Presented to

THE BILTMORE ROOM
By Jiqrty & I/Vel htj

TWENTIETH CENTURY TEXT-BOOKS

PLANTS

A TEXT-BOOK OF BOTANY

BY

JOHN M. COULTER, A.M., Ph.D.

HEAD OF DEPARTMENT OF BOTANY
UNIVERSITY OF CHICAGO

NEW YORK

D. APPLETON AND COMPANY

1910

Copyright, 1899
By D. APPLETON AND COMPANY

PLANTS

A TEXT-BOOK OF BOTANY

PREFATORY NOTE

Although Plant Relations and Plant Structures have
been prepared as independent volumes, chiefly to meet the
needs of those schools which can give but one half year to
Botany, they form together a natural introduction to the
science. With this in view, the simple title Plants seems
suitable, with the understanding that this volume is an
introduction to the study of plants.

Either part of this combined volume may be used first,
according to the views or needs of the teacher. In many
cases it may be wise not to observe the order of the book,
but to organize laboratory work as seems best, and to assign
the appropriate readings wherever they may occur in the
volume. The author is a stickler for independent teaching,
and would not presume to prescribe an order or a method
for teachers. His purpose is simply to offer those facts and
suggestions which may be helpful to them in organizing
and presenting their work. He would urge that intelligent
contact with plants is the essential thing; that a clear
understanding of a few large facts is better than the collec-
tion of numerous small ones ; and that " getting through "
should never sacrifice the leisure needed for digestion.

The two parts of this work are indexed separately, and
references to indexes are to be made at the end of each part.

John M. Coulter.

The University of Chicago, November, 1899.

TWENTIETH CENTURY TEXT- BOOKS

PLANT RELATIONS

A FIRST BOOK OF BOTANY

BY

JOHN M. COULTER, A.M., Ph.D

HEAD PROFESSOR OF BOTANY
UNIVERSITY OF CHICAGO

THIRD EDITION REVISED

NEW YORK

D. APPLETON AND COMPANY

1910

Copyright, 1899, 1904,
By D. APPLETON AND COMPANY.

PEEFACE.

The methods of teaching botany in secondary schools
are very diverse, and in so far as they express the experience
of successful teachers, they are worthy of careful considera- .
tion. As the overwhelming factor in successful teaching
is the teacher, methods are of secondary importance, and
may well vary. It is the purpose of the present work to
contribute another suggestion as to the method of teach-
ing botany in secondary schools. The author does not
intend to criticise other methods of teaching, for each
teacher has his own best method, but it may be well to
state the principles which underlie the preparation of this
work.

The botany is divided into two parts, each representing
work for half a year. The two books are independent,
and opinions may differ as to which should precede. The
first book, herewith presented, is dominated by Ecology,
and also contains certain fundamentals of Physiology that
are naturally suggested. The second book will be domi-
nated by Morphology, but plant structure, function, and
classification will be developed together in an attempt to
trace the evolution of the plant kingdom. In the judg-
ment of the author Ecology should precede Morphology,
but this order brings to Ecology no knowledge of plant
structures and plant groups, which is of course unfortu-
nate. The advantages which seem to overbalance this dis-
advantage are as follows :

1. The study of the most evident life-relations of
plants gives a proper conception of the place of plants in

VI PREFACE.

nature, a fitting background for subsequent more detailed
studies.

2. Such a view of the plant kingdom is certainly of the
most permanent value to those who can give but a half
year to botany, for the large problems of Ecology are con-
stantly presented in subsequent experience, when details
of structure would be forgotten.

3. The work in Ecology herein suggested demands lit-
tle or no use of the compound microscope, an instrument
ill adapted to first contacts with nature.

The second book will demand the use of the compound
microscope, and those schools which possess such an equip-
ment may prefer to use that part first or exclusively.

In reference to the use of this part something should
be said, although such cautions are reiterated in almost
every recent publication. A separate pamphlet containing
" Suggestions to Teachers " who use this book has been
prepared, but a few general statements may be made here.
This book is intended to present a connected, readable
account of some of the fundamental facts of botany, and
may serve to give a certain amount of information. If it
performs no other service in the schools, however, its pur-
pose will be defeated. It is entirely too compact for any
such use, for great subjects, which should involve a large
amount of observation, are often merely suggested. It is
intended to serve as a supplement to three far more im-
portant factors : (1) the teacher, who must amplify and
suggest at every point ; (2) the laboratory, which must
bring the pupil face to face with plants and their struc-
tures ; (3) field-work, which must relate the facts observed
in the laboratory to their actual place in nature, and must
bring new facts to notice which can be observed nowhere
else. Taking the results obtained from these three fac-
tors, the book seeks to organize them, and to suggest
explanations. It seeks to do this in two ways : (1) by
means of the text, which is intended to be clear and un-

PREFACE. Vll

technical, but compact ; (2) by means of the illustrations,
which must be studied as carefully as the text, as they are
only second in importance to the actual material. Espe-
cially is this true in reference to the landscapes, many of
which cannot be made a part of experience.

Thanks are due to various members of the botanical
staff of the University, who have been of great service in
offering suggestions and in preparing illustrations. In
this first book I would especially acknowledge the aid of
Professor Charles R. Barnes and Dr. Henry C. Cowles.

The professional botanist who may critically examine
this first book knows that Ecology is still a mass of incho-
ate facts, concerning which we may be said to be making
preliminary guesses. It seems to be true, nevertheless,
that these facts represent the things best adapted for pres-
entation in elementary work. The author has been com-
pelled to depend upon the writings of Warming and of
Kerner for this fundamental material. From the work of
the latter, and from the recent splendid volume of Sehim-
per, most useful illustrations have been obtained. The
number of original illustrations is large, but those obtained
elsewhere are properly credited. John M. Coulter.

The University of Chicago, May, 1899.

PEEFACE TO THE SECOND EDITION.

In this edition the first eleven chapters remain practi-
cally as they were, with the exception of such corrections
and additions as could be made upon the plates, and a few
changes of illustrations. The remaining chapters, however,
dealing with plant societies, are essentially recast both in
text and illustrations. Especially is this true of the meso-
phyte and halophyte societies. This has been made neces-
sary by the recent rapid development of the subject, by a
larger field experience, and by the availability of more suit-
able illustrations. J. M. C.
The University of Chicago, May, 1901.

Vlll PREFACE.

PREFACE TO THE THIRD EDITION.

During the last three years the science of Botany has
made rapid progress, both in the addition of new facts
and in changed points of view. Some of this progress
affects Plant Relations, and it is recorded in this third
edition so far as it can be without a complete rewriting
of the volume. Changes will be found, therefore, in state-
ments of fact, in points of view, in terminology, in illus-
trations, and also in the addition of new material.

John M. Coulter.
The University of Chicago, July, 1904.

CONTENTS.

CHATTER PAGE

I. — Introduction . 1

II. — Foliage leaves : The light-relation ... 6
III. — Foliage leaves : Function, structure, and protection 28

IV. — Shoots 53

V.— Roots 89

VI. — Reproductive organs 109

VII. — Flowers and insects ....... 123

VIII. — An individual plant in all of its relations . . 138

IX. — The struggle for existence 142

X. — The nutrition of plants 149

XI. — Plant associations: Ecological factors , , , 162

XII. — Hydrophyte associations 170

XIII. — Xerophyte associations ..,.»,, 193

XIV. — Mesophyte associations ...,,., 233

Index • • . 259

BOTANY

PART I.— PLANT RELATIONS

CHAPTER I.

INTRODUCTION.

1. General relations. — Plants form the natural covering
of the earth's surface. So generally is this true that a land
surface without plants seems remarkable. Not only do
plants cover the land, but they abound in waters as well,
both fresh and salt waters. They are wonderfully varied in
size, ranging from huge trees to forms so minute that the
microscope must be used to discover them. They are also
exceedingly variable in form, as may be seen by comparing
trees, lilies, ferns, mosses, mushrooms, lichens, and the
green thready growths {alga) found in water.

2. Plant associations.— One of the most noticeable facts
in reference to plants is that they do not form a monot-
onous covering for the earth's surface, but that there are
forests in one place, thickets in another, meadows in
another, swamp growths in another, etc. In this way the
general appearance of vegetation is exceedingly varied,
and each appearance tells of certain conditions of living.
These groups of plants living together in similar conditions,
as trees and other plants in a forest, or grasses and other
plants in a meadow, are known as plant associations. These

2 PLANT RELATIONS.

associations are as numerous as are the conditions of living,
and it may be said that each association has its own special
regulations, which admit certain plants and exclude others.
The study of plant associations, to determine their conditions
of living, is one of the chief purposes of botanical field work.

3. Plants as living things. — Before engaging in a study
of associations, however, one must discover in a general way
how the individual plant lives, for the plant covering of the
earth's surface is a living one, and plants must always be
thought of as living and at work. They are as much alive
as are animals, and so far as mere living is concerned they
live in much the same way. Nor must it be supposed that
animals move and plants do not, for while more animals than
plants have the power of moving from place to place, some
plants have this power, and those that do not can move cer-
tain parts. The more we know of living things the more is
it evident that life processes are alike in them all, whether
plants or animals. In fact, there are some living things
about which we are uncertain whether to regard them as
plants or animals.

4. The plant body. — Every plant has a body, which may
be alike throughout or may be made up of a number of
different parts. When the green thready plants (algai), so
common in fresh water, are examined, the body looks like
a simple thread, without any special parts ; but the body of
a lily is made up of such dissimilar parts as root, stem,
leaf, and flower (see Figs. 75, 144, 155, 174). The plant
without these special parts is said to be simple, the plant
with them is called complex. The simple plant lives in
the same way and does the same kind of work, so far as
living is concerned, as does the complex plant. The differ-
ence is that in the case of the simple plant its whole body
does every kind of work ; while in the complex plant
different kinds of work are done by different regions of the
body, and these regions come to look unlike when differ-
ent shapes are better suited to different work, as in the

INTRODUCTION. 3

case of a leaf and a root, two regions of the body doing
different kinds of work.

5. Plant organs. — These regions of the plant body thus
set apart for special purposes are called organs. The sim-
plest of plants, therefore, do not have distinct organs,
while the complex plants may have several kinds of organs.
All plants are not either very simple or very complex, but
beginning with the simplest plants one may pass to others
not quite so simple, then to others more complex, and so
on gradually until the most complex forms are reached.
This process of becoming more and more complex is known
as differentiation, which simply means the setting apart of
different regions of the body to do different kinds of work.
The advantage of this to the plant becomes plain by using
the common illustration of the difference between a tribe
of savages and a civilized community. The savages all do
the same things, and each savage does everything. In the
civilized community some of the members are farmers,
others bakers, others tailors, others butchers, etc. This is
what is known as " division of labor," and one great advan-
tage it has is that every kind of work is better done. Dif-
ferentiation of organs in a plant means to the plant just
what division of labor means to the community ; it results
in more work, and better work, and new kinds of work.
The very simple plant resembles the savage tribe, the com-
plex plant resembles the civilized community. It must be
understood, however, that in the case of plants the differ-
entiation referred to is one of organs and not of individuals.

6. Plant functions. — Whether plants have many organs,
or few organs, or no organs, it should be remembered that
they are all at work, and are all doing the same essential
things. Although many different kinds of work are being
carried on by plants, they may all be put under two heads,
nutrition and reproduction. Every plant, whether simple
or complex, must care for two things : (1) its own support
(nutrition), and (2) the production of other plants like

2

4 PLANT EELATIONS.

itself (reproduction). To the great work of nutrition many
kinds of work contribute, and the same is true of repro-
duction. Nutrition and reproduction, however, are the
two primary kinds of work, and it is interesting to note
that the first advance in the differentiation of a simple
plant body is to separate the nutritive and reproductive
regions. In the complex plants there are nutritive organs
and reproductive organs ; by which is meant that there are
distinct organs which specially contribute to the work of
nutrition, and others which are specially concerned with
the work of reproduction. The different kinds of work are
conveniently spoken of as functions, each organ having one
or more functions.

7. Life-relations. — In its nutritive and reproductive work
the plant is very dependent upon its surroundings. It
must receive material from the outside and get rid of waste
material ; and it must leave its offspring in as favorable
conditions for living as possible. As a consequence, every
organ holds a definite relation to something outside of it-
self, known as its life-relation. For example, green leaves
are definitely related to light, many roots are related to
soil, certain plants are related to abundant water, some
plants are related to other plants or animals (living as
parasites), etc. A plant with several organs, therefore,
may hold a great variety of life-relations, and it is quite a
complex problem for such a plant to adjust all of its parts
properly to their necessary relations. The study of the
life-relations of plants is a division of Botany known as
Ecology, and presents to us many of the most important
problems of plant life.

It must not be supposed that any plant or organ holds
a perfectly simple life-relation, for it is affected by a great
variety of things. A root, for instance, is affected by light,
gravity, moisture, soil material, contact, etc. Every or-
gan, therefore, must adjust itself to a very complex set of
life-relations, and a plant with several organs has so many

INTRODUCTION. 5

delicate adjustments to care for that it is really impossi-
ble, as yet, for us to explain why all of its parts are placed
just as they are. In the beginning of the study of plants,
only some of the most prominent functions and life-rela-
tions can be considered. In order to do this, it seems bet-
ter to begin with single organs, and afterwards these can
be put together in the construction of the whole plant.

CHAPTER II.

FOLIAGE LEAVES: THE LIGHT-RELATION.

8. Definition. — A foliage leaf is the ordinary green leaf,
and is a very important organ in connection with the work
of nutrition. It must not be thought that the work done by
such a leaf cannot be done by green plants which have no
leaves, as the algae, for example. A leaf is simply an or-
gan set apart to do such work better. In studying the
work of a leaf, therefore, we have certain kinds of work
set apart more distinctly than if they were confused with
other kinds. For this reason the leaf is selected as an in-
troduction to some of the important work carried on by
plants, but it must not be forgotten that a plant does not
need leaves to do this Avork ; they simply enable it to work
more effectively.

9. Position. — It is easily observed that foliage leaves
grow only upon stems, and that the stems which bear them
always expose them to light ; that is, such leaves are aerial
rather than subterranean (see Figs. 1, 75, 174). Many
stems grow underground, and such stems either bear no
foliage leaves, or are so placed that the foliage leaves are
sent above the surface, as in most ferns and many plants of
the early spring (see Figs. 45, 46, 144).

10. Color. — Another fact to be observed is that foliage
leaves have a characteristic green color, a color so universal
that it has come to be associated with plants, and espe-
cially with leaves. It is also evident that this green color
holds some necessary relation to light, for the leaves of
plants grown in the dark, as potatoes sprouting in a cellar,

FOLIAGE LEAVES: THE LIGHT-RELATION. 7

do not develop this color. Even when leaves have devel-
oped the green color they lose it if deprived of light, as is
shown by the process of blanching celery, and by the effect
on the color of grass if a board has lain upon it for
some time. It seems plain, therefore, that the green color
found in working foliage leaves depends upon light for its
existence.

We conclude that at least one of the essential life-rela-
tions of a foliage leaf is what may be called the light-rela-
tion. This seems to explain satisfactorily why such leaves
are not developed in a subterranean position, as are many
stems and most roots, and why plants which produce them
do not grow in the dark, as in caverns. The same green,
and hence the same light-relation, is observed in other
parts of the plant as well, and in plants without leaves, the
only difference being that leaves display it most conspicu-
ously. Another indication that the green color is con-
nected with light may be obtained from the fact that it is
found only in the surface region of plants. If one cuts
across a living twig or into a cactus body, the green color
will be seen only in the outer part of the section. The con-
clusion is that the leaf is a special organ for the light-re-
lation. Plants sometimes grow in such situations that it
would be unsafe for them to display leaves, or at least large
leaves. In such a case the work of the leaves can be thrown
upon the stem. A notable illustration of this is the cactus
plant, which produces no foliage leaves, but whose stem dis-
plays the leaf color.

11. An expanded organ. — Another general fact in refer-
ence to the foliage leaf is that in most cases it is an expanded
organ. This means that it has a great amount of surface
exposed in comparison with its mass. As this form is of
such common occurrence it is safe to conclude that it is in
some way related to the work of the leaf, and that whatever
work the leaf does demands an exposure of surface rather
than thickness of body. It is but another step to say that

8

PLANT RELATIONS.

the amount of work an active leaf can do will depend in
part upon the amount of surface it exposes.

THE LIGHT-RELATION.

12. The general relation. — The ordinary position of the
foliage leaf is more or less horizontal. This enables it to
receive the direct rays of light upon its upper surface. In

this way more rays of
light strike the leaf sur-
face than if it stood ob-
liquely or on edge. It is
often said that leaf blades
are so directed that the
flat surface is at right
angles to the incident
rays of light. Wbile this
may be true of horizon-
tal leaves in a general
way, the observation of
almost any plant will
show that it is a very
general statement, to
which there are numerous
exceptions (see Fig. 1).
Leaves must be arranged
to receive as much light
as possible to help in
their work, but too much
light will destroy the
green substance {chloro-
phyll), which is essential
to the work. The adjust-
ment to light, therefore,
is a delicate one, for
there must be just enough

Fig. 1. The leaves of this plant (Ficus) are
in general horizontal, but it will be seen
that the lower ones are directed down-
ward, and that the leaves become more
horizontal as the stem is ascended. It
will also be seen that the leaves are so
broad that there are few vertical rows.

FOLIAGE LEAVES: THE LIGHT-RELATION. 9

and not too much. The danger from too much light is
not the same in the case of all leaves, even on the same
plant, for some are more shaded than others. Leaves also
have a way of protecting themselves from too intense light
hy their structure, rather than by a change in their posi-
tion. It is evident, therefore, that the exact position which
any particular leaf holds in relation to light depends upon
many circumstances, and cannot be covered by a general
rule, except that it seeks to get all the light it can without
danger.

13. Fixed position. — Leaves differ very much in the power
of adjusting their position to the direction of the light.

Fig. 2. The day and night positions of the leaves of a member (Amicia) of the pea
family. — After Strasburger.

Most leaves when fully grown are in a fixed position and
cannot change it, however unfavorable it may prove to be,
except as they are blown about. Such leaves are said to
ha,\e fixed light positions. This position is determined by
the light conditions that prevailed while the leaf was grow-
ing and able to adjust itself. If these conditions continue,
the resulting fixed position represents the best one that can
be secured under the circumstances. The leaf may not
receive the rays of light directly throughout the whole
period of daylight, but its fixed position is such that it
probably receives more light than it would in any other
position that it could secure.

10

PLANT RELATIONS.

14. Motile leaves. — There are leaves, however, which
have no fixed light position, but are so constructed that
they can shift their position as the direction of the light
changes. Such leaves are not in the same position in the

afternoon as in the
forenoon, and their
night position may be
very different from
either (see Figs. 2, 3a,
35, 1). Some of the
common house plants
show this power. In
the case of the com-
mon Oxalis the night

Fig. 3a. The day position of the leaves of redbud \

(Cercis).— After Arthur. is remarkably different

from the position in light.
If such a plant. is exposed
to the light in a window and
the positions of the leaves
noted, and then turned
half way around, so as to
bring the other side to the
light, the leaves may be
observed to adjust them-
selves gradually to the
changed light-relations.

15. Compass plants. — A
striking illustration of a

special light position is found in the so-called " compass
plants." The best known of these plants is the rosin- weed
of the prairie region. Growing in situations exposed to
intense light, the leaves are turned edgewise, the flat faces
being turned away from the intense rays of midday, and
directed towards the rays of less intensity ; that is, those of

Fig. Zb. The night position of the leaves
of redbud (Cercis). — After Arthur.

FOLIAGE LEAVES; THE LIGHT-RELATION. 11

Fig. 4. Two sensitive plants, showing the motile leaves. The plant to the left lias its
leaves and numerous leaflets expanded ; the one to the right shows the leaflets
folded together and the leaves drooping.— After Kerner,

the morning and evening (see Fig. 170). As a result, the
plane of the leaf lies in a general north and south direc-
tion. It is a significant fact that when the plant grows in
shaded places the leaves do not assume any such position.
It seems evident, therefore, that the position has something
to do with avoiding the danger of too intense light. It

12

PLANT RELATIONS.

Fig. 5. The common prickly lettuce (Lactuca
Scariola), showing the leaves standing edge-
wise, and in a general north and south plane.
—After Arthur and MacDougal.

must not be supposed
that there is any ac-
curacy in the north or
south direction, as the
edgewise position
seems to be the signifi-
cant one. In the ros-
in-weed probably the
north and south direc-
tion is the prevailing
one ; but in the prickly
lettuce, a very common
weed of waste grounds,
and one of the most
striking of the compass
plants, the edgewise
position is frequently
assumed without any
special reference to the
north or south direc-
tion of the apex (see
Fig. 5).

16. Heliotropism. —
The property of leaves
and of other organs
of responding to light
is known as heliotro-
pism, and it is one
of the most important
of those external influ-
ences to which plant
organs respond (see
Figs. 6, 43).

It should be under-
stood clearly that this
is but a slight glimpse

FOLIAGE LEAVES: THE LIGHT-KELAT10.N. 13

Pig. 6. These plants are growing near a window. It will be noticed that the stems
bend strongly towards the light, and that the leaves face the light.

of the most obvious relations of foliage leaves to light, and
that the important part which heliotropism plays, not only
in connection with foliage leaves, but also in connection
with other plant organs, is one of the most important and
extensive subjects of plant physiology.

RELATION OF LEAVES TO ONE ANOTHER.

A. On erect stems.

In view of what has been said, it would seem that the
position of foliage leaves on the stem, and their relation to
one another, must be determined to some extent by the
necessity of a favorable light-relation. It is apparent that
the conditions of the problem are not the same for an erect
as for a horizontal stem.

17. Relation of breadth to number of vertical rows. —
Upon an erect stem it is observed that the leaves are usu-

14

PLANT RELATIONS.

ally arranged in a definite number of vertical rows. It is
to the advantage of the plant for these leaves to shade one
another as little as possible. Therefore, the narrower the
leaves, the more numerous may be the vertical rows (see

Figs. 7. 8) ; and
the broader the
leaves the fewer
the vertical rows
(see Fig. 1). A_
relation exists,
therefore, be-
tween the breadth
of leaves and the
number of verti-
cal rows, and the
meaning of this
becomes plain
when the light-re-
lation is consid-
ered.

18. Relation of
length, to the dis-
tance between
leaves of the same
row. — The leaves
in a vertical row
may be close together or far apart. If they should be close
together and at the same time long, it is evident that they
will shade each other considerably, as the light cannot well
strike in between them and reach the surface of the lower
leaf. Therefore, the closer together the leaves of a verti-
cal row, the shorter are the leaves ; and the farther apart
the leaves of a row, the longer may they be. Short leaves
permit the light to strike between them even if they are
close together on the stem ; and long leaves permit the
same thing only when they are far apart on the stem. A

Fig. 7.

An Easter lily, showing narrow leaves and
numerous vertical rows.

FOLIAGE LEAVES : THE LIGHT-RELATION

15

relation is to be observed, therefore, between the length
of leaves and their distance apart in the same vertical row.
The same kind of relation can be observed in reference
to the breadth of leaves, for if leaves are not only short but
narrow they can stand very close together. It is thus seen
that the length and breadth of leaves, the number of ver-
tical rows on the stem, and the distance between the leaves

Fig. 8. A dragon-tree, showing narrow leaves extending in all directions, and numer-
ous vertical rows.

of any row, all have to do with the light-relation and are
answers to the problem of shading.

19. Elongation of the lower petioles. — There is still
another common arrangement by which an effective light-
relation is secured by leaves which are broad and placed
close together on the stem. In such a case the stalks
(petioles) of the lower leaves become longer than those
above and thus thrust their blades beyond the shadow (see
Fig. 9). It may be noticed that it is very common to

16 PLANT RELATIONS.

find the lowest leaves of a plant the largest and with the
longest petioles, even when the leaves are not very close
together on the stem.

It must not be supposed that by any of these devices
shading is absolutely avoided. This is often impossible and
sometimes undesirable. It simply means that by these

Fig. 9. A plant (Saintpaulia) with the lower petioles elongated, thrusting the blades
beyond the shadow of the upper leaves. A loose rosette.

arrangements the most favorable light-relation is sought by
avoiding too great shading.

20. Direction of leaves. — Not only is the position on the
stem to be observed, but the direction of leaves may result
in a favorable relation to light. It is a very common thing
to find a plant with a cluster of comparatively large leaves
at or near the base, where they are in no danger of shading
other leaves, and with the stem leaves gradually becoming

FOLIAGE LEAVES : THE LIGHT-RELATION.

17

smaller and less horizontal toward the apex of the stem
(see Figs. 10, 13). The common shepherd's purse and the
mullein may be taken as illustrations. By this arrange-
ment all the leaves are very
completely exposed to the
light.

21. The rosette habit. —
The habit of producing a
cluster or rosette of leaves
at the base of the stem is
called the rosette habit.
Often this rosette of leaves
at the base, frequently lying
flat on the ground or on the
rocks, includes the only fo-
liage leaves the plant pro-
duces. It is evident that a
rosette, in which the leaves
must overlap one another
more or less, is not a very
favorable light arrange-
ment, and therefore it must
be that something is being
provided for besides the
light-relation (see Figs. 11,
12, 13). What this is will
appear later, but even in

this comparatively unfavorable light arrangement, there is
evident adjustment to secure the most light possible under
the circumstances. The lowest leaves of the rosette are
the longest, and the upper (or inner) ones become gradu-
ally shorter, so that all the leaves have at least a part
of the surface exposed to light. The overlapped base of
such leaves is not expanded as much as the exposed apex,
and hence they are mostly narrowed at the base and broad
at the apex. This narrowing at the base is sometimes

Fig. 10. A plant (Echeveria) with fleshy
leaves, showing large horizontal ones
at base, and others becoming smaller
and more directed upward as the
stem is ascended.

18

PLANT KELATIONS.

carried so far that most of the part which is covered is
but a stem (petiole) for the upper part (blade) which is
exposed.

In many plants which do not form close rosettes a gen-

Fig. 11. A group of live-for-evers. fllnstratine the rosette habit and the light-relation.
In the rosettes it will be observed how the leaves are fitted together and diminish
in size inwards, so that excessive shading is avoided. The individual leaves also
become narrower where they overlap, and are broadest where they are exposed to
light. In the background is a plant showing leaves in very definite vertical rows.

3ral rosette arrangement of the leaves may be observed by
looking down upon them from above (see Fig. 9), as in some
of the early buttercups which are so low that the large
leaves would seriously shade one another, except that the
lower leaves have longer petioles than the upper, and so
reach beyond the shadow.

FOLIAGE LEAVES: THE LIGHT-RELATION.

19

Fig. 12. Two clumps of rosettes of the house leek (Semperritmrn), the one to the
right showing the compact winter condition, the one to the left with rosettes more
open after being kept indoors for several days.

22. Branched leaves. — Another notable feature of foliage
leaves, which has something to do with the light-relation,
is that on some plants the blade does not consist of one
piece, but is lobed or even broken up into separate pieces.
When the divisions are distinct they are called leaflets, and
every gradation in leaves can be found, from distinct leaf-
lets to lobed leaves, toothed leaves, and finally those whose
margins are not indented at all {entire). This difference
in leaves probably has
more important rea-
sons than the light-
relation, but its sig-
nificance may be ob-
served in this connec-
tion. In those plants
whose leaves are un-
divided, the leaves
generally either di-
minish in size toward
the top of the stem,
or the lower ones de-
velop longer petioles.
In this case the gen-
eral outline of the
3

Fig. 13. The leaves of a bellflower {Campanula),
showing the rosette arrangement. The lower
petioles are successively longer, carrying their
blades beyond the shadow of the blades above.
—After Kebneb.

Fig. 14. A group of leaves, showing how branched leaves overtop each other without
dangerous shading. It will be seen that the larger blades or less-branched leaves
are towards the bottom of the group.

FOLIAGE LEAVES: THE LIGHT- RELATION.

21

plant is conical, a form very common in herbs with entire
or nearly entire leaves. In plants whose leaf blades are
broken up into leaflets {compound or branched leaves),
however, no such diminution in size toward the top of the
stem is necessary (see Fig. 17), though it may frequently

Fig. 15. A plant showing much-branched leaves, which occur in great profusion with-
out cutting off the light from one another.

occur. When a broad blade is broken up into leaflets
the danger of shading is very much less, as the light can
strike through between the upper leaflets and reach the
leaflets below. On the lower leaves there will be splotches
of light and shadow, but they will shift throughout the
day, so that probably a large part of the leaf will receive
light at some time during the day (see Fig. 14). The

22

PLANT RELATIONS.

general outline of such a plant, therefore, is usually not
conical, as in the other case, but cylindrical (see Figs. 4,
15, 16, 22, 45, 83, 90, 155, 162, 174 for branched leaves).

Many other factors enter into the light-relation of foli-
age leaves upon erect stems, but those given may suggest

Fig. 16. A cycad, showing much-branched leaves ana palm-like habit.

observation in this direction, and serve to show that the
arrangement of leaves in reference to light depends upon
many things, and is by no means a fixed and indifferent
thing. The study of any growing plant in reference to this
one relation presents a multitude of problems to those who
know how to observe.

B. On horizontal stems.

23. Examples of horizontal stems, that is, stems exposed
on one side to the direct light, will be found in the case of
many branches of trees, stems prostrate on the ground, and

FOLIAGE LEAVES: THE LIGHT-RELATION.

23

stems against a support, as the ivies. It is only necessary
to notice how the leaves are adjusted to light on an erect
stem, and then to bend the
stem into a horizontal posi-
tion or against a support, to
realize how unfavorable the
same arrangement would
be, and how many new ad-
justments must be made.
The leaf blades must all be
brought to the light side of
the stem, so far as possible,
and those that belong to
the lower side of the stem
must be fitted into the
spaces left by the leaves
which belong to the upper
side. This may be brought
about by the twisting of.
the stem, the twisting of
the petioles, the bending of
the blade on the petiole,
the lengthening of petioles,
or in some other way.
Every horizontal stem has
its own special problems of
leaf adjustment which may
be observed (see Figs. 18,
50).

Sometimes there is not
space enough for the full
development of every blade,
and smaller ones are fitted
into the spaces left by the larger ones (see Fig. 21). This
sometimes results in what are called unequally paired leaves,
where opposite leaves develop one large blade and one small

Fig. 17. A chrysanthemum, showing
lobed leaves, the rising of the petioles
to adjust the blades to light, and the
general cylindrical habit.

24

PLANT RELATIONS.

one. Perhaps the most complete fitting together of leaves
is found in certain ivies, where a regular layer of angular
interlocking leaves is formed, the leaves fitting together like

Fig. 18. A plant (Pellionia) with drooping stems, showing how the leaves are all
brought to the lighted side and fitted together.

the pieces of a mosaic. In fact such an arrangement is
known as the mosaic arrangement, and involves such an
amount of twisting, displacement, elongation of petioles,

26

PLANT RELATIONS.

Fig. 20. A spray of maple, showing the adjustment of the leaves in size and position
of blades and length of petioles to secure exposure to light on a horizontal stem.—
After Kerner.

etc., as to give ample evidence of the effort put forth by
plants to secure a favorable light-relation for their foliage

Fig. 21. Two plants showing adjustment of leaves on a horizontal stem. The plant
to the left is nightshade, in which small blades are fitted into spaces left by the
large ones. The plant to the right is Selaginella, in which small leaves are dis-
tributed along the Bides of the stem, and others arc displayed along the upper sur-
face.— After Kerner.

FOLIAGE LEAVES: THE LIGHT-RELATION.

27

leaves (see Figs. 19, 22). In the case of ordinary shade trees
every direction of branch may be found, and the resulting
adjustment of leaves noted (see Fig. 20).

Looking up into a tree in full foliage, it will be noticed
that the horizontal branches are comparatively bare be-

Pig. 22. A mosaic of fern (Adiantum) leaflets.

neath, while the leaf blades have been carried to the upper
side and have assumed a mosaic arrangement.

Sprays of maidenhair fern (see Fig. 22) show a remark-
able amount of adjustment of the leaflets to the light side.
Another group of fern-plants, known as club-mosses, luis
horizontal stems clothed with numerous very small leaves.
These leaves may be seen taking advantage of all the space
on the lighted side (see Fig. 21).

CHAPTER III.

FOLIAGE LEAVES: FUNCTION, STRUCTURE, AND PROTEC-
TION.

A. Functions of foliage leaves.

24. Functions in general. — We have observed that foliage
leaves are light-related organs, and that this relation is an
important one is evident from the various kinds of adjust-
ment used to secure it. We infer, therefore, that for some
important function of these leaves light is necessary. It
would be hasty to suppose that light is necessary for every
kind of work done by a foliage leaf, for some forms of work
might be carried on by the leaf that light neither helps nor
hinders. Foliage leaves are not confined to one function,
but are concerned in a variety of processes, all of which
have to do with the great work of nutrition. Among the
variety of functions which belong to foliage leaves some of
the most important may be selected for mention. It will
be possible to do little more than indicate these functions
until the plant with all its organs is considered, but some
evidence can be obtained that various processes are taking
place in the foliage leaf.

25. Photosynthesis. — The most important function of the
foliage leaf may be detected by a simple experiment. If
an actively growing water plant submerged in water in a
glass vessel be exposed to bright light, bubbles may be seen
coming from the leaf surfaces and rising through the water
(see Eig. 23). The water is merely a device by which the
bubbles of gas may be seen. If the plant is very active the

FOLIAGE LEAVES: FUNCTION, STEDCTUEE, ETC.

29

bubbles are numerous. That this activity holds a definite
relation to light may be proved by shading the vessel con-
taining the plant. When the light is diminished the bub-
bles diminish in number, and when sufficiently darkened

Fig. 23. An experiment to illustrate the giving off of oxygen in the process of photo-
synthesis.

the bubbles will cease entirely. If now the vessel be again
illuminated, the bubbles will reappear, and the rapidity
with which the bubbles are formed will indicate in a rough
way the activity of the process. That this gas being given
off is mainly oxygen may be proved by collecting the

30 PLANT RELATIONS.

bubbles (by inverting over the plants a large funnel and
leading them into a test tube), and testing it in the usual
way.

Some very important things are learned by this experi-
ment. It is evident that some process is going on within
the leaves which needs light and which results in giving off
oxygen. It is further evident that as oxygen is eliminated,
the process indicated is dealing with substances which
contain more oxygen than is needed. The amount of
oxygen given off may be taken as the measure of the work.
The more oxygen, the more work ; and, as we have observed,
the more light, the more oxygen ; and no light, no oxygen.
Therefore, light must be essential to the work of which the
elimination of oxygen is an external indication. That this
process, whatever it may be, is so essentially related to
light, suggests the idea that it is the special process which
demands that the leaf shall be a light-related organ. If so,
it is a dominating kind of work, as it chiefly determines
the life-relations of foliage leaves.

The process thus indicated is known as photosynthesis,
and the name suggests that it has to do with the arrange-
ment of material with the help of light. It is really a pro-
cess of food manufacture, by which raw materials are made
into plant food. This process is an exceedingly important
one, for upon it depend the lives of all plants and animals.
The foliage leaves may be considered, therefore, as special
organs of photosynthesis. They are special organs, not ex-
clusive organs, for any green tissue, whether on stem or fruit
or any part of the plant body, may do the same work. It
is at once apparent, also, that during the night the process
of photosynthesis is not going on, and therefore during the
night oxygen is not being given off.

Another part of this process is not so easily observed, but
is so closely related to the elimination of oxygen that it
must be mentioned. Carbon dioxide occurs in the air to
which the foliage leaves are exposed. It is given off from

FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 31

our lungs in breathing, and also comes off from burning
wood or coal. It is a common waste product, being a com-
bination of carbon and oxygen so intimate that the two
elements are separated from one another with great dif-
ficulty. During the process of photosynthesis it has been
discovered that carbon dioxide is being absorbed from the
air by the leaves. As this gas is absorbed chiefly by green
parts and in the light, in just the conditions in which oxy-
gen is being given off, it is natural to connect the two, and
to infer that the process of photosynthesis involves not only
the green color and the light, but also the absorption of
carbon dioxide and the elimination of oxygen.

When we observe that carbon dioxide is a combination
of carbon and oxygen, it seems reasonable to suppose that
the carbon and oxygen are separated from one another in
the plant, and that the carbon is retained and the oxygen
given back to the air. The process of photosynthesis may
be partially defined, therefore, as the breaking up of carbon
dioxide by the green parts of the plants in the presence of
light, the retention of the carbon, and the elimination of
the oxygen. The carbon retained is combined into real
plant food, in a way to be described later. We may con-
sider photosynthesis as the most important function of the
foliage leaf, of which the absorption of carbon dioxide and
the evolution of oxygen are external indications ; and that
light and chlorophyll are in some way essentially connected
with it.

26. Transpiration. — One of the easiest things to observe
in connection with a working leaf is the fact that it gives
off moisture. A simple experiment may demonstrate this.
If a glass vessel (bell jar) be inverted over a small active
plant the moisture is seen to condense on the glass, and
even to trickle down the sides. A still more convenient way
to demonstrate this is to select a single vigorous leaf with
a good petiole ; pass the petiole through a perforated card-
board resting upon a tumbler containing water, and invert

32 PLANT RELATIONS.

a second tumbler over the blade of the leaf, which projects
above the cardboard (see Pig. 24). It will be observed that
moisture given off from the surface of the working leaf is
condensed on the inner surface of the inverted tumbler.
The cardboard is to shut off evaporation from the water
in the lower tumbler.

When the amount of water given off by a single leaf is
noted, some vague idea may be formed as to the amount of
moisture given off by a great mass of vegetation, such as a
meadow or a forest. It is evident that green plants at
work are contributing a very large amount of moisture to
the air in the form of water vapor, moisture which has
been absorbed by some region of the plant. The foli-
age leaf, therefore, may be regarded as an organ of
transpiration, not that the leaves alone are engaged in
transpiration, for many parts of the plant do the same
thing, but because the foliage leaves are the chief seat of
transpiration.

In case the leaves are submerged, as is true of many
plants, it is evident that transpiration is practically checked,
for the leaves are already bathed with water, and under such
circumstances water vapor is not given off. It is evident
that under such circumstances leaf work must be carried
on without transpiration. In some cases, as in certain
grasses, fuchsias, etc., drops of water are extruded at the
apex of the leaf, or at the tips of the teeth. This process
is called guttation, and by means of it a good deal of
water passes from the leaf. It is specially used by shade
plants, which live in conditions which do not favor tran-
spiration.

27. Respiration. — Another kind of work also may be
detected in the foliage leaf, but not so easily described.
In fact it escaped the general attention of botanists much
longer than did photosynthesis and transpiration. It is
work that goes on so long as the leaf is alive, never ceasing
day or night. The external indication of it is the absorption

Pig. 24. Experiment illustrating transpiration.

34 PLANT RELATIONS.

of oxygen and the giving out of carbon dioxide. It will be
noted at once that this is. exactly the reverse of what takes
place in photosynthesis. During the day, therefore, carbon
dioxide and oxygen are both being absorbed and evolved.
It will also be noted that the taking in of oxygen and the
giving out of carbon dioxide is just the sort of exchange
which takes place in our own respiration. In fact this pro-
cess is also called respiration in plants. It does not depend
upon light, for it goes on in the dark. It does not depend
upon chlorophyll, for it goes on in plants and parts of plants
which are not green. It is not peculiar to leaves, but goes
on in every living part of the plant. A process which goes
on without interruption in all living plants and animals
must be very closely related to their living. We conclude,
therefore, that while photosynthesis is peculiar to green
plants, and only takes place in them when light is present,
respiration is necessary to all plants in all conditions, and
that when it ceases life must soon cease. The fact is,
respiration supplies the energy which enables the living
substance to work.

Once it was thought that plants differ from animals
in the fact that plants absorb carbon dioxide and give off
oxygen, while animals absorb oxygen and give off carbon
dioxide. It is seen now that there is no such difference,
but that respiration (absorption of oxygen and evolution of
carbon dioxide) is common to both plants and animals.
The difference is that green plants have the added work of
photosynthesis.

We must also think of the foliage leaf, therefore, as a
respiring organ, because very much of such work is done
by it, but it must be remembered that respiration is going
on in every living part of the plant.

This by no means completes the list of functions that
might be made out for foliage leaves, but it serves to indi-
cate both their peculiar work (photosynthesis) and the fact
that they are doing other kinds of work as well.

FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 35

B. Structure of foliage leaves.

28. Gross structure. — It is evident that the essential part
of a foliage leaf is its expanded portion or blade. Often the

leaf is all blade (see Figs. 7,
8, 18) ; frequently there is a
longer or shorter leaf-stalk
{petiole) which helps to put

Fig. 25. Two types of leaf venation. The figure to the left is a leaf of Solomon's
seal (Polygonalum), and shows the principal veins parallel, the very minute cross
veinlets being invisible to the naked eye, being a monocotyl type. The figure to
the Tight is a leaf of a willow, and shows netted veins, the main central vein (mid-
rib) sending out a series of parallel branches, which are connected with one another
by a network of veinlets, being a dicotyl type. — After Ettingshatjsen.

the blade into better light-relation (see Figs. 1, 9, 17, 20,
2G); and sometimes there are little leaf -like appendages {stip-
ules) on the petiole where it joins the stem, whose func-
tion is not always clear. Upon examining the blade it

is seen to consist of a green substance through which a
4

36

PLANT RELATIONS.

framework of veins is variously arranged. The large veins
which enter the blade send off smaller branches, and these
send off still smaller ones, until the smallest veinlets are

invisible, and the
framework is a
close network of
branching veins.
This is plainly
shown by a "skel-
eton " leaf, one
which has been so
treated that all
the green sub-
stance has disap-
peared, and only
the network of
veins remains. It
will be noticed
that in some
leaves the veins
and veinlets are
very prominent,
in others only
the main veins
are prominent,
while in some it
is hard to detect
any veins (see
Figs. 25, 26).

29. Significance
of leaf veins. — It
is clear that the
framework of veins is doing at least two things for the
blade: (1) it mechanically supports the spread out green sub-
stance ; and (2) it conducts material to and from the green
substance. So complete is the network of veins that this

Fig. 26. A leaf of hawthorn, showing a short petiole, and
a broad toothed blade with a conspicuous network of
veins. Note the relation between the veins and the
teeth. — After Strasburger.

FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 37

support and conduction are very perfect (see Fig. 27). It
is also clear that the green substance thus supported and
supplied with material is the important part of the leaf, the
part that demands the light-relation. Study the various
plans of the vein systems in Figs. 3, 9, 13, 18, 19, 20, 21,
25, 26, 51, 70, 76, 82, 83, 92, 161.

Fie. 27. A plant (Fittonia) whose leaves show a network of veins, and also an adjust-
ment to one another to form a mosaic.

30. Epidermis. — If a thick leaf be taken, such as that
of a hyacinth, it will be found possible to peel off from
its surface a delicate transparent skin (epidermis). This
epidermis completely covers the leaf, and generally shows
no green color. It is a protective covering, but at the same
time it must not completely shut off the green substance
beneath from the outside. It is found, therefore, that
three important parts of an ordinary foliage leaf are : (1)

38

PLANT RELATIONS.

Fig. 28. Cells of the epidermis
of Maranta, showing the
interlocking walls, and a
stoma (a) with its two guard-
cells.

a network of veins ; (2) a green substance (mesophyll) in

the meshes of the network ; and (3) over all an epidermis.
31. Stomata. — If a compound microscope is used, some

very important additional facts may be discovered. The

thin, transparent epidermis is
found to be made up of a layer of
cells which fit closely together,
sometimes dovetailing with each
other. Curious openings in the
epidermis will also be discovered,
sometimes in very great numbers.
Guarding each opening are two
crescent-shaped cells, known as
guard-cells, and between • them a
slit-like opening leads through the
epidermis. The whole apparatus
is known as a stoma (plural
stomata), which really means

" mouth/' of which the guard-cells might be called the

lips (see Figs. 28, 29). Sometimes stomata are found'only

on the under side of the leaf, sometimes

only on the upper side, and sometimes on

both sides.

One important fact about stomata is that

the guard-cells can change their shape, and

so regulate the size of the opening. It is not

certain just why the guard-cells change their

shape and just what stomata do for leaves.

They are often called " breathing pores," but

a better name would be air pores. Stomata

are not peculiar to the epidermis of foliage

leaves, for they are found in the epidermis

of any green part, as stems, young fruit,

etc. It is evident, therefore, that they hold

an important relation to green tissue which

is covered by epidermis. Also, if we examine

Fig. 29. A single
stoma from the
epidermis of a
lily leaf, show-
i n g the two
guard-cells full
of chlorophyll,
and the small
slit-like opening
between.

FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 39

foliage leaves and other green parts of plants which live
submerged in water, we find that the epidermis contains
no stomata. Therefore, stomata hold a definite relation
to green parts covered by epidermis only when this epider-
mis is exposed to the air.

It would seem that the stomata supply open passage-
ways for material from the green tissue through the epider-
mis to the air, or from the air to the green tissue, or both.
It will be remembered, however, that quite a number of
substances are taken into the leaf and given out from it,
so that it is hard to determine whether the stomata are
specially for any one of these movements. For instance,
the leaf gives out moisture in transpiration, oxygen in
photosynthesis, and carbon dioxide in respiration ; while it
takes in carbon dioxide in photosynthesis, and oxygen in
respiration. It is thought that stomata specially favor
transpiration, and that they also much facilitate the en-
trance of carbon dioxide.

32. Mesophyll. — If a cross-section be made of an ordi-
nary foliage leaf, such as that of a lily, the three leaf
regions can be seen in their proper relation to each other.
Bounding the section above and below is the layer of trans-
parent epidermal cells, pierced here and there by stomata,
marked by their peculiar guard-cells. Between the epi-
dermal layers is the green tissue, known as the mesophyll,
made up of cells which contain numerous small green
bodies which give color to the whole leaf, and are known as
chlorophyll bodies or chloroplasts.

The mesophyll cells are usually arranged differently in
the upper and lower regions of the leaf. In the upper
region the cells are elongated and stand upright, present-
ing their narrow ends to the upper leaf surface, forming
the palisade tissue. In the lower region the cells are irreg-
ular, and so loosely arranged as to leave passageways for air
between, forming the spongy tissue. The air spaces among
the cells communicate with one another, so that a system of

40

PLANT RELATIONS.

air chambers extends throughout the spongy mesophyll.
It is into this system of air chambers that the stomata
open, and so they are put into direct communication with
the mesophyll or working cells. The peculiar arrangement
of the upper mesophyll, to form the palisade tissue, has to
do with the fact that that surface of the leaf is exposed to
the direct rays of light. This light, so necessary to the
mesophyll, is also dangerous for at least two reasons. If

Fig. 30. A section through the leaf of lily, showing upper epidermis (ue), lower epi-
dermis (fe) with its stomata (st), mesophyll (dotted cells) composed of the palisade
region ^p) and the spongy region (sp) with airspaces among the cells, and two
veins (v) cut across.

the light is too intense it may destroy the chlorophyll, and
the heated air may dry out the cells. The narrow ends of
the cells present less exposure, and the depth of the cells
permits greater freedom of movement to the chloroplasts.

33. Veins. — In the cross-section of the leaf there will
also be seen here and there, embedded in the mesophyll,
the cut ends of the veinlets, made up partly of thick-
walled cells, which hold the leaf in shape and conduct
material to and from the mesophyll (see Fig. 30).

FOLIAGE LEAVES: FUNCTION, STRUCTUKE, ETC. 41

C. Leaf protection.

34. Need of protection. — Such an important organ as
the leaf, with its delicate active cells well displayed, is ex-
posed to numerous dangers. Chief among these dangers
are intense light, drought, and cold. All leaves are not
exposed to these dangers. For example, plants which grow
in the shade are not in danger from intense light ; many

water plants are not in danger
from drought ; and plants of
the tropical lowlands are in no

US. us.

Fig. 31. Sections through leaves of the same plant, showing the effect of exposure to
light upon the structure of the mesophyll. In both cases os indicates upper surface,
and us under surface. In the section at the left the growing leaf was exposed to
direct and intense sunlight, and, as a consequence, all of the mesophyll cells have
assumed the protected or palisade position. In the section at the right the leaf was
grown in the shade, and none of the mesophyll cells have organized in palisade
fashion. — After Stahl.

danger from cold. The danger from all these sources is be-
cause of the large surface with no great thickness of body,
and the protection against all of them is practically the
same. Most of the forms of protection can be reduced
to two general plans: (1) the development of protective
structures between the endangered mesophyll and the air ;
(2) the diminution of the exposed surface.

35. Protective structures. — The palisade arrangement of
mesophyll may be regarded as an adaptation for protection,

42

PLANT KELATIONS.

but it usually occurs, and does not necessarily imply ex-
treme conditions of any kind. However, palisade tissue of
unusually narrow and elongated cells, or forming two 01

Fia. 32. Section through a portion of the leaf of the yew (Taxus), showing cuticle
(c), epidermis (e), and the upper portion of the palisade cells (p).

three layers, indicates exposure to intense light or drouth,
and is very characteristic of alpine and desert plants. The
accompanying illustration (Fig. 31) shows in a striking
way the effect of light intensity upon the structure of the
mesophyll, by contrasting leaves of the same plant exposed
to the extreme conditions of light and shade.

The most usual structural adaptations, however, are
connected with the epidermis. The outer walls of the epi-
dermal cells may become thickened, sometimes excessively

so ; the other epidermal
walls may also become
more or less thickened ;
or even what seems to
be more than one epi-
dermal layer is found
protecting the meso-
phyll. If the outer

Fig. 33. Section through a portion of the leaf walls of the epidermal

of carnation, showing the heavy cuticle («/) 11 nn-n + ln-n a +/\

. , , . ,, -., ., .Cello U U II I 1 11 U c to

formed by the outer walls of the epidermal

cells (ep). Through the cuticle a passageway thicken, the outer re-

leads to the stoma, whose two guard-cells are p-jon of the thick Wall

seen lying between the two epidermal cells °

shown in the figure. Below the epidermal lOSeS its Structure

cells some of the palisade cells (pal) are shown o„J forms the CUticl&

containing chloroplasts, and below the stoma . , .

is seen the air chamber into which it opens. W n 1 C n IS One Ol tne

FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 43

best protective substances (see Fig.
32). Sometimes this cuticle be-
comes so thick that the passage-
ways through it leading down to
the stomata become regular canals
(see Fig. 33).

Another very common protective
structure upon leaves is to be found
in the great variety of hairs de-
veloped by the epidermis. These
may form but a slightly downy
covering, or the leaf may be cov-
ered by a woolly or felt-like mass
so that the epidermis is entirely
concealed. The common mullein
is a good illustration of a felt-
covered leaf (see Fig. 36). In cold
or dry regions the hairy covering
of leaves is very noticeable, often
giving Jiem a brilliant silky white or bronze look (see
Figs. 34, 35). Sometimes, instead of a hair-like cover-
ing, the epidermis develops scales of various patterns,
often overlapping, and forming an excellent protection
(see Fig. 37). In all these cases it should be remembered
that these hairs and scales may serve other purposes also,
and may even be of no use whatever to the plant.
36. Diminution

of exposed surface. —

It will be impossible
to give more than a
feAv illustrations of
this large subject.
In very dry regions
it has always been
noticed that the
leaves are small and

Fig. 34. A hair from the leaf
of Potentilla. It is seen
to grow out from the epi-
dermis.

Fig. 35. A section through the leaf of bush clover
! Lespedeza), showing upper and lower epidermis,
palisade cells, and cells of the spongy region.
The lower epidermis produces numerous hairs
which bend sharply and lie along the leaf surface
(appressed), forming a close covering.

44

PLANT RELATIONS.

Fig. 36. A branching hair from the leaf of common mullein. The illustration shows
the form, but not the many-celled structure of the hair.

comparatively thick, although they may be very numerous
(see Figs. 4, 172). In this way each leaf exposes a small

surface to the dry-
ing air and intense
sunlight. In our
southwestern dry
regions the cactus
abounds, plants
which have reduced
their leaves so much
that they are no
longer used for
chlorophyll work,
and are not usually
recognized as leaves.
In their stead the
globular or cylin-
drical or flattened
stems are green and

Fig. 37. A scale from the leaf of Shepherdia. TheBe

scales overlap and form a complete covering. CIO leal WOl'K (rigs.

Pig. 39. A group of cactus forms (slender cylindrical, columnar,
and globular), all of them spiny and without leaves ; an agave in
front ; clusters of yucca flowers in the background.

FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 47

38, 39, 40, 185, 18G, 187, 188). In the same regions the
agaves and yuccas retain their leaves, but they become so
thick that they serve as water reservoirs (see Figs. 38, 39,

Fig. 40. A globular cactus, showing the ribbed stem, the strong spines, and the entire
absence of leaves.

189). In all these cases this reduced surface is supple-
mented by palisade tissue, very thick epidermal Avails, and
an abundant cuticle.

37. Rosette arrangement. — The rosette arrangement of
leaves is a very common method of protection used by

48

PLANT RELATIONS.

small plants growing in exposed situations, as bare rocks
and sandy ground. The cluster of leaves, flat upon the
ground, or nearly so, and more or less overlapping, is very
effectively arranged for resisting intense light or drought
or cold (see Figs. 11, 12, 48).

38. Protective positions. — In other cases, a position is
assumed by the leaves which directs their flat
surfaces so that they are not exposed to the
most intense rays of light. The so-called " com-

Pig. 41. A leaf of a sensitive plant in two conditions. In the figure to the left the
leaf is fully expanded, with its four main divisions and numerous leaflets well
spread. In the figure to the right is shown the same leaf after it has been
"shocked" by a sudden touch, or by sudden heat, or in some other way. The
leaflets have been thrown together forward and upward ; the four main divisions
have been moved together ; and the main leaf-stalk has been directed sharply
downward. The whole change has very much reduced the surface of exposure. —
After Duchartbe.

pass plants/' already mentioned, are illustrations of this,
the leaves standing edgewise and receiving on their surface
the less intense rays of light (see Figs. 5, 170). In the
dry regions of Australia the leaves on many of the forest
trees and shrubs have this characteristic edgewise position,
known as the profile position, giving to the foliage a very
curious appearance.

Some leaves have the power of shifting their position
according to their needs, directing their flat surfaces to-
ward the light, or more or less inclining them, according

Fie. 42. The telegraph -p\mX(I)mnodium gyrans). Each leaf is made up of three
leaflets, a large terminal one, and a pair of small lateral ones. In the lowest figure
the large leaflets are spread out in their day position ; in the central figure they are
turned sharply downward in their night position. The name of the plant refers to
the peculiar and constant motion of the pair of lateral leaflets, each one of which
describes a curve with a jerking motion, like the second-hand of a watch, as
indicated in the uppermost figure.

50

PLANT RELATIONS.

to the danger. Perhaps the most completely adapted
leaves of this kind are those of the "sensitive plants/'
whose leaves respond to various external influences by
changing their positions. The common sensitive plant
abounds in dry regions, and may be taken as a type of
such plants (see Figs. 4, 41, 171). The leaves are divided
into very numerous small leaflets, sometimes very small,
which stretch in pairs along the leaf branches. When
drought approaches, some of the pairs of leaflets fold to-
gether, slightly reduc-
ing the surface expo-
sure. As the drought
continues, more leaflets
fold together, then still
others, until finally all
the leaflets may be
folded together, and the
leaves themselves may
bend against the stem.
It is like a sailing vessel
gradually taking in sail
as a storm approaches, until finally nothing is exposed,
and the vessel weathers the storm by presenting only bare
poles. Sensitive plants can thus regulate the exposed sur-
face very exactly to the need.

Such motile leaves not only behave in this manner at the
coming of drought, but the positions of the leaflets are
shifted throughout the day in reference to light, and at
night a very characteristic position is assumed (see Figs. 2,
3, 42), once called a " sleeping position." One danger from
night exposure may come from the radiation of heat which
might chill the leaves too much ; but the night position
may have no such meaning. The leaflets of Oxalis have
been referred to (see §14). Similar changes in the direc-
tion of the leaf planes at the coming of night may be
observed in most of the Leguminosce, even the common

Fio. 43. Cotyledons of squash seedling, show-
ing positions in light (left figure) and in
darkness (right figure).— After Atkinson.

FOLIAGE LEAVES : FUNCTION, STRUCTURE, ETC. 51

white clover displaying it. It can be observed that the
expanded seed leaves {cotyledons) of many young germinat-
ing plants shift their positions at night (see .Fig. 43), often
assuming a vertical position which brings them in contact
with one another, and also covers the stem bud (plumule).

Certain leaves with well-developed
protective structures are able to en-
dure the winter, as in the case of
the so-called evergreens. In the
case of juniper, however, the winter
and summer positions of the leaves
are quite different (see Fig. 44). In
the winter the leaves lie close against
the stem and overlap one another;
while with the coming of summer
conditions they become widely
spreading.

39. Protection against rain. — It is
also necessary for leaves to avoid
becoming wet by rain. If the water
is allowed to soak in there is danger
of filling the stomata and interfering
with the air exchanges. Hence it
will be noticed that most leaves are
able to shed water, partly by their
positions, partly by their structure.
In many plants the leaves are so ar-
ranged that the water runs off towards the stem and so
reaches the main root system ; in other plants the rain is
shed outwards, as from the eaves of a house.

Some of the structures which prevent the rain from
soaking in are a smooth epidermis, a cuticle layer, waxy
secretions, felt-like coverings, etc. Interesting experi-
ments may be performed with different leaves to test their
power of shedding water. If a gentle spray of water is
allowed to -play upon different plants, it will be observed

Fig. 44. Two twigs of juni
per, showing the ordi.
nary summer and winter
positions assumed by the
leaves. The ordinary pro-
tected winter position of
the leaves is shown by A:
while in B, in response tn
summer conditions, the
leaves have spread apart
and have become freelyex*
.—After Warming.

52 PLANT RELATIONS.

that the water glances off at once from the surfaces of
some leaves, runs off more slowly from others, and may be
more or less retained by others.

In this same connection it should be noticed that in
most horizontal leaves the two surfaces differ more or less
in appearance, the upper usually being smoother than the
lower, and the stomata occurring in larger numbers, some-
times exclusively, upon the under surface. While these
differences doubtless have a more important meaning than
protection against wetting, they are also suggestive in this
connection.

CHAPTER IV.

SHOOTS.

40. General characters. — The term shoot is used to include
both stem and leaves. Among the lower plants, such as
the algse and toadstools, there is no distinct stem and leaf.
In such plants the working body is spoken of as the thallus,
which does the work done by both stem and leaf in the
higher plants. These two kinds of work are separated in
the higher plants, and the shoot is differentiated into stem
and leaves.

41. Life-relation. — In seeking to discover the essential
life-relation of the stem, it is evident that it is not neces-
sarily a light-relation, as in the case of the foliage leaf,
for many stems are subterranean. Also, in general, the
stem is not an expanded organ, as is the ordinary foli-
age leaf. This indicates that whatever may be its essential
life-relation it has little to do with exposure of surface.
It becomes plain that the stem is the great leaf-bearing
organ, and that its life-relation is a leaf-relation. Often
stems branch, and this increases their power of producing
leaves.

In classifying stems, therefore, it seems natural to use
the kind of leaves they bear. From this standpoint there
are three prominent kinds of stems : (1) those bearing foli-
age leaves ; (2) those bearing scale leaves ; and (3) those
bearing floral leaves. There are some peculiar forms of
stems which do not bear leaves of any kind, but they need
not be included in this general view.

54 PLANT RELATIONS.

A. Stems bearing foliage leaves.

42. General character. — As the purpose of this stem is to
display foliage leaves, and as it has been discovered that the
essential life-relation of foliage leaves is the light-relation,
it follows that a stem of this type must be able to relate its
leaves to light. It is, therefore, commonly aerial, and that
it may properly display the leaves it is generally elongated,
with its joints {nodes) bearing the leaves well separated (see
Figs. 1, 4, 18, 20).

The foliage-bearing stem is generally the most conspicu-
ous part of the plant and gives style to the whole body.
One's impression of the forms of most plants is obtained
from the foliage-bearing stems. Such stems have great
range in size and length of life, from minute size and very
short life to huge trees which may endure for centuries.
Branching is also quite a feature of foliage-bearing stems ;
and when it occurs it is evident that the power of display-
ing foliage is correspondingly increased. Certain promi-
nent types of foliage-bearing stems may be considered.

43. The subterranean type. — It may seem strange to in-
clude any subterranean stem with those that bear foliage,
as such a stem seems to be away from any light-relation.
Ordinarily subterranean stems send foliage-bearing branches
above the surface, and such stems are not to be classed as
foliage-bearing stems. But often the only stem possessed
by the plant is subterranean, and no branches are sent to
the surface. In such cases only foliage leaves appear above
ground, and they come directly from the subterranean stem.
The ordinary ferns furnish a conspicuous illustration of
this habit, all that is seen of them above ground being the
characteristic leaves, the commonly called " stem " being
only the petiole of the leaf (see Figs. 45, 46, 144). Many
seed plants can also be found which show the same habit,
especially those which flower early in the spring. This
cannot be regarded as a very favorable type of stem for

Fig. 45. A fern (Agpidium), showing three large branching leaves coming from a hori-
zontal subterranean stem (rootstock) : growing leaves are also shown, which are
gradually unrolling. The stem, young leaves, and petioles of the large leaves are
thickly covered with protecting hairs. The stem gives rise to numerous small roots
from its lower surface. The figure marked 3 represents the under surface of a
portion of the leaf, showing seven groups of spore cases ; at 5 is represented a
section through one of these groups, showing how the spore cases are attached and
protected by a flap ; while at 6 is represented a single spore case opening and dis-
charging its spores, the heavy spring-like ring extending along the back and over
the top.— After Wossidlo.

56 PLANT RELATIONS.

leaf display, and as a rule such stems do not produce
many foliage leaves, but the leaves are apt to be large.

Fig. 46. A common fern, showing the underground stem (rootstock), which sends the
few large foliage leaves above the surface.— After Atkinson.

The subterranean position is a good one, however, for
purposes of protection against cold or drought, and when
the foliage leaves are killed new ones can be put out by

SHOOTS.

57

the protected stem. This position is also taken advantage
of for comparatively safe food storage, and such stems are
apt to become more or less thickened and distorted by this
food deposit.

44. The procumbent type. — In this case the main body
of the stem lies more or less prostrate, although the advanc-
ing tip is usually erect. Such stems may spread in all
directions, and become interwoven into
a mat or carpet. They are found
especially on sterile and exposed soil,

Fig. 47. A strawberry plant, showing a runner which has devel-
oped a new plant, which in turn has sent out another run-
ner.— After Seubert.

and there may be an important relation between this fact and
their habit, as there may not be sufficient building material
for erect stems, and the erect position might result in too
much exposure to light, or heat, or wind, etc. Whatever
may be the cause of the procumbent habit, it has its advan-
tages. As compared with the erect stem, there is economy
of building material, for the rigid structures to enable it to
stand upright are not necessary. On the other hand, such
a stem loses in its power to display leaves. Instead of
being free to put out its leaves in every direction, one side
is against the ground, and the space for leaves is diminished
at least one-half. All the leaves it bears are necessarily
directed towards the free side (see Fig. 18).

We may be sure, however, that any disadvantage com-
ing from this unfavorable position for leaf display is over-
balanced by advantages in other respects. The position is

58

PLANT RELATIONS.

certainly one of protection, and it has a further advantage
in the way of migration and vegetative propagation. As
the stem advances over the ground, roots strike out of the
nodes into the soil. In this way fresh anchorage and new
soil supplies are secured ; the old parts of the stem may

i. 48. Two plants of a saxifrage, showing rosette habit, and also the numerous
runners sent out from the base, which strike root at tip and produce new plants.
— After Kerner.

die, but the newer portions have their soil connection and
continue to live. 80 effective is this habit for this kind of
propagation that plants with erect stems often make use of
it, sending out from near the base special prostrate branches,
which advance over the ground and form new plants.
A very familiar illustration is furnished by the straw-
berry plant, which sends out peculiar naked "runners"
to strike root and form new plants, which then become

SHOOTS.

59

independent plants by the dying of the runners (see Figs.
47, 48).

45. The floating type. — In this case the stems are sus-
tained by water. Numerous illustrations can be found in
small inland lakes and slow-moving streams (see Fig. 49).
Beneath the water these stems often seem quite erect, but

Fig. 49. A submerged plant {Ceratophyllum) with floating stems, showing
joints bearing finely divided leaves.

the stem

when taken out they collapse, lacking the buoyant powei
of the water. Growing free and more or less upright in
the water, they seem to have all the freedom of erect sterna
in displaying foliage leaves, and at the same time they
are not called upon to build rigid structures. Economy
of building material and entire freedom to display foliage
would seem to be a happy combination for plants. It must
be noticed, however, that another very important condition
is introduced. To reach the leaf surfaces the light must
pass through the water, and this diminishes its intensity so

60

PLANT RELATIONS.

greatly that the working power of the leaves is reduced.
At no very great depth of water a limit is reached, beyond
which the light is no longer able to be of service to the
leaves in their work. Hence it is that water plants are

restricted to the surface of the
water, or to shoal places ; and in
such places vegetation is very
abundant. Water is so serious
an impediment to light that very
many plants bring their working
leaves to the surface and float
them, as seen in water lilies, thus
obtaining light of undiminished
intensity.

46. The climbing type. — Climb-
ing stems are developed especially
in the tropics, where the vegeta-
tion is so dense and overshadow-
ing that many stems have learned
to climb upon the bodies of other
plants, and so spread their leaves
in better light (see Figs. 50, 55,
98, 212). Great woody vines
fairly interlace the vegetation of
tropical forests, and are known
as "lianas/' or "lianes." The
same habit is noticeable, also, in
our temperate vegetation, but it
is by no means bo extensively dis-
played as in the tropics. There
are a good many forms of climb-
ing stems. Remembering that
the habit refers to one stem de-

Fio. 50. A vine or liana climbing

the trunk of a tree. The leaves pending upon another IOT

are ail adjusted to face the light mechanical support, we may in-

and to avoid shading one an- ,

other as far as possible. elude many hedge plants m the

SHOOTS. 61

list of climbers. In this case the stems are too weak to
stand alone, but by interlacing with one another they may
keep an upright position. There are stems, also, which
climb by twining about their support, as the hop vine and

Fig. 51. A cluster of smilax, showing the tendrils which enable it to climb, and also
the prickles. — After Kerner.

morning glory ; others which put out tendrils to grasp the
support (see Figs. 51, 52), as the grapevine and star
cucumber ; and still others which climb by sending out
suckers to act as holdfasts, as the woodbine (see Figs. 53,
54). In all these cases there is an attempt to reach towards

62 PLANT RELATIONS.

the light without developing such structures in the stem
as would enable it to stand upright.

47. The erect type.— This type seems altogether the
best adapted for the proper display of foliage leaves. Leaves

Fig. 52. Passion-flower vines climbing supports by means of tendrils, which may be
seen more or less extended or coiled. The two types of leaves upon a single stem
may also be noted.

can be sent out in all directions and carried upward to-
wards the light ; but it is at the expense of developing an
elaborate mechanical system to enable the stem to retain
this position. There is an interesting relation between
these erect bodies and zones of temperature. At high alti-

SHOOTS.

63

Fig. 53. Woodbine (Ampelopsis) in a deciduous forest. The tree trunks are almost
covered by the dense masses of woodbine, whose leaves are adjusted so as to form
compact mosaics. A lower stratum of vegetation is visible, composed of shrubs
and tall herbs, showing that the forest is somewhat open. — After ScHistrER.

tudes or latitudes the subter-
ranean and prostrate types of
foliage-bearing stems are most
common ; and as one passes to
lower altitudes or latitudes the
erect stems become more nu-
merous and more lofty. Among
stems of the erect type the tree
is the most impressive, and it
has developed into a great vari-
ety of forms or "habits." Any
one recognizes the great differ-
ence in the habits of the pine
and the elm (see Figs. 56,
57, 58, 59), and many of our

Fig. 54. A portion of a woodbine
{Ampelopsis). The stem tendrils
have attached themselves to a
smooth wall by means of disk-like
suckers. — After Strasburger.

Fig. 55. A liana in the Botanic Garden at Peradenyia, Ceylon. — After Schihper.

Fig. 5$. A tree of the pine type (larch), showing the continuous central shaft and
the horizontal branches, which tend to become more upright towards the top of
the tree. The general outline is distinctly conical. The larch is peculiar among
such trees in periodically shedding its leaves

Fig. 57. A pine tree, showing the central shaft and also the bunching of the
needle leaves toward the tips of the branches where there is the best exposure
to light.

SHOOTS.

67

common trees may be known, even at a distance, by their
characteristic habits (see Figs. 60, 01, G2). The difficulty
of the mechanical problems solved by these huge bodies
is very great. They maintain form and position and en-
dure tremendous pressure and strain.

Fig. 58. An elm in its winter condition, showing the absence of a continuous central
shaft, the main stem soon breaking up into branches, and giving a spreading top
On each side in the background are trees of the pine type, showing the central
shaft and conical outline.

68 PLANT RELATIONS.

48. Relation to light. — As stems bearing foliage leaves
hold a special relation to light, it is necessary to speak of
the influence of light upon their direction, the response to

Fig. 59. An elm in foliage, showing the breaking up of the trunk into branches and
the spreading top.

which is known as heliotropism, already referred to under
foliage leaves. In the case of an erect stem the tendency
is to grow towards the source of light (see Figs. 1, 64).

SHOOTS. 69

This has the general result of placing the leaf blades at
right angles to the rays of light, and in this respect the
heliotropism of the stem aids in securing a favorable leaf
position (see Figs. 63, 63a). Prostrate stems are differently-
affected by the light, however, being directed transversely
to the rays of light. The same is true of many foliage

■

F ■■ .Ayv v ^i^Sj~i^=^

\kir

1

?WBfcZ * <awK S^r^ y j^ if t'<?5C^*>^S'^

"^^^^^X^ki

M

Fig. 60. An oak in its winter condition, showing the wide branching. The various
directions of the branches have been determined by the light-relations.

branches, as may be seen by observing almost any tree in
which the lower branches are in the general transverse posi-
tion. These branches generally tend to turn upwards when
they are beyond the region of shading. Subterranean stems
are also mostly horizontal, but they are out of the influence
of light, and under the influence of gravity, the response to
which is known as geotropism, which guides them into the
transverse position. The climbing stem, like the erect one,

70

PLANT RELATIONS.

Fig. 61. Cottonwoods, in winter condition, on a sand dune, showing the branching
habit, and the tendency to grow in groups.

grows towards the light, while floating stems may be
either erect or transverse.

B. Stems hearing scale leaves.

49. General character. — A scale leaf is one which does
not serve as foliage, as it does not develop the necessary
chlorophyll. This means that it does not need such an
exposure of surface, and hence scale leaves are usually much
smaller, and certainly are more inconspicuous than foliage
leaves. A good illustration of scale leaves is furnished by
the ordinary scaly buds of trees, in which the covering of
overlapping scaly leaves is very conspicuous (see Fig. G5).
As there is no development of chlorophyll in such leaves,

SHOOTS.

71

they do not need to be exposed to the light. Stems bearing
only scale leaves, therefore, hold no necessary light-relation,
and may be subterranean as well as aerial. For the same

h'lo. 62. A group of weeping birches, showing the branching habit and the peculiar
hanging branchlets. The trunks also show the habit of birch bark in peeling off
in bands around the stem.

reason scale leaves do not need to be separated from one
another, but may overlap, as in the buds referred to.

Sometimes scale leaves occur so intermixed with foliage

Fig. 63.

Sunflowers with the upper part of the stem eharpJy bent towards the light,
giving the leaves better exposure.— After Sch affner.

SHOOTS.

73

leaves that no peculiar stem type is developed. In the
pines scale leaves are found abundantly on the stems which
are developed for foliage purposes. In fact, the main stem
axes of pines bear only scale leaves, while short spur-like
branches bear the characteristic needles, or foliage leaves,
but the form of the
stem is controlled
by the needs of the
foliage. Some very
distinct types of
scale-bearing stems
may be noted.

50. The bud type.
— In this case the
nodes bearing the
leaves remain close
together, not sepa-
rating, as is neces-
sary in ordinary
foliage-bearing
stems, and the
leaves overlap. In
a stem of this char-
acter the later joints
may become sepa-
rated and bear foli-
age leaves, so that
one finds scale leaves
the same stem axis.

Fig. 63a. Cotyledons of castor-oil bean ; the seedling
to the left showing the ordinary position of the
cotyledons, the one to the right showing the curva-
ture of the stem in response to light from one
side. — After Atkinson.

below and foliage leaves above on
This is always true in the case of
branch buds, in which the scale leaves serve the purpose
of protection, and are aerial, not because they need a
light-relation, but because they are protecting young foli-
age leaves which do.

Sometimes the scale leaves of this bud type of stem do
not serve so much for protection as for food storage, and
become fleshy. Ordinary bulbs, such as those of lilies, etc.,

74

PLANT RELATIONS.

Fig. 64. An araucarian pine, showing the
central shaft, and the regular clusters of
branches spreading in every direction and
bearing numerous small leaves. The low-
ermost branches extend downwards and
are the largest, while those above become
more horizontal and smaller. These dif-
ferences in the size and direction of the
branches secure the largest light expo-
sure.

are of this character ;
and as the main pur-
pose is food storage
the most favorable
position is a subter-
ranean one (see Fig.
66). Sometimes such
scale leaves become
very broad and not
merely overlap but en-
wrap one another, as
in the case of the
onion.

51. The tuber type.
— The ordinary potato
may be taken as an il-
lustration (see Fig.
07). The minute scale
leaves, to be found at
the "eyes" of the
potato, do not overlap,
which means that the
stem joints are farther
apart than in the bud
type. The whole form
of the stem results
from its use as a place
of food storage, and
hence such stems are
generally subterra-
nean. Food storage,
subterranean position,
and reduced scale
leaves are facts which
seem to follow each
other naturally.

SHOOTS.

75

52. The rootstock type. — This is prob-
ably the most common form of subter-
ranean stem. It is elongated, as are foli-
age stems, and hence the scale leaves
are well separated. It is prominently
used for food storage, and is also admirably
adapted for subterranean migration (see
Fig. 68). It can do for the plant, in the
way of migration, what prostrate foliage-
bearing stems do, and is in a more protected
position. Advancing beneath the ground,
it sends up a succession of branches
to the surface. It is a very efficient
method for the ''spreading" of plants,
and is extensively used by grasses in cov-
ering areas and forming turf. The persist-
ent continuance of the worst weeds is often
due to this habit (see Figs. 69, 70). It

is impossible

Fig. 65. Branch buds
of elm. Three buds
(£) with their over-
lapping scales are
shown, each just
above the scar (b)
of an old leaf. —
After Behrens.

Fig. 66. A bulb, made up of overlap-
ping scales, which are fleshy on
account of food storage. — After
Gray.

to remove

all of the

indefinitely

branching

rootstocks

from the soil,

and any fragments that remain

are able to send up fresh crops

of aerial branches.

53. Alternation of rest and
activity. — In all of the three
stem types just mentioned, it
is important to note that they
are associated with a remark-
able alternation between rest
and vigorous activity. From
the branch buds the new leaves

76

PLANT RELATIONS.

emerge with
great rapidity,
and trees be-
come covered
with new foliage
in a few days.
From the sub-
terranean stems
the aerial parts
come up so
speedily that the
surface of the
ground seems to
be covered suddenly with young vegetation. This sudden
change from comparative rest to great activity has been
well spok*en of as the " awakening " of vegetation.

Fig. 67. A potato plant, showing the subterranean tubers.—
After Strasbubger.

C. Stems bearing floral leaves.

54. The flower. — The so-called
"flowers" which certain plants
produce represent another type of
shoot, being stems with peculiar
leaves. So attractive are flowers
that they have been very much
studied ; and this fact has led
many people to believe that flowers
are the only parts of plants worth
studying. Aside from the fact
that a great many plants do not
produce flowers, even in those
that do the flowers are connected
with only one of the plant pro-
cesses, that of reproduction.
Every one knows that flowers are
exceedingly variable, and names

Fig. 68. The rootstock of Solo-
mon's seal ; from the under side
roots are developed ; and on the
upper side are seen the scars
which mark the positions of the
successive aerial branches which
bear the leaves. The advanc-
ing tip is protected by scales
(forming a bud), and the posi-
tions of previous buds are in-
dicated by groups of ring-like
scars which mark the attach-
ment of former scales. Advanc-
ing in front and dying behind
such a rootetock may give rise
to an indefinite succession of
aerial plants.— After Gray.

SHOOTS.

77

have been given to every kind of variation, so that their
study is often not much more than learning the definitions
of names. However, if we seek to discover the life-rela-
tions of flowers we find that they may be stated very simply.
55. Life-relations. — The flower is to produce seed. It
must not only put itself into proper relation to do this, but

Fig. 69. The rootstock of a rash (Juncus), showing how it advances beneath the
ground and sends above the surface a succession of branches. The breaking up
of such a rootstock only results in so many separate individuals.— After Cowles.

there must also be some arrangement for putting the seeds
into proper conditions for developing new plants. In the
production of seed it is necessary for the flower to secure a
transfer of certain yellowish, powdery bodies which it pro-
duces, known as pollen or pollen-grains, to the organ in
which the seeds are produced, known as the pistil. This
transfer is called pollination. One of the important things,
therefore, in connection with the flower, is for it to put

78

PLANT RELATIONS.

Fig. 70. An alpine willow, showing a strong rootstock developing aerial branches
and roots, and capable of long life and extensive migration.— After Schimper.

itself into such relations that it may secure pollination.

Besides pollination,
which is necessary
to the production of
seeds, there must be
an arrangement for
seed dispersal. It
is always well for
seeds to be scattered,
so as to be separated
from one another
and from the parent
plant. The two
great external prob-

Fig. 71. A flower of peony, showing the four sets of ° t x m

floral organs : k, the sepals, together called the lems ill Connection

calyx ; c, the petals, together called the corolla ; with the flower
a, the numerous stamens; g, the two carpels, .

which contain the ovules.— After Strasburger. therefore, are polll-

SHOOTS.

79

nation and seed-dispersal. It
is necessary to call attention
to certain peculiar features of
this type of stem.

56. Structures. — The joints
of the stem do not spread
apart, so that the peculiar
leaves are kept close together,
usually forming a rosette-like
cluster (see Fig. 71). These
leaves are of four kinds : the
lowest (outermost) ones (indi-
vidually sepals, collectively
calyx) mostly resemble small
foliage leaves ; the next higher
(inner) set (individually petals,
collectively corolla) are usually
the most conspicuous, delicate
in texture and brightly col-
ored ; the third set (stamens)
produces the pollen ; the
highest (innermost) set (car-
pels) form the pistil and pro-
duce the ovules, which are to
become seeds. These four sets
may not all be present in the
same flower ; the members of
the same set may be more or
less blended with one another,
forming tubes, urns, etc. (see
Figs. 72, 73, 74) ; or the dif-
ferent members may be modi-
fied in the greatest variety of
ways.

Another peculiarity of this
type of stem is that when the

Fig. 72. A group of flowers of the rose
family. The one at the top (Polen-
tilla) shows three broad sepals,
much smaller petals alternating
with them, a group of stamens, and
a large receptacle bearing numer-
ous small carpels. The central one
{Alchemilla) shows the tips of two
small sepals, three larger petals
united below, stamens arising from
the rim of the urn, and a single pe-
culiar pistil. The lowest flower (the
common apple) shows the sepals,
petals, stamens, and three styles,
all arising from the ovary part of
the pistil.— After Focke.

80

PLANT RELATIONS.

Pit;. 73. A flower of the tobacco plant : a, a complete flower, showing the calyx with
its sepals blended below, the funnelforni corolla made up of united petals, and the
stamens just showing at the mouth of the corolla tube ; b, acorolla tube split open
and showing the five stamens attached to it near the base ; c, a pistil made up of
two blended carpels, the bulbous base (containing the ovules) being the ovary, the
long stalk-like portion the style, and the knob at the top the stigma.— After
Strasburger.

last set of floral leaves {carpels) appear, the growth of the
stem in length is checked and the cluster of floral leaves

n b c d e

Pig. 7i. A group of flower forms: a, a flower of harebell, showing a bell-shaped
corolla composed of five petals ; b, a flower of phlox, showing a tubular corolla
with its five petals distinct above and sharply spreading ; c, a flower of dead-nettle,
showing an irregular corolla with its five petals forming two lips above the funnel-
form base ; d, a flower of toad-flax, showing a two-lipped corolla, and also a spur
formed by the base of the corolla ; e, a flower of the snapdragon, showing the two
lips of the corolla closed.— After Gray.

SHOOTS. 81

appears to be upon the end of the stem axis. It is usual,
also, for the short stem bearing the floral leaves to broaden

Fig. 75. The Star-of -Bethlehem (Ornilhogalum), showing the loose cluster of flowers
at the end of the stem. The leaves and stem arise from a bulb, which produces a
cluster of roots below.— After Strasburgek.

at the apex and form what is called a receptacle, upon which
the close set floral leaves stand.

Although many floral stems are produced singly, it is

82 PLANT RELATIONS.

very common for them to branch, so that the flowers appear
in clusters, sometimes loose and spray-like, sometimes com-
pact (see Figs. 75, 76, 77). For example, the common

Fig. 76. A flower cluster from a walnut tree.— After Strasburger.

dandelion "flower" is really a compact head of flowers.
All of this branching has in view better arrangements for
pollination or for seed-distribution, or for both.

The subject of pollination and seed-distribution will be
considered under the head of reproduction.

SHOOTS.

83

STRUCTURE AND FUNCTION" OF THE STEM.

57. Stem structure. — The aerial foliage stem is the most
favorable for studying stem structure, as it is not distorted
by its position or by being a depository for food. If an
active twig of an ordinary woody plant be cut across, it will

Fig.

Flower clusters of an umbellifer (Sium).— After Strasburger.

be seen that it is made up of four general regions (see Fig.
78): (1) an outer protecting layer, which may be stripped
off as a thin skin, the epidermis ; (2) within the epider-
mis a zone, generally green, the cortex ; (3) an inner zone
of wood or vessels, known as the vascular region ; (4) a
central pith.

58. Dicotyledons and Conifers. — Sometimes the vessels
7

84

PLANT RELATIONS.

Fig. 78. Section across a young twig of box
elder, showing the four stem regions : e,
epidermis, represented by the heavy bound-
ing line ; c, cortex ; w, vascular cylinder ;
p, pith.

are arranged in a hollow
cylinder, just inside of
the cortex, leaving what
is called pith in the
center (see Fig. 78).
Sometimes the pith dis-
appears in older stems or
parts of stems and leaves
the stem hollow. When
the vessels are arranged
in this way and the stem
lives more than a year, it
can increase in diameter
by adding new vessels
outside of the old. In

the case of trees these additions appear in cross-section like

a series of concentric rings, and as there is usually but one

growth period during the year, they are often called annual

rings (see Fig. 79), and the age of a tree is often estimated

by counting them.

This method of ascer-
taining the age of a

tree is not absolutely

certain, as there may

be more than one

growth period in some

years. In the case of

trees and shrubs the

epidermis is replaced

on the older parts by

layers of cork, which

sometimes becomes

verv thick and makes FlG- 79# Section across a twig of box eider three

years old, showing three annual rings, or growth

Up tne Outer part 01 rings, in the vascular cylinder. The radiating

what is COmmonlv lines (»») which cross the vascular region (w) rep.

, . . , . resent the pith rays, the principal ones extending

Called bark. from the pith to the cortex (c).

SHOOTS.

85

Stems which increase in diameter mostly belong to the
great groups called Dicotyledons and Conifers. To the
former belong most of our common trees, such as maple,
oak, beech, hickory, etc. (see Figs. 58, 59, 60, 61), as
well as the great majority of common herbs; to the latter
belong the pines, hemlocks, etc. (see Figs. 56, 57, 198
to 201). This annual increase in diameter enables the
tree to put out an increased number of branches and
hence foliage leaves each year, so
that its capacity for leaf work be-
comes greater year after year. A
reason for this is that the stem is
conducting important food sup-
plies to the leaves, and if it in-
creases in diameter it can conduct
more supplies each year and give
work to more leaves.

59. Monocotyledons. — In other
stems, however, the vessels are
arranged differently in the central
region. Instead of forming a hol-
low cylinder enclosing a pith, they
are scattered through the central
region, as may be seen in the cross-
section of a corn-stalk (see Fig.

