.

Seedlings of the Grass
Type. — The seedlings of
all Grasses are so similar
in type that their essential
features may be learned
by studying the seedling
stage of Corn. From Fig-
ure 95, showing the de-
velopment of the Corn
seedling, it is seen that the
radicle develops directly

FIG. 96. — A later stage of the Corn seed-
ling, g, ground line; p, plumule; a, first
node with permanent root system; 6, portion
of stem between the first node and kernel;
k, kernel; r, radicle or primary root; s, sec-
ondary roots of the primary root system;
d, permanent root system; c, coleoptile.
About half natural size.

downward, forming the
first root called primary
root from which secondary
roots arise as branches.
However, not all second-
ary roots arise at this time

from the radicle, for some
often grow out from the stem just above or below the cotyledon.
The plumule, although developing more slowly at first than the
radicle, soon breaks through its sheath-like covering (coleoptile)
and rapidly elevates its leaves to the light. As the plumule is

SEEDLINGS OF THE GRASS TYPE 103

unfolding its first leaves to the light, a zone, called a node, is
formed at its base about 2 inches under the surface of the soil,
and from this node and others soon forming above it, there arise
roots of a much larger and stronger type than those formed from
the radicle and from the stem in the region of the cotyledon.
These secondary roots, which are outgrowths of the plumule since
they arise from its nodes, constitute the permanent root system,
which as the name suggests remains active as an anchoring and

FIG. 97. — Diagram showing the effect of planting Corn at different depths.
<7, ground line; p, permanent root system, which always develops at about the
same distance under the surface; a, temporary region of the stem, which is
much longer in deep planting; k, kernel; t, temporary root system. Modified
from "Elementary Principles of Agriculture" by Ferguson and Lewis.

absorptive system as long as the plant lives. After the permanent
roots are established (about 10 days after planting) the first
roots, which are known as the temporary roots since they serve
the plant only till the permanent roots are established, develop
no further and remain as vestigial structures until they finally
disappear.

Also included among the temporary structures is the portion
of stem between the first node and kernel. (Fig. 96.) During
the early stage of germination, this stem portion performs two

104

GERMINATION OF SEEDS: SEEDLINGS

important functions: (1) by its elongation the plumule is assisted
in reaching above the soil; and (2) through it the endosperm
and substances absorbed by the temporary roots reach the plu-

FIG. 98. — Seedling of Wheat after the permanent root system is estab-
lished, g, ground line; p, permanent root system; a, temporary stem por-
tion; k, grain; t, temporary root system. About half natural size.

mule. But after the permanent roots are well established, there
is no longer any need for this stem region, which now being with-
out a function makes no further development. Nevertheless

SEEDLINGS OF THE GRASS TYPE

105

in connection with it, there is a principle which is reckoned
with in growing certain plants of the Grass type. According
to the depth of planting this temporary stem region is long or
short. (Fig. 97.) This is due to the fact that the first node and,
consequently, the first of the permanent roots are always estab-
lished about the same distance under the surface of the soil, re-
gardless of the depth at which the seed was planted. Therefore,

FIG. 99. — Stages in the development of the Onion seedling. A, section
through an Onion seed showing endosperm (en) and embryo (e) with the
hypocotyl (h) and cotyledon (c) indicated. B, seed germinating; g, ground
line; s, seed; c, cotyledon; h, hypocotyl; r, radicle. C, seedling more de-
veloped; c, cotyledon which is being pulled out of the seed; h, hypocotyl;
r, radicle; /, first leaf. D, a later stage of the seedling with cotyledon free
from the seed and permanent root system (p) developing.

a deep permanent root system, which is often desirable in order
that the plant may withstand drought, is not secured by deep
planting — a fact which has been well demonstrated in case of
Corn and the small grains. Moreover, if the seed is planted too
deeply, its food and energy may be exhausted before the plumule
reaches the light, in which case the seedling is unable to continue
its development.

However, after the permanent roots are established they may

106

GERMINATION OF SEEDS: SEEDLINGS

be put deeper in the soil by adding dirt around the plant. In
semi-arid regions where a deep permanent root system is desired,
the ground is often listed, that is, plowed into deep furrows, and
the Corn planted in the bottom of the furrows. Then as the

furrows are gradually
filled in cultivation, the
permanent roots are
buried deeper in the
soil, where there is a
chance for moisture
during drought. In this
same connection, one
can see some advan-
tage in drilling small
grains in that the roots
of the plants will be
buried deeper as the
dirt from the ridges is
carried into the drill
furrows during rains
and thaws.

In the small grains,
such as Wheat, Oats,
Barley, etc., although
the temporary system
is just as prominent as
in Corn, there is, how-
ever, a difference of
minor importance to be
noted in the number
of primary roots, which
is one in Corn but two
in

FIG. 100. — Stages in the development of a
Common Bean seedling. A, the cotyledons
(c) being pulled out of the ground by the hy-
pocotyl (h). t, testa; r, radicle; a, root hairs;
g, ground line. B, the hypocotyl has straight-
ened, and the cotyledons have shed the testa
and spread apart, thus giving freedom to the
plumule (p). C, stage with plumule develop-

ing stem and leaves (I), root system much en-
larged by secondary roots (s), and cotyledons
(c) shrinking through loss of stored food.

or more in the small
grains. (Fig. 98.)

The presence of the
temporary system, although occurring in other plants, is a not-
able feature of the Grass seedlings. Another feature to be noted
is that the cotyledon remains where the seed was placed in
planting, that is, it is not pushed up out of the soil by an elong-
ating hypocotyl.

SEEDLINGS OF THE COMMON BEAN TYPE 107

Onion Seedling. — The seedling of the Onion represents
another type of monocotyledonous seedlings. In this type the
hypocotyl elongates and pushes the cotyledon above ground.
(Fig. 99.) As in the Grass seedlings, the primary root system
is temporary — a feature quite common in Monocotyledons, al-
though in some it lives much longer than in others.

Seedlings of the Common Bean Type. — The seedling of the
Common Bean is representative of those dicotyledonous seedlings
in which the cotyledons through the elongation of the hypocotyl
are carried above ground, sometimes several inches or even a
foot in some Beans. Squashes,
Cucumbers, Pumpkins, Melons,
Radishes, Turnips, Castor Bean,
Maples, Ashes, Clover, Alfalfa,
etc., besides many of the Beans
have this type of seedling. In
seedlings of this type the first root
system is usually the permanent
one and soon firmly anchors the
hypocotyl which then by an arch-
ing movement pulls the cotyle-
dons out of the ground in such a
way that they offer the least re-
sistance in passing through the FIG. 101. — Squash seed germi-
soil and afford the most protec- nating, showing the peg by which
tion for the delicate plumule. the seed coat is held while the
(Fig. WO.) In some cases, as in ^yledons are pulled out of the
. . seed coat by the arch of the hy-

the Melons and Pumpkins, the p0cotyl. Somewhat reduced,
hypocotyl also assists in casting

off the seed coat, in which case the arch of the hypocotyl pulls
the cotyledons out of the seed coat while the latter structure is
held in place by a peg-like structure of the hypocotyl. (Fig. 101.)
In most cases, however, the seed coat is torn and gradually
pushed off by the growth of the seedling. Since the first root
system is usually the permanent one, its depth is closely related
to the depth of planting.

The plumule remains small and enclosed between the coty-
ledons until pulled out of the soil. Then by a straightening of
the hypocotyl arch and the spreading of the cotyledons, it is fully
exposed to the light, where it develops all of the plant above the

108

GERMINATION OF SEEDS: SEEDLINGS

cotyledons. Thus most of the stem and all of the leaves, flowers,
and fruit of the adult stage are produced by the plumule.

The cotyledons, which are commonly fleshy in these seedlings,
enlarge after reaching the light and their color changes to
green, which with the presence of stomata indicates that they
function to some extent like ordinary leaves in the manufacture

of foods. However, it
is only a short time till
most of them, espe-
cially the fleshy ones,
begin to show shrink-
age which continues as
the food is used for
growth, until much
shriveled and dried
they fall from .the
plant. In some cases,
as in the Buckwheat
and Castor Bean where
the seeds are albumin-
ous, the thin cotyledons
are more leaf-like and
function like ordina^
leaves for a consider-
able time, although in
arrangement, shape, or
size they are never just
like ordinary leaves and
never so long-lived.
(Fig. 102.} Where the
cotyledons are large,
much force is required
to pull them through

the soil, and, consequently, when the ground is hard or covered
with a crust, seedlings of this type often fail to develop.

As to how the development of both radicle and plumule pro-
ceeds until the adult stage is reached, that depends much upon
the kind of plant. In most cases the radicle forms a central root
which, although prominent at first, may be much obscured in the
adult stage by large secondary roots developing from the base of

FIG. 102. — Seedling of Castor Bean, in which
the cotyledons persist and function like leaves
for some time.

SEEDLINGS OF THE PEA TYPE

109

the stem. In some plants, as in Red Clover and Alfalfa, the
radicle forms a prominent tap-root which enables the plant to
penetrate deeply into the soil in its adult stage.

In the Morning Glory, where the stem called the vine may be
many feet in length, there is extreme elongation of the plumule.
On the other hand, as in some Clovers and Alfalfa, the plumule and
hypocotyl form a short thick stem, called the crown, which is barely

FIG. 103. — Development of a Red Clover seedling. A, cotyledons being
pulled out of the ground by the hypocotyl (h); r, radicle; a, root hairs; t,
testa; c, cotyledons; g, ground line. B, a more advanced stage, showing some
development of the plumule (p); 6, first real leaf; d, second real leaf. C, a
later stage, showing that the plumule has formed more leaves (e) but has
elongated very little.

above the surface of the ground, and from which the branches
arise that bear the leaves, flowers, and fruit. (Fig. 103.)

Seedlings of the Pea Type. — The seedlings of the Pea and
Scarlet Runner Bean represent those dicotyledonous seedlings in
which the hypocotyl remains short. Thus the cotyledons remain
underground and the plumule is pushed to the surface by the
elongation of the stem of the epicotyl just as occurs in the Grass
seedlings. But in these seedlings, in contrast to those of the

110

GERMINATION OF SEEDS: SEEDLINGS

Grass type, both the stem of the epicotyl and the primary root
system are usually permanent. In many seedlings of this type,
the cotyledons are probably so much distorted in connection with
food storage, that they could not function as leaves if raised to the
light. Again, it is claimed that these seedlings can come up
through harder ground by not having to raise their cotyledons.
(Fig. 104.)

FIG. 104. — Seedlings of the Pea, showing how the seedling develops and
the effect of different depths of planting, p, plumule; a, stem portion of
epicotyl; g, ground line; r, radicle. The seedling at the right is so deep in
the soil that it is unable to push the plumule out of the ground.

Size of Seedlings. — There is no feature in which seedlings
vary more than in size. This might be illustrated by placing the
seedling of Timothy or Clover by the side of a Coconut seedling.
In general, the size of the seedling corresponds to the size of the
seed. The size of seedlings is reckoned with in our methods of
planting different seeds. Thus seeds, like Corn and Beans, are
planted several inches deep in the soil, while seeds, like those of
Lettuce, Clover, and Timothy, are sown on the surface, and cov-
ered only lightly if at all. In small seedlings there is not enough
food to enable the plant to reach through thick layers of soil.
Tests have shown that not many Clover seedlings get through
the soil when the seeds are planted even 2 inches in depth.

SUMMARY OF SEEDLINGS 111

Summary of Seedlings. — Seedlings of Flowering Plants are
either monocotyledonous or dicotyledonous on the basis of the
number of cotyledons. Among the Monocotyledons the tem-
porary root system is a prominent feature, and the cotyledon may
remain in the ground as in the Grasses or be raised to the light as
in the Onion. In Dicotyledons the first root system is usually
the permanent one and may consist mainly of a tap-root or of
many roots nearly equal in size. In many Dicotyledons the coty-
ledons are raised to the light where they function to some extent
like ordinary leaves. The fleshy ones, however, lose their stored
food in a short time and fall from the plant. In some cases, as
the seedlings of Buckwheat, Morning Glory, and Cotton illustrate,
the cotyledons become more leaf-like and persist longer, although
they are always easily distinguished from true leaves. In some
Dicotyledons the cotyledons remain in the soil and the plumule is
raised to the light by the elongation of the stem of the epicotyl.

CHAPTER VII

CELLS AND TISSUES

Structure and Function of Cells

Position of the Cell in Plant Life. — Before proceeding to the
study of the adult stage of the plant more must be known about
the cell. If with a sharp razor a very thin section from any part
of a plant is made and observed with a microscope, it will appear
to be divided into many small divisions. A section through the
growing portion of a root looks like Figure 105. These little

divisions with what they
contain are the cells. Cells
vary much in shape and are
so small that usually four or
five hundred of them could
be laid side by side on a line
not more than an inch in
length. They are rarely more
than i^v of an inch, and
sometimes less than TTrV^ of
an inch in diameter. Al-
though cells are so exceed-
FIG. 105. — A small portion of a length- ingly small, nevertheless, it
wise section through the growing region is within them that all life
of a root showing the cells. Very much processes take place. For
enlarged' this reason cells are often

defined as the units of all plant and animal life. Plants need
phosphate, nitrates, etc., because the cells must have them. All
the problems of the plant relating to the soil, light, temperature,
etc., are problems of the cell. The plant is made up of a countless
number of cells and the activities of the plant are simply the sum
of the activities of the many cells of which the plant is composed.
Discovery of the Cell and Its Structures. — That plants and
animals are composed of cells was not revealed until the inven-
tion of the microscope, which, although very rude in its con-

112

PROTOPLASM 113

struction and efficiency as compared with microscopes of today,
was beginning to be employed by Robert Hooke (1635-1703)
and others in the seventeenth century in the study of plants.
Robert Hooke, one of the earliest to study plants with the
microscope, examined thin sections of cork, and found the cork
to be composed of numerous small compartments which he
called cells on account of their rough resemblance to the cells
of a. honeycomb. Of course in dead tissue like cork the cell
contents are absent, and Robert Hooke saw only the cell walls
enclosing the spaces from which the active substance of the
cells had departed. However, through investigations which fol-
lowed those of these earliest investigators with the microscope,
it gradually came to be recognized that the important part of
the cell is the substance which fills the compartments. By ex-
tending the study to many kinds of tissues of both plants and
animals, it was finally recognized that the substance filling the
cell is the only living substance in plants and animals and the
substance which builds the cell wall and the entire organism.
Various names were at first applied to this substance before the
term protoplasm suggested by Hugo Von Mohl was adopted by
both botanists and zoologists. As the word protoplasm, which
is a combination of protos (first) and plasma (thing formed), sig-
nifies, this substance was considered the first organic substance
formed from the inorganic materials taken into the plant. The
idea that the protoplasm is the essential substance and that the
cell is the unit of plant and animal structure was quite thoroughly
elaborated by Schleiden (1838) and Schwann (1839) and became
generally accepted.

Protoplasm. — The protoplasm, as already noted, is the living
substance of plants and animals. The protoplasm of an indi-
vidual cell is often called a protoplast. Protoplasm is a fluid
substance which varies much in its consistency, sometimes being
a thin viscous fluid like the white of an egg, and sometimes being
more dense and compactly organized. Chemical analyses show
that protoplasm has the composition of protein, although such
analyses necessarily kill the protoplasm and consequently do not
give us a true knowledge of the protoplasm as it is while living.
Although the protoplasm of higher plants usually exhibits no
motion except when dividing, there are cases, however, as in the
hairs of the Pumpkin and Wandering Jew, where the protoplasm,

114

CELLS AND TISSUES

when under the microscope, can be seen streaming around the cell
wall or across the cell from side to side or end to end.

The protoplasm consists of a number of structures which differ in
organization, and each of which has one or more special functions.
(Fig. 106.) One of the most conspicuous of these structures is the
nucleus, which is a comparatively compact protoplasmic body,
usually spherical in shape. Although usually centrally located
in actively growing cells, the nucleus commonly has a lateral posi-
tion in old cells. The nucleus is enclosed by a membrane, called

the nuclear membrane, and is
filled with a liquid known as
nuclear sap, which consists of
water and dissolved substances.
However, nuclear sap is usually
colorless and, therefore, not vis-
ible. Within the nucleus also
occur one or more small globu-
lar bodies known as nucleoli
(singular nucleolus) and much
chunky or granular material
known as chromatin, which is
regarded as the most impor-
tant part of the nucleus and is

FIG. 106.-A growing cell. «,, cell SO named because 'li stains SO
wall; c, cytoplasm; v, vacuoles filled readily when stains are applied
with cell sap; TO, nucleus; a, nucleoli; to the cell. Around the mi-
m, nuclear membrane; g, chromatin cleus and filling up the general

cayity ^^ the cell waR ig

that portion of the protoplasm,
known as cytoplasm, which is a loose spongy structure full of many
cavities called vacuoles. The vacuoles are filled with a liquid called
cell sap, which like the nuclear sap consists of water containing
dissolved sugars, salts, and other substances. The border of
the cytoplasm is in contact with the cell wall and is modified
into a membrane known as the cell membrane, which, since it is
closely applied to the cell wall, can not ordinarily be seen until
the cell is bathed in salt water or some other solution strong
enough to shrink the protoplasm, so that the cell membrane is
drawn away from the wall where it can be seen. (Fig. 107.)
Within the cytoplasm commonly occur a number of small bodies

granules. Enlarged about five hun-

CELL WALL

115

known as plastids, which are masses of cytoplasm but denser than
ordinary cytoplasm. (Fig. 108.) They often develop pigments
as in case of leaves, stems, and other green organs where they
develop chlorophyll, the pigment upon which the green color of
these organs depends. Plastids
containing chlorophyll are called
chloroplasts and are very impor-
tant structures because they have
so much to do with making plant
food. Plastids which occur in
the petals of some flowers have
yellow or red pigments. Plastids FlG. 107. — Cells with protoplasm
which are colorless, having no (p) shrunken to show the cell mem-
pigments at all, are called leuco- brane, which is represented by the
plasts. Starch grains and other dotted line ^rounding the proto-
small bodies (chondriosomes) not p a

shown in our figure are also commonly present in the cytoplasm.
Cell Wall. — The cell wall is formed by the protoplasm and
may be variously modified by it. In actively growing cells the
wall is thin and composed of cellulose — a substance which allows
j! the wall to stretch as the protoplasm ex-

pands in growth. As the cell develops, the
protoplasm in many cases thickens the cell
wall by depositing new layers of material,
which may be of cellulose or of some other
substance better adapted to the function
which the cell is to perform. In nearly all
plants but in trees more especially some
cells deposit lignin in their walls, thus be-
coming the wood cells which give rigidity
to the plant and which we use in the form
of lumber. In the bark of trees, Potato
skins, and other structures for protection,
fat-like substances are deposited in the
walls of the cells which then are known as
cork. Sometimes, as in the so-called bast fibers, which are the
strengthening fibers especially prominent in Flax and Hemp, the
walls are extremely thickened with cellulose. The same is true
in Date seeds and Ivory Nuts where the walls are extremely
thickened with cellulose to be used as a food during germination.

FIG. 108.— Cell from
a leaf, w, cell wall;
n, nucleus; v, a large
vacuole in the cyto-
plasm; ch, chloroplasts.

116

CELLS AND TISSUES

There are many ways in which cell walls are modified as will be
seen in the study of tissues.

Processes Involved in Cell Activity. — The chief of cell activi-
ties is growth which will be discussed in connection with the differ-
ent plant organs. But growth, besides being much under the
influence of external conditions, such as temperature and light,

depends upon metabolism — the proc-
ess by which materials are changed
into forms which have to do with
growth. In connection with growth,
metabolism, and other physiological
processes of the cell, osmosis and res-
piration are involved, both of which
were shown to be important proc-
esses in seed germination. In germi-
nation and other physiological proc-
esses they are important because of
their connection with the other proc-
esses of cells. Osmosis is a physical
process which occurs wherever two
liquids differing in concentration are
separated by a membrane which they
wet, and hence is not a cell activity
except in so far as the protoplasm
controls it when occurring in connec-
tion with the cell. Respiration, on
the other hand, is a physiological
process and only occurs in connec-
tion with protoplasm.

Osmosis. — Osmosis may be de-
nned as that kind of diffusion by
which liquids pass through mem-
branes and its principles can be best

FIG. 109. — Experiment dem-
onstrating osmosis. The pig's
bladder was filled with a sugar
solution and then the tube was
attached. The water from the
jar was drawn into the bladder
and the solution in the bladder
forced up the tube.

understood by the study of an illustration as shown in Figure
109. Thus if a pig's bladder, filled with a sugar solution and
having a long glass tube fastened in its neck, is submerged in
a jar of water, water will pass in and force the solution up the
glass tube. If, on the other hand, a sugar or salt solution stronger
than the one in the bladder be placed in the jar, the water slowly
passes out of the bladder. Thus the water passes from the weaker

OSMOSIS 117

solution through the membrane to the stronger. Sometimes
some of the dissolved substances may pass through the membrane,
but often the membrane permits only the water to pass, in which
case it is known as a semi-permeable membrane. In case of the
pig's bladder not much sugar is allowed to pass through its wall,
which is, therefore, semi-permeable in reference to this particular
solution. When a membrane will allow a dissolved substance
to pass, it is said to be permeable to that substance. Most
membranes are permeable to some substances and impermeable
to others.

The causes of the movement of the water or other solvents from
the less dense to the denser solution are not thoroughly under-
stood. Some think that it is due to the affinity of the dissolved
substances for the solvent, which is pulled to the substances with
a force increasing with the amount of the substances in solution.
Others think that it is due to the checking of the diffusive power
of the dissolved substances by the membrane, which permits the
two liquids to approach an equilibrium only through the passing
of more of the solvent to the denser solution.

In comparing osmosis in the cell with the illustration, the cell
membrane corresponds to the wall of the pig's bladder, the cell
sap to the solution within the bladder, and the solutions around
the cell correspond to the water or solutions in the jar. If the
cell sap in denser than the solution on the outside of the cell mem-
brane, then water with those dissolved substances to which the
membrane is permeable will pass in; but, on the other hand, if the
cell sap is less dense than the solution without, water and prob-
ably some dissolved substances will pass out. Thus the passing
of liquids through the cell membrane from a less dense to a denser
liquid is also the chief feature of osmosis in cells. It should also
be noted in connection with osmosis in cells: (1) that the more
the two solutions separated by the cell membrane differ in con-
centration, the more rapid is the process of osmosis ; and (2) that
the solvent, which is water in case of cells, passes through the
membrane independently of its dissolved substances, which are
either carried along or left behind according to whether or not
the membrane is permeable to them.

However, it is only in principle and not in practice that osmosis
as demonstrated with the pig's bladder is identical with that in
the cell. In the first place, instead of a solution containing only

118 CELLS AND TISSUES

one dissolved substance, both the cell sap and the solution
around the cell usually carry in solution a number of substances,
each of which in its osmotic influence is independent of the others,
although the osmotic influences of all are combined in determin-
ing the osmotic force of the solution. In the second place, the cell
membrane is a living membrane and, therefore, able to alter its
permeability, so that it may be permeable to certain substances
at one time but not at another. Another peculiar feature of pro-
toplasm is that substances are often allowed to pass in .more
readily than out. Thus root hairs, which take in many sub-
stances from the soil, do not allow the sugars and many other
substances in their cell sap to pass out. If the cells of a red Beet
are laid in a strong salt or sugar solution, the water will pass out
but the coloring matter will be retained. Furthermore, when
some cells are placed in very dilute solutions of dyes as methylene
blue, the dye accumulates in the cell sap, which, therefore, be-
comes much more colored than the surrounding solution. In
this way various kinds of substances which are allowed to pass in
more readily than out may become more concentrated in the
cell sap than in the solution without.

It is now seen that by osmosis cells obtain their water supply
which they pull from the soil, surrounding cells, conductive tracts,
or whatever surroundings they may have that puts them in con-
tact with water. Furthermore, the more concentrated their cell
sap, the more forcibly and rapidly they can draw water from their
surroundings. Osmosis, although chiefly concerned with supply-
ing cells with water, assists some in supplying cells with dissolved
minerals, sugars, and other substances, which the cell membrane
permits to be carried in with the water. But in connection with
osmosis substances may pass into and out of cells by the same
principles which are active in ordinary diffusion. Thus if sub-
stances are less concentrated in the cell sap than without and the
membrane is permeable to them, they will diffuse to the cell sap,
more or less independently of the movement of water, although if
the water is moving in the same direction the substances will
move more rapidly. Likewise substances diffuse out of cells
when more concentrated within than without, provided the cell
membrane is permeable to them.

Pressure Within the Cell. — In the case of the pig's bladder,
it is seen that the flow of water into the interior increases the

CHARACTER OF THE CELL MEMBRANE AFTER DEATH 119

amount of solution within until some of the solution is forced
up the tube. The solution rises in the tube because the increase
in the amount of solution within the bladder requires more space,
and is, therefore, accompanied by an increase in pressure against
the wall of the bladder. This pressure, which in this case de-
pends upon the concentration of the sugar solution in the bladder,
might become so great as to burst the bladder, if no tube for an
outlet were provided. This pressure, known as osmotic pressure,
has been found to follow quite well the laws governing gas pres-
sure. Consequently, if the number of molecules of the dissolved
substance contained in a certain volume of the solution is known,
the osmotic pressure can be calculated. Thus 342 grams of Cane
sugar in 1 liter of solution (called a gram-molecular solution) will
exert a pressure of about 22.3 atmospheres or 336 Ibs. and in
whatever proportion the number of grams is increased or de-
creased, the pressure is altered in a similar proportion. Osmotic
pressure in cells, where it is called turgor pressure, is usually not
less than 50 Ibs. and often more than 100 Ibs. The rigidity of
organs such as leaves, soft stems, and roots is largely due to turgor
pressure, as can be easily shown by immersing strips of a fresh
Beet or Radish in a strong solution where they lose water and
become flaccid. The wilting in leaves when exposed to excessive
evaporation is due to the loss of turgor pressure, which occurs
whenever cells lose water more rapidly than they absorb it. The
preservative value of such substances as salts and sugars when
applied to meats and fruits depends largely upon the withdrawal
of water from the micro-organism, so that they can not become
active. Wilted cells, if not dead, will also draw in water and again
become turgid when put in contact with moisture. In this way
flowers are revived by placing their stems in water, and Cucum-
bers, Lettuce, and Celery are made crisp by putting them in cold
water. Sometimes, as the pollen of some plants illustrates when
immersed in water, the pressure becomes so great that the cells
burst. Even fruits, such as Plums, sometimes burst on the trees
from this cause when the weather is warm and moist.

The Character of the Cell Membrane After Death. — With the
death of the cell, the cell membrane ceases to be an osmotic mem-
brane and thus becomes permeable to all substances in solution.
After the cell membrane is dead substances pass through it,
either into or out of the cell, almost as easily as through a piece of

120 CELLS AND TISSUES

cloth. Consequently, osmotic pressure is lost when cells die and
the substances ordinarily retained are allowed to diffuse out.
This is easily demonstrated by soaking plant tissues in water be-
fore and after death. Thus, if from a fresh red Beet a strip is cut,
washed thoroughly so as to remove the contents of the injured
cells, and then soaked in water at a temperature not destructive
to the life of the cell, it will be found that the pigment, sugar, and
other substances of the cell are retained ; but if the strips are put
in water hot enough to kill the cells, then the pigment, sugar, and
other cell substances diffuse out into the water. That pools in
which dead leaves fall soon become colored is a common observa-
tion. The fact has significance for the farmer who has learned
by experience that, when hay that is down is caught in a rain,
more of the elements are washed from the cured hay than from
that more recently mowed and hence still partly green.

Nature of Plant Food. — Besides oxygen, which is chiefly used
in respiration, various substances, such as water, sugar, acids,
salts, and carbon dioxide, enter the protoplasm where most of
them have some use related to the growth of the plant. But as to
whether or not all should be considered as plant foods, not all
students of plants agree; for, although all of these substances
have to undergo transformations in becoming cell structures,
some are more nearly ready for use than others. This may be
illustrated by comparing sugar with carbon dioxide and water.
In the leaves or wherever chlorophyll is present, carbon dioxide
and water have their elements dissociated and combined in such
a way as to form sugar which can be used directly for respiration
or by minor chemical changes be transformed into cell walls.
Thus sugar, since it is more nearly ready for use, may be called a
food and the carbon dioxide and water may be called elements from
which food is made. Likewise protein, which is closely related to
protoplasm, may be regarded as a food, while the mineral salts,
such as nitrates, phosphates, sulfates, etc., which are necessary in
the formation of proteins, may be egarded as the elements from
which food is made. Some investigators restrict the term plant
food to the more complex substances, such as sugars, starch, pro-
teins, fats, and amino acids, while others include some of the sim-
pler elements, especially the mineral salts. In this presentation
water, carbon dioxide, and the mineral salts are regarded as ele-
ments used in the formation of foods,

RESPIRATION 121

Respiration

The general features of respiration were discussed in connec-
tion with seed germination where respiration is not only promi-
nent but also must be reckoned with in understanding the ger-
minative process. There it was stated that respiration takes
place only within the cell and that it is comparable to ordinary
combustion in that it is an oxidation process resulting in the
breaking down of substances into simpler elements with the re-
lease of potential energy.

It is a well known fact that whenever carbon and oxygen are
united energy is released. This is the principle employed in heat-
ing plants, steam engines, etc. where energy in the form of heat is
obtained through the union of oxygen with the carbon in the coal,
wood, or some other combustible substance. If sugars, starches
or other substances containing carbon were used for fuel, the same
results would be obtained. In the cell, however, since most of
the energy released is used in protoplasmic movements, and in
chemical changes involved in enlarging cell walls, making more
protoplasm, etc., not much is exhibited as heat, although enough
that all living plant parts are generally a little warmer than their
surroundings, sometimes 2 or 3 degrees in case of large flowers
and often much more in germinating seeds. Again, although
the rate of respiration increases with the temperature up to a
certain point, respiration proceeds in a lower temperature than
does ordinary combustion. In fact, a temperature high enough
to start the combustion of most substances is entirely too high
for respiration, which in most plants ceases before 60° C. is
reached. Also in respiration the process of oxidation is initiated
and kept going by enzymes or directly by the protoplasm, while
there are no such agents involved in combustion. Thus, although
similar in results, in operation respiration is very different from
combustion.

In combustion there is a constant ratio between the oxygen
used and the carbon dioxide produced. Thus in the combustion
of Grape sugar, as illustrated by the formula C6Hi206 + 6 O2 =

6C02 + 6 H2O, the ratio a ~ 2 is 1. In respiration, however,

O O2

although the ratio is often unity, it varies much, sometimes being
greater and sometimes much less than unity. In germinating

122 CELLS AND TISSUES

seeds, tubers, and bulbs containing starch and sugars, and in
many other plant structures, the volume of oxygen consumed
during active respiration is equal to that of the carbon dioxide
given off; but in the germination of seeds containing fats and
fatty oils, the volume of oxygen consumed is greater than that of
the carbon dioxide given off, in which case some of the oxygen
is apparently used in changing the fats and fatty oils to other
forms of food having a larger proportion of oxygen.

In higher plants the substances oxidized are organic compounds
including the sugars, fats, proteins, organic acids, and probably
the protoplasm itself. Some lower forms of organisms oxidize
inorganic compounds. Some Bacteria obtain energy by oxidizing
the ammonia of ammonia salts to nitrites, while others obtain
energy by oxidizing the nitrites to nitrates. Various other sub-
stances, such as hydrogen sulphide and iron, are oxidized by
certain Bacteria to secure energy.

There are some forms of respiration which can continue when
oxygen is excluded and the one of them best known is fermen-
tation, which is prominent in the Yeast Plant and other fer-
menting organisms. When proceeding in the absence of oxygen,
such forms of respiration are known as anaerobic respiration, that
is, respiration in the absence of air. In fact, some micro-organ-
isms can not carry on their processes well except in the absence of
air. One kind of anaerobic respiration, which is very similar to
if not identical with fermentation, can be detected in seeds, fruits,
and all living plant parts when oxygen is excluded, so that the
process is not obscured by ordinary respiration. This kind of
respiration is considered by some to be the initial stage of ordinary
respiration, thus being closely related to it.

The peculiar feature about fermentation in the absence of air
is that oxidation of carbon continues with the release of energy
and the production of carbon dioxide, although no oxygen is ob-
tainable from without. Furthermore, fermentation, whether in
the presence or absence of air, differs from combustion and ordi-
nary respiration in the completeness with which the substances
involved are broken down. This may be illustrated in the case
of the fermentation of sugar by Yeast, in which case, as shown by
the equation C6Hi2O6 = 2 C02 + 2 C2H60, the molecule of sugar
is broken into 2 molecules of carbon dioxide and 2 of alcohol,
while in case of combustion and often in respiration the molecule

CELL MULTIPLICATION 123

of sugar is broken into carbon dioxide and water, as shown in the
equation CeH^Oe + 6 O2 = 6 CO2 + 6 H2O. In respiration the
breaking of the sugar into carbon dioxide and alcohol is probably
the first step which is then followed by the breaking of the alcohol
into carbon dioxide and water. From the equation in case of the
fermentation of sugar it is seen that the energy is obtained by
uniting the oxygen and carbon, both of which are present in the
molecule of sugar. Thus by the use of the oxygen within the
compound broken down, some oxidation can occur when there is
no oxygen available from without.

Instead of alcohol other substances may be produced by fer-
mentation according to the nature of the fermenting organism
and the kind of compound fermented. Thus in the fermentation
of cider by certain kinds of Bacteria alcohol is first produced and
later acetic acid. In the souring of milk the Bacteria break the
milk sugar into lactic acid. Although sugars are the substances
involved most in fermentation, other compounds are known to be
involved. Even decay, caused principally by Molds and Bac-
teria, is regarded as a kind of fermentation, in which case many
kinds of substances are involved.

The injury caused by Fungi and Bacteria is often due largely to
the by-products of their respiration and growth. Partly in this
way Fungi damage or destroy plants upon which they live. Many
of the Bacteria associated with diseases produce poisons known
as toxins which cause injury or death in animals and sometimes
in plants. To combat some of these toxins antitoxins are used.

Thus respiration whether aerobic or anaerobic is that oxidation
process by which cells secure energy to carry on their work. Any
condition , such as a low or high temperature, absence of food, or
lack of oxygen, which hinders respiration, holds cell activity in
check and thus impedes plant growth. Furthermore, due to the
liberation of heat and moisture which may become destructive
when allowed to accumulate, respiration must be reckoned with
in the storing of plant products.

Cell Multiplication

As previously stated (page 112), cells are exceedingly small
structures and a small size seems preferable in both plants and
animals where numerous small cells rather than a few large ones

124 CELLS AND TISSUES

is the rule. Consequently, as a cell grows, a size is soon attained
at which division must occur. By division the cell becomes two
cells of half the parent size, and each of the new cells has all of the
structures of the parent cell and the ability to repeat the proc-
esses of growth and division.

It is by the growth, division, and differentiation of cells that
both plants and animals become adult individuals. In the ferti-
lized egg, the first stage of an individual's existence, cell division
begins usually in a few hours after fertilization and continues
throughout the life of the plant, although interrupted at various
times. Although the cell divisions are countless in number in the
higher plants, they all proceed in the same way throughout the
plant, except in the anther and ovary where a peculiar type of
division to be discussed later occurs.

In some simple plants, as Bacteria and the Yeast Plant where
cell division is of a simple type, the processes of division may oc-
cupy only a few minutes, but in the higher plants where cell divi-
sion is more complex, the processes of division often require two
or more hours, and so far as we know the processes are continuous
throughout the entire period. Most of this time is occupied by
the division of the chromatin about which cell division centers.

Although cell division consists of a continuous series of events,
a few stages in the process, as shown in Figure 110, will suffice to
give an understanding of cell division as it occurs in the higher
plants. Thus starting with the chromatin in a granular condi-
tion and scattered through the nucleus, the first step in division
is the organization of this chromatin into a thread which then is
segmented into segments known as chromosomes. The number
of chromosomes into which the thread segments is definite for
each plant or animal, although varying much in different species,
ranging from two in some worms to more than one hundred in
some Ferns. However, in many of our common plants and ani-
mals the number ranges from sixteen to forty-eight. In man
there are forty-six or forty-eight, in Tomatoes twenty-four, and
in Wheat sixteen. The chromosomes, which have no definite
arrangement when first formed, soon arrange themselves in a
plane across the cell. As they assume this arrangement, the
nuclear membrane disappears, thus allowing the chromosomes to
come in contact with the fibers, known as spindle fibers, which
seem to be special provisions of the cytoplasm for bringing about

CELL MULTIPLICATION

125

the distribution of the chromosomes. At this stage it becomes
apparent that each chromosome consists of two pieces or halves
each apparently having split longitudinally. The halves of each
chromosome now separate, pass to opposite ends of the cell where
the new nuclei are formed. Thus each new nucleus gets as many
halves, which soon grow to full size chromosomes, as there were
chromosomes in the parent cell. As the new nuclei are forming

FIG. 110. — Cell division, a, cell in resting stage, b, chromatin formed
into a thread, c, the thread of chromatin broken into segments called chromo-
somes, d, chromosomes arranged across the cell for division. Notice the
threads called spindle fibers running through the cell and that the nuclear
membrane has disappeared, e, chromosomes have split and the halves are
passing to opposite ends of the cell. /, chromosomes have reached the points
where they are to form new nuclei, g and h, new nuclei and cross wall be-
tween them forming. After Stevens.

a cross wall is formed, which divides the cytoplasm, and cell divi-
sion is now complete. Instead of one cell there are now two,
each of which after growing to full size will divide in the same
manner as the parent cell.

Except in certain regions where cell multiplication is the spe-
cial function, most cells of the plant sooner or later lose their
ability to grow and divide as a result of their modifications
which adapt them to their special functions. Thus after cells are

126

CELLS AND TISSUES

thoroughly modified for protection, absorption, strength, conduc-
tion, food-making, etc., in most cases growth and division ceases.
This brings us to the tissues which are groups of cells so modified
as to be adapted to special functions and upon which the various
activities of the plant depend.

General View of Tissues

The most important tissues of Seed Plants are those which have
to do with growth, protection, support, conduction, secretions,
absorption, food manufacture, food storage, and reproduction.

FIG. 111. — A, lengthwise section through a tip of a stem, showing the
apical meristem (ra) from which branches (6) are arising and from which cam-
bium (c) and other tissues are being formed below. B, cross section of a
stem, showing the cambium and its position in reference to other tissues.

Tissues Connected with Growth. — Since the cells of most tis-
sues are no longer capable of growth and division after completing
their modifications, there must be provided at certain places in
the plant groups or bands of cells which retain their ability to
grow and divide throughout the life of the plant, for otherwise
the growth of the plant would soon cease. Such cells, forming
the meristematic tissues or meristems (from the Greek word mean-
ing "to divide ")> are present at the stem and root tips and in the
cambium, where their chief function is the multiplication of cells,

PROTECTIVE TISSUES 127

so that the different tissues may be enlarged as the growth of the
plant demands. By means of a meristem at their tips, roots
and stems elongate, and by means of the cambium they in-
crease in circumference. However, the meristems at the growing
apices are the first sources of all other tissues, even of the cam-
bium, and for this reason are known as the primary meristems.
(Fig. 111.)

Meristematic cells are characterized by having thin cellulose
walls, large nuclei, and dense cytoplasm — features which enable
the cell to grow and divide rapidly. Closely related to the meri-
stematic cells are the parenchyma cells, Which also in most cases
have thin cellulose walls but are less active in dividing. Paren-
chyma cells occur scattered throughout the various plant tissues
and constitute the food-making tissues of leaves and stems, and
most of the pith of plants.

Protective Tissues. — For protection against destructive agen-
cies plants have their outer cells modified into protective tissues,

J-

FIG. 112. — A, epidermis of a leaf showing epidermal cells (e) with their
outer cutinized walls (c). B, the flesh (j) and rind of a Jonathan Apple show-
ing the thick, cutinized, outer walls (c) of the epidermal cells (e). Much
enlarged.

such as epidermis, corky rind, and bark, which lessen evaporation
and prevent the entrance of destructive organisms. The most
common protective tissue is the epidermis which consists of one
or more layers of cells forming a jacket about the plant. The
outer walls of the exterior layer of epidermal cells are usually
thickened and contain a waxy substance called cutin which makes
them waterproof. (Fig. 112.) Most plant organs are at first pro-
tected by an epidermis, but in the older portions of stems and roots
the epidermis is often replaced by cork tissue, which is usually

128

CELLS AND TISSUES

much thicker and more protective than an epidermis. (Fig. 113.)
The cork covering may be more or less flexible, as the rind of an

Irish Potato or Sweet Potato, or
harder and more brittle, as in the
bark of trees, where it reaches its
extreme thickness. Cork tissue con-
sists of dead cells in the walls of
which there is deposited a waxy
substance much like cutin but called
suberin to which much of the pro-
tective character of cork is due.
Cork coverings afford more protec-
tion than an epidermis, but on ac-
count of their opaqueness, they are
not suitable except where it is not

necessary for light to penetrate to
FIG. 113. — A small portion J

of a section through an Irish the mner tlSSUes'
Potato, r, rind composed of a The protection afforded by an

number of layers of cork cells, epidermis and cork is often brought

s, tissue filled with food. Highly to our notice in case of fruits, tubers,

magnified. and fleghy rootg Tmig Apples,

Oranges, and most fruits which may be kept a long time, if

uninjured, soon decay when their rinds

are broken. The efficiency of a corky

rind to protect against the loss of water

is shown by the experiment in which a

peeled Irish Potato lost sixty times as

much water in 48 hours as an unpeeled

one of equal weight.

Furthermore, cork tissue has an ad-
ditional function in the healing of
wounds where, by the development of
a callus-like mass of cork, the open-
ing of the wound is closed and the chyma cells from the stem
break in the protective covering of of a Dock (Rumex) showing
the plant thereby repaired. It is im-
portant to recognize this fact in prun- at the angles-

1,1 n j.i_ berlam.

ing where the promptness as well as the

thoroughness of the healing depends much upon how the wound
is made.

FIG. 114. — Some collen-

the cells thickened mainly
After Cham-

STRENGTHENING TISSUES

129

Strengthening Tissues. — In order to endure the strains to
which they are exposed, both stems and roots must have strength-
ening tissues so as to be tough and rigid. Strengthening cells,
although of different types, have much thickened walls and in
most cases are much elongated.

In one kind of strengthening tissue, known as collenchyma,
which often occurs in the younger regions of stems, the cell
walls are thickened chiefly at the angles, thus leaving thin
portions in the side walls through which the protoplasm receives
enough materials to maintain life in spite of the modifications.
(Fig.

FIG. 115. — Bast fibers of Flax. A, a portion of a cross section of a Flax
stem, showing the bast fibers, e, epidermis; b, bast fibers; w, woody part of
the stem; p, pith. B, longitudinal view of a number of bast fibers. Much
enlarged.

A kind of strengthening tissue, in which the cell walls are quite
evenly thickened with cellulose, occurs in the older regions of
stems between the epidermis and woody cylinder, and consists
of bast fibers, the fibers upon which the value of Flax, Hemp, etc.
as fiber plants depends. Fig. 115.) Bast fibers are much elon-
gated cells and so spliced that they form thread-like fibers which
are easily combined into larger fibers for making linen cloth,
twine, ropes, and other textiles. Bast fibers may occur also in
leaves and roots where they are usually not so prominent, how-
ever, as in stems.

In the woody portions of plants, especially in all trees except
the evergreens, there occur along with the conductive tissues
wood fibers, in which the walls of the much elongated cells are not

130

CELLS AND TISSUES

only much thickened but also made woody — a feature in which
they differ from collenchyma and bast fibers, where the thicken-
^ ings are mainly of cellulose. (Fig. 116.} Where

the wood fibers are abundant, as in Oaks, the wood
is compact. Likewise, due to a greater number of
wood fibers, fall wood is more compact than spring
wood.

FIG. 116. — A
wood fiber, con-
sisting of a much
elongated cell
with thick
woody walls.

FIG. 117. — Very much enlarged lengthwise section
through an Alfalfa stem, showing the conductive and food-
making tissues of the stem. /, tracheae (commonly called
xylem), which constitute the water-conducting tissue;
p, the conductive tissue (commonly called phloem), which
conducts the food made by the leaves; c, the food-making
and storage tissue (cortex) just under the epidermis (e).
The cells of the cortex contain chloroplasts (ch). a,
cambium.

Conductive Tissues. — The conductive tissues of plants are of
two kinds, xylem and phloem, which occurring together form the
vascular bundles through which water, mineral salts, and foods
are distributed to all parts of the plant. (Fig. 117.) The xylem
is devoted chiefly to carrying water with what it may have in
solution and the phloem to carrying foods. Furthermore, the
xylem and phloem differ in that the conductive cells of the former
are empty while the conductive cells of the latter retain their pro-
toplasm. In Conifers, such as Pines, Firs, etc., the water-conduct-
ing cells have tapering ends and do not form a continuous series.
They have peculiar pits in their walls, known as bordered pits,
through which the liquids pass from cell to cell. They are com-
monly known as tracheids, meaning " trachea-like." Other plants
have tracheids, but tracheids with bordered pits are characteristic
of Conifers. The tracheids are also important strengthening as

ABSORBING TISSUES

131

well as conductive tissue. (Fig. 118.) In Flowering Plants,
although tracheids are present, the water-conducting tissue is
composed mainly of cells which fit
together end to end and thus form a
continuous series. The end walls of
the cells of the series are resorbed and
thus are formed continuous tubes,
called ducts, vessels, or tracheae, the
last name referring to their resem-
blance to the human trachea. In the
phloem, the main conductive tissue is
composed of the sieve tubes, which are
so named because of the perforations
in their walls. Unlike tracheae, which
have thickened woody areas in their
walls, sieve tubes have thin cellulose
walls and retain their protoplasm.
With the sieve tubes usually occur
thin-walled elongated cells, known as
companion cells, and parenchyma cells,
both of which aid in conduction.

Absorbing Tissues. — In the higher
plants, where the plant body is dif-
ferentiated into roots, stem, and leaves, the roots are especially
devoted to absorption. In case of soil roots, the root hairs,

FIG. 118. — Tracheids from
wood of Pine, showing the
tapering ends and the bor-
dered pits (p). After Cham-
berlain.

FIG. 119. — A, root hairs, the absorptive structures of roots, as they appear
in a surface view of the tip of a root. B, cross section of a root, showing that
the root hairs (h) are projections of the epidermal cells (e).

132

CELLS AND TISSUES

which spread into the soil where they take up water by means
of osmosis, are the chief absorptive structures. (Fig. 119.) There
are some plants, however, which live on other plants, in which
case the root tissues absorb directly from the tissues with which
they are in contact. In some cases the leaves absorb, as in
the Sundew (Drosera), Venus's Flytrap (Dioncea muscipula),
and Pitcher Plants (Sarracenia), where the eaves are especially
constructed for catching and absorbing insects.

Food-making Tissues. — The principal food-making organs are
the leaves where the cells are provided with chloroplasts and so
arranged that they can obtain the raw materials from which foods

FIG. 120. — Cross section of the leaf. /, food-making tissue; e, epidermis;
v, cross section of a vein.

are made. (Fig. 120.) However, food-manufacture is not lim-
ited to leaves, for all green stems have just under their epidermis
a band of green cells, known as the cortex, in which food is manu-
factured as long as light and air are not excluded. (Fig. 117.).

Storage Tissues. — Any living cell usually contains some
stored food, but there are cells which have food storage as their
chief function. This is true in the endosperm and fleshy cotyle-
dons of seeds, and in Irish Potatoes, Sweet Potatoes, and in other
tubers and roots where the cells enlarge and become packed with
food. In the pith some water is usually stored and often much
food, as is well known in the case of Sugar Cane and Sorghum in
which the pith contains much sugar. Throughout the wood of
trees there are thin-walled living cells, forming the medullary rays,
which function as a storage tissue. In Maple trees the sugar
occurring in the spring sap comes from the starch which was

REPRODUCTIVE TISSUES 133

stored chiefly in the medullary rays during the previous season.
For water storage some plants have special tissues, while others
like the Cacti store it throughout the plant body.

Secretory Tissues. — Secretory tissues, although not so essential
and no so common among plants as the other tissues discussed,
perform an important function in some cases. Most showy
flowers have secreting tissues, known as necta glands, located at
the base of the corolla or calyx. (Fig. 121.} These glands secrete
the nectar, which, by attracting insects, aids in securing cross-
pollination. Furthermore, honey is made from nectar, and the
value of a plant as a bee-plant depends upon the amount and qual-
ity of nectar secreted by its nectar glands. On the leaves, stems,
or fruits of many plants, such
as Mints, Oranges, Lemons, etc.,
there are glands whose secre-
tions give the plan!; a peculiar
fragrance. In he stems and
leaves of Conifers occur long
tubes or ducts, known as resin
ducts, which are lined with secre-
tory cells that secrete resin from »-
which pine tar, rosin, turpen-
tine, and other valuable prod-
ucts are made. Much like the FIG. 121. — A Buckwheat flower
resin ducts are the milk or lac- with sepals removed from one side
tiferous vessels of the Milkweeds to show the nectar glands (n). After
(Asclepiadaceae), Spurges (Eu- H- Mtiller-

phorbiaceae) , Dogbanes Apocynaceae) , and other plant families
where milk-like secretions occur. There are numerous secretions
many of which, however, are secreted by cells in which secreting
is not the special function.

Reproductive Tissues. — Reproductive structures are of two
kinds, sexual and asexual. Any portion of a plant, as a bud,
tuber, stem, or root which may function in producing new plants,
is regarded as an asexual reproductive structure. Some plants,
as Irish Potatoes, Sweet Potatoes, and Strawberries illustrate,
are quite generally propagated asexually.

In the higher plants the sexual reproductive tissues are those
of the flower, and more especially those of the stamens and pistils
with which the student is familiar. Although the eggs and sperms

134 CELLS AND TISSUES

are the chief reproductive cells, all the tissues of the stamens and
pistils are related to fertilization, which is the chief feature in
sexual reproduction.

In plants like Ferns, Mosses, and Algae, where there are no
flowers, the sex cells are commonly borne in special organs, called
sex organs, which are so constructed as to favor fertilization.

Summary of the Cell and Tissues. — The cell is the unit of
plant and animal life. It contains the living substance, known
as protoplasm, which is usually enclosed in a cell wall. The pro-
toplasm is composed of a nucleus and cytoplasm. Cells receive
water, food, and mineral elements through osmosis and ordi-
nary diffusion, and obtain energy through respiration. Cells
multiply by division. Cell multiplication is accompanied by
cell modifications which result in the differentiation of the cells
into tissues. Some tissues consist of only the modified cell
walls, the protoplasm having died and disappeared after the
modifications of the walls are complete. The higher plants have
many tissues, each of which has one or more functions. The
meristematic tissues enable the plant to continue growing; epi-
dermal and cork tissues protect the plant from drying and from
attacks of destructive organisms; collenchyma tissue, bast fibers,
and wood fibers enable the plant to support itself in a favor-
able position in spite of the force of gravity and winds; vas-
cular bundles, like the circulatory system of animals, supply the
other tissues with materials ; the absorbing tissues take from the
soil, or other substrata, the water and dissolved substances which
the plant must have; the storage tissues hold the water or food
in reserve for future use. The stored water is used during dry
seasons, and the stored food is used for the growth of new plants,
as in case of seeds, tubers, etc., or for the new growth of leaves
and flowers at the end of a dormant period, as in case of trees.
The food-making tissues furnish the food which all parts of the
plant must have. Secretory tissues assist in cross-pollination
by providing secretions which attract insects, furnish nectar from
which honey is made, and give us many other products, such as
resin, turpentine, etc.

CHAPTER VIII

ROOTS
General Features of Roots

The higher plants consist of roots and shoots. The roots are
generally underground structures, while the shoot is the aerial
portion consisting of the stem with its leaves, buds, flowers, and
fruit. Plants like the Algae, which live in the water where all parts
of the plant can absorb directly from the surroundings, do not
need roots, although they often have structures known as hold-
fasts which anchor them ; but holdfasts are too simple in struc-
ture to be called roots. Even in the Mosses, which are mainly
land plants, instead of roots there are hair-like structures, called
rhizoids, which anchor the plant to the substratum. True roots
are complex structures and are characteristic of Ferns and Seed
Plants.

Although we think of roots as underground structures, there
are, however, a few plants having roots adapted to living in other
situations, as in the water, air, or the tissues of other plants.
But with few exceptions our cultivated plants depend upon soil
roots, which, therefore, deserve most attention.

Being underground structures, soil roots normally arise from
the stem's base, from which they radiate by elongating and grow-
ing new branches, which in turn branch and rebranch until the
soil about the plant is quite thoroughly invaded by its root sys-
tem. Usually a plant's root system, tapering into numerous
branches almost hair-like in size, is more branched and spreads
farther horizontally than its stem system. The profuseness
with which roots branch is well shown by the estimated root
length of some plants. Thus the length of all the roots of a single
Wheat or Oat plant, laid end to end, is estimated at 1600 feet, or
more than a quarter of a mile. For a vigorous Corn plant the
estimated root length is more than a mile. Certainly in some
trees the root length would much exceed that of Corn.

The size of a plant's root system, in general, varies with that of

135

136 ROOTS

the shoot, for the larger the shoot, the larger the root system nec-
essary to supply the adequate amount of water and mineral mat-
ter, and to furnish sufficient anchorage. As to the size of the
roots of a plant, that depends upon the size of the shoot, the num-
ber of roots, and the distances of roots from the stem. The re-
lation of the size of roots to that of the shoot is well shown in case
of trees, where the roots directly connected with the stem and
known as main roots increase in diameter from a few millimeters
often to a foot or more as the shoot passes from the seedling to the
mature stage. Where there is only one main root, as in Alfalfa
and the Dandelion, its size is directly in proportion to the size of
the shoot, usually being as large or even larger in diameter than
the short stem of the crown. On the other hand, when the roots
leading from the stem are numerous, as in the Grasses and
numerous other plants, all are relatively small. As to the size
of a branch root, that depends much upon its distance from the
stem or main root, for all roots branch and rebranch until the
branches are fibrous-like, usually being a millimeter or less in
diameter at their tips. It is in connection with these fiber-like
branches, which are the absorptive regions, that roots show most
uniformity; for the roots of all plants taper down to these fiber-
like branches, which are practically uniform in size for all plants.
This uniformity in size is probably due to the fact that only
roots with a very small diameter are efficient absorbers.

The texture of roots is always soft at the tips where the cells
active in division, elongation, and absorption have thin cellulose
walls, which readily yield to pressure or strains. But not far back
of the absorptive region there are formed strengthening fibers,
which afford a toughness that enables the root to endure the
strains in connection with its anchorage function. Furthermore,
roots, in their older regions, are covered with cork which adds
firmness to the texture. In shrubs and trees the roots, in their
older regions, become as woody and just as hard as the stems.
' As to duration, roots may be short-lived, serving the plant only
in the seedling stage, as in case of temporary roots, or they may
last as long as the plant, as in case of permanent roots. The life
of permanent roots is one, two, or many years according to
whether or not the plant is annual, biennial, or perennial.

Interdependence of Shoot and Root. — Upon the roots the
shoot depends for water, mineral matter, and anchorage, while

INTERDEPENDENCE OF SHOOT AND ROOT 137

upon the shoot the root system depends for food. Neither
could survive without the other. Moreover, if either is hindered
in its development, the other likewise will be stunted. For this
reason when pots in case of potted plants prevent the further
development of the root system, the growth of the shoot is checked
and the plant has to be repotted. Again, the cutting away of the
roots of a shade tree in excavating for a sewer or sidewalk often
kills the tree due to diminished water supply. A number of in-
stances can be cited to show the dependence of the roots upon the
shoot. For example, it is well known that the roots of Asparagus
will not make a good growth unless the shoots are allowed to
grow during a part of the summer, in order that food may be pro-
vided for the growth of the roots. Furthermore, it is a common
practice in eradicating such weeds as Canada Thistle and Quack
Grass, to starve the underground structures by keeping down the
shoots. To enable plants to establish a good root system, in
order that there may be a well developed shoot, is one of the
chief aims in cultivation.

The most necessary material absorbed by plants is water, which
is supplied almost entirely by the roots in higher plants, and
serves at least a half dozen different purposes. First, water is
necessary, and in large quantities too, to prevent the shoot from
becoming dried out through loss of water to the surrounding air.
Leaves and also stems, unless the latter are well covered with
bark, are constantly having water evaporated from them and,
unless this loss is compensated, the shoot will soon die. Second,
water enables the cells to maintain their turgidity, which main-
tains the leaves and other soft tissues of the shoot in a rigid posi-
tion, and thus in a position suitable for work. Third, water is an
essential constituent of sugar, starch, and other foods made by the
shoot. Fourth, water as the plant's solvent is the medium through
which substances in solution are distributed through the plant.
Thus through water as a medium, the mineral elements of the soil
and the foods made in the leaves are carried to all parts of the
plant. Fifth, it is in the form of a solution in water that sub-
stances in the plant react chemically with each other. Sixth,
water is an important constituent of protoplasm, cell walls, and
other plant structures, usually being more than 90 per cent of
their fresh weight. Thus it is no wonder that plants must have
water or they soon perish.

138

ROOTS

In anchoring the shoot most soil roots perform an important
function, except in those plants with stems prostrate on the ground
or climbing supports. In plants with upright stems, as in trees,
the strains due to winds and gravity when the plant is bearing
foliage and fruit is often enormous. However, the root system
is usually able to hold the plant in place, although the strains
may break off branches or even the main stem. It is by spread-
ing laterally and profusely branching, that roots become so firmly
attached to large masses of soil that they can endure enormous
strains.

In addition to anchoring the plant and furnishing it water and
mineral matter, in many plants the roots function as storage or-
gans, in which some of the food made by the shoot each year is

stored for use in the development
of new shoots each succeeding year.
This function is especially obvious
in many plants which die down in
the fall and grow up again in the
spring.

Thus the root depends upon the
shoot for food while the shoot de-
pends upon the root: (1) for water
and mineral matter; (2) for an-
chorage; and (3) often as a storage

Types of Root Systems. — There
are various irregularities among
root systems, due to the altera-
tions which a root system must
make in adjusting itself to obstruc-
tions and the uneven distribution
of water and mineral matter in the
soil. For this reason root systems
are less symmetrical than shoots.
However, despite these irregulari-
ties there are some inherent differ-
ences that are so regular as to be typical of certain plants.

In the Corn, Wheat, Oats, and Grasses in general, there is the
type of root system, known as the fibrous root system, in which
there are no dominant main roots, but all roots are small and with

FIG. 122. — The fibrous roots
of Corn.

TYPES OF ROOT SYSTEMS

139

their numerous fine branches form a system resembling a fine
brush when the dirt is washed away. (Fig. 122.) This type of
root system is common among weeds, trees, and
many cultivated plants. In addition to the un-
derground roots, some of the Grasses, as Corn
illustrates, develop prop or brace roots, which
grow out from nodes above the ground into
which they finally reach to afford additional an-
chorage. However, brace roots are not neces-
sarily an accompaniment
of fibrous root systems,
for they may occur in
connection with other
kinds of root systems.

The tap-root system, in
which there is one large
main root from which
small lateral branches
arise, is typical of the
Alfalfa, Red Clover,
Beets, Dandelion, and
numerous other plants.
Tap-roots usually grow
directly downward, pen-
etrating into the deeper
layers of the soil where
more moisture is avail-
able. (Fig. 123.) For
this reason, the tap-root
system is best adapted for dry regions
and is, therefore, characteristic of drought
resistant plants. Although the tap-root
is more common among herbaceous
plants, it occurs, nevertheless, among
trees, where it often interferes with trans-
planting, as in case of Hickories, Oaks,
and Maples. (Fig. 124.)

Tap-roots are also convenient storage

FIG. 123. —
Alfalfa, a plant
with a promi-
nent tap-root.

FIG. 124.— Young Shell-
bark Hickory, showing the
tap-root. After Farmers'
Bulletin 178, U. S. Dept.
of Agriculture.

organs in which food is stored for the growth of the new shoot
the next year. This fact is well illustrated in Alfalfa, Clover,

140

ROOTS

FIG. 125. — Sugar Beet, a
plant with a fleshy tap-root.

and the Dandelion, where the shoots die down in the fall to be
followed by new ones in the spring. Thus the tap-root system
is well adapted to the perennial habit. In some plants, as

Radishes, Beets, Carrots, Turnips,
etc., where the storage function is
quite prominent, the tap-root is
tender and of much importance as
a vegetable. (Fig. 125.) From some
fleshy roots valuable products are
extracted, notably the Sugar Beet
from which most of our sugar is ob-
tained.

Plants having prominent tap-roots
with short lateral roots can be grown
close together without injury. Due
to this fact and to the size of the
shoot, such plants as Clover, Alfalfa,
Beets, and others with the tap-root
system grow well when crowded.
The fascicled root-system, consisting of a cluster of roots all of
which are much enlarged in connection with the storage of food,
is characteristic of a few plants of which the Sweet Potato and
Dahlia are two that are well known. (Fig. 126.)

Adventitious roots, so named because of their occurrence in un-
accustomed places, may be
mentioned here, although
the classification pertains
to the place of occurrence
and not to any peculiar fea-
ture of the root itself; for
any root, whether fleshy or
fibrous, developing from
leaves or from stem regions
where roots are not nor-
mally present is called ad-
ventitious. All roots may
be regarded as adventitious
except those, known as the primary ones, which develop directly
from the radicle of the embryo.

The ability of many shoots to develop roots from various re-

FIG. 126. — A portion of a Sweet Potato
plant, showing the fascicled roots.

DEPTH AND SPREAD OF ROOT SYSTEMS 141

gions of the stem is of much service in plant propagation. When
canes of some varieties of Raspberries bend over and touch the
ground, they become rooted at their tips. If the canes are cut, the
rooted tip then becomes a new plant. This is a common method
of propagating Raspberries. If the branches of the Grape Vine,
or, if many of our shrubs are bent to the ground and a portion
covered with soil, roots will develop on the buried portion, which
thereby becomes a means of obtaining new plants. Geraniums,
Coleus, Roses, and many other plants are propagated by cutting
off branches and setting them in moist sand where they develop
adventitious roots and become new plants.

Depth and Spread of Root Systems.1 — Roots must go deep
enough and spread far enough laterally to meet the demands of
the plant for absorption and anchorage, both of which in general
must conform to the size of the shoot. On this account, trees
need a deeper and wider root system than a Corn plant. But
aside from these differences which relate to the size of the shoot,
root systems of different plants differ in the depth and spread
according to : (1) the conditions of the soil in relation to moisture,
mineral matter, and air; (2) the type of root system; and (3) the
difference in the disposition of the roots of different plants, al-
though similar in type.

Roots, like all other plant portions containing living cells,
must have oxygen for respiration. For this reason the region of
the soil just under the surface where air is accessible is more fav-
orable for root activity than the deeper soil regions. Besides,
more of the necessary mineral matter is available in the surface
layers of the soil. Consequently, root systems increase by ex-
tending proportionately much more laterally than downward,
except in cases where there is extensive development of a tap-
root, as in such plants as Alfalfa and the Mesquite.

Studies made of the roots of Corn show that under ordinary
conditions the roots extend laterally, most of them being only
from 3 to 6 inches under the surface, until they reach a distance
of about 1J feet from the plant, and then they extend downward
as well as laterally, often having a depth of 3 or 4 feet when the

1 The Roots of Plants. Bulletin 127, Kansas Agr. Exp. Sta. Root Sys-
tems of Field Crops. Bulletin 64, N. Dakota Agr. Exp. Sta. Corn, its Habit
of Root Growth, Methods of Planting and Cultivating, Notes on Ears and
Stools or Suckers. Bulletin 5, Minnesota Agr. Exp. Sta.

142 ROOTS

plant is mature. In the Irish Potato the roots may reach a depth
of 3 feet after extending laterally 2 or 3 feet, but for most of their
length they are within a few inches of the surface. It may now
be seen that deep cultivation may injure Corn, Potatoes, and
other plants with a similar root habit by tearing away the roots.
In Wheat, Oats, and Barley, the root systems do not extend so
far laterally as in Corn but deeper into the soil, reaching a depth of
4 to 5 feet. Flax and Kafir Corn have fibrous root systems which
feed mainly from the surface soil. Plants with shallow roots are
called surface feeders, and are considered " hard on the land," be-
cause they exhaust the moisture and mineral matter in the sur-
face soil.

In trees, although the lateral roots may reach a length of nearly
100 feet, they still remain near the surface. In the Soft Maple,
lateral roots 80 feet in length have been found to range in depth
from 8 inches to 2 feet. In old Apple trees the lateral roots,
which may be 60 feet or more in length, usually have a depth
ranging from 2 to 5 feet. Since trees by the surface habit of their
roots take the moisture and mineral matter from near the surface,
it is clear why crops do not grow well around them even when
not affected by their shade.

Some fruit trees, such as the Cherry and Pear, send their roots
several feet into the soil and, therefore, require a deeper soil than
some other kinds of fruit trees. The Quince, commonly used as
a stock on which to graft Pears, has a shallow root system, and so
has the ' ' Paradise ' ' Apple on which Apples are often grafted. The
fact that Pears grafted on Quinces or Common Apples grafted on
the "Paradise/" Apple bear younger than they do when grown on
their own roots, shows that the shoot and root system are very
closely related in their activities. The deep root systems occur
generally in connection with tap-roots, which sometimes reach
extraordinary depths. Also the lateral roots in a tap-root system
are usually well under the surface. For example, in Sugar Beets
the lateral roots are 6 inches or more under the surface, and,
therefore, not usually disturbed by deep cultivation. On account
of having a deep root system, Beets require a deep and well
loosened soil.

As to the depth reached by tap-roots, 5 or 6 feet is common in
Alfalfa, and a depth of 31 feet has been recorded. The tap-root
of the Mesquite, which is a native of desert regions, has been

CELLULAR ANATOMY OF THE ROOT TIP

143

known to reach a depth of 60 feet. Plants with the Alfalfa type
of root system are not only drought resistant, but also loosen the
subsoil, which is thereby put in better condition for those plants
with roots less able to penetrate a hard subsoil.

Root Structure

As the student already knows, plant organs consist of tissues,
each of which on account of the peculiar structure, of its cells is
especially adapted to do a certain kind of work.
In roots there are tissues to perform the following
functions: (1) protection; (2) growth; (3) absorp-
tion; (4) conduction; and (5) strengthening. Root
tips show on their surface rather distinct regions,
which differ in color, texture, or some other feature
that can be seen without a microscope. Often, but
not always, the small protective cap, which is the
actual end of the root, can be identified by its
brownish color. The smooth whitish zone, which is
usually a conspicuous region of the tip, is where
cell multiplication and growth are most prominent.
Just back of this is the absorptive zone, bearing
numerous root hairs which are more conspicuous
when grown in moist air or moss where there are no
soil particles to influence their shape. (Fig. 127.)
Back of the absorptive region, where protective and
strengthening tissues are becoming prominent, the
root is firmer in texture and darker in color; and
these features become more prominent with age, as is well dem-
onstrated in shrubs and trees where the older parts of roots are
woody and covered with thick bark.

Cellular Anatomy of the Root Tip. — If with the aid of a micro-
scope a lengthwise section through a root tip is studied, more may
be learned about the character of the different tissues. (Fig. 128.)
The root cap now appears as a well defined structure, consisting
of many cells loosely joined into a covering, which is thickest
directly over the end of the root. Next to the root cap is the
zone of cells active in division and constituting meristematic tissue.
Back of this is the growth zone, in which the chief activity is cell
enlargement to which the elongation of the root is due. Other

FIG. 127.
— Tip region
of a root of
Red Clover,
showing root
hairs.

144

ROOTS

FIG. 128. — Longitudinal sections through root of Onion at the following
regions. A, through the tip showing the root cap and meristematic zone.
B, through the zone of elongation. C, through absorptive region or hair zone.
Highly magnified.

CELLULAR ANATOMY OF THE ROOT TIP

145

regions of the root increase in diameter, but almost all elongation
takes place in the growth zone, as shown in Figure 129. The
meristematic zone is thus so situated that the new cells formed
by it may be added both to the root cap, the thickness of which
is thereby maintained in spite of its being rapidly worn away on
its outer surface, and to the growth zone, the older portions of
which are constantly taking on the fea-
tures of the absorptive zone just behind.
The growth zone merges imperceptibly
into the absorptive zone where the fol-
lowing tissues become quite well defined:
(1) a surface layer of cells constituting
the epidermis which has most to do with
absorption, the special absorptive agents
being the root hairs, which, as the section
shows, are merely projections of the epi-
dermal cells; (2) a broad band of cells
just beneath the epidermis and constitut-
ing the cortex; and (3) a group of con-
ductive tissues forming a central cylinder,
known as the vascular cylinder.

It is to be noted that the epidermis of
roots, unlike that of leaves and stems,
has no cutinized walls and contains no
stomata or other openings for the entrance
of air, although so many active cells re-
quire much oxygen for respiration. How-
ever, openings are not necessary, for the
uncutinized walls offer practically no re-
sistance to the passage of water, which
usually carries in solution oxygen enough
to support quite active respiration.

Through the development of the root hairs the absorptive
surface of the root system is much increased, and may be thereby
increased from five to six times in Corn, about twelve times in
Barley and as much as eighteen times in some other plants. All
root hairs are able to absorb regardless of their size, which ranges
from a slight bulge near the growth zone of the root to often
more than an inch farther back. They live only a few days,
but, as they die off behind, new oaes form ahead, and in this

FIG. 129. — The radi-
cle of a Corn seedling
marked to show the re-
gions of elongation. A,
radicle just after being
divided into spaces of
about -aV of an inch in
width. B, radicle sev-
eral hours after mark-
ing, showing the region
where elongation is tak-
ing place. Modified from
Andrews.

146 ROOTS

way the absorptive zone moves along just behind the advancing
tip. The root hairs grow out more or less at right angles to the
surface of the root, and are able on account of their flexible
slimy walls to push through small openings and around the soil
particles against which they flatten and to which they become
glued fast, thereby coming in close contact with the water films
around the soil particles. Why the region of elongation is near
the tip is now clear, for, if behind the absorptive zone, the hair
region would be pushed ahead and the root hairs thereby torn
away.

FIG. 130. — Cross section of a root through the absorbing region, x,
xylem; p, phloem; a, pericycb; e, endodermis; c, cortex; h, root hairs.
Highly magnified.

The cortex, consisting of many layers of parenchyma cells,
transports the substances absorbed by the root hairs to the con-
ductive tissues, and in fleshy roots also serves as a storage region.

The vascular cylinder contains the conductive tissues, notably
the xylem and phloem. The xylem and phloem and their posi-
tions in reference to each other are best seen in a cross section of
a root, as shown in Figure 130. The xylem occupies the cen-
ter and has strands radiating from the center like the spokes of
a wheel. Between the spokes of the xylem and near their outer
ends are the phloem strands. Inasmuch as the absorbed sub-
stances are carried to the shoot by the xylem, this alternate ar-

ANATOMY OF THE OLDER PORTIONS OF THE ROOT 147

rangement of xylem and phloem shows adaptation, in that it per-
mits the absorbed substances to reach the xylem without passing
through the phloem. The vascular cylinder is bordered by a
chain of cells, known as the pericycle. The pericycle joins the
endodermis or starch sheath which is the chain of cells forming
the innermost layer of the cortex. Aside from the fact that
branches or lateral roots develop from the pericycle, the functions
of the endodermis and pericycle in roots are not well understood.

Anatomy of the Older Portions of the Root. — Not far back of
the hair zone, as indicated by the brownish color and the slough-
ing off of the epidermis with its dead root hairs, there appear
some anatomical changes, such as the formation of a corky cover-
ing, enlargement of conductive and strengthening tissues, and
the development of branches. As this region of the root becomes
older, these anatomical changes become more prominent.

Since the epidermis behind the hair zone dies and falls away,
absorption is limited to the tip region of the root. Accompany-
ing the death of the epidermis, a
protective tissue is developed by the
layers of cells beneath. Usually the
cells just beneath become cutinized
and take the place of the epidermis
as a covering. In Grass roots the
layers of cells just beneath the epi-
dermis thicken their walls, thereby
forming over the root a hard woody
rind similar to that of Grass stems.
Commonly in roots there is also
formed in the region of the pericycle
a meristematic band of cells, known
as the cork cambium, which by divid-
ing parallel to the surface of the root
adds layers of cork on its outer side

FIG. 131. — Diagram of a
lengthwise section through the
region of the root back of the
hair zone, showing the changes
in the epidermis and cortex, e,
epidermis dead and sloughing
off; k, cork cambium on the in-

ner side with cork and dead cor-
and cortex cells on its inner side, thus tex between it and the epidermis;
forming a protective covering and c' secondary cortex; v, vascular
a secondary cortex which it also en- cylinder' Highly magnified-
larges as the root grows older. Since cork is impervious to water
and the foods contained, by the formation of cork in the region
of the pericycle, the first or primary cprtex has its conductive
connections with the vascular bundles cut off and its death

148 ROOTS

must follow. (Fig. 131.) The dead epidermis and cortex form
the outer portion of the bark, which thickens as cork is added by
the cork cambium, and in roots living a number of years, like
those of shrubs and trees, may become quite thick, and broken
and furrowed, as in the large roots of trees. As a provision for
strength, fiber-like strands of strengthening tissue are commonly
formed in the secondary cortex.

In order that the vascular cylinder may have adequate con-
ductive capacity in the older portions of the root, it, too, must
enlarge, for as the absorptive surface of the root increases ahead
by the multiplication of branches, not only is there an increase
in the amount of absorbed substances which the xylem must
carry to the shoot, but also an increase in the amount of food
which the phloem must carry to feed the greater number of
branches of the root. In the formation of xylem in roots, the
portions first formed are the radiating strands or spokes which
enlarge by developing toward the center where they usually come
together, thus forming the solid central core of xylem as shown
in Figure 132. However, in some plants, as in Corn and many
other Monocotyledons, the xylem strands never come together,
and consequently a central pith is left, around which the strands
of xylem are arranged. In all roots the xylem strands are at first
enlarged by this centripetal development. In some short-lived
roots of Dicotyledons and in the roots of most Monocotyledons,
the enlargement of the xylem is due to this centripetal develop-
ment and the development of new vascular bundles between the
old ones as the root becomes older. In Dicotyledons and also
in the Gymnosperms, the group to which Pines, Firs, Spruces, etc.,
belong, the vascular cylinders of most roots are increased also
as shown in Figure 132. It is seen in A of Figure 132 that the
phloem and xylem are not in contact. Lying between them
are cells which have not been modified into definite tissues.
Some of these cells become meristematic and so arranged as to
form a continuous band of cambium, known as the cambium
ring, which by curving outward passes on the outside of the
xylem, and by curving inward passes on the inside of the phloem,
thus separating the xylem and phloem regions of the vascular
cylinder as shown at B. The cambium cells, in the main, divide
parallel to the surface of the root, and divide in such a way that
the layers of new cells on the inside of the cambium are about

ANATOMY OF THE OLDER PORTIONS OF THE ROOT 149

equal in number to those on the outside. The new cells next to
the xylem become modified into xylem, at first filling in between
the strands of the primary xylem, and the cells formed next to
the phloem are changed into phloem. In this way the vascular

FIG. 132. — Diagrams showing how the xylem and phloem of roots are
increased. A, cross section of a root showing xylem (x) and phloem (p)
before the cambium is formed. B, cross section of root showing xylem (x)
and phloem (p) after the cambium ring (c) is formed. C, cross section show-
ing the xylem (x} and phloem (p) shown in A and B and the new xylem (a)
and new phloem (6) which have been formed by the cambium (c). Adapted
from J. M. Coulter.

cylinder is increased in diameter as shown at C. In roots which
live many years, like those of trees, the layers of xylem formed
each year form annual rings like those occurring in woody stems;
and the outer layers of the phloem, with the cortex and other
tissues outside of the phloem, constitute a bark like that of
woody stems. In fact, it is only by the presence of their early
root anatomy that sections of such roots can be told from sec-
tions of stems. In some fleshy roots, as Beets illustrate, a num-
ber of cambium rings form outside of the first one, and the
growth resulting from each appears as a ring when a cross-section
of the root is made.

150

ROOTS

Another anatomical feature connected with the older portions
of roots is the development of branches, which begin to develop
some distance back of the hair zone and
in the way shown in Figure 133. In Seed
Plants the branch roots, which are called
secondary, tertiary, and so on according to
their distance from the main root, develop
from the pericycle and usually in the re-
gion closest to the xylem. In forming a
new branch, a few cells of the pericycle in
the region where the branch is to appear
begin to divide parallel to the surface of
the root. The new cells at first appear as
a slight elevation on the pericycle, but by
rapid growth this elevation of cells soon
pushes through the cortex and other over-
lying tissues, and becomes a branch with
vascular cylinder and other tissues con-
tinuous with those of the root of which it
is a branch. Of course the farther from
the root tip, the older and more fully de-
veloped are the branches.

One important feature in connection with
the branching habit is that, when the end
of a root is cut away, the remaining por-
tion is stimulated to develop branches. It
is due to the ability of roots to branch,
that trees and other plants with their roots
heavily pruned in transplanting are usually
able to provide a new root system and become established in
their new location.

FIG. 133. — Length-
wise section through
a root, showing how
branches arise. The
branches (6) originate
in the region of the
vascular cylinder and
push through the cor-
tex, finally reaching
the exterior.

Factors Influencing the Direction of Growth in Roots

Roots and stems respond very differently in respect to gravity.
Primary roots grow toward the center of gravity, while most
stems grow in the opposite direction. This earth influence is
known as geotropism. Geo comes from a word meaning earth
and tropism means turning. So the word, geotropism, means
earth-turning, and refers to the turning of the root and stem in

FACTORS INFLUENCING THE DIRECTION OF GROWTH 151

response to the influence of gravity. Primary roots are positively
geotropic (growing toward the earth's center), while most stems
are negatively geotropic (growing directly away from the earth) .
Lateral roots, as well as most branches of stems, grow more or less

FIG. 134. — The radicle or primary root of the Sunflower growing downward
in response to gravity. After Osterhout.

horizontally and, when strictly horizontal, are neither positively
nor negatively geotropic. What is shown in Figure 134?

Roots, especially primary roots, are sensitive to moisture and
grow towards it when more moisture is needed. The tropism
induced by water is called hydrotropism. Most roots are to a
greater or less extent positively hydrotropic. Notice what is
shown in Figure 135. In response to the water influence, the
roots of most cultivated plants grow deeper in the soil during

FIG. 135. — An experiment to show the effect of moisture upon the direc-
tion of the growth of roots. The box containing moist sawdust in which the
Corn is planted has a bottom of wire netting. After the roots grew through
the meshes, thus coming in contact with dry air, they changed their direction
and grew along the bottom of the box, thus keeping in contact with moisture.
Adapted from Osterhout.

a dry season than during a wet season. When there is abun-
dance of moisture in the soil, Corn roots may grow within 2
inches or less of the surface, but are 3 inches or more under the
surface when there is a lack of moisture, and usually penetrate

152 ROOTS

the soil to a greater depth. The roots of Willows and Poplars
will extend long distances in response to moisture. When these
trees grow near a well, their roots often grow down the sides of
the well until the water is reached. In seeking water and air the
roots of trees and weeds grow into drain tiles and sewers, often
clogging them.

Stems, in general, grow toward the light, while most roots
shun the light. Roots are said to be negatively heliotropic, while
stems are positively heliotropic. A erotropism, growth toward those
regions of the soil where air is more plentiful, Chemotropism,
growth toward certain substances, and Traumatropism, growth
away from injurious bodies, are other movements of roots.

The Soil as the Home of Roots

In the most general meaning of the term, the soil is that upper-
most layer of the earth's crust in which, by means of their root
systems, plants are able to obtain the substances necessary for
growth. However, in agriculture, the term soil is often applied
to the layer which is tilled, and the term subsoil to that which
lies beneath. Although the term soil is used in different ways,
we usually think of the soil as extending down to where the
dark color changes to a light, due to the absence of humus. The
depth of the soil varies greatly in different localities, ranging
from a few inches to several feet.

As to origin, the soil is fundamentally pulverized rock of which
there are a number of kinds, such as granite, limestone, sandstone,
shales, etc., each of which* gives some special property to the soil.
Various agencies, such as wind, water, ice, chemicals, tempera-
ture variations, and plants are active in breaking all rocks into a
pulverized form. They may be very finely pulverized into clay,
as the silicates are, or left in the form of fine sand, coarse sand,
or gravel.

The rock constituents of any bit of soil, even of the finest clays,
are exceedingly various in size and shape as a microscopical
examination shows. The irregularity in size and shape makes
it impossible for the particles to pack closely, and thus insures
the open spaces which are estimated to be from 25 to 50 per cent
of the volume of cultivated soils. (Fig. 136.) The spaces are
exceedingly important, for they permit the circulation of water

WATER, AIR, AND HUMUS OF THE SOIL

153

and air which the roots and micro-organisms of the soil must have.
Although not tightly packed, the particles adhere to each other
when moist, and this feature and the weight of the soil enable
roots to obtain a firm anchorage.

FIG. 136. — Diagram of two root hairs and the soil around them. The
soil particles are shaded and the light area around each soil particle repre-
sents a film of water. The large light areas among the soil particles are air
Modified from J. G. Coulter.

Water, Air, and Humus of the Soil. — To the plant the water
of the soil has two important functions. First, it is the reservoir
upon which the plant depends for water. Second, water is the
solvent in which the soil substances become dissolved before
entering the plant.

The amount of soil water varies for different kinds of soils, and
for the same soil at different times. Thus garden soil rich in
humus or a very heavy clay soil will hold two or three times as
much water as a sandy soil. Just after a heavy rain soils are
saturated with water, that is, all of the spaces are filled. But
much of this water, known as free water, gradually sinks away
toward the center of the earth in response to its own weight, leav-
ing the pores partially empty. The water then remaining in the
pores consists chiefly of capillary water, which is held in the pores
by the force of capillarity. In addition to the capillary water,
which does not respond to the influence of gravity, there is also
the hygroscopic water, which remains, after the capillary water is
removed, as a thin film around each particle and so firmly held

154 ROOTS

that it can not be driven off except by the application of heat.
The driest of "air dry': soils still contain considerable hygroscopic
water, as shown by their loss in weight when they are further
dried in an oven.

Plants depend mainly upon capillary water, although in some
cases, especially in soils with high hygroscopic power, some hygro-
scopic water may be available to the plant. When soils are satu-
rated, as after heavy rains or in bogs and swamps, there is more
water present than the plant needs, and, besides, the air which
roots must have is driven from the soil pores. Experiments have
shown that, in general, a soil is best adapted to plant growth
when the water present is not more than 60 per cent of the amount
required for saturation, or, in other words, when about two-fifths
of the pores are open for the circulation of air.

The forces which resist the pulling away of hygroscopic and
capillary water from the soil particles tend to keep the water
equally distributed. Thus as water is lost from the pores in the
surface soil, either by evaporation or root absorption, it is replaced
by water moving up from below through the force of capillarity.
Consequently, as a root absorbs, the movement of water toward
it from all around enables it to obtain water from regions several
feet away. In fact, capillarity has been known to raise water to
a height of 10 feet in one kind of soil. Again, in hygroscopic
water the thin films, which are like stretched rubber around the
soil particles, are connected where the soil particles touch, and to
compensate the greater water loss one film may have over others,
there is such a movement of water between the films that all for
a considerable distance around share in the loss. Thus, due to
the forces of capillarity and the surface tension of hygroscopic
films, soil water tends to move to the point where it is being
absorbed. It is now clear why the soil becomes so evenly dry
around a plant.

Air in the soil is necessary for the respiration of roots and
micro-organisms. It is also of use in oxidizing poisonous sub-
stances which result from the decay of organic matter in the soil,
so that their poisonous effects on roots are destroyed.

Humus consists of organic matter in a state of decomposition.
When only partially decayed as in some bogs where it accumu-
lates in large quantities, it forms peat. It gives to soils the dark
color which is characteristic of good soils, such as loams where it

SOIL MICRO-ORGANISMS

155

occurs well mixed with sand and clay. It adds to the soil various
organic substances, some useful and some harmful, enables the
soil to retain more moisture, makes the soil light, and makes the
soil a suitable place for micro-organisms to live.

Soil Micro-organisms. — The soil is the home of innumerable
organisms, some plants and some animals, all of which are related
to soil fertility. They are of three kinds, Fungi, Bacteria, and
Protozoa. These organisms, influencing the soil fertility and
having their activities in turn influenced by the soil conditions,
add to the complexity of soils, which
are still far from being understood.

Many Molds occur in the soil, where
their thread-like filaments, like those
of Bread Mold, aid in breaking up the
organic matter into soluble compounds.
Besides Molds, there are other kinds
of Fungi which act on the soil con-
stituents, as in case of Toadstools
which invade the soil with their root-
like filaments. Furthermore, some
Fungi are so intimately connected with
the roots of some plants as to replace
the root hairs. In this case the Fun-
gus weaves around the root a close

FIG. 137. — A Mycorrhizaon
a rootlet of the Beech. The
felt-like mass of mycelial
covering of filaments, thus forming threads closely enwraps the
with the root the structure, known as root tip, extending back to
the mycorrhiza, in which the filaments

beyond the hair zone and

of the Fungus absorb water and solu- Sp7,ding intA°fttht,S°u Uke

... „ root hairs. After Frank,

ble substances, which afterward are

transmitted to the root. (Fig. 137.) Pines, Beeches, Oaks, Blue-
berries, and Orchids are some of the more familiar plants in
which the mycorhizas occur. Plants, like Blueberries, are so
dependent upon Mycorhizas that they can not be grown unless
the proper Fungus is present in the soil.

The Bacteria, the smallest of all living organisms and well
known in connection with diseases of animals and plants, are ex-
ceedingly abundant in soils, often many millions being present
per cubic centimeter of soil. Like the Fungi, the Bacteria of the
soil are dependent in their development upon the presence of
some organic food material, such as decaying plant or -animal

156 ROOTS

matter. They are of various kinds and perform various func-
tions, the most important of which has to do with making
nitrogen available for root absorption. The importance of the
Bacteria concerned with providing available nitrogen in the soil
is due to the fact that nitrogen is an indispensable constituent of
protoplasm, and, although composing four-fifths of the air, it is
only available when it occurs in the form of a soluble salt in the
soil where it is taken in through the roots. To maintain in the
soil an adequate supply of available nitrogen, which is constantly
being lost by the removal of the crop and through drainage, is an
important problem in maintaining soil fertility. The Bacteria
provide available nitrogen in two ways. First, some kinds act
on the ammonia which is usually abundant where there is humus,
forming nitrates, which are soluble, and in which form the nitrogen
is available to our higher plants. Second, certain kinds of Bac-
teria use the free nitrogen of the air in forming the nitrogenous
compounds of their bodies, which after death release these com-
pounds to the soil, and in this way the soil has its nitrogen in-
creased. Thus while some Bacteria change the nitrogenous
substances already present in the soil into forms which can be
used as a source of nitrogen by the higher plants, these forms, by
adding nitrogenous compounds through the rapid multiplication
and early death of their bodies, actually increase the nitrogen
content of the soil, and for this reason are of most importance in
maintaining the soil fertility. Although many of these Bacteria
which fix the free nitrogen of the air live free in the soil, there are
some, however, which live in the roots of some of the higher
plants, especially in those of Clover, Alfalfa, Beans, and other
Legumes. In this case they live in the nodules formed on the
roots, and the relation between the Bacteria and the higher plant
is said to be one of symbiosis, a name applied to such an intimate
association of organisms. In this case both organisms are bene-
fitted; for the Bacteria obtain some food from the higher plant
and the latter obtains nitrogenous compounds from the dead
bodies of the Bacteria. (Fig. 138.)

However, not all kinds of soil Bacteria are so indispensable,
for there are some which have harmful effects, in that they tend
to lessen the nitrogen content of the soil by so thoroughly de-
composing the nitrogenous compounds that the nitrogen escapes
from the soil as free nitrogen or as ammonia gas. Among so

SOIL SOLUTION

157

many kinds of soil Bacteria, breaking up compounds in different
ways to secure energy and food, not all results of their activi-
ties in the soil can be expected to be desirable ones.

The Protozoa, which are small one-celled animals of which
the Amoeba is one type, are abundant in rich soils, where
they are thought to exert a harmful in-
fluence on the soil fertility by feeding
on the Bacteria. The evidence for this
accusation is that soils are more fertile
after being subjected to temperatures
or poisons which kill the Protozoa but
leave the Bacteria unharmed.

Soil Solution. — The soil water and
the various mineral matters and or-
ganic substances dissolved in it consti-
tute the soil solution. The dissolved
organic substances are of use to the soil
micro-organisms, but it is mainly water
and mineral matters that higher plants
need to obtain from the soil solution.
The most important of the mineral ele-
ments for crops are nitrogen, phosphorus ,
potassium, sulphur, calcium, iron, and
magnesium. These occur in compounds known as mineral salts,
which, although very essential to plant growth, are present in
very small quantities, usually constituting less than one per
cent of the best of soil solutions. Of these, nitrogen, phos-
phorus, and potassium are in most demand by crops, and the
ones most likely to be lacking. Consequently, in maintaining
soil fertility, the chief problem is to conserve and restore these
elements. The value of artificial fertilizers and manures de-
pends chiefly upon the amount of these elements contained.
In most soils, iron, sulphur, and magnesium are present in
sufficient quantities. Calcium must always be present to
neutralize the acids, for both roots and soil Bacteria are very
sensitive to acids. Calcium is added to the soil in the form of
lime or limestone. On the other hand, when soils contain too
much of an alkali, such as sodium carbonate, plants will not
do well until the condition is changed by the addition of gypsum
or some other substance capable of breaking up the alkali.

FIG. 138. — Nodules on
the roots of a Pea. Modi-
fied from Palladin.

158

ROOTS

Each of the mineral salts which plants require, apparently,
is so specially related to the nutrition of the plant, that not one
of them can be omitted, although all others are present in

suitable quantities. This fact is dem-
onstrated by growing plants with their
roots in distilled water to which the dif-
ferent mineral salts can be added in such
proportions as the experiment demands.
When the salts are added in such pro-
portions that the solution imitates a soil
solution, such as ordinary spring or well
water, many herbaceous plants are able
to grow in it till they have flowered and
produced seed. In fact, aside from the
lack of anchorage and having to supply
their roots with oxygen from the shoot,
plants may do almost as well as when
FIG. 139.— Water cultures rooted in the soil. For some plants the
of Buckwheat, showing ef- water culture gives good results, when
feet of the lack of the dif- the saltg are in such a prOportion that
ferent mineral elements: „ ,.. c ., •,

1, with all the elements; 2 llters °f.the S°lu- **

2, without potassium; 3, tlon contain 1 gram
with soda instead of potash; of potassium nitrate,
4, without calcium; 5, with- j. gram of iron phos-
out nitrates or ammonia phat6j i gram of cal.

salts. . 1r , j !

cmm sulfate, and t

gram of magnesium sulfate. The results of
omitting some of these salts are shown in
Figure 139.

Root Absorption. — For the process of os-
mosis upon which the entrance of water into

the root depends, the epidermal cells of the FIG. 140. Root

root tip are especially fitted. By means of hair showing the thin
the root hairs they have a large surface in layer of protoplasm
contact with the soil solution. Having thin
cellulose walls against which their protoplasm
is spread out into a thin lining, root hairs afford an easy entrance
of water into their large vacuoles. (Fig. 140-) As the student
already knows from his acquaintance with osmosis, the entrance
of water into the epidermal cell depends upon the concentration

FACTORS THAT HINDER ABSORPTION 159

of the substances in solution in the cell sap. Water is drawn
into the root hairs only when the density of the cell sap is
sufficient to exert an osmotic force that overcomes the osmotic
force of the solution without and the forces by which the soil
holds on to the water. On the other hand, when the forces
without are stronger than the osmotic force of the cell sap,
then water will be drawn out of the root. This latter condition,
which is likely to be disastrous to the plant, occurs when there
is an excessive amount of mineral salts in the soil solution, or
when the soil becomes so dry that the forces with which the
soil holds on to the water become so great as to overcome the
osmotic force of the cell sap. By watering plants with nutrient
solutions which are too strong, the soil solution may become so
concentrated as to injure the plants.

The entrance of the dissolved mineral salts into the root hairs
depends chiefly upon two things: First, the cell membrane must
be permeable to them. Second, the membrane being permeable
to them, they pass into the root hairs by the laws of diffu-
sion. Thus, if a salt is more concentrated without than within
the root hairs, it passes into them until it is as plentiful within
as without. Also substances may diffuse out of the root hairs
when more concentrated within than without. Although the
movement of the salts may be influenced in rate by the move-
ment of the water, experiments show that the amount of min-
eral salts which enter the plant is quite independent of the
amount of water absorbed. However, in being alive, the cell
membrane presents some features not found in connection with
dead membranes. One peculiar feature is that it can alter
its permeability from time to time, and another is that, al-
though it allows many substances to pass in, it allows very
few to pass out. In being permeable to some substances and
not to others, roots are thereby able to exercise selective ab-
sorption, which in general favors the entrance of the more useful
substances, although roots by no means keep out all harmful
substances.

From the epidermal cells, the water and mineral salts pass
through the cortex to the xylem vessels through which they reach
the shoot. (Fig. 141.)

