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This book was presented by

Frederick L. Wellman

I.e. STATE UNIVERSITY D.H HII.l LIBRARY

S00273402

This book is due on the date indicated
below and is subject to an overdue fine
as posted at the Circulation Desk.

DEC G 7
UEU 0 0

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NATURE AND
DEVELOPMENT OF PLANTS

BY

CARLTON C. CURTIS, A.M., Ph.D.

Professor in Botany in Columbia University

FOURTH EDITION-REVISED

NEW YORK

HENRY HOLT AND COMPANY

1915

Copyrighted 1907, 1910, 1914, 1915 by
Henry Holt axd Company

,„^ PRESS OF

THE NEW ERA PRINTING COMPANY
LANCASTER, PA.

PREFACE

In the present volume an attempt has been made to present
the more important aspects of botanical science, not only as
they are related to pure biological science, but also as they are
connected and associated with our daily life and interests. The
general reader will find here a discussion of the laws and prin-
ciples that control and direct plant life and of the importance
of this life. The author also has had in mind the difficulties
that the student experiences in entering a new field. To make
the subject more approachable, many of the technical terms have
been eliminated and an easily comprehended terminology has
been maintained throughout the book. The text is designed
especially for the benefit of the student who is beginning the
subject. We trust that a study of it will bring him to the class
room prepared for a discussion of the topics and we also trust
that this work of preparation will tax him to the full measure
of his intellectual capacity. The author is old fashioned in his
ideas of education. Work that simply entertains or imparts
information and that does not create the sufi^erings associated
with mental effort can be of little permanent value or make for
any considerable development. Finally the book will fall short
of its real purpose unless used in the laboratory in connection
with the plants themselves. Detailed directions are not given
for the laboratory work — these of necessity must vary with the
instructor and with the locality — but the text gives the student
sufficient guidance and understanding to enable him to find (nit
for himself the principles and facts in the material to \\ liich he
has been directed.

Acknowledgments are due to Professors C. B. Atwcll, J. H.
Schafifner, F. D. Kern, Ira D. Cardiff, Jean Broadiiurst, C. A.
Darling, A. H. Chivers, B. O. Dodge, R. C. Benedict, (i. W.
Martin and several others for valuable suggestions, in the prepa-

VI

PREFACE

fV

ration of various portions of the text. The author desires to
express his especial indebtedness to his good friend, Mr. H. O.
Hanson, for the preparation of many of the illustrations; to Mr.
W. A. Andrews, for microscopical studies from which draw^ings
have been made, and to Miss Mary L. Williams, for the execu-
tion of many of the studies. Special mention is also made of the
author's indebtedness to the assistance and collaboration of his
former pupils and present associates, Drs. E. Altenburg and H.

J. MuUer.

Carlton C. Curtis.
Columbia University,
June, 1915.

CONTENTS

PAGE

Introduction i

PART I

Nature of Plants

I. The Leaf 7

II. The Root 46

III. The Stem 71

IV. The Flower, Fruit and Seedling 115

PART II

The Development of Plants

V. Classification of Plants 146

VI. Thallophyta 149

VII. Bryophyta 269

VIII. Pteridophyta 311

IX. Spermatophyta 35^

X. Angiospermae (Spermatophyta concluded) 378

NATURE AND DEVELOPMENT OF PLANTS

INTRODUCTION

I. The Nature of the Plant.— It Is a familiar fact that most
plants have stems, leaves and roots and that these organs are
variously modified. Why have such organs been developed?
Why do they assume such varied forms and arrangements? How
does it come about that the stem reaches up in the air and that
the branches and leaves are arranged in a very definite order
while the root penetrates the soil and grows toward moisture and
other soil material? Is a plant, like an animal, conscious of its
surroundings, seeking suitable foods and avoiding unfavorable
conditions? It is evident to the casual observer that the plant
is sensitive to its surroundings; some plant organs bend toward
a light that we recognize with difficulty or react to a touch that
we cannot appreciate. Has the plant a nervous system and
faculties like ourselves that enable it to adjust itself to its sur-
roundings? To answer these questions it will be necessary to

f%-"' -J

Fig. I. Diagram of a cell showing its three dimensions; mesh- work of
granular cytoplasm which incloses colorless cell sap, the vacuoles, and a denser
dark body, the nucleus.

consider the nature of the living substance of the plant. The
plant body is composed of cells that are like boxes of living
matter having length, breadth, and thickness (Fig. i). While
the cells assume various forms and perform different work or

2 THE LIVING SUBSTANCE OF PLANTS

functions, they all possess cell walls which contain during the
life of the cell a substance termed protoplasm. This substance
resembles somewhat the white of an egg, being viscid and rather
tenacious. The protoplasm is not of uniform consistency. The
larger portion of it is finely granular and is called the cytoplasm
while a denser rounded part is termed the nucleus. Other bodies,
plastids, also denser than the cytoplasm are of common occur-
rence. Such plastids as contain a green pigment, chlorophyll, are
called chloroplastids or chloroplasts, and these produce the green

Fig. 2. Structure of the cell: A, cell ditch-moss, Philotria; n, nucleus,
V, vacuole, ch, chloroplast. B, cell of carrot; ch, yellowish chromoplasts, n,
nucleus. C, reddish chromoplasts from rose hip. D, cells of Begonia; s,
starch grains, c, crystals of lime. E, leucoplast, /, of potato forming a starch
grain.

color of the vegetation (Fig. 2, A). Still other plastids, the
chromoplasts, contain red or yellow pigments that give the color
to many fruits and flowers, as the tomato, rose-hip, squash, nas-
turtium, etc. (Fig. 2, B, C). Colorless plastids, leucoplasts, often

INTRODUCTION 3

occur in cells hidden from the Hght. One group of these leuco-
plasts forms the starch that appears in underground storage
organs as in the potato (Fig. 2, E). Usually spaces or vacuoles,
that appear as cavities also occur in the granular cytoplasm
(Fig. 2, A, v). In reality they are filled with watery solutions of
various substances, the so-called cell sap. Many other structures
(Fig. 2, D) appear in the protoplasm to which attention will be
called later. It is well to remember that these visible structures
do not represent the real composition of the protoplasm. A
variety of units beyond the range of visibility, grow, multiply,
and build up the various structures which we recognize as con-
stituting the protoplasm. The cell walls make up the body of
the plant and give stability to its various organs but the living
part of the cell is the protoplasm. This substance reaches
through delicate pores in the cell wall to the protoplasm of
adjacent cells so that all the living substance of the entire plant
body is in contact and forms one united mass.

2. The Nature of the Living Substance of the Plant. — The pro-
toplasm possesses most remarkable powers. It can absorb vari-
ous fluids and gases, decompose them into simple elements, re-
unite them into foods or discharge from the cell such substances
as are not required. Furthermore the protoplasm effects those
changes termed growth by transforming the foods into the sub-
stances that compose the cell walls and other parts of the cell,
as the protoplasm itself. How are these changes brought about?
Every substance is composed of elements. Water consists of
two elements, hydrogen and oxygen. The elements that compose
any substances are held together with great energy, owing to their
mutual attraction for one another. When two different sub-
stances are brought together, it may happen that the attraction
of certain elements of one substance is greater for one or more
of the elements of the other substance than for its own elements.
The result is, that the elements will be torn away from their
respective substances and united into new combinations. We
say that a decomposition and a re-combination has been effected,
or, that a chemical reaction has occurred. We see an illustration
of these chemical changes when iron-ore is heated with charcoal.

4 NATURE OF THE LIVING SUBSTANCE

The ore is composed of the elements iron and oxygen and the
charcoal consists of carbon. In the presence of heat the carbon
has a stronger attraction for the oxygen than the iron has. Con-
sequently the oxygen is drawn away from the iron and unites
with the carbon, forming a new combination of carbon and oxy-
gen, and leaving the iron free. This decomposition and re-com-
bination is only a part of the chemical change that takes place,
but it will serve to illustrate the nature of many chemical re-
actions. The energy that was required to hold the oxygen and
iron together begins to be set free as soon as the decomposition
starts and it contributes to the rise of temperature during the
reaction. The living substance of the plant is composed of a
great variety of chemical substances that are very readily de-
composed by gases and other substances which it absorbs. Light,
heat, gravitation and moisture are even more powerful in pro-
ducing these changes. These substances and forces are often
referred to as stimuli (sing, stimulus) because they start the
chemical changes referred to above. So the various substances
and forces in nature stimulate or cause chemical changes in the
living substance of the plant and the energy thus set free is used
by the protoplasm in the performance of its work. The cells of
a seed have no power of their own to grow or perform any duty.
It is not until a certain amount of heat, moisture, gases or other
stimulating forces have acted upon the protoplasm or substances
contained in it and so aroused chemical changes that it is fur-
nished with the requisite energy to begin growth. Furthermore,
these forces exert a very definite influence upon the protoplasm
and cause it to accomplish very definite results. This is due to
the fact that not all portions of the protoplasm of the plant body
are equally influenced by, or we may say equally sensitive to
heat, light, and other forces and consequently the various parts
of the plants do not respond alike. The force of gravity, for
example, acts upon some stems and the living substance is in-
fluenced by this stimulus so that a growth is aroused that bends
the stem into an upright position. The protoplasm in the cells
of the root, however, is so constituted that the stimulus of gravity
causes a growth that bends the root down into the soil. Light

NATURE OF PLANTS 5

stimulates the mustard plant in such a way that the stem grows
toward the light and the root away from it. So the living sub-
stance of the plant while forming one united whole varies in its
sensitiveness in various parts of the plant. In this respect the
various parts may be compared to the receiving stations of a
wireless telegraphy system. The receiving stations may have
instruments sensitive to certain intensities of electrical currents
and other currents do not affect them. So the various organs of
the plant are sensitive, each in its own peculiar way, to certain
stimuli. As a result of this adjustment the various parts of the
plant, being attuned or sensitive to certain forces, are led or
induced to grow so that they come into the most helpful and
beneficial relation to these forces. Thus the leaves, branches,
and roots are stimulated to grow and develop so that each part
becomes properly related to light, moisture, gravity, etc. The
broad blades of the majority of leaves are exposed to the direct
rays of light. This arrangement is of the greatest benefit be-
cause they are using the light in the manufacture of foods. This
position is assumed, however, because they are sensitive to light,
gravitation and other forces which direct and cause the arrange-
ment. This is the most noteworthy feature about the sensitive-
ness of the protoplasm. Forces stimulate to growth in such a
way that the results are helpful and the greatest good comes to
the plant. In other words the reactions are purposive. These
growths are so elaborate and beneficial that they often appear as
the result of reason and will. The bending of the root into the
soil brings it into contact with water and other foods; the tendril
coils about a branch and binds the plant firmly to the support.
The plant, however, does not direct these adjustments. These
reactions do not involve consciousness. These movements and
all others are absolutely directed and controlled by the various
forces which act upon the sensitive protoplasm. A definite re-
action follows a stimulation which the plant has no power to
alter or control. The root will bend down into a dish of mercury
with the same energy and directness as that with which it pene-
trates the soil, and the tendril will clasp your finger or a branch
of its own body as firmly as a serviceable support.

AIM OF THE STUDY

We shall now proceed to study the plant from this standpoint,
Wiving attention to the fitness or adaptation of the leaf, root and
/stem, to the conditions under which the plant lives, and to the
^work which it performs; and finally we will be interested to
) learn how the seed is formed and how the plant lives from year
/to year.

PART I
THE NATURE OF PLANTS

CHAPTER I

THE LEAF

3. The Work of the Leaf. — Each plant has a character or a
personality of its own. What is it that gives this individuality
to the plant? When the work that it performs is understood it
will be seen that this character is very largely due to the adapta-
tion of the leaves to the performance of certain duties. The

Fig. 3. Forms of leaves: A, leaf of white birch with netted veins — p,
petiole; b, blade. B, leaf of Solomon's seal with parallel venation and blade
clasping stem without petiole.

leaves bring the plant into harmony with its surroundings and
give to it a subtle individuality owing to the perfection of their
arrangements, structures and forms for the work in hand. The

7

8 THE WORK OF THE LEAF

more important work performed by the leaves is the construction
of foods, the giving off of water or transpiration, and breathing
or respiration. The magnitude of this work far exceeds the
energy expended in all the industries of the world. The leaves,
however, accomplish this work so quietly and economically that
most people are scarcely conscious of it.

In order to understand the purpose of the petiole and green
blade (Fig. 3) or the significance of the various forms and
arrangements of leaves it will be necessary to examine the struc-
ture of the leaf and see the character of the apparatus that is
used in the performance of its work. The blades of the majority

Fig. 4. Blade of lilac leaf cut across showing the more compact arrange-
ment of cells upon the upper side of the leaf.

of leaves are flat. A section through such a blade, cut as in Fig.
4 so that we can look into the end of the blade, shows that the
leaf is composed of a complicated arrangement of cells. Fig. 5
is a greatly enlarged view of Fig. 4 taken at X.

4. The Epidermis. — It is now seen that a layer of compact cells
(Fig. 5, e) surrounds the leaf on all sides. In fact such a layer
covers all parts of the plant body. This layer of cells, the epi-
dermis, is provided with minute openings or stomata (sing,
stoma), Fig. 5, s. The stomata are especially abundant in the
epidermis on the under surface of the leaf and often quite lacking
from the upper surface. A better idea of these openings and of
the epidermal cells may be gained by stripping off the epidermis

NATURE OF PLANTS

Fig. 5. Greatly enlarged view of region A' in Fig. 4: e, epidermis; s, stoma;
p, palisade mesophyll; ch, chloroplast; sp, spongy mesophyll; i, intercellular
spaces; v, small vein cut across; /, end of vein seen from the side, consisting
of elongated and banded cells (tracheids); col, collecting cells that transfer
material to and from the palisade cells. — H. O. Hanson.

Fig. 6. Surface view of epidermis of lilac leaf: g, guard cell; s, stoma; ch,
chloroplasts; «, nucleus. — H. O. Hanson.

10

STRUCTURE OF THE LEAF

from such leaves as the live-for-ever, blue flag or hyacinth. The
cells of the epidermis are very compactly put together and may be
stripped off by means of a penknife as a thin white skin. Ex-
amined under a microscope, such preparations show the stomata
as minute openings guarded by two rather lens-shaped cells,
termed the guard cells (Fig. 6). Chloroplasts usually occur
only in the guard cells, the other epidermal cells having a watery,
colorless cell content. In addition to these features. Fig. 7 shows
that the outer portion of the walls of the epidermal cells is modi-
fied so as to form a delicate, skin-like layer over the outer sur-

FiG. 7. Fig. 8.

Fig. 7. Section across a stoma shown in Fig. 6: s, stoma; g, guard cell;
c, cuticle; a, air chamber. — H. O. Hanson.

Fig. 8. Hairs from the epidermis of squash leaf.

face of the epidermis. This part of the cell wall is called the
cuticle. It contains a fatty substance, cutin, that renders the
epidermis nearly impervious to gases and fluids, so that the leaf
is entirely surrounded by a water- and gas-proof coat. In many
leaves hairs of various forms grow out from_the epidermal cells
(Fig. 8).

5. The Mesophyll. — The cells within the epidermis have, for
the most parts, a greenish color due to numerous green bodies,
chloroplasts, contained in the cells (Fig. 5, ch). These green
cells of the leaf, termed the mesophyll, consist of one or more
layers of rather elongated and compact cells on the upper side
of the leaf (the palisade mesophyll, Fig. 5, p), and below they

NATURE OF PLANTS n

are joined to rather irregular and loosely arranged cells (the
spongy mesophyll, Fig. 5, sp). Numerous air spaces extend in
all directions through this spongy mass of cells reaching up to
and between the palisade cells and down to the stomata (Fig.
5, i). Bundles of small cells also appear here and there between
the palisade and spongy mesophyll (Fig. 5, v, I). These struc-
tures are the so-called veins that appear as fine lines in the
majority of leaves (Fig. 3). These veins or vascular bundles
branch again and again and so extend to all parts of the leaf.
In Fig. 5, V one of the veins has been cut square across while
at / the end of a small vein is shown, having been cut in half
lengthwise. The walls of the mesophyll cells are smooth, thin,
and elastic and composed of a substance called cellulose. Thin
places or pores are often formed in the walls which appear as
openings owing to the extreme thinness of the wall at such places.
The term parenchyma is applied to all cells possessing the char-
acter of walls noted in the mesophyll cells. In the vascular
bundle, however, a portion of the cells have thicker walls which
are more rigid, hard and woody. Such cells are often referred to
as prosenchyma.

What is the significance of the peculiar and varied arrange-
ments of the cells in the leaf? We naturally anticipate that these
different structures are adapted to performance of special duties.
Attention will first be directed to the work performed by the
green portion of the leaf, the mesophyll. Similar cells containing
chloroplasts are of common occurrence in various parts of the
plant, being often found in stems and unripe fruits. In the leaf
such a tissue is termed mesophyll but a more general term, Chlor-
enchyma, includes all chlorophyll-bearing tissue wherever found.

6: The First Function of the Leaf, Photosynthesis.— Chlor-
enchyma is the most important tissue in the plant since it has
for its special function or work the construction of carbohydrates,
such as sugars and starches, which are among the most impor-
tant foods of the plant. The construction of these carbohydrates
by the chlorenchyma is called photosynthesis. This work easily
ranks as the leading wonder of the world because its processes
are so complicated and of such magnitude and importance in the

12 NATURE OF CARBOHYDRATES

economy of this world of ours. It will be seen that this tissue
not only keeps the air in a wholesome condition for breathing
and so makes life possible upon the earth but that all animal life
is absolutely dependent, either directly or indirectly, for food
upon the products built up by the chlorenchyma. Let us see
how these foods are formed. Various substances are absorbed
by the roots, and the vascular bundles, which extend throughout
the plant body, transfer them to the chlorenchyma. This ab-
sorbed material is in large part water, but various mineral sub-
stances are dissolved in it. Various gases, as carbon dioxide,
also find their way through the stoma to the chlorenchyma. All
of these absorbed substances are composed of elements. Thus
water consists of two elements, hydrogen and oxygen, in the pro-
portion of two parts of hydrogen to one part oxygen. This com-
position is indicated by the abbreviation, H2O. So the gas,
carbon dioxide, contains one part of carbon and two parts of oxy-
gen, indicated thus CO2. Chloroplasts have the power under
suitable conditions of decomposing or tearing apart these com-
pounds and of reuniting their elements into new compounds that
are quite different from the original ones. In this way CO2
and H2O are decomposed and the elements reunited into complex
compounds or foods consisting of carbon, hydrogen and oxygen.
Such complex foods as cane sugar, consisting of twelve parts of
carbon, twenty-two of hydrogen, and eleven of oxygen are formed
in this way. This very common food is expressed by the formula
C12H22O11. So also other foods are formed, as grape sugar
C6H12O6 and starch usually indicated as some multiple of CeHioOs.
The cellulose walls of the parenchyma cells have the same com-
position as starch. It will be seen that all these foods have
carbon united to hydrogen and oxygen in the same proportion
as it exists in water. Cane sugar has carbon united to eleven
times H2O, and starch five times H2O. It was supposed that
these substances were a combination of carbon and H2O. For
this reason they were called carbohydrates. This name is still
applied to these foods although it is known that the base is not
H2O but the hydroxyl OH. The actual changes that are effected
in the formation of these various carbohydrates are not known.

NATURE OF PLANTS 13

It has been suggested that CO2 and H.O enter into solution form-
ing carbonic acid (H2CO3). This change is represented by the
equation, CO2 + H2O = H2CO3. By giving off one part of
oxygen this acid is changed to formic acid (CH2O2), i. e., H2CO3
= CH2O2 +0. A similar decomposition is eiTected in the formic
acid which results in the formation of formaldehyde (CH2O),
i. e., CH2O2 = CH2O + O. The two parts of oxygen pass
from the cells and escape through the stomata as a gas. In
the presence of certain alkalies and acids, formaldehyde increases
the number of elements which compose it, thus six times CH.O
would give CeHnOe or grape sugar. This is but a theory based
upon the facts that CO2 and H2O do unite in nature to form
H2CO3 and that this acid may decompose as stated above, and
finally that formaldehyde can be detected in plant cells and it
is possible to produce sugar from this substance by treating it
with alkalies and acids. All that can be definitely stated about
the changes going on in the leaf, during the formation of the
carbohydrates relates solely to the beginning and end of the
process. Water and carbon dioxide enter the chlorenchyma
cells. A series of changes follows, the nature of which can only
be conjectured. Finally, as the end result, sugar appears in
the cells and oxygen is set free, escaping as a gas.

These complex changes are brought about in the leaf in so
subtle a way that we are not conscious of them or of the great
amount of energy that is required to effect them. It requires
the energy expressed by a temperature of 1300° C. to decompose
CO2 into its elements. Where does the plant obtain the energy
to bring about the decompositions and recomposition? This
work is accomplished by the energy- of the sunlight acting upon
the chloroplasts. The chloroplasts of the seed plants are minute,
rather lens-shaped grains, and increase in number by the division
of the plastid into two equal parts (Fig. 9). These bodies are
denser portions of the cytoplasm and like it are nearly colorless.
In the presence of light a green, oily substance, chlorophyll, is
formed in the plastids, thus producing their green color. Chloro-
phyll is rarely formed except in the presence of light. Plants
grown in cellars or in the dark are of a pale color owing to the

14 NATURE OF CHLOROPHYLL

absence of chlorophyll in the plastids. Similarly green plants
lose their chlorophyll when placed in the dark. This is the
principle employed in the blanching of celery. The stalks are
surrounded by earth or the light is excluded from them by some
other means; chlorophyll disappears from the plastids and fails
to develop in the new cells that are formed in the dark. Plants
in which the chlorophyll is not developed are said to be etiolated.
The plastids are in the cells, however, for if an etiolated seed-
ling is removed from the dark to the light the chlorophyll will
begin to appear in a few hours. The greening of potatoes when
removed from the ground and left in a strong light is a familiar
example of this. It is evident that the formation of chlorophyll
is usually closely connected with the action of light. Another
important point regarding the influence of light is the fact that
such a food as starch is not usually found in the leaf in the
absence of light. This may be easily verified by cutting the
initials of your name in rather large letters in a strip of black
paper or smooth tin-foil and fastening the strip in the early
morning to the upper surface of a well-sunned leaf of a starch
forming plant. After a few hours remove the leaf from the plant
and place it in alcohol. This will dissolve the chlorophyll and
the leaf will become quite white in a few hours. Now place the
leaf in a tincture of iodine which turns starch to a blue or blue-
black color. Wherever the light was excluded from the leaf
there is only a pale yellow color but all portions of the sunned
leaf show an abundance of starch as revealed by the blue or blue-
black coloration. Consequently the letters appear black on a
yellow background which marks the limits of the strip of paper.
No starch can be detected as a rule in leaves taken from plants
that have been growing in the dark for twenty-four hours.
Plastids that do not contain chlorophyll, examples of which are
found in the cells forming the white bands or blotches in many
variegated plants, do not form starch in the light. It is evident
from these facts that the chlorophyll and light co-operate in
some way in the formation of starch. It is known that chloro-
phyll absorbs a certain portion of the sunlight. If a beam of
sunlight is caused to pass through a prism it is separated into

NATURE OF PLANTS 15

seven colors, i. e., red, orange, yellow, green, blue, indigo and
violet. When, however, a beam first passes through an alcoholic
solution of chlorophyll or through a green leaf a portion of certain
colors and all of other colors do not appear. The chlorophyll has
taken up or absorbed certain of the light rays especially portions
of the red, blue and violet. It is reasonable to suppose that the
energy in these rays is utilized in part by the plastids in effecting
the changes noted above in the formation of foods. The sun
gives to the earth great quantities of energy in the form of light.
A portion of this light is taken up by the chlorophyll, and the
plastid uses the energy of the light to bring about the recombi-
nation of CO2 and H2O and the formation of sugars, starches,
and other carbohydrates. Because of the importance of the
sunlight in the formation of carbohydrates this process is called
photosynthesis. This term means the putting together by means
of light.

7. The Magnitude of the Work of Photosynthesis. — Carbon is
an important element in the composition of plant foods and also
in the walls of the cells. It forms one half of the dry weight of
the plant. All the carbon appearing in the plant is derived solely
from the CO2 in the air, of which it forms a very small part, only
about three parts in ten thousand. Furthermore the carbon
comprises only 3/11 by weight of the CO2, 8/1 1 being oxygen.
A square meter of leaf surface, however, can withdraw all the
CO2 from 2500 liters of air in one hour. Such a volume of air
would be a space one meter square and 2^-2 meters high. This
would furnish sufficient carbon for the construction of one gram
of starch. In this way such large quantities of CO2 are drawn
into the chlorenchyma cells as to make possible each >car the
harvest and the renewal of vegetation of the earth. In the
United States alone in this w^ay there was in 1914 built up over
734 millions of pounds of cotton, 891 million bushels of wheat.
1 139 million bushels of oats, etc., representing a value for all
crops of 7 billion dollars. Our leading crop is corn which amounted
to nearly 2^ billion bushels and valued at about i^i billion
dollars. The gold and silver coin and bullion of the country are
not of greater value. This corn came up and matured in 120

i6 NATURE OF PROTEIDS

days, consequently these coin plants were manufactUi^ng food
at the rate of over 15 million a day. These figures represent only
a small portion of the work performed, since the grain is but a
part of the plant, and furthermore the larger portion of the land
is covered with forests and other forms of vegetation. The fact
must not be overlooked that this process of photosynthesis is of
vital importance to our welfare in another way. Owing to the
large volumes of CO2 that are constantly formed by fires and the
respiration of animals such an excess of this gas would accumu-
late in the atmosphere that all animal life would eventually cease
were it not for its absorption in photosynthesis. So balanced
are the rates of formation and absorption, however, that the
percentage existing in the air does not materially vary.

8. The Construction of Proteids. — A second group of foods
formed by plants are called proteids. These differ from the car-
bohydrates in that they contain nitrogen in addition to carbon,
hydrogen and oxygen. They are more complex compounds than
the carbohydrates and may contain sulphur and phosphorus in
addition to the four elements mentioned above. Less is known
about their formation than of the carbohydrates. It is probable
that they are largely formed in the leaves and by a process simi-
lar to that of photosynthesis. The sulphates, phosphates, and
nitrates absorbed from the soil are decomposed and the elements
of nitrogen, sulphur or phosphorus are united to simple carbon
compounds and complex proteids are the result. Light probably
does not co-operate directly in this construction although it
may do so indirectly. These foods are formed in much smaller
quantities than the carbohydrates but they are of the greatest
importance in the nourishment of the plant. This is especially
true as regards the living substance, protoplasm, which resembles
somewhat in composition some of the more complex proteids.

9. The Distribution of the Foods. — Let us now consider what
becomes of these foods. A small part is consumed on the spot
by the manufacturing cells themselves, a larger portion is trans-
ported through the vascular bundles to all the living and grow-
ing cells of the plant body, but as the plant approaches the com-
pletion of its annual growth a larger and larger part of the food

NATURE OF PLANTS 17

is transferred to special parts of the plant, such as buds, roots,
seeds. Here it is stored as a reserve food to meet the needs of
the plant at such times as it is not able to manufacture food.
This transfer of foods is slow and consequently the rapidly con-
structed sugars gradually accumulate in the chlorenchyma during
the day. This would result in the saturation of the cells with
sugar and so stop the work of photosynthesis were it not for the
fact that the chloroplasts quickly change the sugar to insoluble
starch, thus leaving the cells free to receive more sugar. If
chloroplasts of well-sunned, starch-forming leaves are examined
in the afternoon, they will be found to contain minute glistening
bodies, the starch grains (Fig. 9, s). The chloroplasts have the
power to absorb sugar and secrete starch. The construction of

Fig. 9. Greatly enlarged chloroplasts from leaf of moss: a, plastid with
three starch grains, s; b, plastid elongating preparatory to division; c, d,
later stages in division.

sugars and their transformation into starch is effected with sur-
prising rapidity. If some plants of pond scum are placed in the
dark for twenty-four hours so that all sugars and starches may
be consumed, starch will re-appear in the cells in these plants in
from three to five minutes after their return to the light. Con-
sider the absorptions, decompositions, recompositions, and trans-
formations that have been effected in this short time. Is it
surprising that the exact nature of the changes effected in the
manufacture of foods is not known? It is evident as a result of
the rapid formation of sugar that starch must accumulate in
the plastids during the day. At night sugar is no longer formed
but the transfer of food continues as in the day. The insoluble
starch is now dissolved bv means of a ferment or enz\me into

i8

STORAGE OF FOODS

sugar so that by morning the cells are quite emptied of the sugar
and ready for another day's work. This accumulation and trans-
fer of foods can easily be demonstrated by cutting off in the
afternoon leaves from clover, bean or other starch-forming plant
and placing them in alcohol. When blanched and tested with
iodine the blue or blue-black color shows that they have accumu-
lated a large amount of starch. Test in the same way leaves
taken from these plants early in the morning. No starch re-
action will be seen because the cells have been freed from their
starch during the night. The formation of starch is often used
as a test for photosynthesis, but it must be borne in mind that
photosynthesis first results in the formation of various sugars
and that starch only appears when the sugars have reached a
certain percentage in the cells and indeed that some plants do
not form starch at all. So when starch appears in a leaf it is
simply the measure of the excess of carbohydrate manufactor
over transportation. Furthermore starch will form in the cells
in the dark if a plant is supplied with an excess of carbohydrate.
10. The Storage of Foods. — It has been stated that the foods
not required for the nourishment and growth of the plant are
stored in the region of buds, the growing portions of stems,

Fig. 10. Storage food: A, starch grains in cell of potato — n, nucleus.
B, cells of bean, the smaller particles being proteid grains.

in bulbs, in roots, in seeds, and in fruits. These are the so-
called storage foods and they remain in these regions for con-
siderable periods or until the conditions are favorable for a re-
newal of growth. They generally assume much more definite

NATURE OF PLANTS

19

forms than the products constructed in the chlorencliyma which
were designed for immediate use or translocation to other parts.
If we examine a potato the cells will seem to be filled with
very regularly constructed starch grains (Fig. 10, A). In the
pea seed and in wheat and other grains proteid granules are
associated with the starch grains (Fig. 10, B). These starch
grains are formed by rather colorless plastids called leucoplasts
(Fig. 3, E) that closely resemble the chloroplasts. The starch
grain first appears in the leucoplast as a minute point. Its strati-
fied structure is due to the successive deposits of starch by the
leucoplasts. If the grain is built up equally on all sides by the
leucoplasts it will possess regular and concentric strata as in the
bean (Fig. 11, ^). If the bulk of the leucoplast lies on one
side of the grain, this side of the grain will receive more material
and consequently the starch grain will become more or less one-

FiG. II. Fig. 12.

Fig. II. Starch grains: A, from bean. B, from potato. C, compound
grain from potato.

Fig. 12. Section of the outer portion of a grain of wheat : p, cells containing
proteid grains. The larger cells are filled with starch. — After Strasburgcr.

sided as in the potato (Fig. 11, B). Frequently two or more
grains originate in one leucoplast and compound grains result
(Fig. II, C). Proteids and other foods are likewise transported
in solution to the storage organs where they may or may not
be deposited in solid form. This is usually efTected directly by
the protoplasm of the cell without the aid of any plastid. Ani-
mals have learned to use the foods stored in these organs just
as does the plant. It is interesting, however, to note that we

20 NATURE OF ENZYMES

cannot use them as economically as the plant does. In the
wheat, for example, the proteid material is largely confined to
the outer portion of the grain (Fig. 12). This layer is of a
darker color and in the "best flour" it is excluded in the milling
process, leaving the flour very white. So the most nutritious
part of the wheat does not appear in white bread, but it should be
remembered that white bread is better than the much exploited
"whole wheat" products because the proteid material is digest-
ible to only a slight degree. These resei-ve foods are made
available to the plant in the same way as to the animal. They
are first put into solutions by ferments or enzymes. Ptyalin in
the saliva of animals and diastase in the plant are common ex-
amples of enzymes which change starch into sugar. So there are
enzymes which transform each reserve food and render it ca-
pable of transport and incorporation into the substance of the
plant body. These enzymes are formed by the living substance
of certain cells and resemble in many ways the living matter
itself. They are most extraordinary in their action and behavior.
The decomposition of the substances upon which they react is
effected by the mere presence of the ferment — at least there is
no permanent union between ferment and the substance. Conse-
quently a very small amount of a ferment may effect a decom-
position of an almost unlimited amount of a given substance.
More commonly water enters into the composition of the sub-
stance as a result of the presence of the enzyme. As a result its
stability is upset and it breaks down into simpler compounds.
This decomposition due to the incorporation of water is termed
for this reason hydrolysis. We say that diastase hydrolyzes
starch into simpler compounds, as maltose and dextrines.
Some enzymes appear to effect decompositions without the co-
operation of other substances. Still more remarkable is the
action of these ferments in that some of them appear to have a
reverse action, i. e., the power of building up again complex
material from the parts into which they have separated it. In
many ways enzymes behave like living matter, being quickened
in their activities by suitable temperatures, the concentration
of the fluid or by the presence of certain acids and alkalies; or

NATURE OF PLANTS 21

retarded by solutions containing copper or mercury which also
poison living matter. The distribution of foods made possible
by enzymes has been compared to the currency of a country;
the storage foods are the bank reserve while the sugars and other
solutions are the circulating currency.

11. Significance of the Leaf Structure. — We now begin to
understand the significance of the structure and form of the leaf.
The broad blade is a device to catch the energy of the sunlight.
The stomata afford access of CO2 and escape of oxygen. The
intercellular spaces in the spongy mesophyll all lead to the
stomata, thus bringing about a quick distribution of the gases
and they also increase the area for the absorption of CO2 and
excretion of oxygen to such an extent that the internal surface
of the leaf exceeds the external many times. It will be noticed
that the cells of the spongy mesophyll vary in character. This is
because of the different functions that they perform. The col-
lecting cells (Fig. 5, col) are closely applied to the palisade cells
and collect the sugars and other foods from them while the elon-
gated conducting cells transport this food to the cells of the
vascular bundles.

12. The Second Function of the Leaf. — Respiration or breath-
ing is a second function of leaves. While work of this kind is
performed by all living cells it may be considered at this point
because in the leaves the nature of the work is \cr\ well illus-
trated. The w^ork of respiration is usually associated with the
absorption of oxygen and the giving off of CO2. Animals and
plants are constantly taking in oxygen and giving off CO2. This
interchange of gases is the reverse of that occurring in photo-
synthesis. Furthermore respiration goes on all the time when
conditions are suitable for life while photosynthesis only is pos-
sible in the light. Let us consider the significance of this inter-
change of gases. Why do animals and plants breathe.-* The
construction of the foods to which attention has been called may
well be termed the storing up of the energy of the sunlight.
When the chlorophyll absorbs certain rays of light it docs not
destroy any of the energy in those rays. The energy is changed
to another form and it still exists in the sugars and other com-

22 NATURE OF RESPIRATION

pounds that are built up through its power. The energy is
locked up, so to speak, in the foods and finally it is lodged in the
compounds that are constructed from the foods, i. e., in the
living substance and in the tissues of the plant. If now any of
these compounds are decomposed into their constituent parts
then the energy will be set free unimpaired. Oxygen is the
principal agent in carrying on this decomposition. It has a
great attraction for nearly all the elements occurring in the plant
and so it is able to detach one element from another and so
effect decomposition and the liberation of the locked up energy.
You are not to think of the oxygen entering directly into combi-
nation with the foods or with the substances formed from the
foods but rather with the decomposition products that are
brought about by the living matter. It is probable that respira-
tion starts in the living substance. Some agent, other than
oxygen and possibly enzymic in nature, inaugurates these
decompositions and then oxygen comes in at some stage in this
breaking down process and makes possible a further reduction.
As evidence that the oxygen does not combine directly with the
more complex substances mention may be made of the fact that
the absorption of oxygen is quite independent of the amount of
oxygen present. The plant respires at the same rate in an
atmosphere of pure oxygen as in an atmosphere containing one
twentieth part of the oxygen in the air. Furthermore the
amount of oxygen absorbed is independent of the amount of
foods stored in the plant. So respiration is not a simple and
direct process like combustion where the percentage of oxygen
and the amount of material present determine the reaction.

In the majority of cases oxygen causes a continuation of the
decomposition processes until such simple substances as CO2
and HO compounds, such as water, are formed — in a word
until the very substances utilized in photosynthesis are formed.
Some of these simple products, such as H2O and CO2, escape
from the stomata as gas and water vapor. Accordingly respira-
tion is usually indicated by the absorption of oxygen and the giv-
ing off of CO2 and H2O. Some plants, however, have sufficient
oxygen in their tissues to enable them to breathe for considerable

NATURE OF PLANTS 23

periods without absorbing oxygen from the air and on the other
hand in some plants CO2 is not always given oil because the
decomposition is not carried so far as to result in the formation
of so simple a product. We eat in order to gain possession of
the energy locked up in the foods. We breathe in order that the
oxygen may assist in the decomposition of these compounds and
set free this energy which gives us power to work and move
and keep our bodies warm. The plant lives in the same way.
It only differs from the animal in that it has the added power to
build up the complex food compounds from crude material.
By the decomposition of the products formed from these foods it
gains energy to grow and carry on its other vital functions.
The work of respiration is carried on more economically by green
plants than by animals since in the animal the CO2 escapes in the
breath as a waste product, while the plant uses this CO2 during
the day time for the construction of foods. Consequently the
escape of CO2 can only be observed during the night and cannot be
detected in the light unless an examination of very rapidly grow-
ing organs be made. If a jar be nearly filled with opening buds
of dandelions or rapidly growing shoots and then closed air tight,
sufficient CO2 will be respired in a few hours to extinguish a light
that is lowered into the jar. In such instances as these very
rapid respiration is necessar}- to furnish the required encrg\' for
growth and the volumes of CO2 expired exceed many times the
volume of CO2 utilized in photosynthesis. A handful of ger-
minating peas or beans placed in a closed jar for a few hours better
illustrates the giving off of CO2 because here there is no green
tissue to absorb any of the CO2. Plants are often considered
unhealthful in sleeping rooms at night because of their cxlialation
of CO2. It is well to remember that the amount of CO2 expired
by a plant is small and that a gas jet would furnish more CO2 to
the air than a window full of plants. A square meter of leaf
surface gives off about .12 gm. (60 c.c.) of carbon dioxide per
hour at 20° C.

We are now in a position to understand ilie imi->ortance of
photosynthesis and respiration. Photosynthesis keeps tlie air
pure for breathing, decomposes the simple inorganic compounds

24 AMOUNT OF TRANSPIRATION

and recombines them into foods which represent a certain amount
of stored-up energy. Respiration breaks down the products
formed from these foods and Hberates the energy necessary for
the growth and activities of the plant.

13. The Third Function of the Leaf, Transpiration. — This
function refers to the giving off of water by the plant. While
other parts of the plant assist in transpiration, the leaf is the
principal organ upon which this very considerable work devolves.
Water is given off from the plant as a vapor and for this reason
transpiration is a more familiar phenomenon than photosynthesis
and respiration, where we are dealing with an interchange of
invisible gases. We see the vapor from plants growing in a
window precipitated on the cool window panes in the form of
drops. On hot summer days the leaves of plants droop. This
is because they have transpired so much water that their cells
are no longer distended by the water, consequently the cells shrink
and the leaves contract or wilt. When plants are covered by a
bell jar or placed in a tight glass jar the water given off soon
saturates the air and collects in drops on the sides of the jar.
The amount of water transpired by a plant is surprisingly large
and it is probably safe to state that usually it amounts daily
during the hot summer months to more than the plant's weight.
An oak with seven hundred thousand leaves was estimated by
Ward to transpire from June to October 244,695 pounds of water.
A birch with 200,000 leaves transpired 700 to 900 gallons on hot
summer days. This means that an acre of such trees would give
off in the course of the season 3,168,000 pounds of water. From
careful measurements of the amount of water given off by grass
plants it has been calculated that six and one-half tons per
acre may be transpired daily during the summer. It is estimated
that from 200 to 500 lbs. of water is transpired in the construction
of one pound of dry substance and that a square meter of leaf
surface evaporates about 50 gms. per hour.

The question naturally arises why are such large volumes of
water transpired? This depends in part upon the size of the loaf
and external conditions, the size of the leaf being determined by
the relation of transpiration to photosynthesis. Conditions mak-

NATURE OF PLANTS 25

ing possible vigorous photosyntheses would result in extensive
growth and leaf development. An extensive leaf area, however,
may not necessitate a heavy transpiration; the amount of water
given off will depend upon external conditions. In warm moist
climates conditions are favorable for photosynthesis but not for
transpiration which is reduced in amount by tlie moist air.
So growth is vigorous, the largest leaves are developed and
transpiration is feeble. In warm but dry climates, on the other
hand, while conditions are favorable for photosynthesis the dry
air promotes excessive transpiration. This interferes with
photosynthesis, consequently very small leaves develop and the
volume of water transpired is correspondingly reduced. Under
conditions of moderate temperature and moisture as in most
temperate climates, the leaf assumes a size intermediate between
these two extremes because these conditions favor vigorous
photos^^nthesis and the resulting growth is not materially checked
by transpiration. So we see that the extensive transpiration in
temperate regions is in part the result of the large leaf develop-
ment and that this leaf area is dependent upon a vigorous photo-
synthesis.

Only a small part of the water absorbed by the plant is re-
quired to furnish the necessary mineral substances and the water
utilized in its growth. The surplus is forced out of the cells and
finds its way in the form of vapor along the intercellular spaces
in the spongy mesophyll through the stomata to the air. The
transpiration of these large volumes of water must be of impor-
tance in keeping down the temperature of the plant during the
burning summer heat. Transpiration is often compared to the
evaporation of water from a dish. While it is controlled to a
limited extent in the same manner as evaporation, it should he
borne in mind that the giving off of water is intimately associated
with the vital activities of the cells and that the loss of water is
to a degree under the control of the plant. The cuticle, which is
practically impervious to water and gases, extends as a coat over
all parts of the plant body, and materially assists in controlling
the amount of transpiration. The thickness of this protective
coating depends upon external conditions, being developed in

26 THE STOMATA

proportion to the ratio of transpiration to absorption. So we
find that it becomes thicker as the volume of water given ofT
comes nearer and nearer to the volume absorbed. In this way it
serves as a very effective check in preventing a fatal loss of water.
As a result of this water-proof coat the vapor can only escape from
the leaf through the stomata. These minute openings are one
of the most interesting features of the leaf. When there is an
adequate supply of water the elliptical guard cells are drawn apart
and give free exit to vapors and gases. However in cases of
drought when a continued loss of water would prove harmful to
the plant, or when an interchange of gases is no longer required
then the guard cells close and so offer such a barrier against
further loss of water that only the severest conditions, such as
prolonged heat and drought, can overcome. It has been claimed
that the stomata do not regulate transpiration because it has
been observed in some cases that the stomata are not closed when
wilting begins or fully opened at the time of maximum transpira-
tion. Further observation is required to settle this question and
for the present we must believe that so elaborate a mechanism
as the guard cells is of significance. The stomata in the majority
of cases range in size from .0002 sq. mm. to .0008 sq. mm. so that
in comparison a needle prick would appear as a huge hole, but
they are so numerous, 40 to 300 to the sq. mm., as to comprise
about I per cent, to 3 per cent, of the area of the leaf surface.
It might be questioned if this extent of opening is sufficient to
permit the entrance and exit of the large volumes of gases and
water handled by the plant. It has been shown that a mem-
brane pierced by sufficiently small openings which are separated
from one another by a distance of at least ten times their diam-
eter, permits diffusion of gases as readily as though there were
no membrane at all. The structure of the epidermis is an
admirable example of such a perforated membrane and it is so
perfectly adjusted to the work in hand that it could accomplish
much more than the necessities of the plant demand.

14. Noteworthy Features of Leaves. — Now that some idea has
been gained of the nature and extent of the work performed by
the leaves Ave are prepared to comprehend the meaning of the

NATURE OF PLANTS 27

large extent of the leaf surface and of the arrangement, siruciure
and modification of the leaves. The broad blades are devices for
gathering or absorbing as much of the sunlight as possible and
they increase the surface of the plant many hundreds of times.
Contrast the extent of surface of a tree in full foliage with the
bare branches of the winter. A mature maple develops annually
from one to two thousand square yards of leaf surface.

One of the most noteworthy features about the leaves is their
arrangement or "hang" on the branches. In some plants they
are arranged in two vertical rows, in other instances three, four,
five, eight or more rows. By fastening a thread to a leaf and
winding it about the stem so as to touch the petiole of each suc-
ceeding leaf the arrangement of the leaves becomes more obvious.
Some plants have their leaves opposite in two rows or ranks,
others opposite in four ranks, each succeeding set of leaves being
at right angles to the lower set. This latter arrangement is called
decussate (Fig. 13). More commonly the leaves are spirally
arranged and the thread passed once, twice, thrice, five, eight,
etc., times around the stem before a leaf is reached that is exactly
over the one from which we started (Fig. 14). The number of
leaves passed before reaching one that stands over the first leaf
indicates the number of rows or ranks of leaves on the stem. This
variation in the arrangement of the leaves is simply a device to
bring the leaves into the light and prevent the shading of one leaf
by another. In opposite two ranked leaves each succeeding leaf
is over the one below it but the cutting off of the light is pre-
vented by lengthening the intemodes and thus separating the
leaves. The decussate arrangement of the leaves is a more eco-
nomical arrangement because more leaves can be developed on
a given length of stem without danger of shading. The spiral
distribution of leaves is still a better device since each succeeding
leaf is placed a little above and to one side of the next lower leaf.
By this means the maximum of leaf surface can be exposed to
the light in a given length of stem. No one can look at the leaves
of the maples, dogwoods, ailanthus, creeping and climbing vines,
etc., and not be impressed with the fact that the leaves are
arranged so as to catch the most favorable light without shading

28

LEAF ARRANGEMENT

or interfering with one another. Furthermore the arrangement
is such as to occupy practically all the space about the stem that

Fig. 13. Fig. 14.

Fig. 13. Branch of Forsythia, leaves decussate, in four rows.
Fig. 14. Branch of poplar with leaves spirally arranged in five rows.

receives light. This becomes very noticeable if we look directly
down upon a shoot or better if we stand under a tree and look
up into the branches. It will be seen that very little direct sun-
light finds its w^ay through the branch, so nicely are the leaves
adjusted to each other. Some plants like the hickories, catalpa,
etc., occupy all the available space with a few large leaves at the
tips of the branches. In other cases, as in the willows, some
lilies, etc., the same result is accomplished by many small leaves
that can be arranged along the stem for considerable distances
without shading. The nicety of leaf arrangements is especially
noticeable in many horizontal and creeping stems. In such cases
the leaves can only be exposed on the sides of the stem and con-
sequently the stem may become twisted or more usually the

NATURE OF PLANTS

2Q

petioles are variously elongated and curved in order to bring the
leaves into proper relation to the light (Fig. 15). Notice the

Fig. 15. Horizontal branch of Forsythia with leaves in two rows owing to
the alternate twisting of each intcrnode, assisted by the curving of the petioles.
Compare Fig. 13.

change in leaf arrangement in horizontal and erect stems of maple.
In an example like the maple if the leaves could not change their
position they would all be standing edgewise to the light on hori-
zontal branches and therefore receive little of it. They not only
place their blades at right angles to the light but owing to the
greater elongation of each succeeding petiole from the apex
toward the base of the stem, all the leaves are arranged one be-
yond another so as to overlap very little. This same device is
noticed in many plants that produce their leaves close to the
ground in rosettes, as mullein, wood betony, plantain, etc. (Fig.

f^

Fig. 16. Leaf rosette of evening primrose. Each leaf blade has a longer
petiole than the one above it, so that the leaves are not shaded ^^• ""- •"•.''!■'

16). Compare the erect and horizontal branches of a variety i>l
plants noting by what devices the leaves are brought into the

30

FORM OF LEAVES

light. Especially instructive are the leaf arrangements of plants
growing in windows or creeping over trellises, etc., where compli-
cated twisting and elongation of stems and petioles are necessary
to adjust the leaves to the one-sided illumination. The angular
leaves of some begonias furnish excellent illustrations of this.
Perhaps so many plants bear angular leaves because they can be
better adjusted and fitted together without loss of space than
would be the case in rounded leaves, and smaller leaves can also
be more advantageously introduced between the larger ones
(Fig. 17). The lobing and branching of leaves has become a
characteristic of many plants because such variations permit the

Fig. 17. Leaf of Aralia. Note the angular shape of the leaflets and the
smaller ones filling the space between the larger ones.

illumination of a larger leaf surface. This is particularly notice-
able in our oaks where the outer leaves are often deeply lobed,
thus permitting considerable light to pass through to the under-
lying leaves. Observe also that these lower leaves are larger and
less lobed, thus catching as much as possible of this rather feeble
light (Fig. 18). In many plants the lobing extends quite to the
middle of the leaf and the lobes are often attached to the midvein
or midrib by a petiole. In this latter case the leaf is said to be
compound (Fig. 19). All such modifications permit the develop-
ment of numerous leaves upon the branches without the danger
of shading. If there is still any doubt as to the perfection of this
light catching arrangement of the leaves, try to substitute the
somewhat similar leaves of two different trees as the birch and elm

NATURE OF PLANTS

31

or the water beech (Carpimis) and the beech, noting how the
leaves of one fit when placed upon the branch of the other. This
subject of leaf form and arrangement may be summed up by the
statement that these features are devices for exposing the
maximum leaf surface without one leaf interfering with another.
This is in accord with the fact that light is practically of no value
in photosynthesis after passing through two leaves.

Another interesting feature about the leaf is its relation to the

Fig. 18. Fig. 19.

Fig. 18. Leaves of red oak: A, sunned leaf. B, shaded leaf.
Fig. 19. Compound leaves: A, red ash. B, horse chestnut.

intensities of light. Some plants demand tlie full intensities of
sunlight while others can tolerate only a small fraction of it.
Some lichens will grow in a light only 1/156 part of the full in-
tensity while many plants, as the grasses, will endure the strongest
illumination. Beech, maple and spruce are tolerant of shade and
oak, hickory and chestnut are intolerant. This relation of plants
to light is an important factor in controlling their associations
and we frequently speak of plants as sun loving, partially so,
shade, and deep shade plants.

The coloration of leaves is often of significance. In addition
to the green color, of which we now have some iindcrst uuluig.

32 LEAF COLORATION

a variety of other colors often appears. One of the most common
is red. Young shoots of the rose, grape vine and so much of the
early spring vegetation are tinged with red. So in the fall our
vegetation takes on a greater wealth of coloration than is seen
in any other land. And finally there are leaves variegated with
yellow, white and red, colored fruits and other organs, and the
endless hues of the flowers. These colors are caused by pigments
or by chromoplasts (p. 2) that are developed in certain cells and
which either transmit or reflect to the eye their particular colors.
The white blotches of variegated leaves are caused by the absence
of chlorophyll, thus allowing the light to pass practically un-
changed, or in some cases these areas are characterized by large
intercellular spaces filled with air. Such a structure would tend
to reflect the light and so contribute to the white appearance of
the area (p. 39). In some cases these colors may be of service to
the plant. The red probably functions as a screen to young or-
gans, shielding them from the intense light that would otherwise
decompose the forming chlorophyll and otherwise interfere with
the vital processes. Red is also a strong absorber of the heat
rays in light, and in some cases it may be of service in ensuring a
higher temperature in the organs and so expedite their work.
Many plants, some tradescantias, hawkweeds and spatter dock,
have in their mature leaves layers of cells, frequently the low^er
epidermis, filled with a red pigment. So many evergreens assume
a brown or reddish hue in the winter owing to the formation of a
reddish pigment. However, too much emphasis should not be
given to this matter of coloration. The colors of flowers, to be
sure, are of service in guiding insects when close at hand, to the
proper approach to the flower, and there is probably a significance
in the coloration of the algae (p. 173) but in general it is not
possible at present to offer an explanation for the variety of colors
that characterize so many leaves, especially noticeable in tropical
plants, and fruits and other organs, as the radish, and beet.
The copper beech grows slower than our green-leaved beech
and so doubtless in the case of many plants that have survived
not because of their coloration but in spite of it. In many
instances the colors are due to the acidity or alkalinity of the

NATURE OF PLANTS 33

cell sap. Red cells often Indicate acidity and they become blue
in weak potash solutions. So violet and blue indicate an al-
kaline condition and such cells change to red in weak acid solu-
tions. This is essentially the explanation of the changing tints
in the autumn. The changing character of the cell sap is at-
tended with a gradual decomposition of the complex chloroi)liyll
and other materials that results in the formation of a series of
substances that are characterized by a variet>' of colors. Here
again the matter of coloration has been over emphasized. While
nitrogen, phosphorus and potassium are transported from the
leaves before they fall, the coloration does not effect so complete
an emptying of the leaves as was previously believed, if indeed it
is of any service.

15. The Cause of Leaf Arrangement. — How is the perfection
in the arrangement of the leaves accomplished? The leaf or
any other organ does not remain in a fixed position during its
growth and development but owing to the fact that first one side
and then an adjoining side of the organ is growing faster than
any other part it comes about that the organ is bent from side to
side by the more rapidly growing cells and the apex of the organ
is often caused to travel through a rather irregular circle. These
growing organs are sensitive to light, gravity, moisture and other
stimuli. As a result of these movements the organ is brought
into relations with various intensities of light, heat, etc. In cer-
tain positions the stimuli are not favorable and they cause it to
grow away from this position while in other positions the stimuli
act in the most favorable way and the organ is stimulated to its
best growth. So during the development of the orv^^an it is
brought into varied relations with the forces which affect it and
as a result of this experience the leaf or other organ is directed
by the stimuli and at maturity finally comes to rest in a fi.xed
position that is the most advantageous. The leaf ordinarily so
reacts to these stimuli that its blade is exposed at right angles
to the rays of light, but in the iris, many grasses and rushes
the blades are nearly erect. Quite a large numluT of plants l)ear
their leaves edgewise. The giant trees of Australia (Eucalyptus)
and the so-called compass plants are famili.ir txanipK's (Fig.
3

34

LEAF ADJUSTMENT

20). Possibly these blades are driven into this position because
they are more sensitive and the direct sunlight upon the broad
surface of the blade would be injurious. This certainly appears
to be the case in young leaves when emerging from the bud.
Note the character of the foldings and the erect positions of such
leaves (Fig. 21). The leaves of the horse chestnut assume at

Fig. 20.

Fig. 21.

Fig. 20. Shoot of wild lettuce, Lactuca, with leaves turned edgewise to
the light.

Fig. 21. Position of young leaves of hickory: A, scales of bud curving
back showing the tightly folded leaves. B, later stage, leaves unfolding but
still erect.

least three different positions during their growth. The folding
of such leaves and their erect positions expose less surface. They
consequently lose less heat and moisture, and the delicate grow-
ing cells are better protected against the intense sunlight. You
must notice as the cells mature and become better protectet that
their irritability changes and they respond in a different way
to light and gravity, etc. , They begin to unfold and turn away
from their erect positions and at maturity assume a fixed posi-
tion to light, etc., that is quite different from the original posi-

NATURE OF PLANTS 35

tion. The position of the leaves of our pines and several other
evergreens is not controlled by light and as a consequence they
assume a variety of angles to the incident rays.

The leaves of many plants do not have a fixed position but
show more or less motility during their entire life. Interesting
examples are seen in the sensitive plants and in nirmbers of
the bean and oxalis families where marked changes result in
the position of the leaves from alterations in the intensity of
light, temperature, or moisture. Many of these plants fold their
leaves when the temperature falls at sundown. Many flowers
close in the same way. These changes are popularly termed sleep
movements. They are caused not only by the changes in the
intensity of the light but also by changes in temperature and
moisture. In the case of these leaves the cells in tin- sw( lien
organ (the pulvinus) at the base of each leaf are kept full of
water so long as the temperature and external conditions exert a
suitable stimulus upon the plant, but when the conditions are
unfavorable certain of the cells on one side of the pulvinus lose
water and contract while the cells on the opposite side of the
pulvinus remain rigid; consequently, the leaves droop and fold
in various ways. This is due in part to the fact that the con-
tracted cells of the pulvinus are no longer capable of assisting in
the support of the leaf and also to the fact that the rigid cells con-
tinue to exert a pressure which bends the pulvinus towards the
side where the contracted cells are situated (Fig. 22). This fold-
ing of the leaves materially reduces the area exix)sed to the
atmosphere and consequently will lessen transpiration. Many of
our sensitive clovers {Meibomia, Lespedeza), sensitive peas (Cas-
sia), sensitive plant (Mimosa), are able to check the excessive
loss of water on hot dry days by the folding of their leaves. As
soon as the loss of water becomes detrimental some of the leaves
begin to fold and the reduction of leaf surfuc e continues until the
loss of water by transpiration is met b>' root absori)tion. \'ery
commonly leaves that do not have this power of adjustment are
able to accomplish the same results by a rolling of the leaf. This
is particularly noticeable among the grasses, which, on hot, dry
days, roll up their leaves so tightly as to rhange the appear-

36

NATURE OF THE EPIDERMIS

ance of the plants. Observe a corn field during a dry period.
The interesting feature of all this is that these reactions and
adjustments are purposive but not intelligent. The movements
are not called forth by consciousness but by stimuli to which the
irritable living substance is attuned or sensitive.

1 6. The Significance of Certain External Leaf Structures, —
Many features connected with the structure of the leaf furnish
admirable illustrations of the fitness of the leaf for the perform-

FiG. 22. Leaf position of the sensitive plant, Mimosa: A, in light. B, in
darkness. The same movements occur as a result of unfavorable temperatures
and humidity.

ance of its work. The stimulus of light and moisture have a
marked influence upon the external structure and form of leaves.
The epidermis is strikingly modified by such forces. In shade
plants the epidermis consists of a rather delicate layer of cells
with very thin cuticle. This gives sufficient protection to such
plants, but in leaves exposed to intense sunlight and a hot and
dry air, the epidermal cells become greatly thickened and often
of two or more rows, while the cuticle may often form the larger
part of the outer cell wall or even extend in between the cells
(Fig. 23, c). Cell walls that are filled with cutin in this manner
are said to be cutinized. This development of cutin gives the
tough leathery aspect to many leaves. Such features are partic-
ularly noticeable in desert plants, in long-lived leaves of many

NATURE OF PLANTS 37

evergreens, as laurels and rhododendrons, in the needles of cone-
bearing trees, and in all plants that are exposed to the hot, dry air
of summer or the drying winds of winter. Such leaves may be
further protected and strengthened by thick-wailed elon^^ated
cells, sterome (Fig. 23, st). Coatings of wax, mucilage and
lime are also frequently developed upon the cuticle to further
reinforce the impermeability of the epidermis.

The stomata are also so developed as to meet the conditions
under which the plant grows. In dorsiventral leaves they are
more numerous on the under side of the leaves because they are
less liable to be filled with water by rains and with dust which
would prevent the interchange of gases. The plugging of the

Fig. 23, Cross-section of the outer cells of a leaf of pine, showing the firm
character of the outer cells of the tough leaf: s, stoma; e, epidermis; c, cuticle;
st, stereome; m, mesophyll cells,

stomata by dust is one of the causes of the sickliness of plants in
homes. The arrangement of the stomata on the under side of the
leaf is also of especial advantage because the direct light does not
fall upon them and cause an excessive loss of water. The stomata
of floating leaves, however, are upon the upper surface and their
stoppage with water is prevented by waxy coatings, as ran l)e
easily demonstrated by dipping a leaf of a water lily or si^iittT
dock in the water. When the leaf is remo\ed the watt-r runs
off of the waxed surface without wetting it. Some leaves are
more or less erect, as the cattails, rushes and grasses, and these
have the stomata developed more or less evenly on Ixjtli surfaces.
Doubtless the intense light of midday is not beneficial to these
leaves and the blade of the leaf is consequently placed parallel

38

LOCATION OF STOMATA

with the sun's rays. This arrangement of the leaf permits a
direct illumination of the leaf only in the morning and afternoon
when the intensity of the light is feebler.

The position and character of the stomata in relation to exter-
nal conditions show many interesting relations. Plants living in
the shade or in the presence of an abundant soil-moisture develop

Fig. 24. Cross-section of a leaf of the inch plant, Tradescantia, showing
the delicate character of the cells and the raised stoma of this shade plant.

the stomata on a level with the leaf surface (Fig. 24), because
there is no necessity of conserving the water supply. For the
same reason some aquatics have lost altogether the power of
closing their stomata. On the other hand plants that are exposed
to arid conditions or drying winds develop the stomata well
below the surface of the leaf, as in the cactus and in the needles
of conifers (Fig. 23, s), or in furrows, as in certain grasses, or
at the bottom of minute pores, as in the oleander. These de-
pressions remove the stomata from the dry winds and prevent
the direct contact of the moist air in the leaf with the dry atmos-
phere. The pores in the oleander contain hairs which would
check transpiration just as a plug of cotton in the neck of a
flask would lessen the evaporation of the fluid in the flask. The
chief purpose of this arrangement, however, is to prevent the
stopping of the stomata with water. This plant grows naturally
along the banks of streams where it is subject almost nightly to
heavy dews. When the stomata become filled with dew it re-

NATURE OF PLANTS 39

quires several hours of sunshine to drive the moisture from these
capillary openings. Consequently during this time there could
be no interchange of gases and the vital functions of the leaves
would be largely stopped. Many devices appear that prevent
water entering the stomata. Attention has been called to the
development of stomata on the under surface of leaves and to
waxy coatings as protective features of this nature. Every one
is familiar with the waxy appearance or bloom of many fruits,
as the plum; or of leaves, as the cabbage; and of stems, as the
raspberry. Water is unable to spread over the wax and enter
the stomata because a thin layer of air clings to it. The silvery
appearance of many grass leaves and of the jewel-weed when
immersed in water is due to this thin film of air which reflects
the light. Accordingly the water does not really touch the leaf
and you notice that it is quite dry when it is removed from the
water save for a few drops which readily run off. Protection
against wetting is also obtained by the development of coatings
of hairs. This is noticeable in many Alpine and polar plants
where they are constantly exposed to hea\y fogs, dews, and rains.
Plant hairs do not always serve this purpose. Desert plants
and those living in dry localities are frequently characterized by
hairy coatings. Such plants are frequently termed xerophytes
in contradistinction to aquatics or hydrophytes and to plants
living in moderately moist soils, which are called mesophytes.
The hairy coatings of xerophytes give them their familiar silvery
or hoary appearance as seen in the sage-brush, dusty miller, and
mullein. The hairs are usually empty tubes or cells which re-
flect a large portion of the intense light and heat and consequently
materially lessen the loss of water, just as straw sprinkled upon
the ground keeps the earth below moist and cool. In many in-
stances plant hairs develop mucilage or resinous secretions which
protect as with a varnish growing regions as buds or young shoots
and leaves of birches, alders, poplars, and peach. In this con-
nection it should be noted that dense hairy coatings reduce the
loss of water only when the plant is exposed to air currents —
not In still air. Cutin, wax and resinous deposits retard trans-
piration under all conditions. Oil and poisonous substances arc

40 ARRANGEMENT OF THE MESOPHYLL

also formed which repel by disagreeable odors or tastes the
attacks of animals. The nettles are shunned because of the
burning sting produced by the plant hairs. The upper portion
of these hairs (Fig. 25) is practically a delicate glass tube since
the walls are filled with silica. The basal portion of the hair is
composed of soft, yielding cellulose so that the fluid collecting
in the hair distends this portion of it. The knob at the end is
fastened to the hair by so thin a ring of wall, as seen in Fig. 25,
B, that the least touch breaks it off. In this way the hair is

Fig. 25. Stinging hair of nettle: A, portion of epidermis bearing hair.
B, tip of hair enlarged, showing easily detachable knob. — I. D. Cardiff.

transformed into a veritable hypodermic needle which easily
punctures the skin while the distended basal portion contracts and
forces out some of the fluid. The burning sensation or sting
that immediately follows the puncture of the skin is caused by
the injection of formic acid while a variety of other poisons pro-
duce the subsequent irritations. These poisons are so powerful
in some of the East Indian nettles as to produce serious results,
even a tetanus.

17. Significance of Certain Internal Leaf Structures. — The
changes or modifications produced in the chlorenchyma by vari-
ous stimuli are quite as striking as in the case of the epidermis.
The palisade cells are an excellent illustration of this point.
These cells are developed as a result of the stimulating influence
of light (Fig. 26). The photosynthetic activity caused by the
light increases the pressure of the fluids in the cells. This

NATURE OF PLANTS

41

tends to round them out. They are unable to expand laterally
owing to the adjacent cells but are free to extend towards the
looser cells below, and so they become lon^^t-r and palisaded.
Leaves growing in the deep shade show little indication of palisade

Fig. 26. Fig. 27.

Fig. 26. Section of a leaf of Rhododendron. Note the compact palisade
tissue which results from intense hght.

Fig. 27. Section of a leaf of skunk cabbage, Spathyema. Note the poorly
developed palisade tissue and the loose arrangement of the cells of this plant
which lives in moist, shaded places.

structure. Shade and moisture plants are not obliged to conserve
the amount of water received and these two forces, feeble light
and moisture, produce a larger and looser arrangement of tissues
which is favorable to an interchange of gases (Fig. 27V Par-
ticularly is this noticeable in aquatics as in the water lilies and
many rushes, etc., where the tissue is loose and six)ngy and
permits a ready circulation of gases from the leaves to the roots
and to all parts of the plant body even when submerged. This
loose arrangement of tissues also renders water j^lants very
buoyant and consequently they are less liable to injury from
the currents of the water. Aquatic plants have little need of
strengthening and conducting tissues because they arc supfx^rted

42 LEAF FALL

by the water. Neither do they require elaborate bundles for the
conduction of the crude foods which may be absorbed by nearly
all parts of the plant body. We now understand why shade leaves
are thin and broad and soft whereas leaves of desert plants and
those exposed to severe drying winds of summer or winter are
thick, compact, firmer and often leathery and hairy.

One of the most interesting adaptive features of leaves is seen
in the leaf fall of our deciduous trees and shrubs. In the tropics
the leaves remain on the perennial plants often for long periods,
but in temperate climates the severe winters necessitate the
annual dropping of the leaves except in a comparatively few
evergreens where the thick leathery leaves are able to endure
such conditions. In our deciduous plants, when the conditions
are no longer favorable for the performance of leaf work, the
cells at the base of the petiole begin to divide and a delicate
layer of cells, the separating layer, is formed across the petiole
(Fig. 28). Various conditions induce this growth for there is
more or less leaf fall at all seasons of the year. Doubtless lack
of nutriment has much to do with it. If a leafy branch of horse
chestnut is placed between moist paper in a few days the separa-
ting layer will have formed and the leaf will have dropped from
the branch. The innermost delicate cells of the separating layer
may become changed into cork cells just before or directly after
leaf fall while the outer cells of this layer may break down
through mucilaginous modification or they may become rounded
off so that water collects in the intercellular spaces. Thus the
leaf is attached to the stem chiefly by the veins since the delicate
separating layer offers little support. If now the water in the
intercellular spaces of the separating layer should freeze, the
expansion of the water as it freezes would so rupture the remain-
ing tissue that we would have the familiar sight of the leaves
falling in a shower in the morning after a frost — either of their
own weight or with the slightest breeze. It will also be noticed
that owing to the formation of the cork layer or the drying up
of the delicate cells of the separating layer, the scar formed by
the fall of the leaf is nicely healed and closed against any loss of
fluids or the entrance of any organism (Fig. 28, C).

NATURE OF PLANTS

43

The casting off of the leaves reduces the area of the plant
that is exposed to the unfavorable conditions to a minimum.
This reduction of the surface is effected in one way or another
by a great variety of plants. Much of our spring vegetation
is possible, by reason of it. The spring beauties, anemones, fawn
lilies, jack-in-the-pulpits, etc., practically complete their growth

Fig. 28. Leaf fall: A, branch of horse-chestnut showing scar formed by
the fall of the leaf. The dots on the scar show the position of the vascular
bundles that are finally broken by the weight of the leaf. At the left the
base of the petiole is shown. B, diagram of a section through a twig of hickory
— s, separating layer at base of petiole; v, vascular bundles. C, enlarged
view of the separating layer — c, cork cells that heal the wound caused by the
fall of the leaf. The granular cells are the outer region of the separating
layer, and they are beginning to break down as seen in the upper jxirt of the
figure, at x, thus causing the fall of the loaf; st, cells of the stem containing
starch.

before the larger summer forms appear. I )uring this short pcricxl
sufficient food is manufactured by the leaves to mature the seeds
and fill the storage organs in the underground l)un)s and stems.

44 NATURE OF DESERT PLANTS

When the larger forms appear later In the season these plants
are overshadowed, their leaves wither, and they are reduced to
the small bulbs and stems hidden in the earth. In this condition
they remain until the next spring when the abundance of stored
food enables them to complete their growth before other com-
peting forms appear.

This ability to reduce the surface of the plant body makes
possible the existence of much of the plant life of arid and desert
regions. During the short rainy seasons the stored foods enable
the plant to quickly put forth the leaves which in turn manu-
facture the foods which are stored in seeds, stem and roots.
Then practically all trace of these plants is destroyed by the
drought. Only the seeds or a greatly reduced portion of the
plant remain alive and this is often further protected by being
hidden in the ground. A return of favorable conditions quickly
awakens these plants to growth. Mr. Dan Beard relates an
interesting experience in the desert regions of Texas that illus-
trates this feature of plant life. At the close of the dry season
this territory was a bare, sun-cracked plain swept by hot, dry
winds. Not a green leaf was to be seen. Heavy rains trans-
formed this level tract of land into an inland sea. In a few days
the water disappeared and one morning he was surprised to see
green blades appearing everyw^here and in a few days the barren
earth was covered with an almost tropical profusion of vegeta-
tion of bright flowers. Many desert plants to be sure develop
permanent aerial parts but here, too, the most striking feature
is the extreme reduction of the organs. Compare the areas of
the Spanish bayonet, aloes, grease wood and sage with that of
our leafy plants. The cactus represents one of the extreme forms
of reduction. The leaves have been dispensed with entirely and
are not represented save possibly by the spines. Consequently
the work of the leaves devolves upon the stems. While desert
plants receive very meager amounts of water, this is so effectually
conserved that the plants are usually rich in water. Drinking
water is frequently obtained in desert regions from cacti by
pounding up the pulpy interior and squeezing out the water.
Animals are well aware of this rich storehouse and will eagerly

NATURE OF PLANTS 45

devour the cacti after the spines are burned ofT. The develop-
ment of these storage cells accounts for tlie fleshy character of
many alkaHne, saline, and desert plants, many of which depend
upon organic compounds lodged in their cells for the retention of
water rather than upon the development of cutin. Curiously
enough there are many examples of xerophytic plants living in
bogs and marshes, as, for example, the rushes and sedges, the
horse tail ferns (Eqiiisetum), the lamb kill {Kalmia) and leather
leaf {Chaynccdaplme), etc. The cause of the association of these
plants with aquatic forms is not known. In exposed moors and
heaths these reduced forms would have decided ad\'antage
because of their protection against drying w^inds. Our sedges and
rushes are exposed to very intense heat and light which may
possibly cause so heavy a transpiration that these plants are not
able to meet the loss by root absorption. This is the more
probable because in many instances the absorption of water by
plants living under such conditions is slow owing to lack of
oxygen and to the concentration of the water in which the roots
grow^

CHAPTER II

THE ROOT

1 8. Character of Primitive Plants. — The first forms of life
upon the earth were doubtless unattached unicellular plants that
lived in the water or moist places. All the varied functions of
the plant body, as the absorption of gases and material from
the soil and the manufacture of organic compounds, etc., were
performed alike by each cell. This was easily accomplished
owing to the simplicity of the organism and also because the plant
was surrounded by water which contained all the substances re-
quired in the construction of foods. Perhaps the first change
produced in these plants by their surroundings was a modification
of a part of a cell so that it served as an organ of attachment, or
root, anchoring the plant to the substratum. As the plant con-
tinued to change and become more complex, and especially as it
became more and more subject to drier conditions, special organs
for the absorption of materials became a necessity. This change
from an aquatic to a terrestrial life finally left the root alone in
touch with the crude materials. Thus the root which at first was
only an anchoring organ, came later to function also as the prin-
cipal absorbing organ of the plant. Next to the leaf the root
commands our attention because of its fitness for the accomplish-
ment of this work. Practically all the water required by land
plants, and all the elements, save carbon and oxygen^ utilized in
the construction of foods are absorbed by the roots.

19. Root Hairs. — We will first be interested to examine the
structure and nature of the absorbing apparatus of the root. If
seeds of radish, mustard, or other plants are germinated upon
moist sand or moist blotting paper it will be seen when the roots
have attained a length of several cm. that a portion of each root
is covered with delicate hairs (Fig. 29, A), There are several
interesting features about these root hairs. In the first place they
are extremely delicate tubular outgrowths of the epidermal cells,

46

NATURE OF PLANTS

47

scarcely more than one millimeter in length (Fi^. 30). Note
that they always begin to grow a few mm. back from the root
tip and at the opposite end of the zone of root hairs the lubes are
withering or dying off, leaving the older portion of the root
covered only with the epidermis. If a root is marked into 2 mm.
spaces with India ink it will be found after 12 to 15 hours that
only the portion below the root hairs is elongating. The first
space from the^ip has grown very little, the second and third

Fig. 29. Fig. 30.

Fig. 29. The absorbing surface of roots: A, seedling of mustard showing
extent of surface covered by root hairs. B, an older branching root, the
shaded areas near the tip are due to particles of earth clinging to the root hairs.
The remaining portions of the root are free hairs and take little or no part in
absorption.

Fig. 30. Origin and character of root hairs: A, first appearance of a root
hair due to the pushing out or growth of a definite portion of the surface of an
epidermal cell. B, older hairs that have become irregular through coming
in contact with soil particles.

much more, and in the other spaces there is a gradual retarding
of the growth until 8 to 12 mm. from the tip elongation has
ceased. It is at the point where elongation has ceased that the
root hairs begin to appear, but the total number existing on the
root at any one time does not materially vary owing to the fact
that they are dying off farther back on the root with the same
rapidity that characterizes their formation near the tip (Pig.
29, B). Before the significance of these peculiar features can be

48 NATURE OF SOILS

understood it will be necessary to consider the nature of soils
and the relation of the substances absorbed by the plant to the
soils.

20. The Nature of Soils. — The soil is composed of minute
particles derived from decaying rocks and of organic particles
or humus derived from decaying plants and animals. The
mineral particles furnish several of the Important crude plant
foods, as calcium, magnesium, potassium, pho^horus, sulphur,
iron and small amounts of other elements. These substances
occur in great abundance, comprising over thirteen per cent, of
the earth's crust. It is a singular fact that two of the most
abundant substances, silicon and aluminium, are utilized by
only comparatively few plants, although it will be seen later on
that the latter element is indirectly of great service to plant life.
The remaining elements used by the plant, oxygen, nitrogen,
carbon, hydrogen, are equally abundant. The first three may be
obtained from the air while the hydrogen is a constituent of water
and other compounds. It is an interesting fact, however, if
rocks containing all these mineral elements should be ground up
into the form of a good soil and the other necessary substances be
added thereto that It would not support plant life at all. The
reason Is that these elements exist In the rocks In a form that the
plants can not utilize. They are largely combined with an ele-
ment silicon, of which substance pure sand or quartz is largely
composed. These compounds are termed silicates. The plant
can only utilize these elements when they are combined with some
element, other than silicon, and to which oxygen has been added.
The potassium, magnesium, etc., must be combined with nitro-
gen, phosphorus, sulphur or carbon to which oxygen is also added
In order to make a substance that the plant can utilize. These
crude foods of the plant are termed nitrates, phosphates, sul-
phates and carbonates. It is only In this form that the necessary
elements can be absorbed, silicates are rarely of service. The
question naturally arises: how do the silicates become trans-
formed into suitable compounds and how do the rocks become
changed into the form of a soil. Several factors are concerned
in the transformation and the reactions are complicated and
very slowly effected.

NATURE OF PLANTS 49

Water is the most important force in starlin^r tlicsc changes,
tt dissolves, that is removes, several of the elements that api)ear
in the silicates. Note also when water has taken up certain of
these elements and compounds that its dissolving power is greatly
increased. For example when water has absorbed carbon dioxide
its power of dissolving calcium is increased 25 times. Further-
more the removal of an element by the water means that new
chemical compounds are formed and these are frequently of such
a nature as to lead to further chemical reactions. We say that
the dissolving action of water upsets the chemical stability of
the substances composing the rocks. Certain gases of the air,
particularly oxygen, also enter into union with some of the rock
elements, making them more soluable. So you can think of a
series of chemical struggles going on between the silicon and
other elements and compounds for the possession of the potas-
sium, calcium, etc., and that in this way these latter elements
are sloAvly removed from the silicon and united into new com-
pounds suitable for plants.-

The removal of these elements from the rock minerals forms
cavities and as a consequence the rock tends to become honey-
combed and finally begins to crumble and so approaches a soil
condition. Two forces are therefore involved in soil making:
the one chemical which is termed decomposition and the other
mechanical, termed disintegration. Disintegration or the crumb-
ling of the rocks is hastened by a number of factors, esp>ecially
the force of running water. The power of water to break up the
rock substance and distribute the particles thus formed is one
of the most important factors in soil formation. Changes in
temperature also constantly cause the contraction and expansion
of the rock material and of the water occupying any of the sjxices
in the rock and so assist in the disintegration. Finally the force
of the wind and the grinding action of moving ice are important
factors in wearing away the rock surface and reducing it to a soil
condition. By these slow and intricate processes comjxiuntls
for plant uses are formed and the rocks are reduced to the condi-
tion of a soil. Such a soil, however, does not contain suflicient
amounts of the crude food material to support tho -.r.lm.rv

50 PROPERTIES OF SOILS

plant. So we find very simple plants, as the lichens and certain
mosses, occupying such primitive soils. They are able to live
upon such soils because they are small and slow growing and
therefore demand meager amounts of the crude foods. Note
the character of the first vegetation that appears on the slopes
of mountains below the solid rock. These simple plants, how-
ever, are very important factors in effecting further decomposition
of the minerals in the soil. Through the chemical activity of
their absorbing organs and especially through the decay of the
dead parts of these plants new compounds are formed so that
more elaborate plants, as certain ferns, are now able to live upon
the soil. These latter plants in turn carry on the work until
the rock material is sufficiently changed to make a good soil.
Do not imagine that the work of soil formation stops here. You
are to think of every growing plant performing the same work
as noted in the lichens and mosses.

The soil is one of the most remarkable things in this world of
ours and we must consider two of its more important features.
The first of its important characteristics is its power to retain
moisture. Water exists in the soil in three forms, — as hygro-
scopic, as capillary and as gravitational water. The first form
is the water that condenses from the atmosphere as extremely
delicate films about the air-dry soil particles. This water is not
available to the plant because it is held so firmly by the soil
particles. It comprises only a small per cent, of the total water
in the soil. The capillary water also exists as films about the
soil particles but it is not retained so tenaciously and it is this
water that furnishes the chief supply to land plants. Capillary
water may amount to lo per cent, of the dry weight of coarse
sand up to 25 per cent, in clay soils. Gravitational water is not
retained by the soil particles and it is therefore free to run down
through the pores and spaces in the soil under the control of
gravity. This water in the upper portions of the soil is not of
value to the majority of plants because it fills the pore spaces in
the soil and so deprives the roots of the air which is necessary
to their existence. When gravitational water exists a few feet
below the surface it often becomes of great value as a source of
supply for the capillary water.

NATURE OF PLANTS

51

Let us now consider how the root is related to the soil water.
You have seen that the soil particles have an attraction for water.
This attraction is due to a force termed surface tension. Each
soil particle has a definite surface tension or capacity for retaining
water that is dependent upon the extent of its surface. The
retention of the hygroscopic water uses up a small portion of the
particle's capacity and then it adds capillary water until no more
can be retained. We say that the particle is in a saturated state.
It is evident that the last water added to the particle will not be
held very firmly and it is this water that the root hair can readily
absorb. But now follows a very remarkable movement of
water. No sooner has a particle lost some of its water than it is
able to replenish its supply from adjacent particles that are in a
saturated state. This transfer of water from particle to particle
goes on for surprising distances, depending upon the amount of
the surface tension. So we find particles four to six feet away
from the root hair contributing a portion of their capillary- water
to replace the water thus absorbed. So you can think of the
capillary water moving as films from particle to particle towards
the points of absorption. It is for this reason that the gravita-
tional water may be of great value. If it is only a few feet below
the absorbing roots It will furnish a constant supply of capillary
water to the overlying soil particles. So one of the most im-
portant features about a soil is the size of its particles. The
term texture refers to this condition of a soil and we speak of fine
or coarse textured soils. The finer the particles the greater the
surface exposed in a given unit of space; and consequentK' the
greater the surface tension and water-holding capacit\. A
cubic foot of coarse sand can not retain as much water as a cubic
foot of clay because the clay particles are 200 times smaller than
the sand particles. A fine textured soil may lose some of its
capacity to retain water owing to the compacting of Its particles
into larger ones. It is evident that two separate ixartlcles possess
more surface area than when combined into one particle. This
relation of the particles to one another is termed structure and
we see that soil structure is of equal Importance with soil texture.
Now we can undesrtand the chief advantage of tillage. Plowing

52 SOIL ABSORPTION

and harrowing break up the compact structure and tend to keep
the soil in a fine textural condition. It is surprising to note the
amount of surface exposed by the particles of various types of
soils. A coarse sand where the particles range from .i to .5 mm.
in diameter has no less than 40,000 sq. ft. of surface exposed in
each cu. ft. of soil and a clay soil where the particles are .005 mm.
in diameter or smaller has over 140 thousand sq. ft. of surface
per cu. ft.

A second noteworthy feature of soil particles is their power to
retain certain elements that are brought in contact with them
in solutions. These elements are retained in some instances
much after the manner of film water but in other cases retention
is effected through chemical reactions. You see an illustration
of this property of vsoils when colored solutions, such as the
drainage w^ater from stables, after passing through a soil comes
out quite colorless. The old type of charcoal filter purified
drinking w^ater in this way. It is an important fact that many
of the important elements and compounds utilized by the plant,
such as potassium, ammonium, magnesium, calcium, phos-
phorus, etc., are retained in this way; and they are retained so
firmly that only small percentages of them are leached out and
lost through the action of running water. Certain aluminum
compounds (see above), also iron, calcium and the humus com-
pounds derived from dead plants and animals are very important
factors in retaining the above mentioned elements.

21. Relation of the Root Hairs to the Soil Particles. — We are
now prepared to understand the significance of the structures
noted in the root and their relation to the physical properties of
the soil. In the first place we see that these microscopic root
hairs can readily penetrate any cavity and they are so delicate
and soft that they become moulded about any soil particle in
their line of growth (Fig. 30, B). So firmly are they attached
to these particles that it is impossible to detach them without
injuring the root hairs (Fig. 29, B). This arrangement admir-
ably adapts them to gathering up the water and other crude
material in the soil. The tubes are filled with organic acids and
other substances which have an affinity for w^ater and such earth

NATURE OF PLANTS 53

substances as the plant requires. In this way the materials in
the soils are drawn through the delicate walls of the root hairs,
a process termed osmosis. Some of the acids in the root hairs
may also reach through the cell walls and so come in contact with
the insoluble substances, dissolving them and thus rendering
them capable of absorption by osmosis. Certain it is that CO2
is given off by the root hairs and as we have seen, this gas enter-
ing into the film water would enable it to dissolve calcium — a
fact easily demonstrated by allowing the roots to grow over the
clean surface of oyster shells or pieces of polished marble. Deli-
cate grooves in the marble show where the roots have dissolved
the calcium in the course of their growth. Some roots show an
acid reaction, as is indicated by their ability to turn blue litmus
paper red ; but all roots do not react in this way and it is impossible
to state whether acids, other than carbonic acid which would
result from the elimination of CO2, are excreted.

We are now prepared to understand why the root hairs are
developed just back of the zone of elongation.. Any disturbance
of these delicate absorbing organs, such as the extension of that
part of the root to which they are attached, would result in
tearing and killing them.

22. Familiar Facts About Roots. — The character of the root
and its relation to the physical properties of the soil explain many
familiar phenomena of plant life. We see why plants must be
repotted. The root hairs, which onh- li\-e for a few da\'s or a
few weeks, are constantly being formed back of the growing
zone and dying off behind. Tliis would result in not onK* placing
the new root hairs in new soil but they would also \)v removed
in a measure from any harmful substances that may have arisen
through the activity of the older root hairs. In this connection
note should be made of the suggestive work iliat is being ac-
complished today tending to show tliat roots gi\e olT substances,
probably of organic nature, which are toxic in character and so
render the soils unfit for plant growth. These substances are
readily oxidized and rendered harmless. The presence of oxy-
gen and certain fertilizers increases the oxidizing power of the
roots. While this theory is far from being established it is jws-

54 RELATION OF ROOT TO SOIL

sible that we may have a new explanation for the sterile condition
of some soils; also plowing and the use of fertilizers may have a
new meaning. Consequently when a potted plant shows signs
of starvation it is removed from the jar, the tangled mass of roots
next to the jar is cut off together with a portion of the earth and
then repotted with fresh earth. In this way room is given for
the rootlets to grow out into the earth and develop a new ab-
sorbing surface and additional materials are placed at the disposal
of the root. This relation of the root hairs to the soil particles
explains to us why transplanted seedlings usually wilt. Many
of the root hairs have been killed and the seedling does not revive
until new root hairs have been developed. So also it is clear
why most shrubs and trees should be transplanted in September
or October or in the early spring in order to permit the rootlets
which renew their growth very early in the spring to establish
in the soil new absorbing surfaces before the leaves develop and
bring about an excessive loss of water through transpiration.
Why are plants often pruned or cut back in transplanting?
Why is nursery stock frequently transplanted, generally a year
before it is to be permanently planted?

You have often noticed that fields of grain and other crops turn
yellow and die during a long rainy season. This is due to the
fact that the air spaces between the earth particles become filled
with water and there is no longer possible an interchange of gases.
At least two-fifths of the space in these capillary pores should be
filled with air to insure healthy plants. This is particularly
noticeable in clay soils which are referred to as cold, wet, and
sour soils. The reason of this is that the earth particles of such
soils are very fine and, owing to their enormous surface, they
hold the water with great tenacity. In this way acids from
decaying vegetation as well as other substances accumulate in
excess and render the soil unwholesome. Therefore heavy clay
soils, which are richer in plant foods than other soils, require an
admixture of sand and an adequate drainage to render them
sufficiently porous for a proper circulation of air. Sandy soils
are made of coarser particles and therefore looser. For this
reason the capillary water is not retained for any considerable

NATURE OF PLANTS 55

time and the soil is properly aerated. Injudicious sprinkling of
lawns, gardens, or potted plants will result in the formal ion of a
compact surface crust. This acts exactly like a clay soil and
draws the water away from the region of the roots to the surface
of the soil where it is lost by evaporation. Potted plants are
best watered by standing the jar in the water until the soil is
wet. Further watering is not required until the jar sounds
hollow when tapped. So gardens and lawns should Ije thor-
oughly soaked during the night. The ordinary sprinkling only
moistens the surface and leaves the soil in a worse condition for
holding the moisture than before, because on drying it bakes
and the hard crust draws up the water to the surface where the
dry air readily evaporates it. A light shower in the summer time
may produce the same injurious efTect. On the other hand when
the surface of the soil is loose and air dry, so that the capillary
water does not extend from the surface to the deeper strata,
then a shower may do harm in quite another way. By supplying
the upper particles with films of water it reestablishes capillary
connection with the lower lying particles so that the film water
now begins to move again towards the surface of the soil. This
explains why the top soil should be harrowed and hoed during
periods of inadequate rainfall. The loosely arranged particles
quickly become dry and the capillary films leading to the water
supply below are thus broken at the surface and further loss of
water through evaporation is in part checked. A mulch of straw
and leaves upon the surface of a soil has the same result as
harrowing. Sometimes it is desirable to have a compact surface
layer of soil, as for example in sowing a field to grain or grass.
This is accomplished by rolling the field, which not only gives a
smooth surface to facilitate the cutting of the crop but also makes
a compact stratum at the surface which draws the moisture up
to the grains, causing them to grow. Otherwise the surface of
the soil containing the grains would be so dried out by the winds
often in a single day, as to prevent their sprouting.

23. Extent of the Root Surface.— It may seem surprising that
the roots are able to take up the large volumes of water that are
lost daily through transpiration. The spread of the root, how-

56 ROOT GROWTH

ever is much more extensive than we think, usually quite equal
to that of the branches, so that the absorbing portion of the root
is beneath the drip of the branches or exposed to the rains and
dews. Furthermore, when we come to add together the lengths
of the numerous rootlets, the extent of the total root system is
still more surprising. The total length of the roots of an oat
plant has been estimated at 154 feet; of a corn plant, at 1320 feet;
and of a squash plant at fifteen miles. These roots in ordinary
soil reach a depth of from three to five feet, but in dry regions
sometimes much greater depths are attained; for example, alfalfa
31 feet and mesquite 60 feet. The most important increase in
the extent of the root system is due to the root hairs, 480 having
been estimated on .01 in. of a root of corn 1/17 in. in diameter,
and 230 on a square mm. of a pea root. By this means the ab-
sorbing surface of roots may be increased from five to ten times.
24. The Structure of the Root. — You have noticed that only
the apical portion of the root elongates and it may be well to first
examine the structure of this region in order to see how grow^th
is brought about and what changes are effected by it. The tip
of the root is composed of very delicate cells. Certain of these
cells are rapidly dividing by forming new walls through the
middle of the cells, thus dividing each cell into two new cells.
The division of a cell is effected in a very elaborate manner. The
first indication of this growth appears in the nucleus. This organ
of the cell enlarges (Fig. 31, ^) and w^e now see that it has a very
complex structure, consisting of a protoplasmic mass surrounded
by a delicate membrane and traversed by a network of delicate
threads (linin) and containing one or more large granules (nu-
cleoli). The most important portions of the nucleus are the
minute particles (chromatin) which are associated with the linin
threads. It has been supposed that this substance controls to a
considerable extent the activity of the cell and determines the
kind of cell that shall be formed and the work that it shall per-
form. As the nucleus enlarges the chromatin increases in amount
and often forms a ribbon-like structure (Fig. 31, B) which finally
contracts and divides, thus forming numerous small masses of
chromatin, called chromosomes (Fig. 31, C). At this stage in the

NATURE OF PLANTS

^/

division delicate colorless strands begin to appear and ultinuitely
grow into a spindle as shown in Fig. 31, D. There are two sets of
these strands or fibrillse; an inner and an outer series. The outer
fibrillar, in some way not understood, arrange the chromosomes in
the center or equator of the spindle (Fig. 31, D) where each

Fig. 31. Cell division in root of corn: A, cell with nucleus enlarging pre-
liminary to division — /, linin; ch, chromatin. The central dark body is the
nucleolus. B, later stage, the chromatin has increased and appears as a ribbon-
like skein. C, formation of the chromosomes, cr. D, formation of the spindle
and the arrangement of the chromosomes in the center of the spindle —
I. D. Cardiff.

chromosome divides by a longitudinal division into two ecjual
parts. The fibrillae now separate the two halves of each chromo-
some and pull them to the opposite poles of the spindle so that
each pole receives a half of every chromosome (Fig. 32, E).
The chromosomes now become rearranged at the poles and two
new nuclei are gradually formed like the original nucleus (Fig.
32, F, G). In the meantime the inner fihrillx of the spindle have
shortened and become thicker at the equator (Fig. 32, G). Tliis
thickening goes on assisted by the addition of new fibrilhe at
either side of the spindle until a delicate line (the cell wall) reaches
across the old cell (Fig. 32, H), and the division of the mother
cell into two daughter cells is completed. In tiiis way new cells
are being constantly added to the root. It will be noticed, if a
longitudinal section through the middle of the root is examined,
that various regions of the elongating root may be recognized
owing to the difference in the character of their growth. In such
a section (Fig. 33) we see that the tip of the root is covered with

58

STRUCTURE OF THE ROOT

a mantle of cells, the root cap. This cap protects the delicate
cells within like a thimble so that they are not exposed or injured
as the root extends through the soil. At the tip of the root, just
within the root cap, the cells are actively dividing and adding
new cells to the end of the root and some cells are also added

Fig. 32. Later stages in the division of the cell: E, the fibrillae pulling
the separated halves of the chromosomes to the opposite poles of the spindle.
The inner series of fibrillae are now seen at the equator between the two
groups of chromosomes. F, the chromosomes arranged at the poles and the
inner fibrillae increasing in size and number. G, the fibrillae have increased
in number until they nearly reach the opposite walls of the mother cell. Their
thickening at the equator is the first indication of the wall separating the two
new cells. H, position of the new wall clearly indicated. — I. D. Cardiff.

to the inner side of the root cap (Fig. 34). Owing to this unique
arrangement it does not matter if the outer cells of the cap are
injured or destroyed as the root pushes through the soil, because
the cap is constantly being renewed from within, and so always
furnishes adequate protection to the delicate cells within. The
cells that are added to the tip of the root divide several times
after their formation, so we find that the tip of the root for a
distance of one or two mm. is composed of small cells that are
in a process of division but that are enlarging to only a slight
degree (Fig. 34). This region is called for this reason the for-
mative region of the root. Back of the formative region for a
distance of tw^o or four mm. the cells are dividing to a less degree
but are elongating very rapidly and changing in form (Fig. 35).
This is the region of rapid elongation. Still further back elonga-
tion has ceased, but the walls of the cells are becoming thicker

NATURE OF PLANTS

59

and the cells are changing in character, so that they perform
different duties. This is illustrated in the cross section of the
root (Fig. 36), where we see that the outer cells have become
modified, forming an epidermis with root hairs
for absorption. Within the epidermis is a broad
zone of cells, the cortex, often used for the stor-
age of manufactured foods, while in the center
of the stem are the vascular bundles. The woody
portion of the bundle, or xylem, radiates out-
ward from the center and the soft portion, or
phloem, alternates with it (Fig. 36, x, p). The
materials absorbed from the soil are largely
transported up to the stem and leaves through
the xylem, and the foods manufactured by the
leaves reach the root by means of the phloem
cells. The branches of the root originate in a
very curious way from the cells just outside the
xylem. These cells by repeated divisions form
lateral roots which gradually destroy the tissues
in their way and finally grow out to the surface
of the root (Fig. 37). By this arrangement they
are provided with a root cap and fully prepared
to enter the soil on emerging from the old
root.

25. The Transport of Water in the Root. — The inner la>er of
the cortex, the endodermis (Fig. 36, end), consists of a layer of
cells which, in the older part of the roots, forms a very compact
and more or less cutinized ring of cells. This tissue is supposed
to function in preventing an undue loss of water from the con-
ducting strands of xylem to the cortex. It should be stated here
that the root hairs not only absorb fluids but also force out the
absorbed substances into the adjoining cells of the cortex. All
living cells have this power of absorbing and expressing fluids.
So it comes about that the absorbed fluids are forcecj by the root
hairs into the cortical cells and by them they are passed on to the
cells of the xylem \\ hen by some force not known they are drawn
up the stem into the leaves. The force exerted by these nnriad

Fig. 33. Dia-
gram of a section
taken through
the center of a
corn root: r, root
cap; e, epider-
mis; c, cortex; p,
central region.

60

ROOT STRUCTURE

Fig. 34. Portion of the formative region of the root taken at a in Fig. 33:
r, root cap; v, cells which by repeated division add new cells to end of root
and to inside of root cap. Note the numerous dividing cells and their uni-
formity in size. — I. D. Cardiff.

'.T

w

\

Fig. 35. Portion of the region of elongation of the root taken at b in Fig.
33. Note the larger size of the cells, increase in cell sap as indicated by the
larger vacuoles and absence of cell division; e, epidermis; c, cortical region;
p, central region. — I. D. Cardiff.

NATURE OF PLANTS 6i

number of cells in the root exerts a pressure that often amounts
to considerably more than one atmosphere, i. e., 15 lbs. to the
square inch. It is the steady expressing of fluids by these cells
that causes the familiar phenomena of the "bleeding," or flow
of water from stumps in the spring; likewise the "bleeding," of
injured branches, or the flow of sap, for in the stem the living

Fig. 36. Cross-section of root taken above section shown in Fig. 33: e,
epidermis with root hairs; c, cortex bounded on inner side by endodermis.
end. Within is the central region containing vascular bundles; x, xylem; p,
phloem. — I. D. Cardiff.

cells are constantly absorbing and giving oft* water as in tlie root.
In the summer, stumps and stems do not "bleed" as a rule be-
cause the water lost by transpiration nearly empties the cells,
whereas in the spring before the leaves ai^pear tlu'\- become filled
with fluid.

