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Full text of "Botany for agricultural students"

BOTANY 

FOR 

AGRICULTURAL STUDENTS 



BY 

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 



COPYRIGHT, 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. 

1 



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 

5 



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, 

9 



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 p lants 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 f rom 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 . M uch 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 



12 



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 



13 




s- 



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- O ne 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, k nown as per f ec t O r 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, 




14 



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 ( mean i ng o f two households ") . 
than those of the Apple. Smaller p.. ... , , . 

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 




A 

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 



16 



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 q ulte 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 



17 




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% 



18 



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 



19 




-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. 



20 



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 



21 



-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 



23 



..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 f am in ar representatives have a 
wings are bent down, exposing the h f npoll ij ar 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. 



24 



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 



25 



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 



26 



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 sha P e 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. g uch hi g hly modified flowers cont rast 

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 



28 



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.) 





B 

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. 



32 



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 



a 





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 P arts of fche P istil indi cated. 

duce no kernels. Some varieties > 7 ary 1 ; *' stigma; ' style * 

f , , ,. , 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- 

33 




34 



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 



35 




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. 



36 



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 



38 



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< r ing 

. 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 



40 



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 ^ k a 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 



42 



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 



44 



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 
1C. 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 



51 



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 



52 



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 

55 



56 



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 



57 



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 



59 



-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 
tyP e - 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 



63 




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 ca P> 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 * h e 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 richness 1 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 



65 



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 milling 1 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. 



66 



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 



67 



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- pin e seed 
nous seeds prevail, while both types are about tioned len s thwis e 
equally common among Dicotyledons. ? w p ^ ' 

Resting Period, Vitality, and Longevity of Seeds !? ed germim f " 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 




a 

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. Experiments 1 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. 



70 



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 



73 



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 



74 



SEEDS AND FRUITS 



PER CENT OF GERMINATION FOR EACH OF 6 YEARS 
OF STORAGE 



Seed. 


lyr. 


2yr. 


3yr. 


4yr. 


5 yr. 


6yr. 


Wheat 


80 


82.3 


77.3 


37 


15 


6 


Oats 


90 2 


93 


78.2 


67 


54 


29.5 


Barley 


97 


91 


78.5 


36 


19.5 


7.5 


Peas 


94 


95 


88 


64 


64 


6 


Flax 


81 


82 


75 


49 


26 


24 

















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 



75 



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 15 WAA ^ 



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, 

o 

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



76 



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 




WM 

<!> $ 

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 



77 



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.) 




S 



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



78 



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 



79 



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 c le; /, 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. 




80 



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 



81 



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. 





B 



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- 



82 



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 



83 



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 




a 



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.) 



84 



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 



85 




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 TT/ 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- 



86 



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 



90 



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. 


42 


240 


180 


55 


144 


32 


75 


56 


24 


87 


48 


24 


102.6 


48 


24 


111.2 


80 





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. 


44 


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. 



92 



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 table 1 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 


6 


90 


Alsike Clover ... ... 


6 


90 


White Clover 
Alfalfa 


6 
6 


90 
90 


Timothy 


6 


96 


Bluegrass (Kentucky) 


28 


80 


Millet 


5 


95 


Wheat 


3 


95 


Oats .... 


3 


93 


Barley . 


3 


95 


Flax 


3 


95 


Corn 


5 


92 



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, 




B 



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 p0 cotyl. 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 TTr V^ 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 C 6 Hi 2 06 + 6 O 2 = 

6C0 2 + 6 H 2 O, the ratio a ~ 2 is 1. In respiration, however, 

O O 2 

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 C 6 Hi 2 O 6 = 2 C0 2 + 2 C 2 H 6 0, 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 O 2 = 6 CO 2 + 6 H 2 O. 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 




e 



g 



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- 

B 

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 ch y ma 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 an g les - 

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 ' secondar y cortex; v, vascular 
a secondary cortex which it also en- cylinder ' Highly ma g nified - 
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- Sp 7, ding int A ft th t, 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- wa ter culture gives good results, when 
feet of the lack of the dif- the saltg are in such a prO portion 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 F IG . 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 

^C 



\ 




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 ^ n v i ng 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 havin g 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 i engthw i se 

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- stem 5 r > roots ' /> flesh y 
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, gi ons 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 P kntS are knOWn 
are attached. Modi- * as res ^ n 9 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 ) anc j (&) i s 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," s how- 
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 j n ^ cage ^ 
of the But ternut and Box-elder . 