Factors that Hinder Absorption. — The forces concerned in
capillarity and surface films increase as the water of the soil de-

160

ROOTS

creases. They retard absorption and may become so great as to
actually prevent it. The wilting of plants when the soil becomes
dry is not due to the fact that there is no water in the soil, but to
the lact that the roots can not pull the water, known as the un-
available water, away from the soil particles. It has been found

FIG. 141. — Lengthwise section through a root, showing the way the water
and mineral substances of the soil reach the vascular bundles, e, epidermis
with root hairs; c, cortex; a, endodermis; p, pericycle; x, xylem of vascular
bundle. The arrows indicate the way the water and dissolved substances
pass to the vascular bundles. After MacDougal.

that most plants wilt when the soil moisture is reduced to 4.6 per
cent in medium fertile garden soil, 7.8 per cent in sandy loam, and
49.7 per cent in peaty soil. From these figures it is seen that the
amount of unavailable water depends much upon the kind of soil.
As shown in the table below, it also depends much upon the kind
of plant, for plants differ widely in their ability to pull water
away from the soil particles.

UNAVAILABLE WATER FOR DIFFERENT PLANTS IN LOAM SOIL

Plant.

Cabbage

Corn

Oats

Asparagus

Lettuce

Cucumber

Pondweed (a water plant)

Unavailable water.

Per cent.

5.8

5.9

6.2

7.0

8.5

10.8

24.8

SUBSTANCES GIVEN OFF BY ROOTS 161

On the other hand, when the soil water is so plentiful that air is
excluded from the soil, root growth is retarded and absorption
thereby hindered. For this reason wet lands have to be drained.

In retarding growth as well as slowing down osmosis, a low
temperature of the soil retards absorption. This fact is related
to winter killing, which is sometimes due to the fact that the roots
can not absorb water from the cold or frozen soil about them as
rapidly as it is lost from the shoot.

There are other factors, such as the presence of alkalies and
certain acids in the soil which hinder absorption by their injurious
effects on roots.

Root Pressure. — The absorptive action of roots sometimes
manifests itself in forcing water through the stem, acting much
like the pump which forces the water through the city's water
mains. This pressure exerted by roots is known as root or sap
pressure, and is one cause of the so-called bleeding of plants when
they are injured. In most plants of the temperate regions, root
pressure is only evident in the spring when the plants are not
losing much water by evaporation and are gorged with sap.
When Grapes are pruned in the spring, they usually bleed pro-
fusely. A vigorous European Grape will sometimes bleed a liter
per day. A Maple tree may exude from 5-8 liters in a day.

Measurements show that sap pressure is often several pounds
to the square inch. In the following table the pressure is recorded
in millimeters of mercury (760 millimeters of mercury = 1 at-
mosphere or about 15 Ibs. of pressure to the square inch).

Red Currant 358

Sugar Maple 1033

Black Birch 2040

European Grape 860

Substances Given off by Roots. — Roots give off carbon diox-
ide. The milky appearance of lime water in which roots are
grown is evidence of this fact (page 97). In the soil the carbon
dioxide unites with the water, forming carbonic acid, which has
an important dissolving action upon the soil minerals. This fact
is demonstrated by the etching effect roots have when grown in
contact with the surface of polished marble.

There is much evidence that roots also give off oxidizing en-
zymes whereby the poisonous substances of the soil are oxidized
to harmless forms.

162 ROOTS

It has been demonstrated in case of some plants that roots ex-
crete poisonous substances which tend to impede further root
activity. These deleterious substances, with those formed
through the decomposition of roots and other organic matter, may
be responsible for much of the soil sterility that is so commonly
attributed to the lack of the necessary mineral salts. In fact,
some think that the value of fertilizers depends mainly upon their
neutralizing effect of these deleterious substances. The improve-
ment of the soil, when fields are allowed to lie fallow, is, at least,
partly due to the disappearance of these deleterious substances
through oxidation. It seems that in many cases the deleterious
substances are more poisonous to the roots of plants of the same
kind, and this may help explain the value of crop rotation.

Water, Air, and Parasitic Roots

Water Roots. — When branches of some herbaceous plants are
cut off and set in water, roots develop from the submerged por-
tion. Branches of the Geranium and
Wandering Jew root readily in water and
will grow for a long time in ordinary
river or well water. The twigs of Willows

FlG 142 Lemna a w^ develop water roots when set in

floating water plant, which water. Willows, growing on the edge of
has only water roots, ponds and streams, develop roots which
Slightly magnified. After penetrate the soil and also roots which
Stevens. dangle in the water. There are a number

of small Seed Plants, like the Duckweeds, which float on the
surface of the water and have no roots other than water roots.

(Fig. 142.}

Air Roots. — Some plants depending upon soil roots also de-
velop air roots. The brace roots of Corn are at first air roots and
later enter the soil. Some climbing plants, like the Poison Ivy,
develop air roots which attach the plant to the support. Many
Orchids and some plants of the Pineapple family grow supported
on other plants and have only air roots. The Tillandsia, called
Spanish Moss, although not a Moss at all, is very common in
southern regions, growing on the branches of trees with its roots
dangling in the air.

Air roots differ in structure from soil roots. Air roots, unless

PARASITIC ROOTS 163

they are growing in wet shady places, are not in a good position
for absorption. Air roots of climbers, as in the Poison Ivy, do no
absorbing, and serve only to attach the plant to the support.
Those air roots that absorb usually have no root hairs, and the
absorbing is done by a sponge-like mantle of cells, called velamen,
covering the root. In some cases, as in many tropical climbers,
the air roots reach to the ground or to cup-shaped leaves where
water is obtained. The air roots of the Orchids which live on
damp tree trunks or rocks of tropical countries take up moisture
when there is rain or dew. Such plants, called epiphytes, nourish
without the assistance of soil roots.

Parasitic Roots. — There are a large number of plants, called
parasites, that depend upon other plants for food. The Dodder is
dependent upon other plants
for its food and obtains it
by sending roots into the
plant upon which it is grow-
ing. Dodder has no food-
making pigment and the
young seedling soon perishes
unless it can obtain food
from some other plant. The FlG m _ A) Dodder (Cuscuia Euro_
thread-like seedlings are sen- pcea^ nving on a Hop Vine; B, diagram-
sitive to touch and coil about matic drawing of a cross section of the
weeds, Clover, Alfalfa, or Hop Vine showing the roots of the Dodder
other plants which they may having penetrated the tissues of the Hop
chance to hit in their growth. Vine' After Kerner-
If the plant has suitable food, then the Dodder grows roots into
its tissues and absorbs food from it. Clover, Flax, and Alfalfa
are attacked in this way and much injured by Dodder. Dodder is
considered a destructive weed, and seed containing only a little
Dodder seed is undesirable for seeding. (Fig. 143.)

The Mistletoe lives upon trees, the roots penetrating the
branches and withdrawing the necessary foods. Many plants,
such as the Beech Drop, Broom Rape, etc., live on the roots of
other plants.

Propagation by Roots

The production of new plants from seeds, stems, leaves, or roots
is called plant propagation. Since roots readily produce adven-
titious buds which can develop into new plants, they are much

164 ROOTS

used in the propagation of some kinds of plants. For example,
Sweet Potatoes, which rarely produce seed, are propagated by
means of the shoots which develop from the fleshy roots. The roots
are planted early in the spring in specially prepared beds, usually
hot beds, where they develop buds which grow into stems bear-
ing leaves and roots, as shown in Figure 144- These young plants
(slips) are broken loose from the potato and planted in the field
after all danger of frost is passed. The abundance of stored food
enables each potato to produce many slips.

FIG. 144. — Sprouting of the Sweet Potato. A, potato with sprouts in
different stages of development. B, sprout, or slip, broken loose from the
potato and ready to be set out.

From the roots of the Red Raspberry l and some Blackberries,
new stems called suckers grow up. These suckers with a small
portion of the parent root are used in starting new plantations.
The larger roots of the Raspberry and Blackberry are often dug
up in the fall, cut into pieces, and stored until spring when they
are planted in the field. From these root segments new plants
are produced. Roses are often propagated by root cuttings.
When plants can be propagated either by root cuttings or by
seed, it is generally better to use cuttings, because plants obtained
from cuttings usually grow faster and are more likely to be like the
parent plant than they are when grown from seed.

1 Raspberries. Farmers' Bulletin 213, U. S. Dept. Agr. Culture of Small
Fruits. Bulletin 105, Oregon Agr. Exp. Sta. Dewberry growing. Bulletin
136, Colorado Agr. Exp. Sta.

PROPAGATION BY ROOTS 165

Some trees, like the Black Locust and Silver-leaved Poplar,
are troublesome because of the readiness with which sprouts
spring up from the roots. Some of the weeds, such as Canada
Thistle and Field Sorrel, spread by developing new plants from
buds on their spreading roots. When the Dandelion is cut off
several inches under the ground, it often grows up again from
the portion of the root left in the ground.

CHAPTER IX

STEMS
Characteristic Features and Types of Stems

The stem, usually consisting of trunk and branches, is the fun-
damental part of the shoot. Upon the stem the other structures
of the shoot, such as leaves, flowers, and fruit, depend for their
support in the air and sunlight — the position most favorable for
leaf-activity, pollination, and scattering of seed and fruit.

Roots, stems, and leaves are intimately related in their activi-
ties, and the efficiency of one affects the efficiency of the others.
The productivity of most of the cultivated plants depends not
only upon a good root system, but also upon a good stem system.
In some plants, such as Beets, Turnips, Radishes, Lettuce, and a
few others which have very short stems during much of their life,
not so much importance is attached to the stem, but even these
plants, in order to complete their life cycle, must eventually
develop stems upon which to bear flowers and seeds. Among
such plants as the trees and grains, the stem is very important.
The value of a tree for shade, lumber, or fruit depends largely
upon the character of the stem. Likewise, a Corn or Wheat plant
with a well developed stem is able to produce larger ears or a
better head than a plant with a stem poorly developed.

In comparing stems with roots the following things may be
stated. First, stems bear leaves and flowers, while roots do not.
Second, stems are divided into nodes and internodes but roots are
not. Third, stems branch at the nodes, while in roots branches
arise anywhere. Fourth, in stems pith is nearly always present,
while in roots it is usually absent.

The nodes are the narrow zones, often more or less swollen, at
which the leaves and buds as well as the branches arise. The
internodes are the zones between the nodes. The division of the
stem into nodes and internodes is quite noticeable in the stems of
Corn and other Grasses, where the nodes divide the stem into
distinct segments. By the elongation of the internodes, the

166

BRANCHING OF STEMS

167

leaves are separated and better exposed to the light. If the in-
ternodes are short, as in the stem of the Dandelion, the leaves are
much crowded. Also in such plants as Beets, Radishes, Turnips,
and Lettuce the stem at first has short internodes and the leaves
are much crowded.

On the ends of branches as well as in the axils of leaves, occur
the buds which have much to do with the growth of stems. The
stem elongates by the development , ^

of new nodes and internodes from
the terminal buds, while branches
develop from the buds occurring in
the axils of the leaves.

Branching of Stems. — Since
branches develop from the buds
located in the axils of the leaves,
the arrangement of branches tends
to follow the leaf arrangement.
Plants having two leaves at a node
and on opposite sides of the stem,
as in the Maple, tend to have
branches with the opposite arrange-
ment. Likewise, plants with leaves
occurring one at a node and on
alternate sides of the stem tend to
have the alternate arrangement of
branches, as Elms illustrate.

The amount of branching varies
much among plants. Among herba-
ceous plants the stems of many of
the Grasses branch very little and
are called simple stems, while in
some plants, as Clover and Alfalfa
illustrate, there is very much branching. Branching reaches its
maximum among the trees, where often there is branching and
rebranching until the youngest branches are so numerous and
small, as in the Elms and Birches, that the tree may be some-
what brush-like in appearance.

Branching is directly related to leaf display, for it not only
enables the plant to bear more leaves, but makes a better exposure
to sunlight possible. Branching is also related to flower and fruit

FIG. 145. — Pines, showing
the excurrent type of stem.
After Fink.

168

STEMS

production, for a well branched tree can produce more flowers and
fruit than one that is less branched, provided the food supply is
sufficient. In plants used for forage, such as Clover and Alfalfa,
the amount of hay produced by a plant depends largely upon the
extent of branching.

In some plants, as in the Pine shown in Figure 145, the stem
system consists of a main axis and many lateral branches, forming

what is known as the excur-
rent type of stem. In others,
as in the Elm shown in
Figure 146, the main stem is
divided into two or more
branches, which are soon lost
in numerous branches, form-
ing the deliquescent type of
stem. Among fruit trees and
forest trees, there is so much
difference in habits of branch-
ing that many kinds of trees
can be identified by their
branching habit.

Work Done by Stems. —
There are four important
functions of stems. They
support the aerial structures,
conduct materials, make
food, and serve as regions of
storage.

The s upp or ting function

FIG. 146. — Elm tree, showing deli-
quescent type of stem.

consists in carrying the
weight of the leaves, flowers,
and fruit, and in elevating
them to a position most favorable for performing their func-
tions. There is strong competition among plants for light, and
it is through the elongation of the stem that plants lift their
leaves higher in the air and often escape the shade of neigh-
boring plants. Some plants, such as the Grape, Poison Ivy,
Morning Glory, Beans, and Peas, which have weak stems, secure
better light by climbing a support, such as a wall, fence, or the
stems of other plants.

WORK DONE BY STEMS 169

As a Conductive structure the stem occupies an important posi-
tion, for through it the leaves and roots exchange materials.
Consequently, the vascular bundles, forming a continuous con-
ductive system from roots to leaves, are prominent structures in
stems. Through the conductive system the leaves receive water
and mineral salts from the soil and the roots receive the food made
in the leaves. For this reason any injury to the stem, such as
girdling, which severs the conductive system is likely to seriously
injure the plant. In fact, girdling is a common method employed
in killing trees.

In the manufacture of plant foods stems may assume consider-
able importance, although seldom so much as leaves, which have
food-making as their primary function. Being well exposed to
light and well provided with chlorophyll, leaves are especially
adapted to carry on photosynthesis — the process by which food is
manufactured. However, any portion of a plant containing chlo-
rophyll to which sunlight and air are accessible can make food,
and the stems of practically all plants that make their own food
have some portions that are green and, therefore, able in some de-
gree to carry on photosynthesis. For example, the young twigs
of trees are almost as green as the leaves and no doubt make con-
siderable food. As the twigs grow older, the green layer is cov-
ered by bark, which excludes the light that is necessary for
photosynthesis. In the Box-elder, Sassafras, and some other
trees, not only the young twigs but portions of the older branches
are green, and probably able to make food. In most of our
short-lived plants, such as Corn, Sorghum, Kafir Corn, Tomatoes,
Melons, Clover, Alfalfa, Beans, etc., the entire stem is green and
able to carry on photosynthesis. In some plants, such as the
Cacti, which have no leaves, all of the food must be made by the
stem. In the garden Asparagus the leaves are scale-like and food
is made chiefly by the stem and its many, small, lateral branches.
Some plants which have scale-like leaves, have green lateral
branches so expanded as to resemble leaves, as the Smilax
(Myrsiphyllum) , common in greenhouses, illustrates. Such
branches are called Cladophylls. (Fig. llfl^)

As to the storage function of stems, there is much difference
among plants, but in nearly all stems there is some accumulation
of substances, such as water, sugars, and starch. During the wet
season the stems of some Cacti take up large amounts of water,

170

STEMS

which supply the plant during the dry season. In the stems of
Sorghum and Sugar Cane, so much sugar is accumulated and re-
tained that these plants are grown for the sugar which they afford.
In the stems of trees- much food is stored in the form of starch,

and when transferred to grow-
ing regions during early spring,
it is changed to sugar, in which
form it occurs in solution in
the sap of the tree. The so-
called maple sap obtained from
the Sugar Maple is a good illus-
tration of sap which contains
much stored food in the form
of sugar. In early spring be-
fore the leaves appear, the trees
are so gorged with sap that it
can be drawn off by boring into
the wood and inserting spiles.
This sugar comes from reserve
food accumulated when the
leaves are active, and serves as
FIG. 147. — A branch of Myrsiphyl- a supply for the growth of new
lum, showing the cladophylls (a), and f ^ h beginning of the

the scale-like leaves (6).

growing season.

Some stems, notably those of the Irish Potato, contain large
amounts of starch on account of which they are valuable for food.
Another tuber-like stem similar to that of the Irish Potato is pro-
duced by the Jerusalem Artichoke — a plant of the Sunflower
type and often grown on account of the food value of its under-
ground tubers.

Many of the early spring plants, such as Spring Beauty, Dutch-
man's Breeches, Wind Flower, some Violets, and many other
plants having a supply of food at hand can spring up quickly,
flower, and accumulate another supply of food before the sunlight
is excluded by the forest foliage. Such plants, being seen only
in April or early May, have what is called the vernal habit, i.e ,
they live their life cycle in the spring of the year. The food
reserve of stems has much to do with the vernal habit.

Classes of Stems. — There are many ways in which stems may
be classified. Stems are classified as monocotyledonous or dicoty-

CLASSES OF STEMS 171

ledonous, according to whether or not they belong to Monocoty-
ledons or Dicotyledons. However, it is more in structure than
in external characters that these two types of stems present
important differences.

As to hardness stems are often classified as either herbaceous or
woody. Stems that are typically herbaceous, like those of Clover,
Alfalfa, Tomatoes, and others which develop very little woody
tissue, are soft and short-lived, usually living but one year. It is
among trees, where the amount of woody tissue reaches its maxi-
mum, that the best examples of woody stems occur. However,
between herbaceous and woody stems there is no distinct line of
division, for most herbaceous stems are woody in their older
regions and all woody stems are herbaceous in their younger
regions. The terms, herbaceous and woody, refer, therefore, to
the amount of woody tissue present, and not to the presence or
absence of it.

As to length of life stems may be classified into annuals, bien-
nials, and perennials. Annual stems live but one growing season.
The stems of most herbaceous plants are annuals, dying down to
the ground either before or after frost comes, as in case of most
vegetables, weeds, and Grasses. But annual stems and annual
plants must not be confused, for many plants, like Alfalfa, Quack
Grass, and Canada Thistle, which live many years, thus being
perennial in habit, have annual stems which grow up in the spring
and die down in the fall. When the plant is annual, roots, stem,
and all other parts die at the end of the growing season, and
the plant must be started anew from seed.

In plants, such as Turnips, Carrots, and Beets, which require
two years to complete their life cycle, and are, therefore, known
as biennials, the stem remains short during the first growing season,
forming a mere crown at the top of the root. During the second
growing season, stems develop which bear flowers and seeds,
and then the entire plant dies. In some biennials, as Cabbage
and Rape illustrate, the stem is prominent during the first season,
although it elongates much more during the second season in
preparation for bearing flowers and seeds, as shown in Figure 148.
In Red Clover, Sweet Clover, and many weeds with the biennial
habit, the portion of the stem known as the crown is biennial,
while the branches arising from the crown are annuals.

Perennial stems, so described because they live year after

172

STEMS

year, are typical of shrubs and trees, although they occur among
herbaceous plants, notably in the Ferns, Sedges, and Grasses
where the underground stem, which is well protected by a
covering of earth, is able to persist for many years.

As to position stems are clas-
sified into aerial, submerged,
and underground. Submerged
stems are of least importance,
being characteristic of plants
which grow in lakes or slug-
gish streams, where the plant
is often supported by the
buoyant power of the water
rather than by its stem sys-
tem. Aerial stems are of
most importance to us, al-
though there are some valu-
able underground stems.

FIG. 148. — Two stages in the development of a Cabbage plant. A,
plant at the beginning of the second season's growth with flowering stem
pushing out of the head. B, Cabbage plant in flower near the end of the
second growing season, a, scars left by the falling of the leaves of the head.

Aerial Stems. — Most of our cultivated plants as well as most
weeds have aerial stems. Since aerial stems keep above ground,
they are best adapted to expose leaves to the air and sunlight.
Aerial stems may be erect, prostrate, or climbing.

AERIAL STEMS 173

The erect stem is the common type of aerial stem, and is best
adapted for leaf display. Having the erect position, it can
branch and display leaves on all sides, and by elongation can lift
its leaves above the shade of other plants.

Erect stems show a striking contrast to primary roots in the
way they respond to gravity and light. While the main axis of
primary roots is positively geotropic and negatively heliotropic,
the trunk of erect stems behaves in the opposite way, thus grow-
ing away from the center of the earth and toward the light. But,
like lateral roots, the branches of the shoot tend to take a hori-
zontal or plagiotropic position, in which they appear indifferent to
both light and gravity. However, this indifference to gravity
and light on the part of the branches of the shoot seems to depend
upon influences exerted by other parts of the stem; for, if the
upper part of the shoot is removed, then the horizontal branches
remaining show a strong tendency to become erect.

Erect stems, being wholly self-supporting and much exposed
to winds, surpass other stems in amount of strengthening tissue
developed. From this type of stems where woody tissue reaches
its maximum development, as in trees, we obtain timber. How-
ever, erect stems are not always sufficiently strong to endure the
strains to which they are exposed, as is well known in case of
grains where the so-called " lodging " often occurs.

In size erect stems surpass all others. The most remarkable
erect stems are those of the giant Sequoias, one of which is
shown in Figure 149. These giant trees, which are natives of
the western mountains, may attain a height of 400 feet, a cir-
cumference of more than 100 feet, and live to be more than 4000
years old.

The prostrate stems of Pumpkins, Melons, Squashes, Cucumbers,
Strawberries, and Sweet Potatoes are well known to the student.
They are not strong enough to maintain an erect position, and
lie stretched upon the ground. Prostrate stems are common
among such weeds as the Five-fingers and Spurges. Some plants,
as Crab Grass and Buffalo Grass illustrate, have both erect and
prostrate stems. In this case the erect stems arise as branches
from the prostrate ones.

The prostrate position is not a good one for leaf display; for
leaves can be displayed only on the upper side and not all around
as in erect stems. Prostrate stems are also not able to escape

174

STEMS

the shade of surrounding plants and, therefore, thrive best where
erect plants are few, as on sandy beaches or river banks.

However, prostrate stems have some distinct advantages over
erect ones. Since they do not have to support much weight, they

FIG. 149. — One of the giant trees of the West, called
"Grizzly Giant." From Forestry Bulletin.

can grow very long and slender without developing much strength-
ening tissue. Furthermore, they are not so much exposed to loss
of water by evaporation as erect stems are. The prostrate posi-
tion is also more favorable for vegetative propagation, for as the

CLIMBING STEMS

175

stems grow over the ground, the nodes may produce roots from
their lower and stems from their upper surface, and thus new
plants are started which become independent by the death of
the parent stem. This method of propagation is common in the
Strawberry where the prostrate stems, known as runners, pro-
duce roots at their tips and start new plants which soon become
independent by the death of the runner. (Fig. 150.) Some of
the Grasses and weeds are able to spread very rapidly by this
method of propagation.

This disposition of nodes to grow roots and start new plants is
an important feature in the propagation of plants. Not only the

FIG. 150. — Prostrate stem (runner) of the Strawberry producing
new plants at the nodes.

nodes of prostrate stems will .do this, but the nodes of most stems
are able to produce roots as well as branches and leaves, if placed
in proper conditions. Much use is made of this feature in prop-
agating many of our useful plants as we shall see later.

Climbing Stems. — Some familiar examples of climbing stems
are those of the Pea, Grape, Hop, Woodbine, Poison Ivy, and
Morning Glory. Climbing stems, like prostrate stems, grow long
and slender, and are not strong enough to support themselves in
an erect position. They raise themselves into the light by climb-
ing a support, such as a fence, wall, or some erect plant. Some
kinds of Beans having climbing stems are often planted with the
Corn, so that they may have the Corn stems for support, or when
planted alone, each plant is provided with a stake for a support.
Sweet .Peas, Hops, and most Grapes are other familiar plants re-
quiring supports. The Woodbine and some wild Grapes are
quite notable climbers, often climbing to the tops of tall trees.

176

STEMS

(Fig. 151.) Many of the most notable climbers are in the trop-
ical regions.

Climbing stems have no more space for the display of leaves
than prostrate stems have, because one-half of the space for leaf
display is cut off by the support; but the climbing position is
much better than the prostrate position for escaping the shade
-:-.-;:.. • - * ....„,.-_. °f other plants.

One interesting feature
of climbing plants is their
different ways of climb-
ing a support. The Bean,
Morning Glory, and Hop
climb by twining around
the support. They are
called twiners. These
plants can not climb a
wall, for they must have
a support which they can
wrap about. (Fig. 152.)
The Sweet Pea and Grape
Vine illustrate climbing
by means of tendrils
which hook about the sup-
port. Tendrils are usually
modified leaves or stems,
although sometimes of
doubtful origin. (Fig.
153.) In some tendril
climbers, as in the Japan
Ivy, the tendrils have
swollen ends which flatten
against a wall or other
supports, where they se-
crete a mucilaginous substance by which they are able to hold
on tenaciously. In case of the English Ivy, the plant is held
to the wall by roots which are as efficient as tendrils. The
Virginia Creeper climbs by means of both roots and tendrils. In
being able to climb vertical walls of stone or brick, the Ivies
are well adapted for wall vines for which they are much used.
(Fig. 154.)

FIG. 151. — A Grape Vine climbing over
a dead Elm tree.

UNDERGROUND STEMS

177

Underground Stems. — The Potato, Onion, and Artichoke are
familiar examples of underground stems. Many of the plants
grown in the greenhouse and
on the lawn for decoration,
such as the Lilies, Hyacinth,
Tulip, Crocus, etc., have un-
derground stems. This type
of stem is common among
plants with the vernal habit.
Many of our useful Grasses,
as Red Top, Kentucky and
Canada Blue Grass, Orchard
Grass, and others have peren-
nial subterranean stems from
which aerial stems are sent up
each year. Grasses of this
type live many years and are
the Grasses which produce
our permanent pastures.
Grasses of this type are also
chosen for lawns, because their
spreading underground stems
produce a compact sod and
send up a thick aerial growth.
Quack Grass, Johnson Grass,
some Morning Glories, Poi-
son Ivy, and many other
weeds have underground
stems, and it is due to this
feature that such weeds are
hard to eradicate. Cutting
off the aerial stems of these
weeds does not kill the plant;
for the underground portion
still lives and is able to send
up more aerial stems.

FIG. 152. — Morning Glory twining
around a Corn stalk.

Underground stems are least adapted for displaying leaves and
bearing flowers, and they must either produce leaves and flower
stalks long enough to reach above ground or grow branches which
become aerial stems upon which the leaves, flowers, and fruit are

178 STEMS

borne. In most cases aerial stems are produced, and the leaves
of the underground stem are mere scales.

Although the underground stems are the least adapted for leaf
display, they have some advantages that aerial stems do not have.

FIG. 153. Smilax climbing over bushes by means of tendrils.
After Kerner.

They are much less exposed to drying and freezing, and escape
being pastured off by stock. They are safe places for the storage
of food, and most underground stems do have much reserve food,
which is used in the growth of new aerial shoots at the opening of
each growing season. Herbaceous plants are able to persist for
many years, if they have an underground stem from which new
shoots may arise each year. In other words, an underground
stem is one of the features that makes it possible for herbaceous
plants to be perennials. The underground position is an advan-
tageous one for vegetative propagation, because not only are the
nodes favorably located for establishing roots, but the supply of
reserve food and protection from drying and freezing makes it
possible for even small segments of underground stems to live
and develop separate plants. When an underground stem like
that of Quack Grass is hoed to pieces, each segment, if it has a

UNDERGROUND STEMS

179

node, will develop a new plant. Weeds of this type are multi-
plied rather than destroyed by plowing and discing.

Underground stems may be much elongated or they may be
short and thick. In their subterranean habit, they resemble

roots, and one may easily
mistake some types of them
for roots, unless the stem
characters are well in mind.
However, the presence of
nodes, internodes, and
leaves, although the latter
are usually scale-like, serve
to identify the underground
structure bearing them as
a stem. For example, the
so-called eyes of the Irish
Potato are buds and are lo-
cated in the axils of small
scales which mark the nodes
of the tuber. (Fig. 155.)
On some the scale-like

FIG. 154. — A Woodbine (Ampelopsis
climbing a stone wall, a, tendrils.

FIG. 155. — Irish Potato.
e, eyes; s, scale leaves.

leaves are large and fleshy, while on others they are very incon-
spicuous. Underground stems differ so much that they have
been classified into rhizomes or rootstocks, tubers, bulbs, and corms.
Rhizomes are very much elongated underground stems. They
are so named because of their resemblance to roots (the word
rhizome meaning root-like). They are commonly called root-
stocks. The rhizome is one of the most common forms of under-

180

STEMS

ground stems, being the kind of underground stem most common
among Grasses and weeds. Many wild plants, such as the Ferns,
May Apple or Mandrake, Solomon's Seal, Iris, Water Lily, and
others, have rhizomes. (Fig. 156.)

FIG. 156. — Rhizome of the Mandrake (Podophyllum), showing aerial
shoot and two scars (a) left by previous shoots.

One striking feature is the difference in the way that rhizomes
and their aerial shoots respond to gravity and light. While their
aerial shoots grow toward the light and away from the earth, the

UNDERGROUND STEMS 181

rhizome elongates horizontally under the surface of the ground,
neither seeking the light nor growing away from the earth.

Rhizomes grow best at certain depths in the soil, and, if the
depth is changed by adding or removing soil from over them, they
will grow up or down until the required depth is reached. By a
covering of manure or straw, the rhizomes of Quack Grass and
some other weeds may be induced to grow to the surface or even
out of the ground. Such weeds are sometimes eradicated by
removing the covering and exposing the rhizomes to drying and
freezing after they have been induced to grow
to the surface.

Rhizomes elongate and push forward
through the soil by growth at one end. It
is near this growing end that the aerial por-
tions are produced from season to season.
As the rhizomes push forward, the older por-
tions behind die away, and if the rhizome is
branched, as many of them are, the branches
become separated and form independent
rhizomes. The creeping and branching habits
of rhizomes are important features for vege-
tative propagation. Rhizomes are able to

creep through a soil which is already well ,.IG' . ' ™ss

.,,'., , section of an Onion

occupied by other plants, and consequently, above and iengthwise

plants having rhizomes are able to spread section below, c, main
where there is no chance for seed to develop, bud; 6, small buds; s,

The tuber occurs among plants where cer- stem5 r> roots' /> fleshy
tain regions of the undergound stem or its scales* After Andrews-
branches become much enlarged in connection with food storage.
The most familiar tuber is the Irish Potato. The nodes are
marked by the scale-like leaves in the axils of which occur the
small buds or eyes. The presence of nodes identifies the Potato
tuber as a stem structure. It is the stem portion of tubers that
is prominent, the leaves and buds being small. Another tuber
with nodes more prominent than in the Irish Potato and also of
some value for food is the Jerusalem Artichoke.

In bulbs the leaves or leaf bases are more prominent than the
stem, which is short, erect, and enclosed by the leaf structures.
Most of the food is stored in the leaf structures rather than in the
stem. Some common bulbs are those of the Onion, Lily, Hya-

182

STEMS

cinth, and Tulip. The edible portion of the Onion bulb consists
mainly of the fleshy scale-like leaves, in which the food has been
stored. (Fig. 157.)

Not all bulbs, however, are produced underground, for small

Onion bulbs, called bulb-
lets, often replace flower
buds in the common
Onion. These small bulbs
are sometimes known as
"Onion sets." Some Lilies
also produce small bulbs
in the leaf axils. Such
bulbs, although they re-
semble underground bulbs
in structure, are not
formed in connection with
underground stems.

Corms are very short,
solid, vertical, under-
ground stems, usually
bearing roots on their
lower and leaves and buds
on their upper surface.
However, buds may arise
anywhere and roots are
sometimes present at the
upper end of the corm, as
in the Jack-in-the-pulpit.
Corms are usually marked
by scar-like rings left by
the decay of former leaves.
From the buds of the old
corm new corms develop. (Fig. 158.) The most familiar corms
are those of the Indian Turnip, Crocus, Timothy, Cyclamen, and
Gladiolus.

General Structure of Stems

Stems have a cylindrical shape, which is associated with the
circular arrangement of their strengthening tissue. By being
arranged in a circle and near the periphery of the stem, the

FIG. 158. — A corm of Gladiolus, showing
young corms developing at the base of the
old one.

GENERAL STRUCTURE OF STEMS

183

strengthening tissues assume a tube-like arrangement, which is well
known to engineers as the arrangement in which the most strength
with a given amount of material can be secured. The truth of
this principle is demonstrated in the construction of bicycle
frames, where much strength with little weight is secured by using
large tubes instead of solid rods. Again, having the cylindrical
form, stems can be equally resistant to strains from all directions.

Stems taper and also decrease in age from the base of the trunk
to the end of the twigs where
the stem tissues are in process
of formation from the apical
meristems. The apical meri-
stems are also known as pri-
mary meristems because they
form other meristems, notably
the cambiums. It is on the
new elongation at the tips of
the stem, that the leaves appear
anew each year. The nodes,
the regions where leaves and
buds occur, are separated by
the elongation of the inter-
nodes, and in this way the
leaves, which are younger and
more crowded the nearer the
tip, are separated and exposed
to the light. In most annual
stems the nodes are all formed
very early, and elongation
thereafter consists chiefly in
the lengthening of the internodes, which thereby separate the
leaves so that they can unfold and expand to their mature size.
Thus, as shown in Figure 159, the nodes and internodes of a Corn
stem are all present in a Corn seedling two or three weeks old.

In most herbaceous stems, where there is no need of corky bark
and almost the entire stem is leaf bearing, the stem is active
throughout in the manufacture of food. But in perennials, such as
shrubs and trees, photosynthesis is limited to the young branches
where the leaves are borne and the light is not excluded from the
green cortex of the branches by a corky covering. In passing

FIG. 159. — Lengthwise section
through the stem of a Corn seedling,
showing the apical meristems (m), the
nodes (n), and the short internodes (i).

184

STEMS

from the leaf bearing portion of a branch to the regions behind
where food manufacture is being abandoned, the following struc-
tural features are plainly seen.

First, there are the leaf scars, left where
the leaves fell away, and interesting because
of the way they are formed. (Fig. 160.) As
the time approaches for leaves to fall, a cork-
like layer, known as the absciss layer, forms
across the base of the leaf, severing the direct
connections of the leaf with the twig and re-
maining as a covering over the scar after the
leaf falls. The absciss layer closes the open-
ings which would otherwise be left by the
falling of the leaf, and thereby prevents the
entrance of destructive organisms into the
twig. It is in connection with leaves which
still remain on the tree after the absciss layer
is formed that the various autumn colors oc-
cur due to changes in the dying leaf tissues.

Second, there are the lens-shaped dots,
known as lenticels, which, although common
on the branches of all woody plants, are espe-
cially conspicuous on
the branches of the
Cherry and Birch.
(Fig. 161.) The for-
mation of lenticels accompanies the forma-
tion of bark. In the young twig, where
the protective covering is an epidermis,
air is supplied to the tissues beneath
through the slit-like openings of the sto-
mata; but, as the twig becomes older and
bark is formed, the stomata are replaced
by lenticels. Lenticels are stomata dis-
torted and transformed in structure by the
development of bark. Just beneath each
stoma, instead of cork, there is formed a
loose mass of cells, and this loose mass of
cells is pushed up into the opening of the stoma, as shown in
Figure 162, rupturing the stoma and surrounding cells and thus

FIG. 160. Twig of
the White Walnut,
showing leaf scars (a) .

FIG. 161. — Twig of Birch,
showing lenticels.

GENERAL STRUCTURE OF STEMS

185

forming a lenticel. Through this loose mass of cells air easily
circulates and reaches the tissues beneath. As the twig increases
in diameter and the bark is stretched, the lenticels are enlarged
and, when they remain visible on the older bark, form the char-
acteristic bands as on the older bark of Cherries and Birches.

ph. ."

FIG. 162. — Section through a lenticel of the Elder (Sambucus nigra). e,
epidermis; ph, cork cambium or phellogen; I, loosely joined or packing cells
of the lenticel; pi, cambium of the lenticel. Much magnified. After Stras-
burger.

Third, as the twig becomes older, the bark increases in thick-
ness, cutting off the light from the green tissue beneath, which,
consequently, loses its green color and no longer functions in the
manufacture of food.

On each leaf scar there are dots, which are the severed ends of
the vascular bundles, known as leaf traces, that branched off from
the vascular cylinder of the stem to enter the leaf, where by pro-
fusely branching they form the veins and numerous veinlets of
the leaf. In turning aside to enter the leaf, the leaf traces leave
a gap in the vascular cylinder of the stem, and around this gap the
vascular bundles of the bud in the axil of the leaf connect with
the vascular cylinder of the stem. (Fig. 163.) Thus through
the branching and rejoining of bundles at the nodes, a plant's
vascular system becomes quite complex, looking like Figure 164-

Stem tips are not covered by caps, as root tips are. The
actual tips of stems are the meristematic tissues. During the

186

STEMS

dormant period, primary meristems are usually protected by bud
scales, while, during their active period, they receive considerable
protection from the young leaves, which, although developing
laterally and behind the tips,
project forward and are usually
so crowded and folded together
that they hide the stem tips.

Behind the stem tips the cells
formed from the primary meri-
stems begin to elongate and
modify into tissues and con-
tinue to do so until transformed
into the mature tissues of the
older parts of the stem. Stem
tissues differ: (1) in some im-

FIG. 163. — Lengthwise section
through the apical region of a stem
with two leaf stalks and the buds in
their axils included, showing the con-
nections of the vascular bundles of
leaves and of axillary buds or branches
with the vascular cylinder of the stem.
The vascular cylinder is represented
by shaded strands on each side of the
pith, the light area in the center of
the stem. Redrawn from Sargent.

FIG. 164. — Diagram of the vascu-
lar cylinder of the young stem of
Clematis viticella, showing by means
of dark lines the branching of the
vascular bundles at the nodes to sup-
ply the leaves and branches with bun-
dles. Modified from Nageli.

portant ways according to whether the stem is monocotyledonous
or dicotyledonous; and (2) in minor ways according to whether
the stem is herbaceous or woody. Thus in trees the tissues of
the herbaceous tips differ some from those in the older regions
where corky bark and other woody features are developed.

STRUCTURE OF MONOCOTYLEDONOUS STEMS 187

Structure of Monocotyledonous Stems

The most useful of the Monocotyledons are the Grasses of
which the Bamboos are the largest representatives. The Lilies,
Asparagus, and Palms are some other Monocotyledons that are
familiar. Nearly all Monocotyledons are herbaceous, although
there are a few, notably the Palms and Bamboos, that are woody.

The characteristic arrangement of the tissues of monocotyle-
donous stems, as they appear to the naked eye, can be seen in the

FIG. 165. — Cross section of a Corn stem, a, rind; v, vascular
bundles; p, pith; e, epidermis.

cross section of a Corn stem, as shown in Figure 165. In this sec-
tion three features are prominent. First, there is the rind-like
portion, forming the outer region of the stem and affording pro-
tection and strength. The cells of this outer region contain
chlorophyll and also function to some extent like leaves in the
manufacture of food. Second, there is the pith, left white in our
drawing and filling the entire cavity within the rind. Third, there
are the vascular bundles (shown by dots) which, although scattered
throughout the pith, are more numerous near the rind, thus tend-
ing to form a hollow column, which, as previously pointed out, is
the best arrangement for strength. In monocotyledonous stems
the tissues are run together and consequently are not so grouped

188

STEMS

as to form distinct regions, such as pith, vascular cylinder, and
cortex, which are more or less distinct regions in Dicotyledons.

When cross sections of the Corn stem are studied with the
microscope, such anatomical features as shown in Figure 166
may be seen. The cells of the rind are rectangular in shape, con-
sist of a number of rows, and their walls are thickened and made
woody for strength. The woody feature of the rind is character-
istic of Grasses and Sedges, being much less prominent in other
monocotyledonous stems, as, for example, in Lilies and Aspara-
gus. The outer row of cells of
the rind constitute the epider-
mis, although in the Grasses the
epidermal cells differ very little
from other rind cells, except that
they have silica and cutin de-
posited in their outer walls.
The vascular bundles, contain-
ing numerous cells, show three
or four large openings which are
the large vessels of the xylem.
Besides the large size of the pith
cells as shown in the drawing,
other features not shown, such
as their storage function and
their being so loosely joined as
to form a spongy filling for the
stem, should be mentioned.
To study the complex structure of a vascular bundle, we must
turn to a more highly magnified cross section of the bundle as
shown in Figure 167. The vascular bundle consists of strength-
ening and conductive tissues, the latter of which is composed
of the xylem and phloem, — the chief structures of all vascular
bundles. In respect to the character of the vessels composing
them, xylem and phloem show much uniformity throughout Flow-
ering Plants.

In the xylem the conductive tissues consist mainly of large ves-
sels, known as spiral, annular, or pitted vessels according to the
character of the thickenings in their walls, as partly shown in Fig-
ure 168 and more fully shown in Figure 169. The woody thicken-
ings, which strengthen the cellulose walls of the vessels so that

FIG. 166. — A portion of a cross
section of a Corn stem much en-
larged, a, epidermis; 6, the band of
strengthening cells under the epider-
mis and often called cortex; v, vascu-
lar bundles; e, pith. After Stevens.

STRUCTURE OF MONOCOTYLEDONOUS STEMS

189

they do not collapse under the pressure of surrounding tissues,
may form rings as in annular vessels, spirals as in spiral vessels,
or be more generally distributed over the wall, leaving only
small unthickened areas which constitute the pits characteristic

FIG. 167. — Cross section of a vascular bundle of Corn highly magnified,
s, strengthening tissue; p, phloem consisting of sieve vessels (e) and companion
cells (c); x, xylem consisting of annular vessel (a), spiral vessel (h) and pitted
vessels (i); b, parenchyma cells.

of pitted vessels. The xylem vessels are free from protoplasm
and are composed of cells joined in series with end walls resorbed.
They are known as tracheae, and are quite tube-like in struc-
ture and function. Through them the water and mineral salts
from the roots are carried, some reaching the leaves and buds
while much leaks out through the cellulose portions of the walls
to supply the tissues of the stem. Around the vessels are the
thin-walled parenchyma cells which may function some in con-
duction.

In the phloem there are sieve vessels and companion cells. The
sieve vessels are composed of cells joined in series and so named
because of the perforated areas occurring in their end and side

190

STEMS

FIG. 168. — Lengthwise section through a vascular bundle of Corn, the
knife splitting the bundle as shown by the line (o) in Figure 167, thus missing
the pitted vessels, x, xylem showing spiral vessel (h) and annular vessels
(a) which have been so torn by the growth of the stem that only the rings
are left; p, phloem consisting of sieve vessels (e) and companion cells (c);
s, strengthening tissue. Highly magnified.

b c

FIG. 169. — Vascular elements common among Ferns and Seed Plants, a,
spiral vessel; b, annular vessel; c, pitted vessel; d, reticulated vessel; e, sca-
lariform vessel; /, elements of the phloem showing sieve vessel with sieve
plate (/O, and companion cell (c). Highly magnified, a and 6, after Bonnier
and Leclerc Du Sablon; c, after DeBary; d, modified from Barnes; e, modi-
fied from Cowles; and/, from Strasburger.

STRUCTURE OF MONOCOTYLEDONOUS STEMS 191

walls. The sieve vessels, assisted by the companion cells, which
are also thin-walled, elongated, living cells, conduct the foods
manufactured in the leaves, such as proteins and the carbohy-
drates of which sugar is the chief one. The strengthening cells,
which are more numerous at the outer margin of the xylem and
phloem, form a sheath around the vascular bundle. One peculiar
feature of the vascular bundles of Monocotyledons is that there
is no provision whereby the bundle can increase its tissues, and
for this reason it is known as a closed vascular bundle. In mono-
cotyledonous stems, where there is no special provision for growth

A B

FIG. 170. — Cross sections of a Barley stem. A, section across the en-
tire stem showing the hollow (h) and the outer region (o) in which the vascular
bundles occur. B, a section of the outer region much enlarged, r, rind com-
posed of strengthening cells; v, vascular bundles.

in diameter, growth is mainly in length, and often results in the
development of extremely slender trunks, like those of Palms and
Bamboos.

In many Grasses the stems are hollow throughout the inter-
nodes, as shown in Figure 170, in which case the vascular
bundles are limited to a zone just within the rind. In most
Monocotyledons not belonging to the Grass or Sedge family,
the outer region of the stem is less firm in texture and in a few
Monocotyledons, as in the Yuccas and Dragon Tree, some of
the cells in the outer region of the stem divide like a cambium,
adding cells which form new vascular bundles and other tissues.
In this way the Dragon Tree may continue to grow in diameter
for thousands of years and attain a diameter of many feet.

192

STEMS

Closed vascular bundles and their scattered arrangement are
the chief distinguishing features of the anatomy of monocotyle-
donous stems.

Structure of Herbaceous Dicotyledonous Stems

Herbaceous Dicotyledons constitute an important group, for
they include many forage plants, notably the Clovers and Alfalfa,
some important fiber plants as Flax and Hemp, most vegetables,
and many greenhouse plants. In the tropical countries there
are a few Gymnosperms that are herbaceous, but in general

features their anatomy is
quite similar to that of
herbaceous Dicotyledons.

All stems of the herba-
ceous dicotyledonous type,
whether they are stems
strictly herbaceous through-
out or only the young
branches of woody stems,
have pith, vascular cylinder,
and cortex which occupy well
separated regions when well
FIG. 171. — Diagram of a cross section developed. Cross sections
of a well developed herbaceous stem, show- appear to the naked eye
ing the epidermis (a); band of tissue (6) about as shown in Figure
composed of cortex and phloem; xylem 171 The epidermis, cortex
cylinder (c): and pith (d).

outer zone, while the xylem forms the woody cylinder just within
the soft zone, and encloses the pith, which occupies the center of
the stem. In order to trace the development and study the
anatomy of the different tissues, we must turn to highly mag-
nified sections as shown in Figure 172.

The Cortex, which is the larger part of the outer zone of tissues,
is covered by the epidermis, and includes the starch sheath as its
innermost layer. Just under the epidermis some of the cells of
the cortex are transformed into collenchyma cells, which are par-
ticularly abundant in the angles of the stem shown in the Figure
but more generally distributed around the stem in many other
plants. The collenchyma cells, often noticeable in sections on
account of their whitish glistening appearance, have much thick-

STRUCTURE OF HERBACEOUS DICOTYLEDONOUS STEMS 193

FIG. 172. — Diagrams of highly magnified sections of an Alfalfa stem.
A, both cross sectional and lengthwise views of the tissues near the tip of
the stem, a, epidermis; Z, collenchyma; e, chlorenchyma of the cortex; s,
starch sheath; i, pericycle; 6, bast fibers; t, conductive portion of the phloem
containing the sieve tubes and companion cells; c, cambium; x, xylem; and
p, pith; v, vascular bundle. B, section farther from the tip, showing cambium
ring and- the closing together of the bundles. Lettering as above.

194

STEMS

ened but elastic walls. Being formed early, they are of much
importance in affording strength to the young regions of the stem
where bast fibers and woody tissues are not yet well formed.
Most of the cortex is made up of thin-walled parenchyma cells,
known as chlorenchyma, since they contain chloroplasts and func-
tion like the cells of leaves in the manufacture of food, being sup-
plied with air through the stomata of the epidermis. The starch
sheath, comparable to the endodermis in roots, is not distinct
from the other cells of the cortex in most stems. Its function is

FIG. 173. — A portion of a cross section from near the base of an Alfalfa
stem, x, xylem, which has formed a compact cylinder; p, pith; c, cam-
bium; t, phloem; e, cortex; a, epidermis. Highly magnified.

in dispute. Some think that its function is to conduct carbohy-
drates, while others think that it is the tissue which perceives
geotropic stimuli, and is thus responsible for the direction that
stems take in response to gravity.

The vascular cylinder, consisting of vascular bundles so joined
as to form a compact cylinder in the older regions of the stem, as
shown in Figure 173, at first consists of separate vascular bundles
having a circular arrangement about the stem and widely sepa-
rated by bands of pith. At the outer border of each mass of
phloem are bast fibers, often called sckrenchyma fibers, — an im-

STRUCTURE OF HERBACEOUS DICOTYLEDONOUS STEMS 195

portant strengthening tissue common to all dicotyledonous stems.
Centerward and matching each mass of phloem, is a mass of xylem,
wedge-shaped in outline with point towards the center of the
stem. This opposite arrangement of phloem and xylem contrasts
with the arrangement in roots, where phloem and xylem alternate.
Between the phloem and xylem is the cambium, the meristematic |
tissue whereby the vascular tissues can be increased indefinitely.
Vascular bundles provided with cambium are called open bundles,
and are characteristic of Dicotyledons and Gymnosperms,
whether herbaceous or woody. During further development,
the cambiums of the different vascular bundles extend through

FIG. 174. — Lengthwise section through a vascular bundle of a herba-
ceous dicotyledonous stem, x, xylem showing pitted, annular, spiral and
scalariform vessels; p, phloem showing sieve vessels and companion cells;
c, cambium. Highly magnified. Modified from Hanson.

the intervening pith and connect to form the continuous cam-
bium ring. Then due to the activity of the cambium ring in the
formation of other vascular bundles between those first formed
and in the enlargement of all, the intervening pith, excepting
narrow strands of it called pith rays, is crowded out, and finally
a compact vascular cylinder as shown in Figure 178 is formed.
In many herbaceous Dicotyledons, such as the Giant Ragweed
and others that grow rapidly, the cambium is so active in adding
new xylem on its inner side and new phloem on its outer side
that both phloem and xylem constitute zones of considerable
thickness at the end of one summer's growth. The zone of
xylem is often so prominent that the basal portions of such stems
are considered woody.

196 STEMS

The vascular bundles of all Dicotyledons are very sim-
ilar to those of Monocotyledons in structure and function
of conductive vessels, but differ essentially in having cambium.
(Fig. 174-) The conductive tissue of the xylem consists
chiefly of annular, spiral, pitted, and scalariform vessels — the
latter being so named because the thickened areas, separated
by slit-like thin areas, are so arranged, one above another, as
to resemble the rounds of a ladder. As in Monocotyledons, the
xylem vessels, probably assisted by the neighboring parenchyma
cells, are the passage ways through which the water and
dissolved substances absorbed by the roots are distributed
throughout the shoot. In addition to sieve tubes and companion

FIG. 175. — Cross section of a Flax stem, a, epidermis; d, bast fibers;
c, cambium; p, phloem; x, xylem, h, pith. Enlarged.

cells, the phloem of Dicotyledons generally contains many thin-
walled parenchyma cells, which serve in conducting the carbohy-
drates and also as storage places for proteins. The sieve tubes
and companion cells conduct the proteins and a part of the carbo-
hydrates. The bast fibers, which commonly occur in connection
with the phloem of all Dicotyledons, are tough flexible strands
adapted to afford strength. In fiber plants, such as Flax and
Hemp, the bast fibers are well developed and their importance
in the manufacture of fabrics, as the manufacture of linen from
Flax, is well known. (Fig. 175.)

In contrast to the stems of Monocotyledons, the stems of Di-
cotyledons and Gymnosperms have as their distinctive features
the circular arrangement of vascular bundles and the presence

STRUCTURE OF WOODY STEMS

197

of cambium. The stems of Dicotyledons and Gymnosperms,
since they increase in diameter by the addition of new layers
of xylem or wood on the outside of that previously formed,
are called exogenous stems. The stems of Monocotyledons are
called endogenous — a term adopted when botanists had the erro-
neous notion that monocotyledonous stems grow by the addition
of new tissues on the inside of the older ones.

Structure of Woody Stems

Woody stems, characteristic of the shrubs and trees of
Dicotyledons and Gymnosperms, are fundamentally the same
in structure as herbaceous dicotyledonous stems, for the

FIG. 176. — Diagrammatic drawing of a cross section of a young Apple
twig, e, epidermis; d, cortex; 6, bast fibers; p, phloem; c, cambium; x,
xylem; a, pith.

circular arrangement of vascular bundles and presence of
cambium are likewise their distinctive structural features.
They, too, are exogenous. Their herbaceous tips, being similar
in structure to the herbaceous dicotyledonous stems just
described, need no special attention. (Fig. 176.) Aside from

198

STEMS

the lenticels already mentioned, the features most peculiar to
woody stems are the annual rings of the woody cylinder and the
corky bark which replaces the epidermis and some or all of the
cortex. Also the medullary rays are commonly better developed
in woody stems than in herbaceous stems. These features are
directly associated with the perennial habit and the capacity to
add new layers of xylem and phloem each year and thus increase

FIG. 177. — Cross section of an Oak branch from a region nine years old.
o, outer corky bark; i, inner bark; c, cambium; a, annual rings; m, medul-
lary rays; p, pith.

in diameter. In well developed woody stems, as shown in Figure
177, there are three regions, bark, woody cylinder, and pith, al-
though the latter is often so small in amount as to appear absent.
The bark, consisting of outer and inner bark, the latter of which
contains the active phloem, extends centerward to the cambium,
which, although distinctly separating the bark and wood, is so
inconspicuous, except under the microscope, that bark and wood
appear directly joined. The annual rings are the circles in the
wood, and the medullary rays show as radiating lines travers-

STRUCTURE OF WOODY STEMS

199

ing the bark and wood, reaching part way or all the way to the
pith.

The bark, characteristic of woody plants, is originated by the
cork cambium which forms as an inner layer of the epidermis
or in the cortex beneath. (Fig. 178.) As the branch increases
in diameter, the epidermis seldom grows in proportion, but usu-

FIG. 178. — Diagrammatic drawing -of a cross section of an apple twig
after completing two seasons growth, e, epidermis sloughing off; fc, cork;
h, cork cambium; i, inner cortex; n, the phloem formed the first season;
p, phloem formed the second season; c, cambium; x, xylem formed the
first season; y, xylem formed the second season.

ally dies and sloughs off, and its protective function is assumed by
the cork formed beneath and gradually thickened as the stem
grows older. In some cases the cork cambium produces cortex
cells on its inner side as well as cork on its outer side, in which
case the cortex is increased in thickness. Since cork is imper-
vious to water, the tissues on its outside, having their water
supply cut off, soon die and with the epidermis and cork form

200

STEMS

the dead outer bark. In a few trees like the Beech and Fir the
original cork cambium may renew its activity year after year, but
usually the cork cambium is replaced each year by a new one
formed just beneath. The inner bark consists of the inner cortex
and the elements of the phloem made up of sieve tubes, com-
panion cells, parenchyma cells, and bast fibers. After years of
growth the outer layers of phloem die and thus on trunks of trees
of much age, the inner living bark contains only the inner layers

FIG. 179. — Cross section through the stem of a Black Oak, showing
heartwood and sapwood. From Pinchot, U. S. Dept. of Agr.

of phloem, the older layers of phloem having become a part of the
outer bark. Due to the addition of cork and the increase of the
phloem and woody cylinder in thickness, the bark, which is un-
able to increase in circumference except in a few cases, as in
Beeches, is usually broken and slowly exfoliated. It is usually
broken into furrows, which are thought to serve the same purpose
as lenticels in letting air into the stem tissues beneath.

The woody cylinder, consisting of the xylems of numerous vas-
cular bundles closely joined, functions chiefly in the conduction

STRUCTURE OF WOODY STEMS

201

of the water and mineral salts supplied by the roots. However,
much stored food in trees is transferred to the growing regions
through the xylem. This is especially true in the early spring as
well known in the case of Maples where the xylem carries a large
amount of sugar in the spring sap. In Gymnosperms the xylem
consists chiefly of tracheids through which the water ascends by
passing from one cell to another through the thin places of the
bordered pits which are provided in their cell walls. In woody

FIG. 180. — Piece of a stem of Scotch Fir four years old, showing the
medullary rays as they appear in cross, radial, longitudinal, and tangential
longitudinal sections of the stem. Enlarged six times. After Strasburger.

Dicotyledons the xylem consists of tracheae of the annular,
spiral, pitted, and scalariform type, among which are inter-
mingled wood fibers, wood parenchyma, and some tracheids.

The xylem tissues formed in the spring, when there is need for
rapid transportation of water and dissolved substances to the ex-
panding tips, consist of large cells with large cavities and thus
give to the spring wood that open, porous character which con-
trasts so much with the compact character of the fall wood that
the annual rings result. Annual rings, as their name indicates,
are formed usually only one each year and consequently their
number indicates quite well the age of the tree. Since the cam-

202

STEMS

bium adds new phloem on its outside at the same time that it adds
new xylem within, annual rings occur in the bark as well as in
the wood; but in the bark where the tissues are soft and there-
fore crushed, the annual rings are either indistinct or obliterated.
. In some woody stems having many annual rings, only the outer
annual rings which constitute the sap wood, recognizable by its

FIG. 181. — A piece of an oak board, e, end of the board showing medul-
lary rays (m) and annual rings (r) ; a, edge of the board showing the medul-
lary rays (m) ; 6, broad surface of the board showing the annual rings con-
sisting of light and dark bands.

light color, are active in conducting. (Fig. 179.) Sap wood is
often called the living wood because, although much of it is dead,
the cells of the medullary rays and wood parenchyma are alive,
while the heart wood is practically all dead. Heart wood is usu-
ally recognized by its dark color due to deposits of various sub-
stances, principally in the cell walls.

The medullary rays are also formed by the cambium and are
of two kinds: (1) those extending from pith into bark and known

STRUCTURE OF WOODY STEMS 203

as primary rays; and (2) those reaching only part way through
the wood and known as secondary rays. (Fig. 180.) The medul-
lary rays are composed of thin-walled living cells, which function
in food storage and in the transportation of materials laterally
through wood and bark. They are narrow plates of cells, one or
only a few cells in thickness, and extend up and down through
the stem only very short distances as may be ascertained in
Figure 180.

Both annual rings and medullary rays have an economic im-
portance in connection with lumber, where they form the beauti-
ful figures on the surface of cabinet woods. When lumber is
quarter sawed, that is, sawed so that the broad surface of the board
is parallel with the medullary rays, then its beauty is due to the
medullary rays which form the smooth-looking blotches as shown
on the edge of the board in Figure 181. When plain sawed, that
is, sawed at right angles to the medullary rays, the beauty of the
board is due to the figures formed by the annual rings as shown
on the broad surface of the board in Figure 181 .

In summarizing, corky bark, annual rings, and prominent
medullary rays may be stated as the distinguishing features of
woody stems. Like herbaceous dicotyledonous stems, they are
characterized by the circular arrangement of vascular bundles
and presence of cambium — features which distinguish them
from monocotyledonous stems where the vascular bundles have
the scattered arrangement and cambium is absent.

CHAPTER X

...a

..b

BUDS; GROWTH OF STEMS; PRUNING; PROPA-
GATION BY STEMS

Buds

Nature of Buds. — Buds contain a partially developed portion
of a stem with leaves and also flowers, when present, in an em-
bryonic state. A close study of buds, like those of fruit trees,
shows that the stem portion contained is
very short and that the leaves and flowers,
although they may be seen with a micro-
scope of low power or often with the naked
eye, are very rudimentary. Buds are often
defined as undeveloped shoots. The most
important thing about a bud is that it con-
tains the meristematic tissues upon which
growth in length (primary growth) and the
formation of new leaves and flowers depend.
For this reason, when the bud on the end of
a branch is removed, the branch can grow no
more in length at that point. (Fig. 182.}

Buds are common to all plants, but they

wise section through a are most noticeable in perennials, such as
Hickory bud. a, furry trees which have dormant periods occurring
inner scales; 6, outer during the winter season in temperate re-
scales; I, folded leaf; m, gions or during dry seasons in warm coun-
apical meristem; r. re- , • r^-i i i f , u T ,

gion to which the sies tneS" .The buds °f theSC PkntS are knOWn
are attached. Modi- *as res^n9 buds and are usually covered with
fied from Andrews. scales which protect the inner portions from
drying and other destructive agencies. The
scales overlap, forming a covering of more than one layer, and
are often made more protective by becoming hairy or waxy. Bud
scales are closely related to leaves and, in most cases, are simply
modified leaves. Sometimes, however, they are modified stipules
which are leaf appendages.

204

FIG. 182. — Length-

OPENING OF BUDS

205

In plants, like annuals and those that live in the tropics, the
buds usually have no protective scales and are called naked buds.
Scaly . buds are characteristic of plants which
must pass through seasons that are unfavor-
able for growth, and may be considered a
device for maintaining partially developed
stem portions in a protected state, and in
readiness to assume rapid growth at the
opening of the growing season.

Opening of Buds. —
The bud scales are forced
open by the growth of
the young shoot within.
The resumption of
growth by the parts en-
closed is first shown by FlG ig3. — Flower
the swelling of the bud. bud of the Pear, in
When the young shoot which the flowers are
resumes growth at the pushing the scales
beginning of the grow-
ing season, it grows with
remarkable rapidity and
in a few days pushes out of its scaly cover-
ing. (Fig. 183.) After the shoot has es-
caped, the scales usually fall off, leaving a
scar about the branch at their place of at-
tachment. The bud has now disappeared
and in its place there is a new growth bear-
ing leaves or flowers, or sometimes both.

The scars left by the scales remain until
the bark is sufficiently developed to obscure
them, and serve to indicate the age of the
different regions of a branch. In Figure

of different ages as in- 1$4, the portion beyond the scar (a) is the
dicated by the scars re- last season's growth. The portion between
suiting from the falling (a) ancj (&) is two years old, and the por-
tion between (&) and (c) is three years of
age. Thus the age of a given region of
a branch is indicated by the scars on the branch as well as by
the annual rings in its woody cylinder.

--— C

FIG. 184. — Plum
branch showing regions

away of the bud scales.
Described in text.

206

BUDS

FIG. 185.—
Branch of the

Position of Buds. — Buds are either terminal, located at the
tip of the stem : or lateral, occupying positions on the side of the
stem. (Fig. 185.) The plumule is the first ter-
minal bud of the seedling. The terminal bud is
usually larger and stronger than the lateral ones,
and its shoot usually makes more growth than the
shoots of lateral buds.

Lateral buds usually occur in the
leaf axils and when so located are
called axillary buds. In many plants
extra buds called accessory buds oc-
cur, which may stand just above the
axillary bud, as in the Butternut, or
on either side of it, as in the Box-elder.
(Fig. 186.)

Buds, called adventitious buds, often
spring from stems, from roots, or

even from leaves with- ilckory,"show-
out any definite order, ing large ter-
In propagation by cut- minal bud (0
tings or layers, adven- and smaller
titious buds often have lateralbuds«>-
an important part in the formation
of roots, and sometimes in the for-
mation of stems. Thus in the propa-
gation of Sweet Potatoes, adventitious
buds are depended upon to develop
the new plants. In Figure 187 is
shown the sprouts springing from the

adventitious buds on the stump of
*•«• 186.- Accessory buds ^ ^^ wmQW jn ^ cage ^
of the But ternut and Box-elder .

A, twig of Butternut; (, ter- sProuts are harvested after they be-
minal bud; a, accessory buds; come large enough to be woven into
x, axillary bud; I, leaf scar, baskets, and a new lot of sprouts is

B, accessory buds (a) and axil- then produced from other adventitious
lary bud (*) of the Box-elder.

After Bergen.

many crops of stems from one stump.
On the other hand, adventitious buds are often a source of
trouble, as in the clearing of ground where the sprouts develop-
ing from the adventitious buds on the stumps and roots tend to

WHAT BUDS CONTAIN

207

reoccupy the ground from which the trees and shrubs have been
removed. However, in case of some valuable trees like the
Chestnut, the sprouting habit is utilized in the production of
a new crop of trees. (Fig. 188.} In some forage plants, as
Alfalfa illustrates, a number of
crops of hay can be obtained
each year because of the contin-
uous development of adventi-
tious buds on the crown or basal
portion of the stem. (Fig. 189.)

What Buds Contain. — Some
buds contain only flowers, some
only leaves, while some contain
both flowers and leaves. Buds
are called flower buds, leaf buds,
or mixed buds according to what
they contain. In such fruit trees
as the Apricot and Peach, the
buds contain only flowers or
only leaves, while in the Apple
and Pear the buds contain both
flowers and leaves, or leaves
only. (Figs. 190 and 191.)

Flower buds, or fruit buds as
they are often called, are usually
broader and more rounded than
leaf buds and can often be iden-
tified by their position on the
branch. For example, in the
Peach and often in the Apricot
the fruit buds are lateral buds
on the current season's growth,
while in the Apple and Pear they
are usually the terminal buds
of the stunted lateral branches

FIG. 187. — Basket Willow from
which many crops of branches are
obtained through the development
of adventitious buds.

called fruit spurs which are located on those portions of the
larger branches two or more years of age. In Cherries and Plums
the fruit buds occur in clusters on the sides of the spurs. In
grapes the flowers occur on the sides of the current spring shoots.
The shape and place of appearance of fruit buds varies much in

208

BUDS

the different classes of fruits, and it is important that one should
know their location and time of formation, for such information
is valuable in deciding how and when to cultivate and prune.

FIG. 188. — Chestnut sprouts growing from stumps.
After Gifford Pinchot.

Formation of Buds. — The buds of plants which have a rest
period are formed during one season, lie dormant during the rest
period, and open at the beginning of the next growing season.
Thus the buds of our fruit trees, which produced flowers and
leafy shoots this year, were formed last year. As the new shoots
develop each year, new buds are formed in the axils of the leaves
and at the apex of branches, and in these buds are the flowers
and leaves which appear the following year.

FORMATION OF BUDS

209

A study 1 of the development of the buds of our fruit trees has
shown that the parts of a bud are formed during the summer and
fall and are often so well developed before frost comes that the
flowers and leaves may be identified if sections of the bu"ds are

FIG. 189. — Alfalfa plant, showing development of branches on the crown.
h, a main branch of the crown; s, stumps of branches which have been mowed
off; n, new branches.

studied with the microscope. Thus the character or content of
the buds of our fruit trees is determined several months before the
buds open. The appearance of a heavy bloom in the orchard
means that the conditions prevailing during the previous summer
and fall favored the formation of flower buds. It is common
observation that fruit trees bloom more profusely some seasons
than others. Evidently there are certain conditions which favor
the formation of flower buds and by controlling these conditions
one can control to a certain extent the fruitfulness of a tree.

1 Fruit-bud Formation and Development.
Virginia Agr. Exp. Sta., 1909-1910.

Annual Report, pp. 159-205,

210

BUDS

The formation of flower buds is known to be closely related to
the food supply.1 Flower buds are formed in greatest abundance
when there is more reserve food than is needed for growth. When
a plarit is growing rapidly and using all the food as fast as the

FIG. 190. — Fruit buds of the
Apricot, in which case a fruit bud
contains a single flower and no
leaves. After Bailey.

leaves make it, few flower
buds are formed. Further-
more, if a tree has exhausted
its food supply in producing
a heavy crop of fruit, not
many flower buds are
formed, and as a result the
tree will bear very little fruit
the following year. Any con-
dition that leads to an ac-
cumulation of reserve food,
such as checking growth by
the removal of terminal buds
or by cutting down the water
supply from the roots, favors
the formation of flower buds.

FIG. 191. — Twig of the Crab Apple
at time of blooming. The terminal shoot
(a) has developed from a leaf bud, no
flowers being produced, while the lateral
shoots (6) have come from mixed buds,
both leaves and flowers having been
produced.

1 Studies in Fruit Bud Formation. Technical Bulletin 9, New Hampshire
College Agr. Exp. Sta., 1915.

Some Effects of Pruning, Root Pruning, Ringing and Stripping on the For-
mation of Fruit Buds on Dwarf Apple Trees. Technical Bulletin 5, Virginia
Agr. Exp. Sta., 1915.

ACTIVE AND DORMANT BUDS

211

Occasionally ringing is employed to induce the formation of fruit
buds, in which case a narrow ring of bark is removed from the
trunk or branches in order to sever the phloem, and thus, by
cutting off the escape of the foods to the roots, bring about their
accumulation in the branches. Favorable
conditions for food formation in the leaves,
such as light and free circulation of air
and the addition of soil fertilizers, also
have an effect upon the formation of
fruit buds.

Active and Dormant Buds. — Many
more buds are produced than can develop
into branches, for, if all buds were to de-
velop, branches would be so numerous
and crowded that none of them could do
well. The food supply and proper light
relations permit the expansion of only a
few buds. Consequently, many buds lie
dormant one or more seasons or through-
out the life of the plant. Usually the
more terminally located a bud is, the more
likely it is to be active. Thus the ter-
minal buds of the main branches are less
likely to be dormant than the terminal
buds of the branches less prominent, and
of the lateral buds often a large per cent "^Zr^ ~f5=""=
remain dormant. An examination of the

branches of most trees shows many leaf ^FlG' 19'2-~Sweet

.,, , ,11 i-i , iM • Cherry, a type of tree in

scars with dormant buds which most likely which terminal growth is

will remain dormant and finally become prominent, resulting in the
obscured by the thickening of the bark, development of a central
just as many others have. shaft called a leader.

The dormancy of buds seems to be due After Ll H' Bailey'
to checks imposed upon them from without and not to condi-
tions within the bud, for most dormant buds can be induced
to become active by the removal of the active buds. Thus
when the terminal buds of branches are removed, some of the
dormant lateral buds become active. Use is made of this prin-
ciple in inducing shade trees and fruit trees to acquire certain
desirable shapes.

212

BUDS

In most cases the terminal bud of the main branch is largest
and its shoot makes the most growth during a growing season,
sometimes producing a growth of several feet in a season, while
growth from buds not so terminally located is usually much

The shape of a tree depends much upon the relative develop-
ment of main and lateral branches. When terminal growth is
very strong, lateral growth is weak and the tree develops a cen-

FIG. 193. — Sour Cherry, a tree which has strong lateral growth and
consequently no leaders. After L. H. Bailey.

tral stem, called leader, with lateral branches more or less sup-
pressed. This kind of growth is common among Poplars, Pines,
and even some fruit trees have it, as the Sweet Cherry in Figure
192 illustrates. Trees with this habit of growth tend to grow tall
and slender. To induce such trees to grow low and bushy the
terminal buds must be removed, so that lateral branches will de-
velop. When terminal growth is weak, lateral growth is stronger,
and the tree is commonly much branched and leaders are absent,
as the Sour Cherry in Figure 193 illustrates. This habit of
growth is characteristic of Maples and many other trees. There

REGIONS OF GROWTH 213

are, however, some plants, like the Lilac shown in Figure 194,
in which the terminal bud is replaced by two lateral ones,
but such is not the rule among plants.

Growth of Stems

Phases of Growth. — In growth three
things occur: (1) the addition of new
cells by the meristematic tissues; (2) the
elongation of cells; and (3) their modifi-
cation into tissues. The first phase must
precede the other two, but elongation
and modification accompany each other,
for cells begin to modify into tissues be-
fore completing their elongation. In
short-lived plants, such as annuals, the
first phase is most prominent in the seed-
ling stage, during which most of the cells
upon which growth in length depends are
formed from the apical meristems. In
Corn most of the cells are formed during
the first three or four weeks of growth.
During the remainder of the growth pe-
riod the cells elongate and modify into
the tissues of the mature stem. In per- FIG. 194. — Branch of
ennials the three phases are repeated tne Lilac, showing the ter-
each year as is well illustrated by the minal buds replaced by two

, , . ,. , -r> , . lateral ones,

yearly growth of trees. But even in

trees most of the cells which have to do with the growth in
length are formed in the buds during the previous year; and to
their remarkably rapid elongation is due the conspicuous phase
of spring growth in which the shoot elongates and leaves and
flowers expand into almost full size in a few days.

Regions of Growth. — The principal regions of growth are at
the apices of stems, where growth in length occurs by the addi-
tion of new nodes and internodes, and at the cambium layer,
where growth in diameter takes place. In such stems as those of
the Grasses, the basal portion of each internode functions for some
time as a meristem and thereby aids in the growth in length of the
internode. It is due to this feature that Corn stems, before they

214

GROWTH OF STEMS

reach maturity, are easily broken off just above the node. Fur-
thermore, in having this meristematic
zone, stems of the Grasses, when blown
down, are able to become partially erect
by bending in the region of the node due
to a more rapid growth of this region on
its lower side.

Since the stem segments are added
in succession at the apex, a stem soon
comes to have segments in various stages
of development, for while those at the
apex are just beginning to elongate, those
at the stem's base may have completed
their elongation and formation of tissues.
This feature is illustrated in Figure 195,
although none of the segments are yet
mature.

Primary and Secondary Growth. — In
both stems and roots, apical growth,
since from it the tissues of the stem and
root first originate, is called primary
growth, while growth from the cambium
is known as secondary growth because it
is chiefly concerned with adding more
tissues of the same kind to those already
formed from the apical meristems. Tis-
sues are also called primary or secondary
according to whether they originated
from the primary meristems or from the
cambium.

Character and Rate of Growth in
Stems. — Since elongation or enlarge-
ment is the most conspicuous phase of
growth, it is employed in determining
the character and rate of growth. Al-
FIG. 195. — Lengthwise though the most conspicuous, neverthe-

section through the stem

of a Corn plant, the plant being about two feet high, I, leaves; t, tassel;

r, region of stem where internodes have not elongated, a, internodes which

have undergone the most elongation; 6, meristematic region at the base of

the internodes.

CHARACTER AND RATE OF GROWTH IN STEMS 215

less it is so slow, except in a few cases, that it is imperceptible
to the unaided eye; and, therefore, to directly observe it, the
growing organ must be watched under the microscope. How-
ever, in measuring growth in large organs, such as stems, leaves,
and roots, other methods that are more convenient are usually
employed. Thus by marking a stem into segments as shown

FIG. 196. — Stem of a seedling marked to show the regions of most elongation.
A, stem just after marking. B, stem after a few hours growth.

in Figure 196 and observing the spread of the marks apart,
one can easily determine what part of the stem is most active
in elongation. Special kinds of apparatus run by clockwork,
one of which is known as the auxograph (meaning "growth
writer") and another as auxanometer (meaning "growth meas-
urer"), have been so devised that the rate and fluctuations
in growth are recorded by a pea which indicates the character of
the growth throughout a considerable period by curvatures in the

216

GROWTH OF STEMS

line which it makes on the paper Carried around on a revolving
drum. (Fig. 197.) Such an apparatus has the advantage in that
one can see just how growth proceeds at any period during day or
night, if the apparatus is so manipulated that the hour at which
any part of the line is made can be determined. Measurements
by such an apparatus show that the rate of growth of an organ
is not uniform, but, beginning slowly, it gradually rises to a point
where growth is most rapid and then gradually falls away, finally

FIG. 197. — Auxanometer in operation. As the plant elongates, the small
pulley (w) revolves, revolving with it the large pulley (r) which magnifies
the motion and transmits it to the marker (z) that marks on the drum (t).
The drum is revolved by the apparatus (k) at its base and this apparatus
is connected with the clock (w). After Pfeffer.

ceasing as the organ approaches maturity. This mode of enlarg-
ing, which is commonly known as the grand period, is character-
istic not only of stems, but also of fruits, flowers, leaves, and roots.
The fundamental cause of the grand period in any organ is due to
the fact that cells themselves enlarge in this way. Unlike leaves,
flowers, and fruit where the expansion is quite even throughout,
stems expand by each internode going through its grand period
independently of the other internodes. Thus between the upper
internodes in which the grand period is just beginning and those

CHARACTER AND RATE OF GROWTH IN STEMS 217

toward the stem's base where the grand period is over, there are
internodes in various stages of the grand period. Due to the
overlapping of the grand periods of the different internodes, the
elongation of the stem as a whole is quite uniform. In roots
the grand period is passed through very quickly and is evident
only near the tip.

The rate of growth also depends much upon the kind of plant
and upon moisture, temperature, and light. While some plants,
like Corn and Giant Ragweeds, grow to a height of six feet or more
in three or four months, the seedlings of some Pines and Oaks
grow only a few inches during an entire season. Some vines like
the Hop plant may grow a stem more than twenty-five feet in
length in one growing season. Most weeds grow more rapidly
than cultivated plants and, if let alone, soon exceed them and cut
off the light. Measurements have shown that some kinds of
Beans and Peas can elongate about two inches and Wheat about
four inches in forty-eight hours. In perennials, such as trees,
growth is very rapid in the spring, after which it slows down dur-
ing the remainder of the season.

The moisture of the soil and air is an important factor in growth.
It is common knowledge that plants are checked in growth when
the ground becomes dry. The moisture of the air, although not
of use to the plant in the same way that the soil moisture is,
checks the evaporation from the plant and thereby influences
growth. When the atmosphere is full of water, as on "muggy"
days, there is not much evaporation and the cells easily retain the
high turgor pressure upon which rapid growth depends. It is
partly due to the greater humidity at night that many plants
grow faster then than in the day time. That the cells of plants
are often more turgid at night than in the day time is shown by the
fact that soft stems, like those of Corn and Sorghum, are more
flexible and not so easily broken oft7 in the latter part of the day as
they are at night or in the morning. For this reason, the after-
noon, when the cells are least turgid, is the best time to lay-by
Corn. The function of water in enabling cells to stretch is an
important one, for enlargement consists in stretching the proto-
plasm and cell walls without much increase at first in dry weight.
Thus the dry weight of an internode of a stem is about the same
at the end as at the beginning of the grand period, although the
size may increase many times. In fact seedlings, before they

218

GROWTH OF STEMS

become active in the manufacture of food, often have a dry
weight less than that of the seed.

Temperature is the most important factor in growth and, just
as in the germination of seeds, the minimum, maximum, and

FIG. 198. — Two Potato plants, one of which was grown in the dark and
the other in the light. A, plant grown in the dark. B, plant grown in the
light. After Pfeffer.

optimum temperature for growth vary with the kind of plant
as shown in the table on next page. For example, the optimum
temperature is between 90° and 95° for Corn, between 80° and
85° for Barley, and about 70° for White Mustard, one of the
weeds. Thus when the days and nights are so cool that such

CHARACTER AND RATE OF GROWTH IN STEMS 219

plants as Corn, Beans, and Pumpkins grow slowly, White Mus-
tard and other plants with a low optimum grow rapidly. In gen-
eral, arctic plants have a lower optimum than tropical plants, and
consequently plants transferred from one region to the other

FIG. 199. — Pines grown much crowded and consequently producing slender
trunks. From Bulletin 24, North Carolina Geological and Economic Survey.

usually do not thrive until they become acclimated, that is,
until the plant's protoplasm becomes adjusted to the tempera-
ture of the region.

GROWTH TEMPERATURES IN FAHRENHEIT

Plant.

Minimum.

Optimum.

Maximum.

Barley

Deg.
41

Deg.

83-84

Deg.

99-100

White Mustard

69-70

82-83

Scarlet Runner Bean

49-50

92-93

115-116

Corn

49-50

92-93

115-116

Pumpkin

56-57

92-93

115-116

220

GROWTH OF STEMS

Indirectly light is very essential for growth because of its im-
portance in the manufacture of plant foods. But directly light
has little effect, unless it is intense, and then it checks growth.
That most plants grow faster at night than in day time is well
known; and, although much of the increase in the rate of growth
at night is due to the greater humidity of the air, some is due to
the absence of the inhibitive effect that the sun's rays have on

FIG. 200. — Pines growing in the open where their trunks are short and
much branched. From Bulletin 24, North Carolina Geological and Economic
Survey.

growth. In Bacteria, where the protoplasm is not protected by
pigments, the sun's rays so inhibit growth that they have an im-
portant germicidal effect.

On the other hand, if plants do not have sufficient light, they
are affected in various ways. For example, when plants are grown
in the dark, as the Potatoes in Figure 198 illustrate, the stems are
excessively elongated, the leaves are abnormal, and the plant
lacks chlorophyll, on which account the plant is said to be etio-
lated. Even plants grown in the shade, having the light only par-
tially cut off, are usually taller and more slender than plants

PRUNING 221

grown in the light. Thus many forest trees which have short,
thick, and much branched trunks, when growing in pastures, grow
tall slender stems with branches only at their tops when grown
in forests where they are much shaded. It is for this reason that
most forest trees grow trunks more valuable for lumber when
grown in thick stands. (Figs. 199 and 200.) This principle is
observed in growing Sorghum and Corn chiefly for fodder, in which
case the plants are grown in thick stands, so that their stems will
be finer and, therefore, better for feed. Such a response to shade
is often an advantage to plants, for it is through the elongation
of their stems that plants compete for light by endeavoring to
raise their leaves above the shade of neighboring plants.

Also the development of stem tissues is more or less influenced
by light. Stems grown in diminished light do not have their
mechanical tissues so well developed. For example, when grain
plants receive insufficient light on account of being much crowded,
they have commonly weak stems and are likely to lodge. The
bast fibers of flax are finer when the plants are thick on the
ground, and when flax is grown for fibers it is commonly grown
in thick stands.

Pruning

Pruning consists in cutting away portions of the plant and is
done for reasons too numerous for more than a few to be men-
tioned here.

First, trees that tend to grow tall and slender may be induced
to acquire a low thick top by subjecting them to the process called
"heading-in," which consists in pruning the main branches so that
growth in height is checked and a good development of lateral
branches is induced. This method is often used in controlling
the shape of shade and fruit trees. It is by this means that hedges
are made to grow low and dense and thus capable of turning stock
when used for fences.

Second, often, as in case of fruit trees, pruning has for its pur-
pose the checking of growth which has been so thoroughly ex-
hausting the food supply as to result in a shortage of fruit buds.
In this case growth is checked by removing the terminal buds
from the leaders and the food supply thereby conserved.

Third, plants are sometimes pruned to delay maturity. For
example, in growing Sweet Peas the young pods are pinched off

222

PRUNING

so as to conserve the food material and thereby prolong the flower-
ing period of the plant. In contrast to this practice, often in
case of nursery trees, the leaves are stripped off so as to cut off
the food supply and thereby hasten maturity in order that the
trees may be in a better condition to stand the winter.

Fourth, fruit trees are often pruned to induce the development
of an open head so as to secure better lighting for the interior
branches. Such pruning is necessary in trees with heads so com-
pact that the interior branches are not able to function properly
in the manufacture of food or in bearing fruit because of the lack
of light.

Fifth, when fruit trees are set out, it is necessary to prune the
top to safeguard the trees against injuries from excessive evapo-

FIG. 201. — A, tree just received from nursery. B, same tree with top
and roots pruned in preparation for setting in the ground. From Alfred
Gaskill.

ration. Since the trees have their absorbing power much reduced
through the loss of many roots broken and cut away in trans-
planting, the development of a large leaf surface must be prevented
or the intake at the roots and outgo at the leaves will not be prop-
erly balanced. (Fig. 201.)

Sixth, the appearance of a tree as well as its protection against
further injury requires the removal of dead and diseased branches.
One can do much toward preventing some plant diseases, such
as Fire Blight and Black Knot, from spreading to healthy
trees by removing and burning the diseased branches of affected
trees.

PRUNING

223

Seventh, by a severe pruning of the top, trees which are beginning
to fail from general debility are often rejuvenated. This kind of
pruning, which is characterized as severe because so much of the
top is removed, is known as "pruning for wood." By the removal

FIG. 202. — An Apple tree which has been severely pruned, its main
branches having been cut back. After G. H. Powell.

of much of the top the balance between the top and roots
is upset, and as a result a much larger supply of water and
mineral salts is received by the remaining branches, which con-
sequently become invigorated and much more active in growth.
(Fig. 202.)

224

PRUNING

B C

FIG. 203. — Twigs pruned,

Wounds and their Healing. — The

removal of a branch exposes the stem
tissues, and makes an opening where
destructive organisms, which may
injure or even destroy the plant, can
enter. Unless wounds are quickly
healed over, the plant will suffer.

Smote tissues that are much spe-
cialized, such as wood and corky
bark, have lost their ability to grow,
the meristematic tissues or cambiums
showing the cuts at different dis- must be depended upon to heal the
tances from the bud. A, the cut i -re ,1 T, • <•

, , u j r> IT wound. If the conditions are favor-

is too far from the bud. B, the

cut is so near the bud that the able for growth, the cambiums and
bud is probably injured. C, the the cells newly formed from them
cut is at the proper distance from develop a mass of tissue known as
Whyarethecutsmade the callus, which spreads over the
wound and forms a cap-like covering.
The development of the callus depends very much upon the
nature of the wound and
where it is made. The cut
should be made with a sharp
tool, and so made that the
stem will not be split. When
a small branch is cut off, the
cut should be made just above
a bud, as shown in Figure 203,
so that the leaves developed
from this bud will supply food
for the formation of the callus.
If the wound is too far above
a bud, or if the cut is so close
that the bud is destroyed,
then there will be a dead
stump which will not heal.
Side branches should be FIG. 204. — An example of bad prun-
pruned close to the main mg, showing the dead stubs of branches
, , ^, , . which may lead to the destruction of

branch, so that the cambium the tree After Bailey

of the main branch can heal

the wound. In Figure 204 is shown an example of improper

CUTTINGS

225

FIG. 205. — An example of a wound
so made that a callus is closing over it.
After Bailey.

pruning, in which case there is a stump which will not heal and its
decay may result in the destruction of the tree. In Figure 205
is shown a cut made in the
proper way. In this case a MJr, Vw

callus is forming and enclos-
ing the wound.

The Propagation l of Plants
by Means of Stems

Some plants, of which the
Irish Potato is a familiar ex-
ample, are propagated almost
entirely by planting portions
of their stems, which are ca-
pable of developing roots and
shoots from their nodes.
(Fig. 206.) A notable exam-
ple in Southern countries is
the Sugar Cane, which is propagated by planting sections of
stalks from which new plants develop. In the propagation of
fruit trees, Grapes, Cranberries, Roses, Geraniums, Carnations,
and many other plants, stems are used, although not always in
the same way.

Propagation by stems is often preferable to propagation by
seeds, because by the former method the new plants are more
likely to be of the parent type. This fact is demonstrated in
propagating Apple trees, which seldom come true, from seeds, but
do when propagated by grafting. Another advantage of propa-
gation by stems is that new plants can be obtained in less time
than by seeds. By means of cuttings new Geraniums or Carna-
tions of considerable size are obtained in a few weeks. Propa-
gation by stems may be by cuttings, layering, grafting, or budding.

Cuttings. — In the study of prostrate and underground stems,
it was noted that nodes of stems can develop roots as well as
shoots. This makes it possible for a portion of a stem to become
an independent plant under proper conditions. Consequently,
many plants are reproduced by setting detached portions of their

1 The propagation of plants. Farmers' Bulletin 157, U. S. Dept. of Agri-
culture.

226 THE PROPAGATION OF PLANTS BY MEANS OF STEMS

stems in soil, sand, or water where they develop roots and become
as self-supporting as the parent plant. Such detached portions
are known as cuttings and consist of a small portion of a stem, as
Figure 207 illustrates, or only of a leaf, as in the propagation of

FIG. 207. — Geranium cut-
FIG. 206. — The Irish Potato, showing new ' ting, showing the roots devel-
plants developing from the eyes. oping at the cut end.

Begonias and a few other plants having fleshy leaves as shown in
Figure 208. Among cultivated herbaceous plants which are
propagated by cuttings, the Irish Potato, Geranium, Carnation,
and Coleus are familiar examples. In Southern countries the use
of cuttings is well illustrated in the propagation of Sugar Cane,
as shown in Figures 209 and 210. Other plants of the Grass fam-
ily, as Johnson Grass and Bermuda Grass, are sometimes propa-
gated by cutting the underground stems into short pieces, which
are used in setting fields to grass. Unintentionally, but often to

GRAFTING

227

his sorrow, the farmer helps bad weeds, such as Quack Grass and
Marsh Smartweed (Polygonum Muhlenbergii) , to spread by scat-
tering portions of their underground stems while putting in and
cultivating crops.

Cuttings, known as hard-wood cuttings, are commonly employed
in propagating such woody plants as the Grape, Currant, Goose-
berry, Willows, Poplars,
and many ornamental
shrubs. They may be
made in different ways as
shown in Figure 211, but
in each case they must
have at least one bud.

Layering. — A layer is
a branch which is put in
contact with the soil and
induced to develop roots
and branches while still
in contact with the parent
plant. After a layer has

developed roots and FIG. 208. — The Life Plant
branches, it is separated ^« developing young plants on the

margins of the leaves. About one-half natu-
from the parent and be- ral gize

comes an independent

plant. There are different methods of layering, but usually the
branches are bent to the ground and covered with dirt. In
layering Grapes, a vine is stretched along in a shallow trench
and buried throughout its entire length as shown in Figure 212.
Raspberries and many shrubs are propagated by layering.

Grafting. — Grafting is the common method used in propa-
gating fruit trees, and consists in so joining parts of different
plants that they unite their tissues and live together as one plant.
In grafting there are two members involved, the stock and don
or scion. The stock, which may be a root, stump, or almost the
entire shoot, is the member which remains in contact with the soil,
while the cion is the portion of a shoot, usually a twig or branch,
which is to be made to grow on the stock. Since only growing
tissues, such as the cambiums, are able to unite and heal wounds,
it is necessary in grafting to have the cambiums of the stock and
cion so adjusted that they can become grown together and thus

228 THE PROPAGATION OF PLANTS BY MEANS OF STEMS

form a perfect union. (Fig. 213.} However, only plants closely
related can be successfully grafted, for in the protoplasms of un-
related plants there are factors, probably differences in chemical
nature, which prevent the union of the cambiums.

FIG. 209. — Cuttings of Sugar Cane. A, cutting, showing two nodes and
a bud at each node. B, cutting, showing a new plant which has developed
from a bud at the node. Adapted from N. A. Cobb.

When grafting is successful, the cion becomes as closely related
to the activities of the stock as ordinary branches are. Through
the stock the cion receives water and mineral elements from the
soil, while the stock receives some of the foods made by the leaves
of the cion. However, with all of this close connection, the nature
of both stock and cion remains in most cases practically unchanged
and each, therefore, continues to produce fruit unchanged in type.
This feature is important for two reasons. First, it enables one
to combine the desirable features of two plants into one individual
where the desirable features, although remaining unchanged in
nature, may assist each other in functioning. Some fruit trees
bear delicious fruit, but on account of poor root systems or other

GRAFTING 229

causes they are not hardy. On the other hand, some trees are
hardy but produce poor fruits. Now by grafting cions from the
trees bearing delicious fruits on the hardy trees as stocks, one may
obtain individuals that are hardy and at the same time bear

FIG. 210. — Cuttings of Sugar Cane being properly placed in the trenches,
after which they are covered by dragging dirt into the trenches. After
N. A. Cobb.

delicious fruits. Second, it enables one to preserve bud sports,
which are individual branches that show qualities strikingly dif-
ferent from other branches of the same plant. Since bud sports
rarely take root from cuttings or come true from seed, grafting
is usually the only way of preserving them; and so important
are bud sports that most of the best varieties of such fruits as
Apples, Pears, and Oranges have originated as sports, which, after
being grafted on stocks, became trees which by further grafting
have been multiplied.

Often minor influences of the stock on the cion, such as dwarfing,
hastening the fruiting period, or altering the time of blossoming,
are desirable, and are obtained by grafting the cion on suitable
stocks. For example, Pears are dwarfed and fruit at an earlier
age when grafted on the Quince. Apples are influenced in the
same way when grafted on the so-called " Paradise " stock, a name

230 THE PROPAGATION OF PLANTS BY MEANS OF STEMS

given to certain surface-rooting dwarf varieties of Apples. By
grafting Pears on Pear stocks raised from seed or by grafting
Apples on stocks raised from the seed of the Crab Apple, larger
and longer-lived trees, which do not fruit so soon, are secured. It

is claimed that in some cases
the quality of the fruit is
changed, having more sugar
or more acid according to the
nature of the stock. One of
the most interesting and for a
long time a very puzzling re-
sult of grafting is the chimera,
which arises when a bud de-
velops from the wound callus
in such a way that the tissues
of both cion and stock grow
out together to form the
branch. The tissues of the
members may grow out side
by side, in which case each
member forms a side of the
branch, or the tissues of the
members may be so related
to each other that one mem-
ber forms the core and the
other the covering of the
branch. In either case both members may be represented in the
leaves, flowers, and fruit of the branch and be the cause of very
peculiar combinations of characters. For example, in Apples one
side of the fruit may be of one variety and the other side of an-
other variety. In grafting together Tomatoes and the Black
Nightshade, the latter of which has small black fruits, chimeras
in which one member formed the core and the other the covering
of the branch have been obtained. As a result very queer fruits
have been produced. Some resembled tomatoes in size but had
the black skin of the Nightshade berry, while others were similar
in size to the small berry of the Nightshade but had the yellow
or red skin of the Tomato. Also in the character of the leaves
and flowers, these chimeras presented queer combinations. By
a study of chimeras produced experimentally, as those of the

FIG. 211. — Hardwood cuttings,
a, simple cutting; 6, heal cutting;
c, mallet cutting; d, single-eye cutting.
After L. C. Corbett.

BUDDING 231

Tomato and Nightshade just described, an explanation has been
obtained for some so-called graft-hybrids, one of note being the
Cytisus Adami which was produced many years ago by grafting to-
gether two shrubs, one having purple and the other yellow flowers.

FIG. 212. — Layering of the grape vine. The vine has been bent to the
ground and covered, and from it roots and shoots are developing.