26. The Sensitiveness of the Root.- AW nia\ now ask how
does this elaborate root mechanism become so perfectly adjusted
to the soil. If the root of a pea or bean seedling is ]ilaced hori-
zontally in sawdust, after one or two hours it will begin to
curve down toward the earth center. No matter in what position
it is placed the result is always the same. We are so familiar
with the downward e:rowth of roots into tlie soil that we ne\iT

62

SENSITIVENESS OF THE ROOT

stop to consider how they gain their sense of direction. Gravity
is the stimulus that acts upon the irritable protoplasm of the cells
and so brings about a growth reaction that sends the root in the
right direction. When roots are placed in a horizontal position
and slowly revolved on their longitudinal axes, no curvature re-
sults, since all parts of the cells are stimulated alike; but when
allowed to rest a curvature results because possibly heavier par-
ticles in the cells fall to the lower sides of the cells and so pro-

end

Fig. 37. Cross-section of a root of lupine showing the origin of the lateral
rootlets. Lettering as in Fig. 36. — H. O. Hanson.

duce an irritation through their unusual position. The cells in
the first millimeter and a half of the root tip, or possibly in the
root cap, are sensitive principally to the stimulus of gravity —
the other cells much less so. If this region of the root is care-
fully removed with a very sharp razor the root is no longer
capable of responding to gravity although it may curve in
various directions owing to the irritation produced by cutting.
Furthermore, while the tip alone perceives the stimulus of
gravity, the curvature occurs two or three mm. back of the
tip, i. e., in the region of rapid elongation, so that we have the
transmission of the impulse somewhat after the manner of our
own nervous system. To be sure there are no specially con-

NATURE OF PLANTS 63

structed cells for conveying this impulse comparable to the nerves,
but it has been shown that chemical changes are set up that ex-
tend from the tip to the region of curvature where a more ex-
tended growth of the cells on the upper side of the root is induced
than on the lower side. This results in bending the root down
into the soil. These reactions apply to the main or tap root.
The lateral branches, or secondary roots, tend to grow more or
less at right angles to the stimulus of gravity, while the short
tertiary branches radiate out in all directions. Why the stimulus
of gravity causes a more extended growth on one side of the tap
root than on the other or why the lateral roots are so differ-
ently affected can not yet be answered, but the nicety of these
reactions in enabling the roots to reach in all directions and
thoroughly explore the soil in their quest for food, is very ap-
parent. The cells of the root tip are also sensitive to touch or
contact. This enables the root to avoid obstacles and grow
around rocks and stones. In this case the irritation of the root
tip produces a more considerable growth in the zone of curva-
ture on the side where the root is irritated with the result that
the tip of the root is bent from the object. In this way roots
work their way through the soil and avoid obstacles and grow
around rocks, for as soon as the irritation of the obstruction is
removed by the curvature, gravity will cause the root to grow
down again.

The cells of the root tip also react to light intensities and to
definite percentages of moisture and mineral foods. Roots avoid
light and as a consequence they grow down into the earth where
they are directed in their growth, owing to the peculiar sensitive-
ness of their cells, so that they come to lie in soils containing
suitable moisture and crude food materials. Every one is fa-
miliar with the fact that roots are caused to curve towards and
follow the course of decaying tree trunks, being stimulated by
the moisture and food contained in them. Numerous instances
might be cited where roots are attracted to wells, drains, etc.,
even surmounting considerable obstructions in order to reach the
water. This localization of the sensitiveness to all external
conditions that affect the root at the very tip is altogether ad-

64 ROOT FUNCTIONS

mirable, because it enables it at the very start to come into the
most helpful relations to its surroundings. It was a very clever
fancy of Darwin to compare this localization to a brain center
as found in certain low orders of animals.

27. Other Root Relations. — While 70 per cent, of our plants
develop roots adapted to absorbing substances from the soil
there are several interesting modifications of the structures noted
above. The roots of aquatics may not develop root hairs be-
cause the roots are so finely divided and delicate as to enable
them to absorb their crude materials directly from the water.
Many plants develop roots in the air. These are sometimes
simply clinging devices, as in the poison ivy; or they may be true
absorbing organs, in some cases reaching down to the ground
or to cup-shaped leaves that hold water, as in many tropical
climbing plants. One of the most interesting of these modifi-
cations is found in the orchids (Fig. 291) that live on the damp
tree trunks or on dripping rocks of tropical countries. Here the
root is covered by a thick mantle of cells that are capable of hold-
ing the water that conies to them either in the form of dew or
drainage or rain. Such plants, called epiphytes, are able to
flourish in the air without the assistance of soil roots. Quite a
large number of plants, parasites, depend entirely, or in part, upon
other plants for their foods. The mistletoe is an example of this.
The sticky seeds are carried by birds to the branches of oaks and
other trees where they germinate, the roots penetrating the
branches and withdrawing the necessary foods. The mistletoe
withdraws largely crude materials from the branch because it
has pale green leaves and is therefore capable of manufacturing
some food on its own account. Our common dodder {Cuscuta)
is entirely dependent upon other plants and illustrates the results
that follow from the disuse of any function. It has lost its chloro-
plasts, the leaves are reduced to inconspicuous scales and the stem
resembles a coarse yellow string while the root has disappeared
entirely. The seeds produce a small thread-like shoot which soon
perishes unless it comes in contact with a plant upon which it
can live. These thread-like plants are sensitive to touch and so
coil about the branches of any plant that they may chance to hit

NATURE OF PLANTS 65

in their growth. If this plant happens to contain fcxxls suiialjle
to the dodder then a second stimukis is aroused which causes root-
like branches to form. These organs penetrate the brandies upon
which the dodder is growing and absorb foods from it. Many
plants live as parasites on the roots of a variety of herbaceous and
woody plants, as the beech drop, orobranches, broom rape, etc.

One of the most remarkable features of roots and one of the
most important in the economy of the earth is seen in that large
family of bean or leguminous plants that are characterized by
flowers and fruit like those of the pea. Minute plants, bacteria,
gain access to the roots of these plants through the root hairs
and cause wart-like enlargements, tubercles, on the roots (Fig.
38). These bacteria have the power of combining the nitrogen
of the air with other elements, termed the fixation of nitrogen,
and so forming a nitrogen-bearing compound that can be ab-
sorbed by the plant. To give an idea of the importance and
work of these bacteria, it is estimated that an acre of clover
yielding two tons contains 100 lbs. of nitrogen — 74 lbs. of which
has been derived from the air. The fixation of this amount of
nitrogen by the bacteria in the roots of the clover would nearly
suffice for the production of 50 bushels of corn and 30 busliels
of wheat. Nitrogen is one of the very essential elements rc-
5

66 NITROGEN FIXATION

quired in the construction of the living substance of the plant.
Strangely enough, though the atmosphere contains over 70 per
cent, of this gas, the plant has no way of utilizing it directly but
only in compounds, such as ammonia and nitrates, etc. Com-
pounds of this kind exist in very meager quantities upon the
earth. The only sources of supply of any consequence are the
decaying animal and vegetable life; manures; small amounts
that are formed in the air and that are then carried to the earth
in rains; the guano deposits, now practically consumed; and the
saltpeter beds of Peru and Chile, which will be exhausted by 1925.
Consequently widespread alarm has arisen lest famines ultimately
result through lack of these nitrogen compounds, and the situ-
ation becomes the more serious since from one half to two
thirds of the nitrogen compounds placed on the soils are lost an-
nually in various ways, especially by the leaching out of these
substances by rains and drainage waters. Naturally of late
years much attention has been directed to devices for uniting
the nitrogen of the air with other elements in order to find a sub-
stitute for the rapidly disappearing nitrogenous compounds.
Working upon the fact that nitrogen compounds are formed in
the atmosphere by electrical discharges, several countries are
now manufacturing by electrical processes large quantities of
nitric compounds. One of the valuable substances thus formed
is calcium nitrate. Nitrogen is first combined with oxygen by
utilizing the high temperature of the electric arc (2500 to 3000°
C.) and the nitric oxide gas thus formed is passed through milk
of lime, thus forming calcium nitrate. Another nitrogen com-
pound, valuable for certain soils, is calcium cyanimid. It is
formed by passing nitrogen into closed retorts containing pow-
dered calcium carbide heated to a temperature of 1100° C.
Under this temperature the calcium carbide (this substance
forms acetylene gas when placed in water) unites with the
nitrogen, forming calcium cyanimid and carbon. The cyani-
mid slowly decomposes in the soil, yielding ammonia which can
be either directly or indirectly utilized by the plant. While
the value and general utility of these artificially formed nitrogen
compounds has not been thoroughly tested, it is not to be ques-

NATURE OF PLANTS 67

tioned that products and processes will be improved so as to meet
the rapidly increasing demands for nitrate.

Much study has also been given to the nitrogen-fixing bacteria
as a further source of supply for nitrogen compounds. Failure
attended the numerous attempts to cultivate these important
plants with practical results, although our own department of
agriculture has met Avith a limited success in making cultures of
bacteria that are sold to farmers for mixing with their clover or
other leguminous seeds. See Bulletin 71, 1905, U. S. Dept. of
Agriculture. There are many difficulties in the way of intro-
ducing these forms, such as keeping them alive, supplying the
proper conditions for their growth and the variety of forms that
must be dealt with. Although there is possibly but one species
of these bacteria, they have become so changed through associ-
ation with the various kinds of leguminous plants that there are
numerous varieties, each adapted to one or a few species of plants.
Soils in one section may have the forms suitable for one kind of
plant and be totally deficient in forms required for other plants.
At present the only sure way of inoculating sterile soils is by
mixing with them soils containing the desired bacteria — a labori-
ous and expensive method. Quite recently ver>' promising
results have been obtained by growing the desired bacteria in
soils until they have so multiplied and permeated it that a small
amount of this soil will suffice to inoculate a large area. With-
out doubt means will be devised for cuftivating the various forms
of these bacteria so that they may be sold for mixing with the
soil or seeds. Thus the farmer will be able to raise a croj^ that
contains a large amount of nitrogen derived from the air and by
plowing under this crop or better still, if conditions permit, by
feeding it to stock and utilizing the manure, he will be able to
add a valuable nitrogenous fertilizer to the soil. In this way the
soil is prepared for another crop, as wheat, which is (Ki)eiukMit
for nitrogen upon nitrogenous compouiuls. This alternation of
crops is of prime importance in successful farming. I'^arh kind of
plant takes from the soil certain elements in large amounts and
for this reason the alternating or succeeding crop should Ik? of
such a kind as to require other elements. As these elements are

68 NATURE OF MYCORRHIZA

removed from the soil suitable fertilizers containing nitrogen,
phosphorus, lime, potash, etc., must be added to the soil to
replace the absorbed crude foods.

The majority of plants do not absorb materials from the soil
in so direct a way as is illustrated in Fig. 29. They are de-
pendent for certain of their materials at least upon a low group
of plants termed fungi. These fungi more commonly consist of
delicate cobwebby threads (termed hyphse), such as are seen on
mouldy bread. This fungal group, known collectively as
mycorrhiza, comprises a number of forms or species, only a few
of which have as yet been identified. These delicate plants
spread through the soil and their hyphse enter either the outer
cells of the roots or form a mantle of more or less closely inter-
woven threads about them or the hyphae may sustain both of
these relations to the root. It has been demonstrated in many
instances that they transfer to the plant various substances
derived from the humus. Certain forms, probably a much larger
number than is now know^n, have the power to fix free nitrogen
much after the manner of the bacteria. It is also probable that
they sometimes take over the normal absorbing function of the
root hairs for frequently roots associated with Mycorrhiza develop
root hairs sparingly or not at all. This relationship is not
always one sided in its benefits for the fungus receives carbo-
hydrate material from the green plant. This state where two
or more plants live together is termed symbiosis. All stages in
the symbiotic relationship of the fungi and the green plants may
be seen. The green plant may not be benefited or even injured
by the fungus. This symbotic relationship is termed para-
sitism. The symbionts may be equally benefited and not all
injurious the one to the other. This relationship is termed
commensalism. The majority of our poplars, willows, beeches,
heaths, orchids and evergreens have attained this state and do
not flourish in soils Avhere suitable fungi do not abound. Per-
haps this explains why it is so difficult to transplant certain
shrubs and trees. The fungi are easily injured and do not be-
come established in the new soil soon enough to keep the plant
alive. This mutual relationship has gone so far in some species

NATURE OF PLANTS 69

that the seeds are unable to grow unless they come in contact
with the hyph^e of the fungus. Finally some plants have become
so adapted to these fungi as to receive all necessary foods from
them. Accordingly, their green leaves and roots have largely
disappeared since they are no longer of service. Examples of
this are seen in the white Indian pipe and pine-sap and in the
coral root orchid. In these latter examples the relationship of
the two plants is completely turned about for we see that the
plant has become a parasite upon the fungus.

28. Roots as Store-houses for Foods. — In all the cases hereto-
fore considered the root has functioned in one way or another as
an organ for the temporary reception and transmission of materi-
als from the soil. It may also serve in other capacities, one of
the more important of which is as a storage organ. Such roots
become fleshy and filled with foods and are of great economic
importance, furnishing a variety of nutritious vegetables, as the
sweet potato, beet, turnip, etc. Many of these valuable plants
are biennials. During the first season the plant develops only
leaves and stores up food in its fleshy roots which is utilized in
the following season in the production of flowers and seeds, after
which the plant perishes. It is noteworthy that the biennial
habit has been induced in many of these plants by cultivation. If
they are left to themselves in a wild condition they will soon
revert to an annual growth, i. e., producing seed and perishing
during the season. Perennial plants li\e on from >c'ar to \ear.
As to whether some plants behave as annuals, biennials, or
perennials depends in some cases upon the place or time of
planting. The castor bean with us is an annual but in warm
countries it becomes a perennial. Winter wheat on the other
hand has acquired a biennial habit owing to the late planting of
summer wheat types.

29. Anchoring and Supporting Roots. — Roots also play a very
important role in anchoring and supporting the plant. Excellent
examples of the supporting roots are seen in the Indian corn
where numerous roots spring from the stem a short distance from
the ground and reach out on all sides like gu>' ropes steadying
the plant in the ground. Similar devices appear in our elms.

70 BINDING ACTION OF ROOTS

maples, and beeches where enlargements of the roots at the base
of the trunk rise up like girders, bracing the tree against winds.
More marked illustrations appear in many tropical plants, as the
stilt roots of the mangrove and screw pines, and the buttressing
roots of one of the Indian rubber plants (Ficus), etc. In the
ban^^an tree of India the branches have an almost unlimited
lateral growth owing to the fact that they are supported by a
succession of roots that reach from the branches to the ground.
Kerner cites an example of one of these trees wdth 300 large and
3,000 small prop roots. This tree sheltered a village of 100 native
huts and an army of 5,000 men.

30. Binding Action of Roots. — The landscape would be con-
stantly subject to great change either by erosion or through the
action of winds w^ere it not for the solidifying and binding action
of roots upon the soil. The difficulty of breaking up the prairies
and the turf of abandoned fields or meadows indicates the extent
and completeness of the ramifications and interweaving of the
roots. Miles of sandy reaches are Held from shifting through
the restraining action of roots and stems of grasses and other
plants. The little town at the end of Cape Cod would have been
submerged long ago by the shifting sand dunes had not suitable
plants been planted to hold in check the loose sands.

Roots not only bind the soil together but the older portion of
the root usually possesses the power of contraction. This pro-
perty results in the pulling dow^n and fixing of the stem in the
ground. You must have often wondered how stems and bulbs
become so deeply buried in the soil although the seeds are
scattered on the surface of the soil. This is also w^ell illustrated
in the tips of raspberry bushes which come in contact with the
soil through the bending of the stalk. Roots strike out from the
tip and when thoroughly established in the earth the olde^
portions contract and bury the tip in the soil. In the same
way seedlings of various plants are slowly pulled down into the
earth so that they finally become planted at considerable depths.
It is this contraction of the roots that keep the slowly elongating
stems of the dandelion, dock and many other plants buried in
the soil and that bind many creeping plants such as certain
clovers and knot weeds firmly to the earth.

CHAPTER III

THE STEM

31. The Function of the Stem.— The stem has for its chief
function the production and display of the leaves and roots and
the conduction of the materials which these organs are especially
concerned in handling. It serves as a connection between them,
carrying up the material absorbed by the roots and distributing
the various substances received from the leaf. Like the traffic
of a city this material is received at many stations and trans-
ported along various channels to many points. At one place
some of it is used in the manufacture of food, at another point
material is required for the nourishment and construction of the
cells. Here a portion is stored, or again, there is the useless
or waste material to be carried away.

32. Character of the Stem. — In order to comprehend the nature
of the work performed by the stem it will be necessary to ex-
amine its external form and its internal structure. While stems
vary greatly in character we may gain a general understanding
of their more important features by studying some wood\' twigs
or branches as they appear in winter (Fig. 39). The most strik-
ing features in such twigs are the buds, the leaf scars (see page 43),
lines of fine scars forming rings about the stems at varying inter-
val?, minute roundish or lense-like elevations (the lenticcls), and
finally the character of the branching. The buds of the hickor>',
horse chestnut, Norway maple, lilac, cherry, etc., are excellent
for study. In these buds, as in most examples, the outer part
consists of a series of leathery, closely overlapping scales or
modified leaves. These function to protect the delicate parts
within against injury and especially against loss of water. By
carefully removing the scales with a sharp pen knife we find
within the ordinary green leaves, stem and also in some buds
the flowers that will appear next year. These organs are so
small and crowded that they appear to arise from a single point.

72 CHARACTER OF BUDS

They have, however, the same arrangement as in the mature
shoot of the summer time and you should observe for a few days
in the spring the opening of a variety of buds, noting how this
crowded arrangement of the organs of the bud gives place to
that of the mature shoot. The formation of buds possibly came
about owing to climatic changes. The earlier vegetation of the
earth was subject to a uniform tropical climate but the formation
of high mountain ranges and other factors caused cold currents
of air to sweep over the earth and also produced variations in the
humidity of the atmosphere. These factors resulted in producing
a season favorable for growth and one in which growth would
be stunted or checked. The latter condition induced many
changes in the plant, of which the bud is an important example.
The great majority of woody plants are characterized by these
two phases: a period of rapid growth up to about the middle
of July, during which time all the organs of the season as well as
the buds are formed, after which they continue to manufacture
food and store it up in the buds and other organs until September
or October; and a period of dormancy in which the plant is in a
resting condition until the spring. Such a method of growth is
termed definite and should be distinguished from the indefinite
growth of a few plants such as the raspberries, locusts, some
honeysuckles, etc., where the growth goes on until the fall frosts,
with the result that the more recently formed, delicate parts are
killed off each year. But even in these cases we have essentially
the same rhythm of growth as in the first case because the older
portions of the shoot have perfected their tissues and developed
their buds and so prepared for the dormant period.

Whatever may be the causes that have resulted in the forma-
tion of the bud, certainly its closely united scales which are often
reinforced with resinous, mucilaginous or hairy coatings, are an
admirable device for protecting the delicate parts within against
winds which would rob them of water at a time when none could
be obtained from the soil. It is quite natural to think of these
devices as protection against cold, but strangely there is no adap-
tation known among plants that serves primarily as a protection
against low temperatures, and there is no cold upon the earth

NATURE OF PLANTS

/o

^

Fig. 39. Types of stems: A, branch of hickory showing a large terminal
bud and several small lateral buds that were developed in the axils of the
leaves of the past season. The branch is three years old, as shown by the
three rings, r, r' , r" , of the bud scars which mark the successive positions of
the terminal bud during the past three seasons; /, leaf scars. B, branch of
pear — after Bailey. The ring at i shows the position of a bud which produced
the next year a pear, as is indicated by the large scar and swollen branch at a.
A short lateral branch was also developed the same season with its terminal
bud at 2. The next year this bud produced a branch that extended to 3.
bearing several leaves (note leaf scars) and of course axillary buds — one of
which, h, we see grew in the following seasons. No fruit was developed this
season. The bud formed at 3 behaved the next season very much as the one
previously noted at i, forming a pear at a' and a short lateral branch that
reached to 4. Note also that the axillary bud, h, of the previous sc-ason grew
a little and its subsequent history can be followed by the annual bud scars.
In the following season bud 4 developed a vigorous shoot reaching to 5. No
fruit was formed but three buds survived, h', b", b'", thus nearly duplicating
the growth of bud 2. Bud 5 develops the next season fruit at a" and also
two lateral shoots that extend to 6 and 7 respectively. The three buds below
5 had only a feeble growth during this and the succeeding seasons and bud b'"
evidently perished during its first period of growth. Bud 6 produces the next
year a pear at a'" and a lateral shoot reaching to 8 and bud 7 at the same

74 NATURE OF BUDS

so intense that it may -not be endured by many plants, if this
were the only unfavorable factor.

Buds are usually found singly in the axils of the leaves and as
a rule only a few that are favorably situated as regards the light
ever develop. The development of the buds brings about the
characteristic appearance or habit of the plant. This growth is
not controlled alone by the favorable position of the buds but
also by the interaction or correlation of all the growing parts.
As a result in some trees we find a single terminal bud that is
larger and better developed than all others. This bud will pro-
duce the longest shoots and consequently such trees will have
spire-like, stems as in the spruces, larches, etc. In other cases
this same type of growth may continue for a time but eventually
terminal buds of equal vigor will be developed upon several
branches and consequently equal growths or diffuse types of
branching will result as in the maples, elms, etc. Buds that are
not favorably located generally perish after a few years but not
infrequently they remain alive and become overgrown by the
increase of the stem. In such cases the bud grows slowly and
maintains itself near the surface of the wood. These buds grow-
ing and often branching in the wood produce those curlings and
twistings in the grain that are commonly known as bird's-eye
wood. Somewhat similar markings are also produced by minute
outgrowths on the surface of the wood. These do not have,
however, the darkened center characteristic of the curl caused
by the bud growth. Dormant buds sometimes develop into
normal branches, as when a portion of a tree is removed and

time only develops a leafy shoot reaching to 9. Buds 8 and 9 both produce
fruit the next season (a*'", a^^) and short lateral branches reaching to 10 and 11.
These buds during the present season only formed leafy shoots.

Buds are formed in the axils of all the leaves that appear on the shoots of a
season's growth, but notice that but few or none at all develop. Some have
a thrifty growth, others remain small and perish after one or more years.
This is true of the fruit. Some of the pears mature, as is indicated by size
of the scars, while others drop off after one or more months. Note also the
almost regular alternation in the production of shoots bearing fruit and leaf
shoots. Bud 6 is the only exception to this succession. Observe that the
branches assume different positions and that the extent of the elongation of
the shoot from year to year varies. Can you explain these facts?

NATURE OF PLANTS 75

consequently an additional supply of food stimulates them to
growth. Buds may also arise upon any part of the stem, root
or leaf. These are the so-called adventitious buds and their
formation upon roots often accounts for the colonial habit of
many trees, plants and shrubs, as in the poplars, Ailanthus,
sumacs, etc. They appear to rise naturally in some cases, but
in other instances their formation is due to the stimulus of a
wound or some other cause and like the dormant bud they serve
to prolong the life of the plant. Richards has shown when
living cells are exposed to the oxygen of the air, owing to a
wound, that these cells are stimulated as a result to renewed
activity. This doubtless explains the formation of callus and
the healing of wounds, as well as the formation of adventitious
buds in many cases. Common examples of buds due to wounds
are seen in the vigorous shoots that spring up from the stumps
of hard wood trees, pollarded willows, etc. Some buds become
fleshy owing to the storage of the food and dropping from the
plants serve to propagate new individuals. Examples of this
are seen in the fleshy buds on the tips of the branches or in the
axils of the leaves of the stone-crop and some lilies and in the
flower clusters of some onions. Many aquatic plants have the
habit of forming similar buds on the approach of winter. These
being compact and heavy with food sink to the bottom of the
ponds in the fall and renew their growth in the spring.

The rapid unfolding of the bud in the spring is a constant
source of surprise but when we recall that it contains usually
an abundant supply of food and practically all the organs that
will appear on the stem during the season we can understand
how the bud opens and elongates into a shoot bearing leaves
and flowers during the first few weeks of spring. The part of
the stem bearing the scale leaves of the bud does not elongate
materially since these leaves arc of service onh- during the winter
or dry season and soon fall ofl" after the o|X'ning of the bud.
Consequently the scars formed by the fall of these protective
scales form a ring (Fig. 39, r) each year about the stem which
marks the position of each successive bud. These scars are
therefore known as annual rings and by counting the number of

76 REGIONS OF THE STEM

these rings on a twig you can ascertain its age and observe the
extent of elongation during each season. The minute dots or
lenticels consist of loosely arranged cells that permit an inter-
change of gases between the living cells within the stem and the
atmosphere, functioning in much the same way as do the stomata
of the leaves.

We are noAv interested to learn something of the Internal char-
acter of the stem. By cutting with a sharp knife across the stem
between the terminal bud and the first annual ring we see that
there are several regions in the stem. On the outside is a very
thin brown layer, largely composed of cork cells, which prevents
loss of water and which in old stems becomes quite thick and
variously split. Within is a zone of rather delicate tissue. The
outer part of this region, termed the cortex contains more or less
chlorenchyma— at least this is true of small stems — and functions
as in the leaf; while the inner rather colorless portion of this
region serves to conduct foods manufactured by the leaves and
it also frequently serves as a storehouse. The third zone is made
up of thick-walled, compact cells, regularly arranged. This is
the wood or xylem portion of the stem. It gives stability, con-
ducts the water and crude materials absorbed from the soil and
also serves as a storage organ for the reserve foods of the plant.
The center of the stem, known as the pith, consists of delicate
cells which conduct water for a short time and soon die.

33. The Anatomy of the Stem. — We are now desirous of seeing
how this complex structure of the stem comes about and of
studying in more detail the character and operations of the
apparatus.

If thin sections are made across the stems of seedlings, as of
the castor bean, the tissues will resemble the arrangement shown
In Fig. 40. It should be stated that the vascular bundles (Fig.
40, v) do not appear as separate strands in the earliest stages of
the development of the stem. This vascular tissue appears
rather as a more or less continuous circle of tissue surrounding
the pith. Very early in the development of the stem this circle
of vascular tissue is broken into segments or vascular bundles.
This separation of the ring of vascular tissue into segments is

NATURE OF PLANTS 77

caused by strands of it here and there extending out into the
leaves, thus forming gaps in the circle. All seedling stems show
this arrangement of tissues and consecjuentK- there appears
more or less clearly three regions; an epidermis, a cortex, and a
central region surrounded by the cortex, an arrangement already
noticed in the roots. The epidermis does not matcrialh- differ in
structure, modification, or function from that already noted in the
leaf, see page 8. The cortex is largely composed of parenchyma
and extends from the epidermis to the endodermis which it in-
cludes. The endodermis is not so well marked as in the nxjt
and owing to the rapid growth of the tissues in this region it is

Fig. 40. Cross-section of young stem of castor bean: c, cpiderniis; c, corlt-x;
p, pith or inner portion of central cylinder; i\ vascular huucilc, arranged in
outer part of central cylinder. — H. O. Hanson.

often impossible to detect it. The cortex assists the leaves in
photosynthesis, the outer portion as a rule being well supplied
with chloroplasts. In old stems that become co\-ercd with a
thick layer of bark the chlorophyll quite disappears, owing
doubtless to its exclusion from air and light. The cortex also
serves as a storehouse for foods. Particularly is this true of the
endodermis and adjoining cells wliidi .uc often temporary

78

STEM STRUCTURE

receiving stations for the carbohydrates during their transport
through the stem. The cells in the outer portion of the cortex
frequently become thickened and more or less elongated to give
strength to the stem. One of the most common modifications of
this kind is shown in Fig. 41, ^. This tissue, collenchyma, is
characterized by the thickening of the cells at the angles or on
all sides and by the silvery luster of the walls. The walls,
though very elastic and tough, are capable of growth and so
they are especially adapted to the support of the young elongating
stems. In older stems, where elongation has ceased, other
greatly elongated cells, called stereome fibers or sclerenchyma

B

(r ' ^^'

! ^

ilil

Fig. 41. The stereome or strengthening cells of the cortex: A , collenchyma
cells in growing stem of Begonia, showing above the cells in cross-section and
below in longitudinal section. B, a group of elongated thick-walled cells,
called sclerenchyma fibers or stereome fibers, from the stereome of mature
flax stem.

fibers, are often formed. These cells have thick walls and taper-
ing ends which interlock and bind the cells very firmly together
(Fig. 41, B). These strengthening cells form a compact zone
about the stem or they may be arranged in separate bundles.
In this latter case they often appear to the eye as light colored
bands extending along the surface of the stems of many herba-
ceous plants. The central region of the stem is characterized
by a mass of parenchyma with several vascular bundles arranged
in a circle (Fig. 40). The central mass of parenchyma is called
the pith.

34. The Vascular Bundle. — The vascular bundle contains three
distinct regions; an inner thick- walled portion (the wood or xy-

NATURE OF PLANTS

79

lem), an outer thin-walled portion (phloem), and between these
two regions a delicate layer of cells, the cambium (Fig. 42).
The transport of all substances is largely confined to the vas-
cular bundles, the xylem conducting principally the crude ma-
terials while the bulk of the organic substances passes through
the phloem. We are now interested to study the character of
these cells and note their adaptation to the work in liand. In
the xylem occur various large spaces, the vessels or ducts (Fig.
42, v), and smaller spaces, wood cells of different kinds. The

Fig. 42. One of the vascular bundles shown in Fig. 40 cniargcHl: .v, xylem;
V, vessels or ducts; p, phloem; s, sieve tube; ac, accompanying cell; c, cam-
bium; st, stereome. — H. O. Hanson.

cells of the phloem are much smaller, thinner walled and less
numerous than those of the xylem. Therefore very thin sec-
tions are necessary in order to make clear all the different tissues.
Sections from a squash stem may well be studied for this purpose
because the various cells of the phloem are comparatively large

8o

STRUCTURE OF BUNDLE

and easily distinguishable. It should be stated that the bundle
of the squash is peculiar in that a phloem region is developed on
both sides of the xylem. The phloem, like the xylem, is also
characterized by large cells, which are here known as sieve tubes,
because these cells are tube-like structures with the cross walls
perforated like a sieve (Fig. 4.2, s). A small cell, the accom-
panying cell, is associated with the sieve tube. Usually a vary-
ing amount of parenchyma also occurs in the phloem and often
thick walled stereome fibers (Fig. 42, st). Between the xylem
and phloem is a region of very delicate and regularly constructed
cells, the cambium (Fig. 42, c). The growth of the bundle in
diameter and, in fact, of the entire stem is brought about very
largely by the formation of new cells through the division of the

Fig. 43. Longitudinal section of the bundle shown in Fig. 42: x, xylem;
ph, phloem; p, pith; v-v'", annular, spiral, scalariform and pitted vessels;
c, cambium; s, sieve tubes; ac, accompanying cells; st, stereome. — H. O.
Hanson.

cells in the cambium. The cross section of the bundle reveals
the arrangement and distribution of the various tissues but it
will be necessary to examine a section taken parallel with the
length of the stem, i. e., a longitudinal section, in order to arrive
at an understanding of the structure and character of the cells
themselves. Fig. 43 shows such a section of a bundle. The

NATURE OF PLANTS 8i

vessels are now seen to be tubular structures with peculiar
thickenings of their inner walls that assume the form of rings
(annular vessels), spirals (spiral vessels), or the spirals may
branch more or less (reticulate vessels), and often to such an
extent that they cover the entire surface of the wall with the
exception of numerous small spaces, pores, thus forming the
pitted vessels (Fig. 43, v'"). These peculiar sculpturings prevent
the crushing in and closing of these tubes and the annular and
spiral thickenings also permit considerable elongation of such
ducts which would not be possible if the thickening were more
uniform over the wall. It will be noticed that the later formed
ducts are characterized by reticulate or pitted walls since this type
of thickening must occur after the elongation has taken place.
The ducts are composed of elongated cells but owing to the
absorption of the majority of the cross walls the ducts finally
come to resemble hollow tubes that often run for considerable
distances through the stem without any cross walls at all. The
smaller cells of the xylem assume various shapes. Some of
them, the tracheids, have pointed ends and are characterized by
markings similar to the ducts. Strengthening fibers, similar to
those noted in the cortex, are of common occurrence and also
short cells with blunt ends, wood parenchyma. The ducts and
tracheids soon lose their cell contents but continue to function
in the conduction of water. The wood parenchyma and other
living cells retain their vitality for several years and generally
function as storage cells. It is not improbable that they may
furnish some of the energy required to force the water through
the ducts and tracheids. It is significant that living tissue is
always associated with these water-conducting cells of the stem.
Passing now to the phloem we see that the sieve tubes are made
up of elongated cells with perforated or sieve-like cross walls (I''ig.
43, s). These minute openings adapt these cells to the transport
of albuminous substances which do not readily diffuse through
cell walls. The accompanying cells (Fig. 43, ac) which are cut
off from the sieve tubes probably assist them in this work. At
any rate they have abundant cell contents and contain nuclei,
whereas, singularly enough, the nuclei of thc^ sit'\r lubes soon dis-
6

S2 STRUCTURE OF BUNDLE

appear although the rest of the protoplasm of the cells remains
active for one or more seasons. Elongated parenchyma cells are
associated with the sieve tubes and accompanying cells, serving
chiefly for the transport and the temporary storage of the more
readily diffusible carbohydrates. Fibrous cells, stereome, are
also of common occurrence in the phloem (Fig. 43, st). The
cambium is composed of very delicate cells, owing to the fact
that in this region cell division is taking place and the cells
consequently have not yet attained their characteristic form (Fig.
43, c). We may best gain an idea of the growth of the cambium
if we consider it as a single layer of cells between the xylem and
phloem. Each of these cells divides into two daughter cells
(Fig. 44), usually the inner cell {i. e., the cell next to the xylem)
develops into a duct, tracheid, or other element of the xylem,
while the outer cell, after increasing to the original size of the
cambium cells, divides into two cells as at first. This process
is repeated again and again, usually the inner cell growing into
one of the cells of the xylem while the outer cell retains the

X 3^^ ^Qr

Fig. 44. Diagram showing the mode of division of the cambium cells.
The cambium cell is shaded to distinguish it from the cells derived from it.
Note in the last division at the right that the inner daughter cell becomes
the cambium cell while the outer develops into a phloem cell.

power of further division. Less commonly the reverse method
of growth takes place and we have the outer cell enlarging and
forming one of the cells of the phloem while the inner cell acts
as a cambium cell. In this way new cells are added to the xylem
and phloem and the enlargement of the vascular bundle is
effected.

35. The Conducting System of the Plant. — These vascular
bundles extend from the root up through the stem and branches
to the leaf where they divide again and again, reaching all parts
of it. In this way the bundles become much reduced in size,
the free end of the vein often consisting of a single tracheid to

NATURE OF PLANTS 83

conduct the crude materials to the surrounding cells and elon-
gated parenchyma cells to collect the manufactured foods (Fig,
5, /). We now see how admirably the vascular bundles are
adapted to the transport of fluids. The xylem carries the sub-
stances received from the root hairs to all parts of the plant
body quickly and with a minimum expenditure of energy because
of the elongated character of its cells and the absorption of many
of the cross walls of its cells. So the manufactured foods lo-
cated at any point in stem or leaf are gathered up and distributed
by the phloem, the more soluble and dilTusible substances being
handled in part by the thin-walled parenchyma, while non-
difTusible, albuminous substances can readily be transported
from cell to cell through the perforations of the sieve tubes.

36. The Strengthening Tissues of the Stem. — Attention may
be directed at this point to the perfection of the arrangements
that give rigidity to the stem. The delicate cells of very young
stems or of any other part of the plant are distended by fluids
which they absorb. In this way a very considerable force, known
as the turgor of the cell, is exerted that may amount to over 200
pounds to the square inch. This pressure at first gives sufficient
rigidity to the rapidly elongating stem, but as it increases in

Fig. 45. Common form of girder.

size and the strain upon it becomes more considerable, thick-
walled collenchyma cells are formed that keep pace in their
growth with the elongation of the stem. When clongatit)!! finally
ceases, tough fibers of stereome appear which are able to meet
the increasing strain upon the stem due to the formation and
development of its various organs. These strengthening tissues,
with which must also be included the tissues of the xylem, are
arranged with the same mechanical effects as are employed in
the construction of buildings, bridges, etc. In bending a beam

84

STRENGTHENING DEVICES

the strain falls especially on the convex and concave surfaces.
The cells will be compressed on the concave side and stretched
on the convex side, while the tissues in the center will be subject
to the least disturbance. For this reason girders are strength-
ened on the surfaces receiving the strain and weakened in the
center (Fig. 45). So also given a certain amount of material

Fig. 46. Cross-section of stems showing arrangement of strengthening
tissues: A, burdock with strands of collenchyma — st, at periphery; v, vas-
cular bundles. B, sweet clover, showing a similar arrangement. C, moon-
seed showing cylinder of stereome fibers outside of vascular bundles. D,
rush with strands of stereome fibers at periphery of stem and also on inner
and outer sides of the bundles; v, vascular bundles; st, stereome.

this is more effectively distributed in the form of a hollow column
than in a solid column since less strain falls upon the center of
the column. Therefore in stems we find the strengthening tissue

NATURE OF PLANTS 85

very economically distributed at the periphery either as bands
or strands of coUenchyma and stereome fibers and these struc-
tures are frequently reinforced by additional mechanical tissue
in the phloem or just outside of it (Fig. 46). The vascular
bundles themselves frequently furnish excellent examples (jf the
girder, masses of stereome being found on the outside of the
phloem and on the inside of the xylem (Fig. 46, D). The resis-
tance of these tissues to cutting and breaking affords abundant
evidence of their hardness and rigidity. It may be surprising
to note that the sustaining strength of the stereome fibers is
equal to that of the best wrought iron or hammered steel while
their ductility is ten to fifteen times that of iron. The superior
quality of these tissues and the perfection of their arrangement
result in structures that can not be approximated in any of our
buildings. The height of the tallest chimneys scarcely exceeds
the diameter of their bases more than fifteen times but many
stems of rushes and grasses exceed the diameter of their bases
from 200 to 500 times. It would be impossible with aii\- metal
to construct a column of the same length and weight and ha\'ing
the same rigidity, elasticity, and resisting power as these stems.
We gain an idea of the toughness and durability of these fibers
when we consider that linen, rope, matting, etc., are manufac-
tured from them. In stems that increase greatly in diameter,
as our trees, the mechanical tissues are confined to the wood or
xylem. These are to be sure nearh' solid colunms, but e\cn
here there is sufficient strengthening tissue in the xylem at the
periphery of the stem to support the trunk, as is often attested
by the sturdy character of trees that have become hollow through
decay.

37. The Secondary Growth of the Stem. — The arrangement
of the tissues as outlined above, termed the i:)rimary growth,
remains practically unchanged in some animal plants but in such
forms as live on from year to year or that increase matrrially
in diameter there results extensive alterations in the structure
of the stems owing to the formation of new tissues, especially
in the region of the cortex and in the vascular bundles. The
epidermis furnishes sufficient protection to such stems as do not

86 NATURE OF CORK TISSUE

increase materially in size, such as the majority of our annual
plants; but in long lived stems, as shrubs and trees, where growth
goes on from 3'ear to year the epidermis is not able to keep pace
with the growth.

38. Cork Tissue. — To meet this condition a new tissue, the
cork, is developed from certain cells called the cork cambium,
usually situated near the epidermis (Fig. 47). The cells of the
cork cambium divide much after the manner noted in the cam-
bium of the vascular bundle, but there is this difference, the outer
cell of the two daughter cells becomes a cork cell while the inner
cell remains capable of further division. Only rarely does the
outer daughter cell act as the cambium cell while the inner cell
develops into one of the cells of the cortex. As soon as the
cork cells have reached their growth, a substance called suber

Fig. 47. Cross-section of the outer part of the stem of geranium. The
cork cambium, c, originating in the cells immediately below the epidermis.

begins to be deposited in their walls. This completely changes
the properties of the cellulose walls and renders them impervious
to fluids and gases. So the cork cells have the same physical
properties as the cuticle of the epidermis and owing to the
continued activity of the cork cambium they are able to keep
pace with the growth of the stem. It is evident that the cork
cells must die as soon as they become impervious to fluids, and
it must also follow that all cells lying outside of these cork cells
will die since no fluids can reach them from the vascular bundles.
These dead cork cells give the characteristic aspect to the outer
bark of trees and we would naturally come to think of the coarse,
dark bark as composed of rather thick cells. As a rule, however,
the cork cells are quite delicate and compactly put together (Fig.
48) and the dark color is more usually due to a discoloration of
their walls or to the dried remains of the cell contents. The

NATURE OF PLANTS

87

furrows and seams that occur in the bark of most trees are caused
by the continued activity of the cork cambium and the cambium
of the vascular bundles which adds each season new cells to the
stem and so pushes out the cork cells. In this way it comes about

Fig. 48. Fig. 49.

Fig. 48. Cross-section of cells of bottle cork showing the delicate character
of cork cells.

Fig. 49. Scale bark of pitch pine. The crescent-like lines in the bark
show the successive positions of the cork cambium.

that the cork cells are pushed further and further from the center
of the stem and since they are not capable of dividing they are
pulled apart, forming the characteristic furrows in the bark.
It will often be noticed that the bark of some trees, see the cone-
bearing trees, sycamore, etc., cleaves off in shell-like scales.
This is due to the formation of a new cambium that joins on to

Fig. 50. Cross-section of the outer part of a stem showing the early
development of a lenticel. Note the irregular character and loose arrange-
ment of the cells below the stoma and the cork cambium, c, extending out on
either side of the lenticel.

the old cambium in the form of crescents (Fig. 49). Conse-
quently as this bark is pushed out it breaks along these successive
crescent shaped cambiums and finally cleavts off in scales. In

NATURE OF LENTICELS

some trees the cork cells break off about as fast as they are formed,
so that a comparatively thin layer of cork cells remains attached
to the trunk. Such trees have a smooth bark, as in the beech.
In other cases the cork cells are formed in great abundance,
and owing to the adhesion of the cells, thick layers of cork are
formed, as in the oaks, and giant trees of the Pacific.

It is evident that this mantle of impervious cork cells would
tend to prevent the access of the atmosphere to the stem. We
have seen that all living cells respire. In many instances the air
spaces extending from the leaves to all regions of the stem are

Fig. 51. Fig. 52.

Fig. 51. Later development of a lenticel: c, cork cambium; i, intercel-
lular space.

Fig. 52. Surface view of lenticels: A, lenticels on branch of horse-chestnut
appearing as minute brownish swellings. B, old lenticels on white birch
appearing as dark lens-shaped streaks.

sufficient to bring about an adequate interchange of gases but in
young stems where growth is vigorous and where therefore
respiration is considerable, special devices are required to bring
the living cells of the stem into more direct communication with
the air. This work is effectively accomplished in herbaceous
stems by the stomata, but in stems characterized by the forma-
tion of cork it is noticed that the cells just below the stomata
begin to divide and form a rather loose mass of cells that lift up
and finally rupture the tissues about the stomata, thus forming a
small, lens-shaped outgrowth on the surface of the young stem,
called a lenticel (Figs. 50; 52, A). Soon this growth becomes

NATURE OF PLANTS 8q

localized in a layer of cells situated on the inside of the Icniicel
(Fig. 51). This layer of cells is a part of the cork cambium,
but strangely enough only loose cells are added to the lenticel
during each year's growth and consequently a passage way is
kept open to the living cells within the stem. The lenticels
appear as minute points upon the surface of young stems but
upon old trunks they often become greatly elongated, fcjrniing
the characteristic bands on the bark of the birch and cherry, etc.
(Fig. 52, B). In bottle cork, derived from the cork oak of the
Mediterranean, the lenticels appear as minute lines often errone-
ously referred to as worm holes. Usually as the cork layer
increases in thickness the cortical cells become less active, lose
their chlorophyll and the cork cambium closes the lenticels b>'
forming compact cork cells instead of the loose cells of the
lenticels. This same closure of the lentiels is frequently scene
in the fall but the renewal of growth in the spring results in the
formation of new loosely related cells which rupture the over-
lying cork layer and so open the lenticels. So it seems probable
that the lenticels arise owing to the active respiration and
transpiration of the cortical cells and, when these functions di-
minish, that the lenticels become closed and fmallv- fail to develop
further.

39. The Cambium Cylinder. — Let us now consider the changes
that are effected in the vascular bundles through the activity of
the cambium. The increase in the diameter of the stems of
many annual plants and especially of our trees and shrubs is
largely brought about by the formation of new cells derived
from the cambium. While the vascular i)undles are very small
it will be noticed that the parenchyma cells separating the vascu-
lar bundles begin to divide so as to form a line of cells connecting
the cambium of each bundle (Fig. 53). In some cases these
divisions are at first somewhat irregular but soon the growth
results in the formation of cells with parallel walls and in this
way a line of regular cells is formed which in cross section apix\ir
as a band or ring of cells but in longitudinal extent they constitute
a cylinder. These cells are therefore termed the cambium cylin-
der and they continue to divide as already not.d in ihc r.imbium

90

THE ANNUAL GROWTH

of the bundles (Fig. 54). Consequently new cells are now added
not alone to the vascular bundles but also along the entire extent

Fig. 53. Cross-section of a stem of castor bean showing the formation of
the cambium between two vascular bundles: x, xylem; ph, phloem; c, cam-
bium of the bundle. The faint lines, ic, are the first divisions of the parenchyma
cells between the bundles that result in the formation of the cambium cylinder.

of the cambium cylinder. This growth results in the formation
of a layer of xylem on the inside of the cambium cylinder and a
layer of phloem on the outside, and so brings about the principal

cam

Fig. 54. Cross-section of a stem pf castor bean in which the formation of
the cambium cylinder, cam, as a ring of regular cells, has been completed;
p, pith; I', vascular bundles; st, stereome; c, cortex. Compare Fig. 40. —
H. O. Hanson.

NATURE OF PLANTS

91

enlargement of the stem (Fig. 55). This growth is repeated
each spring in such plants as continue to enlarge from year to
year, as in the shrubs and trees. The majority of all the cells
that are to be formed in a year's growth are cut off from the
cambium cylinder early in the spring, usually by the last of April.
The majority of these cells become xylem cells while comparatixily
only a few of the cells are added to the phloem. Consequently the

Fig. 55. Cross-section of stem of castor bean three weeks okli-r than one
shown in Fig. 54. Note the changes that have occurred in the stem and
especially the numerous cells that have been added to the xylem. ck, cork;
ph, phloem; cam, cambium cylinder. — U. O. Hanson.

xylem increases faster than the phloem and forms tlu' bulk of the
tissues of the stem. The amount of the xylem added to the stem
each year is generally indicated by bands or annual rings (Fig.
56). This is due to the fact that the first cells formed in the
spring have thinner walls and often contain a great many ducts,
whereas the later formed cells are for the most part small and pro-

92

ANNUAL RINGS

vided with thicker walls. So there is a sharp contrast between
the small thick-walled cells of the summer wood and the thinner
walls and larger cells of the spring wood (Fig. 57). This ap-
pears to the eye as a band, the annual ring (Fig. 56). The
difference in the structure of the cells of the spring and summer
wood is doubtless due to the large volumes of water that are
transported to the rapidly growing shoots, flowers, and leaves
each spring. Later in the season a lessened volume of water

Fig. 56. Fig. 57.

Fig. 56. Diagram of a cross-section of a stem of black oak four years old;
p, pith; I, 2, 3, 4, annual rings of xylem; c, cambium cylinder; ph, phloem;
cr, cortex; ck, cork; m, medullary rays.

Fig. 57. Magnified view of a portion of one of the bands of black oak in
Fig. 56, showing the thick-walled summer wood succeeded by the thinner-
walled cells and vessels. This association of cells causes the banded appear-
ance of the annual rings of growth, m, medullary rays; v, vessels in the spring
wood.

produces firmer and denser tissues. The age of a tree can gener-
ally be ascertained by counting the annual rings. However, two
rings may be formed in one season owing to the checking of
the growth by fire, severe drouth, depredations of insects and
the subsequent recovery and renewal of growth. These annual
rings also reveal the life history of the tree, broad bands indi-
cating favorable seasons and narrow bands telling of fires,

NATURE OF PLANTS

93

drought, transplanting, and other factors that hiiiit the annual
growth. They also show that the development of the plant is
subject to the same rhythm of growth as is seen in"the animal.

Fig. 58. Diagram of a three-year-old stem of basswoocl cut so as to show
the structure in cross-section, C, in radial section, R, and in tangential stHTiion,
T; c, cortcjc: cy^^^jork; ph, phloem with darker bands of stcreomc; ntc, medullary
ray in cross-section; mr, ray m radial section; vit, ray in tangential stxnion;
/, lenticels; cam, cambium cylinder; />. i^U]i»--Il. O. Hanson.

Namely];the thickness of the rings increases yearl>- uj) to a certain
age, after which time there is a gradual ri'tardation. So the
tree has its youth, maturity and old age.

94

MEDULLARY RAY

40. The Medullary Ray. — Another characteristic of most
woody stems as seen in cross section is the series of delicate
lines, the medullary rays, that radiate from the center of the
stem (Fig. 58, mc). Some extend from near the pith through the
phloem and others originate in the various annual zones of the
xylem and extend partially through the phloem. When magni-
fied these medullary rays are seen to consist of rather thin-
walled, oblong cells and varying in width from one to a few cells
(Figs. 57, m\ 58, mc). In a longitudinal section cut parallel
to the rays, called a radial section, they appear as bars of oblong
cells running at right angles to the elongated cells of the xylem
(Figs. 58, mr] 59, A), while in longitudinal sections cut at right

Fig. 59. Relation of the medullary rays to the xylem cells in pitch pine:
A , radial section showing the elongated cells or tracheids, t, marked by circular,
thin places on bordered pores, p; m, medullary ray of two cells accompanied
by tracheids, mt. B, tangential section showing medullary rays one cell
broad and three to nine high — p, bordered pores.

angles to the rays, called a tangential section, we see that the
cells of the rays are arranged in a lens-shaped group (Figs. 58, mt;
59, B). The medullary rays are of great importance in the dis-
tribution and often also in the storage of foods. Their structure
and arrangement in the stem are admirably adapted for this work.
Their thin walls can readily withdraw from the ascending current
in the xylem as much water as is needed for the phloem and cro-

NATURE OF PLANTS 95

tex. Trachelds are often associated with the medullar) rays to
further increase their absorbing power (Fig. 59, A, vit). Par-
ticularly is this true in the spring wood where the ray cells also
often become greatly enlarged in order to absorb the large volume
of water required at this time of year. In the same way the
carbohydrates and albuminous substances transported through
the phloem are withdrawn through the medullary cells and con-
veyed to the active cambium and the growing cells of xylem.

The value of oak and other woods for interior decoration is
materiall}^ increased by the medullary rays. The trees are sawed
into timber in such a way as to expose the medullary rays to the
best advantage, a process called quartering (Fig. 60). Conse-

Fig. 60. Diagram showing a common method of sawing in producing
quartered oak. The log is first squared to remove the sap wood and then a
number of boards, depending upon the size and grain of the tree, are removed
at a and b. These are the best boards obtained from the log, being broad and
approximately radial and therefore showing the medullary rays to good ad-
vantage. The remaining portions of the log are cut radially, as shown at c.
One of the ends of quartered lumber is oblique and must be squared, as shown
in the diagram.

quently the rays appear as flecks and bands upon the surface of
the board and vary greatly in character owing to the angle of
cutting and to the irregularities of the trunk. The medullary
rays can always be detected in the grain of wood because they
run across or at right angles to the xylem cells (Fig. 58, mr).
As a rule the wood cells of the stem remain alive for only a few
years. This fact is usually apparent in any log or stump where it
will be seen that the outer annular rings are of a lighter color, the
sap wood, and the older parts are of a darker color, the heart
wood. The sap wood is active in the transport of water and
contains many living cells. For this reason it is not so valuable

96 OTHER TYPES OF STEMS

for lumber. The heart wood Is composed of dead cells and
while the most serviceable for building purposes, it is of little
use to the tree as we often see a vigorous tree whose heart wood
has been largely destroyed by decay. We ought not to leave this
vascular system of the plant without again emphasizing the per-
fection of it. First there is the completeness of the system, ex-
tending from the root to all parts of the leaf. As soon as the root
hairs begin to form and function there is already at that point the
beginnings of one of the terminals of the system while the other
terminal is also appearing in the leaves while they are forming and
still concealed in the buds. Then the elongated cells, those of
the xylem being in addition largely without living cell contents,
ensures a rapid transport of material. The weak feature of this
system is the terminals for here the materials must not only pass
through the numerous walls of the short cells but also through
the still more impermable membranes of the living matter of
these cells. But once the material is landed in the elongated
cells, then follows a rapid transport. Therefore it is the develop-
ment of this system that makes possible the stature of plants.
Without it, plants must remain small and in close reach of their
crude supply.

41. Other Types of Stems. — The cone bearing trees, such as
the pines and spruces, have essentially the same arrangement
of tissues and mode of growth as noted above in the dicotyledons.
The xylem, however, consists entirely of tracheids (Fig. 59)
with the exception of a few small spiral vessels that are formed
as the first cells of the vascular bundles.

The monocotyledons, plants distinguished usually by their
parallel veined leaves and single seed leaf, like the palms, lilies,
grasses, etc., are characterized by stems that do not increase
materially in diameter. Growth is largely confined to the top
of the stem and consequently it can only elongate, forming a
very regular, columnar trunk. The reason of this is apparent
when cross sections of such stems are examined (Fig. 46, D).
The vascular bundles are more or less scattered throughout the
stem. The cause of this arrangement is seen in the seedling
stage of the plant. Here we see the vascular bundles arising

NATURE OF PLANTS

97

through the opening up of the vascular cyHnder just as in the
case of the dicotyledones, page 76. The vascular bundles thus
formed, however, do not long pursue a uniform course through
the stem, some soon turning into the pith and others bending
into the cortex. It is also especially noteworthy that there is
usually no cambium separating the xylem and phloem. It will
be seen by examining Fig. 61, that the tissues of the bundles are

Fig. 61. Cross-section of a single vascular bundle of corn stem: ph,
phloem; x, small cells of the xylem; v, vessels of xylem; st, stereome that
forms a sheath about the bundle; p, parenchyma of the stem.

of a similar character and have the same arrangement as in the
dicotyledons (Fig. 42) save for the absence of the cambium which
prevents the addition of new cells to the xylem and phloem.
Consequently there can be no considerable increase in the dia-
meter of the stem and no necessity therefore for the protective
layer of cork cells.

98

ELONGATION OF THE STEM

42. Apical Growth of the Stem.— All stems are characterized
by a more or less extended apical growth. The changes pro-
duced in stems by the growth of the cambium only result in
increasing their diameters but the elongation of all stems is
effected by the formation of new cells at the apex of the stem.
Fig. 62 shows the general features to be noticed in the tip of
a stem as seen in longitudinal section. New cells are being
formed at the apex by division just as in the case of the root.
Below this formative region is the zone of elongation in which
the cells gradually change in character so that the cortical and
central regions are apparent and in certain elongated cells we
note the first indication of the vascular bundles (Fig. 62, x).

Fig. 62. Longitudinal section of the tip of a growing stem: e, epidermis
extending over surface of the entire tip; a, formative region; b, upper portion
of the zone of elongation; c, cortex; x, cells of the central region that by
further growth form the vascular bundles; /, first appearance of the leaves.

The leaves also originate in this region through the active divi-
sion of the superficial cells of the stem. The branches of the
stem develop somewhat later in the axils of the leaves, and more
deeply located cells in the cortex co-operate in their formation.
This relationship of the leaves and branches to the stem is shown
in Fig. 63, which is a diagram of an elongating stem, showing the
relation of the apical region of the stem (a in Fig. 62), to the
lower and older portion. The cells in these young leaves and
branches by rapid division and growth soon form the character-

NATURE OF PLANTS

99

istic tissues already noted in the leaves and su-nis; while a cor-
responding growth in the cortex and adjacent regions results in
the formation of vascular bundles that connect the vascular
bundles of the stem with those of the leaves and branches. When
a leaf falls off, the ends of these vascular bundles can be easily
seen in the leaf scar (Fig. 28, A) but owing to their minuteness

Fig. 63. Diagram of the tip of a stem as seen in longitudinal section:
a, formative region corresponding to the part shown in Fig. 61; /, b, leaves
and branches in various stages of development; v, vascular bundles; c, cortex;
p, pith.

it is not an easy matter to trace them through the stem to the
point where they join on to the bundles of the stem. In the
branch, however, owing to its size and woody character, the
union with the stem is very manifest. Fig. 64 shows several
very small branches that continued to keep pace for one or more

100

RELATION OF BRANCH TO STEM

years with the growth of the stem, but eventually they were
killed by the overhanging branches and in time became over-
grown with the annual layers of xylem. If the branch is favor-
ably situated so that it continues to live then a cone-like struc-
ture is developed (Fig. 64, a) since the branch increases in girth
each year in the same manner as the stem. These branches run-
ning through the xylem are the cause of knots that appear in
lumber. If the branch is living the tissues of stem and branch

Fig. 64. Section through the trunk of basswood showing relation of
branches to main stem. In the upper portion of the figure are three branches
that were killed after a few years growth' by shading and that have been
overgrown by the annual rings of the wood. The branch shown at a has
remained alive and increased in size after the manner of the main stem.

are closely bound together and we have a solid knot, but it not
infrequently happens that branches after living several years die
and begin to decay before they are overgrown. Such a limb will
produce in the lumber a knot that is often black and shaky or it
may fall out, forming a knot hole. The lower branches of trees
growing in a forest are quickly pruned off by the shade of the
upper branches. This causes straight, clean trunks to develop
and the lumber is free of knots.

It follows from what has been said above that in pruning trees
the limbs should be cut off close to the trunk. Painting the
the cut the cut surface does not promote the healing of the wound
though it may exclude organisms causing decay. When a wound

NATURE OF PLANTS

lOI

is made that extends across the cambium as in the cutting off of a
Hmb or by a cut into the trunk that removes the tissues as far as
the xylem it will be noticed that the exposed cambium is stimula-
ted to from a mass of delicate cells, the callus, that gradually ex-
tend over the wound if this is not too large. While the callus is still
forming its outer cells become changed to cork cells which join
onto the old cork while a new cambium layer is also developed on
the inner side of the callus, which unites with the old cambium,
thus continuing the cambium of the stem over the surface of the
wound (Fig. 65, ^). In this way wounds are covered or healed

Fig. 65. Section of a portion of a stem showing the heaHng of a wound
caused by the cutting off of a lateral branch: A, formation of the callus, cl,
owing to the renewed activity of the living cells exposed by the wound; c,
cortex; cm, cambium; x, region of the xylem. B, a similar stem three years
later — c, cortex; cm, cambium; x', xylem added since the wound was made;
0, position of the cambium at the time the branch was cut off; .v, original
xylem of the stem. C, trunk from which three branches have been removed,
showing the gradual covering of the wounds by new tissue.

and the tissue exposed by the cut is gradually buried deeper by
the annual addition of new tissue derived from the cambium (Fig.
65, B, C). Grafting is made possible owing to the formation of
callus that unites the tissues of the scion and stock (Fig. 66).
While grafting is performed in a variety of wa>s the process
consists essentially in all cases of bringing the cambium and
cortex of the cutting or scion in contact wllh ibc ((.rresiHMKling

102 PRUNING AND GRAFTING

region of the stock. The junction of the scion with the stock is
sealed with grafting wax to prevent decay and the drying out
of the exposed cells while the callus is joining together the corre-
sponding tissues in the two parts. When the wound is healed
only a slight ring or swelling indicates the point where the scion
was inserted — so complete is the union of the vascular and cor-
tical regions. It is evident that pruning and grafting should be
done at a time when the cambium is most active, i. e., in the
spring. In the case of fruit trees it is now the practice to defer
the pruning until some time in June, because the cambium is

Fig. 66. A common method of grafting: A, insertion of two scions into
cleft of stock at cambium region. B, wound protected with wax to prevent
drying out of tissues. — After L. H. Bailey.

still active at this season and especially since the buds are form-
ing. By removing that portion of the branch which would only
produce leaf buds a better development of the flower buds is
ensured.

Although the organic union between scion and stock is com-
plete, each part continues a separate and distinct existence. For
example, a scion from a plum grafted upon a branch of a peach
tree will develop a shoot bearing plums and the peach tree will
also continue its normal growth. Ordinarily only closely related
forms may be grafted, as tomatoes and potatoes, apples and
quinces, etc. It must be inferred from the above statement
that there is no interaction between stock and scion. Usually
this is not sufficient to change materially the character of either
but there are numerous instances showing that the chemical com-
position of the cells or even the form, coloration, and productive-

NATURE OF PLANTS 103

ness of scion may be modified by the stock. Fruit j^owers take
advantage of this in grafting the cuttings from sccdhngs that
would not bear for several years upon fruiting trees, with the
result that the scion comes into fruitfulncss the next season.
In the same way it is claimed that seedlings may be made to
bear earlier than they normally would by grafting them with buds
from fruiting trees.

43. Comparison of Stem and Root. — It will be noticed that the
structure of the growing tip of the stem presents several striking
departures from that of the root. In the first place a protective
structure such as the root cap is not required (Fig. 62). The
leaves, and consequently the branches are formed with great
regularity from the superficial tissues of the stem, whereas the
rootlets are deep seated in origin and irregularly distributed.
Furthermore, the elongation of the stem is distributed over
several centimeters, while in the root it is confined to a few milli-
meters. The leaves and branches are distributed over the entire
zone of elongation, while lateral roots do not appear until elonga-
tion has ceased. This prolonged growth of the tip of stems is of
great advantage in bringing about the proper adjustment of
leaves and branches. These organs come into sharp competition
with each other as well as with other plants, and so long as growth
continues there is a possibility for them to become adjusted to the
light without interfering with one another. The mode of elonga-
tion of stems and roots is essentially alike. Each has a formative
region at the apex characterized by active cell formation and
slight elongation, back of this is the zone of rapid elongation and
following this is the zone in which elongation is gradually ceas-
ing and the tissues are assuming their characteristic forms and
functions. It is also important to note that the growth of the
cells on one side of the stem or root is faster at a certain time
than that of the other cells. This will cause the stem to be bent
towards the opposite side. This condition of rapid elongation
does not continue on one side for any considerable time but
slowly moves from point to point around tlie stem so that the
stem is successively bent from side to side, thus causing the apex
to travel in an irregular circle. By these constant movements,

104 SENSITIVENESS OF STEMS

nutation, the stems and roots are more fully exposed to the vari-
ous forces that influence their development and consequently
they adjust themselves to the best advantage (see page 33).

As a rule the irritability or sensitiveness of the stem is not so
localized as in the case of the root. It is evident that it is an
advantage to have the entire growing surface of the stem sensi-
tive to all those stimuli that influence its growth, because by this
means it will be so influenced as to bring its leaves and branches
into proper relations with the surroundings. In some plants to
be sure the sensitiveness to stimuli is quite as localized as in the
root; as, for example, the tip of the cotyledons of many grasses
which have an appreciation of light almost the equal of our eyes.
These cotyledons will turn toward a light too feeble to enable us
to read the time of day on a watch. The significant fact to be
noted here is that this keenness and localization directs the plant
into a suitable place as soon as it emerges from the soil. Grass
plants grow in thick colonies and therefore the stems come into
sharp competition. Consequently it is a necessity that the young
shoot on emerging from the soil should be able to appreciate the
feeblest light and so be able to direct its growth through the
smallest openings.

The reactions of the stem to various stimuli, such as light,
gravity, etc., are quite as purposive as those noted in the root.
Lateral light, as in the case of plants growing in windows, acts
as a stimulus and causes a more considerable growth of the cells
on the darker sides of the stem. This results in bending the
stems towards the light and consequently in the exposure of the
broad blades of the leaves to the light. So, also, when erect stems
are placed in a horizontal position the stimulus of gravity awakens
a more active growth on the underside of the stem, thus bending
the tip into an erect position for securing light. Not all stems
and, indeed, not all parts of the same plant body are similarly
influenced by these forces. Many plants tend to arrange their
stems at right angles to the action of gravity and light, as in the
case of creeping and underground stems. So also the lateral
branches are inclined at various angles to the stem. These ad-
justments are not to be looked upon solely as the direct response

NATURE OF PLANTS 105

to one or more stimuli but as the resultant of numerous interac-
tions aroused in the plant by various stimuli. The entire ])lant
responds as a unit to these forces. Stimulation and the conse-
quent growth in one part has its effects upon all other parts. As
a result of this interaction there comes about a correlation in the
growth of all the parts that is expressed in the symmetry of the
plant body and in the perfect adjustment of roots, stem and
branches, and leaves to its needs. Any window garden, forest,
or field furnishes abundant evidence of the complicated and
varied nature of these reactions. An illustration of this is
afforded in the removal of the terminal axis of a tree. An ad-
joining lateral branch that heretofore has been stimulated to a
more or less horizontal growth, now is so directed in its develop-
ment that it ultimately becomes the terminal axis of the tree.
So some parts are constantly being overshadowed and choked
out while others are directed into favorable positions and perform
the work of the destroyed parts. In this way there is brought
about that weaving and twisting of stems and branches and the
adjustment of leaves that so often suggest a conscious effort to
reach favorable positions.

44. Modifications of the Stem. — In the above discussion at-
tention has been called to the more common characteristics of
the stem. It is well to remember, however, that variation is
the law in the plant world. No two plants are alike. The
environment of the plant is constantly stimulating it and caus-
ing it to vary. Frequently the variations are so minute as to
escape our attention or totally distinct forms may arise. These
departures from the parent t}'pe ma^' be of no ad wantage to the
plant and it may therefore perish. On the other hand the varia-
tions may be of such a character as to enable the plant to more
successfully complete with other plants and as a consequence the
plant with its helpful variations will survive. So it hiis come
about that these changes going on during the past ages have
resulted in many remarkable modifications. A few of the modi-
fications of the stem that are of esj^ecial achantage to the plant
and therefore spoken of as adaptive variations will be considered
in the following paragraphs.

io6

NATURE OF CLIMBING STEMS

45. The Climbing Type of Stems. — Climbing stems are char-
acterized by being rather small and having greatly elongated
internodes. Stems grown in the dark show a similar develop-
ment. Perhaps this variation that we call the climbing type
has been brought about by competition with larger plants. The
feeble light has stimulated these stems so that they attain a very
extended growth and finally are able to reach the light and
display their leaves. Many variations in the structure and sensi-
tiveness of the stem are associated with this elongation, all of
which are designed in one way or another to enable the stem
to reach the light. One type of these varia-
tions is seen in twining stems, as the morn-
ing glory, bean, hop, etc. Young twining
plants behave quite like the ordinary plant.
The stems are erect and actively nutating,
the apex traveling through a rather large
circle in one to three hours. When a cer-
tain height has been reached the stems are
stimulated by gravity so that their upper
portions grow more or less horizontally
(Fig. 67). This position is a decided ad-
vantage since the stem is now revolved
through a larger circle and has a greater
chance of coming into contact with an ob-
ject about which it can twine. As soon as

the stem comes into contact with any sup-

FiG. 67. Twining , •, ... .,, . ^ . 1

habit of wild bean. Note P^^^ '^^ nutation Will cause It to wind or

the horizontal position of twine about it. The contact also acts as

the upper portion of the a Stimulus, causing the stem to bend more

stem which results in the
apex nutating through a
wider circle.

energetically. Certainly in many plants the
size and roughness of the support as well
as other features, are important factors in
inducing the twining. At first the coils are merely horizontal
but owing to the elongation of the stem these coils are gradually
pushed upwards and become steep and very firmly bound around
the support. Twisting of the stems and reflexed bristles often
assist in anchoring the plant to its support. It is interesting to

NATURE OF PLANTS u>-;

note that In the majority of cases the stem twines about the sup-
port In left hand spirals or clockwise; less frequently right hand
spirals are formed, as In the hop, some honeysuckles and knot
weeds. Why the growth of the stem results in left or right hand
twining is not known but the fact remains that the plants can not
be induced to change their method of twining.

46. The Tendril Type of Stem. — One of the most Interesting
variations of climbing plants Is seen in those stems that climb
by means of tendrils. In this type almost any part of the plant
may become modified so as to act as a tendril and bind the plant
to a support; e. g., the petioles of nasturtium and clematis,
stipule-like outgrowths In smilax, midrib of leaf in many members
of the bean family, or modified branches as In the grape vine, etc.
The ordinary tendrils as we see them In the gourd, passion flowiT,
etc., are highly specialized stems that are very sensitive to touch.
These organs are rapidly nutating, frequently completing tvvo
or more circles In an hour, and as they approach maturity they
often become slightly curved and hooked (Fig. 68, A). Their
sensitiveness to contact, which is often localized on the concave
upper third of the tendril, also becomes so keen In some forms
that a thread weighing one-fiftieth of a gram will cause a cur\a-
ture of the tendril when placed upon it. In fact a much smaller
weight If caused to vibrate avIU act as a stimulus, so that we
have here an Illustration of sensitiveness to weight that is keener
than our own. When the sensitive part of a tendril is stroked
by a pencil or comes In contact with any hard object, this acts
as a stimulus and causes a very rapid growth upon the con\ex
side of the tendril. As a result of this growth tlie tip of the
tendril curves, forming a complete loop in from a few seconds
to a few minutes. The contact of the tip of a tendril with any
object acts as a continued stimulus and the tendril soon becomes
tightly wound around It. As soon as tlie tip is firmly bound to
the support a new impulse is frequently transmitted to the free
portions of the tendril which In a day or so begin to assume the
form of coils that are often reversed in the middle (Fig. 68, B).
By this unique device tendril bearing stems are firmly fastened to
their supports but at the same time the coiling of the tendrils

io8

NATURE OF TENDRILS

acts like a spring permitting considerable disturbance of the
vine without danger of breaking. This same device is copied
in making the attachment of the wires to the telephones and
other pieces of apparatus. From many standpoints the tendril
bearing stem is an admirable one. It is evident that such stems
can reach the light more directly and economically than twiners.
This doubtless accounts for their greater abundance and common

Fig. 68. Fig. 69.

Fig. 68. Tendrils of the bur cucumber: A, hooked tendrils in receptive
state. B, apical portion of tendril coiled about a branch and the remaining
portion of the tendril forming reversed coils, thus drawing the vine to the
supporting branch. — H. O. Hanson.

Fig. 69. Branch of Japanese ivy attached to wall by means of tendrils
with adhesive discs. — J. O. Hanson.

occurrence. It is also important to note that the tendrils are
especially abundant at the end of the branches where they reach
out considerably beyond the young leaves. Owing to the sway-
ing of the long free shoot of the climber in the wind and the
nutations of the tendril and shoot these sensitive organs are

NATURE OF PLANTS

{')•>

brought into contact with branch after branch and llius the \ inc
is led up to the crown of the tree or to the top of the vegeta-
tion. The Virginia creeper, Boston or Japanese ivy, etc., have
pecuHar tendrils that grow away from the light and form ad-
hesive discs, instead of coils, that bind the vine to the support
(Fig. 69). These discs are caused by the growth of the cells at
the tip of the tendril. As soon as the tendril hits the support
upon which the plant is growing, the cells at the tip are stimulated
to grow out and fit into every little irregularity and groove in
the substratum. Indeed in some vines these discs become in this
way so thoroughly cemented to the crevices and irregularities of
the support as to resemble bits of melted wax. It is very in-
teresting to watch the results of the stimulation induced by one
of these tendrils striking a wall or other support. The formation
and cementing of the disc are accomplished in about four days
in the case of the Virginia creeper. Some plants like the poison
and English ivy and many tropical plants climb by means of
roots. These organs, like the disc forming tendril, avoid the
light and seek the dark nooks and comers in the bark of trees or
rocks over which the plants grow.

47. Prostrate and Creeping Stems. — It is interesting to note
that a large number of plants having rather small and weak
stems like the climbers, have never acquired the habit of climb-
ing. The young shoots of these plants are often erect at first
but soon the weak stems bend over and become prostrate. While
these procumbent and creeping stems are at a disadvantage in
that they are obliged to arrange their leaves in one i)lane, on
the other hand they are not obliged to build up the elaborate
structures that are necessary for the support of the erect stems.
Consequently plants of this class are of common occurrence in
old fields and unfertile soils since they can subsist on a meager
supply of food. They are also ])rotected against injury and
therefore adapted to wind swept sandy plains, to rocky hills and
mountains, and to districts visited by hea\y snows. The ma-
jority of these plants also have a ver>' decided achantage owing
to the fact that certain nodes form roots and buds that develop
into new^ plants. In some cases, stolons, only the terminal l.iid

no RHIZOME TYPE OF STEMS

develops in this way and with the dying off of the old stem a
new plant is established, as in the houseleek (Fig. 70). In other
cases, runners, almost any node may form buds and roots which
become separate plants by the decay of the old stem, as in
the strawberry, gill-over-the-ground, cinquefoil, etc. These new
plants repeat the mode of growth of the parent plant and in

7%:

''^^Ip

Fig. 70. Houseleek forming buds at the end of short branches or stolons.

this way many prostrate stems spread out over the soil, estab-
lishing new plants in wider and wider circles. If you will ob-
serve the number of new plants established and the distances
raversed by some of these creeping stems each year you willt
see the reason for the common occurrence of the large mats and
colonies of plants with prostrate stems.

48. The Rhizome Type of Stems. — In a third type, the stems
have become so modified that they respond to the various stimuli
in quite a different way from the forms noted above. They
creep along in the soil and frequently resemble roots more than
stems. For this reason they are called rootstocks or rhizomes.
They are however, real stems, as is attested by the numerous
leaves and often erect branches that spring from their nodes
as seen in the ferns, sweet flag, cat tail, grasses, etc. Plants of
this type are well protected against drought and fires, and like
prostrate stems they are adapted to establishing new plants as
is apparent in grassy meadows, colonies of golden-rod and daisies
or reedy banks of cat tails, sedges, etc. These stems are also
well adapted to propagating new plants because they generally
serve as storage organs for food and are therefore often of a
fleshy character (Fig. 71). For these reasons it is sometimes
very difficult to eradicate plants of this type. This is very well
illustrated in the quack grass, often a troublesome pest in cul-

NATURE OF PLANTS

III

tivated land. Plowing and hoeing only serves to break up the
rhizome into numerous parts each of which may develop buds
and roots from their nodes and so establish new phints.

49. The Condensed Type of Stems. — In man>- cases we find
that the food is localized in special regions of the rhizome which
consequently become enlarged and rather fleshy, such enlarge-
ments appearing at the end of the rhizome or scattered along its
entire length. Such modified parts of a rhizome are called tubers,
e. g., the potato and Jerusalem artichoke (Fig. 72). The potato

Fig. 71. Fig. 72.

Fig. 71. Rhizome of Solomon's seal with aerial shoot just cmor^iiiK from
the ground. The seal-like scars mark the successive shoots produced during
the past three years.

Fig. 72. Formation of tubers: A, old potato or tuber with two shoots
reaching up into the air and from the base of these shoots, rhizomes have been
formed that are developing new tubers. B, mature tuber with spirally ar-
ranged buds, the so-called "eyes" of the potato.

is formed by the storage of foods in certain parts ol the rather
small rhizomes that branch out from the stem of the plant.
The "eyes" are the buds that develop at each node of the rhizome
and each is capable of forming a shooc, although but a few of
them so function, as is the case in an ordinary branch of a tree.
By passing a thread around a potato so that it touches each
successive bud you will see that there is the same arrangement of
the buds as appears in the leafy stem. There is considerable
evidence to show that the tuber form of rhizome originated
through association with fungi. Thus in the potato the rhizomes
appear to have a vigorous normal growth through the soil until

112 CONDENSED TYPE OF STEMS

they are infested with fungi. This checks their elongation and
possibly accounts for the change in their mode of growth, which
is now characterized by the enlargement of the infested region
and the accumulation of starch. It is w^ell to remember that the
organs of many of these plants are exceedingly plastic and con-
trolled to a remarkable degree by conditions. As a single instance
of this fact note the leafy stems originating from the "eyes" of
the potato are inhibited in their elongation by strong light and
dry air. A potato grown on a moist surface in intense light and
dry air develops short cactus like stems; but grown in the dark
and in moist air they elongate vigorously after which their
nature changes, for if now exposed to light they develop as the
ordinary leafy stem. This is one reason for planting tubers
several inches in the soil.

A great variety of plants develop only very short stems that
are more or less buried in the ground. This type of stem modi-
fication has many of the advantages of the rhizome but not its
power to spread through the soil and so spread the plant. The
vital parts of such plants are only slightly exposed and conse-
quently suffer little from grazing animals or other sources of
injury, as in plantains, dandelions, etc. Frequently these short
stems are associated with an abundance of foods that are stored
in roots or other organs. Such plants can quickly send up and
mature their flower stalks or leafy stems and thus avoid un-
favorable conditions, as drought, competition with larger vege-
tation that will appear later, etc. These facts account for the
common occurrence of this type of stem in arid regions where
there are short rainy seasons. This habit has been made very
conspicuous by cultivation in many plants, as the carrot, turnip,
radish, and beet. But in nature the very short, almost flat stems
and fleshy roots are also of common occurrence, e. g., the wild
carrot, wood betony, dandelion, etc. In many cases the short
stem itself is the storage organ and consequently it becomes en-
larged and fleshy, as in the Jack-in-the-pulpit, garden crocus,
spring beauty, etc. (Fig. 73, A). These short erect stems are
termed corms and are suggestive of the tuber. In other plants
the bases of leaves attached to the short stems function for the

NATURE OF PLANTS

II

storage of food, as in the bulb type of stems of lilies, onions, etc.
(Fig. 73, B).

It is noteworthy that these special types of stems, the runner,
rhizome, bulb, etc., are the most common. They represent the
more recently evolved forms of stems and because of their ad-
vantages they have become the dominant forms. The geological

Fig. 73. Shortened types of stems: A, conn of jack-in-the-pulpit. At
left surface view showing lateral buds, roots and sheathing leaf arising from
top of shortened stem. At right sectional view with folded leaf, /, in bud at
apex of stem. B, bulb'type of shortened stems. At left bulb of onion showing
the ensheathing leaves which are swollen at their bases with food, thus forming
the bulb. At right, section of a bulb of hyacinth showing the fleshy loaves
attached to the very short stem and in the center of the bulb a flower cluster.

ancestors of our seed plants were woody and tree like. The
uniform climatic conditions of these past periods favored the
tree type of stem with its admirable devices for loaf display.
With the appearance of climatic changes upon the earth there
resulted modifications in the stem: the medullary rays tended to

114 EVOLUTION OF STEM TYPES

increase in size and so serve as a storehouse, thus meeting the
greater need of reserve material; the cambium also tended to
form more and more parenchymatous tissue so that the stem
became less woody and more of a storage organ. So you can
think of the stem becoming reduced in size and finally prostrate
through lack of supporting material. The contact of the stem
with the soil exposed it to a variety of new factors which lead to
further modifications, some of which have survived as seen in
the runner, rhizome, corm as well as in the annual and biennial
habit of plants. It is a striking fact that the majority of our
primitive plants are woody and tree like (pines, willows and oaks)
while the more recently evolved and higher types (daisies,
dandelions and golden-rods) are characterized by various forms
of these modified stems.

CHAPTER IV

THE FLOWER, FRUIT AND SEEDLING

50. The Structure of the Flower. — We now come to the most
interesting feature in the nature and life of the plant. All the
energy of the plant is directed towards the perpetuation of its
kind. Numerous examples have been noted where this is ac-
complished by bulbs, rootstocks, buds, etc. Among our common
plants, however, new individuals are usually produced from seeds
that are formed in the flowers. We will first examine the struc-
ture of a flower and see how it is adapted to the performance of
this important work. A flower is a highly modified stem of the
bud type. This is very noticeable before the flower opens or is
in bloom (Fig. 74, ^). A series of leaves usually of a greenish
color envelop the delicate parts within and protect them, as in the
case of the bud, against drying winds. These green leaves are
known as the calyx and each individual leaf is called a sepal. As
the bud opens a number of organs are disclosed, particularly
noticeable are a set of more delicate and variously colored leaves,
the corolla, each leaf of which is called a petal (Fig. 74, B). The
corolla is of service to the flower in many ways. Like the calyx
it may assist in protecting the other organs. Especialh' is it of
importance in guarding them against dews and rains. For this
reason the calyx and corolla, collectively called the floral envelop
or perianth, in many flowers often close at night or on rainy
days or the perianth may be so developed or inclined as to pre-
vent the rain from falling into it. It will be seen later in the
work that the perianth is of the greatest importance in assisting
insects to visit the flowers in such a wa>' that seeds ma>- be pro-
duced. Within the perianth are two kinds of peculiar organs,
the central flask-like ones are called pistils or, if united, carj^cls
and surrounding them are the stamens (Fig. 74, B). These
two kinds of organs are collectively known as sporophylls since
their special work is to produce certain cells called spores. The

1 15

Ii6

NATURE OF THE SPOROPHYLLS

sporophylls are the important organs of the flower. The calyx
and corolla may be lacking but no seed can be formed without
the sporophylls.

51. The Structure of the Sporophylls. — Let us examine the
structure of these organs and see how they co-operate in the
formation of the seed. The stamens consist of a lobed sack or
anther which is usually supported upon a stalk or filament (Fig.

1^

Fig. 74. Flower of the stonecrop, Sedum: A, bud stage of flower — ca,
calyx showing three sepals. B, open flower — p, petals of corolla; s, stamens;
c, pistils or carpels. C, stamen, consisting of a four-lobed anther supported
on delicate stalk or filament. D, pistil or carpel — 0, ovary; s, style which
terminates in a small knob or stigma. E, longitudinal section of ovary showing
row of ovules attached to wall of ovary. F, cross-section of ovary, ovules in
two rows.

74, C). If a young anther is cut across four cavities, contain-
ing minute or dust-like grains, the microspores, will be seen
(Fig. 75, A). The microspores, also called pollen spores or
grains, are minute cells, provided with a cell wall often variously
sculptured and contain a nucleus and protoplasm like an ordinary
cell (Fig. 79). At maturity the anthers break open in a manner

NATURE OF PLANTS 117

that varies in different plants, exposing the microspores to the
air (Fig. 75, B). The pistil or carpel is quite different in char-
acter. At or near the top is a more or less modified part, the
stigma, below which is an elongated stalk, the style, that broadens
out into a base or ovary. By longitudinal and transverse
sections of a pistil we gain a better idea of its real character
(Fig. 74, D-F). The ovary is now seen to contain a cavity
from the walls of which one or more rather spherical bodies,
the ovules, are developed. The structure of the ovule is very
complicated. If a thin section is made through the center of
one of them it will be seen to contain usually but one large spore,
therefore called the megaspore (Fig. 76, mg). This spore is

Fig. 75. Structure of the anther: A, diagram of the anther cut across,
showing the four cavities, sporangia (sing, sporangium), filled with spores.
B, cross-section of a mature anther. The tissue about the spores has broken
down, thus forming two cavities and at the left the breaking open of the wall
of the anther is shown.

developed in a mass of tissue, the nucellus, and the whole is
surrounded by one or two coats, the integument, which grows
up from the base of the ovule but never quite surrounds it,
thus leaving a small opening or micropyle (Fig. 76, mi). The
megaspore, unlike the microspore, is never set free but is retained
permanently in the ovule.

It is not known how the sporophylls have originated but there
are indications that they may have arisen through the modifica-
tion of leaves for the purpose of forming spores instead of con-
structing foods. It is for this reasion that these organs are
called sporophylls, or spore-forming leaves, from phylon, a leaf.
Those forming small spores are sometimes called microsporo-
phylls instead of stamens and those forming large spores mega-
sporophylls instead of pistils or carpels. Very man>- modifica-
tions of the sporophylls will be noted later in the work.

Ii8

GERMINATION OF THE SPORES

52. The Germination of the Microspore and Megaspore.— The

question now naturally arises as to the meaning of these peculiar
structures. The important part of the sporophyll is the spore.
A spore is a cell that is capable of germinating or growing under
favorable conditions and thus producing some kind of a new
organism, or we may say a new plant. You are always to think
of a spore as having this power. In case of the spores under
consideration very rudimentary plants are produced that con-
sist of onh' a few cells. These minute plants are very important,

Fig. 76. Fig. 77.

Fig. 76. Section of the ovule of the Canada lily (greatly magnified):
mi, micropyle; i, integuments; n, nucellus, here consisting of a single layer
of cells, but it often forms a large mass of cells; mg, megaspore with large
nucleus in center. This spore is about to divide;/, stalk or funiculus which
attaches the ovule to the wall of the ovary.

Fig. 77. Germination of the megaspore: A, first division of the nucleus
of the megaspore. B, second stage in the germination, four nuclei being
formed. C, final stage in the division of the nuclei.

however, because they contain the male and female cells, or
gametes, so called because they unite, thus forming a new spore,
the gametospore, quite different from the micro- and mega-
spores. So let us not lose sight of the fact that these apparently
insignificant micro- and mega-spores germinate and produce
minute plants that are quite distinct from the plants that bore

NATURE OF PLANTS 119

them. We shall first follow the germination of the mcgaspore
and note the character of the plant that develops from it. As
has been stated this spore is never discharged from the ovule
within which its entire growth occurs. The first indications of
germination are seen in the enlargement of the spore and the
division of its nucleus into two nuclei, which are pushed to either
end of the spore owing to the accumulation of water in the
central region of the spore (Fig. 77, A). The food for the nour-
ishment of the growing megaspore is derived from the cells of the
nucellus which are dissolved and absorbed as the megaspore
increases in size, and food is also conducted to it from the parent
plant by means of the funiculus. Each of the two nuclei at the
ends of the spore divides again, thus forming two nuclei at either
end. This process is repeated once more so that we now find four
nuclei at the ends of the spore which has become somewhat elon-
gated (Fig. 77, B, C). The so-called polar nuclei, a nucleus
from each end, now move toward the center of the spore and
fuse, forming one nucleus. This completes the germination of
the megaspore and we see that it has grown into a sac-like plant
consisting of seven cells, or nuclei imbedded in a mass of proto-
plasm (Fig. 78). The cells of this minute plant differ in char-
acter and perform very different duties. For example, the larger
cell toward the micropylar end of the plant is the female cell or
Qgg cell. It will be called the female gamete because it unites
with another cell, the male gamete. The two small cells near tlie
female gamete are the synergidae or cells that assist in the
nourishment and later development of the female gamete. The
two cells fusing in the center of the plant form the endosperm
nucleus which later by repeated divisions produces a mass of
nourishing cells that fill the space within the sac. The throe
cells at the bottom of the sac, known as the antipodal cells, may
increase in number and take part in the later growth, but as a
rule they are disorganized and used for food. They are often
provided with cell walls, the other cells consisting only of a
nucleus and a varying amount of cytoplasm. This plant formed
by the germination and growth of the megaspore is called the
female gametophyte, because it contains the female gamete, and

120 GERMINATION OF THE MICROSPORE.

you will see that a corresponding plant appears in the ferns and
other groups of plants.

Returning now to the microspore, we find that its germination
also often begins while in its sporangium but in other cases
growth begins after it has escaped from the sporangium and has

Fig. 78. The germination of the megaspore complete. The plant thus
formed consists of a female gamete or egg cell, 9 , below which are two syner-
gidae and in the center are the two uniting polar nuclei, p, while at the opposite
end of the sac are three antipodal cells, a; mi, micropyle; i, integuments; /,
stalk or funiculus in which a vascular bundle, v, has been developed to trans-
port food from the plant to the ovule.

been carried by the wind or some insect visiting the flower for
food, to the stigma of the pistil. The stigma is admirably
adapted to hold the microspores, being provided with minute out-
growths, and frequently sugary solutions are exuded which fasten
the spores to the stigma. The real importance of these sugary

NATURE OF PLANTS 121

solutions is to nourish the microspores. It is cvick-iii that tliese
minute dust-Hke cells can not contain sufficient f(xxl to bring
about any considerable growth. It is noteworthy tliat the
strength and nature of the sugar solutions varies in different
stigmas and that the microspores are often able to grow only
in solutions of such strength or in the presence of such substances
as are found on the stigmas of their own kind of plant. So it
results that the microspores are often unable to grow when
carried to the stigmas of a different kind of plant from them-
selves. The germination of the microspore results in the division
of its nucleus and the formation of a large and a small cell known
as the tube and antheridial cell respectively (Fig. 79, A, B).
The latter cell divides once, forming two cells, called the male
gametes (Fig. 79, C). This stage in the germination of the
microspore is reached either in its sporangium or after being
transferred to the stigma. Nourished by the foods of the stigma
the microspore continues its growth, the tube cell ruptures
the outer wall of the spore and forms a tube-like growth. This
tube cell grows down between the cells of the style into the
cavity of the ovary where it usually curves out and enters the
ovule by way of the micropyle. It now works its wa\- throut;h
the tissues of the nucellus to the female gametophyte, into the
cavity of which it enters by dissolving the wall. In the mean-
time the two male gametes have been carried down by the
streaming movement of the cytoplasm to the end of the tube
cell as it enters the female gametophyte (Fig. 79, D). The
question will naturally be asked, what directs the peculiar growth
of this minute plant? In the first place the tube cell is repelled
by the oxygen of the atmosphere, so as soon as it appears, it is
directed away from the atmosphere into the stigma. Its course
down the style to the ovary is very largely controlled !)>■ lines of
loose tissue through which it can easily work its way and also
by foods which are deposited in the cells just ahead of the tube
cell so that it is led along rather straight pathways to the ovary.
The curving out of the tube to the micropyle and its subsequent
growth down into the female gametophyte can only be accounted
for by some chemical stimulus that is located in the ovule and

122 FECUNDATION OR FERTILIZATION

female gametophyte. A similar attraction brings about a move-
ment of the cytoplasm which carries the male gametes to the
end of the tube cell. It will be seen that this peculiar plant
(Fig. 79, D) consisting of a long tubular growth with three
naked cells is more rudimentary than the plant derived from the
megaspore. Because this plant produces the male cells or male

I

Fig. 79. Germination of the microspore: A, mature microspore. B, first
stage in its germination — t, tube cell; a, antheridial cell. C, end of cell divi-
sion— t, tube cell; a, antheridial cell forming two male gametes. D, diagram
showing the formation of the tube which grows through the style and finally
reaches the female gametophyte. The two male gametes, g, are shown passing
down the tube; t, tube nucleus.

gametes we call it the male gametophyte. So we see that the
flower forms two kinds of spores and that these germinate and
form two kinds of plants, a male and a female.

53. Fecundation or Fertilization.— As soon as the develop-
ment of these two plants is completed a very remarkable change
takes place which starts an entirely new growth that is quite
independent of the development of the male and female gameto-
phyte. The end of the tube cell becomes distended through the
accumulation of material and finally ruptures, discharging the
male gametes into the female gametophyte. The chemical com-
position of the female gamete is such that one of the male cells
is attracted to it (Fig. 80) and finally the two gametes unite
forming a single cell. This body is a sexually formed spore or
gametospore and like the ordinary spore has the power to germi-

NATURE OF PLANTS

123

nate and produce a plant. The microspores and mcgaspores,
however, were not formed by the union of sexual cells or gametes.
Therefore we may distinguish them as asexual spores or simply
spores. It has frequently been observed that the endosperm
nucleus also attracts the other male gamete and causes a similar
fusion with it (Fig. 80). The union of the male anrl female

Fig. 80. The micropylar end of an ovule of Canada lily (sectional view),
showing the process of fertilization or fecundation. The tube, /, has grown
into the female gametophyte and ruptured, discharging the two male gametes.
One, cf , is seen fusing with the female gamete, 9 , and the other one, c", is
uniting with the two polar cells, thus forming the endosperm nucleus; s, one
of the synergids; i, integuments.

gametes is called fertilization or fecundation. Unless fertilization
is effected the growth outlined above usually ends the history of
the flower. But if fecundation is effected then a gametosiK)re
is formed that is capable of germinating and producing a plant
whose growth and development are attained with most remark-
able changes of the ovule and often of the surrounding parts.
54. The Germination of the Gametospore and the Formation
of Seed. — The first indication of the germination and growth of
the gametospore is seen directly after fertilization. It becomes

124 GERMINATION OF THE GAMETOSPORE

surrounded by a cell wall and attached to one side of the female
gametophyte, which is henceforth called the embryo sac (Fig.
8i, ^). By a series of divisions the gametospore now forms a

Fig, 8i. Germination of the gametospore of peppergrass, Lepidium: A:
micropylar end of the embryo sac showing that the gametospore, g (fertilized
female gamete), has developed a cell wall and become attached to the wall
of the embryo sac. Compare Fig. 80. B, later stage in the germination — e,
embryo cell; 5, suspensory cells; en, endosperm cells. C, the embryo cell
has formed two cells. D, later stage. The embryo cell by further division
has formed a spherical mass of cells, here shown in section. Note the appear-
ance of an outer layer of cells, the epidermis, and a central or stem region,
s, a few of the suspensory cells. E, still later growth. Two growing regions,
the cotyledons are appearing on the side of the stem. F, the micropylar end of
the embryo sac, showing the stage of development where the parts of a small
plant can be clearly recognized — c, cotyledons; st, stem ending in root, r,
to which is still attached the supensory cells; en, endosperm cells, now pro-
vided with walls, are being absorbed as the plant or embryo enlarges. Note
that the embryo and embryo sac are slightly bent to the left. This curvature
is due to the fact that [the ovule in a great many plants becomes curved in
its development with the result that the embryo sac assumes a curved or
U-shaped form. This form of the ovule results in a complete bending over
of the cotyledons against the stem, as shown in the next figure.

NATURE OF PLANTS 125

number of more or less elongated cells, the so-called suspcnsor,
and a terminal cell or embryo cell (Fig. 81, B, V). In the
meantime the endosperm cell has divided again and again and
the resulting naked nuclei have become arranged around the
sides of the embryo sac (Fig. 81, B, en). This formation of
endosperm cells continues until they more or less fill the embryo
sac and they also become surrounded with cell walls. These
endosperm cells are filled with food and serve to nourish the
germinating gametospore just as the nucellus nourished the
female gametophyte. While the suspensory cells usually soon
cease to grow, the embryo cell divides very actively, as shown
in Fig. 81, C~E, and soon there is a clear indication of a minute
plant with root, stem and leaves (Fig. 81, F). This minute
plant developed by the germination of the gametospore, is called
the embryo. In the case of the plant under consideration the
embryo, when fully formed consists of a stem with two laterally
placed leaves, the cotyledons, and a root. The region of the
stem above the attachment of the cotyledons is known as the
plumule and frequently assumes the form of a minute bud. The
region of the stem below the cotyledons is termed the hypocotyl
and the root and root cap appear at its lower end (Fig. 82, A).
The ovule containing the mature embryo is called a seed. The
interesting feature about the seed is the fact that it is a structure
in which growth has ceased and that it is capable, owing to a
remarkable series of devices that will be noted directly, of re-
maining in this condition, i. e., dormant often for one or more
years.

Great variation characterizes the germination of the gameto-
spore in the different groups of plants, but the de^•olopmcnt out-
lined above is fairly characteristic of those plants that form an
embryo with two cotyledons. Such plants are called for this
reason dicotyledons. Other plants develop an embryo with but
one cotyledon, as our grasses, lilies, etc., and for this reason they
are called monocotyledons. Further variations will be seen in
cone-bearing trees, ferns, etc.

It must be borne in mind that the formation ot the seed is
also attended with profound changes in the stnuture of the

126

STRUCTURE OF THE SEED

ovule, and often in the pistil and surrounding parts. Frequently
the endosperm grows so extensively as to absorb and replace the
cells of the nucellus and thus comes to occupy all the space within
the coats of the integument, as in the castor bean, morning glory,
onion, etc. (Fig. 85, B). The embryo mayjemain comparatively

B

Fig. 82. Structure of the seed: A, section of an ovule of peppergrass in
which the growth of the embryo is nearly complete — -/, stalk or funiculus
attaching ovule to wall of ovary; mi, micropyle; i, integument; en, remains
of endosperm. The embryo consists of a stem which is differentiated into
a hypocotyl, hy, that ends below in a root, r, and root cap, and above in
the plumule, pi, and two cotyledons, c, which curve over and lie one upon the
other. V, vascular bundles which extend up through the stem into the coty-
ledons where they subdivide, forming a network of veins. B, section of seed
of water lily, after Conard — e, embryo surrounded by a layer of endosperm
cells; mg, cells of the nucellus; i, integument.

small as in the cases just cited or it may in turn absorb and
replace all the cells of the endosperm and so come to occupy
the space within the integument, as in the bean, pea, etc. (Figs.
82, A; 83, D). Various other arrangements may result; as for
example, there may be remains of the nucellus and endosperm
with the embryo (Fig. 82, B). The integument also undergoes
various changes during these growths, often becoming hard or pa-

NATURE OF PLANTS 127

pery or provided with hairs, hooks, spines, and smooth or pitted.
Compare the bean, catalpa, cotton, milkweed, etc. Attention
must also be directed to the growths that occur outside of the
seed that are induced by fertiHzation and the germination of the
gametospore. Very frequently the pistil becomes tough and
hard and invests the seed like a second integument, as in wheat,
corn, acorn, etc.; again it forms a sac al)out the seed as in the
sedges and buttercups; or it may enlarge, forming the pods of
peas and beans or the capsules of lilies. In other cases it be-
comes fleshy throughout as in the tomato and currant, or only
the outer walls are fleshy while the inner part forms the pit or
stone as in the peach and plum. Frequently wings and hooks
grow out from the pistil as in the mapel, Ailanthus, etc. Even
the end of the stem, the receptacle, which bears the various
organs of the flower, may be induced to grow, as in the apple,
where it surrounds the pistils (the core) with a fleshy coat, or
the receptacle may enlarge, as in the strawberry, forming the
fleshy part of the fruit. In other cases outgrowths below the
flower develop, forming the burr of the chestnut, clotbur, beech-
nut, and the cup of the acorn. The entire growth that is asso-
ciated with the formation of the seed is termed the fruit and we
see that it may include a variety of modified organs. The term
seed, however, refers only to the modified ovule and its embr\o.
Later in the work it will be noted that these growths are often
devices to bring about a distribution of the seed or to protect it
during its dormant period.