80). Such stems belong mostly to a great group of plants
known as Monocotyledons, to which belong palms, grasses,
lilies, etc. For the most part such stems do not increase in
diameter, hence there is no branching and no increased
foliage from year to year. A palm well illustrates this
habit, with its columnar, unbranching trunk, and its crown
of foliage leaves, which are about the same in number from
year to year (see Figs. 81, 82).

60. Ferns. — The same is true of the stems of most fern-
plants, as the vessels of the central region are so arranged
that there can be no diameter increase, though the ar-

Fig. 80. A corn-stalk, showing
cross-section and longitudinal
section. The dots represent
the scattered bundles of ves-
sels, which in the longitudinal
section are seen to be long
fiber-like strands.

Fig. 81. A date palm, showing the unbranched columnar trunk covered with old leaf
bases, and with a cluster of huge active leaves at the top, only the lowest portions
of which are shown. Two of the very heavy fruit clusters are also shown.

SHOOTS.

87

rangement is very different from that found in Monocotyle-
dons. It will be noticed Iioav similar in general appearance
is the habit of the tree fern and that of the palm (see Fig.
83).

61. Lower plants. — In the cuse of moss-plants, and such
algse and fungi as develop stems, the stems are very much

Fig. 82. A palm of the palmetto type (fan palm), with low stem and a crown of large

leaves.

simpler in construction, but they serve the same general
purpose.

62. Conduction by the stem. — Aside from the work of
producing leaves and furnishing mechanical support, the
stem is a great conducting region of the plant. This sub-
ject will be considered in Chapter X., under the general
head of "The Nutrition of Plants."

Pig. 83. A group of tropical plants. To the left of the center is a tree fern, with its
Blender columnar stem and crown of large leaves. The large-leaved plants to the
right are bananas (monocotyledons).

CHAPTER V.

ROOTS.

63. General character. — The root is a third prominent
plant organ, and it presents even a greater variety of rela-
tions than leaf or stem. In whatever relation it is found
it is either an absorbent organ or a holdfast, and very often
both. For such work no light-relation is necessary, as in
the case of foliage leaves ; and there is no leaf-relation, as
in the case of stems. Roots related to the soil may be
taken as an illustration.

It is evident that a soil root anchors the plant in the
soil, and also absorbs water from the soil. If absorption is
considered, it is further evident that the amount of it will
depend in some measure upon the amount of surface which
the roots expose to the soil. We have already noticed that
the foliage leaf has the same problem of exposure, and it
solves it by becoming an expanded organ. The question
may be fairly asked, therefore, why are not roots expanded
organs ? The receiving of rays of light, and the absorbing
of water are very different in their demands. In the former
case a flat surface is demanded, in the latter tubular pro-
cesses. The increase of surface in the root, therefore, is
obtained not by expanding the organ, but by multiplying
it. Besides, to obtain the soil water the roots must burrow
in every direction, and must send out their delicate thread-
like branches to come in contact with as much soil as pos-
sible. Furthermore, in soil roots absorption is not the only
thing to consider, for the roots act as holdfasts and must
grapple the soil. This is certainly done far more effectively

90

PLANT RELATIONS.

by numerous thread-like processes spreading in every direc-
tion than by flat, expanded processes.

It should also be noted that as soil roots are subterra-
nean they are used often for the storage of food, as in the
case of many subterranean stems. Certain prominent root
types may be noted as follows :

64. Soil roots. — These roots push into the ground with

great energy,
and their ab-
sorbing sur-
faces are en-
tirely covered.
Only the young-
est parts of a
root system
absorb actively,
the older parts
transporting
the absorbed
material to the
stem, and help-
ing to grip the
soil. The soil
root is the most
common root
type, being
used by the great majority of seed plants and fern plants,
and among the moss plants the very simple root-like pro-
cesses are mostly soil-related. To such roots the water of
the soil presents itself either as free water — that is, water
that can be drained away — or as films of water adhering to
each soil particle, often called water of adhesion. To come
in contact with this water, not only does the root system
usually branch profusely in every direction, but the youngest
branches develop abundant absorbing hairs, or root hairs
(see Fig. 84), which crowd in among the soil particles and

Fig. 84. Root tips of corn, showing root hairs and their
position in reference to the growing tip : 1, in soil (higher
up the hair6 become much more abundant and longer) ;
2, in moist air.

KOOTS.

91

absorb moisture from them.

Fig. 85. Apparatus to show the influence
of water (hydrotropism) upon the direc-
tion of roots. The ends (a) of the box
have hooks for hanging, while the box
proper is a cylinder or trough of wire
netting and is filled with damp sawdust.
In the sawdust are planted peas (g),
whose roots (h. i, fc, m) first descend until
they emerge from the damp sawdust, but
soon turn back toward it. — After Sachs.

By these root hairs the ab-
sorbing surface, and hence
the amount of absorption,
is greatly increased. Indi-
vidual root hairs do not last
very long, but new ones are
constantly appearing just
behind the advancing root
tips, and the old ones are
as constantly disappearing.
(1) Geotropism and hy-
drotropism.— Many outside
influences affect roots in
the direction of their
growth, and as soil roots
are especially favorable for
observing these influences,
two prominent ones may
be mentioned. The influ-
ence of gravity, or the earth
influence, is very strong
in directing the soil root.

Fig. 86.

A raspberry plant, whose stem has been bent down to the soil and has
"struck root."— After Beal.

92 PLANT RELATIONS.

As is well known, when a seed germinates the tip that is to
develop the root turns towards the earth, even if it has
come from the seed in some other direction. This response
to gravity by the plant is known as geotropism. Another
directing influence is moisture, the response to which is

Fig. 87. A section through the leaf-stalk of a yellow pond-lily (Nuphar), showing the
numerous conspicuous air passages (s) by means of which the parts under water
are aerated ; h, internal hairs projecting into the air passages ; v, the much
reduced and comparatively few vascular bundles.

known as hydrotropism. By means of this the root is di-
rected towards the most favorable water supply in the soil.
Ordinarily, geotropism and hydrotropism direct the root
in the same general way, and so reinforce each other ; but
the following experiment may be arranged, which will
separate these two influences. Bore several small holes in
the bottom of a box, suspended as indicated in Fig. 85,
and cover the bottom and surround the box with blotting
paper. Pass the root tips of several germinated seeds

ROOTS.

93

through the holes, so that the seeds rest on the paper, and
the root tips hang through the holes. If the paper is kept
moist germination will continue, but geotropism will direct
the root tips downwards, and hydrotropism (response to
the moist paper) will direct them upwards. In this way
they will pursue a devious course, now directed by one
response and now by the other.

If a root system be examined it will be found that when
there is a main axis {tap
root) it is directed
steadily downwards,
while the branches are
directed differently.
This indicates that all
parts of a root system
are not alike in their
response to these influ-
ences. Several other
influences are also con-
cerned in directing soil
roots, and the path of
any root branch is a
result of all of them.
How variable they are
may be seen by the
numerous directions in
which the branches

travel, and the whole root system preserves the record of
these numerous paths.

(2) The pull on the stem. — Another root property may
be noted in connection with the soil root, namely the pull
on the stem. When a strawberry runner strikes root at
tip (see Fig. 47), the roots, after they obtain anchorage in
the soil, pull the tip a little beneath the surface, as if they
had gripped the soil and then slightly contracted. The
same thing may be observed in the process known as

Fig. 88. A section through the stem of a water-
wort (Matine), showing the remarkably large
and regularly arranged air passages for root
aeration. The single reduced vascular bundle
is central and connected with the small cor-
tex by thin plates of cells which radiate like
the spokes of a wheel. — After Schenck.

94

l'LANT RELATIONS.

"layering," by which a stem, as a bramble, is bent down
and covered with soil. The covered joints strike root, and
the pulling follows (see Fig. 86). A very plain illustration
of this pulling by roots can be obtained from many tuberous
plants. Tubers, bulbs, rootstocks, etc., are underground
structures which have been observed to bury themselves
deeper and deeper in the soil. This is effected by the young

Fig. 89. Section through the leaf of a quillwort (Isoetes), showing the four large air
chambers («). the central vascular region (b), and the very poorly developed cortex.

roots which they continue to put forth. These roots grip
the soil, then contract, and the tuber is pulled a little deeper.
The compact tuber known as the Indian turnip (" Jack-in-
the-pulpit ") has been found to bury itself very deeply and
rapidly, and this may be observed by transplanting a young
and vigorous tuber into a pot of loose soil.

(3) Soil dangers. — In this connection certain soil dan-
gers and the response of the roots should be noted. The
soil may become poor in water or poor in certain essential
materials, and this results in an extension of the root sys-

KOOTS.

95

tern, as if seeking for water and the essential materials.
Sometimes the root system becomes remarkably extensive,
visiting a large amount of soil in order to procure the
necessary supplies. Sometimes the soil is poor in heat, and
root activity is interfered with. In such cases it is very
common to find the leaves
massed against the soil, thus
slightly checking the loss of
heat.

Most soil roots also need free
air, and when water covers the
soil the supply is cut off. In
many cases there is some Avay
by which a supply of free air
may be brought down into the
roots from the parts above
water ; sometimes by large air
passages in leaves and stems
(see Figs. 87, 88, 89, 90) ; some-
times by developing special root
structures which rise above the
water level, as prominently
shown by the cypress in the
development of knees. These
knees are outgrowths from roots
beneath the water of the cypress
swamp, and rise above the water level, thus reaching the
air and aerating the root system (see Fig. 91). It has been
shown that if the water rises so high as to flood the knees
for any length of time the trees will die, but it does not
follow that this is the chief reason for their development.

65. Water roots. — A very different type of root is devel-
oped if it is exposed to free water, without any soil relation.
If a stem is floating, clusters of whitish thread-like roots
usually put out from it and dangle in the water. If the water
level sinks so as to bring the tips of these roots to the mucky

Fig. 90. Longitudinal section
through a young quillwort leaf,
showing that the four air cham-
bers shown in Fig. 80 are not con-
tinuous passages, but that there
are four vertical rows of promi-
nent chambers. The plates of
cells separating the chambers in
a vertical row very soon become
dead and full of air. In addition
to the work of aeration these air
chambers are very serviceable in
enabling the leaves to float when
they break off and carry the com-
paratively heavy spore cases.

a 5

S s

a m

a m a

o s a

.9 2 ft

ROOTS.

97

imssH

Pia. 92.

A tropical aroid (Anthuriicm), Bhowing its large leaves, and bunches of
aerial roots.

soil they usually do not penetrate or enter into any soil re-
lation. Such pure water roots may be found dangling from
the under surface of the common duck weeds, which often
cover the surface of stagnant water with their minute,
green, disk -like bodies.

98

PLANT RELATIONS.

Plants which ordinarily develop soil roots, if brought
into proper water relations, may develop water roots. For
instance, willows or other stream bank plants may be so
close to the water that some of the root system enters it.
In such cases the numerous clustered roots show their water

Fig. 93. An orchid, showing aerial roots.

character. Sometimes root systems developing in the soil
may enter tile drains, when water roots will develop in such
clusters as to choke the drain. The same bunching of water
roots may be noticed when a hyacinth bulb is grown in a
vessel of water.

66. Air roots. — In certain parts of the tropics the air is
so moist that it is possible for some plants to obtain suffi-

KOOTS.

99

cient moisture from this source, without any soil-relation
or water-relation. Among these plants the orchids are
most notable, and they may be observed in almost any
greenhouse. Clinging to the trunks of trees, usually imi-
tated in the greenhouse by nests of sticks, they send out
long roots which dangle in the moist air (see Figs. 93, 94).
It is necessary to have some special absorbing arrange-
ment, and in the orchids this
is usually provided by the de-
velopment of a sponge-like
tissue about the root known
as the velamen, which greed-
ily absorbs the dew or water
trickling down the plant. See
also Figs. 92, 95, 96, 97.

67. Clinging roots. — These
roots are developed to fasten
the plant body to some sup-
port, and do no work of ab-
sorption (see Fig. 98). Very
common illustrations may be
obtained from the ivies, the
trumpet creeper, etc. These
roots cling to various supports,
stone walls, tree trunks, etc.,
by sending minute tendril-
like branches into the crevices. The sea-weeds (alga?)
develop grasping structures extensively, a large majority
of them being anchored to rocks or to some rigid support
beneath the water, and their bodies floating free. The
root-like processes by which this anchorage is secured are
very prominent in many of the common marine sea-weeds
(see Fig. 157).

68. Prop roots. — Some roots are developed to prop
stems or wide-spreading branches. In swampy ground, or
in tropical forests, it is very common to find the base of

8

Fig. 94. An orchid, showing
roots and thick leaves.

aerial

Fig. a5.

Astaghorn fern (Platycerlum), an aerial plant of the tropics. About it is a
vine, which shows the leaves adjusted to the lighted side.

Fig. 96. Selaginella, showing dangling rhizophores and finely divided leaves.

Fig. 97. Live oaks, in the Gulf States, upon which are growing masses of long moss
or black moss ( Tlllandsia), a common aerial plant.

Fig. 98. A tropical forest, showing the cord-like holdfasts developed by an epi-
phyte, Which pass around the tree trunks like tightly bound ropes. After
Kekneh.

ROOTS

103

tree trunks buttressed by such roots which extend out over
and beneath the surface, and divide the area about the tree
into a series of irregular chambers (see Fig. 100). Some-

Pig. 99. A screw-pine (Pandanus), from the Indian Ocean region, showing the
prominent prop roots put out near the base.

times a stem, either inclined or with a poorly developed
primary root system, puts out prop roots which support
it, as in the screw-pine (see Tig. 99). A notable case is

r;f ,; ••■ ■ ■■■■- *t-*\.

106

PLANT RELATIONS.

that of the banyan tree, whose wide-spreading branches
are supported by prop roots, which are sometimes very
numerous (see Fig. 101). The immense banyans usually

illustrated are
especially culti-
vated as sacred
trees, the prop
roots being as-
sisted in pene-
trating the soil.
There is record
of such a tree in
Ceylon with 350
large and 3,000
small prop roots,
able to cover a
village of 100
huts.

69. Parasites.
— Besides the
roots mentioned
above, certain
plants develop
root-like p r o-
cesses which re-
late themtohosts.
A host is a liv-
ing plant or
animal upon
which some
other plant or
animal is living
as a parasite.
The parasite gets its supplies from the host, and must be
related to it properly. If the parasite grows upon the
surface of its host, it must penetrate the body to obtain

Fig. 102. A dodder plant parasitic on a willow twig. The
leafless dodder twines about the willow, and sends out
sucking processes which penetrate and absorb. — After
Strasburger.

ROOTS.

107

food supplies.
Therefore, pro-
cesses are devel-
oped which pene-
trate and absorb.
The mistletoe and
dodder are seed-
plants which have
this habit, and
both have such
processes (see Figs.
102, 103). This
habit is much more
extensively devel-
oped, however, in
a low group of
plants known as
the fungi. Many
of these parasitic
fungi live upon
plants and animals,
common illustrations being the mildews of lilac leaves and
many other plants, the rust of wheat, the smut of corn, etc.

70. Root structure.
— In the lowest groups
of plants (alga3, fungi,
and moss-plants) true
roots are not formed,
but very simple struc-
tures, generally hair-
like (see Fig. 104). In
fern-plants and seed-
plants, however, the
root is a complex
structure, so different
from the root-like pro-

Fig. 103. A section showing the living connection
between dodder and a golden rod upon which it is
growing. The penetrating and absorbing organ (h)
has passed through the cortex (c), the vascular
zone (6), and is disorganizing the pith (p).

Pig. 104. Section through the thallus of a liver-
wort (Marchantla), showing the hair-like pro-
cesses (rhizoids) which come from the under
surface and act as roots in gripping and ab-
sorbing. In the epidermis of the upper surface
a chimney-like opening is seen, leading into
a chamber containing cells with chloroplasts.

108

PLANT RELATIONS.

cesses of the lower groups that it is regarded as the only
true root. It is quite uniform in structure, consisting of

a tough and fibrous
central axis surround-
ed by a spongy region
(Fig. 105). The
tough axis is made
up mostly of ves-
sels, so called be-
cause they conduct
material, and is called
the vascular axis.
The outer more
spongy region is the
cortex, which covers
the vascular axis like
a thick skin.

One of the pecu-
liarities of the root is
that the branches
come from the vascu-
lar axis and burrow
through the cortex,
so that when the lat-
ter is peeled off the
branches are left at-
tached to the axis,
and the cortex shows
the holes through
which they passed.

Another pecu-
liarity of the root is
that it elongates only by growth at the tip, and in the soil
this delicate growing tip is protected by a little cap of cells,
known as the root-cap.

Pig. 105. A longitudinal section through the root
tip of spiderwort, showing the central vascular
axis (pi), surrounded by the cortex (p), outside
of the cortex the epidermis (e) which disappears
in the older parts of the root, and the promi-
nent root-cap (c).

CHAPTER VI.

REPRODUCTIVE ORGANS.

It will be remembered that nutrition and reproduction
are the two great functions of plants. In discussing
foliage leaves, stems, and roots, they were used as illustra-
tions of nutritive organs, so far as their external relations
are concerned. We shall now
briefly study the reproductive
organs from the same point
of view, not describing the
processes of reproduction, but
some of the external relations.
— 71. Vegetative multiplica-
tion.— Among the very lowest
plants no special organs of
reproduction are developed,
but most plants have them.
There is a kind of reproduc-
tion by which a portion of
the parent body is set apart to
produce a new plant, as when
a strawberry runner produces
a new strawberry plant, or
when a willow twig or a grape
cutting is planted and produces new plants, or when a potato
tuber (a subterranean stem) produces new potato plants, or
when pieces of Begonia leaves are used to start new Begonias.
This is known as vegetative multiplication, a kind of repro-
duction which does not use special reproductive organs.

Fig. 106. A group of spores : A,
spores from a common mold (a
fungus), which are so minute and
light that they are carried about by
the air ; B, two spores from a com-
mon alga {Ulothrix), which can
swim by means of the hair-like
processes; C, the conspicuous dotted
cell is a spore developed by a com-
mon mildew (a fungus), which is
carried about by currents of air.

110

PLANT RELATIONS.

72. Spore reproduction. — Besides vegetative multiplica-
tion most plants develop special reproductive bodies,
known as spores, and this kind of reproduction is known
spore reproduction. These spores are very simple
bodies, but have the power of producing new individuals.
There are two great groups of spores, differing from each
other not at all in their powers, but in the method of their
production by the parent plant. One kind of spore is

produced by dividing
certain organs of the
parent ; in the other
case two special bodies
of the parent blend
together to form the
spore. Although they
are both spores, for
convenience we may
call the first kind
spores (see Figs. 106,
109), and the second
kind eggs (see Fig.
107).* The two special
bodies which blend to-
gether to form an egg
are called gamei&s (see
Figs. 107, 108, 109). These terms are necessary to any
discussion of the external relations. Most plants develop
both spores and eggs, but they are not always equally con-
spicuous. Among the algae, both spores and eggs are prom-
inent ; among certain fungi the same is true, but many
fungi are not known to produce eggs ; among moss-plants
the spores are prominent and abundant, but the egg is
concealed and not generally noticed. What has been said

* It is recognized that this spore is really a fertilized egg, but in
the absence of any accurate simple word, the term egg is used for con-
venience. •

Fig. 107. Fragments of a common alga (Spi-
rogyra). Portions of two threads are shown,
which have been joined together by the grow-
ing of connecting tubes. In the upper thread
four cells are shown, three of which contain
eggs (s), while the cell marked g, and its mate
of the other thread each contain a gamete,
the lower one of which will pass through the
tube, blend with the upper one, and form
another egg.

REPRODUCTIVE ORGANS.

Ill

of the moss-plants is still more true
of the fern-plants ; while among
the seed-plants certain spores {pol-
len grains) are conspicuous (see
Fig. 110), but the eggs can be ob-
served only by special manipulation
in the laboratory. Seeds are neither
spores nor eggs, but peculiar repro-
ductive bodies which the hidden
egg has helped to produce.

73. Germination. — Spores and
eggs are expected to germinate ;
that is, to begin the development
of a new plant. This germination
needs certain external conditions,
prominent among which are defi-
nite amounts of heat, moisture,
and oxygen, and sometimes light.
Conditions of germination may be
observed most easily in connection
with seeds. It must be understood,
however, that what is called the
germination of seeds is something

Fig. 108. A portion of the
body of a common alga
(QSclogonhtm), showing
gametes of very unequal size
and activity ; a very large
one (o) is lying in a globular
cell, and a very small one is
entering the cell, another
similar one (s) being just
outside. The two small
gametes have hair-like pro-
cesses and can swim freely.
The small and large gam-
etes unite and form an egg.

Fig. 109. A group of swim-
ming cells : -4, a spore of
(Edogonium (an alga) ;
B, spores of Ulothrix (an
alga) ; C, a gamete of
Equiselum (horse-tail or
scouring rush).

very different from the germination
of spores and eggs. In the latter
cases, germination includes the very
beginnings of the young plant. In
the case of a seed, germination begun
by an egg has been checked, and
seed germination is its renewal. In
other words, an egg has germinated
and produced a young plant called
the "embryo," and the germination
of the seed simply consists in the
continued growth and the escape of
this embryo.

112

PLANT RELATIONS.

Fig. 110. A pollen grain (spore) from the
pine, which develops wings (w) to assis*
in its transportation by currents of air.

It is evident that for
the germination of seeds
light is not an essential
condition, for they may
germinate in the light or
in the dark ; but the need
of heat, moisture, and
oxygen is very apparent.
The amount of heat re-
quired for germination
varies widely with different
seeds, some germinating
at much lower tempera-
tures than others. Every

kind of seed, or spore, or egg has a special temperature

range, below which and above which

it caunot germinate. The two limits

of the range may be called the

lowest and highest points, but be-
tween the two there is a best point

of temperature for germination. The

same general fact is true in reference

to the moisture supply.

74. Dispersal of reproductive bodies.

— Among the most striking external

relations, however, are those con-
nected with the dispersal of spores,

gametes, and seeds. Spores and

seeds must be carried away from the

parent plant, and separated from

each other, out of the reach of

rivalry for nutritive material ; and

gametes must come together and

blend to form the eggs. Conspicuous (Epiiobium) opening and

among the means of transfer are the exposing its plumed seeds

° which are transported by

following. the wind.-After Bkal.

REPRODUCTIVE ORGANS.

lib

75. Dispersal by locomotion. — The common method of
locomotion is by means of movable hairs (cilia) developed
upon the reproductive body, which propel it through the
water (see Fig. 109).
Swimming spores are
very common among
the algse, and at least
one of the gametes
in algee, moss-plants,
and fern-plants has
the power of swim-
ming by means of
cilia.

7G. Dispersal by
water. — It is very
common for repro-
ductive bodies to be
transported by cur-
rents of water. The
spores of many water
plants of all groups,
not constructed for
locomotion, are thus
floated about. This
method of transfer is
also very common
among seeds. Many
seeds are buoyant, or
become so after soak-
ing in water, and
may be carried to
great distances by

currents. For this reason the plants growing upon the
banks or flood-plains of streams may have come from a
wide area. Many seeds can even endure prolonged soak-
ing in sea-water, and then germinate. Darwin estimated

Fig. 112. The upper figure to the left is ar opening
pod of fireweed discharging its plumed seeds.
The lower figure represents the seed-like fruits
of Clematis with their long tail-like plumes.—
After Kerner.

114

PLANT RELATIONS.

that at least
fourteen per
cent, of the
seeds of any
country can re-
tain their vital-
ity in sea-water
for twenty-
eight days. At
the ordinary
rate of move-
ment of ocean
currents, this
length of time
would permit
such seeds to
be transported
over a thou-
sand miles,
thus making
possible a very great range in distribution.

77. Dispersal of spores by air. — This is one of the most
common methods of transport-
ing spores and seeds. In most
cases spores are sufficiently
small and light to be trans-
ported by the gentlest move-
ments of air. Among the
fungi this is a very common
method of spore dispersal (see
Fig. 106), and it is extensively
used in scattering the spores
of moss-plants, fern-plants (see
Fig. 45), and seed-plants.
Among seed-plants this is one FlG- 114- 8eed-like fraitf> of «««*>

, , , .-IT ,. wltn plumes for dispersal by air.—

method of pollination, the After kbbnbb.

Fig. 118. A ripe dandelion head, showing the mass of
plumes, a few seed-like fruits with their plumes still
attached to the receptacle, and two fallen off.— After
Kerner.

REPRODUCTIVE ORGANS

115

Fig. 115. A winged seed of Bignonia— After Strasburger.

spores called pollen
and occasionally
falling upon the
right spot for
germination.
With such an
agent of transfer
the pollen must
be very light and
powdery, and
also very abun-
dant, for it must
come down al-
most like rain to be

grains being scattered by the wind,

Fig. 117.

Winged fruit of
Kerner.
9

Fig. 116. Winged fruit of maple.— After Kerner.

certain of reaching the right places.
Among the gymno-
sperms (pines, hem-
locks, etc.) this is the
exclusive method of
pollination, and when a
pine forest is shedding
pollen the air is full of
the spores, which may
be carried to a great
distance before being
deposited. Occasional

Ptelea.— After

116

PLANT RELATIONS.

Fig. 118. Winged fruit of
Ailanlhus.— After Ker-

NEK.

reports of "showers of sulphur" have
arisen from an especially heavy fall of
pollen that has been carried far from
some gymnosperm forest. In the case
of pines and their near relatives, the
pollen spores are assisted in their dis-
persal through the air by developing a
pair of brpad wings from the outer
coat of the spore (see Fig. 110). This
same method of pollination — that is,
carrying the pollen spores by currents
of air — is also used by many mono-
cotyledons, such as grasses ; and by
many dicotyledons, such as our most
common forest trees
(oak, hickory, chest-
nut, etc.).

78. Dispersal of
seeds by air. — Many
seeds are carried
about in various ways
by currents of air
without any special
adaptation. Wings
and plumes of very
many and often very
beautiful patterns
are exceedingly com-
mon in connection
with seeds or seed-
like fruits (see Figs.
115, 116, 117, 118,
119). Wings are de-
veloped by the fruit
of maples and of «_,*.,. ^ ,m„ , , ♦».

r Fig. 119. Fruit of basewood ( Tiha), showing the

ash, and by the Seeds peculiar wing formed by a leaf .—After Kerner.

REPRODUCTIVE ORGANS.

117

Pig. 120. A common tumbleweed (Cyclolomd).

of pine and catalpa. Plumes and tufts of hairs are devel-
oped by the seed-like fruits of dandelion, thistle, and very
many of their relatives, and by the seeds of the milkweed
(see Figs. Ill, 112, 113, 111). On plains, or level stretches,
where winds are
strong, a curious
habit of seed dis-
persal has been de-
veloped by certain
plants known as
" tumbleweeds " or
" field rollers. "
These plants are
profusely branching
annuals with a small

Fis. 121. The 3-valved fruit of violet discharging
root system in a its eeeds.-After Beal.

118

PLANT RELATIONS.

Fig. 122. A fruit of witch
hazel discharging its
seeds.— After Beai.

light or sandy soil (see Fig. 120).
When the work of the season is over,
and the absorbing rootlets have
shriveled, the plant is easily broken
from its roots by a gust of wind,
and is trundled along the surface like
a light wicker ball, the ripe seed ves-
sels, dropping their seeds by the way.
In case of an obstruction, such as a
fence, great masses of these tumble-
weeds may often be seen lodged
against the windward side.

79. Discharge of spores. — In many
plants the distribution of spores and
seeds is not provided for by any of

the methods just mentioned, but the vessels containing

them are so constructed that they are discharged with

more or less violence and are some-
what scattered.

Many spore cases, especially those

of the lower plants, burst irregularly,

and with sufficient violence to throw

out spores. In the liverworts pecu-
liar cells, called elaters or "jumpers,"

are formed among the spores, and

when the wall of the spore case is

ruptured the elaters are liberated,

and by their active motion assist in

discharging the spores.

In most of the true mosses the

spore case opens by pushing off a

lid at the apex, which exposes a

delicate fringe of teeth covering the

mouth of the urn-like case. These

teeth bend in and out of the open

spore case as they become moist or

Fig. 123. A pod of wild bean
bursting, the two valves
violently twisting and dis-
charging the seeds.— After
Beai..

REPRODUCTIVE ORGANS.

119

dry, and are of considerable service

in the discharge of spores.

In the common ferns a heavy

spring-like ring of cells encircles

the delicate-walled spore case.

When the wall becomes dry and

comparatively brittle the spring

straightens with considerable force,

the delicate wall is suddenly torn,

and in the recoil the spores are dis-
charged (see Fig. 45).

Even in the case of the pollen-
spores of seed-plants, a special layer

of the wall of the pollen-sac usually

develops as a spring-like layer, which

assists in opening widely the sac
when the wall be-
gins to yield along
the line of break-
ing.

80. Discharge of
seeds. — While seeds are generally carried
away from the parent plant by the agency
of water currents or air currents, as al-
ready noted, or by animals, in some in-
stances there is a mechanical discharge
provided for in the structure of the seed-
case. In such plants as the witch hazel
and violet, the walls of the seed-vessel
press upon the contained seeds, so that
when rupture occurs the seeds are pinched

fig. 125. a fruit of out, as a moist apple-seed is discharged
begsar ilc!L8' by being pressed between the thumb and

snowing the two •' ° r

barbed append- finger (see Figs. 121, 122). In the touch-
ages which lay me-not a strain is developed in the wall

hold of animals. l

— After beai>- of the seed-vessel, so that at rupture it

Fig. 124. Fruits of Spanish
needle, showing barbed ap-
pendages for grappling.
The figure to the left is one
of the fruits enlarged.—
After Kerner.

120

PLANT RELATIONS.

Fig. 126. The fruit of carrot, showing
the grappling appendages. — After
Beal.

suddenly curls up and throws the seeds (see Fig. 123). The
squirting cucumber is so named because it becomes very
much distended with water, which is finally forcibly ejected
along with the mass of seed. An " artillery plant " common

in cultivation discharges its
seeds with considerable vio-
lence. ; while the detonations
resulting from the explosions
of the seed-vessels of Hura
crepitans, the "monkey's din-
ner bell," are often remarked
by travelers in tropical
forests.

81. Dispersal of seeds by animals. — Only a few illustra-
tions can be given of this very large subject. Water birds
are great carriers of seeds which are contained in the mud
clinging to their feet and legs. This mud from the borders
of ponds is usually completely filled with seeds and spores
of various plants. One has no conception of the number
until they are actually com-
puted. The following ex-
tract from Darwin's Origin
of Species illustrates this
point :

' ' I took, in February, three
tablespoonf uls of mud from three
different points beneath water,
on the edge of a little pond. This mud when dried weighed only 6|
ounces ; I kept it covered up in my study for six months, pulling up
and counting each plant as it grew ; the plants were of many kinds,
and were altogether 537 in number ; and yet the viscid mud was all
contained in a breakfast cup ! "

Water birds are generally high and strong fliers, and the
seeds and spores may thus be transported to the margins of
distant ponds or lakes, and so very widely dispersed.

In many cases seeds or fruits develop grappling append-

Fig. 127. The fruit of cocklebur, showing
the grappling appendages. —After Beal.

REPRODUCTIVE ORGANS.

121

ages of various kinds, which lay hold of animals brushing
past, and so the seeds are dispersed. Common illustrations
are Spanish needles, beggar ticks, stick seeds, burdock, etc.
Study Figs. 124, 125, 126, 127, 128, 129, 130.

Fib. 128. Fruits with grappling appendages. That to the left is agrimony ; that to
the right is Galium. — After Kerner.

In still other cases the fruit becomes pulpy, and attrac-
tive as food to certain birds or mammals. Many of the
seeds (such as those of grapes) may be able to resist the
attacks of the digestive fluids and escape from the alimen-
tary tract in a condition to germinate. As if to attract the
attention of fruit-eating animals, fleshy fruits usually
become brightly col-
ored when ripe, so that
they are plainly seen
in contrast with the
foliage.

82. Dispersal of pol-
len spores by insects. —
The transfer of pollen,
the name applied to FlG- 129- Fruits with §raPPlins appendages.

rr The figure to the left is cocklebur ; that to the

Certain spores Of Seed- right is burdock.— After Kerner.

122

PLANT RELATIONS.

plants, is known as pollination, and
the two chief agents of this transfer
are currents of air and insects. In
§77 the transfer by currents of air
was noted, such plants being known
as anemophilous plants. Such plants
seldom produce what are generally
recognized as true flowers. All those
seed-plants which produce more or
less showy flowers, however, are in
some way related to the visits of
insects to bring about pollination,
and are known as entomophilous
plants. This relation between "In-
sects and flowers is so important and so extensive that it
will be treated in a separate chapter.

Fig. 130 A head of fruits of
burdock, showing the
grappling appendages. —
After Beal.

CHAPTEE VII.

FLOWERS AND INSECTS.

83. Insects as agents of pollination. — The use of insects
as agents of pollen transfer is very extensive, and is the pre-
vailing method of pollination among monocotyledons and
dicotyledons. All ordinary flowers, as usually recognized,
are related in some way to pollination by insects, but it
must not be supposed that they are always successful in
securing it. This mutually helpful relation between flow-
ers and insects is a very wonderful one, and in some cases
it has become so intimate that they cannot exist without
each other. Flowers have been modified in every way to be
adapted to insect visits, and insects have been variously
adapted to flowers.

84. Self-pollination and cross- pollination. — The advantage
of this relation to the flower is to secure pollination. The
pollen may be transferred to the carpel of its own flower,
or to the carpel of some other flower. The former is known
as self-pollination, the latter as cross-pollination. In the
case of cross-pollination the two flowers concerned may be
upon the same plant, or upon different plants, which may
be quite distant from one another. It would seem that
cross-pollination is the preferred method, as flowers are so
commonly arranged to secure it.

85. Advantage to insects. — The advantage of this relation
to the insect is to secure food. This the flower provides
either in the form of nectar or pollen ; and insects visiting
flowers may be divided roughly into the two groups of
nectar-feeding insects, represented by butterflies and moths,

124 PLANT RELATIONS.

and pollen-feeding insects, represented by the numerous
bees and wasps. When pollen is provided as food, the
amount of it is far in excess of the needs of pollination.
The presence of these supplies of food is made known to
the insect by the display of color in connection wUh the
flowers, by odor, or by form. It should be said that the
attraction of insects by color has been doubted recently, as
certain experiments have suggested that some of the com-
mon flower-visiting insects are color-blind, but remarkably
keen-scented. However this may be for some insects, it
seems to be sufficiently established that many insects rec-
ognize their feeding ground by the display of color.

86. Suitable and unsuitable insects. — It is evident that
all insects desiring nectar or pollen for food are not suit-
able for the work of pollination. For instance, the ordi-
nary ants are fond of such food, but as they walk from plant
to plant the pollen dusted upon them is in great danger of
being brushed off and lost. The most favorable insect is
the flying one, that can pass from flower to flower through
the air. It will be seen, therefore, that the flower must not
only secure the visits of suitable insects, but must guard
against the depredations of unsuitable ones.

87. Danger of self-pollination. — There is still another
problem which insect-pollinating flowers must solve. If
cross-pollination is more advantageous to the plant than
self-pollination, the latter should be prevented so far as
possible. As the stamens and carpels are usually close to-
gether in the same flower, the danger of self-pollination is
constantly present in many flowers. In those plants which
have stamen-producing flowers upon one plant and carpel-
producing flowers upon another, there is no such danger.

88. Problems of pollination. — In most insect-pollinating
flowers, therefore, there are three problems : (1) to prevent
self-pollination, (2) to secure the visits of suitable insects,
and (3) to ward off the visits of unsuitable insects. It
must not be supposed that flowers are uniformly successful

FLOWERS AND INSECTS.

125

in solving these problems. They often fail, but succeed
often enough to make the effort worth while.

89. Preventing self-pollination. — It is evident that this
danger arises only in those flowers in which the stamens
and carpels are associ
ated, bat their separa- ^. ' ^

tion in different flowers
may be considered as
one method of prevent-
ing self-pollination. In
order to understand the
various arrangements to
be considered, it is nec-
essary to explain that
the carpel does not re-
ceive the pollen indif-
ferently over its whole
surface. There is one
definite region organ-
ized, known as the
stigma, upon which the
pollen must be deposited
if it is to do its work.
Usually this is at the
most projecting point
of the carpel, very often
at the end of a stalk-
like prolongation from
the ovary (the bulbous
part of the carpel),
known as the style;
sometimes it may run down one side of the style. When
the stigma is ready to receive pollen it has upon it a
sweetish, sticky fluid, which holds and feeds the pollen.
In this condition the stigma is said to be mature ; and the
pollen is mature when it is being shed, that is, ready to fall

Fig. 131. Parts of the flower of rose acacia
(Robiniahispidd). In 1 the keel is shown pro-
jecting from the hairy calyx, the other more
showy parts of the corolla having been re-
moved. Within the keel are the stamens
and the carpel, as seen in 3. The keel forms
the natural landing place of a visiting bee,
whose weight depresses the keel and causes
the tip of the style to protrude, as shown in
2. This style tip bears pollen upon it,
caught among the hairs, seen in 3, and as it
strikes the body of the bee some pollen is
brushed ofiE. If the bee has previously visited
another flower and received some pollen, it
will be seen that the stigma, at the very tip
of the style, striking the body first, will very
probably receive some of it. The nectar pit
is shown in 3, at the base of the uppermost
stamen.— After Gray.

126

PLANT RELATIONS.

out of the pollen-sacs or to be removed from them. The
devices used by flowers containing both stamens and carpels
to prevent self-pollination are very numerous, but most
of them may be included under the three following heads :
(1) Position. — In these cases the
pollen and stigma are ready at the same
time, but their position in reference to
each other, or in reference to some con-
formation of the flower, makes it un-
likely that the pollen will fall upon the
stigma. The stigma may be placed
above or beyond the pollen sacs, or the
two may be separated by some mechan-
ical obstruction, resulting in much of
the irregularity of flowers.

In the flowers of the rose acacia and
its relatives, the several stamens and
the single carpel are in a cluster, en-
closed in the keel of the flower. The
stigma is at the summit of the style,
and projects somewhat beyond the
pollen-sacs shedding pollen. Also there
is often a rosette of hairs, or bristles,
just beneath the stigma, which acts as
a barrier to the pollen (see Fig. 131).

In the iris, or common flag, each
stamen is in a sort of pocket between
the petal and the petal-like style, while
the stigmatic surface is on the top of a
flap, or shelf, which the style sends out
as a roof to the pocket. With such an
arrangement, it would seem impossible
for the pollen to reach the stigma un-
aided (see Fig. 132).

In the orchids, remarkable for their
strange and beautiful flowers, there are

Fig. 132. A portion of
the flower of an iris,
or flag. The single
stamen shown is
standing between the
petal to the right and
the petal-like style to
the left. Near the
top of this style the
stigmatic shelf is
seen extending to the
right, which must
receive the pollen
upon its upper sur-
face. The nectar
pit is at the junc-
tion of the petal and
stamen. While ob-
taining the nectar the
insect brushes the
pollen-bearing part
of the stamen, and
pollen is lodged upon
its body. In visiting
the next flower and
entering the stamen
chamber the stig-
matic shelf is apt to
be brushed. — After
Gray.

FLOWERS AND INSECTS.

127

usually two pollen-sacs, and stretched between them is the
stigmatic surface. In this case, however, the pollen grains
are not dry and powdery, but cling together in a mass, and
cannot escape from the sac without being pulled out (see
Fig. 133). The same sort of pollen is developed by the
milkweeds.

(2) Consecutive maturity. — In these cases the pollen and

Fig. 133. A flower of an orchid (Habenaria). At 1 the complete flower is shown,
with three sepals behind, and three petals in front, the lowest one of which has
developed a long strap-shaped portion, and a still longer spur portion, the opening
to which is seen at the base of the strap. At the bottom of this long spur is the
nectar, which is reached by the long proboscis of a moth. The two pollen sacs of
the single stamen are seen in the centre of the flower, diverging downwards, and
between them stretches the stigma surface. The relation between pollen sacs and
stigma surface is more clearly shown in 2. Within each pollen sac is a mass of
sticky pollen, ending below in a sticky disk, which may be seen in 1 and 2. When
the moth thrusts his proboscis into the nectar tube, his head is against the stig-
matic surface and also against the disks. When he removes his head the disks
stick fast and the pollen masses are dragged out. In 3 a pollen mass (a) is
shown sticking to each eye of a moth. Upon visiting another flower these pollen
masses are thrust against the stigmatic surface and pollination is effected.— After

GBA7.

128

PLANT RELATIONS.

stigma of the same flower are not mature at the same time.
It is evident that this is a very effective method of prevent-
ing self-pollination. When the pollen is being shed the
stigma is not ready to receive, or when the stigma is ready
to receive the pollen is not ready to be shed. In some
cases the pollen is ready first, in other cases the stigma,
the former condition being called prota?idry, the latter
protogyny. This is a very common method of preventing

self-pollination, and is
usually not associated with
irregularity.

The ordinary figwort may
be taken as an example of
protogyny. When the flow-
ers first open, the style, bear-
ing the stigma at its tip, is
found protruding from the
urn -like flower, while the
four stamens are curved
down into the tube, and are
not ready to shed their pollen.
At some later time the style
bearing the stigma wilts,
and the stamens straighten
up and protrude from the tube. In this way, first the
receptive stigma, and afterwards the shedding pollen-sacs,
occupy the same position.

Protandry is even more common, and many illustrations
can be obtained. For example, the showy flowers of the
common firewee'd, or great willow herb, when first opened
display their eight shedding stamens prominently, the style
being sharply curved downward and backward, carrying
the four stigma lobes well out of the way. Later, the
stamens bend away, and the style straightens up and ex-
poses its stigma lobes, now receptive (see Fig. 134).

(3) Difference in pollen. — In these cases there are at

Fig. 134. Flowers of fireweed (Epi-
lobium), showing protandry. In 1 the
stamens are thrust forward, and the
style is sharply turned downward and
backward. In 2 the style is thrust
forward, with its stigmatic branches
spread. An insect in passing from 1
to 2 will almost certainly transfer pol-
len from the stamens of 1 to the stig-
mas of 2.— After Gray.

FLOWERS AND INSECTS.

129

least two forms of flowers, which differ from one another
in the relative lengths of their stamens and styles. In the
accompanying illustrations of Houstonia (see Fig. 135) it
is to be noticed that in one flower the stamens are short
and included in the tube, and the style is long and pro-
jecting, with the four stigmas exposed well above the
tube. In the
other flower the
relative lengths
are exactly re-
versed, the
style being
short and in-
cluded in the
tube, and the
stamens long
and projecting.
It appears that
the pollen from
the short sta-
mens is most
effective upon
the stigmas of
the short styles,
and that the
pollen from the
long stamens is
most effective
upon the stig-
mas of the long styles ; and as short stamens and long
styles, or long stamens and short styles, are associated in
the same flower, the pollen must be transferred to some
other flower to find its appropriate stigma. This means
that there is a difference between the pollen of the short
stamens and that of the long ones.

In some cases there are three forms of flowers, as in one

Pig. 135. Flowers of Houstonia, showing two forms of
flowers. In 1 there are short stamens and a long style ;
in 2 long stamens and short style. An insect visiting 1
will receive a band of pollen about the front part of its
body ; upon visiting 2 this band will rub against the
stigmas, and a fresh pollen band will be received upon
the hinder part of the body, which, upon visiting another
flower like No. 1, will brush against the stigmas.—
After Gray.

130

PLANT RELATIONS.

of the common loosestrifes. Each flower has stamens of
two lengths, which, with the style, makes possible three
combinations. One flower has short stamens, middle-length
stamens, and long style ; another has sh^rt stamens, middle-
length style, and long stamens ; the third has short style,
middle-length stamens, and long stamens. In these cases
also the stigmas are intended to receive pollen from stamens

Fig. 136. Yucca and Pronuba. In the lower figure to the right an opened flower
shows the pendent ovary with the stigma region at its apex. The upper figure to
the right shows the position of Pronuba when collecting pollen. The figure to the
left represents a cluster of capsules of Yucca, which shows the perforations made
by the larvae of Pronuba in escaping.— After Riley and Trelease.

of their own length, and a transfer of pollen from flower to
flower is necessary.

90. Self-pollination. — In considering these three general
methods of preventing self-pollination, it must not be sup-
posed that self-pollination is never provided for. It is pro-
vided for more extensively than was once supposed. It is
found that many plants, such as violets, in addition to the
usual showy, insect-pollinated flowers, produce flowers that
are not at all showy, in fact do not open, and are often not
prominently placed. The fact that these flowers are often
closed has suggested for them the name cleistogamous

FLOWERS AND INSECTS. 131

flowers. In these flowers self-pollination is a necessity, and
is found to be very effective in producing seed.

91. Yucca and Pronuba. — There can be no doubt, also,
that there is a great deal of self-pollination effected in
flowers adapted for pollination by insects, and that the in-
sects themselves are often responsible for it. But in the
remarkable case of Yucca and Pronuba there is a definite
arrangement for self-pollination by means of an insect (see
Fig. 136). Yucca is a plant of the southwestern arid regions
of North America, and Pronuba is a moth. The plant and
the moth are very dependent upon each other. The bell-
shaped flowers of Yucca hang in great terminal clusters, with
six hanging stamens, and a central ovary ribbed lengthwise,
and with a funnel-shaped opening at its apex, which is the
stigma. The numerous ovules occur in lines beneath the
furrows. During the day the small female Pronuba rests
quietly within the flower, but at dusk becomes very active.
She travels down the stamens, and resting on the open
pollen-sac scoops out the somewhat sticky pollen with her
front legs. Holding the little mass of pollen she runs to
the ovary, stands astride one of the furrows, and pierc-
ing through the wall with her ovipositor, deposits an egg
in an ovule. After depositing several eggs she runs to the
apex of the ovary and begins to crowd the mass of pollen
she has collected into the funnel-like stigma. These actions
are repeated several times, until many eggs are deposited
and repeated pollination has been effected. As a result of
all this the flower is pollinated, and seeds are formed which
develop abundant nourishment for the moth larvae, which
become mature and bore their way out through the wall of
the capsule (Fig. 136).

92. Securing cross-pollination. — In very many ways flow-
ers are adapted to the visits of suitable insects. In ob-
taining nectar or pollen as food, the visiting insect receives
pollen on some part of its body which will be likely to
come in contact with the stigma of the next flower visited.

10

Fig 137. A clump of lady-slippers (Cypripedium), showing the habit of the plant
and the general structure of the flower.— After Gibson.

FLOWERS AND INSECTS.

133

Illustrations of this process may be taken from the flowers
already described in connection with the prevention of
self-pollination.

In the flowers of the pea family, such as the rose acacia
(see Fig. 131), it will be
noticed that the stamens
and pistil are concealed
within the keel, which
forms the natural land-
ing place for the bees
which are used in pol-
lination. This keel is
so inserted that the
weight of the insect de-
presses it, and the tip
of the style comes in
contact with its body.
Not only does the
stigma strike the body,
but by the glancing
blow the surface of the
style is rubbed against
the insect, and on this
style, below the stigma,
the pollen has been de-
posited and is rubbed
off against the insect.
At the next flower
visited the stigma is
likely to strike the pol-
len obtained from the previous flower, and the style will
deposit a new supply of pollen.

In the flower of the common flag (see Fig. 132) the nectar
is deposited in a pit at the bottom of the chamber formed
by each style and petal. In this chamber the stamen is
found, and more or less roofing it over is the flap, or shelf,

Fi<j. 138. Flower of Cypripedium, showing the
flap overhanging the opening of the pouch,
into which a bee is crowding its way. The
small figure to the right shows a side view of
the flap ; that to the left a view beneath the
flap, showing the two dark anthers, and be-
tween them, further down (forward), the
stigma surface. — After Gibson.

134

PLANT RELATIONS.

upon the upper surface of which the stigma is developed.
As the insect crowds its way into this narrowing chamber,
its body is dusted by the pollen, and as it visits the next
flower and thrusts aside the stigmatic shelf, it is apt to
deposit upon it some of the pollen previously received.

The story of pollination in connection with the orchids
is still more complicated (see Fig. 133). Taking an ordi-
nary orchid for illustration, the details are as follows. Each
of the two pollen masses terminates in a sticky disk or
button ; between them extends the concave stigma sur-
face, at the bottom of which is the opening into the long
tube-like spur in which the nectar is
found. Such a flower is adapted to
the large moths, with long probosces
which can reach the bottom of the
tube. As the moth thrusts its pro-
boscis into the tube, its head touches
the sticky button on each side, so that
when it flies away these buttons stick
to its head, sometimes directly to its
eyes, and the pollen masses are torn
out. These masses are then carried
to the next flower and are thrust
against the stigma in the attempt to get the nectar.

In the lady-slipper (Cypripedium), another orchid, the
flowers have a conspicuous pouch (see Fig. 137), in which
the nectar is secreted. A peculiar structure, like a flap,
overhangs the opening of the pouch, beneath which are the
two anthers, and between them the stigmatic surface (see
Fig. 138). Into the pouch a bee crowds its way and be-
comes imprisoned (see Fig. 139). The nectar which the
bee obtains is in the bottom of the pouch (see Fig. 140).
When escaping, the bee moves towards the opening over-
hung by the flap and rubs first against the stigmatic sur-
face (see Fig. 141), and then against the anthers, receiving
pollen on its back (see Fig. 142). A visit to another flower

Fig. 139. A bee imprisoned
in the pouch (partly cut
away) of Cypripedium.
—After Gibson.

FLOWERS AND INSECTS.

135

Fig. 140.

A bee obtaining nectar in the pouch of
C'ypripedium.— After Gibson.

will result in rubbing some of the pollen upon the stigma,
and in receiving more pollen for another flower.

In cases of protandry, as the common figwort, flowers
in the two condi-
tions will be visited
by the pollinating
insect, and as the
shedding stamens
and receptive stig-
mas occupy the
same relative posi-
tion, the pollen
from one flower
will be carried to the stigma of another. It is evident that
exactly the same methods prevail in the case of protogyny,
as the fire weed (see Fig. 134).

The Houstonia (see Fig. 135), in which there are sta-
mens and styles of different lengths, is visited by insects

whose bodies fill
the tube and pro-
trude above it. In
visiting flowers of
both kinds, one re-
gion of the body
receives pollen
from the short sta-
mens, and another
region from the
long stamens. In
this way the insect
will carry about two bands of pollen, which come in con-
tact with the corresponding stigmas. When there are three
forms of flowers, as mentioned in the case of one of the
loosestrifes, the insect receives three pollen bands, one for
each of the three sets of stigmas.

93. Warding off unsuitable insects. — Prominent among

Fig. 141. A bee escaping from the pouch of Cypri-
pedium, and coming in contact with the stigma.
Advancing a little further the bee will come in con-
tact with the anthers and receive pollen. — After
Gibson.

136

PLANT RELATIONS.

the unsuitable insects, which Kerner calls " unbidden
guests," are ants, and adaptations for reducing their visits
to a minimum may be taken as illustrations.

(1) Hairs. — A common device for turning back ants,
and other creeping insects, is a barrier of hair on the stem,
or in the flower cluster, or in the flower.

(2) Glandular secretions. — In some cases a sticky
secretion is exuded from the surface of plants, which

effectively stops
the smaller creep-
ing insects. In
certain species of
catch-fly a sticky
ring girdles each
joint of the stem.

(3) Isolation. —
The leaves of cer-
tain plants form
water reservoirs
about the stem.
To ascend such a
stem, therefore, a
creeping insect
must cross a series
of such reservoirs.
Teasel furnishes a
common illustration, the opposite leaves being united at
the base and forming a series of cups. More extensive
water reservoirs are found in Bilbergia and Ravenala
(" traveler's tree "), whose flower clusters are protected by
reservoirs formed by the rosettes of leaves, which creeping
insects cannot cross.

(4) Latex. — This is a milky secretion found in some
plants, as in milkweeds. Caoutchouc is a latex secretion
of certain tropical trees. When latex is exposed to the
air it stiffens immediately, becoming sticky and finally

•^t

Fig. 142. A bee escaping from the pouch of Cypri-
pedium, and rubbing against an anther. — After
Gibson.

FLOWERS AND INSECTS. 137

hard. In the flower clusters of many latex-secreting
plants the epidermis of the stem is very smooth and deli-
cate, and easily pierced by the claws of ants and other
creeping insects who seek to maintain footing on the
smooth surface. Wherever the epidermis is pierced the
latex gushes out, and by its stiffening and hardening glues
the insect fast.

(5) Protective forms. — In some cases the structure of
the flower prevents the access of small creeping insects to
the pollen or to the nectar. In the common snapdragon
the two lips are firmly closed (see Fig. 74), and they can be
forced apart only by some heavy insect, as the bumble-bee,
alighting upon the projecting lower lip, all lighter insects
being excluded. In many species of Pentstemon, one of
the stamens does not develop pollen sacs, b.ut lies like a bar
across the mouth of the pit in which the nectar is secreted.
Through the crevices left by this bar the thin proboscis of
a moth or butterfly can pass, but not the whole body of a
creeping insect. Very numerous adaptations of this kind
may be observed in different flowers.

(6) Protective closure. — Certain flowers are closed at
certain hours of the day, when there is the chief danger
from creeping insects. For instance, the evening prim-
roses open at dusk, after the deposit of dew, when ants are
not abroad ; and at the same time they secure the visits of
moths, which are night-fliers.

Numerous other adaptations to hinder the visits of
unsuitable insects may be observed, but those given will
serve as illustrations. In all cases it must be understood
that these so-called " adaptations " have not been produced
to ward off insects, but that having appeared from one
cause or another they have proved to be useful in this
particular.

CHAPTER VIII.

AN INDIVIDUAL PLANT IN ALL OF ITS RELATIONS.

For the purpose of summarizing the general life-rela-
tions detailed in the preceding chapters, it will be useful to
apply them in the case of a single plant. Taking a com-
mon seed-plant as an illustration, and following its history
from the germination of the seed, certain general facts
become evident in its relations to the external world.

94. Germination of the seed. — The most obvious needs of
the seed for germination are certain amounts of moisture
and heat. In order to secure these to the best advantage,
the seed is usually very definitely related to the soil, either
upon it and covered by moisture and heat-retaining debris,
or embedded in it. Along with the demand for heat and
moisture is one for air (supplying oxygen), which is essen-
tial to life. The relation which germinating seeds need,
therefore, is one which not only secures moisture and heat
advantageously, but permits a free circulation of air.

95. Direction of the root. — The first part of the young
plantlet to emerge from the seed is the tip of the axis
which is to develop the root system. It at once shows a
response to the earth influence (geotropism) and to the
moisture influence (hydrotropism), for whatever the direc-
tion of emergence from the seed, a curvature is developed
which directs the tip towards and finally into the soil (see
Fig. 143). When the soil is penetrated the primary root
may continue to grow vigorously downward, showing a
strong geotropic tendency, and forming what is known
as the tap-root, from which lateral roots arise, which are

138

AN INDIVIDUAL PLANT IN ALL OF ITS RELATIONS. 139

much more influenced in direction by other external
causes, especially the presence of moisture. As a rule,
the soil is not perfectly uniform, and contact with different
substances induces curvatures, and as a result of these and
other causes, the root system may become very intricate,
which is extremely favor- -p „

able for absorbing and
gripping.

96. Direction of the stem.
— As soon as the stem tip
is extricated from the seed,
it shows a response to the
light influence (heliotro-
pism), being guided in a
general way towards the
light (see Fig. 143a).
Direction towards the
light, the source of the in-
fluence, is spoken of as
positive heliotropism, as
distinguished from direc-
tion away from the light,
called negative heliotro-
pism. If the main axis
continues to develop, it
continues to show this posi-
tive heliotropism strongly,
but the branches may show
every variation from positive to transverse heliotropism ;
that is, a direction transverse to the direction of the rays
of light. In some plants certain stems, as stolons, run-
ners, etc., show strong transverse heliotropism, while other
stems, as rootstocks, etc., show a strong transverse geot-
ropism.

97. Direction of foliage leaves. — The general direction of
foliage leaves on an erect stem is transversely heliotropic ;

Fig. 143. Germination of the seed of
arbor-vitse (Tkaja). B shows the
emergence of the axis (?•) which is to
develop the root, and its turning to-
wards the soil. C shows a later stage,
in which the root (r) has been some-
what developed, and the stem of the
embryo (Jl) is developing a curve pre-
paratory to pulling out the seed leaves
(cotyledons). E shows the young plant-
let entirely free from the seed, with its
root (/■) extending into the soil, its stem
(h) erect, and its first leaves (c) hori-
zontally spread.— After Strasburger.

140

PLANT RELATIONS.

if necessary, the parts of the leaf or the stem itself twisting
to allow the blade to assume this position. The danger of
the leaves shading one another is reduced to a minimum by
the elongation of internodes, the spiral arrangement, short-
ening and changing direction upwards, or lobing.

This outlines the general nutritive relations, the roots

Fig. 143a. Germination of the garden bean, showing the arch of the seedling stem
above ground, its pull on the seed to extricate the cotyledons and plumule, and
the final straightening of the stem and expansion of the young leaves. — After
Atkinson.

and leaves being favorably placed for absorption, and the
latter also favorably placed for photosynthesis. It is im-
portant to study the behavior of various plants in the
germination of the seed, for in a comparatively short period
all of the important external relations of the vegetative
organs are established. Seeds should be selected that ger-
minate rapidly, and that represent different great groups,
such as squash, bean, corn, etc., and these observations
should be extended as far as possible by including the obser-
vation of seedlings in nature.

AN INDIVIDUAL PLANT IN ALL OF ITS RELATIONS. 141

98. Placing of flowers. — The purposes of the flower seem
to be served best by exposed positions, and consequently
flowers appear mostly at the extremities of stems and
branches, a position evidently favorable to pollination and
seed dispersal. The flowers thus exposed are very com-
monly massed, or, if not, the single flower is apt to be large
and conspicuous. The various devices for protecting nec-
tar and pollen against too great moisture, and the more
delicate structures against chill ; for securing the visits of
suitable insects, and warding off unsuitable insects ; and
for dispersing the seeds, need not be repeated.

99. Branch buds. — If the plant under examination be a
tree or shrub, branch buds will be observed to be developed
during the growing season (see Fig. 65). This device for
protecting growing tips through a season of dangerous cold
is very familiar to those living in the temperate regions.
The internodes do not elongate, hence the leaves overlap;
they develop little or no chlorophyll, and become scales.
The protection afforded by these overlapping scales is often
increased by the development of hairs, or by the secretion
of mucilage or gum.

CHAPTER IX.

THE STRUGGLE FOR EXISTENCE.

100. Definition. — The phrase "struggle for existence"
has come to mean, so far as plants are concerned, that it is
usually impossible for them to secure ideal relations, and
that they must encounter unfavorable conditions. The
proper light and heat relations may be difficult to obtain,
and also the proper relations to food material. It often
happens, also, that conditions once fairly favorable may be-
come unfavorable. Also, multitudes of plants are trying
to take ]30ssession of the same conditions. All this leads
to the so-called "struggle," and vastly more plants fail
than succeed. Before considering the organization of plant
associations, it will be helpful to consider some of the
possible changes in conditions, and the effect on plants.

101. Decrease of water. — This is probably the most com-
mon factor to fluctuate in the environment of a plant.
Along the borders of streams and ponds, and in swampy
places, the variation in the water is very noticeable, but the
same thing is true of soils in general. However, the change
chiefly referred to is that which is permanent, and which
compels plants not merely to tide over a drouth, but to
face a permanent decrease in the water supply.

Around the margins of ponds are very commonly seen
fringes of such plants as bulrushes, cat-tail flags, reed-
grasses, etc., standing in shoal water. As these plants
partially decay, their bodies and the entangled silt from
the land presently accumulate to such an extent that there
is no more standing water, and the water supply for the

THE STRUGGLE FOR EXISTENCE. 143

bulrushes and their associates has permanently decreased
below the favorable amount. In this way certain lake
margins gradually encroach upon the water, and in so
doing the water supply is permanently diminished for many
plants. By the same process, smaller lakelets are gradually
being converted into bogs, and the bogs in turn into drier
ground, and these unfavorable changes in water supply are
a menace to many plants.

The operations of man, also, have been very effective in
diminishing the water supply for plants. Drainage, which
is so extensively practiced, while it may make the water-
supply more favorable for the plants which man desires, cer-
tainly makes it very unfavorable for many other plants.
The clearing of forests has a similar result. The forest
soil is receptive and retentive in reference to water, and is
somewhat like a great sponge, steadily supplying the streams
which drain it. The removal of the forest destroys much
of this power. The water is not held and gradually doled
out, but rushes off in a flood ; hence, the streams which
drain the cleared area are alternately flooded and dried up.
This results in a much less total supply of water available
for the use of plants.

102. Decrease of light. — It is very common to observe
tall, rank vegetation shading lower forms, and seriously
interfering with the light supply. If the rank vegetation
is rather temporary, the low plants may learn to precede or
follow it, and so avoid the shading ; but if the over-shading
vegetation is a forest growth, shading becomes permanent.
In the case of deciduous trees, which drop their leaves at the
close of the growing season and put out a fresh crop in the
spring, there is an interval in the early spring, before the
leaves are fully developed, during which low plants may
secure a good exposure to light (see Fig. 144). In such
places one finds an abundance of "spring flowers," but later
in the season the low plants become very scarce. This
effective over-shading is not common to all forests, for

Pig. 144. A common spring plant (dog-tooth violet) which grows in deciduous
forests. The large mottled leaves and the conspicuous flowers are sent rapidly
above the surface from the subterranean bulb (see cut in the left lower corner)
where are also seen dissected out some petals and stamens and the pistil.

THE STRUGGLE FOR EXISTENCE. 145

there are "light forests," such as the oak forest, which
permit much low vegetation, as well as the shade forests,
such as beech forests, which permit very little.

In the forest regions of the tropics, however, the shad-
ing is jaermanent, since there is no annual fall of leaves.
In such conditions the climbing habit has been extensively
cultivated.

103. Change in temperature. — In regions outside of the
tropics the annual change of temperature is . a very im-
portant factor in the life of plants, and they have provided
for it in one way or another. In tracing the history of
plants, however, back into what are called "geological
times," we discover that there have been relatively per-
manent changes in temperature. Now and then glacial
conditions prevailed, during which regions before temperate
or even tropical were subjected to arctic conditions. It is
very evident that such permanent changes of temperature
must have had an immense influence upon plant life.

104. Change in soil composition. — One of the most ex-
tensive agencies in changing the compositions of soils in
certain regions has been the movement of glaciers of conti-
nental extent, which have deposited soil material over very
extensive areas. Areas within reach of occasional floods,
also, may have the soil much changed in character by the
new deposits. Shifting dunes are billow-like masses of
sand, developed and kept in motion by strong prevailing
winds, and often encroach upon other areas. Besides these
changes in the character of soil by natural agencies, the
various operations of man have been influential. Clearing,
draining, fertilizing, all change the character of the soil,
both in its chemical composition and its physical properties.

105. Devastating animals. — The ravages of animals form
an important factor in the life of many plants. For example,
grazing animals are wholesale destroyers of vegetation, and
may seriously affect the plant life of an area. The various
leaf feeders among insects have frequently done a vast

146 PLANT RELATIONS.

amount of damage to plants. Many burrowing animals
attack subterranean parts of plants, and interfere seriously
with their occupation of an area.

Various protective adaptations against such attacks have
been pointed out, but this subject probably has been much
exaggerated. The occurrence of hairs, prickles, thorns,
and spiny growths upon many plants may discourage the
attacks of animals, but it would be rash to assume that
these protections have been developed because of the danger
of such attacks. One of the families of plants most com-
pletely protected in this way is the great cactus family,
chiefly inhabiting the arid regions of southwestern United
States and Mexico. In such a region succulent vegetation
is at a premium, and it is doubtless true that the armor of
thorns and bristles reduces the amount of destruction.

In addition to armor, the acrid or bitter secretions of
certain plants or certain parts of plants would have a
tendency to ward off the attacks of animals.

106. Plant rivalry. — It is evident that there must be
rivalry among plants in occupying an area, and that those
plants which can most nearly utilize identical conditions
will be the most intense rivals. For example, a great many
young oaks may start up over an area, and it is evident
that the individuals must come into sharp competition with
one another, and that but few of them succeed in establish-
ing themselves permanently. This is rivalry between in-
dividuals of the same kind ; but some other kind of trees,
as the beech, may come into competition with the oak, and
another form of rivalry will appear.

As a consequence of plant rivalry, the different plants
which finally succeed in taking possession of an area are
apt to be dissimilar, and a plant association is usually made
up of plants which represent widely different regions of the
plant kingdom. It is sometimes said that any well-devel-
oped plant association is an epitome of the plant kingdom.

A familiar illustration of plant rivalry may be observed

THE STRUGGLE FOR EXISTENCE. 147

in the case of what are called "weeds." Every one is fa-
miliar with the fact that if cultivated ground is neglected
these undesirable plants will invade it vigorously and seri-
ously affect the development of plants under cultivation.

107. Adaptation. — When the changes mentioned above
occur in the environment of plants to such an extent as
to make the conditions for living very unfavorable, one
of three things is likely to occur, adaptation, migration,
or destruction.

The change in conditions may come slowly enough, and
certain plants may be able to endure it long enough to
adjust themselves to it. Such an adjustment may involve
changes in structure, and probably no plants are plastic
enough to adjust themselves to extreme and sudden changes
which are to be comparatively permanent. There are
plants, such as the common cress, which may be called
amphibious, which can live in the water or out of it without
change of structure, but this is endurance rather than
adaptation. Many plants, however, can pass slowly into
different conditions, such as drier soil, denser shade, etc.,
and corresponding changes in their structure may be noted.
Very often, however, such plants are given no opportunity
to adjust themselves to the new conditions, as the area is
apt to be invaded by plants already better adapted. While
adaptation may be regarded as a real result of changed con-
ditions, it would seem to be by no means the common one.

108. Migration. — This is a very common result of

changed conditions. Plants migrate as truly as animals,

though, of course, their migration is from generation to

generation. It is evident, however, that migration cannot

be universal, for barriers of various kinds may forbid it.

In general, these barriers represent unfavorable conditions

for living. If a plant area with good soil is surrounded by

a sterile area, the latter would form an efficient barrier to

migration from the former. Plants of the lowlands could

not cross mountains to escape from unfavorable conditions.
11

148 PLANT KELATIONS.

To make migration possible, therefore, it is necessary for
the conditions to be favorable for the migrating plants in
some direction. In the case of bulrushes, cat-tail flags,
etc., growing in the shoal water of a lake margin, the
building up of soil about them results in unfavorable con-
ditions. As a consequence, they migrate further into the
lake. If the lake happens to be a small one, the filling up
process may finally obliterate it, and a time will come when
such forms as bulrushes and flags will find it impossible to
migrate.

In glacial times very many arctic plants migrated south-
ward, especially along the mountain systems, and many
alpine plants moved to lower ground. When warmer con-
ditions returned, many plants that had been driven south
returned towards the north, and the arctic and alpine plants
retreated to the north and up the mountains. The history
of plants is full of migrations, compelled by changed con-
ditions and permitted in various directions. It must be
remembered, also, that migrations often result in changes
of structure.

109. Destruction. — Probably this is by far the most com-
mon result of greatly changed conditions. Even if plants
adapt themselves to changed conditions, or migrate, their
structure may be so changed that they will seem like quite
different plants. In this way old forms gradually disappear
and new ones take their places.

CHAPTER X.

THE NUTRITION OP PLANTS.

110. Physiology. — In the previous chapters plants have
been considered in reference to their surroundings. It
was observed that various organs of nutrition hold certain
life-relations, but it is essential to discover what these rela-
tions mean to the life of the plant. The study of plants
from the standpoint of their life-relations has been called
Ecology ; the study of the life-processes of plants is called
Physiology. These two points of view may be illustrated
by comparing them to two points of view for the study of
man. Man may be studied in reference to his relation to
his fellow-men and to the character of the country in which
he lives ; or his bodily processes may be studied, such as
digestion, circulation, respiration, etc. The former cor-
responds to Ecology, the latter is Physiology.

All of the ecological relations that have been mentioned
find their meaning in the physiology of the plant, for life-
relations have in view life-processes. The subject of plant
physiology is a very complex one, and it would be impossi-
ble in an elementary work to present more than a few very
general facts. Certain facts in reference to plant move-
ments, an important physiological subject, have been men-
tioned in connection with life-relations, but it seems neces-
sary to make some special mention of nutrition.

111. Significance of chlorophyll. — Probably the most im-
portant fact to observe in reference to the nutrition of
plants is that some plants are green or have green parts,
while others, such as toadstools, do not show this green

150 PLANT EELATIONS.

color. It has been stated that this green color is due to
the presence of a coloring matter known as chlorophyll
(see §12). The two groups may be spoken of, therefore,
as (1) green plants and (2) plants without chlorophyll.
The presence of chlorophyll makes it possible for the plants
containing it to manufacture their own food out of such
materials as water, soil material, and gases. For this
reason, green plants may be entirely independent of all
other living things, so far as their food supply is concerned.

Plants without chlorophyll, however, are unable to
manufacture food out of such materials, and must obtain
it already manufactured in the bodies of other plants or
animals. For this reason, they are dependent upon other
living things for their food supply, just as are animals. It
is evident that plants without chlorophyll may obtain this
food supply either from the living bodies of plants and ani-
mals, in which case they are called parasites, or they may
obtain it from the substances derived from the bodies of
plants and animals, in which case they are called sapro-
phytes. For example, the rust which attacks the wheat,
and is found upon the leaves and stems of the living plant,
is a parasite ; while the mould which often develops on stale
bread is a saprophyte. Some plants without chlorophyll
can live either as parasites or saprophytes, while others are
always one or the other. By far the largest number of
parasites and saprophytes belong to the group of low plants
called fungi, and when fungi are referred to, it must be
understood that it means the greatest group of plants with-
out chlorophyll.

112. Photosynthesis. — The nutritive processes in green
plants are the same as mother plants, and in addition there
is in green plants the peculiar process known as photosyn-
thesis (see §25). In plants with foliage leaves, these are
the chief organs for this work. It must be remembered,
however, that leaves are not necessary for photosynthesis,
for plants without leaves, such as algaj, perform it. The

THE NUTRITION OF PLANTS. 151

essential thing is green tissue exposed to light, but in this
brief account an ordinary leafy plant growing in the soil
will be considered.

As the leaves are the active structures in the work of
photosynthesis, the raw materials necessary must be brought
to them. In a general way, these materials are carbon di-
oxide and water. The gas exists diffused through the
atmosphere, and so is in contact with the leaves. It also
occurs dissolved in the water of the soil, but the gas used
is absorbed from the air by the leaves. The supply of
water, on the other hand, in soil-related plants, is obtained
from the soil. The root system absorbs this water, which
then ascends the stem and is distributed to the leaves.

(1) Ascent of water. — The water does not move up-
wards through all parts of the stem, but is restricted to a
certain definite region. This region is easily recognized as
the woody part of stems. Sometimes separate strands of
wood, looking like fibers, may be seen running lengthwise
through the stem ; sometimes the fibrous strands are packed
so close together that they form a compact woody mass, as
in shrubs and trees. In the case of most trees new wood is
made each year, through which the water moves. Hence
the very common distinction is made between sap-wood,
through which the water is moving, and heart-wood, which
the water current has abandoned. Just how the water
ascends through these woody fibers, especially in tall trees,
is a matter of much discussion, and cannot be regarded as
definitely known. In any event, it should be remembered
that these woody fibers are not like the open veins and
arteries of animal bodies, and no "circulation" is possible.
These same woody strands are seen branching throughout
the leaves, forming the so-called vein system, and it is evi-
dent, therefore, that they form a continuous route from
roots to leaves.

It is easy to demonstrate the ascent of water in the
stem, and the path it takes, by a simple experiment. If

152 PLANT RELATIONS.

an active stem be cut and plunged into water stained with
an aniline color called eosin,* the ascending water will stain
its pathway. After some time sections through the stem
will show that the water has traveled upwards through it,
and the stain will point out the region of the stem used in
the movement.

In general, therefore, the carbon dioxide is absorbed
directly from the air by the leaves, and the water is ab-
sorbed by the root from the soil, and moves upwards through
the stem into the leaves. An interesting fact about these
raw materials is that they are very common waste products.
They are waste products because in most life-processes they
cannot be taken to pieces and used. The fact that they

can be used in photosynthesis
shows that it is a very re-
markable life process.

(2) Chloroplasts. — Having
obtained some knowledge of
the raw materials used in
photosynthesis, and their

Fig. 145. Some mesopnyll cells from x . .

the leaf of i^Monia, showing chloro- SOlirceS, it is necessary to

plasts- consider the plant machinery

arranged for the work. In the working leaf cells it is
discovered that the color is due to the presence of very
small green bodies, known as chlorophyll bodies or chloro-
plasts (see Fig. 145). These consist of the living substance,
known as protoplasm, and the green stain called chloro-
phyll ; therefore, each chloroplast is a living body (plastid)
stained green. It is in these chloroplasts that the work ol
photosynthesis is done. In order that they may work it
is necessary for them to obtain a supply of energy from
some outside source, and the source used in nature is sun-
light. The green stain (chlorophyll) seems to be used in
absorbing the necessary energy from sunlight, and the

* The commoner grades of red ink are usually solutions of eosin.

THE NUTRITION OF PLANTS. 153

plastid uses this energy in the work of photosynthesis. It
is evident, therefore, that photosynthesis goes on only in
the sunlight, and is suspended entirely at night. It is
found that any intense light can be used as a substitute
for sunlight, and plants have been observed to carry on
the work of photosynthesis in the presence of electric
light.

(3) Result of photosynthesis. — The result of this work
can be stated only in a very general way. Carbon dioxide
is composed of two elements, carbon and oxygen, in the
proportion one part of carbon to two parts of oxygen.
Water is also composed of two elements, hydrogen and oxy-
gen. In photosynthesis the elements composing these sub-
stances are separated from one another, and recombined in
a new way. In the process a certain amount of oxygen is
liberated, just as much as was in the carbon dioxide, and a
new substance is formed, known as a carbohydrate. The
oxygen set free escapes from the plant, and may be re-
garded as waste product in the process of photosynthesis.
It will be remembered that the external changes in this
process are the absorption of carbon dioxide and the giving
off of oxygen (see §25).

(4) Carbohydrates and proteids. — The carbohydrate
formed is an organic substance ; that is, a substance made
in nature only by life processes. It is the same kind of
substance as sugar or starch, and all are known as carbohy-
drates ; that is, substances composed of carbon, and of hy-
drogen and oxygen in the same proportion as in water.
The work of photosynthesis, therefore, is to form carbohy-
drates. The carbohydrates, such as sugar and starch, rep-
resent but one type of food material. Proteids represent
another prominent type, substances which contain carbon,
hydrogen, and oxygen, as do carbohydrates, but which also
contain other elements, notably nitrogen, sulphur, and
phosphorus. The white of an egg may be taken as an ex-
ample of proteids. They seem to be made from the carbo-

154 PLANT RELATIONS.

hydrates, the nitrogen, sulphur, and other necessary
additional elements being obtained from soil substances
dissolved in the water which is absorbed and conveyed
to the leaves.

113. Transpiration. — The water which is absorbed by the
roots and passes to the leaves is much more abundant than
is needed in the process of photosynthesis. It should be re-
membered that the water is not only used as a raw material
for food manufacture, but also acts as a solvent of the soil
materials that are passing into the plant. The water in
excess of the small amount used in food manufacture is
given off from the plant in the form of water vapor, the
process being already referred to as transpiration (see §26).

114. Digestion. — Carbohydrates and proteids may be re-
garded as prominent types of plant food which green
plants are able to manufacture. These foods are trans-
ported through the plant to regions where work is going on,
and if there is a greater supply of food than is needed for
the working regions, the excess is stored up in some part
of the plant. As a rule, green plants are able to manufac-
ture much more food than they use, and it is upon this ex-
cess that other plants and animals live. In the transfer of
foods through the plant certain changes are often neces-
sary. For example, starch is insoluble, and hence cannot
be carried about in solution. It is necessary to transform
it into sugar, which is soluble. These changes, made to
facilitate the transfer of foods, represent digestion.

115. Assimilation. — When food in some form has reached
a working region, it is organized into the living substance
of the plant, known as protoplasm, and the protoplasm
builds the plant structure. This process of organizing the
food into the living substance is known as assimilation.

116. Respiration. — The formation of foods, their diges-
tion and assimilation are all preparatory to the process of
respiration, which may be called the use of assimilated
food. The whole working power of the plant depends

THE NUTRITION OF PLANTS. 155

upon respiration, which means the absorption of oxygen by
the protoplasm, the breaking down of protoplasm, and the
giving off of carbon dioxide and water as wastes. The im-

Fig, 146. The common Northern pitcher plant. The hollow leaves, each with a hood
and a wing, form a rosette, from the center of which arise the flower stalks.—
After Keener.

portance of this process may be realized when it is remem-
bered that there is the same need in our own living, as it
is essential for us also to " breathe in " oxygen, and as a
result we " breathe out " carbon dioxide and water. This
breaking down or " oxidizing " of protoplasm releases the

156

PLANT RELATIONS.

power by which the work of the plant is carried on (see
§27).

117. Summary of life-processes. — To summarize the nu-
tritive life-processes in green plants, therefore, photosyn-
thesis manufactures carbohydrates,
the materials used being carbon
dioxide and water, the work being
done by the chloroplast with the
aid of light ; the manufacture of
proteids uses these carbohydrates,
and also substances containing
nitrogen, sulphur, etc.; digestion
puts the insoluble carbohydrates
and the proteids into a soluble
form for transfer through the
plant ; assimilation converts this
food material into the living sub-
stance of the plant, protoplasm ;
respiration is the oxidizing of the
protoplasm which enables the
plant to work, oxygen being ab-
sorbed, and carbon dioxide and
water vapor being given off in
the process.

118. Plants without chlorophyll.
— Eemembering the life-processes
described under green plants, it is
evident that plants without chlo-

Fig 147 The Southern pitcher ] H calmot do the WQrk of

plant, showing the funnelform r J

and winged pitcher, and the photosynthesis. This HieUllS that

overarching hood with transia- they c.limot manufacture carbo-

cent spots. — After Keknek. j

hydrates, and that they must de-
pend upon other plants or animals for this important food.
Mushrooms, puff-balls, moulds, mildews, rusts, dodder,
corpse plants, beech drops, etc., may be taken as illustra-
tions of such plants.

THE NUTRITION OF PLANTS.

157

Although plants without chlorophyll cannot manufac-
ture carbohydrates, the other processes, proteid manufac-
ture, digestion, assimilation, and respiration, are carried on.
It is true, however, that in obtaining carbohydrates from
other plants and ani-
mals, proteids are ob-
tained also, so that
proteid manufacture
is not so prominent as
in green plants.

119. "Carnivorous"
plants. — This name has
been given to plants
which have developed
the curious habit of
capturing insects and
using them for food.
They are green plants
and, therefore, can man-
ufacture carbohydrates.
But they live in soil
poor in nitrogen com-
pounds, and hence pro-
teid formation is inter-
fered with. The bodies
of captured insects sup-
plement the proteid
supply, and the plants
have come to depend
upon them. Many, if
not all of these carniv-
orous plants, secrete a
digestive substance
which acts upon the

bodies of the captured insects very much as the diges-
tive substances of the alimentary canal act upon proteids

Fig. 148. The Californian pitcher plant (Darling-
torda), showing twisted and winged pitcher,
the overarching hood with translucent spots,
and the fish-tail appendage to the hood
which is attractive to flying insects.— After
Kerner.

158

PLANT RELATIONS.

swallowed by animals. Some common illustrations are as
follows :

(1) Pitcher plants. — In these plants the leaves form
tubes, or urns, of various forms, which contain water, and
to which insects are attracted and drowned (see Fig. 146).
A pitcher plant common throughout the Southern States
may be taken as a type (see Fig. 147). The leaves are
shaped like slender, hollow cones, and rise in a tuft from

the swampy ground.
The mouth of this
conical urn is over-
arched and shaded
by a hood, in which
are translucent spots,
like small windows.
Around the mouth
of the urn are
glands, which se-
crete a sweet liquid
{nectar), and nectar
drops form a trail
down the outside of
the urn. Inside, just
below the rim of the
urn, is a glazed zone,
so smooth that insects
cannot walk upon it.
Below the glazed zone
is another zone,
thickly set with stiff,
downward-pointing hairs, and below this is the liquid in
the bottom of the urn.

If a fly is attracted by the nectar drops upon this curious
leaf, it naturally follows the trail up to the rim of the urn,
where the nectar is abundant. If it attempts to descend
within the urn, it slips on the glazed zone, and falls into

Fig. 149.

A sun-dew, showing rosette habit of
the insect-catching leaves.

THE NUTE1TI0N OF PLANTS.

159

the water, and if it attempts to escape by crawling up the
sides of the urn, the thicket of downward-pointing hairs
prevents. If it seeks to fly away from the rim, it flies
towards the translucent spots in the hood, which look like
the way of escape, as the direction of entrance is in the
shadow of the hood. Pounding against the hood, the fly
falls into the tube. This Southern pitcher plant is known

Fig. 150. Two leaves of a sun-dew. The one to the right has its glandular hairs
fully expanded ; the one to the left shows half of the hairs bending inward, in the
position assumed when an insect has been captured. — After Kerner.

as a great fly-catcher, and the urns are often well supplied
with the decaying bodies of these insects.

A much larger Californian pitcher plant has still more
elaborate contrivances for attracting insects (see Fig. 148).

(2) Drosera. — The droseras are commonly known as
" sun-dews," and grow in swampy regions, the leaves form-
ing small rosettes on the ground (see Fig. 149). In one
form the leaf blade is round, and the margin is beset by
prominent bristle-like hairs, each with a globular gland at
its tip (see Fig. 150). Shorter gland-bearing hairs are

160

PLANT RELATIONS.

scattered also over the inner surface of the blade. These
glands excrete a clear, sticky fluid, which hangs to them in
drops like dew-drops. If a small insect becomes entangled

Fig. 151. Plants of Dioncea, showing the rosette habit of the leaves with terminal
traps, and the erect flowering stem.— After Kerner.

in the sticky drop, the hair begins to curve inward, and
presently presses its victim down upon the surface of the
blade. In the case of larger insects, several of the marginal
hairs may join together in holding it, or the whole blade
may become more or less rolled inward.

THE NUTRITION OF PLANTS.

161

(3) Dioncea. — This is one of the most famous and re-
markable of ily-catching plants (see Fig. 151). It is found
in sandy swamps near Wilmington, North Carolina. The
leaf blade is constructed like a steel trap, the two halves
snapping together, and the marginal bristles interlocking
like the teeth of a trap (see Fig. 152). A few sensitive
hairs, like feelers, arc

developed on the leaf ^dk\Ww'k

surface, and when one
of these is touched by
a small flying or hover-
ing insect, the trap
snaps shut and the in-
sect is caught. Only
after digestion does the
trap open again.

There are certain
green plants, not called
carnivorous plants,
which show the same
general habit of sup-
plementing their food
supply, and so reduc-
ing the necessity of
food manufacture.
The mistletoe is a
green plant, growing
upon certain trees, from

which it obtains some food, supplementing that which it
is able to manufacture.

Fig. 152. Three leaves of Dioticea, showing
the details of the trap in the leaves to right
and left, and the central trap in the act of
capturing an insect.

CHAPTER XL

PLANT ASSOCIATIONS: ECOLOGICAL. FACTORS.

120. Definition of a plant association. — From the previous
chapters it has been learned that every complex plant is a
combination of organs, and that each organ is related in
some special way to its environment. It follows, there-
fore, that the whole plant, made up of organs, holds a very
complex relation with its environment. The stem demands
certain things, the root other things, and the leaves still
others. To satisfy all of these demands, so far as possible,
the whole plant is delicately adjusted.

The earth's surface presents very diverse conditions in
reference to plant life, and as plants are grouped according
to these conditions, this leads to definite associations of
plants, those adapted to the same general conditions being
apt to live together. Such an assemblage of plants living
together in similar conditions is & plant association, the con-
ditions forbidding other plants. It must not be understood
that all plants affecting the same conditions will be found liv-
ing together. For example, a meadow of a certain type will
not contain all the kinds of grasses associated with that type.
Certain grasses will be found in one meadow, and other
grasses will be found in other meadows of the same type.