A, twig of Butternut; (, ter- s P routs 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- t h en 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 oft 7 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 


32 


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 M Jr, 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 



HA 




FIG. 213. Cleft Grafting. A, cion; B, cions inserted in cleft of stock; 
C, the wound covered with wax. After G. C. Brackett. 




d 




e 



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 t h at t h e 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, 




c 

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. 




B 



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 a 1 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 lon s er than the u PP er 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 sma ller 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 


o 


Alfalfa (Medicago sativa) . 


160 


160 


Bean (Phaseolus vulgaris). 


281 


40 


Tomato (Lycopersicum esculentum) 


130 


12 


Cherry 


160 





Pumpkin (Cucurbita pepo) 


269 


28 


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 J IG ' f; T Chloren- 
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 P lasts ^- v is the Iar 8 e 
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 C 6 Hi 2 O 6 , 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 6C0 2 + 6H 2 = C 6 H ]2 O6 + 6O 2 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 60 2 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 60 2 . 

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 C0 2 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 showin S the relation 
be demonstrated as shown <* P h 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 a Ppl ied - The area P r - 




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 sta rch. 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 23 g 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, in g 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 dioxid e- 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 Be gonia leaf , showing water 
the supply furnished from St0 rage 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 F 10 - 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 in g 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 
(C 6 Hi 05)n indicates. In the formula (CeHioC^n each combina- 
tion C 6 HioO5, 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 H 2 O = C 6 Hi O 5 . 
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 Ci 2 H 22 On, 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 C 6 H 12 O 6 + C 6 H 12 O 6 - H 2 O = C 12 H 22 O n . 

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 (C 6 Hi O 5 )n but they differ in the num- 
&, starch grain of Wheat; ber of combinations, C 6 HioO 5 , contained in 
c, starch grain of Corn. their molecules . Their exact difference in 

structure is not known, for the number of combinations, C 6 HioO 5 , 
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 t lvor y " ut ? showin ; * e ex ' 

M . , . treme thickening with hemi- 

plant oils and have an important place C eUulose, 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 to P| 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 



in 

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 C 4 H 8 03N2 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 C 2 oH 2 7NOn, and when decomposed it yields glu- 
cose (CeH^Oe), hydrocyanic acid (HCN), and benzaldehyde 
(C 6 H 5 CHO). Glucosides vary much in composition and con- 
sequently in the products which they yield when decomposed. 
Thus the glucoside coniferin (Ci 6 H 2 2O 8 ), 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 (C4oH 56 ) 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) sa it S) and other substances in solu- 
vulgans. n, nucleus ti ^ The cytoplasm the pro toplasm 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 

p7aVt 0be V^o7 walh Water and solutions enter the P rot - 
From Strasburger. P lasm 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. 




D 



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 




a 



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 A t 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 



or 



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- 
leg 2 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 Blight 1 (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. 





B 

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. 




e 



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 . 3 17. - 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 th j s disease. In Georgia 
the egtimated logs in p ea ches 

. . . . 

and Plums caused b y 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 




D 

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. 

i 



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 




a 



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 species 1 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- ^ 3 40. -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 9 t 
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. 



% 
-. 



j 



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 



B 




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. 




B 



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 




m 



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 live 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. 



or 



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) 



so 



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 _ 




me 



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 



m 




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 (X T V); 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 T V ) ; 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 4 44. - 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 Art i choke , 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 O r 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 y ields 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 (X T V). 



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. 





^y 

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 F 2 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 F 2 generation were allowed to self-ferti- 
lize, and from the seeds obtained the F s 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 F 2 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 F 3 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 F 3 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 (F 3 generation), the dwarfs and one-third of tails of the F 2 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 F 2 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 F 2 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 F 2 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 a 2 + 2 ab + b z 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 F 2 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 





d 



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 F 2 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 F 2 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- 



1 




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 F z 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 
F 2 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. 




a 



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*; s