55. The Seed and its Growth.— Spores of all kinds germinate
and produce new plants. Seeds do not germinate. Tliey simply
continue the growth of the embryo or plant which h.is been tem-
porarily stopped. The falsely called germination of the seed is
but the awakening or renewal of the growth of this plant. The
seed is essentially a young plant that is supplied with a certain
amount of food and that has temporarily stopped growing. The
seed may be looked upon as one of the many adaptations found
among plants to ensure their distribution and perj^et nation.
From a seed a single plant is developed that may produce many,
often thousands of seeds, i. e., plantlets. We haw .iln -.kK- noted

128 STRUCTURE OF THE SEED

that these may be widely distributed and so spread and multiply
the number of plants. Some seeds are so fine that they float away
as a dust, as in the orchids. Other seeds and fruits are provided
with hairs and wings that serve to buoy them up or promote
their transport by winds, as in the case of pine seeds, milk-
weed, willow, and the fruits of maple, dandelion, etc. In other
cases spines and hooks or mucilage glands are developed that
attach them to any passing animal and so bring about a con-
siderable distribution. Note the common devices of this nature,
as in the burdock, stick tight, beggar lice, agrimony or the glands
on the nightshade (Circaea) and twin flower. So also seeds
are scattered by the spring of elastic stems and explosive fruits,
as in many lilies, witch hazel, violet, touch-me-not, etc. But
especially may seeds be looked upon as an adaptation to tide
the plant over seasons unfavorable for growth. In this respect
they may be compared to buds, which they also resemble in their
renewal of growth when conditions are again favorable. There
are some noteworthy exceptions to this statement, as in the man-
grove, certain oaks, and many grasses. The seeds of the former
plant begin their growth while still attached to the tree and a
similar growth has also been observed in the acorns of the
white oak. Grain often sprouts while in the sheaves and the
seeds from green tomatoes produce earlier and larger fruits.
However, the majority of seeds and fruits, as well as buds, do
not renew their growth immediately after their formation. A
longer or shorter resting or dormant period is required during
which time chemical changes occur that render the foods soluble
and therefore available to the young plant or embryo. For ex-
ample, in many seeds and buds the storage foods are in the form
of starch. This food is insoluble and can not be used by the
plant until it has been acted upon by an enzyme which changes
it into a soluble and readily diffusible sugar. These enzymes are
slowly formed during the resting period and a renewal of growth
is not possible until they have begun the transformation of the
insoluble foods into soluble forms. The changes thus effected
are so considerable that growing tubers, root stocks and seeds
often have a sweet taste owing to the considerable amount of

NATURE OF PLANTS 129

sugar present. So the seed may be looked upon as a variation
in the growth of the plant that enables it to multiply and dis-
tribute its kind and to meet the various problems in connection
with its existence, such as drought and sometimes even fires.

56. Conditions Necessary for Growth. — The external condi-
tions necessary for starting the growth of the embryo are a
suitable temperature, moisture, and the oxygen of the atmos-
phere. Light only in rare instances appears to be a contributing
factor in the renewal of growth. Some seeds will sprout at near
the freezing point, while others require a temperature between
10° and 17° C. It is interesting to note that the rising tempera-
ture together with other factors as the season advances from
early spring bring out a regular succession in the awakening of
various kinds of seeds. A suitable amount of moisture must
also be supplied to the seed to assist in the solution and dilTusion
of the foods to the embryo. This is very necessary since the
seed contains very little water. In fact the seed owes its vitality
and power to resist long drought and severe cold to its dry con-
dition. Seeds soaked in water cannot endure such extremes as in
a dry state. Excess of water, however, is unfavorable to the
renewal of growth, because it excludes the oxygen of the air.
This is the cause of the failure to grow and of the decay of the
seeds planted in the spring when the season is rainy. The air
spaces in the soil become filled with water and as a consequence
oxygen can not gain access to the seed and co-operate in those
chemical changes that not only supply some of the foods to the
embryo but also provide it with the energy for growth. An
examination of a few seeds will show some of the more important
structural features that adapt them to the conditions that they
have to meet.

In the case of the bean the integument is tough and wouUl
appear to offer a rather effectual barrier to the entrance of water
and air. You have noted that a minute opening, the micro-
pyle, exists in the integument. This opening is seen near the
scar or hilum that marks the point where the seed was attached
by a minute stem to the walls of the ovary (Fig. 83, B). This
opening permits the entrance of the air and water as does the
9

I30

NATURE OF THE SEED

looser structure of the hilum. This can easily be demonstrated
by placing beans in water, when the spread of the water from the
region of the micropyle will be indicated by the swelling and
wrinkling of the integument (Fig. 83, C). After a time the
entire integument becomes quite soft and water is readily drawn
through it owing to the fact that the storage foods in the embryo
are in close contact with it and draw the water through by
Carefully removing the integument from a soaked bean

osmosis.

^^^ Cs^

Fig. 83. Fruit and seed of the bean: A, mature fruit or pod: 5, style;
c, remains of calyx; 0, ovary. At left the pod has been opened, showing four
seeds attached to the walls of the ovary by funiculus. B, two views of a
seed — h, hilum or scar marking attachment of funiculus to seed; w, micropyle.
C, wrinkling of seed coat caused by the entrance of water through the micro-
pyle and hilum. D, embryo of seed after removal of integument — c, coty-
ledons. At right one of the cotyledons has been removed to show the plumule,
ep, and the hypocotyl, hy. E, half of the membranous integument.

we note that the embryo is well developed, consisting of two
fleshy cotyledons and a short stem (Fig. 83, D, E). The plumule
is well developed, bearing two leaves, and the short hypocotyl
and root are enclosed in a sheath formed from the integument.
Note that the hypocotyl and root are situated near the micropyle.
How does this arrangement work to the advantage of the em-
bryo? Allow the seed to germinate and it will be noted that the
absorption of water first causes the embryo to expand and rupture
the integument. This is immediately followed by the growth and
elongation of the lower part of the hypocotyl which results in the
pushing out of the root (Fig. 84, A). Thus it happens that the
very part of the embryo that shows the first signs of growth is in

NATURE OF PLANTS

131

close proximity to the water as it enters the seed and doubtless
the sheath also assists in retaining water in this region. We are
already familiar with the properties of the root which direct its
growth in such a way that no matter how the seed may be placed
the root will ultimately grow into the soil and toward water and
the soil foods, page 61. It is noteworthy that many seeds are so
fashioned that they naturally assume such a position on the
ground as to bring the root as it emerges into direct contact with
the soil. Such an arrangement is equally advantageous for the
absorption of water. How many seeds can you find that will
naturally lie upon the soil in such a way as to cause the root on

Fig. 84. Renewal of growth of the bean seed: A, basal region of h\ ptxrotyl
elongating and pushing the root into the soil. B, upper region of the hypo-
cotyl elongating, lifting the cotyledons and epicotyl above the soil. C, hypo-
cotyl erect and epicotyl expanding, forming the first normal leaves.

emerging to be directed away from the earth? Thus at the \i r>
start the growth of the seed is of such a character that the young
plant is anchored to the soil by the root and brought into projx^r
relation with the water and other materials necessary for the
construction of foods. As soon as this work is well under way
the upper end of the hypocotyl begins to elongate, pushing up
into the air cotyledons and i^dunuile (Fig. 84, B). Here we

132 GROWTH OF THE SEED

see that the upper end of the hypocotyl reacts to gravity like a
stem and consequently grows away from the earth. This growth
is attended with a curving and constant nutation of the hypo-
cotyl which enables it to work its way through the ground to
better advantage than would be the case if it were pushed pas-
sively up. Danvin compares this work of the seedling in emerg-
ing from the soil to the struggles of a man who is prostrate and
attempting to arise with a heavy load on his back. The ability
of the hypocotyl to overcome obstacles in its way and crowd
through small openings is due to the power generated in the
enlarging cells. In this way a force is often created that may
exceed 80 pounds to the square inch, over five times the atmos-
pheric pressure. In many seeds only the region above the coty-
ledons elongates, leaving the cotyledons in the ground, as in the
pea, acorns, walnuts, etc. The elongating region is called the
epicotyl. This behavior is doubtless associated with the storage
of food. In some seeds the cotyledons serve only as storage or-
gans and in such cases the position of advantage is in the soil
where they are better protected and in contact with moisture
which will assist in the solution and diffusion of the foods. In
many cases the cotyledons perform a dual function, first as storage
organs and later expanding and developing into the first green
leaves of the plant, as in the squash and many of our common
plants. In any of these cases it is evident that the young plant
requires a certain amount of food to enable it to get established
in the soil. It is not until considerable growth has taken place
that it develops its first green leaves and so begins to be self
supporting. In the case of the bean and in many other seeds and
fruits this need is amply met by the abundance of food that is
stored in the fleshy cotyledons (Fig. 84, C). These organs re-
main attached to the plant even after it is well established, and it
is only after the food has been withdrawn that the shriveled
cotyledons drop off. As the young plant becomes established
the plumule unfolds like a bud and develops the stem and leaves
and finally the flowers and fruit with which you are familiar.

The castor bean shows quite a variation from the structure
of the bean. The hilum and micropyle are more or less con-

NATURE OF PLANTS

133

cealed by a fleshy outgrowth, the caruncle (Fig. 85, .4). Remov-
ing the hard integument we find within a while, oily mass of
cells which are attached to the horn-like coat only at the hilum.
Cutting across this mass of cells it will be seen that two thin
leaves are imbedded in the center. Taking two other seeds and
cutting them longitudinally, one at right angles and the other par-
allel to the leaves we see that the embryo consists of a very small
stem and root and two delicate leaves imbedded in an oily tissue,
the endosperm (Fig. 85, B). In the bean the seed consisted of

Fig. 85. Seed and seedling of the castor bean: A, the seed with fleshy
outgrowth, the caruncle, c, at base. B, section of seed. At right showing
the two cotyledons, c, and short hypocotyl in center of the endosperm, en.
At left the seed is split parallel to the surface of the delicate white cotyledon.
C, early stage in the growth of the seedling. The hard intei^unicnt is being
thrown off but the cotyledons are still entirely buried in the endosix-rm. D,
later stage. The cotyledons, c, still absorbing food from the endosptTm. en,
are beginning to appear and to develop chlorophyll. E, the cotyledons and
plumule are expanding, the endosperm being entirely abs..rl.<il.

an embryo and integument. In the castor bean, uw ciulosix^rm
forms the bulk of the seed, since it has been only partially con-
sumed by the embryo. It would naturally be inferred that the
hard coat would prevent the access of water and gases. Observe,

134 GROWTH OF THE CASTOR BEAN

however, that the caruncle readily absorbs water and that the
embryo and endosperm are in direct contact with this region
of the seed. Many devices will be observed among seeds serv-
ing to convey water to the growing parts. Note the mucilag-
inous coats on flax, quince, many mustard seeds, etc., or the
spongy rinds of the fruits of the walnut, cocoanut, etc. It is
well worth your time to submerge in water colored a deep red
with eosin widely different seeds and examine them from time
to time, noting the period required for the penetration of water
and the lines along which the water enters and is distributed.
It will be found that many seeds and fruits exclude water and
oxygen very effectively for one or more years with the result
that the renewal of growth is delayed until the decay of the
parts permits their entrance. The breaking of the integument
would appear as a serious obstacle to the growth of the embryo.
However, the power generated by the cells owing to their ex-
pansion through the absorption of water is quite sufficient to rup-
ture the seed coat. The early growth of the embryo of the castor
bean is similar to that of the bean. The elongation of the lower
end of the hypocotyl pushes the root into the soil, while the
upper end of the hypocotyl, arching and twisting as it elongates,
works the cotyledons and endosperm through the soil into the
air (Fig. 85, C). In this seed the food is without the cotyle-
dons. Consequently, although the cotyledons gradually enlarge
and become green, they remain in contact with the endosperm
until all its food is absorbed and but a papery skin remains
(Fig. 85, D).

A grain of corn will illustrate another modification of the
seed. In this case the pistil remains in such close contact with
the seed that it appears to form an additional integument. Such
a fruit is called a grain and may be compared to a bean pod with
a single seed. We can still see upon the grain (Fig. 86, A)
at 5 the position of the style while at p appears the remains of
the stem that fastened the sporophyll to the cob. The long
threads, often called the silk, that project from the ears of corn
are greatly enlongated stigmas for catching the microspores.
While the outer parts of the grain of corn are very hard, if it

NATURE OF PLANTS

35

is treated with the eosin solution referred to above it will be
seen that at first the water enters by way of the little stalk just
as did the food that was supplied to the growing grain. In
this way water is supplied directly to the root of the embryo
which lies near the stalk as shown in Fig. 86, B. If two seeds
are soaked until soft and then sectioned so as to cut one across
and the other longitudinally througli the embryo it will be ^rcn

Fig. 86. Grain and seedling of corn: A, two views of a grain — s, point of
attachment of style; p, stalk attaching the grain to cob; emb, location of
embryo. B, longitudinal section of a grain. The embryo consists of a
plumule, pi, root, r, and scutellum, sc. eji, endosperm. C, cross-section of
a grain showing endosperm, en, surrounding the embryo, emb. D, diagram
of the outer cells of the scutellum, showing the elongated cells that bring
about a solution and absorption of the foods stored in the endosperm. K-G,
stages in the growth of the seedling.

that the endosperm forms the larger part of the grain and mai
the embryo is relatively small, consisting of a stem and root, a
plumule of several closely wound leaves, and a shiekl-like organ,
the scutellum (Fig. 86, B, C). The entire embryo may easily be
removed from the endosperm with a knife if the seed is thoroughly
soaked. Unlike the two preceding examples the corn is a mono-
cotyledon and many regard the scutellum as a modified coty-
ledon. This organ forms the enzymes which put into soluble

136 ALTERNATION OF GENERATIONS

form the food in the endosperm and it also functions as an
absorbing organ of the embryo (Fig. 86, D). Similar devices
for absorbing the storage foods and transferring them to the
embryo are seen in the wheat, onion, date, and cocoanut. The
early growth of the embryo goes on as in the preceding examples.
The root first emerges and anchors the grain in the soil. The
plumxule of closely rolled leaves forms a sharp bodkin that reaches
up through the soil with ease. This modification of the plumule
would appear to be of a decided advantage and you will notice
that this method of emerging from the soil is followed by a great
many plants with underground stems; see the large leaves of the
skunk cabbage, may apple, Solomon's seal, etc. The rest of
the grain remains in the ground as in the case of the pea.

57. The Two Phases in the Life of a Plant. — As has been
stated the plumule unfolds in all these cases like a bud and
gradually develops into the mature plant. This may be effected
in one year or several years may be required to complete the
work. At some time in this growth flowers will appear bearing
sporophylls and spores and thus we arrive again at the starting
point in the life history of a plant. There are two distinct phases
or generations in the life of a plant. The formation of the
megaspores and microspores marks the beginning of the sexual
generation or gametophyte which ends when the male and female
gametes have been developed. The formation of the gameto-
spore marks the beginning of the asexual generation or sporo-
phyte which ends in the sporophylls when those cells, called the
spore mother cells, appear which produce the micro- and mega-
spores. These two generations are also sharply distinguished
from each other by the number of chromosomes, page 56, which
their cells contain. The cells of the gametophyte generation
contain only one-half the number of chromosomes found in the
cells of the sporophyte generation. If the nuclei of male and
female gametes contain twelve chromosomes (the number varies
in different plants) the gametospore which marks the beginning
of he atsexual generation, will contain twenty-four chromosomes.
This number will be found in all the cells of the embryo and
continue to characterize the subsequent development of the sporo-

NATURE OF PLANTS 137

phyte. When, however, the spore mother cells appear in the
anthers and ovules it is to be noted that these cells in dividing
to form the micro- and mega-spores reduce the number of
chromosomes by one-half so that the nuclei of these spores and of
all cells derived from them contain but twelve chromosomes.
For this reason the division of the spore mother cell is referred
to as the reducing division. The significance of this difference
in the number of chromosomes is not known but it furnishes a
basis for the sharp separation of the two generations.

A sexual and an asexual phase or generation characterize
the life history of nearly all the higher plants and the succes-
sion of these two phases is called the alternation of generation.
In the case of the bean we have as the most important features
of the sporophyte or asexual generation, (i) the gametospore,
(2) the seed, and (3) the flowering bean plant. The micro-
scopic plants that comprise the gametoph\'te or sexual genera-
tion are characterized by (i) the micro- and mega-spores which
develop respectively into (2) a tubular growth of three cells
and (3) a sac-like growth of seven cells, each of which contains
one or more (4) sexual cells or gametes. These two phases are
called respectively the sporophyte or asexual generation, and the
gametophyte or sexual generation, since in the first phase mother
cells are developed which form spores, i. e., this is the spore
forming or asexual generation and in the second phase gametes
or sexual cells are produced.

58. Significance of Fertilization. — Let us now stop to con-
sider the meaning of the complicated process that we have termed
fertilization. No satisfying explanation has been offered. Some
see in these changes only a process of nutrition and chemical
stimulation. The gametes are lacking in certain materials that
are essential to their further growth. According to this view
the male gamete is the complement of the female and by the
union of the two all the substances are supplied that arc neces-
sary for growth.

It has also been suggested that the growth culminating in
fertilization is attended with the removal of impurities from the
sexual cells. Every cell has its growth, maturity, and senescence

138 PRINCIPLES OF HEREDITY

when it loses its power for further growth and division. The
reduction division and the subsequent formation of the gametes
results in the removal of substances that prevent their continued
activity and as a consequence they are restored to a youthful
condition which appears in the growth of the gametospore.

Much attention has been directed in recent years to the process
of fertilization as a method of controlling the character of off-
spring. You have already noted, page 56, that granular bodies
or chromosomes are constant features of every nucleus. These
complex bodies determine the type of plant that shall be de-
veloped. In other words the chromosomes contain the hereditary
substances, termed factors, that cause each plant to develop in a
certain definite way or to follow a certain pattern in its growth.
The chromosomes derived from the parent plants cause the off-
spring to resemble them in growth. The micro- and mega-spores
are formed in the sporophylls, and these spores must therefore
contain the same kind of chromosomes or factors as the plants
bearing the sporophylls, i. e., the parent plant, since they have
been derived directly from the parent plant by cell division.
Therefore gametes derived from the germinating spores must
also contain the same factors as the parent plants. In fertiliza-
tion we have the fusion of the gametes. This means that the
hereditary substances in the gametospore wull be of a dual
character since it has been derived from the male and female
gametes (Fig. 80). Consequently there is now lodged in one
body, the gametospore, all the possibilities of growth that each
gamete inherited from the parent plant.

Let us now examine some of the evidence indicating that the
hereditary substances or factors are distributed in a very definite
way from parent to offspring and that these factors are associ-
ated with the chromosomes. In the common garden plant, four-
o'clock, we find some plants bearing only w^hite flowers ; others
only red flowers. Here we have evidence of two factors; the one
causing the white color, the other causing the red color. If these
two plants be crossed the resulting offspring is pink or inter-
mediate between the two parents. This offspring is a hybrid,
meaning thereby an individual derived from parents that differ

I

NATURE OF PLANTS

I .V >

from one another in one or more factors. If now one of ilie
hybrid four-o'clocks is crossed with the pure wliite, one half of
the resulting offspring will be pure white and one half pink
like the original hybrid. The reason for this variation in the
offspring will be apparent by studying the diagram, Fig. SO A.
Here we see that each parent {RR and WW) produced gametes
having the same color factor as itself and when two .i^amctes

/^e^

T^cirerffs

(W W J

r)o^/>(vv)

\^ 7 ^r^^W- w w)

Fig. 86^1. Diagram showing distribution of the fiictors for red \R) and
for white ( PF) in the cross of the red and white four-o'clock; and the subsequent
cross of the pink hybrid with the white form.

derived from these parents unite the resulting hybrid (AMO
must contain the color factors of both parents. Note, howc\er,
that the hybrid produces gametes like those of the parents, u c,
gametes with the red factor or with the white factor. This is
due to the fact that in the formation of the gametes there is a
separating out of the factors so that the gametes are never
hybrid. They only contain the factors of one or the other of
the original parents. The lower part of the diagram show< '

I40 TRANSMISSION OF FACTORS

the results of crossing the pink hybrid (RW) with the pure
white (WW) come about. The white form can only produce
gametes with the white factor but we have seen that the hybrid
produces gametes, one half of which contain the white factor and
one half the red factor. Therefore in the cross between the pink
hybrid and the pure white there are only two possible combina-
tions of gametes. A gamete from the hybrid bearing the white
factor may fuse with a gamete from the white four o'clock, giving
a pure white offspring or a gamete from the hybrid bearing the
red factor may fuse with a gamete from the white four o'clock,
giving a pink hybrid.

It is now known that the hereditary substances or factors are
associated with the chromosomes and that they are separated
in a very definite way during the cell divisions that result in the
formation of the gametes. These chromosomes are associated
in pairs and in the division of the cell one member of a pair goes
to one daughter cell and the other member of this pair goes to
the other daughter cell. If now the factor for red is in one member
of a pair of chromosomes and the factor for white is in the other
member of this pair, it must follow in the above cell division that
one daughter cell w^Ill contain the red factor and the other
daughter cell will contain the white factor. Therefore the
gametes derived from these two daughter cells must contain
respectively the red factor and the white factor. This distri-
bution of the factors will be more manifest by examining the
accompanying diagram, Fig. S6 B. In figure i is shown a
nucleus with three pairs of chromosomes, one pair very small,
another somewhat larger and a third pair in which the factors for
red and white are indicated by the letters R and W. Of course
it is not to be supposed that any factor is visible to the eye. In
the second figure the two members of each pair of chromosomes
is seen drawing together preparatory to cell division. In the
third figure cell division is under w^ay and the chromosomes still
in pairs have been drawn to the center of the spindle, and in the
fourth figure we see a member of each pair being pulled to the
opposite pole of the spindle, where they will be organized into
two daughter nuclei. The later appearance of these two daughter

NATURE OF PLANTS

141

nuclei is shown at the right and left of figure four. In tins way
the red factor comes to be lodged in one of the daughter cells
and the white factor in the other daughter cell. This separation
of the factors that appear in the two members of a pair of chromo-
somes is termed the segregation of the factors. It is one of the
most important features in the cell divisions that result in the

Fig. 865. Diagram of a nucleus showing mode of division in gamete
formation. See text.

formation of the gametes. The members of a pair of chromo-
somes are termed homologous chromosomes and the contrasting
factors which they bear, in this case red and white, are named
allelomorphs. We see that it is the behavior of the homologous
chromosomes and their allelomorphic factors that determine tlio
nature of the gametes.

The fundamental features to note in the above discussion is
first that the factors remain distinct and show no indication of
fusing with one another. The red and white factors apfxvar
unchanged in the gametes no matter how man\- crossings have
been effected. The second feature is that these factors, though
associated in pairs in the homologous chromosomes, undergo at a

142 PLANT BREEDING

certain stage of cell division a separation or segregation. These
two principles of non-mixing and of segregation of factors were
discovered by Mendel and they are among the most important
and fundamental facts in biological science. As a single illustra-
tion of this note that the success in breeding is solely based upon
Mendel's principles of heredity. Desirable characters of one
individual may by crossing be associated with the desirable
characters of another individual and in the same way undesirable
traits may be eliminated. These new combinations of char-
acters may be effected in some cases with such certainty that
horticulturists have advertised new forms, describing their
characters and qualities before they have been bred. Late fruit-
ing grains have been made to mature earlier, species susceptible
to cold have become hardy, as in the case of wheat, which may
now be cultivated in Canada many miles north of its former range,
and non-resistant forms have become immune to disease. The
annual loss from a single plant disease, one of the rusts, is esti-
mated at 500 million dollars. In the same way the percentage
composition of the reserve food in grains has been changed and
the resulting yield greatly increased.

Another important feature in the transmission of the hereditary
factors is illustrated in the crossing of a blue-flowered tall sweet
pea with a red-flowered dwarf sweet pea. In this case we have
two pairs of allelomorphic factors instead of one as in the four-
o'clock. One member of a pair of chromosomes contains the
factor for blue and the other homologous member of this pair
contains the factor for red. In another pair of chromosomes
there is lodged in the same way the factors for tallness and
dwarfness. When these two plants are crossed we find that the
factors segregate as in the first example but the hybrids resulting
from the cross are never intermediate between the two parents.
It will be noticed in a hybrid from this cross containing the factors
for tallness and dwarfness that the plant is always tall like the
tall parent; so in a hybrid where blue and red are associated the
flower is always blue. It would look as though the factors for
dwarfness and redness in these cases had been obliterated.
Such, however, is not the case. They have been inhibited in

NATURE OF PLANTS

14

their action in some way by the opposite allcloniorpliic lacLor
and reappear in the offspring when hyljrid tall hlue sweet peas
are crossed among themselves. In cases of this nature where
certain factors are suppressed and others are active, the inactive
factors are termed recessive and the active ones are termed
dominant. Tallness and blue are dominant factors, dwarfness
and red are recessive factors.

In the above examples we see that the hereditary factors are
segregated and distributed to the gametes in a ver\' defmite
way. There is one important exception to this rule. A large
number of instances is known where one factor is usually or
always associated with another factor. That is, these two
factors are not separated from one another in the formation of
the gamete and consequently both are transmitted to the
gamete. Factors that behave in this way are said to be linked
and linkage is to be contrasted with the segregation mentioned
above. One of the most striking illustrations of linkage is seen
in color blindness. The factor for this characteristic is always
linked with the factor for sex. Therefore this factor is distributed
differently to the two sexes. The offspring of a color-blind male
and a normal female w^ould not show this characteristic at all
because the sons are absolutely normal and the daughters, though
having the factor for color blindness, do not show it, since this
factor is recessive to the normal sight factor. The daughters,
therefore, can alone transmit this factor, and one half of their sons
by a normal male will be color blind.

In the above examples very simple cases have been taken in
order to bring out more clearly the principles involved. We
have spoken of only one factor being lodged in a chromosome. I r
is now known that many factors may be associated with each
chromosome and the evidence at present makes ver>' probable
the belief that these factors are arranged in a line in the chromo-
some and are separated from each other by definite distances.
The factors have also been spoken of as though the\' were the
sole cause of the characters. The factor for redyi'ss is merely
the deciding cause of this color. A great many factors are
really necessary to cause the production of a '.'i\'"^ .liiu- uf«r,

144 ORIGIN OF PLANT VARIATIONS

some being absolutely necessary, others having only slight effect
upon it. The character is merely the end product of a series of
reactions. Therefore you are not to think of these characters
as definite chemical substances that are all lodged in the indi-
vidual at the start of its life and that maintain their identity
unchanged throughout its development. At the start of an
organism a definite number of factors are involved — just enough
to give direction to its growth and to cause it to follow a
pattern of development that is peculiar to its kind. As growth
goes on owing to the interaction of these factors and also owing to
the action of the environment characters are constantly created
and here and there in the organism characters appear that give
us ocular evidence of the presence of the factors which are the
deciding cause of a color, of a form, of a size, etc.

It will be seen from the above discussion that no cross can
result in a new creation. All the work in breeding shows that
the crosses result merely in new combinations of the old factors
that already existed in the parent stock. How then does vari-
ation come about and new characters arise? de Vries has
shown that one or more new characters may suddenly appear in
the offspring through the influence of unknown causes. These
sudden changes are termed mutations. It appears reasonable
that the gradual accumulations of these mutations going on
over a long period of time may have been the principal cause of
the enormous number of variations that are seen in plant and
animal life.

What then is the purpose of fertilization? We see that it is a
process for effecting crossing and so bringing about a new combi-
nation of the characters and the variations that appear in the
parent plants. Some of these new combinations will be of no
value, others will give their possessors an advantage because
they adapt them to the conditions under which they live.
These latter forms will therefore tend to survive and to crowd
out the less favored. This is natural selection, the environment
bringing about the survival of the fit. As long as the environ-
ment remains constant and natural selection has picked out forms

NATURE OF PLANTS ,,

that are adapted to it. there is no need of farther crossine
Indeed we see many plants that flourish without crossing Owing
to the changes, however, that have occurred on the earth since
plants first appeared there has been a necessity for fertili/alion
in order that new combinations might arise and adapt the
ofTsprmg to the new conditions.

10

PART II
DEVELOPMENT OF PLANTS

CHAPTER V

CLASSIFICATION OF PLANTS

59. The Method of Classifying Plants. — We have now arrived
at an understanding of the nature of the plant and the work
which it performs. In this study the most complex forms have
been largely considered. There are, however, a great variety of
very simple forms ranging from single-celled plants that often
form the green coatings on trees up to the complex types. How
have these different forms come about and what relation do they
sustain to one another? We will also be interested to compare
the life histories of these different forms and their modes of life.
In approaching this subject it is first necessary to gain a know-
ledge of the system employed in grouping or classifying plants.
Obviously many plants are closely related as the red, white,
scarlet oaks. Such a group of closely related plants is called a
genus (plural, genera) and the different kinds of individuals
which compose the group are known as species. The red oak is
one of the species of the oak genus. In scientific work the Latin
or Greek names are used to designate the genera and species.
In the case of the oak the genus is known as Quercus, an old
Latin name for oak, and the red oak species as rubra, meaning
red. Both the name of the species and the genus are employed
in naming a plant and consequently the scientific name of the
red oak is Quercus rubra. You will frequently see a letter or one
or more abbreviations after the scientific name, indicating the
person or persons who are responsible for giving the name now
in use. Thus Quercus rubra L. indicates that this name was

146

DEVELOPMENT OF PLANTS 147

given by Linnaeus. So in other cases we can say white oak
or Quercus alba, scarlet oak or Qiiercus coccinea, etc. The oak
has several characters in common with the chestnut and beech-
nut genera, as for instance the fruit in each case is associated
with outgrowths that appear as the burr in the chestnut and
beech and as a cup in the oak. On account of tliesc and other
common characteristics these genera are supposed to be related
and are therefore grouped together in one family. A family
is composed of allied or related genera. The name of the family
is derived from some characteristic genus in the group. In
this case it happens to be the beech or Fagns. To distinguish
a family from a genus the termination aceae is added to the base
of the generic word, in this example making the family name
Fagaceae. So we have the three allied genera, oak, beech, and
chestnut forming the beech family or Fagaceae. In the same
way it will be found that families are related and joined together
into a still larger group known as the order. The flowers of
the birch, alder, water beech, and hazel genera are very similar
and consequently form a family known as the birch family or
Betulaceae, Betula being the name of the birch genus. It will
also be noticed that there is a similarity between the flowers of
the Betulaceae and Fagaceae. This relationship is expressed by
placing them in the same order known as the beech or Fagalcs,
the termination ales being employed to distinguish the order just
as aceae characterized the family. So also orders are related
and grouped into classes. For example, the Fagales and many
other orders are characterized by having seeds with two cot\le-
dons. Several other orders have seeds with but one cotyledon.
This relationship is expressed by grouping the orders into two
classes, Dicotyledones and Monocotyledones. In both of tlu-se
classes the seeds are inclosed within the pistil, but in the cone-
bearing trees the seeds are exposed on a flat scale-like organ. All
seeds producing plants belong to one or the other of these two
groups. So we have two subdivisions, the Angiosix^niae, mean-
ing seeds inclosed, and the Gymnospermae, meaning naked seeds.
If now we stop to consider still larger groups than subdivisions,
it will be noted that many plants do not produce seeds, as in the

148 RELATIONSHIP OF PLANTS

case of the ferns and mosses, etc. So a final grouping of plants
into divisions or subkingdoms may be made. All seed-bearing
plants form the division Spermatophyta, meaning seed plants;
the ferns comprise the division Pteridophyta, meaning fern plants,
etc. The termination of phyta, from phyton a plant, distin-
guishes the division from the other groups. To repeat, the
division is composed of subdivisions which in turn are made
up of classes. Frequently the division contains only classes.
Classes consist of related orders. The orders are divided into
families which include allied genera and these latter groups com-
prise closely related individuals or species. The vegetation of the
earth is separable into four widely differing divisions: I., The
Thallophyta, including among others the fungi and algae; II.,
The Bryophyta or moss plants; III., Pteridophyta or fern plants;
IV., The Spermatophyta or seed plants. The character and
relationship of these groups will be considered in the following
pages.

CHAPTER VI
DIVISION I. THALLOPHYTA

60. Classification of the Thallophyta. — The Thallophyta com-
prise a multitude of plants that include the most primitive and
simple forms of vegetation upon the earth. They range in size
from single microscopic cells to forms that arc comparable in
bulk to some of our shrubs and trees. They are all of simple
structure and do not possess roots, stems and leaves in the sense
of the seed plants. Such a type of plant body is called a thallus
and it assumes a variety of forms. The absorption and manu-
facture of foods is carried on, as a rule, by any and all of the
cells and this is equally true of the reproductive process. This
Division is not a natural one in that some of the subdivisions are
not related so far as we know. Several of the groups represent
distinct lines of development and they are placed together
simply because they are all simple forms of plant life. So it is
impossible to give in a short space a general idea of the nature
and character of this division which includes many groups of
widely different forms. For the purpose of stud\' the Thallo-
phyta may be divided into five subdivisions: — (i) Myxonncetcs
or Slime Moulds; (2) Schizophyta or Bacteria and Blue green
Algae; (3) Diatomeae or Diatoms; (4) Euphyceae or Algae;
(5) Eumycetes or Fungi.

Subdivision i. Myxomycetes or Shme Moulds

61. The Life History of a Slime Mould. — In one stage of their
life the slime moulds have a motility and mode of feeding sug-
gestive of some of the lower animals, wliile on the otlur liand
the final stage of their existence is more suggestive oi \hv fungi.
For this reason it has often been suggested that lhe\ are inter-
mediate between plant and animal life. These jilants are widely
distributed over the earth and may frequently be found on de-
caying logs and rotting twigs and leaves in forests. A common

149

150

LIFE OF A SLIME MOULD

form, resembling a miniature, brownish puffball, is often seen on
stumps and fallen logs (Fig. 87). Other kinds are illustrated in
Fig. 88. They range in size from scarcely a pin head to nearly
a foot in diameter and from spherical to cylindrical and cake-like
masses. Not infrequently they are of great beauty owing to their
coloration and lace-like structures. These small sacs or spor-
angia (sing, sporangium) as they are commonly called, repre-
sent but one stage in the life of the slime moulds. If we begin
with an examination of the dust that floats away from these

-^-M^

'Mj

Fig. 87. Fig. 88.

Fig. 87. The sporangial stage of two common slime moulds: A, Ly co-
gala. B, Arcyria. C, sporangia rupturing, hair-like structures (the capil-
litium) and ppores protruding. D, sporangia emptied.

Fig 88. Open types of sporangia: A, Stemonith, at left a single spo-
rangium enlarged, showing net-like structure formed by capillitium radiating
from the central stalk of the sporangium. B, Cribaria.

sacs every time they are tapped, the life history will be found to
be about as follows: Under the microscope the particles of dust
are seen to be minute cells or spores (Fig. 89, ^). You may think
of a cell as a cube, a sphere or as assuming almost any form,

DEVELOPMENT OF PLANTS

151

frequently greatly elongated. It is bounded by a wall and within
is the living substance or protoplasm. The more inipcjrtant parts
of the living substance are a viscid rather watery material termed
the cytoplasm and a minute denser body, the nucleus, which is
immersed in the cytoplasm. A spore is a special kliul nf k-W

Fig. 89.

Fig. 90.

Fig. 89. Mobile phase in life of slime mould: .4, group of spores. B,
germination of a spore. C, two forms of zoospores greatly enlarged. The
right-hand one mobile owing to a streaming movement of its protoplasm
and the left-hand one moves about owing to the rapid movements of the
cilium, c. D, association of the zoospores preliminary to the formation of
a Plasmodium. E, Plasmodium. The arrow indicates the direction of the
streaming movement.

Fig. 90. Character of the capillitium: A, in Arcyria. B, in Trichia, por-
tion of thread enlarged on the left hand.

that is capable of germinating under favorable conditions and
producing a plant. In this case it is advisal)le when licrniiiiating
the spores to place them in a nutrient solution made by boiling
in water bits of wood upon which the sporangia were growing.
Germination will begin in about one day. As the spore wall
ruptures the contents escapes as a naked bit of protoplasm
(Fig. 89, B) which possesses a singular power of motion. This
motility is due either to the slow crcci>ing or streaming motion

152 MOVEMENTS OF THE SLIME MOULDS

of the soft plastic body or more commonly the body may possess
a delicate thread or cilium (plu. cilia) (Fig. 89, C). These
ciliated bodies are called zoospores. The cilium is a highly
sensitive organ which by rapid and rhythmical movements,
somewhat like the movements of the arms in swimming, beats the
water and thus drives the body along. While in this condition
the zoospores are rapidly multiplied owing to the repeated split-
ting of the little bodies into two similar parts. Finally this
greatly increased number of individuals begins to come together
in small groups (Fig. 89, D) which in turn merge and so form
large slimy masses termed plasmodia (sing, plasmodium) which
are quite destitute of all walls and consist of a great number of
nuclei surrounded by a watery cytoplasm (Fig. 89, E). You will
often find this plasmodial stage of the slime mould on the under
side of rotting limbs or bits of wood in the forest as a sticky mass
resembling in consistency the white of an egg. The mucilaginous
character of the plasmodium accounts for the name slime mould
popularly applied to these plants, also for the term myxomycetes,
from myxos, slime, and myces, mould or fungus.

The Plasmodium possesses a sensitiveness or capability of
responding to external stimuli to a degree that is remarkable
when we consider the extreme simplicity of these plants. For
example, it avoids too strong a light and slowly moves toward
moisture and food. This accounts for your failure ordinarily
to notice it since it seeks the darkness and food in decaying
logs or the under surface of sticks and leaves. The motion is
brought about by little arm-like branches that flow from the
jelly-like plasmodium and finally the entire plasmodium will
slowly follow along these lines with a complicated streaming
movement (Fig. 89, E). If bits of wood containing some of
the Plasmodium are placed in a damp chamber in the dark after
a time the plasmodium will be seen creeping up the sides of the
moist dish in a complex skein-like mass or if a glass slide, down
which a very meager amount of water is slowly allowed to trickle,
is brought in contact with the plasmodium it may be observed
creeping up the slide toward the source of the water supply.
The slime moulds do not contain chlorophyll and are therefore

DEVELOPMENT OF PLANTS i^:^

dependent upon organic food. Plants that feed in this way are
called saprophytes. Since the Plasmodium is not surrounded by
a cell wall feeding becomes a simple matter. The jelly-like
mass engulfs the decaying particles or other foods and after
digesting the nourishing portions leaves behind the worthless
parts as it creeps along. As the plasmodium approaches the
final stage of its life, its nature appears to change comi)letely
for now it avoids moisture and seeks the light. It crc'e[)s to
the surface of the wood or leaves in which it has been growing and
forms the characteristic bodies seen in Figs. 87, 88. These
sporangia are formed by the outer part of the plasmodium
hardening into a wall, while each of the inclosed nuclei, which
have greatly increased in number by division, becomes sur-
rounded by a wall, thus forming the spores. UsualK- a portion
of the substance of the young sporangium is transformed into
simple or branching threads or tubes, collectively called the capil-
litium (Fig. 90). In several of the genera the capillitium forms
a network within the delicate walls of the sporangia and owing
to the early breaking down of the wall, the feathery frame alone
remains (Fig. 88). These threads are hygroscopic and their con-
stant motion assists in stirring up the spores and exposing them
gradually to the wind as soon as the wall of the sporangium
ruptures. A somewhat simpler type of slime mould lives as a
parasite in the roots of turnip, cabbage and cauliflower, producing
a destructive disease known as clubroot.

There are several groups of low types of plant and animal life
that are suggestive of relationship with the slime moulds. The
most important among these, the myxobacteriales, have Inrn
made known by Thaxter. They are minute plants, ver>' sugges-
tive of the next group, the bacteria, but associated in definite
structures that resemble the sporangia of the slime moulds and
also of certain fungi. On the other hand, certain aquatic forms,
as Protomyxa, with a life history very similar to that of the
slime moulds, intergrade almost perfectly towards simple ani-
mal types, as the protozoans, of which tlu' common amoeba is
an example.

Thus we see that the life history of these plaii'< '< ' ^'■'■\ Mmnlr

154 NATURE OF BACTERIA

one. By the formation of sporangia numerous dust-like spores
are produced that are capable of germinating and forming
zoospores that Increase rapidly by division. The aggregation of
the zoospores results In the formation of the plasmodlum which
after a time completes the life history by creeping upon suitable
dry objects and forming sporangia. The mingling of the zoo-
spores In the formation of the plasmodlum is not a sexual process,
since the nuclei do not fuse, but Olive has shown that the spores
are formed In the same manner as In plants characterized by
sexual reproduction and that therefore there must be a sexual
fusion of the nuclei at some time In the life history of the plas-
modlum. The structure and life history of these plants Is so
simple that It Is possible that they may have been derived from
forms allied to the first forms of life upon the earth. The
common occurrence among simple animals and plants of motile
stages or zoospores In their life history certainly suggests that
possibly such forms may represent the first appearance of living
matter.

Subdivision 2. Schizophyta or Bacteria and Blue Green Algae

As In the preceding group these forms are extremely simple
and possess some characters suggestive of the lower forms of
animal life. They have received the name of Schizophyta, mean-
ing splitting plants, owing to their common method of repro-
duction by division of the cell into two equal parts. There are
two Important classes of Schizophyta: A, Bacteria; B, The Blue
Green Algae.

Class A. Bacteria

62. The Structure and Nature of Bacteria. — Bacteria are com-
monly known by such vague terms as microbes and germs. They
are, however, unicellular plants that are of almost universal dis-
tribution, though more abundant about dwellings and less com-
mon In cold countries, at high altitudes and on the sea. They
include the smallest and simplest forms of plants, ranging from
scarcely 1/50,000 in. to 1/10,000 in. in diameter (Fig. 91). Such
forms would have many times more room in a drop of water
than a whale would find in New York harbor. So minute and

DEVELOPMENT OF PLANTS

00

simple in structure are the bacteria that the real nature of the
plant body is somewhat a matter of dispute. The i)lants are
unicellular and surrounded by a delicate tliln wall which in-
closes a colorless and slightly granular protoplasm (Pig. 91,
A, i). There is no nucleus comparable to that of the higher

Fig. 91. Forms of Bacteria: A, Bacillus suhtilis, a form common in hay
infusions, i, motile state; 2, cells with spores; 3, slimy mass of bacteria,
the zooglea condition, that appears on the surface of infusions, cooked ^ cg-
etables, etc. B, Spirillum. C, a coccus form that appears in pus. D, mobile
and spore stage of lock-jaw bacillus.

plants, although indications of it are seen in a few scattered chro-
matin grains. The cell content is very simple and totally hicking
in plastids and other differentiations with which you are familiar.
The slimy appearance of bacteria not liveable where tlu-N t^row
together in colonies is due to the mucilaginous excretion from
their bodies which is often brightly colored. Bacteria range
from globular to rod-like and curved forms (Fig. 9O. Many
are motile by means of cilia, as in the case of the zo. spores of
the Myxomycetes, which project singly or in tufts from the ends
of the cells or in varying numbers from all sides.

(a) Reproduction of Bacteria. — This is a simpler ])rocess than
in the slime moulds and much more rapidly elTected when the
conditions are favorable for growth. This process consists of

156 EXCLUSION OF BACTERIA

forming a partition through the middle of a cell when the two
daughter cells thus formed separate at once or they may remain
attached, forming ultimately a chain of cells, since these two new
cells continue to grow and repeat the dividing process. This
method of multiplication goes on with great rapidity, frequently
within a half hour, so that many millions of plants may be formed
in a day from a single bacterium. This accounts for the rapidity
with which solutions, as beef tea, in which bacteria thrivel
become turbid and also for the gelatinous masses and wrinkled
scum that appears upon cooked foods and soups, if allow^ed to
stand undisturbed for a day or more (Fig. 91, A, 3). Conn
estimates that a single bacterium, if allowed to develop under
perfect conditions, would produce in three days a mass of bac-
teria weighing 4,700 tons. When the conditions are unfavorable
for growth many species of bacteria have another form of repro-
duction and form small bodies, called spores, within the cell
(Fig. 91, A, D). These spores are exceedingly resistant to un-
favorable conditions but germinate readily, producing bacteria
when conditions are again suitable for growth. Some spores will
stand boiling for more than an hour. This peculiarity of the
spore, while of great advantage to the bacteria, renders them very
difficult to exterminate.

(6) Exclusion of Bacteria. — ^The various devices for preserving
fruits and meats and sterilizing utensils and instruments in
surgery is based upon some method of killing these organisms
and preventing the entrance of others. This is the purpose of
boiling and canning foods and of salting or drying fruits and
meats. The boiling, if thorough, effectually sterilizes, while the
canning excludes all organisms and the material will remain un-
changed indefinitely. Smoking or drying, with addition of salt
in the case of meat and of sugar in fruits, is effectual since
bacteria flourish only in the presence of moisture. This is the
reason that seeds and other structures are not speedily destroyed
by bacteria. Seeds are naturally dry and protected by coats that
tend to exclude the moisture for some time. Sugar is used as a
preservative because bacteria cannot endure over 40 or 50 per
cent, of it. Salt for the same reason is useful in preserving

DEVELOPMENT OF PLANTS 157

meat because it is injurious to bacterial life and smr^king also
introduces volatile products that prevent their growtli. So too
acids are generally harmful to bacteria. Vou have heard much
regarding the use of salicylic acid, benzoate of soda, etc., in pre-
serving foods and certain beverages. Whether these substances
are harmful to us or not, certain it is that they are fatal to the
existence of bacteria. So also meats, fruits and vegetables are
kept in a wholesome condition for considerable periods in ice
boxes and for months in cold storage since the bacteria grow
more slowly as the temperature is lowered and their activity
ceases at the freezing point. It should be remembered, however,
that no amount of cold will kill all of the bacteria and that fcx^d
spoils very quickly when removed from cold storage.

(c) Economic Importance of Bacteria. — Insignificant and simple
as are the forms and structures of bacteria it is safe to say that
no plants are so vitally related to our welfare. Numerous forms
are instrumental in effecting changes in various substances termed
fermentation; others assist in the decay of dead plants and
animals; a comparatively small number of species build up or
construct substances that are of vital importance to plant life;
finally many diseases of plants and animals are due to bacteria.
The work performed in all the above mentioned processes, save
the third, is of the same nature. These plants have the power
of breaking down into simpler compounds the various sub-
stances with which they may be brought in ct)ntact. Thus
vinegar is formed through the decomposition of certain alcohols
into simpler substances, as acetic acid and a gas, carbon dioxide.
These changes are effected by substances, termed ferments or
enzymes, that are excreted by the bacteria. These enzymes are
very remarkable compounds. They do not enter into permanent
union with any substance but by their presence cause it to break
down into some of its component parts. When this decompo-
sition is attended with the liberation of gas, as in certain sugar
solutions, w^e term the process fermentation but when in an
exactly similar way bacteria break down the substances compos-
ing the bodies of plants and animals we term the process decay.
So also in disease bacteria simply cause the <?.' ..mi^.^miK.n of

^s

158 BACTERIA AND FERMENTATION

certain of the compounds that make up the body. In some cases
these simpler products that result from the activity of bacteria
are very poisonous and in disease it is these substances, toxins,
that often cause the principal danger. Let us now consider the
more important of these operations.

1. Fermentation. — A very large group of bacteria, in common
with some fungi, live upon sugars and other carbohydrates,
causing a decomposition known as fermentation. Vinegar is
due to the decomposition of the alcohol in cider and other weak
alcoholic solution by numerous species of bacteria which appear
as slimy masses and are popularly known as mother of vinegar.
Milk sours and coagulates through the agency of bacteria which
reduce the sugar to lactic acid, which in turn coagulates casein.
The beverages known as zoolack, kum.iss, etc., are milk prepara-
tions soured by special kinds of bacteria or yeast. The Metch-
nikoff tablets contain bacteria that are especially active in this
way and they are also supposed to live in the lower alimentary
tract where they form decomposition products that are injurious
to the harmful bacteria that live in this same region. So also
butyric acid, necessary in the manufacture of cheese, is produced
by bacteria. It will be seen from this that the products of de-
composition are not necessarily harmful. It is interesting to
note that some of the flavors of cheese and of high grade butter
are due to the products of decomposition and the excretions from
bacteria. It is evident that this might be true in the case of
limburger cheese and rancid butter but also remember that species
of bacteria are cultivated and used to impart certain flavors to
cheese and also to a less extent to butter. It is altogether
probable that this will become a common practice in our dairies.

2. Decay. — The great majority of bacteria live as saprophytes
upon dead plants and animals. They begin their work of

y^ decomposition immediately upon the death of the organism.
>>- In the case of plants they are generally assisted at first by
^ various fungi. There are numerous classes in this group, each

kind doing a special work in bringing about the decay. The
first steps in the decomposition are quickly performed and then
the simpler compounds, though mostly still quite complex, are

DEVELOPMENT OF PLANTS 159

acted upon by an entirely different class of bacteria that carry
on the work to a further stage of decay. So you can think of
many different kinds of bacteria each taking up the work where
it was left by its predecessor and carrying it a step fartluT. In
this way the complex compounds that make up the Hvinj^^ organ-
ism are broken down into very simple ones, such as water (H2O)
or gases as carbon dioxide (CO2), ammonia (NH3), methane
(CH4), sulphureted hydrogen (HoS), nitrogen (N), etc. It is
not to be understood that all of these simpler substances arise
only in the final stage of decay. Some of them may arise very
early in the process, as ammonia and carbon dioxide, or they
may be set free at later stages in the decay. Some of these
gases are the cause of the foul odors associated with decay and it
should be added that certain of the rather simpler products of
decomposition, termed ptomaines, are exceedingly poisonous.
The ptomaines found in fish, cheese, ice cream, are decomposition
products formed by the bacteria of decay. Unfortunately these
substances are without taste or odor.

This process of decay is of importance from two standpoints.
First, it prevents the accumulation of organic matter upon the
earth. Were it not for these changes every plant and animal
would remain unchanged after death and so prevent further
continuation of life. Second, we see that the successive steps
in decay result in the formation of simple compounds that are
either directly or indirectly utilized by plants in the formation
of their tissues. Thus H2O and CO2 are used by the plant in
forming sugars and starches which furnish the materials that
go to make up the cell walls and also assist in building up the
living substance. So also the N and the S that we have seen
set free as elements or simple compounds are utilized in the
formation of the living matter of the plant, though not directly,
as we will see below.

3. Construction or Synthesis. — In sharp contrast to tiie alK)ve
mentioned forms stands a rather small group of bacteria that
possess the remarkable power of taking certain of the simpler
products of decomposition and adding other elements to them.
These forms, therefore, are quite different in thi-ir work from other

i6o NITRIFYING BACTERIA

bacteria. They build up substances instead of breaking them
down. This work of combining two or more elements is termed
synthesis and we may term these forms the synthetic bacteria.
These bacteria are of great economic importance and those kinds
that have the power of combining nitrogen into a form in which
it can be used by plants are of first importance. Nitrogen, as it
appears in the process of decay is in a form that the plant can uti-
lize either not at all or only to a limited extent. There are several
bacteria that have the power of combining nitrogen with other
elements and so constructing a nitrogen compound, termed a
nitrate, that is the most valuable crude food product of plants.
Among these forms may first be mentioned the nitrifying bacteria.
They have the power of adding another element to nitrogen
compounds. They are able to add oxygen (O) to ammonia
(NH3), that is to oxidize it, and thus transform it into a nitrate.
This synthesis is accomplished in a truly remarkable manner;
no less than two forms working together are required to accom-
plish the work. The first form adds two atoms of oxygen to
the NH compound, thus changing it to a nitrous or nitrite state
while the second form adds to the nitrite an additional atom
of oxygen, thus completing the formation of the nitric or nitrate
condition. The reactions caused by these two forms may be
expressed as follows:

2NH3 + 3O2 = 2H2O -t- 2HNO2 (nitrous form of nitrogen),
2HNO2 +02= 2HNO3 (nitric form of nitrogen).
These two organisms work together, each doing its special work,
and they are so efiftcient that a solution of ammonia poured upon
soil will show no trace of the ammonia in the solution after it has
percolated through the soil. Fully 65 per cent, of the nitri-
fication is effected in the upper twelve inches of the soil and very
little below a few feet. It is probable that the Chilian saltpeter
beds, previously referred to, are the remains of inland seas
where great growths of seaweeds accumulated. Owing to the
drying out of these bodies of water and the decay of the vegeta-
tion ammonia was formed which became changed to nitrates as
outlined above. Were it not for these organisms the ammonia
compounds would quickly escape from the soil as a gas and thus

{

DEVELOPMENT OF PLANTS l6i

the principal source of nitrogen in nature for tlie plant would be
lost.

You have noticed that free nitrogen (N) may also appear in
decomposition. A few forms of bacteria ha\e been discovered
that are able to synthesize this gaseous element into a compound
that is of use to plants in a manner quite as remarkable as in the
case of the nitrifying bacteria. Because they are able to fix the
free gaseous element nitrogen in a stable compound these bacteria
are referred to as the nitrogen-fixing bacteria — in contradis-
tinction to the nitrifying bacteria that only act upon nitrogen
when it is already in combination, as in ammonia. The most
important of the nitrogen-fixing bacteria are those that live in
the roots of plants belonging to the bean family. They cause
minute gall-like swellings, termed tubercles, upon the roots of
beans, clovers, alfalfa, etc., and are able to combine the free
nitrogen of the air into a nitrogenous compound suitable for the
nourishment of the plant. These bacteria are of inestimable
value in maintaining the fertility of the soil (see page 65).
Less important economically than the tubercle-forming bacteria
but more wonderful biologically are those forms thdt live in the
soil quite independent of higher plants and effect a somewhat
similar fixation of free nitrogen. In this work they are associated
with two other forms that take no part in the fixation of nitrogen
but simply remove the oxygen and probably the organic matter,
for only in the absence of these substances can these remarkable
bacteria live. In this manner they are able to fix the nitrogen
and build up complex compounds, probably of a i^roteid nature,
which are subsequently decomposed and e\entually con\'ert«.>d
into available nitrates by other bacteria.

As a final example of these synthesizing bacteria mention
should be made of the iron and especially of the sulphur barti-ria.
They are able to oxidize simple compounds of iron and sulphur
so that deposits of these minerals may often be seen as slimy
white, red or yellow masses in springs and streams. The sulphur
bacteria are of especial interest because they are able to oxidize
one of the products of decomposition, H2S, into a sulphate — one
of the valuable crude plant foods. These plants are practically
II

I

i62 SIGNIFICANCE OF BACTERIA

independent of organic matter for their support, deriving the
necessary energy for their growth from the decomposition of
inorganic matter. This also is absolutely the case with the
nitrifying bacteria which are unable to live upon organic matter
and it should be added that these latter forms effect the same
decomposition of carbon dioxide as noted in photosynthesis
(see page 12). This is an important fact, for we see that there
are two methods for effecting the storing up of energy. Green
plants are the principal agents in this work. They build up,
synthesize, such substances as sugars by utilizing the energy of
the sunlight — for this reason the process is termed photosynthesis.
Certain bacteria also build up compounds by utilizing the energy
of chemical reactions. This process is therefore termed chemo-
synthesis.

We should now stop for a moment and consider what the real
significance of these bacterial forms is. Green plants build up
complex substances from certain elements and simple com-
pounds. In the first steps of this work they may be assisted by
certain bacteria and fungi. These complex compounds are now
reduced by other bacteria to their original state so that they may
be used again by the green plant. This in a word is the story
of the organic world. Certain substances in the air and in the
soil are going through an endless rotation or cycle of chemical
changes — ever changing from simple to complex, from a com-
plex state back to their simple form. So then the green plant is a
temporary carrier of certain elements that, owing to their combi-
nations, represent a certain amount of stored up energy. We
and the plant and the bacteria utilize these substances in the
same way. We eat them, i, e., decompose them. This is
practically the only way that life can be maintained and the end
result of this eating, this decomposition, is that the elements
making up the compounds are returned unchanged to the earth,
to go again through the cycle of changes.

4. Disease. — In contrast to the kinds of bacteria mentioned
above there is another group that live as parasites on plants
and animals producing disease either by destroying the tissue
and sapping the vitality or by the production of poisonous

DEVELOPMENT OF PLANTS 163

compounds, toxins. Among the more terrible of these infec-
tious bacteria may be mentioned'those producing consumption.
These affect especially the lungs of animals and cause, according
to data now available, from 150 to 200 thousand deaths annually
in the United States. It is altogether probable with complete
returns that the death rate will exceed by 50 thousand the higlier
figure mentioned above. Consumption is no longer considered
so fatal a disease as formerly. A person of ordinary constitution,
if suitably nourished, can be cured by living in the open air.
This pest could probably be wiped out witliin ten years, if the
habit of spitting could be stopped. The spores and plants are
readily carried away in the air from the dried sputum to Ix;
inhaled again and so spread the disease. The bacteria causing
lockjaw, Of tetanus, are especially abundant in the soil in certain
localities. Through wounds they are carried into the s\stem.
These forms, like others previously noted, are peculiar in that
they can not grow in the presence of oxygen. Conse(juentl\- the
dangers of infection are great in deep wounds. An innnediatc
cauterization of the entire surface of the wound will kill the or-
ganisms. Other well-known bacterial diseases are j^neumonia,
diphtheria, erysipelas, Asiatic cholera, t>phoid, and sj)K"nic
fevers and grippe. It should be noted that many diseases,
as smallpox, malaria, hydrophobia, yellow fever and probably
scarlet fever, etc., are due to a low order of microscopic animal
life. Disease-producing bacteria are either localized, as in the
lungs, throat, intestines, or are generally distributed throughout
the system. The symptoms of the disease are largely due to
the products of decomposition caused by tlie growth of tlu- bac-
teria or by secretions from the bacteria themselves, all of whicli
act as poisons and are commonly referred to as toxins. In
other cases the toxins are formed by the infected animal owing
to the disturbance of its growth and nutrition b\ the bacteria in
its tissues. In the same way bacteria are the cause of many
diseases among the plants, as in corn, melons, many truits, etc.
See Bacteria in Relation to Plant Disease, by Hrwin V. Smith.

These bacteria are constantly finding their wa\- into our iuKlies
by means of the breath but so long as the system is in a ht-althv

i64 BACTERIA OF DISEASE

condition there is only slight danger of the organism gaining a
foothold. Just how the system is able to combat the growth of
these plants is now the subject of earnest investigation. Certain
diseases, as mumps, measles, whooping-cough, smallpox, etc.,
usually occur but once and it is probable that substances are
formed in the body as a result of the presence of the bacteria or
animal organisms that prevent a second growth of the germs and
the individual thus becomes immune to further attacks. The
formation of such a substance is known to occur in the case of
diphtheria. The bacteria of this disease occur in the upper air
passages of the throat and secrete a poison or toxine that affects
the entire body. Gradually the system forms a substance called
an antitoxine that stops the growth and eventually kills the bac-
teria. The vigor of the system and the virulence of the bacteria
decide whether the disease shall prove fatal before the anti-
toxines are formed in sufficient quantity to destroy the bacteria.
Note that these disease-producing bacteria vary in their power
to produce toxine. Sometimes, as in epidemics, they are so
virulent as to attack with fatal results the strongest individuals.
In other cases the bacteria appear to be degenerate and only pro-
duce mild symptoms of the disease. This is the principle of
vaccination. The little animals causing smallpox are cultivated
under conditions that weaken them and render them less virile.
Consequently when they are introduced into the system they pro-
duce only mild symptoms of the disease. The history of the
study of bacteria constitutes one of the most interesting and
fascinating pages in science. Pasteur was the first to demon-
strate that fermentations and putrefactions, and later that
certain contagious diseases, were due to bacteria and animal
organisms. His work is now commemorated by a tablet on the
rue Pasteur in Paris with the inscription: Here stood Pasteur's
laboratory. 1857 Fermentations; i860 Spontaneous Genera-
tion; 1865 Diseases of Wines and Beers; i§8i Virus and Vaccini;
1888 Silk-worm Distempers; 1 864-1 888 Hydrophobic Remedies.
To-day every state and government has it corps of workers and
every city its expert bacteriologists who are studying the nature
of bacteria and their relation to plants and animals.

DEVELOPMENT OF PLANTS 165

Class B. Blue Green Algae or Cyanoi'iivc eae
63. The Structure and Nature of the Cyanophyceae.— These
plants are very simple organisms that have main features in
common with the bacteria. They are unicellular plants, al-
though the cells are more commonly joined into rows, forming a
thread or filament (Fig. 92, A, B). In some cases these filaments
may be regarded as multicellular plants since they branch and
because the various cells perform different functions. They form
slimy, blue-green, black, yellow or violet masses and, like the
bacteria, appear to be dependent to a degree on organic material.
At least they are especially abundant in the presence of decaying
organic matter, as in the drainage from stables and watering
troughs, or in pools, puddles and damp places where there is an
abundance of filth and decay. The enormous increase of these
plants often produces a discoloration of the water, as in the Red
Sea, and they are often the cause of the foul odors of muddy
ponds and streams and reservoirs and of the pollution commonly
know^n as water-bloom or working of ponds. Like the bacteria
also they are associated with varying amounts of gelatinous sub-
stances derived from their walls or excreted from the cells and in
both cases the walls may become colored with various pigments.
Doubtless the mucilaginous character of these plants enables
them to retain moisture and so adapts them to dr>er conditions
than would otherwise be possible. The Cyanophyceae differ
radically from the bacteria in possessing chlorophyll which is dis-
tributed in the outer portion of the protoplasm, and in the |X)s-
session of a more definite nucleus. A blue pigment, ph>coc\anine,
is associated with the chlorophyll and for this reason they are
called the blue-green algae or cyanophyceae. Cell division fol-
lows the ordinary method noted in the higher plants, at least this
appears to be true in those cases that have been accurately
studied. These plants accordingly show a decided advance over
the preceding forms not only in the dilTerentiation of the cells
but especially because they are capable, to a degree at legist , of
manufacturing food from inorganic substances by reason of their
chlorophyll.

In addition to the multiplication of indi\icluals by cell di\i-

166 REPRODUCTION OF THE CYANOPHYCEAE

sion, as in the case of the bacteria, many of the thread-like forms
have another interesting method of increasing their numbers.
A few of the cells, termed a hormogonium, become separated

Fig. 92. Fig. 93.

Fig. 92. Forms of the Cyanophyceae : A, Gleocapsa. At the right a cell
has divided, but the two daughter cells are held together in a gelatinous
mass. Above numerous divisions have occurred, but all the cells are sur-
rounded by a mucilaginous envelope. B, Nostoc. Below appear the gelat-
inous, spherical masses of the plants as they appear floating upon the sur-
face or resting on the bottom of ponds. At the right a plant enlarged —
h, heterocyst; s, thick- walled resting cells or spores. At the left a spore has
germinated, producing five cells. C, Rividaria. At left gelatinous mass of
plants attached to stem of water plant. At right view of a few of the plants
• — hr, hormogonium; li, heterocyst.

Fig. 93. One of the most common forms of the Cyanophyceae, Oscillatoria.
The different sizes of the cells show that cell division may occur in any of
the cells of the filament — e, a decaying cell which will ultimately free the
cells below it as a hormogonium.

DEVELOPMENT OF PLANTS 167

from the other cells and frequently possess for a lime a slight
motility that enables them to move away from the i)arint fila-
ment (Fig. 92, hr). In some species one or several raihcr larj^e,
colorless cells, heterocysts, appear in the filament and ma>- serve
to separate it into hormogonia (Pig. 92, //). In Oscillatoria
(Fig. 93) the filaments into which these hormogonia grow per-
manently retain their power of motion. No cilia have been
observed for a certainty in any of these forms and the cause of
the motion has been ascribed to the interchange of fluids attend-
ant upon the absorption and digestion of foods. A simple type
of spore production is also to be found in the majorit>' of forms.
This consists merely of a slight enlargement and thickening of
the walls of the ordinary cells (Fig. 92, s). These simple spores
are able to tide the plant over unfavorable conditions and germi-
nate, forming new plants when conditions are again suitable.

The Bacteria and Cyanophyceae are evidently very primitive
and ancient forms of life. The fact that they frequently occur
in hot springs and that they can endure greater extremes of
heat than higher plants may indicate that they are related to
forms that appeared upon the earth when just such conditions
existed and at a time when the environment was not suitable
for the development of higher types. It has been stated on
page 162 that certain bacteria are independent of organic foods
and that they affect the decomposition of CO2 as in photosyn-
thesis while in the purple and in the red sulphur bacteria we have
pigmented forms that are perhaps in a transition state to chloro-
phyll-bearing plants. It seems altogether probable that the
earliest forms of life could build up organic compounds without
chlorophyll and in such forms as these bacteria we Ikuc perhaps
an illustration of a tendency towards the acquisition of chloro
phyll as seen in the higher plants.

Subdivision 3. Diatomaceae or Diatoms
64. The Nature and Structure of Diatoms. The diatoms arc

among the most common and widely distributed plants (Fig.
94). While microscopic, they exist in such large mnnbers as
to form the familiar brown coatings on the bottom of ponds and

i68

FORMS OF DIATOMS

sluggish streams and often render sticks and stones in the water
slippery with slimy deposits. They are widely distributed on
damp soil and in fresh and salt waters. Note should also be

FiG. 94; Common forms of diatoms: A, fan-shaped colonies of diatoms,
Licmophora, attached by gelatinous stalks to seaweed. The repeated divi-
sion of a solitary diatom gradually builds up the fan-like colonies. B, Tahel-
laria, forming colonies grouped in zig-zag blocks. C, Melosira, cylindrical
pill-box-like diatoms, arranged in chains. Below end view of the diatom.
D, a species of Navicula arranged in gelatinous branches. At right enlarged
view of a branch in which the diatoms glide back and forth. — After Wm.
Smith.

made of the peculiar species that float upon the surface of the
ocean and form the larger part of that organic life known as

DEVELOPMENT OF PLANTS

[69

the plankton (Fig. 95). This is especially ahiuulant in northern
waters and has been compared to great pasture lands, since it
furnishes the principal food of surface feeding fishes and cjiher
marine life. The Diatoms are unicellular plants and exhibit

Fig. 95. Fig. 96.

Fig. 95. Plankton forms of diatoms: A, Coscinodiscus, B, Planktottitlla.
Below seen from side. — After Gran.

Fig. 96. Structure of the diatom, Pinnularia: .1, valve vit-w. B, girdle
view. C, cross-section, the two dark bands showing the position of the chro-
moplasts in the diatom — p, pore in wall which appears as a line in .1, running
from the ends towards the center of the valve. — After Lauterborn.

a great variety of forms as circular, elllplical. rod or wedge-
shaped, curved or straight. They are equally variable as regards
their association. Some are solitary and free swinmiing, others
are attached by stalks; some form bands or ribbons or zigzag

170

STRUCTURE OF DIATOMS

chains, Avhile others are imbedded in a gelatinous mass of a
more or less regular form (Fig. 94, D). The cells are covered
by two valves, one of which overlaps the other like the cover
of a box. Therefore a diatom presents two quite distinct ap-
pearances— the top or valve view and the side or girdle view
(Figs. 96, 97). This difference is further intensified by the
sculpturing of fine lines that appear upon the walls. An inter-
esting feature about these valves is the fact that they are com-

FiG. 97. Structure of Cymbella: A, valve view. B, cross-section show-
ing the difference of the two sides of the diatom. C, the two girdle views.
—After Pfitzer.

pletely infiltrated with silica, a substance resembling glass. If
a diatom is burned in a flame on a strip of platinum or placed
in acid to remove the organic substance the appearance of the
valves remains unchanged.

The glass-like valves are quite transparent and it can be readily
observed that the cell contents is much more highly differenti-
ated than in the preceding group. The chlorophyll is deposited
in plastids of definite form, although this color is often masked
by a brown pigment which causes the characteristic appearance
of these plants when associated in masses. The oil drops seen
in the cells are the product of photosynthesis — starch not being
formed. Some species, however, can live upon decaying or-
ganic matter and in consequence contain colorless plastids. In
the free swimming forms the motion consists of an irregular

DEVELOPMENT OF PLANTS

171

gliding movement and this is supposed to be due to ilie expan-
sion and contraction of minute strands of protoplasm that pro-
ject through the pores of the valves (Fig. 96, C, p).

(a) Reproduction of the Diatoms. — It will naturally l)e asked
how can these plants living in glass houses, grow? As the valves
become changed to silica naturally any increase in size must
cease. Nevertheless the cells reproduce with great rapidity and
in a very interesting manner. Through the growth of the living
substance the valves are pushed apart and the cell contents di-
vides, forming two diatoms with but one \alve each. A new
valve that fits into the old valve is soon developed on the un-
covered side of each diatom and two complete diatoms are thus
formed (Fig. 98) which may become free at once or remain at-

]

Fig. 98. Diagram illustrating the divisions of a diatom and the resulting
reduction in size. The brackets connect the two diatoms that were formed
by each successive division. — After Pfitzer.

tached and by further division form the odd groupings or colo-
nies shown in Fig. 94. Since the valves are of unequal size, i. e.,
one fitting into the other, the two diatoms formed !)>• the division
must vary in size and it will be seen that the majority of the off-
spring will become greatly reduced in size as a result of tlie
rapid and repeated divisions. That this reduction nuiy not go
on too far, when a certain minimum size has been reached the
cell contents, with or without dividing, throws ofT the \alves
entirely and grows to the full size of the diatom, wlun new
valves are formed and the diatom is ready to repeat the dividing
process. In some cases we find quite a difterent method of re-
production. Two diatoms become enclosed in a jelK-like mass,
into which the cell contents, with or without dixiding, is dis-
charged. These naked cells now unite in pairs and the bodies
thus formed finally grow into diatoms (Fig. 99)- This latter

172

REPRODUCTION OF DIATOMS

method of reproduction is termed sexual reproduction. The two
fusing cells are looked upon as male and female cells or gametes,
although in these simple plants there is no indication of sex that
would enable us to recognize them as such. In the preceding
groups, multiplication in numbers or reproduction has been
effected by the division of the cells or by the formation of more
or less modified cells, termed spores. All such methods of repro-
duction are called asexual, since but one cell is utilized in the
process. In sexual reproduction a cell is formed by the union
of the contents of two cells. The nuclei and protoplasmic con-
tents of each unite so completely that a cell with but one nucleus
and protoplasmic contents results. These sexually produced
cells are called gametospores because they are formed by the

'%: A

B

Fig. 99. Sexual reproduction of Pinnularia: A two diatoms, enveloped
in a mass of jelly. The valves have been thrown off and the content of
each diatom has divided into two sexual cells or gametes. B, the fusion of
the gametes. — After Karsten.

union of two gametes and are capable of forming a new plant
in the same way as do spores. We may distinguish the spores
derived from single cells as asexually formed spores or we may
call them simply spores, while the spores formed by the fusion
of the two cells or gametes may be termed sexually formed spores
or gametospores. Both kinds of spores are devices to enable
the plant to increase in number or to meet some other problem

DEVELOPMENT OF PLANTS 173

in life, such as conditions unfavorable for growth. The spores
are especially to be looked upon as devices to bring about a rapid
increase in the number of individuals while the gamctospores
more usually serve to carry the plant over periods of drought or
extremes of temperature which w^ould prove fatal to the plant.
For this reason the gametospores are often provided with thick
walls and dense cell contents which enable them to remain in a
dormant state until conditions are favorable for growth. The
gametospore is often called a resting spore for this reason.

So rapidly do the diatoms multiply that the larger part of the
soil and rocks in many localities is composed of their remains.
The constant casting off of the valves through reproduction or
death results in vast deposits, knowm as silicious earth, on the
bottom of ponds and in the sea. Beds of silicious earth formed in
this way are often seen in districts where ponds and lakes have
dried up. Sometimes these beds reach enormous proportions,
the city of Richmond, Va., being built upon such a deposit.
Some of the w-estern beds are quite 300 feet in thickness and >et
it takes about 40 million of these plants to make a cubic inch.
Silicious earth is used as polishing powders and as absorbents in
the manufacture of some explosives.

Subdivision 4. Euphyceae or Algae
65. General Features. — This subdivision includes a large num-
ber of plants that live chiefly in fresh or salt water. These plants,
popularly known as Algae, vary greatly in form and structure
and range from microscopic unicellular forms to some of the larg-
est and most highly constructed plants found among the Thallo-
phyta. Their advance over preceding groups appears especially
in their well-marked walls and distinct plastids and nucki
(Fig. 100, 104). Chlorophyll is always present in the cells,
although in certain groups it is masked by brown or red j)igmenls.
These latter pigments are supposed to adapt the plant to var>'-
ing intensities of light. This is supported b\' the fact that the
marine Algae exhibit a zonal distribution in the water. Forms
living near the surface of the water are predominantly green, at
depths where they are alternately exposed and covered by the

174 ORDERS OF GREEN ALGAE

tide, they are more commonly brown, while in the shade of the
brown Algae, or below tidal limits, red forms occur. The
Algae appear to have developed along three lines, which are
indicated by the green, brown and red colors, although the basis
for this classification rests upon structural and reproductive
characters. These three classes are: A, Green Algae or Chloro-
phyceae; B, Brown Algae or Phaeophyceae ; C, Red Algae or
Rhodophyceae.

Class A. Green Algae or Chlorophyceae

66. The More Important Orders of the Chlorophyceae. — The

Chlorophyceae are one of the most interesting groups of the
Algae, because they contain primitive forms that are at least
suggestive of low animal types and they also exhibit a gradual
modification of the plant body and reproductive processes that
help us to understand how variations arose and how complex
types have been evolved from very simple forms. These plants
are doubtless the most primitive survivors of a line from which
the higher land plants have been derived. Starting with uni-
cellular motile forms, the green algae appear to have diverged
along several lines. Among the more important of these may be
mentioned the following orders: (a) Volvocales, largely uni-
cellular green Algae; (b) The Zygnematales or conjugating green
Algae; (c) Chaetophorales or filamentous green Algae; (d)
Siphonales or tubular green Algae.

67. Order a. Volvocales or Unicellular Green Algae. — The
lower members of this order represent a very primitive type of
plant. Their motility, certain features of their life history and
delicate cell walls are suggestive of some of the preceding groups,
as well as of low forms of animal life. Sphaerella (Fig. 100) is
a familiar example of this group, often appearing in rock hollows
as blood-red stains. If some of this red material is examined
in w^ater, the plants appear as spherical cells with dense red pro-
toplasmic contents and thick walls (Fig. 100, A). This con-
dition represents the resting state of the plant. If the plants,
after being dried, are allowed to stand in water for a few hours,
the nucleus will divide, forming usually from 4 to 16 daughter

DEVELOPMENT OF PLANTS

/o

cells (Fig. 100, B) which escape by the rupturliii; of the old
mother wall and swim actively about h\ means of two cilia.
These cells, or zoospores, remain motile for var>in^ periods,
during which time they increase in size, but finally tlie rilia arc

Fig. 100. Stages in the life history of Sphaerella: A, resting state of the
plant. B, first division. C, second division, the four cells are about to
escape from the mother plant. D, one of the cells of C after escaping. This
is a zoospore of the first generation. E, zoospore at rest. F, forming four
new zoospores. G, one of these zoospores of the second generation. Note
that the red material, represented by the shaded area in the center of the
cell, has become greatly reduced and that the wall is becoming distended and
separated from the granular cytoplasm. H, third resting stage. /, cell
dividing. K, zoospore of third generation with greatly distended cell wall
and small red area. Delicate strands of cytoplasm connect the cell wall
and the central protoplasmic body. L, a resting cell dividing into a larije
number of zoospores which arc consequently smaller. — After Hazen.

retracted and the plants remain in a (luiescent state for a short
period (Fig. lOO, E). A division of the nucleus now results
again in the formation of two or more zoospores 0*'.^'- ^^x^. ^0
which repeat the life history noted above. This metluxi of
reproduction may occur again and again and thus rapidly bring
about a great increase in the number of plants. During tho

176 LIFE HISTORY OF SPHAERELLA

divisions, the red coloring matter is usually rapidly replaced by
chlorophyll until but a red speck remains and the zoospores soon
appear as rather ovate green cells surrounded by delicate walls
which become widely separated from the chlorophyll owing to
the accumulation of water (Fig. lOO, G). Nearly all zoospores
that occur among the algae are characterized by a small red body,
known as the eye spot, which is located near the ciliated end of
the zoospore. It has been supposed that this body is sensitive
to light and gives the zoospore a sense of direction. While this
is questionable, since the eye spot may be lacking (as in Sphae-
rella), it is certain that these beautiful bodies swim about, cili-
ated end foremost, with a rotary motion from right to left and
adjust themselves to a suitable illumination. It can easily be
demonstrated that the zoospores of Sphaerella are keenly sensi-
tive to different intensities of light by placing them in a glass
dish by a window when the zoospores will congregate on the
illuminated side unless the light is too intense, when a reverse
action takes place.

Thus we see that the life of the plants is largely a motile one,
each generation being characterized by a short resting stage
during which division occurs and a longer motile zoospore stage.
In fact, the zoospore may be looked upon as the original state
of these plants, while the resting condition is a departure due
to changes in the environment or the condition of the organism.
This common occurrence of motility in the lower types of life
indicates that possibly such was the condition of the first life upon
the earth.

(a) Conditions Affecting the Life of Sphaerella. — Changes in
the surroundings sometimes produce remarkable variations in
the life of the plant. This fact is well illustrated in Sphaerella.
If the water dries up, a thick wall is formed about the central
protoplasmic body which becomes dense and of a deep red color
while the delicace distended wall and cilia disappear. This varia-
tion adapts the plant to conditions unfavorable to growth, such
as drought and severe temperature. The so-called red snow is
due to a certain species of these plants that are swept off from
the rocks in this resting condition by the winter winds and falling

DEVELOPMENT OF PLANTS 177

to the earth produces red streaks in the snow. Hazen has
been able to show that a most suggestive change in the mode of
life of the plants may also be induced by low temperature or by
a reduction of the volume of water in which they live. Under
such conditions, bodies are formed that resemble zoospores, but
that are not capable of motion owing to the lack of cilia. This
helps us to understand how the motionless or stationary- forms
of plant life came about and how the motile stage became less
and less conspicuous. Owing to such causes as these and many
others, plants become stationary. At first, perhaps, simply an
aggregate of motionless cells, but finally there resulted chains or
filaments of cells owing to the repeated division of the plants in
one plane. By division in tw^o planes, membranous expansions
resulted and the more complex types arose by the division of
the cells in three planes and by modification of the cells.

One other feature appears in the life of the Sphaerclla that
indicates clearly how sexual reproduction came about. When
the conditions for growth are unfavorable, as for example, through
lack of moisture or low temperatures, there result plants that
are not so well nourished, or, at least they appear to lack some
of the qualifications that characterize the plants growing under
favorable conditions. , The same condition often arises in these
plants after several generations have been formed. Such plants,
which may be characterized as weaker, or, at least, lacking in
certain material, do not give rise to the ordinary zor)six)rcs,
but to much smaller though similar bodies that are lacking in
cell w^alls (Fig. 100, L). These small zoiisporcs usualh' perish
or produce small plants unless two of them meet, when a fusion
may occur and thus a plant is formed that is capable of reiH\Uing
the life history of Sphaerella. By suitable nourishment, how-
ever, these small zoospores may sometimes be made to develop
into the normal plant. It is very evident that these small zoo-
spores are lacking in some substance that is essential to their
growth. This is rarely furnished to them in nature exce[)t when
two of the bodies fuse. From numerous examples appearing
among different groups of Algae, it appears that si'xuality arose
in this way. Owing to certeiin conditi(ms of liglit, temixTUunv

178 ORIGIN OF SEX

food, etc., zoospores were formed that were Incapable of further
growth. But, by the union of two of these zoospores, a cell was
formed possessed of renewed vigor and capabilities of growth.
So we can think of the sexual cells or gametes as zoospores
that are lacking In the materials essential to growth and of the
sexual process as a union of the two bodies for the purpose of
bringing together the missing material and supplying the neces-
sary energy for growth.

A closely allied genus, Chlamydomonas (Fig. loi) has a life
history very similar to that of Sphaerella but the formation of
the gametes reveals a variation that gives us an understanding
of how these bodies came to differ and finally became distin-
guishable as male and female gametes. In some of the species
of Chlamydomonas, to be sure, the gametes are similar and can
not be distinguished as male or female (Fig. loi, B). This
stage of sexuality is called isogamous, meaning similar gametes.
In other species, there is a decided variation in the size of the
gametes, certain cells or plants producing large and less active
cells that are termed female gametes, while other plants produce
numerous minute male gametes that swim actively about (Fig.
loi, C, D). This stage of sexuality, where the gametes differ
in size is called heterogamous, meaning unequal gametes. This
variation In the size of the gametes which we call the differentia-
tion of sex Is due to the amount and nature of the material stored
in them, the male gametes being formed In a cell in larger num-
bers than in the case of the females. These heterogamous
gametes do not differ in nature from the Isogamous gametes.
They are also lacking In some substance that renders them In-
capable of growth unless fusion of two gametes is effected.

(6) Colonial Unicellular Algae. — The various stages In the
evolution of sex are also well illustrated in several genera of
unicellular green algae that live together in colonies. These
colonial forms are also of Interest in that they help us to under-
stand how unicellular plants became associated — at first quite
independent of one another — and this led to the dependence of
one plant upon another and finally to a distribution of work
among the plants of the colony, so that certain plants or cells

DEVELOPMENT OF PLANTS

179

would be largely concerned in reproduction and others in the
manufacture of foods, etc. This is essentially the state reached
by our higher plants which may be spoken of as a colony of cells.
Pandorina shows the earlier phases of the differentiation of the
gamete?. This plant is really a spherical colony of al)OUt 16, prac-

FiG. loi. Fig. 102.

Fig. ioi. Features in the life history of Chlamydomonas: A, character
of the motile plant. B, conjugation of isogamous gametes. C, a plant
dividing to form numerous small male gametes. D, a plant forming two large
female gametes. E, male and female gametes about to conjugate.

Fig. 102. Features in the life history of Pandorina: A, a colony of plants.
B, each plant of the colony dividing to form a new colon>'. C, the plants of
a colony escaping as gametes. D, the conjugation of two gametes of un-
equal size. £, later stage in the conjugation. F, gamctospore or resting
spore. G, large zoospore formed from the gametospore. //, a colonx- fi>rmed
by the division of the zoospore, G.

tically independent, motile plants, each one similar to ^piuicrclla
but held together in a mucilaginous mass (F'ig. 102. .1). These
beautiful colonies multiply by each plant or cell (ii\ i(hiiv; into lO
daughter cells which finally become an independent colony
(Fig. 102, B). The gametes are formed in essentially the same
way as the colonies, save that the 16 daughter cells finally escape
as free swimming bodies (Fig. 102, C). The modur plants or
cells producing these gametes difi'er in size, owinu, (loiibtless, to

i8o DIFFERENTIATION OF SEX

the amount of nutriment which they receive. Consequently,
the gametes formed from them must also differ in size. It is
also possible that the mother plants may produce gametes of
different sizes because some divide a larger number of times than
others. The larger the number of divisions in a cell, the smaller
will be the gametes. It is interesting to note that these gametes
fuse quite regardless of their size, although there may be a slight
tendency for the small and more active gametes to fuse with
the larger and slower moving ones. Thus we have an illustra-
tion of the first appearance of the differentiation of sex although
the sexuality of the gametes is not thoroughly established. The
gametospore, resulting from the union of the gametes, develops
a thick cell wall, the contents assumes a reddish color and it
passes into a resting stage (Fig. 102, F). When conditions are
favorable, it germinates as shown in Fig. 102, G-H. in the
beautiful spherical colonies of Eudorina and Volvox (Fig. 103)
the differentiation of the gametes is complete. Certain cells
produce but one large female gamete that remains motionless
in the mother cell while other cells form numerous small motile
male gametes which are yellowish in color. In Volvox, over
200 male gametes may be formed from a single cell (Fig. 103,
D, E). Volvox also shows that the association of plants in
colonies resulted in an interdependence among the plants and
also in a specialization in the work performed by these plants.
Fig. 103, C, shows that the thousands of plants that make up the
colony are in communication with one another and they doubt-
less give and receive material in accordance with the needs of
the colony. Furthermore a few of the cells of the colony receive
more nourishment than the others and these cells increase in size,
divide and form new colonies as in Pandorina. Finally there is
to be noticed that certain cells function in the production of
gametes. So we see here in a simple way some of those speci-
alizations and distributions of labor that will appear as the most
characteristic features in higher forms.

The tendency among the algae to form gametes of different
sizes is manifestly of great advantage. The great number of
male cells increases the chances of fusion of the gametes and their

DEVELOPMENT OF PLANTS

i8i

smallness promotes their movements through the water. Note
also that these results are obtained without an>' additional
expenditure of material — the 200 male gametes being j)n)duced
from a cell no larger than SphacreUa where only a few are found.
The increase in the size of the female gamete renders her move-
ments more difficult, but the storage of food in this gamete pro-
vides for the better nourishment of the next generation. Finally
the nourishing function becomes so strongly dcvelojxd that the
female ceases to move at all and remains protected in the mother

Fig. 103. Features in the life history of Volvox: A, a coloin- which may
contain as many as 20,000 plants. The three central groups arc young col-
onies which may arise by the repeated division of any of the plants. B, side
view of one of the plants showing the canal-like connections with the neigh-
boring plants. C, surface view of a plant with canals radiating out to the
adjoining plants. D, a plant enlarging and forming a single motionless female
gamete. E, a plant forming numerous small male gametes. F, male gametes
enlarged. '

cell. Thus we see that sexuality may have arisen among plants
as the result of definite stimuli as heat, moisture, light, food and
other conditions that occur in the environment. These stimuli
produce enfeebled zoospores that are incapable ordinarily of
growth unless a fusion of two of them is effected. So sexuality

1 82

MOTIONLESS UNICELLULAR ALGAE

arose from the asexual state of the plant as a result of the influence
of the environment and differentiation of sex appeared owing to
the variation in the amount and kind of material in the gamete
producing cells.

(c) Motio7iless Unicellular Algae. — In contrast to these motile
plants, there is a large group of the Volvocales that have lost
their motility and spend a large part of their life as stationary
plants. We have already noticed how this condition may have

Fig. 104. Common forms of stationary unicellular green algae: A, Pleu-
rococcus showing stages in the division of the plants, the nucleus and the
irregular chloroplnst. B, Scenedesmus. Below three of the plants are shown
dividing. C, Ankistrodesmus. At right three plants in process of division.

come about, owing to the temperature and moisture conditions
that were unfavorable for the motile existence. Pleurococcus
(Fig. 104, A) is one of the most common of the green algae
and illustrates many of the features in the life history of the
stationary forms. These plants often cause the green coating
that appears upon the shaded or moist side of tree trunks,
fences and buildings. If a bit of this material is examined under

DEVELOPMENT OP^ PLANTS 183

the microscope, it will be seen that the phmts are rather si)herical
cells provided with nucleus, cytoplasm, chloroplasts and well
marked walls. This latter feature is characteristic of stationary
algae and forms a sharp contrast to the delicate walls of the
motile forms. These plants increase in numbers by the forma-
tion of a wall through the middle of the (x-11. Tlie two daughter
cells or plants grow to the size of tlie mother plant and repeat the
process in division by forming a wall at right angles to the old
wall. It will also be noticed that cell division may occur in
three planes. The plants adhere together for a considerable
period, or become detached shortly after the division of the
mother cell. In this way they multiply with extreme rapidity
and so come to form extensive green coatings. It has been
noted in allied genera and reported in certain species of Plcuro-
cccais also, that changes of temperature and amount of water,
etc., will cause the contents of the cell to divide and form zoo-
spores which are of the same nature and behave in the same way
as in Sphaerella. Thus we see how changes of the environment
may cause these stationary plants to return to the motile condi-
tion of simpler types. It is interesting to note in this connection
that Livingston was able to change the stationary- condition of
certain algae to the motile condition at will by diluting the
solutions in which they were living. Fig. 104, B, C, illustrates
several other forms of stationary green algae that are common
in the drinking water and in ponds and streams. Interesting
forms for study may be obtained by tying a woolen sack to
a water faucet and allowing the water to run into it lor several
hours. Invert the sack into a glass, washing olT the material that
collects on the inside of the sack in the water of the glass. By
means of a pipette, a drop of the material that collects at the
bottom or sides of the glass or forms on the surface of the water
may be transferred to a slide and examined under a microscoi^e.
Some of these forms are free, some attached and others aijgrc-
gated in a mucilaginous mass.

There are a great many of the stationar>' genera tiiat siiow the
same habit of uniting into colonies that we noticed among the
motile genera. These colonies may be ver>' simple and com-

i84

HYDRODICTYON

posed of a few loosely aggregated plants forming flat discs or
star-shaped plates (Fig. 105, ^). In other cases, we find spher-
ical and net-like colonies. The water net, Hydrodictyon, is a
common illustration of this latter arrangement (Fig. 105, D, E).
These plants occur in ponds and sluggish streams, forming sac-
like nets from a few inches to a foot in length, that are constructed

Fig. 105, Colonial forms of unicellular green algae: A, Pediastrum, the
plants of the colony being arranged in a flat plate. B, a view of the outer
cells of the colony showing the formation of a new colony. C, one of these
new colonies. D, a plant of the water net containing a young colony. E,
enlarged view of one of the meshes of a net showing the geometrical arrange-
ment of the plants.

in a very regular manner. The reproduction is quite character-
istic of many of the Volvocales. Zoospores are formed in large
numbers in any of the plants and after a period of motility, they
come to rest and arrange themselves into a new net inside of the
mother plant (Fig. 105, D). The walls of the mother plant soon
break down and the new colony is set free. Under different
conditions, smaller but similar bodies are formed which escape
from the mother plant. These bodies are gametes and incapable
of growth unless a fusion is effected between two of them.

DEVELOPMENT OF PLANTS

IH;

Especial interest attaches to the study of this i)hint because
Klebs was able to change the reproductive character of the plant
by subjecting it to certain definite conditions. By exposing the
plants to bright light, slight increase of temperature and in-
organic culture solutions of definite composition, zoospores are
produced. On the other liand, plants growing in solution of
cane sugar in subdued light or darkness formed gametes.

These reactions are of great importance in helping us to under-
stand the nature of plant life. They demonstrate that defuiite
factors may change motile plants to a stationary- condition and
in Ilydrodictyon we see that measurable factors, as light, heat
and concentration of the solution may bring about the motile
condition of the plant and may also cause a variation in the
motile condition which we call sexualit}-.

68. Order b. Zygnematales or Conjugating Green Algae. —
This order includes the most common and attnictixc forms of
green algae (Figs. lo6, io8). They are of very general occur-

FiG. 1 06. Two common forms of the Zygncmatalc'
71, nucleus; p, pyrenoid. B, Zygncvia.

A , Spirogyra-

rence in the still waters of ponds and streams, whcTt' tlu>- oftt-ii
form the large floating and frothy green masses pc)j)ularl>- known
as pond scum or frog spittle. They can usually be recognized
by their slimy nature which is due to the copious exudation of a
gelatinous substance from the cell walls. In one group of this
order the plants are multi-cellular (Fig. 106), the cells being
attached end to end and forming a filament— though it should be
stated that each cell is practically independent in its life process

i86 REPRODUCTION OF SPIROGYRA

and the organism may therefore be regarded as a colony. Such
a type of plant would have originated in the case of Pleurococcus
if the two daughter cells had remained attached and continued
to divide in but one plane and parallel to the first division wall.
This is exactly the mode of growth in these filamentous plants.
Any of the cells may divide into two daughter cells which grow
to the original size of the mother cells. In this way the length
of the plant is increased. The most characteristic feature about
the Zygnematales is the remarkably large and attractive chloro-
plasts. In Spirogyra (Fig. io6, A) these appear as bands, from
one to several in each cell, spirally arranged just within the cell
wall. In Zygnema (Fig. io6, B) they assume the form of stars
and in other genera they appear as plates, bars and variously
modified bodies. Imbedded in the plastids are denser proto-
plasmic bodies, the pyrenoids, that appear to be connected with
the secretion of starch ; at least a layer of starch can be detected
about the pyrenoids by testing with iodine (Fig. io6, pr).

The filaments are fragile and readily become dissociated into
smaller portions. In this way, the plants may multiply, since the
fragments continue to grow after the manner of the original
filament. Singularly, there is no indication of a return to the
motile condition in the life history of these plants. No zoospores
or motile gametes are formed. Sometimes the walls of a cell
become thickened and after a dormant period, as through the
winter, may germinate and grow into a new plant. With this
rather rare exception, the asexual reproduction of the plant by
spores does not occur. The sexual reproduction is of a peculiar
character. This is very well illustrated in Spirogyra. When
filaments of this plant chance to lie side by side, under certain
conditions, tube-like processes from opposite cells begin to grow
out towards each other (Fig. 107, A). These tubes finally meet,
when the walls at the point of contact are absorbed, forming an
open passage between the two cells. While this growth has been
going on, the contents of the two cells have contracted some-
what and finally the contents of one cell passes through the tube
and fuses with the contents of the other cell. Several modi-
fications of the reproduction process outlined above will occa-

DEVELOPMENT OF 1>LAMS 187

sionally be noted, such as the development of ganicio^jxjres in
either filament or the fusion of the adjacent cells of a filament by
means of lateral tubular outgrowth. In otlicr genera, there is a
considerable modification in the process, as in some sfK-cies of
Zygnema, where the fusion is effected in the middle of the tul)e.

Fig. 107. Sexual reproduction of Spirogyra: A, in lower portion of figure
the formation of the tubes between the opposite cells of the filaments is shown.
Above the contents of two cells have united with two cells of another filament*
forming two gametospores. B, germination of a gami-tosporr.

This is only a different form of the sexual process wiili wliich
you are familiar, the difference being, that but one gamete is
formed in each cell and these bodies are motionless or at least
not provided with cilia and do not escape from the mother plant.
When the fusion of the gametes has been etTected the resulting

i88 GERMINATION OF THE GAMETOSPORE

gametospore becomes surrounded by a thick wall and in this
resting condition it tides the plant over seasons, such as the
winter or drought, unfavorable for the growth of the plant.
When conditions return that permit the growth of the plant,
the gametospore germinates by rupturing the outer wall and pro-
truding the inner wall of the spore as a delicate tube (Fig. 107, B).
Early in this germination the nucleus divides, forming four cells,
the outermost of which only continues to grow and so ultimately
forms the new plant. Note that the gametospore, therefore,

Fig. 108. Common forms of Desmids: A, Closterium. B, Micrasterias,
C, Xanthidium. D, Staurastrum. At left top view, at right side view. E.
Desmidium forming a chain of plants. At right, end view of chain.

does not grow directly into the parent type of plant. This be-
havior of the gametospore is one of the most fundamental
features in plant life. It appears in the preceding groups but
it is mentioned now for the first time because we see here a clear
demonstration of its character. The gametospore is different
in its nature and possibilities of growth from any one of the cells
of the parent plant. It can not produce the spirogyra plant.
It can only produce four cells and from one of these cells there
will develop the spirogyra plant. Further consideration of this
feature in the life history of the plant will be deferred to later

DEVELOPMENT OF PLANTS

189

examples, where it will become still more obvious. The loss of
motility in the reproductive process in this order may possibly
be associated with their exposure to terrestrial conditions.
They are often exposed to the soil, owing to the drying up of the
water and it has been suggested that they have consequently lost
their motility. Their mucilaginous coatings would be of great
service in enabling them to meet such conditions by retaining
moisture, and indeed, they are often able to fl(jurish in many
damp places without the aid of surface water.