As a result of this graft and further grafting, shrubs having some
branches bearing purple flowers and others bearing yellow flowers
were obtained. Even a flower might be part purple and part yel-
low. For a long time some thought these strange plants were true
hybrids, but now we are quite sure that they are only chimeras.
Budding. — Budding is similar to grafting, the principal differ-
ence being in the character of the cion. In budding, instead of
twigs or branches, only a small strip of bark bearing a bud is used.
This strip of bark, which is cut so that it has cambium on its
inner face, is inserted into the young bark of the stock in such
a way that the cambiums can unite. A study of Figure 214 will
show how the bud is inserted. After a T-shaped cut is made in
the young bark of the stock, the bark on the edges of the cut is
lifted and the cion is slipped in, the lifted bark on each side
holding it in place. After the cion is in place, it is fastened more
firmly by wrapping strings around the stem just above and below
the inserted bud. Peaches are quite commonly propagated by
budding and sometimes Apples, Pears, and other fruit trees are
propagated in this way.

232 THE PROPAGATION OF PLANTS BY MEANS OF STEMS

FIG. 213. — Cleft Grafting. A, cion; B, cions inserted in cleft of stock;
C, the wound covered with wax. After G. C. Brackett.

FIG. 214. — Budding, a, opening of bark for insertion of bud; 6, removing
the bud; c, inserting the bud; d, bud inserted; e, bud properly wrapped.
After G. C. Brackett.

CHAPTER XI

LEAVES
Characteristic Feature of Leaves

Ordinary green leaves, known as foliage leaves, may be defined
as the food-making organs of the plant. The green cortex of
stems makes some food, but usually the greater part of it is made
in the leaves. Leaves are constructed especially for utilizing the
carbon dioxide of the air and the water brought up from the roots
in the manufacture of sugar. Although starch may sometimes
be the first food product formed by the leaves from carbon dioxide
and water, the evidence indicates that ordinarily the first product
is sugar from which starch is formed later. Sugar then may be
regarded as the fundamental plant food, since from it or from the
proteins of which it is the chief element, plants build by chemical
transformations all of their structures and organic materials of
whatever kind. Leaves transform sugar, when it is abundant,
into starch which serves as a storage form of sugar. Although
proteins can be made in any living part of the plant, it has been
demonstrated that leaves are very active in the formation of this
food. Although proteins may be regarded as a secondary food
since they depend upon sugar as their chief foundational ele-
ment, they are exceedingly important because from them the pro-
toplasm, the living substance of the cell, is formed. Proteins,
although of various kinds, are formed by combining chemically
the mineral elements of the soil, such as nitrogen, sulphur, and
phosphorus, with the elements of sugar. Since leaves manu-
facture sugar and are well supplied with the mineral elements,
they are well equipped for the manufacture of proteins.

The efficiency of green tissue in making sugar depends upon
exposure to light and air, and the foliage leaf may be considered
a device for securing good exposure of green tissue. The elevat-
ing of leaves into light and air by the stem, and their arrangement,
position, form, and structure are related to the problem of
securing suitable exposure, and thus to food manufacture.

233

234

LEAVES

The variations in form and structure of leaves is so great that
they are often used in classifying plants, and for this purpose
many technical terms have been devised to describe these varia-
tions. Since most of these variations concern only those who are
interested especially in the classification of plants, only the most
common ones will be considered in this presentation.

Primary and Secondary Leaves. — Leaves may be divided into
primary and secondary. The cotyledons are examples of primary
leaves. The cotyledons are parts of the
embryo and hence precede the stem in
development, while the leaves developing
later and called secondary leaves arise
from the stem. The secondary leaves are
usually numerous, while the primary leaves
are few in number. Primary leaves are
usually short lived and often fall away as
soon as their stored food is exhausted. Gen-
erally they disappear while the plant is still
quite small. Consequently the leaves of
plants that attract attention are the second-
ary ones, and when the term leaves is used,
secondary leaves are usually meant.

Development. — Leaves develop upon the
sides of the growing points of stems and
FIG. 215. — Leaf of first appear as mere swellings, the smallest
the Apple. 6, blade; swellings being near the apex. It follows
P, petiole; s, stipules; then that the oldest leaves are at the base
r> of the stem or twig. Thus in a Corn stalk,

for example, the leaves decrease in age from the lowest leaf on
the stalk to the highest. Swellings similar to those that become
leaves appear later just above the leaf swellings, and these
become the buds which appear in the axils of the developed
leaves. In woody plants which prepare for a rest period, the
leaves are partly developed during the previous season, and rest
in the bud in a miniature form until the following spring when
they burst from the bud scales and in a few days complete their
development.

Parts of a Leaf. — In a typical foliage leaf, such as that of the
Apple shown in Figure 215, there are three parts: the expanded
portion or blade; the leaf stalk, called petiole, which supports the

LEAF BLADE 235

blade and makes connection with the twig; and a pair of small
leaf -like appendages at the base of the petiole, known as stipules.
The portion of the leaf at the point of contact with the twig or
stem is called the leaf base. The leaf base is generally enlarged
so as to form a sort of cushion by which the leaf is attached to
the stem.

The leaves of most plants are not typical, but have one or
more parts lacking. The stipules are very frequently absent.
The leaves of the Thistle, Wild Let-
tuce, Mullein, and many other plants
have no petioles, the blade being
directly attached to the stem. Such
leaves are said to be sessile (mean-
ing sitting). (Fig. 216.) In Corn,
Wheat, Oats, and Grasses in general
the leaves have no petioles and the
leaf base is much expanded and
enwraps the stalk completely for a
considerable distance above the node.
A leaf base enwrapping or sheathing FlG- 216- ~ Sessile leaf of a
the stem as just described for the

Grass type of leaf is called a leaf sheath. At the juncture of
the blade with the sheath in the Grass type of leaf occurs an
outgrowth which fits closely to the stem and is known as the
ligule or rain guard. In the Corn and some other plants of
the Grass type small projections, known as auricles, occur at
the base of the blade. (Fig. 217.) Leaves designated as
perfoliate have their blades so joined around the stem that
the stem appears to pass through the leaf as shown in
Figure 218.

Leaf Blade. — In general, the leaf blade is expanded into a
broad thin structure; but all gradations exist between such forms
and those that are thick and fleshy or even cylindrical.

The border of the blade, called margin, may be smooth or
quite irregular, and the character of the leaf margin is one of the
features used in classifying plants. When the margin is smooth,
as that of the Corn leaf, it is said to be entire. Irregular margins
differ much in the form and depth of the indentations, as illus-
trated in Figure 219. The margin may be cut up by many small
notches, as the margin of the Apple leaf shown in Figure 215, or

236

LEAVES

FIG. 217. — A portion of a Corn plant showing two leaves, a, leaf blade;
s, leaf base called leaf sheath; w, auricles; I, ligule or rain guard.

FIG. 218. — Cup Plant (Silphium perfoliatum), a plant with perfoliate leaves.

EXPOSURE TO LIGHT 237

the notches may be very deep and divide the blade into lobes, as
the leaves of the Gooseberry, Cotton, Dandelion, some Oaks,
Maples, and many other plants illustrate. In some cases the
blade is so divided that it is made up of independent portions
united to a common stalk, each independent portion being called
a leaflet. Many familiar plants, such as Clover, Alfalfa, Vetches,

FIG. 219. — A, Margins of leaves, a, serrate; 6, dentate; c, crenate;
d, undulate; e, sinuate. B, lobed leaf of Grape. C, pinnately compound
leaf of Black Locust. A, after Gray.

the Walnut, Ash, Locust, and Sumach, have leaves divided into
leaflets. The number of leaflets into which the leaves of different
plants are divided varies widely. In the leaves of Clover and
Alfalfa three leaflets are common, while leaves of the Black
Walnut often have twenty or more leaflets. (Fig. 220.} Leaves
divided into leaflets are said to be compound, while those less
divided are called simple.

Leaflets resemble simple leaves and in case of some compound
leaves it is possible for one to mistake the axis to which the leaf-
lets are attached for a branch of the stem and the leaflets for
leaves. However, since buds occur only in the axils of leaves,
one can tell whether the leaf-like structure is a leaf or a leaflet
by the presence or absence of a bud in its axil.

Exposure to Light. — Unless the leaf is properly exposed to
light, it can not be an efficient food-maker. It is not always a
problem of securing enough light, but often one of escaping light
that is too intense; for too intense light often injures leaves and
consequently checks them in their work. The adjustment to

238

LEAVES

light, therefore, is a delicate one, and many leaves do not have
the proper amount of light.

The more or less horizontal position which the leaves of many
plants assume enables them to receive the direct and most in-
tense rays on their upper surface. Leaves in this position receive
more light rays than those having the oblique or vertical position.

FIG. 220. — Leaves divided into leaflets. A, leaf of Alfalfa with three
leaflets. B, Walnut leaf having many leaflets. I, leaflets; p, petiole; s,
stipules; 6, bud.

The separation of leaves through the elongation of the internodes
is another means of securing better exposure. For example, dur-
ing the early growth of the Corn plant, the leaves are closely
packed around the growing point of the stem and only the outer
ends of the blades are well exposed. But through the elongation
of the internodes, all of the leaves are finally separated, so that at
the time the tassel and ears appear all portions of the leaves
receive light.

The way leaves are arranged on the stem is a1 so an important
feature in securing proper exposure. There are three common
arrangements, alternate, opposite, and whorled. In the alternate
arrangement, there is but one leaf at a node and they appear to
alternate, first on one side of the stem, then on a different side.

EXPOSURE TO LIGHT

239

The leaves of Corn and other Grasses are good examples of the
alternate arrangement. In Corn, for example, the second leaf
appears at the next node above and on the opposite side of the
stem from the first leaf, and the third leaf appears at the third
node and almost directly over the first leaf. Usually on account
of a slight twisting of the stem, the leaf blades do not occur

FIG. 221. — Tobacco, a plant with the alternate arrangement of leaves.

After Hayes.

directly over each other, but extend in slightly different direc-
tions, so that the lower leaves are not directly in the shade of the
upper ones. In fruit trees and many other plants having the alter-
nate arrangement, the second leaf is not quite on the opposite
side from the first and neither is the third leaf usually over the
first. (Fig. 221 .) The leaves are so arranged that no large open
spaces appear in looking in from the end of the twig as shown in

240

LEAVES

FIG. 222. — End view of an Apple twig, showing the leaves alternately
arranged and so located in reference to each other that all receive light.

FIG. 223. — A Wild Sunflower
with opposite arrangement of
leaves. After Bailey.

FIG. 224.— Sweethearts (Galium
Aparine), one of the weeds having
leaves in whorls. After Beal.
Mich. Agr. Exp. Sta.

EXPOSURE TO LIGHT

241

Figure 222. Many trees as well as many herbaceous plants,
such as Cotton, Clover, Alfalfa, Tomatoes, Potatoes, Buckwheat,
and Flax, have the alternate arrangement of leaves. In the
opposite arrangement two leaves appear at each node on opposite
sides of the stem, and neighboring
pairs are set more or less at right
angles to each other, so that as one
looks down from above each pair
of leaves alternates in position with
the pair above and with the pair
below it as shown in Figure 223.
The opposite arrangement is also
common among both woody and
herbaceous plants. In the whorled

arrangement more than two leaves FIQ ^ _ Dandelion viewed
occur at a node, as illustrated m from above> The leaves form a
Figure 22 4 ^ In this arrangement rosette and the lower leaves are
the leaves are also so placed as to much lonser than the uPPer ones-
shade each other as little as possible. Aft(

In plants, like the Dandelion and Plantain, which have very short
stems bearing many leaves, the leaves form a mat, called a rosette,

on the surface of the
ground. It is readily seen
that leaves so closely
crowded as they are in the
rosette must shade each
other considerably, but
they have the advantage
of being exposed less than
those on elongated stems
to the loss of water by
transpiration. In the
rosette much shading is
eliminated by a difference
in length of petioles, for the outer and under leaves of the rosette
have longer petioles which push their blades beyond those of the
upper leaves, and in this way they escape the shade of the leaves
above. This feature is noticeable in the rosette of the Dandelion
shown in Figure 225. Another arrangement of leaves which is
favorable to light exposure is called a leaf mosaic, being so named

FIG. 226. — Nasturtiums showing mosaic
arrangement of leaves.

242 LEAVES

from the fact that the edges of the leaves, as viewed from above,
fit together like the little tiles of a real mosaic. The fitting
together in this way is the best arrangement for the individual
leaves in a large mass to receive light. (Fig. 226.} A general
mosaic arrangement of leaves may be observed in connection with
almost every broad leaved plant, but is most noticeable in the
Ivies where their mosaic of leaves often completely cover the
surface of a. wall. In case of stems exposed to direct light on
only one side, as the horizontal branches of trees, and stems
prostrate on the ground or in contact with a support, such as
Cucumbers, Melons, and climbing vines, the petioles of those

FIG. 227. — Maple twig, showing mosaic arrangement of leaves.

leaves on the under side of the stem usually curve so as to bring
the blades to the light. For example, in looking up into a tree
in full foliage, one will notice that the horizontal branches are
comparatively bare underneath, the leaf blades being displayed
on the upper side as a mosaic. (Fig. 227.)

When plants receive light from only one side, as plants grown
in a room near a window, the entire plant usually bends toward
the light, thus bringing the leaf blades into a better position for
exposure. (Fig. 228.)

General Structure of Leaves

Although diverse in form and arrangement, foliage leaves
show much uniformity in structure, being so constructed as to be
adapted to the function of food-making. In general, they have

EXPOSURE TO LIGHT

243

FIG. 228. — Geranium growing near a window, toward which it is bending
and thereby bringing the leaves in a better position in reference to light.

three kinds of tissues. First, there are the conductive tissues
which bring the water and mineral salts to the leaf and carry
away the manufactured foods. Second, there is the protective
tissue consisting of epidermis which protects the delicate tissues
within the leaf against drying, intense light, the entrance of
destructive organisms, and to some extent gives rigidity to the

244

LEAVES

leaf. In some cases there are special strengthening tissues
developed within the leaf, either in connection with the conduc-
tive tissues or separately. Third, most important of all is the
food-making tissue, known as the mesophyll, because it fills the
interior of the leaf. The green mesophyll is usually called chloren-
chyma because of its green color.

The Conductive Tissues. — The conductive tissues of leaves
consist of vascular tissues similar to those of the vascular bundles

FIG. 229. — A, leaf of Solomon's Seal, showing parallel veining; B, leaf of
Willow, showing net veining. After Ettinghausen.

of stems and roots. They constitute the veins. The veins are
simply branches of the vascular bundles of the leaf trace, and
the leaf trace is a branch of the vascular cylinder of the stem.
Thus through the direct connection of the vascular tissues of the
leaves with those of the stem, which in turn are in direct connection
with the vascular tissues of the roots, all parts of the plant are
brought into close communication for the exchange of materials.

EPIDERMIS 245

The veins run through the mesophyll of the leaf and form a
frame-work, which with its numerous fine branches, known as
veinlets, resembles a fine-meshed net when a leaf is held up to the
light. The finest veinlets can be seen only with the aid of the
microscope. It is by this profuse branching of the veins and
veinlets that all parts of the mesophyll are brought into direct
contact or close relation with the conductive tissues. Although
the larger veins are often thicker than the leaf and form prominent
ridges on its under side, they taper down to the veinlets which
are well buried within the mesophyll.

The character of the veining, known as venation, differs con-
siderably in different leaves and there are two types of venation
of some prominence. (Fig. 229.) One is the parallel-veined type,
in which there are a number of parallel principal veins with
obscure cross veins. This type is familiar in Corn leaves and
is characteristic of monocotyledonous plants in general. The
other is the net-veined type, in which there is one or only a few
principal veins and their branches so fork and join each other
that a quite noticeable network of veins and veinlets is formed
as Maple or Oak leaves will illustrate. This type is characteristic
of Dicotyledons. Many leaves have one large primary vein
called midrib. Some leaves have a number of primary veins,
which are then called nerves, and a leaf is described as three-
nerved, five-nerved, or whatever the number may be.

Epidermis. — The epidermis forms a continuous covering over
the leaf except where it is broken by the openings of the stomata.
The stomata, although microscopical in size, afford the openings
necessary for the exchange of gases between the interior of the
leaf and the outside air. The epidermis is usually one layer of
cells in thickness, but in some leaves, especially those of dry
regions, it is often thicker. Except in the cells of the stomata,
the epidermis usually contains no pigments, although it may
appear to have since the green color of the mesophyll beneath
readily shows through it. Sometimes the epidermis contains a
red pigment, called anthocyan, which causes a part or all of the
leaf to be red. Red pigment is often noticeable in the leaves of
Sorghum and is common in some greenhouse plants of which
the Wandering Jew is a familiar example. The epidermis when
smooth has the appearance of having been greased, due to the
deposits of cutin in its outer cell walls. Cutin usually forms a

246

LEAVES

thin film called cuticle on the outer surface of the epidermis.
Being a waxy substance and impervious to water, it makes the
epidermis more protective against the loss of water. Sometimes,
as in Cabbage, a waxy substance that can be easily rubbed off is
deposited on the outside of the epidermis. Frequently, as the
common Mullein and some Thistles illustrate, the epidermis
develops hairs, which are sometimes so long and dense as to give
the leaf a white woolly appearance. Some leaves, as those of the
Mints illustrate, have glands that secrete fluids to which the odor
of the plant is due. Some plants are cultivated on account of
the commercial value of their glandular secretions. In many
cases the epidermal secretions of leaves, if not unpleasant to
the sense of smell, are to the taste, and therefore may protect
plants against being eaten by stock. In fact all of the epidermal

modifications are sup-
posed to be related to the
protection of the plant
in one way or another.

Mesophyll. — The
mesophyll, as the term
suggests, occupies the
middle region of the leaf
and its distinctive fea-
ture is its green color
upon which the power
to manufacture food de-
pends. It is soft spongy

FIG. 230. -A much enlarged surface view tissue and is composed

of the lo^er epidermis of a Bean leaf, a, of a number of layers of

epidermal cells; s, stomata; g, guard cells; cells which surround the

t, slit-like opening between the guard cells smaller conductive

through which gases pass. tractg and fin the gpaces

between. It is so delicate in structure and so closely joined
to the epidermis that in most leaves it is difficult to remove the
epidermis without tearing away some of the mesophyll.

Cellular Structure of Leaves

To learn the finer structural features of leaves, a microscope
must be employed, so that the cells of the different leaf tissues
may be studied.

CELLULAR STRUCTURE OF LEAVES

247

A surface view of a small portion of epidermis stripped off and
highly magnified is shown in Figure 230. The epidermal cells in
this view are irregular in shape, but so closely fitted together
that no openings occur except through the stomata. A stoma
(singular of stomata) is a definite structure, consisting of two
curved cells, known as guard cells, which are so fitted together
as to enclose a slit-like opening. The guard cells are so named
because they regulate the size of the opening. Some plants,
such as the Grasses of which Corn is
a familiar example, have a peculiar
type of guard cells as Figure 231
shows. In this case the guard cells
are enlarged at the ends, and re-
semble dumbbells in shape. How-
ever, this difference in shape seems
to have nothing to do with their
behavior, for they open and close
their slit-like opening just as the
ordinary type of guard cells is able
to do.

By changes taking place within
the guard cells, the stomata are
opened and closed, but the causes
of such changes are not definitely

known. The guard cells have

IT , , j ji • -j FIG. 231. — A much enlarged

chloroplasts, and there is consider- gurface ^ of ^ epidermig of

able evidence that the chloroplasts Corn, showing one stoma. g,
have something to do with bringing guard cells; t, slit-like opening;
about these changes. Since chloro- e, epidermal cells. The chloro-
plasts make sugar and have the plasts are in the ends of the guard
power to transform sugar into starch

or starch into sugar, it is evident that they can alter the concentra-
tion of the sugar in the cell sap and in this way alter the turgor pres-
sure of the guard cells. For example, if the chloroplasts of the
guard cells manufacture much sugar which is allowed to concen-
trate in the cell sap, then by the principle of osmosis the guard
cells draw in water forcibly and develop a high internal pressure
which tends to expand them and alter their shape. On the other
hand, if the chloroplasts remove the sugar from the cell sap by
changing it into starch, which is insoluble, the result may be

248 LEAVES

that the guard cells then tend to shrink through lack or loss of
water, since their power to draw in and retain water decreases
with the loss of dissolved substances from their cell sap. Re-
gardless of what the chloroplasts have to do with it, it is obvious
that when the guard cells are swollen with water they bow out,
that is, curve away from each other and make the slit larger.
On the other hand, when the guard cells are shrunken through
the loss of water, they straighten and make the slit smaller.
Hence the stomata tend to open when the water supply is abun-
dant and close when water is scarce.

The importance to the plant of closing the stomata when water
is scarce is apparent, for much water can be lost through open
stomata. It would seem, therefore, that the guard cells regulate
the loss of water from the plant and this they do to some extent.
However, it has been found that stomata open in light and close
in dark, and this tendency of light to open, conflicts with the
tendency of water shortage to close them; for it is during bright
hot daytime when the light stimulus to open is probably strongest,
that there is the greatest shortage of water. That the guard
cells open and close just when they should in order to control
water loss is much doubted. The most important feature of
stomata is that they permit exchange of gases.

Leaves having the horizontal position have their stomata much
more abundant on the under surface; often they are not found at
all on the upper surface. On leaves that stand more or less erect,
as those of the Grass family and Carnations, the stomata are
about equally distributed on both sides, and on leaves which lie
on the surface of the water, like those of the Water Lily, they
occur only on the upper side. The location of the stomata on
the under surface of horizontal leaves is an advantage to the
plant, since here the stomata are less likely to become choked
with water during rains, and also less water is lost through them
by evaporation.

The number of stomata varies much with different plants, but
about sixty thousand to the square inch is a fair average. On
the leaves of some plants there may be as many as four hundred
thousand to the square inch. In the table on the next page are
given the number of stomata found on a square millimeter of leaf
surface of some common plants.

CELLULAR STRUCTURE OF LEAVES

249

NUMBER AND DISTRIBUTION OF STOMATA PER SQUARE
MILLIMETER OF LEAF SURFACE

Plant

Lower Surfaee

Upper Surface

Lilac (Syringa vulgaris).

330

Alfalfa (Medicago sativa) .

160

Bean (Phaseolus vulgaris).

281

Tomato (Lycopersicum esculentum)

130

Cherry

160

Pumpkin (Cucurbita pepo)

269

Oats (A vena sativa)

\ 27

j 48

Corn (Zea Mays)

) 23

f 158

I 25
I 94

I 68

\ 52

Although stomata are most numerous on leaves, they occur in
Flowering Plants wherever there is green tissue to be supplied
with gases. They are common on fruits, green twigs of trees,
and are present on nearly all parts of the aerial stems of herba-
ceous plants. On the older twigs and trunks of trees, the stomata
are represented by the lenticels which are the structures into
which stomata are transformed as the stem becomes enclosed in
bark. The stomata are distorted and transformed into lenticels
partly by the stretching of the bark and partly by the tissue
which grows up from beneath and crowds into the stomatal
openings.

In order to get a view of the epidermis in cross section and to
study the chlorenchyma and veins of a leaf, a thin section must
be made and highly magnified as shown in Figure 232. In this
view an ordinary epidermal cell is rectangular, has a large central
cavity separated from the cell walls by only a thin layer of proto-
plasm, and has the outer wall more thickened than those within.
The continuity of the epidermis is interrupted by the stomata,
each of which opens into an air chamber in the mesophyll just
beneath.

The chlorenchyma is composed of thin-walled cells, having
thin layers of protoplasm in which the characteristic green bodies
(chloroplasts) are located. In most horizontal leaves, the cells
of the chlorenchyma are differentiated into two distinct groups,
the palisade and the spongy tissue. The palisade tissue is next to
the upper epidermis and consists of one or more rows of compact
elongated cells in which chloroplasts are especially abundant.

250

LEAVES

In leaves having an oblique or vertical position, palisade tissue
may be present also on the lower side. The spongy tissue, having
fewer chloroplasts and so characterized on account of its loose
structure, occupies the region between the palisade tissue and
lower epidermis or the region between the palisade tissues when
there is a lower palisade tissue present. It consists of cells irregu-

FIG. 232. — Cross section of a Tomato leaf, e, upper epidermis; c, cuti-
cle; p, palisade cells; s, spongy cells; d, lower epidermis; st, stoma; g, guard
cells of the stoma; h, stomatal chamber; v, vein; w, parenchyma sheath of
the vein. The small bodies shown in the palisade and spongy cells are the
chloroplasts.

lar in shape and so loosely joined as to provide a system of air
spaces which extend in all directions reaching from the stomata
into the palisade tissues. In function, which is the manufacture
of food, the palisade and spongy mesophylls are identical.

Structurally chlorenchyma cells are well adapted to their
function. Their thin cellulose walls permit water and sub-
stances in solution to pass in or out readily. They have proto-
plasm, which, as in all living cells, is the substance endowed with
life and, therefore, able to regulate its activities. The cytoplasm
(the name applied to all of the protoplasm except the nucleus)
only partially fills the cell cavity, forming only a peripheral
layer. In this peripheral layer the nucleus and also the chloro-
plasts are located. Such an arrangement of the protoplasm

CELLULAR STRUCTURE OF LEAVES

251

places the chloroplasts around the cell wall where they are well
exposed to light, and provides a large central vacuole which
accommodates a large quantity of cell sap consisting of water in
which sugar, carbon dioxide, oxygen, mineral salts, and other
substances related to the activities of the cell are dissolved.
(Fig. 233.) Through the layer of protoplasm, the outer border
of which behaves as an osmotic membrane, the cell sap osmoti-
cally pulls in water from the veins or surrounding cells, and in
this way develops a pressure which dis-
tends and gives rigidity to the cell. Its
cells being rigid, the leaf is rigid and ex-
panded to the light. That this pressure
or turgor within the cells gives rigidity
to the leaf is shown by the fact that leaves
wilt when water is so scarce that the cells
can not maintain their internal pressure.

The chloroplasts, usually oval in shape
in Flowering Plants, consist of two sub-
stances. First, the chloroplast has a body
which consists of cytoplasm denser than
ordinary cytoplasm and known as a
plastid. Plastids multiply by constrict-
ing into two equal parts, and are as color-
less as cytoplasm unless they develop JIG' f; T Chloren-
0 7 ' | . chyma cell of a leaf, show-

pigments. Second, there is the chloro- ing wall (w) and layer of

phyll which is the green pigment that protoplasm (p) containing
saturates the plastid, which is then known the nucleus (n) and chloro-
as a hloroplastid or by the shorter term Plasts ^- v is the Iar8e
chloroplast. In the higher plants the central vacuole-
chlorophyll is developed by the plastids and does not occur
except in connection with these bodies. Plastids are common
in all parts of the plant. In regions where they develop no
pigments/the formation of starch from the sugar present is their
chief function. They are even abundant in underground organs,
such as fleshy stems and roots, which store starch.

The presence of chlorophyll depends mainly upon exposure
to light. That chlorophyll disappears in the absence of light is
well demonstrated by the fact that leaves lose their green color
when light is excluded for a time. Thus Grass under a board
or covered with dirt becomes yellow. On the other hand, when

252 LEAVES

leaves lacking chlorophyll, as those of plants allowed to develop
in the dark, are brought to the light, chlorophyll develops. Even
some underground structures, as Potato tubers, will develop chlo-
rophyll when exposed to the sun. Hence the development of
chlorophyll as well as its functioning depends upon the presence
of light. Although the body of the chloroplast can make starch
regardless of the presence of pigment or light, its power to make
sugar depends upon the presence of chlorophyll and light.

The veins in cross section show as colorless often glistening areas
in the mesophyll. In the central region of a vein are the two
conductive tissues, the xylem and phloem. The xylem, consisting
of large, empty, tube-like vessels with spiral, annular, and other
kinds of thickenings in their walls, occupies the upper region of the
vein. The xylem carries the water and mineral elements to the
leaf tissues. In the lower region of the vein is the phloem made
up of small thin- walled cells. The phloem carries away the
proteins and some of the sugar made by the leaves. The bundle
sheath, consisting of a chain of cells having large cavities and well
adapted to conduction, forms a sheath-like covering around the
vein. Through the bundle sheath much of the sugar is carried
away from the leaf.

The Manufacture of Food by Leaves

Sugar, starch, and proteins are formed in leaves, but it is the
manufacture of sugar that is the special function of leaves.
There are various kinds of sugar, but there is considerable evidence
that grape sugar, having the formula C6Hi2O6, is the chief one
formed in leaves. From this sugar as a basis other kinds of sugar,
of which cane sugar (C^H^On) is a common one, can be formed by
minor chemical changes. The formation of grape sugar is a syn-
thetic process and, since light is necessary, the process is called
photosynthesis.

Of all plant processes, photosynthesis is the most important, for
upon sugar as an indispensable constituent the formation of other
kinds of food either directly or indirectly depends. Thus without
the formation of sugar, such foods as starch, fats, and proteins
could not be formed, and consequently neither plants nor animals
could exist. In considering photosynthesis there are two main
topics: first, the nature of the process in reference to the mate-
rials used, the work of the chloroplasts, and the function of light;

THE MANUFACTURE OF FOOD BY LEAVES 253

and second, the various factors which modify the rate of photo-
synthesis.

As the student already knows, carbon dioxide and water
furnish the elements from which sugar is synthesized. Th3
carbon dioxide is obtained from the air through the stomata,
while the water is brought up from the roots through the vascular
system, which through its numerous fine divisions in the meso-
phyll supplies either directly or indirectly all of the chlorenchyma
cells. The carbon dioxide is dissolved in the water with which
it passes into the cells and comes in contact with the chloroplasts
where the photosynthetic process takes place. The details of
the process involved in forming sugar from carbon dioxide and
water are not well known; but, leaving out the intermediate steps,
the equation 6C02 + 6H20 = C6H]2O6 + 6O2 represents the
nature of the process. From the equation it is seen that there
are as many molecules of oxygen liberated as molecules of carbon
dioxide used. Whether all or only a part of the 602 liberated
for each molecule of sugar formed comes from the carbon dioxide
is not known. It is possible that only the carbon of the carbon
dioxide is used, in which case all of the oxygen liberated comes
from the carbon dioxide, or it may be that both water and carbon
dioxide have their constituents dissociated and some oxygen from
each is included in the 602.

Since photosynthesis removes carbon dioxide from the air to
which it returns an equal amount of oxygen, it is obvious that
it purifies the air and makes it more wholesome for animal life;
for animals in their respiration use oxygen and liberate carbon
dioxide, which, if allowed to accumulate, becomes injurious to
animals. Not only ordinary respiration of both plants and
animals but also fermentation, ordinary combustion, and all
other processes which use oxygen and liberate carbon dioxide
have their effects on the air counteracted by photosynthesis. On
the other hand, the oxidation processes maintain the supply of
carbon dioxide for photosynthesis. Thus photosynthesis and
the oxidation processes tend to support each other.

Photosynthesis takes place in the chloroplast, but the exact
function of either the body or the chlorophyll of the chloroplast
is not known. It is generally believed that the chief function of
the chlorophyll is to provide energy for the process; and this it
does by transforming the sun's rays into available forms of

254 LEAVES

energy. The need of energy for photosynthesis is easy to under-
stand. The combining of the elements of carbon dioxide and
water into sugar is preceded by a process of dissociation in which
carbon dioxide and probably water are in part at least separated
into their elements. But carbon dioxide and water are very
stable compounds, and to separate them into their atoms requires
much energy. To force the atoms of C02 to separate requires an
energy expressed by a temperature of 1300° C. It is obvious
that sunlight will not decompose carbon dioxide and water; for,
if so, these elements would be decomposed in the air. Therefore,
the chlorophyll must change the sun's rays into a form of energy
which is available for bringing about these dissociations. How-
ever, this energy consumed in bringing about these dissocia-
tions is not lost, but is stored in the sugar as latent energy to be
released when the sugar or the compounds formed from sugar
are broken into simpler compounds or into carbon dioxide and
water. Thus another relation of photosynthesis to respiration
and other oxidation processes now appears. Photosynthesis
stores the sun's energy in chemical compounds which, when
broken into simpler compounds by respiration, become a source
of energy for all other plant or animal activities. It is also the
sun's energy that is released when coal, wood, oil, and other plant
or animal products are burned. Thus the chloroplasts, enabled
by their chlorophyll to utilize the sun's energy, stand out as the
plant structures upon which our supply of both food and energy
depends.

The utilization of only certain rays of the sun accounts for the
color of leaves. When chlorophyll is boiled out of leaves with
alcohol and the solution is viewed with a spectroscope, it is seen
that the red and blue rays are absorbed while most of the green
rays are allowed to pass through. This experiment demonstrates
that chlorophyll uses the red and blue rays for energy and allows
the green rays to escape. Thus leaves are green because from
them only green rays come to our eyes.

By imagining a chloroplast as a factory, the process of
photosynthesis may be summarized in the following way: the
chlorophyll is the machinery by which sunlight, the source of
power, is applied to the work; carbon dioxide and water are the
raw materials; sugar is the product synthesized; and oxygen is
a by-product. The veins are the lines of transportation which

THE MANUFACTURE OF FOOD BY LEAVES 255

bring up the water from the roots and carry away the manufac-
tured products to all parts of the plant.

The formation of starch, although common in leaves, does not
depend upon the presence of light except in so far as light is
necessary in providing sugar; for starch is formed abundantly in
many roots, tubers, and other structures where light is excluded.
Starch, as its formula (CeHioC^n shows, is very similar to sugar of
which it is considered a storage form. Consequently its abun-
dance in leaves where sugar is being formed is to be expected.
Sugar is changed to starch not only to make room for more sugar,
but also to prevent injuries that may result from its accumulation.
According to the laws of osmosis, as the sugar content of the cell
sap of the chlorenchyma cells increases, their internal pressure
increases. Consequently when the chloroplasts are very active,
the changing of the sugar into starch, which is insoluble in the
cell sap, is necessary to prevent the internal pressure of the
chlorenchyma cells from becoming so high that there is danger
of bursting.

The transformation of sugar into starch not only prevents the
accumulation of the sugar from interfering with the process of
photosynthesis, but also enables the plant to have in storage
food which can be drawn upon when conditions are unfavorable
for photosynthesis. Thus at night when photosynthesis is inac-
tive, the starch in the leaves is changed to sugar and carried to those
regions where it is needed for growth, and in this way the plant
is able to maintain its growth at night as well as in the daytime.

However, starch is not stored in all parts of the plant so
temporarily as in foliage leaves. In some organs, such as
seeds, fleshy roots, tubers, and stems of trees, starch is stored to
remain as a food supply for next season's growth. Since the
starch stored in all parts of the plant is transformed sugar which
is made mostly in the leaves, the dependence of such structures
as seeds, roots, and tubers upon leaves is obvious; for it is only
as the leaves supply the sugar that these storage structures can
form starch.

The amount of starch formed in foliage leaves is closely
related to the rate of photosynthesis. In general, the more
active the process of photosynthesis, the greater the amount of
starch formed. For this reason the amount of starch present in
leaves can be used in determining the rate of photosynthesis.

256

LEAVES

Starch occurs in the form of starch grains, which are light in
color and have a characteristic shape and structure as shown in
Figure 234- When starch grains are treated with iodine, they
turn dark blue, and this color test can be applied directly to the
leaf to indicate the amount of starch present and, therefore, the
rate of photosynthesis. In applying the test, the leaf is first
treated with hot alcohol to remove the chlorophyll. The leaf,

FIG. 234. — Starch grains from a Potato tuber. A, simple grain; B,
half-compound grain; C and D compound grains, c, hilum. Enlarged
540 times. After Strasburger.

now almost white, is immersed in the iodine solution which turns
it blue, if starch is present, with the depth of blue roughly indicat-
ing the amount of starch present. If no starch is present, then
the leaf takes only the brownish color of the iodine solution.
This test is of considerable service in experiments on photosyn-
thesis as its application in Figure 235 shows.

Proteins are made in leaves, but in what part of the leaf
they are made is not known. The main evidence that they
are formed in leaves is that large quantities of them are being
continuously carried away through the veins to the stem. That
light is essential in the formation of proteins is doubtful, for
there is considerable evidence that the energy employed in their
synthesis comes from chemical action and not directly from sun-
light. Although proteins are of many kinds, all are formed by

FACTORS INFLUENCING PHOTOSYNTHESIS

257

combining the elements of sugar, which is the foundational sub-
stance, with nitrogen, sulphur, and phosphorus derived from the
mineral salts of the soil. Even if the construction of proteins in
leaves does not depend upon light, it is obvious that leaves are
well equipped for such
work, since they manufac-
ture sugar and the water
brought up from the soil
supplies them with an
abundance of mineral salts.

Factors Influencing
Photosynthesis. — The
factors influencing photo-
synthesis are light, temper-
ature, moisture, and
amount of chlorophyll.

That light is absolutely
essential for photosynthesis
is easily demonstrated by
applying the iodine tests to
two sets of leaves after one
set has been kept in the
dark and the other in the
light for a few days. Even
by shading only a portion
of a leaf the necessity of
light for photosynthesis can FlG- 235- ~ A leaf showinS the relation
be demonstrated as shown <* Ph°Whesis to light as indicated by

the amount of starch formed. After cover-
in figure 235. * or photo- ing the area represented by the light band,
synthesis sunlight is best, the leaf was left exposed to the sunlight for
although some photosyn- a few hours, then removed from the plant
thesis will take place in and the iodine test aPplied- The area Pr°-

artificial light that has a

tected has no starch while the areas exposed
are quite dark, due to the presence of much

suitable intensity. It has starch. Adapted from Palladin.

been demonstrated in

greenhouses that some plants, at least, carry on photosynthesis at

night if the proper kind of electric light is provided. For many

plants the direct rays of the sun are too intense, in which case

photosynthesis is most active in strong diffuse light. It is

partly for this reason that Pineapples, Tobacco, Potatoes, Cotton,

258 LEAVES

Lettuce, and some other plants grow better in some localities
under the shade afforded by slats or light cotton cloth. In green-
houses during the summer months it is usually necessary to
protect the plants against the intense rays of the sun either by
painting the glass or by some other means. Of course in shading
plants not only more favorable light for photosynthesis is often
provided, but the plants are also benefitted by being protected
from intense heat, excessive evaporation, and from hail and winds.
In many plants, as those of the Grass family, which seem to thrive
well under the direct rays of the sun, the surfaces of the leaves
slant so as to shun the intensity of the direct rays.

On the other hand, it is very common for leaves to be so
situated that they do not receive enough light. This is commonly
true of the lower leaves of the small grains, Clover, Alfalfa, and
other plants grown in thick stands. Often the leaves on the
interior branches of trees do not receive sufficient light. It is
for this reason that fruit trees with open heads have better light
relations for their interior branches than is afforded by trees with
a compact head.

Plants growing in the house are usually insufficiently lighted,
especially if they are not very near a window. The problem of
overcoming so far as possible the insufficient lighting in green-
houses during the winter months is of chief concern in greenhouse
construction, determining largely the quality, thickness, and
shape of the glass, and the nature of the frame.

What should be considered active photosynthesis, -as deter-
mined by the amount of starch produced per unit of time, varies
widely with different plants. However, investigations show that
a number of plants can produce 1 gram of starch per square
meter of leaf surface per hour under conditions favorable to active
photosynthesis. At this rate a leaf area of a square meter can
produce 10 grams of starch in a day of 10 hours. To do this, all
of the carbon dioxide would be taken from 250 cubic meters of
air. Carrying the calculation further in regard to the use of
carbon dioxide, it has been estimated that a yield of 300 bushels
of potatoes on an acre involves, including tops and all, about
5400 pounds of dry substance, and to form this, all of the carbon
dioxide over this acre to a height of 1§ miles would be used,
provided no carbon dioxide were added to the air in the mean-
time. This estimate emphasizes the importance of respiration,

FACTORS INFLUENCING PHOTOSYNTHESIS 259

combustion, and all oxidation processes in maintaining the supply
of carbon dioxide for photosynthesis. Roughly estimated, 150
square meters of leaf area will use up in one summer all of the
carbon dioxide which an average man produces through respira-
tion in one year.

When one considers that the amount of carbon dioxide in the
air is only about 0.03 per cent, that is, about
3 parts in 10,000 parts of air, it is surpris-
ing that plants can make sugar as rapidly
as they do. Sometimes, as around cities
with many factories, the per cent of carbon
dioxide may be a little higher but it is
always exceedingly low. Of course carbon
dioxide is present in solution in the soil
water; but it is easily demonstrated that
this carbon dioxide is of practically no help
to plants in photosynthesis. To compensate
for the limited amount of carbon dioxide,
it is obvious that leaves need broad surfaces
and a thorough distribution of chlorophyll,
so that their absorbing surface may be
large. However, with all of these adjust-
ments of the plant, it has been demonstrated FIQ 23g Leaf

that the normal supply of carbon dioxide showing the effect on
is often insufficient for the maximum photosynthesis of clos-
amount of photosynthesis; for some plants, ing tne stomata. The

when surrounded by air in which the stomata on the under
f ! ,..,.. , surface of the white area

amount of carbon dioxide is increased up , , ,

were closed by covering

to 1 per cent, show a corresponding rise in the epidermis with vase-
photosynthetic activity. line, thus filling the

Since stomata are the openings through stomata and excluding
which carbon dioxide enters the leaf, their carbon dioxide- After
number per area of leaf surface and the
extent to which they are open affect the amount of this gas that
reaches the mesophyll. That photosynthesis is inhibited when
stomata are closed is demonstrated by the experiment shown in
Figure 236. The experiment shows the necessity of keeping the
stomata free from dust and other bodies, such as spores of plants
and secretions of insects, that clog the stomatal openings. It is
for this reason that we are advised to cover house plants with a

260 LEAVES

thin cloth while sweeping. Also for this reason it is well to spray
with clean water or even wash the leaves of plants with clean
rags, so as to open any stomata that may be clogged. Plants
are often much injured by the ctogging of their stomata, as in
case of hedges along roadsides or plants around cement factories.

As for other plant processes, there is an optimum temperature
at which photosynthesis is most active, and above or below this
temperature photosynthesis diminishes. The optimum tempera-
ture, although varying considerably for different plants, is not
far from 80° (Fahrenheit) for most plants in our region. Tem-
peratures unfavorable for photosynthesis not only affect the
yield of crops but also may lengthen the time required for
maturity, as in case of Corn when the summer is cool.

Since water is one of the materials for making sugar, it must
be present in sufficient quantities to supply this demand. Fur-
thermore, the lack of water tends to cause the stomata to close
and may thereby diminish the amount of carbon dioxide entering
the leaf. In some cases; as in Corn, the lack of water causes the
leaves to roll, in which case there is not a good exposure to light.

For the most active photosynthesis an abundance of chloro-
plasts well supplied with chlorophyll is also necessary. As farmers
know, Corn pale in color does not grow so rapidly as Corn that
is dark green.

Transpiration from Plants

Transpiration is the loss of water in the form of vapor from
living plants. Transpiration, although similar in many ways to
ordinary evaporation, differs from the latter process in that it is
modified by the structures and vital activities of the plant. By
transpiration plants are almost constantly losing water to the air.
It is for this reason that shoots quickly wilt when their connec-
tions with roots are severed, so that they receive no water from
the soil to compensate for the loss of water to the air. The
rapidity with which green grass or weeds wilt when mowed on a
hot day is a matter of common observation. Transpiration is
not limited to leaves; but all parts of plants above ground are
exposed to transpiration. Fruits and seeds, although usually
jacketed in a rather heavy covering, lose water during storage.
Even during winter, the buds, twigs, and branches of trees are
continuously losing water to the air. However, the leaves, on

TRANSPIRATION FROM PLANTS 261

account of many openings and the .exposure of much surface, are
the regions where water is lost most rapidly. Transpiration is
the chief enemy of plants and is an important factor in determin-
ing the form, structure, and distribution of plants.

The loss of water is not so much under the plant's control as
photosynthesis and respiration are. Unless the air about the
plant is already saturated with moisture — and it seldom is — it
will take up water wherever water is available, and the moist tis-
sues of plants are available sources of moisture. The air circulat-
ing through the inter-cellular spaces of the leaf receives moisture
from the tissues, and consequently its moisture content becomes
greater than that of the air outside of the leaf. But according
to the law of diffusion, the water-vapor diffuses from the air
within through the stomata to the drier air without, and this
diffusion continues as long as the air within the leaf receives
sufficient moisture from the tissues to maintain a moisture con-
tent greater than that of the air without. The more the moisture
content of the air within and without differs, the more rapidly
the plant loses water.

Transpiration can be easily demonstrated by enclosing a
potted plant in a bell jar, taking the precaution to cover all
evaporating surfaces, except the plant, with rubber cloth or wax.
It can be demonstrated also by enclosing a branch of a plant in a
flask. In a short time moisture collects on the glass, at first as a
mist which may later form into drops and run down the sides
of the jar or flask. (Fig. 237.) This indicates that the plant
loses water to the air, which consequently becomes so nearly
saturated that moisture is condensed on the glass. If a plant
with pot protected from evaporation is exposed to transpiration
and weighed at intervals, the loss in weight due to the loss of
water through transpiration is quite marked.

The amount of water transpired, although varying much with
conditions and in different plants, is always a large proportion of
the amount absorbed. Despite the fact that much water is
used by the plant in making sugar and other compounds and in
maintaining the turgor of cells, much the larger proportion of
the water taken in by the roots passes through the plant and out
into the air. The amount of water transpired under various
conditions ranges from almost zero up to 300 grams or more per
square meter of leaf area per hour. For this unit of leaf area per

262

LEAVES

hour, transpiration in greenhouses often drops to 10 grams or less
at night and rises to 50 and often to 100 or more grams during
the day. For plants outside where there is more exposure to
transpiration the variation is much greater.

As compared with the dry weight produced, the amount of
water transpired by the plant is surprising. It has been esti-

FIG. 237. — Branch of a plant enclosed in a flask in which the air has
become so moist through transpiration from the enclosed leaves that mois-
ture has condensed on the flask.

mated that in the Central United States about 425 pounds of
water are transpired for each pound of dry matter produced by
the plant. It is stated that for the production of one pound of
dry matter, Corn requires 272, Potatoes 423, Red Clover 453,
and Oats 557 pounds of water. Calculated on the same basis,
the production of one acre of Oats of average yield requires 945
tons of water. According to estimates, an Apple tree having
thirty years of growth may lose on an average of 250 pounds of
water per day, or possibly 18 tons of water during a growing
season. An orchard of 40 such trees would transpire about 700
tons in a season. It has been estimated that even an acre of
Grass may transpire from 500 to 700 tons of water during a
season. Now, if an orchard is in sod, then there is the loss of

ADVANTAGES OF TRANSPIRATION 263

water from both trees and Grass. These estimates show the
importance of maintaining for plants a suitable supply of moisture
in the soil.

Conditions Affecting Transpiration. — The humidity of the
air, temperature, light, and velocity of wind influence trans-
piration.

The Humidity of the air is an important factor in transpira-
tion. Other conditions remaining constant, transpiration, in
general, increases with the dryness of the air. For this reason
hay cures quickly when the atmosphere is dry. It is also during
hot days when the air is dry that plants are most likely to wilt.

Since heat hastens evaporation, transpiration usually rises with
the temperature of the surrounding air. Also light, such as the
bright sunshine that is common on hot days, is an important
factor in raising the temperature of leaves, which thereby have
their transpiration increased. In bright sunlight, a large per
cent of the light absorbed by leaves is changed to heat, which may
raise the temperature of the leaf to 10° or 15° C. higher than the
temperature of the surrounding air; and this surplus of heat
induces a more rapid vaporization of the water within the leaf.

The velocity of the wind is an important factor in transpira-
tion; for it is well known that the movement of the air has an
important effect on the rate of evaporation. Thus wind moving
30 miles an hour evaporates water about 6 times as rapidly as
calm air. It is for this reason that muddy roads dry more rapidly
on windy days. When the air is calm, the air about the plant
becomes more nearly saturated and consequently ceases to take
water from the plant so rapidly; but when the air is dry and
rapidly moving, the plant is constantly enveloped in dry air
which permits very little diminution in the rate of transpiration.
When winds are both hot and dry, they are very destructive
to plants. The dry hot winds of some of the Western states
sometimes rob plants of water so rapidly that crops are killed in
a few hours.

Advantages of Transpiration. — Transpiration is an advantage
to the plant in two ways. First, it is an important factor in main-
taining the flow of water and dissolved substances from the roots
to the leaves and other portions of the shoot. Second, by lower-
ing the temperature of plants, it often prevents injury from
excessive heat.

264 LEAVES

As the student well knows, the movement of water and dis-
solved substances into and out of living cells is in accordance with
the laws that govern the passage of liquids through membranes.
But in passing from roots to leaves and other parts of the shoot,
the water with the substances in solution passes through the
tube-like xylem vessels, which are composed of the cell walls of
dead cells, and in such cells, with cell membrane and all parts
of the protoplasm absent, the structural features upon which
osmosis depends are not present. Of course throughout the stem
and roots the osmotic activity of living cells around the xylem
may have something to do with the movement of liquids through
the vessels, but this force combined with capillarity and root
pressure seems entirely inadequate to carry water from the roots
to the tops of tall trees. That transpiration has much to do with
the movement of water through the xylem vessels has been quite
well demonstrated by a number of experiments.

A column of water, due to the coherence of the water mole-
cules, holds together much like a thread or rope. The coherence
of water molecules is shown by the way water drops maintain
themselves when hanging on the end of a pipette or on the eave
of a building where, by accumulating and freezing while still
clinging, they form icicles. It has been demonstrated that even
very small columns of water, like those reaching from roots to
the leaves through the xylem vessels, are able to endure heavy
strains without breaking. Regarding the columns of water
through the vessels as small but tough threads with one end in
contact with the soil water at the roots and the other end in
contact with the cell sap in the mesophyll cells of the leaf, it is
evident that whenever water becomes scarce in the mesophyll
cells through transpiration, then by osmosis these columns of
water will be pulled in until the cells of the mesophyll are so filled
with water and their cell sap so diluted that they no longer have
the osmotic force to overcome the resistance of the water columns.
But since transpiration is practically continuous, although varying
much in rate at different times, the water columns are drawn into
the cells of the mesophyll almost continuously, and hence the
apparently continuous flow of water and dissolved substances
through the xylem of plants. Thus, transpiration, by removing the
water from the cells of the leaf and thereby causing the dissolved
substances in the sap of these cells to become more concentrated,

DANGERS RESULTING FROM TRANSPIRATION 265

brings about the osmotic force by which the cells of the leaf draw
in the water columns. The energy contributed by transpiration is
really the heat-energy involved in changing water into vapor, in
which form the water escapes from the plant. Such seems to be
the relation of transpiration to the ascent of sap, but what other
factors are involved and to what extent we have no definite knowl-
edge, and, therefore, we may attribute too much to transpiration.

It was once generally believed that the flow of water through
the plant is necessary to transport the mineral elements of the
soil to the different regions of the shoot, and that the amount of
the mineral elements reaching the leaves and other parts of the
shoot is directly related to the amount of water flowing through
the plant and, therefore, to transpiration. But some experiments
indicate that in some cases, at least, the process of diffusion by
which the mineral elements and other substances in solution pass
to those regions where they are less concentrated, regardless of
the movement of the water in which they are dissolved, can supply
the mineral elements to different parts of the shoot as rapidly as
needed. In fact, in case of Tobacco plants, analyses have shown
that plants grown in the shade may have a higher mineral content
than plants grown exposed to excessive transpiration. In other
words, the plants through which the least water flows may take the
most mineral from the soil. However, since the water carries the
dissolved substances along in its current, the movement of water
through the plant tends to aid diffusion in the distribution of the
elements in solution.

Since transpiration, like evaporation, is a cooling process, it
often prevents leaves from becoming overheated. Sometimes
bright sunshine, following a summer shower which has filled the
air with moisture, results in the leaf injury known as scalding.
Under these conditions, transpiration is checked and the tempera-
ture of the leaf becomes too high. As a large part of the sunlight
is changed into heat by the leaf, the heat accumulates very
rapidly in bright sunshine. It has been found in the case of
some leaves that the excess of heat, if transpiration be stopped,
may raise the internal temperature of the leaf to the death point
in a few minutes. Transpiration, therefore, rids the leaf of the
dangerous excess of heat.

Dangers Resulting from Transpiration. — So long as water
from the roots can be supplied as rapidly as water is lost by

266

LEAVES

transpiration, the plant is not in danger. But it is not uncommon
to see Corn with leaves rolled and Potatoes, Cotton, Clover, and
other plants wilted during dry hot days. These plants are losing
water faster than it can be replaced from the roots. These
plants are in danger because their living cells are becoming dry,
and too much' drying results in death. More plants die on
account of transpiration than anything else.

The important thing for the plant is the maintenance of a
proper balance between supply and loss of water. The plant can
endure rapid transpiration, if a copious supply of water is coming
up from the roots; but, if the ground is dry about the roots, the
root system small, or water hard to obtain from the soil, as is the

FIG. 238. — A portion of a cross
section through a node of Sugar
Cane, showing rods of wax secreted
by the epidermis. Enlarged many
times. After De Bary.

FIG. 239. — A portion of a sec-
tion through a Mullein leaf, show-
ing the epidermis with its branched
hairs. After Andrews.

case in soils that are cold or frozen, then even a small amount of
transpiration may be injurious.

Protection against Injuries Resulting from Transpiration. —
Plants may be protected against the injurious effects of trans-
piration by having their transpiring surface modified, or by
having the soil moisture increased or conserved.

There are various ways in which plants modify their transpiring
surface. Some plants, such as the Carnation, Pine, and many
plants of the desert, have the epidermis of their leaves covered
with a heavy layer of cutin. Sometimes, as in Cabbage, Sugar
Cane, and Wheat, the epidermis is covered with a waxy bloom.
(Fig. 288.) Many plants are protected by a covering of hairs.
(Fig. 239.) Some plants, such as the Cacti of the desert, have
reduced their leaves to mere spines which offer only little trans-

PROTECTION AGAINST TRANSPIRATION

267

piring surface. (Fig. 240.) By reducing the number of stomata,
as in many Grasses, or by sinking the stomata in special epider-
mal cavities, as in the Carnation, transpiration is reduced.

FIG. 240. — A globular cactus, an example of a plant having leaves
replaced by spines. After J. M. Coulter.

Sometimes, as in the Corn, the rolling of the leaves decreases
the surface exposed and lessens transpiration. (Fig. 24-1-} The

FIG. 241. — Cross section of a Corn leaf. I, lower epidermis; w, upper
epidermis. Notice that the cells are larger on the upper side than on the
lower side of the leaf. The cells of the upper epidermis, being larger, shrink
more than those of the lower epidermis, and thus cause the rolling of the
leaf in dry weather. Much enlarged.

leaves may have an edgewise position and thereby avoid the
direct rays of the midday sun, as Wild Lettuce illustrates.

The shedding of leaves from the plant is an important means of
protection. Many of our trees shed some of their leaves during a

268 LEAVES

summer drought and thereby decrease their transpiring surface.
Of course this is not a protection to the leaves but to the plant.
Most trees of the temperate region shed all their leaves in autumn.
Such trees are known as deciduous. This shedding of leaves in
autumn protects the plant against transpiration during winter.
Even with leaves absent, trees are sometimes killed by trans-
piration from buds and twigs. The killing by transpiration in
winter is not due to a great water loss, but to the inability of the
roots to furnish water to compensate for the loss. Since the roots
of most trees are not far below the surface, a deep freeze may
freeze the water about them. Even when the soil is cold, roots
take up water slowly, and when the water is frozen into ice, they
can not absorb it at all. With only a little water furnished by
the roots, a small amount of transpiration may be sufficient to
cause the death of the cells in the buds and twigs.

In transplanting trees, it is usually necessary to prune the top,
because the root system has been partly broken and cut away,
and consequently is not able to furnish enough water to compen-
sate for the amount transpired from a shoot of normal size.
Pruning the top results in fewer leaves and hence less transpiring
surface. Even after trees have been transplanted and well
established, a reduction of the transpiring surface by pruning the
top is often helpful, but usually the pruning of such trees has
other purposes as pointed out in the study of buds. However,
since the leaves are food-making organs, only a limited number
of them can be removed or the plant will suffer from starvation.

Supplying moisture to the soil protects against injuries result-
ing from transpiration. Plants in the greenhouse must have the
soil about their roots kept moist by watering. In the dry
western regions water is supplied to the soil by methods of
irrigation.

Much can be done in protecting against transpiration by
conserving the moisture of the soil. If an orchard is in sod,
many tons of water will be lost from the soil through the trans-
piration of the grass. By plowing and keeping the ground free
from grass and weeds, the water of the soil is conserved for the
fruit trees. In regions where dry farming is practiced, the
ground is fallowed during one year and then seeded the second
year. Fallowing consists in keeping the ground plowed and well
harrowed, so that the surface will be covered with a mulch and

RESPIRATION 269

be free from vegetation. This treatment allows the water from
snows and rains to soak into the soil readily and also prevents
much loss of water through evaporation. This stored-up water
is then used by the crop during the second year.

Many plants which live in dry regions have regions of water
storage. For example, the
Cacti store much water in
their stems and this storage
enables them to withstand
very dry periods. Some
plants, like the Begonia, have
special cells in their leaves
for the storage of water.
(Fig. 242.) In many plants,
as in Corn, much water is
stored in the pith.

Thus it is seen that trans-
piration is helpful when the

, T i FIG. 242. — A small portion of a cross

water lost does not exceed section of a Begonia leaf , showing water
the supply furnished from St0rage cells (s) and chlorenchyma (/).
the roots; also that the rate

of transpiration depends much upon temperature, humidity of
the air, light, and velocity of the wind; and that the dangers of
transpiration may be overcome by modifying the transpiring
surface or by maintaining an adequate supply of water in the
soil or in storage regions of the plant.

Respiration

Although respiration is a fundamental process in all living
cells, leaves afford a good place for observing the outward signs
of it. Most of the oxygen used in the respiration of the plant
enters at the leaves from which place it is carried to all partis of
the plant. Also through the leaves much of the carbon dioxide
produced by respiration escapes to the air. Respiration and
photosynthesis, although occurring together in leaves, are wholly
separate processes as shown by the ways in which they differ.
First, photosynthesis occurs in chloroplasts and is a synthetic proc-
ess, in which the elements of carbon dioxide and water are built
into compounds with the storage of latent energy, while respira-
tion occurs in all parts of the protoplasms and is a process in which

270 LEAVES

compounds are broken into their constituents, usually through
oxidation, with the result that the latent energy is released to be
used in various kinds of work. Second', photosynthesis uses
carbon dioxide and releases oxygen, while respiration uses oxygen
and releases carbon dioxide. Hence one liberates the gas which
the other uses, and in this way the two processes tend to sup-
port each other. When both processes are active at the same
time, as during the day, each process tends to obscure the other
by using the gases liberated before these gases escape from the
leaf. However, when photosynthesis is active, the amount of
carbon dioxide used and oxygen liberated is so much greater than
the gaseous exchanges of respiration that the latter process is
entirely obscured. On this account, botanists once thought that
respiration was a process performed only by animals and that the
plant breathes in a way just opposite from that of animals. Of
course further investigations showed that plants respire just the
same as animals do, but in addition green plants carry on photo-
synthesis which, when active, so much obscures respiration that
the latter process had escaped notice. Third, photosynthesis
depends upon the presence of light, while respiration is inde-
pendent of light, being active at night as well as in the daytime.
At night when there is no photosynthesis to obscure respiration,
plants take in oxygen and liberate carbon dioxide just as animals
do, and the notion once prevalent that plants purify the air is
only true of them when they are engaged in photosynthesis.

Respiration and transpiration, although influencing each other
to some extent, are also distinct processes. Since respiration
liberates energy, some of which is in the form of heat, respiration
may increase transpiration by raising the temperature of the leaf.
Furthermore, respiration in breaking down compounds releases
water in the form of vapor, in which form it readily escapes to the
air. On the other hand, when transpiration reduces the water
content of cells so much as to interfere with the activities of the
protoplasm, then respiration may be retarded.

Special Forms of Leaves

In contrast to the leaves which we have been studying, there
are some leaves which have become so modified as to resemble
ordinary leaves very little. Some have become so changed that

SPECIAL FORMS OF LEAVES

271

FIG. 243. — A portion of a Sweet Pea, showing one leaf (Z), a portion
of which is transformed into a tendril (t).

they have lost much or all of their power to make food, and have
become apparently useless or have taken on other functions.

A very common modified form of the
leaf is the scale. The most familiar
example of scales is furnished by the
buds of shrubs and trees, where they
form a projection for the inner vital
portions of the bud. These scales are
considered leaves which have been pre-
vented from developing by being so
closely crowded in the overlapping ar-
rangement. The leaves of underground
stems, which do not get to the light,
appear as small scale-like bodies with-
out green tissue, and apparently have
no function. Sometimes scales are
fleshy and are used for food storage, as
in Lily bulbs, Onions, etc. In the Asparagus the leaves are
scale-like and the food-making is mostly done by the stem,

FIG. 244. — A branch of a
Barberry, showing the leaves
transformed into thorns.

272 LEAVES

Leaves may sometimes develop into tendrils, either the entire
leaf or only a part of it becoming tendrils, as in the Sweet Pea.
(Fig. 243.)

In the Barberry and some other shrubs, the leaves are so modi-

FIG. 245. — Pitcher Plant, showing pitcher-like leaves (I).

FIG. 246. — Sundew, showing the leaves which catch and digest insects.

fied as to form thorns. (Fig. %44>) Sometimes, as in the Com-
mon Locust, only a portion of the leaf is devoted to the formation
of thorns.

The most interesting special forms of leaves are those adapted

USES OF THE PHOTOSYNTHETIC FOOD

273

to catching insects. Plants with such leaves are often called
" carnivorous plants " or " insectivorous plants. " The "Pitcher
Plants " are so named because the leaves form tubes or urns of
various forms, which contain water, and to these pitchers insects
are attracted and then drowned. (Fig. 245.) The plants known
as "Sundews" have their leaves spread on the ground and
clothed with secreting hairs.
(Fig. 246.) These secre-
tions not only entangle in-
sects but digest them. In
the "Venus Flytrap," por-
tions of the leaves work like
steel traps and hold the in-
sects fast until digested.
(Fig. 247.}

Uses of the Photosynthetic
Food

In plants, as in animals,
the chemical processes upon
which growth and other vital
activities depend are both
constructive and destruc-
tive. While the simpler ele-
ments are being transformed F10- 247.— Venus Flytrap, showing

into complex substances, *he leaves which open and close in catch-

, , , , / ing insect?

complex substances through

respiration and other destructive processes are being broken into
simpler substances. These chemical transformations constitute
metabolism, and are said to be anabolic when constructive and
catabolic when destructive. Through the plant's metabolic
processes numerous substances are formed, of which protoplasm,
proteins, sugars, starches, fats, oils, hemicellulose, amino-com-
pounds, cellulose, wood, cutin, suberin, enzymes, acids, tannins,
glucosides, and alkaloids are common ones. All of these sub-
stances are thought to be of some use to the plant, although the
exact function of some of them is not definitely known.

The photosynthetic grape sugar, since there is much evidence
that in most cases it is the chief food formed by photosynthesis,
may be regarded as a foundational food; for the photosynthetic

274 LEAVES

sugar furnishes either all or an important part of the constituents
of which all other plant substances are composed. Even if light
is essential to the formation of proteins, which are formed so
abundantly in leaves and carried away like the photosynthetic
sugar to other parts of the plant for use, sugar is an important
constituent of proteins and is therefore a basal substance in their
formation. Since animals obtain their food either directly or
indirectly from plants, it is evident that the photosynthetic sugar
is also the basal food for animals.

The various metabolic changes in the plant have to do with
providing living protoplasm, a frame work, reserve foods, secre-
tions, and energy.

Protoplasm. — It is in connection with protoplasm, the living
substance of both plants and animals, that the metabolic processes
occur. Protoplasm not only transforms substances enclosed
within it but by means of enzymes which it secretes it is also able
to act on substances with which it is not in contact. Within the
protoplasm sugar is synthesized by the chloroplasts, and starch,
proteins, fats, and many other plant products are constructed.
At the same time substances are being constructed in the proto-
plasm, substances are also being decomposed, so that within the
protoplasm substances resulting from both constructive and
destructive processes are always present. So it is not at all
strange that many kinds of substances are present in the proto-
plasm. Some, like starch and some proteins, are insoluble, while
many, like the sugars and acids, are dissolved in the nuclear and
cell sap. That protoplasm can transform substances with which
it is not in contact is well illustrated in seeds, where enzymes
secreted by the embryo diffuse out to the endosperm and trans-
form it into soluble forms of food.

One of the important constructive processes of protoplasm is the
formation of protoplasm. As the plant grows and more cells are
formed, more of the elements of chromatin, nucleoli, cytoplasm,
and all other protoplasmic structures must be formed.

As to the chemical composition of protoplasm in its living
state, we have no definite knowledge. Chemical analyses of
dead protoplasm show that a large number of substances are
present, of which proteins are the largest and most essential part.
The different kinds of proteins vary in composition, but all are
composed chiefly of carbon, hydrogen, oxygen, and nitrogen. In

FRAMEWORK OF THE PLANT

275

addition to these elements, most proteins contain a small amount
of sulphur and some proteins also contain a small amount of
phosphorus. By chemically combining in certain proportions
the carbon, hydrogen, and oxygen of the sugar with the nitrogen,
sulphur, and phosphorus obtained from the soil, proteins are
formed, but as to how the proteins are transformed into living
protoplasm no one knows.

Framework of the Plant. — Protoplasm, since it is a semi-
fluid, has no definite shape except when enclosed in a framework.
It is by means of a frame-
work that higher plants are
able to so shape themselves
as to be ad justed to the soil,
air, and sunlight.

The cell walls constitute
the framework. They are
so joined as to divide the
plant into the compart-
ments in which the pro-
toplasts or individual
masses of protoplasm re-
side. The fact is, how-
ever, that each cell is en-
closed by walls of its own;
but the adjacent walls of FIG. 248.— Cells with protoplasm shrunken,
neighboring cells are usu- so that the fine strands of protoplasm extend-
ally so closely joined that ing through the cell walls and connecting
the cells appear to be sep- neighboring protoplasts may be seen. Highly
arated by a single wall.

Through very small pores in the cell walls, the protoplasts are
commonly connected by small protoplasmic strands, which afford
a means of communication between the protoplasts of neighbor-
ing cells. (Fig. 248.)

The primary substance of which cell walls are formed is
cellulose, a substance closely related to sugar as its formula
(C6Hi005)n indicates. In the formula (CeHioC^n each combina-
tion C6HioO5, of which an unknown (n) number are combined in
forming cellulose, is a molecule of sugar minus a molecule of water
as may be seen from the equation C&H^Oe — H2O = C6Hi0O5.
Thus the formation of cellulose involves no other elements than

276 LEAVES

those of grape sugar and only slight changes in their pro-
portion.

Cellulose is a suitable material for cell walls; for its elasticity
permits cells to enlarge, and its permeability allows water
and solutions of food to reach the protoplasm. In actively
growing cells thin cellulose walls are essential, so that the
cells can enlarge; but when the cells are to afford strength,
as in case of bast fibers, then the cellulose is so deposited as to
form thick walls. Commonly when an important function of
the cell walls is to afford strength, another substance called
lignin is formed from the sugar and combined with the cellulose,
thus forming the wood characteristic of the trunks of trees and
shrubs but also common in herbaceous plants. Also in the shells
of some nuts and the coats of many seeds, lignified walls are
common. Woody walls, like cellulose walls, are permeable to
water and solutions, and for this reason are not adapted for
protective coverings, where the prevention of loss of water is an
important function. In forming waterproof walls, a portion
of the sugar or cellulose is converted into fatty or wax-like
substances known as cutin and suberin. Cutin is common in the
outer walls of epidermal cells, while suberin occurs throughout
the walls of cork.

Occasionally in the formation of cell walls, some of the sugar
is converted into substances which swell and become mucilaginous
when wet, as the seed coats of Flax and some Mustards illustrate.

Some other substances which are formed from sugar and asso-
ciated with cellulose, lignin, cutin, and suberin in cell walls are the
pectic compounds. The pectic substances (pectin, pectose, and
pectic acids), although much like cellulose, are more easily decom-
posed by certain acids and alkalies. They are often combined with
minerals, and one of the mineral compounds, known as calcium
pectate, is the chief substance of the middle portion (middle
lamella) or oldest portion of walls separating cells.

Besides the various substances formed from sugar, cell walls
are often infiltrated with mineral matters, notably silica, and
these minerals often add much strength to the frame work.

Cellulose and its closely allied compounds serve man in many
ways. From cellulose paper is made, and long cellulose fibers, as
those of Cotton and Flax, are woven into clothing. The wood
of plants is the source of lumber. Being oxidizable, cellulose

RESERVE FOODS 277

and its compounds serve as fuel, either in the form of wood, peat,
or coal. Man converts cellulose into celluloid, artificial silks,
artificial rubber, and powerful explosives, such as gun cotton.

Reserve Foods. — The photosynthetic sugar, which is not used
immediately as food, is stored in seeds, stems, roots, and tem-
porarily in leaves for use at some future time. The reserve foods
into which the excess of photosynthetic sugar is transformed are
of various forms, but are chiefly of three general classes — carbo-
hydrates, fats, and proteins.

The carbohydrates include the sugars, starches, and hemi-
celluloses, and are so named because their proportion of hydrogen
and oxygen, being the same as in water, suggested that they were
compounds of carbon and water.

Sugars are of various kinds, but only a few occur in considerable
quantities in the plant. Grape sugar, fruit sugar, and cane sugar,
the most important of the sugars, are commonly present in the
sap of plants. Grape sugar, called glucose or dextrose, and fruit
sugar, called fructose or levulose, are the simplest of the sugars.
They have the same formula CeH^Oe, but differ in the arrange-
ment of atoms. Both are found in all parts of plants, but usually
one is more abundant than the other. In sweet fruits and the
nectar of flowers, fruit sugar is usually more abundant than either
glucose or cane sugar, while in Sugar Cane, where both occur
along with cane sugar, glucose is more abundant than fructose.
Much glucose accumulates in the stems of Corn and other Grasses.
Both glucose and fructose are produced not only synthetically,
but also through the decomposition of some of the more complex
carbohydrates. Thus when Cane sugar is boiled with hydro-
chloric acid, glucose and fructose in equal amounts are produced.

Cane sugar, called sucrose or saccharose, is the sugar of most
service to man. It is present in the sap of most plants and
accumulates in great abundance in Sugar Cane, Sorghum, Beets,
and the Sugar Maple. From the stems of Sugar Cane and the
roots of Sugar Beets many million tons of cane sugar are extracted
each year. A molecule of cane sugar, as represented by the
formula Ci2H22On, contains a molecule of glucose and one of
fructose with a molecule of water dropped in making the com-
bination. The formation of cane sugar is represented by the
equation C6H12O6 + C6H12O6 - H2O = C12H22On.

Another sugar, known as Maltose and having the same formula

278 LEAVES

as cane sugar but differing in arrangement of atoms, occurs in
germinating seeds, and is especially abundant in germinating
Barley. It is formed when starch is broken into its components.
Maltose, when broken into its constituents, gives rise to two
molecules of glucose.

The starches are the most abundant storage forms of food into
which the photosynthetic sugar is converted. They occur in all
parts of the plant, but are especially abundant in seeds, tubers,
and fleshy roots, where they are extensively used by man for food.
The food value of Corn, Wheat, Rice, and other Cereals, and of
Potatoes depends mainly upon the starch which they contain.

The starches, unlike the sugars, are in-
soluble and occur in the form of definitely
shaped bodies, known as starch grains,
which vary much in shape, size, and mark-
ings in different plants. (Fig. 249.) The
size, shape, and markings of the starch
grains are so characteristic of many plants
that by a study of the starch grains in a
mixture of ground vegetable products, the
constituents can often be determined and
, - the adulterants thereby detected.

FIG 249 —a starch Starch and cellulose have the same for-
grain of Irish 'potato; mula (C6Hi0O5)n but they differ in the num-
&, starch grain of Wheat; ber of combinations, C6HioO5, contained in
c, starch grain of Corn. their molecules. Their exact difference in

structure is not known, for the number of combinations, C6HioO5,
contained in a molecule of either starch or cellulose has not been
determined. They differ in physical properties as well as chemi-
cally. The starches are readily converted back into sugar, in
which form they are used by the plant. The starches are broken
into simpler compounds by a group of enzymes, and the prod-
ucts formed are dextrin, maltose, and glucose according to the
extent to which the starch molecules are broken up.

The hemi-celluloses are quite prominent in some seeds, and
occur in many other places in the plant. They are much like
ordinary cellulose, but, being easily converted into sugar, they
are available sources of food and on this account are often called
reserve celluloses. They are usually very hard substances and
are deposited as extra layers on the cell walls. The hardness of

RESERVE FOODS

279

Date seeds and Ivory nuts is due to the hemi-celluloses, in which
form these seeds store their reserve food. (Fig. 250.)

The fats are prominent storage forms of food in a number of
plants and usually occur in the form of oils, known as fatty oils.
They contain the same elements as the carbohydrates, but have
much less oxygen in proportion to carbon, and the hydrogen and
oxygen are not present in the proportion to form water. The
vegetable fats are chiefly compounds of
glycerine and fatty acids. Palmitic,
oleic, and stearic acids are the fatty
acids commonly found in fats, but there
are many others, and the character cf the
fat formed depends much upon the kind
of fatty acid entering into the combina-
tion. The fatty oils are usually stored
in the seeds, but often in the flesh of
fruits and other portions of the plant.
Olive oil which is pressed out of the fruit
of the Olive, Corn oil from the embryos
of Corn, Cotton-seed oil from Cotton seed,
Coconut oil from Coconuts, Linseed oil
from Flax seed, and Castor oil from

seeds of the Castor Bean are well known tlvory "ut? showin« ; *e ex'

M . , . treme thickening with hemi-

plant oils and have an important place CeUulose, which is deposited

in our industries. in layers forming striations.

The fatty oils are usually present in The walls are perforated by

the storage cells in the form of globules, the canals through which

and like starch are insoluble in the cell the P™toP|asmic Brands

_. . . . ii.t-i pass. Highly magnified,

sap. Before being consumed by the plant

as food, they are converted by means of enzymes into simpler
and soluble forms, such as glycerine and fatty acids.

The vegetable fats are not only important animal foods, but
are used in many ways by man in the industries. Rape oil and
Corn oil are much used for lubricating machinery. Linseed oil
is extensively used as a solvent for paint. Palm oil, Cotton-seed
oil, and Coconut oil are made into substitutes for butter, besides
being used in many other ways. Castor oil, besides being used
as a medicine, lubricant, and illuminant, is used in manufacturing
dyes. The use of Olive oil in foods is well known. Many of the
vegetable oils are used in making the best soaps.

FIG. 250.— Cell walls of

280

LEAVES

The vegetable proteins are of many kinds and they vary greatly
in physical and chemical properties. They occur as crystals,

granules, or in solution
the vacuoles of the

protoplasm, or in inti-
mate association with
the protoplasm. They
are present to some ex-
tent in all plant cells,
but are more prominent
as storage products in
seeds, where they are
usually associated with
starch and fats. Some-
times, as in the aleurone
layer of the cereals,
there is little else but
(Fig. 251.}

'01

FIG. 251. — Cross section through grain of
wheat (Triticum vulgar e); p, pericarp; t, testa;
al, aleurone layer containing numerous protein proteins,
grains ; TO, nucleus; am, starch grains. Enlarged The Legumes store con-
240 times. After Strasburger. siderable quantities of

proteins, and for this reason some of them, especially the Beans
and Peas, are very desirable
for food. (Fig. 252.}

Proteins differ chiefly from
the carbohydrates and fats
in that they contain nitro-
gen. They are known as
nitrogenous foods. In
addition to nitrogen they
usually contain sulphur and
sometimes phosphorus; but
nitrogen is the chief mineral
constituent. The proteins
are extremely complex, as the
formula CreoHns^isO^Ss
for one of them indicates.

The steps in the process
by which the photosynthetic
sugar and the mineral elements are formed into proteins are not well
known; but it seems clear that the elements of the sugar are first

FIG. 252. — Section from a cotyledon
of a Pea, showing a few cells; i, intercellular
space; am, starch grains; al, aleurone
grains; TO, nucleus. Enlarged 240 times.
After Strasburger.

SECRETIONS 281

combined with nitrogen to form amino-compounds or amides,
which then combine with themselves and other mineral elements
to form proteins. The proteins, like the starches and celluloses,
are thus supposed to be built up by the combining of simpler
compounds. That amino-compounds are involved in the for-
mation of proteins is suggested by the fact that they are nearly
always present in plants, and also by the fact that they are pro-
duced when proteins are decomposed. Asparagin C4H803N2 is
one of the most common amino-compounds found in plants, but
a number of others, such as arginin, tyrosin, leucin, and trypto-
phane, are often found in considerable quantities in the germinat-
ing seeds or seedlings of plants.

Plants form many kinds of proteins, but most of them belong
to one of the general classes — albumins, globulins, glutelins,
gliadins, or nucleo-proteins. The albumins, the proteins of
which the white of an egg is composed, are represented in Peas
by legumelin and in Wheat and other cereals by leucosin. The
globulins are common in the Legumes, legumin being the chief
one. The globulins are probably the most abundant of the re-
serve proteins in all seeds except cereals. Glutenin found in Wheat
and oryzenin in Rice belong to the glutelins. The gliadins are
found in the cereals. Gluten, the substance upon which the tenac-
ity of dough depends, consists chiefly of glutelins and gliadins.
The nucleoproteins occur in the nucleus of the cell where they
are an important constituent of chromatin. Most proteins are
insoluble in the cell sap, and need to be digested by enzymes into
soluble and diffusible forms, such as proteoses, peptones, or amino-
compounds, before they can be moved through the plant.

Secretions. — A large number of plant substances, differing
widely in both composition and function, are often classed as
secretions. The important classes of secretions are volatile oils,
glucosides, alkaloids, pigments, and enzymes. Some of the
secretions accumulate in glands, which have special cells for
secreting and often cavities provided for holding the secretions,
as the glands on the leaves of the Mints and in the skin of Oranges
illustrate. Some accumulate in long ducts like those in the stems
and leaves of the Milkweeds and Pines and are secreted by the
cells around these ducts. Many of the secretions, however, are
formed in cells, in which secreting is only a minor function, and
are usually found in solution in the vacuoles of the protoplasm.

282 LEAVES

The volatile oils differ not only chemically from fatty oils, but
also in being volatile. They cause most of the odors of plants.
Oil of Peppermint, Sassafras, Cinnamon, Cloves, Cedar, and the
oil of the Orange rind are familiar volatile oils. A number of
uses to the plant have been assigned to the various volatile oils,
such as protection against destructive organisms, attraction of
insects in the pollination of flowers, and serving as a storage form
of food. Their pleasant odors and tastes add charm to the
flowers of many garden plants, and before the chemists learned
to make many of them, plants were our chief source of the per-
fumes and essences of commerce. Their composition suggests
their origin from the photosynthetic sugar, since nearly all of
them contain only carbon and hydrogen, or only carbon, hydrogen,
and oxygen.

Closely related to the volatile oils are a number of substances,
such as the Pine resins, 'india rubber, gutta percha, camphor, and
asafetida, which are important commercially, although their use
to the plant is not definitely known. From the Pine resins, which
are found in the resin ducts of Pines, turpentine, pine tar, rosin,
and pitch are obtained. India rubber is the prepared milk-juice
obtained from a number of trees of tropical countries. Gutta
percha, used in making surgical instruments, in filling teeth, and
in a number of other ways, is obtained also from the milk-juice
or latex of a number of plants. Camphor is obtained from a
number of tropical trees and asafetida from a group of herba-
ceous plants.

Glucosides are complex substances and are so named because
many of them contain glucose as one of their constituents. They
may be considered storage forms of food since they yield a sugar
when broken down. Amygdalin, the bitter substance in the
seeds of the Bitter Almond, is one of the best known glucosides.
Its formula is C2oH27NOn, and when decomposed it yields glu-
cose (CeH^Oe), hydrocyanic acid (HCN), and benzaldehyde
(C6H5CHO). Glucosides vary much in composition and con-
sequently in the products which they yield when decomposed.
Thus the glucoside coniferin (Ci6H22O8), found in coniferous trees
and Asparagus, yields glucose and coniferyl alcohol (CioH^Os)
when decomposed.

Glucosides occur in all parts of the plant and are especially
abundant in the parenchyma cells of roots, stems, and leaves.

SECRETIONS 283

On account of the hydrocyanic acid or other poisonous substances
contained, many of the glucosides are poisonous. For example,
the saponins, which are present in Corn Cockle and Cow Cockle,
are poisonous and make the seeds of these plants very objection-
able impurities of the small grains. The mustards, a number
of which have poisonous seeds, contain sinigrin, a poisonous
glucoside. In the seeds of some Beans, as the Burma Bean, there
is phaseolunatin, a poisonous glucoside.

Although it is not known just how glucosides are formed or
their exact function to the plant, their structure shows that the
photosynthetic sugar furnishes most of the elements. Some
of them may be directly synthesized, while others may result
from the decomposition of more complex compounds. For the
decomposition of each kind of glucoside there seems to be a special
enzyme.

The alkaloids constitute another group of substances whose
origin and function are obscure. Some of the familiar ones are
caffein and thein in Coffee and Tea, nicotine in Tobacco, morphine
from the Poppy, quinine from the bark of the Cinchona tree, and
strychnine from the seeds of Nux vomica. In containing C, H, O, N,
they show a close relationship to the ammo-compounds. The
alkaloids are probably protective substances, since they are
often unpleasant to the taste and the most poisonous group of
the plant substances. In the preparation of drugs, the alkaloids
have a very important place. They are the plant poisons which
commonly poison stock in pastures and often people get them
by mistake. In Poison Hemlock (Conium maculatum) and
Water Hemlock (Cicuta maculata) there is the poisonous alkaloid,
known as conin, which is poisonous to stock and man. In the
Nightshade family, of which Tomatoes and Irish Potatoes are
representatives, there are a number of plants which contain
atr opine and solanin, which are poisonous alkaloids. There is
a large number of plants, many of which are common, that are
poisonous on account of the alkaloids contained. The ptomaines,
the poisonous substances which Bacteria produce in the decom-
position of meats, are alkaloids.

Pigments are the substances upon which the colors in plants
depend. Their origin is obscure and in some cases their function
is not known. The one most prominent is chlorophyll with the
formula often given as Css^Oe^Mg. Associated with chloro-

284 LEAVES

phyll and probably decomposition products of chlorophyll, are
carotin (C4oH56) and xanthophyll (C^H^A), which are usually
yellow or orange. Carotin is so named because of its abundance
in the root of the Carrot. These pigments are present in fruits,
flowers, and autumn leaves, where they produce the yellow and
orange colors. Anthocyan, a pigment whose formula is not well
known, occurs dissolved in the cell sap and is the basis of the
reds, purples, and blues in plants, being red when the cell sap is
acid and blue when alkaline. Besides having an important place
in determining the color of flowers and fruits, it often occurs in
leaves and 'stems.

In addition to the manufacture of food, which is the function
of chlorophyll, the pigments, by producing the showy colors of
flowers, assist in pollination. They also add to the attractiveness
of fruits and thereby assist in the dissemination of seeds. Often
the yellow, red, and blue pigments are prominent in leaves and
other structures where they seem to have no function.

The enzymes are of many kinds and most of the . metabolic
changes in cells involve the action of enzymes. They are the
most general secretions of protoplasm and - in all living cells
enzymes of some kind are present. So far as chemical analyses
have been able to determine, they are similar to proteins in
composition. They occur dissolved in the cell sap or in intimate
relation with the protoplasm, but often diffuse out of the cell and
attack surrounding substances.

Enzymes are specific in their action and hence there are almost
as many kinds of enzymes as there are kinds of substances to be
acted upon. There is a class of enzymes which acts on proteins,
one that acts on carbohydrates, and another that acts on fats.
In addition there are enzymes which act on glucosides and other
substances of minor importance. The enzymes, called proteases,
which act on proteins in plants, are of two classes — ereptases and
peptases. Peptases have been found in a number of plants but
they are not so generally present as the ereptases are. Ereptases
apparently break up proteins more completely than the peptases
do. Bromelin found in the Pineapple and papain in the Papaw
(Carica papaya) are two well known peptases. Papain is used
in making digestive tablets. The proteases break the proteins
into soluble forms, such as proteoses, peptones, and amino-
compounds, that can be translocated and used as food.

SECRETIONS

285

A number of enzymes, such as diastase, invertase,. maltase,
zymase, and cytase, are involved in the digestion of the carbo-
hydrates. Diastase, which is especially abundant in germinating
seeds, changes starch into sugar. Invertase converts cane sugar
into glucose and fructose, and maltase converts maltose into
glucose. Zymase, well known as a secretion of the Yeast plant,
converts sugar into alcohol and carbon dioxide. Cytase breaks
cellulose into simpler compounds.

The Upases digest the fats by changing them into fatty acids
and glycerine, in which form the fats can be moved in the plant
and consumed as food.

For the glucosides there is also a group of enzymes. For
amygdalin there is amygdalase,
and for other glucosides certain
other enzymes which decom-
pose them.

Although we know that en-
zymes have much to do with
the metabolic changes in plants,
our knowledge of enzymes is
comparatively meager. Inves-
tigations on the kinds of en- FlG- 253- — Cells containing crystals

i ,-, ' , . -, of calcium oxalate.

zymes and their particular

functions are much needed and are receiving much attention
by plant chemists.

Other secretions so far omitted from our list are the acids
and the tannins. Malic acid, oxalic acid, citric acid, tartaric acid,
and a number of others, known as fruit acids, which function in
determining the taste of fruits, are very important to man and
assist in seed dissemination when they make the fruit more
pleasing to the taste. Some of the organic acids often form
compounds with minerals and form crystals. Crystals of cal-
cium oxalate are quite common in plant cells. (Fig. 253.} The
tannins are bitter astringent substances as any one who has
tasted a green Persimmon well knows. They occur throughout
plants, but are more abundant in the bark. Tannins harden the
gelatine in skins, and before the chemists provided substitutes bark
was extensively used in tanning leather. Due to their astringent
and antiseptic properties, it is thought that they protect the plant
against the action of organisms, such as Fungi and Bacteria.

286 LEAVES

Stored. in all forms of food there is latent energy, which is trans-
formed sunlight, and through respiration the foods are broken
into simpler compounds with the release of energy, which is
utilized in the various kinds of work of the plant.

Summary

Leaves may be classed as primary and secondary. The
primary leaves, represented by the cotyledons, are mainly storage
organs and are usually short-lived. The secondary leaves form
the foliage of plants and are the food-making organs. The form
and arrangement of leaves vary much, but usually result in the
best exposure to light.

The chief tissues of the leaf are the epidermis consisting of pro-
tective cells and stomata, the mesophyll containing the working
cells, and the veins, which give strength and supply other tissues
with water and salts and carry away the manufactured products.

The processes taking place in leaves are photosynthesis-, trans-
piration, and respiration. Leaves are especially adapted to
photosynthesis because of their green tissue and exposure to
sunlight. Upon photosynthesis the carbohydrate supply of the
world depends. Respiration is not peculiar to leaves; for it
takes place in all living cells, but can be easily observed in leaves.
Leaves are much exposed to transpiration, which may benefit
the plant or result in injury. The amount of water lost through
transpiration depends upon the character of the transpiring
surface, temperature, humidity of the air, light, and velocity of
wind. A plant may be protected against the dangers of trans-
piration by having its transpiring surface modified or by being
able to supply water from the soil or storage organs in sufficient
quantities to meet the loss through transpiration.

Leaves may have become modified into special forms, such as
scales, tendrils, thorns, pitchers, or traps.

Through metabolism the photosynthetic sugar is chemically
combined with mineral elements to form proteins and proto-
plasm, transformed into materials which constitute a frame-
work, changed into storage foods and into various other kinds
of plant products. Through respiration, a phase of metabolism,
plants obtain chemical energy by releasing the latent energy of
sugar or of the compounds of which it is a part.

PART II

PLANTS AS TO KINDS, RELATIONSHIPS, EVO-
LUTION, AND HEREDITY

CHAPTER XII
INTRODUCTION

Aside from some references to Gymnosperms, and to Yeast,
Bacteria, and a few other simple plants, Part I is devoted almost
entirely to a study of the Morphology and Physiology of the Flow-
ering Plants. The Flowering Plants deserve more attention than
other groups, because they are the most highly developed, most
attractive, and are the chief source of food, fibers, and many other
products related to the welfare of mankind. But in addition to
the Flowering Plants, the Plant Kingdom also includes many
kinds of plants which do not have flowers. In fact, not much
more than half of the 233,000 or more species of known plants are
Flowering Plants. About us are many kinds of plants w^hich do
not have flowers and some of them are also of much economic
importance. The Gymnosperms, the group to which Pines,
Spruces, Firs, and some other trees valuable for timber belong, do
not have true flowers but have seeds, and are almost as highly
developed as the Flowering Plants. The Flowering Plants and
Gymnosperms constitute the group called Seed Plants. But
there are many kinds of plants, which are often referred to as the
simpler plants, that do not even have seeds and some of these are
of much economic importance. Well known among the simpler
plants are the Ferns, Mosses, Algae, Fungi, and Bacteria. Both
the Fungi and the Bacteria are important economic groups.
The Fungi cause most of the plant diseases and consequently
much destruction and loss among cultivated plants. Many of
the Bacteria are indispensable to Agriculture, for they decompose
organic compounds, increase the nitrogen of the soil, and do other
things that are related to the soil fertility. On the other hand,
the Bacteria cause most of the animal diseases and these forms
we have to combat.

Some of the simpler forms, like the Bacteria and many Algae,
are unicellular plants and hence are extremely simple, while some,
as the Ferns illustrate, have complex plant bodies and are com-

289

290 INTRODUCTION

paratively well developed. In addition to the many kinds of
plants now living, many other kinds once existed but are now
known only by their fossils.

Among the kinds of plants, including both the living and fossil
forms, there are almost all degrees of complexity, ranging from
the simplest unicellular plants to the most highly developed
Flowering Plants. Although varying widely in complexity, the
various kinds of plants are evidently related as a study of their
structures and habits reveals. Scientists believe that the living
forms have come from previously existing forms and hence are
related through a common ancestry. They have originated
through the process known as evolution, which assumes that the
first plants on earth were extremely simple and from these simple
forms the more complex forms arose. In response to a changing
environment or due to changes arising wholly within, the simple
forms gave rise to more complex forms, which in turn gave rise to
forms still more complex. Thus through slow changes involving
millions of years the highly developed forms were evolved.

Evolution has generally been progressive, giving rise to forms
with higher organization, greater perfection of parts, and in-
creased efficiency of function. Sometimes, however, evolution
has been retrogressive, and forms have been reduced to simpler
forms, through becoming more simply organized and less efficient
in function. For example, in this way the Fungi are supposed to
have arisen from the Algae. Progressive evolution has not been
direct from the simplest to the highest organisms, but has been
along many lines which, although usually progressive, have been
more or less divergent and this accounts for many kinds of
organisms among both plants and animals. Animals and plants
can be distinguished in their higher forms on the basis of loco-
motion, methods of getting food, character of the skeleton, and
so on, but in their simpler forms animals and plants are not easily
distinguished, and this fact suggests that plants and animals
arose as diverging lines from the same preexisting organism. A
diagram of evolution in plants looks like a tree with many
branches. The trunk represents the main line and the branches
the diverging lines of evolution. The lowest branches with their
sub-branches represent the groups of the simplest plants and
their relationships. The groups of Seed Plants and their rela-
tionships are represented by the topmost branches, and the

CLASSIFICATION OF PLANTS 291

branches representing other groups are so located as to show the
relative complexity and relationships of the various other groups
included in the Plant Kingdom.

The origin of a plant from simpler previously existing forms is
known as phylogeny, while the series of changes which a plant
or any living being passes through in attaining a mature con-
dition is called ontogeny. Plants are classified in a number of
ways, but chiefly upon their phylogenetic relationships. An-
other basis of considerable importance upon which plants are
classified pertains to their place of living and adjustments to
environment. Relationships of this kind are ecological.

Part II is devoted, although briefly: first, to a study of the
structure, habits, economic importance, and phylogenetic rela-
tionships of the plants below the Flowering Plants; second, to
a study of some of the important groups of Flowering Plants as
to their phylogenetic relationships and economic importance;
third, to a consideration of plants as to their ecological relation-
ship ; and fourth, to a special study of evolution, heredity, and
the breeding of plants.

Classification of Plants. — On the basis of descent or phylo-
genetic relationships the Plant Kingdom is divided into four
groups called divisions. Divisions are divided into classes,
classes into orders, orders into families, families into genera, and
genera into species. Species are the units in the phylogenetic
classification. Species are aggregates of individuals which are
alike in their important characteristics and are therefore regarded
as the same in kind. Of course the individuals of a species may
vary in a number of minor features. Thus White Oaks vary
much in size, shape, and a number of other ways, but in essential
features all White Oaks are alike and all belong to the species
White Oak. Sometimes some of the individuals of a species
show some important differences, and then the species is sub-
divided, the subdivisions being called varieties, strains, or races.
Also for a similar reason it sometimes happens that orders are
subdivided into suborders, classes into subclasses, and families
into subfamilies.

The scientific names of the phylogenetic groups are Greek or
Latin terms and commonly express some characteristic of the
group. With some exceptions due to historical causes, the
groups are named according to a rather definite plan. The names

292 INTRODUCTION

of the divisions end in -phyta, commonly written -phyte, from
the Greek word phyton which means plant. The names of classes
commonly end in -ineae or -eae and are usually derived from some
important group included. Thus the Lycopodineae is the class
containing the Lycopods, and the Filicineae is the class contain-
ing the Ferns.