The rivalry of closely related plants living in the same
association is apt to be intense, on account of their similar
demands, and unrelated plants are able to live together with
the least rivalry. A plant association, therefore, may con-
tain a wide representation of the plant kingdom, from
plants of low rank to those of high rank.
162

PLANT ASSOCIATIONS: ECOLOGICAL FACTORS. 163

Before considering some of the common associations, it
is necessary to note some of the conditions that deter-
mine plant associations. Those things in the environment
of the plant which influence the organization of an associa-
tion are known as ecological factors.

121. Water. — Water is certainly one of the most im-
portant conditions in the environment of a plant, and has
great influence in determining the organization of associa-
tions. If all plants are considered, it will be noted that the
amount of water to which they are exposed is exceedingly
variable. At one extreme are those plants which are com-
pletely submerged ; at the other extreme are those plants
of arid regions which can obtain very little water ; and be-
tween these extremes there is every gradation in the amount
of available water. Among the most striking adaptations
of plants are those for living in the presence of a great
amount of water, and those for guarding against its lack.

One of the first things to consider in connection with
any plant association is the amount of water supply. It is
not merely a question of its total annual amount, but of its
distribution through the year. Is it supplied somewhat
uniformly, or is there alternating flood and drought ? The
nature of the water supply is also important. Are there
surface channels or subterranean channels, or does the
whole supply come in the form of rain and snow which
fall upon the area?

Another important fact to consider in connection with
the water supply has to do with the structure of the soil.
There is what may be called a water level in soils, and it is
important to note the depth of this level beneath the sur-
face. In some coils it is very near the surface ; in others,
such as sandy soils, it may be some distance beneath the
surface.

Xot only do the amount of water and the depth of the
water level help to determine plant associations, but also the
substances that the water contains. Two areas may have
12

164 PLANT RELATIONS.

the same amount of water and the same water level, but if
the substances dissolved in the water differ in certain par-
ticulars, two entirely distinct associations may result.

122. Heat. — The general temperature of an area is im-
portant to consider, but it is evident that differences of
temperature are not so local as differences in the water sup-
ply, and therefore this factor is not so important in the
organization of the plant associations of any given neigh-
borhood as is the water factor. Even in the distribution
of plants over the surface of the earth, however, the water
factor is probably more important than the heat factor. The
range of temperature which the plant kingdom, as a whole,
can endure during active work may be stated in a general
way as from 0° to 50° 0. ; that is, from the freezing point
of water to 122° Fahr. There are certain plants that can
work at higher temperatures, notably certain algas growing
in hot springs, but they may be regarded as exceptions. It
must be remembered that the range of temperature given
is for plants actively at work, and does not include the tem-
perature which many plants are able to endure in a specially
protected but very inactive condition. For example, many
plants of the temperate regions endure a winter tempera-
ture which is frequently lower than the freezing point of
water, but it is a question of endurance and not of work

It must not be supposed that all plants can work equally
well throughout the whole range of temperature given, for
they differ widely in this regard. Tropical plants, for in-
stance, accustomed to a certain limited range of high tem-
perature, cannot work continuously at the lower tempera-
tures. For each kind of plant there is what may be called
a zero point, below which it is not in the habit of working.

While it is important to note the general temperature
of an area throughout the year, it is also necessary to note
its distribution. Two regions may have presumably the
same amount of heat through the year, but if in the one case
it is uniformly distributed, and in the other great extremes

PLANT ASSOCIATIONS: ECOLOGICAL FACTORS. 165

of temperature occur, the same plants will not be found in
both. It is, perhaps, most important to note the tempera-
ture during certain critical periods in the life of plants,
such as the flowering period of seed-plants.

Although the temperature problem may be compara-
tively uniform over any given area, the effect of it may be
noted in the succession of plants through the growing sea-
son. In our temperate regions the spring plants and summer
plants and autumn plants differ decidedly from one another.
It is evident that the spring plants can endure greater
cold than the summer plants, and the succession of flowers
will indicate somewhat these relations of temperature.

It should be remarked, also, that not only is the tem-
perature of the air to be noted, but also that of the soil.
These two temperatures may differ by several degrees, and
the soil temperature especially affects root activity, and
hence is a very important factor to discover.

At this point it is possible to call attention to the effect
of the combination of ecological factors. For instance, in
reference to the occurrence of plants in any association, the
water factor and the heat factor cannot be considered each
by itself, but must be taken in combination. For example,
if in a given area there is a combination of maximum heat
and minimum water, the result will be a desert, and only
certain specially adapted plants can exist. It is evident
that the great heat increases the transpiration, and trans-
piration when the supply of water is very meager is pecu-
liarly dangerous. Plants which exist in such conditions,
therefore, must be specially adapted for controlling tran-
spiration. On the other hand, if in any area the combina-
tion is maximum heat and maximum water, the result will
be the most luxuriant vegetation on the earth, such as
grows in the rainy tropics. It is evident that the possible
combinations of the water and heat factors may be very
numerous, and that it is such combinations that chiefly
affect plant associations.

166 PLANT RELATIONS.

123. Soil. — The soil factor is not merely important to
consider in connection with those plants directly related
to the soil, but is a factor for all plants, as it determines
the substances which the water contains. There are two
things to be considered in connection with the soil, namely,
its chemical composition and its physical properties. Per-
haps the physical properties arc more important from the
standpoint of soil-related plants than the chemical com-
position, although both the chemical and physical nature
of the soil are so bound up together that they need not be
considered separately here. The physical properties of the
soil, which are important to plants, are chiefly those which
relate to the water supply. It is always important to de-
termine how receptive a soil is. Does it take in water
easily or not ? It is also necessary to determine how re-
tentive it is ; it may receive water readily, but it may not
retain it.

For convenience in ordinary field work with plants,
soils may be divided roughly into six classes : (1) rock,
which means solid uncrumbled rock, upon which certain
plants are able to grow ; (2) sand, which has small water
capacity, that is, it may receive water readily enough, but
does not retain it ; (3) lime soil ; (4) clay, which has great
water capacity ; (5) humus, which is rich in the products
of plant and animal decay ; ((5) salt soil, in which the water
contains certain salts, and is generally spoken of as alka-
line. These divisions in a rough way indicate both the
structure of the soil and its chemical composition. Not
only should the kinds of soil on an area be determined,
but their depth is an important consideration. It is
very common to find one of these soils overlying another
one, and this relation between the two will have a very
important effect. For instance, if a sand soil is found
lying over a clay soil, the result will be that the sand soil
will retain far more water than it would alone. If a humus
soil in one area overlies a sand soil, and in another area

PLANT ASSOCIATIONS: ECOLOGICAL tfAOTOKS. 1(J7

overlies a clay soil, the humus will differ very much in the
two cases in reference to water.

The soil cover should also be considered. The common
soil covers are snow, fallen leaves, and living plants. It
will be noticed that all these covers tend to diminish the
loss of heat from the soil, as Avell as the access of heat to
the soil. In other words, a good soil cover will very much
diminish the extremes of temperature. All this tends to
increase the retention of water.

124. Light. — It is known that light is essential for the
peculiar work of green plants. However, all green plants
cannot have an equal amount of light, and some have
learned to live with a less amount than others. While
no sharp line can be drawn between green plants which
use intense light, and those which use less intense light,
we still recognize in a general way what are called light
plants and shade plants. We know that certain plants
are chiefly found in situations where they can be exposed
freely to light, and that other plants, as a rule, are found
in shady situations.

Starting with this idea, we find that plants grow in
strata. In a forest association, for example, the tall trees
represent the highest stratum ; below this there may be a
stratum of shrubs, then tall herbs, then low herbs, then
forms like mosses and lichens growing close to the ground.
In any plant association it is important to note the num-
ber of these strata. It may be that the highest stratum
shades so densely that many of the other strata are not
represented at all. An illustration of this can be obtained
from a dense beech forest.

125. Wind. — It is generally known that wind has a dry-
ing effect, and, therefore, it increases the transpiration of
plants and tends to impoverish them in water. This fac-
tor is especially conspicuous in regions where there are pre-
vailing winds, such as near the seacoast, around the great
lakes, and on the prairies and plains. In all such regions

168 PLANT RELATIONS.

the plants have been compelled to adapt themselves to this
loss of water ; and in some regions the prevailing winds
are so constant and violent that the force of the wind itself
has influenced the appearance of the vegetation, giving
what is called a characteristic physiognomy to the area.

These five factors have been selected from a much
larger number that might be enumerated, but they may
be regarded as among the most important ones. It will be
noticed that these factors may be combined in all sorts
of ways, so that an almost endless series of combinations
seems to be possible. This will give some idea as to the
possible number of plant associations, for they may be as
numerous as are the combinations of these factors.

126. The great groups of associations. — It is possible to
reduce the very numerous associations to three or four
great groups. For convenience, the water factor is chiefly
used for this classification. It results in a convenient
classification, but one that is certainly more or less arti-
ficial. The selection of any one factor from among the
many for the purpose of classification never results in a
very natural classification when the combination of factors
determines the group. However, for general purposes, the
usual classification on the basis of water supply will be
used. On this basis there are three great groups of asso-
ciations, as follows :

(1) Hydrophytes. — The name means " water plants," and
suggests that such associations are at that extreme of the
water supply where it is very abundant. Such plants may
grow in the water, or in very wet soil, but in any event
they are exposed to a large amount of water.

(2) Xerophytes. — The name means " drouth plants," and
suggests the other extreme of the water supply. True
xerophytes are exposed to dry soil and dry atmosphere.

(3) Mesophytes. — Between the two extremes of the
water supply there is a great middle region of medium
water supply, and plants that occupy it are known as

PLANT ASSOCIATIONS: ECOLOGICAL FACTORS. 169

mesophytes, the plants of medium conditions. It is evi-
dent that mesophytes gradually pass into hydrophytes on
the one side, and into xerophytes on the other ; but it is
also evident that mesophyte associations have the greatest
range of water supply, extending from a large amount of
water to a very small amount.

It should be understood that these three groups of
associations, which are distinguished from one another by
the amount of the water supply, are artificial groups rather
than natural ones, for they bring together unrelated asso-
ciations, and often separate those that are closely related.
For example, a swampy meadow is put among hydrophyte
associations by this classification ; and it may shade into an
ordinary meadow, which belongs among the mesophytes.
Probably the largest fact that may be used in grouping
plant associations is that certain associations are so situ-
ated that they seek for the most part to reduce transpira-
tion, and that others are so situated that they seek for the
most part to increase transpiration.

However, the factors that determine associations are
so numerous that they cannot be presented in an elementary
book, and the simpler artificial grouping given above will
serve to introduce the associations to observation.

CHAPTER XII.

HYDROPHYTE ASSOCIATIONS.

127. General character. — Hydrophytes are related to
abundant water, either throughout their whole structure
or in part of their structure. It is a well-known fact that
hydrophytes are among the most cosmopolitan of plants,
and hydrophyte associations in one part of the world look
very much like hydrophyte associations in any other
region. It is probable that the abundant water makes the
conditions more uniform.

It is evident that for those plants, or plant parts, which
are submerged, the water affects the heat factor by dimin-
ishing the extremes. It also affects the light factor, in so
far as the light must pass through the water to reach the
chlorophyll-containing parts, as light is diminished in in-
tensity by passing through the water. Before considering
a few hydrophyte associations, it is necessary to note the
prominent hydrophyte adaptations.

128. Adaptations. — In order that the illustration may be
as simple as possible, a complex plant completely exposed
to water is selected, for it is evident that the relations of a
swamp plant, with its roots in water and its stem and leaves
exposed to air, are complicated. A number of adaptations
may be noted in connection Avith the submerged or floating
plant.

(1) Thin-walled epidermis. — In the case of the soil-re-
lated plants, the water supply comes mainly from the soil,
and the root system is constructed to absorb it. In the
case of the water plant under consideration, however, the

HYDROPHYTE ASSOCIATIONS.

171

whole plant body is exposed to the water supply, and there-
fore absorption may take place through the whole surface
rather than at any particular region such as the root. In
order that this may be done, however, it is necessary for
the epidermis to have thin walls, which is usually not the
case in epidermis exposed
to the air, where a certain
amount of protection is
needed in the way of
thickening.

(2) Roots much reduced
or wanting. — It must be
evident that if water is
being absorbed by the
whole free surface of the
plant, there is not so
much need for a special
root region for absorp-
tion. Therefore, in such
water plants the root sys-
tem may be much re-
duced, or may even disap-
pear entirely. It is often
retained, however, to act
as a holdfast, rather than
as an absorbent organ, for
most water plants anchor
themselves to some sup-
port.

(3) Reduction of water-conducting tissues. — In the ordi-
nary soil-related plants, not only is an absorbing root sys-
tem necessary, but also a conducting system, to carry the
water absorbed from the roots to the leaves and elsewhere.
It has already been noted that this conducting system takes
the form of woody strands. It is evident that if water
is being absorbed by the whole surface of the plant, the

Fig. 153. Fragment of a common seaweed
(Fitcus), showing the body with forking
branching and bladder-like air cavities.—
After Lueks^en.

172

PLANT RELATIONS.

work of conduction is not so extensive or definite, and
therefore in such water plants the woody bundles are not
so prominently developed as in land plants.

(4) Reduction of mechanical tissues. — In the case of
ordinary land plants, certain firm tissues are developed so

Fig. 154. Gulfweed (jSargassum), showing the thallns differentiated into stem-like and
leaf -like portions, and also the bladder-like floats.- Alter Bennktt and Mhkkay.

that the plant may maintain its form. These supporting
tissues reach their culmination in such forms us trees,
where massive bodies are able to stand upright. It is evi-
dent that in the water there is no such need for rigid sup-
porting tissues, as the buoyant power of water helps to
support the plant. This fact may be illustrated by taking

HYDKOPHYTE ASSOCIATIONS.

173

out of water submerged plants which seem to be upright,
with all their parts properly spread out. When removed they
collapse, not being able to support themselves in any way.
(5) Development of air cavities. — The presence of air in
the bodies of water plants is necessary for two reasons: (1),

,f .- %

Fig. 155. Bladderwort, showing the numerous bladders which float the plant, the
finely divided water leaves, and the erect flowering stems. The bladders are also
effective "insect traps," Utricalaria being one of the "carnivorous plants."
— After Kerner.

to aerate the plant ; (2), to increase its buoyancy. In most
complex water plants there must be some arrangement for
the distribution of air containing oxygen. This usually
takes the form of air chambers and passageways in the
body of the plant (see Figs. 87, 88, 89, 156). Of course
such air chambers increase the buoyancy of the body.
Sometimes, however, a special buoyancy is provided for
by the development of regular floats, which are bladder-

174 PLANT RELATIONS.

like bodies (see Figs. 153, 154). These floats are very com-
mon among certain of the seaweeds, and are fonnd among
higher plants, as the utricularias or bladderworts, which
have received their name from the numerous bladders
developed in connection with their bodies (see Fig. 155).

129. The two groups of associations. — The hydrophyte
associations may be put into two great divisions. True
h i/drophytes are those in which the contents and tempera-
ture of the water are favorable to plant activity; while
xerophytic hydrophytes are those in which the contents
and temperature of the water are unfavorable to plant
activity, and the structures of the plants are adapted to
reduce transpiration, resembling in this feature the struc-
tures displayed by the true xerophytes (see §155).

I. True hydrophytes.
A. Free-swimming associations.

130. Definition. — In these associations there is the largest
exposure to water, and no relation at all to the nutrient or
mechanical support of the soil, the plants being completely
supported by the water. They may be either submerged
or floating, and they are free to move either by locomotion
or by water currents. Two prominent associations are
selected as types.

131. The plankton. — This term is used to designate the
minute organisms, both plants and animals, that are found
in the water. The plankton is composed of individuals
invisible to the naked eye, but taken together they repre-
sent an enormous organic mass. The plankton associa-
tions are especially well represented in the colder oceanic
waters, but they are not absent from any waters. Among
the most prominent plants in these associations are the
diatoms. Diatoms are minute plants of various forms, and
all have a wall very full of silica. This makes their bodies

HYDROPHYTE ASSOCIATIONS.

175

extremely enduring, and therefore diatoms are often found
in great deposits in the rocks, in some cases forming the
whole mass of rock. Associated with the diatoms are
numerous other plant and animal forms.

132. Pond associations. — The word pond is used to indi-
cate stagnant or slow-moving waters. In such waters free-
swimming plants of all groups are associated. Of course
the alga? arc well represented, but even the highest plants
are repre-
sented by the
duckweeds,
which are very
commonl y
seen in the
form of small
green disks
floating on the
surface of the
water, which
they frequent-
ly cover with
great masses

(see Fig. 150). It should be observed that the floating and
submerged positions result in a difference in light-relations.
The floating forms may be regarded as light forms, being
exposed to the greatest amount of light. The submerged
forms are shade plants, and the shading becomes greater
as the depth of the water is greater. It must not be sup-
posed that submerged plants can live at any depth, for
soon a limit is reached, beyond which the light is not
intense enough to enable plants to work.

It has been noticed that this complete water habit has
affected plants in many ways. For instance, the duck-
weeds are related to land plants with root, stem, and leaves,
but they have lost the distinction between stem and leaf,
and the body is merely a flat leaf-like disk floating upon

Fig. 156. A section through the body of a duckweed (Lemnd),
showing the air spaces (a) which make it buoyant, the
origin (?■) of the simple dangling root, and the pockets
(s and I) from which new plants bud out, and in which
flowers are developed.

176 PLANT RELATIONS.

the water, with a few roots dangling from the under side,
or with no roots at all (see Fig. 156). This same duck-
weed also shows some interesting modifications in its hab-
its of reproduction. Although related to plants which pro-
duce flowers and make seed, the duckweeds have almost
lost the power of producing flowers, and when they do
produce them, seeds are very seldom formed. In other
words, the ordinary method of reproduction employed
by flowering plants has been more or less abandoned.
Replacing this method of reproduction is a great power
of vegetative propagation. From the disk-like body of
the plant other disk-like bodies bud out, and this bud-
ding continues until a large group of disks, more or
less connected with each other, may be formed. These
plants also form what are known as winter buds — well
protected bud-like bodies which sink to the bottom of
the pond when the floating plants are destroyed, and
remain protected by the mucky bottom until the waters
become warm again in the next growing season.

In examining the pond associations, therefore, attention
should be paid to the floating forms and the submerged
forms, and also to the varying depths of the latter. It
will also be noted that the leaves of floating forms are
comparatively broad, while those of submerged forms are
narrow.

B. Anchored associations.

133. Definition. — These are associations fixed to the soil
but with submerged or floating leaves. In this case there
is still great exposure to water, but there is also a definite
soil relation. Two prominent associations are selected
from this group for illustration.

134. Rock associations. — The term rock is used in this
connection in a very general way, meaning simply some firm
support beneath the water ; it is just as likely to be a stick

HYDROPHYTE ASSOCIATIONS.

177

as a stone. Probably the most prominent group of plants
affecting these conditions are algae, both fresh water and
marine. In the fresh waters very many of the algas will be

'V

r^

Pig. 157

A group of marine seaweeds (Laminarias). Note the various habits of the
plant body and the root-like holdfasts.— After Rerner.

found anchored to some support. The largest display of
such forms, however, is found among the marine alga?,
which abound along all seacoasts (see Fig. 157). It will

Fig. 158. A natural, but nearly overgrown, lily pond. The lily pads may be seen
rising more or leas above the water where they are thickest. The forest growth
in the background is probably a tamarack (larch) swamp. It is to be noticed that
as the lily pond loses its water it is being invaded by the coarse sedge and grass
growth of a swamp. Between the lily pond and the forest is a swamp-thicket.
At least four distinct associations are represented in this view. A fifth is prob-
ably represented in the form of plants of the reed-swamp type, which form a
transition between the lily pond and the swamp-thicket.

HYDROPHYTE ASSOCIATIONS. 179

be noticed that the habit of anchorage demands the
development of special organs of attachment, which usu-
ally take the form of root-like structures, often associated
with sucker-like disks. Associated with the anchoring
structures is often a development of floats, which is es-
pecially characteristic of seaweeds, enabling the working
body to float freely in the water (see Figs. 153, 154). It is
evident that while free-swimming forms may be suitable
for stagnant waters, anchored forms are better adapted for
moving waters. Therefore, where there are currents of
water, or wave action, the anchored forms predominate.
The ability to live in moving waters, and often in those
that become violently agitated, has its advantage to the
plant in the more rapidly renewed food material. In such
a situation free-swimming forms would soon be stranded
or disposed of in quieter waters.

In the case of the marine seaweeds there is an interest-
ing relation between the depth of the water and the color
of the plants. While the fresh water alga? are prevailingly
green, it will be remembered that the prevailing colors of
the alga? of the seashore are brown and red. The brown
often passes into some shade of yellow, and the red may
merge into purple or violet, but in general the two types of
color may be called brown and red. It has been noticed
that the brown forms are found at less depth than the red
forms, so that in a general way there are two zones of dis-
tribution in relation to depth, the red zone being the lower
one and the yellow zone the upper. Just what this means
in the economy of the plants is not clear, but it has been
suggested that the yellow and the red colors assist the
chlorophyll in its work, which is more or less interfered
with by the diminished intensity of the light passing
through sea water.

135. Loose soil associations. — This phrase is used merely
to contrast with rock associations, referring to the fact that
the anchorage is not merely for mechanical support, but that
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there is a definite relation to soil in which roots or root-like
structures are embedded. Associations of this type contain
the greatest variety of plants of all ranks. In these asso-
ciations are found alga?, mosses, fern plants, pondweeds,
water lilies., etc. (see Figs. 158, 159, 160, 161). Pondweeds
and water lilies may be taken as convenient types of high
grade plants which grow in such conditions.

In the first place, it will be noticed that they are in-
clined to social growths, great numbers of individuals
growing together and forming what are known as lily
ponds or pondweed beds, although in the small lakes of
the interior where pondweeds abound in masses, they are
more commonly known as " pickerel beds." If the petiole
of a lily pad be traced down under the water, it will be
found to arise from an intricate mass of thick, knotted
stems, So extensively do these stems (rootstocks) in the
mucky bottom branch that they are able to give rise to
close set masses of leaves.

Water lilies and pondweeds may also be compared to
show the effect of the floating habit in contrast with the
submerged habit. The leaves of water lilies float on the
surface, and therefore are broad ; and being exposed to light
are a vivid green, indicating the abundant development of
chlorophyll. Many of the pondweeds, however, are com-
pletely submerged. As one floats over one of these " pick-
erel beds," the leafy plants may be seen at considerable
depths, and have a pallid, translucent look. It will be
seen that in these cases the leaf forms are narrow rather
than broad, often being ribbon-like, or in some submerged
plants even cut up into thread-like forms. It is evident
that such narrow leaf forms can respond more easily to
water movements than broad forms. The pallid look of
these submerged leaves indicates that there has not been
an abundant development of chlorophyll. Some pondweeds,
however, have both types of leaves, some being submerged
and others floating. In these cases it is interesting to notice

Fig 160 A group of pondweeds. The stems are sustained in an erect position by
the water, and the narrow leaves are exposed to a light whose intensity is dimin-
ished by passing through the water.— After Keener,

HYDKOPHYTE ASSOCIATIONS. 183

the corresponding change of form ; on the same individual
the submerged leaves are very narrow, or divided into very-
narrow lobes, while the floating ones are broad (see Fig.
162). The relation of the plant to the water, therefore, has
determined the leaf form. The advantage of the floating
habit of leaves is not merely a better relation to light, but
the carbon dioxide used in photosynthesis and the oxygen
used in respiration may be obtained freely from the air,
rather than from the water. It will also be noticed that
these water plants usually send their flowers to the surface,
indicating that such a position is more favorable for the
work of the flower than a submerged position. Any asso-
ciation of this type will furnish abundant material for
observation, and it is, perhaps, the most valuable type of
association for study that has been mentioned so far.

C. Swamp associations.

136. Definition. — In swamp associations the plants are
rooted in water, or in soils rich in water, but the stems
bearing the leaves rise above the surface. Among the
hydrophytes, swamp plants are least exposed to water, and
as the stem and its leaves are exposed to the air, there is
no such reduction of the root system and of conducting
and mechanical tissues as in the other hydrophytes. Also
the epidermis is not thin, and there is no development of
floats to increase the buoyancy. However, the root must
be aerated, and hence air chambers and passageways are
abundant. In ordinary cases the air is admitted through
openings in the epidermis of the stem and leaves, and so
enters the air-passageways that are continuous to the roots.
It has been claimed that a still more elaborate arrangement
for root-aeration exists in the so-called " knees " of cypress
swamps, which are special growths from the submerged
root system and rise above the surface of the water (see
Fig. 91). It has been shown that if such swamps are
flooded above the level of the knees many of the trees are

^zr

Fig 161 Eel grass (Vallisneria), a common pondweed plant. The plants are
anchored and the foliage is submerged. The carpel-bearing flowers are carried to
the surface on long stalks which allow a variable depth of water. The stamen-
bearing flowers remain submerged, as indicated near the lower left corner, the
flowers breaking away and rising to the surface, where they float and effect pollina-
tion.—After Kerneb.

HYDKOPHYTE ASSOCIATIONS. 185

killed, but that aeration of the root system occurs through
the knees remains to be proved.

Another habit of swamp plants is called turf-building,
which means that new individuals arise from older ones,
and so a dense mat of roots and rootstocks is formed. Very
prominent among these turf-building swamp plants are

Fig. 162. Two leaves of a water buttercup, showing the difference in the forms of
submerged ami aerial leaves on the same plant, the former being much more
finely divided. — After Strasburger.

the sedges. Some of the prominent swamp societies may be
enumerated as follows :

137. Reed swamps. — The reed-swamp plants arc tall wand-
like forms, which grow in rather deep, still water (see Fig.
163). Prominent as types are the cat-tail Hag. bulrushes,
and reed grasses. Such an assemblage of forms usually
characterizes the shallow margins of small lakes and ponds.
In such places the different plants are apt to lie arranged
according to depth, the bulrushes standing in the deepest
water, and behind them the reed grasses, and then the

186

PLANT RELATIONS.

cat-tails. This regular arrangement in zones is so often
interfered with, however, that it is not always evident.

The reed-swamp associations have been called " the pio-
neers of land vegetation," for their bodies and the detritus

Fig. 163. A reed swamp, fringing the low shore of a lake or a sluggish stream. The
plants are tall and wand-like, and all are monocotyls. Three types are prominent,
the reed grasses (the tallest), the cat-tails (at the right), and the bulrushes (a group
standing out in deeper water near the middle of the fringing growth). The plant
iu the foreground at the extreme right is the arrow-leaf (Sagittaria), recognized
by its characteristic leaves.— After Keener.

make the water more and more shallow, until finally the
reed plants are compelled to migrate into deeper water
(see §108). In this way small lakes and ponds may be
completely reclaimed, and become converted first into
ordinary swamps, and finally into wet meadows. Instances

HYDROPHYTE ASSOCIATIONS. 187

of nearly reclaimed ponds may be noticed, where bul-
rushes, cat-tail flags, and reed grasses still occupy certain
wet spots, but are shut off from further migration. The
social growth of these plants, brought about by extensive
root stock development, is especially favorable for detain-
ing detritus and building a land surface.

Keed-swamp plants also have in general a tall and un-
branched habit of body. They may be bare and leafless,
with a terminal cluster of flowers, as in the bulrushes ; or
the wand-like stems may bear long, linear leaves, as in the
cat-tails ; or the stem may be a tall stalk with two rows of
narrow leaves, as in the reed grasses. 2so more character-
istic group of forms is found in any association. Of course,
associated with these forms are also free and fixed hydro-
phytes, that characterize the other associations.

138. Swamps. — The word is used to include the ordinary
meadow-like expanses of swampy ground, but does not in-
clude such associations as peat bogs. There is less water
than in the case of the reed swamps, and often very little
standing water. One of the peculiarities of the swamp
is that the water is rich in available soil materials used
in food manufacture, notably the nitrates from which
nitrogen is obtained for proteid manufacture. In such
conditions, therefore, the vegetation is dense, and the soil
is black with the humus derived from the decaying plant
bodies.

Typical swamps border the reed swamps on the land
side, and slowly encroach upon them as the reed plants
build up land. Probably the most characteristic plant
forms of the swamp association are the sedges, and asso-
ciated with them are certain coarse grasses. These give
the meadow-like aspect to the swamp, although these
grass-like forms are very coarse. Along with the domi-
nant sedges and grasses are numerous other plants adapted
to such conditions, such as some of the buttercups. It
would be impracticable to give a list of swamp plants, as

188 PLANT RELATIONS.

the forms associated with sedges and grasses may vary
widely in different associations (Fig. 164).

In almost all swamps there is a lower stratum of vege-
tation than that formed by the sedges. This lower stratum
is made of certain swamp mosses, which grow in very dense
masses. Towards the north, where the temperature con-
ditions are not so favorable for the sedge stratum, it may
be lacking almost entirely, and only the lower moss stratum
left. In these cases the swamp becomes little more than a
great bed of moss, and it is in such conditions that peat
may be formed.

139. Swamp-thickets. — Swamp-thickets are very closely
associated with swamps, and are doubtless derived from
them. If a swamp, with its sedge stratum and moss
stratum, be invaded by shrubs or low trees, it becomes a
swamp-thicket. It will be noticed that these shrubs and
trees are of very uniform type, being mainly willows, alders,
birches, etc. Such willow and alder thickets are very com-
mon in high latitudes.

II. Xerophytic hydrophytes.

A. Fresh-water associations.

140. Sphagnum-moors. — The sphagnum-moor is a very
peculiar type of swamp association. It is so named because
the common bog or peat moss, known as sphagnum, gives
a peculiar stamp to the whole area. Sphagnums are large,
pale mosses, whose lower parts die, and whose upper parts
continue to live and put out new branches, so that a dense
turf is formed. In walking over such a bog the moss turf
seems springy, and sometimes trembles so as to suggest
the name " quaking bog." These are the great peat-form-
ing bogs. It is interesting to know what conditions keep
the swamp plants out of the sphagnum-moor. The plants
of the sphagnum-moor seem to be entirely different from

HYDROPHYTE ASSOCIATIONS. 189

those of the swamp association, although the amount of
water is approximately the same. Xot only are the plants
different in the sphagnum-moor, but they are not so
numerous, and, with the exception of the moss, do not
grow so densely. Creeping plants are common ; certain
kinds of sedges and grasses are found, but generally not
those of the swamps ; while heaths and orchids are espe-
cially abundant. It is in these sphagnum-moors, also, that
the curious forms of carnivorous plants are developed, among
which the pitcher plants, droseras, and dionaeas have been
described. In considering this strange collection of forms,
it is evident that there must be some peculiarity in the
conditions of living. Heaths and orchids are well-known
mycorhiza forms ; the carnivorous plants are so named
because they capture insects to supplement their food
supply ; while the peculiar sphagnum mosses replace the
mosses of ordinary swamps. "What causes have resulted in
an association of such marked physiognomy are unknown,
but the subject is attracting much attention.

It seems evident that the problem is one of absorption
and that some condition is interfering with this important
function. One conclusion, based upon experimental work,
is that the greater coldness of the bog water is the cause of
diminished absorption, for the difference between the tem-
perature of bog water and of other waters is quite remark-
able. Another conclusion is that certain salts dissolved in
the bog water tend to check the power of absorption. In
any event, the assemblage of bog plants is made up of
forms that have learned to live with a diminished power of
absorption.

It is usually stated that the water of the sphagnum-
moor is very poor in the food materials which are abundant
in the water of swamps, and that there is a special lack of
the materials which are used in the manufacture of pro-
teids. If this is true, it would be necessary to obtain some
proteid material already formed, and this might account

190 PLANT RELATIONS.

for the carnivorous habit and for the sphagnum mosses.
Of course it would also account to a certain extent for the
exclusion of the characteristic swamp plants. It is a well-
known fact that bodies of men and animals that have
become submerged in sphagnum-bogs may not decay, but
have been found preserved after a very long period. This
will also indicate why such bogs are especially favorable for
peat formation.

These two associations, therefore, may be contrasted as
follows : The swamp is rich in plant food, and is character-
ized chiefly by grassy plants ; the sphagnum-moor is poor
in food material and unfavorable to absorptiou, and is char-
acterized chiefly by sphagnum moss. It will be noted that
peat may be formed in connection with both, but in the
swamp the plant forms cannot be distinguished in the peat,
as they have been more or less disorganized through decay,
while in the peat of the sphagnum-moor the plant forms
are well preserved. The peat of the swamp, also, yields a
great amount of ash, for the swamp is rich in soil materials,
while the peat of the sphagnum-moor yields very little ash.

141. Swamp-forests. — It was noted that the special
types of shrub or tree growth associated with the swamp
conditions are willows, alders, birches, etc. In the same
way there is a peculiar tree type associated with the
sphagnum-moor. It is very common to have a sphagnum
area occupied by trees, and the area becomes a swamp
forest, rather than a sphagnum-moor. The chief tree
type which occupies such conditions is the conifer type,
popularly known as the evergreens. The swamp forests,
therefore, with a sphagnum-moor foundation, are made up
of larches, certain hemlocks and pines, junipers, etc., and
towards the south the cypress comes in (see Fig. 1G5).
The larch is a very common swamp tree of the northern
regions, where such an area is commonly called a "tama-
rack swamp" (see Fig. 158). The larch forests are apt to
be in the form of small patches, while the larger swamp

192

PLANT RELATIONS.

forests are made of dense growths of hemlocks, pines, etc.
In the densest of these forests the shade is so complete
that there may be very few associated plants occurring in
strata between the sphagnum moss and the trees. In the
larch forests, however, the undergrowth may be very dense.

B. Salt-water associations.

142. Mangrove swamps. — This is certainly the most
vigorous of the salt-water associations. Mangrove swamps
occur along flat tropical seacoasts, where the waters are quiet.

m

Fig. 166. A mangrove forest advancing into the water.— After Schtmper.

The mangrove is a tree of curious habit, which advances
slowly out into the water and extends back landwards as
low woods or thickets (see Figs. 166, 167). The whole
surroundings appear forbidding, for the water is sluggish
and mucky, covered with scum, rich in bacteria, and with
bubbles constantly breaking upon the surface from decay-
ing matter beneath the water. The mangrove has the pe-

14

192b PLANT RELATIONS.

culiarity of germinating its seeds while still upon the tree,
so that embryos hang from the trees, and then drop like
plumb-bobs into the muck beneath, where they stick fast
and are immediately in a condition to establish themselves.
In these mangrove swamps the species are few, and the
adaptations chiefly in the way of developing various kinds
of holdfasts for anchoring in the uncertain soil, and also
various devices for carrying air to the submerged parts.

143. Beach marshes and meadows.— The salt marshes
and meadows near the seacoast are very well known. They
lie beyond the reach of ordinary flood tide, but the waters
are brackish. In these marshes and meadows occur certain
characteristic salt-water grasses and sedges. Such forms
being the dominant type give the general appearance of
a coarse meadow. Very characteristic of such associations
are also certain succulents, such as samphire {Salicornia),
sea blite (Suceda), saltwort (Salsola), etc. In fact, this
succulent character seems to be a direct response to the
saline conditions. The difference between a marsh and
meadow is simply a question of the amount of water.

CHAPTER XIII.

XEROPHYTE ASSOCIATIONS.

144. General character. — Strongly contrasted with the
hydrophytes are the xerophytes, which are adapted to dry
air and soil. The xerophytic conditions may be regarded
in general as drouth conditions. It is not necessary for
the air and soil to be dry throughout the year to develop
xerophytic conditions. These conditions may be put under
three heads : (1) possible drouth, in which a season of
drouth may occur at irregular intervals, or in some seasons
may not occur at all ; (2) periodic drouth, in which there
is a drouth period as definite as the winter period in cer-
tain regions ; (3) perennial drouth, in which the dry con-
ditions are constant, and the region is distinctly an arid
or desert region.

However xerophytic conditions may occur, the problem
of the plant is always one of water supply, and many strik-
ing structures have been developed to answer it. Plants
in such conditions must provide, therefore, for two things :
(1) collection and retention of water, and (2) prevention of
its loss. It is evident that in these drouth conditions the
loss of water through transpiration (see §26) tends to be
much increased. This tendency in the presence of a very
meager water supply is a menace to the life of the plant,
for it is impossible to stop transpiration entirely, as it
must take place so long as the plant is alive. The adapta-
tions on the part of the plant, therefore, are directed
towards the regulation of transpiration, that it may occur

194 PLANT KELATIONS.

sufficiently for the life-processes, but that it may not be
wasteful to the point of danger.

The regulation of transpiration may be accomplished
in two general ways. It will be remembered that the
amount of transpiration holds some relation to the
amount of leaf exposure or exposure of green tissue.
Therefore, if the amount of leaf exposure be diminished,
the total amount of transpiration will be reduced. Another
general way for regulating transpiration is to protect
the exposed surface in some way so that the water does
not escape so easily. In a word, therefore, the general
method is to reduce the extent of exposed surface or to
protect it. It must be understood that plants do not differ
from each other in adopting one or the other of these
methods, for both are very commonly used by the same
plant.

Adaptations.

145. Complete desiccation. — Some plants have a very re-
markable power of completely drying up during the drouth
period, and then reviving upon the return of moisture.
This power is strikingly illustrated among the lichens and
mosses, some of which can become so dry that they may be
crumbled into powder, but revive when moisture reaches
them. A group of club mosses, popularly known as " res-
urrection plants," illustrates this same power. The dried
up nest-like bodies of these plants are common in the
markets, and when they are placed in a bowl of water they
expand and may renew their activity. In such cases it can
hardly be said that there is any special effort on the part of
the plant to resist drouth, for it seems to yield completely
to the dry conditions and loses its moisture. The power
of reviving, after being completely dried out, is an offset,
however, for protective structures.

146. Periodic reduction of surface. — In regions of periodic

XEKOPIIYTE ASSOCIATIONS.

195

drouth it is very com-
mon for plants to
diminish the exposed
surface in a very de-
cided way. In such
cases there is what
may be called a peri-
odic surface decrease.
For example, annual
plants remarkably
diminish their ex-
posed surface at the
period of drouth by
being represented
only by well-pro-
tected seeds. The
whole exposed sur-
face of the plant,
root, stem, and leaves,
has disappeared, and
the seed preserves the
plant through the
drouth.

Little less remark-
able is the so-called
geophilous habit. In '
this case the whole of
the plant surface ex-
posed to the air dis-
appears, and only
underground parts,
such as bulbs, tu-
bers, etc., persist (see
Figs. 45, 46, 66, 67,
68, 69, 70, 75, 144,
168, 169). At the re-

Fig. 168. The bloodroot (Sanguinaria), showing
the subterranean rootstock sending leaves and
flower above the surface. — After Atkinson.

196

PLANT RELATIONS.

Fig. 169. The spring
beauty ( Claytonia),
showing subterranean
tuber-like stem sending leaf and flower-bearing
stem above the surface.— After Atkinson.

turn of the moist season
these underground parts
develop new exposed
surfaces. In such cases
it may be said that at
the coming of the drouth
the plant seeks a sub-
terranean retreat.

A little less decrease
of exposed surface is
shown by the deciduous
habit. It is known that
certain trees and shrubs,
whose bodies remain
exposed to the drouth,
shed their leaves and
thus very greatly reduce
the amount of exposure :
with the return of mois-
ture, new leaves are put
forth. It will be re-
marked, in this connec-
tion, that the same
habits serve just as well
to bridge over a period
of cold as a period of
drouth, and perhaps
they are more familiar
in connection with the
cold period than in con-
nection with the drouth
period.

147. Temporary reduc-
tion of surface. — While
the habits above have to
do with regular drouth

XEKOPHYTE ASSOCIATIONS. 197

periods, there are other habits by which a temporary re-
duction of surface may be secured. For instance, at the
approach of a period of drouth, it is very easy to observe
certain leaves rolling up in various ways. As a leaf be-
comes rolled up, it is evident that its exposed surface is
reduced. The behavior of grass leaves, under such cir-
cumstances, is very easily noted. A comparison of the grass
blades upon a well-watered lawn with those upon a dried-up
lawn will show that in the former case the leaves are flat,
and in the latter more or less rolled up. The same habit
is also very easily observed in connection with the larger-
leaved mosses, which are very apt to encounter drouth
periods.

148. Fixed light position. — In general, when leaves have
reached maturity, they are unable to change their position
in reference to light, having obtained what is known as a
fixed light position. During the growth of the leaf, how-
ever, there may be changes in direction so that the fixed
light position will depend upon the light direction during
growth. The position finally attained is an expression of
the attempt to secure sufficient, but not too much light
(see §13). The most noteworthy fixed positions of leaves
are those which have been developed in intense light.
A very common position in such cases is the profile posi-
tion, in which the leaf apex or margin is directed upwards,
and the two surfaces are more freely exposed to the morn-
ing and evening rays — that is, the rays of low intensity —
than to those of midday.

Illustrations of leaves with one edge directed upwards
can be obtained from the so-called compass plants. Prob-
ably most common among these are the rosin-weed of the
prairie region, and the prickly lettuce, which is an intro-
duced plant very common in waste ground (see Fig. 170).
Such plants received their popular name from the fact that
many of the leaves, when edgewise, point approximately
north and south, but this direction is very indefinite. It is

198

PLANT KELATIONS.

evident that such a position avoids exposure of the leaf
surface to the noon rays, but obtains for these same sur-
faces the morning and evening rays. If these plants are
developed in the shade, the " compass" habit does not

Fig. 170. Two compass plants. The two figures to the left represent the same plant
{Silphium) viewed from the east and from the south. The two figures to the right
represent the same relative positions of the leaves of Lactuca. — After Kerner.

appear (see §15). The profile position is :i very common
one for the leaves of Australian plants, a fact which gives
much of the vegetation a peculiar appearance. All these
positions are serviceable in diminishing the loss of water,
which would occur with exposure to more intense light.
149. Motile leaves. — Although in most plants the mature

XEROPHYTE ASSOCIATIONS.

199

leaves are in a fixed position, there are certain ones whose
leaves are able to perform movements according to the need.
Mention has been made already of such forms as Oxalis
(see §1-4), whose leaves change their position readily in
reference to light. Motile leaves have been developed most
extensively among the Leguminosce, the family to which

Fig. 171. Two twigs of a sensitive plant. The one to the left shows the numerous
small leaflets in their expanded position ; the one to the right shows the greatly
reduced surface, the leaflets folded together, the main leaf branches having
approached one another, and the main leaf-stalk having bent sharply downwards.
—After Strasburger.

belong peas, etc. In this family are the so-called " sen-
sitive plants," which have received their popular name
from their sensitive response to light as well as to other
influences (see Fig. 171). The acacia and mimosa forms
are the most notable sensitive plants, and tire especially
developed in arid regions. The leaves are usually very
large, but are so much branched that each leaf is com-
posed of very numerous small leaflets. Each leaflet has

200

PLANT RELATIONS.

the power of independent motion, or the whole leaf may
move. If there is danger from exposure to drouth, some
of the leaflets will be observed to fold together ; in case

Fig. 172. A heath plant (Erica), showing low, bushy growth and small leaves.

the danger is prolonged, more leaflets will fold together ;
and if the danger persists, the surface of exposure will be
still further reduced, until the whole plant may have its
leaves completely folded up. In this way the amount of

XEEOPHYTE ASSOCIATIONS. 201

reduction of the exposed surface may be accurately regu-
lated to suit the need (see §38).

150. Reduced leaves. — In regions that are rather per-
manently dry, it is observed that the plants in general pro-
duce smaller leaves than in other regions (see Fig. 173).
That this holds a direct relation to the dry conditions is

Fig. 173. Leaves from the common basswood (Tilia), showing the effect of environ-
ment ; those at the right being from a tree growing in a river bottom (mesophyte
conditions) ; those at the left being from a tree growing upon a dune, where it is
exposed to intense light, heat, cold, and wind. Not only are the former larger,
but they are much thinner. The leaves from the dune tree are strikingly smaller,
much thicker, and more compact. — After Cowles.

evident from the fact that the same plant often produces
smaller leaves in xerophytic conditions than in moist con-
ditions. One of the most striking features of an arid
region is the absence of large, showy leaves (see Fig. 172).
These reduced leaves are of various forms, such as the
needle leaves of pines, or the thread-like leaves of certain
sedges and grasses, or the narrow leaves with inrolled
margins such as is common in many heath plants. The

202

PLANT RELATIONS.

Fig. 174. Two species of Achillea on different soils. The one to the left was grown
In drier conditions and shows an abundant development of hairs.— After

SCHIMPER.

extreme of leaf reduction has been reached by the cactus
plants, whose leaves,, so far as foliage is concerned, have
disappeared entirely, and the leaf work is 40ne by the

XEKOPIIYTE ASSOCIATIONS.

203

surface of the globular, cylindrical, or flattened stems (see
§36).

151. Hairy coverings. — A covering of hairs is an effective
sun screen, and it is very common to find plants of xerophyte
regions character-
istically hairy (see
§35). The hairs
are dead struc-
tures, and within
them there is air.
This causes them
to reflect the light,
and hence to ap-
pear white or
nearly so. This
reflection of light
by the hairs dimin-
ishes the amount
which reaches the
working region of
the plant (see Fig.
174).

152.Bodyhabit.
— Besides the va-
rious devices for
diminishing ex-
posure or leaf sur-
face, and hence
loss of water,
enumerated above,
the whole habit of
the plant may em-
phasize the same purpose. In dry regions it is to be observed
that dwarf growths prevail, so that the plant as a whole
does not present such an exposure to the dry air as in
regions of greater moisture (see Fig. 175). Also the pros-

Fig. 175. Two plants of a common scouring rush {Equi-
setum), showing the effect of environment ; the long,
unbranched one having grown in normal mesophyte
conditions ; the short, bushy branching, more slender
form having grown on the dunes (xerophyte condi-
tions).— After Cowles.

204

PLANT RELATIONS.

trate or creeping habit is a much less exposed one in such
regions than the erect habit. In the same manner, the very
characteristic rosette habit, with its cluster of overlapping
leaves close against the ground, tends to diminish loss of
water through transpiration.

One of the most common results of xerophytic conditions
upon body habit is the development of thorns and spiny

Fig. 176. Young plants of Euphorbia splendem, showing a development of thorns
characteristic of the plants of dry regions.

processes. As a consequence, the vegetation of dry regions
is characteristically spiny. In many cases these spiny pro-
cesses can be made to develop into ordinary stems or leaves
in the presence of more favorable water conditions. It is
probable, therefore, that such structures represent reduc-
tions in the growth of certain regions, caused by the unfavor-
able conditions. Incidentally these thorns and spiny pro-
cesses are probably of great service as a protection to plants
in regions where vegetation is peculiarly exposed to the

XEK0PI1YTE ASSOCIATIONS.

205

ravages of animals (see §105). Examine Figs. 176, 177,
178, 179, 180, 181.

153. Anatomical adaptations. — It is in connection with
the xerophytes that some of the most striking anatomical
adaptations have been
developed. In such
conditions the epider-
mis is apt to be cov-
ered by layers of
cuticle, which are de-
veloped by the walls
of the epidermal cells,
and being constantly
formed beneath, the
cuticle may become
very thick. This
forms a very efficient
protective covering,
and has a tendency to
diminish the loss of
water (see §35). It is
also to be observed
that among xerophytes
there is a strong de-
velopment of palisade
tissue. The working
cells of the leaves next
to the exposed surface
are elongated, and are
directed endwise to

the surface. In this way only the ends of the elongated
cells are exposed, and as such cells stand very closely to-
gether, there is no drying air between them. In some
cases there may be more than one of these palisade rows
(see §32). It has been observed that the chloroplasts in
these palisade cells are able to assume various positions in

b a.

Fig. 177. Two plants of common gorge or furze
( Vlex), showing the effect of environment : b
is a plant grown in moist conditions ; a is a
plant grown in dry conditions, the leaves and
branches having been almost entirely developed
as thorns. — After Lotheixer.

206

PLANT RELATIONS.

the cell, so that when
the light is very intense
they move to the more
shaded depths of the
cell, and when it be-
comes less intense they
move to the more ex-
ternal regions of the
cell (see Fig. 182).
The stomata, or air
pores, which are devel-
oped in the epidermis,
are also great regulators
of transpiration, as has
been mentioned already
(see §31).

154. Water reservoirs.
— In xero-
phytes at- /

tent ion
must be
given not
only to the
regulation of transpiration, but also to the
storage of water, as it is received at rare inter-
vals. It is very common to find a certain re-
gion of the plant body given over to this work,
forming what is known as water tissue. In
many leaves this water tissue may be distin-
guished from the ordinary working cells by
being a group of colorless cells (see Figs. 183,
184, 185). In plants of the drier regions leaves
may become thick and fleshy through acting
as water reservoirs, as in the case of the agave,
sedums, etc. Fleshy or ' ' succulent " leaves
are regarded as adaptations of prime impor-

Fig. 178. A brancli of Cytisus, showing the
reduced leaves and thorny branches.— After
Kerner.

Fig. 179. A
leaf of traga-
canth, show-
ing the re-
duced leaf-
lets and the
thorn -like
tip.— After
Kerner.

XEEOPHYTE ASSOCIATIONS.

2o7

tance in xerophytic conditions. In
the cactns plants the peculiar stems
have become great reservoirs of
moisture. The globular body may
be taken to represent the most com-
plete answer to this general problem,
as it is the form of body by which
the least amount of surface may be
exposed and the greatest amount of
water storage secured. In the case
of fleshy leaves and fleshy bodies it
has long been noticed that they not
only contain water, but also have a
great power of re-

Fig. 180. A fragment of bar-
berry, showing the thorns.
— After Kerner.

Fig. 181. Twig of com-
mon locust, showing
the thorns.— After
Kerner.

15

taining it. Plant
collectors have found much difficulty in
drying these fleshy forms, some of which
seem to be able to retain their moisture in-
definitely, even in the driest conditions.
155. Xerophytic structure. — The adap-
tations given above are generally found
in plants growing in drouth conditions,
and they all imply an effort to diminish
transpiration. It must not be supposed,
however, that only plants living in
drouth conditions show these adapta-
tions. Such adaptations result in what
is known as the xerophytic structure,
and such a structure may appear even
in plants growing in hydrophyte condi-
tions. For example, the bulrush grows
in shallow water, and is a prominent
member of one of the hydrophyte asso-
ciations (see §137) ; and yet it has a re-
markably xerophytic structure. This is
probably due to the fact that although it

208

PLANT KELATIONS.

stands in the water its stem is exposed
to a heat that is often intense.

The ordinary prairie (see §169) i3
included among mesophyte associa-
tions on account of the rich, well-
watered soil; and yet many of the
plants are very xerophytic in struc-
ture, probably on account of the pre-
vailing dry winds.

The ordinary sphagnum-bog (see
§140), or " peat-bog," is included
among hydrophyte associations. It
has an abundance of water, and is not
exposed to blazing heat, as in the case
of the bulrushes, or to drying wind, as
in the case of prairie plants ; and yet
its plants show a xerophytic structure.
The cause for this has not yet been
determined, although several sugges-
tions have been made.

It is evident, therefore, that xero-
phytic structures are not necessarily
confined to xerophytic situations. It
is probably true that all associations
that show xerophytic
structures belong to-
gether more naturally
than do the associa-
tions that are grouped
according to the water
supply.

Associations.

Xo attempt will be
made to classify these
very numerous associa-

Fig. 182. Cells from the leaf
of a quillwort (Isoetes).
The light is striking the
cells from the direction of
one looking at the illus-
tration. If it be some-
what diffuse the chloro-
plasts distribute them-
selves through the shal-
low cell, as in the cell to
the left. If the light be
intense, the chloroplasts
move to the wall and as-
sume positions less ex-
posed, as in the cell to
the right.

Fig. 183. A section through a Begonia leaf , show-
ing the epidermis (ep) above and below, the
water-storage tissue (ws) above and below, and
the central chlorophyll region (as).

XEKOPHYTE ASSOCIATIONS.

209

Fig. 184. A section through a fleshy leaf (Clinid), show-
ing the chlorophyll region on the outside (shaded and
marked as), and the large interior water-storage region
(ws).

tions, but a few
prominent illus-
trations will be
given.

156. Rock as-
sociations.— Vari-
ous plants are
able to live upon
exposed rock sur-
faces, and there-
fore form distinct
associations of xe-
rophytes. In gen-
eral they are lichens, mosses, and crevice plants (see Fig.
186). The crevice plants are those
which send their roots into the rock
crevices and so gain a foothold.
The crevice plants also commonly
show a rosette habit, the rosette of
overlapping leaves being against the
rock face, and therefore in the most
favorable position for checking loss
of Avater.

157. Sand associations. — In gen-
eral, sand associations may be roughly
grouped as beach associations, dune
associations, and sandy field associa-
tions. These three hold a certain
definite relation to one another.
This natural relationship appears on
the borders of the large lakes, and on
seacoasts. The beach is nearest the
water, the dunes are next, and be-
hind them stretch the sandy fields.
When the three types are thus asso-
ciated, the plants of the different

Fig. 185. A section through
a leaf of an epiphyte,
showing a very large de-
velopment of water tissue
between the upper epi-
dermis and the chloro-
phyll region, which is
restricted to near the
under surface of the leaf.
—After Schimper.

210

PLANT RELATIONS.

areas pass gradually into one another. It is very common
to find the dunes omitted in the series, and to have the
beaches pass gradually into the sandy fields.

The beach association is usually quite characteristic, and
in general it is a poor flora, the beach being characteristic-
ally bare. The plants that grow in such conditions are apt
to occur in tufts, or are creeping plants. It is evident that

Fig. 186. A rock covered with lichens.

while the water may seem to be abundant, it disappears
quickly, so that plants must adapt themselves to a dry
condition of the soil, which is poor and with little or no
accumulation of humus. At the same time, the exposure
to intense light i.s extreme. This combination results in a
poor display of individuals and of species. Here and there
along beaches, where special conditions have favored the
accumulation of humus, dense vegetation may spring up,
but it should not be confused with the ordinary beach type.

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212

PLANT RELATIONS.

The dune associations are subjected to very peculiar
conditions. Dunes are billows of sand that have been de-
veloped by prevailing winds, and in many cases they are
continually changing their form and are frequently moving

Fig. 188. A sandy field type, showing the development of vegetation upon an old
beach. The vegetation is low, often tufted and heath-like, being composed chiefly
of grasses, bearberry (Arctostaphylos) and Hudsonia. In the background to
the right is a conifer forest, and between it and the old beach is seen a dense mass
of bearberry, a very characteristic heath plant, and forming here what is called a
transition zone between the beach and the forest.— After Cowles.

landward (see Fig. 187). The moving dunes should be
distinguished from the fixed ones, where the billow form is
retained, but the dunes have ceased their motion. In the
case of the active dunes a peculiar type of vegetation is de-
manded. As is to be expected, the flora is very scanty, and

214 PLANT RELATIONS.

has two remarkably developed characters. The plants are
what are known as " sand-binders/' that is, the underground
structures become extremely developed, reaching to great
distances horizontally and vertically, so that one is always
surprised at the extent of the underground system. This
wide searching for water results in giving the plants a deep
anchorage in the shifting soil, and at the same time helps
to prevent the shifting. As soon as enough of the sand-
binders have established tbemselves, a shifting dune becomes
a fixed one. Another characteristic that must be strongly
developed by these plants is the ability to grow up through
the sand after they have been engulfed. The plants of the
shifting dunes are often buried as the dune shifts, and
unless the burial has been too deep, they are able to continue
their development until leaves may be exposed to the air.
In this way plants have often developed a length of stem
which is far beyond anything they attain when growing in
ordinary conditions.

The sandy field associations are represented by a much
more abundant flora than the beach or the dune associa-
tions, the general character being tufted grasses and low
shrubby growths (see Fig. 188).

158. Shrubby heaths. — The shrubby heaths are very
characteristic of the more northern regions, and are closely
related to the sandy field associations. The heath soil is
apt to be a mixture of coarse sand, or gravel and rock, with
an occasional deposit of humus, and would be regarded
in general as a sterile soil. The flora of the shrubby
heaths shows well-marked strata, the upper one being the
low shrubby plants of the heath family, most prominent
among which are huckleberries and bearberries (see Fig.
172). The lower stratum is made up of mosses and
Lichens. A branching lichen, usually spoken of as the
"reindeer moss," often occurs in immense patches on
such heaths. While these shrubby heaths occur most
extensively towards the north, small areas showing the

216 PLANT RELATIONS.

same general character are common in almost all temper-
ate regions.

159. Plains. — Under this head are included great areas
in the interior of continents, where dry air and wind
prevail. The plains of the United States extend from
about the one hundredth meridian westward to the foot-
hills of the Rocky Mountains. Similar great areas are
represented by the steppes of Siberia, and in the interior of
all continents. These regions have been regarded as semi-
desert areas, but they are found for the most part to be
far from the real desert conditions. They are certainly
areas of comparative dryness, on account of the dry winds
which prevail.

Taking the plains of the United States as a type, a very
characteristic plant physiognomy is presented (see Fig.
189). In general, there is a meadow-like expanse, but the
vegetation is much more sparse than in meadows, and is
much more dense than in deserts. The two characteristic
plant forms are the bunch grasses, that is, grasses which
grow in great tufts ; and low grayish shrubs, predomi-
nantly " sage brush." Under the shelter of the sage brush
or other bush forms, many low herbs succeed in growing.
In such areas the growing season is very short, during
which time the vegetation looks vigorous and fresh ; but
during the rest of the year it is very dull. In some parts
the plain is dry enough to permit the growth of the prickly-
pear cactus (Opuntia), which may take possession of ex-
tensive areas (see Fig. 190).

Usually there are two rest periods during the year,
developed by the summer drouth and the winter cold. As
a consequence, the plants of the area are partly spring
plants, which are apt to be very brilliant in flower ; and
partly the later, deep-rooted forms. Over such areas the
transportation of seeds by the wind is very prominent, as
the force of the wind and the freedom of its sweep make
possible very wide distribution. It is in such areas that

£ K

Fig. 192. Two plants of the giant cactus. Note the fluted, clumsy branching, leaf-
less bodies growing from the rocky, sterile soil characteristic of cactus deserts.
Certain dry-ground grasses and low, shrubby plants with small leaves may be seer
in the foreground.

220

PLANT KELATIONS.

the tumbleweed habit is prominently developed. Certain
low and densely branching plants are lightly rooted in
the soil, so that at the close of their growing period they
are easily broken off by the wind, and are rolled to great

Fie. 194. Tree-like yuccas from the arid regions of Africa, showing the very numel
OU6 thick and pointed, sword-like leaves.

222 PLANT RELATIONS.

distances. Where some barrier, such as a fence, lies across
the I ruck of the wind, those tumbleweeds may accumulate
in great masses. This tumbling over the surface results
in an extensive scattering of seeds (see Fig. L20).

The prairies, so characteristic of the United States, are
regarded by some as belonging to the plains. They cer-
tainly are closely related to them in origin, but can hardly
be regarded as being included in xerophyte conditions, as
the conditions of water supply and soil are characteristically
mesophyte, under which head they will be considered.

160. Cactus deserts.— In passing southward on the
plains of the United States, it is to be noted that the con-
ditions become more and more xerophytic, and that the
bunch grasses and sage brush, peculiar to the true plains,
gradually merge into the cactus desert, which represents
a region whose conditions are intermediate between true
plains and true deserts (see Fig. 191). In the United States
this characteristic desert region begins to appear in West-
ern Texas, New Mexico, Arizona, and Southern California,
and stretches far down into the Mexican possessions. This
vast arid region has developed a peculiar flora, which con-
tains most highly specialized xerophytic forms. The va-
rious cactus forms may be taken as most characteristic,
and associated with them are the agaves and the yuccas.
Not only are the adaptations for checking transpiration
and for retaining water of the most extreme kind, but
there is also developed a remarkable armature. It is evi-
dent that such succulent bodies as these plants present
might speedily disappear through the attacks of animals,
were it not for the armor of spines and bristles and rigid
walls. Study Figs. 38, 39, 40, 192, 193, 194.

161. Subtropical deserts. — In such areas xerophyte con-
ditions reach the greatest extreme in the combination of
maximum heat and minimum water supply. It is evident
that such a combination is almost too difficult for plants
to endure. That the very scanty vegetation is due to lack

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224 PLANT RELATIONS.

of water, and not to lack of proper materials in the soil, is
shown by the fact that where water does occur oases are
developed, in which luxuriant vegetation is found.

The desert which extends from Egypt across Arabia may
be regarded as a typical one. It is to be noted that the
vegetation is so scanty that the soil is the conspicuous
feature, and really gives the characteristic physiognomy
(see Fig. 1!)6). Accordingly the appearance of the deserts
will depend upon whether the desert soil is rocky, or of
small stones, or gravel (as in the Desert of Sahara), or of
red clay, or of the dune type. As is to be expected, such
vegetation as does occur is of the tuft and bunch type, as
developed by certain grasses, or of the low irregular bush
type (see Fig. 195).

In the South African deserts certain remarkable plants
hiive been noted which have attained a certain amount of
protection through mimicry, rather than by means of armor,
as in the case of the cactus forms. Some of these plants
resemble the ordinary stones lying about upon the desert.
With the subtropical deserts should not be confused such
areas as those about the Dead Sea, or in the Death's Valley
in Southern California, as the barrenness of these areas is
due to the strongly alkaline soils, and therefore they be-
long to the saline areas.

162. Thickets. — The xerophyte thicket is the most
strongly developed of all thicket growths. Mention has
been made of willow and alder thickets in hydrophyte con-
ditions, but these are not to be compared in real thicket
characters with the xerophyte thickets. These thickets
are especially developed in the tropics and subtropics, and
may be described as growths which are scraggy, thorny,
and impenetrable. Warming speaks of these thickets as
"the unsuccessful attempt of Nature to form a forest."
Evidently the conditions are not quite favorable for for-
est development, and an extensive thicket is the result.
Such thickets are well developed in Texas, where they are

Fir;. 198. A xerophyte conifer forest in the mountains. The peculiar conifer habit
of body is recognized, the trees finding foothold in the crevices of rocks or in
areas of rock debris.

XEROPHYTE ASSOCIATIONS. 227

spoken of as " chaparral." These chaparrals are notably-
composed of mesquit bushes, acacias and mimosas of vari-
ous sorts, and other plants. Similar thickets in Africa and
Australia are frequently spoken of as "bush " or " scrub."
In all of these cases the thicket has the same general type,
and probably represents one of the most forbidding areas
for travel.

163. Forests. — The xerophyte forest associations maybe
roughly characterized under three general heads :

(1) Coniferous forests. — These forests are very common
in xerophyte conditions to the north, and also in the more
sterile regions towards the south (see Figs. 198 to 201).
They are generally spoken of as evergreen forests, although
the name is not distinctive. These forests are of several
types, such as true pine forests, in which pines are the
prevailing trees and the shade is not dense ; the fir and
hemlock forests, which are relatively dark ; and the mixed
forests, in which there is a mingling of various conifers.
In such forests the soil is often very bare, and such under-
growth as does occur is largely composed of perennial
plants. Many characteristic shrubs with fleshy fruits oc-
cur, such as huckleberries, bearberries, junipers, etc. It
will be noted that in these forests a characteristic adapta-
tion to xerophyte conditions is the development of needle
leaves, which are not only narrow, thus presenting a small
exposure of surface, but also have heavy walls, which
further prevents excessive transpiration.

(2) Foliage forests.— -These are more characteristic of
tropical and subtropical xerophyte regions. Illustrations
may be obtained from the eucalyptus, a characteristic
Australian forest tree, the live oaks, oleanders, etc. It
will be noticed that in these cases the leaves are not so
narrow as the needles of conifers, but are generally lance-
shaped, and stiff and leathery, indicating heavy walls to
reduce transpiration.

(3) Leafless forests.— In Java and other oriental regions

Fig. 199.— A xerophyte conifer forest in the Cumberland Mountains of Tennessee.
The table mountain pines find footholds in crevices of the rocks.

Fig. 200. A pine forest, showing the slender, tall, continuous trunks and compara-
tively little undergrowth.— After Schimpeu.

XEEOPHYTE ASSOCIATIONS. 231

areas of dry naked soil are sometimes occupied by forest
growths that show no development of leaves, the tree-
like forms appearing continually bare. The oriental leaf-
less tree form is mostly a Casuarina. Bordering the Gulf
of California, both in Mexico proper and in Lower Cali-
fornia, there are leafless forests composed of various kinds
of giant cactus (see Fig. 192), t known as the "cardon
forests." These leafless forests represent the most extreme
xerophyte conditions occupied by plant forms that may
be regarded as trees.

164. Salt steppes. — In addition to the xerophyte associa-
tions enumerated above, in which the water, though scanty,
is fresh, the two following may be considered. The soil
and air are relatively dry, as in ordinary xerophytic condi-
tions, but the water is more or less saturated with common
salt or alkaline salts. The salt steppes are interior arid
wastes, which probably mark the position of old sea basins.
In the United States one of the most extensive of the salt
steppes is in the Great Salt Lake basin (see Fig. 202). It
is here that members of the chenopod family are especially
at home, such as greasewoods, seablights, samphires, etc.,
for more than any other plants have they learned to endure
such extremely unfavorable conditions. An extensive alka-
line waste in the United States is that known as the Bad
Lands, which stretches over certain portions of Nebraska
and Dakota, and in which the waters are strongly alkaline.
165. Salt and alkaline deserts. — In these areas the water
supply reaches its minimum, and therefore the water be-
comes saturated with the characteristic salts of the soil.
ISTo worse combination for plant activity can be imagined
than the combination of minimum water and maximum
salts. In consequence, such areas are almost, if not abso-
lutely, devoid of vegetation. As illustrations, the exten-
sive desert of the Dead Sea region and the Death's Valley
in Southern California may be cited.

232

PLANT KELAT10NS

165a. Alpine deserts. — In alpine regions a distinct desert
type of vegetation appears upon the drier areas, especially
above timber-line. It is in direct contrast with the alpine
meadow (§168), which is developed in situations where the

snow can lie. On the steeper slopes there is no accumula-
tion of snow, and the scanty vegetation has a distinctly
xerophytic character (Fig. 203).

CHAPTER XIV.

MESOPHYTE ASSOCIATIONS.

1G6. General characters. — Mesophytes make up tne com-
mon vegetation of temperate regions, the vegetation most
commonly met and studied. The conditions of moisture
are medium, precipitation is in general evenly distributed,
and the soil is rich in humus. The conditions are not ex-
treme, and therefore special adaptations, such as are neces-
sary for xerophyte or hydrophyte conditions, do not appear.
This may be regarded as the normal plant condition. It
is certainly the arable condition, and most adapted to the
plants which men seek to cultivate. When for purposes
of cultivation xerophyte areas are irrigated, or hydrophyte
areas are drained, it is simply to bring them into mesophyte
conditions.

In looking over a mesophyte area and contrasting it
with a xerophyte area, one of the first things evident is that
the former is far richer in leaf forms. It is in the meso-
phyte conditions that foliage leaves show their remarkable
diversity. In hydrophyte and xerophyte areas they are apt
to be more or less monotonous in form. Another contrast
is found in the dense growth over mesophyte areas, much
more so than in xerophyte regions, and even more dense
than in hydrophyte areas.

Among the mesophyte associations must be included not
merely the natural ones, but those new associations which
have been formed under the influence of man, and which
do not appear among xerophyte and hydrophyte associations.

233

MESOPHYTE ASSOCIATIONS. . 235

These new associations have been formed by the introduc-
tion of weeds and culture plants.

167. The two groups of associations. — Two very promi-
nent types of associations are included here under the meso-
phytes, although they are probably as distinct from one
another as are the mesophyte and xerophyte associations.
One group is composed of low vegetation, notably the com-
mon grasses and herbs ; the other is a higher woody vegeta-
tion, composed of shrubs and trees. The most character-
istic types under each one of these divisions are noted as
follows :

A. Grass and herb associations.

It should not be inferred from this title that most
grasses are not herbs, but it is convenient to consider
grasses and ordinary herb forms separately.

108. Arctic and alpine carpets. — These are dense mats of
low vegetation occurring beyond forest growth in arctic
regions, and above the tree limit in high mountains. These
carpet-like growths are a notable feature of such regions.
In such positions the growing season is very short, and the
temperature is quite low at times, especially at night. It
is evident, therefore, that there must be provision for rapid
growth, and also for preventing dangerous radiation of
heat, which might chill the active plant below the point of
safety. It is further evident that the short season and the
low temperature form a combination which prevents the
growth of trees or shrubs, or even tall herbs, because the
season is too short for them to reach a protected condition,
and their more exposed young structures are not in a posi-
tion to withstand the daily fall of temperature.

These carpets of vegetation are notably fresh-looking,
indicating rapid growth ; green, indicating an abundance
of chlorophyll and great activity ; thick, as they are
mostly perennials, developed from abundant underground
structures ; low, on account of the short season and low

Fig. 204. Two plants of a rock-rose (Helianthemum), showing the effect of low
ground and alpine conditions. The low-ground plant (a) shows an open habit,
and elongated stems with comparatively large and well-separated leaves. The
same plant in alpine conditions is drawn to the same scale in 6, and magnified in
c, the very short and compact habit heing in striking contrast with that of the low-
ground form.— After Bonnier.

MESOPHYTE ASSOCIATIONS. 237

temperature ; and soft, the low stature and short life not
involving the development of specially rigid structures for
support or resistance. In such conditions, as would be
expected, annuals are in the minority, the plants being
mostly perennial and geophilous. Geophilous plants are
those which have the habit of disappearing underground
when protection is needed. This is probably the best adap-
tation for total disappearance from the surface and for rapid
reappearance (see §146). In such conditions, also, rosette
forms are very common, the overlapping leaves of the rosette
closely pressed to the ground diminishing the loss of heat
by radiation. It has also been noticed that these arctic and
alpine carpets show intense color in their flowers, and often
a remarkable size of flower in proportion to the rest of the
plant. Wherever the area is relatively moist, the carpet is
prevailingly a grass mat ; in the drier and sandier spots the
herbs predominate (see Figs. 202, 203).

In the case of plants which can grow both in the low
ground and in the alpine region, a remarkable adaptation
of the plant body to the different conditions may be noted.
The difference in appearance is sometimes so great that it
is hard to realize that the two plants belong to the same
species (see Fig. 204).

169. Meadows. — This term must be restricted to natural
meadow areas, and should not be confused with those arti-
ficial areas under the control of man, which are commonly
mowed. The appearance of such an area hardly needs defi-
nition, as it is a well-known mixture of grasses and flower-
ing herbs, the former usually being the predominant type.
Such meadow-like expanses are common in connection with
forest areas (see Fig. 205), but they are most character-
istically developed on flood-plains along streams. In most
cases the local meadow is probably an ephemeral society, to
be replaced by forest growth.

The greatest meadows of the United States are the well-
known prairies, which extend from the Missouri eastward

MESOPHYTE ASSOCIATIONS. 239

to the forest regions of Illinois and Indiana (see Fig. 206).
The prairie is regarded by some as a xerophytic area, and
this is a natural conclusion when one examines only the
structures of the plants which occupy it. It is certainly a
transition area between the plains of the West and the true
mesophytic areas of the East, and there is a general tran-
sition from the more xerophytic western prairies to the
more mesophytic eastern prairies. Moreover, in the east-
ern part of the prairie region there is locally every grada-
tion between the strongly mesophytic type of the low ground
to the more xerophytic type of the high ground.

The vegetation of the prairies in general is composed
of tufted grasses and perennial flowering herbs. Unfortu-
nately, most of the natural prairie has disappeared, to be
replaced by farms, and the characteristic prairie forms are
not easily seen. The flowering herbs are often very tall and
coarse, but with brilliant flowers, such as species of aster,
goldenrod, rosin-weed, indigo plant, lupine, bush clover, etc.
The most characteristic of these forms show their xero-
phytic adaptations by their rigidity and roughness.

The origin of the prairie has long been a vexed question,
which has usually taken the form of an inquiry into the
conditions which forbid the growth of forests. Prairies are
at least of two kinds. Some are edaphic — that is, they are
due to local soil conditions. Such prairies are character-
istic of the eastern prairie region, and even appear in scat-
tered patches throughout the forest region as far east as
Ohio, Kentucky, etc. They are probably best explained as
representing old swamp areas, which at a still more ancient
time were ponds or lakes. All the prairies of the Chicago
area are evidently edaphic, being associated with former
extensions of Lake Michigan. Other prairies are climatic —
that is, they are due to general climatic conditions. Such
prairies are characteristic of the western prairie region,
merging into the plains, and are more puzzling than the
edaphic prairies. Among the several explanations sug-
17

k-L

MESOPHYTE ASSOCIATIONS. 241

gested perhaps that which refers the western prairies to
the prevailing dry winds is the most prominent.

The extensive plains of the West develop the strong
and dry winds which prevail over this prairie region, and
this brings about extremes of heat and drouth, in spite of
the character of the soil. In such conditions a tree in a
germinating condition could not establish itself. If it is
protected through this tender period it can maintain itself
afterward, but the drying winds forbid any plant with a
prolonged and sensitive juvenile period. These prairies,
therefore, would represent a sort of broad beach between
the western plains and the eastern prairies and forests.

What seems to be a confirmation of this view may be
observed in certain north and south valleys in the Missouri
region which lies on the border between plains and prairies.
The eastern slopes of such valleys, exposed to the wind
from the plains, are without trees ; while on the western
slopes, protected from this wind, trees occur.

Probably the oldest explanation of such prairies is the
occurrence of prairie fires, but this would appear to be toe
local a cause for what seems to be a continental feature.
Eecently, however, the fire theory has been revived, and
evidence has been brought forward to show that in some
places, at least, a forest growth would appear if fire and
stock were kept out. In fact, the claim is made that Ne-
braska is becoming gradually forest-clad.

170. Pastures. — This term is applied to areas drier than
natural meadows, and includes the meadows formed or con-
trolled by man (see Fig. 207). They may be natural, or
derived from natural meadow areas, or from forest clear-
ings ; therefore they are often maintained in conditions
which, if not interfered with, would not produce a meadow.
In general, the pasture differs from the natural meadow in
being drier, a fact often due to drainage, and in develop-
ing lower and more open vegetation. Naturally the plant
forms are prevailingly grasses, and their cultivation is the

242

PLANT RELATIONS.

purpose of the artificial pasture, but the meadow tendency
is shown by the coming in of perennial weeds. The inva-
sion of pastures by weeds suggests many interesting ques-
tions. Are the weeds natives or foreigners? Are they

Fig. 207. A juniper heath interspersed with pastures. The growths of juniper are
very dense, excluding all other vegetation, and the grass or pasture areas are too
dry to form real meadows. — After Cowles.

annuals or perennials ? What is the relative success of the
different invaders, and why are some more successful than
others ? A study of pastures will also reveal the fact that
there is great difference in the vegetation of mowed and
grazed pastures. The same effects are noted when natural
meadows are used for grazing.

B. Woody associations.

These associations include the various shrub and tree
assemblages of mesophyte areas, assemblages entirely dis-
tinct from the grass and herb associations,

MESOPHYTE ASSOCIATIONS. 243

171. Thickets. — The mesophyte thickets are not so
abundant or impenetrable as the xeropbyte thickets. They
seem to be developed usually as forerunners of forest vege-
tation. An illustration of this fact may be obtained by
noting the succession of plauts which appear on a cleared
area. After such an area has been cleared of its trees, by
cutting or by fire, it is overrun by herbs that develop
rapidly from the seed. Sometimes these herbs are tall
and with showy flowers, as the so-called fire-weed or great
willow herb. Following the herb associations there is a
gradual invasion of coarser herbs and shrubby plants,
forming thickets, and finally a forest growth may appear
again.

In arctic and alpine mesophyte regions the willow is
the great thicket plant, often covering large areas, but in
temperate regions willow thickets are confined to stream
banks and boggy places, being the characteristic hydro-
phyte thicket form.

The upland and flood-plain mesophyte thickets of tem-
perate regions are different in character. For example,
the upland thicket of the Northern States very commonly
contains hazel, birch, and aspen as dominant plants ; while
the flood-plain thicket is apt to contain, in addition to
these, prominent growths of haws and wild crab-apples.
In this same region pure thickets frequently occur — that
is, thickets in which a single form is the prevailing type,
as pure hazel thickets on uplands, or pure haw thickets on
flood-plains.

In the Southern States the plants enumerated above
may not be the characteristic mesophyte thicket plants.
For example, in Kentucky and Tennessee the dominant
thicket plants are persimmon, locust, redbud, and
sassafras.

172. Forests of temperate regions. — Deciduous forests
are especially characteristic of temperate regions. The
deciduous habit, that is, the habit of shedding leaves at a

244

PLANT RELATIONS.

certain period, is an adaptation to climate. In the tem-
perate regions the adaptation is in response to the winter
cold, when a vast reduction of delicate exposed surface is
necessary. Instead of protecting delicate leaf structures
from the severe cold of winter, these plants have formed
the habit of dropping them and putting out new leaves
when the favorable season returns.

It is instructive to notice how differently the conifers
(pines, etc.) and the deciduous trees (oaks, maples, etc.) have

answered the problem of adaptation
to the cold of winter. The conifers
have protected their leaves, giving
them a small surface and heavy
walls. In this way protection has
been secured at the expense of
working power during the season
of work. Eeduced surface and
thick walls are both obstacles to
leaf work. On the other hand,
the deciduous trees have devel-
oped the working power of their
leaves to the greatest extent, giving
them large surface exposure and
comparatively delicate walls. It
is out of the question to protect
such an amount of surface during
the winter, and hence the decidu-
ous habit. The conifers are saved
the annual renewal of leaves, but
lose in working power; the de-
ciduous trees must renew their leaves annually, but gain
greatly in working power.

It should be remarked that leaves do not fall because
they are broken off, but that in a certain sense it is a
process of growing off. Often at the base of the leaves,
where the separation is to occur, a cleavage region is

Fig. 208

A section through the
base of a leaf of horse-chestnut
preparing to fall off at the end
of the growing season. A
cleavage plate (s) has devel-
oped between the woody bun-
dle (b) and the surface. Pres-
ently this reaches the surface,
and only the woody strand
fastens the leaf to the stem.

MESOPHYTE ASSOCIATIONS. 245

gradually developed until the leaf is entirely separated
from the stem except by a woody strand or two, which is
easily broken (see Fig. 208). In this way the scar which
remains has really been formed before the leaf falls.

In this process of sloughing off leaves, the plant cannot
afford to lose the living substance present in the working
leaves. This substance, during the preparation for the
fall, has been gradually withdrawn into the permanent
parts of the plant.

It will be noticed that in general deciduous leaves are
thin, exceedingly variable in form, and in a general hori-
zontal position, nor do they have the firm, leathery texture
of the xerophyte leaves. All this indicates great leaf ac-
tivity, for, the necessity of protection being removed, the
leaf is not impeded in its work by the development of pro-
tective structures.

One of the most prominent features associated with the
deciduous habit is the autumnal coloration. The vivid
colors which appear in the leaves of many trees, just before
the time of falling, is a phenomenon which has attracted a
great deal of attention, but although it is so prominent, the
causes for it are very obscure. It will be noticed that this
autumnal coloration consists in the development of various
shades of two typical colors, yellow and red. These colors
are often associated together in the same leaf, and some-
times a leaf may show a pure color.

The two colors hold a very different relation in the leaf
cell. It is known that the yellow is due to the breaking
down of chlorophyll, so that the chloroplasts, which are
green when active, become yellow when disorganizing, and
finally bleach out entirely. That yellow may indicate a
post mortem change of chlorophyll may be noticed in con-
nection with the blanching of celery, in which the leaves
and upper part of the stem may be green, the green may
shade gradually into yellow, and finally into the pure white
of complete blanching.

246 PLANT EELATIONS.

The red shades, however, do not seem to hold any such
relation to the disorganization of chlorophyll. The red
coloring matter appears as a stain in the cell sap, so that
what might be called the atmosphere of the active cell is
suffused with red. Certain experiments upon plant colors
have indicated that the presence of the red color slightly
increases the temperature by absorbing more heat. This
has suggested that the red color may be a slight protec-
tion to the living substance, which has ceased working
and which is in danger of exposure to cold. If this be
true, it may be that the same explanation will cover the
case of the red flush so conspicuous in buds and young
leaves in the early spring. It must not be supposed that
the need of protection has developed the color, but that
since it is developed it may be of some such service to the
plant. The whole subject, however, is too indefinite and
obscure to be presented in any other form than as a bare
suggestion.

Even the conditions which determine autumnal colora-
tion have not been made out certainly. To many the au-
tumnal coloration is associated with the coming of frost,
which simply means a reduction of temperature ; others
associate it with diminishing water supply ; still others
associate it with the change in the direction of the rays of
light, which are more oblique in autumn than during the
active growing season. It is certainly true that the colors
are far more brilliant in certain years than in others, and
that the coloration must be connected in some way with
the food relations of the plants. Eecent experiments have
shown that the red coloration is largely dependent upon low
temperature, which affects certain of the food-stuffs, and
the red stain is one of the products.

The autumnal colors are notably striking in American
forests on account of the fact that in these forests there is
the greatest display of species, and hence not only are more
colors produced, but they are usually strikingly associated.

MESOPHYTE ASSOCIATIONS. 247

Not only is protection during the cold period secured
by deciduous forests through the falling of leaves, but the
development of scaly buds is an adaptation to the same
end. By means of these overlapping, often hairy, and even
varnished structures, delicate growing tips are protected
during the cold season. The development of cork, also, on
the older parts, is a measure of protection.

Although the trees are the dominant plants of a forest as-
sociation, it must not be forgotten that numerous other forms
are associated with them. At a lower level stand the shrubs,
below these the tall herbs, then the low herbs and grasses,
and finally close to the soil mosses and lichens occur.
These different strata, as they are called, represent differ-
ent habits in reference to light, the lower strata being made
up of shade plants as compared with the upper strata. In
fact, the shade habit has become so established in many
plants of the lower strata that they depend upon the pres-
ence of the overshadowing strata, and could not live with-
out them.

The vernal habit is also an interesting feature of decidu-
ous forests. It is a matter of common observation that the
rich display of " spring flowers " occurs in forests and wooded
glens before the trees come into full foliage. The working
season of these vernal plants is before the dense foliage of
the forest shuts off the light. Accordingly, they are mostly
geophilous in habit (see §116), sending up their shoots or
leaves with great rapidity from underground tubers, root-
stocks, etc., and completing their vegetative work in the
short period during which the light is available. After the
forest leaves are fully developed the spring flowers disap-
pear, waiting in their subterranean retreats for the next
short period of activity. Two prominent forms of the ver-
nal habit may be observed. The leaves may appear before
the flowers, as in Erythronium and Hydrophyllum ; or they
may appear after the flowers, as in Hepatica and Sanguinaria.
One of the wild leeks {Allium tricoccum) has developed a

248 PLANT EELATIONS.

very interesting modification. It sends up its rosette of
large and very active leaves during the vernal season, and
when these have disappeared the flowers are developed in
the forest shade. The significance of this is that while
the leaves must have the light for their work, the flowers
can develop just as well in the shade.

As in the case of thickets, deciduous forests may be
pure or mixed. A very common type of pure forest is the
beech forest, which is a characteristic dark forest. The
wide-spreading branches of neighboring beeches overlap
each other, so as to form dense shade. As a consequence,
in a pure beech forest there is little or no undergrowth ; in
fact, no lower strata of vegetation until the lowest ones are
reached, made up of grasses and mosses. Another type of
pure forest, which belongs to the drier regions, is the oak
forest, which forms a sharp contrast to the beech, in that
it is a light forest, permitting access of light for lower
strata of plants. Hence in such a forest there is usually more
or less undergrowth, consisting of shrubs, etc., which may
develop regular thickets. The typical American deciduous
forest, however, is the great mixed forest, made up of many
varieties of trees, such as beech, oak, elm, walnut, hickory,
gum, maple, etc.

The deciduous forests may be roughly grouped as up-
land and flood-plain forests, the former being less luxuriant
and containing fewer types, the latter being the highest ex-
pression of forest development in its region. A few general
illustrations may be given as follows :

In northern Illinois the upland forest is mostly made
up of three forms, white and red oaks and shellbark hick-
ory ; while the flood-plain forest contains twenty to twenty-
five tree forms, prominent among which are the elms (white
and slippery), linden (bass wood), cottonwood, ash, silver
maple, box elder, walnut, and willows (see Fig. 211).