The Desmids. — A second group of the Zygnematales are strictly
unicellular plants known as Desmids (Fig. 108). They are the
most attractive of unicellular plants and are of conmion occur-
rence associated with coarse algae. The desmids are elaborately
and variously fashioned but can readily be recognized by the

j^:^^^^^ //'

T K ^,:T

Wm

^n ^

^^•r-jii

Fig. 109. Reproduction of Cosmarium: A, a sexual rcproducti(jn, sIkav-
ing the elongation of the isthmus, i, and its gradual enlargement and divi-
sion to form the new lobes of the desmids. B, sexual reproduction. At
the left the cell contents of two desmids fusing to form a gametosjK)re. On
the right the mature gamctospore covered with a spiny coat.

fact that they consist of two similar hah'es (Fig. 109, .1). The
structure of the cells and the sexual method of reproduction
is essentially the same as in the filamentous forms which have
doubtless given rise to these unicellular j^lants. The asexual

190 SPECIALIZATION IN ULOTHRIX

method of reproduction, however, is rather peculiar. The region
connecting the two halves (Fig. 109, A, i), called the isthmus,
becomes somewhat elongated and swollen. Soon a constriction
appears midway between these two halves which deepens until
the desmid is cut in half. Each desmid now consists of one of
the original halves and one half of the isthmus. This latter
part gradual!}' enlarges and forms the second half of the desmid.
Owing to the gelatinous character of the cell walls, the desmids
often become aggregated in masses and form chains of cells
(Fig. 108, E).

69. Order c. Chaetophorales or Filamentous Green Algae. —
Here belong a great array of green algae that grow attached to
various objects and appear as simple or more usually as branched
filaments which frequently end in hair-like tips. They are of
common occurrence at the bottom or margins of springs, wells,
streams and ponds. Ulothrix may be taken as an example of
one of the simple forms of the Chaetophorales. Fig. no shows
that it consists of a simple chain of cells that divide and so bring
about the growth of the plant as in Spirogyra. However, it is
important to note that we have here a higher type of vegetative
structure than in any of the algae heretofore studied. This
advance appears in the one or more rather colorless cells at the
base of the plant which serve to anchor it to the substratum. In
other words, there is apparent a distribution of labor or the per-
formance of definite duties by special parts or cells of the plant
body. The evolution of higher forms comes about in much this
way. So long as each cell of the plant performs the same func-
tion, there can be little variation in the form and structure. But
as these cells are differently stimulated by their environment,
they tend to vary in character and so become adapted to the per-
formance of different functions. Perhaps in Ulothrix the con-
tact of the substratum acted as a stimulus, as Pierce found to
be the case in many algae. Thus the plant comes to consist of
different kinds of cells, each of which performs a different kind
of work. The complexity of the plant is due to the variations of
its cells which are thereby adapted to the performance of special
duties. In Ulothrix, we have one of the simplest modifications

DEVELOPMENT OF PLANTS

191

of this nature, one or more of the basal cells hecomin.i,^ sli^^htly
changed in form and contents and so adapted in an( horin^ the
plant. The other cells of the plant are praclicalh- alike, each con-
taining a single nucleus and chloroplast and therefore capable of
manufacturing food and forming the reproductive bodies which
are of the same simple type as noted in Sphaerella .

(a) Reproduction of Ulothrix. — The contents of an\- of the
green cells may divide into two or more cells which esca[)e as

10.

Fi(.. III.
10. Lower portion of Ulothrix, the basal cells are somewhat nuHli-

Fig.

Fig.

fied and the lowest one acts as an anchoring organ. Each of the up|K>r crils
contains a girdle-like chloroplast.

Fig. III. Asexual reproduction of Ulothrix: A, a few cells of a fila-
ment in the upper cells of which the formation and escape of the larRe zcxi-
spores are shown, while in the lowest cells a large number of small Z(x>s|K>rc»
appear. B, a large zoospore. C, a young plant formed by /i. P, a small
zoospore. E, a young plant formed from D. — After West.

zoospores through an opening formed in the cell wall (I'ig. m,
A). The cells in which the zotJspores are f<'r"i<'l .«r.- .allrd

192 ASEXUAL REPRODUCTION OF ULOTHRIX

sporangia. The zoospores resemble the motile plants noted
above, but after a short motile period they return to the station-
ary condition and grow into new plants (Fig. iii, C). In this
way, the number of plants is rapidly multiplied. The point
must not be overlooked that this phase of the life history is the
same as that of Sphaerella, the difference appearing in the
shortening of the motile period and in the lengthening of the
non -motile period. During the resting stage the cells of Ulothrix
to be sure may divide in one plane forming a filament but re-
member also that non-motile cells of Ulothrix msiy divide, forming
a mass of cells that become detached or loosely associated — the
so-called palmelloid state — which behavior we have seen sug-
gested in the non-motile cells of Sphaerella. It should be stated
that the asexual method of reproduction may be looked upon
as a means of bringing about a rapid increase in the number
of individuals of a species while the conditions for growth are
favorable. When new conditions arise, such as changes of tem-
perature, food, etc., that would interfere with the normal growth
of the individuals then a modification of the contents of the cells
and its mode of division results. The cells of Ulothrix as a con-
sequence of such alterations divide into a larger number of motile
bodies which are consequently smaller than in the case of zoo-
spores (Fig. 112, A). These bodies may behave as gametes and
unite, forming gametospores. The cells in which gametes are
formed are called gametangia, sing, gametangium. Thus, Ulo-
thrix is controlled in the same manner as Chlamydomonas and
Hydrodictyon. It is interesting to note, however, that these
small bodies frequently behave as zoospores and develop into
small and weak plants, which fact accounts for the common asso-
ciation in this genus of puny and vigorous plants (Fig. iii, C, £).
These small bodies of Ulothrix represent a curious intermediate
condition between a zoospore and a gamete where the sexual
character is not strongly enough developed to overcome com-
pletely the zoospore character. Thus, we see again in Ulothrix
that definite environmental conditions caused motile bodies to be
produced that are lacking in some essential substance and conse-
quently do not have the energy for growth, or at least can only

DEVELOPMENT OF PLANTS

193

develop into feeble plants. This weakness is overcome by the
fusion of two gametes which results in the formation of a cell
or gametospore with renewed energy for growth. \\r also note
how these departures in the behavior of the organism are o{
advantage to it. The zoospores effect a distribution of the
species while the gametospore ensures a continuity of the species
by tiding it over unfavorable conditions.

The gametospore germinates after a period of rest somewhat
as in the case of Spirogyra. The contents of the spore, instead
of giving rise directly to a new plant, divides, f(jrming several
zotispores (Fig. 112, E, F) which after a period of motility

Fig. 112. Sexual reproduction of Ulothrix: A, a few cells of a filament
showing the formation and escape of the gametes. B, gamete. C, D, stages
in the union of the gametes. E, gametospore. F, gametospore germin-
ating and forming the sporophyte or asexual generation, which is a sai -like
like plant containing several zoospores.

become attached to some object and grow into a new i)lani.
The cause of this peculiar behavior of the germinating gameto-
spore is not known. Some change has occurred in it that
renders it incapable of developing directly into a mw plant.
This variation in the growth of the gametospore marks the
beginning of one of the most important departures in the evo-
lution of plant life and it will become more and more significant
as the work proceeds. It is evident that the formation of

13

194 NATURE OF THE GAMETOSPORE

several zoospores, each of which produces a new plant, is a de-
cided advance over those types in which the gametospore develops
directly into but one new plant. Note also that in Ulothrix there
are two kinds of zoospores, one formed from the Ulothrix plant
and the other from the gametospore, the former being frequently
called zoogonidia to distinguish them from the latter. This dis-
tinction is of great importance, because most plants have two
distinct stages or generations in their life history. One is repre-
sented by the plant that bears the sexual cells or gametes and
therefore called the sexual plant or sexual generation, or, in short,
the gametophyte. The real nature of this plant is to produce
gametes, although it may for a time produce zoospores. The
other stage is derived from the gametospore and is called the
asexual plant, asexual generation or sporophyte, since this plant
can only produce zoospores. Among the green algae this dis-
tinction is not very manifest, the Ulothrix plant for example being
the sexual plant or gametophyte and the gametospore or the
little plant derived from it (Fig. 112, F) is the asexual plant or
sporophyte, in this case being merely a single cell. It is very
manifest, however, that this single cell or plant is decidedly
different from the Ulothrix plant because it cannot develop into
the Ulothrix plant, but can only produce zoospores. So we see
that there are two generations in the life history and that they
differ radically in their natures and possibilities of growth. In
higher types of plant life it will be seen that the gametospore
tends to develop into a more and more complex body or plant
and it will be one of the most intresting features of the work to
watch the evolution of this asexual plant.

The higher members of the Chaetophorales illustrate the same
gradual differentiation of the gametes, as has been noted in the
motile forms of the Volvocales. The female gametes become
distinguishable because of their larger size and shorter period of
motility, while the male gametes are small, owing to the large
numbers that are found in a cell or because of the small size of
the cells in which they are developed. Finally, in several of the
genera we find but a single female gamete which remains motion-
less in the cell. This condition is very well illustrated in one of
the genera of the next order.

J\^Y^6ff\U^\io^r y j^;^0lO(/a^

DEVELOPMENT OF PLANTS

195

70. Order d. Siphonales or Tubular Green Algae.-— This

order Includes a lari^^e number of odd forms that are Idamentous
in character and they differ from al^ae previously noterl in that
the filaments contain numerous nuclei, but with rare exceptions
no cell partitions. Such plants are called coenocytes. They
assume various forms and often resemble a small plant with stem,
root and leaf, but in all these cases the plant is essentially a huge
cell or tube without partitions and containing numerous nuclei.
Most of the Siphonales are marine, but one genus, Vauchcria, is
well represented in shallow streams, damp places and on the
earth of flower pots in greenhouses, where it forms rather coarse
green felt-like masses. The plant body consists of long tubular
threads, often branching and anchored to the ground by colorless

Fig. 113. Structure and asexual reproduction of Vaucheria: A, portion
of a plant showing the branching tubular filament and colorless root-like
outgrowth, r, B, end of a filament enlarging to form a zoospore. C, zoospore.
D, germination of a zoospore.

outgrowths (Fig. 113, A). The protoplasm forms a thick lining
layer on the inner wall of the filament and embedded in it are
numerous minute chloroplasts, nuclei and oil drops. A water\-
cell sap fills the center of the tube. Partitions onl>- appear when
reproductive bodies are formed or to close 1 wdimd in (.isc of

196 SEXUAL REPRODUCTION OF VAUCHERIA

injury to the filaments. Asexual reproduction is effected by very
large zoospores which are developed in the enlarged extremities of
the filaments (Fig. ii^, B). These tips are cut off by a transverse
wall and a single zoospore escapes through an opening in the
tip of the sporangium thus formed. The entire surface of the
zoospore is clothed with cilia arranged in pairs, each pair being
associated with a nucleus so that the zoospore resembles the
motile colonies previously noted (Fig. 113, C). After a very
short motile period, the zoospore comes to rest and grows into
the characteristic tubular plant (Fig. 113, D). When the plants
are exposed to too dry conditions, the tips of the filaments often

Fig. 114. Sexual reproduction of Vaucheria: A, portion of a filament that
has formed two branches which have grown into a male, an, and female, og,
gametangia. B, later stage, the gametangia have opened, permitting the
escape of the male gametes and the fertilization of female gamete. C, gam-
etospore detached from the filament.

enlarge and finally become detached as motionless spores that
germinate when conditions are again favorable.

The male and female gametes are produced in gametangia that
are formed from short branches. The males are developed in
large numbers in curved branches that become cut off from the
filament by a cross wall (Fig. 114, A) and the gametes finally
escape through an opening that forms at the apex of the branch.
A gametangium that produces clearly differentiated male gametes
is called an antheridium (plu. antheridia) and the gametes are
frequently called antherozoids or sperms. A single female

DEVELOPMENT OF PLANTS

197

gamete is found in a similar branch which becomes rather egg-
shaped and at maturity opens at the beaked end (Pig. 114, B).
A single-celled female gametangium is called an oo.uonluin and
the female gamete is frequently referred to as the o()si)here or
egg. The male gametes are discharged a few minutes after the
oogonium opens, when they swarm al)oul tlie open end of the
female gametangium and readily enter it. As soon as fusion has
been effected the gametospore becomes invested with a thick wall
and in this condition can endure a limited drought. In germi-
nating it develops directly into a new plant. This feature of the

Fig. 115. Growth and asexual reproduction of Oedogoyiium: A, young
plant showing basal cell modified as a hold-fast. B, two cells of a filanu-nt,
in one of which a zoospore is forming and the other cell has opened, jht-
mitting the escape of the zoospore shown at C. D, zoospore at rest. It
becomes attached to an object at its colorless, ciliated ind. K, later stage
in the germination of the zoospore. — After West.

life history of Vaucher'ia, therefore, appears as of a mori: i)rimiti\e
nature than in the case of Ulothrix. In tlie following studies nou
will repeatedly notice variations of this nature. In ilu- (k'Ntloj)-
ment of plants advances are often confined to one or another
feature of their organism which may become highly develofx^d
and specialized; at the same time one or more features may be
retained that have been subject to little or no \ariation and
which, therefore, remain in a iM"iniitive state.

71. Other Members of the Green Algae.- Tlure are several

198

HIGHER TYPES OF REPRODUCTION

orders of the Chlorophyceae, some of which are marine, that
cannot be considered at this time. Two genera, however, de-
serve attention because they show significant advances in the
evolution of plant life. In Oedogonium, a member of a small
order, the Oedogoniales, we find a still higher form of the sexual
reproductive process. These plants are of very common occur-
rence in ditches, streams and springs where their filaments form
for a time greenish masses and coatings upon various objects,
but finally become detached and free-floating (Fig. 115, A).
Any of the cells of a filament, save the basal one, may form a
single zoospore, which are large pear-shaped bodies with a narrow

Fig. 116. Sexual reproduction of Oedogonium: A, portion of a filament
in which a female gamete has been formed — 0, opening in cell wall for en-
trance of male gamete. B, portion of filament showing formation and es-
cape of male gametes. C, gametospore free from mother cell. The germi-
nation of this spore results in the rupture of its outer wall and the protrusion
of the contents of the spore as four cells which are for a time retained by the
delicate inner wall of the spore, as shown in D. E, the four cells have become
mobile zoospores and the delicate inner wall of the gametospore is greatly dis-
tended and about to rupture. F, G, stages in the germination of the zoospore.
— ^After Hirn.

colorless end around the base of which arise a circle of numerous
cilia (Fig. 115, B, C). These zoospores develop into new plants
(Fig. 115, D, E) as in previous cases. The sexual reproduction
presents a high degree of specialization. The male gametes are

DEVELOPMENT OF PLANTS

199

usually formed in pairs in small cells and arc similar to but
smaller than the zoospores, while the female gametes are de-
veloped singly in the ordinary cells of a filament which become
greatly enlarged and spherical or void in form (Fig. 116, A, B).
The female gamete is motionless but the colorless region on one
side of the gamete certainly suggests the idea that this body is

Fig. 117. Fig. 118.

Fig. 117. Coleochaete, showing the radiating filaments and hair-Hlcc out-
growths of the cells.

Fig. 118. Sexual reproduction of Coleochaete: A, end of a filament bearing
the male, an, and female, og, reproductive organs. B, the gametosix^re is
being enveloped by the adjacent filaments of the plant. C, gametospore
germinating. Eight cells have been formed, rupturing the walls of the gameto-
spore.— After Oltmann.

related to the zoospores and male gametes, but tiuu 11 lia> K'>i
its cilia. At maturity, openings of various kinds appear in the
oogonium through which the male gametes enter and one fus^^s
with the female (Fig. 116, A, 0). The thick-walled gametosiK>re
that results from this union is set free by the decay of the oogonial
walls and germinates after a resting period ver>' much as in the

200 ADVANCED TYPE OF REPRODUCTION

case of Ulothrix. The contents of the gametospore is extruded
through the ruptured wall as a delicate sac and then divides,
forming four zoospores which produce new plants (Fig. Ii6, C-G)
— thus we see again an illustration of the peculiar nature of the
gametospore as in Spirogyra and Ulothrix. The question will
naturally arise as to the force that brings these and other gametes
together. The frequently observed aggregation or sw^arming of
the male gametes about the females renders probable the view
that some substances, as organic acids, etc., are developed in the
gametes that serve to draw them together whenever they come
within a certain distance of each other.

The consideration of Coleochaete, a plant of the same order as
Ulothrix, has been deferred to this point because it presents
several interesting departures from previous types which indi-
cate that a higher point has been attained in some respects by
this plant than by any other of the green algae. The filaments of
Coleochaete have a pronounced apical growth and are usually
associated together in a radiate manner, forming small discs or
cushion-like masses on the stems and leaves of water plants (Fig.
117). The advance of this type is indicated not only by the
localization of growth at definite points, but also by the formation
of the zoospores and gametes in definite regions, i. e., in special
cells which are usually located at the end of the filaments. The
zoospores are produced singly from such cells, and from smaller
pear-shaped cells single male gametes are formed. The female
gametes are developed singly in large flask-shaped cells, access
to which is afforded by an opening that appears at the end of
the long neck of the flask (Fig. 118, A). The gametospore de-
velops a cell wall and becomes enveloped by the adjoining cells
of the filaments (Fig. 118, 5). In this condition the winter is
passed and in the spring it germinates, forming neither a plant
like the parent type nor zoospores as in Ulothrix and Oedogonium,
but instead, the gametospore forms a number of cells. This
growth ruptures the coat of the spore and finally from each cell a
rather irregular zoospore (Fig. 118, c) is derived that develops
into a small plant. The plants thus formed multiply solely by
zoospores until finally, after several generations, larger plants

DEVELOPMENT OF PLANTS 201

are produced that bear gametes, tlius coniplcliiiK the Hfe
history.

72. Noteworthy Features of the Chlorophyceae. Two features
in the study of green algae should he kept clearly in mind,
because they are closely connected with the tendencies that will
appear in the development of the mosses. First : The ganicto-
spore is essentially a dormant or resting cell that tides the life
of the plant over the conditions unfavorable for growth. St»cond :
In passing from lower to higher types, the gametosporc tends
to vary in its nature and possibilities of growth. At first it
forms directly a sexual or gamete-bearing plant as in SphaercUa
but in higher types, it develops a generation of zoospores from
which the sexual plant is derived as in Ulothrix and Oedogonium.
Finally in Coleochaete, we find a further advance in that^ several
non-motile cells are formed by the gametospore. These cells,
to be sure, are essentially like the zoospore because they develop
directly into zoospores, but this condition emphasizes the fact
that the nature of the gametospore is steadily departing from
that of the gamete-bearing plant.

This development could be summarized by sa\ing that there
is a tendency to separate the life history of the plant into two
stages ; a sexual or gametophyte generation and an asexual sporo-
phyte generation. These two generations follow each other in
the life history of all the higher plants. This peculiar relation-
ship is called the alternation of generations and the reason for
it will be more apparent when we come to study the mosses.

Class B. The Brown Algae or Piiaeopiivceak

73. General Features. — With few exceptions the Phaeoph> v v ..v
are marine plants. They are commonh' known as brown algae,
owing to the brownish pigments which conceal in a measure
the chlorophyll, thus producing their characteristic brown or
yellow color. They are of common occurrence along rocky
shores and attain enormous dimensions in the northern and
southern seas and on the Pacific Coast. As a rule, the plant
body is better differentiated than in the green algae and shows
a higher type of specialization than yet seen. It would appear

202 SIMPLER BROWN ALGAE

probable that the brown algae have been derived from green
algae but there is no satisfactory evidence of a series of unicellu-
lar forms leading up to the rather complex types that are repre-
sentative of the simplest of the Phaeophyceae. The majority
of these lower forms have already reached the filamentous stage
and many of the higher genera exhibit a differentiation of the
plant body suggestive of the higher plants, as for example, a
definite axis with branches and leaf-like outgrowths and root-
like organs that anchor the plants to the substratum. Likewise,
the tissues of these higher types often reveal many of the features
already noted in the epidermis, cortex and central region of
terrestrial plants. The life history of the brown algae indicates
that they have undergone the same evolution as the Chloro-
ph3^ceae. For purposes of comparison we will consider only
two groups of the brown algae.

(a) The Simpler Brown Algae. — Ectocarpiis may be taken as
an example of this group. It is a filamentous branching form
that is almost universally distributed along the sea shore (Fig.
119). The zoospores are developed in certain cells of the upper
branches. It is important to note in the brown algae, that
the reproductive cells are generally confined to special parts
of the plant and not promiscuously developed as in the green
algae. This marks a decided advance in the evolution of the
plant. The tissues are becoming more specialized and the work
performed by the plant is apportioned to special cells and organs.
The zoospores have essentially the same structure and mode of
growing into new plants as noted in Ulothrix. The cilia, how-
ever, are laterally attached and this is a characteristic feature
of the zoospores of the Phaeophyceae generally (Fig. 119, A, g).

The sexual reproduction of Ectocarpus exhibits some of the
most instructive variations to be found in all the algae. The
gametes are formed in gametangia that become divided into
great numbers of very small cubical cells, each one of which
produces a single gamete (Fig. 119, B). Thus the origin of
the gametes is more specialized than in the case of the green
algae where numerous g metes were formed in a single cell.
You are to remember this feature of the gametangiu be ause

DEVELOPMENT OF PLANTS

203

nn,e of its-Characteristics wil reappear in liryophyta Tl>e
tr "n£ ro. tl: ;l:e JL. ./..erentiati.. as

rangia, .f; «, zoospore <="'^;S^<'- .^^^^'^^^d by s'^' ral still .notilc g..mc.«

i. Ulo^nri. since they --^-^ ^'r^rcrildtt
forming the plant, or they may behave as gan

204 SEXUAL REPRODUCTION OF ECTOCARPUS

form a gametospore. The nature and character of these bodies is
so imperfectly estabHshed that their behavior is entirely deter-
mined by external conditions. Low temperatures and bright
light tend to develop these bodies as zoospores, whereas, high
temperatures cause them to behave as gametes. (See Pandorina
and Hydrodictyon.) Other species of Ectocarpus show an ad-
vance over this stage. The gametes are alike, but it has been
observed that certain ones, the female gametes, have a shorter
period of motility and after coming to rest they attract the still
motile gametes, the males, and cause one to fuse with them
(Fig. 119, C, D). This is the simplest distinction that can be
pointed to as indicating a difference in the nature of the gametes
which we call sex. This variation in the period of motility of
the gametes must be due to an essential difference in the material
or substance of which they are composed, although there is no
external evidence of this. Certain species of this same genus
also reveal a variation in the size of the cells from which the
gametes are derived. In these species, the sex is clearly indi-
cated by the larger size of the female cell and it is noticeable
that there is a decided tendency for the smaller gametes to fuse
with the larger and more slowly moving females. It is also
worthy of note as these gametes become differentiated and their
sex more evident that they are less able to behave as zoospores
and grow directly into new plants. When the gametes produced
by the plant are all alike they may readily be made to grow into
new plants, but this is rarely the case where they can be dis-
tinguished as male and female. Thus, in Ectocarpus, we have
a most remarkable series of variations that indicate how sexuality
has arisen from the asexual condition and also how sex finally
became characterized by a shorter motile period in the female
gamete. The character of the female finally became more pro-
nounced, owing to its better nourishment and consequent increase
in size and slower movements. A more perfect illustration of
the evolution of sex is not found in nature.

The differentiation of reproductive parts, as will be frequently
noted, does not keep pace necessarily with the evolution of the
plant body. It is evident that sexuality has arisen independenlty

DEVELOPMENT OF PLANTS

205

no different levels in the plant world and in groiii»> in no wise
cantected. We have noted the same state of sexuality in some
of the motile green algae, i. c, in Chlamydomonas and in non-
motile forms as Ilydrodidyon and Ulolhrix, and now a^ain in
Ectocarpiis although these plants arc widel>- sei)araled as far as
relationship is concerned and exhibit a marked \ariati()n in the
development of the plant body. It is also frecjuently to be
noted that plants may get along very well indeed with onl\- a
sexual or an asexual method of reproduction.

{h) The Coarser Brown Algae, the Kelps. — Under this head
we may consider two groups, illustrated by the keli)s and rock-
weeds. The kelps are related to Ectocarpiis and include tlie
largest and most highly organized forms of all the algae. Indeed
some of these forms are quite comparable in size with our shrubs
and trees. The Laminarias (Fig. 120, A) of our Atlantic coast

Fig. 120. Three of the larger brown algae: A, Laviiiiaria. B. Lcs^otm

C, Macrocystis.

have stalked blades ten to twenty feet long. The i^reat bladder
kelps of the Pacific, Nereocystis and Macrocystis, attain ^nwt
dimensions, the latter genus reaching a length of 5ck) to i><h) feet
and Lessonia with trunk-like stems and leaf-like segments forms
veritable submerged forests in the Antarctic Ocean (I-'ig. 120,
B, C). These highly organized plants, howe\er. do not show
any advance in sexual reproduction over the simplest fc^rm noted
in Ectocarpiis. Much attention has been directed to the kelps

206

COARSER BROWN ALGAE

as a source of potassium (see page 48), from 23 to 48 per cent,
of their dry weight being potassium chloride. It is estimated
that there is between 600 and 800 sq. miles of these plants on our
Pacific coast and that they should yield annually one million
tons of potassium chloride, valued at 40 million dollars.

The rock weed or bladder wrack {Fucus) and the gulf weed
(Sargassum) are representatives of the second group, Fucaceae,
that contain the most specialized of the brown algae (Fig. 121).
The Sargassum with its stem and leaf-like organs which may
become modified into air sacs and reproductive organs bears the
closest external resemblance to the higher plants of any of the
algae (Fig. 121, A). It forms the major portion of that floating

r»^ .

Fig. 121. Two common forms of the Fucaceae: A, Sargassum, the stem-
like axis bearing air sacs, s, and leaf-Hke organs; g, reproductive branches. B,
Fucus — s, air sacs; g, reproductive branch. C, young plant.

vegetation in the Atlantic known as the Sargasso Sea. The
bladder wracks may be found firmly attached to the rocks by
disc-like holdfasts in almost all colder, temperate and northern
seas (Fig. 121, B). The elongation of the flat leathery stems
is largely localized in a terminal cell and results in a regular

I

DEVELOPMENT OF PLANTS 2(>7

forking of the stem Into two equal i)arts, a niethocJ of branching
called dichotomy in contradistinction to the axial branching
characteristic of the majority of our llowcrin^^ plants. Fucus,
like many of the gross brown algae, contains air cavities or
bladders which buoy it up in the water; this feature accounts
for its popular name of bladder wrack. A cross secli(m of the
stem shows that the tissues of these plants have attained a con-
siderable differentiation (Fig. 122, A) as is attested bv a rudi-

FiG. 122. Structural features of Fucus: A, cross-section of a i>ortion of
the central stem-like part of the plant, showing an epidermal, e, cortical,
cr, and central region, c. B, section of one of the cavities that ai^KMrs to
the eye as a dot. See Fig. 121, B, g. This cavity contains only male game-
tangia. C, section of a cavity from another plant contains only female game-
tangia. — After Oltmann.

mentary epidermal, cortical and central region, the latter oltcn
containing well-marked sieve tubes. These elongated cells of
the central region promote the rapid distril)ution of materials
and doubtless account in part for the size obtained b>- the kelps
and rockweeds.

208

SEXUAL REPRODUCTION OF FUCUS

The most characteristic feature of these plants, however, and
the one separating them sharply from the Laminarias appears
in their method of reproduction. None of the Fucaceae develop
zoospores, although an asexual reproduction may be effected by
the detachment of small branches from the plants. Sexual re-
production is a step in advance of any of the preceding types
of the brown algae, in that the female gamete becomes still
larger and loses entirely the power of motion. The reproductive
organs, or gametangia, are developed in specialized branches or
enlarged tips of the thallus. In Fucus (Fig. 121, g), these organs
are contained in small pits or cavities that appear as minute
points, or as dots when the enlarged tip of a branch is held up
to the light. A magnified section taken through such a branch
shows the nature of the cavities (Fig. 122, B, C). In some
species, the male and female gametangia are found in the same
cavity, or they may occur separately and on different plants.
The male gametes are developed in enormous numbers in numer-
ous little sacs, or antheridia, borne on branching filaments of cells

Fig. 123. One of the branching filaments from Fig. 122, B, greatly en-
larged. Some of the antheridia, an, have discharged the male gametes, which
are still retained in the inner wall of the antheridium, as at h. At a this wall
is ruptured, freeing the gametes.

which grow out from the sides of the cavities (Fig. 123). The
female gametes are produced in larger sacs or oogonia, which
are each supported on a single cell and associated with hair-like
chains of cells, paraphyses (Fig. 124, A). Usually eight gametes
are developed in each oogonium. The gametes when first dis-
charged from the antheridia and oogonia are enclosed in a delicate

DEVELOPMENT OF PLANTS

209

cell wall which later ruptures and sets them free (Fig. 124, B).
The female gametes are without cilia and as they float away in
the water they appear to attract the male gametes which swarm
about them and finally fertilization is effected by one of the male
gametes working down to the nucleus of the female and fusing
with it (Fig. 124, C). In an allied form not only is an attractive
substance formed in the female gamete, but probaljh- a repellent

Fig. 124. Female gametangia: A, greatly enlarged view of one of the
GOgonia shown in Fig. 122, C. The oogonium is dividing and forming the
female gametes; p, paraphyses. B, the female gametes discharged but still
retained in the inner wall of the oogonium. C, greatly enlarged view of a
gamete which is surrounded by male gametes, some of which are seen as
dark bodies penetrating the cytoplasm of the female gamete. D, early stage
in the germination of the gametospore. See later stage. Fig. 121, C. — After
Thuret.

one after fertilization is effected, for Farmer observed thai the
male cells swarm about the female cells for a time and then sud-
denly swim away "like a flock of frightened birds." As soon as
fertilization has been effected, the gametospores become invested
with a cell wall and attached to the rocks. (\11 diNi-icn now
proceeds rapidly and soon establishes the characteristic thallus
of the plant (Figs. 124, D\ 121, C). Fiicus is a very jm-.U^^'- tilitir
14

2IO THE RED ALGAE

and the reproduction process can be observed at almost any time.
After the plants have been exposed for several hours by the low-
tide the male and female gametes will often be found forming
orange yellow and olive green drops at the mouth of the cavities.
In this condition the entire process of fecundation and early stages
of germination can be studied by transferring a bit of these two
fluids to a drop of sea w^ater on a slide and studying them under a
microscope. The differentiation of the gametes of Fucus is an
interesting one because it represents the stage where the female
gamete has become motionless, but is not retained in the mother
cell as in Vaucheria and Oedogonium.

No resting spores are found among the brown algae. This
may be connected with the more uniform conditions that ob-
tain in the sea where also they are not exposed to the dangers
of desiccation as in fresh water forms. It is notew-orthy, that
although the reproductive organs are, on the w'hole, more com-
plex, fertilization is of a more primitive character than among
many of the green algae, in that the fusion of the gametes is
effected outside of the gametangia.

Class C. Red Algae or Rhodophyceae
74. General Features. — The Red Algae are largely marine
plants. Unlike the brown algae, they reach their greatest de-
velopment and abundance in the warmer waters of the temperate
and tropical seas and are usually found attached to various ob-
jects below tidal marks. Their red pigments probably adapt
them to the feeble illumination of the deep w-aters in w-hich they
generally occur. They range through a great variety of forms,
from delicate filaments or flattened ribbon-like bodies to struc-
tures wdth cylindrical axes and leaf-like branches (Fig. 125).
The elegant symmetry of their branching together with the deli-
cacy of structure and richness of coloration has always attracted
attention and made them the most familiar of all the algae.
They are popularly though inaccurately known as sea mosses.

Reproduction of the Red Algae. — The reproduction of the
Rhodophyceae, particularly the sexual method, presents so many
modifications and specializations that only the general features

DEVELOPMENT OF PLANTS 211

can be pointed out. Singularly both the spores and ^^mctes
have lost their motility. The sexual method of rej)rodueiion is
the most complicated among all the algae, and e\'en in the
simplest form is far in advance of any nutiiod heretofore noted.

§||^>•/fe^||i..

Fig. 125. Common forms of the Red Algae: A, a leaf-like form, DeUs-
seria. B, a branching thread-like form, Ceramium. C, a branching form
covered with delicate hairs, Dasya.

The female gametangium resembles a flask with a long neck and
is usually developed at the end of a branch (Fig. 126, .4), see
Coleochaete. The single female gamete is found at the base of
the flask. The male gametes are produced singl>' in small cells
also at the ends of branches and they are often closely aggregated
in dense clusters (Figs. 126, an\ 127, A, B). After the discharge
of the male gamete, they are carried b>- the currents of the
water to the long tube of the female organ. The wall of the tube
at the point of contact is now absorbed and the male gamete
passes down the tube and fuses with the female. The gameto-
spore does not germinate and produce a new plant similar to the
one that bore the gametes. On the other hand, it produces a
number of branches (Fig. 126, B, C), the terminal cells of which
develop as spores (Fig. 126, D). The mode of sexual repro-

212 SEXUAL REPRODUCTION OF RED ALGAE

ductlon outlined above is complicated in the majority of the
red algae, owing to the fusion of the germinating gametospore
with adjacent cells that contain storage foods and from certain
of these storage cells the characteristic spore producing branches
are developed. In this way, the formation of a larger number
of spores is made possible by a single fecundation. Frequently

Fig. 126. Sexual reproduction in Nemalion: A, tip of branch bearing
female gametangium, c, and cluster of male gametangia, an, from some of
which the motionless male gametes are escaping. B, first division of the
germinating gametospore, g. C, later stage showing the early formation
of the branches from which the spores will be developed. D, spores forming
at the ends of the numerous branches.

a sac-like structure, the cystocarp, is developed about the spores
owing to the outgrowths of the adjacent cells (Fig. 127, C). Atten-
tion should be fixed upon the form of the female gametangium and
the sac-like structure of the cystocarp because you will see these
features reappear in certain groups of the fungi.

The spores formed from the growth of the gametospore are
finally set free and grow into plants resembling the gamete
bearing red algae but singularly these plants only produce
spores — never gametangia. These spores are formed in fours
in a single cell (Fig. 128) and therefore called tetraspores. These
spore-forming cells arise singly on the surface or in the tissues

DEVELOPMENT OF PLANTS

213

Fig. 127. Features in the reproduction of Polysiphonia: A, branch of
the plant bearing clusters of male gametangia, an. B, one of the clusters
enlarged, showing the numerous small gametangia. C, branch on which the
spores derived from the gametospores are enveloped by a sac. From one
of these bodies, called cystocarps, the spores are escaping.

of the plant, or they may be associated in conspicuous groups.
The spores escape from the mother cell as naked bodies and

Fig. 128. A branching filament of Callithamniou, showing the spore
mother cells forming tctraspores.

214 LIFE HISTORY OF ALGAE

finally secrete cell walls and develop into new plants. These
plants, however, normally bear only gametangia and not tetra-
spores. So in this life history we have two similar plants appear-
ing but they are manifestly different in nature as one produces
oprastetres and the other gametangia.

The red algae are of considerable economic importance.
Irish moss, CJiondrus, is used in the manufacture of jellies, and
agar-agar is obtained from several species of algae. Many tons
of various kinds of red algae are annually dried and consumed
for food in the East. The sw^allow's nest, of which you have
heard so much as an article of food in the Orient, is constructed
of algae.

75. Significant Features in Life History of the Algae. — The
germination of the gametospore in the Rhodophyceae is a note-
worthy departure in two respects. Unlike previous cases, it is
retained on the plant, where it is nourished during its germination
and grows practically as a parasite. This relation of the gameto-
spore to the mother plant will become more noticeable among
the fungi and the mosses and lead to pronounced changes in the
life history of the plant. In the second place, we notice that the
gametospore in the red algae produces a number of cells before
the spores are formed. In the green and brown algae it either
developed directly into a plant similar to the mother plant, as in
Vaucheria and Fucus, or zoospores w^ere first formed w^hich de-
veloped into a plant similar to the parent, as in Ulothrix and
Oedogonium. What is the significance of this variation in the
germination of the gametospore? Attention has been directed
in the study of Spirogyra and Ulothrix, p. 193, to the fact that
there are two phases in the life history of a plant, a spore-bearing
phase or generation, and a sexual phase or generation. This is
doubtless true of all forms characterized by a sexual reproduction.
On page 136 you have noticed that these two phases or genera-
tions are also sharply distinguishable by the number of chromo-
somes appearing in the nuclei of the cells, the sexual generation
having only half the number found in the asexual. This change
in the number of chromosomes occurs at two points in the life
history, first, when the gametes unite to form the gametospore,

DEVELOPMENT OF PLANTS

215

at which time the number of chromosomes is (loul^lfd, and,
secondly, when the spore mother cell of the asexual Kt-neration
divides to form the four spores, tlie number of cliromosomes is
reduced by one-half. The two generations arc generally not
apparent in the Green and Brown Algae, because the asexual
generation is greatly reduced and often does not produce spores,
as in Vaticheria, etc., where the asexual generation consists of a
single cell, the gametospore. This gamctospore, however, must
have twice the number of chromosomes as the mother j^lant, since
it is formed by the union of the chromatic substance of two
gametes and it will doubtless be found that its first divisions in
germination are attended with a reduction of the chromosomes
to the original number found in the mother plant. This remark
applies also to Ulothrix and Oedogonium, where it is reasonable
to suppose that the formation of the four zoospores by the germi-
nation of the gametospore results in the reduction of the chromo-
somes to the number found in the mother plant. This has been
found to be the case by Allen in the division of the germinating
gametospore of Coleochaete, and Yamanouchi reports that the
reduction of the chromosomes in the Red Algae does not occur
until the mother Ciell divides to form the tetraspores (Fig. 128)
and the same reduction has been reported in Spirogyra in the
germination of the gametospore. The red algae give us the
best illustration of this relationship yet seen. Here the gameto-
spore germinates, producing a small spore-bearing plant, enclosed
in the cystocarp, which is parasitic upon the sexual plant. This
plant is a part of the asexual generation, as all its cells contain
the double number of chromosomes. The spores from tlie
cystocarp form plants externally similar to the sexual plants,
but these plants are really different, being characterized b\- cells
with the double number of chromosomes and also l)>- tiie fact
that they bear tetraspores. The formation of the telrasiwrcs,
however, is attended with a reduction of the chromosomes by
one-half, and these spores produce plants characterized by the
reduced number of chromosomes and also by ilu- tact ili.it they
bear the sexual organs. So in the Red Algae we have very
clearly brought before us the two generations in ilu- life of the

2i6 THE TRUE FUNGI

plant, the sexual generation, characterized by the reduced
number of chromosomes and the production of the sexual organs,
and the asexual generation, characterized by the double number
of chromosomes which appear in two spore-bearing plants, i. e.,
the minute plant in the cystocarp and the tetraspore-bearing
plant. The distinction between the sexual and asexual genera-
tion is emphasized at this point because we see in the algae how
it gradually became more and more conspicuous, and in the
mosses and succeeding groups it will be noticed that the relation
of these two generations is intimately associated with the evolu-
tion of the higher types of plant life.

^^rj Subdivision 5. Eumycetes or True Fungi

76. The Nature of Fungi. — The Fungi are the largest group
of the Thallophyta and include such familiar forms as moulds,
mildews, toadstools and mushrooms. The absence of chloro-
phyll is the most striking feature of these plants. They are
unable therefore to form sugar, starch and other foods from the
elements of the soil and air and must obtain them already manu-
factured. Consequently, they are either saprophytes living upon
decaying organic matter, or parasites preying* upon living organ-
isms. It is quite possible that this mode of life is responsible
for the disappearance of the chlorophyll. You have seen that
the accumulation of sugar in the chlorophyll bearing cells stops
their activity. So here the absorption of organic material from
plants and animals may have had the same result and so led to
the disappearance of chlorophyll. The majority of fungi procure
their food from decaying plant and animal matter, and in this
relation many of them are of the same economic importance as
the saprophytic bacteria. Other forms exist as parasites upon
living plants and animals. In this relation fungi cause almost
incalculable loss annually to the country and frequently crops
over large areas are ruined b}^ their depredations. The parasitic
forms gain access to the host through a wound, stoma or the
delicate tissues of seedlings or young parts of the plant, and in
other cases the fungus forms a ferment that dissolves the cell
walls and thus opens the way for the entrance of the pest. The

DEVELOPMENT OF PLAM S 217

cultivation of plants has doubtless weakened their resistance in
many cases to the attack of the parasites, and like ourselves the
plant may inherit a weaker constitution that is more subject to
disease. Nearly every state now employs experts to study the
diseases caused by these fungi and to devise means {ot killing
them and to develop more resistant plants.

The fungi are of very simple structure because they li\c ujkui
foods already manufactured for them. There is no longer a
necessity for a plant body that is complex, because each part is
adapted to the performance of one or another of the many func-
tions that cooperate in the construction and distribution of the
organic substances. Consequently, the fungi do not exhibit
many of the characteristics of chlorophyll-bearing plants. Their
cell walls are thin and inclose a watery, colorless protoplasm in
which are usually dispersed many small nuclei (Fig. I2()). W'hat-

FlG. 129. A few cells from a branching filament of green mouUi, Pent-
cillium, showing the granular character of the cytoplasms and the absence
of plastids. The colorless areas in the cells, vacuoles, contain principally
water.

ever form the plant body may assume, it will usually be found to
consist of filaments of delicate cells or tubular growths, called
hyphae (sing, hypha) . These fine filaments or tubes are the essen-
tial portion of any fungus and as they spread over the substance
upon which they feed they form a branching and intenvovcn
mass of threads collectively known as the m> r(>liuni. This struc-
ture is well illustrated in the hyphae that sj^read over bread and
fruits, forming a cobwebby mass or mycelium that is commonly
known as mould or mildew. Bodies of ^'arious forms arise from
the mycelium which are often mistaken for the fungus itself, as
the mushroom, but this mushroom sustains the same relation to

2i8 CLASSIFICATION OF FUNGI

the fungus proper (the mycelium) as does an apple to the apple
tree. The mushroom is simply a rather compactly interwoven
mass of hyphae that arise from a mycelium in the ground.
We shall expect to find remarkable departures in the fungi
from previous types because of their peculiar manner of ob-
taining food and also because they are largely terrestrial plants
and therefore exposed to a much wider series of conditions than
in the case of the algae. This will tend to cause variations and
so bring about new structures that are in harmony with the con-
ditions under w^hich the plants live. As a rule, these variations
have been so extensive and have so completely changed the char-
acter of the plffht as to render it impossible to state whence many
of the groups have been derived or what relationship they sus-
tain to 'one another. It may be stated, however, that the lower
fungi show unmistakable evidence of relationship with certain
of the green algae, while an intermediate class have evidently
branched off from the red algae. In nearly all cases it w411 be
seen that their parasitic and saprophytic mode of life has led to
reduction and degeneration. The fungi may be divided into
three classes: A, Phycomycetes or Alga-like Fungi; B, Ascomy-
cetes or Sac Fungi; C, Basidiomycetes or Basidia-bearing Fungi.

Class A. Phycomycetes

77. Alga-like Fungi. — Some of these plants show such a strik-
ing resemblance to certain algae, both in the structure of the
plant body and in their reproductive processes, that they are
called the Phycomycetes from phycos, alga, and myces, mould.
Attention will be called to three orders of this group.

78. Order a. Saprolegniales or Water Moulds. — These fungi
are either saprophytes, living upon dead animals and plants,
or parasites. In this latter relation one form causes a destructive
disease in fish while another genus produces the damping off and
decay of seedlings. They are all microscopic plants and many
are aquatic. Common examples of this order are seen in the
whitish masses that form around decaying insects in the water
and in fluffy or mould-like outgrowths that often appear upon
the bodies of fish in aquaria. The plant body consists of branch-

DEVELOPMENT OF PLANTS

219

ing tubular threads Avithout partitions but conlaininj^ numerous
nuclei and thus resembling Vaucheria save for the absence of
chloroplasts (Fig. 130). The sporangia are also formed by the
cutting off of the tip of one of the l^ranches by a transverse walL
The contents of a sporangium, however, generally breaks up
into a very large number of biciliate zoospores (Fig. 130, C).
In the species that cause so much damage to fish, the spores
come to rest upon the fish and form tubular outgrowths that
readily penetrate the tissues of the fish, especially where a scale
has been rubbed off. Some of the hyphae also extend outward

Fig. 130. Features in the life history of the water moulds: .1, fly in-
fected with Saprolegnia, showing the tubular threads of the fungus radi-
ating out from its body. B, tip of a tube magnified, showing an early stage
in the formation of the sporangium. C, sporangium discharging zoosjK>res.
D, Reproductive organs of a related fungus, Achlya — 0, oogonium contain,
ing four female gametes; an, antheridium with tubes penetrating iM'igdnium
for transport of male gametes.

from the body, causing the mould-like blotches on the iiilroti-d
fish, and from the tips of these branches the sporangia mentioned
above are formed. Most of the species of Saprolc^ma also form
sexual organs as in Vaucheria, from one to sewral gametes Ix'ing
formed in an oogonium (Fig. 130, D). Ihe antheridia, however,
develop several non-motile male gametes. This peculiarit\- of

220 REPRODUCTION OF A WATER MOULD

the gametes is perhaps due to their exposure to atmospheric
conditions, as would be the case when growing upon terrestrial
or floating organic matter. The lack of water for the transport
of the male gametes is nicely met by a tubular outgrowth of
the antheridium which penetrates the oogonium when it ruptures,
discharging the male close to the female gametes (Fig. 130, D,
an) . In the majority of the species, singularly enough, the female
gametes germinate without being fertilized. The gametospore
germinates as in Vaucheria.

It should be stated that, in related forms, large multicillate
zoospores, like those of Vaucheria, are formed and also motile
male gametes. It is also important to note that there are several
parasitic algae closely related to these alga-like fungi and this
remark applies to the Sac Fungi as well. Their parasitic habit
and partial loss of chlorophyll clearly indicate that they are in a
transition state from the algae to the fungi. It is therefore
safe to state that in this group we have evidence of the derivation

Fig. 131. Hyphae of Peronospora extending through the tissues of a plant
and absorbing food from the cells by means of haustoria, h.

of the fungi from the algae. This helps us to understand how
fungi arose. Plants and animals live upon organic materials.
These substances through chemical and physical reactions cause
the absorbing organs of the plant to grow towards them. Conse-
quently the green plant though capable of forming organic sub-
stances may through chemical stimulation be brought into con-
tact with these materials in other plants and animals and as a
result it becomes a fungus.

79. Order b. Peronosporales. Downy Mildews and White
Rusts. — These forms are parasites upon the higher plants espe-

DEVELOPMENT OF PLANTS 221

daily and include two of llic most destructive fun^q, the ix>tato
and the grape vine bliglit. PJiytophthora infestans causes the
potato rot. This disease was widespread in the eastern Lnited
States in 1901, causing the entire loss of the crop in some sections.
It would be difficult to imagine a pest more perfectly adai)ted to a
destructive career. Let us begin the life history with the j^'crmi-
nation of a spore which has fallen upon a leaf. This spore ff)rms
a hypha which enters the leaf and cjuickh- spreads through its
tissues, sending into the cells short lateral branches called haus-
toria (Fig. 131), which absorb the cell contents and thus supply
the fungus with food. In severe cases this produces a withering
and decay of the leaf. The hyphae continue their growth into
the stems and all parts of the plant, thus causing the black dis-
coloration of the potato and its early decay in bad cases of in-
fection. This habit of many parasitic fungi of establishing them-
selves in those organs of the plant which live on from >ear to
year, is one of the most serious difficulties in combating these
pests. For with the renewal of growth of these organs the h>phae
spread and reestablish the disease. Thus the planting of infected
potatoes is sure to result in the appearance of the potato blight if
conditions are favorable. As soon as the mycelium has become
well established in the leaf, numerous branching hyphae extend
out through the stomata and form at their tips little sacs or
sporangia (Fig. 132, A). The formation of the sporangia is
effected in a few hours, when they drop off and are carried b>- the
wind to other plants, where they germinate at once, forming a
tube that penetrates the leaf and rapidly si)reads the disease.
If the sporangia chance to fall upon leaves that are wet b\' dew
or rain, the contents breaks up into several zoospores (Fig. 132,
B-E) which finally come to rest and develop the characteristic
tubular hyphae of the fungi. This behavior of the sporangium is
doubtless a survival of the zoospore stage seen in the algae. It
is equally suggestive that definite changes in tlie environment,
as the dry air, causes the sporangium to germinate as a non-
motile spore, whereas the presence of water causes the sp(»rangium
to produce several zoospores. The potato blight is largely
confined in our country to the northeastern states and usually it

222 REPRODUCTION OF THE DOWNY MILDEWS

does not appear until the latter part of July. It only becomes of
serious importance in sultry weather, when a short period of
rain will result in the formation of the sporangia and whole
fields will be devastated in a few days.

m^.% i

Fig. 132. Fig. 133.

Fig. 132. Asexual reproduction of the mildew: A, hyphae of Plasmo-
para emerging from a stoma and bearing numerous sporangia. B, enlarged
view of sporangium of Peronospora germinating on a dry leaf. In this case
the sporangium behaves as a spore sending out a hypha that will penetrate
the tissues of the leaf. C, sporangium of Phytophthora germinating in the
water and forming zoospores. D, zoospore enlarged. E, zoospore has come
to rest and is forming a tube that will penetrate the tissues of the leaf as
in the case of B.

Fig. 133. Reproduction of the white rust, Albugo: A, asexual stage,
showing several erect hyphae forming spores. B, sexual stage which is
characteristic of the Peronosporales in general — o, gametangium contain-
ing a single female gamete which is being penetrated by tube from male
gametangium, an. — After Wager.

Very similar in character is the grape blight, Plasmopara viti-
cola. This disease causes a browning of the leaves and the stunt-
ing of the fruit, which become brown or .gray, and, finally, the

DEVELOPMEx\T OF PLANTS 22;,

death of the infected parts. This pest caiisifl the ab.indonnKnt
of entire vineyards before the method of kiUin^r the fundus by
spraying the phmts with copper salts was chsroNCTrd. In Ixjth
of these pests and in aUied genera the sporangia-bearing h>phae
are produced in such numbers as to cause a downy, mould-Hke
appearance on the leaves, thus accounting for their popular
name. Downy Mildews.

In a related genus. Albugo, the sporangia are formed in' ilie
repeated cutting off of the tips of the hyphae, as shown in Eig.
133, A. In this way, not one, but a chain of sporangia, are
formed from the ends of the hyphae which do not project from
the leaf, but grow up in dense masses just under the epidermis,
producing glistening white blotches or blisters on the leaves.
This growth finally ruptures the epidermis wlien the spores are
scattered by the wind and germinate as in the preceding cases.
This fungus, known as white rust, is very common on mustards,
pigweed and other plants.

The sexual reproduction of the Peronosporales is suggestive of
Vaucheria. Gametangia are cut ofif from the ends of the hyphae,
as shown in Fig. 133, B. The male gamete gains access to the
female gamete, which is usually formed singly, by means of a tulx;
as in Saprolegnia (Fig. 133, B, an). The thick-walled gameto-
spore, as in many of the algae, tides the plant over the winter, and
being set free by the decay of the surrounding tissues it germi-
nates in the spring, starting anew the life of the pest. It may
germinate directly (see Vaucheria) into the fungus, or zoospores
are first produced, as in Oedogonium.

These two orders are more suggestive of the algae than an\-
others that we shall study and it is well to note the modifications
that have been induced in these plants as a result of their change
from aquatic to terrestrial conditions. Removed from the water,
special root-like organs and haustoria are evoUed for the absorp-
tion of foods. The absence of water brings about a lack of motil-
ity in the male gametes and the formation of a tube to conduct
them to the female. For the same reason, the zoospores arc re-
duced to light motionless spores that are developed ui>on elon-
gated hyphae that expose them to the air currents f«»r disinbuf ion.

224

REPRODUCTION OF BLACKMOULDS

The ability of these spores, or sporangia, to produce zoospores is
doubtless the survival of a trait inherited from their algal an-
cestors.

80. Order c. Mucorales or Black Moulds. — These are among
the most common of the fungi and they are almost sure to appear
upon any cooked food or decaying matter that is exposed to the
air even for a very short time (Fig. 134). The mycelium has
practically the same structure as noted in the preceding groups,
but the black moulds have lost all motile reproductive bodies and
their relationship to any group of the algae is not known. Some
of the hyphae of the mycelium creep over the food supply and
send into it short branches which serve as organs of absorption
while other rather thicker hyphae grow away from the mycelium
and reach up into the air (Fig. 134). The tips of these erect

Fig. 134. Habit of growth of the black mould, Rhizopus: s, sporangia;
r, absorbing branches of the mycelium.

hyphae enlarge owing to the accumulation of protoplasm and
numerous nuclei in them, and finally become spherical, forming
the sporangia. The contents of the young sporangia differentiate
into a denser peripheral portion containing the bulk of the nuclei
and a more watery central and basal region. A layer of large,
round vacuoles now appears between these two regions and,
owing to the flattening out and fusion of these vacuoles, a cleft
is formed that ultimately separates the peripheral from the central
protoplasm. Next furrows advancing inward from the sporangial
wall and outward from the cleft divide the peripheral protoplasm
into numerous small portions, each part containing several nuclei.
These bodies round off, secrete a rather thick smoky black wall

DEVELOPMENT OF PLANTS 225

and so become spores (Fig. 135). The numerous sporan^^a filled
with dark spores are the principal cause of the black color of these
fungi.

During the development of the spores a wall is constructed
over the central protoplasm, thus often forming a dome-like
structure in the sporangium known as the columella (Fig. 135, E).
The walls of the sporangia (save at the region in contact with
the stalk) readily dissolve in the presence of moisture, owing
to their mucilaginous character, and thus allow the spores to
float off in the air as an invisible dust. The spores will germi-
nate at once and produce a new plant under suitable conditions
of moisture and food, or they will retain their vitality for months
if kept dry. As the spores are disseminated, the dome-like struc-

FlG. 135. A, B, early stages in the development of the sporangium of
Rhizopus. C, the formation of the vacuoles which separate the denser peri-
pheral protoplasm from the more watery central region. D, the vacuoles
are flattening out and the denser protoplasm is becoming separated into
multinucleate segment, E, the multinucleate segments are rounding off to
form the spores ai^d a wall has formed over the columella, f, the sporan-
gium has ruptured, permitting the scattering of the spores and the columella
has formed an umbrella-like structure owing to the loss of its water)- con-
tents.— After Swingle.

ture in the sporangium relaxes, owing to the loss of its waier>'
contents, and assumes an umbrella shape, as seen in h'ig. 135. F.
You can readily understand why these black moulds are so com-
mon by counting the number of sporangia on a small bit of myce-
lium and then estimating the number of spores in a sporangium.
So numerous and light are the spores that they are carried every-
where. It is only necessary to expose a bit of moist bread for a
few moments to the air and then enclose it in a damj) chamlxr to
secure a luxuriant crop of these plants. An interesting variation

226

SPORANGIUM OF PHILOBOLUS

in this mode of scattering the spores is seen in a related form,
Piloholus, which is of common occurrence upon horse dung.
Here the wall of the sporangium, unlike Rhizopus, is quite firm
and becomes mucilaginous only at the point of contact with the
stalk that bears it. Owing to the accumulation of water in the
stalk such a pressure is finally set up as to rupture it at the
mucilaginous point and so the sporangium with its contained
spores is hurled considerable distance — often quite one meter.
The sporangia bearing stalks are also sensitive to light. If
a glass jar covered with black paper so as to exclude all light is

Fig. 136. Sexual reproduction in the black mould: A, the meeting of
the tips of two hyphae. B, later stage, the lower part of the figure shows
the cutting off of the tips by transverse walls and in the upper part of the
figure the fusion of the contents of the two gametangia thus formed has be-
gun. C, mature gametospore.

placed over a colony of these plants and a minute opening is now
made in the paper, you will be surprised to note with what
accuracy each sporangium is shot off and hits this opening.
The results are the same no matter where you make the opening.
Under certain conditions the sexual method of reproduction is
effected by the union of two club-shaped hyphae, as shown in
Fig. 136, A. As these hyphae meet, the tip of each branch is
cut off by a wall and the contents of the two tips fuse, forming a

DEVELOPMENT OF PLAN'IS

227

thick-walled gamctospore (Fig. 136, B, C). After a resting
period, this gametospore grows directly into a new mycelium.
In some of the black moulds Blakeslee has made known that
reproduction is only effected by the union of ]i\j)h;i(* frf)m
difTerent plants which must difTcr therefore in their nature, just
as you found to be the case in some species of Spirogyra.

81. A Fly Fungus. — An interesting form allied to the nmcors
is seen in the parasitic fungus Empiisa, that produrcs an epi-

FiG. 137. A fungus, Empusa, parasitic upon Hies: .1, lly surrounded hy
a mass of discharged sporangia. B, enlarged view of several h>phae, sh«>wing
the discharge of the sporangia which are surrounded by a mucilaginous sub-
stance.— After Brefeld.

demic among flies at certain seasons when tlie\- may be m-cu dead
and clinging to the woodwork and window panes surroundeii by
a white halo (Fig. 137, A). This appearance is caused by the
discharge of numerous white sporangia that are found at the
ends of the hyphac that project from all sides of the fly's Ixxiy
(Fig. 137, B). If, In- chance, a i\y sliould rome within r.uigc

228 THE SAC FUNGI

of these discharging sporangia, it would become infested and
later, branches of the hyphae would project from its body and
repeat the process of sporangia formation.

Class B. Ascomycetes or Sac Fungi
82. General Characters. — The Ascomycetes are the largest and
most variable group of the fungi. They are illustrated by the
powdery mildews which affect the leaves of a variety of plants;
the brown and blue moulds that occur on preserves, decaying
fruit, old leather, etc. (Fig. 138); the cup fungi and morels
(Figs. 139, 141); the black knot of the cherry and plum trees
(Fig. 149), and the ergot of rye, etc.

The plant body or mycelium resembles in structure that of
preceding orders, but its hyphae are composed of numerous cells
instead of being tubular or with few cross walls as in the Phyco-
mycetes (Fig. 138). The sporangia (Fig. 152) are reduced in

Fig. 138. The green mould, Penicillium, one of the most common of the
Sac Fungi. The hyphae of the branching mycelium is composed of cells
and the spores or conidia are formed in chains that are arranged in brush-
like clusters at the ends of the erect hyphae.

size and contain but a single spore each, which is never dis-
charged from the sporangium. This body will be referred to
in the future as a spore or conidium (plu. conidia).

The gametangia are suggestive of those noted in the red algae
(Fig. 140), though in many forms they appear to be greatly re-
duced or even lacking. The fusion of the gametes results in the
formation of a gametospore which germinates at once, forming in

DEVELOPMENT OF PLANTS

229

the simplest cases one or more sacs. These bodies arc known as
asci (sing, ascus) and they contain more frequently eight spores,
called ascospores (Fig. 160), wliich must he distinguished from
the spores or conidia mentioned above. The gametospore germi-
nates as a parasite on the mother plant as in the red algae.
The sexual process results, in the majority of forms, not only
in the formation of asci, but numerous ]iyi:)hae from the my-
celium are also stimulated to growth and become associated
with the asci in various ways, recalling the cystocarps of the red
algae. As a result fruit bodies of various forms, called ascocarps
or perithecia, are developed which often become the conspicuous
part of the fungus (Figs. 155-158). The ascospores germinate
immediately or after a period of rest and form a new mycelium.
It should be stated that the ascocarps have not been connected,
in many cases, with sexual processes, and it is inferred that sexu-
ality has been lost along wuth other characters as a consequence
of the degeneracy resulting from parasitic and saprophytic
habits. Only a few of the more important orders will be dis-
cussed.

83. Order a. Pezizales or the Cup Fungi. — These plants are
characterized by the formation of flesh}-, leathery or gelatinous

Fig. 139. One of the common cup fungi, order Pezizales, with broadly
opened ascocarps. Common upon rich hunuis soil and decaying wood.

cup-like ascocarps that range in size from mere specks to forms
four or five inches in diameter (Fig. 139). The mycelium lives
upon the humus in the ground or on decaying j)lants and ap-
parently in this and the next order frcc|uently develops the asco-
carps directly without a reproductive process. In Pyronema
and many other forms, however, Dodge has shown that reprodur-

230

REPRODUCTION OF THE PEZIZALES

tive organs are formed that are very suggestive of those of the
red algae. The female gametangium in the simpler forms has
an enlarged basal region with a long tubular outgrowth sug-
gestive of Nemalion (Fig. 140). In the majority of forms this
organ consists of several cells and often becomes coiled and
greatly extended. The male gametangium is a somewhat
enlarged hypha. These organs are developed here and there on
the mycelium or several may be associated in a compact group.

Fig. 140. Sexual reproduction of one of the cup fungi, Pyronema: A,
the flask-shaped female gametangium, 0, is seen in various stages of fusion
with the male gametangium, an. This is effected by the curvature of the tu-
bular outgrowth. B, the gametospores, g, germinating and forming branching
hyphae which bear the asci, as. These asci are associated with hyphae or
paraphyses, p, that arise from the mycelium. The asci and paraphyses
constitute the hymenium. — After Harper.

Singularly the union between these two sex organs is effected
by the female — its tubular part curves towards the male game-
tangium and effects a fusion with it. As to whether the male
gametes pass over and unite with the females as a result of the
fusion of the gametangia no general statement can be made
at present, though in some forms there is evidence to indicate
that such in the case. The basal portion of the female game-
tangium, or one of its cells in cases where the gametangium con-
sists of several cells, behaves as though fertilization had been

DEVELOPMENT OF PLANTS 2^1

effected and a gametospore had been fornicd. ii ^aTininaies by
developing a number of erect sac-like hyphae or asci (Fi^^ 140, B).
Adjoining hyphae grow up among the asci and around them,
forming a cup-like structure resembling that shown in Fig. 1.^9.
A section through one of these cups reveals the asci intermingled
with hyphae, also called paraphyses, in the form of a layer or
stratum. Such an association of spore-bearing organs and para-
physes is called a hymenium. The spores are discharged to con-
siderable distances, owing to the accumulation oi fluids in the
asci which finally rupture at their aj^ices. Thcx ;ire fornu-d
in such enormous numbers that under favorable conditions they
may be seen passing off as faint puffs of smoke. The bright
scarlet cups of Sarcoscypha, common in the spring on decaying
sticks and the gray cups of Peziza growing upon the ground
and decaying wood, arc familiar examples of the order.

84. Relationship of the Cup Fungi. — The Pezizales may be
regarded as the typical representatives of the Ascomycetes.
The sexual reproductive process as outlined above is fairly
representative of the other groups of ascomycetes and will not
be referred to again except in orders where noteworthy departures
occur. The formation of spores or conidia are characteristic
features of many of the groups but owing to the exceedingly
diverse character and origin of these bodies attention will only
be directed to some of the more familiar and common examples
as illustrated in the two last orders. Diverging from these typi-
cal ascomycetes are several lines of fungi that show varying de-
grees of relationship to them. As an example of this divergence
attention may first be directed to the Helvellales.

85. Order b. Helvellales. — These are fleshy forms like the
cup fungi and grow upon the ground, but the reproductive proce*^
results in the development of oddh' shaped ascoccu-jis witii the
hymenium developed upon the upper exposed surface. Several
common forms of these ascocarps are illustrated in Fig. 141.

The morel or Morchella, in which the hymenium is spread over
the pitted or honey-combed surface of tlie ascocarp, is one of
the most highly prized of the edible fungi. The meml)ers of
this order are largely saprophytic and often attain considerable

232 THE ASCO-LICHENS

size, forms of Morchella occasionally reaching the height of a
foot and some species of Gyromitra weigh over a pound.

86. The Asco-lichens. — A second line of departure from the
Pezizales includes a large group of plants known as the lichen.
The great majority of these forms show strong evidence of rela-

Fig. 141. Common forms of the Helvellales: A, the morel, Morchella,
surface view at left and in section at right. The asci and paraphyses form
a hymenium over the honeycomb surface. B, Leotia, a small gelatinous
form of a light, bluish-green color. C, Geoglossum, a black tongue-like form.
In B and C the hymenium is confined to the upper enlarged portion of the
fungus. Both are common in boggy ground.

tionship with the cup fungi in their reproductive processes and
it should be added that the sex organs are more suggestive of
the red algae than in any other group. A few species belong to
the third class of fungi, the basidiomycetes, but these plants show
the same general features as the asco-lichens and therefore a
consideration of the entire group may be taken up at this point.
These remarkable plants are of almost universal distribution upon
tree trunks, rocks, old fences and buildings, and upon the bare
earth, where they form variously colored incrustations or leaf-
like branching bodies (Fig. 142).

The lichen is one of the most extraordinary plants in the
vegetable kingdom, since it is a union of two separate plants, a
fungus and an alga. Naturally the relationship of the lichens
to other groups of plants has been a matter of dispute, some

DEVELOPMENT OI- PLANTS

233

regarding them as constituting an independcni division, and by
others they are looked upon as fungi. The fart that ilic fungus
forms the bulk of the lichen and lives ui)on ilie alga somewhat
after the manner of a parasite and is usually alone capable of
forming reproductive spore bodies would lead to the latter

Fig. 142. Common species of Lichens: A, an erect branching form,
Cladonia — as, ascocarps. B, a foliaccous lichen, Sticta. C, Parmelia spread-
ing over the bark of tree. The centrally-placed ascocarps are surrounded
by smaller pycnidia.

position. However, all degrees of relationship appear between
the alga and fungus — from stages where their association is of
little or no consequence to stages where their existance is abso-
lutely dependent upon their association. The members of this
curious co-partnership are largely ascomycetes and blue-green
algae. Numerous species in each of these two groups of plants
have become accustomed to living together. The formation of
a lichen comes about through the attachment of a hypha of some
fungus to one or more algal cells, as shown in Fig. 143, B. In
this way, the food manufactured by the algae is absorbed os-
motically by the hyphae. In some cases, the fungus obtains
its food by means of haustoria which penetrate the algal cells.
By the continued growth of the fungus and algae there finally
results an interwoven mass of hyphae about the algae. The
walls of the fungus are sufficiently transparent to permit the
entrance of light and the color of the green algae can readily
be seen when the lichen is moistened.

(o) Structure of the Lichen.— The bulk of the lichen is more
usually composed of hyphae which show <-Mn<ul.r .l.h- ni-nlanf v

234 STRUCTURE OF LICHENS

in their growth and the majority of the algae also are usually
confined to a definite zone near the sunned surface of the lichen.
Thus, in Fig. 143, A, which represents a cross-section of a lichen,
it will be seen that the fungus forms a rather firm layer at the
top and bottom of the thallus, while the algae are distributed near
the upper surface, where they are exposed to the light, and can
therefore carry on photosynthesis. Numerous hyphae projecting
from the under surface of the lichen serve to anchor it to the
substratum and also assist, doubtless, in the absorption of the
earth substances. In some of the gelatinous lichens the fungi
and algae are more promiscuously arranged.

While the fungus is dependent lipon the foods manufactured
by the algae, the latter are also benefited to an extent by this
arrangement since they are protected by the strata of hyphae, and
they are also provided with water and crude material which the
fungus readily takes up and holds. This mutually helpful rela-
tionship of two organisms is a form of symbiosis, termed com-
mensalism. The strange feature about this co-partnership is the
marked change produced in the nature of the two symbionts. As
long as they are independent of each other, very special conditions
are necessary for their welfare, but associated, they form the most
resistant plants known. This accounts for their distribution from
the equator to the pole and their association upon crystalline
rocks, baked earth, bark of trees and other places where no other
plant life is possible. Under unfavorable conditions the lichen
becomes dry and brittle, in which resting condition it is able to
meet any extreme temperature and drought. With the return of
suitable moisture and heat they become leathery or gelatinous
and renew their growth with considerable rapidity. Thus they
live on from year to year, but owing to the exposed places in which
they are usually found, their growing periods are frequently very
short and their total annual growth may not exceed a few milli-
meters— see soils, p. 50.

{b) Reproduction of Lichens. — Reproduction of the lichens is
brought about by means of fragments that are readily detached
when the lichens are dry and brittle, or by means of soredia.
These latter bodies consist of a few algal cells intertwined with

DEVELOPMENT OF PLANTS

235

hyphae, in fact, a miniature lichen (Fig. 143, C). In some sfK-cies
the soredia form a rather powdery or granular coaling on the
upper surface of the lichen, and in other cases they are develo|)ed
within the lichen. These bodies are easil>' scattered hy the
wind when the lichens are dr>' and under favorable conditions
grow into new lichens. Reproduction is also affected by means of
ascospores that are developed as in tlie cup fungi. The frinaie

Fig. 143.

Fig. 144.

Fig. 143. Structure of the Lichen: A, section of a lichen, showing the
compact arrangement of the hyphae at the top and bottom, also the anchor-
ing fungal threads on the underside and the dark algal cells, a, near the top.
B, enlarged view of the algae to show their relation to the hyphae. C, dia-
gram of one of the powdery particles, sorodium, appearing upon certain
lichens, showing the hyphae and algae. These bodies are scattered by the
wind and form new lichens.

Fig. 144. Sexual reproduction of the lichen: A, section of an ast-ocarp,
the hymenium appearing as a dark band in the mouth of the cup. B, en-
larged view of the asci, a, and paraphyses, p, of the hymenium.

gametangium is a coiled organ consisting of many cells as in
some Pezizales but the male gametes are developed in ascnearp-
like bodies where they are formed at the ends of numerous
minute hyphae— in their origin, therefore, being strikingly sug-
gestive of the red algae. In one form, ic should be added that a
fusion between male and female gametangia has been n-tw.n.-d

236 IMPORTANCE OF LICHENS

The ascocarps derived from this reproductive process assume a
great variety of shapes (Fig. 142) but they all show the hymenial
layer of asci and paraphyses (Fig. 144) or of basidia and para-
physes in the few forms belonging to basidio-lichens.

The ascospores germinate readily, producing hyphae which,
however, soon perish unless they chance to meet an alga with
which they can live. It should be noted that these fungi appear-
ing in lichens cannot associate indiscriminately with any species
of algae. Each species of fungus Is adapted to one or more species
of algae and is unable to live with any other form. As may be
imagined, the real nature of the lichen was for a long time mis-
understood, the algae even being looked upon as spore bodies.
The bitter dispute over the question was finally settled when a
lichen was produced artificially by bringing together a suitable
fungus and alga.

(c) Economic Value. — Lichens are of considerable economic
importance in the world. Their service in hastening the decay
and transformation of rock material has already been referred to.
In many sections they form the characteristic vegetation of the
country, as In certain alpine and desert regions and in the north-
ern barrens, where they ajfford rich pasturage to reindeer and
caribou. Their abundant growth in mountainous districts and
subsequent distribution by winds and rains accounts for the
showers of manna in biblical history. Certain species are still
used in northern regions to lengthen out a meager food supply.
Litmus, employed in acid testing, and various pigments are
derived from lichens, as were also the famous purple and blue
dyes of the East.

87. A Third Line Possibly Related to the Pezizales. — The
following orders may not represent a single line of descent but
there are certain features in their life history indicating that
they may represent a line of departure from the Pezizales.
These forms are mostly minute; parasitic, or saprophytic upon
dead plants. The more epiphytic position of these forms may
have been a factor in the reduction in size. Certainly the
lar.ger fleshy forms of the preceding orders are terrestrial In
habitat. There are two quite distinct groups In this alliance,

DEVELOPMEXT OF PLAN TS

237

the one characterized by ascocarps that are only shj^hily ojK'ned
so that the asci are not as freely exposed to the air as in the
preceding orders and the other with closed ascocarps so that the
asci are exposed only on the decay of the fruit IxkIn. The

D ^

Fig. 145. ii<'- u*'-

Fig. 145. Head of rye infested by the parasite, Clax'iccps, which has
transformed several of the grains into black masses of myci-lium known as
sclerotia.

Fig. 146. Various phases in the Hfe history of Claviceps: A, a young
grain or pistil infested with the parasite. B, enlarged view of the mycelium
as it appears on the surface of the pistil, showing the formation of numerous
spores. C, the hyphae of a sclerotium growing out and forming several
purplish stalks, each capped with knob-like clusters of ascocarps. D, en-
larged sectional view of one of these knobs, showing numerous ascocarps
on the periphery. E, one of the ascocarps enlarged, as aaci. P an ascus
containing eight thread-like ascospores.

238 LIFE HISTORY OF CLAVICEPS

first group is further characterized by the fact that the ascocarps
are frequently associated with a more or less conspicuous growth
of the mycelium in which the ascocarps are imbedded in varying
degrees. This growth is known as the stroma and it may form
a rather compact mass of hyphae. Only two orders can be con-
sidered in each of these two groups.

88. Order c. Hypocreales. — These fungi are distinguished by
their rather fleshy or membranous ascocarps and stroma, which
range in color from white to yellow, purple, scarlet and brown.
Numerous species are saprophytic while others are parasitic
upon higher plants, fungi and insects. Claviceps is a common
example of this group, causing the disease known as ergot in the
flowers of rye and other grasses (Fig. 145). The mycelium at
first spreads over the outer part of the pistil, rapidly forming
spores (Fig. 146, A, B) and exuding a sweet slimy juice, honey
dew, which is eagerly eaten by flies. In this way the spores are
carried away to infest other plants. The mycelium finally
completely absorbs the substance of the grain and grows into a
hard blue-black body several times larger than the grain (Fig.
145). This body, known as the sclerotium, remains dormant
during the winter and in the spring groups of hyphae at various
points in the sclerotium grow out into rose-colored stalks that
terminate in globular heads (Fig. 146, C). These stalked bodies
represent the stromata as it appears in this genus. Numerous
ascocarps with minute openings are developed in the outer part
of globular head of the stroma (Fig. 146, D). The ascospores
germinate in the spring and infest the flow^ers of the grain. The
Chinese wonder, Cordyceps, is a related parasite that attacks
caterpillars, larvae of beetles and truffles. In the case of the
forms living upon insects, the parasite does not usually appear
until the spring, when they are in the pupa or cocoon stage.
At this time, the mycelium which flourishes in the tissues of the
host, sends up club-like bodies (Fig. 147), that bear the ascocarps
as in the case of the ergot. Among the important pests included
in this order are those causing the wilt disease of cotton, cowpea
and watermelon; the disease on currant, apple and pear trees;
nad the claviceps of grain.

DEVELOPMENT OF PLANTS 239

89. Order d. Sphaeriales or Black Fungi.— This is ilic largest
group of the ascomycetes, over 2,000 sjK'cies being known in the
United States alone. Scarcely a fallen twig or bit of old wcxxi
can be examined without rcvealiniL; llu- luinule ascocarps wiiieh

■"««=>,

Fig. 147. The stroma of Cordyceps emerging from the pupa of a moth
and forming a club-like organ with numerous ascocarps, as, in its apical
region.

more commonly are hard and black in contradistinction to the
Hypocreales (Figs. 148; 151, A). Many of these fungi are quite
conspicuous since the ascocarps are formed in large compact
masses and also because they are often associated with a more or
less conspicuous stroma (Figs. 149; 151, D). The majority of
the genera are saprophytic upon dead and decaying vegetation,
though some of them are destructive parasites. The black knot,
Plowrightia, the cause of a serious disease to plum and cherry
trees, illustrates very w^ell the characteristics of this order. The
mycelium grows in the cambium and cortical regions of the
branches, causing the bark to split open in the spring when spore
bearing hyphae extend up into the air forming a velvety coating
(Fig. 149, c). By the approach of winter, this mycelium has
grown into the familiar black knotty mass in which are de-
veloped numerous ascocarp-like bodies (Fig. 150). Tlu" six>rcs
from these ascocarps are carried by the wind in the early spring
to other branches and probably infest the budding trees. Other
conspicuous forms are Xylaria and Daldinia, which de\elop an
extensive stroma on stumps and trees that contains numerous
ascocarps (Fig. 151). In Hypoxylofi, the stroma containing the

240

FORMS OF THE BLACK FUNGI

ascocarps breaks through the bark of a large variety of trees
and shrubs in the form of spherical or cake-like masses (Fig.
148). Among the more serious pests may be mentioned the very
destructive black rot of grapes, apple and pear scab; bitter rot
of apples; the s3xamore blight, and the chestnut bark disease.

^U

^.

Fig. 149.

A

Fig. 150.

Fig. 148. A common black fungus, Hypoxylon: A, habit of the fungus
as it appears on dead branches and logs. The round black bodies are an
association of the mycelium, stroma, and numerous ascocarps. B, a single
ascus enlarged, showing character of the ascospores.

Fig. 149. The black knot. Plowrightia, infecting a branch of cherry. At
the bottom of the branch is shown the early summer or spore-bearing stage,
c, and above a black warty mass of ascocarps, as, produced the previous
season.

Fig. 150. A, several ascocarps enlarged, taken from region, as, in Fig.
149, B, diagram of an ascocarp as seen in section, showing the asci and the
opening for the escape of the ascospores.

This and the following order represent possibly the consumma-
tion of those tendencies that we saw appearing in the third line
of departure from the Pezizales. As stated they are characterized
by their closed ascocarps.

90. Order e. Aspergillales or Blue-green and Brown Fungi. —
The Aspergillales includes perhaps the most widely distributed
and familiar examples of the fungi. They occur as blue-green or
brown moulds upon almost any organic matter, forming a deli-
cate mycelium from which are developed numerous erect hyphae.

DEVELOPMENT OF PLANTS

241

In the common blue mould, Penicillium, these erect In i)hae are
broom like (Fig. 152, B) and the spores are formed from the
tips of the branches vary much after the manner of the budding
of the yeast cells. The tip of a branch buds out into a spherical
cell that is finally cut off from the stalk, thus forming the siKjre
(Fig. 152, C). This process is repeated again and again just

Fig. 151. Other common forms of the Spheriales: .1, habit of Ilysterio-
graphium, on a dead twig. B, ascocarps enlarged. C, ascus enlarged, show-
ing character of ascosporos. D, Daldinia. E, section of the same, showing
that the stroma forms a concentric stratum of ascocarps, as, each year. F,
Xylaria. G, the same with branch cut off to show the layer of ascocarps
on the periphery of the stroma.

below each successive spore and in this way chains of spores
are formed. It will be noticed that many single spores are found
instead of a large sporangium which contains many spores as
in the case of Mucor. These spores are sometimes regarded
as sporangia which have become reduced in size and contain but
a single spore. In Aspergillus, a fungus common ujjon preserves
and upon herbarium plants that have not been sutTiciently dried,
the spores are developed from very short branches that arise
from a bulbous swelling at the apex of the eroci h> phae (F*ig.
152, A). The color of the spores, as in Miicor, is the cause of
the characteristic blue or brown color of the funei. The^e ex-
16

242 SEXUAL REPRODUCTION OF ASPERGILLALES

tremely small and numerous spores float off in the air as an
invisible dust and quickly germinate under favorable conditions,
forming new mycelia.

The male and female gametangia have departed considerably
from the form shown in the cup fungi. They now appear as
short, slightly modified branches of the hyphae and have lost
the tubular outgrowth so characteristic of the Pezizales and red

Fig. 152. Formation of spores or conidia in the Aspergillales: A, spore-
bearing hypha of Aspergillus. B, hypha of green mould, Penicillium. C,
one of the terminal branches of B enlarged, showing manner of spore formation.

algae. However they become closely intertwined (Fig. 153)
and the ends of the branches meet, thus the mingling of the
gametes is made possible through a dissolution of the separating
walls at the tips of the gametangia. The resulting gametospore
remains attached to the parent plant and germinates at once,
forming an irregular hyphal outgrowth (Fig. 153, B, s) which
becomes completely overgrown by the hyphae of the mycelium
(Fig. 153, B, C). Thus is formed a solid ascocarp or perithecium
(plu. perithecia), that appears to the eye as a minute grain of
sand. During this growth numerous lateral branches arise on
the hyphae derived from the gametospore and become trans-

DEVELOPMENT OF PLANTS

'M3

formed into asci as shown in Fig. 154, A, B. The ascocarps
finally decay and set free the ascospores which develop a new
plant or mycelium. Thus the entire life history in these forms
as in other fungi is suggestive of the algae. While conditions are

Fig. 153. Sexual reproduction of the Aspergillalcs and the formation of
the ascocarp or perithecium: A, meeting of the male, an, and female, 0,
gametangia. B, early stage in the development of the ascocarp. The gamet-
ospore has formed a series of branches, s, which are being surrounded by hyphae
(unshaded in the figure) from the mycelium. C, later stage seen in section.
The gametospore has formed a much-branched body, s, which is surrounded
by a closely interwoven mass of hyphae that appear in section as cells. —
After Brefeld.

favorable for growth these plants are rapidh' nuihiplied .ind dis-
seminated by conidia. Changed conditions cause the develop-
ment of sexual organs and the consequent formation of the game-
tospore. The gametospore germinates at once, living on the
parent plant like a parasite just as in the case of the red algae.
Note also another resemblance, the gametospore first develops a
mass of cells, in this instance of a hyphal character, on which
later the spore-containing asci arise as lateral branches.

The truffles are a curious group of related fun.ui th.it li\e for
the most part entirely under ground. The niNcelium of many
forms is supposed to live in contact with the roots of oaks and
other trees as a mycorhiza. The fleshy tuber-like ascocarp, often
as large as a walnut, is a higliK- prized delicacy in EurojX' where
dogs and pigs are trained to locate the truffles by smell. This
industry amounts to more than f 1,000,000 annually in France
and Italy.

244

THE POWDERY MILDEWS

91. Order f. Perisporiales or Powdery Mildews. — This name
is given to a common and widely distributed group of largely
parasitic fungi that form cobwebby mycelia on the under surface
of the leaves of the elm, maple, willow, lilac, rhododendron

Fig. 154. Further development of the ascocarp: A, sectional view, show-
ing the branches, s, derived from the germinating gametospore, that are
forming numerous lateral branchlets. B, one of the branchlets enlarged,
showing how it divides into cells which round off, forming the asci, as. C,
ascospore. D, germinating ascospore. — After Brefeld.

Virginia creeper, etc. (Fig. 155), and a great variety of herba-
ceous plants, as the dandelion and cocklebur. The majority of
them do not seriously interfere with the health of the plant, but
others produce serious diseases in young cherry and plum trees,
hop vines, gooseberry, etc. These parasites are external and
obtain their food by means of short branches, haustoria, which
dissolve the cell wall and absorb the cell contents as shown in
Fig. 156, h. The members of this order are of special interest
in that they throw considerable light upon the nature of para-
sitism. Frequently one and the same form will grow upon a
wide variety of plants, just as we will see to be the case in another
group, i. e., the rusts. Erysiphe graminis, for example, is found
upon barley, wheat, rye, oats and several genera of grasses.
But the form growing upon any one of these kinds of plants will

DEVELOPMENT OF PLANTS

-M5

not infest any of the others. It would seem iluit tlie jiarasite
becomes changed and specialized, although there is no visible evi-
dence of this, and so after a time is able to live upon but one
kind of plant. These specialized forms are termed biological
species, in contradistinction to the general or morphological
species which include them all. Evidently tlu-n- is something
within the plant that not only attracts these parasites, but also
changes them, so that after a time they ran nnl\ live upon the

yC

Fig. 155. Fig. 156.

Fig. 155. Appearance of one of the powdery mildews, Uncinula, on leaf
or elm.

Fig. 156. Enlarged view of the myceUum, ascocarp, etc., of one of the
mildews, Erysiphe: c, erect hyphae forming spores or conidia; /;, hausloria
penetrating epidermis of leaf; a, ascocarp or perithecium.

plant having these substances. If this material is absent from
the plant then it is immune and it has been shown in a few cases
where individual plants were not subject to a plant disease that
this was due to the lack of a substance which the infest(xl jilants
had or to the presence of a new substance which was reiK'Ucnt to
the parasite. The relation between parasite and host is strikingly
brought out by Massee's experiment, in which he claims that a
purely saprophytic fungus was induced to become a destructive
parasite upon the leaves of a species of Begonia b>- injecting the
leaves with a sugar solution. The fungus flourished u\kh\ the
leaves treated in this way and produced spores. These six>rcs
were sown upon leaves similarly treated and this was repeated for

246

REPRODUCTION OF PERISPORIALES

twelve generations, at which time he states that the spores would
grow upon the untreated leaves. Perhaps this is the explanation
of some epidemics or the occasional sudden appearance of a plant
disease. A variety of cirucmstances might cause plants to form
substances attractive to the parasite or to fail to develop repellent

Fig. 157. Sexual reproduction of a powdery, mildew: A meeting of the
male, aji, and female, o, gametangia. B, fertilization, the male gamete, m,
is seen approaching the female. C, section of young ascocarp showing the
early germination of the gametospore, which has become surrounded by
hyphae derived from the mycelium. D, later stage, the gametospore has
developed several cells and the second cell from the end, as, will produce
the ascus. — After Harper.

materials. In either case they would become susceptible to the
disease.

The spores are formed in chains (Fig. 156, c) from the end
of the erect hyphae that project from the surface of the leaf in
thick masses, causing the powdery appearance and the popular
name of these parasites. These spores germinate quickly and
rapidly spread the fungus.

The reproductive organs have in this group become still
further reduced and appear as short branches as shown in Fig.
157, A. The solution of the walls at the point of contact of
these organs permits the male gamete to pass over and fuse
with the female (Fig. 157, B). The growth of the gametospore
forms a limited number of cells and one of them, usually the

DEVELOPMENT OF PLANTS 247

second from the end, will develop one or several asci (Fig. 157,
C, D). As in Pemcilliiim, this growth becomes envelofK'd by a
mass of hyphae that originate from the hyphae bearing the
gametospore and from adjacent strands of the nucelium. These
ascocarps appear at maturity as black specks and in the majority
of forms they are provided with hair-like outgrowths that are
very regular and characteristic of the genera (Fig. 158). The
ends of these hairs are rather mucilaginous and may assist in
the dissemination of the ascocarps. The ascospores are resting

.\

- 7

^§^.Af^^

Fig. 158. Forms of ascocarps found among the powdery mildews: .1,
Phyllactinia with needle-like appendages enlarged at the base. B, Micro-
sphaera, appendages dichotomous at apex. C, Uncinida, apix^ndagcs coiled
at apex. D, Erysiphe without appendages and crushed to show escaping
asci. E, an ascus containing six ascospores.

spores adapted to enduring drought and cold as in Pcnicillium,
which they resemble in their discharge and germination.

92. Reduced Ascomycetes. — Space will only permit the
consideration of two other groups from this enormous alliance of
the Ascomycetes. These forms are here considered partly
because of their economic importance and partly because they
illustrate the reduction that may go on in the plant body. 'I'hcse
two groups of fungi have become so changed and simplifuHJ in
their structure and life history as to render impo-MMr a cuess
as to their relationship to other forms.

248

REDUCED ASCOMYCETES

92A. Order g. Exoascales or Peach Curl. — This group in-
cludes a small number of parastic fungi that are especially destruc-
tive to peach and plum trees, causing distortion of the leaves
and fruit, known as leaf curl and bladder plum (Fig. 159). There
is apparently no trace of a reproductive process, the mycelium
spreading through the leaf develops directly a rudimentary
hymenium of numerous asci beneath the cuticle, which is finally

Fig. 159. Fig. 160.

Fig. 159. A branch from peach tree, showing the distortion of the leaves
caused by the fungus, Exoascus.

Fig. 160. Section of a leaf showing numerous cells rupturing the cuticle
and developing into asci, as.

ruptured by their growth (Fig. 160). The ascospores are dis-
charged into the air by the bursting of the asci and carried by
the wind to other plants. The damage in the United States to
peach trees alone is estimated at $2,000,000 to $3,000,000
annually.

93. Order h. Yeast or Saccharomycetes. — These fungi are
unicellular plants with little suggestion of mycelial growth.
Ordinarily they consist of rather oval cells which multiply rapidly
by a budding suggestive of conidia formation in preceding orders
(Fig. 161). The new cells that push out from the side of the
mother cell readily drop off, but in rapid growth they may
remain attached in chains (Fig. 161, D). Under certain condi-
tions, as the exhaustion of the food supply, the cells become
transformed into asci and the contents of each cell rounds ofT
into one or more ascospores (Fig. 161, E). The ascospores are

DEVELOPMENT OF PLANTS

-Mv

freed by the decay of the ascus and when concHiions are favcjrable,
grow into the characteristic yeast cells, as shown in Fig. i6i, F.

(a) Fermentatio7i.- — These microscopic plants must be num-
bered among those plants that are of the greatest economic value.
Their importance is due to the fact that they decompose sugars
upon which they feed into COo and alcohol, a change called fer-
mentation. The extensive brewing and distilling industries all

Fig. i6i. The yeast plant, Saccharomyccs: A, single plain, ly, yuxui
producing three buds. C, section of two plants showing buds and nuclear
division. Z), chain of plants due to rapid budding and growth. E, forma-
tion of ascospores. F, germination of an ascosporc and the formation of
new plants by budding, — After Wager.

over the world are dependent upon the growth and peculiar action
of these microscopic plants. When yeast plants are i>laced in
solutions containing sugar in the form of molasses or prepara-
tions of rye, corn, barley, potatoes, etc., and slightly warmed, the
growth of the yeast produces a vigorous fermentation. CarlKjn
dioxide rises to the surface, forming a froth)' scum while the alco-
hol accumulates in the fluid. Beers and ales are fermented bever-
ages of this nature, while whiskies, brandies, alcohol, etc., are
obtained from the fermented mass by removing a part of the
water by distillation. Wines and cider are weak alcoholic bev-

250 THE NATURE OF YEAST

erages formed by the yeast plant from the sugars in the juices
expressed from grapes and apples. In this case, the yeast is
generally allowed to find its way naturally into the fluids. So
minute are these plants that they are distributed in the air
throughout the world and no weak sugar solution can be exposed
for any length of time without being inoculated by them — a fact
that has been apparently taken advantage of by man in all times
and places. When these fermented beverages are bottled before
the decomposition of the sugar is complete, then a further gen-
eration of CO2 sets up a pressure in the bottle or cask that causes
the popping of the cork when the bottle is opened and the sparkle
of the fluid owing to the escaping gas. These forms show the
same range of variability in the work which they perform as
noted in the bacteria of decay. In the case of cider our govern-
ment experts have separated several forms of yeast that are now
offered for distribution and which give distinctive flavors to
cider. So in brewing and bread making enough has been done
to show that there are numerous forms of yeast that differ
materially in their power to induce fermentation as well as in
the nature of the fermentations that they bring about.

(b) Bread Making. — The most important use of the yeast plant
is its application to the "raising" of bread. Flour contains, in
addition to starch, a little sugar, and this amount is increased in
bread making by a ferment, diastase, in the flour which changes
starch into sugar. The flour is mixed with water containing
yeast plants into a dough and placed in a warm place, when it
begins to rise. This means that the yeast plants begin to grow
and decompose the sugar into CO2 and alcohol. The dough
prevents the escape of the gas which collects in bubbles in the
dough, causing it to swell. Baking further expands the gas and
also drives off the water and alcohol, leaving the bread light and
porous. There are many forms of yeast plants which differ in
their power of producing fermentations and in the flavor which
they impart. Consequently, different forms are used for differ-
ent purposes. Bread yeast is a form that has been selected be-
cause of the quickness of its action and the flavor that it imparts
to bread. Bread yeast is grown in large vats and put up with
starch in cakes, known as compressed yeast.

DEVELOPMENT OF PLANTS 251

Class C. Basidiomycetes or HAsii)iA-F(jkMi\(; Fingi

94. General Features. — This class contains two iinporiant
groups of fungi; one of which inchidcs very deslriKiive para-
sites, and the other comi)rises those conspicuous and fainihar
saprophytes known as nuishrooms, bracket fungi, puff balls,
etc. The striking feature of tlie class is the formation of spores
on club-like hyphae called basidia (sing, basidium) (Fig. 171, D).
These organs are often developed in rather complex outgrowths
of the mycelium which may be fleshy or w^oody, as in the mush-
room and bracket fungi (Figs. 170; 173, B). There is iif) known
sexual reproduction, save possibly in the first order mentioned
below w^here a vegetative union of cells appears to ha\'e been
substituted for a fertilization process which was originalh- like
that seen in the red algae. The more important orders are the
following :

95. Order a. Uredinales or Rusts. — These parasites are well
known by the streaks and blotches of yellow or black rust which
they produce on the leaves and stems of a great variet>' of plants.
About 2,000 species are known in the United States. They are
among the most destructive parasites, causing great damage to
wheat, oats, apples, quinces, roses, carnations, etc. The yearly
loss from grain rust alone is estimated at considerably over
$18,000,000 in the United States. They exhibit a degree of
variation not paralleled among other plants, more than U\c dif-
ferent kinds of spores being formed by some species in their
life history. This is due, doubtless, to the influence of the
climate, the spores varying with the season (spring, sunnner and
fall) and the plants upon which the fungus grows, for one and the
same fungus may grow^ upon different plants, pnxlucing one or
more kinds of spores on each. Forms infesting dilYerent sixties
of plants are said to be heteroecious and those living ujion but one
species are termed autoecious.

{a) The Life History of Puccinia.—Scvcnxl si)ecies of this
genus infest wheat and illustrate the many forms that may
appear in the life history of a rust. ( )ne phase of the life of this
parasite appears upon the leaves of the barberry. During May
and June the mycelium growing in the leaves forms rounchsh

252

DEVELOPMENT OF A RUST

bodies which rupture the epidermis and finally open out into cups
filled with chains of yellowish spores. An examination of
Fig. 162 shows that these spores are formed in rows at the end of
hyphae and surrounded by a layer of rather thick- walled hyphae.
This stage of the rust is known as the cluster cup or aecial stage
and the spores are called aeciospores. Often smaller spore-bear-

FiG. 162. Cluster cups as seen in section of leaf of spring beauty, Clay-
tonia. At right one of the cups is ruptured, exposing the aeciospores.
Below a small cup, pycnium, is discharging pycniospores that are possibly
functionless male gametes.

ing cups, known as pycnia (sing, pycnium), are associated with
this phase of the fungus. These small spores, while capable of
germinating, do not appear to enter into the life history of the
fungus by producing a new parasite. They have been looked
upon as male gametes that originally effected fertilization in a
female organ from which developed the spore-bearing cluster cup,
the process being similar to that noted in the Red Algae. On
the other hand they are regarded by some authorities as spores
that have lost their power to germinate and so breed the fungus
asexually. The process of spore formation in the cluster cup,
as will be seen from the outline given below, is so different from
that of the algae and is known in so few forms that any interpre-
tation of the phenomena should be deferred for the present.

DEVELOPMENT OF PLANTS

253

The spore-producing hyphae in the young aecidia form a rather
loose stratum of somewhat elongated cells (Fig. 162, A, i). These
cells divide, forming an upper series of cells which are smaller
and narrower than the lower or basal cells. These upper cells are
sterile, ultimately disappearing, and they have been coniixircd

Fig. 162.4. Spore formation in a cluster cup: i, appearance of the hyphae
in a young cup. 2, one of the cells from Fig. i, after it has divided into
the sterile cell and the large basal cell. 3, early stage in the fusion of the
basal cells of two adjacent hyphae. 4, the two nuclei of the fused cells have
divided and two of them are passing to the base of the cells and two into the
fused region of the cells. 5, the upper portion of the fused area is cut off,
forming the spore mother cell. This cell will divide once, forming a spore
and a small sterile cell. 6, a chain of spores and sterile'cells is being formed
as noted in 5. — After Christman.

to remnants of the tubular outgrowth of the female gametan.uium
noted in the red algae.

The basal cells enlarge, become inclined towards one anuthcr
in pairs and finally meet at their upper ends, the walls dissolving
at the point of contact. The nuclei of the two cells now pass
to the fused region of the cells and divide, forming four nuclei.
Two of these wander back into the base of their resjx^ctive cells,
while the other two pass to the upper portion of the fused area

254 FORMATION OF SPORES IN THE RUSTS

and become separated from the basal nuclei by a transverse
wall (Fig. 162, A, 3-5). The upper cell formed in this manner is
the spore mother cell and it divides at once, forming an aeciospore
and a small intercalary cell. As soon as this process is completed
the two nuclei at the base of the fused cells move up again into
the fused portion of the cells, which region has elongated in the
meantime, and divided as before. In this way a chain of binu-
cleate aeciospores is formed, separated by small intercalary cells
(Fig. 162, A, 6) that soon disintegrate.