The names of orders end in -ales, and orders are commonly
named from some prominent family included. Thus the Resales
are named from the Rosaceae, the Rose family. Names of families
usually end in -aceae and are commonly derived from some
prominent genus, as for example the Liliaceae, which is the Lily
family. The names of genera are Latin nouns in the nominative
case. Thus Quercus is the Oak genus, Pyrus, the Apple genus,
and Acer, the Maple genus. Species have two names, the name
of the genus and the name that distinguishes the species. For
example, Quercus alba is the White Oak, while Quercus rubra is
the Red Oak. Quercus is the name of the genus, and the terms
alba (the Latin term for white) and rubra (the Latin term for red)
name the species. In English we simply change the terms about
and say White Oak instead of Oak White.

The Divisions of the Plant Kingdom. — The phylogenetic
divisions of the Plant Kingdom arranged in phylogenetic
order are Thallophytes, Bryophytes, Pteridophytes, and Spermato-
phytes.

The Thallophytes are the simplest plants and are regarded as
the lowest and most primitive from the standpoint of evolution.
The word means thallus plants. As previously stated the
ending -phyte always means plant. Thallus refers to the fact
that the plant body has a simple organization. It is not differen-
tiated into roots, stem, and leaves. Bacteria, Toadstools, and
Algae are familiar Thallophytes. The plant body of some of
them consists of a single cell, which is the simplest plant body
possible.

The Bryophytes are so named because they are chiefly Moss
plants. Besides the Mosses, they also include the Liverworts.
The Bryophytes have better organized plant bodies than the
Thallophytes and are, therefore, considered higher in the scale
of evolution.

The Pteridophytes are so named because they include the Fern
plants. Most Pteridophytes are Ferns, but this group includes

SIMPLE THALLOPHYTES WITH SPERM ATOPHYTES 293

some plants that are not true Ferns. The Pteridophytes made
much advancement in developing tissues and organs. They
have roots, stems, and leaves, and for this reason are regarded as
more highly developed than the Bryophytes.

The Spermatophytes are the Seed Plants. With this group we
are most familiar, since to this group belong the trees, shrubs,
and most of the familiar herbaceous plants. It is the seed, which
is one of their contributions to evolution, that makes many of
them so useful. In this group occurs the greatest display of
tissues and organs.

The Spermatophytes consist of two subdivisions, Gymnosperms
and Angiosperms:

The Gymnosperms (Gymnospermae), as the term signifies, do
not have their seeds enclosed. These are the evergreens, such
as Pines, Cedars, Spruces, Hemlocks, Firs, etc.

The Angiosperms (Angiospermae), as the term signifies, have
their seeds enclosed. This refers to the enclosing of the seed in
an ovary. Nearly all of the cultivated plants belong in this
group. They contribute the fruits.

A Comparison of Simple Thallophytes with Spermatophytes. —
One striking difference between the simplest Thallophytes and
the Spermatophytes is in the number of cells of which the plant
is composed. The simplest Thallophytes are unicellular, while
the Spermatophytes are extremely multicellular. A second
striking difference between the plants of the two divisions is in
the differentiation and specialization of cells which are thereby
fitted to perform special functions. In unicellular Thallophytes
one cell performs all of the different kinds of work that the plant
has to do, while in Spermatophytes there is a division of labor
among the cells; that is, Spermatophytes have tissues, which
are groups of cells especially adapted to do particular kinds of
work.

As the cells of multicellular plants become differentiated into
tissues and thus specialized in function, they lose the ability to
exist independently. Many unicellular plants can live inde-
pendently of other cells, but in Spermatophytes, the life of a
cell in most cases depends upon the proper adjustment of the cell
to the vital processes of other cells of the plant body. Thus the
ability of a cell to perform many functions is lost in becoming
adapted to perform one function well.

294 INTRODUCTION

In organization the cells of Spermatophytes do not differ essen-
tially from those of most Thallophytes. Excepting in the very
lowest forms, the cellular structures of Thallophytes are similar
to those of the Spermatophytes as a study of the unicellular
Thallophyte in Figure 25^. will show. This one-celled plant is
composed of protoplasm, which is the living substance, and a
wall, which encloses the protoplasm. The protoplasm, as in the
cells of higher plants, consists of nucleus and cytoplasm. The
nucleus, usually globular in shape, is en-
closed by a nuclear membrane and contains
one or more nucleoli (small globular bodies)
and chromatin (the chunky or granular sub-
stance scattered about in the nucleus). In
FIG. 254. — A addition to nucleoli and chromatin, the nu-
one-celled Thallo- cleus contains nuclear sap (water containing
phyte, Pleurococcus sugar) saitS) and other substances in solu-
vulgans. n, nucleus ti ^ The cytoplasm the protoplasm out-
showing nuclear

membrane, chro- S1(^e °f tne nucleus, is vacuolate and has its
matin, and a nucle- outer border so modified as to form a mem-
olus. c, cytoplasm, brane, which, unless the protoplasm is
in which there is a shrunken, is tightly pressed against the cell

p7aVt0beV^°o7 walh Water and solutions enter the Prot°-
From Strasburger. Plasm through this cell membrane by the
processes of osmosis and diffusion. All of
these cellular structures have practically the same function here as
in the cells of the higher plants. In this particular unicellular plant
there is a chloroplast, which, like the chloroplasts in the food-
making cells of leaves, is a special protoplasmic body saturated
with a green pigment (chlorophyll), which enables it by utilizing
the sunlight to carry on photosynthesis, that is, to form sugar
from carbon dioxide and water.

Although consisting of a single cell, this plant performs most of
the functions which the most highly organized plants perform,
but in a simpler way. In absorbing water and mineral elements
directly from its surroundings, it performs the function of roots.
In carrying on photosynthesis, it performs the function of leaves.
By dividing it gives rise to new individuals and thereby performs
the function of reproduction, which is the function of flowers. In
such a simple plant there is no function comparable to that of
a stem, for there are no distant parts, such as leaves and roots,

SIMPLE THALLOPHYTES WITH SPERM ATOPHYTES 295

to be connected and no definite position which the plant must
maintain.

It is now clear that in passing from the unicellular condition
to the Spermatophyte stage, evolution was along the following
lines: First, plants became multicellular ; second, the cells con-
stituting a multicellular plant became somewhat differentiated as
to function and structure; third, as plants became more multi-
cellular, there was further differentiation which eventually re-
sulted in the establishment of definite structures or organs fitted
to efficiently perform special functions. Such structures in their
most highly organized form are the leaves organized for the
manufacture of plant food, the roots organized for absorbing
and for anchoring the plant, the flowers organized for reproduc-
tion, and the stem organized to support leaves, flowers, and fruit
in the air and sunshine.

Of course the -organs as they occur in Spermatophytes did
not arise suddenly, but they, too, underwent a gradual process of
evolution, at first arising as simple structures and gradually
becoming more complex and better defined. Through the
Thallophytes, Bryophytes, Pteridophytes, and Spermatophytes,
including both living and extinct forms, the organs characteristic
of the highest type of Spermatophytes gradually arose.

CHAPTER XIII

THALLOPHYTES

Algae (Thallophytes with a Food-making Pigment)

General Characteristics. — The Algae are a familiar group of
Thallophytes, for in nearly every lake, pond, and stream, and
along the sea coast some forms of them can be found. They
commonly appear in fresh water as a green scum or as floating
mats of green threads on or near the surface of the water. They
often occur in abundance in watering troughs, and sometimes
become troublesome by clogging sewers and water mains. Along
the sea coast occur the large brown and red forms known as
Seaweeds.

Algae are of some economic importance. The Seaweeds are
much used as food in some countries, especially in Japan, and
from some Seaweeds iodine and potassium are extracted. Along
the Pacific Coast of the United States, Seaweeds are an impor-
tant source of potassium for fertilizers. However, the interest
in the study of Algae is not due so much to their economic
importance as it is to the fact that a knowledge of them is
essential to an understanding of the evolution of the higher
plant forms.

Although Algae are water plants, not all Algae live in the water,
for there are some forms which live on moist soil or rocks where
water is easily obtained, and a few exceptional forms, such as
those that live on the bark of trees, have very dry surroundings
much of the time. Algae differ from other groups of Thallophytes
in having food-making pigments by which they make their car-
bohydrates. Consequently, they are not saprophytes or para-
sites, that is, plants which have to depend directly upon other
plants for food, but are equipped to live independently. Among
them there is a wide range of variation in plant body and
methods of reproduction, and four groups of Algae are commonly
recognized — Blue-green, Green, Brown, and Red Algae.

296

BLUE-GREEN ALGAE 297

Blue-green Algae. Cyanophyceae

The Blue-green Algae are the simplest forms of Algae and are
the simplest known plants that make their own food. They
are so named because of their bluish green color which is due
to the presence of chlorophyll and a blue pigment called Phyco-
cyanin. Although their size is microscopical, they form aggre-
gations that are often quite conspicuous. There are about 1200
species of Blue-green Algae, and they are widely distributed,
occurring nearly everywhere in fresh and salt water and also on
wet soil, rocks, and logs. On wet surfaces they form bluish
green slimy layers or jelly-like lumps, and in sluggish streams
and ponds they form bluish green scums or mats which float on
or near the surface of the water. They thrive best where there
is organic matter and consequently prefer stagnant to running
waters. Some forms are so resistant to heat that they can live
in hot springs where the temperature is near the boiling point
of water. Some, called endophytes, live in the cavities of some of
the more highly organized plants, such as the Liverworts and
Ferns. Some are associated with Fungi in the formation of
Lichens. The Blue-green Algae are of only slight economic im-
portance. When allowed to accumulate, they impart offensive
odors to water supplies, but are easily controlled by use of
copper salts. It is claimed that livestock are sometimes killed
by drinking water that has become foul with Blue-green Algae.

The plant body in the Blue-green Algae is a single cell or a
colony of cells so joined as to form a filament or plate. When
cell division is in only one direction and the cells formed do not
separate, then as a result of a number of successive cell divisions
a chain or filament of cells is formed. When cell division is in
more than one direction and the cells do not separate, then
colonies of other shapes are formed. Colonies, although they
may resemble multicellular plants, are aggregates of essentially
independent cells. One notable feature of the plant body of the
Blue-green Algae is the secretion of a gelatinous substance which
forms a sheath about the plant. As plants grow and multiply,
the gelatinous secretion accumulates and commonly forms a
matrix which holds the plants together in slimy layers or jolly-
like lumps. The gelatinous sheath holds water and thus protects
the plants from drying out. Another notable feature of this

298

THALLOPHYTES

group pertains to the organization of the cell. In a few of the
most highly developed forms the protoplast is pretty well organ-
ized, but in most Blue-green Algae the nucleus and cytoplasm
are not clearly differentiated and there are no chloroplasts. The
chlorophyll and other pigments are diffused through the cyto-
plasm and sometimes throughout the entire protoplast.

A simple form of Blue-green Algae is Gleocapsa shown in Figure
255. This plant, which lives mostly on wet rocks, consists of a
single globular cell with a rather prominent gelatinous sheath
and is about as simple as a plant can possibly be. By the divi-

FIG. 255. — Gleocapsa, one of the
simplest of the Blue-green Algae. A,
single individual enclosed in a heavy
gelatinous sheath and beginning to
divide. B and C show how the
plants as they multiply are held to-
gether by the gelatinous sheath.
X 540. After Strasburger.

FIG. 256. — Portions of three fil-
aments of Oscillatoria. At the left
one cell in the filament has died,
resulting in segmenting the fila-
ment. X 540.

sion of the cell new individuals are formed, which are held
together in loose aggregations by the gelatinous secretion from
their walls.

One of the common colonial forms is Oscillatoria, of which there
are about 100 species (Fig. 256). They form bluish green felt-
like mats in fresh and salt water, and bluish green layers on moist
soil. The colony is a filament, consisting of a large number of
short cylindrical cells joined end to end and enveloped in a thin
gelatinous sheath. Usually the filaments occur together in large
numbers, and often there is enough of the gelatinous secretion to
hold them together in loose aggregations. A characteristic
feature of the plant, as the name suggests, is the swaying and
revolving movement of the filament, which sometimes resembles

BLUE-GREEN ALGAE

299

a tiny worm in its creeping and bending to one side and then the
other. This movement indicates that the cells of the colony of
Oscillatoria work together as a unit and thus the many-celled
colony takes on the character of a many-celled plant where the
cells are closely associated in the activities of the plant.

Another filamentous form (Fig. 257) is Nostoc, which is common
in fresh water and on moist soil. In this plant the cells are
rounded and the filament re-
sembles a chain of beads. Nostoc
secretes an extraordinary amount
of gelatinous substance and forms
jelly-like lumps in which a large
number of the plants are held.
These jelly-like masses are often
more or less rounded, and are of
various sizes up to that of a
marble or even larger. When
growing on soil, they often swell
up and glisten after a rain, on
which account they have been
called " fallen stars."

In Nostoc there is some differ-
entiation of cells. At intervals
in the filament ordinary working
cells enlarge, lose their contents,
and thicken their walls. Being
larger in size and almost colorless,
they are quite distinct from the
other cells of the filament, and
thus divide the filament into sec*
tions called harmogonia. These special cells, called heterocysts,
seem to be concerned with the multiplication of filaments, for
it has been observed that the harmogonia break loose at the
heterocysts, wriggle out through the jelly-like matrix, and de-
velop new filaments.

Another special kind of cell formed in Nostoc is the resting cell,
which is formed when periods unfavorable for the growth of the
plant appear. In this case certain cells of the filament enlarge,
accumulate food, and thicken their walls. These cells are able
to endure cold, drought, and other conditions which are destruc-

FIG. 257. — Nostoc. At the
left are jelly-like lumps of Nostoc
consisting of numerous colonies.
About natural size. At the right
is a single colony, showing the
gelatinous sheath and the hetero-
cysts, the large cells shown empty,
which segment the filament into
harmogonia. X 540.

300

THALLOPHYTES

tive to the ordinary cells of the filament; and, when favorable
conditions for growth return, the protoplast of the resting cell
breaks through the heavy wall and develops a new filament.

In Rivularia (Fig. 258), another filamentous form, the filament
is apparently differentiated into a basal and apical region. A
heterocyst is the basal cell and the cells decrease in size toward
the apex, so that the filament has a whip-like
appearance.

Besides the features just mentioned in con-
nection with the plant body, there are some
other minor ones which some particular species
of Blue-green Algae have. For example, in
one species the cells of the colony arrange
themselves so as to maintain a regular rec-
tangle. In some forms the colony forms a
branched filament.

Food is manufactured, and water and min-
eral matters are absorbed by these simple
plants in essentially the same way as in the
more complex plants, but each cell must
manufacture food and absorb water and
mineral matters for itself. Since these plants
live in water or on a moist substratum, they
are able to absorb water and mineral matters
from their immediate surroundings. Having
chlorophyll, they are able to carry on photo-
synthesis and thereby provide themselves
with carbohydrates. Although the function
of phycocyanin is not known, it is probable
that it assists some in connection with photo-
synthesis. Sometimes there is an additional reddish pigment
developed, which may have something to do with enabling the
plant to utilize the sun's rays in the manufacture of food. The
reddish pigment is so abundant in a few forms that the plants
appear red in mass, as in one group which forms floating colonies
in salt water and has given the name to the Red Sea.

Reproduction in the Blue-green Algae is chiefly by cell division.
They form no sex cells and, therefore, depend entirely upon
vegetative methods of reproduction. By cell division new cells
are formed, which may, according to the species, separate as new

FIG. 258. — A
single colony of
Rivularia consist-
ing of a large hete-
rocyst and many
vegetative cells
which decrease in
size away from the
heterocyst. X 540.

GREEN ALGAE 301

plants, as in Gleocapsa, or remain as a part of a close colony, as
in Oscillatoria and other forms where the cells of a colony are
closely associated. In filamentous forms the method of multi-
plying filaments by means of harmogonia may be classed as a
method of reproduction. In this case a filament breaks into
segments which separate and establish new filaments. The
filament may be segmented by heterocysts or by the death of
ordinary working cells.

The simplicity of plant body, cellular structures, and methods
of reproduction makes the Cyanophyceae the simplest of all
groups of independent plants now in existence. The absence
of chloroplasts and a well-defined nucleus and cytoplasm clearly
distinguishes them from other groups of independent plants.
But in the group some advancement is shown. The formation
of a colony in which the cells are closely associated looks forward
toward the formation of multicellular plants in which the cells
are very intimately associated. Also the differentiation of the
cells of a colony into ordinary working cells, heterocysts, and
resting cells suggests the differentiation of cells in multicellular
plants into tissues.

Green Algae (Chlorophyceae)

The Green Algae are the Algae most commonly seen in our lakes,
ponds, and streams. They usually have only one pigment, chloro-
phyll, and their green or yellow-green color is usually quite
distinct from that of the Blue-green Algae. Some of the Green
Algae are microscopic and some form colonies or multicellular
plant bodies that are clearly visible to the naked eye. Although
they are small plants, large numbers of them commonly occur
together, forming scums or tangles of filaments that are conspicu-
ous. Most of them live in the water but some live on moist
earth, rocks, or wood, and a few forms can endure periods of
drought. A few forms live in salt water, but nearly all are fresh
water plants.

The Green Algae differ from the Blue-green Algae not only
in color but also in a number of other ways. Gelatinous sub-
stances are secreted in abundance only in the lowest forms of
the group, and consequently Green Algae do not commonly
form gelatinous masses. They have chloroplasts, and the

302 THALLOPHYTES

nucleus is well organized and quite distinct from the cytoplasm.
In some, the cells of the colony have their protoplasts joined
by protoplasmic strands and have thus become so closely asso-
ciated that they constitute a multicellular plant. Some repro-
duce entirely by cell division, but many of them have more
specialized methods of reproduction. Many Green Algae form
swimming cells called zoospores, each of which is able to pro-
duce a new plant directly. Others also form gametes or sex
cells which fuse and form a cell that develops a new plant either
directly or indirectly. The simplest gametes occurring in the
group are alike as to size, structure, and behavior and are called
isogametes. When isogametes pair and fuse, a cell called a
zygospore or zygote is formed, and this spore may form zoospores
or develop a new plant directly. The fusion of similar gametes
is called conjugation. The more advanced Green Algae form
morphologically unlike gametes called heterogametes, of which the
large ones are called eggs and the small ones are called sperms.
The spore formed by the fusion of an egg and a sperm, that is, by
the fusion of unlike gametes, is called an oospore and the fusion is
called fertilization. Often the gametes are produced in special
organs called sex organs. It is evident that the Green Algae
resemble the higher plants much more than do the Blue-green
Algae and are, therefore, considered more advanced. It is sup-
posed that from plants like the Green Algae the higher plants
have come.

Among the Green Algae there is much diversity in character
of plant body and methods of reproduction. About 9000
species are known and these are commonly grouped into five
orders — Volvocales, Protococcales, Confervales, Conjugates, and
Siphonales.

Unicellular Motile Green Algae (Volvocales). — These Green
Algae are regarded as one-celled plants, although some of them
form colonies of considerable size and complexity. They live in
the water and chiefly in fresh water. Their vegetative cells have
cilia and swim about like the lower animals. It is this motile
habit that distinguishes them from other Green Algae. On ac-
count of their motility and some animal-like structural features,
they are sometimes regarded as animals. They are microscopic
plants, but some of them form colonies that are sometimes visible
to the naked eye.

CHLAMYDOMONAS

303

Chlamydomonas. — In Chlamydomonas (Fig. 259), which is
regarded as one of the simplest of the Volvocales, the habit of
colony formation is lacking and, therefore, each individual swims
about independently. This plant is common in fresh water and
when seen swimming about under the microscope might be mis-
taken for a protozoan, a one-celled animal which it resembles.

The Plant body consists of a more or less globular protoplast
closely invested by a thin membrane through which the two long
cilia project at the forward end.
There is a large cup-shaped chloro-
plast, in which there is a protein
body called pyrenoid. The nucleus
is in the cup of the chloroplast; at
the base of the cilia are two con-
tractile vacuoles; and not far from
these is the red pigment spot or
eye spot which is supposed to be
sensitive to light and, therefore, of
some use in directing the movements
of the individual. In certain species
a bright red pigment is often so
abundant that, when the plants are
numerous, they cause pools to ap-
pear red and, when blown over the
snow, produce the " red snow " of
arctic and alpine regions.

In the number of cells constituting
the plant body, Chlamydomonas is

as simple as any of the Blue-green Algae, but in having a chloro-
plast and well-defined nucleus and cytoplasm, it shows considerable
advancement.

Reproduction takes place by means of zoospores and gametes.
In forming zoospores the plant becomes quiescent and the proto-
plast divides into two or more ciliated cells which are miniatures
of the parent. These daughter cells or zoospores escape from
the mother cell and enlarge to the parent size. Under certain
conditions the protoplast may form many small zoospore-like
cells which escape from the mother plant and fuse in pairs to
form resting zygospores which later form new plants. Since
these small zoospore-like cells fuse, they are gametes or sex cells

FIG. 259. — Chlamydomonas,
a simple motile Green Algae.
At the left an individual, show-
ing the cilia, the large cup-
shaped chloroplast (c) con-
taining a pyrenoid, the nucleus
(n), the two pulsating vacuoles
(p), and the red pigment spot
represented by a black dot near
the pulsating vacuoles. At
the right an individual which
has formed two zoospores.
X300.

304 THALLOPHYTES

and, since they are alike, they are isogametes. The zygospore,
a spore formed by the fusion of similar gametes as the prefix
(zygo) suggests, is commonly a well-protected spore and, there-
fore, able to resist conditions that are destructive to the zo-
ospores or vegetative cells of the plant. The approach of unfavor-
able conditions commonly induces the formation of gametes and
zygospores. The zygospore remains dormant until favorable
conditions return and then produces a new plant. The zygo-
spore is, therefore, a stage in the round of life in which the plant
is able to survive unfavorable conditions.

Gametes are supposed to be zoospores that are too small to
function alone. By pairing and fusing, the energies of two
gametes are combined in a zygospore which is able to produce
a new plant that neither of the gametes could produce alone.
Thus the zygospore may also be regarded as a cell in which gam-
etes combine their energies, so that they may be effective in pro-
ducing new plants. There are two things which indicate that
gametes are miniature zoospores. First, gametes and zoospores
grade into each other in size. Second, it has been observed that
small zoospores may fuse and, therefore, behave as gametes when
poorly nourished, or grow directly into new plants and, therefore,
function as zoospores when well nourished. Thus a zoospore-like
cell may be a zoospore or gamete according to conditions. Such
is the evidence supporting the theory that sexuality arose through
the fusion of zoospores which, on account of size, or conditions
of light, temperature, food, etc., were unable to function alone.
From this simple isogamous sexuality the more complex heterog-
amous forms of sexuality have followed. Even in some forms
of Chlamydomonas, the gametes pairing often differ some in size
and, therefore, suggest heterogamous sexuality.

Pandorina. — The colony, which is one of the notable features
of the Volvocales, varies widely in different genera, ranging from
16 cells or less up to 20,000 or more. Pandorina, shown in Figure
260, is one of the forms producing simple colonies.

The cells or individuals of which the colony of Pandorina is
formed are similar in structure to Chlamydomonas. Commonly
the spherical colony consists of 16 individuals, held together in
a mucilaginous matrix.

Reproduction differs in some ways from that of Chlamydomonas
on account of the colony formation. Any individual of the

VOLVOX

305

colony may divide into 16 zoospore-like cells which remain
together, escape from the mother colony, and thus become a new
colony. The gametes are formed in essentially the same way as
the individuals of the new colonies, but they separate and thus
swim about independently after leaving the mother colony.
When the zygospore germinates, as shown in Figure 260, there
results a new colony which has only to grow to adult size.

d ' e

FIG. 260. — Pandorina morum. a, Motile colony ordinarily consisting of
sixteen motile cells (X 475); 6, colony in which the cells have formed
daughter colonies (X 475); c, two gametes fusing; d, zygospore; e, zygo-
spore germinating and forming a new colony. Redrawn with modifications
from Oersted.

Among the gametes there is often considerable variation in size and
motility, some being smaller and more active than others. The
gametes pair and fuse regardless of their size, and, when gametes
that are unlike happen to pair, there is a suggestion of heterog-
amy, although there is no distinct differentiation of gametes as
occurs in plants where heterogamy is well established.

Volvox. — The highest expression of colony formation is reached
in forms like Volvox (Fig. 261), where the colony contains thou-
sands of individuals held together in a gelatinous matrix and
so arranged as to form a hollow sphere. The colonies of Volvox
are often as large as a pin head and hence visible to the naked eye.
The two cilia of each individual project from the colony, and by
the lashing of the cilia the colony moves through the water by a
revolving motion. One can often see them slowly moving about
in ditches, ponds, and sometimes in tanks in greenhouses. A
microscopical study of the colony shows that the individuals of
the colony are connected by protoplasmic strands, and hence so

306

THALLOPHYTES

vitally related that the colony of Volvox may be regarded as a
multicellular individual rather than a colony.

Reproduction presents some interesting features. At first all
cells of the colony are alike, but later considerable differentiation
among cells occurs. Some cells of the colony enlarge and pass

FIG. 261. — Volvox. In the colony (Volvox aureus} the smaller cells bear-
ing two cilia are the vegetative cells, the enlarged cells (a) contain sperms,
and the enlarged cells (o), varying in size and stages of development, are
eggs (X 300). Below and at the right is a sperm, and below and at the
left is a oospore of Volvox globator. After West.

into the hollow of the sphere where they form new colonies which
escape and grow to adult size. Sexual reproduction in Volvox
is heterogamous, for two distinct kinds of gametes are involved.
Some of the cells enlarge, lose their cilia, and become filled with
food. They are the female gametes or eggs. Other cells of the
colony form numerous small motile gametes or sperms which seek
the eggs and fuse with them. Fertilization, as this fusing is
called since the gametes are differentiated into eggs and sperms,

PLEUROCOCCUS

307

occurs within the hollow of the sphere. The spore formed from
the fusion, now known as an oospore (meaning eggspore), forms a
new colony upon germination.

There is much advantage gained by differentiating gametes.
The egg, owing to its size and loss of motility, can store much
food for the next generation. The smallness of sperms makes it
possible for large numbers of them to be produced, and promotes
their movements through water.

In summarizing the Volvocales, the following features are the
notable ones. The plant body consists of a single motile cell
having a chloroplast and well-defined nucleus and cytoplasm.
Some swim about independently, but the formation of colonies
is a marked feature of the group, and the colonies range from
simple to complex ones. By the division of cells new individuals
and new colonies are formed. Sexual reproduction advances
from isogamy to heterogamy. As
in the Blue-green Algae, the forma-
tion of colonies is a step toward
the formation of multicellular in-
dividuals.

Unicellular Non-motile Green
Algae (Protococcales). — In con-
trast to the Volvocales, the absence
of cilia, except on reproductive cells,
is a notable feature of this group.
Some plants of this order are very
common on damp soil, walls, and on
the bark of trees, where they are
often exposed to long periods of
drought. Most of the group are
aquatic and occur mainly in fresh
water. Some enter into the forma-
tion of Lichens. Others are endo-
phytic, living in the intercellular
spaces of other plants, and some give the green color to certain
animals, such as the hydra and fresh-water sponge, which eat
them. They show considerable variation in their habit of form-
ing colonies and in methods of reproduction.

Pleurococcus. — Pleurococcus (Fig. 262}, often called Protococ-
cus, is the simplest plant of the group, and may be regarded as

FIG. 262. — Pleurococcus vulr
gari's. Above, a single plant
consisting of a single cell with a
definite wall, well defined nucleus,
and large lobed chloroplast; be-
low, left, plants dividing; and
below, right, a group of four
separate plants. X 540. After
Strasburger.

308

THALLOPHYTES

one of the simplest of the Green Algae. It forms green coatings,
resembling green paint, on flower pots, damp earth or walls, and
on the trunks of trees. It is a single, globu-
lar, non-motile cell. It has a definite wall,
a large lobed chloroplast suggesting several
chloroplasts, and its nucleus and cytoplasm
are well defined. It reproduces entirely by
cell division, thus forming no zoospores or
gametes. They are small plants and a mass
of them perceptible to the eye consists of
numerous individuals. They divide rapidly
when conditions are favorable, and daughter
cells recently formed and not yet separated
are usually seen when a mass of individuals
is observed with the microscope.

Scenedesmus. -- This form, which is
common in fresh water, is often classed with
the Protococcales. The individuals form
simple colonies with the individuals usually
arranged in a row as shown in Figure 263. There are no zoospores

FIG. 263. — Scene-
desmus. Above, a
colony of Scenedesmus
quadricanda consisting
of four cells arranged
in a row; below, a cell
of the old colony form-
ing a new colony.
X 600. Drawn from
West.

FIG. 264. — Pediastrum boryanum. At the left, the plate-like colony of
cells, some of which have formed zoospores and from one of which the
zoospores are escaping; at the right, zoospores arranging themselves into
a new colony. X about 400. From Braun.

or gametes, but reproduction is effected by the division of each
cell into daughter cells which escape as a new colony.

HYDRODICTYON

309

Pediastrum. — A more complicated colony occurs in Pedi-
astrum (Fig. 264}, another form common in ponds and other quiet
waters in warm weather. The cells, which are quite numerous
in some species, form plate-like colonies in which marginal cells
differ in form from those within.

Both zoospores and gametes are produced in this form. Any
cell may form zoospores, which escape from the mother cell
enclosed in a membrane and then arrange themselves into a new
colony. Instead of zoospores the cells may form gametes, which

FIG. 265. — Water-net, Hydrodictyon reticulatum. a, portion of a net
(X about 2); b, a cell which has formed zoospores; c, the zoospores formed
into a small net within the mother cell; d, a cell in which gametes have
formed; at the left of the opening through which the gametes are escaping
two gametes are shown fusing.

resemble zoospores but are smaller and more numerous. The
gametes, since they are alike, form zygospores, and each zygo-
spore upon germination produces a new colony.

Hydrodictyon. — This is the remarkable Water-net, in which
the cylindrical colonies, often a yard or more in length, comprise
thousands of cells so joined as to enclose polygonal meshes and
thus form a net as Figure 265 shows. These massive colonies,
buoyed up by bubbles of oxygen caught within them, often form
extensive floating mats in lakes, ponds, and sluggish streams.

New nets may arise from zoospores or from zygospores. When
a cell reaches a certain size and other conditions are right, its
protoplast divides into thousands of zoospores. These zoospores
do not escape but, after swimming about for a time in the mother

310

THALLOPHYTES

cell, they so arrange themselves and grow together at points of
contact as to form a miniature net. Through the softening and
decay of the wall of the mother cell, the small net is set free
and by the mere enlargement of its cells becomes a colony of
adult size. The gametes are isogamous and are formed in great
numbers by certain cells. As many as 100,000 of them may be
produced within a cell. Almost as soon as formed they escape

from the mother cell and begin to pair
and fuse. The zygospore produces
zoospores which at first pass into a rest-
ing stage and later from new nets.

Thus in the Protococcales the individ-
uals may remain separate or form colo-
nies which are exceedingly complex in
the higher forms. In the simplest forms,
asPleurococcus illustrates, reproduction
is by cell division in which the parent
divides to form two new plants, but in
the higher forms there is reproduction
by zoospores and isogametes. Since
their sexuality does not reach the heter-
ogamous condition, they are not so ad-
vanced in this respect as the Volvocales
are, but they lack motility and this
feature is characteristic of the higher
plants, which are adapted to live on land
rather than in the water.

Confervoid Algae (Confervales). — The Confervales or Con-
fervoid Algae are among the most familiar of the Green Algae.
Their plant bodies are usually filaments, commonly consisting
of much elongated cylindrical cells closely joined end to end in
a single row. The filaments may be several inches in length
and in some forms much branched. In a few forms the plant
body is plate-like instead of filamentous, as the Sea Lettuce illus-
trates (Fig. 266). The Confervales are common in lakes, ponds,
streams, and water troughs, where many of them grow attached
and form green hair-like fringes about rocks and other ob-
jects. More than 700 species of them are known, and there
is considerable variation in plant body and methods of repro-
duction.

FIG. 266. — Sea Lettuce
(Ulva), a Confervoid Alga
having a plate-like plant
body. This plate-like plant
body is two layers of cells
in thickness, bright green,
and resembles a leaf in
form. Natural size. Re-
drawn from Bessey.

ULOTHRIX

311

Ulothrix. — Ulothrix (Fig. 267) is one of the simpler forms of
the group, and its filaments, an inch or two in length, form bright
green fringes about stones and other objects in lakes, ponds,
streams, and troughs. There is some differentiation within the
filament, for the basal cell is modified into a holdfast by which
the filament is attached to a support. The other cells are alike
and each contains one nucleus and a large encircling chloroplast.

FIG. 267. — Ulothrix zonata. A, portion of a filament, showing the hold-
fast and a number of vegetative cells; B, portion of a filament, show-
ing three cells containing gametes; C, a portion of a filament, showing
gametes escaping at b, and zoospores formed at a and escaping at c; D, a new
filament developing from a zoospore, the character of which is shown at z.
At g gametes are shown fusing to form zygospores. At zy a zygospore, just
after the fusion of the gametes and when fully mature, is shown. A zygo-
spore which has germinated and produced four zoospores is shown at y.
X 200-300. Redrawn with modification from Coulter arid from Strasburger.

The plant reproduces asexually by four-ciliate zoospores, and
sexually by two-ciliate isogametes. The zoospores are formed usu-
ally two or more in a cell. They escape together from the mother
cell enclosed in a membrane, but soon separate and after swim-
ming about for a short time become attached to some object by
the ciliated end and by growth and cell division become new
filaments. Some cells produce gametes, which, besides having
only two cilia, are muoh smaller and more numerous than zo-
ospores. After escaping, the gametes fuse in pairs to form resting
zygospores. Upon germination, the zygospore does not pro-

312

THALLOPHYTES

duce a new plant directly but, as in Hydrodictyon, produces
a number of zoospores each of which produces a new plant.
Thus, instead of one, a number of new plants arise from the
zygospore, a feature of advantage in the multiplication of new
plants.

Another form, similar in a number of ways to Ulothrix, is

Cladophora which has long
branched filaments that
form long, green, hair-like
tufts, which, with one end
anchored to a stone or some
other object, wave back and
forth in moving streams.
The cells are multinucleate
and contain many chloro-
plasts. Reproduction is by
zoospores and isogametes,
but ^the zygospore develops
a new plant directly.

Oedogonium. — This form
(Fig. 268], common in lakes
and ponds, is similar to Ulo-
thrix in the character of the
filament, but shows marked
advancement in methods of
reproduction. The z o -
ospores, formed only one in
a cell and consequently very
large, have numerous cilia
forming a crown at the for-
ward end. Sexual reproduc-
tion is distinctly heteroga-

FIG. 268. — Oedogonium. A , a portion
of a filament of Oedogonium echinosper-
mum, showing some vegetative cells and
oogonium above and some antheridia be-
low from which sperms are escaping; B, a
portion of a female filament of OEDOGO-
NIUM HUNTIL showing oogonia and
two dwarf male plants attached near the

oogonia; C, zoospores of an Oedogonium
escaping from the cells of the filament.
X about 300. Drawn from Wolle.

mous. The eggs, which are
large and packed with food,
are borne in much enlarged
cells called oogonia. Each oogonium bears one egg and is simply
a transformed vegetative cell of the filament. Other small cells
produce the sperms which resemble the zoospores except in size.
The sperms swim to the oogonia, enter, and fertilize the eggs and
thick-walled resting oospores are then formed. Upon germina-

COLEOCHAETE

313

tion the oospore forms four zoospores, each of which develops a
new filament. In some forms of Oedogonium there are both
male and female filaments. In some species the male plants are
miniature filaments and attach themselves to the female plants,
where they produce sperms in their terminal cells.

Coleochaete. — This form (Fig. 269), found growing attached
to water plants, has a disk-shaped plant body and also presents
some new features in connection with its reproduction. Like
Oedogonium it reproduces by zoospores and sexually by oospores.
One of the new features is
the development of a case
around the oospore by the
adjacent cells. This fea-
ture suggests a close rela-
tionship of this form to the
higher Algae, where the
formation of a case around
the immediate product of
the oospore is a prevalent
feature. The second new
feature is that the oospore
upon germination develops
neither a plant nor zo-
ospores, but a structure
consisting of several cells
each of which develops a

FIG. 269. — Coleochaete scutata. A, the
plate-like plant body with two oogonia
developed (X 25) ; B, thick-walled oospore
surrounded by vegetative cells (much en-
larged) ; C, a much enlarged section through
the oospore and its jacket of sterile cells,

showing the multicellular body produced
by the oospore, each cell of which pro-
duces a zoospore. Redrawn from Wolle,
Atkinson, and Altmanris.

zoospore from which a new
plant arises. Thus between
fertilization and the de-
velopment of new plants,
there is introduced a new structural stage and one that is char-
acteristic of higher plants. These new features with others have
led to the theory that the higher plants have evolved from Algae
of the type of Coleochaete.

In having multicellular plant bodies and more advanced
methods of reproduction, the Confervales, as a group, show
advancement over the preceding groups. The plant body is a
simple filament, branched filament, or a disk-shaped structure.
Sexual reproduction, which is isogamous in the lower forms,
advances to heterogamy where the two kinds of gametes occur in

314

THALLOPHYTES

special cells and often on different plants. Also in the higher
forms, the introduction of a case around the oospore, and a new
structural stage between fertilization and the formation of new
plants, suggests a relationship to the higher plants. On the other
hand, the simpler forms resemble some of the Protococcales from
which the Confervales have probably been evolved.

Conjugating Algae (Conjugates). — This group is so named
because of the peculiar conjugating habit, in which the contents
of two cells fuse to form zygospores. Some are unicellular but
many are filamentous. They include Spirogyra and others that

FIG. 270. — Desmids. a and 6, two common species of Desmids highly
magnified; at the right of c, a Desmid dividing, and at the left of c, each
daughter cell resulting from the division developing a new half; at d, the pro-
toplasts of two Desmids are escaping and conjugating. Redrawn from Curtis.

are very common nearly everywhere in fresh water. They are
free floating, and the filamentous forms often form extensive
floating mats, which are buoyed up by the oxygen entangled
among the filaments. Some, owing to the shape and arrange-
ment of their chloroplasts, are attractive plants under the micro-
scope. One peculiar feature of the group is that, although the
plants are aquatic, there are no ciliated cells of any kind.

Desmids. — The simplest of the Conjugates are the Desmids,
which are unicellular floating plants that exhibit a variety of
shapes arid some are extremely beautiful (Fig. 270}. They are

SPIROGYRA

315

abundant fresh water plants, and in the examination of other
forms of fresh water Algae with the microscope one usually finds
some Desmids present. The cell is peculiar in being organized
into symmetrical halves, which are separated by a constriction
that forms an isthmus. The nucleus is in the isthmus, and in
each half there is a chloroplast and a number of pyrenoids.

They reproduce in two ways, by cell division and by zygo-
spores. In multiplying by cell
division, the cell divides at the
isthmus, the halves separate, and
the portion of the isthmus re-
maining to each half develops a
new half and thus a new individ-
ual is formed. In sexual repro-
duction the cells pair and the
protoplasts, which escape through
ruptures at the isthmus, fuse and
form a zygospore. Sometimes
the cells after pairing become con-
nected by a tube through which
the protoplasts reach each other.
In either case the entire proto-
plasts of cells conjugate.

Spirogyra. -- Spirogyra (Fig.
271), very common in ponds,
sluggish streams, and watering
troughs, is the most familiar fila-
mentous form of the Conjugates
and the one most commonly
studied in elementary classes. It
gets its name from its large and
beautiful spiral chloroplasts. Its
cells are all alike and it pro-
duces no zoospores. Its sexual reproduction, in which the gam-
etes reach each other through tubes, is its important feature.
Under certain conditions, filaments pair and line up side by side.
In this position, the cells of the filaments grow toward each
other in tubular projections which unite and form open passage
ways between the cells of the paired filaments. The protoplasts
of one filament pass through these tubes and fuse with the pro-

FIG. 271. — A species of Spiro-
gyra. A, a portion of a filament
showing a vegetative cell with its
spiral chloroplasts (c) and nucleus
(n) (X 100); B, filaments conju-
gating and two zygospores (z)
fully formed; C, a zygospore
germinating and producing a new
filament (X 150). A and B from
nature, and C from Wolle.

316 THALLOPHYTES

toplasts of the other filament as shown in Figure 271. The
protoplasts are unlike, for one migrates while the other does
not. In behavior the migrating protoplasts may be regarded
as sperms and the passive ones as eggs, although they show no
differentiation in size or structure. Also, the filament which
loses its protoplasts may be regarded as male and the receiving
filament as a female individual. The zygospore builds about
itself a heavy wall and at the end of a rest period develops
directly into a new filament.

It is now seen that the Conjugales stand quite apart from the
previous groups in having no zoospores or swimming gametes,

FIG. 272. — A species of Vaucheria (Vaucheria sessilis), showing the
coenocytic habit of the filament, the oogonia at o, the antheridia at a, and the
sperms escaping from an antheridium and entering an oogonium at s. X 75.
Partly drawn from nature and partly diagrammatic.

and in having a peculiar kind of conjugation, in which entire
protoplasts fuse and commonly reach each other through tubes.
Although the gametes are alike in size and structure, they show
some differentiation in the way they behave. The group is
considered a highly specialized one.

Tubular Algae (Siphonales) . — These Algae, of which there are
about 300 species, are so named because the plant body, no matter
how long and thread-like, has no cross walls and, therefore,
resembles a tube filled with protoplasm. The protoplasm con-
tains many nuclei and many chloroplasts, and may be regarded
as a much elongated multinucleate cell or as a filament with cross
walls omitted. Such a plant body is called a coenocyte. The
majority of the Siphonales are marine forms, living in warm seas,

VAUCHERIA

317

but there are a number of species living in fresh water and some
on moist shaded soil.

Vaucheria. — Vaucheria (Fig. 272) is one form of the Sipho-
nales that is common in fresh water and on moist shaded soil.
The long filaments, usually much coarser than those of Spiro-
gyra and usually branched, interlace and form felt-like masses,
on which account Vaucheria is often called Green Felt. The
green or yellowish green felt-
like mats of the species grow-
ing on moist soil are common
in flowerpots and on and under
the benches in greenhouses.
Other species are common in
ponds and sluggish streams.

Vaucheria forms zoospores
and heterogametes. In form-
ing a zoospore a portion of pro-
toplasm at the end of the fila-
ment is cut off from the rest
by a cross wall. This severed
mass of protoplasm escapes
from the filament as a multi-
nucleate and multiciliate zo-
ospore, large enough to be seen

y ' FIG. 273. — Botrydium granulatum.

with the naked eye. After At the left, the vegetative plant body,
swimming about for a time showing the root-like projections be-
the zoospore comes to rest and low and the balloon-like top above
elongates into a new filament, ground; at the right, a plant in which

Sexual reproduction shows Zo6spo^ have *°rmed an d are ^f
. ing; between the enlarged plants,

advancement in that the plants about natural size. Drawn with
gametes are borne in well-de- modifications from Wolle.
fined sex organs, which are

special structures for bearing sex cells. The oogonium, oval in
shape, bears one large egg, and the antheridium containing many
sperms is near it and is the end cell of a short curved branch. The
sperms escape, reach the egg through a special opening in the
oogonium, and one of them fertilizes the egg. The heavy-walled
oospore upon germination forms a new filament directly.

There are, however, some Siphonales in which sexual reproduc-
tion is of a simpler type. For example, in Botrydium (Fig. 273),

318 THALLOPHYTES

a form with a small balloon-shaped plant body, which is commonly
found projecting from moist soil, there are no sex organs and the
gametes are alike.

The Siphonales are most peculiar in having a tube-like plant
body. In the production of well-defined sex organs they show
considerable advancement in sexual reproduction.

Summary of Green Algae. — In having chloroplasts and well-
defined nucleus and cytoplasm, the cells of the Green Algae are
more advanced than those of the Blue-green Algae. In the
simplest forms the plant body is a single cell, either motile or
non-motile, and the uniting of cells into colonies is a prominent
feature. In the higher forms the plant body is multicellular and
in the form of a filament or disk. The multicellular forms have
not only established the habit of cells living together to form a
complex plant body but have also to some extent differentiated
cells. These habits look forward toward the formation of plants
consisting of a countless number of cells, which are differentiated
into tissues, such as occur in the higher plants. They introduced
sex cells which were at first alike, and later differentiated the sex
cells, thus introducing eggs and sperms, the sex cells of the higher
plants. They also introduced sex organs which are prominent
structures in the Mosses and Ferns.

Brown Algae (Phaeophyceae)

These Algae are marine forms, occurring on all sea coasts but
more abundantly in the cooler waters. They have two pigments,
chlorophyll and a brown pigment called fucoxanthin, but the
brown pigment obscures the green one and determines the color
of the plant. Both pigments probably help in the manufacture
of food, although the exact function of fucoxanthin is not known.
These Algae grow anchored by holdfasts to the rocks and their
bodies are so tough and leathery that they are not injured by the
beating of the waves.

Although a few are simple filaments, most of them are much
more complex than any of the Green Algae. The plant body of
the majority of them not only consists of a greater number of
cells, but there is more differentiation among the cells than in the
Green Algae. In the largest of them the plant body often attains
a length of several hundred feet. As shown in Figure 274, the

LAMINARIAS

319

plant body commonly consists of a stalk bearing leaf-like branches
and attached to a support by root-like holdfasts. One might
think such a plant too complex to be classed as a Thallophyte,
for, according to definition, a Thallophyte is a plant not differen-
tiated into roots, stem, and leaves. However, when the struc-
ture of these parts that so much resemble roots, stems, and leaves
is studied, one finds that they are too simple in structure to be
classed as such organs, although they mark a notable advance-
ment over the Green Algae in the differentiation of the plant body.
Some have special swollen regions called air bladders, which help

FIG. 274. — One of the Brown Algae, Macrocystis, showing the root-like
holdfasts, the stem-like axis, and the leaf-like blades. Much reduced. Re-
drawn with modifications from Harvey.

the plant to float, and in connection with reproduction there is
much differentiation shown by some forms.

There are about 1000 species of Brown Algae known and these
are divided into two groups, one of which comprises the Kelps
and closely related forms, and the other, the Rockweeds and Gulf-
weeds.

Kelps and Closely Related Forms (Phaeosporales). — This
order comprises a number of families of which the Laminarias or
Kelps are the largest forms.

Laminarias. — These are the largest of Algae, and include such
conspicuous forms as Nereocystis, Postelsia, or Sea Palm, and
the huge Macrocystis (Fig. 274), which is sometimes more than
200 feet in length. It is from the massive plant bodies of the

320

THALLOPHYTES

Kelps that much potassium for fertilizers is obtained (Fig. 275).
Although the Kelps have massive and complex plant bodies,
their reproduction, so far as known, is not so complete as that of
some Green Algae. Their reproduction is sexual and the small
ciliated gametes are borne in special cells which occur in patches

FIG. 275. — Harvesting Kelp on the Pacific Coast. From Report 100,
U. S. Dept. of Agriculture.

on the surfaces of the leaf-like branches. The zygospore develops
a new plant directly.

Ectocarpus. — This form (Fig. 276), although belonging to the
same order, contrasts strikingly in size with the Kelps, for it is a
slender filamentous form not much larger than some of the Green
Algae. This form also shows some interesting features in connec-
tion with reproduction which is effected through the production
of both zoospores and gametes.

The zoospores are produced in certain cells which become trans-
formed into sporangia. In forming a sporangium, a single cell
within the filament or at the end of a branch usually enlarges
and its protoplasm divides up into zoospores. The zoospores
bear their cilia laterally and not terminally as in the Green
Algae, but function in the same way by growing directly into
new plants.

ROCKWEEDS AND GULFWEEDS (FUCALES)

321

The gametes are produced in a multicellular structure known
as a gametangium, and, as in case of a sporangium, the gametan-
gium may be formed from a cell within the filament or from a
terminal cell on a short lateral branch. The small cubical cells
composing a gametangium are packed closely together and each

FIG. 276. — Portions of two fila-
ments of Ectocarpus, showing repro-
duction. At s are shown spo-
rangia, and between the filaments,
a zoospore. At g are shown a game-
tangium and a single sperm and two
sperms fusing at the right . Redrawn
with modifications from Curtis.

FIG. 277. — Plant body of
Fucus vesiculosus (X I). In
the swollen tips are the con-
ceptacles in which the sex
organs occur, and at various
places occur bladder - like
floats.

produces a biciliate zoospore-like gamete. After escaping the
gametes fuse in pairs and form zygospores. In this plant such
gradations occur between sporangia and gametangia and between
zoospores and gametes, as to afford considerable support for the
theory that gametes are simply zoospores which are too small
to function alone.

Rockweeds and Gulf weeds (Fucales). — In this order the
plant body may reach a meter in length, but is usually much

322

THALLOPHYTES

smaller. Although strictly aquatic, they produce no zoospores
and their sexual reproduction is much specialized.

Rockweeds. — These are very common Seaweeds and are
especially abundant on rocky shores. The plant body, sometimes
a foot or more in length, is much branched and has bladder-
like floats and commonly special reproductive structures. The
Rockweeds are common in fish markets, being used as a packing

FIG. 278. — Reproduction in Fucus vesiculosus. a, section through a
swollen tip, showing sections through some of the conceptacles; 6, much
enlarged section through an oogonial conceptacle, showing the pore-like open-
ing to the exterior and the oogonia within; c, a similar section through a
conceptacle containing antheridia which appear as small bodies on the fila-
ments projecting from the walls of the conceptacle; d, antheridia much en-
larged and one antheridium shedding its sperms; e, oogonium from which
the eggs are escaping; /, sperms swarming around an egg.

in the shipment of crabs and other shell fish. Along the west
coast of South America and also in other countries, Fucus is used
for food by the inhabitants, and it is also used as a fertilizer and
as a source of iodine.

Fucus vesiculosus, one of the commonest of the Rockweeds, will
serve to illustrate the character of the plant body and the peculiar
features of reproduction, the former being shown in Figure 277
and the latter in Figure 278. The gametes are differentiated

GULFWEEDS (SARGASSUM)

323

and are borne in sex organs, which are also quite unlike. The sex
organs are borne in hollow conceptacles, which occur in large
numbers in the swollen tips of the branches. Each conceptacle
opens to the exterior by a pore-like opening. In this species the
male and female sex organs do not occur together in the same
conceptacles as they do in some species. The oogonium is a
large, globular, stalked cell and in this species contains eight eggs,
but the number ranges from one to eight in other species of
Fucus. The antheridia are borne
on the lateral branches of much-
branched filaments, which pro-
ject from the wall of the concep-
tacle. They are oval cells which
produce numerous sperms. A
curious feature of Fucus is that
the eggs as well as the sperms are
discharged from the conceptacle
before fertilization. The eggs
while passively floating about
are surrounded by swarms of
sperms, which sometimes, by their
vigorous movements, give the
eggs a rotary motion. In fertil-
ization one sperm, after pene-
trating the cytoplasm of the egg,
fuses with the egg nucleus and
thus an oospore is formed which
develops a new plant.

Gulfweeds (Sargassum).—
The Gulfweeds, well known in
connection with the Sargasso Sea, are sometimes a meter in
length and are more differentiated than the Rockweeds (Fig.
279}. In form the stalks and leaf -like branches resemble very
much the stems and leaves of the higher plants, although they
are very different in structure. The stems are at first anchored
by root-like holdfasts and bear many stalked air bladders, which
buoy up the plant when attached and float it when torn free.
Other short, thick, axillary branches contain the conceptacles.
So far as known their reproduction is similar to that of the Rock-
weeds.

FIG. 279. — A portion of a plant
of Sargassum vulgare, showing the
floats and the stem- and leaf-like
structures. X \.

324 THALLOPHYTES

It is now evident that among the Brown Algae there is more
differentiation of plant body and more specialization in sexuality
than among the Green Algae. Except in their lowest forms, they
show no affinities with the Green Algae and consequently are
supposed to have originated independently and probably from
such organisms as gave rise to the Green Algae. Unlike the Green
Algae they show no evidence of having led to higher forms.

Red Algae (Rhodophyceae)

These Algae are mainly marine forms, although some forms
occur in streams. Besides the green they have a red pigment
called phycoerythrin which determines their color. Some also

FIG. 280. — A finely branched Red Alga. Natural size.

have a blue pigment, phycocyanin. They are not so bulky as the
Brown Algae, but they exceed them in number of species and are
much more diversified in form. Some are mere filaments no
larger than Vaucheria. They live mostly below low water mark
and often at depths of more than 100 feet in clear waters.

The plant body, commonly only a few inches in length, is flat,
thin, and flexible and usually much branched. Some kinds are
finely branched, as the Sea Mosses noted also for their beautiful
colors of red, violet, and purple (Fig. 280). As in the Brown

RED ALGAE (RHODOPHYCEAE)

325

Algae, the plant body is commonly differentiated into parts
similar in form, although not in structure, to the roots, stems,
and leaves of the higher plants. The cells are commonly ar-
ranged in such definite lines that the plant body has the appear-
ance of a bundle of closely joined simple filaments. The evident
protoplasmic connections between cells and the gelatinization of
cell walls are other notable features.

FIG. 281. — Irish Moss, Chondrus crispus, much used for food. Natural size.

The life history of some of them is quite complex. They have
spores, but their spores and likewise their gametes have no cilia,
a curious feature since these plants are wholly aquatic. The
female sex organ is multicellular and more complex than the sex
organs of the Brown Algae.

Several forms of the Red Algae are of economic importance.
Some are used as food, being dried and kept for long periods
The gelatinous material obtained from Red Algae forms a delicacy

326

THALLOPHYTES

much desired by some people. The form called Irish Moss,
shown in Figure 281, is collected in large quantities and employed
in the manufacture of jelly, which is used directly as food and as
the basis for the preparation of other foods. Agar-agar, which
is used as a medium in which Bacteria and Fungi are grown, is a
gelatinous product obtained from Red Algae.

Nemalion. — This plant is one of the simpler forms of Red
Algae. The plant body is a rather soft, cord-like, branching

FIG. 282. — Reproduction in Nemalion. A, a portion of Nemalion, bearing
antheridia at a and at the right a procarp consisting of carpogonium (c) and
trichogyne (f) to the tip of which two sperms have become attached; B,
fertilized carpogonium beginning to develop branches on the ends of which
the carpospores are borne; C, mature cystocarp consisting exteriorly of carpo-
spores. X 100-150. Redrawn with modifications from Bornet.

structure, composed of a large number of filaments, which are
held together by a stiff gelatinous substance. There is a central
axis of delicate threads and an outer region consisting of short
branches pointing outward.

Nemalion produces both spores and gametes (Fig. 282). The
two kinds of reproduction are intimately related, for the produc-
tion of spores follows closely as a result of fertilization.

The female sex organ, which is a very peculiar structure in the
Red Algae, is called a procarp. In Nemalion the procarp is borne

POLYSIPHONIA 327

on the end of a branch and at first apparently consists of two
cells, a basal one called carpogonium and a much elongated
terminal one called trichogyne. The two cells are not separated
by a wall and the nucleus soon disappears from the trichogyne
and the two cells then appear as a single one with a bulbous base
and a hair-like extension. The carpogonium corresponds to the
oogonium in other Algae, for it contains a protoplast which func-
tions as an egg.

The antheria, which are borne in clusters at the ends of short
branches, are single cells, and the protoplast of each a'htheridium
becomes binucleate and functions as a sperm. After these
binucleate protoplasts are discharged from the antheridia, they
depend upon water currents to carry them to the female sex
organs as they have no cilia. When they come in contact with
the trichogyne, the two walls in contact are resorbed, and the two
male nuclei of the sperm pass into the trichogyne through the
perforation. A number of sperms may discharge their nuclei
into the same trichogyne, but only one male nucleus passes on
into the carpogonium and fuses with the female nucleus. After
fertilization, the carpogonium develops numerous short filaments,
each of which bears a spore, called a carpospore, at its tip. The
carpospores, short filaments, and the carpogonium together con-
stitute the structure known as ,a cystocarp. The carpospores
upon germination develop sexual plants, thus completing the life
history.

Polysiphonia. — This plant (Fig. 283} is a representative of
the complex forms of Red Algae. It is a much-branched complex
filament and is so named because it has a central row of elongated
cells (axial siphon), enclosed by peripheral cells. This plant
presents much differentiation and ordinarily a life history in-
volves three types of individuals — male, female, and sexless
plants.

The male plants bear their antheridia on very short lateral
branches which arise from the axial siphon and bear the an-
theridia somewhat laterally on their tips. The protoplast of an
antheridium contains only one nucleus and is not discharged as
in Nemalion, but the antheridium breaks off bodily and is floated
to the trichogyne.

The female plant produces a procarp more complex than that
of Nemalion. The procarp consists of other cells in addition to

328

THALLOPHYTES

the carpogonium and trichogyne. The pericentral cell, the large
cell of the axis from which the carpogonium arises and the vege-
tative cells, known as auxiliary cells, surrounding the carpogo-
nium take part in forming the cystocarp and are therefore con-
sidered a part of the procarp. So in polysiphonia a procarp

FIG. 283. — Polysiphonia violacea. A, a part of a plant showing the branch-
ing and multicellular character of the filament ( X 75) ; B, a branch bearing
antheridia, some of which have broken away (X 400); C, branch bearing
a procarp consisting of carpogonium and adjacent cells at c and trichogyne
(0 to the tip of which a sperm is attached ( X 500) ; D, branch bearing a
mature cystocarp (cy) from which carpospores are shown escaping through
an opening in the jacket of the cystocarp (X 75); at the right is a part
of a tetrasporic plant bearing three tetrasporangia (X 100).

consists of trichogyne, carpogonium, pericentral cell, and auxiliary
cells.

After fertilization, which is essentially the same as in Nemalion,
the carpogonium, pericentral cell, and auxiliary cells unite in
forming a large chamber from which lobes arise, and on the ends
of these lobes the carpospores are produced. In the meantime
vegetative cells, growing up from below, form a jacket which en-
closes the spore-bearing structure, thus completing the formation
of the cystocarp (meaning a fruit case), which in this plant is a
genuine cystocarp. From this cystocarp the carpospores escape
and upon germination produce an asexual or tetrasporic plant.

FLAGELLATES 329

The tetrasporic plant in character of plant body is very similar
to the sex plants. On it no sex organs occur. It bears spores
known as tetraspores, so named because the number occurring in
a sporangium is four. Why the plant is called a tetrasporic plant
is now clear. The sporangia, which have a one-celled stalk, arise
laterally from the axial siphon and push their way through the
peripheral cells. The tetraspores escape and upon germination
give rise to plants that bear sex organs, either antheridia or
procarps.

This type of life history in which sexual plants alternate with
asexual plants is a feature of considerable significance because
it is a feature characteristic of plants above Thallophytes. It
is known as " alternation of generations" and its significance will
be explained in the groups where it is a well-established feature.
The alternation of generations, the cystocarp, and more complex
female sex organs are the chief features introduced by the Red
Algae.

When Polysiphonia is compared with some of the simplest
forms of Algae, as some of the one-celled Blue-green Algae or even
Pleurococcus, it is obvious that the Algae made notable advance-
ments. The plant body, a single cell in the simplest forms,
becomes a multicellular plant body showing considerable differ-
entiation of parts as to form and function in the higher forms.
Reproduction, accomplished entirely by cell division in the
simplest Algae, gradually becomes more complex through the
groups, involving zoospores, gametes, the differentiation of
gametes, and the development of sex organs.

There is very little reliable data as to how each group of Algae
arose. It is not probable that they arose from each other, but
probably all have developed independently from some unknown
ancestor. Regardless of how they arose, the groups mark in a
general way some of the steps in the evolution of the higher
plants.

Some Alga-like Thallophytes not Definitely Classified

There are three groups of alga-like plants, the Flagellates, Di-
atoms, and Stoneworts, which have not been definitely classified.

Flagellates. — These are free-swimming unicellular organisms
of fresh water. They have both plant and animal features, on
which account they are regarded as intermediate between the

330

THALLOPHYTES

plant and animal kingdoms. The protoplast is naked or in-
vested by a membrane which usually contains no cellulose.
They are commonly abundant in stagnant water and among
Green Algae some are usually present.

Euglena represented in Figure 284 is one of the most common
of the 300 or more species and will serve to show the structure
and habits of the group. Euglena is quite commonly seen

swimming about under the
microscope when Algae are be-
ing examined. The slender
unicellular body bears a long
terminal flagellum, has a chlo-
roplast, eye-spot, and pulsating
vacuole. These structures are
characteristic of the Algae,
such as Volvocales and also of
protozoa, the one-celled ani-
mals. No sexuality is known,
and multiplication is effected
by longitudinal fission, a
method characteristic of the
lower animals. At the ap-
proach of unfavorable condi-
tions, as in autumn, it trans-
forms itself into a thick-walled
resting spore which germinates
and produces one or more new
plants when favorable condi-
tions return. Although it

FIG. 284. — A common species of
Euglena (Euglena gracilis). At the
left, an adult individual, showing the
flagellum, the pulsating vacuole (p\
the chloroplast (c), and the nucleus
(n) ( X 650) ; at the right and below,
Euglena in the spore stage (X 1000);
at the right and above, a spore germi-
nating and producing four new indi-
viduals (X 1000). Redrawn from
Zumstein.

usually makes its own food,
sometimes Euglena loses its

chlorophyll and lives on organic solutions as a saprophyte, thus
demonstrating that the saprophytic may readily originate from
the independent habit.

Many of the Flagellates change their forms readily like the
Amoeba. Sometimes the individuals form colonies of various
shapes and often variously branched.

Such features as the possession of chlorophyll and the forma-
tion of thick-walled resting spores suggest a relationship of the
Flagellates to plants, while their swimming habits, amoeboid

DIATOMS

331

movements, reproduction by longitudinal fission, and such
structures as contractile vacuoles and red pigment spots suggest
a relationship to the animal kingdom. Consequently, they are
regarded as a transition group between plants and animals.

Diatoms. — These one-celled plants are often classed with the
Brown Algae on account of their brown pigment, although they
differ from the Brown Algae in a number of ways. The Diatoms
are a vast assemblage of plants varying widely in form and
occurring in vast numbers in fresh water, salt water, and on damp
soil. They float or swim commonly on the surface of water and
often in such vast numbers as to form a scum. They form a large

FIG. 285. — Diatoms of various kinds (X 30-200). In cases where a pair
of individuals equal in length are shown, two views of the same Diatom are
included. From Kerner.

part of the floating plankton or free-swimming organic world on
the surface of the ocean. Many occur as fossils and their silicified
walls form a large part of the deposits of siliceous earth in which
form they are used in the manufacture of dynamite, scouring
powders, etc. Some are free-swimming while others are attached
by stalks.

The plant body is microscopical and may have most any shape
imaginable as may be seen from Figure 285. The cell wall,
consisting largely of silica, is very rigid and durable and is com-
posed of halves which fit together one over the other much like
the two parts of a pill box. The walls of some are delicately
but beautifully marked with fine cross lines, which make certain
Diatoms suitable objects for testing the definition of microscopes.

332

THALLOPHYTES

Usually there is also a longitudinal line, which is a fissure or series
of fine pores through which fine threads of protoplasm project
and serve like cilia in locomotion. The halves of the box-like
shell of a Diatom are called valves and the appearance of a
Diatom depends much upon whether the face of the valve (the
valve side), or the side showing the joining of the valves (the
girdle side) is seen (Fig. 286). The protoplast usually has a large
central vacuole with the nucleus suspended in
the center by small strands of cytoplasm.

Cell division is the chief method of repro-
duction. The cell usually divides lengthwise
and in such a way that the valves separate
with the daughter protoplasts. Each daughter
protoplast then develops a new valve on the
naked side. In connection with this, a pecul-
iar situation arises. The new valve de-
veloped always fits within the old one and
consequently there is a gradual reduction in
the size of the individuals as division con-
tinues, for at each division the daughter
protoplast with the smaller valve is necessarily
smaller than in the preceding division. How-
ever, it has been found that the protoplasts
shed their walls when reduction in size has
reached a certain degree and in a naked
condition grow to full size and then enclose
themselves in new valves. This naked pro-
toplast is called an auxospore (meaning en-
larging spore).

It is in connection with these naked proto-
plasts that the sexual act occurs. Sometimes
the protoplasts of contiguous cells conjugate
and sometimes the four daughter protoplasts of two contiguous
cells escape and conjugate in pairs. The zygospore usually
enlarges and then encloses itself in valves.

Thus Diatoms are one-celled and conjugate like some Green
Algae, have the color of Brown Algae but have no zoospores or
gametes like the Brown Algae.

Stone worts. — The Stoneworts constitute the group scientifi-
cally known as the Char ales. Some classify the Stoneworts as

FIG. 286. — A
common Diatom,
Navicula viridis,
with valve side
shown at the left
and the girdle side
at the right. In
the view of the
girdle side one valve
is seen to fit over
the other. From
Strasburger.

STONEWORTS

333

Green Algae because they are green, while others regard them as
so different from any of the Algae as to put them in a separate
class. They grow in fresh and brackish waters and often form
dense masses of vegetation covering large areas. They grow

•

FIG. 287. — Chara fragilis. A, part of a plant, showing nodes, internodes,
and the two kinds of branches (natural size) ; B, part of a plant, showing a
node bearing sex organs, the oogonium enclosed in its jacket being at o and
the antheridium with its shield-shaped wall cells shown at a ( X 25) ; C, wall
cell of the antheridium, showing the stalk-like projection at the end of which
are borne the filaments in the cells of which the sperms are produced (X
about 50) ; at the left of C, two cells of a filament in which the sperms are
formed, and a single sperm below. Redrawn from Sachs and Thuret.

attached to the bottom and are often so incrusted with calcium
carbonate that they are rough and brittle as the name Stoneworts
suggests.

The plant body has a much branched stem-like axis quite
distinctly differentiated into nodes and internodes (Fig. 287).

334 THALLOPHYTES

From the nodes the branches arise in whorls and some branches
resemble leaves, while others elongate much more and resemble
the main axis.

Their reproduction may be illustrated by following that of
Chara. There are no asexual spores, but the plant is propagated
vegetatively by special tuber-like branches which separate from
the nodes and grow into new plants.

Sexual reproduction involves complex structures which are not
typical of Algae and which are the most distinguishing features
of the Stoneworts. Both antheridia and oogonia (Fig. 287} are
complex structures. Due to their size and color the sex organs
are visible to the unaided eye. Both are developed at the nodes
and often close together.

The antheridium is an orange or reddish globular body with
a wall composed of eight triangular plate-like cells. From the
inner side of each of the wall cells there projects toward the
center of the antheridium a much elongated cell which bears a
terminal cell. The terminal cell, known as head cell, divides into
a number of cells and each of these produces a pair of long fila-
ments. Each filament consists of about 200 cells, each of which
forms a single sperm. When an antheridium is fully formed, its
interior is a tangle of filaments and the sperm output amounts
to many thousands. The sperm is a much elongated ciliated
structure, resembling the sperms of some of the more complex
plants more than those of ordinary Algae.

The oogonium with its jacket is larger and more elongated
than the antheridium. The oogonia are often yellow but are not
so brightly colored as the antheridia. An oogonium contains one
large egg and much stored food in the form of starch and oil.
The oogonium is closely invested by cells which grow up from the
cells below and, as they elongate, wind spirally around the
oogonium, forming a close jacket around it and a crown at
its top.

In fertilization the cells of the jacket spread apart at the
crown, so that the sperms can enter. After fertilization the
jacket hardens, and thus forms a nut-like case for the oospore.
When the oospore germinates, it does not form a new plant
directly but first forms a filament of cells, and the adult plant
arises as a branch from this filament. This feature is prominent
in the Bryophytes.

STONEWORTS 335

It is now evident that the Stoneworts are very different from
the ordinary Green Algae, differing from them in structure of
plant body, character of sex organs, type of sperms, and life
history. They resemble some of the more complex forms of
plants more than they resemble the Algae.

CHAPTER XIV
THALLOPHYTES (Continued)

Myxomycetes and Bacteria (Thallophytes lacking food-making

pigments)

There are three groups of Thallophytes — the Myxomycetes,
Bacteria, and Fungi — which are characterized by the lack of
food-making pigments. Having no chlorophyll or other food-
making pigments, they are unable to carry on photosynthesis and
consequently must depend upon other organisms for their food.
Many obtain their food from the decaying bodies of other or-
ganisms, while others attack living organisms.

As to how these plants arose, we are not certain. Although
some are the simplest of plants, they must have been preceded by
green plants, for otherwise there would have been no food for
them. They are no doubt degenerate forms of green plants,
having lost their food-making pigments as a result of their acquir-
ing the habit of taking food from other organisms. As will be
seen later in the study of these groups, the Bacteria have some of
the features of the Blue-green Algae, while the Fungi present a
number of features found in the Green or the higher groups of
Algae. But for the Myxomycetes we have no definite suggestions
of any relationships with other groups of plants.

Being dependent plants, these Thallophytes are supposed to
have evoluted backward, rather than forward. The Fungi, the
most complex of the group, present nothing new over the Algae
in the way of evolution. To the evolutionists these groups offer
very little that is of interest. They concern us chiefly because
of their economic importance.

Myxomycetes (Slime Molds)

The plant body of the Myxomycetes, commonly called Slime
Molds, consists of a large slimy mass of protoplasm not enclosed
by cell walls, and hence the term myxomycetes from myxos

336

MYXOMYCETES

337

FIG. 288. — Plasmodium of a Myxo-
mycete growing on wood. X about |.

(meaning slime) and myces (meaning mold or fungus) (Fig.
This naked mass of protoplasm is called a plasmodium. It is a
semi-liquid and is found flowing out of the cracks of rotten logs
and stumps, forming white or colored doughy-like masses. They
are often found creeping out of the cracks of old plank walks, out
of decayed bark, or out of
apple pumice around a cider
mill. Some of the Myxomy-
cetes are parasites, living in
the tissues of higher plants
and often causing much in-
jury.

The plasmodium is multi-
nucleate and is able, by put-
ting out and withdrawing regions of its body, to move about like
a gigantic Amoeba. Sometimes the plasmodium" breaks up into
many smaller portions which are able, by means of cilia or flagella,
to move about like the low forms of animals. The Myxomycetes

have the characteristics of both
plants and animals, and opin-
ions differ as to whether they
should be classed as plants or
animals.

Their method of obtaining food
consists chiefly in digesting the
substances found in other
plants. Those forms which
live on dead organisms are
able to utilize the carbohy-

FIG. 289. — 4, Myxomycete,
Stemonitis, in which the plasmodium
has been transformed into slender
stalked sporangia (sp) which bear
numerous spores (s).

drates remaining in decaying
organic matter, while those
attacking living plants prey
upon the tissues of the plant
attacked. Those forms living on dead organisms are called sapro-
phytes, while those forms living on living organisms are called
parasites. The living organism attacked is called the host.

Reproduction in the Myxomycetes is asexual. The first indi-
cation of reproduction in most of the saprophytic forms is the
appearance of upward projections on the surface of the plas-
modium. Into these projections, which are at first hollow

338

THALLOPHYTES

structures, varying in shape according to the species, the remain-
ing protoplasm of the plasmodium passes until they are filled.
Often nearly the entire plasmodium is used in forming and filling

FIG. 290. — Various Myxomycetes, showing various types of sporangia.
The large sporangium at the left and the third one from the left, below, have
shed the spores, and the capillitium, the lace-like framework of the sporangium,
is plainly visible. The larger ones are larger than natural size, the smaller
ones are reduced. From Kerner.

the projections. The protoplasm filling the upper part of each
projection forms numerous, small, globular spores with heavy

FIG. 291. — Spores of a Myxomycete germinating and producing motile
animal-like bodies which usually multiply and later fuse to form a plasmo-
dium. Much enlarged. From Woronin.

walls, and thus the projection becomes a stalked sporangium
(Fig. 289). In the interior of the sporangium there is often a
lace-like framework, called capillitium, which assists through its
hygroscopic movements in the shedding of the spores (Fig. 290).

SOME MYXOMYCETES OF ECONOMIC IMPORTANCE 339

After the spores are mature, the wall of the sporangium breaks
open and the spores are scattered far and near by wind, animals,
and other agencies. When the spores fall on a suitable object
and conditions are right, the protoplasm breaks out of the heavy
wall and either grows directly into a new plasmodium, or pro-
duces cilia, swims about, and multiplies like the simple one-celled
forms of animals (Fig. 291), the plasmodium being formed later
by the fusion of these animal-like bodies.

Some Myxomycetes of Economic Importance

Most of the Myxomycetes are saprophytes and consequently
the group is not so important economically as the Bacteria and
Fungi. Of course the saprophytic forms are of some importance

FIG. 292. — Cabbage plants attacked by the Club Root Myxomycete
(Plasmodiophora Brassicae) which causes wart-like distortions. From Woronin.

because they disintegrate organic matter and make it soluble, so
that it can soak into the soil .and be used by higher plants.
There are, however, a few parasitic forms which attack some of
our useful plants and cause considerable trouble and loss.

340

THALLOPHYTES

Club Root of Cabbage.1 — This is a disease of Cabbage caused
by a parasitic Myxomycete. The Myxomycete gains entrance
through the roots and lives upon -the cells of the plant. The
presence of the parasite causes the wart-like developments on the
roots and stem of the Cabbage, and so injures the plant that
no head is produced and even death often results (Figure 292).

Within the cells of the
Cabbage the plasmodia live
and form spores (Figure
293). When liberated
through the decay of the
Cabbage, the spores are
carried by water, animals,
or wind to other plants.
The spores may lie in the
ground and infect plants in
succeeding years. This dis-
ease is not only destructive
to Cabbage but often at-
tacks Turnips, Radishes,
Rutabagas, and Cauli-
flower. The important fea-
ture in controlling the
disease consists in prevent-
functioning by burning infected plants,
sulphur, and rotation of

FIG. 293. — Cross section of a root
of Cabbage affected with Club Root,
showing the plasmodia (p) within the
tissues. From Woronin.

lime

ing the spores from
treating the soil with
crops.

Powdery scab of the Irish Potato.2 — This disease is caused by
one or more kinds of Myxomycetes which enter the tubers and
roots of the Irish Potato and destroy the tissues (Fig. 294)- The
Amoeba-like plasmodia live in the cells, which, due to the presence

1 Cabbage Club Root in Virginia. Bulletin 191, Virginia Agr. Exp. Sta.,
1911.

Studies on Club Root. Bulletin 175, Vermont Agr. Exp. Sta., 1913.
Studies on Clubroot of Cruciferous Plants. Bulletin 387, Cornell Uni-
versity Agr. Exp. Sta., 1917.

2 Powdery Scab (Spongospora subterranea) of Potatoes. Bulletin 82,
U. S. Dept. Agr., 1914.

Powdery Scab of Potatoes. Bulletin 227, Maine Agr. Exp. Sta., 1914.
Spongospora subterranea and Phoma tuberosa on the Irish Potato, Vol. 7,
No. 5, pp. 213-254, Jour. Agr. Research, U. S. Dept. Agr., 1916.

BACTERIA

341

of the organism, develop abnormally, producing scabby formations
which constitute the scabby areas on the tuber or root. The
plasmodia are finally transformed into spores which are liberated
as powdery masses as the infected tissues die and the spore masses
break open. It has been
found that the spores can
live in the ground for a num-
ber of years and may also
live adhering to the rind of
the Potato. Treating seed
Potatoes with weak solu-
tions of formaldehyde or
corrosive sublimate to kill
the spores adhering to the
tubers, and rotating crops,
so that the Potatoes are
not planted in infected soil
are means of controlling the
disease.

FIG. 294. — An Irish Potato attacked by
a Myxomycete, Spongospora svbterranea.

Bacteria

Bacteria, of which there are 1400 or more species, are the
smallest of plants, and their study requires microscopes of a very
high power of magnification. Some spherical forms, visible only
through the best microscopes, are less than 0.0005 of a millimeter
in diameter, and some Bacteria are known to exist that are
ultramicroscopic, that is, too small to be seen with the best
microscopes. They are present almost everywhere, occurring
in the soil, in water, in the air, and in all organic bodies living or
dead. Although so insignificant in size, they are of great im-
portance, because the service of some forms is indispensable to
our welfare, while the forms which cause diseases are destructive
to both plants and animals. The disease-producing forms are
commonly known as germs or microbes. So numerous and
important are these simple plants that their study now forms a
special subject called Bacteriology.

The plant body of the Bacteria consists of a single cell. Bac-
teria are of three general forms: coccus forms, which are globular;
bacillus forms, in which the shape is rod-like; and spirillum
forms, in which the plant body is a curved rod (Fig, 295).

342

THALLOPHYTES

Many of the Bacteria are provided with cilia or terminal flagella,
which enable them to move about independently. The cilia are
distributed over the body in various ways and are extremely
difficult to detect. Some of the motile forms are quite active

and motility is one of the fea-
tures suggesting that Bacteria
are animals. Their cell walls
are more or less slimy, and their
protoplasm is not definitely or-
ganized into nucleus and cyto-
plasm. These features with
their power of resistance suggest
a relationship with the Blue-
green Algae. They possess no
chlorophyll and are almost ex-
clusively parasites or sapro-
phytes. The ability of the
protoplasm to endure extreme
cold, high temperatures, and
drying even surpasses that of
the Blue-green Algae. Besides
remaining separate or forming
filaments, Bacteria commonly have another stage in which numer-
ous individuals are held together in masses or colonies by a matrix
of gelatinous substance formed from their walls. This stage
is known as the zoogloea stage (Fig. 296). These colonies form
the characteristic pellicles on nutrient media, as on the water in
which hay, Beans, Peas, or other organic substances are decay-
ing, and on bouillon and various solid media (Fig. 297). When
food is scarce or other conditions unfavorable, some forms shrink
their protoplasm and enclose it in an inner heavy wall, thus form-
ing what is called a spore. Enclosed in this heavy wall, they are
inactive and extremely resistant to cold, heat, and drying. When
transferred by wind or other agents to a suitable medium, they
shed the heavy wall and become active again.

Their method of getting food is essentially the same as in the
Myxomycetes. Since they live on or within the food supply,
they are in direct contact with the food material, and have only
to change it to a soluble form and absorb it through their walls.
They secrete enzymes which change insoluble foods to soluble

FIG. 295. — Some forms of Bac-
teria. At the right and above, a
coccus form; a bacillus just below;
and a spirillum form at the bottom.
At the left, above, a chain of bacilli;
and, below, bacteria in the spore
stage. Very highly magnified.

BACTERIA

343

forms and as a result of their activity various substances are
produced, the accumulation of which check their activity.
Some forms, called anaerobic, get along better without air, while
others, called aerobic, must have air.

Their reproduction is accomplished by cell division, which is
not so complex and takes place more rapidly than in the cells of

FIG. 296. — Bacillus sublilis, a Bacterium of decay. Above, the active
form (X 1500); at the left, below, spore stage (X 800); at the right, below,
the zoogloea stage (X 500).

the higher plants. Cell division is so rapid that the progeny of
one individual often runs into many millions in twenty-four
hours. The new individuals may separate immediately after
division is complete or cling together in filaments. Sometimes
in shrinking the protoplasm and enclosing it in an inner heavy
wall in preparation for the resting stage, the protoplasm divides
and each separate mass of protoplasm forms a spore. Since each
spore is an individual in a dormant protected state, the formation
of more than one spore results in the multiplication of individuals.

344 THALLOPHYTES

But a Bacterium commonly forms only one spore, and the
formation of spores, therefore, does not generally result in the
multiplication of individuals. The spore stage is apparently for
protection rather than for multiplication. In the spore stage
Bacteria can live where there is no food and when heat and

FIG. 297. — A petri dish containing agar upon which are colonies of the
Bacterium (Actinomyces chromogenus) which attacks Irish Potatoes, causing
scabby areas. The white spots are the colonies. From Bulletin 184, Ver-
mont Agr. Exp. Sta.

cold are much too extreme for ordinary life. In this condition
some Bacteria can endure boiling water for an hour or longer as
well as extremely low temperatures. It is the spores of Bacteria
that are hard to kill in sterilizing media and other substances.
In the spore stage Bacteria can retain their vitality for months
or years and, floating about with the dust in the air, reach all
kinds of situations.

Some Bacteria of Economic Importance

Bacteria of Decay. — The Bacteria of decay live on dead
organisms. By their activity dead organisms, such as straw,
weeds, corn stalks, logs, and carcasses of animals, are decomposed
into simpler and soluble compounds, in which form they return
to the earth and become available nutrients for plants. Their
activity helps to rid the earth's surface of debris and to pre-

BACTERIA OF NITRIFICATION AND NITROGEN FIXATION 345

vent the soil from becoming depleted of plant nutrients. Of
course they attack meats, canned fruits, and many other things
which we do not wish to have decomposed, but the good they do
more than compensates the harm. Methods, such as cold stor-
age, applications of salt and other preservatives, and canning, are
employed in checking or preventing the activity of Bacteria in
foods. In cold storage the temperature is too low for them to be
active. Salt solutions keep them dormant by extracting water
from them. In canning those present are killed by heat, and by
sealing the cans others are prevented from entering. Alcohol,
formaldehyde, carbolic acid, etc., are useful in preventing bac-
terial action in materials not intended for food.

Bacteria of Fermentation. — These Bacteria attack carbohy-
drates and break them into simpler substances, such as alcohol,
lactic acid, acetic acid, butyric
acid, etc. A few forms are
shown in Figure 298. The
product produced depends
upon the substance attacked
and the kind of Bacteria at
work. For example in the
fermenting of cider, some Bac-
teria break the sugar into FlG- 298- ~ Bacteria of fermenta-
alcohol and carbon dioxide, ^ a, 6, and c vinegar Bacteria;

d, Bacteria that ferment milk; e,

while others attack the alco- butyric acid Bacteria. X 1000. Re-
hol, changing it into acetic drawn from Fisher,
acid. All the forms working

together change the cider into vinegar. After the vinegar Bac-
teria become inactive, due to the exhaustion of the food supply
or the accumulation of the fermented products, they form the
well-known mother of vinegar, which consists of the Bacteria
held together in a gelatinous matrix. In milk, certain kinds of
Bacteria attack the milk sugar and change it into lactic acid.
Another kind produces butyric acid in butter, turning it rancid.

Bacteria of Nitrification and Nitrogen Fixation. — In the soil
there are some kinds of Bacteria that change certain nitrog-
enous compounds of manure and other organic matter into
nitrates in which form the nitrogen is available for crops. The
advantage to the Bacteria is that they secure energy in this
way from these compounds, while the advantage to the soil is

346

THALLOPHYTES

that the nitrogen of the compounds decomposed is put in an
available form for other plants. The Bacteria use the chemical
energy derived from the oxidation of organic compounds in per-
forming the work involved in building up their bodies. There
are a few exceptional forms of soil Bacteria which can actually

make their own food, and
this they do by using this
chemical energy, as green
plants do sunlight, in the
construction of foods from
carbon dioxide and water.
There are other kinds of
soil Bacteria which have
the power of actually in-
creasing the nitrogen in the
soil. They incorporate the
gaseous nitrogen of the air
into nitrogen compounds,
which they use in building
up their own bodies, and
when their bodies decay,
these nitrogenous com-
pounds are added to the
soil, which is thereby en-
riched. Some kinds of
these Bacteria live inde-
pendently in the soil, while
some kinds are associated
with higher plants, espe-
cially the Legumes, such as

FIG. 299. — A young Red Clover plant,
showing the root nodules that are associated
with the nitrogen fixing Bacteria. From
Farmer's Bulletin 435, U. S. Dept. of Agri-
culture.

Clover, Alfalfa, Beans, etc.
(Fig. 299}. They enter the
roots of these plants, and,
as a result of the attack,
the roots form nodules in
which the Bacteria live and carry on their work of fixing nitro-
gen. It is due to their association with these Bacteria that the
Legumes are important in enriching the soil.

Pathogenic bacteria. — These are the disease-producing forms.
They prey upon both animals and plants. The disease is the

BLACK-ROT OF CABBAGE

347

result of their direct attack upon the tissues, or of the poisons,
called toxins, which they excrete. They produce most of the
diseases of human beings, such as erysipelas, tetanus, diphtheria,
tuberculosis, typhoid fever, pneumonia,
cholera, etc. (Fig. 300). Among our
domesticated animals, such diseases as
hog cholera, splenic fever, glanders,
black-leg, etc., are caused by Bacteria.
The righting of these forms, either to
exclude, destroy, or neutralize them, is
the business of modern medicine and
surgery. Besides the dangerous forms
which attack animals, there are numer-
ous harmless forms constantly present
throughout the alimentary canal.

Among plants the disease-producing
Bacteria are almost as busy as among
animals. Not only the tender herbaceous plants but even the
trees are attacked, and the loss caused every year is large.

FIG. 300.— Some patho-
genic Bacteria. a, pus
cocci; b, erysipelas cocci;
c, Bacteria causing diph-
theria; d, typhoid bacilli.
X 1500. Redrawn from
Fisher.

FIG. 301. — Potato tuber affected with the Potato Scab caused by a
Bacterium, Actinomyces chromogenus. From Bulletin 184, Vermont Agr.
Exp. Sta.

Black-rot of Cabbage. — This disease occurs on Cabbage,
Turnips, and other plants of this family. The Bacteria enter
through the openings of the leaf and advance through the
vascular bundles. They are able to destroy cell walls as well

348

THALLOPHYTES

as the living content of the cells. Their presence in the leaf
causes a blackening of the veins and a yellowing of the mesophyll.
The disease may spread to the stem, -where it clogs the vascular
bundles and destroys tissues. Plants attacked lose their leaves
and are dwarfed or killed.

Potato Scab.1 — There are a number of organisms which at-
tack the Irish Potato and cause scabby areas and the decay of

the tuber. Among this
group of organisms pro-
ducing scab there is one
of the higher forms of
Bacteria scientifically
called Actinomyces chromo-
genus (Fig. 801). Among
other bacterial diseases of
the Irish Potato, Black-
leg2 is of considerable im-
portance, especially in the
Southern States.

Pear Blight.3 — This dis-
ease occurs on many fruit
trees, but is more serious
on Pears and Apples. It
is often called Fire Blight
or Blossom Blight. The
Bacteria enter the young
twigs, usually through the
flowers, and attack the
FIG. 302. — Fire blight on the Pear, cambium and cortex. The
The tip of the branch is being killed by tips of the twigs with their
the Bacteria. After Whetzel & Stewart. flowers and leaves soon

wilt, and in a few weeks

blacken and die. Sometimes when the attack is quite general,
scarcely a flower tip of an infected tree escapes. This not
only results in loss of fruit, but the tree is often so disabled
that death results. Figure 302 shows a Pear twig severely

1 Potato Scab. Bulletin 184, Vermont Agr. Exp. Sta., 1914.

2 Potato Tuber Diseases. Farmer's Bulletin 544, U. S. Dept. Agr., 1913.

3 Fire Blight Disease in Nursery Stock. Bulletin 329, Cornell University
Agr. Exp. Sta., 1913.

CROWN GALL

349

attacked. The Bacteria pass the winter in the infected regions,
which are sources of further infection. When growth begins in
the spring, a gummy substance carrying the Bacteria exudes
from these dead portions, and insects visiting the exudations
carry the disease to other trees. How would you combat Pear
Blight?

Crown Gall. — This disease is common on fruit trees, and
occurs on Roses, Blackberries, Alfalfa, and a number of other
plants. The presence of the Bac-
teria causes an abnormal develop-
ment of the infected tissues, resulting
in the formation of cancer-like swell-
ings. The disease may occur on
any portion of the plant but is com-
mon on the roots or on the stem
near the surface of the ground. In
general, nursery stock is more readily
affected than older trees. Plants
affected are much dwarfed or killed
(Fig. 303).

Beans, Tomatoes, Potatoes, Sugar
Cane, Cotton, and most of our eco-
nomic plants have some form of
bacterial disease, but a further study
of the disease-producing forms must
be left to courses in Bacteriology
and Pathology.

FIG. 303. — Crown Gall on
the Cherry tree. The cancer-
like swellings are due to the
presence of Bacteria. After
Bulletin 235, California Agr.
Exp. Sta.

In summarizing, the following fea-
tures should be noted. Bacteria are
the smallest of plants, and their
plant body consists of a single cell
with protoplast poorly organized.
The plant body may be globular
or rod-shaped, either straight or curved. Some have cilia or
flagella and are therefore motile. They are remarkably re-
sistant, especially in the spore stage. With the exception of a
few forms, they are saprophytes or parasites. The disease-
producing forms are very destructive to both animals and plants.
They reproduce by rapid cell division and are spread, partly by
wind, partly by water currents, partly by their own locomotion,

350 THALLOPHYTES

and partly by the movements of animals with which they are
associated. Their structural simplicity, power of resistance, and
gelatinization of walls suggest a relationship to the Blue-green
Algae.

In connection with Bacteria another group of organisms, known
as the Myxobacteria, is commonly discussed. As the name sug-
gests, the myxobacteria resemble both the Bacteria and Myxo-
cytes. They differ from the Bacteria in that they form colonies,
which in some cases are definite and elaborate in form. Some
form colonies resembling a stalk bearing a group of sporangia at
its summit. The life histories of the individuals of a colony are
essentially the same as those of the Bacteria, differing chiefly in
that the resting cells or spores form rod-like cells which escape
and assemble to organize the colony.

CHAPTER XV

THALLOPHYTES (Concluded)

Fungi (Thallophytes Lacking Food-making Pigments)

General Discussion. — The Fungi are a very large group of
Thallophytes. There are thousands of different kinds of Fungi.
Most people know some of the common forms, such as Toadstools,
Mushrooms, and Puffballs, and those who live on the farm are
probably acquainted with the Rusts and Smuts of our common
cereals. Most of the plant diseases are caused by Fungi. Like
the Slime Molds and Bacteria, they have no food-making
pigment and consequently are either saprophytes or parasites.
They attack both animals and plants. Plant Pathology, which
is a study of plant diseases, devotes some time to the study of
Slime Molds and Bacteria, but is concerned mainly with the
Fungi. They attack vegetables, grains, fiber plants, fruits, fruit
trees, and forest and shade trees. The destruction which they
cause is enormous. Some of the parasitic forms, however, are
harmless, and many of the saprophytic forms are beneficial.

It is generally supposed that the Fungi are derived from the
Algae, having lost their chlorophyll and independent living.
Some of them have plant bodies, zoospores, sex organs, and sex
cells similar to those of the Green Algae, while some have sex
organs resembling those of the Red Algae, but have no resemblance
in other features. Some have become -so modified by their de-
pendent habit of living that they have lost all of their alga-like
features. They have made no advancement in evolution, for
there is less differentiation of plant body in this group than
in the Algae, and methods of reproduction show no improvement,
but often are simpler than those of the Algae or have been lost
entirely. Those botanists who study plants mainly from the
standpoint of evolution devote very little time to the Fungi
because they have contributed nothing to evolution. But from
the economic standpoint, the Fungi are an exceedingly important

351

352 THALLOPHYTES

group. A knowledge of their plant bodies, methods of reproduc-
tion, and how they injure other plants is essential for working
out methods of controlling the destructive forms.

The plant body of. Fungi consists of a mass of colorless branch-
ing threads or filaments, and is called a mycelium (plural mycelia) .
The individual threads are called hyphae (singular hypha) . The
hyphae constituting a mycelium may be loosely interwoven,
forming a structure resembling a delicate cobweb, as in the Bread
Mold, or they may be woven into a compact body having a
definite shape, such as occurs in Toadstools and Mushrooms.
The mycelium must be in direct contact with its food supply,
which is called the substratum.

Sometimes, especially in the case of parasites, special short
branches are formed which penetrate the host and absorb food
material. These special absorbing branches are called haustoria,
meaning "absorbers."

Hyphae are modified in various ways for reproduction. Some
produce spores, which are sometimes borne in sporangia and
sometimes openly on the end or sides of the hyphae. Some are
modified into organs for bearing sex cells. These various modi-
fications for reproduction will be learned as the different groups
and their types are studied.

Divisions of Fungi. — The Fungi are so much modified by
their peculiar life habits that they have either lost or disguised
the structures which prove most helpful in the classification of the
Algae. The Fungi are divided into four large subdivisions, but
the life histories of only three of the subdivisions are well known.

The constant termination of the group names is mycetes, a
Greek word meaning "Fungi." To this name is added a prefix
which is intended to indicate some important character of the
group. The three subdivisions in which the life histories are
known are named as follows: (1) Phycomycetes (Alga-like
Fungi) "phyeo" coming from "phykos," meaning Seaweed and
suggesting the water habits of this group; (2) Ascomycetes (Sac
Fungi), so named because they bear spores in small sacs called
asci (singular ascus); and (3) Basidiomycetes, Fungi which bear
spores on small club-shaped hyphae called basidia (singular basid-
ium). To the Basidiomycetes belong such familiar forms as
the Toadstools, Mushrooms, Rusts, and Smuts. The fourth
group is known as the Fungi Imperfecti. They are those Fungi

WATER MOLD (SAPROLEGNIA)

353

which are not known to have a life history of the type character-
istic of either of the other subdivisions. Their life histories so
far as they are known are imperfect.

Phycomycetes (Alga-like Fungi)

This group, as their name suggests, resembles the Algae, but it
is a large assemblage of plants which vary widely in both struc-
ture and habit. Some of them live
in the water and have zoospores
and sex organs similar to those of
the Green Algae, while some have
lost their water habits and nearly
all of their algal features. There
are three principal orders — Water
Molds (Saprolegniales) , Downy
Mildews ( Peronosporales) , and
True Molds (Mucorales).