Farther south, from central Illinois, Indiana, and Ohio
southward, as well as in the Alleghanies, the flood-plain for-

250 PLANT RELATIONS.

ests are the richest known, containing, in addition to the
forms enumerated above, such prominent trees as the syca-
more, beech, hackberry, honey locust, coffee tree, sugar
maple, tulip tree, buckeye, etc.

In Michigan and Wisconsin the upland forests consist
prominently of beech, sugar maple, and hemlock, a charac-
teristic mixture of deciduous and evergreen trees ; while
the flood-plain forests are scarcely at all developed.

In the Alleghany region and New England the upland
forests are very extensive and complicated, grading from
the rich flood-plain forests of the lower levels on the one
hand, to the strictly xerophytic forests (pines and black
oaks) of the higher levels on the other hand, and dominated
by various oaks (especially white, red, and chestnut oaks),
chestnuts, and hickories (see Figs. 209, 210).

The flood-plain forests of New England are not so rich
as those of the Alleghany region and Central West, the
dominant forms being elms, linden, ash, maples, sycamore,
tulip tree, etc.

173. Tropical forests. — The tropical forests may be
grouped under two general heads : (1) the evergreen forests,
and (2) the deciduous or monsoon forests. The former are
characterized by continuous moisture, and are most largely
developed in the East Indies and along the Amazon and its
tributaries in South America. The deciduous tropical for-
ests are characterized by having a period of relative dry-
ness, during which the leaves are shed, and usually border
the evergreen forests.

A. Evergreen forests. — These rainy forests of the tropics
may be regarded, as Warming says, " as the climax of the
world's vegetation," for the conditions in which they are de-
veloped favor constant plant activity at the highest possible
pressure. Such great forest growths are found within the
region of the trade winds, where there is heavy rainfall,
great heat, and rich black soil. So abundant is the precipi-
tation that the air is often saturated and the plants drip

252 PLANT RELATIONS.

with moisture. In such conditions pure forests may oc-
cur, characterized by such tree forms as the tree ferns,
palms, or bamboos. Only the great mixed tropical forest
will be considered. The main characteristics are as fol-
lows :

(1) Absence of simultaneous periodicity. — Perhaps the
most striking feature, in contrast with the deciduous for-
ests, is that there is no regular period for the develop-
ment or fall of leaves. Leaf activity is possible through-
out the year, and there is no time of bare forest, or of
forests just putting out leaves. This does not mean that
the leaves persist indefinitely, but that there is no regular
time for their fall and formation. Leaves are continually
being shed and formed, but the trees always appear in full
foliage.

(2) Density of growth. — Such an area is remarkably
filled with vegetation stratum, after stratum occurring,
resulting in gigantic jungles. The higher strata may be
made up of trees of different heights, below them are shrubs
of varying heights, then tall and low herbs, and finally
mosses and liverworts. Among these close-set standing
forms, great vines or lianas climb and bind the standing
vegetation into an inextricable tangle (see Figs. 55, 212).
In addition to these, hosts of aerial plants find lodging
places upon the tree-trunks and vines (see Fig. 213). These
rainy forests of the tropics furnish the very best conditions
for the development of the numerous epiphytic orchids,
bromelias, etc. In such conditions also numerous sapro-
phytes occur. Such an assemblage of vegetation is to be
found nowhere else.

(3) Number of species. — Not only is there an immense
number of individuals, but an extraordinary number of
species occur. A list of plants growing in these forests
would show a remarkable representation of the plant
kingdom.

(4) Forms of trees. — The dense vegetation results in

MESOPHYTE ASSOCIATIONS.

255

straight leafless tree-trunks, so that the leaves of trees are
mainly clustered at the tops of high branches. The shade
is so dense and the interference is so great that the devel-
opment of low branches is impossible. It is common, also,

Fig. 213. A group of aerial plants (epiphytes) from a tropical forest. Note the vari-
ous habits of the epiphytes attached to the tree-trunks, and the dangling roots.—
After Schimper.

for the larger trees to develop a system of buttresses near
the base, and also frequently to send out prop roots (see
Figs. 100, 101).

(5) Absence of bud scales. — In the deciduous forest bud
scales are necessary to protect the tender growing tips dur-
ing the period of cold. The same device would be suffi-
18

256

PLANT RELATIONS.

cient to protect against a period of drouth. In the tropical
forest there is danger neither from cold nor drouth, and in
such conditions bud scales are not developed, and the buds
remain naked and unprotected.

(6) Devices against too abundant rain. — The abundance
of rain is in danger of checking transpiration, and as this

process is essential to plant
activity, there are often
found devices to prevent
the leaves from becoming
saturated. Many leaves
have cuticles so smooth
and glazed that the water
glances off without soaking
in ; in other cases a velvety
covering of hairs answers
the same purpose; in still
other cases leaves are gut-
ter-pointed, that is, the tip
is prolonged as a sort of
gutter, and the veins are
depressed, the whole sur-
face of the leaf resembling
a drainage system, so that
the rain is conducted rap-
idly from the surface (see
Fig. 214). These are only
a few illustrations of many
devices against dangerous
wetting.

B. Deciduous or mon-
soon forests. — In these for-
ests the same general habits prevail as in the rainy evergreen
forests, but to a less degree. For example, the epiphytes
and lianas are present, but they are not so numerous or
conspicuous. The striking difference, however, is the

Fig. 214. A gutter-pointed leaf from a
tropical plant.— After Schimper.

MESOPHYTE ASSOCIATIONS. 257

deciduous habit, developed apparently by the regular
recurrence of a relatively dry period, although it may be
very short. Such forests are usually adjacent to the ever-
green forests, much as upland forests are adjacent to flood-
plain forests.

INDEX TO PLANT RELATIONS.

[The italicized numbers indicate that the subject is illustrated on the page cited.
In such case the subject may be referred to only in the illustration, or it may be
referred to also in the test.]

Acacia, 199.

Achillea, 202.

Adaptation, 147.

Adiantum, 27.

Aeration, 92, 93, 95, 183.

Agave, 45, 47.

Agrimony, 121.

Ailanthus, 116.

Air, 95, 98, 114, 122, 138.

Air cavities, 171, 172, 173, 175.

Air passages, 92, 93, 94, 95.

Air plants, 97, 98, 99, 100, 101,

246.
Air roots, 97, 98, 99, 100.
Alchemilla, 79.
Algae, 1, 2, 87, 99, 107, 109, 110,

111, 113, 150, 171, 172, 177.
Alkaline deserts, 255.
Alpine plants, 148, 231, 232, 233.
Amicia, 9.
Arapelopsis, 63.
Aneraophilous, 122.
Animals, 119, 121, 122, 123, 145,

205.
Annual habit, 195.
Annual rings, 84.
Anthurium, 97.
Apple, 79.
Araucaria, 74.

Arbor vitas, 139.
Arctic plants, 148, 232.
Arrow-leaf, 186.
Artillery plant, 120.
Ash, 116.
Aspidium, 55.
Assimilation, 154, 156.
Autumn coloration, 241.

B

Bacteria, 189.
Banana, 88.
Banyan, 105, 106.
Barberry, 207.
Bark, 84.

Basswood, 116, 201.
Beach, 209, 210.
Beach marshes, 253.
Beach meadows, 253
Beach pea, 118.
Bean, 140.
Bearberry, 212, 214.
Beech drops, 157.
Beech forest, 145, 243.
Beggar ticks, 119, 121.
Begonia, 25, 208.
Bellflower, 19, 80.
Bidens, 119.
Bignonia, 115.

260

INDEX.

Bilbergia, 136.

Birches, 71.

Black moss, 96, 101.

Bladderwort, 178.

Blade, 35.

Bloodroot, 195.

Bogs, 143.

Box elder, 84.

Bramble, 94.

Branched leaves, 19, 20, 21, 23.

Buds, 70, 73, 75, 141, 247.

Bulbs, 73, 75, 81.

Bulrush, 142, 148, 185, 186, 207.

Burdock, 121, 122.

Bush, 227.

Bush clover, 43.

Buttercup, 185.

Buttresses, 103, 104.

C

Cactus deserts, 217, 222.

Cactus forms, 45, 46, 47, 146, 202,

207, 215, 216, 217, 218, 219, 222.
Calyx, 78, 79, 80, 125.
Campanula, 19, 80.
Caoutchouc, 136.
Carbohydrates, 153, 156.
Carbon, 153.

Carbon dioxide, 30, 151, 153.
Cardon forests, 231.
Carnation, 4%-
Carnivorous plants, 155, 156, 157,

173, 189.
Carpel, 78, 79, 80, 125.
Carrot, 120.
Castor-oil bean, 73.
Casuarina, 231.
Catalpa, 117.
Catchfly, 136.

Cat-tail flag, 142, 148, 185, 186.
Cercis, 10.

Change in temperature, 145.
Chaparral, 227.
Chlorophyll, 6, 8, 149, 152.
Chloroplasts, 39, 107, 152, 205, 208,

209, 245.
Chrysanthemum, 23.
Cilia, 109, 111.
Claytonia, 196.
Cleistogamous, 130.
Clematis, 113.
Climbing stems, 60, 61, 62, 63, 64,

102.
Clinging roots, 99, 102.
Clinia, 209.
Cocklebur, 120, 121.
Compass plants, 10, 12, 197, 198.
Compound leaves, 19, 20, 21, 23.
Conducting tissue, 171.
Conifer forests, 226, 227, 228, 229,

230.
Conifers, 83, 190, 191, 225, 226.
Cork, 247.
Corn, 85, 90.
Corolla, 78, 79, 80.
Cortex, 83, 84, 93, 94, 107, 108.
Cottonwood, 70.
Cotyledons, 50, 51, 73, 139, 140.
Crevice plants, 94, 209.
Cuticle, 42, 205.
Cycad, 22.
Cycloloma, 117.
Cypress knees, 95, 96, 183.
Cypripedium, 132, 133, 134, 185,

136.
Cytisus, 206.

D

Dandelion, 82, 114, 117.
Darlingtonia, 157.
Date palm, 86.
Dead-nettle, 80.

INDEX.

261

Deciduous forests, 243.

Deciduous habit, 143, 196, 243, 244.

Deserts, 221, 222, 223.

Desiccation, 194.

Desmodium gyrans, 49.

Destruction of plants, 148.

Diatoms, 174.

Dicotyledons, 35, 83, 116.

Differentiation, 3.

Digestion, 154, 156.

Dionasa, 160, 161.

Dodder, 106, 107, 157.

Dog-tooth violet, 144.

Dragon tree, 15.

Drainage, 143, 145.

Drosera, 158, 159.

Drouth, 193.

Duckweed, 97, 175.

Dunes, 145, 201, 209, 211, 212.

Dwarf growths, 203.

E

Easter lily, 14.

Echeveria, 17.

Ecological factors, 163.

Ecology, 4, 149.

Eel grass, I84.

Egg, 110, 111.

Elaters, 118.

Elatine, 93.

Elm, 63, 67, 68, 75.

Embryo, 111, 139.

Entomophilous, 122, 123.

Epidermis, 37, 40, 41, 4?, 83, 84,

107, 170, 205, 208, 209.
Epilobium, 112, 113, 128, 135.
Epiphyte, 209.
Equisetum, 111, 203.
Erect stems, 62, 65, 66, 67, 68, 69,

70, 71.
Erica, 200.

Erythronium, 144.
Euphorbia, 204.

F

Ferns, 55, 56, 85, 88, 100, 111, 113,

119.
Ficus, 8.

Figwort, 128, 135.
Fireweed, 112, 113, 128, 135, 243.
Fittonia, 37, 152.
Fixed light position, 197.
Flag, 126, 133.
Floating stems, 59.
Floats, 171, 172, 173.
Flowers, 76, 78, 140.
Foliage forests, 227.
Foliage leaves, 6, 28, 139.
Forest clearing, 143, 145.
Forests, 190, 226, 227, 228, 229.
Fruit, 113, 114, 115, 116, 117, 118,

119, 120, 121, 122.
Pucus, 171.
Functions, 3.
Fungi, 87, 107, 109, 110.
Furze, 205.

G

Galium, 121.

Gamete, 110, 111, 112, 113.

Geophilous habit, 55, 56, 73, 74, 75,

76, 77, 78, 81, 195, 196, 237.
Geotropism, 69, 91, 138.
Germination, 111, 138, 139, 140.
Gorse, 205.
Grape vine, 61.
Grass, 187, 197, 216, 236.
Gravity, 91.
Guard cells, 38.
Gymnosperms, 115.

262

INDEX.

H

Habenaria, 127.

Hairs, 43, 92, 136, 146, 202, 203.

Harebell, 19, 80.

Hawthorn, 36.

Heart-wood, 151.

Heat, 112, 138, 145, 164.

Heath plants, 189, 200, 214.

Helianthemum, 236.

Heliotropism, 12, 13, 68, 72, 78,

139.
Hemlock, 190.
Horse-chestnut, 244-
Hosts, 106.
House leek, 19.
Houstonia, 129, 135.
Huckleberry, 214.
Hudsonia, 212.
Hura crepitans, 120.
Hydrophytes, 168, 170, 174.
Hydrotropism, 91, 138.

Insects and flowers, 123.
Iris, 126, 133.
Isoetes, 94, 95, 208.
Ivy, 99.

Juncus, 77.
Juniper, 51,

V#.

Lactuca, 12, 197, 198.
Lady-slipper, 132, 133,
136.

134, 135,

Lakes, 143, 148.

Laminaria, 177.

Larch, 178, 190.

Latex, 136.

Leafless forests, 227.

Leaflet, 19.

Leaf-relation, 53.

Lemna, 97.

Lespedeza, 43.

Lianas, 60, 61, 62, 63, 64, 102, 245,

246.
Lichens, 194, 209, 214.
Life-relations, 4, 7, 8, 53, 77.
Light, 143, 167, 197.
Light-relation, 7, 8.
Lily, 38, 40.
Live-for-ever, 18.
Live oak, 101.
Liverworts, 118.
Locomotion, 113.
Locust, 207.
Long moss, 96, 101.
Loosestrife, 130, 135.
Lotus, 180.

M

Mangroves, 192, 192a.
Maple, 26, 115, 116.
Maranta, 38.
Marchantia, 107.
Meadows, 237, 238.
Mechanical tissue, 172.
Mesophyll, 38, 39, 40 41, 42, 152.
Mesophytes, 168, 233.
Migration, 58, 75, 147.
Mildew, 109, 157.
Milkweed, 117.
Mimosa, 199.
Mistletoe, 107.
Mold, 109.

Monocotyledons, 35, 85, 88, 116,
186.

INDEX.

263

Moors, 187, 188.

Mosaic arrangement, 25, 27, 37.

Mosses, 87, 107, 110, 113, 118, 188,

194, 209, 214.
Motile leaves, 9, 10, 11, 49, 198,

199.
Mould, 109.
Mullein, 43, 44.
Mushrooms, 157.

N

Nectar, 123, 158.
Nelumbium, 180.
Nicotiana, 80.
Nightshade, 26.
Nitrogen, 153.
Nodes, 54.
Nuphar, 92.
Nutrition, 3, 149.
Nymphaea, 178, 180.

0

Oak, 69, 101.

Oak forest, 145, 248.

GMogonium, 111.

Orchids, 98, 99, 126, 127, 132, 133,

134, 135, 136, 189.
Organs, 3.
Ornithogalum, 81.
Ovary, 79, 80, 125.
Ovules, 78, 79, SO.
Oxalis, 10, 50, 199.
Oxygen, 29, 138, 153.

Palisade tissue, 39, 40, 42, 205.
Palms, 86, 87, 230.
Pandanus, 103.

Parasites, 106, 150.

Passion vine, 62.

Pastures, 241, 242.

Pellionia, 24.

Pentstemon, 137.

Peony, 78.

Petals, 78, 79, 80.

Petioles, 15, 26, 35, 55.

Phlox, 80.

Photosynthesis, 28, 29, 150, 152,

153, 156.
Physiology, 149.
Pickerel weed, 181, 182.
Pines, 63, 65, 66, 112, 115, 117, 190,

227, 229, 230.
Pirus, 79.
Pistil, 77, 79, 80.
Pitcher plant, 155, 156, 157, 158.
Pith, 83, 84, 107.
Plains, 213, 215, 216.
Plankton, 174.
Plant body, 2.
Plastid, 152.
Platycerium, 100.
Plumes, 112, 113, 114, 116, 117.
Plumule, 51, 140.
Pollen, 77, 111, 112, 115, 121,

123.
Pollination, 77, 115, 122, 123.
Polygonatum, 35.
Ponds, 142, 175, 178, 180, I84.
Pondweed, 176, 181, 182.
Potato, 74, 76.
Potentilla, 43, 79.
Prairies, 208, 222, 237, 240.
Prickles, 146.

Prickly lettuce, 12, 197, 198.
Primrose, 137.
Procumbent stem, 57.
Profile position, 197, 198.
Pronuba, 130, 131.
Prop roots, 99, 103, 104, 105, 106,

255.

264

INDEX.

Protandry, 128, 135.

Protection of leaves, 9, 10, 11, 12,

41, 42, 43, 48, 49.
Proteids, 153, 156, 189.
Protogyny, 128, 135.
Protoplasm, 154, 156.
Ptelea, 115.
Puff-balls, 157.

Quillwort, 94, 95, 208.

R

Rain, 51, 256.

Ranunculus, 185.

Raspberry, 91.

Receptacle, 79, 81, 114.

Redbud, 10.

Reed grass, 142, 185, 186.

Reed swamps, 185.

Reproduction, 3, 109.

Respiration, 32, 154, 156.

Rhizoids, 107.

Rivalry, 146.

Robinia, 125, 126, 133, 207.

Rock-rose, 236.

Rock associations, 209, 210.

Roots, 89, 90, 95, 98, 99, 138, 139,

171.
Root-cap, 108.
Root-hairs, 90.
Rootstalk, 55, 56, 75, 76, 77, 78,

195.
Rose acacia, 125, 126, 133.
Rosette habit, 16, 17, 18, 19, 47, 94,

158, 160, 209, 237.
Rosinweed, 10, 197, 198.
Rubber tree, 104.
Runners, 57, 93.
Rusts, 157.

S

Sage brush, 216.

Sagittaria, 186.

Saintpaulia, 16.

Salt deserts, 231.

Salt steppes, 231.

Sand associations, 209.

Sandy fields, 209, 212.

Sanguinaria, 195.

Saprophytes, 150, 189.

Sap-wood, 151.

Sargassum, 172.

Sarracenia, 155, 156, 158.

Saxifrage, 58.

Scale leaves, 70, 75.

Scales, 141.

Scouring rush, 203.

Screw pine, 103.

Scrub, 227.

Seaweeds, 1, 2, 87, 99.

Sedges, 187.

Seed-dispersal, 112, 113, 114, 116,

117, 118, 119, 120.
Seed-plants, 111, 119, 121.
Seeds, 111, 112, 113, 115, 138, 139,

140.
Selaginella, 26, 100, 194.
Sempervivum, 19.
Senecio, 114.

Sensitive plants, 11, 48, 50, 199.
Sepals, 78, 79, 80.
Shepherdia, 44-
Shoots, 53.

Silphium, 10, 197, 198.
Smilax, 61.
Snapdragon, 80, 137.
Soil. 90, 94, 145, 151, 166, 214,

224.
Solomon's seal, 35, 76.
Spanish needle, 119, 121.
Sphagnum, 188.

INDEX.

265

Sphagnum-bogs, 208.

Sphagnum- moors, 188.

Spines, 146, 204.

Spirogyra, 110.

Spongy tissue, 39, 1ft.

Spore case, 55, 118, 119.

Spore-dispersal, 109, 111, 112, 113,

114, 118.
Spores, 109, 110, 111, 112.
Spring beauty, 196.
Spring plants, 143, 144.
Squash seedlings, 50.
Squirting cucumber, 120.
Staghorn fern, 100.
Stamens, 78, 79, 80, 125.
Starch, 153.
Star cucumber, 61.
Star-of-Bethlehem, 81.
Stem, 54, 83, 139.
Steppes, 216.
Stigma, 80, 125.
Stipules, 35.
Stomata, 38, 40, 206.
Strawberry plant, 57, 58, 93.
Struggle for existence, 142.
Style, 80, 125.
Subterranean stems, 54, 55, 56, 76,

77, 78.
Succulent plants, 222.
Sugar, 153.
Sundew, 158, 159.
Sunflower, 72.
Swamp-forest, 190, 191.
Swamps, 183, 187.
Swamp-thickets, 188.

Tamarack, 178, 190.
Tap root, 93.
Taxus, 42.

Teasel, 136.

Telegraph plant, 49.

Temperature, 145.

Tendrils, 61, 62, 63.

Thallus, 107.

Thickets, 188, 224, 243.

Thistle, 117.

Thorns, 146, 204, 205, 206, 207.

224.
Thuja, 139.
Tilia, 116, 201.
Tillandsia, 96, 101.
Toad-flax, 80.
Toadstools, 149.
Tobacco, 80.
Touch-me-not, 119.
Tragacanth, 206.

Transpiration, 31, 33, 154, 193,256.
Tropical forest, 254-
Trumpet creeper, 99.
Tubers, 74, 76, 196.
Tumbleweeds, 117, 220.
Turf-building, 185.

Ulex, 205.
Ulothrix, 109, 111.
Utricularia, 173, 174.

Vallisneria, 184-

Vascular bundles, 83, 84, 92,

94, 107, 108, 151, 171.
Vegetative multiplication, 109.
Veins, 35, 36, 37, 40, 151.
Velamen, 99.
Venation, 35, 36, 37.
Victoria, 180.
Violet, 117, 119.

266

INDEX.

W

Walnut, 82.

Water, 90, 92, 94, 95, 113, 138,
142, 151, 163, 193, 206, 250.

Water lily, 178, 180, 181.

Water reservoirs, 206, 208, 209.

Weeds, 147.

Willow, 35, 243.

Wind, 95, 98, 114, 122, .167.

Wings, 112, 115, 116.

Witch hazel, 118, 119.
Woodbine, 61, 63.

Xerophytes, 168, 193, 208.
Xerophytic structure, 207.

Yew, 42.

Yucca, 45, 47, 130, 131,

TWENTIETH CENTURY TEXT-BOOKS

PLANT STRUCTURES

A SECOND BOOK OF BOTANY

BY

JOHN M. COULTER, A.M., Ph.D.

HEAD OF DEPARTMENT OF BOTANY
UNIVERSITY OF CHICAGO

SECOND EDITION REVISED

NEW YORK
D. APPLETON AND COMPANY

1910

Copyright, 1899, 1904,
Bt D. APPLETON AND COMPANY

PKEFACE

In the preface to Plant Relations the author gave his
reasons for suggesting that the ecological standpoint is best
adapted for the first contact with plants. It may be, how-
ever, that many teachers will prefer to begin with the mor-
phological standpoint, as given in the present book. Eec-
ognizing this fact, Plant Structures has been made an
independent volume that may precede or follow the other,
or may provide a brief course of botanical study in itself.

Although in the present volume Morphology is the domi-
nant subject, it seems wise to give a somewhat general view
of plants, and therefore Physiology, Ecology, and Taxonomy
are included in a general way. For fear that Physiology
and Ecology may be lost sight of as distinct subjects, and
to introduce important topics not included in the body of
the work, short chapters are devoted to them, which seek
to bring together the main facts, and to call attention to
the larger fields.

This book is not a laboratory guide, but is for reading
and study in connection with laboratory work. An accom-
panying pamphlet for teachers gives helpful suggestions
to those who are not already familiar with its scope and
purpose. It is not expected that all the forms and sub-
jects presented in the text can be included in the labora-
tory exercises, but it is believed that the book will prove a
useful companion in connection with such exercises. It
is very necessary to co-ordinate the results of laboratory
work, to refer to a larger range of material than can be
handled, and to develop some philosophical conception of

^ PREFACE

the plant kingdom. The learning of methods and the
collection of facts are fundamental processes, but they
must be supplemented by information and ideas to be of
most service.

The author does not believe in the use of technical
terms unless absolutely necessary, for they lead frequently
to mistaking definitions of words for knowledge of things.
But it is necessary to introduce the student not merely to
the main facts but also to the literature of botany. Ac-
cordingly, the most commonly used technical terms are
introduced, often two or three for the same thing, but it
is hoped that familiarity with them will enable the student
to read any ordinary botanical text. Care has been taken
to give definitions and derivations, and to call repeated
attention to synonymous terms, so that there may be no
confusion. The chaotic state of morphological terminology
tempted the author to formulate or accept some consistent
scheme of terms ; but it was felt that this would impose
upon the student too great difficulty in reading far more
important current texts.

Chapters I-XII form a connected whole, presenting the
general story of the evolution of plants from the lowest to
the highest. The remaining chapters are supplementary,
and can be used as time or inclination permits, but it is the
judgment of the author that they should be included if
possible. The flower is so conspicuous and important a
feature in connection with the highest plants, that Chapter
XIII seems to be a fitting sequel to the preceding chapters.
It also seems desirable to develop some knowledge of the
great Angiosperm families, as presented in Chapter XIV,
since they are the most conspicuous members of every flora.
In this connection, the author has been in the habit of
directing the examination of characteristic flowers, and of
teaching the use of ordinary taxonomic manuals. Chap-
ter XV deals with anatomical matters, but the structures
included are so bound up with the form and work of plants

PREFACE

Vll

that it seems important to find a place for them even in an
elementary work. The reason for Chapters XVI and XVII
has been stated already, and even if Plant Relations is stud-
ied, Chapter XVII will be useful either as a review or as an
introduction. In the chapter on Plant Physiology the
author has been guided by Noll's excellent resume in the
" Strasburger " Botany.

The illustrations have been entirely in the charge of
Dr. Otis W. Caldwell, who for several years has conducted
in the University, and in a most efficient way, such labo-
ratory work as this volume implies. Many original illus-
trations have been prepared by him, and under his direction
by Messrs. S. M. Coulter, B. A. Goldberger, W. J. G. Land,
and A. C. Moore, and some are credited to Dr. Chamberlain
and Dr. Cowles, of the University, but it is a matter of
regret that pressure of work and time limitation have for-
bidden a still greater number. The authors of the original
illustrations are cited, and where illustrations have been
obtained elsewhere the sources are indicated.

The author would again call attention to the fact that
this book is merely intended to serve as a compact supple-
ment to three far more important factors : the teacher, the
laboratory, and field work. John M. Coulter.

The University of Chicago, November, 1899.

PREFACE TO THE REVISED EDITION

During the last five years the science of Botany has

made rapid progress, both in the addition of new facts and

in changed points of view. Some of this progress affects

Plant Structures, and it is recorded in this revised edition

so far as it can be without a complete rewriting of the

volume. Changes will be found, therefore, in statements

of fact, in points of view, in terminology, in illustrations,

and also in the addition of new material.

John M. Coulter.

The University of Chicago, April, 1904,
19

CONTENTS

CHAPTER PAGE

I. — Introduction 1

II. — Thallophytes : Alg^e 4

III. — The evolution of sex 12

IV. — The great groups of Alg^e ... 17

V. — Thallophytes : Fungi 48

VI. — The food of plants 83

VII. — Bryophytes 93

VIII. — The great groups of Bryophytes 109

IX. — Pteridophytes 128

X. — The great groups of Pteridophytes .... 155

XL — Spermatophytes : Gymnosperms 171

XII. — Spermatophytes : Angiosperms 195

XIII.— The flower 218

XIV. — Monocotyledons and dicotyledons 232

XV. — Differentiation of tissues 280

XVI. — Plant physiology 297

XVII. — Plant ecology 311

Glossary 329

Index 337

BOTANY

PART II.— PLANT STRUCTURES

CHAPTEE I

INTRODUCTION

1. Differences in structure. — It is evident, even to the
casual observer, that plants differ very much in structure.
They differ not merely in form and size, but also in com-
plexity. Some plants are simple, others are complex, and
the former are regarded as of lower rank.

Beginning with the simplest plants — that is, those of
lowest rank — one can pass by almost insensible grada-
tions to those of highest rank. At certain points in this
advance notable interruptions of the continuity are dis-
covered, structures, and hence certain habits of work, chang-
ing decidedly, and these breaks enable one to organize the
vast array of plants into groups. Some of the breaks ap-
pear to be more important than others, and opinions may
differ as to those of chief importance, but it is customary
to select three of them as indicating the division of the
plant kingdom into four great groups.

2. The great groups. — The four great groups may be
indicated here, but it must be remembered that their names
mean nothing until plants representing them have been
studied. It will be noticed that all the names have the

1

2 PLANT STRUCTURES

constant termination phytes, which is a Greek word mean-
ing " plants." The prefix in each case is also a Greek word
intended to indicate the kind of plants.

(1) Thallophytes. — The name means "thallus plants,"
but just what a "thallus" is can not well be explained
until some of the plants have been examined. In this
great group are included some of the simplest forms,
known as Algce and Fungi, the former represented by green
thready growths in fresh water and the great host of sea-
weeds, the latter by moulds, mushrooms, etc.

(2) Bryophytes. — The name means " moss plants," and
suggests very definitely the forms which are included.
Every one knows mosses in a general way, but associated
with them in this great group are the allied liverworts,
which are very common but not so generally known.

(3) Pteridophytes. — The name means " fern plants," and
ferns are well known. Not all Pteridophytes, however, are
ferns, for associated with them are the horsetails (scouring
rushes) and the club mosses.

(4) Spermatophytes. — The name means " seed plants " —
that is, those plants which produce seeds. In a general
way these are the most familiar plants, and are commonly
spoken of as " flowering plants." They are the highest in
rank and the most conspicuous, and hence have received
much attention. In former times the study of botany in
the schools was restricted to the examination of this one
group, to the entire neglect of the other three great groups.

3. Increasing complexity. — At the very outset it is well
to remember that the Thallophytes contain the simplest
plants — those whose bodies have developed no organs for
special work, and that as one advances through higher
Thallophytes, Bryophytes, and Pteridophytes, there is a con-
stant increase in the complexity of the plant body, until in
the Spermatophytes it becomes most highly organized, with
numerous structures set apart for special work, just as in the
highest animals limbs, eyes, ears, bones, muscles, nerves, etc..

INTRODUCTION 3

are set apart for special work. The increasing complexity
is usually spoken of as differentiation — that is, the setting
apart of structures for different kinds of work. Hence the
Bryophytes are said to be more highly differentiated than
the Thallophytes, and the Spermatophytes are regarded as
the most highly differentiated group of plants.

4. Nutrition and reproduction. — However variable plants
may be in complexity, they all do the same general kind of
work. Increasing complexity simply means an attempt to
do this work more effectively. It is plant work that makes
plant structures significant, and hence in this book no at-
tempt will be made to separate them. All the work of
plants may be put under two heads, nutrition and repro-
duction, the former including all those processes by which
a plant maintains itself, the latter those processes by which
it produces new plants. In the lowest plants, these two
great kinds of work, or functions, as they are called, are
not set apart in different regions of the body, but usually
the first step toward differentiation is to set apart the re-
productive function from the nutritive, and to develop
special reproductive organs which are entirely distinct from
the general nutritive body.

5. The evolution of plants. — It is generally supposed that
the more complex plants have descended from the simpler
ones ; that the Bryophytes have been derived from the Thallo-
phytes, and so on. All the groups, therefore, are supposed
to be related among themselves in some way, and it is one
of the great problems of botany to discover these relation-
ships. This theory of the relationship of plant groups is
known as the theory of descent, or more generally as evo-
lution. To understand any higher group one must study
the lower ones related to it, and therefore the attempt of
this book will be to trace the evolution of the plant king-
dom, by beginning with the simplest forms and noting the
gradual increase in complexity until the highest forms are
reached.

CHAPTEK II

THALLOPHYTES : ALG-53

6. General characters. — Thallophytes are the simplest of
plants, often so small as to escape general observation, but
sometimes with large bodies. They occur everywhere in
large numbers, and are of special interest as representing
the beginnings of the plant kingdom. In this group also
there are organized all of the principal activities of plants,
so that a study of Thallophytes furnishes a clew to the
structures and functions of the higher, more complex
groups.

The word "thallus" refers to the nutritive body, or
vegetative body, as it is often called. This body does not
differentiate special nutritive organs, such as the leaves and
roots of higher plants, but all of its regions are alike. Its
natural position also is not erect, but prone. While most
Thallophytes have thallus bodies, in some of them, as in
certain marine forms, the nutritive body differentiates into
regions which resemble leaves, stems, and roots ; also cer-
tain Bryophytes have thallus bodies. The thallus body,
therefore, is not always a distinctive mark of Thallophytes,
but must be supplemented by other characters to determine
the group.

7. Algae and Fungi.— It is convenient to separate Thallo-
phytes into two great divisions, known as Algcs and Fungi.
It should be known that this is a very general division and
not a technical one, for there are groups of Thallophytes
which can not be regarded as strictly either Algae or Fungi,
but for the present these groups may be included.

THALLOPHYTES: ALG^E 5

The great distinction between these two divisions of
Thallophytes is that the Algae contain chlorophyll and the
Fungi do not. Chlorophyll is the characteristic green color-
ing matter found in plants, the word meaning " leaf green."
It may be thought that to use this coloring material as the
basis of such an important division is somewhat superficial,
but it should be known that the presence of chlorophyll gives
a peculiar power — one which affects the whole structure
of the nutritive body and the habit of life. The presence
of chlorophyll means that the plant can make its own food,
can live independent of other plants and animals. Algae,
therefore, are the independent Thallophytes, so far as their
food is concerned, for they can manufacture it out of the
inorganic materials about them.

The Fungi, on the other hand, contain no chlorophyll,
can not manufacture food from inorganic material, and
hence must obtain it already manufactured by plants or
animals. In this sense they are dependent upon other or-
ganisms, and this dependence has led to great changes in
structure and habit of life.

It is supposed that Fungi have descended from Algae —
that is, that they were once Algae, which gradually acquired
the habit of obtaining food already manufactured, lost their
chlorophyll, and became absolutely dependent and more or
less modified in structure. Fungi may be regarded, there-
fore, as reduced relatives of the Algae, of equal rank so far
as birth and structure go, but of very different habits.

ALG.E

8. General characters. — As already defined, Algae are
Thallophytes which contain chlorophyll, and are therefore
able to manufacture food from inorganic material. They
are known in general as " seaweeds," although there are
fresh-water forms as well as marine. They are exceedingly
variable in size, ranging from forms visible only by means

6 PLANT STRUCT DEES

of the compound microscope to marine forms with enor-
mously bulky bodies. In general they are hydrophytes — that
is, plants adapted to life in water or in very moist places.
The special interest connected with the group is that it is
supposed to be the ancestral group of the plant kingdom —
the one from which the higher groups have been more or
less directly derived. In this regard they differ from the
Fungi, which are not supposed to be responsible for any
higher groups.

9. The subdivisions. — Although all the Algae contain
chlorophyll, some of them do not appear green. In some
of them another coloring matter is associated with the chlo-
rophyll and may mask it entirely. Advantage is taken of
these color associations to separate Algae into subdivisions.
As these colors are accompanied by constant differences in
structure and work, the distinction on the basis of colors is
more real than it might appear. Upon this basis four sub-
divisions may be made. The constant termination phycew,
which appears in the names, is a Greek word meaning " sea-
weed," which is the common name for Algae ; while the pre-
fix in each case is the Greek name for the color which char-
acterizes the group.

The four subdivisions are as follows : (1) Cyanophycece,
or " Blue Algae," but usually called " Blue-green Algae," as the
characteristic blue does not entirely mask the green, and
the general tint is bluish-green ; (2) Ohlorophycece, or " Green
Algae," in which there is no special coloring matter associ-
ated with the chlorophyll ; (3) Phceophycece, or " Brown
Algae " ; and (4) Rhodophycece, or " Ked Algae."

It should be remarked that probably the Cyanophyceae
do not belong with the other groups, but it is convenient to
present them in this connection.

10. The plant body. — By this phrase is meant the nutri-
tive or vegetative body. There is in plants a unit of struc-
ture known as the cell. The bodies of the simplest plants
consist of but one cell, while the bodies of the most com-

THALLOPHYTES: ALG^E

Mzz-

B

G-'

plex plants consist of very many cells. It is necessary to
know something of the ordinary living plant cell before the
bodies of Algae or any other plant bodies can be under-
stood.

Such a cell if free is approximately spherical in outline,
(Fig. 6), but if pressed upon by contiguous cells may become
variously modified in form
(Fig. 1). Bounding it there
is a thin, elastic wall, com-
posed of a substance called
cellulose. The cell wall,
therefore, forms a delicate
sac, which contains the liv-
ing substance known as pro-
toplasm. This is the sub-
stance which manifests life,
and is the only substance
in the plant which is alive.
It is the protoplasm which
has organized the cellulose
wall about itself, and which
does all the plant work. It
is a fluid substance which
varies much in its consistence, sometimes being a thin vis-
cous fluid, like the white of an egg, sometimes much more
dense and compactly organized.

The protoplasm of the cell is organized into various
structures which are called organs of the cell, each organ
having one or more special functions. One of the most
conspicuous organs of the living cell is the single nucleus, a
comparatively compact and usually spherical protoplasmic
body, and generally centrally placed within the cell (Fig. 1).
All about the nucleus, and filling up the general cavity
within the cell wall, is an organized mass of much thinner
protoplasm, known as cytoplasm. The cytoplasm seems to
form the general background or matrix of the cell, and the

Fig. 1. Cells from a moss leaf, showing
nucleus (B) in which there is a nucle-
olus, cytoplasm (C), and chloroplasts
(A). — Caldwell.

3 PLANT STRUCTURES

nucleus lies imbedded within it (Fig. 1). Every working
cell consists of at least cytoplasm and nucleus. Sometimes
the cellulose wall is absent, and the cell then consists sim-
ply of a nucleus with more or less cytoplasm organized
about it, and is said to be naked.

Another protoplasmic organ of the cell, very conspicuous
among the Algae and other groups, is the plastid. Plastids
are relatively compact bodies, commonly spherical, variable
in number, and lie imbedded in the cytoplasm. There are
various kinds of plastids, the most common being the one
which contains the chlorophyll and hence is stained green.
The chlorophyll-containing plastid is known as the chloro-
plastid, or chloroplast (Fig. 1). An ordinary alga-cell, there-
fore, consists of a cell wall, within which the protoplasm is
organized into cytoplasm, nucleus, and chloroplasts.

The bodies of the simplest Algae consist of one such
cell, and it may be regarded as the simplest form of plant
body. Starting with such forms, one direction of advance
in complexity is to organize several such cells into a loose
row, which resembles a chain (Fig. 4) ; in other forms the
cells in a row become more compacted and flattened, form-
ing a simple filament (Figs. 2, 5) ; in still other forms the
original filament puts out branches like itself, producing
a branching filament (Fig. 8). These filamentous bodies
are very characteristic of the Algae.

Starting again with the one-celled body, another line of
advance is for several cells to organize in two directions,
forming a plate of cells. Still another line of advance is for
the cells to organize in three directions, forming a mass of
cells.

The bodies of Algae, therefore, may be said to be one-
celled in the simplest forms, and in the most complex forms
they become filaments, plates, or masses of cells.

11. Reproduction. — In addition to the work of nutrition,
the plant body must organize for reproduction. Just as the
nutritive body begins in the lowest forms with a single cell

THALLOPHYTES: ALG^E o,

and becomes more complex in the higher forms, so repro-
duction begins in very simple fashion and gradually be-
comes more complex. Two general types of reproduction
are employed by the Algae, and all other plants. They are
as follows :

(1) Vegetative multiplication. — This is the only type of
reproduction employed by the lowest Algae, but it persists
in all higher groups even when the other method has been
introduced. In this type no special reproductive bodies are
formed, but the ordinary vegetative body is used for the
purpose. For example, if the body consists of one cell, that
cell cuts itself into two, each half grows and rounds off as
a distinct cell, and two new bodies appear where there was
one before (Figs. 3, 6). This process of cell division is very
complicated and important, involving a division of nucleus
and cytoplasm so that the new cells may be organized just
as was the old one. Wherever ordinary nutritive cells are
used directly to produce new plant bodies the process is
vegetative multiplication. This method of reproduction may
be indicated by a formula as follows : P — P — P — P — P, in
which P stands for the plant, the formula indicating that
a succession of plants may arise directly from one another
without the interposition of any special structure.

(2) Spores. — Spores are cells which are specially organ-
ized to reproduce, and are not at all concerned in the nutri-
tive work of the plant. Spores are all alike in their power
of reproduction, but they are formed in two very distinct
ways. It must be remembered that these two types of
spores are alike in power but different in origin.

Asexual spores. — These cells are formed by cell divi-
sion. A cell of the plant body is selected for the purpose,
and usually its contents divide and form a variable number
of new cells within the old one (Fig. 2, B). These new cells
are asexual spores, and the cell which has formed them
within itself is known as the mother cell. This peculiar
kind of cell division, which does not involve the wall of the

IQ PLANT STRUCTURES

old cell, is often called internal division, to distinguish it
from fission, which involves the wall of the old cell, and is
the ordinary method of cell division in nutritive cells.

If the mother cell which produces the spores is different
from the other cells of the plant body it is called the sporan-
gium, which means " spore vessel." Often a cell is nutri-
tive for a time and afterward becomes a mother cell, in
which case it is said to function as a sporangium. The wall
of a sporangium usually opens, and the spores are dis-
charged, thus being free to produce new plants. Various
names have been given to asexual spores to indicate certain
peculiarities. As Alga? are mostly surrounded by water,
the characteristic asexual spore in the group is one that
can swim by means of minute hair-like processes or cilia,
which have the power of lashing the water (Fig. 7, C).
These ciliated spores are known as zoospores, or " animal-
like spores," referring to their power of locomotion ; some-
times they are called sivimming spores, or swarm spores. It
must be remembered that all of these terms refer to the
same thing, a swimming asexual spore.

This method of reproduction may be indicated by a for-
mula as follows : P — o — P — o — P — o — P, which indi-
cates that new plants are not produced directly from the
old ones, as in vegetative multiplication, but that between
the successive generations there is the asexual spore.

Sexual spores. — These cells are formed by cell union,
two cells fusing together to form the spore. This process
of forming a spore by the fusion of two cells is called the
sexual process, and the two special cells (sexual cells) thus
used are known as gametes (Fig. 2, C, d, e). It must be
noticed that gametes are not spores, for they are not able
alone to produce a new plant ; it is only after two of them
have fused and formed a new cell, the spore, that a plant
can be produced. The spore thus formed does not differ
in its power from the asexual spore, but it differs very
much in its method of origin.

THALLOPHYTES : ALG^E H

The gametes are organized within a mother cell, and if
this cell is distinct from the other cells of the plant it is
called a gametangium, which means " gamete vessel."

This method of reproduction may be indicated by a for-
mula as follows : P = ° > o — P = ° > o — P = ° > o — P,
which indicates that two special cells (gametes) are pro-
duced by the plant, that these two fuse to form one (sexual
spore), which then produces a new plant.

It must not be supposed that if a plant uses one of these
three methods of reproduction (vegetative multiplication,
asexual spores, sexual spores) it does not employ the other
two. All three methods may be employed by the same
plant, so that new plants may arise from it in three differ-
ent ways.

CHAPTER III

THE EVOLUTION OF SEX

12. The general problem. — In the last chapter it was re-
marked that the simplest Algae reproduce only by vegetative
multiplication, the ordinary cell division (fission) of nutri-
tive cells multiplying cells and hence individuals. Among
other low Algae asexual spores are added to fission as a
method of reproduction, the spores being also formed by
cell division, generally internal division. In higher forms
gametes appear, and a new method of reproduction, the
sexual, is added to the other two.

Sexual reproduction is so important a process in all
plants except the lowest, that it is of interest to discover
how it may have originated, and how it developed into its
highest form. Among the Algae the origin and develop-
ment of the sexual process seems to be plainly suggested ;
and as all other plant groups have probably been derived
more or less directly from Algae, what has been accom-
plished for the sexual process in this lowest group was
probably done for the whole plant kingdom.

13. The origin of gametes. — One of the best Algae to
illustrate the possible origin of gametes is a common fresh-
water form known as Ulothrix (Fig. 2). The body consists
of a simple filament composed of a single row of short
cells (Fig. 2, A). Each cell contains a nucleus, and a
single large chloroplast which has the form of a thick cyl-
inder investing the rest of the cell contents. Through the
microscope, if the focus is upon the center of the cell,
an optical section of the cylinder is obtained, the chloro-

12

THE EVOLUTION OF SEX

13

plast appearing as a thick green mass on each side of the
central nucleus. As no other color appears, it is evident
that Ulothrix is one of the Chlorophyceae.

A—

Fig. 2. Ulothrix, a Conferva form. A, base of filament, showing lowest holdfast
cell and five vegetative cells, each with its single conspicuous cylindrical chloro-
plast (seen in section) inclosing a nucleus; B, four cells containing numerous
small zoospores, the others emptied; C, fragment of a filament showing one cell
(a) containing four zoospores, another zoospore (b) displaying four cilia at its
pointed end and just having escaped from its cell, another cell (<*) from which
most of the small biciliate gametes have escaped, gametes pairing (d), and the
resulting zygotes (e) ; D, beginning of new filament from zoospore ; E, feeble
filaments formed by the small zoospores ; F, zygote growing after rest; G,
zoospores produced by zygote.— Caldwell, except F and G, which are after

DODEL-PORT.

The cells are all alike, excepting that the lowest one of
the filament is mostly colorless, and is elongated and more
or less modified to act as a holdfast, anchoring the filament
to some firm support. With this exception the cells are all
nutritive ; but any one of them has the power of organizing
for reproduction. This indicates that at first nutritive and
20

14

PLANT STRUCTURES

reproductive cells are not distinctly differentiated, but that
the same cell may be nutritive at one time and reproductive
at another.

In suitable conditions certain cells of the filament will
be observed organizing within themselves new cells by
internal division (Fig. 2, C, a, b). The method of forma-
tion at once suggests that the new cells are asexual spores,
and the mother cell which produces them is acting as a
sporangium. The spores escape into the water through an
opening formed in the wall of the mother cell, and each is
observed to have four cilia at the pointed end, by means of
which it swims, and hence it is a zoospore or swarm spore.
After swimming about for a time, the zoospores " settle
down," lose their cilia, and begin to develop a new filament
like that from which they came (Fig. 2, D).

Other cells of the same filament also act as mother cells,
but the cells which they produce are more numerous, hence
smaller in size than the zoospores, and they have but two
cilia (Fig. 2, C, c). They also escape into the water and
swim about, except in size and in number of cilia resem-
bling the zoospores. In general they seem to be unable to
act as the zoospores in the formation of new filaments, but
occasionally one of them forms a filament much smaller
than the ordinary one (Fig. 2, E). This indicates that
they may be zoospores reduced in size, and unable to act as
the larger ones. The important fact, however, is that
these smaller swimming cells come together in pairs, each
pair fusing into one cell (Fig. 2, C, d, e). The cells thus
formed have the power of producing new filaments more or
less directly.

It is evident that this is a sexual act, that the cell pro-
duced by fusion is a sexual spore, that the two cells which
fuse are gametes, and that the mother cell which produces
them acts as a gametangium. Cases of this kind suggest
that the gametes or sex cells have been derived from zoo-
spores, and that asexual spores have given rise to sex cells.

THE EVOLUTION OF SEX

15

The appearance of sex cells (gametes) is but one step in the
evolution of sex. It represents the attainment of sexuality,
but the process becomes much more highly developed.

14. Isogamy. — When gametes first appear, in some such
way as has been described, the two which fuse seem to be
exactly alike. They resemble each other in size and activ-
ity, and in every structure which can be distinguished.
This fact is indicated by the word isogamy, which means
" similar gametes," and those plants whose pairing gametes
are similar, as Ulothrix, are said to be isogamous.

The act of fusing of similar gametes is usually called
conjugation, which means a " yoking together " of similar
bodies. Of course it is a sexual process, but the name is
convenient as indicating not merely the process, but also an
important character of the gametes. The sexual spore
which results from this act of conjugation is called the
zygote or zygospore, meaning " yoked spore."

In isogamy it is evident that while sexuality has been
attained there is no distinction between sexes, as obtains in
the higher plants. It may be called a unisexual condition,
as opposed to a bisexual one. The next problem in the
evolution of sex, therefore, is to discover how a bisexual
condition has been derived from a unisexual or isogamous
one.

15. Heterogamy. — Beginning with isogamous forms, a
series of plants can be selected illustrating how the pairing
gametes gradually became unlike. One of them becomes
less active and larger, until finally it is entirely passive and
very many times larger than its mate (Fig. 7). The other
retains its small size and increases in activity. The pairing
gametes thus become very much differentiated, the larger
passive one being the female gamete, the smaller active one
the male gamete. This condition is indicated by the word
heterogamy, which means " dissimilar gametes," and those
plants whose pairing gametes are dissimilar are said to be
heterogamous.

16 PLANT STRUCTURES

In order to distinguish them the large and passive female
gamete is called the oosphere, which means " egg sphere,"
or it is called the egg ; the small but active male gamete is
variously called the spermatozoid, the antherozoid, or simply
the sperm. In this book egg and sperm will be used, the
names of similar structures in animals.

In isogamous plants the mother cells (gametangia)
which produce the gametes are alike ; but in heterogamous
plants the gametes are so unlike that the gametangia which
produce them become unlike. Accordingly they have re-
ceived distinctive names, the gametangium which produces
the sperms being called the antheridium, that producing the
egg being called the oogonium (Fig. 10).

The act of fusing of sperm and egg is called fertiliza-
tion, which is the common form of the sexual process. The
sexual spore which results from fertilization is known as the
oospore or " egg-spore," sometimes called the fertilized egg.

It is evident that heterogamous plants are bisexual, and
bisexuality is not only attained among Algae, but it prevails
among all higher plants. Among the lowest forms there is
only vegetative multiplication ; higher forms added sexu-
ality ; then still higher forms became bisexual.

16. Summary. — Isogamous forms produce gametangia,
which produce similar gametes, which by conjugation form
zygotes. Heterogamous forms produce antheridia and
oogonia, which produce sperms and eggs, which by fertiliza-
tion form oospores.

CHAPTEE IV

THE GREAT GROUPS OF ALG^J

17. General characters. — The Algae are distinguished
among Thallophytes by the presence of chlorophyll. It
was stated in a previous chapter that in three of the four
great groups another coloring matter is associated with the
chlorophyll, and that this fact is made the basis of a division
into Blue-green Algae (Cyanophyceae), Green Algae (Chloro-
phyceae), Brown Algae (Phaeophyceae), and Eed Algae (Ehodo-
phyceae). In our limited space it will be impossible to do
more than mention a few representatives of each group,
but they will serve to illustrate the prominent facts.

1. Cyanophycese {Blue-green Algce)

18. Glceocapsa. — These forms may be found forming
blue-green or olive-green patches on damp tree-trunks, rock,
walls, etc. By means of the microscope these patches are
seen to be composed of multitudes of spherical cells, each
representing a complete Glceocapsa body. One of the pecuU
iarities of the body is that the cell wall becomes mucilagi-
nous, swells, and forms a jelly-like matrix about the work-
ing cell. Each cell divides in the ordinary way, two new
Glceocapsa individuals being formed, this method of vegeta-
tive multiplication being the only form of reproduction
(Fig. 3).

When new cells are formed in this way the swollen
mucilaginous walls are apt to hold them together, so that
presently a number of cells or individuals are found lying

18

PLANT STRUCTURES

Fig. 3. Olceocapsa, a blue-
green alga, showing
single cells, and small
groups which have been
formed by division and
are held together by the
enveloping mucilage. —
Caldwell.

together imbedded in the jelly-like matrix formed by the
wall material (Fig. 3). These imbedded groups of individ-
uals are spoken of as colonies, and as
colonies become large they break up
into new colonies, the individual cells
composing them continuing to divide
and form new individuals. This rep-
resents a very simple life history, in
fact a simpler one could hardly be
imagined.

19. Nostoc. — These forms occur in
jelly-like masses in damp places. If
the jelly be examined it will be found
to contain imbedded in it numerous
cells like those of Glceocapsa, but they
are strung together to form chains of
varying lengths (Fig. 4). The jelly in
which these chains are imbedded is the
same as that found in Glceocapsa, being
formed by the cell walls becoming mucilaginous and swollen.
One notable fact is that all the cells in the chain are not
alike, for at irregu-
lar intervals there oc-
cur larger colorless
cells, an illustration
of the differentiation
of cells. These larger
cells are known as het-
erocysts (Fig. 4, A),
which simply means
"other cells." It is
observed that when
the chain breaks up

into fragments each pTG. 4. Nostoc, a bine-green alga, showing the

fragment is COmT)OSed chain-like filaments, and the heterocysts (4)

„ , n i which determine the breaking up of the chain

oi the cells between -Caldwell.

THE GREAT GROUPS OF ALG/E

19

two heterocysts. The fragments wriggle out of the jelly
matrix and start new colonies of chains, each cell dividing
to increase the length of the chain. This cell division,
to form new cells, is the characteristic method of repro-
duction.

At the approach of unfavorable conditions certain cells
of the chain become thick-walled and well-protected. These
cells which endure the cold or other hardships, and upon
the return of favorable conditions produce new chains of
3ells, are often called spores, but they are better called
1 resting cells."

20. Oscillatoria. — These forms are found as bluish-green
slippery masses on wet rocks, or on damp soil, or freely
floating. They are simple filaments, composed of very short
flattened cells (Fig. 5), and the name
Oscillatoria refers to the fact that they
exhibit a peculiar oscillating move-
ment. These motile filaments are iso-
lated, not being held together in a
jelly-like matrix as are the chains of
Nostoc, but the wall develops a cer-
tain amount of mucilage, which gives
the slippery feeliDg and sometimes
forms a thin mucilaginous sheath
about the row of cells.

The cells of a filament are all alike,
except that the terminal cell has its
free surface rounded. If a filament
breaks and a new cell surface ex-
posed, it at once becomes rounded.
If a single cell of the filament is
freed from all the rest, both flattened ends become rounded,
and the cell becomes spherical or nearly so. These facts
indicate at least two important things : (1) that the cell
wall is elastic, so that it can be made to change its form,
and (2) that it is pressed upon from within, so that if free

Fia. 5. Oscillatoria, a
blue-green alga, show-
ing a group of filaments
{A), and a single fila-
ment more enlarged (B).
— Caldwell.

20 PLANT STRUCTURES

it will bulge outward. In all active living cells there is
this pressure upon the wall from within.

Each cell of the Oscillatoria filament has the power of
dividing, thus forming new cells and elongating the fila-
ment. A filament may break up into fragments of varying
lengths, and each fragment by cell division organizes a new
filament. Here again reproduction is by means of vegeta-
tive multiplication.

21. Conclusions. — Taking Gloeocapsa, Nostoc, and Oscil-
latoria as representatives of the group Cyanophycese, or
" green slimes," we may come to some conclusions concern-
ing the group in general. The plant body is very simple,
consisting of single cells, or chains and filaments of cells.
Although in Nostoc and Oscillatoria the cells are organized
into chains and filaments, each cell seems to be able to live
and act independently, and the chain and filament seem to
be little more than colonies of individual cells. In this
sense, all of these plants may be regarded as one-celled.

Differentiation is exhibited in the appearance of hetero-
cysts in Nostoc, peculiar cells which seem to be connected
in some way with the breaking up of filamentous colonies,
although the Oscillatoria filament breaks up without them.

The power of motion is also well exhibited by the group,
the free filaments of Oscillatoria moving almost continu-
ally, and the imbedded chains of Nostoc at times moving to
escape from the restraining mucilage.

The whole group also shows a strong tendency in the
cell-wall material to become converted into mucilage and
much swollen, a tendency which reaches an extreme expres-
sion in such forms as Nostoc and Gloeocapsa.

Another distinguishing mark is that reproduction is
exclusively by means of vegetative multiplication, through
ordinary cell division or fission, which takes place very
freely. Individual cells are organized with heavy resistant
walls to enable them to endure the winter or other unfavor-
able conditions, and to start a new series of individuals

THE GREAT GEOUPS OF ALGJS

21

upon the return of favorable conditions. These may be
regarded as resting cells. So notable is the fact of repro-
duction by fission that Cyanophycese are often separated
from the other groups of Alga? and spoken of as " Fission
Algae," which put in technical form becomes Schizophyceae.
In this particular, and in several others mentioned above,
they resemble the " Fission Fungi " (Schizomycetes), com-
monly called "bacteria," so closely that they are often
associated with them in a common group called " Fis-
sion plants " (Schizophytes), distinct from the ordinary
Algae and Fungi.

2. Chlorophyce^; (Green Algce).

22. Pleurococcus. — This may be taken as a type of one-
celled Green Algae. It is most commonly found in masses
covering damp tree-trunks, etc., and looking like a green
stain. These fine-
ly granular green
masses are found
to be made up
of multitudes of
spherical cells re-
sembling those of
Gloeocapsa, except
that there is no
blue with the chlo-
rophyll, and the
cells are not im-
bedded in such
jelly-like masses.
The cells may be
solitary, or may
cling together in

colonies of various sizes (Fig. 6). Like Glmocapsa, a cell
divides and forms two new cells, the only reproduction

Pig. 6. Pleurococcus, a one-celled green alga : A, show-
ing the adnlt form with its nucleus ; B, C, D, E,
various stages of division (fission) in producing new
cells ; F, colonies of cells which have remained in
contact. —C ALDWELL.

22 PLANT STRUCTURES

being of this simple kind. It is evident, therefore, that the
group Chlorophyceae begins with forms just as simple as
are to be found among the Cyanophyceas.

Pleurococvus is used to represent the group of Protococ-
cus forms, one-celled forms which constitute one of the
subdivisions of the Green Algge. It should be said that
Pleurococcus is possibly not a Protococcus form, but may
be a reduced member of some higher group ; but it is so
common, and represents so well a typical one-celled green
alga, that it is used in this connection. It should be
known, also, that while the simplest Protococcus forms re-
produce only by fission, others add to this the other meth-
ods of reproduction.

23. Ulothrix. — This form was described in § 13. It
is very common in fresh waters, being recognized easily by
its simple filaments composed of short squarish cells, each
cell containing a single conspicuous cylindrical chloroplast
(Fig. 2). This plant uses cell division to multiply the cells
of a filament, and to develop new filaments from fragments
of old ones ; but it also produces asexual spores in the form
of zoospores, and gametes which conjugate and form zygotes.
Both zoospores and zygotes have the power of germination —
that is, the power to begin the development of a new plant.
In the germination of the zygote a new filament is not pro-
duced directly, but there are formed within it zoospores,
each of which produces a new filament (Fig. 2, F, G). All
three kinds of reproduction are represented, therefore, but
the sexual method is the low type called isogamy, the pair-
ing gametes being alike.

Ulothrix is taken as a representative of the Conferva
forms, the most characteristic group of Chlorophyceae.
All the Conferva forms, however, are not isogamous, as will
be illustrated by the next example.

24t. (Edogonium. — This is a very common green alga,
found in fresh waters (Fig. 7). The filaments are long and
simple, the lowest cell acting as a holdfast, as in Ulothrix

Wmm

\m

i Sff '4i

w\

iJk i

Pj

i* t .'

m

Fig. 7. (Edogonium nodosum, a Conferva form : A, portion of a filament showing a
vegetative cell with its nucleus (dh an oogonium (a) filled by an egg packed with
food material, a second oogonium (c) containing a fertilized egg or oospore as
shown by the heavy wall, and two antheridia (ft), each containing two sperms; B,
another filament snowing antheridia (a) from which two sperms (b) have escaped,
a vegetative cell with its nucleus, and an oogonium which a sperm (e) has entered
and Is coming in contact with the egg whose nucleus (d) may be seen; C, a zoo-
spore which has been formed in a vegetative cell, showing the crown of cilia and
the clear apex, as in the sperms; J), a zoospore producing a new filament, putting
ont a holdfast at base and elongating; E, a further stage of development; F. the
four zoospores formed by the oospore when it germinates -Caldwell, except
Cand F, which are after Pringshbim.

24 PLANT STRUCTURES

(§13). The other cells are longer than in Ulothrix, each
cell containing a single nucleus and apparently several
chloroplasts, but really there is but one large complex
chloroplast.

The cells of the filament have the power of division,
thus increasing the length of the filament. Any cell also
may act as a sporangium, the contents of a mother cell
organizing a single large asexual spore, which is a zoospore.
The zoospore escapes from the mother cell into the water,
and at its more pointed clear end there is a little crown of
cilia, by means of which it swims about rapidly (Fig. 7, 0).
After moving about for a time the zoospore comes to rest,
attaches itself by its clear end to some support, elongates,
begins to divide, and develops a new filament (Fig. 7, D, E).

Other cells of the filament become very different from
the ordinary cells, swelling out into globular form (Fig. 7,
A, B), and each such cell organizes within itself a single
large egg (oosphere). As the egg is a female gamete, the
large globular cell which produces it, and which is differen-
tiated from the other cells of the body, is the oogonium.
A perforation in the oogonium wall is formed for the
entrance of sperms.

Other cells in the same filament, or in some other fila-
ment, are observed to differ from the ordinary cells in
being much shorter, as though an ordinary cell had been
divided several times without subsequent elongation (Fig.
7, A, /, B, a). In each of these short cells one or two
sperms are organized, and therefore each short cell is an
antheridium. When the sperms are set free they are seen
to resemble very small zoospores, having the same little
crown of cilia at one end.

The sperms swim actively about in the vicinity of the
oogonia, and sooner or later one enters the oogonium
through the perforation provided in the wall, and fuses
with the egg (Fig. 7, B, c). As a result of this act of fer-
tilization an oospore is formed, which organizes a firm wall

THE GREAT GROUPS OF ALG.E

25

about itself. This firm wall indicates that the oospore is
not to germinate immediately, but is to pass into a resting
condition. Spores which form heavy walls and pass into
the resting con-
dition are often
spoken of as " rest-
ing spores," and it
is very common
for the zygotes
and oospores to
be resting spores.
These resting
spores enable the
plant to endure
through unfavor-
able conditions,
such as failure of
food supply, cold,
drought, etc.
When favorable
conditions return,
the protected rest-
ing spore is ready
for germination.

When the
oospore of (Edogo-

Pio. 8. Cladophora, a branching green alga, a very
small part of the plant being shown. The branches
arise at the upper ends of cells, and the cells are
ccenocytic.— Caldwbli..

nium germinates

it does not develop directly into a new filament, but the
contents become organized into four zoospores (Fig. 7, F),
which escape, and each zoospore develops a filament. In
this way each oospore may give rise to four filaments.

It is evident that (Edogonium is a heterogamous plant,
and is another one of the Conferva forms. Conferva bodies
are not always simple filaments, as are those of Ulothrix
and (Edogonium, but they are sometimes extensively branch-
ing filaments, as in Cladophora, a green alga very common

26

PLANT STRUCTURES

in rivers and lakes (Fig. 8). The cells are long and densely
crowded with chloroplasts ; and in certain cells at the tips
of branches large numbers of zoospores are formed, which
have two cilia at the pointed end, and hence are said to be
biciliate.

25. Vaucheria. — This is one of the most common of the
Green Algae, found in felt-like masses of coarse filaments in
shallow water and on muddy banks, and often called " green

Pig. 9. Vaucheria geminata, a Siphon form, showing a portion of the coenocytic
body {A) which has sent out a branch at the tip of which a sporangium (B)
formed, within which a large zoospore was organized, and from which (Z>) it is
discharged later as a large multiciliate body ( C), which then begins the develop-
ment of a new coenocytic body (E ).— Caldwell.

felt." The filament is very long, and usually branches ex-
tensively, but its great peculiarity is that there is no parti-
tion wall in the whole body, which forms one long continuous
cavity (Figs. 9, 11). This is sometimes spoken of as a one-
celled body, but it is a mistake. Imbedded in the exten-
sive cytoplasm mass, which fills the whole cavity, there are
not only very numerous chloroplasts, but also numerous
nuclei. As has been said, a single nucleus with some cyto-

THE GREAT GROUPS OF ALG^E 27

plasm organized about it is a cell, whether it has a wall or
not. Therefore the body of Vaucheria is made up of as
many cells as there are nuclei, cells whose protoplasmic
structures have not been kept separate by cell walls. Such
a body, made up of numerous cells, but with no partitions,
is called a camocyte, or it is said to be ccenocytic. Vaucheria
represents a great group of Chlorophyceae whose members
have ccenocytic bodies, and on this account they are called
the Siphon forms.

Vaucheria produces very large zoospores. The tip of a
branch becomes separated from the rest of the body by a
partition and thus acts as a sporangium (Fig. 9, B). In
this improvised sporangium the whole of the contents or-
ganize a single large zoospore, which is ciliated all over,
escapes by squeezing through a perforation in the wall
(Fig. 9, (7), swims about for a time, and finally
develops another Vaucheria body (Figs. 9, £J, 10).
It should be said that this large body, called
a zoospore and acting like one, is really a
mass of small biciliate zoospores, just as the

Fig. 10. A young Vaucheria germinating from a
spore (sp), and showing the holdfast (w).—
After Sachs.

apparently one-celled vegetative body is really composed of
many cells. In this large compound zoospore there are
many nuclei, and in connection with each nucleus two cilia
are developed. Each nucleus with its cytoplasm and two
cilia represents a small biciliate zoospore, such as those of
Cladophora, § 24.

Antheridia and oogonia are also developed. In a com-
mon form these two sex organs appear as short special
branches developed on the side of the large ccenocytic body,

28

PLANT STRUCTURES

and cut off from the general cavity by partition walls (Fig.
11). The oogonium becomes a globular cell, which usually

Fig. 11. Yaucheria sesgilis, a Siphon form, show-
ing a portion of the ccenocytic body, an an-
theridial branch (A) with an empty anthe-
ridium (a) at its tip, and an oogonium (B)
containing an oospore (c) and showing the
opening (/) through which the sperms passed
to reach the egg.— Caldwell.

develops a perforated beak for
the entrance of the sperms, and
organizes within itself a single
large egg (Fig. 11, B). The an-
theridium is a much smaller cell,
within which numerous very small
sperms are formed (Fig. 11, A, a).
The sperms are discharged, swarm
about the oogonium, and finally
one passes through the beak and
fuses with the egg, the result be-
ing an oospore. The oospore or-
ganizes a thick wall and becomes
a resting spore.

It is evident that Vaucheria is heterogamous, but all the
other Siphon forms are isogamous, of which Botrydium may
be taken as an illustration (Fig. 12).

26. Spirogyra. — This is one of the commonest of the
"pond scums," occurring in slippery and often frothy
masses of delicate filaments floating in still water or about

Fig. 12. Botrydium, one of the
Siphon forms of green algae,
the whole body containing
one continuous cavity, with
a bulbous, chlorophyll-con-
taining portion, and root-
like branches which pene-
trate the mud in which
the plant grows. — Cald-
well.

THE GREAT GRODPS OF ALGM

29

springs. The filaments are simple, and are not anchored
by a special basal cell, as in Ulothrix and (Edogonitwi. The

Fig. 13. Spirogyra, a Conjugate form, showing one complete cell and portions of
two others. The band-like chloroplasts extend in a spiral from one end of the
cell to the other, in them are imbedded nodule-like bodies (pyrenoids), and near
the center of the cell the nucleus is swung by radiating strands of cytoplasm.—
Caldwell.

cells contain remarkable chloroplasts, which are bands pass-
ing spirally about within the cell wall. These bands may

Pig. 14. Spirogyra, showing conjugation : A, conjugating tubes approaching each
other; B, tubes in contact but end walls not absorbed: C, tube complete and con-
tents of one cell passing through; D, a completed zygospore.— Caldwell.
21

30

PLANT STRUCT URES

be solitary or several in a cell, and form very striking and
conspicuous objects (Figs. 13, 14).

Spirogyra and its associates are further peculiar in pro-
ducing no asexual spores, and also in the method of sexual
reproduction. Two adjacent filaments put out tubular
processes toward one another. A cell of one filament sends
out a process which seeks to meet a corresponding process
from a cell of the other filament. When the tips of two
such processes come together, the end walls disappear,

Fig. 15. Spirogyra, showing some common exceptions. At A two cells have been
connected by a tube, but without fusion a zygote has been organized in each cell;
also, the upper cell to the left has attempted to conjugate with the cell to the
right. At B there are cells from three filaments, the cells of the central one hav-
ing conjugated with both of the others.— Caldwell.

and a continuous tube extending between the two cells is
organized (Figs. 14, 15). When many of the cells of two
parallel filaments become thus united, the appearance is
that of a ladder, with the filaments as the side pieces, and
the connecting tubes as the rounds.

While the connecting tube is being developed the con-
tents of the two cells are organizing, and after the comple-
tion of the tube the contents of one cell pass through and
enter the other cell, fuse with its contents, and a sexual

THE GREAT GROUPS OF ALGiE

31

spore is organized. As the gametes
look alike, the process is conjuga-
tion, and the sex spore is a zygote,
which, with its heavy wall, is rec-
ognized to be a resting spore. At
the beginning of each growing
season, the well-protected zygotes
which have endured the winter
germinate directly into new Spi-
rogyra filaments.

On account of this peculiar
style of sexual reproduction, in
which gametes are not discharged,
but reach each other through spe-
cial tubes, Spirogyra and its allies
are called Conjugate forms — that
is, forms whose bodies are " yoked
together " during the fusion of the
gametes.

In some of the Conjugate forms
the zygote is formed in the connect-
ing tube (Fig. 16, A), and some-
times zygotes are formed without
conjugation (Fig. 16, B). Among
the Conjugate forms the Desmids
are of great interest and beauty,
being one-celled, the cells being
organized into two distinct halves
(Fig. 17).

27. Conclusions. — The Green
Alga?, as indicated by the illustra-
tions given above, include simple
one-celled forms which reproduce
by fission, but they are chiefly fila-
mentous forms, simple or branching. These filamentous
bodies either have the cells separated from one another

Fig. 16. Two Conjugate forms :
A (Mmigeotia), showing for-
mation of zygote in conjuga-
ting tube ; B, C (Gonatone-
ma), showing formation of
zygote without conjugation.
— After Wittrock.

32

PLANT STRUCTURES

by walls, or they are ccenocytic, as in the Siphon forms.
The characteristic asexual spores are zoospores, but these
may be wanting, as in the Conjugate forms. In addition
to asexual reproduction, both isogamy and heterogamy are
developed, and both zygotes and oospores are resting spores.

Fig. 17. A group of Desmids, one-celled Conjugate forms, showing various pat-
terns, and the cells organized into distinct halves. — After Kerner.

The Green Algae are of special interest in connection
with the evolution of higher plants, which are supposed by
some to have been derived from them.

3. Ph^ophyce^ (Brown Algm)

28. General characters. — The Blue-green Algae and the
Green Algae are characteristic of fresh water, but the Brown
Algae, or " kelps," are almost all marine, being very charac-

THE GREAT GROUPS OF ALG^E

33

teristic coast forms. All of them are anchored by holdfasts,
which are sometimes highly developed root-like structures ;
and the yellow, brown, or olive-green floating
bodies are buoyed in the water usually by the
aid of floats or air-bladders, which are often
very conspicuous. The kelps are most highly
developed in the colder waters, and form much
of the " wrack," " tangle," etc., of the coasts.
The group is well adapted to
live exposed to waves and cur-
rents with its strong holdfasts,
air-bladders, and tough leathery
bodies. Certain Brown Algae, as
Ectocarpus (Fig. 18), are of great
interest on account of their pos-
sible relation to the evolution of
higher plants. It is
in this group that we
have found our only
suggestions as to the
origin of the complex
sex-organs occurring
in Bryophytes and
Pteridophytes.

29. The plant
body. — There is very
great diversity in the
structure of the
plant body. Some
of them, as Ed oca r-
pus (Fig. 18), are fil-
amentous forms, like
the Confervas among
the Green Algae, but
others are very much more complex. The thallus of Lam-
inaria is like a huge floating leaf, frequently nine to ten

Fig. 18. A brown alga (Ectocarpus), showing a
body consisting of a simple filament which puts
out branches (A), some sporangia (B) contain-
ing zoospores, and gametangia (C) containing
gametes.— Caldwell.

ess

Fio. 18a. A group of brown seaweeds (Laminarias). Note the various habits of
the plant body with its leaf -like thallus and root-like holdfasts. -After Keener.

THE GREAT GROUPS OF ALG^E

35

feet long, whose stalk develops root-like holdfasts (Fig. 18a).
The largest body is developed by an Antarctic Laminaria
form, which rises to the surface from a sloping bottom with
a floating thallus six hundred to nine hundred feet long.
Other forms rise from the sea bottom like trees, with
thick trunks, numerous branches, and leaf-like appendages.

The common Fucus,
or " rock weed," is rib-
bon-form and constantly
branches by forking at
the tip (Fig. 19). This
method of branching is
called dichotomous, as dis-
tinct from that in which
branches are put out
from the sides of the axis
{monopodial). The swol-
len air-bladders distrib-
uted throughout the body
are very conspicuous.

The most differenti-
ated thallus is that of
Sargassum (Fig. 20), or
" gulf weed," in which
there are slender branch-
ing stem-like axes bearing
lateral members of various
kinds, some of them like
ordinary foliage leaves ;
others are floats or air-
bladders, which sometimes

resemble clusters of berries ; and other branches bear the
sex organs. All of these structures are but different regions
of a branching thallus. Sargassum forms are often torn
from their anchorage by the waves and carried away from
the coast by currents, collecting in the great sea eddies

Fig. 19. Fragment of a common brown
alga (Fucus), showing the body with
dichotomous branching and bladder-like
air-bladders.— After Luerssen.

36

PLANT STRUCTURES

produced by oceanic currents and forming the so-called
" Sargasso seas," as that of the North Atlantic.

Fig. 20. A portion of a brown alga (Sargassum), showing the thallus differentiated
into stem-like and leaf-like portions, and also the bladder-like floats.— After Ben-
nett and Mukkay.

30. Reproduction. — The two main groups of Brown Algae
differ from each other in their reproduction. One, repre-
sented by the Laminarias aud a majority of the forms, pro-
duces zoospores and is isogamous (Fig. 18). The zoospores
and gametes are peculiar in having the two cilia attached
at one side rather than at an end ; and they resemble each
other very closely, except that the gametes fuse in pairs and
form zygotes.

a

1,V. ^ jr

F \

S

V*

fcV>

Pig. 21. Sexual reproduction of Fswaw, showing the eight eggs (six in sight) dis-
charged from the oogonium and surrounded by a membrane (.4), eggs liberated
from the membrane (E), antheridinm containing sperms ( C), the discharged lat-
erally biciliate sperms Iff), and eggs surrounded by swarming sperms (F, H ).—
After Singer.

38

PLANT STRUCTUKES

The other group, represented by Fucus (Fig. 21), pro-
duces no asexual spores, but is heterogamous. A single
oogonium usually forms eight eggs (Fig. 21, A), which are
discharged and float freely in the water (Fig. 21, E). The
antheridia (Fig. 21, C) produce numerous minute laterally
biciliate sperms, which are discharged (Fig. 21, G), swim in
great numbers about the large eggs (Fig. 21, F, H), and
finally one fuses with an egg, and an oospore is formed.
As the sperms swarm very actively about the egg and
impinge against it they often set it rotating. Both an-
theridia and oogonia are formed in cavities of the thallus.

4. Khodophyce^: (Red Algce)
31. General characters. — On account of their red colora-
tion these forms are often called Floridece. They are mostly

marine forms, and are
anchored by holdfasts
of various kinds. They
belong to the deepest
waters in which Algae
grow, and it is probable
that the red coloring
matter which character-
izes them is associated
with the depth at which
they live. The Red
Algae are also a high-
ly specialized line, and
will be mentioned very
briefly..

32. The plant body.
— The Eed Algae, in
general, are more deli-
cate than the Brown
Algae, or kelps, their graceful forms, delicate texture, and
brightly tinted bodies (shades of red, violet, dark purple,

^ig. 22. A red alga (Gigartina), showing
branching habit, and "fruit bodies." —
After Schenck.

Fig. 34. A red alga (Dasya), showing a finely divided thallus body.
Caldwell.

Fig. 25. A red alga (Rabdonia), showing holdfasts and branching thallus body.—

Caldwell.

Fig. 20. A red alga (Plilota), whose branching body resembles moss. —
Caldwell.

THE GREAT GROUPS OF ALG^E

43

and reddish-brown) making them very attractive. They
show the greatest variety of forms, branching filaments,
ribbons, and filmy plates prevailing, sometimes branching
very profusely and delicately, and resembling mosses of
fine texture (Figs. 22, 23, 24, 25, 26). The differentiation
of the thallus into root and stem and leaf-like structures
is also common, as in the Brown Algae.

33. Reproduction. — Red Algae are very peculiar in both
their asexual and sexual reproduction. A sporangium pro-
duces just four asexual spores, but they have no cilia and
no power of motion. They
can not be called zoospores,
therefore, and as each spo-

Fig. 27. A red alga (Callilhamnion), show-
ing sporangium (A), and the tetraspores
discharged (B).— After Thurkt.

Fig. 28. A red alga (Nemaliori) ; A,
sexual branches, showing antheri-
dia<a), oogonium (o) with its trich-
ogyne (t), to which are attached two
spennatia (g); B, beginning of a
cystocarp (o), the trichogyne (I) still
showing; C, an almost mature cys-
tocarp (o), with the disorganizing
trichogyne (t). — After Knt.

rangium always produces just
four, they have been called
tetraspores (Fig. 27).

Red Algas are also heterog-
amous, but the sexual process has been so much and so
variously modified that it is very poorly understood. The
antheridia (Fig. 28, A, a) develop sperms which, like the
tetraspores, have no cilia and no power of motion. To dis-

44

PLANT STRUCTURES

B

?0^

tinguish them from the ciliated sperms, or spermatozoids,
which have the power of locomotion, these motionless male
gametes of the Eed Algae are usually called spermatid
(singular, spermatium) (Fig. 28, A, s).

The oogonium is very pe-
culiar, being differentiated
into two regions, a bulbous
base and a hair-like process
(trichogyne), the whole struc-
ture resembling a flask with a
long, narrow neck, excepting
that it is closed (Fig. 28, A,
o, t). Within the bulbous part
fertilization usually takes
place ; a spermatium attaches
itself to the trichogyne (Fig.
28, A, s) ; at the point of
contact the two walls become
perforated, and the contents
of the spermatium thus enter
the trichogyne, and so reach
the bulbous base of the oogo-
nium. The above account rep-
resents the very simplest con-
ditions of the process of fer-
tilization in this group, and
gives no idea of the great and
puzzling complexity exhibited
by the majority of forms.

After fertilization the trich-
ogyne wilts, and the bulbous
base in one way or another
develops a conspicuous struc-
ture called the cystocarp (Figs. 28, 29), which is a case con-
taining asexual spores ; in other words, a spore case, or kind
of sporangium. In the life history of a red alga, there-

Fig- 29. A branch of Polysiphonia,
one of the red algae, showing the
rows of cells composing the body
(.4), small branches or hairs (B),
and a cystocarp (C) with escaping
spores (Z>) which have no cilia (car-
pospores).— Caldwell.

THE GREAT GROUPS OF ALG.E

45

fore, two sorts of asexual spores are produced : (1) the
tetraspores, developed in ordinary sporangia ; and (2) the
carpospores, developed in the cystocarp, which has been
produced as the result of fertilization.

OTHER CHLOROPHYLL-CONTAINING THALLOPHYTES

34. Diatoms. — These are peculiar one-celled forms, which
occur in very great abundance in fresh and salt waters.

Fig. 30. A group of Diatoms : c and d. top and side views of the same form; e, colony
of stalked forms attached to an alga;/ and g, top and side views of the form shown
at e; A, a colony; i, a colony, the top and side view shown at A;.— After Kerner.

They are either free-swimming or attached by gelatinous
stalks; solitary, or connected in bands or chains, or im-
bedded in gelatinous tubes or masses. In form they are
rod-shaped, boat-shaped, elliptical, wedge-shaped, straight
or curved (Fig. 30).
22

46

PLANT STRUCTURES

The chief peculiarity is that the wall is composed of two
valves, one of which fits into the other like the two parts of
a pill box. This wall is so impregnated with silica that it
is practically indestructible, and siliceous skeletons of dia-
toms are preserved abundantly in certain rock deposits.
They multiply by cell division in a peculiar way, and some
of them have been observed to con-
jugate.

They occur in such numbers in the
ocean that they form a large part of
the free-swimming forms on the sur-
face of the sea, and doubtless showers
of the siliceous skeletons are constant-
ly falling on the sea bottom. There
are certain deposits known as "si-
liceous earths," which are simply
masses of fossil diatoms.

Diatoms have been variously placed
in schemes of classification. Some
have put them among the Brown
Algae because they contain a brown
coloring matter; others have placed
them in the Conjugate forms among
the Green Algae on account of the
occasional conjugation that has been
observed. They are so different from
other forms, however, that it seems
best to keep them separate from all
other Algae.

35. Characeae. — These are common-
ly called " stoneworts," and are often
included as a group of Green Algae,
as they seem to be Thallophytes, and
have no other coloring matter than
chlorophyll. However, they are so peculiar that they are
better kept by themselves among the Algae. They are such

Pig. 31. A common Chara,
showing tip of main axis.
—After Strasburger.

THE GREAT GEODPS OF ALG^E 4f

specialized forms, and are so much more highly organized
than all other Algge, that they will be passed over here with
a bare mention. They grow in fresh or brackish waters,
fixed to the bottom, and forming great masses. The cylin-
drical stems are jointed, the joints sending out circles of
branches, which repeat the jointed and branching habit
(Fig. 31).

The walls become incrusted with a deposit of lime,
which makes the plants harsh and brittle, and has sug-
gested the name " stoneworts." In addition to the highly
organized nutritive body, the antheridia and oogonia are
peculiarly complex, being entirely unlike the simple sex
organs of the other Algae.

CHAPTEE V

THALLOPHYTES : FUNGI

36. General characters. — In general, Fungi include Thai-
lophytes which do not contain chlorophyll. From this fact
it follows that they can not manufacture food entirely out
of inorganic material, but are dependent for it upon other
plants or animals. This food is obtained in two general
ways, either (1) directly from the living bodies of plants or
animals, or (2) from dead bodies or the products of living
bodies. In the first case, in which living bodies are at-
tacked, the attacking fungus is called a parasite, and the
plant or animal attacked is called the host. In the second
case, in which living bodies are not attacked, the fungus is
called a saprophyte. Some Fungi can live only as parasites,
or as saprophytes, but some can live in either way.

Fungi form a very large assemblage of plants, much
more numerous than the Algae. As many of the parasites
attack and injure useful plants and animals, producing
many of the so-called " diseases," they are forms of great
interest. Governments and Experiment Stations have ex-
pended a great deal of money in studying the injurious
parasitic Fungi, and in trying to discover some method of
destroying them or of preventing their attacks. Many of
the parasitic forms, however, are harmless ; while many of
the saprophytic forms are decidedly beneficial.

It is generally supposed that the Fungi are derived from
the Algse, having lost their chlorophyll and power of inde-
pendent living. Some of them resemble certain Algae so
closely that the connection seems very plain; but others
48

THALLOPHYTES: FUNGI

49

have been so modified by their parasitic and saprophytic
habits that they have lost all likeness to the Algae, and
their connection with them is very obscure.

37. The plant body. — Discarding certain problematical
forms, to be mentioned later, the bodies of all true Fungi
are organized upon a uniform general plan, to which they
can all be referred (Fig. 32). A set of colorless branching

Pig. 32. A diagrammatic representation of Mucor, showing the profusely branching
mycelium, and three vertical hyphre (sporophores), sporangia forming on b and c.
—After Zopf.

filaments, either isolated or interwoven, forms the main
working body, and is called the mycelium. The interweav-
ing may be very loose, the mycelium looking like a delicate
cobweb ; or it may be close and compact, forming a felt-like
mass, as may often be seen in connection with preserved
fruits. The individual threads are called hyplim (singular,
liypha) or hyphal threads. The mycelium is in contact with
its source of food supply, which is called the substratum.

50 PLANT STRUCTURES

From the hyplial threads composing the mycelium verti-
cal ascending branches arise, which are set apart to produce
the asexual spores, which are scattered and produce new
mycelia. These branches are called ascending liyphce or
sporophores, meaning " spore bearers."

Sometimes, especially in the case of parasites, special
descending branches are formed, which penetrate the sub-
stratum or host and absorb the food material. These spe-
cial absorbing branches are called haustoria, meaning " ab-
sorbers."