As soon as the cluster cups are ruptured the aeciospores are
scattered by the wind, and singularly, will only germinate and
infest the seedlings and young leaves of the wheat in which they
develop a mycelium similar to that of the barberry, but note that
the cells of this mycelium are binucleate like the aeciospores.
However, this mycelium forms in place of cluster cups, groups of
erect hyphae at the ends of which single reddish spores are formed
(Fig. 163, C). This growth ruptures the epidermis, exposing
the spores in rusty lines of blotches (Fig. 163, B). These yellow-
brown blotches account for the popular name of Rusts given to
this group of fungi. This is the summer or uredinal stage of the
parasite. The spores are known as urediniospores. This phase
of the fungus is a very destructive one, for the spores are formed
in great numbers and provided with thin walls. They are widely
distributed by the wind and germinate at once (Fig. 164, A)
on other wheat plants which soon show the rusty brown streaks
of urediniospores. In this way, the pest spreads with great rap-
idity from a single center of infection. This formation of ure-
diniospores goes on during the summer, sapping the vitality of
the plant and, in severe cases of infection, materially interferes
with the maturing of the grain.

Later in the season, this same mycelium forms in the leaves
of the wheat quite a different type of spore. They are formed in
the same manner as the urediniospores but are provided with
thick dark walls and from one to several spores are developed at
the end of the hyphae (Fig. 163, D). Consequently, when the
epidermis is ruptured, these spores form rusty black blotches on
the leaves. This third stage is known as the telial, since it ends

DEVELOPMENT OF PLANTS

255

the season's growth. These spores, called tc'H(jsi)orc's, arc resting
spores and tide the fungus over tlie winter. Furthermore these
spores are uninucleate. The two nuclei which have character-
ized all cells from the aecial stage on, actually fuse, forminj^^ one
nucleus as the teliospores mature. Some regard this as a dchiNed
fertilization. The teliospores germinate in the sj)riHg (juite

}fifi:|

Fig. 163. Fig. 164.

Fig. 163. The summer and fall stages of a rust, Puccinia: A, rust blotches
on leaf of wheat. B, portion of leaf magnified, showing rupturing of the
epidermis due to the formation of spores. C, urediniospores or the summer
spores which effect a rapid distribution of the parasite during the summer.
D, teliospores or fall spores which are dormant during the winter.

Fig. 164. Germination of uredinio- and tclio-spores: A, the thin-wallcd
urediniospore sending out hyphae from thin places in its wall. This is ef-
fected as soon as it is carried by the wind to a moist leaf. 13, teliosporc ger-
minating in the spring and forming a short hypha, from the end i>f whirh four
cells have been cut off that are forming the basidiospores, b.

independent of any plant and being deixMideut wyxm llu- lood
stored in the spore, they only form short hyphae which usually
become divided into four cells (Fig. 164, B). This structure is
known as the basidium. Each cell of the basidiuin sends out a
delicate tube into the end of which the cell contents passes, thus

255 FORMS OF THE RUSTS

forming a small spore, known as the basidiospore. This basldial
stage completes the life history of the fungus for the basidiospores
are carried to the leaves of the barberry and begin again the life
cycle of the parasite by forming the cluster cups. It is held by
many that this sequence in spore formation represents an alter-
nation of generation. According to this view the aecium is the
result of a sexual process. Therefore the aeciospore would be
the beginning of the sporophyte or asexual generation, which
terminates in the teliospore. The germination of the teliospore
marks the beginning of the sexual generation and it is interesting
to note that we have four cells produced as in the tetraspores of
the red algae.

It is not surprising that this story was not unravelled for a
long time and that these different stages of the parasite were
known as distinct species. The first clue to the relationship was
gained in England where it was observed that the wheat fields
to the leeward of the barberry bushes were especially infested with
rust. For this reason, a law was passed early in the history of
Massachusetts compelling the destruction of the barberry bushes.
This suggestion of relationship between the cluster-cup stage of
the barberry and the rust of the wheat finally led to the inocu-
lation of wheat plants with aeciospores and this resulted after
a week or more in the appearance of the characteristic rusty
streaks on the leaves of the wheat.

(b) Features in the Life History of other Rusts. — Considerable
variation characterizes these rusts, not all of them having so
elaborate a life history as that outlined above. One of the most
common species of Puccinia affecting the wheat is perennial in
the wheat and possibly in other grasses where it produces ure-
diniospores in the spring, thus, the aecial, telial and basidial
stages are eliminated. In the apple rust, the uredinial stage is
missing. This disease produces on the leaves of various members
of the apple family, yellow patches in which are formed tube-like
cluster cups (Fig. 165, A). The aeciospores are only capable of
infesting the juniper, in the branches of which they produce
gall-like swellings (Fig. 165, B) known as cedar apples and also
sometimes bushy outgrowths known as witches' brooms. In the

DEVELOPMENT OF PLANTS

'57

spring, variously shaped masses of h>[)liae hearing nunu-rous
teliospores radiate out from these galls and in tiie early spring
rains these strands swell up, forming conspicuous yellow, jelly-
like masses. The basidiospores are developed in the jelly and
infest the leaves of the apple, thorn, shadbush, etc. In some
of the genera of rusts all the stages appear upon the same plant
as in the May apple and jack-in-thc-pulpit. In the early spring

^FiG. 165. A rust, Gymnosporangtiwi, that infests the juniper and mem-
bers of the apple family: A, cluster cups on leaf of thorn apple. B, tclio-
spore stage on red cedar, juniper.

the stems and leaves may often be seen infected with the yellow
cluster cups which are followed later by the dark-colored teli-
ospores. In other cases, teliospores only are produced, as in the
hollyhock, and this was doubtless the original form of the spore.
Numerous physiological species have been reported among the
rusts as in the case of the powdery mildews, and in both groups
we find some of these specialized forms able to grow upon dilTer-
ent plants if an Intermediate plant is used to "bridge" them over.
Thus a physiological species that grows normally upon the oat
but not upon rye or wheat may be made to do so b\- first growing
it upon barley. Spores derived from the barle>' will now infest
rye or wheat.

96. Order b. Ustilaginales or the Smuts.— TlRse para>iies
^^ -more primitive than the rusts to which they are doubtless
17

258

FORMS OF SMUTS

related. They produce very generally but one form of spore that
may possibly be compared to the teliospore. They are com-
mon and exceedingly destructive parasites, affecting especially
the flowers and fruits of corn and other cereals as wheat, oats
and barley (Fig. i66). The damage b}- smuts to the corn crop

Fig. i66. Fig. 167.

Fig. 166. A common smut, Ustilago, transforming the kernels of corn
into sooty black pustules.

Fig. 167. The formation and germination of the spore of a smut: A,
the formation of the spores from the mycelium in the kernel of corn. B,
germination of a spore and the appearance of the basidiospore.

of the United States exceeds $2,000,000 annually. In the case
of corn, the parasite keeps pace with the growth of the plant
without producing serious damage until the flowers appear, when
the mycelium increases greatly in the affected ears and "tassels"
producing sometimes enormous malformation, especially in the
ears, which appear as glistening white blisters or pustules. Later,
these bodies change to a sooty black, owing to the transformation
of the cells of the mycelium (Fig. 167, ^) into black greasy spores.
These spores are scattered in clouds upon the breaking of the
pustules and germinate much after the manner of the telio-
spores of the rusts, infesting the young and delicate part«^ cf

DEVELOPMENT OF PLANTS 259

other corn plants by means of their basitli(jspores (V'l^. 167, B).
In other cases, the teliospores germinate in the sprinp:, infesting
the seedlings. This appears to be always the case in the de-
structive wheat, rye, oat and barley smuts. Infection only
takes place when the spores come in contact with the seed . ( 'on-
sequently, in these cases, the fungus can more rea(hl>- bi- fought
by treating the seed with some fungicide as formahne and copix»r
sulphate.

97. Order c. Agaricales or Mushrooms and Toadstools.—
This is the most familiar group of the basidiomycetes and its
12,000 odd species are commonly referred to as mushrooms and
toadstools. These fungi are largely saprophx'tes, li\ing upon
the humus in the soil and upon decaying wood. Se\eral of the
genera are exceedingly destructive to trees and cause great hjss
to the lumber industry. In the majority of cases, the fungi
appear unable to attack the living portion of the trees and only
thrive upon dead tissues, as the heart wood, wliicli they quickly
disorganize and render worthless. As in all the ])rc'ce(Hng groups
of fungi, the real plant body is a delicate mycelium that spreads
through the soil or decaying substances, as may readilx* be dem-
onstrated by splitting open a tree infested with one of the fungi
(Fig. 168). The "spawn" that is sold in seed stores for plant-
ing in mushroom beds is a dried mass of decaying leaves and
straw mixed with earth, in which the nncelium of tlie nuishrooni
has been allowed to grow. Often the h\phae of the m>(\hinn
lose their delicate character and become woven togetlu-r, forming
rather dense w^oody strands or plates, a dexclopment frecjuently
seen in the timbers of mines and under the bark of decaying trees.
In certain species the mycelium is phosphorescent and the cause
of the pale light that sometimes apjiears upon moist deca>ing
wood, the so-called fox wood. Tlie nurrliuin l)ear> at \ariuus
places the complex fleshy or woody IkkIv conunonly known as
the mushroom or toadstool. If the mycelium grows in a n-gular
manner, radiating outward in all directions, then the nuishrooms
will have a similar arrangement. The older central iK)rtions of
the mycelium will finally die off, while the newer i>ortions con-
tinue to radiate outward and produce the nuishrooms, which con-

26o

DEVELOPMENT OF A MUSHROOM

sequently appear in a more or less regular circle. In this way,
the fairy rings, which are often held in superstitious awe, come
about.

The Mushroom or Toadstool. — The common umbrella type of
mushroom consists of a stalk or stipe and a cap or pileus, on

Fig. i68.

Fig. 1 68. The mycelium of one of the Agaricales forming white masses
as its spreads through wood.

Fig. 169. Development of a mushroom: 3, early appearance of the mush-
room as a ball of hyphae on the strands of the mycelium, i, section of one
of these spherical masses of hyphae, showing the circular openings in which
the gills are developed. 2, a later stage with the gills formed and the velum,
vl, appearing as a delicate membrane.

the underside of which are located radiating plates or gills (Fig.
170, A). This structure originates on a strand of the mycelium
as a very small ball composed of a mass of interw^oven hyphae
(Fig. 169, 3). Soon, however, the hyphae of this mass begin
to grow in a very regular manner. At first the growth results
in the formation of a cavity in the upper part of the ball that
extends completely around it (Fig. 169, i). Hyphae now grow

DEVELOPMENT OF PLANTS

261

down into this cavity in regular lines, forming the radiating
plates or gills noted in Fig. 170. This development divides the
ball of hyphae into an upper part or pileus and a basal region,
the stalk or stipe. As this growth proceeds the mass of hyphae
extending from the margin of the pileus to the stijx,* becomes

P

Fig. 170. Habit of a poisonous mushroom, AmaniUi: A, the mature
mushroom — s, stipe; p, pileus; g, gills; a, annulus; v, volva, a jurt of which
appears in patches on the top of the pileus. C, young form of the Amantta,
the volva beginning to break. D, later development, the volva completely
ruptured, disclosing the pileus, stipe and velum, vl. — H. O. Hanson.

drawn out and ultimately forms a rather thin membrane known
as the velum (Fig. 169, 2, vl). This entire growth is often
enveloped by a sheath-like mass of hyphae, the volva (Fig. 170,
C, D). At this stage of development, the \-oung mushroom is

262 GROWTH OF A MUSHROOM

rather spherical in shape and so small that it is quite concealed
in the ground or by surrounding vegetation. When conditions
are favorable for further growth, as after a rain, each cell of this
miniature fungus absorbs moisture and rapidly expands, thus
causing the mushroom to spring up as by magic and reach its
full growth in a few hours. As the stipe elongates, the pileus
spreads out like an umbrella, rupturing the veil, a part of which
clings to the stalk (Fig. 170, A, a) in the form of a ring, the
annulus, and a part may also hang in the form of ragged fila-
ments from the edge of the pileus. If a volva is formed, this
is also ruptured by the elongation of the stipe, forming a cup at
the base of the stipe, and usually portions also remain attached to
the top of the pileus as scales and patches (Fig. 170, A, D).
The structure of the mushroom is very simple. The stipe con-
sists of a mass of nearly parallel hyphae that show little modi-
fication save at the surface of the stem, where they are sometimes
more compactly arranged (Fig. 171, A, B). By cutting across
the gills, so that we can look into the ends of them, it will be
seen that the hyphae extend down the center of the gills and also
continually radiate out on either side, forming a compact layer
of rather elongated cells on the surface of the gills, known as the
hymenium (Fig. 171, C). A magnified view of a portion of this
hymenium shows that it is composed of paraphyses and basidia
(Fig. 171, D). These so-called paraphyses are potential basidia
and continue to develope as such during the life of the mushroom.
The basidia are not divided as in the smuts and rusts, but the
spores are formed in the same manner at the end of two or four
small tubes that grow out from the apex of the basidia. It
should be stated that a series of intermediate forms exist that
connect the divided basidia with the present form. The spores
are mature and begin to drop off as soon as the pileus opens,
when they are scattered by the wind and develop under favorable
conditions into a new mycelium. If the pileus of a freshly
opened mushroom is placed on a sheet of gray paper and tightly
covered wuth a dish, so as to exclude all air currents, the spores
will fall directly upon the paper and in a few hours form an
exact copy or a spore print of the gill arrangement. There

DEVELOPMENT OF PLANTS

263

are several very widely distributed and familiar families of the
Agaricales, 'distinguished by the arranj^emcnt and distribution of
the hymenium.

A. Thelephoraceae. — These fungi form mfml.r.mcu-. l.•:.t].,•r^•

Fig. 171. Structure of a mushroom: .1 ami B, cross ami longitudinal
sections of a portion of the stipe, showing the character and arrangement
of/the hyphae that make up the mushroom. C, tangential view of the gills
— p, pileus; h, hymenium appearing as a dark band on the surface of the
gills. D, a portion of the hymenium enlarged— 6, basitlia; pa, ixiraphyscs;
s, basidiospores.

or woody incrustations or shell-like strucliirrs (»r branching
bodies on soil or wood (Fig. 172, A, B). The hymenium forms
a smooth or slightly wrinkled surfac^^ oii ih<- tnultr side or ex-
posed surface of the fungus.

264

FAMILIES OF AGARICALES

B. Clavariaceae or Coral Fungi. — This family includes the fairy
clubs and coral-like fleshy masses of various colors (Fig. 172, C).
The hymenium covers the branches.

C. Hydnaceae or Prickly Fungi. — These fungi form masses of
widely various forms, but provided with spine-like outgrowths
upon which the hymenium is developed (Fig. 172, D).

\C

c^ ^s

Fig. 172. Forms of the Agaricales: A, Stereum, top view. B, underside,
showing the characteristic smooth hymenial surface of the Telephoraceae.

C, a coral fungus, Clavaria, with hymenium confined to the tips of the branches.

D, a prickly fungus, Hydnum, with the hymenium on spine-like projections.

D. Polyporaceae or Pore Fungi. — This group includes largely
leathery, woody or corky fungi in which the hymenium is de-
veloped on the surface of pores. They contain some of the most
destructive of the timber-destroying fungi. Many of the genera
of this family form shelf-like outgrowths on trees and are there-
fore known as bracket fungi (Fig. 173, B-D). Professor Duller
estimated that a single pore in one species of this group which he
found growing upon a decaying elm, produced no less than
1,700,000 spores and that the entire bracket, about 250 sq. cm.
in area, formed over eleven billion spores. The combined annual
output of the ten brackets growing upon this tree exceeded by
fifty times the population of the globe. This enormous spore

DEVELOPMENT OF PLANTS

265

production is characteristic of nearly all of the A^aricalcs and it
is not surprising that a bit of dead wood or piece of lumber can
not be exposed without being infested with the sy)ores.

E. Boletaceae or Fleshy Pore Fungi. — The members of this
family are generally characterized by a stalk and a pileus which

Fig. 173. Pore forms of the Agaricales: A, Boletus, showing flesii\ \)<^vv-
bearing layer, p. B, top view of a woody, bracket form, Eljvingia. The
concentric lines represent the annual growth. C, section of a similar form,
showing three layers of pores that represent three year's growth. D, en-
larged view of under surface, showing one of the pores with hymenial layer.

bears pores, the latter being easily separable as a layer from the
pileus (Fig. 173, ^)-

F. Agaricaceae or GUI Fungi. — These fungi more commonly
assume an umbrella form, although some of them are of the
shelving bracket type (Fig. 170). The hymcnium is arranged
on the surface of gills or plates. Man>- of them arc highly
prized for the table, though they contain comparativch- little
nourishment, and must be regarded as relishes rather than fotxls.
While the majority of the 5,000 species of the famih' are edible,
some of them contain the most deadly poisons. No rule can be
given that will enable the collector to separate these fungi into
the two mythical groups of poisonous toadstools and edible
mushrooms. Each species must be known individually beft^re it
is safe to use them for food. All forms with a vulvii should l^e
most carefully identified, because this is a feature of the deadly
amanitas (Fig. 170), which are among the most poisonous plants
known.

266 CHARACTERISTICS OF THE PUFF BALLS

The remaining orders of Basidiomycetes are characterized by
the concealment of the basidia in cavities and are for this reason
collectively known as the Gasteromycetes, meaning stomach
fungi. Among the more common orders may be mentioned:

98. Order d. Lycoperdales or Puff Balls. — These familiar

Fig. 174. Cluster of common puff balls, Ly coper don. At left three older
ones have opened, permitting discharge of basidiospores.

fungi are developed, as in the Agaricales, on strands of the
mycelium, which often form extensive net-like threads in rotten
stumps, logs, sawdust and humus (Fig. 174). The pufT balls
vary in size from a pea to over a foot in diameter. When young,
they consist of white cheesy masses of hyphae which form in

"^ Fig. 175. Diagram of a section of one of the puffballs, showing the thick
skin of periderm and the irregular cavities which are lined with basidia. At
the base the larger, sterile cavities of the stipe are shown.

the interior of the puff ball a series of irregular cavities lined with
basidia and on the exterior, a rather firm skin or periderm (Fig.
175). At maturity, the inner hyphae break up, leaving only a

DEVELOPMENT OF PLANTS] 267

dusty mass of spores and in somes cases firmer hyphae, the capil-
litlum. The skin ruptures in various ways, often Ijy one or
more pores at the top, and the least touch now causes the spores
to sift out in smoke-Hke puffs, hence the popular name of puff
balls. In the earth stars (Fig. 176) the outer layer of the jx^ri-

FiG. 176. One of the puff-balls popularly known as "earth stars," show-
ing the outer periderm splitting into star-like sections and the inner jK-ri-
derm opening by a pore.

derm splits into rather regular star-like segments or vahes which
are hygroscopic. In damp weather these valves roll back, in
some species to such an extent as to lift the puff ball from the
ground, when it may be set rolling by the wind and thus bring
about a better discharge of the spores.

Fig. 177. A common bird's-ncst-fungus, Crucihulum: A, habit of fungus
on branch of hickory. B, one of the cups in section, showing the Xo\\v^\ hyphae

surrounding the spore-bearing cavities.

99. Order e. Nidulariales or Bird's-Nest Fungi. — These

minute and curious fungi may be found growing: upon twigs or
upon bare ground in old fields, or upon dried dung {\''\^- 177. -'H-

268

DEVELOPMENT OF PHALLALES

They differ chiefly from the puff balls in that the spore-bearing
cavities are surrounded by tougher hyphae. Consequently,
when the periderm of these little cup-shaped bodies opens, these
tougher parts appear as minute eggs in a nest (Fig. 177, B).

100. Order f. Phallales or Stink Horns. — These fungi first
appear as egg-like structures on rather coarse strands of the
mycelium which traverse decaying vegetation. These bodies
consist of a white skin-like periderm which encloses a stipe and

Fig. 178. A common form of the Phallales, Phallus: A, the so-called egg-
stage which in B, sectional view, is seen to consist of an outer periderm, p,
and within is a central stipe, s, capped with a pileus. The spore-bearing,
cavities of the pileus are shaded. C, stipe and pileus have emerged from the
periderm, which forms an irregular sac about the base of the stipe. D, later
stage after the spore-bearing layer has dissolved, revealing the cavities on
surface of pileus.

pileus (Fig. 178, A, B). The basidia are formed in honeycomb-
like cavities on the outer surface of the pileus. As soon as the
spores are matured the stipe quickly elongates, rupturing the
periderm and lifting the pileus into the air (Fig. 178, C). In
structure and coloration, the Phallales are among the most attrac-
tive of the Basidiomycetes, but the majority of the forms are
regarded with aversion, since the spore-bearing layer melts
down into a slimy mass which emits a carrion-like stench.

CHAPTER VII
DIVISION II. BRYOPHYTA. THE LIVICRWORTS AND MOSSES

loi. Adaptive Features of the Bryophyta. — \Vc have now
reached the point in the evolution of the pkxnt where it has left
the aquatic condition of its algal ancestors and become largely
terrestrial. As has been stated, the fungi do not enter into the
scheme of evolution of higher types. They are looked upon
as a side line, derived doubtless from various groups of the algae,
that have undergone a peculiar series of variation owing to their
parasitic and saprophytic habits and that are in no way connected
with higher plants. In the Bryophyta we take up again the line
of ascent. Naturally we would expect that a change from the
aquatic to terrestrial conditions would be attended with profound
changes, tending to adapt the plants to the terrestrial conditions.
The results of the stimulus of the new surroundings are seen in
the production of hair-like outgrowths, rhizoids, that anchor the
plant to the substratum and bring it into proper relation with the
soil. So long as the plant lived in the water any of its cells might
serve as absorbing organs, but it is evident that the provision
must now be made for procuring the crude materials which
are no longer in direct contact with the plant. The exposure of
the plant to the atmosphere results also in the formation of a
cuticle and of mucilage-producing cells which tend to retain the
moisture and so adapt the plant to its drier environment. It
will also be noted that modifications are beginning to ai)ix\ir in
the arrangement of the chlorophyll-bearing cells, with the result
that they are sometimes distributed in the same favorable jxisi-
tions for photosynthesis, as you have noted in the higher plants.
However, the Bryophyta are only imperfectly adapted to ter-
restrial conditions and the entire plant body often assists in the
absorption of moisture and mineral substances. For this reason
most of them are moisture and shade i)lants, some indeed being
still aquatic, and they remain rather small and inconspicuous,

269

270 CHARACTERISTICS OF THE BRYOPHYTA

owing to the fact that they have not as yet varied and produced
tissues that can supply them with an adequate amount of mois-
ture and protect them against the cHmatic changes. Owing to
their habit of growing together in colonies, they often become
conspicuous and cover the bare earth, logs and tree trunks
with swards and mats that constitute one of the most attractive
features of the forest vegetation, and in northern regions they
often form the most characteristic feature of the vegetation
over large districts.

102. General Characteristics of the Bryophyta. — Certain
groups of the Bryophyta are characterized by thalloid plant
bodies, i. e., without distinction of stem, root or leaf, while many
have developed into creeping or erect plants with leaf-like organs.
All are distinguished by a marked localization of the growing
zone, the elongation of the plant being due to the repeated divi-
sions of a single apical cell. The reproduction of the Bryophyta
shows such marked departures that we realize that a wide gulf
separates even the simplest of them from the Thallophyta.
Notable among these departures is the absence of zoospores.
Only in a single genus are nonciliated bodies formed and dis-
charged from certain cells as in the case of zoospores. In this
connection it is worth your time to look back and note the
trend of plant life. The lowest forms of the green algae were
essentially motile. Then the stationary condition arose and
the motile state represented by the zoospores became less and
less conspicuous. In the bryophytes we perhaps have in the
spores, referred to above, an interesting example of the last
trace of the motile phase of plant life. The suppression by the
sexuafplant of the spore method of reproduction is due to several
causes. They are able to increase their numbers in a vegetative
manner. Special branches or ordinary shoots become detached
from the parent plant by decay of the older parts or by other
means and develop into new plants. Buds, called gemmae,
consisting of one or more cells, are also very commonly formed
on various parts of the plant and becoming detached grow into
new plants (Fig. i86). Possibly also the higher organization of
the Bryophyta, which renders them more capable of meeting

DEVELOPMENT OF PLANTS 271

conditions that would be fatal to the lower forms, tended to
make them independent of spore reproduction.V. But especially
has this change been brought about by the fact that the ganicto-
spore in germinating develops an asexual plant or generation with
enormous possibilities in the way of spore production, so that the
increase and distribution of individuals is now fulh- i)rovidcd for
by the asexual plant and consequently largeh- remcned from the
sexual plant. This shifting of the responsibility for the increase
in number of individuals from the sexual to the asexual generation
is one of the most significant departures in the evolution of plants.
We will be interested to note how this change came about and
how it steadily gains in importance in the remaining divisions.

The sexual reproduction of the bryophytes reveals a scries of
variations that are only remotely suggestive of the algae, but that
indicate very clearly relationship with higher plants. These
features will be considered in the discussion of the various groups.
There are two classes of Bryophyta: A, The Hepaticae or Li\er-
worts; B, the Musci or Mosses.

Class A. Hepaticae or Liverworts

103. Classification of the Liverworts. — While these plants are
the simplest and least conspicuous of the Bryophyta, they are the
most interesting, because among them will be found reminders of
the algae and at the same time they present many features that
are suggestive of the mosses and ferns. They occur in very
moist places and in deep woods, on rotting logs and moist shady
banks. A few are aquatic, floating uix)n still ponds and streams,
and some have become adapted to dry conditions. There are
three orders: (a) Marchantiales or Thallose Liverworts; (6)
Jungermaniales or Leafy Liverworts; (c) Anthocerotalcs or
Horned Liverworts.

104. Order a. Marchantiales or Thallose Hepatics.- Ihc
liverworts of this order are characterizrd b\- llat, prostrate and
rather fleshy bodies which usual !>• grow upon tlu' earth, to which
they are attached by numerous rhizoids. Owing to tiie frecjuent
division of the apical cell of the thallus into two equal parts, there
result two growing points which thus produce the equal forking

272

STRUCTURE OF RICCIOCARPUS

or dichotomous branching of the thallus, so characteristic of
these plants (Fig. 179, A, B). The appearance of many of these
hepatics is suggestive of the algae. Especially is this true of the
aquatic Ricciocarpus and Riccia.

(a) Structure of Ricciocarpus. — An examination of the struc-
ture of one of these will show, however, that extensive changes
have been induced in even the simplest forms. The new stimuli
to which the terrestrial conditions expose them cause a remark-
able series of transformations In the cells that are cut off from the

Fig. 179. Forms of semiaquatic Marchantiales: A, Riccia, showing the
dichotomous branching of the thallus. B, Ricciocarpus. The sexual or-
gans are concealed in furrows that appear as radiating lines in the center
of the branches. C, diagram of a cross-section of a branch, showing the
male gametangia in the bottom of one of the furrows.

apical cell. The upper cells of the thallus, as soon as they are
formed at the growing point, are exposed directly to the air and
light, and they develop chlorophyll and grow up into vertical
rows or plates just as you have already noticed in the palisade
chlorenchyma of the leaf (Fig. 180, ^). At an early period thege
rows of green cells become separated so that air spaces arise
between them, and thus the chlorophyll-bearing cells are brought
into direct contact with the atmosphere and enabled to carry on
photosynthesis to the best advantage. The terminal cells of
these rows enlarge considerably and form a rudimentary epider-
mis. It is also interesting to note that the direct contact of the
cells with the atmosphere results for the first time in the forma-

DEVELOPMENT OF PLANTS

273

tion of a cuticle. In most species the air spaces arising among the
upper cells of the thallus increase greatly in size and the epidermal
cells by vertical divisions keep pace with this enlargement and
thus arch over the air cavities, forming, however, a small (jpening
in the epidermal layer that permits a free circulation <>{ the air
(Fig. 180, B). These openings are suggestive of stomata and
function in the same w^ay, though they have originated in an
entirely different manner. The lower cells of the thallus are
compact and doubtless serve as storage cells, being nearly or
quite destitute of chlorophyll. The stimulus of the soil causes

Fig. 180. Structure of the thallus of Ricciocarpus: A, section of the
thallus, showing the apical cell, x, forming cells that by further division de-
velop into plates of cells separated by air spaces, j. At the left the plates
thus formed are seen curving over the apical cell. J5, an older portion of
the upper part of the thallus. The air spaces, _;, are greatly enlarged and
the upper cells of the vertical plates have divided, arching over the air spaces
but leaving small openings which permit the entrance of air for ph"''><v'i-
thesis.— I. D. Cardiff.

the lower epidermal cells of the thallus to form numerous smooth
or pitted rhizoids that are suggestive of the root-hairs of tin- hiulur
plants, anchoring the plant to the substratum and assisting in the
absorption of the earth substances. Plates of cells similar in
origin to those occurring on the u\)\wv surface are also found
on the under side of the thallus (Fig. 180, A). These plates
curve up around the growing point and dcnibtlcss protect it
against drought, in which work they are assisted by the mucilage
18

274

REPRODUCTION OF RICCIOCARPUS

cells or glands. The development of these glandular cells on the
plates, as well as on the other parts of the thallus, adapt the
plants to the drier terrestrial conditions to which they are
exposed, as was the case with the mucilaginous walls of the
Schizomycetes and Z^^genmatales.

(b) Sexual Reproduction of Ricciocarpus. — The gametes are
produced in more complex gametangia than we have as yet seen.
These organs are developed upon the upper surface of the thallus

Fig. i8i. The origin and structure of the male gametangia or antheridia:
A, section of the thallus, showing the apical cell, x, and the early stages,
a, h, in the development of the antheridia. B, older antheridium with cells
dividing vertically. C, later stage in which the wall cells are differentiated.
D, mature antheridium of Marchantia, showing the numerous cells that
develop the male gametes and the wall cells, w. E, greatly enlarged view
of a male gamete after discharge from the antheridium.

and in some species appear as lines radiating from the center of
the plant. The male gametangium originates from one of the
superficial cells of the thallus, which at first continues to divide
transversely after the manner of the vertical plates of chlorophyll-
bearing cells (Fig. i8i, A). The cells of these vertical plates
soon begin to divide vertically and thus form an elliptical mass
of cells (Fig. iSi, B, C). As this growth goes on the outer or
wall cells become larger than the others and generally develop
chlorophyll, while the inner cells divide repeatedly and become

DEVELOPMENT OF PLANTS 275

very numerous with dense granular contents (Fig. iHi, D). Each
of the latter cells produces a single male gamete that is motile by
means of two cilia (Fig. 181, E). This complex gametangium is
suggestive of the structure we have seen in Ectocarpus (Fig. 119,
B). The male gametangium is commonly called the antheridium,
and the male gametes are often termed si)erins (»r antherozoids.
The gametes are discharged from the antheridium owing to the
absorption of water by the walls of the cells in the ui)per portion of
the antheridium which become mucilaginous at maturity and ex-
panding, rupture this region of the antheridium, thus permitting
the discharge of its contents. The walls of the gamete producing
cells also become mucilaginous and by the absorption of moisture
they may assist in the extrusion of the gametes. The j)ressure of
the surrounding tissues upon the antheridium is also an important
factor in this discharge. The mucilaginous mass that is extruded
from the antheridium in this manner, rapidly dissolves in the
water and sets free the male gametes or sperms. The female
gametangium is also quite different from the single-celled game-
tangia which w^e have noticed among the algae and fungi. It
originates as in the case of the antheridium, but de\elops into a
flask-shaped body, consisting of a long neck of several rows of
cells surrounding a central row, called the canal cells, and an en-
larged basal region in w^hich is developed the female gamete (Fig.
182). The female gametangium is commonh- known as the
archegonium and the female gamete is sometimes called the
oosphere or egg. The female gamete presents essentially the
same characteristics as seen in man>- of the algae, as Vaucheria,
Oedogonium, etc., but it is surrounded by a jacket of cell which
constitutes the basal portion of the archegonium instead of being
contained in a single cell. Possibly the archegonium has come
about from a multicellular gametangium as seen in ilu- brown
algae, owing to the sterilization of all but one of the cells. .Arche-
gonia, with two or more gamete-like cells, are sometimes found
in the mosses and ferns and there is evidence tending to show
that the primitive gametangia contained both male and female
gametes. As soon as the female gamete is formed, the apical or
lip cells of the neck open in the same manner as noted in the

276

ARCHEGONIUM OF RICCIOCARPUS

antheridium and the canal cells become mucilaginous, thus
forming a passage way to the female gamete (Fig. 182, F). The
male gametes, attracted, It is supposed, by cane sugar, developed
in the archegonlum, swim down the canal of the neck, and one
unites with the female gamete, as shown In Fig. 182, F, where

Fig. 182. Development of the female gametangium or archegonium:
A-D, stages in the development of the archegonium from a single cell. E,
nearly mature archegonium just before the female gamete is formed — en,
canal cells. F, lip cells have opened and the canal cells have dissolved, thus
forming a passage way to female gamete. A male gamete has entered and
is seen fusing with the nucleus of the large female gamete. — After Garber.

the two gametes are seen fusing. Note that mucilage retained
in the neck protects the female gamete against loss of water and
also that the attenuated form of the male is of advantage In
enabling it to work Its way through this substance.

(c) Germination of the Gametospore and Spore Formation. — '
The gametospore, resulting from this fusion, becomes surrounded
by a cell wall, but does not function as a resting spore as in some
of the algae. It germinates at once and by repeated divisions of

DEVELOPMENT OF PLANTS 277

its nucleus forms a globular mass of cells, the capsule, wiihin the
archegonium which keeps pace with its growth (Fig. 183, A, B).
The wall cells of this capsule are soon cJistinguishal)le, owing to
their watery contents and shape, from the other cells, which arc
rather cubical and densely granular. The growth and division
of the granular cells continues until about 400 have been formed,
when they round off, become separated from one another and
increase greatly in size (Fig. 183, C). These large r.Hs, r.ill,<|

■:-n vA

Fig. 183. Germination of the ganu'tosport": -1, lias^il portion of an arche-
gonium, showing the germinating gametospore in the lour-cell stage, B,
later growth, forming a capsule with wall cells, w, which inclose large gran-
ular cells. C, a portion of the capsule, showing spore niotht-r cells roun' m .•
ofif and floating in the fluid of the capsule. D, the mother cells dJM ...s
and forming four spores each. — II. O. Hanson.

Spore mother cells, form four spores each, as in the tctrasporcs
of the red algae (Fig. 183, D). The delicate walls of the cap-
sule break down as soon as the spor<>^ in- m nnnd. leaving them

278

THE SPORES OF RICCIOCARPUS

free in the archegonium, and later they are set free by the decay
of the latter organ. These spores in some forms are provided
with thick walls and are, therefore, resting spores adapted to
carrying the plant over unfavorable conditions for growth. In
other cases the spores have thin walls and germinate at once.
The advantage of the transference of the resting stage from a
single gametospore to the numerous spores derived from the
gametospore is manifest.

{d) Germination of the Spore. — The germination of the spore
is usually indicated by the formation of chlorophyll, and this is

Fig. 184. Germination of the spores: A, spore. 5, first division of the
germ tube. C, early form of the thallus, due to the formation and subse-
quent division of the apical cell.

followed by the rupture of the outer spore coat and the protru-
sion of the inner as a delicate papilla or germ tube (Fig. 184).
Usually from this tube a small, hair-like outgrowth is soon
formed which penetrates the soil as the first rhizoid. The germ
tube continues to elongate and often forms a chain of cells by
successive transverse divisions, but eventually by oblique divi-
sions an apical cell is formed that develops the characteristic
thallus (Fig. 184, C). Thus we arrive again at the starting point
in the life history of these simple plants.

(e) Noteworthy Departures in the Life History. — It should be
stated that the gametophyte of certain species of the Marchan-

DEVELOPMENT OF PLANTS 279

tiales may live for long periods without producing tlie gametes.
In fact, it frequently multiplies by means of buds and branches
which become detached and grow directly into new plants. How-
ever, as soon as conditions are favorable the sexual organs and
gametes will appear. There are several features in this hisi(jry
that must be kept clearly in mind. In tlic first place, tiie
gametospore is not discharged from the i)lant as in the case of the
green or brown algae, but remains permanenth- in the arche-
gonium, where it continues to be nourished by the plant during
its germination and the formation of its spores. Therefore, it
develops essentially as a parasite upon the plant, as is the case
among the red algae. This retention and nourishment of the
germinating gametospore in the plant is perliaps the most
important of any of the variations that appear in plant life.
Owing to this relationship the gametospore attains a larger growth
with increased power of spore production. In Ricciocarpus the
capsule is about 500 times as large as the gametospore. In this
way the development of many new plants is made possible by a
single fusion of gametes. This is a very significant feature in
terrestrial plants, since the fusion of the gametes is effected with
more difficulty, owing to the absence of aquatic conditions.
Without doubt this change was induced by the transference of
foods from the sexual to the asexual plant deri\ed from the
gametospore. This loss of food gradually prevented the sexual
plant from producing spores or other bodies designed to multiply
its numbers and we will finally see that the sole work of the sexual
plant is limited to the production of gametes. Note also that
the gametospore not only has a larger growth but a longer life.
This is of the utmost importance because it became exi)osed in
this way to a new series of stimuli that affected it jirofoundly and
that resulted in the evolution of the higher types of plants.
Among the green algae the actual germination of the gameto-
spore is limited to a few hours at the most and this is effected in
the water, where the conditions are exceptionally uniform. Tlie
germination of the gametospore in the case of the li\erworts is
prolonged over several weeks, and more important still is the
fact that this growth occurs on the land where it is exposed to a

280 THE TWO PHASES IN PLANT LIFE

wide range of stimuli that cause it to var}^ in a most remarkable
manner. So we must bear in mind the character of this rather
simple growth derived from the gametospore of Ricciocarpus and
compare it with the variations that appear in the succeeding
forms.

It is also evident that the fusion of the gametes in the simple
liverworts results in the production of a cell, the gametospore,
that is radically different in its nature and possibilities of growth
from any of the cells of the parent plant or liverwort. This was
not apparent in the simpler forms of the algae, w^here the gameto-
spore produced directly a plant like the parent, as in Vaucheria
and Fucus. In Spirogyra, Ulothrix, Oedogonium and Coleo-
chaete, we saw the first indication of the real nature of the game-
tospore. It did not develop into a plant like the parent, but
uopdecdr cells or zoospores. The reason for this difference in
behavior is possibly due to the higher organization of the gameto-
spore. In the lower forms its first divisions result in the forma-
tion of cells that are of the same nature as the cells of the parent
plant, but in higher forms its composition is more complex and as
a consequence it forms a number of cells before cells like those of
the parent plant are developed. For example, in the Red Algae,
certain of the fungi and in Ricciocarpus the gametospore gives rise
to a varying number of cells that are different from those of the
parent plant, but finally spore mother cells are formed from which
are derived spores that are of the same nature as those of the
parent plant and that consequently produce new plants like the
parent. As has been stated on page 138, one difference between
the cells derived from the gametospore and those of the parent
plant is to be found in the number of chromosomes which they
contain, the former having twice as many chromosomes as the
latter. The doubling of the chromosomes occurs when the two
gametes unite to form the gametospore and this number is re-
tained during the germination of the gametospore until the divi-
sion of the spore mother cells, which results in the formation of
spores with only one half the number of chromosomes. In the
lower algae this reduction must occur in the first divisions of the
germinating gametospore, which therefore corresponds to the

DEVELOPMENT OF PLANTS 281

spore mother cell, but in the Rid Algae and Rkciocarpus a con-
siderable growth intervenes before the spore mother cells apfx-ar
and the reduction of the chromosomes takes place. In other
words, as we ascend the scale of plant life, the formal ion oi the
spore mother cells and the reduction of the chromosomes is pre-
ceded by an ever-increasing growth of the gametospore. This
postponement in the formation of the spore mother cells, owing
to the larger and larger growth of the gametospore, will steadily
progress in the following studies.

These simple liverworts, like Ricciocarpus, show very clearly
two phases or generations in their life history'. The thallose

Fig. 185. Diagram of the life history of Ricciocarpus. The upix-r ix>r-
tion of the figure represents the sexual generation and the lower portion,
the asexual. The former generation begins with the formation of the spores,
sp, from the mother-cell and ends with the formation of the gametes, g. The
asexual generation begins \vith the gametospore, gm, and ends with the sjwrc
mother cells, sm.

plant is the gametophyte or sexual generation because it lK»ars
sexual cells or gametes. The capsule is the sporophyte or asex-
ual generation because it can only produce spores. The gameto-
phyte begins with the spore and ends with the formation of the
gametes. The sporophyte begins with the gametospore and ends
with the division of the spore mother rrll (Fig. 185). It may
appear to you now as strange to regard the few cells of the cap-
sule, the majority of which become spore mother cells, as a plant.
But w«^ ^all directly see this microscoi)ic plant assuming larger
proT[89. Diagl? a result of its better nourishment and the stimuli
t- buds or^posed. It will form a larger mti.1 I.nvcr numU^r

282 STRUCTURE OF MARCHANTIA

of cells and the spore mother cells will not appear until late In Its
growth and they will be confined to definite regions and only con-
stitute a small part of It. We now call the gametophyte the plant
because it Is many times larger than the sporophyte, but we shall
see the sporophyte become quite as conspicuous as the gameto-
phyte, as in the mosses, and finally It will become much larger,
as in the ferns. Then we shall call the sporophyte the plant.

(/) Structure and Life History of Marchantia. — Let us now
examine a higher type of the Marchantiales, as Marchantia, and
note the advances that have been made over simple forms like
Ricciocarpus. Marchantia (Fig. i86) Is of common occurrence

,-v-'--5v^

"S'

'>-5=

^r- '"-'--^

■r-^-'^r^

'r@ '-^^

^^^- :

;:<;~.^^;

&'

•:?© "-

"*

Fig. i86. Fig. 187.

Fig. 186. Thallus of Marchantia bearing three cup-like organs that con-
tain buds or gemmae and also four erect branches that contain the anther-
idia in their upper surfaces in radiating lines.

Fig. 187. Portion of the surface of the thallus of Marchantia, enlarged
showing the rhomboidal air chambers and air pores.

in moist places, often appearing in greenhouses on the earth of
flower pots, and It also forms luxuriant beds on the damp ground,
especially where logs and brush have been burnt. The thallus
shows the same general features that w^e have noticed in Riccio-
carpus, being rather fleshy and creeping over the ground, to which
it Is attached by numerous rhizolds. The simple air chambers
noted in Ricciocarpus are much enlarged and appear as diamond-
shaped or rhomboidal plates on the surface of the thallus (Fig.
187). Thin sections across the thallus show that these air
chambers have a complex structure (Fig. 188). Thev '-mate
in the upper cells of the thallus and as they enlarge -^^o^s 01 ^^g
covered by a well-developed epidermis which fo-^ponas to tn

DEVELOPMENT OP^ PLANTS

283

like pore over the center of each cavity. From ilie l)on<jm of
the chamber numerous delicate chlorophyll-bearing cells arise.
This arrangement of the tissues is again suggestive of the chior-
enchyma of the leaf and it is manifestly i)rolective and adapted
to photosynthesis. The structure of the lower cells of the thallus

Fig. 188, Section through the center of the thallus of Marchantia, show
ing one of the air chambers and chimney-like pores in the epidermis — ch-
palisade-like chlorenchyma arising from bottom of air chamber. The lower
cells of the thallus are nearly colorless and filled with watery solutions or
mucilage, r, rhizoids; /, leaf-like plates of cells.

and the distribution of the rhizoids and ventral plates are essen-
tially as in Ricciocarpiis.

Marchantia, as in many of the liverworts and some mosses,
multiplies extensively b}^ means of buds or gemmae. Tlu'\- are

Fig.

Diagram of a section of one of the cups shtnvn in Fig. 186 — g,

buds or gemmae associated with small glandular •-•"-.

284

REPRODUCTION OF MARCHANTIA

borne in cup-shaped receptacles and consist of small lense-shaped
masses of cells which are freed from the thallus by the swelling
of the mucilage secreted by glandular hairs growing among them
(Fig. 189). These minute bodies grow directly into new plants
and so serve to rapidly multiply the plant.

The reproductive organs appear upon different plants which

a—

Fig. 190. Thallus of Marchantia bearing several erect branches with
lobed or umbrella-like tops. The archegonia are situated on the underside
of the umbrella, between the lobes, a, the early appearance of a branch,
in which condition fertilization is effected.

may therefore be distinguished as antheridial or male plants and
archegonial or female plants (Figs. 186, 190). Such a distribu-
tion of the sexual organs is termed dioecious, meaning in two
households, whereas Ricciocarpus and others are said to be mon-
oecious, because the sexual organs are developed upon the same
plant. The antheridia and archegonia are developed, as in Ric-
ciocarpus, upon the upper surface of the thallus, but they are
confined to special portions of it which appear at first as mush-
room-shaped outgrowths (¥\g. 190, a). These outgrowths, how-

DEVELOPMENT OF PLANTS

28>

ever, are but modified branches of the lliallus and l)ear the
sexual organs upon their upper surfaces in Hnes radiating from
the center of the branches. FertiHzation is effected, as in all
forms, by means of dews and rains which flood these organs with
water. An examination of a young an hegonial l)ran( li will show
how admirably it is constructed to ensure fertilization. The
archegonia radiating in lines have their necks strongly rurvcfl

Fig.

Fig. 191. P'iG. 192.

[91. Section of an archegonial branch similar to a, Fig. 190 — ar,

archegonia; in, involucre or curtain that hangs down on cither side of the
rows of archegonia; ac, air chambers; th, thallus.

Fig. 192. Section of a young antheridial branch: an, anthcridia sunken
in cavities of the branch, which is also provided with air chambers similar
to those of the normal thallus. Some of the anthcridia have discharged
their gametes, as at x.

upwards and away from the center of the branch. These lines
of archegonia are separated by ridges which thus act as a water-
shed, deflecting into the upturned necks any water containing
gametes that may chance to fall upon them. After fertilization
these mushroom-shaped branches elongate, raising the sexual
organs up into the air, where the antheridial branch assumes the
form of a lobed disc, while the archegonial branch terminates in
an umbrella-like structure (Figs. 186, 190). Owing to the ex-
tended growth of this latter branch, the archegonia come to lie
on the under side of the structure between the fmger-like out-
growths, where they are completeh- hidden and jirotecteti by
fringed curtains, the involucre, that hang down from the fingers

286

SPOROPHYTE OF MARCHANTIA

(Figs. 191, 195, yl). The antheridia retain their original position
upon the branch, where they appear in cavities, as shown in
Fig. 192.

The most important and significant departure in Marchantia is
seen in the germination of the gametospore which divides into an
inner and outer cell as in Ricciocarpus (Fig. 193, B). The inner

Fig. 193. Germination of the gametospore: A, section of a mature ar-
chegonium with canal cells dissolved, thus forming a passageway to the
large female gamete, g, B, sectional view of base of archegonium, show-
ing the germinating gametospore in two-cell stage. The perianth, p, is
seen growing up about the archegonium. C, later stage in growth of the
gametospore. The lower cell shown in B is forming stalk cells, while the
outer cell has produced densely granular cells that will later by further di-
vision form spore mother cells and elaters.

cell, however, by a series of divisions, forms a rudimentary stalk
and the lower part of it, known as the foot, comes into close
contact with the tissues of the plant from which it absorbs nour-
ishment (Figs. 193, 194). The outer cell divides, forming a
spherical mass of cells that resembles the capsule of Ricciocarpus.
Some of these cells of the capsule develop as spore mother cells,

DEVELOPMENT OF PLANTS

287

while others, known as elaters, elongate greatly and serve to con-
duct the foods absorbed by the foot to the spore mother cells, and
finally they become spirally thickened (Fig. 194, C). These
elaters arise through the sterilization of certain of the spore-

FiG. 194. Structure of the nearly mature sporophyte: A, continuation
of the growth in Fig. 193, C, showing the formation of a foot, /, stalk, and
capsules, c, which contain elongated dark cells, the elaters, and the spores.
The archegonium has been ruptured by the elongation of the stalk and is
not shown in the figure, p, perianth. B, enlargement of the base of A,
showing the attachment of the foot (indicated by darker lines) to the tissues
of the antheridial stalk. C, an elater and spores.

Fig. 195. Archegonial branch with mature sporophytes: .1 , branch with
the dark capsules of several sporophytes projecting beyond the curtains of
the involucre. B, diagram of a branch as seen in section. On the right
one of the capsules of a sporophyte ruptured, exposing the elaters and spores.
On the left the curtain-like involucre only is shown.

288

SPOROPHYTE OF MARCHANTIA

producing cells. This is an important departure from Riccio-
carpus and we see here the beginning of the tendency towards
the sterilization of sporogeneous cells that becomes more and
more pronounced in higher forms and that
plays an important part in the evolution of
plant life. During the germination and growth
of the gametospore, a delicate membrane (the
perianth) grows up about the archegonium
and doubtless assists the involucre in pro-
tecting it against drying winds ( Fig. 193,
p). When the spores are mature, the cells
of the stalk elongate, rupture the archego-
nium and push the capsule beyond the cur-
tains of the perianth and involucre, so that
it is exposed to the air (Fig. 195). The cap-
sule now ruptures, exposing the spores and
elaters to the air. The elaters, a word mean-
ing lifters, are very hygroscopic; they coil and
uncoil with the least change in the humidity
of the air and thus doubtless assist in the grad-
ual exposure of the spores to the air currents.
Fig. 196. One of The spores germinate as in the lower liver-
simpler thalloid J un- ^.^^^g ^j^^g ^^ g^^ ^j^^^ ^^^ sporophyte,
germaniales, FaUavi-
cinia, showing the consisting of a foot, steraand capsule, is more

rhizoidal growth on complex and larger than in Ricciocarpus.
the under surface of -pj^ig jg doubtless due to the better nourish-
ment which it receives as a result of the de-
velopment of a more efficient absorbing organ,
the foot.

105. Order b. Jungermaniales or Leafy
Hepatics. — By far the larger number of
hepatics belong to this order. They are
especially abundant in moist tropical countries, where they often
cover the stems and leaves with a rich vegetation, and with us
they are of common occurrence on dripping rocks and in deep
woods on the moist bark of trees and decaying logs or damp
earth. In the lower form the thallus is very simple and delicate

the thallus and a
mature sporophyte
arising from a cup-
like perianth which
is surrounded at its
base with involu-
crate leaves. — H. O.
Hanson.

DEVELOPMENT OF PLANTS

289

(Fig. 196). In other genera there are indications of a Icjljin^^ in
the thallus which becomes more pronounced in some forms and
leads by gradual gradations to genera in which the lobes appear
as distinct leafy organs arranged in two rows upon a stem-Hke

Fig. 197. One of the leafy Jungermanialcs, Porella: A, branch of the
plant bearing several sporophytes. B, under surface of a branch, showing
the lobing of the leaves and a row of minute scale-hke leaves. C, portion
of a branch bearing an archegonium surrounded by cup-like perianth with
minute involucrate leaves at base. D, branch with cone-like anthcridial
branchlets. At the left a single leaf is shown with globular antheridium
in its axis. E, section of a branch similar to C. The archegonium is seen
surrounded by the perianth and below the involucrate leaves. The sjxjro-
phyte is nearly mature and ready to elongate. It consists of a round cap-
sule containing elaters and spore mother cells that are dividing to form four
spores each. Below the capsule is the stalk or seta which ends in a foot
buried in the tissues of the branch. At the right is an unfertilized arche-
gonium that shows the original position and size of this organ. — H. O. Hanson.
19

290 STRUCTURE OF LEAFY HEPATICS

axis (Fig. 197). Usually a third row of rudimentary leaves,
associated with rhizoids, appears upon the under side of the thal-
lus (Fig. 197, B). Although the Jungermaniales tend to become
highly modified into a leafy body the tissues remain practically
unchanged in all forms. Even in the leafy genera, which com-
prise the majority- of forms in this order, there is little evidence
of such a differentiation of the tissues as characterize the Mar-
chantiales. Without doubt, the simple thallose Jungermaniales
are the most primitive forms of the hepatics and probably stand
nearer the ancestral type than any other form. The Marchan-
tiales represent a line in which the tissues of the thallus have
attained a considerable degree of differentiation, but in the
present order, the evolution led to a marked change of form with
but slight modification of the tissues. This is apparent in the
highest forms where the leaves, consisting of little more than a
single layer of chlorophyll-bearing cells, are arranged in two
rather oblique rows upon delicate stems. The under surfaces
of these leaves are generally lobed and often form sacs con-
taining water (Fig. 197, B). This peculiarity of the leaves,
together with the rudimentary third row^ of leaves that are
associated with the rhizoids, makes a sharp contrast between
the upper 'and lower surfaces and serves to distmgui^ them
from the mosses, with which they are often com used . 1 n fact,
they are often called scale mosses, owing to the moss-like ap-
pearance of their dorsal surface and the close contact formed
with bark or other surfaces over which they creep. This habit
of pressing the leaves against the substratum tends to retain
the moisture and also enables the leaves to absorb it directly
through their delicate cell walls. The overlapping of the
leaves in many genera, as well as their lobing, would serve the
same purpose and enable the plants to live under drier conditions
than would otherwise be possible. Doubtless, these departures
have been of great advantage to the plants and in part account
for the common occurrenx:e of these leafy forms.j ■ It should be
stated that the leafy hepatics are regarded as forms that have
been evolved from the simple thallose forms in quite recent
geological times and owing to their better adaptation to present

i

DEVELOPMENT OF PLANTS 291

conditions they have become the most numerous of all the
Hepaticae. The cause of the differentiation of ilu- tissues in
the two groups is doubtless explained by their relation to mois-
ture. The leafy hepatics are easily wetted and the entire i)lant
body can readily absorb moisture. Therefore the tissues remain
delicate and fewer rhizoids are required to absorb the moisture.
The Marchantiales are not wettable, and as a consequence they
must develop wick-like strands of rhizoids as well as the elabo-
rate storage tissues of the thallus. Both groui)s, and indeed all
hepatics, must remain prostrate, because they are dcfx-ndent
upon surface water and have not as yet developed an absorbing
and conducting apparatus that will permit of any other position.
Reproductive Features of the Jungermaniales . — Asexual repro-
duction is of the same vegetative type as has been seen in the
preceding groups. The archegonia and antheridia are also of
essentially the same character as seen in Marchantiale^ They
are borne upon the dorsal surface of the thallus or upon more
or less modified branches, the archegonia often arising upon
the apex of the branch and the antheridia appearing as rather
^^^ spherical bodies in the axis of the leaves (Fig. 197, C, D). The
archegonia are developed in rather conspicuous cup-like or leafy
outgrowths of the thallus. This structure, termed the jx^rianth
(Fig. 197, C), assumes very characteristic forms in tlie different
genera and is generally associated with modified lea\es, the
involucre. By these devices the archegonia gain the same
protection against drying out as was secured to them b>- the
cavities in which they were developed in the preceding group.
The gametospore develops after the manner noted in Marchatitia,
forming a capsule with elaters, but its stalk or seta reaches much
larger dimensions, owing doubtless to the weli-(le\ cloiu-d to<.)t
(Fig. 197, E). The majority of the genera also are characterized
by having many of the cells of the capsule sterilized so that
the number of spore-producing cells becomes further n-duied.
When the spores have been matured the seta rai)i(lly elongates
to several times its original length, rupturing tlu* archegonium
and lifting the capsule high in the air (Figs. i<)(); n)7. -'0-
The capsule usually breaks open into four \al\es which are hygro-

292 DEVELOPMENT OF ANTHOCEROS

scopic, closing over the spores In damp weather and opening
in dry weather to expose them to the wind. These spores germi-
nate and begin the life history of a new gametophyte. As in
some of the Marchantiales a filamentous alga-like growth is
first formed by the germinating spore before the characteristic
plant is reproduced.

1 06. Order c. Anthocerotales or Horned Liverworts. — This
small group of four genera is the most interesting of any of the
hepatics because it presents features that are suggestive of the
algae and also of a relationship with the mosses. ' The thallus
is of a primitive type, often with simple lobings and therefore,
suggestive of relationship with the simpler Jungermaniales (Fig.
198). A peculiar feature of the order is the occurrence of mucil-

FiG. 198. One of the Anthocerotales, Anthoceros, bearing four pod-shaped
sporophytes, j. The one on the right, s, has opened at the top and is dis-
charging the spores, but elongation and the formation of spores continue
below owing to its basal growth.

age cavities which communicate with the air through small
openings on the under side of the thallus. These cavities are
always occupied by one of the blue-green algae, Nostoc, which
possibly assists the plant in the retention of water owing to their
mucilaginous character. Another interesting feature of the order
is the occurrence usually of a single chloroplast in each cell, as in
Coleochaete and several other genera of the algae. The archegonia
and antheridia, while resembling those of the preceding orders,

DEVELOPMENT OF PLANTS 293

are sunken In the tissues. This arrangement doubtless protected
the reproductive organs just as did the Involucre and perianth
in the preceding groups. v,, - ---.,,^^^

The most suggestive variation of the Anthocerotales appears
in the development of the sporophyte. We have noted that the
asexual generation in forms like Ricciocarpiis was a microscopic
plant consisting of a delicate spore-bearing capsule. In Mar-
chantia the sporophyte is still minute but differentiated into a
foot, stalk and capsule, and In the Jungermanlales this becomes
much more pronounced. In the Anthocerotales the sporophyte
assumes a growth and dIfTerentiatlon that Is much more extensive
and complex and it also presents features that Indicate a rela-
tionship w^ith the mosses. Furthermore It shows certain features
in its development that also arose In the ferns. The germination
of the gametospore gives rise to a rather cylindrical or pod-shaped
body (Fig. 199), in which all the cells have become sterile save a
dome-shaped layer. The basal portion of the sporophyte de-
velops into a massive foot, often provided with rhizoldal-like
outgrowths, which serve as a very efficient absorbing organ.
The upper portion of the sporophyte presents a remarkable series
of differentiations. The outer part of it consists of chlorophyll-
bearing cells in which, for the first time, genuine stomata appear
(Fig. 199, ch). Within this zone of chlorenchyma is a dome-
shaped layer of spore mother cells often alternating with sterile
cells which in some genera develop as elaters. In the center of
the sporophyte is a mass of rather elongated cells, the columella,
which assist in conducting food from the foot to the forming
spores. Directly above the foot Is a region of rapidly di\iding
cells which causes the sporophyte to elongate by basal growth and
push out of the archegonium before the spores are mature. This
is a radical departure from the other liverworts In the mode of
development of the sporophyte and we will see essentially the
same manner of growth occurring In the mosses. As the sporo-
phyte develops, the spores in the upper part of it mature and the
sporophyte splits Into two valves (Fig. 198, s'), thus permitting
the scattering of the spores. The lower portion, however, may
continue to elongate for several months in some species, forming
additional spores.

294

SPOROPHYTE OF ANTHOCEROS

It is evident that this sporophyte would become a self-support-
ing plant if its foot should reach through the gametophyte and
absorb substances from the soil. /In comparing the sporophyte

Fig. 199. Fig. 200.

Fig. 199. Section of a young sporophyte of Anthoceros emerging from
the involucre-like outgrowth of the thallus — sp, dome-shaped spore form-
ing layer of cells; ch, chlorenchyma with stomata; h, foot or absorbing region;
c, region of growth. At right, surface view of stoma.

Fig. 200. A common moss, Funaria: A, two plants with root-like rhi-
zoids at base and radially arranged leaves. Rising above the leaves are
the stalks or setae and capsules of two sporophytes. B, magnified view of
a plant, showing the early appearance of the sporophytes as a delicate stalk
still covered by the enlarging archegonium or calyptra. C, a plant bearing
antheridia in a rosette of leaves at apex of stem. D, enlarged view of the
upper portion of the sporophyte, showing the twisting of the stalk that assists
in sifting the spores through the fringe of teeth, peristome, that encircles the
mouth of the capsule.

in forms like Ricciocarpus, Marchantia and the leafy hepatics
with Anthoceros, we see that it has undergone a gradual evo-
lution and has finally reached a point where it only needs to

DEVELOPMENT OF PLANTS

295

come in contact with the soil to become an independent i)laiit
since it is provided with all the tissues necessary for photosyn-
thesis and absorption. " Note also that the growth of the sporo-
phyte is becoming more prolonged, that the spore mother cells
appear later in its development and comprise but a small i)orlion
of it, and finally, that the sporogenous and sterile tissues alter-
nate, forming a dome-like zone of cells. Structures very sugges-
tive of these features will appear among the mosses and ferns.

Class B. Musci or Mosses

107. General Characteristics. — The mosses are by far the
largest group of the Bryophyta and show marked advances over
the hepatics in the development of the gametophyte and sporo-
phyte. Particularly noticeable is the size and differentiation of
the sporophyte which now becomes among the majority of the
genera of nearly equal importance with the gametophyte (Fig.
200). Variations appear in the mosses that have been successful
in adapting them to a great variety of conditions, ranging from
submerged aquatics to pronounced xerophytes that live upon
exposed rocks. As a result, they are of common occurrence
everywhere. The mosses, however, do not show any variations
that have led to higher types of plants. In fact, they are re-
garded as a highly specialized group, like the Jungcrmaniales,
that have branched off from the hepatics in recent geological
times and become the dominant representatives of the Bryojihyta
owing to their better adaptation to present conditions uj^on the
earth. This recent derivation of the mosses is indicated by the
remarkable uniformity of structure that characterizes the entire
group.

While the plants themselves are rclati\ely small, they often
form large swards or mats, owing to the extensi\e branching
and prolonged growth of the stems and their rapid multiplication
through the dying off of the older parts and the independent
growth of the branches that are thus set free. Owing to this
habit of growth and multiplication the mosses often furnish the
conspicuous features of the vegetation. Especially is this true
in northern regions and in bogs and liarrens \\ lu-re great sections

296 NATURE OF BOG MOSSES

of the country are carpeted with them. The mosses are leafy
stemmed plants like the Jungermaniales, but they usually
possess the decided advantage of having erect stems and radi-
ally arranged leaves (Fig. 200). Even in the prostrate forms
where but two rows of leaves can be developed, the resemblance
to the leafy hepatics is only superficial. The moss leaves are
not lobed and the central portion is usually traversed by a strand
of cells which serve for conduction like the vascular bundles of
higher plants (Fig. 206, B). The differentiation of the tissues
of the stem also shows a marked advance over preceding forms.
A central conducting region of elongated cells that may be
compared to a rudimentary vascular system and a cortical zone,
often with thickened cells and rudimentary epidermis, are fre-
quently to be seen (Fig. 206, A). Asexual reproduction is al-
most entirely confined to the detachment of branches as stated
above. In only a few genera have the formation of gemmae been
noticed. The sexual reproduction and the development of the
sporophyte present some interesting features that will be noticed
in the following orders.

108. Order a. Sphagnales. Bog or Peat Mosses. — A single
genus, Sphagnum, of numerous poorly defined species is the sole
representative of this order and in several respects it occupies
an intermediate position between the hepatics and the mosses
proper. These pale-green mosses (Fig. 201) grow on bogs and
moors and other places where they are subject to drainage con-
taining organic matter, as humic acid, derived from the decay of
plant and animal life. The majority of plants are unable to
endure these conditions, which are popularly referred to as sour,
and as a result, you will always find associated w^th the bog
mosses a rather limited and peculiar variety of plants such as
several genera of heaths, sedges, orchids, pitcher plants {Sar-
racenia) and other insectivorous plants like the sundew (Drosera) .
For some reason the ordinary plant is not able to procure its food
from these sour bogs and this may explain the common occurrence
in such places of insectivorous plants, and of certain trees and
shrubs that are associated with mycorrhiza. The sphagnums
grow luxuriantly in such places, the low^er portions of the stem

DEVELOPMENT OF PLANTS

297

dying off and the upper portion branching and conlinuing the
growth from year to year. In this way, ponds are gradually
covered with a layer of mosses which become rather insecurely
bound together by the subsequent introduction of other plants
whose roots or stems traverse the covering formed by the mosses.
So frail is this covering at first, that a very slight jar will cause
the surface to tremble and for this reason these places are known

Fig. 201. Fig. 202.

Fig. 201. The bog moss, Sphagtium, bearing three sporophytcs and
numerous lateral branches covered with closely overlapping leaves. Note
that some of these branches envelop the stem in wick-like strands.

Fig. 202. Structure of the leaf of Sphagnum: A, section of leaf, showing
its single layer of cells that consists of large empty cells alternating with
small chlorophyll-bearing ones. B, surface view of the cells, the larger empty
cells being marked with spiral bands and often perforated with minute open-
ings.

as quaking bogs. The constant dropping of the deca>ing vege-
tation to the bottom of the ponds, assisted by the drainage
material, gradually fills them, and in time they may become
quite dry. The acid character of these bogs prevents the entrance
of those organisms which promote decay. As a result, plants

298 STRUCTURE OF SPHAGNUM

and animals that fall into these bogs may only partly decay and
remain for ages in a wonderful state of preservation as is shown
by the recovery of skeletons of animals and clothing of men that
belonged to a prehistoric period. It is because of this condition
that the bog mosses are of great economic importance. The
dead portions of the moss plants, as well as that of the associated
plants, do not entirely decay, and consequently there is slowly
formed a compact mass of material rich in carbon. This is cut
or pressed into blocks, forming peat. This material is an im-
portant fuel in Europe and the vast deposits of it in this country
will doubtless be utilized in the future. It has been recently
shown that the material of some of these bogs is of great value
as a source of pulp for paper.

(a) The Structure and Reproduction of Sphagnum. — The leaves
of the bog moss are arranged in compact spirals around the
stems and consist of a single layer of cells as In the leafy hepatics
(Fig. 202, B). These cells, however, are of two kinds, large
and empty cells with spirally thickened walls which are generally
perforated with small pores and very narrow cells containing
chlorophyll. This distribution of the cells explains the pale^
green color characteristic of the sphagnums. Large and spirally
marked cells, similar to those of the leaf, may also occur in the
cortex of the stem. The closel}" packed leaves enable the bog
mosses to take up water like a sponge and the sphagnums are
of considerable commercial value for this reason, being exten-
sively employed by horticulturists to keep plants moist during
shipment. They are also used extensively in stables in place
of straw and they render them almost odorless owing to their
absorption of liquids and gases. Perhaps these large cells may
serve as floats, enabling the plant to bridge over ponds and they
may enable the plant to endure the acid waters in which these
plants grow. In this connection, it is noteworthy that rhizoids
are entirely lacking and that water and other substances are
absorbed by the outer cells of the stems and by certain branches
which hang down in wick-like strands close to the main stem
(Fig. 201).

The antheridia and archegonia are developed much as in the

DEVELOPMENT OF PLANTS

299

leafy hepatics, the former organs appearin^r as stalked bodies
in the axils of the leaves on short cone-like hranclies anrl the
archegonia originate on the tips of short l)ranches. The ganieto-
spore germinates very much as in Anthoccros (Fig. 203, 6).
It does not, however, have as prolonged a grcnvlli, and at nia-

FiG. 203. The sporoph^'tc of Sphagmim: 6, the young sporophyte sepa-
rated from the archegonium with essentially the same dififcrcntiation of
parts as noted in Anthoceros, Fig. 199. 5^, diagram of a later development
of the sporophyte in the archegonium. The enlarged foot, b, is embedded
in the apex of the moss branch, p, and the spores, sp, form a dome-shaped
layer in the upper part of the capsule. 5, the naked stem, p, of the moss
branch, surrounded at base with spirally arranged leaves, has elongated,
lifting the mature sporophyte into the air. The enlargement of the stem,
b, is due to the growth of the foot region of the sporophyte; ca, remains of
the ruptured archegonium or calyptra; o, Hd or opercuhim of the capsule.
— After Schimper.

turity consists of a well-developed foot emhc-dded in ilie tissues
of the gametophyte and a spore-bearing capsule (Fig. 203, 5J).
The stomata and chlorenchyma which were so consj)icu()Us in
Anthoceros are less perfectly represented, hut the spore mother
are cells still developed in a dome-shajH-d zom-. 'i'his rather
minute sporophyte at maturity- barely breaks through the arche-
gonium, known in this condition as the cahjitra, whicii covers
the capsule as a cap. The absence of a conspicuous stalk or seta

300

SPOROPHYTE OF SPHAGNUM

in the sporophyte of Sphagnum is provided for by the elongation
of the upper part of the moss stem which pushes the sporophyte
above the leaves and exposes the capsule to the winds quite as

Fig. 204. Fig. 205.

Fig. 204. Germination of the spore of Sphagnum: A, early growth of
the spore. B. later development — sp, spore; &, bud developing into moss
plant. C, margin of the thallus, showing the origin of the bud. — After Camp-
bell.

Fig. 205. The twisted stalk moss, Funaria: A, two sporophyte-bearing
plants, the remains of the archegonium or calyptra still attached to the cap-
sule on the right. B, enlarged view of plant with the young sporophyte,
still enclosed in the archegonium, just emerging above the leaves. C, the
male plant bearing the antheridia in a conspicuous rosette of leaves. D,
upper portion of the sporophyte, showing the twisting of the stalk or seta
that assist in sifting the spores through the teeth, peristome, that encircle
the mouth of the capsule.

effectually as the seta of the hepatics and mosses (Fig. 203, 5).
The spores are freed by the forcing off of a circular lid, the
operculum, as is the case among the majority of the mosses
(Fig. 203, 0). This is effected by a ring of thin- walled cells,

DEVELOPMENT OF PLANTS 301

the annulus, that lies in the groove just below llie opcrcuhini,
As the walls of the capsule dry out at maturity lhe>' shrink,
compressing the air w^ithin the capsule until the tension ruptures
the delicate ring of cells and throws out the spores with an
audible report. No elaters are associated with tlie spores in any
of the mosses.

The germination of the spores results in the formation of a
short chain or filament of cells which soon develop into a flat
and rather irregular thallus consisting of a single layer of cells
and attached to the ground by numerous rhizoids (Fig. 204).
This thallus may produce from its marginal cells other filaments
which behave like the original one and thus serve to increase the
number of thalli. The archegonia and antheridia are not de-
veloped upon these thalli, but there are first formed as in most
of the Jungermaniales, bud-like growths which develop into leafy
stems of the sphagnum (Fig. 204, b). These stems soon become
independent plants through the decay of the thallus and finally
bear the reproductive organs.

109. Order b. Bryales. The Higher or True Mosses.— The
great majority of plants, commonly known as mosses, belong
to this order. As a rule, these plants are of a higher type than
any of the other Bryophyta (Fig. 205). The stems are frequently
erect wdth radially arranged leaves and show consideral)k' ditttr-
entiation of tissues, as is apparent in the cross-section of the stem
of one of the higher forms, shown in Fig. 206, A. Particularly
noticeable are the thin-walled cells located in the central strand
of the stem which serve to conduct the fluids and are therefore,
comparable to the tracheal tissue of the higher plants. A similar
conducting tissue is often seen in the central strand that appears
in the leaves of the majority of the Bryales (F'ig. 206, B). In
connection with this higher development of the plant, attention
should be directed to the more efficient organs of absorption.
The rhizoids that spring from the base of the erect stems or from
the under side of forms with creeping stems are often larger than
those noted in the hepatics, and generally they are multicellular
and often twisted together into root-like strands (iMg. 206, C).
These features doubtless account in part f(^r the larger size that

302

STRUCTURE OF THE BRYALES

is attained by many of the Bryales. It is noteworthy, however,
that the devices for absorption and conduction of water are not
as yet sufficiently developed to enable these plants to attain
any considerable height. Erect forms rarely exceed a few centi-
meters in height, but prostrate forms creep over the ground in-
definitely. Some mosses are short lived and many are per-
ennial, continuing their apical growth from year to year. Some
can live only in the wettest of places or as submerged aquatics,

Fig. 206. Structural features of the moss plant; A, cross-section through
the basal region of a moss stem, showing rhizoidal outgrowths from epi-
dermis, cortical and central conducting tissues. B, leaf of Mniiim with mid-
vein of elongated conducting cells. C, rhizoids twisted together into root-
like strands. D, cross-section of a leaf of Polytrichiim, showing the partial
folding in of the margins of the leaf to protect the delicate plates of chloro-
phyll-bearing cells against drought.

while others are capable of enduring almost complete desiccation
and revive quickly when moistened. An interesting device of
service in adapting these plants to periods of drought appears
in the leaves of some of the higher forms, as the hair cap moss,
Polytrichum. Plates of green cells parallel to the midvein project
from the upper surface of the leaves, thus greatly increasing the
surface of the leaves for photosynthesis and also serving as water

DEVELOPMENT OF PLAN FS

303

Fig. 207. Male reproductive organs: i, male plants of J'olylrichum
bearing anthericlia -^in cup-like buds. 6, section of apex of stem of Funaria
— an, antheridia;'^,fparaphysis. 7, male gametes, the one at left still en-
veloped in mucilaginous remains of mother cell wall.

9

Fig. 208. Female reproductive organs: 8, l()ni;iiiitliiMl MCtion i>f a|x-x
of stem, showing arrangement of archcgonia. 9, arihcgonium enlarged,
the canal cells just beginning to dissolve — g, the female gamete.

304

SPOROPHYTE OF THE BRYALES

reservoirs (Fig. 206, D) . During drought, this delicate apparatus
is protected by the coiling of the leaves into needle-like rolls
which result in the exposure only of the thickened epidermal cells.
The sexual organs are developed on the apices of the main or
lateral branches and they are protected by modified leaves which
are often colored and form a more or less conspicuous bud or
cup (Fig. 207, i). This is especially true of the antheridial
buds, the plants being monoecious or dioecious. The antheridia
and archegonia are essentially of the same structure as noted in

Fig. 209. Germination of the gametospore: 10, base of the archegonium
in which the gametospore has germinated, forming a mass of cells with apical
growing cell, x. ii, later growth of the gametospore. The sporophyte,
spr, still enveloped by the archegonium, ar, appears as a cylindrical mass
of cells with foot, h, penetrating the stem of the moss plant. At right an
unfertilized archegonium. See Fig. 205, B. — After Sachs.

previous groups and they are usually associated with modified
leaves known as paraphyses (Figs. 207, 6; 208). The germina-
tion of the gametospore and the development of the sporophyte,
while presenting many features in common with the hepatics
and Anthoceros in particular, shows a remarkable series of varia-
tions that are of decided advantage to the plant. The gameto-
spore in its early growth forms a spindle-shaped mass of cells,

I

. DEVELOPMENT OF PLANTS 305

the basal portion of which reaches down into the stem oi tlie
moss plant and forms a well-developed absorbing organ or foot,
while the upper portion elongates by means of an apical cell
(Fig. 209, 10). Later, the growth becomes basal as in Atitho-
ceros. For a time, the archegonium keeps pace with ilie elonga-
tion of the young sporophyte (Fig. 209, 11), but finally it is
ruptured and lifted up as a cap, called the calyptra, on the apex
of the young sporophyte. This is one of the most essential
differences between the mosses and hepatics. In tlie lat ter group,
with the exception of the Anthocerotales, the spores were matured
in the archegonium. In the Bryales, the spores are usually
not formed until the seta has elongated considerably. As the
sporophyte elongates, the upper part enlarges and finally forms
a complex capsule which remains covered for a varying length
of time by the calyptra. The hairy calyptra of Polytrichiim
(this name means many hairs) is due to the development of a
felt of hairs upon the archegonium which forms a protective
covering to the young sporophyte against the loss of water.

The capsule and calyptra assume va^-ious forms and positions.
In some cases the capsule is quite erect and completely covered
by the calyptra (Fig. 210, i), and again it may be more or
less inclined or even pendulous and the calyptra laterally placed
like a cap (Figs. 205, A\ 211, 5). Removing the calyptra >ou
notice that a lid or operculum is situated on top of the capsule
while at the bottom is a somewhat enlarged region, the apophysis,
provided with stomata that communicate with the interior re-
gions of the capsule (Figs. 210, 2; 211, 5). When the opercu-
lum is removed a circle of minute teeth-like structures, the peri-
stome, appears that more or less completely closes the mouth of
the capsule (Fig. 205, D). In some mosses a delicate membrane,
the epiphragm, lies just below the peristome, which assists in
closing the mouth of the capsule (Fig. 210, 2B, ^A). We are
now interested to know the meaning of these structures and to
learn how they have come about. By examining a longitudinal
section of the nearly mature capsule you will see that the spore-
producing tissue is relatively small and appears as a hollow
cylinder. The progressive sterilization that we haw seen going

3o6

CAPSULE OF BRYALES

on has here resulted in the transformation of the upper part of
the dome-shaped zone of sporogenous cells into vegetative cells
(Figs. 210, 3; 211, 6). The sporogenous tissue is usually

sur-

FiG. 210. The mature sporophyte of Polytrichum: i, moss plant bearing
sporophyte. lA, calyptra removed, showing capsule, which has curved to
one side. 2, capsule, the calyptra, 2A, removed, showing the operculum or
lid and the enlarged apophysis, a, at base. 2B, capsule with operculum re-
moved, showing the teeth-like peristome and the epiphragm, which has
been lifted up at one side. 3, section of nearly mature capsule — sp, cyl-
inder of spore-forming cells that is surrounded on its inner and outer sides
by loosely arranged chlorophyll cells; 0, operculum; r, annulus; p, peri-
stome; e, epiphragm; a, apophysis. 3^, stoma of apophysis. 3^, cross-
section of capsule — sp, spore-forming cells. 4/I, portion of mouth of cap-
sule, showing position of peristome and epiphragm in dry weather. 4^,
peristome pressed down on epiphragm, closing the capsule in damp weather.
— H. O. Hanson.

DEVELOPMENT OF PLANTS

307

rounded on the outside, and less commonly on the inside, by
loose chlorophyll-bearing cells which extend down to the base
of the capsule where chlorenchyma and stomata are developed.
This lower region of the capsule, the apophysis, is often enlarged
and provided with air spaces and stomata, thus constituting the
chief photosynthetic apparatus of the sporophyle (Fi^^. 210. :^A ).

F:o. .11. Structure of capsule of ^"--^5' "^^J^ V^ l,;; ^i
removed. 6, section of nearly mature ^f P^^^^-J^,^,;^^:;,^^ .. o,xt.

roundea on outside by loosely arranged ^h oroplnll-bcarm, c . ^,^ ^

culum; .. annulus; p, peristome; a. ^pophys .- J>. ^ ^ .,.^^.,„,, ,,„,
portion of the capsule, showing the ^-^^-^.;^^ he cpi<lcrnus by
of the peristome. ^ which --/"^^f ,,^\;':;;t Hnc should ^
a double row of cells; ^^ spore-formmg c^, he It 1 ^^^^^^ ^^^^^^

four roundish cells at the left. 7. '^ '^^^^^L of the ,K-ristom..- ^•••-
thus forming the inner and outer teclh-hke >ogmcnt^ 1

Sachs.

3o8

THE PERISTOME OF THE CAPSULE

The entire surface of the capsule is protected by a well-developed
epidermis but at the top a lid or operculum is cut off by a ring
of rather delicate mucilage-bearing cells, the annulus, which
swells at maturity and so assists in casting off the operculum
(Fig. 210, 3, r). Below the operculum a layer of cells, known
as the peristome, extends across the capsule and becomes greatly
thickened on its outer and usually on its inner walls. This layer
of cells is firmly attached to the walls of the capsule by rather
short thick-walled cells (Figs. 210, 3, p] 211, 6-8, p). When
the spores are mature, the more delicate cells break down, leaving
little more than a loose mass of spores and the peristome within
the capsule. The cells of the peristome break apart into teeth-

FiG. 212. Germination of the spore: 3, early stage in the germination,
4, character of the branching, alga-Hke filaments that are finally developed
from the spore — r, rhizoids which penetrate the ground; b, bud which will
develop into a leafy moss plant.

like segments (Figs. 211, 7) and being very hygroscopic, co-
operate with the contracting capsule in forcing off the operculum.
After the removal of the operculum the mouth of the capsule is
covered by the peristome which now appears as a fringe of teeth
radiating out from the walls of the capsule (Fig. 210, 4). The
teeth of the peristome are always in multiples of two and form
one or more, usually two layers, accordingly as only the outer or

DEVELOPMENT OF PLANTS 309

both surfaces of the peristome cells are thickened. These teeth
are so variously marked and fashioned that they furnish one of
the most characteristic features of the various genera of the
mosses. The peristome closes the mouth of the capsule in wet
weather, but on dry days it opens in a variety of ingenious ways
in the various species of mosses so as to effect a gradual scattering
of the spores. The seta also contributes in the distribution,
being very elastic and often hygroscopic (Fig. 205, D).

The spores germinate and produce profusely branching chains
or filaments of green cells, called the protonema (Fig. 212). In
a few instances, thalloid structures are developed as in Sphagnum.
The protonema absorbs the earth substances through delicate
rhizoids and spreads over the ground, appearing to the eye as a
green mould or fine algal cells. Goebel presents some evidence
tending to show that the mosses may have been derived from
branching forms of the green algae. Buds finally appear on these
branches at numerous places and develop the leafy stems of
the moss plant (Fig. 212, h). This in part accounts for the dense
colonies so characteristic of the mosses. As soon as the young
plants are established in the soil by means of rhizoids that spring
from the basal parts, the protonema withers away, although in
some genera it is perennial and continues to produce new plants
from year to year.

no. The Relationship of the Bryophyta. — It appears probable
that there have been three lines of variation or evolution among
the Bryophyta that have had their origin in some simple thalloid
form most nearly related to the lower Jungcrnianialcs. One line
has resulted in the highly differentiated thallus of the Marchanti-
ales with its simple sporophyte. Another, the Jungermaniales,
retaining the simple structures of the primitive thallus has modi-
fied its form into a leafy plant and developed a more complex
sporophyte, while the variations of the third line, or Anthocero-
tales, have resulted in slight alterations of the primitive type of
the thallus but in profound modifications of tlie sporophyte. The
variations of the sporophyte in the latter group appear to have
been very advantageous to the present conditions upon the earth
and perhaps led to the mosses. The important feature in this

310 EVOLUTION OF THE SPOROPHYE

evolution, already noted among the algae, is the increasing im-
portance of the sporophyte. Among the green algae it was de-
pendent upon the amount of food stored in the gametospore.
In the Bryophyta, it is better nourished, owing to its essential
parasitic habit. This led to a larger growth and more complex
development and it is especially important to note that the sporo-
phyte became more profoundly modified by the stimuli of external
conditions than did the gametophyte. For example, we see in
the sporophyte of Anthoceros and of the mosses, a plant that is
practically dependent upon the gametophyte only for water and
the earth substances. So the way is prepared for a study of
the next division where the sporophyte finally comes to absorb
its crude materials directly from the soil and thus becomes an
independent self-supporting plant.

CHAPTER VIII

DIVISION III. PTERIDOPHYTA OR FERNS

III. Nature of the Ferns. — The members of this division differ
greatly in character and Hve under a wide range of conditions,
although, like the Bryophyta, they usually frequent moist and
shady places, as glens, ravines and damp woods, or in tropical
countries they often cover the moist rocks and tree trunks with
a luxuriant and attractive vegetation. The most noticeable fea-

Fig. 213. The Christmas fern, Polystichiim, with young leaves uncoiling
in the spring at the tip of the stem and further back the fully developed
leaves of the past season. The older portion of the stem is covered with
the petioles of leaves that have died off and numerous roots are seen arising
from the stem and bases of the petioles, — H. O. Hanson.

311

312 SUPREMACY OF THE SPOROPHYTE

true in the fern is the large development and complex character
of the plant (Fig. 213). This is due to a more efficient absorb-
ing and conducting apparatus, the root and vascular bundles.
This conducting system was foreshadowed in the elongated cells
that appear in the sporophyte of Anthoceros and of the mosses.
Here the elongated cells are much more specialized and are united
into large bundles, termed the vascular bundles, that appear in
the leaves as "veins" and in the stems they are variously related
and often constitute a conspicuous portion of these organs, as
is the case in the higher plants, see page 76. The fern plant does
not correspond to the moss plant. It is the sporophyte which has
become independent of the gametophyte. The sexual generation
is a small thallus, commonly called the prothallium, that is quite
as primitive as the simplest Jungermaniales. Its chief function is
the formation of gametes (page 282) and it is capable of nourish-
ing the sporophyte only for a short time in the majority of ferns

Fig. 214. The thalloid gametophyte or prothallium, p, of a fern from
which a young fern plant is being developed. This sporophyte has already
developed two roots, r, that penetrate the soil, and two leaves, /, are ap-
pearing, thus making the fern at an early age independent of the gameto-
phyte; rh, rhizoids.

(Fig. 214). The gametophyte of the Bryophyta never attained
any considerable proportions owing to its inability to vary and
produce adequate absorbing organs and conducting tissues.
This naturally limited the development of the sporophyte which

DEVELOPMENT OF PLANTS 313

was parasitic upon it. In the Ptcridophyta, however, the
sporophyte is not at such a disadvantage since it actually becomes
independent of the gametophyte owing to the development of a
root w^hich puts it in communication with the earth substances.
This radical departure in its mode of life acted as a profound
stimulus and cooperated in inducing marked variations that were
so beneficial in character as to cause the sporophyte to assume
large proportions and become differentiated into a highly or-
ganized plant. The stem elongates through the repeated divi-
sion of a single apical cell, as in the Bryophyta, but the stem and
also the leaves and roots in addition contain vascular bundles
and an arrangement of tissues already noticed in the higher
plants.

The gametophyte in many of the Pteridophyta is very sugges-
tive of the simpler thalloid hepatics. Owing to the fact that it
is no longer permanently burdened with the nutrition of the
sporophyte, it becomes greatly reduced in size and length of life.
In fact, in several of the more specialized ferns, the gameto-
phyte may be reduced to a few cells and the entire development
may take place within a day. The reproductive organs and the
germination of the gametospore are suggestive of the correspond-
ing features noted in the Bryophyta. The more important orders
of the Pteridophyta are: i. Ophioglossales. 2. Filicales or Com-
mon Ferns. 3. Equisetales or Horsetails. 4. Lycopodiales or
Club Mosses. This sequence is followed because it brings out
more clearly the trends or tendencies that arose step by step in
the evolution of plant life. As a matter of fact the club mosses
appear to be the most primitive forms and they are the most
suggestive of the Bryophyta; while the common ferns are the
most recent in development. Enough has been said to show that
these forms are very remotely related to the ancestors of the Bry-
ophyta and that these four orders of Pteridophyta are very dis-
tantly connected.

Order~i. Ophioglossales

112. General Characters of the Sporophyte. — This order con-
tains three genera which are probably but a remnant of an
earlier and widely distributed group. Only two, Ophioglossum

314 STRUCTURE OF OPHIOGLOSSALES

and Botrychium, are of common occurrence (Fig. 215). They
are of unusual Interest because they present many features sug-
gestive of the Uverworts and also of the more specialized ferns
and seed plants. This remark is not intended as necessarily
implying relationship between the three groups, but that analo-
gous structures have arisen in each, due either to inheritance
from allied ancestors or to the operation of stimuli upon plants
that are only slightly or not at all related. The sporoph^^te
consists of a short stem with thick flesh^^ roots that are associated
with mycorrhiza. The leaves are simple or divided and usually
appear singly, ensheathing the apex of the stem which always
remains in the soil. One of the most remarkable features about
these ferns is the development in the stems, roots and leaves
of the same tissues that we have noted in the higher seed plants.
The vascular bundles have the same origin as in the seed plants.
The woody portion (xylem) of the conducting system appears as
a solid strand in the center of the very young stem and this is
surrounded by the cellulose part (phloem) of the system. Early
in the development of the stem delicate cells (pith) replace the
central cells of the xylem so that the conducting system now
appears as a hollow cylinder. As the stem elongates this cylinder
is broken into segments by certain strands from it passing out
into the leaves. The spaces formed by the departure of these
strands are termed foliar gaps. The segments of the con-
ducting system are termed the vascular bundles and since the
phloem is outside the xylem they are known as collateral bundles.
In some cases these bundles develop a cambium which adds new
cells to the xylem and phloem so that the arrangement of tissues
in the stem is strikingly suggestive of the dicotyledonous stem
with its pith; concentric rings of xylem, cambium and phloem;
medullary rays; cortex; cork; etc. (Fig. 216, 40). It is note-
worthy also that the cells added to the xylem by the cambium
resemble those appearing in the pines. The stem structure of
some of the simpler representatives of the next order resembles
that of the Ophioglossales and it has been suggested that the
ancestors of these two lines of Pteridophyta may have been the
starting point of the seed plants.

DEVELOPMENT OF PLANTS

315

The spores are borne in peculiar modified outgrowths of the
leaves that assume a cylindrical shape in Ophioglosstim and be-
come more or less branched in the species of the Botrychium
(Fig. 215). The structure of these organs presents many fea-
tures that recall the sporophyte of Anthoceros (Figs. 215, B;
199). The outer portion of the cylinder consists of chlorophyll-
bearing cells which communicate with stomata in some of the
forms at least. The spore mother cells are also distributed as

Fig. 215. The two common genera of the Ophioglossales: A, the addcf
tongue fern, Ophioglossum, with single leaf ensheathing the short stem and
producing a spore-bearing spike. B, section of the spike — sp, the spore-
forming cells arranged in groups or sporangia. The spores are exposed by
the breaking apart of the cells between the dark lines. C, stoma from epi-
dermis of spike. D, the grape fern, Botrychium. The leaf is much divided
and also forms a branched spore-bearing organ. E, two sporangia, showing
the manner of opening for discharge of spores.

in Anthoceros^ the essential difference being that they are sepa-
rated by a larger number of sterile cells and a larger number of
spore mother cells are also grouped together, forming rather con-
spicuous sacs or sporangia (Fig. 215, sp). The spores are formed

3i6

NATURE OF THE SPORANGIA

from the mother cells as in the Bryophyta and are discharged
from the sporangia through a transverse cleft (Fig. 215, E). It
is interesting to note in a closely allied order occurring in the

■^.zX-

Xcr T >-\ r

^^^^^^

Fig. 216. Cross-section of a stem of Botrychium: p, pith; x, xylem:
w, medullary ray; c, cambium; ph, phloem; e, endodermis; cr, cortex. —
After Jeffrey.

tropics, that the sporangia are developed directly upon the leaves
(Fig. 217) instead of upon special branches and that they are
sometimes associated together in groups, known as sori (sing.

Fig. 217. Arrangement of the sporangia of an allied order, Marattiales;
A, leaflet of Archangiopteris with sporangia on surface of leaf and arranged
in groups or sori. B, magnified view of a portion of the leaflet. C, section
of leaf, showing two sporangia, the left-hand one in section.

sorus) and provided with thickened cells, the annulus — charac-
ters that will become conspicuous in the next order. These
sporangia originate, however, as in Ophioglossum and open by
transverse clefts, which operation is promoted by the unequal
drying of the thick- and thin-walled cells of the annulate forms,
(a) The Gametophyte. — The gametophyte or sexual genera-

DEVELOPMENT OF PLANTS

317

tion is not often met with, owing to the fact that, Hke the sporo-
phyte, its development is associated with mycorrhizal fungi and
consequently it is usually completely buried in the soil and usually
quite devoid of chlorophyll. The various stages in the germina-
tion of the spores have never been observed, but the mature gam-
etophyte of Botrychium was found a few years ago by Jeffrey

Fig. 218. The gamctophyte and young sporophyte of Botrychium: A,
tuberous appearance of the gametophyte — e, a young sporophyte or em-
bryo developing in one of the archegonia. B, a section of gametophyte —
ar, archegonia; an, antheridia. C, two antheridia in section. D, male
gamete. E, archegonium before dissolution of canal cells. F, gametospore
in two-cell stage of germination. G, young sporophyte with roots and first
leaf developed but still attached by foot to the round gametophyte, gm.
A. F.— After Jeffrey.

and carefully studied. It appeared as a rather tuberous body
that is provided with numerous rhizoids and a growing apical
cell as in the Bryophyta (Fig. 218, A, B). The archegonia and
antheridia are usually borne upon the upper surface of the game-

3i8 NATURE OF THE GAMETOPHYTE

tophyte and in origin and appearance are rather suggestive of
Anthoceros. The antheridia consist of a number of mother cells
quite buried in the tissues of the thallus, being covered by one
or two layers of cells which are destroyed or break open when
the male gametes are mature (Fig. 218, C). These gametes are
quite different from any yet seen, being spirally coiled and pro-
vided with numerous cilia (Fig. 218, D). The archegonia are
similar to those noted in Anthoceros, consisting of a rather short
neck w^ith two central canal cells and a swollen base sunken in
the tissues of the gametophyte (Fig. 218, £). The female gamete
is formed at the base of the canal cells which finally become
mucilaginous and thus form an open passage-way for the entrance
of the male gametes as soon as the lip cells have opened. It has
been shown in the common ferns that malic acid is present in
this mucilaginous substance which strongly attracts the male
gametes so that they crowd into the canal, often completely chok-
ing it. So we see that the gametophyte of this fern (though
differing in form) is essentially of the same character as in the
Bryophyta, its subterranean character and absence of chlorophyll
being due to its association with fungi. It should be stated that
lobes are produced from the end of the tuberous gametophyte
of one of the species which become green on reaching the surface
of the soil and in a closely allied order or tropical ferns, the
sexual generation is normally a green thallus strikingly like some
of the simpler Jungermaniales.

{h) The Germination of the Gametospore. — ^The early develop-
ment of the sporophyte resembles Afithoceros in many respects.
The gametospore divides into two cells (Fig. 218, 7^), from the
lower of which a large foot is formed, and from the upper cell
the short stem and root arise. Later, the first leaf or cotyledon
is formed from the stem. This development of the sporophyte
goes on very slowly within the archegonium where it lives as a
parasite for a long time. Eventually the root ruptures the arche-
gonium or calyptra and comes in contact with the soil. The
cotyledon now grows upward and makes its appearance above
the ground as the first green leaf (Fig. 218, G). The sporophyte
thus becomes a self-supporting plant, although it probably re-

DEVELOPMENT OF PLANTS 319

mains in part dependent upon the ganietophyte for several years.
At this stage of development the young sporoplnte is a very
simple type of fern, but gradually more efficient roots are de-
veloped and each year the stem sends up a larger and larger leaf
until the adult size is reached when the spore-bearing branch is
formed (Fig. 215), thus making possible again a new series of
gametophytes.

(c) Comparison of the Adder Tongue Ferns with Preceding
Groups. — The most noteworthy difference betAveen these simple
ferns and the Bryophyta is the larger development of the sporo-
phyte and its final independence of the gametophyte. This is
doubtless due to the development of true roots which made pos-
sible a continuous and abundant supply of the crude materials
from the soil. This change acted as a stimulus which promoted
variations in the sporophyte while the light and various climatic
factors also assisted to a very marked degree. Among the algae,
the gametophyte is the dominant generation, the sporophyte
being represented often by the gametospore. This is essentially
the relationship among the majority of the Hepaticae where the
gametophyte performs all the work of food construction and the
sporophyte is a minute parasitic plant upon it. In Anthoceros
and the mosses, the sporophyte is longer lived and more highly
organized, but the gametophyte is still the more important gen-
eration as it performs the major portion of the work. Among the
ferns, this relationship and the distribution of labor is completely
turned about. The sporophyte becomes the larger plant and the
principal center of photosynthesis, while the gametophyte re-
mains small and gradually becomes very short lived.

There is also to be noted the longer postponement in the for-
mation of the spores. In certain of the algae the gametospores
produce the spores directly on germinating while in Ricciocarpiis
a small mass of cells is first formed and certain of these cells
soon become spore mother cells. In Marchantia, a larger number
of cells are formed by the gametospore and the spore mother cells
originate after a few weeks in a definite region of the sporophyte.
These features are more noticeable in the Jungermaniales and
especially in the long-lived sporophyte of Anthoceros. Recall

320 DECLINE OF THE GAMETOPHYTE

also the considerable growth of the sporophyte of the Bryales
and the final development of the spore mother cells In a definite
and restricted region In the capsule. In the Pterldophyta, the
sporophyte not only becomes larger and more highly differenti-
ated, but the formation of the spores is often deferred for years
and the spore mother cells are localized in special organs. This
postponement of the spore formation ma}^ appear at first sight
as a disadvantage since the sporophyte is exposed to many dan-
gers during the long period of preparation for Its work. However,
the delay Is more than compensated for by the large number of
spores that are finally formed and also by the fact that the sporo-
phyte does not entirely perish as In previous cases, but lives on,
sending up annually new spore-bearing leaves. It would appear
that a point had at last been reached In the evolution of the
sporophyte where it Is so well organized as to ensure Its exist-
ence. It can consequently with safety defer the formation of
spores until well-developed roots, stems and leaves have been
formed. The independent existence of the sporophyte brings
out very clearly the two phases In the life history of the plant
which we call the alternation of generations. The formation of
the spores marks the beginning of the gametophyte or sexual
generation, which ends with the formation of the gametes. The
formation of the gametospore through the fusion of the gametes
starts the sporophyte or asexual generation which ends with the
division of the spore mother cells.