Water Molds (Saprolegniales). —
These are the Fungi showing closest
affinities with the Algae. In fact,
if some of them had chlorophyll,
they could scarcely be told from
some of the Green Algae. They (Saprolegnia). A, a fly affected
live in the water where they feed with Saprolegnia. The fuzzy ap-
upon the dead bodies of insects, pearance of the fly is due to nu-
fish, and tadpoles. Sometimes they meroushyphae which project from
, ,. . . J the body of the fly. B, tips of

attack living organisms, as the one projecting hyphae which have

called Saprolegnia illustrates. formed zoosporangia. From the

Water Mold (Saprolegnia). — zoosporangium at the right the

There are several kinds of Water zoospores are escaping. C, a

Molds, but the one called Sapro- tip of a projecting hypha bearing

, . , • TT an oogomum containing a number

legnia, shown in Figure 304, is a ofeggg D an o5gonium and an_

very common one. Although com- theridium, the latter of which has
monly a saprophyte on the floating pierced the wall of the oogonium
bodies of dead organisms, it often and thereby enabled the sperms
attacks and kills young fish that to reach the eggs,
are confined in close quarters, on which account it is some-
times very destructive in fish hatcheries. This Fungus, since
it can live on both live and dead organisms, shows that there

FIG. 304. — A Water Mold

354 THALLOPHYTES

is no sharp line of distinction between parasites and saprophytes.
As long as the host is living, the Fungus is a parasite, but upon
the death of the host it becomes a saprophyte. Thus a Fungus
may be a parasite at one time in its life and a saprophyte at
another. Saprolegnia is usually obtained for study by throwing
dead insects or pieces of beefsteak into stagnant water from a
pond, where the objects usually become infected and soon look
like the fly in Figure 304-

The mycelium of Saprolegnia is composed of many branched
hyphae which extend throughout the tissues of the host. The
hyphae are coenocytes. This coenocytic feature suggests a closer
kinship to Vaucheria than to the other Green Algae. After the
mycelium is well established in the host, numerous hyphae, which
cause the fuzzy appearance, protrude from the surface of the
host.

The hyphae within the tissues of the host are able to absorb
food materials directly. They are also able by means of enzymes
to change materials to soluble forms, and in this way the Water
Molds bring about the decay of animal bodies in water.

Many of the hyphae protruding from the host become modified
for reproduction. Some produce zoospores, while others produce
sex organs. The swollen tips of some of the protruding hyphae
are cut off by a cross wall and form sporangia in which are pro-
duced numerous zoospores. These zoospores escape, swim about,
and when in contact with another host produce hyphae that
penetrate and infect the new host.

Oogonia and antheridia are also formed at the ends of hyphae.
The oogonia are spherical and form one and sometimes many
eggs. The antheridia are formed on branches near the oogonia.
The antheridium comes in contact with the oogonium and pierces
its wall with a small tube through which the sperms from the
antheridium pass and fertilize the eggs. As a result of fertiliza-
tion, a heavy-walled oospore is formed, which after rest grows
into a hypha which can penetrate and infect a host.

A peculiar feature in connection with some of the Saprolegnias
is the ability of their eggs to develop without fertilization. In
most plants, unless the egg is fertilized, it will not develop, but
will soon disintegrate and disappear. In some Saprolegnias. the
sperms of the antheridium fail to enter the oogonium, or there
may be no antheridium developed, and still the egg without

THE GRAPE DOWNY MILDEW (PLASMOPARA VITICOLA) 355

fertilization forms an oospore which can germinate. This
peculiar feature, called parthenogenesis, meaning reproduction
by an egg without fertilization, has been mentioned before
(page 50).

The aquatic habit, reproduction by zoospores, and character
of sex organs support the theory that the Water Molds are
degenerate forms with the Green Algae as their ancestors. The

FIG. 305. — Leaf of the Grape, showing the downy areas caused by the
Downy Mildew, Plasmopara Viticola. After W. H. Hein.

coenocytic character of their hyphae suggests a close relationship
with the Siphonales.

Downy Mildews (Peronosporales). — These Fungi, which are
parasites on the higher plants, cause some serious plant diseases
of which the Grape Downy Mildew and Potato Blight are notable
ones, and will serve to illustrate the habits of the group. There
are about 100 species known and the order is so named because
of the downy patches which they produce on the diseased portions
of the host.

The Grape Downy Mildew (Plasmopara Viticola). — The
Downy Mildew of the Grape, shown in Figure 305, is a very

356

THALLOPHYTES

common and important one of the many Downy Mildews and
often causes much loss in grape-growing districts. Its downy
white growth occurs most commonly on the leaves, but the Fun-
gus often attacks the green shoots and fruit (Fig. 306). Some-
times it destroys the fruit crop and weakens the vines.

The mycelium consists of coenocytic hyphae, which extend

through the tissues of the part at-
tacked. The hyphae grow between
cells and send into the cells short
branches (haustoria) which absorb
the cell contents of the host (Fig.
807). The death of the leaf cells re-
sulting from the attack is indicated
by the occurrence of yellow or brown
areas which may involve much of
the leaf. This destruction of leaf
tissue diminishes the carbohydrates

FIG. 306. — A bunch
of Grapes partially de-
stroyed by the Downy
Mildew. From Farmer's
Bulletin 284, U. S. Dept.
of Agriculture.

FIG. 307. — The haustoria of the
Downy Mildew reaching 'into the
cells of the grape, h, hypha; a,
haustoria. From Bulletin 214, Ohio
Agr. Exp. Sta.

furnished by the leaves and as a result both fruit and vine
may suffer. Often, the fruit is directly attacked and de-
stroyed. After the Mildew is well established within the tissues
of the hosts, it sends through the stomata numerous branches
which constitute the superficial downy patches characteristic
of the parasite (Fig. 308). On the tips of these protruding
hyphae are produced small globular bodies known as conidio-
spores or conidia, and the hyphae bearing them are called
conidiophores which means " conidia bearing."

The conidia are really small sporangia which break off and are

POTATO BLIGHT (PHYTOPHTHORA INFESTANS) 357

scattered about like spores. When the conidia germinate,
instead of producing hyphae they produce zoospores, which, after
swimming about for a few minutes, lose their cilia and begin to
produce new hyphae. If favorably located, the new hyphae find
entrance to a leaf through its stomata and start the disease anew.
The oogonia and antheridia resemble those of Saprolegnia, but
are produced on short hyphae
within the tissues of the host.
The oospore has a heavy wall
and is not liberated until the
tissues of the host surrounding
it decay. The oospores are well
fitted to endure winter condi-
tions, and as the dead leaves
are scattered, the oospores con-
tained are also scattered, and
when freed it is probable that
they often start the disease the
following year.

Potato Blight1 (Phytophthora
inf estans) . — This Fungus, com-
monly called the Late Blight of
the Potato, is a near relative of
the Grape Mildew. It attacks
the leaves, stems, and tubers
of the Irish Potato and is very
rapid and destructive in its
work. Figure 309 shows the
leaves of a Potato plant affected
with this disease. Like the

Grape Mildew, after the mycelium is well established in the
host, conidiophores are produced (Fig. 310). The conidia may
grow directly into hyphae or produce zoospores (Fig. 311).

1 Late Blight and Rot of Potatoes. Circular 19, Cornell University Agr.
Exp. Sta.

Investigations of the Potato Fungus, Phytophthora Inf estans. Bulletin 168,
Vermont Agri. Exp. Sta.

Germination and Infection with the Fungus of the Late Blight of Potato
Research Bulletin 37, Wisconsin Agr. Exp. Sta., 1915.

Studies of the Genus Phytophthora. Vol. 8, No. 7, pp. 233-276, Jour.
Agr. Research, U. S. Dept. Agr., 1917.

FIG. 308. — Reproduction in the
Downy Mildew of the Grape, a,
conidiophores bearing conidiospores
on the ends of their branches; b,
conidiospores; c, oospore; z, zo-
ospore. Much enlarged. From
Farmer's Bulletin 284, U. S. Dept.
of Agriculture.

358 THALLOPHYTES

The conidia and zoospores which they produce spread the
disease very rapidly in moist weather. Since the zoospore
is a swimmer, it can function more efficiently during moist
weather. Moist weather also favors the germination of the
conidia. Little is known about the sex organs of the Potato
Blight, but in the form which causes the Bean Blight, the sex

FIG. 309. — Leaf of Irish Potato affected with the Late Blight. From
Bulletin 140, Cornell University.

organs and oospores occur in the seed coat or cotyledons of the
seed, in which case the oospore is planted with the seed. In the
Phytophthora cactorum,1 which is destructive to Ginseng, the sex
organs and oospores have been found in the stem and roots
(Fig. 312).

There are many Downy Mildews which give us trouble. In
fact many of our plants such as Cucumbers, Melons, Beans,
Potatoes, Lettuce, Grapes, etc., are attacked and much damaged
by Downy Mildews. But the study of the Grape Mildew and
Potato Blight has given a general knowledge of their habits. In
combatting the Mildews one must reckon with conidiospores,
zoospores, and oospores.

One is able to check the spread of the disease by spraying the
plants with a solution that is poisonous to conidia and zoospores.

1 Phytophthora Disease of Ginseng. Bulletin 363, Cornell Agr. Exp. Sta.,
1915.

POTATO BLIGHT (PHYTOPHTHORA INFESTANS) 359

A spray that is very commonly used is Bordeaux mixture.1 The
oospores live over winter and may perpetuate the disease from
year to year. Portions of diseased plants containing oospores,
when hauled out in manure or scattered about by the wind, may
be a means of spreading the disease.

In some forms of the Peronosporales as in Albugo or White
Rust, which forms white blisters on
the leaves and stem of the Radish
and other plants of the Mustard
family, both the sex organs and
conidiospores are produced internally.
The hyphae form in clusters under
the epidermis and form conidiospores
in chains which push up the epider-
mis, forming white blisters which
finally rupture and allow the spores
to escape. In this Fungus the
conidiospore produces a number of
zoospores.

In this order Pythium is some-
times included, species of which at-
tack seedlings in greenhouses, causing
the rapid wilting known as damping
off, when moisture and warmth are
abundant. Some species of Pythium
live in the water like the Saproleg-
niales in which order Pythium is
often put, while other species live in
the soil.

In contrast to the Water Molds,
the Downy Mildews are chiefly para-
sitic, much less aquatic and, having introduced the conidia, they
depend less upon water for dissemination. But like the Water
Molds the presence of zoospores and the character of the re-
productive organs suggest a relationship to the Green Algae.

1 The preparation as most commonly made consists of 5 pounds of copper
sulphate and 5 pounds of stone lime dissolved in 50 gallons of water. Potato
Spraying Experiments in 1906. Bulletin 279, New York Agr. Exp. Sta. Cer-
tain Potato Diseases and their Remedies. Bulletin 72, Vermont Agr. Exp.
Sta., 1899.

FIG. 310. — The lower
epidermis of a Potato leaf
showing the conidiophores of
the Late Blight protruding
through the stomata and
bearing conidiospores at the
tips of their branches. Many
times enlarged.

360 THALLOPHYTES

True Molds (Mucorales). — There are a number of Molds
some of which belong to other divisions of the Fungi. The
Molds of this order are characterized by a zygosporic reproduc-
tion, on which account they are called Zygomycetes. Of the
nearly 200 species known, Bread Mold is the most familiar one.

FIG. 311. — Conidia of the Late Blight of the Potato developing zoospores,
and zoospores growing hyphae. X about 400. After Ward.

Bread Mold (Rhizopus nigricans) . — Bread Mold is very
common about homes, producing a fluffy tangle of hyphae on the
surface of bread, fruit, and other favorable nutrient substances
when left exposed (Fig. 313). It is sometimes injurious to
Sweet Potatoes and other vegetables in storage. The fluffy
tangle of hyphae is white while young but becomes dark when old,
owing to the dark color of the mature sporangia.

A strong poison has been found in connection with Rhizopus
nigricans, and it has been suggested that some of the diseases of
stock, such as the " cornstalk disease " and the " horse disease,"
prevalent in some of the Western states, may be due to the toxin
which stock get in moldy fodder or other feed. The toxin
apparently is only effective when introduced into the circulatory
system. This is shown by the fact that rabbits can be fed the
Mold without any injury, but when a little of the sap is expressed
from the mycelium and injected into the blood, the animal dies
almost instantly.

The mycelium consists of numerous coenocytic branching
hyphae. Some of the hyphae penetrate the substratum and
gather food, while others gnrw above the substratum and produce
the visible fluffy mass. The surface hyphae with more or less

BREAD MOLD (RHIZOPUS NIGRICANS)

361

upright growth bear the sporangia, while others running over
the surface of the substratum produce at certain places a new
set of both penetrating and upright hyphae. These runner-like
hyphae are called stolons, and serve to spread the mycelium over
the substratum. The hyphae which penetrate the substratum
are able to change the elements
of the substratum into soluble
forms and absorb them.

The sporangia occur singly on
the hyphae and contain numer-
ous aerial spores, which when
mature are liberated by the
breaking of the sporangial wall.
The spores are nearly always
present, floating about in the air
and resting on objects where
they happen to fall. It is prob-
able that they can live for many
years in the dormant state and
then germinate when they come
in contact with suitable food
material.

The Bread Mold has no sex
organs, but there is a sexual
process which reminds one of
the sexual process in Spirogyra.
Sometimes, as shown in Figure
814, tips of hyphae approach
each other and finally meet. From each hyphae an end cell is cut
off, and these end cells fuse to form heavy walled zygospores.
Upon germination the zygospore produces an erect hypha bearing
a sporangium of the ordinary type, and the aerial spores developed
therein are capable of starting a new series of plants.

Conjugation is only occasionally obtained in Rhizopus nigricans
unless the cultures are made in a certain way. It has been found
that in Rhizopus nigricans there are two kinds of plants, which,
although looking just alike, behave differently. They are called
strains, one being known as the plus (-f) and the other as the
minus ( — ) strain. When either of these occur alone in a culture
then no conjugation takes place, but if both are present then

FIG. 312. — Methods of repro-
duction in the Phytophthora cacto-
rum, which attacks Ginseng. A, sex
organs consisting of oogonium (o)
and antheridium (a). B, conidi-
ospore forming zoospores above,
and a group of zoospores below.
C, conidiospore producing hyphae
directly. Much enlarged. From
Bulletin 363, Cornell University
Agr. Exp. Sta.

362 THALLOPHYTES

there is abundant conjugation and formation of zygospores. In
many laboratories the spores of both strains are kept in stock,
and conjugation is obtained whenever desired by using spores
of both strains in growing the cultures.

Another Mold of this order is PiloboluSj commonly called
Squirting Fungus on account of the way it throws its sporangia.

FIG. 313, — Bread Mold, Rhizopus nigricans. A, piece of bread on which
there is a growth of Mold (X I). B, plant body of Bread Mold, showing
the hyphae (r) which penetrate the bread, the hyphae which grow up and bear
the sporangia (s), and the hyphae (a) (stolons) which grow prostrate on the
surface of the substratum and start new plants. (X about 20.)

It is common on stable manure and resembles Bread Mold. The
hyphae become turgid and swollen just beneath the sporangia
and finally burst, hurling the sporangia with considerable force,
whence the name Squirting Fungus.

In the True Molds, where there are no swimming spores, the
Phycomycetes become entirely aerial, although the coenocytic
plant body and conjugation still suggest a relationship with the
Green Algae. The mycelium, a tangle of hyphae with no definite
shape in Phycomycetes, shows some differentiation into absorbing,
vegetative, and reproductive structures. The chief propagative
structures of the group are zoospores, conidia, and aerial spores.

ASCOMYCETES

363

Oospores and zygospores tide the plant over unfavorable con-
ditions and produce new plants when favorable conditions return.
In combatting the disease-producing Phycomycetes, the control
of zoospores, conidia, and oospores must be considered.

FIG. 314. — Conjugation in Bread Mold. a, b, c, and d are successive
stages in conjugation. At a the short hyphae have just come together, while
at d the zygospore is formed, e, zygospore developing a new hypha bearing
a sporangium. X about 130.

Ascomycetes (Sac Fungi and Lichens)

General Description. — To the Ascomycetes belong the largest
number of Fungi, and most of them are parasites. Many of our
most troublesome diseases are caused by these Fungi. Some,

364 THALLOPHYTES

like the Yeast Plant, and the Molds which help in making cheese,
are useful. Some of the Ascomycetes are used directly as food.
The saprophytic forms are useful in hastening the decay of or-
ganic matter. But the main reason for their study is the desire
to be able to stop the destruction caused by the disease-producing
forms.

The Ascomycetes are so named because of the ascus or sac
which is the characteristic spore-bearing structure of the group.
The ascus is an enlarged end of a hypha which becomes a thin
walled sac in which spores are produced. Any Fungus producing
spores in an ascus is called an Ascomycete. The spores produced
in an ascus are called ascospores. The Ascomycetes have other
spores, but the ascospores are the most general ones.

The Ascomycetes differ from the Phy corny cetes in having no
zoospores and in having hyphae divided by cross walls. Many of
the Ascomycetes have sex organs and differentiated gametes, but
the cell resulting from fusion develops immediately into asci, so
there are no resting oospores to be considered in this group.
Taking care of the ascospores takes care of the results of fertili-
zation.

The Ascomycetes vary widely in character of plant body and
methods of reproduction. In some the plant body is a structure
with a definite form, while in others it is only a scattered mass of
hyphae. In some the plant body is very prominent, but extremely
inconspicuous in others. Some have well-defined sex organs,
while others apparently have abandoned sexual reproduction and
have lost their sex organs. Their sex organs resemble those of
the Red Algae and this is the feature that suggests their relation-
ship to the Algae. There are about 15 orders and 29,000 species
of Ascomycetes. The Morels (Helvellales), Cup Fungi (Pezizales),
Closed Fungi (Pyrenomycetales) , Naked Ascus Fungi (Protodi
scales), Mildews (Peri sporiales), the Blue and Green Molds
(Plectascales) , and the Yeasts (Protoascales) are familiar orders.

The Morels (Helvellales). — Not all of the Fungi of this order
are Morels, but the Morels are the most familiar ones. The
fleshy plant body with a definite form and often so large as to
be quite conspicuous is one of the notable features of the Hel-
vellales. They are mostly saprophytic and the mycelium usually
develops underground where it lives on decaying wood, leaves,
etc. Here belongs the Edible Morel shown on next page.

COMMON EDIBLE MOREL (MORCHELLA ESCULANTA) 365

The Common Edible Morel (Morchella esculenta). — The
common Edible Morel is found in the spring, commonly in May
and early June. It is quite generally collected and used for food.
It is often called a Mushroom, although it is not the cultivated
Mushroom. Morels are usually found in the woods among the
leaves and about old logs and stumps. Often they grow in
clusters as Figure 315 shows. The wrinkled top and supporting
stalk consist of hyphae so massed together as to form a definitely

FIG. 315. — A cluster of Morels, Morchella esculenta (X I). Photographed

by C. M. King.

shaped plant body. The mycelium absorbs food from decaying
organic matter in the earth, and when it is well established in the
soil, the portion above ground is produced. The asci with the
ascospores are produced in the pits of the wrinkled top which is
known as the ascocarp. A small portion of a section through a
pit, as seen under the microscope, is shown in Figure 316. The
asci are numerous and each contains eight ascospores. The asci
with the intermingling sterile hyphae, called paraphyses, consti-
tute a distinct layer, known as the hymenium, on the surface of
the ascocarp. After the spores are mature, the ascocarp decays
and frees the spores which are widely distributed by wind and

366

THALLOPHYTES

other agents. When located on favorable organic matter, the
spores grow directly into new mycelia.

Although Morels spring up quickly, often apparently over
night, much time is required for the development of the sub-
terranean mycelium before the aerial portion is developed. No
sexual reproduction has been discovered
in the Morels, and the only spore known
is the ascospore.

Some other edible Ascomycetes, which
command high prices in Europe, are the
Truffles, which belong in the order Tube-
rales. The distinctive feature of the
Truffles is that the ascocarp occurs
wholly underground. The ascocarp,
which is tuber-like, is closed except
for a small opening and the spores are
released by the decay of its walls. Since
they are underground, they are very
difficult to find, and experts hunt them
by the aid of trained pigs or dogs which
detect them through the sense of smell.
No sexual reproduction has been dis-
covered, but not much is known of their
life cycle.

Cup Fungi (Pezizales). — The Cup
Fungi include many species most of
which are saprophytes. The loose my-
celium develops in decaying rich humus, decaying wood, or leaf
mold, and when well established it produces above the surface an
ascocarp which has the form of a disk, funnel, or cup. Such an
ascocarp is called an apothecium to distinguish it from other types
of ascocarps.

Peziza. — This genus, a species of which is shown in Figure
317, is common in the woods and the cup-shaped apothecium is
sometimes 2 or 3 inches across and often brightly colored. In
one common form the interior of the cup is bright scarlet. The
interior of the apothecium is lined with a hymenium consisting
of parallel, sterile, hyphal threads or paraphyses among which
occur the asci each containing eight spores. By the swelling and
rupturing of the asci the ripe spores are expelled and then scat-

FIG. 316. — Asci (a) of
the Morel, showing the
ascospores (X about 200).
The hypha (p) producing
no spores is called a pa-
raphysis.

PYRONEMA

367

tered by the wind. No sexual reproduction has been discovered
in Peziza, but in Pyronema, a form similar to Peziza, sexual
reproduction has been
discovered and carefully
followed.

Pyronema. — In this
form there are sex organs
and the apothecium de-
velops as a result of
fertilization (Fig. 318).

The female sex organ FlG. 317. - A cluster of Cup Fungi,

resembles that of the Pezizas. x £.

simpler Red Algae, such

as Nemalion. It consists of a globular cell (oogonium) and an
elongated tube-like cell (trichogyne or conjugating tube). The

FIG. 318. — Sexual reproduction in Pyronema confluans. A, the sex organs
at the time of fertilization, showing the antheridia (a) in contact and fusing
with the trichogynes through which the sperms pass to the oogonia (o) ; B, de-
velopment of apothecium, showing the oogonia developing ascogenous hyphae
which are beginning to form asci at the ends of their branches, and the sterile
hyphae (6) which grow up among the ascogenous hyphae and form a large
part of the wall of the apothecium. Highly magnified. After Harper.

antheridium is a somewhat club-shaped terminal cell which
comes in contact with the tip of the trichogyne and fuses with
it. Both oogonium and antheridium are multinucleate. The

368

THALLOPHYTES

numerous nuclei of the antheridium flow into the trichogyne and
pass on into the oogonium where they pair and fuse with the
numerous nuclei of the oogonium. From the fertilized oogonium,
now known as the ascogonium, branches called ascogenous hyphae
are developed and on the ultimate branches of these are produced

the asci. From beneath the
ascogonium sterile hyphae
(hyphae producing no asci)
grow up among the ascoge-
nous hyphae and constitute the
paraphyses of the hymenium.
Other sterile hyphae form the
wall of the cup-shaped plant
body or ascocarp. Usually
several oogonia are involved
in the formation of a single
ascocarp.

Brown Rot of Stone Fruits
(Sclerotinia fructigena). -
This Fungus, shown in Figure
319, is one of the parasitic
forms of the Pezizales. In
some years this Fungus is an
extremely destructive para-
site. It attacks nearly all
stone fruits and in some years
nearly half of the Plum and
Peach crop may be destroyed
by thjs disease. In Georgia
the egtimated logs in peaches

. . . .

and Plums caused by tms
disease in 1900 was between

$500,000 and $700,000. To a

limited extent it attacks the twigs and flowers and does some
damage in this way.

Fruits half size or larger seem to be most susceptible to the
attack of the Fungus. The disease first shows as small decayed
spots, dark brown in color. The fruit decays rapidly and soon
hyphae break through from beneath, forming moldy patches on
the surface. The moldy patches contain conidiophores which

FIG. 319. — Sclerotinia fructigena.
Above, the apothecia developed on a
decayed Plum; at the right, below,
section through an apothecium show-
mg asci and paraphyses; at the lelt,
below, an ascus and paraphysis more
highly magnified. After Duggar.

BLACK KNOT (PLOWRIGHTIA MORBOSA) 369

produce conidiospores abundantly. The conidiospores can live
over till the succeeding season and start the disease anew. The
disease is propagated chiefly by conidiospores. It was a long
time after the disease was known before ascospores were found
and of course it was not then classed as an Ascomycete but was
put into the class Fungi Imperfecti. Apparently ascospores are
often not formed at all, and, when they are, they occur in the
diseased fruits after they have dried up and usually fallen from
the tree. As the fruit decays it dries up into a mummy. In this
dried-up fruit, regardless of whether it is on the ground or on the
tree, the mycelium becomes changed into compact masses called
sclerotia. Later, probably the next spring, upon these sclerotia
are developed bell-shaped apothecia in which the ascospores occur
(Fig. 319). Thus in controlling the disease the destruction of
the mummied fruits as well as spraying to kill the conidiospores
that are sticking to the buds and bark are advised.

The Closed or Black Fungi (Pyrenomycetales). — These
Fungi, of which there are about 11,000 species, include both
parasites and saprophytes. They vary much in form and
manner of growth. They are chiefly characterized by a super-
ficial, compact, black mycelium looking as if it had been charred
by fire. The structure in which the asci are produced is a peri-
thecium, a small commonly flask-shaped cavity with a small
pore-like opening. Many of these Fungi produce destructive
plant diseases, of which the Black Knot, Ergot, and Chestnut
Disease are familiar ones.

Black Knot (Plowrightia morbosa) . — This Fungus occurs on
the twigs of Plum and Cherry trees, producing wart-like excres-
cences as shown at A in Figure 320. The mycelium attacks the
cambium, phloem, and cortex, causing at first an abnormal growth
and later the death of these tissues. As a result of the attack, the
twig is much injured or killed. The attack is often so general that
the entire tree is killed. The wart-like excrescences or knots con-
sist of the mycelium and the abnormally developed tissues of the
host. During the first summer the disease shows as slight swell-
ings, but with the renewed growth of the following spring, the
swellings enlarge rapidly, and during May or June the mycelium
breaks through the bark and forms a dense covering over the sur-
face of the swellings. From the hyphae forming the covering of
the knot numerous erect hyphae arise which give the knot a

370

THALLOPHYTES

velvety appearance. These erect hyphae are conidiophores and
bear conidiospores as shown at B in Figure 320. The conidi-
ospores are scattered by the wind and upon germination grow
directly into hyphae which can penetrate a young shoot and start

the disease anew. In late
summer after the produc-
tion of conidiospores is
over, the knot becomes
black and on its surface
occur numerous small
papillae which are the
flask - shaped perithecia,
opening with a pore and
lined on the inside with
asci as shown at C n
Figure 320. The asco-
spores are mature and
ready to be distributed
early the next spring.

It follows then that the
disease may be spread dur-
ing the early spring by
ascospores or during late
spring and summer by the
conidiospores. The de-
struction of the knots be-
fore the shedding of the

FIG. 320. — Black Knot, Plowrightia
morbosa. A, branch of a Plum, showing the
wart-like excrescences caused by the Fungus;
B, conidiophores producing conidiospores
(X 500), and at the right a conidiospore
germinating; C, two perithecia sectioned
lengthwise, showing the asci and paraphyses
within ( X 50) ; D, asci and paraphyses more
highly magnified.

spores will check the dis-
ease. Bordeaux mixture
applied at proper times is
useful in checking the dis-
ease, but most attention
should be given to the de-
struction of the diseased

branches.

Ergot (Claviceps purpurea and Paspali).1 — Ergot is a parasite
on the young ovaries of the Grasses, being especially common on
Rye and occurring sometimes on Wheat, Barley, and a number of

1 Ergot and Ergotism. Press Bulletin 23, Nebraska Agr. Exp. Sta., 1906.
Life History and Poisonous Properties of Claviceps Paspali. Vol. 7, No. 9,
pp. 401-406, Jour. Agr. Research, U. S. Dept. Agr., 1916.

THE CHESTNUT DISEASE (ENDOTHIA PARASITICA) 371

other Grasses. The ascospores affect the ovaries in early summer.
In the ovary the mycelium develops, using the food material
which the ovary should have. The mycelium produces on the
surface of the ovary numer-
ous conidiophores which
produce conidia abundantly,
and the conidia are dis-
seminated largely by insects
which seek the honey dew
secreted by the mycelium.
After the tissues of the
ovary are destroyed, the
mycelium becomes trans-
formed into a dark, hard,
club-shaped body called
sclerotium which projects
from the spikelet as shown
in Figure 321. These
bodies, which are the so-
called Ergot, contain one or
more alkaloids which are
poisonous to both man and
live stock. Stock are some-
times badly poisoned by
eating Timothy, Red Top,
and other kinds of hay
where Ergot is abundant.
The sclerotia fall to the
ground and pass the winter.
The next spring they de-
velop branches which bear
rose-colored globular heads,
called stromata, in which the
asci are produced in sunken

FIG. 321. — The Ergot Fungus, Clavi-
ceps purpurea. a, head of Rye, showing
projecting sclerotia; 6, a sclerotium which
has developed stalks bearing globular
heads in which the perithecia occur ( X 3) ;
c, section through one of the globular
heads, showing the perithecia (X 15); d,
ascus highly magnified, showing the
spindle-shaped ascospores; e, hypha and
conidia which develop on the surface of
the grain in the early stage of infection.
From Tulasne and Strasburger.

perithecia.

The Chestnut Disease (Endothia parasitica) . — This disease
was introduced from Asia and appeared in New York about
1904. It is very destructive to Chestnut trees, and the estimated
loss in New York City and vicinity is more than $5,000,000.
For the entire United States, the financial loss up to 1911 was

372

THALLOPHYTES

estimated at about $25,000,000. So serious is this disease that
legislatures have made special appropriations for righting it.

The spores are carried by the wind and sometimes by birds
and insects. When the spores reach the bark of the Chestnut,
they develop hyphae which penetrate and kill the phloem and
cambium. The dead bark soon becomes warty with yellowish-
brown pustules in which summer spores in great numbers are

FIG. 322. — Pus-
tules on the bark of
a Chestnut caused
by the Chestnut
Blight Fungus.
From Bulletin 380,
U. S. Dept. Agri-
culture, 1917.

FIG. 323. — Powdery Mildew
on an Apple leaf. The light
areas are due to the presence
ot many superficial hyphae.
From Bulletin 185, Maine Agr.
Exp. Sta.

produced (Fig. 322}. The summer spores are extruded in
threads and spread the disease to other trees. In autumn these
same pustules develop deeply buried perithecia in which the
ascospores (winter spores) develop. The ascospores germinate
the next spring and when carried to other trees start the disease
anew. The mycelium in an affected tree renews its activity each
year and thus continues to spread, usually downward, until the

POWDERY MILDEWS (PERISPORIALES) 373

tree is killed. The deeply buried mycelium is not reached by
sprays, and the total destruction of the infected trees is the only
available method of checking the disease.

Powdery Mildews (Perisporiales) . — This group includes
many Fungi, but they are all very similar in their habits. The
mycelium commonly occurs on the surface of leaves, but some-
times on the stems and fruits of the higher plants. The myce-

FIG. 324. — Powdery Mildew of the Hop. Below, diagrammatic draw-
ing of a section of a Hop leaf, showing the superficial mycelium which has
grown haustoria into the epidermal cells, and produced erect conidiophores
bearing chains of conidia ( X about 50) . Above, epidermal cell, hypha, and
invading haustorium more highly magnified. From Bulletin 328, Cornell
University Agr. Exp. Sta.

lium forms quite noticeable powdery patches. The asci are
produced in closed ascocarps called cleistothecia. In Figure 323
is shown the mildew of the Apple.

The Lilac Mildew (Microsphaera) is the one most commonly
observed of the Mildews. Often in late summer and autumn,
the leaves of the Lilac are so generally covered with the whitish
dusty-looking patches, that the entire bush appears covered with
street dust. But there are also Mildews that occur on fruit
trees, Roses, Gooseberries, Peas, and other cultivated plants,
which do considerable damage. From the superficial hyphae

374

THALLOPHYTES

haustoria are sent into the host. These haustoria absorb food
from the tissues, and often cause considerable injury to the
leaves and fruit.

From the superficial hyphae arise numerous erect conidio-
phores, which pioduce chains of conidiospores (Fig. 324)- The
powdery appearance of the Fungus is due to the ascocarps and
the numerous conidiospores. The conidiospores are distributed
by the wind and, when favorably placed, grow directly into hyphae,
and are the means of producing new growths of the Mildew.
Late in the summer and autumn, the superficial hyphae form

FIG. 325. — At the left, surface of a leaf infected with Powdery Mildew,
showing the superficial mycelium, ascocarps, and conidiophores. At the
right, a cleistothecium broken open, showing the asci which develop within.
From Tulasne and Nature.

globular heavy-walled cleistothecia in which the asci are produced
and which, when mature, appear to the naked eye as black dots
on the surface of the leaf (Fig. 325).

Projecting from the wall of the ascocarp are appendages which
may have variously branched tips. Enclosed within the heavy
wall of the ascocarp, the ascospores pass the winter. When freed
in the spring by the breaking of the ascocarp, the spores may be
blown or carried . about and germinate upon a new host. The
development of the ascocarp is a result of fertilization and the sex
organs, like those of Pyronema, suggest those of the Red Algae.

The ascocarp of the Mildews suggests the cystocarp of the

ASPERGILLUS

375

higher Red Algae, such as Polysiphonia, for as the ascogenous
hyphae develop from the ascogonium, sterile hyphae, growing up
from below the ascogonium, form a compact hard wall which
makes a case for the asci and ascospores, just as the filaments
growing up from below the
carpogonium produce a case
for the carpospores in Poly-
siphonia.

The Blue and Green Molds
(Plectascales) . — S u p e r fi-
cially these Molds resemble
the true Molds discussed
under the Mucorales, but
their spore masses are gen-
erally green or blue, while
those of the true Molds are
black. There are about 250
known species in this order,
but they are saprophytes and
only a few of them are of
much importance. They
bear their ascospores in
closed ascocarps or Cleisto-
thecia. Aspergillus and
Penicillium are two familiar
genera of the order.

Aspergillus. — These Molds
are commonly green on ac-
count of their greenish spore
masses. One form known
as the Herbarium Mold is
troublesome in herbariums

where it attacks specimens that are not well dried. They
often occur along with the true Molds. They will grow
on cheese, leather, wall paper, fruit, hay, silage, and on
most any damp object from which they can obtain nourish-
ment. Some are poisonous and stock are injured and
sometimes killed by eating them in moldy Corn, hay, and
silage.

The loose extensive mycelium runs over and through the

FIG. 326. — A species of Aspergillus.
A, a portion of a mycelium, showing a
conidiophore bearing chains of conidia
(300); B, sex organs coiled about each
other and consisting of hyphse similar
in appearance; C, the cleistothecium
which develops after fertilization and in
which the asci develop (X 200).

376

THALLOPHYTES

substratum, and sends up conidiophores at the ends of which the
conidia are borne in radiating chains as shown in Figure 326.
The spores are scattered mostly by the wind.

The sex organs appear a little later than the conidia and
consist of two short hyphal filaments which come together and
intertwine spirally. One of these filaments represents the oogo-

nium and the other, the antheridium.
After fertilization, ascogenous hyphae
develop from the ascogonium and bear
eight-spored asci at their tips. In the
meantime other hyphae grow up from
below the ascogonium and a closed case
or cleistothecium is
formed, within which
are the asci inter-
mingled amongst
sterile hyphae. The
walls of the asci
finally dissolve, thus
setting the asco-
spores free within
the cleistothecium.
Through the decay

of the wall of the cleistothecium the spores are
finally freed to be scattered by the wind.

Another Ascomycete which sometimes
poisons livestock is the Purple Monascus.
It belongs to another order and is a simpler
Ascomycete than Aspergillus. It is often
present in moldy silage and when fed to live-
stock may cause death. This mold produces
a purple pigment which colors the substratum
upon which the mold lives and distinctly colors
silage attacked by the Mold.

Penicillium. — A common species of Penicillium is the Blue
Mold which develops on shoes or gloves left in damp places, and
on lemons, cheese, etc. It often occurs intermingled with Bread
Mold on bread. The conidia are borne as shown in Figure 327.
Its sexual reproduction is similar to that of Aspergillus and the
cleistothecia are about as large as a coarse grain of sand.

FIG. 327. — A species of
Penicillium, showing conidi-
ophores bearing chains of
conidia.

DOQ

FIG. 328. — *A
naked -ascus Fun-
gus, Taphrina pruni
on a plum, showing
the asci developed
without any cover-
ing on the surface
of the epidermis
(X400). Redrawn
with modifications
from Strasburger.

YEASTS (SACCHAROMYCES)

377

Certain species1 give desirable flavors to some kinds of cheese
and are quite useful in this connection.

Naked-ascus Fungi (Protodiscales). — This is a small group of
parasites which attack seed plants. They produce no ascocarp
and the asci are therefore borne exposed (Fig. 328}. So far as
known they have no sexual reproduction. They are regarded
as simple Ascomycetes. One common species is the Exoascus
deformans, which causes the disease known as Peach Curl. The
mycelium develops in the tissues of the host and forms on the
surface asci which appear as gray pow-
dery films. One species attacks the
young ovaries of Plums, causing the
malformation known as " Bladder
Plums," and one species causes Witches'
Brooms on some of our deciduous trees.

Yeasts (Saccharomyces). — The
Yeasts are very simple Ascomycetes.
In most Yeasts the hyphae are so short
and simple that they appear as single
globular cells The only reason for
calling them Ascomycetes is that under
certain conditions the cells form spores
and then resemble asci (Fig. 329).

On account of their ability to fer-
ment sugars and produce carbon dioxide
and alcohol, they are useful in making
bread and in making alcohol, wine,

beer, and other liquors which contain alcohol. When placed in
dough they grow and work rapidly, arid the carbon dioxide pro-
duced causes the bread to rise. There are many kinds of Yeasts,
and each kind gives a different flavor to the fermented product.
For this reason brewers keep pure cultures of certain kinds of
Yeasts, which give the liquor the desired characteristics.

Their main method of reproduction is by the rapid division of
cells, often called budding, in which small cells are apparently
pinched off from the parent cell. The cells often remain in
contact for some time after being budded off, forming chains of
cells.

1 Cultural Studies of Species of Penicillium. Bulletin 148, Bureau of
Animal Industry, U. S. Dept. Agriculture, 1911.

FIG. 329. — Bread Yeast,

Saccharomyces cerevisiae. a,
single plant (X 600); b, a
plant in the process of bud-
ding; c, plant which has
formed spores; d, plants re-
maining in contact and
forming chains as they are
multiplied by budding.

378

THALLOPHYTES

Other Ascomycetes. — A study of a few types of the Ascomy-
cetes has given a general notion of their habits but no notion
at all of their extensive number. However, with this general
acquaintance, other forms can be easily understood. Some other
common destructive forms are the Apple and Pear Scab 1 (Fig.
330}, the Bitter Rot of Apples 2 (Fig. 331), Peach Mildew,3 Black

FIG. 330. — Apple attacked by Scab, Venturia Pomi. Photographed

by Whetzel.

Rot of Grapes,4 and the Wilt disease of Cotton, Watermelons,
and Cowpeas,5 etc.

Summary of Ascomycetes. — The Ascomycetes have no water
habits and their chief resemblance to the Algae is in the character
of their sex organs and fruiting bodies. The plant body ranges

1 A Contribution to Our Knowledge of Apple Scab. Bulletin 96, Mon-
tana Agr. Col. Exp. Sta., 1914.

2 Bitter Rot of Apples. Bulletin 44, Bur. PI. Ind., U. S. Dept. of Agricul-
ture, 1903.

3 Peach Mildew. Bulletin 107, Colorado Agr. Exp. Sta., 1906.

4 The Control of Black-Rot of Grape. Bulletin 155, Bur. PL Ind., U. S.
Dept. Agriculture, 1909.

5 Wilt Disease of Cotton, Watermelon, and Cowpea. Bulletin 17, Division
of Vegetable Path., U. S. Dept. Agriculture, 1899.

Also see Spraying Practice for Orchard and Garden. Bulletin 127, Iowa
Agr. Exp. Sta., 1912.

LICHENS 379

from a single cell, as in Yeast, to a massive mycelium which in
some cases takes no definite shape while in others it forms a
definitely shaped fruiting body. In parasitic forms the mycelium
sometimes runs through the tissues of the hosts, and sometimes
is chiefly superficial, sending only haustoria into the host.

FIG. 331. — Apple attacked by the Bitter Rot Fungus, Glomerella rufomaculans.

After Alwood.

The spores are of two kinds, conidiospores and ascospores.
The conidiospores are borne free on projecting hyphae, and grow
directly into hyphae upon germination. The ascospores, the
characteristic spores of the group, are borne in asci which are
usually produced within a fruiting body or ascocarp, which may
be an open structure or a closed one.

In controlling the disease-producing forms one must reckon
with conidiospores and ascospores.

Lichens

Lichens are very common structures which form splotches on
stumps, tree trunks, rocks, old boards, etc., and some grow upon
the ground. Figure 332 shows an Apple twig covered with
Lichens. They may appear as a crust covering the support ; or
they may have flat lobed bodies like the one shown in Figure 333;

380

THALLOPHYTES

or they may have slender branching bodies like the one shown
in Figure 334- The slender branches may be erect, prostrate,

or hang in festoons from the
branches of trees or other sup-
ports.

A Lichen, although regarded
as a plant, is a structure formed
by the association of a Fungus
and an Alga. The Fungus in-
volved is in nearly all cases an As-
comycete, and the Alga involved
is nearly always a unicellular
form of the Green Algae or some
form of the Blue-green Algae.
The Fungus is a parasite on the
Alga, obtaining food from the
Alga. The hyphae of the Fungus
get food from the Alga by being

FIG. 332. — Lichens on an Apple
branch. Frpm Bulletin 185, Maine
Agr. Exp. Sta.

injured in most cases.
Figure 335, shows a
meshwork of hyphae
and in the meshes the
cells of the Alga are
held. Usually the hy-
phae are more closely
interwoven in the outer
.region, thus forming a
compact cortical region
which encloses the
looser region within
where the cells of the
Alga are usually more
abundant. On the
under surface filamen-

in close contact, and since the
cells of the Alga are rarely pene-
trated, the Alga apparently is not
A section through a Lichen, as shown in

FIG. 333. — A Lichen with a flat lobed body
growing on bark. The asci are produced in
the small cups. X \.

tous structures are developed which attach the plant body to the
substratum. The mycelium of the Fungus thus constitutes the
framework of the plant body or thallus. i

LICHENS 381

The two plants of this association are of mutual help. The
sponge structure formed by the Fungus holds water for the Alga,
while the Alga makes carbohydrates, some of which can be used
by the Fungus. As a result of this mutual help, the Lichen can
live on dry barren rocks where other plants cannot exist. Neither

FIG. 335. — A much en-
larged section through a
FIG. 334. — A much branched Lichen, showing the fungal
Lichen hanging from the branch hyphae and the globular cells
of a tree. of the Alga.

the Alga nor the Fungus could grow in such places alone, for the
Alga would lack moisture and the Fungus would lack food.
Being so little dependent upon their support for moisture and
food, the Lichens are the pioneers on bare and exposed surfaces.
They hasten the disintegration of rock and start soil formation.
The materials of their dead bodies added to the disintegrate rock
form a soil for other plants.

Lichens multiply vegetatively by small scale-like portions,
called soredia, which separate from the main plant body. Soredia
are small masses of hyphae in which some algal cells are en-
tangled and are capable of growing directly into Lichens.

The fungal member of Lichens usually reproduces by asco-
spores and the algal member by cell division. The asci occur in
ascocarps which appear as small cups or disk-like bodies on the
surface of the plant body (Fig. 336). The sex organs are quite
suggestive of the Red Algae. The antheridia occur on branching
hyphae and are very small cells which break off and function as
sperms. After fertilization, sterile hyphae grow up from below
the ascogonium and form the wall of the ascocarp which finally

382

THALLOPHYTES

breaks through and appears on the surface of the plant body as

a cup or disk.

Besides being the pioneer plants on rocks and other places

where they form soil and thus make it possible for higher plants

to get a start, they are also of some economic importance

in other ways. In northern re-
gions the Lichen known as Rein-
deer Moss is an important food
for animals. Some forms are
used as food by man. Although
not parasites, they sometimes are
harmful to plants upon which
they grow. When growing on
the twigs of fruit trees, they pre-
vent the bark from functioning
properly and also furnish a shelter
for various kinds of destructive
insects.

FIG. 336. — Reproduction in
Lichens by ascospores. Above,
vertical section through a cup
(apothecium), showing asci and
paraphyses; below, asci and pa-
raphyses shown more enlarged.
Redrawn from Schneider.

Basidiomycetes

General Description. — This is
the group of Fungi to which
Toadstools, Mushrooms, Puff-
balls, Rusts, and Smuts belong.
The group scarcely needs an intro-
duction, because such conspicu-
ous forms, as Toadstools, Mush-
rooms, and Puffballs are familiar
to everybody. In number of forms this group is next to the
Ascomycetes. Their characteristic spore-bearing structure is the
basidiwn, which is the enlarged end of a hypha with usually four
slender branches upon which spores are borne, one spore being
borne on the end of each branch. Just as the spores borne in an
ascus are called ascospores and are the characteristic spores of
the Ascomycetes, so those borne on a basidium are called basidi-
ospores and are the characteristic spores of the Basidiomycetes.
The mycelium of many is saprophytic, living in decaying wood,
rotten manure, and other kinds of organic matter. In others,
such as the Rusts, Smuts, and other forms, the mycelium is
parasitic, living upon the tissues of the grains and other higher

GENERAL DESCRIPTION 383

plants. Even the saprophytic forms cause some undesirable
destruction. They often start in the wounds of fruit trees, shade
trees, and forest trees, and the action of their mycelia hastens
decay and may lead to the destruction of the tree.

In many forms the mycelium, after it is well established in the
region of food supply, produces on the surface of the substratum
some kind of a body in which the spores are borne. It will be
recalled that this is the habit of the Morel. This body, since it
bears the spore, is called a sporophore which really means a
"spore-bearing body." It is a term commonly applied to a
spore-bearing hyphae or to any portion or all of the plant body
which has to do with bearing spores. Thus the wrinkled top and
stalk bearing it constitutes the sporophore in the Morel. In the
Toadstools and Mushrooms, the sporophore is often umbrella-
shaped. In some forms which grow on the sides of trees and
stumps, the sporophore resembles a small shelf projecting from
the support, and in this case the sporophore is often hard. In
Puffballs the sporophore is more or less globular. Sporophores
are extremely variable in both shape and texture, and are the
structures by which those Fungi which have them are classified.
The sporophore is the part of the Fungus that attracts attention.
It is the portion that is eaten and called a Mushroom. The
portion of the mycelium which traverses the substratum is usually
hidden, and its presence is not known until the sporophore
appears.

Many of the parasitic Basidiomycetes, like the Smuts and
Rusts, have no conspicuous sporophores, and the presence of the
mycelium is indicated only by the occurrence of unusual struc-
tures on the surface of the host plant. In case of Smut the pres-
ence of the disease is indicated by the appearance of Smut balls,
and in Rusts, by the red or black blisters occurring on the leaves
and stem of the host.

Although the basidiospores are the characteristic spores of the
group, a number of other kinds of spores occur, which in some
cases are more important in reproduction than the basidiospores.
Sexual reproduction has been entirely lost by many of the group,
and in those where it is retained the fusion is between hyphae,
there being no sex organs formed. There are no oospores or
zygospores to be considered in this group.

The Basidiomycetes, of which there are 14,000 or more species,

384

THALLOPHYTES

are divided into a number qf orders. The most familiar orders
are those represented by the Toadstools and Mushrooms (Hy-
menomycetes) , Puff balls (Gasteromycetes), Smuts (Ustilaginales),
and Rusts (Uredinales).

Toadstools and Mushrooms (Hymenomycetes). — This is the
most familiar order to most people, because it includes so many
forms like the Toadstools and Mushrooms, which have conspicu-
ous sporophores. In addition to the Toadstools and Mushrooms,
the order contains some other rather familiar kinds of Fungi.

The Fungi of this order are chiefly
saprophytes, living on decaying wood,
leaf mold, rich humus, and manure.
Often the organic matter upon which
they are living is not visible and they
seem to be growing right out of the
soil. As the name of the order sug-
gests, they have a hymenium, and
the hymenium, which consists of
basidia commonly intermingled with
sterile hyphae, is borne exposed.
Usually the hymenium is on the
under side of the sporophore where it
is protected from rain.

Those of the order having umbrella-
shaped sporophores are popularly
called Toadstools and when edible
they are popularly called Mushrooms.
The term Mushroom, however, is
often applied to Morels and all kinds
of Fungi that are edible. There are

several hundred species of edible Fungi in the United States
and more than one hundred of them are of the Toadstool type.
Some of the Toadstools are deadly poisonous, as the one shown
in Figure 337, and many that are not poisonous are tough,
fibrous, or ill-tasting and hence not edible. Between edible and
non-edible Fungi there are no botanical distinctions or guides.
By experience people have learned that some species are edible
and some non-edible, and many sad accidents have occurred as a
result of not being able to distinguish the poisonous from the
edible ones.

FIG. 337. — A poisonous
Toadstool, Amanita bulbosa.
Xi

TOADSTOOLS AND MUSHROOMS (HYMENOMYCETES) 385

The order is divided scientifically into a number of sub-
groups according to the method of exposing the hymenium. In
the largest and most important group of Hymenomycetes, the
hymenium covers the surface of thin radiating plates called gills.
These Fungi are known as the Agarics or Gill Fungi. To the
Gill Fungi belong most Toadstools and the Field Mushroom
(Agaricus campestris) which is extensively cultivated for market.

FIG. 338. — Stages in the development of the Mushroom, Agaricus cam-
pestris. I, ground line; m, underground portion of mycelium; s, stipe;
p, pileus; g, gills; a, annulus. X I-

On account of their structural complexity the Agarics are re-
garded as highly developed Fungi. They develop as shown in
Figure 338.

Before developing the sporophore, the mycelium becomes well
established in decaying organic matter and this may require
considerable time. In the development of a sporophore, there
first appears on the surface of the substratum a small spherical
body called a button which has a skin-like covering within which
the sporophore is forming. This body elongates very rapidly if

386

THALLOPHYTES

the weather is warm and moist and sometimes the sporophore
attains full size in a few hours. The elongating sporophore
finally breaks through the covering of the button, spreads out its
umbrella-like top, and the characteristic sporophore appears with
remnants of the torn skin-like covering remaining attached.

When mature the sporophore consists of a stalk, called stipe,
and the expanded umbrella-like top, called pileus. On the under

FIG. 339. — Reproductive structures of the Mushroom, Agaricus cam-
pestris. A, the Mushroom with a portion of its pileus cut away to show
the gills, g, gills; s, stipe; a, annulus. B, section through a gill, highly
magnified to show the basidia (b) and the basidiospores (r) . Redrawn from
Leavitt.

side of the pileus are the thin radiating plates or gills bearing the
hymenium in which occur the basidia as shown in Figure 339. A
fragment of the skin-like covering of the button stage commonly
remains attached to the stipe, forming the annulus and in some
forms, as shown in Figure 337, a portion of the covering remains
as a cup at the base of the stipe, forming the volva. Other frag-
ments of the covering often remain as flecks on the outer surface
of the pileus. When the spores are mature, they fall from the

TOADSTOOLS AND MUSHROOMS (HYMENOMYCETES) 387

basidia and may reach the ground directly beneath or be carried
away by the wind. When favorably situated, the spores grow
new mycelia, thus com-
pleting the round of life.
The basidiospore is the
only spore formed and no
sexuality has been dis-
covered.

Small brick-like masses
of organic matter, usually
consisting of manure and
containing myc-elial
threads of the Mushroom
in a dormant state, are
sold on the market, and
used in starting Mush-
room beds, the mycelial
threads contained consti-

tuting the so-called Mush- ^ 340. -The Edible Boletus, a

room spawn. polyporus Fungus. X i

In another rather com-
mon family (Polyporaceae) of the Hymenomycetes, the hymenium

FIG. 341. — A Hydnum, a
Fungus in which the hymenium
is borne on tooth-like projec-
tions. X £.

FIG. 342. — A Basidio-
mycete, Clavaria, with a
much branched sporo-
phore. X |.

lines tubes with pore-like openings. These are known as the Pore
Fungi, and to this family belong some Toadstools, some of which
are edible (Fig. 240), and the Bracket Fungi, which form shelf-

388 THALLOPHYTES

like sporophores on the sides of trees and stumps. In the family
to which the Hydnums belong the hymenium is borne on tooth-
like projections (Fig. 341)- In another family the sporophore
is much branched and the hymenium covers the surface of the
branches (Fig. 342}. As to the texture of the sporophore, that
varies widely in the different families. In some families it is
gelatinous and without definite shape. It is fleshy in the Toad-
stools and Mushrooms and in some of the Bracket Fungi it be-
comes as hard and persistent as wood.

FIG. 343. — Some of the roots and the lower portion of the trunk of an
Apple tree which has been killed by the Toadstools.

Destructive Toadstools and Bracket Fungi. — Some Toad-
stools attack the roots of trees and cause the disease called Root
Rot. This disease occurs on a number of fruit trees, such as the
Apple, Plum, Cherry, and Peach, and on many shrubs and forest
trees. In Figure 843 is shown some Toadstools which have
destroyed an Apple tree. The Toadstools usually cause the death
of the roots, and this results in killing the tree. The mycelia of
the Toadstools probably enter the roots through wounds.

PUFFBALLS AND RELATED FORMS (GASTEROMYCETES) 389

In Figure 344 is a Bracket Fungus which causes a disease
known as White Heart Rot. This disease occurs on fruit trees
and many forest trees. The spore enters through a wound and
starts the mycelium which penetrates and transforms the heart
wood into a white pulpy mass. In Figure 345 is shown another
Bracket Fungus which attacks trees in a similar way and causes
the wood to rot and become reddish brown or black. It produces
the Red Heart Rot. There are many other destructive forms
which concern the forester and horticulturist. They start in

FIG. 344. — One of the Bracket Fungi, Fomes igniarius, living on the trunk
of a living Aspen. It attacks various trees, destroying the wood and causing
much damage. From Bulletin 189, Bureau of Plant Industry, U. S. Dept.
of Agriculture.

wounds where there is some decaying matter, and in pruning it
is necessary to guard against the entrance of these Fungi.

Puffballs and Related Forms (Gasteromycetes) . — On account
of the complexity of their sporophores, the Gasteromycetes are
considered the highest of all Fungi. They are saprophytes,
growing on decaying wood, leaf mold, rich humus, and manure.
They require about the same conditions for growth as do the Toad-
stools and Mushrooms and are often found growing with them.
There are about 700 species, many of which are edible. The
sporophore of these Fungi is usually more or less globular in form
and the hymenium is enclosed.

390

THALLOPHYTES

The most common and familiar members of the order are the
Puffballs, common in the woods and fields, and so named because
when pressed upon the spores puff out in cloud-like masses
(Fig. 346). Some of the Puffballs are a foot or more in diameter
when mature and most of them are edible. The sporophore

FIG. 345. — A Polyporus Fungus, Polyporus sulfureus, on the Red Oak.
It causes the Red Heart Rot of trees. Photo by Dr. W. A. Murrill, N. Y.
Botanical Garden,

develops from a subterranean mycelium, and is differentiated
into an outer region which constitutes a two-layered skin-like
covering (peridiwri) and an interior chambered region (gleba) in
which the basidia intermingled with sterile hyphae occur. Spores
are produced in immense numbers. A Puffball of ordinary size
produces many millions of spores. The spores are dark in color
due to their heavy walls. They escape from the sporophore
through pore-like or slit-like openings in the peridium.

PUFFBALLS AND RELATED FORMS (GASTEROMYCETES) 391

A very interesting Puffball is the Earthstar (Geaster) shown in
Figure 847. In this form the outer layer of the peridium splits
into regular segments and these segments are hygroscopic.
When the segments are wet they bend back and downward and
in this way the outer layer
of the peridium spreads out
like a star. The inner layer
of the peridium opens by an
apical pore and allows the
spores to escape as in other
Puffballs.

The Bird's Nest Fungi
(Fig. 348), which are close
relatives of the Puffballs,
show another interesting fes>

ture. They are small, usu-
ally less than a centimeter
in height and width. They
develop on twigs and sticks
as well as on organic matter

that is quite well decayed. One often finds them growing on the
benches in greenhouses. The chambers of the gleba become

FIG. 346. — Puffballs, Lycoperdons.
Three have opened at the top, thus
allowing the spores produced in the in-
terior to escape. X !•

FIG. 347. — An Earthstar, Geaster. About natural size.

enclosed in walls and separate. After the peridium opens, the
sporophore is cup-shaped and, with the egg-like chambers of the
gleba exposed, resembles a bird's nest full of eggs.

The Stink Horn (Fig. 349), noted for its intolerable odor, is
another Fungus of this order. Its mycelium feeds on decaying

392

THALLOPHYTES

FIG. 348. — A Bird's
Nest Fungus, Nidularia.
About natural size.

organic matter in the ground. The sporophore is at first globose,
but the gleba soon breaks out of the peridium and is elevated to
some distance above ground by an elongating stalk. The spore

masses are slimy and have the odor of
carrion. Certain insects which dissemi-
nate the spores are attracted by the
odor.

Smuts (Ustilaginales). — The Smuts
are parasitic Basidiomycetes. In some
Smuts, the mycelium, although evident
only in local areas, traverses widely
through the host, while in others only
local areas of the host are attacked.
No sporophores, such as characterize

the Toadstools and Puffballs, occur in the Smuts. There are
more than 2000 species of Smuts. They attack chiefly plants
of the Grass family and espe-
cially the cereals, the grains of
which they commonly displace
with powdery black masses of
spores. The financial loss due
to Oat Smut alone has been
estimated to be $10,000,000
annually in the United States.
In addition to the loss due to
the destruction of the cereal
crops and the lowering of their
market price, there is consider-
able loss due to Smut explosions
in thrashing machines. During
the summer of 1914, 300 thrash-
ing machines were blown up or
burned in the Pacific Northwest
by Smut explosions. Smut dust
is highly combustible when dry,
and is probably ignited by static
electricity in the cylinder of the

thrashing machine. The Smuts are particularly destructive
to Oats, Wheat, Rye, and Barley. Corn Smut is exceedingly
common but less destructive.

FIG. 349. — Stink Horn Fungus,
Phallus impudicus. At the right,
vertical section of the Fungus in
early stage of development, showing
the gleba enclosed by the peridium.
At the left, mature stage, showing
the gleba elevated much above the
peridium. X £.

THE SMUT OF OATS 393

The Smut of Oats.1 — The Smut of the Oats is probably the
most common and destructive one of the Smut group. The
mycelium of the Oat Smut gets started in the tissues of the Oat
plant when the latter is in the seedling stage, and at flowering

FIG. 350. — Loose Smut of Oats. Left, normal head; right, head
destroyed by Smut. After Bulletin 112, Minnesota Agr. Exp. Sta.

time it masses in the ovaries, which become swollen and finally
destroyed and replaced by masses of spores (Fig. 350). A

1 The following references will be found helpful in understanding the
smuts and methods of combatting them.

The Grain Smuts. Farmers' Bulletin 75, U. S. Dept. of Agriculture, 1898.

Corn Smut. Annual Report 12, Indiana Agr. Exp. Sta., 1900.

The prevention of Stinking Smut of Wheat and Loose Smut of Oats. Farm-
ers' Bulletin 250, U. S. Dept. Agriculture, 1906.

The Smuts of Grain plants. Bulletin 122, Minnesota Agr, Exp. Sta., 1911.

The Smuts of Wheat, Oats, Barley, and Corn. Farmers' Bulletin 507, U. S.
Dept. Agriculture, 1912.

Bunt or Stinking Smut of Wheat . Bulletin 126, Washington Agr. Exp . Sta.,
1915.

394

THALLOPHYTES

study of the formation of these spores shows that they are not
basidiospores, for they are not formed on basidia. The hyphae
in the smut ball simply divide into cells which separate and
become spores. These spores are the so-called brand spores, the
whole mass of them forming the so-called Smut. The spores are
very heavy-walled and appear black in mass. This kind of a
heavy-walled spore, which is simply a transformed vegetative
cell of the mycelium, is called a chlamydospore, a name referring
to the heavy protective wall. The spore masses break up when
mature and the spores are shed. In han-
dling the grain, especially in thrashing, the
spores escape in dust-like fogs. The spores
pass the winter on the ground, straw, grain,
or wherever they happen to fall. Many
of the spores lodge on the Oat grain, fall-
ing down between the lemma and palea
which enclose the Oat kernel. The follow-
ing spring the chlamydospores germinate,
each producing a small hypha called a pro-
mycelium, on which the basidiospores are pro-
duced. The basidiospores are produced on
the end and sides of the promycelium as
shown in Figure 851. Their number is in-
definite and they often multiply by budding
after the manner of the Yeasts. They are
quite commonly called conidia and often
sporidia, although they are comparable to the
basidiospores of the Toadstools and Puff-
balls. It is on account of the occurrence of
the promycelium, which is regarded as a
basidium, that the Smuts are classed as Basidiomycetes. Once
in contact with a young Oat plant, the basidiospores produce
hyphae, known as infection hyphae, which penetrate the young
plant and start the development of a mycelium.

It has been found that most of the infection in Oat Smut
results from the chlamydospores which are lodged on the grain,
and that by soaking seed Oats in hot water (132° to 133° F.) for
ten to fifteen minutes or in water containing about 1 pint of
40 per cent formalin to 45 gallons of water, the spores can be
killed and much loss to the Oat crop prevented.

FIG. 351. — Ger-
mination of Chlamy-
dospores. At the
left, a spore, and at
the right, a spore
which has germinated
and produced a pro-
mycelium bearing
basidiospores (c). X
about 300.

CORN SMUT

395

The Smut of Oats, Stinking Smut of Wheat, and Covered Smut
of Barley are very similar in habit and require similar treatment.
Sometimes, as in case of the Stinking Smut of Wheat, the infec-
tion of the seedling may be due to spores lodged in the soil as
well as to spores adhering to the kernel.

Loose Smuts of Wheat and Barley. — The Loose Smuts of
Wheat and Barley mature and shed their chlamydospores when
the grain is in flower. These
spores are borne away by the
wind and when falling on the
flowers of their respective
hosts, grow hyphae into the
young kernel. The kernel
continues its development,
but when mature it has con-
cealed within a tiny Smut
plant, which is able, when the
kernel is planted, to resume
its growth and develop in the
grain plant. Much of the
damage from these Smuts
can be avoided by seed selec-
tion. Treatments for these
Smuts must aim at killing
the tiny Smut plants con-
cealed in the seed grain.
Soaking the seed in cold
water five hours and then in

FIG. 352. — Ear of Corn with kernels
destroyed and replaced by masses of
Smut. From Farmers' Bulletin 507, TJ. S.
Dept. of Agriculture.

water 130° F. for ten minutes
is recommended.

Corn Smut. — Corn Smut
is the most conspicuous of
the Smut group. It attacks

all tender regions of the Corn plant but does most damage to the
flowers which become much enlarged and transformed into Smut
balls. Tumor-like developments of the Fungus occur also on the
leaves and stem as well as on the ear and tassel. In Figure 352
is shown an ear in which the kernels are replaced by the tumor-
like masses of the Fungus. These Smut bodies have a thin,
grayish, hyphal covering, and within the chlamydospores are pro-

396 THALLOPHYTES

duced by the division of hyphae as described in Oat Smut.
When the spores are mature, the skin-like covering breaks, thus
allowing the spores to be scattered. Some spores pass the winter
on the old stalks. Others pass the winter on the ground or wher-
ever they happen to fall. In the spring the chlamydospores ger-
minate and produce the promycelia and basidiospores. The
basidiospores are blown to the Corn and are able to grow hyphae
into the tender regions of the plant and start the disease. Treat-
ment of the seed Corn is, therefore, of little value in combatting
Corn Smut. In what way can Corn Smut be controlled?

Rusts (Uredinales) .* — Like the Smuts, the Rusts are internal
parasites and only their spore masses are visible externally.
They are so named on account of the red color of their spore
masses. There are about 2000 species of Rusts and they attack
nearly all kinds of plants but more especially members of the
Grass family. Although regarded as degraded parasites, they
are more complex than the Smuts, for they have more kinds of
spores and many of them have alternating stages upon different
hosts. For example, it is well known that Wheat Rust and the
Common Barberry bush (Berberis vulgaris) are associated. They
are associated because the Wheat Rust lives one stage of its life
cycle on the Wheat and the other on the Barberry. Each kind
of Rust lives on only certain hosts and the alternating hosts are
plants very different in kind, as those of the Wheat Rust
illustrate.

Rusts, although directly affecting only limited areas of tissue
around the places of attack, commonly attack the host in so
many places that they weaken the host and thereby prevent grain
plants from yielding normally. The financial loss to the farmer
due to Rusts is considerably more than that caused by Smuts.
Some years the loss in the United States due to the Black Rust
exceeds $15,000,000. The Black Rust of which six forms are
distinguished is the most important one of the Rusts.

Black Rust of Grain (Puccinia graminis). — The Black Rust,
sometimes called Red Rust, is a dreaded pest on Wheat, Oats,

1 Investigations of Rusts. Bulletin 65, Bureau of Plant Industry, U. S.
Dept. Agriculture, 1904.

Lessons from the Grain Rust Epidemic of 1904. Farmers' Bulletin 219,
U. S. Dept. Agriculture, 1905.

Rust of Cereals. Bulletin 109, South Dakota Agr. Exp. Sta., 1908.

BLACK RUST OF GRAIN (PUCCINIA GRAMINIS) 397

Rye, and Barley, and occurs on other Grasses. The presence of
the mycelium in the host is first known through the appearance of
reddish spots or lines on
the stems and leaves in
late spring or early sum-
mer. The reddish spots
or lines are regions of
spore production. They
are pustules or blister-
like structures caused by
masses of spore-bearing
hyphae which push up
the epidermis until it is
finally ruptured (Fig.
353} . The reddish color
of the pustules is due to
the reddish color of the
spores. These spores are
known as the " summer
spores" or uredospores.
The uredospores, which

FIG. 353. — Wheat Rust as it appears on
Wheat. Left, portion of a Wheat plant,
showing the pustules on the stem and leaf;
right, a much enlarged section through a pus-
tule, showing the summer spores (X 200).

are produced in great
numbers, are scattered by the wind, thus reaching other host

plants into which they grow
hyphae and thereby infect.
They are chiefly responsible
for the rapid spread of the
disease during summer.

Later in the summer, when
the grain is ripening and the
food for the Fungus becomes
scarce, the same mycelia pro-
duce heavy-walled, two-celled
spores, known as winter spores
or teleutospores (Fig. 354).
These spores are dark in color,
giving the pustules a dark ap-
pearance— whence the name

Black Rust. They pass the winter on the straw, ground, or wher-
ever they happen to fall. The following spring, each cell of the

FIG. 354. — A section through a pus-
tule in late summer, showing the winter
spores or teleutospores. X about 200.

398

THALLOPHYTES

teleutospore produces a promycelium bearing the basidiospores,
often called sporidia, as shown in Figure 355. Thus the teleuto-
spore occupies the same position in the life history of Rusts as the
brand spore occupies in the life history of Smuts. The basidio-
spores are scattered by the wind, and in regions where Barberry
bushes grow, they come in contact with the leaves of the Barberry
where they grow and produce mycelia in the leaf tissues.
Upon the Barberry, the mycelia produce on
the under surface of the leaf small cups called
aecidia in which spores are borne in chains
as shown in Figure 356. These spores are
called aecidiospores or cup spores. The
aecidiospores, which are shed in the spring
or early summer, are disseminated by the
wind and start the disease on the grains or
other Grasses, thus completing the life cycle
as it is shown in Figure 357.

In connection with the development of the
aecidiospores there occur on the upper sur-
face of the Barberry leaf very small flask-
shaped cups called spermagonia, in which are
produced very small spores called spermatia
or pycniospores. The spermatia have no
function and the spermagonia and aecidia are
supposed to represent the remnants of a sexual
apparatus which has become functionless.

Thus four kinds of spores are involved in
the complete life cycle of the Black Rust
and a fifth kind occurs. The uredospores
and teleutospores occur on the grains or
other Grasses. The basidiospores are pro-
duced by the teleutospores and no host is required, while aecidio-
spores occur on the Barberry bush.

If the Black Rust must have all of the stages in order to
propagate from year to year, then it seems that there should be
little or no Black Rust in regions where there are no Barberry
bushes, but such is not the case, for the Black Rust occurs
abundantly in fields many miles away from Barberry bushes.
Just how it gets started on the grains in localities where there
are no Barberry bushes is not definitely known. It was once

FIG. 355. — Te-
leutospore having
developed the pro-
mycelia bearing
basidiospores (s).

BLACK RUST OF GRAIN (PUCCINIA GRAMINIS) 399

FIG. 356. — Stage of the Wheat Rust on the Barberry bush, Berberis
vulgaris. Left, leaf of Barberry, showing the affected areas which are red-
dish, much thickened, and contain many cup-like depressions; right, a very
much enlarged section through the affected area of the leaf, showing one of
the cups (c) with chains of aecidiospores ( X 200) , The very small spores at
(p) are the spermatia or pycniospores.

FIG. 357. — Diagram showing the life cycle of the Wheat Rust. A,
wheat plants; B, barberry bush; u, uredospore; t, teleutospore; s, basidio-
spores; a, aecidiospore.

400 THALLOPHYTES

thought that the basidiospores started the disease directly on the
Grass host, but experiments have shown that they will not grow
on this host. Experiments have also shown that uredospores
are ordinarily killed by freezing weather and therefore are rarely
able to live over winter where the temperature goes much below
freezing. It has been suggested that some hyphae may enter the
kernels of the diseased plants and remain dormant until the seed
is planted and then infect the seedling, but this theory is not
generally accepted. Another suggestion is that the wind carries
the uredospores northward from the Southern states where they

FIG. 358. — Apple affected with Cedar Rust. From Technical Bulletin 9t
Virginia Agr. Exp. Sta.

are able to live over winter. It is also probable that the aecidio-
spores may be carried a considerable distance by the wind and
thus reach grain fields not in the immediate vicinity of Barberry
bushes. Then there i$ the probability that the disease may start
on the wild Grasses growing near the Barberry bushes, and be
passed along by the uredospores from one patch of Grass to an-
other until grain fields far away are reached.

No satisfactory preventative for the Black Rust has been dis-
covered. We are not able to control the spores. It is generally
believed that the eradication of all of the Common Barberry
bushes would do much toward eliminating this Rust. The most

CEDAR APPLES AND APPLE RUST (GYMNOSPORANGIUM) 401

hope, however, seems to be in breeding and selecting varieties of
grains which can resist the attack of the Rust, and some progress
has already been made in this direction.

Cedar Apples and Apple Rust (Gymnosporangium).1 — There
are several Rusts belonging to this group, but the one producing
Cedar Apples and the Rust
on Apple trees is the most
common and the most im-
portant of the group. It is
common in nearly every
region where Red Cedars
grow, but does most damage
to fruit trees in the Eastern
and Southern states. It lives
a part of its life cycle on the
Cedar, producing gall-like en-
largements on the branches,
and a part of its life cycle on
the Apple tree where it at-
tacks the leaves and fruit,
often causing much damage
to the fruit (Fig. 358). It
is the gall-like enlargements
on the Cedar tree that are
called Cedar apples, although
they are not apples at all.
In Figure 359 are shown
Cedar apples as they appear
in the winter. In the spring
gummy branches containing
many teleutospores develop on these galls which then look like
the one shown in Figure 360. The teleutospores produce basidio-
spores which are blown to the Apple tree where they start the

1 The Cedar-Apple Fungi and Apple Rust in Iowa. Bulletin 84, Iowa
Agr. Exp. Sta., 1905.

The Life History of the Cedar Rust Fungus Gymnosporangium juniperi-
virginianae. Annual Report 22, pp. 105-113, Nebraska Agr. Exp. Sta., 1909.

Apple Rust and its Control in Wisconsin. Bulletin 257, Wisconsin Agr.
Exp. Sta., 1915.

The Cedar Rust Disease of Apples caused by Gymnosporangium juniperi-
virginianae Schw. Technical Bulletin 9, Virginia Agr. Exp. Sta., 1915.

i m .,; l

FIG. 359. — Cedar Apples on the
Cedar. This is the way the galls look
in winter. From Bulletin 257, Wiscon-
sin Agr. Exp. Sta.

402

THALLOPHYTES

disease on the leaves and fruit. Upon the Apple tree, the aecidia
stage is produced, and the aecidiospores are able to attack the
Cedar and form new galls, thus completing the life cycle as shown
in Figure 361.

Pine Tree Blister-rust (Cronartium ribicola) . — As its name
suggests this Rust attacks Pine trees. It was introduced from

Europe about ten years ago
and has now become a seri-
ous disease in this country.
It has its aecidial stage on
Pines with five leaves in a
fascicle, such as the White
Pine and Sugar Pine, and
has species of Ribes (Goose-
berries and Currants) as the
other host. In this Rust the
aecidial stage is the most de-
structive. The mycelium of
the aecidial stage kills the
cambium and inner bark of
Pines, thus causing the
death of branches and some-
times of the entire tree.

FIG. 360. — A Cedar Apple which has
developed the gelatinous branches con-
taining numerous teleutospores. The
teleutospores produce sporidia or basidio-
spores that attack the Apple tree. These
gelatinous branches develop in the spring
after a rain and while the leaves and
shoots of the Apple are young and easily
attacked. After Bulletin 257, Wisconsin
Agr. Exp. Sta.

Both uredospores and teleu-
tospores are produced on
the infected Currant and
Gooseberry bushes, which
are apparently very little
injured thereby. Pines are
infected through the basid-
iospores. The chief means

of checking the spread of
the disease is through the destruction of the wild Currant and
Gooseberry bushes.

The damage done to Pine trees is serious and since our
Pine forests are valued at many millions of dollars, it is
not surprising that our government has put restrictions
upon the importation of Pines from Europe and has appro-
priated large sums of money to be expended in checking
this disease.

SUMMARY OF BASIDIOMYCETES

403

Asparagus Rust. — Asparagus is often attacked by a Rust
(Pucdnia Asparagi) which is a type of those having but one
host. The uredospores, teleutospores, and aecidiospores all occur
on the Asparagus.

Some other forms of Rusts of some importance occur on Clover,
Alfalfa, Beans, Peas, Beets, Timothy, Corn, Peach trees, etc.

Summary of Basidiomycetes. — Like the Ascomycetes the
Basidiomycetes are parasites or saprophytes on land plants and
have no motile spores. The Basidiomycetes are supposed to

sporidia

teleutospore

aecidiospores

FIG. 361. — Diagram showing life history of the Cedar Rust Fungus. A,
Cedar tree; B, Apple tree. The sporidia from the teleutospores infect the
Apple tree and the aecidiospores produced on the Apple foliage during summer
reinfect the cedars. From Technical Bulletin 9, Virginia Agr. Exp. Sta.

have been evolved from the Ascomycetes, and hence are farthest
removed from the Algae, which they resemble very little.

In such Basidiomycetes as the Toadstools and Puffballs, the
most highly developed sporophores occur, while in the parasitic
Basidiomycetes, as the Smuts and Rusts, the mycelium is scat-
tered through the host and is only visible through the production
of spore masses.

Such forms as the Toadstools, Mushrooms, and Puffballs
reproduce entirely by basidiospores, while in the reproduction
of the Smuts brand and basidiospores are involved, and in the
reproduction of Rusts there are four kinds of functional spores
— uredo-, teleuto-, basidio-, and aecidiospores, — and the non-
functional spermatia.

404

THALLOPHYTES

Fungi Imperfect! (imperfect Fungi)

All Fungi in which the features characteristic of the Phycomy-
cetes, Ascomycetes, or Basidiomycetes have not been discovered
in their life histories are classed as imperfect Fungi. It is a
heterogenous group, containing numerous Fungi varying widely
in characteristics. Investigators think that most of them are
the conidial stages of Ascomycetes in which the Ascogenous

stage has been abandoned or has not
been discovered. Careful investiga-

«4 ^ tions have already discovered that
a number of Fungi which have been
classed as imperfect Fungi have
ascogenous stages and are therefore
J||| Ascomycetes. As investigations go

on no doubt others and probably all
of them will be definitely classified in
the other groups.

The spore commonly known in the
group is the conidiospore and the
character of this spore and the way
it is borne are the chief features upon
which the group is divided into nu-
merous subdivisions.

%
-.»

FIG. 362.— Apple affected
with Apple Blotch caused by
an Imperfect Fungus. From
Bulletin 144, Bureau of Plant
Industry, U. S. Dept. of
Agriculture

Among them are many disease-producing forms, a large number
of which produce serious diseases on cultivated plants. The
Early Blight of the Potato, Leaf Blight of Cotton, Black Rot of
the Sweet Potato, Fruit Spot of Apples, one of the Potato Scabs,
Apple Blotch shown in Figure 362, and numerous other diseases
are produced by these Fungi.

Some special books on Fungi:

HARSHBERGER, JOHN W. A Text-Book of Mycology and Plant Pathology.
STEVENS, F. L. The Fungi which Cause Plant Diseases.
DUGGAR, BENJAMIN M. Fungous Diseases of Plants.
MASSEE, GEORGE. Diseases of Cultivated Plants.
MASSEE, GEORGE AND IVY. Mildews, Rusts, and Smuts.

CHAPTER XVI

BRYOPHYTES (MOSS PLANTS)
Liverworts and Mosses

General Discussion. — In the study of the Myxomycetes,
Bacteria, and Fungi, not much attention was given to evolution-
ary tendencies, for these groups are supposed to be degenerate
forms and have contributed nothing of importance in the way of
evolution. But in taking up the study of the Bryophytes, we
return to the study of evolution which will be emphasized
throughout the remaining groups, the aim being to see how
Flowering Plants could have originated.

The Bryophytes include two large groups of plants — Liver-
worts ( Hepaticae) and Mosses (Musci) — although the term
refers to Mosses. The Mosses are more conspicuous and more
familiar to most people than the Liverworts, but they are no
more important in the study of evolution.

The Bryophytes are of practically no economic importance.
They are of very little value for food and rarely harm other
plants. They make their own food and therefore do not need
to prey upon other plants. The only reason for studying them
is that they have contributed to evolution, and a knowledge of
them is necessary for an understanding of the higher plants.

The Bryophytes are supposed to have originated from the
Algae, and the advancements made by the Algae, such as the
establishment of multicellular plant bodies, food-making by
photosynthesis, development of gametes and sex organs, and the
differentiation of gametes and other cells, are resumed and some
of them carried farther by the Bryophytes.

Most Algae live in the water while the Bryophytes in most
part live on the land. The Bryophytes are considered the first
and most primitive land plants. The Algae are exposed to water
while most Bryophytes are exposed to the drying effects of the
air. Most Algae soon die when removed from the water and
exposed to the air, for they are not protected against loss of water

405

406 BRYOPHYTES (MOSS PLANTS)

and the air soon dries them out. To live on land a plant must be
protected against transpiration, and to become large and erect, a
plant must have structures for connecting it to the ground and
a stem to support it against the wind. It is believed that the
land plants came from Algae, and this means that certain Algae
must have acquired the land habit and in so doing ceased to be
Algae and became Bryophytes. One can imagine that this trans-
formation came about by some Algae gradually becoming more
and more adapted to living on the shore, where they were often
stranded, until finally they became so modified as to be fitted to
live permanently on land.

Liverworts

The Liverworts are thought to be the group that first acquired
the land habit, for, as a group, they are less complex than the
Mosses and are also more like the Algae in their moisture re-
quirements. While many of them live on land, there are some
forms which still live in water, and it is, therefore, in the Liver-
worts that the connection between water forms and land forms
is most evident. Even most of those Liverworts that live on
land are not able to endure dry air and hot sunshine, for, in most
part, they must grow in places that are moist or at least shaded.
But the Liverworts did much toward establishing the land habit,
and it is thought that our strictly land plants originated from
such forms as the Liverworts.

The plant body of most Liverworts is a flat body, known as
a thallus, but in some forms it is differentiated into stem- and
leaf-like structures. The thallus form of plant body, although
varying much in form according to the species, is usually lobed
and often branched. Often the thalli are liver-shaped, and
their shape was once thought to signify that these plants
possess special virtues in the cure of liver diseases — whence
the name Liverworts.

The thallus forms of Liverworts often form mat-like coverings
on moist soil or on moist rocks, such as the sides of a cliff. Those
Liverworts having better differentiated plant bodies and re-
sembling Moss commonly grow on logs and tree trunks in moist
and shady woods. There are about 4000 species of Liverworts
and they vary widely in complexity. They are commonly sub-

THE MARCHANTIAS

407

and

divided into three orders — Marchantiales, Jungermanialei
Anthocerotales.

The Marchantias. — The Marchantiales include the best
known Liverworts, among which are the Marchantias, the most
highly 'specialized Liverworts of this order and the family after
which the order is named. The Marchantia common in the north
temperate regions is Marchantia polymorpha. It grows in moist
places, often occurring abundantly in swampy regions, on shaded
river banks, and on protected rocky ledges. It often gets started

FIG. 363. — A female and a male plant of Marchantia polymorphia, show-
ing the external features of the plant body (about natural size). The two
plants, of which A is the female and B the male plant, differ most noticeably
in the character of the gametophores which are the erect stalks with expanded
tops (conceptacles) on which the sex organs occur, r, rhizoids; c, the gemmae
cups which are concerned with vegetative multiplication.

in greenhouses where it develops and spreads rapidly on moist
soil that is left undisturbed. Being easily obtained, it is one of
the Liverworts most commonly studied in botanical laboratories.

The plant body is shown in Figure 363. The flat, lobed, green
plant body or thallus lies prostrate on the substratum. Often
the plants are so much crowded as to overlap, and form aggrega-
tions that cover the substratum like a carpet.

Single plants are often several inches in length and breadth,
and consist of a number of layers of cells in thickness. On the

408

BRYOPHYTES (MOSS PLANTS)

under surface, cells are differentiated into thread-like structures
called rhizoids, which attach the plant to the substratum. In
the notches about the margin are cells which function like the
meristematic cells of the higher plants, and thus have to do with
the addition of new cells whereby the growth of the plant is
maintained. The cells of the upper region of the thallus are
differentiated into an epidermis, which affords protection against

FIG. 364. — Highly magnified cross sections of a thallus of Marchantia.
A , section through a thick portion of a thallus, showing the following features :
the upper epidermis and the chlorenchyma tissue (chl) just beneath divided
into chambers by partitions (o) ; the layers of cells (p) between the chloren-
chyma and lower epidermis, giving thickness and rigidity to the thallus; and
the lower epidermis with rhizoids (A) and scale-like plates of cells (6). B, sec-
tion near the margin of the thallus and more highly megnified, showing the
following features: upper epidermis (o); a chamber of chlorenchyma tissue
(chi) bounded by the partitions (s) and into which the chimney-like air pore
(sp) opens; the lower epidermis (u); and two layers of supporting tissue (p).
From J. M. Coulter, originally after Goebel.

evaporation, and into tissues which utilize the air and sunlight
in manufacturing carbohydrates. Highly magnified sections
through a thallus are shown in Figure 364- In the epidermis are
many chimney-shaped pores which permit the air to reach the
filaments of food-making cells in the chambers beneath.

On the thalli shown in Figure 363 are also shown some small
cups and some erect stalks with expanded tops. These struo-

THE MARCHANTIAS 409

tures are further differentiations of the plant body, that are not
always present, occurring only during periods of reproduction.
The erect umbrella-shaped structures, which are upgrowths of
the midrib of the thallus, bear the sex organs and are called
gametophores, while the cup-shaped structures have to do with a
vegetative method of reproduction which will be discussed later.

Since the plant lives spread out on a moist substratum, much
of the plant body is in direct contact with moisture and can
absorb water and minerals directly. The rhizoids are not roots
and are of very little service in supplying water and mineral salts.
It is probable that they do nothing more than hold the plant
body to the substratum. The filaments of cells in the air cham-
bers on the upper surface are well provided with chloroplasts and
carry on active photosynthesis which supplies the plant with
carbohydrates. Many of the other cells in the upper region of
the plant body have some chloroplasts and no doubt assist some
in providing food.

In addition to the sexual method of reproduction, there are
two ways of propagating vegetatively or asexually. As the
branches of the thalli develop and push ahead, the older regions
die away and soon the branches become isolated and form sepa-
rate plants. This is known as a vegetative or asexual method of
reproduction because no spores or sex cells are involved. An-
other vegetative method occurs in connection with the cups
which have been pointed out on the surface of the thalli. In
these cups are produced small plates of cells, called gemmae,
which, when splashed out by rain and suitably located on a
substratum, grow directly into new plants. These vegetative
methods remind one of the propagation of Strawberries by run-
ners, or of Geraniums by cuttings.

The sex organs are produced upon the umbrella-like tops or
receptacles of the gametophores. On the under surface of the
much lobed receptacles of the gametophores of the female plants
(A, Fig. 363) occur the female sex organs called archegonia.
When a thin section is made through a female receptacle and
examined under the microscope, the archegonia are seen pro-
jecting from the under surface as shown at A in Figure 365.
Each archegonium consists of many cells so arranged as to form
a long hollow neck and an enlarged hollow base called venter, in
which the large egg is located. It is obvious that an archegonium

410

BRYOPHYTES (MOSS PLANTS)

is much more complex than the oogonium of the algae. In the
less lobed receptacles of the gametophores of the male plants (B,
Fig. 363) occur the antheridia, consisting of a stalk and of a jacket
of cells which encloses a mass of sperms as shown in Figure 366.

FIG. 365. — Highly magnified vertical sections through the expanded tops
or receptacles of female gametophores of Marchantia, showing the sex organs
and sporophytes. A, section through female gametophore, showing the
archegonia (a), each of which consists of a neck and an expanded base called
venter, in which the egg (e) is located. B, section through a female gameto-
phore, showing sporophytes (s), with their sporangia (h), stalks (t), foot (/),
and also showing spores (i) escaping from the sporangium of the sporophyte
at the left.

Since the sperms are produced on one plant and the eggs on an-
other, the sperms have a considerable distance to be carried to
the eggs. The sperms are splashed about during heavy rains, and,
when near an archegonium, they are attracted to the entrance in
the neck by an attractive substance which diffuses out of the
archegonium. The sperms swim down the canal in the neck of
the archegonium and the first one reaching the egg fertilizes it.
The fertilized egg or oospore remains where it was formed, be-
gins to grow and divide rapidly, and soon produces an oblong,
multicellular, brownish body which consists of a stalk that is
attached to the receptacle by an absorbing organ called foot
and bears at the other end a sporangium (B, Fig. 365). The

THE TWO GENERATIONS

411

foot extends into the gametophore and absorbs food which is
supplied to the elongating stalk and developing sporangium.
In the sporangium are produced numerous spores and also
elongated twisted cells called elaters, which assist in scattering
the spores. When the spores are
mature the sporangial wall opens
and the spores are scattered. When
the spores fall on a moist substra-
tum, they germinate and produce
new thallus plants "like the ones
described.

The Two Generations. — The ob-
long body produced by the fertilized
egg, and consisting of foot, stalk,
and sporangium, is regarded as a
plant within itself. When fully
mature it is so small that one must
look closely under the finger-like
lobes to find it. It doesn't look
much like a plant, since it is so
simple and depends upon the
gametophore for food and water,
but it is this plant that differenti-
ates and becomes the conspicuous

plant body of the higher plants. Since it produces spores, it
is called a spore plant or sporophyte. When one is reminded
that a Corn plant or Apple tree is all sporophyte excepting some
microscopical structures within the flowers, then the significance
of this small sporophyte of the Liverworts in relation to the
origin of the higher plants may be realized.

It is obvious that if this little sporophyte is regarded as a plant,
then all of the remainder of Marchantia must be regarded as
another plant. This other plant consists of all that has been
described as the plant body of Marchantia. It consists of the
flat prostrate thallus and the gametophores with the sex organs
and gametes. Since it is the function of this plant to bear
gametes, it is called gametophyte.

It follows then that the complete life cycle of Marchantia in-
volves two plants or generations as illustrated in Figure 367.
The gametophyte generation develops from a spore and produces

FIG. 366.— Highly magnified
vertical section through the
expanded top or conceptacle of
a male gametophore, showing
the antheridia (a) imbedded in
the gametophore and consist-
ing of a short stalk and of a
jacket enclosing numerous cells
which form sperms.

412

BRYOPHYTES (MOSS PLANTS)

gametes, while the sporophyte generation develops from the fer-
tilized egg and produces the spores. It is obvious that this is
alternation of generations. Some of the higher Red Algae, as
illustrated by Polysiphonia, have an alternation of generations

FIG. 367. — A diagram showing the life history of Marchantia. Above
the line (a) is the gametophyte generation and below the line is the sporo-
phyte generation, p, spore; q, spore germinating to form gametophyte;
g, mature gametophytes; o, sex organs; t, gametes; /, fertilized egg or first
cell of the sporophyte; m, fertilized egg dividing; n, mature sporophyte
ready to shed spores which are the first cells of new gametophytes.

in their life cycle, but in Bryophytes this feature is so well estab-
lished that it occurs everywhere in the group and is so evident
that it was in the Bryophytes that the alternation of generations
was first observed. Alternation of generations is also an estab-
lished feature of Pteridophytes and Spermatophytes or all plants
above the Bryophytes.

One of the interesting features in connection with the transition
from the gametophyte generation to the sporophyte generation
is a peculiar kind of cell division known as the reduction division.
It will be recalled that in preparation for cell division the chro-
matin in the nuclei of cells forms into a definite number of chro-
mosomes, the number depending upon the kind of plant. Now
reckoning nuclear content in terms of chromosomes, it is obvious
that since fertilization is a fusion of the nuclear contents of a
sperm and an egg, the number of chromosomes in the nucleus of

THE TWO GENERATIONS

413

the fertilized egg is double that of the sperm or egg. It follows
that, unless the number of chromosomes is reduced somewhere
in the life cycle of the plant, each generation of plants would have
double the number of chromosomes of the preceding generation.
This doubling of the chromosome number in each generation

e f S h

FIG. 368. — Diagrams showing the difference between ordinary cell divi-
sion and the reduction division. To make the diagrams easy to follow only
two chromosomes in each case are represented, but their behavior is typical
of all the chromosomes of the nucleus. One chromosome has been blackened
and the other left white to indicate that they differ in that one consists of
chromatin material of the father parent and the other, of the mother parent
of the individual whose cell division is illustrated by the diagrams. The
upper diagram illustrates the behavior of chromosomes in ordinary cell divi-
sion, showing the chromosomes at a soon after organization, their arrange-
ment on the spindle fibers and splitting lengthwise at b, the separation of the
longitudinal halves at c, and the formation of the new nuclei at d, with each
new nucleus containing a longitudinal half of each of the original chromo-
somes. In the lower diagram, illustrating the behavior of chromosomes
in the reduction division, the chromosomes are paired at e, arranged on the
spindle in pairs at /, separated as whole chromosomes at g, and thus each
nucleus at h receives one chromosome of the pair and not half of each chro-
mosome as in ordinary cell division.

would soon result in a disastrous piling up of chromosomes. In-
vestigations show that the sporophyte has twice the number of
chromosomes of the gametophyte, but that the spores formed by
the sporophyte have the gametophytic number. The transition
is made in the mother cells, that is, in the cells which form the
spores, and by these cells dividing in such a way that the chromo-

414 BRYOPHYTES (MOSS PLANTS)

somes are so distributed that each daughter cell gets only half
of the number of chromosomes or the gametophytic number.
This kind of cell division is called the reduction division and
simply undoes the doubling of chromosomes resulting from fer-
tilization. The diagrams in Figure 368 show how the reduction
division differs from ordinary cell division. Cytologically the
sporophyte begins with the fertilized egg and ends with mother
cells, while the gametophyte begins with the spore and ends with
fertilization. More will be said about the significance of the
reduction division in connection with heredity where it has an
important bearing.

The Riccias. — The genus Riccia, which is often regarded as a
subdivision of the Marchantiales, includes the simplest of Liver-
worts. Some of them are almost entirely aquatic, living sub-

Fia. 369. — One of the Riccias, the simplest of Liverworts. X 4.

merged or floating on the surface of the water, while others live
spread out on moist soil. The plant body is a simple thallus,
smaller and not so well differentiated as the thallus of Marchantia.
(Fig. 369.) No gametophores are developed and the sex organs,
both kinds of which may develop on the same plant, occur in
grooves along the ribs of the thallus. The air pores are not well
developed and sometimes rhizoids are absent. The sporophyte,
which is also much simpler than the sporophyte of the Mar-
chantias, lacks a foot and stalk, and thus consists of only a
sporangium.

When the sporophytes of the Riccias and Marchantias are
compared, it is obvious that much more of the fertilized egg has
been turned into spores in the Riccias than in the Marchantias.
In the Marchantias much of the cell progeny of the fertilized egg,
instead of forming spores, is used in forming a foot, stalk, and
elate rs. Such a diverting of the cells which could form spores

PORELLA

415

into other kinds of work is spoken of as sterilization of sporog-
enous tissue. One can now see that a sporophyte could become
as complex as a Corn plant by becoming more and more multi-
cellular while at the same time most of the cells were used in
forming structures, such as roots, stems, and leaves. In this
way sporophytes became more and more com-
plex until the highest plant forms were pro-
duced.