Such a mycelial body, with its sporophores, and perhaps
haustoria, lies either upon or within a dead substratum in
the case of saprophytes, or upon or within a living plant or
animal in the case of parasites.

38. The subdivisions. — The classification of Fungi is in
confusion on account of lack of knowledge. They are so
much modified by their peculiar life habits that they have
lost or disguised the structures which prove most helpful in
classification among the Alga?. Four groups will be pre-
sented, often made to include all the Fungi, but doubtless
they are insufficient and more or less unnatural.

The constant termination of the group names is mycetes,
a Greek word meaning " fungi." The prefix in each case is
intended to indicate some important character of the group.
The names of the four groups to be presented are as follows :
(1) Phycomycetes (" Alga-Fungi "), referring to the fact
that the forms plainly resemble the Algae ; (2) Ascomycetes
(" Ascus-Fungi ") ; (3) ^Ecidiomycetes ("iEcidium-Fungi ") ;
(4) Basidiomycetes (" Basidium-Fungi "). Just what the
prefixes ascus, cecidium, and basidium mean will be ex-
plained in connection with the groups. The last three
groups are often associated together under the name My-
comycetes, meaning " Fungus-Fungi," to distinguish them
from the Phycomycetes, or " Alga-Fungi," referring to the
fact that they do not resemble the Algge, and are only like
themselves.

THALLOPHYTES: FUNGI

51

One of the ordinary life processes which seems to be
seriously interfered with by the saprophytic and parasitic
habit is the sexual process. At least, while sex organs
and sexual spores are about as evident in Phycomycetes
as in Algas, they are either obscure or wanting in the
Mycomycete groups.

1. Phycomycetes {Alga-Fungi)

39. Saprolegnia. — This is a group of "water-moulds,"
with aquatic habit like the Algae. They live upon the dead
bodies of water plants and animals (Fig. 33), and some-
times attack living fish, one kind being very destructive
to young fish in hatcheries. The hyphse composing the
mycelium are ccenocytes, as in the Siphon forms.

Sporangia are organized at the ends of branches by
forming a partition wall separating the cavity of the tip
from the general cavity (Fig. 33, B). The tip becomes
more or less swollen, and within it are formed numerous
biciliate zoospores, which are discharged into the water
(Fig. 33, 0), swim about for a short time, and rapidly form
new mycelia. The process is very suggestive of Cladophora
and Vaucheria. Oogonia and antheridia are also formed
at the ends of the branches (Fig. 33, F), much as in Vau-
cheria. The oogonia are spherical, and form one and some-
times many eggs (Fig. 33, D, E). The antheridia are
formed on branches near the oogonia. An antheridium
conies in contact with an oogonium, and sends out a deli-
cate tube which pierces the oogonium wall (Fig. 33, F).
Through this tube the contents of the antheridium pass,
fuse with the egg, and a heavy-walled oospore or resting
spore is the result.

It is an interesting fact that sometimes the contents of
an antheridium do not enter an oogonium, or antheridia
may not even be formed, and still the egg, without fertiliza-
tion, forms an oospore, which can germinate. This peculiar

52

PLANT STKUCTUKES

habit is called parthenogenesis, which means reproduction
by an egg without fertilization.

Fig. 33. A common water mould (Saprolegnia) : A, a fly from which mycelial fila-
ments of the parasite are growing; B, tip of a branch organized as a sporangium;
C, sporangium discharging biciliate zoospores; F, oogonium with antheridium in
contact, the tube having penetrated to the egg; D and E, oogonia with several
eggs.— A- C after Thuret. D-F after DeBart.

40. Mucor. — One of the most common of the Mucors, or
" black moulds," forms white furry growths on damp bread,
preserved fruits, manure heaps, etc. It is therefore a
saprophyte, the ccenocytic mycelium branching extensively
through the substratum (Fig. 34).

THALLOPHYTES : FUNGI

53

Erect sporophores arise from it in abundance, and at
the top of each sporophore a globular sporangium is formed,
within which are numerous small asexual spores (Figs. 35,

Fig. 34. Diagram showing mycelium and sporophores of a common Mucor. —
Caldwell.

36). The sporangium wall bursts (Fig. 37), the light spores
are scattered by the wind, and, falling upon a suitable sub-
stratum, germinate and
form new mycelia. It is
evident that these asex-
ual spores are not zoo-
spores, for there is no
water medium and swim-
ming is impossible. This
method of transfer being
impossible, the spores are
scattered by currents of
air, and must be corre-
spondingly light and pow-
dery. They are usually
spoken of simply as
" spores," without any
prefix.

Fig. 35. Forming sporangia of Mucor, show-
ing the swollen tip of the sporophore (A),
and a later stage (B), in which a wall is
formed separating the sporangium from
the rest of the body.— Caldwell.

54

PLANT STRUCTURES

While the ordinary method of reproduction through the
growing season is by means of these rapidly germinating
spores, in certain conditions a sexual process is observed,
by which a heavy-walled sexual spore is formed as a resting
spore, able to outlive unfavorable conditions. Branches
arise from the hyphae of the mycelium just as in the forma-

Fig. 36. Mature sporangium of Mucor, showing
the wall (A), the numerous spores (C), and
the columella (B)— that is, the partition wall
pushed up into the cavity of the sporangium.
—Caldwell.

Fig. 37. Bursted sporangium of
Mucor, the ruptured wall not
being shown, and the loose
spores adhering to the colu-
mella.— Caldwell.

tion of sporophores (Fig. 38). Two contiguous branches
come in contact by their tips (Fig. 38, A), the tips are cut
off from the main coenocytic body by partition walls (Fig.
38, B), the walls in contact disorganize, the contents of
the two tip cells fuse, and a heavy-walled sexual spore is
the result (Fig. 38, C). It is evident that the process is
conjugation, suggesting the Conjugate forms among the

TIIALLOPHYTES: FUNGI

55

Algae ; that the sexual spore is a zygote ; and that the two
pairing tip cells cut off from the main body by partition
walls are gametangia. Mucor, therefore, is isogamous.

Fig. 38. Sexual reproduction of Mucor, showing tips of sex branches meeting (A),
the two gametangia cut off by partition walls (B), and the heavy-walled zygote
( C).— Caldwell.

41. Peronospora. — These are the " downy mildews," very
common parasites on seed plants as hosts, one of the most
common kind attacking grape leaves. The mycelium is cceno-
cytic and entirely internal, ramifying among the tissues
within the leaf, and piercing the living cells with haustoria
which rapidly absorb their contents (Fig. 39). The pres-
ence of the parasite is made known by discolored and

56

PLANT STRUCTURES

finally deadened spots on the leaves, where the tissues have
been killed.

From this internal mycelium numerous sporophores
arise, coming to the surface of the host and securing the
scattering of their
spores, which fall
upon other leaves
and germinate, the
new mycelia pene-
trating among the
tissues and begin-
ning their ravages.
The sporophores, af-
ter rising above the
surface of the leaf,

branch freely ; and many of them rising near together,
they form little velvety patches on the surface, suggesting
the name " downy mildew."

b c

Fig. 39. A branch of Peronospora in contact with
two cells of a host plant, and sending into them
its large haustoria.— After DeBary.

Fig. 40. Peronospora, one of the Phycomycetes, showing at a an oogonium (o) con-
taining an egg, and an antheridium (») in contact; at b the antheridial tube pene-
trating the oogonium and discharging the contents of the antheridium into the
egg; at c the oogonium containing the oospore or resting spore. — After DeBary.

In certain conditions special branches arise from the
mycelium, which organize antheridia and oogonia, and
remain within the host (Fig. 40). The oogonium is of the
usual spherical form, organizing a single egg. The an-

THALLOFHYTES : FUNGI

57

theridium comes in contact with the oogonium, puts out a
tube which pierces the oogonium wall and enters the egg,
into which the contents of the antheridium are discharged,
and fertilization is effected. The result is a heavy-walled
oospore. As the oospores are not for immediate germina-
tion, they are not brought to the surface of the host and
scattered, as are the asexual spores. When they are ready
to germinate, the leaves bearing them have perished and
the oospores are liberated.

42. Conclusions. — The coenocytic bodies of the whole group
are very suggestive of the Siphon forms among Green Alga?,
as is also the method of forming oogonia and antheridia.

The water-moulds, Saprolegnia and its allies, have re-
tained the aquatic habit of the Algae, and their asexual
spores are zoospores. Such forms as Mucor and Perono-
spora, however, have adapted themselves to terrestrial con-
ditions, zoospores are abandoned, and light spores are de-
veloped which can be carried about by currents of air.

In most of them motile gametes are abandoned. Even
in the heterogamous forms sperms are not organized within
the antheridium, but the contents of the antheridium are
discharged through a tube developed by the wall and pene-
trating the oogonium. It should be said, however, that a
few forms in this group develop sperms, which make them
all the more alga-like.

They are both isogamous and heterogamous, both zygotes
and oospores being resting spores. Taking the characters
all together, it seems reasonably clear that the Phycomycetes
are an assemblage of forms derived from Green Algae (Chlo-
rophyceae) of various kinds.

2. Ascomycetes (Ascus- or Sac-Fungi)

43. Mildews. — These are very common parasites, growing
especially upon leaves of seed plants, the mycelium spread-
ing over the surface like a cobweb. A very common mil-

58

PLANT STRUCTURES

dew, Microsphcera, grows on lilac leaves, which nearly
always show the whitish covering after maturity (Fig. 41).
The branching hyphas show numerous partition walls, and
are not ccenocytic as in the Phycomycetes. Small disk-like
haustoria penetrate into the superficial cells of the host,
anchoring the mycelium and absorbing the cell contents.

Sporophores arise, which form asexual spores in a pe-
culiar way. The end of the sporophore rounds off, almost
separating itself from the part below, and becomes a spore
or spore-like body. Below this another organizes in the

same way, then another, until
a chain of spores is developed,
easily broken apart and scat-
tered by the wind. Falling
upon other suitable leaves,
they germinate and form new
mycelia, enabling the fungus
to spread rapidly. This meth-
od of cutting a branch into
sections to form spores is
called abstriction, and the
spores formed in this way
are called conidia, or conidi-
ospores (Fig. 43, B).

At certain times the myce-
lium develops special branches
which develop sex organs, but
they are seldom seen and may
not always occur. An oogo-
nium and an antheridium, of
the usual forms, but probably
without organizing gametes,
come into contact, and as a
result an elaborate structure is developed — the ascocarp,
sometimes called the "spore fruit." These ascocarps ap-
pear on the lilac leaves as minute dark dots, each one being

Fig. 41. Lilac leaf covered with mil-
dew (Mic?-0fi])hcera), the shaded re-
gions representing the mycelium,
and the black dots the ascocarps.—
S. M. Coulter.

THALLOPHYTES: FUNGI

59

£&§?

a little sphere, which suggested the name Microsphcera
(Fig. 41). The heavy wall of the ascocarp bears beauti-
ful branching hair-like appendages (Fig. 42).

Bursting the wall of this spore fruit several very delicate,
bladder-like sacs are extruded, and through the transparent
wall of each sac there may be
seen several spores (Fig. 42).
The ascocarp, therefore, is
a spore case, just as is the
cystocarp of the Red Algae
(§ 33). The delicate sacs
within are the asci, a word
meaning "sacs," and each
ascus is evidently a mother
cell within which asexual
spores are formed. These
spores are distinguished
from other asexual spores
by the name ascospore.

It is these peculiar moth-
er cells, or asci, which give
name to the group, and an

Ascomycete, Ascus-fungus, or Sac-fungus, is one which pro-
duces spores in asci ; and an ascocarp is a spore case which
contains asci.

In the mildews, therefore, there are two kinds of asexual
spores : (1) conidia, formed from a hyphal branch by abstric-
tion, by which the mycelium may spread rapidly; and (2)
ascospores, formed in a mother cell and protected by a heavy
case, so that they may bridge over unfavorable conditions,
and may germinate when liberated and form new mycelia.
The resting stage is not a zygote or an oospore, as in the
Alga? and Phycomycetes, no sexual spore probably being
formed, but a heavy-walled ascocarp.

44. Other forms. — The mildews have been selected as a
simple illustration of Ascomycetes, but the group is a very

Fig. 42. Ascocarp of the lilac mildew,
showing branching appendages and
two asci protruding from the rup-
tured wall and containing ascospores.
— S. M. Coulter.

60

PLANT STRUCTURES

large one, and contains a great variety of forms. All of
them, however, produce spores in asci, but the asci are not
always inclosed by an ascocarp. Here belong the common
blue mould {Penicillium), found on bread, fruit, etc., in
which stage the branching chains of conidia are very con-
spicuous (Fig. 43) ; the truffle-fungi, upon whose subter-

Fig. 43. Penicillium, a common mould: A, mycelium with numerous branching
sporophores bearing conidia; B, apex of a sporophore enlarged, showing branch-
ing and chains of conidia. — After Brefeld.

ranean mycelia ascocarps develop which are known as
" truffles " ; the black fungi, which form the diseases known
as " black knot " of the plum and cherry, the " ergot " of
rye (Fig. 44), and many black wart-like growths upon the
bark of trees ; other forms causing " witches'-brooms " (ab-
normal growths on various trees), " peach curl," etc., the
cup-fungi (Figs. 45, 46), and the edible morels (Fig. 47).

THALLOPHYTES: FUNGI

61

Vie. 44. Head of rye attacked by " er-
got" (a), peculiar grain-like masses
replacing the grains of rye ; also a
mass of "ergot" germinating to
form spores (6).— After Tttlasne.

Fig. 46. A cup-fungus (Pitya) grow-
ing on a spruce (Picea). — After
Rehm.

In some of these forms the ascocarp is completely closed,
as in the lilac mildew; in others it is flask-shaped; in
others, as in the cup-fungi, it is like a cup or disk ; but in
all the spores are inclosed by a delicate sac, the ascus.
23

62

i'LANT STRUCTURES

Here must probably be included the yeast-fungi (Fig.
48), so commonly used to excite alcoholic fermentation.

Fig. 47. The common edible morel (Morrhella
esculenta). The structure shown and used
represents the ascocarp, the depressions of
whose surface are lined with asci contain-
ing ascospores. — After Gibson.

Fig. 48. Yeast cells, reprodu-
cing by budding, and form-
ing chains. — Caldwell.

The " yeast cells " seem to be conidia having a peculiar bud-
ding method of multiplication, and the remarkable power
of exciting alcoholic fermentation in sugary solutions.

3. JEcidiomycetes {JEcidium-Fungi)

45. General characters. — This is a large group of very
destructive parasites known as " rusts " and " smuts." The
rusts attack particularly the leaves of higher plants, pro-
ducing rusty spots, the wheat rust probably being the best
known. The smuts especially attack the grasses, and are
very injurious to cereals, producing in the heads of oats,
barley, wheat, corn, etc., the disease called smut.

THALLOPIIYTES ? FUNGI (53

In some forms an obscure sexual process has been de-
scribed, but it is beyond the reach of ordinary observation.
The iEcidiomycetes do not form an independent and nat-
ural group, but are now generally placed under the Basi-
diomycetes, but they are so unlike the ordinary forms of
that group that they are here kept distinct.

Most of the forms are very polymorphic — that is, a plant
assumes several dissimilar appearances in the course of its
life history. These phases are often so dissimilar that they
have been described as different plants. This polymorphism
is often further complicated by the appearance of different
phases upon entirely different hosts. For example, the
wheat-rust fungus in one stage lives on wheat, and in an-
other »on barberry.

46. Wheat rust. — This is one of the few rusts whose life
histories have been traced, and it may be taken as an illus-
tration of the group.

The mycelium of the fungus is found ramifying among
the leaf and stem tissues of the wheat. While the wheat is
growing this mycelium sends to the surface numerous spo-

cg^^lfllHg^?D lIliJL

Fig. 49. Wheat rust, showing sporophores breaking through the tissues of the host
and bearing summer spores (uredospores).— After H. Marshall Ward.

rophores, each bearing at its apex a reddish spore (Fig. 49).
As the spores occur in great numbers they form the rusty-
looking lines and spots which give name to the disease.
The spores are scattered by currents of air, and falling upon
other plants, germinate very promptly, thus spreading the

64

PLANT STKUCTURES

disease with great rapidity (Fig. 50). Once it was thought
that this completed the life cycle, and the fungus received
the name Uredo. When it was known that this is but one

Fig. 50

-Wheat rust, showing a young hypha forcing its way from the surface of a
leaf down among the nutritive cells.— After H. Marshall Ward.

stage in a polymorphic life history it was called the Uredo-
stage, and the spores uredospores, sometimes "summer
spores."

Fig. 51. Wheat rust, showing the winter spores (teleutospores). — After
H. Marshall Ward.

Toward the end of the summer the same mycelium
develops sporophores which bear an entirely different kind
of spore (Fig. 51). It is two-celled, with a very heavy black

THALLOPHYTES : FUNGI

65

wall, and forms what is called the " black rust," which ap-
pears late in the summer on wheat stubble. These spores
are the resting spores, which last through the winter and
germinate in the following spring. They are called teleuto-
spore.% meaning the " last spores " of the growing season.
They are also called " winter spores," to distinguish them
from the uredospores or " summer spores." At first this
teleutospore-bearing mycelium was not recognized to be
identical with the uredospore-bearing mycelium, and it was
called Puccinia. This name is now
retained for the whole polymorphous
plant, and wheat rust is Puccinia
graminis. This mycelium on the
wheat, with its summer spores and
winter spores, is but one stage in
the life history of wheat rust.

In the spring the teleutospore
germinates, each cell developing a
small few-celled filament (Fig. 52).
From each cell of the filament a
little branch arises which develops
at its tip a small spore, called a spo-
ridium, which means " spore-like."
This little filament, which is not a
parasite, and which bears sporidia,
is a second phase of the wheat rust,
really the first phase of the growing
season.

The sporidia are scattered, fall
upon barberry leaves, germinate, and
develop a mycelium which spreads
through the leaf. This mycelium produces sporophores
which emerge on the under surface of the leaf in the
form of chains of reddish-yellow conidia (Fig. 53). These
chains of conidia are closely packed in cup-like receptacles,
and these reddish-yellow cup-like masses are often called

Fig. 52. Wheat rust, show-
ing a teleutospore germina-
ting and forming a short fil-
ament, from four of whose
cells a spore branch arises,
the lowest one bearing at
its tip a sporidium. — After
H. Marshall Ward.

66

PLANT STRUCTURES

"cluster-cups." This mycelium on the barberry, bearing
cluster-cups, was thought to be a distinct plant, and was

called JEcidium. The
name now is applied to
the cluster-cups, which
are called cecidia, and
the conidia-like spores
which they produce are
known as cecidiospores.

It is the ascidia which
give name to the group,
and ^cidiomycetes are
those Fungi in whose
life history secidia or
cluster-cups appear.

The secidiospores are
scattered by the wind,
fall upon the spring
wheat, germinate, and
develop again the myce-
lium which produces the
rust on the wheat, and
so the life cycle is com-
pleted. There are thus
at least three distinct
stages in the life history
of wheat rust. Begin-
ning with the growing
season they are as fol-
lows : (1) The phase bear-
ing the sporidia, which
is not parasitic ; (2) the
aecidium phase, parasitic
on the barberry; (3) the uredo-teleutospore phase, para-
sitic on the wheat.

In this life cycle at least four kinds of asexual spores

THALLOPHYTES: FUNGI

67

appear : (1) sporidia, which develop the stage on the barber-
ry ; (2) cecidiospores, which develop the stage on the wheat ;
(3) uredospores, which repeat the mycelium on the wheat ; (4)
teleutospores,which last through the winter, and in the spring
produce the stage bearing sporidia. It should be said that
there are other structures of this plant produced on the bar-
berry (Fig. 53), but they are too uncertain to be included here.
The barberry is not absolutely necessary to this life cycle.
In many cases there is no available barberry to act as host,
and the sporidia germinate directly upon the young wheat,
forming the rust-producing mycelium, and the cluster-cup
stage is omitted.

Fig. 54. Two species of "cedar apple" (Gymnosporangium), both on the common
juniper (Juniperus Virginiana).—A after Farlow, B after Engleb and Prantl.

47. Other rusts.— Many rusts have life histories similar
to that of the wheat rust, in others one or more of the
stages are omitted. In very few have the stages been con-

68

PLANT STBUCTUKES

nected together, so that a mycelium bearing uredospores is
called a Uredo, one bearing teleutospores a Puccinia, and
one bearing aecidia an jEcidium ; but what forms of Uredo,
Puccinia, and ^Ecidium belong together in the same life
cycle is very difficult to discover.

Another life cycle which has been discovered is in con-
nection with the " cedar apples " which appear on red
cedar (Fig. 54). In the spring these diseased growths be-
come conspicuous, especially after a rain, when the jelly-
like masses containing the orange-colored spores swell.
This corresponds to the phase which produces rust in
wheat. On the leaves of apple trees, wild crab, hawthorn,
etc., the ascidium stage of the same parasite develops.

4. Basidiomycetes (Basidium-Fungi).
48. General characters. — This group includes the mush-
rooms, toadstools, and puffballs. They are not destructive

parasites, as are many
forms in the preceding
groups, but mostly harm
less and often useful sap-
rophytes. They must
also be regarded as the
most highly organized of
the Fungi. The popular
distinction between toad-
stools and mushrooms is
not borne out by botan-
ical characters, toadstool
and mushroom being the
same thing botanically,
and forming one group,
puffballs forming an-
other.

Fig. 55. The common edible mushroom, As in ^cidiomycetes,

Agaricus campestris.— After Gibson. an obscure Sexual process

THALLOPHYTES: FUNGI

69

is reported. The life history seems simple, but this appar-
ent simplicity may represent a very complicated history.
The structure of the common mushroom (Agaricus) will
serve as an illustration of the group (Fig. 55).

49. A common
mushroom. — The
mycelium, of white
branching threads,
spreads extensively
through the decay-
ing substratum,
and in cultivated
forms is spoken of
as the " spawn."
Upon this myce-
lium little knob-
like protuberances
begin to arise, grow-
ing larger and
larger, until they
are organized into
the so-called
"mushrooms."
The real body of
the plant is the
white thread - like
mycelium, while
the " mushroom "
part seems to rep-
resent a great num-
ber of sporophores
organized together
to form a single
complex spore-
bearing structure.

The mushroom

Fig. 50. A common Agaricus : A, section throngh one
side of pileus, showing sections of the pendent gills;
B. section of a gill more enlarged, showing the cen-
tral tissue, and the broad border formed by the ba-
sidia : C, still more enlarged section of one side of
a gill, showing the club-shaped basidia standing at
right angles to the surface, and sending out a pair
of small branches, each of which bears a single ba-
sidiospore.— After Sachs.

*„•

- I

. -JJ

THALLOPHYTES: FUNGI

VI

has a stalk-like portion, the stipe, at the base of which the
slender mycelial threads look like white rootlets ; and an
expanded, umbrella-like top called the pileus. From the
under surface of the pileus there hang thin radiating plates,
or gills (Fig. 55). Each gill is a mass of interwoven fila-
ments (hyphas), whose tips turn toward the surface and
form a compact layer of end cells (Fig. 56). These end

Fig. 60. A bracket fungus (Polyporus) growing on the trunk of a red oak.—
Caldwell.

cells, forming the surface of the gill, are club-shaped, and
are called basidia. From the broad end of each basidium
two or four delicate branches arise, each bearing a minute
spore, very much as the sporidia appear in the wheat rust.

72

PLANT STRUCTURES

These spores, called basidiospores, shower down from the
gills when ripe, germinate, and produce new mycelia. The
peculiar cell called the basidium gives name to the group
Basidiomycetes.

50. Other forms. — Mushrooms display a great variety of
form and coloration, many of them being very attractive

.(

h\

a / y/f

IV

1 s^Z^i^^

- - J^i^

j&JSf'" \

ISlk -

/ H>B

/■At "*^B

Pig. 61. A toadstool of the bracket form which has grown about blades of grass
without interfering with their activity.— Caldwell.

(Figs. 57, 58, 59). The " pore-fungi " have pore-like depres-
sions for their spores, instead of gills, as in the very com-
mon "bracket-fungus" (Polyporus), which forms hard
shell-like outgrowths on tree-trunks and stumps (Figs. 60,

Pig. 62. The common edible Boletus (B. edu- Fig. 63. Another edible Boletus (B. scro-
lls), in which the gills are replaced by bilaceus).— After Gibson.
pores.— After Gibson.

Fig. 64. The common edible "coral fun- Fig. 65. Hydnum repandum, in which gills
rob" iClavaria).— After Gibson. are replaced by spinous processes; edi-

ble.—After Gibson.

74

PLANT STKUCTUKES

61), and the mushroom-like Boleti (Figs. 62, 63). The
" ear-fungi " form gelatinous, dark-brown, shell-shaped
masses, and the " coral fungi " resemble branching corals
(Fig. 64). The Hydnum forms have spinous processes

instead of gills (Fig.
65). The puffballs or-
ganize globular bodies
(Fig. 06), within which
the spores develop, and
are not liberated until
ripe ; and with them
belong also the "bird's
nest fungus," the "earth
star," the ill-smelling
"stink-horn," etc.

OTHER THALLOPHYTES
WITHOUT CHLOROPHYLL

51 . Slime - moulds. —

These perplexing forms,
named Myxomycetes, do
not seem to be related
to any group of plants,
and it is a question
whether they are to be regarded as plants or animals. The
working body is a mass of naked protoplasm called a plas-
modium, suggesting the term "slime," and slips along like
a gigantic amoeba. They are common in forests, upon
black soil, fallen leaves, and decaying logs, the slimy yel-
low or orange masses ranging from the size of a pinhead
to as large as a man's hand. They are saprophytic, and
are said to engulf food as do the amcebas. So suggestive
of certain low animals is this body and food habit that
slime-moulds have also been called Mycetozoa or " fungus-
animals."

Fig. 66. Puffballs, in which the basidia and
spores are inclosed ; edible. — After Gibson.

THALLOPIIYTES: FUNGI

75

In certain conditions, however, these slimy bodies come
to rest and organize most elaborate and often very beau-
tiful sporangia, full of spores (Fig. 07). These varied
and easily preserved sporangia are used to classify the

Fig. 67. Three common slime moulds (Myxomycetee) on decaying wood: to the
left above, groups of ihe sessile sporangia of Trichia ; to the right above, a group
of the stalked sporangia of Stemonitis, with remnant of old Plasmodium at base;
below, groups of sporangia of Hemiarcyria, with a Plasmodium mass at upper
left hand.— Goldbekqer.

forms. Slime-moulds, or " slime-fungi," therefore, seem
to have animal-like bodies which produce plant-like spo-
rangia.

52. Bacteria. — These are the " Fission-Fungi," or Schizo-
mycetes, and are popularly known as " bacteria," " bacLli,"
" microbes," " germs," etc. They are so important and pe-
culiar in their life habits that their study has developed a
special branch of botany, known as " Bacteriology." In
many ways they resemble the Cyanophyceae, or "Fission-
Algae," so closely that they are often associated with them
in classification (see § 21).

r r

Fig. 68. A group of Bacteria, the bodies being black, and bearing motile cilia in
various ways. A, the two to the left the common hay Bacillus (B. subtilis), the
one to the right a Spirillum ; B, a Coccus form (Planocoecus); C, D, E, species of
Pseudomonas : F, G, species of Bacillus, F being that of typhoid fever; H, Micro-
spira ; J, K, L, M, species of Spirillum.— After Englbr and Prantl.

THALLOPHYTES: FUNGI 77

They are the smallest known living organisms, the one-
celled form which develops on cooked potatoes, bread, milk,
meat, etc., forming a blood-red stain, having a diameter of
but 0.0005 mm. (-^oioT in.). They are of various forms
(Fig. 68), as Coccus forms, single spherical cells ; Bacterium
forms, short rod-shaped cells ; Bacillus forms, longer rod-
shaped cells ; Leptothrix forms, simple filaments ; Spirillum
forms, spiral filaments, etc.

They multiply by cell division with wonderful rapidity,
and also form resting spores for preservation and distri-
bution. They occur everywhere — in the air, in the water,
in the soil, in the bodies of plants and animals ; many of
them harmless, many of them useful, many of them dan-
gerous.

They are intimately concerned with fermentation and
decay, inducing such changes as the souring of fruit juices,
milk, etc., and the development of pus in wounds. What
is called antiseptic surgery is the use of various means to
exclude bacteria and so prevent inflammation and decay.

The pathogenic forms — that is, those which induce dis-
eases of plants and animals — are of great importance, and
means of making them harmless or destroying them are
being searched for constantly. They are the causes of such
diseases as pear-blight and peach-yellows among plants, and
such human diseases as tuberculosis, cholera, diphtheria,
typhoid fever, etc.

LICHENS

53. General character. — Lichens are abundant every-
where, forming various colored splotches on tree-trunks,
rocks, old boards, etc., and growing also upon the ground
(Figs. 69, 70, 71). They have a general greenish-gray color,
but brighter colors may also be observed.

The great interest connected with Lichens is that they are
not single plants, but each Lichen is formed of a fungus and
an alga, living together so intimately as to appear like a single
24

THALLOPHYTES : FUNGI

79

plant. In other words, a Lichen is not an individual, but a
firm of two individuals very unlike each other. This habit

Fio. 70. A common lichen (Physcia) growing on bark, showing the spreading thallus
and the numerous dark disks (apothecia) bearing the asci.— Goi.dberger.

of living together has been called symbiosis, and the indi-
viduals entering into this relation are called symbionts.

Fig. 71. A common foliose lichen {Parmdia) growing upon a board, and showing
apothecia. — Goldberger.

80

PLANT STRUCTURES

If a Lichen be sectioned, the relation between the sym-
bionts will be seen (Fig. 72). The fungus makes the bulk
of the body with its interwoven mycelial threads, in the
meshes of which lie the Algae, sometimes scattered, some-

'WBM

Fig. 72. Section through thallus of a lichen (Slicla), showing holdfasts (r), lower (w)
and upper (o) surfaces, fungus hyphse (m), and enmeshed algae (g). — After Sachs.

times massed. It is these enmeshed Algae, showing through
the transparent mycelium, that give the greenish tint to
the Lichen.

In the case of Lichens the symbionts are thought by
some to be mutually helpful, the alga manufacturing food
for the fungus, and the fungus providing protection and
water containing food materials for the alga. Others do not
recognize any special benefit to the alga, and see in a Lichen
simply a parasitic fungus living on the products of an alga.
In any event the Algae are not destroyed but seem to thrive.
It is discovered that the alga symbiont can live quite inde-

THALLOPHYTES: FUNGI

81

pendently of the fungus. In fact, the enmeshed Algje are
often recognized as identical with forms living independ-
ently, those thus used being various Blue-green, Protococ-
cus, and Conferva forms (see p. 87).

On the other hand, the fungus symbiont has become
quite dependent upon the alga, and its germinating spores
do not develop far unless the young mycelium can lay hold
of suitable Algas. At certain times cup-like or disk-like
bodies appear on the surface of the lichen thallus, with
brown, or black, or more brightly-colored lining (Figs. 70,
71). These bodies are the apothecia, and a section through
them shows that the colored lining is largely made up of
delicate sacs containing spores (Figs. 73, 74). These sacs
are evidently asci, the apothecia correspond to ascocarps,
and the Lichen fungus proves to be an Ascomycete.

Fig. 73. Section through an apothecium of Anaptychia, showing stalk of the cnp
(to), masses of algal cells (g), outer margin of cup (?•), overlapping edge (t, t), layer
of asci (h), and massing of hyphs beneath asci (y).— After Sachs.

Certain Ascomycetes, therefore, have learned to use cer-
tain Algae in this peculiar way, and a Lichen is the result.
Some Basidiomycetes have also learned the same habit, and
form Lichens.

Various forms of Lichen bodies can be distinguished as
follows : (1) Crustaceous Lichens, in which the thallus resem-

82

PLANT STRUCTUKES

bles an incrustation upon its substratum of rock, soil, etc. ;
(2) Foliose Lichens, with flattened, leaf-like, lobed bodies, at-

Fig. 74. Much enlarged section of a portion of the apothecium of Anaptychia, show-
ing the fungus mycelium (m), which is massed above (y), just beneath the layer of
asci U, 2, 3, It), in which spores in various stages of development are shown.—
After Sachs.

tached only at the middle or irregularly to the substratum ;
(3) Fruticose Lichens, with filamentous bodies branching
like shrubs, either erect, pendulous, or prostrate.

CHAPTER VI

THE FOOD OF PLANTS

54. Introductory. — All plants use the same kind of food,
but the Algae and Fungi suggest that they may have very
different ways of obtaining it. The Algss can manufacture
food from raw material, while the Fungi must obtain it
already manufactured. Between these two extreme condi-
tions there are plants which can manufacture food, and at
the same time have formed the habit of supplementing this
by obtaining elsewhere more or less manufactured food.
Besides this, there are plants which have learned to work
together in the matter of food supply, entering into a con-
dition of symbiosis, as described under the Lichens. These
various habits will be presented here briefly.

55. Green plants. — The presence of chlorophyll enables
plants to utilize carbon dioxide (C02), a gas present in the
atmosphere and dissolved in waters, and one of the waste
products given off in the respiration of all living organisms.
This gas is absorbed by green plants, its constituent ele-
ments, carbon and oxygen, are dissociated, and with the ele-
ments obtained from absorbed water (H20) are recombined to
form a carbohydrate (sugar, starch, etc.), which is an organ-
ized food. With this as a basis other foods are formed,
and so the plant can live without help from any other
organism.

This process of utilizing carbon dioxide in the formation
of food is not only a wonderful one, but also very important.
It is wonderful, because carbon dioxide and water, both of
them very refractory substances, are broken up at ordinary

83

84 PLANT STKUCTUKES

temperatures and without any special display of energy. It
is important, because the food of all plants and animals de-
pends upon it, as it is the only known process by which inor-
ganic material can be organized.

The process is called photosynthesis, or photosyntaz,
words indicating that the presence of light is necessary.
The mechanism on the part of the plant is the chloroplast,
which when exposed to light is able to do this work. The
process is often called " carbon assimilation," " chlorophyll
assimilation," " fixation of carbon," etc. It should be noted
that it is not the chlorophyll which does the work, but the
protoplasmic plastid stained green by the chlorophyll. The
chlorophyll manipulates the light in some way so that the
plastid may obtain from it the energy needed for the work.
Further details concerning it may be obtained by reading
§ 112 of Plant Relations.

It is evident that green plants must expose their chloro-
phyll to the light. For this reason the Algae can not live
in deep waters or in dark places. In the case of the large
marine kelps, although they may be anchored in considera-
ble depth of water, their working bodies are floated up
toward the light by air-bladders. In the case of higher
plants, specially organized chlorophyll-bearing organs, the
foliage leaves, are developed.

56. Saprophytes. — Only cells containing chloroplasts can
live independently. In the higher plants, where bodies be-
come large, many living cells are shut away from the light,
and must depend upon the more superficial green cells for
their food supply. The habit of cells depending upon one
another for food, therefore, is a very common one.

When none of the cells of the plant body contain chloro-
phyll, the whole plant becomes dependent, and must live as
a saprophyte or a parasite. In the case of saprophytes dead
bodies or body products are attacked, and sooner or later all
organic matter is attacked and decomposed by them. The
decomposition is a result of the nutritive processes of plants

THE FOOD OF PLANTS

85

without chlorophyll, and were it not for them " the whole
surface of the earth would be covered with a thick deposit
of the animal and plant remains of the past thousands of
years."

The green plants, therefore, are the manufacturers of
organic material, producing far more than they can use,
while the plants without chlorophyll are the destroyers of
organic material. The chief destroyers are the Bacteria
and ordinary Fungi, but some of the higher plants have
also adopted this method of obtaining food. Many ordinary
green plants have the saprophytic habit of absorbing organic
material from rich humus soil ; and some plants (as broom
rapes) are parasitic, attaching their subterranean parts to
those of other plants, becoming what are called "root para-
sites." In cases of mycorhiza (see p. 87), which are now
thought to include great numbers of green plants, it is sup-
posed that some organic material is brought in by the fungus.

57. Parasites. — Certain plants without chlorophyll are
not content to obtain organic material from dead bodies,
but attack living ones. As in the case of saprophytes, the
vast majority of plants which have formed this habit are
Bacteria and ordinary Fungi. Parasites are not only modi-
fied in structure in consequence of the absence of chloro-
phyll, but they have developed means of penetrating their
hosts. Many of them have also cultivated a very selective
habit, restricting themselves to certain plants or animals, or
even to certain organs.

The parasitic habit has also been developed by some of
the higher plants, sometimes completely, sometimes par-
tially. Dodder, for example, is completely parasitic at
maturity (Fig. 75), while mistletoe is only partially so,
doing chlorophyll work and also absorbing from the tree
into which it has sent its haustoria.

That saprophytism and parasitism are both habits grad-
ually acquired is inferred from the number of green plants
which have developed them more or less, as a supplement to

86

PLANT STRUCTURES

the food which they manufacture. The less chlorophyll is
used the less is it developed, and a green plant which is
obtaining the larger amount of its food in a saprophytic

or parasitic way is
on the way to losing
all of its chlorophyll
and becoming a com-
plete saprophyte or
parasite.

Certain of the low-
er Algae are in the
habit of living in the
body cavities of high-
er plants, finding in
such situations the
moisture and protec-
tion which they need.
They may thus have
brought within their
reach some of the
organic products of
the higher plant. If
they can use some of
these, as is very like-
ly, a partially para-
sitic habit is begun,
which may lead to
loss of chlorophyll
and complete para-
sitism.

58. Symbionts. —
Symbiosis means
" living together,"
and two organisms thus related are called symbionts. In
its broadest sense symbiosis includes any sort of depend-
ence between living organisms, from the vine and the tree

Fig. 75. A dodder plant parasitic on a willow twig.
The leafless dodder twines about the willow, and
sends out sucking processes which penetrate and
absorb. — After Strasburger.

THE FOOD OF PLANTS 37

upon which it climbs, to the alga and fungus so intimately
associated in a Lichen as to seem a single plant. In a nar-
rower sense it includes only cases in which there is an inti-
mate organic relation between the symbionts. This would
include parasitism, the parasite and host being the sym-
bionts, and the organic relation certainly being intimate.
In a still narrower sense symbiosis includes only those cases
in which the symbionts are mutually helpful. Tbis fact,
however, is very difficult to determine, and opinions vary
widely as to the mutual advantage in certain cases. How-
ever large a set of phenomena may be included under the
term symbiosis, we use it here in this narrowest sense,
which is often distinguished as mutualism.

(1) Lichens. — A lichen is a complex made up of a fun-
gus and an alga living togetber. It is certain that the fun-
gus cannot live without the alga, but the alga can live
without the fungus. Hence it seems plain that this rela-
tion is not one of mutual helpfulness, but that the fungus
is living upon the alga, as any other parasite lives upon its
host (see § 191).

(2) Mycorhiza. — The name means "root-fungus," and
refers to an association which exists between certain Fungi
of the soil and roots of higher plants. It was formerly
thought that mycorhiza occurred only in connection with
a limited number of higher plants, such as orchids, heaths,
oaks, etc., but more recent study indicates that probably
the large majority of vascular plants (that is, plants with
true roots) possess it, the water plants being excepted (Figs.
149, 150). It has been found that the humus soil of forests
is in large part " a living mass of innumerable filamentous
fungi." It is clearly of advantage to roots to relate them-
selves to this great network of filaments, which are already
in the best relations for absorption, and those plants which
are unable to do this are at a disadvantage in the competi-
tion for the nutrient materials of the forest soil. It is
doubtful whether many vascular green plants can absorb

Pig. 76. Mycorhiza : to the left is the tip of a rootlet of beech enmeshed by the
fungus; A, diagram of longitudinal section of an orchid root, showing the cells
of the cortex (p) filled with hyphse; B, part of longitudinal section of orchid root
much enlarged, showing epidermis (e), outermost cells of the cortex (p) filled with
hyphal threads, which are sending branches into the adjacent cortical cells (a, i).
—After Frank.

<* 8

Ftg. 77. Mycorhiza : A, rootlets of white poplar forming mycorhiza ; B, enlarged

section of single rootlets, showing the hyphae penetrating the cells. — After

KlRNBB.

THE FOOD OF PLANTS

89

enough for their needs from the soil without this assistance,
and, if so, the fungus becomes of vital importance in the
nutrition of such plants. In the case of some of these
plants it seems that the soil fungus is not merely passing
into their bodies the soil water with its dissolved salts, but
is contributing to them organized food, thus diminishing
the amount of necessary food manu-
facture. The delicate branching fila-
ments (hyphae) of the fungus wrap
the rootlets with a mesh of hyphae
and penetrate into the cells, and it
is evident that the fungus obtains
food from the rootlet as a parasite.

(3) Root-tubercles. — On the roots
of many legume plants, as clovers,
peas, beans, etc., little wart -like
outgrowths are frequently found,
known as " root-tubercles " (Fig.
78). It is found that these tuber-
cles are caused by certain Bacteria,
which penetrate the roots and in-
duce these excrescent growths. The
tubercles are found to swarm with
Bacteria, which are doubtless ob-
taining food from the roots of the
host. At the same time, these Bac-
teria have the peculiar power of
laying hold of the free nitrogen of
the air circulating in the soil, and
of supplying it to the host plant
in some usable form. Ordinarily
plants can not use free nitrogen,

although it occurs in the air in such abundance, and this
power of these soil Bacteria is peculiarly interesting.

This habit of clover and its allies explains why they are
useful in what is called " restoring the soil." After ordi-

Fig. 78. Root-tubercles on
Yicia Faba— After Noll.

90

PLANT STRUCTURES

nary crops have exhausted the soil of its nitrogen-contain-
ing salts, and it has become comparatively sterile, clover is
able to grow by obtaining nitrogen from the air through the
root-tubercles. If the crop of clover be " plowed under,"
nitrogen-containing materials which the clover has organ-
ized will be contributed to the soil, which is thus restored
to a condition which will support the ordinary crops again.
This indicates the significance of a very ordinary " rotation
of crops."

(4) Ant-plants, etc. — In symbiosis one of the symbionts
may be an animal. Certain fresh-water polyps and sponges
become green on account of Algae which they harbor with-
in their bodies (Fig. 79). Like
the Lichen-fungus, these animals
are benefited by the presence of
the Algae, which in turn find a
congenial situation for living.
By some this would also be re-
garded as a case of helotism,
the animal enslaving the alga.

Very definite arrangements
are made by certain plants for
harboring ants, which in turn
guard them against the attack
of leaf-cutting insects and oth-
er foes. These plants are called
Myrmecophytes, which means
" ant-plants," or myrmecophilous
plants, which means "plants loving ants." These plants
are mainly in the tropics, and in stem cavities, in hollow
thorns, or elsewhere, they provide dwelling places for tribes
of warlike ants (Fig. 80). In addition to these dwelling
places they provide special kinds of food for the ants.

(5) Floivers and insects. — A very interesting and impor-
tant case of symbiosis is that existing between flowers and
insects. The flowers furnish food to the insects, and the

Fig. 79. A fresh-water polyp (Hy-
dra) attached to a twig and con-
taining algse (6'), which may be
seen through the transparent
body wall (B).— Goldberger.

THE FOOD OF PLANTS

91

latter are used by the flowers as agents of pollination. An
account of this relationship is deferred until seed-plants are

w&*iM

Rv

Fig. 80. An ant plant (Hydnophyturri) from South Java, in which an excrescent
growth provides a habitation for ants.— After Schimpek.

considered, or it may be found, with illustrations, in Plant
Relations, Chapter VII.

92 PLANT STRUCTURES

59. Carnivorous plants. — Certain green plants, growing
in situations poor in nitrogen-containing salts, have learned
to supplement the proteids which they manufacture by cap-
turing and digesting insects. The various devices employed
for securing insects have excited great interest, since they
do not seem to be associated with the ordinary idea of plant
activities. Prominent among these forms are the bladder-
worts, pitcher-plants, sundews, Venus's fly-trap, etc. For
further account and illustrations of these plants see Plant
Relations, § 119.

CHAPTEE VII

BRYOPHYTES (MOSS PLANTS)

60. Summary from Thallophytes. — Before considering the
second great division of plants it is well to recall the most
important facts connected with the Thallophytes, those
things which may be regarded as the contribution of the
Thallophytes to the evolution of the plant kingdom, and
which are in the background when one enters the region of
the Bryophytes.

(1) Increasing complexity of the tody. — Beginning with
single isolated cells, the plant body attains considerable
complexity, in the form of simple or branching filaments,
cell-plates, and cell-masses.

(2) Appearance of spores. — The setting apart of repro-
ductive cells, known as spores, as distinct from nutritive
cells, and of reproductive organs to organize these spores,
represents the first important differentiation of the plant
body into nutritive and reproductive regions.

(3) Differentiation of spores. — After the introduction of
spores they become different in their mode of origin, but
not in their power. The asexual spore, ordinarily formed
by cell division, is followed by the appearance of the sexual
spore, formed by cell union, the act of cell union being
known as the sexual process.

(4) Differentiation of gametes.— At the first appearance
of sex the sexual cells or gametes are alike, but after-
ward they become different in size and activity, the large
passive one being called the egg, the small active one the

25 98

94 PLANT STRUCTURES

sperm, the organs producing the two being known as oogo-
nium and antheridium respectively.

(5) Algm the main line. — The Algae, aquatic in habit,
appear to be the Thallophytes which lead to the Bryophytes
and higher groups, the Fungi being regarded as their de-
generate descendants ; and among the Algae the Chloro-
phyceae seem to be most probable ancestors of higher forms.
It should be remembered that among these Green Algae the
ciliated swimming spore (zoospore) is the characteristic
asexual spore, and the sexual spore (zygote or oospore) is
the resting stage of the plant, to carry it over from one
growing season to the next.

61. General characters of Bryophytes. — The name given
to the group means " moss plants," and the Mosses may be
regarded as the most representative forms. Associated
with them in the group, however, are the Liverworts, and
these two groups are plainly distinguished from the Thallo-
phytes below, and from the Pteridophytes above. Starting
with the structures that the Algae have worked out, the
Bryophytes modify them still further, and make their own
contributions to the evolution of the plant kingdom, so
that Bryophytes become much more complex than Thallo-
phytes.

62. Alternation of generations. — Probably the most im-
portant fact connected with the Bryophytes is the distinct
alternation of generations which they exhibit. So impor-
tant is this fact in connection with the development of the
plant kingdom that its general nature must be clearly under-
stood. Probably the clearest definition may be obtained by
tracing in bare outline the life history of an ordinary moss.

Beginning with the asexual spore, which is not ciliated,
as there is no water in which it can swim, we may imagine
that it has been carried by the wind to some spot suitable
for its germination. It develops a branching filamentous
growth which resembles some of the Conferva forms among
the Green Algae (Fig. 81). It is prostrate, and is a regu-

BRYOPHYTES

95

lar thallus body, not at all resembling the " moss plant "
of ordinary observation, and is not noticed by those una-
ware of its existence.

Presently one or more buds appear on the sides of this
alga-like body (Fig. 81, h). A bud develops into an erect

Pig. 81. Protonema of moss : A, very young protonema, showing spore (S) which
has germinated it; B, older protonema, showing branching habit, remains of
spore («), rhizoids (r), and buds (6) of leafy branches (gametophores).— After
Muller and Thurgau.

stalk upon which are numerous small leaves (Figs. 82, 102).
This leafy stalk is the " moss plant " of ordinary observa-
tion, and it will be noticed that it is simply an erect leafy
branch from the prostrate alga-like body.

At the top of this leafy branch sex-organs appear, cor-
responding to the antheridia and oogonia of the Algae, and
within them there are sperms and eggs. A sperm and egg
fuse and an oospore is formed at the summit of the leafy
branch.

The oospore is not a resting spore, but germinates im-
mediately, forming a structure entirely unlike the moss

96

PLANT STKDCTUEES

,rh

Pig. 82. A common mos8
(Po lytrichu m commune),
showing the leafy gameto-
phore with rhizoids (rA),
and two sporophytes (sporo-
gonia), with seta (s), calyp-
tra (c), and operculum (d),
the calyptra having been re-
moved.—After Schenck.

plant from which it came. This new
leafless body consists of a slender
stalk bearing at its summit an urn-
like case in which are developed nu-
merous asexual spores (Figs. 82, 107).
This whole structure is often called
the "spore fruit," and its stalk is
imbedded at base in the summit of
the leafy branch, thus obtaining firm
anchorage and absorbing what nour-
ishment it needs, but no more a part
of the leafy branch than is a para-
site a part of the host.

When the asexual spores, pro-
duced by the "spore fruit," germi-
nate, they reproduce the alga-like
body with which we began, and the
life cycle is completed.

In examining this life history, it
is apparent that each spore produces
a different structure. The asexual
spore produces the alga-like body
with its erect leafy branch, while
the oospore produces the " spore
fruit" with its leafless stalk and
spore case. These two structures,
one produced by the asexual spore,
the other by the oospore, appear in
alternating succession, and this is
what is meant by alternation of gen-
erations.

These two "generations" differ
strikingly from one another in the
spores which they produce. The
generation composed of alga -like
body and erect leafy branch pro-

BRYOPHYTES

97

duces only sexual spores (oospores), and therefore pro-
duces sex organs and gametes. It is known, therefore,
as the gametophyte — that is, "the gamete plant."

The generation which consists of the "spore fruit" —
that is, leafless stalk and spore case — produces only asexual
spores, and is called the sporophyte — that is, "the spore
plant."

Alternation of generations, therefore, means the alter-
nation of a gametophyte and a sporophyte in completing a
life history. Instead of having the same body produce both
asexual and sexual spores, as in most of the Algae, the two
kinds of spores are separated upon different structures,
known as "generations." It is evident that the gameto-
phyte is the sexual generation, and the sporophyte the
asexual one ; and it should be kept clearly in mind that
the asexual spore always produces the gametophyte, and
the sexual spore the sporophyte. In other words, each
spore produces not its own generation, but the other one.

The relation between the two alternating generations
may be indicated clearly by the following formula, in
which Gr and S are used for gametophyte and sporophyte
respectively :

G=g>o— S— o— G=g>o— S— o— G, etc.

The formula indicates that the gametophyte produces
two gametes (sperm and egg), which fuse to form an oospore,
which produces the sporophyte, which produces an asexual
spore, which produces a gametophyte, etc.

That alternation of generations is of great advantage is
evidenced by the fact that it appears in all higher plants.
It must not be supposed that it appears first in the Bryo-
phytes, for its beginnings may be seen among the Thallo-
phytes. The Bryophytes, however, first display it fully
organized and without exception. Just what this alterna-
tion does for plants may not be fully known, but one
advantage seems prominent. By means of it many gameto-
phytes may result from a single oospore ; in other words,

98 PLANT STRUCTURES

it multiplies the product of the sexual spore. A glance at
the formula given above shows that if there were no sporo-
phyte (S) the oospore would produce but one gametophyte
(G). By introducing the sporophyte, however, as many
gametophytes may result from a single oospore as there are
asexual spores produced by the sporophyte, which usually
produces a very great number.

In reference to the sporophytes and gametophytes of
Bryophytes two peculiarities may be mentioned at this
point : (1) the sporophyte is dependent upon the gameto-
phyte for its nourishment, and remains attached to it ;
(2) the gametophyte is the special chlorophyll -generation,
and hence is the more conspicuous. It follows that, in a
general way, the sporophyte of the Bryophytes only pro-
duces spores, while the gametophyte both produces gametes
and does chlorophyll work.

It is important also to note that the protected resting
stage in the life history is not the sexual spore, as in the
Algae, but is the asexual spore in connection with the
sporophyte. These spores have a protecting wall, are
scattered, and may remain for some time without germi-
nation.

If the ordinary terms in reference to Mosses be fitted
to the facts given above, it is evident that the "moss
plant " is the leafy branch of the gametophyte ; that
the " moss fruit " is the sporophyte ; and that the alga-
like part of the gametophyte has escaped attention and
a popular name.

The names now given to the different structures which
appear in this life history are as follows : The alga-like part
of the gametophyte is the protonema, the leafy branch is
the gametophore ("gamete-bearer") ; the whole sporophyte
is the sporogonium (a name given to this peculiar leafless
sporophyte of Bryophytes), the stalk-like portion is the
seta, the part of it imbedded in the gametophore is the
foot, and the urn-like spore-case is the capsule.

BKYOPHYTES

99

63. The antheridium. — The male organ of the Bryophytes
is called an antheridium, just as among Thallophytes, but
it has a very different structure. In general among the

Fig. 83. Sex organs of a common moss (Funaria): the group to the right represents
an antheridium (A) discharging from its apes a mass of sperm mother cells (a), a
single mother cell with its sperm (b), and a single sperm (c), showing body and
two cilia; the group to the left represents an archegonial cluster at summit of
stem (A), showing archegonia (a), and paraphyses and leaf sections (b), and also a
single archegonium IB), with venter (b) containing egg and ventral canal cell, and
neck (A) containing the disorganizing axial row (neck canal cells).— After Sach6.

Thallophytes it is a single cell (mother cell), and may be
called a simple antheridium, but in the Bryophytes it is a
many-celled organ, and may be regarded as a compound
antheridium. It is usually a stalked, club-shaped, or oval to

100

PLANT STRUCTURES

globular body (Figs. 83, 84, 103). A section through this
body shows it to consist of a single layer of cells, which
forms the wall of the antheridium, and within this a com-
pact mass of small cubical (square in section) cells, within
each one of which there is formed a single sperm (Fig. 84).
These cubical cells are evidently moth-
er cells, and to distinguish them from
others they are called sperm mother cells.
An antheridium, therefore, aside from
its stalk, is a mass of sperm mother
cells surrounded by a wall consisting
of one layer of cells.

The sperm is a very small cell with
two long cilia (Fig. 83). The two
parts are spoken of as "body" and
cilia, and the body may be straight or
somewhat curved. These small bicili-
ate sperms are one of the distinguish-
ing marks of the Bryophytes. The
existence of male gametes in the form
of ciliated sperms indicates that fertil-
ization can take place only in the pres-
ence of water, so that while the plant
has become terrestrial, and its asexual spores have respond-
ed to the new conditions and are no longer ciliated, its
sexual process is conducted as among the Green Algae. It
must not be supposed, however, that any great amount of
water is necessary to enable sperms to swim, even a film
of dew often answering the purpose.

When the mature antheridia are wet they are opened
at the apex and discharge the mother cells in a mass (Figs.
83, 105, E), the walls of the mother cells become mucilagi-
nous, and the sperms escaping swim actively about and are
attracted to the organ containing the egg.

64. The archegonium. — This name is given to the female
sex organ, and it is very different from the oogonium of

Fig. 84. Antheridium of
a liverwort in section,
showing single layer
of wall cells surround-
ing the mass of moth-
er cells. — After Stras-

BURGER.

BEYOPHYTES 101

Thallophytes. Instead of being a single mother cell, it is
a many-celled structure, shaped like a flask (Figs. 83, 98).
The neck of the flask is more or less elongated, and within
the bulbous base (venter) the single egg is organized. The
archegonium, made up of neck and venter, consists mostly
of a single layer of cells. This hollow flask is solid at first,
there being a central vertical row of cells surrounded by
the single layer just referred to. All of the cells of this
axial row, except the lowest one, disorganize and leave a
passageway down through the neck. The lowest one of
the row, which lies in the venter of the archegonium, or-
ganizes the egg. In this way there is formed in the arche-
gonium an open passageway through the neck to the egg
lying in the venter.

To this neck the swimming sperms are attracted, enter
and pass down it, one of them fuses with the egg, and this
act of fertilization results in an oospore.

Archegonia and antheridia are supposed to have been
derived from a many-celled gametangium, such as occurs
in certain Brown Algae (Fig. 18). The presence of the
archegonia is one strong and unvarying distinction between
Thallophytes and Bryophytes. Pteridophytes also have
archegonia, and so characteristic an organ is it that Bryo-
phytes and Pteridophytes are spoken of together as Arche-
goniates.

65. Germination of the oospore. — The oospore in Bryo-
phytes is not a resting spore, but germinates immediately
by cell division, forming the sporophyte embryo, which
presently develops into the mature sporophyte (Fig. 85,^4).
The lower part of the embryo develops the foot, which ob-
tains a firm anchorage in the gametophore by the latter
growing up around it (Fig. 85, 2?, C). The upper part of
the embryo develops the seta and capsule. As the embryo
increases in size, the venter of the archegonium grows also,
forming what is called the calyptra ; and in true Mosses
the embryo presently breaks loose the calyptra at its base

102

PLANT STKUCTUEES

and carries it upward perched on the top of the capsule like
a loose cap or hood (Figs. 82, c, 107), which sooner or later

falls off. As stated be-
fore, the mature struc-
ture developed from
the oospore is called a
sporogonium, a form of
sporophyte peculiar to
the Bryophytes.

66. The sporogonium.
— In its fullest devel-
ojjment the sporogoni-
um is differentiated
into the three regions,
foot, seta, and capsule
(Figs. 82, 107) ; but in
some forms the seta
may be lacking, and
in others the foot also,
the sporogonium in this
last case being only the
capsule or spore case,
which, after all, is the
essential part of any
sporogonium.

At first the capsule
is solid, and its cells
are all alike. Later a
group of cells within
begins to differ in ap-
pearance from those
about them, being set
apart for the produc-
tion of spores. This
initial group of spore-producing cells is called the arclie-
sporium, a word meaning "the beginning of spores." It

Fig. 85. Sporogonium of Fvnaria: A, an em-
bryo sporogonium (/,/'), developing within
the venter (b, b) of an archegonium ; B, C,
tips of leafy shoots bearing young eporo-
gonia, pushing up calyptra (c) and archego-
nium neck (h), and sending the foot down
into the apex of the gametophore. — After
Goebel.

BKYOPHYTES 103

does not follow that the archesporial cells themselves pro-
duce spores, but that the spores are to appear sooner or
later in their progeny. Usually the archesporial cells
divide and form a larger mass of spore-producing cells.
Such cells are known as sporogenous ("spore-producing")
cells, or the group is spoken of as sporogenous tissue. Spo-
rogenous cells may divide more or less, and the cells of the
last division are mother cells, those which directly produce
the spores. The usual sequence, therefore, is archesporial
cells (archesporium), sporogenous cells, and mother cells ;
but it must be remembered that they all may be referred
to as sporogenous cells.

Each mother cell organizes within itself four spores,
the group being known as a tetrad. In Bryophytes and
the higher groups asexual spores are always produced in
tetrads. After the spores are formed the walls of the
mother cells disorganize, and the spores are left lying loose
in a cavity which was formerly occupied by the sporoge-
nous tissue. All mother cells do not always organize spores.
In some cases some of them are used up in supplying nour-
ishment to those which form spores. Such mother cells are
said to function as nutritive cells. In other cases, certain
mother cells become much modified in form, being organ-
ized into elongated, spirally-banded cells called elaters (Figs.
97, 101), meaning "drivers" or "hurlers." These elaters
lie among the loose ripe spores, are discharged with them,
and by their jerking movements assist in scattering them.

The cells of the sporogonium which do not enter into
the formation of the archesporium, and are not sporoge-
nous, are said to be sterile, and are often spoken of as
sterile tissue. Every sporogonium, therefore, is made up
of sporogenous tissue and sterile tissue, and the differences
found among the sporogonia of Bryophytes depend upon
the relative display of these two tissues.

The sporogonium is a very important structure from
the standpoint of evolution, for it represents the conspicu-

104

PLANT STKUCTUKES

ous part of the higher plants. The "fern plant," and
the herbs, shrubs, and trees among " flowering plants "
correspond to the sporogonium of Bryophytes, and not to
the leafy branch (gametophore) or "moss plant." Conse-
quently the evolution of the sporogonium through the
Bryophytes is traced with a great deal of interest. It may
be outlined as follows :

In a liverwort called Riccia the simplest sporogonium
is found. It is a globular capsule, without seta or foot

Pig. 86. Diagrammatic sections of sporogonia of liverworts: A. Riccia, the whole
capsule being archesporium except the sterile wall layer; B, Marchantia, one
half the capsule being sterile, the archesporium restricted to the other half; D,
Anthoceros, archesporium still more restricted, being dome-shaped and capping a
central sterile tissue, the columella (col).— After Goebel.

(Fig. 86, A). The only sterile tissue is the single layer of
cells forming the wall, all the cells within the wall be-
longing to the archesporium. The ripe sporogonium,
therefore, is nothing but a thin-walled spore case. It is
well to note that the sporophyte thus begins as a spore
case, and* that any additional structures that it may de-
velop later are secondary.

In another liverwort (Marchantia) the entire lower half
of the sporogonium is sterile, while in the upper half there

BKYOrHYTES

105

is a single layer of sterile cells as a wall about the arche-
sporium, which is composed of all the remaining cells of the
upper half (Fig. 86, B). It will be noted that the sterile
tissue in this sporogonium has encroached upon the arche-
sporium, which is restricted to one half of the body. In
this case the archesporium has the form of a hemisphere.

In another liverwort (Jungermannia) the archesporium
is still more restricted (Fig. 87). The sterile tissue is organ-

Fig. 87. Diagrammatic section of spo-
rogonium of a Jungermannia form,
showing differentiation into foot,
seta, and capsule, the archesporium
restricted to upper part of sporogo-
nium.—After Goebel.

Fig. 88. Section through sporogonium of
Sphagnum, showing capsule (k) with
old archegonium neck (ah), calyptra (ca),
dome-shaped mass of sporogenous tissue
(spo), and columella (co), also the bulb-
ous foot (spf) imbedded in the pseudo-
podium (ps).— After Schimper.

ized into a foot and a seta, and the archesporium is a com-
paratively small mass of cells in the upper part of the
sporogonium.

In another liverwort (Anthoceros) the sterile tissue or-
ganizes foot and seta, and the archesporium is still more
restricted (Fig. 86, D). Instead of a solid hemispherical

106

PLANT STEUCTUEES

mass, it is a dome-shaped mass, the inner cells of the hemi-
sphere having become sterile. This central group of sterile
cells which is surrounded by the ar-
chesporium is called the columella,
which means "a small column."

In a moss called Sphagnum there
is the same dome-shaped archespori-
um with the columella, as in An-
thoceros, but it is relatively smaller
on account of the more abundant
sterile tissue (Fig. 88).

In the highest Mosses the arche-
sporium becomes very small as com-
pared with the sterile tissue (Fig.
89). A foot, a long seta, and an
elaborate capsule are organized from
the sterile tissue, while the arche-
sporium is shaped like the walls of
a barrel, as though the dome-shaped
archesporium of Sphagnum or An-
thoceros had become sterile at the
apex. In this way the columella is
continued through the capsule, and
is not capped by the archesporium.

This series indicates that after
the sporogonium begins as a simple
spore case {Riccia), its tendency is
to increase sterile tissue and to re-
strict sporogenous tissue, using the
sterile tissue in the formation of the
organs of the sporogonium body, as
foot, seta, capsule walls, etc.

Among the Green Algae there is
a form known as Coleoclmte, whose
body resembles those of the sim-
plest Liverworts (Fig. 90). When

Fig. 89. Young sporogoni-
um of a true moss, show-
ing foot, seta, and young
capsule, in which the ar-
chesporium (darker por-
tion) is barrel-shaped, and
through it the columella is
continuous with the lid. —
After Campbell.

BRYOPHYTES 107

its oospores germinate there is formed a globular mass of
cells, every one of which is a spore mother cell (Fig. 90, C).
If an outer layer of mother cells should become sterile and
form a wall about the others, such a spore case as that of

Fig. 90.— Coleochate, one of the green algse : A, a portion of the thallus, showing
oogonia with trichogynes (og), antheridia (an), and two enlarged biciliate sperms
(z); B, a fertilized oogonium containing oospore and invested by a tissue (r)
which has developed after fertilization ; C, an oospore which has germinated
and formed a mass of cells (probably a sporophyte), each one of which organizes
a biciliate zoospore (Z>).— After Pringsheim.

Riccia would be the result (Fig. 86, A). For such reasons
many believe that the Liverworts have been derived from
such forms as Coleochcete.

67. The gametophyte. — Having considered the sporo-
phyte body as represented by the sporogonium, we must
consider the gametophyte body as represented by proto-
nema and leafy branch (gametophore). The gametophyte
results from the germination of an asexual spore, and in
the Mosses it is differentiated into protonema and leafy
gametophore (Figs. 81, 82, 102). Like the sporophyte,

IQg PLANT STKUCTUKES

however, it shows an interesting evolution from its sim-
plest condition in the Liverworts to its most complex con-
dition in the true Mosses.

In the Liverworts the spore develops a flat thallus body,
one plate of cells or more in thickness, which generally
branches dichotomously (see § 29) and forms a more or less
extensive body (Fig. 92). This thallus is the gametophyte,
there being no differentiation into protonema and leafy
branch.

In the simpler Liverworts the sex organs (antheridia
and archegonia) are scattered over the back of this thallus
(Fig. 92). In other forms they become collected in certain
definite regions of the thallus. In other forms these defi-
nite sexual regions become differentiated from the rest of
the thallus as disks. In other forms these disks, bearing
the sex organs, become short-stalked, and in others long-
stalked, until a regular branch arises from the thallus
body (Figs. 96, 97). This erect branch, bearing the sex or-
gans, is, of course, a gametophore, but it is leafless, the
thallus body doing the chlorophyll work.

In the Sphagnum Mosses the spore develops the same
kind of flat thallus (Fig. 104), but the gametophore be-
comes leafy, sharing the chlorophyll work with the thallus.
In the true Mosses most of the chlorophyll work is done by
the leafy gametophore, and the flat thallus is reduced to
branching filaments (the protonema) (Fig. 102).

The protonema of the true Mosses, therefore, corre-
sponds to the flat thallus of the Liverworts and Sphagnum,
while the leafy branch corresponds to the leafless gameto-
phore found in some Liverworts. It also seems evident
that the gametophore was originally set apart to bear sex
organs, and that the leaves which appear upon it in the
Mosses are subsequent structures.

CHAPTER VIII

THE GREAT GROUPS OF BRYOPHYTES

Hepatic^e {Liverworts)

68. General character. — Liverworts live in a variety of
conditions, some floating on the water, many in damp
places, and many on the bark of trees. In general they are
moisture-loving plants (hydrophytes), though some can en-
dure great dryness. The gametophyte body is prostrate,
though there may be erect and leafless gametophores.

This prostrate habit develops a dorsiventral body — that
is, one whose two surfaces {dorsal and ventral) are exposed
to different conditions and become unlike in structure. In
Liverworts the ventral surface is against the substratum,
and puts out hair-like processes {rhizoids) for anchorage
and possibly absorption. The dorsal region is exposed to
the light and its cells develop chlorophyll. If the thallus
is thin, chlorophyll is developed in all the cells ; if it be so
thick that the light is cut off from the ventral cells, the
thallus is differentiated into a green dorsal region doing the
chlorophyll work, and a colorless ventral region producing
anchoring rhizoids. This latter represents a simple differ-
entiation of the nutritive body into working regions, the
ventral region absorbing material and conducting it to the
green dorsal cells which use it in making food.

There seem to have been at least three main lines of
development among Liverworts, each beginning in forms
with a very simple thallus, and developing in different di-
rections. They are briefly indicated as follows :
26 109

110

PLANT STRUCTURES

69. Marchantia forms. — In this line the simple thallus
gradually becomes changed into a very complex one. The

thallus retains its simple
outlines, but becomes thick
and differentiated in tissues
(groups of similar cells).
The line may be distin-
guished, therefore, as one
in which the differentia-
tion of the tissues of the
gametophyte is emphasized
(Figs. 91-93). In Mar-
chantia proper the thallus
becomes very complex, and
it may be taken as an illus-
tration.

The thallus is so thick

that there are very distinct

green dorsal and colorless

ventral regions (Fig. 94). The latter puts out numerous

rhizoids and scales from the single layer of epidermal cells.

Above the ventral epidermis are several layers of colorless

Fig. 91. A very small species of Jticcia,
one of the Marchantia forms : A, a
group of thallus bodies slightly en-
larged ; J3, section of a thallus, show-
ing rhizoids and two sporogonia im-
bedded and communicating with the
outside by tubular passages in the
thallus. — After Stbasburger.

Fig. 92. Ricciocarpus, a Marchantia form, showing numerous rhizoids from ventral
surface, the dichotomous branching, and the position of the sporogonia on the
dorsal surface along the "midribs."— Goldberger.

Fig. 93. Two common liverworts : to the left is Conocephalus, a Marchantia form,
showing rhizoids, dichotomous branching, and the conspicuous rhombic areas
(areolae) on the dorsal surface ; to the right is Anthoceros, with its simple thallus
and pod-like sporogonia. — Goldberger.

Fig. 94. Cross-sections of thallus of Marchantia : A, section from thicker part of
thallus, where supporting tissue (p) is abundant, and showing lower epidermis
giving rise to rhizoids (h) and plates (b), also chlorophyll tissue (chl) organized
into chambers by partitions (o)\ B, section near margin of thallus more magnified,
showing lower epidermis, two layers of supporting tissue {p) with reticulate walls,
a single chlorophyll chamber with its bounding walls (s) and containing short,
often branching filaments whose cells contain chloroplasts (chl), overarching
upper epidermis (o) pierced by a large chimney-like air-pore (sp).— After Goebel.

Fig. 95. Section through cupule of Marchantia, showing wall in which are chloro-
phyll-bearing air-chambers with air-pores, and gemmae (a) in various stages of
development.— Dodel- Port.

Fig. 96. Marchantia polymorpha : the lower figure represents a gametophyte bear-
ing a mature antheridial branch (d), some young antheridial branches, and also
some cupules with toothed margins, in which the gemmae may be seen ; the
upper figure represents a partial section through the antheridial disk, and shows
antheridia within the antheridial cavities (a, b, c, d, «,/).— After Knt.

THE GEEAT GEOUPS OF BEYOPHYTES

113

cells more or less modified for conduction. Above these
the dorsal region is organized into a series of large air cham-
bers, into which project chlorophyll-containing cells in the

Fig. 97. Marchantia polymorpha, a common liverwort : 1, thallus, with rhizoids,
bearing a mature archegonial branch (/) and several younger ones (a, b, c, d, «);
2 and 3, dorsal and ventral views of archegonial disk; h and 5, young sporophyte
(sporogonium) embryos; o, more mature sporogonium still within enlarged venter
of archegonium; 7, mature sporogonium discharging spores; 8, three spores and
an elater. — After Knt.

form of short branching filaments. Overarching the air
chambers is the dorsal epidermis, and piercing through it
into each air chamber is a conspicuous air pore (Fig. 94, B).

114

PLANT STRUCT UKES

The air chambers are outlined on the surface as small

rhombic areas (areola), each containing a single air pore.

Peculiar reproductive bodies are also developed upon

the dorsal surface of Marchantia for vegetative multiplica-

Fig. 98. Marchantia poiymorpha : 1, partial section through archegonial disk, show-
ing archegonia with long necke, and venters containing eggs; 9, young archego-
nium showing axial row; 10, superficial view at later stage; 11. mature archego-
nium, with axial row disorganized and leaving an open passage to the large egg;
1$, cross-section of venter; 13, cross-section of neck.— After Knt.

tion. Little cups (cupules) appear, and in them are numer-
ous short-stalked bodies (gemmce), which are round and
flat (biscuit-shaped) and many-celled (Figs. 95, 96). The

THE GREAT GROUPS OF BRYOPHYTES H5

gemmae fall off and develop new thallus bodies, making
rapid multiplication possible.

Marchantia also possess remarkably prominent gameto-
phores, or "sexual branches" as they are often called.
In this case the gametophores are differentiated, one bear-
ing only antheridia (Fig. 96), and known as the "anthe-
ridial branch," the other bearing only archegonia (Figs. 97,
98), and known as the "archegonial branch." The scal-
loped antheridial disk and the star-shaped archegonial disk,
each borne up by the stalk-like gametophore, are seen in the
illustrations. Not only are the gametophores sexually dif-
ferentiated, but as only one appears on each thallus, the thal-
lus bodies are sexually differentiated. When the two sex
organs appear upon different individuals, the plant is said to
be dioecious, meaning " two households " ; when they both
appear upon the same individual, the plant is monoecious,
meaning " one household." Some of the Bryophytes are mo-
noecious, and some of them are dioecious (as Marchantia).

Another distinguishing mark of the line of Marchantia
forms is that the capsule-like sporogonium opens irregu-
larly to discharge its spores (Fig. 97, 7).

70. Jungermannia forms. — This is the greatest line of
the Liverworts, the forms being much more numerous
than in the other lines. They grow in damp places ; or in
drier situations on rocks, ground, or tree-trunks ; or in the
tropics also on the leaves of forest plants. They are gen-
erally delicate plants, and resemble small Mosses, many of
them doubtless being commonly mistaken for Mosses.

This resemblance to Mosses suggests one of the chief
features of the line. Beginning with a simple thallus, as
in the Marchantia line, the structure of the thallus re-
mains simple, there being no such differentiation of tissues
as in the Marchantia line ; but the form of the thallus
becomes much modified (Figs. 99, 100). Instead of a flat
thallus with even outline, the body is organized into a cen-
tral stem-like axis bearing two rows of small, often crowded

116

PLANT STRUCTUKES

leaves. There are really three rows of leaves, but the third
is on the ventral side against the substratum, and is often
so much modified as not to look like the other leaves. In
consequence of this the Jungermannia forms are usually
called "leafy liverworts," to distinguish them from the

Fig. 99. Two liverworts, both Jungermannia forms: to the left is Blasia, which re-
tains the thallus form but has lobed margins; to the right is Scapania, with dis-
tinct leaves and sporogonia (^4).— Goldbergeb.

other Liverworts, which are "thallose." They are also
often called "scale mosses," on account of their moss-like
appearance and their small scale-like leaves.

The line may be distinguished, therefore, as one in
which the differentiation of the form of the gametophyte
is emphasized. Another distinguishing mark is that the
sporogonium has a prominent seta, and the capsule splits
down into four pieces (valves) when opening to discharge
the spores (Fig. 100, 0).

71. Anthoceros forms. — This line contains comparatively
few forms, but they are of great interest, as they are sup-
posed to represent forms which have given rise to the

THE GREAT GROUPS OF BRYOPHYTES H7

Pig. 100. Species of Lfpidozia. a genua of leafy liverworts, showing different leaf
forms, and in A and (' the dehiscence of the sporogonium hy four valves. In C
rhizoids are evident; and in B, D, and E the three rows of leaves are seen, the
leaves of the ventral row being comparatively small.— After Englbk and Pkantl.

Mosses, and possibly to the Pteridophytes also. The
thallus is very simple, being differentiated neither in
structure nor form, as in the two other lines ; but the

118

PLANT STRUCTUKES

special development has been in connection with the
sporogonium (Figs. 93, 101).

This complex sporogonium (sporophyte) has a large
bulbous foot imbedded in the simple thallus, while
above there arises a long pod-like capsule. The com-
plex walls of this cap-
sule contain chlorophyll
and air pores, so that
the sporogonium is or-
ganized for chlorophyll
work. If it could send
absorbing roots into the
soil, this sporophyte
could live independent
of the gametophyte. In
opening to discharge
spores the pod-like cap-
sule splits down into
two valves.

Another peculiarity

of the Antlioceros forms

is in connection with

m jj fa the antheridia and arch-

fcj vk*_ 'Oh. W egonia. These organs,

instead of growing out
free from the body of the
thallus, as in other Liv-
erworts, are imbedded in
it. The significance of
this peculiarity lies in
the fact that it is a char-
acter which belongs to
the Pteridophytes.
The chief direction of development of the three liv-
erwort lines may be summed up briefly as follows : The
Marchantia line has differentiated the structure of the

Fig. 101. Anthoceros gracilis : A, several
gametophyte8, on which sporogonia have
developed ; B, an enlarged sporogonium,
showing its elongated character and de-
hiscence by two valves leaving exposed
the slender columella on the surface o'
which are the spores; C, D, E, F, ela-
ters of various forms ; G, spores. — After
Schipfner.

THE GREAT GEOUPS OF BRYOPHYTES H9

gametophyte ; the Jungermannia line has differentiated
the form of the gametophyte ; the Anthoceros line has
differentiated the structure of the sporophyte. It should
be remembered that other characters also serve to distin-
guish the lines from one another.

Musci (Mosses)

72. General character. — Mosses are highly specialized
plants, probably derived from Liverworts, the numerous
forms being adapted to all conditions, from submerged to
very dry, being most abundantly displayed in temperate
and arctic regions. Many of them may be dried out com-
pletely and then revived in the presence of moisture, as is
true of many Lichens and Liverworts, with which forms
Mosses are very commonly associated.

They also have great power of vegetative multiplica-
tion, new leafy shoots putting out from old ones and from
the protonema indefinitely, thus forming thick carpets and
masses. Bog mosses often completely fill up bogs or small
ponds and lakes with a dense growth, which dies below
and continues to grow above as long as the conditions are
favorable. These quaking bogs or "mosses," as they are
sometimes called, furnish very treacherous footing unless
rendered firmer by other plants. In these moss-filled bog?
the water shuts off the lower strata of moss from complete
disorganization, and they become modified into a coaly sub«
stance called peat, which may accumulate to considerable
thickness by the continued upward growth of the mass of
moss.

The gametophyte body is differentiated into two very
distinct regions : (1) the prostrate dorsiventral thallus,
which is called protonema in this group, and which may be
either a broad flat thallus (Fig. 104) or a set of branching
filaments (Figs. 81, 102) ; (2) the erect leafy branch or
gametophore (Fig. 82). This erect branch is said to be

120

PLANT STRUCTURES

radial, in contrast with the dorsiventral thallus, referring
to the fact that it is exposed to similar conditions all
around, and its organs are arranged about a central axis
like the parts of a radiate animal. This position is much

more favorable for the
chlorophyll work than
the dorsiventral posi-
tion, as the special
chlorophyll organs
(leaves) can be spread
out to the light freely
in all directions.

It should be re-
marked that the gam-
etophyte in all groups
of plants is a thallus,
doing its chlorophyll
work, when it does
any, in a dorsiventral
position ; the only ex-
ception being the ra-
dial leafy branch that
arises from the thal-
lus of Mosses. From
Mosses onward the
gametophyte becomes
less conspicuous, so
that the prominent
leafy plants of the
higher groups hold no
relation to the little erect leafy branch of the Mosses,
which is put out by the gametophyte, and which is the
best the gametophyte ever does toward getting into a bet-
ter position for chlorophyll work.

The leafy branch of the Mosses usually becomes inde-
pendent of the thallus by putting out rhizoids at its base

Fig. 102. A moss Uiryum), showing base of a
leafy branch (gamctophore) attached to the
protonema. and having sent out rhizoids. On
the protonemal filament to the right and be-
low is the young bud of another leafy branch.

— MfJLLER.

THE GKEAT GROUPS OF BKYOPHYTES

121

(Fig. 102), the thallus part dying. Sometimes, however,
the filamentous protonema is very persistent, and gives rise
to a perennial succession of leafy branches.

A ,J»

Fig. 103. Tip of leafy branch of a moss (Funaria), bearing a cluster of sex organs,
showing an old antheridium (A), a younger one (_B), some of the curious associated
hairs (p), and leaf sections (/).— After Campbell.

At the summit of the leafy gametophore, either upon
the main axis or upon a lateral branch, the antheridia and
archegonia are borne (Figs. 83, 103). Often the leaves at
the summit become modified in form and arranged to form

122

PLANT STKUCTUKES

a rosette, in the center of which are the sex organs. This
rosette is often called the "moss flower," but it holds no
relation to the flower of Seed-plants, and the phrase should
not be used. A rosette may contain but one kind of sex
organ (Figs. 83, 103), or it may contain both kinds, for
Mosses are both dioecious and monoecious. The two prin-
cipal groups are as follows :

73. Sphagnum forms. — These are large and pallid bog
mosses, found abundantly in marshy ground, especially of
temperate and arctic regions, and are conspicuous peat-
formers (Fig. 105, A). The leaves and gametophore axis

are of peculiar struc-
ture to enable them
to suck up and hold
a large amount of wa-
ter. This abundant
water - storage tissue
and the comparative-
ly poor display of
chlorophyll - contain-
ing cells gives the
peculiar pallid ap-
pearance.

They resemble the
Liverworts in the
broad thallus body
of the gametophyte,
from which the large
leafy gametophore
arises (Fig. 104).
They also resemble
Anthoceros forms in the sporogonium, the archesporium
being a dome-shaped mass (Fig. 105, C). On the other
hand, they resemble the true Mosses, not only in the leafy
gametophore, but also in the fact that the capsule opens
at the apex by a circular lid, called the operculum (Fig.

Fig. 104. Thallus body of gametophyte of Sphag-
num, giving rise to rhizoids (r) and buds (k)
which develop into the large leafy branches
(gametophores).— After Campbell.

THE GREAT GROUPS OF BRYOPHYTES

123

105, D), which means a "cover" or "lid." This may
serve to illustrate what is called an "intermediate" or
"transition" type, Sphagnum showing characters which
ally it to^ Anthoceros forms on the one side, and to true
Mosses on the other.

A peculiar feature of the sporogonium is that it has no
long stalk-like seta, as have the true Mosses, although it
appears to have one. This false appearance arises from the

Fig. 105. Sphagnum : A, a leafy branch (gametophore) bearing four mature sporo-
gonia; B, archegonium in whose venter a young embryo sporophyte (em) is de-
veloping; C, section of a young sporogonium (sporophyte), showing the bulbous
foot (spf) imbedded in the apex of the pseudopodium (ps), the capsule (k), the
columella (co) capped by the dome-shaped archesporium (spo), a portion of the
calyptra (ca), and the old archegonium neck (ah); D, branch bearing mature
sporogonium and showing pseudopodium (ps), capsule (k), and operculum (d); E,
antberidium discharging sperms; F, a single sperm, showing coiled body and two
cilia. — After Schimper.

fact that the axis of the gametophore is prolonged above
its leafy portion, the prolongation resembling the seta of
an ordinary moss (Fig. 105, D). This prolongation is

124

PLANT STRUCTURES

called a pseudopodium, or " false stalk," and in the top of
it is imbedded the foot of the sporogonium carrying the
globular capsule (Fig. 105, C).

74. True Mosses. — This immense and most highly organ-
ized Bryophyte group contains the great majority of the
Mosses, which are sometimes called the Bryum forms, to
distinguish them from the Sphagnum forms. They are

Fig. 106. Different stages in the development of the leafy gametophore from the pro-
tonema of a common moss (Fimaria): A, the first few cells and a rhizoid (r); B,
C, later stages, showing apical cell (;) and young leaves (2); D, later stage much
less magnified, showing protonemal filaments and the young gametophore (gam)
— After Camtbell.

the representative Bryophytes, the only group vying with
them being the leafy Liverworts, or Jungermannia forms.
They grow in all conditions of moisture, from actual sub-
mergence in water to dry rocks, and they also form exten-
sive peat deposits in bogs.

The thallus body of the gametophyte is made up of
branching filaments (Figs. 81, 102), those exposed to the

THE GREAT GROUPS OF BRYOPHYTES

125

light containing chlorophyll, and those in the substratum
being colorless and acting as rhizoids. The leafy gameto-
phores are often highly organized (Figs. 102, 106), the
leaves and stems showing a certain amount of differentia-
tion of tissues.

It is the sporophyte, however, which shows the great-
est amount of serialization (Fig. 107). The sporogonium

Fig. 107. A common moss (Funaria): in the center is the leafy shoot (gametophore),
with rhizoids, several leaves, and a sporogonium (sporophyte), with a long seta,
capsule, and at its tip the calyptra {cal); to the right a capsule with calyptra re-
moved, showing the operculum (o); to the left a young sporogonium pushing up
the calyptra from the leafy shoot.— After Campbell.

has a foot and a long slender seta, but the capsule is espe-
cially complex. The archesporium is reduced to a small
hollow cylinder (Fig. 88), the capsule wall is most elabo-
rately constructed, and the columella runs through the
27

Fig. 108. Longitudinal section of moss capsule
(Funaria), showing its complex character:
d, operculum; p, peristome: c, c', columel-
la; «, sporogenous tissue ; outside of s the
complex wall consisting of layers of cells
and large open spaces (h) traversed by
strands of tissue.— After Goebel.

Pig. 110. Sporogonia of Gtimmia, from all of
which the operculum has fallen, displaying
the peristome teeth : A, position of the teeth
when dry; B, position when moist. — After
Kekner.