Why has not the gametophyte varied as well as the sporophyte?
Perhaps the simplicity of its structure has become fixed by in-
heritance from a long line of algal ancestors. The occurrence
in the Bryophyta and Pterldophyta of motile male gametes and
the consequent necessity of water for fertilization, points to the
inheritance of this peculiarity from aquatic ancestors. It is evi-
dent that there can be no considerable modification of the gameto-
phyte, as long as this feature Is retained, without greatly decreas-
ing the possibilities of fertilization. Consequently the gameto-
phyte must remain of necessity a primitive structure. The
sporophyte, on the other hand, was accustomed from the first to
terrestrial conditions, or to -such conditions as were unfavorable

DEVELOPMENT OF PLANTS 321

to the growth of the gametophyte. Among the algae we have
noticed that it may carry the plant over the more or less com-
plete drying up of the water or unfavorable temperatures. Per-
haps these exposures to a variety of stimuli which the gameto-
phyte never experienced or in a lesser degree enabled the sporo-
phyte to respond with more profound variations when it became
parasitic upon the gametophyte and exposed directly to the
light and air as was the case among the Bryophyta. Certainly
we know that abundance of foods, light, temperatures, etc.,
are among the important stimuli in causing variations. The
simple sporophyte of the Bryophyta was exposed to just such
forces as these and steadily gained in complexity until in the
Bryales it nearly equaled in importance the gametophyte. The
sporophyte of the Pteridophyta, owing to the development of the
root, experiences a new stimulus, i. e., that of the soil which, co-
operating with the stimuli of the light, etc., causes it to assume
much larger proportions than the gametophyte and become the
dominant generation in the life history of the plant. Perhaps
another factor played an important role in securing the supremacy
of the sporophyte. We have seen in paragraph 58, The Signifi-
cance of Fertilization, that the sporophyte is the result of the
fusion of two gametes w^hich may vary in character. The con-
stant introduction of variant characters into the asexual genera-
tion in this way may have cooperated in its increasing complexity.

Order 2. Filicales or Common Ferns

113. General Features. — The great majority of plants popu-
arly known as ferns belong to this order. They are a highly
specialized group that have branched off from some primitive
fern stock in recent geological times and owing to their variations
being highly adapted to present conditions upon the earth, they
lave become very numerous and widely distributed — probably
exceeding 3000 species. In temperate climates the majority
live upon the ground in moist and shady regions and some are
aquatic or xerophytic. They attain their greatest abundance
in the mountainous district of tropical countries where they occur
in astonishing profusion and variety upon the moist rocks and

322

LEAVES OF FI Lie ALES

trunks of trees, as well as upon the earth. The leaves are the
most striking feature of the Filicales. They are usually large and
divided and are characterized by being coiled when young (Fig.
219). This is due to the stronger growth of the outer cells which
causes an inward rolling of the leaf. The growth of the leaf is
very slow, often requiring three years for its formation in tem-

FiG. 219. Christmas fern, Polystichum, with prostrate stem bearing at
the tip young coiled leaves covered with chafify scales and further back large
leaves of the previous season. The older portions of the stem are covered
with the petioles of the dead leaves. — H. O. Hanson.

perate regions. During the season preceding its expansion, the
petiole and blade are completely formed and appear in crosier-
like coils more or less covered with chaffy scales (Fig. 220, c).
This development enables the leaves to expand with surprising
rapidity in the spring when the more rapid enlargement of the
cells on the inner side of the leaf cause it to uncoil. There is

DEVELOPMENT OF PLANTS

323

another feature about these leaves that compels our admiration.
Comparatively few leaves are produced annually from the tip
of each stem — in the preceding order usually but one. This

Fig. 220. Stem in early spring freed from all of its leaves save the young
ones, c, near the tip: r, roots; v, vascular bundles in base of petiole; x, region
from which the cortex has been removed to show the vascular bundles from
the leaves uniting to form a lattice work.

appears to be the rule in underground stems, since the difficulty
and danger of sending the tender young leaves up through the
soil is minimized by the development of one or a few leaves which

Fig. 221. Cross-section of fern stem: v, a concentric vascular strand, the
large cells of the xylem being surrounded by the phloem. Each strand is
surrounded by a compact layer of cells, the endodcrmis; st, stercome; cr,
cortical region.

form large blades after reaching the air. In the two remaining
orders of Pteridophyta, branches of the underground stems rise
above the ground and these bear numerous small leaves, also a
rule for stems of this kind. The tissues of the leaves are differ-

324

STRUCTURE OF THE STEM

entiated into an epidermis, stomata, chlorenchyma and vascular
bundles as in the higher plants (Fig. 223). The stems are more
usually prostrate, creeping rhizomes that branch sparingly and
so gradually give rise to colonies of ferns. One of the most
attractive features of certain tropical districts is the tree ferns
with erect stems of palm-like appearance which lift their great
crowns of leaves 30 to 50 feet in the air. The vascular bundles
are more commonly of the concentric type (Fig. 221). Perhaps
we should repeat that a bundle is composed of two types of elon-
gated cells, thick walled woody cells (the xylem) and thin walled

Fig. 222. Arrangements of the sporangia: A, lobe of leaf of Dryopteris
with sporangia grouped in circular sori, s. B, sorus enlarged, showing the
shield-like membrane or indusium, in, covering the sporangia, sp. C, lobe
of leaf of Asplenium with elongated sori, s.

cellulose cells (the phloem). In the fern t^^pe of concentric
bundle the phloem surrounds the xylem. These bundles are
united in the stem in a variety of ways. In the simplest case
we find a central core of xylem surrounded by phloem which
gives off strands or vascular bundles to the leaves and roots. In
the majority of ferns this core of xylem is modified by the develop-
ment of rather delicate pith-like cells in its center, consequently
it appears as a ring in cross section or as a cylinder in longitudinal
view. This ring is broken into segments owing to the fact that

DEVELOPMENT OF PLANTS 325

portions of it turn out here and there, thus forming the vascular
bundles that extend into the leaves. Such an arrangement is
seen in Botrychium (Fig. 216). These spaces formed in the ring,
called foliar gaps, are often quite large so that the cylinder of
vascular tissue in the stem presents the appearance of a lattice
work — note the vascular tissue at the right-hand end of the stem
in Fig. 220. In cross section such a cylinder would appear as a
series of more or less widely separated strands (Fig. 221). This
latter figure shows that strands arising from the lattice-like
cylinder may also extend into the pith-like region — note the three
large strands in Fig. 221, v. This explains the confusing
arrangement of strands often seen in the cross section of fern
stems and previously noted in monocotyledons, p. 96. Certain
cells of the cortex and pith often become modified into strengthen-
ing cells of stereome (Fig. 221, st). Roots arise near the base
of the leaves, and in some of the tree ferns form a thick mat-like
covering on the stems. They originate from the endodermis
of the bundles and possess a root cap and radial arrangement of
the vascular bundles as in higher plants.

(a) Structure and Character of the Sporangia. — The sporangia,
instead of being produced in the tissues of special branches as
in Ophioglossum, are borne in curiously constructed capsules,
usually situated on the under surface of the ordinary green
leaves (Fig. 222). The sporangia-bearing leaves are usually
called sporophylls, meaning spore-bearing leaves. In some cases
the sporophylls are highly modified, being entirely given up to
spore production and therefore quite different from the green
leaves (Fig. 227). The sporangia are usually associated in
groups or sori (sing, sorus) on the vascular bundles and pro-
tected by a membranous outgrowth of the epidermis, known
as the indusium (Fig. 222, B). Each sporangium originates
usually from a single epidermal cell, which by repeated divisions
(Fig. 223) produces a capsule or sporangium that contains the
spores. A sporangium of the shield fern contains 48 spores and
there are fully 100 sporangia to a sorus and 20 sori on each lobe
or pinna of the leaf. A well-developed leaf would have on an
average 50 pinnae and a healthy plant would l^car about 10

326

SPORANGIA OF FILICALES

leaves. Therefore, Bower estimates that the shield fern produces
annually upwards of 50 million spores. This makes a striking
comparison when we consider the spore production in the Bryo-

FiG. 223. Section of a leaf of Woodwardia, showing two sori: i, indusium;
sp, sporangia arising from the epidermis and in various stages of develop-
ment. Note the epidermis, stoma, 5, palisade and spongy chlorenchyma,
and vascular bundles, as in higher plants.

phyta, and it is evident that ample provision is made for the
maintenance of the race notwithstanding the postponement and
specialization in spore production.

The sporangia vary considerably in structure in the various

Fig. 224. Character of sporangia: A, simple type of sporangium of
Osmunda with rudimentary annulus, an, of a few thickened cells. B, com-
mon type of sporangium — an, annulus; /, lip cells.

genera. In the simplest forms, they consist of a uniform layer of
wall cells inclosing a mass of cells, the majority of which become
spore mother cells, producing four spores each. In other cases,
a few of the wall cells are thickened, forming a rudimentary

DEVELOPMENT OF PLANTS 327

annulus (Fig. 224, A), but in the majority of our common ferns,
the sporangium consists of a dcHcatc stalk, supporting a rather
spherical spore-bearing capsule (Fig. 224, B). The walls of this
capsule consist of a single layer of thin-walled cells except for a
row of thickened cells, the annulus, that extends from the stalk
over the capsule nearly to the opposite side (Fig. 224, j5, an).
At the latter point, the annulus ends in a few rather weakened
cells, two of which, the lip cells, are conspicuous for their larger
size (Fig. 224, B, I). It is to be noticed that the cells of the an-
nulus are thickened on their inner and radial walls while the outer
walls remain comparatively thin. When the spores are mature
and lie loose in the capsule, the cells of the annulus that have
been filled with water up to this time begin to dry out. As the
volume of water in these cells lessens through evaporation, the
cohesion of the water pulls upon the thin outer walls, causing
them to bend in, assuming a U-shaped appearance. This tends
to pull the two radial walls of each cell together and shorten
the outer circumference of the annulus. This contraction pulls
open the sporangium at the weak lip cells, the split extending
back through the lateral cells of the capsule which is finally
drawn back to such an extent that the annulus becomes nearly
straight. Finally, when the water is nearly withdrawn from the
cells, the tension becomes so great that air is drawn in through
the thin walls of the cells. The entrance of air breaks the water
adhering to the walls and which had kept them pulled in as
described above and consequently each cell returns instantly to
its original form. This causes the capsule to snap back to near
its original position with a quick jerk, thus throwing out the
spores to a considerable distance. This motion of the capsule
can be approximated by mounting sporangia that have been
soaked in a drop of water and watching them under a micro-
scope while a drop of glycerine placed in touch with one side
of the mount is drawn in with a filter paper applied to the oppo-
site side. The denser glycerine will draw the water out of the
cells by osmosis and set up a tension in the cells of the annulus
just as did the evaporation of the water.

The form of the sorus and its relation to the indusium and

328

FORMS OF FILICALES

particularly the structure of the annulus are important charac-
teristics to be observed in identifying the ferns. In the flowering
ferns, Osmunda, the large slight-stalked sporangia are pro-
vided with a rudimentary annulus of a few thickened cells lo-
cated at one side of the capsule (Fig. 224, A). The entire leaf
or certain leaflets are completely covered with the sporangia,
little more than the vascular bundles remaining (Fig. 225, A).

Fig. ?25. Common forms of the Filicales: A, the flowering fern, Os-
munda, showing below two green leaflets and above two sporangia-bearing
leaflets. At left a cluster of sporangia magnified. The first leaves in the
spring only bear sporangia; those appearing later have only green leaves.
B, chain fern, Woodwardia. C, Christmas fern, Polystichum. D, bladder
fern, Felix. E, hay-scented fern, Dennstaedtia — s, sorus enlarged. F, Woodsia
— After Sprague.

Osmunda belongs to the lowest group of the Filicales and shows
certain analogies in the arrangement and structure of its collateral
vascular bundles with the Ophioglossales. The majority of
ferns in temperate climates are characterized by sporangia of
the type shown in Fig. 224, B, and they are distinguished by
the form and arrangement of the sori and indusia. The shield
fern, Dryopteris, has a rather circular or curved sorus covered

DEVELOPMENT OF PLANTS

329

by an indusium that is laterally attached at a single point (Fig.
222, A). The Christmas fern, Polystichum, has a circular sorus
with centrally attached indusium (Fig. 225, C). The spleenwort
fern, Asplenium, is characterized by elongated sori arranged
obliquely to the midrib upon the upper side of the veinlets.
The indusium is attached on one side of the sorus along its entire
length (Fig. 222, B). The chain fern. Woodwardia, differs
from Asplenium in having the sori arranged in chain-like rows

Fig. 226. Ferns without indusia or possessing false ones: A, leaflet of
bracken fern, Pteridium. B, maiden-hair fern, Adiantnm. C, polypod fern,
Polypodium. — After Sprague.

parallel to its midrib (Fig. 225, B). In several genera of ferns
the indusium is partly or entirely inferior. Thus in the bladder
fern, Filix, the partly inferior indusium covers the circular sorus
like a hood (Fig. 225, D), while it is wholly inferior in the hay-
scented fern, Dennstaedtia, forming a cup (Fig. 225, E) and in
Woodsia the indusium is roundish or star-like (Fig. 225, F).

330

GAMETOPHYTE OF FILICALES

Several ferns are distinguished by false indusia that are formed
by the more or less modified margins of the leaf. In the bracken,
Pteridmm, the entire membranous margin of the leaf curves over
the closely crowded sori (Fig. 226, A), and in the maiden-hair
fern, Adiantum, the sporangia are at the ends of the veins and
covered by reflexed portions of the leaf (Fig. 226, B). The
indusia are lacking in some forms, as in the beech fern, Phegoteris,
and in the polypody, Poly podium (Fig. 226, C). These genera
are distinguished by the fact, among others, that the leaves of
Polypodium drop off, leaving a scar as in our deciduous trees.

Fig. 227. The sensitive fern, Onoclea: A, portion of normal green leaf.
B, a spore-bearing leaf. C, two views of one of the round lobes of B, showing
the views and the sori on inner side of the lobe. — After Bailey.

This feature is possibly due to its more or less epiphytic habit.
This is the only really xerophytic fern of temperate regions.
The sporophyll of the sensitive fern, Onoclea, bears a striking
outward resemblance to Botrychium, but the sporangia-like bodies
are really leaf lobes rolled up and each bears several round
sori on its inner side (Fig. 227). In this and several other genera,
see Osmunda, the work of photosynthesis is given over to large
green leaves that do not produce sporangia.

(b) The Gametophyte. — In the majority of ferns the spore
germinates by rupturing the outer coats and producing a germ
tube from which one or more delicate rhizoids are cut off. The
germ tube elongates, forming a short chain of cells which soon
develop by apical growth into a flat thalloid structure, commonly
called the prothallium, that is attached to the ground by numer-

DEVELOPMENT OF PLANTS

331

ous rhizoids (Fig. 228). This gametophyte often becomes heart-
shaped (Fig. 228, C), owing to the more rapid growth of the
cells that are cut off from the apical cell. In some genera
branched filamentous or narrowly thalloid growths are developed
that resemble the protonema of the mosses or the thallose hepat-

FiG. 228. Fig. 229.

Fig. 228. Gametophyte of the Filicales: A, germination of the spore.
B, early appearance of the thalloid structure of the gametophyte owing to
the formation of an apical cell, x. C, mature gametoph>'te — aw, anthcridia;
ar, archegonia: r, rhizoids; v, apical cell or growing point.

Fig. 229. Structure in reproductive organs: A, antheridium as seen in
section, just before the discharge of the gametes. B, male gamete. C,
section view of archcgonium — g, female gamete.

ics. The gametophyte usually lives but a few months, although
in some species they may endure for years, multiphing exten-
sively by gemmae, and so form conspicuous mats upon the moist
trunks and rocks. The archegonia and antheridia are usually
borne upon the same gametophyte. Some genera, however, are

332 THE YOUNG SPOROPHYTE

strictly dioecious, producing small antheridial or male gameto-
phytes and larger archegonial or female gametophytes. Small
male gametophytes occur not uncommonly among any of the
genera, owing doubtless to their poorer nourishment. The
sexual organs are developed upon the under side of the gameto-
phyte (Fig. 228, C), probably because this position is of advantage
in keeping them in contact with any water that may fall upon the
earth ^and thus preventing their drying out. The antheridia
develop before the archegonia, often appearing while the gameto-
phyte is quite small. They usually project from the surface of
the thallus as spherical bodies covered with a single layer of
chlorophyll-bearing cells which inclose usually from thirty-two
to sixty-four gamete-bearing cells (Fig. 229, A), The male
gametes are discharged as in the Bryophyta — the swelling of the
antheridium causes the rupture or throwing off of the apical cell
and the extrusion of the inner cells as a mucilaginous mass. The
gametes are large spirally-coiled bodies provided with numerous
cilia and the larger posterior coils enclose a delicate sac containing
the remains of the nourishment stored in the mother cell (Fig.
229, B). The archegonia are developed on the older prothallia,
usually just back of the growing point (Fig. 228, C). The neck
of the archegonia, consisting of four rows of cells, projects from
the under surface of the prothallium and is usually curved back
from the growing point, while the basal portion containing the
female gamete remains buried in the tissues of the prothallium
(Fig. 229, C). When the female gamete is mature the canal
cells become mucilaginous and the lip cells open as in the arche-
gonium in the mosses.

(c) Germination of the Gametospore. — The gametospore ger-
minates in a week or more after fertilization, usually dividing very
regularly first by a vertical and later by a transverse division into
four cells. Each of these cells forms a primary organ of the young
sporophyte. Of the two outer cells, the one near to the grow-
ing point of the prothallium (Fig. 230, A,c)hy repeated divisions
produces the first leaf or cotyledon, while the other one forms
the first root (Fig. 230, A, r). Of the two lower cells, the one
directly below the leaf cell gives rise to the stem (Fig. 230, A, s)

DEVELOPMENT OF PLANTS

333

and the other one to the foot (Fig. 230, A, f). The growth
of these four cells results at first in the formation of a rather
globular sporophyte, but soon the growing point of the young

Fig. 230. Germination of the gametospore: A, section of archegonium,
after fertilization, showing the four-celled stage of the germinating gameto
spore. The cell r by repeated division forms the first root, c forms the first
leaf, s forms the stem and / the foot; x, apical cell of prothallium. B, later
stage. The young sporophyte rupturing the archegonium or calyptra-
lettering as in ^.

root grows through the calyptra, turns downward and pene-
trates the soil (Fig. 230, B). This is followed by emergence

Fig. 231. Older sporophyte that has developed two roots, r, and is un-
folding the second leaf, /, but still attached to the withering gametophyte
or prothallium, p; rh, rhizoids.

334 EQUISETALES OR HORSETAILS

of the cotyledon, which, however, curves upward and spreads
out its blade to the light (Fig. 231). Note in Fig. 230 that the
necessities of the plant have determined the position of these
four organs. The foot is formed in contact with the bulk of the
food in the prothallium, the root next the ground which it reaches
at once, the leaf easily reaches the light since the root has already
ruptured the calyptra, and since the leaf is near the margin of
the prothallium, and finally the stem is in a position of advantage
(Fig. 230, B) for growing straight up or horizontally. The young
sporophyte, which has up to this time drawn its nourishment by
means of the foot from the gametophyte, is now in a position to
care for itself. The prothallium soon withers, leaving the sporo-
phyte an independent and self-supporting plant. The first leaf is
small and usually bears little resemblance to those of the mature
plant, but as the stem elongates, new leaves are formed which
gradually become larger and each succeeding one resembles more
closely the adult form. The foot disappears with the withering
of the gametophyte, and this is soon the fate of the primary root,
but numerous secondary roots are formed along the stems as it
continues to creep along on or near the surface of the soil. This
growth goes on from two to several years before the plant is
prepared to develop sporangia upon the leaves, page 282.

Order 3. Equisetales. The Horsetails

114. General Characters. — This small group of Pteridophyta,
comprising but a single genus of about twenty-five species, is but
a remnant of an extensive group of plants that flourished in the
coal period and that formed conspicuous features of the vegeta-
tion at that time. The structure of these earlier forms has been
so perfectly preserved in fossil remains that they give a better
idea of the relationship of the group than could be obtained from
the living plants. Forms allied to Equisetum doubtless formed
during the coal period of the earth large forests and attained a
height of sixty to ninety feet and perhaps three feet in diameter.
The species that survive to-day are rush-like plants that rarely
exceed a foot in height, though a single tropical form supports
its delicate stem upon other vegetation and so attains a length

DEVELOPMENT OF PLANTS

335

of over thirty-five feet. Species of Eqidsetum are of common
occurrence in nearly all countries, living in shallow ponds,
swamps, and marshes or drier soils.

At first sight they show little suggestion of fern relationship
(Fig. 232) and impress one as being singularly out of harmony

Fig 232. A common horsetail, Equisetum arvense: a, the green branch-
ing plant that lives through the summer-/, scale leaves; r, rhizome or under-
ground stem with tuberous storage organs, b, early spring shoot that bears
a spike or strobilus of modified spore-bearing leaves, sp. This stem is ot a
light brown color and withers after the spores are shed.— H. O. Hanson.

with our common plants, as though they were indeed relics of
a past age. The large leaves, which were so characteristic of
the Filicales, are reduced to minute papery scales in this order

336 STRUCTURE OF EQUISETUM

and take little or no part in photosynthesis. These scale leaves
are arranged with great regularity at the nodes of the stem, and
owing to their close association they grow together, forming a
papery sheath with teeth-like points about the stem (Fig. 232, I).
It is noteworthy that in allied fossil forms large chlorophyll-
bearing leaves occurred.

The stems are of two kinds, subterranean rhizomes that branch

Fig. 233. Cross-section of a portion of the stem showing its grooved
character and stereome confied to the ridges: a, air spaces; e, endodermis,
inside of which are shown three bundles.

extensively through the soil, and aerial stems that arise as
branches from the rhizomes. The aerial stem is simple or
branched and is characterized by nodes made conspicuous by the
sheathing teeth-like leaves and strongly furrowed internodes. In
such species as branch, these organs originate with great regular-
ity in the axils of the leaves and perforating the sheathing leaves
produce a bushy symmetry that caused the name of Equisetum
or horsetail to be applied to these plants. The epidermal cells
are hard and rough, owing to the abundant deposit of silica in
the cell walls. For this reason certain species were used in early
times for scouring purposes and so they became popularly known
as scouring rushes. Complete silicious casts of the epidermal

DEVELOPMENT OF PLANTS 337

walls and stomata can be obtained by treating the tissue as noted
in the diatoms. The arrangement of the leaves and branches and
the distribution of the vascular bundles and other tissues presents
a mathematical regularity unexcelled in any plant. As in the
Bryophyta, the elongation of the stem is effected by a single
apical cell which cuts off with extreme regularity the cells from
which the various tissues and organs of the plant are formed.
The very rudimentary vascular bundles are arranged around a
large pith (Fig. 233). They rarely show secondary growth
through the activities of a cambium, although this was a marked
feature of many of the extinct species. These bundles lie directly
below each ridge of the stem and run downward through the
internode to the next lower node, where they divide into two equal
parts. One of these branches joins a similar branch from the
adjacent bundle and continues straight down to the next node,
where the branching is repeated. The rudimentary bundles of
the leaves also join on to the bundles of the stem at the node.
Owing to this regularity of branching, the leaves and vascular
bundles alternate in each succeeding node and the vascular system
assumes a cylindrical form composed of oblong six-sided figures.
Thick-walled stereome cells are developed in the ridges and con-
stitute the principal mechanical tissue of the stem, while the
stomata and the chlorophyll apparatus are largely confined to
the grooves. The stems are very light, owing to absorption of
the larger part of the pith and certain regions of the cortex (Fig.
233, cl)-

(a) The Spore-bearing Leaves or Sporophylh. — The sporangia
are formed only upon special organs, probably leaves or sporo-
phylls, that are arranged in a compact cone or strobilus (plu.
strobili) at the end of the stem (Fig. 232. sp). In some species the
strobilus is borne upon a special branch that does not contain
chlorophyll, though variously colored, and that withers away as
soon as the spores are shed (Fig. 232, b). In other cases the stro-
bilus appears at the tip of the ordinary green shoot. This vari-
ation is doubtless associated with the season of the year at which
the strobili appear. The species characterized by the special
branches produce these earl}- in the spring when the temperatures
22

338 SPOROPHYLLS OF EQUISETUM

are not favorable for the work of the green shoots. Note in this
connection the significance of the coloration. Species with the
strobili upon the ordinary plants form these structures later in
the season when the temperatures are favorable for photosyn-
thesis. The development of the sporangia upon specialized
sporophylls that are grouped together at definite points on the
plant is one of the significant departures that appears in the evo-
lution of the plant. This distribution of labor that is seea in the
setting aside of certain groups of leaves for spore production
will appear in all the succeeding groups ; and the arrangement and
structure of the sporophylls will steadily become more and more
complex until a point is reached where they are popularly called
a flower, although this term can be just as correctly applied to
the strobilus appearing in Egidsetum and succeeding groups.
The sporophylls of Equisetum originate at the nodes, as in the
case of the scale leaves, but as they enlarge the apical portion
spreads out like a shield. The internodes do not elongate and
separate the sporophylls to any considerable extent, consequently
the shields become six-sided through contact with the adjacent
sporophylls.

{h) Character of Spora^tgia and Spores. — The sporangia appear
as rather elongated sacs on the under side of the shields (Fig.
234, 2) and at maturity open by a longitudinal cleft. The
structure of the spores is of especial interest. The outer wall
of these spores is thickened in spiral bands, and owing to the
dissolution of the thin portion of the wall separating these bands,
the outer coat uncoils and appears as four bands, with spoon-like
ends, attached to the spore at one point (Fig. 234, 3). These
bands or elaters are very hygroscopic and their movements assist
in rupturing the sporangium, but their special significance is seen
in the fact that the elaters become entangled and so several spores
are cariied away together by the wind. The meaning of this
arrangement will appear directly.

(c) Germination of the Spore. — The gametophyte produced from
these spores (Fig. 235) is an irregularly lobed thallus more
suggestive of the irregular thallus of an hepatic or the leaves
of a moss plant, or the lobed gametophyte of certain species of

DEVELOPMENT OF PLANTS

339

the Ophioglossales than of the prothalHum of the Filicalcs. The
most important feature about the gametophytes is the fact that
they are as a rule dioecious and of two sizes, the smaller ones
bearing only antheridia and the larger only archegonia. This
difference in the nature of the gametophytes is largely due to
nutrition as noted among the Filicales, the well-nourished ones
being the female. In fact antheridia may appear upon the female
gametophyte as a result of insufficient nourishment during its

Fig. 234. Fig. 235.

Fig. 234. Sporophylls and spores of Egtiisetum: 2, sporophylls viewed
from outer and inner side, showing form and attachment of sporangia and
the central stalk attaching the sporophyll to the strobilus. 3, spores with
elaters expanded in a and partially coiled in b.

Fig. 235. Female gametophyte of Eqiiisetum bearing several archegonia
and leaf-like lobes. At right male gamete. — After Sadebeck.

later development. Thus we see that the germination of these
spores, which are apparently exactly alike, is controlled by a defi-
nite stimulus, just as was the case in the formation of zoospores
and gametes among the lower green algae (page 185). It is
noteworthy that some of the extinct species of this group actually
stored more food in certain spores than in others, so that they
came to differ somewhat in size. This habit is well established
in some of the living Filicales and will also be noted in the next
order. As a consequence of this tendency, the nature of the
gametophyte developed from the spore is no longer a matter of
chance. The larger spores, called megaspores, by reason of the
more abundant food produce female gametophytes, while the
smaller spores, microspores, form small gametophytes bearing
only antheridia. The significance of this tendency to produce

340 REPRODUCTION OF EQUISETUM

megaspores and microspores will be seen in the disussion of the
fourth order, but attention is called to it here because the Equise-
tales show very clearly how such a condition came about. It
would appear probable that the spores of Eqidsetum, although
exactly alike as far as we can see, must have already undergone
some physiological change which predisposes them to develop
either male or female gametophytes. If this is not the case it
would be difficult to explain the common occurrence of dioecious
gametophytes in Equisetum and the rarity of such an occurrence
among the Filicales. It would appear reasonable to suppose that
the living substance in the spore of the Filicales is not so highly
organized and therefore not so readily influenced by external con-
ditions, while the composition of the spore of Equisetum is of
such a nature that the amount of food placed at its disposal pro-
foundly affects its germination and development. Perhaps this
physiological differentiation of the spores led to their appro-
priating different amounts of food during their formation in the
sporangium, and so they finally came to be distinguished as
large and small spores, as noted above. The antheridia and
archegonia present essentially the same features as were noted
in the Filicales, and fertilization is effected in the same manner-
(d) The Germination of the Gametospore. — The most note-
worthy departure in the germination of the gametospore is seen
in the limited growth of the stem. The early stages are similar
to those of the Filicales, but after the stei^ has formed a few
nodes with three leaves each, it is replaced by a stem that devel-
ops at its base. This second stem attains a somewhat larger size,
but is finally replaced in the same manner by a third shoot. In
this way several stems are formed until finally one is developed
that penetrates the ground and forms the characteristic rhizome
of the mature sporophyte. These plants with their scale leaves
reduced to protective organs, sunken stomata and chlorenchyma
confined to the grooves of the internodes, present an extreme
form of xerophytic structure and form a sharp contrast with the
large and usually thin-leaves Filicales. While they occur in dry
localities, to which conditions their structures admirably adapt
them, they are of more common occurrence in moist and wet

DEVELOPMENT OF PLANTS

341

places. This peculiar distribution of the species of Eqiiisehim
has not been explained. It is evident that these plants, like the
rushes and sedges of our marshes and shallow ponds, are often
exposed to intense heat and light, Avhich would cause an excessive
transpiration. Possibly these plants are not able to absorb
water rapidly owing to the limited amount of conducting tissues
and to the exclusion of the atmosphere by the water which sur-
rounds the roots (see pages 45, 54), consequently they are at
the same disadvantage and require the same protective devices
as plants living in arid localities.

115.

Order 4. Lycopodiales. The Club Mosses
General Characters. — The members of this group are

popularly known as the club mosses owing to their small moss-
like leaves and the arrangement of the spore-bearing leaves into

Fig. 236. A common club moss, Lycopodium annotinum, with creeping
stem and erect branches covered with small moss-like leaves: s, strobilus;
r, roots. — H. O. Hanson,

342 THE LYCOPODIALES

club-like stroblli or cones (Fig. 236) . The gametophyte or sexual
generation is also suggestive of the mosses. Particularly is this
irue of the male gamete, which is small and not spirally coiled
andjt is apparentl}^ biciliate (Fig. 240).

The Lycopodiales are largely tropical, though a small number
of^forms are of common occurrence in temperate regions. The
stems are erect or creeping and rather small, but owing to their
prolonged growth and extensive branching they often form con-

FiG. 237. Cross-section of a portion of the stem of Lycopodium, showing
the centrally arranged vascular bundles.

spicuous and attractive colonies. The tissues of the stem do
not materially differ from those of the common fern, though the
vascular bundles from the leaves generally unite in the stem to
form a central core of xylem surrounded by phloem. This
solid core of xylem may be broken up in various ways by the
development in it of masses of pith-like cells. Sometimes these
pith cells form plates alternating with the xylem (Fig. 237).
The work performed by the leaves, as in the Equisetales, is
usually of two kinds, namely, photosynthesis by the green foliage
'leaves and spore production by the leaves that usually form

DEVELOPMENT OF PLANTS

343

strobili at the tips of certain branches. But a single sporangium
is associated with each of these sporophylls (Fig. 239).

The club mosses are a very much larger group than the Equise-
tales, but like the latter group, they are a remnant of a highly
developed and widely distributed race. Fossil remains indicate
that the ancient allies of these plants were conspicuous features
of an earlier vegetation, with palm-like trunks 100 feet in height
and three feet in diameter, and bearing a crown of long narrow
leaves that attained a length of three feet. They reached their

Fig. 238. Fig. 239.

Fig. 238. Phylloglossum Dnimmondi. — After Pritzcl.

Fig. 239. Strobilus and sporophylls of Lycopodium: 2, strobilus. 3, a
leaf or sporophyll from the strobilus enlarged and showing attached spo-
rangium, 4, a spore greatly magnified.

greatest abundance in the coal age and thence gradually declined,
being crowded out by the more specialized seed plants. There
are two important families of the Lycopodiales: i, Lycopodiaceae;
2, Selaginellaceae.

116. Family i. Lycopodiaceae. — With but one exception the
members of this family belong to the genus Lycopodium, com-
monly known as the club moss, ground or running pine, ground

344 SIGNIFICANCE OF PHYLLOGLOSSUM

fir or hemlock (Fig. 236). These plants are very well repre-
sented in our open woods, where they often form conspicuous
colonies owing to the extensive branching and prolonged growth
of the stems which creep over or through the ground, sending
up numerous, erect branches and giving off roots that branch
with great regularity. Various species of Lycopodium are ex-
tensively gathered for decorations since the aerial portions of
the stems are thickly clothed with small moss-like leaves. The
second genus of the family (PhyUoglossum) contains but one
species, which is found in New. Zealand and Australia. It is of
interest because it is the most simple fern known (Fig. 238),
consisting of a few narrow leaves, rudimentary strobilus and
poorly developed tissues. The simple strobilus of this plant is
suggestive of the sporophyte of Anthoceros and it also recalls the
spore-bearing spike of Ophioglossum. If the sterile cells separat-
ing the spore-forming cells in Anthoceros had increased in number
so as to project somewhat and thus form rudimentary leaf -like
outgrowths, then it Avould have resembled the strobilus of PhyU
loglossum. To be sure the sporangia of the latter plant are borne
upon the leaves but in the next family they arise from the stem
as in Anthoceros. Ophioglossum shows the increase of the sterile
cells between the spore-bearing cells but this did not result in
leaf development; so it occupies an intermediate position in this
trend of development. It is certainly a striking fact that these
three groups, though widely separated, exhibit the same type of
development.

The sporangia of the Lycopodiaceae are large considering the
size of the leaf, and usually appear on specialized leaves that are
arranged in strobili on the ends of erect branches (Fig. 239).
The sporangia open by a longitudinal cleft and the spores are
produced in such quantities in some of the forms that they are
of commercial importance, being used for flashlight effects and
sold as lycopodium powder.

(a) The Gametophyte of the Lycopodiaceae. — The most inter-
esting feature about the club mosses is the sexual generation,
because it indicates that these forms may have been derived from
the same ancestry as the mosses or at least from a very primitive

DEVELOPMENT OF PLANTS

345

stock. In certain species the spores produce a rather cyhndrical
erect gametophyte which terminates in a number of radially-
arranged green leaf-like lobes, among which the archegonia and
antheridia are produced (Fig. 240). This structure has been
compared to a miniature leafy moss plant. This suggestion of
relationship is strengthened by the resemblance of the male
gametes to those of the mosses, the sperms, so far as they have
been observed, being small, almost straight bodies, and provided
with two cilia (Fig. 240). The archegonia may also be more
primitive than those of the ferns in possessing several neck canal

Fig. 240. Gametophyte of Lyco podium as seen in section: an, antheridia;
ar, archegonia. The young sporophytc or embryo developed in one of the
archegonia consists of a foot, /; a root, r; and a stem, st, bearing a leaf on
either side; s, the suspensor, which has pushed the sporophyte early in its
development into the tissues of the gametophyte. Above a single male
gamete is figured. — After Bruchmann.

cells and five instead of four rows of neck cells. The gameto-
phyte appears in all species examined to be associated with
symbiotic fungi, as in Ophioglossales. The spores will not de-
velop unless they come in contact with a suitable fungus and this
has led in many forms to a subterranean and chlorophylless
development of the, gametophyte.

ih) The Germination of the Gametospore. — The germination of
the gametospore is characterized by new departures that will
continue on through the seed plants. The gametospore enlarges
and divides into two cells by a transverse wall. The outer of

346 EMBRYO OF LYCOPODIUM

the two daughter cells takes no part in the formation of the
sporophyte, though elongating and often dividing several times.
These cells are termed the suspensor (Fig. 240, s) and function
in pushing the lower daughter cell down into the nourishing tissue
of the gametophyte. The lower daughter cell, by a series of
divisions that at first resemble the common ferns, forms the
young sporophyte. This consists of a stem with one or two
cotyledons, a massive foot and finally, at a late period in the
development in the sporophyte, of a root (Fig. 240). The tardy
development of the root has been cited as an indication of the
origin of the lycopods from very primitive ancestors, in which
the formation of the root had not become established. It may
also be due to the abundant food stored in the gametophyte and
hence the development of the root might well be delayed until
this store is in part exhausted. The growth of the sporophyte is
very slow and it remains as a parasite upon the gametophyte for a
long time, even for years in some of the subterranean forms
(see Ophioglossales) . The elongation of the stem and root is
effected by the division of several cells rather than by one apical
cell, as in previous cases, a feature to be noted in the seed plants.
117. Family 2. Selaginellaceae. — This family includes but a
single genus, Selaginella, of over 600 species. Only a few forms
occur in the temperate regions, the majority being confined to
tropical countries, where they often form one of the most attrac-
tive features of the forest vegetation owing to the symmetry of
their branching and the rare delicacy of their foliage (Fig. 241).
For these reasons they are extensively cultivated and familiar
objects in conservatories and florists' shops. The so-called res-
urrection plant, Selaginella lepidophylla, lives in the very arid
sections of the southw^estern United States, and during drought
reduces its surface to a nest-like ball by rolling up its branches
into tight coils. In this condition it appears as a brownish dead
mass. When moistened, the absorption of water causes the
branches to quickly uncoil and also renders the tissues trans-
lucent, so that the green color of the chloroplasts can be seen.
These reactions occur even in the dead plants and so create the
impression that they have returned to life. Many of the species

DEVELOPMENT OF PLANTS 347

of Selaginella have a creeping habit Hke that of Lycopodium,
and the aerial stems are covered with leaves arranged in several
rows, very frequently in four rows of two large and two smaller
leaves (Fig. 241, i^). Usually but a single chloroplast appears
in each cell, as already noticed in several algae and in the Antho-

FiG. 241. A common cultivated Selaginella: 1, habit of the plant — s,
strobili; b, a branch bearing roots, r. lA, portion of stem, showing leaf ar-
rangement.— H. O. Hanson.

cerotales. The roots are frequently developed from the end of
naked branches that extend from the stems to the ground (Fig.
241, r).

(a) Sporangia and Spores. — The sporangia arc borne on the
stems in the axils of the leaves which form a strobilus, as in
Lycopodium. However, they differ radically from those of the
Lycopodium, in that two kinds of spores are formed, small spores
or microspores and larger ones or megaspores. The formation
of two kinds of spores, or heterospory, is not confined to the
Lycopodiales. It occurs among certain genera of the Filicales

348

SPOROPHYLLS OF SELAGINELLA

and, as has been stated, characterizes some of the fossil Equise-
tales. The megaspores are generally formed at the base of the
strobilus and the microspores occupy the upper sporangia (Fig.
242). The sporangia are called microsporangia and megaspor-
angia accordingly as they contain small or large spores and for
the same reason the leaves may be designated as micro- and

Fig. 242. Sporophylls and spores of Selaginella: 2, strobilus with lower
sporophylls separated for spore dissemination. 3, megasporophyll with
sporangium containing four megaspores. 4, microsporophyll. 5, mega-
spore enlarged. 6, microspore equally magnified. 6A, more enlarged view
of the microspore. The triangular surfaces of the spores show that these
spores have been formed in tetrads from a spore mother cell as in previous
groups.

mega-sporophylls. The two kinds of spores originate in the
same manner as previously noted and the difference in size is
due to the amount of food which they receive. In the case of
the microspores, the numerous mother cells of the sporangia form
four spores each in the usual manner: but in the megasporangia
only one of the mother cells divides in this manner, and in some
cases only one spore is formed. The other mother cells do not
develop and are ultimately consumed in nourishing the mega-
spores. As a result of the large amount of food transferred to
the megasporangium, both it and especially the one to four
spores become much larger than the others (Fig. 242, 3-6).
The physiological differentiation of the spores noted in Equisetum

DEVELOPMENT OF PLANTS 349

has here gone a step further. They are so constituted that they
now appropriate all the food in their respective sporangia.

(b) The Germination of the Spores. — The most important and
suggestive feature in the life history of Selaginella appears in
the germination of the spores. The spores germinate in the
sporangia and not after being shed, as in the other orders. This
growth is somewhat different from previous cases in that at first
there is no outgrowth of the spore but simply the formation of a
varying number of nuclei within the spore. Later these nuclei
develop cell w^alls and subsequent growth ruptures the spore.
In this way the microspore forms two cells while in the sporangia,
a small one often compared to a remnant of the male gameto-
phyte or prothallium and a large cell which ultimately forms a
single antheridium. At about this stage of germination the
microsporangium opens by a vertical cleft, permitting the scatter-
ing of the spores. The larger cell of the microspores now forms
a central mass of gamete mother cells which are surrounded by a
single layer of wall cells (Fig. 243, i). The male gametes are of
the same character as noted in Lycopodium and are set free
by the disintegration of the wall cells. The megaspores begin to
germinate even before they have reached their full size. The
nucleus of a megaspore divides repeatedly, forming numerous
nuclei which become arranged about the walls of the young spore.
Subsequently walls are formed about the nuclei at the apex of
the spore (Fig. 243, 2) and by further division a mass of cells
results, in the outer part of which the first divisions of the arche-
gonia arise. At this stage of development, or earlier, the mega-
sporangium opens by a vertical cleft permitting the discharge of
the spores. It must be borne in mind that this origin of the
gametophyte is of a radically different nature from any case pre-
viously noted. Heretofore the gametophyte always had an inde-
pendent existence and the sporophyte for varying periods of time
was a parasite upon it. In Selaginella you note the beginning
of a reversal of this relation, for the sporangia remain green
and continue to nourish the spores during their germination; in
other words, during the early stages in the development of the
gametoph^'te. We see that the gametophyte is becoming para-

350

GAMETOPHYTE OF SELAGINELLA

sitic upon the sporophyte (see page 281). After the spores are
shed they complete the growth of the gametophyte by forming
the archegonia and filHng the space within the spores with a soUd
mass of cells. This growth ruptures the spore walls at the apical
regions, thus exposing the archegonia to the male gametes (Fig.
243, 3). The archegonia are rudimentary structures consisting

Fig. 243. Gametophyte and young sporophyte of Selaginella: i, section
of a microspore that has nearly completed its germination — p, a single cell
that is possibly a remnant of the large gametophyte or prothallium of pre-
vious groups of ferns; aw, antheridium consisting of a layer of wall cells
which enclose the gamete mother cells. lA, a male gamete. 2, section
of a megaspore, showing the stage of germination that is usually attained
in the sporangium. 3, mature female gametophyte that has ruptured the
wall of the megaspore, thus exposing the archegonia, ar, one of which has
developed a young sporophyte or embryo with root, r; stem bearing two
leaves, st, foot, /, and the suspensor, s. 4, young sporophyte with root,
stem and leaves emerging from the gametophyte.

of but two neck cells, and a single canal cell leads to the female
gamete.

It is evident that these variations are of great advantage in
ensuring the perpetuation of the species. Especially is the
nourishment of the young gametophyte by the highly organized
sporophyte a distinct gain. In previous types the formation of
the gametophyte was dependent upon favorable conditions, such
as moisture, light, temperature, etc., and it should be said that

DEVELOPMENT OF PLANTS 351

only in rare cases do the spores fall in such favorable situations
and meet with such suitable conditions as to enable them to
mature archegonia and antheridia, and so provide for a new sporo-
phyte. In Selaginella the formation of the gametophyte is en-
sured by the food supplied to it by the sporophyte. Thus the
germination of the spore is practically independent of its sur-
roundings and it can complete its growth under a variety of condi-
tions, even in the dark. It should also be noted that this relation-
ship of the two generations results in a marked reduction in the
size of both the male and female gametophyte, since they are no
longer burdened with the work of food construction. This re-
duction will proceed very rapidly as we advance through the
remaining groups.

(c) Germination of the Gametospore. — The germination of the
gametospore is very much like that of Lycopodium. The sus-
pensor is much longer and the young sporophyte consists of
a stem bearing two cotyledons, a well-developed foot and a root
that is developed after the other organs (Fig. 243, 3). The foot
continues to absorb food from the gametophyte after the stem
and root have emerged, and in this condition the relation of the
sporophyte to the gametophyte is strikingly suggestive of a
sprouting seed (Fig. 243, 4). In this connection it should be
remembered that the young sporophyte of certain species remains
in the gametophyte during the winter or during a drought in a
resting or dormant condition, thus resembling very closely the
seed structure to be seen in the next group. It may also be
stated that fertilization and even the formation of the young
sporophyte may begin before the spores are discharged from the
sporangium, thus helping us to understand how it came about
that the megaspores were permanently retained in the sporan-
gium as is the case among seed plants. In one of the large
groups of Pteridophyta that have become extinct, it is note-
worthy that both the gametophyte and young sporophyte were
retained in the sporangium so that the seed-like structures were
formed .

CHAPTER IX

DIVISION IV. SPERMATOPHYTA OR SEED PLANTS

1 1 8. General Characters. — This is the largest group of the
vegetable kingdom and includes all our common trees, shrubs
and herbs. The relationship of these plants is very imperfectly
understood and the various classes, orders, etc., into which they
are divided are in part artificial and do not therefore represent
completely the alliances of the groups. You have noticed that
the sporophyte is the most important feature in the life history
of the fern and that the gametophyte, subordinate from the start,
becomes very inconspicuous in some of the forms. This in-
equality of the two generations is more noticeable in the seed
plants where a progressive series of variations result in a more and
more complex external and internal differentiation of the various
organs of the sporophyte and in a steady reduction of the gameto-
phyte. Heterospory that appeared in several groups of the Pteri-
dophyta, becomes a constant characteristic of the seed plants.
The spores originate much as in the preceding group but the
megaspore is nourished and permanently retained in the sporan-
gium where it not only forms the female gametophyte (see Selagi-
nella) but also develops the young sporophyte. At this point,
the most characteristic feature of the Spermatophyta is seen.
The young sporophyte or embryo usually ceases to grow while
still in the gametophyte and passes into a resting or dormant con-
dition and the sporangium which has become variously modified
for the purpose of protecting the embryo is discharged from the
sporophyte. This sporangium containing the embryo is called
the seed. Contrast with these features two important differences
that appeared in Selaginella. First: The spores of these ferns
are retained and nourished in the sporangia only during the
early development of the gametophytes. Second: The young
sporophyte of Selaginella — it may also be called the embryo —
does not normally pass into a resting state, but stead il grows on

352

DEVELOPMENT OF PLANTS 353

nto the mature ^porophyte. The seed plants arc divided into
two subdivisions based upon the relation of the megasporangia
CO the sporophylls: I. Gymnospermae, with sporangia, hence
with the seed, on the surface of the sporophyll. 2. Angiospcrmae,
with sporangia, hence with the seed, enclosed by the sporophyll.
[t should be stated that this association of these two subdivisions

s artificial, the Gymnosperms being more nearly related to the
Pteridophyta than to the Angiosperms.

Subdivision i. Gymnospermae. Plants with Naked Seeds

119. Origin of the Gymnospermae. — This group is principally
represented to-day by the cone-bearing trees. In very ancient
geological times, several groups of primitive gymnosperms, now
extinct, flourished and formed an extensive and varied forest
vegetation. Fossil remains of these various groups have been
A'onderfully well preserved and show that they were undoubtedly
related to the Pteridophyta. The modern group of gymnosperms
!ias been derived from these ancient lines and but a remnant
las survived the changes occurring upon the earth and the com-
petition with the more highly specialized Angiospcrmae. A few
;^enera, however, are well represented to-day and they are of
^^reat commercial importance as lumber. The sporophylls of
che gymnosperms are arranged in a strobilus as in the Equise-
tales and Lycopodiales, but the microspores and megaspores are
not associated, being developed in separate strobili. Attention
will be directed to only two of the more important orders: The
Cycadales or Cycads and the Pinales or cone-bearing trees.

120. Order a. Cycadales or Cycads. — These plants are strictly
cropical, though two genera are subtropical, Zamia or coontie
'n Florida, and Cycas, often called the sago palm, in China and
Japan. Many of them are extensively cultivated in green-houses
Dwing to their peculiar growth and attractive foliage (Fig. 244).
The cycads include less than 100 species of what was in geolog-
cal times, a very extensive alliance. They are of special in-
terest as howing unmistakable fern characters and because they
are the most primitive of our extant seed plants. The stems
of these plants are rather tuberous, though certain species are

354

THE CYCADS

decidedly palm-like in appearance, with stems 12 to 60 feet high.
The vascular bundles are collateral and arranged around a large
pith as noted in Botrychium. A slight enlargement of the stem
is brought about owing to the weak growth of the cambium of
these bundles, but in some genera the principal increase is effected
by the formation of new bundles outside of those first formed.
Fern types of bundles also occur in the leaves, cortex and strobili
of certain species. The foliage leaves are large and leathery and
form a rosette, alternating with scale leaves, at the apex of the

fefe:^:::^

-i^^i^

Fig. 244. Zamia, a cycad common in southern Florida, with strobilus of
megasporophylls. — H. O. Hanson.

Stem. The bases of these leaves form an armor-like plate over
the surface of the stems. In certain genera, the young leaves
are coiled as in the ferns.

(a) The Sporophylls and Sporangia of the Cycads. — The spor-
angia are borne on more or less modified leaves arranged in
large strobili at the top of the stem, though rarely on the apex.
Unlike preceding cases these strobili are of two kinds, the one
bearing only megasporangia, the other microsporangia. Further-
more these two kinds of strobili are on different plants. The
sporangia are either scattered over the sporophyll or arranged in

DEVELOPMENT OF PLANTS

355

groups suggestive of the sori of the ferns, in some forms even
showing a rudimentary annuUis. There is a considerable vari-
ation in the form of the sporophylls and the distribution of the
sporangia. For example, in Cycas, the megasporophylls are
loosely associated and only slightly modified, the sporangia being
developed on their margins (Fig. 245, 3^3). The microsporo-

FiG. 245. Sporophylls and sporangia of the Cycads: 2, strobilus of Zamia.
2A, cross-section of strobilus of Zamia, showing arrangement of microsporo-
phylls. 2B, microsporophyll enlarged, showing sporangia arranged in sori.
2C, sorus of three sporangia that have opened. 2D, microsporophyll of
Cycas. 3, cross-section of a strobilus of Zamia, showing arrangement of
megasporophylls. t,A, megasporophyll enlarged with two sporangia. The
one on the right shown in section; ms, megasporc; t, integument; sp, spor-
angium; m, micropyle. t,B, megasporophyll of Cycas with laterally ar-
ranged megasporangia. — H. O. Hanson.

phylls are small and more compactly arranged, the sporangia
being associated in sori on the lower surface of the sporophylls
(Fig. 245, 2D). In Zamia, the strobili and sporophylls are quite
suggestive of Eqiiisetum, the sporangia being developed on the
inner side of shield-like sporophylls (Fig. 245, 2-3.I).

The microspores originate by the division of certain cells just
beneath the epidermis and are discharged from the sporangium
very much as in the Ophioglossales. The megasporangia.

356

THE MEGASPORANGIUM

however, present several new departures that must be borne
in mind. A rather thick coat or integument covers the sporan-
gium save for a small opening called the micro pyle (Fig. 245, 3^!).
The integument will be a feature in all the succeeding groups and
it has been compared to a highly modified indusium. It origi-
nates, however, from the base of the sporangium rather than
from the epidermis as in the common forms. As in Selaginella,
only one of the mother cells takes part in the formation of spores
and but one megaspore (instead of four) is formed. This mega-
spore does not escape from the sporangium, owing to the large

Fig. 246. Section of a megasporangium of Zamia, showing the female
gametophyte just before fertilization: i, integument differentiating into a
fleshy outer and hard inner coat; m, micropyle; p, cavity in which three
microspores are shown germinating; g, female gametophyte which has de-
veloped from the single megaspore. At the micropylar end of the gameto-
phyte are two archegonia opening into the archegonial chamber, ar. Two
neck cells lead to large female gamete, 0; sp, remains of the sporangium not
yet consumed by the growth of the gametophyte. At the right a few of the
jacket cells which supply the female gamete with food are enlarged. — After
Webber.

number of cells surrounding it and consequently it does not form
a thick protective coat. It has been suggested that these cells
enveloping the megaspore are due to the transformation of the
spore mother cells, which make up the bulk of the fern sporan-

DEVELOPMENT OF PLANTS 357

glum, into nourishing cells. This structure of the megasporan-
gium is characteristic of all the spermatoph>'ta and the arrange-
ment is of decided advantage since it provides the megaspore
during its formation and germination with an abundance of
food and moisture.

(b) The Gametophyte of the Cycads. — The megaspore germi-
nates as in Selaginella. Numerous free nuclei are first formed
that are arranged around the walls of the spore and by the
later formation of walls a tissue is developed that gradually
fills the enlarging spore. Archegonia now appear at the micro-
pylar end of this gametophyte and become sunken in pits, called
the archegonial chamber, owing to a continued growth of the
adjacent cells of the gametophyte (Fig. 246). While this female
gametophyte is very suggestive of the one noted in Selaginella
it should be observed that the archegonia are more simple in
structure, usually possessing but two neck cells and that the
central canal cells of the neck are entirely lacking. The female
gamete is also much larger (Fig. 246, 0) than any heretofore
noticed and it is nourished by specialized cells, the so-called
jacket cells, that surround it and supply it through minute pores
with an abundance of food.

It is evident that the buried position of the archegonia must
necessitate some new departure in order to bring the male ga-
metes to the archegonia. The microspores have already begun
their germination at the time when they are discharged from the
sporangium and being very light, they are carried by the winds
to strobili containing megasporangia. When the megasporo-
phylls are mature, they spread apart slightly, permitting the
microspores to rattle down to the sporangia where they are
caught, at least in some forms, by a mucilaginous secretion
from the micropyle. This substance in drying contracts and
pulls the spores in large numbers through the micropyle into
a chamber formed in the upper part of the sporangium (Fig.
246, p). As has been stated, the microspore begins to germinate
in its own sporangium and by the time that it reaches the cavity
of the megasporangium its nucleus has already divided twice,
forming a rudimentary male gametophyte of three cells that may

358

MALE GAMETOPHYTE OF ZAMIA

be compared with that of Selaginella (Fig. 247, A)\ the ceir(g)
representing the small cell, and the cell (a) corresponding to the
large antheridial cell of the gametophyte of Selaginella (Fig. 243,
i), while the cell {t) called the tube cell, represents a new depar-
ture in the evolution of the male gametophyte. The tube cell

Fig. 247. Male gametophyte of Zamia: A, stage of germination of the
microspore attained in the sporangium. See text for explanation of figures.
B, formation of tube for absorbing of food from megasporangium. C, spore
end of gametophyte showing the antheridial cell dividing into a body cell,
h, and a single wall cell, w. D, the body cell has divided, forming two cells
which become the male gametes. E, spore end of male gametophyte, show-
ing the spirally ciliated gametes. — After Webber.

grows out into the tissues of the megasporangium, forming a
tubular structure, often branching extensively, and absorbs food
for the nourishment of the antheridial cell (Fig. 247, B). This
latter cell finally divides, forming a rudimentary antheridium con-
sisting of but a single wall cell {w) and a body cell {h). The body

DEVELOPMENT OF PLANTS 359

cell usually divides but once forming two cells, in each of which
are organized two huge male gametes with spirally arranged
cilia (Fig. 247, C-E). Owing to the growth of the numerous tube
cells, the tLssues above the archegonial chamber become dis-
organized and absorbed so that the end of the gametophyte
containing the gametes bends Into the cavity thus formed and
comes into close proximity to the archegonia (Fig. 246). Owing
to the swelling of the cells {g) and {w) assisted also by the accu-
mulation of fluids in the tube, the male gametophyte is ruptured
at its basal or spore end and the male gametes are discharged into
the archegonial chamber where they swim about in the fluids
probably emitted 'from the tubes at the time of their discharge.
This fluid has been observed by Chamberlain to cause a shrinking
of the neck cells In one species of Dioon and so we see how the
male gametes may enter the archegonia.

The most noticeable departure in the sexual generation of
the cycads is the very considerable reduction of the male game-
tophyte which consists of a delicate tube containing a few cells |
from one of which the male gametes are formed as in the ferns.
You also notice that the gametophytes have become entirely
parasitic upon the sporophyte, the male gametophyte at first upon
the sporophyte which bore it and later upon the megasporangium.
Attention may be called to a suggestive departure In the develop-
ment of the female gametophyte that has been observed In one
of the cycads. In case fertilization is not effected, the gameto-
phyte ruptures the sporangium and projects as a green tissue, as
in certain heterosporous ferns.

(c) Development of the Sporophyte. — The germination of the
gametospore differs In two Important respects from the ferns.
Its nucleus gives rise by repeated divisions to numerous nuclei
which arrange themselves around the walls of the gametospore,
becoming especially numerous at Its lower end, and finally form
cell walls (Fig. 248, A). This structure is called the i:)ro-embryo.
From the lower cells of the pro-embryo is organized a massive
suspensor (Fig. 248, B), which pushes the outermost cells of the
pro-embryo deep Into the tissues of the gametophyte where they
develop the young sporophyte or eml)r\o, which consists of a

36o

THE EMBRYO OF CYCADS

rudimentary root, stem and two cotyledons (Fig. 248, C). In
the ferns the gametospore divided into two cells which by further
division formed the sporophyte. The second departure now ap-
pears. The sporophyte or embryo ceases to grow and it is
admirably protected during its resting period by the integument
which has become differentiated into a stony middle layer and into
a fleshy inner and outer layer. This completed structure, con-

FiG. 248. Development of the sporophyte or embryo of Cycas: A, sec-
tion of archegonium, showing a large number of cells lining the walls of the
germinating gametospore. This is the pro-embryo stage. B, later stage
in which the pro-embryo has developed a suspensor, s, which pushes the
embryo-forming cells, e, into the nourishing tissues of the gametophyte. C,
still later development, showing the greatly coiled suspensor and the young
sporophyte or embryo, e, with two cotyledons. D, section of a seed of Zamia:
i, integument which is fleshy without and stone-like within; w, micropyle;
g, gametophyte; e, dormant sporophyte or embryo. — A-C. — After Treub.

sisting of the modified integument, the embryo, and the gameto-
phyte, which has increased in size until it has absorbed all the
tissues of the sporangium, we call the seed (Fig. 248, D). At this
stage of development, the seed falls from the strobilus and may
remain in a dormant condition for years.

The formation of the seed is the most characteristic feature
of the Spermatophyta and the most important advance that is
to be noted in the evolution of plant life. This vaiiation has
given the seed plants an advantage over all other terrestrial
groups and made them the dominant plants upon the earth. Not

DEVELOPMENT OF PLANTS

361

only are the male and female gametophytes better provided for
than in Selaginella, owing to their complete parasitism on the
well developed sporophytes, but the formation of the young
sporophyte or embryo is also ensured by the abundance of food
placed at its disposal. The most important advantage, however,
possessed by the seed plant appears in the modification of the
integument which renders it impervious to gases and fluids.
This change so effectually seals up the embryo as to protect it
for years in some cases against conditions unfavorable for growth.
The modification of the integument was doubtless the chief
factor that led to the seed habit, since it cuts off the supply of

Fig. 249, Renewal of growth of the sporophyte or embryo in the seed:
7, section of the seed, showing the base of the cotyledons extending from the
seed, thus pushing out the stem and root, the latter organ (shown in part)
curving down into the ground. The free ends of the cotyledons are enlarging
as they absorb the food stored in the gametophyte. 6, seedling six months
old with first normal leaf. — After Sachs.

oxygen and fluids from the sporoph3'te or embryo and thus
stops its growth. When conditions are favorable for growth
(see page 129), the integument becomes permeable to fluids and
gases and the embryo continues its development. The cotyle-
dons remain in the seed to absorb the food stored in the gameto-
phyte, but their basal portions elongate, pushing out the root
which soon becomes established in the soil, while the stem which
is carried out with the root extends up into the air, and de-

362 THE CONE-BEARING TREES

velops the leaves (Fig. 249). This renewal of growth of the
embryo, or sprouting of the seed, is often erroneously termed the
germination of the seed.

The maidenhair tree, Ginkgo biloba, a native of China and
Japan and now extensively cultivated throughout the world, is
the sole survivor of another ancient order of gymnosperms that
have many points in common with the cycads and ferns. The
tall shaft of the stem, wide-spreading branches and the arrange-
ment and character of the tissues are more suggestive of the next
order, the cone-bearing trees. The leaves, however, are fern-
like in appearance and are deciduous, unlike most of the Gymno-
spermae. The arrangement of the sporangia, the development of
the gametophytes, and fertilization are of the same nature
as in the cycads. It is noteworthy that fertilization may be
effected after the megasporangia have fallen, thus approaching
the condition seen in Selaginella.

121. Order b. Finales or Cone-bearing Trees. — This order is
by far the largest group of gymnosperms and includes our well-
known evergreen or cone-bearing trees. They are largely con-
fined to temperate regions and are more numerous in the northern
than in the southern hemisphere. The shores of the Pacific are
especially favorable for their development and on our western
coast they attain dimensions equalled only by the giant Eucalyp-
tus of Australia. Though less ancient in origin than the cycads,
they have declined in the same way as the latter order and only a
remnant of this very extensive group survives today. However,
their variations have been more beneficial than in the case of the
cycads and as a result nearly 300 species are adapted to present
conditions upon the earth and constitute an important part, and
commercially the most valuable part of our forest vegetation.
(a) Structural and Adaptive Features of the Finales. — One of
the variations that is of great benefit to many of these plants
is their habit of forming buds (see page 71), which appear in this
order in more perfect form than any of the preceding. By
means of the scale leaves of the buds all the organs that are to
be produced each season are protected against drying winds.
The terminal bud of the tree is frequently the most vigorous and

DEVELOPMENT OF PLANTS

363

as a result spirc-likc stems with very symmetrically arranged
branches are often formed. The growth of the Pinales is usually
prolonged and vigorous, the age of some of the giant trees of
California being estimated at over 5,000 years, and they may
attain the height of over 300 feet and a diameter of 30 feet. The
elongation of the stem is effected by a group of actively dividing
cells at the apex, while the increase of girth is brought about
by the activities of a cambium, as in the dicotyledons (see page
89). In fact, all the features seen in the cross-section of the
stem as the cork, cortex, phloem, cambium, xylem with its an-
nual rings of growth and medullary rays and the pith are sugges-
tive of the dicotyledons, save that the tissues are more simple.
This is particularly noticeable in the xylem, which is composed

Fig. 250. A, radial section of the xylem of Finns — /, tracheids; p, border
pores or thin places in cell wall to promote transfer of fluids; m, medullary
ray of two cells accompanied by tracheids, mt. B, tangential section, show-
ing medullary rays, one cell broad and from throe to nine cells high, p, bordered
pores, C, cross-section of the outer cells of pine leaf, showing heavy epi-
dermal cells, e; well-developed cuticle, c; sunken stoma, s; strengthening
stereome cells, st, and chlorenchyma, m.

of very regularly formed tracheids with peculiar bordered pores
on their radial walls to promote the rapid transfer of fluids to
and from the medullary rays (Fig. 250, A, B). The character-
istic uniform and even grain of pine, spruce, etc., is due to the
simplicity of the tracheids which compose the wood. This struc-

364 STRUCTURE OF THE FINALES

ture of the wood explains the ease with which the timber is
worked and the common name of "soft wood" that is appHed
to it. The leaves are xerophytic in character and usually rather
small and often needle-Kke. The strongly thickened epidermis
is strengthened with strands of stereome that give them a leathery
character (Fig. 250, C). The stomata are sunken and the
centrally-placed vascular bundles communicate with the rather
irregular chlorenchyma cells by means of tracheids, and thin-
walled parenchyma cells instead of small veins as in the higher
plants. The leaves in the majority of the genera remain on the
branches for several years, and for this reason the members of
the Finales are commonly called "evergreens."

These plants generally flourish in localities where there is an
abundance of moisture and at first sight the modifications of the
leaves noted above are not in harmony with such surroundings,
being decidedly characteristic of plants growing in arid regions.
The minute tracheids which constitute the conducting system of
the Finales cannot, however, transport the fluids absorbed from
the soil as rapidly as the large cells noted among the dicotyledons.
Consequently smaller amounts of Avater are placed at the disposal
of the leaves, and were it not for the fact that they are admirably
adapted to lessen transpiration these plants would doubtless have
disappeared from the earth long ago. The ability of the leaves
to diminish the loss of water may also account for the common
occurrence of the cone-bearing trees in northern and mountainous
districts where the soils are cold and root absorption is therefore
lessened and where the vegetation is especially exposed to the
drying winds of winter (see page 38).

Nearly all the Finales are characterized by the presence of
resin passages or ducts in the stems and leaves. Whenever the
stems are cut or injured these passages pour out a thick resinous
liquid that effectually heals the wound. This doubtless explains
in part the freedom of these trees from the attack of insects and
wood-destroying fungi. It may possibly account for the inabil-
ity of many of these trees to sprout from the stump when cut.
Hardwood forests will very generally perpetuate themselves by
means of sprouts that arise from the stumps, but our evergreen

DEVELOPMENT OF PLANTS

36 =

forests are rapidly disappearing, because with few exceptions the
trees are able only to develop from seeds. Possibly the rapid
covering of living cells of the stump by the resinous substances
excludes the atmosphere so effectually as to prevent any further
growth (page 75). This resinous liquid is obtained in large
quantities from certain species of pine by removing the bark and
a little of the wood from the side of the trees when the liquid
exudes and is caught in a pan placed at the bottom of the cut
surface or in a pocket made in the tree. This substance is re-
moved from time to time and distilled, yielding turpentine and
resin. Balsam is a resin obtained from the balsam fir (Abies),
(b) The Sporangia of the Finales, — The sporangia resemble
those of the cycads in origin and structure and are developed
upon sporophylls that are usually grouped in strobili (Figs. 251,

Fig. 251. Microsporophylls of the pine: i, branch of pitch pine with
leaves spirally arranged in fascicles of three and also with several strobili,
St. At tip of branch the bud, 6, is shown elongating and bearing numerous
fascicles of leaves that are still enclosed by paper bracts. 2, surface and side
view of microsporophyll, showing arrangement of sporangia, sp. 2a, micro-
sporophyll of juniper bearing three sporangia. — II. O. Hanson.

252). The microsporangia are formed on rather small, delicate
and often brightly colored sporophylls that perish as soon as the
spores are scattered (Fig. 251, 2). The microsporophylls bear
one or several sporangia on their under surface and are hygro-

366

SPOROPHYLLS OF PINALES

scopic, curving away from each other when the sporangia are
ripe, thus permitting the opening of the sporangia and the gradual
distribution of the spores by the wind and closing when mois-
tened to protect the spores against wetting. In some of these
genera the outer coat of the microspore is lifted away from the
inner coat in such a way as to form two sac-like outgrowths
which render them more buoyant and adapted to distribution by
the wind (Fig. 254). The miscrospores are produced in such
enormous numbers that the ground in the vicinity of the ever-

FiG. 252. Strobili of megasporophylls: i, a branch that has developed
in the spring from a bud. The fascicles of leaves are just emerging from
their papery bracts and at the right is shown a strobilus, st, consisting of
minute spine-tipped scales. 2, growth of strobilus shown in i, twelve months
later. 3, appearance of strobilus shown in 2, six months later. — H. O. Hanson.

green forests is often covered with the yellow spores, the so-
called "sulphur snow." This extravagant formation of micro-
spores is a necessity owing to the small chance of any one of the
microspores being carried to the megasporophylls and so ensur-
ing fertilization. Were it not for the fact that the microspores
need be carried only comparatively short distances, this order

DEVELOPMENT OF PLANTS 367

would doubtless become extinct. The gymnosperms, however,
live in colonies and the strobili of mega- and micro-sporophylls
usually occur upon the same plant, thus necessitating but a short
transfer of microspores. The megasporophylls usually endure
for a year or more and become quite large, forming the leathery
or woody scales of the cone- or strobilus (Fig. 252). The mega-
sporangia are usually developed on the upper side of the sporo-
phyll (Fig. 253, A), the number formed varying in the different
genera. In the majority of the Pinales, the sporangia appear
upon curious outgrowths which become large and form the
conspicuous scales of the cone. This structure is generally
regarded as a greatly modified branch or shoot with which two
sporophylls are fused. It generally arises from the axil of a
bract just below it and with which it may be more or less united —
see the spruces, pines, firs, etc. (Fig. 253, B). The origin and
structure of the megasporangium and megaspore are essentially
as noted in the cycads. There is only slight indication, however,
in a few of the forms of the development of a cavity for the
reception of the microspores. Several mother cells may form
megaspores, but only one of these megaspores ever develops and
germinates.

(c) The Gametophyte of the Pinales and Fertilization. — The
female gametophyte is formed as previously noted in the cycads.
By the repeated divisions of the nucleus of the megaspore numer-
ous free nuclei are formed that become arranged around the walls
of the megaspore. Later cell walls are developed about these
nuclei, and by further division the entire space within the enlarg-
ing spore is filled with tissue (Fig. 253, C). A few or a large
number of archegonia are developed at the micropylar end of
the gametophyte. The archegonium consists of two or several
neck cells, and a large female gamete which is surrounded by
nourishing cells as in the cycads. It is noteworthy in a few of the
forms that the archegonia begin to appear very carl>' in this
growth outlined above and that the bulk of the female gameto-
phyte is developed subsequent to their origin. The possible
significance of this will be more apparent in the Angiosperms,
where we will see the female gamete arising with the third division

368

GAMETOPHYTE OF FINALES

of the nucleus of the megaspore, the gametophytic tissue being
developed at a later period. You have noticed that there is no
chamber formed above the archegonia, and it would be a natural
inference that the male gametophyte must be of a somewhat
diflferent character from that of the cycads in order to met this

Fig. 253. Megasporangia of pine: A, scale-like outgrowth of sporoph^-ll
bearing two sporangia. B, section of A, taken through one of the sporangia,
showing a small bract, s, and large scale-like outgrowth which bears the
sporangia, sp. The sporangium is surrounded by an integument, i, and con-
tains a single megaspore, m, as in the Cycads. mc, microspores that have
been drawn through the micropyle to the sporangium, into which their tube
cells are growing. C, section of megasporangium enlarged and diagrammatic,
showing the female gametophyte at the time of fertilization — g, female gamet-
ophyte, showing two archegonia with large female gametes, fg. At the left
the tube of the microspore, mc, has entered the neck of the archegonium and
the two male gametes have been discharged into the large sac of the female
gamete. On the right the tube cell with male gametes is seen approaching
the neck of the archegonium. i, integument; sp, remains of sporangium
not disorganized by the gametophyte; / shows the disorganization caused by
the tube cells as they grow towards the archegonia.

DEVELOPMENT OF PLANTS

369

new departure. The microspores which have already begun to
germinate when discharged from their sporangia are carried by
the wind to the megasporophylls which are sHghtly spread apart
at this time, permitting the microspores to rattle down to the
megasporangia. The microspores fall into the micropyle either
by reason of the position and construction of the megasporangia,
which may be so placed that the spores naturally roll down into
the micropyle, or the microspores may be drawn through the
micropyle by mucilaginous excretions as in the cycads. The

Fig. 254. Male gametophyte of the pine: 6, section of microspore, show-
ing the two air sacs, s, formed by the lifting up of the outer wall of the spore.

8, stage of germination of the spore at time of discharge from its sporangium
— t, tube cell; a, antherldial cell, above which are seen two black lines, the
remains of cells formed by earlier division of the nucleus of the microspore.

9, continuation of germination of the microspore after reaching the mega-
sporangium — /, tube cell forming a branching tube that disorganizes and ab-
sorbs the cells of the megasporangium. The antheridial cell (a) of 8 ha-
divided into a wall cell, w, and a body cell, b. 10, end of tube cell as it aps
proaches the female gamete — m, m, male gametes. The nuclei of the tube
cell and wall cell are also seen (somewhat disorganized) in the end of the tube.
— After Coulter and Chamberlain.

early stages in the germination of the microspore are essentially
as in Cycas, though genera ly more reduced. The microspore on
reaching the megasporangium has germinated, forming a tube

24

370 MALE GAMETOPHYTE OF FINALES

cell and an antheridial cell, but the one or two cells, which corre-
spond to the cell (g) of the gametophyte noted in the cycads, are
quickly disorganized and appear as faint lines (Fig. 254, 8).
In some forms several of these prothallial cells are formed so that
the male gametophyte is more primitive in this respect than the
cycads. On the other hand other groups do not apparently
form a prothallial cell at all. So the tendency is towards the
elimination of these cells because they are no longer of service
in setting free the gametes. The tube cell develops as in the
cycads, but it is directed in its growth so that one of its branches
finally reaches one of the archegonia, when it pushes aside the
neck cells and fuses with the cell membrane of the large female
gamete (Figs. 253, C; 255, A). In the meantime the antheridial
cell has divided into a wall cell and body cell (Fig. 254, 9), and
two motionless male cells are formed from the body cell. It is
to be noted here that the body cell does not divide, forming cells
in which male gametes are organized, as in all preceding cases.
It forms two free nuclei and these with their associated cytoplasm
function as gametes. It is now apparent why the tube cell
extends to the female gamete and also why the archegonial
chamber is not developed. The male gametes and the wall cell,
now quite unattached, are carried down to the end of the tube
(Fig. 254, 10) by the cytoplasmic currents and pass into the cyto-
plasm of the female gamete through an opening that is formed
in the end of the tube. Chemical attraction now draws one of
the male gametes to the nucleus of the female and their fusion
results in the formation of the gametospore. The remaining
cells of the male gametophyte are apparently disorganized
(Fig. 255, A). Several instances have been reported where the
male cells differ in size, indicating the loss of function in one of
these cells. On the other hand in those groups where the
archegonia are closely associated and where, therefore, the tube
cell may spread over more than one archegonium, no such reduc-
tion appears. The functions performed by the tube cells in the
Finales will be a constant characteristic of all the members of
the next subdivision or Angiospermae, and it should be noted that
this important modification of the male gametophyte probably

DEVELOPMENT OF PLANTS

371

arose at first as an al)sorbing organ to supply the antlieridial cell
with food, as is the case in the cycads. In the more advanced
types the archegonia became more effectually enclosed in the
sporangial tissues and the tube cell assumed in addition the
function of a conduit for the male gametes which lost their

/-.^,

D

Fig. 255. Development of the pro-embryo of pine: A, section of an ar-
chegonium at time of fertilization. One of the male gametes, cf , is seen
fusing with the female, 9. The second male gamete, cf, the tube nucleus,
and the wall cell are also shown near the neck of the archegonium. B, the
gametospore has germinated, forming four ceils, which are passing to the
upper end of the sac. C, the cells shown in B, arranged at end of sac and
in process of division (only two cells in this sectional view). D, later stage,
the pro-embryo of four plates of cells. E, the second plate or suspensory
cells, s, of the pro-embryo elongating, thus pushing the embryo-forming cells,
e, into the tissues of the female gametophyte. — A after Ferguson; B-E after
Coulter and Chamberlain.

motility as a result of this new method of transport to the
female gamete.

(d) Development of the Sporophyte. — The germination of the
gametospore of the pine will illustrate the more important
features in the process. The nucleus of the gametospore divides
a var^dng number of times, commonly forming four nuclei (Fig.
255, B), which pass to the upper end of the spore and arrange

1

372

EMBRYO OF FINALES

themselves in a plate. By successive division these cells are
increased until usually four plates of four cells each are formed

Fig. 256. Development of the young sporophyte or embryo: 2, diagram
of a section of a megasporangium, showing the formation of elongated sus-
pensory cells and numerous embryos, e; i, integument; sp, sporangium nearly
consumed by the growth of the gametophyte; g, gametophyte, the central
portion of it disorganized by growth of embryos, e. ^A, section of a nearly
mature embryo; s, suspensor; r, root cap; c, cotyledons; sf, stem. 35, external
view of embryo.

which become surrounded by cell walls save in the case of the
lower plate, which Js in direct contact with the rich food of the
spore (Fig. 255, C-D). This meager growth corresponds to the

Fig. 257. Sectional view of pine seed: i, hard integument; g, gameto-
phyte, often called the endosperm, which has completely consumed the spor-
angial tissues. The embryo consists of a root, r, ensheathed in a large root cap
cotyledons, c, and stem, s.

DEVELOPMENT OF PLANTS 373

pro-embryo of Cycas (Fig. 248, A). In this connection It should
be noticed that only a portion of the gametospore, as In the
cycads, is used In forming the embryo. In all the preceding
groups, as the mosses, ferns, etc., the gametospore in germinating
behaves as an ordinary cell, dividing into two cells which con-
tinue the process. In the Finales the larger portion of the
gametospore serves as a storehouse for food, cell formation being
confined to the upper end of it (Fig. 255, B, C). With one ex-
ception this is the only group characterized by this peculiar ger-
mination which is more suggestive of the growth of the animal
^gg where a large portion of the ^gg is reserve food for the nourish-
ment of the embryo. The nourishment of the female gamete
by the jacket cells is also suggestive of certain animals in which
the ^gg cells are formed in a similar manner. The embryo is
developed from the uppermost plate of cells which are pushed Into
the cells of the gametophyte by the elongation of the suspensory
cells just below them (Fig. 255, £). Each of these four cells
may form an embryo (Fig. 256, 2) or they function collectively
in this growth. The result is the same in all cases, rarely more
than one embryo being found In a seed. The mature embryo
consists of a stem bearing two or several laterally-placed coty-
ledons and a root with root cap (Fig. 256, 3). Attending this
formation of the embryo, pronounced changes occur in the
sporangium. It steadily increases in size and the integuments
become modified into a hard coat, or the outer layer may be
pulpy, as in some of the cycads. The gametophyte also increases
in size, and as the embryo matures it absorbs all the tissues of the
sporangium, thus filling the space within the Integument (Fig.
257). The cells of the female gametophyte are often called the
"endosperm." By these growths the seed is formed and pre-
pared for its dormant state, as in the preceding order. In some
cases the integument forms a membranous outgrowth, which
assists in the distribution of the seeds (Fig. 258, C). It Is inter-
esting to note that the seeds are so attached to these wings that
they rotate in falling to the ground, thus retarding their fall and
making possible a wider flight. The stimulus resulting from the
fusion of the gametes also produces extensive changes outside of

374

THE SEED OF FINALES

the sporangia. The sporophylls or their outgrowths often be-
come greatly enlarged, forming the hard, woody scales of the
cones (Fig. 258, B), or they may become fleshy and fuse, forming
a berry-like fruit, as in the juniper (Fig. 260, 7). When this
growth has been completed, the woody sporophylls or scales of
the strobilus dry out, and becoming hygroscopic, they spread
apart on dry days, thus permitting the scattering of the seeds
by the winds (Fig. 258, A). Many of the fleshy fruits are eaten
by birds and the hard nut-like seeds are distributed in this w^ay.

Fig. 258. A, mature strobilus of pine with open scales to permit the
scattering of the seeds. B, scale from strobilus showing the winged seeds
developed from the two sporangia. C, a seed with wing-like outgrowth,
as it escapes from strobilus.

The stages in the development of the seed, outlined above, are
very much prolonged in many of the pines. The microspores on
reaching the megasporangia, in the spring, develop only a short
tube cell during the first season and not until about the first of
July of the following season are the male and female gametes
mature and ready for fertilization. The seeds are matured during
the following season, over two years after the appearance of
the strobilus. Accordingly, three stages in the development
of the strobilus of megasporophylls may be seen on certain
species of pine in the early summer — very small ones that received
the microspores in the spring, larger ones in which fertilization
and the formation of the embryo is being effected and older stro-
bili in which the seeds are approaching maturity (Fig. 252).
When the conditions are favorable the embryo renews its growth.

DEVELOPMENT OF PLANTS

375

The root is pushed out through the micropyle and bends down
into the soil, the hard integument often being cracked open by
the SwelHng of the cells. The cotyledons remain within the seed
until they have absorbed all the food from the gametophyte,
when they are withdrawn and become erect (Fig. 259).

Fig. 259. Renewal of growth of the embryo. At the left a seed has been
cut across to show the relation of parts during the early growth. The in-
tegument has been ruptured by the swelling of the seed and the protrusion
of the root which is curving down into the soil. The cotyledons, c, remain
in the seed absorbing the food from the gametophyte or endosperm, g. At
the right a later growth with the cotyledons partially withdrawn from the
seed after the absorption of its food.

(e) The Principal Genera of Finales. — The more important
genera of Pinales may be distinguished as follows: Pinus or
pine, leaves long and needle-like, borne in fascicles on short stems
that are quite concealed by papery sheathing scales (Fig. 251, i).
Larix or larch, short needle-like leaves clustered in tufts on short
lateral branches. This is the only northern member of the order
with deciduous leaves. Picea or spruce, leaves angled or four-
sided, radiating from all sides of stem, petioles remaining on
branchlet after leaves fall, thus causing the rough appearance of
the branchlets (Fig. 260, 9). Tsiiga or hemlock, leaves fiat in
two rows, the petiole remaining on tlie stem after fall of leaf;

376

FORMS OF FINALES

the strobilus is ovoid with rather leathery scales. The spruce
and hemlock have pendent strobili. Abies or fir, leaves flat in
two rows, the stems being smooth after leaf fall, strobili erect on
the branches. Taxodium or bald cypress, leaves flat in two rows

Fig. 260. Common examples of the Finales: 5, Thuja or arbor vitae.
6, Strobilus of Chamaecyparis or southern white cedar. 7, strobilus of Ju7ii-
perus or red cedar with fleshy scales fused into a berry-like fruit. 8, branch
of Taxus or yew. The seeds are produced singly in the axils of leaves on short
lateral branches and nearly enveloped by a thick fleshy cup that becomes
bright red. 9, Picea or spruce.

and deciduous, strobilus globose with thick, spirally-arranged and
shield-like scales. Thuja, arbor vitae, or northern white cedar,
leaves scale-like, closely appressed to the branches in four rows,
strobili ovoid with flat tough scales (Fig. 260, 5). Chamaecyperus

DEVELOPMENT OF PLANTS 377

or southern white cedar, leaves resembling Thuja, but cones
globose with shield-like opposite scales (Fig. 260, 6). Jtiniperus
or juniper, pointed scale-like leaves, opposite or in whorls on
stem, scales of strobili becoming fleshy and fusing to form a
berry-like fruit (Fig. 260, 7).

CHAPTER X

SUBDIVISION 2. ANGIOSPERMAE. PLANTS WITH ENCLOSED

SEEDS

122. Origin of the Angiospermae. — These plants, hke tke
Gymnospermae, produce seeds, but they differ so essentially from
the latter group in the character and structure of the sporophyte
and gametophyte as to justify their separation into a distinct
division. They are retained here as a group coordinate with
the gymnosperms out of deference to common usage. It has
been noticed that the seed habit arose quite independently of
the Spermatophyta (see page 351) and it appears probable that
the Angiospermae represent an independent line of development.
Though the most recently evolved plants, their origin is uncertain
and the meager evidence points to their derivation from a stock
related to the more primative Filicales rather than to the Gym-
nospermae. The variations of this modern group of plants have
been many and so successful that they have crowded out many
of the lower forms and become the dominant plants upon the
earth. They exceed all other groups combined in variety and
number of forms, approximately, 125,000 species — certainly a
striking contrast to the other vascular plants w^hich comprise
about 450 gymnosperms and 4000 pteridophytes. The angio-
sperms are adapted by their variations to practically all con-
ditions that will sustain life, ranging from aquatics to xerophytes,
from terrestrials to epiphytes and from photosynthetic to sapro-
phytes and parasitic plants. The important features of the
plant body have been considered in the opening chapters of the
book.

123. The Sporophylls of Angiosperms. — The flower is often
considered as one of the most characteristic features of the
angiosperms, but this structure contains as its essential organs
one or more sporophylls and the term flower could be applied
quite as well to the association of these organs in the Pterido-
phyta and gymnosperms.

378

DEVELOPMENT OF PLANTS

379

The sporophylls, like all organs of the j^lant, show a wide range
of variation and they are generally associated with more or less
modified leaf-like organs which serve to protect them. These
leaf-like organs are known as the floral envelope or perianth and
doubtless arose in many forms through the sterilization and modi-
fication of the sporophylls (Fig. 261 A, i). The microsporo-
phylls, often called stamens, usually consist of a stalk or fila-
ment and a four-lobed spore-bearing part, the anther (Fig. 261 A,
2). In cross-section, the anther is seen to consist of four spo-

FiG. 261 A. Flower and sporophylls of Angiospcrnis: i, flower of Sedum
with leaf-like perianth, p; microsporophylls, s; megasporophylls, c. 2,
microsporophyll of the buttercup, showing four-lobed anther and filament.

3, diagram of a cross-section of an anther, showing the breaking down of the
tissue about the four sporangia and the beginning of the opening of the anther.

4, one of the sporangia from a young anther, as seen in cross-section — m,
spore mother cells. The large cells surrounding the mother cells are nour-
ishing cells, known as the tapetum, and disorganize as the spores mature.
At the right a mother cell forming four microspores, the upper one being
characteristic of dicotyledons and the lower of monocotyledons.

380

SPOROPHYLLS OF ANGIOSPERMS

rangia in which the microspores originate in fours as in the
preceding groups (Fig. 261 A, 3, 4). At maturity, the two
sporangia on each side of the anther usually merge into one
cavity, owing to the breaking down of the intervening tissue.
The anther opens in a variety of ways, as by slits and pores,
permitting the scattering of the spores (Fig. 261 A, 3). The

Fig. 261B. Megasporophyll and megasporangium : i, megasporophyll of
buttercup — s, stigma; st, style; 0, ovary. 2, longitudinal section of mega-
sporophyll— mg, megasporangium; m, megaspore, the mother cell having
formed four spores in series, but the end one only develops; i, integument;
mi, micropyle.

microsporophylls are often externally quite suggestive of those
noted in certain of the Pinales, but the megasporophylls are
essentially different from any other group considered, in that
the sporangia are inclosed in a cavity formed in the sporophyll.
This peculiarity is one of the important characteristics of the
angiosperms, as the name indicates. The megasporophyll, often
termed the pistil or carpel, is usually a rather elongated, flask-
shaped organ with a hollow swollen base and consists of a stigma,
style and ovary (Fig. 261 B, i) within which are produced the
megasporangia or ovules. Such a structure would result if the
edges of the leaf-like megasporophylls of previous groups were
inroUed so as to form a closed organ. The megaspores originate

DEVELOPMENT OF PLANTS

381

in these sporangia as already noted in the g^-mnospcrms and
usually but one megaspore is developed in each sporangium (Fig.
261^,2).

124. Development of the Flower of Angiosperms. — The sporo-
phylls are variousl}^ associated in groups tliat are commonly
called flowers. In its simplest form, the flower may be defined
as a minute branch or receptacle bearing one or more sporo-
phylls. Such a type is illustrated in the cat- tail (Fig. 262, B, C)

Fig. 262. Forms of primitive flowers: A, inflorescence of Typha or cat-
tail— mi, region bearing only flowers with microsporophylls; mg, flowers with
megasporophylls; h, bract. B, flower consisting of two microsporophylls
which are sessile on a short stalk that has numerous hairs. C, flower consist-
ing of one megasporophyll — s, stigma; 0, ovary surrounded with haiirs. D
early appearance of the inflorescence of Salix or willow. E, inflorescence
bearing only megasporophylls. F, flower, of a single megasporophyll with
forked stigma — -h, bract; n, nectar gland. G, inflorescence bearing only
microsporophylls. H, flower of two microsporophylls.

where the flower consists of one or a few sporophylls associated
with hairs, and also in the willow where the sporophylls are
developed in the axil of a minute bract (Fig. 262, 7^, H). It
should be noted in the cat-tail that numerous spirally arranged

382 PRIMITIVE FLOWERS

hairs are associated with the sporophylls. These are supposed
to represent sterile sporophylls and this is borne out by the fact
that primitive flowers are characterized by just such an arrange-
ment of their sporophylls, and also by the fact that in an allied
genus spirally arranged sporophylls actually occur. These simple
types of flowers are often developed in large numbers upon an
elongated stem (Fig. 262, A, D-G) and are rather suggestive
of a strobilus although the individual flower really corresponds to
the strobilus, as will be seen especially in those types where
the sporophylls are numerous and arranged spirally upon the
receptacle of the flower (Fig. 264). A group or cluster of flowers
is called an Inflorescence in contradistinction to the solitary
flower developed at the end of a branch or stem. It Is notice-
able in these primitive anglosperms that the micro- and mega-
sporophylls are usually borne in separate flowers or inflorescences
(compare Finales) which are developed on separate plants as
in the willow or on different parts of the same plant as in the
cat-tail. This arrangement is probably associated with the
fact that the advantages of crossing or the transfer of the micro-
spores of one flower to the megasporophylls of another is effected
by the wind. In higher types, which Include the great majority
of anglosperms, the micro- and mega-sporophylls are developed
in the same flower which is therefore said to be perfect since it
contains both of the organs essential for seed production (Fig.
261 A, i). A type like the willow is termed imperfect because
the flower lacks one kind of sporophyll. Crossing Is effected
in the perfect type of flower by the earlier ripening of the mlcro-
or mega-sporophylls and often also, by the arrangement of the
organs of the flower which is of such a nature that the microspores
cannot readily reach the mega-sporophylls of the same flower.
Insects are usually the agents for the transport of the micro-
spores in such cases. Flowers In which the microspores mature
and are shed from the anthers before the stigmas of the mega-
sporophylls are ready to receive them, are called protandrous,
meaning that the microspores w^hich develop the male gametes,
are the first to mature. If the mega-sporophylls become recep-
tive before the anthers open, the flower is said to be protogynous,

DEVELOPMENT OF PLANTS

383

meaning that the pistil matures first. It must also be borne in
mind, although the devices for effecting a crossing are almost
universal among the various groui:)s of angiosperms, that there
are equally elaborate provisions for the transference of the
microspores of perfect flowers to the stigmas of their own flower.
This is called autogamy and would appear to be a provision for
setting seed in case crossing fails.

From the above discussion we might characterize the primitive

Fig. 263. Development of the perianth: -1, inllorescencc of Qnercus or
oak — mi, inflorescence with flowers bearing only microsporophylls; mg,
inflorescence with flowers bearing megasporophylls. B, flower of oak, en-
larged, consisting of several microsporophylls and a perianth of minute scale-
like organs. C, flower of Erythronium or fawn lily. The perianth of six con-
spicuous leaf-like organs. D, flower of Melandrytim or day pink — ca, calyx of
green sepals; c, corolla of five delicate petals.

flowers as consisting usually of a large and indefinite number of
sporophylls, spirally arranged upon the receptacle which is suf-
ficiently elongated as to permit the separate attachment of each
organ. Following the development of this type of flower, there
appeared as the next advance, minute outgrowths about the
sporophylls, known as the perianth. In its simplest form this

384

EVOLUTION OF THE FLOWER

consists of a few scales as in the sweet flag, oak, etc. (Fig. 263, B),
but in higher forms, the perianth appears as the conspicuous
leafy portion of the flower as in the Hly (Fig. 263, C). Finally
flowers appear in which the leaves of the perianth become dif-
ferentiated into an outer calyx composed of several green sepals
and a corolla of larger, more delicate and often brightly-colored
leaves, called the petals (Fig. 263, D). We have now reached a
point where the flower is said to be complete, consisting of all
the organs that are normally associated in the flower.

Fig. 264. Fig. 265.

Fig. 264. Flower of strawberry with elongated receptacle bearing nu-
merous spirally arranged sporophylls: A, open flower. B, flower in section,
showing arrangement of parts upon receptacle, r, which forms a shallow cup
at base bearing the perianth and microsporophylls. C, the fruit or enlarged
receptacle bearing the minute spirally-arranged megasporophylls.

Fig. 265. Flowers with shortened receptacles: A, flower of Pyrola with
calyx and corolla arranged in whorls or cycles. B, section of flower, showing
all the organs in cycles. C, flower of geranium. D, flower with corolla
removed to show the coherence of the five megasporophylls that results from
the shortening of the receptacle.

As stated above the simpler type of flower is characterized
by numerous sporophylls, spirally arranged and separately at-
tached to an elongated receptacle. One of the most important
variations that appeared in the evolution of the flower is asso-
ciated with the shortening of this receptacle. This is brought

DEVELOPMENT OF PLANTS

^^0

about by the checking of the apical growth of the receptacle and
is often associated with a more or less extended growth of its
basal region. These changes affected the flower in a most pro-
found way. A crowding resulted, organs were reduced in number
and their spiral arrangement upon the receptacle became so
flattened that the various sets of organs appear to arise in whorls
or cycles (Fig. 265, A, B). These cyclic flowers are characteristic
of all the higher orders of angiosperms and no one character is of

\ <■'

Fig. 266. Forms of adhesion that result from shortening of receptacle:
A, flower of rose. B, section of flower, showing the lower portion of recep-
tacle forming a cup about the megasporophylls, mg, and bearing the other
organs of the flower. C, inflorescence of comfrey, Symphytum. D, flower
enlarged in section to show adhesion of microsporophylls, ;;//, to the tubular
corolla.

more value in enabling us to state whether a plant is of high or
low rank. As the cyclic habit became established, so the number
of organs in each whorl became constant. Thus at a certain
point in the evolution of the monocotyledons you will find the
organs usually in threes, whereas in cyclic dicotyledons whorls
with four or five organs are the rule.

The crowding of the organs on the receptacle also caused them
to develope e7t masse (as a unit), not as separate organs. This

386 EVOLUTION OF THE FLOWER

tendency is especially noticeable in the megasporophylls, which
very frequently form a compound megasporophyll (Fig. 265,
C, D), and in the same way the sepals and petals may appear as
a more or less tubular calyx and corolla (Fig. 266, C). The
crowding also led to the mass growth of the organs of adjacent
sets or whorls. This is often seen in the case of the petals and
microsporophylls, the latter organs appearing to arise from the
corolla (Fig. 266, D).

The receptacle plays an important role in all cases of mass
growth. As stated above, its apical portion ceases to elongate
at an early period while the basal part continues active, forming a
cup about the ovaries. The sepals, petals, and even the micro-
sporophylls are often so associated with this basal growth of
the receptacle that they arise en masse about the ovaries. This
growth may be so slight that a careful examination of a section
of a flower is required to detect it or a conspicuous cup-like struc-
ture may be formed, as in the rose (Fig. 266, A, B). This type
of flower is termed perigynous, meaning that the receptacle and
other organs form a more or less conspicuous cup about the
ovaries. In simpler types, as the spiral flowers, it can be seen
that the megasporophylls arise at the top of the receptacle and
that each of the other organs arises at a point just below the
next inner one. Such flowers are called for this reason hypo-
gynous, meaning below the ovaries.

Very frequently the basal growth of the receptacle also involves
the ovaries which becomes distinct from the mass growth at
various stages in their development. Consequently the other
organs of the flower appear to arise from the sides or from the
top of the ovaries; the flowers being partially or completely
epigynous, meaning that the organs of the flower are developed
upon the ovary (Fig. 267, B, D). Often in completely epigynous
flowers the receptacle elongates very slightly and the basal
portion grows up around the apex forming a cup-like cavity
which is roofed over by the megasporophylls. The sporangia
or ovules usually arise from the walls of the cavity thus formed
and not from the walls of the megasporophylls at the top of the
cavity (Fig. 267, D). The calyx, corolla and microsporophylls

DEVELOPMENT OF PLANTS

387

arise from the top of the structure thus formed and these organs
may be developed separately or there may be varying degrees of
mass growth as noted in the perig^nous flower.

Another feature to be noted in connection with the evolution
of the flower is its symmetry. In the lower types, the organs

Fig. 267. Fig. 268.

Fig. 267. Adhesion due to basal growth of receptacle: A, flower of saxi-
frage. B, section of flower, showing megasporophyll partially inclosed by
receptacle. C, inflorescence of red currant. D, section of flower, showing
the receptacle forming a sporangial cavity that is covered at the top by the
megasporophylls. The other organs arise from the top of this structure,,
the microsporophylls adhering to the corolla.

Fig. 268. Irregular or zygomorphic flower of honeysuckle: ca, calyx;
c, corolla of five unequal cohering petals, s, stigma.

of a set are alike and radially arranged about the center of the
flower (Fig. 267, A). This is the regular or actinomorphic type
of flower, meaning radially symmetrical. In many of the orders
of angiosperms one or more members of a set are different from
the others, thus destroying the radial symmetry (Fig. 268).
This is the irregular or zygomorphic type of flower, meaning
yoke-form. Such flowers can be cut into two similar halves in
but one plane.