Porella. — This Liverwort belongs to the
Jungermaniales, which order contains the
largest number of Liverworts.

The Jungermaniales vary widely in their
moisture requirements, some being able to
live in dry situations. They are especially
abundant in the tropics where they grow on
the trunks of trees, on leaves of other plants,
and on the ground. Some have thallose game-
tophytes like the Marchantiales, while others,
known as foliose forms, have gametophytes
that are differentiated into leaf- and stem-
like structures and resemble the Mosses.

Porella is one of the foliose forms of the
Jungermaniales and is common on the trunks
of trees and fallen logs in the north temperate
regions. The character of the gametophyte is shown in Figure 370.
It has a slender, creeping, branched, stem-like axis bearing two hor-
izontal rows of larger leaves on the dorsal surface and one horizontal
row of smaller leaves on the ventral surface. Although much more
differentiated as to form, the gametophyte of Porella is much less
differentiated as to tissues than the gametophyte of the Marchantias.

The two kinds of sex organs may occur on the same plant or on
different plants. The Archegonia occur in groups on the ends
of short lateral branches. The antheridia occur in the axils of the
leaves of certain branches which can be identified by the closely
imbricated leaves.

The sporophyte has a long stalk and the sporangium splits
into four valves which spread out and allow the spores to escape.
There is more sterilization of sporogenous tissue and a more
definite provision for the shedding of spores than in the sporo-
phyte of the Marchantias.

FIG. 370. — A
branch of Porella,
a foliose Liverwort
of the Jungermani-
ales. X 3.

416

BRYOPHYTES (MOSS PLANTS)

Thus as compared with the Marchantiales, the Jungermaniales
have gametophytes more differentiated in form but less in struc-
ture, and have sporophytes characterized by a greater sterilization
of sporogenous tissue.

Anthoceros. — Anthoceros is a representative of the Antho-
cerotales which is a very small group of inconspicuous Liverworts.
Anthoceros and its allied forms are the most interesting of all
Liverworts, because their structure suggests the steps by which
Pteridophytes, the Fern group, could have originated from the
Bryophytes. Anthoceros grows spread out like some of the

Riccias and is common on moist
soil in north temperate regions
(Fig. 371). The gametophyte is
a simple thallus, much simpler
than that of the Marchantias.
The sex organs develop in sunken
areas on the top surface of the
thallus.

The remarkable feature is the
sporophyte, which differs in a
number of ways from the spo-
rophytes of other Liverworts.
In the first place the sporo-
phyte is green, which means that
it is supplied with chloroplasts
and is thereby able to make

food for itself, although it has to depend upon the gameto-
phyte for water and mineral salts. This feature suggests the
independent sporophyte of the Pteridophytes. The epidermis
of the sporophyte even contains stomata for allowing the air
to reach the green tissues beneath as in the leaves of higher
plants. Evidently, if this sporophyte had roots, it could live
independently of the gametophyte. In the second place there
is a core or central axis of sterile tissue called columetta extend-
ing lengthwise through the sporophyte, and bands of spore-
forming tissue alternate with bands of sterile tissue around this
columella. The columella is a characteristic feature of Moss
sporophytes, and in this way the Anthocerotales relate the
Liverworts to Mosses. If one imagines the bands of sterile
tissue which alternate with the bands of spore-forming tissue

FIG. 371. — Anthoceros, showing
a gametophyte (g) bearing sporo-
phytes (s). X about 2.

TRUE MOSSES (BRYALES) 417

growing out so as to form leaves, then a leafy sporophyte like
those of Pteridophytes would be formed. In the third place
there is a meristematic group of cells at the base of the sporophyte
by which growth and production of spores are maintained for a
period of time.

Mosses

General Description. — In general, Mosses do not need so much
moisture as Liverworts do, and are, therefore, more generally
distributed. They are common in moist places and some inhabit
bogs and streams, but Mosses are also very common in dry
places. They live on tree trunks, logs, stumps, rocks, soil, and in
bogs and fresh water. In fact one can find Mosses nearly every-
where. They often mass together in clumps and cushion-like
masses which hold water much like a sponge. Many Mosses,
especially those growing in dry places, can become dried out and
then revive when they become moist again. The Mosses as a
group have better differentiated gametophytes and sporophytes
than the Liverworts.

The Mosses are divided into three groups, Sphagnales, Andrea-
les, and Bryales. The Sphagnales are the Sphagnums, which live
in bogs where the accumulation of their plant bodies forms peat.
The Andreales are a very small group of siliceous rock Mosses
which will receive no further discussion, although they are inter-
esting because they present a combination of characters which
relate them to the Sphagnales, Bryales, and also to Liverworts.
The group containing the vast assemblage of our most familiar
Mosses is the Bryales. The Bryales, known also as the True
Mosses, are the most highly organized of the Mosses.

True Mosses (Bryales). — The most conspicuous part of the
Moss plant is the gametophyte, which looks like Figure 372. It
consists of a leafy stem attached to the substratum by rhizoids.
In some Mosses the leafy stem is prostrate, but in many it grows
erect. The leaves of the Moss plant, like the leaves of the foliose
Liverworts, are quite simple. In most part they are only one
cell in thickness. They have no stomata and no palisade or
spongy tissues. Although they are called leaves, it is obvious
that they are not like the leaves of the higher plants. But their
cells contain chloroplasts and they make carbohydrates just as
the leaves of the higher plants do. Stomata, palisade, and

418

BRYOPHYTES (MOSS PLANTS)

spongy tissues cannot occur and are not needed until leaves be-
come more than one cell in thickness. The stem is also quite
simple in structure, and is not dif-
ferentiated into the tissues which
characterize the stems of higher
plants.

The sporophyte is commonly much
larger than that of the Liverworts

—r.

FIG. 372. — Thegame-
tophyte of a Moss, con-
sisting of stem- (s) and
leaf-like structures (Z),
and rhizoids (r) which
attach it to the sub-
stratum. X about 2.

FIG. 373. — The two
generations of Moss, g,
gametophyte genera-
.tion; a, sporophyte gen-
eration; s, sporangium
of the sporophyte.

and it can be seen usually at a considerable distance projecting
from the top of the gametophyte. A plant bearing a sporophyte
looks like Figure 373.

TRUE MOSSES (BRYALES)

419

Most parts of the Moss absorb water and salts directly. Even
the leaves are probably able to absorb. The leaves carry on
active photosynthesis and supply the carbohydrates. No vascu-
lar bundles occur, but in many Mosses there are strands of elon-
gated cells which assist in conducting and distributing the foods.
The erect habit and the radiate arrangement of the leaves on the
stem enable the plant to make the best use of light.

Knowing that the leafy green plant is the gametophyte, one
knows where to look for the sex organs. They are produced on

FIG. 374. — The sex organs of Moss. A, highly magnified vertical sec-
tion through the apical region of the stem of a gametophyte, showing arche-
gonia (a) with eggs at (e). B, a similar section through a plant bearing
antheridia (t). Sperms escaping from an antheridium and one sperm much
enlarged are shown at s.

the upper end of the stem and are quite well surrounded and hid-
den by the upper leaves. If one carefully pulls off the terminal
leaves from plants that are in the reproductive condition, the
sex organs'may be found. They stand erect on the stem tip and
are so large that they can^be seen with a magnifier of very low
power. The antheridia can sometimes be seen without any
magnifier. The archegonia are flask-shaped and have very long
necks, while the antheridia are club-shaped (Fig. 374). In many
Mosses both sex organs occur on the same plant, but in the one
shown in the Figure they occur on separate plants. The male

420

BRYOPHYTES (MOSS PLANTS)

plants of some Mosses can be identified by a small terminal cup
in which the antheridia are produced.

The antheridia produce numerous swimming sperms, and, when
there is suitable moisture, the sperms reach the archegonia, swim
down the long necks into the venters, and fertilize the eggs.
The fertilized egg begins to grow almost immediately after fer-
tilization, and like the fertilized egg of the Liverworts, it develops
in the place in which it was formed. By rapid growth and cell
division, it soon forms a spindle-shaped body with one end called
foot pushing into the stem of the gametophyte to absorb food,

FIG. 375. — A protonema of Moss (X 50). Buds which develop leafy
gametophores are shown at 6.

and the other end pushing into the air, forming a stalk called seta
which bears a sporangium at its upper end in which the spores
are produced. As the sporophyte develops, the venter about the
young sporophyte and also the neck of the archegonium enlarge.
Finally the venter is ruptured and the enlarged archegonium is
carried up by the sporophyte, forming a pointed cap on the top
of the sporangium. When the spores are shed and fall on a
moist soil, they produce new gametophytes. However, the
spore does not grow a leafy plant directly, but first produces
an Alga-like filament which branches and creeps over the
substratum (Fig. 375). From bud-like structures on this fila-
ment, the leafy green plants grow, thus completing the lif e

TRUE MOSSES (BRYALES)

421

cycle as shown in Figure 376. The Alga-like filament called
protonema is comparable to the thallus of the Marchantias, and
the leafy plants to the gametophores. Although the leafy plants
or gametophores of Moss are not all of the gametophyte, they are
the conspicuous part of it, the protonemas being microscopic in
size. One protonema may produce many buds, and, therefore,
many gametophores.

In Moss the two generations are more noticeable than in the
Liverworts. The gametophytes with their leafy gametophores
present more differentiation than is the rule among Liverworts.

FIG. 376. — Diagram of the life cycle of Moss, p, protonemas from
which the gametophores (gr) have arisen; a and 6, the sex organs with a
sperm shown passing from antheridium to archegonium; c sporophyte which
the fertilized egg produces; s, spores which grow new protonemas and thus
the life cycle is. completed.

The sporophyte, consisting of a large sporangium supported on a
long stalk, or seta, is usually quite conspicuous. It is more multi-
cellular and has carried the sterilization of sporogenous tissue
farther than the sporophytes of most Liverworts have. Not
only is it larger and more multicellular, but it also shows more
differentiation than the sporophytes of Liverworts. The seta
is so differentiated as to have a central strand of elongated
cells for conduction. The sporangium of the Moss sporophyte
develops at its top a special lid-like structure (operculum) for
opening, and often special tooth-like structures (peristome) are
produced just under the lid and assist in scattering the spores.

422 BRYOPHYTES (MOSS PLANTS)

In the sporangium there is a columella or axis of sterile tissue,
and in the sporangial wall air spaces and filaments of green
tissue are provided. In some Mosses the base of the capsule,
called apophysis, is devoted to food-making rather than to
the formation of spores, in which case there is much chlorophyll
tissue and many stomata present. This feature is quite impor-
tant as was pointed out in Anthoceros, because it looks forward
to the independence of the sporophyte; for, if the sporophyte
can make carbohydrates for itself, it then needs only roots to
absorb water and mineral salts, in order to live independently of
the gametophyte.

The gametophytes of the Mosses have a remarkable power of
propagating vegetatively. Since the sperms depend upon water
for transportation and the sex organs are borne above the moist
substratum, fertilization rarely occurs in some Mosses, which,
therefore, must depend largely upon vegetative propagation.
There are a number of ways by which they propagate vegeta-
tively. First, by the isolation of branches through the death of
the older axes; second, the cells of the protonema sometimes
separate, become restive, and later from each resting cell a new
protonema is developed ; third, from the leaves and stems of the
gametophore new protonemas are often developed; and fourth,
some Mosses develop gemmae which are commonly borne at the
summit of the leafy gametophore.

The Sphagnums (Sphagnales). — The genus Sphagnum in-
cludes all of the Mosses of this order. There are about 250
species, and they occur mostly in temperate and arctic regions.
They live chiefly in bogs and are commonly called Bog or Peat
Mosses. Their slender, branched, leafy gametophores (Fig. 877)
are pale in color due to the fact that many of the leaf cells as well
as many of the outer cells of the stem are empty except for the
water and air which they hold, thus containing no chloroplasts.
It is due to the ability of these much enlarged empty cells to
take up and retain water by capillarity that Sphagnum retains
moisture so well when used in germinating boxes or for moist
packing around plants. The gametophores are commonly
creeping, turning up only at the ends, and they usually form close
mats, which gradually thicken by growth above and eventually
fill up bogs. Due to the indefinite growth at the tips, gameto-
phores may attain great length and age. In bogs where, due to

THE SPHAGNUMS (SPHAGNALES)

423

the lack of drainage, organic acids accumulate and prevent the
action of Molds and Bacteria, the dead remains of Sphagnum and
accompanying plants do not decay, but are finally transformed
into peat, which is a valuable
fuel in some countries, espe-
cially in Ireland.

Both antheridia and arch-
egonia are stalked and are
produced on branches. The
sex organs differ from those of
the Bryales in their develop-
ment but are quite similar in
appearance when mature.

The sporophyte differs from
the sporophyte of the Bryales
in having only a very short
seta, which is only a neck be-
tween the foot and the capsule.
In connection with this fea-
ture there occurs another
characteristic feature known

as the pseudopodium. The

FIG. 377. — The gametophyte and
sphorophyte of Sphagnium. At the
left, gametophyte of Sphagnium; at
the right, a sporophyte and the pseudo-
podium; between, a vertical section
through the sporophyte, showing the
short rounded foot, the short neck-like
seta, and the globular sporangium in

pseudopodium, which replaces
the seta in function, is formed
by the elongation of the axis
of the gametophore just be-
neath the sporophyte, which
is thereby carried up as if it
were on an elongating seta.
Another peculiar feature of

which the spores are borne in a cavity
forming an arch over the columella.

the sporophyte is that the

columella does not extend entirely to the top of the spor-
angium as in Bryales, but the sporogenous tissue arches over
the columella. In this respect the sporophyte is like that of
Anthoceros.

When the spores germinate, instead of producing a filamentous
protonema, they produce a flat thallus that resembles a Liver-
wort, and from buds on this thallus the leafy gametophores arise.
When studied in detail one finds that Sphagnum has a number
of features characteristic of Liverworts and a number that are

424 BRYOPHYTES (MOSS PLANTS)

characteristic of the Bryales, while it has some that belong to
neither. It is often called a synthetic form, for it combines the
characters of Liverworts and True Mosses.

Summary of Bryophytes. — The Bryophytes show progress
over the Algae in a number of ways. First, the Bryophytes
established the land habit, which meant the establishment of a
plant body that was adapted to live and function in the air rather
than in the water. In establishing the land habit the plant body
had to develop tissues to protect against transpiration/ sex cells
had to be jacketed, and sex organs, now called antheridia and
archegonia, consequently became multicellular, and tissues for
utilizing the carbon dioxide of the air and sunlight in making food
had to be provided. Second, although alternation of generations
is quite prominent in some of the higher Algae, it is a very dis-
tinct feature throughout the Bryophytes. Both gametophyte
and sporophyte generations show considerable advancement
from the simplest Liverworts, where the gametophyte is a small
flat thallus and the sporophyte merely a sporangium, to the
highest of the Mosses, where there is a leafy gametophore and a
sporophyte with a well developed seta and a sporangium having
an operculum, peristome, columella, aerenchyma, and food-
making tissues.

It should be noticed, however, that, although the Bryophytes
adopted the land habit, they have a swimming sperm which puts
a limit on the size of gametophytes, for swimming sperms can
travel only short distances and only when water is present. In
Mosses a.nd the more complex Liverworts, there is much evidence
that a large percentage of the sperms are not able to reach the
archegonia. But the spore, since it is protected against drying
and can, therefore, be transported by the wind, puts no limit on
the size of the sporophyte. This means that the higher plants
must consist chiefly of the sporophytic generation.

CHAPTER XVII
PTERIDOPHYTES (FERN PLANTS)

General Discussion. — Ferns are much larger plants than
Bryophytes and consequently are much better known by the
general public. In the woods Ferns are common and often they
can be found in the fields. On account of their large, attractive,
feather-like leaves, they are common house plants and are ex-
tensively grown in greenhouses. Most Ferns require a moist or
shady region, but some are able to grow in dry situations.

In studying the different layers of rock which form the earth's
crust, many Pteridophytes are found preserved. In the layer
of rock from which coal is obtained, Pteridophyte fossils are very
abundant. These fossils show that Pteridophytes were at one
time much more abundant than now. Some of these ancient
forms were like trees in size and resembled Seed Plants more than
any of the present forms do. Although the forms that made
most advancement toward Seed Plants have long been extinct,
the forms which now exist show us some of the lines along which
progress was made.

In beginning the study of Pteridophytes, one should have in
mind the features contributed by the Bryophytes, because the
Pteridophytes are supposed to have come from forms like the
Bryophytes, although we are not able to connect them up with
any of the existing forms of Bryophytes. From forms like the
Bryophytes, the Pteridophytes inherited the land-habit. They
not only inherited those features which enable plants to live,
work, and reproduce in the air, but they have improved upon
these features, so that in general they are better fitted to li«ve on
land than most of the Bryophytes. They have the alternation
of generation which the Bryophytes so firmly established and
have carrie<i the sterilization of sporogenous tissue so far that the
sporophyte is a massive and well differentiated plant body.
Probably, instead of speaking of it as sterilization of sporogenous
tissue, it would be clearer to say that the fertilized egg now pro-

425

426 PTERIDOPHYTES (FERN PLANTS)

duces an enormous number of cells which go to form vegetative
tissues of various kinds, before sporogenous tissue is produced.
Thus by delaying the formation of sporogenous tissue, the sporo-
phyte of Pteridophytes has become more and more massive and
at the same time with its larger number of cells has formed more
kinds of tissues than occur in the sporophytes of Bryophytes.
It is the sporophyte, which is the plant that we call the Fern,
that is the conspicuous generation in the Pteridophytes. The
gametophytes in most cases are quite small and generally simpler
than the gametophytes of most Liverworts. In passing from the
Bryophytes, where the sporophyte is small, dependent, and rela-
tively simple, to the Pteridophytes, where the sporophyte is so
many times larger and differentiated into roots, stems, and leaves
so that it lives independently, one is struck with the big jump
between the two groups. In the absence of forms to bridge over
this gap, the relation between the Bryophytes and Pteridophytes
is obscure. The sporophyte with its roots, stem, and leaves is
now well advanced toward Seed Plants.

Although the Pteridophytes are known as the Fern group,
there are many Pteridophytes, of which Horsetails and Club
Mosses are familiar ones, that are not really Ferns. The True
Ferns are the most highly specialized and much the largest group
of the Pteridophytes, but in order to get a notion of the most
important features contributed toward Seed Plants by Pterido-
phytes, a study of the Ferns should be followed by a study of
some other groups of Pteridophytes.

Filicales

The Filicales are composed of the True Ferns and the Water
Ferns. The latter are small forms living in the water or mud and
are supposed to be an aquatic branch of the True Ferris. Al-
though the Water Ferns present some features of interest to
special morphologists, they will receive no attention in this brief
discussion. The True Ferns, which are the most abundant and
familiar of all Pteridophytes, are even more abundant in the
tropics than in the temperate regions. In the tropics the sporo-
phytes of some grow so large as to be called Tree Ferns.

Sporophyte. — Since the gametophyte is very inconspicuous,
the sporophyte, or the plant known as the Fern, is the only genera-

SPOROPHYTE

427

tion of the Fern which people in general know (Fig. 378).
There is much range in size of Fern sporophytes, from very small
plants jike some that are common in our woods, to those as high
as a man's head, and to the Tree Ferns of the tropics and green-
houses that may reach a height of forty feet or more.

The stems of a few Ferns are erect and may become large like
the trunk of a tree, as the Tree Ferns illustrate (Fig. 379), but in

FIG. 378. — A fern sporophyte. r, roots; s, stem; a, young
fronds unfolding; I, mature fronds. After Wossidlo.

our common Ferns, the stems remain a few inches under the sur-
face of the ground and, as they elpngate and push horizontally
through the soil, leaves are produced from the upper and roots
from the lower surface. They are called rootstocks or rhizomes,
both terms referring to the root-like feature of growing under
the ground.

The stems of Fern sporophytes are woody and have many of

428 PTERIDOPHYTES (FERN PLANTS)

the structures characteristic of the stems of Seed Plants and are,
therefore, not merely stems in appearance as the stem-like struc-
tures developed by the gametophytes of Mosses and some Liver-
worts are. It remained for the sporophyte generation to develop

FIG. 379. — A Tree Fern. After Bailey.

a real stem. At the tip of the Fern sporophyte there is a meriste-
matic region which by the rapid growth and division of its cells
elongates the stem. Just behind the advancing tip new roots
and leaves are developed and stem tissues are formed. A cross
section of a stem, as shown in Figure 380, shows an epidermis,
cortex, vascular cylinder, and pith — tissues characteristic of
the stems of Seed Plants.

The roots too are true roots and are not simple structures like
the rhizoids of gametophytes. They have a root cap, region of
growth and elongation, epidermis, root hairs, cortex, and vascu-
lar cylinder, thus having the features characteristic of the roots
of Seed Plants.

The leaves, although true leaves, are generally called fronds, a,
term formerly applied to them because they were considered a
combination of leaf and stem. Fern leaves are usually much
branched and are easily identified by the way their veins branch
and by the way they develop in the spring. Their veins branch
by forking; that is, a vein divides into two veins of equal size

SPOROPHYTE

429

(dichotomous branching); and the leaves develop in the spring
by unrolling from the base, much like unrolling a bolt of cloth,
until their final length is reached (circinate vernation) . They have
epidermis, stomata, and chlorenchyma or food-making tissue,
and through their veins run well developed vascular bundles.

FIG. 380. — A cross section of a Fern stem, showing the epidermis (e), the
cortex (c), the vascular cylinder (v), and the pith (p).

The sporangia occur in the rusty looking spots, called sori
(singular sorus), which are formed at certain times on the under
surface of the leaves (B, Fig. 381}. Each sorus has a membrane-
like covering called indusium, under which the sporangia are
protected. By making a thin cross section of a leaf, so that the
section passes through a sorus, the sporangia then appear under
the low power of the microscope as shown at C in Figure 381 . A
number of sporangia occur in a sorus, but the number varies in
different Ferns. The sporangia are usually stalked and flattened,
and around the margin there is a row of heavy walled cells form-
ing the annulus, which assists in opening the sporangia and
scattering the spores (D, Fig. 381).

430

PTERIDOPHYTES (FERN PLANTS)

The character of the sporangia and the way they are borne vary
much in different Ferns and are much used in the classification of

Ferns. In the lowest group
of the True Ferns the spor-
angia are borne in syn-
angia, which are apparently
composed of united spor-
angia. In some Ferns the
sporangia are borne singly.
In some the sori have no
true indusium, but the edge
of the leaf folds over and
protects the sporangia.
Then in the shape of the
sporangia, presence or ab-
sence of an annulus, the
location of the annulus,
and in the number of
spores borne in a sporan-
gium, there are important
differences among Ferns.
Again there are two ways
in which sporangia begin
their development. In
some Ferns, known as
eusporangiates, both epi-
dermal and sub-epidermal
cells of the leaf are involved
in forming the sporangia,
while in Ferns, known as
leptosporangiates, the spor-
angia are formed entirely
from the epidermal cells of
the leaf.

In some of the True
Ferns the sporangia are not
borne on ordinary leaves,

FIG. 381. — A sporophyte and spore-
producing structures of a True Fern. A,
a Fern sporophyte, showing roots (r),
stem (st), and a leaf (/) (X about $). #>
an enlarged view of the under surface of
a Fern leaf, bearing sori (so). C, highly
magnified section through a Fern leaf
and sorus, with section of leaf shown at
I, sporangia at sp, and indusium at i. D,
a much enlarged view of a sporangium,
showing annulus a and method of opening
to allow the spores (s) to escape.

in which case the sporo-
phyte is differentiated into vegetative and spore-bearing regions.
Sometimes some of the leaflets are devoted entirely to bearing

GAMETOPHYTE

431

spores as in the Interrupted Fern (Osmunda Claytonia) (Fig. 382).
In some like the Sensitive Fern (Onoclea sensibilis), common along
roadsides and in wet meadows, there are two distinctly different
kinds of fronds, one of which is entirely devoted to bearing spores
and the other entirely to vegetative
work (Fig. 383). This separation of
spore-bearing and vegetative tissues
is adhered to more closely in some
other Pteridophytes than in the
True Ferns, and it is a feature

FIG. 382. — A portion of a leaf of the
Interrupted Fern (Osmunda Claytonia),
showing a pair of vegetative leaflets above
and below and between them two pairs of
spore-bearing leaflets.

FIG. 383. — The Sensitive
Fern (Onoclea sensibilis),
showing a vegetative frond
at the left and a spore-bear-
ing frond at the right.

of considerable significance because it is characteristic of Seed
Plants.

Gametophyte. — When the spores are shed and fall in moist
places, the protoplasm breaks the spore wall and begins the de-
velopment which results in the production of a gametophyte.
In True Ferns a short tube with one or more rhizoids at the spore

432

PTERIDOPHYTES (FERN PLANTS)

end is first produced. The development of this tube, called germ
tube, is germination. The germ tube soon reaches its full length,
and then it begins to broaden at the outer end and a tiny, green,
heart-shaped gametophyte is produced (Fig. 384).
The gametophyte resembles the thallus of the
simplest Liverworts. When mature it has a
cushion-like central axis where the rhizoids and
sex organs are developed, and wing-like margins
consisting of a single layer of cells. The game-
tophyte is called a prothallus, the term referring
to the fact that it is thallus-like in form and
precedes the sporophyte in reproduction. In and around Fern
beds in greenhouses Fern gametophytes are quite common on the

FIG. 384.—
Three Fern
gametophytes
shown about
natural size.

FIG. 385. — An enlarged view of the
under surface of a Fern gametophyte,
showing the archegonia (a), the antheridia
(6), and the rhizoids (r).

FIG. 386. — A Fern
gametophyte (0) bearing
a young sporophyte (s)
with leaf at I and root
at r.

damp walls, damp soil, and on the sides of flower pots. Oc-
casionally they can be found out of doors about Ferns growing
in moist shady places. They lie flat on the substratum, and the
sex organs are borne underneath where there is moisture for the

GAMETOPHYTE

433

swimming sperms (Fig. 385). The chimney-shaped archegonia
are near the notch of the prothallus, and the globular anther-
idia are in the region of the rhizoids. In some Ferns the male
and female sex organs are on different gametophytes.

The sperms are active swimmers and reach the egg by swim-
ming down the neck of the archegonium which, like the arche-
gonia of Bryophytes, opens at the top when the egg is ready for
fertilization. From the neck of the archegonium, a substance is
also discharged, which chemically attracts the sperms.

FIG. 387. — A diagram of the life cycle of a Fern. A, sporophyte bearing
sori in which the sporangia occur. B, a gametophyte, a product of a spore
and the generation bearing the gametes, the sperms of which are shown
passing from the antheridia to the archegonia. C, gametophyte bearing a
sporophyte, which soon becomes independent and like the one at A.

The fertilized egg immediately grows into a sporophyte, which
lives on the gametophyte only until it has roots and' leaves
sufficiently developed to support itself (Fig. 386). After the
sporophyte reaches maturity, sori are developed and the life cycle
is completed (Fig. 387). Among a group of gametophytes one
usually finds sporophytes in various stages of development and
greenhouse attendants sometimes collect and pot the young
sporophytes growing in unfavorable places, so that they mature
and thereby increase their stock of Ferns. Usually, however,

434

PTERIDOPHYTES (FERN PLANTS)

Ferns are propagated vegetatively in greenhouses, and out of doors,
where conditions are usually unfavorable for the development of
their delicate gametophytes, many Ferns propagate almost en-

FIG. 388. — A Moonwort (Botrychium Virginianum). X about £.

tirely vegetatively. Some propagate by runners, many by the
branching and segmenting of the rhizome, some by buds which
fall from the leaves to the ground where they develop new plants,
and some by the leaves bending over and taking root at their tips.

EQUISETALES (HORSETAILS)

435

Some Plants Resembling True Ferns. — Some plants which
resemble the True Ferns, although they belong to another group,
are the Botrychiums or Moonworts that are common in the woods
(Fig. 388). They have an underground stem which sends up
leaves that have a finely divided vegetative portion and a spore-
bearing portion that much resembles clusters of small grapes.

FIG. 389. — A section through the tuber-like gametophyte of Botrychium,
showing one archegonium and a number of antheridia in the upper surface.
X about 10.

It is, however, in their gametophyte generation that they differ
most from True Ferns. Their gametophytes are tuberous sub-
terranean structures bearing the sex organs on the upper surface,
and associated with the gametophytes there is always an
endophytic Fungus (Fig. 389).

Equisetales (Horsetails)

In ancient times, as shown by their fossils in coal and other
kinds of rock, the Equisetales were very abundant, but the only
surviving group is the Horsetails. Their slender stems, often
called Joint Grass, are common in meadows, in moist places in
the woods and along roadsides. There are about 25 species of
Equisetum. There is Equisetum palustre common in swamps,
Equisetum pratense and Equisetum arvense common in meadows
and fields, and so on. Those growing in meadows and fields
are often troublesome weeds. They are widely distributed over
North America and also occur on other continents. They range
in height from a few inches to several feet. It is reported that
one form in the West Indies and Chili sometimes reaches a height
of 40 feet, but in our region 3 or 4 feet is a good height. The
Equisetums are also called Scouring Rushes because their stems
contain silica which is used in making scouring powders.

436

PTERIDOPHYTES (FERN PLANTS)

Sporophyte. — The sporophyte consists of a horizontal, much
branched, underground stem from which two kinds of aerial
branches or shoots arise (Fig. 390). One kind of shoot bears
spores and is called a fertile shoot, while the other kind does only
vegetative work and is called a sterile shoot. Both kinds of

FIG. 390. — Equisetum arvense. A, a portion of the underground stem
with two fertile or spore-bearing shoots, each of which bears a strobilus (d)
(X 5). B, a portion of a sterile or vegetative shoot (X i). C, asporophore,
showing the stalk and umbrella-like top on the under surface of which are the
sporangia (e) (X 6). Below, at the right, are shown spores, one with elaters
coiled about the spore and the other with elaters uncoiled (X about 15).

shoots are formed under the ground in the fall in most Equise-
tums and are thus ready to elongate and appear above ground
early in the spring. On both kinds of shoots the leaves are mere
scales, which are so joined as to form a sheath at each node. The
sterile shoots produce whorls of slender branches at the nodes and
are so finely branched as to resemble a horse's tail — whence the
name Horsetails. The food is made by the green cortex of the

GAMETOPHYTES 437

aerial shoots in the epidermis of which are stomata through
which carbon dioxide and oxygen reach the cortex.

The fertile branch commonly appears first in the spring, and
in some common forms of Equisetum bears no side branches,
thus having only whorls of scale-like leaves at the nodes. At the
apex of the fertile branch is borne the strobilus (plural strobili)
which is so named because of its resemblance to a cone such as
occurs in Pines (A, Fig. 390). The strobilus consists of a central
axis (the prolongation of the axis of the branch) to which are
attached the stalked shield-shaped structures or sporangiophores,
so named because they bear sporangia (C, Fig. 390). Some re-
gard the sporangiophores as modified leaves and, therefore, call
them sporophylls, which means spore-bearing leaves, but until
their relation to leaves is definitely determined, sporangiophore
is the safer term. Under the shield-shaped top of the sporan-
giophores are borne the sporangia, ranging from five to ten in
number on each sporangiophore. The spores are provided with
ribbon-like appendages, called elaters, which become entangled
and thus cause the spores to fall in clumps. The spores, although
alike in size, are physiologically different, for some of them pro-
duce only male while others produce only female gametophytes.
In some species of Equisetum the fertile branch dies after the
spores are shed, but in others the strobilus falls off and the branch
continues to elongate, becomes green, and makes food during the
remainder of the growing season.

There are two notable features presented by the sporophytes
of the Equisetums. One is the differentiation of the aerial por-
tion of the stem into sterile and fertile shoots. The second is the
aggregation of sporogenous tissue into a strobilus. The sterile
branch is a means by which sporangia can be elevated, so that the
spores are in a good position to be scattered. The strobilus is
supposed to be the forerunner of the flower, which likewise is a
structure consisting essentially of aggregates of sporogenous tis-
sue, for the pollen grains are spores, and also in the ovules there
are spores developed.

Gametophytes. — In the Equisetums the gametophytes are
much more reduced than in the True Ferns (Fig. 391). They are
so small that one needs a lens to identify them. Unless conditions
are very favorable, they are not able to survive out of doors, and
consequently the Equisetums are propagated principally vegeta-

438 PTERIDOPHYTES (FERN PLANTS)

lively. The gametophytes are small, green, ribbon-like bodies and
lie flat on the surface of the substratum. The male gametophyte
is the smaller and is one cell in thickness. It bears the antheridia
on the tips of the lobes or on the margin. The female gametophyte

FIG. 391. — The gametophytes of Equisetum arvense. A, female gameto-
phyte, showing one archegonium (ar) (X about 20). B, male gametophyte
with four antheridia shown ($ ) (X about 40).

forms a cushion, a number of cells in thickness, on the upper sur-
face of which the archegonia are borne.

The multiciliate sperms, after being set free from neighboring
male gametophytes, swim to the archegonia and down their necks
to the eggs. The fertilized egg begins to develop immediately
and continues until a new sporophyte is formed, and the life cycle
is thus completed.

Lycopodiales (Club Mosses)

About one-eighth of the living Pteridophytes are Club Mosses.
They are commonly divided into four groups — Lycopodium
Phylloglossum, Selaginella, and Isoetes — but a study of the
Lycopodiums and the Selaginellas will serve to give a general
notion of the Club Mosses.

The Club Mosses, although not Mosses at all, get their name
from their Moss-like stem and their club-shaped appearance due
to the large terminal strobili which some have.

Lycopodium. — There are several hundred species of Lycopo-
diums, and they are widely distributed, occurring in both hemi-
spheres and from the torrid to the frigid zones. They prefer
shady places and some are aquatic.

SPOROPHYTE

439

Sporophyte. — The sporophytes vary considerably in the dif-
ferent species, but consist of a stem simple or branched, bearing
numerous small leaves (Fig. 392). In numerous species com-
mon in temperate America the stems trail over the ground.
These species are often used for decorations at Christmas time
and are called Ground Pines, probably from the appearance of
their foliage, although they are not Pines at all.

One of the notable features of the sporophyte has to do with a
suggestion as to the origin of
the strobilus. In the simplest
forms all leaves are alike and
sporangia occur in the axils of
the leaves on most any part
of the stem. These leaves do
the vegetative work and in
addition are sporophylls in so
far as they bear sporangia.
In the more advanced sporo-
phytes of Lycopodium only
certain leaves bear sporangia,
and these leaves differ consid-
erably in form as well as in
function from the other leaves.
They are located at the top of
the stem, forming the close
aggregation or strobilus. In
such forms it is obvious that
there are two distinct kinds of
leaves — sporophylls and vege-
tative leaves. In intermedi-
ate forms one can find sporo-
phytes in which the leaves are

all alike but some bear sporangia while some do not, and often
leaves bearing rudimentary sporangia can be found. These
facts have suggested that all leaves were at first spore-bearing and
that foliage leaves are sterilized sporophylls. According to this
theory, the simplest condition is one in which all leaves bear
sporangia, and the differentiation of foliage leaves and sporo-
phylls came about by sterilizing the leaves from below until the
spore-bearing leaves were finally limited to the top of the stem.

FIG. 392. — Lycopodium complana-
tum, showing vegetative branches and
clusters of terminal strobili (X I). At
the left of the strobili is an enlarged
view of a sporophyll. showing the spor-
angium. Below the sporophyll are
shown some spores highly magnified.
Redrawn from Britton & Brown.

440

PTERIDOPHYTES (FERN PLANTS)

The strobilus, therefore, arose as a result of differentiating the
leaves in function and aggregating the sporophylls. Differing in
function, sporophylls and vegetative leaves would come to differ
in form. One can see considerable advantage in this to the plant.
It permits a large amount of leaf tissue to be devoted entirely to
the manufacture of food, while the sporophylls, since they are
not depended upon for food, can be much crowded, and as a result
many spores can be produced on a small region. In scattering
the spores there is also an advantage in having the sporophylls
at the top of the stem.

Gametophyte. — When the spores fall to the ground and
germinate, they develop fleshy gametophytes consisting usually
of a tuberous subterranean portion from which small, aerial,
green lobes arise on which the sex organs are produced. Within

FIG. 393. — The sporophyte of a Selaginella. After J. M. Coulter.

the tissues of the gametophyte there lives a filamentous Fungus,
and thus it is seen that the gametophyte resembles the gameto-
phyte of Botrychium 'in a number of ways.

The fertilized egg begins to develop immediately after fertiliza-
tion, and the young sporophyte is soon formed and the life cycle
thus completed.

Selaginella. — The Selaginellas, known as Little Club Mosses,
are widely distributed over the world and are common in con-
servatories where they are grown under the benches, in pots, and
in hanging baskets for their decorative effect.

SPOROPHYTE

441

Sporophyte. — The sporophytes are delicate plants with leafy
much branched stems (Fig. 398). The strobili occur on the ends
of the branches, and the sporophylls somewhat resemble the foli-
age leaves, but are usually smaller and more compact (Fig. S9Jf).

One notable feature is that there are two kinds of spores pro-
duced. In Bryophytes, True Ferns, Horsetails, and Lycopo-

•vf _

FIG. 394. — The vegetative and spore-bearing structures of the sporo-
phyte of Selaginella. A, a shoot of Selaginella, showing the stem, vegetative
leaves, and the strobili (st) at the ends of the branches (X 2). B, a micro-
sporophyll, showing the microsporangium (m) which has opened to allow the
microspores to escape (X about 10). At the right of the microsporophyll
are shown two microspores (s) (X 50) . C, megasporophyll with megasporan-
gium (me) open, thus exposing the four megaspores and permitting the micro-
spores to come in contact with the megaspores. Below the megasporophyll
are shown two megaspores (ri) ( X about 20) . D, lengthwise section through
a portion of a shoot, showing the position of the two kinds of sporangia in
relation to the leaves, and also the relative sizes of the two kinds of spores
(X 15). Partly from Dodel-Port and partly from nature.

442 PTERIDOPHYTES (FERN PLANTS)

diums the spores are alike as to size, although in some cases they
differ in the kinds of gametophytes produced. Other Pterido-
phytes differentiated spore-bearing and vegetative tissues, but
the Selaginellas have differentiated spores both in size and func-
tion. The larger spores, which are many times larger than the
smaller ones, produce only female, while the smaller ones produce
only male gametophytes. The two kinds of spores are borne
in separate sporangia which also differ in size. The prefixes,
micro, meaning little, and mega or macro, meaning large, are used
to designate these spores and also the sporangia and sporophylls
which bear them. Thus we speak of microspores and megaspores,
microsporangia and megasporangia, and microsporophylls and mega-
sporophylls (B and C, Fig. 394).

This habit of producing two kinds of spores in regard to size is
called heterospory (meaning different spores), while the habit of
producing spores alike in size is called homospory (meaning same
spores). The introduction of heterospory by Selaginella is a
significant feature because all Seed Plants are heterosporous. In
Seed Plants the pollen grains are microspores and within the
ovules occur the megaspores.

Gametophytes. — The second notable feature which Selagi-
nella presents is that the gametophytes are so much reduced that
they develop within the spores, where food and protection are
provided. Thus in Selaginella there are no green independent
gametophytes as we have been used to in other Pteridophytes
and in Bryophytes, but the gametophyte now lives on the sporo-
phyte just as the sporophyte of the Bryophytes lives on the
gametophyte. This, also, is a feature that is characteristic of
Seed Plants.

The male gametophyte is extremely simple, consisting of one
vegetative cell and a simple antheridium containing only a few
sperms, each of which nas two slender cilia (C, Fig. 395). In
developing, the male gametophyte breaks the spore wall, so that
a crack is produced through which the sperms escape.

The megaspores germinate and form the female gametophytes
while still in the sporangium, and this is a third feature that is
characteristic of Seed Plants. The female gametophyte is much
larger than the male gametophyte (A and B, Fig. 395). Its much
larger size is permitted by the greater size of the megaspore and
is also necessary because the female gametophyte must support

GAMETOPHYTES 443

the young sporophyte until it becomes self-supporting. The
female gametophyte therefore consists of many cells when mature
and bears a number of archegonia on the portion exposed by the
opening forced in the spore wall by the expansion of the game-
tophyte.

Previous to fertilization, the male gametophytes, each still,
except for a small slit-like opening, encased in the wall of the

FIG. 395. — The gametophytes and young sporophyte of Selaginella. A,
a megaspore containing a female gametophyte with the portion bearing the
archegonia exposed by the slit-like opening in the spore wall (X 100). B,
section through a megaspore, showing the spore wall (w) and female game-
tophyte (g) with one archegonium (a) with neck and egg (e) visible (X 100).
C, a section through an antheridium, showing the small prothallial cell at the
base and the wall cells which enclose the sperms within, one of which is shown
fully mature at the left ( X 500) . D, a young sporophyte with stem at s and
root at r and foot extending into the gametophyte which is still enclosed in
the spore wall (m). From Atkinson and nature.

microspore, fall out or are blown out of the microsporangia, which
open when the spores are mature, and fall or are carried by the
wind to the megasporangia where the female gametophytes are
developing. Here the sperms escape, and reach the archegonia,
which are accessible through the slit-like openings in the walls of
the megasporangia and megaspores. The fertilized egg develops
immediately into a sporophyte. Often the female gametophyte
remains in the megasporangium until the weight of the young
sporophyte tumbles it out. After the young sporophyte becomes

444 PTERIDOPHYTES (FERN PLANTS)

established in the soil and reaches maturity, strobili are produced,
and thus the life cycle is completed. Thus besides having an
independent complex sporophyte, the Selaginellas protect their
gametophytes and this is an additional adjustment to the land
habit.

Summary of Pteridophytes. — They present a number of
features characteristic of Seed Plants. They have an inde-
pendent sporophyte with well developed roots, stems, and leaves,
which in general have the same tissues that are characteristic
of these organs in Seed Plants. The second important feature
is the differentiation of vegetative and spore-bearing tissues.
This gave rise to the strobilus which is regarded as the forerunner
of the flower. The third important feature is the introduction
of heterospory and the production of gametophytes within the
spore wall. Heterospory and dependent gametophytes made the
origin of the seed possible. The fourth feature is the retention
of the female gametophyte within the megasporangium during
fertilization. This also is a seed-like feature.

CHAPTER XVIII

SPERMATOPHYTES (SEED PLANTS)
Gymnosperms (seed not enclosed)

Spermatophytes or Seed Plants constitute the fourth large
division of plants. They are the most highly developed plants,
and, therefore, in them we find the final achievement of plant
evolution. Their distinguishing feature is the seed, although
they have other notable features not found in the groups pre-
viously studied. The notable features of Pteridophytes, such as
sporophylls, strobili, heterospory, dependent gametophytes that
are developed within the spore wall, and the retention of the
megaspore in the megasporangium, are retained by the Sperma-
tophytes and to these they have added new features. Because
of the seed, lumber, fibers, and numerous other products ob-
tained from them, the Spermatophytes surpass all other divisions
of plants in economic importance. They are also very numerous,
and on account of the large size of their sporophytes they are our
most conspicuous plants.

The Spermatophytes are divided into two groups, — the Gym-
nosperms and Angiosperms. As the names suggest, the Gym-
nosperms bear their seeds exposed while Angiosperms bear them
enclosed, but the two groups differ also in other features as will
be noted later.

The Gymnosperms are more primitive than the Angiosperms
and are, therefore, more like the Pteridophytes, the group from
which Seed Plants are supposed to have originated. The groups
of Gymnosperms most like Pteridophytes are now extinct and
hence are known only by their fossils. Some of these extinct
forms resembled Ferns so much that they are called Pterido-
sperms, a term which means " Ferns with seeds." Thus Gymno-
sperms connect more closely with the Pteridophytes than the
latter group does with the Bryophytes. The Gymnosperms still
in existence are divided into a number of groups, but a study of
the Cycads and Pines will give a notion of the general features
characteristic of Gymnosperms.

445

446

SPERMATOPHYTES (SEED PLANTS)

Cycads

Of the Gymnosperms now in existence, the Cycads bear most
resemblance to the Ferns. In leaf and stem characters, some of
them could easily be mistaken for Ferns (Fig. 396). There are
nearly one hundred species of Cycads. They are tropical plants
but are grown nearly everywhere in greenhouses. One of the

FIG. 396. — A Cycad, showing the finely divided leaves and the short thick
trunk with its rough covering of leaf bases. After J. M. Coulter.

forms (Cycas revoluta) common in cultivation is often labeled
" Sago Palm " because its leaves resemble those of some of the
Palms.

Sporophyte. — The sporophyte has a tuberous or columnar
stem at the top of which are borne the large, much branched,
fern-like leaves. The stems are covered by the leaf-bases which
remain after the leaves fall. In some Cycads, where the stem is
subterranean, the plant is small, but in others with columnar
stems, the plant may reach a height of 50 feet or more.

Strobili. — The strobili are borne near the apex of the stem of
which they are really branches, and are of two kinds — staminate
and ovulate. The staminate strobili are simply microstrobili,
that is, strobili in which only microsporophylls and microspo-

STROBILI

447

rangia are produced. The name, however, suggests the likeness
of the microsporophylls to the stamens of Flowering Plants.
The ovulate strobili are strobili in which only megasporophylls
and megasporangia occur. The term ovulate suggests the like-
ness of the megasporangium to the ovule of Flowering Plants.
The megasporangia are now called ovules because they remain
closed, so that the female gametophyte is at no time exposed.

It is obvious that the Cycads have carried the differentiation
of structures farther than the Selaginellas have. In Cycads, not

FIG. 397. — Staminate strobilus and microsporophylls in Cycads. At the
left, a staminate strobilus of a Cycad (Dioori); at the right, microsporo-
phylls from two different Cycads, showing difference in shape, and the way
the sporangia are borne. After Chamberlain and Richard.

only spores, sporangia, and sporophylls are differentiated, but
there is also a differentiation of strobili.

The strobili of Cycads are much larger than those of Selaginella
or Lycopodium, and the sporophylls are usually very different
from the foliage leaves. In some Cycads the strobili are a foot or
more in length and several inches in diameter.

In the staminate strobili, the sporophylls are closely crowded
and practically have no resemblance to foliage leaves. They
vary considerably in shape in different Cycads, but have an
outer, expanded, sterile portion and bear the microsporangia,
usually grouped in sori, on their under surface (Fig. 397}.

The ovulate strobili are often much larger than the staminate
strobili. The megasporophylls are usually closely crowded, and

448

SPERMATOPHYTES (SEED PLANTS)

when they are short and fleshy, they fit together like the kernels
on an ear of Corn. The ovules are borne separately near the
base of the megasporophyll and as shown in Figure 398. In some
Cycads each megasporophyll bears only two ovules, while in
others, as Figure 398 shows, a larger number may be present. In
some Cycads the megasporophylls are much branched like foliage
leaves, and the sporangia appear to be transformed lower branches
or pinnae. Megasporophylls of this type suggest the relation-
ship of sporophylls to foliage leaves.

The young megasporangium or ovule contains four megaspores,
which are enclosed by two distinct coverings of sterile tissue.
The inner covering is the nucellus, which surrounds and encloses

FIG. 398. — Ovulate strobilus and megasporophylls in Cycads. At the
left, an ovulate strobilus; at the right, two types of megasporophylls, show-
ing the ovules (o).

the megaspores, and the outer one is the integument, which grows
up from the base of the ovule and forms a covering over the
nucellus. The integument is a protection for the nucellus, and,
when the ovule develops into a seed, it is transformed into a seed
coat. At the outer end of the megasporangium where the integu-
ment closes over the nucellus, a small opening or micropyle is
left which leads into a cavity, called the pollen chamber, into
which a beak-like portion of the nucellus projects (Fig. 399).

Female Gametophyte. — Only one of the four megaspores in
the megasporangium develops. The other three disappear and
all of the space and food is therefore given over to the develop-
ment of one gametophyte. The megaspore germinates in the

FEMALE GAMETOPHYTE

449

sporangium as in Selaginella, but a new feature of the Cycads is
that the megasporangium does not open to allow the megaspore
to be exposed, and therefore the female gametophyte remains
permanently enclosed in the sporangium. The developing female
gametophyte uses most of the nucellus for food and thereby
makes room for itself. When the gametophyte is mature the

FIG. 399. — Section through a Cycad ovule containing a mature gameto-
phyte. /, female gametophyte with two archegonia (a) shown; m, micro-
spores developing tubes, and male gametophytes; n, nucellus; i, integument;
p, pollen chamber into which the micropyle shown just above opens. Re-
drawn from Webber.

nucellus is so nearly used up that it is reduced to a thin layer,
except at the micropylar end where a beak-like portion remains.
A female gametophyte when fully formed consists of a large num-
ber of cells, most of which form a nutritive tissue for the devel-
oping sporophyte and are therefore spoken of as endosperm,
although the endosperm of Cycads is not the same in origin as the
endosperm of Angiosperms. The archegonia, usually several in

450 SPERMATOPHYTES (SEED PLANTS)

number, are produced at the micropylar end, and have much
shorter necks and are simpler in other ways than the archegonia
of Pteridophytes. The eggs are large and the most conspicuous
part of the archegonia. A section through an ovule ready for
fertilization looks like the one shown in Figure 399.

Male Gametophyte. — The microspores or pollen grains, as they
may now be called since they have to be transferred to the ovule
before they can function, usually contain three-celled gameto-
phytes at the time of their shedding, and in this condition they
reach the megasporangium, pass through the micropyle, and reach
the pollen chamber, where they are in contact with the beak of
the nucellus. In this position the three-celled gametophyte,
which consists of a vegetative, generative, and tube cell, com-
pletes its development. The miscrospore develops tubes which
branch and penetrate the beak of the nucellus in various direc-
tions, and function as absorptive structures. Finally, the beak
of the nucellus breaks down and thereby a passage way to
the archegonia is provided. Meanwhile the generative cell
enters one of the pollen tubes and passes farther into the pollen
chamber where it divides, forming a stalk cell and a body cell,
the latter of which forms the sperms, usually two in number.
The sperms bear a large number of cilia, and after escaping from
the pollen tube they swim through the watery solution present
in the chamber and thereby reach the archegonia and finally the
eggs.

Thus, when the male gametophyte is mature, it consists of only
four -cells besides the sperms, and there is no structure formed that
resembles an antheridium. In addition to the absence of an
antheridium, it should also be noted that pollination and the
growth of tubes are other new features which occur in connection
with the male gametophytes of Cycads. It is obvious that the
introduction of pollination and the growth of pollen tubes must
accompany the permanent enclosing of the female gametophyte
in the megasporangium.

Seed. — The seed is another new feature of the Cycads. After
fertilization, a young sporophyte (embryo) is developed and is
pushed well down into the nutritive tissue of the gametophyte
by a filament of celh (suspensor). During fertilization and the
development of the embryo, the ovule continues to grow and
the integument becomes pulpy, while the outer region of the re-

PINES (PINACEAE)

451

maining portion of the nucellus hardens, so that the seed when
mature resembles some of the stone fruits, such as the' Plum,
although it is a seed and not a fruit.

It is obvious that a seed is simply a transformed megaspo-
rangium. In the Cycads a seed is a megasporangium which has
its outer portions modified for protection and contains within
a female gametophyte bearing a
young sporophyte. Thus the re-
duction of the female gametophyte
through the Pteridophytes and
finally its retention in the mega-
sporangium in the Cycads so that
the young sporophyte also develops
within the megasporangium were
important steps in the evolution of
the seed.

<-¥ Although the Cycads resemble
Ferns in having swimming sperms,
and in having leaves and stems that
are Fern-like, they contrast with
them in such new features as differ-
entiation of strobili, simpler ga-
metophytes, pollination, growth of
pollen tubes, and the seed.

Pines (Pinaceae)

The Pines are a subdivision of the
Pine family (Pinaceae) . In addition
to the Pines, the Pine family in-
cludes the Spruces, Firs, Hemlocks,
Larches, Cedars, Redwood, Cypress,
and others. The Pine family is an
exceedingly important one because it includes a large proportion
of the trees from which lumber is obtained. The Pine family
belongs to the order of Conifers (Coniferales), so named because
of the cones which they bear. Not all of them, however, bear
dry cones like the Pines, for some have fleshy fruit-like structures,
as the berry-like structures of the Junipers illustrate. All of the
representatives of the Pine family are interesting, but a study of
their life history will be limited to that of the Pine.

FIG. 400. — Pine sporophytes.
After Miss Hay den.

452

SPERMATOPHYTES (SEED PLANTS)

Sporophyte. — The sporophytes of the Pines are mostly large
and in some cases are of huge dimensions. Some species of Pine
attain a height 6f 150 feet or more. It is characteristic of Pine
trees to have a main trunk and comparatively small lateral
branches. The main branches are usually in clusters, and in
some Pines, unless closely inspected, one might mistake the
branches to be in whorls. There is a gradual reduction in length

of branches from below up-
ward, so that trees grown
in the open have a conical
shape (Fig. 400.)

The needle-like leaves
are usually borne in groups
or fascicles of two, three,
or five leaves according to
the species. The duration
of leaves varies according
to the species and condi-
tions, but Pines shed only
a part of their leaves at a
time and hence are always
green.

Strobili. — The strobili,
as in the Cycads, are of
two kinds — staminate and
ovulate (Fig. 401). The
staminate and ovulate
strobili occur separately,
on the same trees, or on
different trees.

The staminate strobili or
cones (Fig. 402) are pro-
duced in clusters and in the Northern states may be seen in May or
early June. They vary in size in different species, sometimes at-
taining a length of half an inch or more, but in many species they
are much smaller. They expand from the buds in a few days,
soon shed their pollen and disappear, usually persisting only a few
weeks. A microstrobilus is, in reality, a modified branch con-
sisting of a main axis bearing scale-like microsporophylls or
Stamens, which are arranged spirally and closely crowded. On

FIG. 401. — A branch of a Pine, show-
ing an ovulate strobilus at a and a cluster
of staminate strobili at b.

STROBILI

453

the back or lower side of the microsporophylls are the micro-
sporangia, usually two, and each contains numerous microspores.
Nearly opposite each other on the microspore are two air-sacs
whereby the spores are easily carried by the wind. When the
spores are mature, the microsporangia or pollen sacs open by
longitudinal slits, and the pollen shatters out, often like small

FIG. 402. The staminate structures of the Pine. A, cluster of staminate
strobili ( X about f ) . B, a staminate strobilus enlarged, showing the arrange-
ment of the microsporophylls. C, a microsporophyll, showing the two
sporangia (m) ; D, microspore showing the two wings and two cells of the male
gametophyte.

clouds of dust. The wind carries the pollen about, and some
reaches the ovulate strobili, but much the larger part of it is
wasted. Sometimes pollen accumulates on walks under Pines
that are shedding their pollen until the walks look as if they had
been sprinkled with finely powdered sulphur.

The ovulate strobili or cones appear near the tips of the new
growths in early spring. Usually they are smaller when they
first appear than the staminate cones, but they persist and, after
a growth of two seasons, become the conspicuous scaly cones so
familiar on or about pine trees. Sometimes several occur to-

454 SPERMATOPHYTES (SEED PLANTS)

gether, but they do not form close clusters as the staminate
cones do.

The scales of the ovulate cones are considered too complex to
be called sporophylls, for each scale consists of an ovuliferous
scale (ovule-bearing scale) and a bract, the two being partly united.
Some morphologists think that the ovuliferous scale itself repre-
sents two sporophylls fused together. The megasporangia or

FIG. 403. — The ovulate structures of the Pine. A, branch bearing four
ovulate strobili, B, ovulate strobilus, showing the arrangement of scales
( X about 2) ; C, a view of the inner or upper side of a scale, showing the two
sporangia (s).

ovules, two in number, are borne on the upper side and at the
base of the ovuliferous scale (Fig. 403). The scales are spirally
arranged and closely crowded, but during pollination they spread
apart, and the pollen can slide in between them and reach the
ovules. After pollination the scales close together again, and the
cone is made water-tight by a secretion of resin. After pollina-
tion the cone also changes from the vertical to the nodding
position.

The ovules consist of an integument and a nucellus, and deeply
buried within the nucellus the four megaspores occur. The
ovules are arranged one on each side of the median line of the
scale, with the micropyles pointing downward. The integument

FEMALE GAMETOPHYTE

455

extends beyond the nucellus, and its free margin flares open, thus
forming an open micropyle that leads into the pollen chamber.

Female Gametophyte. — Although four megaspores are formed
in the megasporangium, only one of them develops a gameto-
phyte, the others being destroyed and used for food by the one
that develops. During the first season the surviving megaspore
enlarges and becomes multinucleate. With the megaspore in this

FIG. 404. — Development of the ovule and pollen tubes in the Pine. C,
section through an ovuliferous scale, showing the bract behind and a section
of an ovule (s) on its inner face, the megaspore being shown at m; D, an
ovule with female gametophyte (/) mature, showing eggs at e, ovule wall
consisting of nucellus and integument at w, and pollen grains growing tubes
through the nucellus at (p).

condition the ovule passes the winter. Early next spring growth
is resumed, and by about the first of June of the second season
the gametophyte is complete, consisting of 250 or more cells and
bearing a number of archegonia (usually two to five) at the
micropylar end (Fig. 40 4-) The eggs are usually ready for
fertilization about the first of June of the second season. While
the female gametophyte is developing, the male gametophyte is
completing its development in the pollen chamber and the pollen

456 SPERMATOPHYTES (SEED PLANTS)

tube is eating its way through the nucellus to the female
gametophyte.

Male Gametophyte. — The male gametophyte forms within
the pollen grain and its tube. At the time of pollination the male
gametophyte commonly consists of four cells — two prothallial
or vegetative cells, a generative cell, and a tube cell. At least one
of the prothallial cells usually disintegrates and disappears early
in the development of the gametophyte. This is the condition
of the male gametophyte when the pollen is carried to the ovulate
cone. Upon reaching the ovulate cones the pollen grains fall
down to the base of the scales in the region of the ovules, and

FIG. 405. — Seed structures of the Pine. A, a mature ovulate strobilus
with scales spread apart to allow the seeds to escape. B, a view of the inner
side of a scale, showing the two seeds when mature. The wings of the seeds
are a part of the scale and did not develop from the ovule. C, section through
a pine seed, showing the female gametophyte (g), embryo (e), and seed coat (w) .

some lodge at the mouth of the micropyles, where they are caught
in a drop of a mucilaginous secretion and drawn in close to the
tip of the nucellus. In this position the pollen grains begin to
develop tubes, which by means of an enzyme dissolve the nucel-
lar tissue, using it as food and at the same time making a way for
themselves. Cold weather finally checks the growth of the
pollen tubes, and the male gametophytes now rest over winter.
Early the next spring the pollen tube resumes its growth toward
the archegonia, and the generative cell passes into the pollen tube
and divides, forming two cells, one of which divides and forms the
two sperms which now have the pollen tube as a passageway to
the archegonia. The sperms reach the archegonia about the

SEED

457

middle of June of the second season and fertilization soon follows.
In addition to its simplicity the notable features of the male
gametophyte are that the sperms have no cilia and that they are
conducted to the archegonia by the pollen tube.

Seed. — The fertilized egg at first forms tiers of cells, which
constitute a long filament, called a suspensor, at the end of which
the embryo develops deeply imbedded in the nutritive tissue of
the female gametophyte. When mature the embryo is still
surrounded by much gametophytic tissue called endosperm.

While the embryo or the young sporophyte is developing, the
ovule and the entire cone continue to enlarge. The integument
is transformed into a seed coat, and when mature the seed sepa-

Iree

FIG. 406. — Diagram of the life cycle of the Pine. Starting with the tree
at the left, the two kinds of strobili are shown at a and 6, the two kinds of
sporophylls and their sporangia at c and d, the two kinds of spores at e and /,
the gametophytes at g, the mature seed at h, from the embryo of which a
new tree develops.

rates from the ovulate scale with a long membraneous wing,
which enables the seed to float in the air (Fig. 405.} Pine
seeds, although usually smaller, are similar in general structure
to the seeds of Cycads. They contain a female gametophyte
bearing a young sporophyte and a protective covering composed
of the integument and the nucellus, the latter persisting as a
membrane about the gametophyte or endosperm.

The scales of the ovulate strobilus continue their development
until the seeds are mature and remain tightly closed so that the
seeds are well protected. After the cone is mature, the scales dry

458 SPERMATOPHYTES (SEED PLANTS)

and spread apart and the seeds fall out. Although the seeds are
protected between the scales, they are not enclosed as the seeds of
a Bean or an Apple are. They are on the outside of the structure
which bears them, — whence the name Gymnosperms.

The seeds are dispersed by the wind and usually do not germi-
nate until the next spring after dispersal. In germination the
axis (hypocotyl) of the sporophyte elongates, forming an arch
and drawing the cotyledons out of the ground, and at the same
time the tap-root at the lower end of the hypocotyl becomes
established in the soil. By the straightening of the hypocotyl
the green cotyledons are lifted into the air and sunlight, and the
sporophyte soon becomes independent of the seed. After a
number of years of growth, it begins to bear strobili, thus com-
pleting the life cycle of the Pine as shown in Figure 406.

In summarizing it should be noted that the Pines have two
kinds of strobili, reduced gametophytes, pollination, and pollen
tubes, features which were pointed out as the notable ones of the
Cycads. But in contrast with the Cycads the Pines have more
massive sporophytes with leaves bearing no resemblance to those
of Ferns, and also the Pines have abandoned swimming sperms
and conduct the sperms to the eggs through pollen tubes.

In pines the cones mature the second fall after pollination, but
in some genera of the pine family, as the Spruces illustrate, sexual
reproduction proceeds more rapidly, although similar in nature,
and the cones mature the fall following pollination.

CHAPTER XIX

SPERMATOPHYTES (Continued)

Angiosperms (Seeds Enclosed)

General Characteristics. — The Angiosperms are the most
highly evolved group of the plant kingdom, being the most per-
fectly adapted to terrestrial conditions. They also surpass all
other groups in economic importance, for they include the large
majority of our cultivated plants. Our dependence upon the
grains and fruits and upon forage, root, and tuber crops attests
the economic importance of the Angiosperms. The Angiosperms
probably have more species than any other group of plants and
show more variations. Approximately 125,000 species are
known. They form the most conspicuous part of our vegetation,
for not only most of our cultivated plants but nearly all weeds
are Angiosperms. The origin of the Angiosperms is not known,
but they probably arose from some Fern-like plants as the Gym-
nosperms did. The Angiosperms, as the name suggests, are
characterized by having their seeds enclosed. The enclosure is
the ovary, which is one of the notable features of Angiosperms.
Another notable feature is the flower, which is regarded as a
special type of strobilus. They also differ from the Gymno-
sperms in having more reduced gametophytes. Both male and
female gametophytes consist of only a few cells and have lost all
traces of sex organs. Since the gametophytes are microscopical,
most people are acquainted with only the sporophytes of Angio-
sperms. In character of roots, stems, leaves, flowers, seeds, and
fruits, there are numerous variations in Angiosperms, but, since
Part I of this book is devoted chiefly to these variations, the dis-
cussion will now be limited to the characteristic features of the
group and to such features as characterize the families of most
economic importance.

The Flower. — The flower (Fig. 407), consisting of a perianth
(calyx and corolla), stamens, anj^e or more pistils, is a structure

459"

460

SPERMATOPHYTES (SEED PLANTS)

characteristic of Angiosperms. The stamens are microsporo-
phylls and the pistils are megasporophylls. A typical flower is,
therefore, essentially an association of sporophylls surrounded
by a perianth, and, in so far as a flower is an association of sporo-
phylls, it does not differ
fundamentally from a stro-
bilus. In passing from the
simplest Angiosperms,
where there are flowers
that have no perianth, to
those Angiosperms having
typical flowers, all grada-
tions between a typical
strobilis and a typical
flower can be found. It is,
therefore, impossible to de-
fine a flower so as to in-
clude the flowers of all
Angiosperms and at the
same time separate the
flower from the strobilus.
The flowers of Angiosperms
and the strobili of the Gym-
nosperms and P t e r i d o-
phytes differ in the char-
acter of their sporophylls
more than in any other
feature.

Perianth. — The peri-
anth, usually consisting of
both sepals and petals, not
only protects the sporo-
phylls during their develop-
ment but also serves in
pollination, which in Angiosperms is done largely by insects.
At the base of the perianth occur nectar glands, which are further
adaptations to insect pollination. The perianth seems to have
arisen in two ways. In some cases there is evidence that the
parts of the perianth are modified sporophylls, while in other
cases they are apparently modified foliage leaves.

FIG. 407. — The floral structures of a
typical flower. The floral structures com-
prise a perianth (a) composed of calyx
and corolla, a number of microsporophylls
or stamens each consisting of anther (e)
and filament (c), and a pistil (6) composed
of one or more megasporophylls with the
megasporangia or ovules (d) enclosed in an
ovary.

STAMEN

461

Stamen. — The stamen (microsporophyll) has its pollen sacs
(microsporangia), usually four in number, joined into the struc-
ture called anther. The pollen grains (microspores) are numer-
ous in each sac and are formed before the flower opens. Like
the spores of Gymnosperms, Pteridophytes, and Bryophytes,
they are formed by special cells known as mother cells of which
there are many in each pollen sac as shown at A in Figure 408.
These mother cells also divide by the reduction division, that is,
by the kind of cell division in which the daughter nuclei get only
half the sporophytic number of chromosomes. The mother cells

FIG. 408. — The spore mother cells of Angiosperms. A, cross section of
a young anther, showing the microspore mother cells (ra) . B, section through
an ovule, showing the megaspore mother cell (m) . Both are highly magnified.

are formed and undergo the reduction division while the flowers
are still small buds. Immediately following the division of the
mother cell, the daughter nuclei resulting from this division
divide and consequently there are four spores or pollen grains
formed from each mother cell. The four spores constituting
the progeny of a mother cell are called a tetrad. The cells of the
tetrad commonly cling together for a short time after they are
formed, but soon separate and each becomes a pollen grain. The
pollen grains are in reality the one-celled stages of the male
gametophytes, since they have the reduced or gametophytic
number of chromosomes. Usually before the pollen grain
leaves the anther its nucleus divides, forming a tube and genera-
tive nucleus. In this condition the pollen grain is carried to the

462

SPERMATOPHYTES (SEED PLANTS)

stigma where the male gametophyte completes its development.
The history of the pollen is shown in the upper diagram of
Figure 409.

The Pistil. — A pistil consists of one or more megasporophylls
(carpels). The megasporophyll is usually organized into an

FIG. 409. — The formation of the spores and gametophytes in Angiosperms.
The upper diagram shows the origin of the pollen grains and male gameto-
phytes. a, cross section of a young anther, showing the mother cells; 6, a
mother cell beginning to divide; c, the first division of the mother cell com-
pleted; d, the second division of the mother cell completed, resulting in a tetrad
of daughter cells; e, cells of the tetrad separated and fully formed pollen grains;
/, pollen grain with male gametophyte developed, showing tube nucleus at t,
and sperms at s.

The lower diagram shows the formation of megaspores and female game-
tophyte. g, section through an ovule, showing the megaspore mother cell
with chromatin in a thread in preparation for the reduction division; h, a sec-
tion through an ovule, showing the four megaspores resulting from the two
successive divisions of the megaspore mother cell; i, section through an
ovule showing the mature female gametophyte which is formed by the sur-
viving megaspore.

ovary, style, and stigma. In compound pistils, where a number
of carpels are present, the ovaries are usually joined, thus form-
ing a compound 6vary, and often the styles and sometimes the
stigmas are also joined.

The ovary, which is the enclosure for the megasporangia or
ovules, is one of the notable features of Angiosperms. With the
ovules enclosed the pollen cannot come in contact with the ovules
as it does in Gymnosperms, so the stigma, another characteristic

L THE PISTIL 463

structure of the Angiosperms, had to be introduced. That the
ovary gives the Angiosperms a special economic importance is
attested by the fact that our fruits are either ripened ovaries or
ripened ovaries plus closely related parts. Within the ovary
occur the cavities or locules in which are borne the megasporangia
or ovules, varying in number and also in the way they are at-
tached in different Angiosperms.

The ovule is generally borne on a stalk (funiculus), and the
chief structure of the ovule is the nucellus, which in most Angio-
sperms is enclosed by two integuments, an inner and an outer one.
As in Gymnosperms, the integuments do not completely close
over the top of the nucellus, but leave a small opening (micropyle) .

Usually the ovule curves as it develops and the micropyle is
brought around to near the base of the ovule. This position of
the micropyle is a favorable one for the entrance of the pollen
tube. There are terms used to indicate the amount of curving
ovules undergo in their development. Ovules that remain'
straight are orthotropous. Those that double clear back upon
themselves are anatropous. Those turning only part way back
upon themselves are campylotropous. Within the nucellus is
formed the megaspore mother cell (B, Fig. 408), which also
divides by two successive divisions in one of which the number of
chromosomes is reduced to the gametophytic number. A mega-
spore, therefore, produces four megaspores, each of which is com-
parable to a pollen grain. Although the megaspores are formed
while the flowers are mere buds, they are formed later than the
pollen grains. As in the Gymnosperms, in most Angiosperms
only one of the megaspores develops into a gametophyte, although
among Monocotyledons, there are cases in which more than one
or all of the megaspores apparently take part in forming the one
gametophyte. The lower diagram in Figure 409 gives the usual
history of the megaspores.

The female gametophyte is very much reduced, consisting of
only a few nuclei and naked cells in a small mass of cytoplasm.
In most Angiosperms the female gametophyte is developed in
the following way. The megaspore first enlarges by digesting
and using the other three megaspores and the adjoining cells of
the nucellus as food. Then as the megaspore further enlarges
the nucleus divides, and the daughter nuclei pass to opposite
ends of the embryo sac which is the term now applied to the

464

SPERMATOPHYTES (SEED PLANTS)

region enclosed within the cell membrane of the germinating
megaspore. In this position nuclear division follows until there
are four nuclei at each end. The megaspore has now become the
female gametophyte consisting of eight nuclei, four at the
micropylar and four at the opposite end, known as the chalazal
or antipodal end of the embryo sac. After the stage with eight
nuclei is reached, then the organization of the female gameto-
phyte begins as shown in Figure J^IO. A nucleus called polar

nucleus from each end of the
embryo sac moves toward
the center of the sac until the
two come in contact. Some-
times they fuse soon after
coming in contact to form
the primary endosperm nu-
cleus, but often they remain
in contact until fertilization
and then fuse at the same
time they fuse with the
sperm to form the endo-
sperm nucleus. The three
nuclei and adjacent cyto-
plasm at the micropylar end
are organized into three
naked cells, the inner one
being the egg and the other
two the synergids. The three
nuclei at the antipodal end
and known as antipodals
usually disappear early, but
in some Angiosperms they
become organized with the adjacent cytoplasm into cells that
seem to have an absorptive function. The female gametophyte
is now organized and ready for fertilization. When compared
with the female gametophyte of tfre Pine, its remarkable reduc-
tion in number of cells, the absence of archegonia, and the forma-
tion of a nucleus for providing endosperm are notable features.
Male Gametophyte and Fertilization. — On the stigma the
pollen grain develops a tube which by means of enzymes eats its
way through the stigma, style, and ovule into the embryo sac.

FIG. 410. — Organization of the female
gametophyte in Red Clover. At the left,
a section through the nucellus, showing
eight nuclei of the female gametophyte
with four nuclei at each end of the em-
bryo sac. At the right, the gametophyte
fully organized, showing the antipodals
at a, the polars at p, the egg at e, and
the synergids at s.

MALE GAMETOPHYTE AND FERTILIZATION

465

The pollen tube lives as a parasite on the structures through
which it passes, using their tissues as food for growth and mak-
ing a passageway for itself at the same time. The growth of the
pollen tube is directed by the tube nucleus which maintains a
position near the end of the tube. Soon after the pollen tube is
well started, the generative nucleus passes from the pollen grain
into the tube and later divides, forming two
sperms which are carried along with the con-
tents of the tube to the embryo sac. The
male gametophyte, consisting of tube nucleus
and two sperms, is now complete. In some
plants, however, the formation of the sperms
occurs before the development of the tube is
begun.

When the tube reaches the embryo sac and
comes in contact with its contents, the mem-
brane enclosing the tube is destroyed, and the
tube nucleus, sperms, and other contents of
the tube flow into the embryo sac. The con-
tents of the embryo sac apparently destroy
the tube nucleus, for it soon disappears, while
the sperms apparently thrive. Since there
are no cell walls in the embryo sac, the sperms
are free to move about. As to how they are
moved is not known, for they have no cilia,
but one very soon reaches the nucleus of the
egg and the other the polar nuclei or the
primary endosperm nucleus, with which they
come in contact and fuse. Since there are two
fusions, one with the egg nucleus and the
other with the polar nuclei or the primary
endosperm nucleus, there are two fertilizations
or double fertilization, and this also is a notable
feature of Angiosperms (Fig. 1+1 1). Of course fertilization is
difficult to follow and has been seen in only a comparatively few
Angiosperms. It is therefore possible that many times the
second sperm does not fuse with the polars or the primary
endosperm nucleus, but double fertilization has been found so
generally in the Angiosperms whose fertilization has been studied
that it is believed to be quite universal among Angiosperm. In

FIG. 411. —An
embryo sac of a
Lily, showing
double fertilization.
At the upper end of
the sac the egg (e)
and a sperm (s) are
shown fusing, and
near the center of
the sac the second
sperm (s) is shown
fusing with the two
polar nuclei (p).

466

SPERMATOPHYTES (SEED PLANTS)

connection with double fertilization it should be noted that the
endosperm nucleus contains the contents of three nuclei, since
it is a product of a triple fusion, involving a sperm and the two
polar nuclei.

Embryo. — The first cells produced by the division of the fer-
tilized egg form a filament which pushes down into the embryo
sac. This filament is called the proembryo. The terminal cell
of the proembryo develops the embryo, while the remainder of
the filament remains as a stalk called suspensor. After the termi-

FIG. 412. — Development of the embryo and endosperm in the Shepherd's
Purse. A, section through ovule with embryo and endosperm in early stage
of development, showing the proembryo which consists of the suspensor (6)
and the terminal three-celled embryo (a), and also showing the endosperm (c)
as a chain of free nuclei around the wall of the embryo sac. B, the same as
A, excepting that the proembryo and endosperm are more developed. C,
section through a mature seed showing the seed coat (s), and the mature
embryo with cotyledons at h, plumule at p, hypocotyl at e, and radicle at d.

nal cell divides a number of times, the parts of the embryo begin
to be differentiated. In Dicotyledons two lobes appear at the
end farthest from the micropyle and these become the two coty-
ledons characteristic of dicotyledonous Angiosperms. Between
the cotyledons the plumule is formed, while the axis of the embryo
below the cotyledons is differentiated into the hypocotyl, which
is the main part of the axis, and the radicle at its lower end
(Fig. 412).

The embryos of monocotyledonous Angiosperms have a radicle,
hypocotyl, plumule, but only one cotyledon. They also differ

POLYEMBRYONY

467

from the embryos of Dicotyledons in the relative positions of the
cotyledon and plumule. Although the cotyledon apparently
arises laterally, it soon becomes terminal and the plumule appears
to develop on the side of the em-
bryo (Fig. 413).

Parthenogenesis. -- Partheno-
genesis, which is the develop-
ment of an embryo from a sup-
posedly unfertilized egg, occurs
in a number of Angiosperms.
In the Dandelion (Taraxacum),
Meadow Rue (Thalidrum), Ever-
lasting (Antennaria) , Apples,
Pears, Quinces, and a few other
plants parthenogenesis is known
to occur. In cases which have
been investigated cytologically, it
has been found that the mother
cell in the ovule omits the reduc-
tion division, and, therefore, the
cell which occupies the position'of
an egg has the sporophytic num-
ber of chromosomes and fertiliza-
tion is not necessary. Since par-
thenogenetic plants show no re-
sults of crossing in the offspring
when cross-pollinated, partheno-
genesis may be a source of disap-
pointment to the plant breeder.

Parthenocarpy. — Parthenocarpy is the development of fruit
without fertilization and is quite common among Angiosperms.
Bananas, seedless Oranges, and seedless Currants are familiar
examples of parthenocarpic plants. Sometimes Apples develop
without seeds, and some varieties of Cucumbers develop fruits
without pollination.

Polyembryony. — In a few Angiosperms, of which one of the
Onions (Allium) is a notable example, a number of embryos may
be developed in the same embryo sac or around it. The syner-
gids and antipodals have been known to develop embryos, and
sometimes some of the cells of the nucellus around the embryo

FIG. 413. — A monocotyledon-
ous embryo as typified by that of
Corn. The cotyledon (c) appears
terminal and the plumule (p} as
arising from the side of the em-
bryo.

468 SPERMATOPHYTES (SEED PLANTS)

sac develop like buds and form embryos, in which case, of course,
there is no fertilization. Polyembryony may also be a source of
annoyance to plant breeders, for if plants that are used in cross-
ing develop polyembryonous seeds, the offspring arising from
these seeds may develop from embryos that were formed by the
budding of the nucellus, in which case the embryos have only the
characteristics of the mother plant. For example, in crossing
different strains of Tobacco, in some cases the plants arising
from the seeds obtained by crossing are not hybrids but like the
mother plant. Some think this may be due to parthenogenesis
and others attribute it to polyembryony.

Endosperm. — While the embryo is developing, the endosperm
nucleus is dividing and its accompanying cytoplasm is increasing..
The free nuclei at first form in a chain around the wall and then
multiply towards the center. Cell walls are finally formed and in
these cells food is stored. In some Angiosperms the endosperm
is taken up by the embryo almost as rapidly as formed and stored
in the cotyledons, while in other Angiosperms most of the en-
dosperm remains outside of the embryo until the seed germi-
nates.

Since the endosperm nucleus contains the contents of a sperm,
the character of the endosperm of a seed is often determined by
the sperm. Thus, as in case of Corn where the endosperm re-
mains outside of the embryo, the color and other characteristics
of the - endosperm are often like the pollen parent and not at
all like those of the mother parent. This feature called xenia
has already been referred to. In some seeds, in addition to the
formation of endosperm, the portion of the nucellus remaining
becomes stored with food and forms what is known as peri-
sperm.

Seed Coat. — As the embryo and endosperm develop, the
ovule enlarges rapidly, and at the same time the embryo sac de-
stroys much or all of the nucellus and frequently a part or all
of the inner integument. Consequently the seed coat consists
chiefly of the outer integument, which is usually very much mod-
ified for protection.

It is obvious that the seeds of Angiosperms differ considerably
from the seeds of Gymnosperms, for the female gametophyte of
Angiosperms is soon destroyed after fertilization by the develop-
ing embryo and endosperm, and consequently there is no gameto-

SEED COAT

469

phytic tissue in the seeds of Angiosperms comparable to that
in the seeds of Gymnosperms. The endosperm in the seeds of
Gymnosperms is simply the portion of the gametophyte that

FIG. 414. — The life cycle of Angiosperms illustrated by the life cycle of
Red Clover. At the left in the line above, a branch of Red Clover with
heads of flowers ( X £) ; next, a vertical section through a flower, showing the
floral structures; at the right, a section of an anther, a pollen grain, and a
pollen grain with tube and male gametophyte developed. At the left in the
line below, an ovule with female gametophyte mature and pollen tubes en-
tering through the micropyle; next, embryo and endosperm forming; next,
seed mature from the embryo of which the new plant at the right develops.

remains, but in Angiosperms the endosperm develops after the
gametophyte is formed and from a nucleus formed by the fusion
of three other nuclei, one of which came from the male gameto-
phyte.

470 SPERMATOPHYTES (SEED PLANTS)

Notable Features of Angiosperms. — In contrast to Gymno-
sperms, the Angiosperms have the flower; a megasporophyll con-
sisting of an ovary, in which the megasporangia are enclosed, and
of a stigma to receive the pollen; more reduced gametophytes;
and endosperm nucleus and double fertilization. The life cycle
of an Angiosperm is shown in Figure 414-

CHAPTER XX

CLASSIFICATION OF ANGIOSPERMS AND SOME

OF THEIR FAMILIES OF MOST ECONOMIC

IMPORTANCE

Classification. — The Angiosperms are so numerous and vary
so widely that their classification is not at all settled. Ray, a
noted English botanist (1628-1705), divided the Angiosperms
into two sub-classes — Monocotyledons and Dicotyledons —
on the basis of the number of cotyledons. There are also other
features which are used in distinguishing these two groups,
such as the number of floral structures composing the flower,
the venation of the leaves, the arrangement of the vascular
bundles in the stem, and the presence or absence of cambium.
Thus leaves with parallel veins, the parts of the flower in threes
or sixes, the scattered arrangement of vascular bundles in the
stem, and closed vascular bundles are characteristic of Mono-
cotyledons, while leaves with net-veins, floral parts in fours or
fives, vascular bundles arranged in a circle so as to enclose the
pith, and indefinite growth by means of a cambium are charac-
teristic of Dicotyledons. As to whether the Monocotyledons
arose from the Dicotyledons, or the Dictyledons from the
Monocotyledons is a question that botanists are not able to
answer satisfactorily. However, recent studies of the young
embryos of some of the Monocotyledons show that there are
two cotyledons present, one of which is very rudimentary.
This discovery with other structural and historical features has
given rise to the view that the monocotyledonous condition
arose from the dicotyledonous condition through the suppres-
sion of one of the cotyledons. This means that the growth
which first becomes evident at the top of the developing embryo
as two points, each of which develops into a cotyledon in Dicoty-
ledons, became concentrated into the development of only one
point which consequently develops the single large cotyledon
characteristic of Monocotyledons.

471

472 ANGIOSPERMS

Upon differences which pertain chiefly to the flowers, the
Monocotyledons and Dicotyledons are subdivided into many
groups.

The Monocotyledons are subdivided into 8 or 10 orders which
are in turn subdivided into about 42 families. The families are
subdivided into many genera and the genera into species of
which there are about 25,000.

The Dicotyledons, of which there are more than 100,000 spe-
cies, include most of the Angiosperms, being more than four
times as numerous as the Monocotyledons. The Dicotyledons
are divided into two large subdivisions — the Archichlamydeae
and the Sympetalae.

The Archichlamydeae have a corolla of separate petals or no
corolla at all. They include about 180 families and 61,000
species of Dicotyledons. They are grouped into two classes,
one of which has apetalous flowers, that is, flowers without
petals, and the other of which has polypetalous flowers, that is,
flowers with petals present and free from each other.

The Sympetalae include those Dicotyledons in which the
petals are more or less united. There are about 50 families
and 42,000 species of the Sympetalae.

In arranging the orders and families taxonomists have en-
deavored to follow an evolutionary sequence. The rank of an
order or family depends chiefly upon the organization of its
flowers. Flowers most like a typical strobilus, that is, resem-
bling most the strobili of Gymnosperms, are regarded as the
simplest of flowers. Thus a flower without any perianth is
simpler than one with a perianth. Also a flower with parts
arranged spirally, thus having parts arranged like the sporophylls
in a strobilus, is considered simpler than one with parts having a
cyclic arrangement. Again flowers having petals joined or carpels
united are considered more advanced than flowers in which these
parts are separate. Thus the Sympetalae, since they have united
petals, are considered more advanced than the Archichlamydeae
which have separate petals or no petals at all. In respect to
these evolutionary tendencies the orders and families of both
Dicotyledons and Monocotyledons form an ascending series.

Most of the families of the Angiosperms have some species
of economic importance, but some families are much more
notable than others for their species related to man's welfare.

(WILLOW FAMILY SALICACEAE)

473

The species may concern us because they are useful for food,
fibers, lumber, medicine, etc., or because they are weeds which
hinder the growth of cultivated plants, poison live stock, or do
damage in other ways.

Beginning with one of the lower families of the Dicotyledons,
a number of families of Angiosperms having species of consid-
erable economic importance are discussed in the following pages.

FIG. 415. — The flowers of a Willow. Above, at the left, a staminate
catkin, and below, at the left, a staminate flower, showing the bract and sta-
mens; above, at the right, a pistillate catkin, and below, at the right, a pistil-
late flower, showing the bract and pistil. After Burns and Otis.

Archichlamydeae

Apetalae

Willow Family (Salicaceae) . — This family, although it is not
the lowest family of the Dicotyledons, stands well toward
the bottom of the series. To this family belong the Willows
and Poplars. The flowers are unisexual and simple in type.
The plants are dioecious and bear their apetalous flowers in
scaly spikes or catkins (Fig. 415), A flower consists of a pistil

474

ANGIOSPERMS

or of a number of stamens borne in the axil of a small scale or
bract.

The Weeping Willow, so named because of its drooping
branches, is cultivated for its beauty. The growing of Basket
Willows for sprouts, which are woven into baskets, chairs, and

other articles, is an industry
of considerable importance.
Willows are easily propagated,
taking root readily when
transplanted or from cuttings.
They grow especially well near
water and are often planted
along river banks where they
prevent the cutting away of
the banks by floods. A num-
ber of the Poplars, such as
the Aspens, Balm of Gilead,
and Cottonwood, are culti-
vated for shade. The Cot-
tonwood grows to be a very
large tree and is of some
value for lumber. Both Wil-
lows and Poplars are used in
making medicinal charcoal,
and a number of substances,
such as salicin, populin,
tannin, and a volatile oil are
obtained from their bark.

Walnut Family (Juglan-
daceae). — This family com-
prises the Walnuts and Hick-
ories. The Walnuts and

Hickories are monoecious, and their flowers are generally apetal-
ous, although in some cases the pistillate flowers have petals
The staminate flowers are borne in catkins, while the pistillate
flowers are borne singly or in small clusters (Fig. 416)-

The White Walnut (Juglans cinered), called Butternut, and the
Black Walnut (Juglans nigra) are the most common Walnuts in
the United States. The European Walnut (Juglans regia), not-
able for its delicately flavored nuts, is grown in California and

FIG. 416. — The flowers and fruit
of the Black Walnut. At the left, a
branch bearing a catkin of staminate
flowers below and two pistillate flowers
above (Xf). At the right, above, a
pistillate flower, showing the pistil
enclosed in bracts which form the husk
of the fruit; next, below, a staminate
flower, showing the bracts and the
stamens; at the bottom, a fruit
After Burns and Otis.

BIRCH FAMILY

475

the Southern States, and some other species occur in certain
parts of the United States.

The nuts are rich in oil, which is expressed and used as food
and in painting. The nuts are common on the market and are
of considerable importance as food. The wood of the White
and Black Walnut is much
used for furniture and cab-
inet work. The wood of the
Black Walnut is probably
the most valuable wood of
the North American forest.
It is a durable wood, takes
a fine polish, and is much
sought for furniture, gun-
stocks, and for cabinet
work.

There are a number of
species of Hickories, and
the Pecans and several other
species bear nuts having
considerable value for food.
Hickory wood is very tough,
and on account of its
strength, elasticity, and
lightness, it is the best wood
for spokes of buggy and
wagon wheels and for ax
handles. It is also the best
wood for fuel.

Birch Family (Betulaceae).
— To this family belong the

FIG. 417. — The flowers and fruit of
the Cherry Birch. At the left, above,
a flowering branchlet bearing two stam-
inate catkins at the left and one pis-
tillate catkin at the right ( X \] ; at the
right, above, a pistillate flower and just
below a staminate flower; at the left,
below, a pistillate catkin in fruit and at
the right, below, a single fruit. After
Burns and Otis.

Birches, Hazelnuts, Iron-
woods, and Alders. They are trees or shrubs and, except in
rare cases, are monoecious with the staminate flowers borne in
catkins, and the pistillate flowers borne in clusters, in spikes,
or scaly catkins (Fig. 417)- The fruit is a one-seeded nut,
which in the Hazel is of some value for food. The Birches, of
which there are many species, are the most important genera
in this family. They are much used for shade and ornamental
trees, and the wood is used for furniture, barrel hoops, shoe pegs,

476*

ANGIOSPERMS

FIG. 418. — The flowers and
fruit of the Red Oak. Above,
a flowering branchlet bearing a
cluster of staminate catkins be-
low and solitary pistillate flowers
above ( X ^) ; at the right, above,
a pistillate flower, and just be-
low, a staminate flower; at the
bottom, a mature fruit, showing
the matured ovary and the cu-
pule (natural size) . After Burns
and Otis.

From the Cork Oak the cork

spools, and paper pulp. The bark
of the Paper Birch was employed
by the Indians for canoes, baskets,
cups, and for sheathing wigwams.

Beech and Oak Family (Faga-
ceae). — This family includes the
Beeches, Chestnuts, and Oaks.

The plants of this family are
monoecious trees or shrubs with
staminate flowers in catkins or
clusters, and pistillate flowers soli-
tary or slightly clustered (Fig. 418).
The fruit is a one-seeded nut
partly or entirely enclosed by a
covering called cupule, which is
formed by bracts that develop at
the base of the ovary and grow
up over it.

The nuts of the Chestnut are
common on the market and are of
considerable value for food. Beech
nuts contain much oil and are a
good feed for hogs. From the
Oaks, of which there are a large
number of species, a large propor-
tion of our hardwood is obtained.
The beautiful figures which Oak
lumber can be made to show make
it a valuable wood for furniture,
inside finishing of buildings, and for
cabinet work. Beech wood is very
hard and is used considerably for
hardwood floors and in the manu-
facture of furniture. Chestnut wood
is soft but durable and is used for
fences and buildings. The bark of
Oak and Chestnut trees is rich in
tannin and at one time was the
source of tannin for tanning hides,
of commerce is obtained (Fig.

BEECH AND OAK FAMILY (FAGACEAE)

477

FIG. 419. — Stripping cork from the Cork Oak. After Lecomte.

FIG. 420. — The flowers and fruit of the Red Mulberry. Above, from left
to right, a spike of staminate flowers, a 'spike of pistillate flowers, and a pis-
tillate spike in fruit (natural size); at the bottom, a staminate and pistillate
flower much enlarged. After Burns and Otis.

478

ANGIOSPERMS

Elm Family (Urticaceae). — The Elm family includes about
1500 species of herbs, shrubs, and trees. Besides the Elms this
family includes the Mulberries, Figs, Hemps, Hops, Nettles,
tropical Bread Fruits and a number of others less important.

The apetalous flowers are mostly unisexual. The flowers
are usually borne in loose or catkin-like clusters (Fig. 420). In
the Fig the flowers are produced in hol-
low receptacles, which with the ovaries
within form the well-known fleshy fruits
of the Fig (Fig. 421). The fruits in this
family vary much in size, form, and
texture. In the Elms the fruits are
winged and depend upon the wind for
dissemination.

The Elms are very popular shade trees,
and their wood is used for flooring, hubs,
barrels, sills, posts, and railroad ties.
The multiple fleshy fruits of the Mul-
berries are edible, and the leaves of Mul-
berries constitute the food for silkworms.
The Hemps are well-known fiber plants,
and the Hop Vine is extensively grown
for its fruits, which are used in brswing beer and at one time
were used in making bread. The Rubber Plant, so common
in greenhouses and homes, belongs to this family and is one
of a number of plants that yield the invaluable rubber from
their milky juice.

Buckwheat Family (Polygonaceae). — The plants of this
family are mostly herbs, distinguished by their swollen nodes,
sheathing stipules, and simple flowers in clusters (Fig. 422).
The Smartweeds and Knotweeds, which are extremely common
around gardens and in waste places, are well-known plants of
this family. The fruit, in most cases, is an achene which is
usually angled and sometimes winged. In case of Buckwheat,
which is an important cereal crop, the starchy achene is ground
into flour. Some of them, as the Rhubarb and Sorrel, contain
acid in the leaves or stem. The family includes a number of
weeds of which the Docks, I^ield or Sheep Sorrel (Fig. 423),
Black Bindweed, Climbing False Buckwheat, and the Smart-
weeds are common ones.

FIG. 421. — Pistillate
flowers of the Fig, show-
ing the flowers borne in
a hollow receptacle.

BUCKWHEAT FAMILY (POLYGON ACE AE)

479

FIG. 422. — A Smartweed (Polygonum Muhlenbergii), one of the trouble-
some weeds, showing the sheathed nodes and terminal spikes of flowers ( X |),
and also showing a flower and a fruit much enlarged. This plant has both
underground and aerial stems.

FIG. 423. — Field or Sheep Sorrel (Rumex Acetosella), showing the underground
and aerial stems, the halberd-shaped leaves, and terminal spikes of flowers. X|.

480

ANGIOSPERMS

Goosefoot Family (Chenopodiaceae). — This family contains
many plants, chiefly herbs and most of which are weeds. The

FIG. 424. — Russian Thistle (Salsola Kali, var. tenuifolia). At the left,
an entire plant, showing the tap-root and character of the stem (XTV); at
the right, a portion of a plant, showing the leaves and flowers about natural
size. Modified from Oswald and from Beal.

flowers are small and usually greenish. The Spinach and Beets

are well-known pot herbs of this
family, and also from Beets most
of our sugar is now obtained.
Among the many that are classed
as weeds, the Russian Thistle
(Fig. 424) is the most noted one.
Belonging to the same order is the
Amaranth family, which contains
some ornamental plants and a
number of common weeds. Of those
that are ornamental, the Cocks-
comb, Prince's Feather, and Bache-
lor's Button, grown in gardens for
their highly colored flower clusters,
are common ones. The Pigweed,
and Tumble weed (Fig. 425), com-
mon in gardens, truck patches,
and waste places, are the most
troublesome weeds of this family.
Pink Family (Caryophyllaceae). — This family contains many

species, which are chiefly herbs of the temperate regions. The

FIG. 425. — The Tumble Weed

(Amaranthus graedzans), showing
the general character of the plant.
X*.