Fig. 109. Partial longitudinal
section through a moss cap-
sule : A, younger capsule,
showing wall cells (a), cells
of columella (i), and sporog-
enous cells (su) ; B, some-
what older capsule, a and i
same as before, and sm the
spore mother cells. — After
Goebel.

THE GREAT GROUPS CF BRYOPHYTES ^27

center of the capsule to the lid-like operculum (Figs. 108,
109). When the operculum falls off the capsule is left like
an urn full of spores, and at the mouth of the urn there is
usually displayed a set of slender, often very beautiful teeth
(Fig. 110), converging from the circumference toward the
center, and called the peristome, meaning " about the
mouth." These teeth are hygroscopic, and by bending
inward and outward help to discharge the spores.

CHAPTEK IX

PTERIDOPHYTES (FERN PLANTS)

75. Summary from Bryophytes. — In introducing the Bryo-
phytes a summary from the Thallophytes was given (see §
60), indicating certain important things which that group
has contributed to the evolution of the plant kingdom.
In introducing the Pteridophytes it is well to notice certain
important additions made by the Bryophytes.

(1) Alternation of generations. — The great fact of alter-
nating sexual (gametophyte) and sexless (sporophyte) gen-
erations is first clearly expressed by the Bryophytes, although
its beginnings are to be found among the Thallophytes.
Each generation produces one kind of spore, from which is
developed the other generation.

(2) Gametophyte the chlorophyll generation. — On account
of this fact the food is chiefly manufactured by the gameto-
phyte, which is therefore the more conspicuous generation.
When a moss or a liverwort is spoken of, therefore, the
gametophyte is usually referred to.

(3) Gametophyte and sporophyte not independent. — The
sporophyte is mainly dependent upon the gametophyte for
its nutrition, and remains attached to it, being commonly
called the sporogonium, and its only function is to produce
spores.

(4) Differentiation of thalhis into stem and leaves. —
This appears incompletely in the leafy Liverworts (Junger-
mannia forms) and much more clearly in the erect and
radial leafy branch (gametophore) of the Mosses.

128

PTERIDOPHYTES 129

(5) Many-celled sex o?'gans. — The antheridia and the
flask-shaped archegonia are very characteristic of Bryo-
phytes as contrasted with Thallophytes.

76. General characters of Pteridophytes. — The name means
" fern plants/"' and the Ferns are the most numerous and the
most representative forms of the group. Associated with
them, however, are the Horsetails (Scouring rushes) and
the Club-mosses. By many the Pteridophytes are thought
to have been derived from such Liverworts as the Antho-
ceros forms, while some think that they may possibly have
been derived directly from the Green Alga?. Whatever
their origin, they are very distinct from Bryophytes.

One of the very important facts is the appearance of
the vascular system, which means a "system of vessels,"
organized for conducting material through the plant body.
The appearance of this system marks some such epoch in
the evolution of plants as is marked in animals by the
appearance of the- "backbone." As animals are often
grouped as "vertebrates" and "invertebrates," plants are
often grouped as "vascular plants" and "non-vascular
plants," the former being the Pteridophytes and Spermato-
phytes, the latter being the Thallophytes and Bryophytes.
Pteridophytes are of great interest, therefore, as being the
first vascular plants.

77. Alternation of generations. — This alternation con-
tinues in the Pteridophytes, but is even more distinct than
in the Bryophytes, the gametophyte and sporophyte be-
coming independent of one another. An outline of the life
history of an ordinary fern will illustrate this fact, and will
serve also to point out the prominent structures. Upon the
lower surface of the leaves of an ordinary fern dark spots
or lines are often seen. These are found to yield spores,
with which the life history may be begun.

When such a spore germinates it gives rise to a small,
green, heart-shaped thallus, resembling a delicate and sim-
ple liverwort (Fig. Ill, A). Upon this thallus antheridia

130

PLANT STRUCT UEES

and archegonia appear, so that it is evidently a gameto*
phyte. This gametophyte escapes ordinary attention, as it
is usually very small, and lies prostrate upon the substra-
tum. It has received the name prothallium or prothattus,
so that when the term prothallium is used the gametophyte
of Pteridophytes is generally referred to ; just as when the
term sporogonium is used the sporophyte of the Bryophytes
is referred to. Within an archegonium borne upon this little
prothallium an oospore is formed. When the oospore ger-

Fig. 111. Prothallium of a common fern (A$pidium): A, ventral surface, showing
rhizoids (rA), antheridia {an), and archegonia (.«/•) ; B, ventral surface of an older
gametophyte, showing rhizoids (rh) and young sporophyte with root (w) and leaf
(6).— After Schenck.

minates it develops the large leafy plant ordinarily spoken
of as " the fern," with its subterranean stem, from which
roots descend, and from which large branching leaves rise
above the surface of the ground (Fig. Ill, B). It is in
this complex body that the vascular system appears. No
sex organs are developed upon it, but the leaves bear numer-
ous sporangia full of asexual spores. This complex vascular
plart, therefore, is a sporophyte, and corresponds in this
life history to the sporogonium of the Bryophytes. This

PTEEIDOPHYTES 131

completes the life cycle, as the asexual spores develop the
prothallium again.

In contrasting this life history with that of Bryophytes
several important differences are discovered. The most
striking one is that the sporophyte has become a large,
leafy, vascular, and independent structure, not at all re-
sembling its representative (the sporogonium) among the
Bryophytes.

Also the gametophyte is much less prominent than the
gametophytes of the larger Liverworts and Mosses. If
Ferns have been derived from the Liverworts, therefore, it
is probable that they came from those with very simple
bodies rather than from those in which the gametophyte
had become large and complex. The conspicuous leafy
branch of the Mosses, commonly called "the moss plant,"
corresponds to nothing in the Pteridophytes, the prothal-
lium representing only the protonema part of the gameto-
phyte of the true Mosses.

The small size of the gametophyte seems to be associ-
ated with the fact that the chlorophyll work has been
transferred to the sporophyte, which hereafter remains the
conspicuous generation. The " fern plant " of ordinary
observation, therefore, is the sporophyte ; while the " moss
plant " is a leafy branch of the gametophyte..

Another important contrast indicated is that in Bryo-
phytes the sporophyte is dependent upon the gametophyte
for its nutrition, remaining attached to it ; while in most
of the Pteridophytes both generations are independent
green plants, the leafy sporophyte remaining attached to
the small gametophyte only while beginning its growth
(Fig. Ill, B).

Among the Ferns some interesting exceptions to this
method of alternation have been observed. Under certain
conditions a leafy sporophyte may sprout directly from the
prothallium (gametophyte) instead of from an oospore.
This is called apogamy, meaning " without the sexual act/'

132

PLANT STKUCTUKES

Under certain other conditions prothallia are observed to
sprout directly from the leafy sporophyte instead of from
a spore. This is called apospory, meaning "without a
spore."

78. The gametophyte. — The prothallium, like a simple
liverwort, is a dorsi ventral body, and puts out numerous

Fig. 112. Stag-horn fern (Plalycerium grande), an epiphytic tropical form, showing
the two forms of leaves: a and b, young sterile leaves; c, leaves bearing spo-
rangia ; d, an old sterile leaf. — Caldwell.

rhizoids from its ventral surface (Fig. 111). It is so thin
that all the cells contain chlorophyll, and it is usually short-
lived. In rare cases it becomes quite large and permanent,

Fig. 113. Archegoniuru of Pteris at the time of fertilization, showing tissue of gam-
etophyte (A), the cells forming the neck (B), the passageway formed by the dis-
organization of the canal cells (C), and the egg (Z» lying exposed in the venter.
— Caldwell.

Fig. 114. Antheridium of Pteris (B), showing wall cells (</), opening for escape of
sperm mother cells (e), escaped mother cells (c), sperms free from mother cells (6),
showing spiral and multiciliate character. — Caldwell.

134

PLANT STKUCTUKES

being a conspicuous object in connection with the sporo-
phyte.

At the bottom of the conspicuous notch in the prothal-
lium is the growing point,
representing the apex of the
plant. This notch is always
a conspicuous feature.

The antheridia and arch-
egonia are usually developed
on the under surface of the
prothallium (Fig. Ill, A),
and differ from those of all
Bryophytes, except the An-
tlwceros forms, in being sunk
in the tissue of the prothal-
lium and opening on the sur-

Pig. 115. Development of gametophyte
of Pteris: the figure to the left shows
the old spore ( B), the rhizoid ( C), and
the t'nallus (A); that to the right is
older, showing the same parts, and
also the apical cell (B). — Caldwell.

Fig. 116. Young gametophyte of Pteris,
showing old spore wall ( /?), rhizoids
(£')> apical cell (D), a young anther-
idium (E), and an older one in which
sperms have organized (F). — Cald-
well.

PTEKIDOPHYTES

135

face, more or less of the neck of the archegonium projecting
(Fig. 113). The eggs are not different from those formed
within the archegonia of Bryophytes, but the sperms are
very different. The Bryophyte sperm has a small body and
two long cilia, while the Pteridophyte sperm has a long
spirally coiled body, blunt behind and tapering to a point in
front, where numerous cilia are developed (Fig. 114). It
is, therefore, a large, spirally-coiled, multiciliate sperm, and
is quite characteristic of all Pteridophytes excepting the
Club-mosses. It is evident that a certain amount of water
is necessary for fertilization — in fact, it is needed not only

Fig. 117. Sections of portions of the gametophyte of Pteris, showing development
of archegonium: A, young stage, showing cells which develop the neck (a), and
the cell from which the egg cell and canal cells develop (6); B, an older stage,
showing neck cells (a), neck canal cell (6), and cell from which is derived the egg
cell, and the ventral canal cell (c); C, a still older stage, showing increased num-
ber of neck cells (a), two neck canal cells (6), the ventral canal cell (c), and the
cell in which the egg is organized (d).— Caldwell.

by the swimming sperm, but also to cause the opening of
the antheridium and of the archegonium neck. There
seems to be a relation between the necessity of water for
fertilization and a prostrate, easily moistened gametophyte.
Prothallia are either monoecious or dioecious (see § 69).
When the prothallia are developing (Fig. 115) the anther-

Fig. 118. A fern (Aspidivm), showing three large branching leaves coming from a
horizontal subterranean stem (rootstock): young leaves are also shown, which
show circinate vernation. The stem, young leaves, and petioles of the large
leaves are thickly covered with protecting hairs. The stem gives rise to numerous
small roots from its lower surface. The figure marked 3 represents the under sur-
face of a portion of the leaf, showing seven sori with shield-like indusia; at 5 is
represented a section through a sorus. showing the sporangia attached and pro-
tected by the indusium; while at 6" is represented a single sporangium opening
and discharging its spores, the heavy annulus extending along the back and over
the top.— After Wossidlo.

TTERIDOrHYTES

137

idia begin to appear very early (Fig. 116), and later the
archegonia (Fig. 117). If the prothallium is poorly nour-
ished, only antheridia appear ; it needs to be well developed
and nourished to develop archegonia. There seems to be
a very definite relation, therefore, between nutrition and
the development of the two sex organs, a fact which must
be remembered in connection with the development of
heterospory.

79. The sporophyte. — This complex body is differentiated
into root, stem, and leaf, and is more highly organized
than any plant body heretofore mentioned (Fig. 118). The
development of this body and its three great working regions
must be considered separately.

(1) Development of embryo. — The oospore, from which
the sporophyte develops, rests in the venter of the arche-
gonium, which at this stage resembles a depression in the

Pig. 119. Embryos of a common fern {Pterin): A, young embryo, showing direction
of basal wall (/), and of second walls (II), which organize quadrants, each of
which subsequently develops into foot (/), root (w), leaf (b), and stem (*■); B, an
older embryo, in which the four regions (lettered as in A) have developed further,
also showing venter of archegonium (aw), and some tissue of the prothallium (pr).
—A after Kienitz-CJerlopp; B after IIofmeister.

lower surface of the prothallium (Fig. 119, B). It germi-
nates at once, as in Bryophytes, not being a resting spore
as in Green Algae. The resting stage, as in the Bryophytes,

J38 PLANT STRUCTURES

is in connection with the asexual spores, which may be
kept for a long time and then germinated.

The first step in germination is for the oospore to di-
vide into two cells, forming a two-celled embryo. In the
ordinary Ferns this first dividing wall is at right angles to
the surface of the prothallium, and is called the basal wall
(Fig. 119, A). One of the two cells, therefore, is anterior
(toward the notch of the prothallium), and the other is
posterior.

The two cells next divide by forming walls at right
angles to the basal wall, and a four-celled embryo is the
result. This is called the "quadrant stage" of the em-
bryo, as each one of the four cells is like the quadrant of a
sphere.

With the appearance of the quadrant, four body regions
are organized, each cell by its subsequent divisions giving
rise to a distinct working region (Fig. 119, A). Two of the
cells are inner (away from the substratum) ; also one of the
inner and one of the outer (toward the substratum) cells
are anterior ; while the two other inner and outer cells are
posterior. The anterior outer cell develops the first leaf of
the embryo, generally called the cotyledon (Fig. 119, b) ; the
anterior inner cell develops the stem (Fig. 119, s) ; the pos-
terior outer cell develops the first (primary) root (Fig.
119, w) ; the posterior inner cell develops a special organ
for the use of the embryo, called the foot (Fig. 119, /).
The foot remains in close contact with the prothallium and
absorbs nourishment from it for the young embryo. When
the young sporophyte has developed enough to become in-
dependent the foot disappears. It is therefore spoken of
as a temporary organ of the embryo. It is necessary for the
leaf to emerge from beneath the prothallium, and it may
be seen usually curving upward through the notch. The
other parts remain subterranean.

(2) The root. — The primary root organized by one of
the quadrants of the embryo is a temporary affair (Figs.

PTERIDOPHYTES 139

111, 119), as it is in an unfavorable position in reference to
the dorsiventral stem, which puts out a series of more favor-
ably placed secondary roots into the soil (Fig. 118). The
mature leafy sporophyte, therefore, has neither foot nor
primary root, the product of two of the quadrants of the
embryo having disappeared.

The secondary roots put out by the stem are small, and
do not organize an extensive system, but they are interest-
ing as representing the first appearance of true roots, which
therefore come in with the vascular system. In the lower
groups the root function of absorption is not assumed by
any special organ, unless rhizoids sometimes absorb ; but
true roots are complex in structure and contain vessels.

(3) The stem. — In most of the Ferns the stem is sub-
terranean and dorsiventral (Fig. 118), but in the "tree
ferns " of the tropics it forms an erect, aerial shaft bearing
a crown of leaves (Fig. 120). In the other groups of Pteri-
dophytes there are also aerial stems, both erect and pros-
trate. The stem is complex in structure, the cells being
organized into different " tissue systems," prominent among
which is the vascular system. These tissue systems of vas-
cular plants are described in Chapter XV.

The appearance of the vascular system in connection
with the leafy sporophyte is worthy of note. The leaves
are special organs for chlorophyll work, and must receive
the raw material from air and soil or water. The leaves
of the moss gametophyte are very small and simple affairs,
and can be supplied with material by using very little ap-
paratus. In the leafy sporophyte, however, the leaves are
very prominent structures, capable of doing a great deal
of work. To such working structures material must be
brought rapidly in quantity, and manufactured food ma-
terial must be carried away, and therefore a special con-
ducting apparatus is needed. This is supplied by the vas-
cular system. These vessels extend continuously from root-
tips, through the stem, and out into the leaves, where they

Fie. 120. A group of tropical plants. To the left of the center is a tree fern, with its
slender columnar stem and crown of large leaves. The large-leaved plants to the
right are bananas (Monocotyledons).— From " Plant Relations."

PTEKIDOPHYTES

141

are spoken of as "leaf veins." Large working leaves and
a vascular system, therefore, belong together and appear
together; and the vascular plants are also the plants with
leafy sporophytes.

(4) The leaf. — Leaves are devices for spreading out
green tissue to the light, and in the Ferns they are usually
large. There is a stalk-like portion (petiole) which rises
from the subterranean stem, and a broad expanded portion
(blade) exposed to the light and air (Fig. 118). In Ferns
the blade is usually much branched, being cut up into
segments of various sizes and forms.

The essential structure consists of an expansion of
green tissue (mesophyll), through which strands of the
vascular system (veins) branch, forming a supporting
framework, and over all a compact layer of protecting
cells (epidermis). A surface
view of the epidermis shows
that it is pierced by numer-
ous peculiar pores, called
stomata, meaning " mouths."
The surface view of a stoma
shows two crescentic cells
(guard cells) in contact at
the ends and leaving be-
tween them a lens-shaped
opening (Fig. 121).

A cross-section through
a leaf gives a good view of
the three regions (Fig. 122).
Above and below is the col-
orless epidermis, pierced
here and there by stomata ;
between the epidermal lay-
ers the cells of the mesophyll are packed; and among
the mesophyll cells there may be seen here and there the
cut ends of the veins. The leaf is usually a dorsiventral
28

Fig. 121. Some epidermal cells from
leaf of Pteris, showing the inter-
locking walls and three stomata, the
guard cells containing chloroplasts.
— Land.

142

PLANT STEUCTUEES

organ, its two surfaces being differently related to light.
To this different relation the mesophyll cells respond in
their arrangement. Those in contact with the upper epi-
dermis become elongated and set endwise close together,
forming the palisade tissue; those below are loosely ar-

Fio. 122. Cross-section through a portion of the leaf of Pteris, showing the heavy-
walled epidermis above and below, two stomata in the lower epidermis (one on
each side of the center) opening into intercellular passages, the mesophyll cells
containing chloroplasts, the upper row arranged in palisade fashion, the other
cells loosely arranged (spongy mesophyll) and leaving large intercellular passages,
and in the center a section of a veinlet (vascular bundle), the xylem being repre-
sented by the central group of heavy-walled cells.— Land.

ranged, leaving numerous intercellular spaces, forming
the spongy tissue. These spaces form a system of inter-
cellular passageways among the working mesophyll cells,
putting them into communication with the outer air
through the stomata. The freedom of this communication

PTERIDOPHYTES 143

is regulated by the guard cells of the stomata, which come
together or shrink apart as occasion requires, thus dimin-
ishing or enlarging the opening between them. The sto-
mata have well been called "automatic gateways" to the
system of intercellular passageways.

One of the peculiarities of ordinary fern leaves is
that the vein system branches dichotomously, the forking
veins being very conspicuous (Figs. 123-126). Another
fern habit is that the leaves in expanding seem to unroll
from the base, as though they had been rolled from the
apex downward, the apex being in the center of the roll
(Fig. 118). This habit is spoken of as circinate, from a
word meaning "circle" or "coil," and circinate leaves
when unrolling have a crozier-like tip. The arrangement
of leaves in bud is called vernation ("spring condition"),
and therefore the Ferns are said to have circinate verna-
tion. The combination of dichotomous venation and cir-
cinate vernation is very characteristic of Ferns.

80. Sporangia. — Among Thallophytes sporangia are usu-
ally simple, mostly consisting of a single mother cell ; among
Bryophytes simple sporangia do not exist, and in connec-
tion with the usually complex capsule of the sporogonium
the name is dropped ; but among Pteridophytes distinct
sporangia again appear. They are not simple mother cells,
but many-celled bodies. Their structure varies in different
groups of Pteridophytes, but those of ordinary Ferns may
be taken as an illustration.

The sporangia are borne by the leaves, generally upon
the under surface, and are usually closely associated with
the veins and organized into groups of definite form, known
as sort. A sorus may be round or elongated, and is usually
covered by a delicate flap (indusium) which arises from the
epidermis (Figs. 118, 123, 124). Occasionally the sori are
extended along the under surface of the margin of the leaf,
as in maidenhair fern (Adiantum), and the common brake
(Pteris), in which case they are protected by the inrolled

Fig. 12.3. Fragrant shield fern (.Aspid-

ium fragrans), showing general
habit, and to the left (a) the under
surface of a leaflet bearing sori
covered by shield-like indusia.—
After Marion Satterlee.

Fig. 124. The bladder fern ( Cyst&ptt its bulb-
ifera), showing general habit, and to the
right (a) the under surface of a leaflet,
showing the dichotomous venation, and
five sori protected by pouch-like indusia.
—After Marion Satterlee.

PTEKLDOPHYTES

145

margin (Figs. 125, 126), which may be called a "false in-
dusium."

It is evident that such leaves are doing two distinct
kinds of work — chlorophyll work and spore formation.
This is true of most of the ordinary Ferns, but some of
them show a tendency to di-
vide the work. Certain leaves,
or certain leaf-branches, pro-
duce spores and do no chloro-
phyll work, while others do
chlorophyll work and produce
no spores. This differentia-
tion in the leaves or leaf-re-
gions is indicated by appro-
priate names. Those leaves
which produce only spores are
called sporophylls, meaning
"spore leaves," while the leaf
branches thus set apart are
called sporophyll branches.
Those leaves which only do
chlorophyll work are called fo-
liage leaves ; and such branch-
es are foliage branches. As
sporophylls are not called upon
for chlorophyll work they often

become much modified, being much more compact, and not
at all resembling the foliage leaves. Such a differentiation
may be seen in the ostrich fern and sensitive fern ( Onoclea)
(Figs. 127, 128), the climbing fern (Lygodium), the royal
fern (Osmunda), the moonwort (Botrycliium) (Fig. 129),
and the adder's tongue {Ophioglossum) (Fig. 130).

An ordinary fern sporangium consists of a slender stalk
and a bulbous top which is the spore case (Fig. 118, 6).
This case has a delicate wall formed of a single layer of
cells, and extending around it from the stalk and nearly to

Fig. 135. Leaflets of two common
ferns : A, the common brake
{Pterin); B, maidenhair (Adian-
tum); both showing sori borne
at the margin and protected by
the infolded margin, which thus
forms a false indusium. — Cald-
well.

146 PLANT STRUCTDKES

the stalk again, like a meridian line about a globe, is a row of
peculiar cells with thick walls, forming a heavy ring, called
the annitlus. The annulus is like a bent spring, and when
the delicate wall becomes yielding the spring straightens
violently, the wall is torn, and in the recoil the spores are
discharged with considerable force (Fig. 131). This dis-

Fig. 126.— The purple cliff brake (Pelkea atropurpurea), showing general habit, and
at a a single leaflet showing the dichotomous venation and the infolded margin
covering the eori.— After Marion Satterlee.

charge of fern spores may be seen by placing some sporangia
upon a moist slide, and under a low power watching them
as they dry and burst.

Within this sporangium the archesporium (see § 66)
consists of a single cell, which by division finally produces

PTEEIDOPHYTES

147

numerous mother cells, in each of which a tetrad of spores
is formed. The disorganization of the walls of the mother

sUsLAMAJV

Fig. 127. The ostrich fern ( Onoctea strutkiopteris), showing differentiation of foliage
leaf (a) and sporophyll (6).— After Marion Satterlee.

cells sets the spores free in the cavity of the sporangium,
and ready for discharge.

Fig. 128. The sensitive fern (Onoclea senHbilis), showing differentiation of foliage
leaves and sporophylls. — From "Field, Forest, and Wayside Flowers."

PTERIDOPHYTES

149

Among the Bryophytes the sporogenous tissue appears
very early in the development of the sporogonium, the pro-
duction of spores being its only function ; also there is a

At

^

^

Fig. 129. A moonwort (Botrychi-
tiDi), showing the leaf differen-
tiated into foliage and sporophyll
branches.— After Strasburger.

Fig. 130. The adder's tongue (Ophiogloamm
vulgatum), showing two leaves, each
with a foliage branch and a much longer
sporophyll branch.— After Marion Sat-

TERLEE.

150

PLANT STRUCT ORES

tendency to restrict the sporogenous tissue and increase the
sterile tissue. It will be observed that with the introduc-
tion of the leafy sporophyte among the Pteridophytes the
sporangia appear much later in its development, sometimes
not appearing for several years, as though they are of

Fig. 131. A series showing the dehiscence of a fern sporangium, the rupture of the
wall, the straightening anil bending back of the annulus, and the recoil.— After
Atkinson.

secondary importance as compared with chlorophyll work ;
and that the sporogenous tissue is far more restricted, the
sporangia forming a very small part of the bulk of the
sporophyte body.

PTERIDOPHYTES 15}

81. Heterospory. — This phenomenon appears first among
Pteridophytes, but it is not characteristic of them, being en-
tirely absent from the true Ferns, which far outnumber all
other Pteridophytes. Its chief interest lies in the fact that
it is universal among the Spermatophytes, and that it rep-
resents the change which leads to the appearance of that
high group. It is impossible to understand the greatest
group of plants, therefore, without knowing something
about heterospory. As it begins in simple fashion among
Pteridophytes, and is probably the greatest contribution
they have made to the evolution of the plant kingdom,
unless it be the leafy sporophyte, it is best explained here.

In the ordinary Ferns all the spores in the sporangia
are alike, and when they germinate each spore produces a
prothallium upon which both antheridia and archegonia
appear. It has been remarked, however, that some pro-
thallia are dioecious — that is, some bear only antheridia
and others bear only archegonia. In this case it is evident
that the spores in the sporangium, although they may ap-
pear alike, produce different kinds of prothallia, which
may be called male and female, as each is distinguished by
the sex organ which it produces. As archegonia are only
produced by well-nourished prothallia, it seems fair to sup-
pose that the larger spores will produce female prothallia,
and the smaller ones male prothallia.

This condition of things seems to have developed finally
into a permanent and decided difference in the size of the
spores, some being quite small and others relatively large,
the small ones producing male gametophytes (prothallia
with antheridia), and the large ones female gametophytes
(prothallia with archegonia). When asexual spores differ
thus permanently in size, and give rise to gametophytes of
different sexes, we have the condition called heterospory
(" spores different "), and such plants are called heterospo-
rous (Fig. 139). In contrast with heterosporous plants, those
in which the asexual spores appear alike are called homos-

152 PLANT STRUCTURES

porous, or sometimes isosporous, both terms meaning
"spores similar." The corresponding noun form is liomos-
pory or isospory. Bryophytes and most Pteridophytes are
homosporous, while some Pteridophytes and all Spermato-
phytes are heterosporous.

It is convenient to distinguish by suitable names the
two kinds of asexual spores produced by the sporangia of
heterosporous plants (Fig. 139). The large ones are called
megaspores, or by some writers macrospores, both terms
meaning " large spores"; the small ones are called micro-
spores, or " small spores." It should be remembered that
megaspores always produce female gametophytes, and mi-
crospores male gametophytes.

This differentiation does not end with the spores, but
soon involves the sporangia (Fig. 139). Some sporangia
produce only megaspores, and are called megasporangia ;
others produce only microspores, and are called microspo-
rangia. It is important to note that while microsporangia
usually produce numerous microspores, the megasporangia
produce much fewer megaspores, the tendency being to
diminish the number and increase the size, until finally
there are megasporangia which produce but a single large
functioning megaspore.

The differentiation goes still further. If the sporangia
are born upon sporophylls, the sporophylls themselves may
differentiate, some bearing only megasporangia, and others
only microsporangia, the former being called megasporo-
phylls, the latter microsporophylls. In such a case the
sequence is as follows : megasporophylls produce megaspo-
rangia, which produce megaspores, which in germination
produce the female gametophytes (prothallia with archego-
nia) ; while the microsporophylls produce microsporangia,
which produce microspores, which in germination produce
male gametophytes (prothallia with antheridia).

A formula may indicate the life history of a heteros-
porous plant. The formula of homosporous plants with

PTER1D0PHYTES

153

alternation of generations (Bryophytes and most Pterido-
phytes) was given as follows (§ 62) :

G=g > o— S— o— G=8 > o— S— o— G=g > o— S, etc.

In the case of heterosporous plants (some Pteridophytes
and all Spermatophytes) it would be modified as follows :
S=8>o— S=8=g=g>o— S=8=g=8>o— S, etc.

In this case two gametophytes are involved, one pro-
ducing a sperm, the other an egg, which fuse and form the
oospore, which in germination produces the sporophyte,
which produces two kinds of asexual spores (megaspores
and microspores), which in germination produce the two
gametophytes again.

One additional fact connected with heterospory should
be mentioned, and that is the great reduction of the gam-
etophyte. In the homosporous ferns the spore develops
a small but free and independent prothallium which pro-
duces both sex organs. When in heterosporous plants this
work of producing sex organs is divided between two gam-
etophytes they become very much reduced in size and lose
their freedom and independence. They are so small that
they do not escape entirely, if at all, from the embrace of
the spores which produce them, and are mainly dependent
for their nourishment upon the food stored up in the spores
(Figs. 140, 141). As the spore is produced by the sporo-
phyte, heterospory brings about a condition in which the
gametophyte is dependent upon the sporophyte, an exact
reversal of the condition in Bryophytes.

The relative importance of the gametophyte and the
sporophyte throughout the plant kingdom may be roughly
indicated by the accompanying diagram, in which the

shaded part of the parallelogram represents the gameto-
phyte and the unshaded part the sporophyte. Among the

154

PLANT STKUCTUKES

lowest plants the gametophyte is represented by the whole
plant structure. When the sporophyte first appears it is
dependent upon the gametophyte (some Thallophytes and
the Bryophytes), and is relatively inconspicuous. Later
the sporophyte becomes independent (most Pteridophytes),
the gametophyte being relatively inconspicuous. Finally
(heterosporous Pteridophytes) the gametophyte becomes
dependent upon the sporophyte, and in Spermatophytes is
so inconspicuous and concealed that it is only observed by
means of laboratory appliances, while the sporophyte is the
whole plant of ordinary observation.

CHAPTER X

THE GREAT GROUPS OF PTERIDOPHYTES

82. The great groups. — At least three independent lines
of Pteridophytes are recognized : (1) Filicales (Ferns),
(2) Equisetales (Scouring rushes, Horsetails), and (3) Ly-
copodiales (Club-mosses). The Ferns are much the most
abundant, the Club-mosses are represented by a few hun-
dred forms, while the Horsetails include only about twenty-
five species. These three great groups are so unlike that
they hardly seem to belong together in the same division
of the plant kingdom.

Filicales {Ferns)

83. General characters. — The Ferns were used in the
preceding chapter as types of Pteridophytes, so that little
need be added. They well deserve to stand as types, as
they contain about four thousand of the four thousand five
hundred species belonging to Pteridophytes. Although
found in considerable numbers in temperate regions, their
chief display is in the tropics, where they form a striking
and characteristic feature of the vegetation. In the trop-
ics not only are great masses of the low forms to be seen,
from those with delicate and filmy moss like leaves to those
with huge leaves, but also tree forms with cylindrical
trunks encased by the rough remnants of fallen leaves and
sometimes rising to a height of thirty-five to forty-five
feet, with a great crown of leaves fifteen to twenty feet
long (Fig. 120).

155

THE GREAT GROUPS OF PTERIDOPHYTES 157

There are also epiphytic forms (air plants) — that is,
those which perch " upon other plants " but derive no
nourishment from them (Fig. 112). This habit belongs
chiefly to the warm and moist tropics, where the plants
can absorb sufficient moisture from the air without send-
ing roots into the soil. In this way many of the tropical
ferns are found growing upon living and dead trees and
other plants. In the temperate regions the chief epi-
phytes are Lichens, Liverworts, and Mosses, the Ferns be-
ing chiefly found in moist woods and ravines (Fig. 132),
although a number grow in comparatively dry and exposed
situations, sometimes covering extensive areas, as the com-
mon brake (Pteris) (Fig. 125).

The Filicales differ from the other groups of Pterido-
phytes chiefly in having few large leaves, which do chloro-
phyll work and bear sporangia. In a few of them there is a
differentiation of functions in foliage branches and sporo-
phyll branches (Figs. 127-130), but even this is excep-
tional. Another distinction is that the stems are un-
branched.

84. Origin of sporangia. — An important feature in the
Ferns is the origin of the sporangia. In some of them a
sporangium is developed from a single epidermal cell of
the leaf, and is an entirely superficial and generally stalked
affair (Fig. 118, 5) ; in others the sporangium in its devel-
opment involves several epidermal and deeper cells of the
leaf, and is more or less of an imbedded affair. In the first
case the ferns are said to be leptosporangiate ; in the sec-
ond case they are eusporangiate.

The leptosporangiate Ferns are overwhelmingly abun-
dant as compared with the Eusporangiates. Back in the
Coal-measures, however, there was an abundant fern vege-
tation which was probably all eusporangiate. The Lep-
tosporangiates seem to be the modern Ferns, the once
abundant Eusporangiates being represented now in the
temperate regions only by such forms as moonwort (Bo-
29

158

PLANT STKDCTUKES

trychium) (Fig. 129) and adder's tongue (Ophioglossum)
(Fig. 130). It is important to note, however, that the
Horsetails and Club-mosses are Eusporangiates, as well as
all the Seed-plants.

Another small but interesting group of Ferns includes
the "Water-ferns," floating forms or sometimes on muddy
flats. The common Marsilia may be taken as a type (Fig.

133). The slender creeping stem
sends down numerous roots into
the mucky soil, and at intervals
gives rise to a comparatively large
leaf. This leaf has a long erect
petiole and a blade of four spread-

•• ■, ft HIV

Fig. 133. — A water-fern (Marsilia),
showing horizontal stem, with
descending roots, and ascend-
ing leaves ; a, a yonng leaf
showing circinate vernation ;
s,«,sporophyll branches ("spo-
rocarps ").— After Bischofp.

J?

Fio. 134. One of the floating water-ferns (Sal-
vinia), showing side view (.4) and view from
above (2?). The dangling root-like processes
are the modified submerged leaves. In A,
near the top of the cluster of submerged
leaves, some sporophyll branches ("sporo-
carps") may be seen. — After Bischofp.

ing wedge-shaped leaflets like a "four-leaved clover."
dichotomous venation and circinate vernation at once
gest the fern alliance. From near the base of the petiole

The
sug-

THE GREAT GROUPS OF PTERIDOPHYTES 159

another leaf branch arises, in which the blade is modified
as a sporophyll. In this case the sporophyll incloses the
sporangia and becomes hard and nut-like. Another com-
mon form is the floating Salvinia (Fig. 134). The chief
interest lies in the fact that the water-ferns are heteros-
porous. As they are leptosporangiate they are thought
to have been derived from the ordinary leptosporangiate
Ferns, which are homosporous.

Three fern groups are thus outlined : (1) homosporous-
eusporangiate forms, now almost extinct ; (2) homosporous-
leptosporangiate forms, the great overwhelming modern
group, not only of Filicales but also of Pteridophytes, well
called true Ferns, and thought to be derived from the pre-
ceding group ; and (3) heterosporous-leptosporangiate
forms, the water-ferns, thought to be derived from the pre-
ceding group.

Equisetales ( Horsetails or Scouring rushes)

85. General characters. — The twenty-five forms now rep-
resenting this great group belong to a single genus (Equise-
ttim, meaning " horsetail "), but they are but the linger-
ing remnants of an abundant flora which lived in the time
of the Coal-measures, and helped to form the forest vegeta-
tion. The living forms are small and inconspicuous, but
very characteristic in appearance. They grow in moist or
dry places, sometimes in great abundance (Fig. 135).

The stem is slender and conspicuously jointed, the joints
separating easily ; it is also green and fluted with small
longitudinal ridges ; and there is such an abundant deposit
of silica in the epidermis that the plants feel rough. This
last property suggested its former use in scouring, and its
name " scouring rush." At each joint is a sheath of minute
leaves, more or less coalesced, the individual leaves some-
times being indicated only by minute teeth. This arrange-
ment of leaves in a circle about the joint is called the cyclic

Fig. 135. Equisetum arvense, a common horsetail: /, three fertile shoots rising from
the dorsiventral stem, showing the cycles of coalesced scale-leaves at the joints
and the terminal strobili with numerous sporophylls, that at a being mature; 2,
a sterile shoot from the same stem, showing branching; 3, a single peltate sporo-
phyll bearing sporangia; U, view of sporophyll from beneath, showing dehiscence
of sporangia; 5, 6, 7. spores, showing the unwinding of the outer coat, which aids
in dispersal. —After Wossidlo.

THE GEEAT GROUPS OE PTERIDOPHYTES

161

arrangement, or sometimes the whorled arrangement, each
such set of leaves being called a cycle or a whorl. These
leaves contain no chlorophyll and have evidently abandoned
chlorophyll work, which is carried on by the green stem.
Such leaves are known as scales, to distinguish them from
foliage leaves. The stem is either simple or profusely
branched (Fig. 135).

86. The strobilus. — One of the distinguishing characters
of the group is that chlorophyll-work and spore-formation
are completely differentiated. Although the foliage leaves

Fig. 136. Dicecious gainetophytes of Equisetum : A, the female gametophyte, show-
ing branching, rhizolds, and an archegonium {ar)\ B, the male gametophyte,
showing several antheridia ( <5 ).— After Campbell.

are reduced to scales, and the chlorophyll-work is done by
the stem, there are well-organized sporophylls. The sporo-
phylls are grouped close together at the end of the stem in
a compact conical cluster which is called a strobilus, the
Latin name for "pine cone," which this cluster of sporor
phylls resembles (Fig. 135).

Each sporophyll consists of a stalk-like portion and a
shield-like (peltate) top. Beneath the shield hang the

162 PLANT STKDCTUKES

sporangia, which produce spores of but one kind, hence
these plants are homosporous ; and as the sporangia origi-
nate in eusporangiate fashion, Equisetum has the homospo-
rous-eusporangiate combination shown by one of the Fern
groups. It is interesting to know, however, that some of
the ancient, more highly organized members of this group
were heterosporous, and that the present forms have
dioecious gametophytes (Fig. 136).

Lycopodiales ( Club-mosses)

87. General characters. — This group is now represented
by about five hundred species, most of which belong to
the two genera Lycopodium and Selaginella, the latter
being much the larger genus. The plants have slender,
branching, prostrate, or erect stems completely clothed
with small foliage leaves, having a general moss-like
appearance (Fig. 137). Often the erect branches are
terminated by conspicuous conical or cylindrical strobili,
which are the " clubs " that enter into the name ". Club-
mosses." There is also a certain kind of resemblance
to miniature pines, so that the name " Ground-pines " is
sometimes used.

Lycopodiales were once much more abundant than now,
and more highly organized, forming a conspicuous part of
the forest vegetation of the Coal-measures.

One of the distinguishing marks of the group is that the
sperm does not resemble that of the other Pteridophytes,
but is of the Bryophyte type (Fig. 140, F). That is, it
consists of a small body with two cilia, instead of a large
spirally coiled body with many cilia. Another distinguish-
ing character is that there is but a single sporangium pro-
duced by each sporophyll (Fig. 137). This is in marked
contrast with the Filicales, whose leaves bear very numer-
ous sporangia, and with the Equisetales, whose sporophylls
bear several sporangia.

THE GREAT GROUPS OF PTERIDOPHYTES

163

Fig. 137. A common club-moss {Lycopodimn clavatum): 1, the whole plant, showing
horizontal stem giving rise to roots and to erect branches bearing strobili; 2, a
single sporophyll with its sporangium; 3, spores, much magnified.— After Wos-

88. Lycopodimn. — This genus contains fewer forms than
the other, but they are larger and coarser and more charac-
teristic of the temperate regions, being the ordinary Club-
mosses (Fig. 137). They also more commonly display
conspicuous and distinct strobili, although there is every

164

PLANT STRUCTUKES

gradation between ordinary foliage leaves and distinct
sporophylls.

The sporangia are borne either by distinct sporophylls
or by the ordinary foliage leaves near the summit of the
stem. At the base of each of these leaves, or sporophylls,
on the upper side, is a single sporangium (Fig. 137). The
sporangia are eusporangiate in origin, and as the spores are
all alike, Lycopodium has the same homosporous-eusporan-
giate combination noted in Equisetales and in one of the
groups of Filicales.

89. Selaginella. — This large genus contains the smaller,
more delicate Club-mosses, often being called the " little
Club-mosses." They are especially displayed in the trop-

Fia. 138. Selaginella, showing general spray-like habit, and dangling leafless stems
which strike root (rhizophores). — From " Plant Relations.1'

ics, and are common in greenhouses as delicate, mossy,
decorative plants (Fig. 138). In general the sporophylls
are not different from the ordinary leaves (Fig. 139), but
sometimes they are modified, though not so distinct as in
certain species of Lycopodium

THE GREAT GROUPS OF PTERIDOPHYTES

165

The solitary sporangium appears in the axils (upper
angles formed by the leaves with the stem) of the leaves
and sporophylls, but arise from the stem instead of the

Fig. 139. Sdaginella Martensii: A, branch bearing strobili; B, a microsporophyll
with a microsporangium, showing microspores through a rupture in the wall; C,
a megasporophyll with a megasporangium ; D, megaspores : E, microspores.—

GOLDBEKGER.

166

PLANT STRUCTURES

leaf (Fig. 139). This is important as showing that sporan-
gia may be produced by stems as well as by leaves, those
being produced by leaves being called foliar, and those by
stem cauline.

The most important fact in connection with Selaginella,
however, is that it is heterosporous. Megasporangia, each
usually containing but four megaspores, are found in the
axils of a few of the lower leaves of the strobilus, and more
numerous microsporangia occur in the upper axils, con-
taining very many microspores (Fig. 139). The character
of the gametophytes of heterosporous Pteridophytes may
be well illustrated by those of Selaginella.

The microspore germinates and forms a male gameto-
phyte so small that it is entirely included within the spore

w J0

Fig. 140. Male gametophyte of Selaginella: in each case p is the prothallial cell, w
the wall cells of the antheridium, s the sperm tissue: F, the biciliate sperms.—
After Bklajepp.

wall (Fig. 140). A single small cell is all that represents
the ordinary cells of the prothallium, while all the rest is
an antheridium, consisting of a wall of a few cells sur-
rounding numerous sperm mother cells. In the presence

THE GREAT GROUPS OF PTERIDOPHYTES

167

of water the antheridium wall breaks down, as also do the
walls of the mother cells, and the small biciliate sperms
are set free.

The much larger megaspores germinate and become
filled with a mass of numerous nutritive cells, representing
the ordinary cells of a prothallium (Fig. 141). The spore
wall is broken by this growing prothallium, a part of which
thus protrudes and becomes exposed, although the main
part of it is still invested by the old megaspore wall. In
this exposed portion
of the female gameto-
phyte the archegonia
appear, and thus be-
come accessible to the
sperms. In the case
of Isoetes (see § 90)
the reduction of the
female gametophyte is
even greater, as it does
not project from the
megaspore wall at all,
and the archegonia
are made accessible
through cracks in the
wall immediately over
them.

The embryo of Se-
laginella is also impor-
tant to consider. Be-
ginning its development in the venter of the archegonium,
it first lies upon the exposed margin of the prothallium,
while the mass of nutritive cells lie deep within the mega-
spore (Fig. 141, embv embj. It first develops an elongated
cell, or row of cells, which thrusts the embryo cell deeper
among the nutritive cells. This cell or row of cells, formed
by the embryo to place the real embryo cell in better rela-

spm

Pig. 141. Female gametophyte of a Selaginella :
spm, wall of megaspore ; pr, gametophyte ;
ar, an archegonium ; embx and emb^ em-
bryo sporophytes ; et, suspensors ; the gam-
etophyte has developed a few rhizoids.—
After Pfepfeb.

168

PLANT STRUCTURES

tion to its food supply, is called the suspensor, and is a
temporary organ of the embryo (Figs. 141, 142, et). At
the end of the suspensor the real embryo develops, and
when its regions become organized it shows the following
parts : (1) a large foot buried among the nutritive cells of
the prothallium and absorbing nourishment ; (2) a root
stretching out toward the substratum ; (3) a stem extend-

Fig. 142. Embryo of Selaginella removed from the gametophyte, showing suspensor
(et), root-tip (w), foot (/), cotyledons (bl), stem-tip {sty, and ligules (lig).— After
Pfeffer.

ing in the other direction, and bearing just behind its tip
(4) a pair of opposite leaves (cotyledons) (Fig. 142).

As the sporangia of Selaginella are eusporangiate, this
genus has the heterosporous-eusporangiate combination — a
combination not mentioned heretofore, and being of special
interest as it is the combination which belongs to all the
Spermatophytes. For this and other reasons, Selaginella
is one of the Pteridophyte forms which has attracted
special attention, as possibly representing one of the an-
cestral forms of the Seed-plants.

THE GREAT GROUPS OF PTERIDOPHYTES

169

90. Isoetes. — This little group of aquatic plants, known
as "quillworts," is very puzzling as to its relationships
among Pteridophytes. By some it is put with the Ferns,
forming a distinct division of Filicales ; by others it is put

Fig. 143. A common quillwort (Isoetes lacus-
tris), showing cluster of roots dichoto-
mously branching, and cluster of leaves
each enlarged at base and inclosing a sin-
gle sporangium.— After Schenck.

Fig. 144. Sperm of Isoetes, show-
ing spiral body and seven long
cilia arising from the beak. —
After Belajeff.

with the Club-mosses, and is associated with Selaginella.
It resembles a bunch of fine grass growing in shoal water
or in mud, but the leaves enlarge at the base and overlap
one another and the very short tuberous stem (Fig. 143).
Within each enlarged leaf base a single sporangium is
formed, and the cluster contains both megasporangia and
microsporangia. The sporangia are eusporangiate, and
therefore Isoetes shares with Selaginella the distinction of

^70 PLANT STRUCTURES

having the heterosporous-eusporangiate combination, which
is a feature of the Seed-plants.

The embryo is also peculiar, and is so suggestive of the
embryo of the Monocotyledons (see § 114) among Seed-
plants that some regard it as possibly representing the
ancestral forms of that group of Spermatophytes. The
peculiarity lies in the fact that at one end of the axis of the
embryo is a root, and at the other the first leaf (cotyledon),
while the stem tip rises as a lateral outgrowth. This is
exactly the distinctive feature of the embryo of Monocoty-
ledons.

The greatest obstacle in the way of associating these
quillworts with the Club-mosses is the fact that their sperms
are of the large and spirally coiled multiciliate type which
belongs to Filicales and Equisetales (Fig. 144), and not at
all the small biciliate type which characterizes the Club-
mosses (Fig. 140). To sum up, the short unbranched stem
with comparatively few large leaves, and the coiled multi-
ciliate sperm, suggest Filicales ; while the solitary spo-
rangia and the heterosporous-eusporangiate character sug-
gest Selaginella.

CHAPTER XI

SPERMATOPHYTES : GYMNOSPERMS

91. Summary from Pteridophytes. — In considering the
important contributions of Pteridophytes to the evolution
of the plant kingdom the following seem worthy of note :

(1) Prominence of sporophyte and development of vascu-
lar system. — This prominence is associated with the display
of leaves for chlorophyll work, and the leaves necessitate
the work of conduction, which is arranged for by the vas-
cular system. This fact is true of the whole group.

(2) Differentiation of sporophylls. — The appearance of
sporophylls as distinct from foliage leaves, and their or-
ganization into the cluster known as the strobilus, are facts
of prime importance. This differentiation appears more or
less in all the great groups, but the strobilus is distinct only
in Horsetails and Club-mosses.

(3) Introduction of heterospory and reduction of gameto-
pliytes. — Heterospory appears independently in all of the
three great groups — in the water-ferns among the Fili-
cales, in the ancient horsetails among the Equisetales, and
in Selagifiella and Isoetes among Lycopodiales. All the
other Pteridophytes, and therefore the great majority of
them, are homosporous. The importance of the appear-
ance of heterospory lies in the fact that it leads to the
development of Spermatophytes, and associated with it is
a great reduction of the gametophytes, which project little,
if at all, from the spores which produce them.

92. Summary of the four groups. — It may be well in this
connection to give certain prominent characters which will

171

1^2 PLANT STRUCTURES

serve to distinguish the four great groups of plants. It
must not be supposed that these are the only characters,
or even the most important ones in every case, but they
are convenient for our purpose. Two characters are given
for each of the first three groups — one a positive character
which belongs to it, the other a negative character which
distinguishes it from the group above, and becomes the
positive character of that group.

(1) Tliallopliytes. — Thallus body, but no archegonia.

(2) Bryophytes. — Archegonia, but no vascular system.

(3) Pteridophytes. — Vascular system, but no seeds.

(4) Spermatophytes. — Seeds.

93. General characters of Spermatophytes. — This is the
greatest group of plants in rank and in display. So con-
spicuous are they, and so much do they enter into our
experience, that they have often been studied as "botany,"
to the exclusion of the other groups. The lower groups
are not meiely necessary to fill out any general view of the
plant kingdom, but they are absolutely essential to an
understanding of the structures of the highest group.

This great dominant group has received a variety of
names. Sometimes they are called Anthophytes, meaning
"Flowering plants," with the idea that they are distin-
guished by the production of " flowers." A flower is diffi-
cult to define, but in the popular sense all Spermatophytes
do not produce flowers, while in another sense the strobilus
of Pteridophytes is a flower. Hence the flower does not
accurately limit the group, and the name Anthophytes is
not in general use. Much more commonly the group is
called Phanerogams (sometimes corrupted into Phaenogams
or even Phenogams), meaning "evident sexual reproduc-
tion." At the time this name was proposed all the other
groups were called Cryptogams, meaning "hidden sexual
reproduction." It is a curious fact that the names ought
to have been reversed, for sexual reproduction is much more
evident in Cryptogams than in Phanerogams, the mistake

SPERMATOCYTES: GYMNOSPERMS 173

arising from the fact that what were supposed to be sexual
organs in Phanerogams have proved not to be such. The
name Phanerogam, therefore, is being generally abandoned ;
but the name Cryptogam is a useful one when the lower
groups are to be referred to ; and the Pteridophytes are
still very frequently called the Vascular Crypt ogams. The
most distinguishing mark of the group seems to be the
production of seeds, and hence the name Spermatophytes,
or " Seed-plants," is coming into general use.

The seed can be better defined after its development
has been described, but it results from the fact that in this
group the single megaspore is never discharged from its
megasporangium, but germinates just where it is devel-
oped. The great fact connected with the group, therefore,
is the retention of the megaspore, which results in a seed.
The full meaning of this will appear later.

There are two very independent lines of Seed-plants,
the Gymnosperms and the Anglo sperms. The first name
means "naked seeds," referring to the fact that the seeds
are always exposed ; the second means " inclosed seeds,"
as the seeds are inclosed in a seed vessel.

Gymnosperms

94. General characters. — The most familiar Gymnosperms
in temperate regions are the pines, spruces, hemlocks,
cedars, etc., the group so commonly called "evergreens."
It is an ancient tree group, for its representatives were
associated with the giant club-mosses and horsetails in
the forest vegetation of the Coal-measures. Only about
four hundred species exist to-day as a remnant of its for-
mer display, although the pines still form extensive forests.
The group is so diversified in its structure that all forms
can not be included in a single description. The common
pine (Pinus), therefore, will be taken as a type, to show
the general Gymnosperm character.
30

174 PLANT STRUCTURES

95. The plant body. — The great body of the plant, often
forming a large tree, is the sporophyte ; in fact, the
gametophytes are not visible to ordinary observation. It
should be remembered that the sporophyte is distinctly a
sexless generation, and that it develops no sex organs.
This great sporophyte body is elaborately organized for
nutritive work, with its roots, stems, and leaves. These
organs are very complex in structure, being made up of
various tissue systems that are organized for special kinds
of work. The leaves are the most variable organs, being
differentiated into three distinct kinds — (1) foliage leaves,
(2) scales, and (3) sporophylls.

96. Sporophylls. — The sporophylls are leaves set apart to
produce sporangia, and in the pine they are arranged in
a strobilus, as in the Horsetails and Club-mosses. As
the group is heterosporous, however, there are two kinds
of sporophylls and two kinds of strobili. One kind of
strobilus is made up of megasporophylls bearing mega-
sporangia ; the other is made up of microsporophylls bear-
ing microsporangia. These strobili are often spoken of as
the " flowers " of the pine, but if these are flowers, so are
the strobili of Horsetails and Club-mosses.

97. Microsporophylls. — In the pines the strobilus com-
posed of microsporophylls is comparatively small (Figs.
145, d, 164). Each sporophyll is like a scale leaf, is nar-
rowed at the base, and upon the lower surface are borne
two prominent sporangia, which of course are microspo-
rangia, and contain microspores (Fig. 146).

These structures of Seed-plants all received names
before they were identified with the corresponding struc-
tures of the lower groups. The microsporophyll was called a
stamen, the microsporangia pollen-sacs, and the microspores
pollen grains, or simply pollen. These names are still very
convenient to use in connection with the Spermatophytes,
but it should be remembered that they are simply other
names for structures found in the lower groups.

Fig. 145. Mnus Laricio, showing tip of branch bearing needle-leaves, scale-leaves,
and cones (strobili): a, very young carpellate cones, at time of pollination, borne
at tip of the young shoot upon which' new leaves are appearing; b, carpellate cones
one year old; c, carpellate cones two years old, the scales spreading and shedding
the seeds; d, young shoot bearing a cluster of staminate cones.— Caldwell.

176

PLANT STRUCTURES

The strobilus composed of microsporophylls may be
called the staminate strobilus — that is, one composed of
stamens ; it is often called the staminate cone, " cone "
being the English translation of the word "strobilus."
Frequently the staminate cone is spoken of as the "male
cone," as it was once supposed that the stamen is the

Fig. 146. Staminate cone (strobilus) of pine (Pinus): A, section of cone, showing
microsporophylls (stamens) bearing microsporangia; B, longitudinal section of a
single stamen, showing the large sporangium beneath ; C, cross-section of a sta-
men, showing the two sporangia; D, a single microspore (pollen grain) much en-
larged, showing the two wings, and a male gametophyte of two cells, the lower
and larger (wall cell) developing the pollen tube, the upper and smaller (genera-
tive cell) giving rise to the sperms.— After Strasburgeb.

male organ. This name should, of course, be abandoned,
as the stamen is now known to be a microsporophyll, which
is an organ produced by the sporophyte, which never pro-
duces sex organs. It should be borne distinctly in mind
that the stamen is not a sex organ, for the literature of
botany is full of this old assumption, and the beginner is in

SPEKMATOPHYTES : UYMNOKPERMS

177

danger of becoming confused and of forgetting that pollen
grains are asexual spores.

98. Megasporophylls— The strobili composed of mega-
sporophylls become much larger than the others, forming

Fig. 147. Firms sylvestris, showing mature cone partly sectioned, and showing car-
pels (sq, sq1, sq*) with seeds in their axils (g), in which the embryos (em) may be
distinguished; A, a young carpel with two megasporangia; B, an old carpel with
mature seeds (ch), the micropyle being below (M). — After Bessey.

the well-known cones so characteristic of pines and their
allies (Figs. 145, «, b, c, 163). Each sporophyll is some-
what leaf -like, and at its base upon the upper side are two
megasporangia (Fig. 147). It is these sporangia which are
peculiar in each producing and retaining a solitary large
megaspore. This megaspore resembles a sac-like cavity in

178

PLANT STRUCTURES

the body of the sporangium (Fig. 148, d), and was at first
not recognized as being a spore.

These structures had also received names before they
were identified with the corresponding structures of the
lower groups. The megasporophyll was called a carpel,
the megasporangia ovules, and the megaspore an embryo-
sac, because the young embryo was observed to develop
within it (Fig. 147, em).

The strobilus of megasporophylls, therefore, may be
called the carpellate strobilus or carpellate cone. As the
carpel enters into the organization of a structure known as
the pistil, to be described later, the cone is often called
the pistillate cone. As the staminate cone is sometimes
wrongly called a "male cone," so the carpellate cone is
wrongly called a "female cone," the
old idea being that the carpel with
its ovules represented the female sex
organ.

The structure of the megaspo-
rangium, or ovule, must be known.
The main body is the nucellus (Figs.
148, c, 149, nc) ; this sends out from
near its base an outer membrane
{integument) which is distinct above
(Figs. 148 b, 149 i), covering the main
part of the nucellus and projecting
beyond its apex as a prominent neck,
the passage through which to the apex
of the nucellus is called the micropyle
("little gate") (Fig. 148, a). Cen-
trally placed within the body of the
nucellus is the conspicuous cavity
called the embryo-sac (Fig. 148, d),
in reality the retained megaspore.
The relations between integument, micropyle, nucellus,
and embryo-sac should be kept clearly in mind. In the

Fig. 148. Diagram of the
carpel structures of pine,
showing the heavy scale
(A) which bears the
ovule (2?), in which are
seen the micropyle (a),
integument (&), nucellus
(c), embryo sac or mega-
spore (d). — Moorb.

SPERMAT0PI1YTES : GYMNOSPERMS

179

pine the micropyle is directed downward, toward the base
of the sporophyll (Figs. 147, 148).

99. Female gametophyte. — The female gametophyte is
always produced by the germination of a megaspore, and
therefore it should be
produced by the so-
called embryo-sac with-
in the ovule. This im-
bedded megaspore ger-
minates, just as does
the megaspore of Se-
laginetta or Isoetes, by
cell division becoming
filled with a compact
mass of nutritive tissue
representing the ordi-
nary cells of the female
prothallium (Fig. 149,
e). This prothallium
naturally does not
protrude beyond the
boundary of the mega-
spore wall, being com-
pletely surrounded by
the tissues of the
sporangium. It must
be evident that this
gametophyte is abso-
lutely dependent upon
the sporophyte for its
nutrition, and remains
not merely attached to
it, but is actually im-
bedded within its tis-
sues like an internal parasite. So conspicuous a tissue
within the ovule, as well as in the seed into which the

Fig. 149. Diagrammatic section through ovule
(megasporangium) of spruce (Picea), showing
integument (i), nucellus (nc), endosperm or
female gametophyte (e) which fills the large
megaspore imbedded in the nucellus, two
archegonia («) with short neck (c) and venter
containing the egg (o), and position of ger-
minating pollen grains or microspores (p)
whose tubes (t) penetrate the nucellus tissue
and reach the archegonia.— After Schimper.

180 PLANT STRUCTURES

ovule develops, did not escape early attention, and it was
called endosperm, meaning "within the seed." The endo-
sperm of Gymnosperms, therefore, is the female gameto-
phyte.

At the margin of the endosperm nearest the micropyle
regular flask-shaped archegonia are developed (Fig. 149, a),
making it sure that the endosperm is a female gameto-
phyte. It is evident that the necks of these archegonia
(Fig. 149, c) are shut away from the approach of sperms by
swimming, and that some new method of approach must be
developed.

100. Male gametophyte. — The microspores are developed
in the sporangium in the usual tetrad fashion, and are pro-
duced and scattered in very great abundance. It will be
remembered that the male gametophyte developed by the
microspore of Selaginella is contained entirely within the
spore, and consists of a single ordinary prothallial cell
and one antheridium (see § 89). In the pine it is no bet-
ter developed. One or two small cells appear, which may
be regarded as representing prothallial cells, while the rest
of the gametophyte seems to be a single antheridium (Fig.
146, D). At first this antheridium seems to consist of a
large cell called the wall cell, and a small one called the
generative cell. Sooner or later the generative cell divides
and forms two small cells, one of which divides again and
forms two cells called male cells, which seem to represent
the sperm mother cells of lower plants. The three active
cells of the completed antheridium, therefore, are the wall
cell, with a prominent nucleus, and two small male cells
which are free in the large wall cell.

These sperm mother cells (male cells) do not form
sperms within them, as there is no water connection be-
tween them and the archegonia, and a new method of
transfer is provided. This is done by the wall cell, which
develops a tube, known as the pollen-tube. Into this tube
the male cells enter, and as it penetrates among the cells

SPEEMATOPHYTES : GYMNOSPEEMS

181

which shut off the archegonia it carries the male cells
along, and so they are brought to the archegonia (Fig. 150).

Fig. 150. Tip of pollen tube of pine,
showing the two male cells (.4, B),
two nuclei ( C) which accompany
them, and the numerous food
granules (Z>) : the tip of the tube
is just about to enter the neck of
the archegonium.— Caldwell.

Fig. 151. Pollen tube passing through the
neck of an archegonium of spruce (Picea),
and containing near its tip the two male
nuclei, which are to be discharged into the
egg whose cytoplasm the tube is just en-
tering.— After Strasburger.

101. Fertilization. — Before fertilization can take place
the pollen-grains (microspores) must be brought as near as
possible to the female gametophyte with its archegonia.
The spores are formed in very great abundance, are dry
and powdery, and are scattered far and wide by the wind.
In the pines and their allies the pollen-grains are winged
(Fig. 146, D), so that they are well organized for wind dis-
tribution. This transfer of pollen is called pollination, and
those plants that use the wind as an agent of transfer are
said to be anemophilotis, or "wind-loving."

The pollen must reach the ovule, and to insure this it
must fall like rain. To aid in catching the falling pollen
the scale-like carpels of the cone spread apart, the pollen
grains slide down their sloping surfaces and collect in a

182

PLANT STRUCTURES

little drift at the bottom of each carpel, where the ovules
are found (Fig. 147, A, B). The flaring lips of the micro-
pyle roll inward and outward as they are dry or moist, and
by this motion some of the pollen-grains are caught and
pressed down upon the apex of the nucellus.

In this position the pollen-tube develops, crowds its
way among the cells of the nucellus, reaches the wall of
the embryo-sac, and penetrating that, reaches the necks
of the archegonia (Fig. 149, p, t) ; crowding into them
(Fig. 151), the tip of the tube opens, the male cells are

an

■

mmsm

Fig. 152. Fertilization in spruce (Picea): B is an egg, in the tip of which a pollen
tube (p) has entered and has discharged into the cytoplasm a male nucleus (m),
which is to unite with the egg (female) nucleus (on); 0, a later stage in which the
two nuclei are uniting. — After Schimper.

discharged, one male cell fuses with the egg (Fig. 152),
and fertilization is accomplished, an oospore being formed
in the venter of the archegonium.

It will be noticed that the cell which acts as a male
gamete is really the sperm mother cell, which does not
organize a sperm in the absence of a water connection.
This peculiar method of transferring the male cells by
means of a special tube developed by the antheridium is

SPEEMATOPHYTES: GYMNOSPEKMS

183

called siphonogamy, which means " sexual reproduction by
means of a tube." So important is this character among
Spermatophytes that some have proposed to call the group
Siphonogams.

102. Development of the embryo. — The oospore when
formed lies at the surface of the endosperm (female gameto-
phyte) nearest to the micropyle. As the endosperm is to
supply nourishment to the em-
bryo, this position is not the
most favorable. Therefore, as
in Selaginella, the oospore first
develops a suspensor, which in
pine and its allies becomes very
long and often tortuous (Fig.
153, A, s). At the tip of the
suspensor the cell or cells (em-
bryo cells) which are to develop
the embryo are carried (Fig. 153,
A, ha), and thus become deeply
buried, about centrally placed,
in the endosperm.

Several suspensors may start
from as many archegonia in the
same ovule, and several embryos
may begin to develop, but as a
rule only one survives, and the
solitary completed embryo (Fig.
153, B) lies centrally imbedded

in the endosperm (Fig. 153a). The development of more
than one embryo in a megasporangium (ovule) is called
polyembryony, a phenomenon natural to Gymnosperms with
their several archegonia upon a single gametophyte.

103. The seed. — While the embryo is developing some
important changes are taking place in the ovule outside
of the endosperm. The most noteworthy is the develop-
ment of a special tissue that forms a hard bony covering,

Fig. 153. Embryos of pine: A,
very young embryos (ka) at the
tips of long and contorted sus-
pensors (s) ; B, older embryo,
showing attachment to suspen-
sor («), the extensive root sheath
(tvh), root tip (ws), stem tip
(v), and cotyledons (c).— After
Strasburger.

184

PLANT STRUCTURES

known as the seed coat, or testa (Fig. 153a). The devel-
opment of this testa hermetically seals the structures with-
in, further development and activity
are checked, and the living cells pass
into the resting condition. This pro-
tected structure with its dormant cells
is the seed.

In a certain sense the seed is a transformed ovule (mega-
sporangium), but this is true only as to its outer configura-

Fig. 153a. Pine seed.

tZrZjT'

Fig. 154.

Pine seedlings, showing the long hypocotyl and the numerous cotyledons,
with the old seed case still attached.— After Atkinson.

SPERMATOPHYTES: GYMNOSPERMS

185

tion. If the internal structures be considered it is much
more. It is made up of structures belonging to three gen-
erations, as follows : (1) The old sporophyte is represented
by seed coat and nucellus, (2) the endosperm is a gameto-
phyte, while (3) the embryo is a young sporophyte. It can
hardly be said that the seed is simple in structure, or that
any real conception of it can be obtained without approach-
ing it by way of the lower groups.

The organization of the seed checks the growth of the
embryo, and this development within the seed is known as

Fig. 155. A cycad, showing the palm-like habit, with much branched leaves and
scaly stem.— From " Plant Relations."

the intra-seminal development. In this condition the em-
bryo may continue for a very long time, and it is a ques-
tion whether it is death or suspended animation. Is a seed
alive ? is not an easy question to answer, for it may be kept
in a dried-out condition for years, and then when placed
in suitable conditions awaken and put forth a living plant.

SPERMATOPHYTES : GYMNOSPERMS

187

This " awakening " of the seed is spoken of as its " ger-
mination," but this must not be confused with the germi-
nation of a spore, which is real germination. In the case
of the seed an oospore has germinated and formed an embryo,
which stops growing for a time, and then resumes it. This
resumption of growth is not germination, but is what

Fig. 157. Tip of pollen tube of Cycas rexoluta. containing the two spiral, multiciliate
sperms. — After Ikeno.

happens when a seed is said to "germinate." This second
period of development is known as the extra-seminal, for it
is inaugurated by the escape of the sporophyte from the
seed (Fig. 154).

104. The great groups of Gymnosperms. — There are at
least four living groups of Gymnosperms, and two or three

Fig. 158. A pine {Finns) showing the central shaft and also the bunching of the
needle leaves toward the tips of the branches.— From " Plant Relations."

SPERMATOPHYTES : GYMNOSPERMS

189

extinct ones. The groups differ so widely from one an-
other in habit as to show that Gymnosperms can be very
much diversified. They are all woody forms, but they may
be trailing or straggling
shrubs, gigantic trees, or
high-climbing vines ; and
their leaves may be nee-
dle-like, broad, or "fern-
like." For our purpose it
will be only necessary to
define the two most prom-
inent groups.

105. Cycads. — Cycads
are tropical, fern - like
forms, with large branched
(compound) leaves. The
stem is either a columnar
shaft crowned with a ro-
sette of great branching
leaves, with the general
habit of tree-ferns and
palms (Figs. 155, 156) ;
or they are like great tu-
bers, crowned in the same
way. In ancient times
(the Mesozoic) they were
very abundant, forming
a conspicuous feature of
the vegetation, but now
they are represented only
by about eighty forms
scattered through both
the oriental and occiden-
tal tropics.

They are very fern-
like in structure as well
31

Fig. 159. The giant redwood {Sequoia gi-
gantea) of California : the relative size
is indicated by the figure of a man stand-
ing at the right.— After Williamson.

190

PLANT STRUCTURES

as in appearance, but they produce seeds and must be
associated with Spermatophytes, and as the seed is ex-
posed they are Gymnosperms. A discovery has been made

recently that strikingly
JIkK emphasizes their fern-
like structure. In fer-
tilization a pollen-tube
develops, as described
for pine and its allies,
but the male cells
(sperm mother - cells)
which it contains or-
ganize sperms, and
these sperms are of
the coiled multiciliate
type (Fig. 157) charac-
teristic of all the Pter-
idophytes except Club-
mosses. This associa-
tion of the old ciliated
sperm habit with the
new pollen-tube habit
is a very interesting in-
termediate or transition
condition. It should be
said that these sperms
have been actually found
in but few species of
the Cycads, but there
are reasons for suppos-
ing that they may be
found in all. Another
one of the Gymnosperm

Fig. 160. An araucarian pine (Araucaria), ^ *

showing the central shaft, and the regular groups, represented tO-

cycles of branches spreading in every direc- ^ay only by the COm-
tion and bearing numerous small leaves.— . . , . ,

From " Plant Relations." monly cultivated maid-

SPERM ATOPHYTES: GYMMOSPEKMS

191

enhair tree (Gingko), with broad dichotomously veined
leaves, also develops multiciliate sperms.

The testa of the seed, instead of being entirely hard as
described for pine and its allies, develops in two layers, the
inner hard and bony, and the outer pulpy, making the ripe
fruit resemble a plum.

106. Conifers. — This is the great modern Gymnosperm
group, and is characteristic of the temperate regions, where
it forms great forests. Some of the forms are widely dis-
tributed, as the great genus of pines (Finns) (Fig. 158),
while some are now very much restricted, although for-
merly very widely distributed, as the gigantic redwoods
(Sequoia) of the Pacific slope (Fig. 159). The habit of
the body is quite charac-
teristic, a central shaft
extending continuously to
the very top, while the
lateral branches spread
horizontally, with dimin-
ishing length to the top,
forming a conical outline
(Figs. 160, 162). This
habit of firs, pines, etc.,
gives them an appearance
very distinct from that of
other trees.

Another peculiar fea-
ture is furnished by the
characteristic "needle-
leaves," which seem to be

poorly adapted for foliage. These leaves have small spread
of surface and very heavy protecting walls, and show
adaptation for enduring hard conditions (Fig. 161). As
they have no regular period of falling, the trees are always
clothed with them, and have been called " evergreens."
There are some notable exceptions to this, however, as in

Fig. 161. — Cross-section of a needle-leaf of
pine, showing epidermis (e) in which
there are sunken stomata (sp), heavy-
walled hypodermal tissue («s) which
gives rigidity, the mesophyll region (p)
in which a few resin-ducts (h) are seen,
and the central region {stele) in which
two vascular bundles are developed.—
After Sachs.

Fig. 162. A larch (Larix), showing the continuous central shaft and horizontal
branches, the general outline being distinctly conical. The larch is peculiar
among Conifers in periodically shedding its leaves.— From " Plant Relations."

SPERMATOPHYTES: GYMNOSPERMS

193

the case of the common larch or tamarack, which sheds
its leaves every season (Fig. 162). There are Conifers,
also, which do not produce needle-leaves, as in the com-
mon arbor-vitae, whose leaves consist of small closely-over-
lapping scale-like bodies
(Fig. 163).

The two types of leaf
arrangement may also be
noted. In most Conifers
the leaves are arranged
along the stem in spiral
fashion, no two leaves
being at the same level.
This is known as the spi-
ral or alternate arrange-
ment. In other forms, as
the cypresses, the leaves
are in cycles, as was men-
tioned in connection with
the Horsetails, the ar-
rangement being known
as the cyclic or whorled.

The character which
gives name to the group
is the "cone" — that is,
the prominent carpellate
cone which becomes so

Fig. 163. Arbor-vitae (Thuja), showing a
branch with scaly overlapping leaves,
and some carpellate cones (strobili).—
After Eichler.

conspicuous in connec-
tion with the ripening of
the seeds. These cones
generally ripen dry and

hard (Figs. 145, 147, 163), but sometimes, as in junipers,
they become pulpy (Fig. 164), the whole cone forming the
so-called "berry."

There are two great groups of Conifers. One, repre-
sented by the pines, has true cones which conceal the

194 PLANT STKUCTUKES

ovules, and the seeds ripen dry. The other, represented
by the yews, has exposed ovules, and the seed either ripens
fleshy or has a fleshy investment.

Fig. 164. The common juniper {Juniperus communis); the branch to the left bearing
staminate strobili; that to the right bearing staminate strobili above and carpel-
late strobili below, which latter have matured into the fleshy, berry-like fruit,
— After Berg and Schmidt.

CHAPTER XII

SPERMATOPHYTES : ANGIOSPERMS

107. Summary of Gymnosperms. — Before beginning An-
giosperms it is well to state clearly the characters of Gym-
nosperms which have set them apart as a distinct group of
Sperniatophytes, and which serve to contrast them with
Angiosperms.

(1) The microspore (pollen-grain) by wind-pollination
is brought into contact with the megasporangium (ovule),
and there develops the pollen-tube, which penetrates the
nucellus. This contact between pollen and ovule implies
an exposed or naked ovule and hence seed, and therefore
the name " Gymnosperm."

(2) The female gametophyte (endosperm) is well organ-
ized before fertilization.

(3) The female gametophyte produces archegonia.

108. General characters of Angiosperms. — This is the great-
est group of plants, both in numbers and importance, being
estimated to contain about 100,000 species, and forming
the most conspicuous part of the vegetation of the earth.
It is essentially a modern group, replacing the Gymnosperms
which were formerly the dominant Seed-plants, and in the
variety of their display exceeding all other groups. The
name of the group is suggested by the fact that the seeds
are inclosed in a seed case, in contrast with the exposed
seeds of the Gymnosperms.

These are also the true flowering plants, and the ap-
pearance of true flowers means the development of an

195

196

PLANT STRUCTURES

elaborate symbiotic relation between flowers and insects,
through which pollination is secured. In Angiosperms,
therefore, the wind is abandoned as an agent of pollen
transfer and insects are used ; and in passing from Gym-
nosperms to Angiosperms one passes from anemophilous to
entomophilous ("insect-loving") plants. This does not
mean that all Angiosperms are entomophilous, for some are
still wind-pollinated, but that the group is prevailingly ento-
mophilous. This fact, more than anything else, has re-
sulted in a vast variety in the structure of flowers, so char-
acteristic of the group.

109. The plant body. — This of course is a sporophyte,
the gametophytes being minute and concealed, as in Gym-
nosperms. The sporophyte represents the greatest possible
variety in habit, size, and duration, from minute floating
forms to gigantic trees ; herbs, shrubs, trees ; erect, pros-
trate, climbing ; aquatic, terrestrial, epiphytic ; from a few
days to centuries in duration.

Koots, stems, and leaves are more elaborate and vari-
ously organized for work than in other groups, and the
whole structure represents the high-
est organization the plant body has
attained. As in the Gymnosperms,
the leaf is the most variously used
organ, showing at least four distinct
modifications : (1) foliage leaves, (2)
scales, (3) sporophylls, and (4) floral
leaves. The first three are present
in Gymnosperms, and even in Pteri-
dophytes, but floral leaves are pecul-
iar to Angiosperms, making the true
flower, and being associated with en-
tomophily.

110. Microsporophylls. — The micro-
sporophyll of Angiosperms is more
definitely known as a " stamen " than

Fig. 165. Stamens of hen-
bane (Hyoscyamue) : A,
front view, showing fila-
ment (/) and anther {p)\
B, back view, showing
the connective (c) be-
tween the pollen-sacs.
—After Schimper.

SPERMATOPHYTES : ANGIOSPERMS

197

that of Gymnosperms, and has lost any semblance to a leaf.
It consists of a stalk-like portion, the filament; and a
sporangia - bearing portion, the anther (Figs. 165, 167«).

Fig. 166. Cross-section of anther of thorn apple (Datura), showing the four imbedded
sporangia (a, p) containing microspores; the pair on each side will merge and
dehisce along the depression between them for the discharge of pollen. — After
Frank.

The filament may be long or short, slender or broad, or
variously modified, or even wanting. The anther is simply
the region of the sporophyll which bears sporangia, and is

Fig. 167. Diagrammatic cross-sections of anthers: A, younger stage, showing the
four imbedded sporangia, the contents of two removed, but the other two con-
taining pollen mother cells (p?n) surrounded by the tapetum «); B, an older stage,
in which the microspores (pollen grains) are mature, and the pair of sporangia on
each side are merging together to form a single pollen-sac with longitudinal
dehiscence.— After Baillon and Luerssen.

therefore a composite of sporophyll and sporangia and is
often of uncertain limitation. Such a term is convenient,
but is not exact or scientific.

198

PLANT STEOCTUEES

If a young anther be sectioned transversely four sporan-
gia will be found imbedded beneath the epidermis, a pair
on each side of the axis (Figs. 166, 167). When they reach
maturity, the paired sporangia on each side usually merge to-
gether, forming two spore-containing cavities (Fig. 167, B).
These are generally called " pollen-sacs," and each anther is
said to consist of two pollen-sacs, although each sac is made
up of two merged sporangia, and is not the equivalent of the
pollen-sac in Gymnosperms, which is a single sporangium.

Fig. 167a, Various forms of stamens: A, from Solanum, showing dehiscence by
terminal pores; B, from Arbutus, showing anthers with terminal pores and
"horns"; C, from Berberis; B, from Atherosperma, showing dehiscence by
uplifted valves; E, from Aquilegia, showing longitudinal dehiscence; F, from
Popowia. showing pollen-sacs near the middle of the stamen.— After Enuler
and Prantl.

SPERMATOPHYTES: ANGIOSPEKMS

199

The opening of the pollen-sac to discharge its pollen-
grains (microspores) is called dehiscence, which means "a
splitting open," and the methods of
dehiscence are various (Fig. 167a).
By far the most common method
is for the wall of each sac to split
lengthwise (Fig. 1G8), which is
called longitudinal dehiscence; an-
other is for each sac to open by a
terminal pore (Fig. 167«), in which
case it may be prolonged above into
a tube.

111. Megasporophylls. — These
are the so-called " carpels " of Seed-
plants, and in Angiosperms they
are organized in various ways, but
always so as to inclose the mega-
sporangia (ovules). In the simplest

cases each carpel is independent (Fig. 169, A), and is dif-
ferentiated into three regions : (1) a hollow bulbous base,

which contains the

Fig. 168. Cross - section of
anther of a lily (Butomus),
showing the separating walls
between the members of each
pair of sporangia broken
down at z, forming a con-
tinuous cavity (pollen sac)
which opens by a longitudi-
nal slit.— After Sachs.

the

Fig. 169. Types of pistils : A, three simple pistils
(apocarpous), each showing ovary and style tipped
with stigma ; B, a compound pistil (syncarpous),
showing ovary (/), separate styles (g), and stigmas
(n) ; C, a compound pistil (syncarpous), showing
ovary (/), single style (g), and stigma (n).— After
Berg and Schmidt.

ovules and is
real seed case,
known as the
ovary ; (2) sur-
mounting this is a
slender more or less
elongated process,
the style; and (3)
usually at or near
the apex of the style
a special receptive
surface for the pol-
len, the stigma.

In other cases
several carpels to-

200

PLANT STKUCTUKES

gether form a common ovary, while the styles may also
combine to form one style (Fig. 169, C), or they may remain
more or less distinct (Fig. 169, B). Such an ovary may
contain a single chamber, as if the carpels had united edge
to edge (Fig. 170, A) ; or it may contain as many chambers
as there are constituent carpels (Fig. 170, B), as though
each carpel had formed its own ovary before coalescence.
In ordinary phrase an ovary is either "one-celled" or
" several-celled," but as the word " cell " has a very differ-
ent application, the ovary chamber had better be called a
loculus, meaning "a compartment." Ovaries,

Fig. 170. Diagrammatic sections of ovaries: A, cross-section of an ovary with one
loculus and three carpels, the three sets of ovules' said to be attached to the wall
(parietal); B, cross-section of an ovary with three loculi and three carpels, the
ovules being in the center (central) ; C\ longitudinal section of B, showing ovules
attached to free axis (" free central '1). — After Schimpeb.

therefore, may have one loculus or several loculi. Where
there are several loculi each one usually represents a con-
stituent carpel (Fig. 170, B) ; where there is one loculus
the ovary may comprise one carpel (Fig. 169, A), or several
(Fig. 170, A).

There is a very convenient but not a scientific word,
which stands for any organization of the ovary and the
accompanying parts, and that is pistil. A pistil may be
one carpel (Fig. 169, A), or it may be several carpels or-
ganized together (Fig. 169, B, C), the former case being a
simple pistil, the latter a compound pistil. In other words,

SPERMATOPHYTES : ANGIOSPEEMS

201

any organization of carpels which ap-
pears as a single organ with one ovary
is a pistil.

The ovules (megasporangia) are
developed within the ovary (Fig. 170)
either from the carpel wall, when they
are foliar, or from the stem axis which
ends within the ovary, when they are
cauline (see § 89). They are similar
in structure to those of Gymnosperms,
with integument and micropyle, nu-
cellus, and embryo -sac (megaspore),
except that there are often two integu-
ments, an outer and an inner (Fig.
171).

112. The male gametophyte. — When the pollen-grain
(microspore) germinates there is formed within it the sim-
plest known gametophyte (Fig. 172). No trace of the

Fig. 171. A diagrammatic
section of an ovule of
Angiosperms, showing
outer integument (ai),
inner integument (ii),
micropyle (m), nucellus
(k), and embryo sac or
megaspore (em).— After
Sacks.

Fig. 172. Germination of microspore (pollen grain) in duckweed (Lemna): A, mature
spore with its nucleus; B, nucleus of spore dividing; C, two nuclei resulting from
the division; D, a large and small cell following the nuclear division, forming the
two-celled male gametophyte; E, division of smaller cell (generative) to form the
two male cells; F, the two male cells completed and lying near the large tube
nucleus. — Caldwell.

202

PLANT STRUCTURES

ordinary nutritive cells of the gametophyte remains, and
the whole structure seems to represent a single antherid-
ium. At first it consists of two cells, the large wall cell
and the small free generative cell (Fig. 172, D). Later

the generative cell di-
vides (Fig. 172, E),
either while in the
pollen -grain or after
entrance into the pol-
len-tube, and two male
cells (sperm mother-
cells) are formed (Fig.
172, F), which do not
organize sperms, but
which function direct-
ly as gametes.

When pollination
occurs, and the pollen
has been transferred
from the pollen-sacs
to the stigma, it is de-
tained by the minute
papillae of the stig-
matic surface, which
also excretes a sweet-
ish sticky fluid. This
fluid is a nutrient so-
lution for the micro-
spores, which begin to
put out their tubes.
A pollen-tube pene-
trates through the
stigmatic surface, en-
ters among the tissues
of the style, which is sometimes very long, slowly or rap-
idly traverses the length of the style supplied with food by

Fig. 173. Diagram of a longitudinal section through
a carpel, to illustrate fertilization with all parts
in place : s, stigma ; g, style ; o, ovary ; ai, ii,
outer and inner integuments; n, base of nucel-
lus ; /, funiculus ; b, antipodal cells ; c, endo-
sperm nucleus; *, egg and one synergid; p, pol-
len-tube, having grown from stigma and passed
through the micropyle (m) to the egg. — After
Lubussen.

SPERMATOPHYTES : ANGIOSPEKMS

203

its cells but not penetrating them, enters the cavity of the
ovary, passes through the micropyle of an ovule, penetrates
the tissues of the nucellus (if any), and finally reaches and
pierces the wall of the embryo-sac, within which is the egg
awaiting fertilization (Fig. 173).

This remarkable ability of the pollen-tube to make its
way through so much tissue, directly to the micropyle of
an inclosed ovule, can only be explained by supposing that
it is under the guidance of some strong attraction.

113. The female gametophyte. — The megaspore (embryo-
sac) occupies the same position in the ovule as in Gymno-
sperms, but its germination is remarkably modified. The
development of the female gametophyte, the act of f ertil-

Fig. 174. Lilium PMladelphicum : to the left a young megasporangium (ovule),
showing integuments ( C), nucellus {A), and megaspore (B) containing a large nu-
cleus. To the right a megaspore whose nucleus is undergoing the first division
in the formation of the gametophyte.— Caldwell.

ization, and the development of endosperm are the three
subjects to be considered. If fertilization is not accom-
plished the endosperm is usually not developed.

Development— The megaspore nucleus divides (Fig.
174), and one nucleus passes to each end of the embryo-

204

PLANT STKUCTUEES

sac (Fig. 175, at left). Each of these nuclei divide (Fig.
175, at right), and two nuclei appear at each end of the
sac (Fig. 175, at middle). Each of the four nuclei divide

Fig. 175. Lilium Philadelphicum : to the left is an embryo-sac with a gametophyte
nucleus in each end; to the right these two nuclei are dividing to form the two
nuclei shown in each end of the sac in the middle figure.— Caldwell.

(Fig. 176, at left), and four nuclei appear at each end (Fig.
176, at middle). When eight nuclei have appeared, nuclear
division stops. Then a remarkable phenomenon occurs.
One nucleus from each end, the two being called " polar
nuclei," moves toward the center of the sac, the two meet
and fuse (Fig. 176, at right, C), and a single large nucleus
is the result.

The three nuclei at the end of the sac nearest the micro-
pyle are organized into cells, each being definitely sur-
rounded by cytoplasm, but there is no wall and the cells
remain naked but distinct. These three cells constitute
the egg-apparatus (Fig. 176, at right, A), the central one,
which usually hangs lower in the sac than the others, being
the egg, the two others being the synergids, or "helpers."
Here, therefore, is an egg without an archegonium, a dis-
tinguishing feature of Angiosperms.