It must not be understood that evolution of the flower has

388 GAMETOPHYTE OF ANGIOSPERMS

progressed steadily through the various changes outHned above
and that consequently in the following lessons we can begin with
the most primitive type and proceed by regular steps to the
highest forms. The various orders of angiosperms have doubt-
less been derived from several distinct stocks and they have
not only varied in different degrees but especially will it be noted
that some orders have a tendency to emphasize certain forms of
these variations, while in other alliances, the variations will
proceed along quite different lines. These various modifications
have been retained because they were of advantage to the plant
(p. 144). The cause of the variations is unknown. Similar
lines of variations also appear among the insects, and singularly
these modifications of the flower and insect have been mutually
beneficial the one to the other. In the simpler forms of flowers
the microspores are carried by the wind, as in the gymnosperms.
In the higher types odor and nectar glands appear and bright
colors w^hich serve to attract insects. This type of flower becomes
modified so that it is adapted to special types of insects — all
others being excluded. In this w^ay the microspores are carried
with greater certainty from one flower to another of the same
kind.

125. The Gametophyte of the Angiospermae. — The angio-
sperms have developed along quite distinct lines, but they show
such a remarkable uniformity in the development and character
of the gametophyte generation that this feature of their life
history may be considered at this point, as it applies to all forms.
The megasporangium, also called the ovule, originates in the
cavity of the ovary at various points know^n as the placenta (Fig.
269, A). The structure of the sporangium and the formation
of the megaspore is very similar to that of the gymnosperms.
More often two integuments are formed and the sporangium or
the stalk which supports it becomes curved so that it very fre-
quently is turned completely over (Fig. 269, B). The germina-
tion of the megaspore results in a female gametophyte which is
very much more reduced than in the case of the gymnosperms.
As the megaspore enlarges, disorganizing the cells of the sporan-
gium, also called the nucellus, its nucleus divides and the daugh-

DEVELOPMENT OF PLANTS

389

ter nuclei move to each end of the spore (Fig. 270, A). Each
of these nuclei di\'ides twice, forming four nuclei at either end
of the spore (Fig. 270, B, C). This generally completes the
nuclear divisions in the gametophyte. Only a few cases are

Fig. 269. Development of the megasporangium and megaspore: A, sec-
tional view of the pepper-grass, Lepidium. This flower has two cohering
sporophylls, only a portion of the right-hand one being shown, s, stigma
with protruding cells to receive the microspores; mg, megasporangium con-
taining a single spore mother cell, inc. Two integuments, i, are growing up
about the sporangium. B, later stage of development, the megasporangium,
mg, becoming inverted and completely covered by the integument. The
mother cell of A has formed four daughter cells in series and not in tetrads
as in the Pteridophyta. The innermost cell of the series, ms, only matures
as a megaspore; m, micropyle;/, stalk or funiculus of sporangium.

known where a larger number of nuclei are formed — compare
with the gymnosperms. A nucleus from each of these groups,
called the polar nuclei, now approach each other and fuse,
forming a single large nucleus that is usually called the endo-
sperm nucleus. This rudimentary growth represents the female
gametophyte. The three outer or micropylar cells are not pro-

390

GAMETOPHYTE OF ANGIOSPERMS

vided with walls and consist of a rather larger cell, the female
gamete, and two nourishing cells, the synergids or helpers (Fig.
271). The inner group or antipodal cells usually have walls and
they are either soon disorganized and absorbed by the enlarging
gametophyte or they may remain as permanent features of the
gametophyte for a long time and even increase greatly in number,
serving to nourish the gametophyte by absorbing food from the
sporangium. The endosperm nucleus plays a very important
role in the development of the sporophyte, for as soon as fertiliza-

FiG. 270, Germination of the megaspore: A, first division of the mega-
spore. B, second division of the nuclei. C, final division of the nuclei.

tion has been effected it forms by repeated division a mass of cells
that completely fills the entire space within the enlarging spore,
thus providing food for the nourishment of the sporophyte. At
first sight it would appear impossible to compare the various cells
of this peculiar gametophyte with the tissues of the female gam-
etophyte of the gymnosperms or ferns. It has been suggested
that the female gamete and the synergids are the remains of three
archegonia, only one of which is usually capable of being fertilized,
and that the antipodal cells are a remnant of the numerous vege-
tative cells of the gametophyte. The peculiar formation of the
endosperm nucleus through the fusion of the two polar cells is
looked upon as a nourishing device to give the endosperm nucleus

DEVELOPMENT OF PLANTS 391

the power to grow and form a tissue that supplements the anti-
podal cells and so takes the place of the nourishing gametophyte
of the gymnosperms. According to this view, the endosperm is
a delayed prothallial growth which does not take place until
fertilization is effected. There is considerable evidence to justify

Fig. 271. Section of a megasporangium of lily, showing the mature female
gametophyte: 9, female gamete, below which are two synergids; />, the
two polar nuclei uniting to form the endosperm nucleus; a, antipodal cells;
mi, micropyle; i, integuments; /, funiculus in which a vascular bundle, i\
has been formed to transport foods to the sporangium.

this conclusion. In the Ptcridophyta the archegonia are formed
at the close of the prothallial development. Among the species
of the Gymnospermae there are several examples indicating that
the archegonia are formed at earlier and earlier stages in the
development of the gametophyte (p. 367). Possibly we have in
the Angiospermae a final condition where the homologucs of the

392 GAMETOPHYTE OF ANGIOSPERMS

archegonia are developed at the very start of the gametophytic
growth. The nicety of such a sequence of development is alto-
gether admirable. In the ferns, if fertilization is not effected,
the prothallial growth is wasted, but in the angiosperms there is
no prothallial growth without fertilization.

The male gametophyte presents several features suggestive of
the gymnosperms. The germination of the microspores usually
begins within the sporophylls, and by the time that they are shed
and carried to the stigma, their nuclei have already divided once
and each spore consists of a large tube cell and an antheridial cell
(Fig. 272, B). There is very rarely a trace of the vegetative

/>1 C B ^^^

Fig. 272. Germination of the microspore: A, mature microspore of lily,
By first stage of germination — t, tube cell; a, antheridial cell. C, final divi-
sion, in this case effected while in the microsporangium — t, tube cell; a, an-
theridial cell forming directly two male gametes. D, diagram showing the
formation of the tube which grows down the style and finally reaches the
female gametophyte. The two male gametes, g, are shown passing down
the tube; t, tube nucleus. All the figures in sectional view.

cells of the gametophyte, as in the cycads and pines, since the
necessity for these cells no longer exists. The stigma is generally
provided with minute outgrowths or papillae derived from the
epidermal cells which serve to hold the microspores (Fig. 269, s).
These cells of the stigma usually secrete a sugary solution which
nourishes the microspores and causes a continuation of their ger-
mination. It is noteworthy that these microspores may be made
to germinate by placing them in sugar solutions, but the approxi-

DEVELOPMENT OF PLANTS 393

mate strength of the solution on the stigma must be determined
in order to prepare a solution suitable for their development.
The tube cell, stimulated by the secretion of the stigma, ruptures
the outer wall of the spore, which is provided with one or more
thin places to favor this growth, and protrudes as a delicate tube.
This tube, owing to the fact that it is repelled by the oxygen of
the air, grows down into the tissues of the style which are really
a continuation of the stigma. These tissues of the style are usu-
ally looser and provided with abundant foods which are deposited
in the cells just ahead of the elongating tube to nourish and direct
it in its growth. In this way the tube is directed down the style
to the cavity of the ovary where, owing to the attractive influence
of the organic substances in the sporangium, possibly in the syn-
ergids, it usually turns out into the cavity of the ovary, enters
the micropyle and works its way through the sporangium, and,
unlike the Pinales, enters the female gametophyte generally
alongside of one of the synergids (Fig. 273). The antheridial cell
usually divides, forming directly two motionless male cells during
the elongation of the tube, and in other cases these two cells are
already formed when the microspores are discharged from their
sporangia. In none of these cases is there any indication in the
division of the antheridial cell of the formation of a wall cell as in
the gymnosperms, so that the male gametophyte of the Spermato-
phyta presents a very regular series of reductions from the cycads
to the pines and thence to the angiosperms, where it consists of a
tubular growth containing three naked cells (Fig. 272, D). This
development of the male gametophyte requires from a day to
several months and is quite independent, apparently, of the dis-
tance that it has to traverse in reaching the female gametophyte.
The male cells are often somewhat elongated and even spirally
coiled and carried to the end of the tube, as in the Pinales.

126. Fertilization.— The end of the tube finally ruptures, owing
to the tension of the fluids that gradually accumulate in it, and
the male cells are forcibly expelled into the sac-like cavity
of the female gametophyte (Fig. 273). One of the male cells
passes over to and fuses with the female gamete, thus forming
the gametospore; and the other male cell unites with the polar

394

SPOROPHYTE OF ANGIOSPERMS

nuclei, thus forming the endosperm nucleus through a triple
fusion. While this process is probably in the nature of a rein-
forcement, enabling the endosperm nucleus to perform its work,
it is noteworthy that the qualities of the male parent are trans-

FiG. 273, Section of the micropylar end of the megasporangium, show-
ing the process of fertilization. The tube, t, has passed through the micro-
pyle, entered the female gametophyte and ruptured, discharging the male
cells. One, cf, is shown fusing with the female gamete, 9, and the other
one, (^', is uniting with the two polar nuclei, thus making a triple fusion in
the formation of the endosperm nucleus; s, one of the synergids; i, integu-
ments.

mitted to the endosperm cell just as though this fusion were a
sexual process.

127. The Germination of the Gametospore. — After fertiliza-
tion the endosperm nucleus divides repeatedly, and usually the
resulting nuclei become arranged about the walls of the sac-like
gametophyte which may now be called the embryo sac (Fig.
274). Later the endosperm cells develop walls and by further
division completely fill the embryo sac with cells. This mass of
cells is called the endosperm and the method of its development
is exactly similar to that of the female gametophyte of the gym-

DEVELOPMENT OF PLANTS

395

nosperm, and it serves the same purpose, namely, to nourish the
young sporophyte or embryo. Recall, however, that it originates
in a different manner and at a different time in the life cycle.
The germination of the gametospore and the formation of the
embryo vary so greatly that only a very general statement can
be made. Following fertilization, the gametospore becomes sur-
rounded by a cell wall and attached to the wall of the embryo
sac (Fig. 275, A). It now begins to elongate and its nucleus
divides several times, forming a row of cells (Fig. 275, A, B).
This is the proembryo, the terminal cell being known as the
embryo cell and the remaining cells as the suspensor. Less

Fig. 274. Sectional view of megasporophylls of Lepidium shortly after
fertilization (see Fig. 269); en, early development of endosperm cells about
wall of embryo sac; p, young sporophyte developing from gametospore;
mi, micropyle; s, stigma;/, funiculus.

commonly the proembryo is a mass of cells without any differ-
entiation. This appears as a radical departure from the gymno-
sperms where the formation of free nuclei characterized the
germination of the gametospore. But in passing from the cycads
to the pines there is a steady decline in the number of free cells
formed and indeed in one case in the latter group the proembryo

396

SPOROPHYTE OF ANGIOSPERMS

consists of a row of cells. The synergids, which are partly con-
sumed during the entrance of the tube cells and the process of
fertilization, usually become entirely disorganized and absorbed
during these early stages of germination. The embryo is formed
by the repeated divisions of the embryo cell of the pro-embryo

B A

Fig. 275. Stages in the germination of the gametospore of Lepidtum,
sectional view: A, micropylar end of embryo sac, showing the enlarging
gametospore provided with cell wall and attached to wall of sac. B, later
growth — s, suspensor; e, embryo cell; en, endosperm cells. C, pro-embryo
after first division of embryo cell. D, further divisions of embryo cell, show-
ing formation of an epidermis and a central stem region. E, later growth,
two growing regions, the cotyledons, appearing on the sides of the stem. F,
microp^dar end of the embryo sac in which the embryo cell has formed a small
plant or embryo, consisting of two cotyledons, c; stem, st, which terminates
in the root, r; the endosperm cells, en, are being absorbed by the enlarging
embryo; s, suspensor.

assisted to a varying extent by one or more of the adjoining cells
of the suspensor (Fig. 275, C-F). The remaining cells of the
suspensor are ultimately disorganized or they may increase
greatly in size and themselves become the principal means of

DEVELOPMENT OF PLANTS 397

absorbing food and transferring it to the embryo. The structure
of the mature embryo varies greatly. In some genera, as in the
orchids, Indian pipe, etc., it remains rudimentary, consisting of
only a few cells. Among the monocotyledons, the embryo cell
frequently produces the single cotyledon, while the next under-
lying cell of the suspensor forms the root and the laterally-placed
growing point of the stem (Fig. 279). The pro-embryo of dico-
tyledons is frequently a filament of cells of varying length and
the embryo cell, by a regular series of divisions, gives rise to the
stem, two laterally-placed cotyledons and all of the root, save the
tip, which is formed from the cells adjoining the embryo cell
(Fig. 276, A).

128. The Fruit and Seed. — Various changes occur in the
sporangium during the growth of the embryo. More frequently,
perhaps, the embryo sac enlarges, absorbing all the cells within
the integument, and it becomes filled with endosperm cells. In
a case like this, the embryo either remains small and embedded
in the endosperm (Fig. 279), or the embryo may entirely con-
sume the endosperm, the food in this case being stored in the
cotyledons (Fig. 276, A). Less commonly, the embryo sac ab-
sorbs only a portion of the sporangial tissue (often called the
perisperm) and consequently the embryo is associated with a
varying amount of endosperm, which in turn is surrounded with
sporangial cells or perisperm, as in the water lily (Fig. 276, B).
The integument usually undergoes pronounced changes during
this growth, becoming hard and tough to protect the parts within,
and often developing appendages of various kinds, as hairs and
wings, to promote the distribution of the seed (see gymnosperms,
page 374).

The stimulus of fertilization extends beyond the changes
wrought in the sporangium. This is particularly noticeable in
the megasporophyll and often in adjacent parts which keep pace
with the growth of the sporangium and often undergo remarkable
transformations. The result of this total growth is called the
fruit, while the term seed is restricted to the modified sporangium
with its integument and embryo. The megasporophyll may form
a firm coat that is closely attached to the seed, as in the corn and

398

SEED AND FRUIT OF ANGIOSPERMS

other grasses. Such a fruit is called a grain. Again the tough
walls of the megasporophyll are free from the seed, as in the
buttercup, forming a fruit known as the akene. The sporophylls
may become papery or hard and split open to scatter the seed.
Where a single megasporoph^^U behaves in this way and opens by
two' valves, the fruit is called a pod or legume, example the bean,

f

Fig. 276. Seed structure: A, section of a nearly mature seed of Lepid-
ium. The stem of the embryo is differentiated below into a hypocotyl, hy^
and above into an epicotyl, pi, commonly known in the seed as the plumule,
r, root with root cap; c, the two cotyledons, which are bent over, lying one
upon the other; v, vascular bundles extending through the stem into the
cotyledons, where they form a network of veins; en, remains of endosperm.
B, section of seed of water lily (after Conard) — e, embryo, surrounded by a
layer of endosperm cells; mg, cells of the megasporangium; i, integument.

or if by one valve the fruit is termed a follicle, example the
peony. Frequently wing-like processes develop from the mega-
sporophyll, as in the maple and ailanthus, which are of service in
distribution and in other ways, as in the manufacture of foods,
these organs often being green during the development of the
embryo. Such fruits are known as key fruits or samaras. In
other cases the sporophyll becomes fleshy, forming a berry as in
the currant or the inner layer forms a pit or stone, while the outer

DEVELOPMENT OF PLANTS 399

layer forms the pulp and skin, as in the cherry and peach, fruits
known as drupes. In many fruits the receptacle becomes fleshy
and forms the major portion of the fruit, as in the fig, apple,
strawberry, etc., and in the pineapple, the axis of the inflores-
cence, as well as parts of the flower, become fleshy. These vari-
ous devices serve primarily to preserve the life of the embryo
and some of them also assist in distributing the seed.

When conditions are favorable for growth, which may not be
till several years after the seeds have been scattered, the embryo
renews its growth. The region below the cotyledons, the hypo-
cotyl, or base of the cotyledon itself, elongates and pushes out
the root which grows down into the soil, and by a later growth
the stem tip, with or without the cotyledons which may remain
In the seed, elongates and develops the characteristic stem,
branches, leaves and sporophylls of the parent plant. The struc-
ture and variations of the mature sporophyte have been presented
in the first part of the work (pages 7-145), and will be further
considered in the following studies. The terms sporangium,
microsporophyll and megasporophyll have been retained up to
this point in the work in order to keep clearly before you the
progress and relationship of the variations that have attended the
evolution of plant life. Now that we have arrived at the highest
and largest group of plants, where the sporophylls have become
highly modified and curiously associated in clusters called flowers,
more familiar names will be used in discussing them, i. e., ovule
for the megasporangium and its integument, stamen for micro-
sporophyll and pistil for megasporophyll. The terms "carpel"
and "pistil" are synonymous when these organs a:e distinct — •
i. e., free from one another; but when they cohere to form a
compound pistil the term "carpel" is employed to indicate the
number of pistils that are united, as in the willow the pistil is
composed of two carpels, in the lily of three carpels, etc.

While the sexual generation, mode of reproduction, and the
character of the tissues are very similar in all angiosperms, two
very distinct lines of variation or classes are to be noted: A,
the Monocotyledones, represented by the grasses, lilies and or-
chids; B, the Dicotyledones, represented by our common trees,
roses, mints and daisies.

400 MONOCOTYLEDONES

Class A. Monocotyledones

129. General Characteristics. — This group of angiosperms con-
tains about 25,000 species that constitute a very natural alliance,
owing to the uniformity and simplicity of their structures. These
features may be due to the uniform conditions under which these
plants live. The majority of Monocotyledones are moisture-
loving plants and are therefore exposed to very constant condi-
tions, which would naturally result in less stimulation and conse-
quent variation than in the case of plants exposed to the varying
conditions of drier soils. Some, to be sure, have become adapted
to dry and even arid regions, owing to peculiar modifications of
their leaves or stems, as in the grasses, certain bulbous plants, etc.

The leaves are smooth and simple, lance-shaped or linear in
outline, sessile and often attached to the stem by sheathing bases.
The veins do not end in free branches on the margins of the
leaves, and as a result of this closed venation the leaves are usu-
ally entire and destitute of teeth or lobed margins. In many
instances the principal veins are quite parallel, but whatever the
arrangement, the prominent veins are connected by very minute
veinlets that form an inconspicuous network or reticulation
throughout all parts of the leaf (Fig. 277). The stems are com-
posed largely of parenchyma, through which are scattered numer-
ous vascular bundles as in some ferns (Fig. 278) — see page 324.
These bundles rarely develop a cambium (Fig. 60) and conse-
quently the stem does not increase materially in diameter, and
usually it is columnar in appearance. In many instances the
stems are reduced in size and are subterranean, the bulb and
rhizome being common forms of stems which send up annually
short-lived aerial branches. As a rule the stems do not branch,
owing to the failure of the buds in the axils of the leaves to
develop.

The flowers are also of rather simple and uniform structure,
in the simplest cases being imperfect and consisting of either
spirally arranged stamens or pistils without perianth. In the
higher types spiral flowers with perianth and both kinds of sporo-
phylls appear, and these give place to forms in which the organs
are arranged in successive whorls of three parts each, the latter

DEVELOPMENT OF PLANTS

401

type of flower being especially common. Perigynous, epig>^nous
and irregular flowers are of less common occurrence. The most
conspicuous feature of the monocotyledons is the embryo, which
has a single terminal cotyledon and a laterally developed stem tip

I

Fig.

Fig. 278.

Fig. 277. Leaf of Solomon's seal with closed venation, entire margins,
and sessile upon the stem, i. e., without petiole.

Fig. 278. Cross-section of stem of rush: v, vascular bundles; st, stereome.

(Fig. 279). Generally the root and stem tip are set free from
the seed by the elongation of the basal portion of the cotyledon,
while the cotyledon remains in contact with the endosperm as a
food-absorbing organ, being often highly modified to perform
this work. An examination of a few of the more important
orders will give a better idea of the characteristics and \'ariations
of the monocotyledons.

130. Pandanales, the Cat-tail Order. — The cat-tail, TypJia,
may be taken as an illustration of this order (Fig. 280). These
plants live in wet and marshy places and present several varia-
tions that adapt them to such conditions. The main stem is a
rhizome that branches through the mud and sends up each spring
leafy stems. The rhizome grows on from \ear to year, and
owing to the decay of the older portions, the branches become
independent plants. In this way a single plant may spread and
26

402

THE PANDALES

thickly populate an extensive marsh. The aerial stems live but
for a year and are rather weak, but owing to the sheathing bases
of the leaves they become sufficiently rigid to support a heavy
foliage and withstand the winds. Notice also the extreme light-
ness of these organs. This is due to the larger air spaces which
also permit a ready interchange of gases from the leaves to all
parts of the plant, even throughout the submerged rhizomes.
You would naturally expect to find this structure in all aquatics
since the roots at least are submerged and all living cells require
an interchange of gases (see page 54). The leaves are long

mi

Fig. 279. Sectional view of seed of Veltheimia. The embryo consisting
of a large cotyledon, c, and laterally placed stem, s, below which is the root,
e, endosperm; mi, micropyle;/, funiculus.

and narrow and point nearly straight up in the air. This pre-
vents shading and permits the association of the plants in dense
colonies. The leaves are covered with a waxy coating or bloom
to prevent the adhesion of water and the plugging of the stomata.
This device is often to be seen on the leaves of plants that are
subject to heavy dews or rains. Moisture is frequently not
evaporated from the leaves until near midday, and if the stomata
become filled with water, there can be no interchange of gases
for photosynthesis during this time.

The flowers are of a very primitive t^-pe, consisting of naked

DEVELOPMENT OF PLANTS

403

sporophylls arranged in a compact inflorescence that assumes a
spike-like structure at the tip of the stem. The sporophylls are
protected until mature by modified sheathing leaves (Fig. 280, A),
Each flower in the upper portion of the spike contains only two
or three stamens supported upon a short stalk, which is asso-
ciated with hair-like outgrowths (Fig. 280, B), while the lower
flowers of the spike bear a single pistil each (Fig. 280, C), which

Fig. 280.

Fig. 281.

Fig. 280. Inflorescence and fruit of Typha: A, inflorescence — h, pro-
tecting bracts curving away from the sporophylls; mi, region of staminate
flowers; mg, region of pistillate flowers. B, staminate flower consisting of
two stamens sessile upon a short stem. C, pistillate flower of one carpel —
5, stigma; 0, ovary containing a single ovule. D, appearance of A in the
fall — mi, region occupied by staminate flowers; mg, the pistils have increased
greatly in size during the ripening of the seed.

Fig. 281. The mature fruit of Typha: 5, remains of the stigma; 0, ovary;
P, elongated pedicel bearing numerous hairs. Compare Fig. 280, C.

consists of a large flat stigma, style and ovary containing a single
ovule and supported upon a hairy stalk or pedicel. Note the

404 THE PANDANALES

association of numerous hairs with both the stamens and pistils.
These organs are supposed to be sterile sporophylls so that we
have here a very good illustration of a primitive type of flower.
The microspores are carried to the stigmas of the pistils by the
wind.

Such wind pollinated flowers are characterized by several fea-
tures well illustrated in the cat-tail. The microspores must be
produced in large numbers since the chance of one reaching the
stigma of another plant rapidly decreases as the distance tra-
versed by the spores increases. This probably accounts for the
growth of such plants in dense colonies since the close proximity
of the plants greatly increases the chance of the microspores
reaching the stigmas. The Finales, grasses, willows, oaks, etc.,
are other illustrations of a very large series of plants, some
10,000 in number, that have a similar habit. Wind pollinated
flowers are inconspicuous and simple in structure. You will
notice that showy perianths, nectar and perfume glands are
developed only in such flowers as utilize insects for the distri-
bution of the microspores. The stigmas of these flowers are
usually large and hairy or brush-like and conspicuously exposed
so as to increase the chances of catching the microspores, thus
reducing the dangers of this rather risky method of crossing.
You will also observe that wind pollinated flowers are usually
characterized by having imperfect flowers, the stamens and pistils
being developed on different parts of the plant, or on different
plants. By this arrangement, the advantages of crossing are
secured and it also happens that the microspores are either
scattered before the stigmas of neighboring flowers are mature,
or, more frequently after they have withered and are therefore
no longer capable of catching and nourishing the spores. In
this way it comes about that the microspores, even when close
to the pistillate flowers as in Typha, are often only of service
when carried to some earlier flow^ering plant whose stigmas are
mature. In the cat-tail, the microspores are shed a day or so
before the stigmas on the same spike are mature and so there
must result the benefit that comes from crossing two more or
less widely separated plants (page 144). The stamens soon

DEVELOPMENT OF PLANTS 405

perish after the discharge of their spores, but the pistils Increase
greatly In size during the ripening of the seed and form con-
spicuous brown cylinders (Fig. 280, D). As winter approaches,
the pedicel below the ovary and the hairs attached to It become
greatly elongated and the sporophyll is thus transformed Into
a very stable parachute that is easily detached from the spike
and capable of floating the seed in even the lightest winds (Fig.
281). If this fruit chances to fall In a marsh, the torpedo-like
seed after a time falls from the ruptured ovary and sinks in the
water. When the growth of the seed Is renewed, the base of
the cotyledon elongates, pushes off the lid at the end of the seed
and curves down so as to bring the root of the embryo in con-
tact with the mud. Hairs now develop from the lower part of
the cotyledon, anchoring the young plant to the ground while
the tip of the cotyledon remains in the seed until the food is
absorbed, when it is withdrawn. In the meantime, roots have
developed and penetrated the soil and the stem tip begins to
elongate, lifting up the cotyledon and making possible the for-
mation of other leaves. The curious screw-pines, Pandanits, a
large group from the oriental tropics and frequently seen In con-
servatories, belong to this order, also the bur reed {Sparganiiim),
common about marshes and water ways, which Is the highest
member of the order.

131. Graminales, the Grass and Sedge Order. — This Is one of
the largest groups of the Spermatophyta, some 7,000 species
being known, and in number of Individuals it exceeds all others
(Fig. 282). These plants are almost universally distributed
over the earth and have become adapted to almost every condi-
tion of climate and soil. The ability of these plants, which are of
aquatic origin, to establish themselves upon the drier and more
diversified land surface doubtless promoted variations and ac-
counts in part for the enormous display of forms. The Grami-
nales and the palms (Princlpales) which belong to an order
related to Typha are about the only groups of monocotyledons
that are associated In sufficient numbers to form striking features
of the vegetation of a country. This is particularly true of the
Graminales in all lands, where they constitute the principal vege-

4o6

THE GRAM IN ALES

tation of hills, plains, meadows and marshes. These plants are
also of the greatest economic importance, furnishing a variety of
foods, as wheat, r^^^e, barley, oats, rice, corn, hay, etc. The
bamboo also belongs to the order. The stem in the majority
of forms is a rhizome (as in Typha), which branches extensively
through the soil and sends up numerous aerial branches (Fig. 282,
B). These interwoven rhizomes, with their numerous roots,
form the firm swards of meadows and prairies. The aerial stems
are models of mechanical construction. The ability of the long

Fig. 282. Habit of growth of one of the grasses: A, aerial stem termi-
nating in a branched inflorescence. B, underground stem or rhizome send-
ing up a new shoot from one of the nodes. C, section through the node of
a stem that has been placed horizontal, showing the sheathing leaf base, /,
and the beginning of the upward curvature of the stem.

hollow stems of the grasses to support the heavy head of grain
must appeal to every one. An examination of Fig. 282 will
show you how the stem is reinforced by the long sheathing base
of the leaves while tough, elastic strands of stereome fibers are
so distributed as to give the best mechanical support. An un-

DEVELOPMENT OF PLANTS

407

usual localization of the growing regions also characterizes the
grasses which enables them to lift up again^their stems if they
by any means become prostrate. In case the stem becomes pros-
trate, the stimulus of gravity causes a renewaPof growth in the
cells at the base of the node on the side of thc^stem next to the
ground, thus causing the stem to curve up (Fig. 282, C). You
can easily demonstrate this peculiar localization of growth by
cutting off a growing stem of grass and placing^it horizontally
with one end embedded in moist sand and noting the curvature

Fig. 283. Inflorescence of a grass: i, tip of stem with (lowers arranged
in spike-like inflorescences. 2, a single spike enlarged at time of flowering.
4, diagram showing structure of spike. At base two sterile bracts, above
three flowers, each enclosed by an outer firm bract and an inner more delicate
bract. /, lodicules. At apex of spike a sterile flower. 5, another species
of grass, showing the scattering of the microspores.

of the node after a day. The leaves show wide variation in
structure and are doubtless one of the factors that have enabled
these plants to live under a variety of conditions. For example,
where the plants are exposed to drought or drying winds the

4o8 THE GRAMINALES

epidermal cells are very much thickened, stomata often sunken
in narrow furrows and practically all species have the power of
rolling up the leaves in dry seasons and thus reducing the leaf
surface and lessening transpiration.

The flowers are exceptionally alike in structure and show about
the same state of floral development. While resembling those
of Typlia in some particulars, they present several features
that indicate a decided advance over the previous group. Fig.
283 illustrates the common types of inflorescence found among
the grasses. The flowers are arranged on elongated branches,
but instead of a large bract ensheathing the inflorescence (see
Typlia), each flower is inclosed by one or more small bracts
so arranged as to form a spike-like structure of overlapping
bracts (Fig. 283, 2). The two kinds of sporophylls may be
arranged in separate spikes or on different parts of the same
spike, or, as in many grasses, the stamens and pistils are developed
in the same flower. One or more of the lower bracts of a spike
are usually without flowers and above them are one to several
flowers enclosed by secondary bracts (Fig. 283, 4). Removing
the secondary bracts, the flower proper is seen (Fig. 284, 3^,
3jB). This consists in many of the grasses of three stamens
and one pistil and usually two small scale-like organs, the lodi-
cules, which assist by their expansion in forcing open the pro-
tecting bracts at the time of flowering. The stamen consists
of a long filament attached to the middle of the anther so that
the latter organ can swing at the end of the filament, an arrange-
ment known as the versatile anther. The stigma is often
brightly colored and of a delicate feathery character, indicating
that the plants are wind pollinated. The ovary contains a
single ovule. The manner of flowering of the numerous genera
of this order varies. In many cases the stigmas are first ex-
truded from the bracts and can therefore be crossed only with
the microspores from some earlier flowering plant. In other
instances, stigmas and anthers are extended together and in
some genera no injury results from the transfer of the micro-
spores to the stigmas of the same flower. It is worth any one's
time to note the time of flowering, the region on the spike where

DEVELOPMENT OF PLANTS

409

it begins and the manner of opening of the bracts and extension
of anther and stigma. Many open between 6 and 7 A. M. on
pleasant days. Owing to the swelling of tlie huHcules, or of
the base of the bracts, these latter organs are forced apart, per-

i-f

Fig. 284. Fig. 285.

Fig. 284. Flower and fruit of grass: 2,A, a single flower with the two
enveloping bracts opened, exposing the stamens and pistil with feathery
stigmas. 3jB, flower with outer firm bract removed — /, lodiculcs; st, stigma.
6, mature fruit or grain — e, region of embryo. 7, section through base of
grain, showing the root, stem leaves, and scutellum, sc, or absorbing organ
of the embryo; en, endosperm. yA, diagram of a few of the outer cells of
the scutellum, sc.

Fig. 285. Inflorescence of one of the sedges, Carex: p, spike of pistillate
flowers, each pistil is surrounded by a papery sac, through which the style
and stigma protrude; s, spike bearing staminate flowers. Note the triangu-
lar stem, a characteristic of this large genus of over 1,000 species.

mitting the extension of the stigma and anthers. The lilamcnt
quickly elongates and soon curves so as to allow the anther to
swing back and forth in the wind. The two lobes of the anther
now curve apart and open at their ends by a narrow slit, forming

410 THE ARALES

a cup in which the microspores collect (Fig. 283, 5). As the
rocking of the anther in the wind sifts out the spores, more
rattle down into the cup and so a very gradual scattering of the
spores is effected. This entire process may be effected in a few
minutes, and in the case of the wheat each flower is said to lasc
for onl}^ 15 minutes. The Graminales includes two families:
The Graminaceae, or grass family, and the Cyperaceae or sedge
family. The grass family is by far the more important and is
distinguished by the usually hollow aerial stems, two rows of
leaves with sheathing bases split and the fruit is generally a
grain (Fig. 284, 6). The sedges are largely paludose and of
little value, the stems and leaves being often silicified. They
usually have solid stems, three rows of leaves with sheathing
bases entire, more simple flowers and the fruit is usually an
akene (Fig. 285).

132. Arales, the Aroid Order. — These plants are largely tropi-
cal and are characterized by large and generally net-veined leaves
and showy inflorescences and fruit (Fig. 286). They present
an odd series of striking forms quite different from other mono-
cotyledons. Many of them are familiar plants owing to their
extensive cultivation in green-houses, as the calla lily (Richardia),
Caladium, often w^th variegated leaves, and the gigantic leaves of
Dracontium and Colocasia, known as the elephant ear, and the
curiously perforated leaves of the Mo?istera. Many species of
Anthurium are climbers and reach to the top of the highest
trees, sending out with great regularity from the successive nodes,
naked branches that may reach the ground and form roots.
The order is represented in temperate regions by a few very
familiar genera as jack-in-the-pulpit (Arisaema), skunk cabbage
(Spathyema) , golden club {Orontium), sweet flag (Acorns), water
arum (Calla). The duckweeds that float upon nearly every
pond are extremely reduced allies of the order, representing the
smallest seed plants known, Wolffia being an oval, rootless plant
scarcely one millimeter in diameter.

The inflorescence is usually covered, as in Typha, with a bract
known as the spathe. This organ is variously colored and often
the most attractive feature of the plant. The coloration and the

DEVELOPMENT OF PLANTS

411

odors and glands that are developed in some of the forms indicate
that these plants are pollinated by insects. The flowers are
arranged upon a fleshy axis, the spadix, and in the simplest forms
are quite comparable to those of Typha, consisting of one or
a few stamens or a single pistil (Fig. 286, B-D). These im-

FiG. 286. Inflorescence of the Arales: A, habit of the jack-in-the-pulpit.
B, diagram of the inflorescence with spathe, s, opened on one side to show
the spadix, sp, bearing staminate flowers at the base. C, a pistillate flower
consisting of a single naked carpel. D, a staminate flower of four two-lobed
anthers. £, section of the inflorescence of one of the arums — p, compart-
ment containing pistillate flowers; s, staminate flowers.

perfect flowers ma^' be developed on dilYerent parts of the same
spadix or on difYerent spadices. In more advanced types, the
flowers are perfect, containing one or several pistils that may be
united and stamens which are surrounded by a very rudimentary

412 THE ARALES

perianth. The relation of these plants to insects is a very inter-
esting one. The brightly colored spathe serves as an allurement
and it also protects the microspores against wetting. The spores
of comparatively few plants can endure w^ater and it will be
interesting to observe in the following lessons the devices that
appear to guard against this danger. The spathe also affords
shelter to the insect and the temperature is higher within the
spathe since the food is being rapidly oxidized while the spores
are forming. The temperature of the larger spadices may ap-
proach blood heat, being lo to 25° C. higher than the outside
air. Insects are not slow to avail themselves of these advantages
and they also find an abundance of food in the microspores and
glands that are often developed upon the spathe. Many aroids
are characterized by the foul odors of decaying flesh and it is
noteworthy that the coloration of the spathe is in singular har^
mony with these odors, resembling the hues of putrid meat. As
a result of these variations, the lower orders of insects visit these
flowers in great numbers and unwittingly serve in the transfer
of the microspores; 250 carrion beetles and over 1,000 midges
have been reported as taken from a single spathe. You can
readily demonstrate that small insects are attracted to the spathe
by examining the inflorescence of the skunk cabbage and jack-
in-the-pulpit.

Common Examples of the Arales. — In the skunk cabbage
{Spathyema foetida), which is of common occurrence along
muddy streams, the inflorescence may appear as early as Febru-
ary, while the leaves are still rolled up into a compact bodkin to
enable them to penetrate the soil. Later the leaves unroll and
become very large, being good types of aroid leaves with broad
blades, strongly net-veined and evidently adapted to abundant
moisture and rapid transpiration. The spathe is shell-like, mot-
tled with purple, green and yellow and encloses a fleshy oblong
spadix that is entirely covered with flowers. You will find that
small insects are attracted to these spathes in great numbers and
spiders often take advantage of this fact and spin their webs
out of sight in the darker recesses of the spathe. In this plant,
the flowers are decidedly in advance of those noted in Typha,

DEVELOPMENT OF PLANTS 413

being perfect, and in addition containing a perianth of four
sepals which arch over the sporophylls. The crossing of the
flowers is effected in a variety of ways. In some cases the in-
conspicuous stigmas first push out from between the sepals so
that microspores must be brought from another earlier flowering
plant. In other cases, the stamens at the top of the spadix are
first extruded and shed their spores; while at the bottom of the
spadix the reverse condition obtains, the stigmas being in a re-
ceptive condition. This might result in a crossing of the upper
flowers with the lower, and later, when the lower flowers put
out their stamens and the upper their stigmas, insects could effect
a reverse crossing. As the seeds mature the spadix and sepals
become large and spongy, enclosing the ovaries which are finally
set free by the decay of the spadix, as fleshy berries. This fruit
is admirably adapted to the aquatic habit of these plants and
readily floats in the water, owing to the spongy outer tissues, and
so bring about the distribution of the seeds. The jack-in-the-
pulpit has a surer method for effecting a crossing, since the
sporophylls are usually borne on separate spadices. You will
often find, however, spadices with a few stamens situated just
above the pistillate flowers. The flowers are very simple, con-
sisting of a single naked pistil or of four nearly sessile stamens
(Fig. 286, C-D). The fruit is a shining bright red berry.

In some of the tropical genera the insects are held in the
spathe by hairs that extend obliquely downwards, thus permit-
ting the entrance but blocking the exit of the insects until cross-
ing of the flowers has been effected. In one of the species of
Arum there are two sets of hairs that divide the spathe into two
compartments containing respectively staminate and pistillate
flowers (Fig. 286, E). As soon as the spathe opens the insect
ladened with microspores from another flower makes his way to
the lower chamber, where the stigmas arc in a receptive condi-
tion. Here he is held a prisoner and rubs the microspores upon
the stigmas as he wanders about in the compartment. When the
microspores in the upper chamber begin to discharge, the lower
set of hairs wither, giving him entrance to this chamber, where
he feeds upon the microspores and becomes covered with them.

414

THE LILIALES

N

Finally the upper set of hairs wither and he is free to leave the
spathe and repeat his work in another inflorescence.

133. Liliales, the Lily Order. — The members of this order
comprise nearly 5,000 species that are widely distributed and
extensively cultivated for their show^y flowers. We have now
reached a point in the evolution of the flower where it has become
perfect and the protective spathe of preceding orders is replaced
by a well-developed perianth. The floral axis also becomes short-

FiG. 287. The fawn lily, Erythronium americaniim: A, habit of the plant.
5, the bulb, showing the origin of the stem and leaves shown in A; r, run-
ners that penetrate the soil forming new bulbs at their tips. C, pistil of
three carpels, at the right the fruit, a capsule, opening to scatter the seeds.

ened and we pass from the spiral series of flowers with a vari-
able number of organs to the cyclic flowers with a definite
number of organs arranged in whorls. In the lily order there
are five whorls of organs of three members each, the stamens
being arranged in two whorls. Note also that this crowding
usually results in a compound pistil of three carpels (Fig. 287,
C). These plants are largely perennial, with underground
stems in the form of bulbs or rhizomes. This feature adapts

DEVELOPMENT OF PLANTS 415

many of them to dry and steppe regions, from which source many
of our cultivated hHes have been obtained. It will be seen that
this habit of storing food in underground stems enables these
plants to develop their leaves, flowers and fruit during the short
rainy season, after which the entire aerial portion withers away
and their life lies dormant in the buried stems. This haljit is
equally serviceable if these plants come into competition with
larger forms, as in forests where phmts with bulbs and rhizomes
may complete their annual growth before the grosser vegetation
that would crowd them out is fail ly started (see page 44) .

(a) The Fawn Lily, Erythronium americamim. — This species
may be examined as typical of the order (Fig. 287). This plant
has received the atrocious name of adder's tongue, which is
offensive and far-fetched, and also of dog-tooth violet, although
it is not a violet at all. Burroughs has suggested the very appro-
priate name of trout lily, since the mottled leaves often form
conspicuous beds on shady banks of streams; but to those who
have experienced the spring time in the north country, the term
fawm lily seems singularly appropriate. The leaves of the fawn
lily spring from deep-seated bulbs that are formed in a peculiar
way. The seed germinates on the surface of the soil and forms
a very small bulb and a single grass-like leaf. During each suc-
ceeding season a larger leaf and bulb are formed, and w^hen of
sufficient size, the bulb sends out one or more runners that pene-
trate the soil and develop new bulbs at their tips (Fig. 287, B).
In this way the bulbs become deep-seated and rapidly increase
in numbers, and after several years they attain sufficient size to
develop two leaves and a flower. The mottled leaves have the
same habit of rolling up in emerging from the ground, as noted
in the skunk cabbage. It is to be observed that the position
assumed by the mature leaf of many plants is often strikingly
correlated with the extent of the root system. In the Liliales
generally, which do not have extensive lateral roots, the hang of
the leaves is such as to direct the water that falls upon them
towards the center of the plant, a feature doubtless of consider-
able advantage to plants living in semi-arid regions. The flower
is of a decidedly higher type than any previously studied and it

4i6

THE LILIALES

presents several features of special interest The fl • ,

ally placed orslightly pendulous. TheSan^ s " '*'''■

developed, consisting of two whorls of th u '^°"^P"^"°"sly

these organs are not as rfJlt dff ''"""'''" ""^"^ *''°"^h
corolla. The stamens n re ,1 ' ^■'^^'•^"t'^ted into calyx and

n.e™wseach:rt;:\ e :;r^^^^^ '-. r t-'- °^ ^^-

^^ a compound pistil. The :;^LtTtrstif .^^l;;

Fig. 288.

Lower forms of the Liiialpc- /t .
.ng grass-like appearance of the stem and i„1' '°'""'°" '"'^- ^'"'"''' ^ho"-
showing a ,i„ type. C. whL'rnrbA ""^^r ^-/°— ;-^^'''
show.ng the partial coherence of the carpus Tr-prf.itt^T.-pTK

DEVELOPMENT OF PLANTS 417

with each other, so that the flower lias three planes of symmetry. "^

At maturity the walls of the ovary become i)aper>' and split Tx
down the side, thus freeing the seeds. This form of fruit is
known as the capsule, and is of very common occurrence in the
order (Fig. 287, C). The development of a conspicuous perianth
is a noteworthy departure. This structure protects the micro-
spores and replaces the bracts and spathe of preceding orders,
but owing to its peculiar coloration and form, it also serves as
an attraction to special kinds of insects. In the preceding,^^ ^
forms that attracted insects, the inducements were usually ^"^ yt,
in the nature of shelter and foods, which were largely the micro-
spores that were offered freely to all. Such flowers are princi-
pally visited by a low order of stupid flies and beetles that are
rather promiscuous feeders and are quite as likely to go from
a flower of one species to that of a difl'erent kind, or to some
other object and so defeat the principal object of the flower.
With the development of the perianth, however, the flower is
equipped with a device that primarily serves to exclude these less
desirable visitors and to attract the more intelligent ones. In
Erythronium the perianth is in a horizontal position, so that it
offers a natural landing place for the insect, and the inducement
is a sugary solution secreted by nectar glands concealed at the
base of the perianth. It is an easy task for the more intelligent
long-tongued insects, like the bees, wasps and butterflies, to reach
the nectar in this type of flower. Such insects come to know by
experience how to gather the food from a particular form of
flower and consequently they will often confine their attention to
a single species during their entire flight; consequently the flower
by excluding the less intelligent and slothful insects is more cer-
tain of being properly crossed. The development of nectar and
also of odor glands is among the important variations that
appear in the evolution of the flower. It is chiefly by means of
the perfumes derived from these organs that the insect is directed
to the flowers. The coloration is also of service, when the insect
is near to the flower, thus supplementing the perfume glands by
directing him to the proper entrance. Some of the larger and
more brightly colored members of this and other orders appa-

4i8 THE LILIALES

rently depend entirely upon the attraction of their colors, while
other forms rely upon perfumes, the flowers being small and in-
conspicuous and often hidden. So we have reached a point in
the evolution of the flower where it presents a number of varia-
tions that are adapted and of benefit to special kinds of insects.
Insects have likewise varied and some have become of special use
to flowers owing to their peculiar form. In this way certain in-
sects and flowers have become dependent upon each other, and
as a result of their mutually beneficial variations, the flowering
plants and insects have greatly increased in numbers and exceeded
all other groups in the plant and animal kingdoms (p. 388).

(b) Other Forms of the Lily Order. — This order comprises
an extensive series of plants that present nearly all the variations
that occur in the flower. At the bottom of the series is the Rush
family with grass-like stems and leaves (Fig. 288, A, B). The
flower has the same structure as noted in Erythronium, but the
perianth is composed of small scale-like leaves destitute of nectar
glands, and as you would expect, is wind pollinated. The Bunch
flower family is a somewhat higher type. The perianth is some-
what larger and more delicate then in the rushes and the carpels
are often but partially fused (Fig. 288, C, D). The flowers gain
in conspicuousness by being associated in compact inflores-
cences, but do not develop as a rule the bright colors characteris-
tic of large flowers. The family is represented by Tofieldia,
swamp pink (Helonias), blazing star {Chamaelirium) , Zygade-
nus, hellebore (Veratrum), bellwort (Uvularia), autumn crocus,
etc. The Lily of the Valley family is characterized by rather
larger flowers than those of the previous family, and they are less
clustered and often provided with tubular perianths owning to
the mass growth of the perianth segments. The fruit is a berry,
unlike the majority of the families of this order. Here belong
the asparagus, the false and true Solomon's seal, Indian cucum-
ber, Trillium, and the closely related Smilax, etc. The Lily
family includes nearly one half of the members of the order and
occupies a medium position in the evolution of the group. The
flowers are large and of the Erythronium type. Here belong
some of the most showy of our native and cultivated plants; the

DEVELOPMENT OF PLANTS

419

day lily, Erythronium, tulips, hyacinths, lilies, Fritillaria, onion,
aloes and Spanish bayonets ( Yucca) of arid regions. Among
the higher genera of the lily family the organs of the perianth
often form a tubular structure. In the Amaryllis family we
find the same type of flower and fruit as in the lily family, but
the basal growth of the receptacle has enveloped the ovary so
that* the flower has become epigynous (Fig. 289, A). This

Fig. 289. Advanced forms of the Liliales: A, Narcissus with inferior
ovary, 0. The si v sepals cohere at their base, forming a tube and they also
develop an outgrowth at the mouth of this tube, which surrounds the anthers
and stigma like a cup. B, flower of Iris. C, section of same, showing in-
ferior ovary, 0; stigma, g; and anther, a.

large family furnishes a number of showy flowers, as the jonquil
and daffodil (Narcissus), amaryllis, snowflake, Criniim, star
grass, century pant, etc. The flowers of some genera become
irregular through the unequal development of certain leaves of
the perianth. The Iris family marks the culmination of the
variations noted in the order. The types of flower and fruit are
like those of the previous family, but the crowding on the recep-
tacle has resulted in the obliteration of one whorl of stamens.
The structure of the flowers often shows a series of variations that
adapt them to insect visitors and crossing, as is well illustrated

420 THE SCITAMINALES

in the iris (Fig. 289, B, C). The styles have a very unusual
form, resembHng a leafy organ with the stigma on the upper side
of a small projecting shelf. Beneath each of these curving styles
is a stamen. The insect naturally alights upon the broad leaf
of the outer whorl of the perianth and the peculiar coloration of
this leaf probably directs him to the nectar secreted at the base
of the perianth. In reaching this food he crowds down into the
tube and rubs off the microspores upon his back. In leaving the
flower he cannot hit the stigma, but in visiting another flower it
can readily be seen that he will deposit some of the spores upon
the stigma situated upon the top of the shelf, thus effecting a
crossing. This order includes a large number of showy culti-
vated forms, as the iris, fleur-de-lis, crocus, gladiolus, Friesia,
blue-eyed grass, etc.

134. The Scitaminales. — This order is confined largely to the
tropics and includes such familiar plants as the banana, the trav-
eler's tree, gfnger plant, canna, etc. It is mentioned here as fur-
nishing an interesting illustration of the variations that often
appear in epigynous flowers as a result of the crowding of the
organs upon the receptacle. Tendencies toward irregularity ap-
peared in the epigynous families of the Liliales, but in this
order these variations become very marked and make less abrupt
the transition to the next order. The flowers of the banana are
suggestive of the amaryllis family, though somewhat irregular.
All of the perianth leaves but one are united and only five per-
fect stamens are developed. These flowers are united into groups
in the axils of bracts that form large buds at the ends of the stem.

I ' As the bud elongates, the basal or lower bract is first curved
back from the bud, thus exposing the flower cluster, and this
expansion goes on until all the flowers are exposed. The lowest
flowers in the inflorescence are imperfect, containing only pistils,
the central are perfect and the upper ones bear only stamens.

4 I Only the ovaries of the lower flowers of the inflorescence develop,
each "hand" of bananas representing the matured ovaries of the
flowers in the axil of the bract. Singularly, seeds do not develo
in the edible banana, although you can see minute dark-colored
ovules forming three radiating lines in a cross-section of the

DEVELOPMENT OF PLANTS

421

fruit. The ability of these plants to propagate themselves by
means of buds developed on the underground rhizomes may have
resulted in the loss of the seed habit. A great many plants are
so successful in propagating themselves by buds, bulbs, runners,
etc., that they have ceased to produce seed. In the higher mem-
bers of the order, as in the ginger and canna families, the flowers
become very irregular through the unequal development of the
leaves of the perianth or in some forms on account of the abor-

FiG. 290. Flower of Canna, the showy petal-like organs being modified
stamens (staminodia) while the perianth proper, p, is reduced to green bract-
like organs: /, labellum; an, anther on petal-like organ; s, stigma; 0, ovary.
B, section of flower, showing ovary, 0, the modified stamen, an, and stigma,
s, the other organs being removed,

tion or transformation of all the stamens save one into petal-like'^- —
organs termed staminodia. Usually one of the leaves of the
perianth, or one or more of the staminodia, are highly modified
and known as the labellum. This organ is so placed as to afford
a natural landing place for the insects visiting the flower and also
to necessitate crossing (Fig. 290). Observe a bee visiting the
flowers of the canna and determine the significance of the position
and movement of the labellum and its relation to the stamen and
stigma.

135. Orchidales, the Orchid Order. — The orchids are the

422 THE ORCHIDALES

highest group of the monocotyledons and their flowers are a
source of wonder and admiration, owing to the singular beauty
and delicacy of their mechanical construction. Variation in this
order has occurred on a gigantic scale, resulting in a larger num-
ber of species (over 7,000) than is found in any of the preceding
orders. Nevertheless these elaborate variations have not been
very successful in enabling them to compete with other plants,
and as a result the orchids are rather rare and not at all com-
parable in number of individuals with the lilies and grasses.
Though more widely distributed than any of the other monocoty-
ledons, their variations have adapted them as a rule to peculiar
conditions. They are especially abundant in the mountainous
districts of the tropics, where they more commonly appear as
epiphytes upon the trunks of trees and in the crevices of rocks.
Such conditions are met by the development of a thick mantle
of cells about the aerial roots, the velamen, which absorbs the
moisture from the air and doubtless the enlargement of the leaf
base in some of these plants into a bulbous storage organ enables
them to anticipate in this way the heavy demands that will be
made upon them in the flowering season (Fig. 291). The ter- ^
restrial forms are largely parasitic or saprophytic and associated
with mycorrhiza, and this has resulted in some of the forms in the
suppression of various organs of the plant as the primary root or
even of the entire root system, as is illustrated in the coral root
orchid, where the leaves have also become reduced to mere scales
and the chlorophyll has disappeared. The dependence upon
fungal life has reached such a stage in some forms that the seeds
do not germinate unless hyphae come in contact with them
(p. 69). The organs of the flowers are subject to such remark-
able variations that the various parts may appear at first some-
what difficult to recognize. It is evident that the same line
of variation noted in the higher families of the Liliales has been
continued, for the perianth is arranged upon a compound ovary,
but its parts are often sharply differentiated into calyx and
corolla. The sepals and petals comprising these two whorls/,-
differ greatly in form, but especially to be noted is the oddly CGfn^
structed petal known as the labellum (Figs. 292, 293, /). This

DEVELOPMENT OF PLANTS

423

organ is the most striking feature of the orchid and it assumes
an almost endless variety of forms and colorations, being a goo4^'
illustration of Wallace's law that the most highly modified part
shows the greatest variation in coloration. In Cypripedium (Fig.
292, A) the labellum assumes the form of a moccasin, in other
genera it resembles a vase, boat, tongue, body of insect, etc. It

Fig. 291. An epiphytic orchid growing upon the branch of a tree. The
coarse roots, r, are surrounded by a mantle of cells which takes up the mois-
ture from the atmosphere, h, storage organs formed from the base of the
leaves, enabling the plant to produce flowers and fruit. The smaller stalks,
a, are the shriveled remains of these organs after flowering and fruiting.

is entire or variously lobed, slit, fringed and often prolonged into
a tube for the concealment of nectar. The stamens and stigmas
are reduced in number and greatly modified, the former organs
usually being reduced to one and so fused with the style that the
anther is sessile upon it (Fig. 292, B). One or two of the stig-
mas are generally modified into a mucilage-secreting organ, called
the rostellum (Fig. 293, A). The microspores are usually united

424

THE ORCHIDALES

into a waxy mass, pollinium (pi. pollinia) which is attached to
a sticky part of the rostellum (Fig. 293, B, C). The remarkable
feature about these gaudy flowers is the relation that the label-
lum, anther, rostellum and stigma sustain to each other. The
position of these organs is such that an insect visiting the flower
touches with some part of his body the sticky part of the rostel-
lum and the pollinia are thus made fast to him and carried to the
stigma of another flower. These devices are so elaborate in
many orchids that the microspores can only reach the stigma
through the agency of an insect. In the lower types of orchids,
as the moccasin flower (Fig. 292, B), a somewhat different

Fig. 292. A simple type of the Orchidales: A, the moccasin flower, Cy-
pripedium — /, labellum; p, the two unmodified petals; s, sepals, two being
united below the labellum; h, bract, partially concealing the inferior ovary.
B, section of the flower — /, labellum; s, stigma; an, anther; st, shield-like
sterile stamen covering the two anthers and stigma; 0, ovary; h, bract.

arrangement is found. The bee enters the opening in the upper
part of the labellum and feeds upon the glands distributed along
the bottom. In leaving the flower he forces his way through
the small opening on either side of the style and so he first comes
into contact with the stigma and later with the anthers, which are
two in number and located back of the stigma. In this flower
the anthers are surrounded by a sticky mass and the microspores
are thus fastened to the insect's body to be carried to another
flower. In the higher types of orchids, the insect probing for the
nectar touches the sticky discs of the rostellum, to which the

DEVELOPMENT OF PLANTS

425

pollinia are attached, and in this \va\' they are fastened to his
body and drawn out of the open anthers wlien he leaves the
flower (Fig. 293, C). In some cases the polhnia quickly curve
after being withdrawn from the anther, with the result that this
change of position brings them into line with the stigma of the
next flower visited. In other genera certain cells are irritable
and in a high state of tension. A touch causes an explosion that
results in hurling out the pollinia which always land on the end to
which the sticky disc is attached and so become fastened to the

Fig. 293. Higher type of the Orchidalcs: A, flower of Orchis — /, label-
lum; p, the two unmodified petals; s, sepals; r, rostellum to which the two
pollen masses, pollinia, in the two-lobed anther, an, are attached by sticky
discs; st, stigma. B, enlarged lateral view of the anther which has opened,
exposing the pollinia. C, one of the pollinia withdrawn from the anther show-
ing adhesive disc at base which is attached to the rostellum. At right a
pollinium enlarged, showing the small masses of microspores, massula, that
may be separately detached from the pollinium. — After Warming.

insect. (See Darwin's Fertilization of Orchids for a discussion
of the multiplicity of these arrangements.)

The seeds are among the most rudimentary of the Spermato-
phyta, consisting of an undifferentiated, few-celled embryo with-
out endosperm and surrounded by a delicate bladder-like integu-
ment. They are exceedingly small and produced in enormous
numbers; in fact, the seeds of the rattlesnake orchid, Peramium,
weigh but two millionths of a grain each and float in the air like

426 DICOTYLEDONES

dust particles. The fringed orchis (BlepharigloUis) , Arethusa,
ladies' tresses (Gyrostachys) , rattlesnake plantain (Peramium),
grass pink (Limodorum) , rose pogonia, showy orchis (Galeorchis) ,
moccasin flower {Cypripedium) are among the common and
more showy of our native orchids.

Class B. Dicotyledones

136. General Characters. — The structure of these plants is
more complex than that of the monocotyledons and their varia-
tions have been more extensive and successful, over 100,000
species being known. For this reason they are adapted to a
greater range of conditions and have become the dominant plants,
forming the conspicuous and characteristic features of the vege-
tation of the earth.

The dicotyledons may be short-lived annual plants, or per-
ennials, and they include a great variety of climbers, epiphytes,
parasites, and saprophytes and comparatively few aquatics. The
most extreme forms of xerophytes are also found in this group.
The leaves are highly differentiated, usually consisting of a
blade and petiole which is often associated with small leaf-like
organs, the stipules (Fig. 294). The blade varies in form and
is foten characterized by teeth and various forms of lobing.
This is due to the fact that the veins usually differ, from those of
the monocotyledons in that they repeatedly branch, becoming
smaller and smaller and thus forming a network or reticulated
venation w^ith free ends on the margins and other parts of the leaf.
The stems are markedly different from the previous class, owing
to the arrangement of the vascular tissues in a circle about the
pith (p. 76) and the formation of a cambium cylinder which
brings about an increase in the diameter of the stem (Fig. 295).
This arrangement gives the plant a great advantage, permitting
the extensive system of branching that characterizes the group
and the consequent increase in the display of foliage.

The flower is subject to the same modifications as noted in the
monocotyledons. Among the lower orders, the flowers are quite
as simple as those of the primitive monocotyledons and the de-
velopment of imperfect and wind pollinated flowers is of common

DEVELOPMENT OF PLANTS

427

occurrence. The spiral arrangement of the numerous organs
of the flower will also be noted. The majority of the orders of
the dicotyledons, however, are characterized by perfect, cyclic
flowers and the various sets of organs usually consist of four or
five members each. The perianth, when present, is generally
differentiated into a green calyx and a variously colored corolla.

Fig. 294. Fig. 295.

Fig. 294. Leaf of white birch, the blade, b, traversed by a network of
veins that end in free branches (the margin irregularly toothed or dentate)
and supported upon a petiole, p.

Fig. 295. Diagram of a cross-section of a stem of black oak four years
old: p, pith; i, 2, 3, 4, annual rings of xylem; c, cambium cylinder; ph, phloem;
c, cortex; ck, cork; m, medullary rays.

The stamens are more frequently arranged In one or two whorls,
equalling or twice the number of the sepals, and the pistils usually
form a single whorl, equalling or less than the number of sepals.
The crowding of the various organs, and the lateral growth of
the receptacle results in the reduction in the number of organs
and in their mass growth so that in the higher types the calyx
and corolla become more or less tubular and the carpels form a
compound ovary. The basal growtli of tlie receptacle also causes
the frequent occurrence of the perig}nous and epigynous t>pes of
flowers. Irregularity of the flower is more common than in the

428

EMBRYO OF DICOTYLEDONES

monocotyledons and this feature will be seen to characterize the
higher members of many of the orders.

The most characteristic feature of the dicotyledons is the
embryo which usually consists of a root, stem, and two laterally
attached cotyledons. The region of the stem above the attach-
ment of the cotyledons is known as the epicotyl and frequently ap-
pears as a minute bud, the plumule. The region of the stem below
the cotyledons, the hypocotyl, terminates in the root (Fig. 296).

t B

Fig. 296. Structure of dicotyledonous seeds: A, nearly mature seed of
Lepidium. The embryo consists of the hypocotyl, hy, ending below in the
root, r, and the root cap and above the epicotyl, pi. Two cotyledons, c,
arise laterally from the stem; /, funiculus; mi, micropyle; in, integuments;
en, remains of endosperm. B, section of seed of water lily — e, embryo with
two cotyledons attached laterally to the minute stem of the embryo and sur-
rounded by a layer of endosperm cells; mg, sporangial cells or perisperm; i,
integument.

The elongation of the basal portion of the hypocotyl frees the
root from the seed and the growth of the upper region of the
hypocotyl pushes up into the air the cotyledons and growing point
or epicotyl. The formation of the stem is sometimes due to
the elongation of the epicotyl alone, the cotyledons frequently
containing the storage foods for the nourishment of the young
plant and remaining buried in the seed.

DEVELOPMENT OF PLANTS 429

In endospermous seeds it should be noted that the cotyledons
are not developed as digestive and absorbing organs as observed
in the monocotyledons (page 401). The primary root often per-
sists, forming the main or tap root of the plant, which manner of
growth is not so common in the monocotyledons. The Dicoty-
ledons include two rather distinct series: a. The Choripetalae,
distinguished by their free petals or lack of perianth, b. The
Sympetalae, with united petals.

Series a. Choripetalae

137. General Characters. — This group comprises about 61,000
species and includes the majority of the trees and shrubs found
in the temperate regions. The flowers as a rule are of a simple
type but exceedingly variable in structure, so that it is not pos-
sible to separate them into so sharply characterized orders as in
case of the monocotyledons, or in the following group of Sym-
petalae. Furthermore, these orders doubtless represent many
parallel lines of development that are imperfectly understood.
For this reason, the order of the presentation in the following
pages does not attempt to represent the real relationship of the
groups.

138. Salicales, the Willow and Poplar Order. — The willows
are almost universally distributed along Avatcr ways, as though
demanding for their existence onl}- light and water. The poplars
are less restricted and some can endure moderately arid condi-
tions. Few plants have greater vitality. A bit of a twig and
often a portion of a root is capable of developing buds and so
starting the shoot. Trees are often pointed out that have origi-
nated through the careless sticking of a twig in the soil and
several of the willows are naturally propagated by their twigs
which are easily broken off by the winds. Their unusual abilit}'
to form numerous buds is well shown in the pollarded willows
where the branches have been cut back and a large number of
shoots develop about the wound. Perhaps the stimulus of the
wound also awakens some of the dormant buds (page 75). The
flowers, as in Typha, are arranged on an elongated axis forming
a compact inflorescence known as an anient or catkin (Fig. 297,

430

THE SALICALES

C, E). The distinguishing feature of this kind of flower cluster
is seen in the scale or bract that protects each flower (Fig. 297,

D, F). The aments and also the leaves are concealed in buds
that^re peculiar in that they are protected by a boat-shaped
scale (Fig. 297, A). These plants flower very early in the spring
and as the ament emerges from the bud the overlapping bracts
with their hairy coats form the "pussy willow" stage of the

Fig. 297. Flowers and seed of the willow (Salix): A, winter appearance
of a flowering twig, each boat-shaped scale concealing an ament. B, pussy
willow stage of flowering, the aments emerging from the scales and exposing
the hairy bracts that conceal the flowers. C, ament of pistillate flowers in
full bloom. D, pistillate flower, consisting of a compound pistil of two car-
pels— b, bract; n, nectar gland. E, ament of staminate flowers. F, stam-
inate flower of two stamens. G, ament of mature pistils which are opening
to discharge the seeds. H-I, successive stages in the opening of the pistil.
/, a seed with circle of hairs at base forming a parachute for dissemination.

inflorescence (Fig. 297, B). Soon the bracts spread apart, ex-
posing the flowers which are nearly as primitive as the simplest
of the monocotyledons, being without perianth, imperfect and
usually the two kinds of sporophylls are arranged on different
plants. The pistil is compound and composed of two carpels,
each of which contains numerous ovules, so that the pistillate

^^

J

DEVELOPMENT OF PLANTS 431

flowers are not as primitive as those of the cat-tails, where the
pistil is simple (Fig. 297, Z)). The staminate flowers show the
same primitive characters (Fig. 297, F), consisting of one or
more naked stamens. The absence of showy perianth, the ex-
travagant production of microspores and the formation of the _
flowers before the leaves become large and so interfere with the
distribution of the microspores are all characteristics of wind polli-
nated flowers. It is noteworthy, however, that nectar glands
are developed in the flowers of the willow (F'ig. 297, D, n) and
that the microspores are sticky. Perhaps we have here an illus- /

tration of one of the earliest variations of the flower thai served asl-

-4Jie_DEctar iriands

\

an allurement to insects. Certaiiil^, tho n^rtar_^gm.nds and the \
conspicuous display of microsporophylls are a very efficient
attraction, as is attested by the variety of insects that swarm
about the aments.

The most efflcient factor in the distribution of the willows and
poplars is found in the seed. The pistils mature in the early
summer, when the two carpels spread apart (Fig. 297, G-I),
permitting the discharge of the seeds, which are provided with
a circle of hair at the base (Fig. 297, J). This parachute is
not so nicely constructed as in Typha, but it is so efficient that^^-^:;;;;
myriad numbers of minute seeds are carried a considerable . ^ U
distance and cover everything in the neighborhood of the trees \ \^v^
with their lint-like masses. The name cottonwood is popularly ^*v^
applied to several of the poplars because of the cotton-like clusters
of seeds that emerge from their aments.

139. Fagales, the Beech Order. — This order includes many
of the most important hard-wood trees of the temperate regions,
comprising the family of the birches, with such representatives
as the American hornbeam (Carpimis), hop hornbeam (Ostrya),
hazel (Corylus), birch (Betula), alder (Abius) and the beech
family, which includes the chestnut {Castanca), beech (Fagus)
and oak {Ouerciis). The inflorescence is more commonly an
anient as in the preceding order (Figs. 298, A ; 299, A), although
in the hazel and in the beech family only one or a few pistils are
developed in the bud-like clusters of overlapping bracts (Fig.
299» P)- The flowers are imperfect and the two kinds of sporo-

432

THE FAGALES

phylls are usually developed upon the same plant. Several bracts
are usually associated with sporophylls (Figs. 298, B-F; 299,
B-E) so that the flowers are of a higher type than the willows.
The innermost of these bracts is often of a delicate structure and
has been referred to as a primitive form of the calyx (Fig. 298,
B, pr) and when present in the pistillate flowers it adheres to

wm? '1 W"

Fig. 298. Flowers and fruits of the birch family, order Fagales: A, in-
florescence of hornbeam (Carpinus) — s, staminate ament; p, pistillate ament.
B, staminate clusters from ament of alder (Alnus), each flower consisting
of four stamens attached to a four-parted perianth, pr. C, upper view of
the same cluster, showing the numerous bracts associated with the flowers.
D, fruit of Carpinus attached to the greatly enlarged three-lobed bract. D,
fruit cluster of hazel (Corylus) — b, bract encircling the ovary and within a
more delicate bract, pr, adnate to the ovary; s, stigma. F, fruit of Corylus,
the bract, b, in E has grown out into a tubular beaked structure that com-
pletely envelops the nut.

the ovary (Figs. 298, E, pr; 300, B, pr). The pistils are pro-
vided with long delicate stigmas and are compound, containing
several ovules but only one usually matures. The wall of the
ovary develops into a tough coat about the seed, forming a fruit

DEVELOPMENT OF PLANTS 433

known as the nut. Certain bracts of the ilowcr increase greatly
in size as the ovary matures and form a conspicuous part of the
fruit. Thus in the hornbeam (Fig. 298, D), one of the bracts
develops into a large, three-lobed green leaf, in the hop horn-
beam the bract forms a papery sac about the nut, in the hazel
a leafy husk (Fig. 298), in the birch And alders a woody peg-
like structure. In the chestnut and beech the pistils are com-
pletely enveloped by prickly bracts or outgrowths that form the
bur (Fig. 300, A-C), while in the oak these structures only cover
the lower portion of the ovary, forming the cup (Fig. 299, C-E).
The flowers of this order are typical of those that are wind polli-
nated. Note the small and simple flowers, absence of showy
perianth, nectar, and odor glands, dry and light microspores
lavishly produced to ensure seed formation and the delicate
bushy stigmas for catching the spores. The stigmas appear a
day or so before the adjoining microspores are being shed so
that a crossing from an earlier flowering plant is necessitated.
These plants form the larger part of our deciduous forests and
their association in colonics is doubtless connected with the dis-
tribution of pollen by the wind, as is also the appearance of the
flowers before the leaves are fully developed. The positions as-
sumed by the staminate aments is of service in protecting the
microspores against wetting and also to assist in their distribu-
tion. During the winter, these aments are quite erect but as the
flowering stage approaches they become pendulous, the larger
bracts protecting the sporophylls, like the shingles on a roof
(Fig. 298, A). These bracts, being hygroscopic, remain closed
during wet weather but on dry days they curve back, each bract
forming a shelf which serves to catch the spores when no winds
are stirring and thus prc\'cnt their falling to the ground.

140. Other Tree Orders Suggestive of the Fagales. — Closely
allied to the Fagales is the order of the walnuts, Juglandales,
including the black walnut and butternut (Juglatis) and the hick-
ories {Ilicoria, Fig. 301). These trees are of very common
occurrence in the northern United States and are characterized
by aromatic oils and large compound leaves. The flowers are
very similar in structure and arrangement to those of the beech
28

434

ORDERS RELATED TO FAGALES

order, but at maturity the outer part of the wall of the ovary
becomes transformed into a fleshy rind and the inner portion
forms a hard shell. In the walnut this pulpy, aromatic rind
about the nut is finally destroyed by decay, while in the hickories

^r "

Fig. 300.

Fig. 299. The beech family, order Fagales: A, inflorescence of oak {Quer-
cus) — s, staminate ament; p, pistillate inflorescence. B, staminate flower
surrounded by a perianth of slightly united bracts. C, pistillate flowers
with numerous bracts surrounding base of ovary. D, section of flower, the
pistil being composed of three carpels and the inner bracts adnate to the
ovary. E, fruit of oak, the cup consisting of the modified outer bracts shown
in C and D and the nut has developed from the ovary and one of its ovules.

Fig. 300. Flower and fruit of the beech (Fagus), order Fagales: A, pis-
tillate inflorescence, the three-lobed stigmas projecting beyond the bracts.
B, section of the inflorescence — pr, inner bracts or perianth surrounded by
an outer spiny set. C, the fruit, the outer bracts of B have become hard
and spiny and are splitting into four valves, exposing the three-angled nuts.

the rind becomes leathery and splits into valves, freeing the nut
(Fig. 301, C).
The order of the nettles, Urticales, also has many points in

DEVELOPMENT OF PLANTS

435

common with the beech order and contains several of our com-
mon trees as the elm {Ulmtis, Fig. 302), hackberry (Celtis), mul-
berry (Morns), osage orange {Toxylon), numennis tropical
forms, as the India rubber trees, banyan tree, etc., as well as a
variety of valuable herbaceous plants, as the hop and hemp.
These plants very generally contain a milky juice, latex, which

':^m

B

Fig. 301. Flower and fruit of the Juglandalcs: A, inflorescence of but-
ternut— s, staminate ament; p, pistillate inflorescence. The butternut and
black walnut (Juglans) may be recognized by the chambered pith, as shown
at bottom of twig. B, section of a pistillate flower — pr, perianth adnata to
the two carpels composing the pistil; s, stigma. C, fruit of hickory, the
outer part of ovary wall splitting into four valves and exposing the hard inner
part, the shell of the nut.

in the ca-e of Ficiis and Castilloa, two of the india rubber trees,
furnishes the raw material of india rubber and in the cow tree
of South America yields a saccharine, nutritious milky juice.
Tough stereome fibers are characteristic features in many of
these plants and furnish the hemp derived from the Cafinabis,
and in Japan paper is manufactured from the fibers of the paper

436

ORDERS HIGHER THAN FAGALES

mulberry. The leaves are often provided with rough hairs which
sometimes contain irritating acids, as in the nettles (page 40).
The flowers are of a somewhat higher type than in the preceding
orders, and associated in a variety of dense inflorescence. They
are rarely perfect, though rudimentary (sterile) stamens may
be developed with fertile pistils or vice versa, thus giving the
appearance of perfect flowers (Fig. 302, B-D). The calyx does

%-^-'

Fig. 302.

Fig. 303.

Fig. 302. Inflorescence and fruit of elm, order Urticales: A, twig of elm
bearing principally staminate flowers. B, a staminate flower enlarged, show-
ing lobed perianth enclosing numerous stamens. C, pistillate flower, two-
lobed stigma projecting from perianth. D, section of pistillate flower, show-
ing aborted (sterile) stamens and pistil (fertile) of two carpels, as indicated
by the two stigmas. E, the fruit with perianth still attached. A wing has
grown out from the sides of the ovary.

Fig. 303. A simple type of the Chenopodiales: A, shoot of Mexican tea
{Cheno podium), showing character of inflorescence, i7i. B, flower enlarged,
showing the perianth, stamens and pistil of three cohering carpels.

not adhere to the ovary as in the Fagales, though the sepals often
cohere (Fig. 302, D). The ovary usually matures as a nut, but
in many cases a kind of drupe (page 399) is formed, owing to
the perianth becoming fleshy and enveloping the nut. The fruit
of the mulberry- Is an aggregation of drupes. In the fig the stem
envelops the minute curious flowers and becomes fleshy, forming
the edible portion of the fruit.

141. Orders Showing an Advance over Preceding Forms. —

DEVELOPMENT OF PLANTS

437

The buckwheat order, Polygonalcs, containing such famiHar
weeds as the sorrel, dock (Ricmex), and knotweed or smartweed
(Polygonum), has perfect regular flowers with a distinct perianth.
These characters also appear in the allied goosefoot order, Cheno-
podiales, with its great array of common weeds, as the goosefoot
(C heno podium) , orache (Atriplex), tumbleweed and pigweed

Fig. 304. Advanced type of the Chenopodiales: A, shoot of Melandry-
num bearing flower with perianth differentiated into calyx, ca, and corolla ^
c. B, section of flower, showing the relation of calyx to corolla and the con-
cealment of the nectar glands at the base of the ovary. Access to the corolla
tube is guarded by an outgrowth on the petals, as shown in C, which, assisted
by the styles or stamens, so eff^ectually closes the mouth of the tube that
only insects with long tongues can reach the honey.

{Amaranthus) . The flowers are arranged in loose clusters and
are for the most part small and wind pollinated (Fig. 303). How-
ever, it is noteworthy that some of the genera are adapted to
insects, the calyx becoming larger and brightly colored and nectar
glands are associated with it. In the higher members of the
order, as in the purslane and pink families (which include the
spring beauty, Claylouia), the perianth becomes differentiated
into a calyx and showy corolla, and the flowers have become in a
marked degree adapted to insect visitors (Fig. 304). These

438

[J THE;RANALES

orders form a natural transition from the primitive flowers of
the willows and beeches . toNfhe large flowers of the next order
with their showy perianths, though the structure of the flower
is not very indicative of a relationship between them,

142. Ranales, the Buttercup or Crowfoot Order. — This large
and interesting order includes a great variety of our common
plants, herbs, and J;^ees, as the white and yellow water lilies
(Nymphaea ^;and 'pastalia), buttercups {Ranunculus), marsh
marigold (Cg///?(2), windflower {Anemone), Hepatica, rue {Thalic-
tru}n) X^i^i^^^h'me {Aqicilegia), larkspur {Delphinium), monks-

(V Fig. 305. A common type of the Ranales, Ranunculus repens: A, habit
'Of the plant. B, early stage of flowering, the stamens clustered about the
stigmas. C, late stage of flowering, nearly all the stamens bent over towards
tne petals, having discharged their spores. D, petal with nectar gland at
base.' E, fruit consisting of numerous spirally-arranged akenes.

hood {Aconitum), may apple {Podophyllum), magnolia, tulip tree
{Liriodendron) , Sassafras, spice bush {Benzoin), etc. In this
order we have again reached the point, just as in the monocoty-
ledons (see Liliales), where the flowers are more usually solitary
and conspicuous, owing to the development of large, showy
perianths. While the perianth is more differentiated than in the
lilies it is noteworthy in the majority of the forms that the flower
has not reached the state where the calyx is clearly separable from
the corolla. The flowers are very simple, as is indicated by
the regular and hypogynous arrangement of the parts (Fig.

DEVELOPMENT OF PLANTS 439

305). The receptacle is frequently elongated and consequently
the organs of the flower, especially the sporophylls, are numerous
and indefinite in number and spirally arranged. Were it not
for the perianth, such a type of flower would be ([uite as primi-
tive as any of the preceding orders.

(a) The Buttercup, Ranunculus. — The flower of the buttercup
illustrates the more characteristic features of the Ranales (Fig.
305, A, B). Here the perianth is apparently differentiated
into calyx and corolla. The petals, however, are sometimes
regarded as stamens that have become highly modified for the
production of nectar. The sporophylls are indefinite in number
and spirally arranged. It is interesting to note the relationship
of the various parts of this simple flower and their behavior during
the period of blooming. When the flower first opens, the anthers
are still closed and clustered around the receptive stigmas, afford-
ing a natural landing place for the insects (Fig. 305, B). Thus
crossing may only be effected with the spores carried from older
flow^ers. At a later period the outermost filaments elongate,
curving over towards the petals, so that as their anthers discharge
there is little chance of autogamy (Fig. 305, C). This position
also brings the anther into the pathway leading to the nectar
gland at the base of the petal (Fig. 305, D), so that the insect
in probing for the nectar is sure to receive some of the sticky
microspores on his body. Each day, for about a week, succes-
sive series of stamens behave in this manner, and it is not until
the innermost anthers are discharging on the last day of bloom-
ing of the flower that there is an opportunity' for autogam\'. In
many members of this and other orders you will find that the
stigmas and anthers of the innermost stamens actually meet,
owing to the peculiar curvature of the styles and filaments, thus
ensuring autogamy in case crossing has failed. It should be
borne in mind that the devices for effecting autogamy, as a last
resort, are quite as general and often as elaborate as the pro-
visions for bringing about crossing. The microspores are pro-
tected against dews and rains by the folding of the perianth and
also by the downward curvature of the pedicel at night, whereas on
each succeeding bright day the perianth opens again and the

^40

THE RANALES

flower becomes nearly or quite erect and turned towards the
light, so that it is in a position from which the insect will naturally
come. The opening of the flower is due to a growth in the morn-
ing of the basal, inner portion of the petals thus bending them
away from the center of the flower and thus effecting the opening;
while a corresponding growth towards evening of the basal outer
part of the petal bends them towards the center of the flower.
This growth slowly increases the size of the flowers, as may read-
ily be observed by comparing the freshly opened flowers of the
anemone, buttercup, etc., with those several days old. All these
features are of common occurrence in a great variety of orders.
The furit of the buttercup is an akene, each carpel containing but
a single ovule (Figs. 305, E; 261 B).

(b) Some Variations of the Order. — Many variations of this
simple structure appear in other members of the order. The

Fig. 306. Modifications appearing in the Ranales: A, flower of Helle-
borus — n, nectar glands due to modifications of stamens. B, columbine —
n, honey leaves due to modifications of the petals or possibly of the stamens.

carpels may be reduced to one, as in sassafras and other groups,
and in the green hellebore (Fig. 306, A) they are partially com-
pounded and even united with the receptacle in some water lilies,
etc. (flowers perigynous and epigynous). The stamens are some-
times few in number and they are frequently modified into nectar
glands and assume a variety of odd shapes (Fig. 306, A). The
showy nectar-bearing petals, honey leaves, of many of the genera
are regarded by some as modified stamens, as in the monkshood,
columbine, larkspur, etc. (Fig. 306, B).

DEVELOPMENT OF PLANTS

441

The parts of the perianth, wliicli are more frefjuenth' in threes
or indefinite in number than in fives, are also subject to consider-
able variation. Very frequently the corolla is not developed or
partially suppressed, and the sepals become colored and petaloid,
as in the marsh marigold, hepatica, certain windflowers, rue,
spice bush, larkspur, etc. In a few of the genera the perianth
becomes greatly modified and even irregular, as in the columl)ine
(Fig. 306, B, w), where five of the petals, often regarded as
modified stamens, are transformed into tubular hone}' leaves or

Fig. 307. Highest type of the Ranales: A, inflorescence of monkshood.
B, section of a flower — s, helmet-like sepal enclosing nectar organ, n. C,
flower of larkspur. D, section of flower — s, spurred sepal enclosing two-
spurred nectar organs, n. E, fruit of larkspur consisting of 3 pistils maturing
as follicles,

into spurs and hoods, as in the larkspur and monkshood (Fig.
307, B, D). These flowers illustrate ver\' well the progressive
coloration that is often associated with the variation of the peri-
anth. The simpler forms are usually yellow or white, while
flowers with more highly modified parts are pink and pale blue
(columbine), blue in higher tyi:)es (larkspur) and ulira-marine
blue in the most irregular and highly modified type (monks-
hood). Any of these flowers may take on by reversion the lower

442 THE PAPAVERALES

grades of color so that a blue-colored form may vary and its
offspring may produce white or yellow flowers, as in the colum-
bine. The remarkable brilliant yellow of the flowers, as seen in
the buttercup and marsh marigold, etc., has been regarded as the
primitive coloration of the perianth, and it is to be noted that
the flowers showing reversion to this color assume a duller
yellow. This order furnishes a great variety of showy orna-
mental plants, but none with fragrant flowers. Many contain
acrid juices and poisonous alkaloids, some of which are of medic-
inal value, as aconite, hydrastin, helleborine, etc. Others are of
value for the volatile oils, as sassafras, cinnamon, camphor, nut-
meg, etc. The simple structure of the more characteristic flowers
indicates that this order is a very ancient one and the pronounced
tendency to vary would suggest that possibly many of the higher
orders of the Choripetalae have been derived from this group.
The origin of the monocotyledons from this order has also been
suggested, owing to the structural features of the embryo and
vascular bundles of certain groups.

143. Papaverales, the Poppy Order. — This group includes two
very well known families: (i) The poppies, with such familiar
plants as the poppy (Papaver), sea poppy (Glaucium), celandine
(Chelidonium), soldier's cap {Bicuculla), fumitory (Adhimia),
corydalis (Capnoides) (Fig. 308). (2) The mustards, includ-
ing the peppergrass (Lepidmm), hedge mustard {Sisymbrium) ,
white and black mustard (Sinapis and Brassica), yellow rocket
(Barbarea), cress (Roripa), toothwort (Dentaria), shepherd's
purse (Bursa), whitlow grass (Draba), rock cress (Arabis) (Fig.
309). The families of the caper and mignonette are also in-
cluded in the order. These plants are closely related to the
Ranales, as is indicated by their simple and showy flowers, the
various organs usually being in multiples of two and quite dis-
tinct (flowers hypogynous) save in the case of the carpels. These
organs are compound as in the water lilies and more commonly
reduced to two, thus forming a sharp contrast with the preceding
order. Note also that there is a sharp distinction between calyx
and corolla.

(a) The Poppy Family, Papaveraceae. — The Bloodroot, San-

DEVELOPMENT OF PLANTS

443

guinaria, is typical of the simpler nieinbers of this family (I^g.
308). These plants form colonies in rather open, rich woods
owing to their fleshy rhizomes, and like many plants with storage
organs, flower very early in the spring. The flowers endure for
about two days, but the largc-lobed leaves are consi)icuous fea-
tures of the forest carpet for several weeks wliile tlH-\' are manu-

FiG. 308. Examples of the poppy family, order Papaverales: .4, habit
of the bloodroot at time of flowering — r, rhizome; /, leaf folded about flower
stalk. B, leaf unfolded, C, section of flower, showing its hypogynous struc-
ture with numerous stamens and pistil of two carpels. D, flower of fumitory
(Adlumia) — s, sepal; p, inflated petals; w, winged petals which conceal the
sporophylls.

facturing the food for the next season. The large leaves are
tightly rolled about the solitary flower and enveloped in a papery
sheath as they emerge from the soil. A juice, latex, is found in
the majority of the plants of this order, and in this example it is of
a blood-red color, thus giving the latin and popular name to the

444 THE PAPAVERALES

plant, Sangtiinaria, or bloodroot. The flowers are of the simple
open type noted in the buttercup, having two sepals (a character-
istic of the family) which fall with the opening of the flower,
petals 8-12, stamens indefinite and a compound pistil of two
carpels, as is indicated by the two stigmas and two rows of ovules
(Fig. 308, C). The stigmas are receptive as soon as the flow^er
opens, but the anthers remain closed, and so the spores must be
carried from an older flower. The petals close during the late
afternoon and when opened on the following day the anthers
shed their spores and complete the flowering.

In higher types of the poppies we find the flowers becoming
irregular and the various parts reduced in numbers. Thus, in
the soldier's cap (Fig. 308, D), there are two scale-like sepals,
four petals in two pairs, the two outer heart-shaped and spurred
at the base, while the two inner are narrow and winged on the
back, enclosing the sporophylls. The six stamens are somewhat
united and arranged into two groups opposite the spurred petals.
The carpel has the same structure as in the bloodroot. It is
evident that the nectar concealed in this closed type of flower
can only be secured by some long-tongued insect. Find out how
the bee procures the honey, the purpose of the wings on the
narrow petals and the position of the nectar glands. Is the
stigma mature before the anthers open? The structure of this
higher type of the poppies makes the transition to the mustard
family a simple one.

{h) The Ahistard Family, Cruciferae. — Here we find the
sepals and petals four in number, and the latter arranged cross-
wise (Fig. 309, Ay B), thus explaining the family name, Crucif-
erae. The petals usually have rather long claws (Fig. 309, D)
that are so associated with the sepals as to form a narrow tubular
perianth that effectually conceals the nectar glands situated at
the base of the ovary. There are usually six stamens, but the
two outer ones are shorter than the others (Fig. 309, C). The
ovary is divided into two compartments by a membranous parti-
tion. The fruit is generally a capsule that usually opens by two
valves, as seen in Fig. 309, F. This large family of over 1,800
species is almost universally distributed over the earth and of

DEVELOPMENT OF PLANTS

445

very common occurrence. These plants with tlieir small white
or yellow flowers aggregated in conspicuous inflorescences are
among the most familiar weeds of waste places, fence rows and
barren fields. The flowers are of a more specialized type than
in the open blossoms of the buttercups and poppies, and present
a variety of interesting devices that can only be hinted at in this
lesson. More commonly, perhaps, the stigmas are receptive as
soon as the flower opens and the anthers are still closed, thus

Fig. 309. Examples of the mustard family, order Papaverales: .1, branch
of Brassica, one of the turnips. B, a flower enlarged. C, flower in section.
D, a petal, showing narrow claw and broad blade. E, pistil. F, fruit, the
valves of the two carpels opened to show the seeds.

ensuring crossing. The short stamens are more freciuenth' util-
ized only for crossing since they are below the stigma and so
placed as to be in line with the nectar glands that are often located
in sac-like enlargements of the perianth. The four long stamens

446 THE PAPAVERALES

are variously related to the stigmas. Sometimes they are below
the stigma and when open are of service only for crossing, but
later they elongate and bear some of the spores to the stigma.
The reverse position of the sporophylls also occurs, but the out-
ward opening of the anthers keeps the microspores from dropping
by chance on the stigma until the stigma is finally lifted up to
the anther by the elongation of the style, where It Is sure to be
dusted with spores, as the anthers open wider and wider. Certain
species are characterized by an extraordinary twisting of the
filaments, so that for a time the anther is turned away and bent
over towards the perianth so as to be in line with the nectar
glands, and later, at the close of the flowering period, a reversal of
this bending brings the anther in contact with the stigma. These
illustrations are sufficient to show that the flowers have many
arrangements and exact movements that result in crossing, or,
if this fails, in autogamy. You would naturally expect that
specialized flowers like the mustards would show higher types of
coloration. This is indeed the case in some forms where pink and
purple colors appear, but in the majority of the species the un-
favorable situations in which they grow have resulted in a reduc-
tion in the size of the flowers and also In a decline In color to
white and pale yellow. It is noteworthy that the flowers of the
radish when cultivated on barren soil develop white, but upon
rich soil, pink. These minute flowers have a decided advantage
in being grouped In compact inflorescences, a device that you
will see copied again and again in the higher orders because it
ensures crossing and makes them conspicuous. It should be
stated that several of these plants get along very well without
crossing.

Many products of great commercial value are derived from
various members of this order. Opium, from which morphine
and other alkaloids are obtained, is the dried latex obtained from
the Incisions of the unripe capsules of the Turkish poppy. The
majority of these plants contain acrid or peppery juices that
render certain parts of the plant or their seeds of value as spices,
oils, foods, etc. Many of them are biennial, forming a close
rosette of leaves the first year and flowers and fruit the second.

DEVELOPMENT OF PLANTS 447

By cultivation an abnormal devulopment of one part or another
of the plant has been induced. This feature is well illustrated
in the cabbage, where the elongated stem of the wild plant has
become shortened by cultivation and covered with fleshy leaves,
forming a large bud or head. Brussels sprouts are a modifica-
tion of the cabbage in which the stem becomes more elongated
and covered with numerous small heads. The great variet}^ of
kales are really headless cabbages in which the leaves remain free
from one another, assuming a variety of forms. The cauliflower
is a variety of the cabbage in which the inflorescence has been
transformed into a fleshy mass of tissue, and in the kohlrabi the
stem becomes swollen and herbaceous. The turnip is a related
species of the cabbage genus in which the underground portion
of the plant is modified. Radish, cress, horse-radish, caper,
spices and oils of mustard, etc., are other products of the order, as
well as many cultivated flowers, as the wallflower, stock, mig-
nonette, etc.

144. Resales, the Rose Order. — This enormous order, com-
prising over 14,000 species, is better known than any other, not
only because of its great array of common field plants, but espe-
cially because of the large variety of our cultivated fruits and
flowers that belong to it. The culti\'ated currant, blackbcrr\',
strawberry, apple, pear, cherry, peach, plum, pea, bean, h^'dran-
gea, syringa, rose, spiraea, wistaria, laburnum, as well as many
native trees and shrubs, as the liquid-ambar, sycamore, witch-
hazel, locusts, etc., belong to the rose order.*

These plants may be looked upon as the most typical of the
Choripetalae, just as the Liliales were the most representative of
the monocotyledons. The parts of the flower are more com-
monly in fives and cyclic (Fig. 310, .4) though the spiral ar-
rangement still persists in nearly every family of the order. The
most distinguishing feature of the group is seen in the basal
growth of the receptacle, thus lifting up the corolla and stamens
about the ovary (perigynous flowers) and this growth also fre-
quently results in the epigynous type of flower. The simple
forms of flowers that appear in man\- of tlie families are very
suggestive of the Ranales, the sporoph>lls being free and often

448

THE ROSALES

spirally arranged. Thus, in the live-forever (Sediim), we have a
flower that has always been cited as the typical flower of the angi-
osperms. It is very regular with five whorls of alternating parts
of five members each, i. e.: 5 sepals, 5 petals, 5 to 10 stamens
(one or two whorls) and usually 5 pistils (Fig. 310, ^). This type

Fig. 310. Simple forms of the Rosales: A, flower of Sedum, showing the
radial symmetry of the flower and five organs in each whorl. B, grass of
Parnassus (Parnassia), a member of a closely-allied family. C, flower on
first day of bloom, stamens converging over the pistil and encircled by row
of modified stamens. D, section of flower, showing slight growth of base of
receptacle and consequent adhesion to perianth. This flower is protandrous
and the stamens in shedding the spores straighten up one by one on succeed-
ing days and curve back towards the petals. The section shows three positions
assumed by the stamens. You may calculate the significance of these features.

closely resembles the arrangement of parts in the buttercups, save
for the slight adhesion of the calyx and receptacle (Fig. 310, D).
These fleshy plants as illustrated in the houseleek (Fig. 69), hen
and chickens, etc., are very common forms of xerophytes.

DEVELOPMENT OF PLANTS

449

Owing to their ability to retain moisture (page 45) they can exist
in the crevices of rocks and upon dr>' soil, while the formation of
roots from a bit of stem or even a leaf, as well as their habit of
producing buds on short stems that afterw^ard become detaclied
cmd grow into new plants, brings about a sure distribution and
explains their popular name, live-forever.

(a) Some Variations of the Rose Order. — The active growth
:>{ the base of the receptacle and the consequent crowding result
n an interesting series of departures from the simple type noted

Fig. 311. Higher forms of the Rosalcs: A, flower of the saxifrage. B,
lower in section, showing the partial adhesion of receptacle to ovary. C.
nflorescence of currant {Rihes). D, section of flower, showing receptacle
'orming the cavity for the ovules which is roofed over by the carpels, epi-
^ynous flower.

above that lead in very regular gradations to the highest forms
of the order. In the higher family of the saxifrages, for example,
w^hich includes the saxifrage, false mitorwort (Tiarella), heu-
^hera, bishop's cap {Mitella), golden saxifrage {ChrysospJenium) ,
the receptacle generally adheres to the ovary (Fig. 311, A, B).
30 that the flower is, in a measure, epigynous and the carpels are
reduced to two and partly fused. Passing to the families of the
hydrangeas, syringas (Philadelphus), currants (Ribes), etc., the
receptacle and carpels are quite fused (Fig. 311, C, D) and

450

THE ROSALES

the flower Is strictly epigynous. In the rose family, which is
very closely connected w^ith the preceding group, this story of
change in the evolution of the flower is repeated. In the simpler
forms the mass growth of the receptacle and calyx, forms a cup-
like structure (perig^^nous flower). This cup may be rather
broad and shallow as in the strawberry, cinquefoil and black-
berry (Fig. 312), but in higher types the receptacle more or less
completely surrounds the pistils as in the spiraea, avens, rose,
agrimony (Fig. 313). In the apple family, including the apple,

Fig. 312. Fig. 313.

Fig. 312. A member of the rose family with the simple structure of the
saxifrages: A, flower of the strawberry (Fragaria). B, section of flower
showing slight adhesion of receptacle to calyx and the spiral arrangement of
the sporophylls. C, the fruit, akenes spirally arranged on the enlarged and
fleshy receptacle.

Fig. 313. Higher forms of the rose family: C, flower of Agrimonia. D,
section of flower, showing the bristle-cove ed receptacle completely surround'
ing the pistils.

peach, quince, shadbush, thornapple, hawthorn, the ovules are
completely enveloped by the receptacle (Fig. 314, A-C) as in
the currants, while in the plum family, with its plum, prune,
cherry, peach, almond, apricot members, the pistils are reduced
to one and are quite distinct from the cup-like receptacle (Fig.

The senna family has essentially the same type of flower as
the plum save that the pistil usually contains many seeds (nor-

DEVELOPMENT OF PLANTS

451

mally but one develops in the plum) and splits at maturity into
two valves, a form of fruit called a pod. For example, in the
honey locust (Gleditsia) and the Kenluck\' coffee bean (Gymno-
cladtis), the receptacle forms a shallow cup which bears the regu-
lar sepals, petals and usually ten stamens about a single pistil
(Fig. 315, A). The flowers are realh' monoecious, but in other
respects are very suggestive of the plum flower. In the coffee
bean tree, however, the petals are not quite equal and this irre-
gularity becomes more noticeable in the sensitive pea (Cassia)

A

Fig. 314. Flowers and fruits of the apple and plum families: .1, inllor-
escence of the apple {Malus). B, section of llower, showing adhesion of
receptacle to the ovary, epigynous flower. C, sections of the fruit — c, car-
pels of the pistil; r, fleshy receptacle. D, flower of cherry (Prunus). E,
section of flower — r, cup-like receptacle which falls off as fruit matures. F,
fruit in section, showing the outer part of the wall of the ovary as a fleshy
rind and the inner part forming the stone or pit which enclosed a single seed.

(Fig. 315, B-D). In the redbud, or Judas tree (Cercis), the
petals are very irregular, two of them being insecurely united into
a boat-like structure, known as the keel, which encloses the ten
distinct stamens and single ])islil, while two lateralK- placed
petals, the wings, inclose the fifth petal known as the standard
(Fig. 315, E, F). These three examples from the senna family

452

THE ROSALES

make a very easy transition from the regular flower of the plum
to the highly modified flowers of the pea family.

(b) The Pea Family, Papilionaceae. — The pea family (Fig.
316) is the highest of the rose order and the largest family, with
one exception, of all the angiosperms, comprising over 11,000
species. Here we find the same type of flower as in the redbud.

Fig. 315. Development of the irregular type of flower in the rose order.
A, regular flower of the honey locust {Gleditsia). At left staminate flower:
At right pistillate with single pistil, c, which develops into long flat pod, s.
rudimentary stamens. B, flower of Cassia, showing slightly irregular corolla,
C, section of flower — s, stamen; c, pistil. D, the fruit or pod. E, irregular
flower of redbud (Cercis) — c, calyx; k, keel enclosing stamens and pistil; w,
wings which arch over the standard, 5. F, corolla removed, showing the ten
stamens surrounding the simple pistil, c.