CROWFOOT OR BUTTERCUP FAMILY

481

plants are like those of the preceding families in character of
the ovary and seeds but differ from them in having a perianth
differentiated into a showy corolla
and a large calyx (Fig. 426). They
are regarded as a transition group
between the Apetalae and Poly-
petalae. Among them are some
garden favorites, such as the Carna-
tions, Pinks, Sweet Williams, and
Lychnis, and also some weeds of
which the Chickweeds, Corn Cockle,
Cow-herb, and Bouncing Bet are
common ones.

Polypetalae

FIG. 426. — A portion of a
plant of Corn" Cockle (Agro-
stemma Githago) (X^). The
flowers have a perianth consist-
ing of a calyx and showy corolla.
Modified from Beal.

As previously stated the Poly-
petalae have petals and the petals
are generally separate. The colored
corolla is usually distinct from the
green calyx, and the flowers are pol-
linated chiefly by insects. Among
the lower families of the Polypeta-
lae, as the Buttercups (Ranunculaceae) illustrate, the flower usu-
ally has numerous stamens and a number
of separate pistils. The calyx and corolla
are also attached below the stamens and
pistils or, in other words, the flowers are
hypogynous. In passing to the more ad-
vanced families of the Polypetalae, the
number of stamens and carpels become
more definite, and assume the cyclic ar-
rangement. There is also a tendency for
the carpels to join and a tendency of the
flower toward epigyny in the higher
families.

Crowfoot or Buttercup Family (Ranun-
culaceae). — This family includes numer-
ous species, mostly herbs, having in common separate petals,
and separate sepals. The stamens and commonly the carpels

FIG. 427. — A flower
of a Buttercup, showing
the many stamens and
pistils. X 2.

482

ANGIOSPERMS

are numerous, but indefinite in number and separate (Fig. 427).
A few of the well-known plants of this family are the Anemone,
Clematis, Larkspur, Columbine, Hepatica, Marsh Marigold, and
Peony. The Wolfsbane or Aconite, which contains the virulent
poison aconite, and the Golden Seal, which yields the drug hydras-
tis, are medicinal plants of considerable importance. Belonging

FIG. 428. — American-grown Camphor trees. From Yearbook, U. S.

Dept. Agr.

to other families grouped in the same order with the Buttercups,
are the Magnolias, trees and shrubs noted for their large flowers
and including the Tulip tree, a noted timber tree. Also the
Barberries, the tropical Nutmeg tree, and the Laurels belong
to the same order. The Laurels include such plants as the Sas-
safras, Cinnamon and Camphor tree (Fig. 428).

Mustard Family (Cruciferae) . — The flowers of this family
generally have four sepals, four petals, and six stamens. The

ROSE FAMILY

483

pistil forms a pod known as the silique. The four petals, when
opened out, suggest the Greek cross, — whence the name Cru-
ciferae (Fig. 429).

To this family belong such useful plants as the Cabbage,
Turnip, Kohlrabi, Brussels Sprouts, and Rape.

A number of plants of this family, such as Peppergrass,
Shepherd's Purse, White and Black Mustard, Tumbling Mus-
tard, Indian Mustard, and Charlock are weeds. Their seeds

FIG. 429. — The character of the
plant, flowers, and fruit of the Black
Mustard (Brassica nigrd). At the
right, a plant in flower (X^j), and a
mature pod about natural size ; at the
left, above, a flower, and below, an
open pod. After Vasey and Nature.

FIG. 430. —
One of the Pop-
pies, showing
the character of
the flowers and
pod. After Le-
comte.

are troublesome impurities in commercial seeds, and the seeds
of some are poisonous.

Associated with the Mustard family is the Poppy family
(Papaveraceae), characterized by a milky juice and represented
by Bloodroot, common in the woods, and by the California
Poppy from the juice of which opium is obtained (Fig. 430).

Rose Family (Rosaceae). — To this family belong about 2000
species of herbs, shrubs, and trees. In most plants of this
family, there is an indefinite number of stamens and one to
many separate carpels. The flowers of Strawberries and Black-
berries, for example, have many stamens and many separate

484 ANGIOSPERMS

pistils, and the number of each is indefinite, while in the Apple
there are generally five united carpels, and in the Peach, Plum,
Cherry, and Almond the number of carpels has settled down to
one. There is also a noticeable tendency toward epigyny, for
perigyny, which is common in the family, is a step toward
epigyny (Fig. l$i}. The Rose family is the family of fruits.
It includes Apples, Pears, Peaches, Plums, Apricots, Cherries,

FIG. 431. — Some flowers of the Rose family. At the left, a Strawberry
flower, which has many stamens and pistils and is hypogynous; next, a flower
of an Agrimony, and, at the extreme right, a Pear flower, both of which are
perigynous and have few pistils with ovaries joined.

Quinces, Strawberries, Blackberries, Raspberries, and some
others. No one can estimate what this family contributes to
the welfare of mankind.

Some, like the Roses, Spireas, and Hawthornes, are impor-
tant ornamental plants. Some of them, as the Cinquefoils
or Five-fingers and the Agrimonies, are weeds. The Five-
fingers grow in fields and crowd out other plants, while the
Agrimonies grow in pastures, and their spiny fruits get in the
wool and hair of live stock.

Closely related to the Rose family is the Saxifrage family
(Saxifragaceae) , the family to which the Gooseberry, Currant,
Syringa, and Hydrangea belong.

Pea Family (Leguminosae) . — The Pea family, which includes
about 7000 species of herbs, shrubs, and trees, is the largest
group of the Archichlamydeae. The flowers are hypogynous
or somewhat perigynous, and the parts of the calyx and corolla
are generally in fives. The stamens are usually 10, and 9 or all
of them are joined. The petals are often irregular, as those of
the Beans and Peas illustrate, and also show a tendency to

PEA FAMILY

485

unite (Fig. 432). In the uniting of some of the petals, the plants
of the Pea family suggest those of the Sympetalae which is
considered the most advanced group of Dicotyledons. Also
irregularity in the shape or size of sepals or petals is considered
an advanced feature. The pistil consists of one carpel and be-
comes the one-celled fruit called legume, which is characteristic
of the family.

The Beans, Peas, Peanuts, Soy Beans, Cow-peas, Clovers,
Alfalfas, and Vetches make this
family a noted one. The value
of Beans, Peanuts, and Peas as
food for man, and of the others
mentioned for forage and the
improvement of the soil are well
known to the student. The Pea-
nut is peculiar in that it forces
its pods underground to ripen.
Although Peanuts are not so
important for food as Beans and
Peas, several millions of bushels
of them are grown in the United
States per year. From some
of the leguminous plants medici-
nal substances, dyestuffs, gum
arabic, licorice, logwood, copal
varnish, and other useful sub-
stances are obtained. Some of
the leguminous trees, as the
Black Locust, Honey Locust,
and a number of trees in the
tropics and sub-tropics, furnish
fine cabinet woods.

Thirty or more leguminous plants are classed as weeds.
Some, like the Loco- weeds and some of the Lupines, are poison-
ous to live stock and cause considerable trouble in pastures in
the Western states. Some, like the Rabbit-foot Clover, are very-
hairy and when eaten by stock, the hairs often collect in balls
and clog the intestines. In the Tick Trefoils, of which there are
many species, the fruits are commonly spiny and are trouble-
some to wool-growers.

FIG. 432. — Flowers and fruit of
the Common Locust (Robinia
Pseudo-Acacia}. At the right, a
raceme of flowers, showing the
irregular corollas ( X |) ; at the left,
above, a flower with a portion of
corolla removed to show the diadel-
phous stamens; at the left, below,
a mature pod or legume (X|).
After Burns and Otis.

486

ANGIOSPERMS

Spurge Family (Euphorbiaceae) . -- The Spurge family con-
tains many species, many of which are tropical. The flowers
are commonly small, hypogynous, and unisexual. The perianth
is usually simple and sometimes absent. The stamens range
from one to many, and the pistil is composed of three united
carpels (Fig. 4^3). The plants usually contain a milky juice,
which in many species is poisonous. A few of them are common
weeds, usually growing prostrate in gardens and truck patches.

FIG. 433. — Flowers and fruit of the Flowering
Spurge (Euphorbia corollatd). At the right, a por-
tion of a plant in flower; above, at the left, a
flower cluster consisting of one pistillate flower and
a number of staminate flowers enclosed by an in-
volucre (i) bearing appendages resembling petals;
at the right of the flower cluster, a single stami-
nate flower with anther at a; below, at the left,
a flower cluster with staminate flowers removed
to show the pistillate flower; below, at the right, a
pistillate flower in fruit, showing the ovary (c), the
stigma (s), and the involucre (i}. In part after
Bergen and Caldwell.

FIG. 434. —The Hevea
tree, one of the plants
from the milk- juice of
which India rubber is ob-
tained. After Lecomte.

The Castor Bean, from which castor oil is obtained, is one of the
large species of our region. Some are trees, as for example the
Hevea tree (Fig. 4$4) of South America from which India rubber
is obtained. Tapioca is obtained from the Cassava plant, a
plant of the Spurge family and native of Brazil. A number are
useful for medicine, and some, as the Castor Bean, Poinsetta,
and some others, are ornamental plants.

Between the Pea family and the Spurge family is usually
placed the Flax family (Linaceae) to which the cultivated Flax

MALLOW FAMILY

487

belongs, and the Rue family (Rutacea), the family of citrous
fruits, such as Oranges, Lemons, Tangerines, Grapefruit, and
others.

Maple Family (Aceraceae). — This family is composed chiefly
of the Maples, valuable trees for shade, lumber, and yielding a
sweet sap from which maple syrup and sugar are obtained.
Closely related to the Maples are the Buckeyes which are also
important shade trees.

Mallow Family (Malvaceae) . — This family is a notable one
chiefly because it includes the Cotton plant (Fig. 43$) > The

FIG. 435. — A Cotton Plant, showing the general character of the plant.
X about 3^. After Orton.

flowers have five sepals and five petals. The stamens are
numerous and united, and the pistil is composed of a number
of carpels united at the base. The sepals are also partly united
(Fig. 9).

Cotton surpasses all other plants of the family in value.
To this family also belongs the Theobroma Cacao, a small tree
which yields cocoa and chocolate. The Shrubby Althaea and
Hollyhock are of some importance as ornamental plants, while
the Indian Mallow (Albutilon Theophrasti) , Flower-of-an-hour
(Hibiscus Trionum), and a few others are more or less trouble-
some weeds.

488

ANGIOSPERMS

Parsley Family (Umbelliferae) . — The Parsley family com-
prises about 1300 species. The
small epigynous flowers are
borne in umbels, — whence the
name of the family (Fig. 436).
The stamens and parts of the
calyx and corolla are five. The
pistil consists of two partly
united carpels which separate
in the fruit. Carrots, Parsnips,
Celery, and Fennel are mem-
bers of this family.

This family also contains some
bad weeds. The poison Hemlock
(Conium maculatum) and Water
Hemlock (Cituta maculata) are
two very poisonous plants,
which often grow in pastures
where livestock eat them and
are killed. The Wild Carrot
(Fig. 437) is troublesome in pas-
tures, meadows, and grain fields where it crowds out other plants.

FIG. 436. — Flowers and fruit of
the Wild Carrot (Daucus Carota). At
the left, a portion of a plant bearing
umbels of flowers and fruit; at the
right, flowers and a fruit much en-
larged to show their structure.

FIG. 437. — A meadow taken by the Wild Carrot.

HEATH FAMILY

489

Sympetalae

Among the fifty or more families of the Sympetalae, there are
some families of considerable economic importance. As previ-
ously stated, the Sympetalae are characterized by a gamopet-
alous corolla. Also the ova-
ries are commonly inferior.
Their flowers are commonly
showy and insect pollinated.

Heath Family (Ericaceae).

- The plants of the Heath

family are mostly shrubs,

and they are distributed

from the polar regions to the

FIG. 438. — One of the
Bindweeds (Convolvulvus
sepium), showing the corolla
composed of united petals.

FIG. 439. — Alfalfa
Dodder twining about an
Alfalfa plant and drawing
nourishment from it by
means of parasitic roots
(X£). Below, at the
right, also a fruit, called
capsule, of the Dodder is
shown much enlarged.

tropical forests. The flowers are usually regular, and both calyx
and corolla are 4-5 lobed. The stamens are as many or twice
as many as the lobes of the calyx or corolla, and the flowers are
hypogynous or perigynous.

Some, as the Cranberries, Blueberries, and Huckleberries, pro-
duce berries that are valuable fruits. The Heath family also
includes some highly prized ornamental shrubs, such as the Rho-

490

ANGIOSPERMS

FIG. 440. — A portion of a Tomato
plant bearing flowers and fruits, and also
a flower enlarged to show the structure
of the flower.

dodendrons and Heathers.
The Trailing Arbutus
(Epigaea), which is the
favorite spring flower
wherever it grows, and the
Madrona, one of the most
beautiful trees of the
Pacific coast, belong to this
family.

Sweet Potato Family
(Convolvulaceae). — The
plants of this family are
chiefly trailing or twining
herbs. Their flowers, as

those of the Morning Glory

. J
illustrate, are otten quite

showy. They have five
stamens, and their calyx
and corolla are composed
of five parts (Fig. 438).
There is usually one pistil
with two or three locules
in the ovary.

The Sweet Potato is of
considerable value for food
and is quite extensively
grown in a number of
states. A number of
plants of this family are
weeds, of which the Morn-
ing Glory (Ipomoea), Bind-
weeds, and Dodders (Fig.
439) are the chief ones.
The Morning Glory and
Bindweeds twine around

cultivated plants, cutting
Qff ^ ^ and often

breaking them down. The
Bindweeds are extremely hard to eradicate because of their
spreading roots and rootstocks which propagate the plants

FIG. 441. — A portion of a Jimson
Weed bearing flowers and fruit. Both
sepals and petals are joined most their

NIGHTSHADE FAMILY

491

very rapidly. The Dodders are parasitic plants and do much
damage in Clover, Alfalfa, and Flax fields, where they twine
about the plants and grow their roots into their stems and
rob them of their food.

Nightshade Family (Solonaceae) . — This family is the one to
which the Irish Potato, Tomato, and Tobacco belong. Some
authors give the number of species as about 1700. Both the
five sepals and five petals
are more or less joined
(Fig. 440). The stamens
are five and usually inserted
on the corolla. The Irish
Potato (Solanum tuberosum)
is probably the most im-
portant plant of this group
and Tobacco (Nicotiana
Tabacum) next. Some years
the potato crop in the
United States is more than
300,000,000 bushels. New
York is the chief potato
growing state, although
many potatoes are grown in
Michigan, Wisconsin, and
Pennsylvania.

The Tomato (Ly coper si-
cum esculentum) , when first
introduced from tropical
America as an ornamental FlG- 442. -A portion of the Horse

plant, was considered poison- *ettle! sl™ fowers and fruits and

the spiny character of the plant (Xf).
ous, but now its fruits are After Dewey.
important vegetables.

In some of the Southern states, as Kentucky, North Caro-
lina, and Virginia, Tobacco is one of the leading agricultural
products, while in many other states it is grown in considerable
quantities. Some other cultivated plants of this family are the
Egg Plant, Cayenne Pepper, Petunia, and Belladonna.

To this family belong a number of weeds, some of which are
quite troublesome. The Black Nightshade (Solanum nigrum)
and Jimson Weed (Datura Stramonium) (Fig. 441) are common

492

ANGIOSPERMS

weeds that are poisonous. The Horse Nettle (Solarium caro-
linense) (Fig. 442} is troublesome on account of its spiny stems,
and it has a deep rootstock, which is difficult to eradicate.
Another troublesome weed of this family is the Buffalo Bur

(Solanum rostratum) (Fig.
443), which has spiny fruits
that catch into the wool and
hair of livestock.

The Madder Family
(Rubiaceae) . — This is one
of the largest families of the
Dicotyledons. There are
more than 4000 species be-
longing to this family, but
the majority of them are
tropical. They include
herbs, shrubs, and trees.
Their flowers are epigynous,

X^w/X ! and the stamens and lobes

TT of the calyx and corolla

are the same in number
(usually 4-5).

The Coffee tree (Fig. 444),
which is grown extensively
in Brazil, Arabia, and Java,
is the most important plant
of the family. The fruit
(Fig. 445} of the Coffee tree
is a cherry-like drupe containing two seeds, and these seeds are
the coffee of commerce. The Cinchona tree, growing wild in
the Andes and cultivated in India, furnishes Cinchona bark
from which quinine is made.

Gourd Family (Gucurbitaceae). — This family includes the
Gourds, Pumpkins, Squashes, Melons, and Cucumbers. The
flowers are epigynous, and the plants are monoecious or dioecious
(Fig. 6). The stamens are usually more or less united. In
our region there are only a few species of this family and none
of much importance except those mentioned above.

Composite Family (Compositae) . — This is an immense fam-
ily and is of world- wide distribution. It is the highest group

FIG. 443. — A plant of the Buffalo
Bur bearing flowers and fruits, showing
the character of the plant ( X TV ) ; and a
single flower, showing the prickly calyx
and gamopetalous corolla. After Dewey.

COMPOSITE FAMILY

493

of Dicotyledons. The most conspicuous character of the
family is the grouping of the flowers into a compact head, which
is surrounded by bracts
forming the structure
called involucre (Fig.
446). The flowers are
epigynous, the corolla
is usually tubular or
strap-shaped, and the
five stamens are in-
serted on the corolla
and usually have their
anthers united in a tube
around the style. The
calyx is often a tuft of
hairs (pappus) . They
have developed very
effective means of dis-
seminating their seeds.
In manjr, as the Dande-

lion and Thistles illus- FlQ 444. - A Coffee tree in fruit,

trate, the pappus forms After Lecomte.

a parachute-like ar-
rangement, which enables the fruit to be easily transported by

the wind. In others, as the
Burdock, Cocklebur, and
Spanish Needles illustrate,
the fruits have hooks or
spines, which catch onto pass-
ing animals.

Although the family is a
large one, it contains only a
FIG. 445. — Flower, fruit, and seeds few food plants, of which Let-
of the Coffee. At the left, a flower, tuce, Chicory, Oyster plant,
and at the right, a fruit with the upper the Globe Artichoke, and
portion of the ovary removed to show T , A , . , , ,,

the two seeds. After Karsten. Jerusalem Artichoke are the

chief ones. Some, as Arnica,

Boneset, Camomile, Dandelion, Tansy, and Wormwood, are
used some for Medicine, and from the seeds of the Sunflower oil
is extracted.

494

ANGIOSPERMS

The family includes a number of ornamental plants, of which
the Cornflower (also called Bachelor's Button), Marguerite, China
Aster, Chrysanthemum, Cosmos, Dahlia, and English Daisy are
familiar ones.

In number of weeds which it includes this family surpasses all
others. About one hundred plants of this family are classed

FIG. 446. — The Marguer-
ite, one of the Composites,
showing the flowers grouped
into a compact head and sur-
rounded by an involucre. In
this composite a head con-
tains two kinds of flowers —
ligulate flowers, one of which
is shown at the left, and tu-
bular flowers, one of which is
shown at the right of the
heads. After Lecomte.

FIG. 447. — Canada
Thistle, showing a horizontal
root and an aerial stem in
flower (X|), and also show-
ing single fruits or achenes,
one of which is shown with-
out pappus and slightly en-
larged.

as weeds, although not many of them are bad weeds. The
Canada Thistle (Fig. 44?) is probably the worst weed of the
family. It spreads rapidly by spreading roots or rootstocks
and soon takes possession of pastures and meadows and gives
considerable trouble in cultivated ground. On account of the
spreading underground structures which propagate readily when
cut into pieces, the plant is exceedingly hard to eradicate. Some
of the other Thistles are also quite troublesome in some regions.

GRASS FAMILY

495

Some other well-known weeds of the family are the Cockle-
burs, Ragweeds, Ironweeds, Spanish Needles, Wild Lettuce,
and Beggar-ticks.

Monocotyledons

Among Monocotyledons about 25,000 species are recognized,
which are distributed among 42 families. They are less than
one-fourth as numerous as the Dicotyledons. As previously
stated, Monocotyledons differ from
Dicotyledons in having flowers with
parts usually in threes or sixes, leaves
with parallel veins except in rare cases,
and vascular bundles with the scattered
arrangement. The Monocotyledons
contain a few families of economic im-
portance and one family that surpasses
all other groups of Angiosperms in
number of valuable food plants.

Cat-tail Family ( Typhaceae) . — This
family is mentioned because it includes
the. simplest of the Monocotyledons.
They are aquatic plants, growing in
groups in swamps and wet places. Some
get as high as one's head, and in the
late summer and fall, when their in-
florescences resembling a cat's tail are
well formed, they are conspicuous plants
(Fig. 44$) • The flowers are monoecious
and have neither calyx nor corolla (Fig.
449)\ The pistil is composed of one
carpel containing one locule and only
one ovule. The st animate flowers are

borne at the top and the pistillate flowers b'elow on the spike.
The pistil is supported by a stalk or stipe which develops hairs
that become* the brown down of the fruit. The stamens are at-
tached directly to the axis of the spike and are intermixed with
hairs. As to whether the simple flowers of the Cat-tails are primi-
tive or are reduced forms of more complex flowers is not known.

Grass Family (Gramineae) . — The Grasses constitute one of
the largest families of Angiosperms and are widely distributed

FIG. 448. — The com-
mon Cat-tail (Thypha
latifolia), showing the
terminal spikes of flowers
consisting of staminate
flowers above and pistil-
late flowers below

496

ANGIOSPERMS

over the earth. The many valuable plants, such as Corn,
Wheat, Oats, Barley, Rye, Rice, Millet, Sugar Cane, Sorghum,

Blue-Grass, Timothy, and
others that are included, make
the Grass family the most im-
portant family of Angiosperms.
Except the Bamboos, which
are shrubs or trees, the Grasses
are herbaceous. Some have
unisexual (Fig. 14, 16), while
others have bisexual flowers
(Fig. 18), and the flowers are
FIG. 449. — The flowers of the com- commonly arranged in spikes
mon Cat-tail. At the left, a staminate Or panicles. Their chief char-
flower: at the right, a pistillate flower. ... ,, ,, .

actenstics are that their essen-
tial reproductive organs are enclosed in bracts and that they
have a nut-like fruit called a grain or cariopsis.

FIG. 450. — Sugar Cane. After Lecomte.

Besides the grains upon which mankind depends so much for
food and the Sugar Cane (Fig. 450), which is grown extensively

PALM FAMILY

497

in the Southern states, West Indies, Hawaii, and Java for sugar
and molasses, there are the meadow and pasture Grasses which
are highly important to man. The Grasses most valuable for
hay are Timothy and Redtop, while the Kentucky Blue Grass,
Bermuda Grass, Brome Grass, Meadow Fescue, and Rye Grass
are some of the Grasses useful for pasture.

A large number of the Grasses are weeds, of which the Sand
Bur, Foxtails, Chess, Wild Oats, Quack Grass, and Darnel are
some of the worst ones. In the Southern states Johnson Grass,
although useful for hay and pasture, is regarded as a weed

FIG. 451. — A group of Date Palms.

because it spreads so rapidly by rootstocks to regions where it
is not wanted and is so hard to eradicate. Quack Grass, which
spreads by rootstocks, is a bad weed in some of the Northern
states. The Wild Oats is troublesome in grain fields, and grain
containing the seeds of Wild Oats in considerable quantities is
usually docked. The seeds of Darnel are poisonous and, when
ground with Wheat, make the flour unwholesome, for which
reason Darnel is a bad weed.

Palm Family (Palmaceae) . — This is about the only family of
Monocotyledons that contains trees. Nearly all of the family

498

ANGIOSPERMS

are tropical or subtropical, but they are quite extensively grown
in greenhouses everywhere. The flowers have three sepals, three

petals, three to six stamens,
and a pistil commonly of three
united carpels. The flowers
are borne on a spadix and en-
closed in a spathe. The fruit
is sometimes a berry, as in the
Date, or nut-like, as in the
Coconut. A number of the
palms are valuable plants in
the region where they grow.
The Date Palm (Fig. 451)

yields the dates of the market.

452. — A portion of a Lily in mu /^ i. T» i • ^^ ^

flower and also a single flower, show- The CoCOnufc Palm yields the
ing the perianth consisting of six Coconuts of the market and is
similar parts. After Lecomte. probably one of the most use-

ful Palms to the natives, fur-
nishing food, clothing, utensils of all kinds, building materials,
etc. The Sago Palms yield Sago, which is prepared by washing
out the starch from the stems. A
tree 15 years old will sometimes yield
800 Ibs. of starch. The Oil Palm of
West Africa yields a fruit from which
palm oil is obtained.

Lily Family (Liliaceae) . — The Lilies
have a perianth of six parts and six
stamens (Fig. 452). The pistil usually
consists of three united carpels. Their
flowers are often showy, and many of
them, as the true Lilies, Hyacinths,
Star of Bethlehem, Tulip, Day Lily,
and Lily of the Valley, are ornamental
plants. The Onion and Asparagus
are common articles of food. Some,
as the Aloe, Smilax, Colchicum, and
Veratrum, yield valuable medicines.
One, called New Zealand Flax, is a valuable fiber plant. In
another family closely related to the Lily family are the
Agaves, of which the Century Plant (Fig. 453) is a familiar

r- -

FIG. 453. — A Century
Plant, one of the Agaves,
showing the thick leaves and
shape of the plant (XTV).

ORCHID FAMILY

499

one in our region. The Agaves are very important plants in
Mexico where the natives obtain from them pulque, a fermented
drink, mescal, a distilled drink resembling rum, and various
fibers as sisal hemp and henequen.

In connection with families noted for fibers and not pre-
viously referred to, there is the Linden family of the Dicotyle-
dons from which Jute is obtained and the
Banana-like plant of the Banana family
from the leaf stalks of which Manila hemp
is obtained.

Orchid Family (Orchidaceae) . — This
family includes the most highly developed
Monocotyledons, and if the Monocoty-
ledons are higher than the Dicotyledons,
then the plants of this family are the most
highly developed of the plant kingdom,
In the Orchids the flowers are highly
specialized, often very showy, and present
interesting mechanisms to secure cross-
pollination. The family includes numer-
ous species and nearly one-fourth of the
Monocotyledons. The most highly spe-
cialized ones are tropical and occur only
in greenhouses in the temperate regions.

The flowers are epigynous and show extreme irregularity
(Fig. 454}' One of the petals called the lip varies much in shape
and is usually very different from the other petals. The one
or two stamens join with the style of the pistil to form the column.
In most cases the pollen sticks together, forming masses called
pollinia, and it is in these masses that the pollen is carried from
one flower to another by insects.

FIG. 454. — A flower of
an Orchid (Cypripedium),
showing the irregularity
among the parts of the
perianth, and an insect
entering the pouch-like
structure of the corolla.
After Gibson.

CHAPTER XXI

ECOLOGICAL CLASSIFICATION OF PLANTS
Nature of Ecology

It is common observation that certain kinds of plants live
only in certain places. Thus regions distinct in type, such as
ponds, bogs, shady ravines, dry hillsides, etc., have distinct
types of vegetation. The plants in ponds and bogs are ad-
justed to much water, in shady ravines to shade and moist
air, and on dry exposed hillsides the plants are adjusted to hot
sunshine and dry soil. Certain kinds of plants are therefore
adjusted to a certain environment which is known as their
habitat. In order to thrive, a plant must be able to compete
with other plants and endure the hardships which the environ-
ment imposes upon it. It must be adjusted to the range of
temperature, amount of light and moisture, conditions of the
soil, surrounding plants and animals, etc. Plant Ecology is
the science which treats of the adjustments and distribution
of plants in relation to the various environmental factors.
Throughout the preceding chapters Ecology has been touched
upon repeatedly, for the adjustment of leaves and stems to
light, the storage of food in tubers and seeds for the next gen-
eration, the adjustments of flowers to various kinds of pollina-
tion, the parasitic and saprophytic habits, the adjustments for
living in the water or air, etc., really belong to Ecology. In
the classification of plants phylogenetically, which is emphasized
in the previous chapters of Part II, the basis of classification is
kinship, but in classifying plants ecologically the basis is ad-
justment to environment, and plants varying widely in their
phylogenetic relationships occur together in the same ecological
class. For example, Thallophytes, Bryophytes, Pteridophytes,
and Spermatophytes occur together in some ecological classes
of water plants.

Many of the problems of Agriculture have to do with the
securing of strains or varieties of crop plants better adjusted

500

WARMTH 501

to a particular environment. We are constantly striving to
find the Apples, Pears, and other fruits best adjusted to the
environmental factors of different regions. One of the objects
in the breeding of Citrous fruits has been to procure varieties
less sensitive to cold, so that Citrous fruits can be grown farther
north and consequently over a larger area. Much time and
energy has been spent in obtaining strains of Cotton resistant
to the insect pests and other unfavorable environmental factors
of the Southern states. In the Northern states, where the
growing season is short, one of the problems in connection with
the raising of Corn is to secure varieties that can mature before
frost. . The securing of drought resistant plants for dry regions,
of plants resistant to the diseases prevalent in the different
agricultural regions, of pasture Grasses best adapted to a given
region, of trees adapted to grow in a given region for shade or on
a given area that is to be reforested are some of the many other
agricultural problems that have to do with adjustment of plants
to their environment and hence are ecological.

Ecological Factors

The various environmental features to which plants and ani-
mals must adjust themselves are called ecological factors. The
chief ecological factors are water, heat, light, soil, wind, and
associated plants or animals.

Water. — This is one of the most important ecological factors.
The amount of water to which various plants are adj usted varies
from complete submergence to perpetual drought. Most Algae
live completely submerged in water, while Cacti are adjusted
to the drought of deserts. Most crop plants require a medium
amount of water in the soil, and an excess or lack of water re-
tards their growth. But among crop plants there is also much
variation in the amount of water necessary for living. For
example, the Sorghums are more resistant to drought than
Corn, while some varieties of Rice require flooding.

Warmth. — All kinds of plants are adjusted to certain ranges
of temperature. For example, Wheat and Oats require less
warmth than Corn, and hence can be grown farther north.
There are great zones of plants corresponding to the great
zones of temperature. Thus the arctic, temperate, and tropi-

502 ECOLOGICAL CLASSIFICATION OF PLANTS

cal zones are distinguished by their kinds of plants as well as
by their difference in temperature. When the temperature is
extremely low, as in the polar ice regions, or extremely hot, as
in some deserts, very few or no plants at all are able to live.
Even on the same area, as in a woods or a field, if the plants are
not disturbed, one can observe the effect of the heat factor in
the succession of plants through the growing season, the spring
plants being very different from the summer and autumn
plants. To secure crop plants adapted to the temperatures of
the different agricultural regions is also one of the problems of
Agriculture.

Light. — Not all plants in an association can receive the same
amount of light, and some plants are so adjusted that they can live
in the shade. They are known as shade plants, and the Ferns,
common in the woods, are examples of such plants. But even
in an association of herbaceous plants, as in a field of weeds,
many small plants grow among and in the shade of the taller
ones. Some plants, like the Pumpkins and Melons which grow
well along with Corn, have a very large leaf surface which may
compensate for the lack of light. Plants climb other plants or
walls, grow tall erect stems, and adjust themselves to neigh-
boring plants in various other ways in order to obtain sufficient
light.

Soil. — The soil in regard to its chemical and physical prop-
erties determines largely the kinds of plants that can grow in a
given region. Thus the plants on a sandy beach or sand dune
differ from those on a clay or loam soil. The chemical elements
of a soil and its power to retain water both have a determining
effect upon the growth of plants. Some plants, like Alfalfa and
some of the Clovers, are more sensitive than the grains to acids
in the soil. Some weeds, like the Sheep Sorrel, grow best in an
acid soil. Some plants require more potash, nitrates, or some
other element than other plants. Even water plants are some-
what dependent upon the soil, for the minerals in a pond or
lake are carried in from the soil. One of the chief problems of
Agriculture consists in putting the soil in a suitable condition
for plants and in choosing plants adapted to the different types
of soil.

Wind. — The wind tends to dry out plants by increasing their
transpiration, while at the same time it is an important agent

ASSOCIATED PLANTS AND ANIMALS 503

in pollination and dissemination of fruits and seeds. In
regions where there are strong prevailing winds only such plants
as are adapted to regulate transpiration can grow. Most of our
early flowering plants, as the Pines, Oaks, Beeches, and Poplars,
are pollinated by the wind, and some of our crop plants, as Corn
illustrates, depend largely upon the wind for pollination. For
the wide dissemination of the fruits and seeds of many of the
common weeds and of some cultivated plants, and also for the
spreading of some fungous diseases the wind is responsible.

Associated Plants and Animals. — A plant must compete
with surrounding plants and often with animals for existence.
It is common observation that most crop plants will not do well
under the shade of trees. The trees cut off the light and make
the soil too dry for the crop plants. On the other hand, there
are plants which require shade and hence grow best in the woods.
In some cases plants are benefited while in other cases they are
injured through the association of their roots with the roots of
other kinds of plants. For example, when Corn and Clover
are grown together, experiments indicate that Corn does better
than when it is grown alone. One experimenter grew Oats,
Barley, Buckwheat, Wheat, and Flax in pots with and without
the underground shoots of Canada Thistle and found all except
Buckwheat to grow better with the Canada Thistle than alone.
He repeated the experiment, using a young Elm tree instead
of the Canada Thistle, and found that all grew more poorly with
the Elm tree than alone. In Jutland it is found that Spruce
trees grow well on waste areas if their roots can associate with
those of the Mountain Pine. If there are no Mountain Pines
present, the Spruces will not grow. If the Pines are present
but are cut before the Spruces get well started, the Spruces die
or make a poor growth. No doubt much injury to crops caused
by weeds is due to the antagonistic effects of their root systems.
The association of certain kinds of nitrogen-fixing Bacteria
with the roots of legumes and of parasitic plants with their
hosts are familiar examples of a very intimate relation of the
life processes of one plant with those of another. In competing
for light, as previously pointed out, plants must adjust themselves
to each other in various ways. Climbing plants, in securing a
better position in reference to light for themselves, frequently
injure the plant which they climb. For example, Morning

504 ECOLOGICAL CLASSIFICATION OF PLANTS

Glories and Bindweeds cut off the light and break plants down,
and Grape vines often injure the trees over which they spread.

In a number of ways plants are adjusted to animals. The
presence of thorns, stinging hairs, and bitter juices may pro-
tect plants against destruction by animals. Insect pollina-
tion is a notable example of the dependence of plants upon ani-
mals. The flowers of some plants are so adjusted that they
require insects and often certain species of insects to pollinate
them. Thus bees are required to cross-pollinate Red Clover,
and Sweet Clover and Alfalfa, although they do not require
cross-pollination, require insects which can trip their flowers,
so that the pollen can get on the stigma. It is also recognized
that bees are essential to good pollination in orchards. Orchids
and Yuccas are two of the most notable examples of plants
which have flowers so constructed that only certain types of
insects can pollinate them. In such cases it is obvious that
propagation by seeds depends upon the presence of the insects
which are required to pollinate the flowers. For securing the
dissemination of their seeds, plants are adjusted to animals in
a number of ways, but chiefly by developing hooked or spiny
fruits or seeds which cling to the coats of animals.

The above factors with minor ones largely determine the
modifications and distribution of plants. These factors work
together and not singly, and the combinations of factors are
numerous. According to their adjustment to the ecological
factors, plants fall into groups or classes known as societies.
Thus all plants adjusted to a water habitat belong to a
hydrophytic society and are called Hydrophytes, while those
adjusted to a drought habitat belong to a xerophytic society
and are known as Xerophytes.

Ecological Societies

Since the ecological factors and their combinations vary
widely, there are many different habitats and hence many eco-
logical societies. With reference to the water factor plants are
grouped into Hydrophytic, Mesophytic, and Xerophytic soci-
eties.

Hydrophytic Societies. — These are the societies of water
plants called Hydrophytes and include plants which live sub-

HYDROPHYTIC SOCIETIES 505

merged, standing in the water, or floating on the surface of the
water. As previously stated, they include plants from various
phylogenetic groups. Many are Thallophytes, as the Algae
illustrate, while some, like the Pond Lilies, Duckweeds, Pond-
weeds, Eelgrass, and others, are Angiosperms. Not only repre-
sentatives of the lowest and highest divisions' of the Plant
Kingdom, but also some Bryophytes and Pteridophytes are
included in these societies.

The hydrophytes are adapted in various ways to living in the
water. In the Algae the unicellular and filamentous bodies with
all cells thin-walled afford the maximum amount of surface for
absorbing gases and minerals from the water and for absorbing
the light that reaches them. The more massive Algae are com-
monly so anchored that they are aerated through wave action,
and many are provided with floats or air chambers whereby they
float near the surface where there are more gases and light than
at greater depths. The more complex Hydrophytes, such as
the Seed Plants, that live chiefly submerged in the water have a
thin-walled epidermis, so that all parts of the plant can absorb,
and water-conducting tissues are feebly developed. Since they
depend upon the buoyant power of the water for support, the
root system is commonly reduced or even wanting, and their
mechanical tissues are not so well developed as those of Seed
Plants that live on land. Usually such plants collapse when
taken out of the water. Some, like the Pond Lilies, raise their
leaves to the surface of the water where they receive good
light, while others, as the Pondweeds and Eelgrass illustrate, are
wholly submerged and are able to get along with the little light
that reaches them. The submerged forms even bear their flow-
ers under water. Among the Hydrophytic societies there are
the free-swimming, pondweed, and swamp societies.

The free- swimming societies are made up of such plants as
the Diatoms, Algae, Duckweeds, and other plants which float
in stagnant or slow-moving water.

In the pondweed societies the plants are anchored, but their
bodies are submerged or floating (Fig. 455). To this society
belong the Water Lilies, Pondweeds, Water Ferns, Marine Algae,
some fresh-water Algae, and some species of Mosses.

Swamp societies consist of water plants which have leaf-bear-
ing stems reaching above the surface of the water. Some typical

506

ECOLOGICAL CLASSIFICATION OF PLANTS

plants of swamp societies are the Sagittarias, Bulrushes, Cat-tails,
Rushes, Sedges, and Reedgrasses, which form fringes around
ponds and lakes (Figs. J+56 and Jfil). Some trees, such as Wil-
lows, Poplars, Birches, and Alders, are common in swamp societies.
In a swamp of the bog type, Sphagnum Moss, Orchids, and some
trees, such as the Tamarack, Pine, and Hemlock, are character-
istic plants.

Aside from Rice, which is a Hydrophyte during a part of its

FIG. 455. — A pond in which are growing Water Lilies, plants typical of a
Pond-weed society. After C. M. King.

development, the hydrophytic societies are not noted for plants
important economically.

Mesophytic Societies. — The mesophytic societies comprise
the common vegetation. They require a medium amount of
moisture and a fertile soil. To these societies belong our culti-
vated plants, weeds, and deciduous forests. The mesophytic
condition is the arable condition and is the normal or optimum
condition for plants. If a hydrophytic area is to be cultivated,
it must be drained and made mesophytic.

MESOPHYTIC SOCIETIES

507

FIG. 456. — A swamp society consisting chiefly of Sagittarias and Sedges.
After C. M. King.

FIG. 457. — A swamp society in which Cat-tails are dominant.
After C. M. King.

508 ECOLOGICAL CLASSIFICATION OF PLANTS

Mesophytes, in contrast to Hydrophytes, are exposed much
more to the drying effect of the air and consequently are better
protected against transpiration. They need better root systems
for absorption and anchorage and also have better developed
conductive and mechanical tissues. There are many types of
mesophytic societies.

Meadows and prairies are mesophytic societies in which trees
are absent, and the dominant plants are, therefore, grasses and
other herbaceous plants (Fig. 4&?). The most important of

FIG. 458. — A prairie, a mesophytic society in which trees are absent.

the woody mesophytic societies are the deciduous forests com-
posed of Maples, Beeches, Oaks, Tulips, Elms, Walnuts, and
other valuable trees (Fig. 459). In such forests grow also char-
acteristic societies of herbaceous plants. The thicket, composed
of small woody plants, such as Willows, Birches, Alders, Hazel
bushes, etc., is another woody mesophytic society. The most
remarkable of the mesophytic societies are the rainy tropical
forests, where, due to a heavy rainfall and great heat, vegeta-
tion reaches its climax, and gigantic jungles are developed, com-
posed of trees of various heights, shrubs of all sizes, tall and
low herbs, all bound together in a great tangle by vines and
covered by numerous epiphytes.

XEROPHYTIC SOCIETIES

509

FIG. 459. — A deciduous forest, a mesophytic society consisting of Bass-
wood, Birches, Elms, Maples, and Oaks, under which grow many herbaceous
plants. After C. M. King.

Xerophytic Societies. — These are the societies adapted to
drought. Among xerophytic plants there are various adapta-
tions to drought, such as sunken stomata, hairy epidermis, re-
duction of leaf surface, deep tap-roots, reservoirs within the
leaves or other parts of the plant for holding water, edgewise
position or rolling of leaves, bridging over the period of drought
in the form of seeds or subterranean structures, etc.

Among the xerophytic societies are the rock societies, composed
chiefly of Lichens (Fig. 460) and Mosses which grow on dry and
exposed rocks; desert and dry plain societies (Fig. 461) where such
plants as Cacti, Sage Brush, Agaves, and Yuccas dominate; xero-
phytic thickets, composed of a dense mass of bushes and repre-
sented by the chaparral of the Southwest; and the xerophytic
forests, in which Pines, Spruces, and Firs, adapted to mountain
slopes and gravel ridges, occur.

510

ECOLOGICAL CLASSIFICATION OF PLANTS

In Asia, Africa, and North America, there is much land that
is xerophytic. Much of the Southwestern part of the United
States is xerophytic. One of the important problems in Agri-
culture is to bring xerophytic
areas into cultivation. This
may be done by making these
areas mesophytic through irri-
gation or by securing crop
plants through selection or
breeding that are drought re-
sistant, that is, able to grow
under xerophytic conditions.

FIG. 460. — A xerophytic society,
consisting of Lichens growing on a
bare rock. After Bailey.

Plant Succession

One society of plants com-
monly prepares the way for
another. For example, the Lichens and Mosses, growing on bare
rocks, disintegrate the rocks and form soil in which other plants
can get a start (Fig. 460). Ponds and lakes are gradually, filled up

L-- '

FIG. 461 — A desert xerophytic society consisting chiefly of Sage Brush
and Yuccas. After R. G. Kirby.

through the growth of pond societies until they are transformed
into swamps, in which the Pond Lilies, Pondweeds, Eelgrass,
and other representatives of pond societies are replaced by
Rushes, Sedges, Sagittaraes, Cat-tails, Reeds, True Flags, and

PLANT SUCCESSION 511

other representatives of swamp societies. Through the growth
of the swamp societies, the swamp is finally so filled up that it
is transformed into a mesophytic area, and the plants of the
swamp societies are succeeded by Mesophytes (Figs. J$2 and
463). It is obvious that the hydrophytic societies have been
exceedingly important factors in transforming lakes, ponds, and
old river beds into tillable land, and the fertile soil of such

FIG. 462. — A succession of plant societies, showing transition from hydro-
phytic to mesophytic societies. The successive societies are as follows:
Pond Lily Society, Sedge Society at the margin of the pond and grading into
a Swamp Grass Society further back, a shrub society still further back, and
finally in the background a mesophytic forest society. From Coulter, photo,
by Lewis.

areas is largely due to the humus added through the decay of
the hydrophytic societies. On sand dunes, beaches, ground
cleared and allowed to grow up again, and most everywhere one
can observe plant succession. On sand dunes around the Great
Lakes, for example, Poplars are succeeded by Pines, which are
in turn succeeded by Oaks and other deciduous trees.

Studies of successions and societies give us very useful in-

512

ECOLOGICAL CLASSIFICATION OF PLANTS

formation as to what plants can be successfully grown on' a
given area. There are instances, as in case of some of the wild
lands of the West, where a study of the societies of wild plants
has suggested the kind of crop plants best adapted to the condi-
tions. It is quite probable that more extended studies in Ecol-
ogy in connection with soil analyses will reveal such a close

FIG. 463. — A lake which is being rapidly filled up by the accumulation
of vegetable matter. Swamp societies consisting of clumps of Rushes, Sedges,
and Sagittarias are most conspicuous about the water. Further back are
swamp Grasses grading into mesophytic Grasses, and finally on the ridge,
as shown by the corn field and trees, a typical mesophytic condition prevails.
After C. M. King.

association of plant societies and the chemical and physical
characteristics of soils that the chemical and physical differ-
ences of soils on different farms or in different parts of the same
farm may be quite accurately judged by observing the societies
of weeds and other .wild plants. In reforesting a given area it
is very essential to take into consideration the plant societies
adapted to the region. For example, it would be unwise to plant
Pines on bare sand dunes, or Maples where Black Oaks, which
grow in much drier situations than Maples, prevail.

CHAPTER XXII

EVOLUTION
Meaning and Theories of Evolution

Meaning of Evolution. — Throughout the preceding discus-
sion of plant groups evolution was assumed. It was assumed
that the more complex forms of plants have come from the
simpler ones by gradual changes which involved the modifi-
cation of structures and the introduction of new structures.
It was also assumed that some simpler plants are reduced forms
of more complex plants. Hence evolution, which is usually
forward, that is, leading to more advanced forms, may be
backward, leading to simpler forms. According to the theory
of evolution, the organisms which first inhabited the earth were
extremely simple, and the various forms of plants and animals
which we now have are their modified descendants.

The evidences of evolution in both plants and animals are
obtained by studying the structure, development, and behavior
of living forms, and the structure of ancient forms preserved as
fossils. Evolution takes place too slowly to be observed or
demonstrated, and hence our conclusions about it are only
inferences.

There are two kinds of evolution — organic and inorganic.
Organic evolution is confined to living things including animals
as well as plants. However, inorganic things are also constantly
changing, and hence evolution applies to all nature and not
merely to living things. The physical features of the earth are
constantly changing. Mountains are being worn down, while
there are other regions where the land is becoming more ele-
vated. Areas that are now land were seas or a part of the ocean
at one time. A study of fossil plants shows that climates have
changed, so that regions once tropical no\t have a temperate
or arctic climate. Rivers and valleys are constantly changing
and thus altering the landscape. Also the planets, like our own
earth, and even the stars, are changing externally and internally.

513

514 EVOLUTION

Again, the phenomenon of radioactivity teaches us that the
atoms composing one chemical substance are transformed into
those of another, and thus chemical substances are built up by
the process of evolution. There seems to be but one thing that
is constant and that is constant change.

Evolution and the Doctrine of Special Creation. — The theory
of evolution is directly opposed to the " Doctrine of Special
Creation." The " Doctrine of Special Creation " is based upon
a literal interpretation of the account of creation given in the
Bible and, therefore, assumes that all things were created at the
beginning of the world. According to the " Doctrine of Special
Creation " plants, animals, mountains, oceans, planets, and stars
were created in the beginning by the Creator and have remained
constant in all fundamental features until the present time.
This means that Angiosperms and all other groups of plants,
however simple or complex, did not come from simpler forms,
but were made in the beginning and, therefore, have been in
existence practically ever since the world began. It is thus
seen that the " Doctrine of Special Creation " is not at all in
harmony with the theory of evolution, for the latter theory as-
sumes that in the early history of the world there were only very
simple organisms, and that from them through changes involving
many millions of years the complex forms have been derived.

Although a few believed in evolution even as far back as the
Greek philosophers, the " Doctrine of Special Creation " pre-
vailed until comparatively a short time ago. During the last
150 years, the theory of evolution has gradually gained favor,
and, since Charles Darwin's time, it has gained such supremacy
that it is now a fundamental conception not only in botany
and zoology but also in such subjects as history, philosophy,
sociology, and theology.

Theories in Regard to Evolution. — The most difficult thing
about evolution is its explanation. One can trace connections
between the higher and lower forms, bub to explain just in what
way one form arose from another is not at all easy. Scientists
are all convinced of^the reality of evolution, but the forces or
factors which bring about evolution are still under discussion.

Earliest Theories. — Perhaps the oldest explanation of evo-
lution was that of Erasmus Darwin, Goethe of Germany, and a
few others of their time. According to the explanation of these

LAMARCK'S EXPLANATION 515

early scientists, plants and animals are changed by their en-
vironment, and these changes are then inherited by the offspring
and retained as long as the environment remains constant. For
example, according to this explanation, a plant, originally smooth
but having been induced to become hairy through exposure to
a drier climate, will impart the hairy feature to the offspring,
which will maintain the modification until the environment so
changes that hairiness is lost. In this way the origin of new char-
acters and new species was explained. This assumes of course
that a change in any part of the plant is recorded in the sperms
and eggs of the plant and, therefore, transmitted to the progeny.

Lamarck's Explanation. — Lamarck, a noted French natur-
alist whose views were published first in 1801 and in an en-
larged form in 1809, offered the explanation which he called
" Appetency," meaning desire. His explanation was based upon
the observation that the organs of men and other animals are
enlarged and strengthened by use and particularly by con-
scious use. For example, it is common observation that one's
muscles enlarge and become stronger with proper use, while,
on the other hand, with lack of use they decrease in size and
strength. Long continued disuse may even result in the loss
of an organ.

There are three ideas involved in Lamarck's explanation.
First, his explanation assumes that the environment of animals
and plants has been constantly changing, so that they have been
constantly subjected to changes in temperature, moisture, light,
nutrition, etc. Second, he believed that all living things have
come from preexisting forms as a result of changes which were
responses to changes in the environment that made a new mode
of life necessary. Thus the neck of the giraffe lengthened as
a result of the animal's effort to reach leaves on high branches.
Also the form of the body of reptiles, such as snakes, which
glide over the ground and conceal themselves in the Grass, is
due to the mode of life which these animals have adopted. By
repeated efforts of the animal to elongate in order to pass
through small spaces, the body became extremely elongated and
very narrow, and since long legs would raise the body too high
from the ground and short legs would not move them rapidly
enough, legs, vestiges of which are still in their plan of organiza-
tion, were finally lost and another means of transportation was

516 EVOLUTION

evolved. Although Lamarck's explanation, as based upon the
use and disuse of organs, applies particularly to -animals, he also
offered an explanation for evolution in plants. In case of plants,
in which there is no conscious effort as in animals, he assumed
that changes in the environment affect the body of a plant
directly and induce modifications which may become sufficiently
pronounced to characterize a new species. Since species are
the units of all other groupings of plants or animals, the origin
of new species results in the origin of new genera, new families,
new orders, and so on. It is, therefore, obvious that accounting
for the origin of species accounts for the origin of all those
differences upon which the various groupings of organisms are
based.

Third, Lamarck believed that whatever changes a plant or
animal made in the form, structure, or function of its body
were inherited by the offspring. To the succeeding generation
each generation transmits what it inherited and whatever addi-
tional modifications it may take on. In this way modifications
which are only slight at first may become more pronounced in
succeeding generations, if the conditions remain constant, until
finally plants or animals so different from their ancestors as to
form new species may arise. He did not claim that all individ-
uals taking on new modifications survive but only those pos-
sessing changes that fit them most perfectly to their environ-
ment.

Lamarck's explanation is unsatisfactory in a number of ways.
In the first place both observations and experiments furnish
much evidence that the effects of use and disuse are seldom, if
at all, inheritable and hence have no permanency such as the
characters of species have. If the effects of use and disuse are
not transmitted, the hypothesis that the effects of use and dis-
use may accumulate from generation to generation also lacks
support. Also, in case of plants, more recent investigations show
that modifications that are direct responses to the environment
are not generally, if at all, inheritable. In the second place his
explanation does not account for the desire of the animal to
change its habits, but simply assumes that animals change in
their desires, and that such changes are also transmitted.

Darwin's Explanation. — Although Charles Darwin (Fig. 464}
was neither the first to believe in evolution nor the first to

VARIATION

517

attempt to explain it, his arguments for evolution were much
more convincing than those of his predecessors, and his explana-
tion was so well based upon facts and so well organized that it
was widely accepted. His book, the Origin of Species, published
in 1859, in which he set forth his argument for evolution and its
explanation, was based
upon 20 years of careful
observat:on, experiments,
and thought, and is one
of the greatest books ever
published. To this book
is largely due the accept-
ance of the doctrine of
evolution as based upon
natural selection, and
modern biology is said to
date from this book. The
two fundamental concep-
tions of this book are:

(1) that the process of
creation is evolution; and

(2) that the process of
evolution is based upon
natural selection.

Natural Selection. -
According to Darwin's
theory of natural selec-
tion, the individuals of the plant and animal world are in-
volved in continuous competition in nature, and only those
best adapted to their surroundings survive. Thus through
the destruction of those individuals not able to survive in the
struggle for existence, a selective process, which permits only
certain individuals out of a large number to live and propa-
gate, is thereby established. Darwin* s theory of natural selec-
tion involves five fundamental conceptions, — variation, inheri-
tance, fitness for environment, struggle for existence, and survival
of the fittest.

Variation. — Variation is the fundamental fact in Darwin's
theory of natural selection. By variations is meant the devia-
tions of organisms from a type chosen as a standard of compari-

FIG. 464. — Charles Darwin, the noted
scientist, to whose work the establishment
of the theory of evolution by natural selec-
tion is chiefly due.

518

EVOLUTION

son. Thus if one should select a certain height, a certain num-
ber of flowers per plant, etc., as typical of a given species and as
standards of comparison, deviations of individuals from these
standards are variations. Darwin was a careful student of

FIG. 465. — Heads of Timothy selected from a field of Timothy to
show variation in form and size of heads. After Clark.

J9 20 2?

FIG. 466. — Variation in length of ears selected from a field of Black
Mexican Sweet Corn. After East.

variations and observed that variations in structure and func-
tion are so common that no two individual organisms are ex-
actly alike. In a field of grain or in a group of any of the culti-
vated or wild plants no two individuals can be found that are
exactly alike (Figs. 465 and 466)- They differ in height, shape,

INHERITANCE 519

number of leaves or flowers, time of maturing, character of root
system, etc. The individuals of a group of organisms not only
differ from each other, but they also differ just as strikingly
from their parents. Even the organs of the same plant, such
as leaves, vary widely (Fig. 1+61}. Many of the variations are
conspicuous, while others are discovered only through close
inspection. Some variations better adjust the individual to its

FIG. 467. — A number of Mulberry leaves selected from the same tree
to show variation in form.

surroundings, others are detrimental, and some are neither use-
ful nor detrimental. Darwin's idea was that variations, which
are innumerable and various in kind, afford ample material
upon which natural selection can work. Why plants and ani-
mals vary, Darwin did not attempt to explain.

Inheritance. — Although it is common observation that off-
spring and parents differ, it is also common observation that
there are always some fundamental resemblances. Thus the
offspring of a given variety of Corn are the same in variety as

520 EVOLUTION

the parent and not of another kind, although there may be much
difference in size of plants, length of ears, size and shape of ker-
nels, etc. In spite of variations there is inheritance which is
an imparting of something by the parents to the offspring,
which as a result develop some of the parental features. Dar-
win also thought that not only the resemblances to parents, but
also whatever variations the offspring may have are inheritable.
Assuming that variations are transmitted to succeeding gen-
erations in which they may become more pronounced, he could
explain the origin of new characters and new species. If a
variation is such as to better adapt individuals to their en-
vironment, those individuals in which the variation is most
pronounced have a better opportunity to survive and propagate
in greater numbers than individuals in which the variation is
less pronounced. Thus through many successive generations in
which the individuals with the variation most pronounced are
more favored than other individuals, the variation is intensified
by natural selection and eventually becomes a character of a new
species. Darwin drew this conclusion from some of his experi-
ments as well as from general observations of plants and animals
under domestication. He observed that through selection both
plants and animals changed much under domestication. Starting
with wild forms of plants, he was able by selecting the variants
from the progeny for a number of generations to obtain indi-
viduals differing strikingly from the original wild forms. He
found that continuous selection gradually built up the selected
character until the desired result was obtained. He concluded
that in nature such a selective process was brought about by
the competition among individuals for existence; but, while
any character might be built up by artificial selection, only those
which enable the individual to withstand competition are built
up in nature.

Fitness for Environment. — It is common observation that
different kinds of plants and animals require different condi-
tions under which to live. As pointed out in the discussion of
Ecology, some plants and animals live submerged in the water,
but most plants and animals can live only on land. The Cacti
are so constructed that they can endure the drought of the
desert, but Corn, Wheat, and most plants require a medium
amount of moisture. Again some plants which are well pro-

STRUGGLE FOR EXISTENCE 521

tected by hairs, like the Mullein, can thrive in the open on dry
hillsides, while there are other plants that can grow only in the
shade or moist ravines. The polar bear is adjusted to live
where the climate is cold, while the elephant is adjusted to a
tropical climate. Colored people can live better than white
people under the tropical sun because of the black pigment in
their skin. There are countless ways in which plants and ani-
mals show adjustments to particular kinds of environment, and
according to Darwin these adjustments are largely the results
of natural selection.

Struggle for Existence. — That there is an immense struggle
for existence in which vast numbers of both plants and animals
perish is easily demonstrated. The number of seeds produced
by a plant and the number of offspring that mature are com-
monly very different. For example, one plant of the Russian
Thistle, one of the common weeds, produces from 20,000 to
200,000 seeds. Taking 25,000 seeds to a plant as a moderate
estimate, the offspring of one plant would number 15,625,000,-
000,000 in the third generation, if all the seeds grew. Allowing
one square foot per plant, the plants of the third generation
would cover more than 500,000 square miles. At this rate of
multiplication, there would be no room in the United States
for anything else in a few years. But the number of plants that
develop is exceedingly small in comparison with the number of
seeds produced. Many of the seeds are destroyed before they
germinate, but many plants start that do not complete their
development. Many are killed by insects, and many are
crowded out by more vigorous individuals. On an area three
feet long and two feet wide which had been dug and cleared so
that the seedlings were not choked by other plants, Darwin
found that out of a total of 357 seedlings no less than 295 were
destroyed by slugs and insects. He also measured off a small
area of turf which had long been mown and allowed the plants
to grow. Out of the 20 species of plants growing on this small
plot of turf nine species, some of which were fully grown, were
crowded out by the more vigorous species and perished. The
same process of elimination is going on among animals. Even
in case of the elephant, which is a very slow breeder, Darwin
showed that the progeny of a single pair would number
19,000,000 in less than 800 years, if all survived. But since the

522 EVOLUTION

number of elephants in the world remains practically the same,
many must perish for every one that survives.

Survival of the Fittest. — If only a few of the vast number of
living things which come into the world survive, then the ques-
tion as to what individuals survive arises. Darwin's answer
is that the survivors are those individuals having those varia-
tions which adjust them most perfectly to their surroundings.
Thus a vigorous growing plant will soon get out of the shade of
its neighbors and will also get a larger proportion of the water
and mineral substances of the soil. It will therefore succeed in
crowding out the less vigorous plants around it. Also the
plant that succeeds in developing the best protective structures
against drought, heat, bright sunshine, or animals will have an
advantage over the less fortunate ones, and will therefore sur-
vive while the others perish. The better adaptation may be
in the production of more seeds, in better methods of dissemi-
nating the seeds, or in numerous other ways. Among animals
the strongest, fleetest, most cunning, or best equipped for
fighting are the fittest and survive to the expense of those not
so well equipped for the struggle. To this process of natural
selection Herbert Spencer applied the phrase " survival of the
fittest." It is obvious, however, that this process in nature is
more a rejection of the unfit than a selection of the fit, and ib
has been suggested that " natural rejection " would be a more
appropriate phrase for the process than " natural selection."
The rejection of the unfit makes room for the fit to live and
perpetuate their variations through offspring.

Although the variations which are useful may be slight at
first, through successive generations of selection Darwin as-
sumed that they become more marked and finally new char-
acters are established and thus new species formed. Of course
there are many variations which are neither of advantage nor of
disadvantage to the individual. According to Darwin only
those variations that are of advantage or disadvantage to the
individual are affected by natural selection.

Objections to Darwinism. — Darwin's theory aroused bitter
antagonism among theologians because they thought it elimi-
nated God from the plan of Creation. Darwin was even accused
of teaching that man descended from monkeys. But Dar-
win's theory neither eliminates God nor does it teach that man

OBJECTIONS TO DARWINISM 523

descended from monkeys, and theologians gradually came to
see that a plan in which the multitudinous forms are evolved
is just as noble a conception of God as the " Doctrine of Special
Creation."

The scientists offered a number of objections, some of which
later investigations have answered. They said that, if evolu-
tion by natural selection is now in progress, one should be
able to see one species forming from another, but such has never
been observed. The absence of forms connecting two related
species, and the presence of many apparently useless characters
among both plants and animals they said were not accounted
for. Again, according to geologists and astronomers, the world
has not been in existence long enough for the present forms to
be evolved by natural selection. The discovery of mutations
enables us to answer the above objections as will be noted
later.

A number of questions in regard to his theory remain to be
answered satisfactorily and are receiving much attention at
the present time.

First, as was stated under Lamarck's explanation of evolu-
tion, there is much evidence that acquired characters, that is,
characters which an individual does not get from its parents
but takes on during its lifetime, are seldom if at all inherited.
This strikes at Darwin's assumption that useful variations are
finally established as characters through inheritance and selec-
tion.

Second, it is claimed that by the selection of slight variations
new species cannot be formed. The individuals of a species
can be changed, but they can never be changed so much as to
form a new species. In support of this objection, it is claimed
that through centuries of artificial selection, plant and animal
breeders have not been able to produce new species.

Third, although the theory assumes that variations are
selected because of their advantage to the individual, it also
assumes that useful variations may be built up through the
selection of variations which are so slight at first that they
give the individuals having them no advantage over individuals
lacking them. Also the causes of variations Darwin left to his
successors to explain, and variations are still a subject of much
investigation and discussion.

524

EVOLUTION

Experimental Evolution

The early students of science studied plants and animals
simply by observing them in the field. They made no effort to
control the conditions under which the plants or animals were
growing. But one can see that, in order to draw definite con-
clusions in regard to the inheritable factors in plants, the ances-
try of the plants must be known, their pollination controlled so
as to know definitely the parents of the progeny, and the various
factors that affect the growth of the plants must be taken into

account. Likewise, in the study
of animals in reference to prob-
lems of evolution it is essen-
tial to control their breeding
and often the conditions to
which they are exposed. The
early scientists had poorly
equipped laboratories or none at
all, and science then was chiefly
a description of nature and was
called "Natural History."
Darwin and a few of his con-
temporaries put considerable
emphasis on the experimental
method, but since Darwin's time
the experimental method has
been especially emphasized with
the result that rapid strides
have been made in interpreting
and explaining facts.

Hugo De Vries. — Hugo De Vries (Fig. 468), director of the
Botanic Garden in Amsterdam, Holland, was among the first
to apply the experimental method to the study of evolution.
Starting with seed selected from plants which he thought were
pure, that is, not mixed with another variety, he grew a large
number of generations, which, by carefully preventing cross-
pollination, were kept pure to the parent type. In order to
make accurate comparisons and thereby detect variations from
the parents, such as the dropping of parental characters or the
taking on of new ones, he not only kept careful records but also

FIG. 468. — Hugo De Vries, whose
mutation theory is one of the most
important contributions to the study
of evolution since Darwin's time. He
has also made valuable contributions
to our knowledge of osmosis.

NATURE OF CONTINUOUS VARIATION 525

preserved specimens of each generation. At the same time he
also carefully observed plants in the field, and, when one was
found that showed extraordinary features, the experimental
method was applied to it. He was especially interested in vari-
ations, and his most notable contribution is on this subject.
He demonstrated by -his painstaking work that there are two
kinds of variations — continuous or fluctuating and discontinu-
ous or saltative variations. Discontinuous variations are also
called mutations and are those extreme variations which sud-
denly arise and remain fixed, that is, they are transmitted to
succeeding generations.

Nature of Continuous Variation. — Continuous variations are
the most common kind of variations. They are simply the fluc-
tuations that individuals show in size, shape, color, and other
characters. Thus red flowers vary in degree of redness, leaves
vary in shape and size, seeds vary in number per pod as well
as in shape and size, plants differ in height, shape, method of
branching, and so on. Continuous variations are chiefly due
to differences in the environment, such as differences in sun-
light, food and 'water supply, temperature, and influences ex-
erted by one organism upon another. According to the work
of De Vries and other investigators, they are not inheritable
and, therefore, are constantly changing with the conditions that
cause them. They fluctuate around a mean or average which
remains practically constant, and, above or below this average,
the individuals varying gradually grow less in number as vari-
ability departs more and more from the average until a limit
in each direction is reached. Continuous variations follow the
law of Quetelet, the Belgian anthropologist, who found that
variability follows the law of probability. -Small divergences
from the average are numerous, while larger ones are less numer-
ous, and the larger the less numerous they are. If, for example,
a bushel or any quantity of ears of Corn are separated into
piles according to length, there will be one length which will
include the greatest number and, above or below this length,
which is known as the average, the piles will decrease in size
as the length of ears in each pile are greater or less than the
average. This is well illustrated in Figure 469 in which 82 ears
of Corn with extreme lengths 4.5 and 9 inches are arranged in
10 piles according to size. The same fact is illustrated in Figure

526

EVOLUTION

470, in which case beans are assorted according to size. Of course
where size is the variation considered, the space occupied by
each class of individuals will vary more than the number of
individuals in each class, for the smaller individuals occupy less

FIG. 469. — Quetelet's law of continuous or fluctuating variability, demon-
strated by arranging 82 ears of Corn in piles according to size. The ears were
taken from unselected material from a field of Corn. The length and number
of ears in each pile are given in figures below. Notice that the piles decrease
each way from the pile (highest pile) containing the ears of average length,
decreasing accordingly as the length of ears in each pile is greater or less than
the average length. After Blakeslee.

FIG. 470. — A demonstration of Quetelet's law of continuous variation in
the size of the seeds of a Common Bean. The seeds are grouped with refer-
ence to length. The longest column contains those of average length and
the columns to the right or left of it are shorter accordingly as the length of
Beans in each column are greater or less than the average length. Redrawn
from De Vries.

space than the larger ones. All kinds of continuous variations,
such as number of flowers per plant, number of seeds per pod,
weight of seeds, height of plant, size and shape of leaves, etc.,
distribute themselves around an average in the same general way
as illustrated by size in case of Beans and ears of Corn.

DISCONTINUOUS VARIATIONS OR MUTATIONS 527

When a number of generations of offspring are considered,
the continuous variations in each generation tend to fluctuate
around an average that is common for all the generations.
Consequently the selection of continuous variations seldom im-
proves the average, and, if any improvement is obtained in this
way, it is soon lost when selection is discontinued. The average
yield of sugar in Sugar Beets has been improved and is main-
tained by selecting for seed the plants having the highest sugar
content, but the improvement is lost when selection is discon-
tinued. Johannsen has demonstrated that, if one starts with a
pure line, that is, with the offspring of a single individual pro-
duced by self-fertilization, and keeps the generations pure by
preventing cross-pollination, the average of a fluctuating continu-
ous variation cannot be increased. He clearly demonstrated this
with Beans, which self-pollinate and hence remain pure. He at-
tempted to increase the average size of the seeds by selecting
the largest seeds of each generation of a certain variety of Beans
for planting. He continued this for a number of generations,
but obtained no increase in the average size of the seeds. Simi-
lar results have been obtained by other investigators in attempt-
ing to intensify certain desirable variations in Wheat, Oats, and
in pure lines of other plants. For example, an effort to increase
the yield in a strain of Oats by selecting each year the best
yielding plants for seed gave practically no increase in yield
after a number of years of selection.

Discontinuous Variations or Mutations. — That there are
two kinds of variations was observed by Darwin and others
before and after him, but De Vries was the first to show the
importance of discontinuous variations in evolution. He formu-
lated the theory that discontinuous variations and not con-
tinuous variations furnish the material for natural selection.
Discontinuous variations differ from continuous variations in
that they arise suddenly, are usually of marked character, and
breed true, that is, they are passed on to the offspring. Dis-
continuous variations De Vries called mutations. A plant that
gives rise to mutations is said to mutate, and a plant arising by
mutation is called a mutant. De Vries's theory of evolution is
known as the mutation theory, and its fundamental conception
is that species are formed by the selection of mutations and not
by the selection of continuous variations.

528

EVOLUTION

We now believe that our cultivated plants afford many ex-
amples of mutations. Strawberries without runners have sud-
denly arisen among plants with runners and have bred true from
seed. The Beseler Oats, a beardless variety, originated from a few
plants found in a field of bearded Oats. Also a number of choice
new varieties of Wheat started from one or a few plants which

FIG. 471. — The Wild Cabbage and some of the forms that are supposed to
be mutants of the Wild Cabbage. A, Wild Cabbage; B, Kohlrabi; C, Cauli-
flower; D, Cabbage; E, Welsh or Savoy Cabbage; F, Brussels Sprouts.
After Smalian.

suddenly appeared differing in characters from the other plants
of the field. The Cauliflower and Kohlrabi (Fig. 471) were
raised from isolated monstrosities of the wild Cabbage (Brassica
oleracea). Green Roses, green Dahlias, seedless Oranges, seed-
less Bananas, and varieties of the Boston Fern with finely divided
leaves are other examples of mutations. Sometimes a bud may
mutate, giving rise to a bud sport. In this way the Nectarine,
which has a fruit resembling that of the Peach but lacking

MUTATION IN THE EVENING PRIMROSE

529

the fuzz of a peach, sometimes arises as a branch of the Peach
tree.

Mutation in the Evening Primrose. — The Evening Primrose
is especially noted because it has furnished much of the material
upon which the mutation theory is founded. One of the diffi-
culties in finding mutations is that any given species does not
mutate all of the time but only at occasional periods. In
1886 De Vries began to search for species that were in the
mutating period. The
American Evening Prim-
rose (Oenothera Lamarck-
iana) (Fig. 472), also
known as Lamarck's
Evening Primrose, proved
to be the species for which
he was searching. He
found a large number of
plants of this species
growing in an abandoned
potato field at Hilversum,
near Amsterdam. Among
them he found some un-
known and very distinct
forms which apparently
had come from seeds of
the normal American
Evening Primrose. Seeds
were secured from the
normal plants, and cul-
tures were begun in the
Botanical Garden at the
University of Amsterdam. From the first sowing he obtained
another new form. Through a series of pedigree cultures in-
volving a number of generations, quite a number of distinct
forms were obtained. Some of these distinctly new forms ap-
peared repeatedly in the cultures, while others appeared only
once, but they all bred true, thus producing offspring like them-
selves. These new forms did not arise gradually but appeared
suddenly and were so distinct from the American Evening
Primrose, their parent, as to be called new species.

FIG. 472. — Lamarck's Evening Primrose
(Oenothera Lamarckiana) , a mutating
species. After De Vries.

530

EVOLUTION

The mutations involved various kinds of characters. One of
the new forms, Oenothera brevistylis, had, among other distinc-
tive characters, a much shorter style than the Oenothera La-
marckiana. Another new form, Oenothera laevifolia, had smooth
leaves and much prettier foliage than the parent type, and its
petals were not notched. One of the finest and rarest of the
mutants was the Oenothera gigas (Fig. IflS), which was stronger,
bigger, and more heavily built than the parent. Others of the
' new forms differed from

the parent in other char-
acters.

As to what a new form
is, can be determined only
by breeding it and its
parents in pedigree cul-
tures. A new form may
result from crossing be-
tween two parents with
different characters and,
therefore, be a hybrid.
The new form may be
due to the fact that the
parent is a hybrid or de-
scendant of a hybrid and
consequently does not
breed true, thus producing
offspring different from
the parent type. If the
parents are pure, and the
new form is not a hybrid,
it must be a mutant or a
fluctuating variant. If it breeds true, it is a mutant; other-
wise, it is a fluctuating variant.

By growing the parents of the new forms in pedigree cultures
for a number of generations, he found that they bred true,
except for the new forms which occasionally appeared, and
concluded that the parents were pure. Since the parents of
the new forms were carefully pollinated artificially, so as to
prevent crossing between parents differing in characters, the new
forms were not the result of hybridizing. They were either

FIG. 473. — The Giant Evening Prim-
rose (Oenothera gigas}, one of the mutants
from Lamarck's Evening Primrose. After
De Vries.

CAUSES OF VARIATION 531

mutants or fluctuating variants and, when the test of breeding
true was applied, they proved to be mutants. Thus De Vries
had obtained mutations under experimental conditions and was
ready to announce the mutation theory.

The Mutation Theory and Darwinism. — The mutation the-
ory does not disturb the theory of evolution by natural selection,
but holds a different view as to the material upon which natural
selection works. According to Darwinism, most all kinds of
variations are inheritable, and even though slight at first, they can
become intensified through generations of selection and finally
become distinct characters of new species. According to the
mutation theory, only mutations are inheritable, and they are
not built up through generations of selection, but arise suddenly,
in full force, and breed true thereafter. Thus according to the
mutation theory, new species arise at one bound, and all that nat-
ural selection has to do is to determine whether they survive or
perish. If the mutation is such that the new species is well
adapted to its surroundings, then, according to the law of the
survival of the fittest, it will survive; otherwise, it will likely
perish.

The mutation theory explains a number of the early objections
to the theory of natural selection. It accounts for the fact that
species have so many characters which are apparently of no
value in fitting the individual to live. It is obvious that a
species may have characters of no importance as well as useful
ones, if new characters arise at a bound in full force, and their
presence does not depend upon their having met the test of fitting
the species to live through generations of selection. The muta-
tion theory accounts for the absence of intermediate forms or
so-called connecting links between species. Evolution by mu-
tations requires less time than evolution by the natural selec-
tion of fluctuations, and in this way the theory answers the
objection to the lack of time.

Causes of Variations. — There are various causes which have
to do with bringing about variations in both plants and ani-
mals, many of which are not understood. Numerous varia-
tions are due directly to differences in food supply, climatic
conditions, and other external factors to which the individual
is exposed. Thus a plant well situated in reference to light, so
that its leaves can make carbohydrates abundantly, is likely to

532 EVOLUTION

be larger than a plant that is shaded. Likewise, the amount of
soil moisture and mineral matter available, the amount of
transpiration to which the plant is exposed, the temperature of
the soil and atmosphere, and the velocity of winds, and the
amount of competition with other plants are also common en-
vironmental factors that cause variations. Variations which
are responses to variations in the environment are seldom, if
at all, inheritable and hence are fluctuating variations. Fluctu-
ating variations in individuals may be -due also to fluctuating
variations in parents. Thus, if a parent plant is poorly nour-
ished, its seeds may be poorly developed and produce offspring
that vary from the ordinary type in size and vigor.

Fluctuating variations also arise as a result of sex. Plants
produced by vegetative propagation are often less vigorous than
those grown from seed. The fusion of a sperm and an egg in
fertilization often results in a rejuvenating effect which is mani-
fested in a more vigorous offspring. The relationship of the
sperm and egg involved in fertilization has an important bearing
upon the variations in the offspring. For example, in Corn the
offspring resulting from self-fertilization is not nearly so vigor-
ous as offspring resulting from a fertilization in which the sperm
and egg come from different parents, although the parents are
the same in type. In addition to fluctuating variations and
mutations, there are those numerous differences among indi-
viduals due to heredity, such as occur in the offspring when
parents differing in variety or species are crossed. In connec-
tion with these differences due to heredity, some fluctuating
variations, such as variations in size and vigor, commonly occur.

Mutations are apparently caused by changes within the in-
dividual, and, although environmental factors may have much
to do with bringing them about, they are not direct responses
to variations in the environment. Usually they involve the en-
tire constitution of the individual, the gametes as well as vegeta-
tive structures, while fluctuating variations usually involve only
vegetative structures.

Somatoplasm and Germ-plasm. — Weismann, a German bi-
ologist, and his followers hold the theory that a plant or animal
consists of two kinds of protoplasms, which act more or less
independently of each other. The protoplasm of which sperms
and eggs are formed they call germ-plasm, while all protoplasm

SOMATOPLASM AND GERM-PLASM 533

that does not have to do directly with forming sex cells is called
somatoplasm. Thus the protoplasm of all vegetative structures
of plants, such as leaves, roots, and stems, is somatoplasm.
Even the parts of a flower, excepting the protoplasm immedi-
ately involved in the production of sex cells, is somatoplasm.
According to Weismannism, the characters of a species are de-
termined by certain units or factors within the germ-plasm
and the germ-plasm remains practically the same from genera-
tion to generation in respect to the factors contained, although
the factors may change in reference to each other. He holds
that changes in the somatoplasm, such as those variations in
leaves, roots, and stems that occur in response to environmental
influences, are not imparted to the germ-plasm and consequently
are not inheritable. This theory that the germ-plasm remains
practically the same throughout generations is known as the con-
tinuity of the germ-plasm. Since modifications in the somato-
plasm leave no trace of themselves in the germ-plasm, it is obvi-
ous that, according to Weismann's view, characters cannot be
acquired. This means that any trait which an individual does
not inherit but acquires during its life time in response to envi-
ronment is not transmitted to the offspring. Thus, if a parent
acquires great skill as a musician, mathematician, or in other
lines, the children of this parent inherit none of this acquired
ability. Also, in case of plants, the particular modifications
which individuals take on during their life time disappear with
the individuals.

In accounting for the inheritable changes occurring in indi-
viduals and generations, Weismann tells us that the units or
factors in the germ-plasm are changing in relation to each
other, and these changes account for the origin of new char-
acters. Some units may become stronger and others weaker,
and they may combine in various ways. Such changes may be
induced by external conditions, such as poor nourishment,
drought, competition, etc., but the character resulting there-
from may be of any kind, and hence only by chance is it of such
a nature as to adjust the individual to its environment. For
example, the changes induced in the units of germ-plasm by an
environmental factor, such as drought, may result in a change
in the color of the flower, length of style, arrangement of leaves,
etc. Thus the character resulting from the change induced by

534 EVOLUTION

the environment may show no adaptive connection whatever
with the external influence inducing the change. Another cause
of changes among the units in the germ-plasm Weismann attrib-
utes to interactions upon each other of different units brought
together through fertilization.

According to Weismann's theory of the constitution of organ-
isms, fluctuating variations are due to changes only in the
somatoplasm, while mutations are due to shifts among the
factors in the germ-plasm. Mutations arise as a result of changes
within the individual, while fluctuating variations usually arise in
response to influences external to the individual/ Although the
external influences causing fluctuating variations may at the same
time so indirectly influence the germ-plasm as to cause mutations,
the fluctuating variations themselves involve only the somato-
plasm, and hence are not recorded in the germ-plasm. For
example, a lack of moisture may cause such fluctuating variations
as a reduction in size of leaves, fruit, and in number of flowers
produced per plant, and at the same time induce such changes in
the germ-plasm that a mutation in color of flowers, length of
style, etc., may appear in the offspring, but the fluctuating varia-
tions disappear with the vegetative structures involved.

Weismann's theories have not been sufficiently demonstrated,
and there are some objections to them. Although the evidence
seems to be against the inheritance of acquired characters, this
question is not yet settled.

CHAPTER XXIII

HEREDITY
General Features of Heredity

Nature of Heredity. — The constancy of species of plants
arid animals through successive generations depends upon the
fact that the individuals of each generation are fundamentally
like their parents. Thus the offspring of Sweet Corn have the
characters of Sweet Corn, and the offspring of Flint Corn have
the characters of Flint Corn. Even when parents differing in
one or more fundamental characters are crossed, the characters
of both parents will appear somewhere in future generations.
The transmission from parent to offspring of similarities in
structure and function is heredity. But heredity means more
than the transmission of similarities. In the study of varia-
tions it was learned that no two individuals are alike, and hence
offspring, although fundamentally similar and like their parents,
always have individual differences. Thus every plant or ani-
mal, besides resembling its parents, brothers, and sisters, has its
own peculiar features which give it individuality. In a field of
a given variety of Corn, although the plants have the characters
of the parent variety, they differ in height of stalk, length and
shape of ear, depth of kernel, time of maturing, and in many
other ways. Most of these differences are fluctuating varia-
tions, while some may be due to something inherited. In the
study of heredity not only the resemblances but also the differ-
ences must be accounted for. Heredity means the transmission
of fundamental resemblances with differences in detail.

The Physical Basis of Heredity. — It is obvious that parents
do not actually transmit characters to the offspring, but trans-
mit something that causes the characters to appear in the
offspring. For example, a red flowered Sweet Pea does not
transmit a red color to the flowers of its offspring, but transmits
something through the sperm and egg that causes a red color

535

536 HEREDITY

to appear in the flowers of its progeny. The exact nature of
the factors or substances which are responsible for the appear-
ance of characters is not well understood, but it is quite evident
that they are protoplasmic substances. That they are proto-
plasmic substances is probably more easily demonstrated in
the lower than in the higher organisms. In the reproduction of
simple one-celled plants, like Pleurococcus, the plotoplasm of
the parent divides, and each half of the parent becomes a new
individual. The parent thus disappears in the formation of
new individuals or progeny, which are at first merely segments
of the parent. The new individuals, as they develop to normal
size, develop in full the features characteristic of the parent.
They separate soon after they are formed, develop a lobed chlo-
roplast, enlarge and thicken their walls with cellulose, and retain
a globular form. These are the constant characteristics by which
we know Pleurococcus, despite the fact that there are numerous
other ways the plant might develop. It might, for example,
form a filament like that of Spirogyra, develop ribbon-like
chloroplasts, and enclose itself in a woody wall. The charac-
ters of Pleurococcus are the results of the way the protoplasm
works, for the protoplasm forms the chloroplasts, the cell walls,
and is responsible for the separation and shape of the cells. It
is obvious that the characters of this simple plant are what
they are and are constant because the protoplasm has a dispo-
sition to work only in certain ways and retains this particular
disposition as it passes from generation to generation.

In higher plants and animals the parent does not divide and
each half go to form a new individual, but only a small part of
the parent, a sperm and an egg, are transmitted to the offspring.
Hence the sperm and egg must contain all of the protoplasmic
constituents necessary for producing in the offspring the char-
acters of the parents. But the sperm consists almost entirely
of a nucleus, and hence it is believed that the material upon
which the development of characters depends is within the
nucleus, and occurs in connection with the chromatin, which, in
the form of chromosomes, behaves in such a regular way during
cell division as to suggest a definite relation to heredity. These
protoplasmic constituents upon which the development of char-
acters depends, Weismann called determinants, and some other
biologists call them genes.

NEED OF EXPERIMENTAL STUDY 537

Active and Latent Genes. — Not all of the genes inherited
manifest themselves, for many lie dormant and consequently
cause no corresponding characters to develop. Sometimes they
lie dormant for a generation or more and then become active.
On this account offspring may have ancestral characters which
the parents did not have. Certain genes for male characters
are always latent in females. Thus a mother may transmit to
her son genes for a beard like her father's, although she develops
no beard herself. It is obvious that one cannot accurately judge
the constitution of a plant or animal by the characters present,
for judgment based upon external appearances takes no account
of the latent genes. Further breeding is necessary to determine
ohese. Latent genes becoming active is, therefore, one of fche
reasons why offspring differ from their parents.

Importance of the Study of Heredity. — The study of her-
edity occupies an important position among the sciences. The
study of heredity has for its aim the discovery of the laws of
heredity, and a knowledge of these laws is essential in many
lines of work. The improvement of our crop plants and domes-
tic animals through breeding depends upon the laws of heredity.
The science of sociology and all sciences dealing with the physical,
moral, and spiritual welfare of humanity are more or less con-
cerned with the laws of heredity. Even in the realm of medi-
cine the laws of heredity are taken into account.

Experimental Study of Heredity

Need of Experimental Study. — In the discussion of evolution
it was shown that the application of the experimental method
to the study of evolution has added much to our knowledge
of this subject. We had no definite knowledge of the kinds of
variations and their relation to natural selection until they
were studied by the experimental method. In the study of
heredity the experimental method is as essential as in the study
of evolution. Heredity is so apparent in both plants and ani-
mals that we are all convinced of its reality. That children
resemble their parents in habits, disposition, color of eyes and
hair, and other features are so noticeable that they are matters
of common observations. Casual observations, however, fall
short of giving us a knowledge of the laws of heredity. The

538 HEREDITY

discovery of the laws of heredity requires carefully planned and
systematic work. One must obtain a more or less accurate
knowledge of the history of the parents, control the breeding
so as to know the exact parents of the offspring, carefully study
each individual of the offspring so as to discover the hereditary
relationship between parents and offspring and between the
different individuals of the offspring. One must also take into
account the conditions which affect the plants or animals of the
experiment. Carefully recorded facts obtained by experimental
study involving a number of generations and various kinds of
plants and animals afford a basis for conclusions concerning the
laws of heredity. The experimental study of heredity as just
described is known as genetics, a subject in its infancy but one
of the most popular and most promising of the sciences.

Biometry. — Before the experimental method of studying
heredity came into common use, the statistical method was
employed. The investigators who study heredity by the sta-
tistical method of recording data are called biometricians. In-
stead of dealing with the variations of single individuals, they
deal with the average variations of a mass of individuals or
populations. It is a method of discovering how masses of
individuals behave through a series of generations, and not a
method of discovering how individuals behave. For example,
they determine whether or not the average yield, average
height, or average weight of a population is remaining constant
or shifting, and such information is often valuable. By keeping
a record of the average yield per acre of different strains of
Wheat for a number of years, the strain yielding best can be
determined. By such records one can also detemine whether
or not newly introduced strains or varieties hold up in yield.

The biometricians formulated some laws of heredity. Francis
Galton, who was one of the foremost of the biometricians, formu-
lated a law of heredity and announced it in 1897. This law
states that to the total heritage of the offspring the parents on
an average contribute f , the grandparents J, and the great grand-
parents |, and so on, the total heritage being taken as unity.

The objection to the method employed by the biometricians
is that it does not pay enough attention to the variations of
individuals. A mass of individuals, such as a field of Wheat
or Corn grown from the purest market seed, is a mixture of in-

GREGOR MENDEL

539

dividuals differing in heritage, and not much can be determined
concerning the laws of heredity from such a mixture. The
average means nothing unless the individuals measured or
counted are alike in their heritage, and the only way to be sure
that the individuals of a mass or population are homogeneous in
constitution is to pedigree them, that is, grow them all from a
common stock. The importance of pedigree cultures is well
shown in Mendel's work.

Gregor Mendel. — Gregor Mendel (1822-1884) (Fig. 474),
was an Austrian monk and abbott in the monastery of Brunn,
where he conducted his ex-
periments in the Cloister
Garden. He loved plants
and loved to experiment with
them. Although he studied
heredity only as a pastime,
his laws of heredity and his
experimental method of in-
vestigating them are two of
the most important contribu-
tions ever made to biological
science.

Mendel's success was due to
the clearness with which he
thought out the problem. He
knew the works of other in-
vestigators of heredity, and
attributed their failure to
reach definite conclusions to
a want of precise and con-
tinued analysis. To obtain
definite results he saw that it was necessary to start with pure
material, to consider each character separately, and to keep the
different generations distinctly separate. He also realized that
the progeny of each individual must be recorded separately.
Such ideas were new in Mendel's time, but he felt certain that
experiments carried on in this systematic way would give regu-
lar results and lead to definite conclusions.

Mendel saw that most could be accomplished by crossing
plants of different varieties or species and observing the be-

FIG. 474. — Gregor Mendel, whose
theory of inheritance is the most im-
portant contribution ever made to our
knowledge of heredity.

540 HEREDITY

havior of the hybrid offspring in successive generations. His
plan was to cross plants differing in one or a few outstanding
characters, such as the color of flowers, height of plant, color and
shape of seeds, etc., and determine the laws governing the appear-
ance of these characters in the hybrid offspring.

Material Chosen. — Mendel used great care in the selection
of the plants to be used in the experiment, for he realized that
the success of any experiment depends upon the choice of the
most suitable material. After careful consideration he chose
the Edible or Garden Pea as the chief plant with which to work.
The Garden Pea proved to be the most suitable plant because:
(1) it matures in a short time; (2) varieties in cultivation are
distinguished by striking characters recognizable without trouble;
(3) the plants are self-fertilized, and hence plants chosen for
parents would be pure, that is, they would have no hybrid
blood in them, and the hybrids and their offspring would not be
disturbed by crossing in the successive generations.

MendePs Experiments. — Mendel investigated pairs of char-
acters separately and in relation to each other, and extended his
investigations to include many pairs of characters, in order to see
if all appeared in the successive generations of offspring accord-
ing to the same law. His method of procedure may be shown
by describing his experiments with tall and dwarf Peas. A tall
Pea, having a height of 6-7 feet was crossed with a dwarf Pea,
having a height f to 1| feet. By means of forceps or other in-
struments and before the flowers were open, the anthers were re-
moved from the flowers of the plant selected as the mother plant,
and pollen from the pollen parent was applied to the stigma.
The hybrid seeds developed by the mother plant as a result of the
crossing were carefully collected. These cross-bred seeds were
planted and produced the first hybrid generation of plants, known
as the Fi generation in our modern terminology. The height of
each individual of this generation was carefully noted, and each
individual was compared with 'the parents in respect to tallness
or dwarfness. The individuals of this generation were allowed
to self-fertilize, and the seeds of each individual were collected
and planted separately. From these seeds he grew the second
generation or F2 generation according to modern terminology.
The individuals of the progeny of each of the FI plants were
carefully compared with their parents and grandparents in re-

MENDEL'S DISCOVERIES 541

spect to tallness and dwarf ness and the facts carefully recorded.
The individuals of the F2 generation were allowed to self-ferti-
lize, and from the seeds obtained the Fs generation was grown,
and the individuals in this generation were studied in the same
careful way as those of the previous generations. Throughout
a number of generations the behavior of tallness and dwarfness
was carefully recorded. In this way he studied many pairs of
characters such as: (1) shape of pod (whether simply inflated
or deeply constricted between the seeds); (2) color of unripe
pod (whether green or yellow); (3) distribution of flowers on
the stem (whether distributed along the axis of the plant or
bunched at the top); (4) color of cotyledons (whether yellow
or green); (5) shape of seeds (whether round or wrinkled);
and (6) color of seed coat (whether gray or brown, with or
without violet spots, or white).

Mendel's Discoveries. — Mendel found that in most cases the
different pairs of characters investigated behaved in the same
way and appeared in a regular way in the successive generations.
Furthermore, it made no difference as to which variety was used
as the mother parent. In case of tallness and dwarfness, all the
plants of the first or FI generation were tall. They were all like
the tall parent. In the second or F2 generation there were both
tall and dwarf plants, but there were three times as many tall
plants as dwarf ones, the tails and the dwarfs thus occurring
in the ratio of 3 : 1. The offspring of the dwarfs were all dwarfs
in the third or F3 generation and in all' succeeding generations.
The dwarfs, therefore, were pure for dwarfness, that is, they
had nc factors or genes for tallness in them. One out of every
three tall plants also bred true and, therefore, proved to be
pure for tallness, but two out of every three tall ones gave three
times as many tall ones as dwarfs or a ratio of 3 : 1, thus being
apparently the same in constitution as each of the individuals of
the FI generation. They evidently contained factors or genes
for both tallness and dwarfness. The dwarfs and one-third of
the tall ones of the F3 progeny bred true, while two-thirds of
the tall ones again bred as in the previous generation, giving the
ratio 3:1, and two-thirds of the tall ones being impure. This
proved to be a constant way of behaving throughout genera-
tions. The character of the individuals of the different gen-
erations are shown in Figure 475. Thus by the further breed-

542

HEREDITY

FIG. 475. — A diagram illustrating Mendel's discovery concerning the in-
heritance of tallness and dwarfness in the Garden Pea. At the top are the
parents of the cross, the tall variety at the left and the dwarf variety at the
right. In the line immediately below is the first (Fi) generation, all plants of
which are tall and thus like the tall parent, and each of which upon being self-
fertilized produced a progeny (F% generation) consisting of tall and dwarf
plants in the ratio of 3 : 1, as shown in the third line. As shown in the lower
line (F3 generation), the dwarfs and one-third of tails of the F2 generation bred
true, that is, they produced progeny like themselves, while two-thirds of the
tall ones produced a progeny consisting of tall and dwarf plants in the ratio
of 3 : 1.

SEGREGATION AND PURITY OF GAMETES 543

•

ing of the second hybrid generation, it was found that although
the tails and dwarfs appeared in the ratio of 3 : 1, there were in
reality three kinds of plants, pure tails, impure tails, and pure
dwarfs, occurring in the ratio 1:2:1, and that the impure tails
always produced three kinds of plants in the same ratio of 1 : 2 : 1 .
Law of Dominance. — It is obvious that tallness dominated
dwarfness in the hybrid Peas, and this accounts for the fact that
all of the first generation were tall, although all of them had genes
for dwarfness as well as for tallness in them. It also explains why
the impure tall ones in succeeding generations were tall, although
they had genes for dwarfness in them. In extending his investi-
gations to other pairs of characters, Mendel found that smooth-

D X R first parent generation

D(R) first hybrid generation

ID 2D(R) IR second hybrid generation

~1 T~ ~T~

D ID 2D(R) IR R third hybrid generation

FIG. 476. — Diagram illustrating the constitution of the individuals of
the first, second, and third hybrid generation with reference to dominant
(D) and recessive characters (R).

ness of seeds dominated wrinkledness, yellow color of cotyledons
dominated green, and so on. Such pairs of contrasting characters
are called allelomorphs, and the one dominating is known as the
dominant and the other as the recessive character. Mendel's
law of dominance may be stated as follows: When pairs of
contrasting characters are combined in a cross, one character
behaves as a dominant while the other behaves as a recessive.
Representing the dominant character by D and the recessive
by R, the behavior of dominant and recessive characters are as
shown in the diagram in Figure J+16.