SPERMATOPHYTES: ANGIOSPERMS

205

The three nuclei at the other end of the sac are also or-
ganized into cells, and usually have walls. These cells are
known as antipodal cells (Fig. 176, at right,
B). The large nucleus near the center of A

L

the sac, formed by the fusion of the two

Fig. 176. Lilium Philadelphicxim, showing last stages of germination of megaspore
before fertilization: the embryo sac to the left contains the pair of nuclei in each
end in a state of division preparatory to the stage represented by the middle figure,
in which there are four nuclei at each end; the figure to the right shows an embryo-
sac containing a gametophyte about ready for fertilization, with the egg apparatus
(A) composed of the two synergids and egg (central and lower), the three antipo-
dal cells (B), and the two polar nuclei fusing ( C) to form the primary endosperm
nucleus.— Caldwell.

polar nuclei, is known as the primary endosperm nucleus
or the definitive nucleus.
32

206

PLANT STRUCTURES

Fig. 177. Fertilization in the cotton plant,
a Dicotyledon, showing the pollen tube (P)
passing through the micropyle and con-
taining a single sperm (male cell), and hav-
ing entered the embryo-sac is in contact
with one of the synergids (<S*) on its way to
the egg (22).— After Duggar.

Fertilization. — The
pollen-tube, carrying the
two male cells, has passed
down the style and en-
tered the micropyle (Fig.
173). It then reaches the
wall of the embryo -sac,
pierces it, and is in con-
tact with the egg-appa-
ratus (Fig. 177). When
it comes near the egg, the
tip of the tube breaks and
the two male cells are dis-
charged into the embryo-
sac. One male cell passes
to the egg and the two
nuclei fuse, the resulting
cell being the oospore,
which develops the em-
bryo. The other male
cell passes to the endo-
sperm nucleus and fuses
with it, the cell resulting
from this triple fusion of
a male cell and two polar
nuclei developing th e
endosperm (Fig. 178).
These two simultaneous
acts of fertilization are
spoken of as " double fer-
tilization."

Endosperm. — After
fertilization, the primary
endosperm nucleus begins
a series of divisions, and
as a result the sac becomes

SPERM ATOFIIYTES : AN GIOS PERMS

207

more or less filled with nutritive
cells, which are often organized
into a compact tissue (Fig. 179).
These nutritive cells do not cor-
respond to the endosperm of
Gynmosperms, although they re-
receive the same name. In Gym-
nosperms the endosperm is main-
ly formed before fertilization and
is the nutritive body of the female
gametophyte ; while in Arigio-
sperms it is formed after fertiliza-
tion and is probably not a part
of the gametophyte. As the
endosperm of Angiosperms is a
product of what appears to be
fertilization, it would seem
proper to regard it as sporo-

phyte tissue, but its real character is still under discussion.

The antipodal cells probably represent nutritive cells

of the gametophyte. Sometimes they disappear very soon

after they are formed; but sometimes they become very

Fig. 178. End of embryo-sac of
Silphii/m, showing double fer-
tilization : sy, eynergid, the
other having been destroyed by
the pollen-tube ; o, egg with
coiled male cell («/>,) lying
against its nucleus ; e, endo-
sperm cell, with large coiled
male cell (sp9) lying against it.
—After Land.

Fig. 179. One end of the embryo-sac in wake-robin (Trillium), showing endosperm
(shaded cells) in which a young embryo is imbedded. — After Atkinson.

208

PLANT STEUCTUKES

active and even divide and form a considerable amount of
tissue, which usually nourishes the embryo until endosperm
tissue is developed, and then becomes disorganized; or
even invades the tissue of the nucellus.

114. Development of embryo. — While the endosperm is
forming, the oospore has germinated and the sporophyte
embryo is developing (Fig. 180). Usually a suspensor, more
or less distinct, but never so prominent as in Gymnosperms,

is formed ; at the end of it the
embryo is developed (Fig. 181),
which, when completed, is more
or less surrounded by nourish-
ing endosperm (Fig. 183).

The two groups of Angio-
sperms differ widely in the struc-
ture of the embryo. In Mono-
cotyledons the axis of the em-
bryo develops the root-tip at one
end and the " seed-leaf " (coty-
ledon) at the other, the stem-tip
arising from the side of the axis
as a lateral member (Fig. 182).
This relation of organs recalls
the embryo of Isoetes (see § 90).
Naturally there can be but one
cotyledon under such circum-
stances, and the group has been
named Monocotyledons.

In Dicotyledons the axis of
the embryo develops the root-tip at one end and the stem-
tip at the other, the cotyledons (usually two) appearing as
a pair of opposite lateral members on either side of the
stem-tip (Fig. 181). This recalls the relation of parts in
the embryo of Selaginella (see § 89). As the cotyledons
are lateral members their number may vary. In Gymno-
sperms, whose embryos are of this type, there are often

Fig. 180. Curved embryo-sac of
arrowhead (Sagitlaria), show-
ing in the upper right end a
young embryo, in the other
end the antipodal cells cutoff
by a partition, and scattered
through the sac a few free en-
dosperm cells. — After Schafp-
ner.

SPERMATOPHYTES . ANGIOSPERMS

209

several cotyledons in a cycle (Fig. 154) ; and in Dicotyle-
dons there may be one or several cotyledons ; but as a pair
of opposite cotyledons is almost without exception in the
group, it is named Dicotyledons.

The axis of the embryo between the root-tip and the
cotyledons is called the hypocotyl (Figs. 154, 193, 194), which

Fig. 181. Development of embryo of shepherd's purse (Capsella), a Dicotyledon:
beginning with /, tbs youngest stage, and following the sequence to VI, the old-
est stage, v represents the suspensor, c the cotyledons, s the stem-tip, w the root,
h the root-cap. Note the root-tip at one end of the axis and the stem-tip at the
other between the cotyledons. — After Hanstein.

means " under the cotyledon," a region which shows pecul-
iar activity in connection with the escape of the embryo
from the seed. Formerly it was called either caulicle or
radicle. In Dicotyledons the stem-tip between the coty-

210

PLANT STKUCTURES

ledons often organizes the rudiments of subsequent leaves,
forming a little bud which is called the plumule.

Embryos differ much as to com-
pleteness of their development within
the seed. In some plants, especially
those which are parasitic or sapro-
phytic, the embryo is merely a small
mass of cells, without any organiza-
tion of root, stem, or leaf. In many
cases the embryo becomes highly de-
veloped, the endosperm being used
up and the cotyledons stuffed with
food material, the plumule contain-
ing several well - organized young
leaves, and the embryo completely
filling the seed cavity. The com-
mon bean is a good illustration of
this last case, the whole seed within
the integument consisting of the two
large, fleshy cotyledons, between
which lie the hypocotyl and a plu-
mule of several leaves.

115. The seed. — As in Gymno-
sperms, while the processes above
described are taking place within
the ovule, the tissue is developing
that forms the hard seed-coat or testa (Fig. 183). When
this hard coat is fully developed, the activities within
cease, and the whole structure passes into that condition of
suspended animation which is so little understood, and
which may continue for a long time.

The testa is variously developed in seeds, sometimes
being smooth and glistening, sometimes pitted, sometimes
rough with warts or ridges. Sometimes prominent append-
ages are produced which assist in seed-dispersal, as the
wings in Catalpa or Bignonia (Fig. 184), or the tufts of

Fig. 182. Young embryo of
water plantain (Alisma), a
Monocotyledon, the root
being organized at one
end (next the suspensor),
the single cotyledon (C)
at the other, and the stem-
tip arising from a lateral
notch (v). — After Han-
stein.

SPERMATOCYTES: ANGIOSPERMS

211

Fig. 183. The two figures to the left are seeds of violet, one showing the black, hard
testa, the other being sectioned and showing testa, endosperm, and imbedded
embryo; the figure to the right is a section of a pepper fruit (Piper), showing
modified ovary wall (pc), seed testa (sc), nucellus tissue (p), endosperm (en), and
embryo (em).— After Baillon.

hair on the seeds of milkweed, cotton, or fireweed (Fig.
185). For a fuller account of the methods of seed-dispersal
see Plant Relations, Chapter VI.

Fig. 184. A winged seed of Bignonia.— After Strasbitrger.

116. The fruit.— The effect of fertilization is felt beyond
the boundaries of the ovule, which forms the seed. The
ovary is also involved, and becomes more or less modified.
It enlarges more or less, sometimes becoming remarkably
enlarged. It also changes in structure, often becoming
hard or parchment-like. In case it contains several or
numerous seeds, it is organized to open in some way and
discharge them, as in the ordinary pods and capsules (Fig.
185). In case there is but one seed, the modified ovary

212

PLANT STRUCTURES

wall may invest it as closely as another
integument, and a seed-like fruit is
the result — a fruit which never opens
and is practically a seed. Such a
fruit is known as an akene, and is
very characteristic of the greatest
Angiosperm family, the Composite,
to which sunflowers, asters, golden-
rods, daisies, thistles, dandelions,
etc., belong. Dry fruits which do
not open to discharge the seed often
bear appendages to aid in dispersal
by wind (Figs. 186, 187), or by animals
(Fig. 188).

Capsules, pods, and akenes are said
to be dry fruits, but in many cases
fruits ripen fleshy. In the peach,
plum, cherry, and all ordinary " stone
fruits," the modified ovary wall or-
ganizes two layers, the inner being
very hard, forming the "stone," the
outer being pulpy (Fig. 189), or vari-
ously modified (Fig. 190). In the true berries, as the
grape, currant, tomato, etc., the whole ovary becomes a
thin-skinned pulpy mass in which the seeds are imbedded.

In some cases
the effect of ferti-
lization in chang-
ing structure is
felt beyond the
ovary. In the ap-
ple, pear, quince,
and such fruits,
the pulpy part is
the modified

Calyx (one Of the Fiq. 186. .Winged fruit of maple.— After Keener.

Fig. 185. A pod of firevveed
{Epilobium) opening and
exposing its plumed seeds
which are transported by
the wind.— After Beal.

SPERMATOPHYTES: ANGIOSPERMS

213

floral leaves), the ovary and its contained seeds being repre-
sented by the "core." In other cases, the end of the stem
bearing the ovaries (receptacle) becomes enlarged and
pulpy, as in the strawberry (Fig. 191). This effect some-
times involves even
more than the
parts of a single
flower, a whole
flower-cluster,
with its axis and
bracts, becoming
an enlarged pulpy
mass, as in the
pineapple (Fig.
192).

The term
"fruit," therefore,

Fig. 187. A ripe dandelion head, showing the mass of
plumes, a few seed-like fruits (akenes) with their
plumes still attached to the receptacle, and two
frllen off.— After Kerner.

Fig. 188. An akene of beg-
gar ticks, showing the two
barbed appendages which
lay hold of animals.— Af-
ter Beal.

Fig. 189. To the left a section of a peach (fruit),
showing pulp and stone formed from ovary wall,
and the contained seed (kernel) ; to the right
the fruit of almond, which ripens dry.— After
Gray.

PLANT STRUCTURES

•y indefinite one, so far as the structures it includes
concerned. It is simply an effect which follows fer-
tilization, and involves more or less of the structures adja-

Fig. 190. Fruit of nutmeg (Myristica) : A, section of fruit, showing seed within the
heavy wall ; B, section of seed, showing peculiar convoluted and hard endosperm
(m) in which an embryo (n) is imbedded.— After Berg and Schmidt.

cent to the seeds. As has been seen, this effect may extend
only to the ovary wall, or it may include the calyx, or it
may be specially directed toward the
receptacle, or it may embrace a whole
flower-cluster. It is what is called a
physiological effect rather than a defi-
nite morphological structure.

117. Germination of the seed. — It
has been pointed out (§ 103) that the
so-called "germination of the seed"
is not true germination like that of
spores. It is the awakening and es-
cape of the young sporophyte, which
has long before passed through its
germination stage.

By various devices seeds are sepa-
rated from the parent plant, are dis-
persed more or less widely, and find
lodgment. If the lodgment is suitable, there are many
devices for burial, such as twisting stalks and awns, bur-

Fig. 191. Fruit of straw-
berry, showing the per-
sistent calyx, and the en-
larged pulpy receptacle
in which numerous sim-
ple and dry fruits (a-
kenes) are imbedded. —
After Bailey.

SPERMATOPHVTES: ANGIOSPEKMS

215

rowing animals, etc. The period of rest may be long or
short, but sooner or later, under the influence of moisture,
suitable temperature, and oxygen the quiescent seed begins
to show signs of life.

The sporophyte within begins to grow, and the seed
coat is broken or penetrated through some thin spot or

\s * A/,//

Pig. 192. Pineapple: A, the cluster of fruits forming the so-called "fruit"; B, single
flower, showing small calyx and more prominent corolla; C, section of flower,
showing the floral organs arising above the ovary (epigynous).— A, B after Koch;
C after Lb Maout and Decaisnb.

opening. The root-tip emerges first, is protruded still
farther by the rapid elongation of the hypocotyl, soon
curves toward the earth, penetrates the soil, and sending
out rootlets, becomes anchored. After anchorage in the

216

PLANT STRUCTURES

soil, the hypocotyl again rapidly elongates and develops a
strong arch, one of whose limbs is anchored, and the other
is pulling upon the cotyledons (Fig. 193). This pull finally
frees the cotyledons, the hypocotyl straightens, the cotyle-

Fig. 193. Germination of the garden bean, showing the arch of the hypocotyl above
ground, its pull on the seed to extricate the cotyledons and plumule, and the final
straightening of the stem and expansion of the young leaves.— After Atkinson.

dons are spread out to the air and light, and the young
sporophyte has become independent (Fig. 194).

In the grain of corn and other cereals, so often used in
the laboratory as typical Monocotyledons, but really excep-
tional ones, the embryo escapes easily, as it is placed on
one side of the seed near the surface. The hypocotyl and
stem split the thin covering, and the much-modified cotyle-
don is left within the grain to absorb nourishment.

In some cases the cotyledons do not escape from the
seed, either being distorted with food storage (oak, buck-
eye, etc.), or being retained to absorb nourishment from
the endosperm (palms, grasses, etc.). In such cases the
stem-tip is liberated by the elongation of the petioles of the

SPERMATOPHYTES : ANGIOSPERMS

217

cotyledons, and the seed coat containing the cotyledons
remains like a lateral appendage upon the straightened axis.

It is also to be observed in
many cases that the young root
system, after gripping the soil,
contracts, drawing the young
plant deeper into the ground.

118. Summary from Angio-
sperms. — At the beginning of this
chapter (§ 107) the characters of
the Gymnosj)erms were summar-
ized which distinguished them
from Angiosperms, whose con-
trasting characters may be stated
as follows :

(1) The microspore (pollen-
grain), chiefly by insect pollina-
tion, is brought into contact with
the stigma, which is a receptive

. region on the surface of the car-
pel, and there develops the pollen-
tube, which penetrates the style
to reach the ovary cavity which
contains the ovules (megasporan-
gia). The impossibility of con-
tact between pollen and ovule im-
plies inclosed ovules and hence
seeds, and therefore the name
" Angiosperm."

(2) The female gametophyte
at the time of fertilization con-
sists of only a few free nuclei and
cells, usually seven in number.

(3) The female gametophyte produces no archegonia,
but a single naked egg.

Fig. 194. Seedling of hornbeam
(Oarpintts), Bhowing primary
root (hiv) bearing rootlets (ato)
upon which are numerous
root hairs (r), hypocotyl (A),
cotyledons (c), young stem
(<?), and first (I) and second
(/') true leaves.— After Sciiim-

PER.

CHAPTER XIII

THE FLOWER

119. General characters. — In general the flower may be
regarded as a modified branch of the sporophyte stem bear-
ing sporophylls and usually floral leaves. Its representa-
tive among the Pteridophytes and Gymnosperms is the stro-
bilus, which has sporophylls but not floral leaves. Among
Angiosperms it begins in a simple and somewhat indefinite
way, gradually becomes more complex and modified, until
it appears as an elaborate structure very efficient for its
purpose.

This evolution of the flower has proceeded along many
lines, and has resulted in endless diversity of structure.
These diversities are largely used in the classification of
Angiosperms, as it is supposed that near relatives are indi-
cated by similar floral structures, as well as by other fea-
tures. The significance of these diversities is supposed to
be connected with securing proper pollination, chiefly by
insects, and favorable seed distribution.

Although the evolution of flowers has proceeded along
several lines simultaneously, now one feature and now
another being emphasized, it will be clearer to trace some
of the important lines separately.

120. Floral leaves. — In the simplest flowers floral leaves
do not appear, and the flower is represented only by the
sporophylls. Both kinds of sporophylls may be associated,
in which case the flower is said to be 'perfect (Fig. 195) ; or
they may not both occur in the same flower, in which case
one flower is staminate and the other pistillate (Fig. 196).

218

THE FLOWER

219

When the floral leaves first appear in connection with
the sporophylls they are inconspicuous, scale-like bodies.
In higher forms they become more prominent and inclose

Fig. 195. Lizard's tail (Saururus): A, tip of branch
bearing leaves and elongated cluster of flowers;

B, a single naked flower from A, showing sta-
mens and four spreading and stigmatic styles;

C, flower from another species, showing sub-
tending bract, absence of floral leaves, seven
stamens, and a syncarpous pistil ; the flowers
naked and perfect.— After Engler.

Fig. 196. Naked flowers of dif-
ferent willows ISalix), each
from the axil of a bract :
a, b, c, staminate flowers ;
d, e, /, pistillate flowers, the
pistil composed of two car-
pels (syncarpous). — After
Warming.

Fig. 197. Flower of calamus
(Acorus), showing simple
perianth, stamens, and syn-
carpous pistil : a hypogynous
flower without differentiation
of calyx and corolla.— After
Engler.

Fig. 199. Common flax (Limtm) :
A. entire flower, showing calyx
and corolla ; I?, floral leaves re-
moved, showing stamens and
syncarpous pistil ; C, a mature
Fig. 198. Flowers of elm (I'lmt/s) : A, branch capsule splitting open. —After

bearing clusters of flow-ers and scaly buds ; Schimper.

B, single flower, showing simple perianth
and stamens, being a staminate flower; V,

flower showing perianth, stamens, and the two divergent styles stigmatic on inner
surface, being a perfect flower; D, section through perfect flower, showing peri-
anth, stamens, and pistil with two loculi each with a single ovule. — After Englek.

Fig. 200. A flower of peony, showing the four sets of floral organs: k, the sepals, to-
gether called the calyx; c, the petals, together called the corolla; a, the numerous
stamens; g, the two carpels, which contain the ovules.— After Strasburger.

THE FLOWER

221

the young sporophylls, but they are all alike, forming what
is called the perianth (Figs. 197, 198).

In still higher forms the perianth differentiates, the
inner floral leaves become more delicate in texture, larger
and generally brightly colored (Fig. 199, ^1). The outer
set may remain scale-like, or become like small foliage
leaves. When the dif-
ferentiation of the peri-
anth is distinct, the
outer set of floral leaves
is called the calyx, each
leaf being a sepal ; the
inner set is the corolla,
each leaf being a petal
(Fig. 200). Sometimes,
as in the lily, all the
floral leaves become
uniformly large and
brightly colored, in
which case the term
perianth is retained
(Fig. 201). In other
cases, the calyx may be
the large and colored
set, but whenever there
is a clear distinction
between sets, the outer
is the calyx, the inner
the corolla.

Both floral sets may
not appear, and it has
become the custom to
regard the missing set
as the corolla, such
flowers being called
apetalous, meaning
33

Fig. 201. —An easter-lily. a Monocotyledon,
showing perianth (a), stamens (b), stigma (c),
flower bud (d), and a carpel after the peri-
anth has fallen (I), with its knob-like stigma,
long style, and slender ovary.— Caldwell.

222 PLANT STRUCTURES

" without petals." It is not always possible to tell whether
a flower is apetalous — that is, whether it has lost a floral
set which it once had — or is simply one whose perianth has
not yet differentiated, in which case it would be a "primi-
tive type."

The line of evolution, therefore, extends from flowers
without floral leaves, or naked flowers, to those with a dis-
tinctly differentiated calyx and corolla.

121. Spiral to cyclic flowers. — In the simplest flowers the
sporophylls and floral leaves (if any) are distributed about
an elongated axis in a spiral, like a succession of leaves.
That part of the axis which bears the floral organs is for
convenience called the receptacle (Fig. 202). As the recep-

.»

Fig. 202. A buttercup (Ranunculus): a, complete flower, showing sepals, petals, sta-
mens, and head of numerous carpels on a large receptacle; b, section showing
relation of parts; a hypogynous, polypetalous, apocarpous, actinomorphic flower.
—After Baillon.

tacle is elongated and capable of continued growth, an in-
definite number of each floral organ may appear, especially
of the sporophylls. With the spiral arrangement, there-
fore, there is no definiteness in the number of floral organs ;
there may be one or very many floral leaves, or stamens, or
carpels. The spiral arrangement and indefinite numbers
are features of the ordinary strobilus, and therefore such
flowers are regarded as more primitive than the others.

In higher forms the receptacle becomes shorter, the
spiral more closely coiled, until finally the sets of organs

THE FLOWER 223

appear to be thrown into rosettes or cycles. This change
does not necessarily affect all the parts simultaneously.
For example, in the common buttercup the sepals and
petals are nearly in cycles, while the carpels are spirally
arranged and indefinitely numerous on the head-like recep-
tacle (Fig. 202). On the other hand, in the common water-

Fig. 203. Flower of water-lily (Nymphcea), showing numerous petals and stamens. —
After Caspart.

lily the petals and stamens are spiral, and indefinitely re-
peated, while the sepals and carpels are approximately
cyclic (Fig. 203).

Finally, in the highest forms, all the floral organs are
in definite cycles, and there is no indefinite repetition of
any part. All through this evolution from the spiral to the
cyclic arrangement there is constantly appearing a tend-
ency to "settle down "to certain definite numbers. When
the complete cyclic arrangement is finally established these
numbers are established, and they are characteristic of
great groups. In cyclic Monocotyledons there are nearly
always just three organs in each cycle, forming what is
called a trimerous flower (Fig. 204) ; while in cyclic Dicot-

224

PLANT STKUCTUKES

yledons the number five prevails, hut often four appears,
forming pentamerous or tetramerous flowers (Fig. 199).
This does not mean that there are necessarily just three,
four, or five of each organ in the flower, for there may be
two or more cycles of some one organ. For example, in the
common lily there are six floral leaves in two sets, six sta-
mens in two sets, and three carpels (Fig. 204).

In the cyclic flowers it is also to be noted that each set
alternates with the next set outside (Fig. 204). The petals

are not directly opposite the se-
pals, but are opposite the spaces
between sepals ; the stamens in
^J^<w>'>^ \ turn alternate with the petals; if

f c9/^\u> i\ there is a second set of stamens,

\ /v, V&}) „ ' 1 it alternates with the outer set,

and so on. If two adjacent sets
are found opposing one another,
it is usually due to the fact that
a set between has disappeared.
For example, if a set of stamens
is opposite the set of petals, either
an outer stamen set or an inner
petal set has disappeared.

This line of evolution, there-
fore, extends from flowers whose
parts are spirally arranged upon
an elongated receptacle and in-
definite in number, to those whose parts are in cycles and
definite in number.

122. Hypogynous to epigynous flowers. — In the simpler
flowers the sepals, petals, and stamens arise from beneath
the ovary (Figs. 197, 202, 205, 1). As in such cases the
ovary or ovaries may be seen distinctly above the origin
(insertion) of the other parts, such a flower is often said to
have a "superior ovary." The more usual term, however,
is hypogynous, meaning in effect " under the ovary," refer-

Fig. 304. Diagram of such a
flower as the lily, showing re-
lation of parts : uppermost
organ is the bract in the axil
of which the flower occurs ;
black dot below indicates po-
sition of stem ; floral parts in
threes and in five alternating
cycles (two stamen Bets), being
a trimerous. pentacyclic flow-
er.— After Schimper.

THE FLOWER

225

ring to the fact that the insertion of the other parts is
under the ovary.

Hypogyny is very largely displayed among flowers, but
there is to be observed a tendency in some to carry the
insertion of the outer parts higher up. When the outer
parts arise from the rim of an urn-like outgrowth from the

Fig. 905. Flowers of Rose family: 1, a hypogynous
flower of PotentUla, sepals, petals, and stamens
arising from beneath the head of carpels; 2, a
perigynous flower of Alchemilla, sepals, petals,
and stamens arising from rim of urn-like pro-
longation of the receptacle, which surrounds the
carpel ; 3, an epigynous flower of the common
apple, in which all the parts seem to arise from
the top of the ovary, two of whose loculi are
seen.— After Focke.

receptacle, which surrounds the pistil or pistils, the flower
is said to be perigynous (Figs. 205, #, 206), meaning " around
the pistil." Finally, the insertion is carried above the ovary,
and sepals, petals, and stamens seem to arise from the top
of the ovary (Fig. 205, 3), such a flower being epigynous,
the outer parts appearing "'upon the ovary." In such a
case the ovary does not appear within the flower, but below
it (Figs. 205, 252, 261), and the flower is often said to have
an "inferior ovary."

123. Apocarpous to syncarpous flowers. — In the simpler
flowers the carpels are entirely distinct, each carpel organ-

226

PLANT STRUCTURES

izing a simple pistil, a single flower containing as many
pistils as there are carpels, as in the buttercups (Figs.
200, 202). Such a flower is said to be apocarpous, meaning
"carpels separate." There is a very strong tendency,

Fig. 206. Sweet-scented shrub (Calycanthus): A, tip of branch bearing flowers; B,
section through flower, showing numerous floral leaves, stamens, and carpels, and
also the development of the receptacle about the carpels, making a perigynous
flower.— After Thiebatjlt.

however, for the carpels of a flower to organize together
and form a single compound pistil. In such a flower there
may be several carpels, but they all appear as one organ
(Figs. 195, (7, 197, 198, D, 199, B), and the flower is said
to be syncarpous, meaning "carpels together."

124. Polypetalous to sympetalous flowers. — The tendency
for parts of the same set to coalesce is not confined to the
carpels. Sepals often coalesce (Fig. 208), and sometimes
stamens, but the coalescence of petals seems to be more
important. Among the lower forms the petals are entirely
separated (Figs. 199, A, 202, 203, 207), a condition which

THE FLOWER

227

has received a variety of names, but
probably the most common is poly-
pet alous, meaning "petals many,"
although eleutheropetalotis, meaning
"petals free," is much more to the
point.

In the highest Angiosperms, how-
ever, the petals are coalesced, form-
ing a more or less tubular organ
(Figs. 208-210). Such flowers are
said to be sympetalous, meaning
"petals united." The words gamo-
petalous and monopetalous are also

much used, but all three words refer to the same condition
of the flower. Often the sympetalous corolla is differenti-

Fio. 207. Flower of straw-
berry, showing sepals, pet-
als, numerous stamens,
and head of carpels ; the
flower is actinomorphic,
hypogynous, and with no
coalescence of parts. — Af-
ter Bailey.

Fig. 208. A flower of the tobacco plant: a, a complete flower, showing the calyx with
its sepals blended below, the ftinnelform corolla made up of united petals, and the
stamens just showing at the mouth of the corolla tube; b, a corolla tube split open
and showing the five stamens attached to it near the base; c, a syncarpous pistil
made up of two carpels, showing ovary, style, and stigma.— After Strasburger.

228

PLANT STRUCTUKES

ated into two regions (Fig. 210, b), a more or less tubular
portion, the tube, and a more or less flaring portion, the limb.

125. Actinomorphic
to zygomorphic flow-
ers.— In the simpler
flowers all the mem-
bers of any one cycle
are alike ; the petals
are all alike, the
stamens are all alike,
etc. Looking at the
center of the flower,
all the parts are re-
peated about it like
the parts of a radi-
ate animal. Such a
flower is actinomor-
phic, meaning "ra-
diate," and is often
called a " regular
flower." Although
the term actinomor-
phic •strictly applies to all the floral organs, it is especially
noteworthy in connection with the corolla, whose changes
will be noted.

Pig. 209. Flower of morning-glory (Ipomcea), with
sympetalous corolla split open, showing the five
attached stamens, and the superior ovary with
prominent style and stigma ; the flower is hy-
pogynous, sympetalous, and actinomorphic. —
After Mkissner.

Fig. 210. A group of sympetalous flower forms: a, a flower of harebell, showing a
bell-shaped corolla; b, a flower of phlox, showing a tube and spreading limb; c, a
flower of dead-nettle, showing a zygomorphic two-lipped corolla; d, a flower of
toad-flax, showing a two-lipped corolla, and also a spur formed by the base of the
corolla; e, a flower of the snapdragon, showing the two lips of the corolla closed.
—After Gray.

THE FLOWER

229

In many cases the petals are not all alike, and the radi
ate character, with its similar parts repeated about a cen
ter, is lost. In the
common violet, for
example, one of the
petals develops a spur
(Fig. 211) ; in the
sweet pea the petals
are remarkably un-
like, one being broad
and erect, two small-
er and drooping
downward, and the
other two much modi-
fied to form together
a boat-like structure
which incloses the
sporophylls. Such flowers are called zygomorphic, meaning
" yoke-form," and they are often called " irregular flowers."

When zygomorphic flowers are also sympetalous the
corolla is often curiously shaped. A very common form

Fig. 811. The pansy (Viola tricolor) : A, section
showing sepals (/, I'), petals (c) one of which
produces a spur (cs), the flower being zygomor-
phic; B, mature fruit (a capsule) and persistent
calyx (k); C, the three boat-shaped valves of
the fruit open, most of the seeds (s) having
been discharged.— After Sachs.

Fig. 212. Flower of a mint (Mentha aquatica): A, the entire flower, showing calyx
of united sepals, unequal petals, stamens, and style with two stigma lobes; B, a
corolla split open, showing petals united and the four stamens attached to the
tube; the flower is sympetalous and zygomorphic— After Werming.

230

PLANT STRDCTUKES

Fig. 213. Flower of a Labiate (Teucrium),
showing the calyx of coalesced sepals,
the sympetalous and two-lipped (bilabi-
ate) corolla with three petals (middle one
largest) in the lower lip and two small
ones in the upper, and the stamens and
style emerging through a slit on the up-
per side of the tube; a sympetalous and
zygomorphic flower.— After Briquet.

is the bilabiate, or " two-lipped," in which two of the petals
usually organize to form one lip, and the other three form

the other lip (Figs. 210,
c, d, e, 212, 213). The two
lips may be nearly equal,
the upper may stand high
or overarch the lower, the
lower may project more or
less conspicuously, etc.

126. Inflorescence.—
Very often flowers are soli-
tary, either on the end of
a stem or branch (Figs.
231, 236), or in the axil
of a leaf (Fig. 258). But
such cases grade insensibly into others where a definite
region of the plant is set aside to produce flowers (Figs.
253, 260). Such a region forms what is called the inflo-
rescence. The various ways in which flowers are arranged
in an inflorescence have received technical names, but they
do not enter into our purpose here. They are simply dif-
ferent ways in which plants seek to display their flowers
so as to favor pollination and seed distribution.

There are several tendencies, however, which may be
noted. Some groups incline to loose clusters, either elon-
gated (Fig. 260) or flat-topped (Fig. 253) ; others prefer
large and often solitary flowers (Fig. 258) to a cluster of
smaller ones ; but in the highest groups there is a distinct
tendency to reduce the size of the flowers, increase their
number, and mass them into a compact cluster. This ten-
dency reaches its highest expression in the greatest family
of the Angiosperms, the Composite, of which the sunflower
or dandelion can be taken as an illustration (Figs. 261, 262),
in which numerous small flowers are closely packed together
in a compact cluster which resembles a single large flower.
It does not follow that all very compact inflorescences in-

THE FLOWER 231

dicate plants of high rank, for the cat-tail flag (Fig. 221)
and many grasses have very compact inflorescences, and
they are supposed to be plants of low rank. It is to be
noted, however, that the very highest groups have settled
upon this as the best type of inflorescence.

127. Summary. — In tracing the evolution of flowers,
therefore, the following tendencies become evident : (1)
from naked flowers to those with distinct calyx and corolla ;
(2) from spiral arrangement and indefinite numbers to cyclic
arrangement and definite numbers ; (3) from hypogynous
to epigynous flowers ; (4) from apocarpous to syncarpous
pistils ; (5) from polypetalous to sympetalous corollas ; (6)
from actinomorphic or regular to zygomorphic or irregular
flowers ; (7) from loose to compact inflorescences.

These various lines appear in all stages of advancement
in different flowers, so that it would be impossible to deter-
mine the relative rank in all cases. However, if a flower
is naked, spiral, with indefinite numbers, hypogynous, and
apocarpous, it would certainly rank very low. On the con-
trary, the flowers of the Composite have calyx and corolla,
are cyclic, epigynous, syncarpous, sympetalous, often zygo-
morphic, and are in a remarkably compact inflorescence,
indicating the highest possible combination of characters.

128. Flowers and insects. — The adaptations between
flowers and insects, by which the former secure pollination
and the latter food, are endless. Many Angiosperm flowers,
especially those of the lower groups, are still anemophilous,
as are the Gymnosperms, but most of them, by the presence
of color, odor, and nectar, indicate an adaptation to the
visits of insects. This wonderful chapter in the history of
plants will be found discussed, with illustrations, in Plant
Relations, Chapter VII.

CHAPTER XIV

MONOCOTYLEDONS AND DICOTYLEDONS

129. Contrasting characters. — The two great groups of
Angiosperms are quite distinct, and there is usually no dif-
ficulty in recognizing them. The monocotyledons are
usually regarded as the older and the simpler forms, and
are represented by about twenty thousand species. The
Dicotyledons are much more abundant and diversified, con-
taining about eighty thousand species, and form the domi-
nant vegetation almost everywhere.
The chief contrasting characters
may be stated as follows :

Monocotyledons. — (1) Embryo
with terminal cotyledon and lat-
eral stem-tip. This character is
practically without exception.

(2) Vascular bundles of stem
scattered (Fig. 214). This means
that there is no annual increase in
the diameter of the woody stems,
and no extensive branching, but
to this there are some exceptions.

(3) Leaf veins forming a closed
system (Fig. 215, figure to left).
As a rule there is an evident set

of veins which run approximately parallel, and intricately
branching between them is a system of minute veinlets not
readily seen. The vein system does not end freely in the
232

Fig. 214. Section of stem of
corn, showing the scattered
bundles, indicated by black
dots in cross-section, and by
lines in longitudinal section.
—From "Plant Relations."

MONOCOTYLEDONS AND DICOTYLEDONS 233

margin of the leaf, but forms a " closed venation/' so that
the leaves usually have an even (entire) margin. There

are some notable exceptions
to this character.

(4) Cyclic flowers trim-
erous. The " three-parted "

Pig. 215. Two types of leaf venation: the figure to the left is from Solomon's seal,
a Monocotyledon, and shows the principal veins parallel, the very minute cross
veinlets heing invisible to the naked eye; that to the right is from a willow, a
Dicotyledon, and shows netted veins, the main central vein (midrib) sending out
a series of parallel branches, which are connected with one another by a network
of veinlets. — After Ettingshausen.

flowers of cyclic Monocotyledons are quite characteristic,
but there are some trimerous Dicotyledons.

Dicotyledons. — (1) Embryo with lateral cotyledons and
terminal stem-tip.

(2) Vascular bundles of stem forming a hollow cylinder
(Fig. 216, w). This means an annual increase in the diam-

234

PLANT STRUCTUKES

Fig. 216. Section across a young twig of
box elder, showing the four stem regions:
e, epidermis, represented by the heavy
bounding line; c, cortex; w, vascular cyl-
inder; p, pith.— From "Plant Relations."

eter of woody stems (Fig.
217, w), and a possible
increase of the branch
system and foliage dis-
play each year.

(3) Leaf veins form-
ing an open system (Fig.
215, figure to right).
The network of smaller
veinlets between the
larger veins is usually
very evident, especially
on the under surface of
the leaf, suggesting the
name "net- veined"
leaves, in contrast to the " parallel -veined " leaves of Mono-
cotyledons. The vein system ends freely in the margin of
the leaf, forming an " open venation." In consequence of
this, although the leaf
may remain entire, it
very commonly be-
comes toothed, lobed,
and divided in various
ways. Two main types
of venation may be
noted, which influence
the form of leaves. In
one case a single very
prominent vein {rib)
runs through the mid-
dle of the blade, and
is called the midrib.
From this all the mi-
nor veins arise as
branches (Figs. 218,
219), and such a leaf

Fig. 217. Section across a twig of box elder
three years old, showing three annual rings,
or growth rings, in the vascular cylinder; the
radiating lines (m) which cross the vascular
region (w) represent the pith rays, the princi-
pal ones extending from the pith to the cor-
tex (c).— From " Plant Relations."

MONOCOTYLEDONS AND DICOTYLEDONS

235

is said to be pinnate or pinnately veined, and inclines to
elongated forms. In the other case several ribs of eqnal
prominence enter the blade and diverge through it (Fig.
218). Such a leaf is palmate or palmately veined, and in-
clines to broad forms.

(4) Cyclic flowers pentamerous or tetramerous. The
flowers " in fives " are greatly in the majority, but some

Fig. 218. Leaves showing pinnate and palmate branching; the one to the left is from
sumach, that to the right from buckeye. — Caldwell.

very prominent families have flowers " in fours." There
are also dicotyledonous families with flowers "in threes,"
and some with flowers " in twos."

It should be remembered that no one of the above char-
acters, unless it be the character of the embryo, should be
depended upon absolutely to distinguish these two groups.

236

PLANT STRUCTURES

It is the combination of characters which determines a
group.

Monocotyledons

130. Introductory. — This great group gives evidence of
several distinct lines of development, distinguished by what
may be called the working out of different ideas. In this
way numerous families have resulted — that is, groups of

Fig. 219.

A leaf of honey locust, to show twice pinnate branching (bipinnate leaf).—
Caldwell.

forms which seem to belong together on account of similar
structures. This similarity of structure is taken to mean
relationship. A family, therefore, is made up of a group
of nearly related forms. Opinions may differ as to what
forms are so nearly related that they deserve to consti-
tute a distinct family. A single family of some botanists
may be " split up " into two or more families by others.
Despite this diversity of opinion, most of the families are
fairly well recognized.

MONOCOTYLEDONS AND DICOTYLEDONS 237

Within a family there are smaller groups, indicating
closer relationships, known as genera (singular, genus).
For example, in the great family to which the asters belong,
the different asters resemble one another more than they do
any other members of the family, and hence are grouped
together in a genus Aster. In the same family the golden-
rods are grouped together in the genus Solidago. The
different kinds of Aster or of Solidago are called species
(singular also species). A group of related species, there-
fore, forms a genus ; and a group of related genera forms a
family.

The technical name of a plant is the combination of its
generic and specific names, the former always being written
first. For example, Quercus alba is the name of the com-
mon white oak, Quercus being the name of the genus to
which all oaks belong, and alba the specific name which
distinguishes this oak from other oaks. Xo other names
are necessary, as no two genera of plants can bear the same
name.

In the Monocotyledons about forty families are recog-
nized, containing numerous genera, and among these
genera the twenty thousand species are distributed. It is
evident that it will be impossible to consider such a vast
array of forms, even the families being too numerous to
mention. A few important families will be mentioned,
which will serve to illustrate the group.

131. Pondweeds. — These are submerged aquatics, found
in most fresh waters (some are marine), and are regarded
as among the simplest Monocotyledons. They are slender,
branching herbs, growing under water, but often having
floating leaves, and sending the simple flowers or flower
clusters above the surface for pollination and seed-distri-
bution. The common pondweed (Potamogeton) contains
numerous species (Fig. 220), while Naias (naiads) and
Zannichellia (horned pondweed) are common genera in
ponds and slow waters.
34

238

PLANT STEUCTUKES

The simple character of these forms is indicated by their
aquatic habit and also by their flowers, which are mostly
naked and with few sporophylls. A flower may consist of
a single stamen, or a single carpel ; or there may be several
stamens and carpels associated, but without any coalescence
(Fig. 220, B).

In the same general line with the pondweeds, but with
more complex flowers, are the genera Sagittaria (arrow-

Fiq. 220. Pondweed (Potamogeton): A, branch with cluster (spike) of simple flowers,
showing also the broad floating leaves and the narrow submerged ones; B, a sin-
gle flower, showing the inconspicuous perianth lobes (c), the short stamens (a),
and the two short styles with conspicuous stigmatic surfaces. — A after Reichen-
bach; B after Le Maout and Decaisnb.

Fig. 221. Cat-tails {Typha\ showing the dense spikes of very simple flowers, each
showing two regions, the lower the pistillate flowers, the upper the staminate.—
From ""*Field, Forest, and Wayside Flowers."

240

PLANT STRUCTURES

leaf) and Alisma (water-plantain), in which there is a dis-
tinct calyx and corolla. The genus Typlia (cat-tail) is also
an aquatic or marsh form of very simple type, the flow-
ers being in dense
cylindrical clusters
(spikes), the upper
flowers consisting of
stamens, the lower of
carpels, thus forming
two very distinct re-
gions of the spike
(Fig. 221).

132. Grasses. —
This is one of the
largest and probably
one of the most use-
ful groups of plants,
as well as one of the
most peculiar. It is
world-wide in its dis-
tribution, and is re-
markable in its dis-
play of individuals,
often growing so
densely over large
areas as to form a
close turf. If the
grass -like sedges be
associated with them
there are about six
thousand species,
representing nearly
one third of the Mon-
ocotyledons. Here
belong the various
cereals, sugar canes,

Fig. 222. A common meadow grass (Festuca) : A,
portion of flower cluster (spikelet), showing the
bracts, in the axils of two of which flowers are
exposed ; B, a single flower with its envelop-
ing bract, showing three stamens, and a pistil
whose ovary bears two style branches with much
branched stigmas.— After Stkasburger.

MONOCOTYLEDONS AND DICOTYLEDONS 241

bamboos, and pasture grasses, all of them immensely use-
ful plants.

The flowers are very simple, having no evident perianth
(Fig. 222). Most commonly a flower consists of three sta-
mens, surrounding a single carpel, whose ovary ripens into
the grain, the characteristic seed-like fruit of the group.
The stamens, however, may be of any number from one to
six. The flowers, therefore, are naked, with indefinite num-
bers, and hypogynous, indicating a comparatively simple
type. It is also noteworthy that the group is anemophilous.

One of the noteworthy features of the group is the
prominent development of peculiar leaves (bracts) in con-
nection with the flowers. Each flower is completely pro-
tected or even inclosed by one of these bracts, and as the
bracts usually overlap one another the flowers are invisible
until the bracts spread apart and permit the long dangling
stamens to show themselves. These bracts form the so-
called "chaff" of wheat and other cereals, where they
persist and more or less envelop the grain (ripened ovary).
As they are usually called glumes, the grasses and sedges
are said to be glumaceous plants.

Grasses are not always lowly plants, for in the tropics
the bamboos and canes form growths that may well be
called forests. The grasses constitute the family Graminece,
and the sedges the family Cyperacece.

133. Palms. — More than one thousand species of palms
are grouped in the family Palmacew. These are the tree
Monocotyledons, and are very characteristic of the tropics,
only the palmetto getting as far north as our Gulf States.
The habit of body is like that of tree-ferns and Cycads, a
tall unbranched columnar trunk bearing at its summit a
crown of huge leaves which are pinnate or palmate in char-
acter, and often splitting so as to appear lobed or compound
(Figs. 223, 224).

The flower clusters are usually very large (Fig. 223),
and each cluster at first is inclosed in a huge bract, which

Fig. 223. A date palm, showing the unbranched columnar trunk covered with old leaf
bases, and with a cluster of huge pinnate leaves at the top, only the lowest por-
tions of which are shown ; two of the very heavy fruit clusters are also shown. —
From " Plant Relations."

MONOCOTYLEDONS AND DICOTYLEDONS

243

is often hard. Usually a perianth is present, but with no
differentiation of calyx and corolla, and the flower parts are
quite definitely in " threes," so that the cyclic arrangement
with the characteristic Monocotyledon number appears.

Fio.

224. A fan palm, with low stem and crown of large palmate leaves, which have
split so as to appear palmately branched.— From " Plant Relations."

134. Aroids. — This is a group of nearly one thousand
species, most of them belonging to the family A racece. In
our flora the Indian turnip or Jack-in-the-pulpit (Arismma)
(Fig. 225), sweetflag (Acorns), and skunk-cabbage (Symplo-
carpus), may be taken as representatives ; while the culti-
vated Calla-lily is perhaps even better known. The great
display of aroids, however, is in the tropics, where they are
endlessly modified in form and structure, and are erect, or
climbing, or epiphytic.

244

PLANT STKUCTUKES

The flowers are usually very simple, often being naked,
with two to nine stamens, and one to four carpels (Fig.

Fig. 235. Jack-in-the-pulpit (Ariscema), showing the overarching spathes; in one
case a side view shows the naked tip of the projecting spadix.— After Atkinson.

197). They are inconspicuous and closely set upon the
lower part of a fleshy axis, which is naked above and often

MONOCOTYLEDONS AND DICOTYLEDONS

245

modified in a remarkable way into a club-shaped or tail-like
often brightly colored affair. This singular flower-cluster
with its fleshy axis is called a spadix. The flowers often
include but one sort of sporophyll, and staminate and
pistillate flowers hold different positions upon the spadix
(Fig. 226).

The spadix is enveloped by a great bract, which sur-
rounds and overarches like a large loose hood, and is called
the spathe. The spathe is exceedingly
variable in form, and is often conspic-
uously colored, forming in the Calla-
lily the conspicuous white part, within
which the spadix may be seen, near the
base of which the flowers are found.
In Jack-in-the-pulpit (Fig. 225) it is
the overarching spathe which suggests
the " pulpit." The spadix and spathe
are the characteristic features of the
group, and the spathe is variously
modified in form, structure, and color
for insect pollination, as is the peri-
anth of other entomophilous groups.

Aroids are further peculiar in hav-
ing broad net-veined leaves of the Di-
cotyledon type. Altogether they form
a remarkably distinct group of Mon-
ocotyledons.

135. Lilies.— The lily and its allies are usually regarded
as the typical Monocotyledon forms. The perianth is
fully developed, and is very conspicuous, either undifferen-
tiated or with distinct calyx and corolla, and the flower is
well organized for insect pollination. The flowers are either
solitary or few in a cluster and correspondingly large, or in
more compact clusters and smaller. In any event, the
perianth is the conspicuous thing, rather than spathes or
glumes.

Fig. 226. Spadix of an
Arum, with spathe re-
moved, showing cluster
of naked pistillate flow-
ers at base, just above
a cluster of staminate
flowers, and the club-
shaped tip of the spa-
dix.— After Wossidlo.

246

PLANT STRUCTURES

In the general lily alliance, composed of eight or nine
families, there are more than four thousand species, repre-
senting about one fifth of all the Monocotyledons, and they
are distributed everywhere. They are almost all terrestrial
herbs, and are prominently geophilous ("earth -lovers") —

that is, they develop
bulbs, rootstocks, etc.,
which enable them to
disappear from above
the surface during un-
favorable conditions
(cold or drought), and
then to reappear rap-
idly upon the return
of favorable conditions
(Figs. 227, 228, 231,
233).

In the regular lily
family (Liliacece) the
flowers are hypogy-
nous and actinomor-
phic (Fig. 231), the
six perianth parts are
mostly alike and some-
times sympetalous (as
in the lily-of-the-val-
ley, hyacinth, easter
lily) (Figs. 201, 229),
the stamens are usu-
al ly six (two sets),
and the three carpels are syncarpous (Figs. 204, 230).
This is a higher combination of floral characters than
any of the preceding groups presents. Hypogyny and
actinomorphy are low, but a conspicuous perianth, syn-
carpy, and occasional sympetaly indicate considerable ad-
vancement.

Fig. 227. Wake-robin (Trillium), showing root-
stock, from which two branches arise, each bear-
ing a cycle (whorl) of three leaves and a single
trhnerous flower.— After Atkinson.

MONOCOTYLEDONS AND DICOTYLEDONS

247

In the amaryllis family (Amaryllidacece), a higher fam-
ily of the same general line, represented by species of Nar-
cissus (jonquils, daffodils, etc.), Agave, etc., the flowers
are distinctly epigynous.

Fig. 228. Star-of -Bethlehem ( Ornilhogalum) : a, entire plant with tuberous base and
trimerous flowers; b, a single flower; c, portion of flower showing relation of
parts, perianth lobes and stamens arising from beneath the prominent ovary (by-
pogynous); d, mature fruit; «, section of the syncarpous ovary, showing the three
carpels and loculi.— After Schimper.

In the iris family (Iridacece), the most highly specialized
family of the lily line, and represented by the various spe-

Fig. 239. The Japan lily, showing a tubular perianth, the parts of the perianth
distinct above.— From "Field, Forest, and Wayside Flowers."

MONOCOTYLEDONS AND DICOTYLEDONS

249

cies of Iris (flags) (Fig. 232), Crocus, Gladiolus (Figs. 233,
234), etc., the flowers are not only epigynous, but some of
them are zygomorphic.
When a plant has
reached both epigyny
and zygomorphy in its
flowers, it may be re-
garded as of high rank.

136. Orchids.— In
number of species this
(Orchidacece) is the
greatest family among
the Monocotyledons,
the species being vari-
ously estimated from
six thousand to ten
thousand, representing
between one third and
one half of all known
Monocotyledons. In display of individuals, however, the
orchids are not to be compared with the grasses, or even
with lilies, for the various species are what are called "rare
plants" — that is, not extensively distributed, and often
very much restricted. Although there are some beautiful
orchids in temperate regions, as species of Habenaria (rein-
orchis) (Fig. 235), Pogonia, Calopogon, Calypso, Cypripe-
dium (lady-slipper, or moccasin flower) (Fig. 236), etc.,
by far the greatest display and diversity are in the tropics,
where many of them are brilliantly flowered epiphytes
(Fig. 237).

Orchids are the most highly specialized of Monocoty-
ledons, and their brilliant coloration and bizarre forms are
associated with marvelous adaptation for insect visitation
(see Plant Relations, pp. 134, 135). The flowers are epigy-
nous and strongly zygomorphic. One of the petals is re-
markably modified, forming a conspicuous lip which is

Fig. 230. Diagrammatic cross-section of ovary
of Lilium PhUadelphicum, showing the three
loculi, in each of which are two ovules (mega-
sporangia); -4, ovule; B, integuments; C, nu-
cellus ; D, embryo-sac (megaspore). — Cald-
well.

Fig. 231. The common dog-tooth violet, showing the large mottled leaves and con-
spicuous flowers which are sent rapidly above the surface from the subterranean
bulb (see cut in the left lower corner), also some petals and stamens and the pistil
dissected out.— From " Plant Relations."

MONOCOTYLEDONS AND DICOTYLEDONS

251

modified in a great variety of
ways, and a prominent, often
very long, sjnir, in the bottom of
which nectar is secreted, which
must be reached by the proboscis
of an insect (Fig. 235). The
stamens are reduced to one or
two, and welded with the style

Fiq. 232. Flower of flag (Iris),
showing some of the sepals
and petals, one of the three
stamens, and the distinctly in-
ferior ovary, being an epigy-
nous flower.— After Grat.

Fig. 234. Flower cluster of Gla-
diolus, showing somewhat zygo-
morphic flowers.— Caldwell.

Fig. 233. Gladiolus, showing tuberous subter-
ranean stem from which roots descend, grass-
like leaves, and somewhat zygomorphic flow-
ers.—After Reichenbach.

252

PLANT STRUCTURES

and stigma tic surface into an indistinguishable mass in
the center of the flowers. The pollen-grains in each sac
are sticky and cohere in a club-shaped mass {pollinium),
which is pulled out and carried to another flower by the

Fig. 235. A flower of an orchid (Habena-
riar. at 1 the complete flower is shown,
with three sepals behind and three pet-
als in front, the lowest one of which has
developed a long strap-shaped portion
(lip) and a still longer spur portion, the
opening to which is seen at the base of
the strap, and behind the spur the long
inferior ovary (epigynous character) ;
the two pollen sacs of the single stamen
are seen in the center of the flower, di-
verging downward, and between them
stretches the stigma surface ; the rela-
tion between pollen sacs and stigma sur-
face is shown in 2 ; within each pollen
sac is a mass of sticky pollen (pollini-
um), ending below in a sticky disk,

which may be seen in 1 and 2 ; in 3 a pollen mass (a) is shown sticking to each

eye of a moth.— After Gray.

visiting insect. The whole structure indicates a very
highly specialized type, elaborately organized for insect
pollination.

Another interesting epigynous and zygomorphic trop-
ical group, but not so elaborate as the orchids, is repre-
sented by the cannas and bananas (Fig. 120), common in
cultivation as foliage plants, and the aromatic gingers.

From the simple pondweeds to the complex orchids the
evolution of the Monocotyledons has proceeded, and be-
tween them many prominent and successful families have
been worked out.

Fig. 236. A clump of lady-slippers (Cypripedium), showing the habic of the plant
and the general structure of the zygomorphic flower.— After Gibson.
35

254

PLANT STRUCTURES

Fig. 237. A group of orchids ( Cattleya), showing the very zygomorphic flowers, the
lip being well shown in the flower to the left (lowest petal).— Caldwell.

Dicotyledons

137. Introductory. — Dicotyledons form the greatest group
of plants in rank and in numbers, being the most highly
organized, and containing about eighty thousand species.
They represent the dominant and successful vegetation in
all regions, and are especially in the preponderance in tem-
perate regions. They are herbs, shrubs, and trees, of every
variety of size and habit, and the rich display of leaf forms
is notably conspicuous.

Two great groups of Dicotyledons are recognized, the
Archichlamydece and the Sympetalce. In the former there
is either no perianth or its parts are separate (polypeta-
lous) ; in the latter the corolla is sympetalous. The Archi-
chlamydeae are the simpler forms, beginning in as simple a
fashion as do the Monocotyledons ; while the Sympetalas

MONOCOTYLEDONS AND DICOTYLEDONS

255

are evidently derived from them and become the most
highly organized of all plants. The two groups each con-
tain about forty thousand species, but the Archichlamydese
contain about one hundred and sixty families, and the
Sympetalse about fifty.

To present over two hundred families, containing about
eighty thousand species, is clearly impossible, and a very
few of the prominent ones will be selected for illustrations.

Archichlamydem

138. Poplars and their allies. — This great alliance repre-
sents nearly five thousand species, and seems to form an
isolated group. It is a notable tree assemblage, and appar-
ently the most primitive and ancient group of Dicotyledons,
containing the most important deciduous forest forms of

Fig. 238. An oak in winter condition.— From " Plant Kelations."

256

TLANT STRUCTURES

temperate regions, for here belong the oak (Fig. 238), hick«
ory, walnut, chestnut, beech, poplar, birch, elm (Figs. 198,
239), willow (Fig. 240), etc. The primitive character is in-
dicated not merely by the floral structures, but also by the
general anemophilous habit.

In the poplar (Populus) and its allied form, the willow
(Salix), the flowers are naked and hypogynous (Fig. 196),

Fig. 239. An elm in foliage.— From "Plant Relations."

MONOCOTYLEDONS AND DICOTYLEDONS

257

the stamens are indefinite in number (two to thirty), and
the pistil is syncarpous (two carpels). The stamens and

Fig. 240. Flower clusters of willow (amenta); that to the left is pistillate, the other
staminate.— After Warming.

pistils are not only separated in different flowers, but upon
different plants, some plants being staminate and others
pistillate (Fig. 240). The flowers are clustered upon a long
axis, and each one is
protected by a promi-
nent bract. It is these
scaly bracts which
give character to the
cluster, which is called
an ament or catkin,
and the plants which
produce such clusters
are said to be amenta-
ceous. These aments
of poplars, "pussy
willows," and the
alders and birches are
very familiar objects
(Figs. 240, 241).

Fig. 241. Aments of alder (Alnus) : a, branch
with staminate aments (n), pistillate aments
(«i), and a young bud (£); b, pistillate ament at
time of discharging seeds, showing the promi-
nent bracts.— After Warming.

258

PLANT STRUCTURES

The only advanced character in the flowers as described
above is the syncarpous pistil, but in the great allied pepper
family (Piperacece) of the tropics, with its one thousand
species, and most nearly represented in our flora by the

Pig. 242. Ovule of hornbeam (Carpimis), showing chalazogamy: m, the micropyle;
pt, the pollen tube, which may be traced to its entrance into the embryo-sac at its
antipodal end, and thence upward through the sac toward the egg.— After Mart
Ewabt.

lizard-tail (Saururus) of the swamps (Fig. 195), the flowers
are not merely naked, but also apocarpous, and the whole
structure is much like that of the simplest Monocotyle-

MONOCOTYLEDONS AND DICOTYLEDONS 259

dons. The peppers seem to represent the simplest of the
Dicotyledons, and this great line may have begun with
some such forms.

A very interesting fact in connection with the fertiliza-
tion of certain amentaceous plants has been discovered.
In birch, alder, walnut, hornbeam, and some others, the
pollen-tube does not eiiter the ovule by way of the micro-
pyle, but pierces through in the region of the base of the
ovule and so penetrates to the embryo-sac (Fig. 242). As
the region of the ovule where integument and nucellus are
not distinguishable is called the chalaza, this phenomenon
is known as chalazogamy, meaning "fertilization through
the chalaza."

139. Buttercups and their allies. — This is a great assem-
blage of terrestrial herbs, including nearly five thousand
species, and is thought by many to be the great stock from
which most of the higher Dicotyledons have been derived.
The alliance includes the water-lilies, buttercups, and pop-
pies, the specialized mustards, and certain notable tree
forms, as magnolias, custard-apples, and the tropical laurels
with one thousand species represented in our flora only
by the sassafras. Here also is the strange group of " car-
nivprous" plants (Sarracenia, Drosera, Dionma, etc.). The
group is distinctly entomophilous, in striking contrast with
the preceding one.

Taking the buttercup {Ranunculus) as a type (Fig. 202),
the flower is hypogynous, the calyx and the corolla are dis-
tinctly differentiated and actinomorphic, and adapted for
insect-pollination, but the spiral arrangement and indefinite
numbers are very apparent, notably in connection with the
apocarpous pistils, which are very numerous upon a promi-
nent receptacle, but involving more or less all the parts.
The stamens are also very numerous (Figs. 200, 243, 244).
In the water-lilies the petals and stamens are indefinitely
numerous (Fig. 203), and in the poppies there is no definite
number. In many of the forms, however, in connection

Pig. 343. Marsh marigold (Caltha), a member of the Buttercup family, also showing
floral diagram, in which the floral leaves are five, but the stamens and apocarpous
pistils are indefinitely numerous.— After Atkinson.

Fig. 244. Zygomorphic flower of larkspur
(Delphinium), with sepals removed, show-
ing two petals with prominent spurs, and
numerous stamens. — After Baillon.

Fig. 245. Diagram of the zygomorphic
flower of larkspur (Delphinium), show-
ing the spur developed by a sepal and
inclosing the two petal spurs. — After
Baillon.

MONOCOTYLEDONS AND DICOTYLEDONS

261

with one or more of the parts, the Dicotyl number (five)
appears (Figs. 243, 245), but with no special constancy.

In certain genera of the buttercup family (Ranuncula-
cecp) zygomorphy appears, as in the larkspur (Delphinium)
with its spurred petals and sepals (Figs. 244, 245), and the
monkshood (Aconitum) with its hooded sepal ; and in the

Fig. 246. The common cabbage (Brassica), a member of the mustard family: A,
flower cluster, showing buds at tip, open flowers below with four spreading petals,
and forming pods below ; B. mature pod, with the persistent style; C, pod opening
by two valves, and showing seeds attached to the false partition.— After Warming.

water-lily family (Nymplmacem) and poppy family (Pajxi-
veracece) syncarpy appears. In this alliance, also, belong
the sweet-scented shrubs (Calycanthus), with their perigy-
nous flowers containing numerous parts (Fig. 206).

262

PLANT STRUCTURES

Fig. 347. Diagram of crucifer
flower, showing the relations
of parts ; four sepals, four
petals, six stamens, and one
carpel with a false partition.
—After Warming.

The most specialized large group in this alliance is
the mustard family (Cruciferce) , with twelve hundred
species, to which belong the mustards, cresses, shep-
herd's purse, peppergrass, radish, cabbage (Fig. 246), etc.
The sepals are four in two sets, the
petals four in one set, the stamens
six with two short ones in an outer
set and four long ones in an inner
set, and one pistil whose ovary be-
comes divided into two loculi by
what is called a "false partition"
(Figs. 246, C, 247), and usually be-
comes an elongated pod (Fig. 246,
Ay B). This specialized structure
of the flower distinctly marks the
family, whose name is suggested
by the fact that the four spreading
petals often form a Maltese cross (Fig. 246, A). The pecul-
iar stamen character, four long and two short stamens, is
called tetradynamous ("four strong").

140. Roses. — This family (Rosacem) of one thousand
species is one of the best known and most useful groups of
the temperate regions. In it are such forms as Spircea,
five-finger (Poten-
tilla), strawberry
(Fragaria) (Figs.
191, 207), raspberry
(Fig. 248), and
blackberry [Bu-
bus), rose (Rosa),
hawthorn (CratcB-
gus), apple, and
pear (Pirus) (Fig.
249), plum, cherry,
almond, and peach
(Prunus).

Fig. 248. The common raspberry: the figure to the
left showing flower-stalk, calyx, old stamens
(«), and prominent receptacle, from which the
"fruit" (a cluster of small stone fruits, each
representing a carpel) has been removed.— After
Bailey.

MONOCOTYLEDONS AND DICOTYLEDONS

2G3

Many of the true roses have a strong resemblance (Fig.
207) to the buttercups (Ranunculus), with their hypogy-
nous regular flowers, and indefinite number of stamens and
carpels, but the sepals and petals are much more frequently
five, the Dicotyl number being better established. The

Fig. 249. The common pear (Pints communis), showing hraneh with flowers (1). sec-
tion of a flower (x?) showing its epigynous character, section of fruit i.n showing
the thickened calyx outside of the ovary or "core" (indicated by dotted outline},
and flower diagram (i) showing all the organs in fives except the stamens.— After
Wossidlo.

whole family remains actinomorphic, but perigyny and
epigyny appear in certain forms (Fig. 205), giving rise to
the peculiar fruit (pome) of apples and pears (Fig. 249), in
which the calyx and ovary ripen together. Another spe-
cialized group of roses is that which develops the stone-

264

PLANT STRUCTURES

fruits (drupes), as apricots, peaches (Fig. 189), plums,
cherries.

141. Legumes. — This is far the greatest family (Legumi-
nosce) of the Archichlamydeae, containing about seven thou-
sand species, distributed everywhere and of every habit. It
is the great zygomorphic group of the Archichlamydeas,
being elaborately adapted to insect pollination. The more

Fig. 250. A legume plant (Lotus), showing flowering branch (1), a single flower (2)
showing zygomorphic corolla, the cluster of ten stamens (8) which with the carpel
is included in the keel, the solitary carpel (h) which develops into the pod or le-
gume (5), the petals (6) dissected apart and showing standard (a), wings (b), and
the two lower petals (c) which fold together to form the keel, and the floral dia-
gram (7).— After Wossidlo.

primitive forms of the Leguminosge, the mimosas, acacias
(Fig. 251), etc., very much resemble true roses and the but-
tercups, with their hypogynous regular flowers and nu-
merous stamens, but the vast majority are Papilio forms
with very irregular (zygomorphic) flowers and few stamens

MONOCOTYLEDONS AND DICOTYLEDONS

265

(Fig. 250). The petals are very dissimilar, the upper one
(standard) being the largest, and erect or spreading, the two
lateral ones (icings) oblique and descending, the two lower
ones coherent by their edges to form a projecting boat-shaped
body (heel), which
incloses the sta-
mens and pistil.
From a fancied re-
semblance to a but-
terfly such flowers
are said to be papil-
ionaceous.

The whole fam-
ily is further char-
acterized by the sin-
gle carpel, which
after fertilization
develops a pod
(Fig. 250, 5), which
often becomes re-
markably large as
compared with the
carpel. It is this
peculiar pod (le-
gume) which has
given to the family
its technical name
Leguminosce and
the common name
"Legumes."

Well-known members of the family are lupine ( Lupi-
nus), clover (Trifolium), locust (Robinia), Wistaria, pea
(Pisum), bean (Phaseolus), tragacanth (Astragalus), vetch
(Vicia), redbud (Cercis), senna (Cassia), honey-locust
(Gleditschia), indigo (Indigo/era), sensitive-plants (Acacia,
Mimosa, etc.) (Fig. 251), etc.

Fig. 251. A sensitive-plant (Acacia), showing the
flowers with inconspicuous petals and very nu-
merous stamens, and the pinnately branched sen-
sitive leaves. — After Meter and Schumann.

266

PLANT STRUCTURES

142. Umbellifers. — This is the most highly organized
family ( Umbelliferce) of the Archichlamydeae, which may
be said to extend from Peppers to Umbellifers. The Le-
gumes adopt zygomorphy, but remain hypogynous ; and in
some of the Koses epigyny appears ; but the Umbellifers
with their fifteen hundred species are all distinctly epigy-

Fig. 252. The common carrot (Dauciis Carota): A. branch bearing the compound
umbels; B, a single epigynous flower, showing inferior ovary, five spreading
petals, five stamens alternating with the petals, and the two styles of the bicarpel-
lary pistil; C, section of flower, showing relation of parts, and also the minute
sepals near the top of the ovary and just beneath the other parts. — After Warming.

nous (Fig. 252, B, C), being one of the very few epigy-
nous families among the Archichlamydeae. In addition
to epigyny, the cyclic arrangement and definite Dicotyl
number is established, there being five sepals, five petals,
five stamens, and two carpels, the highest known floral

MONOCOTYLEDONS AND DICOTYLEDONS

267

formula, and one that appears among the highest Sym-
petalae.

The name of the family is suggested by the character-
istic inflorescence, which is also of advanced type. The
flowers are reduced in
size and massed in flat- /0§?fii
topped clusters called
umbels (Figs. 252, A, 253).
The branches of the clus-
ter arise in cycles from
the axis like the braces
of an umbrella. As a re-
sult of the close approxi-
mation of the flowers the
sepals are much reduced
in size and often obsolete
(Fig. 252, G).

The Umbellifers are
mainly perennial herbs of
the north temperate re-
gions, forming a very dis-
tinct family, and contain-
ing the following familiar
forms : carrot (Daucus)
(Fig. 252), parsnip (Pasti-
naca), hemlock (Conium)
(Fig. 253), pepper-and-
salt (Erigenia), caraway
(Carum), fennel (Fcenic-
ulum), coriander (Cori-
andrum), celery (Api-
um), parsley {Petroseli-
num), etc. Allied to the
Umbellifers are the Ara-
lias (Araliacece), and the
Dogwoods (Cornacece).

Fig. 253. Hemlock ( Conium), an Umbellifer,
showing the umbels, with the principal
rays rising from a cycle of bracts (invo-
lucre), and each bearing at its summit a
secondary umbel with its cycle of second-
ary bracts (involucel).— After Schimpeb.

268 PLANT STRUCTURES

Sympetalm

143. Introductory. — These are the highest and the most
recent Dicotyledons. While they contain numerous shrubs
and trees in the tropics, they are by no means such a shrub
and tree group in the temperate regions as are the Archi-
chlamydeae. The flowers are constantly cyclic, the num-
ber five or four is established, and the corolla is sympeta-
lous, the stamens usually being borne upon its tube (Figs.
208, 209, 212).

There are two well-defined groups of Sympetalae, distin-
guished from one another by the number of cycles and the
number of carpels in the flower. The group containing
the lower forms is pentacyclic, meaning " cycles five," there
being two sets of stamens. In it also there are five carpels,
the floral formula being, Sepals 5, Petals 5, Stamens 5 + 5,
Carpels 5. As the carpels are the same in number as the
other parts, the flowers are called isocarpic, meaning " car-
pels same." The group is named either Pentacyclai or Iso-
carpce, and contains about ten families and 4,000 species.

The higher groups, containing about forty families and
36,000 species, is tetracyclic, meaning " cycles four," and
anisocarpic, meaning "carpels not the same," the floral
formula being, Sepals 5, Petals 5, Stamens 5, Carpels 2.
The group name, therefore, is Tetracyclce or Anisocarpce.

144. Heaths. —The Heath family {Ericacece) and its allies
represent about two thousand species. They are mostly
shrubs, sometimes trailing, and are displayed chiefly in
temperate and arctic or alpine regions, in cold and damp
or dry places, often being prominent vegetation in bogs
and heaths, to which latter they give name (Fig. 254). The
flowers are pentacyclic and isocarpic, as well as mostly hyp-
ogynous and actinomorphic. It is interesting to note that
some forms are not sympetalous, the petals being distinct,
showing a close relationship to the Archichlamydeae. One
of the marked characteristics of the group is the dehiscence

MONOCOTYLEDONS AND DICOTYLEDONS 269

of the pollen-sacs by terminal pores, which are often pro-
longed into tubes (Fig. 255).

Fig. 254. Characteristic heath plants: A, B, C, Lyonia, showing sympetalous flowers
and single style from the lobed syncarpous ovary; D, two forms of Casdope,
showing trailing habit, small overlapping leaves, and sympetalous flowers, but in
the smaller form the petals are almost distinct. — After Drude.

Common representatives of the family are as follows :
huckleberry {Gaylussacia) , cranberry and blueberry ( Vac-
cinium), bearberry (Arctostaphylos), trailing arbutus {Epi-
36

270

PLANT STRUCTURES

gcea), wintergreen (Gaultheria), heather (Calluna), moun-
tain laurel (Kalmia), Azalea, Rhododendron (Fig. 256),
Indian pipe (Monotropa), etc.

Fig. 255. Flowers of heath plants (Erica), showing complete flowers (.4), the sta-
mens with "two-horned" anthers which discharge pollen throngh terminal pores,
and the lobed syncarpous ovary with single style and prominent terminal stigma
(B, C, D).— After Drude.

145. Convolvulus forms. — The well-known morning-glory
(Ipomcea) (Fig. 209) may be taken as a type of the Convol-

MONOCOTYLEDONS AND DICOTYLEDONS

271

vulus family (Convolvulacece). Allied with it are Polemo-
nium and Phlox (Fig. 210, b) (Polemomacew), the gentians
(Ge)itianacece), and the dog-banes (Apocynacem) (Fig. 257).
It is here that the regular sympetalous flower reaches its
highest expression in the form of conspicuous tubes, f un-

Fig. 256. A cluster of Rhododendron flowers.— After Hooker.

nels (Fig. 258), trumpets, etc. The flowers are tetracyclic
and anisocarpic, besides being hypogynous and actinomor-
phic. These regular tubular forms represent about five
thousand species, and contain many of the best-known
flowers.

272

PLANT STRUCTURES

146. Labiates. — This great family {Labiatce) and its alli-
ances represent more than ten thousand species. Tne con-
spicuous feature is the
zygomorphic flower, dif-
fering in this regard from
the Convolvulus forms,
which they resemble
being tetracyclic and ani-
socarpic, as well as hypogy-
nous. The irregularity
consists in organizing the
mouth of the sympetalous
corolla into two "lips,"
resulting in the labiate or

Fig. 257. A common dogbane (Apocynum). — From "Field, Forest, and Wayside

Flowers."

Fig 258 The hedge bindweed ( Con wlvulm I, showing the twining habit and the con-
spicuous funnelform corollas.-From " Field, Forest, and Wayside Flowers.

274

PLANT STRUCTURES

bilabiate structure (Fig. 210, c, d, e), and suggesting the
name of the dominant family. The upper lip usually con-
tains two petals, and the lower three ; the two lips are some-
times widely separated, and sometimes in close contact, and
differ widely in relative prominence.

Associated with zygomorphy in this group is a frequent
reduction in the number of stamens, which are often four
(Fig. 212) or two. The whole structure is highly special-
ized for the visits of insects, and this great zygomorphic
alliance holds the same
relative position among
Sympetalae as is held
by the zygomorphic Le-
gumes among Archi-
chlamydeae.

In the mint family,
as the Labiates are often
called, there are about
two thousand seven hun-
dred species, including
mint {Mentha) (Fig.
212), dittany (Cunila),
hyssop {Hyssopus), mar-
joram {Origanum),

Fig. 259. Flowers of dead nettle (La-
mium) : A, entire bilabiate flower ;
B, section of flower, showing rela-
tion of parts.— After Warming.

Fig. 260. A labiate plant (Teucrium), show-
ing branch with flower clusters (^4), and
side view of a few flowers (B), showing
their bilabiate character. — After Briquet.

MONOCOTYLEDONS AND DICOTYLEDONS 275

thyme {Thymus), balm {Melissa), sage {Salvia), catnip
(Nepeta), skullcap {Scutellaria), horehound {Marrubium),
lavender {Lavandula), rosemary {Rosmarinus), dead nettle
{Lamium) (Fig. 259), Teucrium (Figs. 213, 260), etc., a
remarkable series of aromatic forms.

Allied is the Nightshade family {Solanaceai), with fif-
teen hundred species, containing such common forms as
the nightshades and potato {Solatium), tomato {Lycoper-
sicum), tobacco {Nicotiana) (Fig. 208), etc., in which the
corolla is actinomorphic or nearly so ; also the great Fig-
wort family {Scrophulariacece) , with two thousand species,
represented by mullein ( Verbascum), snapdragon {Antir-
rhinum) (Fig. 210, e), toad-flax {Linaria) (Fig. 210, d),
Pentstemon, speedwell ( Veronica), Gerardia, painted cup
{Castilleia), etc.; also the Verbena family ( Verbenacece),
with over seven hundred species ; and the two hundred
plantains {Plant aginacem), etc.

147. Composites. — This greatest and ranking family
( Compositce) of Angiosperms is estimated to contain at least
twelve thousand species, containing more than one seventh
of all known Dicotyledons and more than one tenth of all
Seed-plants. Not only is it the greatest family, but it is
the youngest. Composites are distributed everywhere, but
are most numerous in temperate regions, and are mostly
herbs.

The name of the family suggests the most conspicuous
feature — namely, the remarkably complete organization of
the numerous small flowers into a compact head which
resembles a single flower, formerly called a "compound
flower." Taking the head of an Arnica as a type (Fig.
261), the outermost set of organs consists of more or less
leaf -like bracts or scales {involucre), which resemble sepals ;
within these is a circle of flowers with conspicuous yellow
corollas {rays), which are zygomorphic, being split above
the tubular base and flattened into a strap-shaped body,
and much resembling petals (Fig. 261, A, D) ; within the

Fig. 261. Flowers of Arnica: .4, lower part of stem, and upper part bearing a
head, in which are seen the conspicuous rays and the disk: I), Bingle ray flower,
showing the corolla, tubular at base and strap-shaped above, the two-parted style,
the tuft of pappus hairs, aud the inferior ovary which develops into a seed-like
fruit (akene); E. single disk flower, showing tubular corolla with spreading limb,
the two-parted style emerging from the top of the Stamen tube, the prominent
pappus, and the inferior ovary or akene; C, a single stamen. — After Hoffman.
276

MONOCOTYLEDONS AND DICOTYLEDONS

277

ray-flowers is the broad expanse supplied by a very much
broadened axis, and known as the disk (Fig. 261, A), which
is closely packed with very numerous small and regular
tubular flowers, known as dish-flowers (Fig. 261, e).

Fig. 262. The common dandelion ( Taraxacum): 1, two flower stalks; in one the head
is closed, showing the double involucre, the inner erect, the outer reflexed, in the
other the head open, showing that all the flowers are strap-shaped; S, a single
flower shewing inferior ovary, pappus, corolla, stamen tube, and two-parted style;
;;, a mature akene; 4, a head from which all but one of the akenes have been re-
moved, showing the pitted receptacle and the prominent pappus beak.— After
Strasburuer.

The division of labor among the flowers of a single head
is plainly marked, and sometimes it becomes quite com-
plex. The closely packed flowers have resulted in modify-
ing the sepals extremely. Sometimes they disappear en-

278

PLANT STKUCTUKES

tirely ; sometimes they become a tuft of delicate hairs, as
in Arnica (Fig. 261, D, E), thistle (Cnicus), and dandelion
(Taraxacum) (Fig. 263), surmounting the seed-like akene
and aiding in its transportation through the air ; sometimes
they are converted into two or more tooth-like and often

Fig. 263. Flowers of dandelion, showing action of style in removing pollen from the
stamen tube: /, style having elongated through the tube and carrying pollen; 2,
style branches beginning to recurve; 3, style branches completely recurved.-^
From "Field, Forest, and Wayside Flowers."

barbed processes arising from the akene, as in tickseed
(Coreopsis) and beggar-ticks (Fig. 188) or Spanish needles
(Bidens), to lay hold of passing animals ; sometimes they
become beautifully plumose bristles, as in the blazing star
(Liatris) ; sometimes they simply form a more or less con-
spicuous cup or set of scales crowning the akene. In all
of these modifications the calyx is called pappus.

The stamens within the corolla are organized into a
tube by their coalescent anthers (Fig. 263), and discharge
their pollen within, which is carried to the surface of the

MONOCOTYLEDONS AND DICOTYLEDONS 279

head and exposed by the swab-like rising of the style (Fig.
263). The head is thus smeared with pollen, and visiting
insects can not fail to distribute it over the head or carry
it to some other head.

In the dandelion and its allies the flowers of the disk
are like the ray-flowers, the corolla being zygomorphic and
strap-shaped (Figs. 262, 263).

The combination of characters is sympetalous, tetracyc-
lic, and anisocarpic flowers, which are epigynous and often
zygomorphic, with stamens organized into a tube and calyx
modified into a pappus, and numerous flowers organized
into a compact involucrate head in which there is more or
less division of labor. There is no group of plants that
shows such high organization, and the Compositae seem to
deserve the distinction of the highest family of the plant
kingdom.

The well-known forms are too numerous to mention,
but among them, in addition to those already mentioned,
there are iron-weed ( Vernonia), Aster, daisy (Bellis),
goldenrod {Solidago), rosin-weed and compass-plant {Silph-
ium), sunflower (Helianthus), Chrysanthemum, ragweed
{Ambrosia), cocklebur {Xanthium), ox-eye daisy (Leucan-
themum), tansy (Tanacetum), wormwood and sage-brush
{Artemisia), lettuce {Lactuca), etc.

CHAPTER XV

DIFFERENTIATION OF TISSUES

148. Introductory. — Among the simplest Thallophytes
the cells forming the body are practically all alike, both as
to form and work. What one cell does all do, and there
is very little dependence of cells upon one another. As
plant bodies become larger this condition of things can not
continue, as all of the cells can not be put into the same
relations. In such a body certain cells can be related to
the external food supply only through other cells, and the
body becomes differentiated. In fact, the relating of cells
to one another and to the external food-supply makes large
bodies possible.

The first differentiation of the plant body is that which
separates nutritive cells from reproductive cells, and this is
accomplished quite completely among the Thallophytes.
The differentiation of the tissues of the nutritive body,
however, is that which specially concerns us in this chapter.

A tissue is an aggregation of similar cells doing similar
work. Among the Thallophytes the nutritive body is prac-
tically one tissue, although in some of the larger Thallo-
phytes the outer and the inner cells differ somewhat. This
primitive tissue, composed of cells with thin walls and ac-
tive protoplasm, and to be regarded as the parent tissue, is
called parenchyma.

Among the Bryophytes, in the leafy gametophore and

in the sporogonium, there is often developed considerable

dissimilarity among the cells forming the nutritive body,

but the cells may all still be regarded as parenchyma. It

280

DIFFERENTIATION OF TISSUES 281

is in the sporophyte of the Pteridophytes and Spermato-
phytes that this differentiation of tissues becomes extreme,
and tissues are organized which differ decidedly from
parenchyma. This differentiation means division of labor,
and the more highly organized the body the more tissues
there are.

All the other tissues are derived from parenchyma, and
as the work of nutrition and of reproduction is always
retained by the parenchyma cells, the derived tissues are
for mechanical rather
than for vital purposes.
There is a long list of
these derived and me-
chanical tissues, some of
them being of general
occurrence, and others
more restricted, and
there is every gradation

between them and the Fig. 2&4. Parenchyma and sclerenchyma from

parenchyma from which the 8tem of **""<*• in cross-section.-CHAM-

r J BERLAIX.

they have come. \Ve

shall note only a few which are distinctly differentiated

and which are common to all vascular plants.

149. Parenchyma. — The parenchyma of the vascular plants
is typically made up of cells which have thin walls and whose
three dimensions are approximately equal (Figs. 264, 265),
though sometimes they are elongated. Until abandoned,
such cells contain very active protoplasm, and it is in them
that nutritive work and cell division are carried on. So
long as these cells retain the power of cell division the
tissue is called meristem, or it is said to be meristematic,
from a Greek word meaning " to divide." When the cells
stop dividing, the tissue is said to be permanent. The
growing points of organs, as stems, roots, and leaves, are
composed of parenchyma which is meristematic (Figs. 266,
274), and meristem occurs wherever growth is going on.

282

PLANT STEUCTUKES

150. Mestome and stereome. — When the plant body be-
corned complex a conductive system is necessary, so that
the different regions of the body may be put into communi-
cation. The material absorbed
by the roots must be carried to
the leaves, and the food manu-
factured in the leaves must
be carried to regions of growth
and storage. This business of
transportation is provided for
by the specially organized ves-
sels referred to in preceding
chapters, and all conducting tis-
sue, of whatever kind, is spoken
of collectively as mestome.

If a complex body is to main-
tain its form, and especially if
it is to stand upright and be-
come large, it must develop
structures rigid enough to fur-
nish mechanical support. All
the tissues which serve this pur-
pose are collectively known as
stereome.

The sporophyte body of
Pteridophytes and Spermato-
phytes, therefore, is mostly
made up of living and working parenchyma, which is
traversed by mechanical mestome and stereome.

151. Dicotyl and Conifer stems. — The stems of these two
groups are so nearly alike in general plan that they may
be considered together. In fact, the resemblances were
once thought to be so important that these two groups
were put together and kept distinct from Monocotyledons ;
but this was before the gametophyte structures were
known to bear very different testimony.

Fig. 265. Same tissues as in pre-
ceding figure, in longitudinal sec-
tion, the parenchyma showing
nuclei. — Chamberlain.

DIFFEKEMTlATION OF TISSUES

283

At the apex of the growing stem there is a group of
active meristem cells, from which all the tissues are de-
rived (Fig. 266). This group is known as the apical group.
Below the apical group the tissues and regions of~Ehestem
begin to appear, and still farther down they become dis-
tinctly differentiated, passing into permanent tissue, the
apical group by its
divisions continually
adding to them and
increasing the stem
in length.

Just behind the
apical group, the
cells begin to give the
appearance of being
organized into three
great embryonic re-
gions, the cells still
remaining meristem-
atic (Fig. 266). At
the surface there is a
single layer of cells
distinct from those

within, known as the dermatogen, or " skin-producer," as
farther down, where it becomes permanent tissue, it is the
epidermis. In the center of the embryonic region there
is organized a solid cylinder of cells, distinct from those
around it, and called the plerome, meaning " that which
fills up." Farther down, where the plerome passes into
permanent tissue, it is called the central cylinder or stele
("column"). Between the plerome and dermatogen is
a tissue region called the periblem, meaning " that which
is put around," and when it becomes permanent tissue it
is called the cortex, meaning "bark" or "rind."

Putting these facts together, the general statement is
that at the apex there is the apical group of meristem cells ;

Fig. 266. Section through growing point of stem of
Bippuris : below the growing point, composed
of a uniform meristem tissue, the three embry-
onic regions are outlined, showing the dermato-
gen (d, d), the central plerome (p,p), and be-
tween them the periblem. — After De Bart.

284

PLANT STRUCTURES

below them are the three embryonic regions, dermatogen,
periblem, and plerome ; and farther below these three
regions pass into permanent tissue, organizing the epider-
mis, cortex, and stele. The three embryonic regions are
usually not so distinct in the Conifer stem as in the Dico-
tyl stem, but both stems have epidermis, cortex, and stele.
Epidermis. — The epidermis is a protective layer, whose
cells do not become so much modified but that they may
be regarded as parenchyma. It gives rise also to super-
ficial parts, as hairs, etc. In the case of trees, the epidermis
does not usually keep up with the increasing diameter, and
disappears. This puts the work of protection upon the
cortex, which organizes a superficial tissue called cork, a
prominent part of the structure known as bark.