The standard in this famil}^, however, incloses the wings and
the stamens, usually ten in number, may be distinct as noted
above or united by their filaments into a sheath about the soli-
tary pistil; more frequently a single stamen remains free, an
arrangement called diadelphous (two brotherhoods). This type
of flower is called papilionaceous from its fancied resemblance

DEVELOPMENT OF PLANTS

453

to a certain genus of butlertly, Papilio. Tlie nujre important
characteristics of this form of flower are very well illustrated
in the pea (Fig. 316). The standard is the conspicuous and
most highly-colored organ of the flower, overlapping the two
wings which in turn practically cover the keel. By carefully
removing the keel and wings, it will be seen that these organs
are attached to the calyx and receptacle by rather narrow claws
(Fig. 316, C) and that they are also locked together by a little
process on each wing that fits into a groove on the keel. Nine
of the filaments, the tenth being free, form a sheath about the

Fig. 316. Structure of the sweet pea (Lathyrns): A, flower of the pea
— c, calyx; s, standard enclosing the two wings, ic; k, keel. B, section of
flower, showing the sporophylls concealed in the keel. C, one of the wings.
D, perianth removed, showing relation of sporophylls.

ovary which terminates In a st3lar brush of upwardh' pointing
hairs and in a stigma. The anthers and style are confined in
the tip of the keel (Fig, 316, J5, D). The significance of these
features will be discovered If you watch a bee probing into the
sheath of filaments after the nectar secreted at the base of the
ovary. The standard serves to attract and also direct his ap-
proach to the flower so that he alights at the proper place, i. e.^
on the keel and wings. The weight of his body causes a slight
depression of the keel and the stylar brush sweeps some of the
microspores upon his body. It should be added that the flower
is ready for the insect as soon as opened since the spores are
discharged before the flower blooms. Only a portion of the
spores are swept out by the Insect and the dusting may be re-
peated on several visitors. The stigma docs not mature until a

454 THE ROSALES

later period and it is evident that there is a good chance of
autogamy as soon as the stigma is mature. Some genera are
always autogamous, but it has been observed in many cases that
the spores grow more vigorously upon the stigma of another
flower than upon the stigma of their own flower, so that if the
stigma receives spores from its own and other flowers, the latter
spores would develop better and effect fertilization. This is
called the prepotency of foreign spores. In some species, au-
togamy is impossible owing to the failure of the spores to germi-
nate except when transported to other flowers. There was a
good illustration of this fact in Australia when the English
clover was introduced and flourished exceedingly well, but pro-
duced no seed until bees from England were introduced for
crossing. Plants transported to foreign countries are often
sterile owing to the absence of the insect to which their varia-
tions are adapted. There are many variations of the mechanism
shown in the pea. In some genera the style and stigma act like
a piston, pushing out some of the spores from the apex of the
keel with each visit of the insect and in some of these forms the
wings are unlocked by the insect when the stamens that have been
retained in the keel under considerable pressure are liberated by
the dropping of the keel and spring up, scattering the spores with
an explosive effect.

In many of these plants the pod splits at maturity into two
valves, each of which twists with a sudden snap in opposite direc-
tions, hurling the seeds to a considerable distance (Fig. 317, A).
This reaction is due to the tension set up by the difference in
the drying of the three strata of cells which compose the walls
of the pod. The Papilionaceae contain very many nutritious
food plants as the pea, bean and lentils, and also forage plants
as the clover, alfalfa and vetches. In temperate regions the
majority of the forms are herbaceous, only a few trees and
shrubs as the locust (Robinia), broom (Cytissus), dye weed
(Genista), false indigo (Amorpha) belong to the family, but
the tropics are richly represented by a great variety of woody
forms that are valuable for lumber, resins and dyes, as the
red sandalwood, licorice root, gum tragacanth, balsam of tolu.

DEVELOPMENT OF PLANTS

455

indigo, etc. Related to the pea family are the curious vSensitive
plant {Mimosa) and the acacias. One of the species of the
Acacia develops enormous spines that are tunnelled by ants
that were supposed 'to protect the trees against leaf-destn)>ing
insects. Gum arable is obtained from African and Australian
species of Acacia.

One of the most interesting features of the Rosales is the.
variety of changes to which the receptacle and ovaries are subject
in the ripening of the fruit. In the majority of cases, the pistil
ripens as an akene or follicle (splitting along one side to free

Fig. 317, Fruits of the pea family: .1, fruit or pod of the ground nut,
showing the manner of seed dissemination by the snapping back of the vah'es
with a twisting motion. B, fruit or lomentum of tick-trefoil {Meihomia) ,
The lomentum breaks into as many nut-like parts as there are seeds in the
fruit. C, hooked bristles on the surface of lomentum.

the seeds) or as a pod, without any considerable modification in
the form of the ovary. In many instances, however, this growth
is attended with pronounced alterations of the parts as in the
currants and gooseberries where the receptacle forms the ovary
and the entire structure becomes succulent, forming a berry.
The same organs in the witch-hazel develop into a horn-like
capsule, the walls of which, after splitting open, contract and
pinch out the hard, smooth seed with great force. In the rasp-
berries, the ovaries are transformed into drupes which may be
lifted off from the convex receptacle like a thimlile, while in

456 SAPINDALES

the blackberries, the drupes remain attached to the receptacle
which becomes the most edible portion of the fruit. The at-
tractive portion of the strawberry is the enlarged succulent re-
ceptacle while the objectionable hard particles on its surface are
the akenes. The receptacle surrounds the pistils in the agrimony
and rose, but in the former genus it is hard and covered with
hooked bristles for distribution of the nut-like fruit, while in the
rose it is fleshy and often brightly colored, serving in some cases
as food for birds and thus effecting seed distribution (Fig. 313,
B, D). The most pronounced changes are seen in the apple and
plum families. In the former group, the carpels become tough,
forming the "core" (Fig. 314, C) and the edible part is the
greatly enlarged receptacle. The plum, cherry, peach, etc., rep-
resent the ovary alone, the cup-like portion of the receptacle is
cast off as the fruit matures and the ovary becomes a drupe
(Fig. 314, F). In the pea family, the ovary usually becomes
a pod with elastic valves but it is also various^ modified. In
the peanut, after the withering of the flower, the pistil is thrust
into the ground where it develops as a pod that does not open
at all. In other genera, the pod is nut-like as in the clovers,
spirally coiled in alfalfa, or separable into nut-like joints that
are provided with hooks as in the tick trefoil {Meihomia) (Fig.
317, B). Doubtless these methods of distribution, the well-
developed seeds with their exceptionally firm integuments, the
symbiotic relation of these plants with the bacteria (page 161)
and the elaborate mechanism of the iregular flowers have been
the causes that have led to the abundance and wide distribution
of these plants. It will be noticed in the following orders as in
the preceding Orchidales, that the irregular and more highly
constructed, flowers are generally represented by a great number
of genera and, barring some weakness, as for example, the poorly
developed embryos of the orchids and their peculiar habitats, by
a great number of individuals in each species.

145. Sapindales, the Soapberry Order. — This group includes
principally shrubs and trees, as the box, sumac {Rhus), smoke
tree (Cotinus), holly (Ilex), burning bush (Euonymus), climb-
ing bittersweet (Celastrus), maple (Acer), horse-chestnut and

DEVELOPMENT OF PLANTS

457

buckeye {Aesculus), etc. The llowers are more coninionl>' small,
of a yellow-green color and varioUvsly grouped into inflorescences
(Fig. 318, A). We noticed in the rose order as the dominant
characteristic the tendency towards the development of the basal
region of the receptacle which was associated with the checking
of its apical growth. In the Sapindales a similar shortening
of the axis without its basal growth leads to quite a different
series of variations. As a result of the 'crowding of the parts
of the flower upon the shortened receptacle, the distinguishing

Fig. 318. A common form of the Sapindales: A, inflorescence of sugar
maple {Acer saccharum) — s, staminate flowers: p, pistillate flowers. B,
staminate (right-hand) and pistillate flower enlarged. C, section of a pis-
tillate flower, showing the two sterile stamens and nectar disc at base of fila-
ments. D, section of ovary, showing early development of the wings of the
fruit and the perpendicular ovules with micropyle pointing down.

features of the order are seen in the reduction of the number
of the organs of the flower, a tendency towards the c\clic arrange-
ment of parts and an adhesion of the carpels. For example,
the sepals and petals are four to five in number, usually distinct
and the corolla may be entirely suppressed. There are two
whorls of stamens, five to eight in luiniber, rarely ten, while
the pistils are reduced in number, ranging from two to more
commonly three or rarely five. Thus we see that the spiral

458 THE SAPINDALES

type of flower so often noticed in the rose and preceding orders
has become reduced as a rule to the cyclic type and that five-
numerous sets of organs are not of common occurrence. This
reduction of the flower and the suppression of parts is well
illustrated in the maples (Fig. 318). The sepals form a five-
lobed calyx. The petals are often suppressed and a nectar disc
is developed outside the stamens, a distinguishing feature of the
order (Fig. 318, C). The two whorls of stamens are suppressed
to a varying degree so that from four to eight commonly appear
and the pistils are normally reduced to two. More frequently the
flowers are imperfect, either the pistils or stamens of each
flower being aborted (Fig. 318, B). These two kinds of imper-
fect flowers are arranged on the same or different trees and they
are adapted to small short-tongued lapping insects which visit
these open types of flowers. The intelligent long-tongued bees
and butterflies generally avoid such flowers, having learned by
experience with colors and odors that a surer supply of food is
to be found in those flowers that conceal their nectar and so
exclude the promiscuous crowd of insects that swarm about the
simpler types. It should be stated that the development of
imperfect or incomplete flowers appearing in many of the orders
is not to be looked upon as a primitive condition, though common
in the lower monocotyledons and dicotyledons, since, throughout
the Angiospermae, forms will constantly appear in which one or
another set of organs fails to develop. The formation of wind
pollinated flowers, however, as in the box maple, is a return to a
primitive condition.

The stimulation of fertilization results in a green wing-like
outgrowth on each of the ovaries that assists at first in the manu-
facture of food for the embryo and later becomes a dry, mem-
branous organ for seed distribution. This fruit, known as a
schizocarp or samara, is at first partly loosened from its support
and remains attached only by a small stalk (Fig. 319, B), which
requires a rather strong wind to snap it. Thus the fruit is
freed under conditions that will result in the widest dissemina-
tion of the seed. The development of the chlorophyll-bearing
tissue in immature fruits to assist in the work of food produc-

DEVELOPMENT OP^ PLANTS

459

tion is an economical arrangement of tissues often to be seen.
It is noteworthy that these green fruits are often protected by
bitter, acrid juices and poisonous proi)erties that finalh- give place
to attractive flavors, odors and colors, variations that are of
considerable assistance to seed protection and distribution.

Fig. 319. Fruit of the maple: A, mature fruit of red majile (Acer ruhruju).
B, schizocarp of Norway maple. The fruit split in half is still attached to
the receptacle by delicate stalks.

Many of the members of this order contain acid or poisonous
juices, as in the scarlet fruit of the sumac, which were a source
of acetic acid to the early settlers of this country, or poisonous
oils, as in the poison ivy or poison oak {Rhus radicans) and the
poison sumac {R. Vernix). The former species is a climbing
vine or sometimes a shrubby plant with leaves divided into three
leaflets and with nut-like fruits (Fig. 320, B), while the ix)ison
sumac is an erect coarse shrub ten to fifteen feet high, with large
pinnate leaves with reddish petioles and fruit clusters, as in the
poison ivy (Fig. 320, A). Both of these plants contain volatile
oils that cause the poisoning. The oil ma>- readily be removed
by washing in water containing baking soda, which saponifies the
oil, or in alcohol which dissolves it. With alcohol the washing
must be thorough in order not to spread the infection. Applica-
tions of alcohol containing sugar of lead (50 or 70 per cent,
alcohol) are also recommended, which treatment is followed by

460

GERANIALES— RHAMNALES

thorough washing in alcohol. As In the preceding orders, the
lower members of this group are characterized by a regular alter-
nation of the members of each whorl, but in the higher types the
sets of organs vary in number and the flowers become irregular,
as in the horse-chestnut and balsam weed or touch-me-not.

146. Orders Suggestive of the Sapindales. — The geranium
order, Geraniales, shows essentially the same type and range of
variation in the flower as the Sapindales, but there is this rather
singular difference, namely, that the ovules, usually pendulous,

Fig. 320. Two poisonous species of the Sapindales: .4, Rhus vernix, poison
sumac. B, Rhus radicans, poison ivy.

are always turned away from the axis of the ovary with micro-
pyle directed upwards, while in the Sapindales the opposite ar-
rangement is to be found, micropyle pointing down (Figs. 318, D\
321, D). This order ranges from the regular flowers of the
oxalis, flax and geranium families to irregular forms like the
nasturtium, milkwort {Polygala), etc. Many of these plants are
known by their peculiar juices, oils, gums, as in the spurges
{Euphorbia), castor bean (Ricinus), citron, lemon, orange, etc.
The fruit of the lemon and orange is a berry in which the outer
part of the ovary becomes leathery. The juicy pulp, which en-
velops the seeds, is formed from hair-like structures that arise
on the inner side of the carpellary walls and by degrees entirely
fill them. The navel orange is a chance variation in which a

DEVELOPMENT OF PLANTS

461

secondary receptacle, bearing coni})artmenls like the normal fruit,
is introduced. This never reaches very large dimensions and
may readily be seen at one end of the orange. The original
stock from which the navel oranges grown in our country have
been derived, was obtained from Brazil.

The Buckthorn order, Rhamnales, is very closely allied to the

Fig. 321. Flowers of the Geraniales and Rhamnales: A, inflorescence
of Geranium. B, nearly mature fruit consisting of five united carpels. C,
discharge of the seed. The carpels snap apart, owing to the tension set up by
the drying out of the tissues. D, section of pistil, showing the ovules with
micropyle directed upwards. E, flower of the grape (Vitis), a common form
of the Rhamnales — ca, calyx, reduced to a rim; c, corolla, which opens at
base and falls off as a cap; n, nectar glands within stamens. Compare Sapin-
dales. F, corolla free from the receptacle. G, flower freed from corolla,
ovary in section, showing micropyle pointing down as in Sapindales.

Sapindales and includes such familiar plants as the buckthorn
(Rha^nmis) , Jersey tea (Ceanothiis), used in revolutionary times
as a substitute for tea, the grape ( Vitis), Japanese ivies, \'irginia
creeper {Parthenocissiis), etc. The minute green or white flower
shows a further reduction in parts, the petals not only being fre-
quently suppressed, but the stamens are reduced to one whorl

462

THE MYRTALES

and placed opposite the sepals, whereas they alternate with the
sepals in the Sapindales (Fig. 331, E-G), and the nectary is
within the filaments.

147. Myrtales, the Myrtle Order. — This order marks a decided
advance in the evolution of the flower over preceding groups,
owing to the mass growth of the calyx and receptacle and usually
of the ovary, the flowers being perigynous and usually epigynous.
Another important character is that the flowers are always cyclic,
the organs being regularly arranged in five whorls of four or less

Fig. 322. Lower form of the Myrtales, flower perigynous: A, flower of
Lythrum. B, flower in section — c, lobe of the tubular calyx, from the margin
of which arise the petals, p. This flower has the long form of style and the
short and medium form of stamens. C, flower with medium style and with
short and long stamens. D, flower with short style and with medium and
long stamens.

commonly of five members each, though the number of stamens
may be greatly increased by the splitting of the original number
of the set. The petals are frequently suppressed and the calyx
is often highly colored, a variation to be seen in many groups when
the corolla is wanting. The carpels, as a rule, contain numerous
ovules (Figs. 322, 323). The Myrtales is an important tropical
order and represented with us by many familiar plants, such as
the meadow beauty {Rhexia), willow-herb {Epilobitim), the large

DEVELOPMENT OF PLANTS 463

evening primrose family, with one of \vhieh, Oowthcra (I-'ig. 323,
A, B), deVries worked out liis theory of mutation, and the
common ditch and pond aquatics — the water milfoil (Myrio-
phyllum) and mermaid weed (Froserpinaca).

The purple loosestrife {Lythrum) illustrates the characters of
the lower meml)ers of the order before ei)ig\ny has heccjme estab-
lished (Fig. 322). The wand-like branches of this introduced
plant with their terminal spikes of purple flowers are becoming
rather common around the borders of marshes and water ways.
The cup formed by the calyx and receptacle bears on its rim
alternately with calyx lobes the rather twisted petals and at its
base eight to twelve stamens. The pistil is composed of two
carpels. This flower has become historic because of the atten-
tion that has been given to its devices for crossing. Fig. 322
shows that the sporophylls are of three lengths, short, medium
and long, an arrangement called heterostyly. It can readily be
seen that the three different lengths of the styles correspond
exactly with the position of the anthers, consequently whatever
part of the proboscis or body of the bee or butterfly in \'isiting
these flowers comes in contact with one of the sets of open
anthers, exactly the same region of their bodies will touch the
stigmas as soon as they visit another flower with a corresponding
length of style. The spores from the three lengths of stamens
differ in size and color, and it has been demonstrated that better
results follow when crossing is effected between sporophylls of
the same length, i. e., long with long, short with short, etc.
Heterostyly arose in many of the orders, as among some of the
knotweeds, buttercups and rose families and in the gentians,
primroses, forget-me-not, etc.

A higher and more characteristic t>'pe of the order is seen in
the Oenothera and in the great willow-herb {Clmniaoicrion, Fig.
323). This latter plant flourishes in rather dry soils, forming-
large colonies by its underground stems and gaining in con-
spicuousness through its terminal racemes of large deep purple
flowers. The parts of the flower are in fours. The receptacle
forms the ovary and the mass growth of perianth and stamens
results in the development of a long tube which forms at its

464

THE MYRTALES

mouth the Hnear segments of the calyx alternating with the
round spreading petals and the stamens. At the time of the
opening of the flower the eight anthers are shedding their spores
and are in line with the nectaries, while the lobed stigma is
closed and bent backward (Fig. 323, C, s). A day later the
stigmas assume the position shown in the lower flowers, the lobes
curving backward so as to lie in the pathway leading to the
nectaries (Fig. 323, C, 0). It is evident that these positions

Fig. 323. Higher forms of the Myrtales, flowers epigynous: A, flower
of Oenothera — 0, ovary. B, enlarged sectional view of flower, showing the
receptacle enveloping the ovules. The receptacle grows above the ovary,
forming a tube, f, that bears at its summit the lobes of the calyx, c, the petals,
p, and stamens. C, inflorescence of the great willow herb (Chamaenerion)
— s, closed stigma in young flowers; 0, opened stigmas in older flowers; a,
stigma touching anthers in withered flowers. D, capsule opening and dis- ^
charging the seeds. E, a seed enlarged. — C after Kerner,

must necessitate a crossing if the flowers receive the proper vis-
itors. If, for any reason, crossing should fail autogamy results,
owing to the continued curvature of the stigmas, which are finally
brought into contact with the anthers, in which work the positions
of the flowers and stamens cooperate, as shown in the lowest

DEVELOPMENT OF PLANTS 465

flower (Fig. 323, C, a). The microspores are fastened together
in threads by a sticky substance, viscin, so that they adhere to
the insect's body and are drawn out from the anthers in net-Hke
skeins. This may be tested by applying the moistened finger to
the opened anthers; this is also true of the related plants, as the
fuchsia, evening primrose, firewood {Epilobium), enchanter's
nightshade (Circaea).

The numerous seeds developed in the capsule are provided
at the end with tufts of long white hair (Fig. 323, D, £), which
readily transport them and explains the quick appearance of these
plants in forest lands that have been devastated by fire or cutting.
In addition to the native plants mentioned above, many well-
known tropical plants are represented in this order, as the man-
grove of our southern swamps and the large myrtle family, often
characterized by leathery leaves and aromatic oils. This family
furnishes the clove, which is a flower bud, the pomegranate,
guava, bay rum, the Brazil nut and the eucalyptus trees, some of
which (Australian) attain the greatest height of any tree, over
"our hundred feet, with a diameter of twent^'-four feet.

148. Umbellales, the Carrot Order. — This order marks the
consummation in the reduction and mass growth of parts that we
have seen steadily progressing through the various orders of the
Choripetalae. Notice that the strictly epig^mous, cyclic flowers
^Fig. 324, C, D) are now reduced to four whorls, the five lobes
of the calyx alternating with the five petals and these in turn
with the four to five stamens. The reduction also appears in the
;Distil which usually consists of two carpels that contain but a

>ingle ovule each. The order contains two rather small families,
■he ginseng family (Araliaceae) represented by the sarsaparilla
or spikenard (Aralia), the ginseng {Panax) and the dogwood

amily (Cornaceae), including the pepperidge or sour gum
trees {Nyssa), dogwood or cornel {Coruus); and the large carrot
family (Umbelliferae) of 2,100 species. The carrot family
contains many familiar native and cultivated plants which may
be recognized by the hollow internodes of the stems, leaves vari-

)usly lobed and attached by conspicuous sheathing petioles,

peculiar odors derived from oils and resins, small flowers that are
30

466

THE UMBELLALES

usually white or yellow and grouped into flat-topped inflores-
cences (umbels), which are usually surrounded by bracts, called
the involucre (Fig. 324, A, B). These features are well seen in
the wild carrot which has become a troublesome weed in pasture
lands and meadows. The calyx lobes are very small, a feature
likely to be seen in any epigynous flower. The incurved petals
alternate with the five stamens and in the center of the flower, the

vi^ -'^r-}^^ *V'''^

Fig. 324. A common form of the Umbellales: A, stems of wild carrot
(Daucus) with flowers arranged in compound umbels. B, inflorescence with
all the umbels removed but one — in, bracts of the involucre. C, forms of
flowers, the one on the right being a flower from the margin of the umbel.
The corolla is enlarged and irregular, thus adding to the conspicuousness
of the inflorescence. D, corolla partially removed to show the epigynous
character of the flower and the cushion-like nectary at the base of the styles;
0, ovary. E, fruit splitting into two nutlets. F, portion of hollow stem
with sheathing of leaf base.

two Styles broaden out into a conspicuous nectary above the
ovaries. The fruit is a schizocarp, splitting into two nut-like
parts after the manner of the maple (Fig. 324, E). The flowers

DEVELOPMENT OF PLANTS 467

on the outside of the umbel become somewhat irregular, (Aving to
the unequal enlargement of the outermost petals. These small
flowers that individually cannot be seen at a distance of a few feet
become the most conspicuous feature of our fields owing to their
association in large numbers into compact, lace-like um])els. The
aggregation of flowers has been noticed in various groups, as in
the mustards, various families of the Rosales, in the Sapindales
and buckthorns, of which latter simple regular type the carrot
flower would appear to be a natural sequence (compare Figs.
321, G; 324, D). But in no group has so successful and varied
an arrangement of the flowers been achieved. The advantages
of this umbellate arrangement are apparent. The crowding of
the flowers is attended with the reduction in their size and in
saving of material while they have gained in the number of
flowers and in conspicuousness. This arrangement also increases
the chances of seed production since the insect in a single visit
may cross a score of flowers in crawling over the umbel. Autog-
amy is prevented at first by the difl"erence in the maturation of
the anthers and stigmas, frequently one or more days interven-
ing between these two conditions and in many genera the flowers
are imperfect, the stamens, and pistils being arranged on difi"er-
ent parts of the umbel and so further assist in crossing. But
even if the great variety of small lapping insects that swarm over
these flowers were absent the construction of the umbel is such
that the flowers are usually able to effect pollination unassisted.
The variety of devices for the accomplishment of this work is
without parallel in any other group. The opening of the flowers
of an umbel proceeds either from the circumference towards
the center or from the center outwards. In some genera, as
Eryngium, the flowers on the margin open first and the stigmas
are ready for crossing while the anthers are closed and bent
down upon the petals. On the following day, the inner adjoining
set of flowers is in the same condition while the anthers of the
marginal flowers have been lifted up by their filaments so that
they reach out and come in contact with the stigmas of the ad-
jacent inner flowers, the curvature of the styles often assisting in
bringing the two organs together. In this way, ample oppor-

468

THE UMBELLALES

tunity for crossing with the older flowers of another umbel is
first given and later a crossing between the adjacent flowers of
the same umbel is almost sure to result. In other cases, as the
snakeroot {Sanicula) a few of the simple flowers are perfect
while the remaining flowers of the umbel bear only stamens.
The perfect flowers are the first to open, being in the same con-
dition as the marginal flowers of the preceding example. Later
the filaments straighten out, curve away from the stigmas and
the anthers discharge their spores and finally drop off. Now the
adjoining flowers open and extend their filaments so that the
anthers are pushed over to the stigmas of the perfect flowers. In
the chervil, a similar crossing is effected after the perfect flowers
have shed their anthers by the numerous imperfect flowers grow-
ing above them and extending their anthers so that the micro-
spores will drop straight down upon the stigmas (Fig. 325).

Fig. 325. Flowers of chervil: A, perfect flowers only in bloom. The
anthers are discharging their spores before the stigmas are receptive. B,
later stage — the staminate flowers are now in bloom and shedding their spores
upon the mature stigmas of the perfect flowers. — After Kerner.

In the water parsnip {Stum), and beaked parsley (Anthricsus) ,
etc., there are two kinds of umbels, one containing principally
perfect flowers and the other only staminate flowers. The per-
fect flowers blossom first, and shed their spores and anthers be-
fore the stigmas are receptive. After all the anthers have been
shed, the stigmas mature and continue in this condition for two
days, during which time the umbels of imperfect flowers have
grown above them and assume such a position that when the
anthers finally open there results a rain of microspores upon the
stigmas below.

The members of the carrot family are characterized by the

DEVELOPMENT OF PLANTS 469

presence of essential oils and resins in the roots, stems, leaves or
especially in the fruits which usualK' contain oil cavities between
the ribs of the ovary. These substances are the sources of the
peculiar odors that make easy the recognition of these plants.
These oils give the commercial value to several fruits, as in the
caraway, parsley, fennel, anise, coriander. Se\-eral species con-
tain very poisonous alkaloids, as the water hemlock (Ciciita),
among the most poisonous of our native plants, and the hemlock
{Conitim) which perhaps furnished the potion drunk by Socrates.
Several valuable food plants, as the carrot, parsnip, celer}', par-
sley, are cultivated species of the family.

Series h. Synipetalae.

149. General Characteristics. — This group contains the most
common and abundant of our flowering plants, approximating
42,000 species, and ranks as the most specialized of the angio-
sperms. The variations of these plants have been very success-
ful, especially to be noticed are the development of underground
stems, special types of flower structures and the massing of the
flowers in dense inflorescences. As a result of these advantages,,
they exceed all other groups in the number of individuals, though
they comprise fewer orders as a consequence of the striking
uniformity in the floral structure.

While a few of the simpler meml)ers of this group have dis-
tinct petals, the crowding of the organs on the receptacle has
resulted, in nearly all the forms, in the sympetalous type' of
corolla, in the cyclic arrangement of the parts, and in the reduc-
tion in the number of stamens, so that each whorl does not exceed
the number of petals, and more frequently one whorl of stamens
is partly or entirely suppressed. The pistils are usually less
numerous than the petals. The flowers range from regular
hypogynous forms to epigynous and irregular l\pes through the
same series of variations, as noted in the Choripetalae. The
Sympetalae are the most recently evohed of the Angiospermae,
as is apparent from their uniformity of structure and they
appear to be adapted to temperate and northern conditions where
they have preempted the open country and flourished exceedingly,

470

ERICALES

forming the most characteristic features of the herbaceous flora.
Aquatics are of rare occurrence and but few tree forms appear;
notably the ash, persimmon, catalpa, paulownia, etc. Heath-
hke shrubs, however, are widely distributed in northern regions.
The invasion of the tropics w4th its favorable conditions has led
to an enormous increase in some of the groups and the develop-
ment also of a great variety of woody plants, as trees and climbers.
150. Ericales, the Heath Order. — -This order is the simplest of
the Sympetalae and includes a great variety of plants that are
largely northern in their distribution. Many are cultivated, as
the azaleas, rhododendrons and laurels (Kalmia), and others are
familiar plants of bogs and woods as the sweet-pepper bush

Fig. 326. A simple form of the Ericales: A, the shin leaf (Pyrola) with
flower just opened. B, flower with part of perianth removed to show the
retention of the anthers by the petals. C, later stage of flowering. The
anthers have been released by the spreading of the petals and the flower
stalk has inclined so that the spores sprinkle down on the stigma. D, flower
in section, showing the relation of parts in autogamy. — After Kerner.

(Clethra), wintergreen (Chimaphila), Labrador tea (Ledum),
Rhodora, Leucothoe, rosemary (Andromeda), leather leaf (Cham-
aedaphne), arbutus (Epigaea), checkerberry (Gaultheria) , bear-
berry (Arctostaphylos) , huckleberry (Gaylussacia) , blueberry
(Vaccinium), heather (Calluna), cranberry (Oxycoccus). The
flowers of the heaths are very characteristic and sharply dis-

DEVELOPMENT OF PLANTS 471

tingulshed from other orders. More'^frequenll}' they consist of
five regular whorls of five members each, anthers usually opening
by pores and often provided with two horn-like appendages, and
the pistil is composed of five carpels, the ovary maturing as a
capsule or berry (Fig. 326). Tlie separate origin of each \\ liorl fjf
organs is an important feature of the order as contrasted with
other sympetalae, each set being as a rule separately attached to
the receptacle. While the sympetalous corolla is a step in ad-
vance of previous types, it is evident that these regular hypogy-
nous flowers are in other respects of a rather simple character.
This feature is further illustrated in some of the genera like
Clethra, Monotropa and Pyrola where the free or slightly fused
petals form a very natural transition from the Choripetalae
(Figs. 327, A\ 326).

The simpler forms of flowers are illustrated in Pyrola with
its regular hypogynous and polypetalous flowers, though some
species show a slight tendency to sympetaly. The stigmas are
mature with the opening of the flower while the anthers are
bent back out of the way, their filaments being held under con-
siderable tension by the petals (Fig. 326, A, B). A bee laden
with spores would efi"ect a crossing as soon as he alighted upon
the flower, but in probing after the nectar he presses back the
petals, thus releasing the anthers which snap down, spilling the
spores upon him. Doubtless autogamy results in many of these
forms if crossing fails owing to the expansion of the petals
which would release the stamens, and the drooping of the flower
which would bring the falling spores in line with the stigma
(Fig. 326, C, D).

Other genera show varying degrees of sympetaly but in the
majority of cases, tubular corollas appear (Fig. 327, C) as in the
Leucothoe, bearberry, Andromeda, blueberries, etc. The majority
of such forms have horned anthers which are shaken by the
insect and so assist in sifting out the spores. In the blueberry
family, we note that a mass growth of the receptacle and o\-ary
has begun (Fig. 327, D) while in Rhodora, rhododendron and
azaleas another line of variation appears in the irregular corolla
(Fig. 327, F). In the two latter genera, the styles and stamens

472

THE ERICALES

extend far out, furnishing a natural landing place for hovering
animals as moths and humming birds. In this position the
spores cannot be sifted out and we find them fastened together
In fours, just as they were formed In the mother cell, by sticky
threads which cause them to adhere on the insect's body in
fringe-like masses (Fig. 327, D). One of the lower basidiomy-
cetes (Exobasidium) infests the leaves and flowers of the azalea
and other genera, causing large watery outgrowths that are

Fig. 327. Common forms of Ericales: A, Indian pipe {Monotropa).
Leaves reduced to scales and without chlorophyll, owing to saprophytic
habit of plant. B, section of flowers, showing polypetalous corolla. C,
inflorescence of the blue berry {Vaccinium). D, section of flower, showing
sympetalous corolla and epigynous type of flower. E, a stamen enlarged,
opening by pores at end of anther. F, azalea, showing irregular type of flower.
G, spores attached by viscid threads.

often eaten under the impression that they are the fruit of the
plant. The members of this order are adapted to a wide range
of conditions though more characteristic of north temperate and

I

DEVELOPMENT OF PLANTS 473

arctic regions where they constitute the conspicuous features of
the tundras, bogs, heaths and moors. Tliese plants are often
characterized by their thick leathery, evergreen leaves of pro-
nounced xerophytic character and their common distribution
in wet moors and bogs presents a most puzzling problem in the
association of plants. Many of the heaths have been cultiva<-ed
from very early times and they have lent themselves so readily
to crossing that perhaps no order furnishes so many forms for
decorative and landscape effects. Several yield valuable fruits,
as the blueberries, huckleberries and cranberries.

151. Orders of a Higher Type than the Ericales. — The prim-
rose order (Primulales) forms a very natural transition from the
Ericales. As the next step in advance, we note the slight associ-
ation of the stamens and the corolla (Fig. 328, A) and their
frequent reduction in number. The occurrence of vStaminodia,
page 421, in certain forms marks the first transition to the
reduction of the number of stamens. The pistil is generally
composed of five carpels forming a capsule without partitions
(unilocular) and containing a central placenta, bearing numerous
seeds, that is attached only to the base of the ovary (Fig. 328, A).
This is the so-called free central placenta and only occurs else-
where in the small bladderwort family of aquatics. This order
includes the loosestrife {Lysimachia and Steironema), star flower
(Trientalis), shooting star (Dodecatheon) and cultivated forms of
the cyclamen and primroses.

Proceeding to the gentian order (Gentianales), it will be
noted that the flowers are variable and not as clearly character-
ized, the corolla being sometimes wanting or pol>petalous. The
reduction of the stamens to a single whorl and the pistils usually
to two in number indicates points of advance that are to be
associated with the twisting of the petals in the bud and the
opposite arrangement of the leaves as distinguishing features
of the order (Fig. 328, B-D). Note should be made of the fact
that the two pistils are often quite distinct, doubtless a sur\i\al
of a more primitive condition. The Gentianales include the ash
{Fraxinns), lilac (Syringa), Forsythia, olive {Olea), valued for
fruit and oil, fringe tree (Chionanthus), privet {Ligustnim), jas-

474

THE POLEMONIALES

mine, Strychnos, yielding the alkaloid strychnine, oleander
(Nerium), the gentians, marsh pink (Sabbatia), the aquatic float-
ing heart {Limnanthemiim) and the highly specialized plants of
the dogbane and milkweed families. The latter family is dis-
tinguished by the microspores being united into club-shaped
poUinia that are attached, one from each of the two adjoining
anthers, to a peculiar hook, so that the leg of the insect becomes
caught in the hook and withdraws the poUinia in pairs.

m.

^ ^k m

Fig. 328. Primulales and Gentlanales: A, flower of loosestrife {Steiro-
nema). Above flower in section, showing cohesion of single row of stamens
to base of corolla and the numerous ovules on a free central placenta. These
are the most important characteristics of the order. B, fringed gentian
{Gentiana), showing leaf arrangement and twisting of petals in bud, b, char-
acteristics of the order. C, section of flower. D, fruit, the two carpels
separated.

152. Polemoniales, or Phlox Order. — This group is the richest
in the number of species, over 14,600, of any order of angio-

DEVELOPMENT OF PLANTS

475

sperms. The advance over previous orders is seen in the more
prolonged mass growth of the corolla and stamens, the filaments
appearing to rise at a higher point on the corolla (Fig. 330, D)
and the stamens are reduced to a single whorl, and frequently less
than five in numl^er. The pistils are completely compound and
usually composed of but two carpels. This reduction is asso-

FiG. 329. Fig. 330.

Fig. 329. Flower of the morning-glory (Ipomoea), showing the tubular
corolla characteristic of the Polemoniales.

Fig. 330. Boraginaceae: C, inflorescence of comfrey (Symphytum). Note
the coiled inflorescence, a, a feature of this family, D, section of flower,
showing the deeply four-lobed ovary and the stamens cohering high on the
corolla and alternating with small tongue-like scales.

ciatcd with a pronounced irregularity of the corolla in the higher
families and a high degree of specialization in the construction
of the flower which in part accounts for the occurrence of the large
number of individuals. The order is noted for its great number
of showy flowers and the large tubular corollas which attain their
highest perfection in several of the families.

(a) The More Important Families of the Phlox Order. — The
flowers of the lower families are regular, as in the morning-
glory, sweet potato, dodder (Cnscuta), a yellow thread-like para-

476 THE POLEMONIALES

site that twines about various plants, and the phlox (Fig. 329).
The flowers of the rough-leaved borages (Boraginaceae) are
also regular but they are distinguished by having the ovary of
the two carpels deeply lobed so that the fruit appears as four
nutlets and also by their coiled Inflorescences as shown in Fig.
330. This family includes the heliotrope, hound's-tongue {Cyno-
glossum), forget-me-not (Myosotis), comfrey {Symphytum), and
the blueweed (Echium) in which the corolla becomes irregular.
The mint family, Labiatae, is world-wide in its distribution
and the largest of the order with 3,000 species. These plants
are sharply characterized by square stems, opposite leaves, aro-
matic oils, nut-like fruits as in the borages, but the corolla is
very irregular and usually two-lipped. The stamens, usually
two short and two long ones, together with the two lobed style
are more frequently concealed under the upper lip of the corolla
(Fig. 331). The flowers are generally so placed that the lower
lip serves as a landing place for insects and in entering the
flower, the top of their bodies rubs against the stigmas or anthers.
Crossing Is secured by the difference in the time of the matura-
tion of the anthers and stigmas, assisted by a variety of move-
ments, such as the alternate curving down of the anthers and
stigmas which brings first one set of organs and then another
into the pathway of the insect. One form of this arrangement
is seen In the sage, where there are but two strangely modified
stamens, the two anther lobes being connected by a long-curved
rod which is fastened near one end to a short filament so that
the anther becomes a lever with the filament as a fulcrum (Fig.
332). Only the anther lobes concealed under the upper lip are
fertile. As the bee enters the flower, he pushes up the short
arm of the lever, thus causing the long arm to swing down,
pressing the fertile anther upon his back and dusting him with
spores. Later, the style which is at first immature and above
this apparatus, grows out and curves down and at maturity
comes to lie in such a position as to rub upon the back of a
visiting insect at the same point as the anthers. The peculiar
forms of the corolla, the relation of the anthers and stigmas and
the frequent occurrence of hairs and bristles that direct the In-

DEVELOPMENT OF PLANTS

477

Fig. 331. Fig. 332.

Fig. 331. A common species of the mint family: A, inflorescence of the
skull cap (Scutellaria). Note the square stem, opposite leaves. Why are
all the flowers facing one way? B, flower enlarged, showing the two-lobed
un:ler lip and the three-lobed upper lip which conceals the sporophylls. C,
section of the flower. Ovary four-lobed, stamens cohering with the corolla
and anthers concealed with the stigma beneath upper lip. Purpose of the
crest, c, on the calyx?

Fig. 332. Flower of the sage (Salvia): A, flower after the anthers have
shed their spores. The two-lobed stigma is bending down into the position
occupied by them. B, sectional view of the flower, showing four-lobed ovary
with nectar glands at the base, stigma not receptive and bent back. As the
insect enters the flower he pushes against the sterile lobe of the anther, /, and
thus causes the fertile lobe, a, to swing down upon his back. /, filament of
anther.

478 THE POLEMONIALES

sect with the greatest precision to particular parts of the flower
with a view to crossing, mark this family of the mints as the
most specialized of the order. A fine series of devices are seen
in such common mints as the blue curls {Tricho sterna), skull-
cap {Scutellaria), hyssop (Agastache), ground ivy {Glecoma),
catmint (Nepeta), selfheal (Prtmella), motherwort {Leo7iurus),
dead nettle (Lamium), hedge nettle (Stachys), sage (Salvia),
bergamot (Monarda), pennyroyal (Hedeoma), mountain mint
(Kolleia), bugle weed (Lycopus), etc. Many of these plants
are of commercial importance. Mentha yields valuable oils as
spearmint, peppermint, and menthol and from other genera are
derived oils used in perfumery, medicine and as condiments,
as rosemary, lavender, origanum, thyme. Several are cultivated
for their flowers and foliage, as the coleus, monarda, stachys, etc.
The large family of figworts, Scrophulariaceae, with its 2,400
species, closely resembles the mints, but is distinguished by the
ovary being undivided and containing many seeds on a central
axis (Fig. 333, C). While the simpler forms have almost regular
corollas and five stamens, the higher forms are bilabiate with
two to four stamens as in the mints. This family contains some
of the most showy of our cultivated plants, as the foxglove
(Dasystoma and Digitalis), Gerardia, snapdragon (Antirrhi-
fium), rattlebox {RJiinanthiis) , toadflax (Linaria), Paulownia,
beardtongue (Pentstemon) , monkey flower {Mimultts) ; and the
allied Catalpa and trumpet creeper; also many other attractive
plants that are not cultivated as the mullen (Verbascum), fig-
wort (Scrophularia) , turtle head (Chelone), hedge hyssop (Gra-
tiola), speedwell {Veronica), painted cup {Castilleja) , lousewort
{Pedicularis) , cow- wheat {Melampyncm) . These plants are
among the most characteristic features of our flora, the family
being largely confined in its distribution to the north temperate
regions. The flower of the toadflax {Linaria) illustrates a com-
mon type of the figwort family (Fig. 333). Such flowers are
said to be personate, that is bilabiate but with the under lip
arched so as to meet the upper lip and entirely closing the mouth
of the corolla. This arrangement very effectively protects the
microspores and conceals the nectar and it requires a rather

DEVELOPMENT OF PLANTS

479

muscular insect alighting upon tlie knohhcd lower lip to force
his way into the flower. It is worth any one's time to sit by
this plant and examine the mechanism of the flower while the
bee is at work. The sporophylls present the same variety of
arrangements for crossing as noted in the mints. The nectar
glands are situated at the base of the carpels but the nectar does
not remain upon the glands as in the majority of flowers, but
runs down through a narrow duct between the filaments and
collects in the spur-like prolongation of the corolla (Fig. 333, s).

f-Aflif-'t' ■^■■:- ~

\l^:

Fig. 333. Examples of the figwort family: A, inflorescence of the toad
flax {Linaria). B, flower viewed from beneath, showing the under lip arching
up against the upper, two-lobed lip — s, nectar spur of the corolla. C, section
of the flower viewed from the side, showing the undivided ovary with central
ovules. Note the stigma and anthers concealed at the lips of the corolla.
D, sectional view of flower of Rhinayithus with the four stamens arranged
in pairs. E, appearance of the stamens as viewed from the mouth of the
corolla. The filaments have been pressed apart, thus separating the opened
anthers.

The yellow rattlebox {RhinanthuSy Fig. 333, D, E) shows an-
other form of the flower, common in this family. The general
arrangement of the floral organs is the same as in Linaria, but
the anthers are in pairs so that when the>- open, owing to the
rigidity of the filaments or the pressure of the upper lip, they

48o THE POLEMONIALES

are kept tightly pressed together hke a pair of sugar tongs,
thus preventing the shedding of the spores. Owing to the form
of the corolla and often because of numerous hairs and bristles,
the insect is directed in such a definite wa}^ to the flower that he
comes in contact with appendages of the anthers, or causes a
slight deflection of the rigid filaments or of the lip of the corolla,
any of which movements are suflicient to separate the anthers
and bring down upon him a shower of spores. These flowers
are largely protog>mous so that at first only a crossing is possible.
In many species, autogamy results from the downward bending
of the corolla and style, accompanied by a loosening of the anthers
so that during the last days of flowering the spores may fall
directly upon the stigma.

Many of these flowers are characterized by mottlings and
blotches. It appears to be a rather general law that the most
highly modified parts are variegated in this manner — compare the
mints and orchids. Less specialized flowers present an associa-
tion of various colors or veinings, a feature that is illustrated
in many families, as violets, peas, geraniums, etc., whereas the
simplest types of flowers are usually of a uniform color. The
fig^vorts have been called a suspicious group, and if not actually
poisonous, none at least serve as food. Several are medicinal,
as Digitalis, Veronica, Gratiola, etc. Many are parasitic, as the
foxglove, Gerardia, eyebright (Euphrasia) and the related and
reduced brownish rapes (Thalesia and Orobanche), beech drops
(Leptamnium) and squawroot (Conopholis) .

The curious family of the bladderworts is closely related to
the fig^vorts. The bladderwort {Utricularia) is a very common
aquatic in still waters and ponds. Certain leaves become modi-
fied into elaborate sacs with trapdoors for enticing and capturing
small insects which die and decay in these prisons and are ulti-
mately absorbed and so contribute to the nourishment of the
plant. The butterwort {Pingiiiciila) is another member of this
family found in the northern countries and also in our southern
states. It has more the appearance of a violet with a rosette
of leaves which are provided with glands that secrete a viscid
substance for the capture of insects and also digestive fluids.

DEVELOPMENT OF PLANTS

481

The contact of the insect acts as a stimulus, causing the leaves
to roll up and so bringing to bear upon him a larger surface of
digestive glands. One of the species of Pinguicula is used in
northern countries to curdle milk in place of "rennet" which
contains the similar digesti\'e fluids of calves' stomachs.

The potato family (Solanaceae) shows many of the irregu-
larities of the fig^vorts, but the majority of its 1,700 species have
regular flowers as illustrated in Fig. 334. While the family

Ma-.,

V.

^

Fig. 334. Flower of the potato {Solanum tuberosum), showing the stamens
inserted on the tube of the corolla and encircling the style.

is largely tropical, many of the species are cultivated, as the
potato, eggplant, tomato, cayenne pepper (Capsicum). Poison-
ous, acrid and narcotic properties are characteristic features of
the family. Belladonna and atropine from Atropa, stramonium
from Datura and nicotine from Nicotiana tahacum are character-
istic drugs. Ground cherry (Physalis), nightshade {Solanum),
Petunia, etc., are cultivated forms.

153. A Transitional Order. — It would appear that all the
changes possible in the hypog>'nous type of flowers had been
wrought in the members of the Polemoniales and that the next
step in advance must be to the epigynous flower. This is seen
to be the case with the madder order, Rubiales. Here the
flowers are epigynous and the parts are generally in fives though
the carpels vary greatly in number. The simplicity in the struc-
ture of the flower and often also the form of the inflorescence,
as in the elderberry {Sambucus), bedstraw {Galium) and arrow-
wood ( Viburnum) is strikingly suggestive of the Umbellales
(Figs. 335, A, B] 324). It appears that each step in advance
is attained in a very simple way and that only gradually are
31

482

THE RUBIALES

alterations made that lead up to the specialized types. Look
back over the preceding groups and orders and note that many
of the irregular types of flowers are preceded by a long series
of regular forms. So in the madder order, this new departure
of the flower is attended with such a simplicity of structure as
to render difficult the separation of some of the genera from the
open flowers of the Umbellales which have also arrived at the

Fig. 335. Simpler forms of the Rubiales: A, inflorescence of the arrow-
wood (Viburnum). B, simple, epigynous flower of the elderberry (Sam-
hucus) — 0, ovary. C, the twin flower (Linnaea).

same stage of development, page 465. This would not necessarily
imply a relationship, since it repeatedly happens both among
plants and animals that identically similar structures arise in
groups in no way related. From these simple flowers, that may
be no more sympetalous than certain genera of the carrot order,
we pass to more pronounced tubular forms in the bluets (Hous-
tonia), buttonbush (Cephalanthus), tw^in flower {Linnaea, Fig.
335 » C), snowberry (Symphoricarpus) and finally to irregula/
and even labiate types as in the honeysuckle (Lonicera, Fig.
268), valerian, teasel (Dipsacus) and scabious (Fig. 336). It
is noteworthy that in the three latter genera the flowers become
massed in a dense inflorescence, known as a head, which are sub-

DEVELOPMENT OF PLANTS

483

tended by modified leaves, the involucre (Fig. 336) and the pistils
are reduced to a single fertile carpel with two-lobed style. These
variations will become very prominent in the next order. The
Rubiales are an important tropical group and furnish the coffee
(Coffea) and the cinchonas which yield such drugs as quinine
and calisaya.

Fig. 336. Advanced forms of the Rubiales: A, inflorescence of Scabiosa,
at the right showing the involucre, in. B, a single flower enlarged, show-
ing the somewhat irregular corolla. At the right the sectional view of the
flower shows the calyx, ca, terminating in bristle-like teeth; br, bract-like
cup surrounding the calyx. C, irregular flower of valerian — n, nectar sac;
ca, rudimentary calyx, which matures in the fruit as a mass of delicate feath-
ery organs, known as the pappus.

154. Campanulales, the Bellflower Order. — This order marks
the culmination of the tendencies that we have seen steadily
progressing through the monocotyledons and the dicotyledons.
The variations have been of so peculiar and successful a nature
that no group of plants are so widely distributed and in the
more specialized families of so common occurrence. Over
14,500 species are known. The parts of the epigynous flowers
are arranged in four whorls of usually five members each. The

484

THE CAMPANULALES

anthers are aggregated {and usually cohere, forming a sheath
about the style which is frequently covered with hairs and acts
like a piston rod in the cylinder of anthers. The flowers are
protandrous with few exceptions, the anthers opening on their
inner sides and discharging the spores upon the style which later
sweeps them out as it elongates. The pistils are generally re-
duced to a single pne-ovuled carpel that ripens as an akene.
Leaving out of consideration the gourds which include the
melons, pumpkins, cucumber and gourd, as a family of uncertain

Fig. 337. Lower forms of the Campanulales: A, inflorescence of the
bellflower (Campanula). B, section of a young flower. The hairy style
is pushing up between the encircling anthers and sweeping the spores out
of them. Note the closed stigmatic lobes. C, older flower. The anthers
are withering and the stigmas are curving back towards the spore-covered
style. D, flower of Lobelia. The tubular corolla is opened at one side, per,
mitting the style and encircling stamens, g, to protrude. E, section of flower-
showing' the anthers, a, united about the style and stigma. F, relation of
stigma to anthers. At right the section shows the anthers cohering about the
bushy style, which acts later as a brush sweeping out the spores. At the left
the style has grown beyond the anthers and the stigmatic lobes are spreading
apart.

DEVELOPMENT OF PLANTS

485

alliance, we find a very natural sequence leading to the higher
families. For example the lower members of the bell-flower
family have nearly or quite regular flowers, parts in five, anthers
grouped about the style but not united, fruit a capsule with many
seeds as in the harebell {Campanula) and Specularia (Fig. 337,
A, C). In the Lobelia the corolla usually becomes irregular
and slit down one side, thus approaching the form assumed in
the highest groups of the order. The anthers arc united about
the style which is two lobed and provided with a whorl of hairs
to sweep the spores from the cylinder of anthers. The fruit is
a capsule of two carpels (Fig. 337, D-E). This brings us to
that great group of familiar plants that were formerly known as
the Compositae, but that are now separated into the chicory
family (Cichoriaceae) of 1,400 species, the ragweed family
(Ambrosiaceae) of 55 species, and the thistle family (Cardu-
aceae) of over 11,000 species. The most conspicuous feature
of these three families is the aggregation of numerous small

I

Fig. 338. Habit of the dandelion (Taraxacum), an example of the Cicho-
raceae: a, appearance of the inflorescence or head; b, appearance of the head
during the ripening of the seed; in, involucre; c, appearance of the fruit.

486

THE CAMPANULALES

flowers in heads subtended by one or more rows of bracts that
form a calyx-Hke involucre (Fig. 338, in). This type of in-
florescence might readily be mistaken for a single floAver as the
buttercup, rose, etc. This tendency to group the flowers in
heads and compact clusters has been attained in several orders,
notably the mustards, peas, Umbelliferae, mints, scrophularias,
and especially in the teasel family, page 482. But in no group
has the aggregation been so successful and coupled with such
efficient types of flowers. Leaving out of consideration the de-
generate ragweeds the individual flowers of a head are very

Fig. 339. Flowers and fruit of Taraxacum: A, sectional view of inflor-
escence— in, involucre. The flowers in the center of the head not as yet in
bloom. B, an unopened flower. The thread-like calyx (pappus) and corolla
arising from the ovary, 0. C, corolla opening. D, later stage, the style has
elongated, sweeping the spores from the sheath of anthers, an, and the two
stigmatic lobes are beginning to open. E, flower in full bloom, the stigma
lobes recurving. 7^, mature fruit. The pappus is lifted up on a long, slender
outgrowth of the ovary, 0.

uniform in structure. They are epigynous, parts usually in fives,
calyx wanting or more often appearing as tufts of hairs, plumose
or barbed bristles, and known as the pappus (Figs. 339, 341).
The corolla is tubular or partially split open, forming a strap-

DEVELOPMENT OF PLANTS 487

shaped blade known as the Hgulate corolla (Fijr. 339, R), The
anthers are united about the style which is often hairy and pushes
out the microspores like a piston (Fi^. 341, B-D). The style
is two-lobed, the stigma usualh' appearing as a line on the inner
surface of the lobes and the single, one-ovuled carpel matures
as an akene. Scale-like bracts, the chaff, are often associated
with the flowers (Fig. 341, B). The nectar formed from the
glands at the base of the style is concealed at the bottom of the
corolla tube.

(a) The Chicory Family. — This family is distinguished by all
the flowers of the head being ligulate (Fig. 339) and mostly yel-
low in color and by the possession of milky, bitter or acrid juices.
Here belong the goat's beard (Adopogofi), hawkbit (Leontodon),
sow thistle (Sonchiis), hawkweed (Hieracitim), rattlesnake root
(Nabalus), as well as several introduced and native plants that
are cultivated, as the dandelion (Taraxacum), species of chicory
(Cichorium), salsify (Tragopogon), lettuce {Lactuca). The char-
acter of the head, flower and fruit of this family are well shown
in the dandelion (Fig. 339). All the flowers are ligulate, the
five lobes frequently to be seen on the margin of the corolla
indicating the number of petals, and the calyx assumes the form
of minute hairs (Fig. 339, B-E). The five filaments adhere to
the tube of the corolla while the anthers form a C3'linder about
the style and discharge their spores before the flower opens. The
style begins to elongate as soon as the flower opens and pushes
out the spores which may be seen as little piles of dust at the
top of the anthers in freshly opened flowers. The filaments of
this genus are sensitive to touch and curve outward, pulling down
the sheath of anthers, thus assisting in sweeping out the spores
and exposing the style when an insect irritates them in his (|uest
for nectar. This device appears in other genera and is sometimes
the only means of exposing the microspores and styles. Autog-
amy is at first impossible owing to the fact that the two lobes of
the style are closely pressed together efTectually excluding the
microspores from its inner, stigmatic, surface. Later the style
grows considerably above the anthers and the lobes separate, ex-
posing the stigmas for crossing (Fig. 339, E). The association

THE CAMPANULALES

of this type of flower is more efl^ective than any as yet noticed,
and it would appear impossible for any of the numerous insects
which frequent these flowers to miss crossing a score of -them at
a single visit. Crossing between the flowers of a head is effected
in some genera b^^ the movements of the flowers. The inner
leaves of the involucre fold over the flowers at night and in rainy
weather, protecting them like a perianth. This results in crowd-

FiG. 340. Inftorescence of the bur-marigold (Bidens), a common ray form
of flower of the thistle family: in, involucre.

ing of the flowers together, and the stigmas are often brought in
contact with the spores that have been dusted upon the various
parts of adjoining flowers. The outward curving of the stylar
lobes may also bring the stigmas in contact with the spore-covered
parts of adjoining flowers w^ith a like result. Autogamy is
brought about in the dandelion, as in nearly all other members
of the order, b}^ the curvature of the lobes of the style which
continue to bend back until the stigmatic surface is brought in
contact with the spore-covered style (Fig. 341, D). The bloom-
ing of the flowers progresses from the margin to the center of
the head so that during several days new sets of flowers are ex-
posed for crossing. When the period of bloom is passed the
involucre closes over the head and remains in this condition until
the fruit is mature (Fig. 338, b). During this period, the stalk

DEVELOPMENT OF PLANTS 489

bearing the head elongates considerably, so as to lift the fruit
above the surrounding vegetation — the extent of its elongation
depends upon the height of this vegetation. The corolla withers
away and the hair-like calyx grows out into a white, delicate
pappus, which is lifted upon a beak-like outgrowth of the ovary
(Fig. 339, F). The fruit is now mature and in a position f(jr dis-
tribution. The involucre opens, the hairs of the pappus expand,
loosening the akenes so that the least touch or breath of air floats
off the fruit as the most perfect type of parachute (Fig. 338, c).

(b) The Thistle Family, Cardtiaceae. — This is the largest
family of the angiospcrms. The flowers have the same arrange-
ment and structure as in the Cichoriaceae, save that the corollas
are either all tul^ular, as in the thistle and ironweed (Fig. 342),
or the marginal flowers of the head may be ligulate, thus in-
creasing the conspicuousness of the inflorescence, as in the asters,
daisy, etc. (Figs. 340, 341). The marginal ligulate flowers are
termed ray flowers to distinguish them from the inner tubular or
disc flowers. The ray flowers may be sterile, though more fre-
quently imperfect and provided with pistils. The calyx may be
wanting, but more often assumes the form of silky or plumose
pappus or of membranous scales for wind transportation or of
barbed bristles of various kinds for distribution by animals. In
the case of the burdock (Arctium) the involucre is covered with
hooks. The Carduaceae includes a great array of plants, many
of which are among our troublesome weeds: Ironweed (Ver-
nonia), thoroughwort {FAipatoriiim), blazing star {Lacinaria),
golden-rod (Solidago), aster, fleabane {Erigeron), cat's-paw
(Antennaria) , everlasting {Gnaphaliitm) , rosinweed and com-
pass plant {Silphiiim), Spanish needles (Bidens), daisy (Chrys-
anthe^mim), groundsel (Senecio), thistle (Carduus). Cultivated
for their showy flowers: Tickseed (Coreopsis) , cone flower (Rud-
beckia), sunflower (Ilelianthus), cornflower (Cefitaurea), ciner-
arias, etc. Medicinal: Wormwood (Artemisia), one species of
which yields absinth, milfoil (Achillea), colt's foot (Tussilago),
tans}^ (Tanacetum), chamomile (Anthemis), arnica, burdock
(Arctium).

The ragweed family, Ambrosiaceae, is a small group, princi-

490

THE CAMPANULALES

pally North American in its distribution, that have become sepa-
rated from the Carduaceae through degeneration. The heads
contain a few greatly reduced Avind pollinated flowers that are
always imperfect and generally lacking in calyx or corolla, or

Fig. 341. Flowers and fruit of Bidens: A, sectional view of the inflor-
escence— r, ray flowers; d, disc flowers; in, bracts of the involucre. B, disc
flower before opening, the shaded region showing the position of the anthers
in the corolla — c, calyx in the form of downwardly barbed bristles; b, bract
or chaff associated with the disc flowers. C, early stage in the opening of
the flower. The stamens have been lifted beyond the mouth of the opened
corolla by the growth of their filaments and the style is elongating, pushing
out the spores, which appear in little piles at the top of the anthers. D, last
stage in the bloom of flowers. The style has grown beyond the anthers and
the stigmatic lobes have reflexed, touching the spore-covered style. E, a
sterile ray flower, much less enlarged. F, the fruit, showing the barbed pappus
for dissemination.

DEVELOPMENT OF PLANTS

491

both, and characterized by free anthers. The more famihar are:
The marsh elder (/m), ragAveed (Ambrosia), and cloth-bur and
cockle bur {XantJiium).

These three alliances are apparently the most recently evolved
of the angiosperms, and owing to their numerous variations that
have been so successful In meeting the present conditions upon

,':^'- ^

Fig. 342. A common tubular flower of the thistle family: A, inflores-
cence of the ironweed (Vernonia). B, sectional view of the inflorescence,
only the outer flowers in bloom. C, enlarged view of one of the flowers — c,
calyx or pappus. D, fruit.

the earth, the}^ have become the dominant plants the world over.
Their success can be traced to a variety of causes, as the occur-
rence in many genera of perennial underground stems, which
make them to a degree. independent of climate conditions and the
depredations of grazing animals. A great many are avoided
because of their protective armor of prickles, bitter, acrid juices
or oils and resins. Especially to be noted is the seed-like fruit
with its numerous devices for distribution. All these features,
among others, explain their extensive distribution in open coun-
tries. Another advantage appears in the structure of the flower
and the inflorescence. The highest type of a tiower, as in the case
of a machine, is the one that is most efficient. The tiower is
not so elaborate as the orchid or as those of some other groups,
but it is superior because it accomplishes its work with greater
certainty and more economically. Note the significance of
these facts — the ovary of the orchid consists of three carpels

492 THE CAMPANULALES

which may produce thousands of seeds, whereas the simple
ovary of one of the Compositae contains but a single seed.
Reduction has been made in every part without loss of con-
spicuousness and variations have also appeared that practically
eliminate the uncertainties of crossing and autogamy, and con-
sequently the formation and distribution of the seed is ensured.
In a word every evolutionary tendency that you have noticed
in the development of the flower finds its expression in this
group of plants.

INDEX

Abies, 376

Absorption, 12, 20, 51, 54

Acer, 457, 459

Accompanying cells, 79, 80

Achlya, 219

Actinomorphic, 387

Adhesion, 385, 471

Adhesive discs, 108, 109

Adiantum, 329

Adlumia, 443

Aecial stage, 252

Aeciospores, 252

Agaricaceae, 259, 260

Agaricales, 259

Agrimony, 450

Air chambers, 273, 282, 283, 336,

Akene, 398, 438, 440

Albugo, 222, 223

Alder, 432

Algae, 173

blue-green, 164, 233

brown, 201

colonial, 178

conjugating green, 185

filamentous green, 190

green, 174

red, 210

tubular green, 195

unicellular green, 174
Alga-like fungi, 218
Allen, 215
Allelomorph, 141
Alnus, 432

Alternation of crops, 67
Alternation of generation, 136,
194, 201, 211-216, 256, 271,
281, 312, 319, 352
Amanita, 261
Amaryllis family, 419
Ambrosiaceac, 485, 489
Ament, 429, 430
Ammonia, 66, 159, 160

Angiospcrmae, 378
Ankistrodesmus, 182
Annual rings, of scars, 75, 73

of xylem, 91, 92
Annuals, 69
Annulus, 261, 262, 300, 301, 306, 307,

316, 326, 327, 355
Anther, 116, 117, 379
Anther, versatile, 408
Antheridial cell, 121, 122, 358, 369,

370, 392
Antheridium, antheridia, 196, 208

274. 303
Antherozoids, 196, 275
Anthoceros. 292, 294
402 Anthocerotales, 292

Antipodal cells, 119, 120, 390
Antitoxin, 164
Apophysis, 305, 306
Apple, 450, 451

rust, 256, 257
Aquatic plants, 39, 41, 64
Arales, 410
Aralia, 30, 465
Araliaceae, 465
Archangiopteris, 316
Archegonium, 275, 276, 303, 350
Arcyria, 150, 151
Aroid order, 410
Arrow-wood, 481, 482
Arum, 411, 413
Ascocarp, 229
Ascomycetes, 228
Asco-lichens, 232
137, Ascopores, 229
279, Ascus, asci, 229, 230

Asexual generation, 136, 137, 194,

204, 214, 256, 271, 279, 281, 319,

320
Asexual spore, 123, 172
Aspcrgillales, 240
Aspergillus, 241, 242

493

494

INDEX

Asplenium, 324
Autoecious, 251
Autogamy, 383, 439, 446, 454, 470,

480, 488
Azalea, 471, 472

Bacillus, 155
Bacteria, 154, 155

economic importance of, 157

exclusion of, 156

of decay, 157, 158

of disease, 157, 162

iron, 161

nitrifying, 65, 67, 160

nitrogen fixing, 161

sulphur, 161

synthetic, 159
Banana, 420
Banyan tree, 70
Barberry rust, 251
Bark, 86, 87
Basidial stage, 256
Basidiomycetes, 251
Basidiospore, 255, 256
Basidium, basidia, 251, 263, 255, 258,

262, 263
Basswood stem, 100
Bean seed, 130, 131
Beard, 44
Beech, 431, 434

ordei, 431
Begonia, 2
Bellflower, 484

order, 483
Berry, 398
Bidens, 488, 490
Biennials, 69
Bilabiate flowers, 478
Birch leaf, 7, 427

family, 147, 431, 432
Black knot, 239, 240
Bladderwort, 480
Bladderwrack, 206
Blade, 7, 8, 21, 27, 426
Blakeslee, 227
Bleeding, of plants, 61
Blood root, 442, 443
Blueberry, 471, 472

Blue green algae, 165

Body cell, 358, 369, 370

Bogs, 296, 473

Boletaceae, 265

Boletus, 265

Borages, 476

Boraginaceae, 475, 476

Botrychium, 314, 315, 316, 317

Bower, 326

Brassica, 445

Bread making, 20, 250

Bryales, 301

Bryophyta, 269

Buckthorn order, 461

Buckwheat order, 437

Buds, 71-75, 73, 362

adventitious, 75

axillary, 74
Bulbs, 113
Buller, 264

Bunch flower family, 418
Bur-marigold, 488, 490
Burroughs. 415
Buttercup, 438

order, 438
Butternut, 433, 435
Butterwort, 480

Cabbage, 447
Cactus, 44

Calcium cyanamide, 66
Callithamnion, 213
Callus, 75, loi
Calyptra, 294, 299, 305
Calyx, 115, 116, 384
Cambium, 79, 80, 82, 89, 314

cylinder, 89, 90, 91, 314, 426
Campanula, 484
Campanulales, 483
Canal cells, 276, 303, 317, 318, 345
Canna, 421

Capillitium, 150, 151, 153
Capsule, 277, 305, 306, 307, 326, 414

417
Carbohydrates, 11, 12
Carbon, 15

Carbon dioxide, 12, 15, 21, 49, 159
Carbonic acid, 13

INDEX

495

Carduaccac, 485, 489
Carex, 409
Carpel. 115, 116, 399
Carpinus, 432
Carrot, order, 465

family, 465

wild, 466
Caruncle, 132, 133
Cassia, 35, 451.452
Castor bean, 76, 77, 90, 91, 133
Catkin, 429, 430
Cat-tail, 381, 401, 403

order, 401
Cedar apple, 256, 257
Celery, blanching of, 14
Cell, crystals, 2

division of, 56, 57, 58

intercalary, 253, 254

nature of, i

sap, I, 3r 33

wall, I, 57, 58
Cellulose, 1 1
Ceraniium, 211
Cercis, 451, 452
Chaetophoralcs, 190
Chaff, 487, 490
Chamaccyparis, 376
Chamacnerion, 463, 464
Characters, nature of, 144
Chemical reaction, 3
Chemo-synthesis, 162
Chenopodiales, 436, 437
Chenopodium, 436
Cherry, 450, 451
Chervil, 468

Chicory family, 485, 487
Chlamydomonas, 178, 179
Chlorenchyma, 11, 12, 76
Chlorophyceac, 174
Chlorophyll, 13-15, 167
Chloroplasts, 2, 9, 10, 12, 13, 17
Chondrus, 214
Choripetalac, 429
Chromatin, 56, 57
Chromoplasts, 2, 32
Chromosome, 56, 57

reduction of, 136, 137, 214-216,
280

Chromosome, homologous, 141

Cichoraceae, 485, 487

Cilium, 151, 152

Cladonia, 233

Class, 147

Classification of plants, 146

Clavaria. 264

Clavariaceae, 264

Claviceps, 237, 238

Climbing plants, 106

Closterium, 188

Club moss, 341

Club root, 153

Cluster-cup, 252

Cohesion, 385

Coenocytes, 195

Coleochaetes, 199, 200

Collateral bundles, 314, 354

Collecting cells, 9, 21

Collenchyma, 78, 83, 84

Colors of flowers, 417, 423, 441, 446.

480
Columbine, 440
Columella, 225, 293, 294
Comfrey, 385. 475, 476
Commensalism, 68, 234
Compass plant, 33, 34
Compositae, 485
Concentric bundles, 323, 324
Conducting system, 82, 94, 312
Cone, 337. 342, 366
Conidium, 228
Conn, 156
Consumption, 163
Contact, reaction to, 63, 106, 107
Cordyceps, 238, 239
Cork, 42, 76, 86, 87, 314-315
Cork cambium, 86, 87
Corm, 112, 113
Corn, root of, 57

seedling of, 134, 135
Corolla, 115, 116, 384
Cortex, 59, 60, 61, 76, 314-315
Corylus, 432
Coscinodiscus, 169
Cosmarium, 189
Cotyledon, 124, 125, 126, 131, 132

133. 318, 332, 360, 398, 428

496

INDEX

Creeping stems, 109

Cribrai a, 150

Crossing, 115, 138, 382, 404, 412, 413,

445. 446, 463. 467. 476, 487
Crowfoot order, 438
Crucibulum, 267
Cruciferae, 444, 445
Currant, 387, 449
Cuticle, 10, 25, 36, 269
Cutin, 10, 36
Cyanophyceae, 165, 166
Cycadales, 353
Cycads, 353
Cycas, 353, 355
Cyclic flowers, 384, 385
Cymbella, 170
Cyperaceae, 410
Cypripedium, 424
Cystocarp, 212, 213
Cytoplasm, 2

Daldinia, 239, 241

Dandelion, 485, 486

Darwin, 64, 132

Dasya, 211

Daucus, 466

Decay, 158

Deciduous, 42

Decompositions, in living substance,

22-23
Decussate leaf arrangement. 27, 28
Delesseria, 211
Dennstaedtia, 328
Desert plants, 44
Desmidium, 188
Desmids, 188, 189
De Vries, 144, 463
Diadelphous, 452
Diastase, 20, 250
Diatomaceae, 167
Diatoms, 167, 168
Dichotomy, 207, 271, 272
Dicotyledons, 96, 125, 426
Dioecious, 284, 339
Diphtheria, 164
Disease, 162 ■
Disc flower, 489, 490
Divisions, 148

Dodder, 64
Dodge, 229
Dominant factor, 143
Dx-upe, 398
Dryopteris, 324
Duckweed, 410
Ducts, 79

Earth star, 267

Ectocarpus, 202, 203

Egg cell, 119, 120, 197, 275

Elaters, 286, 287, 338, 339

Elderberry, 481, 482

Elfvingia, 265

Elm, 436

Embryo, 124, 125, 359, 360, 396

cell, 124, 125, 395, 396

sac, 124, 394, 395
Empusa, 227

Endodermis, 59, 61, 77, 316, 325
Endosperm, 124, 125, 133, 373, 391,
394. 398

nucleus, 119, 120, 123, 389, 390,

394

Energy of plant, 13, 15, 23, 162

Enzyme, 17, 20, 128, 157

Epicotyl, 131, 132, 398. 428

Epidermis, 8, 9, 26, 36, 272

Epigynous, 386, 387

Epiphragm, 305, 306

Epiphyte, 64, 422

Equisetales, 334

Equisetum, 335, 339

Ergot, 237, 238

Ericales, 470

Erysiphe, 244, 254, 247

Erythronium, 383, 414, 41 S

Etiolation, 14

Eucalyptus, 33

Eumycetes, 216

Euphyceae, 173

Evaporation, 25

Evening primrose, 29, 463, 464

Evolution of Algae, 174, 190
of Angiospermae, 378
of Bryophyta, 269, 290, 295, 309
of Eumycetes (true fungi), 218,
220

INDEX

497

Evolution of llowcr, 381, 388, 426
of Gymnospermae, 353
of Pteridophyta, 312, 319
of sex, 178, 180, 194, 198, 204,

340
of sexuality, 171, 177, 181, 185,
204

Excretion, 20, 22, 53

Exoascales, 248

Exoascus, 248

Exobasidium, 472

Eye spot, 175, 176

Factor, hereditary, 138

distribution of, 139

dominant, 143

nature of, 144

recessive, 143
Fagales, 431, 432
Fagus, 434
Fairy rings, 259. 260
Family, 147
Farmer, 209
Fecundation, 122, 123
Felix, 328

Fermentation, 157, 158, 249
Ferments, see enzymes.
Ferns, 311

adder tongue, 314, 315

Christmas, 311, 322

common, 321

flowering, 328

grape, 315

shield, 324, 325
Fertilization, 122, 123, 209, 276, 393

significance of, 137, 144
Fertilizers, of soil. 66
Fibrillac, of spindle, 57, 58
Ficus, 435
Figwort, 478
Filament, 116, 165, 379
Filicales, 321
Flower, 338, 378, 381

coloration of, 417, 423, 441, 480

cyclic, 384, 385

development of, 381

perfect and imperfect, 382

spiral, 383
32

Flower, structure of, 115, 116
Foliar gaps, 314, 325
Follicle, 398
Food of plants, il, 48

distribution of, 16

storage of, 17, 18
Foods, formation of, 12
Foot, 286, 287
Formaldehyde, 13
Formative regions, 58, 60, 98, 99
Formic acid, 13, 40
Forsythia, 28, 20
Fragaria, 450
Fruit, 127, 374, 397-399
Fucus, 206, 207, 208, 209
Fumitory, 443
Funaria, 294. 300, 303, 307
Fungus, 216

alga-like, 218

basidia-forming, 251

bird's nest, 267

black, 239, 240

blue-green, 240

brown, 240

coral, 264

cup, 229

fly, 227

gill, 263, 265

pore, 264, 265

prickly, 264

sac, 228
Funiculus, 118, 389, 395

Gamctangium, 192, 196, 202, 203,

208
Gamete, 118, 172, 178, 179, 187, 192,

198, 200, 211, 275
Gamete, female, 118, 120, 178, 389
male, 118, 121, 122, 178, 318,
332, 342, 358, 392
Gametophyte generation, 136, 194,
215, 274, 276, 281, 310.
312, 316-317, 320, 330,
339. 345. 349. 388
reduction of, 313, 349-351-
359. 393
female, 119, 120, 332, 339, 349,
357, 367. 368, 388, 389

498

INDEX

Gametophy te , male, 122, 332, 339,

349. 357, 369, 392
Gametospore, 118, 122, 172, 201

germination of, 123, 124, 187,
188, 193, 197, 198, 200, 213,
214, 215, 276, 277, 280, 286,
293, 304. 308, 309, 318, 332,
333, 340, 345, 351, 359, 37i-
395
Gasteromycetes, 266
Gemmae, 270, 331

Generations, alternation of, 136, 137,
194, 201, 211-216, 256, 271, 279,
281, 319, 352
Gentian, 474

order, 473
Gentianales, 473
Genus, 146
Geoglossum, 232
Geraniales, 460
Geranium, 384, 461

order, 460
Germ tube, 278
Gills, 260, 261, 263
Ginkgo, 362
Ginseng family, 465
Gleditsia, 451, 452
Gleocapsa, 166
Goebel, 309
Goosefoot order, 437
Grafting, loi, 102
Grain, 134, 135, 397, 398, 409
Graminaceae, 410
Graminales, 405
Grape, 461

blight, 222

Grass, 406, 407, 409

family, 410

order, 405

Gravity, effect on plant, 62, 104, 106,

132, 407
Ground nut, 455
Growth, apical, 56, 98, 200, 270, 304,

313 _
conditions of, 72, 74, 129
definite, 72
nature of, 3
of stem, 89, 103, 106

Growth of root, 56

relation of chemical changes to, 4

rhythm of, 93
Growing point, 56, 98, 270, 313, 346
Guard cells, 9, 10, 26
Gulf weed, 206
Gymnospermae, 353
Gymnosporangium, 256, 257

Hairs, plant, 10, 38, 39, 40

root, 47, 46, 52
Haustoria, 220, 221
Hazel, 432
Hazen, 177
Head, 482, 485
Heath order, 470
Heartwood, 95
Helleborus, 440
Helvellales, 231, 232
Hepaticae, 271
Hepatics, leafy, 288
Hepatics, thallose, 271, 272
Hereditary substance, 138
Heterocyst, 166, 167
Heteroecious, 251
Heterogamous, 178
Heterospory, 347, 352
Heterostyly, 462, 463
Hickor>^, fruit of, 435

branch of, 73

leaves of, 34
Hilum, 129, 130
Honey locust, 451, 452
Honeysuckle, 387
Hop, twining, 106
Hormogonium, 166
Horizontal branches, 28, 29
Hornbeam, 432
Horsechestnut, 31, 34
Horsetail, 334
Hybrid, 138
Hydnaceae, 264
Hydnum, 264
Hydrodictyon, 184
Hydrolysis, 20
Hydrophytes, 39
Hymenium, 230, 231, 262, 263
Hypha, 68, 217

INDEX

499

Hypocotyl, 125, 126, 398, 428
Hypocreales, 238
Hypogynous, 386
Hypoxylon, 239, 240
Hysteriographium, 241

Immunity, 164, 245
Indian-pipe, 69, 472
Individuality of the plant, 7
Indusium, 324, 325, 328, 329
Inflorescence, 381, 382
Insectivorous plants, 296, 480
Insects, crossing by, 115, 388, 412,

413. 417. 419. 421, 424. 439.

445. 453. 471, 476, 479

sight and sense of smell, 417

Integument, 117, 118, 356, 361, 398,

425, 428
Intercellular spaces, 9, 11, 21, 25, 32,

273
Involucre, 285, 287, 466, 483
Ipomoea, 475
Iris, 419
Iris, family, 419
Ironweed, 491
Iron bacteria, 161
Irritability, 33, 61, 104
Isogamous, 178
Isthmus, 189, 190

Jacket-cells, 356, 357
Jack-in-the-pulpit, 411
Jeffrey, 317
Judas tree, 451
Juglandales, 433
Juglans, 435
Juncus, 416
Jungermanialcs, 288
Juniperus, 376, 377
rust, 256, 257

Keel, 451-453
Kelps, 205
Kerner, 70
Key fruit, 398, 459
Klebs, 185
Knot, 100

black, 239, 240

Labelluni, 421. 422, 423, 424, 425

Labiatae, 476

Lactuca, 33, 34

Laminaria, 205

Larix, 375

Larkspur, 441

Lateral roots, 59, 62, 63

Latex, 435, 443

Lathyrus, 453

Leaf, arrangement of, 27-36

color, 32

compound, 30, 31

fall of, 42, 43

forms of, 7, 30, 31

growth of, 33

lobing of, 30, 31

movement, 33-36

reduction of, 43, 44

size of, 24, 25

structure of, 7-1 1, 21, 36-42

work of, 11-26
Leaf-scar, 43
Legume, 65, 398
Lenticel, 71, 76, 87, 88
Leotia, 232
Lepidium, 124, 389, 395, 396, 398,

428
Lespedeza, 35
Lessonia, 205
Leucoplasts, 2, 19
Lichens, 232

Asco-, 232, 233
Licmophora, 168
Light, 4, 13-14, 17, 31, 36, 41, 63.

104, 106, 112, 176
Ligulate flowers, 487
Lilac leaf, 9
Liliales, 414
Lily, fawn, 383, 414

of the Valley family, 418

family, 418
Lily order, 414
Linaria, 478, 479
Linin, 56, 57
Linkage of factors, 143
Linnaca, 482
Live -forever, 448-489
Liverworts. 271

500

INDEX

Liverworts, horned, 292
Living substance, i-6
Livingston, 183
Lobelia, 484, 485
Lockjaw, bacillus, 155, 163
Lodicules, 407, 408
Lomentum, 455
Loosestrife, 473, 474
purple, 462, 463
Lycogala, 150
Lycoperdales, 266, 267
Lycoperdon, 266
Lycopodiaceae, 343
Lycopodiales, 341
Lycopodium, 341, 342, 343, 345
Lythrum, 462, 463

Macrocystis, 205

Madder order, 481

Malus, 451

Maple, 457, 458, 459

Marattalies, 516

Marchantia, 274, 282, 284, 285, 286,

287
Marchantiales, 271
Massee, 245
Massula, 425

Mechanical structure, 78, 83
Medullary ray, 92, 93, 94, 314-315
Megasporangia, 348, 356, 380, 388,

389, 391
Megaspore, 117, 118, 339, 347, 348,
367, 368, 380. 388, 390
germination of, 118, 119
Megasporophyll, 117, 348, 355, 367,

379. 380, 389, 397
Meibomia, 455
Melandryum, 383, 437
Melosira, 168
Mendel, 142
Mendel's Law, 142
Mesophyll, 9, 10
Mesophytes, 39
Metchnikoff, 158
Micropyle, 117, 118, 355, 356, 368,

380
Microsphaera, 247
Microsporangia, 348, 365, 379

Microspore, 116, 339, 347, 348

germination of, 120, 122, 349,
350, 357. 358, 369. 392, 454
Microsporophyl, 117, 348, 355, 365

379
Micrasterias, 188
Mildews, downy, 220, 223

powdery, 244, 245
Mimosa, 35, 36
Minerals in soils, 48
Mint family, 476
Mistletoe, 64
Mnium, 302
Moccasin flower, 422
Monkshood flower, 441
Monocotyledons, 96, 125, 400
Monoecious, 284
Monotropa, 472
Morchella, 231, 232
Morel, 231, 232
Morning-glory, 475
Mosaics, 32
Moss, 295

bog or peat, 296

chloroplasts, 17

scale, 290

true, 301
Mould, black, 224, 225, 226

brown, 240

blue-green, 217, 240
Mould, water, 218
Movements, gases, 12, 21, 45

leaf, 33-36

root, 6 1

stem, 104

water, 24, 50, 54
Mucorales, 224
Mushrooms, 217, 259, 260
Musci, 295
Mustard family, 444
Mutation, 144
Mycelium, 21"]

Mycorrhiza, 68, 243, 296, 317, 422
Myrtales, 462
Myrtle order, 462
Myxomycetes, 149, 150, 151
Myxobacteriales, 153

INDEX

501

Narcissus, 419

Nectar glands, 381, 388, 404, 430,

431.437. 439.440,457, 458
Nemalion, 213
Nereocystis, 205
Netted veined leaf, 426, 427
Nettle, 40, 434
Nidularialcs, 267
Nitrate, 66, 160
Nitrite, 160
Nitrogen, 16, 65, 160
Nostoc, 166, 292
Nucleoli, 56, 57
Nucleus, I, 2, 56
Nut, 432, 433
Nutations, 103, 104, 106

Oak, 31, 383, 427, 434

Odors, 412, 417, 433

Oedogonium, 197, 198

Oenothera, 29, 463, 464

Olive, 154

Onoclea, 330

Oogonium, 197, 208

Oosphere, 197, 275

Operculum, 299, 300, 305, 306

Ophioglossalcs, 313

Ophioglossum, 313, 315

Orange, 460

Orchid order, 64, 421

Orchidales, 421, 423

Orchis, 425

Order, 147

Oscillatoria, 166, 167

Osmosis, 53

Osmunda, 326, 328

Ovary, 116, 117, 380

Ovule, 116, 117, 118, 380, 388, 389,

399
Oxygen, 3, 13, 15, 21, 22, 75, 121

Palisade mesophyll, 9, 10, 21, 40, 41

Pallavicinia, 288

Palms, 405

Palmelloid state, 192

Pandanales, 401

Pandorina, 179

Papaverales, 442

Papavcraceae, 442

Papillae of stigma, 389, 392

Papilionaceae, 452

Papilionaceous flower, 453

Pappus, 483, 486

Paraphyscs, 208, 209, 230, 231, 262
263, 303

Parasites, 64, 68, 216, 244, 245, 350

Parenchyma, 11, 77, 81

Parmclia, 233

Parnassia, 448

Pasteur, 164

Pea, family, 452
sweet, 453

Peach curl, 248

Pear, branch of, 73

Peat, 298

moss, 296

Pedicel, 403

Pediastrum, 184

Peniclllium, 217, 228, 242

Perennials, 69

Perianth, 115, 116, 286, 287, 288, 379,

383
Periderm, 266, 267, 268
Perigynous, 385, 386
Perisperm, 397
Perisporialcs, 244
Peristome, 294, 305, 306, 307
Perithecia, 229
Peronospora, 220, 223
Peronosporales, 220
Personata flower, 478
Petals, 115, 116, 384
Petiole, 7, 8, 29, 42, 43, 107, 426

427
Peziza, 231
Pezizales, 229
Phaeophyceae, 201
Phallales, 268
Phallus, 268
Phegopteris, 330
Philotria, 2

Phloem, 59, 79, 91, 314-315
Phosphorescence, 259
Phlox order, 474
Phosphorus, 16
Photosynthesis, 11-16, 23

502

INDEX

Photosynthesis, economic importance

of, 15-16
Phycocyanin, 165
Phycomycetes, 218
Phyllactinia, 247
Phylloglossum, 343, 344
Phytophthora, 221, 223
Picea, 375, 376
Pierce, 190
Pigments, 32
Pileus, 260
Pilobolus, 226
Pinales, 362

classification of, 375
Pine, 365, 366, 368, 369, 371, 372, 374,

375
wood, 94, 363
Pinguicula, 480
Pinnularia, 169, 172
Pistil, 115, 116, 399
Pit, 398

Pith, 76, 78, 314-315, 324, 342
Placenta, 388, 389
Placenta, free central, 474
Plankton, 168, 169
Planktoniella, 169
Plant, nature of, i

cell, I
Plasmodium, 151, 152
Plasmopara, 221, 222
Plastids, 2, 17
Pleurococcus, 182
Plowrightia, 239, 240
Plumule, 125, 126, 398, 428
Pod, 127, 130, 398, 451, 454, 455
Poison Ivy, 459, 460
Poisonous secretions, 39, 40, 53, 265,

442, 459, 469, 481
Poison sumac, 459, 460
Polar nucleus, 118, 119, 389
Polemoniales, 474
Pollen grains, 116, 120, 122
Pollinium, 424, 425, 474
Polygonales, 437
Polypodium, 329
Polyporaceae, 264
Polysiphonia, 212
Polystichum, 311, 322, 328

Polytrichum, 302, 303, 306

Pond scum, 185

Poplar, leaf arrangement, 28

order, 429
Poppy, family, 442

order, 442
Pore, II, 80, 81, 94, 264, 265, 283, 363
Porella, 289
Potato, 481

rot, 221
Prepotency, of foreign spores, 454
Primitive plants, 46, 154
Primrose order, 473
Primulales, 473

Pro-embryo, 359, 360, 371, 395, 396
Prosenchyma, 11
Protandrous, 283
Prostrate stems, 109
Protection of microspores, 412
Proteids, 16
Proteid grains, 18, 19
Prothallium, 312, 317, 330, 331, 370
Protogynouf, 283
Protomyxa, 153
Protonema, 308, 309
Protoplasm, function of, 3
Protoplasm, nature of, 2, 3
Protoplasmic connection of cells, 3
Pruning, lOO, 102
Prunus, 451
Pteridophyta, 311
Pteridium, 329
Ptomaines, 159
Puccinia, 251, 252, 253, 255
Puff ball, 266, 267
Pulvinus, 35

Purposive reactions, 5, 36, 104
Pussy Willow, 430
Pustules, 258
Pycnium, 252

Pycnidium, pycnidia, 233, 252
Pycnidiospores, 252
Pyrenoids, 185, 186
Pyrola, 384, 470, 471
Pyronema, 229, 230

Quarter-sawed oak, 95
Quercus, 146, 383, 434

INDEX

503

Ragweed family, 485, 489
Ranales, 438, 440
Ranunculus, 438. 439
Ray, flower, 489, 490

medullary, 92, 93, 94
Rattlebox, yellow, 479
Recessive factor, 143
Receptacle, 381-386, 439
Red algae, 210, 218
Red bud, 451
Red snow, 176
Regeneration of stems, 364
Respiration, 8, 21, 54, 88
Reproduction, no, 136
Resin, 39, 364
Resurrection plant, 346
Rhamnales, 461
Rhinanthus, 479
Rhizoids, 269, 273
Rhizomes, no, in, 323, 324
Rhizopus, 224, 225, 226
Rhododendron, leaf, 41
Rhodophyceae, 210, 211
Rhus, 460
Ribcs, 449
Riccia, 272
Ricciocarpus, 272, 273, 274, 276, 278,

281
Richards, 75

Ring, annual, 73, 75, 91, 92
Rivularia, 166
Rock, decomposition of, 49

disintegration of, 49
Rockweed, 206
Root, I, 4, 46, 298, 314

aerial, 64

anchoring and supporting, 69

bacteria on, 65

bending of, 62, 63

binding action. 70

cap, 58, 60, 398

contraction of, 70

growth of, 47, 56-58, 63

hairs, 46, 47, 52

lateral, 59, 62, 63

modification of, 64, 109

sensitiveness of, 61, 109

storage organs of, 69

Root, structure of, 56, 59, 60, 61

surface of, 55

toxic substances from, 53
Rootstock, no, in
Rosales, 447
Rosette, 29
Rose, 385

family ; 450

order, 447
Rostellum, 423, 425
Rubiales, 481
Rush, 401, 416, 418
Rusts, 251

white, 220, 223

Saccharomycetes, 248, 249
Sac fungi, 228
Sage, 477
Salicales, 429
Salix, 381, 430
Salvia, 477
Saltpeter, 66, 160
Samara, 398, 458, 459
Sambucus, 481, 482
Sanguinaria, 442, 443
Sap, 3. 33. 69, 71, 83
Sapindales, 456
Saprolegnia, 219, 222, 223
Saprolegniales, 218
Saprophytes, 153, 217
Sapwood, 95
Sargassum, 206
Saxifrage, 387, 449
Scabiosa, 481, 482
Scenedesmus, 182
Schizocarp, 458, 459
Schizophyta, 154
Scitaminales, 420
Sclerenchyma, 78
Sclerotium, 237, 238
Scouring rush, 336
Scrophulariaceae, 478
Scutellaria, 477
Scutellum, 135, 409
Secondary growth, 85
Sedge family, 410
Sedge order, 405
Sedum, 116. 379, 448

504

INDEX

Seed, 351, 352, 360, 372, 397, 398

development of, 125, 126, 130

forms of, 126, 127

functions of, 127

growth of, 127-136
Seedling, castor bean, 132, 133

corn, 134, 135
Seed plant, 352
Segregation of factors, 141
Selaginella, 346, 347, 348, 350
Selaginellaceae, 346
Senna family, 450
Sensitive pea, 451, 452

plant, 35, 36, 455
Sensitiveness of leaf, 33

of protoplasm, 4, 5, 152

of root, 61

of stem, 104, 106

of zoospores, 176
Sepals, 384, 441
Seta, 289, 291, 309
Sex, evolution of, 178, 180, 181, 182,

194, 198, 204
Sexual generation, 136, 137, 279
Sexuality, nature of, 171, 172, 177,

181, 185, 204
Shade plants, 41, 44
Shin leaf, 470, 471
Sieve tubes, 79, 80
Silicates, 48, 170, 173, 336
Silk, of corn, 134
Siphonales, 195
Skull-cap, 477
vSkunk cabbage, 412
Sleep movements, 35
Slime mould, 149, 150, 151
Smallpox, 163, 164
Smuts, 257
Soapberry order, 456
Soils, care of, 54, 55

formation of, 48-50

nature of, 50-52

retention of substances by, 52

sterility of, 53-55

structure of, 51

texture of, 51
Solanum, 481
Soldier's cap, 443, 444

Solomon's seal, leaf, 7, 401
Soredium, 234, 235
Sorus, 316, 324, 325, 355
Spadix, 411
Spathe, 410, 411
Spathyema, 412
Spathyema, leaf structure, 41
Species, 146

biological, 245
Sperm. 196, 275
Spermatophyta, 352
Sphaerella, 174, 175
Sphaeriales, 239, 240
Sphagnales, 296
Sphagnum, 296, 297, 299, 300
Spindle, of cell, 57, 58
Spiral leaf arrangement, 27, 28
Spirillum, 155
Spirogyra, 185, 186, 187
Spleenwort, 329
Spongy mesoph\dl, 9, ii, 21
Sporangia, 117, 150, 192, 224, 225,

315, 325, 326
Spore, 115, 118, 151, 270

differentiation of, 339

germination of, 278, 300, 301,

330,331,338,349
Sporophylls, 115, 116, 117, 337, 338,

379, 381
Sporophyte generation, 136, 194, 215,

256, 281
Sporophyte, development of, 281,
286, 291, 293, 304, 310, 312, 318,
319, 332-334, 346, 359, 371, 395,
396, 399
Squash, leaf, 10
stem, 79, 80
Stamen, 115, 116, 379, 399
Staminodia, 421, 473
Standard, 451-453
Starch, 2, 12, 17, 18, 19

test for, 14
Staurastrum, 188
Steironema, 473, 474
Stem, branches of, 99, 100
dicotyledon, 84, 90, 93
function of, 71
growth of, 89, 90, 91, 92, 93, 98, 99

INDEX

505

Stem, healing of, loi
modification of, 105
monocotyledons, 84, 96, 400
sensitiveness of, 104
strengthening tissues of, 83
structure of, 71, 73, 77, 90, 91,
92. 93

Stemonitis, 150

Stereome, 37, 38, 39, 78, 79, 84, 85,

323, 325
Stereum, 264
Sticta, 233

Stigma, 116, 117, 380
Stimuli, nature and effects of, 4

reaction to, 5, 33-36, 40, 61-63,
104,^106, 121
Stink horns, 268
Stipe, 260
Stipules, 426
Stolon, 109, no

Stoma, 8, 9, 10, 21, 25, 26, 37, 88, 293
Stone fruit, 398
Storage organs, 18, 59, 77, no, in,

113. 132
Strawberry, 384, 450
Streaming of protoplasm, 151, 152
Strobilus, 335, 337. 344, 347, 353.

365. 366, 374
Stroma, 238
Style, 116, 117, 380
Suber, 86
Subdivision, 147
Subkingdom, 148
Sugar, 12, 16, 17, 120
Sulphur, 16

bacteria, 161
Surface reductions, 44

tension, 51
Suspensor, 124, 125, 345, 346, 360,

371, 395. 396
Symbiosis, 68, 234
Sympetalae, 469
Symphytum, 385, 475, 476
Synergid, 118, 119, 120, 390, 391
Synthesis, 159, 160, 162

Tabellaria, 168
Tapetum, 379

Taraxacum. 485, 486

Taxodium, 376

Taxus, 376

Teeth, of peristome, 307, 308

Tclial stage, 254. 255

Teliospores, 254, 255

Teliospore stage, 255

Tendrils, 107, 108

Tetraspores, 213

Thallophyta, classification of, 149

Thallus, 149, 270

Thaxter, 153

Thelephoraceae, 263

Thistle family, 485, 489

Thuja, 376

Tissue, strengthening, 83

Toadflax, 478, 479

Toadstools, 259, 260

Toxins, 158, 162, 163

Tracheids, 9, 81, 94, 363

Tradescantia, stoma, 38

Transpiration, 8, 24, 35

Transport of water, 59, 79, 134

Tree ferns, 325

Truffles, 243

Tsuga, 375

Tube, cell, 121, 122, 358, 369-371

392, 393
Tubercles, 65, 161
Tuberculosis, 163
Tubers, in
Turgor, 83
Twigs, 73

Twining stems, 106
Typha, 381, 401, 403

Ulothrix, 190, 191, 193
Umbel, 466
Umbel lales, 465
Uncinula, 245, 247
Unilocular pistil, 473
Uredinial stage, 254, 255
Uredinales, 251
Urediniospoies, 254, 255
Urticales, 434, 436
Ustilaginales, 257
Ustilago, 258
Utricularia, 480

5o6

INDEX

Vaccinium, 472

Vacuoles, i, 2, 3, 60, 217

Valerian, 483

Valves, of diatoms, 170

Vapor, from plants, 24

Variation, 105, 144, 176, 190, 197,

218, 280, 312-313. 320, 388
Variegated plants, 14, 32
Vascular bundles, 11, 12, 16, 76, 77,

78, 79, 80, 97, 312, 314, 323, 324,

337, 342, 354
Vaucheria, 195, 196
Veins, 7, 9, 11, 301, 302, 312
Velamen, 422
Veltheimia, 402
Velum, 260, 261
Venation, 7, 400, 401, 426, 427
Veratrum, 416
Vernonia, 491
Versatile anthers, 408
Vessels, 79, 80
Viburnum, 481, 482
Vitis, 461
Volva, 261
Volvocales, 174
Volvox, 180, 181

Wallace's Law, 423

Wall cells, 277, 318, 358, 392

Walnut order, 433

Ward, 24

Water-bloom, 165

Water, action on rock, 49

capillary, 50

composition of, 12

gravitational, 50

hygroscopic, 50

lily, 398, 438

mould, 218

net, 184

Watering of plants, 55
Wax of epidermis, 37, 39
Weeds, 445
Willow, 381, 430

order, 429
Wilting, cause of, 24
Wind, pollination by, 388, 404, 433,

458
Winged fruits, 373, 458
Wings, of flowers, 451-453
Witches' broom, 256
Wolfifia, 410
Wood, heart, 95

oak, 95, 427

pine, 363

sap, 95

sections, 93, 94, 95

spring, 91, 92, 95

summer, 91, 92
W^oodsia, 328
Woodwardia, 326, 328
Work of plants, 15
Working of ponds, 165

Xanthidium, 188

Xerophytes, 39, 45

Xylaria, 239, 241

Xylem, 59, 76, 78, 79, 91, 314-315

Yamanouchi, 215
Yeast, 248, 249
Yucca, 419

Zamia, 353, 354, 355, 356
Zooglea, 155
Zoospore, 151, 152
Zygnema, 185, 186
Zygnematales, 185
Zygomorphic, 387

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