Segregation and Purity of Gametes. — Since the pure tall and
pure dwarf plants of the second generation and succeeding
generations showed no tendency to produce anything but pure
tall or pure dwarf plants, they evidently had no genes or parts
of genes for the contrasting characters. The genes for con-

544 HEREDITY

trasting characters must have separated as units, and in the
separation no straggling part of a gene was left associated with
the gene for the contrasting character. To this complete sep-
aration of genes and consequently contrasting characters the
term segregation was applied.

The complete segregation ol characters also implies a purity
of gametes. The constitution of an individual depends upon
what the sperm and egg introduced into the fertilized egg from
which the individual developed. Thus, if a plant is pure for
tallness, the sperm and egg involved in the fertilization resulting
in the production of this plant could not have contained genes
for dwarfness. Of the two kinds of genes, they contained only
those for tallness. The same is true in case of dwarfness or any
other one of a pair of contrasting characters. This really means
that in a fertilization resulting in the production of a plant
pure for a character both the sperm and egg have genes for the
same contrasting character, and that an individual pure in re-
spect to a character, therefore, is one that has inherited from
both parents genes for the same character. In other words, a
plant pure for a character is one that receives a double dose of
genes for this character. On the other hand, plants, like the
impure tall ones, have received genes for the dominant character
from one parent and genes for the recessive character from the
other parent, and hence they have only a single dose of genes
for either of the characters. Such a plant we now speak of as
being heterozygous, while plants having a double dose of genes
and hence pure for a character are regarded as homozygous.
Since plants pure for a character breed true, their gametes must
be alike in respect to genes contained. The descendants of a
homozygous parent propagated entirely by self-fertilization are
of course pure and constitute what is known as a pure line.

After tracing the behavior of single pairs of characters through
successive generations, he took up the study of two or more pairs
of contrasting characters, the aim being to determine how pairs
of contrasting characters behave in respect to each other. For
example, he crossed Peas characterized by smooth yellow seeds
with Peas characterized by wrinkled green seeds. In this
case he was dealing with two pairs of characters, smooth and
wrinkled, and yellow and green, with smooth and yellow as
dominants. He found that each pair of contrasting characters

SEGREGATION AND PURITY OF GAMETES 545

behaved independently of each other, but all possible combina-
tions of them could be obtained. The F2 generation of seeds
contained smooth and yellow, wrinkled and yellow, smooth and
green, and wrinkled and green seeds, and each kind of seeds
occurred in a definite proportion of about 9 smooth and yel-
low: 3 wrinkled and yellow: 3 smooth and green: 1 wrinkled
and green. The wrinkled green seeds were pure recessives and
bred true, and 1 out of 9 of the smooth j^ellow seeds was a pure
dominant and thus bred true. All of the other seeds were not
pure and various combinations again occurred in their offspring.
The combinations and the number of individuals in each com-
bination that occurred in the F2 generation were in accord with
mathematical laws governing combinations. Representing the
dominants, smooth and yellow, by large S and large Y, and the
recessives, wrinkled and green, by small w and small g, the com-
binations of S and 117 are SS + 2 Sw + ww, and the combina-
tions of Y and g are YY + 2 Yg + gg. These combinations are
simply the pure dominants, impure dominants, and recessives in
the ratio of 1:2:1 which occurs when a pair of contrasting
characters is considered separately. Now (SS + 2 Sw + ww)
(YY + 2 Yg + gg) = SSYY + 2 SYYw + YYww + 2 YgSS +
4 YSwg -f 2 Ygww -f SSgg + 2 ggSw + ggww, which are the dif-
ferent combinations and the relative numbers of individuals
in each combination obtained when two pairs of contrasting
characters were considered in relation to each other. Since the
dominants obscure the recessives, the apparent combinations
with the relative number of individuals in each are 9 dominants,
3 individuals with dominant yellow and recessive wrinkled, 3
individuals with dominant smooth and recessive green, and 1
individual with recessive wrinkled and green. The individuals
having the constitution YYSS, as represented in above formula,
are pure dominants, the individuals having the constitution
wwgg are pure recessive, while the others are not pure. Thus
the laws of mathematics afford a way of expressing what Mendel
discovered concerning the behavior of characters in inheritance.

He crossed Peas having smooth yellow seeds and gray-brown
seed coats with Peas having wrinkled green seeds with white
seed coats, thus employing three pairs of contrasting characters.
He found also in this case that the pairs of contrasting characters
behaved independently of each other, and that the combinations

546 HEREDITY

in the F% generation were of many kinds. The combinations
in this case also agreed quite well with the mathematical laws
of combination, when a large number of the F2 individuals
were taken into account. The kinds of combinations and their
proportions follow quite well the general algebraic formula
(a + 6)?, in which n represents the number of characters in-
volved. Thus (a + 6)2 expanded gives a2 + 2 ab + bz which is
in accord with the 1:2:1 ratio, the ratio expressing the inheri-
tance of two contrasting characters. The formula (a + 6)4 gives
the combinations when plants are crossed that have two pairs
of contrasting characters. Of course the results obtained scarcely
ever exactly agree with the mathematical formula, and the more
individuals taken into account, the closer the agreement.

As a result of his work with a number of pairs of characters,
Mendel showed that by means of repeated artificial fertilization,
the constant characters of different varieties of plants may be
obtained in all of the associations which are possible according
to the mathematical laws of combination. This means that, by
crossing in a certain way, the desirable characters of different
varieties may be brought together, and thereby plants of a more
desirable type produced.

Mendel's Law. — Mendel's discoveries and conclusions con-
cerning the way parental characters appear in hybrids and their
offspring constitute Mendel's law, and his discoveries may be sum-
marized as follows: (1) characters are of two kinds, dominant and
recessive; (2) characters do not blend but behave as units and
separate completely from one another; (3) gametes are, there-
fore, pure, never containing genes for both of a pair of contrast-
ing characters; (4) the offspring of a hybrid consist of dominants
and recessives in the ratio of three dominants to one recessive,
and the recessives and one-third of the dominants breed true,
while two-thirds of the dominants breed as hybrids, producing
offspring consisting of dominants and recessives and again in
the ratio 3:1; (5) when any number of pairs of characters
are considered, each pair behaves independently, and all com-
binations of characters according to the mathematical laws of
combination can be obtained.

After eight years of work, Mendel published an account of
his remarkable discoveries, but unfortunately his publication
remained unnoticed until 1900, thirty-five years after its pub-

INVESTIGATIONS SINCE MENDEL 547

lication and sixteen years after Mendel's death. In 1900 his
paper was discovered simultaneously by three students of
genetics, Correns, De Vries, and Tschermak, who recognized
its importance. Since that time Mendel's law has formed the
basis of all work in genetics.

The Value of Mendel's Discoveries. — Mendel's discoveries
have completely revised our methods of investigating and ideas
concerning heredity. First, Mendel's discoveries have im-
pressed upon us the value of pedigree cultures in investigating
problems of heredity. Second, they afford us laws concerning
the appearance of characters in the offspring, whereby we know
what to expect and can thereby interpret results which were
previously a medley and not understandable. Third, knowing
how characters behave in the offspring when plants or animals
are crossed, we can start in our crossing work with definite
results to be obtained in mind and also plan a definite method
of procedure to obtain the desired results. Fourth, owing to
the discovery of the segregation of characters, we now know
that, in the second generation of hybrids, individuals that are
perfectly pure occur in definite proportions and that purity of
plants and animals in respect to a character does not depend
upon a long series of selections as was formerly the notion.
Fifth, the law of dominance explains why plants or animals
impure in respect to a character may appear just as pure as
pure individuals. Sixth, knowing that some characters are
recessive and are entirely obscured by the contrasting domi-
nant characters, we can now explain the appearance in the
offspring of a character which did not appear in the parents or
even for generations back, and in this way account for many of
the variations in offspring, such as talented offspring of medi-
ocre parents, blue-eyed children of brown-eyed parents, bad
sons of pious preachers, rust-resistant plants of plants suscep-
tible to rust, and so on. Seventh, Mendel's discovery that pairs
of contrasting characters behave independently of each other
but may be combined in various ways makes it possible for us to
improve plants and animals by breeding them in such a way as
to bring the desirable characters of different varieties together
in one individual.

Investigations since Mendel. — Since the discovery of Men-
del's paper, numerous investigators have been applying and

548

HEREDITY

FIG. 477. — Mendelism demonstrated in the inheritance of starchy and
non-starchy endosperm in Corn. In the top row, the ear c shows the immedi-
ate result obtained when the starchy parent (a) and non-starchy parent (6)
are crossed. It is evident that starchness is completely dominant, d, an
ear with F2 kernels resulting from the cross, showing; segregation of starchiness
and non-starchiness. Lower row, ears of plants grown from kernels of d. e, f,
and g, result from planting starchy seeds. One ear out of the three is pure
starchy, h, result from planting non-starchy kernels, showing that the non-
starchy kernels were pure for the recessive character. After East.

INVESTIGATIONS SINCE MENDEL

549

testing out Mendelism. In both plants and animals numerous
pairs of characters have been found to behave in accordance
with Mendelism. In plants alone more than 100 pairs of char-
acters of various kinds have been found to behave according
to the Mendelian conception. Among plants, color and shape of

FIG. 478. — Mendelism demonstrated in the inheritance of color in the
endosperm of Corn, c, ear bearing the F} kernels of the cross between a
(white endosperm) and 6 (yellow endosperm), showing dominance of yellow.
d, ear bearing F2 kernels of the cross, showing segregation of color. After East.

flowers; color, shape, size, and quality of fruit and seeds (Figs.
477 and 478); time required to mature; resistance to disease,
drought, and cold; and many others have been found to follow
Mendel's law. Among live stock Mendelism applies to numerous
characters, such as the presence or absence of horns in cattle, the
color of the hair in cattle and horses, the character of the comb

550

HEREDITY

and feathers in chickens, etc. In man many characters, among
which are insanity and susceptibility to tuberculosis, are known
to behave according to Mendel's law, and Eugenics, which has
to do with applying the laws of heredity in a way to produce a
healthier and a more efficient race of men, has its chief sup-
port in Mendelism.

Mendel's discoveries have already enabled us to make some
notable achievements in the way of improving plants and ani-
mals. By working according to the Mendelian conception,
many desirable varieties of the cereals, much more desirable

FIG. 479. — Height of plants in the F2 generation of 'Tom Thumb Pop
Corn (a dwarf Corn) crossed with Missouri Dent (a large Corn). The plant
at the extreme right is similar in height to the dwarf parent, while the one at
the extreme left is similar in height to the Missouri Dent. After Emerson and

East.

ornamental plants, and various kinds of better fruits have been
developed.

Of course in the extended investigations in Genetics since
1900, many situations have arisen that Mendel did not meet
Cases have arisen in which the Mendelian behavior can be
explained better by assuming that pairs of contrasting characters
are due to the presence and absence of certain factors and not to
dominant and recessive factors. According to the latter hypothe-
sis, the tallness of the tall variety of Peas is due to the presence
of a factor for tallness, while dwarfness in the dwarf variety is

INVESTIGATIONS SINCE MENDEL

551

due to the absence of the factor for tallness. Since the presence
and absence hypothesis explains more cases than the dominant
and recessive hypothesis, it has been generally accepted.

Again Mendel worked chiefly with qualitative characters, which
have been found to behave differently from most quantitative
characters, such as size and weight. For example, in crossing large
and small varieties of Corn, the individuals of the first hybrid gen-

FIG. 480. — Inheritance of length of ears in Corn. The ears Pi are ears
of the parent plants (Tom Thumb Pop Corn at the left and Purple Flint Corn
at the right) chosen to represent the average length of ears of parents. Notice
that the ear of the FI generation is intermediate in length between the paren-
tal ears, while in the Fz generation, as shown by the ears at the left and right
of the FI ear, the length of ears range from that of Tom Thumb Pop to that
of Purple Flint. After East.

eration are intermediate in size between the parents, the size of
neither parent dominating, and in the second hybrid generation
the individuals are of various sizes, ranging from that of the
smaller to that of the larger parent (Figs. 479, 4$0 and ^81).
At first such cases were considered striking exceptions to Men-
del's law. However, a more careful study has led to the view
that quantitative characters do mendelize but commonly depend
upon so many independent factors, each of which is responsible

552

HEREDITY

for a part of the character, that, although they do segregate and
combine according to Mendelism, they form so many kinds of
combinations and thus so many kinds of individuals occur in the
second generation of hybrids that it is difficult to detect Mende-
lian ratios. For example, in the case of crossing the tall and small
varieties of Corn, it is assumed that the tall variety has a number

FIG. 481. — Inheritance of size and beards in Wheat. Parent types
(Turkey X Bluestem at the left and a hybrid No. 143 at the right) and be-
tween the parent types the Fi generation of the Cross. In the FI generation
the heads are somewhat intermediate in size and have short beards. After
Gaines.

of factors for size that are not present in the small variety. Let
us suppose the large variety has four extra factors for size that
are not present in the small variety. These factors may be rep-
resented by A, B, C, and D. Since the tall variety is pure for
its height, its extra height over the small variety is due to the
presence of AABBCCDD, and the corresponding constitution of
the small variety is aabbccdd, in which the small letters represent

INVESTIGATIONS SINCE MENDEL 553

the absence of the extra factors for size. If the tall variety is
32 inches taller than the small variety, each of the factors A, B,
C, and D represents 4 inches in height. Now the gametes of
the tall variety have A BCD in them, while the corresponding
constitution of the gametes of the small variety is abed, and
the fertilized eggs and first generation of hybrids resulting
from the cross between these two varieties have the formula
AaBbCcDd. Now since each factor for height represented by
the large letters is responsible for 4 inches of height, the individ-
uals of the first generation of hybrids should be 16 inches taller
than the small variety but 16 inches shorter than the tall variety.
They are intermediate in height between the two parents. The
hybrids form gametes having the constitution A BCD, aBCD,
abCD, abcD, abed, AbCD and so on, involving all the combina-
tions that can be made with the four pairs of letters. In fertili-
zation all possible combinations of the various kinds of gametes
can take place, and consequently individuals of eight various
sizes can occur in the second generation of hybrids. Thus, if a
gamete with a constitution abed unites with a gamete with the
constitution abcD, the resulting offspring has the constitution
aabbccdD and should be 4 inches higher than the smaller variety
of the parent generation. If a gamete with the constitution
ABCD unites with a gamete having the constitution abCD, the
resulting offspring, which has the factors aAbBCCDD, should
be 24 inches higher than the smaller variety or 8 inches lower
than the taller variety of the parent generations. It is, there-
fore, obvious that due to the various kinds of combinations
that may occur among the gametes, individuals of various
sizes may occur in the second hybrid generation.

Another peculiar situation which has been discovered among
both plants and animals may be illustrated by the behavior of
color in the Andalusian fowl. These fowls are what fanciers
call blue, but when they are bred together the offspring con-
sist of black, blue, and white fowls, and the proportion is accord-
ing to the Mendelian ratio 1:2:1. The black and white fowls
breed true, but the blues breed as before. When black and
white fowls are crossed, blue fowls are obtained. The blue is
therefore a result of a heterozygous condition in which the
factor for black is combined with a factor for white. In this
case the hybrids may be regarded as having a different character

554 HEREDITY

from that of either parent. A similar situation has been dis-
covered in connection with the breeding of Sweet Peas. Cer-
tain white-flowered varieties of Sweet Peas when crossed produce
red-flowered offspring. There are still a number of other situa-
tions that Mendel did meet in his experiments.

To more recent investigators we are also indebted for some
present conceptions of the inheritable constitution of organisms.
Johannsen of Copenhagen, Denmark, who is responsible for the
pure line theory, has done much to establish the theory that
inheritance is due to the reappearance of the same organization
of protoplasm with reference to genes or character units in suc-
cessive generations and not to the transmission of external
characters. The sum total of all the genes in a gamete or fertil-
ized egg Johannsen calls a genotype, while an organism consid-
ered as to its appearance he calls a phenotype. Organisms may
be alike genotypically, that is, alike as to genes but be very
different phenotypically. For example, two Corn plants may
be exactly alike in their genes for size, yield, etc., but due to a
difference in environment differ greatly in these features. On
the other hand, organisms may be different genotypically but
be very similar phenotypically.

Segregation and the Reduction Division. — As previously
stated, the purity of gametes and the segregation of characters
depend upon the separation of the genes for contrasting char-
acters. A plant that is a hybrid for tallness and dwarfness can
not have gametes pure for tallness and dwarfness, unless the
genes for these contrasting characters are separated so as to
appear in different cells. The reduction division, which always
precedes the formation of gametes in both plants and animals,
affords a mechanism by which genes may be segregated. The
constant occurrence of the reduction division and also the fact
that it is the division in which chromosomes are separated,
suggest that it has some vital connection with heredity.

It is generally believed that the genes are associated with the
chromatin of the nucleus and are, therefore, distributed with the
chromosomes to new cells during cell division. The chromatin
of a plant or animal consists of the chromatin contributed by
each of its parents. At each cell division this chromatin is
organized into a definite number of chromosomes, and there is
considerable evidence that the chromatin of each of the parents

INVESTIGATIONS SINCE MENDEL

555

of the plant or animal whose cell is dividing organizes separately
into chromosomes, thus one-half of the number of chromosomes
being .composed of father chromatin and the other half being com-
posed of mother chromatin. This means that the chromosomes
contributed to the offspring by each of the parents maintain
their individuality in the offspring. In vegetative cell division
each chromosome splits longitudinally, and to each new nucleus
there is contributed a half of each chromosome. It is obvious

FIG. 482. — A diagram illustrating the behavior of chromatin in the reduc-
tion division. For convenience the chromatin contributed by the father of
the plant, the division of whose cell the diagram illustrates, is shown black
and the chromatin contributed by the mother plant is shown white. In the
upper line, organization of the chromosomes and their pairing, each pair con-
sisting of one father and one mother chromosome; in the lower line, the dis-
tribution of the chromosomes in the formation of the daughter nuclei. In
this case one of the daughter nuclei receives one father and three mother
chromosomes, while the other daughter nucleus receives one mother and three
father chromosomes, but this is only one of a number of ways of distributing
the chromosomes.

that the vegetative cell division tends to distribute the chromatin
from both parents equally to the new nuclei. But in the reduc-
tion division, as shown in Figure 482, whole chromosomes and
not halves are contributed to each new nucleus, and conse-
quently the new nuclei resulting from the reduction division
receive only half as many chromosomes as the mother cell con-
tained. In the reduction division the chromosomes contributed
to the daughter nuclei may be only those of the mother parent or
only those of the father parent, in which case the daughter nuclei

556 HEREDITY

receive only the genes of one of the parents. OH the other
hand, the daughter nuclei may receive chromosomes of both
parents and in different proportions in different divisions. Again
cytological studies of the reduction division show that there
is a pairing of chromosomes previous to their separation, and
there is evidence that each pair consists of a father and a
mother chromosome. Now, if we assume that chromosomes
pairing carry genes for contrasting characters, then the separa-
tion and distribution of the members of each pair to different
daughter nuclei should result in the segregation of genes for
contrasting characters and in the production of pure gametes.
The trouble with this assumption is that a plant or animal has
so many more pairs of contrasting characters than chromo-
somes, that it is difficult to explain the numerous combinations
that occur when many pairs of contrasting characters are taken
into account. Despite the fact that there are some things
about segregation we are unable to explain by the mechanism
of reduction division, it is generally believed that the two phe-
nomena are vitally related.

The Mendelian Ratio and the Combinations of Gametes. -
It is possible to account for the Mendelian ratio 1:2:1 by
taking into account the probable combinations that may occur
among gametes during fertilization. A hybrid forms two kinds
of gametes equal in number in respect to a pair of contrasting
characters. One kind of sperms and eggs may be represented
by A and the other by B. Now the probable combinations
between the two kinds of sperms and two kinds of eggs in the

A \ /E

self-fertilization of a hybrid are represented by T /\ £ • There

are two chances for A and B to unite to one chance for A to
unite with A or B to unite with B. The probable combinations
and their ratios are,- therefore, A A :2AB : BB or 1:2:1.
If the factors represented by either A or B are dominant, then
3 : 1 is the ratio of the dominant to the recessive offspring.

CHAPTER XXIV
PLANT BREEDING

Plant breeding has to do with the improvement of old plants
and the securing of new ones. Many and various are the aims
of plant breeding. The object may be to improve the yield,
increase the resistance to drought or disease, shorten the period
of development, or secure strains or varieties with new chara-
ters. The two important methods used are selection and hy-
bridization.

In connection with plant breeding, the discoveries of De Vries
and Mendel have proven to be of inestimable value. The dis-
coveries of De Vries have resulted in a better understanding
of the nature of variations and has enabled us to improve plants
by selection much more efficiently. The introduction of Men-
del's methods of investigation and his discoveries concerning
the behavior of characters in hybrid offspring afford a scientific
foundation to the improvement of plants by hybridization. As
a result of Mendel's contributions, we now know much more
about how to proceed, what to expect, and how to interpret the
results obtained in hybridizing.

Selection. — Selection takes advantage of variations. Vari-
ations, as previously noted, are not only due to differences in
gametes and their combinations in fertilization, but also to
differences in temperature, moisture, soil conditions, and other
environmental factors. But only those variations due to factors
which can be transmitted by the gametes are inheritable through
the seed. A Corn plant may be larger or bear larger ears than
the ordinary type because of especially favorable conditions.
It may stand alone in the hill, thus having all of the water and
mineral supply to itself, or it may be growing on ground more
heavily fertilized. This increased size, due to external condi-
tions and not to any special factors in the cells of the plant,
is not transmitted to the offspring, and the embryos in the
kernels of this especially favored plant may have inherited no

557

558

PLANT BREEDING

more for size than the embryos developed by a much smaller
plant which has grown in less favorable conditions. To com-
pare plants as to what is really in their cells, the conditions
under which the plants were grown must be considered. It is
for this reason that it is much better to select seed Corn from the
field than to select it from the crib; for by the first method the
conditions under which the plants developed
can be considered and their genetic consti-
tution better estimated.

In addition to Johannsen's investigations
of inheritance in Beans and a number of
other plants, there are many other experi-
ments that tend to establish the fact that
little is gained by selections in pure lines,
unless mutations happen to occur. For
example, as previously cited a long series of
selections in a pure line of Oats to increase
the yield secured no results. Likewise selec-
tions in pure lines of Wheat to intensity and
fix a desirable variation have given no re-
sults. In each generation plants having the
desirable variation were selected for seed,
but selections carried on this way for a
number of generations did not increase the
average of the variation.

In vegetative propagation, where the pro-
geny does not develop from seed but grows
from vegetative structures taken from the
parent, some variations, such as bud sports,
that are often not inheritable through seed
can be perpetuated. Such is true in such
plants as Strawberries, which are propagated
by runners, and in fruit trees, which are
propagated by grafting. But modifications that are simply
responses to some peculiarity in the environment are not per-
petuated even by vegetative propagation.

Although selection often fails to produce the desired results,
nevertheless, selection is one of the chief means of improving
plants. A mass of plants, such as a field of grain, is a mixture
of individuals most of which are heterozygous for one or more

FIG. 483, — Heads
of Wheat, showing
improvement in size
as a result of a five-
year selection in
which the plants
bearing the largest
heads were selected
each year for seed.
Redrawn from De
Vries.

MASS CULTURE

559

characters. By selecting, through a number of generations, the
most desirable individuals for seed, eventually individuals that are
homozygous and breed true to the desirable character may be
secured, and a better race of plants thereby established. Muta-
tions also afford considerable opportunity to improve plants by
selection. In case individuals appear that are more desirable due
to a mutation, a more desir-
able race of individuals is
immediately secured by
selecting and propagating •
these mutants. Through
selection, races of plants much
more desirable than the ordi-
nary types from which the
plants were selected have
been produced. Grains, for-
age crops, Strawberries,
Blackberries, Melons, fruit
trees, etc., have been im-
proved by selecting and prop-
agating those plants having
more desirable features than
the ordinary types. In this
way plants have been im-
proved in yield (Figs. 4$3 and
4$4), ability to resist drought
and disease, length of period

required for maturing (Fig. 485), and many other ways. There
are two methods used in improving plants by selection, the mass
culture and pedigree culture.

Mass Culture. — This is the oldest method of plant breeding.
This method employs large masses or fields of plants, known as
mass cultures, from which to select. For example, in applying
this method to the breeding of small grains, the plant breeder,
desiring to produce a better yielding race, goes through the field
and selects those plants with heads having the largest number of
grains. From the next year's crop grown from the seed of the
plants selected the previous year, the best yielding plants are
again selected for seed, and year after year he continues to select
until a race more or less constant for high yield is obtained.

FIG. 484. — Heads of Timothy, show-
ing improvement by selection. After
Hays.

560

PLANT BREEDING

This method of selection is productive of good results, but
has some disadvantages. It requires much time and labor as
well as the use of much ground. Since each crop is grown
from seed furnished by many plants selected the previous year,
the progeny of many plants are involved and the yield of a crop

FIG. 485. — Dakota Amber Sargo, a strain that matures much earlier and
is more drought-resistant than the South Dakota No. 341 from which this
new strain was produced by selection. After Dillman.

is the average yield of the descendants of many plants varying
in capacity and heritage for high yield. Many of the plants
in the selection are likely to be heterozygous for the character
and consequently will not breed true. Races obtained by this
method of selection usually lose their desirable features unless
selection is continued.

Pedigree Culture. — The value of pedigree cultures was well
demonstrated by De Vries and Mendel. In the method of
selection by pedigree cultures, a single plant is selected, and
from its progeny, which are carefully guarded, the best indi-

HYBRIDIZATION 561

viduals are selected. After continuing the selection for a few
generations, a race with a certain standard and steadiness is
obtained. The race obtained by this method of selection is
the progeny of a single individual, and its desirable features are
more stable than in most races secured by mass selection. This
method also requires less labor and usually less time than the
method of mass culture. After the race is secured by pedigree
culture, it is usually tested in mass culture to see how it behaves
when grown in masses under ordinary field conditions.

Selection of Mutants. — Many valuable races of plants have
been discovered accidentally and apparently have arisen sud-
denly. The Fultz Wheat comes from a few plants which were
accidentally discovered growing in a field of Lancaster Red.
These few plants, which were smoother and had more beautiful
heads than the Lancaster Red, were saved for seed, and from
these seeds the well-known and valuable race of Fultz Wheat
originated. The Gold Coin Wheat was accidentally found
growing in a field of Mediterranean Wheat. There are a num-
ber of varieties of Wheat, Oats, Barley, and Rye which appar-
ently originated in a similar way.

Many or all of the different cultivated varieties of Dewber-
ries were accidentally found growing wild and were selected
because they showed some desirable features not possessed by
the ordinary type of wild Dewberries. Some may be hybrids,
while others are most likely mutants.

In woody plants, such as fruit trees, the selection of vegeta-
tive mutations known as bud sports, in which a branch may
produce a type of fruit different from the fruit produced by
other branches, often leads to the establishment of new varieties.
By propagating these special branches by grafting, a different
type of tree may be obtained. The Nectarine has already been
mentioned as arising in this way, and there still are other ex-
amples among Peaches, Apples, and other fruits. Greening
Apples often have branches bearing Russet Apples, and Russet
Apples often have branches bearing Greening Apples. There
are, therefore, many instances in which selection has not only
resulted in the securing of better grains, vegetables, fruits, and
ornamental plants, but also in new types.

Hybridization. — The advantage of hybridization is that by
crossing one can combine in the offspring the different desirable

562 PLANT BREEDING

features of the two plants used in the cross. Since Mendel's
discoveries have furnished principles that make it possible to
interpret the behavior of hybrids, one can proceed with consid-
erable certainty. As to just how the factors introduced by the
sperm and egg will manifest themselves in the offspring resulting
from a cross is not known until the offspring appear. The char-
acters, whether they blend or behave as dominants and recessives,
are identified only by observations of the hybrid generations. The
hybrid may be like one parent in some features and like the other
parent in other features, or in size and some other characters it
may be intermediate betwsen the two parents. According to
Mendel's law, we can expect three kinds of individuals in the
F2 generation when one pair of contrasting characters is con-
sidered, and that the pure dominants and pure recessives will
breed true whether one or many pairs of characters are taken
into account. The pure dominants and pure recessives can be
identified by further breeding, and if they prove to be more
desirable than the varieties used in the cross, then by propa-
gating them a more desirable race or variety is established.
In case one wishes to bring together in one individual a number
of desirable characters, some of which are present in one variety
and some in another, the breeding process to obtain the indi-
viduals pure for these characters is complex, as was shown in
the discussion of Mendelism, and the more factors involved, the
more complex is the process. But Mendel's law points the
way of procedure, and it is possible for the patient plant breeder
to so manipulate the breeding through a number of generations
as to finally obtain a combination of the desirable characters in
an individual that will breed true. The desired individual hav-
ing been secured, the new race or variety is practically estab-
lished. Much has been accomplished in improving plants
through hybridization. For example, in this way a much more
desirable race of Wheat has been obtained in England. One
variety of English Wheat, yielding well but producing a poor
grade of flour, was crossed with a variety of Canadian Wheat,
which produces a good grade of flour but does not yield so well
in the English climate as the English variety. The plant breeder
finally succeeded in getting a race having the desirable features
of producing good flour and yielding well in the English climate
By crossing Wheat, having some desirable qualities but sus-

HYBRIDIZATION

563

ceptible to Rust, with Wheat, immune to Rust but less desirable
in other features, a type of Wheat having the Rust resistance
of one parent and the desirable features of the other has been
obtained. Cotton producing longer and better lint has been
obtained by crossing the Sea Island Cotton with the Upland
Cottons. There are many instances in which more desirable
races have been secured through hybridization.

FIG. 486. — The effect of three degrees of relationship in breeding Corn.
Nos. 3 and 4 are pure strains from seed-stock inbred for three years. No. 2
is from a close-fertilized seed-stock, the plants each year being fertilized with
pollen from sister plants grown from the same ear. No. 1 is from seed-stock
that has been cross-fertilized for three years. After Montgomery.

The greatest advantage arising from hybridization is among
plants propagated by vegetative methods, as by tubers, bulbs,
cuttings, layering, grafting, etc.; for in these cases the progeny
is simply a continuation of the hybrid individual and not the
result of the fusion of gametes. Many berries, vegetables, fruit
trees, and ornamental plants are hybrids. By crossing different
kinds of Strawberries, hybrids more desirable than either of the
parents have been obtained, and since they propagate by run-

564

PLANT BREEDING

ners, the hybrid type is maintained generation after generation.
Blackberries, which are propagated vegetatively, have been im-
proved by hybridization. By crossing the cultivated Black-
berry, which has a large black fruit, with a small wild Blackberry,
having a whitish or cream colored fruit, a Blackberry having a
fruit large in size and light in color has been obtained. Many

FIG. 487. — Results of inbreeding and crossing on the size of ears in Corn
Outer ears, result of inbreeding one generation; middle ear, result in the firs
generation of crossing these inbred generations. After East.

•

of the best Plums and other fruit trees are hybrids, and the
hybrid characters are retained by propagating the trees by graft-
ing and budding. Hybrids are also common among Roses, Car-
nations, and other ornamental plants.

Crossing and Vigor of Offspring. — Crossing usually results
in increased vigor, while self-fertilization commonly results in

CROSSING AND VIGOR OF OFFSPRING

565

the loss of vigor in the offspring. Hybrids are usually more
vigorous than their parents. Corn grown from seed resulting
from self-fertilization shows much loss in vigor and consequently
does not yield so well (Figs. 1*86 and 487). The difference in
yield between plants resulting from crossing and plants resulting
from self-fertilization often amounts to several bushels per acre.

FIG. 488. — Increase in size of fruits in Cucumbers as a result of crossing.
a and c show size of fruits borne by the parents and 6, the size of fruits borne
by the first generation of the cross. After Halsted.

Darwin found that Cabbage plants obtained by crossing were
nearly three times the weight of those obtained by self-fertiliza-
tion. In Buckwheat Darwin obtained plants much taller and
about one-fifth better in yield by crossing. In Lettuce, Beets,
Pumpkins, Squashes, Tomatoes, and many other plants (Fig.
488), it has been shown that crossing produces more vigorous
offspring.

INDEX

(The numbers with stars refer to the pages on which the illustrations are given.)

Absciss layer, 184.

Absorption of water, by seeds, 91 ; by

roots, 158; factors that hinder,

159; selective, 159.
Absorptive zone, 143, 144*.
Abundance and Distribution of

plants, 5.

Abutilon Theophrasti, 487.
Accessory buds, 206.
Aceraceae, 487.
Achene, 60, 79, 60*.
Achillea millefolium, 24*.
Acids, 120; malic, 285; oxalic, 285;

citric, 285; tartaric, 285.
Aconite, 482.
Acorn, 80, 83*.
Actinomyces chromogenus, 348, 344*,

347*.

Active buds, 211.
Adventitious buds, 206, 207*.
Adventitious roots, 140.
Aecidiospores, 398, 399*.
Aerial stems, 172.
Aerobic Bacteria, 343.
Aerotropism, 152.
After-ripening, 69.
Agarics, 385, 385*, 386*.
Agaricus campestris, 385, 386*.
Agave, 498, 498*.
Aggregate fruits, 79*, 80.
Agropyron repens, 74.
Air chamber, 249.
Air, in the soil, 153.
Air roots, 162.
Albugo, 359.
Albumins, 281.
Albuminous seeds, 60.
Aleuron layer, 64, 65, 66*, 280*.
Alfalfa Dodder, 489.

Alfalfa, seeds, 74, 87, 74*; fruits, 82;

stem, 193*, 194*; plant, 209*;

leaf, 238*.
Algae, 296.

Blue-green, 297-301.
Green, 301-318.
Brown, 318-324.
Red, 324-329.
Alkaloids, 281, 283.
Allelomorphs, 543, 543*; dominant,

543; recessive, 543.
Alternate arrangement of leaves, 238,

239*, 240*.

Alternation of generations, 412.
Amanita bulbosa, 384*.
Amaranthus albus, 84*.
Amino acids, 120.
Amino compounds, 281.
Amygdalase, 285.
Amygdalin, 282.
Anabolic metabolism, 273.
Anaerobic Bacteria, 343.
Anaerobic respiration, 122.
Analysis of seeds, 74.
Anatomy, described, 2; of leaves,

242-252; of root tip, 143; of the

older portion of the root, 147;

of stems, 182-203.
Anatropous ovule, 463.
Andreales, 417.
Angiosperms, 293, 445; described,

459; life cycle, 469*; classifica-
tion, 471.
Annuals, 171.

Annual rings, 197, 201, 198*, 202*.
Annular vessels, 188, 189*, 190*,

195*.

Annulus, 386, 429, 385*, 386*, 430*.
Anther, 41, 41*.
567

568

INDEX

Antheridium, 322, 326, 328, 410, 312*,
316*, 333*, 411*, 419*, 432*.

Anthoceros, 416, 416*.

Anthocerotales, 407.

Anthocyan, 284.

Antitoxins, 123.

Apetalae, 473.

Apetalous flower, 13, 11*.

Apical meristem, 126*, 183*.

Apophysis, 422.

Apothecium, 366, 367*, 382*.

Apple, flower, 13*; cyme, 31*; apple
pollen, 50; type of fruit, 78; sec-
tion of twig, 199*.

Apple Blotch, 404.

Apple Rust, 401, 400*, 403*.

Apple Scab, 378.

Archegonia, 409, 410*, 419*, 432*,
449*.

Archichlamydeae, described, 472;
families, 473.

Arctium Lappa, fruits, 86*.

Arginin, 281.

Aril, 56, 57*.

Arisaema triphyllum, 26*.

Arrangements, of leaves, 238.

Ascus, 364, 368*, 382*.

Ascocarp, 365.

Ascogenous hyphae, 368, 376, 367*.

Ascogonium, 368.

Ascomycetes, 353, 363.

Ascospore, 364, 366*.

Asparagus, 160, 271.

Asparagus Rust, 403.

Aspergillus, 375, 375*.

Associated plants and animals, 503.

Atropine, 283.

Auricles, 235.

Auxanometer, 215, 216*.

Auxograph, 215.

Auxospore, 332.

Available water, 160.

Awn, 22.

Bacillus subtilis, 343*.

Bacteria, 155, 156, 289, 336, 157*;
described, 341; coccus forms,
341, 342*; bacillus forms, 341,

342*; spirillum forms," 341, 342*;

reproduction, 343; of decay, 344;

of fermentation, 345, 345*; of

nitrification, 345, 346*; patho-
genic, 346, 347*.
Balm of Gilead, 474.
Banner, 24.
Barberry, 396, 399*.
Bark, 127; described, 199.
Basidium, 382, 386*.
Basidiomycetes, 352; described, 382.
Basidiospore, 383, 386*, 399*.
Basswood, flower, 10*; fruits, 83*.
Bast fibers, 115, 129, 129*, 193*, 196*,

198*.
Bean, family, 23; seed, 56*, 57*; type

of seeds, 58; seedling, 106*; type

of seedling, 107.
Beech and Oak family, 476; flowers,

476*.

Beggar-ticks, fruits, 86*.
Berberis vulgaris, 396, 399*.
Berry, 77, 77*.
Betulaceae, 475.
Bidens, fruit, 86*.
Biennial, 171.

Bindweeds, 489*; described, 490.
Biometry, 538.

Birch family, 475; flowers, 475*.
Bird's nest Fungus, described, 391,

392*.

Bitter Rot of Apples, 378, 379*.
Blackberry, fruit, 79.
Black Fungi, 369.

Black Knot, 222; described, 369, 370*.
Black leg, 348.

Black oak, section of stem, 200*.
Black-rot of cabbage, 347.
Black-rot of Grapes, 378.
Black-rot of Sweet Potato, 404.
Black Rust of grain, described, 396,

397*, 398*, 399*.
Black Walnut, 474.
Bladder Plums, 377.
Blade of leaf, 234; described, 235:

236*, 237*.

Bleeding of plants, 161.
Blister Rust of Pines, described, 402.

INDEX

569

Blotch of Apples, 404.

Blueberries, 489.

Blue-green Algae, described, 297; re-
production, 300.

Blue Mold, 375; described, 376.

Boletus, 387*.

Boneset, 493.

Bordeaux mixture, 359.

Bordered pits, 130, 131*.

Botany, definition, 1; derivation, 1.

Botrychium Virginianum, 434*; de-
scribed, 435.

Botrydium, 317, 317*.

Brace roots, 139.

Bracket Fungi, 388, 389*, 390*.

Bract, 17, 19*.

Bracted Plantain, seeds, 75*.

Bracts of Grass flowers, 17, 19*, 21*,
22*.

Branching, of roots, 150; of stems,
167.

Brand spores, 394, 394*.

Brassica nigra, 483*.

Brassica Sinapistrwn, 74.

Bread Mold, 360, 362*, 363*.

Bread Yeast, 377, 377*.

Bromelin, 284.

Brown Algae, 296; described, 318.

Brown Rot, 368.

Bryales, 417, 418*.

Bryophytes, 292; described, 405;
groups, 405.

Buckwheat family, 478; flowers, 479*.

Buckwheat, flower, 133, 11*; type of
seeds, 59; fruit, 60*; plants, 158*.

Bud, described, 167, 204*, 205*; rest-
ing, 204, 204*; opening, 205;
terminal, 206, 206*; lateral, 206,
206*; contents, 207, 213*; axil-
lary, 206, 206*; accessory, 206,
206*; adventitious, 206, 206*,
207*; formation, 208; flower,
207, 210*; leaf, 207, 210*; mixed,
207, 210*; active, 211; dormant,
211.

Budding, 225; described, 231, 232*;
of Yeast, 377, 377*.

Bulb, 179; described, 181, 181*.

Bundle sheath, 252.
Bunt or Stinking Smut, 393, 395.
Burdock, 493; fruits, 86*.
Buttercup family, 481.
Butternut, 474.

Button stage of Mushroom, 385,
385*.

Caffein, 283.

Calcium, 157, 158*.

Callus, 224, 225*.

Calyx, 11, 10*, 11*, 12*.

Cambium, 126, 126*, 149*, 194*, 195*,

196*, 198*.
Cambium ring, 148.
Camphor trees, 482*.
Campylotropous ovules, 463.
Canada Thistle, 82, 165, 494, 503,

494*.

Capillitium, 337, 337*.
Capillary water, 153.
Carbonic acid, 161.
Carbohydrates, 277.

sugars, 277.

starches, 278, 278*.

hemi-cellulose, 278, 279*.
Carica papaya, 284.
Cariopsis, 62.
Carnations, 10.

Carnivorous plants, 273, 272*, 273*.
Carotin, 284.
Carpels, 15, 14*.
Carpogonium, 327, 326*, 328*.
Carpospore, 327, 326*, 328*.
Caruncle, 57, 57*.
Caryophyllaceae, 480.
Castor Bean, seed, 57; seedling, 108;

use, 486.
Castor oil, 279.
Catabolic metabolism, 275.
Catkin, 30, 473, 29*, 473*.
Cat-tail family, 495; flowers, 495,

495*, 496*.

Causes of variations, 531.
Cedar Apples, 401, 401*, 402*, 403*.
Cedar Rust, 401, 400*.
Cells, 39, 96; position in plant life,
112; discovery, 112; structures,

570

INDEX

112, 113, 114, 114*, 115*; pres-
sure within, 118; size, 112.

Cell activity, processes involved in,
116.

Cell division, described, 124, 125,
413, 125*.

Cell membrane, 114, 115*; character
after death, 119.

Cell multiplication, 123.

Cell sap, 114.

Cell wall, 114; described, 115.

Cellular anatomy, of root tip, 143.

Cellular structure, of leaves, 246.

Cellulose, 115, 275, 276.

Century Plant, 498, 498*.

Chaparral, 509.

Charales, 332, 333*.

Charafragilis, 333*.

Chemotropism, 152.

Chenopodiaceae, 480.

Cherry Birch, flowers, 475*.

Chestnut, sprouts, 208*; disease, 271,
272*; nuts, 476.

Chimera, 230.

Chlamydomonas, 303, 304, 303*.

Chlamydospore, 394, 394*.

Chlorenchyma, 194, 249, 250, 193*,
251*; palisade tissue, 249, 250*;
spongy tissue, 249, 250*, 251*.

Chlorophyceae, 301.

Chlorophyll, 115, 294, 115*.

Chloroplast, 115, 247, 249, 251, 247*.

Chondrus crispus, 325*.

Chondriosomes, 115.

Chromatin, 114, 114*.

Chromosome, 124, 125*.

Chrysanthemums, 10.

Cicuta macidata, 283, 488.

Cion, 227, 231, 232*.

Circinate vernation, 429.

Citric acid, 285.

Citrus fruits, 27, 501.

Cladophora, 312.

Cladophyll, 169, 170*.

Classes, 291.

Classification, of plants, 291.

Clavaria, 387*.

Cfaviceps purpurea, 370, 371*.

Cleft grafting, 232.

Cleistothecia, 373, 374*, 375.

Clematis viticella, 186*.

Climbing stems, 175.

Closed bundles, 191, 191*.

Closed Fungi (Pyrenomycetales), 364;

described, 369, 370*, 371*.
Close-pollination, 48.
Clover Dodder, seeds, 75*.
Club Mosses, 438.

Club Root of Cabbage, 339; de-
scribed, 340, 340*.
Coconut oil, 279.
Coenocyte, 316, 316*.
Coffee tree, 492, 493*; flowers and

berry, 492, 493*.
Coleochaete, 313, 313*.
Coleochaete scutata, 313*.
Coleoptile, 63, 102, 63*, 102*.
Coleorhiza, 63, 63*.
Collenchyma, 128*; described, 129,

193*.
Color, of flowers, 10; inheritance in

endosperm of corn, 549*.
Columella, 416.
Column, 26.
Comfrey, fruit, 86*.
Common Barberry, 396.
Common Bean, seedling, 107, 106*.
XDommon Cabbage, flowers, 28*;

plants, 172*, 528*.
Companion cells, 131, 190*, 193*,

195*.

Complete flowers, 13.
Composite family (Compositae), 492,

494*.

Composite flower, 24.
Compound leaf, 237, 237*, 238*.
Compound pistil, 15.
Conceptacle, 323, 322*, 407*, 411*.
Conditions affecting transpiration,

263.
Conductive tissues, 130; of leaves,

244.
Confervoid Algae (Confervales), 302;

described, ^310.
Conidia, 356, 394, 357*, 360*, 373*,

375*.

INDEX

571

Conidiophores, 356, 357*, 359*, 360*,
373*, 374*, 375*.

Conidiospores, 356, 357*, 359*, 360*.

Coniferyl alcohol, 282.

Coniferin, 282.

Conin, 283.

Conium maculatum, 283, 488.

Conjugation, 302; in Bread Mold,
361, 363*.

Conjugating Algae (Conjugales), 302;
described, 314.

Convolvulaceae, 490.

Core, of apple, 78*.

Cork, 128, 199, 199*.

Cork cambium, 147, 199, 147*, 185*,
199*.

Cork Oak, 476, 477*.

Corky bark, 197, 198*.

Corky rind, 127, 127*, 128*.

Corm, 179; described, 182, 182*.

Corn Cockle, 74, 481.

Corn, life cycle, 7*; plant, 16*;
flowers, 17-20, 17*, 18*, 19*;
pistil, 34, 36; stigma, 43*; devel-
opment of seed, 44*; embryo sac,
40, 39*; effect on ear of cross-
pollination, 51; kernel, 62, 63*,
64*; seedling, 102, 103, 101*; fi-
brousroots,138; growth of radical,
145*, 151*; section of seedling,
183*; cross section of stem, 187,
187*, 188*; vascular bundle, 188,
189*, 190*; leaves, 236*; section
of leaf, 267*; oil, 279; smut, 395,
395*.

Corn Smut, 395, 395*.

Corolla, 11, 10*, 12*.

Cortex, 132, 145, 146*, 160*, 188*,
192, 194*, 198*.

Corymb, 28, 30, 30*, 32*; compound,
32*.

Cotton, flower, 14*, 33*; seed, 59,
59*; plant, 487, 487*.

Cotton-seed oil, 279.

Cotyledon, 56, 56*, 63*, 101*, 105*,
106*, 107*, 108*, 109*, 466*

Cow Cockle, 74.

Cow-herb, 481.

Covered Smut of Barley, 395.

Crab Apple, 210.

Cronartium ribicola, 402.

Cross-pollination, 48; compared with
self-pollination, 52, 53.

Crossing and vigor of offspring, 564,
563*, 564*, 565*.

Crowfoot family, 481, 482; flower,
481*, 482*.

Crown, of Red Clover, 109, 109*.

Crown Gall, 349, 349*.

Cruciferae, 482.

Cucumber, section of fruit, 79, 78*.

Cucurbitaceae, 492.

Culture of small fruits, 164.

Cup Fungi (Pezizales), 364; described,
366, 367*.

Cup plant, 236*.

Cup spores, 398.

Curled Dock, seeds, 75*, 76*.

Cuticle, 246, 250*.

Cutin, 245, 276.

Cuttings, 225, 226*, 228*, 229*,
230*.

Cyanophyceae, 297.

Cycad, 445; described, 446-451, 446*.

Cycas revoluta, 446.

Cyme, 28, 31*; scorpoid, 32*; corym-
bose, 32*; typical, 32*.

Cypripedium, 499*.

Cystocarp, 327, 326*, 328*.

Cytase, 285.

Cytisus Adami, 231.

Cytoplasm, 114, 327, 326*, 328*.

Cytology, 2.

Dakota Amber Sargo, 560*.
Dandelion, flowers, 24, 24*; rosette,

241, 241*.
Dangers resulting from transpiration,

265.
Darwin, Erasmus, 514; Charles, 516,

517*.
Darwin's explanation of Evolution,

516.

Darwinism, objections to, 522.
Date seeds, hemi-cellulose in, 279.
Datura Stramonium, 491, 490*.

572

INDEX

Deciduous trees, 268; forests, 508,
509*.

Deliquescent stem, 168, 168*.

Demonstration of osmosis, 116, 116*.

Depth of root systems, 141.

Desmids, 314, 314*.

Destructive Toadstools and Bracket
Fungi, 388.

Determinants, 536.

Development, of ovule into a seed,
43, 43*, 44*, 45*; of Bean seed-
ling, 107, 106*; of corn seedling,
101*, 103*; of Clover seedling,
109, 109*; of embryo in Angio-
sperms, 466, 466*, 467*; of Onion
seedling, 107, 105*; of ovule and
pollen tubes in Pine, 455-457,
455*, 457*; of wheat seedling,
104*.

De Vries, Hugo, 524, 524*.

Dextrose, 277.

Diadelphous stamens, 15, 14*.

Diagrams of inflorescences, 32*.

Diatoms, 231, 231*, 232*.

Dichotomous branching, 429.

Dicotyledons, 170; herbaceous stems,
192, 192*, 193*, 194*, 195*, 196*;
woody stems, 197, 198*, 199*,
200*, 201*, 202*; venation of
leaves, 245, 471, 244*.

Diffusion, of liquids and gases, 95.

Dionaea muscipula, 132, 273*.

Direction of growth in roots, factors
influencing, 150, 151*.

Dioon, staminate strobilus, 447*.

Discovery of the cell, 112.

Dissemination of seeds and fruits, 82;
by animals, 84, 85*, 86*; by
explosive or spring-like mechan-
isms, 87, 87*; by water, 84; by
wind, 82, 83*, 84*.

Distribution of plants, 5.

Diversity of plant forms, 5.

Divisions, in classification, 291; of
Fungi, 252.

Dock, 59, 60, 128*.

Doctrine of Special Creation, 514.

Dodder, 59, 163, 163*, 489.

Dogbane, 133.

Doll rag germinator, 99, 99*.

Dominance, law of, 543; impure, 545;

pure, 545.

Dominant characters, 543.
Dormant buds, 211.
Downy Mildews, 355; of Ginseng,

358, 361*; of Grapes, 355, 355*,

356*; of Potatoes, 357, 257*,

358*, 359*, 360*.
Double fertilization, 43, 465, 465*.
Drosera, 132, 272*.
Dry plain societies, 509.
Duckweeds, 9, 505.
Dutchman's Breeches, 170.

Earthstar, 391, 391*.

Ecbalium Elaterium, 87*.

Ecology, plant, 2; nature, 500.

Ecological factors, 501.

Ecological societies, 504.

Economic Botany, 3.

Ectocarpus, 320, 321*.

Edible Boletus, 387*.

Elaboration of foods into plant
structures, 95.

Elaters, 411.

Elm, 9; family, 478; tree, 168*, 176*.

Embryology, 2.

Embryo sac, 39; of Corn, 40, 39*, 44*;
of oats, 40, 40*, 45*; of Red
Clover, 40, 38*, 42*, 45*; of To-
mato, 43*.

Embryo, development from fertilized
egg, 43, 44, 44*, 45*; of Apple
and Squash, 59*; parts of Bean
embryo, 56, 56*; of Corn, 44*;
parts of corn embryo, 63, 63*,
64*; of Cotton, 59, 59*; dicotyle-
donous type, 466, 466*; function,
55; monocotyledonous type, 466,
467*; of Oats, 45*; of Potato and
Buckwheat, 60*; of Red Clover,
45*; of tomato, 43*.

Endodermis, of roots, 147.

Endogenous development, 197.

Endophytes, 297.

Endothia parasitica, 371, 372*.

INDEX

573

Endosperm, 40; in Bean type of seeds,
58, 59*; of Buckwheat and Flax
type of seeds, 60, 60*; of Corn,
44; of Cycads, 449, 449*; in Grass
type of seeds, 62; in hybrid Corn,
51, 51*; kinds in Corn, 63, 64,
63*; of Oats, 45; of Shepherd's
Purse, 466*; of Tomato, 43*; in-
heritance, 548*, 549*; location
in seed, 58, 60, 468.

Endosperm nucleus, 40, 38*, 39*, 40*.

Envelopes of the flower, 11.

Enzymes, 93, 284.

Epidermis, as an absorptive structure,
145, 146*; cellular structure,
246*, 247*, 250*; as a protective
structure, 127, 127*; of leaves,
245; modifications for protection,
266, 266*.

Epigaea, 490.

Epigynous flowers, 15, 15*.

Epiphytes, 163.

Equisetales, 435-438.

Equisetum arvense, 435, 436*, 438;
palustre, 435; pratense, 435.

Erect stems, 172.

Ergot, 369, 370, 371*.

Ericaceae, 489.

Essential organs of the flower, 11.

Etiolation, 220.

Evolution, plant, 2, 290; meaning and
theories, 513-524; experimental,
524-535.

Euglena gracilis, 230, 230*.

Euphorbiaceae, 486, 486*.

Eusporangiates, 430.

Excretions of roots, 161.

Excurrent stem, 168, 168*.

Exogenous stems, 197.

Explosive mechanisms of seeds, 87,
87*.

Exposure of leaves to light, 237.

Factors, ecological, 501-504; in-
fluencing the direction of growth
in roots, 150, 151*; influencing
photosynthesis, 257-260; that
hinder absorption by roots, 159.

Families, in classification, 291; Cat-
tail, 495; Composite, 492; Beech
and Oak, 476; Birch, 475; Buck-
wheat, 478; Buttercup, 481; Elm,
478; Goosefoot, 480; Gourd, 492;
Grass, 495; Heath, 489; Lily,
498; Madder, 492; Mallow, 487;
Maple, 487; Mustard, 482; Night-
shade, 491; Orchid, 494; Palm,
497; Parsley, 488; Pea, 484;
Pink, 480; Rose, 483; Spurge,
486; Sweet Potato, 490; Walnut,
474; Willow, 473.

Farm Crops, 1, 4.

Fascicled root system, 140, 140*.

Fats, 277.

Fatty oils, 279.

Female gametophyte, of Angio-
sperms, 463, 464*, 465*; of
Cycads, 448, 449*; of Equise-
tum, 438, 438*; of Pines, 455,
455*; of Selaginella, 442, 443*.

Fermentation, 122; Bacteria of, 345,
345*.

Fern gametophyte, 431, 432*, 433*.

Fern plants, 292; described, 425-444;
life cycle, 433*.

Fertilization, 41; described, 42, 42*;
double, 43; effect, 50; in Algae,
302; in Angiosperms, 464; in
Cycads, 450; in Pines, 456.

Fibrous root system, 138, 138*.

Field Dodder, seeds, 75*.

Filament of stamen, 41, 41*.

Filicales, 426-435.

Fitness for environment, 520.

Flagellates, 329.

Flax family, 486.

Flower buds, 207.

Flowers, general characteristics, 9;
apetalous, 13, 11*; arrangement,
26-32; as structures peculiar to
Angiosperms, 459; of Bean
family, 23, 23*; of Beech family,
476, 476*; of Birch family, 475,
475*; color, 10; of Buckwheat
family, 478*, 479*, complete, 13,
10*; of Composites, 24, 24*; of

574

INDEX

Corn, 17, 16*, 17*, 18*, 19*; of
Crowfoot family, 481, 481*; of
Elm family, 478, 477*, 478*;
epigynous, 15, 15*; function, 10;
gamopetalous, 13, 12*; of Goose-
foot family, 480, 480*; of Grass,
17; hypogynous, 16, 15*; incom-
plete, 13, 11*; of Mallow family,
14, 14*; of Mustard family, 482,
483*; of Oats, 20, 20*, 21*; odor,
10; of Orchids, 26, 25*; of Pars-
ley family, 488, 488*; parts, 11,
10*; of Pea- family, 484, 485*;
perigynous, 16, 15*; of Pink
family, 480, 481*; pistillate, 14;
polypetalous, 13, 10*; of Rose
family, 483, 484*; size, 9; some
particular forms, 16; of Spurge
family, 486, 486*; staminate, 13;
unisexual, 13, 12*, 16*, 17*, 18*,
19*; of Walnut family, 474, 474*;
of Wheat, 22, 22*; of Willow
family, 473, 473*.

Flowering glume, 18, 18*.

Flowering plants, 6, 289; life cycle, 6,
7*, 469*.

Fluctuating variations, 525.

Foods, elaboration into plant struc-
tures, 95; manufacture by leaves,
252; nature of, 120; reserve, 277;
use, 273.

Foot of sporophyte, 410, 410*.

Forestry, 1, 4.

Formation of buds, 208.

Foxtail, seeds, 76*.

Framework of plants, 275.

Free-swimming societies, 505.

Free water in the soil, 153.

Fronds, 428, 427*.

Fructose, 277.

Fruits of Flowering plants, nature
and types, 77; aggregate, 80;
berry type, 77, 77*; blackberry
type, 79, 79*; definition, 81;
dissemination, 82-88; Pineapple
type, 80, 80*; pepo type, 79, 78*;
pome type, 78, 78*; multiple, 80;
some other types, 80, 81*, 82*;

stone type, 77*, 78; strawberry

type, 79, 79*.
Fruit sugar, 277.
Fucales, 321-324.
Fucoxanthin, 318.
Function, meaning, 6; of cells, 112; of

flowers, 10; of leaves, 233, 252;

of roots, 136, 137; of seeds, 55,

56; of stems, 168.
Fungi, 289, 336; described, 351-404;

Alga-like, 353-363; basidia, 382-

403; divisions, 352; Imperfect,

404; sac, 363-379.
Fungi Imperfecti, 352.
Funiculus, 38, 38*.

Galium Aparine, 240*.

Galton, Francis, 538.

Gametes, 302; combinations and the
Mendelian ratio, 556; kinds, 302;
segregation and purity, 543.

Gametophyte generation, 411; in
Angiosperms, 463, 464, 462*,
464*; in Cycads, 448, 450, 449*;
in Equisetum, 437, 438*; in
Ferns, 431, 432*, 433*; in Liver-
worts, 411, 412*; in Lycopodium,
440; in Moss, 417, 418*, 421*,
423*; in Pines, 455, 456, 455*; in
Selaginella, 442, 443*.

Gamopetalous flowers, 13, 12*.

Gamosepalous flowers, 13.

Garden Pea, flower and pod, 35*; in-
heritance in, 540, 541, 542*.

Gasteromyces, 384; described, 389.

Geaster, 391, 391*.

Gemmae, 409.

Gemmae cups, 409, 407*.

Genera, in classification, 291.

Genes, 536; active and latent, 537.

Genetics, 538.

Genotype, 554.

Geotropism, 150, 151*.

Geranium, bending toward light,
243*.

Germination of seeds, 89-98; effect
of temperature on rate, 90r*
moisture requirement, 91; neces-

INDEX

575

sary conditions, 89; oxygen re-
quirement, 92; processes, 93-98;
temperature requirement, 89.

Germinators, 99, 99*, 100*.

Germ-plasm, 532.

Giant Evening Primrose, 530, 530*.

Gill Fungi, 385.

Gleba, 390.

Gleocapsa, 298.

Gliadins, 281.

Globulins, 281.

Glomerella rufomaculans, 379, 379*.

Glucose, 277.

Glucosides, 281.

Glumes, 17, 18*; empty or outer, 18,
18*; flowering, 18.

Glutelins, 281.

Giutenin, 281.

Grafting, 225; described, 227, 232*.

Gramineae, 495.

Grape vine, 176, 176*.

Grape Downy Mildew, 355, 355*, 356*,
357*.

Grape sugar, 277.

Grass, flowers, 17-23, 17*, 18*, 19*,
20*, 21*, 22*; family, 495; seed-
lings, 102, 102*, 103*, 104*.

Grass type of seeds, 61-67.

Green Algae, 301-318; confervoid,
310-314; conjugating, 314-316;
in relation to Lichens, 380; tubu-
lar, 316-318; unicellular motile,
302-307; unicellular non-motile,
307-310.

Green Foxtail, seed, 76*.

Green Molds, 364; described, 375,
375*.

Grizzly Giant, 174*.

Growth in roots, factors influencing
the direction, 150, 151*.

Growth of Stems, 213-221; char-
acter and rate, 214; factors in-
fluencing, 217-221 ; grand period,
216; phases, 213; primary and
secondary, 214; regions, 213,
214*.

Guard cells, 247, 246*, 247*.

Gulfweeds, 319; described, 323.

Gymnosperms, 67, 289; described,
445-458; life cycle, 457*; seed
and seedling, 67, 67*.

Gymnosporangium, 401, 400*, 401*,
402*, 403*.

Halberd-shaped leaves, 479*.

Hard seeds, 69.

Hardwood cuttings, 227, 230*.

Harmogonia, 299, 299*.

Haustoria, 352, 356*.

Head, 30, 29*, 32*.

Heart Rot, White and Red, 389.

Heart wood, 202, 200*.

Heath family, 489.

Heathers, 490.

Heliotropism, 152.

Helvellales, 364.

Hemi-celluloses, 278.

Hemlock, 451.

Hepaticae, 405.

Herbaceous stems, 171; structure,
192-197.

Herbarium Mold, 375.

Heredity, 535-556; experimental
study, 537-556; importance of
its study, 537; nature, 535; physi-
cal basis, 535.

Heterocysts, 299, 299*.

Heterogametes, 302.

Heterogamous sexuality, 304.

Heterospory, 442.

Heterozygous, 544.

Hibiscus Trionum, 487.

Hickory, pistillate flower, 81*; species
and economic importance, 475;
tap-root, 139, 139*.

Hilum, of seed, 57, 57*; of starch
grain, 256*.

Histology, 2.

Holdfasts, 135.

Homospory, 442.

Homozygous, 544.

Hooke, Robert, 113.

Horny endosperm, 64, 63*, 64*.

Horse Nettle, 492, 491*.

Horsetails, 426; described, 435.

Horticulture, 1.

576

INDEX

Host, 337.
Huckleberries, 489.
Hugo De Vries, 524, 524*.
Hugo Von Mohl, 113.
Humus, 154.
Hybridization, 561.
Hydrocyanic acid, 282.
Hydrodictyon, 309.
Hydrodictyon reticulatum, 309*.
Hydrophytes, 504.
Hydnum, 388, 387*.
Hydrophytic societies, 504.
Hydrotropism, 151, 151*.
Hygroscopic water, 153.
Hymenium, 365.
Hymenomycetes, 384.
Hypha, 352; infection, 394.
Hypocotyl, of Bean, 56, 56*, 106*;

of Bean type of seedlings, 107;

of Corn, 63; of Onion, 105*; of

Red Clover, 109*; of Squash, 107,

107*.
Hypogynous flowers, 16, 15*, 484*.

Impatiens, 88.

Imperfect Fungi, 352; described, 404.
Inbreeding, results, 564, 563*, 564*.
Incomplete flowers, 13, 11*, 12*.
Indeterminate inflorescence, 28, 28*.
Indian Turnip, 18, 18*.
Induaium, 429, 430*.
Infection hypha, 394.
Inferior ovary, 15, 15*.
Inflorescence, 26; determinate, 28,

32*; indeterminate, 28, 32*.
Inheritance, 517; described, 519; in

ears of Corn, 551*; in heads of

Wheat, 552*.
Inorganic evolution, 513.
Integument, 38, 35*,' 36*, 38*, 39*; of

Cycads, 448, 449*; of Pines, 454,

455*.
Interdependence of shoot and root,

136.
Internodes, 166; of Corn seedling,

183*.

Interrupted Fern, 431, 431*.
Invertase, 285.

Investigations since Mendel, 547.

Involucre, 30, 29*.

Iodine test for starch, 256.

Irish Moss, 326, 325*.

Irish Potato, roots, 142; eyes and scale

leaves, 181, 179*; propagation,

226, 226*.
Iron, a mineral element for crops,

157.

Isoetes, 438.
Isogametes, 302.

Jerusalem Artichoke, 181.
Jimson Weed, 61.
Johnson Grass, 177.
Juglandaceae, 474.
Jungermaniales, 407.

Kafir Corn, 142.

Keel, 24, 23*.

Kelps, 319, 319*, 320*.

Kernel of Corn, 44*; structure, 62, 63,

64*.
Kernel, of Oats, 22, 45*; of Wheat, 65,

65*.

Lactiferous vessels, 133.

Lady's Thumb, seeds, 75.

Lamarck's Evening Primrose, 529.

Lamarck, 515; his explanation of evo-
lution, 515.

Laminarias, 319.

Lamb's Quarter, seeds, 75*, 76*.

Larches, 451.

Large seeded Alfalfa Dodder, 76*.

Lateral buds, 206, 206*.

Lateral flowers, 27.

Law of Dominance, 543.

Layering, 225, 231*.

Leader, 312.

Leaf Blight of Cotton, 404.

Leaf buds, 207.

Leaflet, 237, 237*, 238*.

Leaf traces, 185, 186*.

Leaves, 233-286; auricles, 235, 236*;
base, 235; blade, 235; cellular
structure, 246-252; compound,
237, 237*; development, 234;

INDEX

577

general structure, 242-246; man-
'ufacture of food, 252; margin,
235; mosaic, 241, 241*; parts,
234; perfoliate, 235, 236*; pri-
mary and secondary, 234; of
Corn, 235, 236*; rosette, 241,
241*; sessile, 235, 235*; sheath,
235; simple, 237, 237*; special
forms, 270; stipules, 234, 235;
transpiration, 260; use of the
photosynthetic food, 273.

Legume, 80, 485, 82*, 485*.

Legumelin, 281.

Legumes, 156.

Leguminosae, 58; described, 484.

Legumin, 281.

Lemma, 17, 22, 18*, 19*, 22*.

Lemons, 77, 133.

Lenticels, 184, 184*, 185*.

Leptosporangiate, 430.

Lettuce, prickly, 85; Sea, 310, 310*;
Wild, 495.

Leucin, 281.

Leucoplast, 115.

Leucosin, 281.

Levulose, 277.

Lichens, 363; described, 379-382.

Life Cycle, of Angiosperms, 469*; of
Ferns, 433*; of Flowering Plants,
6, 7*; of Marchantia, 412*; of
Moss, 421*; of Pine, 457*; of
Wheat Rust, 399*; of Cedar
Rust, 403*.

Light, as influencing growth, 220; as
an ecological factor, 502; as
related to leaves, 237; as related
to photosynthesis, 257, 257*; as
related to transpiration, 263.

Lignin, 115, 276.

Ligulate flowers, 253, 24*.

Ligule or rain guard, 235.

Lilac Mildew, 373.

Lily, embryo sac, 465*; family, 498.

Linaceae, 486.

Linseed oil, 279.

Lipase, 94.

Liverworts, 504; described, 406-417.

Loam, 154.

Locules, of anther, 41, 41*; of fruit,
77, 78, 77*, 78*; of ovary, 34, 38*.

Lodicules, 17, 20, 18*, 19*, 22*.

Longevity of seeds, 67; discussed, 71-
74.

Maerocystis, 319, 319*.

Madder family, 492.

Magnesium, 157.

Main roots, 136.

Malic acid, 285.

Mallow family, 487.

Maltase, 285.

Maltose, 277.

Manufacture of food, in leaves, 252-
260; in stems, 169.

Maple, genus, 292; family, 487.

Marchantias, 407-415.

Marchantiales, 407-415.

Marchantia polymorpha, 407-414.

Margin, of leaves, 235.

Marguerite, 494, 494*.

Mass culture, 559.

May Apple, or Mandrake, 180, 180*.

Meadows, 508.

Medicago saliva, fruits, 82*.

Medullary rays, 132, 197, 198*, 201*,
202*.

Megaspore, 442, 441*; formation, 463,
462*; mother cells, 463, 461*,
462*.

Megasporangium, 442, 441*.

Megasporophyll, 442, 441*.

Megastrobilus, 448*.

Melon type of fruit, 79, 78*.

Membrane, of the cell, 114, 115*; of
the nucleus, 114, 114*; semi-
permeable, 117.

Mendel, Gregor, 539, 539*; dis-
coveries, 541, 542*; experiments,
540; law of dominance, 543.

Mendel's law, 546; use in plant breed-
ing, 562; value, 547.

Mendelian ratio and the combination
of gametes, 556.

Mendelism, as demonstrated in Corn,
548*, 549*, 550*, 551*; as demon-
strated in Garden Pea, 541-546,

578

INDEX

542* ; as demonstrated in Wheat,
552*.

Meristem, 126, 204, 126*, 204*.

Meristematic tissue, 126, 126*; of
buds, 204, 204*; of Grass stems,
213, 214*; of root tip, 143, 144*.

Mesophyll, 244; described, 246, 250*.

Mesophytic societies, 504; described,
506.

Metabolism, 116, 273; anabolic and
catabolic, 273.

Micro-organisms of the soil, 155.

Micropyle, 39, 57, 38*, 57*.

Microsphaera, 373.

Microspore, 442, 441*; formation,
461, 462*; mother cells, 461, 461*.

Microsporangium, 442, 441*.

Microsporophyll, 442, 441*.

Microstrobilus, 446, 447*.

Middlings, 65.

Midrib, 245.

Mildews, Downy, 355-360; Powdery,
373-375.

Milkweed, 83, 133, 83*.

Mistletoe, 163. *

Mixed buds, 207.

Moisture, given off in transpiration,
260, 260*; of the soil, 217; re-
quired for germination, 91.

Molds, 155; Blue and Green, 375-377;
True, 360-363; Water, 353-360.

Monadelphous stamens, 15.

Monocotyledons, embryo, 466, 467*;
families, 495; origin, 471; seeds,
66; seedling, 102*, 103*, 104*,
105*; stems, 170; structure of
stems, 187-192.

Monoecious, 14.

Moonworts, 435, 434*, 435*.

Marchdla esculenta, 365, 365*, 366*.

Morels, 364, 365*.

Morning Glory, cotyledons, 111;
plumule, 109; stem, 168, 175,
177*.

Morphine, 283.

Morphology, 2, 289.

Mosaic, of leaves, 241, 241*.

Mosses, 292, 405; described, 417-424.

Mucorales, 360-363.

Mullein, leaf, 266*.

Multiple fruits, 80, 80*.

Multiplication of cells, 123, 125*.

Musci, 405.

Mushrooms, 352, 382, 384, 385*, 386*.

Mustard, Black, 483, 483*; family,

482.

Mutant, 527.
Mutation, 525, 528*; hi Evening

Primrose, 529.
Mutation theory, 527; compared with

Darwinism, 531.
Mycelium, 352.
Mycorhiza, 155, 155*.
Myrsiphyttum, 169.
Myxobacteria, 350.
Myxomycetes, 336; described, 336-

341; economic importance, 339.

Naked Ascus Fungi, 364, 377, 376*.

Naked buds, 205.

Nasturtiums, 241*.

Natural history, 524.

Natural selection, 517,

Navicuia viridis, 332*.

Nectar, 133.

Nectar glands, 133, 133*.

Nectarine, 528, 561.

Nemalion, 326, 326*.

Nereocystis, 319.

Nerves, 245.

Net-veined leaves, 245, 244*.

Nicotine, 283.

Nidularia, 392, 392*.

Nightshade family, 491.

Nitrogen, 133.

Nodes, 166.

Nostoc, 209, 208*.

Nucellus, 38, 448, 454, 35*, 36*, 38*,
449*, 455*.

Nuclear membrane, 114, 114*.

Nuclear sap, 114, 114*.

Nucleolus, 114, 114*.

Nucleo-proteins, 281.

Nucleus, 39; described, 114, 114*;
endosperm, 40, 44, 51; of leaf
cell, 251, 251*; primary en-

INDEX

579

dosperm nucleus, 40, 38*, 39*;

tube, 42, 42*.
Nut type of fruit, 80, 81*.
Nux vomica, 283.

Oak, family, 476; flowers, 476, 476*;

plain sawed, 203*; quarter sawed,

203*; uses, 476, 477*.
Oat, flower, 20, 20*, 21*; kernel, 66;

Smut, 393.

Objections to Darwinism, 522.
Oedogonium, 312, 312*.
Oenothera, brevistylis, 530; gigas,

530, 530*; laevifolia, 530; La-

marckiana, 529, 529*.
Oils, 273; fatty, 279; volatile, 281.
Onion, section of root, 144*; section

of bulb, 181*; seedling, 107, 105*.
Onoclea sensibilis, 431, 431*.
Ontogeny, 291.
Oogonium, 312, 312*.
Oospore, 302.
Open bundles, 195.
Opening of buds, 205, 205*.
Optimum temperature for germina-
tion, 90.
Orchid, family, 499; flowers, 26, 25*,

499*.

Order, in classification, 291.
Organ, 6; essential organs of flowers,

11; sex, 302.
Organism, 6.
Organic evolution, 513.
Origin oj Species, 517.
Orthotropous ovule, 463.
Oryzenin, 281.
Oscillatoria, 298, 298*.
Osmosis, 94; as related to cell activity,

117; described, 116, 116*.
Osmotic pressure, 119.
Osmunda Claytonia, 431, 431*.
Ovary, 33, 33*, 34*; inferior, 15, 15*;

structure, 34, 34*, 35*, 36*;

superior, 16, 15*.
Ovules, 34, 34*, 35*, 36*; of Angio-

sperms, 463; of . Cycads, 448,

448*, 449*; cellular structure, 39,

39*; development to seeds, 43,

43*, 44*, 45*; of Pines, 454, 454*,

455*; parts, 38, 38*; relation to

seeds, 37.
Ovulate strobilus, of Cycads, 446,

447, 448*; of Pines, 453, 454*.
Oxalic acid, 285.
Ox-eye Daisy, 86.
Oxygen, for germination, 92; in

respiration, 121, 269.

Palea, 17, 18*, 19*.
Paleobotany, 3.
Palisade tissue, 249, 250*.
Palmaceae, 497.
Palm family, 497.
Pandorina, 304, 305*.
Panicle, 23, 32; of Oats, 120*.
Papain, 284.
Papaw, 284.
Pappus, 26, 24*.

Parallel-veined leaves, 245, 244*.
Paraphysis, 365, 366*.
Parasite, 337.
Parasitic roots, 163, 163*.
Parenchyma cells, 127.
Parsley family, 488.
Parthenocarpic fruits, 50.
Parthenogenesis, 355, 467.
Parthenogenetic fruits, 50.
Parthenocarpy, 467.
Parts, of a flower, 11, 11*; of a leaf,
234; of a pistil, 33; of a plant, 6.
Pathogenic Bacteria, 346, 347*.
Pea family, 484-486.
Pea type of seedling, 109, 110*.
Pear Blight, 348, 348*.
Peat, 154.
Peat Moss, 422.
Pectic substances, 276.
Pediastrum, 309, 308*.
Pediastrum boryanum, 308*.
Pedicels, 20, 20*.
Pedigree culture, 560.
PeniciUium, 375; described, 376, 376*.
Pepo type of fruit, 79, 78*.
Pepsin, 94.
Peptases, 284.
Peptones, 284.

580

INDEX

Perennial stems, 171.

Perfect flower, 13, 11*.

Perianth, 11, 460, 11*, 460*.

Pericarp, 61, 66*.

Pericentral cell, 328.

Pericycle, 147.

Peridium, 390.

Perigynous flower, 16, 15*.

Perisperm, 46.

Perisporiales, 364; described, 373-

375.

Peristome, 421.
Perithecium, 369, 370*.
Permanent root, 103.
Peronosporales, 353; described, 355-

360.

Petal, 12, 11*.
Petiole, 234.
Peziza, 366, 367*.
Pezizales, 364; described, 366-369.
Phaeophyceae, 318-324.
Phallus impudicus, 391, 392*.
Phellogen, 185*.
Phenotype, 554.
Phloem, 130, 146, 188.
Phosphorus, as a soil constituent, 157.
Photosynthesis, 169; described, 252-

260; factors influencing, 257.
Pltycocyanin, 297, 324.
Phycoerythrin, 324.
Phy corny cetes, 352; described, 353-

363.

Phylogenetic divisions, 292.
Phylogeny, 291.
Phylloglossum, 438.
Physical basis of heredity, 535.
Phytophthora cactorum, 358, 361*.
Phytophthora infestans, 357, 358*,

359*, 360*.

Pigments, 281; described, 283.
Pileus, 386, 386*.
Pilobolus, 362.

Pineapple type of fruit, 80, 80*.
Pine Blister-Rust, 402.
Pines (Pinaceae), 451-458; gameto-

phytes, 455-457; life cycle, 457*;

seed, 457; spoTOphyte, 452, 451*;

strobili, 452-455.

Pink, family, 480; flower, 481, 15*,

481*.

Pistil, 11, 11*; compound, 15; de-
scribed, 14, 462; simple, 15;

structure, 33-37.
Pistillate flower, 14, 12*; of Corn, 18,

19*.

Pitcher Plant, 132, 272*.
Pith, 187, 187*.
Pitted vessels, 188, 190*.
Placenta, 77, 34*.
Plagiotropism, 173.
Plant Breeding, 557-565.
Plant Ecology, 2.
Plant food, nature, 120.
Plant Geography, 3.
Plant Pathology, 2.
Plant Physiology, 2.
Plant succession, 510-512.
Plasmopara Viticola, 355, 355*, 356*,

357*.

Plastids, 115, 251.

Plectascales, 364; described, 375-377.
Pkurococcus vulgaris, 294, 307, 294*,

307*.

Plowrightia morbosa, 369, 370*.
Plum type of fruit, 78, 77*.
Plumule, 56, 466, 56*, 466*.
Podophyllum, 180, 180*.
Poisonous Toadstool, 384*.
Pollen, 41, 41*, 42*; function, 42; in

relation to external factors, 49;

structure, 41, 42*.
PoUen chamber, 448, 449*.
Pollination, 46; agents, 46; kinds, 47;

kinds giving best results, 52;

nature, 46; results, 50; self-,

close-, and cross-, 48.
Polyadelphous flowers, 15.
Poly cotyledons, 67.
Polyembryony, 467.
Polygonaceae, 478-480.
Polygonum Muhlenbergii, 227, 479*.
Polypetalae, 481-489.
Polypetalous flowers, 13.
Polyporaceae, 387.
Polysiphonia, 327, 328*.
Polysiphonia violacea, 328*.

INDEX

581

Pome type of fruit, 78, 78*.

Pond Lily society, 511*.

Pondweed societies, 505.

Porella, 415.

Postelsia, 319.

Potassium, as a soil constituent, 157.

Potato Blight, 357, 358*, 359*, 360*.

Potato scab, 348, 247*.

Powdery Mildews, 373-375; on Apple

leaf, 372; on Hop, 373*.
Powdery Scab of Irish Potato, 340,

341*.

Prairies, 508.

Pressure within cells, 118.
Primary endosperm nucleus, 40.
Primary growth, 214.
Primary leaves, 234.
Primary meristems, 127.
Primary rays, 203, 201*.
Primary root, 102.
Primary veins, 245.
Procarp, 326, 326*.
Processes in cell activity, 116.
Promycelium, 394.
Prop roots, 139.
Propagation, by roots, 163-165; by

stems, 225-233.
Prostrate stems, 172, 173.
Proteases, 284.
Protective tissues, 127.
Protection against transpiration, 266.
Proteins, 256, 280; kinds, 281.
Prothallial cell, 456.
Prothallus, 432, 432*.
Protoascales (Yeasts), 377, 377*.
Protococcales, 302; described, 307.
Protococcus, 307.
Protodiscales, 364; described, 377,

376.*

Protonema, 421.
Protoplasm, 39, 274; described, 113,

114*.

Protoplast, 113.
Protozoa, 155.
Pruning, 221-225.
Pseudopodium, 423.
Pteridophytes, 292; described, 425-

411.

Pteridosperms, 445.
Ptomaines, 283.
Puccinia Asparagi, 403.
Pucdnia graminis, 396-401.
Puffballs, 382; described, 389-392.
Pulsating vacuoles, 303, 303*; in

Euglena, 330, 330*.
Pumpkin, flowers, 12, 12*; length of

root system, 135.
Purity and analysis of seeds, 74.
Pyrenoid, 303, 303*.
Pyrenomycetales, 364; described,

369-373.
Pyronema, 367, 367*.

Quack Grass, 86.
Quarter-sawed Oak, 203, 202*.
Quetelet's law, 525, 526*.
Quince, 142.

Raceme, 28, 28*, 32*.

Rachilla, 20, 21*.

Rachis, 20, 20*, 21*.

Radicle, of Bean, 56, 56*; of Corn, 63,
102, 63*, 101*; development,
466, 467, 466*, 467*.

Ranunculaceae, 481; flower, 481*.

Raphe, 57, 57*.

Rays, primary and secondary, 203.

Receptacle, 12, 10*, 11*.

Recessive character, 543.

Red Algae, 296; described, 324 "329.

Red Clover, flower, 23, 12*, 23*; fer-
tilization in, 42*; hard seeds, 69;
impurities in seeds of, 75*; life
cycle, 469*; ovule, 38*; pistil, 36,
35*; pollination, 47*; seedling,
109, 109*.

Red Heart Rot, 389.

Reduction division, 412, 413*; as re-
lated to segregation, 554, 555*.

Regions of growth, in roots, 145, 149,
145*, 149*; in steins, 213, 214*,
215*.

Reproductive tissues, 133.

Reserve cellulose, 278, 279*.

Reserve foods, 277.

582

INDEX

Resin ducts, 133.

Respiration, 96-98, 121-123; anaero-
bic, 122; as observed in leaves,
269.

Resting buds, 204.

Resting cell, 299.

Resting period of seeds, 67-69.

Results of pollination, 50, 51*.

Reticulated vessel, 190*.

Rhizoids, 135; of Liverworts, 408,
407*, 408*; of Moss, 418*.

Rhizome, 179, 180*; of Ferns, 427,
427*.

Rhizopus nigricans, 360-363.

Rhodophyceae, 324-329.

Riccia, 414, 414*.

Rind, of Corn stem, 187, 187*; of
Potato, 128, 128*.

Ringing, 211.

Rivularia, 300, 300*.

Robert Hooke, 113.

Rockweeds, 322-323.

Root cap, 63, 143, 63*, 144*.

Root hairs, 131, 145, 131*, 146*,
153*.

Root Rot, 388.

Roots, primary, secondary, permanent
and temporary, 102, 103, 101*,
102*, 103*, 104; described, 135-
152; absorption, 158-161; adven-
titious, 140; depth and spread,
141 ; direction of growth, 150-152;
excretions, 161; fascicled, 140,
140*; fibrous, 138, 138*; in prop-
agation, 163-165; in relation to
the soil, 152-158; main, 136; of
Ferns, 427, 428, 427*; pressure,
161; prop or brace, 139; relation
to shoot, 136; tap-root, 139, 139*;
texture, 136; types of root sys-
tems, 138-141; water, air, and
parasitic, 162.

Rootstock, 179, 180*; of Ferns, 427,
427*.

Rosaceae, 483.

Rose, family, 483; flowers, 483, 484*.

Rosette, 241, 241*.

Eubiaceae, 492.

Rumex acetosella, 479*.

Rumex, collenchyma cells in, 128.

Runners, 175, 175*.

Russian Thistle, 480, 480*.

Rusts, 396-403; Apple, 401; of

Asparagus, 403; Black, 396; of

Pines, 402.

Saccharomyces, 377, 377*.

Saccharose or sucrose, 277.

Sac Fungi, 352; described, 363-379.

Sago Palm, 446.

Saponaria Vaccaria, 74.

Saponins, 283.

Saprolegnia, 353, 353*.

Saprolegniales, 353; described, 353-
355.

Saprophytes, 337.

Sap wood, 202.

Sargasso Sea, 323.

Sargassum, 323, 323*.

Scenedesmus, 308, 308*.

Sclerenchyma fibers, 194.

Sderotinia fructigena, 368, 368*.

Sclerotium, 371, 371*.

Scouring Rushes, 435.

Scutellum, 63.

Sea Lettuce, 310, 310*.

Sea Palm, 319.

Seaweeds, 296.

Secondary growth, 214.

Secondary leaves, 234.

Secondary root, 102; origin, 150,
150*.

Secretions of plants, 281.

Secretory tissue, 133, 133*.

Seed coat, 55; function, 56.

Seed plants, 3; described, 445-470.

Seedlings, 101; Common Bean type,
107, 106*; comparative size, 110;
Grass type, 102, 101*, 102*, 103*,
104*; Onion type, 106, 105*; Pea
type, 109, 110*; types, 102-110.

Seeds, dissemination, 82-88; germi-
nation, 89-98; nature, 55; of
Cycads, 450; of Pines, 457; purity
and analysis, 74-77; structure,
56, 57, 56*, 57*; testing germi-

INDEX

583

native capacity, 98-101; types,

58-67.
Segregation and purity of gametes,

543.
Segregation and reduction division,

554.

Selaginella, 438; described, 440-444.
Selection, 557.
Selection of mutants, 561.
Selective absorption, 159.
Self-pollination, 48.
Semi-permeable membrane, 117.
Sepals, 11, 10*, 11*.
Sessile leaf, 235, 235*.
Seta, 420.
Sex cells, 302.
Sex organs, 302.
Sheep Sorrel, 479*.
Sieve tubes, 131.
Silphium perfoliatum, 236*.
Simple stems, 167.
Sinigrin, 283.
Siphonales, 302; described, 316, 316*,

317*.

Smartweed, 479*.
Smuts, 384; described, 392-396.
Soil, as the home of roots, 152-162;

as an ecological factor, 502;

microorganisms, 155; origin, 152;

rock constituents, 152; solution,

157; water, air, and humus, 153.
Solonaceae, 491.
Solanin, 283.

Solatium Carolinense, 492.
Solatium nigrum, 491.
Solanum rostratum, 492.
Solitary flowers, 27.
Solsola Kali, var. tenuifolia, 480*.
Somatoplasm, 532.
Soredia, 381.
Sorus, 429, 430*.
Sour Cherry, 212, 212*.
Spawn of the Mushroom, 387.
Special forms of leaves, 270-273.
Species, in classification, 291.
Spermatophytes, 292; described, 445-

470.
Sperms, 302.

Sphagnales, 417; described, 422-424.

Sphagnum, 422, 423*.

Spike, 17, 29, 29*, 32*.

Spikelets, of Com, 17, 18*, 19*; of
Oats, 20, 20*, 21*; of Wheat, 22,
22*.

Spindle fibers, 124.

Spiral vessel, 188, 189*, 190*.

Spirillum, 341.

Spirogyra, 315, 315*.

Spongospora subterranea, 341.

Spongy tissue^ 249, 250*.

Sporangia, 320, 338.

Sporangiophore, 437.

Spore, 338; of Bacteria, 343, 343*.

Sporidia, 394.

Sporophore, 383.

Sporophylls, 437, 439, 441, 447, 448,
452, 453, 454, 460, 462.

Sporophyte, of Angiosperms, 459-
463; of Cycads, 446, 446*; of
Equisetales, 436-437; of Ferns,
426-431; of Liverworts, 411,
410*; of Lycopodium, 439-440;
of Moss, 418, 418*; of Pines, 452-
455; of SelagineUa, 441, 440*.

Spurge family, 486.

Squash, arrangement of flowers, 27*.

Squirting Cucumber, 88, 87*.

Stamens, 11; described, 41, 41*.

Staminate flower, 13; of Corn, 17, 16*,
17*; of Pumpkin, 12*.

Staminate strobilus, 446, 447, 447*;
of Pines, 452, 452*, 453*.

Standard, 23, 23*.

Starch, as a storage product, 278;
formation, 255; test for, 256.

Starch grains, 256, 256*; structure,
278, 278*.

Starch sheath, 147, 192, 193*.

Starchy endosperm, 64.

Stemonitis, 337*.

Stems, aerial, 172; branching, 167;
characteristic features, 166;
classes, 170; climbing, 175; erect,
173; functions, 168; growth, 213-
221; of Ferns, 427; of herbaceous
Dicotyledons, 192-197; of Mono-

584

INDEX

cotyledons, 187-192; of woody
Dicotyledons, 197-203; pros-
trate, 173; pruning, 221-225;
structure, 192-203; underground,
177; use in propagation, 225-
232.

Stigma, 33, 33*.

Slink Horn Fungus, 392, 392*.

Stinking Smut, 395.

Stipe, 386.

Stipules, 235, 234*.

Stock, 227.

Stolons, 361.

Stomata, 184; function, 247; location,
248, 249; structure, 247, 246.*

Stoneworts, 329; described, 332-335.

Storage tissue, 132.

Strap-shaped flowers, 25, 24*.

Strawberry fruit, 79, 79*.

Strengthening tissue, 129, 128*, 129*,
130*.

Strobilus, 437, 439, 441, 447, 452.

Stromata, 371, 371*.

Structure, of cells, 112-116, 114*,
115*; of herbaceous dicotyle-
donous stems, 192-197; of leaves,
242-252; of monocotyledonous
stems, 187-192; of roots. 143-
150; of stems, 182-203; of woody
stems, 197-203.

Struggle for existence, 517; described,
521.

Strychnine, 283.

Style, 34, 33*.

Suberin, 276.

Sugar cane, plants, 496, 496*; propa-
gation, 226, 228*, 229*.

Sugar, formation, 253; kinds, 277;
transformation into starch, 255.

Sulphur, as a soil constituent, 157.

Summer spores of Rust, 397, 397*.

Sundew, 272.

Survival of the fittest, 517; described,
522.

Suspensor, 466, 466*.

Swamp societies, 505.

Sweet Cherry, 212, 211*.

Sweethearts, 240*.

Sweet Potato family, 490.

Sweet Potato, propagation, 164, 164*.

Symbiosis, 156.

Sympetalae, 472; families, 489-495.

Synangium, 430.

Systematic Botany, 3.

Tap-root, 139, 139*.

Taphrina pruni, 376.

Tassel of Corn, 16*, 17*.

Tartaric acid, 285.

Taxonomy, 3.

Teleutospores, 397, 398*.

Temperature, in relation to germina-
tion, 89, 90; in relation to growth
of stems, 218; in relation to
photosynthesis, 260; in relation
to transpiration, 263.

Temporary roots, 103.

Terminal buds, 206.

Terminal flowers, 27, 27*.

Tertiary roots, 150.

Testa, 55.

Tetraspore, 329.

Tetrasporic plant, 328.

Thalictrum, 467.

Thallophytes, 292; described, 296-
405; Algae, 296-329; Bacteria,
341-350; Basidiomycetes, 382-
404; Blue-green algae, 297-301;
Brown Algae, 318-324; Flag-
ellates, Diatoms, and Stone-
worts, 329-335; Fungi, 351-405;
Green Algae, 301-318; Lichens,
379-382; Myxobacteria, 350;
Red Algae, 324-329; Sac
Fungi, 363-379; Slime molds,
336-341; Water Molds, 353-363.

Thorns, 272, 271*.

Tillandsia, 162.

Timothy heads, showing improve-
ment, 529.

Tissues, general view, 126-134; ab-
sorbing, 131, 131*; conductive,
130, 130*; food-making, 132, 132*;
meristematic, 126, 126*; of
leaves, 242-252; of roots, 146-
150; of stems, 182-203; protec-

INDEX

585

tive, 127, 127*, 128*; reproduc-
tive, 133; secretory, 133, 133*;
storage, 132; strengthening, 129,
129*, 130*.

Toadstools, 382; described, 384; de-
structive, 388.

Tobacco, flower, 13, 12*; leaf arrange-
ment, 239*.

Tomato, described, 491; ovary, 36,
34*, 36*; pistil natural size, 37,
37*.

Torus, 12.

Toxins, 123.

Tracheae, 131.

Tracheids, 130, 131*.

Trailing Arbutus, 490.

Transpiration, 241; advantages, 263;
amount, 262; conditions affect-
ing, 263; dangers, 265; described,
260-269; protection against, 266.

Traumatropsim, 152.

Tree Ferns, 427, 428*.

Trichogyne, 327.

True Ferns, 426.

Truffles, 366.

Tryptophane, 281.

Tube nucleus, 42, 42*.

Tuber, 179, 179*; described, 181.

Tuberales, 366.

Tubular Algae, 316-318.

Tubular flowers, 25, 24*.

Turgor pressure, 119.

Twiners, 176.

Typhaceae, 495.

Tyrosin, 281.

Ulothrix, 311, 311*.
Umbelliferae, 488.
Umbel, 30, 30*, 32*.
Uredinales, 396.
Uredospore, 399.
Urticaceae, 478.

Vacuoles, 114, 114*, 115*, 156*.
Variation, 517, 518*, 519*.

Variation, causes, 531; continuous,
525, 526*; discontinuous, 527.

Vascular bundles, 130, 130*.

Vascular cylinder, 145, 146, 146*.

Vaucheria, 317, 316*.

Vegetative reproduction, 409, 422.

Vein, 244, 244*.

Veinlets, 245, 244*.

Velamen, 163.

Venation, 245; netted, 245, 244*;
parallel, 245, 244*.

Venter, 409, 410*.

Venus Flytrap, 273, 273*

Vernal habit, 170.

Volva, 386, 384*.

Volvocales, 307.

Volvox, 305, 306*.

Walnut family, 474.

Warmth, as an ecological factor, 501.

Water, as an ecological factor, 50) .

Water Ferns, 426.

Water Mold, 353.

White Heart Rot, 389.

White Rust, 359.

Willow family, 473.

Wilt Disease, 378.

Wind, as an ecological factor, 502.

Wing, of corolla, 24, 23*.

Witches' Brooms, 377.

Wood fiber, 129, 130, 130*.

XanthophyU, 284.
Xenia, 51, 51*.

Xerophytic societies, 509; kinds, 510.
Xylem, 130, 130*; of roots, 146, 149,
146*, 149*; of stems, 188, 190*.

Yeasts, 377.

Zooglea stage of Bacteria, 342, 343*.
Zoospores, 302.
Zygospore, 302.
Zygote, 302.
Zymase, 94, 285.-

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