Cortex. — The cortex is characterized by containing
much active parenchyma, or primitive tissue, being the
chief seat of the life activities of the stem. Its superficial
cells, at least, contain chlorophyll and do chlorophyll work,
while its deeper cells are usually temporary storage places

for food. The cortex is also char-
acterized by the development of
stereome, or rigid tissues for me-
chanical support. The stereome
may brace the epidermis, forming
the Jiypodermis ; or it may form
bands and strands within the cor-
tex ; in fact, its amount and ar-
rangement differ widely in differ-
ent plants.

The two principal stereome tis-
sues are collenchyma and scleren-
chyma, meaning " sheath-tissue "
and " hard-tissue " respectively.
In collenchyma the cells are thick-
ened at the angles and have very elastic walls (Fig. 267),
making the tissue well adapted for parts which are growing

Fig. 267. Some collenchyma
cells from the stem of a com-
mon dock (Bitmex), showing
the cells thickened at the
angles. —Ch AMBERLAIN.

DIFFERENTIATION OF TISSUES

285

in length. The chief mechanical tissue for parts which
have stopped growing in length is sclerenchyma (Figs. 264,
265). The cells are thick-walled, and usually elongated
and with tapering ends, including the so-called "fibers."

Fig. 268. Sections through an open collateral vascular bundle from a sunflower stem;
A, cross-section; B. longitudinal section; the letters in both referring to the same
structures; M, pith; X, xylem, containing spiral (8, «') and pitted (t, t') vessels;
C, cambium; P, phloem, containing sieve vessels (sb); b, a mass of bast fibers or
sclerenchyma; ic, pith rays between the bundles; e, the bundle sheath; R, cor-
tex.—After Vines.

Stele.— The characteristic feature of the stele or central
cylinder is the development of the mestome or vascular
37

286

PLANT STRUCTURES

tissues, of which there are two prominent kinds. The
traclieary vessels are for water conduction, and are cells
with heavy walls and usually large diameter (Fig. 268).
The thickening of the walls is not uniform, giving them a
very characteristic appearance, the thickening taking the
form of spiral bands, rings, or reticulations (Fig. 268, B).
Often the reticulation has such close meshes that the cell
wall has the appearance of being covered with thin spots,
and such cells are called " pitted vessels." The vessels with
spirals and rings are usually much smaller in diameter than
the pitted ones. The true traclieary cells are more or less
elongated and without tapering ends, fitting end to end
and forming a continuous longitudinal series, suggesting a
trachea, and hence the name. In the Conifers there are

no true traclieary cells, as in
the Dicotyledons, except a few
small spiral vessels which are
formed at first in the young
stele, but the tracheary tissue
is made up of tracheitis, mean-
ing "trachea -like," differing
from trachea or true traclieary
vessels in having tapering ends
and in not forming a continu-
ous series (Fig. 269). The walls
of these tracheids are "pitted"
in a way which is characteristic
of Gymnosperms, the "pits"
appearing as two concentric
rings, called "bordered pits."

The other prominent mes-
tome tissue developed in the
stele is the sieve vessels, for the
conduction of organized food, chiefly proteids (Fig. 268).
Sieve cells are so named because in their walls special areas
are organized which are perforated like the lid of a pepper-

Fig. 269. Tracheids from wood of
pine, showing tapering ends and
bordered pits.— Chamberlain.

DIFFERENTIATION OF TISSUES 287

box or a " sieve." These perforated areas are the sieve-
plates, and through them the vessels communicate with
one another and with the adjacent tissue.

The tracheary and sieve vessels occur in separate
strands, the tracheary strand being called xylem (" wood *),
the sieve strand phloem ("bark "). A xylem and a phloem
strand are usually organized together to form a vascular
bundle, and it is these fiber-like bundles which are found
traversing the stems of all vascular plants and appearing
conspicuously as the veins of leaves. Among the Dicotyls
and Conifers the vascular bundles appear in the stele in
such a way as to outline a hollow cylinder (Fig. 216), the
xylem of each bundle being toward the center, the phloem
toward the circumference of the stem. The undifferenti-
ated parenchyma of the stele which the vascular cylinder
incloses is called the pith. In older parte of the stem the
pith is often abandoned by the activities of the plant, and
either remains as a dead spongy tissue, or disappears en-
tirely, leaving a hollow stem. Between the bundles form-
ing the vascular cylinder there is also undifferentiated
parenchyma, and as it seems to extend from the pith out
between the bundles like "rays from the sun," the rays
are called pith rays.

Such vascular bundles as described above, in which the
xylem and phloem strands are " side-by-side " upon the same
radius, are called collateral (Fig. 270). One of the pecul-
iarities of the collateral bundles of Dicotyls and Conifers,
however, is that when the two strands of each bundle are
organized some meristem is left between them. This means
that between the strands the work of forming new cells can
go on. Such bundles are said to be open ; and the open
collateral bundle is characteristic of the stems of the Dico-
tyls and Conifers.

The meristem between the xylem and phloem of the
open bundle is called cambium (Figs. 268, 270). The cam-
bium also extends across the pith rays between the bundles,

288

PLANT STRUCTURES

connecting the cambium in the bundles, and thus forming
a cambium cylinder, which separates the xylem and phloem
of the vascular cylinder. This cambium continues the f or-

Fig. 270. Cross-section of open collateral vascular bundle from stem of castor-oil
plant (Bicinus), showing pith cells (to), xylem containing spiral (t) and pitted (g)
vessels, cambium of bundle (c) and of pith rays (cb), phloem containing sieve vea
sels (y), three bundles of bast fibers or sclerenchyma (6), the bundle sheath con-
taining starch grains, and outside of it parenchyma of the cortex (?•)• — After Sachs.

mation of xylem tissue on the one side and phloem tissue
on the other in the bundles, and new parenchyma between
the bundles, and so the stem increases in diameter. If the
stem lives from year to year the addition made by the cam-
bium each season is marked off from that of the previous
season, giving rise to the so-called growth rings or annual
rings, so conspicuous a feature of the cross-section of tree

DIFFERENTIATION OF TISSUES

289

trunks (Fig. 217). This continuous addition to the vessels
increases the capacity of the stem for conduction, and per-
mits the further extension of branches and a larger display
of leaves.

The annual additions to the xylem are added to the in-
creasing mass of wood. The older portions of the xylem
mass are gradually abandoned by the ascending water
("sap"), often change in color, and form the heart-wood.
The younger portion, through which the sap is moving, is
the sap-iuood. It is evident, however, that the annual ad-
ditions to the phloem are not in a position for permanency.
The new phloem is deposited inside of the old, and this, to-
gether with the new xylem, presses upon the old phloem,
which becomes ruptured in various ways, and rapidly or
very gradually peels off, being constantly renewed from
within. It is the protecting layers of cork (see this section
under Cortex), the old phloem, and the new phloem down
to the cambium, which constitute the so-called bark of
trees, a structure exceedingly complex and extremely vari-
able in different trees.

The stele also frequently develops stereome tissue in the
form of sclerenchyma. These thick-walled fibers are often
closely associated with one or both of the vascular strands
of the bundles (Fig. 270), and lead to the old name fibro-
vascular bundles.

To sum up, the stems of Dicotyledons and Conifers are
characterized by the development of a vascular cylinder, in
which the bundles are collateral and open, permitting
increase in diameter, extension of the branch system, and
a continuous increase in leaf display.

152. Monocotyl stems. — In the stems of Monocotyledons
there is the same apical development and differentiation
(Fig. 266). The characteristic difference from the Dicotyl
and Conifer type, just described, is in connection with the
development of the vascular bundles in the stele. Instead
of outlining a hollow cylinder, the bundles are scattered

290

PLANT STRUCTURES

through the stele (Fig. 214). This lack of regularity would
interfere with the organization of a cambium cylinder, and
we find the bundles collateral but closed — that is, with no
meristem left between the xylem and phloem (Fig. 271).

Pig. 271. Cross-section of a closed collateral bundle from the stem of corn, showing
the xylem with annular (r), spiral («), and pitted (g) vessels; the phloem contain-
ing sieve vessels (v), and separated from the xylem by no intervening cambium;
both xylem and phloem surrounded by a mass of sclerenchyma (fibers); and in-
vesting vessels and fibers the parenchyma (p) of the pith-like tissue through
which the bundles are distributed. — After Sachs.

This lack of cambium means that stems living for sev-
eral years do not increase in diameter, but become columnar

DIFFERENTIATION OF TISSUES

291

shafts, as in the palm, rather than much elongated cones.
It also means lack of ability to develop an extending branch
system or to display more numerous leaves each year. The
palm may be taken as a typical result of such a structure,
with its columnar and unbranched trunk, and its foliage
crown containing about the same number of leaves each year.

The lack of regular arrangement of the bundles also
prevents the outlining of a pith region or the organization
of definite pith rays. The failure to increase in diameter
also precludes the necessity of bark, with its protective cork
constantly renewed, and its sloughing-off phloem.

To sum up, the stems of the Monocotyledons are
characterized by the vascular bundles not developing a
cylinder or any regular arrangement, and by collateral and
closed bundles, which do not permit increase in diameter,
or a branch system, or increase in leaf display.

153. Pteridophyte stems. — The stems of Pteridophytes
are quite different from those of Spermatophytes. While
the large Club -mosses {Lyco-
podium) and Isoetes usually
have an apical group of meris-
tem cells, as among the Seed-
plants, the smaller Club-mosses
(Selaginella), Ferns, and Horse-
tails usually have a single api-
cal cell, whose divisions give
rise to all the cells of the stem.
Generally also a dermatogen is
not organized, and in such
cases there is no true epidermis,
the cortex developing the ex-
ternal protective tissue. In the cortex there is usually an
extensive development of stereome, in the form of scleren-
chyma (Fig. 272), the stele furnishing little or none, and
the vascular bundles not adding much to the rigidity, as
they do in the Seed-plants.

Fig. 272. Diagram of tissues in cross-
section of stem of a fern (Pteris),
showing two masses of scleren-
chyma (st), between and about
which are vascular bundles. —
Chamberlain.

292 PLANT STKUCTUKES

In Equisetum and Isoetes the vascular bundles may be
said to be collateral, as in the Seed-plants, but the charac-
teristic Pteridophyte type is very different. In fact, the
vascular masses can hardly be compared with the bundles
of the Seed-plants, although they are called bundles for
convenience. In the stele one or more of these bundles
are organized (Fig. 272), the tracheary vessels (xylem) being
in the center and completely invested by the sieve vessels

Fig. 273. Cross-section of concentric vascular bundle of a fern (P(eris): the single
row of shaded cells investing the others is the bundle sheath ; the large and heavy-
walled cells within constitute the xylem; and between the xylem and the bundle
sheath is the phloem. — Chamberlain.

(phloem). This is called the concentric bundle (Fig. 273),
as distinguished from the collateral bundles of Seed-plants,
and is characteristic of Pteridophyte stems.

DIFFERENTIATION OF TISSUES

293

154. Roots. — True roots appear only in connection with
the vascular plants (Pteridophytes and Spermatophytes) ;

Fig. 274. Section through root-
tip ofPteris: the cell with
a nucleus is the single apical
cell, which in front has cut
off cells which organize the
root-cap. — Chamberlain.

and in all of them the
structure is essential-
ly the same, and quite
different from stem
structure. A single
apical cell (in most
Pteridophytes) (Fig.
274) or an apical
group (in Spermato-
phytes) usually gives
rise to the three em-
bryonic regions — der-
matogen, periblem,
and plerome (Fig.
275). A fourth re-
gion, however, pecul-
iar to root, is usually added. The apical cell or group cuts
off a tissue in front of itself (Fig. 274), known as the calyp-
trogen, or "cap producer," for it organizes the root-cap,
which protects the delicate meristem of the growing point.

Fro. 275. A longitudinal section through the root-
tip of spiderwort, showing the plerome {pi),
surrounded by the periblem ( p), outside of
periblem the epidermis (e) which disappears in
the older parts of the root, and the prominent
root-cap (c). — Land.

294 PLANT STRUCTURES

Another striking feature is that in the stele there is
organized a single solid vascular cylinder, forming a tough
central axis (Fig. 277), from which the usually well-devel-
oped cortex can be peeled off as a thick rind. In this vas-
cular axis, which is called " a bundle " for convenience but
does not represent the bundle of Seed-plant stems, the ar-
rangement of the xylem and phloem is entirely unlike that

r

Fig. 276. Cross-section of the vascular axis of a root, showing radiate type of bundle,
the xylem (p) and phloem (p/n alternating. -Alter Saciib.

found in stems. The xylem is in the center and sends out
a few radiating arms, between which are strands of phloem,
forming the so-called radiate bundle (Fig. 276). This
arrangement brings the tracheary vessels (xylem) to the
surface of the bundle region, which is not true of either
the concentric or collateral bundle. This seems to be asso-
ciated with the fact that the xylem is to receive and conduct
the water absorbed from the soil. It should be said that
this characteristic bundle structure of the root appears only

DIFFERENTIATION OF TISSUES

295

in young and active roots. In older ones certain secondary
changes take place which obscure the structure and result
in a resemblance to the stem.

The origin of branches in
roots is also peculiar. In stems
branches originate at the sur-
face, involving epidermis, cor-
tex, and vascular bundles, such
an origin being called exogenous
("produced outside"); but in
roots branches originate on
the vascular cylinder, burrow
through the cortex, and emerge
at the surface (Fig. 277). If the
cortex be stripped off from a
root with branches, the branches
are left attached to the woody
axis, and the cortex is found
pierced with holes made by the burrowing branches. Such
an origin is called endogenous, meaning " produced within."

me.

Fig. 277. Endogenous origin of
root branches, showing them («)
arising from the central axis (/)
and breaking through the cortex
(?•).— After Vines.

Fig 278 A section through the leaf of lily, (showing upper epidermis {itt i. lower epi-
dermis (M with its stomata(«0, mesophy.ll .dotted cells) composed of the palisade
region (p) and the spongy region («p) with air spaces among the cells, and two
veins (») cut across.— From " Plant Relations."

29G PLANT STRUCTURES

To sum up the peculiarities of the root, it may be said
to develop a root-cap, to have a solid vascular cylinder in
which the xylem and phloem are arranged to form a bundle
of the radiate type, and to branch endogenously.

155. Leaves. — Leaves usually develop from an apical
region in the same general way as do stems and roots,
modified by their common dorsiventral character. Com-
paring the leaf of an ordinary seed-plant with its stem, it
will be noted that the three regions are represented (Fig.
278): (1) the epidermis; (2) the cortex, represented by
the mesophyll ; (3) the stele, represented by the veins.

In the case of collateral bundles, where in the stem the
xylem is always toward the center and the phloem is toward
the circumference, in the leaves the xylem is toward the
upper and the phloem toward the lower surface.

CHAPTER XVI

PLANT PHYSIOLOGY

156. Introductory. — Plants may be studied from several
points of view, each of which has resulted in a distinct
division of Botany. The study of the forms of plants and
their structure is Moephology, and it is this phase of Bot-
any which has been chiefly considered in the previous chap-
ters. The study of plants at work is Physiology, and as
structure is simply preparation for work, the preceding
chapters have contained some Physiology, chiefly in refer-
ence to nutrition and reproduction. The study of the clas-
sification of plants is Taxonomy, and in the preceding
pages the larger groups have been outlined. The study of
plants as to their external relations is Ecology, a subject
which will be presented in the following chapter, and which
is the chief subject of Plant Relations. The study of the
diseases of plants and their remedies is Pathology ; their
study in relation to the interests of man is Economic
Botany.

Besides these general subjects, which apply to all plants,
the different groups form the subjects of special study. The
study of the Morphology, Physiology, or Taxonomy of the
Bacteria is Bacteriology; of the Algge, Algology ; of the
Fungi, Mycology; of the Bryophytes, Bryology; of the
fossil plants, Paleobotany or PaheopJtytology, etc.

In the present chapter it is the purpose to give a very
brief outline of the great subject of Plant Physiology, not
with the expectation of presenting its facts adequately, but
with the hope that the important field thus presented may

297

298 PLANT STRUCTURES

attract to further study. It is merely the opening of a door
to catch a fleeting glimpse.

A common division of the subject presents it under five
heads : (1) Stability of form, (2) Nutrition, (3) Inspira-
tion, (4) Movement, (5) Reproduction.

STABILITY OF FORM

157. Turgidity. — It is a remarkable fact that plants and
parts of plants composed entirely of cells Avith very thin and
delicate walls are rigid enough to maintain their form.
It has already been noted (see § 20) that such active cells
exert an internal pressure upon their walls. This seems to
be due to the active absorption of liquid, which causes the
very elastic walls to stretch, as in the "blowing up " of a
bladder. In this way each gorged and distended cell be-
comes comparatively rigid, and the mass of cells retains its
form. It seems evident that the active protoplasm greedily
pulls liquid through the wall and does not let it escape so
easily. If for any reason the protoplasm of a gorged cell
loses its hold upon the contained liquid the cell collapses.

158. Tension of tissues. — The rigidity which comes to
active parenchyma cells through their turgidity is increased
by the tensions developed by adjaceDt tissues. For exam-
ple, the internal and external tissues of a stem are apt to
increase in volume at different rates ; the faster will pull
upon the slower, and the slower will resist, and thus be-
tween the two a tension is developed which helps to keep
them rigid. This is strikingly shown by splitting a dande-
lion stem, when the inner tissue, relieved somewhat from
the resistance of the outer, elongates and causes the strip
to become strongly curved outward or even coiled. Experi-
ments with strips from active twigs, including the pith,
will usually demonstrate the same curve outward. Tension
of tissues is chiefly developed, of course, where elongation
is taking place.

PLANT PHYSIOLOGY 299

159. Stereome. — When growth is completed, cell walls
lose their elasticity, turgidity becomes less, and therefore
tensions diminish, and rigidity is supplied by special ster-
eome tissues, chief among which is sclerenchyma. An-
other stereome tissue is collenchyma, which on account of
its elastic walls can be used to supplement turgidity and
tension where elongation is still going on. For a fuller
account of stereome tissues see 8 150.

NUTRITION

160. Food. — Plant food must contain carbon (C), hydro-
gen (H), oxygen (0), and nitrogen (N), and also more
or less of other elements, notably sulphur, phosphorus,
potassium, calcium, magnesium, and iron. In the case
of green plants these elements are obtained from inor-
ganic compounds and food is manufactured ; while plants
without chlorophyll obtain their food already organized.
The sources of these elements for green plants are as
follows: Carbon from carbon dioxide (C02) of the air;
hydrogen and oxygen from water (H30) ; and nitrogen
and the other elements from their various salts which
occur in the soil and are dissolved in the water which
enters the plant.

All of these substances must present themselves to
plants in the form of a gas or a liquid, as they must pass
through cell walls ; and the processes of absorption have
to do with the taking in of the gas carbon dioxide and of
water in which the necessary salts are dissolved.

161. Absorption. — Green plants alone will be considered,
as the unusual methods of securing food have been men-
tioned in Chapter VII. For convenience also, only terres-
trial green plants will be referred to, as it is simple to
modify the processes to the aquatic habit, where the sur-
rounding water supplies what is obtained by land plants
from both air and soil.

300 PLANT STRUCTURES

In such plants the carbon dioxide is absorbed directly
from the air by the foliage leaves, whose expanse of surface
is as important for this purpose as for exposing chlorophyll
to light. When the work of foliage leaves is mentioned it
must always be understood that it applies as well to any
green tissue displayed by the plant.

The water, with its dissolved salts, is absorbed from the
soil by the roots. Only the youngest parts of the root-
system can absorb, and the absorbing capacity of these
parts is usually vastly increased by the development of
numerous root hairs just behind the growing tip (Fig. 194).
These root hairs are ephemeral, new ones being continu-
ally put out as the tip advances, and the older ones disap-
pearing. They come in very close contact with the soil
particles, and "suck in" the water which invests each
particle as a film.

162. Transfer of water. — The water and its dissolved salts
absorbed by the root-system must be transferred to the foli-
age leaves, where they are to be used, along with the carbon
dioxide, in the manufacture of food.

Having entered the epidermis of the absorbing rootlets
the water passes on to the cortex, and traversing it enters
the xylem system of the central axis. In some way this
transfer is accompanied by pressure, known as root pres-
sure, which becomes very evident when an active stem is
cut off near the ground. The stump is said to "bleed,"
and sends out water ("sap") as if there were a force
pump in the root-system. This root pressure doubtless
helps to lift the water through the xylem of the root into
the stem, and in low plants may possibly be able to send it
to the leaves, but for most plants this is not possible.

When the water enters the xylem of the root it is in a
continuous system of vessels which extends through the
stem and out into the leaves. The movement of the ab-
sorbed water through the xylem is called the transpiration
current, or very commonly the "ascent of sap." An ex-

PLANT PHYSIOLOGY 3Q|

periment demonstrating this ascent of sap and its route
through the xylem will be found described in Plant Rela-
tions, p. 151. How it is that the transpiration current
moves through the xylem is not certainly known.

163. Transpiration. — When the water carrying dissolved
salts reaches the mesophyll cells, some of the water and all
of the salts are retained for food manufacture. However,
much more water enters the leaves than is needed for food,
this excess having been used for carrying soil salts. When
the soil salts have reached their destination the excess of
water is evaporated from the leaf surface, the process being
called transpiration. For an experiment demonstrating
transpiration see Plant Relations, § 26.

This transpiration is regulated according to the needs
of the plant. If the water is abundant, transpiration is
encouraged ; if the water supply is low, transpiration is
checked. One of the chief ways of regulating is by means
of the very small but exceedingly numerous stomata (see §
79 [4]), Avhose guard cells become turgid or collapse and so
determine the size of the opening between them. It has
been estimated that a leaf of an ordinary sunflower contains
about thirteen million stomata, but the number varies widely
in different plants. In ordinary dorsiventral leaves the sto-
mata are much more abundant upon the lower surface than
upon the upper, from which they may be lacking entirely.
In erect leaves they are distributed equally upon both sur-
faces ; in floating leaves they occur only upon the upper
surface ; in submerged leaves they are lacking entirely.

The amount of water thus evaporated from active
leaves is very great. It is estimated that the leaves of a
sunflower as high as a man evaporate about one quart of
water in a warm day ; and that an average oak tree in its
five active months evaporates about twenty-eight thousand
gallons. If these figures be applied to a meadow or a
forest the result may indicate the large importance of this

process.

38

302 PLANT STRUCTURES

164. Photosynthesis. — This is the process by which car-
bon dioxide and water are "broken up," their elements
recombined to form a carbohydrate, and some oxygen given
off as a waste product, the mechanism being the chloroplasts
and light. It has been sufficiently described in § 55, and
also in Plant Relations, pp. 28 and 150.

165. Formation of proteids. — The carbohydrates formed
by photosynthesis, such as starch, sugar, etc., contain car-
bon, hydrogen, and oxygen. Out of them the living cells
must organize proteids, and in the reconstruction nitrogen
and sulphur, and sometimes phosphorus, are added. This
work goes on both in green cells and other living cells, as
it does not seem to be entirely dependent upon chloroplasts
and light.

166. Transfer of carbohydrates and proteids. — These two
forms of food having been manufactured, they must be
carried to the regions of growth or storage. In order to be
transported they must be in soluble form, and if not already
soluble they must be digested, insoluble starch being con-
verted into soluble sugar, etc. In these digested forms
they are transported to regions where work is going on,
and there they are assimilated — that is, transformed into
the enormously complex working substance protoplasm ;
or they are transported to regions of storage and there they
are reconverted into insoluble storage forms, as starch, etc.

These foods pass through both the cortex and phloem
in every direction, but the long-distance transfer of pro-
teids, as from leaves to roots, seems to be mainly through
the sieve vessels.

RESPIRATION"

167. Respiration. — This is an essential process in plants
as well as in animals, and is really the phenomenon of
"breathing." The external indication of the process is
the absorption of oxygen and the giving out of carbon di-
oxide ; and it goes on in all organs, day and night. When

PLANT PHYSIOLOGY 393

it ceases death ensues sooner or later. By this process
energy, stored up by the processes of nutrition, is liberated,
and with this liberated energy the plant works. It may be
said that oxygen seems to have the power of arousing pro-
toplasm to activity.

It is not sufficient for the air containing oxygen to come
in contact merely with the outer surface of a complex plant,
as its absorption and transfer would be too slow. There
must be an "internal atmosphere" in contact with the
living cells. This is provided for by the intercellular
spaces, which form a labyrinthine system of passageways,
opening at the surface through stomata and lenticels (pores
through bark). In this internal atmosphere the exchange
of oxygen and carbon dioxide is effected, the oxygen being
renewed by diffusion from the outside, and the carbon
dioxide finally escaping by diffusion to the outside.

MOVEMENT

168. Introductory. — In addition to movements of mate-
rial, as described above, plants execute movements depend-
ent upon the activity of protoplasm, which result in change
of position. Naked masses of protoplasm, as the Plas-
modium of slime-moulds (see § 51), advance with a sliding,
snail-like movement upon surfaces ; zoospores and ciliated
sperms swim freely about by means of motile cilia ; while
many low plants, as Bacteria (§ 52), Diatoms (§ 34), Oscil-
laria (§ 20), etc., have the power of locomotion.

When the protoplasm is confined within rigid walls and
tissues, as in most plants, the power of locomotion usually
disappears, and the plants are fixed ; but within active cells
the protoplasm continues to move, streaming back and
forth and about within the confines of the cell.

In the case of complex plants, however, another kind
of movement is apparent, by which parts are moved and
variously directed, sometimes slowly, sometimes with great

304 PLANT STRUCTURES

rapidity. In these cases the part concerned develops a
curvature, and by various curvatures it attains its ultimate
position. These curvatures are not necessarily permanent,
for a perfectly straight stem results from a series of cur-
vatures near its apex. Curvatures may be developed by
unequal growth on the two sides of an organ, or by unequal
turgidity of the cells of the two sides, or by the unequal
power of the cell walls to absorb water.

169. Hygroscopic movements. — These movements are only
exhibited by dry tissues, and hence are not the direct result
of the activity of protoplasm. The dry walls absorb mois-
ture and swell up, and if this absorption of moisture and
its evaporation is unequal on two sides of an organ a curva-
ture will result. In this way many seed vessels are rup-
tured, the sporangia of ferns are opened, the operculum of
mosses is lifted off by the peristome, the hair-like pappus
of certain Composites is spread or collapsed, certain seeds
are dispersed and buried, etc. One of the peculiarities of
this hygroscopic power of certain cells is that the result
may be obtained through the absorption of the moisture of
the air, and the hygroscopic awns of certain fruits have
been used in the manufacture of rough hygrometers
(" measures of moisture ").

170. Growth movements. — Growth itself is a great physi-
ological subject, but certain movements which accompany
it are referred to here. Two kinds of growth movements
are apparent.

One may be called nutation, by which is meant that the
growing tip of an organ does not advance in a straight
line, but bends now toward one side, now toward the other.
In this way the tip describes a curve, which may be a
circle, or an ellipse of varying breadth ; but as the tip is
advancing all the time, the real curve described is a spiral
with circular or elliptical cross-section. The sweep of a
young hop-vine in search of support, or of various tendrils,
may be taken as extreme illustrations, but in most cases

PLANT PHYSIOLOGY 3Q5

the nutation of growing tips only becomes apparent through
prolonged experiment.

The other prominent growth movement is that which
places organs in proper relations for their work, sending
roots into the soil and stems into the air, and directing
leaf planes in various ways. For example, in the germina-
tion of an ordinary seed, in whatever direction the parts
emerge the root curves toward the soil, the stem turns
upward, and the cotyledons spread out horizontally.

The movement of nutation seems to be due largely to
internal causes, while the movements which direct organs
are due largely to external causes known as stimuli. Some
of the prominent responses to stimuli concerned in direct-
ing organs are as follows :

Heliotropism. — In this case the stimulus is light, and
under its influence aerial parts are largely directed. Plants
growing in a window furnish plain illustration of helio-
tropism. In general the stems and petioles curve toward
the light, showing positive heliotropism (Fig. 279) ; the
leaf blades are directed at right angles to the rays of light,
showing transverse heliotropism ; while if there are hold-
fasts or aerial roots they are directed away from the light,
showing negative heliotropism. The thallus bodies of ferns,
liverworts, etc., are transversely heliotropic, as ordinary
leaves, a position best related to chlorophyll work. If the
light is too intense, leaves may assume an edgewise or pro-
file position, a condition well illustrated by the so-called
"compass plants." (See Plant Relations, p. 10.)

Geotropism. — In this case the stimulus is gravity, and
its influence in directing the parts of plants is very great.
All upward growing plants, as ordinary stems, some leaves,
etc., are negatively geotropic, growing away from the center
of gravity. Tap-roots are notable illustrations of positive
geotropism, growing toward the source of gravity with con-
siderable force. Lateral branches from a main or tap-root,
however, are usually transversely geotropic.

Fig. 279. Sunflower stems with the upper part of the stem sharply bent toward the
light, giving the leaves better exposure, the stem showing positive heliotropism. —
After Schaffner.

PLANT PHYSIOLOGY

307

That these influences in directing are very real is testi-
fied to by the fact that when the organs are turned aside
from their proper direction they will curve toward it and
overcome a good deal of resistance to regain it. Although
these curvatures are mainly developed in growing parts,
even mature parts which have been displaced may be
brought back into position. For example, when the stems
of certain plants, notably the grasses, have been prostrated
by wind, etc., they often can resume the erect position under
the influence of negative geotropism, a very strong and even
angular curvature being developed at certain joints.

Hydrotropism. — The influence of moisture is very strong
in directing certain organs, notably absorbing systems.
Roots often wander widely and in every direction under
the guidance of hydrotropism, even against the geotropic
influence. Ordinarily geotropism and hydrotropism act in
the same direction, but it is interesting to dissociate them
so that they may "pull" against one another. For such
an experiment see Plant Relations, p. 91.

Other stimuli. — Other outside stimuli which have a
directive influence upon organs are chemical substances
{cliemotropism), such as direct sperms to the proper female
organ ; heat (thermotropism) ; water currents (rheotropism) ;
mechanical contact, etc. The most noteworthy illus-
trations of the effect of contact are furnished by tendril-
climbers. When a nutating tendril comes in contact with
a support a sharp curvature is developed which grasps it.
In many cases the irritable response goes further, the ten-
dril between the plant axis and the support developing a
spiral coil.

171. Irritable movements. — The great majority of plants
can execute movements only in connection with growth, as
described in the preceding section, and when mature their
parts are fixed and incapable of further adjustment. Cer-
tain plants, however, have developed the power of moving
mature parts, the motile part always being a leaf, such as

308

PLANT STRUCTURES

foliage leaf, stamen, etc. It is interesting to note that these
movements have been cultivated by but few families, nota-
ble among them being the Legumes (§ 141).

These movements of mature organs, some of which are
very rapid, are due to changes in the turgidity of cells. As
already mentioned (§ 157), turgid cells are inflated and
rigid, and when turgidity ceases the cells collapse and the
tissue becomes flaccid. A special organ for varying tur-
gidity, known as the pulvi?ius, is usually associated with
the motile leaves and leaflets. The pulvinus is practically
a mass of parenchyma cells, whose turgidity is made to vary
by various causes, and leaf -movement is the result.

The causes which induce some movements are unknown,
as in the case of Desmodium gyrans (see Plant Relations,
p. 49), whose small lateral leaflets uninterruptedly de-
scribe circles, completing a cycle in one to three minutes.

In other cases the inciting cause is the change from light
to dark, the leaves assuming at night a very dif-
ferent position from that during the day. Dur-
ing the day the leaflets are spread out freely,

Fig. 280. A leaf of a sensitive plant in two conditions: in the figure to the left the leaf
is fully expanded, with its four main divisions and numerous leaflets well spread;
in the figure to the right is shown the same leaf after it has been " shocked " by
a sudden touch, or by sudden heat, or in some other way; the leaflets have been
thrown together forward and upward, the four main divisions have been moved
together, and the main leaf-stalk has been directed sharply downward.— After

DUCH AUTRE.

PLANT PHYSIOLOGY 309

while at night they droop and usually fold together (see
Plant Relations, pp. 9, 10). These are the so-called nycti-
tropic movements or " night movements," which maybe ob-
served in many of the Legumes, as clover, locust, bean, etc.
In still other cases, mechanical irritation induces move-
ment, as sudden contact, heat, injury, etc. Some of the
" carnivorous plants " are notable illustrations of this, es-
pecially Dionma, which snaps its leaves shut like a steel
trap when touched (see Plant Relations, p. 161). Among
the most irritable of plants are the so-called "sensitive
plants," species of Mimosa, Acacia, etc., all of them Le-
gumes. The most commonly cultivated sensitive plant is
Mimosa pudica (Fig. 280), whose sensitiveness to contact
and rapidity of response are remarkable (see Plant Rela-
tions, p. 48).

KEPEODUCTION"

172. Reproduction. — The important function of repro
duction has been considered in connection with the various
plant groups. Among the lowest plants the only method
of reproduction is cell division, which in the complex
forms results in growth. In the more complex plants va-
rious outgrowths or portions of the body, as gemmae, buds,
bulbs, tubers, various branch modifications, etc., furnish
means of propagation. All of these methods are included
under the head of vegetative multiplication, as the plants
are propagated by ordinary vegetative tissues.

When a special cell is organized for reproduction, dis-
tinct from the vegetative cells, it is called a spore, and re-
production by spores is introduced. The first spores devel-
oped seem to have been those produced by the division of
the contents of a mother cell, and are called asexual spores.
These spores are scattered in various ways — by swimming
(zoospores), by floating, by the wind, by insects.

Another type of spore is the sexual spore, formed by
the union of two sexual cells called gametes. The gametes

310 PLANT STKUCTUKES

seem to have been derived from asexual spores. At first
the pairing gametes are alike, but later they become differ-
entiated into sperms or male cells, and eggs or female cells.

With the establishment of alternation of generations,
the asexual spores are restricted to the sporophyte, and the
gametes to the gametophyte. With the further introduction
of heterospory, the male and the female gametes are sepa-
rated upon different gametophytes, which become much
reduced.

With the reduction of the functioning megaspores to
one in a sporangium (ovule), and its retention, the seed is
organized, and the elaborate scheme of insect-pollination
is developed.

CHAPTER XVII

PLANT ECOLOGY

173. Introductory.— Ecology has to do with the external
relations of plants, and forms the principal subject of the
volume entitled Plant Relations, which should be consulted
for fuller descriptions and illustrations. It treats of the
adjustment of plants and their organs to their physical
surroundings, and also their relations with one another
and with animals, and has sometimes been called "plant
sociology."

LIFE RELATIONS

174. Foliage leaves. — The life relation essential to foliage
leaves is the relation to light. This is shown by their
positions and forms, as well as by their behavior when
deprived of light. This light relation suggests the answer
to very many questions concerning leaves. It is not very
important to know the names of different forms and differ-
ent arrangements of leaves, but it is important to observe
that these forms and arrangements are in response to the
light relation.

In general a leaf adjusts its own position and its relation
to its fellows so as to receive the greatest amount of light.
Upon erect stems the leaves occur in vertical rows which
are uniformly spaced about the circumference. If these
rows are numerous the leaves are narrow ; if they are few
the leaves are usually broad. If broad leaves were associ-
ated with numerous rows there would be excessive shading ;

311

312 PLANT STRUCTURES

if narrow leaves were associated with few rows there would
be waste of space.

It is very common to observe the lower leaves of a stem
long-petioled, those above short-petioled, and so on until
the uppermost have sessile blades, thus thrusting the blades
of lower leaves beyond the shadow of the upper leaves.
There may also be a gradual change in the sree and direc-
tion of the leaves, the lower ones being relatively large and
horizontal, and the upper ones gradually smaller and more
directed upward. In the case of branched (compound)
leaves the reduction in the size of the upper leaves is not
so necessary, as the light strikes between the upper leaflets
and reaches those below.

On stems exposed to light only or chiefly on one side,
the leaf blades are thrown to the lighted side in a variety
of ways. In ivies, many prostrate stems, horizontal branches
of trees, etc., the leaves brought to the lighted side are
observed to form regular mosaics, each leaf interfering
with its neighbor as little as possible.

There is often need of protection against too intense
light, against chill, against rain, etc., which is provided
for in a great variety of ways. Coverings of hairs or scales,
the profile position, the temporary shifting of position,
rolling up or folding, reduction in size, etc., are some of
the common methods of protection.

175. Shoots. — The stem is an organ which is mostly
related to the leaves it bears, the stem with its leaves being
the shoot. In the foliage-bearing stems the leaves must be
displayed to the light and air. Such stems may be sub-
terranean, prostrate, floating, climbing, or erect, and all of
these positions have their advantages and disadvantages,
the erect type being the most favorable for foliage display.

In stems which bear scale leaves no light relation is
necessary, so that such shoots may be and often are sub-
terranean, and the leaves may overlap, as in scaly buds
and bulbs. The subterranean position is very favorable

PLANT EC< >!/>(; Y

313

for food storage, and such shoots often become modified as
food depositories, as in bulbs, tubers, rootstocks, etc. In
the scaly buds the structure is used for protection rather
than storage.

The stem bearing floral leaves is the shoot ordinarily
called "the flower," whose structure and work have been
sufficiently described. Its adjustments have in view polli-
nation and seed dispersal, two very great ecological sub-
jects full of interesting details.

176. Roots. — Eoots are absorbent organs or holdfasts or
both, and they enter into a variety of relations. Most
common is the soil relation, and the energetic way in
which such roots penetrate the soil, and search in every
direction for water and absorb it, proves them to be highly
organized members. Then there are roots related to free
water, and others to air, each with its appropriate struc-
ture. More mechanical are the clinging roots (ivies, etc.),
and prop roots (screw pines, banyans, etc.), but their adap-
tation to the peculiar service they render is none the less
interesting.

The above statements concerning leaves, shoots, and
roots should be applied with necessary modifications to the
lower plants which do not produce such organs. The
light relation and its demands are no less real among the
Algae than among Spermatophytes, as well as relations to
air, soil, water, mechanical support, etc.

PLANT ASSOCIATION'S

177. Introductory. — Plants are not scattered at hap-
hazard over the surface of the earth, but are organized into
definite communities. These communities are determined
by the conditions of living — conditions which admit some
plants and forbid others. Such an assemblage of plants
living together in similar conditions is a plant associatio?i.
Closely related plants are the most intense rivals, as they

314 PLANT STRUCTURES

make almost identical demands upon their surroundings.
Hence it is usual for a plant association to be made up of a
large number of unrelated plants.

There are numerous factors which combine to deter-
mine associations, and it is known as yet only in a vague way
how they operate.

178. Ecological factors. — Water. — This is a very impor-
tant factor in the organization of associations, which are
usually local assemblages. Taking plants altogether, the
amount of water to which they are exposed varies from
complete submergence to perpetual drought, but within
this range plants vary widely as to the amount of watei
necessary for living.

Heat. — In considering the general distribution of plants
over the surface of the earth, great zones of plants are out-
lined by zones of temperature ; but in the organization of
local associations in any given area the temperature condi-
tions are nearly uniform. Usually plants work only at
temperatures between 32° and 122° Fahr., but for each
plant there is its own range of temperature, sometimes
extensive, sometimes restricted. Even in plant associations,
however, the effect of the heat factor may be noted in the
succession of plants through the working season, spring
plants being very different from summer and autumn
plants.

Soil. — The great importance of this factor is evident,
even in water plants, for the soil of the drainage area deter-
mines the materials carried by the water. Soil is to be
considered both as to its chemical composition and its
physical properties, the latter chiefly in reference to its
disposition toward water. Soils vary greatly in the power
of receiving and retaining water, sand having a high recep-
tive and low retentive power, and clay just the reverse,
and these factors have large effect upon vegetation.

Light. — All green plants can not receive the same amount
of light. Hence some of them have learned to live with a

PLANT ECOLOGY

315

less amount than others, and are "shade plants" as dis-
tinct from " light plants." In forests and thickets many
of these shade plants are to be seen which would find an
exposed situation hard to endure. In almost every associa-
tion, therefore, plants are arranged in strata, dependent
upon the amount of light they receive, and the number of
these strata and the plants characterizing each stratum are
important factors to note.

Wind. — This is an important factor in regions where
there are strong prevailing winds. Wind has a drying
effect and increases the transpiration of plants, tending to
impoverish them in water. In such conditions only those
plants can live which are well adapted to regulate trans-
piration.

The above five factors are among the most important,
but no single factor determines an association. As each
factor has a large possible range, the combinations of fac-
tors may be very numerous, and it is these combinations
which determine associations. For convenience, however,
associations are usually grouped on the basis of the water
factor, at least three great groups being recognized.

179. Hydrophyte associations.— These are associations of
water plants, the water factor being so conspicuous that the
plants are either submerged or standing in water. A plant
completely exposed to water, submerged, or floating, may
be taken to illustrate the usual adaptations. The epi-
dermal walls are thin, so that water may be absorbed
through the whole surface ; hence the root system is very
commonly reduced or even wanting ; and hence the water-
conducting tissues (xylem) are feebly developed. The tis-
sues for mechanical support (stereome) are feebly devel-
oped, the plant being sustained by the buoyant power of
water. Such a plant, although maintaining its form in
water, collapses upon removal. Very common also is the
development of conspicuous air passages for internal aera-
tion and for increasing buoyancy ; and sometimes a special

316 PLANT STRUCTURES

buoyancy is provided for by the development of bladder-
like floats.

Conspicuous among hydrophyte associations may be
mentioned the following : (1) Free-swimming associations,
in which the plants are entirely sustained by water, and are
free to move either by locomotion or by water currents.
Here belong the "plankton associations," consisting of
minute plants and animals invisible to the naked eye,
conspicuous among the plants being the diatoms ; also the
" pond associations," composed of algae, duckweeds, etc.,
which float in stagnant or slow-moving waters.

(2) Pondweed associations, in which the plants are an-
chored, but their bodies are submerged or floating. Here
belong the "rock associations," consisting of plants an-
chored to some firm support under water, the most conspic-
uous forms being the numerous fresh-water and marine
algae, among which there are often elaborate systems of
holdfasts and floats. The " loose-soil associations " are dis-
tinguished by imbedding their roots or root-like processes
in the mucky soil of the bottom (Figs. 281, 282). The wa-
ter lilies with their broad floating leaves, the pondweeds or
pickerel weeds with their narrow submerged leaves, are
conspicuous illustrations, associated with which are algae,
mosses, water ferns, etc.

(3) Swamp associations, in which the plants are rooted
in water, or in soil rich in water, but the leaf-bearing stems
rise above the surface. The conspicuous swamp associations
are "reed swamps," characterized by bulrushes, cat-tails
and reed-grasses (Figs. 283, 284), tall wand-like Monocoty-
ledons, usually forming a fringe about the shallow margins
of small lakes and ponds ; " swamp-moors," the ordinary
swamps, marshes, bogs, etc., and dominated by coarse
sedges and grasses (Fig. 282) ; " swamp-thickets," consist-
ing of willows, alders, birches, etc. ; " sphagnum-moors," in
which sphagnummoss predominates, and is accompanied by
numerous peculiar orchids, heaths, carnivorous plants, etc. ;

39

PLANT ECOLOGY

319

"swamp-forests," which are largely coniferous, tamarack
(larch), pine, hemlock, etc., prevailing.

180. Xerophyte associations. — These associations are ex-
posed to the other extreme of the water factor, and are com-
posed of plants adapted to dry air and soil. To meet these

320

PLANT STRUCTURES

drought conditions numerous adaptations have been de-
veloped and are very characteristic of xerophytic plants.
Some of the conspicuous adaptations are as follows : peri-

odic reduction of surface, annuals bridging over a period
of drought in the form of seeds, geophilous plants also dis-
appearing from the surface and persisting in subterranean

■Am

324 PLANT STRUCTURES

parts, deciduous trees and shrubs dropping their leaves,
etc. ; temporary reduction of surface, the leaves rolling up
or folding together in various ways ; profile position, the
leaves standing edgewise and not exposing their flat sur-
faces to the most intense light ; motile leaves which can
shift their position to suit their needs ; small leaves, a very
characteristic feature of xerophytic plants ; coverings of
hair ; dwarf growth ; anatomical adaptations, such as
cuticle, palisade tissue, etc. Probably the most conspicu-
ous adaptation, however, is the organization of "water-
reservoirs," which collect and retain the scanty water sup-
ply, doling it out as the plant needs it.

Some of the prominent associations are as follows :
"rock-associations," composed of plants living upon exposed
rock surfaces, Avails, fences, etc., notably lichens and mosses ;
" sand associations," including beaches, dunes, and sandy
fields ; " shrubby heaths," characterized by heath plants ;
" plains," the great areas of dry air and wind developed in
the interiors of continents; "cactus deserts," still more
arid areas of the Mexican region, where the cactus, agave,
yucca, etc., have learned to live by means of the most ex-
treme xerophytic modifications; " tropical deserts," where
xerophytic conditions reach their extreme in the combina-
tion of maximum heat and minimum water ; " xerophyte
thickets," the most impenetrable of all thicket-growths,
represented by the " chaparral " of the Southwest, and the
" bush " and " scrub " of Africa and Australia ; " xero-
phyte forests," also notably coniferous. (See Figs. 285,
286, 287.)

181. Mesophyte associations. — Mesophytes make up the
common vegetation, the conditions of moisture being me-
dium, and the soil fertile. This is the normal plant condi-
tion, and is the arable condition — that is, best adapted for
the plants which man seeks to cultivate. If a hydrophytic
area is to be cultivated, it is drained and made mesophytic ;
if a xerophytic area is to be cultivated, it is irrigated and

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PLANT ECOLOGY

327

made mesophytic. As contrasted with hydrophyte and
xerophyte associations, the mesophyte associations are far
richer in leaf forms and in general luxuriance. The arti-
ficial associations which have been formed under the influ-
ence of man, through the introduction of weeds and culture
plants, are all mesophytic.

Among the mesophyte grass and herb associations are
the " arctic and alpine carpets," so characteristic of high
latitudes and altitudes where the conditions forbid trees,
shrubs, or even tall herbs ; " meadows," areas dominated by
grasses, the prairies being the greatest meadows, where
grasses and flowering herbs are richly displayed; "pas-
tures," drier and more open than meadows.

Among the woody mesophyte associations are the "thick-
ets," composed of willow, alder, birch, hazel, etc., either
pure or forming a jungle of mixed shrubs, brambles, and
tall herbs ; " deciduous forests," the glory of the temperate
regions, rich in forms and foliage display, with annual fall
of leaves, and exhibiting the remarkable and conspicuous
phenomenon of autumnal coloration; " rainy tropical for-
ests," in the region of trade winds, heavy rainfalls, and
great heat, where the world's vegetation reaches its climax,
and where in a saturated atmosphere gigantic jungles are
developed, composed of trees of various heights, shrubs of
all sizes, tall and low herbs, all bound together in an inex-
tricable tangle by great vines or lianas, and covered by a
luxuriant growth of numerous epiphytes. (See Figs. 288,
289.)

GLOSSARY

[The definitions of a glossary are often unsatisfactory. It is much better to con-
sult the fuller explanations of the text by means of the index. The following glos-
sary includes only frequently recurring technical terms. Those which are found only
in reasonably close association with their explanation are omitted. The number fol-
lowing each definition refers to the page where the term will be found most fully
defined.]

Actinomorphic : applied to a flower in which the parts in each set are

similar ; regular. 228.
Akene : a one-seeded fruit which ripens dry and seed-like. 212.
Alternation of generations : the alternation of gametophyte and

sporophyte in a life history. 94.
Anemophilous : applied to flowers or plants which use the wind as agent

of pollination. 181.
Anisocarpic : applied to a flower whose carpels are fewer than the other

floral organs. 268.
Anther : the sporangium-bearing part of a stamen. 197.
Antheridium : the male organ, producing sperms. 16.
Antipodal cells : in Angiosperms the cells of the female gametophyte

at the opposite end of the embryo-sac from the egg-apparatus.

205.
Apetalous : applied to a flower with no petals. 221.
Apocarpous : applied to a flower whose carpels are free from one an-
other. 226.
Archegonium : the female, egg-producing organ of Bryophytes, Pteri-

dophytes, and Gymnosperms. 100.
Arciiesporium : the first cell or group of cells in the spore-producing

series. 102.
Ascocarp : a special case containing asci. 58.
Ascospore : a spore formed within an ascus. 59.
Ascus: a delicate sac (mother-cell) within which ascospores develop.

59.
Asexual spore : one produced usually by cell-division, at least not by

cell-union. 9.

329

330 GLOSSARY

Calyx : the outer set of floral leaves. 221.

Capsule : in Bryophytes the spore- vessel ; in Angiosperms a dry fruit

which opens to discharge its seeds. 98, 211.
Carpel : the megasporophyll of Sperraatophytes. 178.
Chlorophyll : the green coloring matter of plants. 5.
Chloroplast : the protoplasmic body within the cell which is stained

green by chlorophyll. 7.
Columella : in Bryophytes the sterile tissue of the sporogonium which

is surrounded by the sporogenous tissue. 106.
Conidium : an asexual spore formed by cutting off the tip of the sporo-

phore, or by the division of hyphae. 58.
Conjugation : the union of similar gametes. 15.
Corolla : the inner set of floral leaves. 221.

Cotyledon : the first leaf developed by an embryo sporophyte. 138.
Cyclic : applied to an arrangement of leaves or floral parts in which

two or more appear upon the axis at the same level, forming a cycle,

or whorl, or verticil. 159.

Dehiscence : the opening of an organ to discharge its contents, as in
sporangia, pollen-sacs, capsules, etc. 199.

Dichotomous : applied to a style of branching in which the tip of the
axis forks. 35.

Dicecious : applied to plants in which the two sex-organs are upon dif-
ferent individuals. 115.

Dorsiventral : applied to a body whose two surfaces are differently
exposed, as an ordinary thallus or leaf. 109.

Egg : the female gamete. 16.

Egg-apparatus : in Angiosperms the group of three cells in the embryo-
sac composed of the egg and the two synergids. 204.
Elater : in Liverworts a spore-mother-cell peculiarly modified to aid

in scattering the spores. 103.
Embryo : a plant in the earliest stages of its development from the

spore. 137.
Embryo-sac : the megaspore of Spermatophytes, which later contains

the embryo. 178.
Endosperm : the nourishing tissue developed within the embryo-sac, and

thought to represent the female gametophyte. 180.
Endosperm nucleus : the nucleus of the embryo-sac which gives rise to

the endosperm. 205.
Entomophilous : applied to flowers or plants which use insects as agents

of pollination. 196.

GLOSSARY 331

Epigynous : applied to a flower whose outer parts appear to arise from

the top of the ovary. 225.
Eusporangiate : applied to those Pteridophytes and Spermatophytes

whose sporangia develop from a group of epidermal and deeper

cells. 157.

Family : a group of related plants, usually comprising several genera.

236.
Fertilization : the union of sperm and egg. 16.
Filament : the stalk-like part of a stamen. 197.
Fission: cell - division which includes the wall of the old cell.

10.
Foot : in Bryophytes the part of the sporogonium imbedded in the

gametophore ; in Pteridophytes an organ of the sporophyte embryo

to absorb from the gametophyte. 98, 138.

Gametangium : the organ within which gametes are produced. 11.

Gamete : a sexual cell, which by union with another produces a sexual
spore. 10.

Gametophore : a special branch which bears sex organs. 98.

Gametophyte : in alternation of generations, the generation which bears
the sex organs. 97.

Generative cell: in Spermatophytes the cell of the male gameto-
phyte (within the pollen grain) which gives rise to the male
cells. 180.

Genus : a group of very closely related plants, usually comprising sev-
eral species. 237.

Haustorium : a special organ of a parasite (usually a fungus) for ab-
sorption. 50.

Heterogamous : applied to plants whose pairing gametes are un-
like. 15.

Heterosporous : applied to those higher plants whose sporophyte pro-
duces two forms of asexual spores. 151.

Homosporous: applied to those plants whose sporophyte produces simi-
lar asexual spores. 151.

Host : a plant or animal attacked by a parasite. 48.

Hypha : an individual filament of a mycelium. 49.

Hypocotyl : the axis of the embryo sporophyte between the root-tip and
the cotyledons. 209.

Hypogynous : applied to a flower whose outer parts arise from beneath
the ovary. 224.

332 GLOSSARY

Indusium : in Ferns a flap-like membrane protecting a sorus. 143.

Inflorescence : a flower-cluster. 230.

Insertion : the point of origin of an organ. 224.

Integument : in Spermatophytes a membrane investing the nucellus.

178.
Involucre : a cycle or rosette of bracts beneath a flower-cluster, as in

Umbellifers and Composites. 275.
IsoCARPic : applied to a flower whose carpels equal in number the other

floral organs. 268.
Isogamous : applied to plants whose pairing gametes are similar. 15.

Leptosporangiate : applied to those Ferns whose sporangia develop
from a single epidermal cell. 157.

Male cell : in Spermatophytes the fertilizing cell conducted by the

pollen-tube to the egg. 180.
Megasporangium : a sporangium which produces only megaspores. 152.
Megaspore : in heterosporous plants the large spore which produces a

female gametophyte. 152.
Megasporophyll : a sporophyll which produces only megasporangia.

152.
Mesophyll : the tissue of a leaf between the two epidermal layers which

usually contains chloroplasts. 141.
Microsporangium : a sporangium which produces only microspores.

152.
Microspore : in heterospoi-ous plants the small spore which produces »

male gametophyte. 152.
Microsporophyll : a sporophyll which produces only microsporangia-

152.
Micropyle: the passageway to the nucellus left by the integument.

178.
Moncecious : applied to plants in which the two sex organs are upon

the same individual. 115.
Monopodial : applied to a style of branching in which the branches

arise from the side of the axis. 35.
Mother cell : usually a cell which produces new cells by internal divi-
sion. 9.
Mycelium : the mat of filaments which composes the working body of

a fungus. 49.

Naked flower : one with no floral leaves. 222.
Nucellus : the main body of the ovule. 178.

GLOSSARY

333

Oogonium : the female, egg-producing organ of Thallophytes. 16.

Oosphere : the female gamete, or egg. 16.

Oospore : the sexual spore resulting from fertilization. 16.

Ovary : in Angiosperms the bulbous part of the pistil, which contains

the ovules. 199.
Ovule : the megasporangium of Spermatophytes. 178.

Pappus : the modified calyx of the Composites. 278.

Parasite : a plant which obtains food by attacking living plants or ani-
mals. 48.

Pentacyclic : applied to a flower whose four floral organs are in five
cycles, the stamens being in two cycles. 268.

Perianth : the set of floral leaves when not differentiated into calyx
and corolla. 221.

Periqynous : applied to a flower whose outer parts arise from a cup
surrounding the ovary. 225.

Petal : one of the floral leaves which make up the corolla. 221.

Photosynthesis : the process by which chloroplasts, aided by light,
manufacture carbohydrates from carbon dioxide and water. 84.

Pistil : the central organ of the flower, composed of one or more car-
pels. 200.

Pistillate : applied to flowers with carpels but no stamens. 218.

Pollen : the microspores of Spermatophytes. 174.

Pollen-tube : the tube developed from the wall of the pollen grain
which penetrates to the egg and conducts the male cells. 180.

Pollination : the transfer of pollen from anther to ovule (in Gymno-
sperms) or stigma (in Angiosperms). 181.

Polypetalous : applied to flowers whose petals are free from one an-
other. 227.

Prothallium : the gametophyte of Ferns. 130.

Protonema : the thallus portion of the gametophyte of Mosses. 98.

Radial : applied to a body with uniform exposure of surface, and pro-
ducing similar organs about a common center. 120.

Receptacle : in Angiosperms that part of the stem which is more or
less modified to support the parts of the flower. 222.

Rhizoid : a hair-like process developed by the lower plants and by inde-
pendent gametophytes to act as a holdfast or absorbing organ, or
both. 109.

Saprophyte : a plant which obtains food from the dead bodies or body
products of plants or animals. 48.
40

334 GLOSSARY

Scale : a leaf without chlorophyll, and usually reduced in size.
161.

Sepal : one of the floral leaves which make up the calyx. 221.

Seta : in Bryophytes the stalk-like portion of the sporogoniuni. 98.

Sexual spore : one produced by the union of gametes. 10.

Species : plants so nearly alike that they all might have come from a
single parent. 237.

Sperm : the male gamete. 16.

Spiral : applied to an arrangement of leaves or floral parts in which
no two appear upon the axis at the same level ; often called alter-
nate. 193.

Sporangium : the organ within which asexual spores are produced (ex-
cept in Bryophytes). 10.

Spore : a cell set apart for reproduction. 9.

Sporogonium : the leafless sporophyte of Bryophytes. 98.

Sporophore : a special branch bearing asexual spores. 49.

Sporophyll : a leaf set apart to produce sporangia. 145.

Sporophyte : in alternation of generations, the generation which pro-
duces the asexual spores. 97.

Stamen : the microsporophyll of Spermatophytes. 174.

Staminate : applied to a flower with stamens but no carpels. 218.

Stigma : in Angiosperms that portion of the carpel (usually of the style)
prepared to receive pollen. 199.

Stoma (pi. Stomata) : an epidermal organ for regulating the communi-
cation between green tissue and the air. 141.

Strobilus : a cone-like cluster of sporophylls. 161.

Style : the stalk-like prolongation from the ovary which bears the
stigma. 199.

Suspensor : in heterosporous plants an organ of the sporophyte embryo
which places it in a more favorable position in reference to food
supply. 168.

Symbiont : an organism which enters into the condition of symbio-
sis. 79.

Symbiosis : usually applied to the condition in which two different
organisms live together in intimate and mutually helpful rela-
tions. 79.

Sympetalous : applied to a flower whose petals have coalesced.
227.

Syncarpous : applied to a flower whose carpels have coalesced.
226.

Synergid : in Angiosperms one of the pair of cells associated with the
egg to form the egg-apparatus. 204.

GLOSSARY 335

Testa : the hard coat of the seed. 184.

Tetracyclic : applied to a flower whose four floral organs are in four

cycles. 268.
Tetrad : a group of four spores produced by a mother-cell. 103.

Zoospore : a motile asexual spore. 10.

Zygomorphic : applied to a flower in which the parts in one or more

sets are not similar ; irregular. 229.
Zygote : the sexual spore resulting from conjugation. 15.

INDEX TO PLANT STRUCTURES

[The italicized numbers indicate that the subject is illustrated upon the page cited.
In such case the subject may be referred to only in the illustration, or it may be
referred to also in the text.]

Absorption, 299.
Acacia, 265.
Aconitum, 261.
Acorus, 2 19, 243.
Aetinomorphy, 228.
Adder's tongue : see Ophioglossum.
Adiantura, 143, 145.
jEcidiomyeetes, 50, 62.
^Ecidiospore, 66.
iEcidium, 66.
Agaricus, 68, 69.
Agave, 247.
Air pore : see Stoma.
Akene, 212, 213, 214, 276, 277.
Alchemilla, 225.
Alder : see Alnus.
Algae, 4, 5, 17.
Alisma, 210, 240.
Almond : see Prunus.
Alnus, 257.

Alternation of generations, 94, 129.
Amanita, 70.
Amaryllidaceae, 247.
Amaryllis family : see Amarylli-
daceae.
Ambrosia, 279.
Ament, 257.
Anaptychia, 81, 82.

Anemophilous, 181.
Angiosperms, 173, 195, 217.
Anisocarpae, 268.
Annulus, ISti, 146, 150.
Anther, 196, 197, 199.
Antheridium, 16, 99, 100, 112, 121,

133, 134, 161, 166.
Antherozoid, 16.

Anthoceros, 104, 105, 111, 116, 118.
Anthophytes, 172.
Antipodal cells, 202, 205, 208.
Antirrhinum, 228, 275.
Ant-plants, 90, 91.
Apical cell, 13 4.
Apical group, 283.
Apium, 267.
Apocarpy, 199. 222, 225.
Apocynaceae, 271.
Apocynum, 272.
Apogamy, 131.
Apospory, 132.
Apothecium, 79, 81, 82.
Apple : see Pirus.
Aquilegia, 198.
Araceae, 243.
Araliaceae, 267.
Araucaria, 190.
Arbor vitas : see Thuja.
Arbutus, 198 : see Epigaea.
Archegoniates, 101.
337

338

INDEX

Archegonium, 99, 100, 113, 114, 133,

135, 161, 167, 179.
Archesporium, 102, 104, 105, 146.
Archichlamydea?, 255.
Arctostaphylos, 269.
Areolae, 111, 114.
Arisaema, 243, 244.
Arnica, 275, 276, 278.
Aroids, 243.
Artemisia, 279.
Arum, 245.
Ascoearp, 58, 59.
Ascomycetes, 50, 57.
Ascospore, 59.
Aseus, 59.
Asexual spore, 9.
Aspidium, 130, 136, 144.
Assimilation, 302.
Aster, 279.
Astragalus, 265.
Atherosperma, 198.
Azalea, 270.

B

Bacillus, 76.

Bacteria, 21, 75. 76.

Balm : see Melissa.

Banana, 140.

Bark, 284, 289.

Basidiomycetes, 50, 68

Basidiospore, 69, 72.

Basidium, 69, 71.

Bean : see Phaseolus.

Bearberry : see Arctostaphylos.

Beech, 256.

Bellis, 279.

Berberis, 198.

Bidens, 278.

Beggar-ticks, 213.

Bignonia, 211.

Birch. 256.

Blackberry : see Rubus.

Black knot, 60.
Black mould, 52.
Blasia, 116.

Blueberry : see Vaccinium.
Blue-green algae, 6, 17.
Blue mould, 60.
Boiet is, 73, 74.
Botrychium, 145, 149.
Botrydium, 28.
Box elder, 234.
Bracket fungus, 72.
Brake: see Pteris.
Brassica, 261.
Bryophytes, 2, 93, 172.
Brown algae, 6, 32.
Bryum, 120, 124.
Buckeye, 235.
Butomus, 199.

Buttercup : see Ranunculus.
Buttercup family : see Ranuncu-
laceae.

C

Cabbage : see Brassica.

Calamus : see Acorus.

Calla-lily, 243.

Callithamnion, 43.

Callophyllis, 39.

Call una, 270.

Calopogon, 249.

Caltha, 260.

Cal yean thus, 226, 261.

Calypso, 249.

Calyptra, 102, 125.

Calyptrogen, 293.

Calyx, 220, 221.

Cambium, 285, 287, 288.

Capsella, 209, 293.

Capsule, 98, 123, 125, 126, 211, 212.

Caraway : see Carum.

Carbohydrate, 302.

Carbon dioxide, 83.

INDEX

339

Carnivorous plants, 92.

Carpel, 177, 178, 199, 219, 220.

Carpinus, 217, 258.

Carpospore, 44, 45.

Carrot : see Daucus.

Carum, 267.

Cassia, 265.

Cassiope, 269.

Castilleia, 275.

Catkin, 257.

Catnip : see Nepeta.

Cat-tail : see Typha.

Cattleya, 254.

Caulicle, 209.

Cauline, 166.

Cedar apple, 67, 68.

Celery : see Apium.

Cell, 6, 7.

Cellulose, 7.

Cercis, 265.

Chalazogamy, 258, 259.

Characeae, 46.

Chemotropism, 307.

Cherry : see Primus.

Chestnut, 256.

Chlorophyceae, 6, 21.

Chlorophyll, 5, 83.

Chloroplast, 7, 8.

Chrysanthemum, 279.

Cilia, 10.

Circinate, 136, 143.

Cladophora, 25.

Clavaria, 73.

Climbing fern : see Lygodium.

Closed bundle, 290.

Clover : see Trifolium.

Club mosses, 162.

Cnicus, 278.

Cocklebur : see Xanthium.

Coenocyte, 27.

Coleochaete, 106, 107.

Collateral bundle, 287.

Collenchyma, 284.

Columella, 104, 105, 106, 126.

Compass plant : see Silphium.

Composite, 275.

Composites, 275, 276, 277, 278.

Concentric bundle, 292.

Conferva forms, 22.

Conidia, 58, 60.

Conifers, 191, 282.

Conium, 267.

Conjugate forms, 31.

Conjugation, 15.

Connective, 196.

Conocephalus, 111.

Convolvulacete, 271.

Convolvulus forms, 270-

Convolvulus, 273.

Coprinus, 70.

Coral fungus, 73, 74.

Coreopsis, 278.

Coriandrum, 267.

Cork, 284.

Corn, 216, 282, 290.

Cornaceae, 267.

Corolla, 220, 221.

Cortex, 283, 284, 288.

Cotton, 206.

Cotyledon, 137, 138, 168, 184, 209,

210, 216, 217.
Cranberry: see Vaccinium.
Crataegus, 262.
Crocus, 249.
Crucifer, 262.
Cruciferae, 262.
Cryptogams, 172.
Cunila, 274.
Cup fungus, 60, 61.
Cupule, 112, 114.
Cyanophyceae. 6 17.
Cycads, 185, 186, 187, 189.
Cyclic, 159, 193.
Cyperaceae, 241.

340

INDEX

Cypripedium, 249, 253.
Cystocarp, 43, 44-
Cystopteris, 78, 144.
Cytoplasm, 7.

D

Daisy : see Bellis.

Dandelion : see Taraxacum.

Dasya, 40.

Datura, 197.

Daucus, 266, 267.

Dead-nettle, 228.

Definitive nucleus: see Endosperm

nucleus.
Dehiscence, 198, 199.
Delphinium, 260, 261.
Dermatogen, 283.
Desmids, 31, 32.
Desmodium, 308.
Diatoms, 45.
Dichotomous, 35.
Dicotyledons, 208, 233, 254, 282.
Differentiation, 3, 280.
Dogbane: see Apocynum.
Dog-tooth violet : see Erythronium.
Dogwood family : see Cornaceae.
Dorsiventral, 109.
Downy mildew, 55.
Drupe, 264.
Digestion, 302.
Dioecious, 115.
Disk, 276, 277.
Dodder, 86.

E

Ear-fungus, 74.
Easter lily, 221.
Ecology, 297, 311.
Economic botany, 297.
Ectocarpus, 33.
Edogonium, 22, 23.
Egg, 16, 202, 204, 205, 206.

Egg-apparatus, 204, 205, 206.

Elater, 103, 113, 118.

Elm : see Ulmus.

Embryo, 137, 167, 168, 170, 183, 207,

208, 209, 210, 211.
Embryo-sac, 178, 179, 201, 203, 208.
Endosperm, 179, 180, 207, 208, 211.
Endosperm nucleus, 202, 205.
Entomophilous, 196.
Epidermis, 141, 142, 191, 283, 284,

295.
Epigaea, 269.
Epigyny, 224, 225.
Epilobium, 212.
Epiphyte, 157.
Equisetales, 159.
Equisetum, 159, 160, 161.
Ergot, 60, 61.
Erica, 270.
Ericaceae, 268.
Erigenia, 267.
Erythronium, 250.
Eusporangiate, 157.
Evolution, 3.

F

Fennel : see Fceniculum.

Ferns, 155, 156.

Fertilization, 16, 181, 206, 207.

Festuca, 240.

Figwort family : see Scrophula-

riaceae.
Filament, 8, 196, 197.
Filicales, 155.
Fireweed : see Epilobium.
Fission, 10.
Flax : see Linum.
Floral leaves, 218.
Florideae, 38.
Flower, 218.
Flowering plants, 172.
Fceniculum, 267.

INDEX

341

Foliar, 166.

Food, 83, 299.

Foot, 98, 102, 137, 138, 168.

Fragaria, 214, 227, 262.

Fruit, 211, 212, 213, 214, 215.

Fucus, 35, 37.

Funaria, 99, 102, 121, 124, 125, 126.

Fungi, 4, 48.

G

Gametangium, 11.

Gamete, 10, 12.

Gametophore, 98, 112, 120, 124.

Garnetophyte, 97, 107, 132, 134, 161,

166, 167, 176, 179, 180, 201, 203,

204, 205.
Gaultheria, 270.
Gaylussacia, 269.
Gemma, 112, 114.
Generative cell, 180, 201.
Gentianaceae, 271.
Geophilous, 246.
Geotropism, 305.
Gerardia, 275.
Germination, 187, 214.
Gigartina, 38.
Gills, 71.
Ginkgo, 191.
Gladiolus, 249, 251.
Gleditschia, 236, 265.
Gloeocapsa, 17, 18.
Glume, 241.

Goldenrod : see Solidago.
Gonatonema, 31.
Gramineae, 241.
Green algae, 6. 21.
Green plants, 83.
Green slimes, 20.
Grimmia, 126.
Growth movement, 304.
Growth ring, 234.
Grain, 241.

Grasses, 240.

Grass family : see Gramineae.
Gymnosperms, 171, 173, 195.
Gymnosporangium, 67.

H

Habenaria, 249, 252.

Harebell, 228.

Haustoria, 50.

Hazel : see Carpinus.

Heart-wood, 289.

Heat, 314.

Heath family : see Ericaceae.

Heaths, 268, 269, 270.

HeliauLhus, 279, 285, 306.

Heliotropism, 305.

Hemiarcyria, 75.

Hemlock : see Conium.

Henbane : see Hyoscyamus.

Hepaticae, 109.

Heterocyst, 18.

Heterogamy, 15.

Heterospory, 151.

Hickory, 256.

Hippuris, 283.

Homospory, 151.

Honey locust : see Gleditschia.

Horehound : see Marrubium.

Hornbeam : see Carpinus.

Horsetail, 159.

Host, 48.

Huckleberry: see Gaylussacia.

Hydnum, 73, 74.

Hydra, 90.

Hydrophytes, 6, 315.

Hydrophytum, 91.

Hydrotropism, 307.

Hygroscopic movement, 304.

Hyoscyamus, 196.

Hypha, 49.

Hypocotyl, 184, 209, 216, 217.

342

INDEX

Hypodermis, 284.
Hypogyny, 224, 225.
Hyssopus, 274.

I

Indigo : see Indigofera.

Indigofera, 265.

Indusium, 136, 143, 144.

Inflorescence, 230.

Insects and flowers, 90.

Integument, 178, 179, 201, 202, 203.

Involucre, 267, 275, 277.

Ipomoea, 228, 270.

Iridaceae, 247.

Iris, 248, 251.

Iris family : see Iridaceae.

Irritable movement, 307.

Isocarpae, 268.

Isoetes, 169.

Isogamy, 15.

Japan lily, 248.

Jungermannia, 105, 115, 116, 117.

Juniper, 194-

K

Kalmia. 270.

Labiata?, 272.

Labiates, 272.

Lactuca, 279.

Laminaria, 33, 34-

Lamium, 274, 275.

Larch : see Larix.

Larix, 192.

Larkspur : see Delphinium.

Laurel : see Kalmia.

Lavandula, 275.

Leaf, 141, 142, 295, 296, 311.

Legumes, 250, 251, 264.

Leguminosae, 264.

Lemna, 201.

Lepidozia. 117.

Lepiota, 70.

Leptosporaugiate, 157.

Lettuce : see Lactuca.

Leucanthemum, 279.

Liatris, 278.

Lichens, 77, 78, 79, 87.

Life relations, 311.

Light, 314.

Ligule. 168.

Liliaceae, 246.

Lilies, 245.

Lilium, 203, 204, 205, 207, 224, 249,

295.
Lily: see Lilium.
Lily family : see Liliaceae.
Linaria, 228, 275.
Linum, 220.
Liverworts, 109.
Loculus, 200.
Locust : see Robinia.
Lotus, 264.
Lupinus, 265.
Lycopersicum, 275.
Lycopodiales, 162.
Lycopodium, 162, 163.
Lygodium, 145.
Lyonia, 269.

M

Macrospore, 152.

Maidenhair fern : see Adiantum.

Male cell, 180, 181, 201, 206, 207.

Maple, 212.

Marasmius, 70.

Marchantia, 104, 110, 111, 112, 113,

114.
Marguerite : see Leucanthemum.
Marjoram : see Origanum.
Marrubium, 275.
Marsh marigold : see Caltha.

INDEX

343

Marsilia, 158.

Megasporangium, 152, 177, 179.
Megaspore, 152, 165, 167, 179, 201,

203.
Megasporophyll, 152, 165, 177, 199.
Melissa, 275.
Mentha, 229, 274.
Meristem, 281.

Mesophyll, 141, 142, 191, 295.
Mesophytes, 324.
Mestome, 282.

Micropyle, 17S, 201, 202, 206.
Microspira, 76.
Microspha^ra, 5S.
Microsporangium, 152, 176, 197.
Microspore, 152, 165, 166, 179, 197,

201.
Microsporophyll, 152, 165, 174,196,

198.
Midrib, 234.
Mildews, 57.
Mimosa, 265, 308, 309.
Mint : see Mentha.
Mint family : see Labiatae.
Monocotyledons, 208, 232, 236, 289.
Monoecious, 115.
Monopodial, 35.
Monotropa, 270.
Moonwort : see Botrychium.
Morels, 60, 62.

Morning-glory : see Tpomcea.
Morphology, 297.
Mosses, 93, 119, 124.
Mother cell, 9.
Mougeotia, 31.
Movement, 303.
Mucor, 49, 52, 53, 54, 55.
Mullein : see Verbascum.
Musci, 119.
Mushrooms, 68.

Mustard family : see Cruciferae.
Mycelium, 49.

Mycomycetes, 50.
Mycorrhiza, 87, 88.
Myristica, 214.
Myrmecophytes, 90, 91.
Myxomycetes, 74, 75.

N
Naias, 237.
Narcissus, 247.
Nemalion, 43.
Nepeta, 275.
Nicotiana, 227, 275.
Nightshade family : see Solanaceae.
Nostoc, 18.

Nucellus, 178, 179, 201, 202, 203.
Nucleus, 7.
Nutation, 304.
Nutmeg, 214.
Nutrition, 3, 299.
Nyctitropic movement, 309.
Nyrnphaeaceae, 261.

O
Oak, 255, 256.

(Edogoniuin : see Edogonium.
Onoclea, 145, 147, 148.
Oogonium, 16.
Oosphere, 16.
Oospore, 16, 101.
Open bundle, 287.
Operculum, 122, 125.
Ophioglossum, 145, 149.
Orchidacea?, 249.
Orchids, 249, 252, 253, 254.
Orchid family: see Orchidacea?.
Origanum, 274.
Ornithogalum, 247.
Oscillatoria, 19.
Osmunda, 145, 156.
Ostrich fern : see Onoclea.
Ovary, 199, 200, 202.
Ovule, 178, 179, 201, 203.

344

INDEX

Palisade tissue, 142, 295.

Palmaceae, 241.

Palm family : see Palmaceae.

Palms, 241, gjg, 243.

Papaveraceae, 261.

Pappus, 276, 277, 27S.

Parasites, 48, 85.

Parenchyma, 280, 281, 282, 288.

Parmelia, 79.

Parsley: see Petroselinum.

Parsley family : see Umbellif era?.

Parsnip : see Pastinaca.

Parthenogenesis, 52.

Pastinaca, 267.

Pathology, 297.

Pea : see Pisum.

Peach : see Prunus.

Peach curl, 60.

Pea family : see Leguminosae.

Pear : see Pirus.

Peat, 119.

Pellaea, 146.

Penicillium, 60.

Pentacycke, 268.

Pentstemon, 275.

Peony, 220.

Pepper. 211, 258.

Pepper family : see Piperaceae.

Perianth, 219, 220, 221.

Periblem, 283.

Perigyny, 225, 226.

Peristome, 126, 127.

Peronospora, 55, 56.

Petal, 220, 221.

Petiole, 141.

Petroselinum, 267.

Phaeophyceae, 6, 32.

Phanerogams, 172.

Phaseolus, 216, 265.

Phloem, 285, 287, 288, 290, 292, 294.

Phlox, 228, 271.

Photosyntax, 84.

Photosynthesis, 84, 302.

Phycomycetes, 50, 51.

Physcia, 79.

Physiology, 297.

Picea, 179, 181, 182. "

Pileus, 71.

Pine : see Pinus.

Pineapple, 215.

Pinus, 173, 175, 176, 177, 178, 181,

183, 184, 188, 191, 286.
Piperaceae, 258.
Pirus, 225, 262, 263.
Pistil, 199, 200, 219, 220.
Pisum, 265.
Pith, 285, 287, 288.
Planococcus, 76.
Plantaginacea?, 275.
Plant body, 6.
Plant associations, 313.
Plasmodium, 74, 75.
Plastid, 7, 8.
Platycerium, 132.
Plerome, 283.
Pleurococcus, 21.
Plum : see Prunus.
Plumule, 210.
Pod, 211, 212.
Pogonia, 249.
Polemoniacea?, 271.
Polemonium, 271.
Pollen, 174, 176, 197, 201.
Pollen-tube, 179, 180, 181, 187, 202,

206, 207.
Pollination, 181.
Polyembryony, 183.
Polymorphism, 63.
Polypetaly, 226.
Polyporus, 71, 72.
Polysiphonia, 44-
Polytrichum, 96.

INDEX

345

Pome, 263.

Pondweeds, 237.

Poplars, 255.

Popowia, 198.

Poppy, 261.

Poppy family : see Papaveracea?.

Populus, 256.

Pore-fungus, 72.

Potamogeton, 237, 238.

Potato : see Solatium.

Potentilla, 225, 262.

Proteid, 302.

Prothallium, 130, 132, 134.

Protococcus forms, 22.

Protonema, 95, 98.

Protoplasm, 7.

Prunus, 2 13, 262.

Pseudomonas, 76.

Pseudopodium, 105, 123, 124.

Pteridophytes, 2, 128, 172, 291.

Pteris, 133, 134, 135, 137, 141, 1&,

143, 145, 281, 291, 292, 293.
Ptilota, 42.

Puccinia, 63, 64, 65, 66.
Puff-balls, 68, 74.
Pulvinus, 308.

Q
Quillwort : see Isoetes.

R

Rabdonia, 41-
Radiate bundle, 294.
Radicle, 209.
Radish, 120.

Ragweed : see Ambrosia.
Ranunculaceas, 261.
Ranunculus, 222. 259.
Raspberry : see Rubus.
Rays, 275, 276.
Receptacle. 222.
Red algae, 6, 38.

Redbud : see Cercis.

Redwood : see Sequoia.

Reproduction, 3, 8, 309.

Respiration, 302.

Rheotropism, 307.

Rhizoid, 109, 110, 134.

Rhizophores, I64.

Rhododendron, 270, 271.

Rhodophyceas, 6, 38.

Riccia, 104, 110.

Ricciocarpus, 110.

Ricinus, 288.

Robinia, 265.

Root, 138, 217, 293, 294, 313.

Root-cap, 293.

Root-fungus, 87, 88.

Root-hairs, 217, 300.

Root-pressure, 300.

Root-tubercles, 89.

Rosacea?, 262.

Rose family : see Rosacea?.

Rosin-weed : see Silphium.

Rosmarinus, 275.

Royal fern : see Osmunda.

Rubus, 262.

Rumex, 284.

Rust, 62, 63, 64, 65, 66.

S
Sac-fungi, 57.
Sage : see Salvia.
Sage-brush : see Artemisia.
Sagittaria, 208, 338.
Salix, 219, 233, 256, 257.
Salvia, 275.
Salvinia, 158.
Saprolegnia, 51, 52.
Saprophyte, 48, 84.
Sap-wood, 289.
Sargassum, 35. 36.
Saururus, 219, 258.
Scales, 161.

346

INDEX

Scapania, 116.
Schizomycetes, 21.
Schizophytes, 21.
Sclerenchyma, 281, 282, 284, 285,

288, 290, 291.
Scouring rush, 159.
Scrophulariacea?, 275.
Scutellaria, 275.
Sedge family: see Cyperaceae.
Seed, 183, 184, 210, 211, 212, 214.
Selaginella, 162, 164, 165, 166, 168.
Sensitive fern : see Onoclea.
Sensitive-plant : see Acacia.
Sepal, 220, 221.
Sequoia, 189.
Seta, 98, 125.
Sex, 12.

Sexual spore, 10.
Shepherd's purse : see Capsella.
Shield fern : see Aspidium.
Shoot, 312.

Sieve vessels, 285, 286.
Silphium, 279.
Siphon forms, 27.
Siphonogams, 183.
Siphonogamy, 183.
Slime moulds, 74, 75.
Smut, 62.

Snapdragon : see Antirrhinum.
Soil, 314.
Solanaeea^ 275.
Solanum, 198, 275.
Solidago, 279.
Solomon's seal, 233.
Sorus, 136, 143, I44.
Spadix, 244, 245.
Spathe, 244, 245.
Sperm, 16, 100, 133, 135, 162, 166,

169, 187, 190.
Spermatia, 43, 44.
Spermatophytes, 2, 171, 172.
Spermatozoid, 16.

Sperm mother cell, 100.

Sphagnum, 105, 106, 122, 123.

Spike, 240.

Spirasa, 262.

Spiral, 193.

Spirillum, 76.

Spirogyra, 28, 29, 30.

Spongy tissue, 142.

Sporangium, 10, 136, 143, 145, 150,

157, 163, 179.
Spore, 9.
Sporidium, 65.
Sporogenous tissue, 103.
Sporogonium, 98, 102, 104, 105, 106,

125, 126.
Sporophore, 4$, 50.
Sporophyll, 145, 147, 148, 149, 174,

176.
Sporophyte, 97, 102, 137.
Spruce : see Picea.
Stability of form, 298.
Stamen, 174, 176, 196, 198, 219,

220.
Stele, 191, 283, 285.
Stem, 139, 282, 289, 291, 312.
Stemonitis, 75.
Stereome, 282, 299.
Sterile tissue, 103.
Sticta, 80.
Stigma, 199, 202.
Stomata, 141, I42, 191, 295, 301.
Strawberry : see Fragaria.
Strobilus, 160, 161, 163, 165, 174,

175, 176, 193, 194.
Style, 199, 202.
Substratum, 49.
Sumach, 235.

Sunflower : see Helianthus.
Suspensor, 167, 168, 183, 209,

210.
Symbiont, 79, 86.
Symbiosis, 79, 86.

INDEX

347

Sy in petals?, 268.
Synipetaly, 226, ">27.
Symplocarpus, ',i43.
Syncarpy, 199, 219, 225.
Synergid, 202, 2'>4, 205, 206.

T

Tanacetum, 279.
Tansy : see Tanacetum.
Taraxacum, 213, 277, 278.
Taxonomy, 297.
Teleutospore, 64, 65.
Tension of tissues, 298.
Testa, 184, 211.
Tetracycla\ 268.
Tetrad, 103.
Tetraspore, 43.
Teucrium, 230, 274, 275.
Thallophytes, 2, 4, 172.
Thermotropism, 307.
Thistle : see Cnicus.
Thorn apple : see Datura.
Thuja, 193.
Thymus, 274.
Tickseed : see Coreopsis.
Tissues, 280.
Toad-flax : see Linaria.
Toadstools, 68.
Tobacco : see Nicotiana.
Tomato : see Lycopersicum.
Tracheae, 285, 286.
Tracheids, 286.
Transfer of water, 300.
Transpiration, 301.
Tree fern, 140.
Trichia, 75.
Trichogyne, 43, 44.
Trillium, 207, 246, 265.
Truffles, 60.
Turgidity, 298.
Typha, 239, 240.

D

Umbel, 266, 267.
Uinbelliferae, 266.
Umbellifers, 266.
Ulmus, 210, 256.
Ulothrix, 12, 13, 22.
Uredo, 64.
Uredospore, 63, 64.

Vaccinium, 269.

Vascular bundle, 232, 234, 287, 291.

Vascular cylinder, 234, 287.

Vascular system, 129, 139.

Vaucheria, 26, 27, 28.

Vegetative multiplication, 9.

Veins, 141, 142.

Venation, 233.

Verbascum, 275.

Verbenacea?, 275.

Vernation, 143.

Vernonia, 279.

Veronica, 275.

Vicia, 265.

Violet, 211, 229.

W

Wall cell, 180.

Walnut, 256.

Water, 83, 314.

Water ferns, 158.

Water-lily, 223, 261.

Water-lily family : see Nymphaea-

ceae.
Water moulds, 51.
Wheat rust, 63, 64, 65, 66.
Willow : see Salix.
Wind, 315.
Wintergreen : see Gaultheria.

348

INDEX

Wistaria, 265.
Witches'-broora, 60.
Wormwood : see Artemisia.

X

Xanthium, 279.

Xerophytes, 319.

Xylem, 285, 287, 288, 290, 292, 294.

Yeast, 62.

Zannichellia, 237.
Zoospore, 10.
Zygomorphy, 228,
Zygospore, 15.
Zygote, 15.

THE END

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